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Perspective  |  Open Access  |  10 Aug 2021

Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies

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Ageing Neur Dis 2021;1:6.
10.20517/and.2021.06 |  © The Author(s) 2021.
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The role of norepinephrine (NE) in the pathogenesis of Parkinson’s disease (PD) has not been well investigated until recently. The purpose of this perspective article is to review evidence supporting the idea that dysfunction of the locus coeruleus (LC)/NE system in the brain may be fundamentally linked to the pathogenesis of PD. Compelling evidence demonstrates that loss of NE neurons in the LC is sufficient to initiate chronic neuroinflammation, resulting in a progressive and sequential loss of neuronal populations in the brain. This article summarizes the critical role of both microglial and neuronal NADPH oxidase 2 (NOX2), the superoxide and reactive oxygen species generating enzyme, as an important regulator of chronic neuroinflammation. Moreover, NOX2 inhibitors show high efficacy in halting chronic neuroinflammation, oxidative damage, and neurodegeneration in several animal PD models. This line of research offers a promising disease-modifying therapeutic strategy for PD.


Parkinson’s diseases, progressive neurodegeneration, chronic neuroinflammation, locus coeruleus, noradrenergic system, motor/nonmotor symptoms


Parkinson’s disease (PD) is a neurological disorder characterized by progressive neurodegeneration in the nigrostriatal system, resulting in the development of progressive movement disorders[1]. Pathological examination revealed cytoplasmic inclusions known as Lewy bodies or Lewy neurites in the survival dopaminergic (DA) neurons[2,3]. About 15% of PD cases occur in familial clusters at early age[4], which are attributed to mutations in genes, including parkin, leucine-rich repeat kinase 2, and α-synuclein[5]. By contrast, the remaining PD cases are sporadic and may represent the final outcome of a complex set of interactions among the innate vulnerability of DA system, genetic predisposition, and environmental toxins exposure[6]. Exposures to infectious agents, pesticides, or heavy metals in humans increase the risk of acquiring PD[7-13]. We and others have proposed that exposures to these risk factors trigger neuroinflammation, which plays a key role in the pathogenesis of PD[14]. However, this concept has not been proved until recently[15,16].

Microglia and astroglia are the two major types of glial cells involved in the initiation and maintenance of neuroinflammation. Microglia, the resident macrophages in the brain[17], play critical roles in the programmed elimination of neural cells in the early stage of neuronal development[18,19]. As the brain’s main immune cells, microglia can rapidly be activated in response to brain injuries and immunological stimuli[20-23]. Activated microglia undergo morphological and functional changes[20] and increase the expression of many surface molecules[24,25]. Activated microglia release a variety of immune factors to recruit more cells and phagocytize foreign substances. In normal physiological conditions, microglia exerts beneficial functions in immune surveillance and depletion of noxious stimuli. By contrast, in pathological conditions, such as chronic inflammation in the brain, microglia can cause neurotoxicity and significantly lead to neurodegeneration.

Different from microglia, astroglia are not derived from immune cell lineage, but they are essential to the integrity and function of the brain[26]. Besides serving as an component of the blood-brain barrier (BBB), astroglia provide physical support and nutrition to neurons, buffer excess neurotransmitters, and maintain ionic homeostasis[26]. Astroglia also become activated under immunologic challenges or brain injuries[27,28]. Activated astroglia secrete a host of neurotrophic factors, such as BDNF, GDNF, and NGF[29,30], which are crucial for the survival of neurons. It has been reported that many anticonvulsant drugs exert potent neuroprotection through astroglia-derived neurotrophic factors[31]. These findings suggest that astroglia are promising targets for developing novel therapies for PD.

Interactions among microglia and astroglia are an important yet not fully studied area. It was found that, in response to immunologic challenges, activation of astroglia often depends on the presence of microglia. Secreted immune factors from prior activated microglia can act and turn astroglia into different phenotypes depending on the immune conditions. In physiological condition, increased release of neurotrophic factors, such as GDNF, BDNF, and NGF, benefits neuronal survival[32,33]. By contrast, neurotoxic reactive astrocytes (A1 astroglia) induced by activated microglia can exaggerate neurotoxicity in pathological condition[34]. Since the role of astroglia in inflammation-related neurodegeneration is less well-documented, this review mainly focuses on the role of microglia in neuroinflammation and neurodegeneration.

Scope of this article

Recent research revealed that low-grade, chronic neuroinflammation is a key to cause progressive neurodegeneration[35,36]. However, the detailed mechanisms involved in the onset and maintenance of chronic neuroinflammation and related neurodegeneration still require additional studies. Emerging evidence suggests critical roles of central norepinephrine (NE) in the pathogenesis of disease progression and manifestations of a variety of nonmotor dysfunctions in PD patients. Thus, this perspective article focuses on the following three aspects.

Neuroinflammation-based rodent PD models

We review several toxin-elicited PD models, which show some of the cardinal characteristics of observed in PD patients, such as chronic neuroinflammation, sequential neurodegeneration, and progressive motor and nonmotor dysfunction.

Roles of central NE in neuroinflammation

Based on common features observed from inflammation-based animal models, we discuss immune factors involved in the initiation and maintenance of low-grade neuroinflammation. The possibility that the loss of locus coeruleus-norepinephrine (LC/NE) neurons may be the focal initiating point in producing a similar pattern of progressive caudal-rostral degeneration by various toxins is evaluated. Furthermore, cellular and molecular mechanisms underlying chronic neuroinflammation-induced progressive neurodegeneration are discussed.

Molecular mechanisms of anti-inflammatory and neuroprotective functions of NE

Anti-inflammatory therapy for neurodegenerative diseases has been emerging as a promising disease-modifying therapeutic strategy. We review the current status in the development of PD therapies by focusing on drugs that affect the NE system.


Disease progression in PD patients

One of the cardinal characteristics of PD is the progressive nature. However, the mechanism of PD progression remains unclear. Currently, PD therapies are limited to symptoms relief, while disease-modifying therapies aimed at stopping PD progression are still lacking. The understanding of PD progression has been greatly facilitated by both basic and clinical research. Braak’s group was the first to document a caudal-rostral pattern of disease progression[37]. In PD patients, neuronal loss starts from the lower brain (raphe nucleus, LC, and olfactory bulb) and gradually affects the higher centers of the brain[38]. Further studies uncovered that peripheral inflammation occurs years before PD patients show movement dysfunction. The proposed route of disease progression originating from the gut and spreading to the brain fits well with the symptom progression of PD patients[39]. Before symptoms of movement disorder are observed, gut dysfunction, such as constipation and other premotor disorders, including smell loss, sleep disorder, and other autonomic dysfunction, are often found in patients with PD. Recently, creating animal models mimicking the pattern of neurodegeneration observed in PD patients and investigating its underlying mechanisms has become a widely pursued research area.

Role of neuroinflammation in disease progression

Accumulating evidence strongly indicates that brain inflammation plays a critical role in progressive neurodegeneration. Both gene-mutated and toxin-induced animal PD models have been generated to investigate neurodegenerative pattern and associated motor and nonmotor behavioral changes. This perspective article focuses on only a few commonly accepted toxin-elicited animal PD models, which are inflammation based and show some of the cardinal progressive features observed in PD patients.

LPS model

Peripheral inflammation induces neuroinflammation and neurodegeneration

Most rodent PD models, including those generated by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) or 6-hydroxydopamine, display acute toxicity within days but fail to recapitulate the delayed, progressive pattern of DA neurodegeneration. To investigate whether chronic neuroinflammation plays a role in the progressive neurodegeneration of PD, several environmental risk factors implicated in the pathogenesis of PD (e.g., pesticides, herbicides, and infectious agents) and whether they could recapitulate the delayed, progressive nature of PD have been determined in rodents by our lab and others[35,40,41]. Gram-negative bacterial endotoxin LPS is one of the commonly used toxins. Following a systemic injection of lipopolysaccharide (LPS, 5 mg/kg), mice show delayed, progressive neurodegeneration of DA neurons[35]. Further studies indicate that this model not only provides an excellent tool for studying the role of neuroinflammation-related neuronal damage but also serves as a useful platform for exploring drug therapies in PD. Clinically, the relevance of the LPS model for PD is supported by several case reports, in which a significant correlation between infections and the risk of developing PD was found[42].

To investigate the role of gene-environment interactions in the etiology of PD, we created an accelerated rodent PD model by LPS in transgenic mice over-expressing mutant human α-synuclein (A53T). After a single intraperitoneal injection of LPS (1 mg/kg) in seven-month-old male mutant A53T mice (Tg mice) and wild type controls (WT mice), the delayed, progressive degeneration of nigral DA neurons was observed in Tg mice, but not in WT mice[43]. After five months of LPS treatment, Tg mice lost more than half of their nigral DA neurons, while the striatal TH levels were reduced by a comparable degree. By contrast, LPS-induced neuronal damage was not observed in WT mice or saline-injected Tg mice. These results demonstrate synergistic neurotoxicity of LPS and A53T α-synuclein overexpression, thus strongly indicating the critical role of gene-environment interactions in PD. Selective DA degeneration was assessed by immunofluorescence double-labeling with antibodies against TH and Neu-N[43]. An about 52% decrease in nigral DA neurons was found in LPS-injected Tg mice, whereas only 9.2% of non-DA neurons were lost. Collectively, this two-hit PD model recapitulated the signature lesion of PD by its chronic, progressive, and selective neurodegeneration of nigral DA neurons.

LPS-elicited chronic neuroinflammation exerts progressive ascending neurodegeneration and behavioral changes

The involvement of neuroinflammation in the pathogenesis of PD was identified decades ago. Positron emission tomography (PET) imaging reveals prominent and heterogeneous neuroinflammation in the brains of patients with PD[44,45]. Strong evidence indicates that LPS-induced chronic neuroinflammation is sufficient to not only induce nigral DA neurodegeneration[35,46,47] but also drive progressive loss of other vulnerable neuronal populations outside the basal ganglia. Mechanistically, LPS-generated sub-lethal septicemia in the periphery is able to activate microglia, resulting in low-grade chronic neuroinflammation in the brain for the remaining lifetime of the mice[35]. The pattern of delayed neurodegeneration in this model is dissimilar to that of the intracranial LPS model that produces acute neurodegeneration. Chronic neuroinflammation elicited by systemic LPS injection enables steady production of cytotoxic factors to damage bystander neurons. In turn, damaged/dying neurons can re-active neighboring microglia through the release of danger-associated molecular pattern (DAMP), forming a self-propelling cycle that eventually leads to sustained neuronal damage[48] .

How does a single intraperitoneal injection of LPS induce long-lasting brain inflammation and progressive neuronal loss?

An intriguing question arises: why can a single injection of LPS produce such robust and long-lasting effects in the brain, since the half-life of LPS in mouse blood is only approximately 12 h[49]? It is well-documented that, under the physiological conditions, very minimal LPS can enter to the brain due to the poor passage through BBB[50]. Therefore, LPS-induced neuroinflammation appears to be an indirect effect. Studies showed that a single intraperitoneal LPS injection initially produced large amounts of proinflammatory cytokines or chemokines from Kupffer cells, the resident macrophage-like cells in the liver[51]. We found that the levels of cytokines in blood were greatly elevated at early times but declined to basal levels by 6-9 h. Remarkably, proinflammatory levels and microglial activation sustained in the brain for up to 10 months. Further mechanistic studies revealed that blood immune factors can pass through BBB[52]. After entering to brain parenchyma, these proinflammatory factors can activate microglia to continually produce more cytokines, reactive oxygen, and nitrogen species and other cytotoxic factors[35]. These microglia-generated toxic factors cause neuronal damage to release DAMPs, which further reactivate microglia. Through this process, a vicious cycle is formed to maintain neuroinflammation and cause additional neuronal loss [Figure 1].

Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies

Figure 1. How does a single ip injection of LPS induce long-lasting brain inflammation and produce progressive neuronal loss? LPS reaches the liver via the portal vein circulation and causes it to secrete large amounts of various cytokines, such as TNFα. Some of these cytokines can pass through BBB by receptor-mediated mechanisms to activate microglia and produce additional proinflammatory factors to cause neuron damage. DAMP released from damaged neurons further reactivate microglia to from a self-propelling vicious cycle to maintain chronic neuroinflammation and lead to neurodegeneration. (This figure was modified from our previous paper with permission[136]).

To further investigate the mechanism of transition of chronic neuroinflammation from the periphery to the brain, mice were administered with TNFα peripherally. The results show that both TNFα and LPS injections elevated the production proinflammatory factors (TNFα, MCP-1, and IL-1β) in the brain. Further, mice deficient in TNFR1/R22/2 receptors failed to show brain neuroinflammation in response to LPS and TNFα challenges, supporting that TNFα was one of the critical factors in bridging inflammation from the periphery to the CNS. Long-lasting and enhanced microglial activation, indicated by the immunohistochemical analysis of brain sections with anti-Iba-1 or anti-CD11b antibodies, was observed in brain regions, such as the SN, hippocampus, and motor cortex. The LPS-elicited long-lasting inflammatory and neurotoxic effects in the brain were consistent with previous findings. Exposed to MPTP, a selective DA neurotoxicant, in humans[53] and monkeys[54], led to sustained microglial activation up to years after exposure. In addition, in utero LPS exposure during a critical window of development (E11) rendered a 30% loss of nigral DA neurons in offspring at the age of seven months[55]. Taken together, these findings suggest that early or brief exposure to toxins/toxicants can induce chronic, self-propelling neuroinflammation and lead to progressive neurodegeneration.

Rotenone model

Rotenone, a previously widely used pesticide, reproduces Parkinsonism associated with increased risk for PD. Since the first publication by Greenamyre et al.[8], rotenone has been commonly used as a tool to create a rodent PD model[8]. Chronic rotenone exposure in rodents induces key features of Parkinsonism[56]. Mechanistically, rotenone is believed to impair mitochondrial complex I[57,58] and microtubule-based transport of neurotransmitter vesicles[59,60]. Although the role of mitochondrial complex I deficits has been demonstrated in rotenone-induced Parkinsonism[8,56], inhibition of mitochondrial complex I appears not to be the only mechanism for rotenone-induced DA degeneration[61]. A mouse strain lacking functional Ndufs4, a gene encoding a subunit required for complete assembly and function of complex I, has been used to further address this issue. Genetic ablation of Ndufs4 gene suppressed complex I activity but did not affect DA neuron survival in midbrain cultures prepared from E14 mice[61].

The involvement of microglia in mediating rotenone-elicited neurotoxicity has also been reported. In midbrain neuron and glia cultures, rotenone showed much higher potency in reducing the survival of DA neurons than that in neuron-enriched cultures[62]. Further studies revealed that microglial NADPH oxidase 2 (NOX2)-derived superoxide markedly exacerbated DA degeneration in rotenone-treated cultures[63], suggesting that microglial NOX2 is an alternative target of rotenone. This finding was further confirmed by a study showing that rotenone directly interacted with the catalytic gp91phox subunit of NOX2[64].

The involvement of microglia in generating NOX2-dependent superoxide in rotenone-treated neuron-glial cultures further suggests a critical role of neuroinflammation in rotenone-induced neurotoxicity. Indeed, a recent report indicated that daily intraperitoneal injections of rotenone for three weeks produce microglia-dependent neuroinflammation in mouse brain[65,66]. Moreover, not only was neuronal loss observed in the SN area, but it also showed a greater loss of LC/NE neurons[65]. Mechanistic studies unraveled that the integrin Mac1/NOX2 complex is a major pathway coupling the production of superoxide and neuroinflammation in rotenone-treated mice[65]. These results provide a novel insight into the pathogenesis of rotenone-induced neurodegeneration. However, how the microglial NOX2 activation is related to the inhibition of mitochondria dysfunction in the rotenone PD model is an interesting but not yet studied question.

Paraquat/maneb model

Paraquat and maneb, two pesticides used in agriculture, are commonly used in many of the same crops. Epidemiological studies revealed an increased risk of PD in human when exposed to combined paraquat and maneb compared with either alone. Paraquat and maneb cotreatment has been widely employed to model PD in rodents. Systemic administration of combined paraquat and maneb led to synergistic damage to nigrostriatal DA neurons and reduction of motor activities in mice[41]. In addition to the DA system, paraquat and maneb co-exposure also damaged neurons in other brain regions and followed a time-dependent ascending neurodegenerative pattern. We recently reported that paraquat and maneb co-treated mice displayed loss of LC/NE and nigral DA neurons at four weeks after exposure, which was two weeks earlier than that of hippocampal and cortical neurodegeneration[40,67,68]. Consistent with sequential neurodegeneration, paraquat and maneb co-exposure induced gait abnormality and cognitive decline in mice at four and six weeks after treatment, respectively[40,67]. Interestingly, inhibition of microglial activation and production of inflammatory factors by targeting CD11b, the α-chain of Mac-1, or NOX2 significantly mitigated combined paraquat- and maneb-induced neurodegeneration and behavioral abnormalities in mice[40,68,69]. Furthermore, attenuated neuronal damage in paraquat- and maneb-treated mice was also observed once these mice were co-administered with taurine, a major intracellular free β-amino acid with potent anti-inflammatory capacity[67,70,71]. Altogether, these findings suggest that microglia-mediated neuroinflammation contributes to progressive neurodegeneration in this two-pesticide-induced mouse PD model.


Chronic neuroinflammation and progressive neurodegeneration can be generated by various toxins with different modes of action

Animal PD models described in previous section are generated by a variety of toxins with different chemical structures and modes of action. In general, neuroinflammation can be generated by two ways: (1) agents that are infectious, such as microorganism or endotoxins; and (2) chemicals that are not infectious, such as rotenone, paraquat/maneb, or DSP-4, a selective NE neurotoxicant (see below). Despite their differences in initiating neuroinflammation, these toxins somehow produced a similar pattern of neurodegeneration. The pattern of ascending caudal-rostral neurodegeneration generated by a single systemic injection of LPS or DSP-4, (or repeated injections of rotenone) is of the utmost importance for two reasons: (1) it resembles the pattern of neurodegeneration observed in PD patients; and (2) it indicates that a common mechanism is operative to drive a similar pattern of neurodegeneration produced by various toxins, even if they are different in chemical structures and modes of action. Elucidation of this common pathway would greatly advance our understanding of the etiology and pathogenesis of PD. Therefore, rodent PD models generated by LPS (infectious) or DSP-4 (non-infectious) could be useful to investigate possible mechanisms underlying the similar ascending sequential pattern of neurodegeneration induced under different pathological conditions.

Loss of LC/NE neurons is the focal point in producing similar patterns of progressive caudal-rostral degeneration by various toxins

As mentioned above, despite high chemical disparity and toxicological actions, exposure to various toxins/toxicants produces similar patterns of neurodegeneration in mouse brain. Immunochemical analysis reveals a sequential caudal-rostral fashion: neuronal degeneration is first found in the brain stem region, such as LC, followed by neurons in the SN and thalamus, and lastly observed in the hippocampus and cortical regions[36,65,67,72]. Based on these observations, as well as our previous reports indicating anti-inflammatory and neuroprotective functions of NE[73], a logical hypothesis was proposed that loss of LC/NE neurons may be the critical focal point for producing similar patterns of progressive caudal-rostral degeneration by various toxins. Recent progress in this area of research has greatly advanced our understanding of the roles of NE in neurodegenerative diseases, particularly in PD. We review evidence supporting this hypothesis and discuss potential clinical implications of NE dysfunction in PD below.

NE deletion by DSP-4 elicits progressive neurodegeneration

The early loss of LC/NE neurons induced by LPS suggests a possibility that depletion of central NE is a key for progressive neurodegeneration in this neuroinflammatory PD mouse model and even possibly in PD patients. To test this hypothesis, the NE-depleting toxin DSP-4 was used. A single injection of DSP-4 (50 mg/kg; ip) reduced tissue levels of NE (ranging from 55% to 80%) one day after injection in NE-innervated regions, such as the midbrain, motor cortex, and hippocampus. Brain NE levels remained significantly reduced for up to four months, but they slowly returned to normal by 10 months post injection. Depletion of brain NE levels was accompanied by a time-dependent sequential loss of neurons: as expected, a more than 60% decrease in LC neurons was found one day after DSP-4 treatment. Time-dependent decreases in nigral DA neurons were observed at 4, 7, and 10 months after DSP-4 injection, in comparison to age-matched vehicle controls[36,72]. DSP-4 also led to reduction of Neu-N-positive neurons in the motor cortex and hippocampus, but not in caudate/putamen and ventral tegmentum area 10 months later [Figure 2]. DSP-4-induced neurodegeneration was accompanied by decreased metabolism of glucose detected by PET imaging with [18F]-FDG. The reduced glucose levels were observed in the olfactory bulb, thalamus, hindbrain, midbrain, hippocampus, and across all cerebral cortices at 10 months in DSP-4 injected mice[36], implicating putative neurodegeneration in these brain regions. Again, it is interesting to note that no change of glucose utilization was observed in the cerebellum or the caudate/putamen.

Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies

Figure 2. DSP-4 injection causes progressive neuronal loss along the gut-brain axis. DSP-4-induced chronic inflammatory models display progressive ascending neuronal loss along a caudal-rostral axis, which recapitulates the spatiotemporal order of neurodegeneration in PD. Furthermore, the colon is an early site affected after injection with DSP-4[137]. α-synuclein pathology and enteric neuronal loss were initially found in the large intestine at one month, while neurodegeneration in the brain was observed a few months later, indicating progressive neurodegeneration occurs along the gut-brain axis.

One salient finding of these studies is that the pattern of neurodegeneration in both LPS and DSP-4 models approximate the spatiotemporal progression of neuronal loss in PD. Following the degeneration of LC/NE neurons, both models show significant loss of DA neurons in the SN, yet without affecting DA-neurons in the VTA region. Cortical[74,75] and hippocampal atrophy[76,77], which are often observed in the late-stages of PD, were also found months after LPS or DSP-4 injection. In agreement with neurocircuit degeneration, both LPS- and DSP-4-injected mice displayed behavioral dysfunction, including motor deficits[35] and a variety of nonmotor phenotypes[72] [Figure 2].

These findings approximate the neurodegeneration found in PD patients. The selective neurodegeneration pattern revealed a strong correlation between the concentration of NE and the vulnerability of the intrinsic neurons in LC/NE neuron-innervated regions in response to different toxins/toxicants, such as LPS, DSP-4, rotenone, paraquat, etc.[36,65,67,72]. Together, these findings strongly suggest that loss of LC/NE neurons play a pivotal role in producing a similar pattern of progressive caudal-rostral degeneration.

Comparison of LPS, DSP-4, rotenone, and paraquat/maneb models

Different toxins produce neurodegeneration with distinct modes of action. However, based on the initial cell types targeted, most toxins used for modeling PD can be generally classified into three groups.

Cell non-autonomous mechanism

Pathogen associated molecular pattern agents such as microorganisms, endotoxins, or proinflammatory cytokines belong to this class. The primary target cells are microglia in the CNS. Upon the activation of microglia, large amounts of cytokines are released and produce a high degree of acute neuroinflammation to combat the infectious agents. However, over-production of immune factors also causes collateral bystander neuronal damage. Subsequent release of DAMP substances from injured neurons in turn triggers reactive microgliosis through the activation of the MAC-1 receptor, which further activates microglial NOX2, increases the production of superoxide/ROS, and causes additional inflammation and neuronal death. Thus, a vicious cycle becomes operative to cause delayed and progressive neurodegeneration[78] [Figure 3].

Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies

Figure 3. Comparison of LPS, DSP-4, rotenone, and paraquat/maneb. This figure illustrates that toxins produce neurotoxicity through different mechanisms: (1) LPS by activating microglia; (2) DSP-4 by directly damaging LC /NE neurons; and (3) rotenone and paraquat/maneb by exert directing neurotoxicity in high concentrations while in lower concentrations causing activation of microglia. However, between neuronal damage and reactivation of microglia, eventually, these toxins all generate a self-propelling vicious cycle to keep chronic neuroinflammation continued and drive progressive. (This figure was modified from our previous paper with permission[138]).

Cell autonomous mechanism

Common toxins used in animal PD models such as MPTP, 6-hydroxydopamine, and the aforementioned DSP-4 belong to this class. Initially, these toxins are selectively taken up by neurons and directly cause neuronal damage. Different from the LPS model, these direct-acting toxins usually cause neuronal loss within days without causing acute inflammation during the initial stage. If the damaged neurons are able to secrete enough DAMP to trigger reactive microgliosis, then the vicious cycle will start and drive inflammation-based progressive neurodegeneration[78] [Figure 3].

Mixed-mode mechanism

Many environmental risk factors, such as pesticides, herbicides, fungicides, and heavy metals, display mixed modes of action in causing neurodegeneration. In vitro studies revealed that mixed-mode agents may target different cell types depending on toxin concentrations. Rotenone serves as a prototype agent for illustrating this class of toxins. In high concentrations, rotenone can directly damage neurons in neuron-enriched cultures by inhibiting mitochondrial complex I. By contrast, rotenone at low concentrations is not sufficient to directly damage neurons, but it exerts neurotoxicity through microglial activation in neuron-glial cultures[62]. Microglia-dependent neurotoxicity of rotenone has also been reported in an animal study[65] [Figure 3].

Prolonged microglial activation plays a key role in DSP-4-elicited neurotoxicity

A consistent pattern of progressive, ascending, and sequential loss of brain neurons was found in different models of NE-deficient mice, which is similar to that of LPS-treated mice. These findings align with the idea that loss of LC/NE neurons could play a key role in the subsequent neuron loss in other brain regions. To address this question, the time course study of microglial activation after DSP-4 injection was performed. Immunocytochemical analysis using CD11b, a marker for microglial activation, revealed that DSP-4 induced time-delayed microglial activation. Enhanced CD11b-immunoreactivity was not observed until seven days after injection, peaked at two weeks, and remained elevated for up to ten months in NE heavily innervated regions, such as SN, hippocampus, and cortex, but not in the caudate/putamen[36]. PET analysis using [18F]-PBR translocator protein as a ligand for neuroinflammation in DSP-4-injected mice showed similar patterns of increased microglial activation at 10 months after injection[36]. Genetic studies using DBH conditional KO mice showed a long-lasting increase in microglial activation compared with WT mice (Song et al., unpublished observations). Putting all the evidence together, a clear pattern emerges, indicating a high degree of correlation of prolonged microglial activation and neuronal loss in LC/NE-innervated regions in both DBH-genetic knock-out and NE-depleted mice. These findings clearly demonstrate a crucial role of LC/NE in the pathogenesis of PD.

Why is LC/NE particularly vulnerable to the insults of a variety of toxins?

LC/NE neurons are more susceptible to oxidative damage following injections of a variety of toxins/toxicants: LPS, rotenone, paraquat, maneb, etc.[79,80]. In PD, the reduced level of NE following LC/NE degeneration is closely correlated with the development of a series of prodromal and nonmotor symptoms[81-84]. It has been reported that depletion of brain NE significantly enhanced neuronal loss in many rodent PD models, including LPS, MPTP, 6-OHDA, and combined paraquat and maneb models[73,85-90]. These findings were further confirmed by our recent studies on both NE-depleted and DBH-deficient conditional knock-out mice[36,72].

To further address the question, we explored the differential vulnerability among various groups of neurons in response to toxic insults. It is generally believed that distinct nuclei respond differently to microenvironments under chronic exposure to oxidative stress and may lead to PD with age[91]. The most vulnerable neuronal populations likely share three intrinsic features: (1) coexist with a large quantity of active microglia[92,93]; (2) impaired antioxidant buffering capabilities[94,95]; and (3) greater energetic demands in neurons with long-axon projections, multi-synaptic neurotransmission, and pacemaker firing[96,97]. In a DSP-4-treated chronic neuroinflammatory mouse PD model, the superoxide/ROS productions were significantly increased in LC and SN in comparison to age-matched vehicle control. However, the appearance of oxidative injuries in the cortex and hippocampus was not observed until a few months later. When antioxidant systems in those nuclei are overwhelmed by too much oxidative stress, it results in the irreversible dysfunction of mitochondria and cell death[80,98-101]. Thus, the vulnerability to oxidative injuries in different brain regions seems to be the driving force for a discrete, sequential spatiotemporal pattern of neurodegeneration[102]. Indeed, the PET with [18F]-Fluorodeoxyglucose {[18F]-FDG} study clearly showed the high basal levels of glucose consumption in olfactory bulb, thalamus, midbrain, and hindbrain regions in control mice[36,72]. Moreover, the drastic increase in microglial activation, as measured by [18F]-PBR111 uptake, was found in the same brain areas after different toxins challenge[36,72]. Taken together, these results further support the idea that the energy demand and neuronal susceptibility are the key factors that lead to the subsequent oxidative injury-related neurodegeneration in the caudo-rostral order.

Dysfunction of noradrenergic system exacerbates inflammation-based ascending sequential neurodegeneration and behavioral deficits

Besides producing the sequential caudal-rostral pattern of neurodegeneration, noradrenergic dysfunction is associated with both motor and nonmotor behavioral changes in mice. Since the level of NE content reduces with aging, so it is thought to be associated with the appearance of a wide range of nonmotor symptoms as well as contributing to the neurodegenerative process. We hypothesized that selective pre-depletion of NE in an LPS-induced chronic neuroinflammatory mouse PD model may not only accelerate the disease progression but also expedite PD-like nonmotor and motor symptoms. Indeed, we found that mice pre-treated with DSP-4 significantly potentiated LPS-induced neurodegeneration in different brain regions in a sequential, ascending, and time-dependent pattern, such as SN, hippocampus, and motor cortex, but spared in VTA and striatum[72]. Most importantly, aligned with the enhanced neurodegeneration, this “two-hit” model also displayed greater deficits of both nonmotor (e.g., hyposmia, constipation, anxiety, sociability, exaggerated startle response, and impaired learning) and motor (e.g., decreased rotarod activity, grip strength, and gait disturbance) symptoms in a progressive fashion[72]. It is interesting to comment on the clinical relevance of loss of LC/NE neurons in nonmotor dysfunctions of PD patients. The prodromal nonmotor PD symptoms, such as GI disturbance, constipation, orthostatic hypotension, anxiety, and loss of sociability, are likely related to the early loss of LC/NE neurons since adrenergic neurons directly control the autonomic nervous system regulating these functions. Furthermore, the loss of cognition ability in the late stage of PD patients may be related to dysfunction of higher centers, such as the hippocampus and cortex, which are heavily innervated by LC/NE neurons[103,104]. Our DSP-4/LPS mouse PD model recapitulates many nonmotor dysfunctions in a similar temporal fashion[72]. Our mechanistic study demonstrating the relationship among the loss of LC/NE function, chronic neuroinflammation, and neurodegeneration lends strong support for a pivotal role of the LC/NE system in the pathogenesis of PD. Taken together, this novel “two-hit” dosing regimen not only revealed a critical role of early LC lesion in the pathogenesis of PD but also provided an accelerated PD model that recapitulates both PD-like sequential neurodegeneration and progressive appearance of motor/nonmotor symptoms[72].

Molecular mechanism of anti-inflammatory and neuroprotective functions of NE

Besides functioning as a neurotransmitter, NE has also been well-studied in the periphery for its anti-inflammatory capacities[105-108]. We hypothesized a lesion of NE neurons may disrupt brain immune homeostasis results in chronic neuroinflammation and subsequent neurodegeneration. Previous in vitro studies demonstrated that NE in micromolar concentrations or higher exert neuroprotective effects[109-111]. Interestingly, a recent report showed that sub-micromolar concentrations of NE (10-9-10-6 M) also exert anti-inflammatory and neuroprotective effects in LPS-treated midbrain neuron-glial cultures[73]. The reason for using lower concentrations of NE was that, while micromolar NE can be reached in synaptic junctions[112], sub-micromolar concentrations of NE are probably more relevant for studying its extra-synaptic effects. In the brain, most of NE will be either re-taken up by nerve terminals or undergo enzymatic breakdown. Therefore, it was reasoned that the remaining NE, which escapes from both processes, is capable of acting on the surrounding microglia even at less than micromolar concentrations[113] [Figure 4]. Further studies demonstrated that sub-micromolar NE exerts neuroprotective effects by way of reducing the release of a series of pro-inflammatory cytokines (e.g., IL-1β, IL-6, and TNFα) and free radicals (e.g., superoxide/ROS, nitric oxide, etc.) from LPS-treated microglia cultures[73].

Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies

Figure 4. Initial loss of NE/LC neurons resulted from either LPS or DSP-4 injection renders neurons in the NE neuron-innervated regions more susceptible to inflammation-related damage. In normal condition (Left), NE released from the presynaptic terminals of LC/NE neurons performs multiple functions through different ways of transmission. During synaptic transmission, released NE functions as a neuromodulator by directly acting on postsynaptic β2-receptors to modulate the function of postsynaptic neurons. In volume transmission, extra-synaptic NE, diffused out of the synapse or released from dendrites, can act on other neighboring cells, such as microglia. NE exerts anti-inflammatory and neuroprotective functions through the inhibition of microglial NOX2. In pathological condition (Right), reduced NE release from LC/NE neurons not only disrupts the synaptic transmission, but also renders surrounding microglia prone to activation to release proinflammatory immune factors, leading to neuronal damage. Thus, we hypothesize that dysfunction of LC/NE neurons after LPS or DSP-4 injection renders neurons more sensitive to inflammation/oxidative insults and initiates neurodegeneration.

A novel β2-AR-independent pathway mediating sub-micromolar NE-induced anti-inflammatory effect: inhibition of microglial NOX2-produced superoxide

On immune cells, β2-Adrenergic receptor (β2-AR) plays a critical role in mediating the NE-elicited anti-inflammatory effect by suppressing the release of pro-inflammatory factors via activation of the cAMP/protein kinase A pathway. It is generally accepted that β2-AR mediates the anti-inflammatory effect of micromolar concentrations of NE. It is interesting to find that a novel β2-AR-independent pathway may mediate the sub-micromolar NE-elicited anti-inflammatory effect. We demonstrated that LPS-induced superoxide production was significantly inhibited by NE in a dose-dependent manner in primary midbrain neuron-glial cultures[73]. Two NE optical isomers were used to investigate the important role of ARs in inhibiting NE-derived superoxide production. Surprisingly, the active isomer (-)-NE showed over 100-fold AR-binding affinity than that of inactive isomer (+)-NE[114,115]. However, both (+)-NE and (-)-NE were found equipotent in inhibiting superoxide production in LPS-treated mixed-glia cultures. The AR-independent inhibitory function of both NE isomers on superoxide production was further confirmed in a low AR-expressing cell line, COS7 cells, treated with phorbol myristate acetate (PMA)[116-118]. As expected, after transfected with NOX2, both isomers exerted a comparable inhibitory capacity on PMA-induced superoxide in COS7 cells[119]. Moreover, the AR-independent inhibitory capacity of both NE isomers has also been confirmed in mouse mixed-glia cultures with genetically depleting ARs[73].

Besides β2-AR, the possibility of the involvement of other types of ARs in regulating NE-elicited reduction of superoxide production has also been studied. However, blocking α1 and/or β1 ARs by pretreating with their non-selective antagonists (phentolamine and propranolol, respectively) failed to show any changes in NE-induced superoxide reduction in LPS-treated mixed-glia cultures[120]. Moreover, inhibition of PKA, a common enzyme in ARs signal transduction pathways, again failed to affect NE-induced superoxide production[73]. Altogether, these findings reveal that NOX2 plays a critical role in regulating sub-micromolar NE-elicited microglial deactivation. It should be emphasized here that β2-AR is still activated by micromolar NE. In fact, our previous report showed that salmeterol, a long-acting β2 adrenergic receptor agonist, exerts a neuroprotective effect against LPS-elicited DA neuron damage mediated through the β2-AR/β-arrestin pathway[120].


Recent studies revealed a critical role of microglial NOX2-derived ROS in initiating neuroinflammation-mediated oxidative damage and progressive neurodegeneration[121]. Neuroinflammation has been widely accepted as a crucial contributor to progressive neurodegeneration in a broad spectrum of neurodegenerative diseases[78,122,123]. Microglia can be activated by a wide range of stimuli that are able to disrupt brain homeostasis, such as infection, ischemia, trauma, toxic insults, or autoimmune injury.

Once activated, microglia release innumerable cytotoxic factors, including cytokines, chemokines, proteases, excitatory amino acids, eicosanoids, and ROS. NOX2-derived superoxide has been recognized as one of the most crucial players in chronic progressive neurodegeneration[78,122,123]. Those microglial NOX2-derived ROS (H2O2 and peroxynitrite) can directly enter neurons, resulting in impaired mitochondrial integrity, reduced ATP production, and increased mitochondria-derived ROS. They also cause a series of damages to enzymes and other proteins through oxidation, nitration, aggregation, or accumulation (e.g., a-synuclein). By dysfunction of the ubiquitin-proteasome system, ROS will not only reduce protein degradation but also exaggerate abnormal protein accumulation. Moreover, the impaired redox-sensitive signal transduction, products of oxidated DNA, RNA, and lipids, and/or ROS-induced autophagy also play a role in oxidative neuronal damages during neuroinflammation[10,124,125].

Role of dysregulated NOX2 in PD

It has been reported that the increase in microglial NOX2 was found in the SN of both PD patients and mouse PD models[126]. In line with those pathological examinations, a crucial role of microglial NOX2 activation in driving DA neurodegeneration has also been extensively studied[78,127]. For example, in a microglia and DA neuron co-culture system, the mis-folded α-synuclein is able to kill DA neurons by activating microglial NOX2 to release ROS[128]. Moreover, the presence of microglia exacerbates DA neurodegeneration following diverse challenges, including fMLP and LPS, angiotensin II and nanometer-sized diesel exhaust particles, PD-producing neurotoxins (6-OHDA, MPTP, and MPP+), and PD-associated pesticides (paraquat and rotenone); such neurodegeneration could be alleviated by NOX2 deletion, diphenyleneiodonium (DPI), or apocynin[129]. In addition, the release of DAMPs and other cellular components to the extracellular space, such as high-mobility group box 1, the active form of matrix metalloproteinase-3, or aggregated α-synuclein, could trigger reactive microgliosis and release NOX2-dependent ROS production, which further facilitates DA neurodegeneration[46]. In a MPTP-induced mouse PD model, minocycline-induced neuroprotective effects were achieved by inhibition of microglial activation and membrane translocation of p67phox[130]. Furthermore, the neurotoxic effects induced by either systemic administration of MPTP or intra-nigral injection of LPS were significantly suppressed in NOX2-deficient mice in comparison to WT mice[131].

NOX2 is a prime target for anti-inflammatory therapy

Chronic aberrant neuroinflammation, a ubiquitous feature among a variety of neurodegenerative diseases, has been targeted as a disease-modifying strategy for halting the diseases progression[120,132-134]. However, little progress has been made on the ground due to the lack of knowledge pinpointing the immune factors released during chronic neuroinflammation. Recent studies suggest that blocking the superoxide/ROS-generating enzyme NOX2 ameliorates neuroinflammation and reduces neurodegeneration[132].

A NOX2 inhibitor DPI has served as a useful tool to demonstrate the advantages of targeting NOX2 as a prime target for therapy. DPI is a widely used NOX2 inhibitor. However, commonly used concentrations (1-10 µM) of DPI are highly toxic in cell cultures and animals, thus preventing its use in humans. We discovered that an ultra-low dose of DPI (10 ng/kg/day) displayed potent anti-inflammatory and neuroprotective effects in LPS-treated mice[132]. Furthermore, post-treatment of DPI to LPS-treated mice that already shown marked loss of nigral DA neurons and motor symptoms could effectively stop the remaining neuronal population from degeneration and largely restore motor functions[132]. The dopaminergic neuroprotective effects of low-dose DPI, even in a post-treatment regimen, were also detected in an MPTP-induced mouse PD model[132]. Recent studies using similar post-treatment regimens demonstrated the same efficacy of DPI in DSP-4 injected mice[36]. DPI greatly reduced microglial activation, decreased oxidative stress, and most importantly protected DA neurons. Collectively, these findings suggest that DPI is effective at protecting neurons in either infectious agent (LPS)- or non-infectious agent (DSP-4)-induced mouse PD models, suggesting that targeting NOX2 can be a novel and promising therapeutic strategy for PD.

In addition to targeting microglial NOX2, the use of β-AR agonists has also been tried as a potential therapy for PD. It is worth noting that, in LPS-injected mice, post-treatment with the β2 adrenergic receptor (β2-AR) agonist salmeterol significantly rescued DA neurons and improved motor function deficits[120]. Results from these animal studies corroborate a recently published human study. A meta-analysis showed that asthmatic patients prescribed with salbutamol, a β2-AR agonist, had significantly reduced lifetime risk of developing PD[135].


This review provides clear and convincing evidence to demonstrate that low-grade chronic neuroinflammation is a key factor leading to the progressive neurodegeneration in PD. The reduction of brain NE resulted from the lesion of LC/NE neurons is sufficient to initiate and maintain chronic neuroinflammation, which is associated with progressive, massive, and sequential loss of vulnerable neurons that are sensitive to oxidative damage. Dysregulated microglial NOX2 plays a critical role in generating and maintaining chronic neuroinflammation, oxidative stress, and subsequent neurodegeneration among vulnerable brain regions. NOX2 may serve as a prime target for developing promising disease-modifying therapeutic strategies for PD.


Authors’ contributions

Preparing the first draft: Wang QS, Hong JS

Made substantial contributions to conception and revised manuscript: Song S

Give critical comments: Jiang LL

Availability of data and material

Not applicable.

Financial support and sponsorship


Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.


© The Author(s) 2021.


1. Jellinger KA. The pathology of Parkinson’s disease. Adv Neurol 2001;86:55-72.

2. Holdorff B. Friedrich Heinrich Lewy (1885-1950) and his work. J Hist Neurosci 2002;11:19-28.

3. Schiller F. Fritz Lewy and his bodies. J Hist Neurosci 2000;9:148-51.

4. Mizuno Y, Hattori N, Kitada T, et al. Familial Parkinson’s disease. Alpha-synuclein and parkin. Adv Neurol 2001;86:13-21.

5. Jiang H, Wu YC, Nakamura M, et al. Parkinson’s disease genetic mutations increase cell susceptibility to stress: mutant alpha-synuclein enhances H2O2- and Sin-1-induced cell death. Neurobiol Aging 2007;28:1709-17.

6. Tanner CM. Is the cause of Parkinson’s disease environmental or hereditary? Adv Neurol 2003;91:133-42.

7. Rock RB, Peterson PK. Microglia as a pharmacological target in infectious and inflammatory diseases of the brain. J Neuroimmune Pharmacol 2006;1:117-26.

8. Betarbet R, Sherer TB, MacKenzie G, et al. Chronic systemic pesticide exposure reproduces features of Parkinson’s disease. Nat Neurosci 2000;3:1301-6.

9. Zheng W, Fu SX, Dydak U, Cowan DM. Biomarkers of manganese intoxication. Neurotoxicology 2011;32:1-8.

10. Gao HM, Hong JS. Gene-environment interactions: key to unraveling the mystery of Parkinson’s disease. Prog Neurobiol 2011;94:1-19.

11. Jang H, Boltz D, Sturm-Ramirez K, et al. Highly pathogenic H5N1 influenza virus can enter the central nervous system and induce neuroinflammation and neurodegeneration. Proc Natl Acad Sci U S A 2009;106:14063-8.

12. Jang H, Boltz D, McClaren J, et al. Inflammatory effects of highly pathogenic H5N1 influenza virus infection in the CNS of mice. J Neurosci 2012;32:1545-59.

13. Guilarte TR. Manganese and Parkinson’s disease: a critical review and new findings. Environ Health Perspect 2010;118:1071-80.

14. McGeer PL, Yasojima K, McGeer EG. Inflammation in Parkinson’s disease. Adv Neurol 2001;86:83-9.

15. Block ML, Zecca L, Hong JS. Microglia-mediated neurotoxicity: uncovering the molecular mechanisms. Nat Rev Neurosci 2007;8:57-69.

16. Gao HM, Zhou H, Hong JS. NADPH oxidases: novel therapeutic targets for neurodegenerative diseases. Trends Pharmacol Sci 2012;33:295-303.

17. del Rio-Hortega P. Cytology and cellular pathology of the nervous system. Arch Intern Med 1932;50:508.

18. Barron KD. The microglial cell. A historical review. J Neurol Sci 1995;134 Suppl:57-68.

19. Milligan CE, Cunningham TJ, Levitt P. Differential immunochemical markers reveal the normal distribution of brain macrophages and microglia in the developing rat brain. J Comp Neurol 1991;314:125-35.

20. Kreutzberg GW. Microglia: a sensor for pathological events in the CNS. Trends Neurosci 1996;19:312-8.

21. Liu B, Hong JS. Role of microglia in inflammation-mediated neurodegenerative diseases: mechanisms and strategies for therapeutic intervention. J Pharmacol Exp Ther 2003;304:1-7.

22. Streit WJ, Graeber MB, Kreutzberg GW. Functional plasticity of microglia: a review. Glia 1988;1:301-7.

23. Streit WJ, Walter SA, Pennell NA. Reactive microgliosis. Prog Neurobiol 1999;57:563-81.

24. Graeber MB, Streit WJ, Kreutzberg GW. The microglial cytoskeleton: vimentin is localized within activated cells in situ. J Neurocytol 1988;17:573-80.

25. Oehmichen W, Gencic M. Experimental studies on kinetics and functions of monuclear phagozytes of the central nervous system. Acta Neuropathol Suppl (Berl) 1975;Suppl 6:285-90.

26. Verkhratsky A, Nedergaard M. Physiology of astroglia. Physiol Rev 2018;98:239-389.

27. Aloisi F. The role of microglia and astrocytes in CNS immune surveillance and immunopathology. Adv Exp Med Biol 1999;468:123-33.

28. Tacconi MT. Neuronal death: is there a role for astrocytes? Neurochem Res 1998;23:759-65.

29. Pavlov VA, Wang H, Czura CJ, Friedman SG, Tracey KJ. The cholinergic anti-inflammatory pathway: a missing link in neuroimmunomodulation. Mol Med 2003;9:125-34.

30. Lindsay RM. Neurotrophic growth factors and neurodegenerative diseases: therapeutic potential of the neurotrophins and ciliary neurotrophic factor. Neurobiol Aging 1994;15:249-51.

31. Chen PS, Peng GS, Li G, et al. Valproate protects dopaminergic neurons in midbrain neuron/glia cultures by stimulating the release of neurotrophic factors from astrocytes. Molecular psychiatry 2006;11:1116-25.

32. Chen SH, Oyarzabal EA, Sung YF, et al. Microglial regulation of immunological and neuroprotective functions of astroglia. Glia 2015;63:118-31.

33. Chen PS, Wang CC, Bortner CD, et al. Valproic acid and other histone deacetylase inhibitors induce microglial apoptosis and attenuate lipopolysaccharide-induced dopaminergic neurotoxicity. Neuroscience 2007;149:203-12.

34. Liddelow SA, Guttenplan KA, Clarke LE, et al. Neurotoxic reactive astrocytes are induced by activated microglia. Nature 2017;541:481-7.

35. Qin L, Wu X, Block ML, et al. Systemic LPS causes chronic neuroinflammation and progressive neurodegeneration. Glia 2007;55:453-62.

36. Song S, Jiang L, Oyarzabal EA, et al. Loss of brain norepinephrine elicits neuroinflammation-mediated oxidative injury and selective caudo-rostral neurodegeneration. Mol Neurobiol 2019;56:2653-69.

37. Tredici K, Braak H. To stage, or not to stage. Curr Opin Neurobiol 2020;61:10-22.

38. Tredici K, Braak H. Review: Sporadic Parkinson’s disease: development and distribution of α-synuclein pathology. Neuropathol Appl Neurobiol 2016;42:33-50.

39. Itzhaki RF, Lathe R, Balin BJ, et al. Microbes and Alzheimer’s Disease. J Alzheimers Dis 2016;51:979-84.

40. Hou L, Sun F, Huang R, et al. Inhibition of NADPH oxidase by apocynin prevents learning and memory deficits in a mouse Parkinson’s disease model. Redox Biol 2019;22:101134.

41. Thiruchelvam M, Richfield EK, Baggs RB, Tank AW, Cory-Slechta DA. The nigrostriatal dopaminergic system as a preferential target of repeated exposures to combined paraquat and maneb: implications for Parkinson’s disease. J Neurosci 2000;20:9207-14.

42. Vlajinac HD, Sipetic SB, Maksimovic JM, et al. Environmental factors and Parkinson’s disease: a case-control study in Belgrade, Serbia. Int J Neurosci 2010;120:361-7.

43. Gao HM, Zhang F, Zhou H, et al. Neuroinflammation and alpha-synuclein dysfunction potentiate each other, driving chronic progression of neurodegeneration in a mouse model of Parkinson’s disease. Environ Health Perspect 2011;119:807-14.

44. Brown GC, Neher JJ. Microglial phagocytosis of live neurons. Nat Rev Neurosci 2014;15:209-16.

45. Edison P, Ahmed I, Fan Z, et al. Microglia, amyloid, and glucose metabolism in Parkinson’s disease with and without dementia. Neuropsychopharmacology 2013;38:938-49.

46. Gao HM, Zhou H, Zhang F, et al. HMGB1 acts on microglia Mac1 to mediate chronic neuroinflammation that drives progressive neurodegeneration. J Neurosci 2011;31:1081-92.

47. Qin L, Liu Y, Hong JS, Crews FT. NADPH oxidase and aging drive microglial activation, oxidative stress, and dopaminergic neurodegeneration following systemic LPS administration. Glia 2013;61:855-68.

48. Chen SH, Oyarzabal EA, Hong JS. Critical role of the Mac1/NOX2 pathway in mediating reactive microgliosis-generated chronic neuroinflammation and progressive neurodegeneration. Curr Opin Pharmacol 2016;26:54-60.

49. Huang H, Liu T, Rose JL, Stevens RL, Hoyt DG. Sensitivity of mice to lipopolysaccharide is increased by a high saturated fat and cholesterol diet. J Inflamm (Lond) 2007;4:22.

50. Nadeau S, Rivest S. Regulation of the gene encoding tumor necrosis factor alpha (TNF-alpha) in the rat brain and pituitary in response in different models of systemic immune challenge. J Neuropathol Exp Neurol 1999;58:61-77.

51. Kumins NH, Hunt J, Gamelli RL, Filkins JP. Partial hepatectomy reduces the endotoxin-induced peak circulating level of tumor necrosis factor in rats. Shock 1996;5:385-8.

52. Pan W, Ding Y, Yu Y, et al. Stroke upregulates TNFalpha transport across the blood-brain barrier. Exp Neurol 2006;198:222-33.

53. Langston JW, Forno LS, Tetrud J, et al. Evidence of active nerve cell degeneration in the substantia nigra of humans years after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine exposure. Ann Neurol 1999;46:598-605.

54. McGeer PL, Schwab C, Parent A, Doudet D. Presence of reactive microglia in monkey substantia nigra years after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration. Ann Neurol 2003;54:599-604.

55. Ling Z, Gayle DA, Ma SY, et al. In utero bacterial endotoxin exposure causes loss of tyrosine hydroxylase neurons in the postnatal rat midbrain. Mov Disord 2002;17:116-24.

56. Alam M, Schmidt WJ. Rotenone destroys dopaminergic neurons and induces parkinsonian symptoms in rats. Behav Brain Res 2002;136:317-24.

57. Greenamyre JT, MacKenzie G, Peng TI, Stephans SE. Mitochondrial dysfunction in Parkinson’s disease. Biochem Soc Symp 1999;66:85-97.

58. Jenner P. Parkinson’s disease, pesticides and mitochondrial dysfunction. Trends Neurosci 2001;24:245-7.

59. Marshall LE, Himes RH. Rotenone inhibition of tubulin self-assembly. Biochim Biophys Acta 1978;543:590-4.

60. Ren Y, Feng J. Rotenone selectively kills serotonergic neurons through a microtubule-dependent mechanism. J Neurochem 2007;103:303-11.

61. Choi WS, Kruse SE, Palmiter RD, Xia Z. Mitochondrial complex I inhibition is not required for dopaminergic neuron death induced by rotenone, MPP+, or paraquat. Proc Natl Acad Sci U S A 2008;105:15136-41.

62. Gao HM, Hong JS, Zhang W, Liu B. Distinct role for microglia in rotenone-induced degeneration of dopaminergic neurons. J Neurosci 2002;22:782-90.

63. Gao HM, Liu B, Hong JS. Critical role for microglial NADPH oxidase in rotenone-induced degeneration of dopaminergic neurons. J Neurosci 2003;23:6181-7.

64. Zhou H, Zhang F, Chen SH, et al. Rotenone activates phagocyte NADPH oxidase by binding to its membrane subunit gp91phox. Free Radic Biol Med 2012;52:303-13.

65. Jing L, Hou L, Zhang D, et al. Microglial Activation Mediates Noradrenergic Locus Coeruleus Neurodegeneration via Complement Receptor 3 in a Rotenone-Induced Parkinson’s Disease Mouse Model. J Inflamm Res 2021;14:1341-56.

66. Zhang D, Li S, Hou L, et al. Microglial activation contributes to cognitive impairments in rotenone-induced mouse Parkinson’s disease model. J Neuroinflammation 2021;18:4.

67. Che Y, Hou L, Sun F, et al. Taurine protects dopaminergic neurons in a mouse Parkinson’s disease model through inhibition of microglial M1 polarization. Cell Death Dis 2018;9:435.

68. Hou L, Zhang C, Wang K, et al. Paraquat and maneb co-exposure induces noradrenergic locus coeruleus neurodegeneration through NADPH oxidase-mediated microglial activation. Toxicology 2017;380:1-10.

69. Hou L, Qu X, Qiu X, et al. Integrin CD11b mediates locus coeruleus noradrenergic neurodegeneration in a mouse Parkinson’s disease model. J Neuroinflammation 2020;17:148.

70. Hou L, Che Y, Sun F, Wang Q. Taurine protects noradrenergic locus coeruleus neurons in a mouse Parkinson’s disease model by inhibiting microglial M1 polarization. Amino Acids 2018;50:547-56.

71. Wang K, Shi Y, Liu W, Liu S, Sun MZ. Taurine improves neuron injuries and cognitive impairment in a mouse Parkinson’s disease model through inhibition of microglial activation. Neurotoxicology 2021;83:129-36.

72. Song S, Wang Q, Jiang L, et al. Noradrenergic dysfunction accelerates LPS-elicited inflammation-related ascending sequential neurodegeneration and deficits in non-motor/motor functions. Brain Behav Immun 2019;81:374-87.

73. Jiang L, Chen SH, Chu CH, et al. A novel role of microglial NADPH oxidase in mediating extra-synaptic function of norepinephrine in regulating brain immune homeostasis. Glia 2015;63:1057-72.

74. Braak H, Rub U, Gai WP, Del Tredici K. Idiopathic Parkinson’s disease: possible routes by which vulnerable neuronal types may be subject to neuroinvasion by an unknown pathogen. J Neural Transm (Vienna) 2003;110:517-36.

75. Hilker R, Thomas AV, Klein JC, et al. Dementia in Parkinson disease: functional imaging of cholinergic and dopaminergic pathways. Neurology 2005;65:1716-22.

76. Dickson DW, Schmidt ML, Lee VM, et al. Immunoreactivity profile of hippocampal CA2/3 neurites in diffuse Lewy body disease. Acta Neuropathol 1994;87:269-76.

77. Pereira JB, Junque C, Bartres-Faz D, et al. Regional vulnerability of hippocampal subfields and memory deficits in Parkinson’s disease. Hippocampus 2013;23:720-8.

78. Gao HM, Hong JS. Why neurodegenerative diseases are progressive: uncontrolled inflammation drives disease progression. Trends Immunol 2008;29:357-65.

79. Elstner M, Muller SK, Leidolt L, et al. Neuromelanin, neurotransmitter status and brainstem location determine the differential vulnerability of catecholaminergic neurons to mitochondrial DNA deletions. Mol Brain 2011;4:43.

80. Sanchez-Padilla J, Guzman JN, Ilijic E, et al. Mitochondrial oxidant stress in locus coeruleus is regulated by activity and nitric oxide synthase. Nat Neurosci 2014;17:832-40.

81. Borodovitsyna O, Flamini M, Chandler D. Noradrenergic Modulation of Cognition in Health and Disease. Neural Plast 2017;2017:6031478.

82. Jellinger KA. Pathology of Parkinson’s disease. Changes other than the nigrostriatal pathway. Mol Chem Neuropathol 1991;14:153-97.

83. Tong J, Hornykiewicz O, Kish SJ. Inverse relationship between brain noradrenaline level and dopamine loss in Parkinson disease: a possible neuroprotective role for noradrenaline. Arch Neurol 2006;63:1724-8.

84. Zarow C, Lyness SA, Mortimer JA, Chui HC. Neuronal loss is greater in the locus coeruleus than nucleus basalis and substantia nigra in Alzheimer and Parkinson diseases. Arch Neurol 2003;60:337-41.

85. Fornai F, Alessandri MG, Torracca MT, Bassi L, Corsini GU. Effects of noradrenergic lesions on MPTP/MPP+ kinetics and MPTP-induced nigrostriatal dopamine depletions. J Pharmacol Exp Ther 1997;283:100-7.

86. Lookingland KJ, Chapin DS, McKay DW, Moore KE. Comparative effects of the neurotoxins N-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) and 6-hydroxydopamine on hypothalamic noradrenergic, dopaminergic and 5-hydroxytryptaminergic neurons in the male rat. Brain Res 1986;365:228-34.

87. Ostock CY, Lindenbach D, Goldenberg AA, Kampton E, Bishop C. Effects of noradrenergic denervation by anti-DBH-saporin on behavioral responsivity to L-DOPA in the hemi-parkinsonian rat. Behav Brain Res 2014;270:75-85.

88. Heneka MT, Galea E, Gavriluyk V, et al. Noradrenergic depletion potentiates beta -amyloid-induced cortical inflammation: implications for Alzheimer’s disease. J Neurosci 2002;22:2434-42.

89. Perez V, Sosti V, Rubio A, et al. Noradrenergic modulation of the motor response induced by long-term levodopa administration in Parkinsonian rats. J Neural Transm (Vienna) 2009;116:867-74.

90. Hou L, Sun F, Sun W, Zhang L, Wang Q. Lesion of the Locus Coeruleus Damages Learning and Memory Performance in Paraquat and Maneb-induced Mouse Parkinson’s Disease Model. Neuroscience 2019;419:129-40.

91. Sanders LH, Timothy Greenamyre J. Oxidative damage to macromolecules in human Parkinson disease and the rotenone model. Free Radic Biol Med 2013;62:111-20.

92. Kim WG, Mohney RP, Wilson B, et al. Regional difference in susceptibility to lipopolysaccharide-induced neurotoxicity in the rat brain: role of microglia. J Neurosci 2000;20:6309-16.

93. Yang TT, Lin C, Hsu CT, et al. Differential distribution and activation of microglia in the brain of male C57BL/6J mice. Brain Struct Funct 2013;218:1051-60.

94. Smeyne M, Smeyne RJ. Glutathione metabolism and Parkinson’s disease. Free Radic Biol Med 2013;62:13-25.

95. Wang X, Michaelis EK. Selective neuronal vulnerability to oxidative stress in the brain. Front Aging Neurosci 2010;2:12.

96. Surmeier DJ, Obeso JA, Halliday GM. Selective neuronal vulnerability in Parkinson disease. Nat Rev Neurosci 2017;18:101-13.

97. Surmeier DJ, Schumacker PT. Calcium, bioenergetics, and neuronal vulnerability in Parkinson’s disease. J Biol Chem 2013;288:10736-41.

98. Burbulla LF, Song P, Mazzulli JR, et al. Dopamine oxidation mediates mitochondrial and lysosomal dysfunction in Parkinson’s disease. Science 2017;357:1255-61.

99. Goldberg JA, Guzman JN, Estep CM, et al. Calcium entry induces mitochondrial oxidant stress in vagal neurons at risk in Parkinson’s disease. Nat Neurosci 2012;15:1414-21.

100. Guzman JN, Sanchez-Padilla J, Wokosin D, et al. Oxidant stress evoked by pacemaking in dopaminergic neurons is attenuated by DJ-1. Nature 2010;468:696-700.

101. Surmeier DJ, Guzman JN, Sanchez-Padilla J, Schumacker PT. The role of calcium and mitochondrial oxidant stress in the loss of substantia nigra pars compacta dopaminergic neurons in Parkinson’s disease. Neuroscience 2011;198:221-31.

102. Wang Q, Oyarzabal EA, Song S, et al. Locus coeruleus neurons are most sensitive to chronic neuroinflammation-induced neurodegeneration. Brain Behav Immun 2020;87:359-68.

103. Uribe C, Segura B, Baggio HC, et al. Progression of Parkinson’s disease patients’ subtypes based on cortical thinning: 4-year follow-up. Parkinsonism Relat Disord 2019;64:286-92.

104. Robertson SD, Plummer NW, de Marchena J, Jensen P. Developmental origins of central norepinephrine neuron diversity. Nat Neurosci 2013;16:1016-23.

105. Kin NW, Sanders VM. It takes nerve to tell T and B cells what to do. J Leukoc Biol 2006;79:1093-104.

106. Kohm AP, Sanders VM. Norepinephrine and beta 2-adrenergic receptor stimulation regulate CD4+ T and B lymphocyte function in vitro and in vivo. Pharmacol Rev 2001;53:487-525.

107. Severn A, Rapson NT, Hunter CA, Liew FY. Regulation of tumor necrosis factor production by adrenaline and beta-adrenergic agonists. J Immunol 1992;148:3441-5.

108. der Poll T, Jansen J, Endert E, Sauerwein HP, van Deventer SJ. Noradrenaline inhibits lipopolysaccharide-induced tumor necrosis factor and interleukin 6 production in human whole blood. Infect Immun 1994;62:2046-50.

109. Heneka MT, Nadrigny F, Regen T, et al. Locus ceruleus controls Alzheimer’s disease pathology by modulating microglial functions through norepinephrine. Proc Natl Acad Sci U S A 2010;107:6058-63.

110. Troadec JD, Marien M, Darios F, et al. Noradrenaline provides long-term protection to dopaminergic neurons by reducing oxidative stress. J Neurochem 2001;79:200-10.

111. Troadec JD, Marien M, Mourlevat S, et al. Activation of the mitogen-activated protein kinase (ERK(1/2)) signaling pathway by cyclic AMP potentiates the neuroprotective effect of the neurotransmitter noradrenaline on dopaminergic neurons. Mol Pharmacol 2002;62:1043-52.

112. Abercrombie ED, Zigmond MJ. Partial injury to central noradrenergic neurons: reduction of tissue norepinephrine content is greater than reduction of extracellular norepinephrine measured by microdialysis. J Neurosci 1989;9:4062-7.

113. Gresch PJ, Sved AF, Zigmond MJ, Finlay JM. Local influence of endogenous norepinephrine on extracellular dopamine in rat medial prefrontal cortex. J Neurochem 1995;65:111-6.

114. Bylund DB, Snyder SH. Beta adrenergic receptor binding in membrane preparations from mammalian brain. Mol Pharmacol 1976;12:568-80.

115. Deupree JD, Kennedy RH. Stereospecific (--)-[3H]norepinephrine binding to bovine hypothalamus. Possible identification of the catecholamine uptake site in synaptic vesicles. Biochim Biophys Acta 1979;582:470-85.

116. Regan JW, Kobilka TS, Yang-Feng TL, et al. Cloning and expression of a human kidney cDNA for an alpha 2-adrenergic receptor subtype. Proc Natl Acad Sci U S A 1988;85:6301-5.

117. Schwinn DA, Lomasney JW, Lorenz W, et al. Molecular cloning and expression of the cDNA for a novel alpha 1-adrenergic receptor subtype. J Biol Chem 1990;265:8183-9.

118. Strader CD, Sigal IS, Register RB, et al. Identification of residues required for ligand binding to the beta-adrenergic receptor. Proc Natl Acad Sci U S A 1987;84:4384-8.

119. Mizrahi A, Berdichevsky Y, Ugolev Y, et al. Assembly of the phagocyte NADPH oxidase complex: chimeric constructs derived from the cytosolic components as tools for exploring structure-function relationships. J Leukoc Biol 2006;79:881-95.

120. Qian L, Wu HM, Chen SH, et al. beta2-adrenergic receptor activation prevents rodent dopaminergic neurotoxicity by inhibiting microglia via a novel signaling pathway. J Immunol 2011;186:4443-54.

121. Hou L, Zhang L, Hong JS, et al. Nicotinamide Adenine Dinucleotide Phosphate Oxidase and Neurodegenerative Diseases: Mechanisms and Therapy. Antioxid Redox Signal 2020;33:374-93.

122. Philips T, Robberecht W. Neuroinflammation in amyotrophic lateral sclerosis: role of glial activation in motor neuron disease. Lancet Neurol 2011;10:253-63.

123. Glass CK, Saijo K, Winner B, Marchetto MC, Gage FH. Mechanisms underlying inflammation in neurodegeneration. Cell 2010;140:918-34.

124. Barnham KJ, Masters CL, Bush AI. Neurodegenerative diseases and oxidative stress. Nat Rev Drug Discov 2004;3:205-14.

125. Zhou C, Huang Y, Przedborski S. Oxidative stress in Parkinson’s disease: a mechanism of pathogenic and therapeutic significance. Ann N Y Acad Sci 2008;1147:93-104.

126. Wu DC, Teismann P, Tieu K, et al. NADPH oxidase mediates oxidative stress in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson’s disease. Proc Natl Acad Sci U S A 2003;100:6145-50.

127. Wu DC, Re DB, Nagai M, Ischiropoulos H, Przedborski S. The inflammatory NADPH oxidase enzyme modulates motor neuron degeneration in amyotrophic lateral sclerosis mice. Proc Natl Acad Sci U S A 2006;103:12132-7.

128. Zhang W, Wang T, Pei Z, et al. Aggregated alpha-synuclein activates microglia: a process leading to disease progression in Parkinson’s disease. FASEB J 2005;19:533-42.

129. Gao X, Hu X, Qian L, et al. Formyl-methionyl-leucyl-phenylalanine-induced dopaminergic neurotoxicity via microglial activation: a mediator between peripheral infection and neurodegeneration? Environ Health Perspect 2008;116:593-8.

130. Wu DC, Jackson-Lewis V, Vila M, et al. Blockade of microglial activation is neuroprotective in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson disease. J Neurosci 2002;22:1763-71.

131. Zhang W, Wang T, Qin L, et al. Neuroprotective effect of dextromethorphan in the MPTP Parkinson’s disease model: role of NADPH oxidase. FASEB J 2004;18:589-91.

132. Wang Q, Qian L, Chen SH, et al. Post-treatment with an ultra-low dose of NADPH oxidase inhibitor diphenyleneiodonium attenuates disease progression in multiple Parkinson’s disease models. Brain 2015;138:1247-62.

133. Gilgun-Sherki Y, Melamed E, Offen D. Anti-inflammatory drugs in the treatment of neurodegenerative diseases: current state. Curr Pharm Des 2006;12:3509-19.

134. Wyss-Coray T, Mucke L. Inflammation in neurodegenerative disease--a double-edged sword. Neuron 2002;35:419-32.

135. Mittal S, Bjornevik K, Im DS, et al. beta2-Adrenoreceptor is a regulator of the alpha-synuclein gene driving risk of Parkinson’s disease. Science 2017;357:891-8.

136. Qian L, Flood PM, Hong JS. Neuroinflammation is a key player in Parkinson's disease and a prime target for therapy. J Neural Transm 2010;117:971-9.

137. Song S, Liu J, Zhang F, Hong JS. Norepinephrine depleting toxin DSP-4 and LPS alter gut microbiota and induce neurotoxicity in α-synuclein mutant mice. Sci Rep 2020;10:15054.

138. Block ML, Hong JS. Microglia and inflammation-mediated neurodegeneration: multiple triggers with a common mechanism. Prog Neurobiol 2005;76:77-98.

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Wang Q, Song S, Jiang L, Hong JS. Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies. Ageing Neur Dis 2021;1:6.

AMA Style

Wang Q, Song S, Jiang L, Hong JS. Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies. Ageing and Neurodegenerative Diseases. 2021; 1(1): 6.

Chicago/Turabian Style

Qingshan Wang, Sheng Song, Lulu Jiang, Jau-Shyong Hong. 2021. "Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies" Ageing and Neurodegenerative Diseases. 1, no.1: 6.

ACS Style

Wang, Q.; Song S.; Jiang L.; Hong J.S. Interplay among norepinephrine, NOX2, and neuroinflammation: key players in Parkinson's disease and prime targets for therapies. Ageing. Neur. Dis. 2021, 1, 6.

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