1. Introduction
Parkinson’s disease (PD) is a progressive neurodegenerative disease mainly characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Consequently, dopamine (DA) deficiency in the striatum causes motor symptoms, such as akinesia/bradykinesia, tremor, rigidity, and postural instability. PD patients also exhibit non-motor symptoms, such as hyposmia, autonomic disturbance, depression, and REM sleep behavior disorder (RBD), which precedes motor symptoms [
1]. Pathologically, neurodegeneration accompanied by an accumulation of α-synuclein, Lewy bodies and neurites, is observed in the central and peripheral nervous systems of sporadic PD patients [
2,
3]. Currently, it is hypothesized that PD pathology propagates from the enteric nervous system (ENS) to the central nervous system (CNS) via the vagal nerve [
4,
5]. Although the mechanism of neurodegeneration in PD remains unknown, various pathogenic factors, such as oxidative stress, neuroinflammation, α-synuclein toxicity and mitochondrial impairment, and neuron vulnerability, are thought to drive apoptosis. In addition, there is a consensus that non-neuronal cells contribute to the progression of PD pathology [
6,
7,
8].
Astrocytes are the most numerous glial cells in the CNS and surrounding neurons. Astrocytes have been shown to protect neurons by the release of neurotrophic factors, the production of antioxidants, and the disposal of neuronal waste products, including aggregated α-synuclein and damaged mitochondria [
9,
10]. In the brains of patients with neurodegenerative diseases, astrocytes are known to migrate and become hypertrophic, that is, reactive astrocytes. The role of these reactive astrocytes is controversial; it is not clear whether these cells are neuroprotective or neurotoxic [
11]. Recently, the classification of reactive astrocytes has been proposed as harmful A1 and protective A2 astrocytes [
12]. It is reported that A1 astrocytes lose their characteristic functions, including neuroprotective and supportive property, and contribute to neuronal death in PD [
12]. On the other hand, we previously demonstrated that reactive astrocytes produce antioxidative molecules in response to oxidative stress and protect dopaminergic neurons in PD models [
13]; however, we did not characterize the types of reactive astrocytes. In this review, we summarize the interaction of astrocytes and dopaminergic neurons, and discuss the pathogenic role of astrocytes in PD. We also review the therapeutic strategies used to prevent dopaminergic neurodegeneration by upregulating astrocytic neuroprotective properties. Understanding the multiple functions of astrocytes and the interaction of astrocytes and neurons in neurological conditions will aid in the development of a disease-modifying drug for PD in future studies.
5. Astrocyte Dysfunction in Parkinsonism
The loss of normal astrocyte roles (loss of function) is implicated in PD pathogenesis. Astrocytes construct the BBB in cooperation with endothelial cells and pericytes. The BBB disruption is thought to contribute to PD progression, because increased BBB permeability is linked to the infiltration of systemic inflammatory mediators into the brain, which can promote microglial activation and dopaminergic neurodegeneration. Gray et al. [
143] reported an increase in the permeability of the BBB in the putamen of patients with PD. Astrocyte end-feet cover over 99% of cerebral capillaries and express various receptors, transporters and channels. Astrocytes are known to play a pivotal role in BBB maintenance by modifying the formation of tight junction in endothelial cells via the release of regulatory factors, such as TGF-α and GDNF, and regulate brain microvascular permeability via astrocyte-endothelial communication [
144]. Thus, astrocytes act as “gate keepers” to prevent peripheral immune cell infiltration into the brain. Accordingly, impairment in astrocyte-endothelial communication could be involved in the increased BBB permeability observed in PD patients. In addition, the regulation of water permeability by astrocytic water channels, aquaporin-4 (AQP4), in the BBB plays an important role in several physiological conditions in the CNS. Recently, the expression and regulation of AQP4 have been studied in several pathological situations, suggesting that AQP4 participates in the onset and progression of PD [
145]. Xue et al. [
146] reported that AQP4-deficient mice displayed significantly stronger microglial inflammatory responses and a markedly greater loss of dopaminergic neurons after MPTP injection. The mechanism is that TGF-β1, a key suppressive cytokine in PD onset and development, was decreased in AQP4 deficient mice after MPTP treatment, which resulted from the impairment of its generation by astrocytes. In addition, Sun et al. [
147] demonstrated that AQP4 modulates astrocyte-to-microglia communication in neuroinflammation; AQP4 knockout mice exhibited gliosis (astrocytosis and microgliosis) in PD models produced by chronic MPTP injections, accompanied by an increase in the production of IL-1β and TNF-α in the midbrain.
EAAT2 dysfunction leads to extracellular glutamate accumulation and the onset of excitotoxicity due to excessive stimulation of excitatory amino acid receptors, which is associated with many neurodegenerative diseases. A recent study demonstrated that astrocytic GLT-1 deficiency in the SNpc induced parkinsonian phenotypes, progressive motor deficits and nigral DA neuronal death in mice [
148]; GLT-1 knockdown induced the activation of astrocytes and microglia in the SNpc. These findings suggest that GLT-1 dysfunction contributes to PD pathogenesis. Glutamine plays an important role in brain energy metabolism and as a precursor for the synthesis of neurotransmitter glutamate and γ-aminobutyric acid. In particular, glutamine transport between astrocytes and neurons is a key factor in the glutamate-glutamine cycle. Manganese (Mn) is known to lead to neurological disorders characterized by early psychotic symptoms, frequently followed by chronic symptoms common to PD [
149]. Previous studies found that Mn exposure disrupted astrocytic glutamine transporter expression and function [
150,
151]. Thus, the Mn-induced reduction of glutamine transport in astrocytes disrupts glutamate homeostasis and diminishes its availability to neurons, which may lead to impairment in glutamatergic neurotransmission and neurotoxicity.
As mentioned above, the antioxidative properties of astrocytes are extremely important for neuronal survival, especially for DA neurons. The Nrf2 signaling pathway followed by the induction of antioxidants, such as GSH and MTs, is essential in antioxidative defense. Therefore, the dysfunction of the Nrf2 system in astrocytes easily leads to dopaminergic neurodegeneration (
Figure 1). Innamorato et al. [
152] found that Nrf2 knockout mice showed exacerbated gliosis and dopaminergic neurodegeneration after injection with MPTP. L’Episcopo et al. [
153] also found that aging downregulated Nrf2 expression and its activation in response to MPTP exposure. Furthermore, Lastres-Becker et al. [
154] reported that human α-synuclein-expressing Nrf2 knockout mice exhibited exacerbated α-synuclein aggregation and the degeneration of nigral dopaminergic neurons, which is correlated with neuroinflammation and gliosis. Similarly, Nrf2 deficiency induced the microglial activation and production of inflammatory cytokines. As described above, the disruption of Nrf2 signaling in astrocytes leads to a reduction in GSH synthesis, which is linked to GSH depletion in neighboring neurons. Chinta et al. [
155] reported that the depletion of GSH within dopaminergic neurons promoted a reduction in mitochondrial complex I activity and age-related dopaminergic neurodegeneration. These findings suggest that the reduction of the GSH-supplying ability of astrocytes could result in the selective inhibition of mitochondrial complex I and dopaminergic neuronal damage. MTs are strong antioxidative molecules produced by astrocytes. MTs also detoxify metal toxicity and regulate metal homeostasis. It has been reported that MTs are overexpressed in PD brains, in which iron accumulates [
64]. Furthermore, recent studies have demonstrated that MTs may influence α-synuclein aggregation in PD [
156]. Cu accumulates in the brain with aging and binds to α-synuclein, which initiates α-synuclein aggregation [
157,
158]. MTs bind Cu with a high affinity, suggesting that MTs play a role in Cu homeostasis [
159]. Several studies have reported that MTs prevented the Cu-induced aggregation of α-synuclein [
160,
161]. Taken together, the findings suggest that the dysregulation of MTs may disturb metal homeostasis, leading to neurotoxicity. Indeed, we previously reported aggravated neurodegeneration in the nigrostriatal pathway in MT-1/-2 knockout PD model mice [
162,
163].
Various studies have demonstrated that the knockout or mutation of genes related to early-onset AR-PD promotes astrocyte dysfunction (loss of function) resulting in dopaminergic neurodegeneration. DJ-1 is a redox-sensitive protein with multiple functions, including mitochondrial physiology, protein transcription, proteasome regulation, and chaperone activity [
164]. DJ-1 gene deletions and point mutations have been identified as one of the causes of AR-PD (PARK7) [
77]. As mentioned above, DJ-1 is mainly expressed in astrocytes in the human brain, and acts as a sensor of oxidative stress [
79,
80]. Reactive astrocytes induce DJ-1 expression in response to oxidative stress and release the protein extracellularly to protect neurons [
81,
165]. Several studies have demonstrated that DJ-1 deficiency or mutation in astrocytes impairs astrocyte-mediated neuroprotection in parkinsonian models [
166,
167,
168]. In addition, DJ-1 deficiency impairs glutamate uptake into astrocytes by altering EAAT2 expression [
169], and reduces mitochondrial motility in astrocytes [
166]. Moreover, DJ-1 selectively influences TLR4 to regulate astrocyte inflammation; DJ-1 deficient astrocytes induce proinflammatory mediators, such as COX-2 and IL-6 [
170]. It has also been reported that IL-1β decreases the levels of DJ-1 and parkin in astrocytes [
171].
Parkin (PARK2), an E3 ubiquitin ligase, forms a complex with PINK1 and DJ-1, and promotes the degradation of unfolded or misfolded proteins [
172]. In addition, parkin and PINK1 have been identified as essential proteins for the removal of damaged mitochondria via autophagy (mitophagy) [
96,
97,
98]. Therefore, parkin deficiency induces aberrant ubiquitination and compromised mitochondrial integrity, leading to neuronal dysfunction and degeneration. Constitutive parkin expression is higher in neurons than in astrocytes. However, astrocytes increase parkin expression during unfolded protein stress [
173]. Solano et al. [
174] examined the effects of parkin protein loss on the response of astrocytes to oxidative stress in parkin-knockout mice. As a result, parkin deficiency was found to impair GSH synthesis in astrocytes and showed the neurodegenerative pathogenesis of parkin-linked AR-PD. These results suggest that parkin deficiency results in the abnormal function of astrocytes, making dopaminergic neurons vulnerable to oxidative stress. In addition, it has also been reported that parkin-knockout astrocytes exhibit increased ER stress, cytokine production, and decreased astrocytic secretion of neurotrophic factors [
175]. Moreover, severe mitochondrial damage was observed in mesencephalic astrocytes from parkin-knockout mice [
176].
PINK1 is a serine/threonine kinase, whose mutation is associated with AR-PD (PARK6) [
177]. PINK1 contributes to astrocyte development and proliferation, facilitating the autophagic degradation of damaged mitochondria [
178,
179]. PINK1 also regulates astrocytic inflammatory response, where the loss of PINK1 increases iNOS, NO, TNF-α, and IL-1β in astrocytes under neuroinflammatory conditions [
180]. Moreover, it has been reported that PINK1-knockout astrocytes exhibit proliferation defects, which appeared to be linked to mitochondrial defects, increased intracellular ROS levels, decreased glucose-uptake capacity, and decreased ATP production [
179]. These observations suggest that PINK1 deficiency promotes astrocytic dysfunction. In addition, parkin and PINK1 cooperate to degrade impaired mitochondria. Therefore, the dysfunction of PINK1-parkin-mediated mitophagy in astrocytes can lead to the disruption of mitochondrial maintenance.
Mutation within LRRK2 is a common cause of autosomal-dominant PD (PARK8). Although the physiological functions of LRRK2 remain unclear, it has been reported that the protein localizes to vesicular structures such as endosomes and lysosomes, suggesting that LRRK2 functions as a regulator of the endolysosomal system [
181]. Correlations between mutant LRRK2 and several pathogenic mechanisms linked to PD progression have been reported, with mutations within LRRK2 altering autophagy and, consequently, the accumulation of α-synuclein [
182,
183,
184]. LRRK2 is constitutively expressed in not only neurons but also glial cells, astrocytes and microglia, in the human brain [
185,
186]. Astrocytic LRRK2 is also involved in the autophagy-lysosome pathway as well as neurons. Henry et al. [
187] demonstrated that LRRK2 regulates lysosome size, number and function in astrocytes. The expression of LRRK2 G2019S, the most common pathological mutation, produced enlarged lysosomes and diminished the lysosomal capacity of astrocytes. As mentioned above, a recent study demonstrated that iPSC-derived astrocytes from familial mutant LRRK2 G2019S PD patients exhibited dysfunctional chaperone-mediated autophagy, impaired macroautophagy, and progressive α-synuclein accumulation, which caused dopaminergic neurodegeneration [
113]. Taken together, these findings indicate that the dysfunction of the autophagy-lysosome pathway in astrocytes may be implicated in PD pathogenesis.
ATP13A2 is a transmembrane lysosomal P5-type ATPase, whose mutation results in the Kufor-Rakeb Syndrome, a form of AR-PD (PARK9) [
188]. Missense or truncation in the Atp13a2 gene impairs lysosomal function. Qiao et al. [
189] reported that ATP13A2 deficiency induced astrocytic inflammation via NLRP3 inflammasome activation, exacerbating dopaminergic neuronal damage after MPTP treatment. In addition, a recent study demonstrated that the loss of ATP13A2 function in astrocytes could contribute to neuronal α-synuclein pathology by using iPSC-derived dopaminergic neurons and astrocytes from healthy subjects and patients carrying mutations in lysosomal ATP13A2 [
190]. The uptake and degradation of α-synuclein were reduced in ATP13A2-mutated astrocytes, which resulted in increased α-synuclein transmission between DA neurons.
Gaucher disease (GD) is a lysosomal storage disorder caused by a mutation in the glucocerebrosidase 1 (GBA1) gene, resulting in the deficiency of the enzyme glucocerebrosidase (GCase). Recently, patients with PD or the related disorder dementia with Lewy bodies were found to be more likely to carry a mutation in GBA1 compared to controls [
191]. Aflaki et al. [
192] reported that iPSC-derived astrocytes from GD patients showed deficient GCase activity, levels, and substrate accumulation. In addition, astrocytes mutated within GBA1 manifested broad deficits in lysosomal function and immune dysfunction [
193]. Furthermore, the GBA1-mutated astrocytes exhibited impaired cathepsin D activity, leading to α-synuclein accumulation, and inflammatory response. Thus, GBA1-mutated astrocytes appear to play a role in α-synuclein accumulation, contributing to neuroinflammation.
To date, neurotoxins, such as MPTP and 6-OHDA, have been used to produce PD models. In MPTP models, astrocytic activation occurs in parallel to dopaminergic cell death. Meanwhile, GFAP-expression remains upregulated even after most dopaminergic neurons have died [
194], indicating that astrocytic reactions occur after neuronal cell death [
195]. Astrocytes take up MPTP and convert it into neurotoxic 1-methyl-4-phenylpyridinium (MPP
+), which inhibits the mitochondrial complex I [
196]. Because MPP
+ is taken up into neurons via the DAT [
16], neurotoxins are able to attack dopaminergic neurons specifically. Since astrocytes also express DAT, it is suggested that MPP
+ also affects astrocytes. Boyalla et al. [
197] demonstrated that MPP
+ decreased the ATP levels and increased ROS in astrocytes. These data suggest that MPP
+ impairs energy production in astrocytes and increases oxidative stress, leading to neuronal damage. In addition, Chuang et al. [
198] also reported that MPP
+ induced astrocyte apoptosis and oxidative stress. Another neurotoxin, 6-OHDA, is also taken up by dopaminergic neurons via DAT, and then oxidized to produce radicals, thereby inhibiting mitochondrial complex I [
199]. Therefore, 6-OHDA could also be captured by astrocytes, affecting their function. Indeed, Gupta et al. [
200] demonstrated that 6-OHDA decreased mitochondrial dehydrogenase activity and mitochondrial membrane potential, and increased ROS levels, caspase-3 mRNA level, chromatin condensation, and DNA damage, which induced apoptotic cell death in astrocytes. Taken together, these findings suggest that neurotoxin-induced dopaminergic neurodegeneration could occur as a result of astrocyte dysfunction, which can cause oxidative stress, neuroinflammation and excitotoxicity.
7. Conclusions
Impaired astrocytes contribute to various pathogenic factors in PD, such as oxidative stress, neuroinflammation, α-synuclein toxicity, and mitochondrial impairment. Therefore, the normalization of astrocytic dysfunction or the upregulation of neuroprotective ability represents a therapeutic approach to prevent progressive neurodegeneration in PD. However, neurodegeneration accompanied by α-synuclein accumulation is observed in the central and peripheral nervous system in PD patients, and the mechanism underlying selective neuronal death in specific brain areas remains unknown. Astrocytes were recently recognized as morphologically and functionally diverse cells, with a reactivity that varies according to the brain region [
240]. In addition, the diverse roles of astrocytes include modulation of neuronal survival via neuron-astrocyte interactions. In recent studies, we identified regional differences in the response of astrocytes and the induction of antioxidative molecules in astrocytes against oxidative stress, and found that region-specific features of astrocytes lead to region-specific vulnerability of neurons [
241]. cDNA microarray analysis showed that 6-OHDA treatment upregulated the expression of Nrf2 and Nrf2-regulating molecules related to GSH synthesis in striatal astrocytes but not mesencephalic astrocytes. In addition, midbrain neurons co-cultured with striatal astrocytes were found to be more resistant to 6-OHDA than those with mesencephalic astrocytes. Furthermore, astrocyte conditioned media from 6-OHDA-treated striatal astrocytes showed a greater protective effect on 6-OHDA-induced neurotoxicity and oxidative stress than that from mesencephalic astrocytes. These results suggest that the region-specific response of astrocytes could determine neuronal vulnerability. Furthermore, with regards to astrocyte-microglia interactions, such as activated microglia-induced A1 astrocyte in neuroinflammation, regional differences in astrocyte-microglia interactions and their response in neurological conditions may contribute to the region-specific neuronal death observed in PD pathology.
PD is a complex, multi-system, neurodegenerative disorder, with PD patients exhibiting not only motor symptoms due to the loss of nigrostriatal dopaminergic neurons but also non-motor symptoms that precede motor symptoms [
1]. Gastrointestinal dysfunction is a particularly prominent non-motor symptom of PD. Constipation appears approximately 10 to 20 years prior to the presentation of motor symptoms [
242]. Pathologically, α-synuclein deposition is also observed in the peripheral nervous system of sporadic PD patients, including the intestinal myenteric plexus, gastric mucosa, cardiac sympathetic nerve, and skin nerve [
243]. It is currently hypothesized that PD pathology propagates from the ENS to the CNS via the vagal nerve [
4]. The ENS controls the gastrointestinal tract on the local level, in which astrocyte-like GFAP-positive stellate-shaped enteric glial cells (EGC) are found. EGC express various receptors for neurotransmitters and neuromodulators that allow them to respond to transmitters released from neurons [
244]. Thus, the intimate physical association between enteric glia and neurons is highly reminiscent of the relationship between astrocytes and neurons in the CNS. EGC dysfunction in pathological conditions has been attributed to abnormal intestinal motor activity, which in turns causes constipation. In a recent study, we observed a reduction of GFAP-positive enteric glia in the intestine of a novel parkinsonian model produced by chronic systemic exposure to a low dose of rotenone (2.5 mg/kg/day) for 4 weeks [
245]. The PD model also exhibited neurodegeneration of the intestinal myenteric plexus, accompanied by α-synuclein accumulation and gastrointestinal dysfunction [
246]. In addition, the expression of MT-1/-2 in GFAP-positive enteric glia cultured cells from the intestine was found to decrease after exposure to rotenone. Furthermore, treatment with the coffee components caffeic acid and chlorogenic acid was found to inhibit the rotenone-induced reduction of MT/1-/2 expression in enteric glia and neurodegeneration in the myenteric plexus [
245]. These results suggest that the upregulation of antioxidative molecules in the EGC prevents neurodegeneration in the ENS. Taken together, these findings suggest that astrocytes (astrocyte-like glial cells) contribute to neuronal function and survival not only in the CNS but also ENS. As a result, this review provides insights for the design of new therapies aimed at providing neuroprotection against neurological disorders.