Background
Parkinson’s disease (PD) is a common neurological disorder characterised by progressive degeneration of dopaminergic (DA) neurons and the formation of cytoplasmic inclusions called Lewy bodies in the substantia nigra pars compacta (SNc). The resulting disruption of DA neurotransmission in the basal ganglia produces progressive extrapyramidal motor symptoms. A range of pathogenic mechanisms causing DA neuronal death has been proposed [
1]. Recently, accumulating evidence suggests important roles for non-neuronal cells, especially astrocytes, in DA neuron degeneration [
2].
Astrocytes are the most abundant glial cell type in the central nervous system and play crucial roles in brain homeostasis, providing metabolic, electrical, and structural support for surrounding neurons in both normal and pathological conditions [
3]. Astrocytes are heterogeneous in their functions and morphologies, depending on their location, subtype, and developmental stage [
4]. Following pathological brain insult, astrocytes undergo a dynamic transformation called reactive astrogliosis. The functions of reactive astrocytes are controversial, and they have been reported to play both neuroprotective and neurodegenerative roles, providing another example of their heterogeneity [
5]. The roles of astrocytes largely depend on the molecules they release into and take up from the extracellular space. Recently, it has been proposed that two types of reactive astrocytes, harmful A1 and protective A2 types, should be recognised based on genetic classification [
6,
7].
In PD, astrocytes accumulate α-synuclein in their cytoplasm, and the distribution of such cells parallels that of Lewy bodies [
8]. Animal PD models have shown that the accumulation of α-synuclein aggregates in astrocytes promotes their secretion of proinflammatory cytokines and chemokines, resulting in microglial activation [
9,
10]. In contrast, astrocytes exert neuroprotective functions by releasing a variety of trophic factors, such as glial cell-derived neurotrophic factor (GDNF) [
11] and brain-derived neurotrophic factor (BDNF) [
12]. Thus, because astrocytes can both facilitate and prevent neuronal damage, their precise roles in PD remain uncertain.
Striatal DA terminal loss is an early and dominant feature of PD, suggesting that PD pathology may begin in the terminals and progress retrogradely to neuron bodies in the SNc [
13]. However, mechanisms for this retrograde degeneration (‘dying back’) are not well characterised. Studies in a 6-hydroxydopamine (6-OHDA)-induced rat model of PD showed that levels of Cx30, but not of Cx43, the two major gap junction connexins (Cxs) in astrocytes, were increased in the striatum of 6-OHDA rats, suggesting that Cx30 levels may be associated with the disease process [
14]. Based on the elevated Cx30 immunoreactivity around the vessels in 6-OHDA-treated mice, the authors hypothesised that Cx30 contributes to neurometabolic coupling via its channel-mediated energy transportation [
14]. Recently, however, non-channel functions of Cx30 have been investigated [
15], and it has been shown that Cx30 can alter astrocyte morphology and modulate their functions, such as synaptic transmission [
16]. Thus, the precise roles of Cx30 in DA neurodegeneration remain to be established, especially in terms of non-channel-mediated functions.
We therefore aimed to determine the roles of Cx30 in the pathomechanisms of PD using a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) PD model in Cx30 knockout (KO) mice. The goals of this study were to clarify (i) whether the distribution or levels of Cx30 and Cx43 in astrocytes of the nigrostriatal system are altered in response to MPTP, (ii) whether Cx30-deficient mice are sensitive to MPTP toxicity, and (iii) whether the lack of Cx30 induces astrocyte modulation, particularly A1 and A2 astrocytes, under conditions of MPTP toxicity.
Methods
Ethical statement
The experimental procedures were designed to minimise the number of animals used as well as animal suffering. All animal experiments were carried out according to the guidelines for the proper conduct of animal experiments published by the Science Council of Japan, and ethical approval for the study was granted by the Animal Care and Use Committee of Kyushu University (#No. A29-179). The Animal Research: Reporting of In Vivo Experiments (ARRIVE) guidelines for animal research were followed.
Animals
Male heterozygous Cx30 KO mice were obtained from the European Mouse Mutant Archive [
17]. Their spermatozoa were used to fertilise C57BL/6 oocytes in vitro. Heterozygous mice were interbred to obtain homozygous Cx30 KO and wild-type (WT) mice. Homozygous non-mutant mice were used as WT controls to ensure the control of the genetic background. All mice were provided food and water ad libitum and kept under a 12-h light/dark cycle in a specific pathogen-free room at the Biomedical Research Laboratory Station for Collaborative Research I of Kyushu University.
MPTP treatment
Eight-week-old male WT and Cx30 KO mice were intraperitoneally injected with 20 mg/kg of free base MPTP-HCl (Tokyo Chemical Industry, Tokyo, Japan) four times at 2 h intervals, and were sacrificed on days 1 and 7 after the last injection.
Gene expression microarrays
Total RNA was isolated from the striatum of each animal on day 1 after the last MPTP injection using TRIzol reagent (Thermo Fisher Scientific, Waltham, MA, USA) and purified using the SV Total RNA Isolation System (Promega Corporation, Madison, WI, USA). Total RNA (50 ng) was amplified and labelled with amplification and labelling kits (Agilent Technologies, Santa Clara, CA, USA) and then hybridised to a 60K Agilent 60-mer oligomicroarray (Agilent Technologies). The hybridised microarray slides were scanned using an Agilent Scanner, and the relative hybridisation intensities and background hybridisation values were calculated using Agilent Feature Extraction software (9.5.1.1). Gene set enrichment analysis (GSEA) (
www.broadinstitute.org/gsea) was performed to determine the enrichment score (ES), which indicates the degree to which each gene set is overrepresented at the top or bottom of a ranked list of genes. The false discovery rate (FDR) is the estimated probability of an ES representing a false-positive finding. An ES with a normalised
p value less than 0.05 by an empirical phenotype-based permutation test and an FDR less than 0.25 was considered to be significant. We also calculated
Z-scores and ratios from the normalised signal intensities of each probe for comparison.
Z-scores are the number of standard deviations from the mean of log-scaled signal intensities. Ratios are non-log-scaled fold changes in signal intensities. We established criteria of
Z-score ≥ 2.0 and ratio ≥ 1.5 for upregulated genes and
Z-score ≤ − 2.0 and ratio ≤ 0.66 for downregulated genes. To determine significantly overrepresented categories of KEGG pathways, we used the tools and datasets provided at the Database for Annotation, Visualisation and Integrated Discovery (DAVID) (
http://david.abcc.ncifcrf.gov/home.jsp). The raw data from this study have been submitted to the Gene Expression Omnibus (accession number: GSE113693).
Immunohistochemistry
Mice were euthanised and transcardially perfused with 4% paraformaldehyde. The brains were removed, fixed in 4% paraformaldehyde overnight, cryopreserved in 30% sucrose in phosphate-buffered saline (PBS), and stored at − 80 °C until analysis. For fluorescent immunostaining, 40-μm-thick sections were cut and incubated overnight at 4 °C with the primary antibodies against Cx30, Cx43, glial fibrillary acidic protein (GFAP), and ionised calcium-binding adapter molecule 1 (Iba1) (Additional file
1: Table S1) and then with an Alexa Fluor 488- or 594-conjugated secondary antibody for 1 h. The sections were mounted in mounting medium containing 4′,6-diamidino-2-phenylindole (DAPI) (Vector Laboratories, Burlingame, CA, USA) and visualised by confocal laser microscopy (Nikon A1, Nikon, Tokyo, Japan). For colorimetric immunostaining, the sections were incubated with primary antibodies against tyrosine hydroxylase (TH), dopamine transporter (DAT), GFAP, and Iba1 (Additional file
1: Table S1) followed by a rabbit or mouse Vectastain Elite ABC HRP kit (Vector Laboratories) and ImmPACT DAB Peroxidase Substrate (Vector Laboratories). We used Nikon NIS-Elements software (Nikon) to calculate the number of Iba1-positive cells, Cx30 dots, and Cx43 dots in the unilateral striata according to the previously reported procedures [
18,
19]. The values from the three sections for each animal were averaged.
In situ hybridisation of S100a10 and Gdnf
Mice were euthanised and perfused as described above. The brains were removed, fixed with G-Fix (Genostaff, Tokyo, Japan), and embedded in paraffin on a CT-Pro20 system (Genostaff) using G-Nox (Genostaff), which is a less toxic organic solvent than xylene. The brains were cut into 8-μm-thick sections and stained as follows with an in situ hybridisation (ISH) Reagent Kit (Genostaff) according to the manufacturer’s instructions. The tissue sections were deparaffinised with G-Nox and rehydrated through a graded ethanol series to PBS. The sections were fixed with 10% formalin in PBS for 30 min at 37 °C, washed with distilled water, placed in 0.2 N HCl for 10 min at 37 °C, washed in PBS, treated with 4 μg/ml proteinase K (Wako Pure Chemical Industries, Osaka, Japan) in PBS for 10 min at 37 °C, and washed again in PBS. The sections were then placed in a Coplin jar containing 1× G-Wash (Genostaff; equivalent to 1× saline sodium citrate buffer). Hybridisation was performed by incubation with probes for
S100a10 and
Gdnf (Additional file
1: Table S2) at 250 ng/ml in G-Hybo-L (Genostaff) for 16 h at 60 °C. The sections were then washed in 1× G-Wash for 10 min at 60 °C and incubated in 50% formamide in 1× G-Wash for 10 min at 60 °C. The sections were washed twice in 1× G-Wash for 10 min at 60 °C, twice in 0.1× G-Wash for 10 min at 60 °C, and twice in 0.1% Tween-20 in Tris-buffered saline (TBST) at room temperature. The sections were then incubated with 1× G-Block (Genostaff) for 15 min at room temperature and with alkaline phosphatase-coupled anti-digoxigenin (Roche Diagnostics, Mannheim, Germany) diluted 1:2000 with 1× 50 G-Block (Genostaff) in TBST for 1 h at room temperature. The sections were washed twice in TBST and then incubated in 100 mM NaCl, 50 mM MgCl
2, 0.1% Tween-20, 100 mM Tris-HCl, and pH 9.5. Colour development was performed by incubation with nitro blue tetrazolium and 5-bromo-4-chloro-3′-indolyphosphate solution (Sigma-Aldrich, St. Louis, MO, USA) overnight followed by washing in PBS.
Following ISH, S100β protein was detected by IHC. The sections were incubated with 0.3% H
2O
2 in PBS for 30 min to block endogenous peroxidase and then incubated with G-Block (Genostaff) and Avidin/Biotin Blocking reagent (Vector Laboratories). The sections were then incubated successively with anti-S100β antibody (Abcam) at 4 °C overnight, biotin-conjugated goat anti-rabbit antibody (Dako, Santa Clara, CA, USA) for 30 min at room temperature, and peroxidase-conjugated streptavidin (Nichirei, Tokyo, Japan) for 5 min. Peroxidase activity was visualised by incubation with diaminobenzidine, and the sections were mounted with G-Mount (Genostaff). Images of the bilateral striata were captured using a Leica DM2500 microscope with a × 40 objective (Leica Microsystems, Wetzlar, Germany). Finally, the number of cells double positive for S100β protein and either
Gdnf or
S100a10 mRNA was counted. The primer sequences specific for
Gdnf and
S100a10 are listed in Additional file
1: Table S2.
Stereology
The total numbers of TH-, GFAP-, and Iba1-positive cells in unilateral SNcs were measured stereologically using an optical fractionator method as previously described [
20‐
22]. Every fourth section through the SNc was analysed using Stereo Investigator software (Stereo Investigator 10.0; MicroBrightField, Williston, VT, USA). Immunolabelled cells were counted by the optical fractionator method (× 40 objective; counting frame, 100 × 100 μm; sampling grid, 200 × 200 μm; counting frame thickness, 10 μm). The coefficient of error (Gundersen,
m = 1) for cell count estimation was less than 0.15 for each animal.
Western blotting of Cx30, Cx43, and GFAP in striatum specimens
Striata were rapidly isolated and homogenised on ice in a radioimmunoprecipitation buffer containing a protease inhibitor cocktail, 0.5% sodium dodecyl sulphate (Nacalai Tesque, Kyoto, Japan), and PhosSTOP phosphatase inhibitor cocktail (Roche Diagnostics). Lysates were incubated on ice for 30 min, sonicated, and then centrifuged at 4 °C for 10 min at 10,000×g. Supernatants were removed, and protein concentrations were determined using a BCA protein assay kit (Thermo Fisher Scientific). The samples were mixed with Laemmli buffer with (Cx43 and GFAP) or without (Cx30) heating to 95 °C for 5 min. Proteins were separated by polyacrylamide gel electrophoresis (12% for Cx30 and 7.5–15% gradient for Cx43 and GFAP) and blotted onto polyvinyl difluoride membranes. The membranes were blocked with 3% nonfat milk (for Cx30) or Blocking One (Nacalai Tesque; for Cx43 and GFAP) and then incubated with anti-Cx30 (1: 500; Invitrogen), anti-Cx43 (1: 10,000; Abcam), or anti-GFAP antibodies (1: 2000; STEMCELL Technologies) overnight at 4 °C or with anti-β-actin antibody (1: 20,000; Sigma-Aldrich) for 1 h at room temperature. The membranes were then incubated with a horseradish peroxidase-conjugated secondary antibody for 1 h at room temperature. The immunoreactive protein bands were visualised by enhanced chemiluminescence (ECL Prime, GE Healthcare Bio-Sciences, Uppsala, Sweden). Band intensities were measured using the ChemiDoc™ XRS system (Bio-Rad Laboratories) and normalised to β-actin levels.
Analysis of TH- and DAT-positive fibre density
The density of TH- and DAT-positive fibres in the striatum was assessed in the sections between Bregma + 0.62 and − 0.10 mm, as previously reported [
23]. Every fourth section was immunolabelled, to give a total of six sections analysed per animal. Four to six images per section were captured using an Olympus B51 microscope with a × 100 objective (Olympus Corp., Tokyo, Japan). The optical density was measured using ImageJ software (National Institutes of Health, Bethesda, MD, USA), followed by background subtraction of the corpus callosum.
RNA extraction and quantitative reverse transcription polymerase chain reaction (RT-PCR) of Cx30, Cx43, S100a10, and Gdnf mRNA in the striatum
Total RNA was isolated from the striata using an RNA purification kit (Qiagen, Hilden, Germany), and cDNA was synthesised using ReverTra Ace qPCR RT Master Mix with gDNA Remover (Toyobo, Osaka, Japan). Quantitative PCR was performed using an Applied Biosystems 7500 Real-Time PCR System (Thermo Fisher Scientific) with TaqMan Gene Expression Master Mix (Thermo Fisher Scientific) and TaqMan Gene Expression Assays (Cx30, Mm00433661_s1; Cx43, Mm01179639_s1; S100a10, Mm00501457_m1; Gdnf, Mm01285715_m1; Thermo Fisher Scientific). Glyceraldehyde 3-phosphate dehydrogenase (Gapdh, Mm99999915_g1) was amplified as an internal control. The ∆∆CT efficiency correction method was used to calculate relative mRNA levels.
Measurement of GDNF protein level in the striatum by enzyme-linked immunosorbent assay (ELISA)
The striata were rapidly isolated and homogenised on ice in a radioimmunoprecipitation buffer containing a protease inhibitor cocktail (Nacalai Tesque). Lysates were incubated on ice for 30 min, sonicated, and centrifuged at 4 °C for 10 min at 10,000×g. Supernatants were removed, and protein concentrations were determined using a BCA protein assay kit (Thermo Fisher Scientific). GDNF was measured using a specific ELISA kit (Affymetrix, Santa Clara, CA, USA).
Measurement of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) levels by high-performance liquid chromatography with electrochemical detection (HPLC-ECD)
The striata were weighed and stored at − 80 °C until analysis. Frozen striata were sonicated in 300 μl of ice-cold 0.1 N perchloric acid and 0.2 mM sodium bisulphite solution and centrifuged at 4 °C for 20 min at 20,000×g. The supernatant was filtered and analysed by HPLC-ECD. Aliquots (10 μl) of each sample were separated on a C18 reverse phase column (Capcell Pak column, 150 × 4.6 mm, Shiseido, Tokyo, Japan) and detected with an ECD system consisting of a Coulochem III controller (ESA, Inc., Chelmsford, MA, USA) fitted with a guard cell (M5020) and an analytical cell set (M5011). The guard cell was set at 450 mV, electrode 1 at 50 mV, and electrode 2 at 400 mV. The mobile phase was phosphate buffer containing heptanesulfonic acid, and the flow rate was 0.8 ml/min.
Measurement of striatal 1 methyl-4-phenylpyridinium (MPP+) levels
Mice were injected once with MPTP and euthanised 2 h later. The brains were removed, and the striata were weighed, placed in 100 μl of tissue buffer (0.1 M phosphate-citric acid buffer, pH 2.5, containing 20% methanol), and centrifuged for 1 min at 13,000×g. Aliquots (2 μl) of each supernatant sample were applied to a liquid chromatography-tandem quadrupole mass spectrometer (LC-MS/MS) with electrospray ionisation in positive mode. LC was performed on a Waters UltraPerformance LC system (Waters Corporation, Milford, MA, USA) with a Waters BEH C18 column (2.1 mm × 100 mm) maintained at 40 °C. The mobile phase consisted of 0.1% formic acid in water (solvent A) and acetonitrile (solvent B). Separation was performed starting at 5% B increasing to 95% B over 5 min. The flow rate was 0.3 ml/min.
Statistical analysis
All values are expressed as the means ± standard error of mean (SEM). Differences were analysed using two-way analysis of variance (ANOVA) for more than two groups, Dunnett’s t test for two groups, and Pearson’s chi-square test for comparison of mortality rates. When ANOVA showed significant differences, pairwise comparisons were assessed using the Tukey–Kramer post hoc test. The null hypothesis was rejected at the 0.05 level.
Discussion
The main findings of the present study are as follows: (1) Cx30 expression in astrocytes was markedly upregulated in the striatum and SNc 7 days after MPTP administration in WT mice. (2) Cx30 deficiency modestly increased basal Cx43 levels and MPTP-upregulated Cx43 levels on day 1 after administration, but not on day 7 after administration. (3) Cx30 deficiency accelerated MPTP-induced DA neuron loss but had no effect on MPP+ production or DAT intensity in the striatum. (4) Cx30 deficiency reduced the responses of A2 and pan-reactive astrocytes on day 1 after MPTP treatment and decreased the expression of S100a10 mRNA, Gdnf mRNA, and GDNF protein levels in the striatum on day 1, but not day 7, after MPTP treatment. (5) Cx30 deficiency partly suppressed GFAP upregulation and astrogliosis in the striatum and SNc on day 7 after MPTP treatment but did not change microglial responses. These results suggest that the hypersensitivity of Cx30 KO mice to MPTP is mainly due to the alterations in the acute responses of astrocytes.
Upregulation of both Cx30 and Cx43 in WT mice upon MPTP treatment can be either neuroprotective or neurotoxic. Increased DA neuron loss in Cx30-deficient mice indicates that the expression and upregulation of Cx30 are beneficial in the acute MPTP PD model. It has been reported that Cx30, but not Cx43, can compensate for other Cxs in the hippocampus [
35]. Indeed, although Cx30 KO mice showed increased basal expression of Cx43 in the striatum and showed increased upregulation of Cx43 at 1 day after MPTP treatment compared with WT mice, this was not sufficient to compensate for the absence of Cx30. In another PD model, Cx30 was upregulated in the striatum after treatment with 6-OHDA, whereas Cx43 was not [
14]. Collectively, these observations suggest that Cx30 is mainly involved in astrocytic neuroprotection, at least in these neurotoxin-induced PD models. Because the deficiency of Cx30 did not interfere with the MPTP-induced increase in Cx43, Cx30-mediated neuroprotection likely occurs through a Cx43-independent mechanism.
Both Cx30 and Cx43 are abundant in the perivascular astrocyte foot processes [
36,
37], while Cx30 is also a major Cx in astrocyte processes around neurons in the grey matter [
37]. Cx30 thus appears to be a component of the astrocytic metabolic network, providing an activity-dependent intercellular pathway for the delivery of energy sources, such as glucose and lactate, from the blood vessels to neurons [
14,
38,
39]. This is consistent with the fact that Cx30 ablation reduces energy trafficking [
38]. MPTP induced marked perivascular upregulation of Cx30 in the striatum in our study, similar to the previous observations with 6-OHDA [
14]. This indicates that the increased transfer of energy sources from the blood vessels to neurons via astrocyte Cx30 channels may be partly responsible for neuronal survival after exposure to neurotoxic compounds, such as by MPTP and 6-OHDA, which inhibit the mitochondrial respiratory chain [
40,
41].
Non-channel functions of Cx30 might also contribute to enhanced neuronal survival after MPTP treatment. Many channel-independent functions of Cxs in cell growth, migration, apoptosis, and signalling have been reported [
16,
42], including inhibition of DNA synthesis and subsequent effects on gene expression [
42,
43]. We evaluated the effects of Cx30 deficiency on gene expression patterns in the striatum of MPTP-treated animals and found that upregulation of neuroprotective A2 and pan-reactive astrocyte genes was markedly attenuated in Cx30 KO mice compared with that in WT mice. Upregulation of A2 astrocyte gene expression in WT mice in response to MPTP was similar to that observed in a brain infarct model [
6]. Because neuronal injury by MPTP and acute ischaemia by middle cerebral artery occlusion both involve acute energy failure [
44], it seems reasonable to propose that both insults predominantly induce A2 astrocyte gene expression. Attenuation of the neuroprotective A2 astrocyte response by Cx30 deficiency may well contribute to MPTP hypersensitivity in the Cx30 KO mice.
In murine MPTP models, dopaminergic terminal loss occurs earlier and to a greater extent than cell body loss [
23,
45], as seen in human PD pathology [
13]. We detected no significant difference in DAT intensity between WT and Cx30 KO mice, indicating that Cx30 deficiency does not upregulate DAT expression in DA neuron terminals. MPP
+ concentrations in the striatum were also not significantly different in the two mouse strains, suggesting that MPP
+ uptake by DA neuron terminals, and the subsequent damage is comparable in the presence and absence of Cx30. MPP
+ is produced from MPTP in astrocytes and reduces their viability in a concentration-dependent manner [
46]. It is possible that Cx30 KO astrocytes may be more vulnerable than WT astrocytes to MPP
+. Since striatal astrocytes reportedly have protective functions on DA neuron terminals [
47,
48], such heightened vulnerability may reduce the ability of Cx30 KO astrocytes to protect DA neurons. Our pathway enrichment analysis identified alterations in the expression of genes related to the axon guidance pathway, including netrins, ephrins, semaphorins, and their receptors. These molecules not only regulate axon guidance during brain development but also are involved in axon regeneration after brain injury in adults [
49]. DA axon regeneration occurs in the striatum after MPTP or 6-OHDA treatment [
50,
51], and ephrin signalling has been reported to influence DA neurogenesis in adult PD animal models [
52,
53]. Multiple single nucleotide polymorphisms in axon guidance pathway genes are known to confer PD susceptibility [
54]. Since astrocyte-secreted proteins and signals, such as netrins, ephrins, and semaphorins, act as axon guidance cues [
55], altered expression of these genes may reduce the recovery from dying-back degeneration of DA neurons in the striatum of MPTP
-treated Cx30 KO mice. Further studies will be required to clarify this issue.
We found that MPTP induced upregulation of the A2 astrocyte-related gene
S100a10 on day 1 after treatment in both WT and Cx30 KO mice, but the magnitude of the response was smaller in the Cx30 KO mice. However, there were no genotype- or treatment-related differences in
S100a10 expression at day 7 after MPTP administration, suggesting that the acute upregulation of
S100a10 may be beneficial for neuronal survival. S100A10 is a member of the S100 protein family and is expressed in numerous cell types, including astrocytes [
6]. In the present study, we confirmed the expression of
S100a10 mRNA in striatal astrocytes by ISH. Although the precise functions of astrocytic S100A10 remain to be established, its functions may be similar to the non-channel functions of Cx30 [
56‐
58]. S100A10 is required for membrane repair [
56], cell proliferation [
57], and inhibition of cell apoptosis by interaction with a Bcl-xL/Bcl-2-associated death promoter [
58]. Thus, it is possible that S100A10 may promote the survival and proliferation of astrocytes upon MPTP exposure, thereby supporting the survival of neurons through secretion of neuroprotective factors. This hypothesis is supported by the observation that the number of GFAP-positive astrocytes was significantly decreased in Cx30 KO mice compared with WT mice at 7 days after MPTP treatment.
One such astrocyte-protective molecule is GDNF. Although
Gdnf is not listed in the A2 astrocyte-related genes, GDNF is constitutively expressed by striatal astrocytes [
31,
47] and is indispensable for DA neuron survival in adulthood [
48]
. Thus, we asked whether Cx30 deficiency might influence astrocytic-protective functions by molecules other than those reported to be A2 astrocyte-related genes. We found that basal levels of
Gdnf mRNA and GDNF protein in the striatum were similar in WT and Cx30 KO mice, but their levels were significantly lower in Cx30 KO mice than in WT mice on day 1 after MPTP treatment. This acute reduction in striatal GDNF could be one reason for decreased DA neuron survival in MPTP-treated Cx30 KO mice. We confirmed the expression of
Gdnf mRNA in astrocytes by ISH, consistent with previous reports in the 6-OHDA PD model [
31]. Since we also found that GDNF was expressed in other cell types in the striatum, it is possible that Cx30 loss may also influence GDNF synthesis in those cells, perhaps as a consequence of the hyporeactivity of A2 astrocytes. One of the pan-reactive astrocyte genes that showed no change in the expression in response to MPTP in Cx30 KO mice was
Cp, which encodes ceruloplasmin. This protein has properties similar to those of GDNF, including constitutive expression in the striatum [
59]. In addition, ablation of ceruloplasmin leads to nigrostriatal degeneration, and its supplementation restores DA neurons [
60]. Based on these observations, we suggest that Cx30 deficiency in astrocytes reduces constitutive expression of
Gdnf and other survival factor genes, directly in striatal astrocytes and indirectly in neurons, which promotes the death of DA neurons.
Activated microglia found in the PD striatum and SNc are proposed to play an important role in neuroinflammation, which accelerates disease progression [
61]. In our acute MPTP PD model, we observed widespread microglial activation in the nigrostriatal system. However, no difference was detected in microglia numbers or in M1- and M2-related gene expression between WT and Cx30 KO mice at 1 day after MPTP administration, when the microglial response was at its peak. Therefore, A2 astrocyte and pan-reactive astrocyte responses in the striatum may be more critical than the microglial response for ‘dying-back’ cell death of DA neurons in the SNc, at least in acute neurotoxin PD models. The contributions of astroglia and microglia to DA neuron loss in chronic PD models, however, remain to be determined. In our hands, Cx30 KO mice displayed attenuated A2 astrocyte-related
S100a10 upregulation and constitutive GDNF production on day 1, but not on day 7, after MPTP treatment, whereas Cx30/Cx43 upregulation was markedly enhanced at 7 days after MPTP administration in WT mice. Cx43 was also strongly upregulated 7 days after MPTP administration in Cx30 KO mice. We therefore suggest that
Gdnf,
S100a10, and other A2 astrocyte-related gene products may act as protective factors at a very early stage of MPTP exposure, while astroglial Cx channels may play roles at later times.
This study has several limitations. First, we examined the striatum and SNc only on days 1 and 7 after MPTP administration. Thus, investigation of additional time points may clarify the precise times at which neuroprotective factors operate. Second, we assessed differential gene and protein expression using whole dissected striatal tissues and not purified astrocytes. Although we confirmed that
Gdnf and
S100a10 mRNA are expressed in striatal astrocytes, a quantitative comparison of GDNF and S100A10 expression in astrocytes, neurons, and other cell types will be informative. Third, we did not examine the channel functions of striatal Cxs in situ. Finally, we used an acute MPTP treatment regimen, whereas DA neuronal degeneration in PD patients occurs over a much longer time frame. However, because axonal degeneration precedes cell death even after acute MPTP treatment [
23,
45], we believe that our results provide important insights into the role of astrocytes in dying-back degeneration of DA neurons in PD.