Decrease in ITGA7 Levels Is Associated with an Increase in α-Synuclein Levels in an MPTP-Induced Parkinson’s Disease Mouse Model and SH-SY5Y Cells

We investigated the potential association between integrin α7 (ITGA7) and alpha-synuclein (α-syn) in a methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson’s disease (PD) mouse model. Tyrosine hydroxylase (TH), ITGA7, and α-syn expression in the substantia nigra (SN) of the brain were observed to examine the pathological characteristics of PD. To determine the relationship between ITGA7 and PD, the expression of TH and α-syn was investigated after ITGA7 siRNA knockdown in SH-SY5Y cells. The ITGA7 microarray signal was decreased in the SN of the MPTP group, indicating reduced ITGA7 expression compared to that in the control. The expression patterns of ITGA7 in the control group and those of α-syn in the MPTP group were similar on immunohistochemical staining. Reduction in ITGA7 expression by ITGA7 siRNA administration induced a decrease in TH expression and an increase in α-syn expression in SH-SY5Y cells. The decreased expression of ITGA7 significantly decreased the expression of bcl2 and increased the bax/bcl2 ratio in SH-SY5Y cells. These results suggest that reduced ITGA7 expression may be related to increased α-syn expression and apoptosis of dopaminergic cells in an MPTP-induced PD mouse model. To the best of our knowledge, this is the first study to show an association between ITGA7 and PD.


Introduction
As life expectancy and the prevalence of degenerative brain diseases increase, the number of patients with Parkinson's disease (PD), the second most common degenerative brain disease after Alzheimer's disease, is increasing rapidly [1]. The main symptoms of PD are motor function-related symptoms, including slow movement, tremor, rigidity, and postural instability [2]. PD is characterized by the loss of dopamine-secreting neurons in the substantia nigra (SN) located in the midbrain, but the cause of this decrease is still unknown [3]. The major histopathological feature of PD is the deposition of Lewy bodies, and the major protein component of these intracellular deposits is a fibrillar aggregate of α-synuclein (α-syn) [4]. Increased expression of α-syn was observed in both patients with PD and PD animal models and is reported to be closely related to PD pathology [5]. Self-aggregation of these proteins leads to the formation of an amphipathic helical structure, which may result in membrane association [6].
Integrin α7 (ITGA7), also known as the integrin subunit alpha 7 and integrin alpha 7 chain 3, encodes an extracellular matrix (ECM)-binding protein [7]. Integrins are a major family of cell surface receptors that mediate ECM adhesion and are closely implicated in the regulation of various cellular functions, including embryonic development, tumor 2 of 10 cell growth and metastasis, and programmed cell death [8]. This protein functions as a receptor for laminin-1, a basement membrane protein on the surface of skeletal myoblasts and muscle fibers [9].
Abnormal integrin expression is associated with several human diseases [10]. Defects in ITGA7 are associated with congenital myopathy [11]. ITGA7-deficient mice display significant hyperplasia, hypertrophy of arteries and arterioles, and malformation of skeletal muscles [12,13]. In addition, ITGA7 deficiency is common in muscular dystrophy and myopathy [14]. α-syn and integrin were first reported in 2005, in a case of MSA, another synucleinopathy [15], and there are also recent studies reporting integrins and α-syn [16,17].
Based on these previous studies, we hypothesized that ITGA7 expression is involved in the pathological changes in PD. The main symptoms of PD are motor function-related symptoms; therefore, we speculated that the motor function-related symptoms of PD may be related to myopathy due to a decrease in ITGA7 expression. Herein, we report that ITGA7 expression is reduced in the SN in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism mouse model. We further showed that the reduction in ITGA7 expression induces an increase in α-syn expression in SH-SY5Y cells treated with MPP+. To the best of our knowledge, this is the first study to show an association between ITGA7 and PD.

Reduction in Tyrosine Hydroxylase Expression in a Mouse Model of Chronic MPTP-Induced PD
MPTP and saline were injected intraperitoneally once daily in the control (CTL) and MPTP groups, respectively. After four weeks, to confirm that a chronic MPTP-induced PD mouse model was established, we analyzed the changes in tyrosine hydroxylase (TH) expression in the SN and striatum (ST). The expression of TH was significantly decreased in MPTP mice (Figure 1c,d) compared to that in the CTL mice in both the SN and ST. Similar to the results of the Western blot analysis, it was confirmed that the expression of TH was significantly reduced in the SN and ST (p < 0.005) treated with MPTP ( Figure 1). family of cell surface receptors that mediate ECM adhesion and are closely implicated in the regulation of various cellular functions, including embryonic development, tumor cell growth and metastasis, and programmed cell death [8]. This protein functions as a receptor for laminin-1, a basement membrane protein on the surface of skeletal myoblasts and muscle fibers [9]. Abnormal integrin expression is associated with several human diseases [10]. Defects in ITGA7 are associated with congenital myopathy [11]. ITGA7-deficient mice display significant hyperplasia, hypertrophy of arteries and arterioles, and malformation of skeletal muscles [12,13]. In addition, ITGA7 deficiency is common in muscular dystrophy and myopathy [14]. α-syn and integrin were first reported in 2005, in a case of MSA, another synucleinopathy [15], and there are also recent studies reporting integrins and α-syn [16,17].
Based on these previous studies, we hypothesized that ITGA7 expression is involved in the pathological changes in PD. The main symptoms of PD are motor function-related symptoms; therefore, we speculated that the motor function-related symptoms of PD may be related to myopathy due to a decrease in ITGA7 expression. Herein, we report that ITGA7 expression is reduced in the SN in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism mouse model. We further showed that the reduction in ITGA7 expression induces an increase in α-syn expression in SH-SY5Y cells treated with MPP+. To the best of our knowledge, this is the first study to show an association between ITGA7 and PD.

Reduction in Tyrosine Hydroxylase Expression in a Mouse Model of Chronic MPTP-Induced PD
MPTP and saline were injected intraperitoneally once daily in the control (CTL) and MPTP groups, respectively. After four weeks, to confirm that a chronic MPTP-induced PD mouse model was established, we analyzed the changes in tyrosine hydroxylase (TH) expression in the SN and striatum (ST). The expression of TH was significantly decreased in MPTP mice (Figure 1c,d) compared to that in the CTL mice in both the SN and ST. Similar to the results of the Western blot analysis, it was confirmed that the expression of TH was significantly reduced in the SN and ST (p < 0.005) treated with MPTP ( Figure 1).  Western blot analyses (e) showed that TH expression was significantly decreased in MPTP-treated mice (f). ** denotes p < 0.005 compared to CTL. All values are expressed as mean ± standard error, and statistical analysis was performed using Student's t-test.

Microarray Analysis and Western Blot Analysis of ITGA7 in the Substantia Nigra
Microarray signal analysis of the SN confirmed that the ITGA7 signal in MPTP-treated mice was decreased compared to that in CTL (p < 0.05, n = 2, Figure 2b). Therefore, when ITGA7 expression was evaluated by Western blot analysis, it was confirmed that the expression of ITGA7 was significantly decreased in the MPTP group compared to that in the CTL group (p < 0.05, n = 3, Figure 2a,b). α-syn expression was evaluated using Western blot analysis, and it was confirmed that the expression of α-syn was significantly increased in the MPTP group compared to that in the CTL group (p < 0.05, n = 3, Figure 2a,b). expression decreased in the ST and SN regions of the brains of MPTP-treated mice. Western blot analyses (e) showed that TH expression was significantly decreased in MPTP-treated mice (f). ** denotes p < 0.005 compared to CTL. All values are expressed as mean ± standard error, and statistical analysis was performed using Student's t-test.

Microarray Analysis and Western Blot Analysis of ITGA7 in the Substantia Nigra
Microarray signal analysis of the SN confirmed that the ITGA7 signal in MPTPtreated mice was decreased compared to that in CTL (p < 0.05, n = 2, Figure 2b). Therefore, when ITGA7 expression was evaluated by Western blot analysis, it was confirmed that the expression of ITGA7 was significantly decreased in the MPTP group compared to that in the CTL group (p < 0.05, n = 3, Figure 2a,b). α-syn expression was evaluated using Western blot analysis, and it was confirmed that the expression of α-syn was significantly increased in the MPTP group compared to that in the CTL group (p < 0.05, n = 3, Figure 2a  Histogram shows a significant decrease in ITGA7 expression, detected by Western blot analysis, and the decreased signal of microarray in the MPTP group compared to that in the CTL group. Histogram shows a significant increase in α-syn in the MPTP group compared to that in the CTL group. * denotes p < 0.05 compared to CTL. All values are expressed as mean ± standard error, and statistical analysis was performed using Student's t-test.

Reduction in ITGA7 Expression and Increase in α-Syn Expression in the Substantia Nigra in a Mouse Model of Chronic MPTP-Induced PD
Immunohistochemical analysis (IHC) showed that the expression of ITGA7 in the SN was significantly decreased in the MPTP group (Figure 3c,d) when compared to that in the CTL group (Figure 3a,b). Conversely, the expression of α-syn was remarkably increased in the MPTP group (Figure 3e-h). Although the expression of ITGA7 and α-syn was inversely proportional, the patterns were similar. Histogram shows a significant decrease in ITGA7 expression, detected by Western blot analysis, and the decreased signal of microarray in the MPTP group compared to that in the CTL group. Histogram shows a significant increase in α-syn in the MPTP group compared to that in the CTL group. * denotes p < 0.05 compared to CTL. All values are expressed as mean ± standard error, and statistical analysis was performed using Student's t-test.

Reduction in ITGA7 Expression and Increase in α-Syn Expression in the Substantia Nigra in a Mouse Model of Chronic MPTP-Induced PD
Immunohistochemical analysis (IHC) showed that the expression of ITGA7 in the SN was significantly decreased in the MPTP group (Figure 3c,d) when compared to that in the CTL group (Figure 3a,b). Conversely, the expression of α-syn was remarkably increased in the MPTP group (Figure 3e-h). Although the expression of ITGA7 and α-syn was inversely proportional, the patterns were similar.

Immunofluorescence Analysis of ITGA7 Co-Localized with a α-Syn in the Substantia Nigra
Immunofluorescence analysis of ITGA7 and α-syn revealed co-localization and showed that ITGA7 expression was stronger in the CTL group ( Figure 4a) than in the MPTP group ( Figure 4f). Conversely, α-syn was more strongly expressed in the MPTP group than in the CTL group (Figure 4b,g). When ITGA7 and α-syn were merged (Figure 4c

Immunofluorescence Analysis of ITGA7 Co-Localized with a α-Syn in the Substantia Nigra
Immunofluorescence analysis of ITGA7 and α-syn revealed co-localization and showed that ITGA7 expression was stronger in the CTL group (Figure 4a) than in the MPTP group (Figure 4f). Conversely, α-syn was more strongly expressed in the MPTP group than in the CTL group (Figure 4b  . Immunofluorescence images of ITGA7 co-localized with α-syn in the SN in an MPTP-induced Parkinson's disease mouse model. SN regions were immunofluorescently labeled with anti-ITGA7 (b,g) and anti-α-syn (a,f) antibodies by using Rhodamine Avidin (a,f, red) and were then double immunolabeled with ITGA7 antibodies by using Fluorescein Avidin (b,g, green). The middle panels (c,h) show merged images of the individual α-syn (a,f) and ITGA7 (b,g) panels. In the control group (c), ITGA7 was more strongly expressed (green) and in the MPTP group (h), intense red fluorescence was observed. (d,i) show merged images with DAPI. Images of (e) and (j) show enlarged images of the square in (d) and (i). (scale bar, 10 µm).

Immunofluorescence Analysis of ITGA7 Co-Localized with a α-Syn in the Substantia Nigra
Immunofluorescence analysis of ITGA7 and α-syn revealed co-localization and showed that ITGA7 expression was stronger in the CTL group ( Figure 4a) than in the MPTP group ( Figure 4f). Conversely, α-syn was more strongly expressed in the MPTP group than in the CTL group (Figure 4b,g). When ITGA7 and α-syn were merged ( Figure  4c,d,h,i), the expression of ITGA7 was stronger in the CTL group (Figure 4d), whereas the expression of α-syn was stronger in the MPTP group (Figure 4i).

Immunofluorescence Analysis of ITGA7 Revealed Co-Localization with α-Syn in SH-SY5Y Cells
Immunofluorescence analysis of ITGA7 and α-syn revealed their co-localization, and the expression of ITGA7 was stronger in the CTL than in MPP+-treated SH-SY5Y cells

Figure 5. (a)
Western immunoblot analysis showed that the administration of ITGA7 siRNA decreased ITGA7 and TH expression but increased α-synuclein (α-syn) expression in the SH-SY5Y cells. (b) NC, negative control siRNA treatment (100 nM for 24 h); 10, ITGA7 siRNA treatment (10 nM for 24 h); 100, ITGA7 siRNA treatment (100 nM for 24 h). * denotes p < 0.05 compared to NC. All values are expressed as mean ± standard error, and statistical analysis was performed using ANOVA.

Figure 6. (a)
Western immunoblot analysis revealed that the administration of ITGA7 siRNA significantly decreased bcl2 expression and increased the bax/bcl2 ratio in SH-SY5Y cells. (b) NC, negative control siRNA treatment (100 nM for 24 h); 10, ITGA7 siRNA treatment (10 nM for 24 h); 100, ITGA7 siRNA treatment (100 nM for 24 h). * denotes p < 0.05 and ** p < 0.001 compared to NC. All values are expressed as mean ± standard error, and statistical analysis was performed using ANOVA.

Immunofluorescence Analysis of ITGA7 Revealed Co-Localization with α-Syn in SH-SY5Y Cells
Immunofluorescence analysis of ITGA7 and α-syn revealed their co-localization, and the expression of ITGA7 was stronger in the CTL than in MPP+-treated SH-SY5Y cells

Immunofluorescence Analysis of ITGA7 Revealed Co-Localization with α-Syn in SH-SY5Y Cells
Immunofluorescence analysis of ITGA7 and α-syn revealed their co-localization, and the expression of ITGA7 was stronger in the CTL than in MPP+-treated SH-SY5Y cells (Figure 7b,f). Conversely, the expression of α-syn was stronger in the MPP+-treated group than in the CTL group (Figure 7a,e). When α-syn and ITGA7 were combined, ITGA7 expression was strong in the CTL group and α-syn was strongly expressed in the MPP+treated group (Figure 7c,g).
( Figure 7b,f). Conversely, the expression of α-syn was stronger in the MPP+-treated group than in the CTL group (Figure 7a,e). When α-syn and ITGA7 were combined, ITGA7 expression was strong in the CTL group and α-syn was strongly expressed in the MPP+treated group (Figure 7c,g).

Discussion
It has previously been reported that an increase in α-syn expression is an important step in the pathology of PD [18,19], and our results suggest that one of the causes of this increase may be a decrease in ITGA7 expression. The mechanism by which dopaminergic cell destruction occurs following increased α-syn expression has been reported to be related to its effect on membrane fusion machinery, which regulates neurotransmitter release [20,21]. α-syn is highly concentrated at the presynaptic nerve terminals in both soluble and membrane-bound states [22,23]. Membrane binding by α-syn is likely physiologically important because α-syn remodels membranes [24]. Moreover, membranes were found to be important for the neuropathological effects of α-syn. Importantly, several studies have shown the involvement of aberrant α-syn-membrane interactions in cytotoxicity [19]. The self-aggregation of α-syn promotes membrane disruption, which can result from the formation of an amphipathic helical structure of α-syn [6]. Modern views of PD encompass the cell-to-cell trans-synaptic spread of α-syn [25,26] and the existence of many α-syn polymorphs that might be responsible for different synucleinopathies [27,28]. The cell membrane containing integrins is thought to play an important role in the intercellular synaptic diffusion of α-syn. Although studies have shown the link between PD and integrins [15,29], studies in Chinese [30], Swedes [31], and Taiwanese [32] reported that ITGA8 was not associated with PD.
ITGA7 encodes the ECM-binding protein.
Integrins are a major family of cell surface receptors that mediate ECM adhesion and are strongly implicated in the regulation of various cellular functions, including programmed cell death [33]. Consistent with these previous studies, our results demonstrated that decreased ITGA7 expression induced by ITGA7 siRNA administration decreased TH expression in SH-SY5Y cells but increased αsyn expression. In addition, the decreased expression of ITGA7 reduced the expression of bcl2 and significantly increased the bax/bcl2 ratio. Increased bax induces apoptosis by

Discussion
It has previously been reported that an increase in α-syn expression is an important step in the pathology of PD [18,19], and our results suggest that one of the causes of this increase may be a decrease in ITGA7 expression. The mechanism by which dopaminergic cell destruction occurs following increased α-syn expression has been reported to be related to its effect on membrane fusion machinery, which regulates neurotransmitter release [20,21]. α-syn is highly concentrated at the presynaptic nerve terminals in both soluble and membrane-bound states [22,23]. Membrane binding by α-syn is likely physiologically important because α-syn remodels membranes [24]. Moreover, membranes were found to be important for the neuropathological effects of α-syn. Importantly, several studies have shown the involvement of aberrant α-syn-membrane interactions in cytotoxicity [19]. The self-aggregation of α-syn promotes membrane disruption, which can result from the formation of an amphipathic helical structure of α-syn [6]. Modern views of PD encompass the cell-to-cell trans-synaptic spread of α-syn [25,26] and the existence of many α-syn polymorphs that might be responsible for different synucleinopathies [27,28]. The cell membrane containing integrins is thought to play an important role in the intercellular synaptic diffusion of α-syn. Although studies have shown the link between PD and integrins [15,29], studies in Chinese [30], Swedes [31], and Taiwanese [32] reported that ITGA8 was not associated with PD.
ITGA7 encodes the ECM-binding protein.
Integrins are a major family of cell surface receptors that mediate ECM adhesion and are strongly implicated in the regulation of various cellular functions, including programmed cell death [33]. Consistent with these previous studies, our results demonstrated that decreased ITGA7 expression induced by ITGA7 siRNA administration decreased TH expression in SH-SY5Y cells but increased α-syn expression. In addition, the decreased expression of ITGA7 reduced the expression of bcl2 and significantly increased the bax/bcl2 ratio. Increased bax induces apoptosis by stimulating the release of pro-apoptotic proteins such as cytochrome C from the mitochondrial intermembrane space to the cytoplasm [34]. As bcl2, an anti-apoptotic protein, can inhibit Ca 2+ or bax-mediated cytochrome C release, a decrease in bcl2 plays an important role in inducing apoptosis [35]. These results suggest that the decreased expression of ITGA7 could be associated with the increased expression of α-syn and apoptosis of dopaminergic cells. In addition, since the protein encoded by ITGA7 is involved in the membrane and the aggregated form of α-syn promotes membrane destruction, an increase in α-syn expression due to a decrease in ITGA7 expression may contribute to transmembrane pathogenesis. In particular, ITGA7 may be involved in the mechanism by which α-syn acts on the membrane. Immunofluorescent analysis of the SN regions demonstrated that ITGA7 and α-syn were localized near the membrane, and the MPTP group showed stronger α-syn expression and weaker ITGA7 expression than the CTL group.
Considering that the increase in cell membrane α-syn expression leads to the destruction of dopaminergic cells [36], the increase in α-syn expression may be caused by a decrease in the expression of ITGA7, which is present in the cell membrane.
In the IHC results of this experiment, the expression pattern of ITGA7 in the CTL group was quite similar to that of α-syn expressed in the MPTP group. The decrease in ITGA7 expression induced by ITGA7 siRNA in SH-SY5Y cells confirmed that the expression of α-syn was inversely increased. Based on these results, we believe that additional research on whether α-syn expression increases to replace the decrease in ITGA7 expression is needed.

MPTP-Induced Parkinson's Disease Mouse Model
The study protocol was approved by the Institutional Animal Care and Use Committee (IACUC) of Sangji University (IACUC 2018-6). Four-week-old male C57BL/6 mice (20-22 g; DBL, Daejeon, Korea) were used. The mice were divided into two groups: control (CTL) and MPTP groups. The mice in the CTL group were administered 0.9% (100 µL) saline intraperitoneally once every 24 h for 4 weeks. In the MPTP group, MPTP-HCl (20 mg/kg of free base) in 0.9% (100 µL) saline was intraperitoneally injected once a day for 4 weeks at 24-h intervals.

Immunofluorescence
After incubation with the primary antibodies and biotinylated anti-mouse IgG, each section was treated with fluorescein avidin DCS (Vector Laboratories, Burlington, ON, Canada). The sections were then treated with an avidin/biotin blocking kit and M.O.M mouse Ig blocking reagent (Vector Laboratories), after which they were stained with antiα-syn or anti-ITGA7 IgG at 4 • C overnight. Each section was treated with biotinylated anti-mouse IgG, followed by incubation with rhodamine D (Vector Laboratories, Burlington, ON, Canada). Photographic documentation was performed using a Nikon X-cite series 120Q microscope (Nikon, Tokyo, Japan), and the exposure parameters were the same for each group of samples.

Statistical Analysis
Statistical analysis was performed using Student's t-test or analysis of variance (ANOVA) with SPSS 25 (SPSS Inc., Chicago, IL, USA) software. All values are shown as the mean ± standard error.

Conclusions
In this study, ITGA7 expression was found to be reduced in the SN of an MPTPinduced Parkinsonism mouse model. In addition, the reduced expression of ITGA7 may be related to the increased expression of α-syn and apoptosis of dopaminergic cells. Our results suggest that a decrease in ITGA7 expression may be one of the causes of this increase in α-syn expression, which induces PD pathology.