MiR-7, miR-153 and miR-203a-3p possess binding sites to the 3′ UTR of the SNCA gene. The expression of miR-7 is significantly reduced in the SNpc of PD patients (McMillan et al., 2017). It has been shown that over-expression of both miR-7 and miR-153 in the human embryonic kidney cell line, HEK293 cells and cortical neurons lead to significant reduction of SNCA mRNA and protein expression levels meanwhile miR-7 knockdown induces over-expression of α-synuclein protein level (Doxakis, 2010; McMillan et al., 2017). Both miRNAs regulate SNCA protein levels via a different pathway in which miR-7 inhibits its translation while mir-153 degrades the mRNA (Doxakis, 2010). Je and Kim (2017) demonstrated that exposure of 1-methyl-4-phenylpyridinium (MPP⁺, the active metabolite of MPTP) to the HEK293 cells relieved the miRNA-mediated translational suppression of SNCA, via MPP⁺-mediated mitochondrial reactive oxygen species (ROS) production. Interestingly, the over-expression of miR-7 and miR-153 in cortical neurons reduced MPP⁺-induced neurotoxicity by increasing neuronal viability, up-regulation of pro-survival BCL-2 protein and in-activation of pro-apoptotic caspase-3 (Fragkouli and Doxakis, 2014).

Another miRNA targeting SNCA, miR-203a-3p expression was down-regulated in MMP⁺-treated human dopaminergic neuroblastoma, SH-SY5Y cells (Jiang et al., 2020). MMP⁺ treatment led to reduce cell proliferation and induced apoptosis in the SH-SY5Y cells that then led to enhance expression of SNCA, p53 and cleaved Caspase-3 proteins of which were inhibited by the up-regulation of miR-203a-3p (Jiang et al., 2020). Other miRNAs that were identified as regulators of SNCA expression include the miR-30b, miR-34b/c, miR-214, and miR-433 (Wang et al., 2015; Consales et al., 2018; Tarale et al., 2018; Shen et al., 2020).

Mutations in the PRKN, encoding Parkin (an important E3 ubiquitin-protein ligase that mediates the elimination of damaged mitochondria via mitophagy), cause an autosomal recessive form of early onset PD (Kim et al., 2012; Zanellati et al., 2015). Mitochondrial dysfunction and the down-regulation of Parkin in the MPTP-treated mice and MPP⁺-treated SH-SY5Y cells (Zhou et al., 2020). Furthermore, Parkin protein expression was significantly down-regulated following miR-103a-3p over-expression while Parkin protein expression was up-regulated following miR-103a-3p knockout. This suggest that miR-103a-3p able to regulate Parkin expression, by binding to the 3′-UTR of Parkin mRNA (Zhou et al., 2020).

In a rotenone-induced PD rat model, miR-146a, a known inflammation regulatory miRNAs was identified as the most up-regulated miRNAs (Jauhari et al., 2020). The exposure of rotenone led to the activation of NF-κβ and induces the transcription of miR-146a that is responsible for the down-regulation of Parkin protein level (Jauhari et al., 2020). In contrast, down-regulation of miR-146a expression led to the inhibition of NF-κβ phosphorylation and increased Parkin level in the rotenone-treated SH-SY5Y cells (Jauhari et al., 2020).

PRKN was also identified as a target of other miRNAs such as miR-181a and miR-218. Over-expression of miR-181a significantly reduced Parkin mRNA and protein level, while inhibition of miR-181a increased Parkin expression (Cheng et al., 2016). In addition to PRKN, miR-181a also targets genes that control neurite growth namely SMAD1 and SMAD5 transcription factors (Hegarty et al., 2018). Inhibition of miR-181a expression in SH-SY5Y cells led to elevation of Smad1/5 proteins, as well as the neurite length and neurite branching (Hegarty et al., 2018). Similarly, down-regulation of miR-181a-5p confers neuroprotection to MPP⁺-treated cells by enhancing cell viability and suppression of cell apoptosis, LDH activity and inflammation (Guo and Hua, 2020). The data on miR-181a dysregulation reported in PD patients and axonal degeneration of DA neurons in early stages of PD suggest that inhibition of miR-181a may be a promising therapeutic approach for PD. MiR-218 also targets PRKN and negatively regulates PINK1/PRKN-mediated mitophagy. Over-expression of miR-218 down-regulated PRKN expression, leading to dysregulated mitophagy (Di Rita et al., 2020). However, miR-218 expression was found to be reduced in the patients and animal models (Xing et al., 2020; Ma et al., 2021).

Homozygous and compound heterozygous mutations in the PINK1 gene are associated with the early onset of PD (Krohn et al., 2020). The PINK1 gene encodes a PTEN-induced serine-threonine kinase 1 located in the mitochondria in which the mutations can lead to respiratory chain dysfunction and impairment in ATP production (Rango et al., 2020). MiR-27a and miR-27b suppressed the expression of PINK1 by direct binding to the 3′UTR of its mRNA (Kim J. et al., 2016). Thus, miR-27a/b inhibited accumulation of PINK1, which in turn prevented the translocation of Parkin to the mitochondria following mitochondrial damage hence leads to the inhibition of lysosomal degradation of damaged mitochondria (Kim J. et al., 2016). In addition to miR-27a/b, miR-140 has also been reported to target PINK1. Amyloid β-derived diffusible ligands-induced neurons transfected with miR-140 mimic exhibited up-regulated miR-140 and down-regulated PINK1 expressions. Reciprocal effects were seen following miR-140 inhibitor transfection (Liang et al., 2021).

The LRRK2 gene mutations are associated with familial and sporadic forms of PD (Gopalai et al., 2014). The LRRK2 protein level increased with no significant changes in its mRNA levels in the frontal cortex of sporadic PD patients hence suggesting the involvement of miRNAs in suppressing post-transcriptional regulation of LRRK2 (Cho et al., 2013). There were reduced miR-205 expression in PD patients, and the reduction showed inverse correlation between the expression of LRRK2 protein and miR-205 (Cho et al., 2013). Similarly, over-expression of miR-205 in HEK293T cells suppressed LRRK2 protein expression in vitro, while inhibition of miR-205 enhanced LRRK2 protein expression (Cho et al., 2013).

MiR-599 expression was down-regulated both in vivo [lipopolysaccharide (LPS) administered mice] and in vitro (MPP⁺-treated SH-SY5Y cells) PD models, which was associated with increased LRRK2 expression (Wu et al., 2019). Over-expression of miR-599 was shown to down-regulate LRRK2 expression while knockdown of miR-599 up-regulated LRRK2 expression in the MPP⁺-treated SH-SY5Y cells (Wu et al., 2019).

DJ-1 (PARK7) is identified as a recessive familial PD gene in which loss of this gene resulted in the increased the susceptibility of cells to the damaging effect of oxidative stress, leading to the early onset of PD (Irrcher et al., 2010). MiR-494 binds to the 3′ UTR of DJ-1 and leads to the inhibition of DJ-1 transcription (Xiong et al., 2014). Over-expression of miR-494 in 3T3 cells resulted in decreased DJ-1 protein level and increased ROS production without changing the mRNA expression (Xiong et al., 2014). Similarly, over-expression of miR-494 in mice leads to a decrease in DJ-1 expression in the SNpc following MPTP administration, alongside the exacerbated degeneration of DA neurons in the SNpc and significant motor impairment (Xiong et al., 2014; Geng et al., 2018).

The miR-4639-5p level was up-regulated in the plasma of PD patients whereby miR-4639-5p negatively regulates DJ-1 in the post-transcriptional level (Chen et al., 2017). The up-regulation of miR-4639-5p led to the down-regulation of DJ-1 protein level hence resulted in severe oxidative stress and neuronal death in MPP⁺- and rotenone-treated SH-SY5Y cells (Chen et al., 2017). MiR-145-3p and miR-874 are among other miRNAs that were predicted to regulate DJ-1 expression, and both were shown to be highly expressed in the saliva of PD patients (Chen et al., 2020).

The expression of both miR-34b and miR-34c were shown to be down-regulated in various brain region of PD patients including the putamen, amygdala, substantia nigra, frontal cortex, and cerebellum (Minones-Moyano et al., 2011; Villar-Menendez et al., 2014). Inhibition of miR-34b/c expression in SH-SY5Y cells lead to reduced cell viability, mitochondrial dysfunction and increased ROS generation in the cells (Minones-Moyano et al., 2011). In addition, miR-34b/c inhibition could also lead to a significant reduction in DJ-1 and Parkin protein levels in which these proteins are associated with familial forms of PD (Minones-Moyano et al., 2011). In another study, miR-34b/c inhibition increased the α-synuclein levels and stimulate the aggregate formation, possibly contributing to PD pathogenesis (Kabaria et al., 2015).

MiR-126 was up-regulated in the SNpc DA neurons from PD patients (Kim W. et al., 2014). Over-expression of miR-126 compromised the survival of primary DA neurons and SH-SY5Y cells hence increasing their vulnerability to 6-OHDA toxicity (Kim W. et al., 2014). In addition to DA neurotoxins, miR-126 over-expression also increases neuronal vulnerability to staurosporine (a non-selective protein kinase inhibitor) and Alzheimer’s disease-associated amyloid β 1-42 peptides (Kim W. et al., 2016). The miR-126 over-expressing cells showed reduced expression of p85β, IRS-1 and SPRED1, reduced phosphorylated AKT and extracellular signal-regulated kinase (ERK) protein levels. These results suggest that miR-126 down-regulates insulin/IGF-1/phosphatidylinositol-3-kinase (PI3K)/AKT and ERK signaling cascades in promoting neuronal death in PD (Kim W. et al., 2014).

The expression of miR-128 decreased in the MPTP-induced PD mice (Zhou et al., 2018; Zhang G. et al., 2020). MiR-128 protects DA neurons from apoptosis and up-regulates the expression of Excitatory Amino Acid Transporter 4 (EAAT4) by binding to the axis inhibition protein 1 (AXIN1) (Zhou et al., 2018). Further study reported that the increased HIF-1α/miR-128-3p inhibited apoptosis of hippocampal neurons through Wnt/β-catenin signaling pathway activation via the suppression of AXIN1 (Zhang G. et al., 2020).

The expression of miR-200a was significantly up-regulated in the pheochromocytoma cell line PC12 cells and SH-SY5Y cells following exposure to MPP⁺ that leads to the induction of oxidative stress and cell apoptosis (Salimian et al., 2018; Talepoor Ardakani et al., 2019). Additionally, the transcript level of SIRT1 (a miR-200a target) showed significant down-regulation in the MPP⁺-treated cells, confirming that SIRT1 may induce DA neuronal apoptosis via P53 and FOXO signaling pathways (Salimian et al., 2018). In contrast, inhibition of miR-200a reduced the apoptosis and increased DA neuronal population in 6-OHDA-treated PD rats, via inhibition of the cAMP/PKA signaling pathway (Wu et al., 2018).

MiR-216a over-expression has shown to provide protection against MPP⁺-induced neurotoxicity in SH-SY5Y cells by significantly increase neuronal viability following MPP⁺ treatment (Yang et al., 2020). This was also associated with reduced neuronal apoptosis, ROS level and lipid peroxidation (Yang et al., 2020). MiR-216a promotes neuronal survival by down-regulating Bax that is involved as apoptotic regulators (Yang et al., 2020). The up-regulation of miR-216a also exerts its neuroprotective effects against ischemic brain injury by targeting JAK2/STAT3 pathways (Tian et al., 2018).

MiR-221 expression was significantly down-regulated in 6-OHDA-treated PC12 cells (Li et al., 2018). Over-expression of miR-221 cells treated with 6-OHDA and MPP⁺ significantly promotes cell viability and proliferation while inhibited cell apoptosis (Li et al., 2018; Oh et al., 2018). The brains of DJ-1^–/– mouse and DJ-1 knock down SH-SY5Y cells demonstrated down-regulation of miR-221 (Oh et al., 2018). Additionally, miR-221 down-regulated the expression of pro-apoptotic proteins (bcl-2-like protein 11, bcl2 modifying factor, and bcl2 interacting protein 3-like) at basal conditions thus prevented the oxidative stress-induced increase of bcl-2-like protein 11 (Oh et al., 2018). Based on these findings, the modulation of miR-221 by DJ-1 could contribute to the pathogenesis of autosomal recessive inherited PD or sporadic PD, resulted from the loss-of-function mutations in DJ-1.

The expression of miR-326 was shown to be down-regulated in MPTP-induced PD mice (Zhao et al., 2019). MiR-326 over-expression ameliorates the motor dysfunction and the structural abnormality of the SNpc (increase content of DA, SOD, GSH-Px, and TH positive expression) in PD mice (Zhang et al., 2019). In addition, miR-326 overexpression also inhibits numerous immunomodulatory factors such as TNF-α, IL-1, IL-6, and INF-γ. Mice treated with miR-326 mimic inhibits expression of iNOS and neuronal apoptosis and promotes autophagy of DA neurons (Zhang et al., 2019; Zhao et al., 2019). Recent evidence also showed that miR-326 may play an important role as the key suppressor in the development of PD due to the suppression of pyroptotic cell death with the activation of NLRP3 inflammasome. It was shown that silencing the lncRNA homeobox transcript antisense RNA resulted in significant inhibition of neuronal damage through the suppression of NLRP3-mediated pyroptosis via the regulation of miR-326/ELAVL1 axis in PD model (Zhang Q. et al., 2020).

The serum miR-29 level was significantly down-regulated in PD patients, and were correlated with the disease severity (Bai et al., 2017). Over-expression of miR-29c attenuated DA neuronal death and α-synuclein aggregation in the SNpc of MPTP-treated mice. Furthermore, miR-29c exhibited anti-inflammatory properties by ameliorated the elevated pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6) and apoptosis in the MPTP-treated mice (Wang et al., 2020b). Additionally, over-expression of miR-29c suppressed the LPS-induced microglial activation, via modulation of NOD-like receptor protein 3 (NLRP3) inflammasome by targeting nuclear factor of activated T-cells (NFAT) 5 (Wang et al., 2020a).

The expression of miR-124 decreased in the midbrain of the MPTP-induced PD mouse model (associated with reduced DA neurons, and decreased tyrosine hydroxylase and DA transporter expressions), MPP⁺-treated SH-SY5Y and MN9D cells (Kanagaraj et al., 2014; Liu et al., 2017). The reduced miR-124 expression increased the expression of calpain 1, leading to the activation of the p25/cdk5 pathway, which is known to be involved in the DA neuronal loss in PD (Kanagaraj et al., 2014). Additionally, miR-124 suppression increased cell autophagy-related protein expression (LC3 II and Beclin 1), increased phosphorylated-AMPK level while decreased p-mTOR expression suggest that miR-124 regulates DA neuronal apoptosis and autophagy by regulating the AMPK/mTOR pathway (Gong et al., 2016). In contrast, up-regulation of miR-124 through administration of miR-124 in the MPTP-treated mice attenuated the loss of DA neurons and striatal DA level caused by MPTP toxicity (Wang H. et al., 2016). Additionally, over-expression of miR-124 protects SH-SY5Y cells from cell death caused by microglial activation following LPS exposure suggesting the miR-124 role in the inhibition of neuroinflammation (Yao et al., 2018). In addition, over-expression of miR-124 effectively attenuated the expression of pro-inflammatory cytokines (iNOS, IL-6, and TNF-α) and promote the secretion of neuroprotective factors (TGF-β1 and IL-10) in LPS-induced immortalized murine microglial cell line BV2 cells (Yao et al., 2018). Further study suggested that miR-124 inhibits neuroinflammation by targeting sequestosome 1 (p62), phospho-p38 mitogen-activated protein kinases and autophagy (Yao et al., 2019).

Exposure of MPP⁺ to SH-SY5Y cells leads to a significant down-regulation of miR-135b (Zhang et al., 2017). Over-expression of miR-135b was able to attenuate neurotoxicity effects of MPP⁺: increased in cell viability, suppressed apoptosis, reduced level of cleaved caspase 3 (an apoptotic marker), and inhibited MPP⁺-induced TNF-α and IL-1β secretion (Zhang et al., 2017). Over-expression of miR-135b suppressed MPP⁺-induced increased quantity of pyroptotic cells and protein levels of various pyroptotic related genes (TXNIP, NLRP3, Caspase-1, ASC, GSDMD^Nterm and IL-1β) in the SH-SY5Y and PC-12 cells (Zeng et al., 2019).

MiR-155 demonstrated enhanced expression in the SNpc of PD mice model produced by adeno-associated-virus-mediated expression of α-synuclein (Thome et al., 2016). The deletion of miR-155 in mice prevents α-synuclein-associated neurodegeneration of DA neurons, which corresponds to the reduced microgliosis (Thome et al., 2016). Prolong exposure of TNF-α (PD patients demonstrated increased TNF-α level in the serum and CSF) to SH-SY5Y cells induced apoptosis, dysregulation of complex I and mitochondrial oxidative stress (Prajapati et al., 2015). TNF-α exposure also resulted in the up-regulation of numerous miRNAs, including miR-155, which may target nuclear encoded subunits of mitochondrial complex-I (NADH:Ubiquinone Oxidoreductase Core Subunit V1) (Prajapati et al., 2015). In addition, DJ-1 deficiency increased the expression of miR-155 in microglia and astrocytes, which down-regulates the expression of suppressor of cytokine signaling 1 (SOCS1) (Kim J. H. et al., 2014). Taken together, miR-155 modulates microglia-mediated inflammation by down-regulating SOCS1 expression and increasing the production of cytokine and nitric oxide. This leads to inefficient termination of STAT1-induced inflammatory response, which may cause neuronal death (Cardoso et al., 2012; Kim J. H. et al., 2014).