MicroRNAs in central nervous system disorders: current advances in pathogenesis and treatment

MicroRNAs (miRNAs) are a class of short, non-coding, regulatory RNA molecules that function as post transcriptional regulators of gene expression. Altered expression of multiple miRNAs was found to be extensively involved in the pathogenesis of different neurological disorders including Alzheimer’s disease, Parkinson’s disease, stroke, epilepsy, multiple sclerosis, amyotrophic lateral sclerosis, and Huntington’s disease. miRNAs are implicated in the pathogenesis of excitotoxicity, apoptosis, oxidative stress, inflammation, neurogenesis, angiogenesis, and blood–brain barrier protection. Consequently, miRNAs can serve as biomarkers for different neurological disorders. In recent years, advances in the miRNA field led to identification of potentially novel prospects in the development of new therapies for incurable CNS disorders. MiRNA-based therapeutics include miRNA mimics and inhibitors that can decrease or increase the expression of target genes. Better understanding of the mechanisms by which miRNAs are implicated in the pathogenesis of neurological disorders may provide novel targets to researchers for innovative therapeutic strategies.


Introduction
MiRNAs are a class of short, non-coding RNA molecules that contain 19-24 nucleotides. They usually regulate gene expression at the messenger RNA (mRNA) level [1]. There is strong evidence that miRNAs have a role in different cellular processes including neural cells proliferation and differentiation, cell specification, cellular metabolism [1,2]. miRNA are implicated in the pathogenesis of excitotoxicity, apoptosis, oxidative stress, inflammation, neurogenesis, angiogenesis, and blood-brain barrier protection [3]. Therefore, it is not surprising that miRNAs have emerged as key regulators of pathophysiology of different neurological disorders including Alzheimer's disease, Parkinson's disease, stroke, epilepsy, multiple sclerosis, amyotrophic lateral sclerosis, and Huntington's disease [4].
MiRNAs are released as circulating molecules into body fluids such as CSF, blood, and urine and therefore, they may be valuable biomarkers for detecting early onset neurodegenerative disorders. MiRNAs have the potential to be therapeutic molecules, where miRNA inhibitors and mimics can be used to target pathological upregulated and down-regulated miRNAs [5].
The objective of this review is to provide a brief synopsis about the role of miRNAs as key regulators and novel therapeutic targets in different neurological disorders including Alzheimer's disease, Parkinson's disease, stroke, epilepsy, multiple sclerosis, Amyotrophic lateral sclerosis, and Huntington's disease.

Main text miRNA biogenesis
MiRNA genes are transcribed by RNA polymerase II into primary miRNA (pri-miRNA) transcripts. The Drosha (a class 2 ribonuclease III enzyme) and DiGeorge syndrome critical region 8 (DGCR8, an RNA-binding protein) cleave the pri-miRNA into precursor miRNA (pre-miRNA). Exportin-5 binds to the pre-miRNA and helps their export into the cytoplasm. Dicer, also known as endoribonuclease Dicer, is an enzyme that cleaves the pre-miRNA into double strand RNA. One of the two strands of the miRNA duplex is incorporated into Argonaute (AGO) proteins to form the miRNA inducing silencing complex (miRISC) which leads to either translational repression or degradation of the target mRNA. The other miRNA strand is degraded [6].

miRNA mimics and inhibitors
MiRNA mimics are synthetic short double-stranded oligonucleotides imitating miRNA precursors. Once they are introduced into cells, they are recognized by miRNA biogenesis machinery and then processed accordingly [7]. MiRNA inhibitors (antagomirs) inhibit the interaction between miRNA and the miR-ISC or between the miRISC and its target mRNAs. They block the translation of mRNA into protein or induce its destruction. MiRNA-based therapeutics include miRNA mimics and inhibitors that can decrease or increase the expression of target genes [8].

Mechanisms of miRNA-based therapeutics for neurological disorders Excitotoxicity
Several miRNAs were found to attenuate excitotoxicity after ischemic stroke. Overexpression of miR-223 in hippocampal neurons protected them from neuronal death following transient global ischemia through decreasing the levels of NMDAR subunit 2B and glutamate receptor 2 (GluR2) and halting NMDA-induced calcium influx [9]. MiR-181a inhibitor attenuated astrocyte dysfunction and hindered the decrease of glutamate transporter 1 resulting in enhancing the survival of hippocampal neurons [10].

Many
miRNA-based therapeutics have antiinflammatory effect. Their anti-inflammatory actions involve the suppression of cytokines secretion, astrocytes activation, and leukocyte extravasation. Inhibition of miR-15a/16-1 or overexpression of miR-122 and miR-22 decreased the levels of the following inflammatory cytokines: TNF-a, IL-6, COX-2, VCAM-1, and iNOS in ischemic brain [23,24]. Exosomemediated delivery of miR-124-3p reduced tissue inflammation and induced M2 microglia polarization after traumatic brain injury. The function of M2 microglia is to downregulate the inflammatory pathway, thus promoting tissue repair [24].
Neurogenesis miRNAs were found to modulate neurogenesis in cerebral cortex and spinal cord in stroke, spinal cord injury and traumatic brain injury. Overexpression of the miR-17-92 cluster in the subventricular zone (SVZ) significantly enhanced neurogenesis and promoted the proliferation of neural stem cells after acute ischemic stroke [27]. miR-20a inhibitor increased neurogenesis and enhanced the survival of motor neurons in mice following spinal cord injury through increasing expression of the miR-20a target gene neurogenin 1 [28].

Angiogenesis
Several miRNAs were found to modulate angiogenesis in stroke, spinal cord injury and traumatic brain injury.

Blood brain barrier/blood spinal cord barrier (BBB/BSCB) protection
The inflammatory cascade following damage to the BBB or the BSCB can be modulated by some miRNAs. Downregulating miR-150 alleviated BBB disruption after ischemic stroke through increasing claudin-5 and stabilization of TJ protein ZO-1,191. Anti-mir-130 and anti-miR-320 upregulated the expression of aquaporins following ischemic stroke, which are implicated in the clearance of cerebral edema [33,34]. MiR-320a mimic attenuated BBB disruption, ameliorated spinal cord reperfusion, and decreased water content in the spinal cord through suppressing the expression of AQP1 [35].

MiRNAs dysregulation in neurological disorders
MiRNAs dysregulation in Alzheimer's disease Altered expression of some miRNA in patients suffering from AD suggests that miRNA may have a crucial regulatory role on the mechanisms involved in the pathogenesis of AD, including beta-amyloid (Aβ) metabolism by modulating the activity of β-secretases such as BACE1 and tau aggregation leading to neurofibrillary tangle (NTF) formation ( Fig. 1) [36,37].

MiRNA dysregulation in Parkinson's disease
Multiple microRNAs were reported to control αsynuclein aggregation either by direct regulation, or by chaperon-mediated autophagy and their downregulation may contribute to α-synuclein-mediated neurotoxicity in PD [38][39][40], (Fig. 2). In PD, there was upregulation of some of these microRNAs [41]. Some microRNAs were reported to be implicated in neuroinflammation, such as miR-124 and miR-146a (anti-inflammatory) and miR-155 (pro-inflammatory) [42]. MiR-124 enhanced the survival of dopaminergic neurons and attenuated microglial activation in MPTP model of PD [43].

miRNAs dysregulation in ischemic stroke
Several reports demonstrated that miRNAs have distinct expression patterns that modulate pathophysiological process of stroke [44]. Tan and colleagues carried out miRNA profiling from blood of young patients with stroke and identified characteristic patterns in ischemic stroke [45]. Moreover, anti-miR-320a led to a reduction of infarct size in ischemic stroke with a concomitant increase in aquaporins-1 and 4 mRNA [33]. After subjecting rat brains to MCAO then reperfusion for 24 or 48 h, 114 miRNAs were detected in ischemic brain samples. Among them, 82 and 106 transcripts were detected in the 24-h and 48-h reperfusion [46]. The pattern of miRNA profile in patients with small artery stroke is distinctly different from that of large artery stroke. Therefore, microRNA profiling can be used to predict the stroke subtypes [47]. Some miRNAs can be used as novel biomarkers for diagnosis and prognosis in acute ischemic stroke, and some were investigated as potential therapeutic targets [47,48], (Fig. 3).

MiRNAs dysregulation in epilepsy
There is strong evidence that miRNAs dysregulation were linked to the mechanisms of epileptogenesis through regulating ion channels, inflammatory response, synaptic plasticity, and neuronal apoptosis. Some miR-NAs were reported to influences molecular and cellular pathways implicated in epilepsy, including oxidative stress, inflammation, immune responses, cell differentiation, migration, and proliferation [49][50][51] (Fig. 4). Targeting these miRNAs is a challenge for future strategies for anti-epileptogenesis therapy. Yuan, Huang [52] found that silencing miR-132 had a neuroprotective effect on epileptic mouse models through regulating the morphology and electrophysiology of dendritic spines. The expression of miR-22, miR-34a, miR-21, and miR-125a in blood and the hippocampus were found to be changed 24 hours after the onset of status epilepticus [53].

MiRNA dysregulation multiple sclerosis
MiRNAs dysregulation display strong association with multiple sclerosis (MS). Several MiRNAs were reported to be consistently upregulated in MS patients including miR-142-3p, miR-145, miR-146a/b, miR-22, miR-155, miR223/-3p, miR-584, and miR-326. Overexpression of these miRNAs in MS patients suggests their implication in the pathogenic inflammatory process observed in MS. miR-155 was one of the most consistently dysregulated miRNA in MS. It has a role in disruption of the blood-brain barrier, immune cell activation and neurodegeneration [54,55]. Upregulation of miR-155 was significantly reduced by immunomodulatory medications such as glatiramer acetate (GA), supporting its potential role in the pathogenic pro-inflammatory process [56,57]. miR-146 has also been reported to be upregulated in the blood and CNS lesions of MS patients [57].
miR-17, miR-21, miR-320, and miR-150 exhibited different patterns across the compartments in MS patients with predominant upregulation in the CNS lesions and downregulation in the immune tissue [55,58,59]. Interestingly, It has been reported that miR-21 exhibit both pro-and anti-inflammatory functions. It is upregulated in the active disease and downregulated in remission state and in secondary progressive MS [60].
On the other hand, members of the miR-103, miR-548 miR-15, and let-7 families were consistently downregulated in MS patients. Let-7 and miR-548 family members were exclusively dysregulated in the immune compartment while miR-103 family and miR-15a/b were downregulated in all cellular compartments apart from regulatory T cells [55]. Dysregulation of different MiR-NAs targeting the inflammatory activity of various immune cells was shown in Fig. 5 [55,58,61,62].
MiR-155 is upregulated in both sporadic and familial ALS patients, and inhibiting it in the brains of SOD1G93A mice model increases survival [63]. miR-34a is dysregulated in ALS. It regulates X-linked inhibitor of apoptosis (XIAP) and Sirtuin 1 (SIRT1), which is protective against oxidative stress-induced apoptosis [65,66]. ALS patients were also found to have reduced expression of miR-34a, which is rescued by treatment with enoxacin [65]. Thus, microRNA biogenesis stimulating drugs can be potentially used in treatment of ALS [67].

MiRNA-based therapeutics
miRNA-based therapeutics include the pharmacological agents that exert neuroprotection through the regulation of functional miRNAs. Hydrogen gas was found to regulate oxidative stress via upregulating miR-21 [72]. VELCADE or bortezomib, which is used for treatment of multiple myelomas and mantle cell lymphoma, exerts neuroprotective effect against cerebral ischemia through upregulation of miR-146a [73]. Ferulic acid improves functional recovery in spinal cord injury by inhibiting miR-590 [74]. Additionally, there are other pharmacological agents including trimetazidine (TMZ) [75], acetylbritannilactone (ABL) [76], hydrogen sulfide (H2S) [77], and nicorandil [78] that depend on the modulation of specific miRNAs. So, blocking the modulation of these miRNAs can completely abolish the neuroprotective effects of these agents against CNS injuries (Fig. 6).

Conclusion
There is strong evidence that clearly demonstrates the association between miRNA dysregulation and neurodegenerative diseases. miRNA-based therapeutics have become one of the most promising strategies in treatment of incurable neurological disorders. Further researches are needed to identify candidate miRNAs, clarify how they exert their effects, design pharmacological formulations and delivery methods that can cross the BBB to target brain tissue, and develop methods to decrease off target effects.