Spinal cord injury (SCI) is a serious and debilitating condition that currently is one of the most challenging of medical problems, as it exhibits a high rate of disability and death [1]. Globally, there are over 27 million people living with SCI, with approximately one million new cases each year. Among them, falls and car accidents are the main causes [2]. Recently, the trend globally for SCI is for it to affect younger people, mostly under 30 years of age [3]. The occurrence of irreversible motor and sensory impairment following SCI has implications for the whole person [4]. Many studies have also shown that SCI patients are more likely to suffer from depression or anxiety than normal people [5, 6, 7]. Additionally, the financial cost per SCI patient is generally higher [8]. It is evident that SCI can place a serious burden on the patient physically, psychologically, and financially. However, currently the pathological mechanisms of spinal cord injury are known to be complex and undefined, and clinical diagnosis relies mainly on physical examination, imaging, and relevant biochemical indicators [9]. Most importantly, the treatment of SCI is limited, currently focused on surgical relief of compression, and reduction of secondary pathologies, as well as the use of hormonal drugs to reduce inflammation and swelling [10]. Therefore, it is of great importance to identify and study the pathogenesis of SCI and develop new treatment methods and tools.

miRNAs are RNAs of 21–25 amino acids in length that do not encode proteins [11, 12]. Intracellularly they regulate gene expression by binding to the 3’-untranslated region (UTR) of messenger RNA (mRNA) to either inhibit translation or induce degradation of the target mRNA [13]. Further, it has been shown that miRNAs influence both various physiological processes during development and tissue homeostasis by regulating the expression of approximately 90% of human genes [14]. Recently miRNAs have been widely studied for their role in various human diseases including tumours [15], haematological diseases [16], cardiovascular diseases [17], and central nervous system diseases [18]. Currently, studies in animal models [19] and bioinformatics [20] analyses have preliminarily demonstrated that alterations in miRNA expression have an impact on key processes in the pathophysiology of SCI. In-depth studies of miRNAs may also generate novel approaches to the treatment of SCI.

In this review, initially the close relationship between miRNAs and SCI is summarized and their potential to treat SCI through multiple pathways briefly outlined. Three miRNAs are then described that are more closely related to SCI, have been studied more frequently, and have potential applications for SCI treatment. Finally, current approaches to miRNA-based drug therapy for SCI and current issues of clinical translation are discussed and future directions for miRNA research in SCI are examined.

Expression of miRNAs in the rat spinal cord is exceptionally abundant, with one study showing that approximately 77% of identified mature rat miRNAs are expressed there [21]. Tang et al. [22] identified a total of 3361 miRNAs expressed in the spinal cord of adult rats. Additionally, the spatial distribution of miRNAs in the spinal cord varied. For example, miRNA-9 is more highly expressed in the dorsal sacral medulla, whereas, miRNA-124a/125b is more highly expressed in the ventral aspect [23]. SCI models can be classified according to the mechanism of injury, such as avulsion of the spinal brachial plexus, contusion of the spinal cord, ischaemia and reperfusion of the spinal cord, compression of the spinal cord, dislocation, transection, or chemical injury [24]. As SCI models of three injury mechanisms—avulsion of the spinal brachial plexus, ischemia-reperfusion of the spinal cord, contusion of the spinal cord, and spinal cord transection—are currently well studied, the miRNA expression patterns of these four injury-dependent mechanisms are briefly outlined here. The first is the miRNA expression pattern after spinal brachial plexus nerve avulsion. After such unilateral nerve avulsions in adult rats, the injured side was compared to the contralateral side. It was found that on day three after injury, 55 miRNAs were up-regulated and 24 were down-regulated. More significant up-regulations included miRNA-201-5p and miRNA-142-5p, while more significant down-regulation included miRNA-34a-3p and miRNA-324-5p. Up-regulation of 36 miRNAs including miRNA-363-3p and down-regulation of 23 miRNAs including miRNA-147 were observed at day 14 after injury. Additionally, 11 miRNAs, including miRNA-21-5p, continued to increase in expression after SCI, while only miRNA-466c-3p continued to decrease in expression after SCI. In comparison, 16 miRNAs including miRNA-18a showed persistent and significant changes at both days three and fourteen after SCI [22] (Fig. 1). Thirteen miRNAs were aberrantly expressed 24 hours after spinal cord ischemia-reperfusion. while 12 miRNAs including miRNA-331-5p were upregulated and miRNA-3084b-5p was downregulated. Forty-eight hours after spinal cord ischemia-reperfusion, 105 miRNAs showed differential expression. This included the upregulation of 44 miRNAs including miRNA-140-5p and the downregulation of 61 miRNAs including miRNA-129-2-3p. Only miRNA-22-3p was significantly upregulated at both 24 and 48 hours following reperfusion [25]. Studies of significant miRNA dysregulation after spinal cord contusion have recently been extensively reported [26, 27, 28]. Liu et al. [21] published an earlier report on miRNA expression analysis after contusion SCI in rats. Of the 46 miRNAs examined, 30 miRNAs were found to be increased and 16 miRNAs were found to be decreased after spinal cord contusion. Additionally, miRNA-21 was found to be elevated in the serum of patients with spinal cord contusion immediately during the acute phase of injury and peaked on day seven following SCI, before decreasing to normal control levels [27]. He et al. [29] reported the miRNA expression profile of the rat spinal cord 3 days after transection. Compared with the sham-operated group, the expression of 42 miRNAs, including miRNA-124, was down-regulated by 2-fold and the expression of 42 miRNAs, including miRNA-182, was up-regulated by more than 2-fold. In addition, miRNA-326, miRNA-30b-5p, miRNA-10a-5p and miRNA-127-3p were more than 4-fold down-regulated. Since the main function of miRNAs is the regulation of gene expression products, it is inferred that miRNA expression patterns after SCI may fall into three broad categories: (1) increased after injury, (2) decreased after injury, and (3) possibly bi-directional changes after injury. These expression patterns may also regulate gene expression products at different pathophysiological stages after SCI. This has illuminating implications for elucidation of the pathogenesis of SCI and identification of new targets for the treatment of SCI. The pathological process of SCI is currently divided into a primary injury phase and secondary injury phase. The primary injury phase is mainly the compression, contusion, and transection of the spinal cord, which are mechanical injuries. while the secondary injury phase is typically the period of biological effects such as inflammation, neuronal cell death, and destruction of neurological functions [30]. As miRNA is a molecule that typically regulates gene expression products, it may play a key role in the secondary damage phase of SCI. In recent years a number of miRNAs have been identified as important regulators of SCI, which can influence the pathophysiological processes of SCI through a variety of pathways. miRNA-21, miRNA-212-3p, and miRNA-26a inhibit neuronal apoptosis through the PTEN/AKT pathway thereby contributing to the recovery of motor function after SCI in rats [31, 32, 33]. miRNA-940, miRNA-182, miRNA-488, and miRNA-543-5p are involved in the NF-KB pathway to inhibit the release of pro-inflammatory factors such as tumour necrosis factor-$\alpha$ (TNF-$\alpha$) and interleukin-1$\beta$ (IL-1$\beta$) and subsequently regulate the inflammatory response after SCI in rats [34, 35, 36, 37]. miRNA-411, miRNA-129-5p, miRNA-9-5p, and miRNA-7a inhibit apoptosis of neuronal cells after SCI in rats [38, 39, 40, 41]. miRNA-466c-3p and miRNA-155 are involved in the regulation of mitochondrial function after SCI in rats [42, 43]. miRNA-99a and miRNA-672-3p regulate oxidative stress induced after SCI in rats [44, 45]. miRNA-125a and miRNA-216a-5p regulate M1/M2 polarization in microglia by targeting IRF5 (Recombinant Interferon Regulatory Factor 5) and TLR4 (Toll-Like Receptor 4), respectively, ultimately inhibiting the inflammatory response after SCI in rats [46, 47]. Many of the basic experiments described above have shown that miRNAs are extensively involved in various repair pathways of SCI. However, miRNAs may not contribute to SCI recovery through a single pathway. Many studies are now highlighting the role of complex regulatory networks among various non-coding RNAs in human diseases [48, 49, 50]. miRNA regulation through a single pathway may only be part of a large network, which may point the way to future research on non-coding RNAs. Although most of the experimental studies are currently at the animal testing stage, the conclusions of further clinical trials are unclear and there are no detailed and complete reports on the analysis of abnormal miRNA expression profiles in SCI patients. However, the data from the aforementioned animal models of SCI may largely reflect the condition of SCI patients. It is thus clear that miRNAs have shown initial potential for the treatment of SCI.

Fig. 1.

Map of miRNA changes over time after unilateral and contralateral spinal cord injury in rats.

Although the effect of miRNAs in treating human diseases, including SCI, remains to be elucidated, there is growing evidence that miRNAs represent a new class of drug targets.

4.1 Currently, Two Types of miRNA-Based Therapies have been Developed

(1) miRNA mimics and (2) miRNA inhibitors. The former is an exogenous synthetic miRNA mimic that acts specifically on its target RNA to silence the endogenous RNA and thereby attenuate the protein expression product of the unfavourable gene. The latter is a synthetic inhibitor of a miRNA that binds to the endogenous miRNA through specific targeting and weakens the silencing effect of the endogenous miRNA on the favourable gene and promotes the expression of the protein product of the favourable gene [73] (Fig. 2).

Fig. 2.

Schematic diagram of miRNA-based therapy.

In summary, (1) miRNAs are likely to provide a new approach to SCI treatment. However, the animal models chosen for most of the current experiments demonstrating the recovery-promoting effects of miRNAs on SCI are non-primate and future studies will likely also be conducted in non-human primates. (2) Most of the RNAs present in organisms are non-coding RNAs [99], such as miRNA, circRNA, lncRNA, etc. As research progresses the interactions of these non-coding RNAs and the complex network of regulatory relationships are found to remain largely unknown. It supports the notion that ideal future miRNA-based therapies should focus on the regulatory network of non-coding RNAs. In 2018, a related study found that miRNA-7 and miRNA-671 cooperated to build a complex regulatory network to regulate brain function in mice [100]. However, similar studies have been rarely reported in recent years in SCI as well as other diseases. Future studies could therefore focus on the regulatory network of non-coding RNAs. (3) Although miRNAs have been shown to be useful for the alleviation of SCI in animal models, there are still many issues in clinical application. (i) How to effectively avoid the off-target effect of miRNAs and thus avoid the possible side effects of miRNAs acting on other target genes. (ii) How to choose the appropriate vector and delivery method, the appropriate dosing window, and the appropriate dose to ensure the maximum benefit of miRNA as a drug for SCI. (iii) How miRNA-based drugs can bypass the BBB or BSCB. Although much work remains to be done to develop miRNAs for application in clinical applications, the resolution of related issues will certainly enhance the development of this emerging field.

ZZD wrote and revised the manuscript; YHC designed and critically revised the manuscript, and all authors read and approved the final manuscript.

Not applicable.

We are grateful to two anonymous reviewers for their critical comments on the manuscript.

This research was funded by Shaanxi Provincial Education Department Project, grant number: 14JK1835.

The authors declare no conflict of interest.