MicroRNAs (miRNAs) have recently emerged as a new class of modulators of gene expression. miRNAs control protein synthesis by targeting mRNAs for translational repression or degradation at the posttranscriptional level. These noncoding RNAs are endogenous, single-stranded molecules approximately 22 nucleotides in length and have roles in multiple facets of immunity, from regulation of development of key cellular players to activation and function in immune responses. Recent studies have shown that dysregulation of miRNAs involved in immune responses leads to autoimmunity. Multiple sclerosis (MS) serves as an example of a chronic and organ-specific autoimmune disease in which miRNAs modulate immune responses in the peripheral immune compartment and the neuroinflammatory process in the brain. For MS, miRNAs have the potential to serve as modifying drugs. In this review, we summarize current knowledge of miRNA biogenesis and mode of action and the diverse roles of miRNAs in modulating the immune and inflammatory responses. We also review the role of miRNAs in autoimmunity, focusing on emerging data regarding miRNA expression patterns in MS. Finally, we discuss the potential of miRNAs as a disease marker and a novel therapeutic target in MS. Better understanding of the role of miRNAs in MS will improve our knowledge of the pathogenesis of this disease.
MicroRNAs (miRNAs) represent a class of noncoding RNA molecules that play pivotal roles in cellular and developmental processes by regulating gene expression at the posttranscriptional level. miRNAs are endogenous, evolutionarily conserved, single-stranded RNAs approximately 22 nucleotides in length that suppress the expression of protein-coding genes by directing translational repression through base-pairing with complementary messenger RNA (mRNA)and/or by promoting degradation of target mRNA degradation [
The regulation of mammalian immune responses by miRNAs is a concept currently evidenced by rapidly accumulating data [
All miRNAs are processed and maturated through a complex biogenesis process involving multiple protein catalysts, accessory proteins, and macromolecular complexes following a coordinated series of events. The reader is referred to excellent recent reviews for detailed discussions of miRNA biogenesis and its regulation [
miRNA genes are transcribed in the form of Pri-miRNA. The DGCR8-Drosha complex processes in the form Pre-miRNA followed by transport into cytoplasm by Exportin-5. In cytoplasm, Pre-miRNA is processed by Dicer into miRNA duplex. Of miRNA duplex, one strand is loaded into RISC complex, which functions for either mRNA degradation or translational repression.
Inflammation has been reported to regulate miRNA biogenesis; TLR ligands, antigens, or cytokines can modulate miRNA expression level through regulation of specific transcription factors [
Information about miRNA and target expression patterns can help to assess the likelihood that a predicted miRNA-target relationship is relevant
Clearly, both innate and adaptive immune responses are extremely highly regulated. Recent work from a number of laboratories has revealed that miRNAs play an important role in this intricate system (Table
miRNA in immune functions.
miRNA | Expressing cells | Functions | Targets |
---|---|---|---|
Let-7e | macrophages | Innate immune response | TLR4 |
miR-9 | myeloid cells | Immune response | NFK B1 |
miR-17-5p | myeloid cells | monocyte proliferation and differentiation | RUNX1 |
miR-17-92 | B and T cells | B and T cell development | BIM, PTEN |
miR-21 | myeloid cells | macrophage activation | IL12a, PTEN, PDCD4 |
miR-34 | DC and B cells | Myeloid DC differentiation | FOXP1, JAG1, WNT1 |
miR-125b | monocyte | Innate immune response, TLR signaling | TNF- |
miR-126 | HSC | expansion of progenitor cells | HOXA9, PLK2 |
miR-132 | monocyte | Innate immune response | not determined |
miR-142 | Treg cell | Suppresor function of Treg cells | AC9 |
miR-146a | monocyte | Innate immune response, TLR signaling | IRAK-1, IRAK-2, TRAF6 |
miR-150 | B and T cells | mature B-cell production, T-cell activation | Myb |
miR-155 | B and T cells, DC | Innate and adaptive immune response | AID, BACH1, CEBPB, CSFR |
macrophages germinal center response | c-MAF, FADD, IKK, JARID2, | ||
Ig G class-switch | PU.1, Ripk1, SOCS, TAB2 | ||
Peripheral T cell development | |||
miR-181a | T cells | T cell receptor signaling | AID, BCL2, CD69, DUSP5 |
B cell development | DUSP6, PTPN22, SHP2 | ||
miR-181b | macrophages, B cells | B cell class switch | AID |
miR-196b | HSC | Hematopoietic stem-cell homeostasis | HOX |
miR-223 | myeloid cells | Granulopoiesis | MEF2C |
miR-326 | T cells | TH-17 cells development | ETS1 |
miR-424 | myeloid cells | monocyte differentiation and maturation |
AC9: adenylate cyclase 9; AID: Activation-Induced Cytidine Deaminase; BACH1: BTB and CNC homology 1, basic leucine zipper transcription factor 1; BCL2: B-cell lymphoma 2; BIM: BCL2-like 11; CEBPB: CCAAT/enhancer-binding protein beta; CSFR: Colony stimulating factor receptor; c-MAF: musculoaponeurotic fibrosarcoma oncogene homolog; DC: dendritic cell; DUSP5: Dual specificity protein phosphatase 5; DUSP6: Dual specificity protein phosphatase 6; ETS1: v-ets erythroblastosis virus E26 oncogene homolog 1; FADD: Fas-Associated protein with Death Domain; HSC: haematopoetic stem cell; HOX: Homeobox protein; HOXA9: Homeobox protein Hox-A9; FOXP1: Forkhead box P1; IKK: inhibitor of NF-kappaB kinase; IL12a: Interleukin-12 subunit alpha; IRAK-1: Interleukin-1 receptor-associated kinase 1; IRAK-2: Interleukin-1 receptor-associated kinase 2; JAG1: jagged 1; JARID2: Jumonji; Myb: Myb oncogene-like; MEF2C: Myocyte-specific enhancer factor 2C; NFIA: Nuclear factor 1 A-type; PDCD4: Programmed cell death protein 4; PTEN: phosphatase and tensin homolog; PLK2: pololike kinase 2; PTPN22: Tyrosine-protein phosphatase non-receptor type 22; PU.1: spleen focus forming virus (SFFV) proviral integration oncogene spi1; Ripk1: Receptor-interacting serine/threonine-protein kinase 1; RUNX1: Runt-related transcription factor; SHP2: SH2 domain containing protein thyrosine phosphatase; SOCS: Suppressor of cytokine signaling; TAB2: TAK1-associated binding protein 2 TRAF6: TNF receptor associated factor-6; TLR: Toll-like receptor; WNT1: wingless-related MMTV integration site 1.
miRNAs have an important role in regulating stem cell self-renewal and differentiation by repressing the translation of selected mRNAs in stem cells and differentiating into daughter cells. Such a role has been shown in embryonic stem cells, germline stem cells and various somatic tissue stem cells [
One of the first miRNAs described to have a role in immune cell development was miR-181a which is highly expressed in thymus cells and expressed at lower levels in the heart, lymph nodes, and bone marrow [
The adaptive or acquired immune system involves the selective recognition and removal of nonself by the TCRs on T cells and antibodies produced by B cells. The maturation, proliferation, differentiation, and activation of T and B cells are complex processes tightly controlled at different levels including miRNA-mediated posttranscriptional gene regulation [
The development of T cells in the thymus and their activation in the periphery are controlled by complex protein signalling networks that are subject to regulation by miRNAs [
Recent data have also indicated a role for miRNAs in the differentiation of T cells into distinct effector T helper cell subsets. miR-155 has an important role in the mammalian immune system, specifically in regulating T helper cell differentiation and the germinal center reaction to produce an optimal T cell-dependent antibody response [
miR-155 deficiency in Treg cells results in increased suppressor of cytokine signaling 1 (SOCS1) expression accompanied by impaired activation of signal transducer and activator of transcription 5 (STAT5) transcription factor in response to limiting amounts of IL-2. Forkhead box P3- (Foxp3-) dependent regulation of miR155 maintains competitive fitness of Treg cell subsets by targeting SOCS1 [
The generation of B cells that express high affinity antigen receptors involves two main stages: antigen-independent development in the bone marrow and antigen-dependent selection in the secondary lymphoid organs, both of which are associated with dynamic regulation by miRNAs [
In contrast, mice with a conditional deletion of Dicer in B cells had a complete block in B cell development [
The contribution of miRNAs in the antigen-driven stages of the humoral response in secondary lymphoid organs has also been described [
The innate immune response provides the initial defense against infection by external pathogens and is predominantly mediated via myeloid cells such as macrophages, DCs, monocytes, neutrophils, as well as natural killer (NK) cells. The presence of pathogens is commonly detected by tissue APCs such as macrophages and DCs via families of pattern recognition receptors that bind nonself-antigens such as microbial products. Many families of pattern recognition receptors have been identified, although the best characterised are the TLR which are composed of 11 members and the interleukin IL-1 receptors which have 10 members. On ligation, the APC is activated by the Nuclear factor kappa B (NF-
Several studies have shown that transcription factors involved in monocytopoiesis are regulated by, and/or regulate, specific miRNAs, which indicates a connection between these molecular species during development [
Neutrophils arise from granulocyte-monocyte progenitors under the influence of the transcription factor growth factor independent 1 (GfI1) [
miRNAs regulate distinct aspects of DC biology and so are involved in the crucial connection between innate and adaptive immune responses. miR-34 and miR-21 have been shown to be important for human myeloid-derived DC differentiation by targeting the mRNAs encoding Jagged1 and WNT1 [
miRNAs have been implicated in the development and function of NK cells which are important components of immune surveillance against cancer and viral infection [
Numerous studies clearly demonstrate that miRNAs play an essential role in the regulation of various aspects of innate immunity, including the regulation of direct microbial killing, the production of cytokines, and antigen presentation by MHC molecules. All of these mechanisms are important for host defense and are instrumental in initiating antigen-specific responses by cells of the adaptive immune system [
Both the induction and repression of miRNA expression in response to inflammatory stimuli can influence several biological processes and exert pro- or antinflammatory effects [
The downregulation of miR-125b appears to be necessary in macrophages to prevent suppression of Tumor necrosis factor-alpha (TNF-
The roles of miRNAs are only beginning to be explored in the context of autoimmunity, in which they may be involved in regulating immune responses against self-tissues [
In 2007, the involvement of miRNA in a new pathway regulating autoimmunity was discovered in T lymphocytes in the sanroque mouse [
Further support for a causal relationship between specific miRNAs and the onset of autoimmunity has come from studies involving miR-17–92 overexpression in mice [
Emerging evidence has demonstrated that miRNAs are differentially expressed in autoimmune diseases and miRNA regulation may impact in the development or prevention of autoimmunity. miRNA dysregulation is linked to autoimmune diseases that include rheumatoid arthritis, systemic lupus erythematosus, primary biliary cirrhosis, ulcerative colitis, psoriasis, Idiopathic thrombocytopenic purpura, primary Sjögren’s syndrome, and MS.
These molecules have also been shown to be useful as diagnostic and prognostic indicators of disease type and severity [
Clinical characteristics along with pathological heterogeneity make MS appealing to study many aspects of miRNAs in an organ-specific autoimmune disease, such as their potential as diagnostic or prognostic biomarkers and their role in pathogenesis of autoimmunity, neuroinflammation, and organ dysfunction. Thus, we will focus on the involvement of specific miRNAs in MS pathogenesis following the general overview of the immunopathobiology of the disease.
MS is a chronic inflammatory demyelinating disease of the CNS that primarily affects young adults. Prevalence rates for MS vary between 2 and 160 per 100,000 in different countries, and more than 2 million individuals are affected by this disease worldwide [
Experimental allergic encephalomyelitis (EAE) serves as the primary and most widely used animal model for MS and can be induced in susceptible rodent strains by active immunization of myelin antigens [
The clinical course of MS varies, with 80% of patients presenting with episodes of disability followed by a period of recovery classified as relapsing-remitting while 10%–15% exhibit a more progressive disease without remission, namely, primary progressive [
There is consensus that a dysregulated immune system plays a critical role in the pathogenesis of MS. Relapses are driven by the adaptive immune system and involve waves of Th1, Th17, and CD8+ cells that infiltrate the CNS and provoke an attack. These cells are modulated by Treg and B cells. MS is initiated and maintained by continuous migration of inflammatory immune cells from the periphery into the target organ. The three ways that lymphocytes can enter the CNS include entry from the bloodstream across the choroid plexus into the Cerebrospinal fluid (CSF), from the blood in the subanachroid space into the CSF, or directly into the parenchyma under permissible conditions, such as inflammation, controlled by cell adhesion molecules and cytokines [
Among cells isolated from the inflammatory infiltrate in actively demyelinating MS lesions, approximately 10% are T cells [
Most TCRs are composed of two linked polypeptides,
Defects in Treg-cell function have been described in MS, and a major goal of MS immunotherapy is to induce regulatory cells in a physiological fashion [
MS is generally thought to be a T cell-mediated immune disease although there is an important role of humoral immunity in pathogenesis of MS. Intrathecal antibody synthesis is a hallmark of the disease process and, in most of cases, consists of oligoclonal IgG production [
The innate immune system consists of monocytes, dendritic cells, and microglia. The innate immune system plays an important role in the immunopathogenesis of MS. The secondary progressive phase of MS has been believed to be related to neurodegenerative changes in the CNS [
Macrophages are the major MHC Class II positive cell in the CSF. Macrophages in EAE have an integral role in initiating disease, and depletion of macrophages significantly inhibits disease [
Microglial cells seem to be crucial for maintaining autoimmune responses in the CNS. It has been demonstrated that both a microglial cell-specific deficiency of CD40 expression and a transient inactivation of microglial cells reduce disease severity [
Astrocytes also express MHC Class II after IFN
The identification of MS susceptibility loci, of which at least 15 have a primary function in immunological systems, favors early immune dysregulation followed by secondary neurodegenerative processes [
In MS, both soluble factors and surface molecules could participate in neurodegeneration. Besides injurious proinflammatory molecules, proapoptotic factors produced by T cells, including FasL, granzyme B, soluble TNF-related apoptosis-inducing ligand (TRAIL), glutamate, nitric oxide, and free radicals, are possible mediators of injury [
Axonal injury can be directly caused by immune cells. CD4+ and CD8+ T-cell subsets, once activated, are highly neurotoxic. These effects are mediated through a variety of contact-dependent mechanisms involving cell surface molecules such as FasL, LFA-1, and CD40. Th1 and Th17 proinflammatory classes of CD4+ T cells are neurotoxic whereas the anti-inflammatory Th2 subset is not [
Many recent studies provide a link between miRNA function and neurodegeneration [
Within the CNS, myelin is produced by oligodendrocytes. Developmentally, the oligodendrocyte lineage arises from subventricular zone progenitors that give rise to oligodendrocyte progenitor cells (OPCs), which divide and migrate throughout the CNS before terminally differentiating to generate mature oligodendrocytes which myelinate receptive axons [
Remyelination following CNS demyelination restores rapid saltatory conduction of action potentials and contributes to the maintenance of axonal integrity [
Selective deletion of miRNA-processing enzyme, Dicer, in oligodendrocyte lineage cells results in severe myelinating deficits despite an expansion of the oligodendrocyte progenitor pool [
Dysfunction of the BBB is a major hallmark of MS and may impair tissue homoeostasis, which may have effects on disease progression, repair mechanisms, and drug delivery [
Little is known about what drives the differential control of the immune system in MS patients compared to unaffected individuals. Thus, it is important to reveal the aberrant miRNA expression profiling in MS patients. To our knowledge there have been only seven publications investigating the role of miRNAs in MS, six of which focus on the immune system in MS and the other on active and inactive MS lesions (Table
Differential miRNA expression in Multiple Sclerosis.
Sampe type | Number of patients and disease status | Specificity of patients and treatment | Number of tested miRNA | Results | Target genes | Reference |
---|---|---|---|---|---|---|
Whole bood | 59 MS (18 PP, 17 SP, 24 RR) and 37 controls | Causian No IMT | 733 | miR-17 and miR-20a downregulated | ND | Cox |
CD4+CD25+ | 12 MS (RR) and 14 controls | No IMT | 723 | miR-106b, MiR-19a, MiR-19b and miR-25 upregulated | TGF | De Santis |
CD4+, CD8+, B | 8 MS (RR) and 10 controls (microarray) | No IMT | 365 | miR-17-5p upregulated in CD4+ cells | ND | Lindberg |
15 MS (RR) | ||||||
and 10 controls (qPCR) | ||||||
Peripheral blood leukocytes | 43 MS (RR) | Chinese | ND | miR 326 upreguated in CD4+ cells | Ets-1 | Du |
40 control | miR-326 promotes Th-17 differentiation | |||||
11 NMO | ||||||
Whole bood | 20 MS (RR) | glatiramer acetate (9) | 866 | miR-145 upregulated in MS | ND | Keller |
19 controls | interferon-b (10) | |||||
Whole bood | 21 MS (9 remission, 4 relaps) | ND | 364 | miR-18b and miR-599 upregulated in relapse | interleukine signaling | Otaegui |
8 control | miR-96 upregulated in remission | Wnt, glutamate | ||||
Brain tissue | 20 MS (16 active, 5 inactive) | ND | 365 | miR-34a, miR-155 and miR-326 upregulated in active lesions | CD47 | Junker |
9 controls |
Ets-1: v-ets erythroblastosis virus E26 oncogene homolog 1; IMT: Immunmodulatory treatment; MS: Multiple sclerosis; ND: not determined; NMO: neuromyeliis optica; PP: primary progressive; RR: Relapsing remitting, Secondary progressive.
Studies in peripheral blood mononuclear cells (PBMCs) of patients with MS revealed different expression patterns compared to control individuals. Using qPCR, a pilot study of the expression of 346 miRNAs in PBMCs obtained from a small number of MS patients during relapse and remission, versus healthy controls, demonstrated differences in gene expression patterns not only between the MS patients and healthy controls but also between patients with and without active disease [
The recent study by Du et al. [
Although it is known that specific miRNAs are involved in each step of the maturation of pluripotent hematopoietic stem cells into the various blood cell lineages including B and T cells [
It is known that Tregs play a key role in the autoimmune balance and their improper function may facilitate the expansion of autoreactive T cell clones. CD4+CD25+Foxp3+ Treg cells play a pivotal role in the maintenance of self-tolerance and controlling autoimmunity [
Recent evidence has been provided for a potential functional defect of CD4+CD25+Foxp3+ Treg cells in patients with RRMS [
A recent
Altered miRNA profiles detected in MS active lesions may reflect the presence of infiltrating immune cells, changes in brain resident cells such as glial cells, or both. MiRNA profiling in isolated cells by laser capture microdissection from active and inactive MS lesions showed that the most prominently upregulated miRNAs in active MS lesions, miR-155, miR-650, miR-34a, and miR-326 were detected in both microdissected astrocytes and infiltrating immune cells [
Although Junker et al. have focused on only three upregulated miRNAs in active MS lesions (i.e., miR-155, miR-326, and miR-34a) and their common target CD47, other upregulated miRNAs, especially miR-146a and miR-34a deserve further mention. These miRNAs are known to modulate immune responses in different ways. They are also implicated in other CNS disorders accompanied by chronic neuroinflammatory conditions such as epilepsy, Alzheimer’s disease (AD), and schizophrenia. miR-34a upregulation was determined in peripheral blood cells of sporadic AD patients, cerebral cortex of APPswe/PSDeltaE9 mice, and prefrontal cortex of schizophrenic patients [
miR-146a has been recently identified as a potentially endogenous regulator of TLR and cytokine receptor signalling, suggesting a link between miRNAs and human inflammatory diseases [
Recently, specific miRNAs have been shown to be significantly upregulated in response to cytokine stress and in affected regions of AD brain. The brain-enriched miRNA-146a is currently thought to be a key regulator of the immune and inflammatory signaling systems in both health and disease [
The established role of miR-146a in innate immunity responses may also contribute to the pathogenesis of neuroinflammatory CNS diseases such as MS and AD. miRNA-146a controls TLR and cytokine signaling through a negative feedback regulation loop involving downregulation of TRAF6 and IRAK1 levels [
The miRNA profiling of microglial cells in both unstimulated and stimulated conditions has not been reported. Our preliminary study using microarray and qPCR revealed that the expression levels of a set of miRNA were deregulated upon LPS stimulation in N9 murine microglial cell line (Table
Significantly altered miRNAs upon stimulation LPS in N9 microglial cells.
MicroRNA | Microarray | qPCR | Targets | ||
Fold change | Fold change | ||||
mmu-miR-105 | 0.35 | .02 | ns | not determined | |
mmu-miR-125b-3p | 2.81 | ns | 0.16 | .047 | IL-1 |
mmu-miR-191 | 3.12 | ns | 0.21 | .032 | CCL9, CRP, IL-6, TLR-3 |
mmu-miR-193* | 0.26 | .03 | 0.28 | .047 | CCL6, IL-10, IL-12R |
mmu-miR-208a | 3.01 | ns | 0.12 | .015 | CD8, IL-18BP, IL-24 |
mmu-miR-224 | 3.73 | ns | 0.09 | .033 | CD53, CXCL-14, IL-11 |
mmu-miR-297c* | 0.31 | ns | 0.12 | .033 | not determined |
mmu-miR-324-3p | 0.33 | ns | 0.18 | .049 | not determined |
mmu-miR-376c | 2.99 | .01 | ns | not determined | |
mmu-miR-421 | 0.35 | ns | 0.03 | .033 | not determined |
mmu-miR-431* | 4.62 | ns | 0.22 | .034 | CD5, CD81, DICER, IRAK1, TRAP 1 |
mmu-miR-669g | 3.48 | ns | 0.15 | .015 | not determined |
mmu-miR-1190 | 0.28 | .01 | 0.12 | .016 | not determined |
mmu-miR-1894-5p | 0.34 | ns | 0.08 | .017 | not determined |
CCL9: chemokine (C-C) motif ligand 9; CXCL-14: chemokine (C-X-C) motif ligand 14; CRP: c reactive protein; IL-1
In patients with MS, intensive efforts are directed at identifying biomarkers in body fluids related to underlying disease mechanisms, disease activity and progression, and therapeutic response [
While we have only just begun to gain insights into miRNA biology, their apparent association with the onset and progression of human diseases such as MS has produced great interest in assessing the feasibility of therapeutic regulation of miRNAs [
The field of study of miRNAs is a very rapidly evolving new field in molecular biology. miRNAs are important regulators of gene expression, and they function by repressing specific target genes at the posttranscriptional level. miRNA-mediated regulation is essential for immune homeostasis and the prevention of autoimmune diseases. miRNA expression is tightly regulated during hematopoiesis and lymphoid cell differentiation and disruption of the entire miRNA network or specific miRNAs may lead to dysregulated immune responses. Abnormalities in miRNA expression related to inflammatory cytokines, Th17 and Treg cells, as well as B cells have been described in several autoimmune diseases. Emerging evidence suggests that miRNA dysregulation may contribute to the pathogenesis of MS. In the near future, further understanding of the role of miRNAs in intracellular signaling, the expression of proteins involved in immune responses, modulation of cytokines and chemokines, adhesion and costimulatory molecules and the interplay between the immune system and CNS should help to define the role of miRNAs in autoimmunity, and provide an exciting framework for developing new biomarkers and new therapeutic interventions in MS. It is reasonable to assume that future studies concerning the function of miRNAs involved in immune responses will extend our understanding about the complex regulatory networks in autoimmune diseases and MS. These efforts might allow the invention of novel strategies for the treatment of MS. miRNAs are promising reliable biomarkers of human diseases due to their stability being less susceptible to chemical modification and RNase degradation. Although there is much to be learned in the field, the role of miRNAs in regulating a great variety of targets and, as a consequence, multiple pathways makes their use in diagnostics a powerful tool to be exploited for early detection of MS, assessment for risk disease, and monitoring both disease progression and therapeutic responses to disease-modifying drugs.
miRNAs have recently emerged as a new class of modulators of gene expression at the posttranscriptional level and are thought to play a critical role in many biological processes.
miRNAs are involved in the development, maturation, and the functions of immune cells, which suggest that they are implicated in the development of autoimmune diseases.
Changes of expression of some miRNAs have been reported in autoimmune pathologies such as rheumatoid arthritis, systemic lupus erythematosus, and MS.
MS serves an example of a chronic and organ-specific autoimmune disease in which miRNAs modulate immune responses in the peripheral immune compartment and the neuroinflammatory process in the brain.
The differential expression of miRNAs and their role in MS have been investigated by several studies.
miRNAs have the potential to serve as biomarkers for the assessment of disease activity and therapeutic response to disease-modifying drugs in MS.
The authors declare no competing financial interests.
The authors thank Professor Anne Frary for critical reading of the manuscript for English.