Increase in NF-κB-sensitive miRNA-146a and miRNA-155 in multiple sclerosis (MS) and pro-inflammatory neurodegeneration

Deidre J. Devier, Jesus F. Lovera and Walter J. Lukiw* 1 Department of Cell Biology and Anatomy, Louisiana State University Health Sciences Center, New Orleans, LA, USA 2 Department of Neurology, Louisiana State University Health Sciences Center, New Orleans, LA, USA 3 Neuroscience Center of Excellence, Louisiana State University Health Sciences Center, New Orleans, LA, USA 4 Department of Ophthalmology, Louisiana State University Health Sciences Center, New Orleans, LA, USA *Correspondence: wlukiw@lsuhsc.edu


OVERVIEW
Multiple sclerosis (MS) is a complex, debilitating, immunopathologic disease of the human central nervous system (CNS) characterized by chronic systemic inflammation, alterations in innate-immune signaling, progressive demyelination and axonal loss. Currently there is no effective treatment or cure for MS. Recent data indicate that common molecular-genetic mechanisms involving a select group of NF-kB-sensitive microRNAs are shared by most MS patients, and their mechanism of pathogenic action is becoming increasingly understood. This brief "Opinion" paper will highlight some recently clarified roles for two NF-kBregulated, pro-inflammatory microRNAs, miRNA-146a and miRNA-155, in the MS disease process. We will also advance an opinion on how anti-NF-kB, anti-miRNA and/or related therapeutic strategies may be beneficial in the clinical management of MS and other progressive CNS diseases exhibiting inflammatory neurodegeneration.

MULTIPLE SCLEROSIS-INCIDENCE, SYMPTOMS, AND AUTOIMMUNITY
To understand the contribution of miRNA-146a and miRNA-155 to the etiopathogenesis of MS we will briefly highlight some salient features of this autoimmune disease. MS is globally the most common immunopathologic disorder affecting the human CNS: (i) with about 2.5 million affected worldwide (approximately 30 cases/100,000 globally; http://www.nationalmssociety.org/); (ii) with a variable incidence of frequency in different regions of the world (Melcon et al., 2014); (iii) with a variable incidence of occurrence amongst different human populations (Høglund and Maghazachi, 2014); (iv) with significant heterogeneity in the clinical phenotype (Sturm et al., 2014; http://www. nationalmssociety.org/); and (v) with low concordance rates in monozygotic twins, suggesting the involvement of complex heritable, epigenetic, microbial and/or environmental factors (Küçükali et al., 2014;Ma et al., 2014;Melcon et al., 2014). MS is generally characterized by abnormal responses of the immune system directed against glial-derived CNS myelin which normally sheaths, insulates and protects axons and nerve bundles (Küçükali et al., 2014;Melcon et al., 2014). MS exhibits (i) "sclerotic" or lipoprotein-enriched scar tissue nodules associated with demyelinated axons where normal electrical activities become progressively disorganized (Baranzini, 2014;Küçükali et al., 2014); and (ii) a wide variety of clinical symptoms that include muscle weakness, spasticity, loss of balance, sensory deficit and fatigue, dizziness, and vertigo, bowel, bladder and visual problems, depression and cognitive and psychiatric changes; approximately 90% of MS individuals become ultimately disabled (Baranzini, 2014;Guo et al., 2014;Harris and Sadiq, 2014). Consistent observations at the pathogenic and molecular-genetic level indicate five main highly interactive characteristics of MS: (i) a progressive demyelination whose extent correlates to MS severity; (ii) axonal swelling and macrophage activation; (iii) a T-cell mediated inflammatory response that subsequently triggers immune cells to release pro-inflammatory cytokines such as IL-1β; (iv) permeability changes in the blood-brain barrier (Kamphuis et al., 2015); and (v) increases in proinflammatory microRNA and related pathogenic biomarkers (Haghikia et al., 2012;Meinl and Meister, 2012;Danborg et al., 2014;Harris and Sadiq, 2014;Küçükali et al., 2014;Ma et al., 2014;Sturm et al., 2014;Kamphuis et al., 2015;see below). Specific gene mutations linked to MS include a cluster at human chromosome 6, part of the so-called "autoimmunome" network, which serves as the major histocompatibility complex (MHC) locus; interestingly this genetic locus is also implicated in the autoimmune disease type 1 diabetes and systemic lupus erythematosis (Baranzini, 2014;Gourraud et al., 2014;Sturm et al., 2014). Epidemiological evidence collectively indicates that MS is an immunopathologic disorder initiated by exogenous factors including microbes, possibly of viral origin, vaccines or unknown environmental factors in susceptible individuals genetically predisposed for MS (Gilden, 2005;Küçükali et al., 2014;Ma et al., 2014;Sturm et al., 2014). Indeed, heterogeneity in the MS clinical course and low twin concordance rates implicate multiple, complex, environmental and epigenetic factors that contribute to MS pathogenesis and most recently, a potential contribution by inducible species of CNS microRNAs (Haghikia et al., 2012;Lopez-Ramirez et al., 2014;Ma et al., 2014;Zhang et al., 2014;Kroesen et al., 2015).
One major mode of action in the CNS is for inducible, up-regulated miR-NAs to decrease their target mRNA levels and hence decrease gene expression Guo et al., 2010). Concurrent induction of both NF-kB (p50/p65) and pro-inflammatory miRNAs in stressed human primary brain cells has identified a group of pathogenic miRNAs under NF-kB (p50/p65) transcriptional control and these include miRNA-146a and miRNA-155 (Lukiw, 2012a;Zhang et al., 2014;Zhao et al., 2014). Interestingly, these same miRNAs have been found to be increased in sporadic Alzheimer's disease (AD) tissues which exhibit (i) significant global up-regulation of NF-kB in AD-affected anatomical regions (Lukiw and Bazan, 1998;Lukiw, 2012b); (ii) down-regulation in the expression of innate-immune markers such as the IL-1β receptor-associated kinase 1 (IRAK-1; with a concurrent surge in IRAK-2; Cui et al., 2010); and (iii) a progressive inflammatory degeneration (Latta et al., 2014). Different species of pro-inflammatory miRNAs in different CNS compartments may contribute to similar degenerative pathologies -for example miRNA-146a and miRNA-155 have slightly different effects on inflammatory gene expression in human brain and retina (see below; Lukiw et al., 2012;Ma et al., 2014). It is also important to point out that there appears to be some heterogeneity in miRNA abundance, complexity and related biomarkers amongst different human populations with the same neurological disorder, however recent evidence suggests important common, underlying pathogenic roles for miRNA-146a and miRNA-155 throughout the MS disease process (Meinl and Meister, 2012;Kutty et al., 2013;Harris and Sadiq, 2014;Ma et al., 2014).

ANTI-miRNA vs. anti-NF-kB STRATEGIES
In recent efforts to neutralize NF-kBtriggered, miRNA-mediated pathologies, the use of anti-miRNA and anti-NF-kB strategies have worked surprisingly well both in vitro and in vivo in animal experimentation. For example anti-miRNA-146a and/or anti-miRNA-155 LNA-protected oligonucleotides administered individually or as combinatorial cocktails exhibited significant efficacy in cytokine-stressed human primary neuronal-glial cell cocultures and in EAE in reducing aberrant AD-and MS-related pro-inflammatory signaling Murugaiyan et al., 2011;Lukiw et al., 2012;Lopez-Ramirez et al., 2014). Very recently miRNA-155 up-regulation that altered junctional organization and permeability of the blood-brain barrier in MS murine models was prevented using inhibition of endogenous miRNA-155 (Lopez-Ramirez et al., 2014;Kamphuis et al., 2015). Equally efficacious appear to be the use of anti-NF-kB remedial strategies; the current number of NF-kB inhibitors now exceeds 900 and the use of combined anti-miRNA and NF-kB inhibitors, and how and when to use them therapeutically, has been recently addressed (Gilmore and Herscovitch, 2006;Lukiw, 2012aLukiw, ,b, 2013Gibson, 2014). In our view pathogenic miRNA-146a and miRNA-155 up-regulation in several progressive immunodeficiency and/or pro-inflammatory disorders of the CNS indicates that (i) knowledge of the disease mechanism in one neurological disorder may shed some light on a similar disease mechanism in a related CNS disease; (ii) differential anti-miRNA and/or anti-NF-kB therapeutic strategies, perhaps using combinatorial cocktails, should be useful in the clinical management of neurological disorders such as MS; and (iii) multiple NF-kB inhibitors, perhaps combined with multiple anti-miRNA oligonucleotides and current MS pharmacological drugs including dimethyl fumarate and steroids may be tailored to suit each individual MS case in the expanding arena of personalized medicine (Lukiw, 2012a,b;Gotovac et al., 2014;Harris and Sadiq, 2014;Latta et al., 2014).

CONCLUDING REMARKS
Our understanding of NF-kB-regulated miRNAs and their abundance and complexity are revolutionizing our perceptions and ideas on gene expression in CNS aging and disease. It is our opinion that (i) progressive inflammatory neurodegeneration in MS involves NF-kB-regulated miRNA-146a and miRNA-155 and perhaps other pathogenic miRNAs which are inducible; (ii) targeting of inflammation-relevant gene expression by miRNA-146a and/or miRNA-155 suggest pathogenic pathways of MS may be in common with other kinds of human CNS degenerations; (iii) altered miRNA expression patterns in MS may be useful both diagnostically and in the design of novel therapeutic approaches; and (iv) either alone