The Macrophage Migration Inhibitory Factor (MIF) is apro-inflammatory cytokine expressed by a variety of cell types includingepithelial, endothelial and immune cells (Stosic-Grujicic, 2009). Unlike othercytokines, MIF is constitutively expressed and stored in intracellular poolsand does not require de novo protein synthesis before secretion.
MIF possessesproperties of cytokine, enzyme, endocrine molecule and chaperon-like protein.It binds to the cell-surface receptor CD74 and to the intracellular receptorJAB1(Cvetkovic, 2006).MIF is a homotrimer of 12.5 kDa subunits, withoxidoreductase and tautomerase activity (Stosic-Grujicic, 2009). Substratesfor the enzymatic activity of MIF are represented by phenylpyruvic acid,p-hydroxyphenylpyruvic acid,3,4-dihydroxyphenylaminechrome, and norepinephrinechrome. Because of itshomotrimeric structure, there are multiple binding sites for potentialinhibitors of MIF that could disrupt itstertiary structure and inhibit its enzymatic activity or the binding to CD74and other ligands.
The roles of MIF in immunologic response regulationare diverse. As a product of cells of the innate immune system, MIF actsthrough enhancement of TLR4 expression, phagocytosis, intracellular killing,nitric oxide, H2O2 and TNF-? production in macrophages, thus representing animportant factor in the protection of the host against various infectiousagents. Through induction of IL-12 and inhibition of IL-10 synthesis, MIFfavors Th1 immune response (Cvetkovic, 2006). A regulatory role of MIF has been observed in MIF genedeficient cells. It has been shown that MIF deficiency attenuatesleukocyte–endothelial cell interactions (Gregory, 2004), as well as theexpression and function of IL-1 and TNF receptors (Toh, 2006), thus providingfurther molecular evidence for the critical role of MIF in autoimmune andinflammatory states.
Several data suggest a key role of MIF in thepathogenesis of Multiple Sclerosis (MS). In mice with EAE, an animal model ofMS, MIF was found to be upregulated in the affected tissue (Baker, 1991).Immunoneutralization or genetic depletion of MIF (Denkinger, 2003; Powell,2005) reduced the severity of the disease by impairing the migration ofautoreactive T cells to the CNS and down-regulation of inflammatory cytokineproduction. Moreover, intraspinal microinjection of MIF resulted in theupregulation of inflammatory mediators in microglia, which was sufficient torestore EAE-mediated inflammatory pathology in MIF-deficient mice (Cox, 2013).The inhibition of MIF actions by usage of neutralizinganti-MIF antibodies has also proven therapeutically effective (Denkinger, 2003;Powell, 2005). Indeed, MIF blockadedecreases the expression of VCAM-1 in the CNS, and impairs the homing ofneuroantigen-specific T cells to this site. Moreover, MIF blockade reduces theclonal size of the autoantigen-specific Th1 cells, and increases theiractivation threshold (Denkinger, 2003).In clinical studies, enhanced levels of MIF wereobserved in serum and in cerebrospinal fluid of patients with active/relapsedMS (Niino, 2005).
In particular, Niino and collaborators found that theconcentration of MIF in CSF samples was significantly elevated in relapsedcases of MS compared with control samples. In addition, Cox et al. (Cox, 2013)observed that MIF is highly expressed in human active MS lesions. The biological effects of MIF are predominatelymediated through its primary receptor, CD74 (Simons D. et al.,2011). Thecomprehensive analysis recently shows that MIF controls the activation of CD74(Pantouris G.
et al.,2015).The binding of MIF to its receptor complex CD74/CD44leads to the activation of the extracellular signal regulated kinase (ERK) 1and 2 in the mitogen-activated protein kinase (MAPK) pathway, and thePI3K/Akt/SRC signal transduction cascade (Lue H.
et al., 2007; Shi X. etal.
,2006), which, in turn, increase cell proliferation, decrease cellapoptosis, and enhance cell migration (Meyer-Siegler KL. Et al., 2006; Lee CY.Et al., 2012;).In addition to activating the type-II receptor CD74.(MIF) exhibits chemokine-like activities through non-cognate interactions withthe chemokine receptors CXCR2 and CXCR4.The activation of MIF-CXCR2 and -CXCR4 axes promotesrecruitment of leukocytes and contributes to the promotion of other biologicalactivities.
As for the MIF-CXCR2 interaction it has been found to commit apseudo-ELR and an N-like motif, with respect to the interaction of CXCR4 andMIF nothing has been discovered about it.MIF activity also manifests itself through the bindingof three receptors: the CD74-CD44, CXCR2 and CXCR4 complex. The role of MIFswith these three different receptors is the answer to the various biologicalactivities associated with MIF. MIF is also unique because it is the onlyprotein that activates both an ELR + and ELR-chemocin receptor. Therelationships leading to the biological function between MIF and CXCR2 or CD74(Kraemer, S.et al,2011; Pantouris, G.2015; Weber, C.2008) have been studied,but interactions with CXCR4 are not yet known.
In 1990, a new protein (DDT or MIF-2) was obtained bypurification of mouse melanocytes during an investigation into the regulationof mammalian melanogenesis. This molecule has been defined as dopachrometautomerase according to its tautomerase activity on dopachrome (Aroca, P. etal.,1990). It has been reported that dopachrome tautomerase increases theamount of melanin formed by tyrosin-1-tyrosine melanoma. Subsequently, twomelanin-forming cells (Tsukamoto, K.
et al,1992; Winder, A. J. et al,1993) havebeen defined as two l-isomers linked to dopachrome tautomerase membrane,tyrosinase-related protein-1 and -2 (TRP-1, TRP-2). Both enzymes catalyze theisomerization of L-dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA).These isomers were obtained by purification of a specific enzyme for thetautomerization of D-dopachrome, which was called D-dopachrome tautomerase(Odh, G. et al,1993). The D-DT enzymatic activity has been found in the malerat, localized in the liver, kidneys, spleen and also in the human at the levelof melanoma cells, liver cells and ultimately in the blood (Bjork, P.
etal.,1996). Similar to MIF, MIF-2 is strongly expressed in most tissues (Merk Met al.,2011). These two proteins work cooperatively and it has been shown thatneutralizing MIF-2 in vivo leads to significantly decrease of inflammation(Merk M et al.,2011).
MIF as MIF-2 is also stored in cytosol in preformedstorage pools that allows rapid release of both proteins on different stressstimuli (Burger-Kentischer A., 2002;Merk M.2012). It is not common that theMIF-2 secretion mode is caused by the so-called non-classic path for secrecyand is based on the lack of a sequence of signals that usually averageGolgi-dependent secretion (H. Sugimoto, M. et al.
, 1999). There are currentlyseveral stimuli known to stimulate MIF and MIF-2 secretion, such as LPS,inflammation, hypoxia, hyperoxia, ischemia and reperfusion, hormone, surgicalstress and mitogenic factors (M. Merk, et al. 2011). Rapid secretion from MIFand MIF-2 cellular pools is therefore useful for serving as indicators in awide range of critical illnesses.
D-DT and MIF are considered homologous as they sharethe identity of 35% (M. Merk, et al. 2011).
From a structural point of view,the D-DT topology resembles that of MIF and bends to form an homotrimer withextended contacts between the subunits through the lateral bands of the betaleaf . The link between D-DT and CD74 also exhibits high affinity (D-DT: KD =5.42 109 M vs MIF: KD = 1.40 109 M) (M. Merk, et al. 2011) and competes withMIF to bind to the receptor, suggesting that the two homologues could share thesame binding site on CD74.
As a MIF, D-DT is able to activate ERK1 / 2 MAPkinase activation in similar signaling responses.Despite the structural and functional knowledge of MIFand D-DT, molecular parameters that regulate the interaction of these twocytokines with their shared receptor are not fully understood.From a genomic point of view, the macrophage migrationinhibitory factor gene MIF, located on 22q11.2, encodes a multifunctionalcytokine, MIF. However, the MIF-antisense transcription, called MIF-AS, is anovel unknown lncRNA. Studies on the biological function of MIF-AS and its rolehave not yet been reported. Single nucleotide polymorphism (SNP), caused by a singlenucleotide variation, mainly refers to the genomic DNA sequence polymorphism.Recent testimonies have confirmed that SNPs in lncRNAs (long non coding RNAs)can influence its biological mRNA formation processes, which can lead to theaberration of its interactors (Zhu Z.
et al., 2012; Li L. et al.
, 2013)Multiple sclerosis (MS) is one of the most commonchronic inflammatory diseases of the central nervous system leading todemyelination and neurodegeneration. MS is a variable condition and thesymptoms depend on which areas of the central nervous system have beenaffected. There is no set pattern to MS and every patients has a different setof symptoms, which vary from time to time and can change in severity andduration, even in the same person. Increasing body of data supports thehypothesis that MIF is located at an upstream position in the events leading topossible dysregulated immunoinflammatory responses leading to autoimmunereactions and may therefore represent a new potential target for the treatmentof autoimmune diseases. Indeed, it has been reported in independent studiesthat MIF plays a key role in the pathogenesis of both organ-specific andsystemic autoimmune diseases (Stosic-Grujicic et al.
, 2009).The presentstudy aims for a better elucidation of the involvement of MIF in modulating theencephalitogenic immune responses underlying MS and to the development ofanti-MIF based therapeutic strategies.