The ubiquitin ligase Mdm2 controls oligodendrocyte maturation by intertwining mTOR with G protein-coupled receptor kinase 2 in the regulation of GPR17 receptor desensitization

Abstract

During oligodendrocyte precursor cell (OPC) differentiation, defective control of the membrane receptor GPR17 has been suggested to block cell maturation and impair remyelination under demyelinating conditions. After the immature oligodendrocyte stage, to enable cells to complete maturation, GPR17 is physiologically down-regulated via phosphorylation/desensitization by G protein-coupled receptor kinases (GRKs); conversely, GRKs are regulated by the "mammalian target of rapamycin" mTOR. However, how GRKs and mTOR are connected to each other in modulating GPR17 function and oligodendrogenesis has remained elusive. Here we show, for the first time, a role for Murine double minute 2 (Mdm2), a ligase previously involved in ubiquitination/degradation of the onco-suppressor p53 protein. In maturing OPCs, both rapamycin and Nutlin-3, a small molecule inhibitor of Mdm2-p53 interactions, increased GRK2 sequestration by Mdm2, leading to impaired GPR17 down-regulation and OPC maturation block. Thus, Mdm2 intertwines mTOR with GRK2 in regulating GPR17 and oligodendrogenesis and represents a novel actor in myelination.

Differential local tissue permissiveness influences the final fate of GPR17‐expressing oligodendrocyte precursors in two distinct models of demyelination

“…However, when inflammation becomes stronger and chronic (as it occurs when EAE is overtly symptomatic), this beneficial effect is turned into a detrimental condition (Marchetti & Abbracchio 2005) that worsens rather than favor, remyelination. In line with this hypothesis, the strong inflammatory milieu associated to the degenerative conditions mentioned above is accompanied by GPR17 upregulation (Fumagalli et al, 2015), which prevents terminal OPC maturation and remyelination.…”

“…Previous studies have shown that, in the brain, GPR17 acts as an intrinsic regulator of this process: indeed, it is necessary to start OPC differentiation but, after a certain stage, it has to be turned down to allow oligodendrocyte terminal maturation (Fumagalli et al, 2015; Fumagalli, Lecca, Coppolino, Parravicini, & Abbracchio, 2017). These features have led to the proposal that GPR17 represents a new potential target for remyelination therapies in diseases characterized by myelin disruption such as multiple sclerosis.…”

“…As already mentioned, while GPR17 is needed to start differentiation, its chronic overexpression due to impaired physiological downregulation has been associated to OPC blockade and impaired remyelination (Fumagalli et al, 2015; Ou et al, 2016). Besides EAE (the present study), several other distinct models of brain disease, including stroke, trauma, Alzheimer's (Fumagalli et al, 2016) have been associated to persistent GPR17 upregulation and impaired progression along the oligodendrocyte lineage, eventually resulting in dysfunctional repair.…”

“…To detail the final fate of the GPR17‐expressing cells and to unveil whether they indeed become functionally mature myelinating oligodendrocytes, we took advantage of the first inducible fluorescent GPR17 reporter mouse line for fate mapping studies (GPR17‐iCreER T2 xCAG‐eGFP mice), where, upon tamoxifen treatment, all GPR17‐expressing cells at that specific moment become permanently fluorescent, and can be traced throughout life (Viganò et al, 2016). Since, as already mentioned, GPR17 is only transiently expressed by OPCs and is no longer present in mature myelinating oligodendrocytes (Lecca et al, 2008; Fumagalli et al, 2011; Fumagalli et al, 2015) this transgenic mouse line represents the only possible means to follow in vivo the final destiny of the GPR17 + pool of OPCs, even after cells have physiologically downregulated the receptor along their differentiation pathway.…”

“…In the intact brain and spinal cord, GPR17 is specifically expressed by a subset of early bipolar NG2 positive (GPR17 + ‐NG2 + ) OPCs, accompanies OPC maturation up to immature/pre‐oligodendrocytes and is then downregulated before terminal maturation (Lecca et al, 2008; Ceruti et al, 2009). Any alterations in this precise expression pattern result in myelination defects (Chen et al, 2009; Fumagalli et al, 2011; Fumagalli et al, 2015). Pathologically increased levels of GPR17 have been found after focal lysolecithin induced demyelination (Boda et al, 2011), following brain ischemia in rodents subjected to middle cerebral artery occlusion (MCAO) (Ciana et al, 2006; Lecca et al, 2008) and after traumatic brain injury in both rodents (Boda et al, 2011) and human subjects (Franke et al, 2013).…”