Unveiling oligodendrocyte precursor cell heterogeneity: Focus on brain´s regional differences and on the G Protein coupled receptor GPR17.

Oligodendrocyte Precursor Cells (OPCs, also referred to NG2 cells) have been proposed as the fourth glial population of the adult brain besides astrocytes, microglia and oligodendrocytes. In the past 30 years the interest in this class of cells has raised and led to important discoveries. It is now well known that adult OPCs are cycling precursors in the adult central nervous system (CNS), that have the ability to generate mature, and subsequently myelinating oligodendrocytes during development but also in the adulthood. Interestingly OPCs showed also the potential to generate astrocytes and neurons, both in vitro and in vivo. OPCs are not only multipotential but they also display the unique property, among glial cells, to form synapses with neurons. All these multiple functions raised the hypothesis that adult OPCs consist of a heterogeneous population of cells devoted to different tasks. The idea of heterogeneity, among OPCs, has been raised also for their differentiation properties. Previous studies indeed showed a regional difference in the rate of OPCs differentiation. However since the mechanisms of differentiation of adult OPCs have not been yet fully resolved, it is not clear whether this observation is determined by intrinsic differences among adult OPCs (supporting the thesis of heterogeneity), or by diverse environmental cues (fostering, on the contrary, the idea of homogeneity among adult OPCs, at least in terms of differentiation). In this study we aimed to unveil the diversity between adult OPCs with respect to differentiation properties and to the expression of the G-protein coupled receptor GPR17. To understand if the diversity in differentiation properties between grey matter (GM) and white matter (WM) OPCs is due to intrinsic determinants or environmental cues, we set up a series of transplantation experiments of adult OPCs derived from GM or WM into the adult brain WM or GM. Transplanted cells showed a good rate of survival and the ability to differentiate. Interestingly, only WM cells differentiated in both cortical areas, whereas GM cells differentiate only in the WM. These results suggest that both intrinsic determinants and environmental cues are important in the fate decision of OPCs. With the aim to find possible candidates of such difference, we focused the study on the expression and possible function of the G protein receptor GPR17 (previously shown to be involved in postnatal OPCs differentiation), in the oligodendrocyte lineage of the adult brain. GPR17 is expressed only in a precise window of OPCs differentiation. A fraction of adult OPCs indeed transiently expressed the receptor, that is maintained on the cell membrane until a premyelinating stage. Proliferation and label-retaining studies pointed out that GPR17 expression starts immediately after adult OPCs division, suggesting that GPR17 labels a specific postmitotic stage of OPCs. With this deep characterization of GPR17 expression and relation to adult OPCs proliferation, we provided a new valuable tool to study a specific phase of the oligodendrocyte lineage. However, from our analysis is still not clear whether all OPCs are transiently expressing GPR17 or if the receptor labels only a subpopulation of OPCs. To solve this issue, the transgenic mouse line (GPR17-iCreERT2), that has been generated in parallel during this study, will be of great help. Moreover, the GPR17-iCreERT2 mouse model will be extremely useful to characterize the expression of GPR17 in other tissues besides the CNS. The unique possibility, to exploit this line to specifically knock out or overexpress other proteins, in cells expressing GPR17, will also increase the knowledge regarding GPR17 function in different systems. In conclusion, we provided new insights in the differentiation properties of adult cortical OPCs, new tools to study in detail the function of GPR17 in OPCs differentiation and to define the pattern of expression of GPR17 and its role in different tissues