Regulation of the P2Y-like receptor GPR17 by agonist induced desensitization

The P2Y-like G-protein-coupled receptor (GPCR) GPR17 is structurally and phylogenetically related to the already known P2Y receptors for extracellular nucleotides and the cysteinyl leukotriene 1 receptor and cysteinyl leukotriene 2 receptor for arachidonic acid-derived cysteinyl-leukotrienes. This receptor seems to act as both a classical ligand-activated GPCR responding to both uracil nucleotides (UDP, UDP glucose and UDP-galactose) and cysteinyl-leukotrienes (LTC4, LTD4 and LTE4) and as a direct, negative regulator of the CysLT1 receptor. In intact brain, GPR17 is normally present on neurons and on a subset of oligodendrocyte precursor cells. In pathological conditions, this receptor plays a role in inducing early necrotic death inside the lesion, but also participates in the subsequent remodeling and repair of the lesioned area, occurring in the days and weeks after injury. Oligodendrocyte myelination starts when GPR17 expression is downregulated: in vitro, GPR17 overexpression not only blocks differentiation of neural progenitor cells into oligodendro-cytes, but also inhibits terminal differentiation of primary OPCs. The fact that GPR17 expression is downregulated during active myelinogenesis and upregulated in demyelinating lesions raises the possibility that sustained expression of this receptor may negatively regulate oligodendrocyte myelination. In order to shed light on GPR17 receptor-mediated intracellular pathway and regulatory mechanisms, during my PhD I investigated: • Intracellular signaling pathways involved in GPR17 regulatory mechanisms: in particular THE ROLE OF GRKs, β-arrestin1/2, ERK and CREB interactions in GPR17 desensitization and internalization; • The role of Akt/mTOR/ Mdm2 and GRK2 intercellular pathway in OPCs myelination and differentiation; • Pharmacological profile of new GPR17 ligands. My PhD work led to the identification of new potential intracellular pathways activated during GPR17 desensitization and new promising GPR17 ligands , which could represent a starting point for the design of innovative therapies in neurodegenerative diseases. Specifically, the results demonstrate that, the two classes of chemically- and metabolically-unrelated ligands recruiter different GRK isoforms to regulate GPR17 functional responses and activate distinct intracellular signaling pathways to control OPC differentiation program. Furthermore, in rat primary OPCs, inhibition of mTOR by RAPA reduces GRK2 levels and prevents GPR17 down-regulation/desensitization, strongly inhibiting OPC maturation. Overall these data confirm GPR17 as a promising target for the treatment of demyelinating diseases.