Abstract
Abstract Background and Purpose: Gpr17 is an orphan receptor involved in
the process of myelination due to its ability to inhibit the maturation
of oligodendrocyte progenitor cells into myelinating oligodendrocytes.
Despite multiple claims that the biological ligand has been identified,
it remains an orphan receptor. Experimental Approach: Seventy-seven
oxysterols were screened in a cell-free [35S]- GTPgS binding assay
using membranes from cells expressing Gpr17. The positive hits were
characterised using cAMP, IP1, and calcium mobilisation assays, with
results confirmed in rat primary oligodendrocytes. Rat and pig brain
extracts were separated by HPLC chromatography and endogenous
activator(s) were identified in receptor activation assays. Gene
expression studies of Gpr17 and Cyp46a1, the enzymes responsible for the
conversion of cholesterol into specific oxysterols, were performed using
quantitative real time PCR. Key Results: Eight oxysterols were able to
stimulate Gpr17 activity, including the brain cholesterol,
24(S)-hydroxycholesterol. A specific brain fraction from rat and pig
extracts containing 24S-HC activates Gpr17 in vitro assays. Expression
of Gpr17 during mouse brain development correlates with the expression
of Cyp46a1 and the levels of 24S-HC itself. Other active oxysterols have
low brain concentrations below effective ranges. Conclusions and
Implications: Oxysterols, including but not limited to 24S-HC, could be
physiological activators for Gpr17 and thus potentially regulate OPC
differentiation and myelination through activation of the receptor.