Quantitative Proteomics Reveal an Altered Pattern of Protein Expression in Brain Tissue from Mice Lacking GPR37 and GPR37L1

GPR37 and GPR37L1 are glia-enriched G protein-coupled receptors
that have been implicated in several neurological and neurodegenerative
diseases. To gain insight into the potential molecular mechanisms
by which GPR37 and GPR37L1 regulate cellular physiology, proteomic
analyses of whole mouse brain tissue from wild-type (WT) versus GPR37/GPR37L1
double knockout (DKO) mice were performed in order to identify proteins
regulated by the absence versus presence of these receptors (data
are available via ProteomeXchange with identifier PXD015202). These
analyses revealed a number of proteins that were significantly increased
or decreased by the absence of GPR37 and GPR37L1. One of the most
decreased proteins in the DKO versus WT brain tissue was S100A5, a
calcium-binding protein, and the reduction of S100A5 expression in
KO brain tissue was validated via Western blot. Coexpression of S100A5
with either GPR37 or GPR37L1 in HEK293T cells did not result in any
change in S100A5 expression but did robustly increase secretion of
S100A5. To dissect the mechanism by which S100A5 secretion was enhanced,
cells coexpressing S100A5 with the receptors were treated with different
pharmacological reagents. These studies revealed that calcium is essential
for the secretion of S100A5 downstream of GPR37 and GPR37L1 signaling,
as treatment with BAPTA–AM, an intracellular Ca2+ chelator, reduced S100A5 secretion from transfected HEK293T cells.
Collectively, these findings provide a panoramic view of proteomic
changes resulting from loss of GPR37 and GPR37L1 and also impart mechanistic
insight into the regulation of S100A5 by these receptors, thereby
shedding light on the functions of GPR37 and GPR37L1 in brain tissue.