GPR37 (also known as Pael-R) and GPR37L1 are orphan G protein-coupled receptors that are almost exclusively expressed in the nervous system. We screened these receptors for potential activation by various orphan neuropeptides, and these screens yielded a single positive hit: prosaptide, which promoted the endocytosis of GPR37 and GPR37L1, bound to both receptors and activated signaling in a GPR37- and GPR37L1-dependent manner. Prosaptide stimulation of cells transfected with GPR37 or GPR37L1 induced the phosphorylation of ERK in a pertussis toxin-sensitive manner, stimulated 35 S-GTPγS binding, and promoted the inhibition of forskolin-stimulated cAMP production. Because prosaptide is the active fragment of the secreted neuroprotective and glioprotective factor prosaposin (also known as sulfated glycoprotein-1), we purified full-length prosaposin and found that it also stimulated GPR37 and GPR37L1 signaling. Moreover, both prosaptide and prosaposin were found to protect primary astrocytes against oxidative stress, with these protective effects being attenuated by siRNA-mediated knockdown of endogenous astrocytic GPR37 or GPR37L1. These data reveal that GPR37 and GPR37L1 are receptors for the neuroprotective and glioprotective factors prosaptide and prosaposin.

Highlights d Cell-surface IR translocates to the nucleus and associates with promoters genome-wide d IR interaction with DNA is mediated by coregulator HCF-1 and transcription factors d IR associates with Pol II and regulates gene expression d Target genes are characteristic of insulin functions in physiology and disease

Prosaposin (also known as SGP-1) is an intriguing multifunctional protein that plays roles both intracellularly, as a regulator of lysosomal enzyme function, and extracellularly, as a secreted factor with neuroprotective and glioprotective effects. Following secretion, prosaposin can undergo endocytosis via an interaction with the low-density lipoprotein-related receptor 1 (LRP1). The ability of secreted prosaposin to promote protective effects in the nervous system is known to involve activation of G proteins, and the orphan G protein-coupled receptors GPR37 and GPR37L1 have recently been shown to mediate signaling induced by both prosaposin and a fragment of prosaposin known as prosaptide. In this review, we describe recent advances in our understanding of prosaposin, its receptors and their importance in the nervous system.

GPR37, also known as the parkin-associated endothelin-like receptor (Pael-R), is an orphan G protein-coupled receptor (GPCR) that exhibits poor plasma membrane expression when expressed in most cell types. We sought to find ways to enhance GPR37 trafficking to the cell surface in order to facilitate studies of GPR37 functional activity in heterologous cells. In truncation studies, we found that removing the GPR37 N-terminus (NT) dramatically enhanced the receptor's plasma membrane insertion. Further studies on sequential NT truncations revealed that removal of the first 210 amino acids increased surface expression nearly as much as removal of the entire NT. In studies examining the effects of co-expression of GPR37 with a variety of other GPCRs, we observed significant increases in GPR37 surface expression when the receptor was co-expressed with the adenosine receptor A 2A R or the dopamine receptor D 2 R. Co-immunoprecipitation experiments revealed that full-length GPR37 and, to a greater extent, the truncated GPR37 were capable of robustly associating with D 2 R, resulting in modestly-altered D 2 R affinity for both agonists and antagonists. In studies examining potential interactions of GPR37 with PDZ scaffolds, we observed a specific interaction between GPR37 and syntenin-1, which resulted in a dramatic increase in GPR37 surface expression in HEK-293 cells. These findings reveal three independent approaches -N-terminal truncation, coexpression with other receptors and co-expression with syntenin-1 -by which GPR37 surface trafficking in heterologous cells can be greatly enhanced to facilitate functional studies on this orphan receptor.The orphan G-protein coupled receptor (GPCR) GPR37, also known as the parkin-associated endothelin-like receptor (Pael-R), is highly expressed in the mammalian central nervous system, but its function still remains largely unknown. GPR37 is most closely related to another CNS-enriched orphan receptor known as GPR37-like 1 (GPR37L1), and both orphans share significant sequence homology with the endothelin B receptor and other related peptideactivated GPCRs (1-5). However, none of the mammalian peptides tested so far, including endothelins, bombesin and others, have produced activation of any signaling pathways in heterologous cells or Xenopus oocytes expressing either GPR37 or GPR37L1 (2-5). A peptide called "head activator" (HA), which is derived from the freshwater coelenterate Hydra, has been reported to be capable of activating GPR37 (6), but no peptide equivalent to HA has been definitively identified in vertebrates. Thus, GPR37 and GPR37L1 must still be considered orphan GPCRs at the present time.A major stumbling block impeding progress in understanding the ligand binding and signaling of GPR37 is the receptor's poor trafficking to the plasma membrane in most heterologous cell lines. GPR37 is commonly misfolded and therefore aggregated in the endoplasmic reticulum † This work was supported by the Pharmacological Sciences training grant T32 GM008602, the NIH and the...

Progressive myoclonus epilepsies (PMEs) are disorders characterized by myoclonic and generalized seizures with progressive neurological deterioration. While several genetic causes for PMEs have been identified, the underlying causes remain unknown for a substantial portion of cases. Here we describe several affected individuals from a large, consanguineous family presenting with a novel PME in which symptoms begin in adolescence and result in death by early adulthood. Whole exome analyses revealed that affected individuals have a homozygous variant in GPR37L1 (c.1047G>T [Lys349Asp]), an orphan G protein-coupled receptor (GPCR) expressed predominantly in the brain. In vitro studies demonstrated that the K349N substitution in GPR37L1 did not grossly alter receptor expression, surface trafficking or constitutive signaling in transfected cells. However, in vivo studies revealed that a complete loss of Gpr37L1 function in mice results in increased seizure susceptibility. Mice lacking the related receptor Gpr37 also exhibited an increase in seizure susceptibility, while genetic deletion of both receptors resulted in an even more dramatic increase in vulnerability to seizures. These findings provide evidence linking GPR37L1 and GPR37 to seizure etiology and demonstrate an association between a GPR37L1 variant and a novel progressive myoclonus epilepsy.