Research ArticleGPCR SIGNALING

Metalloprotease cleavage of the N terminus of the orphan G protein–coupled receptor GPR37L1 reduces its constitutive activity

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Sci. Signal.  12 Apr 2016:
Vol. 9, Issue 423, pp. ra36
DOI: 10.1126/scisignal.aad1089

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Turning off a constitutively active GPCR

In animals, most G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptors (GPCRs) are inactive under basal conditions and require either binding of a ligand, isomerization of their bound ligand by light, or cleavage of a tethered ligand to become activated. Coleman et al. found that GPR37L1, which is a GPCR abundant in the cerebellum, signaled to Gαs and stimulated cyclic AMP signaling in the absence of any exogenously added ligand when expressed in a cultured cell line and assayed in the presence of normal growth medium. Furthermore, cleaved fragments of this receptor are detectable in human cerebral spinal fluid, and biochemical analysis revealed a long and short form of the receptor. The short form, which resulted from cleavage of the N terminus, was inactive and was the predominant form detected in rodent cerebellum. Metalloprotease inhibitors blocked the cleavage of the N terminus, suggesting that signals that regulate the activity of these proteases, rather than ligand-binding events, control the signaling mediated by this GPCR. How widespread this type of GPCR regulatory mechanism is and whether the cleaved fragments have bioactivity remain interesting questions.

Abstract

Little is known about the pharmacology or physiology of GPR37L1, a G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor that is abundant in the cerebellum. Mice deficient in this receptor exhibit precocious cerebellar development and hypertension. We showed that GPR37L1 coupled to the G protein Gαs when heterologously expressed in cultured cells in the absence of any added ligand, whereas a mutant receptor that lacked the amino terminus was inactive. Conversely, inhibition of ADAMs (a disintegrin and metalloproteases) enhanced receptor activity, indicating that the presence of the amino terminus is necessary for GPR37L1 signaling. Metalloprotease-dependent processing of GPR37L1 was evident in rodent cerebellum, where we detected predominantly the cleaved, inactive form. However, comparison of the accumulation of cAMP (adenosine 3′,5′-monophosphate) in response to phosphodiesterase inhibition in cerebellar slice preparations from wild-type and GPR37L1-null mice showed that some constitutive signaling remained in the wild-type mice. In reporter assays of Gαs or Gαi signaling, the synthetic, prosaposin-derived peptide prosaptide (TX14A) did not increase GPR37L1 activity. Our data indicate that GPR37L1 may be a constitutively active receptor, or perhaps its ligand is present under the conditions that we used for analysis, and that the activity of this receptor is instead controlled by signals that regulate metalloprotease activity in the tissue.

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