Adhesion G protein-coupled receptors (aGPCRs) comprise the second largest yet least studied class of the GPCR superfamily. aGPCRs are involved in many developmental processes, immune and synaptic functions, but the mode of their signal transduction is unclear. Here, we show that a short peptide sequence (termed the Stachel sequence) within the ectodomain of two aGPCRs, GPR126 and GPR133, functions as a tethered agonist. Upon structural changes within the receptor ectodomain, this intramolecular agonist is exposed to the 7-transmembrane helix domain, which triggers G-protein activation. Our studies show high specificity of a given Stachel sequence for its receptor. Finally, the function of Gpr126 is abrogated in zebrafish with a mutated Stachel sequence, and signaling is restored in hypomorphic gpr126 zebrafish mutants upon exogenous Stachel peptide application. These findings illuminate a previously unknown mode of aGPCR activation, and can initiate the development of specific ligands for this currently untargeted GPCR family.

SUMMARY Myelin ensheathes axons to allow rapid propagation of action potentials and proper nervous system function. In the peripheral nervous system, Schwann cells (SCs) radially sort axons into a 1:1 relationship before wrapping an axonal segment to form myelin. SC myelination requires the adhesion G protein-coupled receptor GPR126, which undergoes autoproteolytic cleavage into an N-terminal fragment (NTF) and a 7-transmembrane-containing C-terminal fragment (CTF). Here, we show that GPR126 has domain-specific functions in SC development whereby the NTF is necessary and sufficient for axon sorting while the CTF promotes wrapping through cAMP elevation. These biphasic roles of GPR126 are governed by interactions with Laminin-211, which we define as a novel ligand for GPR126 that modulates receptor signaling via a tethered agonist. Our work suggests a model in which Laminin-211 mediates GPR126-induced cAMP levels to control early and late stages of SC development.

The myelin sheath surrounding axons ensures that nerve impulses travel quickly and efficiently, allowing for the proper function of the vertebrate nervous system. We previously showed that the adhesion G-protein-coupled receptor (aGPCR) Gpr126 is essential for peripheral nervous system myelination, although the molecular mechanisms by which Gpr126 functions were incompletely understood. aGPCRs are a significantly understudied protein class, and it was unknown whether Gpr126 couples to G-proteins. Here, we analyze Dhh Cre ; Gpr126 fl/fl conditional mutants, and show that Gpr126 functions in Schwann cells (SCs) for radial sorting of axons and myelination. Furthermore, we demonstrate that elevation of cAMP levels or protein kinase A activation suppresses myelin defects in Gpr126 mouse mutants and that cAMP levels are reduced in conditional Gpr126 mutant peripheral nerve. Finally, we show that GPR126 directly increases cAMP by coupling to heterotrimeric G-proteins. Together, these data support a model in which Gpr126 functions in SCs for proper development and myelination and provide evidence that these functions are mediated via G-protein-signaling pathways.

The authors note that, due to a printer's error, references 41-50 appeared incorrectly. The corrected references follow. The authors note: "Our paper unfortunately missed reference to an earlier suggestion of the T6 structure (43). This work entitled 'A hypothetical dense 3,4-connected carbon net and related B 2 C and CN 2 nets built from 1,4-cyclohexadienoid units' by M. J. Bucknum and R. Hoffmann was published in J Am Chem Soc 116: 11456-11464 (1994), where the electronic structure of a hypothetical 3,4-connected tetragonal allotrope of carbon is discussed. The results in this article are consistent with what we find. The same group had also suggested a metallic carbon structure (44) that was published in J Am Chem Soc 105: 4831-4832 (1983), which we also missed to cite. We thank Prof. Hoffmann for bringing these papers to our attention."The complete references appear below. www.pnas.org/cgi

Ablation of the cellular prion protein PrPC leads to a chronic demyelinating polyneuropathy (CDP) affecting Schwann cells. Neuron-restricted PrPC expression prevents the disease1, suggesting that it acts in trans through an unidentified Schwann cell receptor. We found that the cAMP concentration in PrPC-deficient sciatic nerves is reduced, suggesting the involvement of a G protein-coupled receptor (GPCR). The amino-terminal “flexible tail” (FT, residues 23-120) of PrPC triggered a concentration-dependent cAMP increase in primary Schwann cells, in the Schwann-cell line SW10, and in Hek293T cells overexpressing the GPCR Gpr126/Adgrg6. In contrast, naïve HEK293T cells and HEK293T cells expressing several other GPCRs did not react to the FT, and ablation of Gpr126 from SW10 cells abolished the FT-induced cAMP response. The FT contains a polycationic cluster (KKRPKPG) similar to the GPRGKPG motif of the Gpr126 agonist, type-IV collagen2 (Col4). A KKRPKPG-containing PrPC-derived peptide (FT23-50) sufficed to induce a Gpr126-dependent cAMP response in cells and mice, and improved myelination in hypomorphic Gpr126 zebrafish mutants. Substitution of the cationic residues with alanines abolished the biological activity of both FT23-50 and the respective Col4 peptide. We conclude that PrPC promotes myelin homeostasis through FT-mediated Gpr126 agonism. Besides clarifying the physiological role of PrPC, these observations are relevant to the pathogenesis of demyelinating polyneuropathies, common debilitating diseases with limited therapeutic options.