α-Latrotoxin Receptor CIRL/Latrophilin 1 (CL1) Defines an Unusual Family of Ubiquitous G-protein-linked Receptors

Abstract

␣-Latrotoxin, a potent excitatory neurotoxin, binds to two receptors: a G-protein-coupled receptor called CIRL/latrophilin 1 (CL1) and a cell-surface protein called neurexin I␣. We now show that CL1 belongs to a family of closely related receptors called CL1, CL2, and CL3. CLs exhibit an unusual multidomain structure with similar alternative splicing and large extra-and intracellular sequences. CLs share domains with other G-protein-coupled receptors, lectins, and olfactomedins/ myocilin. In addition, CLs contain a novel, widespread cysteine-rich domain that may direct endoproteolytic processing of CLs during transport to the cell surface. Although the mRNAs for CLs are enriched in brain, CLs are ubiquitously expressed in all tissues. To examine how binding of ␣-latrotoxin to CL1 triggers exocytosis, we used PC12 cells transfected with human growth hormone. Ca 2؉ -dependent secretion of human growth hormone from transfected PC12 cells was triggered by KCl depolarization or ␣-latrotoxin and was inhibited by tetanus toxin and by phenylarsine oxide, a phosphoinositide kinase inhibitor. When CL1 was transfected into PC12 cells, their response to ␣-latrotoxin was sensitized dramatically. A similar sensitization to ␣-latrotoxin was observed with different splice variants of CL1, whereas CL2 and CL3 were inactive in this assay. A truncated form of CL1 that contains only a single transmembrane region and presumably is unable to mediate G-proteinsignaling was as active as wild type CL1 in ␣-latrotoxintriggered exocytosis. Our data show that CL1, CL2, and CL3 perform a general and ubiquitous function as Gprotein-coupled receptors in cellular signaling. In addition, CL1 serves a specialized role as an ␣-latrotoxin receptor that does not require G-protein-signaling for triggering exocytosis. This suggests that as an ␣-latrotoxin receptor, CL1 recruits ␣-latrotoxin to target membranes without participating in exocytosis directly.

Protein-tyrosine Phosphatase-ς Is a Novel Member of the Functional Family of α-Latrotoxin Receptors

“…We may thus conclude that the direct interaction of the toxin with the phosphatase catalytic domains is unlikely and that the PTP -mediated signaling is not essential for the ␣-latrotoxin stimulatory effect in chromaffin cells. Essentially similar data have been reported previously for CIRL and neurexin exogenously expressed in chromaffin and PC12 cells (14,15). Thus, at least in secretory cells, ␣-latrotoxin receptors serve as targeting sites that attract the toxin and facilitate its partial insertion into the cell membrane that ultimately results in the stimulation of exocytosis via channel formation and/or another yet unknown mechanism.…”

“…In two other ␣-latrotoxin-binding proteins, neurexin and CIRL, the toxin-binding sites are located within the extracellular domains (14,15). It has been shown that the extracellular region of PTP can be cleaved off and released into the medium (41).…”

“…Therefore, the ␣-latrotoxin receptors have been long viewed as specific markers for the presynaptic release sites. Two neuronal ␣-latrotoxin-binding proteins were described previously, neurexin (10,11) and CIRL 1 (calcium-independent receptor of ␣-latrotoxin), also called latrophilin, lectomedin, and CL (12,13,14). Although they do not share any sequence homology, either of them can serve as a functionally active receptor of ␣-latrotoxin (12,15).…”

“…Although they do not share any sequence homology, either of them can serve as a functionally active receptor of ␣-latrotoxin (12,15). Their signal-transducing membranes and intracellular domains are not required to support the stimulatory effect of ␣-latrotoxin in secretory cells (14,16). It was therefore proposed that these receptors serve as targeting sites to bring the toxin to the necessary location on the cell surface.…”

Mutant α-Latrotoxin (LTXN4C) Does Not Form Pores and Causes Secretion by Receptor Stimulation

“…Secretion of Catecholamines from Bovine Chromaffin Cells-LTX is known to efficiently stimulate exocytosis of LDCV containing catecholamines and hormones in chromaffin and PC12 cells; this action is strictly Ca 2ϩ -dependent (6,36).…”