G-Protein-Coupled Receptor

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Background a-Latrotoxin is a specific presynaptic neurotoxin contained in black widow spider venom. It stimulates spontaneous release of various neurotransmitters in vertebrates. It also causes degeneration of the poisoned nerve terminals over the long term. Interestingly, a-latrotoxin's stimulatory action in neurons does not require extracellular calcium. It was therefore hypothesized that its effects involve a novel mechanism of activation of neurosecretion which does not depend on a classical calcium-signaling pathway.  It has been convincingly shown that the stimulatory effect of a-latrotoxin requires the presence of its high-affinity receptors on the neuronal membrane. Previously, neurexin Ia was identified as a neuronal a-latrotoxin receptor. However, neurexin Ia binds a-latrotoxin only in the presence of calcium and therefore cannot be critically important in the stimulation of neurosecretion by a-latrotoxin which does not require calcium. The identification of the molecule via which a-latrotoxin mediates neurotransmitter release would shed light on the critical problem in neurobiology, the nature of biochemical machinery responsible for regulation of synaptic vesicle release at presynaptic nerve terminals, and allow the development of drugs that could modulate neurotransmitter release in various diseases. Description Dr. Petrenko and colleagues have discovered a novel neuronal G-protein-coupled receptor which binds a-latrotoxin with high affinity. It is an orphan receptor in that the actual endogenous ligand for it is not known. This molecule, termed CIRL for calcium independent receptor of a-latrotoxin, binds to the toxin independently of calcium. CIRL is a novel member of the secretin receptor family of G-protein-coupled receptors. In contrast with other known serpentine receptors, CIRL consists of two large subunits that are the result of endogenous proteolytic cleavage of a precursor polypeptide. The extracellular p120 subunit has a pronounced multi-domain structure with regions homologous to sea urchin lectin, olfactomedin, mucin, neuropeptide receptors and a family of large orphan G-protein-coupled receptors. The second subunit of CIRL, p85, is an integral membrane protein with seven membrane-spanning segments and a large C-terminal cytoplasmic domain with proline-rich clusters. The importance of CIRL in regulation of neurotransmitter release is supported by the following findings: CIRL is a target of extremely potent natural secretagogue, a-latrotoxin. CIRL is exclusively expressed in the nervous system, with highest concentrations in the regions enriched in synaptic contacts. CIRL interacts with syntaxin and synaptotagmin, components of the neuronal exocytotic machinery. Over-expression of CIRL in chromaffin cells results in the inhibition of ATP-dependent stage of calcium-stimulated secretion. Applications CIRL represents a novel orphan G-protein-coupled receptor with physiological role as regulator of neurosecretion. CIRL is a potential drug target and can be used to search for natural and synthetic agonists and antagonists which may find application as regulators of neurotransmitter release. Applications of this technology could also be developed for use of anti-CIRL antibodies and DNA probes in diagnostic tests for various neurological diseases, including schizophrenia, Alzheimer’s, and Huntington’s diseases, as well as for brain cancer, where CIRL may be mutated or its expression changed.