Potential role of caveolin-1-positive domains in the regulation of the acetylcholine receptor's activable pool: Implications in the pathogenesis of a novel congenital myasthenic syndrome

Abstract

Cholesterol modulates the plasmalemma's biophysical properties and influences the function and trafficking of membrane proteins. A fundamental phenomenon that remains obscure is how the plasmalemma's lipid composition regulates the activatable pool of membrane receptors. An outstanding model to study this phenomenon is the nicotinic acetylcholine receptor (nAChR), since the nAChR activatable pool has been estimated to be but a small fraction of the receptors present in the plasmalemma. Studies on the effect of cholesterol depletion in the function of the Torpedo californica nAChR, using the lipid-exposed nAChR mutation (αC418W) that produces a congenital myasthenic syndrome (CMS), demonstrated that cholesterol depletion causes a remarkable increase in the αC418W nAChR's macroscopic current whereas not in the wild-type (WT). A variety of approaches were used to define the mechanism responsible for the cholesterol depletion mediated-increase in the αC418W nAChR's macroscopic current. The present study suggests that a substantial fraction of the αC418W nAChRs is located in caveolin-1-positive domains, "trapped" in a non-activatable state, and that membrane cholesterol depletion results in the relocation of these receptors to the activatable pool. Co-fractionation and co-immunoprecipitation of the αC418W nAChR and the membrane raft protein caveolin-1 (cav1) support the notion that interactions at lipid-exposed domains regulate the partition of the receptor into membrane raft microdomains. These results have potential implications as a novel mechanism to fine-tune cholinergic transmission in the nervous system and in the pathogenesis associated to the αC418W nAChR.

Tryptophan scanning mutagenesis reveals distortions in the helical structure of the δM4 transmembrane domain of theTorpedo californicanicotinic acetylcholine receptor

“…The partitioning of mutant nAChRs into caveolae that is favored by the introduction of the CBM may contribute to the reduced expression levels observed for the P463W mutant nAChR, as is the case for the Torpedo αC418W mutant nAChR. 46 The increase in nAChR expression levels caused by a tryptophan substitution at position Ser-457 may be due to a favorable californica nAChR δM4 TMD displays a spring motion between its different conformational states, similar to the recently proposed spring model for the αM3 TMD of the Mus musculus nAChR. 24 Helical net diagrams.…”

“…the first lipid-exposed mutation that has been shown to produce a Slow Channel Congenital Myasthenic Syndrome (SCCMS) in a patient. 46,54,55 Further studies are underway to determine if the P463W mutant nAChR is cholesterol-sensitive. Taken together, the functional data of the δM4 mutant nAChRs has demonstrated that this TMD plays an important role in the mechanism of channel gating of the Torpedo nAChR.…”