Untangling Direct and Domain-Mediated Interactions Between Nicotinic Acetylcholine Receptors in DHA-Rich Membranes

Abstract

At the neuromuscular junction (NMJ), the nicotinic acetylcholine receptor (nAChR) self-associates to give rise to rapid muscle movement. While lipid domains have maintained nAChR aggregates in-vitro, their specific roles in nAChR clustering are currently unknown. In the present study, we carried out coarse-grained molecular dynamics simulations (CG-MD) of 1-4 nAChR molecules in two membrane environments: One mixture containing domainforming, homoacidic lipids, and a second mixture consisting of heteroacidic lipids. Spontaneous dimerization of nAChRs was up to ten times more likely in domain-forming membranes; however, the effect was not significant in fourprotein systems, suggesting that lipid domains are less critical to nAChR oligomerization when protein concentration is higher. With regard to lipid preferences, nAChRs consistently partitioned into liquid-disordered domains occupied by the omega-3 (ω-3) fatty acid, Docosahexaenoic acid (DHA); enrichment of DHA boundary lipids increased with protein concentration, particularly in homoacidic membranes. This result suggests dimer formation blocks access of saturated chains and cholesterol, but not polyunsaturated chains, to boundary lipid sites.

Spontaneous Lipid Binding to the Nicotinic Acetylcholine Receptor in a Native Membrane

“…As in our previous work 70,78,79 , the two-dimensional density distribution ρ B of the beads within a given lipid species B around the protein was calculated on a polar grid:…”

“…Each pLGIC has ten of each site (five in the outer leaflet and five in the lower leaflet) for a total of twenty sites (Figure 1B). The boundaries for each site were drawn to correspond to the localized binding hot spots observed for heteroacidic membranes 79 , and are non-overlapping. Inter-subunit sites include bins with angular components between the M1 and M3 alpha-helices of two adjacent subunits, and a radial component satisfying 10 < r ≤ 32Å.…”

“…In non-domain forming model membranes composed of heteroacidic lipids, two classes of five-fold symmetric sites emerged: an intersubunit site and the M4 site (Figure 1B). Cholesterol and saturated chains were enriched at the inter-subunit interfaces and n-3 PUFA acyl-chains were enriched around the M4 helices 79 . These results were consistent with binding to minimize packing defects: the rigid lipids could fill in the concave regions at the intersubunit sites while the flexible chains would easily deform around the "star points" of the M4 helices.…”

“…Coarse-grained MD simulations have been used previously to probe interactions of pLGICs with propofol 77 as well as spontaneous lipid binding in model membranes 70,78,79 . In previous work, we found that nAChR embedded in multiple domain-forming model membranes partitioned to the PUFA-rich liquid disordered domain 78 , rather than to the cholesterol-rich liquid-ordered or "raft" domain that was suggested by cholesterol modulation.…”

Special Issue: Membrane and Receptor Dynamics

“…Even for large receptor complexes such as the nicotinic acetylcholine receptor (nAChR), the structure and function has been reported to be regulated by lipid effects, although the molecular mechanisms underlying cluster formation are currently not very clear. Woods et al (2019) have investigated the nAChR interactions in domain-forming acidic lipid membranes via coarse-grained molecular dynamics simulations. The results suggest that protein concentration plays a deciding role in nAChR oligomerization in lipid domains.…”