Interactions between the Nicotinic and Endocannabinoid Receptors at the Plasma Membrane

The pleomorphic cholesterol sensing motifs of transmembrane proteins

Millions of years of phylogenetic evolution have shaped the crosstalk between sterols and membrane-embedded proteins. This lengthy process, which began before the appearance of eukaryotic cells, has sculpted the two types of molecules to cover a wide spectrum of structural interconnectedness, ranging from rapid touch-and-go hits of low-affinity between surfaces to stronger lock-and-key type structural contacts. The former usually involve relatively loose contacts between linear amino acid sequences on the membrane-exposed transmembrane domains of the protein, readily accessible to the sterols as they briefly visit clefts between adjacent transmembrane segments while in rapid exchange with the bulk lipid bilayer. This operational mode is probably the most ancestral one, since it was already present in primitive bacteria interacting with hopanoid lipids. At the other end of this spectrum are more complex cholesterol binding sites that have required the acquisition of complex 3D non-sequential segments of the membrane protein to establish stereochemically elaborate 3D designs complementary to the rough and smooth surfaces of the eukaryotic neutral lipid, cholesterol. This short review explores cholesterol-membrane protein interactions using membrane protein paradigms having in common their participation in intercellular communications neurotransmission, hormone signalling, amino acid/neurotransmitter transport- and in cancer.

Adult consequences of repeated nicotine and Δ9-tetrahydrocannabinol (THC) vapor inhalation in adolescent rats

RATIONALE

Use of electronic drug delivery systems (EDDS, "e-cigarettes") to ingest nicotine and Δ9-tetrahydrocannabinol (THC) has surged in adolescents in the USA; five times as many high-school seniors vape nicotine daily using tobacco. At the same time, 19.5% of seniors use cannabis at least monthly, with 12% using EDDS to deliver it.

OBJECTIVES

This study was conducted to examine the impact of repeated adolescent vapor inhalation of nicotine and THC in rats.

METHODS

Female Sprague-Dawley rats were exposed to 30-min sessions of vapor inhalation, twice daily, from post-natal day (PND) 31 to PND 40. Conditions included vapor from the propylene glycol (PG) vehicle, nicotine (60 mg/mL in the PG), THC (100 mg/mL in the PG), or the combination of nicotine (60 mg/mL) and THC (100 mg/mL). Rats were assessed on wheel activity, heroin anti-nociception and nicotine and heroin vapor volitional exposure during adulthood.

RESULTS

Nicotine-exposed rats exhibited few differences as adults, but were less sensitive to anti-nociceptive effects of heroin (1 mg/kg, s.c.). THC- and THC + nicotine-exposed rats were less spontaneously active, and obtained fewer nicotine vapor deliveries as adults. In contrast, THC-exposed rats obtained volitional heroin vapor at rates indistinguishable from the non-THC-exposed groups. Repeated THC exposure also caused tolerance to temperature-disrupting effects of THC (5 mg/kg, i.p.).

CONCLUSIONS

These studies further confirm that the effects of repeated vapor exposure to THC in adolescence last into early to middle adulthood, including decreased volitional consumption of nicotine. Effects of repeated nicotine in adolescence were comparatively minor.

Structure and function meet at the nicotinic acetylcholine receptor-lipid interface

The nicotinic acetylcholine receptor (nAChR) is a transmembrane protein that mediates fast intercellular communication in response to the endogenous neurotransmitter acetylcholine. It is the best characterized and archetypal molecule in the superfamily of pentameric ligand-gated ion channels (pLGICs). As a typical transmembrane macromolecule, it interacts extensively with its vicinal lipid microenvironment. Experimental evidence provides a wealth of information on receptor-lipid crosstalk: the nAChR exerts influence on its immediate membrane environment and conversely, the lipid moiety modulates ligand binding, affinity state transitions and gating of ion translocation functions of the receptor protein. Recent cryogenic electron microscopy (cryo-EM) studies have unveiled the occurrence of sites for phospholipids and cholesterol on the lipid-exposed regions of neuronal and electroplax nAChRs, confirming early spectroscopic and affinity labeling studies demonstrating the close contact of lipid molecules with the receptor transmembrane segments. This new data provides structural support to the postulated "lipid sensor" ability displayed by the outer ring of M4 transmembrane domains and their modulatory role on nAChR function, as we postulated a decade ago. Borrowing from the best characterized nAChR, the electroplax (muscle-type) receptor, and exploiting new structural information on the neuronal nAChR, it is now possible to achieve an improved depiction of these sites. In combination with site-directed mutagenesis, single-channel electrophysiology, and molecular dynamics studies, the new structural information delivers a more comprehensive portrayal of these lipid-sensitive loci, providing mechanistic explanations for their ability to modulate nAChR properties and raising the possibility of targetting them in disease.