Influence of lipid bilayer on the structure of the muscle-type nicotinic acetylcholine receptor

Cholesterol´s role in membrane organization and nicotinic acetylcholine receptor function: Implications for aging and Alzheimer's disease

Biological membranes are complex entities composed of various molecules exhibiting lateral and transbilayer lipid asymmetries, along with a selective spatial distribution of different membrane proteins. This dynamic orchestration is crucial for proper physiological functions, undergoes changes with aging, and is disturbed in several neurological disorders. In this review, we analyze the impact of disruption in this equilibrium on physiological aging and the onset of pathological conditions. Alzheimer´s disease (AD) is a multifactorial neurodegenerative disorder in the elderly, characterized by the increased presence of the Aβ peptide, which supports the amyloid hypothesis of the disease. However, AD also involves a progressive loss of cholinergic innervation, leading to the cholinergic hypothesis of the disease. Nicotinic acetylcholine receptors (nAChRs) are transmembrane proteins, and Aβ peptides, their oligomeric and fibrillar species, which increase in hydrophobicity as they develop, interact with membranes. Therefore, a membrane hypothesis of the disease emerges as a bridge between the other two. Here, we discuss the impact of the membrane environment, through direct or indirect mechanisms, on cholinergic signaling and Aβ formation and subsequent incorporation into the membrane, with a special focus on the crucial role of cholesterol in these processes.

The ABC transporter MsbA in a dozen environments

High-resolution structure determination of membrane proteins typically requires reconstitution into artificial membrane mimics. The choice of the specific membrane substitute can strongly affect the protein's specific activity, stability, and conformational spectrum, potentially leading to errors or misinterpretation during analysis. The bacterial ATP-binding cassette transporter MsbA is a prominent example of such environment-specific bias. Here, we present a systematic analysis of the conformational spectrum of MsbA, stabilized in a dozen environments, using cryoelectron microscopy (cryo-EM), and show pronounced feedback between the membrane mimetics and the transporter. Detergents generally favor wide inward-facing conformations while nanodiscs induce narrower conformations. Notably, only in three tested environments, MsbA samples the full movement of the nucleotide-binding domains, including narrow and wide conformations. We expect this study to serve as a blueprint for other membrane proteins, even where a structural reaction to the hydrophobic environment is not directly visible but still critical for the proteins' function.

Structures of the human adult muscle-type nicotinic receptor in resting and desensitized states

Muscle-type nicotinic acetylcholine receptor (AChR) is the key signaling molecule in neuromuscular junctions. Here, we present the structures of full-length human adult receptors in complex with Fab35 in α-bungarotoxin (αBuTx)-bound resting states and ACh-bound desensitized states. In addition to identifying the conformational changes during recovery from desensitization, we also used electrophysiology to probe the effects of eight previously unstudied AChR genetic variants found in patients with congenital myasthenic syndrome (CMS), revealing they cause either slow- or fast-channel CMS characterized by prolonged or abbreviated ion channel bursts. The combined kinetic and structural data offer a better understanding of both the AChR state transition and the pathogenic mechanisms of disease variants.

Single-particle cryogenic electron microscopy structure determination for membrane proteins

Membrane proteins are crucial to many cellular functions but are notoriously difficult for structural studies due to their instability outside their natural environment and their amphipathic nature with dual hydrophobic and hydrophilic regions. Single-particle cryogenic electron microscopy (cryo-EM) has emerged as a transformative approach, providing near-atomic-resolution structures without the need for crystallization. This review discusses advancements in cryo-EM, emphasizing membrane sample preparation and data processing techniques. It explores innovations in capturing membrane protein structures within native environments, analyzing their dynamics, binding partner interactions, lipid associations, and responses to electrochemical gradients. These developments continue to enhance our understanding of these vital biomolecules, advancing the contributions of structural biology for basic and translational biomedicine.