Membrane Interactions and Toxicity by Misfolded Protein Oligomers

Abstract

The conversion of otherwise soluble proteins into insoluble amyloid aggregates is associated with a range of neurodegenerative disorders, including Alzheimer's and Parkinson's diseases, as well as non-neuropathic conditions such as type II diabetes and systemic amyloidoses. It is increasingly evident that the most pernicious species among those forming during protein aggregation are small prefibrillar oligomers. In this review, we describe the recent progress in the characterization of the cellular and molecular interactions by toxic misfolded protein oligomers. A fundamental interaction by these aggregates involves biological membranes, resulting in two major model mechanisms at the onset of the cellular toxicity. These include the membrane disruption model, resulting in calcium imbalance, mitochondrial dysfunction and intracellular reactive oxygen species, and the direct interaction with membrane proteins, leading to the alteration of their native function. A key challenge remains in the characterization of transient interactions involving heterogeneous protein aggregates. Solving this task is crucial in the quest of identifying suitable therapeutic approaches to suppress the cellular toxicity in protein misfolding diseases.

Keywords: amyloid fibrils; cellular toxicity; membrane interaction; protein misfolding; receptor binding.

FIGURE 1

Aberrant interactions between misfolded protein aggregates and biological membranes. Toxic protein oligomers can bind strongly biological membranes, including plasma and mitochondrial membranes, as well as membrane proteins. Perturbation and pore formation (A) of the lipid bilayer by misfolded protein aggregates may generate membrane disruption and ultimately its permeabilization (B), leading to aberrant processes such as mitochondrial dysfunction, intracellular ROS and release of cytochrome c. In addition, interactions of misfolded protein aggregates with membrane receptors (C,D) and mitochondrial receptors (E) result in loss of function (e.g., VDACs, α3-NKA) or overactivation (e.g., mGluRs, NMDARs), with consequent alteration of the cellular viability, including calcium gradient imbalance and cellular and organelle dysfunction. Indeed, most of the interactions of misfolded protein aggregates with cellular membranes and membrane proteins have a common downstream effect on the Ca²⁺-dependent toxicity. In this scheme, interactions and processes by protein oligomers at the surface of biological membranes are shown. (A) Pore formation. (B) Membrane permeabilization. (C) Interaction with Ionotropic/metabotropic glutamate receptors. (D) Interaction with plasma membrane receptors (e.g., α7 nAChR, α3-NKA, etc.). (E) Mitochondrial receptors interactions (VDACs, mPTP, etc.). (F) Intracellular ROS production. Protein aggregates are schematically shown in purple, Ca^{2 +} ions in orange and neurotransmitter molecules in blue.

FIGURE 2

Disruption of biological membranes by misfolded protein oligomers. A variety of mechanisms of membrane disruption by misfolded protein oligomers were proposed including (A) partial insertion of the oligomers into the hydrophobic region of the membrane, (B) pore formation, (C) membrane thinning and redistribution of lipids through a detergent-like mechanism (Sciacca et al., 2018). The consequent loss of membrane integrity results in aberrant Ca²⁺ influx, the generation of ROS and the alteration of signaling pathways, ultimately inducing cell death in a Ca²⁺-dependent manner. (D) In the mitochondria, preferential binding to cardiolipin (depicted as yellow lipids) by misfolded protein oligomers promotes aberrant protein-membrane interactions. Defects in the mitochondrial membranes lead to ionic gradient imbalances, which in turn interrupt the electron transport chain and compromise the process of mitochondrial respiration. A defective mitochondrial electron transport chain, in association with elevated Ca²⁺ influx result in an increase in intracellular ROS. The loss of mitochondrial membrane potential signals the release of cytochrome c.

FIGURE 3

Aberrant interactions of misfolded protein oligomers with membrane receptors. Misfolded protein oligomers are able to bind a variety of proteins on the plasma and mitochondrial membranes, resulting in the alteration of their functions. These interactions may result in overactivation and hence gain-of-function (e.g., mGluR5s or NMDARs) whereas internalization (e.g., AMPARs, α7 AChR) and clustering of transporters (e.g., α3-NKA) may lead to loss-of-function. The binding to the receptors has been shown to alter the ionic gradients and generate an excess of intracellular and mitochondrial Ca²⁺. The resulting opening of the mPTP causes the depolarization of the mitochondria, which triggers the release of cytochrome c and consequent signaling for apoptosis. Disruption of the electron transport chain by direct binding of oligomers to the respiratory complexes exacerbates the calcium-mediated toxicity through the unregulated generation of ROS. In this schematic figure, interactions with (A) ionotropic/metabotropic glutamate receptors, (B) nicotinic receptors (e.g., NKA transporter) and (C) mitochondrial receptors (e.g., TOM20), are shown in association with (D) the resulting ROS generation.