Pore Forming Protein Induced Biomembrane Reorganization and Dynamics: A Focused Review

Emerging Designs and Applications for Biomembrane Biosensors

Nature has inspired the development of biomimetic membrane sensors in which the functionalities of biological molecules, such as proteins and lipids, are harnessed for sensing applications. This review provides an overview of the recent developments for biomembrane sensors compatible with either bulk or planar sensing applications, namely using lipid vesicles or supported lipid bilayers, respectively. We first describe the individual components required for these sensing platforms and the design principles that are considered when constructing them, and we segue into recent applications being implemented across multiple fields. Our goal for this review is to illustrate the versatility of nature's biomembrane toolbox and simultaneously highlight how biosensor platforms can be enhanced by harnessing it.

Peptide-Induced Fusion of Dynamic Membrane Nanodomains: Implications in a Viral Entry

Enveloped viruses infect host cells via protein-mediated membrane fusion. However, insights into the microscopic rearrangement induced by the viral proteins and peptides have not yet emerged. Here, we report a new methodology to extract viral fusion peptide (FP)-mediated biomembrane dynamical nanodomain fusion parameter, λ, based on stimulated emission depletion microscopy coupled with fluorescence correlation spectroscopy. We also define another dynamical parameter membrane gradient, defined in terms of the ratio of average lipid diffusion coefficients across dynamic crossover length scales, ξ. Significantly, we observe that λ as well as these mobility gradients are larger in the stiffer liquid-ordered (Lo) phase compared to the liquid-disordered phase and are more effective at the smaller nanodomain interfaces, which are only present in the Lo phase. The results could possibly help to resolve a long-standing puzzle about the enhanced fusogenicity of FP in the Lo phase. Results obtained from the diffusion results have been correlated with the human immunodeficiency virus gp41 FP-induced membrane fusion.

Membrane binding and pore formation is Ca 2+ -dependent for the Clostridioides difficile binary toxin

The C. difficile binary toxin (CDT) enters host cells via endosomal delivery like many other 'AB'-type binary toxins. In this study, the cell-binding component of CDT, termed CDTb, was found to bind and form pores in lipid bilayers upon depleting free Ca 2+ ion concentrations, and not by lowering pH, as found for other binary toxins (i.e., anthrax). Cryoelectron microscopy, nuclear magnetic resonance spectroscopy, surface plasmon resonance, electrochemical impedance spectroscopy, CDT toxicity studies, and site directed mutagenesis show that dissociation of Ca 2+ from a single site in receptor binding domain 1 (RBD1) of CDTb is consistent with a molecular mechanism in which Ca 2+ dissociation from RBD1 induces a "trigger" via conformational exchange that enables CDTb to bind and form pores in endosomal membrane bilayers as free Ca 2+ concentrations decrease during CDT endosomal delivery.

Pore-Forming Proteins: From Pore Assembly to Structure by Quantitative Single-Molecule Imaging

Pore-forming proteins (PFPs) play a central role in many biological processes related to infection, immunity, cancer, and neurodegeneration. A common feature of PFPs is their ability to form pores that disrupt the membrane permeability barrier and ion homeostasis and generally induce cell death. Some PFPs are part of the genetically encoded machinery of eukaryotic cells that are activated against infection by pathogens or in physiological programs to carry out regulated cell death. PFPs organize into supramolecular transmembrane complexes that perforate membranes through a multistep process involving membrane insertion, protein oligomerization, and finally pore formation. However, the exact mechanism of pore formation varies from PFP to PFP, resulting in different pore structures with different functionalities. Here, we review recent insights into the molecular mechanisms by which PFPs permeabilize membranes and recent methodological advances in their characterization in artificial and cellular membranes. In particular, we focus on single-molecule imaging techniques as powerful tools to unravel the molecular mechanistic details of pore assembly that are often obscured by ensemble measurements, and to determine pore structure and functionality. Uncovering the mechanistic elements of pore formation is critical for understanding the physiological role of PFPs and developing therapeutic approaches.

Visualizing the Domino-Like Prepore-to-Pore Transition of Streptolysin O by High-Speed AFM

Pore-forming proteins (PFPs) are produced by various organisms, including pathogenic bacteria, and form pores within the target cell membrane. Streptolysin O (SLO) is a PFP produced by Streptococcus pyogenes and forms high-order oligomers on the membrane surface. In this prepore state, multiple α-helices in domain 3 of each subunit exist as unfolded structures and transiently interact with each other. They subsequently transition into transmembrane β-hairpins (TMHs) and form pores with diameters of 20-30 nm. However, in this pore formation process, the trigger of the transition in a subunit and collaboration between subunits remains elusive. Here, I observed the dynamic pore formation process using high-speed atomic force microscopy. During the oligomer transition process, each subunit was sequentially inserted into the membrane, propagating along the oligomer in a domino-like fashion (chain reaction). This process also occurred on hybrid oligomers containing wildtype and mutant subunits, which cannot insert into the membrane because of an introduced disulfide bond. Furthermore, propagation still occurred when an excessive force was added to hybrid oligomers in the prepore state. Based on the observed chain reactions, I estimate the free energies and forces that trigger the transition in a subunit. Furthermore, I hypothesize that the collaboration between subunits is related to the structure of their TMH regions and interactions between TMH-TMH and TMH-lipid molecules.

Endosomal Escape of Bioactives Deployed via Nanocarriers: Insights Into the Design of Polymeric Micelles

Cytoplasmic delivery of bioactives requires the use of strategies such as active transport, electroporation, or the use of nanocarriers such as polymeric nanoparticles, liposomes, micelles, and dendrimers. It is essential to deliver bioactive molecules in the cytoplasm to achieve targeted effects by enabling organelle targeting. One of the biggest bottlenecks in the successful cytoplasmic delivery of bioactives through nanocarriers is their sequestration in the endosomes that leads to the degradation of drugs by progressing to lysosomes. In this review, we discussed mechanisms by which nanocarriers are endocytosed, the mechanisms of endosomal escape, and more importantly, the strategies that can be and have been employed for their escape from the endosomes are summarized. Like other nanocarriers, polymeric micelles can be designed for endosomal escape, however, a careful control is needed in their design to balance between the possible toxicity and endosomal escape efficiency. Keeping this in view, polyion complex micelles, and polymers that have the ability to escape the endosome, are fully discussed. Finally, we provided some perspectives for designing the polymeric micelles for efficient cytoplasmic delivery of bioactive agents through endosomal escape.

Membrane Dynamics and Remodelling in Response to the Action of the Membrane-Damaging Pore-Forming Toxins

Pore-forming protein toxins (PFTs) represent a diverse class of membrane-damaging proteins that are produced by a wide variety of organisms. PFT-mediated membrane perforation is largely governed by the chemical composition and the physical properties of the plasma membranes. The interaction between the PFTs with the target membranes is critical for the initiation of the pore-formation process, and can lead to discrete membrane reorganization events that further aids in the process of pore-formation. Punching holes on the plasma membranes by the PFTs interferes with the cellular homeostasis by disrupting the ion-balance inside the cells that in turn can turn on multiple signalling cascades required to restore membrane integrity and cellular homeostasis. In this review, we discuss the physicochemical attributes of the plasma membranes associated with the pore-formation processes by the PFTs, and the subsequent membrane remodelling events that may start off the membrane-repair mechanisms.

Coexisting Ordered and Disordered Membrane Phases Have Distinct Modes of Interaction with Disease-Associated Oligomers

Ordered membrane domains are thought to influence the attachment and insertion of toxic amyloid oligomers, and consequently, their toxicity. However, if and how the molecular aspects of this interaction depend on the membrane order is poorly understood. Here we measure the affinity, location, and degree of insertion of the small oligomers of hIAPP (human Islet Amyloid Polypeptide, associated with Type II diabetes) at near-physiological concentrations to adjacent domains of a biphasic lipid bilayer. Using simultaneous atomic force, confocal and fluorescence lifetime microscopy (AFM-FLIM), we find that hIAPP oligomers have a nearly 8-fold higher affinity to the disordered domains over the ordered domains. To probe whether this difference indicates different modes of interaction, we measure the change of lifetime of peptide-attached fluorescent labels induced by soluble fluorescence quenchers and also measure the kinetics of localized photobleaching. We find that in the raft-like ordered domains, the oligomers primarily lie on the aqueous interface with limited membrane penetration. However, in the neighboring disordered domains, their C-termini penetrate deeper into the lipid bilayer. We conclude that local membrane order determines not only the affinity but also the mode of interaction of amyloid oligomers, which may have significant implications for disease mechanisms.