No abstract

Soluble oligomers of amyloidogenic proteins like an amyloid-β (Aβ) peptide are believed to exhibit toxic effects in neurodegenerative diseases. The structural classification of oligomers indicates two fundamentally distinct oligomers, namely, fibrillar and prefibrillar oligomers that are recognized by OC and A11 conformation-specific antibodies, respectively. Previous studies have indicated that the interaction of Aβ oligomers with the lipid membrane is one of the mechanisms by which these oligomers exert their toxic effects in Alzheimer's disease. Here, we report that the orientational ordering of liquid crystals (LC) can be used to study the membrane-induced aggregation of Aβ oligomers at nanomolar concentrations. Our results demonstrate a faster fibrillation kinetics of OCpositive fibrillar Aβ oligomers with the lipid monolayer in comparison to that of the A11positive prefibrillar Aβ oligomers. Our findings suggest a general strategy for distinguishing conformationally distinct soluble oligomers that are formed by a number of amyloidogenic proteins on lipid-decorated aqueous−LC interfaces.

Aqueous interfaces of liquid crystals (LCs) are widely explored in the design of functional interfaces to recapitulate the key aspects of biomolecular interactions in cellular milieu. Herein, using aqueous LC dispersions, we explore the interactions between mitochondrial cardiolipin and membrane-associated cytochrome c which play a pivotal role in the apoptotic signaling cascade. Conventional techniques used to decipher LC ordering at the droplet interface fail to give information about the interactions at a molecular level. Besides, owing to the complexity of LC systems and multiple determinants driving the LC reorientation, accurate analysis of the underlying mechanism responsible for the LC ordering transition remains challenging. Using a combination of atomistic simulations and microscopic and spectroscopic readouts, for the first time, we unveil the lipid−protein interactions that drive the reorientation at the LC droplet interface. The insights from our work are fundamental to the design of these interfaces for a spectrum of interfacial applications.

Amyloids are proteinaceous aggregates, the deposition of which is associated with neurodegenerative diseases, such as Alzheimer's disease. In vitro protein aggregation requires high protein concentration, which is generally far from physiological concentration. Here, we utilize the interfacial properties of liquid crystals (LCs) to monitor the membrane-induced aggregation of a bacterial functional amyloid, curli, at nanomolar concentration. The binding event triggers an orientational transition of the LC, which is accompanied by the appearance of dynamic spatial patterns enabling sensitive detection of lipopolysaccharide (LPS)-mediated protein aggregation. Quantification of LC response shows a sigmoidal time profile, typical of a protein fibrillation assay. Curli is composed of two subunits (CsgA and CsgB) and is expressed on the outer membrane of Gramnegative bacteria containing LPSs endotoxin. CsgA forms the major subunit of curli, which is nucleated by the membranetethered minor subunit CsgB. Using an array of complementary tools, such as polarizing optical microscopy, fluorescence, and atomic force microscopy imaging, we found that the patterned orientation of the LC in response to the binding of curli subunits with LPS corresponds to amyloid fibril formation. Furthermore, using the curli amyloid system, we have successfully demonstrated that membrane-decorated interfaces of LC can be used to study heterotypic cross-seeding in amyloidogenesis. We believe that the LC-based system can be used as a probe to monitor mechanistic details of lipid-induced protein aggregation in the low-concentration regime.

The structural integrity of biological membranes is maintained by membrane proteins embedded in the lipid bilayer. A delicate balance of weak interactions between the lipid bilayer and membrane associated proteins regulates cellular homeostasis and disease states. Recently, there has been a growing interest in the construction of in vitro mimics of biological membranes. This allows the study of multiple facets of complex interactions involving lipids and proteins in a simple environment. In recent years, liquid crystal (LC) interfaces decorated with self-assembled layers of phospholipids have evolved as biomimetic systems for systematic study of lipidprotein interactions. Binding of proteins to these phospholipid-laden fluid interfaces can be coupled to the orientational ordering of LCs. In this minireview, we have surveyed the key investigations of these interactions using LC interfaces as the sensing platform. Micrometer thick films of liquid crystals can report interactions ranging from hydrolysis of lipids by enzymatic peptides to membrane induced amyloid formation.