Aggregation Behavior of Amyloid Beta Peptide Depends Upon the Membrane Lipid Composition.

Protein aggregation plays a crucial role in the development of several neurodegenerative diseases. It is important to understand the aggregation process for the detection of the onset of these diseases. Alzheimer's Disease (AD) is one of the most prevalent neurodegenerative diseases caused by the aggregation of Aß-40 and Aß-42 peptides. The smaller oligomers lead to the formation of protein plaque at the neural membranes leading to memory loss and other disorders. Interestingly, aggregation takes place at the neural membranes, therefore the membrane composition seems to play an important role in the aggregation process. Despite a large number of literatures on the effect of lipid composition on protein aggregation, there are very few concise reviews that highlight the role of membrane composition in protein aggregation. In this review, we have discussed the implication of membrane composition on the aggregation of amyloid beta peptide with a special emphasis on cholesterol. We have further discussed the role of the degree of unsaturation of fatty acids and the participation of apolipoprotein E4 (ApoE4) in the onset of AD.

Phosphatidylglycerol Acts as a Switch for Cholesterol-Dependent Membrane Binding of ApoE Signal Peptide.

The apolipoprotein E (ApoE) signal peptide is a short stretch of N-terminal amino acids that direct the ApoE protein to the endoplasmic reticulum after synthesis. Previous studies have shown that this peptide can bind to lipid membranes in a cholesterol-dependent manner; however, the mechanism of this interaction is yet to be clarified. In this study, we aimed to investigate how the composition of neighboring lipids affects the membrane-binding of the ApoE signal peptide. We found that a negatively charged lipid, such as phosphatidylglycerol, can act as a switch that reduces the binding efficiency of the peptide to cholesterol-rich membranes. Interestingly, phosphatidylethanolamine does not activate the cholesterol-dependent binding of the ApoE signal peptide yet acts synergistically to enhance the cholesterol sensitivity in phosphatidylglycerol-containing membranes. To the best of our knowledge, this is the first report of modulation of the affinity of a peptide for a membrane by a neighboring lipid rather than by the lipid-binding domain of the peptide. Our findings revealed a novel role of lipid diversity in modulating the membrane binding of the ApoE signal peptide and its potential implications in the unidirectional trafficking of a newly synthesized protein from the ribosomes to the endoplasmic reticulum.

Lipid composition dependent binding of apolipoprotein E signal peptide: Importance of membrane cholesterol in protein trafficking.

Soluble secretory and membrane proteins contain a short stretch of signal peptide (SP) at their N-terminal end, which gets cleaved after reaching the destination organelle. However, the importance of SP in protein trafficking is not fully understood. The lipid compositions of cellular organelles are highly heterogeneous, and the preference of SP toward a particular lipid composition might play a key role in unidirectional trafficking of protein. In order to understand the preference of Apolipoprotein E (ApoE) toward endoplasmic reticulum (ER), we have studied the interaction of its SP with membranes of varying lipid compositions. The importance of cholesterol is paramount as subcellular organelles contain differential amount of cholesterol; endoplasmic reticulum (ER) contains the least amount of cholesterol. We have utilized batteries of steady-state and time-resolved fluorescence techniques to understand the affinity of ApoE signal peptide toward membranes of varying lipid compositions. We observed that the ApoE signal peptide binds tightly with membranes devoid of cholesterol, and binding affinity reduces with increasing concentration of membrane cholesterol. Our results clearly suggest the importance of membrane composition in the unidirectional movement of ApoE toward ER. This property of SP can further be utilized for the development of organelle specific cargo delivery.

Fluorescence-based techniques for the detection of the oligomeric status of proteins: implication in amyloidogenic diseases.

Intrinsically disordered proteins (IDPs) have captured attention in the last couple of decades due to their functional roles despite a lack of specific structure. Moreover, these proteins are found to be highly aggregation prone depending on the mutational and environmental changes to which they are subjected. The aggregation of such proteins either in the intracellular context or extracellular matrix is associated with several adverse pathophysiological conditions such as Alzheimer's, Parkinson's, and Huntington's diseases, Spinocerebellar ataxia, and Type-II diabetes. Interestingly, it has been noted that the smaller oligomers formed by IDPs are more toxic to cells than their larger aggregates. This necessitates the development of techniques that can detect the smaller oligomers formed by IDPs for diagnosis of such diseases during their early onset. Fluorescence-based spectroscopic and microscopic techniques are highly effective as compared to other techniques for the evaluation of protein oligomerization, organization, and dynamics. In this review, we discuss several fluorescence-based techniques including fluorescence/Förster resonance energy transfer (FRET), homo-FRET, fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), fluorescence lifetime imaging (FLIM), and photobleaching image correlation spectroscopy (pbICS) that are routinely used to identify protein oligomers in extracellular and intracellular matrices.

Characterization of structural conformers of κ-casein utilizing fluorescence spectroscopy.

The intrinsically disordered proteins (IDPs) belong to an important class of proteins due to their higher structural flexibility and diverse functions. IDPs lack stable three-dimensional structure and exist as structural ensemble in solution. Furthermore, IDPs have been found to be associated with various neurodegenerative diseases like Alzheimer's, Parkinson's, diabetes and spinocerebellar ataxia. Several spectroscopic techniques are being employed to predict the structure of IDPs as the X-ray crystallography is immensely challenging due to their structural dynamism. κ-Casein, an important milk protein containing net negative charge, belongs to the class of IDPs. κ-Casein has been found to assume different structural conformations at various pH and surfactant concentrations. In this work, we have utilized quenching efficiency of ionic liquid (1-butyl-3-methylimidazolium bromide) and energy transfer efficiency between tryptophan and 1,6-Diphenyl-1,3,5-hexatriene (DPH) to evaluate the conformation of κ-casein at various pH and cetyltrimethylammonium bromide (CTAB) concentrations. Our results validated the applicability of above-mentioned methods to determine the polarity of tryptophan environment in κ-casein at different conditions, where κ-casein assumes diverse structural conformations. The present work opens up the possibility to employ quenching properties of ionic liquid in conjunction with energy transfer efficiency between tryptophan and DPH for the elucidation of protein conformation.