Current Protein and Peptide Science - Volume 11, Issue 5, 2010

The growing interest in membrane interactions of amyloidogenic peptides and proteins emanates from the realization that lipids and membranes play important, potentially central, roles in the toxicity and pathological pathways of amyloid diseases. Expanding body of evidence indicates that lipid binding of amyloidogenic peptides, and amyloid peptide association with cellular membranes are critical to the onset and progression of amyloid diseases. Conversely, membranes and membrane components appear to intimately affect the aggregation properties, and consequently the biological functions, of varied amyloidogenic proteins. This special issue of CPPS aims to provide an overview of important aspects on the cutting edge of this field. This Issue encompasses contributions from leading researchers, focusing on central phenomena involving aspects pertaining to lipid- and membrane-interactions of amyloidogenic peptides and proteins. Eckert et al. summarize the large body of experimental evidence pertaining to amyloid-β interactions with lipid bilayers and specific components within the bilayers. Henriques and Castanho focus on the prion system, and discuss the molecular interactions and biological significance of membrane interactions of an amyloidogenic fragment of the prion protein (PrP). Van Rooijen et al. concentrate upon membrane interactions of α-synuclein, in particular the roles of oligomeric species of the protein in induction of toxicity. Stefani expanded on the issue of amyloid oligomers, and provides a comprehensive overview on amyloid polymorphism and the relationship between this often-encountered phenomenon and membrane interactions and cell toxicity. Otzen presents a biophysical analysis of amyloid peptide structural studies carried out in model membrane systems, while Jelinek and Sheynis highlight the occasionally conflicting conclusions drawn from studies using different membrane models and experimental techniques.

Gradual changes in steady-state levels of beta amyloid peptides (Aβ) in the brain are considered as initial step in the amyloid cascade hypothesis of Alzheimer's disease (AD). Aβ is a product of the secretase cleavage of the amyloid precursor protein and there is evidence that the membrane lipid environment may modulate secretase activity and alters its function. Aβ disturbs membrane properties of artificial and isolated biological membranes and of plasma membranes in living cells. Aβ induced changes in membrane fluidity could be explained by physico-chemical interactions of the peptide with membrane components such as cholesterol, phospholipids and gangliosides. Thus, cell membranes may be the location where the neurotoxic cascade of Aβ is initiated. Perturbation of membranes, binding to lipids and alteration of cellular calcium signaling by Aβ have been reported by several studies and these topics are examined in this review.

Prion diseases are a class of fatal neurodegenerative disorders that affect mammals and are characterized by their unique transmissibility and the nature of the infectious agent. When the physiological prion protein (PrPC) becomes corrupted (PrPSc) it accumulates in the brain, promoting infection and self-propagation via recruitment of PrPC. Although with identical sequence, PrPC and PrPSc differ in their physicochemical properties: PrPC is soluble, has an α-helical structure and is sensitive to enzymatic degradation, whereas PrPSc is insoluble, forms β-aggregates and is resistant to proteolysis. The fragment PrP(16-126) possess similar physicochemical and pathological properties to PrPsc, and therefore is commonly used as a model to study pathogenic effects. Although the pathogenicity of prion diseases is still unclear, strong evidences suggest that the cell membrane is relevant not only in infection and propagation of the disease but also in the manifestation of the clinical symptoms. In particular, the fragment PrP(106-126) has been implicated in the perturbation of the membranes and in the manifestation of Prion diseases. However, this is controversial. This review will discuss the effect of PrP(106-126) on the cell membrane based on its effect on model phospholipid bilayers. Different conditions were studied, including membrane charge, viscosity, lipid composition, pH, and ionic strength, revealing that PrP(106- 126) only interacts with lipid membranes at conditions with no physiological relevance. Such findings are here reviewed and correlated with the full-length protein effect.

α-Synuclein is a small neuronal protein that has been implicated to play an important role in Parkinson's disease. Genetic mutations and multiplications in the α-synuclein gene can cause familial forms of the disease. In aggregated fibrillar form, α-synuclein is the main component of Lewy bodies, the intraneuronal inclusion bodies characteristic of Parkinson's disease. The loss of functional dopaminergic neurons in Parkinson's disease may be caused by a gain in toxic function of the protein. Elucidating if this gain of toxic function is related to the aggregation of α-synuclein may be vital in understanding Parkinson's disease. Although there are many ideas on how α-synuclein could be involved in the disease, this review will focus on the amyloid pore hypothesis. This hypothesis assumes that aggregation intermediates or oligomers are more likely to be toxic than monomeric or fibrillar forms of the protein. Oligomeric species are thought to exercise their toxicity through permeabilization of cellular membranes. Membrane pore formation by an oligomeric intermediate might play a role in other neurodegenerative disorders in which protein aggregation and amyloid formation play a role, such as Alzheimer's disease. We will discuss the role of this hypothesis in Parkinson's disease.

The past fifteen years have led to a profound re-consideration of the molecular and cellular basis of amyloid diseases. Since the formulation of the amyloid hypothesis in 1991-1992, increasing interest was initially focused at amyloid fibrils and, subsequently, at their precursors, oligomers and pre-fibrillar aggregates as main culprits of cell impairment and demise, particularly in neurodegenerative diseases with amyloid deposition. In 2002, this concept was generalized by the demonstration that pre-fibrillar aggregates were toxic even when they were grown from proteins not associated with amyloid disease. Presently, the general structural features and polymorphism of amyloid fibrils grown from a range of different peptides and proteins are rather well known; however, in spite of the growing interest in amyloid oligomers as the main source of amyloid toxicity, a better definition of their structural features remains elusive due to their transient nature, remarkable instability, high flexibility and structural heterogeneity possibly resulting in the appearance of polymorphic assemblies. Nevertheless, recent studies have started to unravel this key topic by providing significant insights into some general structural features and conformational polymorphism of amyloid oligomers and the higher order structures they generate. Important clues into the structure-toxicity relation of amyloids, the role performed by natural surfaces in oligomer growth and the molecular basis of oligomer-membrane interaction are also emerging.

Attempts to understand the biophysical foundations and biochemical consequences of protein aggregation process are greatly aided by conditions which provide either robust and reliable reaction conditions or constitute mimics of the physiological conditions. While both anionic surfactants such as SDS and fluorinated alcohols such as TFE are often championed as membrane mimics in one way or another, it is probably fair to say that their greatest advantage is to facilitate protein aggregation under simple and well-defined solvent conditions which are compatible with a plethora of biophysical techniques. In contrast to the biological membrane, whose chemical complexity and physical heterogeneity gives rise to a multitude of possible interactions with proteins, SDS and TFE exert a surprisingly versatile effect on proteins by a combination of two opposing forces: a weakening of protein-protein hydrophobic interactions and a strengthening of inter- and intra-molecular hydrogen bonding. This invariably gives rise to a concentration range (typically 0.5-1 mM SDS and 20-30% TFE) which favours intermolecular β-sheet formation. I discuss a number of examples of this behaviour, and present recent investigations based on a combination of calorimetric, spectroscopic and Small Angle X-ray scattering techniques. Together these provide a structural and stoichiometric picture of the different species involved in SDSmediated protein aggregation, driven by the hydrophobic bonds formed when SDS clusters on different proteins form a contiguous micelle by protein association. Higher-order aggregates are formed by protein regions linking these shared micelles, providing a flexible bead-on-a-string that grows in a step-wise fashion and leads to worm-like fibrillar structures. Despite the unique features displayed in different aggregating systems, there are clear parallels between membranemediated aggregation and aggregation in SDS and TFE in terms of modulation between α-helical and β-sheet structures depending on the ratio between protein and amphiphile.

The growing interest in membrane interactions of amyloidogenic peptides and proteins emanates from the realization that lipids and membranes play important, potentially central, roles in the toxicity and pathological pathways of amyloid diseases. Expanding body of evidence indicates that lipid binding of amyloidogenic peptides and amyloid peptide association with cellular membranes is critical in the onset and progression of amyloid diseases. Advancing the understanding in this field goes hand in hand with application of varied biophysical and biological techniques designed to probe the characteristics and underlying mechanisms of membrane-peptide interactions. This review summarizes experimental approaches and technical aspects employed in recent years for investigating the interaction of amyloid peptides and fibrillar species with lipid bilayers, and the reciprocal contribution of membranes to fibrillation processes and amyloidogenesis.

The famous Kramers rate theory for diffusion-controlled reactions has been extended in numerous ways and successfully applied to many types of reactions. Its application to protein folding reactions has been of particular interest in recent years, as many researchers have performed experiments and simulations to test whether folding reactions are diffusion- controlled, whether the solvent is the source of the reaction friction, and whether the friction-dependence of folding rates generally can provide insight into folding dynamics. These experiments involve many practical difficulties, however. They have also produced some unexpected results. Here we briefly review the Kramers theory for reactions in the presence of strong friction and summarize some of the subtle problems that arise in the application of the theory to protein folding. We discuss how the results of these experiments ultimately point to a significant role for internal friction in protein folding dynamics. Studies of friction in protein folding, far from revealing any weakness in Kramers theory, may actually lead to new approaches for probing diffusional dynamics and energy landscapes in protein folding.