Biology subjects

Sparr, E.

Publications and source records attributed to Sparr, E..

4 recordsLinked to original sources

bioRxiv2025

Structural defects in amyloid-β fibrils drive secondary nucleation

The nucleation of amyloid fibrils from monomeric protein, catalyzed by the surface of existing fibrils, is an important driver of many disorders such as Alzheimers and Parkinsons diseases. The structural basis of this secondary nucleation process, however, is poorly understood. Here, we ask whether secondary nucleation sites are found predominantly at rare growth defects: defects in the fibril core structure generated during their original assembly. We first demonstrate using the specific inhibitor of secondary nucleation, Brichos, that secondary nucleation sites on Alzheimers disease-associated fibrils composed of A{beta}40 and A{beta}42 peptides are rare compared to the number of protein molecules they contain. We then grow A{beta}40 fibrils under conditions designed to eliminate most growth defects while leaving the regular fibril morphology unchanged, and confirm the latter using cryo-electron microscopy. We measure both the ability of these annealed fibrils to promote secondary nucleation and the stoichiometry of their secondary nucleation sites, finding that both are greatly reduced as predicted. Re-analysis of published data for other proteins suggests that fibril growth defects that expose monomer planes or other structural units may also drive secondary nucleation generally, across most or all amyloids. These findings could unlock structure-based drug design of therapeutics that aim to halt amyloid disorders by inhibiting secondary nucleation sites.

biophysics↗

bioRxiv2025

On the transient interactions of α-synuclein in different dimensions

0.-Synuclein (Syn) is a neuronal protein predominantly found in the brain, whose native function seems to be associated with vesicle trafficking. While intrinsically disordered in solution, the first ca. 100 residues adopt an amphipathic -helical structure when the protein adsorbs onto membranes. Additionally, the aggregation of Syn into highly ordered {beta}-sheet rich amyloid fibrils is associated with Parkinso[n]s disease. The different regions of Syn and the interactions between them have been reported to play a key role in the behaviour of the protein in solution, its membrane binding, and its aggregation into fibrils. This study employs photo-induced cross-linking of unmodified proteins (PICUP) to capture and identify the transient contacts of Syn in different conformational states: free in solution, adsorbed to membranes, and aggregated into fibrils. By using tyrosine-to-phenylalanine mutations to block the reactivity of specific amino acid residues, we establish key cross-links in each state. In solution, we identify internal contacts between the N and C termini of monomers, as well as inter-monomer contacts between C termini in oligomers. When Syn is adsorbed to membranes, the internal cross-linking is blocked, while cross-linking between C-terminal regions persists. In fibrils, cross-linking is significantly reduced, primarily occurring between C-terminal residues of adjacent monomers. This work highlights the utility of PICUP for reporting on the transient contacts that occur on the pathways of self- and co-assembly of Syn.

biochemistry↗

bioRxiv2024

On the coupling between membrane bending and stretching in lipid vesicles

The formation of a lipid vesicle from a lamellar phase involves a cost in bending energy of 100-1000 times the thermal energy for values of the membrane bending rigidity{kappa} typical for phospholipid bilayers. The bending rigidity of a bilayer is however a strongly decreasing function of its thickness h, and the bilayer can thus reduce its bending energy by stretching (and thus thinning) the bilayer. In this paper, we construct a simple model to describe this mechanism for the coupling between bending and stretching and analyse its effect on the bending energy and thermal fluctuations of spherical lipid vesicles. We show that the bilayer thinning becomes significant for small vesicles, and for a vesicle with radius R0 [~] 15 nm there is a sizeable thinning of the bilayer compared to the planar state. We furthermore demonstrate how this thinning is associated with a significant decrease in free energy due to the thermally excited bending modes. We argue that this previously unexplored effect can explain the experimentally observed lower limit of achievable vesicle sizes, which eventually become unstable due to the thinning of the bilayer. We also sketch how this effect provides a potential generic mechanism for the strong curvature dependence of protein adsorption to lipid membranes.

biophysics↗

bioRxiv2024

Tipping point in α-synuclein-membrane interactions: stable protein-covered vesicles or amyloid aggregation

-synuclein is a neuronal protein implicated in neurotransmitter release. Its function is thought to critically depend on the dynamic equilibrium between free and membrane-bound protein. -synuclein amyloid formation implicated in Parkinsons Disease was also shown to be modulated by lipid membranes. However, it remains elusive whether -synuclein-related pathology is due to loss-of-function or gain-of-toxic-function. To help address this question, we studied the coupling of the equilibrium between free and membrane-bound -synuclein and membrane-induced amyloid formation - phenomena that are usually treated separately. We present a description of the system on a wide range of length scales and timescales for lipid-to-protein ratio conditions where amyloid formation is either accelerated or inhibited by lipid membranes. We find a clear difference between the dynamics and heterogeneity of the protein-covered membrane interface in the two sets of conditions. In aggregation-accelerating conditions, the membrane interface is dynamic and heterogeneous with rapid exchange between free and membrane-bound protein, and disordered protein segments of varying lengths exposed to solution. All these characteristics of the membrane interface are likely to decrease the free energy barrier for amyloid formation. Conversely, the membrane interface is homogeneous and less dynamic in conditions where amyloid formation is inhibited. Importantly, any factors affecting the equilibrium between free and membrane-bound -synuclein may trigger a change from non-aggregating to aggregating conditions. Altogether, our results highlight a strong coupling of the dynamic equilibrium between the free and membrane-bound -synuclein and membrane-modulated amyloid formation and thus of the physiological function of -synuclein and its aberrant aggregation.

biochemistry↗