Mechanistic Modeling of Amyloid Oligomer and Protofibril Formation in Bovine Insulin

Early phase of amyloid formation, where prefibrillar aggregates such as oligomers and protofibrils are often observed, is crucial for understanding pathogenesis. However, the detailed mechanisms of their formation have been difficult to elucidate because they tend to form transiently and heterogeneously. Here, we found that bovine insulin protofibril formation proceeds in a monodisperse manner, which allowed us to characterize the detailed early aggregation process by light scattering in combination with thioflavin T fluorescence and Fourier transform infrared spectroscopy. The protofibril formation was specific to bovine insulin, whereas no significant aggregation was observed in human insulin. The kinetic analysis combining static and dynamic light scattering data revealed that the protofibril formation process in bovine insulin can be divided into two steps based on fractal dimension. When modeling the experimental data based on Smoluchowski aggregation kinetics, an aggregation scheme consisting of initial fractal aggregation forming spherical oligomers and their subsequent end-to-end association forming protofibrils was clarified. Furthermore, the analysis of temperature and salt concentration dependencies showed that the end-to-end association is the rate-limiting step, involving dehydration. The established model for protofibril formation, wherein oligomers are incorporated as a precursor, provides insight into the molecular mechanism by which protein molecules assemble during the early stage of amyloid formation.

Lysophagy protects against propagation of α-synuclein aggregation through ruptured lysosomal vesicles

The neuron-to-neuron propagation of misfolded α-synuclein (αSyn) aggregates is thought to be key to the pathogenesis of synucleinopathies. Recent studies have shown that extracellular αSyn aggregates taken up by the endosomal-lysosomal system can rupture the lysosomal vesicular membrane; however, it remains unclear whether lysosomal rupture leads to the transmission of αSyn aggregation. Here, we applied cell-based αSyn propagation models to show that ruptured lysosomes are the pathway through which exogenous αSyn aggregates transmit aggregation, and furthermore, this process was prevented by lysophagy, i.e., selective autophagy of damaged lysosomes. αSyn aggregates accumulated predominantly in lysosomes, causing their rupture, and seeded the aggregation of endogenous αSyn, initially around damaged lysosomes. Exogenous αSyn aggregates induced the accumulation of LC3 on lysosomes. This LC3 accumulation was not observed in cells in which a key regulator of autophagy, RB1CC1/FIP200, was knocked out and was confirmed as lysophagy by transmission electron microscopy. Importantly, RB1CC1/FIP200-deficient cells treated with αSyn aggregates had increased numbers of ruptured lysosomes and enhanced propagation of αSyn aggregation. Furthermore, various types of lysosomal damage induced using lysosomotropic reagents, depletion of lysosomal enzymes, or more toxic species of αSyn fibrils also exacerbated the propagation of αSyn aggregation, and impaired lysophagy and lysosomal membrane damage synergistically enhanced propagation. These results indicate that lysophagy prevents exogenous αSyn aggregates from escaping the endosomal-lysosomal system and transmitting aggregation to endogenous cytosolic αSyn via ruptured lysosomal vesicles. Our findings suggest that the progression and severity of synucleinopathies are associated with damage to lysosomal membranes and impaired lysophagy.

Phosphatidylinositol-3,4,5-trisphosphate interacts with alpha-synuclein and initiates its aggregation and formation of Parkinson's disease-related fibril polymorphism

Lipid interaction with α-synuclein (αSyn) has been long implicated in the pathogenesis of Parkinson's disease (PD). However, it has not been fully determined which lipids are involved in the initiation of αSyn aggregation in PD. Here exploiting genetic understanding associating the loss-of-function mutation in Synaptojanin 1 (SYNJ1), a phosphoinositide phosphatase, with familial PD and analysis of postmortem PD brains, we identified a novel lipid molecule involved in the toxic conversion of αSyn and its relation to PD. We first established a SYNJ1 knockout cell model and found SYNJ1 depletion increases the accumulation of pathological αSyn. Lipidomic analysis revealed SYNJ1 depletion elevates the level of its substrate phosphatidylinositol-3,4,5-trisphosphate (PIP₃). We then employed Caenorhabditis elegans model to examine the effect of SYNJ1 defect on the neurotoxicity of αSyn. Mutations in SYNJ1 accelerated the accumulation of αSyn aggregation and induced locomotory defects in the nematodes. These results indicate that functional loss of SYNJ1 promotes the pathological aggregation of αSyn via the dysregulation of its substrate PIP₃, leading to the aggravation of αSyn-mediated neurodegeneration. Treatment of cultured cell line and primary neurons with PIP₃ itself or with PIP₃ phosphatase inhibitor resulted in intracellular formation of αSyn inclusions. Indeed, in vitro protein-lipid overlay assay validated that phosphoinositides, especially PIP₃, strongly interact with αSyn. Furthermore, the aggregation assay revealed that PIP₃ not only accelerates the fibrillation of αSyn, but also induces the formation of fibrils sharing conformational and biochemical characteristics similar to the fibrils amplified from the brains of PD patients. Notably, the immunohistochemical and lipidomic analyses on postmortem brain of patients with sporadic PD showed increased PIP₃ level and its colocalization with αSyn. Taken together, PIP₃ dysregulation promotes the pathological aggregation of αSyn and increases the risk of developing PD, and PIP₃ represents a potent target for intervention in PD.

Extracellular α-synuclein drives sphingosine 1-phosphate receptor subtype 1 out of lipid rafts, leading to impaired inhibitory G-protein signaling

α-Synuclein (α-Syn)-positive intracytoplasmic inclusions, known as Lewy bodies, are thought to be involved in the pathogenesis of Lewy body diseases, such as Parkinson's disease (PD). Although growing evidence suggests that cell-to-cell transmission of α-Syn is associated with the progression of PD and that extracellular α-Syn promotes formation of inclusion bodies, its precise mechanism of action in the extracellular space remains unclear. Here, as indicated by both conventional fractionation techniques and FRET-based protein-protein interaction analysis, we demonstrate that extracellular α-Syn causes expulsion of sphingosine 1-phosphate receptor subtype 1 (S1P₁R) from the lipid raft fractions. S1P₁R regulates vesicular trafficking, and its expulsion involved α-Syn binding to membrane-surface gangliosides. Consequently, the S1P₁R became refractory to S1P stimulation required for activating inhibitory G-protein (G_i) in the plasma membranes. Moreover, the extracellular α-Syn also induced uncoupling of the S1P₁R on internal vesicles, resulting in the reduced amount of CD63 molecule (CD63) in the lumen of multivesicular endosomes, together with a decrease in CD63 in the released exosomes from α-Syn-treated cells. Furthermore, cholesterol-depleting agent-induced S1P₁R expulsion from the rafts also resulted in S1P₁R uncoupling. Taken together, these results suggest that extracellular α-Syn-induced expulsion of S1P₁R from lipid rafts promotes the uncoupling of S1P₁R from G_i, thereby blocking subsequent G_i signals, such as inhibition of cargo sorting into exosomal vesicles in multivesicular endosomes. These findings help shed additional light on PD pathogenesis.

DHHC5-mediated palmitoylation of S1P receptor subtype 1 determines G-protein coupling

Sphingosine 1-phosphate (S1P) is a pleiotropic lipid mediator involved in the regulation of immune cell trafficking and vascular permeability acting mainly through G-protein-coupled S1P receptors (S1PRs). However, mechanism underlying how S1PRs are coupled with G-proteins remains unknown. Here we have uncovered that palmitoylation of a prototypical subtype S1P₁R is prerequisite for subsequent inhibitory G-protein (Gi) coupling. We have identified DHHC5 as an enzyme for palmitoylation of S1P₁R. Under basal conditions, S1P₁R was functionally associated with DHHC5 in the plasma membranes (PM) and was fully palmitoylated, enabling Gi coupling. Upon stimulation, the receptor underwent internalisation leaving DHHC5 in PM, resulting in depalmitoylation of S1P₁R. We also revealed that while physiological agonist S1P-induced endocytosed S1P₁R readily recycled back to PM, pharmacological FTY720-P-induced endocytosed S1P₁R-positive vesicles became associated with DHHC5 in the later phase, persistently transmitting Gi signals there. This indicates that FTY720-P switches off the S1P signal in PM, while switching on its signal continuously inside the cells. We propose that DHHC5-mediated palmitoylation of S1P₁R determines Gi coupling and its signalling in a spatio/temporal manner.

Involvement of Gβγ subunits of Gi protein coupled with S1P receptor on multivesicular endosomes in F-actin formation and cargo sorting into exosomes

Exosomes play a critical role in cell-to-cell communication by delivering cargo molecules to recipient cells. However, the mechanism underlying the generation of the exosomal multivesicular endosome (MVE) is one of the mysteries in the field of endosome research. Although sphingolipid metabolites such as ceramide and sphingosine 1-phosphate (S1P) are known to play important roles in MVE formation and maturation, the detailed molecular mechanisms are still unclear. Here, we show that Rho family GTPases, including Cdc42 and Rac1, are constitutively activated on exosomal MVEs and are regulated by S1P signaling as measured by fluorescence resonance energy transfer (FRET)-based conformational changes. Moreover, we detected S1P signaling-induced filamentous actin (F-actin) formation. A selective inhibitor of Gβγ subunits, M119, strongly inhibited both F-actin formation on MVEs and cargo sorting into exosomal intralumenal vesicles of MVEs, both of which were fully rescued by the simultaneous expression of constitutively active Cdc42 and Rac1. Our results shed light on the mechanism underlying exosomal MVE maturation and inform the understanding of the physiological relevance of continuous activation of the S1P receptor and subsequent downstream G protein signaling to Gβγ subunits/Rho family GTPases-regulated F-actin formation on MVEs for cargo sorting into exosomal intralumenal vesicles.