Compartmentalization of soluble endocytic proteins in synaptic vesicle clusters by phase separation

Live-cell quantification reveals viscoelastic regulation of synapsin condensates by α-synuclein

Synapsin and α-synuclein represent a growing list of condensate-forming proteins where the material states of condensates are directly linked to cellular functions (e.g., neurotransmission) and pathology (e.g., neurodegeneration). However, quantifying condensate material properties in living systems has been a substantial challenge. Here, we develop micropipette aspiration and whole-cell patch-clamp (MAPAC), a platform that allows direct material quantification of condensates in live cells. We find 10,000-fold variations in the viscoelasticity of synapsin condensates, regulated by the partitioning of α-synuclein, a marker for synucleinopathies. Through in vitro reconstitutions, we identify multiple molecular factors that distinctly regulate the viscosity, interfacial tension, and maturation of synapsin condensates, confirming the cellular roles of α-synuclein. Overall, our study provides unprecedented quantitative insights into the material properties of neuronal condensates and reveals a crucial role of α-synuclein in regulating condensate viscoelasticity. Furthermore, we envision MAPAC applicable to study a broad range of condensates in vivo.

The synaptic vesicle cluster as a controller of pre- and postsynaptic structure and function

The synaptic vesicle cluster (SVC) is an essential component of chemical synapses, which provides neurotransmitter-loaded vesicles during synaptic activity, at the same time as also controlling the local concentrations of numerous exo- and endocytosis cofactors. In addition, the SVC hosts molecules that participate in other aspects of synaptic function, from cytoskeletal components to adhesion proteins, and affects the location and function of organelles such as mitochondria and the endoplasmic reticulum. We argue here that these features extend the functional involvement of the SVC in synapse formation, signalling and plasticity, as well as synapse stabilization and metabolism. We also propose that changes in the size of the SVC coalesce with changes in the postsynaptic compartment, supporting the interplay between pre- and postsynaptic dynamics. Thereby, the SVC could be seen as an 'all-in-one' regulator of synaptic structure and function, which should be investigated in more detail, to reveal molecular mechanisms that control synaptic function and heterogeneity.

Liquid-liquid phase separation in presynaptic nerve terminals

The presynaptic nerve terminal is crucial for transmitting signals to the adjacent cell. To fulfill this role, specific proteins with distinct functions are concentrated in spatially confined areas within the nerve terminals. A recent concept termed liquid-liquid phase separation (LLPS) has provided new insights into how this process may occur. In this review, we aim to summarize the LLPS of proteins in different parts of the presynaptic nerve terminals, including synaptic vesicle (SV) clusters, the active zone (AZ), and the endocytic zone, with an additional focus on neurodegenerative diseases (NDDs), where the functional relevance of these properties is explored. Last, we propose new perspectives and future directions for the role of LLPS in presynaptic nerve terminals.

Live-Cell Quantification Reveals Viscoelastic Regulation of Synapsin Condensates by α-Synuclein

Synapsin and α-synuclein represent a growing list of condensate-forming proteins where the material states of condensates are directly linked to cellular functions (e.g., neurotransmission) and pathology (e.g., neurodegeneration). However, quantifying condensate material properties in living systems has been a significant challenge. To address this, we develop MAPAC (micropipette aspiration and whole-cell patch clamp), a platform that allows direct material quantification of condensates in live cells. We find 10,000-fold variations in the viscoelasticity of synapsin condensates, regulated by the partitioning of α-synuclein, a marker for synucleinopathies. Through in vitro reconstitutions, we identify 4 molecular factors that distinctly regulate the viscosity and interfacial tension of synapsin condensates, verifying the cellular effects of α-synuclein. Overall, our study provides unprecedented quantitative insights into the material properties of neuronal condensates and reveals a crucial role of α-synuclein in regulating condensate viscoelasticity. Furthermore, we envision MAPAC applicable to study a broad range of condensates in vivo.  .

Roles of membrane mechanics-mediated feedback in membrane traffic

Synthesizing the recent progresses, we present our perspectives on how local modulations of membrane curvature, tension, and bending energy define the feedback controls over membrane traffic processes. We speculate the potential mechanisms of, and the control logic behind, the different membrane mechanics-mediated feedback in endocytosis and exo-endocytosis coupling. We elaborate the path forward with the open questions for theoretical considerations and the grand challenges for experimental validations.

VAMP2 regulates phase separation of α-synuclein

α-Synuclein (αSYN), a pivotal synaptic protein implicated in synucleinopathies such as Parkinson's disease and Lewy body dementia, undergoes protein phase separation. We reveal that vesicle-associated membrane protein 2 (VAMP2) orchestrates αSYN phase separation both in vitro and in cells. Electrostatic interactions, specifically mediated by VAMP2 via its juxtamembrane domain and the αSYN C-terminal region, drive phase separation. Condensate formation is specific for R-SNARE VAMP2 and dependent on αSYN lipid membrane binding. Our results delineate a regulatory mechanism for αSYN phase separation in cells. Furthermore, we show that αSYN condensates sequester vesicles and attract complexin-1 and -2, thus supporting a role in synaptic physiology and pathophysiology.

The juxtamembrane linker of synaptotagmin 1 regulates Ca2+ binding via liquid-liquid phase separation

Synaptotagmin (syt) 1, a Ca2+ sensor for synaptic vesicle exocytosis, functions in vivo as a multimer. Syt1 senses Ca2+ via tandem C2-domains that are connected to a single transmembrane domain via a juxtamembrane linker. Here, we show that this linker segment harbors a lysine-rich, intrinsically disordered region that is necessary and sufficient to mediate liquid-liquid phase separation (LLPS). Interestingly, condensate formation negatively regulates the Ca2+-sensitivity of syt1. Moreover, Ca2+ and anionic phospholipids facilitate the observed phase separation, and increases in [Ca2+]i promote the fusion of syt1 droplets in living cells. Together, these observations suggest a condensate-mediated feedback loop that serves to fine-tune the ability of syt1 to trigger release, via alterations in Ca2+ binding activity and potentially through the impact of LLPS on membrane curvature during fusion reactions. In summary, the juxtamembrane linker of syt1 emerges as a regulator of syt1 function by driving self-association via LLPS.

Dipping contacts - a novel type of contact site at the interface between membraneless organelles and membranes

Liquid-liquid phase separation is a major mechanism for organizing macromolecules, particularly proteins with intrinsically disordered regions, in compartments not limited by a membrane or a scaffold. The cell can therefore be perceived as a complex emulsion containing many of these membraneless organelles, also referred to as biomolecular condensates, together with numerous membrane-bound organelles. It is currently unclear how such a complex concoction operates to allow for intracellular trafficking, signaling and metabolic processes to occur with high spatiotemporal precision. Based on experimental observations of synaptic vesicle condensates - a membraneless organelle that is in fact packed with membranes - we present here the framework of dipping contacts: a novel type of contact site between membraneless organelles and membranes. In this Hypothesis, we propose that our framework of dipping contacts can serve as a foundation to investigate the interface that couples the diffusion and material properties of condensates to biochemical processes occurring in membranes. The identity and regulation of this interface is especially critical in the case of neurodegenerative diseases, where aberrant inclusions of misfolded proteins and damaged organelles underlie cellular pathology.

The juxtamembrane linker of synaptotagmin 1 regulates Ca2+ binding via liquid-liquid phase separation

Synaptotagmin (syt) 1, a Ca2+ sensor for synaptic vesicle exocytosis, functions in vivo as a multimer. Syt1 senses Ca2+ via tandem C2-domains that are connected to a single transmembrane domain via a juxtamembrane linker. Here, we show that this linker segment harbors a lysine-rich, intrinsically disordered region that is necessary and sufficient to mediate liquid-liquid phase separation (LLPS). Interestingly, condensate formation negatively regulates the Ca2+-sensitivity of syt1. Moreover, Ca2+ and anionic phospholipids facilitate the observed phase separation, and increases in [Ca2+]i promote the fusion of syt1 droplets in living cells. Together, these observations suggest a condensate-mediated feedback loop that serves to fine-tune the ability of syt1 to trigger release, via alterations in Ca2+ binding activity and potentially through the impact of LLPS on membrane curvature during fusion reactions. In summary, the juxtamembrane linker of syt1 emerges as a regulator of syt1 function by driving self-association via LLPS.

Biology of endophilin and it's role in disease

Endophilin is an evolutionarily conserved family of protein that involves in a range of intracellular membrane dynamics. This family consists of five isoforms, which are distributed in various tissues. Recent studies have shown that Endophilin regulates diseases pathogenesis, including neurodegenerative diseases, tumors, cardiovascular diseases, and autoimmune diseases. In vivo, it regulates different biological functions such as vesicle endocytosis, mitochondrial morphological changes, apoptosis and autophagosome formation. Functional studies confirmed the role of Endophilin in development and progression of these diseases. In this study, we have comprehensively discussed the complex function of Endophilin and how the family contributes to diseases development. It is hoped that this study will provide new ideas for targeting Endophilin in diseases.

Intersectin and Endophilin condensates prime synaptic vesicles for release site replenishment

Neurotransmitter is released from dedicated sites of synaptic vesicle fusion within a synapse. Following fusion, the vacated sites are replenished immediately by new vesicles for subsequent neurotransmission. These replacement vesicles are assumed to be located near release sites and used by chance. Here, we find that replacement vesicles are clustered around this region by Intersectin-1. Specifically, Intersectin-1 forms dynamic molecular condensates with Endophilin A1 near release sites and sequesters vesicles around this region. In the absence of Intersectin-1, vesicles within 20 nm of the plasma membrane are reduced, and consequently, vacated sites cannot be replenished rapidly, leading to depression of synaptic transmission. Similarly, mutations in Intersectin-1 that disrupt Endophilin A1 binding result in similar phenotypes. However, in the absence of Endophilin, this replacement pool of vesicles is available but cannot be accessed, suggesting that Endophilin A1 is needed to mobilize these vesicles. Thus, our work describes a distinct physical region within a synapse where replacement vesicles are harbored for release site replenishment.