Condensate biology of synaptic vesicle clusters

Synapsin Condensation is Governed by Sequence-Encoded Molecular Grammars

Multiple biomolecular condensates coexist at the pre- and post- synapse to enable vesicle dynamics and controlled neurotransmitter release in the brain. In pre-synapses, intrinsically disordered regions (IDRs) of synaptic proteins are drivers of condensation that enable clustering of synaptic vesicles (SVs). Using computational analysis, we show that the IDRs of SV proteins feature evolutionarily conserved non-random compositional biases and sequence patterns. Synapsin-1 is essential for condensation of SVs, and its C-terminal IDR has been shown to be a key driver of condensation. Focusing on this IDR, we dissected the contributions of two conserved features namely the segregation of polar and proline residues along the linear sequence, and the compositional preference for arginine over lysine. Scrambling the blocks of polar and proline residues weakens the driving forces for forming micron-scale condensates. However, the extent of clustering in subsaturated solutions remains equivalent to that of the wild-type synapsin-1. In contrast, substituting arginine with lysine significantly weakens both the driving forces for condensation and the extent of clustering in subsaturated solutions. Co-expression of the scrambled variant of synapsin-1 with synaptophysin results in a gain-of-function phenotype in cells, whereas arginine to lysine substitutions eliminate condensation in cells. We report an emergent consequence of synapsin-1 condensation, which is the generation of interphase pH gradients that is realized via differential partitioning of protons between coexisting phases. This pH gradient is likely to be directly relevant for vesicular ATPase functions and the loading of neurotransmitters. Our studies highlight how conserved IDR grammars serve as drivers of synapsin-1 condensation.

At the crossroads of calcium signaling, protein synthesis and neuropeptide release

By influencing calcium homeostasis, local protein synthesis and the endoplasmic reticulum, a small protein called Rab10 emerges as a crucial cytoplasmic regulator of neuropeptide secretion.

Condensates of synaptic vesicles and synapsin are molecular beacons for actin sequestering and polymerization

Neuronal communication relies on precisely maintained synaptic vesicle (SV) clusters, which assemble via liquid-liquid phase separation (LLPS). This process requires synapsins, the major synaptic phosphoproteins, which are known to bind actin. The reorganization of SVs, synapsins and actin is a hallmark of synaptic activity, but their interplay is still unclear. Here, we combined the reconstitution approaches, expansion microscopy, super-resolution imaging and cryo-electron tomography to dissect the roles of synapsin-SV condensates in the organization of the presynaptic actin cytoskeleton. Our data indicate that LLPS of synapsin initiates actin polymerization, allowing for SV:synapsin:actin assemblies to facilitate the mesoscale organization of SV clusters along axons mimicking the native presynaptic organization in both lamprey and mammalian synapses. Understanding the relationship between the actin network and synapsin-SVs condensates is an essential building block on a roadmap to unravel how coordinated neurotransmission along the axon enables circuit function and behavior.

Integrated transcriptomic hypothalamic-pituitary-ovarian axis network analysis reveals the role of energy availability on egg production in layers

Energy is a crucial component for maintaining egg production in layers. The hypothalamic-pituitary-ovarian (HPO) axis is an energy-sensitive functional axis for follicle development, synthesis, and secretion of reproductive hormones, and plays a key role in modulating sustained ovulation in layers. To investigate the mechanism of integrated network regulation of the HPO axis under energy fluctuation, ninety Hy-line brown layers (265-day-old, 1.92 ± 0.02 kg) were randomly divided into three groups for a 17-day experiment: a control group (Con group) fed ad libitum from days 1 to 17, an energy-deprived group (ED group) that was fed ad libitum from days 1 to 12 and then underwent a fasting period from days 13 to 17 to induce a pause in laying, and a re-fed group (Rf group) that fasted for seven days (specifically, days 1 to 5, day 7, and day 9), had ad libitum access to feed on days 6 and 8, and was continuously fed from days 10 to 17. Each treatment consisted of 10 replicates with 3 birds per replicate. The study found that energy deprivation significantly decreased reproductive performance such as egg laying rate, ovarian index, number of small yellow follicles (SYF), and normal hierarchical follicles (NHIE) (P < 0.05), which recovered after refeeding, indicating the importance of energy availability for sustained ovulation in layers. In addition, estradiol (E2), estradiol to progesterone (E2/P4) ratio, and luteinizing hormone (LH) displayed changes similar to follicle number, whereas follicle-stimulating hormone (FSH) exhibited a contrasting pattern. Transcriptome analysis revealed that energy deprivation downregulated genes related to energy and appetite-regulated neurotransmitter receptors and neuropeptides in the hypothalamus. These signals combined to inhibit gonadotropin-releasing hormone (GnRH) secretion and subsequently downregulated the crucial genes responsible for synthesizing gonadotropins, gonadotropin-releasing hormone receptor (GnRHR), and glycoprotein hormones alpha chain (CGA). Consequently, this suppression of the hypothalamus and pituitary affected ovarian function through ovarian steroidogenesis and the extracellular matrix (ECM)-receptor interaction. These findings suggest that energy deprivation inhibits the function of the HPO axis, leading to impaired follicle development and reduced egg production, and that refeeding can partially restore these indicators.

The α-synuclein seed amplification assay: Interpreting a test of Parkinson's pathology

The α-synuclein seed amplification assay (αSyn-SAA) sensitively detects Lewy pathology, the amyloid state of α-synuclein, in the cerebrospinal fluid (CSF) of patients with Parkinson's disease (PD). The αSyn-SAA harnesses the physics of seeding, whereby a superconcentrated solution of recombinant α-synuclein lowers the thermodynamic threshold (nucleation barrier) for aggregated α-synuclein to act as a nucleation catalyst ("seed") to trigger the precipitation (nucleation) of monomeric α-synuclein into pathology. This laboratory setup increases the signal for identifying a catalyst if one is present in the tissue examined. The result is binary: positive, meaning precipitation occurred, and a catalyst is present, or negative, meaning no precipitation, therefore no catalyst. Since protein precipitation via seeding can only occur at a concentration many-fold higher than the human brain, laboratory-elicited seeding does not mean human brain seeding. We suggest that a positive αSyn-SAA reveals the presence of pathological α-synuclein but not the underlying etiology for the precipitation of monomeric α-synuclein into its pathological form. Thus, a positive αSyn-SAA supports a clinical diagnosis of PD but cannot inform disease pathogenesis, ascertain severity, predict the rate of progression, define biology or biological subtypes, or monitor treatment response.

Cellular Cholesterol Loss Impairs Synaptic Vesicle Mobility via the CAMK2/Synapsin-1 Signaling Pathway

BACKGROUND

Neuronal cholesterol deficiency may contribute to the synaptopathy observed in Alzheimer's disease (AD). However, the underlying mechanisms remain poorly understood. Intact synaptic vesicle (SV) mobility is crucial for normal synaptic function, whereas disrupted SV mobility can trigger the synaptopathy associated with AD. In this study, we investigated whether cellular cholesterol deficiency affects SV mobility, with the aim of identifying the mechanism that links cellular cholesterol loss to synaptopathy in AD.

METHODS

Lentiviruses carrying 3β-hydroxysteroid-Δ24 reductase-complementary DNA (DHCR24-cDNA), DHCR24-short hairpin RNA (DHCR24- shRNA) or empty lentiviral vectors were transfected into SHSY-5Y cells in order to construct DHCR24 knock-down and knock-in models, along with corresponding controls. Filipin III cholesterol staining was employed to visualize membrane and intracellular cholesterol in the different cell models, and fluorescence intensity was assessed using confocal microscopy. Additionally, we performed immunoblotting to quantify the expression of DHCR24, total calmodulin-dependent protein kinase 2 (CAMK-2), p-CAMK2 (T286), caveolin-1, total synapsin-1, phosphorylated synapsin-1 (p-synapsin-1; S605), and synaptophysin in each experimental group.

RESULTS

In DHCR24-silenced cells, the loss of cellular cholesterol caused by knock-down of DCHR24 resulted in a significant decrease in the levels of phosphorylated CAMK2 (p-CAMK2) and phosphorylated synapsin-1 (p-synapsin-1) compared to control cells. The reduction in p-CAMK2 and p-synapsin-1 could disrupt SV mobility, thereby reducing replenishment of the readily releasable pool (RRP) from the reserve pool (RP). Furthermore, cells with DHCR24 knock-down showed downregulation of caveolin-1, a crucial lipid raft marker, compared to control cells. Conversely, elevated cellular cholesterol levels caused by knock-in of DHCR24 reversed the effects of cholesterol deficiency, suggesting that CAMK2-mediated synapsin-1 phosphorylation may be regulated in a lipid raft-associated manner. Additionally, we found that cellular cholesterol loss could significantly downregulate the expression of synaptophysin protein, which is vital for SV biogenesis and synaptic plasticity.

CONCLUSION

These results suggest that depletion of cellular cholesterol following knock-down of DHCR24 can decrease synaptophysin protein expression and impair SV mobility by regulating the CAMK2-meditated synapsin-1 phosphorylation pathway, potentially via a lipid raft-associated mechanism. Our study indicates a critical role for cellular cholesterol deficiency in AD-related synaptopathy, thus highlighting the potential for targeting cellular cholesterol metabolism in therapeutic strategies.

Synaptic Physiology Depends on Electrical Forces and Liquid-Liquid Phase Separation

Pre- and post-synaptic events are regulated by liquid-liquid phase separation and this phenomenon requires multiple electrical forces. Both axonal transport and the organization of postsynaptic excitatory and inhibitory receptors are regulated by LLPS, with its mandatory electrical drivers ultimately determining our cognitive health and capacity.

Biomarkers of mature neuronal differentiation and related diseases

The nervous system regulates perception, cognition and behavioral responses by serving as the body's primary communication system for receiving, regulating and transmitting information. Neurons are the fundamental structures and units of the nervous system. Their differentiation and maturation processes rely on the expression of specific biomarkers. Neuron-specific intracellular markers can be used to determine the degree of neuronal maturation. Neuronal cytoskeletal proteins dictate the shape and structure of neurons, while synaptic plasticity and signaling processes are intricately associated with neuronal synaptic markers. Furthermore, abnormal expression levels of biomarkers can serve as diagnostic indicators for nervous system diseases. This article reviews the markers of mature neuronal differentiation and their relationship with nervous system diseases.

Morphology and intervesicle distances in condensates of synaptic vesicles and synapsin

Synaptic vesicle clusters or pools are functionally important constituents of chemical synapses. In the so-called reserve and the active pools, neurotransmitter-loaded synaptic vesicles (SVs) are stored and conditioned for fusion with the synaptic membrane and subsequent neurotransmitter release during synaptic activity. Vesicle clusters can be considered as so-called membraneless compartments, which form by liquid-liquid phase separation. Synapsin as one of the most abundant synaptic proteins has been identified as a major driver of pool formation. It has been shown to induce liquid-liquid phase separation and form condensates on its own in solution, but also has been shown to integrate vesicles into condensates in vitro. In this process, the intrinsically disordered region of synapsin is believed to play a critical role. Here, we first investigate the solution structure of synapsin and SVs separately by small-angle x-ray scattering. In the limit of low momentum transfer q, the scattering curve for synapsin gives clear indication for supramolecular aggregation (condensation). We then study mixtures of SVs and synapsin-forming condensates, aiming at the morphology and intervesicle distances, i.e., the structure of the condensates in solution. To obtain the structure factor S(q) quantifying intervesicle correlation, we divide the scattering curve of condensates by that of pure SV suspensions. Analysis of S(q) in combination with numerical simulations of cluster aggregation indicates a noncompact fractal-like vesicular fluid with rather short intervesicle distances at the contact sites.

Direct evidence for ultrastructures of the α-synuclein-associated synaptic vesicle pool in presynaptic terminals

SNCA/PARK1 encodes α-synuclein, which is associated with familial Parkinson's disease. Despite its abundance in presynaptic terminals, the aggregation mechanism of α-synuclein and its relationship with Parkinson's disease have not yet been elucidated. Moreover, the ultrastructures of α-synuclein localization sites in neuronal presynaptic terminals remain unclear. Therefore, we herein generated transgenic mice expressing human α-synuclein tagged with mKate2 (hSNCA-mKate2 mice). These mice exhibited normal growth and fertility and had no motor dysfunction relative to their wild-type littermates, even at one year old. α-Synuclein-mKate2 accumulated in presynaptic terminals, particularly between Purkinje cells in the cerebellum and neurons in cerebellar nuclei. α-Synuclein-mKate2 was associated with the presynaptic marker, synaptophysin. In-resin CLEM and immunoelectron or electron microscopy revealed that α-synuclein-mKate2 localized on the surface of synaptic vesicles that were tightly arranged and assembled to form large synaptic pools in the cerebellum with negligible effects on the active zone. These results suggest that α-synuclein-associated ultrastructures in the presynaptic terminals of hSNCA-mKate2 mice reflect the structures of α-synuclein-assembled synaptic vesicle pools, and the size of vesicle pools increased. This transgenic mouse model will be a valuable tool for studying α-synuclein-associated synaptic vesicle pools.

The ataxia-telangiectasia disease protein ATM controls vesicular protein secretion via CHGA and microtubule dynamics via CRMP5

The autosomal recessive disease ataxia-telangiectasia (A-T) presents with cerebellar degeneration, immunodeficiency, radiosensitivity, capillary dilatations, and pulmonary infections. Most symptoms outside the nervous system can be explained by failures of the disease protein ATM as a Ser/Thr-kinase to coordinate DNA damage repair. However, ATM in adult neurons has cytoplasmic localization and vesicle association, where its roles remain unclear. Here, we defined novel ATM protein targets in human neuroblastoma cells, and filtered initial pathogenesis events in ATM-null mouse cerebellum. Profiles of global proteome and phosphoproteomics - both direct ATM/ATR substrates and overall phosphorylation changes - confirmed previous findings for NBN, MRE11, MDC1, CHEK1, EIF4EBP1, AP3B2, PPP2R5C, SYN1 and SLC2A1. Even stronger downregulation of ATM/ATR substrate phosphopeptides after ATM-depletion was documented for CHGA, EXPH5, NBEAL2 and CHMP6 as key factors of protein secretion and endosome dynamics, as well as for CRMP5, DISP2, PHACTR1, PLXNC1, INA and TPX2 as neurite extension factors. Prominent effects on semaphorin-CRMP5-microtubule signals and ATM association with CRMP5 were validated. As a functional consequence, microtubules were stabilized, and neurite retraction ensued. The impact of ATM on secretory granules confirms previous ATM-null cerebellar transcriptome findings. This study provides the first link of A-T neural atrophy to growth cone collapse and aberrant microtubule dynamics.

Autophagy adaptors mediate Parkin-dependent mitophagy by forming sheet-like liquid condensates

During PINK1- and Parkin-mediated mitophagy, autophagy adaptors are recruited to damaged mitochondria to promote their selective degradation. Autophagy adaptors such as optineurin (OPTN) and NDP52 facilitate mitophagy by recruiting the autophagy-initiation machinery, and assisting engulfment of damaged mitochondria through binding to ubiquitinated mitochondrial proteins and autophagosomal ATG8 family proteins. Here, we demonstrate that OPTN and NDP52 form sheet-like phase-separated condensates with liquid-like properties on the surface of ubiquitinated mitochondria. The dynamic and liquid-like nature of OPTN condensates is important for mitophagy activity, because reducing the fluidity of OPTN-ubiquitin condensates suppresses the recruitment of ATG9 vesicles and impairs mitophagy. Based on these results, we propose a dynamic liquid-like, rather than a stoichiometric, model of autophagy adaptors to explain the interactions between autophagic membranes (i.e., ATG9 vesicles and isolation membranes) and mitochondrial membranes during Parkin-mediated mitophagy. This model underscores the importance of liquid-liquid phase separation in facilitating membrane-membrane contacts, likely through the generation of capillary forces.

Synapsin condensation is governed by sequence-encoded molecular grammars

Multiple biomolecular condensates coexist at the pre- and post- synapse to enable vesicle dynamics and controlled neurotransmitter release in the brain. In pre-synapses, intrinsically disordered regions (IDRs) of synaptic proteins are drivers of condensation that enable clustering of synaptic vesicles (SVs). Using computational analysis, we show that the IDRs of SV proteins feature evolutionarily conserved non-random compositional biases and sequence patterns. Synapsin-1 is essential for condensation of SVs, and its C-terminal IDR has been shown to be a key driver of condensation. Focusing on this IDR, we dissected the contributions of two conserved features namely the segregation of polar and proline residues along the linear sequence, and the compositional preference for arginine over lysine. Scrambling the blocks of polar and proline residues weakens the driving forces for forming micron-scale condensates. However, the extent of clustering in subsaturated solutions remains equivalent to that of the wild-type synapsin-1. In contrast, substituting arginine with lysine significantly weakens both the driving forces for condensation and the extent of clustering in subsaturated solutions. Co-expression of the scrambled variant of synapsin-1 with synaptophysin results in a gain-of-function phenotype in cells, whereas arginine to lysine substitutions eliminate condensation. We report an emergent consequence of synapsin-1 condensation, which is the generation of interphase pH gradients realized via differential partitioning of protons between coexisting phases. This pH gradient is likely to be directly relevant for vesicular ATPase functions and the loading of neurotransmitters. Our study highlights how conserved IDR grammars serve as drivers of synapsin-1 condensation.

Synaptotagmin-1 undergoes phase separation to regulate its calcium-sensitive oligomerization

Synaptotagmin-1 (Syt1) is a calcium sensor that regulates synaptic vesicle fusion in synchronous neurotransmitter release. Syt1 interacts with negatively charged lipids and the SNARE complex to control the fusion event. However, it remains incompletely understood how Syt1 mediates Ca2+-trigged synaptic vesicle fusion. Here, we discovered that Syt1 undergoes liquid-liquid phase separation (LLPS) to form condensates both in vitro and in living cells. Syt1 condensates play a role in vesicle attachment to the PM and efficiently recruit SNAREs and complexin, which may facilitate the downstream synaptic vesicle fusion. We observed that Syt1 condensates undergo a liquid-to-gel-like phase transition, reflecting the formation of Syt1 oligomers. The phase transition can be blocked or reversed by Ca2+, confirming the essential role of Ca2+ in Syt1 oligomer disassembly. Finally, we showed that the Syt1 mutations causing Syt1-associated neurodevelopmental disorder impair the Ca2+-driven phase transition. These findings reveal that Syt1 undergoes LLPS and a Ca2+-sensitive phase transition, providing new insights into Syt1-mediated vesicle fusion.

Synaptotagmin-1 in phase: Condensate biology reveals new insights into the synaptic calcium sensor

Two recent papers by Mehta et al. and Zhu et al. in this issue (https://doi.org/10.1083/jcb.202311191) discover that synaptotagmin-1, the primary calcium sensor at the synapse, forms biomolecular condensates, identifying a new layer of regulation in calcium-triggered synaptic vesicle exocytosis.

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.

Biomolecular condensates and disease pathogenesis

Biomolecular condensates or membraneless organelles (MLOs) formed by liquid-liquid phase separation (LLPS) divide intracellular spaces into discrete compartments for specific functions. Dysregulation of LLPS or aberrant phase transition that disturbs the formation or material states of MLOs is closely correlated with neurodegeneration, tumorigenesis, and many other pathological processes. Herein, we summarize the recent progress in development of methods to monitor phase separation and we discuss the biogenesis and function of MLOs formed through phase separation. We then present emerging proof-of-concept examples regarding the disruption of phase separation homeostasis in a diverse array of clinical conditions including neurodegenerative disorders, hearing loss, cancers, and immunological diseases. Finally, we describe the emerging discovery of chemical modulators of phase separation.

Functional and Structural Changes in Diaphragm Neuromuscular Junctions in Early Aging

Age-related impairment of the diaphragm causes respiratory complications. Neuromuscular junction (NMJ) dysfunction can be one of the triggering events in diaphragm weaknesses in old age. Prominent structural and functional alterations in diaphragm NMJs were described in elderly rodents, but NMJ changes in middle age remain unclear. Here, we compared diaphragm muscles from young adult (3 months) and middle-aged (12 months) BALB/c mice. Microelectrode recordings, immunofluorescent staining, electron microscopy, myography, and whole-body plethysmography were used. We revealed presynaptic (i) and postsynaptic (ii) changes. The former (i) included an increase in both action potential propagation velocity and neurotransmitter release evoked by low-, moderate-, and high-frequency activity but a decrease in immunoexpression of synapsin 1 and synaptic vesicle clustering. The latter (ii) consisted of a decrease in currents via nicotinic acetylcholine receptors and the area of their distribution. These NMJ changes correlated with increased contractile responses to moderate- to high-frequency nerve activation. Additionally, we found alterations in the pattern of respiration (an increase in peak inspiratory flow and a tendency of elevation of the tidal volume), which imply increased diaphragm activity in middle-aged mice. We conclude that enhancement of neuromuscular communication (due to presynaptic mechanism) accompanied by improved contractile responses occurs in the diaphragm in early aging.

Tau-mediated synaptic dysfunction is coupled with HCN channelopathy

INTRODUCTION

In tauopathies, altered tau processing correlates with impairments in synaptic density and function. Changes in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels contribute to disease-associated abnormalities in multiple neurodegenerative diseases.

METHODS

To investigate the link between tau and HCN channels, we performed histological, biochemical, ultrastructural, and functional analyses of hippocampal tissues from Alzheimer's disease (AD), age-matched controls, Tau35 mice, and/or Tau35 primary hippocampal neurons.

RESULTS

Expression of specific HCN channels is elevated in post mortem AD hippocampus. Tau35 mice develop progressive abnormalities including increased phosphorylated tau, enhanced HCN channel expression, decreased dendritic branching, reduced synapse density, and vesicle clustering defects. Tau35 primary neurons show increased HCN channel expression enhanced hyperpolarization-induced membrane voltage "sag" and changes in the frequency and kinetics of spontaneous excitatory postsynaptic currents.

DISCUSSION

Our findings are consistent with a model in which pathological changes in tauopathies impact HCN channels to drive network-wide structural and functional synaptic deficits.

HIGHLIGHTS

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are functionally linked to the development of tauopathy. Expression of specific HCN channels is elevated in the hippocampus in Alzheimer's disease and the Tau35 mouse model of tauopathy. Increased expression of HCN channels in Tau35 mice is accompanied by hyperpolarization-induced membrane voltage "sag" demonstrating a detrimental effect of tau abnormalities on HCN channel function. Tau35 expression alters synaptic organization, causing a loosened vesicle clustering phenotype in Tau35 mice.

The Lack of Synapsin Alters Presynaptic Plasticity at Hippocampal Mossy Fibers in Male Mice

Synapsins are highly abundant presynaptic proteins that play a crucial role in neurotransmission and plasticity via the clustering of synaptic vesicles. The synapsin III isoform is usually downregulated after development, but in hippocampal mossy fiber boutons, it persists in adulthood. Mossy fiber boutons express presynaptic forms of short- and long-term plasticity, which are thought to underlie different forms of learning. Previous research on synapsins at this synapse focused on synapsin isoforms I and II. Thus, a complete picture regarding the role of synapsins in mossy fiber plasticity is still missing. Here, we investigated presynaptic plasticity at hippocampal mossy fiber boutons by combining electrophysiological field recordings and transmission electron microscopy in a mouse model lacking all synapsin isoforms. We found decreased short-term plasticity, i.e., decreased facilitation and post-tetanic potentiation, but increased long-term potentiation in male synapsin triple knock-out (KO) mice. At the ultrastructural level, we observed more dispersed vesicles and a higher density of active zones in mossy fiber boutons from KO animals. Our results indicate that all synapsin isoforms are required for fine regulation of short- and long-term presynaptic plasticity at the mossy fiber synapse.

Actin-nucleation promoting factor N-WASP influences alpha-synuclein condensates and pathology

Abnormal intraneuronal accumulation of soluble and insoluble α-synuclein (α-Syn) is one of the main pathological hallmarks of synucleinopathies, such as Parkinson's disease (PD). It has been well documented that the reversible liquid-liquid phase separation of α-Syn can modulate synaptic vesicle condensates at the presynaptic terminals. However, α-Syn can also form liquid-like droplets that may convert into amyloid-enriched hydrogels or fibrillar polymorphs under stressful conditions. To advance our understanding on the mechanisms underlying α-Syn phase transition, we employed a series of unbiased proteomic analyses and found that actin and actin regulators are part of the α-Syn interactome. We focused on Neural Wiskott-Aldrich syndrome protein (N-WASP) because of its association with a rare early-onset familial form of PD. In cultured cells, we demonstrate that N-WASP undergoes phase separation and can be recruited to synapsin 1 liquid-like droplets, whereas it is excluded from α-Syn/synapsin 1 condensates. Consistently, we provide evidence that wsp-1/WASL loss of function alters the number and dynamics of α-Syn inclusions in the nematode Caenorhabditis elegans. Together, our findings indicate that N-WASP expression may create permissive conditions that promote α-Syn condensates and their potentially deleterious conversion into toxic species.

RANBP17 Overexpression Restores Nucleocytoplasmic Transport and Ameliorates Neurodevelopment in Induced DYT1 Dystonia Motor Neurons

DYT1 dystonia is a debilitating neurological movement disorder, and it represents the most frequent and severe form of hereditary primary dystonia. There is currently no cure for this disease due to its unclear pathogenesis. In our previous study utilizing patient-specific motor neurons (MNs), we identified distinct cellular deficits associated with the disease, including a deformed nucleus, disrupted neurodevelopment, and compromised nucleocytoplasmic transport (NCT) functions. However, the precise molecular mechanisms underlying these cellular impairments have remained elusive. In this study, we revealed the genome-wide changes in gene expression in DYT1 MNs through transcriptomic analysis. We found that those dysregulated genes are intricately involved in neurodevelopment and various biological processes. Interestingly, we identified that the expression level of RANBP17, a RAN-binding protein crucial for NCT regulation, exhibited a significant reduction in DYT1 MNs. By manipulating RANBP17 expression, we further demonstrated that RANBP17 plays an important role in facilitating the nuclear transport of both protein and transcript cargos in induced human neurons. Excitingly, the overexpression of RANBP17 emerged as a substantial mitigating factor, effectively restoring impaired NCT activity and rescuing neurodevelopmental deficits observed in DYT1 MNs. These findings shed light on the intricate molecular underpinnings of impaired NCT in DYT1 neurons and provide novel insights into the pathophysiology of DYT1 dystonia, potentially leading to the development of innovative treatment strategies.

Molecular Organization and Regulation of the Mammalian Synapse by the Post-Translational Modification SUMOylation

Neurotransmission occurs within highly specialized compartments forming the active synapse where the complex organization and dynamics of the interactions are tightly orchestrated both in time and space. Post-translational modifications (PTMs) are central to these spatiotemporal regulations to ensure an efficient synaptic transmission. SUMOylation is a dynamic PTM that modulates the interactions between proteins and consequently regulates the conformation, the distribution and the trafficking of the SUMO-target proteins. SUMOylation plays a crucial role in synapse formation and stabilization, as well as in the regulation of synaptic transmission and plasticity. In this review, we summarize the molecular consequences of this protein modification in the structural organization and function of the mammalian synapse. We also outline novel activity-dependent regulation and consequences of the SUMO process and explore how this protein modification can functionally participate in the compartmentalization of both pre- and post-synaptic sites.

Vesicle condensation induced by synapsin: condensate size, geometry, and vesicle shape deformations

We study the formation of vesicle condensates induced by the protein synapsin, as a cell-free model system mimicking vesicle pool formation in the synapse. The system can be considered as an example of liquid-liquid phase separation (LLPS) in biomolecular fluids, where one phase is a complex fluid itself consisting of vesicles and a protein network. We address the pertinent question why the LLPS is self-limiting and stops at a certain size, i.e., why macroscopic phase separation is prevented. Using fluorescence light microscopy, we observe different morphologies of the condensates (aggregates) depending on the protein-to-lipid ratio. Cryogenic electron microscopy then allows us to resolve individual vesicle positions and shapes in a condensate and notably the size and geometry of adhesion zones between vesicles. We hypothesize that the membrane tension induced by already formed adhesion zones then in turn limits the capability of vesicles to bind additional vesicles, resulting in a finite condensate size. In a simple numerical toy model we show that this effect can be accounted for by redistribution of effective binding particles on the vesicle surface, accounting for the synapsin-induced adhesion zone.

Defining a Lewy Body: Running Up the Hill of Shifting Definitions and Evolving Concepts

Lewy bodies (LBs) are pathological hallmarks of Parkinson's disease and dementia with Lewy bodies, characterized by the accumulation of α-synuclein (αSyn) protein in the brain. While LBs were first described a century ago, their formation and morphogenesis mechanisms remain incompletely understood. Here, we present a historical overview of LB definitions and highlight the importance of semantic clarity and precise definitions when describing brain inclusions. Recent breakthroughs in imaging revealed shared features within LB subsets and the enrichment of membrane-bound organelles in these structures, challenging the conventional LB formation model. We discuss the involvement of emerging concepts of liquid-liquid phase separation, where biomolecules demix from a solution to form dense condensates, as a potential LB formation mechanism. Finally, we emphasize the need for the operational definitions of LBs based on morphological characteristics and detection protocols, particularly in studies investigating LB formation mechanisms. A better understanding of LB organization and ultrastructure can contribute to the development of targeted therapeutic strategies for synucleinopathies.

Oxysterols in Central and Peripheral Synaptic Communication

Cholesterol is a key molecule for synaptic transmission, and both central and peripheral synapses are cholesterol rich. During intense neuronal activity, a substantial portion of synaptic cholesterol can be oxidized by either enzymatic or non-enzymatic pathways to form oxysterols, which in turn modulate the activities of neurotransmitter receptors (e.g., NMDA and adrenergic receptors), signaling molecules (nitric oxide synthases, protein kinase C, liver X receptors), and synaptic vesicle cycling involved in neurotransmitters release. 24-Hydroxycholesterol, produced by neurons in the brain, could directly affect neighboring synapses and change neurotransmission. 27-Hydroxycholesterol, which can cross the blood-brain barrier, can alter both synaptogenesis and synaptic plasticity. Increased generation of 25-hydroxycholesterol by activated microglia and macrophages could link inflammatory processes to learning and neuronal regulation. Amyloids and oxidative stress can lead to an increase in the levels of ring-oxidized sterols and some of these oxysterols (4-cholesten-3-one, 5α-cholestan-3-one, 7β-hydroxycholesterol, 7-ketocholesterol) have a high potency to disturb or modulate neurotransmission at both the presynaptic and postsynaptic levels. Overall, oxysterols could be used as "molecular prototypes" for therapeutic approaches. Analogs of 24-hydroxycholesterol (SGE-301, SGE-550, SAGE718) can be used for correction of NMDA receptor hypofunction-related states, whereas inhibitors of cholesterol 24-hydroxylase, cholestane-3β,5α,6β-triol, and cholest-4-en-3-one oxime (olesoxime) can be utilized as potential anti-epileptic drugs and (or) protectors from excitotoxicity.

Icariin rescues developmental BPA exposure induced spatial memory deficits in rats

Bisphenol A (BPA) has been implicated in cognitive impairment. Icariin is the main active ingredient extracted from Epimedium Herb with protective function of nervous system. However, the potential therapeutic effects of Icariin on spatial memory deficits induced by developmental BPA exposure in Sprague-Dawley rats have not been investigated. This study investigated the therapeutic effect of Icariin (10 mg/kg/day, from postnatal day (PND) 21 to PND 60 by gavage) on spatial memory deficits in rat induced by developmental BPA exposure (1 mg/kg/day, from embryonic to PND 60), demonstrating that Icariin can markedly improve spatial memory in BPA-exposed rat. Furthermore, intra-gastric administration of Icariin could attenuate abnormal hippocampal cell dispersion and loss, improved the dendritic spine density and Nissl bodies. Moreover, Icariin reversed BPA induced reduction of frequency of miniature excitatory postsynaptic currents(mEPSC) and decrease of Vesicular glutamate transporter 1(VGlut1). Collectively, Icariin could effectively rescue BPA-induced spatial memory impairment in male rats by preventing cell loss and reduction of dendritic spines in the hippocampus. In addition, we also found that VGlut1 is a critical target in the repair of BPA-induced spatial memory by Icariin. Thus, Icariin may be a promising therapeutic agent to attenuate BPA-induced spatial memory deficits.

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.

Electric Potential at the Interface of Membraneless Organelles Gauged by Graphene

Eukaryotic cells contain membrane-bound and membrane-less organelles that are often in contact with each other. How the interface properties of membrane-less organelles regulate their interactions with membranes remains challenging to assess. Here, we employ graphene-based sensors to investigate the electrostatic properties of synapsin 1, a major synaptic phosphoprotein, either in a single phase or after undergoing phase separation to form synapsin condensates. Using these graphene-based sensors, we discover that synapsin condensates generate strong electrical responses that are otherwise absent when synapsin is present as a single phase. By introducing atomically thin dielectric barriers, we show that the electrical response originates in an electric double layer whose formation governs the interaction between synapsin condensates and graphene. Our data indicate that the interface properties of the same protein are substantially different when the protein is in a single phase versus within a biomolecular condensate, unraveling that condensates can harbor ion potential differences at their interface.

Lipid vesicle pools studied by passive X-ray microrheology

Vesicle pools can form by attractive interaction in a solution, mediated by proteins or divalent ions such as calcium. The pools, which are alternatively also denoted as vesicle clusters, form by liquid-liquid phase separation (LLPS) from an initially homogeneous solution. Due to the short range liquid-like order of vesicles in the pool or cluster, the vesicle-rich phase can also be regarded as a condensate, and one would like to better understand not only the structure of these systems, but also their dynamics. The diffusion of vesicles, in particular, is expected to change when vesicles are arrested in a pool. Here we investigate whether passive microrheology based on X-ray photon correlation spectroscopy (XPCS) is a suitable tool to study model systems of artificial lipid vesicles exhibiting LLPS, and more generally also other heterogeneous biomolecular fluids. We show that by adding highly scattering tracer particles to the solution, valuable information on the single vesicle as well as collective dynamics can be inferred. While the correlation functions reveal freely diffusing tracer particles in solutions at low CaCl[Formula: see text] concentrations, the relaxation rate [Formula: see text] shows a nonlinear dependence on [Formula: see text] at a higher concentration of around 8 mM CaCl[Formula: see text], characterised by two linear regimes with a broad cross-over. We explain this finding based on arrested diffusion in percolating vesicle clusters.

Protein condensates as flexible platforms for membrane traffic

With an essential role in nearly every physiological process and disease state, trafficking vesicles are fundamental to cell biology. Canonical understanding of membrane traffic has been driven by key achievements in structural biology. Nonetheless, discoveries over the past few years progressively point to the critical role of intrinsically disordered domains and proteins, which lack a well-defined secondary structure. From the initiation of endocytosis and the sequestration of synaptic vesicles to the stabilization of endoplasmic reticulum exit sites and the extension of the autophagic cup, flexible protein condensates, rich in intrinsic disorder, are increasingly implicated. While important debates about the physical nature and mechanistic interpretation of these findings remain, the significance of transient, multivalent protein assemblies in membrane traffic is increasingly clear.

Tau forms synaptic nano-biomolecular condensates controlling the dynamic clustering of recycling synaptic vesicles

Neuronal communication relies on the release of neurotransmitters from various populations of synaptic vesicles. Despite displaying vastly different release probabilities and mobilities, the reserve and recycling pool of vesicles co-exist within a single cluster suggesting that small synaptic biomolecular condensates could regulate their nanoscale distribution. Here, we performed a large-scale activity-dependent phosphoproteome analysis of hippocampal neurons in vitro and identified Tau as a highly phosphorylated and disordered candidate protein. Single-molecule super-resolution microscopy revealed that Tau undergoes liquid-liquid phase separation to generate presynaptic nanoclusters whose density and number are regulated by activity. This activity-dependent diffusion process allows Tau to translocate into the presynapse where it forms biomolecular condensates, to selectively control the mobility of recycling vesicles. Tau, therefore, forms presynaptic nano-biomolecular condensates that regulate the nanoscale organization of synaptic vesicles in an activity-dependent manner.

Synthetic Membraneless Droplets for Synaptic-Like Clustering of Lipid Vesicles

In eukaryotic cells, the membraneless organelles (MLOs) formed via liquid-liquid phase separation (LLPS) are found to interact intimately with membranous organelles (MOs). One major mode is the clustering of MOs by MLOs, such as the formation of clusters of synaptic vesicles at nerve terminals mediated by the synapsin-rich MLOs. Aqueous droplets, including complex coacervates and aqueous two-phase systems, have been plausible MLO-mimics to emulate or elucidate biological processes. However, neither of them can cluster lipid vesicles (LVs) like MLOs. In this work, we develop a synthetic droplet assembled from a combination of two different interactions underlying the formation of these two droplets, namely, associative and segregative interactions, which we call segregative-associative (SA) droplets. The SA droplets cluster and disperse LVs recapitulating the key functional features of synapsin condensates, which can be attributed to the weak electrostatic interaction environment provided by SA droplets. This work suggests LLPS with combined segregative and associative interactions as a possible route for synaptic clustering of lipid vesicles and highlights SA droplets as plausible MLO-mimics and models for studying and mimicking related cellular dynamics.

Orchestrating vesicular and nonvesicular membrane dynamics by intrinsically disordered proteins

Compartmentalization by membranes is a common feature of eukaryotic cells and serves to spatiotemporally confine biochemical reactions to control physiology. Membrane-bound organelles such as the endoplasmic reticulum (ER), the Golgi complex, endosomes and lysosomes, and the plasma membrane, continuously exchange material via vesicular carriers. In addition to vesicular trafficking entailing budding, fission, and fusion processes, organelles can form membrane contact sites (MCSs) that enable the nonvesicular exchange of lipids, ions, and metabolites, or the secretion of neurotransmitters via subsequent membrane fusion. Recent data suggest that biomolecule and information transfer via vesicular carriers and via MCSs share common organizational principles and are often mediated by proteins with intrinsically disordered regions (IDRs). Intrinsically disordered proteins (IDPs) can assemble via low-affinity, multivalent interactions to facilitate membrane tethering, deformation, fission, or fusion. Here, we review our current understanding of how IDPs drive the formation of multivalent protein assemblies and protein condensates to orchestrate vesicular and nonvesicular transport with a special focus on presynaptic neurotransmission. We further discuss how dysfunction of IDPs causes disease and outline perspectives for future research.

Synapsin autoantibodies during pregnancy are associated with fetal abnormalities

Anti-neuronal autoantibodies can be transplacentally transferred during pregnancy and may cause detrimental effects on fetal development. It is unclear whether autoantibodies against synapsin-I, one of the most abundant synaptic proteins, are associated with developmental abnormalities in humans. We recruited a cohort of 263 pregnant women and detected serum synapsin-I IgG autoantibodies in 13.3% using cell-based assays. Seropositivity was strongly associated with abnormalities of fetal development including structural defects, intrauterine growth retardation, amniotic fluid disorders and neuropsychiatric developmental diseases in previous children (odds ratios of 3-6.5). Autoantibodies reached the fetal circulation and were mainly of IgG1/IgG3 subclasses. They bound to conformational and linear synapsin-I epitopes, five distinct epitopes were identified using peptide microarrays. The findings indicate that synapsin-I autoantibodies may be clinically useful biomarkers or even directly participate in the disease process of neurodevelopmental disorders, thus being potentially amenable to antibody-targeting interventional strategies in the future.

Synapsin condensation controls synaptic vesicle sequestering and dynamics

Neuronal transmission relies on the regulated secretion of neurotransmitters, which are packed in synaptic vesicles (SVs). Hundreds of SVs accumulate at synaptic boutons. Despite being held together, SVs are highly mobile, so that they can be recruited to the plasma membrane for their rapid release during neuronal activity. However, how such confinement of SVs corroborates with their motility remains unclear. To bridge this gap, we employ ultrafast single-molecule tracking (SMT) in the reconstituted system of native SVs and in living neurons. SVs and synapsin 1, the most highly abundant synaptic protein, form condensates with liquid-like properties. In these condensates, synapsin 1 movement is slowed in both at short (i.e., 60-nm) and long (i.e., several hundred-nm) ranges, suggesting that the SV-synapsin 1 interaction raises the overall packing of the condensate. Furthermore, two-color SMT and super-resolution imaging in living axons demonstrate that synapsin 1 drives the accumulation of SVs in boutons. Even the short intrinsically-disordered fragment of synapsin 1 was sufficient to restore the native SV motility pattern in synapsin triple knock-out animals. Thus, synapsin 1 condensation is sufficient to guarantee reliable confinement and motility of SVs, allowing for the formation of mesoscale domains of SVs at synapses in vivo.

Protein nanocondensates: the next frontier

Over the past decade, myriads of studies have highlighted the central role of protein condensation in subcellular compartmentalization and spatiotemporal organization of biological processes. Conceptually, protein condensation stands at the highest level in protein structure hierarchy, accounting for the assembly of bodies ranging from thousands to billions of molecules and for densities ranging from dense liquids to solid materials. In size, protein condensates range from nanocondensates of hundreds of nanometers (mesoscopic clusters) to phase-separated micron-sized condensates. In this review, we focus on protein nanocondensation, a process that can occur in subsaturated solutions and can nucleate dense liquid phases, crystals, amorphous aggregates, and fibers. We discuss the nanocondensation of proteins in the light of general physical principles and examine the biophysical properties of several outstanding examples of nanocondensation. We conclude that protein nanocondensation cannot be fully explained by the conceptual framework of micron-scale biomolecular condensation. The evolution of nanocondensates through changes in density and order is currently under intense investigation, and this should lead to the development of a general theoretical framework, capable of encompassing the full range of sizes and densities found in protein condensates.

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

Synaptic vesicle (SV) clusters, which reportedly result from synapsin's capacity to undergo liquid-liquid phase separation (LLPS), constitute the structural basis for neurotransmission. Although these clusters contain various endocytic accessory proteins, how endocytic proteins accumulate in SV clusters remains unknown. Here, we report that endophilin A1 (EndoA1), the endocytic scaffold protein, undergoes LLPS under physiologically relevant concentrations at presynaptic terminals. On heterologous expression, EndoA1 facilitates the formation of synapsin condensates and accumulates in SV-like vesicle clusters via synapsin. Moreover, EndoA1 condensates recruit endocytic proteins such as dynamin 1, amphiphysin, and intersectin 1, none of which are recruited in vesicle clusters by synapsin. In cultured neurons, like synapsin, EndoA1 is compartmentalized in SV clusters through LLPS, exhibiting activity-dependent dispersion/reassembly cycles. Thus, beyond its essential function in SV endocytosis, EndoA1 serves an additional structural function by undergoing LLPS, thereby accumulating various endocytic proteins in dynamic SV clusters in concert with synapsin.

Role of SNAREs and Rabs in Myelin Regulation

The myelin sheath is an insulating layer around the nerves of the brain and spinal cord which allows a fast and efficient nerve conduction. Myelin is made of protein and fatty substances and gives protection for the propagation of the electrical impulse. The myelin sheath is formed by oligodendrocytes in the central nervous system (CNS) and by Schwann cells in the peripheral nervous system (PNS). The myelin sheath presents a highly organized structure and expands both radially and longitudinally, but in a different way and with a different composition. Myelin alterations determine the onset of several neuropathies, as the electrical signal can be slowed or stopped. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and ras (rat sarcoma)-associated binding proteins (rabs) have been proved to contribute to several aspects regarding the formation of myelin or dysmyelination. Here, I will describe the role of these proteins in regulating membrane trafficking and nerve conduction, myelin biogenesis and maintenance.

Early Alterations in Structural and Functional Properties in the Neuromuscular Junctions of Mutant FUS Mice

Amyotrophic lateral sclerosis (ALS) is manifested as skeletal muscle denervation, loss of motor neurons and finally severe respiratory failure. Mutations of RNA-binding protein FUS are one of the common genetic reasons of ALS accompanied by a 'dying back' type of degeneration. Using fluorescent approaches and microelectrode recordings, the early structural and functional alterations in diaphragm neuromuscular junctions (NMJs) were studied in mutant FUS mice at the pre-onset stage. Lipid peroxidation and decreased staining with a lipid raft marker were found in the mutant mice. Despite the preservation of the end-plate structure, immunolabeling revealed an increase in levels of presynaptic proteins, SNAP-25 and synapsin 1. The latter can restrain Ca2+-dependent synaptic vesicle mobilization. Indeed, neurotransmitter release upon intense nerve stimulation and its recovery after tetanus and compensatory synaptic vesicle endocytosis were markedly depressed in FUS mice. There was a trend to attenuation of axonal [Ca2+]in increase upon nerve stimulation at 20 Hz. However, no changes in neurotransmitter release and the intraterminal Ca2+ transient in response to low frequency stimulation or in quantal content and the synchrony of neurotransmitter release at low levels of external Ca2+ were detected. At a later stage, shrinking and fragmentation of end plates together with a decrease in presynaptic protein expression and disturbance of the neurotransmitter release timing occurred. Overall, suppression of synaptic vesicle exo-endocytosis upon intense activity probably due to alterations in membrane properties, synapsin 1 levels and Ca2+ kinetics could be an early sign of nascent NMJ pathology, which leads to neuromuscular contact disorganization.