Overlapping role of dynamin isoforms in synaptic vesicle endocytosis

Persistence of quantal synaptic vesicle recycling in virtual absence of dynamins

Dynamins are GTPases required for pinching vesicles off the plasma membrane once a critical curvature is reached during endocytosis. Here, we probed dynamin function in central synapses by depleting all three dynamin isoforms in postnatal hippocampal neurons down to negligible levels. We found a decrease in the propensity of evoked neurotransmission as well as a reduction in synaptic vesicle numbers. Recycling of synaptic vesicles during spontaneous or low levels of evoked activity were largely impervious to dynamin depletion, while retrieval of synaptic vesicle components at higher levels of activity was partially arrested. These results suggest the existence of balancing dynamin-independent mechanisms for synaptic vesicle recycling at central synapses. Classical dynamin-dependent mechanisms are not essential for retrieval of synaptic vesicle proteins after quantal single synaptic vesicle fusion, but they become more relevant for membrane retrieval during intense, sustained neuronal activity. KEY POINTS: Loss of dynamin 2 does not impair synaptic transmission. Loss of all three dynamin isoforms mostly affects evoked neurotransmission. Excitatory synapse function is more susceptible to dynamin loss. Spontaneous neurotransmission is only mildly affected by loss of dynamins. Single synaptic vesicle endocytosis is largely dynamin independent.

The host GTPase Dynamin 2 modulates apical junction structure to control cell-to-cell spread of Listeria monocytogenes

The food-borne pathogen Listeria monocytogenes uses actin-based motility to generate plasma membrane protrusions that mediate the spread of bacteria between host cells. In polarized epithelial cells, efficient protrusion formation by L. monocytogenes requires the secreted bacterial protein InlC, which binds to a carboxyl-terminal Src homology 3 (SH3) domain in the human scaffolding protein Tuba. This interaction antagonizes Tuba, thereby diminishing cortical tension at the apical junctional complex and enhancing L. monocytogenes protrusion formation and spread. Tuba contains five SH3 domains apart from the domain that interacts with InlC. Here, we show that human GTPase Dynamin 2 associates with two SH3 domains in the amino-terminus of Tuba and acts together with this scaffolding protein to control the spread of L. monocytogenes. Genetic or pharmacological inhibition of Dynamin 2 or knockdown of Tuba each restored normal protrusion formation and spread to a bacterial strain deleted for the inlC gene (∆inlC). Dynamin 2 localized to apical junctions in uninfected human cells and protrusions in cells infected with L. monocytogenes. Localization of Dynamin 2 to junctions and protrusions depended on Tuba. Knockdown of Dynamin 2 or Tuba diminished junctional linearity, indicating a role for these proteins in controlling cortical tension. Infection with L. monocytogenes induced InlC-dependent displacement of Dynamin 2 from junctions, suggesting a possible mechanism of antagonism of this GTPase. Collectively, our results show that Dynamin 2 cooperates with Tuba to promote intercellular tension that restricts the spread of ∆inlC Listeria. By expressing InlC, wild-type L. monocytogenes overcomes this restriction.

Effective knockdown-replace gene therapy in a novel mouse model of DNM1 developmental and epileptic encephalopathy

Effective gene therapy for gain-of-function or dominant-negative disease mutations may require eliminating expression of the mutant copy together with wild-type replacement. We evaluated such a knockdown-replace strategy in a mouse model of DNM1 disease, a debilitating and intractable neurodevelopmental epilepsy. To challenge the approach robustly, we expressed a patient-based variant in GABAergic neurons-which resulted in growth delay and lethal seizures evident by postnatal week three-and delivered to newborn pups an AAV9-based vector encoding a ubiquitously expressed, Dnm1-specific interfering RNA (RNAi) bivalently in tail-to-tail configuration with a neuron-specific, RNAi-resistant, codon-optimized Dnm1 cDNA. Pups receiving RNAi or cDNA alone fared no better than untreated pups, whereas the vast majority of mutants receiving modest doses survived with almost full growth recovery. Synaptic recordings of cortical neurons derived from treated pups revealed that significant alterations in transmission from inhibitory to excitatory neurons were rectified by bivalent vector application. To examine the mutant transcriptome and impact of treatment, we used RNA sequencing and functional annotation clustering. Mutants displayed abnormal expression of more than 1,000 genes in highly significant and relevant functional clusters, clusters that were abrogated by treatment. Together these results suggest knockdown-replace as a potentially effective strategy for treating DNM1 and related genetic neurodevelopmental disease.

Dynamin 1xA interacts with Endophilin A1 via its spliced long C-terminus for ultrafast endocytosis

Dynamin 1 mediates fission of endocytic synaptic vesicles in the brain and has two major splice variants, Dyn1xA and Dyn1xB, which are nearly identical apart from the extended C-terminal region of Dyn1xA. Despite a similar set of binding partners, only Dyn1xA is enriched at endocytic zones and accelerates vesicle fission during ultrafast endocytosis. Here, we report that Dyn1xA achieves this localization by preferentially binding to Endophilin A1 through a newly defined binding site within its long C-terminal tail extension. Endophilin A1 binds this site at higher affinity than the previously reported site, and the affinity is determined by amino acids within the Dyn1xA tail but outside the binding site. This interaction is regulated by the phosphorylation state of two serine residues specific to the Dyn1xA variant. Dyn1xA and Endophilin A1 colocalize in patches near the active zone, and mutations disrupting Endophilin A binding to the long tail cause Dyn1xA mislocalization and stalled endocytic pits on the plasma membrane during ultrafast endocytosis. Together, these data suggest that the specificity for ultrafast endocytosis is defined by the phosphorylation-regulated interaction of Endophilin A1 with the C-terminal extension of Dyn1xA.

Membrane remodeling by FAM92A1 during brain development regulates neuronal morphology, synaptic function, and cognition

The Bin/Amphiphysin/Rvs (BAR) domain protein FAM92A1 is a multifunctional protein engaged in regulating mitochondrial ultrastructure and ciliogenesis, but its physiological role in the brain remains unclear. Here, we show that FAM92A1 is expressed in neurons starting from embryonic development. FAM92A1 knockout in mice results in altered brain morphology and age-associated cognitive deficits, potentially due to neuronal degeneration and disrupted synaptic plasticity. Specifically, FAM92A1 deficiency impairs diverse neuronal membrane morphology, including the mitochondrial inner membrane, myelin sheath, and synapses, indicating its roles in membrane remodeling and maintenance. By determining the crystal structure of the FAM92A1 BAR domain, combined with atomistic molecular dynamics simulations, we uncover that FAM92A1 interacts with phosphoinositide- and cardiolipin-containing membranes to induce lipid-clustering and membrane curvature. Altogether, these findings reveal the physiological role of FAM92A1 in the brain, highlighting its impact on synaptic plasticity and neural function through the regulation of membrane remodeling and endocytic processes.

Role of adaptor protein complexes in generating functionally distinct synaptic vesicle pools

The synaptic vesicle (SV) cycle ensures the release of neurotransmitters and the replenishment of SVs to sustain neuronal activity. Multiple endocytosis and sorting pathways contribute to the recapture of the SV membrane and proteins after fusion. Adaptor protein (AP) complexes are among the critical components of the SV retrieval machinery. The canonical clathrin adaptor AP2 ensures the replenishment of most SVs across many neuronal populations. An alternative AP1/AP3-dependent process mediates the formation of a subset of SVs that differ from AP2 vesicles in molecular composition and respond preferentially during higher frequency firing. Furthermore, recent studies show that vesicular transporters for different neurotransmitters depend to a different extent on the AP3 pathway and this affects the release properties of the respective neurotransmitters. This review focuses on the current understanding of the AP-dependent molecular and functional diversity among SVs. We also discuss the contribution of these pathways to the regulation of neurotransmitter release across neuronal populations.

Epigenetic and transcriptional control of adipocyte function by centenarian-associated SIRT6 N308K/A313S mutant

BACKGROUND

Obesity is a major health burden. Preadipocytes proliferate and differentiate in mature adipocytes in the adipogenic process, which could be a potential therapeutic approach for obesity. Deficiency of SIRT6, a stress-responsive protein deacetylase and mono-ADP ribosyltransferase enzyme, blocks adipogenesis. Mutants of SIRT6 (N308K/A313S) were recently linked to the in the long lifespan Ashkenazi Jews. In this study, we aimed to clarify how these new centenarian-associated SIRT6 genetic variants affect adipogenesis at the transcriptional and epigenetic level.

METHODS

We analyzed the role of SIRT6 wild-type (WT) or SIRT6 centenarian-associated mutant (N308K/A313S) overexpression in adipogenesis, by creating stably transduced preadipocyte cell lines using lentivirus on the 3T3-L1 model. Histone post-translational modifications (PTM: acetylation, methylation) and transcriptomic changes were analyzed by mass spectrometry (LC-MS/MS) and RNA-Seq, respectively, in 3T3-L1 adipocytes. In addition, the adipogenic process and related signaling pathways were investigated by bioinformatics and biochemical approaches.

RESULTS

Overexpression of centenarian-associated SIRT6 mutant increased adipogenic differentiation to a similar extent compared to the WT form. However, it triggered distinct histone PTM profiles in mature adipocytes, with significantly higher acetylation levels, and activated divergent transcriptional programs, including those dependent on signaling related to the sympathetic innervation and to PI3K pathway. 3T3-L1 mature adipocytes overexpressing SIRT6 N308K/A313S displayed increased insulin sensitivity in a neuropeptide Y (NPY)-dependent manner.

CONCLUSIONS

SIRT6 N308K/A313S overexpression in mature adipocytes ameliorated glucose sensitivity and impacted sympathetic innervation signaling. These findings highlight the importance of targeting SIRT6 enzymatic activities to regulate the co-morbidities associated with obesity.

The Structural and Functional Integrity of Rod Photoreceptor Ribbon Synapses Depends on Redundant Actions of Dynamins 1 and 3

Vertebrate vision begins with light absorption by rod and cone photoreceptors, which transmit signals from their synaptic terminals to second-order neurons: bipolar and horizontal cells. In mouse rods, there is a single presynaptic ribbon-type active zone at which the release of glutamate occurs tonically in the dark. This tonic glutamatergic signaling requires continuous exo- and endocytosis of synaptic vesicles. At conventional synapses, endocytosis commonly requires dynamins: GTPases encoded by three genes (Dnm1-3), which perform membrane scission. Disrupting endocytosis by dynamin deletions impairs transmission at conventional synapses, but the impact of disrupting endocytosis and the role(s) of specific dynamin isoforms at rod ribbon synapses are understood incompletely. Here, we used cell-specific knock-outs (KOs) of the neuron-specific Dnm1 and Dnm3 to investigate the functional roles of dynamin isoforms in rod photoreceptors in mice of either sex. Analysis of synaptic protein expression, synapse ultrastructure, and retinal function via electroretinograms (ERGs) showed that dynamins 1 and 3 act redundantly and are essential for supporting the structural and functional integrity of rod ribbon synapses. Single Dnm3 KO showed no phenotype, and single Dnm1 KO only modestly reduced synaptic vesicle density without affecting vesicle size and overall synapse integrity, whereas double Dnm1/Dnm3 KO impaired vesicle endocytosis profoundly, causing enlarged vesicles, reduced vesicle density, reduced ERG responses, synaptic terminal degeneration, and disassembly and degeneration of postsynaptic processes. Concurrently, cone function remained intact. These results show the fundamental redundancy of dynamins 1 and 3 in regulating the structure and function of rod ribbon synapses.

Identification of a Novel Homozygous Mutation in MTMR2 Gene Causes Very Rare Charcot-Marie-Tooth Disease Type 4B1

Background

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders involving peripheral nervous system. Charcot-Marie-Tooth disease 4B1 (CMT4B1) is a rare subtype of CMT. CMT4B1 is an axonal demyelinating polyneuropathy with an autosomal recessive mode of inheritance. Patients with CMT4B1 usually manifested with dysfunction of the motor and sensory systems which leads to gradual and progressive muscular weakness and atrophy, starting from the peroneal muscles and finally affecting the distal muscles. Germline mutations in MTMR2 gene causes CMT4B1.

Material and Methods

In this study, we investigated a 4-year-old Chinese boy with gradual and progressive weakness and atrophy of both proximal and distal muscles. The proband's parents did not show any abnormalities. Whole-exome sequencing and Sanger sequencing were performed.

Results

Whole-exome sequencing identified a novel homozygous nonsense mutation (c.118A>T; p.Lys40*) in exon 2 of MTMR2 gene in the proband. This novel mutation leads to the formation of a truncated MTMR2 protein of 39 amino acids instead of the wild- type MTMR2 protein of 643 amino acids. This mutation is predicted to cause the complete loss of the PH-GRAM domain, phosphatase domain, coiled-coil domain, and PDZ-binding motif of the MTMR2 protein. Sanger sequencing revealed that the proband's parents carried the mutation in a heterozygous state. This mutation was absent in 100 healthy control individuals.

Conclusion

This study reports the first mutation in MTMR2 associated with CMT4B1 in a Chinese population. Our study also showed the importance of whole-exome sequencing in identifying candidate genes and disease-causing variants in patients with CMT4B1.

Rho GTPase signaling and mDia facilitate endocytosis via presynaptic actin

Neurotransmission at synapses is mediated by the fusion and subsequent endocytosis of synaptic vesicle membranes. Actin has been suggested to be required for presynaptic endocytosis but the mechanisms that control actin polymerization and its mode of action within presynaptic nerve terminals remain poorly understood. We combine optical recordings of presynaptic membrane dynamics and ultrastructural analysis with genetic and pharmacological manipulations to demonstrate that presynaptic endocytosis is controlled by actin regulatory diaphanous-related formins mDia1/3 and Rho family GTPase signaling in mouse hippocampal neurons. We show that impaired presynaptic actin assembly in the near absence of mDia1/3 and reduced RhoA activity is partly compensated by hyperactivation of Rac1. Inhibition of Rac1 signaling further aggravates impaired presynaptic endocytosis elicited by loss of mDia1/3. Our data suggest that interdependent mDia1/3-Rho and Rac1 signaling pathways cooperatively act to facilitate synaptic vesicle endocytosis by controlling presynaptic F-actin.

Activity-Dependent Transcriptional Program in NGN2+ Neurons Enriched for Genetic Risk for Brain-Related Disorders

BACKGROUND

Converging evidence from large-scale genetic and postmortem studies highlights the role of aberrant neurotransmission and genetic regulation in brain-related disorders. However, identifying neuronal activity-regulated transcriptional programs in the human brain and understanding how changes contribute to disease remain challenging.

METHODS

To better understand how the activity-dependent regulome contributes to risk for brain-related disorders, we profiled the transcriptomic and epigenomic changes following neuronal depolarization in human induced pluripotent stem cell-derived glutamatergic neurons (NGN2) from 6 patients with schizophrenia and 5 control participants.

RESULTS

Multiomic data integration associated global patterns of chromatin accessibility with gene expression and identified enhancer-promoter interactions in glutamatergic neurons. Within 1 hour of potassium chloride-induced depolarization, independent of diagnosis, glutamatergic neurons displayed substantial activity-dependent changes in the expression of genes regulating synaptic function. Depolarization-induced changes in the regulome revealed significant heritability enrichment for schizophrenia and Parkinson's disease, adding to mounting evidence that sequence variation within activation-dependent regulatory elements contributes to the genetic risk for brain-related disorders. Gene coexpression network analysis elucidated interactions among activity-dependent and disease-associated genes and pointed to a key driver (NAV3) that interacted with multiple genes involved in axon guidance.

CONCLUSIONS

Overall, we demonstrated that deciphering the activity-dependent regulome in glutamatergic neurons reveals novel targets for advanced diagnosis and therapy.

Multiple machine-learning tools identifying prognostic biomarkers for acute Myeloid Leukemia

BACKGROUND

Acute Myeloid Leukemia (AML) generally has a relatively low survival rate after treatment. There is an urgent need to find new biomarkers that may improve the survival prognosis of patients. Machine-learning tools are more and more widely used in the screening of biomarkers.

METHODS

Least Absolute Shrinkage and Selection Operator (LASSO), Support Vector Machine-Recursive Feature Elimination (SVM-RFE), Random Forest (RF), eXtreme Gradient Boosting (XGBoost), lrFuncs, IdaProfile, caretFuncs, and nbFuncs models were used to screen key genes closely associated with AML. Then, based on the Cancer Genome Atlas (TCGA), pan-cancer analysis was performed to determine the correlation between important genes and AML or other cancers. Finally, the diagnostic value of important genes for AML was verified in different data sets.

RESULTS

The survival analysis results of the training set showed 26 genes with survival differences. After the intersection of the results of each machine learning method, DNM1, MEIS1, and SUSD3 were selected as key genes for subsequent analysis. The results of the pan-cancer analysis showed that MEIS1 and DNM1 were significantly highly expressed in AML; MEIS1 and SUSD3 are potential risk factors for the prognosis of AML, and DNM1 is a potential protective factor. Three key genes were significantly associated with AML immune subtypes and multiple immune checkpoints in AML. The results of the verification analysis show that DNM1, MEIS1, and SUSD3 have potential diagnostic value for AML.

CONCLUSION

Multiple machine learning methods identified DNM1, MEIS1, and SUSD3 can be regarded as prognostic biomarkers for AML.

Amyloid-Beta Peptides 40 and 42 Employ Distinct Molecular Pathways for Cell Entry and Intracellular Transit at the Blood-Brain Barrier Endothelium

The blood-brain barrier (BBB) plays a critical role in maintaining the equilibrium between amyloid beta (Aβ) levels in blood and the brain by regulating Aβ transport. Our previous publications demonstrated that BBB trafficking of Aβ42 and Aβ40 is distinct and is disrupted under various pathophysiological conditions. However, the intracellular mechanisms that allow BBB endothelium to differentially handle Aβ40 and Aβ42 have not been clearly elucidated. In this study, we identified mechanisms of Aβ endocytosis in polarized human cerebral microvascular endothelial cell monolayers. Our studies demonstrated that Aβ peptides with fluorescent label (F-Aβ) were internalized by BBB endothelial cells via energy, dynamin, and actin-dependent endocytosis. Interestingly, endocytosis of F-Aβ40 but not F-Aβ42 was substantially reduced by clathrin inhibition, whereas F-Aβ42 but not F-Aβ40 endocytosis was reduced by half after inhibiting the caveolae-mediated pathway. Following endocytosis, both isoforms were sorted by the endo-lysosomal system. Although Aβ42 was shown to accumulate more in the lysosomes, which could lead to its higher degradation and/or aggregation at lower lysosomal pH, Aβ40 demonstrated robust accumulation in recycling endosomes, which may facilitate its exocytosis by the endothelial cells. These results provide a mechanistic insight into the selective ability of BBB endothelium to transport Aβ40 versus Aβ42. This knowledge contributes to the understanding of molecular pathways underlying Aβ accumulation in the BBB endothelium and associated BBB dysfunction. Moreover, it allows us to establish mechanistic rationale for altered Aβ40:Aβ42 ratios and anomalous amyloid deposition in the cerebral vasculature as well as brain parenchyma during Alzheimer's disease progression. SIGNIFICANCE STATEMENT: Differential interaction of Aβ40 and Aβ42 isoforms with the blood-brain barrier (BBB) endothelium may contribute to perturbation in Aβ42:Aβ40 ratio, which is associated with Alzheimer's disease (AD) progression and severity. The current study identified distinct molecular pathways by which Aβ40 and Aβ42 are trafficked at the BBB, which regulates equilibrium between blood and brain Aβ levels. These findings provide molecular insights into mechanisms that engender BBB dysfunction and promote Aβ accumulation in AD brain.

Comprehensive analysis of sex differences in the function and ultrastructure of hippocampal presynaptic terminals

Sex differences in the brain, encompassing variations in specific brain structures, size, cognitive function, and synaptic connections, have been identified across numerous species. While previous research has explored sex differences in postsynaptic structures, synaptic plasticity, and hippocampus-dependent functions, the hippocampal presynaptic terminals remain largely uninvestigated. The hippocampus is a critical structure responsible for multiple brain functions. This study examined presynaptic differences in cultured hippocampal neurons derived from male and female mice using a combination of biochemical assays, functional analyses measuring exocytosis and endocytosis of synaptic vesicle proteins, ultrastructural analyses via electron microscopy, and presynaptic Ca2+-specific optical probes. Our findings revealed that female neurons exhibited a higher number of synaptic vesicles at presynaptic terminals compared to male neurons. However, no significant differences were observed in presynaptic protein expression, presynaptic terminal ultrastructure, synaptic vesicle exocytosis and endocytosis, or presynaptic Ca2+ alterations between male and female neurons.

Dynamin 1xA interacts with Endophilin A1 via its spliced long C-terminus for ultrafast endocytosis

Dynamin 1 (Dyn1) has two major splice variants, xA and xB, with unique C-terminal extensions of 20 and 7 amino acids, respectively. Of these, only Dyn1xA is enriched at endocytic zones and accelerates vesicle fission during ultrafast endocytosis. Here, we report that the long tail variant, Dyn1xA, achieves this localization by preferentially binding to Endophilin A through a newly defined Class II binding site overlapping with its extension, at a site spanning the splice boundary. Endophilin binds this site at higher affinity than the previously reported site, and this affinity is determined by amino acids outside the binding sites acting as long distance elements within the xA tail. Their interaction is regulated by the phosphorylation state of two serine residues specific to the xA variant. Dyn1xA and Endophilin colocalize in patches near the active zone of synapses. Mutations selectively disrupting Endophilin binding to the long extension cause Dyn1xA mislocalization along axons. In these mutants, endocytic pits are stalled on the plasma membrane during ultrafast endocytosis. These data suggest that the specificity for ultrafast endocytosis is defined by the phospho-regulated interaction of Endophilin A through a newly identified site of Dyn1xA's long tail.

CNS resilience in the progression of MS

Resilience; Progressive multiple sclerosis; Genomics.

A centronuclear myopathy-causing mutation in dynamin-2 disrupts neuronal morphology and excitatory synaptic transmission in a murine model of the disease

AIMS

Dynamin-2 is a large GTPase, a member of the dynamin superfamily that regulates membrane remodelling and cytoskeleton dynamics. Mutations in the dynamin-2 gene (DNM2) cause autosomal dominant centronuclear myopathy (CNM), a congenital neuromuscular disorder characterised by progressive weakness and atrophy of the skeletal muscles. Cognitive defects have been reported in some DNM2-linked CNM patients suggesting that these mutations can also affect the central nervous system (CNS). Here we studied how a dynamin-2 CNM-causing mutation influences the CNS function.

METHODS

Heterozygous mice harbouring the p.R465W mutation in the dynamin-2 gene (HTZ), the most common causing autosomal dominant CNM, were used as disease model. We evaluated dendritic arborisation and spine density in hippocampal cultured neurons, analysed excitatory synaptic transmission by electrophysiological field recordings in hippocampal slices, and evaluated cognitive function by performing behavioural tests.

RESULTS

HTZ hippocampal neurons exhibited reduced dendritic arborisation and lower spine density than WT neurons, which was reversed by transfecting an interference RNA against the dynamin-2 mutant allele. Additionally, HTZ mice showed defective hippocampal excitatory synaptic transmission and reduced recognition memory compared to the WT condition.

CONCLUSION

Our findings suggest that the dynamin-2 p.R465W mutation perturbs the synaptic and cognitive function in a CNM mouse model and support the idea that this GTPase plays a key role in regulating neuronal morphology and excitatory synaptic transmission in the hippocampus.

"Gearing" up for dynamin-catalyzed membrane fission

Endocytic dynamins self-assemble into helical scaffolds and utilize energy from GTP hydrolysis to constrict and sever tubular membranous necks of budded endocytic intermediates. They bind the membrane using a pleckstrin-homology domain (PHD). The PHD is characterized by four unstructured loops, two of which partially insert into the membrane. Recent studies reveal that loop insertion lowers the bending rigidity of the membrane and that mutations in these two loops produce separable and opposite effects on the efficiency of dynamin-catalyzed membrane fission. Here, we review the current understanding of dynamin-catalyzed membrane fission and attempt to reconcile contrasting notions that have emerged from biochemical and cellular studies evaluating the role of the PHD in this process. We propose that two membrane-inserting loops act as "gears" that define the catalytic efficiency of the dynamin helical scaffold in membrane fission.

The contribution of endocytosis to sensitization of nociceptors and synaptic transmission in nociceptive circuits

ABSTRACT

Chronic pain involves sensitization of nociceptors and synaptic transmission of painful signals in nociceptive circuits in the dorsal horn of the spinal cord. We investigated the contribution of clathrin-dependent endocytosis to sensitization of nociceptors by G protein-coupled receptors (GPCRs) and to synaptic transmission in spinal nociceptive circuits. We determined whether therapeutic targeting of endocytosis could ameliorate pain. mRNA encoding dynamin (Dnm) 1 to 3 and adaptor-associated protein kinase 1 (AAK1), which mediate clathrin-dependent endocytosis, were localized to primary sensory neurons of dorsal root ganglia of mouse and human and to spinal neurons in the dorsal horn of the mouse spinal cord by RNAScope. When injected intrathecally to mice, Dnm and AAK1 siRNA or shRNA knocked down Dnm and AAK1 mRNA in dorsal root ganglia neurons, reversed mechanical and thermal allodynia and hyperalgesia, and normalized nonevoked behavior in preclinical models of inflammatory and neuropathic pain. Intrathecally administered inhibitors of clathrin, Dnm, and AAK1 also reversed allodynia and hyperalgesia. Disruption of clathrin, Dnm, and AAK1 did not affect normal motor functions of behaviors. Patch clamp recordings of dorsal horn neurons revealed that Dnm1 and AAK1 disruption inhibited synaptic transmission between primary sensory neurons and neurons in lamina I/II of the spinal cord dorsal horn by suppressing release of synaptic vesicles from presynaptic primary afferent neurons. Patch clamp recordings from dorsal root ganglion nociceptors indicated that Dnm siRNA prevented sustained GPCR-mediated sensitization of nociceptors. By disrupting synaptic transmission in the spinal cord and blunting sensitization of nociceptors, endocytosis inhibitors offer a therapeutic approach for pain treatment.

Dynamins in human diseases: differential requirement of dynamin activity in distinct tissues

Dynamin, a 100-kDa GTPase, is one of the most-characterized membrane fission machineries catalyzing vesicle release from plasma membrane during endocytosis. The human genome encodes three dynamins: DNM1, DNM2 and DNM3, with high amino acid similarity but distinct expression patterns. Ever since the discoveries of dynamin mutations associated with human diseases in 2005, dynamin has become a paradigm for studying pathogenic mechanisms of mutant proteins from the aspects of structural biology, cell biology, model organisms as well as therapeutic strategy development. Here, we review the diseases and pathogenic mechanisms caused by mutations of DNM1 and DNM2, focusing on the activity requirement and regulation of dynamins in different tissues.

The readily retrievable pool of synaptic vesicles

In the CNS communication between neurons occurs at synapses by secretion of neurotransmitter via exocytosis of synaptic vesicles (SVs) at the active zone. Given the limited number of SVs in presynaptic boutons a fast and efficient recycling of exocytosed membrane and proteins by triggered compensatory endocytosis is required to maintain neurotransmission. Thus, pre-synapses feature a unique tight coupling of exo- and endocytosis in time and space resulting in the reformation of SVs with uniform morphology and well-defined molecular composition. This rapid response requires early stages of endocytosis at the peri-active zone to be well choreographed to ensure reformation of SVs with high fidelity. The pre-synapse can address this challenge by a specialized membrane microcompartment, where a pre-sorted and pre-assembled readily retrievable pool (RRetP) of endocytic membrane patches is formed, consisting of the vesicle cargo, presumably bound within a nucleated Clathrin and adaptor complex. This review considers evidence for the RRetP microcompartment to be the primary organizer of presynaptic triggered compensatory endocytosis.

Interactions between Membraneless Condensates and Membranous Organelles at the Presynapse: A Phase Separation View of Synaptic Vesicle Cycle

Action potential-induced neurotransmitter release in presynaptic boutons involves coordinated actions of a large list of proteins that are associated directly or indirectly with membrane structures including synaptic vesicles and plasma membranes. These proteins are often highly abundant in different synaptic bouton sub-compartments, and they rarely act alone. Instead, these proteins interact with each other forming intricate and distinct molecular complexes. Many of these complexes form condensed clusters on membrane surfaces. This review summarizes findings in recent years showing that many of presynaptic protein complex assemblies are formed via phase separation. These protein condensates extensively interact with lipid membranes via distinct modes, forming various mesoscale structures by different mode of organizations between membraneless condensates and membranous organelles. We discuss that such mesoscale interactions could have deep implications on mobilization, exocytosis, and retrieval of synaptic vesicles.

Short-Term and Long-Term Sensitization Differentially Alters the Composition of an Anterograde Transport Complex in Aplysia

Long-term memory formation requires anterograde transport of proteins from the soma of a neuron to its distal synaptic terminals. This allows new synaptic connections to be grown and existing ones remodeled. However, we do not yet know which proteins are transported to synapses in response to activity and temporal regulation. Here, using quantitative mass spectrometry, we have profiled anterograde protein cargos of a learning-regulated molecular motor protein kinesin [Aplysia kinesin heavy chain 1 (ApKHC1)] following short-term sensitization (STS) and long-term sensitization (LTS) in Aplysia californica Our results reveal enrichment of specific proteins associated with ApKHC1 following both STS and LTS, as well as temporal changes within 1 and 3 h of LTS training. A significant number of proteins enriched in the ApKHC1 complex participate in synaptic function, and, while some are ubiquitously enriched across training conditions, a few are enriched in response to specific training. For instance, factors aiding new synapse formation, such as synaptotagmin-1, dynamin-1, and calmodulin, are differentially enriched in anterograde complexes 1 h after LTS but are depleted 3 h after LTS. Proteins including gelsolin-like protein 2 and sec23A/sec24A, which function in actin filament stabilization and vesicle transport, respectively, are enriched in cargos 3 h after LTS. These results establish that the composition of anterograde transport complexes undergo experience-dependent specific changes and illuminate dynamic changes in the communication between soma and synapse during learning.

Dynamin is primed at endocytic sites for ultrafast endocytosis

Dynamin mediates fission of vesicles from the plasma membrane during endocytosis. Typically, dynamin is recruited from the cytosol to endocytic sites, requiring seconds to tens of seconds. However, ultrafast endocytosis in neurons internalizes vesicles as quickly as 50 ms during synaptic vesicle recycling. Here, we demonstrate that Dynamin 1 is pre-recruited to endocytic sites for ultrafast endocytosis. Specifically, Dynamin 1xA, a splice variant of Dynamin 1, interacts with Syndapin 1 to form molecular condensates on the plasma membrane. Single-particle tracking of Dynamin 1xA molecules confirms the liquid-like property of condensates in vivo. When Dynamin 1xA is mutated to disrupt its interaction with Syndapin 1, the condensates do not form, and consequently, ultrafast endocytosis slows down by 100-fold. Mechanistically, Syndapin 1 acts as an adaptor by binding the plasma membrane and stores Dynamin 1xA at endocytic sites. This cache bypasses the recruitment step and accelerates endocytosis at synapses.

Condensing our understanding of endocytosis

In this issue of Neuron, Imoto et al. report that a splice variant of dynamin (Dyn1xA) interacts with syndapin to form a molecular condensate at the edge of the presynaptic active zone. This enables rapid recruitment of proteins to endocytic sites essential for powering ultrafast endocytosis.

Molecular mechanics underlying flat-to-round membrane budding in live secretory cells

Membrane budding entails forces to transform flat membrane into vesicles essential for cell survival. Accumulated studies have identified coat-proteins (e.g., clathrin) as potential budding factors. However, forces mediating many non-coated membrane buddings remain unclear. By visualizing proteins in mediating endocytic budding in live neuroendocrine cells, performing in vitro protein reconstitution and physical modeling, we discovered how non-coated-membrane budding is mediated: actin filaments and dynamin generate a pulling force transforming flat membrane into Λ-shape; subsequently, dynamin helices surround and constrict Λ-profile's base, transforming Λ- to Ω-profile, and then constrict Ω-profile's pore, converting Ω-profiles to vesicles. These mechanisms control budding speed, vesicle size and number, generating diverse endocytic modes differing in these parameters. Their impact is widespread beyond secretory cells, as the unexpectedly powerful functions of dynamin and actin, previously thought to mediate fission and overcome tension, respectively, may contribute to many dynamin/actin-dependent non-coated-membrane buddings, coated-membrane buddings, and other membrane remodeling processes.

Dynamin 1 controls vesicle size and endocytosis at hippocampal synapses

Following calcium-triggered vesicle exocytosis, endocytosis regenerates vesicles to maintain exocytosis and thus synaptic transmission, which underlies neuronal circuit activities. Although most molecules involved in endocytosis have been identified, it remains rather poorly understood how endocytic machinery regulates vesicle size. Vesicle size, together with the transmitter concentration inside the vesicle, determines the amount of transmitter the vesicle can release, the quantal size, that may control the strength of synaptic transmission. Here, we report that, surprisingly, knockout of the GTPase dynamin 1, the most abundant brain dynamin isoform known to catalyze fission of the membrane pit's neck (the last step of endocytosis), not only significantly slowed endocytosis but also increased the synaptic vesicle diameter by as much as ∼40-64% at cultured hippocampal synapses. Furthermore, dynamin 1 knockout increased the size of membrane pits, the precursor for endocytic vesicle formation. These results suggest an important function of dynamin other than its well-known fission function - control of vesicle size at the pit formation stage.

Identification of Synaptic DGKθ Interactors That Stimulate DGKθ Activity

Lipids and their metabolic enzymes are a critical point of regulation for the membrane curvature required to induce membrane fusion during synaptic vesicle recycling. One such enzyme is diacylglycerol kinase θ (DGKθ), which produces phosphatidic acid (PtdOH) that generates negative membrane curvature. Synapses lacking DGKθ have significantly slower rates of endocytosis, implicating DGKθ as an endocytic regulator. Importantly, DGKθ kinase activity is required for this function. However, protein regulators of DGKθ's kinase activity in neurons have never been identified. In this study, we employed APEX2 proximity labeling and mass spectrometry to identify endogenous interactors of DGKθ in neurons and assayed their ability to modulate its kinase activity. Seven endogenous DGKθ interactors were identified and notably, synaptotagmin-1 (Syt1) increased DGKθ kinase activity 10-fold. This study is the first to validate endogenous DGKθ interactors at the mammalian synapse and suggests a coordinated role between DGKθ-produced PtdOH and Syt1 in synaptic vesicle recycling.

Microtubule assembly by soluble tau impairs vesicle endocytosis and excitatory neurotransmission via dynamin sequestration in Alzheimer's disease mice synapse model

Elevation of soluble wild-type (WT) tau occurs in synaptic compartments in Alzheimer’s disease. We addressed whether tau elevation affects synaptic transmission at the calyx of Held in slices from mice brainstem. Whole-cell loading of WT human tau (h-tau) in presynaptic terminals at 10–20 µM caused microtubule (MT) assembly and activity-dependent rundown of excitatory neurotransmission. Capacitance measurements revealed that the primary target of WT h-tau is vesicle endocytosis. Blocking MT assembly using nocodazole prevented tau-induced impairments of endocytosis and neurotransmission. Immunofluorescence imaging analyses revealed that MT assembly by WT h-tau loading was associated with an increased MT-bound fraction of the endocytic protein dynamin. A synthetic dodecapeptide corresponding to dynamin 1-pleckstrin-homology domain inhibited MT-dynamin interaction and rescued tau-induced impairments of endocytosis and neurotransmission. We conclude that elevation of presynaptic WT tau induces de novo assembly of MTs, thereby sequestering free dynamins. As a result, endocytosis and subsequent vesicle replenishment are impaired, causing activity-dependent rundown of neurotransmission.

Endosomal phosphatidylinositol 3-phosphate controls synaptic vesicle cycling and neurotransmission

Neural circuit function requires mechanisms for controlling neurotransmitter release and the activity of neuronal networks, including modulation by synaptic contacts, synaptic plasticity, and homeostatic scaling. However, how neurons intrinsically monitor and feedback control presynaptic neurotransmitter release and synaptic vesicle (SV) recycling to restrict neuronal network activity remains poorly understood at the molecular level. Here, we investigated the reciprocal interplay between neuronal endosomes, organelles of central importance for the function of synapses, and synaptic activity. We show that elevated neuronal activity represses the synthesis of endosomal lipid phosphatidylinositol 3-phosphate [PI(3)P] by the lipid kinase VPS34. Neuronal activity in turn is regulated by endosomal PI(3)P, the depletion of which reduces neurotransmission as a consequence of perturbed SV endocytosis. We find that this mechanism involves Calpain 2-mediated hyperactivation of Cdk5 downstream of receptor- and activity-dependent calcium influx. Our results unravel an unexpected function for PI(3)P-containing neuronal endosomes in the control of presynaptic vesicle cycling and neurotransmission, which may explain the involvement of the PI(3)P-producing VPS34 kinase in neurological disease and neurodegeneration.

Presynaptic autophagy is coupled to the synaptic vesicle cycle via ATG-9

Autophagy is a cellular degradation pathway essential for neuronal health and function. Autophagosome biogenesis occurs at synapses, is locally regulated, and increases in response to neuronal activity. The mechanisms that couple autophagosome biogenesis to synaptic activity remain unknown. In this study, we determine that trafficking of ATG-9, the only transmembrane protein in the core autophagy pathway, links the synaptic vesicle cycle with autophagy. ATG-9-positive vesicles in C. elegans are generated from the trans-Golgi network via AP-3-dependent budding and delivered to presynaptic sites. At presynaptic sites, ATG-9 undergoes exo-endocytosis in an activity-dependent manner. Mutations that disrupt endocytosis, including a lesion in synaptojanin 1 associated with Parkinson's disease, result in abnormal ATG-9 accumulation at clathrin-rich synaptic foci and defects in activity-induced presynaptic autophagy. Our findings uncover regulated key steps of ATG-9 trafficking at presynaptic sites and provide evidence that ATG-9 exo-endocytosis couples autophagosome biogenesis at presynaptic sites with the activity-dependent synaptic vesicle cycle.

Exosome: A novel neurotransmission modulator or non-canonical neurotransmitter?

Neurotransmission is the electrical impulse-triggered propagation of signals between neurons or between neurons and other cell types such as skeletal muscle cells. Recent studies point out the involvement of exosomes, a type of small bilipid layer-enclosed extracellular vesicles, in regulating neurotransmission. Through horizontally transferring proteins, lipids, and nucleic acids, exosomes can modulate synaptic activities rapidly by controlling neurotransmitter release or progressively by regulating neural plasticity including synapse formation, neurite growth & removal, and axon guidance & elongation. In this review, we summarize the similarities and differences between exosomes and synaptic vesicles in their biogenesis, contents, and release. We also highlight the recent progress made in demonstrating the biological roles of exosome in regulating neurotransmission, and propose a modified model of neurotransmission, in which exosomes act as novel neurotransmitters. Lastly, we provide a comprehensive discussion of the enlightenment of the current knowledge on neurotransmission to the future directions of exosome research.

Role of Clathrin and Dynamin in Clathrin Mediated Endocytosis/Synaptic Vesicle Recycling and Implications in Neurological Diseases

Endocytosis is a process essential to the health and well-being of cell. It is required for the internalisation and sorting of “cargo”—the macromolecules, proteins, receptors and lipids of cell signalling. Clathrin mediated endocytosis (CME) is one of the key processes required for cellular well-being and signalling pathway activation. CME is key role to the recycling of synaptic vesicles [synaptic vesicle recycling (SVR)] in the brain, it is pivotal to signalling across synapses enabling intracellular communication in the sensory and nervous systems. In this review we provide an overview of the general process of CME with a particular focus on two key proteins: clathrin and dynamin that have a central role to play in ensuing successful completion of CME. We examine these two proteins as they are the two endocytotic proteins for which small molecule inhibitors, often of known mechanism of action, have been identified. Inhibition of CME offers the potential to develop therapeutic interventions into conditions involving defects in CME. This review will discuss the roles and the current scope of inhibitors of clathrin and dynamin, providing an insight into how further developments could affect neurological disease treatments.

Synthesis of Phthaladyn-29 and Naphthalimide-10, GTP Site Directed Dynamin GTPase Inhibitors

Herein we describe the detailed synthesis of the dynamin inhibitors Phthaladyn-29 and Napthaladyn-10, and their chemical scaffold matched partner inactive compounds. Combined with the assay data provided, this allows the interrogation of dynamin in vitro and potentially in vivo.

Comprehensive Analysis of Mouse Hippocampal Lysine Acetylome Mediated by Sea Cucumber Peptides Preventing Memory Impairment

Memory impairment is becoming a potential health issue with the delicacy of diet and social stress. Sea cucumber peptides (SCP) prevent memory impairment, as previously reported. In this study, further research was performed using hippocampal lysine-acetylome to explore molecular regulation mechanisms. C57BL/6 mice were treated with scopolamine via intraperitoneal injection to simulate memory impairment. To determine the influence of SCP on the total acetylated-protein level of the hippocampus, acetylated-proteomics was performed. SCP increased the acetylation level of histone (H3 and H4). Meanwhile, for non-histones, the differentially acetylated proteins were involved in multiple memory-related pathways, as shown by KEGG enrichment analysis. Additionally, long-term potentiation was confirmed by western blotting. Finally, a combined analysis of proteome and lysine acetylome revealed that SCP contributed to synaptic vesicle cycle regulation and dopamine metabolism. Consequently, our findings revealed that SCP was potentially neuroprotective by regulating post-transcriptional hippocampal protein acetylation.

An Emerging Role for Phosphoinositides in the Pathophysiology of Parkinson’s Disease

Recent data support an involvement of defects in homeostasis of phosphoinositides (PIPs) in the pathophysiology of Parkinson’s disease (PD). Genetic mutations have been identified in genes encoding for PIP-regulating and PIP-interacting proteins, that are associated with familial and sporadic PD. Many of these proteins are implicated in vesicular membrane trafficking, mechanisms that were recently highlighted for their close associations with PD. PIPs are phosphorylated forms of the membrane phospholipid, phosphatidylinositol. Their composition in the vesicle’s membrane of origin, as well as membrane of destination, controls vesicular membrane trafficking. We review the converging evidence that points to the involvement of PIPs in PD. The review describes PD- and PIP-associated proteins implicated in clathrin-mediated endocytosis and autophagy, and highlights the involvement of α-synuclein in these mechanisms.

Preformed Ω-profile closure and kiss-and-run mediate endocytosis and diverse endocytic modes in neuroendocrine chromaffin cells

Transformation of flat membrane into round vesicles is generally thought to underlie endocytosis and produce speed-, amount-, and vesicle-size-specific endocytic modes. Visualizing depolarization-induced exocytic and endocytic membrane transformation in live neuroendocrine chromaffin cells, we found that flat membrane is transformed into Λ-shaped, Ω-shaped, and O-shaped vesicles via invagination, Λ-base constriction, and Ω-pore constriction, respectively. Surprisingly, endocytic vesicle formation is predominantly from not flat-membrane-to-round-vesicle transformation but calcium-triggered and dynamin-mediated closure of (1) Ω profiles formed before depolarization and (2) fusion pores (called kiss-and-run). Varying calcium influxes control the speed, number, and vesicle size of these pore closures, resulting in speed-specific slow (more than ∼6 s), fast (less than ∼6 s), or ultrafast (<0.6 s) endocytosis, amount-specific compensatory endocytosis (endocytosis = exocytosis) or overshoot endocytosis (endocytosis > exocytosis), and size-specific bulk endocytosis. These findings reveal major membrane transformation mechanisms underlying endocytosis, diverse endocytic modes, and exocytosis-endocytosis coupling, calling for correction of the half-a-century concept that the flat-to-round transformation predominantly mediates endocytosis after physiological stimulation.

Early Changes in Exo- and Endocytosis in the EAE Mouse Model of Multiple Sclerosis Correlate with Decreased Synaptic Ribbon Size and Reduced Ribbon-Associated Vesicle Pools in Rod Photoreceptor Synapses

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system that finally leads to demyelination. Demyelinating optic neuritis is a frequent symptom in MS. Recent studies also revealed synapse dysfunctions in MS patients and MS mouse models. We previously reported alterations of photoreceptor ribbon synapses in the experimental auto-immune encephalomyelitis (EAE) mouse model of MS. In the present study, we found that the previously observed decreased imunosignals of photoreceptor ribbons in early EAE resulted from a decrease in synaptic ribbon size, whereas the number/density of ribbons in photoreceptor synapses remained unchanged. Smaller photoreceptor ribbons are associated with fewer docked and ribbon-associated vesicles. At a functional level, depolarization-evoked exocytosis as monitored by optical recording was diminished even as early as on day 7 after EAE induction. Moreover compensatory, post-depolarization endocytosis was decreased. Decreased post-depolarization endocytosis in early EAE correlated with diminished synaptic enrichment of dynamin3. In contrast, basal endocytosis in photoreceptor synapses of resting non-depolarized retinal slices was increased in early EAE. Increased basal endocytosis correlated with increased de-phosphorylation of dynamin1. Thus, multiple endocytic pathways in photoreceptor synapse are differentially affected in early EAE and likely contribute to the observed synapse pathology in early EAE.

Dynamin deficiency causes insulin secretion failure and hyperglycemia

Pancreatic β cells operate with a high rate of membrane recycling for insulin secretion, yet endocytosis in these cells is not fully understood. We investigate this process in mature mouse β cells by genetically deleting dynamin GTPase, the membrane fission machinery essential for clathrin-mediated endocytosis. Unexpectedly, the mice lacking all three dynamin genes (DNM1, DNM2, DNM3) in their β cells are viable, and their β cells still contain numerous insulin granules. Endocytosis in these β cells is severely impaired, resulting in abnormal endocytic intermediates on the plasma membrane. Although insulin granules are abundant, their release upon glucose stimulation is blunted in both the first and second phases, leading to hyperglycemia and glucose intolerance in mice. Dynamin triple deletion impairs insulin granule exocytosis and decreases intracellular Ca2+ responses and granule docking. The docking defect is correlated with reduced expression of Munc13-1 and RIM1 and reorganization of cortical F-actin in β cells. Collectively, these findings uncover the role of dynamin in dense-core vesicle endocytosis and secretory capacity. Insulin secretion deficiency in the absence of dynamin-mediated endocytosis highlights the risk of impaired membrane trafficking in endocrine failure and diabetes pathogenesis.

Dynamin-2 mediates clathrin-dependent endocytosis for amyloid-β internalization in brain microvascular endothelial cells

Dynamin is recognized as a crucial regulator for membrane fission and has three isoforms in mammals. But the expression patterns of dynamin isoforms and their roles in non-neuronal cells are incompletely understood. In this study, the expression profiles of dynamin isoforms and their roles in endocytosis was investigated in brain endothelial cells. We found that Dyn2 was expressed at highest levels, whereas the expression of Dyn1 and Dyn3 were far less than Dyn2. Live-cell imaging was used to investigate the effects of siRNA-mediated knockdown of individual dynamin isoforms on transferrin uptake, and we found that Dyn2, but not Dyn1 or Dyn3, is required for the endocytosis in brain endothelial cells. Results of dextran uptake assay showed that dynamin isoforms are not involved in the clathrin-independent fluid-phase internalization of brain endothelial cells, suggesting the specificity of the role of Dyn2 in clathrin-dependent endocytosis. Immunofluorescence and electron microscopy analysis showed that Dyn2 co-localizes with clathrin and acts at the late stage of vesicle fission in the process of endocytosis. Further results showed that Dyn2 is necessary for the basolateral-to-apical internalization of amyloid-β into brain endothelial cells. We concluded that Dyn2, but not Dyn1 or Dyn3, mediates the clathrin-dependent endocytosis for amyloid-β internalization particularly from basolateral to apical side into brain endothelial cells.

Current Pharmacologic Strategies for Treatment of Intractable Epilepsy in Children

Epileptic encephalopathy (EE) is a devastating pediatric disease that features medically resistant seizures, which can contribute to global developmental delays. Despite technological advancements in genetics, the neurobiological mechanisms of EEs are not fully understood, leaving few therapeutic options for affected patients. In this review, we introduce the most common EEs in pediatrics (i.e., Ohtahara syndrome, Dravet syndrome, and Lennox-Gastaut syndrome) and their molecular mechanisms that cause excitation/inhibition imbalances. We then discuss some of the essential molecules that are frequently dysregulated in EEs. Specifically, we explore voltage-gated ion channels, synaptic transmission-related proteins, and ligand-gated ion channels in association with the pathophysiology of Ohtahara syndrome, Dravet syndrome, and Lennox-Gastaut syndrome. Finally, we review currently available antiepileptic drugs used to treat seizures in patients with EEs. Since these patients often fail to achieve seizure relief even with the combination therapy, further extensive research efforts to explore the involved molecular mechanisms will be required to develop new drugs for patients with intractable epilepsy.

Dynamin controls neuropeptide secretion by organizing dense-core vesicle fusion sites

Synaptic vesicles (SVs) release neurotransmitters at specialized active zones, but release sites and organizing principles for the other major secretory pathway, neuropeptide/neuromodulator release from dense-core vesicles (DCVs), remain elusive. We identify dynamins, yeast Vps1 orthologs, as DCV fusion site organizers in mammalian neurons. Genetic or pharmacological inactivation of all three dynamins strongly impaired DCV exocytosis, while SV exocytosis remained unaffected. Wild-type dynamin restored normal exocytosis but not guanosine triphosphatase-deficient or membrane-binding mutants that cause neurodevelopmental syndromes. During prolonged stimulation, repeated use of the same DCV fusion location was impaired in dynamin 1-3 triple knockout neurons. The syntaxin-1 staining efficiency, but not its expression level, was reduced. αSNAP (α-soluble N-ethylmaleimide-sensitive factor attachment protein) expression restored this. We conclude that mammalian dynamins organize DCV fusion sites, downstream of αSNAP, by regulating the equilibrium between fusogenic and non-fusogenic syntaxin-1 promoting its availability for SNARE (SNAP receptor) complex formation and DCV exocytosis.

Brain-specific functions of the endocytic machinery

Endocytosis is an essential cellular process required for multiple physiological functions, including communication with the extracellular environment, nutrient uptake, and signaling by the cell surface receptors. In a broad sense, endocytosis is accomplished through either constitutive or ligand-induced invagination of the plasma membrane, which results in the formation of the plasma membrane-retrieved endocytic vesicles, which can either be sent for degradation to the lysosomes or recycled back to the PM. This additional function of endocytosis in membrane retrieval has been adopted by excitable cells, such as neurons, for membrane equilibrium maintenance at synapses. The last two decades were especially productive with respect to the identification of brain-specific functions of the endocytic machinery, which additionally include but not limited to regulation of neuronal differentiation and migration, maintenance of neuron morphology and synaptic plasticity, and prevention of neurotoxic aggregates spreading. In this review, we highlight the current knowledge of brain-specific functions of endocytic machinery with a specific focus on three brain cell types, neuronal progenitor cells, neurons, and glial cells.

CtBP1-Mediated Membrane Fission Contributes to Effective Recycling of Synaptic Vesicles

Compensatory endocytosis of released synaptic vesicles (SVs) relies on coordinated signaling at the lipid-protein interface. Here, we address the synaptic function of C-terminal binding protein 1 (CtBP1), a ubiquitous regulator of gene expression and membrane trafficking in cultured hippocampal neurons. In the absence of CtBP1, synapses form in greater density and show changes in SV distribution and size. The increased basal neurotransmission and enhanced synaptic depression could be attributed to a higher vesicular release probability and a smaller fraction of release-competent SVs, respectively. Rescue experiments with specifically targeted constructs indicate that, while synaptogenesis and release probability are controlled by nuclear CtBP1, the efficient recycling of SVs relies on its synaptic expression. The ability of presynaptic CtBP1 to facilitate compensatory endocytosis depends on its membrane-fission activity and the activation of the lipid-metabolizing enzyme PLD1. Thus, CtBP1 regulates SV recycling by promoting a permissive lipid environment for compensatory endocytosis.

Dauricine Attenuates Spatial Memory Impairment and Alzheimer-Like Pathologies by Enhancing Mitochondrial Function in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by extracellular amyloid plaques composed of β-amyloid (Aβ) and intracellular neurofibrillary tangles containing hyperphosphorylated tau protein. No effective therapy is available for this disease. In this study, we investigated the potential therapeutic effects of dauricine (DAU), a benzyl tetrahydroisoquinoline alkaloid, on AD, and found that DAU administration significantly improved cognitive impairments in 3xTg-AD mice by decreasing Aβ plaques and hyperphosphorylated tau and increasing the hippocampal ATP level. Proteomic and western blot analyses revealed that DAU treatment mainly modified the expression of proteins involved in mitochondrial energy metabolism, such as Aco2, Ndufs1, Cox5a, and SDHB, and that of synapse-related proteins such as Syn1 and Syn2. Pathway analysis revealed that DAU modulated the tricarboxylic acid cycle, synaptic vesicle cycle, glycolysis, and gluconeogenesis in 3xTg-AD mice. Our study suggests that DAU may be a potential drug for the treatment of AD.

Comparative 2D and 3D Ultrastructural Analyses of Dendritic Spines from CA1 Pyramidal Neurons in the Mouse Hippocampus

Three-dimensional (3D) reconstruction from electron microscopy (EM) datasets is a widely used tool that has improved our knowledge of synapse ultrastructure and organization in the brain. Rearrangements of synapse structure following maturation and in synaptic plasticity have been broadly described and, in many cases, the defective architecture of the synapse has been associated to functional impairments. It is therefore important, when studying brain connectivity, to map these rearrangements with the highest accuracy possible, considering the affordability of the different EM approaches to provide solid and reliable data about the structure of such a small complex. The aim of this work is to compare quantitative data from two dimensional (2D) and 3D EM of mouse hippocampal CA1 (apical dendrites), to define whether the results from the two approaches are consistent. We examined asymmetric excitatory synapses focusing on post synaptic density and dendritic spine area and volume as well as spine density, and we compared the results obtained with the two methods. The consistency between the 2D and 3D results questions the need—for many applications—of using volumetric datasets (costly and time consuming in terms of both acquisition and analysis), with respect to the more accessible measurements from 2D EM projections.

Specialised endocytic proteins regulate diverse internalisation mechanisms and signalling outputs in physiology and cancer

Although endocytosis was first described as the process mediating macromolecule or nutrient uptake through the plasma membrane, it is now recognised as a critical component of the cellular infrastructure involved in numerous processes, ranging from receptor signalling, proliferation and migration to polarity and stem cell regulation. To realise these varying roles, endocytosis needs to be finely regulated. Accordingly, multiple endocytic mechanisms exist that require specialised molecular machineries and an array of endocytic adaptor proteins with cell-specific functions. This review provides some examples of specialised functions of endocytic adaptors and other components of the endocytic machinery in different cell physiological processes, and how the alteration of these functions is linked to cancer. In particular, we focus on: (i) cargo selection and endocytic mechanisms linked to different adaptors; (ii) specialised functions in clathrin-mediated versus non-clathrin endocytosis; (iii) differential regulation of endocytic mechanisms by post-translational modification of endocytic proteins; (iv) cell context-dependent expression and function of endocytic proteins. As cases in point, we describe two endocytic protein families, dynamins and epsins. Finally, we discuss how dysregulation of the physiological role of these specialised endocytic proteins is exploited by cancer cells to increase cell proliferation, migration and invasion, leading to anti-apoptotic or pro-metastatic behaviours.

Neurotransmitter Release Site Replenishment and Presynaptic Plasticity

An action potential (AP) triggers neurotransmitter release from synaptic vesicles (SVs) docking to a specialized release site of presynaptic plasma membrane, the active zone (AZ). The AP simultaneously controls the release site replenishment with SV for sustainable synaptic transmission in response to incoming neuronal signals. Although many studies have suggested that the replenishment time is relatively slow, recent studies exploring high speed resolution have revealed SV dynamics with milliseconds timescale after an AP. Accurate regulation is conferred by proteins sensing Ca²⁺ entering through voltage-gated Ca²⁺ channels opened by an AP. This review summarizes how millisecond Ca²⁺ dynamics activate multiple protein cascades for control of the release site replenishment with release-ready SVs that underlie presynaptic short-term plasticity.

α-Synuclein facilitates endocytosis by elevating the steady-state levels of phosphatidylinositol 4,5-bisphosphate

α-Synuclein (α-Syn) is a protein implicated in the pathogenesis of Parkinson's disease (PD). It is an intrinsically disordered protein that binds acidic phospholipids. Growing evidence supports a role for α-Syn in membrane trafficking, including, mechanisms of endocytosis and exocytosis, although the exact role of α-Syn in these mechanisms is currently unclear. Here we investigate the associations of α-Syn with the acidic phosphoinositides (PIPs), phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2). Our results show that α-Syn colocalizes with PIP2 and the phosphorylated active form of the clathrin adaptor protein 2 (AP2) at clathrin-coated pits. Using endocytosis of transferrin as an indicator for clathrin-mediated endocytosis (CME), we find that α-Syn involvement in endocytosis is specifically mediated through PI(4,5)P2 levels on the plasma membrane. In accord with their effects on PI(4,5)P2 levels, the PD associated A30P, E46K, and A53T mutations in α-Syn further enhance CME in neuronal and nonneuronal cells. However, lysine to glutamic acid substitutions at the KTKEGV repeat domain of α-Syn, which interfere with phospholipid binding, are ineffective in enhancing CME. We further show that the rate of synaptic vesicle (SV) endocytosis is differentially affected by the α-Syn mutations and associates with their effects on PI(4,5)P2 levels, however, with the exception of the A30P mutation. This study provides evidence for a critical involvement of PIPs in α-Syn-mediated membrane trafficking.

Dynamin Superfamily at Pre- and Postsynapses: Master Regulators of Synaptic Transmission and Plasticity in Health and Disease

Dynamin superfamily proteins (DSPs) comprise a large group of GTP-ases that orchestrate membrane fusion and fission, and cytoskeleton remodeling in different cell-types. At the central nervous system, they regulate synaptic vesicle recycling and signaling-receptor turnover, allowing the maintenance of synaptic transmission. In the presynapses, these GTP-ases control the recycling of synaptic vesicles influencing the size of the ready-releasable pool and the release of neurotransmitters from nerve terminals, whereas in the postsynapses, they are involved in AMPA-receptor trafficking to and from postsynaptic densities, supporting excitatory synaptic plasticity, and consequently learning and memory formation. In agreement with these relevant roles, an important number of neurological disorders are associated with mutations and/or dysfunction of these GTP-ases. Along the present review we discuss the importance of DSPs at synapses and their implication in different neuropathological contexts.