Activity-Dependent Bulk Synaptic Vesicle Endocytosis-A Fast, High Capacity Membrane Retrieval Mechanism

Central nerve terminals are placed under considerable stress during intense stimulation due to large numbers of synaptic vesicles (SVs) fusing with the plasma membrane. Classical clathrin-dependent SV endocytosis cannot correct for the large increase in nerve terminal surface area in the short term, due to its slow kinetics and low capacity. During such intense stimulation an additional SV retrieval pathway is recruited called bulk endocytosis. Recent studies have shown that bulk endocytosis fulfils all of the physiological requirements to remedy the acute changes in nerve terminal surface area to allow the nerve terminal to continue to function. This review will summarise the recent developments in the field that characterise the physiology of bulk endocytosis which show that it is a fast, activity-dependent and high capacity mechanism that is essential for the function of central nerve terminals

Acute increase of alpha-synuclein inhibits synaptic vesicle recycling evoked during intense stimulation

This work was supported by grants from the NIH/National Institute of Neurological Disorder and Stroke RO1 NS078165 (to J.R.M.), the Morton Cure Paralysis Fund (to J.R.M.), and the Branfman Family Foundation (to J.M.G.) and by a Dorothea Bennett graduate fellowship (to D.J.B.)

VAMP4 is an Essential Cargo Molecule for Activity-Dependent Bulk Endocytosis

SummaryThe accurate formation of synaptic vesicles (SVs) and incorporation of their protein cargo during endocytosis is critical for the maintenance of neurotransmission. During intense neuronal activity, a transient and acute accumulation of SV cargo occurs at the plasma membrane. Activity-dependent bulk endocytosis (ADBE) is the dominant SV endocytosis mode under these conditions; however, it is currently unknown how ADBE mediates cargo retrieval. We examined the retrieval of different SV cargo molecules during intense stimulation using a series of genetically encoded pH-sensitive reporters in neuronal cultures. The retrieval of only one reporter, VAMP4-pHluorin, was perturbed by inhibiting ADBE. This selective recovery was confirmed by the enrichment of endogenous VAMP4 in purified bulk endosomes formed by ADBE. VAMP4 was also essential for ADBE, with a cytoplasmic di-leucine motif being critical for this role. Therefore, VAMP4 is the first identified ADBE cargo and is essential for this endocytosis mode to proceed

Control of synaptic vesicle endocytosis by an extracellular signalling molecule

Signalling cascades control multiple aspects of presynaptic function. Synaptic vesicle endocytosis was assumed to be exempt from modulation, due to its essential role maintaining synaptic vesicle supply and thus neurotransmission. Here we show that brain-derived neurotrophic factor arrests the rephosphorylation of the endocytosis enzyme dynamin I via an inhibition of glycogen synthase kinase 3. This event results in a selective inhibition of activity-dependent bulk endocytosis during high-intensity firing. Furthermore, the continued presence of brain-derived neurotrophic factor alleviates the rundown of neurotransmission during high activity. Thus, synaptic strength can be modulated by extracellular signalling molecules via a direct inhibition of a synaptic vesicle endocytosis mode

Differential requirements for clathrin in receptor-mediated endocytosis and maintenance of synaptic vesicle pools

Clathrin is a coat protein involved in vesicle budding from several membrane-bound compartments within the cell. Here we present an analysis of a temperature-sensitive (ts) mutant of clathrin heavy chain (CHC) in a multicellular animal. As expected Caenorhabditis elegans chc-1(b1025ts) mutant animals are defective in receptor-mediated endocytosis and arrest development soon after being shifted to the restrictive temperature. Steady-state clathrin levels in these mutants are reduced by more than 95% at all temperatures. Hub interactions and membrane associations are lost at the restrictive temperature. chc-1(b1025ts) animals become paralyzed within minutes of exposure to the restrictive temperature because of a defect in the nervous system. Surprisingly synaptic vesicle number is not reduced in chc-1(b1025ts) animals. Consistent with the normal number of vesicles, postsynaptic miniature currents occur at normal frequencies. Taken together, these results indicate that a high level of CHC activity is required for receptor-mediated endocytosis in nonneuronal cells but is largely dispensable for maintenance of synaptic vesicle pools

Neurotrophin-mediated dendrite-to-nucleus signaling revealed by microfluidic compartmentalization of dendrites

Signaling from dendritic synapses to the nucleus regulates important aspects of neuronal function, including synaptic plasticity. The neurotrophin brain-derived neurotrophic factor (BDNF) can induce long-lasting strengthening of synapses in vivo and this effect is dependent on transcription. However, the mechanism of signaling to the nucleus is not well understood. Here we describe a microfluidic culture device to investigate dendrite-to-nucleus signaling. Using these microfluidic devices, we demonstrate that BDNF can act directly on dendrites to elicit an anterograde signal that induces transcription of the immediate early genes, Arc and c-Fos. Induction of Arc is dependent on dendrite- and cell body-derived calcium, whereas induction of c-Fos is calcium-independent. In contrast to retrograde neurotrophin-mediated axon-to-nucleus signaling, which is MEK5-dependent, BDNF-mediated anterograde dendrite-to-nucleus signaling is dependent on MEK1/2. Intriguingly, the activity of TrkB, the BDNF receptor, is required in the cell body for the induction of Arc and c-Fos mediated by dendritically applied BDNF. These results are consistent with the involvement of a signaling endosome-like pathway that conveys BDNF signals from the dendrite to the nucleus