Studies of synaptic vesicle endocytosis in the nematode C. elegans

Synaptic Vesicle Endocytosis in Different Model Systems

Neurotransmission in complex animals depends on a choir of functionally distinct synapses releasing neurotransmitters in a highly coordinated manner. During synaptic signaling, vesicles fuse with the plasma membrane to release their contents. The rate of vesicle fusion is high and can exceed the rate at which synaptic vesicles can be re-supplied by distant sources. Thus, local compensatory endocytosis is needed to replenish the synaptic vesicle pools. Over the last four decades, various experimental methods and model systems have been used to study the cellular and molecular mechanisms underlying synaptic vesicle cycle. Clathrin-mediated endocytosis is thought to be the predominant mechanism for synaptic vesicle recycling. However, recent studies suggest significant contribution from other modes of endocytosis, including fast compensatory endocytosis, activity-dependent bulk endocytosis, ultrafast endocytosis, as well as kiss-and-run. Currently, it is not clear whether a universal model of vesicle recycling exist for all types of synapses. It is possible that each synapse type employs a particular mode of endocytosis. Alternatively, multiple modes of endocytosis operate at the same synapse, and the synapse toggles between different modes depending on its activity level. Here we compile review and research articles based on well-characterized model systems: frog neuromuscular junctions, C. elegans neuromuscular junctions, Drosophila neuromuscular junctions, lamprey reticulospinal giant axons, goldfish retinal ribbon synapses, the calyx of Held, and rodent hippocampal synapses. We will compare these systems in terms of their known modes and kinetics of synaptic vesicle endocytosis, as well as the underlying molecular machineries. We will also provide the future development of this field.

Increased Levels of Extracellular Microvesicle Markers and Decreased Levels of Endocytic/Exocytic Proteins in the Alzheimer's Disease Brain

BACKGROUND

Alzheimer's disease (AD) is a chronic neurodegenerative disorder accounting for more than 50% of all dementia cases. AD neuropathology is characterized by the formation of extracellular plaques and intracellular neurofibrillary tangles consisting of aggregated amyloid-β and tau, respectively. The disease mechanism has only been partially elucidated and is believed to also involve many other proteins.

OBJECTIVE

This study intended to perform a proteomic profiling of post mortem AD brains and compare it with control brains as well as brains from other neurological diseases to gain insight into the disease pathology.

METHODS

Here we used label-free shotgun mass spectrometry to analyze temporal neocortex samples from AD, other neurological disorders, and non-demented controls, in order to identify additional proteins that are altered in AD. The mass spectrometry results were verified by antibody suspension bead arrays.

RESULTS

We found 50 proteins with altered levels between AD and control brains. The majority of these proteins were found at lower levels in AD. Pathway analyses revealed that several of the decreased proteins play a role in exocytic and endocytic pathways, whereas several of the increased proteins are related to extracellular vesicles. Using antibody-based analysis, we verified the mass spectrometry results for five representative proteins from this group of proteins (CD9, HSP72, PI42A, TALDO, and VAMP2) and GFAP, a marker for neuroinflammation.

CONCLUSIONS

Several proteins involved in exo-endocytic pathways and extracellular vesicle functions display altered levels in the AD brain. We hypothesize that such changes may result in disturbed cellular clearance and a perturbed cell-to-cell communication that may contribute to neuronal dysfunction and cell death in AD.

Smed-dynA-1 is a planarian nervous system specific dynamin 1 homolog required for normal locomotion

Dynamins are GTPases that are required for separation of vesicles from the plasma membrane and thus are key regulators of endocytosis in eukaryotic cells. This role for dynamin proteins is especially crucial for the proper function of neurons, where they ensure that synaptic vesicles and their neurotransmitter cargo are recycled in the presynaptic cell. Here we have characterized the dynamin protein family in the freshwater planarian Schmidtea mediterranea and showed that it possesses six dynamins with tissue specific expression profiles. Of these six planarian homologs, two are necessary for normal tissue homeostasis, and the loss of another, Smed-dynA-1, leads to an abnormal behavioral phenotype, which we have quantified using automated center of mass tracking. Smed-dynA-1 is primarily expressed in the planarian nervous system and is a functional homolog of the mammalian Dynamin I. The distinct expression profiles of the six dynamin genes makes planarians an interesting new system to reveal novel dynamin functions, which may be determined by their differential tissue localization. The observed complexity of neurotransmitter regulation combined with the tools of quantitative behavioral assays as a functional readout for neuronal activity, renders planarians an ideal system for studying how the nervous system controls behavior.

Endocytosis and intracellular trafficking contribute to necrotic neurodegeneration in C. elegans

Unlike apoptosis, necrotic cell death is characterized by marked loss of plasma membrane integrity. Leakage of cytoplasmic material to the extracellular space contributes to cell demise, and is the cause of acute inflammatory responses, which typically accompany necrosis. The mechanisms underlying plasma membrane damage during necrotic cell death are not well understood. We report that endocytosis is critically required for the execution of necrosis. Depletion of the key endocytic machinery components dynamin, synaptotagmin and endophilin suppresses necrotic neurodegeneration induced by diverse genetic and environmental insults in C. elegans. We used genetically encoded fluorescent markers to monitor the formation and fate of specific types of endosomes during cell death in vivo. Strikingly, we find that the number of early and recycling endosomes increases sharply and transiently upon initiation of necrosis. Endosomes subsequently coalesce around the nucleus and disintegrate during the final stage of necrosis. Interfering with kinesin-mediated endosome trafficking impedes cell death. Endocytosis synergizes with autophagy and lysosomal proteolytic mechanisms to facilitate necrotic neurodegeneration. These findings demonstrate a prominent role for endocytosis in cellular destruction during neurodegeneration, which is likely conserved in metazoans.

Evolution of insect proteomes: insights into synapse organization and synaptic vesicle life cycle

BACKGROUND

The molecular components in synapses that are essential to the life cycle of synaptic vesicles are well characterized. Nonetheless, many aspects of synaptic processes, in particular how they relate to complex behaviour, remain elusive. The genomes of flies, mosquitoes, the honeybee and the beetle are now fully sequenced and span an evolutionary breadth of about 350 million years; this provides a unique opportunity to conduct a comparative genomics study of the synapse.

RESULTS

We compiled a list of 120 gene prototypes that comprise the core of presynaptic structures in insects. Insects lack several scaffolding proteins in the active zone, such as bassoon and piccollo, and the most abundant protein in the mammalian synaptic vesicle, namely synaptophysin. The pattern of evolution of synaptic protein complexes is analyzed. According to this analysis, the components of presynaptic complexes as well as proteins that take part in organelle biogenesis are tightly coordinated. Most synaptic proteins are involved in rich protein interaction networks. Overall, the number of interacting proteins and the degrees of sequence conservation between human and insects are closely correlated. Such a correlation holds for exocytotic but not for endocytotic proteins.

CONCLUSION

This comparative study of human with insects sheds light on the composition and assembly of protein complexes in the synapse. Specifically, the nature of the protein interaction graphs differentiate exocytotic from endocytotic proteins and suggest unique evolutionary constraints for each set. General principles in the design of proteins of the presynaptic site can be inferred from a comparative study of human and insect genomes.

Factors regulating the abundance and localization of synaptobrevin in the plasma membrane

After synaptic vesicle fusion, vesicle proteins must be segregated from plasma membrane proteins and recycled to maintain a functional vesicle pool. We monitored the distribution of synaptobrevin, a vesicle protein required for exocytosis, in Caenorhabditis elegans motor neurons by using a pH-sensitive synaptobrevin GFP fusion protein, synaptopHluorin. We estimated that 30% of synaptobrevin was present in the plasma membrane. By using a panel of endocytosis and exocytosis mutants, we found that the majority of surface synaptobrevin derives from fusion of synaptic vesicles and that, in steady state, synaptobrevin equilibrates throughout the axon. The surface synaptobrevin was enriched near active zones, and its spatial extent was regulated by the clathrin adaptin AP180. These results suggest that there is a plasma membrane reservoir of synaptobrevin that is supplied by the synaptic vesicle cycle and available for retrieval throughout the axon. The size of the reservoir is set by the relative rates of exo- and endocytosis.

Polyunsaturated fatty acids and neurotransmission in Caenorhabditis elegans

Changes in PUFA (polyunsaturated fatty acid) metabolism can cause mental retardation and cognitive impairment. However, it is still unclear why altered levels of PUFAs result in neuronal dysfunction. Recent studies on the nematode Caenorhabditis elegans suggest that PUFA depletion may cause cognitive impairment by compromising communication among neurons. Pharmacological and electrophysiological experiments showed that animals devoid of most PUFAs release abnormally low levels of neurotransmitters. In addition, ultrastructural analysis revealed that synapses in these mutants are severely depleted of synaptic vesicles. The conclusion of these studies is that PUFAs are required to maintain a normal pool of synaptic vesicles at pre-synaptic sites, thus ensuring efficient neurotransmission.

Novel Caenorhabditis elegans unc-119 axon outgrowth defects correlate with behavioral phenotypes that are partially rescued by nonneural unc-119

UNC-119 function is necessary for the correct development of the Caenorhabditis elegans nervous system. Worms mutant for unc-119 exhibit nervous system structural defects, including supernumerary axon branches, defasciculated nerve fibers, and choice point errors. Axons of both mechanosensory (ALM) and chemo- sensory (ASI) neurons have elongation defects within the nerve ring. Expressing unc-119 cDNA in mechanosensory neurons rescues the elongation defect of ALM axons, but expression in ASI neurons does not rescue ASI axon elongation defects. Neither gross movement nor dauer larva formation defects are rescued in either case. However, expressing a construct including introns under the control of the same promoters results in substantial rescue of phenotypic defects. In these cases reporter expression expands to tissues outside those specified by the promoter, notably into head muscles. Surprisingly, expressing an unc-119 cDNA construct under the control of a muscle-specific promoter fully rescues the dauer formation defect and substantially rescues movement. Thus, although UNC-119 normally acts in a cell-autonomous fashion, the cell-nonautonomous rescue of neural function suggests that it either acts at the cell surface or that it can be transported into the cell from the extracellular environment and play its normal role.

Expression profiling of GABAergic motor neurons in Caenorhabditis elegans

Neurons constitute the most diverse cell types and acquire their identity by the activity of particular genetic programs . The GABAergic nervous system in C. elegans consists of 26 neurons that fall into six classes . Animals that are defective in GABAergic neuron function and development display "shrinker" movement , abnormal foraging and defecation . Among the known shrinker genes, unc-25 and unc-47 encode the GABA biosynthetic enzyme glutamic acid decarboxylase and vesicular transporter, respectively . unc-30 encodes a homeodomain protein of the Pitx family and regulates the differentiation of the D-type GABAergic neurons . unc-46 probably functions in presynaptic GABA release , but its identity has not been reported. By cell-based microarray analysis, we identified over 250 genes with enriched expression in GABAergic neurons. The highly enriched gene set included all known genes. In vivo expression study with computational predictions further identified six new genes that are potential transcriptional targets of UNC-30. Behavioral studies of a deletion mutant implicate a function of a nicotinic receptor subunit in D-type neurons. Our analysis demonstrates the utility of neuron-specific genomics in identifying cell-specific genes and regulatory networks.

RNA interference in neuroscience

One of the most exciting findings in recent years has been the discovery of RNA interference (RNAi). RNAi is an important system that allows sequence-specific gene silencing through targeted degradation of mRNA by cognate double-stranded RNA (dsRNA). RNAi plays a role in endogenous cellular processes, such as developmental control and heterochromatin formation, and serves as an antiviral defense mechanism. Recent findings suggest that RNAi and related pathways are involved in protecting the genome against instabilities caused by transposons and repetitive sequences. Several rapidly developing RNAi methodologies provide a new approach for elucidation of gene functions. RNAi technology is also currently being evaluated as a potentially useful method to develop highly specific dsRNA-based gene-silencing therapeutics. In this paper, we review the use of RNAi in neuroscience research and as a possible therapeutic tool for treatment of neurological diseases.

Presynaptic terminal differentiation: transport and assembly

The formation of chemical synapses involves reciprocal induction and independent assembly of pre- and postsynaptic structures. The major events in presynaptic terminal differentiation are the formation of the active zone and the clustering of synaptic vesicles. A number of proteins that are present in the presynaptic active zone have been identified. Recent studies of various mutants have clarified the in vivo functions of some of the main players. Time-lapse imaging studies have captured dynamic and transient events in the transport of synaptic components, and therefore provided insight into the early stages of synaptogenesis.

Endophilin is required for synaptic vesicle endocytosis by localizing synaptojanin

Endophilin is a membrane-associated protein required for endocytosis of synaptic vesicles. Two models have been proposed for endophilin: that it alters lipid composition in order to shape membranes during endocytosis, or that it binds the polyphosphoinositide phosphatase synaptojanin and recruits this phosphatase to membranes. In this study, we demonstrate that the unc-57 gene encodes the Caenorhabditis elegans ortholog of endophilin A. We demonstrate that endophilin is required in C. elegans for synaptic vesicle recycling. Furthermore, the defects observed in endophilin mutants closely resemble those observed in synaptojanin mutants. The electrophysiological phenotype of endophilin and synaptojanin double mutants are virtually identical to the single mutants, demonstrating that endophilin and synaptojanin function in the same pathway. Finally, endophilin is required to stabilize expression of synaptojanin at the synapse. These data suggest that endophilin is an adaptor protein required to localize and stabilize synaptojanin at membranes during synaptic vesicle recycling.

N-Glycosylation Is Essential for Vesicular Targeting of Synaptotagmin 1

Synaptotagmins 1 and 7 are candidate Ca(2+) sensors for exocytosis localized to synaptic vesicles and plasma membranes, respectively. We now show that the N-terminal intraluminal sequence of synaptotagmin 1, when transplanted onto synaptotagmin 7, redirects synaptotagmin 7 from the plasma membrane to secretory vesicles. Conversely, mutation of the N-terminal N-glycosylation site of synaptotagmin 1 redirects synaptotagmin 1 from vesicles to the plasma membrane. In cultured hippocampal neurons, the plasma membrane-localized mutant of synaptotagmin 1 suppressed the readily releasable pool of synaptic vesicles, whereas wild-type synaptotagmin 1 did not. In addition to the intraluminal N-glycosylation site, the cytoplasmic C(2) domains of synaptotagmin 1 were required for correct targeting but could be functionally replaced by the C(2) domains of synaptotagmin 7. Our data suggest that the intravesicular N-glycosylation site of synaptotagmin 1 collaborates with its cytoplasmic C(2) domains in directing synaptotagmin 1 to synaptic vesicles via a novel N-glycosylation-dependent mechanism.

An overview of C. Elegans trafficking mutants

It is almost 40 years since Sydney Brenner introduced Caenorhabditis elegans as a model genetic system. During that time mutants with defects in intracellular trafficking have been identified in a diverse range of screens for abnormalities. This should, of course, come as no surprise as it is hard to imagine any biological process in which the regulated movement of vesicles within the cells is not critical at some step. Almost all of these genes have mammalian homologs, and yet the role of many of these homologs has not been investigated. Perhaps the protein that regulates your favorite trafficking step has already been identified in C. elegans? Here I provide a brief overview of those trafficking mutants identified in C. elegans and where you can read more about them.