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Postsynaptic expression of tetanus toxin light chain blocks synaptogenesis in Drosophila.

ABSTRACT: During the development of the nervous system embryonic neurons are incorporated into neural networks that underlie behaviour. For example, during embryogenesis in Drosophila, motor neurons in every body segment are wired into the circuitry that drives the simple peristaltic locomotion of the larva. Very little is known about the way in which the necessary central synapses are formed in such a network or how their properties are controlled. One possibility is that presynaptic and postsynaptic elements form relatively independently of each other. Alternatively, there might be an interaction between presynaptic and postsynaptic neurons that allows for adjustment and plasticity in the embryonic network. Here we have addressed this issue by analysing the role of synaptic transmission in the formation of synaptic inputs onto identified motorneurons as the locomotor circuitry is assembled in the Drosophila embryo. We targeted the expression of tetanus toxin light chain (TeTxLC) to single identified neurons using the GAL4 system. TeTxLC prevents the evoked release of neurotransmitter by enzymatically cleaving the synaptic-vesicle-associated protein neuronal-Synaptobrevin (n-Syb) [1]. Unexpectedly, we found that the cells that expressed TeTxLC, which were themselves incapable of evoked release, showed a dramatic reduction in synaptic input. We detected this reduction both electrophysiologically and ultrastructurally.

SUBMITTER: Baines RA

PROVIDER: S-EPMC2726806 | biostudies-literature | 1999 Nov

REPOSITORIES: biostudies-literature

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Postsynaptic expression of tetanus toxin light chain blocks synaptogenesis in Drosophila.

Baines R A RA Robinson S G SG Fujioka M M Jaynes J B JB Bate M M

Current biology : CB 19991101 21

During the development of the nervous system embryonic neurons are incorporated into neural networks that underlie behaviour. For example, during embryogenesis in Drosophila, motor neurons in every body segment are wired into the circuitry that drives the simple peristaltic locomotion of the larva. Very little is known about the way in which the necessary central synapses are formed in such a network or how their properties are controlled. One possibility is that presynaptic and postsynaptic ele ...[more]

PMID: 10556094

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Project description:The clostridial neurotoxins (CNTs) comprise tetanus toxin (TT) and botulinum neurotoxin (BoNT [BT]) serotypes (A to G and X) and several recently identified CNT-like proteins, including BT/En and the mosquito BoNT-like toxin Pmp1. CNTs are produced as single proteins cleaved to a light chain (LC) and a heavy chain (HC) connected by an interchain disulfide bond. LC is a zinc metalloprotease (cleaving soluble N-ethylmaleimide-sensitive factor attachment protein receptors [SNAREs]), while HC contains an N-terminal translocation domain (HCN) and a C-terminal receptor binding domain (HCC). HCN-mediated LC translocation is the least understood function of CNT action. Here, ?-lactamase (?lac) was used as a reporter in discovery-based live-cell assays to characterize TT-mediated LC translocation. Directed mutagenesis identified a role for a charged loop (767DKE769) connecting ?15 and ?16 (cis-loop) within HCN in LC translocation; aliphatic substitution inhibited LC translocation but not other toxin functions such as cell binding, intracellular trafficking, or HCN-mediated pore formation. K768 was conserved among the CNTs. In molecular simulations of the HCN with a membrane, the cis-loop did not bind with the cell membrane. Taken together, the results of these studies implicate the cis-loop in LC translocation, independently of pore formation.IMPORTANCE How protein toxins translocate their catalytic domain across a cell membrane is the least understood step in toxin action. This study utilized a reporter, ?-lactamase, that was genetically fused to full-length, nontoxic tetanus toxin (?lac-TT) in discovery-based live-cell assays to study LC translocation. Directed mutagenesis identified a role for K768 in LC translocation. K768 was located between ?15 and ?16 (termed the cis-loop). Cellular assays showed that K768 did not interfere with other toxin functions, including cell binding, intracellular trafficking, and pore formation. The equivalent K768 is conserved among the clostridial neurotoxin family of proteins as a conserved structural motif. The cis-loop appears to contribute to LC translocation.

Project description:Background: Synaptic transmission is required for functional maturation of the CNS and to induce gene transcription involved in neuronal plasticity. Our aim is to identify genes that are transcriptionally regulated by synaptic transmission during development. cDNA microarrays will be used to compare gene expression in normal embryos and embryos with no synaptic transmission. Using the Gal4 UAS system, the tetanus toxin light fragment (TET) will be expressed throughout the nervous system. The effects of TET are well characterised: the synaptic protein n-Synaptobrevin is cleaved, blocking evokes vesicle fusion in TET expressing neurons and as a result synaptic transmission is blocked. Embryos expressing an inactive, mutant version of TET (iTET) show no detectable phenotype. Thus a comparison of gene expression in iTET expressing embryos and TET expressing embryos focuses our screen cleanly and specifically on genes that are regulated by synaptic transmission. At the end of larval life, during metamorphosis, there is a second phase of nervous system development with the same requirement for neural circuit differentiation and maturation as in the embryo. We will compare gene expression in TET expressing and iTET expressing pupae to identify genes which are regulated by synaptic transmission in this second phase of nervous system development. We will focus our analysis on genes which are similarly regulated in both systems. Plan: elav Gal4 (expressed in all the neurons) for the embryos and Heat Shock Gal4 (which allows a temporally controlled expression) for the pupae are used to drive UAS TET and UAS iTET throughout the nervous system. Crosses are set up with flies homozygous for Gal4 or UAS transgenes, so that all the progeny have the same genotype. Total RNA is extracted from 500 carefully staged embryos that have been collected at the time of hatching. Pupae are heat shocked at 24 hours after puparium formation, and heads are then collected 5 days after puparium formation. TET expressing pupae are paralysed but morphologically normal whereas iTET expressing pupae emerge as normal adults. Total RNA will be extracted from 250 heads of each genotype. Probes made from TET and iTET expressing embryos extracts will be hybridised together to the chip and so will be probes made from TET and iTET expressing pupae extracts. Genes regulated in both systems will be compared together.

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Postsynaptic expression of tetanus toxin light chain blocks synaptogenesis in Drosophila.

Publications

Postsynaptic expression of tetanus toxin light chain blocks synaptogenesis in Drosophila.

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