synapse assembly and plasticity!

so i went to a lecture this morning given by Ann Marie Craig, who is currently at UBC, talking about the work she's been doing there and previously at WashU on synapse assembly and plasticity. most of her work was focused on neurexins and neuroligins, which i incidentally did a project on last summer quarter for bio 401. i'm not going to go into detail about these molecules right here and now, but the short of it is that neurexins are expressed on presynaptic neurons and neuroligins on postsynaptic neurons, and the interaction between NXs and NLs is a key interaction in synapse assembly. what makes these guys so difficult to study is the fact that each one has several splice isoforms - especially the neurexins. the long (alpha) isoform of neurexins has 5 splice sites, and the short (beta) isoform has 2 (confusingly called S4 and S5, since they are homologous to the 4th and 5th splice site in alpha-neurexins). so the Craig lab (as i understand from her talk this morning) has shown among other things that the "insert" in splice site 4 (S4) of neurexin 1-beta makes it bind specifically to neuroligin-2, which is specific to GABAergic synapses (which are generally inhibitory), whereas the form without the insert is much more likely to bind to NLs 1, 3 or 4, which are more specific to excitatory/glutamatergic synapses. they also showed that fibroblasts (basic, non-neural cells) co-cultured with hippocampal neurons, and ectopically expressing NX, can induce dendrites to form what she termed "hemipostsynapses" onto the fibroblast, whereas fibroblasts expressing ectopic NL can induce axons to form "hemipresynapses". pretty cool work.

but by far the coolest thing (IMHO) in her talk was some time-lapse imaging they did on cultured neurons with a fluorescently tagged version of CaMKIIa, or calcium/calmodulin-dependent protein kinase II-alpha. (Protein kinases are proteins which add a phosphate group to other proteins, and they are crucial for many intracellular signal transduction events.) CaM kinases have a special domain or subunit, calmodulin, which binds to calcium ions and becomes active. this protein is important in transducing signals from Ca ion concentration into phosphorylation signals... which is key to promoting synapse assembly and potentiation. so the cool data that she showed in the talk was that if a cultured hippocampal neuron with this fluorescently tagged CaMKIIa is stimulated with a "puff" of glutamate/glycine solution, within seconds of application, the (previously uniformly located) CaMKIIa clusters at synapses, and this wave of CaM movement propagates (in some cases) across the neuron, from dendrite through soma to axon. so what is going on? Calcium binding to the CaMK is somehow triggering it to relocate. what exactly is going on, she couldn't offer any ideas... but it is a cool result and i'll be interested in seeing what else they find out.

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