| Cyclin-dependent kinase-5 (CDK-5) is a serine/threonine kinase with many important functions in neurons, including regulation of development, protein transport, synaptic transmission and plasticity, and neurodegeneration. Aberrant activation of CDK-5 can cause neuronal death, and inhibition of CDK-5 is protective in animal models of Alzheimer's disease, prion protein disease, and Huntington's disease. In this thesis, I use C. elegans to investigate the role of CDK-5 in regulation of neuropeptide trafficking and synaptic transmission. In Chapter 2, I identify a novel role for CDK-5 in the regulation of polarized trafficking of dense-core vesicles. Dense-core vesicles (DCVs) release neuropeptides, which are key regulators of synaptic strength and plasticity, but little is known about the regulation of DCV transport. I utilize a fluorescently-tagged neuropeptide (INS-22::Venus) to visualize DCVs in DA and DB cholinergic neurons, which have a well-defined axon and dendrite. In wild-type animals INS-22::Venus localizes primarily to axons, but cdk-5 mutants have reduced axonal DCVs and increased dendritic DCVs, indicating that CDK-5 promotes the polarized distribution of DCVs to the axon. I find that CDK-5 is not required to establish axon/dendrite polarity, but instead acts at the cell body to inhibit the trafficking of DCVs into the dendrite. Mutations in cytoplasmic dynein (dhc-1) completely block the increase in dendritic DCVs observed in cdk-5 mutants, suggesting that CDK-5 inhibits dynein-mediated trafficking of DCVs into dendrites. In Chapter 3, I identify cell-type specific differences between DA and DB motor neurons. In DA neurons, cdk-5 mutants have increased dendritic DCV markers without a corresponding change in axonal DCVs, suggesting CDK-5 has independent functions in the axon and dendrite. DB motor neurons, in contrast, have decreased axonal DCVs in cdk-5 mutants. Similarly, time-lapse imaging shows that DCV trafficking to the axon is altered in DB neurons, but not DA neurons, in cdk-5 mutants. Finally, I investigate the role of CDK-5 in synaptic transmission at the neuromuscular junction (NMJ) using a paralysis assay based on an inhibitor of acetylcholinesterase, aldicarb. These studies reveal that CDK-5 promotes acetylcholine release at the NMJ and lay the groundwork for future studies to identify novel CDK-5 substrates and regulators that promote synaptic transmission. |
