The Gi/Go-like G alpha protein ODR-3 is strongly and selectively implicated in the function of C. elegans olfactory and nociceptive neurons. Either loss of odr-3 function or overexpression of odr-3 causes severe olfactory defects, and odr-3 function is essential in the ASH neurons that sense noxious chemical and mechanical stimuli. In the nociceptive neurons, ODR-3 may interact with OSM-9, a channel similar to the mammalian capsaicin receptor implicated in pain sensation; in AWC olfactory neurons, ODR-3 may interact with another signal transduction pathway. ODR-3 exhibits an unexpected ability to regulate morphogenesis of the olfactory cilia. In odr-3 null mutants, the fan-like AWC cilia take on a filamentous morphology like normal AWA cilia, whereas ODR-3 overexpression in AWA transforms its filamentous cilia into a fan-like morphology.
During synapse formation, presynaptic axon outgrowth is terminated, presynaptic clusters of vesicles are associated with active zone proteins, and active zones are aligned with postsynaptic neurotransmitter receptors. We report here the identification of a novel serine/threonine kinase, SAD-1, that regulates several aspects of presynaptic differentiation in C. elegans. In sad-1 mutant animals presynaptic vesicle clusters in sensory neurons and motor neurons are diffuse and disorganized. Sensory axons fail to terminate in sad-1 mutants, whereas overexpression of SAD-1 causes sensory axons to terminate prematurely. SAD-1 protein is expressed in the nervous system and localizes to synapse-rich regions of the axons. SAD-1 is related to PAR-1, a kinase that regulates cell polarity during asymmetric cell division. Overexpression of SAD-1 causes mislocalization of vesicle proteins to dendrites, suggesting that sad-1 affects axonal-dendritic polarity as well as synaptic development.
Odorant receptors and signaling proteins are localized to sensory cilia on olfactory dendrites. Using a GFP-tagged odorant receptor protein, Caenorhabditis elegans ODR-10, we characterized protein sorting and transport in olfactory neurons in vivo. ODR-10 is transported in rapidly moving dendritic vesicles that shuttle between the cell body and the cilia. Anterograde and retrograde vesicles move at different speeds, suggesting that dendrites have polarized transport mechanisms. Residues immediately after the seventh membrane-spanning domain of ODR-10 are required for localization; these residues are conserved in many G protein-coupled receptors. UNC-101 encodes a mu1 subunit of the AP-1 clathrin adaptor complex. In unc-101 mutants, dendritic vesicles are absent, ODR-10 receptor is evenly distributed over the plasma membrane, and other cilia membrane proteins are also mislocalized, implicating AP-1 in protein sorting to olfactory cilia.
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