For many organisms the ability to transduce light into cellular signals is crucial for survival. Light stimulates DNA repair and metabolism changes in bacteria, avoidance responses in single-cell organisms, attraction responses in plants, and both visual and nonvisual perception in animals. Despite these widely differing responses, in all of nature there are only six known families of proteins that can transduce light. Although the roundworm Caenorhabditis elegans has none of the known light transduction systems, we show here that C. elegans strongly accelerates its locomotion in response to blue or shorter wavelengths of light, with maximal responsiveness to ultraviolet light. Our data suggest that C. elegans uses this light response to escape the lethal doses of sunlight that permeate its habitat. Short-wavelength light drives locomotion by bypassing two critical signals, cyclic adenosine monophosphate (cAMP) and diacylglycerol (DAG), that neurons use to shape and control behaviors. C. elegans mutants lacking these signals are paralyzed and unresponsive to harsh physical stimuli in ambient light, but short-wavelength light rapidly rescues their paralysis and restores normal levels of coordinated locomotion. This light response is mediated by LITE-1, a novel ultraviolet light receptor that acts in neurons and is a member of the invertebrate Gustatory receptor (Gr) family. Heterologous expression of the receptor in muscle cells is sufficient to confer light responsiveness on cells that are normally unresponsive to light. Our results reveal a novel molecular solution for ultraviolet light detection and an unusual sensory modality in C. elegans that is unlike any previously described light response in any organism.

The G␣ q pathway is essential for animal life and is a central pathway for driving locomotion, egg laying, and growth in Caenorhabditis elegans, where it exerts its effects through EGL-8 (phospholipase C␤ [PLC␤]) and at least one other effector. To find the missing effector, we performed forward genetic screens to suppress the slow growth and hyperactive behaviors of mutants with an overactive G␣ q pathway. Four suppressor mutations disrupted the Rho-specific guanine-nucleotide exchange factor (GEF) domain of UNC-73 (Trio). The mutations produce defects in neuronal function, but not neuronal development, that cause sluggish locomotion similar to animals lacking EGL-8 (PLC␤). Strains containing null mutations in both EGL-8 (PLC␤) and UNC-73 (Trio RhoGEF) have strong synthetic phenotypes that phenocopy the arrested growth and near-complete paralysis of G␣ q -null mutants. Using cell-based and biochemical assays, we show that activated C. elegans G␣ q synergizes with Trio RhoGEF to activate RhoA. Activated G␣ q and Trio RhoGEF appear to be part of a signaling complex, because they coimmunoprecipitate when expressed together in cells. Our results show that Trio's Rho-specific GEF domain is a major G␣ q effector that, together with PLC␤, mediates the G␣ q signaling that drives the locomotion, egg laying, and growth of the animal.[Keywords: G␣ q ; Trio; Rho; phospholipase C␤; C. elegans] Supplemental material is available at http://www.genesdev.org.

Uncoordinated movement in Rab2 mutants is caused by impaired retention of cargo on dense core vesicles, not by defective synaptic vesicle release. (Also see the companion article by Sumakovic et al. in this issue.)

C. elegans mutants lacking the dense-core vesicle priming protein UNC-31 (CAPS) share highly similar phenotypes with mutants lacking a neuronal Ga s pathway, including strong paralysis despite exhibiting near normal levels of steady-state acetylcholine release as indicated by drug sensitivity assays. Our genetic analysis shows that UNC-31 and neuronal Ga s are different parts of the same pathway and that the UNC-31/Ga s pathway is functionally distinct from the presynaptic Ga q pathway with which it interacts. UNC-31 acts upstream of Ga s because mutations that activate the Ga s pathway confer similar levels of strongly hyperactive, coordinated locomotion in both unc-31 null and (1) backgrounds. Using cell-specific promoters, we show that both UNC-31 and the Ga s pathway function in cholinergic motor neurons to regulate locomotion rate. Using immunostaining we show that UNC-31 is often concentrated at or near active zones of cholinergic motor neuron synapses. Our data suggest that presynaptic UNC-31 activity, likely acting via dense-core vesicle exocytosis, is required to locally activate the neuronal Ga s pathway near synaptic active zones.A T least two distinct vesicle systems, synaptic vesicles and dense core vesicles, mediate the presynaptic release of neurotransmitters. The synaptic vesicle system mediates the highly localized, rapid release of classical neurotransmitters from active zones. Presynaptic dense core vesicle release does not occur at morphologically defined active zones and is generally associated with neuropeptide release. Little is known about whether and how these two vesicle systems interact at the synapse during the execution of behaviors. Both vesicle systems release their contents in response to electrically induced calcium influx (Martin 2003), and each system has a mechanism for priming vesicles, which is the process by which vesicles become competent to fuse in response to the calcium signal. Synaptic vesicles use an UNC-13-based system (Aravamudan et al. 1999;Augustin et al. 1999;Richmond et al. 1999Richmond et al. , 2001. Dense core vesicles use a CAPS-based system (Grishanin et al. 2004). UNC-13 and CAPS are distantly related as indicated by a shared UNC-13 homology domain (Grishanin et al. 2002).In either system, if vesicles are not primed, they will not release neurotransmitter in response to stimuli, even if calcium enters the synapse or cell (Hay and Martin 1992;Aravamudan et al. 1999;Augustin et al. 1999;Richmond et al. 1999). Although the pathways regulating dense core vesicle priming are poorly understood, previous studies have shown that diacylglycerol (DAG) and cAMP can regulate the readily releasable (primed) pool of synaptic vesicles (Chen and Regehr 1997;Stevens and Sullivan 1998;Waters and Smith 2000;Kidokoro et al. 2004;Virmani et al. 2005). Genetic studies in the model organism Caenorhabditis elegans have identified a network of pathways that appears to regulate priming in the synaptic vesicle system. These studies have suggested a model, the synaptic signal...

Forward genetic screens for mutations that rescue the paralysis of ric-8 (Synembryn) reduction-of-function mutations frequently reveal mutations that cause hyperactivation of one or more components of the Ga s pathway. Here, we report that one of these mutations strongly reduces the function of the Dunce cAMP phosphodiesterase PDE-4 by disrupting a conserved active site residue. Loss of function and neural overexpression of PDE-4 have profound and opposite effects on locomotion rate, but drug-response assays suggest that loss of PDE-4 function does not affect steady-state acetylcholine release or reception. Our genetic analysis suggests that PDE-4 regulates both Ga s -dependent and Ga s -independent cAMP pools in the neurons controlling locomotion rate. By immunostaining, PDE-4 is strongly expressed throughout the nervous system, where it localizes to small regions at the outside boundaries of synaptic vesicle clusters as well as intersynaptic regions. The synaptic subregions containing PDE-4 are distinct from those containing active zones, as indicated by costaining with an antibody against the long form of UNC-13. This highly focal subsynaptic localization suggests that PDE-4 may exert its effects by spatially regulating intrasynaptic cAMP pools.