Rho deep in thought.

Neuronal communication underlies all aspects of brain function, including learning, memory, and consciousness. How neurons communicate is controlled by both the formation of neuronal connections during neural development and the regulation of neuronal activity in the adult brain. Rho GTPases have a well-known role in neuronal development, and recent studies published in Genes & Development (Steven et al. 2005; McMullan et al. 2006) have demonstrated that they also regulate neuronal activity in the adult brain—at least in Caenorhabditis elegans. Rho in C. elegans acts as part of a network of Gαq pathways that increase neuronal activity by regulating both production and destruction of the second messenger diacylglycerol (DAG), which is a regulator of synaptic vesicle release. In this issue of Genes & Development, Williams et al. (2007) demonstrate that Gαq acts via the UNC-73RhoGEF to increase Rho activity in neurons, and thus increase levels of DAG. The targets of DAG are known and, in one case, a pathway stretching from binding of ligand on the cell surface to changes in synaptic vesicle priming has been mapped out

Sensory Regulation of C. elegans Male Mate-Searching Behavior

SummaryHow do animals integrate internal drives and external environmental cues to coordinate behaviors? We address this question by studying mate-searching behavior in C. elegans. C. elegans males explore their environment in search of mates (hermaphrodites) and will leave food if mating partners are absent [1]. However, when mates and food coincide, male exploratory behavior is suppressed and males are retained on the food source [1]. We show that the drive to explore is stimulated by male-specific neurons in the tail, the ray neurons. Periodic contact with the hermaphrodite detected through ray neurons changes the male's behavior during periods of no contact and prevents the male from leaving the food source. The hermaphrodite signal is conveyed by male-specific interneurons that are postsynaptic to the rays and that send processes to the major integrative center in the head. This study identifies key parts of the neural circuit that regulates a sexual appetitive behavior in C. elegans

UNC-13 Interaction with Syntaxin Is Required for Synaptic Transmission

SummaryNeurotransmitter secretion at synapses is controlled by several processes—morphological docking of vesicles at release sites, priming of docked vesicles to make them fusion competent, and calcium-dependent fusion of vesicles with the plasma membrane [1, 2]. In worms, flies, and mice, mutants lacking UNC-13 have defects in vesicle priming [3–5]. Current models propose that UNC-13 primes vesicles by stabilizing Syntaxin's “open” conformation by directly interacting with its amino-terminal regulatory domain [6–8]. However, the functional significance of the UNC-13/Syntaxin interaction has not been tested directly. A truncated protein containing the Munc homology domains (MHD1 and MHD2) and the carboxy-terminal C2 domain partially rescued both the behavioral and secretion defects of unc-13 mutants in C. elegans. A double mutation in MHD2 (F1000A/K1002A) disrupts the UNC-13/Syntaxin interaction. The rate of endogenous synaptic events and the amplitude of nerve-evoked excitatory post-synaptic currents (EPSCs) were both significantly reduced in UNC-13S(F1000A/K1002A). However, the pool of primed (i.e., fusion-competent) vesicles was normal. These results suggest that the UNC-13/Syntaxin interaction is conserved in C. elegans and that, contrary to current models, the UNC-13/Syntaxin interaction is required for nerve-evoked vesicle fusion rather than synaptic-vesicle priming. Thus, UNC-13 may regulate multiple steps of the synaptic-vesicle cycle

The RHO-1 RhoGTPase Modulates Fertility and Multiple Behaviors in Adult C. elegans

The Rho family of small GTPases are essential during early embryonic development making it difficult to study their functions in adult animals. Using inducible transgenes expressing either a constitutively active version of the single C. elegans Rho ortholog, RHO-1, or an inhibitor of endogenous Rho (C3 transferase), we demonstrate multiple defects caused by altering Rho signaling in adult C. elegans. Changes in RHO-1 signaling in cholinergic neurons affected locomotion, pharyngeal pumping and fecundity. Changes in RHO-1 signaling outside the cholinergic neurons resulted in defective defecation, ovulation, and changes in C. elegans body morphology. Finally both increased and decreased RHO-1 signaling in adults resulted in death within hours. The multiple post-developmental roles for Rho in C. elegans demonstrate that RhoA signaling pathways continue to be used post-developmentally and the resulting phenotypes provide an opportunity to further study post-developmental Rho signaling pathways using genetic screens

Behavioral and Immune Responses to Infection Require Gαq- RhoA Signaling in C. elegans

Following pathogen infection the hosts' nervous and immune systems react with coordinated responses to the danger. A key question is how the neuronal and immune responses to pathogens are coordinated, are there common signaling pathways used by both responses? Using C. elegans we show that infection by pathogenic strains of M. nematophilum, but not exposure to avirulent strains, triggers behavioral and immune responses both of which require a conserved Gαq-RhoGEF Trio-Rho signaling pathway. Upon infection signaling by the Gαq pathway within cholinergic motorneurons is necessary and sufficient to increase release of the neurotransmitter acetylcholine and increase locomotion rates and these behavioral changes result in C. elegans leaving lawns of M. nematophilum. In the immune response to infection signaling by the Gαq pathway within rectal epithelial cells is necessary and sufficient to cause changes in cell morphology resulting in tail swelling that limits the infection. These Gαq mediated behavioral and immune responses to infection are separate, act in a cell autonomous fashion and activation of this pathway in the appropriate cells can trigger these responses in the absence of infection. Within the rectal epithelium the Gαq signaling pathway cooperates with a Ras signaling pathway to activate a Raf-ERK-MAPK pathway to trigger the cell morphology changes, whereas in motorneurons Gαq signaling triggers behavioral responses independent of Ras signaling. Thus, a conserved Gαq pathway cooperates with cell specific factors in the nervous and immune systems to produce appropriate responses to pathogen. Thus, our data suggests that ligands for Gq coupled receptors are likely to be part of the signals generated in response to M. nematophilum infection

Rho is a presynaptic activator of neurotransmitter release at pre-existing synapses in C. elegans

Rho GTPases have important roles in neuronal development, but their function in adult neurons is less well understood. We demonstrate that presynaptic changes in Rho activity at Caenorhabditis elegans neuromuscular junctions can radically change animal behavior via modulation of two separate pathways. In one, presynaptic Rho increases acetylcholine (ACh) release by stimulating the accumulation of diacylglycerol (DAG) and the DAG-binding protein UNC-13 at sites of neurotransmitter release; this pathway requires binding of Rho to the DAG kinase DGK-1. A second DGK-1-independent mechanism is revealed by the ability of a Rho inhibitor (C3 transferase) to decrease levels of release even in the absence of DGK-1; this pathway is independent of UNC-13 accumulation at release sites. We do not detect any Rho-induced changes in neuronal morphology or synapse number; thus, Rho facilitates synaptic transmission by a novel mechanism. Surprisingly, many commonly available human RhoA constructs contain an uncharacterized mutation that severely reduces binding of RhoA to DAG kinase. Thus, a role for RhoA in controlling DAG levels is likely to have been underestimated

Gαq-Rho GEF Trio-Rho Signaling is required for aversion to pathogenic <i>M. nematophilum.</i>

<p>Animals were placed equidistant from a two lawns of bacteria (A vs B) and the number of animals on lawns A and B were counted at 30 minutes (solid bars) and at 4 hours (hatched bars). The preference ratio shown is given by the formula [animals at A- animals at B/animals (A+B)]. Wildtype animals have no preference between <i>E. coli</i> (OP50) and pathogenic <i>M. nematophilum</i> at 30 minutes, but at 4 hours they have a strong preference for OP50 E. coli. This preference is abolished if the strain of <i>M. nematophilum</i> is avirulent (UV336) or if animals have a mutation in <i>unc-73</i> or <i>egl-30</i>. Expression of EGL-30 in the motorneurons (MN::EGL-30) or of UNC-73 in all neurons (N::UNC-73) rescued the preference for OP50 in <i>egl-30</i> and <i>unc-73</i> mutants respectively. P values between 0.05 and 0.001 (*), and P values of 0.001 or less (**).</p