The insulin-like signaling pathway is known to regulate fat metabolism, dauer formation, and longevity in Caenorhabditis elegans. Here, we report that this pathway is also involved in salt chemotaxis learning, in which animals previously exposed to a chemoattractive salt under starvation conditions start to show salt avoidance behavior. Mutants of ins-1, daf-2, age-1, pdk-1, and akt-1, which encode the homologs of insulin, insulin/IGF-I receptor, PI 3-kinase, phosphoinositide-dependent kinase, and Akt/PKB, respectively, show severe defects in salt chemotaxis learning. daf-2 and age-1 act in the ASER salt-sensing neuron, and the activity level of the DAF-2/AGE-1 pathway in this neuron determines the extent and orientation of salt chemotaxis. On the other hand, ins-1 acts in AIA interneurons, which receive direct synaptic inputs from sensory neurons and also send synaptic outputs to ASER. These results suggest that INS-1 secreted from AIA interneurons provides feedback to ASER to generate plasticity of chemotaxis.

ASH sensory neurons are required in Caenorhabditis elegans for a wide range of avoidance behaviors in response to chemical repellents, high osmotic solutions and nose touch. The ASH neurons are therefore hypothesized to be polymodal nociceptive neurons. To understand the nature of polymodal sensory response and adaptation at the cellular level, we expressed the calcium indicator protein cameleon in ASH and analyzed intracellular Ca 2 þ responses following stimulation with chemical repellents, osmotic shock and nose touch. We found that a variety of noxious stimuli evoked strong responses in ASH including quinine, denatonium, detergents, heavy metals, both hyper-and hypo-osmotic shock and nose touch. We observed that repeated chemical stimulation led to a reversible reduction in the magnitude of the sensory response, indicating that adaptation occurs within the ASH sensory neuron. A key component of ASH adaptation is GPC-1, a G-protein c-subunit expressed specifically in chemosensory neurons. We hypothesize that G-protein c-subunit heterogeneity provides a mechanism for repellent-specific adaptation, which could facilitate discrimination of a variety of repellents by these polymodal sensory neurons.

Summary Chemotaxis in C. elegans, like chemotaxis in bacteria1, involves a random walk biased by the time derivative of attractant concentration2,3, but how the derivative is computed is unknown. Laser ablations have shown that the strongest deficits in chemotaxis to salts are obtained when the ASE neurons (ASEL and ASER) are killed, indicating that this pair plays a dominant role4. Although these neurons are left-right homologs anatomically, they exhibit marked asymmetries in gene expression and ion preference5–7. Here, using optical recordings of calcium transients in ASE neurons in intact animals, we demonstrate an additional asymmetry: ASEL is an ON-cell, stimulated by increases in NaCl concentration, whereas ASER is an OFF-cell, stimulated by decreases in NaCl concentration. Both responses are reliable yet transient, indicating that ASE neurons report changes in concentration rather than absolute levels. Recordings from synaptic and sensory transduction mutants show that the ON-OFF asymmetry is the result of intrinsic differences between ASE neurons. Unilateral activation experiments indicate that the asymmetry extends to the level of behavioral output: ASEL lengthens bouts of forward locomotion (runs) whereas ASER promotes direction changes (turns). Strikingly, the input and output asymmetries of ASE neurons are precisely those of a simple yet novel neuronal motif for computing the time derivative of chemosensory information, which is the fundamental computation of C. elegans chemotaxis3,8. Evidence for ON and OFF cells in other chemosensory networks9–12 suggests that this motif may be common in animals that navigate by taste and smell.

In the nematode C. elegans, genes encoding components of a putative mechanotransducing channel complex have been identified in screens for light-touch-insensitive mutants. A long-standing question, however, is whether identified MEC proteins act directly in touch transduction or contribute indirectly by maintaining basic mechanoreceptor neuron physiology. In this study, we used the genetically encoded calcium indicator cameleon to record cellular responses of mechanosensory neurons to touch stimuli in intact, behaving nematodes. We defined a gentle touch sensory modality that adapts with a time course of approximately 500 ms and primarily senses motion rather than pressure. The DEG/ENaC channel subunit MEC-4 and channel-associated stomatin MEC-2 are specifically required for neural responses to gentle mechanical stimulation, but do not affect the basic physiology of touch neurons or their in vivo responses to harsh mechanical stimulation. These results distinguish a specific role for the MEC channel proteins in the process of gentle touch mechanosensation.

Chromophore-assisted light inactivation (CALI) is a powerful technique for acute perturbation of biomolecules in a spatio-temporally defined manner in living specimen with reactive oxygen species (ROS). Whereas a chemical photosensitizer including fluorescein must be added to specimens exogenously and cannot be restricted to particular cells or sub-cellular compartments, a genetically-encoded photosensitizer, KillerRed, can be controlled in its expression by tissue specific promoters or subcellular localization tags. Despite of this superiority, KillerRed hasn't yet become a versatile tool because its dimerization tendency prevents fusion with proteins of interest. Here, we report the development of monomeric variant of KillerRed (SuperNova) by direct evolution using random mutagenesis. In contrast to KillerRed, SuperNova in fusion with target proteins shows proper localization. Furthermore, unlike KillerRed, SuperNova expression alone doesn't perturb mitotic cell division. Supernova retains the ability to generate ROS, and hence promote CALI-based functional analysis of target proteins overcoming the major drawbacks of KillerRed.