Small RNAs couple embryonic developmental programs to gut microbes.

Embryogenesis has long been known for its robustness to environmental factors. Although developmental tuning of embryogenesis to the environment experienced by the parent may be beneficial, little is understood on whether and how developmental patterns proactively change. Here, we show that Caenorhabditis elegans undergoes alternative embryogenesis in response to maternal gut microbes. Harmful microbes result in altered endodermal cell divisions; morphological changes, including left-right asymmetric development; double association between intestinal and primordial germ cells; and partial rescue of fecundity. The miR-35 microRNA family, which is controlled by systemic endogenous RNA interference and targets the ß-transducin repeat-containing protein/cell division cycle 25 (CDC25) pathway, transmits intergenerational information to regulate cell divisions and reproduction. Our findings challenge the widespread assumption that C. elegans has an invariant cell lineage that consists of a fixed cell number and provide insights into how organisms optimize embryogenesis to adapt to environmental changes through epigenetic control.

Characterization of putative tachykinin peptides in Caenorhabditis elegans.

Tachykinin-like peptides, such as substance P, neurokinin A, and neurokinin B, are among the earliest discovered and best-studied neuropeptide families, and research on them has contributed greatly to our understanding of the endocrine control of many physiological processes. However, there are still many orphan tachykinin receptor homologs for which cognate ligands have not yet been identified, especially in small invertebrates, such as the nematode Caenorhabditis elegans (C. elegans). We here show that the C. elegans nlp-58 gene encodes putative ligands for the orphan G protein-coupled receptor (GPCR) TKR-1, which is a worm ortholog of tachykinin receptors. We first determine, through an unbiased biochemical screen, that a peptide derived from the NLP-58 preprotein stimulates TKR-1. Three mature peptides that are predicted to be generated from NLP-58 show potent agonist activity against TKR-1. We designate these peptides as C. elegans tachykinin (CeTK)-1, -2, and -3. The CeTK peptides contain the C-terminal sequence GLR-amide, which is shared by tachykinin-like peptides in other invertebrate species. nlp-58 exhibits a strongly restricted expression pattern in several neurons, implying that CeTKs behave as neuropeptides. The discovery of CeTKs provides important information to aid our understanding of tachykinin-like peptides and their functional interaction with GPCRs.

Dynamics of Presynaptic Diacylglycerol in a Sensory Neuron Encode Differences between Past and Current Stimulus Intensity.

Memorizing the intensity of sensory stimuli enables animals to successfully deal with changing environmental conditions and contributes to cognitive functions such as auditory and visual working memory. However, how nervous systems process past and current stimulus intensity is largely unknown at the molecular level. Here, we employ in vivo diacylglycerol (DAG) imaging in the ASER taste neuron of Caenorhabditis elegans and demonstrate that associative learning between ambient salt concentrations and food can be explained by changes in presynaptic DAG. The abundance of DAG is regulated in response to external salt concentration changes via sensory transduction in ASER and can encode differences between past and current salt concentrations. The DAG dynamics are modulated downstream of the synaptic insulin/phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which regulates the behavioral plasticity induced by starvation. These results provide insights into how a single neuron stores past input intensity and generates appropriate behavioral responses.

Luqin-like RYamide peptides regulate food-evoked responses in C. elegans.

Peptide signaling controls many processes involving coordinated actions of multiple organs, such as hormone-mediated appetite regulation. However, the extent to which the mode of action of peptide signaling is conserved in different animals is largely unknown, because many peptides and receptors remain orphan and many undiscovered peptides still exist. Here, we identify two novel Caenorhabditis elegans neuropeptides, LURY-1-1 and LURY-1-2, as endogenous ligands for the neuropeptide receptor-22 (NPR-22). Both peptides derive from the same precursor that is orthologous to invertebrate luqin/arginine-tyrosine-NH2 (RYamide) proneuropeptides. LURY-1 peptides are secreted from two classes of pharyngeal neurons and control food-related processes: feeding, lifespan, egg-laying, and locomotory behavior. We propose that LURY-1 peptides transmit food signals to NPR-22 expressed in feeding pacemaker neurons and a serotonergic neuron. Our results identified a critical role for luqin-like RYamides in feeding-related processes and suggested that peptide-mediated negative feedback is important for satiety regulation in C. elegans.

The intestinal TORC2 signaling pathway contributes to associative learning in Caenorhabditis elegans.

Several types of associative learning are dependent upon the presence or absence of food, and are crucial for the survival of most animals. Target of rapamycin (TOR), a kinase which exists as a component of two complexes, TOR complex 1 (TORC1) and TOR complex 2 (TORC2), is known to act as a nutrient sensor in numerous organisms. However, the in vivo roles of TOR signaling in the nervous system remain largely unclear, partly because its multifunctionality and requirement for survival make it difficult to investigate. Here, using pharmacological inhibitors and genetic analyses, we show that TORC1 and TORC2 contribute to associative learning between salt and food availability in the nematode Caenorhabditis elegans in a process called taste associative learning. Worms migrate to salt concentrations experienced previously during feeding, but they avoid salt concentrations experienced under starvation conditions. Administration of the TOR inhibitor rapamycin causes a behavioral defect after starvation conditioning. Worms lacking either RICT-1 or SINH-1, two TORC2 components, show defects in migration to high salt levels after learning under both fed and starved conditions. We also analyzed the behavioral phenotypes of mutants of the putative TORC1 substrate RSKS-1 (the C. elegans homolog of the mammalian S6 kinase S6K) and the putative TORC2 substrates SGK-1 and PKC-2 (homologs of the serum and glucocorticoid-induced kinase 1, SGK1, and protein kinase C-α, PKC-α, respectively) and found that neuronal RSKS-1 and PKC-2, as well as intestinal SGK-1, are involved in taste associative learning. Our findings shed light on the functions of TOR signaling in behavioral plasticity and provide insight into the mechanisms by which information sensed in the intestine affects the nervous system to modulate food-searching behaviors.

Role of synaptic phosphatidylinositol 3-kinase in a behavioral learning response in C. elegans.

The phosphatidylinositol 3-kinase (PI3K) pathway regulates many cellular functions, but its roles in the nervous system are still poorly understood. We found that a newly discovered insulin receptor isoform, DAF-2c, is translocated from the cell body to the synaptic region of the chemosensory neuron in Caenorhabditis elegans by a conditioning stimulus that induces taste avoidance learning. This translocation is essential for learning and is dependent on the mitogen-activated protein kinase-regulated interaction of CASY-1 (the calsyntenin ortholog) and kinesin-1. The PI3K pathway is required downstream of the receptor. Light-regulated activation of PI3K in the synaptic region, but not in other parts of the cell, switched taste-attractive behavior to taste avoidance, mimicking the effect of conditioning. Thus, synaptic PI3K is crucial for the behavioral switch caused by learning.

S. pombe TORC1 activates the ubiquitin-proteasomal degradation of the meiotic regulator Mei2 in cooperation with Pat1 kinase.

Target of rapamycin (TOR) kinase regulates cell metabolism and growth, acting as a subunit of two multi-protein complexes, TORC1 and TORC2. Known TORC substrates are either kinases or general factors involved in growth control. Here, we show that fission yeast TORC1, which promotes vegetative growth and suppresses sexual development, can phosphorylate Mei2 (a specific factor involved in switching the cell fate) in vitro. Alanine substitutions at the nine Mei2 phosphorylation sites stabilize the protein and promote mating and meiosis in vivo. We found that Mei2 is polyubiquitylated in vivo in a TORC1-dependent manner. Based on these data, we propose that TORC1 contributes to the suppression of sexual development by phosphorylating Mei2, in addition to controlling the cellular metabolic status.

Concentration memory-dependent synaptic plasticity of a taste circuit regulates salt concentration chemotaxis in Caenorhabditis elegans.

It is poorly understood how sensory systems memorize the intensity of sensory stimulus, compare it with a newly sensed stimulus, and regulate the orientation behaviour based on the memory. Here we report that Caenorhabditis elegans memorizes the environmental salt concentration during cultivation and exhibits a strong behavioural preference for this concentration. The right-sided amphid gustatory neuron known as ASER, senses decreases in salt concentration, and this information is transmitted to the postsynaptic AIB interneurons only in the salt concentration range lower than the cultivation concentration. In this range, animals migrate towards higher concentration by promoting turning behaviour upon decreases in salt concentration. These observations provide a mechanism for adjusting the orientation behaviour based on the memory of sensory stimulus using a simple neural circuit.

Reversal of salt preference is directed by the insulin/PI3K and Gq/PKC signaling in Caenorhabditis elegans.

Animals search for foods and decide their behaviors according to previous experience. Caenorhabditis elegans detects chemicals with a limited number of sensory neurons, allowing us to dissect roles of each neuron for innate and learned behaviors. C. elegans is attracted to salt after exposure to the salt (NaCl) with food. In contrast, it learns to avoid the salt after exposure to the salt without food. In salt-attraction behavior, it is known that the ASE taste sensory neurons (ASEL and ASER) play a major role. However, little is known about mechanisms for learned salt avoidance. Here, through dissecting contributions of ASE neurons for salt chemotaxis, we show that both ASEL and ASER generate salt chemotaxis plasticity. In ASER, we have previously shown that the insulin/PI 3-kinase signaling acts for starvation-induced salt chemotaxis plasticity. This study shows that the PI 3-kinase signaling promotes aversive drive of ASER but not of ASEL. Furthermore, the Gq signaling pathway composed of Gqα EGL-30, diacylglycerol, and nPKC (novel protein kinase C) TTX-4 promotes attractive drive of ASER but not of ASEL. A putative salt receptor GCY-22 guanylyl cyclase is required in ASER for both salt attraction and avoidance. Our results suggest that ASEL and ASER use distinct molecular mechanisms to regulate salt chemotaxis plasticity.