Negative regulation and gain control of sensory neurons by the C. elegans calcineurin TAX-6

Structural delineation of the calcineurin-NFAT interaction and its parallels to PP1 targeting interactions

Calcineurin is a phosphoprotein phosphatase that channels intracellular Ca signals into multiple biological pathways. Calcineurin is known to interact directly with its substrate nuclear factor of activated T cells (NFAT or NFATc), with other substrates, and with several targeting and scaffold proteins including AKAP79 and Cabin1/cain. The calcineurin-NFAT interaction depends on recognition of a PxIxIT sequence motif present in NFAT-family proteins and in certain other calcineurin-interacting proteins. Here, we define the structural basis for the interaction of calcineurin with NFAT and with other proteins possessing the PxIxIT motif. The calcineurin-PxIxIT contact has a direct parallel in the contact of protein phosphatase 1 with its regulatory proteins, suggesting that the evolution of these related phosphatases involved local remodelling of an ancestral docking site.

The C. elegans thermosensory neuron AFD responds to warming

The mechanism of temperature sensation is far less understood than the sensory response to other environmental stimuli such as light, odor, and taste. Thermotaxis behavior in C. elegans requires the ability to discriminate temperature differences as small as approximately 0.05 degrees C and to memorize the previously cultivated temperature. The AFD neuron is the only major thermosensory neuron required for the thermotaxis behavior. Genetic analyses have revealed several signal transduction molecules that are required for the sensation and/or memory of temperature information in the AFD neuron, but its physiological properties, such as its ability to sense absolute temperature or temperature change, have been unclear. We show here that the AFD neuron responds to warming. Calcium concentration in the cell body of AFD neuron is increased transiently in response to warming, but not to absolute temperature or to cooling. The transient response requires the activity of the TAX-4 cGMP-gated cation channel, which plays an essential role in the function of the AFD neuron. Interestingly, the AFD neuron further responds to step-like warming above a threshold that is set by temperature memory. We suggest that C. elegans provides an ideal model to genetically and physiologically reveal the molecular mechanism for sensation and memory of temperature information.

G protein-coupled receptor kinase function is essential for chemosensation in C. elegans

G protein-coupled receptors (GPCRs) mediate diverse signaling processes, including olfaction. G protein-coupled receptor kinases (GRKs) are important regulators of G protein signal transduction that specifically phosphorylate activated GPCRs to terminate signaling. Despite previously described roles for GRKs in GPCR signal downregulation, animals lacking C. elegans G protein-coupled receptor kinase-2 (Ce-grk-2) function are not hypersensitive to odorants. Instead, decreased Ce-grk-2 function in adult sensory neurons profoundly disrupts chemosensation, based on both behavioral analysis and Ca(2+) imaging. Although mammalian arrestin proteins cooperate with GRKs in receptor desensitization, loss of C. elegans arrestin-1 (arr-1) does not disrupt chemosensation. Either overexpression of the C. elegans Galpha subunit odr-3 or loss of eat-16, which encodes a regulator of G protein signaling (RGS) protein, restores chemosensation in Ce-grk-2 mutants. These results demonstrate that loss of GRK function can lead to reduced GPCR signal transduction and suggest an important role for RGS proteins in the regulation of chemosensation.

Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development

Calcineurin signaling has been implicated in a broad spectrum of developmental processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, calcium-activated protein phosphatase composed of catalytic and regulatory subunits. The serine/threonine-specific phosphatase functions within a signal transduction pathway that regulates gene expression and biological responses in many developmentally important cell types. Calcineurin signaling was first defined in T lymphocytes as a regulator of nuclear factor of activated T cells (NFAT) transcription factor nuclear translocation and activation. Recent studies have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in cardiovascular and skeletal muscle development in vertebrates. Inhibition, mutation, or forced expression of calcineurin pathway genes result in defects or alterations in cardiomyocyte maturation, heart valve formation, vascular development, skeletal muscle differentiation and fiber-type switching, and cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic studies have demonstrated specific myogenic functions for the phosphatase in their development. The ability to investigate calcineurin signaling pathways in vertebrates and model genetic organisms provides a great potential to more fully comprehend the functions of calcineurin and its interacting genes in heart, blood vessel, and muscle development.

The Caenorhabditis elegans homologue of Down syndrome critical region 1, RCN-1, inhibits multiple functions of the phosphatase calcineurin

A conserved family of calcineurin-regulating proteins whose members have been implicated in several disease models such as Down syndrome, Alzheimer's disease, and cardiac hypertrophy has been identified in several organisms including yeast, mice, and humans. We have characterized Caenorhabditis elegans rcn-1, which belongs to this family of calcineurin regulators, and shows approximately 40% identity with the human homologue DSCR-1. rcn-1 is expressed in hypodermal cells, nerve cords and various neurons, vulva epithelial and muscle cells, marginal cells of the pharynx, and structures of the male tail. rcn-1 expression is upregulated by calcineurin activity. RCN-1 binds to calcineurin A from C.elegans lysate in a calcium-dependent manner, and inhibits bovine calcineurin phosphatase activity dose-dependently. In addition, overexpression of RCN-1 results in calcineurin-deficient phenotypes such as small body size, cuticle defects, fertility defects, slow growth, and serotonin-resistant egg-laying defects. Moreover, phenotypes observed in gain-of-function calcineurin mutant animals were restored to normal by RCN-1 overexpression. These results demonstrate an effective and specific inhibition of calcineurin in vitro as well as in vivo by RCN-1.

Behavioral plasticity in C. elegans: paradigms, circuits, genes

Life in the soil is an intellectual and practical challenge that the nematode Caenorhabditis elegans masters by utilizing 302 neurons. The nervous system assembled by these 302 neurons is capable of executing a variety of behaviors, some of respectable complexity. The simplicity of the nervous system, its thoroughly characterized structure, several sets of well-defined behaviors, and its genetic amenability combined with its isogenic background make C. elegans an attractive model organism to study the genetics of behavior. This review describes several behavioral plasticity paradigms in C. elegans and their underlying neuronal circuits and then goes on to review the forward genetic analysis that has been undertaken to identify genes involved in the execution of these behaviors. Lastly, the review outlines how reverse genetics and genomic approaches can guide the analysis of the role of genes in behavior and why and how they will complement the forward genetic analysis of behavior.

The cyclic GMP-dependent protein kinase EGL-4 regulates olfactory adaptation in C. elegans

Prolonged odor exposure causes a specific, reversible adaptation of olfactory responses. A genetic screen for negative regulators of olfaction uncovered mutations in the cGMP-dependent protein kinase EGL-4 that disrupt olfactory adaptation in C. elegans. G protein-coupled olfactory receptors within the AWC olfactory neuron signal through cGMP and a cGMP-gated channel. The cGMP-dependent kinase functions in AWC neurons during odor exposure to direct adaptation to AWC-sensed odors, suggesting that adaptation is a cell intrinsic process initiated by cGMP. A predicted phosphorylation site on the beta subunit of the cGMP-gated channel is required for adaptation after short odor exposure, suggesting that phosphorylation of signaling molecules generates adaptation at early time points. A predicted nuclear localization signal within EGL-4 is required for adaptation after longer odor exposure, suggesting that nuclear translocation of EGL-4 triggers late forms of adaptation.

Calcineurin, a calcium/calmodulin-dependent protein phosphatase, is involved in movement, fertility, egg laying, and growth in Caenorhabditis elegans

Calcineurin is a Ca(2+)-calmodulin-dependent serine/threonine protein phosphatase that has been implicated in various signaling pathways. Here we report the identification and characterization of calcineurin genes in Caenorhabditis elegans (cna-1 and cnb-1), which share high homology with Drosophila and mammalian calcineurin genes. C. elegans calcineurin binds calcium and functions as a heterodimeric protein phosphatase establishing its biochemical conservation in the nematode. Calcineurin is expressed in hypodermal seam cells, body-wall muscle, vulva muscle, neuronal cells, and in sperm and the spermatheca. cnb-1 mutants showed pleiotropic defects including lethargic movement and delayed egg-laying. Interestingly, these characteristic defects resembled phenotypes observed in gain-of-function mutants of unc-43/Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and goa-1/G(o)-protein alpha-subunit. Double mutants of cnb-1 and unc-43(gf) displayed an apparent synergistic severity of movement and egg-laying defects, suggesting that calcineurin may have an antagonistic role in CaMKII-regulated phosphorylation signaling pathways in C. elegans.