The Claudin Superfamily Protein NSY-4 Biases Lateral Signaling to Generate Left-Right Asymmetry in C. elegans Olfactory Neurons

Abstract

Early in C. elegans development, signaling between bilaterally symmetric AWC olfactory neurons causes them to express different odorant receptor genes. AWC left-right asymmetry is stochastic: in each animal, either the left or the right neuron randomly becomes AWC(ON), and the other neuron becomes AWC(OFF). Here we show that the nsy-4 gene coordinates the lateral signaling that diversifies AWC(ON) and AWC(OFF) neurons. nsy-4 mutants generate 2 AWC(OFF) neurons, as expected if communication between the AWC neurons is lost, whereas overexpression of nsy-4 results in 2 AWC(ON) neurons. nsy-4 encodes a transmembrane protein related to the gamma subunits of voltage-activated calcium channels and the claudin superfamily; it interacts genetically with calcium channels and antagonizes a calcium-to-MAP kinase cascade in the neuron that becomes AWC(ON). Genetic mosaic analysis indicates that nsy-4 functions both cell-autonomously and nonautonomously in signaling between AWC neurons, providing evidence for lateral signaling and feedback that coordinate asymmetric receptor choice.

Transcriptional regulation and stabilization of left–right neuronal identity in C. elegans

“…A complex signaling pathway involving a multicellular gap junction network, claudins, calcium channels, and protein kinases acts in embryogenesis to distinguish the AWC ON and AWC OFF cells (Troemel et al 1999;Sagasti et al 2001;Tanaka-Hino et al 2002;Chuang and Bargmann 2005;Vanhoven et al 2006;Bauer Huang et al 2007;Chuang et al 2007). Previous genetic studies and cell ablations suggest that the AWC OFF identity is a default, or ground state, and AWC ON is an induced state (Troemel et al 1999).…”

“…As expected from previous studies (Troemel et al 1999), AWCL and AWCR were equally likely to enter the str-2 + srsx-3 À AWC ON state or the str-2 À srsx-3 + AWC OFF state. nsy-7(ky630) initiates but fails to maintain AWC ON asymmetry nsy-7(ky630) was isolated from a screen for mutants that fail to express str-2 in either AWC neuron (Vanhoven et al 2006). As adults, these mutants had a typical 2-AWC OFF phenotype in which both AWC neurons expressed srsx-3.…”

“…This element of randomness is unusual in C. elegans, which is otherwise notable for the predictability of its cell fate decisions (Sulston and Horvitz 1977). The initial AWC ON /AWC OFF decision depends on a signaling pathway in which a claudin-like transmembrane protein, NSY-4, and an embryonic gap junction network generated by the innexin NSY-5 induce one AWC to exit the AWC OFF default identity and become AWC ON (Vanhoven et al 2006;Chuang et al 2007). The induced AWC ON neuron then provides feedback to the contralateral neuron to stabilize the AWC OFF identity.…”

Microtubule-based localization of a synaptic calcium-signaling complex is required for left-right neuronal asymmetry in C. elegans

“…Mosaic analysis and statistical analysis were performed as previously described (Bauer Huang et al, 2007;Sagasti et al, 2001;VanHoven et al, 2006). Expected numbers of mosaic animals were predicted from internal control animals, which retained the unc-104p::unc-104; odr-1p::DsRed transgene in both AWC neurons, from the same transgenic line.…”

“…These results suggest that these genes normally promote the default AWC OFF identity (Bauer Huang et al, 2007;Chuang and Bargmann, 2005;Sagasti et al, 2001;Tanaka-Hino et al, 2002;Troemel et al, 1999). An innexin gap junction protein, encoded by nsy-5, and a claudinlike protein, encoded by nsy-4, function in parallel to inhibit the downstream calcium-regulated signaling pathway in the neuron that acquires the AWC ON identity VanHoven et al, 2006). When the activity of nsy-4 or nsy-5 is reduced or lost, mutant animals fail to induce the AWC ON cell, leading to the formation of two AWC OFF cells (2AWC OFF phenotype).…”