Academic journal article Genetics

Genetic Screens for Caenorhabditis Elegans Mutants Defective in Left/Right Asymmetric Neuronal Fate Specification

Academic journal article Genetics

Genetic Screens for Caenorhabditis Elegans Mutants Defective in Left/Right Asymmetric Neuronal Fate Specification

Article excerpt

ABSTRACT

We describe here the results of genetic screens for Caenorhabditis elegans mutants in which a single neuronal fate decision is inappropriately executed. In wild-type animals, the two morphologically bilaterally symmetric gustatory neurons ASE left (ASEL) and ASE right (ASER) undergo a left/right asymmetric diversification in cell fate, manifested by the differential expression of a class of putative chemoreceptors and neuropeptides. Using single cell-specific gfp reporters and screening through a total of almost 120,000 haploid genomes, we isolated 161 mutants that define at least six different classes of mutant phenotypes in which ASEL/R fate is disrupted. Each mutant phenotypic class encompasses one to nine different complementation groups. Besides many alleles of 10 previously described genes, we have identified at least 16 novel "lsy" genes ("laterally symmetric"). Among mutations in known genes, we retrieved four alleles of the miRNA lsy-6 and a gain-of-function mutation in the 3'-UTR of a target of lsy-6, the cog-1 homeobox gene. Using newly found temperature-sensitive alleles of cog-1, we determined that a bistable feedback loop controlling ASEL vs. ASER fate, of which cog-1 is a component, is only transiently required to initiate but not to maintain ASEL and ASER fate. Taken together, our mutant screens identified a broad catalog of genes whose molecular characterization is expected to provide more insight into the complex genetic architecture of a left/right asymmetric neuronal cell fate decision.

APART from expressing a core set of features that distinguish neuronal from nonneuronal cell types, individual cell types in the nervous system express distinct batteries of genes that generate the structural and functional diversity of neuronal cell types. The diversification of cell fate in the developing nervous system presumably relies on the interplay of a host of regulatory factors. Screens for mutant animals defective in neuronal fate specification provide a powerful and unbiased approach to identify these regulatory factors. Such screens have been successfully conducted in various model systems, most prominently flies and worms, yielding valuable insights into the molecular mechanisms that control neuronal fate specification. We describe here genetic screens for neuronal fate mutants in the nematode Caenorhabditis elegans that focus on a cell fate decision executed by a single neuron class, the ASE gustatory neurons. This neuron class is composed of two morphologically bilaterally symmetric neurons, ASE left (ASEL) and ASE right (ASER). ASEL and ASER are the main taste receptor neurons of C. elegans and sense multiple chemosensory cues in a left/right asymmetric manner (Bargmann and Horvitz 1991; Pierce-Shimomura et al. 2001) (our unpublished data). Left/right asymmetric chemosensory functions correlate with the left/right asymmetric expression of a class of putative chemoreceptors encoded by the gcy (guanylyl cyclase) gene family (Yu et al. 1997; Pierce-Shimomura et al. 2001; Ortiz et al. 2006) (Figure 1). These left/right asymmetric features of ASEL and ASER provide a model to understand how functional laterality is superimposed on a morphologically symmetric structure, a hallmark of many nervous systems across phylogeny (Hobert et al. 2002; Sun and Walsh 2006).

We have previously reported the identification of mutants that affect the development of the left/right asymmetry of the ASE neurons (Chang et al. 2003; Johnston and Hobert 2003, 2005; Johnston et al. 2006). These mutants, which we termed lsy mutants (for laterally symmetric) to indicate their role in controlling the fate of two left/right asymmetric neurons, fell into four distinct classes (Figure 1A). In class I mutants, both ASE neurons adopt the fate of the ASEL neuron, with a concomitant loss of ASER cell fate (''2 ASEL'' mutants). In class II mutants, the opposite phenotype is observed, with both neurons expressing ASER fate and losing ASEL fate (''2 ASER'' mutants). …

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