The LIM homeobox gene ceh-14 is required for phasmid function and neurite outgrowth

Transcription factors play key roles in cell fate specification and cell differentiation. Previously, we showed that the LIM homeodomain factor CEH-14 is expressed in the AFD neurons where it is required for thermotaxis behavior in Caenorhabditis elegans. Here, we show that ceh-14 is expressed in the phasmid sensory neurons, PHA and PHB, a number of neurons in the tail, i.e., PHC, DVC, PVC, PVN, PVQ, PVT, PVW and PVR, as well as the touch neurons. Analysis of the promoter region shows that important regulatory elements for the expression in most neurons reside from -4kb to -1.65kb upstream of the start codon. Further, within the first introns are elements for expression in the hypodermis. Phylogenetic footprinting revealed numerous conserved motifs in these regions. In addition to the existing deletion mutation ceh-14(ch3), we isolated a new allele, ceh-14(ch2), in which only one LIM domain is disrupted. The latter mutant allele is partially defective for thermosensation. Analysis of both mutant alleles showed that they are defective in phasmid dye-filling. However, the cell body, dendritic outgrowth and ciliated endings of PHA and PHB appear normal, indicating that ceh-14 is not required for growth. The loss of a LIM domain in the ceh-14(ch2) allele causes a partial loss-of-function phenotype. Examination of the neurites of ALA and tail neurons using a ceh-14::GFP reporter shows abnormal axonal outgrowth and pathfinding.