SUMMARYNeuroligins are postsynaptic cell adhesion proteins that bind specifically to presynaptic membrane proteins called neurexins. Mutations in human neuroligin genes are associated with autism spectrum disorders in some families. The nematode Caenorhabditis elegans has a single neuroligin gene (nlg-1), and approximately a sixth of C. elegans neurons, including some sensory neurons, interneurons and a subset of cholinergic motor neurons, express a neuroligin transcriptional reporter. Neuroligin-deficient mutants of C. elegans are viable, and they do not appear deficient in any major motor functions. However, neuroligin mutants are defective in a subset of sensory behaviors and sensory processing, and are hypersensitive to oxidative stress and mercury compounds; the behavioral deficits are strikingly similar to traits frequently associated with autism spectrum disorders. Our results suggest a possible link between genetic defects in synapse formation or function, and sensitivity to environmental factors in the development of autism spectrum disorders. RESEARCH ARTICLENLG-1 is 26-28% identical (45-47% similar) to the four human neuroligins and is most similar overall (28% identical, 47% similar) to human neuroligin 4 (supplementary material Fig. S1).Through a combination of cDNA sequencing and reverse transcription PCR (RT-PCR) analysis of transcripts, we documented several types of nlg-1 alternative splicing (Figs 1, 2). Exons 13 and 14 are variably present in nlg-1 transcripts; the skipping of these two exons occurs independently, and we have detected transcripts containing only exon 13, only exon 14, both exons, and neither exon. In addition, we have identified tandem alternative splice acceptor sites at the 5Ј-ends of exons 4 and 16, and tandem alternative splice donor sites at the 3Ј-end of exon 14 (Figs 1, 2). If these splicing events are independent, there could be as many as 24 distinct NLG-1 isoforms. nlg-1 is expressed in a subset of neurons and muscle cellsWe used a transgenic transcriptional reporter, with the nlg-1 promoter driving YFP expression (FRM77, Fig. 1), to examine the cellular expression of nlg-1. We found that nlg-1 is expressed in a subset of neurons in C. elegans adults, including ~20 cells in the ventral nerve cord and ~20 cells in the head (Fig. 3). We identified the nlg-1-expressing cells in the ventral nerve cord as the cholinergic VA and DA motor neurons (Fig. 3). We also identified the two AIY and two URB interneurons and the four URA motor neurons in the head, and the two PVD mechanosensory and two HSN motor neurons in the body, as nlg-1-expressing cells. Of these cells, the AIY interneurons are cholinergic (Altun-Gultekin et al., 2001), the PVD neurons are glutamatergic (Lee et al., 1999), and the HSN neurons release both serotonin and acetylcholine (ACh) (Desai et al., 1988;Duerr et al., 2001). Neurotransmitter assignments have not been reported for the remaining nlg-1-expressing neurons; however, they do not express GABAergic, dopaminergic, serotonergic or glutamatergic repo...

The cho-1 gene in Caenorhabditis elegans encodes a high-affinity plasma-membrane choline transporter believed to be rate limiting for acetylcholine (ACh) synthesis in cholinergic nerve terminals. We found that CHO-1 is expressed in most, but not all cholinergic neurons in C. elegans. cho-1 null mutants are viable and exhibit mild deficits in cholinergic behavior; they are slightly resistant to the acetylcholinesterase inhibitor aldicarb, and they exhibit reduced swimming rates in liquid. cho-1 mutants also fail to sustain swimming behavior; over a 33-min time course, cho-1 mutants slow down or stop swimming, whereas wildtype animals sustain the initial rate of swimming over the duration of the experiment. A functional CHO-1TGFP fusion protein rescues these cho-1 mutant phenotypes and is enriched at cholinergic synapses. Although cho-1 mutants clearly exhibit defects in cholinergic behaviors, the loss of cho-1 function has surprisingly mild effects on cholinergic neurotransmission. However, reducing endogenous choline synthesis strongly enhances the phenotype of cho-1 mutants, giving rise to a synthetic uncoordinated phenotype. Our results indicate that both choline transport and de novo synthesis provide choline for ACh synthesis in C. elegans cholinergic neurons.