The
cho-1 gene encodes a high-affinity plasma membrane choline transporter believed to be rate limiting for acetylcholine (ACh) synthesis in cholinergic nerve terminals. Using
cho-1 promoter fusions, we found that
cho-1 is expressed in most, but not all cholinergic neurons. In addition,
cho-1 appears to be expressed in a small set of non-cholinergic neurons in the head and tail. A functional CHO-1::GFP fusion protein rescues the
cho-1 mutant phenotypes, and is enriched at cholinergic synapses.
cho-1 null mutants are viable, but exhibit mild deficits in cholinergic behavior: they are mildly 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-minute 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. Although
cho-1 mutants clearly exhibit defects in cholinergic behaviors, the loss of
cho-1 function has surprisingly mild effects on cholinergic neurotransmission. Other potential sources of choline for ACh synthesis include low-affinity choline transporters, turnover of membrane phospholipids, and de novo synthesis. We found that mutations in other potential choline transporters (i.e.
oct-1 and
chtl-1) do not enhance the
cho-1 mutant phenotype, suggesting that these transporters do not supply choline for ACh synthesis. Similarly, a mutation in the gene encoding phospholipase D, an enzyme required for converting phosphatidylcholine to choline, does not enhance the
cho-1 phenotype (also see Matthies et al., 2006). However, we found that reducing endogenous choline synthesis strongly enhances the phenotype of
cho-1 mutants, giving rise to a synthetic uncoordinated (Unc) phenotype. Our results indicate that both choline transport and de novo synthesis provide choline for acetylcholine synthesis in C. elegans cholinergic neurons.