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Neurotoxicity through abnormal activation of membrane channels is a potential cause of neurodegenerative disease. Here we show that a gain-of-function mutation, deg.3(u662), leads to the degeneration of a small set of neurons in the nematode C. elegans. The deg.3 gene encodes a nicotinic acetylcholine receptor alpha subunit, which in the region of transmembrane domain II is most similar to the neuronal alpha 7 subunits from rat and chicken. The u662 mutation changes a residue in the second transmembrane domain, the domain thought to form the channel pore. A similar change in the equivalent amino acid in the chick protein produces channels that desensitize slowly. Channel hyperactivity may underlie the degenerations seen in the C. elegans deg.3(u662) mutants, since antagonists of nicotinic acetylcholine receptors suppress the deg-3(u662) mutant phenotypes.

In Caenorhabditis elegans, the ric-3 gene is required for the maturation of multiple nicotinic acetylcholine receptors (nAChRs), whereas other neurotransmittergated channels expressed within the same cells are unaffected by the presence of RIC-3. Here we show that RIC-3 is a member of a conserved gene family with representatives in both vertebrates and invertebrates. All members of this family have two transmembrane domains followed by a coiled-coil domain. Expression of the human ric-3 homolog, hric3, like the C. elegans ric-3, enhances C. elegans DEG-3/DES-2, rat α7, and human α7 nAChR-dependent whole-cell current amplitudes in Xenopus leavis oocytes, thus demonstrating functional conservation. However, hric3 also reduces human α4β2 and α3β4 nAChR-dependent whole-cell current amplitudes. Thus, hric3 shows differential effects on human nAChRs unlike the observed uniform effect of ric-3 on C. elegans nAChRs. Moreover, hric3 totally abolished currents evoked by 5-HT₃ serotonin receptors, whereas it barely modified α1 glycine receptor currents. With this caveat, RIC-3 belongs to a conserved family of genes likely to regulate nAChR-mediated transmission throughout evolution. Analysis of transcripts encoded by the hric3 locus shows that it encodes for multiple transcripts, likely to produce multiple hric3 isoforms, and that hric3 is expressed in neurons and muscles, thus enabling its interactions with nAChRs in vivo.

The deg-3 gene from the nematode Caenorhabditis elegans encodes an α subunit of a nicotinic acetylcholine receptor that was first identified by a dominant allele, u662 , which produced neuronal degeneration. Because deg-3 cDNAs contain the SL2 trans-spliced leader, we suggested that deg-3 was transcribed as part of a C. elegans operon. Here we show that des-2 , a gene in which mutations suppress deg-3(u662) , is the upstream gene in that operon. The des-2 gene also encodes an α subunit of a nicotinic acetylcholine receptor. As expected for genes whose mRNAs are formed from a single transcript, both genes have similar expression patterns. This coexpression is functionally important because ( i ) des-2 is needed for the deg-3(u662) degenerations in vivo ; ( ii ) an acetylcholine-gated channel is formed in Xenopus oocytes when both subunits are expressed but not when either is expressed alone; and ( iii ) channel activity, albeit apparently altered from that of the wild-type channel, results from the expression of a u662 -type mutant subunit but, again, only when the wild-type DES-2 subunit is present. Thus, the operon structure appears to regulate the coordinate expression of two channel subunits.