Neuroligin Has Cell-Autonomous As Well As Cell-Non-Autonomous Functions In C. Elegans

Background: Neuroligins are postsynaptic adhesion proteins originally identified by their binding to presynaptic neurexins.  Recent studies suggest that neuroligins function primarily in the maturation and/or maintenance of synapses.  There are four neuroligin genes in humans, and mutations in the genes encoding neuroligin-3 and neuroligin-4 are associated with autism spectrum disorders (ASDs) in some families.  We had previously examined the expression, localization and biological functions of neuroligin in a simple model organism, the nematode Caenorhabditis elegans.  C. elegans has a single neuroligin gene (nlg-1), and we had shown that nlg-1 null mutants are viable and are not deficient in any major motor functions.  However, they are defective in a subset of sensory behaviors and sensory processing.  nlg-1 mutants are also hypersensitive to oxidative stress (i.e., exposure to paraquat); this is an unexpected phenotype for a synaptic mutant.  Like many other stress-sensitive mutants, nlg-1 mutants also have a reduced lifespan and an increased level of oxidative protein damage (Hunter et al., 2010).  All of these mutant phenotypes are rescued by transgenic expression of mammalian neuroligin (human neuroligin-4 or rat neuroligin-1).  The C. elegans and mammalian neuroligins, therefore, appear to be functionally equivalent (including having the ability to prevent or counteract oxidative stress).

Objectives: To analyze the contribution of neuroligin to normal function of specific neurons and of known behavioral circuits in C. elegans. 

Methods: Using standard methods, we engineered a set of constructs in which the nlg-1 cDNA was driven by several different promoters, each of which was specific for a small subset of neurons.  We then expressed these constructs as stable transgenes in nlg-1 null mutants, and we compared the behaviors and toxin sensitivity of the transgenic animals to wild-type animals and to nlg-1 mutants lacking the transgenes.  

Results: Neuroligin is normally expressed in approximately ~20% of C. elegans neurons, including the pair of AIY interneurons.  AIY cells receive direct synaptic input from different types of sensory neurons (e.g., chemosensory, thermosensory, nociceptive), and have been shown to play an important role in the integration of sensory information.  We find that expressing neuroligin only in the AIY interneurons is sufficient to rescue all of the sensory deficits as well as the elevated oxidative stress present in nlg-1 mutants.  In addition, we find that expressing neuroligin ectopically in neurons which do not normally express this protein can also rescue mutant phenotypes.  

Conclusions:  It is both noteworthy and surprising that expression of NLG-1 in only the two AIY neurons is sufficient to rescue all of the mutant phenotypes we examined.  Equally noteworthy and surprising is the phenotypic rescue observed when the only neuroligin in the animal is in cells that normally do not express neuroligin - clearly a non-cell-autonomous effect.