Neuroligin-Deficient Mutants of C. Elegans Have Sensory Deficits and Are Hypersensitive to Oxidative Stress

Background: Neuroligins are postsynaptic cell adhesion proteins originally identified by their binding to presynaptic proteins called neurexins.  Although the interaction between neuroligin and neurexin is capable of inducing synaptogenesis under certain conditions, recent studies suggest that neuroligins function primarily in the maturation and/or maintenance of synapses.  There are four neuroligin genes in humans, and studies have shown that mutations in the genes encoding neuroligin 3 and neuroligin 4 are associated with autism spectrum disorders (ASDs).
Objectives: We examined the expression, localization and biological functions of neuroligin in a simple model organism, the nematode Caenorhabditis elegans.  We are using C. elegans for these studies because it has a simple nervous system, and is well suited for genetic, molecular and behavioral analyses.
Methods: We determined the cellular expression and sub-cellular localization of C. elegans neuroligin using fluorescent reporters and neuroligin::YFP fusion proteins.  We also compared wild-type animals to nlg-1 null mutants using a set of behavioral tests, lifespan measurements, toxicity tests, and quantitation of oxidatively damaged proteins. 
Results: C. elegans has a single neuroligin gene (nlg-1), and approximately one-sixth of C. elegans neurons express a neuroligin transcriptional reporter.  Neuroligin-expressing cells include some sensory neurons, interneurons, and a subset of cholinergic motor neurons, as well as body-wall muscles.  nlg-1 null mutants are viable, and they do not appear deficient in any major motor functions.  However, they are defective in a subset of sensory behaviors and sensory processing; these deficits are strikingly similar to traits frequently associated with ASDs.  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 have a reduced lifespan and an increased level of oxidative damage to proteins.  nlg-1 mutants are also hypersensitive to the toxicity of inorganic (HgCl₂) and organic (thimerosal) mercury compounds and copper compounds, but not to cadmium (CdCl₂). 
Conclusions: The grossly normal structure and function of the nervous system in nlg-1 null mutants are consistent with current models emphasizing the importance of neuroligin in synaptic maintenance, rather than synaptogenesis.  Our data on the sensitivity of nlg-1 mutants to oxidative stress (e.g., paraquat) and mercury compounds support an important model for how both genetic and environmental contributions to a neurological disorder can have a single underlying basis. 
           Furthermore, although several studies have demonstrated a correlation between elevated markers of oxidative stress and ASDs, the precise relationship between autism and oxidative stress is not clear.  Our C. elegans studies demonstrate that loss of the synaptic protein neuroligin is not merely correlated with oxidative stress, but actually causes the oxidative stress.  If oxidative stress is a consequence of aberrant synaptic structure and/or transmission in C. elegans, then perhaps similar defects in humans have similar consequences.  This raises the intriguing possibility that in humans, specific types of neuronal disruption (including mutations affecting synaptic adhesion proteins) may be the cause, and not the result, of oxidative stress.