What can a worm tell us about autism?
It is sometimes challenging to relay the importance of scientific findings about autism to the public, especially when the animal subject of the research is far removed from the people affected with autism spectrum disorders
(ASDs). In this case, however, James Rand, Ph.D. and his colleagues at the Oklahoma Medical Research Foundation have made our task easy. Their new research paper, funded by a grant from Autism Speaks, shows how the humble round worm, Caenorhabditis elegans, can reveal impressive – even surprising – findings that are important to our understanding of the biological underpinnings of autism.
Starting in 2003, a series of studies showed that some individuals with ASD have mutations in genes coding for proteins called "neuroligins" that are involved in the formation of synapses (connection between nerve cells). Since then, mutations of different synapse-related genes have been reported by several investigators using different families of affected individuals. These findings set the stage for the now generally accepted hypothesis that ASD may fundamentally involve alterations in synaptic structure and function.
Using worms to study the effects of autism-related mutations is not as far-fetched as it might seem. Numerous studies have shown that C. elegans synapse proteins are structurally and functionally equivalent to the corresponding mammalian proteins, and it is now well established that C. elegans provides a powerful model for analyzing synapse structure, function and development. The worms that Dr. Rand and his colleagues study have neuroligin in their relatively simple nervous systems, and Dr. Rand's research group has now shown that the neuroligin protein in the worms is quite similar to human neuroligins, and much easier to study, which leads to the question: what would happen if worms had a genetic mutation that completely eliminated the production of neuroligin? In the publication Disease Model & Mechanisms, released this week, the authors have now created and characterized neuroligin deficient worms, finding that animals lacking in this protein not only show discrete neurological and behavioral deficits, they are surprisingly also more susceptible to oxidative stress.
It is difficult to map the triad of symptoms that characterize ASDs to the relatively limited behavioral repertoire of this little (~1 mm) worm. In fact, at first glance, C. elegans with the neuroligin mutation appear to develop and behave normally. Upon closer inspection, however, they show subtle sensory deficits, including a lack of sensitivity to some chemicals and altered processing of simultaneous sensory inputs. Findings of sensory abnormalities mirror many anecdotal reports from families as well as some research on children and adolescents with ASDs. The neuroligin-deficient worms also display a lack of temperature sensitivity. Typical worms prefer a temperature range in which they were raised, but the neuroligin-deficient worms displayed no preference, suggesting that they either do not sense temperature changes or are indifferent to them. These sensory differences are intriguing and indeed familiar to many people with experience in ASDs. More surprising, perhaps, were the differences in metabolism brought on by the neuroligin mutation.
All cells produce free radicals and reactive oxygen species (ROS), which are toxic agents capable of damaging many cellular components (e.g., proteins, lipids and DNA). Although cells normally have robust mechanisms to manage and detoxify these agents, certain types of cellular or metabolic challenges can lead to chronic elevated levels of ROS, a condition known as oxidative stress. Therefore, members of Dr. Rand's research group thought to test whether the worms with the neuroligin mutation were more sensitive to different types of stress. To their surprise, the worms showed a heightened response to the toxic effects of agents that cause or increase oxidative stress, such as copper, paraquat and thimerosal, suggesting that the worms may already be under oxidative stress by virtue of having the neuroligin mutation. The mutants also had increased levels of oxidative damage to their proteins.
The mechanism for this is not yet known, but impaired mitochondria function may be one possibility as mitochondria are a major source of cellular ROS. In fact, the authors showed that worms with the neuroligin mutation show an increase in oxidative stress that is similar to worms with a mutation in a different gene known to affect mitochondria. Alternatively, many cellular and metabolic mechanisms are normally involved in the control of ROS levels, and the loss of neuroligin might somehow compromise one or more of these processes. In sum, using a relatively simple animal model, the researchers were able to identify a novel and completely unexpected connection between the worm equivalent of an autism-associated synaptic mutation, enhanced response to environmental toxins (e.g., paraquat, mercury compounds, etc.), and oxidative stress. These studies thus provide an important example of how both genetic and environmental contributions to a neurological disorder can have a single underlying basis.
There have been a number of reports indicating an increased level of oxidative stress in individuals with autism, and these have led to models that oxidative stress somehow causes or contributes to the development of ASDs. However, it has never been clear how or why autism may be correlated with oxidative stress. Dr. Rand points out that "in C. elegans, loss of the synaptic protein neuroligin is not merely correlated with oxidative stress, but it actually causes the oxidative stress. Therefore, it is plausible that a comparable mechanism might link autism-associated mutations in humans to the autism-associated oxidative phenotypes that have been reported. This suggests that some types of neurological disorders (such as ASDs) in humans might be the cause, rather than the result, of oxidative stress." For the future, Dr. Rand, who is the parent of two sons with ASD, believes that "use of simple model organisms such as C. elegans will help elucidate the precise mechanism by which genetic disruptions of synapse proteins can trigger oxidative stress and will hopefully provide a molecular framework for biochemical and metabolic studies that may identify intervention opportunities."