Genetic Dissection of Molecular Pathways Implicated in Autism

Defining molecular pathways that are dysfunctional in autistic spectrum disorders (ASDs) is key to understanding their pathogenesis. In Alzheimer’s and Parkinson’s Disease, identification of single gene mutations in the 5-10% of genetic cases have revealed core molecular pathways that are altered, including in the larger category of non-genetic cases. A key question is whether a similar molecular pathway will emerge for autism based on the recent identification of defined mutations and de novo genome copy number variations that account for 10-20% of ASDs. Current evidence suggests the disease may result from disruption in synapse formation and synaptic plasticity during development, with several autism-linked mutations in humans mapping to the endosomal pathway. Recent work from our laboratory has indicated that regulation of presynaptic endosomal trafficking plays a key role in synaptic growth regulation by controlling the activity of retrograde synaptic growth signals processed by presynaptic growth receptors. Protein complexes regulating signal transduction are directed to specific subcellular membrane domains along their journey through the endocytic pathway. These temporally and spatially regulated trafficking steps result in distinct signaling properties at various points along this route, due to compartment-specific post-translational modifications and degradative events, or interactions with local binding partners. In neurons, growth factor signaling controls the expansion of synaptic arbors in response to activity and external stimuli, leading to long-lasting changes in synaptic strength and connectivity that underlies learning and memory. The Drosophila larval neuromuscular junction (NMJ) serves as a useful model for synaptic growth, as the muscle surface area expands 100-fold over 4 days of larval development, requiring increased input from its innervating motor neuron to drive contraction. NMJ synaptic arbors expand by adding matched pre- and post-synaptic specializations (termed synaptic boutons), in response to motor neuron synaptic activity, retrograde signals from the muscle to the neuron, and anterograde signals from the neuron to the muscle. Defining the mechanisms by which protein traffic between endosomal intermediates at synapses controls the output of signal transduction pathways leading to synaptic growth is a key area of study. We are using Drosophila to explore the mechanisms by which the newly identified autism-associated endosomal protein, NHE9, couples alterations in neuronal activity to modifications of synaptic connectivity. NHE9 is one of several newly identified genetic links that indicate abnormal endosomal trafficking may lead to autism. Defining the role of NHE9 in endosomal trafficking at synapses will allow us to place this important protein into the emerging picture of a dysfunctional molecular cascade that regulates synaptic plasticity and development that may underlie autism.