A lack of validated preclinical models for exploring cellular mechanisms has limited treatment development in severe autism. In order to develop new preclinical models and therapeutics, rig- orous studies of cell phenotypes and cellular responses to potential agents in patient-derived neurons will be advantageous. The long-term goal of this research is to elucidate the molecular mechanisms underlying the pathology of severe autism that will serve as the basis to develop innovative treatments for these patients with few current therapeutic options. We recently identi- fied a novel mechanism in autism involving endosomal Na+/H+ exchangers (NHEs). We have now generated induced pluripotent stem cell lines (iPSCs) from patients with a range of NHE6 mutations. The objective for this application is to elucidate the role of NHE6 in human axon de- velopment and to test the response of patient-derived neurons to available, mechanism-based agents that may subsequently serve as treatments in patients. NHE6 regulates the efflux of pro- tons from endosomes. In our preliminary studies in human neurons, NHE6 deficiency leads to over-acidification of the endosomal lumen and defects in axon growth and branching. This mechanism is fortuitous because there are a number of well-known FDA-approved agents that target intra-endosomal pH. Our central hypothesis is that deficiency of NHE6 inhibits axon growth and branching due to diminished polarized membrane addition. The rationale for this re- search is that it constitutes the first critical steps toward the development of new treatments for individuals with severe autism and related disorders caused by reductions in NHE6 expression and/or by defects in axonal growth or arborization. We have published studies on NHE6 function in mouse models, yet the transition to patient-derived tissues is warranted at this time. Screening of potential human therapeutic agents is best performed on human (and patient-derived) tis- sues. Three specific aims are proposed: (1) Determine the role of NHE6 in regulating endosome lumen acidity and endosome recycling. (2) Determine the extent to which the function of RAB10-associated SVs in axon growth is perturbed by NHE6 mutation in patient-derived neu- rons. (3) Determine the reversibility of axon growth and branching defects in patient-derived neurons by exogenous growth factors. The proposed research is innovative because we are studying a novel cellular mechanism in autism, namely, regulation of intra-endosomal pH in ax- on growth. This research is significant because it will lead to the development of critically need- ed preclinical models in patient-derived neurons and potential treatments for severe autism and related disorders, an urgent public health problem.