UC Irvine

Autism spectrum disorder (ASD) affects 2% of children and is characterized by impaired social and communication skills together with repetitive, stereotypic behavior. The pathophysiology of ASD is complex due to genetic and environmental heterogeneity, complicating the development of therapies and making diagnosis challenging. Growing evidence supports a role of disrupted Ca2+ signaling in ASD. I developed and applied a high-throughput fluorometric imaging plate reader (FLIPR) assay to monitor agonist-evoked Ca2+ signals in human primary skin fibroblasts. My results indicate that IP3 -mediated Ca2+ release from the endoplasmic reticulum in response to activation of purinergic receptors is significantly depressed in subjects with sporadic, as well as rare syndromic forms of ASD. This was apparent in Ca2+ signals evoked by G protein-coupled receptors and by photoreleased IP3 at the levels of both global and local elementary Ca2+ events, suggesting fundamental defects in IP3R channel activity in ASD.

Given the ubiquitous involvement of IP3R-mediated Ca2+ signaling in neuronal excitability, synaptic plasticity, gene expression and neurodevelopment, I further expanded my findings to a murine model of FXS. Activation of the IP3 cascade via plasma membrane metabotropic receptors did not reveal any Ca2+ signaling deficits in neurons from mice with the FMR1 gene deletion. Glial cells from FXS mice did not demonstrate any sizable difference in response to GPCR activation, or IP3 UV flash uncaging as compared with wild type. Finally, mouse fibroblasts from FXS mice assayed with the high-throughput screen FLIPR, analogous to what was used on the human skin fibroblasts, did not reveal any difference in the IP3-mediated Ca2+ release compared with wild type mice. These findings highlight divergence between animal models and human conditions, and show inadequacy of the murine model in studying the effect of the FMR1 gene mutation on IP3 signaling cascade.

In conclusion, my findings suggest that deficits in IP3-mediated Ca2+ signaling represent a convergent function shared across the spectrum of autistic disorders – whether caused by rare highly penetrant mutations or sporadic forms – and hold promise as a biomarker for diagnosis and novel drug discovery. This work also highlights potential pharmaceutical targets, and identifies Ca2+ screening in human skin fibroblasts as a promising technique for early detection of individuals susceptible to ASD.