Upregulation of Phospho-S6 and Dendritic Overgrowth of Developing Layer V Neurons Are Phenotypes in Common Between Pten+/- and Fmr1-/y Mice
Objectives: Our goal is to identify common cell type(s) and time window(s) in which p-S6 is dysregulated across two different mouse models of autism risk factors, Pten and Fmr1, and to study the cellular phenotypes associated with dysregulated p-S6 in these two models.
Methods: We have used immunohistochemistry and Western blot to analyze the spatiotemporal distribution and levels of p-S6 in the developing cerebral cortex of Pten+/- and Fmr1-/y mice. Furthermore, we have also used genetic labeling along with cellular imaging and reconstruction techniques to analyze neuronal growth, a process that is tightly correlated with p-S6 activity, in these models.
Results: In the developing brain of wild type mice, p-S6 immunoreactivity is differentially enriched across cell types during the first two postnatal weeks. One particularly interesting cell type that shows enrichment for p-S6 during this time window is layer V neurons of the cerebral cortex. We find that p-S6 levels are elevated during development in the cerebral cortex of both Pten+/- and Fmr1-/y mice. Consistent with this, these models exhibit dendritic overgrowth of layer V neurons during development. Moreover, the neuronal overgrowth is not uniformly across cortical cell types since we observed increased cell soma size in layer V neurons (which are enriched for p-S6), but not in layer VI neurons (which are not enriched for p-S6).
Conclusions: Upregulation of p-S6 in layer V neurons during development is a point of convergence for two distinct autism risk factors, Pten and Fmr1. Layer V neurons are the primary cortical output neurons and project to subcortical areas essential for social behavior. Dendritic overgrowth in developing layer V neurons could disrupt the cortical circuit assembly or desynchronized cortical output. Consistent with reports of abnormal layer V dendritic morphology in individuals with ASD, this phenotype may contribute to the pathophysiology of autism risk factors associated with mTOR/p-S6 signaling.