Upregulation of Phospho-S6 and Dendritic Overgrowth of Developing Layer V Neurons Are Phenotypes in Common Between Pten+/- and Fmr1-/y Mice

Background:  PTEN and FMR1 are two examples of susceptibility genes for autism spectrum disorder (ASD) that encode regulators of the PI3K-mTOR pathway. Phosphorylation of ribosomal protein S6 (p-S6) is a downstream readout of activity in the mTOR pathway. Altered levels of p-S6 has been reported in the postmortem cerebral cortex of individuals with autism and in mouse models of autism risk factors. However, it is not known when during development and in which cell types dysregulation of p-S6 signaling may contribute to the symptoms of ASD.  

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.