Mtor/Rptor function globally prevents cortical microcephaly and cell-autonomously promotes postnatal neuron survival in cell type specific manner

The generation of faithful cell-type diversity and correct projection neuron numbers is essential for cerebral cortex development. Corticogenesis is however susceptible to genetic interference of critical signaling pathways, including mutations in Mtor/Rptor that lead to microcephaly. How the loss of Rptor/mTORC1 function affects cortical developmental programs, at single cell level, is still unknown. Here, we utilized Mosaic Analysis with Double Markers (MADM) technology to probe Rptor gene function upon sparse single cell- or global tissue-wide ablation. We found that tissue-wide effects drive the etiology of cortical microcephaly upon loss of Rptor, rather than deficits in projection neuron genesis. Conversely, Rptor function is cell-autonomously required for postnatal projection neuron survival in a highly cell-type-specific manner. Collectively, our results suggest that the fine balance of precise cell-type-specific cell-autonomous Rptor/mTORC1 function in concert with non-cell-autonomous tissue-wide effects is essential for the development of a properly-sized cerebral cortex with accurate projection neuron diversity.