Tox: A multifunctional transcription factor and novel regulator of mammalian corticogenesis

Major efforts are invested to characterize the factors controlling the proliferation of neural stem cells. During mammalian corticogenesis, our group has identified a small pool of genes that are transiently downregulated in the switch of neural stem cells to neurogenic division and reinduced in newborn neurons. Among these switch genes, we found Tox, a transcription factor with hitherto uncharacterized roles in the nervous system. Here, we investigated the role of Tox in corticogenesis by characterizing its expression at the tissue, cellular and temporal level. We found that Tox is regulated by calcineurin/Nfat signalling. Moreover, we combined DNA adenine methyltransferase identification (DamID) with deep sequencing to characterize the chromatin binding properties of Tox including its motif and downstream transcriptional targets including Sox2, Tbr2, Prox1 and other key factors. Finally, we manipulated Tox in the developing brain and validated its multiple roles in promoting neural stem cell proliferation and neurite outgrowth of newborn neurons. Our data provide a valuable resource to study the role of Tox in other tissues and highlight a novel key player in brain development. Synopsis Calcineurin signalling triggers the nuclear transport of Nfat4 in neural stem cells and neurons leading to the activation of the transcription factor Tox. Tox regulates the expression of a number of cell-specific downstream targets to promote stem and progenitor cell self-renewal and to regulate neurite outgrowth and specification of newborn neurons. Tox is a HMG-box transcription factor expressed during mammalian brain development. Tox is expressed in neural stem cells, downregulated in neurogenic progenitors and reinduced in newborn neurons. Tox expression is controlled by the calcineurin/Nfat4 signalling. DamID-Seq identified a GC-rich Tox binding motif and a number of downstream targets that are involved in brain development and neurite outgrowth. In vivo manipulation of Tox expression during embryonic development inhibited neurogenesis, induced the expansion of neural progenitors and altered neurite outgrowth and specification of postmitotic neurons. Tox is a novel regulator of corticogenesis with multiple and diverse roles at the level of progenitors and neurons. A novel neuronal transcription factor promotes renewal in neural stem cells and neurite outgrowth in newborn neurons, but is transiently downregulated while neural stem cells switch to neurogenic divisions.