MyT1 Counteracts the Neural Progenitor Program to Promote Vertebrate Neurogenesis

Cellular differentiation and lineage commitment are considered robust and irreversible processes during development. Recent work has shown that mouse and human fibroblasts can be reprogrammed to a pluripotent state with a combination of four transcription factors. This raised the question of whether transcription factors could directly induce other defined somatic cell fates, and not only an undifferentiated state. We hypothesized that combinatorial expression of neural lineage-specific transcription factors could directly convert fibroblasts into neurons. Starting from a pool of nineteen candidate genes, we identified a combination of only three factors, Ascl1, Brn2, and Myt1l, that suffice to rapidly and efficiently convert mouse embryonic and postnatal fibroblasts into functional neurons in vitro. These induced neuronal (iN) cells express multiple neuron-specific proteins, generate action potentials, and form functional synapses. Generation of iN cells from non-neural lineages could have important implications for studies of neural development, neurological disease modeling, and regenerative medicine.

During the development of the mammalian central nervous system, neural stem cells and their derivative progenitor cells generate neurons by asymmetric and symmetric divisions. The proliferation versus differentiation of these cells and the type of division are closely linked to their epithelial characteristics, notably, their apical-basal polarity and cell-cycle length. Here, we discuss how these features change during development from neuroepithelial to radial glial cells, and how this transition affects cell fate and neurogenesis.

Notch belongs to a family of transmembrane proteins that are widely conserved from flies to vertebrates and are thought to be involved in cell-fate decisions. In Drosophila, the Suppressor of hairless (Su(H)) gene and genes of the Enhancer of split (E(Spl)) complex, which encode proteins of the basic helix-loop-helix type have been implicated in the Notch signalling pathway. Mammalian homologues of E(Spl), such as the mouse Hairy enhancer of split (HES-1), have been isolated. Both HES-1 and the intracellular domain of murine Notch (mNotch) are able to block MyoD-induced myogenesis. Here we show that activated forms of mNotch associate with the human analogue of Su(H), KBF2/RBP-J kappa (refs 8,9) and act as transcriptional activators through the KBF2-binding sites of the HES-1 promoter.