Regulation and Lineage Analysis of Neurog1 in the Developing Spinal Cord
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The bHLH transcription factor Neurog1 is involved in neuronal differentiation and cell-type specification in distinct regions of the developing nervous system. I developed mouse models that efficiently drive expression of GFP or Cre recombinase in all Neurog1 (Ngn1, NeuroD3) domains. Deleting highly conserved sequences from a BAC containing 113kb 5' and 71kb 3' genomic sequence surrounding the Neurog1 coding region allowed the identification of enhancer elements required to drive Neurog1 expression. I show that a 3.8 kb fragment located 4.2 kb 5' of Neurog1 is required for efficient reporter expression in all Neurog1 domains. This sequence contains previously identified enhancer elements for midbrain, hindbrain and dorsal neural tube, and has two sequences conserved from human to fish. A 16kb fragment containing 8.9 kb 5' and 5.2 kb 3' of the Neurog1 coding sequence was not sufficient to drive expression in all domains. Reporter expression was observed in the dorsal neural tube, the midbrain, hindbrain and trigeminal ganglia, but was missing in the olfactory epithelium, dorsal root ganglia, dorsal telencephalon, and ventral neural tube. A 2.3 kb enhancer element located 8 kb 5' of the Neurog1 coding region was identified that is necessary to direct expression in the ventral neural tube. In addition, these mouse models allowed both short-term and long-term lineage analyses. I show that derivatives of Neurog1-expressing progenitor cells in the neural tube largely comprise the interneuron populations dI2, dI6, V0, V1, and V2, and to a lesser extent motorneurons. This is seen in the co-expression of GFP driven by Neurog1 regulatory sequences with the neuronal identity markers Brn3a, Islet1/2, Lhx1/5, Lhx3, Pax2, and Chx10. Genetic fate mapping in vivo using Cre recombinase reveals that although Neurog1-expressing cells primarily give rise to neurons, minor populations of oligodendrocytes and astrocytes are also identified in the lineage by adult stages in the spinal cord. Adding temporal control to the fate mapping strategy demonstrates that the neurons are generated from Neurog1-expressing cells prior to E13, and glial cells after E13, placing Neurog1 in lineage restricted precursor cells during embryogenesis.