Abstract
Proneural basic-helix-loop-helix (bHLH) proteins, such as Neurogenin2 (Ngn2) and Ascl1, are critical regulators at the onset of neuronal differentiation. Endogenously they have largely complementary expression patterns, and have conserved roles in the specification of distinct neuronal subtypes. InXenopusembryos, xNgn2 is the master regulator of primary neurogenesis forming sensory, inter- and motor neurons within the neural plate, while xAscl1 is the master regulator of autonomic neurogenesis, forming noradrenergic neurons in the antero-ventral region of the embryo. Here we characterise neuronal subtype identity of neurons induced by xNgn2 in the ectoderm of wholeXenopusembryos in comparison with xAscl1, and in ectodermal “animal cap” explants. We find that the transcriptional cascades mediating primary and autonomic neuron formation are distinct, and while xNgn2 and xAscl1 can upregulate genes associated with a non-endogenous cascade, this expression is spatially restricted within the embryo. xNgn2 is more potent than xAscl1 at inducing primary neurogenesis as assayed by neural-β-tubulin. In ectoderm of the intact embryo, these induced primary neurons have sensory characteristics with no upregulation of motor neuron markers. In contrast, xNgn2 is able to up-regulate both sensory and motor neuron markers in naïve ectoderm of animal cap explants, suggesting a non-permissive environment for motor identity in the patterned ectoderm of the whole embryo.
Highlights
Proneural proteins are members of the basic-helixloop-helix family of transcription factors, with a conserved role in driving and coordinating the various stages of neurogenesis[1]
We focus on the key Xenopus proneural protein xNgn[2] and define xNgn[2] subtype specification during induced neurogenesis in the ectoderm of whole embryos and in isolated ectodermal “animal cap” explants
Ectopic primary neurons generated show predominantly sensory characteristics with the pattern of sensory marker xHox11L2 mirroring that of N-β-tubulin
Summary
Proneural proteins are members of the basic-helixloop-helix (bHLH) family of transcription factors, with a conserved role in driving and coordinating the various stages of neurogenesis[1]. In Xenopus, xNgn[2] ( known as X-ngnr-1) is the master regulator of primary neurogenesis[4], during which neurons develop in the trigeminal ganglia and in three bilateral stripes within the neural plate to form sensory, inter- and motor neurons for coordination of larval reflexes[5]. This is analogous to the role of mammalian Ngn[2] in specification of sensory and motor neurons in the dorsal root ganglia and ventral spinal cord, respectively[6]. What is less well documented is whether these induced neurons acquire a subtype identity, and whether that identity differs depending on the inducing proneural factor
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