bHLH transcription factor Her5 links patterning to regional inhibition of neurogenesis at the midbrain-hindbrain boundary

Abstract

The midbrain-hindbrain (MH) domain of the vertebrate embryonic neural plate displays a stereotypical profile of neuronal differentiation, organized around a neuron-free zone (`intervening zone', IZ) at the midbrain-hindbrain boundary(MHB). The mechanisms establishing this early pattern of neurogenesis are unknown. We demonstrate that the MHB is globally refractory to neurogenesis,and that forced neurogenesis in this area interferes with the continued expression of genes defining MHB identity. We further show that expression of the zebrafish bHLH Hairy/E(spl)-related factor Her5 prefigures and then precisely delineates the IZ throughout embryonic development. Using morpholino knock-down and conditional gain-of-function assays, we demonstrate that Her5 is essential to prevent neuronal differentiation and promote cell proliferation in a medial compartment of the IZ. We identify one probable target of this activity, the zebrafish Cdk inhibitor p27Xic1. Finally, although the her5 expression domain is determined by anteroposterior patterning cues, we show Her5 does not retroactively influence MH patterning. Together, our results highlight the existence of a mechanism that actively inhibits neurogenesis at the MHB, a process that shapes MH neurogenesis into a pattern of separate neuronal clusters and might ultimately be necessary to maintain MHB integrity. Her5 appears as a partially redundant component of this inhibitory process that helps translate early axial patterning information into a distinct spatiotemporal pattern of neurogenesis and cell proliferation within the MH domain.

Establishment of a proneural field in the inner ear

“…Homozygous mutation of Tbx1 gene in mouse causes birth defects derived from developmental abnormalities of the embryonic pharyngeal sysIt has been shown that hairy controls the pattern of sensory organ formation by repressing achaete-scute (Orenic et al, 1993). In the zebrafish midbrain-hindbrain boundary, mouse olfactory placode and the inter-proneural stripes of Xenopus, Hairy and Enhancer of Split homologues (Hes/Her) repress neural fate (Cau et al, 2000, Geling et al, 2003, Bae et al, 2005. In chick otic cups, Hairy1 (Hes1 in mammals) is expressed restricted in the nonneural domain, disclosing a putative role in repressing proneural fate in the posterior region (Abelló et al, 2007).…”

Xenopus hairy2 functions in neural crest formation by maintaining cells in a mitotic and undifferentiated state

“…Hes1 directly contributes to the promotion of progenitor cell proliferation through transcriptional repression of a cyclin-dependent kinase inhibitor, p27Kip1, in embryonic carcinoma cells (Murata et al, 2005). Zebrafish p27 xic1 expression is negatively regulated by Her5 in the midbrain-hindbrain boundary (Geling et al, 2003). These findings suggested that hes-related genes might be involved in maintaining neural crest stem cells in the mitotic and undifferentiated state.…”

Signalling from hindbrain boundaries regulates neuronal clustering that patterns neurogenesis

“…For example, three neurogenic zones form along the dorsoventral axis of the spinal cord during primary neurogenesis due to inhibition of differentiation in the intervening regions by specific Hes/Her and Zic genes (Brewster et al, 1998;Bae et al, 2005). Similarly, a non-neurogenic zone forms at the midbrainhindbrain boundary due to the inhibitory action of Hes/Her family members (Geling et al, 2003;Geling et al, 2004). In these examples, the expression of transcription factors that inhibit neuronal differentiation appears to be linked to dorsoventral and anteroposterior patterning within the neural epithelium that underlies cell type and regional specification.…”