Early regionalization of the otic placode and its regulation by the Notch signaling pathway

Abstract

Otic neuronal precursors are the first cells to be specified and do so in the anterior domain of the otic placode, the proneural domain. In the present study, we have explored the early events of otic proneural regionalization in relation to the activity of the Notch signaling pathway. The proneural domain was characterized by the expression of Sox3, Fgf10 and members of the Notch pathway such as Delta1, Hes5 and Lunatic Fringe. The complementary non-neural domain expressed two patterning genes, Lmx1b and Iroquois1, and the members of the Notch pathway, Serrate1 and Hairy1. Fate map studies and double injections with DiI/DiO showed that labeled cells remained confined to anterior or posterior territories with limited cell intermingling. To explore whether Notch signaling pathway plays a role in the initial regionalization of the otic placode, Notch activity was blocked by a gamma-secretase inhibitor (DAPT). Notch blockade induced the expansion of non-neural genes, Lmx1 and Iroquois1, into the proneural domain. Combined gene expression and DiI experiments showed that these effects were not due to migration of non-neural cells into the proneural domain, suggesting that Notch activity regulates the expression of non-neural genes. This was further confirmed by the electroporation of a dominant-negative form of the Mastermind-like1 gene that caused the up-regulation of Lmx1 within the proneural domain. In addition, Notch pathway was involved in neuronal precursor selection, probably by a classical mechanism of lateral inhibition. We propose that the regionalization of the otic domain into a proneural and a non-neural territory is a very early event in otic development, and that Notch signaling activity is required to exclude the expression of non-neural genes from the proneural territory.

Jagged 1 regulates the restriction of Sox2 expression in the developing chicken inner ear: a mechanism for sensory organ specification

“…The forced expression of hJag1 in the dorsal domain of the otocyst close to the origin of the endolymphatic sac was ineffective in inducing Sox2 expression . Fate maps show that this domain derives from the dorsal and posterior otic cup (Abello et al, 2007;Bell et al, 2008;Brigande et al, 2000), which is devoid of neurosensory competence and does not express Sox2 (Neves et al, 2007) (Fig. 1).…”

Her9 represses neurogenic fate downstream of Tbx1 and retinoic acid signaling in the inner ear

“…In Tbx1 -/-mice, the neurogenic region of the otic epithelium is expanded, the otocyst is small and the sensory patches do not form (Moraes et al, 2005;Raft et al, 2004;Vitelli et al, 2003;Xu et al, 2007). Our previous studies in chick revealed that Hairy1 (Hes1 homolog) is present in the non-neurogenic territory of the otic vesicle (Abello et al, 2007), suggesting that Hes1 might have an as yet unidentified role in neural patterning of the otic vesicle.…”

Hindbrain signals in otic regionalization: walk on the wild side

“…In an attempt to answer the first question, fate maps of the otic primordium have been performed in different species and at different stages (Brigande et al, 2000;Lang and Fekete, 2001;Kil and Collazo, 2001;Streit, 2002;Kozlowski et al, 1997;Satoh and Fekete, 2005;Abello et al, 2007; reviewed in Kil and Collazo, 2002). However, the results obtained so far fail to give a clear picture.…”