Delta-like 1 and lateral inhibition during hair cell formation in the chicken inner ear: evidence against cis-inhibition

Abstract

SUMMARYThe formation of the salt-and-pepper mosaic of hair cells and supporting cells in the sensory epithelia of the inner ear is regulated by Notch signalling and lateral inhibition, but the dynamics of this process and precise mode of action of delta-like 1 (Dll1) in this context are unclear. Here, we transfected the chicken inner ear with a fluorescent reporter that includes elements of the mammalian Hes5 promoter to monitor Notch activity in the developing sensory patches. The Hes5 reporter was active in proliferating cells and supporting cells, and Dll1 expression was highest in prospective hair cells with low levels of Notch activity, which occasionally contacted more differentiated hair cells. To investigate Dll1 functions we used constructs in which Dll1 expression was either constitutive, regulated by the Hes5 promoter, or induced by doxycycline. In support of the standard lateral inhibition model, both continuous and Hes5-regulated expression of Dll1 promoted hair cell differentiation cell-autonomously (in cis) and inhibited hair cell formation in trans. However, some hair cells formed despite contacting Dll1-overexpressing cells, suggesting that some progenitor cells are insensitive to lateral inhibition. This is not due to the cis-inhibition of Notch activity by Dll1 itself, as induction of Dll1 did not cell-autonomously reduce the activity of the Hes5 reporter in progenitor and supporting cells. Altogether, our results show that Dll1 functions primarily in trans to regulate hair cell production but also that additional mechanisms operate downstream of lateral inhibition to eliminate patterning errors in the sensory epithelia of the inner ear.

Ligand-dependent Notch signaling strength orchestrates lateral induction and lateral inhibition in the developing inner ear

“…This is in agreement with recent results showing that once sensory progenitors start to highly express Atoh1 and subsequently Dl1, they cannot be prevented from becoming hair cells (Driver et al, 2013). This might also underlie the observation by Chrysostomou et al (2012) of the generation of hair cells in direct contact with several neighboring cells expressing high levels of Dl1.…”

“…This situation resembles that found in cis-inhibition of Notch signaling, in which Dl1 ligand in a cell competes with Dl1 ligand in neighboring cells to bind to Notch receptor (Formosa-Jordan and Ibañes, 2014). In the context of inner ear development, cisinhibition does not occur (Chrysostomou et al, 2012), but the competition between Dl1 and Jag1 ligands results in a similar effect on the signal. Our results indicate that, upon Atoh1 expression, Jag1 switches from increasing overall signaling and driving lateral induction to effectively decreasing Notch signaling and facilitating hair cell patterning.…”

Synthetic lateral inhibition governs cell-type bifurcation with robust ratios

“…Although the spatial arrangement of red cells and green cells shown in this study appeared random, the lateral inhibition mechanism is known to create a more regular salt-andpepper pattern in several epithelial cell tissues, such as the inner ear 1,19,20,39 . In fact, our numerical simulation of the aTRLF model with a hexagonal cell arrangement resulted in a regular salt-andpepper pattern (Supplementary Fig.…”

“…A prominent example of the binary cell fate decision through lateral inhibition is the specification of bristle precursor cells in initially homogeneous epithelial cells in Drosophila 3,4,9-11 . In vertebrates, the lateral inhibition mechanism also governs cell fate decisions between neurons and non-neuronal cells in the central nervous system 3,[12][13][14] , secretory cells and absorptive cells in the intestine [15][16][17][18] , and hair cells and supporting cells in the inner ear 3,19,20 .…”

Numb is not a critical regulator of Notch-mediated cell fate decisions in the developing chick inner ear

“…Lateral inhibition regulates the production of otic neuroblasts at early stages of ear development (Adam et al, 1998; Haddon et al, 1998; Abello et al, 2007; Daudet et al, 2007), and controls hair cells vs. supporting cell fate decisions within the embryonic sensory patches (Adam et al, 1998; Lanford et al, 1999; Riley et al, 1999; Zine et al, 2000; Daudet and Lewis, 2005; Chrysostomou et al, 2012). The nascent hair cells express several Notch ligands: Delta1-like 1 (Dll1), Delta-like 3 (Dll3), and Serrate2/Jagged2 (Jag2) and activate Notch in their neighbors, which become supporting cells.…”