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???displayArticle.abstract??? Atoh1 (Math1) was the first gene discovered in ear development that showed no hair cell (HC) differentiation when absent and could induce HC differentiation when misexpressed. These data implied that Atoh1 was both necessary and sufficient for hair cell development. However, other gene mutations also result in loss of initially forming HCs, notably null mutants for Pou4f3, Barhl1, and Gfi1. HC development and maintenance also depend on the expression of other genes (Sox2, Eya1, Gata3, Pax2) and several genes have been identified that can induce HCs when misexpressed (Jag1) or knocked out (Lmo4). In the earAtoh1 is not only expressed in HCs but also in some supporting cells and neurons that do not differentiate into HCs. Simple removal of one gene, Neurod1, can de-repress Atoh1 and turns those neurons into HCs suggesting that Neurod1 blocks Atoh1 function in neurons. Atoh1 expression in inner pillar cells may also be blocked by too many Hes/Hey factors but conversion into HCs has only partially been achieved through Hes/Hey removal. Detailed analysis of cell cycle exit confirmed an apex to base cell cycle exit progression of HCs of the organ of Corti. In contrast, Atoh1 expression progresses from the base toward the apex with a variable delay relative to the cell cycle exit. Most HCs exit the cell cycle and are thus defined as precursors before Atoh1 is expressed. Atoh1 is a potent differentiation factor but can differentiate and maintain HCs only in the ear and when other factors are co-expressed. Upstream factors are essential to regulate Atoh1 level of expression duration while downstream, co-activated by other factors, will define the context of Atoh1 action. We suggest that these insights need to be taken into consideration and approaches beyond the simple Atoh1 expression need to be designed able to generate the radial and longitudinal variations in hair cell types for normal function of the organ of Corti.
Figure 1. The interactions of Sox and bHLH genes in HC differentiation in mice. Experimental data indicate a complex interaction of Sox and bHLH genes in the progression of HC fate commitment and differentiation. (A) Sox2 and Myc genes act downstream of Eya1/Six1 and are essential for proliferation of HC precursor cells to ensure self-renewal of precursors but also commitment to the HC lineage. These precursor genes are turned off in the neurosensory lineage and bHLH genes are activated that antagonizes Sox2. (B) bHLH TFs can form complex interactions in a given cell that can undergo periodic changes in expression levels and their signal can undergo context dependent variation between gene expression and suppression. Data in mice and flies suggest that all proneural TFs compete for the E-proteins (Tcf3,4,12) to form heterodimers for proper binding. Thus, the level of all proneuronal bHLH TFs (here Atoh1 and Neurod1) and available E-proteins as well as their binding preference will determine how much signaling of heterodimers will occur. Importantly, E-proteins can also interact with Hes/Hey factors and the inhibitors of DNA binding (Ids), limiting availability of E-proteins for heterodimerization of proneuronal protein, proportionally to the affinity and concentration of all these interactive partners. In essence, the binding properties and frequency of the binding partners will determine whether a cell is differentiating as a neuron/HC, a supporting/glial cell, or is continuing proliferation as a prosensory precursor. HC, hair cell; SC, supporting cell. Modified after (Fritzsch et al., 2015a).
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