Neurog1 can partially replace Atoh1 to differentiate and maintain hair cells in a disorganized organ of Corti

Abstract

Atoh1, a basic helix-loop-helix (bHLH) transcription factor (TF), is essential for the differentiation of hair cells (HCs), mechanotransducers that convert sound into auditory signals in the mammalian organ of Corti (OC). Previous work demonstrated that replacing mouse Atoh1 with the fly ortholog atonal rescues HC differentiation, indicating functional replacement by other bHLH genes. However, replacing Atoh1 with Neurog1 resulted in reduced HC differentiation compared with transient Atoh1 expression in a 'selfterminating' Atoh1 conditional null mouse (Atoh1-Cre; Atoh1 f/f ). We now show that combining Neurog1 in one allele with removal of floxed Atoh1 in a self-terminating conditional mutant (Atoh1-Cre; Atoh1 f/kiNeurog1 ) mouse results in significantly more differentiated inner HCs and outer HCs that have a prolonged longevity of 9 months compared with Atoh1 self-terminating littermates. Stereocilia bundles are partially disorganized, disoriented and not HC type specific. Replacement of Atoh1 with Neurog1 maintains limited expression of Pou4f3 and Barhl1 and rescues HCs quantitatively, but not qualitatively. OC patterning and supporting cell differentiation are also partially disrupted. Diffusible factors involved in patterning are reduced (Fgf8) and factors involved in cell-cell interactions are affected (Jag1, Hes5). Despite the presence of many HCs with stereocilia these mice are deaf, possibly owing to HC and OC patterning defects. This study provides a novel approach to disrupt OC development through modulating the HC-specific intracellular TF network. The resulting disorganized OC indicates that normally differentiated HCs act as 'self-organizers' for OC development and that Atoh1 plays a crucial role to initiate HC stereocilia differentiation independently of HC viability.

Gene, cell, and organ multiplication drives inner ear evolution

“…The latter remained a possibility given the positive feedback loop Atoh1 protein exerts on its own regulation via specific enhancer binding (Fritzsch et al, 2006a; Helms et al, 2000). To distinguish between these two possibilities, a mouse line was made that combined a delayed deletion of Atoh1 (Pan et al, 2012a) with the misexpression of Neurog1 in the second allele (Jahan et al, 2015b). These mice displayed a surprisingly robust development of nearly all hair cells but in a disorganized organ of Corti.…”

“…The mechanosensory cell specializations to allow sensory cells to develop this exquisite mechanosensation require a number of factors for their normal development such as the basic Helix-Loop-Helix transcription factors Atoh1 (Jahan et al, 2015b; Pan et al, 2012a) or Neurod1 (Jahan et al, 2015a) that is tightly regulated in addition through several other transcription factors such as Pou4f3 (Ikeda et al, 2015; Xiang et al, 2003). Gfi1 (Hertzano et al, 2004) and Barlh1 (Chellappa et al, 2008) to name a few.…”

“…Additional information is needed to understand the development of different types of hair cells. Hair cells show variable stereociliary diameters that clearly are affected by bHLH genes (Jahan et al, 2013, 2015b), but also show the unique stereocilia arrangements of inner and outer hair cells that apparently evolved out of the much simpler stereocilia bundle found in ancestral hair cells of the vestibular type (Lewis et al, 1985). When and how factors that coordinate polarity of hair cells across sensory epithelia (May-Simera et al, 2015) evolved is unclear.…”

“…However, while there is an obvious interaction of neuronal precursor formation with hair cell development (Matei et al, 2005), the detailed nature of it remains unclear at a molecular and cellular level (Jahan et al, 2015b; Raft and Groves, 2015). What is clear is the sequence of activations of transcription factors that result in proliferation of neuronal precursors and their differentiation (Goodrich, 2016).…”

Sensing External and Self-Motion with Hair Cells: A Comparison of the Lateral Line and Vestibular Systems from a Developmental and Evolutionary Perspective

“…This lineage relation would allow cell fate switching between afferent neurons and hair cells, given their common precursor. Indeed, loss of hair cells [Ma et al 2000; Matei et al 2005], cross-regulation of Atoh1 and Neurog1 [Raft and Groves 2015], development of intra- ganglionic hair cells when Neurod1 is removed [Jahan et al 2015c] and the ability of Neurog1 to govern some aspects of hair cell development [Jahan et al 2015b] support the notion of a molecular overlap in inner ear hair cell and sensory neuron lineages [Fritzsch et al 2010]. What remains still unclear is at which stage of placode evolution these lineages diverged.…”