Correct Timing of Proliferation and Differentiation is Necessary for Normal Inner Ear Development and Auditory Hair Cell Viability

Abstract

Background Hearing restoration through hair cell regeneration will require revealing the dynamic interactions between proliferation and differentiation during development to avoid the limited viability of regenerated hair cells. Pax2-Cre N-Myc conditional knockout (CKO) mice highlighted the need of N-Myc for proper neurosensory development and possible redundancy with L-Myc. The late-onset hair cell death in the absence of early N-Myc expression could be due to mis-regulation of genes necessary for neurosensory formation and maintenance, such as Neurod1, Atoh1, Pou4f3, and Barhl1. Results Pax2-Cre N-Myc L-Myc double CKO mice show that proliferation and differentiation are linked together through Myc and in the absence of both Mycs, altered proliferation and differentiation results in morphologically abnormal ears. In particular, the organ of Corti apex is re-patterned into a vestibular-like organization and the base is truncated and fused with the saccule. Conclusions These data indicate that therapeutic approaches to restore hair cells must take into account a dynamic interaction of proliferation and differentiation regulation of basic Helix-Loop-Helix transcription factors in attempts to stably replace lost cochlear hair cells. In addition, our data indicate that Myc is an integral component of the evolutionary transformation process that resulted in the organ of Corti development.

Transcription Factors with Conserved Binding Sites Near ATOH1 on the POU4F3 Gene Enhance the Induction of Cochlear Hair Cells

“…Consistently, overexpression of N-Myc results in increased labyrinthine size through increased proliferation [30]. L-Myc appears to be of lesser importance in inner ear development [31]. These data are consistent with our results that only hN-MYC altered the effects of hATOH1.…”

Inner ear development: building a spiral ganglion and an organ of Corti out of unspecified ectoderm

“…Levels of Gata3 and Sox2 define hair cell differentiation in interaction with Atoh1 (Dabdoub et al, 2008, Duncan and Fritzsch, 2013) and micro RNA is crucial for normal hair cell differentiation (Groves et al, 2013, Kersigo et al, 2011). In contrast to this data, there is no data on the molecular definition of cell cycle exit beyond effects of other bHLH genes such as Neurog1 (Matei et al, 2005) and N-Myc (Kopecky et al, 2013). Clearly, neither gene defines hair cells but plays a yet to be specified role at some point in the precursors of hair cells.…”

“…To emphasize this point about the importance of cell type and positioning, even slight changes in numbers and distribution of outer pillar cells through manipulation of Fgfr3 signaling (Puligilla et al, 2007), or reduction or loss of the inner hair cells in Srrm4 mutants (Nakano et al, 2012) or loss of the outer most row of outer hair cells in Fgf20 mutants (Huh et al, 2012) can render the OC dysfunctional. Attempts to regenerate the OC require an understanding of the signals that position the OC properly and differentiate the right hair cell and supporting cell type in the right position in the correct numbers and correct proximity, aspects of OC development we just begin to understand (Jahan et al, 2013, Kopecky et al, 2013). Given the unique distribution and number of inner pillar cells, it is fair to say that understanding OC development requires minimally a causal explanation on how the position and distribution of the single row of inner pillar cells and inner hair cells (Fig.…”

“…Similar but less profound disorganization of the OC occurs when Atoh1 is prematurely expressed in mutants null for Neurog1 (Matei et al, 2005) or Neurod1 (Jahan et al, 2010b). Manipulation of the cell cycle by mutating the bHLH transcription factor N-Myc results in premature cell exit but delayed expression of Atoh1 and disorganization of apical hair cells so that no OC organization is apparent (Kopecky et al, 2013). Combined, these data point out that timing of cell cycle exit relative to expression of genes that affect timing and level of expression of Atoh1 is crucial for normal OC development (Jahan et al, 2013).…”

Evolving gene regulatory networks into cellular networks guiding adaptive behavior: an outline how single cells could have evolved into a centralized neurosensory system

“…On the one extreme are hair cell precursors that are already committed prior to Atoh1 expression, perhaps through Sox2 (Kiernan, et al, 2005), possibly at the time they exit the cell cycle (Kopecky, et al, 2013), to differentiate as hair cell so that they cannot be reverted into a different fate even if powerful neurogenic factors are expressed (Jahan, et al, 2012). On the other hand are neurons and supporting cells that even in a late stage in their decision making process can be transdifferentiated into hair cells (Jahan, et al, 2010, Mizutari, et al, 2013).…”