Neurod1 suppresses hair cell differentiation in ear ganglia and regulates hair cell subtype development in the cochlea.
BACKGROUND
At least five bHLH genes regulate cell fate determination and differentiation of sensory neurons, hair cells and supporting cells in the mammalian inner ear. Cross-regulation of Atoh1 and Neurog1 results in hair cell changes in Neurog1 null mice although the nature and mechanism of the cross-regulation has not yet been determined. Neurod1, regulated by both Neurog1 and Atoh1, could be the mediator of this cross-regulation.


RESULTS
We used Tg(Pax2-Cre) to conditionally delete Neurod1 in the inner ear. Our data demonstrate for the first time that the absence of Neurod1 results in formation of hair cells within the inner ear sensory ganglia. Three cell types, neural crest derived Schwann cells and mesenchyme derived fibroblasts (neither expresses Neurod1) and inner ear derived neurons (which express Neurod1) constitute inner ear ganglia. The most parsimonious explanation is that Neurod1 suppresses the alternative fate of sensory neurons to develop as hair cells. In the absence of Neurod1, Atoh1 is expressed and differentiates cells within the ganglion into hair cells. We followed up on this effect in ganglia by demonstrating that Neurod1 also regulates differentiation of subtypes of hair cells in the organ of Corti. We show that in Neurod1 conditional null mice there is a premature expression of several genes in the apex of the developing cochlea and outer hair cells are transformed into inner hair cells.


CONCLUSIONS
Our data suggest that the long noted cross-regulation of Atoh1 expression by Neurog1 might actually be mediated in large part by Neurod1. We suggest that Neurod1 is regulated by both Neurog1 and Atoh1 and provides a negative feedback for either gene. Through this and other feedback, Neurod1 suppresses alternate fates of neurons to differentiate as hair cells and regulates hair cell subtypes.
Jahan
Israt
I
Department of Biology, University of Iowa, Iowa City, Iowa, United States of America.
Pan
Ning
N
Kersigo
Jennifer
J
Fritzsch
Bernd
B
D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics / Q000502:physiology*; D002454:Cell Differentiation / Q000235:genetics / Q000502:physiology; D003051:Cochlea / Q000166:cytology* / Q000378:metabolism; D005260:Female; D005455:Fluorescent Antibody Technique; D005838:Genotype; D006198:Hair Cells, Auditory / Q000166:cytology* / Q000378:metabolism; D017403:In Situ Hybridization; D051379:Mice; D018345:Mice, Knockout; D008855:Microscopy, Electron, Scanning; D011247:Pregnancy
D051792:Basic Helix-Loop-Helix Proteins; C501008:Neurod1 protein, mouse