Artificial Induction of Sox21 Regulates Sensory Cell Formation in the Embryonic Chicken Inner Ear

Abstract

During embryonic development, hair cells and support cells in the sensory epithelia of the inner ear derive from progenitors that express Sox2, a member of the SoxB1 family of transcription factors. Sox2 is essential for sensory specification, but high levels of Sox2 expression appear to inhibit hair cell differentiation, suggesting that factors regulating Sox2 activity could be critical for both processes. Antagonistic interactions between SoxB1 and SoxB2 factors are known to regulate cell differentiation in neural tissue, which led us to investigate the potential roles of the SoxB2 member Sox21 during chicken inner ear development. Sox21 is normally expressed by sensory progenitors within vestibular and auditory regions of the early embryonic chicken inner ear. At later stages, Sox21 is differentially expressed in the vestibular and auditory organs. Sox21 is restricted to the support cell layer of the auditory epithelium, while it is enriched in the hair cell layer of the vestibular organs. To test Sox21 function, we used two temporally distinct gain-of-function approaches. Sustained over-expression of Sox21 from early developmental stages prevented prosensory specification, and abolished the formation of both hair cells and support cells. However, later induction of Sox21 expression at the time of hair cell formation in organotypic cultures of vestibular epithelia inhibited endogenous Sox2 expression and Notch activity, and biased progenitor cells towards a hair cell fate. Interestingly, Sox21 did not promote hair cell differentiation in the immature auditory epithelium, which fits with the expression of endogenous Sox21 within mature support cells in this tissue. These results suggest that interactions among endogenous SoxB family transcription factors may regulate sensory cell formation in the inner ear, but in a context-dependent manner.

SoxB2 in sea urchin development: implications in neurogenesis, ciliogenesis and skeletal patterning

“…The function of Sox21 , the SoxB2 vertebrate’s ortholog, has been mostly studied in fish, chicken and mouse. In chicken, it has a neurogenic function and it is expressed in vestibular and auditory organs, being an important regulator of sensory cell differentiation [ 8 , 9 ]. In Xenopus laevis , Sox21 is expressed in several regions of the central nervous system (CNS) and in the sensory organs [ 4 ], similarly to chicken [ 10 ].…”

Can the ‘neuron theory’ be complemented by a universal mechanism for generic neuronal differentiation

“…Mutational inactivation of Sox21 shows only mild effects in hair cells with an increase in their number in some homozygous animals combined with disorganized polarity. In the chick inner ear, which displays a similar Sox21 expression pattern, overexpression at different stages leads to different results (Freeman and Daudet 2012). Retrovirus-mediated misexpression reduces Sox2 and Prox1 (prospero homeobox 1) expression and prevents neuron and hair cell differentiation as analyzed by TuJ1 IHC, which in the chick embryo labels both cell types.…”

Molecular cloning and functional characterisation of chicken Atonal homologue 1: A comparison with human Atoh1

“…In sharp contrast, 92.1% of non‐sensory cells transfected with pCIG.cAtoh1.IRES.nucEGFP developed as hair cells ( n = 419; Figures B′, 3B″ and 3D). Detailed examination of these ectopic hair cells revealed that they possessed stereocilia bundles (the base of which is labelled with HCA; Figure C′), and cytoplasmic localisation of otoferlin revealed the classic flask shape that is observed in mature vestibular hair cells (Figure C″) (Freeman and Daudet, ). We observed the presence of ectopic hair cells in all non‐sensory regions that were transfected; however, it must be noted that transfection was consistently concentrated in the two maculae and the ampullas of the semicircular canals.…”