Identification of putative retinoic acid target genes downstream of mesenchymal Tbx1 during inner ear development

Abstract

The T-box transcription factor Tbx1 is expressed in the otic vesicle and surrounding mesoderm of the periotic mesenchyme (POM) during inner ear development. Mesenchymal Tbx1 is essential for inner ear development, with conditional mutants displaying defects in both the auditory and vestibular systems. We have previously reported that mesodermal Tbx1 loss of function mutants (Mest-KO) have reduced expression of retinoic acid (RA) metabolic genes, Cyp26a1 and Cyp26c1, in the POM, consistent with other studies showing an increase in mesodermal RA reporter expression in Tbx1-/- embryos. However, putative RA effector genes whose expression is altered downstream of increased otic mesenchymal-epithelial RA signaling have remained elusive. Here we report the identification of eighteen retinoic acid responsive genes altered in Mest-KO conditional mutants by microarray gene profiling. Nine were chosen for biological validation including quantitative RT-PCR and in situ hybridization (Otor, Mia, Col2a1, Clu, Adm, Myt1, Dlx3, Itgb3 and Itga2b). This study provides a series of newly identified RA effector genes for inner ear development downstream of mesenchymal Tbx1 that may contribute to the inner ear phenotype observed in Tbx1 loss of function mouse models.

Mutations inMYT1, encoding the myelin transcription factor 1, are a rare cause of OAVS

“…Interestingly, unusual spinal dysraphism observed in patient F2 II-2 might result from ectodermal non-disjunction of the otic placode, a pathogenic mechanism that was hypothesised for OAVS45 and was also observed in rats after exposure to RA 46. Moreover, sensorineural hearing loss of this patient's father (F2 I-2) suggested a possible effect of MYT1 loss of function, considering the role of Myt1 during mouse inner ear development as a downstream RA target 21. We then hypothesised links between RA, MYT1 and RARB where excess of exogenous RA or MYT1 loss of function could lead to OAVS features through deregulation of some RA-related genes.…”

“…Moreover, mutations in genes of RA signalling pathway led to craniofacial anomalies 38–40. Interestingly, MYT1 belongs to RA-induced transcriptome as shown in inner ear development of mesodermal-knock-out Tbx1 conditional mouse mutants21 and in zebrafish early embryogenesis experiments 22. In X. laevis embryos, RA treatment expanded the expression domains of several positive regulators of neurogenesis such as myt1 , nrgn1 and gli3 and directed the neural plate towards a uniform proneural territory 23.…”

A cellular and molecular mosaic establishes growth and differentiation states for cranial sensory neurons

“…Two of these signals, Fgf and RA, can modify cranial placode (41) and neural crest (42, 43) cells. Most studies addressing cranial Fgf or RA signaling in the mouse have relied upon use of constitutive genetic mutants, which have concatenated phenotypes from initial early embryogenesis onward.…”

Cyp1B1 expression patterns in the developing chick inner ear

“…It is well accepted that RA generated from the underlying mesenchyme by RALDH activities participate directly in the specification of the neural tube along its anterior‐to‐posterior and dorsal‐to‐ventral axes . Similarly, RA from the periotic mesenchyme may control the regionalization of the developing otic epithelium in a stage‐dependent manner . In the chick, Raldh2 is expressed in the mesoderm caudal to the otic placode, and an appropriate spatial and temporal concentration of the diffusible RA is necessary for the correct specification of the anterior‐posterior axis of the developing otic anlagen .…”