Mcrs1 plays a role in otic and branchial arch gene expression

Abstract

Branchio-oto-renal (BOR) syndrome presents with inner, middle, and outer ear deformities, hyoid cysts and fistulae, and kidney hypoplasia or absence. Half of BOR cases are caused by mutations in genes encoding the transcription factor SIX1 or its co-factor EYA1. We previously suggested that other Six1 cofactors might be BOR candidates, and subsequently showed that Microspherule 1 (Mcrs1) binds to Six1, reduces Six1-Eya1 transcriptional activation, and during preplacodal development aids to define both the neural plate and preplacodal region. Disruption of Mcrs1 endogenous levels also disrupts otic vesicle gene expression and size. However, the consequences of these changes during larval stages of craniofacial development have yet to be characterized. To alter the levels of Mcrs1 in Xenopus laevis larvae, either mcrs1 mRNA or translation blocking antisense oligonucleotides were microinjected into the neural crest (NC) and cranial placode progenitor blastomeres. Larvae then were assessed by in situ hybridization for changes in gene expression within the otic vesicle (OV) and branchial arches (BA). Increasing Mcrs1 levels did not significantly impact OV or BA gene expression, whereas knockdown of Mcrs1 downregulated key skeletogenesis/NC (dlx5, sox9, tbx1) and cranial placode (six1, irx1) genes in both the OV and BA. In contrast, emx2 expression was upregulated by Mcrs1 knockdown. These results indicate that Mcrs1 is required for the proper level of expression of several OV and BA genes, and is a repressor of emx2, a gene known to regulate proliferation, differentiation, patterning, and migration of cells within the OV. These data indicate that Mcrs1 plays an important role in the development of craniofacial structures that are affected in BOR patients.