Pax2 coordinates epithelial morphogenesis and cell fate in the inner ear

Abstract

Crucial components of the vertebrate eye, ear and nose develop from discrete patches of surface epithelium, called placodes, which fold into spheroids and undergo complex morphogenesis. Little is known about how the changes in cell and tissue shapes are coordinated with the acquisition of cell fates. Here we explore whether these processes are regulated by common transcriptional mechanisms in the developing ear. After specification, inner ear precursors elongate to form the placode, which invaginates and is transformed into the complex structure of the adult ear. We show that the transcription factor Pax2 plays a key role in coordinating otic fate and placode morphogenesis, but appears to regulate each process independently. In the absence of Pax2, otic progenitors not only lose otic marker expression, but also fail to elongate due to the loss of apically localised N-cadherin and N-CAM. In the absence of either N-cadherin or N-CAM otic cells lose apical cell–cell contact and their epithelial shape. While misexpression of Pax2 leads to ectopic activation of both adhesion molecules, it is not sufficient to confer otic identity. These observations suggest that Pax2 controls cell shape independently from cell identity and thus acts as coordinator for these processes.

Pax2 modulates proliferation during specification of the otic and epibranchial placodes

“…While induction of the PPA does not appear to be affected by Pax2 repression, specification of the inner ear and epibranchial placodes are repressed; otic development is reduced and epibranchial neurogenesis blocked. Previous studies using morpholinos to down-regulate Pax2 in the chick have suggested that the repression of otic differentiation results from the inhibition of placodal morphogenesis (Christophorou et al, 2010). However, we do not observe such a dramatic phenotype on PPA cells.…”

“…However, this expansion of the early otic territory does not translate into an expanded otic terrain, or ectopic otic vesicles. Importantly, the transient overexpression in zebrafish does not inhibit endogenous otic vesicle formation while, as we and others have shown, sustained high-level Pax2 over-expression in the chick does (Christophorou et al, 2010). This suggests that the temporal regulation of Pax2 is as important as the exact levels of expression for inner ear and epibranchial formation; too little and the cells of the PPA do not proliferate adequately, whereas too much, or for too long, prevents the differentiation of the PPA into its derivatives.…”

“…The clearest is the effect on the cell cycle, where Pax2 overexpression shows little effect on the rate of division of PPA cells in this experimental design. In addition, the effect on tissue morphogenesis is also different, with Pax2 over-expression frequently affecting otic invagination, as has already been reported (Christophorou et al, 2010). Over-expression data from zebrafish suggest that Pax2 over-expression expands early otic territories (Padanad et al, 2012;McCarroll et al, 2012).…”

“…In chick, reduction of Pax2 does not affect the early induction of the PPA. However, morphogenesis and the integrity of the later otocyst, is disrupted (Christophorou et al, 2010). In zebrafish, the roles of Pax2 and pax8 are slightly different.…”

Spemann organizer gene Goosecoid promotes delamination of neuroblasts from the otic vesicle

“…In zebrafish, chick, and mouse, Pax2 marks the nascent otic placode and is later restricted to the medial half of the otic vesicle (14)(15)(16)(17)(18). Loss of Pax2 and related factor Pax8 compromises epithelial integrity, leading to faulty morphogenesis of the otic vesicle and cell dispersal (17,18).…”