Graded levels of Pax2a and Pax8 regulate cell differentiation during sensory placode formation

Abstract

SUMMARYPax gene haploinsufficiency causes a variety of congenital defects. Renal-coloboma syndrome, resulting from mutations in Pax2, is characterized by kidney hypoplasia, optic nerve malformation, and hearing loss. Although this underscores the importance of Pax gene dosage in normal development, how differential levels of these transcriptional regulators affect cell differentiation and tissue morphogenesis is still poorly understood. We show that differential levels of zebrafish Pax2a and Pax8 modulate commitment and behavior in cells that eventually contribute to the otic vesicle and epibranchial placodes. Initially, a subset of epibranchial placode precursors lie lateral to otic precursors within a single Pax2a/8-positive domain; these cells subsequently move to segregate into distinct placodes. Using lineage-tracing and ablation analyses, we show that cells in the Pax2a/8+ domain become biased towards certain fates at the beginning of somitogenesis. Experiments involving either Pax2a overexpression or partial, combinatorial Pax2a and Pax8 loss of function reveal that high levels of Pax favor otic differentiation whereas low levels increase cell numbers in epibranchial ganglia. In addition, the Fgf and Wnt signaling pathways control Pax2a expression: Fgf is necessary to induce Pax2a, whereas Wnt instructs the high levels of Pax2a that favor otic differentiation. Our studies reveal the importance of Pax levels during sensory placode formation and provide a mechanism by which these levels are controlled.

Pax2 modulates proliferation during specification of the otic and epibranchial placodes

“…Thus, it is possible that Pax2 repression in chick does inhibit Fgf18 expression, similar to its proposed role in zebrafish (Padanad and Riley, 2011). Although the function of Fgf18 in epibranchial formation has yet to be determined, we hypothesize that the more severe epibranchial phenotype is a combination of reduced progenitor production in the PPA, aberrant specification of the epibranchial portion of the PPA (McCarroll et al, 2012), and reduced autocrine Fgf18 action during epibranchial specification.…”

“…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 the zebrafish pax2 expression is expanded anteriorly (Padanad et al, 2012;McCarroll et al, 2012).…”

“…Over-expression data from zebrafish suggest that Pax2 over-expression expands early otic territories (Padanad et al, 2012;McCarroll et al, 2012). In the zebrafish pax2 expression is expanded anteriorly (Padanad et al, 2012;McCarroll et al, 2012). However, this expansion of the early otic territory does not translate into an expanded otic terrain, or ectopic otic vesicles.…”

“…In zebrafish, it has been suggested that this is due, in part, to a cell-autonomous requirement of pax2 in the otic placode controlling the expression of fgf24, which is required for epibranchial induction (Padanad and Riley, 2011) as well as a cell-autonomous role for pax2 in the specification of the zebrafish epibranchial placodes (McCarroll et al, 2012). In the chick, otic development still occurs after Pax2 repression, but epibranchial formation is nonetheless curtailed.…”

“…In zebrafish, recent experiments have suggested a subsequent role for different levels of pax2a in segregating the otic and epibranchial progenitors within the PPA. Wnt signalling acts on the PPA increasing the expression of pax2a in the otic portion of the PPA (McCarroll et al, 2012). In addition, pax2 and pax8 have a noncell-autonomous effect on epibranchial formation via the induction of fgf24 in the inner ear (Padanad and Riley, 2011).…”