Bapx1 regulates patterning in the middle ear: altered regulatory role in the transition from the proximal jaw during vertebrate evolution

Abstract

The middle ear apparatus is composed of three endochondrial ossicles (the stapes, incus and malleus) and two membranous bones, the tympanic ring and the gonium, which act as structural components to anchor the ossicles to the skull. Except for the stapes, these skeletal elements are unique to mammals and are derived from the first and second branchial arches. We show that, in combination with goosecoid (Gsc), the Bapx1 gene defines the structural components of the murine middle ear.During embryogenesis, Bapx1 is expressed in a discrete domain within the mandibular component of the first branchial arch and later in the primordia of middle ear-associated bones, the gonium and tympanic ring. Consistent with the expression pattern of Bapx1, mouse embryos deficient for Bapx1 lack a gonium and display hypoplasia of the anterior end of the tympanic ring. At E10.5, expression of Bapx1partially overlaps that of Gsc and although Gsc is required for development of the entire tympanic ring, the role of Bapx1 is restricted to the specification of the gonium and the anterior tympanic ring. Thus, simple overlapping expression of these two genes appears to account for the patterning of the elements that compose the structural components of the middle ear and suggests that they act in concert.In addition, Bapx1 is expressed both within and surrounding the incus and the malleus. Examination of the malleus shows that the width, but not the length, of this ossicle is decreased in the mutant mice. In non-mammalian jawed vertebrates, the bones homologous to the mammalian middle ear ossicles compose the proximal jaw bones that form the jaw articulation(primary jaw joint). In fish, Bapx1 is responsible for the formation of the joint between the quadrate and articular (homologues of the malleus and incus, respectively) enabling an evolutionary comparison of the role of a regulatory gene in the transition of the proximal jawbones to middle ear ossicles. Contrary to expectations, murine Bapx1 does not affect the articulation of the malleus and incus. We show that this change in role of Bapx1 following the transition to the mammalian ossicle configuration is not due to a change in expression pattern but results from an inability to regulate Gdf5 and Gdf6, two genes predicted to be essential in joint formation.

Evidence for the prepattern/cooption model of vertebrate jaw evolution

“…Bapx labels intermediate first arch CNCCs that form the jaw joint in nonmammalian gnathostomes (5) and the homologous joint between the malleus and incus in mammals (38). Bapx knockdown in zebrafish causes a loss of the jaw joint and jaw jointspecific expression of Gdf5 and Chordin in zebrafish (5).…”

“…However, first-arch Bapx expression in zebrafish is broader than the joint, including nascent chondrocytes in the upper and lower jaws (5). Furthermore, in mouse, Bapx knock-out causes a reduction in the size of the inner ear bones but does not effect the malleusincus joint or joint-specific expression of Gdf5/6 (38). Bapx knockdown also does not effect the formation joints in the posterior pharyngeal arches or elsewhere in the axial or appendicular skeletons of fish or mouse (5,39).…”

Joint formation in the middle ear: Lessons from the mouse and guinea pig

“…Gdf5 and Gdf6 are both expressed in the developing malleal-incudo joint (Settle et al, 2003;Tucker et al, 2004). Mutants for Gdf5 have defects in a subset of joints but the middle ear is unaffected (Storm and Kingsley, 1996).…”

Unraveling new roles for serotonin receptor 2B in development: key findings from Xenopus

“…5C). As the regulatory network involved in the specific expression of Bapx1 in vertebrates is conserved (Newman et al, 1997;Miller et al, 2003;Tucker et al, 2004), it may be hypothesized that 5-HT2B signaling controls Bapx1 expression in mammalian embryos as well. This aspect, to be verified and further to SSRIs such as persistent pulmonary hypertension, cardiac defects and spontaneous abortion (Oberlander et al, 2009;Tuccori et al, 2009;Bellissima et al, 2012).…”