Lmx1a is required for segregation of sensory epithelia and normal ear histogenesis and morphogenesis

Abstract

At E8.5, the LIM-homeodomain factor Lmx1a is expressed throughout the otic placode but becomes developmentally restricted to non-sensory epithelia of the ear (endolymphatic duct, ductus reuniens, cochlea lateral wall). We confirm here that the ears of newborn dreher (Lmx1a dr ) mutants are dysmorphic. Hair cell markers such as Atoh1 and Myo7 reveal for the first time that newborn Lmx1a mutants have only three sensory epithelia: two enlarged canal cristae and one fused epithelium comprising an amalgamation of the cochlea, saccule and utricle, a "cochlear-gravistatic" endorgan. The enlarged anterior canal crista develops by fusion of horizontal and anterior crista whereas the posterior crista fuses with an enlarged papilla neglecta that may extend into the cochlear lateral wall. In the fused endorgan the cochlear region is distinguished from the vestibular region by markers such as Gata3, the presence of a tectorial membrane and cochlea-specific innervation. The cochlea-like apex displays minor disorganization of the hair and supporting cells. This contrasts with the basal half of the cochlear region which shows a vestibular epithelium-like organization of hair cells and supporting cells. The dismorphic features of the cochlea are also reflected in altered gene expression patterns. Fgf8 expression expands from inner hair cells in the apex to most hair cells in the base. Two supporting cell marker proteins, Sox2 and Prox1, also differ in their cellular distribution between the base and the apex. Sox2 expression expands in mutant canal cristae prior to their enlargement and fusion and displays a more diffuse and widespread expression in the base of the cochlear region whereas Prox1 is not detected in the base. These changes in Sox2 and Prox1 expression suggest that Lmx1a expression restricts and sharpens Sox2 expression thereby defining non-sensory and sensory epithelium. The adult Lmx1a mutant organ of Corti showed a loss of cochlear hair cells, suggesting that long term hair cell maintenance is also disrupted in these mutants.

Limited inner ear morphogenesis and neurosensory development are possible in the absence of GATA3

“…This is in contrast to the wild-type ear where the concentrated expression in the lateral wall of the cochlea was previously described ( Fig. 2C; (Nichols et al, 2008). Pax2 is initially expressed in the nonsensory areas of the cochlear duct and later in development in hair cells (Bouchard et al, 2010).…”

“…Thus, GATA3 must be interacting with other factors needed for canal formation that remain to be determined. Some candidate genes could be Bmp4, Jag1, Lmo4, Lmx1a, and Foxg1 because of their known or suspected function in canal development (Nichols et al, 2008). We suspect that the early widespread expression in the growing canal plate and not the later limited expression in the cruciate eminence (Karis et al, 2001) is responsible for the absence of canal formation.…”

“…Moreover, the presumptive cochlear expression was less distinctly organized compared to wild-type embryos. Lmx1a is expressed in the otic placode and necessary for segregation of sensory epithelia (Nichols et al, 2008). In the absence of GATA3, Lmx1a was expressed in the endolymphatic duct but showed diffuse expression in the remaining ear (Fig.…”

Evolution and Development of Hair Cell Polarity and Efferent Function in the Inner Ear

“…Their primordia of sensory epithelia remain fused and show an aberrant hair cell orientation in the atavistic transition zone between the organs. Such PCP defects in Otx1 and Lmx1a Nichols et al, 2008] reveal that epithelial regionalization and the generation of distinct basic hair cell orientation patterns are linked.…”

Connecting Ears to Eye Muscles: Evolution of a ‘Simple' Reflex Arc

“…For example, Otx1 and N-Myc null mice have no horizontal semicircular canal [Kopecky et al, 2011], and loss of Foxg1 abolishes horizontal canal crista differentiation [Pauley et al, 2006;Hwang et al, 2009]. Other factors, such as homeobox genes Fgf s and BMP s [Chang et al, 2008], also play a role in this process, and the inner ear shows dramatic malformations or incomplete segregation of sensory epithelia in many mutants, such as the Lmx1a null mutation [Nichols et al, 2008]. The emerging picture implies a progressive transformation of a simple ear, as found in jawless vertebrates, through formation of multiple recesses, each associated with its own sensory epithelium, into the complex labyrinth of jawed vertebrates [Fritzsch et al, 2013] paralleled by a concurrent segregation of sensory afferent innervation [de Burlet, 1934;Fritzsch et al, 2002].…”