Specification of the otic placode depends on Sox9 function in Xenopus

The vertebrate inner ear develops from a thickening of the embryonic ectoderm, adjacent to the hindbrain, known as the otic placode. All components of the inner ear derive from the embryonic otic placode. Sox proteins form a large class of transcriptional regulators implicated in the control of a variety of developmental processes. One member of this family, Sox9, is expressed in the developing inner ear, but little is known about the early function of Sox9 in this tissue. We report the functional analysis of Sox9 during development of Xenopus inner ear. Sox9 otic expression is initiated shortly after gastrulation in the sensory layer of the ectoderm, in a bilateral patch of cells immediately adjacent to the cranial neural crest. In the otic placode, Sox9 colocalizes with Pax8 one of the earliest gene expressed in response to otic placode inducing signals. Depletion of Sox9 protein in whole embryos using morpholino antisense oligonucleotides causes a dramatic loss of the early otic placode markers Pax8 and Tbx2. Later in embryogenesis, Sox9 morpholino-injected embryos lack a morphologically recognizable otic vesicle and fail to express late otic markers (Tbx2, Bmp4, Otx2 and Wnt3a) that normally exhibit regionalized expression pattern throughout the otocyst. Using a hormone inducible inhibitory mutant of Sox9, we demonstrate that Sox9 function is required for otic placode specification but not for its subsequent patterning. We propose that Sox9 is one of the key regulators of inner ear specification in Xenopus.