Early requirement of the transcriptional activator Sox9 for neural crest specification in Xenopus.

The neural crest is a multipotent population of cells that arises at the neural plate border in the vertebrate embryo. We have previously shown that a member of the Sox family of transcription factors, Sox9, is a regulator of neural crest formation in Xenopus, as Sox9-depleted embryos failed to form neural crest progenitors. Here, we describe experiments that further investigate Sox9 function during neural crest development. Induction of neural crest progenitors in Xenopus is regulated by Wnt signaling. We show that this process is largely dependent on Sox9 function as Wnt-mediated neural crest induction is inhibited in the context of Sox9-depleted embryos. Moreover, we demonstrate that Sox9 functions as a transcriptional activator during neural crest formation. Expression of a construct in which Sox9 DNA-binding domain (HMG box) is fused to the repressor domain of Drosophila engrailed blocked neural crest formation, thereby mimicking the phenotype of Sox9-depleted embryos. Finally, using a hormone-inducible inhibitory mutant of Sox9, lacking the transactivation domain, we show that Sox9 function is required for neural crest specification but not for its subsequent migration.

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.

Sox10 regulates the development of neural crest-derived melanocytes in Xenopus.

The transcription factors of the Sox family play important roles in diverse developmental processes. A number of genetic studies have established that Sox10 is a major regulator of neural crest formation. Here, we report the cloning and functional analysis of the Xenopus Sox10 gene. Sox10 mRNA accumulates during gastrulation at the lateral edges of the neural plate, in the neural crest-forming region. In this tissue, Sox10 expression is regulated by Wnt signaling and colocalizes with two major regulators of neural crest formation, Slug and Sox9. While initially expressed in neural crest cells from all axial levels, at the tailbud stage, Sox10 is downregulated in the cranial neural crest and persists mostly in neural crest cells from the trunk region. Overexpression of Sox10 causes a dramatic expansion of the Slug expression domain. We show that the C-terminal portion of Sox10 is sufficient to mediate this activity. Later during embryogenesis, Sox10-injected embryos show a massive increase in pigment cells (Trp-2-expressing cells). The responsiveness of the embryo to Sox10 overexpression by expansion of the Slug expression domain and ectopic production of Trp-2-positive cells and differentiated melanocytes is lost during gastrulation, as revealed by a hormone-inducible Sox10 construct. These results suggest that Sox10 is involved in the specification of neural crest progenitors fated to form the pigment cell lineage.

The transcription factor Sox9 is required for cranial neural crest development in Xenopus.

The SOX family of transcription factors has been implicated in cell fate specification during embryogenesis. One member of this family, Sox9, has been shown to regulate both chondrogenesis and sex determination in the mouse embryo. Heterozygous mutations in Sox9 result in Campomelic Dysplasia (CD), a lethal human disorder characterized by autosomal XY sex reversal, severe skeletal malformations and several craniofacial defects. Sox9 is also expressed in neural crest progenitors but very little is known about the function of Sox9 in the neural crest. We have cloned the Xenopus homolog of the Sox9 gene. It is expressed maternally and accumulates shortly after gastrulation at the lateral edges of the neural plate, in the neural crest-forming region. As development proceeds, Sox9 expression persists in migrating cranial crest cells as they populate the pharyngeal arches. Depletion of Sox9 protein in developing embryos, using morpholino antisense oligos, causes a dramatic loss of neural crest progenitors and an expansion of the neural plate. Later during embryogenesis, morpholino-treated embryos have a specific loss or reduction of neural crest-derived skeletal elements, mimicking one aspect of the craniofacial defects observed in CD patients. We propose that Sox9 is an essential component of the regulatory pathway that leads to cranial neural crest formation.