The Pax6 gene encodes a transcription factor with a restricted expression in the ventricular zone of the pallium and subpallium. We tested whether the function of Pax6 is necessary for the correct patterning and morphogenesis of the vertebrate telencephalon. Homozygous embryos of the Pax6/Small eye mutant lack functional PAX6 protein because of a point mutation of the gene. In the mutant Small eye embryos we detected a ventralization of the molecular patterning of the telencephalon at two borders, the pallium/subpallium and the lateral/medial ganglionic eminence. The results indicate that Pax6 controls the lateral limit of the expression of Nkx2.1, Shh, and Lhx6 in the prechordal neural tube, the telencephalon. This finding is in agreement with previous studies and supports a model for a common genetic mechanism for modulation of the dorsoventral patterning of the prechordal and epichordal CNS. The pattern defects caused by the loss of Pax6 function result in multiple morphological abnormalities in the Small eye brain: dysgenesis of the piriform, insular, and lateral cortices, the claustrum-endopiriform nucleus, and a failure in the differentiation of a subpopulation of the cortical precursors. Together the results demonstrate that Pax6 has an essential role for the modulation of the dorsoventral patterning of the embryonic telencephalon, influencing thereby the forebrain morphogenesis.

The transcription factors Pax2 and Pax6 are co-expressed in the entire optic vesicle (OV) prior and concomitant with the establishment of distinct neuroretinal, retinal, pigmented-epithelial and optic-stalk progenitor domains, suggesting redundant functions during retinal determination. Pax2; Pax6 compound mutants display a dose-dependent reduction in the expression of the melanocyte determinant Mitf, accompanied by transdifferentiation of retinal pigmented epithelium (RPE) into neuroretina(NR) in Pax2-/-; Pax6+/- embryos,which strongly resembles the phenotype of Mitf-null mutants. In Pax2-/-; Pax6-/- OVs Mitffails to be expressed and NR markers occupy the area that usually represents the Mitf+ RPE domain. Furthermore, both, Pax2 and Pax6 bind to and activate a MITF RPE-promoter element in vitro,whereas prolonged expression of Pax6 in the Pax2-positive optic stalk leads to ectopic Mitf expression and RPE differentiation in vivo. Together,these results demonstrate that the redundant activities of Pax2 and Pax6 direct the determination of RPE, potentially by directly controlling the expression of RPE determinants.

buttonhead (btd) encodes an SP1-like transcription factor required for the generation and specification of Drosophila head segments. We identified a murine btd homolog, termed mouse Btd (mBtd), which can support btd-dependent head development in transgenic fly embryos. Functional studies show that mBtd-deficient mice develop to term and die at birth. They exhibit brain malformations, posterior axial skeleton truncations, and shortened limbs. We present evidence that mBtd is required during early limb development to maintain, but not to initiate Wnt/␤-catenin-dependent FGF, Shh, and BMP-mediated signaling. The data indicate that mBtd represents a novel key player mediating proximodistal outgrowth of the limb.Supplemental material is available at http://www.genesdev.org.Received May 30, 2003; revised version accepted September 5, 2003. buttonhead (btd), empty spiracles (ems), and orthodenticles (otd) are required for anterior head development during Drosophila embryogenesis (Cohen and Jürgens 1991;Finkelstein and Boncinelli 1994). The three genes are expressed in overlapping domains at blastoderm stage. btd is required for development of the mandibular, btd and ems for the intercalary, and the combination of btd, ems, and otd for the antennal segment. ems and otx encode homeodomain transcription factors, and btd encodes an Egr-like zinc finger transcription factor (Wimmer et al. 1993).Homologs of ems and otd have been identified in mouse. Functional analysis revealed that they are required for patterning processes during early embryonic brain development (Simeone 2002). A vertebrate homolog of btd has not yet been identified. However, sequence-related genes with a high degree of similarity were isolated. This includes human and mouse Sp1 (Wimmer et al. 1993;Schöck et al. 1999a) and the btdlike gene bts1 of zebrafish (Tallafuss et al. 2001). Furthermore, BTD and SP1 proteins are capable of binding to the same DNA sites in vitro, share most biochemical properties, and regulate transcription via the same interacting factors (Wimmer et al. 1993;Schöck et al. 1999a). However, transgenes expressing Sp1 or bts1 in place of btd failed to rescue btd mutant embryos. This finding implies that despite the molecular similarities, none of the known vertebrate btd-like genes represents a btd ortholog (Schöck et al. 1999b;Tallafuss et al. 2001).Here we report the identification and the functional analysis of a murine btd homolog, termed mouse Btd (mBtd), showing that its activity rescues head segment formation in btd mutant fly embryos. Loss of mBtd in the mouse embryo causes severe brain defects and truncations of body structures, including the posterior axial skeleton and limbs.We focused our attention on the requirement of mBtd activity on limb development, a process that is initiated through interactions between ectoderm and mesenchyme. The result of these interactions is the formation of the so-called apical ectodermal ridge (AER), a signaling center which is generated through the concerted signaling activities of fibrob...

The genetic cascade that governs left-right (L-R) specification is starting to be elucidated. In the mouse, the lateral asymmetry of the body axis is revealed first by the asymmetric expression of nodal, lefty2 and pitx2 in the left lateral plate mesoderm of the neurulating embryo. Here we describe a novel gene, rotatin, essential for the correct expression of the key L-R specification genes nodal, lefty and Pitx2. Embryos deficient in rotatin show also randomized heart looping and delayed neural tube closure, and fail to undergo the critical morphogenetic step of axial rotation. The amino acid sequence deduced from the cDNA is predicted to contain at least three transmembrane domains. Our results show a novel key player in the genetic cascade that determines L-R specification, and suggest a causal link between this process and axial rotation.

We describe the sequence and expression pattern of Sp5, a novel member of the vertebrate Sp1 transcription factor gene family which consists of at least five members. This gene family is characterized by a highly conserved domain which is formed by three Zn fingers, which bind to the GC box or the GT/CACC box in the promoter of many genes. These boxes are important cis-acting elements required for the expression of the respective genes. In vitro experiments indicate that the Sp1 transcription factors act by influencing the methylation state of the DNA, or by direct interactions with other promoter specific transcription factors. Despite intensive research, the results from in vivo experiments, including targeted gene inactivation, have been difficult to explain. This may be due to possible redundancies and interferences with other transcription factors of this gene family. Here, we report the isolation of the mouse Sp5 gene, a novel Sp1 homolog. Its sequence indicates that Sp5 is a possible link between Sp1 and the closely related BTEB/KLF gene family. We provide detailed information of its highly dynamic expression pattern during mouse embryogenesis in the developing brain, the spinal cord, the trigeminal ganglia, the somites and additional sites outside the nervous system starting from embryonic day 7.25 (E7.25) up to E10.5.