Molecular basis for skeletal variation: insights from developmental genetic studies in mice.

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Expression of members of the Fgf family and their receptors during midfacial development.

Members of the FGF family play diverse roles in patterning, cell proliferation and differentiation during embryogenesis. To begin to address their function during craniofacial development we have analyzed the expression of 18 members of the Fgf family (Fgf1-15, -17, -18 and -20) and the four members of the FGF-receptor family in the prospective midfacial region between E9.5 and E11.5 by whole-mount in situ hybridization. We show that at E9.5, Fgf3, -8, -9, -10 and -17 are broadly expressed in midfacial ectoderm. Concomitant with the outgrowth of the nasal processes at E10.5, expression of Fgf3, -8, -9, -10, -15, -17 and -18 was detected in spatially restricted regions of ectoderm at the edge of the nasal pit and at the oral edge of the medial nasal process. Expression of Fgf8, Fgf9, Fgf10 and Fgf17 was still observed in these domains at E11.5. In contrast to the restricted expression patterns of the ligands, FgfR1 and FgfR2 were broadly expressed in facial mesenchyme and ectoderm, respectively, indicating a wide competence of midfacial tissue to respond to FGF signaling.

FGF signaling is necessary for the specification of the odontogenic mesenchyme.

Tooth development is initiated by signals from the oral ectoderm which induce gene expression required for tooth development in the underlying mesenchyme. In this study, we have used Su5402, an inhibitor of FGF receptor signaling, to analyze the requirement of FGF signaling during early tooth development. We show that FGF signaling is necessary for expression of Pax9, a transcription factor required for development of all teeth, in prospective incisor and molar mesenchyme until E11.0. Expression of the LIM homeobox gene Lhx7 also requires FGF signaling until E11.0 whereas expression of its homologue Lhx6 and the homeobox transcription factor Barx1 already becomes independent of FGF signaling at E10.75. In contrast, ectodermal expression of several genes thought to be important for tooth development was unaffected by the block of FGF signaling. Finally, we show that expression of the TGFbeta antagonist Dan in prospective tooth mesenchyme requires ectodermal signals and can be induced by FGF-soaked beads but is maintained in mandibular explants in the absence of FGF signaling. Together, these results demonstrate that FGF signaling is required for development of both molar and incisor teeth and suggest that specification of tooth mesenchyme involves at least two FGF-dependent steps.

Vertebrate Sprouty genes are induced by FGF signaling and can cause chondrodysplasia when overexpressed.

The Drosophila sprouty gene encodes an antagonist of FGF and EGF signaling whose expression is induced by the signaling pathways that it inhibits. Here we describe a family of vertebrate Sprouty homologs and demonstrate that the regulatory relationship with FGF pathways has been conserved. In both mouse and chick embryos, Sprouty genes are expressed in intimate association with FGF signaling centers. Gain- and loss-of-function experiments demonstrate that FGF signaling induces Sprouty gene expression in various tissues. Sprouty overexpression obtained by infecting the prospective wing territory of the chick embryo with a retrovirus containing a mouse Sprouty gene causes a reduction in limb bud outgrowth and other effects consistent with reduced FGF signaling from the apical ectodermal ridge. At later stages of development in the infected limbs there was a dramatic reduction in skeletal element length due to an inhibition of chondrocyte differentiation. The results provide evidence that vertebrate Sprouty proteins function as FGF-induced feedback inhibitors, and suggest a possible role for Sprouty genes in the pathogenesis of specific human chondrodysplasias caused by activating mutations in Fgfr3.

Pax9-deficient mice lack pharyngeal pouch derivatives and teeth and exhibit craniofacial and limb abnormalities.

Pax genes have been shown to play important roles in mammalian development and organogenesis. Pax9, a member of this transcription factor family, is expressed in somites, pharyngeal pouches, mesenchyme involved in craniofacial, tooth, and limb development, as well as other sites during mouse embryogenesis. To analyze its function in vivo, we generated Pax9 deficient mice and show that Pax9 is essential for the development of a variety of organs and skeletal elements. Homozygous Pax9-mutant mice die shortly after birth, most likely as a consequence of a cleft secondary palate. They lack a thymus, parathyroid glands, and ultimobranchial bodies, organs which are derived from the pharyngeal pouches. In all limbs, a supernumerary preaxial digit is formed, but the flexor of the hindlimb toes is missing. Furthermore, craniofacial and visceral skeletogenesis is disturbed, and all teeth are absent. In Pax9-deficient embryos tooth development is arrested at the bud stage. At this stage, Pax9 is required for the mesenchymal expression of Bmp4, Msx1, and Lef1, suggesting a role for Pax9 in the establishment of the inductive capacity of the tooth mesenchyme. In summary, our analysis shows that Pax9 is a key regulator during the development of a wide range of organ primordia.

Pax genes and organogenesis: Pax9 meets tooth development.

Pax genes encode a family of transcription factors that play key roles during embryogenesis. They are required for the development of a variety of organs including the nervous and muscular system, skeleton, eye, ear, kidney, thymus, and pancreas. Whereas the developmental roles of many of the nine known Pax genes have been analyzed in great detail, a functional analysis of Pax9 has just begun. During mouse embryogenesis, Pax9 exhibits a highly specific expression pattern in derivatives of the foregut endoderm, somites, limb mesenchyme, midbrain, and the cephalic neural crest. In the mandibular arch mesenchyme, the expression of Pax9 marks the prospective sites of tooth development prior to any morphological signs of odontogenesis and is maintained in the developing tooth mesenchyme thereafter. To understand the function of Pax9 during mouse embryogenesis, we recently have created a null allele by gene targeting. Preliminary analyses show that Pax9 is essential for the formation of teeth, and we conclude that Pax9 is required for tooth development to proceed beyond the bud stage. Here, we briefly summarize our current knowledge about Pax genes and introduce Pax9 to the growing family of factors which are involved in tooth development.

Genetic analysis reveals that PAX6 is required for normal transcription of pancreatic hormone genes and islet development.

We present genetic and biochemical evidence that PAX6 is a key regulator of pancreatic islet hormone gene transcription and is required for normal islet development. In embryos homozygous for a mutant allele of the Pax6 gene, Small eye (Sey(Neu)), the numbers of all four types of endocrine cells in the pancreas are decreased significantly, and islet morphology is abnormal. In the remaining islet cells, hormone production, particularly glucagon production, is markedly reduced because of decreased gene transcription. These effects appear to result from a lack of PAX6 protein in the mutant embryos. Biochemical studies identify wild-type PAX6 protein as the transcription factor that binds to a common element in the glucagon, insulin, and somatostatin promoters, and show that PAX6 transactivates the glucagon and insulin promoters.

Antagonistic interactions between FGF and BMP signaling pathways: a mechanism for positioning the sites of tooth formation.

Vertebrate organogenesis is initiated at sites that are often morphologically indistinguishable from the surrounding region. Here we have identified Pax9 as a marker for prospective tooth mesenchyme prior to the first morphological manifestation of odontogenesis. We provide evidence that the sites of Pax9 expression in the mandibular arch are positioned by the combined activity of two signals, one (FGF8) that induces Pax9 expression and the other (BMP2 and BMP4) that prevents this induction. Thus it appears that the position of the teeth is determined by a combination of two different types of signaling molecules produced in wide but overlapping domains rather than by a single localized inducer. We suggest that a similar mechanism may be used for specifying the sites of development of other organs.

Comparative characterization of SecA from the alpha-subclass purple bacterium Rhodobacter capsulatus and Escherichia coli reveals differences in membrane and precursor specificity.

We have cloned the secA gene of the alpha-subclass purple bacterium Rhodobacter capsulatus, a close relative to the mitochondrial ancestor, and purified the protein after expression in Escherichia coli. R. capsulatus SecA contains 904 amino acids with 53% identity to E. coli and 54% identity to Caulobacter crescentus SecA. In contrast to the nearly equal partitioning of E. coli SecA between the cytosol and plasma membrane, R. capsulatus SecA is recovered predominantly from the membrane fraction. A SecA-deficient, cell-free synthesis-translocation system prepared from R. capsulatus is used to demonstrate translocation activity of the purified R. capsulatus SecA. This translocation activity is then compared to that of the E. coli counterpart by using various precursor proteins and inside-out membrane vesicles prepared from both bacteria. We find a preference of the R. capsulatus SecA for the homologous membrane vesicles whereas E. coli SecA is active with either type of membrane. Furthermore, the two SecA proteins clearly select between distinct precursor proteins. In addition, we show here for the first time that a bacterial c-type cytochrome utilizes the canonical, Sec-dependent export pathway.

Expression of avian Pax1 and Pax9 is intrinsically regulated in the pharyngeal endoderm, but depends on environmental influences in the paraxial mesoderm.

Pax1 and Pax9 represent a subfamily of paired-box-containing genes. In vertebrates, Pax1 and Pax9 transcripts have been found specifically in mesodermal tissues and the pharyngeal endoderm. Pax1 expression in the sclerotomes has been shown to be indispensable for proper formation of the axial skeleton, but expression of Pax1 in the endoderm has not been studied in detail. We have cloned the chick homologue of the murine Pax9 gene. Our results show that transcripts of Pax1 and Pax9 are first detectable in the prospective foregut endoderm of headfold-stage avian embryos. Endodermal expression correlates with the highly proliferative zones of the folding foregut and evaginating pharyngeal pouches. In later stages, Pax1 and Pax9 are expressed in overlapping but distinct patterns within the developing sclerotomes and limb buds. From grafting experiments we conclude that activation of pharyngeal Pax1 and Pax9 expression is an intrinsic property of the endoderm, not requiring midline structures or head mesoderm. In contrast, notochord is required to induce Pax1 in competent sclerotomes. Here we show that in vitro there is a cranio-caudal gradient of inductive capacity in the notochord. This coincides with the graded expression of Pax1 and Pax9 along the cranio-caudal axis in 2- to 3-day-old embryos. Furthermore, paraxial head mesoderm shows no competence to express Pax1. Finally, in vitro we find counteracting influences on notochord signaling by lateral tissues (lateral plate, intermediate mesoderm), leading to an inhibition of Sonic hedgehog (Shh) expression in notochord and floor plate, as well as Pax1 and Pax9 expression in sclerotomes. Taken together, our results demonstrate that different mechanisms regulate expression of Pax1 and Pax9 in foregut and sclerotome, but suggest a common function for both genes in the two tissues that is promoting proliferation and preventing fusion of neighboring blastemas.

Function of somite and somitocoele cells in the formation of the vertebral motion segment in avian embryos.

We have studied the distribution of thoracic somite and somitocoele-derived cells using homotopical grafting between quail and chicken embryos and reincubation periods of 2-6 days. Serial sections were evaluated with antibodies against quail cells, quail hemangiopoietic cells and desmin. With the exception of neural crest cells in the cranial sclerotome half, all cells of the operated segment are quail cells derived from a single somite. These cells differentiate into sclerotome, myotome and the anlage of the dermis of the back. After longer reincubation periods, the somite-derived quail cells form the neighboring halves of 2 adjacent vertebral bodies and the intervening (disc-homologous) tissue. Resegmentation is furthermore visible in the lamina and the spinous process. Somite cells also form the articular and transverse processes, and the intertransverse muscle including its insertion to the next cranial transverse process. One thoracic somite forms the proximal part of 1 rib. In more distal parts, 1 somite forms the cranial half of 1 rib and the caudal half of the next cranial rib, and the intercostal muscle and part of the connective tissue. Somite-derived quail cells are found in muscle that bridges over 2 segments cranial and caudal from the operated segment. The craniocaudal distribution of endothelial cells is approximately the same. Somitocoele cells that are located centrally in the epithelial somite express the sclerotome-markers Pax-1 and Pax-9. After 2-3 days of reincubation, grafted thoracic somitocoele cells are found mainly in the cranial part of the caudal sclerotome half. They form an area representing the anlagen of the intervertebral disc and the rib. After longer reincubation periods, the grafted quail somitocoele cells form the intervertebral disc-homologous tissue and the proximal part of the rib. In more distal parts of the rib they are located in the cranial half of 1 rib and the caudal half of the next cranial rib. The somitocoele cells also form the surface of the intervertebral joint, and give rise to a small number of endothelial cells that are found up to 1 segment cranial and caudal to the operation site. Our studies show that resegmentation is found in most parts of the vertebra and in the distal ribs. One somite forms the origin and insertion of the segmental muscle. Therefore, the somite can be regarded as the ancestor of the vertebral motion segment. Somitocoele cells are located centrally both in the epithelial somite and in the vertebral motion segment.

Pax1 is expressed during development of the thymus epithelium and is required for normal T-cell maturation.

Pax1 is a transcriptional regulatory protein expressed during mouse embryogenesis and has been shown to have an important function in vertebral column development. Expression of Pax1 mRNA in the embryonic thymus has been reported previously. Here we show that Pax1 protein expression in thymic epithelial cells can be detected throughout thymic development and in the adult. Expression starts in the early endodermal epithelium lining the foregut region and includes the epithelium of the third pharyngeal pouch, a structure giving rise to part of the thymus epithelium. In early stages of thymus development a large proportion of thymus cells expresses Pax1. With increasing age, the proportion of Pax1-expressing cells is reduced and in the adult mouse only a small fraction of cortical thymic stromal cells retains strong Pax1 expression. Expression of Pax1 in thymus epithelium is necessary for establishing the thymus microenvironment required for normal T cell maturation. Mutations in the Pax-1 gene in undulated mice affect not only the total size of the thymus but also the maturation of thymocytes. The number of thymocytes is reduced about 2- to 5-fold, affecting mainly the CD4+8+ immature and CD4+ mature thymocyte subsets. The expression levels of major thymocyte surface markers remains unchanged with the exception of Thy-1 which was found to be expressed at 3- to 4-fold higher levels.

Characterization and developmental expression of Pax9, a paired-box-containing gene related to Pax1.

Pax9, a recently identified mouse paired-box-containing gene, is highly homologous to Pax1 and belongs to the same subfamily as Pax1, Hup48, PAX9, and pox meso. Two overlapping cDNA clones spanning the entire coding region of Pax9 were isolated and sequenced. A comparison of the Pax1 and -9 protein sequences reveals a high degree of similarity even outside the paired box, while the carboxy-terminus of the two proteins diverges completely. We demonstrate that Pax9 can bind to the e5 sequence from the Drosophila even skipped promoter, which is also recognized by Pax1. We analyzed the expression of Pax9 during embryogenesis of wildtype, Undulated short-tail (Uns), and Danforth's short tail (Sd) mice. In wildtype embryos Pax9 is expressed in the pharyngeal pouches and their derivatives, the developing vertebral column, the tail, the head, and the limbs. Expression of Pax9 is unaffected in Uns embryos, in which the Pax1 gene is deleted, arguing that expression of Pax9 is not dependent on Pax1. The expression of Pax9 is lost in the caudal part of Sd homozygous embryos, suggesting that expression of Pax9 in the vertebral column is dependent on the notochord. These results indicate that both Pax9 and -1 may act in parallel during morphogenesis of the vertebral column.

Fine genetic mapping defines the genetic order of Pax9, Tcf3a, and Acrodysplasia (Adp).

We present here the fine genetic mapping of the proximal part of mouse Chromosome (Chr) 12 between D12Mit54 and D12Mit4. This chromosomal region contains three loci, Pax9, Tcf3a, and Acrodysplasia (Adp), which seem to play an important role in pattern formation during mouse embryogenesis. The Adp mutation, which was created by transgene integration, causes skull, paw, and tail deformities. Pax9, which is expressed in the face, paws, and tail, once qualified as a possible candidate for the Adp locus. We analyzed 997 interspecific backcross progeny for recombination between the markers D12Mit54 and D12Mit4; we recovered 117 recombinants, which were further typed for Pax9, Tcf3a, Adp, D12Mit88, D12Nds1, D12Mit36, and D12Mit34. This study represents the first instance in which all the above loci have been included in a single analysis, thereby allowing unambiguous determination of the genetic order and distance between D12Mit54 and D12Mit4. From our results, we conclude that the Adp locus is distinct from either Pax9 or Tcf3a.

Structure of the membrane channel porin from Rhodopseudomonas blastica at 2.0 A resolution.

The crystal structure of a membrane channel, homotrimeric porin from Rhodopseudomonas blastica has been determined at 2.0 A resolution by multiple isomorphous replacement and structural refinement. The current model has an R-factor of 16.5% and consists of 289 amino acids, 238 water molecules, and 3 detergent molecules per subunit. The partial protein sequence and subsequently the complete DNA sequence were determined. The general architecture is similar to those of the structurally known porins. As a particular feature there are 3 adjacent binding sites for n-alkyl chains at the molecular 3-fold axis. The side chain arrangement in the channel indicates a transverse electric field across each of the 3 pore eyelets, which may explain the discrimination against nonpolar solutes. Moreover, there are 2 significantly ordered girdles of aromatic residues at the nonpolar/polar borderlines of the interface between protein and membrane. Possibly, these residues shield the polypeptide conformation against adverse membrane fluctuations.