Expression and function of Pax 1 during development of the pectoral girdle

An overview of PAX1: Expression, function and regulation in development and diseases

Transcription factors play multifaceted roles in embryonic development and diseases. PAX1, a paired-box transcription factor, has been elucidated to play key roles in multiple tissues during embryonic development by extensive studies. Recently, an emerging role of PAX1 in cancers was clarified. Herein, we summarize the expression and functions of PAX1 in skeletal system and thymus development, as well as cancer biology and outline its cellular and molecular modes of action and the association of PAX1 mutation or dysregulation with human diseases, thus providing insights for the molecular basis of congenital diseases and cancers.

Genetics of scapula and pelvis development: An evolutionary perspective

In tetrapods, the scapular and pelvic girdles perform the important function of anchoring the limbs to the trunk of the body and facilitating the movement of each appendage. This shared function, however, is one of relatively few similarities between the scapula and pelvis, which have significantly different morphologies, evolutionary histories, embryonic origins, and underlying genetic pathways. The scapula evolved in jawless fish prior to the pelvis, and its embryonic development is unique among bones in that it is derived from multiple progenitor cell populations, including the dermomyotome, somatopleure, and neural crest. Conversely, the pelvis evolved several million years later in jawed fish, and it develops from an embryonic somatopleuric cell population. The genetic networks controlling the formation of the pelvis and scapula also share similarities and differences, with a number of genes shaping only one or the other, while other gene products such as PBX transcription factors act as hierarchical developmental regulators of both girdle structures. Here, we provide a detailed review of the cellular processes and genetic networks underlying pelvis and scapula formation in tetrapods, while also highlighting unanswered questions about girdle evolution and development.

Problems in Fish-to-Tetrapod Transition: Genetic Expeditions Into Old Specimens

The fish-to-tetrapod transition is one of the fundamental problems in evolutionary biology. A significant amount of paleontological data has revealed the morphological trajectories of skeletons, such as those of the skull, vertebrae, and appendages in vertebrate history. Shifts in bone differentiation, from dermal to endochondral bones, are key to explaining skeletal transformations during the transition from water to land. However, the genetic underpinnings underlying the evolution of dermal and endochondral bones are largely missing. Recent genetic approaches utilizing model organisms—zebrafish, frogs, chickens, and mice—reveal the molecular mechanisms underlying vertebrate skeletal development and provide new insights for how the skeletal system has evolved. Currently, our experimental horizons to test evolutionary hypotheses are being expanded to non-model organisms with state-of-the-art techniques in molecular biology and imaging. An integration of functional genomics, developmental genetics, and high-resolution CT scanning into evolutionary inquiries allows us to reevaluate our understanding of old specimens. Here, we summarize the current perspectives in genetic programs underlying the development and evolution of the dermal skull roof, shoulder girdle, and appendages. The ratio shifts of dermal and endochondral bones, and its underlying mechanisms, during the fish-to-tetrapod transition are particularly emphasized. Recent studies have suggested the novel cell origins of dermal bones, and the interchangeability between dermal and endochondral bones, obscuring the ontogenetic distinction of these two types of bones. Assimilation of ontogenetic knowledge of dermal and endochondral bones from different structures demands revisions of the prevalent consensus in the evolutionary mechanisms of vertebrate skeletal shifts.

Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland

A bizarre latimeriid coelacanth fish from the Middle Triassic of Switzerland shows skeletal features deviating from the uniform anatomy of coelacanths. The new form is closely related to a modern-looking coelacanth found in the same locality and differences between both are attributed to heterochronic evolution. Most of the modified osteological structures in the new coelacanth have their developmental origin in the skull/trunk interface region in the embryo. Change in the expression of developmental patterning genes, specifically the Pax1/9 genes, may explain a rapid evolution at the origin of the new coelacanth. This species broadens the morphological disparity range within the lineage of these ‘living fossils’ and exemplifies a case of rapid heterochronic evolution likely trigged by minor changes in gene expression.

Evolution of the hominoid scapula and its implications for earliest hominid locomotion

OBJECTIVES

The higher primate scapula has been subject to many explanations of the putative "adaptive value" of its individual traits. However, the shift from the bone's position in above branch quadrupeds to its more posterolateral position in recent hominoids obviously required fundamental changes to its general form. We hypothesize that most features argued to be individually adaptive are more likely secondary consequences of changes in its fundamental bauplan, a view more consistent with modern developmental biology.

MATERIALS AND METHODS

We tested this hypothesis with scapular metrics and angles from a broad anthropoid sample.

RESULTS

Our results support our hypothesis. Contrary to earlier predictions, vertebral border length differs little relative to body size in anthropoids, inferior angle position primarily reflects mediolateral scapular breadth, and supraspinous and infraspinous fossa sizes largely reflect scapular spine orientation. Suspensory taxa have cranially oriented glenoids, whereas slow clamberers and humans do not. Australopithecus most closely resembles the latter.

DISCUSSION

Most scapular features can be explained by only two primary changes: (1) reduction in mediolateral breadth and (2) change in the glenoid position relative to the vertebral border with increased reliance on suspension, which led to a more cranially angled scapular spine. Virtually all other scapular traits appear to be byproducts of these two changes. Based on fossil morphology, hominids¹ were derived from a last common ancestor primarily adapted for clambering and not for suspension. Scapular form in early hominids such as Australopithecus is therefore primitive and largely reflects the genus's general clambering heritage.

The function of DrPax1b gene in the embryonic development of zebrafish

Vertebrate Pax1 gene is a member of Pax gene family and encodes a transcription factor associated with crucial roles in the development of pharyngeal pouch, scletrotome and limb bud. In zebrafish, the genome contains two Pax1 paralogs, DrPax1a and DrPax1b, which share high sequence similarity with other Pax1 genes. To elucidate the function of zebrafish DrPax1b gene, we first examined the gene expression pattern and found that it was mainly expressed in the endodermal pharyngeal pouch, caudal somites, notochord, and fin bud. Then, we performed knockdown experiments using antisense morpholino oligonucleotides, which lead to the defects in the vertebral column, tail, pharyngeal skeleton, and pectoral fin. Additionally, we also found that the mouse MmPax1 mRNA, but not the amphioxus AmphiPax1/9 mRNA, could rescue the MO-induced defects. Furthermore, sequence alignment revealed that the N-terminal region of vertebrate Pax1 and amphioxus Pax1/9 were highly conserved, whereas their C-terminal regions were relatively divergent. However, the chimeric Am(N)Dr(C)Pax1, Mm(N)Dr(C)Pax1 and Dr(N)Mm(C)Pax1 mRNA could partially rescue the defects, while the Dr(N)Am(C)Pax1 mRNA could not. In conclusion, our data demonstrate a conserved function of DrPax1b in the development of the vertebral column, pectoral fin and pharyngeal skeleton formation in zebrafish and also provide critical insight into the functional evolution of Pax1 gene by changing its C-terminal sequence.

On the homology of the shoulder girdle in turtles

The shoulder girdle in turtles is encapsulated in the shell and has a triradiate morphology. Due to its unique configuration among amniotes, many theories have been proposed about the skeletal identities of the projections for the past two centuries. Although the dorsal ramus represents the scapular blade, the ventral two rami remain uncertain. In particular, the ventrorostral process has been compared to a clavicle, an acromion, and a procoracoid based on its morphology, its connectivity to the rest of the skeleton and to muscles, as well as with its ossification center, cell lineage, and gene expression. In making these comparisons, the shoulder girdle skeleton of anurans has often been used as a reference. This review traces the history of the debate on the homology of the shoulder girdle in turtles. And based on the integrative aspects of developmental biology, comparative morphology, and paleontology, we suggest acromion and procoracoid identities for the two ventral processes.

Application of synthetic photostable retinoids induces novel limb and facial phenotypes during chick embryogenesis in vivo

We have recently developed a range of synthetic retinoid analogues which include the compounds EC23 and EC19. They are stable on exposure to light and are predicted to be resistant to the normal metabolic processes involved in the inactivation of retinoids in vivo. Based on the position of the terminal carboxylic acid groups in the compounds we suggest that EC23 is a structural analogue of all-trans retinoic acid (ATRA), and EC19 is an analogue of 13-cis retinoic acid. Their effects on the differentiation of pluripotent stem cells has been previously described in vitro and are consistent with this hypothesis. We present herein the first description of the effects of these molecules in vivo. Retinoids were applied to the anterior limb buds of chicken embryos in ovo via ion-exchange beads. We found that retinoid EC23 produces effects on the wing digits similar to ATRA, but does so at two orders of magnitude lower concentration. When larger quantities of EC23 are applied, a novel phenotype is obtained involving production of multiple digit 1s on the anterior limb. This corresponds to differential effects of ATRA and EC23 on sonic hedgehog (shh) expression in the developing limb bud. With EC23 application we also find digit 1 phenotypes similar to thumb duplications described in the clinical literature. EC23 and ATRA are shown to have effects on the entire proximal–distal axis of the limb, including hitherto undescribed effects on the scapula. This includes suppression of expression of the scapula marker Pax1. EC23 also produces effects similar to those of ATRA on the developing face, producing reductions of the upper beak at concentrations two orders of magnitude lower than ATRA. In contrast, EC19, which is structurally very similar to EC23, has novel, less severe effects on the face and rarely alters limb development. EC19 and ATRA are effective at similar concentrations. These results further demonstrate the ability of retinoids to influence embryonic development. Moreover, EC23 represents a useful new tool to investigate developmental processes and probe the mechanisms underlying congenital abnormalities in vertebrates including man.

Origin of the unique morphology of the shoulder girdle in turtles

The shoulder girdle of turtles has a triradiate morphology. Although its dorsal process represents the scapular blade, the skeletal identities of the two ventral processes remain uncertain. To elucidate the question, developmental patterns of the girdles were compared between Chinese soft-shelled turtles, chickens, and mice. Despite the morphological diversity of adults, the initial primordia of the shoulder girdles showed similar morphological patterns. The ventral two processes developed from the anlagen comparable to those of the acromion and the coracoid in other amniotes. The developmental pattern of the acromion is very similar among embryos, whereas that of the coracoid in mammals differs from that in non-mammals, implying that coracoids are not homologous between non-mammals and mammals. Therefore, amniotes have retained the ancestral pattern of the girdle anlage, and the shoulder girdle of turtles has been achieved through a transformation of the pattern in the late ontogenic period.

Control of pelvic girdle development by genes of the Pbx family and Emx2

Genes expressed in the somatopleuric mesoderm, the embryonic domain giving rise to the vertebrate pelvis, appear important for pelvic girdle formation. Among such genes, Pbx family members and Emx2 were found to genetically interact in hindlimb and pectoral girdle formation. Here, we generated compound mutant embryos carrying combinations of mutated alleles for Pbx1, Pbx2, and Pbx3, as well as Pbx1 and Emx2, to examine potential genetic interactions during pelvic development. Indeed, Pbx genes share overlapping functions and Pbx1 and Emx2 genetically interact in pelvic formation. We show that, in compound Pbx1;Pbx2 and Pbx1;Emx2 mutants, pelvic mesenchymal condensation is markedly perturbed, indicative of an upstream control by these homeoproteins. We establish that expression of Tbx15, Prrx1, and Pax1, among other genes involved in the specification and development of select pelvic structures, is altered in our compound mutants. Lastly, we identify potential Pbx1-Emx2-regulated enhancers for Tbx15, Prrx1, and Pax1, using bioinformatics analyses.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

Transcriptional activity analysis of promoter region of human PAX9 gene under dexamethasone, retinoic acid, and ergocalciferol treatment in MCF-7 and MDPC23

PAX9 gene is a member of the family homeobox of transcription factors and performs important function in development and organogenesis. Mutations in PAX9 coding sequences have been implicated in autosomal dominant oligodontia affecting predominantly permanent molars and second premolars. Previous studies have shown that PAX9 is required for secondary palate development and teratogens have been identified as inducers of a tooth and craniofacial malformations. This work focused on the analysis on the 5'-flanking region of the PAX9 gene studying the influence of retinoic acid, dexamethasone, and vitamin D on the expression of PAX9 by expression constructs that carry the reporter gene luciferase. As results, retinoic acid and dexamethasone showed progressive decrease of PAX9 expression. PAX9-pGL3B1 and PAX9-pGL3B2 promoter was inhibited under the treatment of dexamethasone and ergocalciferol. Retinoic acid and dexamethasone did not alter PAX9-pGL3B3 behavior indicating that sequences present between -1106 and +92 were important for the transcriptional activity of PAX9 promoter. In this study, we characterized the transcriptional activity of specific regions of the PAX9 promoter gene and we demonstrated that retinoic acid and ergocalciferol can modulate the transcriptional activity of PAX9 gene.

Scapula development is governed by genetic interactions of Pbx1 with its family members and with Emx2 via their cooperative control of Alx1

The genetic pathways underlying shoulder blade development are largely unknown, as gene networks controlling limb morphogenesis have limited influence on scapula formation. Analysis of mouse mutants for Pbx and Emx2 genes has suggested their potential roles in girdle development. In this study, by generating compound mutant mice, we examined the genetic control of scapula development by Pbx genes and their functional relationship with Emx2. Analyses of Pbx and Pbx1;Emx2 compound mutants revealed that Pbx genes share overlapping functions in shoulder development and that Pbx1 genetically interacts with Emx2 in this process. Here, we provide a biochemical basis for Pbx1;Emx2 genetic interaction by showing that Pbx1 and Emx2 can bind specific DNA sequences as heterodimers. Moreover, the expression of genes crucial for scapula development is altered in these mutants, indicating that Pbx genes act upstream of essential pathways for scapula formation. In particular, expression of Alx1, an effector of scapula blade patterning, is absent in all compound mutants. We demonstrate that Pbx1 and Emx2 bind in vivo to a conserved sequence upstream of Alx1 and cooperatively activate its transcription via this potential regulatory element. Our results establish an essential role for Pbx1 in genetic interactions with its family members and with Emx2 and delineate novel regulatory networks in shoulder girdle development.

Avian pelvis originates from lateral plate mesoderm and its development requires signals from both ectoderm and paraxial mesoderm

The pelvic girdle is composed of three skeletal elements: ilium, pubis, and ischium. In comparison with other parts of the postcranial skeleton, its development is not well known to date. To elucidate the embryonic origin of the avian pelvic girdle and the signaling centers that control its development, we have performed extirpation and quail-to-chick grafting experiments. The results reveal that the entire pelvic girdle originates from the somatopleure at somite levels 26 to 35. No somitic cell contribution to skeletal elements of the pelvis has been detected. Removal of the surface ectoderm covering the lateral plate mesoderm has revealed that ectodermal signals control the development of the pelvic girdle, especially the formation of the pubis and ischium. The impaired development of the ischium and pubis correlates with the downregulation of Pax1 and Alx4, two transcription factors that control the normal development of the ischium and pubis. Although of somatopleural origin, the development of the ilium depends on somitic signals. Insertion of a barrier between somites and somatopleure disrupts the expression of Emx2 and prevents normal development of the ilium but does not affect the expression of Pax1 or Alx4 and the development of the pubis and ischium. Thus, the development of the ilium, but not of the pubis and ischium, depends on somitic and ectodermal signals.

PAX genes: Roles in development, pathophysiology, and cancer

PAX proteins function as transcription factors and play an essential role in organogenesis during embryonic development in regulating cell proliferation and self-renewal, resistance to apoptosis, migration of embryonic precursor cells, and the coordination of specific differentiation programs. Recent studies have also discovered a role for PAX proteins in specific stem cell or progenitor cell populations, including melanocytes, muscle, and B-cells. The normal functions of the PAX proteins, including apoptosis resistance and repression of terminal differentiation, may be subverted during the progression of a number of specific malignancies. This is supported by the fact that expression of PAX proteins is dysregulated in several different types of tumors, although the precise roles for PAX proteins in cancer are not clearly understood. An emerging hypothesis is that PAX proteins play an essential role in maintaining tissue specific stem cells by inhibiting terminal differentiation and apoptosis and that these functional characteristics may facilitate the development and progression of specific cancers. In this review, we provide a general background to the PAX protein family and focus on specific cells and tissues and the role PAX proteins play within these tissues in terms of development, mature tissue maintenance, and expression in tumors. Understanding the normal developmental pathways regulated by PAX proteins may shed light on potentially parallel pathways shared in tumors, and ultimately result in defining new molecular targets and signaling pathways for the development of novel anti-cancer therapies.

Homology of the reptilian coracoid and a reappraisal of the evolution and development of the amniote pectoral apparatus

As in monotreme mammals, the pectoral apparatus of basal (fossil) amniotes includes two coracoid elements, the procoracoid and metacoracoid. Among extant reptiles the metacoracoid has long been assumed lost; this notion is herein challenged. A comprehensive review of data from numerous sources, including the fossil record, experimental embryology, genetic manipulations and an analysis of morphology at the level cell condensations, supports the conclusion that the metacoracoid gives rise to the majority of the reptilian coracoid. By contrast, the reptilian procoracoid remains as a rudiment that is incorporated as a process of the (meta)coracoid and/or the glenoid region of the scapula early during development, prior to skeletogenesis. Application of this integrated approach corroborates and enhances previous work describing the evolution of the pectoral apparatus in mammals. A revised scenario of amniote coracoid evolution is presented emphasizing the importance of considering cell condensations when evaluating the homology of a skeletal complex.

Differential regulation of avian pelvic girdle development by the limb field ectoderm

Although limb development has been a subject of intense research over the last decades, development of the girdles has been poorly investigated. Particularly, a detailed analysis of pelvic girdle development including functional data is not available to date. Here, we describe the early steps of the formation of mesenchymal and cartilaginous anlagen of the pelvic elements using alcian blue staining in whole mount embryos and serial histological sections, and the expression pattern of several marker genes to provide an operative basis for further research in pelvis development. Moreover, we describe pelvis development after unilateral hindlimb bud amputation and somatopleural ectoderm extirpation. We show for the first time, that ectodermal signals at pre-limb bud stages are required for pelvis formation. We present evidence suggesting that the regulation of ilium development is different from the development of ischium and pubis.

Endoderm development in vertebrates: fate mapping, induction and regional specification

The formation of the vertebrate body plan begins with the differentiation of cells into three germ layers: ectoderm, mesoderm and endoderm. Cells in the endoderm give rise to the epithelial lining of the digestive tract, associated glands and respiratory system. One of the fundamental problems in developmental biology is to elucidate how these three primary germ layers are established from the homologous population of cells in the early blastomere. To address this question, ectoderm and mesoderm development have been extensively analyzed, but study of endoderm development has only begun relatively recently. In this review, we focus on the 'where', 'when' and 'how' of endoderm development in four vertebrate model organisms: the zebrafish, Xenopus, chick and mouse. We discuss the classical fate mapping of the endoderm and the more recent progress in characterizing its induction, segregation and regional specification.

The role of Emx2 during scapula formation

The scapula is subdivided into head, collum, and blade. Due to the expression pattern of Emx2 and the absence of the scapula blade in Emx2 knockout mice, it has been suggested that Emx2 is involved in the formation of the scapula. Micromanipulation experiments revealed that ectoderm ablation over the somites does not affect Emx2 expression but inhibits the formation of the scapula blade indicating that Emx2 is not sufficient to induce scapula blade formation. Furthermore, we show that the formation of the scapula head is dependent, scapula blade formation independent of FGFR-1-mediated signaling. Overexpression of Emx2 does not influence scapula blade formation but leads to the development of an additional posterior digit in the anterior border of the limb. Taken together, the data presented implicate that Emx2 expression is necessary but not sufficient for the development of the scapula blade. It is not a marker for scapula development but rather provides positional information along the proximodistal and anterior-posterior limb axes, whereas the specificity of the developing skeletal elements is determined by the concerted interaction of Emx2 with other factors.

The scapulocoracoid of the Queensland lungfish Neoceratodus forsteri (Dipnoi: Sarcopterygii): morphology, development and evolutionary implications for bony fishes (Osteichthyes)

Among bony fishes, the ontogenetic sequence by which the actinopterygian scapulocoracoid develops has been well described, but that of the sarcopterygian scapulocoracoid is poorly known, as the majority of taxa are only known from fossils. To rectify this, the cartilaginous scapulocoracoid of the extant lungfish Neoceratodus forsteri is examined. In initial stages of its development, the scapulocoracoid of Neoceratodus has a simple rounded shape, and supports the glenoid fossa. It appears nearly contemporaneously with the proximal endochondral element (humerus) of the pectoral fin. Pectoral fin elements develop by segmentation from a continuous field of cartilaginous precursor cells extending distally from the glenoid region of the scapulocoracoid. Subsequent scapulocoracoid development produces a ventromedial process, which is not associated with this field of precursor cells. A dorsal process also develops outside this field. Thus, the scapulocoracoid of Neoceratodus may consist of at least two developmentally distinct regions; (1) the ventromedial being homologous with the coracoid of actinopterygians, tetrapods and other jawed vertebrates and (2) a smaller dorsal process, homologous to the scapular region. The two, together with the glenoid region, give an overall triangular shape. The scapulocoracoids of fossil lungfish and other sarcopterygian fishes are also triangular and are composed of scapular and coracoid regions, rather than the 'buttresses' associated with scapulocoracoids of the Actinopterygii and Tetrapoda.

Meox homeodomain proteins are required for Bapx1 expression in the sclerotome and activate its transcription by direct binding to its promoter

The axial skeleton of vertebrates derives from the sclerotomal compartment of the somites. Genetic analysis has demonstrated that the transcription factors Pax1, Pax9, Meox1, Meox2, and Bapx1 are all required for sclerotomal differentiation. Their hierarchical relationship is, however, poorly understood. Because Bapx1 expression in the somites starts slightly later than that of the Meox genes, we asked whether Bapx1 is one of their downstream targets. Our analysis of Meox1; Meox2 mutant mice supports this hypothesis, as Bapx1 expression in the sclerotome is lost in the absence of both Meox proteins. Using transient-transfection assays, we show that Meox1 activates the Bapx1 promoter in a dose-dependent manner and that this activity is enhanced in the presence of Pax1 and/or Pax9. Furthermore, by electrophoretic mobility shift and chromatin immunoprecipitation experiments, we demonstrate that Meox1 can bind the Bapx1 promoter. The palindromic sequence TAATTA, present in the Bapx1 promoter, binds the Meox1 protein in vitro and is necessary for Meox1-induced transactivation of the Bapx1 promoter. Our data demonstrate that the Meox genes are required for Bapx1 expression in the sclerotome and suggest that the mechanism by which the Meox proteins exert this function is through direct activation of the Bapx1 gene.

Modularity and integration in the hominoid scapula

In this paper, several hypotheses of morphological integration within the hominoid (ape) scapula are tested. In particular, whether the scapula represents a set of developmental tissues sharing tight correlations between constituent parts (i.e., highly integrated) or is more modularly organized (i.e., covariation is greater within regions than between) is tested. Whether the patterns of integration in the scapula have changed over phylogenetic time or in response to selective forces is also examined. Results from two different analyses (matrix correlations and edge deviance) indicate traits comprising the blade and acromion, and to a weaker degree the glenoid, correlate highly with each other. The coracoid exhibits more independence from other parts of the scapula, perhaps reflecting its distinct evolutionary developmental history. Overall, similarity in species-specific patterns of correlation was high between all taxa. Correlation matrix similarity was significantly correlated with functional similarity and morphological distance, but not with phylogenetic distance. These results are congruent with other studies of integration that suggest correlation patterns remain stable over evolutionary time. There are changes associated with phylogeny, but the tight link between functional similarity and phylogenetic distance at this level of comparison presents possible challenges to interpretation. Overall similarities in the pattern of integration in all taxa might be better interpreted as relative strengthening or weakening of trait correlations rather than broadscale changes in the pattern of relationship between developmental regions. Larger sample sizes with greater taxonomic/functional breadth, and finer scale analyses of patterns of correlation are needed to test these hypotheses further.

Undulated short-tail Deletion Mutation in the Mouse Ablates Pax1 and Leads to Ectopic Activation of Neighboring Nkx2-2 in Domains That Normally Express Pax1

Previous studies have indicated that the Undulated short-tail deletion mutation in mouse Pax1 (Pax1Un-s) not only ablates Pax1, but also disturbs a gene or genes nearby Pax1. However, which gene(s) is involved and how the Pax1Un-s phenotype is confined to the Pax1-positive tissues remain unknown. In the present study, we determined the Pax1Un-s deletion interval to be 125 kb and characterized genes around Pax1. We show that the Pax1Un-s mutation affects four physically linked genes within or near the deletion, including Pax1, Nkx2-2, and their potential antisense genes. Remarkably, Nkx2-2 is ectopically activated in the sclerotome and limb buds of Pax1Un-s embryos, both of which normally express Pax1. This result suggests that the Pax1Un-s deletion leads to an illegitimate interaction between remotely located Pax1 enhancers and the Nkx2-2 promoter by disrupting an insulation mechanism between Pax1 and Nkx2-2. Furthermore, we show that expression of Bapx1, a downstream target of Pax1, is more strongly affected in Pax1Un-s mutants than in Pax1-null mutants, suggesting that the ectopic expression of Nkx2-2 interferes with the Pax1-Bapx1 pathway. Taken together, we propose that a combination of a loss-of-function mutation of Pax1 and a gain-of-function mutation of Nkx2-2 is the molecular basis of the Pax1Un-s mutation.

Developmental anomalies of the scapula-the "omo"st forgotten bone

Congenital malformations of the scapula, ranging from complete absence, to abnormal shape and position (Sprengel anomaly) are encountered, not infrequently, in genetic practice. Despite this, little is known of the embryologic origin of the scapula and the relationship of the embryology to the observed birth defects. Standard embryology texts, when discussing the subject at all, generally consider the scapula as part of the upper limb. The pattern of associated birth defects suggests that this is at least an oversimplification and may be inaccurate. Sprengel anomaly is the most frequently encountered malformation of the scapula. It can be seen in isolation, but is often seen in association with other defects that include; scoliosis, hemivertebrae, segmentation abnormalities of vertebrae and ribs (including Klippel-Feil sequence), spina bifida, clavicular abnormalities, renal abnormalities and hypoplasia of the muscles of the neck and shoulder. The ipsilateral limb is usually normal. An unappreciated association between Sprengel anomaly and diastematomyelia of the lumbar spine also raises questions about the embryologic origin of the scapula. 25-50% of Sprengel anomaly patients have an associated omovertebral band or bone that arises from the posterior process of a vertebral body and attaches to the superior angle/medial portion of the scapula. This is felt to be of scapular origin, but the report of at least one patient with an omovertebral bone, not associated with a Sprengel anomaly questions this assumption. Scapuloiliac dysostosis (Kosenow syndrome), a rare skeletal dysplasia, is associated with marked hypoplasia of the scapulae, clavicles and pelvis. Associated anomalies include eye anomalies, rib anomalies and spina bifida. The limbs are normal. Knockout of the Emx2 gene in mice yields a similar skeletal phenotype. Mutations in EMX2 in humans are associated with schizencephaly, not skeletal anomalies. Data on gene expression in the scapula will be reviewed. Based on this information, the author proposes that the scapula arises from 2 or more distinct embryologic anlage under different genetic control than the upper limb.

Cooperation of Hoxa5 and Pax1 genes during formation of the pectoral girdle

Hox and Pax transcription factors are master regulators of skeletal and organ morphogenesis. Some skeletal malformations encountered in Hoxa5 mutants are shared by the undulated (un) mice, which bear a point mutation in the Pax1 gene. To investigate whether Hoxa5 and Pax1 act in common pathways during skeletal development, we analyzed Hoxa5;un compound mutants. Our genetic studies show that Hoxa5 and Pax1 cooperate in the vertebral patterning of the cervicothoracic transition region and in acromion morphogenesis. The dynamics of expression of Hoxa5 and Pax1 in the pectoral girdle region suggest that both genes function in a complementary fashion during acromion formation. Whereas Pax1 is required for the recruitment of acromion precursor cells, Hoxa5 may provide regional cues essential for the correct formation of the acromion by ensuring Pax1 expression at the proper time and position during morphogenesis of the pectoral girdle. Hoxa5 also has a distinctive role in specifying the fate of perichondrial and chondrogenic cell lineages in a Sox9-dependent way.

Agenesis of the scapula in Emx2 homozygous mutants

The shoulder and pelvic girdles represent the proximal bones of the appendicular skeleton that connect the anterior and posterior limbs to the body trunk. Although the limb is a well-known model in developmental biology, the genetic mechanisms controlling the development of the more proximal elements of the appendicular skeleton are still unknown. The knock-out of Pax1 has shown that this gene is involved in patterning the acromion, while the expression pattern candidates Hoxc6 as a gene involved in scapula development. Surprisingly, we have found that scapula and ilium do not develop in Emx2 knock-out mice. In the homozygous mutants, developmental abnormalities of the brain cortex, the most anterior structure of the primary axis of the body, are associated with important defects of the girdles, the more proximal elements of the secondary axis. These abnormalities suggest that the molecular mechanisms patterning the more proximal elements of the limb axis are different from those patterning the rest of appendicular skeleton. While Hox genes specify the different segments of the more distal part of the appendicular skeleton forming the limb, Emx2 is concerned with the more proximal elements constituting the girdles.

The evolution of the pectoral girdle

The pectoral girdle articulates the forelimb with the axial skeleton in all vertebrates with paired anterior appendages. The structure of the pectoral girdle and its position along the axial skeleton has changed significantly during vertebrate evolution. These morphological changes have been well described, but there is little comparative embryology to indicate how these changes may have occurred. It is equally obscure how the muscles that connect the head with the pectoral girdle have maintained appropriate attachments even though these 2 structures have become separated. Here I review the changes in the pectoral girdle across different vertebrate taxa, indicating, where known, the developmental mechanisms underlying these changes. I also suggest how the muscular connections between the head and pectoral girdle have been maintained between these once adjacent bones, displaced during vertebrate evolution.

Association of enhanced expression of gap junctions with in vitro chondrogenic differentiation of rat nasal septal cartilage-released cells following their dedifferentiation and redifferentiation

The nasal septum is an important centre of endochondral ossification during the development of the facial region. Previous studies have shown that it is possible to recapitulate the differentiation programme of 21-day-old rat nasal chondrocytes in vitro. The purpose now was to investigate, in vitro, the cell condensation phase that represents the earliest morphological event associated with cartilage differentiation in skeletal development. The study focuses on the ability of the cells to form condensations before overt differentiation, with special emphasis on gap-junction expression. The gap-junction protein connexin 43 was localized by indirect immunofluorescence as primarily intracellular and, on day 5, at the condensation stage, as spot-like contacts between cells. Intracellular injection of the permeable dye Lucifer yellow led to the staining of up to 20 neighbouring cells, indicating functional gap junctions and coupling. In contrast, treatment of cultures with the gap-junction blocker glycyrrhetinic acid inhibited dye coupling and reduced cartilage differentiation. Northern blotting of connexin 43 mRNA showed a faint band during the first days of culture, with a striking increase after day 4. In addition, the mRNA of the homeodomain-containing gene Cart-1 began to be expressed in prechondrogenic condensations and corresponded to the expression of type II collagen mRNA. These data indicate that the early stage of in vitro chondrocyte differentiation is the formation of cell condensations and the ability to establish cell-to-cell communication. Connexin 43, together with other molecular mechanisms, mediates the condensation phase of chondrogenesis and sets up the optimal environment in which nasal septal cells may terminally differentiate into chondrocytes.

Dual origin and segmental organisation of the avian scapula

Bones of the postcranial skeleton of higher vertebrates originate from either somitic mesoderm or somatopleural layer of the lateral plate mesoderm. Controversy surrounds the origin of the scapula, a major component of the shoulder girdle, with both somitic and lateral plate origins being proposed. Abnormal scapular development has been described in the naturally occurring undulated series of mouse mutants, which has implicated Pax1 in the formation of this bone. Here we addressed the development of the scapula, firstly, by analysing the relationship between Pax1 expression and chondrogenesis and, secondly, by determining the developmental origin of the scapula using chick quail chimeric analysis. We show the following. (1) The scapula develops in a rostral-to-caudal direction and overt chondrification is preceded by an accumulation of Pax1-expressing cells. (2) The scapular head and neck are of lateral plate mesodermal origin. (3) In contrast, the scapular blade is composed of somitic cells. (4) Unlike the Pax1-positive cells of the vertebral column, which are of sclerotomal origin, the Pax1-positive cells of the scapular blade originate from the dermomyotome. (5) Finally, we show that cells of the scapular blade are organised into spatially restricted domains along its rostrocaudal axis in the same order as the somites from which they originated. Our results imply that the scapular blade is an ossifying muscular insertion rather than an original skeletal element, and that the scapular head and neck are homologous to the ‘true coracoid’ of higher vertebrates.

Bone morphogenetic proteins regulate interdigital cell death in the avian embryo

The embryonic limb bud provides an excellent model for analyzing the mechanisms that regulate programmed cell death during development. At the time of digit formation in the developing autopod, the undifferentiated distal mesodermal cells may undergo or chondrogenic differentiation or apoptosis depending whether they are incorporated into the future digital rays or into the interdigital spaces. Both chondrogenesis or apoptosis are induced by local BMPS. However, whereas the chondrogenic-promoting activity of BMPs appears to be regulated through the BMPR-1b receptor, the mechanism by which the BMPs execute the death program remains unknown. The BMP proapoptotic activity requires the expression of members of the msx family of closely related homeobox-containing genes and is finally mediated by caspase activation, but the nature of the caspase(s) directly responsible for the cell death is also unknown. Finally, other growth factors present in the developing autopod at the stages of digit formation such as members of the FGF and TGF beta families modulate the ability of BMPs to induce cell death or chondrogenesis.

Characterization of gill-specific genes of the acorn worm Ptychodera flava

Acorn worms are hemichordate deuterostomes that have remarkable gills thought to be homologous to pharyngeal gills in urochordates and cephalochordates, and pharyngeal pouches in vertebrates. In search of molecular keys to analyzing the origin and evolution of the anterior gut and neck region of the chordate body, the present study isolated cDNA clones for six gill-specific genes, designated PfG1 to PfG6, from Ptychodera flava using differential screening of a cDNA library of RNA from gills. Northern blotting confirmed that these genes were all expressed only in the gills. In situ hybridization showed that the expression of these genes is limited to the endodermally derived columnar epithelium of the pharynx. PfG1 encodes a 42-kDa polypeptide containing sequence similar to D-domains, protein domains characteristic of extracellular proteins. Expression of PfG1 is localized in a delimited pattern along the columnar epithelium of the inner gill apparatus. Expression in the epibranchial ridge appears as two stripes running longitudinally in the epithelium just lateral of the midline. A stripe of expression also appears in a slightly posterior portion on the curve of each band of columnar epithelium on the pharyngeal surface of the secondary gill bars. The five other gill-specific genes, PfG2 to PfG6, encode a family of C-type lectin polypeptides that appear to be secreted proteins. PfG2 to PfG6 are also expressed in the columnar epithelium of the epibranchial ridge as two parallel stripes, but at the lateral margin of the ridge. One of the genes, PfG6, is additionally expressed in the innermost curve of the epithelium on the pharyngeal surface of each secondary gill bar. The localization of expression of PfPax1/9, a gill-specific transcription factor gene, was examined and shown to also be primarily in the endodermal columnar epithelium on the pharyngeal faces of the gill bars. On the secondary gill bars, where PfG1 and PfG6 are also expressed in the columnar epithelium, PfPax1/9 is expressed in the anterior and posterior portions but signal is not evident in the epithelium on the central, innermost curve of the gill bar. The anterior domain of PfPax1/9 expression is more extensive but overlaps the anterior domain of PfG1 expression, whereas its posterior domain of expression is more posterior and complementary to that of PfG6.

Characterization of a novel insertional mouse mutation, kkt: A closely linked modifier of Pax1

We describe a novel transgene insertional mouse mutant with skeletal abnormalities characterized by a kinked tail and severe curvature of the spine. The disrupted locus is designated kkt for "kyphoscoliosis kinked tail." Malformed vertebrae including bilateral ossification centers and premature fusion of the vertebral body to the pedicles are observed along the vertebral column, and the lower thoracic and lumbar vertebrae are the most affected. Some of the homozygous kkt neonates displayed two backward-pointing transverse processes in the sixth lumbar vertebra (L6) that resembled the first sacral vertebra, and some displayed one forward- and one backward-pointing transverse process in L6. The fourth and fifth sternebrae were also fused, and the acromion process of the scapula was missing in kkt mice. The skeletal abnormalities are similar to those observed in the mouse mutant undulated (un). The transgene is integrated at the distal end of chromosome 2 close to the Pax1 gene, as revealed by FISH analysis. However, mutation of the Pax1 gene is responsible for the un phenotype, but the Pax1 gene in the kkt mice is not rearranged or deleted. Pax1 is expressed normally in kkt embryos and in the thymus of mature animals, and there is no mutation in its coding sequence. Thus, the skeletal abnormalities observed in the kkt mutant are not due to a lack of functional Pax1. Mouse genomic sequences flanking the transgene and PAC clones spanning the wild-type kkt locus have been isolated, and reverse Northern analysis showed that the PACs contain transcribed sequence. Compound heterozygotes between un and kkt (un(+/-)/kkt(+/-)) display skeletal abnormalities similar to those of un or kkt homozygotes, but they have multiple lumbar vertebrae with a split vertebral body that is more severe than in homozygous un or kkt neonates. Furthermore, the sternebrae are not fused and no backward-pointing transverse processes are detected in L6. It is therefore apparent that these two mutations do not fully complement each other, and we propose that a gene in the kkt locus possesses a unique role that functions in concert with Pax1 during skeletal development.

Sclerotome induction and differentiation

Inductive events in the development of the sclerotome and their possible underlying mechanisms were reviewed from the primary literature. A brief review of morphological and anatomical aspects of sclerotome development was given. The importance of the notochord and neural tube in sclerotome induction and somite chondrogenesis in vivo and in vitro was established. The functions and patterns of expression of different sclerotome markers were discussed. Shh and Noggin were discussed as two molecules produced by the neural tube and notochord that appear to maintain and initiate the sclerotome, respectively. While the abilities of the axial organs and Shh and Noggin to induce sclerotome marker expression in the somite was not disputed, the exact nature of these inductions was discussed with regard to possible effects on gene expression, effects on cell survival, and physical effects on the cells and it was argued that the fundamental nature of inductive events in the sclerotome is still unknown.

Developmental expression of Pax1/9 genes in urochordate and hemichordate gills: insight into function and evolution of the pharyngeal epithelium

The epithelium of the pharynx contributes to the formation of gills in hemichordates, urochordates, cephalochordates and primitive vertebrates, and is therefore a key structure for understanding developmental mechanisms underlying the establishment of chordate body plans. Pax1- and Pax9-related genes encode transcription factors which are expressed in the pharyngeal region of cephalochordates as well as in the vertebrate pharyngeal pouch epithelium that forms the thymus and parathyroid glands. To explore the molecular basis underlying the occurrence and modifications of the pharyngeal epithelium during evolution, we isolated cDNA clones for Pax1- and Pax9-related genes of urochordates (HrPax1/9 of Halocynthia roretzi and CiPax1/9 of Ciona intestinalis) and a hemichordate (PfPax1/9 of Ptychodera flava) from gill cDNA libraries. Each gene is present as a single copy per haploid genome. All of the cDNAs encode typical paired domains and octapeptides but not a homeodomain, as is also true of other Pax1- and Pax9-related genes. Molecular phylogenetic analysis based on comparison of the paired domain amino-acid sequences suggests that HrPax1/9, CiPax1/9 and PfPax1/9 belong to the Pax1/9 subfamily, and that they are descendants of a single precursor of Pax1/Pax9. Screening of HrPax1/9 cDNA clones yielded six different types of transcripts which were generated by alternative splicing. Northern blot, RT-PCR/Southern and in situ hybridization analyses revealed that HrPax1/9, CiPax1/9 and PfPax1/9 are not expressed during early embryogenesis but are expressed in the epithelia of differentiating gills, suggesting that these genes encode gill-specific transcription factors. The Pax1/9 genes therefore might provide the first developmental genetic corroboration of hypotheses of organ-level homology that unifies hemichordates, urochordates and cephalochordates.

Rib truncations and fusions in the Sp2H mouse reveal a role for Pax3 in specification of the ventro-lateral and posterior parts of the somite

The splotch (Pax3) mouse mutant serves as a model for developmental defects of several types, including defective migration of dermomyotomal cells to form the limb musculature. Here, we describe abnormalities of the ribs, neural arches, and acromion in Sp2H homozygous embryos, indicating a widespread dependence of lateral somite development on Pax3 function. Moreover, the intercostal and body wall muscles, derivatives of the ventrolateral myotome, are also abnormal in Sp2H homozygotes. Pax3 is expressed in the dermomyotome, but not in either the sclerotome or the myotome, raising the possibility that Pax3-dependent inductive influences from the dermomyotome are necessary for early specification of lateral sclerotome and myotome. Support for this idea comes from analysis of gene expression markers of lateral sclerotome (tenascin-C and scleraxis) and myotome (myogenin, MyoD, and Myf5). All exhibit ventrally truncated domains of expression in Sp2H homozygotes, potentially accounting for the rib and intercostal muscle truncations. In contrast, the medial sclerotomal marker Pax1 is expressed normally in mutant embryos, arguing that Pax3 is not required for development of the medial sclerotome. Most of the somitic markers show ectopic expression in anteroposterior and mediolateral dimensions, suggesting a loss of definition of somite boundaries in splotch and explaining the rib and muscle fusions. An exception is Myf5, which is not ectopically expressed in Sp2H homozygotes, consistent with the previous suggestion that Pax3 and Myf5 function in different pathways of skeletal myogenesis. PDGFalpha and its receptor are candidates for mediating signalling between myotome and sclerotome. We find that both genes are misexpressed in Sp2H embryos, suggesting that PDGFalpha/PDGFRalpha may function downstream of Pax3, accounting for the close similarities between the splotch and Patch mutant phenotypes. Our findings point to additional regulatory functions for the Pax3 transcription factor, apart from those already demonstrated for development of the neural tube, neural crest, and dermomyotome.

Expression of the paired-box genes Pax-1 and Pax-9 in limb skeleton development

Vertebrate Pax genes encode a family of transcription factors that play important roles in embryonic patterning and morphogenesis. Two closely related Pax genes, Pax-1 and Pax-9, are associated with early axial and limb skeleton development. To investigate the role of these genes in cartilage formation we have examined the expression profiles of Pax-1 and Pax-9 in developing chick limb mesenchyme in vivo and in vitro. Both transcripts are detected by reverse transcription polymerase chain reaction and Northern blotting throughout chick limb development, from the early bud stages (Hamburger-Hamilton 20-23) to fully patterned appendages (stage 30). Whole-mount in situ hybridization reveals complex, nonoverlapping expression domains of these two genes. Pax-1 transcripts first appear at the anterior proximal margin of the limb buds, while Pax-9 is expressed more distally at what will be the junction of the autopod and the zeugopod. In situ hybridization to serial sections of the girdles reveals that in the pectoral region Pax-1 is expressed proximally in condensed mesenchyme surrounding the junction of the developing scapula, humerus, and coracoid. In the pelvis, Pax-1 is expressed between the femur and the developing acetabulum and along the ventral edge of the ischium; this transcript was also found in the distal hindlimb along the posterior edge of the fibula. Pax-9 transcripts were not detected in the pectoral girdle at any stage, and only weakly in the pelvis along the ventral ischial margin. In the distal parts of both wings and legs, however, Pax-9 is strongly expressed between the anterior embryonic cartilages (e.g., distal radius or tibia) and the anterior ectodermal ridge. The expression of both genes was strongest in undifferentiated cells of precartilage condensations or at the margins of differentiated cartilages, and was absent from cartilage itself. In micromass cultures of chondrifying limb bud mesenchyme expression of Pax-1 and Pax-9 is maintained for up to 3 days in vitro, most strongly at the end of the culture period during chondrogenic differentiation. As seen in vivo, transcripts are found in loose mesenchyme cells at the outer margins of developing cartilage nodules, and are absent from differentiated chondrocytes at the nodule center. Taken together, these investigations extend previous studies of Pax-1 and Pax-9 expression in embryonic limb development while validating limb bud mesenchyme culture as an accessible experimental system for the study of Pax gene function and regulation. Our in vivo and in vitro observations are discussed with reference to 1) the relationship between somitic and limb expression of these two Pax genes, 2) what regulates this expression in different regions of the embryo, and 3) the putative cellular functions of Pax-1 and Pax-9 in embryonic skeletogenesis.

Inhibitory action of BMPs on Pax1 expression and on shoulder girdle formation during limb development

Pax1 expression in vertebrate limb buds is confined to cells in a discrete anterior proximal domain (Timmons et al. [1994] Development 120:2773-2785; Ebensperger et al. [1995] Anat. Embryol. 191:297-310). In dorsoventral patterning of Drosophila, expression of pox meso, an insect gene with high sequence similarity to Pax1, is repressed by decapentaplegic (dpp) in dorsal mesoderm and, thus, is restricted to a discrete ventral domain (Staehling-Hampton et al. [1994] Nature 372:783-786). In the chick wing, cells expressing a vertebrate homolog of dpp, bone morphogenetic protein 4 (Bmp4), abut the Pax1 domain, suggesting a similar relationship between homologous genes in both vertebrates and invertebrates. Here, we show that two BMPs (BMP4, and BMP2, also highly related to dpp) can repress Pax1 in the developing chick wing. Chick wing bud cells expressing Pax1 give rise to the shoulder girdle. Cells in an equivalent position in the mouse forelimb also express Pax1, and Pax1 mutant mice display shoulder girdle defects. Similarly in chick embryos, girdle defects are produced by treatments with signalling molecules that lead to expression of BMPs, which subsequently reduce Pax1 expression in the limb bud. Recently, BMP4 has been shown to inhibit Pax1 expression in the developing trunk (Monsoro-Burq et al. [1996] Development 122:3607-3616) and Pax9 expression in developing teeth (Neubüser et al. [1997] Cell 90:247-255). Thus, a property of BMPs appears to be to regulate pox meso homologs negatively and, thus, limit their expression domains.

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.

Targeted disruption of Pax1 defines its null phenotype and proves haploinsufficiency

The murine paired box-containing gene Pax1 is required for normal development of the vertebral column, the sternum, and the scapula. Previous studies have shown that three natural Pax1 mouse mutants, the undulated alleles, exhibit phenotypes of different severity in these skeletal elements. Nevertheless, these analyses have not clarified whether the semidominant Undulated short-tail (Uns) mutation, in which the complete Pax1 locus is deleted, represents a null allele. Moreover, the analyses of the classical undulated mutants did not allow a conclusion with respect to haploinsufficiency of Pax1. To address both questions we have created a Pax1 null allele in mice by gene targeting. Surprisingly, the phenotype of this defined mutation exhibits clear differences to that of Uns. This result strongly indicates the contribution of additional gene(s) to the Uns mutant phenotype. Furthermore, the phenotype of mice heterozygous for the null allele demonstrates that Pax1 is haploinsufficient in some though not all skeletal elements which express Pax1 during embryonic development.

Differential expression of the chicken Pax-1 and Pax-9 gene: in situ hybridization and immunohistochemical analysis

We report the cloning, partial sequence analysis, and spatiotemporal expression of the chicken Pax-1 (chPax-1) and Pax-9 (chPax-9) gene, two closely related members of the paired box-containing (PAX) gene family. The chPax-1 gene encodes RNAs of 2.0 and 4.3 kb and a 42 kD protein while the gene products of chPax-9 are represented by 1.9 and 3.1 kb transcripts and a 39 kD protein. In situ hybridization and immunohistochemical analyses reveal chPax-1 expression in the developing pectoral girdle, in cells of the ventral part of sclerotomes, in sclerotome cells of the perichordal tube, and, later in development, in sclerotome-derived cells of the intervertebral disks. Other chPax-1 expression domains detected in the mesenchyme surrounding the atlas and axis and in chondrocytes of immature vertebral bodies, so far unreported for mouse Pax-1, correlate with as yet unexplained malformations in the mouse Pax-1 mutant undulated and Undulated-short tail. Overlapping expression of chPax-1 and chPax-9 is detected in epithelial cells of the embryonic and adult thymus and in cells of the developing intervertebral disks. Unlike chPax-1, however, chPax-9 is not expressed in those perichordal sclerotome cells which are thought to give rise to vertebral bodies. Furthermore, chPax-9 gene products are detected in circumscribed areas of mesenchyme in the metatarsus and in entodermal derivatives, i.e., in the lining epithelium of the developing pharynx and of the embryonic and adult esophagus.

Pax-1, a regulator of sclerotome development is induced by notochord and floor plate signals in avian embryos

Pax-1 encodes for a DNA-binding transcriptional activator that was originally discovered in murine embryos using a probe from the Drosophila paired-box-containing gene, gooseberry-distal. We have cloned the avian Pax-1 gene as a basis for experimental studies of the induction of Pax-1 in the paraxial mesoderm. The amino acid sequence of the paired-domain is exactly the same in the quail and mouse, whereas outside the paired-domain there is 61% homology. Starting at about the eight-somite stage, quail Pax-1 is expressed in the paraxial mesoderm in a craniocaudal sequence. The unsegmented paraxial mesoderm and the two most recently formed somites do not express Pax-1. In the epithelial somite, the somitocoele cells and the cells of the ventral two-thirds of the epithelial wall are positive. As soon as the sclerotome is formed, only a subset of sclerotome cells expresses Pax-1. These are the cells that migrate towards the notochord to form the perinotochordal tube. Expression then becomes restricted to the intervertebral discs, the perichondrium of the vertebral bodies and the connective tissue surrounding the spinal ganglia. Additional expression domains are found in the scapula and the pelvic region, distinct areas of the head, and the epithelium of the second to the fourth visceral pouch. In later stages the thymus is positive. In vitro and in vivo experiments show that the notochord induces Pax-1 in the paraxial mesoderm, but limb bud mesoderm is not competent to respond to notochordal signals. Floor plate is also capable of inducing Pax-1 expression in sclerotome cells. Our studies show that in competent cells of the paraxial mesoderm, Pax-1 is a mediator of signals emanating from the notochord and the floor plate.