The paired homeobox gene Uncx4.1 specifies pedicles, transverse processes and proximal ribs of the vertebral column

Histology Atlas of the Developing Mouse Respiratory System From Prenatal Day 9.0 Through Postnatal Day 30

Respiratory diseases are one of the leading causes of death and disability around the world. Mice are commonly used as models of human respiratory disease. Phenotypic analysis of mice with spontaneous, congenital, inherited, or treatment-related respiratory tract abnormalities requires investigators to discriminate normal anatomic features of the respiratory system from those that have been altered by disease. Many publications describe individual aspects of normal respiratory tract development, primarily focusing on morphogenesis of the trachea and lung. However, a single reference providing detailed low- and high-magnification, high-resolution images of routine hematoxylin and eosin (H&E)-stained sections depicting all major structures of the entire developing murine respiratory system does not exist. The purpose of this atlas is to correct this deficiency by establishing one concise reference of high-resolution color photomicrographs from whole-slide scans of H&E-stained tissue sections. The atlas has detailed descriptions and well-annotated images of the developing mouse upper and lower respiratory tracts emphasizing embryonic days (E) 9.0 to 18.5 and major early postnatal events. The selected images illustrate the main structures and events at key developmental stages and thus should help investigators both confirm the chronological age of mouse embryos and distinguish normal morphology as well as structural (cellular and organ) abnormalities.

Antagonistic regulation of homeologous uncx.L and uncx.S genes orchestrates myotome and sclerotome differentiation in the evolutionarily divergent vertebral column of Xenopus laevis

In anurans, the vertebral column diverges widely from that of other tetrapods; yet the molecular mechanisms underlying its morphogenesis remain largely unexplored. In this study, we investigate the role of the homeologous uncx.L and uncx.S genes in the vertebral column morphogenesis of the allotetraploid frog Xenopus laevis. We initiated our study by cloning the uncx orthologous genes in the anuran Xenopus and determining their spatial expression patterns using in situ hybridization. Additionally, we employed gain-of-function and loss-of-function approaches through dexamethasone-inducible uncx constructs and antisense morpholino oligonucleotides, respectively. Comparative analysis of the messenger RNA sequences of homeologous uncx genes revealed that the uncx.L variant lacks the eh1-like repressor domain. Our spatial expression analysis indicated that in the presomitic mesoderm and somites, the transcripts of uncx.L and uncx.S are located in overlapping domains. Alterations in the function of uncx genes significantly impact the development and differentiation of the sclerotome and myotome, resulting in axial skeleton malformations. Our findings suggest a scenario where the homeologous genes uncx.L and uncx.S exhibit antagonistic functions during somitogenesis. Specifically, uncx.S appears to be crucial for sclerotome development and differentiation, while uncx.L primarily influences myotome development. Postallotetraploidization, the uncx.L gene in X. laevis evolved to lose its eh1-like repressor domain, transforming into a "native dominant negative" variant that potentially competes with uncx.S for the same target genes. Finally, the histological analysis revealed that uncx.S expression is necessary for the correct formation of pedicles and neural arch of the vertebrae, and uncx.L is required for trunk muscle development.

DNA methylation clocks for clawed frogs reveal evolutionary conservation of epigenetic aging

To address how conserved DNA methylation-based epigenetic aging is in diverse branches of the tree of life, we generated DNA methylation data from African clawed frogs (Xenopus laevis) and Western clawed frogs (Xenopus tropicalis) and built multiple epigenetic clocks. Dual species clocks were developed that apply to both humans and frogs (human-clawed frog clocks), supporting that epigenetic aging processes are evolutionary conserved outside mammals. Highly conserved positively age-related CpGs are located in neural-developmental genes such as uncx, tfap2d as well as nr4a2 implicated in age-associated disease. We conclude that signatures of epigenetic aging are evolutionary conserved between frogs and mammals and that the associated genes relate to neural processes, altogether opening opportunities to employ Xenopus as a model organism to study aging.

Building a vertebra: Development of the amniote sclerotome

In embryonic development, the vertebral column arises from the sclerotomal compartment of the somites. The sclerotome is a mesenchymal cell mass which can be subdivided into several subpopulations specified by different regulatory mechanisms and giving rise to different parts of the vertebrae like vertebral body, vertebral arch, ribs, and vertebral joints. This review gives a short overview on the molecular and cellular basis of the formation of sclerotomal subdomains and the morphogenesis of their vertebral derivatives.

Spatial transcriptomics reveals a cnidarian segment polarity program in Nematostella vectensis

During early animal evolution, the emergence of axially polarized segments was central to the diversification of complex bilaterian body plans. Nevertheless, precisely how and when segment polarity pathways arose remains obscure. Here, we demonstrate the molecular basis for segment polarization in developing larvae of the sea anemone Nematostella vectensis. Utilizing spatial transcriptomics, we first constructed a 3D gene expression atlas of developing larval segments. Capitalizing on accurate in silico predictions, we identified Lbx and Uncx, conserved homeodomain-containing genes that occupy opposing subsegmental domains under the control of both bone morphogenetic protein (BMP) signaling and the Hox-Gbx cascade. Functionally, Lbx mutagenesis eliminated all molecular evidence of segment polarization at the larval stage and caused an aberrant mirror-symmetric pattern of retractor muscles (RMs) in primary polyps. These results demonstrate the molecular basis for segment polarity in a non-bilaterian animal, suggesting that polarized metameric structures were present in the Cnidaria-Bilateria common ancestor over 600 million years ago.

Spatial transcriptomics reveals a conserved segment polarity program that governs muscle patterning in Nematostella vectensis

During early animal evolution, the emergence of axially-polarized segments was central to the diversification of complex bilaterian body plans. Nevertheless, precisely how and when segment polarity pathways arose remains obscure. Here we demonstrate the molecular basis for segment polarization in developing larvae of the pre-bilaterian sea anemone Nematostella vectensis . Utilizing spatial transcriptomics, we first constructed a 3-D gene expression atlas of developing larval segments. Capitalizing on accurate in silico predictions, we identified Lbx and Uncx, conserved homeodomain-containing genes that occupy opposing subsegmental domains under the control of both BMP signaling and the Hox-Gbx cascade. Functionally, Lbx mutagenesis eliminated all molecular evidence of segment polarization at larval stage and caused an aberrant mirror-symmetric pattern of retractor muscles in primary polyps. These results demonstrate the molecular basis for segment polarity in a pre-bilaterian animal, suggesting that polarized metameric structures were present in the Cnidaria-Bilateria common ancestor over 600 million years ago.

Highlights

Nematostella endomesodermal tissue forms metameric segments and displays a transcriptomic profile similar to that observed in bilaterian mesoderm Construction of a comprehensive 3-D gene expression atlas enables systematic dissection of segmental identity in endomesoderm Lbx and Uncx , two conserved homeobox-containing genes, establish segment polarity in Nematostella The Cnidarian-Bilaterian common ancestor likely possessed the genetic toolkit to generate polarized metameric structures.

Identification of a Functional Susceptibility Variant for Adolescent Idiopathic Scoliosis that Upregulates Early Growth Response 1 (EGR1)-Mediated UNCX Expression

Adolescent idiopathic scoliosis (AIS) is a serious health problem affecting 3% of live births all over the world. Many loci associated with AIS have been identified by previous genome wide association studies, but their biological implication remains mostly unclear. In this study, we evaluated the AIS-associated variants in the 7p22.3 locus by combining in silico, in vitro, and in vivo analyses. rs78148157 was located in an enhancer of UNCX, a homeobox gene and its risk allele upregulated the UNCX expression. A transcription factor, early growth response 1 (EGR1), transactivated the rs78148157-located enhancer and showed a higher binding affinity for the risk allele of rs78148157. Furthermore, zebrafish larvae with UNCX messenger RNA (mRNA) injection developed body curvature and defective neurogenesis in a dose-dependent manner. rs78148157 confers the genetic susceptibility to AIS by enhancing the EGR1-regulated UNCX expression. © 2022 American Society for Bone and Mineral Research (ASBMR).

TGFβ signaling is required for sclerotome resegmentation during development of the spinal column in Gallus gallus

We previously showed the importance of TGFβ signaling in development of the mouse axial skeleton. Here, we provide the first direct evidence that TGFβ signaling is required for resegmentation of the sclerotome using chick embryos. Lipophilic fluorescent tracers, DiO and DiD, were microinjected into adjacent somites of embryos treated with or without TGFβRI inhibitors, SB431542, SB525334 or SD208, at developmental day E2.5 (HH16). Lineage tracing of labeled cells was observed over the course of 4 days until the completion of resegmentation at E6.5 (HH32). Vertebrae were malformed and intervertebral discs were small and misshapen in inhibitor injected embryos. Hypaxial myofibers were also increased in thickness after treatment with the inhibitor. Inhibition of TGFβ signaling resulted in alterations in resegmentation that ranged between full, partial, and slanted shifts in distribution of DiO or DiD labeled cells within vertebrae. Patterning of rostro-caudal markers within sclerotome was disrupted at E3.5 after treatment with TGFβRI inhibitor with rostral domains expressing both rostral and caudal markers. We propose that TGFβ signaling regulates rostro-caudal polarity and subsequent resegmentation in sclerotome during spinal column development.

Characterization of paralogous uncx transcription factor encoding genes in zebrafish

The paired-type homeodomain transcription factor Uncx is involved in multiple processes of embryogenesis in vertebrates. Reasoning that zebrafish genes uncx4.1 and uncx are orthologs of mouse Uncx, we studied their genomic environment and developmental expression. Evolutionary analyses indicate the zebrafish uncx genes as being paralogs deriving from teleost-specific whole-genome duplication. Whole-mount in situ mRNA hybridization of uncx transcripts in zebrafish embryos reveals novel expression domains, confirms those previously known, and suggests sub-functionalization of paralogs. Using genetic mutants and pharmacological inhibitors, we investigate the role of signaling pathways on the expression of zebrafish uncx genes in developing somites. In identifying putative functional role(s) of zebrafish uncx genes, we hypothesized that they encode transcription factors that coordinate growth and innervation of somitic muscles.

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The Uncx4.1 and Uncx genes derive from the teleost-specific whole-genome duplication.

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Uncx genes are expressed during embryogenesis in unique and overlapping domains.

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Uncx gene expression during somite differentiation is regulated by FGF signaling.

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Synteny and expression profiles correlate Uncx genes with axon guidance.

Development of the axial skeleton and intervertebral disc

Development of the axial skeleton is a complex, stepwise process that relies on intricate signaling and coordinated cellular differentiation. Disruptions to this process can result in a myriad of skeletal malformations that range in severity. The notochord and the sclerotome are embryonic tissues that give rise to the major components of the intervertebral discs and the vertebral bodies of the spinal column. Through a number of mouse models and characterization of congenital abnormalities in human patients, various growth factors, transcription factors, and other signaling proteins have been demonstrated to have critical roles in the development of the axial skeleton. Balance between opposing growth factors as well as other environmental cues allows for cell fate specification and divergence of tissue types during development. Furthermore, characterization of progenitor cells for specific cell lineages has furthered the understanding of specific spatiotemporal cues that cells need in order to initiate and complete development of distinct tissues. Identifying specific marker genes that can distinguish between the various embryonic and mature cell types is also of importance. Clinically, understanding developmental clues can aid in the generation of therapeutics for musculoskeletal disease through the process of developmental engineering. Studies into potential stem cell therapies are based on knowledge of the normal processes that occur in the embryo, which can then be applied to stepwise tissue engineering strategies.

IVD Development: Nucleus pulposus development and sclerotome specification

PURPOSE OF REVIEW

Intervertebral discs (IVD) are derived from embryonic notochord and sclerotome. The nucleus pulposus is derived from notochord while other connective tissues of the spine are derived from sclerotome. This manuscript will review the past 5 years of research into IVD development.

RECENT FINDINGS

Over the past several years, advances in understanding the step-wise process that govern development of the nucleus pulposus and the annulus fibrosus have been made. Generation of tissues from induced or embryonic stem cells into nucleus pulposus and paraxial mesoderm derived tissues has been accomplished in vitro using pathways identified in normal development. A balance between BMP and TGF-β signaling as well as transcription factors including Pax1/Pax9, Mkx and Nkx3.2 appear to be very important for cell fate decisions generating tissues of the IVD.

SUMMARY

Understanding how the IVD develops will provide the foundation for future repair, regeneration, and tissue engineering strategies for IVD disease.

Patterning spinal nerves and vertebral bones

A prominent anatomical feature of the peripheral nervous system is the segmentation of mixed (motor, sensory and autonomic) spinal nerves alongside the spinal cord. During early development their axon growth cones avoid the developing vertebral elements by traversing the anterior/cranial half of each somite-derived sclerotome, so ensuring the separation of spinal nerves from vertebral bones as axons extend towards their peripheral targets. A glycoprotein expressed on the surface of posterior half-sclerotome cells confines growth cones to the anterior half-sclerotomes by contact repulsion. A closely similar glycoprotein is expressed in avian and mammalian grey matter, where we hypothesize it may have evolved to regulate neural plasticity in birds and mammals.

Deletion of the sclerotome-enriched lncRNA PEAT augments ribosomal protein expression

To define a complete catalog of the genes that are activated during mouse sclerotome formation, we sequenced RNA from embryonic mouse tissue directed to form sclerotome in culture. In addition to well-known early markers of sclerotome, such as Pax1, Pax9, and the Bapx2/Nkx3-2 homolog Nkx3-1, the long-noncoding RNA PEAT (Pax1 enhancer antisense transcript) was induced in sclerotome-directed samples. Strikingly, PEAT is located just upstream of the Pax1 gene. Using CRISPR/Cas9, we generated a mouse line bearing a complete deletion of the PEAT-transcribed unit. RNA-seq on PEAT mutant embryos showed that loss of PEAT modestly increases bone morphogenetic protein target gene expression and also elevates the expression of a large subset of ribosomal protein mRNAs.

Developmental Patterning as a Quantitative Trait: Genetic Modulation of the Hoxb6 Mutant Skeletal Phenotype

The process of patterning along the anterior-posterior axis in vertebrates is highly conserved. The function of Hox genes in the axis patterning process is particularly well documented for bone development in the vertebral column and the limbs. We here show that Hoxb6, in skeletal elements at the cervico-thoracic junction, controls multiple independent aspects of skeletal pattern, implicating discrete developmental pathways as substrates for this transcription factor. In addition, we demonstrate that Hoxb6 function is subject to modulation by genetic factors. These results establish Hox-controlled skeletal pattern as a quantitative trait modulated by gene-gene interactions, and provide evidence that distinct modifiers influence the function of conserved developmental genes in fundamental patterning processes.

DNA Methylation Landscapes of Human Fetal Development

Remodelling the methylome is a hallmark of mammalian development and cell differentiation. However, current knowledge of DNA methylation dynamics in human tissue specification and organ development largely stems from the extrapolation of studies in vitro and animal models. Here, we report on the DNA methylation landscape using the 450k array of four human tissues (amnion, muscle, adrenal and pancreas) during the first and second trimester of gestation (9,18 and 22 weeks). We show that a tissue-specific signature, constituted by tissue-specific hypomethylated CpG sites, was already present at 9 weeks of gestation (W9). Furthermore, we report large-scale remodelling of DNA methylation from W9 to W22. Gain of DNA methylation preferentially occurred near genes involved in general developmental processes, whereas loss of DNA methylation mapped to genes with tissue-specific functions. Dynamic DNA methylation was associated with enhancers, but not promoters. Comparison of our data with external fetal adrenal, brain and liver revealed striking similarities in the trajectory of DNA methylation during fetal development. The analysis of gene expression data indicated that dynamic DNA methylation was associated with the progressive repression of developmental programs and the activation of genes involved in tissue-specific processes. The DNA methylation landscape of human fetal development provides insight into regulatory elements that guide tissue specification and lead to organ functionality.

Building the backbone: the development and evolution of vertebral patterning

The segmented vertebral column comprises a repeat series of vertebrae, each consisting of two key components: the vertebral body (or centrum) and the vertebral arches. Despite being a defining feature of the vertebrates, much remains to be understood about vertebral development and evolution. Particular controversy surrounds whether vertebral component structures are homologous across vertebrates, how somite and vertebral patterning are connected, and the developmental origin of vertebral bone-mineralizing cells. Here, we assemble evidence from ichthyologists, palaeontologists and developmental biologists to consider these issues. Vertebral arch elements were present in early stem vertebrates, whereas centra arose later. We argue that centra are homologous among jawed vertebrates, and review evidence in teleosts that the notochord plays an instructive role in segmental patterning, alongside the somites, and contributes to mineralization. By clarifying the evolutionary relationship between centra and arches, and their varying modes of skeletal mineralization, we can better appreciate the detailed mechanisms that regulate and diversify vertebral patterning.

Paraxis is required for somite morphogenesis and differentiation in Xenopus laevis

BACKGROUND

In most vertebrates, the segmentation of the paraxial mesoderm involves the formation of metameric units called somites through a mesenchymal-epithelial transition. However, this process is different in Xenopus laevis because it does not form an epithelial somite. Xenopus somitogenesis is characterized by a complex cells rearrangement that requires the coordinated regulation of cell shape, adhesion, and motility. The molecular mechanisms that control these cell behaviors underlying somite formation are little known. Although the Paraxis has been implicated in the epithelialization of somite in chick and mouse, its role in Xenopus somite morphogenesis has not been determined.

RESULTS

Using a morpholino and hormone-inducible construction approaches, we showed that both gain and loss of function of paraxis affect somite elongation, rotation and alignment, causing a severe disorganization of somitic tissue. We further found that depletion or overexpression of paraxis in the somite led to the downregulation or upregulation, respectively, of cell adhesion expression markers. Finally, we demonstrated that paraxis is necessary for the proper expression of myotomal and sclerotomal differentiation markers.

CONCLUSIONS

Our results demonstrate that paraxis regulates the cell rearrangements that take place during the somitogenesis of Xenopus by regulating cell adhesion. Furthermore, paraxis is also required for somite differentiation.

MBTPS1/SKI-1/S1P proprotein convertase is required for ECM signaling and axial elongation during somitogenesis and vertebral development†

Caudal regression syndrome (sacral agenesis), which impairs development of the caudal region of the body, occurs with a frequency of about 2 live births per 100 000 newborns although this incidence rises to 1 in 350 infants born to mothers with gestational diabetes. The lower back and limbs can be affected as well as the genitourinary and gastrointestinal tracts. The axial skeleton is formed during embryogenesis through the process of somitogenesis in which the paraxial mesoderm periodically segments into bilateral tissue blocks, called somites. Somites are the precursors of vertebrae and associated muscle, tendons and dorsal dermis. Vertebral anomalies in caudal regression syndrome may arise through perturbation of somitogenesis or, alternatively, could result from defective bone formation and patterning. We discovered that MBTPS1/SKI-1/S1P, which proteolytically activates a class of transmembrane transcription factors, plays a critical role in somitogenesis and the pathogenesis of lumbar/sacral vertebral anomalies. Conditional deletion of Mbtps1 yields a viable mouse with misshapen, fused and reduced number of lumbar and sacral vertebrae, under-developed hind limb bones and a kinky, shortened tail. We show that Mbtps1 is required to (i) maintain the Fgf8 'wavefront' in the presomitic mesoderm that underpins axial elongation, (ii) sustain the Lfng oscillatory 'clock' activity that governs the periodicity of somite formation and (iii) preserve the composition and character of the somitic extracellular matrix containing fibronectin, fibrillin2 and laminin. Based on this spinal phenotype and known functions of MBTPS1, we reason that loss-of-function mutations in Mbtps1 may cause the etiology of caudal regression syndrome.

Follistatin interacts with Noggin in the development of the axial skeleton

When compared to single mutants for Follistatin or Noggin, we find that double mutants display a dramatic further reduction in trunk cartilage formation, particularly in the vertebral bodies and proximal ribs. Consistent with these observations, expression of the early sclerotome markers Pax1 and Uncx is diminished in Noggin;Follistatin compound mutants. In contrast, Sim1 expression expands medially in double mutants. As the onset of Follistatin expression coincides with sclerotome specification, we argue that the effect of Follistatin occurs after sclerotome induction. We hypothesize that Follistatin aids in maintaining proper somite size, and consequently sclerotome progenitor numbers, by preventing paraxial mesoderm from adopting an intermediate/lateral plate mesodermal fate in the Noggin-deficient state.

Multi-detector thoracic CT findings in cerebro-costo-mandibular syndrome: rib gaps and failure of costo-vertebral separation

Cerebro-costo-mandibular syndrome (CCMS) describes a triad of mandibular hypoplasia, brain dysfunction and posterior rib defects ("rib gaps"). We present the CT imaging for a 2-year-old girl with CCMS that highlights the rib gap defects and shows absent transverse processes with abnormal fusion of the ribs directly to the vertebral bodies. We argue that this is likely to relate to abnormal lateral sclerotome development in embryology, with the failure of normal costo-vertebral junctions compounding impaired thoracic function. The case also highlights the use of CT for specific indications in skeletal dysplasia.

Two-color in situ hybridization of whole-mount mouse embryos

RNA in situ hybridization is a powerful technique used to identify the spatial localization of a specific RNA in a tissue section or whole tissue. In this protocol, we describe a reliable method for two-color in situ hybridization that can be used to accurately assess the expression of multiple genes with contrasting or overlapping expression patterns in whole mouse embryos.

Metameric pattern of intervertebral disc/vertebral body is generated independently of Mesp2/Ripply-mediated rostro-caudal patterning of somites in the mouse embryo

The vertebrae are derived from the sclerotome of somites. Formation of the vertebral body involves a process called resegmentation, by which the caudal half of a sclerotome is combined with the rostral half of the next sclerotome. To elucidate the relationship between resegmentation and rostro-caudal patterning of somite, we used the Uncx4.1-LacZ transgene to characterize the resegmentation process. Our observations suggested that in the thoracic and lumbar vertebrae, the Uncx4.1-expressing caudal sclerotome gave rise to the intervertebral disc (IVD) and rostral portion of the vertebral body (VB). In the cervical vertebrae, the Uncx4.1-expressing caudal sclerotome appeared to contribute to the IVD and both caudal and rostral ends of the VB. This finding suggests that the rostro-caudal gene expression boundary does not necessarily coincide with the resegmentation boundary. This conclusion was supported by analyses of Mesp2 KO and Ripply1/2 double KO embryos lacking rostral and caudal properties, respectively. Resegmentation was not observed in Mesp2 KO embryos, but both the IVD and whole VB were formed from the caudalized sclerotome. Expression analysis of IVD marker genes including Pax1 in the wild-type, Mesp2 KO, and Ripply1/2 DKO embryos also supported the idea that a metameric pattern of IVD/VB is generated independently of Mesp2/Ripply-mediated rostro-caudal patterning of somite. However, in the lumbar region, IVD differentiation appeared to be stimulated by the caudal property and suppressed by the rostral property. Therefore, we propose that rostro-caudal patterning of somites is not a prerequisite for metameric patterning of the IVD and VB, but instead required to stimulate IVD differentiation in the caudal half of the sclerotome.

Dynamic CREB family activity drives segmentation and posterior polarity specification in mammalian somitogenesis

The segmented body plan of vertebrates is prefigured by reiterated embryonic mesodermal structures called somites. In the mouse embryo, timely somite formation from the presomitic mesoderm (PSM) is controlled by the "segmentation clock," a molecular oscillator that triggers progressive waves of Notch activity throughout the PSM. Notch clock activity is suppressed in the posterior PSM by FGF signaling until it crosses a determination front at which its net activity is sufficiently high to effect segmentation. Here, Notch and Wnt signaling directs somite anterior/posterior (A/P) polarity specification and boundary formation via regulation of the segmentation effector gene Mesoderm posterior 2. How Notch and Wnt signaling becomes coordinated at this front is incompletely defined. Here we show that the activity of the cAMP responsive element binding protein (CREB) family of transcription factors exhibits Wnt3a-dependent oscillatory behavior near the determination front and is in unison with Notch activity. Inhibition of CREB family in the mesoderm causes defects in somite segmentation and a loss in somite posterior polarity leading to fusions of vertebrae and ribs. Among the CREB family downstream genes, several are known to be regulated by Wnt3a. Of those, we show that the CREB family occupies a conserved binding site in the promoter region of Delta-like 1, encoding a Notch ligand, in the anterior PSM as a mechanism to specify posterior identity of somites. Together, these data support that the CREB family acts at the determination front to modulate Wnt signaling and strengthen Notch signaling as a means to orchestrate cells for somite segmentation and anterior/posterior patterning.

Characterization of pax1, pax9, and uncx sclerotomal genes during Xenopus laevis embryogenesis

BACKGROUND

The axial skeleton develops from the sclerotome, a mesenchymal cell population derived from somites. Sclerotomal cells migrate from somites to the perinotochordal and perineural space where they differentiate into chondrocytes to form cartilage and bone. In anurans, little is known about the way how the sclerotome changes as development proceeds and how these events are regulated at the molecular level. Pax1, Pax9, and Uncx4.1 genes play a central role in the morphogenesis of the axial skeleton in vertebrates, regulating cell proliferation and chondrogenic specification of the sclerotome.

RESULTS

In this work, we cloned and examined through whole-mount in situ hybridization and reverse transcriptase-polymerase chain reaction the expression patterns of pax1, pax9, and uncx transcription factors in the anuran Xenopus laevis.

CONCLUSIONS

We found that these genes are similarly expressed in the sclerotome and in the pharyngeal pouch. A detailed analysis of the location of these transcripts showed that they are expressed in different subdomains of the sclerotomal compartment and differ from that observed in other vertebrates.

The transcription factor Uncx4.1 acts in a short window of midbrain dopaminergic neuron differentiation

Background

The homeobox containing transcription factor Uncx4.1 is, amongst others, expressed in the mouse midbrain. The early expression of this transcription factor in the mouse, as well as in the chick midbrain, points to a conserved function of Uncx4.1, but so far a functional analysis in this brain territory is missing. The goal of the current study was to analyze in which midbrain neuronal subgroups Uncx4.1 is expressed and to examine whether this factor plays a role in the early development of these neuronal subgroups.

Results

We have shown that Uncx4.1 is expressed in GABAergic, glutamatergic and dopaminergic neurons in the mouse midbrain. In midbrain dopaminergic (mDA) neurons Uncx4.1 expression is particularly high around E11.5 and strongly diminished already at E17.5. The analysis of knockout mice revealed that the loss of Uncx4.1 is accompanied with a 25% decrease in the population of mDA neurons, as marked by tyrosine hydroxylase (TH), dopamine transporter (DAT), Pitx3 and Ngn2. In contrast, the number of glutamatergic Pax6-positive cells was augmented, while the GABAergic neuron population appears not affected in Uncx4.1-deficient embryos.

Conclusion

We conclude that Uncx4.1 is implicated in the development of mDA neurons where it displays a unique temporal expression profile in the early postmitotic stage. Our data indicate that the mechanism underlying the role of Uncx4.1 in mDA development is likely related to differentiation processes in postmitotic stages, and where Ngn2 is engaged. Moreover, Uncx4.1 might play an important role during glutamatergic neuronal differentiation in the mouse midbrain.

Notch signaling and the developing skeleton

Notch signaling is an important regulator of skeletogenesis at multiple developmental stages. The Notch signaling pathway is involved in the promotion of somite segmentation, patterning and differentiation into sclerotome pre-chondrogenic cells to allow for appropriate axial skeleton development. In addition, studies performed in vitro and in vivo demonstrate that Notch signaling suppresses chondrogenic and osteoblastic differentiation and negatively regulates osteoclast formation and proliferation. Through the use of in vitro and in vivo approaches, Notch signaling has been shown to regulate somitogenesis, chondrogenesis, osteoblastogenesis and osteoclastogenesis that ultimately affect skeletogenesis. Dysregulation of Notch signaling results in congenital skeletal malformations that could reveal therapeutic potential.

α5β1 integrin-mediated adhesion to fibronectin is required for axis elongation and somitogenesis in mice

The arginine-glycine-aspartate (RGD) motif in fibronectin (FN) represents the major binding site for α5β1 and αvβ3 integrins. Mice lacking a functional RGD motif in FN (FN^RGE/RGE) or α5 integrin develop identical phenotypes characterized by embryonic lethality and a severely shortened posterior trunk with kinked neural tubes. Here we show that the FN^RGE/RGE embryos arrest both segmentation and axis elongation. The arrest is evident at about E9.0, corresponding to a stage when gastrulation ceases and the tail bud-derived presomitic mesoderm (PSM) induces α5 integrin expression and assumes axis elongation. At this stage cells of the posterior part of the PSM in wild type embryos are tightly coordinated, express somitic oscillator and cyclic genes required for segmentation, and form a tapered tail bud that extends caudally. In contrast, the posterior PSM cells in FN^RGE/RGE embryos lost their tight associations, formed a blunt tail bud unable to extend the body axis, failed to induce the synchronised expression of Notch1 and cyclic genes and cease the formation of new somites. Mechanistically, the interaction of PSM cells with the RGD motif of FN is required for dynamic formation of lamellipodia allowing motility and cell-cell contact formation, as these processes fail when wild type PSM cells are seeded into a FN matrix derived from FN^RGE/RGE fibroblasts. Thus, α5β1-mediated adhesion to FN in the PSM regulates the dynamics of membrane protrusions and cell-to-cell communication essential for elongation and segmentation of the body axis.

Cooperative activity of noggin and gremlin 1 in axial skeleton development

Inductive signals from adjacent tissues initiate differentiation within the somite. In this study, we used mouse embryos mutant for the BMP antagonists noggin (Nog) and gremlin 1 (Grem1) to characterize the effects of BMP signaling on the specification of the sclerotome. We confirmed reduction of Pax1 and Pax9 expression in Nog mutants, but found that Nog;Grem1 double mutants completely fail to initiate sclerotome development. Furthermore, Nog mutants that also lack one allele of Grem1 exhibit a dramatic reduction in axial skeleton relative to animals mutant for Nog alone. By contrast, Pax3, Myf5 and Lbx1 expression indicates that dermomyotome induction occurs in Nog;Grem1 double mutants. Neither conditional Bmpr1a mutation nor treatment with the BMP type I receptor inhibitor dorsomorphin expands sclerotome marker expression, suggesting that BMP antagonists do not have an instructive function in sclerotome specification. Instead, we hypothesize that Nog- and Grem1-mediated inhibition of BMP is permissive for hedgehog (Hh) signal-mediated sclerotome specification. In support of this model, we found that culturing Nog;Grem1 double-mutant embryos with dorsomorphin restores sclerotome, whereas Pax1 expression in smoothened (Smo) mutants is not rescued, suggesting that inhibition of BMP is insufficient to induce sclerotome in the absence of Hh signaling. Confirming the dominant inhibitory effect of BMP signaling, Pax1 expression cannot be rescued in Nog;Grem1 double mutants by forced activation of Smo. We conclude that Nog and Grem1 cooperate to maintain a BMP signaling-free zone that is a crucial prerequisite for Hh-mediated sclerotome induction.

Uncx regulates proliferation of neural progenitor cells and neuronal survival in the olfactory epithelium

Uncx (Phd1, Chx4) is a paired homeobox transcription factor gene. It and its probable functional partners, Tle co-repressors, were expressed by neurally-fated basal progenitor cells and olfactory sensory neurons of the olfactory epithelium. Uncx expression was rare in olfactory epithelia of Ascl1(-/-) mice, but common in Neurog1(-/-) mice. In Uncx(-/-) mice olfactory progenitor cell proliferation, progenitor cell number, olfactory sensory neuron survival, and Umodl1 and Kcnc4 mRNAs were reduced. Evidence of sensory neuron activity and functional connections to the olfactory bulb argue that decreased neuronal survival was not due to loss of trophic support or activity-dependent mechanisms. These data suggest that UNCX acts downstream of neural determination factors to broadly control transcriptional mechanisms used by neural progenitor cells to specify neural phenotypes.

Division of labor during trunk neural crest development

Neural crest cells, the migratory precursors of numerous cell types including the vertebrate peripheral nervous system, arise in the dorsal neural tube and follow prescribed routes into the embryonic periphery. While the timing and location of neural crest migratory pathways has been well documented in the trunk, a comprehensive collection of signals that guides neural crest migration along these paths has only recently been established. In this review, we outline the molecular cascade of events during trunk neural crest development. After describing the sequential routes taken by trunk neural crest cells, we consider the guidance cues that pattern these neural crest trajectories. We pay particular attention to segmental neural crest development and the steps and signals that generate a metameric peripheral nervous system, attempting to reconcile conflicting observations in chick and mouse. Finally, we compare cranial and trunk neural crest development in order to highlight common themes.

Caudal regression in adrenocortical dysplasia (acd) mice is caused by telomere dysfunction with subsequent p53-dependent apoptosis

Adrenocortical dysplasia (acd) is a spontaneous autosomal recessive mouse mutation that exhibits a pleiotropic phenotype with perinatal lethality. Mutant acd embryos have caudal truncation, vertebral segmentation defects, hydronephrosis, and limb hypoplasia, resembling humans with Caudal Regression syndrome. Acd encodes Tpp1, a component of the shelterin complex that maintains telomere integrity, and consequently acd mutant mice have telomere dysfunction and genomic instability. While the association between genomic instability and cancer is well documented, the association between genomic instability and birth defects is unexplored. To determine the relationship between telomere dysfunction and embryonic malformations, we investigated mechanisms leading to the caudal dysgenesis phenotype of acd mutant embryos. We report that the caudal truncation is caused primarily by apoptosis, not altered cell proliferation. We show that the apoptosis and consequent skeletal malformations in acd mutants are dependent upon the p53 pathway by genetic rescue of the limb hypoplasia and vertebral anomalies with p53 null mice. Furthermore, rescue of the acd phenotype by p53 deficiency is a dosage-sensitive process, as acd/acd, p53(-/-) double mutants exhibit preaxial polydactyly. These findings demonstrate that caudal dysgenesis in acd embryos is secondary to p53-dependent apoptosis. Importantly, this study reinforces a significant link between genomic instability and birth defects.

Lack of the mesodermal homeodomain protein MEOX1 disrupts sclerotome polarity and leads to a remodeling of the cranio-cervical joints of the axial skeleton

Meox1 and Meox2 are two related homeodomain transcription factor genes that together are essential for the development of all somite compartments. Here we show that mice homozygous for Meox1 mutations alone have abnormalities that are restricted to the sclerotome and its derivatives. A prominent and consistent phenotype of these mutations is a remodeling of the cranio-cervical joints whose major feature is the assimilation of the atlas into the basioccipital bone so that the skull rests on the axis. These abnormalities can be traced back to changes in the relative rates of cell proliferation in the rostral and caudal sclerotome compartments, and they are associated with alterations in the expression of at least three transcription factor genes, Tbx18, Uncx, and Bapx1. As previously observed for Bapx1, MEOX1 protein occupies evolutionarily conserved promoter regions of Tbx18 and Uncx, suggesting that Meox1 regulates these genes at least in part directly. Hence, Meox1 is part of a regulatory circuit that serves an essential, non-redundant function in the maintenance of rostro-caudal sclerotome polarity and axial skeleton formation.

An evo-devo view on the origin of the backbone: evolutionary development of the vertebrae

Vertebral columns are a group of diverse axial structures that define the vertebrates and provide supportive, locomotive, protective, and other important functions. The embryonic origin of the first vertebral element in this subphylum, the lamprey arcualia, has remained a puzzle for more than a century although much developmental and genetic progress has been made. The comparative approach is a very powerful tool for studying vertebrate morphological variation and understanding how the novel structures were generated during evolution. Here, I first briefly describe the vertebral structures and their developmental processes in major taxa, and then analyze the most recently published data on the basal vertebrates. Finally, an ontogenetic and phylogenetic origin is proposed. The lamprey may have already evolved a sclerotome, which gave rise to arcualia ontogenetically; whole genome duplications likely promoted the establishment of sclerotomal core genetic program by gene co-options.

Extensive molecular differences between anterior- and posterior-half-sclerotomes underlie somite polarity and spinal nerve segmentation

Background

The polarization of somite-derived sclerotomes into anterior and posterior halves underlies vertebral morphogenesis and spinal nerve segmentation. To characterize the full extent of molecular differences that underlie this polarity, we have undertaken a systematic comparison of gene expression between the two sclerotome halves in the mouse embryo.

Results

Several hundred genes are differentially-expressed between the two sclerotome halves, showing that a marked degree of molecular heterogeneity underpins the development of somite polarity.

Conclusion

We have identified a set of genes that warrant further investigation as regulators of somite polarity and vertebral morphogenesis, as well as repellents of spinal axon growth. Moreover the results indicate that, unlike the posterior half-sclerotome, the central region of the anterior-half-sclerotome does not contribute bone and cartilage to the vertebral column, being associated instead with the development of the segmented spinal nerves.

Interpreting neonatal lethal phenotypes in mouse mutants: insights into gene function and human diseases

The mouse represents the model of choice to study the biological function of mammalian genes through mutation of its genome. However, the biggest challenge of mouse geneticists remains the phenotypic analysis of mouse mutants. A survey of mouse mutant databases reveals a surprisingly high number of gene mutations leading to neonatal death. These genetically modified mouse mutants have been instrumental in elucidating gene function and have become important models of congenital human diseases. The main complication when phenotyping mutant mice dying during the neonatal period is the large spectrum of physiological systems whose defects can challenge neonatal survival. Here, we present a comprehensive review of gene mutations leading to neonatal lethality and discuss the impact of these mutations on the major physiological processes critical to mouse newborn survival: parturition, breathing, suckling, and homeostasis. Selected examples of mouse mutants are highlighted to illustrate how the precise identification of the timing and cause of death associated with these physiological processes allows for a more profound understanding of the underlying cellular and molecular defects. This review provides a guide for the analysis of neonatal lethal phenotypes in mutant mice that will be helpful for dissecting out the function of specific genes during mouse development.

Progress in the understanding of the genetic etiology of vertebral segmentation disorders in humans

Vertebral malformations contribute substantially to the pathophysiology of kyphosis and scoliosis, common health problems associated with back and neck pain, disability, cosmetic disfigurement, and functional distress. This review explores (1) recent advances in the understanding of the molecular embryology underlying vertebral development and relevance to elucidation of etiologies of several known human vertebral malformation syndromes; (2) outcomes of molecular studies elucidating genetic contributions to congenital and sporadic vertebral malformation; and (3) complex interrelationships between genetic and environmental factors that contribute to the pathogenesis of isolated syndromic and nonsyndromic congenital vertebral malformation. Discussion includes exploration of the importance of establishing improved classification systems for vertebral malformation, future directions in molecular and genetic research approaches to vertebral malformation, and translational value of research efforts to clinical management and genetic counseling of affected individuals and their families.

T-box protein Tbx18 interacts with the paired box protein Pax3 in the development of the paraxial mesoderm

The compartmentalization of somites along their anterior-posterior axis is crucial to the segmental organization of the vertebral column. Anterior-posterior somite polarity is generated in the anterior presomitic mesoderm by Mesp2 and Delta/Notch signaling and is further maintained by two transcriptional regulators, Uncx4.1 and Tbx18, acting in the posterior and anterior somite compartment, respectively. Here, we report that the paired box transcription factor Pax3 cooperates with the T-box protein Tbx18 in maintaining anterior somite half identity. Our findings that both genes are co-expressed in the anterior presomitic mesoderm and in early somites, that Pax3 and Tbx18 proteins physically interact, and that the loss of Pax3 gene function enhances the vertebral defects (i.e. the gain of vertebral elements derived from posterior somite halves in Tbx18 mutant mice) suggests that the two proteins cooperatively regulate the gene expression program necessary for maintaining anterior-posterior somite polarity. Genetic interaction of Pax3 with Tbx18 and the closely related T-box gene Tbx15 was also observed in the development of the scapula blade, indicating an additional cooperative function for these genes in the paraxial mesoderm.

Mouse Ripply2 is downstream of Wnt3a and is dynamically expressed during somitogenesis

Somites are blocks of mesoderm that form when segment boundaries are periodically generated in the anterior presomitic mesoderm (PSM). Periodicity is thought to be driven by an oscillating Notch-centered segmentation clock, whereas boundaries are spatially positioned by the secreted signaling molecules Wnt3a and Fgf8. We identified the putative transcriptional corepressor Ripply2 as a differentially expressed gene in wild-type and Wnt3a(-/-) embryos. Here, we show that Ripply2 is expressed in the anterior PSM and that it indeed lies downstream of Wnt3a. Dynamic Ripply2 expression in prospective somites S0 and S-I overlaps with the rostral expression of cycling genes in the Notch pathway, suggesting that Ripply2 may be controlled by the segmentation clock. Continued expression of Ripply2 in embryos lacking Hes7, a molecular oscillator in the Notch clock, indicates that Hes7 is not a major regulator of Ripply2. Our data are consistent with Ripply2 functioning as a segment boundary determination gene during mammalian embryogenesis. Developmental

Transcription factors Mesp2 and Paraxis have critical roles in axial musculoskeletal formation

Mesp2 and Paraxis are basic helix-loop-helix (bHLH) -type transcription factors coexpressed in the presomitic mesoderm (PSM) and are required for normal somite formation. Here, we show that Mesp2/Paraxis double-null mice exhibit a distinct phenotype unexpected from either Mesp2 or Paraxis single-null mice. In the posterior region of the body, most of the skeletal components of both the vertebral body and neural arches are severely reduced and only a rudimental lamina and ribs remain, indicating a strong genetic interaction in the sclerotomal cell lineage. However, yeast two-hybrid analyses revealed no direct interaction between Mesp2 and Paraxis. The Mesp2/Paraxis double-null embryo has caudalized somites, revealed by expanded Uncx4.1 expression pattern observed in the Mesp2-null embryo, but the expression level of Uncx4.1 was significantly decreased in mature somites, indicative of hypoplasia of lateral sclerotome derivatives. By focusing on vertebral column formation, we found that expressions of Pax1, Nkx3.1, and Bapx1 are regulated by Paraxis and that Pax9 expression was severely affected in the Mesp2/Paraxis double-null embryo. Furthermore, the expression of Pax3, a crucial factor for hypaxial muscle differentiation, is regulated by both Mesp2 and Paraxis in the anteriormost PSM and nascent somite region. The present data strongly suggest that patterning events by bHLH-type transcription factors have deep impacts on regional chondrogenic and myogenic differentiation of somitic cells, mainly by means of control of Pax genes.

Transcriptional Repression by the T-box Proteins Tbx18 and Tbx15 Depends on Groucho Corepressors

Tbox18 (Tbx18) and Tbox15 (Tbx15) encode a closely related pair of vertebrate-specific T-box (Tbx) transcription factors. Functional analyses in the mouse have proven the requirement of Tbx15 in skin and skeletal development and of Tbx18 in the formation of the vertebral column, the ureter, and the posterior pole of the heart. Despite the accumulation of genetic data concerning the embryological roles of these genes, it is currently unclear how Tbx18 and Tbx15 exert their function on the molecular level. Here, we have initiated a molecular analysis of Tbx18 and Tbx15 proteins and have characterized functional domains for nuclear localization, DNA binding, and transcriptional modulation. We show that both proteins homo- and heterodimerize, bind to various combinations of T half-sites, and repress transcription in a Groucho-dependent manner. Competition with activating T-box proteins may constitute one mode of action as we show that Tbx18 interacts with Gata4 and Nkx2-5 and competes Tbx5-mediated activation of the cardiac Natriuretic peptide precursor type a-promoter and that ectopic expression of Tbx18 down-regulates Tbx6-activated Delta-like 1 expression in the somitic mesoderm in vivo.

Chick Hairy1 protein interacts with Sap18, a component of the Sin3/HDAC transcriptional repressor complex

BACKGROUND

The vertebrate adult axial skeleton, trunk and limb skeletal muscles and dermis of the back all arise from early embryonic structures called somites. Somites are symmetrically positioned flanking the embryo axial structures (neural tube and notochord) and are periodically formed in a anterior-posterior direction from the presomitic mesoderm. The time required to form a somite pair is constant and species-specific. This extraordinary periodicity is proposed to depend on an underlying somitogenesis molecular clock, firstly evidenced by the cyclic expression of the chick hairy1 gene in the unsegmented presomitic mesoderm with a 90 min periodicity, corresponding to the time required to form a somite pair in the chick embryo. The number of hairy1 oscillations at any given moment is proposed to provide the cell with both temporal and positional information along the embryo's anterior-posterior axis. Nevertheless, how this is accomplished and what biological processes are involved is still unknown. Aiming at understanding the molecular events triggered by the somitogenesis clock Hairy1 protein, we have employed the yeast two-hybrid system to identify Hairy1 interaction partners.

RESULTS

Sap18, an adaptor molecule of the Sin3/HDAC transcriptional repressor complex, was found to interact with the C-terminal portion of the Hairy1 protein in a yeast two-hybrid assay and the Hairy1/Sap18 interaction was independently confirmed by co-immunoprecipitation experiments. We have characterized the expression patterns of both sap18 and sin3a genes during chick embryo development, using in situ hybridization experiments. We found that both sap18 and sin3a expression patterns co-localize in vivo with hairy1 expression domains in chick rostral presomitic mesoderm and caudal region of somites.

CONCLUSION

Hairy1 belongs to the hairy-enhancer-of-split family of transcriptional repressor proteins. Our results indicate that during chick somitogenesis Hairy1 may mediate gene transcriptional repression by recruiting the Sin3/HDAC complex, through a direct interaction with the Sap18 adaptor molecule. Moreover, since sap18 and sin3a are not expressed in the PSM territory where hairy1 presents cyclic expression, our study strongly points to different roles for Hairy1 throughout the PSM and in the prospective somite and caudal region of already formed somites.