BMP receptor type IA in limb bud mesenchyme regulates distal outgrowth and patterning

Single-cell profiling of penta- and tetradactyl mouse limb buds identifies mesenchymal progenitors controlling digit numbers and identities

The cellular interactions controlling digit numbers and identities have remained largely elusive. Here, we leverage the anterior digit and identity loss in Grem1 tetradactyl mouse limb buds to identify early specified limb bud mesenchymal progenitor (LMP) populations whose size and distribution is governed by spatial modulation of BMP activity and SHH signaling. Distal-autopodial LMPs (dLMP) express signature genes required for autopod and digit development, and alterations affecting the dLMP population size prefigure the changes in digit numbers that characterize specific congenital malformations. A second, peripheral LMP (pLMP) population is anteriorly biased and reduction/loss of its asymmetric distribution underlies the loss of middle digit asymmetry and identities in Grem1 tetradactyl and pig limb buds. pLMPs depend on BMP activity, while dLMPs require GREM1-mediated BMP antagonism. Taken together, the spatial alterations in GREM1 antagonism in mouse mutant and evolutionarily diversified pig limb buds tunes BMP activity, which impacts dLMP and pLMP populations in an opposing manner.

Creb5 coordinates synovial joint formation with the genesis of articular cartilage

While prior work has established that articular cartilage arises from Prg4-expressing perichondrial cells, it is not clear how this process is specifically restricted to the perichondrium of synovial joints. We document that the transcription factor Creb5 is necessary to initiate the expression of signaling molecules that both direct the formation of synovial joints and guide perichondrial tissue to form articular cartilage instead of bone. Creb5 promotes the generation of articular chondrocytes from perichondrial precursors in part by inducing expression of signaling molecules that block a Wnt5a autoregulatory loop in the perichondrium. Postnatal deletion of Creb5 in the articular cartilage leads to loss of both flat superficial zone articular chondrocytes coupled with a loss of both Prg4 and Wif1 expression in the articular cartilage; and a non-cell autonomous up-regulation of Ctgf. Our findings indicate that Creb5 promotes joint formation and the subsequent development of articular chondrocytes by driving the expression of signaling molecules that both specify the joint interzone and simultaneously inhibit a Wnt5a positive-feedback loop in the perichondrium.

SMAD4 target genes are part of a transcriptional network that integrates the response to BMP and SHH signaling during early limb bud patterning

SMAD4 regulates gene expression in response to BMP and TGFβ signal transduction, and is required for diverse morphogenetic processes, but its target genes have remained largely elusive. Here, we identify the SMAD4 target genes in mouse limb buds using an epitope-tagged Smad4 allele for ChIP-seq analysis in combination with transcription profiling. This analysis shows that SMAD4 predominantly mediates BMP signal transduction during early limb bud development. Unexpectedly, the expression of cholesterol biosynthesis enzymes is precociously downregulated and intracellular cholesterol levels are reduced in Smad4-deficient limb bud mesenchymal progenitors. Most importantly, our analysis reveals a predominant function of SMAD4 in upregulating target genes in the anterior limb bud mesenchyme. Analysis of differentially expressed genes shared between Smad4- and Shh-deficient limb buds corroborates this function of SMAD4 and also reveals the repressive effect of SMAD4 on posterior genes that are upregulated in response to SHH signaling. This analysis uncovers opposing trans-regulatory inputs from SHH- and SMAD4-mediated BMP signal transduction on anterior and posterior gene expression during the digit patterning and outgrowth in early limb buds.

Summary: The transcriptional targets of SMAD4 in early limb buds are identified and the largely opposing impact of BMP and SHH signaling on early digit patterning and outgrowth is revealed.

Neural control of growth and size in the axolotl limb regenerate

The mechanisms that regulate growth and size of the regenerating limb in tetrapods such as the Mexican axolotl are unknown. Upon the completion of the developmental stages of regeneration, when the regenerative organ known as the blastema completes patterning and differentiation, the limb regenerate is proportionally small in size. It then undergoes a phase of regeneration that we have called the ‘tiny-limb’ stage, which is defined by rapid growth until the regenerate reaches the proportionally appropriate size. In the current study we have characterized this growth and have found that signaling from the limb nerves is required for its maintenance. Using the regenerative assay known as the accessory limb model (ALM), we have found that growth and size of the limb positively correlates with nerve abundance. We have additionally developed a new regenerative assay called the neural modified-ALM (NM-ALM), which decouples the source of the nerves from the regenerating host environment. Using the NM-ALM we discovered that non-neural extrinsic factors from differently sized host animals do not play a prominent role in determining the size of the regenerating limb. We have also discovered that the regulation of limb size is not autonomously regulated by the limb nerves. Together, these observations show that the limb nerves provide essential cues to regulate ontogenetic allometric growth and the final size of the regenerating limb.

Humans’ ability to regrow lost or damaged body parts is relatively limited, but some animals, such as the axolotl (a Mexican salamander), can regenerate complex body parts, like legs, many times over their lives. Studying regeneration in these animals could help researchers enhance humans’ abilities to heal. One way to do this is using the Accessory Limb Model (ALM), where scientists wound an axolotl’s leg, and study the additional leg that grows from the wound.

The first stage of limb regeneration creates a new leg that has the right structure and shape. The new leg is very small so the next phase involves growing the leg until its size matches the rest of the animal. This phase must be controlled so that the limb stops growing when it reaches the right size, but how this regulation works is unclear. Previous research suggests that the number of nerves in the new leg could be important.

Wells et al. used a ALM to study how the size of regenerating limbs is controlled. They found that changing the number of nerves connected to the new leg altered its size, with more nerves leading to a larger leg. Next, Wells et al. created a system that used transplanted nerve bundles of different sizes to grow new legs in different sized axolotls. This showed that the size of the resulting leg is controlled by the number of nerves connecting it to the CNS. Wells et al. also showed that nerves can only control regeneration if they remain connected to the central nervous system.

These results explain how size is controlled during limb regeneration in axolotls, highlighting the fact that regrowth is directly controlled by the number of nerves connected to a regenerating leg. Much more work is needed to reveal the details of this process and the signals nerves use to control growth. It will also be important to determine whether this control system is exclusive to axolotls, or whether other animals also use it.

The tissues and regulatory pattern of limb chondrogenesis

Initial limb chondrogenesis offers the first differentiated tissues that resemble the mature skeletal anatomy. It is a developmental progression of three tissues. The limb begins with undifferentiated mesenchyme-1, some of which differentiates into condensations-2, and this tissue then transforms into cartilage-3. Each tissue is identified by physical characteristics of cell density, shape, and extracellular matrix composition. Tissue specific regimes of gene regulation underlie the diagnostic physical and chemical properties of these three tissues. These three tissue based regimes co-exist amid a background of other gene regulatory regimes within the same tissues and time-frame of limb development. The bio-molecular indicators of gene regulation reveal six identifiable patterns. Three of these patterns describe the unique bio-molecular indicators of each of the three tissues. A fourth pattern shares bio-molecular indicators between condensation and cartilage. Finally, a fifth pattern is composed of bio-molecular indicators that are found in undifferentiated mesenchyme prior to any condensation differentiation, then these bio-molecular indicators are upregulated in condensations and downregulated in undifferentiated mesenchyme. The undifferentiated mesenchyme that remains in between the condensations and cartilage, the interdigit, contains a unique set of bio-molecular indicators that exhibit dynamic behaviour during chondrogenesis and therefore argue for its own inclusion as a tissue in its own right and for more study into this process of differentiation.

The benefits differential equations bring to limb development

Systems biology is a large field, offering a number of advantages to a variety of biological disciplines. In limb development, differential-equation based models can provide insightful hypotheses about the gene/protein interactions and tissue differentiation events that form the core of limb development research. Differential equations are like any other communicative tool, with misuse and limitations that can come along with their advantages. Every theory should be critically analyzed to best ascertain whether they reflect the reality in biology as well they claim. Differential equation-based models have consistent features which researchers have drawn upon to aid in more realistic descriptions and hypotheses. Nine features are described that highlight these trade-offs. The advantages range from more detailed descriptions of gene interactions and their consequence and the capacity to model robustness to the incorporation of tissue size and shape. The drawbacks come with the added complication that additional genes and signaling pathways that require additional terms within the mathematical model. They also come in the translation between the mathematical terms of the model, values and matrices, to the real world of genes, proteins, and tissues that constitute limb development. A critical analysis is necessary to ensure that these models effectively expand the understanding of the origins of a diversity of limb anatomy, from evolution to teratology. This article is categorized under: Vertebrate Organogenesis > Musculoskeletal and Vascular Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Establishment of Spatial and Temporal Patterns > Repeating Patterns and Lateral Inhibition.

Developmental mutant mouse models for external genitalia formation

Development of external genitalia and perineum is the subject of developmental biology as well as toxicology and teratology researches. Cloaca forms in the lower (caudal) end of endoderm. Such endodermal epithelia and surrounding mesenchyme interact with various signals to form the external genitalia. External genitalia (the anlage termed as genital tubercle: GT) formation shows prominent sexually dimorphic morphogenesis in late embryonic stages, which is an unexplored developmental research field because of many reasons. External genitalia develop adjacent to the cloaca which develops urethra and corporal bodies. Developmental regulators including growth factor signals are necessary for epithelia-mesenchyme interaction (EMI) in posterior embryos including the cloaca and urethra in the genitalia. In the case of male type urethra, formation of tubular urethra proceeds from the lower (ventral) side of external genitalia as a masculinization process in contrast to the case of female urethra. Mechanisms for its development are not elucidated yet due to the lack of suitable mutant mouse models. Because of the recent progresses of Cre (recombinase)-mediated conditional target gene modification analyses, many developmental regulatory genes become increasingly analyzed. Conditional gene knockout mouse approaches and tissue lineage approaches are expected to offer vital information for such sexually dimorphic developmental processes. This review aims to offer recent updates on the progresses of these emerging developmental processes for the research field of congenital anomalies.

The BMP Pathway and Its Inhibitors in the Skeleton

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor-β family of ligands. BMPs exhibit widespread utility and pleiotropic, context-dependent effects, and the strength and duration of BMP pathway signaling is tightly regulated at numerous levels via mechanisms operating both inside and outside the cell. Defects in the BMP pathway or its regulation underlie multiple human diseases of different organ systems. Yet much remains to be discovered about the BMP pathway in its original context, i.e., the skeleton. In this review, we provide a comprehensive overview of the intricacies of the BMP pathway and its inhibitors in bone development, homeostasis, and disease. We frame the content of the review around major unanswered questions for which incomplete evidence is available. First, we consider the gene regulatory network downstream of BMP signaling in osteoblastogenesis. Next, we examine why some BMP ligands are more osteogenic than others and what factors limit BMP signaling during osteoblastogenesis. Then we consider whether specific BMP pathway components are required for normal skeletal development, and if the pathway exerts endogenous effects in the aging skeleton. Finally, we propose two major areas of need of future study by the field: greater resolution of the gene regulatory network downstream of BMP signaling in the skeleton, and an expanded repertoire of reagents to reliably and specifically inhibit individual BMP pathway components.

Cellular Complexity at the Interface: Challenges in Enthesis Tissue Engineering

The complex heterogeneous cellular environment found in tendon-to-bone interface makes this structure a challenge for interface tissue engineering. Orthopedic surgeons still face some problems associated with the formation of fibrotic tissue or re-tear occurring after surgical re-attachment of tendons to the bony insertion or the application of grafts. Unfortunately, an understanding of the cellular component of enthesis lags far behind of other well-known musculoskeletal interfaces, which blocks the development of new treatment options for the healing and regeneration of this multifaceted junction. In this chapter, the main characteristics of tendon and bone cell populations are introduced, followed by a brief description of the interfacial cellular niche, highlighting molecular mechanisms governing tendon-to-bone attachment and mineralization. Finally, we describe and critically assess some challenges faced concerning the use of cell-based strategies in tendon-to-bone healing and regeneration.

Spatial and Quantitative Detection of BMP Activity in Mouse Embryonic Limb Buds

Modulation of bone morphogenetic protein (BMP) activity is essential to the progression of limb development in the mouse embryo. Genetic disruption of BMP signaling at various stages of limb development causes defects ranging from complete limb agenesis to oligodactyly, polydactyly, webbing, and chondrodysplasia. To probe the state of BMP signaling in early limb buds, we designed two sets of primers to measure both spatially and quantitatively the transcription of nine key genes indicative of canonical BMP activity. One set is used to generate digoxigenin (DIG)-labeled antisense RNA probes for whole-mount mRNA in situ hybridization, while the second set is used for SYBR^® Green-based quantitative PCR on limb bud cDNA. Here we describe step-by-step protocols for both methods around this specific set of genes.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

Differences of RNA Expression in the Tendon According to Anatomic Outcomes in Rotator Cuff Repair

BACKGROUND

Despite increased understanding of the pathophysiology of rotator cuff tears and the evolution of rotator cuff repair, healing failure remains a substantial problem. The critical roles played by biological factors have been emphasized, but little is known of the implications of gene expression profile differences at the time of repair.

PURPOSE

To document the relationship between the perioperative gene expression of healed and unhealed rotator cuffs by RNA microarray analysis.

STUDY DESIGN

Case-control study; Level of evidence, 3.

METHODS

Superior (supraspinatus involvement) and posterosuperior (supraspinatus and infraspinatus involvement) tears were included in the study. Samples of rotator cuff tendons were prospectively collected during rotator cuff surgery. Three samples were harvested at the tendon ends of tears from the anterior, middle (apex), and posterior parts using an arthroscopic punch. Seven patients with an unhealed rotator cuff were matched one-to-one with patients with a healed rotator cuff by sex, age, tear size, and fatty degeneration of rotator cuff muscles. mRNA microarray analysis was used to identify genetic differences between healed and unhealed rotator cuff tendons. Gene ontology and gene association files were obtained from the Gene Ontology Consortium, and the Gene Ontology system in DAVID was used to identify enhanced biological processes.

RESULTS

Microarray analyses identified 262 genes that were differentially expressed by at least 1.5-fold between the healed and unhealed groups. Overall, in the healed group, 103 genes were significantly downregulated, and 159 were significantly upregulated. DAVID Functional Annotation Cluster analysis showed that in the healed group, the genes most upregulated were related to the G protein-coupled receptor protein signaling pathway and to the neurological system. On the other hand, the genes most downregulated were related to immune and inflammatory responses. BMP5 was the gene most upregulated in the healed group, and the majority of downregulated genes were involved in the immune/inflammatory response.

CONCLUSION

The downregulation of inflammatory response genes and the upregulation of cell differentiation genes in torn rotator cuffs at the time of surgery are related to rotator cuff healing. These results provide useful baseline information for future biological studies on rotator cuff healing.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

BMP signalling in skeletal development, disease and repair

Since the identification in 1988 of bone morphogenetic protein 2 (BMP2) as a potent inducer of bone and cartilage formation, BMP superfamily signalling has become one of the most heavily investigated topics in vertebrate skeletal biology. Whereas a large part of this research has focused on the roles of BMP2, BMP4 and BMP7 in the formation and repair of endochondral bone, a large number of BMP superfamily molecules have now been implicated in almost all aspects of bone, cartilage and joint biology. As modulating BMP signalling is currently a major therapeutic target, our rapidly expanding knowledge of how BMP superfamily signalling affects most tissue types of the skeletal system creates enormous potential to translate basic research findings into successful clinical therapies that improve bone mass or quality, ameliorate diseases of skeletal overgrowth, and repair damage to bone and joints. This Review examines the genetic evidence implicating BMP superfamily signalling in vertebrate bone and joint development, discusses a selection of human skeletal disorders associated with altered BMP signalling and summarizes the status of modulating the BMP pathway as a therapeutic target for skeletal trauma and disease.

Tendon-to-bone attachment: from development to maturity

The attachment between tendon and bone occurs across a complex transitional tissue that minimizes stress concentrations and allows for load transfer between muscles and skeleton. This unique tissue cannot be reconstructed following injury, leading to high incidence of recurrent failure and stressing the need for new clinical approaches. This review describes the current understanding of the development and function of the attachment site between tendon and bone. The embryonic attachment unit, namely, the tip of the tendon and the bone eminence into which it is inserted, was recently shown to develop modularly from a unique population of Sox9- and Scx-positive cells, which are distinct from tendon fibroblasts and chondrocytes. The fate and differentiation of these cells is regulated by transforming growth factor beta and bone morphogenetic protein signaling, respectively. Muscle loads are then necessary for the tissue to mature and mineralize. Mineralization of the attachment unit, which occurs postnatally at most sites, is largely controlled by an Indian hedgehog/parathyroid hormone-related protein feedback loop. A number of fundamental questions regarding the development of this remarkable attachment system require further study. These relate to the signaling mechanism that facilitates the formation of an interface with a gradient of cellular and extracellular phenotypes, as well as to the interactions between tendon and bone at the point of attachment.

BMP signaling mediated by constitutively active Activin type 1 receptor (ACVR1) results in ectopic bone formation localized to distal extremity joints

BMP signaling mediated by ACVR1 plays a critical role for development of multiple structures including the cardiovascular and skeletal systems. While deficient ACVR1 signaling impairs normal embryonic development, hyperactive ACVR1 function (R206H in humans and Q207D mutation in mice, ca-ACVR1) results in formation of heterotopic ossification (HO). We developed a mouse line, which conditionally expresses ca-ACVR1 with Nfatc1-Cre(+) transgene. Mutant mice developed ectopic cartilage and bone at the distal joints of the extremities including the interphalangeal joints and hind limb ankles as early as P4 in the absence of trauma or exogenous bone morphogenetic protein (BMP) administration. Micro-CT showed that even at later time points (up to P40), cartilage and bone development persisted at the affected joints most prominently in the ankle. Interestingly, this phenotype was not present in areas of bone outside of the joints - tibia are normal in mutants and littermate controls away from the ankle. These findings demonstrate that this model may allow for further studies of heterotopic ossification, which does not require the use of stem cells, direct trauma or activation with exogenous Cre gene administration.

Modeling digits. Digit patterning is controlled by a Bmp-Sox9-Wnt Turing network modulated by morphogen gradients

During limb development, digits emerge from the undifferentiated mesenchymal tissue that constitutes the limb bud. It has been proposed that this process is controlled by a self-organizing Turing mechanism, whereby diffusible molecules interact to produce a periodic pattern of digital and interdigital fates. However, the identities of the molecules remain unknown. By combining experiments and modeling, we reveal evidence that a Turing network implemented by Bmp, Sox9, and Wnt drives digit specification. We develop a realistic two-dimensional simulation of digit patterning and show that this network, when modulated by morphogen gradients, recapitulates the expression patterns of Sox9 in the wild type and in perturbation experiments. Our systems biology approach reveals how a combination of growth, morphogen gradients, and a self-organizing Turing network can achieve robust and reproducible pattern formation.

Dynamics of BMP signaling in limb bud mesenchyme and polydactyly

Mutations in the Bone Morphogenetic Protein (BMP) pathway are associated with a range of defects in skeletal formation. Genetic analysis of BMP signaling requirements is complicated by the presence of three partially redundant BMPs that are required for multiple stages of limb development. We generated an inducible allele of a BMP inhibitor, Gremlin, which reduces BMP signaling. We show that BMPs act in a dose and time dependent manner in which early reduction of BMPs result in digit loss, while inhibiting overall BMP signaling between E10.5 and E11.5 allows polydactylous digit formation. During this period, inhibiting BMPs extends the duration of FGF signaling. Sox9 is initially expressed in normal digit ray domains but at reduced levels that correlate with the reduction in BMP signaling. The persistence of elevated FGF signaling likely promotes cell proliferation and survival, inhibiting the activation of Sox9 and secondarily, inhibiting the differentiation of Sox9-expressing chondrocytes. Our results provide new insights into the timing and clarify the mechanisms underlying BMP signaling during digit morphogenesis.

To BMP or not to BMP during vertebrate limb bud development

The analysis of vertebrate limb bud development provides insight of general relevance into the signaling networks that underlie the controlled proliferative expansion of large populations of mesenchymal progenitors, cell fate determination and initiation of differentiation. In particular, extensive genetic analysis of mouse and experimental manipulation of chicken limb bud development has revealed the self-regulatory feedback signaling systems that interlink the main morphoregulatory signaling pathways including BMPs and their antagonists. It this review, we showcase the key role of BMPs and their antagonists during limb bud development. This review provides an understanding of the key morphoregulatory interactions that underlie the highly dynamic changes in BMP activity and signal transduction as limb bud development progresses from initiation and setting-up the signaling centers to determination and formation of the chondrogenic primordia for the limb skeletal elements.

Oncogenic deregulation of NKL homeobox gene MSX1 in mantle cell lymphoma

NKL homeobox gene MSX1 is physiologically expressed during embryonic hematopoiesis. Here, we detected MSX1 overexpression in three examples of mantle cell lymphoma (MCL) and one of acute myeloid leukemia (AML) by screening 96 leukemia/lymphoma cell lines via microarray profiling. Moreover, in silico analysis identified significant overexpression of MSX1 in 3% each of patients with MCL and AML, confirming aberrant activity in subsets of both types of malignancies. Comparative expression profiling analysis and subsequent functional studies demonstrated overexpression of histone acetyltransferase PHF16 together with transcription factors FOXC1 and HLXB9 as activators of MSX1 transcription. Additionally, we identified regulation of cyclin D1/CCND1 by MSX1 and its repressive cofactor histone H1C. Fluorescence in situ hybridization in MCL cells showed that t(11;14)(q13;q32) results in detachment of CCND1 from its corresponding repressive MSX1 binding site. Taken together, we uncovered regulators and targets of homeobox gene MSX1 in leukemia/lymphoma cells, supporting the view of a recurrent genetic network that is reactivated in malignant transformation.

Molecular study of a Hoxa2 gain-of-function in chondrogenesis: a model of idiopathic proportionate short stature

In a previous study using transgenic mice ectopically expressing Hoxa2 during chondrogenesis, we associated the animal phenotype to human idiopathic proportionate short stature. Our analysis showed that this overall size reduction was correlated with a negative influence of Hoxa2 at the first step of endochondral ossification. However, the molecular pathways leading to such phenotype are still unknown. Using protein immunodetection and histological techniques comparing transgenic mice to controls, we show here that the persistent expression of Hoxa2 in chondrogenic territories provokes a general down-regulation of the main factors controlling the differentiation cascade, such as Bapx1, Bmp7, Bmpr1a, Ihh, Msx1, Pax9, Sox6, Sox9 and Wnt5a. These data confirm the impairment of chondrogenic differentiation by Hoxa2 overexpression. They also show a selective effect of Hoxa2 on endochondral ossification processes since Gdf5 and Gdf10, and Bmp4 or PthrP were up-regulated and unmodified, respectively. Since Hoxa2 deregulation in mice induces a proportionate short stature phenotype mimicking human idiopathic conditions, our results give an insight into understanding proportionate short stature pathogenesis by highlighting molecular factors whose combined deregulation may be involved in such a disease.

Attenuation of bone morphogenetic protein signaling during amphibian limb development results in the generation of stage-specific defects

The vertebrate limb is one of the most intensively studied organs in the field of developmental biology. Limb development in tetrapod vertebrates is highly conserved and dependent on the interaction of several important molecular pathways. The bone morphogenetic protein (BMP) signaling cascade is one of these pathways and has been shown to be crucial for several aspects of limb development. Here, we have used a Xenopus laevis transgenic line, in which expression of the inhibitor Noggin is under the control of the heat-shock promoter hsp70 to examine the effects of attenuation of BMP signaling at different stages of limb development. Remarkably different phenotypes were produced at different stages, illustrating the varied roles of BMP in development of the limb. Very early limb buds appeared to be refractory to the effects of BMP attenuation, developing normally in most cases. Ectopic limbs were produced by overexpression of Noggin corresponding to a brief window of limb development at about stage 49/50, as recently described by Christen et al. (2012). Attenuation of BMP signaling in stage 51 or 52 tadpoles lead to a reduction in the number of digits formed, resulting in hypodactyly or ectrodactyly, as well as occasional defects in the more proximal tibia-fibula. Finally, inhibition at stage 54 (paddle stage) led to the formation of dramatically shortened digits resulting from loss of distal phalanges. Transcriptome analysis has revealed the possibility that more Noggin-sensitive members of the BMP family could be involved in limb development than previously suspected. Our analysis demonstrates the usefulness of heat-shock-driven gene expression as an effective method for inhibiting a developmental pathway at different times during limb development.

Dual requirement of ectodermal Smad4 during AER formation and termination of feedback signaling in mouse limb buds

BMP signaling is pivotal for normal limb bud development in vertebrate embryos and genetic analysis of receptors and ligands in the mouse revealed their requirement in both mesenchymal and ectodermal limb bud compartments. In this study, we genetically assessed the potential essential functions of SMAD4, a mediator of canonical BMP/TGFß signal transduction, in the mouse limb bud ectoderm. Msx2-Cre was used to conditionally inactivate Smad4 in the ectoderm of fore- and hindlimb buds. In hindlimb buds, the Smad4 inactivation disrupts the establishment and signaling by the apical ectodermal ridge (AER) from early limb bud stages onwards, which results in severe hypoplasia and/or aplasia of zeugo- and autopodal skeletal elements. In contrast, the developmentally later inactivation of Smad4 in forelimb buds does not alter AER formation and signaling, but prolongs epithelial-mesenchymal feedback signaling in advanced limb buds. The late termination of SHH and AER-FGF signaling delays distal progression of digit ray formation and inhibits interdigit apoptosis. In summary, our genetic analysis reveals the temporally and functionally distinct dual requirement of ectodermal Smad4 during initiation and termination of AER signaling.

Molecular anatomy of the developing limb in the coquí frog, Eleutherodactylus coqui

The vertebrate limb demonstrates remarkable similarity in basic organization across phylogenetically disparate groups. To gain further insight into how this morphological similarity is maintained in different developmental contexts, we explored the molecular anatomy of size-reduced embryos of the Puerto Rican coquí frog, Eleutherodactylus coqui. This animal demonstrates direct development, a life-history strategy marked by rapid progression from egg to adult and absence of a free-living, aquatic larva. Nonetheless, coquí exhibits a basal anuran limb structure, with four toes on the forelimb and five toes on the hind limb. We investigated the extent to which coquí limb bud development conforms to the model of limb development derived from amniote studies. Toward this end, we characterized dynamic patterns of expression for 13 critical patterning genes across three principle stages of limb development. As expected, most genes demonstrate expression patterns that are essentially unchanged compared to amniote species. For example, we identified an EcFgf8-expression domain within the apical ectodermal ridge (AER). This expression pattern defines a putatively functional AER signaling domain, despite the absence of a morphological ridge in coquí embryos. However, two genes, EcMeis2 and EcAlx4, demonstrate altered domains of expression, which imply a potential shift in gene function between coquí frogs and amniote model systems. Unexpectedly, several genes thought to be critical for limb patterning in other systems, including EcFgf4, EcWnt3a, EcWnt7a, and EcGremlin, demonstrated no evident expression pattern in the limb at the three stages we analyzed. The absence of EcFgf4 and EcWnt3a expression during limb patterning is perhaps not surprising, given that neither gene is critical for proper limb development in the mouse, based on knockout and expression analyses. In contrast, absence of EcWnt7a and EcGremlin is surprising, given that expression of these molecules appears to be absolutely essential in all other model systems so far examined. Although this analysis substantiates the existence of a core set of ancient limb-patterning molecules, which likely mediate identical functions across highly diverse vertebrate forms, it also reveals remarkable evolutionary flexibility in the genetic control of a conserved morphological pattern across evolutionary time.

Digit patterning during limb development as a result of the BMP-receptor interaction

Turing models have been proposed to explain the emergence of digits during limb development. However, so far the molecular components that would give rise to Turing patterns are elusive. We have recently shown that a particular type of receptor-ligand interaction can give rise to Schnakenberg-type Turing patterns, which reproduce patterning during lung and kidney branching morphogenesis. Recent knockout experiments have identified Smad4 as a key protein in digit patterning. We show here that the BMP-receptor interaction meets the conditions for a Schnakenberg-type Turing pattern, and that the resulting model reproduces available wildtype and mutant data on the expression patterns of BMP, its receptor, and Fgfs in the apical ectodermal ridge (AER) when solved on a realistic 2D domain that we extracted from limb bud images of E11.5 mouse embryos. We propose that receptor-ligand-based mechanisms serve as a molecular basis for the emergence of Turing patterns in many developing tissues.

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity

How muscle diversity is generated in the vertebrate body is poorly understood. In the limb, dorsal and ventral muscle masses constitute the first myogenic diversification, as each gives rise to distinct muscles. Myogenesis initiates after muscle precursor cells (MPCs) have migrated from the somites to the limb bud and populated the prospective muscle masses. Here, we show that Sonic hedgehog (Shh) from the zone of polarizing activity (ZPA) drives myogenesis specifically within the ventral muscle mass. Shh directly induces ventral MPCs to initiate Myf5 transcription and myogenesis through essential Gli-binding sites located in the Myf5 limb enhancer. In the absence of Shh signaling, myogenesis is delayed, MPCs fail to migrate distally, and ventral paw muscles fail to form. Thus, Shh production in the limb ZPA is essential for the spatiotemporal control of myogenesis and coordinates muscle and skeletal development by acting directly to regulate the formation of specific ventral muscles.

Smad4 is required to induce digit ray primordia and to initiate the aggregation and differentiation of chondrogenic progenitors in mouse limb buds

SMAD4 is an essential mediator of canonical TGFβ/BMP signal transduction and we inactivated Smad4 in mouse limb buds from early stages onward to study its functions in the mesenchyme. While this Smad4 inactivation did not alter the early Sox9 distribution, prefiguring the chondrogenic primordia of the stylopod and zeugopod, it disrupted formation of all Sox9-positive digit ray primordia. Specific inactivation of Smad4 during handplate development pointed to its differential requirement for posterior and anterior digit ray primordia. At the cellular level, Smad4 deficiency blocked the aggregation of Sox9-positive progenitors, thereby preventing chondrogenic differentiation as revealed by absence of collagen type II. The progressive loss of SOX9 due to disrupting digit ray primordia and chondrogenesis was paralleled by alterations in genes marking other lineages. This pointed to a general loss of tissue organization and diversion of mutant cells toward non-specific connective tissue. Conditional inactivation of Bmp2 and Bmp4 indicated that the loss of digit ray primordia and increase in connective tissue were predominantly a consequence of disrupting SMAD4-mediated BMP signal transduction. In summary, our analysis reveals that SMAD4 is required to initiate: (1) formation of the Sox9-positive digit ray primordia; and (2) aggregation and chondrogenic differentiation of all limb skeletal elements.

Bone morphogenetic protein receptor type Ia localization causes increased BMP2 signaling in mice exhibiting increased peak bone mass phenotype

Bone morphogenetic protein 2 (BMP2) is a growth factor that initiates osteoblast differentiation. Recent studies show that BMP2 signaling regulates bone mineral density (BMD). BMP2 interacts with BMP receptor type Ia (BMPRIa) and type II receptor leading to the activation of the Smad signaling pathway. BMPRIa must shuttle between distinct plasma membrane domains, enriched of Caveolin-1 alpha and Caveolin-1 beta isoforms, and receptor activation occurs in these domains. Yet it remains unknown whether the molecular mechanism that regulates BMP2 signaling is driving mineralization and BMD. Therefore, the B6.C3H-1-12 congenic mouse model with increased BMD and osteoblast mineralization was utilized in this study. Using the family image correlation spectroscopy, we determined if BMP2 led to a significant re-localization of BMPRIa to caveolae of the alpha/beta isoforms in bone marrow stromal cells (BMSCs) isolated from B6.C3H-1-12 mice compared to the C57BL/6J mice, which served as controls. The control, C57BL/6J mice, was selected due to only 4 Mb of chromosome 1 from the C3H/HeJ mouse was backcrossed to a C57BL/6J background. Using reporter gene assays, the B6.C3H-1-12 BMSCs responded to BMP2 with increased Smad activation. Furthermore, disrupting caveolae reduced the BMP2-induced Smad signaling in BMSCs isolated from B6.C3H-1-12 and C57BL/6J. This study suggests for the first time a regulatory mechanism of BMPRIa signaling at the plasma membrane of BMSCs that (i) associated with genetic differences in the distal Chromosome 1 segment carried by the B6.C3H-1-12 congenic and (ii) contributes to increase BMD of the B6.C3H-1-12 compared to the C57BL/6J control mice.

BMP activity is required for tooth development from the lamina to bud stage

Several Bmp genes are expressed in the developing mouse tooth germ from the initiation to the late-differentiation stages, and play pivotal roles in multiple steps of tooth development. In this study, we investigated the requirement of BMP activity in early tooth development by transgenic overexpression of the extracellular BMP antagonist Noggin. We show that overexpression of Noggin in the dental epithelium at the tooth initiation stage arrests tooth development at the lamina/early-bud stage. This phenotype is coupled with a significantly reduced level of cell proliferation rate and a down-regulation of Cyclin-D1 expression, specifically in the dental epithelium. Despite unaltered expression of genes known to be implicated in early tooth development in the dental mesenchyme and dental epithelium of transgenic embryos, the expression of Pitx2, a molecular marker for the dental epithelium, became down-regulated, suggesting the loss of odontogenic fate in the transgenic dental epithelium. Our results reveal a novel role for BMP signaling in the progression of tooth development from the lamina stage to the bud stage by regulating cell proliferation and by maintaining odontogenic fate of the dental epithelium.

Transient downregulation of Bmp signalling induces extra limbs in vertebrates

Bone morphogenetic protein (Bmp) signalling has been implicated in setting up dorsoventral patterning of the vertebrate limb and in its outgrowth. Here, we present evidence that Bmp signalling or, more precisely, its inhibition also plays a role in limb and fin bud initiation. Temporary inhibition of Bmp signalling either by overexpression of noggin or using a synthetic Bmp inhibitor is sufficient to induce extra limbs in the Xenopus tadpole or exogenous fins in the Danio rerio embryo, respectively. We further show that Bmp signalling acts in parallel with retinoic acid signalling, possibly by inhibiting the known limb-inducing gene wnt2ba.

Cdc42 is required for chondrogenesis and interdigital programmed cell death during limb development

Cdc42, a member of the Rho subfamily of small GTPases, is known to be a regulator of multiple cellular functions, including cytoskeletal organization, cell migration, proliferation, and apoptosis. However, its tissue-specific roles, especially in mammalian limb development, remain unclear. To investigate the physiological function of Cdc42 during limb development, we generated limb bud mesenchyme-specific inactivated Cdc42 (Cdc42(fl/fl); Prx1-Cre) mice. Cdc42(fl/fl); Prx1-Cre mice demonstrated short limbs and body, abnormal calcification of the cranium, cleft palate, disruption of the xiphoid process, and syndactyly. Severe defects were also found in long bone growth plate cartilage, characterized by loss of columnar organization of chondrocytes, and thickening and massive accumulation of hypertrophic chondrocytes, resulting in delayed endochondral bone formation associated with reduced bone growth. In situ hybridization analysis revealed that expressions of Col10 and Mmp13 were reduced in non-resorbed hypertrophic cartilage, indicating that deletion of Cdc42 inhibited their terminal differentiation. Syndactyly in Cdc42(fl/fl); Prx1-Cre mice was caused by fusion of metacarpals and a failure of interdigital programmed cell death (ID-PCD). Whole mount in situ hybridization analysis of limb buds showed that the expression patterns of Sox9 were ectopic, while those of Bmp2, Msx1, and Msx2, known to promote apoptosis in the interdigital mesenchyme, were down-regulated. These results demonstrate that Cdc42 is essential for chondrogenesis and ID-PCD during limb development.

TGF-β and BMP signaling in osteoblast differentiation and bone formation

Transforming growth factor-beta (TGF-β)/bone morphogenic protein (BMP) signaling is involved in a vast majority of cellular processes and is fundamentally important throughout life. TGF-β/BMPs have widely recognized roles in bone formation during mammalian development and exhibit versatile regulatory functions in the body. Signaling transduction by TGF-β/BMPs is specifically through both canonical Smad-dependent pathways (TGF-β/BMP ligands, receptors and Smads) and non-canonical Smad-independent signaling pathway (e.g. p38 mitogen-activated protein kinase pathway, MAPK). Following TGF-β/BMP induction, both the Smad and p38 MAPK pathways converge at the Runx2 gene to control mesenchymal precursor cell differentiation. The coordinated activity of Runx2 and TGF-β/BMP-activated Smads is critical for formation of the skeleton. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of TGF-β/BMP signaling in bone and in the signaling networks underlying osteoblast differentiation and bone formation. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in bone from studies of genetic mouse models and human diseases caused by the disruption of TGF-β/BMP signaling. This review also highlights the different modes of cross-talk between TGF-β/BMP signaling and the signaling pathways of MAPK, Wnt, Hedgehog, Notch, and FGF in osteoblast differentiation and bone formation.

BMPR-II is dispensable for formation of the limb skeleton

Initiation of BMP signaling is dependent upon activation of Type I BMP receptor by constitutively active Type II BMP receptor. Three Type II BMP receptors have been identified; Acvr2a and Acvr2b serve as receptors for BMPs and for activin-like ligands whereas BMPR-II functions only as a BMP receptor. As BMP signaling is required for endochondral ossification and loss of either Acvr2a or Acvr2b is not associated with deficits in limb development, we hypothesized that BMPR-II would be essential for BMP signaling during skeletogenesis. We removed BMPR-II from early limb mesoderm by crossing BMPR-II floxed mice with those carrying the Prx1-Cre transgene. Mice lacking limb expression of BMPR-II have normal skeletons that could not be distinguished from control littermates. From these data, we conclude that BMPR-II is not required for endochondral ossification in the limb where loss of BMPR-II may be compensated by BMP utilization of Acvr2a and Acvr2b.

A systems biology representation of developmental anatomy

The formation of any tissue involves differentiation, cell dynamics and interactions with adjacent tissues. This paper suggests that the complexity of the system as a whole can be represented as a mathematical graph, that is, a set of connected triples of the general form [term] [term]. Computationally, such graphs are widely used for modeling data; visually, they form hierarchies and networks. For morphogenesis, the triples are of the general structure , where nouns cover tissues, molecules and networks and verbs describe processes such as moves, differentiates, grows and apoptoses. The paper considers the general formalism of graphs, where graphs are already used in biology, and how developmental anatomy may be described using this format. Representing morphogenesis as a visual graph is complicated as the formalism has to incorporate tissue types, molecular signals, networks, dynamic processes and some aspects, at least, of tissue geometry. The formation of a capillary sprout is chosen as an example of how this complexity can be represented graphically, with colour used to distinguish tissues and molecules. There are three key benefits, beyond its compactness, in using the graph formalism of morphogenesis to complement experimentation. First, it emphasizes the distributed nature of causality in morphogenesis. Secondly, producing all the triples for the visual graph requires explicit formalization of each aspect of the process, and this, in turn, often exposes gaps in knowledge and so suggests new experiments. Thirdly, once the graph has been formalized, triples can be annotated with associated information or IDs (e.g. cell types, publications, gene-expression data) that link to external online resources that may be regularly updated. Such annotations allow the graph to be viewed as a self-maintaining review. The graph approach sees dynamic processes as the drivers of developmental momentum and, because the same processes are used many times during development, it seems appropriate to view them as modules and their underlying networks as genomic subroutines.

Msx1 and Msx2 in limb mesenchyme modulate digit number and identity

Msx1 and Msx2 encode homeodomain transcription factors that play a crucial role in limb development. However, the limb phenotype of the double Msx1(null/null) Msx2(null/null) mutant is difficult to analyze, particularly along the anteroposterior axis, because of the complex effects of the double mutation on both ectoderm- and mesoderm-derived structures. Namely, in the mutant, formation of the apical ectodermal ridge (AER) is impaired anteriorly and, consequently, the subjacent mesenchyme does not form. Using the Cre/loxP system, we investigated the respective roles of Msx genes in ectoderm and mesoderm by generating conditional mutant embryos with no Msx activity solely in the mesoderm. In these mutants, the integrity of the ectoderm-derived AER was maintained, allowing formation of the anterior mesenchyme. With this strategy, we demonstrate that mesenchymal expression of Msx1 and Msx2 is required for proper Shh and Bmp4 signaling to specify digit number and identity.

Mechanisms of digit formation: Human malformation syndromes tell the story

Identifying the genetic basis of human limb malformation disorders has been instrumental in improving our understanding of limb development. Abnormalities of the hands and/or feet include defects affecting patterning, establishment, elongation, and segmentation of cartilaginous condensations, as well as growth of the individual skeletal elements. While the phenotype of such malformations is highly diverse, the mutations identified to date cluster in genes implicated in a limited number of molecular pathways, namely hedgehog, Wnt, and bone morphogenetic protein. The latter pathway appears to function as a key molecular network regulating different phases of digit and joint development. Studies in animal models not only extended our insight into the pathogenesis of these conditions, but have also contributed to our understanding of the in vivo functions and interactions of these key players. This review is aimed at integrating the current understanding of human digit malformations into the increasing knowledge of the molecular mechanisms of digit development. Developmental Dynamics 240:990-1004, 2011. © 2011 Wiley-Liss, Inc.

Genetic analysis of Hedgehog signaling in ventral body wall development and the onset of omphalocele formation

Background

An omphalocele is one of the major ventral body wall malformations and is characterized by abnormally herniated viscera from the body trunk. It has been frequently found to be associated with other structural malformations, such as genitourinary malformations and digit abnormalities. In spite of its clinical importance, the etiology of omphalocele formation is still controversial. Hedgehog (Hh) signaling is one of the essential growth factor signaling pathways involved in the formation of the limbs and urogenital system. However, the relationship between Hh signaling and ventral body wall formation remains unclear.

Methodology/Principal Findings

To gain insight into the roles of Hh signaling in ventral body wall formation and its malformation, we analyzed phenotypes of mouse mutants of Sonic hedgehog (Shh), GLI-Kruppel family member 3 (Gli3) and Aristaless-like homeobox 4 (Alx4). Introduction of additional Alx4^Lst mutations into the Gli3^Xt/Xt background resulted in various degrees of severe omphalocele and pubic diastasis. In addition, loss of a single Shh allele restored the omphalocele and pubic symphysis of Gli3^Xt/+; Alx4^Lst/Lst embryos. We also observed ectopic Hh activity in the ventral body wall region of Gli3^Xt/Xt embryos. Moreover, tamoxifen-inducible gain-of-function experiments to induce ectopic Hh signaling revealed Hh signal dose-dependent formation of omphaloceles.

Conclusions/Significance

We suggest that one of the possible causes of omphalocele and pubic diastasis is ectopically-induced Hh signaling. To our knowledge, this would be the first demonstration of the involvement of Hh signaling in ventral body wall malformation and the genetic rescue of omphalocele phenotypes.

FGF and ROR2 receptor tyrosine kinase signaling in human skeletal development

Skeletal malformations are among the most frequent developmental disturbances in humans. In the past years, progress has been made in unraveling the molecular mechanisms that govern skeletal development by the use of animal models as well as by the identification of numerous mutations that cause human skeletal syndromes. Receptor tyrosine kinases have critical roles in embryonic development. During formation of the skeletal system, the fibroblast growth factor receptor (FGFR) family plays major roles in the formation of cranial, axial, and appendicular bones. Another player of relevance to skeletal development is the unusual receptor tyrosine kinase ROR2, the function of which is as interesting as it is complex. In this chapter, we review the involvement of FGFR signaling in human skeletal disease and provide an update on the growing knowledge of ROR2.

Disruption of PCP signaling causes limb morphogenesis and skeletal defects and may underlie Robinow syndrome and brachydactyly type B

Brachydactyly type B (BDB1) and Robinow syndrome (RRS) are two skeletal disorders caused by mutations in ROR2, a co-receptor of Wnt5a. Wnt5a/Ror2 can activate multiple branches of non-canonical Wnt signaling, but it is unclear which branch(es) mediates Wnt5a/Ror2 function in limb skeletal development. Here, we provide evidence implicating the planar cell polarity (PCP) pathway as the downstream component of Wnt5a in the limb. We show that a mutation in the mouse PCP gene Vangl2 causes digit defects resembling the clinical phenotypes in BDB1, including loss of phalanges. Halving the dosage of Wnt5a in Vangl2 mutants enhances the severity and penetrance of the digit defects and causes long bone defects reminiscent of RRS, suggesting that Wnt5a and Vangl2 function in the same pathway and disruption of PCP signaling may underlie both BDB1 and RRS. Consistent with a role for PCP signaling in tissue morphogenesis, mutation of Vangl2 alters the shape and dimensions of early limb buds: the width and thickness are increased, whereas the length is decreased. The digit pre-chondrogenic condensates also become wider, thicker and shorter. Interestingly, altered limb bud dimensions in Vangl2 mutants also affect limb growth by perturbing the signaling network that regulates the balance between Fgf and Bmp signaling. Halving the dosage of Bmp4 partially suppresses the loss of phalanges in Vangl2 mutants, supporting the hypothesis that an aberrant increase in Bmp signaling is the cause of the brachydactyly defect. These findings provide novel insight into the signaling mechanisms of Wnt5a/Ror2 and the pathogenesis in BDB1 and RRS.

Bone morphogenetic protein signaling in the developing telencephalon controls formation of the hippocampal dentate gyrus and modifies fear-related behavior

The cortical hem is an embryonic signaling center that generates bone morphogenetic proteins (BMPs) and acts as an organizer for the hippocampus. The role of BMP signaling in hippocampal neurogenesis, however, has not been established. We therefore generated mice that were deficient in Bmpr1b constitutively, and deficient in Bmpr1a conditionally in the dorsal telencephalon. In double mutant male and female mice, the dentate gyrus (DG) was dramatically smaller than in control mice, reflecting decreased production of granule neurons at the peak period of DG neurogenesis. Additionally, the pool of cells that generates new DG neurons throughout life was reduced, commensurate with the smaller size of the DG. Effects of diminished BMP signaling on the cortical hem were at least partly responsible for these defects in DG development. Reduction of the DG and its major extrinsic output to CA3 raised the possibility that the DG was functionally compromised. We therefore looked for behavioral deficits in double mutants and found that the mice were less responsive to fear- or anxiety-provoking stimuli, whether the association of the stimulus with fear or anxiety was learned or innate. Given that no anatomical defects appeared in the double mutant telencephalon outside the DG, our observations support a growing literature that implicates the hippocampus in circuitry mediating fear and anxiety. Our results additionally indicate a requirement for BMP signaling in generating the dorsalmost neuronal lineage of the telencephalon, DG granule neurons, and in the development of the stem cell niche that makes neurons in the adult hippocampus.

The expression of twisted gastrulation in postnatal mouse brain and functional implications

Twisted gastrulation (TWSG1), an extracellular regulator of bone morphogenetic protein (BMP) signaling, is critical for embryonic brain development. Mice deficient in TWSG1 have abnormal forebrain development manifesting as holoprosencephaly. The expression and potential roles of TWSG1 in postnatal brain development are less well understood. We show that Twsg1 is expressed in the adult mouse brain in the choroid plexus (CP), hippocampus, and other regions, with the strongest expression observed in CP. TWSG1 was also detected in a human fetal brain at mid-gestation, with highest levels in the epithelium of CP. Bmp1, Bmp2, Bmp4-Bmp7 as well as BmprIA and BmprII, but not BmprIB, were expressed in CP. BMP antagonists Chordin (Chrd) and Noggin were not detected in CP, however Chrd-like 1 and brain-specific Chrd-like (Brorin) were expressed. Electrophysiological study of synaptic plasticity revealed normal paired-pulse facilitation and long-term potentiation in the CA1 region of hippocampus in Twsg1(-/-) mice. Among the homozygous mutants that survive beyond the first 2 weeks, the prevalence of hydrocephalus was 4.3%, compared to 1.5% in a wild type colony (P=0.0133) between 3 and 10 weeks of life. We detected a high level of BMP signaling in CP in wild type adult mice that was 17-fold higher than in the hippocampus (P=0.005). In contrast, transforming growth factor beta (TGFbeta) signaling was predominant in the hippocampus. Both BMP signaling and the expression of BMP downstream targets Msx1 and Msx2 were reduced in CP in Twsg1(-/-) mice. In summary, we show that Twsg1 is expressed in the adult mouse and human fetal CP. We also show that BMP is a branch of TGFbeta superfamily that is dominant in CP. This presents an interesting avenue for future research in light of the novel roles of CP in neural progenitor differentiation and neuronal repair, especially since TWSG1 appears to be the main regulator of BMP present in CP.

Bone ridge patterning during musculoskeletal assembly is mediated through SCX regulation of Bmp4 at the tendon-skeleton junction

During the assembly of the musculoskeletal system, bone ridges provide a stable anchoring point and stress dissipation for the attachment of muscles via tendons to the skeleton. In this study, we investigate the development of the deltoid tuberosity as a model for bone ridge formation. We show that the deltoid tuberosity develops through endochondral ossification in a two-phase process: initiation is regulated by a signal from the tendons, whereas the subsequent growth phase is muscle dependent. We then show that the transcription factor scleraxis (SCX) regulates Bmp4 in tendon cells at their insertion site. The inhibition of deltoid tuberosity formation and several other bone ridges in embryos in which Bmp4 expression was blocked specifically in Scx-expressing cells implicates BMP4 as a key mediator of tendon effects on bone ridge formation. This study establishes a mechanistic basis for tendon-skeleton regulatory interactions during musculoskeletal assembly and bone secondary patterning.

Antagonistic crosstalk of Wnt/beta-catenin/Bmp signaling within the Apical Ectodermal Ridge (AER) regulates interdigit formation

Digit and interdigit (D/ID) development is one of the important research fields in molecular developmental biology. Interdigital cell death (ICD) is a morphogenetic event which has been considered as an essential process for D/ID formation. Although some growth factors including Bmp and Fgf signaling can modulate ICD, growth factor crosstalk regulating ICD is poorly understood. Wnt canonical pathway and Bmp signal crosstalk has been considered as the essential growth factor crosstalk in organogenesis. To elucidate the crosstalk to regulate the D/ID formation, we analyzed conditional mutant mice with limb bud ectoderm expressing constitutively activated beta-catenin signaling. We showed that modulation of Wnt/beta-catenin signal in the limb ectoderm including the AER regulates ID apoptosis. We also demonstrated that Wnt/beta-catenin signaling in the ectoderm can positively regulate Fgf8 possibly antagonizing the epithelial derived Bmp signaling. Human birth defects for digit abnormalities have been known to be affected by multiple parameters. Elucidation of the potential mechanisms underlying such D/ID development is an urgent medical issue to be solved. This work would be one of the first studies showing essential growth factor cascades in the D/ID formation.

Role of Epiprofin, a zinc-finger transcription factor, in limb development

The formation and maintenance of the apical ectodermal ridge (AER) is critical for the outgrowth and patterning of the vertebrate limb. In the present work, we have investigated the role of Epiprofin (Epfn/Sp6), a member of the SP/KLF transcription factor family that is expressed in the limb ectoderm and the AER, during limb development. Epfn mutant mice have a defective autopod that shows mesoaxial syndactyly in the forelimb and synostosis (bony fusion) in the hindlimb and partial bidorsal digital tips. Epfn mutants also show a defect in the maturation of the AER that appears flat and broad, with a double ridge phenotype. By genetic analysis, we also show that Epfn is controlled by WNT/b-CATENIN signaling in the limb ectoderm. Since the less severe phenotypes of the conditional removal of b-catenin in the limb ectoderm strongly resemble the limb phenotype of Epfn mutants, we propose that EPFN very likely functions as a modulator of WNT signaling in the limb ectoderm.

Patched 1 is a crucial determinant of asymmetry and digit number in the vertebrate limb

The vertebrate hedgehog receptor patched 1 (Ptc1) is crucial for negative regulation of the sonic hedgehog (Shh) pathway during anterior-posterior patterning of the limb. We have conditionally inactivated Ptc1 in the mesenchyme of the mouse limb using Prx1-Cre. This results in constitutive activation of hedgehog (Hh) signalling during the early stages of limb budding. Our data suggest that variations in the timing and efficiency of Cre-mediated excision result in differential forelimb and hindlimb phenotypes. Hindlimbs display polydactyly (gain of digits) and a molecular profile similar to the Gli3 mutant extra-toes. Strikingly, forelimbs are predominantly oligodactylous (displaying a loss of digits), with a symmetrical, mirror-image molecular profile that is consistent with re-specification of the anterior forelimb to a posterior identity. Our data suggest that this is related to very early inactivation of Ptc1 in the forelimb perturbing the gene regulatory networks responsible for both the pre-patterning and the subsequent patterning stages of limb development. These results establish the importance of the downstream consequences of Hh pathway repression, and identify Ptc1 as a key player in limb patterning even prior to the onset of Shh expression.

Essential mesenchymal role of small GTPase Rac1 in interdigital programmed cell death during limb development

Developing vertebrate limbs are often utilized as a model for studying pattern formation and morphogenetic cell death. Herein, we report that conditional deletion of Rac1, a member of the Rho family of proteins, in mouse limb bud mesenchyme led to skeletal deformities in the autopod and soft tissue syndactyly, with the latter caused by a complete absence of interdigital programmed cell death. Furthermore, the lack of interdigital programmed cell death and associated syndactyly was related to down-regulated gene expression of Bmp2, Bmp7, Msx1, and Msx2, which are known to promote apoptosis in the interdigital mesenchyme. Our findings from Rac1 conditional mutants indicate crucial roles for Rac1 in limb bud morphogenesis, especially interdigital programmed cell death.