Induction of the LIM homeobox gene Lmx1 by WNT7a establishes dorsoventral pattern in the vertebrate limb

Classification of dorsal and ventral dimelia in humans

Dorsal dimelia (appearance of dorsal structures on the palmar aspect of the hand) and ventral dimelia (appearance of palmar structures on the dorsal aspect of the hand) are rare congenital hand malformations that occur due to errors of the dorso-ventral axis of development of the limb. The current literature includes numerous cases and there is now sufficient basic science/genetics research on the topic so that a classification of dorsal/ventral dimelia could be proposed. Dorsal dimelia is subclassified into two types: distal (dorsalization of the digits ± distal palm) and proximal (dorsalization of the proximal palm only) types. Ventral dimelia is classified into three types (mild, moderate, and severe) according to the degree of ventralization. The classification is supported by the genetic basis of each subtype.

Planar cell polarity in vertebrate limb morphogenesis

Studies of the vertebrate limb development have contributed significantly to understanding the fundamental mechanisms underlying growth, patterning, and morphogenesis of a complex multicellular organism. In the limb, well-defined signaling centers interact to coordinate limb growth and patterning along the three axes. Recent analyses of live imaging and mathematical modeling have provided evidence that polarized cell behaviors governed by morphogen gradients play an important role in shaping the limb bud. Furthermore, the Wnt/planar cell polarity (PCP) pathway that controls uniformly polarized cell behaviors in a field of cells has emerged to be critical for directional morphogenesis in the developing limb. Directional information coded in the morphogen gradient may be interpreted by responding cells through regulating the activities of PCP components in a Wnt morphogen dose-dependent manner.

Notch signalling is required for the formation of structurally stable muscle fibres in zebrafish

BACKGROUND

Accurate regulation of Notch signalling is central for developmental processes in a variety of tissues, but its function in pectoral fin development in zebrafish is still unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that core elements necessary for a functional Notch pathway are expressed in developing pectoral fins in or near prospective muscle territories. Blocking Notch signalling at different levels of the pathway consistently leads to the formation of thin, wavy, fragmented and mechanically weak muscles fibres and loss of stress fibres in endoskeletal disc cells in pectoral fins. Although the structural muscle genes encoding Desmin and Vinculin are normally transcribed in Notch-disrupted pectoral fins, their proteins levels are severely reduced, suggesting that weak mechanical forces produced by the muscle fibres are unable to stabilize/localize these proteins. Moreover, in Notch signalling disrupted pectoral fins there is a decrease in the number of Pax7-positive cells indicative of a defect in myogenesis.

CONCLUSIONS/SIGNIFICANCE

We propose that by controlling the differentiation of myogenic progenitor cells, Notch signalling might secure the formation of structurally stable muscle fibres in the zebrafish pectoral fin.

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.

The signaling pathways of LMX1B and its role in glomerulosclerosis

LMX1B, a developmental LIM-homeodomain transcription factor, is widely expressed in vertebrate embryos, and it takes part in the development of diverse structures such as limbs, kidneys, eyes, brains, etc. LMX1B contributes to transcriptional regulation of glomerular basement membrane (GBM) collagen expression by podocytes. The normal function of podocytes and the normal morphology of GBM are very important to maintain the healthy renal filtration barrier. Recent discoveries find that the LMX1B gene is pivotal in glomus development and it is implicated in the dysfunction of the podocytes. Here, we review the signal transduction pathways of LMX1B and its role in the pathogenesis of glomerulosclerosis.

Wnt signaling in limb organogenesis

Secreted signaling molecules of the Wnt family have been found to play a central role in controlling embryonic development of a wide range of taxa from Hydra to humans. The most extensively studied Wnt signaling pathway is the canonical Wnt pathway, which controls gene expression by stabilizing beta-catenin, and regulates a multitude of developmental processes. More recently, noncanonical Wnt pathways, which are beta-catenin-independent, have been found to be important developmental regulators. Understanding the mechanisms of Wnt signaling is essential for the development of novel preventive and therapeutic approaches of human diseases. Limb development is a paradigm to study the principles of Wnt signaling in various developmental contexts. In the developing vertebrate limb, Wnt signaling has been shown to have important functions during limb bud initiation, limb outgrowth, early limb patterning, and later limb morphogenesis events. This review provides a brief overview on the diversity of Wnt-dependent signaling events during embryonic development of the vertebrate limb.

Twist2 contributes to termination of limb bud outgrowth and patterning through direct regulation of Grem1

Twist1 has been demonstrated to play critical roles in the early development of neural crest and mesodermally derived tissues including the limb. Twist2 has been less well characterised but its relatively late onset of expression suggests specific roles in the development of a number of organs. Expression of Twist2 within the developing limbs begins after formation of the limb bud and persists within the peripheral mesenchyme until digital rays condense. We have used RCAS-mediated overexpression in chick to investigate the function of Twist2 in limb development. Viral misexpression following injection into the lateral plate mesoderm results in a spectrum of hypoplastic limb phenotypes. These include generalized shortening of the entire limb, fusion of the autopod skeletal elements, loss of individual digits or distal truncation resulting in complete loss of the autopod. These phenotypes appear to result from a premature termination of limb outgrowth and manifest as defective growth in both the proximal-distal and anterior-posterior axes. In situ hybridisation analysis demonstrates that many components of the Shh/Grem1/Fgf regulatory loop that controls early limb growth and patterning are downregulated by Twist2 overexpression. Grem1 has a complementary expression pattern to Twist2 within the limb primordia and co-expression of both Grem1 and Twist2 results in a rescue of the Twist2 overexpression phenotype. We demonstrate that Twist proteins directly repress Grem1 expression via a regulatory element downstream of the open reading frame. These data indicate that Twist2 regulates early limb morphogenesis through a role in terminating the Shh/Grem1/Fgf autoregulatory loop.

Integration of the transcriptional networks regulating limb morphogenesis

The developing limb is one of the best described vertebrate systems for understanding how coordinated gene expression during embryogenesis leads to the structures present in the mature organism. This knowledge, derived from decades of research, is largely based upon gain- and loss-of-function experiments. These studies have provided limited information about how the key signaling pathways interact with each other and the downstream effectors of these pathways. We summarize our current understanding of known genetic interactions in the context of three temporally defined gene regulatory networks. These networks crystallize our current knowledge, depicting a dynamic process involving multiple feedback loops between the ectoderm and mesoderm. At the same time, they highlight the fact that many essential processes are still largely undescribed. Much of the dynamic transcriptional activity occurring during development is regulated by distal cis-regulatory elements. Modern genomic tools have provided new approaches for studying the function of cis-regulatory elements and we discuss the results of these studies in regard to understanding limb development. Ultimately, these genomic techniques will allow scientists to understand how multiple signaling pathways are integrated in space and time to drive gene expression and regulate the formation of the limb.

Sim2 prevents entry into the myogenic program by repressing MyoD transcription during limb embryonic myogenesis

The basic helix-loop-helix transcription factor MyoD is a central actor that triggers the skeletal myogenic program. Cell-autonomous and non-cell-autonomous regulatory pathways must tightly control MyoD expression to ensure correct initiation of the muscle program at different places in the embryo and at different developmental times. In the present study, we have addressed the involvement of Sim2 (single-minded 2) in limb embryonic myogenesis. Sim2 is a bHLH-PAS transcription factor that inhibits transcription by active repression and displays enhanced expression in ventral limb muscle masses during chick and mouse embryonic myogenesis. We have demonstrated that Sim2 is expressed in muscle progenitors that have not entered the myogenic program, in different experimental conditions. MyoD expression is transiently upregulated in limb muscle masses of Sim2(-/-) mice. Conversely, Sim2 gain-of-function experiments in chick and Xenopus embryos showed that Sim2 represses MyoD expression. In addition, we show that Sim2 represses the activity of the mouse MyoD promoter in primary myoblasts and is recruited to the MyoD core enhancer in embryonic mouse limbs. Sim2 expression is non-autonomously and negatively regulated by the dorsalising factor Lmx1b. We propose that Sim2 represses MyoD transcription in limb muscle masses, through Sim2 recruitment to the MyoD core enhancer, in order to prevent premature entry into the myogenic program. This MyoD repression is predominant in ventral limb regions and is likely to contribute to the differential increase of the global mass of ventral muscles versus dorsal muscles.

The transformation suppressor gene Reck is required for postaxial patterning in mouse forelimbs

The membrane-anchored metalloproteinase-regulator RECK has been characterized as a tumor suppressor. Here we report that mice with reduced Reck-expression show limb abnormalities including right-dominant, forelimb-specific defects in postaxial skeletal elements. The forelimb buds of low-Reck mutants have an altered dorsal ectoderm with reduced Wnt7a and Igf2 expression, and hypotrophy in two signaling centers (i.e., ZPA and AER) that are essential for limb outgrowth and patterning. Reck is abundantly expressed in the anterior mesenchyme in normal limb buds; mesenchyme-specific Reck inactivation recapitulates the low-Reck phenotype; and some teratogens downregulate Reck in mesenchymal cells. Our findings illustrate a role for Reck in the mesenchymal-epithelial interactions essential for mammalian development.

Detection of genes regulated by Lmx1b during limb dorsalization

Lmx1b is a homeodomain transcription factor that regulates dorsal identity during limb development. Lmx1b knockout (KO) mice develop distal ventral-ventral limbs. Although induction of Lmx1b is linked to Wnt7a expression in the dorsal limb ectoderm, the downstream targets of Lmx1b that accomplish limb dorsalization are unknown. To identify genes targeted by Lmx1b, we compared gene arrays from Lmx1b KO and wild type mouse limbs during limb dorsalization, i.e., 11.5, 12.5, and 13.5 days post coitum. We identified 54 target genes that were differentially expressed in all three stages. Several skeletal targets, including Emx2, Matrilin1 and Matrilin4, demonstrated a loss of scapular expression in the Lmx1b KO mice, supporting a role for Lmx1b in scapula development. Furthermore, the relative abundance of extracellular matrix-related soft tissue targets regulated by Lmx1b, such as collagens and proteoglycans, suggests a mechanism that includes changes in the extracellular matrix composition to accomplish limb dorsalization. Our study provides the most comprehensive characterization of genes regulated by Lmx1b during limb development to-date and provides targets for further investigation.

Wls-mediated Wnts differentially regulate distal limb patterning and tissue morphogenesis

Wnt proteins are diffusible morphogens that play multiple roles during vertebrate limb development. However, the complexity of Wnt signaling cascades and their overlapping expression prevent us from dissecting their function in limb patterning and tissue morphogenesis. Depletion of the Wntless (Wls) gene, which is required for the secretion of various Wnts, makes it possible to genetically dissect the overall effect of Wnts in limb development. In this study, the Wls gene was conditionally depleted in limb mesenchyme and ectoderm. The loss of mesenchymal Wls prevented the differentiation of distal mesenchyme and arrested limb outgrowth, most likely by affecting Wnt5a function. Meanwhile, the deletion of ectodermal Wls resulted in agenesis of distal limb tissue and premature regression of the distal mesenchyme. These observations suggested that Wnts from the two germ layers differentially regulate the pool of undifferentiated distal limb mesenchyme cells. Cellular behavior analysis revealed that ectodermal Wnts sustain mesenchymal cell proliferation and survival in a manner distinct from Fgf. Ectodermal Wnts were also shown for the first time to be essential for distal tendon/ligament induction, myoblast migration and dermis formation in the limb. These findings provide a comprehensive view of the role of Wnts in limb patterning and tissue morphogenesis.

Mutations in the Notch pathway alter the patterning of multifidus

Clinical studies have suggested that defects in the epaxial muscles, particularly multifidus, may contribute to the etiology of idiopathic scoliosis. While the epaxial muscles and the vertebrae derive from the same embryonic segmentation process, the mechanisms that pattern the multisegmental back muscles are still unclear. The process of segmentation is regulated by the Notch signaling pathway, and mutations in the modulators delta-like 3 (Dll3) and lunatic fringe (Lfng) are genetic models for spinal disorders such as scoliosis. Osteological defects have been characterized in these genetic models, but myological phenotypes have not previously been studied. We analyzed the multifidus muscle in the mouse (Mus musculus) and observed intriguing changes in the cranio-caudal borders of multifidus in Dll3 and Lfng models. Statistical analysis did not find a significant association between the majority of the multifidus anomalies and the vertebral defects, suggesting a previously unappreciated role for Notch signaling in patterning epaxial muscle groups. These findings indicate an additional mechanism by which DLL3 and LFNG may play a role in the etiology of human idiopathic scoliosis.

A LIM-homeobox gene is required for differentiation of Wnt-expressing cells at the posterior end of the planarian body

Planarians have high regenerative ability, which is dependent on pluripotent adult somatic stem cells called neoblasts. Recently, canonical Wnt/β-catenin signaling was shown to be required for posterior specification, and Hedgehog signaling was shown to control anterior-posterior polarity via activation of the Djwnt1/P-1 gene at the posterior end of planarians. Thus, various signaling molecules play an important role in planarian stem cell regulation. However, the molecular mechanisms directly involved in stem cell differentiation have remained unclear. Here, we demonstrate that one of the planarian LIM-homeobox genes, Djislet, is required for the differentiation of Djwnt1/P-1-expressing cells from stem cells at the posterior end. RNA interference (RNAi)-treated planarians of Djislet [Djislet(RNAi)] show a tail-less phenotype. Thus, we speculated that Djislet might be involved in activation of the Wnt signaling pathway in the posterior blastema. When we carefully examined the expression pattern of Djwnt1/P-1 by quantitative real-time PCR during posterior regeneration, we found two phases of Djwnt1/P-1 expression: the first phase was detected in the differentiated cells in the old tissue in the early stage of regeneration and then a second phase was observed in the cells derived from stem cells in the posterior blastema. Interestingly, Djislet is expressed in stem cell-derived DjPiwiA- and Djwnt1/P-1-expressing cells, and Djislet(RNAi) only perturbed the second phase. Thus, we propose that Djislet might act to trigger the differentiation of cells expressing Djwnt1/P-1 from stem cells.

Functional analysis of conserved non-coding regions around the short stature hox gene (shox) in whole zebrafish embryos

BACKGROUND

Mutations in the SHOX gene are responsible for Leri-Weill Dyschondrosteosis, a disorder characterised by mesomelic limb shortening. Recent investigations into regulatory elements surrounding SHOX have shown that deletions of conserved non-coding elements (CNEs) downstream of the SHOX gene produce a phenotype indistinguishable from Leri-Weill Dyschondrosteosis. As this gene is not found in rodents, we used zebrafish as a model to characterise the expression pattern of the shox gene across the whole embryo and characterise the enhancer domains of different CNEs associated with this gene.

METHODOLOGY/PRINCIPAL FINDINGS

Expression of the shox gene in zebrafish was identified using in situ hybridization, with embryos showing expression in the blood, putative heart, hatching gland, brain pharyngeal arch, olfactory epithelium, and fin bud apical ectodermal ridge. By identifying sequences showing 65% identity over at least 40 nucleotides between Fugu, human, dog and opossum we uncovered 35 CNEs around the shox gene. These CNEs were compared with CNEs previously discovered by Sabherwal et al., resulting in the identification of smaller more deeply conserved sub-sequence. Sabherwal et al.'s CNEs were assayed for regulatory function in whole zebrafish embryos resulting in the identification of additional tissues under the regulatory control of these CNEs.

CONCLUSION/SIGNIFICANCE

Our results using whole zebrafish embryos have provided a more comprehensive picture of the expression pattern of the shox gene, and a better understanding of its regulation via deeply conserved noncoding elements. In particular, we identify additional tissues under the regulatory control of previously identified SHOX CNEs. We also demonstrate the importance of these CNEs in evolution by identifying duplicated shox CNEs and more deeply conserved sub-sequences within already identified CNEs.

Pleiotropic patterning response to activation of Shh signaling in the limb apical ectodermal ridge

Sonic hedgehog (Shh) signaling in the limb plays a central role in coordination of limb patterning and outgrowth. Shh expression in the limb is limited to the cells of the zone of polarizing activity (ZPA), located in posterior limb bud mesoderm. Shh is not expressed by limb ectoderm or apical ectodermal ridge (AER), but recent studies suggest a role for AER-Shh signaling in limb patterning. Here, we have examined the effects of activation of Shh signaling in the AER. We find that targeted expression of Shh in the AER activates constitutive Shh signaling throughout the AER and subjacent limb mesoderm, and causes a range of limb patterning defects with progressive severity from mild polydactyly, to polysyndactyly with proximal defects, to severe oligodactyly with phocomelia and partial limb ventralization. Our studies emphasize the importance of control of the timing, level and location of Shh pathway signaling for limb anterior-posterior, proximal-distal, and dorsal-ventral patterning.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

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

HMGB factors are required for posterior digit development through integrating signaling pathway activities

The chromatin factors Hmgb1 and Hmgb2 have critical roles in cellular processes, including transcription and DNA modification. To identify the function of Hmgb genes in embryonic development, we generated double mutants of Hmgb1;Hmgb2 in mice. While double null embryos arrest at E9.5, Hmgb1(-/-) ; Hmgb2(+/-) embryos exhibit a loss of digit5, the most posterior digit, in the forelimb. We show that Hmgb1(-/-) ; Hmgb2(+/-) forelimbs have a reduced level of Shh signaling, as well as a clear downregulation of Wnt and BMP target genes in the posterior region. Moreover, we demonstrate that hmgb1 and hmgb2 in zebrafish embryos enhance Wnt signaling in a variety of tissues, and that double knockdown embryos have reduced Wnt signaling and shh expression in pectoral fin buds. Our data show that Hmgb1 and Hmgb2 function redundantly to enhance Wnt signaling in embryos, and further suggest that integrating Wnt, Shh, and BMP signaling regulates the development of digit5 in forelimbs.

Spatiotemporal changes in cell adhesiveness during vertebrate limb morphogenesis

During vertebrate limb development, various molecules are expressed in the presumptive limb field or the limb bud in a spatiotemporal-specific manner. The combination of these molecules regulates cellular properties that affect limb initiation and its morphogenesis, especially cartilage formation. Cell adhesiveness of the limb mesenchyme is a key factor in the regulation of cell distribution. Differential adhesiveness of mesenchymal cells is first observed between cells in the presumptive limb field and flank region, and the adhesiveness of the cells in the limb field is higher than that of cells in the flank region. In the limb bud, the adhesiveness of mesenchymal cells shows spatiotemporal difference, which reflects the positional identity of the cells. Position-dependent cell adhesiveness is also observed in blastema cells of the regenerating limb. Therefore, local changes in cell adhesiveness are observed during limb development and regeneration, suggesting significant roles for cell adhesiveness in limb morphogenesis.

Developmental biology and classification of congenital anomalies of the hand and upper extremity

Recent investigations into the mechanism of limb development have clarified the roles of several molecules, their pathways, and interactions. Characterization of the molecular pathways that orchestrate limb development has provided insight into the etiology of many limb malformations. In this review, we describe how the insights from developmental biology are related to clinically relevant anomalies and the current classification schemes used to define, categorize, and communicate patterns of upper limb malformations. We advocate an updated classification scheme for upper limb anomalies that incorporates our current molecular perspective of limb development and the pathogenetic basis for malformations using dysmorphology terminology. We anticipate that this scheme will improve the utility of a classification as a basis for diagnosis, treatment, and research.

Identification of genes controlled by LMX1B in E13.5 mouse limbs

During limb development, the dorsal limb mesenchyme expression of the transcription factor LMX1B is required for dorsoventral limb patterning. In mice, Lmx1b mutations result in the mirror-image duplication of ventral limb structures and loss of dorsal limb structures. Heterozygous LMX1B mutations in humans cause the Nail-Patella Syndrome characterized by limb, kidney, and eye developmental defects. We used DNA microarrays to compare the mRNAs in E13.5 mouse Lmx1b mutant and wild-type limbs. We report 14 genes that require Lmx1b for their normal expression in the dorsal limb or the restriction of their expression to the ventral limb.

Dorsal dimelia: report of two cases with an emphasis on the variation of phenotypic expression and a search for candidate causative genes

Dorsal dimelia is a form of duplication along the dorsoventral axis of the developing limb. Previous authors reporting on this rare entity have stated that the essential feature of the deformity is the presence of double or circumferential nail at the tip of the finger and that the aetiology is probably related to a mutation of Engrailed-1 (En-1). In this paper we report on two cases to demonstrate that the deformity in humans may be fully or partially expressed, with or without the double nail deformity, respectively. We also reviewed reported cases in humans and experimental animals and searched our two cases for candidate causative genes within the En-1 pathway.

Vertebrate limb development: moving from classical morphogen gradients to an integrated 4-dimensional patterning system

A wealth of classical embryological manipulation experiments taking mainly advantage of the chicken limb buds identified the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA) as the respective ectodermal and mesenchymal key signaling centers coordinating proximodistal (PD) and anteroposterior (AP) limb axis development. These experiments inspired Wolpert's French flag model, which is a classic among morphogen gradient models. Subsequent molecular and genetic analysis in the mouse identified retinoic acid as proximal signal, and fibroblast growth factors (FGFs) and sonic hedgehog (SHH) as the essential instructive signals produced by AER and ZPA, respectively. Recent studies provide good evidence that progenitors are specified early with respect to their PD and AP fates and that morpho-regulatory signaling is also required for subsequent proliferative expansion of the specified progenitor pools. The determination of particular fates seems to occur rather late and depends on additional signals such as bone morphogenetic proteins (BMPs), which indicates that cells integrate signaling inputs over time and space. The coordinate regulation of PD and AP axis patterning is controlled by an epithelial-mesenchymal feedback signaling system, in which transcriptional regulation of the BMP antagonist Gremlin1 integrates inputs from the BMP, SHH, and FGF pathways. Vertebrate limb-bud development is controlled by a 4-dimensional (4D) patterning system integrating positive and negative regulatory feedback loops, rather than thresholds set by morphogen gradients.

Uncoupling skeletal and connective tissue patterning: conditional deletion in cartilage progenitors reveals cell-autonomous requirements for Lmx1b in dorsal-ventral limb patterning

Integration of muscle, connective tissue and skeletal patterning during development is essential for proper functioning of the musculoskeletal system. How this integration is achieved is poorly understood. There is ample evidence suggesting that skeletal pattern is programmed autonomously, whereas muscle pattern is, for the most part, programmed non-cell-autonomously. Connective tissues depend upon both muscle and skeletal tissues for their proper survival and development. Here, we employed a novel approach to dissect the coordination of musculoskeletal patterning during mouse limb development. Using both conditional gain- and loss-of-function approaches, we selectively deleted or activated the LIM-homeodomain transcription factor Lmx1b in skeletal progenitors using a Sox9-Cre knock-in allele. As Lmx1b is both necessary and sufficient to specify dorsal pattern, this approach allowed us to investigate the effect of selectively deleting or activating Lmx1b in skeletal progenitors on muscle, connective and skeletal tissues during limb development. Our results indicate that whereas Lmx1b activity is required autonomously in skeletal progenitors to direct dorsal pattern, loss or gain of Lmx1b activity in skeletal progenitors has no effect on muscle or connective tissue patterning. Hence, we show for the first time that skeletal and connective tissue patterning can be uncoupled, indicating a degree of autonomy in the formation of the musculoskeletal system.

Building limb morphology through integration of signalling modules

Growth and patterning of the vertebrate limb relies on signals produced by three discrete signalling centres: the Apical Ectodermal Ridge (AER), the Zone of Polarising Activity (ZPA) and the dorsal ectoderm. The molecular identities of these signals and their associated downstream pathways have begun to be uncovered. In this review, we focus on recent work that has highlighted the importance of cross-talk between these signalling centres and how mesenchymal progenitors integrate multiple signalling inputs. We also discuss recent evidence suggesting how modulations of key signalling pathways have been used to generate the morphological diversity seen between different vertebrate limb appendages.

Embryology of the upper limb

An increased understanding of embryogenesis has advanced our fundamental knowledge of limb anomalies. Animal models with similar limb patterning have been used to dissect and manipulate crucial signaling centers that affect limb development and orientation. Experimental embryologists can produce limb anomalies that are similar to the human phenotype encountered in clinical practice. The evaluating physician must possess a basic comprehension of embryogenesis and limb formation to comprehend congenital limb anomalies and to communicate relevant knowledge to the family. This Current Concepts article is intended to provide an update of limb development that is germane to the clinical scenario.

Lmx1a maintains proper neurogenic, sensory, and non-sensory domains in the mammalian inner ear

Lmx1a is a LIM homeodomain-containing transcription factor, which is required for the formation of multiple organs. Lmx1a is broadly expressed in early stages of the developing inner ear, but its expression is soon restricted to the non-sensory regions of the developing ear. In an Lmx1a functional null mutant, dreher (dr(J)/dr(J)), the inner ears lack a non-sensory structure, the endolymphatic duct, and the membranous labyrinth is poorly developed. These phenotypes are consistent with Lmx1a's role as a selector gene. More importantly, while all three primary fates of the inner ear - neural, sensory, and non-sensory - are specified in dr(J)/dr(J), normal boundaries among these tissues are often violated. For example, the neurogenic domain of the ear epithelium, from which cells delaminate to form the cochleovestibular ganglion, is expanded. Within the neurogenic domain, the demarcation between the vestibular and auditory neurogenic domains is most likely disrupted as well, based on the increased numbers of vestibular neuroblasts and ectopic expression of Fgf3, which normally is associated specifically with the vestibular neurogenic region. Furthermore, aberrant and ectopic sensory organs are observed; most striking among these is vestibular-like hair cells located in the cochlear duct.

Hear the Wnt Ror: how melanoma cells adjust to changes in Wnt

The interplay between canonical and non-canonical Wnt pathways in development and tumorigenesis is tightly regulated. In this review we will describe the yin and the yang of canonical and non-canonical Wnt signaling pathways during melanocyte development, and melanoma genesis. Canonical Wnt signaling, represented by Wnts such as Wnt1 and Wnt3A, signals via beta-catenin to promote melanocyte differentiation and tumor development. Non-canonical Wnt signaling, specifically Wnt5A, regulates canonical pathways, and signals to induce melanoma metastasis. This review will focus on the role of Wnt5A during melanoma progression, and its relationship to canonical Wnt signaling.

Developmental expression of apterous/Lhx2/9 in the sepiolid squid Euprymna scolopes supports an ancestral role in neural development

The transcription factors Apterous/Lhx2/9 play many pivotal roles in the development of protostomes and deuterostomes, most notably limb patterning, eye morphogenesis, and brain development. Full-length apterous/lhx2/9 homologs have been isolated from several invertebrate species, but hitherto not from a lophotrochozoan. Here, we report the isolation, characterization, and spatio-temporal expression of apterous in the sepiolid squid Euprymna scolopes. The isolated composite cDNA encodes a hypothetical protein of 448 amino acid residues with a typical LIM-homeodomain (LIM-HD) structure and the greatest overall sequence similarity to vertebrate Lhx2/9 proteins. The Euprymna scolopes apterous (Es-ap) expression patterns provided no indication of a role in the early dorso/ventral patterning or growth of the arm crown that showed expression only in two ventral cords running in parallel inside the arms and tentacles and at the base of the suckers, a region rich in nerve endings and chemosensory neurons. The Es-ap hybridization signal was also conspicuous in the eyes, olfactory organs, optic lobes, and in several lobes of the supraesophageal mass, among these the olfactory and vertical lobes, and paravertical bodies. The observed expression patterns suggest gene involvement in eye morphogenesis and neural wiring of sensory structures, including those for olfaction and vision.

LIM homeobox transcription factors integrate signaling events that control three-dimensional limb patterning and growth

Vertebrate limb development is controlled by three signaling centers that regulate limb patterning and growth along the proximodistal (PD), anteroposterior (AP) and dorsoventral (DV) limb axes. Coordination of limb development along these three axes is achieved by interactions and feedback loops involving the secreted signaling molecules that mediate the activities of these signaling centers. However, it is unknown how these signaling interactions are processed in the responding cells. We have found that distinct LIM homeodomain transcription factors, encoded by the LIM homeobox (LIM-HD) genes Lhx2, Lhx9 and Lmx1b integrate the signaling events that link limb patterning and outgrowth along all three axes. Simultaneous loss of Lhx2 and Lhx9 function resulted in patterning and growth defects along the AP and the PD limb axes. Similar, but more severe, phenotypes were observed when the activities of all three factors, Lmx1b, Lhx2 and Lhx9, were significantly reduced by removing their obligatory co-factor Ldb1. This reveals that the dorsal limb-specific factor Lmx1b can partially compensate for the function of Lhx2 and Lhx9 in regulating AP and PD limb patterning and outgrowth. We further showed that Lhx2 and Lhx9 can fully substitute for each other, and that Lmx1b is partially redundant, in controlling the production of output signals in mesenchymal cells in response to Fgf8 and Shh signaling. Our results indicate that several distinct LIM-HD transcription factors in conjunction with their Ldb1 co-factor serve as common central integrators of distinct signaling interactions and feedback loops to coordinate limb patterning and outgrowth along the PD, AP and DV axes after limb bud formation.

Failure of differentiation part I: Syndactyly

Syndactyly is one of the two most common congenital hand anomalies, the other being polydactyly. Traditionally, syndactyly is considered simple when only skin is involved; complex when there is bone connection; complete when the web involvement includes the nail folds; incomplete or partial when the nail folds are not involved, but when the web depth is distal to its normal position; and complicated when there are multiple tissue abnormalities. This article discusses the various types of syndactyly, the current state of known genetic mechanisms, and the author's preferred surgical techniques for correction.

Duplication

Congenital limb duplications include pre- and post-axial polydactyly, central polydactyly, and the mirror-hand spectrum. Treatment of these duplications constitutes a significant functional and aesthetic challenge for the reconstructive hand surgeon. This article provides an inclusive review of the embryologic and molecular mechanisms underlying these deformities and focuses on their clinical treatment. The anatomic variation, classification, surgical treatment, and outcomes of surgical intervention are reviewed for each of the disorders of duplication.

Specification of motor axon trajectory by ephrin-B:EphB signaling: symmetrical control of axonal patterning in the developing limb

Studies of the innervation of limb muscles by spinal motor neurons have helped to define mechanisms by which axons establish trajectories to their targets. Related motor axons select dorsal or ventral pathways at the base of the limb, raising the question of how these alternate trajectories are specified. EphA signaling has been proposed to control the dorsal trajectory of motor axons in conjunction with other signaling systems, although the respective contributions of each system to motor axon guidance are unclear. We show that the expression of EphB receptors by motor axons, and ephrin-B ligands by limb mesenchymal cells, directs the ventral trajectory of motor axons. Our findings reveal symmetry in the molecular strategies that establish this aspect of nerve-muscle connectivity. The involvement of ephrin:Eph signaling in guiding both sets of motor axons raises the possibility that other signaling systems function primarily to refine or modulate a core Eph signaling program.

Manifold functions of the Nail-Patella Syndrome gene Lmx1b in vertebrate development

The LIM (Lin-1, Isl-1 and Mec-3)-homeodomain transcription factor 1 beta (Lmx1b) is widely expressed in vertebrate embryos, and is implicated in the development of diverse structures such as limbs, kidneys, eyes and brains. LMX1B mutations in humans cause an autosomal dominant inherited disease called nail-patella syndrome (NPS), which is characterized by abnormalities of the arms and legs as well as kidney disease and glaucoma. Expression of Lmx1b in the dorsal compartment of growing limb buds is critical for specification of dorsal limb cell fates and consequently dorsoventral patterning of limbs. In addition, Lmx1b is involved in the differentiation of anterior eye structures, formation of the glomerular basement membrane in kidneys and development of the skeleton, especially calvarial bones. In the central nervous system, Lmx1b controls the inductive activity of isthmic organizer, differentiation and maintenance of central serotonergic neurons, as well as the differentiation and migration of spinal dorsal horn neurons. Although details of the genetic programs involved in these developmental events are largely unknown, it is suggested that Lmx1b plays central roles in fate determination or cell differentiation in these tissues. Sustained expression of Lmx1b in the postnatal and mature mouse brain suggests that it also plays important roles in brain maturation and in the regulation of normal brain functions. This review aims to highlight recent insights into the many activities of Lmx1b in vertebrates.

Lmx1b-expressing cells in the mouse limb bud define a dorsal mesenchymal lineage compartment

The LIM-homeodomain transcription factor lmx1b is a critical regulator of vertebrate dorsal-ventral limb patterning. In the mouse embryo, lmx1b is initially transcribed throughout most, if not all, limb bud cells at early stages, but then rapidly becomes restricted specifically to dorsal mesenchymal cells with a sharp boundary between the dorsal-positive and ventral-negative expression domains. How this expression pattern is initially established is not well understood, nor are mechanism(s) that maintain a sharp dorsal-ventral boundary between lmx1b expressing and nonexpressing cells. Here, we employ a genetic fate mapping approach to establish that the transition from a broad expression domain of lmx1b to a restricted dorsal domain of expression involves selective loss of lmx1b expression in presumptive ventral cells. In addition, we show that once lmx1b expression becomes restricted to dorsal mesenchyme cells, these cells form a lineage-based compartment that prevents mixing between dorsal and ventral cells, consistent with recent fate mapping experiments carried out the chick and mouse (Pearse et al.,2007, Dev Biol 310:388-400; Arques et al.,2007, Development 134:3713-3722). Moreover, lmx1b activity is required to maintain, but not to establish the dorsal mesenchymal compartment likely through a mechanism involving differential cell adhesion. Taken together, our results indicate that lmx1b expressing cell define a dorsal limb bud mesenchymal lineage compartment and that maintenance of this compartment depends on lmx1b function.