TBX3 is essential for establishment of the posterior boundary of anterior genes and upregulation of posterior genes together with HAND2 during the onset of limb bud development.

During limb bud formation, axis polarities are established as evidenced by the spatially restricted expression of key regulator genes. In particular, the mutually antagonistic interaction between the GLI3 repressor and HAND2 results in distinct and non-overlapping anterior-distal Gli3 and posterior Hand2 expression domains. This is a hallmark of the establishment of antero-posterior limb axis polarity, together with spatially restricted expression of homeodomain and other transcriptional regulators. Here, we show that TBX3 is required for establishment of the posterior expression boundary of anterior genes in mouse limb buds. ChIP-seq and differential gene expression analysis of wild-type and mutant limb buds identifies TBX3-specific and shared TBX3-HAND2 target genes. High sensitivity fluorescent whole-mount in situ hybridisation shows that the posterior expression boundaries of anterior genes are positioned by TBX3-mediated repression, which excludes anterior genes such as Gli3, Alx4, Hand1 and Irx3/5 from the posterior limb bud mesenchyme. This exclusion delineates the posterior mesenchymal territory competent to establish the Shh-expressing limb bud organiser. In turn, HAND2 is required for Shh activation and cooperates with TBX3 to upregulate shared posterior identity target genes in early limb buds.

Research Note: SOCS2 contributes to reduction of the third digit during development of the chicken forelimb.

The development of the avian wing pattern has been the subject of heated debate due to its special shape. The Suppressor of cytokine signaling 2 (SOCS2) gene encodes a negative regulator of growth hormone (GH) signaling and bone growth and is known to be strongly expressed in the third digit of chicken forelimbs. These observations suggest that SOCS2 might regulate the morphology of the avian wing, however, the function of SOCS2 in avian limb development remains unknown. Here, we reexamined SOCS2 expression in successive developmental stages of chicken limb development by in situ hybridization (ISH) and describe extended expression from the posterior of the stypolod to the third digit of the forelimbs. We used the RCAS avian retrovirus to overexpress SOCS2 in the developing chicken limb buds, which resulted in reduced or malformed chicken wings while hindlimbs developed normally. Transcriptome sequencing (mRNA-Seq) revealed changes in expression of genes known to be associated with growth and development in forelimbs with overexpressed SOCS2. This study highlights a pivotal role for SOCS2 during the development of the wing in the chicken and provides new insight into molecular mechanisms regulating avian limb development.

Sall genes regulate hindlimb initiation in mouse embryos.

Vertebrate limbs start to develop as paired protrusions from the lateral plate mesoderm at specific locations of the body with forelimb buds developing anteriorly and hindlimb buds posteriorly. During the initiation process, limb progenitor cells maintain active proliferation to form protrusions and start to express Fgf10, which triggers molecular processes for outgrowth and patterning. Although both processes occur in both types of limbs, forelimbs (Tbx5), and hindlimbs (Isl1) utilize distinct transcriptional systems to trigger their development. Here, we report that Sall1 and Sall4, zinc finger transcription factor genes, regulate hindlimb initiation in mouse embryos. Compared to the 100% frequency loss of hindlimb buds in TCre; Isl1 conditional knockouts, Hoxb6Cre; Isl1 conditional knockout causes a hypomorphic phenotype with only approximately 5% of mutants lacking the hindlimb. Our previous study of SALL4 ChIP-seq showed SALL4 enrichment in an Isl1 enhancer, suggesting that SALL4 acts upstream of Isl1. Removing 1 allele of Sall4 from the hypomorphic Hoxb6Cre; Isl1 mutant background caused loss of hindlimbs, but removing both alleles caused an even higher frequency of loss of hindlimbs, suggesting a genetic interaction between Sall4 and Isl1. Furthermore, TCre-mediated conditional double knockouts of Sall1 and Sall4 displayed a loss of expression of hindlimb progenitor markers (Isl1, Pitx1, Tbx4) and failed to develop hindlimbs, demonstrating functional redundancy between Sall1 and Sall4. Our data provides genetic evidence that Sall1 and Sall4 act as master regulators of hindlimb initiation.

The BAF chromatin complex component SMARCC1 does not mediate GLI transcriptional repression of Hedgehog target genes in limb buds.

Transcriptional responses to the Hedgehog (HH) signaling pathway are primarily modulated by GLI repression in the mouse limb. Previous studies suggested a role for the BAF chromatin remodeling complex in mediating GLI repression. Consistent with this possibility, the core BAF complex protein SMARCC1 is present at most active limb enhancers including the majority of GLI enhancers. However, in contrast to GLI repression which reduces chromatin accessibility, SMARCC1 maintains chromatin accessibility at most enhancers, including those bound by GLI. Moreover, SMARCC1 binding at GLI-regulated enhancers occurs independently of GLI3. Consistent with previous studies, some individual GLI target genes are mis-regulated in Smarcc1 conditional knockouts, though most GLI target genes are unaffected. Moreover, SMARCC1 is not necessary for mediating constitutive GLI repression in HH mutant limb buds. We conclude that SMARCC1 does not mediate GLI3 repression, which we propose utilizes alternative chromatin remodeling complexes.

Sall4 restricts glycolytic metabolism in limb buds through transcriptional regulation of glycolytic enzyme genes.

Recent studies illustrate the importance of regulation of cellular metabolism, especially glycolysis and pathways branching from glycolysis, during vertebrate embryo development. For example, glycolysis generates cellular energy ATP. Glucose carbons are also directed to the pentose phosphate pathway, which is needed to sustain anabolic processes in the rapidly growing embryos. However, our understanding of the exact status of glycolytic metabolism as well as genes that regulate glycolytic metabolism are still incomplete. Sall4 is a zinc finger transcription factor that is highly expressed in undifferentiated cells in developing mouse embryos, such as blastocysts and the post-implantation epiblast. TCre; Sall4 conditional knockout mouse embryos exhibit various defects in the posterior part of the body, including hindlimbs. Using transcriptomics approaches, we found that many genes encoding glycolytic enzymes are upregulated in the posterior trunk, including the hindlimb-forming region, of Sall4 conditional knockout mouse embryos. In situ hybridization and qRT-PCR also confirmed upregulation of expression of several glycolytic genes in hindlimb buds. A fraction of those genes are bound by SALL4 at the promoters, gene bodies or distantly-located regions, suggesting that Sall4 directly regulates expression of several glycolytic enzyme genes in hindlimb buds. To further gain insight into the metabolic status associated with the observed changes at the transcriptional level, we performed a comprehensive analysis of metabolite levels in limb buds in wild type and Sall4 conditional knockout embryos by high-resolution mass spectrometry. We found that the levels of metabolic intermediates of glycolysis are lower, but glycolytic end-products pyruvate and lactate did not exhibit differences in Sall4 conditional knockout hindlimb buds. The increased expression of glycolytic genes would have caused accelerated glycolytic flow, resulting in low levels of intermediates. This condition may have prevented intermediates from being re-directed to other pathways, such as the pentose phosphate pathway. Indeed, the change in glycolytic metabolite levels is associated with reduced levels of ATP and metabolites of the pentose phosphate pathway. To further test whether glycolysis regulates limb patterning downstream of Sall4, we conditionally inactivated Hk2, which encodes a rate-limiting enzyme gene in glycolysis and is regulated by Sall4. The TCre; Hk2 conditional knockout hindlimb exhibited a short femur, and a lack of tibia and anterior digits in hindlimbs, which are defects similarly found in the TCre; Sall4 conditional knockout. The similarity of skeletal defects in Sall4 mutants and Hk2 mutants suggests that regulation of glycolysis plays a role in hindlimb patterning. These data suggest that Sall4 restricts glycolysis in limb buds and contributes to patterning and regulation of glucose carbon flow during development of limb buds.

KIF3B promotes a PI3K signaling gradient causing changes in a Shh protein gradient and suppressing polydactyly in mice.

Digit determination in limb buds is driven by a posteriorizing Sonic hedgehog (Shh) protein gradient; however, the mechanism regulating this is unclear. Here, we propose a diffusion-and-trapping hypothesis for Shh gradient formation based on data from the preaxial polydactyly phenotype of KIF3B motor hypomorphic mice. In the limb buds of these mice, a distal-to-proximal gradient of fibroblast growth factor (FGF) and phosphatidylinositol 3-kinase (PI3K) signaling and a posterior-to-anterior gradient of Shh were disorganized. This phenotype was reproduced by transplanting FGF8b-soaked beads. At the subcellular level, KIF3B transported the phosphatase and tensin homolog (PTEN)-like phosphatase Talpid3 to terminate PI3K signaling. High and low PI3K signaling strengths differentially sorted endocytosed Shh toward exosome-like particles and cytonemal punctata, respectively. These results indicate that the Shh-containing particles undergo either the diffusional movement in the periphery or cytonemal trapping in the center and form a spatial gradient along the periphery of developing limb buds.

A limb bud morphogen bites the dust.

Hedgehog signaling has traditionally been considered to be a morphogen for digits. In this issue of Developmental Cell, Zhu et al. show that a brief exposure to Sonic Hedgehog is sufficient for digit specification, and this finding suggests that it is not acting as a direct morphogen but rather as an initiator of this process.

Gene expression analysis of the Xenopus laevis early limb bud proximodistal axis.

BACKGROUND: Limb buds develop as bilateral outgrowths of the lateral plate mesoderm and are patterned along three axes. Current models of proximal to distal patterning of early amniote limb buds suggest that two signals, a distal organizing signal from the apical epithelial ridge (AER, Fgfs) and an opposing proximal (retinoic acid [RA]) act early on pattern this axis. RESULTS: Transcriptional analysis of stage 51 Xenopus laevis hindlimb buds sectioned along the proximal-distal axis showed that the distal region is distinct from the rest of the limb. Expression of capn8.3, a novel calpain, was located in cells immediately flanking the AER. The Wnt antagonist Dkk1 was AER-specific in Xenopus limbs. Two transcription factors, sall1 and zic5, were expressed in distal mesenchyme. Zic5 has no described association with limb development. We also describe expression of two proximal genes, gata5 and tnn, not previously associated with limb development. Differentially expressed genes were associated with Fgf, Wnt, and RA signaling as well as differential cell adhesion and proliferation. CONCLUSIONS: We identify new candidate genes for early proximodistal limb patterning. Our analysis of RA-regulated genes supports a role for transient RA gradients in early limb bud in proximal-to-distal patterning in this anamniote model organism.

The fin roundabout: Slit-Robo and S1P signaling coordinate fin morphogenesis.

Development of vertebrate limbs and fins requires that tissue growth is directed outwards, away from the body. How such directed growth is achieved is a fascinating biological problem. For limb/fin formation and outgrowth, signaling between mesenchymal cells and the overlying epithelium is essential. In particular, the epithelium at the distal margin of the growing limb/fin bud, termed the apical ectodermal ridge (AER), promotes directed outgrowth of the underlying mesenchyme, e.g., by providing polarization cues for mesenchymal cell migration. Several classical signaling pathways, such as fibroblast growth factor (Fgf), hedgehog, and Wnt signaling, are involved in the regulation of the cellular events that shape the limb/fin bud (Iovine, 2007). In this issue of EMBO Reports, Carney and colleagues surprisingly find that the Slit-Robo pathway, which is best known for its function in axon guidance, regulates the polarity of developing zebrafish fins (Mahabaleshwar et al, 2007). Intriguingly, they identify an intricate back and forth of signals between the mesenchyme and the AER. Slit ligands derived from mesenchyme act on Robo receptors in the AER to stimulate the production of sphingosine-1-phosphate, which then acts back on the mesenchyme to regulate cell polarity and orientation.

Newt Hoxa13 has an essential and predominant role in digit formation during development and regeneration.

The 5'Hox genes play crucial roles in limb development and specify regions in the proximal-distal axis of limbs. However, there is no direct genetic evidence that Hox genes are essential for limb development in non-mammalian tetrapods or for limb regeneration. Here, we produced single to quadruple Hox13 paralog mutants using the CRISPR/Cas9 system in newts (Pleurodeles waltl), which have strong regenerative capacities, and also produced germline mutants. We show that Hox13 genes are essential for digit formation in development, as in mice. In addition, Hoxa13 has a predominant role in digit formation, unlike in mice. The predominance is probably due to the restricted expression pattern of Hoxd13 in limb buds and the strong dependence of Hoxd13 expression on Hoxa13. Finally, we demonstrate that Hox13 genes are also necessary for digit formation in limb regeneration. Our findings reveal that the general function of Hox13 genes is conserved between limb development and regeneration, and across taxa. The predominance of Hoxa13 function both in newt limbs and fish fins, but not in mouse limbs, suggests a potential contribution of Hoxa13 function in fin-to-limb transition.

Downregulation of Grem1 expression in the distal limb mesoderm is a necessary precondition for phalanx development.

BACKGROUND: The phalanges are the final skeletal elements to form in the vertebrate limb and their identity is regulated by signaling at the phalanx forming region (PFR) located at the tip of the developing digit ray. Here, we seek to explore the relationship between PFR activity and phalanx morphogenesis, which define the most distal limb skeletal elements, and signals associated with termination of limb outgrowth. RESULTS: As Grem1 is extinguished in the distal chick limb mesoderm, the chondrogenesis marker Aggrecan is up-regulated in the metatarsals and phalanges. Fate mapping confirms that subridge mesoderm cells contribute to the metatarsal and phalanges when subridge Grem1 is down-regulated. Grem1 overexpression specifically blocks chick phalanx development by inhibiting PFR activity. PFR activity and digit development are also disrupted following overexpression of a Gli3 repressor, which results in Grem1 expression in the distal limb and downregulation of Bmpr1b. CONCLUSIONS: Based on expression and fate mapping studies, we propose that downregulation of Grem1 in the distal limb marks the transition from metatarsal to phalanx development. This suggests that downregulation of Grem1 in the distal limb mesoderm is necessary for phalanx development. Grem1 downregulation allows for full PFR activity and phalanx progenitor cell commitment to digit fate.

Time-sequenced transcriptomes of developing distal mouse limb buds: A comparative tissue layer analysis.

BACKGROUND: The development of the amniote limb has been an important model system to study patterning mechanisms and morphogenesis. For proper growth and patterning, it requires the interaction between the distal sub-apical mesenchyme and the apical ectodermal ridge (AER) that involve the separate implementation of coordinated and tissue-specific genetic programs. RESULTS: Here, we produce and analyze the transcriptomes of both distal limb mesenchymal progenitors and the overlying ectodermal cells, following time-coursed dissections that cover from limb bud initiation to fully patterned limbs. The comparison of transcriptomes within each layer as well as between layers over time, allowed the identification of specific transcriptional signatures for each of the developmental stages. Special attention was given to the identification of genes whose transcription dynamics suggest a previously unnoticed role in the context of limb development and also to signaling pathways enriched between layers. CONCLUSION: We interpret the transcriptomic data in light of the known development pattern and we conclude that a major transcriptional transition occurs in distal limb buds between E9.5 and E10.5, coincident with the switch from an early phase continuation of the signature of trunk progenitors, related to the initial proximo distal specification, to a late intrinsic phase of development.

Cell-specific alterations in Pitx1 regulatory landscape activation caused by the loss of a single enhancer.

Developmental genes are frequently controlled by multiple enhancers sharing similar specificities. As a result, deletions of such regulatory elements have often failed to reveal their full function. Here, we use the Pitx1 testbed locus to characterize in detail the regulatory and cellular identity alterations following the deletion of one of its enhancers (Pen). By combining single cell transcriptomics and an in-embryo cell tracing approach, we observe an increased fraction of Pitx1 non/low-expressing cells and a decreased fraction of Pitx1 high-expressing cells. We find that the over-representation of Pitx1 non/low-expressing cells originates from a failure of the Pitx1 locus to coordinate enhancer activities and 3D chromatin changes. This locus mis-activation induces a localized heterochrony and a concurrent loss of irregular connective tissue, eventually leading to a clubfoot phenotype. This data suggests that, in some cases, redundant enhancers may be used to locally enforce a robust activation of their host regulatory landscapes.

HES1 is a novel downstream modifier of the SHH-GLI3 Axis in the development of preaxial polydactyly.

Sonic Hedgehog/GLI3 signaling is critical in regulating digit number, such that Gli3-deficiency results in polydactyly and Shh-deficiency leads to digit number reductions. SHH/GLI3 signaling regulates cell cycle factors controlling mesenchymal cell proliferation, while simultaneously regulating Grem1 to coordinate BMP-induced chondrogenesis. SHH/GLI3 signaling also coordinates the expression of additional genes, however their importance in digit formation remain unknown. Utilizing genetic and molecular approaches, we identified HES1 as a downstream modifier of the SHH/GLI signaling axis capable of inducing preaxial polydactyly (PPD), required for Gli3-deficient PPD, and capable of overcoming digit number constraints of Shh-deficiency. Our data indicate that HES1, a direct SHH/GLI signaling target, induces mesenchymal cell proliferation via suppression of Cdkn1b, while inhibiting chondrogenic genes and the anterior autopod boundary regulator, Pax9. These findings establish HES1 as a critical downstream effector of SHH/GLI3 signaling in the development of PPD.

Spatial regulation by multiple Gremlin1 enhancers provides digit development with cis-regulatory robustness and evolutionary plasticity.

Precise cis-regulatory control of gene expression is essential for normal embryogenesis and tissue development. The BMP antagonist Gremlin1 (Grem1) is a key node in the signalling system that coordinately controls limb bud development. Here, we use mouse reverse genetics to identify the enhancers in the Grem1 genomic landscape and the underlying cis-regulatory logics that orchestrate the spatio-temporal Grem1 expression dynamics during limb bud development. We establish that transcript levels are controlled in an additive manner while spatial regulation requires synergistic interactions among multiple enhancers. Disrupting these interactions shows that altered spatial regulation rather than reduced Grem1 transcript levels prefigures digit fusions and loss. Two of the enhancers are evolutionary ancient and highly conserved from basal fishes to mammals. Analysing these enhancers from different species reveal the substantial spatial plasticity in Grem1 regulation in tetrapods and basal fishes, which provides insights into the fin-to-limb transition and evolutionary diversification of pentadactyl limbs.

Expression and function of Ebf1 gene during chondrogenesis in chick embryo limb buds.

The expression profile of early B-cell factor (Ebf) genes and loss of function experiments denote a crucial role for these genes during the late stage of skeletogenesis. However, little is known regarding the expression and function of these genes during the early stage of skeletogenesis. Therefore, this study aimed to detail the spatiotemporal expression pattern of cEbf1, in comparison to cEbf2 and cEbf3, in chick limb buds and investigate its function during chondrogenesis. cEbf1-3 were co-expressed in the distal mesenchyme from a very early stage and later in the outer perichondrium and the surrounding noncartilaginous mesenchymal cells. Ebf1 loss of function through injection of RCASBP virus-carrying Ebf1 dominant-negative form (ΔEbf1) into the wing buds resulted in shortened skeletal elements with a clear defect in the chondrocyte differentiation program. In RCASBP-ΔEbf1 injected wing, the chondrogenesis was initiated normally but hindered at the maturation stage. Subsequently, the chondrocytes failed to become mature or hypertrophic and the long bone diaphysis was not properly developed. The final phenotype included shorter, thicker, and fused long bones. These phenotypic changes were associated with downregulation of the early [Sox9 and collagen type II (Col2a1)] and the late [alkaline phosphatase (AP)] chondrocytes differentiation markers in the limb buds. These results conclude that cEbf1 could be involved in a molecular cascade that promotes the terminal stages of chondrogenesis in the long bone anlagen.

Tbx4 function during hindlimb development reveals a mechanism that explains the origins of proximal limb defects.

We dissect genetically a gene regulatory network that involves the transcription factors Tbx4, Pitx1 and Isl1 acting cooperatively to establish the hindlimb bud, and identify key differences in the pathways that initiate formation of the hindlimb and forelimb. Using live image analysis of murine limb mesenchyme cells undergoing chondrogenesis in micromass culture, we distinguish a series of changes in cellular behaviours and cohesiveness that are required for chondrogenic precursors to undergo differentiation. Furthermore, we provide evidence that the proximal hindlimb defects observed in Tbx4 mutant mice result from a failure in the early differentiation step of chondroprogenitors into chondrocytes, providing an explanation for the origins of proximally biased limb defects.

Control of mouse limb initiation and antero-posterior patterning by Meis transcription factors.

Meis1 and Meis2 are homeodomain transcription factors that regulate organogenesis through cooperation with Hox proteins. Elimination of Meis genes after limb induction has shown their role in limb proximo-distal patterning; however, limb development in the complete absence of Meis function has not been studied. Here, we report that Meis1/2 inactivation in the lateral plate mesoderm of mouse embryos leads to limb agenesis. Meis and Tbx factors converge in this function, extensively co-binding with Tbx to genomic sites and co-regulating enhancers of Fgf10, a critical factor in limb initiation. Limbs with three deleted Meis alleles show proximal-specific skeletal hypoplasia and agenesis of posterior skeletal elements. This failure in posterior specification results from an early role of Meis factors in establishing the limb antero-posterior prepattern required for Shh activation. Our results demonstrate roles for Meis transcription factors in early limb development and identify their involvement in previously undescribed interaction networks that regulate organogenesis.

Generation of a new mouse line with conditionally activated signaling through the BMP receptor, ACVR1: A tool to characterize pleiotropic roles of BMP functions.

BMP signaling plays pleiotropic roles in various tissues during embryogenesis and after birth. We have previously generated a constitutively activated Acvr1(ca-Acvr1) transgenic mouse line (line L35) through pronuclei injection to investigate impacts of enhanced BMP signaling in a tissue specific manner. However, line L35 shows a restricted expression pattern of the transgene. Here, we generated another ca-Acvr1 transgenic line, line A11, using embryonic stem (ES) transgenesis. The generated line A11 shows distinctive phenotypes from line L35, along with very limited expression levels of the transgene. When the transgene is activated in the neural crest cells in a Cre-dependent manner, line A11 exhibits cleft palate and shorter jaws, while line L35 develops ectopic cartilages and highly hypomorphic facial structures. When activated in limb buds, line A11 develops organized but smaller limb skeletal structures, while line L35 forms disorganized limbs with little mineralization. Additionally, no heterotopic ossification (HO) is identified in line A11 when bred with NFATc1-Cre mice even after induction of tissue injury, which is an established protocol for HO for line L35. Therefore, the newly generated conditional ca-Acvr1 mouse line A11 provides an additional resource to dissect highly context dependent functions of BMP signaling in development and disease.

Overexpression of Limb Bud and Heart Alleviates Sepsis-Induced Acute Lung Injury via Inhibiting the NLRP3 Inflammasome.

OBJECTIVE: Sepsis is a leading cause of acute lung injury (ALI). This study attempted to investigate the effects of limb bud and heart (LBH) on the development of sepsis-induced ALI and its underlying mechanism of action. METHODS: The sepsis-induced ALI mouse model was established by cecal ligation and puncture (CLP). The lung injury score and lung wet/dry weight (W/D) ratio were used to evaluate the lung injury. In vitro, ALI was simulated by lipopolysaccharide (LPS) treatment in A549 cells. The mRNA expression of LBH, NLRP3, ASC, and proinflammatory cytokines was measured by qRT-PCR. The viability of LPS-induced A549 cells was analyzed by MTT assay. Furthermore, western blot was performed to detect the protein expression of LBH, NLRP3, and ASC. LPS-induced A549 cells were treated with MCC950 (NLRP3 inflammasome inhibitor) to confirm the effect of LBH on NLRP3 inflammasome. RESULTS: The mRNA and protein expression of LBH was decreased in sepsis-induced ALI. LBH overexpression reduced the lung injury score, lung W/D ratio, expression of proinflammatory cytokines, and NLRP3 inflammasome activation in sepsis-induced ALI mouse model. Additionally, LBH upregulation increased the viability, while it decreased the proinflammatory cytokine expression and NLRP3 inflammasome activation of LPS-induced A549 cells. Moreover, MCC950 reversed the promoting effects of LBH silencing on proinflammatory cytokine expression and NLRP3 inflammasome activation in LPS-induced A549 cells. CONCLUSIONS: LBH alleviated lung injury in sepsis-induced ALI mouse model by inhibiting inflammation and NLRP3 inflammasome, and restrained the inflammation by inhibiting NLRP3 inflammasome in LPS-induced A549 cells, providing a novel therapeutic target for ALI.