Tbx5 Buffers Inherent Left/Right Asymmetry Ensuring Symmetric Forelimb Formation.

The forelimbs and hindlimbs of vertebrates are bilaterally symmetric. The mechanisms that ensure symmetric limb formation are unknown but they can be disrupted in disease. In Holt-Oram Syndrome (HOS), caused by mutations in TBX5, affected individuals have left-biased upper/forelimb defects. We demonstrate a role for the transcription factor Tbx5 in ensuring the symmetric formation of the left and right forelimb. In our mouse model, bilateral hypomorphic levels of Tbx5 produces asymmetric forelimb defects that are consistently more severe in the left limb than the right, phenocopying the left-biased limb defects seen in HOS patients. In Tbx hypomorphic mutants maintained on an INV mutant background, with situs inversus, the laterality of defects is reversed. Our data demonstrate an early, inherent asymmetry in the left and right limb-forming regions and that threshold levels of Tbx5 are required to overcome this asymmetry to ensure symmetric forelimb formation.

Subdivision of the lateral plate mesoderm and specification of the forelimb and hindlimb forming domains.

The limbs are a significant evolutionary innovation that enabled vertebrates to diversify and colonise new environments. Tetrapods have two pairs of limbs, forelimbs in the upper body and hindlimbs in the lower body. The morphologies of the forelimbs and hindlimbs are distinct, reflecting their specific locomotory functions although they share many common signalling networks that regulate their development. The paired appendages in vertebrates form at fixed positions along the rostral-caudal axis and this occurs as a consequence of earlier subdivision of the lateral plate mesoderm (LPM) into regions with distinct limb forming potential. In this review, we discuss the molecular mechanisms that confer a broad region of the flank with limb-forming potential and its subsequent refinement into distinct forelimb-forming, hindlimb-forming and interlimb territories.

A combination of activation and repression by a colinear Hox code controls forelimb-restricted expression of Tbx5 and reveals Hox protein specificity.

Tight control over gene expression is essential for precision in embryonic development and acquisition of the regulatory elements responsible is the predominant driver for evolution of new structures. Tbx5 and Tbx4, two genes expressed in forelimb and hindlimb-forming regions respectively, play crucial roles in the initiation of limb outgrowth. Evolution of regulatory elements that activate Tbx5 in rostral LPM was essential for the acquisition of forelimbs in vertebrates. We identified such a regulatory element for Tbx5 and demonstrated Hox genes are essential, direct regulators. While the importance of Hox genes in regulating embryonic development is clear, Hox targets and the ways in which each protein executes its specific function are not known. We reveal how nested Hox expression along the rostro-caudal axis restricts Tbx5 expression to forelimb. We demonstrate that Hoxc9, which is expressed in caudal LPM where Tbx5 is not expressed, can form a repressive complex on the Tbx5 forelimb regulatory element. This repressive capacity is limited to Hox proteins expressed in caudal LPM and carried out by two separate protein domains in Hoxc9. Forelimb-restricted expression of Tbx5 and ultimately forelimb formation is therefore achieved through co-option of two characteristics of Hox genes; their colinear expression along the body axis and the functional specificity of different paralogs. Active complexes can be formed by Hox PG proteins present throughout the rostral-caudal LPM while restriction of Tbx5 expression is achieved by superimposing a dominant repressive (Hoxc9) complex that determines the caudal boundary of Tbx5 expression. Our results reveal the regulatory mechanism that ensures emergence of the forelimbs at the correct position along the body. Acquisition of this regulatory element would have been critical for the evolution of limbs in vertebrates and modulation of the factors we have identified can be molecular drivers of the diversity in limb morphology.

Hox genes regulate the onset of Tbx5 expression in the forelimb.

Tbx4 and Tbx5 are two closely related T-box genes that encode transcription factors expressed in the prospective hindlimb and forelimb territories, respectively, of all jawed vertebrates. Despite their striking limb type-restricted expression pattern, we have shown that these genes do not participate in the acquisition of limb type-specific morphologies. Instead, Tbx4 and Tbx5 play similar roles in the initiation of hindlimb and forelimb outgrowth, respectively. We hypothesized that different combinations of Hox proteins expressed in different rostral and caudal domains of the lateral plate mesoderm, where limb induction occurs, might be involved in regulating the limb type-restricted expression of Tbx4 and Tbx5 and in the later determination of limb type-specific morphologies. Here, we identify the minimal regulatory element sufficient for the earliest forelimb-restricted expression of the mouse Tbx5 gene and show that this sequence is Hox responsive. Our results support a mechanism in which Hox genes act upstream of Tbx5 to control the axial position of forelimb formation.

The secreted integrin ligand nephronectin is necessary for forelimb formation in Xenopus tropicalis.

While limb regeneration has been extensively studied in amphibians, little is known about the initial events in limb formation in metamorphosing anurans. The small secreted integrin ligand nephronectin (npnt) is necessary for development of the metanephros in mouse. Although expressed in many tissues, its role in other developmental processes is not well-studied. Here we show that a transgene insertion that disrupts this gene ablates forelimb formation in Xenopus tropicalis. Our results suggest a novel role for integrin signalling in limb development, and represent the first insertional phenotype to be cloned in amphibians.

RA Acts in a Coherent Feed-Forward Mechanism with Tbx5 to Control Limb Bud Induction and Initiation.

The retinoic acid (RA)- and ß-catenin-signaling pathways regulate limb bud induction and initiation; however, their mechanisms of action are not understood and have been disputed. We demonstrate that both pathways are essential and that RA and ß-catenin/TCF/LEF signaling act cooperatively with Hox gene inputs to directly regulate Tbx5 expression. Furthermore, in contrast to previous models, we show that Tbx5 and Tbx4 expression in forelimb and hindlimb, respectively, are not sufficient for limb outgrowth and that input from RA is required. Collectively, our data indicate that RA signaling and Tbx genes act in a coherent feed-forward loop to regulate Fgf10 expression and, as a result, establish a positive feedback loop of FGF signaling between the limb mesenchyme and ectoderm. Our results incorporate RA-, ß-catenin/TCF/LEF-, and FGF-signaling pathways into a regulatory network acting to recruit cells of the embryo flank to become limb precursors.

Fibroblast growth factor 13 is essential for neural differentiation in Xenopus early embryonic development.

In Xenopus embryonic development, the MEK5-ERK5 pathway, one of the MAPK pathways, lies downstream of SoxD and upstream of Xngnr1 in a signaling pathway regulating neural differentiation. It remains unclear, however, how the MEK5-ERK5 pathway is regulated in Xenopus neural development. As SoxD is a transcription factor, we hypothesized that some growth factor should be induced by SoxD and activate the MEK5-ERK5 pathway. As the expression level of fibroblast growth factor 13 (FGF13) is increased by SoxD, we analyzed the function of FGF13 in neural development. Knockdown of FGF13 with antisense morpholino-oligonucleotides (MOs) results in the reduced head structure and inhibition of neural differentiation. FGF13 MOs inhibit the SoxD-induced expression of Xngnr1 and the Xngnr1-induced expression of NeuroD, suggesting that FGF13 is necessary both upstream and downstream of Xngnr1 in neural differentiation. In addition, FGF13 MOs inhibit the activation of the MEK5-ERK5 pathway by dominant-negative bone morphogenetic protein receptor, a mimicker of neural inducers, indicating that FGF13 is involved in the activation of the MEK5-ERK5 pathway. Together, these results identify a role of FGF13 in Xenopus neural differentiation.

Activation of a C-terminal transcriptional activation domain of ERK5 by autophosphorylation.

ERK5 plays a crucial role in many biological processes by regulating transcription. ERK5 has a large C-terminal-half that contains a transcriptional activation domain. However, it has remained unclear how its transcriptional activation activity is regulated. Here, we show that the activated kinase activity of ERK5 is required for the C-terminal-half to enhance the AP-1 activity, and that the activated ERK5 undergoes autophosphorylation on its most C-terminal region. Changing these phosphorylatable threonine and serine residues to unphosphorylatable alanines significantly reduces the transcriptional activation activity of ERK5. Moreover, phosphomimetic mutants of the C-terminal-half of ERK5 without an N-terminal kinase domain are shown to be able to enhance the AP-1 activity in fibroblastic cells. These results reveal the role of the stimulus-induced ERK5 autophosphorylation in regulation of gene expression.

MAPK signalling: ERK5 versus ERK1/2.

Extracellular-signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase (MAPK) family and, similar to ERK1/2, has the Thr-Glu-Tyr (TEY) activation motif. Both ERK5 and ERK1/2 are activated by growth factors and have an important role in the regulation of cell proliferation and cell differentiation. Moreover, both the ERK5 and the ERK1/2 pathways are sensitive to PD98059 and U0126, which are two well-known inhibitors of the ERK pathway. Despite these similarities, recent studies have revealed distinctive features of the ERK5 pathway: ERK5 has a key role in cardiovascular development and neural differentiation; ERK5 nuclear translocation is controlled by its own nuclear localizing and nuclear export activities; and the carboxy-terminal half of ERK5, which follows its kinase catalytic domain, has a unique function.

Requirement of the MEK5-ERK5 pathway for neural differentiation in Xenopus embryonic development.

Although previous studies have identified several key transcription factors in the generation process of the vertebrate nervous system, the intracellular signalling pathways that function in this process have remained unclear. Here we identify the evolutionarily conserved mitogen-activated protein kinase kinase 5 (MEK5)-extracellular signal-regulated kinase 5 (ERK5) pathway as an essential regulator in neural differentiation. Knockdown of Xenopus ERK5 or Xenopus MEK5 with antisense morpholino oligonucleotides results in the reduced head structure and inhibition of neural differentiation. Moreover, forced activation of the MEK5-ERK5 module on its own induces neural differentiation. In addition, we show that the MEK5-ERK5 pathway is necessary for the neuralizing activity of SoxD, a regulator of neural differentiation, and is sufficient for the expression of Xngnr1, a proneural gene. These results show that the MEK5-ERK5 pathway has an essential role in the regulation of neural differentiation downstream of SoxD and upstream of Xngnr1.