The ectodermal control in chick limb development: Wnt-7a, Shh, Bmp-2 and Bmp-4 expression and the effect of FGF-4 on gene expression

Lewis Wolpert (1929-2021)

Lewis Wolpert was a brilliant and inspiring scientist who made hugely significant contributions which underpin and influence our understanding of developmental biology today. He spent his career interested in how the fertilised egg can give rise to the whole embryo (and ultimately the adult) with one head, two arms, two legs, all its organs and importantly how cells become different from each other and how they 'know' what to become. His ideas revolutionised the way developmental biology was perceived and also reinvigorated, in particular, the key question of how pattern formation in embryonic development is achieved. He published over 200 scientific articles and received many accolades over his career for his work and services to science in the UK. These included a CBE (Commander of the Order of the British Empire) from the Queen, being elected a Fellow of the Royal Society and a Fellow of the Royal Society of Literature. He was also a recipient of the Waddington Medal from the British Society for Developmental Biology and was awarded The Royal Society's top honour, the Royal Medal in 2018. Lewis was also a gifted teacher and communicator, including being the author of a textbook on developmental biology used around the world to train the next generation of developmental biologists. This contribution was recognised in 2003, by the award of the Viktor Hamburger Outstanding Educator Award from the Society of Developmental Biology in the USA. Lewis always enjoyed giving talks and lectures, having an infectious and persuasive enthusiasm coupled with a sharp sense of humour. He also published articles in popular science journals (aimed at the public) such as New Scientist, Scientific American and The Scientist. Lewis also wrote several popular science books. He was a passionate advocate for the public understanding of science and was the Chair of The Royal Society/Royal Institution/British Association for the Advancement of Science Committee for Public Understanding of Science (1994-1998). For this contribution he was awarded The Royal Society Michael Faraday Medal for "excellence in communicating science to UK audiences". He presented the prestigious Royal Institution Christmas Lectures in 1986 entitled 'Frankenstein's Quest: development of life'. These lectures, six in total, are presented by leading scientists and aimed at the general public and broadcast on national television. On a personal level, Lewis influenced all who came into contact with him, shaped his students and postdocs careers and instilled in them, and the community as whole, a life-long love of developmental biology.

WNT signaling and cartilage: of mice and men

In adult articular cartilage, the extracellular matrix is maintained by a balance between the degradation and the synthesis of matrix components. Chondrocytes that sparsely reside in the matrix and rarely proliferate are the key cellular mediators for cartilage homeostasis. There are indications for the involvement of the WNT signaling pathway in maintaining articular cartilage. Various WNTs are involved in the subsequent stages of chondrocyte differentiation during development, and deregulation of WNT signaling was observed in cartilage degeneration. Even though gene expression and protein synthesis can be activated upon injury, articular cartilage has a limited ability of self-repair and efforts to regenerate articular cartilage have so far not been successful. Because WNT signaling was found to be involved in the development and maintenance of cartilage as well as in the degeneration of cartilage, interfering with this pathway might contribute to improving cartilage regeneration. However, most of the studies on elucidating the role of WNT signaling in these processes were conducted using in vitro or in vivo animal models. Discrepancies have been found in the role of WNT signaling between chondrocytes of mouse and human origin, and extrapolation of results from mouse models to the human situation remains a challenge. Elucidation of detailed WNT signaling functions will provide knowledge to improve cartilage regeneration.

Mechanoactive tenogenic differentiation of human mesenchymal stem cells

A mesenchymal stem cell (MSC)-seeded collagen gel under static or dynamic tension is a well-established model to study the potential of MSCs in regenerating a tendon- or ligament-like tissue. Using this model, upregulation of fibrillar collagen mRNA expression and protein production has been demonstrated in response to cyclic tensile mechanical stimulation. However, the mechanisms driving MSC tenogenesis (differentiation into tendon or ligament fibroblasts) have not been elucidated. This study investigated the mechanisms of tenogenesis of human bone marrow-derived MSCs in a dynamic, three-dimensional (3D) tissue-engineering model by investigating the effects of cyclic stretching on matrix production and gene expression of candidate tendon and ligament markers. The 3D MSC tenogenesis culture system upregulated scleraxis, but cyclic stretching was required to maintain expression of this putative tendon marker over time. Enhanced tendinous neo-tissue development demonstrated with extracellular matrix staining was largely due to changes in matrix deposition and remodeling activity under dynamic loading conditions, as evidenced by differential regulation of matrix metalloproteinases at a transcriptional level with minimal changes in collagen mRNA levels. Regulation of Wnt gene expression with cyclic stimulation suggested a similar role for Wnt4 versus Wnt5a in tenogenesis as in cartilage development. This first report of the potential involvement of matrix remodeling and Wnt signaling during tenogenesis of human MSCs in a dynamic, 3D tissue-engineering model provides insights into the mechanisms of tenogenesis in a mechanoactive environment and supports the therapeutic potential of adult stem cells.

Abnormal development of the apical ectodermal ridge and polysyndactyly in Megf7-deficient mice

Megf7/Lrp4 is a member of the functionally diverse low-density lipoprotein receptor gene family, a class of ancient and highly conserved cell surface receptors with broad functions in cargo transport and cellular signaling. To gain insight into the as yet unknown biological role of Megf7/Lrp4, we have disrupted the gene in mice. Homozygous Megf7-deficient mice are growth-retarded, with fully penetrant polysyndactyly in their fore and hind limbs, and partially penetrant abnormalities of tooth development. The reason for this developmental abnormality is apparent as early as embryonic day 9.5 when the apical ectodermal ridge (AER), the principal site of Megf7 expression at the distal edge of the embryonic limb bud, forms abnormally in the absence of Megf7. Ectopic expression and aberrant signaling of several molecules involved in limb patterning, including Fgf8, Shh, Bmp2, Bmp4 and Wnt7a, as well as the Wnt- and Bmp-responsive transcription factors Lmx1b and Msx1, result in reduced apoptosis and symmetrical dorsal and ventral expansions of the AER. Abnormal signaling from the AER precedes ectopic chondrocyte condensation and subsequent fusion and duplication of digits in the Megf7 knockouts. Megf7 can antagonize canonical Wnt signaling in vitro. Taken together, these findings are consistent with a role of Megf7 as a modulator of cellular signaling pathways involving Wnts, Bmps, Fgfs and Shh. A similar autosomal recessive defect may also occur in man, where polysyndactyly, in combination with craniofacial abnormalities, is also part of a common genetic syndrome.

Coordinate expression of Fgf8, Otx2, Bmp4, and Shh in the rostral prosencephalon during development of the telencephalic and optic vesicles

Previous studies suggest that Fgf8 has a key role in regulating vertebrate development. In the rostral head of the embryonic chicken, there are increasing numbers of separate Fgf8 domains; these are present in tissues that appear to have previously expressed Otx2. As Fgf8 expression becomes established, Otx2 expression weakens, but remains in cells abutting the Fgf8 expression domain. These Fgf8 expression domains are closely associated with tissues expressing Bmp4 and Shh. Based on analogy with the embryonic limb, we suggest that Fgf8, Bmp4 and Shh function together in patterning regions of the embryonic head. Gene expression changes are particularly prominent in 14-21 somite stage embryos in the rostral forebrain, during early morphogenesis of the telencephalic and optic vesicles, when several new interfaces of Fgf8, Bmp4 and Shh are generated. To gain insights into the functions of fibroblast growth factor 8 (FGF8) in the embryonic forebrain, we studied the effects of implanting beads containing this protein in the dorsal prosencephalon of embryonic day 2 chicken embryos. Ectopic FGF8 had profound effects on morphogenesis of the telencephalic and optic vesicles. It disrupted formation of the optic stalk and caused a transformation of the pigment epithelium into neural retina. Within the telencephalon, FGF8 beads frequently induced a sulcus that had features of an ectopic rostral midline. The sulcus separated the telencephalon into rostral and caudal vesicles. Furthermore, we present evidence that FGF8 can regulate regionalization of the prosencephalon through inhibition of Otx2 and Emx2 expression. Thus, these experiments provide evidence that FGF8 can regulate both morphogenesis and patterning of the rostral prosencephalon (telencephalic and optic vesicles). FGF8 beads can induce midline properties (e.g. a sulcus) and can modulate the specification and differentiation of adjacent tissues. We suggest that some of these effects are through regulating the expression of homeobox genes (Otx2 and Emx2) that are known to participate in forebrain patterning.

Differential response of Shh expression between chick forelimb and hindlimb buds by FGF-4

The interactions of Sonic hedgehog (Shh) and fibroblast growth factor (FGF) play important roles in vertebrate limb pattern formation. In the posterior region of the chick limb bud, Shh and FGF-4 each maintain expression in a positive feedback loop. In the anterior region, Shh can also induce Fgf-4 expression in the anterior apical ectodermal ridge. However, the possibility of Shh induction by FGF protein is unclear. Because many experiments to analyze gene expression have been carried out by using the forelimb bud of the chick embryo, we investigated gene expression of the cells in the anterior region of the chick hindlimb bud after FGF-4 application and compared the results with those for the forelimb bud. When an FGF-4-containing bead was implanted into the anterior region of a stage 20 hindlimb bud, ectopic expression of Shh was induced in the mesenchyme beneath the anterior end of the apical ectodermal ridge at 36 hr after implantation. Subsequent to Shh activation, Hoxd13 was also observed in the anterior-distal region of the limb bud. Furthermore, FGF-4 implantation to the hindlimb bud caused additional digit formation accompanying respecification of positional value in the anterior tissue. Ectopic Shh was induced in cells located distal to the FGF-4 bead, and the cells of the flank region did not contribute to ectopic Shh induction. On the other hand, no ectopic Shh and Hoxd13 expression was detected by grafting an FGF-4 bead into the forelimb bud. Although FGF-4 implantation to the forelimb bud occasionally induced extra digit 2 formation, no embryos had an extra digit 3 or digit 4, and many specimens exhibited normal skeletal pattern. These results demonstrate the difference between the fore- and hindlimb buds in the cell competence of Shh induction in response to FGF-4, suggesting the possibility that the responsiveness of mesenchymal cells in signaling molecules is not the same in the fore- and hindlimb buds.

Role of FGFs in the control of programmed cell death during limb development

We have investigated the role of FGFs in the control of programmed cell death during limb development by analyzing the effects of increasing and blocking FGF signaling in the avian limb bud. BMPs are currently considered as the signals responsible for cell death. Here we show that FGF signaling is also necessary for apoptosis and that the establishment of the areas of cell death is regulated by the convergence of FGF- and BMP-mediated signaling pathways. As previously demonstrated, cell death is inhibited for short intervals (12 hours) after administration of FGFs. However, this initial inhibition is followed (24 hours) by a dramatic increase in cell death, which can be abolished by treatments with a BMP antagonist (Noggin or Gremlin). Conversely, blockage of FGF signaling by applying a specific FGF-inhibitor (SU5402) into the interdigital regions inhibits both physiological cell death and that mediated by exogenous BMPs. Furthermore, FGF receptors 1, 2 and 3 are expressed in the autopodial mesoderm during the regression of the interdigital tissue, and the expression of FGFR3 in the interdigital regions is regulated by FGFs and BMPs in the same fashion as apopotosis. Together our findings indicate that, in the absence of FGF signaling BMPs are not sufficient to trigger apoptosis in the developing limb. Although we provide evidence for a positive influence of FGFs on BMP gene expression, the physiological implication of FGFs in apoptosis appears to result from their requirement for the expression of genes of the apoptotic cascade. We have identified MSX2 and Snail as candidate genes associated with apoptosis the expression of which requires the combined action of FGFs and BMPs.

csal1 is controlled by a combination of FGF and Wnt signals in developing limb buds

While some of the signaling molecules that govern establishment of the limb axis have been characterized, little is known about the downstream effector genes that interpret these signals. In Drosophila, the spalt gene is involved in cell fate determination and pattern formation in different tissues. We have cloned a chick homologue of Drosophila spalt, which we have termed csal1, and this study focuses on the regulation of csal1 expression in the limb bud. csal1 is expressed in limb buds from HH 17 to 26, in both the apical ectodermal ridge and the distal mesenchyme. Signals from the apical ridge are essential for csal1 expression, while the dorsal ectoderm is required for csal1 expression at a distance from the ridge. Our data indicate that both FGF and Wnt signals are required for the regulation of csal1 expression in the limb. Mutations in the human homologue of csal1, termed Hsal1/SALL1, result in a condition known as Townes-Brocks syndrome (TBS), which is characterized by preaxial polydactyly. The developmental expression of csal1 together with the digit phenotype in TBS patients suggests that csal1 may play a role in some aspects of distal patterning.

Mechanism of action in thalidomide teratogenesis

In this commentary, we describe a model to explain the mechanism of the embryopathy of thalidomide. We propose that thalidomide affects the following pathway during development: insulin-like growth factor I (IGF-I) and fibroblast growth factor 2 (FGF-2) stimulation of the transcription of alphav and beta3 integrin subunit genes. The resulting alphavbeta3 integrin dimer stimulates angiogenesis in the developing limb bud, which promotes outgrowth of the bud. The promoters of the IGF-I and FGF-2 genes, the genes for their binding proteins and receptors, as well as the alphav and beta3 genes, lack typical TATA boxes, but instead contain multiple GC boxes (GGGCGG). Thalidomide, or a breakdown product of thalidomide, specifically binds to these GC promoter sites, decreasing transcription efficiency of the associated genes. A cumulative decrease interferes with normal angiogenesis, which results in truncation of the limb. Intercalation into G-rich promoter regions of DNA may explain why certain thalidomide analogs are not teratogenic while retaining their anti-tumor necrosis factor-alpha (TNF-alpha) activity, and suggests that we look elsewhere to explain the action of thalidomide on TNF-alpha. On the other hand, the anti-cancer action of thalidomide may be based on its antiangiogenic action, resulting from specific DNA intercalation. The tissue specificity of thalidomide and its effect against only certain neoplasias may be explained by the fact that various developing tissues and neoplasias depend on different angiogenesis or vasculogenesis pathways, only some of which are thalidomide-sensitive.

Successive formative stages of precartilaginous mesenchymal condensations in vitro: modulation of cell adhesion by Wnt-7A and BMP-2

High-density chick limb bud cell culture is a useful model to study mesenchymal condensatifons and chondrogenesis. Most previous studies have focused on the effects of soluble reagents on terminal chondrogenic differentiation and have not defined the early cellular processes and signaling events. In this study, we defined five successive stages in the differentiation process: 1) dissociated cells, 2) small aggregates, 3) formation of cell clusters, 4) precartilaginous condensations, and 5) cartilage nodule. We used RCAS retrovirus-mediated Wnt-7a gene transduction to test the effect of Wnt-7a on the differentiation process. We found that Wnt-7a suppressed chondrogenic differentiation. Wnt-7a did not inhibit the initiation of condensation formation but blocked the progression of precartilaginous condensations to cartilage nodules. The Wnt-7a-transduced cultures showed characteristics of a less mature culture with persistent expression of NCAM, N-cadherin, wider distribution of integrin beta1 and fibronectin, and suppression of tenascin-C. BMP-2 is known to enhance chondrogenic differentiation in these cultures by promoting cell clusters to form continuous sheet-like precartilaginous condensations. However, cultures exposed to both BMP-2 and Wnt-7a showed inhibition of chondrogenic differentiation. Different signaling molecules such as Wnt-7a and BMP-2 may have antagonistic effects on cartilage differentiation and the gradient of the two molecules may be involved in defining the boundaries of the initial precartilaginous condensation. We propose that the shape of the precartilaginous condensations may be modulated by local concentrations of signaling molecules, such as Wnt-7a and BMP-2, which act to alter cell-substrate and cell-cell adhesions.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

Dorsal-ventral signaling in limb development

In both Drosophila wings and vertebrate limbs, signaling between dorsal and ventral cells establishes an organizer that promotes limb formation. Significant progress has been made recently towards characterizing the signaling interactions that occur at the dorsal-ventral limb border. Studies of chicks have indicated that, as in Drosophila, this signaling process requires the participation of Fringe. Studies of Drosophila have indicated that Fringe functions by inhibiting the ability of Notch to be activated by one ligand, Serrate, while potentiating the ability of Notch to be activated by another ligand, Delta. Recent studies of both Drosophila and vertebrates have also shed new light on the signaling activity of the dorsal-ventral boundary limb organizer, and have highlighted how this organizer is maintained by feedback mechanisms with neighboring cells.