Fibroblast growth factors 2 and 4 stimulate migration of mouse embryonic limb myogenic cells

Building Haplotype-Resolved 3D Genome Maps of Chicken Skeletal Muscle

Haplotype-resolved 3D chromatin architecture related to allelic differences in avian skeletal muscle development has not been addressed so far, although chicken husbandry for meat consumption has been prevalent feature of cultures on every continent for more than thousands of years. Here, high-resolution Hi-C diploid maps (1.2-kb maximum resolution) are generated for skeletal muscle tissues in chicken across three developmental stages (embryonic day 15 to day 30 post-hatching). The sequence features governing spatial arrangement of chromosomes and characterize homolog pairing in the nucleus, are identified. Multi-scale characterization of chromatin reorganization between stages from myogenesis in the fetus to myofiber hypertrophy after hatching show concordant changes in transcriptional regulation by relevant signaling pathways. Further interrogation of parent-of-origin-specific chromatin conformation supported that genomic imprinting is absent in birds. This study also reveals promoter-enhancer interaction (PEI) differences between broiler and layer haplotypes in skeletal muscle development-related genes are related to genetic variation between breeds, however, only a minority of breed-specific variations likely contribute to phenotypic divergence in skeletal muscle potentially via allelic PEI rewiring. Beyond defining the haplotype-specific 3D chromatin architecture in chicken, this study provides a rich resource for investigating allelic regulatory divergence among chicken breeds.

Fibroblast growth factor 2

Fibroblast Growth Factor 2 (FGF2), also known as basic fibroblast growth factor, is a potent stimulator of growth and differentiation in multiple tissues. Its discovery traces back over 50 years ago when it was first isolated from bovine pituitary extracts due to its ability to stimulate fibroblast proliferation. Subsequent studies investigating the genomic structure of FGF2 identified multiple protein isoforms, categorized as the low molecular weight and high molecular weight FGF2. These isoforms arise from alternative translation initiation events and exhibit unique molecular and cellular functions. In this concise review, we aim to provide an overview of what is currently known about the structure, expression, and functions of the FGF2 isoforms within the contexts of development, homeostasis, and disease.

Potential of Soluble Decellularized Extracellular Matrix for Musculoskeletal Tissue Engineering - Comparison of Various Mesenchymal Tissues

Background

It is well studied that preparations of decellularized extracellular matrix (ECM) obtained from mesenchymal tissues can function as biological scaffolds to regenerate injured musculoskeletal tissues. Previously, we reported that soluble decellularized ECMs derived from meniscal tissue demonstrated excellent biocompatibility and produced meniscal regenerate with native meniscal anatomy and biochemical characteristics. We therefore hypothesized that decellularized mesenchymal tissue ECMs from various mesenchymal tissues should exhibit tissue-specific bioactivity. The purpose of this study was to test this hypothesis using porcine tissues, for potential applications in musculoskeletal tissue engineering.

Methods

Nine types of porcine tissue, including cartilage, meniscus, ligament, tendon, muscle, synovium, fat pad, fat, and bone, were decellularized using established methods and solubilized. Although the current trend is to develop tissue specific decellularization protocols, we selected a simple standard protocol across all tissues using Triton X-100 and DNase/RNase after mincing to compare the outcome. The content of sulfated glycosaminoglycan (sGAG) and hydroxyproline were quantified to determine the biochemical composition of each tissue. Along with the concentration of several growth factors, known to be involved in tissue repair and/or maturation, including bFGF, IGF-1, VEGF, and TGF-β1. The effect of soluble ECMs on cell differentiation was explored by combining them with 3D collagen scaffold culturing human synovium derived mesenchymal stem cells (hSMSCs).

Results

The decellularization of each tissue was performed and confirmed both histologically [hematoxylin and eosin (H&E) and 4’,6-diamidino-2-phenylindole (DAPI) staining] and on the basis of dsDNA quantification. The content of hydroxyproline of each tissue was relatively unchanged during the decellularization process when comparing the native and decellularized tissue. Cartilage and meniscus exhibited a significant decrease in sGAG content. The content of hydroxyproline in meniscus-derived ECM was the highest when compared with other tissues, while sGAG content in cartilage was the highest. Interestingly, a tissue-specific composition of most of the growth factors was measured in each soluble decellularized ECM and specific differentiation potential was particularly evident in cartilage, ligament and bone derived ECMs.

Conclusion

In this study, soluble decellularized ECMs exhibited differences based on their tissue of origin and the present results are important going forward in the field of musculoskeletal regeneration therapy.

Cellular dynamics of myogenic cell migration: molecular mechanisms and implications for skeletal muscle cell therapies

Directional cell migration is a critical process underlying morphogenesis and post-natal tissue regeneration. During embryonic myogenesis, migration of skeletal myogenic progenitors is essential to generate the anlagen of limbs, diaphragm and tongue, whereas in post-natal skeletal muscles, migration of muscle satellite (stem) cells towards regions of injury is necessary for repair and regeneration of muscle fibres. Additionally, safe and efficient migration of transplanted cells is critical in cell therapies, both allogeneic and autologous. Although various myogenic cell types have been administered intramuscularly or intravascularly, functional restoration has not been achieved yet in patients with degenerative diseases affecting multiple large muscles. One of the key reasons for this negative outcome is the limited migration of donor cells, which hinders the overall cell engraftment potential. Here, we review mechanisms of myogenic stem/progenitor cell migration during skeletal muscle development and post-natal regeneration. Furthermore, strategies utilised to improve migratory capacity of myogenic cells are examined in order to identify potential treatments that may be applied to future transplantation protocols.

Profiling of human lymphocytes reveals a specific network of protein kinases modulated by endurance training status

To date, the effects of endurance exercise training on lymphocyte physiology at the kinome level are largely unknown. Therefore, the present study used a highly sensitive peptide-based kinase activity profiling approach to investigate if the basal activity of tyrosine (Tyr) and serine/threonine (Ser/Thr) kinases of human lymphocytes is affected by the aerobic endurance training status. Results revealed that the activity of various tyrosine kinases of the FGFR family and ZAP70 was increased, whereas the activity of multiple Ser/Thr kinases such as IKKα, CaMK4, PKAα, PKCα+δ (among others) was decreased in lymphocytes of endurance trained athletes (ET). Moreover, functional associations between several differentially regulated kinases in ET-derived lymphocytes were demonstrated by phylogenetic mapping and network analysis. Especially, Ser/Thr kinases of the AGC-kinase (protein kinase A, G, and C) family represent exercise-sensitive key components within the lymphocytes kinase network that may mediate the long-term effects of endurance training. Furthermore, KEGG (Kyoto Encyclopedia of Genes and Genomes) and Reactome pathway analysis indicate that Ras as well as intracellular signaling by second messengers were found to be enriched in the ET individuals. Overall, our data suggest that endurance exercise training improves the adaptive immune competence by modulating the activity of multiple protein kinases in human lymphocytes.

Developmental mechanisms of migratory muscle precursors in medaka pectoral fin formation

Limb muscles are formed from migratory muscle precursor cells (MMPs) that delaminate from the ventral region of dermomyotomes and migrate into the limb bud. MMPs remain undifferentiated during migration, commencing differentiation into skeletal muscle after arrival in the limb. However, it is still unclear whether the developmental mechanisms of MMPs are conserved in teleost fishes. Here, we investigate the development of pectoral fin muscles in the teleost medaka Oryzias latipes. Expression of the MMP marker lbx1 is first observed in several somites prior to the appearance of fin buds. lbx1-positive cells subsequently move anteriorly and localize in the prospective fin bud region to differentiate into skeletal muscle cells. To address the developmental mechanisms underlying fin muscle formation, we knocked down tbx5, a gene that is required for fin bud formation. tbx5 morphants showed loss of fin buds, whereas lbx1 expression initiated normally in anterior somites. Unlike in normal embryos, expression of lbx1 was not maintained in migrating fin MMPs or within the fin buds. We suggest that fin MMPs appear to undergo two phases in their development, with an initial specification of MMPs occurring independent of fin buds and a second fin bud-dependent phase of MMP migration and proliferation. Our results showed that medaka fin muscle is composed of MMPs. It is suggested that the developmental mechanism of fin muscle formation is conserved in teleost fishes including medaka. Through this study, we also propose new insights into the developmental mechanisms of MMPs in fin bud formation.

Suppression of ERK signalling abolishes primitive endoderm formation but does not promote pluripotency in rabbit embryo

Formation of epiblast (EPI) – the founder line of all embryonic lineages – and extra-embryonic supportive tissues is one of the key events in mammalian development. The prevailing model of early mammalian development is based almost exclusively on the mouse. Here, we provide a comprehensive, stage-by-stage analysis of EPI and extra-embryonic primitive endoderm (PrE) formation during preimplantation development of the rabbit. Although we observed that rabbit embryos have several features in common with mouse embryos, including a stage-related initiation of lineage specification, our results demonstrate the existence of some key differences in lineage specification among mammals. Contrary to the current view, our data suggest that reciprocal repression of GATA6 and NANOG is not fundamental for the initial stages of PrE versus EPI specification in mammals. Furthermore, our results provide insight into the observed discrepancies relating to the role of FGF/ERK signalling in PrE versus EPI specification between mouse and other mammals.

Highlighted Article: A comprehensive analysis of rabbit preimplantation development reveals key differences between rabbit and mouse, with some aspects of lineage specification in rabbit more closely resembling that of human and primate embryos.

Basic Fibroblast Growth Factor 2 Is a Determinant of CD4 T Cell-Airway Smooth Muscle Cell Communication through Membrane Conduits

Activated CD4 T cells connect to airway smooth muscle cells (ASMCs) in vitro via lymphocyte-derived membrane conduits (LMCs) structurally similar to membrane nanotubes with unknown intercellular signals triggering their formation. We examined the structure and function of CD4 T cell-derived LMCs, and we established a role for ASMC-derived basic fibroblast growth factor 2 (FGF2b) and FGF receptor (FGFR)1 in LMC formation. Blocking FGF2b's synthesis and FGFR1 function reduced LMC formation. Mitochondrial flux from ASMCs to T cells was partially FGF2b and FGFR1 dependent. LMC formation by CD4 T cells and mitochondrial transfer from ASMCs was increased in the presence of asthmatic ASMCs that expressed more mRNA for FGF2b compared with normal ASMCs. These observations identify ASMC-derived FGF2b as a factor needed for LMC formation by CD4 T cells, affecting intercellular communication.

bFGF Promotes Migration and Induces Cancer-Associated Fibroblast Differentiation of Mouse Bone Mesenchymal Stem Cells to Promote Tumor Growth

Tumors recruit bone mesenchymal stem cells (BMSCs) to localize to tumor sites, which induces their conversion into cancer-associated fibroblasts (CAFs) that facilitate tumor progression. However, this process is poorly understood on the molecular level. In this study, we found that 4T1 breast cancer cells promoted the migration of BMSCs, and bFGF neutralizing antibody inhibited the migration of BMSCs induced by a tumor-conditioned medium. In addition, exogenous bFGF enhanced the migration of BMSCs in a dose-dependent manner in vitro. Furthermore, BMSCs promoted the proliferation of 4T1 tumor cells under BMSC-conditioned medium and in tumor xenograft model. Dramatically, BMSCs expressed CAF markers and produced collagen in the tumor microenvironment, and this transition was blocked by bFGF antibody. In addition, exogenous bFGF induced CAF differentiation of BMSCs. And bFGF increased phosphorylation of Erk1/2 and Smad3 in BMSCs and Erk inhibitor PD98059 was shown to block bFGF-induced Erk and Smad3 phosphorylation, suggesting that Erk/Smad3 signaling pathway involved in BMSC transdifferentiation induced by bFGF. Collectively, our results indicate that bFGF signaling plays indispensable roles in BMSC recruitment and transdifferentiation into CAFs and the consequent protumor effects, and targeting tumor stroma through bFGF inhibition maybe a promising strategy to suppress tumor progression.

Tissue specific reactions to positional discontinuities in the regenerating axolotl limb

We investigated cellular contributions to intercalary regenerates and 180° supernumerary limbs during axolotl limb regeneration using the cell autonomous GFP marker and exchanged blastemas between white and GFP animals. After distal blastemas were grafted to proximal levels tissues of the intercalary regenerate behaved independently with regard to the law of distal transformation; graft epidermis was replaced by stump epidermis, muscle-derived cells, blood vessels and Schwann cells of the distal blastema moved proximally to the stylopodium and cartilage and dermal cells conformed to the law. After 180° rotation, blastemas showed contributions from stump tissues which failed to alter patterning of the blastema. Supernumerary limbs were composed of stump and graft tissues and extensive contributions of stump tissues generated inversions or duplications of polarity to produce limbs of mixed handedness. Tail skeletal muscle and cardiac muscle broke the law with cells derived from these tissues exhibiting an apparent anteroposterior polarity as they migrated to the anterior side of the blastema. We attribute this behavior to the possible presence of a chemotactic factor from the wound epidermis.

Microfluidic analysis of extracellular matrix-bFGF crosstalk on primary human myoblast chemoproliferation, chemokinesis, and chemotaxis

Exposing myoblasts to basic fibroblast growth factor (bFGF), which is released after muscle injury, results in receptor phosphorylation, faster migration, and increased proliferation. These effects occur on time scales that extend across three orders of magnitude (10(0)-10(3) minutes). Finite element modeling of Transwell assays, which are traditionally used to assess chemotaxis, revealed that the bFGF gradient formed across the membrane pore is short-lived and diminishes 45% within the first minute. Thus, to evaluate bFGF-induced migration over 10(2) minutes, we employed a microfluidic assay capable of producing a stable, linear concentration gradient to perform single-cell analyses of chemokinesis and chemotaxis. We hypothesized that the composition of the underlying extracellular matrix (ECM) may affect the behavioral response of myoblasts to soluble bFGF, as previous work with other cell types has suggested crosstalk between integrin and fibroblast growth factor (FGF) receptors. Consistent with this notion, we found that bFGF significantly reduced the doubling time of myoblasts cultured on laminin but not fibronectin or collagen. Laminin also promoted significantly faster migration speeds (13.4 μm h(-1)) than either fibronectin (10.6 μm h(-1)) or collagen (7.6 μm h(-1)) without bFGF stimulation. Chemokinesis driven by bFGF further increased migration speed in a strictly additive manner, resulting in an average increase of 2.3 μm h(-1) across all ECMs tested. We observed relatively mild chemoattraction (∼67% of myoblast population) in response to bFGF gradients of 3.2 ng mL(-1) mm(-1) regardless of ECM identity. Thus, while ECM-bFGF crosstalk did impact chemoproliferation, it did not have a significant effect on chemokinesis or chemotaxis. These data suggest that the main physiological effect of bFGF on myoblast migration is chemokinesis and that changes in the surrounding ECM, resulting from aging and/or disease may impact muscle regeneration by altering myoblast migration and proliferation.

A Network Map of FGF-1/FGFR Signaling System

Fibroblast growth factor-1 (FGF-1) is a well characterized growth factor among the 22 members of the FGF superfamily in humans. It binds to all the four known FGF receptors and regulates a plethora of functions including cell growth, proliferation, migration, differentiation, and survival in different cell types. FGF-1 is involved in the regulation of diverse physiological processes such as development, angiogenesis, wound healing, adipogenesis, and neurogenesis. Deregulation of FGF-1 signaling is not only implicated in tumorigenesis but also is associated with tumor invasion and metastasis. Given the biomedical significance of FGFs and the fact that individual FGFs have different roles in diverse physiological processes, the analysis of signaling pathways induced by the binding of specific FGFs to their cognate receptors demands more focused efforts. Currently, there are no resources in the public domain that facilitate the analysis of signaling pathways induced by individual FGFs in the FGF/FGFR signaling system. Towards this, we have developed a resource of signaling reactions triggered by FGF-1/FGFR system in various cell types/tissues. The pathway data and the reaction map are made available for download in different community standard data exchange formats through NetPath and NetSlim signaling pathway resources.

Fibroblast growth factor 4 is required but not sufficient for the astrocyte dedifferentiation

Our recent studies demonstrated that mature astrocytes from spinal cord can be reprogrammed in vitro and in vivo to generate neural stem/progenitor cells (NSPCs) following treatment with conditioned medium collected from mechanically injured astrocytes. However, little is known regarding the molecular mechanisms underlying the reprogramming of astrocytes. Here, we show that fibroblast growth factor 4 (FGF4) exerts a critical role in synergistically converting astrocytes into NSPCs that can express multiple neural stem cell markers (nestin and CD133) and are capable of both self-renewal and differentiation into neurons and glia. Lack of FGF4 signals fails to elicit the dedifferentiation of astrocytes towards NSPCs, displaying a substantially lower efficiency in the reprogramming of astrocytes and a slower transition through fate-determined state. These astrocyte-derived NSPCs displayed relatively poor self-renewal and multipotency. More importantly, further investigation suggested that FGF4 is a key molecule necessary for activating PI3K/Akt/p21 signaling cascades, as well as their downstream effectors responsible for directing cell reprogramming towards NSPCs. Collectively, these findings provide a molecular basis for astrocyte dedifferentiation into NSPCs after central nervous system (CNS) injury and imply that FGF4 may be a clinically applicable molecule for in situ neural repair in the CNS disorders.

Mechanisms of muscle injury, repair, and regeneration

Skeletal muscle continuously adapts to changes in its mechanical environment through modifications in gene expression and protein stability that affect its physiological function and mass. However, mechanical stresses commonly exceed the parameters that induce adaptations, producing instead acute injury. Furthermore, the relatively superficial location of many muscles in the body leaves them further vulnerable to acute injuries by exposure to extreme temperatures, contusions, lacerations or toxins. In this article, the molecular, cellular, and mechanical factors that underlie muscle injury and the capacity of muscle to repair and regenerate are presented. Evidence shows that muscle injuries that are caused by eccentric contractions result from direct mechanical damage to myofibrils. However, muscle pathology following other acute injuries is largely attributable to damage to the muscle cell membrane. Many feaures in the injury-repair-regeneration cascade relate to the unregulated influx of calcium through membrane lesions, including: (i) activation of proteases and hydrolases that contribute muscle damage, (ii) activation of enzymes that drive the production of mitogens and motogens for muscle and immune cells involved in injury and repair, and (iii) enabling protein-protein interactions that promote membrane repair. Evidence is also presented to show that the myogenic program that is activated by acute muscle injury and the inflammatory process that follows are highly coordinated, with myeloid cells playing a central role in modulating repair and regeneration. The early-invading, proinflammatory M1 macrophages remove debris caused by injury and express Th1 cytokines that play key roles in regulating the proliferation, migration, and differentiation of satellite cells. The subsequent invasion by anti-inflammatory, M2 macrophages promotes tissue repair and attenuates inflammation. Although this system provides an effective mechanism for muscle repair and regeneration following acute injury, it is dysregulated in chronic injuries. In this article, the process of muscle injury, repair and regeneration that occurs in muscular dystrophy is used as an example of chronic muscle injury, to highlight similarities and differences between the injury and repair processes that occur in acutely and chronically injured muscle.

Promyelocytic leukemia (PML) protein plays important roles in regulating cell adhesion, morphology, proliferation and migration

PML protein plays important roles in regulating cellular homeostasis. It forms PML nuclear bodies (PML-NBs) that act like nuclear relay stations and participate in many cellular functions. In this study, we have examined the proteome of mouse embryonic fibroblasts (MEFs) derived from normal (PML^+/+) and PML knockout (PML^−/−) mice. The aim was to identify proteins that were differentially expressed when MEFs were incapable of producing PML. Using comparative proteomics, total protein were extracted from PML^−/− and PML^+/+ MEFs, resolved by two dimensional electrophoresis (2-DE) gels and the differentially expressed proteins identified by LC-ESI-MS/MS. Nine proteins (PML, NDRG1, CACYBP, CFL1, RSU1, TRIO, CTRO, ANXA4 and UBE2M) were determined to be down-regulated in PML^−/− MEFs. In contrast, ten proteins (CIAPIN1, FAM50A, SUMO2 HSPB1 NSFL1C, PCBP2, YWHAG, STMN1, TPD52L2 and PDAP1) were found up-regulated. Many of these differentially expressed proteins play crucial roles in cell adhesion, migration, morphology and cytokinesis. The protein profiles explain why PML^−/− and PML^+/+ MEFs were morphologically different. In addition, we demonstrated PML^−/− MEFs were less adhesive, proliferated more extensively and migrated significantly slower than PML^+/+ MEFs. NDRG1, a protein that was down-regulated in PML^−/− MEFs, was selected for further investigation. We determined that silencing NDRG1expression in PML^+/+ MEFs increased cell proliferation and inhibited PML expression. Since NDRG expression was suppressed in PML^−/− MEFs, this may explain why these cells proliferate more extensively than PML^+/+ MEFs. Furthermore, silencing NDRG1expression also impaired TGF-β1 signaling by inhibiting SMAD3 phosphorylation.

Gradients of signalling in the developing limb

The developing limb is one of the first systems where it was proposed that a signalling gradient is involved in pattern formation. This gradient for specifying positional information across the antero-posterior axis is based on Sonic hedgehog signalling from the polarizing region. Recent evidence suggests that Sonic hedgehog signalling also specifies positional information across the antero-posterior axis by a timing mechanism acting in parallel with graded signalling. The progress zone model for specifying proximo-distal pattern, involving timing to provide cells with positional information, continues to be challenged, and there is further evidence that graded signalling by retinoic acid specifies the proximal part of the limb. Other recent papers present the first evidence that gradients of signalling by Wnt5a and FGFs govern cell behaviour involved in outgrowth and morphogenesis of the developing limb.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

Fibroblast growth factors: biology, function, and application for tissue regeneration

Fibroblast growth factors (FGFs) that signal through FGF receptors (FGFRs) regulate a broad spectrum of biological functions, including cellular proliferation, survival, migration, and differentiation. The FGF signal pathways are the RAS/MAP kinase pathway, PI3 kinase/AKT pathway, and PLCγ pathway, among which the RAS/MAP kinase pathway is known to be predominant. Several studies have recently implicated the in vitro biological functions of FGFs for tissue regeneration. However, to obtain optimal outcomes in vivo, it is important to enhance the half-life of FGFs and their biological stability. Future applications of FGFs are expected when the biological functions of FGFs are potentiated through the appropriate use of delivery systems and scaffolds. This review will introduce the biology and cellular functions of FGFs and deal with the biomaterials based delivery systems and their current applications for the regeneration of tissues, including skin, blood vessel, muscle, adipose, tendon/ligament, cartilage, bone, tooth, and nerve tissues.

Regulation of EphB2 activation and cell repulsion by feedback control of the MAPK pathway

In this study, we investigated whether the ability of Eph receptor signaling to mediate cell repulsion is antagonized by fibroblast growth factor receptor (FGFR) activation that can promote cell invasion. We find that activation of FGFR1 in EphB2-expressing cells prevents segregation, repulsion, and collapse responses to ephrinB1 ligand. FGFR1 activation leads to increased phosphorylation of unstimulated EphB2, which we show is caused by down-regulation of the leukocyte common antigen–related tyrosine phosphatase receptor that dephosphorylates EphB2. In addition, FGFR1 signaling inhibits further phosphorylation of EphB2 upon stimulation with ephrinB1, and we show that this involves a requirement for the mitogen-activated protein kinase (MAPK) pathway. In the absence of activated FGFR1, EphB2 activates the MAPK pathway, which in turn promotes EphB2 activation in a positive feedback loop. However, after FGFR1 activation, the induction of Sprouty genes inhibits the MAPK pathway downstream of EphB2 and decreases cell repulsion and segregation. These findings reveal a novel feedback loop that promotes EphB2 activation and cell repulsion that is blocked by transcriptional targets of FGFR1.

FGF signals guide migration of mesenchymal cells, control skeletal morphogenesis and regulate gastrulation during sea urchin development

The sea urchin embryo is emerging as an attractive model to study morphogenetic processes such as directed migration of mesenchyme cells and cell sheet invagination, but surprisingly, few of the genes regulating these processes have yet been characterized. We present evidence that FGFA, the first FGF family member characterized in the sea urchin, regulates directed migration of mesenchyme cells, morphogenesis of the skeleton and gastrulation during early development. We found that at blastula stages, FGFA and a novel putative FGF receptor are expressed in a pattern that prefigures morphogenesis of the skeletogenic mesoderm and that suggests that FGFA is one of the elusive signals that guide migration of primary mesenchyme cells (PMCs). We first show that fgfA expression is correlated with abnormal migration and patterning of the PMCs following treatments that perturb specification of the ectoderm along the oral-aboral and animal-vegetal axes. Specification of the ectoderm initiated by Nodal is required to restrict fgfA to the lateral ectoderm, and in the absence of Nodal, fgfA is expressed ectopically throughout most of the ectoderm. Inhibition of either FGFA, FGFR1 or FGFR2 function severely affects morphogenesis of the skeleton. Furthermore,inhibition of FGFA and FGFR1 signaling dramatically delays invagination of the archenteron, prevents regionalization of the gut and abrogates formation of the stomodeum. We identified several genes acting downstream of fgfAin these processes, including the transcription factors pea3 and pax2/5/8 and the signaling molecule sprouty in the lateral ectoderm and SM30 and SM50 in the primary mesenchyme cells. This study identifies the FGF signaling pathway as an essential regulator of gastrulation and directed cell migration in the sea urchin embryo and as a key player in the gene regulatory network directing morphogenesis of the skeleton.

Intrinsic phenotypic diversity of embryonic and fetal myoblasts is revealed by genome-wide gene expression analysis on purified cells

Skeletal muscle development occurs asynchronously and it has been proposed to be dependent upon the generation of temporally distinct populations of myogenic cells. This long-held hypothesis has not been tested directly due to the inability to isolate and analyze purified populations of myoblasts derived from specific stages of prenatal development. Using a mouse strain with the GFP reporter gene targeted into the Myf5 locus, a cell-sorting method was developed for isolating embryonic and fetal myoblasts. The two types of myoblasts show an intrinsic difference in fusion ability, proliferation, differentiation and response to TGFbeta, TPA and BMP-4 in vitro. Microarray and quantitative PCR were used to identify differentially expressed genes both before and after differentiation, thus allowing a precise phenotypic analysis of the two populations. Embryonic and fetal myoblasts differ in the expression of a number of transcription factors and surface molecules, which may control different developmental programs. For example, only embryonic myoblasts express a Hox code along the antero-posterior axis, indicating that they possess direct positional information. Taken together, the data presented here demonstrate that embryonic and fetal myoblasts represent intrinsically different myogenic lineages and provide important information for the understanding of the molecular mechanisms governing skeletal muscle development.

FGF6 in myogenesis

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage. However, the analyses of Fgf6 (-/-) mutant mice gave contradictory results and the role of FGF6 during myogenesis remained largely unclear. Recent reports support the concept that FGF6 has a dual function in muscle regeneration, stimulating myoblast proliferation/migration and muscle differentiation/hypertrophy in a dose-dependent manner. The alternative use of distinct signaling pathways recruiting either FGFR1 or FGFR4 might explain the dual role of FGF6 in myogenesis. A role for FGF6 in the maintenance of a reserve pool of progenitor cells in the skeletal muscle has been also strongly suggested. The aim of this review is to summarize our knowledge on the involvement of FGF6 in myogenesis.

Interleukin-4 improves the migration of human myogenic precursor cells in vitro and in vivo

Different molecules are available to recruit new neighboring myogenic cells to the site of regeneration. Formerly called B cell stimulatory factor-1, IL-4 can now be included in the list of motogenic factors. The present report demonstrates that human IL-4 is not required for fusion between mononucleated myoblasts but is required for myotube maturation. In identifying IL-4 as a pro-migratory agent for myogenic cells, these results provide a mechanism which partly explains IL-4 demonstrated activity during differentiation. Among the different mechanisms by which IL-4 might enhance myoblast migration processes, our results indicate that there are implications of some integrins and of three major components of the fibrinolytic system. Indeed, increases in the amount of active urokinase plasminogen activator and its receptor were observed following an IL-4 treatment, while the plasminogen activator inhibitor-1 decreased. Finally, IL-4 did not modify the amount of cell surface alpha5 integrin but increased the presence of beta3 and beta1 integrins. This integrin modulation might favor myogenic cell migration and its interaction with newly formed myotubes. Therefore, IL-4 co-injection with transplanted myoblasts might be an approach to enhance the migration of transplanted cells for the treatment of a damaged myocardium or of a Duchenne Muscular Dystrophy patient.

Functions and regulations of fibroblast growth factor signaling during embryonic development

Fibroblast growth factors (FGF) are secreted molecules which function through the activation of specific tyrosine kinases receptors, the FGF receptors that transduce the signal by activating different pathways including the Ras/MAP kinase and the phospholipase-C gamma pathways. FGFs are involved in the regulation of many developmental processes including patterning, morphogenesis, differentiation, cell proliferation or migration. Such a diverse set of activities requires a tight control of the transduction signal which is achieved through the induction of different feedback inhibitors such as the Sproutys, Sef and MAP kinase phosphatase 3 which are responsible for the attenuation of FGF signals, limiting FGF activities in time and space.

FGF6 regulates muscle differentiation through a calcineurin-dependent pathway in regenerating soleus of adult mice

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage, but its precise role in vivo remains mostly unclear. Here, using FGF6 (-/-) mice and rescue experiments by injection of recombinant FGF6, we dissected the functional role of FGF6 during in vivo myogenesis. We found that the appearance of myotubes was accelerated during regeneration of the soleus of FGF6 (-/-) mice versus wild type mice. This accelerated differentiation was correlated with increased expression of differentiation markers such as CdkIs and calcineurin, as well as structural markers such as MHCI and slow TnI. We showed that an elevated transcript level for calcineurin Aalpha subunit correlated with a positive regulation of calcineurin A activity in regenerating soleus of the FGF6 (-/-) mice. Cyclin D1 and calcineurin were up- and down-regulated, respectively in a dose-dependent manner upon injection of rhFGF6 in regenerating soleus of the mutant mice. We showed an increase of the number of slow oxidative (type I) myofibers, whereas fast oxidative (type IIa) myofibers were decreased in number in regenerating soleus of FGF6 (-/-) mice versus that of wild type mice. In adult soleus, the number of type I myofibers was also higher in FGF6 (-/-) mice than in wild type mice. Taken together these results evidenced a specific phenotype for soleus of the FGF6 (-/-) mice and led us to propose a model accounting for a specific dose-dependent effect of FGF6 in muscle regeneration. At high doses, FGF6 stimulates the proliferation of the myogenic stem cells, whereas at lower doses it regulates both muscle differentiation and muscle phenotype via a calcineurin-signaling pathway.

FGF-2 signaling is sufficient to induce dermal condensations during feather development

In a previous report, we showed that fibroblast growth factor-2 (FGF-2) is a signal produced by epidermal placode cells during feather development and that this growth factor can induce feathers in scaleless mutant skins that fail to form feathers due to a defective epidermis (Song et al., [1996] Proc Natl Acad Sci USA 93:10246-10249). Here, we test whether FGF-2 is sufficient to induce dermal condensations, structures that normally form under the control of signals from the epidermal placode and are identified on the basis of aggregation of cells, the expression of FGF receptor-1 and bone morphogenetic protein-2 transcripts and the cessation of proliferation of the condensed cells. By using denuded 8-day scaleless dermis as a test system, we have established that FGF-2 is sufficient to induce dermal condensation. We suggest that the primary effect of FGF-2 is an increase in cellular density mediated through cell migration, followed by the expression of dermal condensation-specific genes and cessation of cell proliferation. The FGF-2 effect can be abolished by heparin, suggesting the involvement of heparan sulfate proteoglycans (HSPGs) in growth factor signaling. The spatiotemporal expression of syndecan-3 during feather development suggests that this cell-surface HSPG may be involved in the response of competent embryonic skin dermis to FGF-2.

Fibroblast growth factor signaling during early vertebrate development

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

One day exposure to FGF-2 was sufficient for the regenerative repair of full-thickness defects of articular cartilage in rabbits

OBJECTIVES

Administration of fibroblast growth factor (FGF)-2 for 2 weeks induces a successful cartilaginous repair response in 5-mm full-thickness articular cartilage defects in rabbits. The purpose of this study was to investigate the effects of a short time exposure to FGF-2 on the repair of the defects.

METHODS

Five-mm-diameter cylindrical defects, which do not repair spontaneously, were created in the femoral trochlea of the rabbit knees. The defects were administered sterile saline or FGF-2 (150pg/h) via an osmotic pump for the initial 1 day, 3 days, or 2 weeks, and we assessed the FGF-2 action on the proliferation and migration of mesenchymal cells in the reparative tissue. Using a total of 126 rabbits, we performed three sets of experiments. We also studied the effect of FGF-2 on migration of marrow-derived mesenchymal cells in vitro.

RESULTS

FGF-2 treatment for 1 day or 3 days induced the sequential chondrogenic repair responses that led to successful cartilaginous resurfacing of defects within 8 weeks as well as the 2-week treatment did. We confirmed by a radioisotope study that FGF-2 injected was rapidly eliminated from the defects (a residual ratio of 50% within 30min). The effect of FGF-2 on cultured marrow-derived cells suggested that FGF-2 facilitated the mobilization and migration of replicating mesenchymal cells from bone marrow.

CONCLUSIONS

Only 1 day exposure to FGF-2 is sufficient for induction of the chondrogenic repair response in 5-mm-diameter full-thickness defects of articular cartilage in rabbits. FGF-2 stimulated the recruitment of mesenchymal cells into the defects, which was a limiting step for the induction of cartilage.

mRNA expression of fibroblast growth factors and hepatocyte growth factor in rat plantaris muscle following denervation and compensatory overload

We addressed the question of whether hypertrophy induced by compensatory overload differs according to innervation status, and how fibroblast growth factors (FGF) and hepatocyte growth factor (HGF) mRNAs are expressed in the rat plantaris muscle during overload (OL) and/or denervation. Male Wistar rats were divided into four groups (Normal-Cont, Normal-OL, Denervated-Cont, and Denervated-OL). according to the plantaris denervation and/or overload. Three weeks later, plantaris weight in Denervated-Cont and Denervated-OL was significantly lower than in the Normal-Cont. The muscle weights in the Normal-OL were higher than in the Normal-Cont. The muscle weights in the Denervated-OL were higher than in the Denervated-Cont. Three days after the treatment, FGF-2, FGF-6, FGF-7 and HGF mRNAs in the Normal-OL were significantly higher than those in the Normal-Cont. FGF-2, FGF-6, FGF-7 and HGF mRNAs in the Denervated-OL were also significantly higher after 3 days than those in the Denervated-Cont. After 7 days, FGF-2, FGF-5, FGF-6, FGF-7 and HGF mRNAs were significantly higher in the Normal-OL than those in the Normal-Cont. At 21 days, FGF-1, FGF-6 and HGF mRNA levels were significantly increased. In the Denervated-OL, FGF-2, FGF-7 and HGF mRNAs at 7 days, and FGF-2 mRNA at 21 days were significantly higher than those in the Denervated-Cont. FGF-2 and FGF-6 mRNA levels decreased significantly following denervation; however, FGF-1, FGF-5, FGF-7 and HGF mRNA levels increased and maintained this increase for the 21-days treatment period. Muscle hypertrophy was thus induced by compensatory overload irrespective of innervation status, possibly in association with certain FGFs and HGF. The differential mRNA expression patterns of FGFs and HGF observed following compensatory overload and/or denervation suggest distinct roles for individual FGFs and HGF in muscle hypertrophy and/or atrophy.

Functional analysis of the domains of the C. elegans Ror receptor tyrosine kinase CAM-1

cam-1 encodes a Caenorhabditis elegans orphan receptor tyrosine kinase (RTK) of the Ror family that is required for cell migration and to orient cell polarity. Ror RTKs share a common domain structure. The predicted extracellular region contains immunoglobulin (Ig), cysteine-rich (CRD), and kringle (Kri) domains. Intracellularly are tyrosine kinase (Kin) and serine- and threonine (S/T)-rich domains. To investigate the functional requirement for CAM-1 domains in mediating cell migration, we engineered deletions that remove various domains and assessed the ability of these CAM-1 derivatives to rescue cam-1 mutant phenotypes. We find that the Ig, Kri, Kin, and S/T domains are dispensable for cell migration, but the CRD is required. Surprisingly, the entire intracellular region of CAM-1 is not required for proper cell migration. Most notably, a version of CAM-1 from which all domains besides the CRD and transmembrane domains have been deleted is able to rescue the migration of a single cell type, although not those of other cell types. Our results show that CAM-1 does not function exclusively as a canonical RTK and that it may function, at least in part, to regulate the distribution of a secreted ligand-possibly a Wnt protein.

Reduced mobility of fibroblast growth factor (FGF)-deficient myoblasts might contribute to dystrophic changes in the musculature of FGF2/FGF6/mdx triple-mutant mice

Development and regeneration of muscle tissue is a highly organized, multistep process that requires cell proliferation, migration, differentiation, and maturation. Previous data implicate fibroblast growth factors (FGFs) as critical regulators of these processes, although their precise role in vivo is still not clear. We have explored the consequences of the loss of multiple FGFs (FGF2 and FGF6 in particular) for muscle regeneration in mdx mice, which serve as a model for chronic muscle damage. We show that the combined loss of FGF2 and FGF6 leads to severe dystrophic changes in the musculature. We found that FGF6 mutant myoblasts had decreased migration ability in vivo, whereas wild-type myoblasts migrated normally in a FGF6 mutant environment after transplantation of genetically labeled myoblasts from FGF6 mutants in wild-type mice and vice versa. In addition, retrovirus-mediated expression of dominant-negative versions of Ras and Ral led to a reduced migration of transplanted myoblasts in vivo. We propose that FGFs are critical components of the muscle regeneration machinery that enhance skeletal muscle regeneration, probably by stimulation of muscle stem cell migration.

Intrinsic, Hox-dependent cues determine the fate of skeletal muscle precursors

It is generally held that vertebrate muscle precursors depend totally on environmental cues for their development. We show that instead, somites are predisposed toward a particular myogenic program. This predisposition depends on the somite's axial identity: when flank somites are transformed into limb-level somites, either by shifting somitic boundaries with FGF8 or by overexpressing posterior Hox genes, they readily activate the program typical for migratory limb muscle precursors. The intrinsic control over myogenic programs can only be overridden by FGF4 signals provided by the apical ectodermal ridge of a developing limb.

Regulation of myogenic differentiation in the developing limb bud

The limb myogenic precursors arise by delamination from the lateral dermomyotome in response to signals from the lateral plate mesoderm. They subsequently migrate into the developing limb bud where they switch on the expression of the myogenic regulatory factors, MyoD and Myf5, and coalese to form the dorsal and ventral muscle masses. The myogenic cells subsequently undergo terminal differentiation into slow or fast fibres which have distinct contractile properties determining how a muscle will function. In general, fast fibres contract rapidly with high force and are characterized by the expression of fast myosin heavy chains (MyHC). These fibres are needed for movement. In contrast, slow fibres express slow MyHC, contract slowly and are required for maintenance of posture. This review focuses on the molecular signals that control limb myogenic development from the initial delamination and migration of the premyogenic cells to the ultimate formation of the complex muscle pattern and differentiation of slow and fast fibres.

The formation of skeletal muscle: from somite to limb

During embryogenesis, skeletal muscle forms in the vertebrate limb from progenitor cells originating in the somites. These cells delaminate from the hypaxial edge of the dorsal part of the somite, the dermomyotome, and migrate into the limb bud, where they proliferate, express myogenic determination factors and subsequently differentiate into skeletal muscle. A number of regulatory factors involved in these different steps have been identified. These include Pax3 with its target c-met, Lbx1 and Mox2 as well as the myogenic determination factors Myf5 and MyoD and factors required for differentiation such as Myogenin, Mrf4 and Mef2 isoforms. Mutants for genes such as Lbx1 and Mox2, expressed uniformly in limb muscle progenitors, reveal unexpected differences between fore and hind limb muscles, also indicated by the differential expression of Tbx genes. As development proceeds, a secondary wave of myogenesis takes place, and, postnatally, satellite cells become located under the basal lamina of adult muscle fibres. Satellite cells are thought to be the progenitor cells for adult muscle regeneration, during which similar genes to those which regulate myogenesis in the embryo also play a role. In particular, Pax3 as well as its orthologue Pax7 are important. The origin of secondary/fetal myoblasts and of adult satellite cells is unclear, as is the relation of the latter to so-called SP or stem cell populations, or indeed to potential mesangioblast progenitors, present in blood vessels. The oligoclonal origin of postnatal muscles points to a small number of founder cells, whether or not these have additional origins to the progenitor cells of the somite which form the first skeletal muscles, as discussed here for the embryonic limb.

Establishing the trochlear motor axon trajectory: role of the isthmic organiser and Fgf8

Formation of the trochlear nerve within the anterior hindbrain provides a model system to study a simple axonal projection within the vertebrate central nervous system. We show that trochlear motor neurons are born within the isthmic organiser and also immediately posterior to it in anterior rhombomere 1. Axons of the most anterior cells follow a dorsal projection, which circumnavigates the isthmus, while those of more posterior trochlear neurons project anterodorsally to enter the isthmus. Once within the isthmus, axons form large fascicles that extend to a dorsal exit point. We investigated the possibility that the projection of trochlear axons towards the isthmus and their subsequent growth within that tissue might depend upon chemoattraction. We demonstrate that both isthmic tissue and Fgf8 protein are attractants for trochlear axons in vitro, while ectopic Fgf8 causes turning of these axons away from their normal routes in vivo. Both inhibition of FGF receptor activation and inhibition of Fgf8 function in vitro affect formation of the trochlear projection within explants in a manner consistent with a guidance function of Fgf8 during trochlear axon navigation.

Cell proliferation and movement during early fin regeneration in zebrafish

Cell proliferation and cell movement during early regeneration of zebrafish caudal fins were examined by injecting BrdU and Di-I, respectively. In normal fins of adult fish, a small number of proliferating cells are observed in the epidermis only. Shortly following amputation, epithelial cells covered the wound to form the epidermal cap but did not proliferate. However, by 24 hr, epithelial cells proximal to the level of amputation were strongly labeled with BrdU. Label incorporation was also detected in a few mesenchymal cells. Proliferating cells in the basal epithelial layer were first observed at 48 hr at the level of the newly formed lepidotrichia. At 72 hr, proliferating mesenchymal cells were found distal to the plane of amputation whereas more proximal labeled cells included mainly those located between the lepidotrichia and the basal membrane. When BrdU-injected fins were allowed to regenerate for longer periods, labeled cells were observed in the apical epidermal cap, a location where cells are not thought to proliferate. This result is suggestive of cell migration. Epithelial cells, peripheral to the rays or in the tissue between adjacent rays, were labeled with Di-I and were shown to quickly migrate towards the site of amputation, the cells closer to the wound migrating faster. Amputation also triggered migration of cells of the connective tissue located between the hemirays. Although cell movement was induced up to seven segments proximal from the level of amputation, cells located within two segments from the wound provided the main contribution to the blastema. Thus, cell proliferation and migration contribute to the early regeneration of zebrafish fins.

Functions of the Growth Arrest Specific 1 Gene in the Development of the Mouse Embryo

The growth arrest specific 1 (gas1) gene is highly expressed in quiescent mammalian cells (Schneider et al., 1988, Cell 54, 787-793). Overexpression of gas1 in normal and some cancer cell lines could inhibit G(0)/G(1) transition. Presently, we have examined the functions of this gene in the developing mouse embryo. The spatial-temporal expression patterns for gas1 were established in 8.5- to 14.5-day-old embryos by immunohistochemical staining and in situ hybridization. Gas1 was found heterogeneously expressed in most organ systems including the brain, heart, kidney, limb, lung, and gonad. The antiproliferative effects of gas1 on 10.5 and 12.5 day limb cells were investigated by flow cytometry. In 10.5 day limbs cells, gas1 overexpression could not prevent G(0)/G(1) progression. It was determined that gas1 could only induce growth arrest if p53 was also coexpressed. In contrast, gas1 overexpression alone was able to induce growth arrest in 12.5 day limb cells. We also examined the cell cycle profile of gas1-expressing and nonexpressing cells by immunochemistry and flow cytometry. For 10.5 day Gas1-expressing heart and limb cells, we did not find these cells preferentially distributed at G0/G1, as compared with Gas1-negative cells. However, in the 12.5 day heart and limb, we did find significantly more Gas1-expressing cells distributed at G0/G1 phase than Gas1-negative cells. These results implied that Gas1 alone, during the early stages of development, could not inhibit cell growth. This inhibition was only established when the embryo grew older. We have overexpressed gas1 in subconfluent embryonic limb cells to determine the ability of gas1 to cross-talk with various response elements of important transduction pathways. Specifically, we have examined the interaction of gas1 with Ap-1, NFkappaB, and c-myc responsive elements tagged with a SEAP reporter. In 10.5 day limb cells, gas1 overexpression had little effect on Ap-1, NFkappaB, and c-myc activities. In contrast, gas1 overexpression in 12.5 day limb cells enhanced AP-1 response while it inhibited NFkappaB and c-myc activities. These responses were directly associated with the ability of gas1 to induce growth arrest in embryonic limb cells. In the 12.5 day hindlimb, gas1 was found strongly expressed in the interdigital tissues. We overexpressed gas1 in these tissues and discovered that it promoted interdigital cell death. Our in situ hybridization studies of limb sections and micromass cultures revealed that, during the early stages of chondrogenesis, only cells surrounding the chondrogenic condensations expressed gas1. The gene was only expressed by chondrocytes after the cartilage started to differentiate. To understand the function of gas1 in chondrogenesis, we overexpressed the gene in limb micromass cultures. It was found that cells overexpressing gas1/GFP could not participate in cartilage formation, unlike cells that just express the GFP reporter. We speculated that the reason gas1 was expressed outside the chondrogenic nodules was to restrict cells from being recruited into the nodules and thereby defining the boundary between chondrogenic and nonchondrogenic forming regions.

Cell biology of limb patterning

Of vertebrate organ systems, the developing limb has been especially well characterized. Morphological studies have yielded a wealth of information describing limb outgrowth and have allowed for the identification of a multitude of important factors. In terms of the latter, key signaling pathways are known to control numerous aspects of limb development, including establishment of the early limb field, determination of limb identity, elongation of the limb bud, specification of digit pattern, and sculpting of the digits. Modification of underlying signaling pathways can thus result in dramatic alterations of the limb phenotype, accounting for many of the diverse limb patterns observed in nature. Given this, it is clear that signaling pathways regulate the highly orchestrated and tightly controlled sequence of cellular events necessary for limb outgrowth; however, exactly how molecular signals interface with the cell biology of limb development remains largely a mystery. In this review we first provide an overview of a number of the morphogenetic signaling pathways that have been identified in the developing limb and then review how a subset of these signals are known to modify cell behaviors important for limb outgrowth.

Involvement of Ras and Ral in chemotactic migration of skeletal myoblasts

In skeletal myoblasts, Ras has been considered to be a strong inhibitor of myogenesis. Here, we demonstrate that Ras is involved also in the chemotactic response of skeletal myoblasts. Expression of a dominant-negative mutant of Ras inhibited chemotaxis of C2C12 myoblasts in response to basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), and insulin-like growth factor 1 (IGF-1), key regulators of limb muscle development and skeletal muscle regeneration. A dominant-negative Ral also decreased chemotactic migration by these growth factors, while inhibitors for phosphatidylinositol 3-kinase and mitogen-activated protein kinase kinase (MEK) showed no effect. Activation of the Ras-Ral pathway by expression of an activated mutant of either Ras, the guanine-nucleotide dissociation stimulator for Ral, or Ral resulted in increased motility of myoblasts. The ability of Ral to stimulate motility was reduced by introduction of a mutation which prevents binding to Ral-binding protein 1 or phospholipase D. These results suggest that the Ras-Ral pathway is essential for the migration of myoblasts. Furthermore, we found that Ras and Ral are activated in C2C12 cells by bFGF, HGF and IGF-1 and that the Ral activation is regulated by the Ras- and the intracellular Ca(2+)-mediated pathways. Taken together, our data indicate that Ras and Ral regulate the chemotactic migration of skeletal muscle progenitors.

Influence of FGF4 on digit morphogenesis during limb development in the mouse

Much of what we currently know about digit morphogenesis during limb development is deduced from embryonic studies in the chick. In this study, we used ex utero surgical procedures to study digit morphogenesis during mouse embryogenesis. Our studies reveal some similarities; however, we have found considerable differences in how the chick and the mouse autopods respond to experimentation. First, we are not able to induce ectopic digit formation from interdigital cells as a result of wounding or TGFbeta-1 application in the mouse, in contrast to what is observed in the chick. Second, FGF4, which inhibits the formation of ectopic digits in the chick, induces a digit bifurcation response in the mouse. We demonstrate with cell marking studies that this bifurcation response results from a reorganization of the prechondrogenic tip of the digit rudiment. The FGF4 effect on digit morphogenesis correlates with changes in the expression of a number of genes, including Msx1, Igf2, and the posterior members of the HoxD cluster. In addition, the bifurcation response is digit-specific, being restricted to digit IV. We propose that FGF4 is an endogenous signal essential for skeletal branching morphogenesis in the mouse. This work stresses the existence of major differences between the chick and the mouse in how digit morphogenesis is regulated and is thus consistent with the view that vertebrate digit evolution is a relatively recent event. Finally, we discuss the relationship between the digit IV bifurcation restriction and the placement of the metapterygial axis in the evolution of the tetrapod limb.

Chemotactic migration of mesencephalic neural crest cells in the mouse

We examined the roles of fibroblast growth factor (FGF)-2 and FGF-8 in the migration of mesencephalic mouse neural crest cells. Our in vitro migration assay has shown that FGF-2 (basic FGF) and FGF-8 have chemotactic activity for these cells. Chemotaxis was inhibited by anti-FGF-2 and anti-FGF-8 neutralizing antibodies. In addition, anti-FGF-2 blocked neural crest cell migration in cranial organ cultures. This observation suggests that FGF-2 functions as a chemoattractant in migration of mesencephalic neural crest cells in vivo. In organ culture, the antagonist of FGF binding to a low-affinity fibroblast growth factor receptor (FGFR) heparan sulfate, inositolhexakisphosphate (InsP6), inhibited migration as well. Mesencephalic neural crest cells had high-affinity FGFRs, in particular FGFR-1 and FGFR-3. Thus, the chemotactic activities of FGF-2 can be mediated by the low-affinity FGFR alone or by a combination of low- and high-affinity FGFRs (FGFR-1, FGFR-3, or both). Moreover, differential localization of FGF-2 was found at the mesencephalic axial level of intact embryos during neural crest cell migration. FGF-2 protein expression was predominant in the target regions, in particular the mandibular mesenchyme, that are colonized by mesencephalic neural crest cells. This characteristic distribution supports the notion that FGF-2 acts as a chemoattractant in the mouse embryo that directs mesencephalic neural crest cell migration. Whereas FGF-8 showed chemotactic activity in vitro, neural crest cell dispersion was observed in explants that had been treated with anti-FGF-8 neutralizing antibodies. This result suggests that FGF-8 may not be a chemoattractant in vivo. However, the distribution of neural crest cells in explants treated with anti-FGF-8 differed from that in control explants or in intact embryos. Extreme FGF-2 distribution was observed in the mandibular arch and FGF-8 is expressed in the epithelium. FGF-8 may play a role in mesencephalic neural crest cell migration, and its role may be concerned with the differential localization of FGF-2. To establish this notion, we performed immunohistochemical examination of FGF-2 distribution in explants treated with FGF-8 and analysis of FGF-2 gene expression levels by reverse transcriptase-polymerase chain reaction by using RNA from explants. The data indicate that FGF-2 is distributed throughout the mesenchyme in FGF-8-treated explants and that expression of FGF-2 is promoted by FGF-8. Therefore, we conclude that the expression of FGF-8 in the mandibular arch epithelium is a prerequisite for the differential localization of FGF-2 and that the FGF-2 distribution pattern is essential for chemotaxis of mesencephalic neural crest cell migration.

The role of Lbx1 in migration of muscle precursor cells

The homeobox gene Lbx1 is expressed in migrating hypaxial muscle precursor cells during development. These precursors delaminate from the lateral edge of the dermomyotome and form distinct streams that migrate over large distances, using characteristic paths. The targets of migration are limbs, septum transversum and the floor of the first branchial arch where the cells form skeletal muscle of limbs and shoulders, diaphragm and hypoglossal cord, respectively. We used gene targeting to analyse the function of Lbx1 in the mouse. Myogenic precursor cells delaminate from the dermomyotome in Lbx1 mutants, but migrate in an aberrant manner. Most critically affected are migrating cells that move to the limbs. Precursor cells that reach the dorsal limb field are absent. In the ventral limb, precursors are present but distributed in an abnormal manner. As a consequence, at birth some muscles in the forelimbs are completely lacking (extensor muscles) or reduced in size (flexor muscles). Hindlimb muscles are affected strongly, and distal limb muscles are more affected than proximal ones. Other migrating precursor cells heading towards the floor of the first branchial arch move along the appropriate path in Lbx1 mutants. However, these cells migrate less efficiently and reduced numbers of precursors reach their distal target. At birth, the internal lingual muscle is therefore reduced in size. We suggest that Lbx1 controls the expression of genes that are essential for the recognition or interpretation of cues that guide migrating muscle precursors and maintain their migratory potential.

Cell migration and chick limb development: chemotactic action of FGF-4 and the AER

Experiments have been carried out to investigate the role of the apical ectodermal ridge (AER) and FGF-4 on the control of cell migration during limb bud morphogenesis. By coupling DiI cell labeling with ectopic implantation of FGF-4 microcarrier beads we have found that FGF-4 acts as a potent and specific chemoattractive agent for mesenchymal cells of the limb bud. The response to FGF-4 is dose dependent in both the number of cells stimulated to migrate and the distance migrated. The cell migration response to FGF-4 appears to be independent of the known inductive activity of FGF-4 on Shh gene expression. We investigated the role of the AER in controlling cell migration by characterizing the migration pattern of DiI-labeled subapical cells during normal limb outgrowth and following partial AER removal. Subapical cells within 75 micrometer of the AER migrate to make contact with the AER and are found intermingled with nonlabeled cells. Thus, the progress zone is dynamic with cells constantly altering their neighbor relationships during limb outgrowth. AER removal studies show that cell migration is AER dependent and that subapical cells redirect their path of migration toward a functional AER. These studies indicate that the AER has a chemoattractive function and regulates patterns of cell migration during limb outgrowth. Our results suggest that the chemoattractive activity of the AER is mediated in part by the production of FGF-4.

The role of SF/HGF and c-Met in the development of skeletal muscle

Hypaxial skeletal muscles develop from migratory and non-migratory precursor cells that are generated by the lateral lip of the dermomyotome. Previous work shows that the formation of migratory precursors requires the c-Met and SF/HGF genes. We show here that in mice lacking c-Met or SF/HGF, the initial development of the dermomyotome proceeds appropriately and growth and survival of cells in the dermomyotome are not affected. Migratory precursors are also correctly specified, as monitored by the expression of Lbx1. However, these cells remain aggregated and fail to take up long range migration. We conclude that parallel but independent cues converge on the migratory hypaxial precursors in the dermomyotomal lip after they are laid down: a signal given by SF/HGF that controls the emigration of the precursors, and an as yet unidentified signal that controls Lbx1. SF/HGF and c-Met act in a paracrine manner to control emigration, and migratory cells only dissociate from somites located close to SF/HGF-expressing cells. During long range migration, prolonged receptor-ligand-interaction appears to be required, as SF/HGF is expressed both along the routes and at the target sites of migratory myogenic progenitors. Mice that lack c-Met die during the second part of gestation due to a placental defect. Rescue of the placental defect by aggregation of tetraploid (wild type) and diploid (c-Met−/−) morulae allows development of c-Met mutant animals to term. They lack muscle groups that derive from migratory precursor cells, but display otherwise normal skeletal musculature.

Glycoprotein-340 binds surfactant protein-A (SP-A) and stimulates alveolar macrophage migration in an SP-A-independent manner

Glycoprotein-340 (gp-340) was first identified as a surfactant protein (SP)-D-binding molecule purified from lung lavage of patients with alveolar proteinosis (Holmskov, et al., J. Biol. Chem. 1997;272:13743). In purifying SP-A from proteinosis lavage, we isolated a protein that copurifies with SP-A and SP-D and that was later found by protein sequencing to be gp-340. We have shown that soluble gp-340 binds SP-A in a calcium-dependent manner independent of the lectin activity of SP-A. To examine the functional significance of this interaction, we tested the ability of soluble gp-340 to block SP-A binding to and stimulation of the chemotaxis of alveolar macrophages. We found that gp-340 does not affect the binding of SP-A to alveolar macrophages over a wide range of SP-A concentrations, nor does it inhibit the ability of SP-A to stimulate macrophage chemotaxis. We also found that gp-340 alone stimulates the random migration (chemokinesis) of alveolar macrophages in a manner independent of SP-A-stimulated chemotaxis. These results suggest that gp-340 is not a cell-surface receptor necessary for SP-A stimulation of chemotaxis, and show that gp-340 can directly affect macrophage function.

Morphogenetic movements at gastrulation require the SH2 tyrosine phosphatase Shp2

The SH2 domain-containing tyrosine phosphatase Shp2 plays a pivotal role during the gastrulation of vertebrate embryos. However, because of the complex phenotype observed in mouse mutant embryos, the precise role of Shp2 during development is unclear. To define the specific functions of this phosphatase, Shp2 homozygous mutant embryonic stem cells bearing the Rosa-26 LacZ transgene were isolated and used to perform a chimeric analysis. Here, we show that Shp2 mutant cells amass in the tail bud of embryonic day 10.5 chimeric mouse embryos and that this accumulation begins at the onset of gastrulation. At this early stage, Shp2 mutant cells collect in the primitive streak of the epiblast and thus show deficiencies in their contribution to the mesoderm lineage. In high-contribution chimeras, we show that overaccumulation of Shp2 mutant cells at the posterior end of the embryo results in two abnormal phenotypes: spina bifida and secondary neural tubes. Consistent with a failure to undergo morphogenic movements at gastrulation, Shp2 is required for embryo fibroblast cells to mount a positive chemotactic response to acidic fibroblast growth factor in vitro. Our results demonstrate that Shp2 is required at the initial steps of gastrulation, as nascent mesodermal cells form and migrate away from the primitive streak. The aberrant behavior of Shp2 mutant cells at gastrulation may result from their inability to properly respond to signals initiated by fibroblast growth factors.

Hepatocyte growth factor stimulates chemotactic response in mouse embryonic limb myogenic cells in vitro

In this study we investigate the influence of Hepatocyte Growth Factor (HGF) on the motility of embryonic forelimb myoblasts. Using Blindwell chemotactic chambers, it was found that HGF at concentrations of 1-50 ng/ml dramatically enhanced the ability of myogenic cells to migrate. This stimulatory effect was elicited in a dose-dependent fashion and the effect was reversed with the addition of HGF neutralizing antibodies. A checkerboard analysis was performed and it revealed that HGF's effect on limb myoblast motility was through both chemokinesis and chemotaxis. HGF was also examined for its ability to stimulate myogenic cell proliferation, using MF20 antibody as the myogenic marker. At all concentrations tested, HGF did not stimulate an overall increase in the numbers of MF20-positive myoblasts in culture. To examine the chemokinetic effect of HGF on cell migration in the limb, cells were isolated from the proximal regions of the limb (areas rich in myogenic cells), exposed to HGF, labeled with DiI and transplanted into 11.5 day mouse forelimbs. After 36 h of culture, it was found that DiI-labeled limb cells, pretreated with HGF, migrated significantly further in the limb than labeled cells that have not been exposed to HGF. The chemotactic effect of HGF was also investigated by implanting beads loaded with and without HGF into the 11.5 day limb. Proximal to the beads, DiI-labeled limb cells were also transplanted. It was found that HGF was able to chemotactically attract and direct the migration of DiI-labeled limb cells. Immunohistological staining was performed with HGF antibodies to determine the distribution of HGF in the 11.5 day mouse forelimb. It was found that HGF was strongly expressed by the apical ectodermal ridge (AER), the ectoderm and the mesenchyme directly beneath the AER. Positive staining was also obtained for the myogenic regions. However, the pattern was heterogeneous--punctuated with myogenic cells expressing and not expressing HGF.