Auto and transactivation of FGF expression: potential mechanism for regulation of myogenic differentiation

A continuous myofibroblast precursor cell line from the tail muscle of Australasian snapper (Chrysophrys auratus) that responds to transforming growth factor beta and fibroblast growth factor

Chrysophrys auratus (Australasian snapper) is one of the largest and most valuable finfish from capture fisheries in New Zealand, yet no cell lines from this species are reported in the scientific literature. Here, we describe a muscle-derived cell line initiated from the tail of a juvenile snapper which has been designated CAtmus1PFR (Chrysophrys auratus, tail muscle, Plant & Food Research). The cell line has been passaged over 100 times in 3 years and is considered immortal. Cells are reliant on serum supplementation for proliferation and exhibit a broad thermal profile comparable to the eurythermic nature of C. auratus in vivo. The impact of exogenous growth factors, including insulin-like growth factors I and II (IGF-I and IGF-II), basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGFβ), on cell morphology and proliferation was investigated. Insulin-like growth factors acted as mitogens and had minimal effect on cell morphology. TGFβ exposure resulted in CAtmus1PFR exhibiting a myofibroblast morphology becoming enlarged with actin bundling. This differentiation was confirmed through the expression of smooth muscle actin (sma), an increase in type 1 collagen (col1a) expression, and a loss of motility. Expression of col1a and sma was decreased when cells were exposed to bFGF, and no actin bundling was observed. These data indicate that CAtmus1PFR may be myofibroblastic precursor cells descending from mesenchymal progenitor cells present in the tail muscle myosepta.

Syndecan-1 regulates vascular smooth muscle cell phenotype

OBJECTIVE

We examined the role of syndecan-1 in modulating the phenotype of vascular smooth muscle cells in the context of endogenous inflammatory factors and altered microenvironments that occur in disease or injury-induced vascular remodeling.

METHODS AND RESULTS

Vascular smooth muscle cells (vSMCs) display a continuum of phenotypes that can be altered during vascular remodeling. While the syndecans have emerged as powerful and complex regulators of cell function, their role in controlling vSMC phenotype is unknown. Here, we isolated vSMCs from wild type (WT) and syndecan-1 knockout (S1KO) mice. Gene expression and western blotting studies indicated decreased levels of α-smooth muscle actin (α-SMA), calponin, and other vSMC-specific differentiation markers in S1KO relative to WT cells. The spread area of the S1KO cells was found to be greater than WT cells, with a corresponding increase in focal adhesion formation, Src phosphorylation, and alterations in actin cytoskeletal arrangement. In addition, S1KO led to increased S6RP phosphorylation and decreased AKT and PKC-α phosphorylation. To examine whether these changes were present in vivo, isolated aortae from aged WT and S1KO mice were stained for calponin. Consistent with our in-vitro findings, the WT mice aortae stained higher for calponin relative to S1KO. When exposed to the inflammatory cytokine TNF-α, WT vSMCs had an 80% reduction in syndecan-1 expression. Further, with TNF-α, S1KO vSMCs produced increased pro-inflammatory cytokines relative to WT. Finally, inhibition of interactions between syndecan-1 and integrins αvβ3 and αvβ5 using the inhibitory peptide synstatin appeared to have similar effects on vSMCs as knocking out syndecan-1, with decreased expression of vSMC differentiation markers and increased expression of inflammatory cytokines, receptors, and osteopontin.

CONCLUSIONS

Taken together, our results support that syndecan-1 promotes vSMC differentiation and quiescence. Thus, the presence of syndecan-1 would have a protective effect against vSMC dedifferentiation and this activity is linked to interactions with integrins αvβ3 and αvβ5.

HtrA1 is a novel antagonist controlling fibroblast growth factor (FGF) signaling via cleavage of FGF8

Accumulating evidence suggests that HtrA1 (high-temperature requirement A1) is involved in modulating crucial cellular processes and implicated in life-threatening diseases, such as cancer and neuropathological disorders; however, the exact functions of this protease in vivo remain unknown. Here, we show that loss of HtrA1 function increases fibroblast growth factor 8 (FGF8) mRNA levels and triggers activation of FGF signaling, resulting in dorsalization in zebrafish embryos. Notably, HtrA1 directly cleaves FGF8 in the extracellular region, and this cleavage results in decreased activation of FGF signaling, which is essential for many physiological processes. Therefore, HtrA1 is indispensable for dorsoventral patterning in early zebrafish embryogenesis and serves as a key upstream regulator of FGF signaling through the control of FGF levels. Furthermore, this study offers insight into new strategies to control human diseases associated with HtrA1 and FGF signaling.

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.

Expression of fibroblast growth factor 2 and its receptor during skeletal muscle development from turkeys with different growth rates

Fibroblast growth factor 2 (FGF2) is a key regulator of muscle cell proliferation and differentiation. To address how FGF2 and fibroblast growth factor receptor 1 (FGFR1) gene expression influences skeletal muscle development and growth, pectoralis major muscle was isolated at embryonic days (ED) 14, 16, 18, 20, 22, and 24, and at 1-, 8-, 12-, and 16-week posthatch from a turkey line (F) selected only for increased 16-week body weight and its genetic control line (RBC2). The mRNA levels of FGF2 and FGFR1 were measured by semi-quantitative reverse transcription polymerase chain reaction. Compared to the RBC2 line males, the F line males had higher FGF2 mRNA levels at ED 14 and 16, and higher FGFR1 mRNA levels at ED 18, but down-regulated FGF2 and FGFR1 gene expression at ED 22. Although no FGF2 mRNA was detected in posthatch muscle tissue, the F line turkeys had more FGFR1 gene expression at 8-, 12-, and 16-week posthatch than the RBC2 line turkeys. During myogenic satellite cell proliferation, the F line cells had higher FGF2 and FGFR1 mRNA levels than the RBC2 line cells. The satellite cell responsiveness to FGF2 treatment was evaluated by the ability of the cells to proliferate. The male satellite cells were more responsive to FGF2 stimulation than the female cells in both lines. These results suggest that the F line turkeys have increased FGF2 signaling that may affect muscle cell proliferation and differentiation, which may also lead to an enhancement in muscle development and growth rate.

Differentiation in C(2)C(12) myoblasts depends on the expression of endogenous IGFs and not serum depletion

Myogenic differentiation in vitro has been usually viewed as being negatively controlled by serum mitogens. A depletion of critical serum components from medium has been considered to be essential for permanent withdrawal from the cell cycle and terminal differentiation of myoblasts. Removal of serum mitogens induces the expression of insulin-like growth factors (IGFs), whereas it inhibits that of basic fibroblast growth factor (bFGF) and transforming growth factor (TGF)-beta in myoblasts. These responses of growth factors to medium conditioning seem to be well matched to their functions in proliferation/differentiation. In the present study, we showed that C(2)C(12) myoblasts differentiated actively, even in mitogen-rich medium, and that this medium offered an advantage over mitogen-poor medium in terms of increasing differentiation. Our attention focused on endogenous growth factors, as described above, especially IGFs in mitogen-rich medium. During differentiation, IGF-I and IGF-II mRNA levels increased, but bFGF and TGF-beta(1) mRNAs decreased. Differentiation was commensurable with IGF mRNA levels and suppressed by antisense oligodeoxynucleotides and neutralizing monoclonal antibodies against IGFs. These results suggest that an autocrine/paracrine loop of IGFs, bFGF, and TGF-beta(1) is active in proliferating and differentiating C(2)C(12) cells without a depletion of serum and that endogenous IGFs actively override the negative control of differentiation by serum mitogens.

Misexpression of Fgf-4 in the chick limb inhibits myogenesis by down-regulating Frek expression

Skeletal muscle development involves an initial period of myoblast replication followed by a phase in which some myoblasts continue to proliferate while others undergo terminal differentiation. The latter process involves the permanent cessation of DNA synthesis, activation of muscle-specific gene expression, and fusion of single cells to generate multinucleated muscle fibres. The in vivo signals regulating the progression through all these steps remain unknown. Fibroblast growth factors (Fgfs) and Fgf receptors comprise a large family whose members have been shown to play multiple roles in the development of skeletal muscle in vitro. Exogenously applied Fgfs are able to stimulate proliferation and suppress myogenic differentiation in cell culture. We sought to determine the role played by Fgf-4 during limb myogenesis in vivo. Fgf-4 transcripts are located at both extremities of myotubes whereas the mRNAs of one of the Fgf receptors, Frek, are detected in mononucleated proliferating myoblasts surrounding the multinucleated fibres. Overexpression of mouse Fgf-4 (mFgf-4) using a replication-competent retrovirus, RCAS, leads to a down-regulation of muscle markers followed by an inhibition of terminal differentiation in limb muscles. Using quail/chick transplantations we were able to follow the muscle cells and found a dramatic decrease in their number after exposure to mFgf-4. Interestingly ectopic mFgf-4 down-regulates Frek transcripts in limb muscle areas. We conclude that overexpression of mFgf-4 inhibits myoblast proliferation, probably by down-regulating Frek mRNAs. This suggests a role for Fgf-4, located at the extremities of the myotubes, where it could be responsible for the absence of Frek mRNA in the muscle fibre.

Vascular delay and administration of basic fibroblast growth factor augment latissimus dorsi muscle flap perfusion and function

Ischemia of the distal latissimus dorsi muscle flap occurs when the entire muscle is acutely elevated. Although this level of ischemia may not be critical if the muscle is to be used as a conventional muscle flap, the ischemia causes decreased distal muscle function if it is used for dynamic muscle flap transfer. This experiment was designed to determine whether or not the administration of exogenous basic fibroblast growth factor (bFGF), combined with a sublethal ischemic insult (i.e., vascular delay), would further augment muscle perfusion and function. Both latissimus dorsi muscles of nine canines were subjected to a bipedicle vascular delay procedure immediately followed by thoracodorsal intraarterial injection of 100 microg of bFGF on one side and by intraarterial injection of vehicle on the other. Ten days later, both latissimus dorsi muscles were raised as thoracodorsally based island flaps, with perfusion determined by laser-Doppler fluximetry. The muscles were wrapped around silicone chambers, simulating cardiomyoplasty, and stimulating electrodes were placed around each thoracodorsal nerve. The muscles were then subjected to an experimental protocol to determine muscle contractile function. At the end of the experiment, latissimus dorsi muscle biopsies were obtained for measurement of bFGF expression. The results demonstrated that the administration of 100 microg of bFGF immediately after the vascular delay procedure increases expression of native bFGF. In the distal and middle muscle segments, it also significantly increased muscle perfusion by approximately 20 percent and fatigue resistance by approximately 300 percent. The administration of growth factors may serve as an important adjuvant to surgical procedures using dynamic muscle flap transfers.

Changes in FGF and FGF receptor expression in low-frequency-stimulated rat muscles and rat satellite cell cultures

This study compares effects of chronic electrical stimulation on the expression levels of FGF-1, FGF-2 and their receptors (FGFRI, FGFR4) in rat tibialis anterior (TA) muscle of hypothyroid rat, as well as in satellite cell cultures derived from normal rat TA and soleus (SOL) muscles. In 5-day (5-d)-stimulated hypothyroid TA muscle, FGF-1 and FGF-2 mRNA levels were threefold elevated over control. FGFR1 and FGFR4 mRNAs were twofold and 1.5-fold elevated, respectively. In longer stimulated muscles, FGF-1 and FGFR4 mRNAs returned to basal levels, whereas FGF-2 mRNA remained elevated. FGFR1 mRNA decreased to control levels in 10-d stimulated muscles, but increased again after 20 days of stimulation. SOL- and TA-derived satellite cell cultures were stimulated for 5 days. At this time point, changes in myosin heavy chain isoforms were detectable consisting of increases in MHCI mRNA and decreases in MHCIIb and MHCIId mRNA. The comparison between 5-d-stimulated hypothyroid TA muscle and 5-d-stimulated TA- and SOL-derived satellite cell cultures revealed differences in the expression of FGF-1 and FGF-2, but similar expression levels of FGFR1 and FGFR4. Even though FGF-1 and FGF-2 mRNAs were elevated in the satellite cell cultures, their increases were less pronounced than in the stimulated hypothyroid muscle. Taking into consideration that skeletal muscle contains muscle fibres and various non-muscle tissues, e.g. blood vessels, these results suggest that the latter contribute to the observed increases in FGF-1 and FGF-2 expression in stimulated muscle.

Evidence that acidic fibroblast growth factor promotes maturation of rat satellite-cell-derived myotubes in vitro

Satellite cells isolated from fast tibialis anterior (TA) and slow soleus (SOL) rat muscles were cultivated on matrigel, and treated with acidic fibroblast growth factor (aFGF). The following observations were made: 1) aFGF-treated cultures exhibited enhanced proliferation as mirrored by a twofold increase in DNA content. 2) Compared to the untreated cultures, myotubes in the aFGF cultures were larger; 3) Using reverse transcriptase polymerase chain reaction (RT-PCR) and northern blot analyses, we observed enhanced expression of all adult myosin heavy chain (MHC) isoforms, as well as of myogenin. These findings indicate that, under the culture conditions used, aFGF has a stimulatory effect on proliferation but also on maturation and differentiation of satellite cells. Furthermore, transcript levels of FGF receptor 1 (FGFR1) and 4 (FGFR4) isoforms, as well as of aFGF and bFGF were assessed by RT-PCR. aFGF-treated myotubes displayed increased expression of aFGF and bFGF, suggesting a paracrine effect of exogenous aFGF. In this regard, SOL-derived cultures responded more strongly than TA-derived cultures. The effects of aFGF treatment on the two receptors consisted of a decrease in FGFR1 and an increase in FGFR4 mRNA levels in 5-day-old cultures. In 8-day-old TA cultures, effects of FGF were similar to those in 5-day-old cultures. 8-day FGF-treated SOL cultures treated with FGF for 8 days exhibited higher FGFR1 and FGFR4 mRNA levels than the respective untreated cultures. Compared to 5 day-treated cultures, FGFR1 increased and FGFR4 decreased. This led to a shift in the ratio of FGFR1 to FGFR4 in the FGF-treated cultures which may explain the ability of satellite cells to differentiate under the influence of aFGF.

Basic fibroblast growth factor regulates extracellular matrix and contractile protein expression independent of proliferation in vascular smooth muscle cells

Basic fibroblast growth factor (bFGF) can influence proliferation and differentiation in vascular smooth muscle cells. Basic FGF promotes some features of the synthetic phenotype (proliferation) but is known to inhibit others (collagen synthesis). Whether bFGF availability influences smooth muscle cell phenotype independent of proliferation is not known. The purpose of this study was to determine if the effects of bFGF on extracellular matrix and contractile protein expression are dependent on changes in proliferation. Basic FGF availability was manipulated by adding bFGF to cultured cells or by inhibiting bFGF expression using antisense RNA, and adjusting culture conditions such that proliferation was held constant. Compared to cells cultured in serum alone, smooth muscle alpha-actin and myosin heavy chain expression was markedly reduced by added bFGF, but was not influenced by antisense inhibition of bFGF expression. Under the same conditions, collagen synthesis was inhibited by added bFGF, and was stimulated by reduced bFGF expression. These consequences of altering bFGF availability were not associated with changes in FGF receptor expression. These findings demonstrate that alterations in bFGF availability can regulate smooth muscle cell phenotype independent of proliferation, which may be related to the regulation of smooth muscle cell phenotype in vivo.

Selection for retroviral insertions into regulated genes

Gene traps can be used to monitor faithfully the changes in gene expression accompanying several cellular processes. Here, we present a strategy that combines retroviral gene trap vectors, efficient selection schemes based on fluorescence-activated cell sorting or dominant positive and negative drug selection, and appropriately responsive cell lines in order to enrich for retroviral insertions into regulated genes (i.e., genes participating in cellular differentiation processes and genes induced by growth factors, drugs, or neurotransmitters, etc.). As an example, we applied this approach to the identification of insertions into genes activated by a MyoD protein, using a MyoD-responsive fibroblast line. In a single experiment designed to demonstrate the feasibility of this approach, we have been able to screen thousands of gene trap integrations and to select those that represent direct or indirect targets of MyoD. Distinct patterns of regulation were observed during myogenic determination. Sequences flanking the integrations can be rescued with several approaches, and they can be used to isolate the host genes or can serve as entry points for genome-wide sequencing projects.

Autonomous control of expression of genes for insulin-like growth factors during the proliferation and differentiation of C2C12 mouse myoblasts in serum-free culture

The proliferation and differentiation of skeletal muscle cells in culture are usually controlled by serum components, and the differentiation can be induced by a reduction in the serum concentration. Insulin-like growth factors (IGFs) play a critical role in stimulating myoblast differentiation, and the expression of their genes is controlled by serum factors. We have found that C2C12 myoblasts are capable of proliferation and differentiation even in serum-free medium that does not contain peptide mitogens. During these processes in serum-free medium, the accumulation of mRNAs for IGFs in the cells was observed; and their levels increased with concomitant increases in creatine kinase activity and myotube formation and a decrease in DNA synthesis. Thus, the present results suggest that proliferation and differentiation of C2C12 cells are autonomously controlled and that the increase in the expression of the IGFs may be independent of exogenous components.

The neurotrophic activity of fibroblast growth factor 1 (FGF1) depends on endogenous FGF1 expression and is independent of the mitogen-activated protein kinase cascade pathway

The expression of fibroblast growth factor (FGF) 1, a potent neurotrophic factor, increases during differentiation and remains high in adult neuronal tissues. To examine the importance of this expression on the neuronal phenotype, we have used PC12 cells, a model to study FGF-induced neuronal differentiation. After demonstrating that FGF1 and FGF2 are synthesized by PC12 cells, we investigated if FGF1 expression could be a key element in differentiation. Using the cell signaling pathway to determine the effects of FGF1 alone, FGF1 plus heparin, or a mutated FGF1, we showed an activation to the same extent of mitogen-activated protein (MAP) kinase kinase and MAP kinase (extracellular regulated kinase 1). However, only FGF1 plus heparin could promote PC12 cell differentiation. Thus, the MAP kinase pathway is insufficient to promote differentiation. Analysis of the PC12 cells after the addition of FGF1 plus heparin or FGF2 demonstrated a significant increase in the level of FGF1 expression with the same time course as the appearance of the neuritic extensions. Transfection experiments were performed to enhance constitutivly or after dexamethasone induction the level of FGF1 expression. The degree of differentiation achieved by the cells correlated directly with the amount of FGF1 expressed. The MAP kinase pathway did not appear to be involved. Interestingly, a 5-fold increase in FGF1 in constitutive transfected cells extended dramatically their survival in serum-free medium, suggesting that the rise of FGF1 synthesis during neuronal differentiation is probably linked to their ability to survive in the adult. All of these data demonstrate that, in contrast to the MAP kinase cascade. FGF1 expression is sufficient to induce in PC12 cells both differentiation and survival. It also shows that auto- and trans-activation of FGF1 expression is involved in the differentiation process stimulated by exogenous FGFs through a new pathway which remains to be characterized.

Transforming growth factor-beta isoform expression in insulin-like growth factor stimulated myogenesis

Transforming growth factor betas (TGF-beta s) are the defining members of a super-family of small proteins that are involved in the regulation of development and morphogenesis in a wide array of systems. Previous studies have demonstrated that TGF-beta s both inhibit and, under specialized conditions, induce the differentiation of myoblasts. TGF-beta have been shown to be secreted by mouse C2C12 myoblast cultures undergoing differentiation. Insulin-like growth factors (IGFs) have also been shown to be secreted by myoblasts and to induce myogenesis. This study characterizes the effect of IGF treatment on the expression and secretion of TGF-beta s in the IGF-sensitive L6A1 myoblast line. IGF downregulated the expression of TGF-beta 3 in a concentration-dependent manner at 24 and 48 hours; TGF-beta 1 was not sensitive to IGF treatment at 24 hours but was downregulated by IGFs at 48 hours. This downregulation was mediated by the type 1 IGF receptor and modulated by IGF binding proteins secreted by the myoblasts. Some reexpression of TGF-beta 1 and TGF-beta 3 mRNAs was observed after extensive morphological differentiation had occurred. These results support the hypothesis that IGFs act through the IGF type I receptor as part of a concerted mechanism to modulate expression of the TGF-beta genes, as part of a coordinated set of changes associated with terminal myogenic differentiation.

Basic fibroblast growth factor identified in chronically stimulated cardiomyoplasties

In the presence of myocardial ischemia, chronic electrical stimulation of a latissimus dorsi (LD) cardiomyoplasty enhances extramyocardial collateral blood flow. We postulated that basic fibroblast growth factor (bFGF) may mediate extramyocardial collateral formation. To test this hypothesis, LDs from goats with cardiomyoplasties were probed for the presence of bFGF by Western blot analysis and immunohistochemistry. Three groups were studied: static LD cardiomyoplasty (group 1); LD cardiomyoplasty stimulated at a 2-Hz frequency for 6 weeks (group 2); and LD cardiomyoplasty electrically stimulated and given human recombinant bFGF (group 3). There was no evidence of bFGF in the left LDs of group 1 by Western blot. Basic fibroblast growth factor-like immunoreactive evidence was found in the left LDs of group 2 goats by both Western blot and immunohistochemistry. In the right LDs of group 2, bFGF-like material was found by immunohistochemistry but not by Western blot, which suggests that the tissue concentrations were low (near the limits of detection). The left LDs of group 3 were positive for bFGF by Western blot and immunohistochemistry. Group 3 right LDs were positive for bFGF by immunohistochemistry. Immunohistochemical findings in group 2 indicate that bFGF is present in goat skeletal muscle. Western blot data from groups 1 and 2 suggest that bFGF may be increased in chronically stimulated cardiomyoplasties. From findings in group 3, we conclude that exogenous bFGF does not downregulate, and may upregulate, endogenous production. These results support the possibility that skeletal muscle bFGF is an important factor in extramyocardial collateral formation.

Differential regulation of acidic and basic fibroblast growth factor gene expression in fibroblast growth factor-treated rat aortic smooth muscle cells

The acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) proteins are potent vascular smooth muscle cell (SMC) mitogens that are expressed by endothelial cells and SMCs in vivo. Overexpression of these proteins in transfected cell lines can result in autocrine transformation; therefore, the precise control of fibroblast growth factor gene expression in the vessel wall may be an important mechanism regulating vascular cell growth. In the present study, we demonstrate that bFGF can induce bFGF mRNA expression, but not aFGF mRNA expression, in serum-starved rat aortic SMCs. bFGF autoinduction is maximal at 4 hours, requires de novo RNA and protein synthesis, and is mediated predominantly by a protein kinase C-dependent signaling pathway. Furthermore, aFGF treatment of rat SMCs also increases bFGF mRNA and protein expression; however, aFGF mRNA levels are only slightly modulated. These results suggest that the local release of aFGF or bFGF within the vessel wall could promote a prolonged period of elevated bFGF synthesis. This, in turn, could be of importance in the SMC hyperplasia that occurs in response to vascular injury and during atherosclerotic plaque formation.

Myogenic differentiation triggered by antisense acidic fibroblast growth factor RNA

Acidic fibroblast growth factor (FGF) and related family members regulate differentiation in organisms as diverse as Xenopus laevis and mammals. We utilized a well-characterized model of myogenic development to directly assess the importance of endogenously produced FGF in controlling differentiation. A role for endogenous FGF is suggested by the previous finding that acidic and basic FGF abundance in cultured myocytes decreases during differentiation. In this study we inhibited the endogenous production of FGF in murine Sol 8 myoblasts by using antisense RNA and observed precocious myogenic differentiation. Exogenously supplied acidic FGF rescues this phenotype. Further results suggest that the effect of FGF on myogenic differentiation is mediated in part through inhibition of myogenin expression. These results demonstrate a direct role for endogenously synthesized growth factors in regulating myogenesis and provide support for a general role for related proteins in mammalian development.

Myogenic differentiation triggered by antisense acidic fibroblast growth factor RNA

Acidic fibroblast growth factor (FGF) and related family members regulate differentiation in organisms as diverse as Xenopus laevis and mammals. We utilized a well-characterized model of myogenic development to directly assess the importance of endogenously produced FGF in controlling differentiation. A role for endogenous FGF is suggested by the previous finding that acidic and basic FGF abundance in cultured myocytes decreases during differentiation. In this study we inhibited the endogenous production of FGF in murine Sol 8 myoblasts by using antisense RNA and observed precocious myogenic differentiation. Exogenously supplied acidic FGF rescues this phenotype. Further results suggest that the effect of FGF on myogenic differentiation is mediated in part through inhibition of myogenin expression. These results demonstrate a direct role for endogenously synthesized growth factors in regulating myogenesis and provide support for a general role for related proteins in mammalian development.

FGF-2: apical ectodermal ridge growth signal for chick limb development

The apical ectodermal ridge permits growth and elongation of amniote limb buds; removal causes rapid changes in mesodermal gene expression, patterned cell death, and truncation of the limb. Ectopic fibroblast growth factor (FGF)-2 supplied to the chick apical bud mesoderm after ridge removal will sustain normal gene expression and cell viability, and allow relatively normal limb development. A bioassay for FGFs demonstrated that FGF-2 was the only detectable FGF in chick limb bud extracts. By distribution and bioactivity, FGF-2 is the prime candidate for the chick limb bud apical ridge growth signal.