Evaluation of heparin-binding growth factors in rescuing morphogenesis of heparitinase-treated mouse embryonic lung explants

Manipulation of Glycosaminoglycans Using Synthetic Xylosides to Study Their Roles in Lung Branching Morphogenesis in Ex Vivo Lung Bud Culture System

The primary left and right bronchial buds grow and sprout secondary bronchi, which in turn develop tertiary bronchi, and so on. Branching continues for a total of 6-8 generations in the mouse and for about 23 generations in humans, forming the estimated 50 million branches of the human lung. Thus, patterns of branching are incalculably complex. However, these branches are rarely random, implying that they are under genetic control. Genomic information alone cannot specify the patterning information in terms of where the branching occurs and the direction it grows as well as their size and shape. There is a complex choreography among glycosaminoglycans and growth factors/morphogens that provide a highly complex instructive cues that control lung branching and development of the functional lung. Herein, we describe the use of xylosides in the manipulation of glycosaminoglycan (GAG) biosynthesis and study the effect of xyloside-primed GAGs in the regulation of lung branching events.

Assessment of the nitrofen model of congenital diaphragmatic hernia and of the dysregulated factors involved in pulmonary hypoplasia

PURPOSE

To study pulmonary hypoplasia (PH) associated with congenital diaphragmatic hernia (CDH), investigators have been employing a fetal rat model based on nitrofen administration to dams. Herein, we aimed to: (1) investigate the validity of the model, and (2) synthesize the main biological pathways implicated in the development of PH associated with CDH.

METHODS

Using a defined strategy, we conducted a systematic review of the literature searching for studies reporting the incidence of CDH or factors involved in PH development. We also searched for PH factor interactions, relevance to lung development and to human PH.

RESULTS

Of 335 full-text articles, 116 reported the incidence of CDH after nitrofen exposure or dysregulated factors in the lungs of nitrofen-exposed rat fetuses. CDH incidence: 54% (27-85%) fetuses developed a diaphragmatic defect, whereas the whole litter had PH in varying degrees. Downregulated signaling pathways included FGF/FGFR, BMP/BMPR, Sonic Hedgehog and retinoid acid signaling pathway, resulting in a delay in early epithelial differentiation, immature distal epithelium and dysfunctional mesenchyme.

CONCLUSIONS

The nitrofen model effectively reproduces PH as it disrupts pathways that are critical for lung branching morphogenesis and alveolar differentiation. The low CDH rate confirms that PH is an associated phenomenon rather than the result of mechanical compression alone.

Acellular lung scaffolds direct differentiation of endoderm to functional airway epithelial cells: requirement of matrix-bound HS proteoglycans

Efficient differentiation of pluripotent cells to proximal and distal lung epithelial cell populations remains a challenging task. The 3D extracellular matrix (ECM) scaffold is a key component that regulates the interaction of secreted factors with cells during development by often binding to and limiting their diffusion within local gradients. Here we examined the role of the lung ECM in differentiation of pluripotent cells in vitro and demonstrate the robust inductive capacity of the native lung matrix alone. Extended culture of stem cell-derived definitive endoderm on decellularized lung scaffolds in defined, serum-free medium resulted in differentiation into mature airway epithelia, complete with ciliated cells, club cells, and basal cells with morphological and functional similarities to native airways. Heparitinase I, but not chondroitinase ABC, treatment of scaffolds revealed that the differentiation achieved is dependent on heparan sulfate proteoglycans and its bound factors remaining on decellularized scaffolds.

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Lung scaffolds direct ESC-derived endoderm differentiation to airway epithelia

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ESC-derived airway epithelial cells are functional and resemble native airways

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Differentiation by scaffolds is dependent on matrix heparan sulfate proteoglycans

Midkine: a promising molecule for drug development to treat diseases of the central nervous system

Midkine (MK) is a heparin-binding cytokine, and promotes growth, survival, migration and other activities of target cells. After describing the general properties of MK, this review focuses on MK and MK inhibitors as therapeutics for diseases in the central nervous system. MK is strongly expressed during embryogenesis especially at the midgestation period, but is expressed only at restricted sites in adults. MK expression is induced upon tissue injury such as ischemic brain damage. Since exogenously administered MK or the gene transfer of MK suppresses neuronal cell death in experimental systems, MK has the potential to treat cerebral infarction. MK might become important also in the treatment of neurodegenerative diseases such as Alzheimer's disease. MK is involved in inflammatory diseases by enhancing migration of leukocytes, inducing chemokine production and suppressing regulatory T cells. Since an aptamer to MK suppresses experimental autoimmune encephalitis, MK inhibitors are promising for the treatment of multiple sclerosis. MK is overexpressed in most malignant tumors including glioblastoma, and is involved in tumor invasion. MK inhibitors may be of value in the treatment of glioblastoma. Furthermore, an oncolytic adenovirus, whose replication is under the control of the MK promoter, inhibits the growth of glioblastoma xenografts. MK inhibitors under development include antibodies, aptamers, glycosaminoglycans, peptides and low molecular weight compounds. siRNA and antisense oligoDNA have proved effective against malignant tumors and inflammatory diseases in experimental systems. Practical information concerning the development of MK and MK inhibitors as therapeutics is described in the final part of the review.

Midkine: a novel prognostic biomarker for cancer

Since diagnosis at an early stage still remains a key issue for modern oncology and is crucial for successful cancer therapy, development of sensitive, specific, and non-invasive tumor markers, especially, in serum, is urgently needed. Midkine (MK), a plasma secreted protein, was initially identified in embryonal carcinoma cells at early stages of retinoic acid-induced differentiation. Multiple studies have reported that MK plays important roles in tumor progression, and is highly expressed in various malignant tumors. Because increased serum MK concentrations also have been reported in patients with various tumors, serum MK may have the potential to become a very useful tumor marker. Here, we review and discuss the possibility and usefulness of MK as a novel tumor marker.

Midkine in plasma as a novel breast cancer marker

Midkine, a heparin-binding growth factor, is up-regulated in many types of cancer. The aim of this study was to measure plasma midkine levels in patients with breast cancer and to assess its clinical significance. We examined plasma midkine levels in 95 healthy volunteers, 11 patients with ductal carcinoma in situ (DCIS), 111 patients with primary invasive breast cancer without distant metastasis (PIBC), and 25 patients with distant metastatic breast cancer (MBC), using an automatic immunoasssay analyzer (TOSOH AIA system). In PIBC, we studied the correlation between plasma midkine levels and clinicopathological factors. Immunoreactive midkine was detectable in the plasma of healthy volunteers, and a cut-off level of 750 pg/mL was established. In breast cancer patients, plasma midkine levels were increased above normal values. These elevated levels of midkine were seen in one (9.1%) of 11 patients with DCIS, 36 (32.4%) of 111 patients with PIBC, and 16 (64.0%) of 25 patients with MBC. Increased levels of midkine were correlated with menopausal status (P = 0.0497) and nuclear grade (P = 0.0343) in PIBC. Cancer detection rates based on midkine levels were higher than those based on three conventional markers including CA15-3 (P < 0.0001), CEA (P = 0.0077), and NCCST-439 (P < 0.0001). Detection rates of breast cancer using a combination of two conventional tumor markers (CA15-3/CEA, CA15-3/NCCST-439, or CEA/NCCST-439) with midkine is significantly higher than those using combination of three conventional tumor markers. Midkine may be a useful novel tumor marker for detection of breast cancer, superior to conventional tumor markers.

The angiogenic factor midkine is regulated by dexamethasone and retinoic acid during alveolarization and in alveolar epithelial cells

Background

A precise balance exists between the actions of endogenous glucocorticoids (GC) and retinoids to promote normal lung development, in particular during alveolarization. The mechanisms controlling this balance are largely unknown, but recent evidence suggests that midkine (MK), a retinoic acid-regulated, pro-angiogenic growth factor, may function as a critical regulator. The purpose of this study was to examine regulation of MK by GC and RA during postnatal alveolar formation in rats.

Methods

Newborn rats were treated with dexamethasone (DEX) and/or all-trans-retinoic acid (RA) during the first two weeks of life. Lung morphology was assessed by light microscopy and radial alveolar counts. MK mRNA and protein expression in response to different treatment were determined by Northern and Western blots. In addition, MK protein expression in cultured human alveolar type 2-like cells treated with DEX and RA was also determined.

Results

Lung histology confirmed that DEX treatment inhibited and RA treatment stimulated alveolar formation, whereas concurrent administration of RA with DEX prevented the DEX effects. During normal development, MK expression was maximal during the period of alveolarization from postnatal day 5 (PN5) to PN15. DEX treatment of rat pups decreased, and RA treatment increased lung MK expression, whereas concurrent DEX+RA treatment prevented the DEX-induced decrease in MK expression. Using human alveolar type 2 (AT2)-like cells differentiated in culture, we confirmed that DEX and cAMP decreased, and RA increased MK expression.

Conclusion

We conclude that MK is expressed by AT2 cells, and is differentially regulated by corticosteroid and retinoid treatment in a manner consistent with hormonal effects on alveolarization during postnatal lung development.

Molecular mechanisms of early lung specification and branching morphogenesis

The "hard wiring" encoded within the genome that determines the emergence of the laryngotracheal groove and subsequently early lung branching morphogenesis is mediated by finely regulated, interactive growth factor signaling mechanisms that determine the automaticity of branching, interbranch length, stereotypy of branching, left-right asymmetry, and finally gas diffusion surface area. The extracellular matrix is an important regulator as well as a target for growth factor signaling in lung branching morphogenesis and alveolarization. Coordination not only of epithelial but also endothelial branching morphogenesis determines bronchial branching and the eventual alveolar-capillary interface. Improved prospects for lung protection, repair, regeneration, and engineering will depend on more detailed understanding of these processes. Herein, we concisely review the functionally integrated morphogenetic signaling network comprising the critical bone morphogenetic protein, fibroblast growth factor, Sonic hedgehog, transforming growth factor-beta, vascular endothelial growth factor, and Wnt signaling pathways that specify and drive early embryonic lung morphogenesis.

Midkine is regulated by hypoxia and causes pulmonary vascular remodeling

Midkine (MK) is expressed in a precise temporal-spatial pattern during lung morphogenesis; however, its role in pulmonary homeostasis is unknown. Increased MK staining and mRNA expression were observed in the lungs of hypoxia-susceptible CAST/eiJ mice during hypoxia. MK expression was induced by hypoxia in cell lines in vitro. Because the transcription factor hypoxiainducible factor-1alpha (HIF-1alpha) modulates cellular responses to hypoxia, we tested whether increased expression of MK in the lung was mediated by HIF-1alpha. HIF-1alpha enhanced the transcription of MK, acting on HIF-1alpha regulatory elements located in the MK gene promoter. Site-directed mutagenesis of the 3' HIF response element in the MK promoter blocked the stimulatory effects of HIF-1alpha. To directly assess the role of MK on lung morphogenesis, transgenic mice were generated in which MK was expressed in the respiratory epithelial cells of the developing lung. MK increased muscularization of small pulmonary arteries, increasing alpha-smooth muscle actin and caldesmon staining and the expression of myocardin. MK directly enhanced the expression of myocardin and the smooth muscle-specific genes alpha-smooth muscle actin, calponin, and SM-22 in vascular smooth muscle precursor cells. Expression of MK in the respiratory epithelium is regulated by hypoxia and HIF-1alpha. These data provide a model wherein the respiratory epithelium responds to hypoxia via HIF-1alpha-dependent regulation of MK, enhancing myocardin expression to influence pulmonary vascular gene expression.

Midkine and pleiotrophin in neural development and cancer

The midkine (MK) family consists of only two members, namely heparin-binding growth factors MK and pleiotrophin (PTN). During embryogenesis, MK is highly expressed in the mid-gestational period, whereas PTN expression reaches the maximum level around birth. Both proteins are localized in the radial glial processes of the embryonic brain, along which neural stem cells migrate and differentiate. Zebrafish and Xenopus MK can induce neural tissues. In addition, deposits of MK and/or PTN are found in neurodegenerative diseases, such as Alzheimer's disease and multiple system atrophy. Both molecules are induced in reactive astrocytes by ischemic insults. In this context, it is interesting that LDL receptor-related protein is a receptor for MK and PTN, and this receptor has been implicated in the pathogenesis of Alzheimer's disease. MK and PTN share receptors, and show similar biological activities that include fibrinolytic, anti-apoptotic, mitogenic, transforming, angiogenic, and chemotactic ones. These activities explain how these molecules are involved in carcinogenesis. MK is detected in human carcinoma specimens from pre-cancerous stages to advanced stages. Strong expression of PTN is also detected in several carcinomas, although, in general, MK is expressed more intensely and in a wide range of carcinomas than PTN. The blood MK level is frequently elevated in advanced human carcinomas, decreases after surgical removal of the tumors, and is correlated with prognostic factors. Thus, it is a good market for evaluating the progress of carcinomas. Furthermore, antisense oligonucleotides for MK and ribozymes for PTN show anti-tumor activity. Therefore, MK and PTN are candidate molecular targets for therapy for human carcinomas.

Epithelial-mesenchymal interactions in the developing lung

Classical experiments in embryology have shown that normal growth, morphogenetic patterning, and cellular differentiation in the developing lung depend on interactive signaling between the endodermal epithelium and mesenchyme derived from splanchnic mesoderm. These interactions are mediated by a myriad of diffusible factors that are precisely regulated in their temporal and spatial expression. In this review we first describe factors regulating formation of the embryonic foregut. We then discuss the experiments demonstrating the importance of tissue interactions in lung patterning and differentiation. Finally, we detail the roles that a few key signaling systems-fibroblast growth factors and their receptors, sonic hedgehog and Gli genes, Wnt genes and beta-catenin, and BMP4-play as mediators of epithelial-mesenchymal interactions in the developing lung.

Chondroitin sulfate proteoglycans are required for lung growth and morphogenesis in vitro

Proteoglycans (PGs) have been shown to play a key role in the development of many tissues. We have investigated the role of sulfated PGs in early rat lung development by treating cultured tissues with 30 mM sodium chlorate, a global inhibitor of PG sulfation. Chlorate treatment disrupted growth and branching of embryonic day 13 lung explants. Isolated lung epithelium (LgE) migrated toward and invaded lung mesenchyme (LgM), and chlorate irreversibly suppressed this response. Chlorate also inhibited migration of LgE toward beads soaked in FGF10. Chlorate severely decreased branching morphogenesis in tissue recombinants consisting of LgM plus either LgE or tracheal epithelium (TrE) and decreased expression of surfactant protein C gene (SP-C). Chlorate also reduced bone morphogenetic protein-4 expression in cultured tips and recombinants but had no effect on the expression of clara cell 10-kDa protein (CC10), sonic hedgehog (Shh), FGF10, and FGF receptor 2IIIb. Chlorate reduced the growth of LgE in mesenchyme-free culture but did not affect SP-C expression. In contrast, chlorate inhibited both rudiment growth and the induction of SP-C in mesenchyme-free cultured TrE. Treatment of lung tips and tissue recombinants with chondroitinase ABC abolished branching morphogenesis. Chondroitinase also suppressed growth of TrE in mesenchyme-free culture. Chondroitinase treatment, however, had no effect on the induction of SP-C expression in any of these cultures. These results demonstrate the overall importance of sulfated PGs to normal lung development and demonstrate a dynamic role for chondroitin sulfate PGs in embryonic lung growth and morphogenesis.

Heparan sulfate-FGF10 interactions during lung morphogenesis

Signaling by fibroblast growth factor 10 (FGF10) through FGFR2b is essential for lung development. Heparan sulfates (HS) are major modulators of growth factor binding and signaling present on cell surfaces and extracellular matrices of all tissues. Although recent studies provide evidence that HS are required for FGF-directed tracheal morphogenesis in Drosophila, little is known about the HS role in FGF10-mediated bud formation in the vertebrate lung. Here, we mapped HS expression in the early lung and we investigated how HS interactions with FGF10-FGFR2b influence lung morphogenesis. Our data show that a specific set of HS low in O-sulfates is dynamically expressed in the lung mesenchyme at the sites of prospective budding near Fgf10-expressing areas. In turn, highly sulfated HS are present in basement membranes of branching epithelial tubules. We show that disrupting endogenous gradients of HS or altering HS sulfation in embryonic lung culture systems prevents FGF10 from inducing local responses and markedly alters lung pattern formation and gene expression. Experiments with selectively sulfated heparins indicate that O-sulfated groups in HS are critical for FGF10 signaling activation in the epithelium during lung bud formation, and that the effect of FGF10 in pattern is in part determined by regional distribution of O-sulfated HS. Moreover, we describe expression of a HS 6-O-sulfotransferase preferentially at the tips of branching tubules. Our data suggest that the ability of FGF10 to induce local budding is critically influenced by developmentally regulated regional patterns of HS sulfation.

Thyroid transcription factor (TTF) -1 regulates the expression of midkine (MK) during lung morphogenesis

Midkine (MK) is a 13-kDa heparin-binding growth factor that is thought to mediate developmental processes, including vasculogenesis, cell migration, and proliferation in various organs. To determine whether MK plays a role during lung morphogenesis, immunostaining for MK was assessed in mouse lung from embryonic day (E) 13 to postnatal day (PN) 24. MK was detected in mesenchymal and respiratory epithelial cells of the peripheral mouse lung from E13.0 to E15.5. From E18.5 to PN1, MK was observed primarily in epithelial cells lining conducting airways and peripheral lung saccules. By PN10, expression was no longer observed in respiratory epithelial cells but was readily detected in small blood vessels in the alveolar region of the lung. Although most respiratory epithelial cells uniformly expressed MK before E13.0, MK was restricted to subsets of cells by E18.5, colocalizing with the Clara cell secretory protein (CCSP) marker in conducting airways and with pro-SPC, a marker specific for alveolar type II pneumocytes. By PN10, MK was not detected in respiratory epithelial cells of the conducting airways and was closely associated with capillary networks. The sites of intense MK staining in the respiratory epithelial cells correlated with sites of expression of thyroid transcription factor (TTF) -1, a transcription factor regulating formation and gene expression in the lung parenchyma. TTF-1 enhanced transcription of the mouse MK gene promoter, acting on TTF-1 regulatory elements located in the 5'-region of the gene. Furthermore, MK expression was not detected in lungs of TTF-1 null mice. TTF-1 regulates expression of MK in the lung. The temporal/spatial distribution of midkine is consistent with a potential role in paracrine signaling during lung morphogenesis.

The growth factor midkine is modulated by both glucocorticoid and retinoid in fetal lung development

The glucocorticoids (GC) and retinoids (RA) modulate branching morphogenesis and cytodifferentiation in the developing lung. We investigated downstream target genes that link glucocorticoid stimulation to the achievement of a mature lung in glucocorticoid receptor (GR) knockout mice. All GR(null) mice and approximately 80% of mice homozygous for a hypomorphic allele (GR(hypo)) die shortly after birth of respiratory failure. cDNA microarray analysis showed organ-specific upregulation of the retinoic acid responsive gene midkine (MK) and its chondroitin-sulfate binding partner PG-M/versican at fetal day 18 and at neonatal day 1 in lungs of GR(hypo) mice, and at neonatal day 1 in lungs of GR(null) mice. By contrast, lung MK and PG-M/versican were downregulated in these mice at fetal day 16.5. In situ hybridization studies showed a dramatic decrease in MK and PG-M/versican RNA between days 16.5 and 17.5 in GR(WT) but not in GR(null) mice. Continued diffuse and robust expression of MK protein was observed in GR(null) mice at neonatal day 1. These findings suggest that MK may contribute to the dysmature lung phenotype in GR-deficient mice. Exposure of cultured day 21 fetal rat lung cells to GC downregulated MK, whereas RA enhanced MK expression. Our findings demonstrate the coincident modulation of expression of MK at the same developmental time point by both GC and RA, providing a potential mechanism for the integration of GC and RA effects on fetal lung development.

Midkine, a heparin-binding growth factor, promotes growth and glycosaminoglycan synthesis of endothelial cells through its action on smooth muscle cells in an artificial blood vessel model

To study the interactions between smooth muscle cells and endothelial cells in vitro, we developed an artificial blood vessel model, which consisted of collagen gel containing human aortic smooth muscle cells and human umbilical vein endothelial cells grown on the gel. The blood vessel model was utilized to investigate the role of midkine, a heparin-binding growth factor, in the intercellular interactions that are important in angiogenesis. In the blood vessel model, midkine induced stratification of the endothelial cells and increased their proliferation and glycosaminoglycan synthesis. However,midkine had no effect on the smooth muscle cells or endothelial cells when they were cultured separately. Increased proliferation of the endothelial cells was also attained by coculturing them with smooth muscle cells in the presence of midkine or culturing endothelial cells with the conditioned medium of the smooth muscle cells, which had been treated with midkine. These experiments indicate that the target of midkine was smooth muscle cells, which secreted factor(s) acting on the endothelial cells. We identified interleukin-8 as one such factor; the synthesis of interleukin-8 by the smooth muscle cells was increased by exposure to midkine, and anti-interleukin-8 inhibited the midkine action. Furthermore, interleukin-8 caused stratification of the endothelial cells in the blood vessel model. These results provided evidence that midkine is one of the factors involved in epithelial-mesenchymal interactions.

Midkine is involved in kidney development and in its regulation by retinoids

In the kidney, in which development depends on epithelial-mesenchymal interactions, it has been shown that retinoids modulate nephrogenesis in a dose-dependent manner in vivo and in vitro. Midkine (MK) is a retinoic acid responsive gene for a heparin-binding growth factor. The aim of the present study was therefore to quantify the expression of MK mRNA during renal development in the rat, to analyze the regulation of MK expression by retinoids in vivo and in vitro, and, finally, to study the role of MK in rat metanephric organ cultures. The spatiotemporal expression of MK in fetal kidney was studied. In control rats, MK expression is ubiquitous at gestational day 14, i.e., at the onset of nephrogenesis. On day 16, MK is expressed in the condensed mesenchyme and in early epithelialized mesenchymal derivatives. On gestational day 21, MK is rather localized in the nonmature glomeruli of the renal cortex. In utero exposure to vitamin A deficiency did not modify the specific spatial and temporal expression pattern of MK gene in the metanephros, although a decrease in mRNA expression occurred. In metanephroi explanted from 14-d-old fetuses and cultured in a defined medium, expression of MK mRNA was found to be stimulated when retinoic acid (100 nM) was added in the culture medium. Finally, in vitro nephrogenesis was strongly inhibited in the presence of neutralizing antibodies for MK: the number of nephrons formed in vitro was reduced by approximately 50% without changes in ureteric bud branching morphogenesis. These results indicated that MK is implicated in the regulation of kidney development by retinoids. These results also suggested that MK plays an important role in the molecular cascade of the epithelial conversion of the metanephric blastema.

Chondroitin sulphate proteoglycan is involved in lens vesicle morphogenesis in chick embryos

Proteoglycans have been implicated in the invagination and formation of various embryonal cavitied primordia. In this paper the expression of chondroitin sulphate proteoglycan (CSPG) is analysed in the lens primordium during lens vesicle formation, and demonstrate that this proteoglycan has a specific distribution pattern with regard to invagination and fusion processes in the transformation of placode into lens vesicle. More specifically, CSPG was detected in: (1) the apical surface of lens epithelial cells, where early CSPG expression was observed in the whole of the lens placode whilst in the vesicle phase it was restricted to the posterior epithelium; (2) intense CSPG expression in the basal lamina, which remained constant for the entire period under study; (3) CSPG expression in the intercellular spaces of the lens primordium epithelium, which increased during the invagination of the primordium and which at the vesicle stage was more evident in the posterior epithelium; and (4) CSPG expression on the edges of the lens placode both prior to and during fusion. Treatment with beta- D -xyloside causes significant CSPG depletion in the lens primordium together with severe alterations in the invagination and fusion of the lens vesicle; this leads to the formation of lens primordia which in some cases remain practically flat or show partial invagination defects or fusion disruption. Similar results were obtained by enzyme digestion with chondroitinase AC but not with type II heparinase, which indicates that alterations induced by beta- D -xyloside were due to interference in CSPG synthesis. The findings demonstrate that CSPG is a common component of the lens primordium at the earliest developmental stages during which it undergoes specific modifications. It also includes experimental evidence to show that 'in vivo' CSPG plays an important role in the invagination and fusion processes of the lens primordium.

Fibroblast growth factors as multifunctional signaling factors

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