The fibroblast growth factors

The fibroblast growth factor (FGF) family currently has over a dozen structurally related members. FGF1 is also known as acidic FGF; FGF2 is sometimes called basic FGF; and FGF7 sometimes goes by the name of keratinocyte growth factor. Over a dozen distinct FGF genes are known in vertebrates, and they can generate hundreds of protein isoforms by varying their RNA splicing or initiation codons in different tissues (Lappi 1995). FGFs can activate a set of receptor tyrosine kinases called the fibroblast growth factor receptors (FGFRs). As we will discuss later in this chapter, receptor tyrosine kinases are proteins that extend through the cell membrane (Figure 6.10A). On the extracellular side is the portion of the protein that binds the paracrine factor. On the intracellular side is a dormant tyrosine kinase (i.e., a protein that can phosphorylate another protein by splitting ATP). When the FGF receptor binds an FGF (and only when it binds an FGF), the dormant kinase is activated, and it phosphorylates certain proteins within the responding cell. The proteins are now activated and can perform new functions. FGFs are associated with several developmental functions, including angiogenesis (blood vessel formation), mesoderm formation, and axon extension. While FGFs can often substitute for one another, their expression patterns give them separate functions. FGF2 is especially important in angiogenesis, and FGF8 is important for the development of the midbrain and limbs (Figure 6.10B; Crossley et al. 1996).

Figure 6.10

FGF expression. (A) Structure of a receptor tyrosine kinase. The dormant tyrosine kinase is activated by the binding of FGF by the extracellular portion of the receptor protein. This enzyme activity phosphorylates specific tyrosine residues of certain (more...)

The Hedgehog family

The Hedgehog proteins constitute a family of paracrine factors that are often used by the embryo to induce particular cell types and to create boundaries between tissues. Vertebrates have at least three homologues of the Drosophila hedgehog gene: sonic hedgehog (shh), desert hedgehog (dhh), and indian hedgehog (ihh). Desert hedgehog is expressed in the Sertoli cells of the testes, and mice homozygous for a null allele of dhh exhibit defective spermatogenesis. Indian hedgehog is expressed in the gut and in cartilage and is important in postnatal bone growth (Bitgood and McMahon 1995; Bitgood et al. 1996).

Sonic hedgehog^† is the most widely used of the three vertebrate homologues. Made by the notochord, it is processed so that only the amino-terminal two-thirds of the molecule is secreted. This peptide is responsible for patterning the neural tube such that motor neurons are formed from the ventral neurons and sensory neurons are formed from the dorsal neurons (see Chapter 12; Yamada et al. 1993). Sonic hedgehog is also responsible for patterning the somites so that the portion of the somite closest to the notochord becomes the cartilage of the spine (Fan and Tessier-Lavigne 1994; Johnson et al. 1994). As we will see in later chapters, Sonic hedgehog has been shown to mediate the formation of the left-right axis in chicks, to initiate the anterior-posterior axis in limbs, to induce the regionally specific differentiation of the digestive tube, and to induce feather formation (see Figures 6.11 and 6.6). Sonic hedgehog often works with other paracrine factors, such as Wnt and FGF proteins. In the developing tooth, Sonic hedgehog, FGF4, and other paracrine factors are concentrated in the region where cell interactions are creating the cusps of the teeth (see Figure 13.9; Vaahtokari et al. 1996a).

Figure 6.11

The sonic hedgehog gene is shown by in situ hybridization to be expressed in the 3-day embryonic chick nervous system (red arrow), gut (blue arrow), and limb bud (black arrow). (Photograph courtesy of C. Tabin.)

The Wnt family

The Wnts constitute a family of cysteine-rich glycoproteins. There are at least 15 members of this family in vertebrates. Their name comes from fusing the name of the Drosophila segment polarity gene wingless with the name of one of its vertebrate homologues, integrated. While Sonic hedgehog is important in patterning the ventral portion of the somites (causing the cells to become cartilage), Wnt1 appears to be active in inducing the dorsal cells of the somites to become muscle (McMahon and Bradley 1990; Stern et al. 1995). Wnt proteins also are critical in establishing the polarity of insect and vertebrate limbs, and they are used in several steps of urogenital system development (Figure 6.12).

Figure 6.12

Wnt proteins play several roles in the development of the urogenital organs. Wnt4 is necessary for kidney developnent and for female sex determination. (A) Whole-mount in situ hybirdization of Wnt4 expression in a 14-day mouse embryonic male urogenital (more...)

The TGF-β superfamily

There are over 30 structurally related members of the TGF-b superfamily,^‡ and they regulate some of the most important interactions in development (Figure 6.13). The proteins encoded by TGF-β superfamily genes are processed such that the carboxy-terminal region contains the mature peptide. These peptides are dimerized into homodimers (with themselves) or heterodimers (with other TGF-β peptides) and are secreted from the cell. The TGF-β superfamily includes the TGF-β family, the activin family, the bone morphogenetic proteins (BMPs), the Vg1 family, and other proteins, including glial-derived neurotrophic factor (necessary for kidney and enteric neuron differentiation) and Müllerian inhibitory factor (which is involved in mammalian sex determination).

Figure 6.13

Relationships among members of the TGF-β superfamily. (After Hogan 1996.)

TGF-β family members TGF-β1, 2, 3, and 5 are important in regulating the formation of the extracellular matrix between cells and for regulating cell division (both positively and negatively). TGF-β1 increases the amount of extracellular matrix epithelial cells make (both by stimulating collagen and fibronectin synthesis and by inhibiting matrix degradation). TGF-βs may be critical in controlling where and when epithelia can branch to form the ducts of kidneys, lungs, and salivary glands (Daniel 1989; Hardman et al. 1994; Ritvos et al. 1995). The effects of the individual TGF-β family members are difficult to sort out, because members of the TGF-β family appear to function similarly and can compensate for losses of the others when expressed together. Moreover, targeted deletions of the Tgf-β1 gene in mice are difficult to interpret, since the mother can supply this factor through the placenta and milk (Letterio et al. 1994).

The members of the BMP family were originally discovered by their ability to induce bone formation; hence, they are the bone morphogenetic proteins. Bone formation, however, is only one of their many functions, and they have been found to regulate cell division, apoptosis (programmed cell death), cell migration, and differentiation (Hogan 1996). BMPs can be distinguished from other members of the TGF-β superfamily by their having seven, rather than nine, conserved cysteines in the mature polypeptide. The BMPs include proteins such as Nodal (responsible for left-right axis formation) and BMP4 (important in neural tube polarity, eye development, and cell death; see Figure 4.21 ). (As it turns out, BMP1 is not a member of the family; it is a protease.) The Drosophila Decapentaplegic protein is homologous to the vertebrate BMP4, and human BMP4 can replace the Drosophila homologue, rescuing those flies deficient in Dpp (Padgett et al. 1993).

Other paracrine factors

Although most of the paracrine factors are members of the above-mentioned four families, some have few or no close relatives. Factors such as epidermal growth factor, hepatocyte growth factor, neurotrophins, and stem cell factor are not in the above-mentioned families, but each plays important roles during development. In addition, there are numerous factors involved almost exclusively with developing blood cells: erythropoietin, the cytokines, and the interleukins. These factors will be discussed when we detail blood cell formation in Chapter 14.

Footnotes

*

There is considerable debate as to how far paracrine factors can operate. Activin, for instance, can diffuse over many cell diameters and can induce different sets of genes at different concentrations (Gurdon et al. 1994, 1995). The Vg1, BMP4, and Nodal proteins, however, probably work only on their adjacent neighbors (Jones et al. 1996; Reilly and Melton 1996). These factors may induce the expression of other short-range factors from these neighbors, and a cascade of paracrine inductions can be initiated.

In addition to endocrine, paracrine, and juxtacrine regulation, there is also autocrine regulation. Autocrine regulation occurs when the same cells that secrete paracrine factors also respond to them. In this case, the cell synthesizes a molecule for which it has its own receptor. Although autocrine regulation is not common, it is seen in placental cytotrophoblast cells; these cells synthesize and secrete platelet-derived growth factor, whose receptor is on the cytotrophoblast cell membrane (Goustin et al. 1985). The result is the explosive proliferation of that tissue.

†

Yes, it is named after the Sega Genesis character. The original hedgehog gene was found in Drosophila, in which genes are named after their mutant phenotype. The loss-of-function hedgehog mutation in Drosophila causes the fly embryo to be covered with pointy denticles on its cuticle. Hence, it looks like a hedgehog. The vertebrate hedgehog genes were discovered by searching chick gene libraries with probes that would find sequences similar to that of the fruit fly hedgehog gene. Riddle and his colleagues in Cliff Tabin's laboratory (1993) discovered three genes homologous to the Drosophila hedgehog. Two were named after species of hedgehogs, the third was named after the cartoon character.

‡

TGF stands for transforming growth factor. The designation superfamily is often given when each of the different classes of molecules constitutes a “family.” The members of a superfamily all have similar structures, but are not as close as the molecules within a family are to one another.