For more information on FGF receptor homologs, see the Heartless Evolutionary Homologs site.

Heartless, or DFR1, or FGF-R1 is a second FGF receptor homolog of Drosophila. Heartless mRNA is observed in several imaginal discs. Heartless expression in the wing and leg discs takes place in probable myoblasts, forming a pattern similar to that of twist, a mesodermal gene. The mRNA is also detected in the morphogenetic furrow and its posterior region of the eye disc and around the proliferation center of the brain. These results suggest that DFR1 is involved in the development of mesodermal and neuronal cells constituting the adult body (Emori, 1993). For information on the role of FGF and the FGF receptor in mesoderm, see the Heartless site.

Heartless and Breathless proteins contain, respectively, two and five immunoglobulin-like domains, in the extracellular region, and a split tyrosine kinase domain in the intracellular region. Homology between the two kinase domains is 79%. In early embryos, Heartless RNA expression, requiring both twist and snail proteins, is specific to mesodermal primordium and invaginated mesodermal cells. At later stages, putative muscle precursor cells and cells in the central nervous system (CNS) express DFR1 (Shishido, 1993).

Activation of FGF receptors necessitates ligand induced dimerization. Dimerization of FGF receptors is mediated by either soluble or cell surface-bound heparin sulfate proteoglycans, which in concert with FGFs promote receptor dimerization, activation, and induction of biological responses. The cytoplasmic domain of mammalian FGFR1 contains at least seven tyrosine autophosphorylation sites. Autophosphorylation of two tyrosine residues in the catalytic domain is critical for the kinase activity of FGFR1. Another C-terminal tail tyrosine functions as a binding site for phospholipase C-gamma, and is essential for FGF-induced stimulation of phosphatidylinositol hydrolysis. Analysis of the crystal strucure of the tyrosine kinase domain reveals that residues in the activation loop appear to interfere with substrate peptide binding but not with ATP binding, revealing a general autoinhibitory mechanism for receptor tyrosine kinases (Mohammadi, 1996 and references).

A lipid-anchored Grb2-binding protein links FGF-receptor activation to the Ras/MAPK signaling pathway. Activation of the Ras/MAPK signaling cascade is essential for growth factor-induced cell proliferation and differentiation. A novel protein, designated FRS2 is tyrosine phosphorylated and binds to Grb2/Sos in response to FGF or NGF stimulation. FRS2 is myristylated and this modification is essential for membrane localization, tyrosine phosphorylation, Grb2/Sos recruitment, and MAPK activation. FRS2 functions as a lipid-anchored docking protein that targets signaling molecules to the plasma membrane in response to FGF stimulation to link receptor activation with the MAPK and other signaling pathways essential for cell growth and differentiation. FRS2 is closely related and probably indentical to SNT (suc1-associated neurotrophic factor target), the long-sought target of FGF and NGF receptors (Kouhara, 1997).

The cell adhesion molecules (CAMs) NCAM, N-cadherin, and L1 (See Drosophila Fasciclin 2) are homophilic binding molecules that stimulate axonal growth. It has been postulated that the above CAMs can stimulate axonal growth by activating the fibroblast growth factor receptor (FGFR) in neurons. Activation of NCAM and L1 can lead to phosphorylation of the FGFR. Both this and the neurite outgrowth response stimulated by all three of the above CAMs are lost when a kinase-deleted, dominant negative form of FGFR1 is expressed in PC12 cells. Transgenic mice have been generated that express the dominant negative FGFR under control of the neuron-specific enolase (NSE) promoter. Cerebellar neurons isolated from these mice have also lost their ability to respond to NCAM, N-cadherin, and L1. A peptide inhibitor of phospholipase C gamma (PLCgamma) that inhibits neurite outgrowth stimulated by FGF also inhibits neurite outgrowth stimulated by the CAMs. It is concluded that activation of the FGFR is both necessary and sufficient to account for the ability of the above CAMs to stimulate axonal growth, and that PLCgamma is a key downstream effector of this response (Saffell, 1997).

Lepidopteran ortholog of Drosophila Breathless is a receptor for the baculovirus fibroblast growth factor

The Bombyx mori nucleopolyhedrovirus (BmNPV) encodes a gene homologous to the mammalian fibroblast growth factor (FGF) family. B. mori and Spodoptera frugiperda orthologous genes (Bmbtl and Sfbtl, respectively) of Drosophila melanogaster breathless (btl), encoding a receptor for Branchless/FGF, has been cloned, these genes encode the receptor for a baculovirus-encoded FGF (vFGF). Sequence analysis showed that BmBtl is composed of 856 amino acid residues, which potentially encodes a 97.3-kDa polypeptide and shares structural features and sequence similarities with the FGF receptor family. Reverse transcription-PCR experiments showed that Bmbtl is abundantly expressed in the trachea and midgut in B. mori larvae, with moderate expression observed in the hemocytes and the B. mori cultured cell line BmN. Sf-9 cells were generated that stably express His-tagged BmBtl. Western blot analysis revealed that BmBtl is an approximately 110-kDa protein. Immunoprecipitation experiments showed that BmNPV vFGF markedly phosphorylates BmBtl in Sf-9 cells. In addition, BmBtl overexpression enhances the migration activity for BmNPV vFGF. Furthermore, Sf-9 cells were generated in which Sfbtl was knocked down by transfection with double-strand RNA-expressing plasmids. In these cells, cell motility triggered by vFGF was markedly reduced. These results strongly suggest that the Btl orthologs, BmBtl and SfBtl, are the receptors for vFGF, which mediate vFGF-induced host cell chemotaxis (Katsuma, 2006).

FGF-dependent metabolic control of vascular development

Blood and lymphatic vasculatures are intimately involved in tissue oxygenation and fluid homeostasis maintenance. Assembly of these vascular networks involves sprouting, migration and proliferation of endothelial cells. Recent studies have suggested that changes in cellular metabolism are important to these processes. Although much is known about vascular endothelial growth factor (VEGF)-dependent regulation of vascular development and metabolism, little is understood about the role of fibroblast growth factors (FGFs) in this context. This study identified FGF receptor (FGFR; see Drosophila Breathless) signalling as a critical regulator of vascular development. This is achieved by FGF-dependent control of c-MYC (MYC; see Drosophila Myc) expression that, in turn, regulates expression of the glycolytic enzyme hexokinase 2 (HK2; see Drosophila Hexokinase A). A decrease in HK2 levels in the absence of FGF signalling inputs results in decreased glycolysis, leading to impaired endothelial cell proliferation and migration. Pan-endothelial- and lymphatic-specific Hk2 knockouts phenocopy blood and/or lymphatic vascular defects seen in Fgfr1/Fgfr3 double mutant mice, while HK2 overexpression partly rescues the defects caused by suppression of FGF signalling. Thus, FGF-dependent regulation of endothelial glycolysis is a pivotal process in developmental and adult vascular growth and development (Yu, 2017).

breathless: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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