Non-canonical fibroblast growth factor signalling in angiogenesis

Whereas fibroblast growth factors (FGFs) classically transmit their signals via high-affinity tyrosine kinase receptors (FGFR1-4), recent evidence strongly implicates non-tyrosine kinase receptors (NTKR) or cell-surface FGFR-interacting proteins as important players in FGF signalling. Although NTKR have lower affinity for FGFs in comparison with cognate tyrosine kinase receptors, because of their high abundance they can effectively bind FGFs and produce unique biological effects independent of FGFRs. A prime example of such NTKR is the syndecan family of plasma membrane proteoglycans and, in particular, syndecan-4, which transmits FGF signalling via a protein kinase C alpha pathway. Another NTKR, CIA integrin, functions as an FGF signalling modulator by binding both FGF2 and FGFR1. Yet another NTKR, neural cell adhesion molecule (NCAM), can serve as an FGFR ligand and assemble an FGFR signalling complex in the absence of FGFs. Furthermore, N-cadherin, which has been reported to associate with FGFR, appears to activate FGFR in both ligand (FGF)-dependent and ligand-independent manners. Finally, gangliosides are implicated as a co-receptor system of FGFs. The biological consequence of non-canonical FGF signalling tends to be less discernable compared to the canonical FGFR activation because of the overlap between these two pathways; nevertheless, non-canonical signalling is important and sometimes essential for cellular functions. Given the diversity of FGF activities through embryonic development to adult physiology, the existence of the non-canonical signalling system may account for the different cellular response to the FGF input in different biological contexts. In this review, we will discuss recent findings related to non-canonical FGF signalling with emphasis on the endothelial biology and angiogenesis.