Abstract
BackgroundFibronectin leucine rich transmembrane (FLRT) proteins have dual properties as regulators of cell adhesion and potentiators of fibroblast growth factor (FGF) mediated signalling. The mechanism by which the latter is achieved is still unknown and is the subject of this investigation.Principal FindingsHere we show that FLRT1 is a target for tyrosine phosphorylation mediated by FGFR1 and implicate a non-receptor Src family kinase (SFK). We identify the target tyrosine residues in the cytoplasmic domain of FLRT1 and show that these are not direct substrates for Src kinase suggesting that the SFK may exert effects via potentiation of FGFR1 kinase activity. We show that whilst FLRT1 expression results in a ligand-dependent elevation of MAP kinase activity, a mutant version of FLRT1, defective as an FGFR1 kinase substrate (Y3F-FLRT1), has the property of eliciting ligand-independent chronic activation of the MAP kinase pathway which is suppressed by pharmacological inhibition of either FGFR1 or Src kinase. Functional investigation of FGFR1 and FLRT1 signalling in SH-SY5Y neuroblastoma cells reveals that FLRT1 alone acts to induce a multi-polar phenotype whereas the combination of FLRT1 and FGFR activation, or expression of Y3F-FLRT1, acts to induce neurite outgrowth via MAPK activation. Similar results were obtained in a dendrite outgrowth assay in primary hippocampal neurons. We also show that FGFR1, FLRT1 and activated Src are co-localized and this complex is trafficked toward the soma of the cell. The presence of Y3F-FLRT1 rather than FLRT1 resulted in prolonged localization of this complex within the neuritic arbour.ConclusionsThis study shows that the phosphorylation state of FLRT1, which is itself FGFR1 dependent, may play a critical role in the potentiation of FGFR1 signalling and may also depend on a SFK-dependent phosphorylation mechanism acting via the FGFR. This is consistent with an ‘in vivo’ role for FLRT1 regulation of FGF signalling via SFKs. Furthermore, the phosphorylation-dependent futile cycle mechanism controlling FGFR1 signalling is concurrently crucial for regulation of FLRT1-mediated neurite outgrowth.
Highlights
Knowledge of the architecture of receptor tyrosine kinase signalling pathways is rapidly expanding but much less is known about the mechanisms that shape the spatial and temporal dynamics of signal propagation
This study shows that the phosphorylation state of FLRT1, which is itself FGFR1 dependent, may play a critical role in the potentiation of FGFR1 signalling and may depend on a Src family kinase (SFK)-dependent phosphorylation mechanism acting via the fibroblast growth factor receptor (FGFR)
The phosphorylation-dependent futile cycle mechanism controlling FGFR1 signalling is concurrently crucial for regulation of FLRT1-mediated neurite outgrowth
Summary
Knowledge of the architecture of receptor tyrosine kinase signalling pathways is rapidly expanding but much less is known about the mechanisms that shape the spatial and temporal dynamics of signal propagation. Three members of the FLRT family (FLRT 1–3) have been identified in higher vertebrates from functional screens and in silico searches [6] They exhibit canonical fibronectin and leucine rich repeat motifs in the extracellular domain which mediate the homotypic cell adhesion functions; a single transmembrane domain and a short (,100 amino acid) cytoplasmic domain devoid of overt signalling motifs. FLRT1, the subject of this study, is expressed in adult brain and kidney [6] and, in embryonic development, is localized in the midbrain at the boundary with the hindbrain and in the dorsal diencephelon adjacent to the telencephalon, the eye, dorsal root and trigeminal ganglia and in cells adjacent to the urogenital ridge [4] This pattern overlaps with regions of FGFR and FGF ligand expression suggestive of a specific requirement for interaction of the FGF and FLRT axis in these cell types. The mechanism by which the latter is achieved is still unknown and is the subject of this investigation
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