Background Fibronectin leucine rich transmembrane (FLRT) proteins have dual properties as regulators of cell adhesion and potentiators of fibroblast growth factor (FGF) mediated signalling. kinase pathway which is usually suppressed by pharmacological inhibition of either FGFR1 or Src kinase. Functional investigation of FGFR1 and XL-888 FLRT1 signalling in SH-SY5Y neuroblastoma cells reveals that FLRT1 alone functions to induce a multi-polar phenotype whereas the combination of FLRT1 and FGFR activation, or manifestation of Y3F-FLRT1, functions to induce neurite outgrowth via MAPK activation. Comparable results were obtained in a dendrite outgrowth assay in main hippocampal neurons. We also Col1a1 show that FGFR1, FLRT1 and activated Src are co-localized and this complex is usually trafficked toward the soma of the cell. The presence of Y3F-FLRT1 rather than FLRT1 resulted in long term localization of this complex within the neuritic arbour. Findings This study shows that the phosphorylation state of FLRT1, which is usually itself FGFR1 dependent, may play a crucial role in the potentiation of FGFR1 signalling and may also depend on a SFK-dependent phosphorylation mechanism acting via the FGFR. This is usually consistent with an in vivo role for FLRT1 rules of FGF signalling via SFKs. Furthermore, the phosphorylation-dependent futile cycle mechanism controlling FGFR1 signalling is usually concurrently crucial for rules of FLRT1-mediated neurite outgrowth. Introduction Knowledge of the architecture of receptor tyrosine kinase signalling pathways is usually rapidly expanding but much less is usually known about the mechanisms that shape the spatial and temporal mechanics of transmission propagation. In particular, a number of brokers have been recognized which attenuate or accelerate signalling through downstream pathways  but their mechanisms of action are frequently poorly comprehended. Here we focus on the fibronectin leucine rich transmembrane protein (FLRTs): a subclass of the larger diverse leucine rich repeat (LRR) superfamily  which take action as multifunctional accelerators of fibroblast growth XL-888 factor receptor (FGFR) signalling. We, and others, have exhibited that: users of the FLRT family associate with users of the FGFR family, accentuate FGF-mediated signalling via the Ras/Raf/ERK pathway and play a role in cadherin-dependent homotypic cell adhesion functions , , . A key issue in further understanding the function of FLRTs is usually to determine the inter-relationships between these three cardinal properties. Three users of the FLRT family (FLRT 1C3) have been recognized in higher vertebrates from functional screens and in silico searches . They exhibit canonical fibronectin and leucine rich repeat motifs in the extracellular domain name which mediate the homotypic cell adhesion functions; a single transmembrane domain name and a short (100 amino acid) cytoplasmic domain name devoid of overt signalling motifs. Each FLRT family member exhibits characteristic and restricted patterns of manifestation in the developing embryo , , . FLRT1, the subject of this study, is usually expressed in adult brain and kidney  and, in embryonic development, is usually localized in the midbrain at the boundary with the hindbrain and in the dorsal diencephelon adjacent to the telencephalon, the eye, dorsal main and trigeminal ganglia and in cells adjacent to the urogenital ridge . This pattern overlaps with regions of FGFR and FGF ligand manifestation suggestive of a specific requirement for conversation of the FGF and FLRT axis in these cell types. Indeed a potential role for FLRT action in neuronal function has been proposed from studies of FLRT3 manifestation in neural regeneration models , , . In this work we set out to further understand the functional relationship between FGFR activation and FLRT function via an initial analysis of FGFR-mediated phosphorylation of FLRT1. We show that phosphorylation of FLRT1 in the cytoplasmic domain name modulates the ability of FLRT to activate the MAPK pathway and induce neurite outgrowth. A non-phosphorylated form of FLRT1 acts as a chronic activator of FGFR1 signalling and both signalling propagation and induction of XL-888 neurite outgrowth require the activity of a non-receptor Src family kinase. Results FLRT1 and FGFR1 are co-localized We have previously documented an association between FGFR1 and FLRT1  and we were interested to learn the cellular location(h).