Abstract
Fibroblast growth factor receptor-1 (FGFR1) activity at the plasma membrane is tightly controlled by the availability of co-receptors and competing receptor isoforms. We have previously shown that FGFR1 activity in pancreatic beta-cells modulates a wide range of processes, including lipid metabolism, insulin processing, and cell survival. More recently, we have revealed that co-expression of FGFR5, a receptor isoform that lacks a tyrosine-kinase domain, influences FGFR1 responses. We therefore hypothesized that FGFR5 is a co-receptor to FGFR1 that modulates responses to ligands by forming a receptor heterocomplex with FGFR1. We first show here increased FGFR5 expression in the pancreatic islets of nonobese diabetic (NOD) mice and also in mouse and human islets treated with proinflammatory cytokines. Using siRNA knockdown, we further report that FGFR5 and FGFR1 expression improves beta-cell survival. Co-immunoprecipitation and quantitative live-cell imaging to measure the molecular interaction between FGFR5 and FGFR1 revealed that FGFR5 forms a mixture of ligand-independent homodimers (∼25%) and homotrimers (∼75%) at the plasma membrane. Interestingly, co-expressed FGFR5 and FGFR1 formed heterocomplexes with a 2:1 ratio and subsequently responded to FGF2 by forming FGFR5/FGFR1 signaling complexes with a 4:2 ratio. Taken together, our findings identify FGFR5 as a co-receptor that is up-regulated by inflammation and promotes FGFR1-induced survival, insights that reveal a potential target for intervention during beta-cell pathogenesis.
Highlights
Fibroblast growth factor receptor-1 (FGFR1) activity at the plasma membrane is tightly controlled by the availability of coreceptors and competing receptor isoforms
We revealed that co-expression of FGFR1 and its co-receptor KLB permits pancreatic beta-cells to respond to FGF21, as measured by modulated insulin biosynthesis, citrate and lipid metabolism, and cell survival [2,3,4]
We examined the effect of endogenous FGFR5 and FGFR1 on beta-cell survival using siRNA knockdown (R5 and R1) versus scrambled control
Summary
FGFR5 is up-regulated in beta-cells by inflammatory cytokines to enhance cell survival. These data show that the relative brightness of R5Ven was unaffected by FGF2, suggesting that no further aggregation of FGFR5 is driven by ligand addition (Fig. 3F) Overall, both homoFRET and N&B analysis were entirely consistent with a model where the full-length receptor forms an FGF2-insensitive mixture of homotrimers (ϳ75% of (FGFR5)3) and dimers (ϳ25% of (FGFR5)2) at the cell surface (Fig. 3G). Anisotropy imaging showed higher R5Ven steady-state anisotropy when co-expressed with R1Dark, but these values did not reach monomeric levels (black line), and no further change was induced by the addition of FGF2 (Fig. 4C) These data suggest that FGFR1 only partially dissolves FGFR5 homotrimers to homodimers and that ligand addition does not subsequently aggregate the homodimers, at least not within the Forster distance of ϳ4.95 nm. The anisotropy enhancement curves for R5Ven shifted from exponential to
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