FGF signaling induces ECM degradation and polyp detachment
A previous study of P. damicornis suggested participation of ECM degradation in the polyp-detachment step of polyp bail-out (Wecker et al., 2018). Congruent with that study, we found remarkable upregulation of eight MMP-encoding transcripts during the experiments. MMPs are a family of proteases that function in ECM degradation (Birkedal-Hansen et al., 1993; Kojima, Itoh, Matsumoto, Masuho, & Seiki, 2000). Regulation of these MMPs in polyp bail-out is probably linked to the FGF signaling pathway, since expression profiles of FGF2 and FGFR2 matched those of the two MMP genes in our qPCR assay. The MMP-inducing function of the FGF2/FGFR2 signaling pathway has been documented in many mammalian cells for its involvement in cell migration and angiogenesis (Ardi et al., 2009; Birkedal-Hansen et al., 1993; Pintucci et al., 2003). Intracellular signal transduction from FGF receptors to activation of MMPs likely occurs through Rho family GTPases, suggested by the similarity of expression profiles between RHO and the aforementioned genes in our qPCR assay. Furthermore, upon activation, some MMPs have been shown to induce release of FGF2 (Whitelock, Murdoch, Iozzo, & Underwood, 1996). The irreversibility of coral polyp bail-out, and the remarkable upregulation of FGF2 and MMPs from 6 h to 18 h in our qPCR analysis, can thus be explained by formation of a positive feedback loop between MMPs and FGF signals. Based on these results, we hypothesize that the FGF signaling pathway is triggered under environmental stress and the signal is likely relayed by Rho family GTPases to activate MMPs, which in turn initiate ECM degradation and subsequent polyp detachment (Fig. 4).