Mfge8 Knockout Affects Tail Resorption Speed During Xenopus Metamorphosis.

To elucidate the control mechanism of tail resorption during Xenopus metamorphosis, the expression of mfge8, a macrophage-apoptotic cell bridging molecule that promotes phagocytosis in mammals, was examined. In both Xenopus tropicalis and Xenopus laevis, the mfge8 expression in the tail increased significantly during metamorphosis, reaching its peak at the metamorphic climax, when the tail shortens rapidly. This finding suggests that the up-regulation of mfge8 at metamorphic climax is involved in the clearance of apoptotic tail muscles. To investigate the significance of mfge8 up-regulation, mfge8-deficient X. tropicalis tadpoles were generated using the CRISPR/Cas9 method, and the effects of mfge8-deletion were examined. Delayed tail resorption process was observed in the mfge8-deficient mutants (from 9.8 days [wild type] to 13.2 days [F2 mutant]), suggesting that elevated mfge8 expression during the metamorphic climax contributes to the tail resorption.