(A) Predicted binding site of miR-302 on 3′-UTR of Micu1. (B) HEK293T cells were transfected with both the empty LUC reporter or micu1 3′-UTR reporter (Micu1 3′-UTR-luc) or micu1 3′-UTR reporter with mutation of the miR-302 binding site (Micu1 3′-UTR mut-luc) and an expression plasmid for miR-302. Cell extracts were assayed for LUC expression at 48 hours after transfection. LUC reporter assays showing that miR-302 can repress Micu1 expression through its 3′-UTR. This repression can be reversed by mutation of the miR-302 binding site. (C) Adult Sftpc^CreERT2 or Sftpc^CreERT2; Rosa26^miR-302 mice received two doses of tamoxifen. AT2 cells were purified after 7 days from last tamoxifen treatment, and expression of Micu1 was examined by qPCR. Gene expression was normalized to Rps13, a mitochondrial gene. (D) Transmission electron microscopy of lungs 3 weeks after tamoxifen treatment showed disruptive mitochondrial morphology and cristae structure in Sftpc^CreERT2; Rosa26^miR-302 AT2 cells. Dashed white lines outline mitochondria. (E) Quantification of mitochondria size and cristae numbers per mitochondrion, using ImageJ. (F) Expression of MICU1 and MCU proteins in mouse lungs as analyzed by Western blot. Tom20 was the mitochondrial loading control. (G) Densitometry chart showing the relative protein level of MICU1. Band density was normalized to Tom20. (H) Graph of the ratio of MICU1 to MCU determined by Western blot. Data are presented as mean ± SEM. P values were calculated using 1-way ANOVA (B, G, and H) and Student’s t test (C and E).*P < 0.05; **P < 0.01; ***P < 0.001.