Figure 10. SIRT4 upregulation upon ionizing radiation stress increases L-OPA1 levels and promotes mitochondrial fusion in fibroblasts. (A) Primary human dermal fibroblasts were subjected to γIR (20 Gy) both in either the presence or absence of siRNA duplexes against SIRT4  followed by subcellular visualization of SIRT4 (green), MTC02 (red), and α-Tubulin (yellow) after four days.The mitochondrial profiles (MTC02; black/white) were further visualized as binary confocal pictures. Cellular morphology/size was defined by α-Tubulin staining. Representative images are depicted. (B) Quantification of the mitochondrial mass via MTC02 staining analysis using ImageJ software (Material & Methods and suppl. Material & Methods). (C) Quantitative analysis of mitochondrial fusion/length of mitochondrial tubes (as exemplified in Suppl. Fig. 7) using ImageJ software (Material & Methods and suppl. Material & Methods). To evaluate statistical significance, two-way ANOVA followed by Tukey’s test was performed (*p<0.05; n=13-19 cells analysed by spinning disk confocal microscopy in three experiments).