Figure 8. Aging-associated SIRT4 upregulation leads to a shifted L-OPA1 to S-OPA1 ratio in two different fibroblast senescence models. Primary human dermal fibroblasts were either transfected with miR-15b inhibitors (or control oligonucleotides) or subjected to γ−irradiation (γIR; 20 Gy) (both in the presence or absence of siRNA duplexes against SIRT4)  followed by analysis of OPA1-L and OPA1-S expression by immunoblotting after four days (A) As a control for complete proteolytic processing of L-OPA1 to S-OPA1, fibroblasts were treated with CCCP (10 µM) for two hours. The ratio between the expression levels of L-OPA1 and S-OPA1 was determined by ImageJ-based densitometric analysis in miR-15b inhibitor transfected fibroblasts (B) and cells subjected to γIR (C). To evaluate statistical significance (compared to control), two-way ANOVA followed by Tukey’s test was performed (*p<0.05; n=4).