A novel and robust method for assessing mitochondrial (dys)function in healthy and diseased frozen cardiac tissue

Cardiovascular diseases are often associated with impairment in mitochondrial function detected by reduced mitochondrial oxygen consumption using high-resolution respirometry. However, existing respirometry protocols are limited by the necessity for fresh tissue samples. This study developed a method with tailored substrate-inhibitor titration (TSIT) of mitochondrial electron transport complexes (ETC) to measure mitochondrial function in frozen cardiac samples using high-resolution respirometry. Briefly, acetyl-CoA was added to fuel the tricarboxylic acid (TCA) cycle for NADH production, enabling complex I (CI)-linked respiratory assessment. NADH was then added to measure maximum CI-linked respiratory capacity, followed by rotenone and succinate to assess complex II (CII)-linked respiratory capacity. TSIT detected mitochondrial functional differences between frozen atrial and ventricular tissue, with comparable results as measured in fresh samples. It also detected cardiac mitochondrial dysfunction across various (patho)physiological mouse models (including aging, ischemia reperfusion, obesity, and CI deficiency) as well as in frozen human donor samples, highlighting its clinical potential. Furthermore, we showed the first evidence for supercomplexes (SCs) formation between ETC-SCs and the TCA cycle metabolon, underpinning TSIT feasibility. In conclusion, we established a novel, robust, sensitive and translational method (TSIT) for assessing mitochondrial (dys)function in frozen cardiac samples from various species, enabling flexible analysis of mitochondrial function in both laboratory and clinical settings.