Skeletal muscles account for ~30-40% of the total weight of human body and are responsible for its most important functions, including movement, respiration, thermogenesis, and glucose and protein metabolism. Skeletal muscle damage negatively impacts the whole-body functioning, leading to deterioration of the quality of life and, in severe cases, death. Therefore, timely diagnosis and therapy for skeletal muscle dysfunction are important goals of modern medicine. In this review, we focused on the skeletal troponins that are proteins in the thin filaments of muscle fibers. Skeletal troponins play a key role in regulation of muscle contraction. Biochemical properties of these proteins and their use as biomarkers of skeletal muscle damage are described in this review. One of the most convenient and sensitive methods of protein biomarker measurement in biological liquids is immunochemical analysis; hence, we examined the factors that influence immunochemical detection of skeletal troponins and should be taken into account when developing diagnostic test systems. Also, we reviewed the available data on the skeletal troponin mutations that are considered to be associated with pathologies leading to the development of diseases and discussed utilization of troponins as drug targets for treatment of the skeletal muscle disorders.

Human induced pluripotent stem cell-derived cardiomyocytes (iCMs) provide a powerful platform for investigating cardiac biology. However, structural, metabolic, and electrophysiological immaturity of iCMs limits their capacity to model adult cardiomyocytes. Currently, no universally accepted criteria or protocols for effective iCMs maturation exist. This study aimed to identify practical culture conditions that promote iCMs maturation, thereby generating more physiologically relevant in vitro cardiac models. We evaluated the effects of short- and long-term culture in media supplemented with various stimulatory compounds under 2D conditions, focusing on intracellular content and localization of slow skeletal troponin I (ssTnI) and cardiac troponin I (cTnI) isoforms. Our findings demonstrate that the multicomponent metabolic maturation medium (MM-1) effectively enhances the transition toward a more mature iCM phenotype, as evidenced by increased cTnI expression and formation of cross-striated myofibrils. iCMs cultured in MM-1 more closely resemble adult cardiomyocytes and are compatible with high-resolution single-cell techniques such as electron microscopy and patch-clamp electrophysiology. This work provides a practical and scalable approach for advancing the maturation of iPSC-derived cardiac models, with applications in disease modeling and drug screening.

Troponin I (TnI), together with troponin T (TnT) and troponin C (TnC), forms the troponin complex, a thin filament protein of the striated muscle that plays a key role in regulation of muscle contraction. In humans, TnI is represented by three isoforms: cardiac, which is synthesized only in myocardium, and fast and slow skeletal, which are synthesized in fast- and slow-twitch muscle fibers, respectively. Skeletal TnI isoforms could be used as biomarkers of skeletal muscle damage of various etiologies, including mechanical trauma, myopathies, muscle atrophy (sarcopenia), and rhabdomyolysis. Unlike classical markers of muscle damage, such as creatine kinase or myoglobin, which are also present in other tissues, skeletal TnIs are specific for skeletal muscle. In this study, we developed a panel of monoclonal antibodies for immunochemical detection of skeletal TnI isoforms using Western blotting (sensitivity: 0.01-1 ng per lane), immunohistochemical assays, and fluorescence immunoassays. Some of the designed fluorescence immunoassays enable quantification of fast skeletal (limit of detection [LOD] = 0.07 ng/mL) and slow skeletal (LOD = 0.1 ng/mL) TnI isoforms or both isoforms (LOD = 0.1 ng/ml). Others allow differential detection of binary (with TnC) or ternary (with TnT and TnC) complexes, revealing composition of troponin forms in the human blood.