Nuclei isolation methods fail to accurately assess the subcellular localization and behaviour of proteins in skeletal muscle

<p dir="ltr">Aim: Subcellular fractionation is often used to determine the subcellular localization of proteins, including whether a protein translocates to the nucleus in response to a given stimulus. Examining nuclear proteins in skeletal muscle is difficult because myonuclear proteins are challenging to isolate unless harsh treatments are used. This study aimed to determine the most effective method for isolating and preserving proteins in their native state in skeletal muscle.</p><p dir="ltr">Methods: We compared the ability of detergents, commercially available kit-based and K⁺-based physiological methodologies for isolating myonuclear proteins from resting samples of human muscle by determining the presence of marker proteins for each fraction by western blot analyses.</p><p dir="ltr">Results: We found that following the initial pelleting of nuclei, treatment with 1% Triton-X 100, 1% CHAPS or 0.5% Na-deoxycholate under various ionic conditions resulted in the nuclear proteins being either resistant to isolation or the proteins present behaving aberrantly. The nuclear proteins in brain tissue were also resistant to 1% Triton-X 100 isolation. Here, we demonstrate aberrant behaviour and erroneous localization of proteins using the kit-based method. The aberrant behaviour was the activation of Ca²⁺-dependent protease calpain-3, and the erroneous localization was the presence of calpain-3 and troponin I in the nuclear fraction.</p><p dir="ltr">Conclusion: Our findings indicate that it may not be possible to reliably determine the translocation of proteins between subcellular locations and the nucleus using subcellular fractionation techniques. This study highlights the importance of validating subcellular fractionation methodologies using several subcellular-specific markers and solutions that are physiologically relevant to the intracellular milieu.</p>