Duchenne Muscular Dystrophy (DMD) is a common inherited muscle disease showing chronic inflammation and life-limiting progressive weakness of skeletal and cardiac muscle. Absent dystrophin renders sarcolemma more Ca²⁺ permeable, disturbs signaling and triggers inflammation. Sustained degeneration/regeneration cycles render muscle cyto-architecture susceptible to remodeling. The nuclei of healthy muscles are exclusively peripherally located, but dystrophic muscle fibres often show centrally located nuclei as a sign of fibre regeneration. Moreover, the regenerative capacity of dystrophic muscle exhausts with age. We used Second Harmonic Generation (SHG) and multiphoton fluorescence microscopy in living toe muscle fibres from mdx, wt and transgenic mdx mice expressing a exδ17-48 mini-dystrophin (MinD) for 3D rendering and analysis of nucleus volume (a measure for metabolic activity) and myosin volume (a measure for apparent motor activity). We studied the time course of the nucleus-to-myosin volume ratio (NMV) in single muscle fibres to test the hypothesis that during aging, exhaustion of regenerative capacity (indicated by nuclear volume) was less severe as compared to myosin volume reduction. During maturation, NMV declined in wt and MinD fibres and stabilized at around 3 mo, but NMV in mdx fibres increased with age. Based on NMV, we define a ’biomotoric efficiency’ (BE) that aims to provide a rough measure for the nuclear control over motorprotein architecture (BE = 1/(1+NMV)). BE is close to unity in adult wt and MinD fibres, but smaller in very young and old mdx mice as a result of cell differentiation and cell regeneration. Therefore, BE seems to be a reliable measure for fibre maturation. The measurements of NMV and BE are a fast method for the analysis of regenerative processes in muscles using SHG microscopy. The results are part of morphometric analysis to describe the cellular remodeling during the dystrophic process, which is associated by an increase of ubiquitin level in cytoplasm of dystrophic muscle fibres.

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