Tubular Aggregates as a Marker of Aging in Skeletal Muscle.

Tubular aggregates (TA) are skeletal muscle structures that arise from the progressive accumulation of sarcoplasmic reticulum proteins, mainly with aging. Muscle regeneration plays a role in TA formation. TA quantification may aid in the evaluation of muscle aging and genetic muscle degeneration. TA form over time, appears in aging in normal murine muscles. TA reduction in injured conditions may be due to the degeneration-regeneration process in muscles, with loss of damaged muscle fibers and formation of new fibers that do not present protein aggregation. These new regenerated fibers do not improve the function capacity of the aged muscle. Here, we present a methodology for labeling and identifying tubular aggregates in muscle fibers and also the standardization of its quantification.

Quantifying Muscle Regeneration: Activated Muscle Satellite Cells and New Regenerated Myofibers in Chronic and Acute Degeneration Models.

Regeneration is a remarkable characteristic of the skeletal muscle. Triggered by common lesions, regeneration is stimulated resulting in muscle fiber repair and restoration of muscle homeostasis in normal muscle. In genetic dystrophic muscle, the cycle of degeneration/regeneration is an endless loop that leads to impaired regeneration and substitution of muscle fibers by connective and adipose tissue, causing muscle weakness. Identification and characterization of muscle regeneration steps can help discover potential therapy targets for muscle diseases and aging. Muscle regeneration markers such as the number of satellite cells in the muscle, the proportion of activated satellite cells, and the quantity of regenerating muscle fiber can be quantified using immunolabeling.Here we are presenting a quantitative method to measure muscle regeneration that can be applied to different proposals. To demonstrate the protocol applicability, we used models for acute and chronic muscle injuries. As model of acute degeneration, a wild-type C57BL6 mice with muscle injury induced by electroporation was used, and the muscle was analyzed after 5 and 10 days post-injury. DMDmdx mouse muscle was used as a model of chronic degeneration. The methodologies presented here are among the gold standard methodologies for muscle regeneration analysis and can be easily applied to any type of muscle regeneration study.

Induced degeneration and regeneration in aged muscle reduce tubular aggregates but not muscle function.

Introduction: Tubular aggregates (TA) are skeletal muscle structures that arise from the progressive accumulation of sarcoplasmic reticulum proteins. Cytoplasmic aggregates in muscle fibers have already been observed in mice and humans, mainly during aging and muscle disease processes. However, the effects of muscle regeneration on TA formation have not yet been reported. This study aimed to investigate the relationship between degeneration/regeneration and TA in aged murine models. We investigated the presence and quantity of TA in old males from two murine models with intense muscle degeneration and regeneration. Methods: One murine lineage was a Dmdmdx model of Duchenne muscular dystrophy (n = 6). In the other model, muscle damage was induced by electroporation in C57BL/6J wild-type mice, and analyzed after 5, 15, and 30 days post-electroporation (dpe; n = 15). Regeneration was evaluated based on the quantity of developmental myosin heavy chain (dMyHC)-positive fibers. Results: The frequency of fibers containing TA was higher in aged C57BL/6J (26 ± 8.3%) than in old dystrophic Dmdmdx mice (2.4 ± 2%). Comparing the data from induced degeneration/regeneration in normal mice revealed a reduced proportion of TA-containing fibers after 5 and 30 dpe. Normal aged muscle was able to regenerate and form dMyHC+ fibers, mainly at 5 dpe (0.1 ± 0.1 vs. 16.5 ± 2.6%). However, there was no difference in force or resistance between normal and 30 dpe animals, except for the measurements by the Actimeter device, which showed the worst parameters in the second group. Discussion: Our results suggest that TA also forms in the Dmdmdx muscle but in smaller amounts. The intense degeneration and regeneration of the old dystrophic model resulted in the generation of new muscle fibers with a lower quantity of TA. Data from electroporated wild-type mice support the idea that muscle regeneration leads to a reduction in the amount of TA. We suggest that TA accumulates in muscle fibers throughout physiological aging and that regeneration leads to the formation of new fibers without these structures. In addition, these new fibers do not confer functional benefits to the muscle.