A Novel Method to Quantify Diaphragm Muscle Fiber Type Clustering in the Context of Sarcopenia

Abstract

Sarcopenia is the age‐related decline of skeletal muscle function. Sarcopenia involves loss of motor neurons and the subsequent denervation of muscle fibers. Denervated muscle fibers can be re‐innervated through terminal axonal sprouting of neighboring axons. Given the predominant role of motor neurons in establishing muscle fiber type, we hypothesized that sarcopenia (and the associated partial denervatin effects) result in clustering of muscle fibers of the same type (classified by myosin heavy chain isoform composition). Fiber type specific immunofluorescence histology was used to quantify fiber clustering in murine diaphragm muscle (n=15) at 6 and 24 months of age. Clustering was analyzed using the distance between fibers of the same fiber type as well as quantification of the number of type‐identified fibers in a cluster. There was a 14% decrease to the closest fiber for type I and a 24% increase for type IIx and/or IIb fibers (P<0.001) in old vs. young mice. Additionally, a 34% increase to the 3 closest type IIx and/or IIb fibers was found (P<0.001). Changes in the interfiber distances for type‐identified fibers reveals important effects that consider changes in fiber size and proportion across disease conditions. This novel method of analyzing fiber type clustering may be useful in examining pathophysiological conditions of motor unit loss in neuromuscular disorders, myopathies, dystrophies, injuries, or amyotrophic lateral sclerosis.Supported by R01‐AG044615, T32‐HL105355, and the Mayo Clinic.