Rotator cuff tear reduces muscle fiber specific force production and induces macrophage accumulation and autophagy

Abstract

Summary Full-thickness tears to the rotator cuff can cause severe pain and disability. Untreated tears progress in size and are associated with muscle atrophy and an infiltration of fat to the area, a condition known as “fatty degeneration.” To improve the treatment of rotator cuff tears, a greater understanding of the changes in the contractile properties of muscle fibers and the molecular regulation of fatty degeneration is essential. Using a rat model of rotator cuff injury, we measured the force generating capacity of individual muscle fibers and determined changes in muscle fiber type distribution that develop after a full thickness rotator cuff tear. We also measured the expression of mRNA and miRNA transcripts involved in muscle atrophy, lipid accumulation, and matrix synthesis. We hypothesized that a decrease in specific force of rotator cuff muscle fibers, an accumulation of type IIb fibers, an upregulation in fibrogenic, adipogenic, and inflammatory gene expression occur in torn rotator cuff muscles. Thirty days following rotator cuff tear, we observed a reduction in muscle fiber force production, an induction of fibrogenic, adipogenic and autophagocytic mRNA and miRNA molecules, and a dramatic accumulation of macrophages in areas of fat accumulation.

Histological Evidence of Muscle Degeneration in Advanced Human Rotator Cuff Disease

“…Mendias et al corroborated these findings, and further showed diminished force production in muscle fibers isolated from chronic, full-thickness tears 16 . Animal models of rotator cuff disease, particularly large animal models, are relatively effective in recapitulating this atrophic, fat-accumulating phenotype of early and midstage disease [29][30][31][32][33][34][35] . However, to our knowledge, no previous study involving humans or animals has addressed the existence of degeneration or the tissue-level pattern of fat accumulation within the muscle in the most chronic and advanced stage of rotator cuff disease.…”

Measurement of Maximum Isometric Force Generated by Permeabilized Skeletal Muscle Fibers

“…Examples include studies that have evaluated the effects of aging 12 , exercise 10,13,14 , spaceflight 15 , injury 2,3,16 , drug treatments 17,18 , disease 19 and genetic manipulation 20,21 on fiber structure and function. Due to the ability to directly assess the contractile performance of myofibrils in their native configuration, this technique provides an attractive platform from which to form an understanding of myofibrillar function absent of potentially confounding effects that are present when neuromuscular signal transmission and excitation-induced calcium release are included in the system under study.…”

“…Furthermore, alternative means for exposing the fiber to the various experimental solutions could be employed, including devising a manually-operated system of chambers or a single chamber that allows for rapid filling and emptying of solutions. Finally, while sub-physiological experimental temperatures such as 15 °C are commonly used to improve the reproducibility of mechanical measurements 1,2,3,5,8,12,17,27 . Other approaches to solution composition have been employed with notable success by other research groups and typically make use of published binding constants and computational tools 27,30,31 .…”

“…Measurements of force using permeabilized skeletal muscle fibers are thus valuable when assessing alterations in muscle function observed in vivo. For example, we have used this technique to characterize the force generating capacity of fibers from myostatin deficient mice 1 and to assess the cause of persistent muscle weakness exhibited following chronic rotator cuff tears 2,3 .…”

Muscle Fibers are Injured at the Time of Acute and Chronic Rotator Cuff Repair

“…To determine whether the plasma membrane of muscle fibers from chronically torn rotator cuff muscles is damaged during repair, we used a well-established rat model of full-thickness rotator cuff tears [12,18,24]. Evans Blue Dye (EBD) is a water-soluble, membrane-impermeable dye that accumulates in the cytosol of muscle fibers that have sustained an injury to their plasma membrane [14].…”