Collagen content does not alter the passive mechanical properties of fibrotic skeletal muscle in mdx mice

Abstract

Many skeletal muscle diseases are associated with progressive fibrosis leading to impaired muscle function. Collagen within the extracellular matrix is the primary structural protein providing a mechanical scaffold for cells within tissues. During fibrosis collagen not only increases in amount but also undergoes posttranslational changes that alter its organization that is thought to contribute to tissue stiffness. Little, however, is known about collagen organization in fibrotic muscle and its consequences for function. To investigate the relationship between collagen content and organization with muscle mechanical properties, we studied mdx mice, a model for Duchenne muscular dystrophy (DMD) that undergoes skeletal muscle fibrosis, and age-matched control mice. We determined collagen content both histologically, with picosirius red staining, and biochemically, with hydroxyproline quantification. Collagen content increased in the mdx soleus and diaphragm muscles, which was exacerbated by age in the diaphragm. Collagen packing density, a parameter of collagen organization, was determined using circularly polarized light microscopy of picosirius red-stained sections. Extensor digitorum longus (EDL) and soleus muscle had proportionally less dense collagen in mdx muscle, while the diaphragm did not change packing density. The mdx muscles had compromised strength as expected, yet only the EDL had a significantly increased elastic stiffness. The EDL and diaphragm had increased dynamic stiffness and a change in relative viscosity. Unexpectedly, passive stiffness did not correlate with collagen content and only weakly correlated with collagen organization. We conclude that muscle fibrosis does not lead to increased passive stiffness and that collagen content is not predictive of muscle stiffness.

Low Intensity Training Provokes Adaptations on Muscle Fibrosis of a Muscular Dystrophy Model

“…Fibrosis is a final result of a chronic inflammatory process on dystrophic muscle due to the absence of dystrophin (Smith & Barton;Fontana et al, 2015). We have shown that this modality and intensity of training affected the intramuscular fibrosis in a beneficial way but further investigations are required to explain if LIT has delayed the formation of an intramuscular fibrosis or if it was reorganized by the contraction-induced adaptations on skeletal muscle tissue.…”

Resistance to radial expansion limits muscle strain and work

“…The use of a mathematical model with realistic healthy and pathological connective tissue properties enables us to determine that the increased resistance to radial expansion reduces a muscle capacity to generate work independent of any other changes. Remodeling of the ECM with age (Ramaswamy et al 2011) and disease (Smith and Barton 2014) is often associated with an increase in collagen content (as a proportion of the cross section) and in turn a large decreases in specific force. Therefore, muscles with increased collagen content likely face both a reduction in specific force and shortening capacity, such that the mechanical work output is potentially more compromised than our results indicate (Fig.…”

“…This approach allowed us to input into the model empirical data from studies that have quantified the relative proportion of a muscle cross section that is composed of collagen/ECM versus contractile muscle cells with values ranging from 2 to 20% (Lieber and Ward 2013; Smith and Barton 2014; Wood et al 2014). In addition, our simulations allowed us to vary the initial orientation of collagen fibers to explore the effect of collagen pitch angle on collagen strain.…”

“…9). We also used published values of the Young’s modulus of elasticity (Svensson et al 2010) for collagen fibers and the cross-sectional area of the ECM (Lieber and Ward 2013; Smith and Barton 2014) to calculate the force required to stretch the collagen (Eq. 8).…”

Endogenous Optical Signals Reveal Changes of Elastin and Collagen Organization During Differentiation of Mouse Embryonic Stem Cells

“…Furthermore, while proven histology approaches for both collagen 83,84 and Eln 85,86 are more inclusive of fiber types, they require fixation and potentially physical sectioning before staining, obviating the application of these methods to live samples and limiting the 3D assessment of the microenvironments. Although SHG and MPLSM, in their current form, have limitations, overcoming these deficiencies is the focus of ongoing work by several groups.…”