The role of fibrosis in Duchenne muscular dystrophy

Abstract

Muscular dystrophies such as Duchenne muscular dystrophy (DMD) are usually approached as dysfunctions of the affected skeletal myofibres and their force transmission. Comparatively little attention has been given to the increase in connective tissue (fibrosis) which accompanies these muscular changes. Interestingly, an increase in endomysial tissue is apparent long before any muscular degeneration can be observed. Fibrosis is the result of a reactive or reparative process involving mechanical, humoral and cellular factors. Originating from vulnerable myofibres, muscle cell necrosis and inflammatory processes are present in DMD. Muscular recovery is limited due to the limited number and capacity of satellite cells. Hence, a proactive and multimodal approach is necessary in order to activate protective mechanisms and to hinder catabolic and tissue degrading pathways. Several avenues are discussed in terms of potential antifibrotic therapy approaches. These include pharmaceutical, nutritional, exercise-based and other mechanostimulatory modalities (such as massage or yoga-like stretching) with the intention of exerting an anti-inflammatory and antifibrotic effect on the affected muscular tissues. A preventive intervention at an early age is crucial, based on the early and seemingly non-reversible nature of the fibrotic tissue changes. Since consistent assessment is essential, different measurement technologies are discussed.

Keywords: Duchenne muscular dystrophy; TGF-β1; antifibrotic therapy; endo- and perimysium; extracellular matrix; fibrosis; myostatin.

Figure 1.

Typical age-related progression of muscle infiltration with loose connective tissue.

Figure 2.

Flowchart: from dystrophin deficiency to fibrosis. Membranes lacking the dystroglycan complex are intrinsically vulnerable to mechanical and oxidative stress. Overstrain and overuse lead to activation of stretch-activated cation channels. Influx of sodium and calcium cannot be compensated for by pumps regulating electrolyte balance. Cellular oedema and calcium overload cause depletion of energy supply. Calcium acts as a second messenger and activates a cascade of inflammatory processes. In terms of a vicious cycle, further stress on muscle cell membranes and activation of ion channels is inevitable. Finally, the cellular integrity is unsustainable. Myofibre necrosis and inflammation lead to fibrotic tissue remodelling.

Figure 3.

Fibroblasts as the major mechanoresponsive cells in connective tissue. Mechanostimulation, fibronectin and/or chemical stimuli like reactive oxygen species (ROS) activate fibroblast differentiation from precursor cells, protein expression and transition to mobile fibrocytes and myofibroblasts which are characterised by contractile filaments (α-smooth muscle actin, α-SMA stress fibres). Mechanical stability is generated by focal adhesions which anchor the fibres to cell membrane. Several growth factors which stimulate synthesis of collagen and concomitant compounds are also expressed in response to mechanical loading. The most important examples are transforming growth factor-ß-1 (TGFß-1), insulin-like growth factor-I (IGF-I), platelet-derived growth factor (PDGF) and connective tissue growth factor (CTGF). These factors also induce proliferation of fibroblasts and myofibroblasts which connect via irregular extensions and also adherent junctions. The contractile properties are substantial for fibrotic tissue shrinkage. These mechanisms finally result in increased production of collagen, elastin, hyaluronan, glycoproteins and proteoglycans. Remodelling of the extracellular matrix (ECM) leads to fibrotic conversion of connective tissue and later fatty involution.

Figure 4.

Intracellular water and sodium accumulation in DMD muscle. Calves of a 5 year old DMD boy (left panels) and a 5 year old healthy volunteer (right): conventional T1-weighted 1H-MR images (upper panels); short tau inversion recovery (STIR). ¹H-MR images (middle panels), and 23Na inversion recovery MR images (Na-IR, lower panels). Neither the DMD boy nor the healthy volunteer showed fatty degeneration of the triceps surae muscles. However, an oedema is visible in the calf muscles of the DMD boy that is pronounced in the soleus muscles. The ²³Na IR showed an elevated signal in the calf muscles of the DMD boy compared to the volunteer. Note that the signal of the reference tube containing free 51.3 mM Na⁺ solution (asterisks) is suppressed in the ²³Na IR sequence, while the contralateral reference tube in which 51.3 mM Na⁺ is trapped in 5% agarose gel (circle) is quite visible. Modified after Weber et al. (22).