**4. Mature fibres, skeletal muscle plasticity and the extracellular matrix**

Both, proteomic cataloguing studies of various skeletal muscle specimens [77–82] and the comparative expression profiling of crude skeletal muscle preparations or subcellular fractions [83–86] routinely identify ECM proteins that form the core complexes of the basal lamina, microfibrils and interstitial matrix [87]. **Figure 4** shows a bioinformatics STRING [88] map of core ECM components from skeletal muscles. In mature skeletal muscles, frequently identified ECM molecules include collagens of the interstitial matrix (COL I, COL III and COL V), the microfibrillar collagen isoform COL VI, the non-fibrillar collagen isoform COL IV of the basement membrane, components of cell–ECM adhesion complexes (laminin, fibronectin, integrins, dystrophin, utrophin), regulatory ECM proteins (dermatopontin, nidogen, periostin and osteopontin), SLRP-type proteoglycans (asporin, biglycan, decorin, mimecan, fibromo‐ dulin and lumican), heparan sulfate proteoglycans (syndecan and perlecan), the chondroitin sulphate proteoglycan aggrecan, the PRELP-type proteoglycan prolargin and the neuromus‐ cular junction-specific proteoglycan agrin, as well as matrix metalloproteinases and their inhibitors (MMP-1, MMP-2, MMP-9, MMP-10, MMP-13 and TIMPs) [77–86]. **Figure 4** includes the protein products of the following genes: *BGN, SDC1, HSPG2, AGRN, FN1, DAG1, ACAN, DPT, POSTN, MMP1, MMP2, MMP9, MMP10, MMP13, TIMP1, COL1A1, COL1A2, COL6A1, COL6A2, COL3A1, COL5A1, COL4A4, LAMA2, LAMB1, LAMB2, LAMC1, NID1, ASPN, PRELP, PGS2, LUM, OGN, FMOD, ITGA7, ITGB1.* This encompasses essential members of the various collagen networks found in the basement membrane, the microfibrillar structures and the interstitial matrix. In addition, major proteoglycans, matricellular proteins and matrix metalloproteinases are shown, as well as the interaction sites between sarcolemmal adhesion receptor complexes and the ECM.

Individual skeletal muscles are characterized by their fibre type distribution pattern whereby the proportion of fast-twitching fibres, slow-twitching fibres and hybrid fibres is highly adaptable and changes according to specific physiological, biochemical and/or metabolic demands [89]. Alterations in physical activity affect the molecular and cellular composition of the neuromuscular system, including hypertrophy, i.e. the increase in fibre size and hyper‐ plasia, i.e. the increase in fibre number [90]. The plasticity of the neuromuscular system is a well-established physiological concept. Endurance training is associated with an increased aerobic capacity and elevated utilization of fatty acid oxidation [91]. In contrast, sprint training triggers higher activities of the glycolytic and phosphocreatine pathways and enhances carbohydrate metabolism [92]. Prior to systematic proteomic studies, a large number of biochemical, cell biological and physiological studies have established ECM changes in response to exercise [93–95], including collagens, adhesion receptors, growth factors, matri‐ cellular proteins and matrix metalloproteinases [96–100]. Neuromuscular unloading clearly depresses collagen COL I and COL III production and reloading enhances collagen expression in fast muscles [101]. The mass spectrometric analysis of exercise indicates proteome-wide changes in the graded response of skeletal muscles to physical exercise using different training regimes [102,103]. While moderate-intensity exercise causes a shift to a more fatigue-resistant and a slower-contracting skeletal muscle phenotype, interval-exercise training is associated with changes in post-translational modifications of metabolic enzymes [104–106]. The proteo‐ mic analysis of skeletal muscle plasticity in relation to acute versus chronic exercise was recently determined using human *vastus lateralis* muscle biopsy specimens and label-free LC-MS/MS analysis [107]. While structural and mitochondrial proteins were shown to be increased after long-term exercise, components related to energy metabolism were decreased following short-term exercise. Moderate ECM changes were described for several α-chains of collagen VI, fibronectin and decorin [107].

changes in ECM proteins have recently been studied in relation to human myogenesis and established that altered protein expression underlies the dramatic phenotypic conversion of primary mono-nucleated muscle cells during differentiation to form multi-nucleated myo‐ tubes [76]. The temporal profiling of the human myoblast proteome during *in vitro* differen‐ tiation highlighted the importance of ECM rearrangement during early myogenesis and showed a drastic increase in key ECM components, including several α-isoforms of collagen COL VI and COL XVIII, as well as the heparan sulfate proteoglycan HSPG2 of the basement membrane, the elastin–microfibril interface–located ECM glycoprotein EMILIN2 and nidogen isoform NID2 [76]. These findings confirm the developmental concept that enhanced synthesis of ECM proteins occurs during the transition from myoblasts to syncytial myotubes [72] and that complex interactions at the cell–ECM interface facilitate the fusion of myoblasts [35].

76 Composition and Function of the Extracellular Matrix in the Human Body

**4. Mature fibres, skeletal muscle plasticity and the extracellular matrix**

receptor complexes and the ECM.

Both, proteomic cataloguing studies of various skeletal muscle specimens [77–82] and the comparative expression profiling of crude skeletal muscle preparations or subcellular fractions [83–86] routinely identify ECM proteins that form the core complexes of the basal lamina, microfibrils and interstitial matrix [87]. **Figure 4** shows a bioinformatics STRING [88] map of core ECM components from skeletal muscles. In mature skeletal muscles, frequently identified ECM molecules include collagens of the interstitial matrix (COL I, COL III and COL V), the microfibrillar collagen isoform COL VI, the non-fibrillar collagen isoform COL IV of the basement membrane, components of cell–ECM adhesion complexes (laminin, fibronectin, integrins, dystrophin, utrophin), regulatory ECM proteins (dermatopontin, nidogen, periostin and osteopontin), SLRP-type proteoglycans (asporin, biglycan, decorin, mimecan, fibromo‐ dulin and lumican), heparan sulfate proteoglycans (syndecan and perlecan), the chondroitin sulphate proteoglycan aggrecan, the PRELP-type proteoglycan prolargin and the neuromus‐ cular junction-specific proteoglycan agrin, as well as matrix metalloproteinases and their inhibitors (MMP-1, MMP-2, MMP-9, MMP-10, MMP-13 and TIMPs) [77–86]. **Figure 4** includes the protein products of the following genes: *BGN, SDC1, HSPG2, AGRN, FN1, DAG1, ACAN, DPT, POSTN, MMP1, MMP2, MMP9, MMP10, MMP13, TIMP1, COL1A1, COL1A2, COL6A1, COL6A2, COL3A1, COL5A1, COL4A4, LAMA2, LAMB1, LAMB2, LAMC1, NID1, ASPN, PRELP, PGS2, LUM, OGN, FMOD, ITGA7, ITGB1.* This encompasses essential members of the various collagen networks found in the basement membrane, the microfibrillar structures and the interstitial matrix. In addition, major proteoglycans, matricellular proteins and matrix metalloproteinases are shown, as well as the interaction sites between sarcolemmal adhesion

Individual skeletal muscles are characterized by their fibre type distribution pattern whereby the proportion of fast-twitching fibres, slow-twitching fibres and hybrid fibres is highly adaptable and changes according to specific physiological, biochemical and/or metabolic demands [89]. Alterations in physical activity affect the molecular and cellular composition of the neuromuscular system, including hypertrophy, i.e. the increase in fibre size and hyper‐ plasia, i.e. the increase in fibre number [90]. The plasticity of the neuromuscular system is a

**Figure 4.** Bioinformatics STRING map of major components that form the core of the extracellular matrix (ECM) com‐ plexome. Shown are key proteins belonging to the collagen network of the basal lamina, microfibrils and the intersti‐ tial matrix, proteoglycans, matricellular proteins and matrix metalloproteinases. The interaction sites of the ECM with sarcolemmal adhesion receptor complexes are shown.

An interesting non-physiological system is presented by external chronic low-frequency stimulation of fast muscles. This electro-stimulation method causes the complete activation of all affected motor units to a maximum extent [108]. During fast-to-slow transitions, skeletal muscles show a remarkable adaptation and transform physiologically and biochemically into motor units with an improved resistance to fatigue [109]. Chronic low-frequency stimulated fast muscles are characterized by decreased fibre calibres, an increase in the time-to-peak twitch tension, an increase in half-relaxation time and a significant elevation of aerobicoxidative capacity [110]. The proteomic analysis of continuous electro-stimulation at 10 Hz has demonstrated complex biochemical changes with a significant shift from glycolytic to more aerobic-oxidative metabolism [111,112]. The ECM of transforming skeletal muscle undergoes distinct changes and exhibits increased collagen levels [113,114]. In the stimulated *latissimus dorsi* model for testing the suitability of dynamic cardiomyoplasty to treat heart failure, the collagen content was shown to be significantly elevated in the paced muscle. Although the chronically electro-stimulated muscle increased the level of fatigue resistance, distal regions of the paced *latissimus dorsi* muscle were characterized by muscular atrophy and myofibrosis [114].
