TABLE 1.

Overview of collagenous components of the skeletal muscle extracellular matrix.

Gene(s)	Skeletal muscle RNA expression (protein expression myocytes)*	Protein	Form	Appearance	Role	Source
COL1A1-2	1: 11.0 NX (not detected) 2: 12.1 NX (low)	Collagen type I alpha 1 and 2 chains	Fibrils	Endo-, peri-, and epimysium	Forms strong parallel fibers, confers tensile strength and rigidity	Kovanen (2002)
COL3A1	14.5 NX (not detected)	Collagen type III alpha 1 chain	Fibrils	Endo- and perimysium, myotendinous junction	Forms a loose meshwork of fibers, confers compliance	Kovanen (2002)
COL4A1-6	1: 24.6 NX (not detected) 2: 26.3 NX (high) 3: 17.8 NX (not detected) 4: 7.5 NX (no data) 5: 4.8 NX (no data) 6: 1.0 NX (no data)	Collagen type IV alpha 1-6 chains	Helices	Basal lamina	Produces a network structure, constitutes the basis of the basal lamina	Sanes (2003)
COL5A1-3	1: 5.0 NX (not detected) 2: 3.7 NX (no data) 3: 15.4 NX (medium)	Collagen type V alpha 1-3 chains	Fibrils	Endomysium	Control of collagen fibrillogenesis	Kovanen (2002)
COL6A1-6	1: 20.0 NX (not detected) 2: 30.6 NX (not detected) 3: 31.8 NX (not detected) 4: no data 5: 0.2 NX (low) 6: 0.0 NX (medium)	Collagen type VI alpha 1-6 chains	Beaded filaments	Endo-, peri-, and epimysium (α6-chain) Basal lamina (α3-chain) Myotendinous junction (α5-chain)	Interacts with a large number of molecules and cell surface receptors, maintains muscle functional integrity. Mutations associated with fibrosis and Ullrich, Bethlem or Myosclerosis myopathies	Bönnemann (2011), Sabatelli et al. (2012), Cescon et al. (2015)
COL12A1	21.6 NX (medium)	Collagen type XII alpha 1 chain	FACIT	Endo- and perimysium, myotendinous junction	Linkage protein between fibrillar collagens and other ECM components	Jakobsen et al. (2017)
COL13A1	2.7 NX (not detected)	Collagen type XIII alpha 1 chain	MACIT	Neuromuscular junction	Regulation and formation of neuromuscular synapse. Lack associated with myasthenia	Härönen et al. (2017), Heikkinen et al. (2019)
COL14A1	7.6 NX (not detected)	Collagen type XIV alpha 1 chain	FACIT	Endo- and perimysium, myotendinous junction	Linkage protein between fibrillar collagens and other ECM components. Increases following training at myotendinous junction (protection against strain injury?)	Jakobsen et al. (2017)
COL15A1	12.8 NX (low to medium)	Collagen type XV alpha 1 chain	Multiplexin	Basement membrane	Stabilizes muscle cells and microvessels. Guides motor axons toward muscle targets. Deficiency increases vulnerability to exercise-induced muscle injury and leads to mild forms of myopathies	Eklund et al. (2001), Guillon et al. (2016)
COL18A1	8.1 NX (not detected)	Collagen type XVIII alpha 1 chain	Multiplexin	Basement membrane	May bind growth factors. May link the basement membrane to other basement membrane glycoproteins and endomysium	Gillies and Lieber (2011), Heljasvaara et al. (2017)
COL19A1	3.7 NX (low)	Collagen type XIX alpha 1 chain	FACIT	Basement membrane	Presence at early embryonic stages is relevant for the muscle tissue differentiation. Acts as a cross-bridge between fibrils and other extracellular matrix molecules	Khaleduzzaman et al. (1997), Sumiyoshi et al. (2001)
COL22A1	0.5 NX (not detected)	Collagen type XXII alpha 1 chain	FACIT	Myotendinous junction	Integrates ECM and contributes to mechanical stability of the myotendinous junction. Knockdown of COL22A1 results in dystrophy-like muscle phenotype in zebrafish	Koch et al. (2004), Charvet et al. (2013)

*RNA expression summary shows the consensus RNA-data based on normalized expression (NX) data and protein expression comprises profiles using single as well as independent antibodies directed against different, non-overlapping epitopes on the same protein from the Human Protein Atlas (http://www.proteinatlas.org, Uhlen et al., 2010). FACIT, Fibril Associated Collagen with Interrupted Triple Helices; MACIT: Membrane Associated Collagen with Interrupted Triple Helices. Multiplexin: Collagen with Multiple Triple Helix Interruptions.