Collagen XII: Protecting bone and muscle integrity by organizing collagen fibrils

Abstract

Collagen XII, largest member of the fibril-associated collagens with interrupted triple helix (FACIT) family, assembles from three identical α-chains encoded by the COL12A1 gene. The molecule consists of three threadlike N-terminal noncollagenous NC3 domains, joined by disulfide bonds and a short interrupted collagen triple helix toward the C-terminus. Splice variants differ considerably in size and properties: "small" collagen XIIB (220 kDa subunit) is similar to collagen XIV, whereas collagen XIIA (350 kDa) has a much larger NC3 domain carrying glycosaminoglycan chains. Collagen XII binds to collagen I-containing fibrils via its collagenous domain, whereas its large noncollagenous arms interact with other matrix proteins such as tenascin-X. In dense connective tissues and bone, collagen XII is thought to regulate organization and mechanical properties of collagen fibril bundles. Accordingly, recent findings show that collagen XII mutations cause Ehlers-Danlos/myopathy overlap syndrome associated with skeletal abnormalities and muscle weakness in mice and humans.

Keywords: Bethlem myopathy; Collagen XII; Collagen fiber; Ehlers–Danlos syndrome; Osteogenesis.

Fig. 1. Structure of collagen XII

(A) Domain structure of "large" collagen XIIA and "small" collagen XIIB splice variants. Collagen XIIB misses the entire N-terminal half of the NC3 domain. The alternatively spliced region (8 FN3 and 2 VWA modules) contains sites for heparin-binding and glycosaminoclycan (GAG) attachment. A second heparin-binding site is present in the collagenous domain. (B) A collagen XII molecule with three XIIA subunits as seen in the electron microscope after negative staining. Note three flexible arms ≈90 nm in length corresponding to the NC3 domains, along which globular VWA modules are visible. A thin, ≈70 nm long stiff rod with a kink (arrow), the collagen triple helix, emanates from the central globe connecting NC3 domains. (C) Homo- and heterotrimeric collagen XIIA/XIIB molecules, isolated by immuno-affinity chromatography, as seen after rotary shadowing. XIIA and XIIB subunits of individual molecules were identified from the length of their NC3 domains (≈90 and ≈50 nm, respectively; Watt et al. [1992], Koch et al. [1994]). Images (from top left to bottom right) show collagen XIIA₃, A₂B, AB₂, and B₃, respectively. NC3 domains are labeled with A or B; collagen domains with arrows. Bar, 50 nm. (A, C) Modified and reproduced from Koch et al. (1994) and (B) from Koch et al. (1992).

Fig. 2. Function of collagen XII

(A) Cross-bridging of collagen I-containing fibrils by collagen XII and tenascin-X. Collagen XII interacts with the fibril surface via its collagenous domain; this might be facilitated by a ternary complex with small proteoglycan decorin. NC3 domains of collagen XII extend from the fibril surface and bind tenascin-X, a trimer of subunits with EGF repeats (blue), FN3 domains (purple), and a C-terminal fibrinogen-like globe (pink). Like collagen XII, tenascin-X binds decorin that links it to fibrils. (B) Ultrastructural colocalization of collagen XII and tenascin-X in embryonic dermis. Ultrathin sections were incubated with polyclonal antibodies to collagen XII (left) or tenascin-X (right) followed by colloidal gold-labeled secondary antibody. Note clusters of gold particles associated with cross-striated collagen fibrils in both images. Reproduced from Veit et al. (2006). (C) Disorganized collagen fiber arrangement in bone matrix of collagen XII-deficient mice. Collagen organization was analyzed in femurs of P14 Col12a1^+/+ and Col12a1^−/− mice under polarized light after Sirius red staining. PO, periosteum; CB, cortical bone, BM, bone marrow. (D) X-ray (left) and alizarin red staining (right) of adult Col12a1^+/+ and Col12a1^−/− mice. Note kyphosis of the spine (asterisk) and abnormal spinous processes of lumbar vertebrae (marked on vertebra L3 with arrows) in the Col12a1^−/− mouse. (A, B) Modified and reproduced from Veit et al. (2006). (C, D) Reproduced from Izu et al. (2011). Bars: 50 nm (B); 50 µm (C); 2 cm (D, left); 1 mm (D, right).