Role of collagen XII in skin homeostasis and repair.

Skin integrity and function depends to a large extent on the composition of the extracellular matrix, which regulates tissue organization. Collagen XII is a homotrimer with short collagenous domains that confer binding to the surface of collagen I-containing fibrils and extended flexible arms, which bind to non-collagenous matrix components. Thereby, collagen XII helps to maintain collagen suprastructure and to absorb stress. Mutant or absent collagen XII leads to reduced muscle and bone strength and lax skin, whereas increased collagen XII amounts are observed in tumor stroma, scarring and fibrosis. This study aimed at uncovering in vivo mechanisms by which collagen XII may achieve these contrasting outcomes. We analyzed skin as a model tissue that contains abundant fibrils, composed of collagen I, III and V with collagen XII decorating their surface, and which is subject to mechanical stress. The impact of different collagen XII levels was investigated in collagen XII-deficient (Col12-KO) mice and in mice with collagen XII overexpression in the dermis (Col12-OE). Unchallenged skin of these mice was histologically inconspicuous, but at the ultrastructural level revealed distinct aberrations in collagen network suprastructure. Repair of excisional wounds deviated from controls in both models by delayed healing kinetics, which was, however, caused by completely different mechanisms in the two mouse lines. The disorganized matrix in Col12-KO wounds failed to properly sequester TGFβ, resulting in elevated numbers of myofibroblasts. These are, however, unable to contract and remodel the collagen XII-deficient matrix. Excess of collagen XII, in contrast, promotes persistence of M1-like macrophages in the wound bed, thereby stalling the wounds in an early inflammatory stage of the repair process and delaying healing. Taken together, we demonstrate that collagen XII is a key component that assists in orchestrating proper skin matrix structure, controls growth factor availability and regulates cellular composition and function. Together, these functions are pivotal for re-establishing homeostasis after injury.