Characterization of the function of type XIII collagen in mice; specific roles during cardiovascular development and posnatally in bone modeling

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Type XIII collagen is a type II transmembrane protein which is expressed in many tissues throughout development and adult life. It is located in focal adhesions of cultured cells and in the adhesive structures of tissues such as the myotendinous junctions in muscle, intercalated discs in the heart and the cell-basement membrane interphases. To further characterize the function of this protein, we generated transgenic mice overexpressing it in normal and mutant forms. </p><p>A large in-frame deletion in the COL2 domain of type XIII collagen led to synthesis of truncated α1(XIII) chains in transgenic mice, disrupting the assembly of normal type XIII collagen trimers. Fibroblasts derived from the mutant mice expressed shortened α1(XIII) chains, and no intracellular accumulation of the mutant protein was detected, suggesting that the mutant molecules were expressed on the cell surface. Transgene expression led to an embryonally lethal phenotype in offspring from heterozygous mating at two distinct stages of development. The early phenotype fetuses died due to the lack of chorioallantoic fusion and functioning placenta at 10.5 dpc, while the death of the late phenotype fetuses was caused by cardiac and placental defects around 13.5 dpc. The phenotype resembles closely several other cell adhesion molecule mutants, indicating that type XIII collagen has an essential role in certain adhesive interactions that are necessary for normal development.</p><p>Mice overexpressing type XIII collagen with or without a point mutation developed postnatally an unexpected skeletal phenotype marked by a massive increase in bone mass. The cortical bone cross-sectional area and volumetric bone mineral density were highly increased, but trabecular bone volume was not significantly altered. The bone formation rate was several times higher in the mutant mice than in their normal littermates, while the osteoclast number and resorption activity were normal. Type XIII collagen was expressed highly in primary osteoblasts derived from the transgenic mice. Overexpression of type XIII collagen in osteoblasts enhanced both cell proliferation and differentiation while lack of it had opposite effects. Furthermore, mutant cells responded to mechanical strain differently than wild-type cells. The findings suggest that type XIII collagen has an important role in bone modeling, and it may in particular have a function in coupling the regulation of bone mass to mechanical usage.</p></div>