Collagenous transmembrane proteins: recent insights into biology and pathology.

Is collagen just the workhorse of the extracellular matrix with few roles other than merely giving the body structural platforms? Hardly so. The collagen family has now swelled to at least 27 distinct members and thus is full of promise for functional diversity (1). Granted, some collagens belong to the best known and most abundant structural proteins. However, collagens also control sophisticated organ or tissue functions by means which are unexpected, fascinating, and often bewildering. Like other matrix macromolecules, collagens are functional only after aggregating into tissue suprastructures. They form complex alloys or composites containing not only several collagen types but also other kinds of molecules. Even tiny fractions of “minor” collagens astoundingly control suprastructural assembly and thus cell-matrix interactions dictating cellular activities such as growth, survival, differentiation, gene expression, and metabolism. This minireview will focus on a growing subgroup of collagens, the collagenous transmembrane proteins, which have dual functions as cell surface receptors or as matrix components. If their extracellular domains are cast off by limited proteolysis, they can begin to signal to cells as soluble molecules. Collagenous transmembrane proteins are widely expressed and are involved in cell adhesion, epithelial-mesenchymal interactions during morphogenesis, neuromuscular signaling, and host defense against microbial agents. Correspondingly, they are associated with genetic and acquired human diseases, e.g. epidermolysis bullosa, ectodermal dysplasias, bullous pemphigoid, or Alzheimer disease, and mouse models implicate them in the etiology of further pathological conditions.