Osteoarthritis: a cellular differentiation defect?:

Mutant WISP3 triggers the phenotype shift of articular chondrocytes by promoting sensitivity to IGF-1 hypothesis of spondyloepiphyseal dysplasia tarda with progressive arthropathy (SEDT-PA)

This article introduces the hypothesis that mutant WISP3 (Wnt1 inducible secreted protein-3) triggers the phenotype shift of the chondrocytes, especially in the articular chondrocytes, by promoting sensitivity to IGF-1 (insulin-like growth factor 1), and results in chondrocytes apoptosis in SEDT-PA. SEDT-PA is also referred to as progressive pseudorheumatoid dysplasia (PPD), arthropathy progressive pseudorheumatoid of childhood (APPRC). Evidence for the hypothesis is based on the following indications: (1) SEDT-PA is caused by mutations of the WISP3 gene. WISP3 encodes a domain that bears homology to the amino-terminal domain of the insulin-like growth factor binding proteins (IGFBPs). (2) IGF-1 enhances chondrocyte hypertrophy by insulin-like actions. WISP3 can up-regulate the expression of type II collagen. When chondrocytes become hypertrophic, they reduce the expression of types II and IX collagen. (3) The chondrocytes in the normal articular cartilage maintain a stable phenotype. These cells exhibit no mitotic activity, low matrix synthesis and low degradation. But articular chondrocytes could react to certain stimuli such as IGF-1. (4) The loss of WISP3 expression alters the phenotype of the breast epithelium and promotes motility and invasion. The WISP3-deficient cells are extremely sensitive to the growth stimulatory effects of IGF-1. (5) The action of IGF-I is inhibited by IGFBPs, both in articular chondrocytes and in the normal breast epithelium. In conclusion, the mutant WISP3 lose is the function of inhibiting IGF-1 and disturbs the maintenance of a stable phenotype in articular chondrocytes. So, the articular chondrocytes undergo hypertrophic and terminal differentiation apoptosis. The precise mechanism of WISP3 function during postnatal cartilage growth and homeostasis is not clear yet. This hypothesis provides a new clue on the present mechanism study on SEDT-PA. If verified, this new concept may lead to a novel pathogenesis of SEDT-PA.