Effect of impaired matrix turnover on differentiation of intervertebral disc cells

Abstract

Intervertebral discs (IVDs) are structures connecting vertebral bodies to form the spine. Each consists of a central gelatinous nucleus pulposus (NP), surrounded peripherally by a tough annulus fibrosus (AF), and flanked on either end by an endplate (EP). They confer flexibility and weight-bearing property to the spine.IVD cells are crucial in maintenance of the disc matrix and its proper function. Changes in IVD cellular differentiation, determined by their surrounding microenvironment, may disrupt this homeostasis, hence destroying disc function. Current studies are focused on the cellular outcome associated with degradative changes in microenvironment, yet little is known of the relation between normal matrix turnover and IVD cells. A mouse with an altered degradation site in type II collagen (Col2a1cr/cr) exhibits a cartilage matrix less susceptible to degradation (Gauci, 2008). Given Col2a1 is highly expressed in the IVD, a similar reduction of its turnover is expected in the discs of this mouse. We thus utilize the mouse to address whether cellular changes would result from this. Metachromatic histological analysis (Leung, 2009) is performed on IVD sections at different stages from neonatal (P10) to maturity (1 year). The mutant IVD is histologically similar to the wild type at newborn, but changes are observed at later stages. While a boney EP is already formed in the wild type at 6 weeks, the mutant maintained a cartilaginous EP with chondrocyte-like cells until 6 months, when signs of ossification appeared (Fig. A). Changes in alcian blue staining pattern indicates altered glycoprotein distribution in the inner AF, and the residing cells are more rounded and chondrocyte-like (Fig. B). These changes demonstrate the decreased Col II turnover could lead to cellular differentiation and subsequent morphology differences. Maintenance of IVD cells is dependent on the matrix homeostasis. Our results suggested that impairment of Col II turnover may disturb this balance and influence the differentiation of IVD cells. The effect becomes pronounced at later stages of development. This may be due to gradual accumulation of mutant Col II, altering the matrix composition and subsequently interferes with the IVD cell fate. Modifications of metabolite flow resulting from the structural change of EP in mutant could also contribute to the observed outcome. Staining for molecular markers such as Sox9 and brachyury will better characterize the cellular changes involved. Our work gives implication on the role of matrix turnover in the maintenance of IVD cells.