Background: Calcification of cartilage with BCP crystals is a common finding during osteoarthritis (OA) and is directly linked to the severity of the disease. However, in some cases additionally calcium pyrophosphate dihydrate (CPPD) – crystals can be found. Genetic mutations in the ANKH gene, hypermagnesaemia and haemochromatosis have been described to be the reason for the CPPD crystal formation. Since the mechanisms of the formation of the CPPD crystals and their effects on chondrocytes are not completely understood the aim of this study is to evaluate the molecular mechanisms of the formation of CPPD crystals and their effect on the chondrocytes of the articular cartilage.
Methods: Cartilage samples of patients with chondrocalcinosis and samples of severe OA patients without chondrocalcinosis and healthy cartilage samples served as control. Radiological presence of chondrocalcinosis was evaluated using standard X-ray pictures. The cartilage was stained using von Kossa/Safranin-orange staining. These stainings were used for OA severity scoring using the OARSI-Score. Chondrocyte differentiation was evaluated using Collagen 2 and X, as well as Sox9 and aggrecan as markers for chondrocyte hypertrophic differentiation in immunohistological stainings. TUNEL staining was performed to investigate cell death. The results were validated using qRT-PCR of CPPD and BCP crystal stimulated C28 chondrocytes.
Results: We observed radiological detectable calcification in chondrocalcinosis patients. Interestingly, the presence of CPPD crystals induced collagen 10 expression and thereby hypertrophic differentiation of chondrocytes in both OA and chondrocalcinosis cartilage. Aggrecan and collagen 2 was not reduced in chondrocalcinosis cartilage, but significantly reduced in OA cartilage. TUNEL positive cells were significantly increased in CPPD cartilage, although the OA severity was lower. Q RT-PCR indicated no relevant influence of hypertrophic marker genes by CPPD crystals, whereas BCP crystals significantly induced hypertrophic differentiation.
Discussion: BCP and CPPD crystals seem to trigger differential effects on the chondrocyte phenotype. BCP crystals induce hypertrophic differentiation, which is not induced by CPPD crystals.