Mechanical injury of bovine cartilage explants induces depth-dependent, transient changes in MAP kinase activity associated with apoptosis

Abstract

To characterize mitogen activated protein (MAP) kinase activity and chondrocyte apoptosis in an invitro model of cartilage mechanical injury as a function of tissue depth and time post-injury. Mechanically injured osteochondral explants were assessed for cell viability, MAP kinase and caspase-3 activity over 15 days using immunofluorescence microscopy and Western blot. Zonal distributions of cell viability and apoptosis were quantified in the presence of specific mitogen activated protein kinase inhibitors. Viability rapidly decreased post-injury, most significantly in the superficial zone, with some involvement of the middle and deep zones, which correlated with increased caspase-3 activity. Transient and significant increases in extracellular-regulated protein kinase (ERK) activity were observed in middle and deep zones at 1 and 6 days post-injury, while c-Jun-amino terminal protein kinase activity increased in the deep zone at 1 and 6 days compared to uninjured controls. Changes in p38 activity were particularly pronounced, with significant increases in all three zones 30min post-injury, but only in the middle and deep zones after 1 and 6 days. Inhibition of ERK and p38 increased chondrocyte viability which correlated with decreased apoptosis. Spatiotemporal patterns of MAP kinase signalling in cartilage after mechanical injury strongly correlate with changes in cell viability and chondrocyte apoptosis. Importantly, these signals may be pro-survival or pro-apoptotic depending on zonal location and time post-injury. These data yield mechanistic insights which may improve the diagnosis and treatment of cartilage injuries.