Region-Dependent Aggrecan Degradation Patterns in the Rat Intervertebral Disc Are Affected by Mechanical Loading In Vivo

Study Design. Immunoblotting study to evaluate aggrecan degradation patterns in rat intervertebral discs (IVDs) subjected to mechanical overload. Objective. To evaluate the effects of in vivo dynamic compression overloading on aggrecan degradation products associated with matrix metalloproteinase (MMP) and aggrecanase activity in different regions of the IVD. Summary of Background Data. Aggrecan cleavage at the MMP and aggrecanase sites is an important event in human IVD aging, with distinct cleavage patterns in the anulus and nucleus regions. No such information is available on regional variations in rat IVDs, nor on how such cleavage is affected by mechanical loading. Methods. Sprague-Dawley rats were instrumented with an Ilizarov-type device and subjected to dynamic compression (1 MPa and 1 Hz for 8 hours per day for 8 weeks). Control, sham, and overloaded IVDs were separated by disc region and analyzed for aggrecan degradation products using immunoblotting techniques, with antibodies specific for the aggrecanase and MMP cleavage sites in the interglobular domain of aggrecan. Results. Control IVDs exhibited strong regional variation in aggrecan degradation patterns with minimal degradation products being present in the nucleus pulposus, degradation products associated with aggrecanase cleavage predominating in the inner anulus fibrosus (AF), and degradation products associated with MMP cleavage predominating in the outer AF. Dynamic compression overloading increased the amount of aggrecan degradation products associated with MMP cleavage not only in the AF but also in the nucleus pulposus. Degradation profiles of sham IVDs were similar to control. Conclusion. Aggrecan G1 regions resulting from proteolysis were found to have a strong regionally specific pattern in the rat IVD, which was altered under excessive loading. The shift from aggrecanase to MMP-induced degradation products with dynamic compression overloading suggests that protein degradation and loss can precede major structural disruption in the IVD, and that MMP-induced aggrecan degradation may be a marker of mechanically induced disc degeneration.