Matrix Metalloproteinase-9 is a Diagnostic Marker of Heterotopic Ossification in a Murine Model

Heterotopic ossification (HO) is a serious disorder that occurs when there is aberrant bone morphogenic protein (BMP) signaling in soft tissues. Currently, there are no methods to detect HO beforemineralization occurs. Yet once mineralization occurs, there are no effective treatments, short of surgery, to reverse HO. Herein, we used in vivo molecular imaging and confirmatory ex vivo tissue analyses of an establishedmurine animalmodel of BMP-induced HO to show that matrix metalloproteinase-9 (MMP-9) can be detected as an early-stage biomarker before mineralization. Ex vivo analyses showthat activeMMP-9 protein is significantly elevated within tissues undergoingHOas early as 48 h after BMP induction, with its expression co-localizing to nerves and vessels. In vivo molecular imaging with a dual-labeled near-infrared fluorescence and micro-positron emission tomography (mPET) agent specific to MMP-2/-9 expression paralleled the ex vivo observations and reflected the site of HO formation as detected from microcomputed tomography 7 days later. The results suggest that theMMP-9 is a biomarker of the early extracellular matrix (ECM) re-organization and could be used as an in vivo diagnostic with confirmatory ex vivo tissue analysis for detecting HO or conversely for monitoring the success of tissue-engineered bone implants that employ ECM biology for engraftment.