Matrix metalloproteinase-3 evokes microglial responses and subsequent neuronal apoptosis

Apoptotic cells are rapidly phagocytosed, eliminating intracellular factors that might provoke further immune responses directed at healthy neighboring cells. In the CNS, microglia play a role in the clearance of degenerating neurons that parallels the activity of peripheral macrophages.; In the current study we asked whether signals released from apoptotic neurons evoked a microglial response that would cause degeneration of naïve cells, bypassing the putatively positive effects of microglial phagocytic activity. Interestingly, we found that a soluble signaling macromolecule, which was released from apoptotic PC12 cells in serum-deprived media, evoked BV2 microglial responses including cytokine gene induction and TNF-α release.; An active form of matrix metalloproteinase-3 (MMP-3) was found as a candidate molecule released from apoptotic neurons by using MALDI-TOF analysis. Matrix metalloproteinases (MMPs) are proteolytic enzymes that degrade extracellular macromolecules. Recent evidence has also linked MMPs to the inflammatory response in Alzheimer's disease and multiple sclerosis. Microglial activation by MMP-3 was confirmed by treating BV2 cells with recombinant catalytic domain of MMP-3 (cMMP-3). cMMP-3-treated BV2 cells showed a marked TNF-α induction and release that increased in a time-dependent manner and that was completely blocked by pretreatment with NNGH, an MMP-3 inhibitor.; Signal transduction pathways utilized in MMP-3-mediated activation of BV2 microglia involved ERK1/2 phosphorylation since the MEK1/2-specific inhibitor U0126 completely blocked TNF-α release. Next, the effect of cMMP-3-activated microglia on neuronal degeneration was examined. Naïve NGF-differentiated PC 12 cells exposed to conditioned medium from cMMP-3-treated BV2 cell cultures underwent apoptosis as early as 12 hours. These results showed that apoptotic neurons release signaling molecules at an early stage of apoptosis. MMP-3 is a novel molecule released from apoptotic neurons that provokes microglial activation. The data also suggest that MMP-3 may cause the propagation of neuronal degeneration through ERK-dependent microglial activation.