Stromelysin-1 (MMP-3) expression driven by a macrophage-specific promoter results in reduced viability in transgenic mice

Macrophage expression of matrix degrading metalloproteinases (MMPs) in human atheroma has been found to occur in rupture-prone areas of plaques. To investigate the effect of metalloproteinase activity on plaque stability, we attempted to generate mice that expressed a stromelysin-1 (MMP-3) transgene specifically in macrophages. Promoter sequences taken from a macrophage-tropic lentivirus (visna) were used to drive transgene expression. The transgene construct was expressed in macrophages in vitro and its autoactivation was established by casein zymography. Transgenic mice generated with this construct died at or before birth. No gross anatomical changes were observed in these mice. Embryos arising from a second round of oocyte injections with the transgene were examined at day 16 of gestation. Of the products of conception, 40% resulted in vacant conceptuses. Only one animal of 38 examined carried the transgene and its expression of MMP-3 mRNA at E16 was faintly detected by RT-PCR. When a non-toxic reporter gene, luciferase, was substituted for the MMP-3 cDNA, healthy transgenic mice were produced that expressed the reporter gene in a wide variety of tissue macrophages, including those located in the brain, testis, lung, and thymus. These studies suggest that constitutive expression of MMP-3 in diverse populations of tissue macrophages leads to prenatal or neonatal death in the mouse. It appears likely that more sophisticated transcriptional control of MMP-3 expression will be required in order to generate stromelysin-1 transgenic mice that could be useful models for studying overexpression of this metalloproteinase’s activity in the lesional macrophages of atherosclerotic plaques.