Abstract 207: Comparison of Two Lines of Mice Expressing Smooth Muscle-Targeted Cre Recombinase from Different Promoters

Background The development of mouse lines that drive inducible deletion of conditional alleles specifically in smooth muscle cells (SMC-Cre mice) is a major advance in experimental vascular biology. Two recently reported SMC-Cre lines differ in the promoter that drives Cre expression and in their sex linkage. One line uses a smooth muscle myosin heavy chain (SMMHC) promoter and is Y-chromosome linked; the other uses a smooth muscle actin (SMA) promoter and is autosomal. Performance of these lines has not been compared in the same laboratory. Hypothesis We hypothesized that the 2 lines of SMC-Cre mice would rearrange a floxed allele with equal efficiency and tissue specificity and would have equivalent_and at most low_levels of recombinase activity in vascular (V)SMC in the absence of Cre activation by Tamoxifen. Methods Both SMC-Cre lines were crossed with the R26R reporter line. Cre-positive/R26R-positive mice were injected with Tamoxifen or vehicle (n = 2 - 5/group). Cre-negative/R26R positive mice were injected with Tamoxifen (n = 3 - 4/group). Cre activity was assessed by X-gal stain (n = 2 - 4/group) and qRT-PCR for lacZ (n = 3 - 5/group). Results X-gal stains showed that both lines inducibly rearranged R26R with high efficiency in VSMC of the aorta and carotids. Visceral SMC in the bladder and gastrointestinal tissues of these mice also stained blue, as did VSMC within these organs and within other, SMC-poor organs such as skeletal muscle and heart. Vehicle-injected Cre-positive/R26R-positive mice in both lines had low levels of blue staining in VSMC but high levels of blue staining in SMC of the intestine (25 - 50%) and bladder (> 50%). No blue cells were detected in Cre-negative/R26R-positive mice injected with Tamoxifen. Both lines of Cre-positive/R26R-positive mice showed Tamoxifen-inducible expression of lacZ mRNA in the aorta, carotid, and bladder. Both lines also had essentially undetectable lacZ mRNA in the heart, either in the absence or the presence of Tamoxifen. Conclusions The 2 lines of SMC-Cre mice appear equivalent in their ability to achieve inducible gene deletion specifically in VSMC. Both have low levels of background activity in VSMC in the absence of Tamoxifen. Background Cre activity is higher in visceral than vascular SMC of both lines.