Surprisingly we discovered that among adipose depots Prx Cre

Surprisingly, we discovered that among adipose depots, Prx1-Cre activity is specific for iWAT APCs, particularly in male mice. These results were unexpected, given a prior study that reported widespread recombination in both WAT and BAT, purportedly due to recombination at the progenitor stage (Calo et al., 2010). However, that study utilized a different indicator line, Rosa-lacZ (Soriano, 1999), and did not quantify recombination or determine the identity of the gsk-3 in which recombination was detected. Here, we comprehensively examine stromal cells from multiple depots and quantify recombination in vivo in defined cell populations. These findings are consistent with the immunostaining results of adipose tissues: high levels of recombination in iWAT, much less in gWAT, and little to no recombination in BAT. Finally, previous work has shown the same Cre allele can direct different expression patterns depending on the mouse strain background (Hébert and McConnell, 2000). The mice utilized in our study were maintained in the C57BL/6 background, whereas the prior study utilized a mixed genetic background (Calo et al., 2010). Therefore, some differences in recombination observed between these studies may be the result of background modifiers of Prx1-Cre expression. Our studies do not address whether this recombination pattern reflects physiological expression of Prx1, the activity of isolated Prx1 promoter elements, or results from chromosomal positional effects of this particular transgenic line. The documented expression of Prx1-Cre in the limb bud (Logan et al., 2002) and our own studies of bone marrow cultures (Figure 4C) suggest that experiments with these mice will require careful interpretation. However, for the study of genes primarily essential to adipose tissue function, our results highlight the utility of Prx1-Cre over PdgfRα-Cre, as the restricted activity of the former within adipose depots allows for the study of gene function specifically in iWAT APCs. Previous studies in mice have shown that iWAT transplanted into the visceral cavity is metabolically protective, resulting in decreased overall fat mass, improved glucose homeostasis, and insulin sensitivity, although the reasons for these benefits are not well understood (Tran et al., 2008). The specificity of Prx1-Cre activity could enable the identification of genes responsible for these and other known functional differences among fat depots, including gene expression, adipokine release, APC proliferation, and lipid homeostasis (Tchkonia et al., 2013). Interestingly, another recently characterized Cre line is active in various visceral adipose depots, including gWAT, but not in iWAT or BAT (Chau et al., 2014), and thus will be a useful complementary tool to analyze alongside Prx1-Cre-directed recombination. Finally, inducible (Cre-ER) versions of these lines have been generated using the same or similar promoter elements (Imai et al., 2001; Rivers et al., 2008; Kawanami et al., 2009; Kang et al., 2010). However, even if the same genomic sequences are used, the unique integration site of each transgenic line and mouse genetic background will likely alter the Cre expression pattern from what is reported here. We would recommend that investigators employing these and other lines evaluate the expression patterns to assess the specific Cre recombination profiles. Nevertheless, it will be worthwhile to examine whether increased temporal resolution can further restrict the Cre activity and thus increase their specificity for defined target populations.
Experimental Procedures
Acknowledgments