Brown adipose tissue (BAT) secretes cytokines that influence the function of other tissues. Given the widespread distribution of brown fat depots, we generated BAT-lacking (ΔBAT) mouse models by specifically eliminating brown adipocytes using the Cre-loxP system combined with a floxed-stop DTA cassette. Uncoupling protein 1 (UCP1) is essential for BAT thermogenesis and exhibits a highly restricted expression pattern, so it was chosen to direct BAT-specific Cre recombinase expression. We employed CRISPR-Cas9 to insert an ires-Cre sequence downstream of the UCP1 stop codon, developing the novel knock-in line, Ucp1-CreYH. Ucp1-CreYH and transgenic line TgUcp1-CreEvdr mouse (Ucp1-CreEvdr) were crossed with Ai14-tdTomato and floxed-CPEB2 mice to assess Cre specificity and efficiency. ΔBAT mice were then generated by crossing each Cre line with floxed-stop DTA mice, followed by assessments of locomotor activity, body weight, and glucose tolerance. Although both Cre lines showed cold-enhanced expression, Ucp1-CreEvdr exhibited considerably lower Cre levels in BAT compared to Ucp1-CreYH mice, leading to inefficient ablation of some floxed alleles, such as Cpeb2. Moreover, Ucp1-CreEvdr mice displayed non-specific Cre expression whereas neither line showed evidence of substantial autonomous Cre activity in BAT-resident macrophages. Consequently, ΔBATEvdr mice experienced off-target neuronal ablation, resulting in hyperactive locomotion and reduced body weight. While ΔBATYH mice showed normal locomotor activity and body weight, they had a modest weight gain and altered glucose homeostasis only after high-fat-diet feeding. In conclusion, novel Ucp1-Cre knock-in mouse showed specificity and efficiency for gene manipulation in brown adipocytes, highlighting its application in generating BAT-specific knockout and BAT-depleted mouse models.
Hsu et al. (Tue,) studied this question.