A UCP1-IRES-Cre Knock-In Mouse Enables Specific Brown Adipocyte Targeting Without CNS Off-Target Expression.

Uncoupling protein 1 (UCP1) is predominantly expressed in brown adipocytes, where it plays a central role in thermogenesis and energy metabolism. BAC-transgenic UCP1-Cre mouse lines have been widely used to target thermogenic adipocytes, but ectopic Cre activity in the hypothalamus and peripheral organs has raised concerns regarding specificity. Here, we characterize a novel knock-in UCP1-IRES-Cre mouse model that drives Cre expression from the endogenous UCP1 promoter. UCP1-IRES-Cre mice were crossed with Ai14 reporter mice to visualize Cre-dependent tdTomato expression across adipose depots, brain regions, and peripheral tissues and compared with UCP1 protein localization. Reporter expression in male and female UCP1-IRES-Cre mice closely mirrored endogenous UCP1 expression in brown and white adipose tissue depots, with substantially reduced ectopic activity compared with previously reported BAC-transgenic models. Minimal reporter expression was observed in the hypothalamus and peripheral organs, although robust reporter expression was detected in the choroid plexus despite absent UCP1 protein staining. White adipose depots exhibited broader reporter labeling than UCP1 immunoreactivity, consistent with transient or prior UCP1 promoter activation during development or adult life. Together, these findings demonstrate that the UCP1-IRES-Cre model faithfully recapitulates endogenous UCP1 expression while minimizing off-target Cre activity, providing a knock-in alternative for genetic targeting of thermogenic adipocytes.