The generation and characterization of CTRP9-CreERT2 knock-in mice for targeting endothelial cell muscle and adipose tissues.

Vascular endothelial cells (ECs) exhibit significant diversity in different tissues. To elucidate the distinct physiological roles of individual EC subtypes, genetic mouse lines utilizing the Cre-loxP recombination system are crucial. However, there is still a lack of Cre mouse lines that specifically target muscle ECs. Here, through analysis of publicly available single-cell RNA sequencing datasets, C1q/tumor necrosis factor-related protein-9 (CTRP9) was identified as a highly selective marker for microvascular ECs in muscle and adipose tissues. A CTRP9-CreERT2-tdTomato knock-in mouse line was then generated using CRISPR/Cas9, with tdTomato serving as a reporter for endogenous CTRP9 expression. By breeding these mice with Rosa26LSL-EYFP reporter mice and administering tamoxifen, specific EYFP and tdTomato expression was observed in CD31+ ECs in the heart, skeletal muscle, tongue, and adipose tissue from postnatal to adult stages. The EYFP+ cells were mainly identified as microvascular ECs, rather than arterial or venous ECs. The CTRP9-CreERT2 mouse model allows for precise and effective genetic manipulation of this distinct endothelial population, providing a valuable tool for investigating their roles in muscle metabolism, regeneration, and pathophysiology.