Background Perivascular adipose tissue (PVAT) surrounds most vessels and shares common features with brown adipose tissue (BAT). Whereas adaptive thermogenesis in BAT increases energy expenditure and is beneficial for metabolic diseases, little is known on the role of PVAT in vascular diseases such as atherosclerosis. We hypothesize that the thermogenic function of PVAT regulates intravascular temperature and reduces atherosclerosis. Methods and Results PVAT shares similar structural and proteomics with BAT. We demonstrate that PVAT has thermogenic properties similar to BAT in response to cold stimuli in vivo. Proteomics analysis of the PVAT from mice housed in a cold environment identified differential expression in proteins highly related with cellular metabolic processes. In a mouse model deficient in PPARγ in smooth muscle cells (SMPG KO mice), we uncovered a complete absence of PVAT surrounding the vasculature likely due to PPARγ deletion also in the perivascular adipocyte precursor cells. Lack of PVAT, resulting in loss of its thermogenic activity, impairs vascular homeostasis causing temperature loss and endothelial dysfunction. We further show that cold exposure inhibits atherosclerosis and improves endothelial function in mice with intact PVAT, but not in SMPG KO mice, as a result of impaired lipid clearance. Pro-inflammatory cytokine expression in PVAT is not altered upon cold exposure. Finally, prostacyclin released from PVAT contributes to the vascular protection against endothelial dysfunction. Conclusions PVAT is a vasoactive organ with functional characteristics similar to BAT and is essential for intravascular thermoregulation upon cold acclimation. This thermogenic capacity of PVAT plays an important protective role in the pathogenesis of atherosclerosis.

Hyperpolarization-activated cyclic nucleotide-gated channels (HCN1-4) play a crucial role in the regulation of cell excitability. Importantly, they contribute to spontaneous rhythmic activity in brain and heart. HCN channels are principally activated by membrane hyperpolarization and binding of cAMP. Here, we identify tyrosine phosphorylation by Src kinase as another mechanism affecting channel gating. Inhibition of Src by specific blockers slowed down activation kinetics of native and heterologously expressed HCN channels. The same effect on HCN channel activation was observed in cells cotransfected with a dominant-negative Src mutant. Immunoprecipitation demonstrated that Src binds to and phosphorylates native and heterologously expressed HCN2. Src interacts via its SH3 domain with a sequence of HCN2 encompassing part of the C-linker and the cyclic nucleotide binding domain. We identified a highly conserved tyrosine residue in the C-linker of HCN channels (Tyr 476 in HCN2) that confers modulation by Src. Replacement of this tyrosine by phenylalanine in HCN2 or HCN4 abolished sensitivity to Src inhibitors. Mass spectrometry confirmed that Tyr 476 is phosphorylated by Src. Our results have functional implications for HCN channel gating. Furthermore, they indicate that tyrosine phosphorylation contributes in vivo to the fine tuning of HCN channel activity.