Project Summary – Project I PVAT has been narrowly viewed as a tissue that communicates with the blood vessel through secretions and homing of immune cells. This is `outside-in' communication and is a passive function of PVAT. We hypothesize that to maintain the homeostasis so critical to tissue health, there must be an `inside-out' communication from the formally accepted vessel layers – intima, media, adventitia – to the PVAT that is mechanical in nature. This allows the blood vessel to inform PVAT of its status, such that PVAT can respond appropriately to maintain homeostasis. Thus, if secretions are made by PVAT, this may not be uninformed/passive, but rather in response to messages received from the blood vessel it surrounds. Project I overall hypothesis is that PVAT mechanically coordinates with the blood vessel in control of vascular tone, contributing to (patho)physiological function. We propose investigation of two (2) functions of PVAT that have not been previously considered and which exert profound effects on vascular function. Our first hypothesis is that pressure is transmitted to PVAT through mechanosensitive elements (Aim 1). Of all the adipose tissues in the body, PVAT is primed to be mechanoresponsive because it is exposed to constant pressure. Second, we hypothesize that PVAT has a dynamic mechanical stiffness of its own that reduces vascular stiffness in health (Aim 2). Further, changes in this dynamic stiffness occur with disease (e.g. increased stiffening). We share preliminary data that support both hypotheses. PVAT, in healthy tissue, is integral to the vessel reduced vessel tone and stiffness. Thus, this understudied vessel layer must be considered as a clinically relevant tissue. Two aims focus individually on these new parameters. We will integrate information from other projects in how innervation and neurotransmitters (Project II), immune cells (Project III), and direct influence of stretch on adipocyte/SVF function (Project IV) impact PVAT mechanotransduction (Aim 1) and stiffness (Aim 2) in health. Core B provides high fat (HF) diet-induced hypertension (Dahl S with non-hypertensive Dahl R control) as well as a novel mid-thoracic aorta coarcted model to impose elevated pressure independent of diet. Core C has and will continue to provide RNA sequencing analysis for determination of specific cell types within PVAT that support mechanotransduction. Core D will be of marked assistance in measuring non cellular contributions made by collagens and elastin. When these gaps are filled, our work will justify that PVAT should always be considered an active partner of the formal blood vessel. We stand to redefine what is the formal blood vessel with this new knowledge.