No pharmacological approach has been found to prevent/treat calcific aortic valve disease (CAVD), whose pathophysiological changes include calcification and fibrosis, causing aortic stenosis, often in combination with regurgitation. Evidence shows that pro-inflammatory cytokines are potent stimulators of cardiovascular calcification, while the serotonergic system is a potent stimulator of fibrosis. In particular, serotonin receptor 2B (SR-2B) activation is known to cause structural and mechanical changes in aortic valves, such as fibrosis and thickening, in some cases, dense nodules, though not calcification. Clinical studies also show that serotonin (5- HT) levels are almost twice as high in patients with aortic stenosis, and are significantly associated with progression of aortic stenosis. These findings suggest that SR-2B activation also contributes to the pathogenesis of inflammation-induced CAVD. Interestingly, as confirmed in our preliminary studies, the predominant receptor subtype expressed in murine aortic valve interstitial cells, at baseline, is SR-2A, rather than SR-2B. However, in our additional preliminary studies, we found that it was SR-2B that was dramatically upregulated (> 30-fold) by TNF-a. This induction required BMP-2 signaling and may account for our findings that, although 5-HT augmented TNF-a-induced calcification, it had no effect alone.. We also found that the TNF-a/5-HT-induced calcification occurred via induction of Orai1, a plasma membrane calcium channel, and this calcification was attenuated by specific inhibitors or knockdown of only SR-2B. Additional preliminary studies show that circulating peripheral 5-HT levels were higher in hyperlipidemic (Apoe-/-) compared with wild type mice, and SR-2B expression in the valve cusps was adjacent to cholesterol clefts and calcified lesions in Apoe-/- but not in wild type mice. Based on the collective evidence, we hypothesize that 5-HT in the hyperlipidemic milieu plays a key role in CAVD. We posit that hyperlipidemia increases circulating levels of both 5-HT and inflammatory cytokines, the latter of which induce SR-2B and osteogenesis in valve cusps, and, together, these events lead to CAVD.
In Aim 1, we will determine receptor subtype-specific effects of 5-HT on matrix mechanics under non-inflammatory (SR-2A dominant) as well as on calcification under inflammatory (SR-2B dominant) conditions and on the signaling pathways mediating TNF-a induction of SR-2B and of downstream osteoblastic differentiation.
In Aim 2, we will determine whether 5-HT signaling via SR-2B is critical for inflammation-induced CAVD using 1) a 5-HT-deficient, inflammatory mouse model (Tph1-/-Ldlr-/- Apob100/100), and 2) receptor subtype-specific agonists and antagonists. We will apply cutting edge technology, including laser light sheet fluorescence microscopy; serial in vivo fused 18F-PET-CT imaging; atomic force microscopy; 3-D hydrogel cultures; and implantation of receptor subtype knockdown cells. The proposed experiments will elucidate how 5-HT receptor subtype-specific signaling changes the structure of valve cusps and whether targeting SR-2B opens the path for effective pharmacological-based treatment of CAVD.
In older adults, heart valves often become stiff, scarred and constricted, leading to high morbidity and mortality, with no known pharmacological treatment. Since medicines that affect certain serotonin receptors are known to alter valve stiffness, we will determine whether special combinations of these agents are able to halt and/or reverse this fatal valve disease process.
