Inflammation; Endothelial cell metabolism of tetrahydrobiopterin
Our laboratory has been focused on two research programs. The first is to understand how inflammation, and in particular, the adaptive immune response contributes to hypertension. Several years ago, we found that T cells are essential for the development of hypertension. We have shown that various hypertensive stimuli, including angiotensin II, norepinephrine and DOCA-salt cause activation of T cells and leads to their accumulation in the perivascular fat and kidneys. Our data indicate that T cell-derived cytokines such as IL-17 and TNF-a enhance vasoconstriction and sodium retention, leading to the hypertensive phenotype. Central signals derived from the circumventricular organs contribute to T cell activation, and manipulation of signals from this region affect T cell activation and the eventual elevation in blood pressure caused by angiotensin II. Our current studies are directed toward understanding the specific subtypes of T cells involved in hypertension. We are attempting to understand mechanisms involved in T cell activation in response to hypertensive stimuli. We are attempting to separate the effects of physical forces, such as vascular stretch and neurohumoral effects in this process.
Our second research program deals with endothelial cell metabolism of tetrahydrobiopterin. This critical co-factor for the nitric oxide synthase enzymes is produced via a complex synthetic pathway involving the sequential actions of three enzymes. We discovered a new phosphorylation site (serine 81) on the first of these, GTP cyclohydrolase-1 (GTPCH-1), which increases activity of the enzyme. We have further shown that the physical force of laminar shear causes phosphorylation of GTPCH-1 at serine 81 and thus dramatically increases endothelial levels of tetrahydrobiopterin. Interestingly, oscillatory shear stress, which is commonly associated with accelerated atherosclerosis, does not induce GTPCH-1 phosphorylation or increase tetrahydrobiopterin. This leads to a condition known at NOS uncoupling in cells exposed to oscillatory shear stress. We have gone on to show that these differences in GTPCH-1 phosphorylation and activity exist in vivo in mice in which we can induce flow disturbances in one carotid artery. Further studies have show that this deficiency of tetrahydrobiopterin and NO at sites of disturbed flow contribute to atherosclerosis. Treatment with oral tetrahydrobiopterin dramatically reduces atherosclerosis at these sites in ApoE-/- mice. To further study this, we have made mice in which the phosphorylated serine has been replaced with an aspartate (to mimic phosphorylation) and with an alanine (to inhibit phosphorylation). We plan to use these animals to understand how GTPCH-1 phosphorylation affects vascular function, blood pressure and oxidative events in vivo in future experiments.