Andrew J. Patterson

Dimethylarginine Dimethylaminohydrolase Regulates Nitric Oxide Synthesis: Genetic and Physiological Evidence

Background—NO is a major regulator of cardiovascular physiology that reduces vascular and cardiac contractility. Accumulating evidence indicates that endogenous inhibitors may regulate NOS. The NOS inhibitors asymmetric dimethylarginine (ADMA) and N-monomethylarginine are metabolized by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). This study was designed to determine if increased expression of DDAH could reduce tissue and plasma levels of the NOS inhibitors and thereby increase NO synthesis. Methods and Results—We used gene transfer and transgenic approaches to overexpress human DDAH I in vitro and in vivo. The overexpression of DDAH in cultured endothelial cells in vitro induced a 2-fold increase in NOS activity and NO production. In the hDDAH-1 transgenic mice, we observed ≈2-fold increases in tissue NOS activity and urinary nitrogen oxides, associated with a 2-fold reduction in plasma ADMA. The systolic blood pressure of transgenic mice was 13 mm Hg lower than that of wild-type controls (P <0.05). The systemic vascular resistance and cardiac contractility were decreased in response to the increase in NO production. Conclusions—DDAH I overexpression increases NOS activity in vitro and in vivo. The hDDAH-1 transgenic animal exhibits a reduced systolic blood pressure, systemic vascular resistance, and cardiac stroke volume. This study provides compelling evidence that the elaboration and metabolism of endogenous ADMA plays an important role in regulation of NOS activity.

Apelin signaling antagonizes Ang II effects in mouse models of atherosclerosis.

Apelin and its cognate G protein-coupled receptor APJ constitute a signaling pathway with a positive inotropic effect on cardiac function and a vasodepressor function in the systemic circulation. The apelin-APJ pathway appears to have opposing physiological roles to the renin-angiotensin system. Here we investigated whether the apelin-APJ pathway can directly antagonize vascular disease-related Ang II actions. In ApoE-KO mice, exogenous Ang II induced atherosclerosis and abdominal aortic aneurysm formation; we found that coinfusion of apelin abrogated these effects. Similarly, apelin treatment rescued Ang II-mediated increases in neointimal formation and vascular remodeling in a vein graft model. NO has previously been implicated in the vasodepressor function of apelin; we found that apelin treatment increased NO bioavailability in ApoE-KO mice. Furthermore, infusion of an NO synthase inhibitor blocked the apelin-mediated decrease in atherosclerosis and aneurysm formation. In rat primary aortic smooth muscle cells, apelin inhibited Ang II-mediated transcriptional regulation of multiple targets as measured by reporter assays. In addition, we demonstrated by coimmunoprecipitation and fluorescence resonance energy transfer analysis that the Ang II and apelin receptors interacted physically. Taken together, these findings indicate that apelin signaling can block Ang II actions in vascular disease by increasing NO production and inhibiting Ang II cellular signaling.

Thrombin activatable fibrinolysis inhibitor, a potential regulator of vascular inflammation.

The latent plasma carboxypeptidase thrombin-activable fibrinolysis inhibitor (TAFI) is activated by thrombin/thrombomodulin on the endothelial cell surface, and functions in dampening fibrinolysis. In this study, we examined the effect of activated TAFI (TAFIa) in modulating the proinflammatory functions of bradykinin, complement C5a, and thrombin-cleaved osteopontin. Hydrolysis of bradykinin and C5a and thrombin-cleaved osteopontin peptides by TAFIa was as efficient as that of plasmin-cleaved fibrin peptides, indicating that these are also good substrates for TAFIa. Plasma carboxypeptidase N, generally regarded as the physiological regulator of kinins, was much less efficient than TAFIa. TAFIa abrogated C5a-induced neutrophil activation in vitro. Jurkat cell adhesion to osteopontin was markedly enhanced by thrombin cleavage of osteopontin. This was abolished by TAFIa treatment due to the removal of the C-terminal Arg168 by TAFIa from the exposed SVVYGLR α4β1 integrin-binding site in thrombin-cleaved osteopontin. Thus, thrombin cleavage of osteopontin followed by TAFIa treatment may sequentially up- and down-modulate the pro-inflammatory properties of osteopontin. An engineered anticoagulant thrombin, E229K, was able to activate endogenous plasma TAFI in mice, and E229K thrombin infusion effectively blocked bradykinin-induced hypotension in wild-type, but not in TAFI-deficient, mice in vivo. Our data suggest that TAFIa may have a broad anti-inflammatory role, and its function is not restricted to fibrinolysis.

Overexpression of Dimethylarginine Dimethylaminohydrolase Reduces Tissue Asymmetric Dimethylarginine Levels and Enhances Angiogenesis

Background—This study was designed to determine whether overexpression of the enzyme dimethylarginine dimethylaminohydrolase (DDAH) could enhance angiogenesis by reducing levels of the endogenous nitric oxide synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA). Methods and Results—In DDAH1 transgenic (TG) and wild-type mice (each n=42), the role of DDAH overexpression on angiogenesis was studied by use of the disk angiogenesis system and a murine model of hindlimb ischemia (each n=21). After surgery, animals were treated with either PBS or the NOS inhibitors ADMA or N&ohgr;-nitro-l-arginine methyl ester (L-NAME; each 250 &mgr;mol · kg−1 · d−1) by use of osmotic minipumps (each n=7). L-NAME was chosen to study an inhibitor that is not degraded by DDAH. Neovascularization in the disk angiogenesis system was impaired by both NOS inhibitors; however, TG animals were resistant to the effects of ADMA on neovascularization. Similarly, TG mice were more resistant to the inhibitory effect of ADMA on angioadaptation (angiogenesis and arteriogenesis) after hindlimb ischemia, as assessed by fluorescent microsphere studies and postmortem microangiograms. Enhanced neovascularization and limb perfusion in TG mice were associated with reduced plasma and tissue ADMA levels and enhanced tissue NOS enzyme activity. Conclusions—We describe a novel mechanism by which DDAH regulates postnatal neovascularization. Therapeutic manipulation of DDAH expression or activity may represent a novel approach to restore tissue perfusion.

Signaling from β1‐ and β2‐adrenergic receptors is defined by differential interactions with PDE4

β1‐ and β2‐adrenergic receptors (βARs) are highly homologous, yet they play clearly distinct roles in cardiac physiology and pathology. Myocyte contraction, for instance, is readily stimulated by β1AR but not β2AR signaling, and chronic stimulation of the two receptors has opposing effects on myocyte apoptosis and cell survival. Differences in the assembly of macromolecular signaling complexes may explain the distinct biological outcomes. Here, we demonstrate that β1AR forms a signaling complex with a cAMP‐specific phosphodiesterase (PDE) in a manner inherently different from a β2AR/β‐arrestin/PDE complex reported previously. The β1AR binds a PDE variant, PDE4D8, in a direct manner, and occupancy of the receptor by an agonist causes dissociation of this complex. Conversely, agonist binding to the β2AR is a prerequisite for the recruitment of a complex consisting of β‐arrestin and the PDE4D variant, PDE4D5, to the receptor. We propose that the distinct modes of interaction with PDEs result in divergent cAMP signals in the vicinity of the two receptors, thus, providing an additional layer of complexity to enforce the specificity of β1‐ and β2‐adrenoceptor signaling.

Regulatory T Cells Limit Vascular Endothelial Injury and Prevent Pulmonary Hypertension

Rationale: Pulmonary arterial hypertension (PAH) is an incurable disease associated with viral infections and connective tissue diseases. The relationship between inflammation and disease pathogenesis in these disorders remains poorly understood. Objective: To determine whether immune dysregulation due to absent T-cell populations directly contributes to the development of PAH. Methods and Results: Vascular endothelial growth factor receptor 2 (VEGFR2) blockade induced significant pulmonary endothelial apoptosis in T-cell-deficient rats but not in immune-reconstituted (IR) rats. T cell–lymphopenia in association with VEGFR2 blockade resulted in periarteriolar inflammation with macrophages, and B cells even prior to vascular remodeling and elevated pulmonary pressures. IR prevented early inflammation and attenuated PAH development. IR with either CD8 T cells alone or with CD4-depleted spleen cells was ineffective in preventing PAH, whereas CD4-depleting immunocompetent euthymic animals increased PAH susceptibility. IR with either CD4+CD25hi or CD4+CD25− T cell subsets prior to vascular injury attenuated the development of PAH. IR limited perivascular inflammation and endothelial apoptosis in rat lungs in association with increased FoxP3+, IL-10- and TGF-&bgr;-expressing CD4 cells, and upregulation of pulmonary bone morphogenetic protein receptor type 2 (BMPR2)–expressing cells, a receptor that activates endothelial cell survival pathways. Conclusions: PAH may arise when regulatory T-cell (Treg) activity fails to control endothelial injury. These studies suggest that regulatory T cells normally function to limit vascular injury and may protect against the development of PAH.

Protecting the myocardium: A role for the β2 adrenergic receptor in the heart

ObjectiveThe sympathetic nervous system enhances cardiac muscle function by activating &bgr; adrenergic receptors (&bgr;ARs). Recent studies suggest that chronic &bgr;AR stimulation is detrimental, however, and that it may play a role in the clinical deterioration of patients with congestive heart failure. To examine the impact of chronic &bgr;1AR and &bgr;2AR subtype stimulation individually, we studied the cardiovascular effects of catecholamine infusions in &bgr;AR subtype knockout mice (&bgr;1KO, &bgr;2KO). DesignProspective, randomized, experimental study. SettingAnimal research laboratory. Subjects&bgr;1KO and &bgr;2KO mice and wild-type controls. InterventionsThe animals were subjected to 2 wks of continuous infusion of the &bgr;AR agonist isoproterenol. Analyses of cardiac function and structure were performed during and 3 days after completion of the infusions. Functional studies included graded exercise treadmill testing, in vivo assessments of left ventricular function using Mikro-Tip catheter transducers, right ventricular pressure measurements, and analyses of organ weight to body weight ratios. Structural studies included heart weight measurements, assessments of myocyte ultrastructure using electron microscopy, and in situ terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick-end labeling staining to quantitate myocyte apoptosis. Measurements and Main ResultsWe found that isoproterenol-treated &bgr;2KO mice experienced greater mortality rates (p = .001, chi-square test using Fisher’s exact method) and increased myocyte apoptosis at 3- and 7-day time points (p = .04 and p = .0007, respectively, two-way analysis of variance). ConclusionThe results of this study suggest that in vivo &bgr;2AR activation is antiapoptotic and contributes to myocardial protection.

Nitrous Oxide Produces Antinociceptive Response via α2Band/or α2CAdrenoceptor Subtypes in Mice

BACKGROUND Opiate receptors in the periaqueductal gray region and alpha2 adrenoceptors in the spinal cord of the rat mediate the antinociceptive properties of nitrous oxide (N2O). The availability of genetically altered mice facilitates the detection of the precise protein species involved in the transduction pathway. In this study, the authors establish the similarity between rats and mice in the antinociceptive action of N2O and investigate which alpha2 adrenoceptor subtypes mediate this response. METHODS After obtaining institutional approval, antinociceptive dose-response and time-course to N2O was measured in wild-type and transgenic mice (D79N), with a nonfunctional alpha2A adrenoceptor using tail-flick latency. The antinociceptive effect of N2O was tested after pretreatment systemically with yohimbine (nonselective alpha2 antagonist), naloxone (opiate antagonist), L659,066 (peripheral alpha2-antagonist) and prazosin (alpha2B- and alpha2C-selective antagonist). The tail-flick latency to dexmedetomidine (D-med), a nonselective alpha2 agonist, was tested in wild-type and transgenic mice. RESULTS N2O produced antinociception in both D79N transgenic and wild-type litter mates, although the response was less pronounced in the transgenic mice. Antinociception from N2O decreased over time with continuing exposure, and the decrement was more pronounced in the transgenic mice. The antinociceptive response could be dose dependently antagonized by opiate receptor and selective alpha2B-/alpha2C-receptor antagonists but not by a central nervous system-impermeant alpha2 antagonist (L659,066). Whereas dexmedetomidine exhibited no antinociceptive response in the D79N mice, the robust antinociceptive response in the wild-type litter mates could not be blocked by a selective alpha2B-/alpha2C-receptor antagonist. CONCLUSION These data confirm that the antinociceptive response to an exogenous alpha2-agonist is mediated by an alpha2A adrenoceptor and that there appears to be a role for the alpha2B- or alpha2C-adrenoceptor subtypes, or both, in the analgesic response to N2O.

Vasoregulation at the molecular level: A role for the β1 subunit of the calcium-activated potassium (BK) channel

Essential hypertension is among the most common and most costly medical conditions in the United States. Multiple defects in the kidneys, the vasculature, and the neuro-endocrine system may contribute to the development of this disorder. Within the past decade investigators have identified several molecular components of the vasculature that control tone and influence blood pressure. For example, the large conductance BK type calcium-activated potassium channel has recently been shown to play an important role in maintaining the dynamic equilibrium between vasoconstriction and vasodilation of vascular smooth muscle. Activation of vascular smooth muscle BK channels leads to hyperpolarization of the cell membrane, which causes deactivation of voltage-dependent calcium channels and vasodilation. In this review, we will summarize recently published data focusing on the role of the BK channels accessory beta1 subunit as well as other modulators of BK channel activation that influence vascular tone and blood pressure.

Postischemic Brain Injury Is Attenuated in Mice Lacking the β2-Adrenergic Receptor

BACKGROUND: Several &bgr;-adrenergic receptor (&bgr;AR) antagonists have been shown to have neuroprotective effects against cerebral ischemia. However, clenbuterol, a &bgr;2AR agonist, was shown to have neuroprotective activity by increasing nerve growth factor expression. We used &bgr;2AR knockout mice and a &bgr;2 selective antagonist to test the effect of loss of &bgr;2ARs on outcome from transient focal cerebral ischemia. METHODS: Ischemia was induced by the intraluminal suture method, for 60 min of middle cerebral artery occlusion (MCAO) followed by 24 h reperfusion. Neurological score was determined at 24 h reperfusion and infarct size was determined by cresyl violet or 2,3,5-triphenyltetrazolium chloride staining. &bgr;2AR knockout mice and wild-type congenic FVB/N controls were studied, as well as 2 groups of wild type mice given either ICI 118,551 (0.2 mg/kg) or 0.9% saline intraperitoneally 30 min before MCAO (n = 10 per group). Changes in expression of heat shock protein (Hsp)72 after ischemia were examined by immunohistochemistry and western blots. RESULTS: Compared with wild type littermates, infarct volume was decreased by 22.3% in &bgr;2AR knockout mice (39.7 ± 10.7 mm3 vs 51.0 ± 11.4 mm3, n = 10/group, P = 0.034) after 60 min of MCAO followed by 24 h reperfusion. Pretreatment with a &bgr;2AR selective antagonist, ICI 118,551, also decreased infarct size significantly, by 25.1%, compared with the saline control (32.8 ± 11.9 mm3 vs 43.8 ± 10.3 mm3, n = 10/group, P = 0.041). Neurological scores were also significantly improved in mice lacking the &bgr;2AR or pretreated with ICI 118,551. After cerebral ischemia, total levels of Hsp72 and the number of Hsp72 immunopositive cells were greater in mice lacking &bgr;2 AR. CONCLUSION: Brain injury is reduced and neurological outcome improved after MCAO in mice lacking the &bgr;2AR, or in wild type mice pretreated with a selective &bgr;2AR antagonist. This is consistent with a shift away from prosurvival signaling to prodeath signaling in the presence of &bgr;2AR activation in cerebral ischemia. Protection is associated with higher levels of Hsp72, a known antideath protein. The effect of &bgr;2AR signaling in the setting of cerebral ischemia is complex and warrants further study.