Cognitive Resilience in Brain Health and Dementia Research.

With the expected rise in Alzheimer's disease and related dementias (ADRD) in the coming decades due to the aging population and a lack of effective disease-modifying treatments, there is a need for preventive strategies that may tap into resilience parameters. A wide array of resilience strategies has been proposed including genetics, socioeconomic status, lifestyle modifications, behavioral changes, and management of comorbid disease. These different strategies can be broadly classified as distinguishing between modifiable and non-modifiable risk factors, some of which can be quantified so that their clinical intervention can be effectively accomplished. A clear shift in research focus from dementia risk to addressing disease resistance and resilience is emerging that has provided new potential therapeutic targets. Here we review and summarize the latest investigations of resilience mechanisms and methods of quantifying resilience for clinical research. These approaches include identifying genetic variants that may help identify novel pathways (e.g., lipid metabolism, cellular trafficking, synaptic function, inflammation) for therapeutic treatments and biomarkers for use in a precision medicine-like regimen. In addition, innovative structural and molecular neuroimaging analyses may assist in detecting and quantifying pathological changes well before the onset of clinical symptoms setting up the possibility of primary and secondary prevention trials. Lastly, we summarize recent studies demonstrating the study of resilience in caregivers of persons living with dementia may have direct and indirect impact on the quality of care and patient outcomes.

Agmatine induced NO dependent rat mesenteric artery relaxation and its impairment in salt-sensitive hypertension.

l-Arginine and its decarboxylated product, agmatine are important mediators of NO production and vascular relaxation. However, the underlying mechanisms of their action are not understood. We have investigated the role of arginine and agmatine in resistance vessel relaxation of Sprague-Dawley (SD) and Dahl salt-sensitive hypertensive rats. Second or 3rd-order mesenteric arterioles were cannulated in an organ chamber, pressurized and equilibrated before perfusing intraluminally with agonists. The vessel diameters were measured after mounting on the stage of a microscope fitted with a video camera. The gene expression in Dahl rat vessel homogenates was ascertained by real-time PCR. l-Arginine initiated relaxations (EC50, 5.8±0.7mM; n=9) were inhibited by arginine decarboxylase (ADC) inhibitor, difluoromethylarginine (DFMA) (EC50, 18.3±1.3mM; n=5) suggesting that arginine-induced vessel relaxation was mediated by agmatine formation. Agmatine relaxed the SD rat vessels at significantly lower concentrations (EC50, 138.7±12.1µM; n=22), which was compromised by l-NAME (l-N(G)-nitroarginine methyl ester, an eNOS inhibitor), RX821002 (α-2 AR antagonist) and pertussis toxin (G-protein inhibitor). The agmatine-mediated vessel relaxation from high salt Dahl rats was abolished as compared to that from normal salt rats (EC50, 143.9±23.4µM; n=5). The α-2A AR, α-2B AR and eNOS mRNA expression was downregulated in mesenteric arterioles of high-salt treated Dahl hypertensive rats. These findings demonstrate that agmatine facilitated the relaxation via activation of α-2 adrenergic G-protein coupled receptor and NO synthesis, and this pathway is compromised in salt-sensitive hypertension.

Kinetic analysis of DAF-FM activation by NO: toward calibration of a NO-sensitive fluorescent dye.

Nitric oxide (NO) research in biomedicine has been hampered by the absence of a method that will allow quantitative measurement of NO in biological tissues with high sensitivity and selectivity, and with adequate spatial and temporal resolution. 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) is a NO sensitive fluorescence probe that has been used widely for qualitative assessment of cellular NO production. However, calibration of the fluorescent signal and quantification of NO concentration in cells and tissues using fluorescent probes, have provided significant challenge. In this study we utilize a combination of mathematical modeling and experimentation to elucidate the kinetics of NO/DAF-FM reaction in solution. Modeling and experiments suggest that the slope of fluorescent intensity (FI) can be related to NO concentration according to the equation: ddtFI=2αk(1)NO(2)O(2)DAF-FMkNO+DAF-FM where α is a proportionality coefficient that relates FI to unit concentration of activated DAF-FM, k(1) is the NO oxidation rate constant, and k was estimated to be 4.3±0.6. The FI slope exhibits saturation kinetics with DAF-FM concentration. Interestingly, the effective half-maximum constant (EC(50)) increases proportionally to NO concentration. This result is not in agreement with the proposition that N(2)O(3) is the NO oxidation byproduct that activates DAF-FM. Kinetic analysis suggests that the reactive intermediate should exhibit NO-dependent consumption and thus NO(2)() is a more likely candidate. The derived rate law can be used for the calibration of DAF-FM fluorescence and the quantification of NO concentration in biological tissues.

Glutathiyl radical as an intermediate in glutathione nitrosation.

Nitrosation of thiols is thought to be mediated by dinitrogen trioxide (N(2)O(3)) or by nitrogen dioxide radical (()NO(2)). A kinetic study of glutathione (GSH) nitrosation by NO donors in aerated buffered solutions was undertaken. S-nitrosoglutathione (GSNO) formation was assessed spectrophotometrically and by chemiluminescence. The results suggest an increase in the rate of GSNO formation with an increase in GSH with a half-maximum constant EC(50) that depends on NO concentration. Our observed increase in EC(50) with NO concentration suggests a significant contribution of ()NO(2)-mediated nitrosation with the glutathiyl radical as an intermediate in the production of GSNO.

Nicotine augments glomerular injury in a rat model of acute nephritis.

BACKGROUND/AIMS: Epidemiologic studies suggest that cigarette smoke worsens the progression of renal injury in patients with glomerular diseases. The mechanisms involved have not been elucidated. These studies were designed to determine whether nicotine worsens markers of inflammation including glomerular cell proliferation and fibronectin deposition in an in vivo model of glomerular injury. METHODS: Sprague-Dawley rats were injected with anti-Thy1 antibody and given either tap water or nicotine in the drinking water until sacrifice at day 14. Fibronectin expression was measured by Western blot and immunohistochemistry. COX-2 expression was also determined by Western blot in the kidney cortex of rats treated with nicotine and in cultured human mesangial cells treated with nicotine. RESULTS: Anti-Thy1 antibody administration resulted in a significant increase in the number of cells per glomerulus that was further increased by the administration of nicotine. In nephritic rats, the administration of nicotine significantly increased fibronectin and COX-2 expression. In cultured human mesangial cells we also demonstrated that nicotine increases COX-2 expression and activity and that COX-2 mediates mesangial cell proliferation in response to nicotine. CONCLUSION: Either in vivo or in vitro treatment with nicotine leads to activation of inflammatory mediators and hallmarks of glomerular injury, which may explain the mechanisms involved in the deleterious effects of cigarette smoking on renal disease.

Increased nitrite reductase activity of fetal versus adult ovine hemoglobin.

Growing evidence indicates that nitrite, NO2-, serves as a circulating reservoir of nitric oxide (NO) bioactivity that is activated during physiological and pathological hypoxia. One of the intravascular mechanisms for nitrite conversion to NO is a chemical nitrite reductase activity of deoxyhemoglobin. The rate of NO production from this reaction is increased when hemoglobin is in the R conformation. Because the mammalian fetus exists in a low-oxygen environment compared with the adult and is exposed to episodes of severe ischemia during the normal birthing process, and because fetal hemoglobin assumes the R conformation more readily than adult hemoglobin, we hypothesized that nitrite reduction to NO may be enhanced in the fetal circulation. We found that the reaction was faster for fetal than maternal hemoglobin or blood and that the reactions were fastest at 50-80% oxygen saturation, consistent with an R-state catalysis that is predominant for fetal hemoglobin. Nitrite concentrations were similar in blood taken from chronically instrumented normoxic ewes and their fetuses but were elevated in response to chronic hypoxia. The findings suggest an augmented nitrite reductase activity of fetal hemoglobin and that the production of nitrite may participate in the regulation of vascular NO homeostasis in the fetus.

Receptor-mediated activation of nitric oxide synthesis by arginine in endothelial cells.

Arginine contains the guanidinium group and thus has structural similarity to ligands of imidazoline and alpha-2 adrenoceptors (alpha-2 AR). Therefore, we investigated the possibility that exogenous arginine may act as a ligand for these receptors in human umbilical vein endothelial cells and activate intracellular nitric oxide (NO) synthesis. Idazoxan, a mixed antagonist of imidazoline and alpha-2 adrenoceptors, partly inhibited L-arginine-initiated NO formation as measured by a Griess reaction. Rauwolscine, a highly specific antagonist of alpha-2 AR, at very low concentrations completely inhibited NO formation. Like L-arginine, agmatine (decarboxylated arginine) also activated NO synthesis, however, at much lower concentrations. We found that dexmedetomidine, a specific agonist of alpha-2 AR was very potent in activating cellular NO, thus indicating a possible role for alpha-2 AR in L-arginine-mediated NO synthesis. D-arginine also activated NO production and could be inhibited by imidazoline and alpha-2 AR antagonists, thus indicating nonsubstrate actions of arginine. Pertussis toxin, an inhibitor of G proteins, attenuated L-arginine-mediated NO synthesis, thus indicating mediation via G proteins. L-type Ca(2+) channel blocker nifedipine and phospholipase C inhibitor U73122 inhibited NO formation and thus implicated participation of a second messenger pathway. Finally, in isolated rat gracilis vessels, rauwolscine completely inhibited the L-arginine-initiated vessel relaxation. Taken together, these data provide evidence for binding of arginine to membrane receptor(s), leading to the activation of endothelial NO synthase (eNOS) NO production through a second messenger pathway. These findings provide a previously unrecognized mechanistic explanation for the beneficial effects of L-arginine in the cardiovascular system and thus provide new potential avenues for therapeutic development.

Concerted nitric oxide formation and release from the simultaneous reactions of nitrite with deoxy- and oxyhemoglobin.

Recent studies reveal a novel role for hemoglobin as an allosterically regulated nitrite reductase that may mediate nitric oxide (NO)-dependent signaling along the physiological oxygen gradient. Nitrite reacts with deoxyhemoglobin in an allosteric reaction that generates NO and oxidizes deoxyhemoglobin to methemoglobin. NO then reacts at a nearly diffusion-limited rate with deoxyhemoglobin to form iron-nitrosyl-hemoglobin, which to date has been considered a highly stable adduct and, thus, not a source of bioavailable NO. However, under physiological conditions of partial oxygen saturation, nitrite will also react with oxyhemoglobin, and although this complex autocatalytic reaction has been studied for a century, the interaction of the oxy- and deoxy-reactions and the effects on NO disposition have never been explored. We have now characterized the kinetics of hemoglobin oxidation and NO generation at a range of oxygen partial pressures and found that the deoxy-reaction runs in parallel with and partially inhibits the oxy-reaction. In fact, intermediates in the oxy-reaction oxidize the heme iron of iron-nitrosyl-hemoglobin, a product of the deoxy-reaction, which releases NO from the iron-nitrosyl. This oxidative denitrosylation is particularly striking during cycles of hemoglobin deoxygenation and oxygenation in the presence of nitrite. These chemistries may contribute to the oxygen-dependent disposition of nitrite in red cells by limiting oxidative inactivation of nitrite by oxyhemoglobin, promoting nitrite reduction to NO by deoxyhemoglobin, and releasing free NO from iron-nitrosyl-hemoglobin.

Nitric oxide is consumed, rather than conserved, by reaction with oxyhemoglobin under physiological conditions.

Although irreversible reaction of NO with the oxyheme of hemoglobin (producing nitrate and methemoglobin) is extremely rapid, it has been proposed that, under normoxic conditions, NO binds preferentially to the minority deoxyheme to subsequently form S-nitrosohemoglobin (SNOHb). Thus, the primary reaction would be conservation, rather than consumption, of nitrogen oxide. Data supporting this conclusion were generated by using addition of a small volume of a concentrated aqueous solution of NO to a normoxic hemoglobin solution. Under these conditions, however, extremely rapid reactions can occur before mixing. We have thus compared bolus NO addition to NO generated homogeneously throughout solution by using NO donors, a more physiologically relevant condition. With bolus addition, multiple hemoglobin species are formed (as judged by visible spectroscopy) as well as both nitrite and nitrate. With donor, only nitrate and methemoglobin are formed, stoichiometric with the amount of NO liberated from the donor. Studies with increasing hemoglobin concentrations reveal that the nitrite-forming reaction (which may be NO autoxidation under these conditions) competes with reaction with hemoglobin. SNOHb formation is detectable with either bolus or donor; however, the amounts formed are much smaller than the amount of NO added (less than 1%). We conclude that the reaction of NO with hemoglobin under normoxic conditions results in consumption, rather than conservation, of NO.