Effects of p67phox on the mitochondrial oxidative state in the kidney of Dahl salt-sensitive rats: optical fluorescence 3-D cryoimaging

AW, Dash RK. Computational Modeling of Substrate-Dependent Mitochondrial Respiration and Bioenergetics in the Heart and Kidney Cortex and Outer Medulla

Integrated computational modeling provides a mechanistic and quantitative framework to characterize alterations in mitochondrial respiration and bioenergetics in response to different metabolic substrates in-silico. These alterations play critical roles in the pathogenesis of diseases affecting metabolically active organs such as heart and kidney. Therefore, the present study aimed to develop and validate thermodynamically constrained integrated computational models of mitochondrial respiration and bioenergetics in the heart and kidney cortex and outer medulla (OM). The models incorporated the kinetics of major biochemical reactions and transport processes as well as regulatory mechanisms in the mitochondria of these tissues. Intrinsic model parameters such as Michaelis-Menten constants were fixed at previously estimated values, while extrinsic model parameters such as maximal reaction and transport velocities were estimated separately for each tissue. This was achieved by fitting the model solutions to our recently published respirometry data measured in isolated rat heart and kidney cortex and OM mitochondria utilizing various NADH- and FADH2-linked metabolic substrates. The models were validated by predicting additional respirometry and bioenergetics data, which were not used for estimating the extrinsic model parameters. The models were able to predict tissue-specific and substrate-dependent mitochondrial emergent metabolic system properties such as redox states, enzyme and transporter fluxes, metabolite concentrations, membrane potential, and respiratory control index under diverse physiological and pathological conditions. The models were also able to quantitatively characterize differential regulations of NADH- and FADH2-linked metabolic pathways, which contribute differently toward regulations of oxidative phosphorylation and ATP synthesis in the heart and kidney cortex and OM mitochondria.

Mechanisms of decreased tubular flow-induced nitric oxide in Dahl salt-sensitive rat thick ascending limbs

Dahl salt-sensitive (SS) rat kidneys produce less nitric oxide (NO) than those of salt-resistant (SR) rats. Thick ascending limb (TAL) NO synthase 3 (NOS3) is a major source of renal NO, and luminal flow enhances its activity. We hypothesized that flow-induced NO is reduced in TALs from SS rats primarily due to NOS uncoupling and diminished NOS3 expression rather than scavenging. Rats were fed normal-salt (NS) or high-salt (HS) diets. We measured flow-induced NO and superoxide in perfused TALs and performed Western blots of renal outer medullas. For rats on NS, flow-induced NO was 35 ± 6 arbitrary units (AU)/min in TALs from SR rats but only 11 ± 2 AU/min in TALs from SS (P < 0.008). The superoxide scavenger tempol decreased the difference in flow-induced NO between strains by about 36% (P < 0.020). The NOS inhibitor N-nitro-l-arginine methyl ester (l-NAME) decreased flow-induced superoxide by 36 ± 8% in TALs from SS rats (P < 0.02) but had no effect in TALs from SR rats. NOS3 expression was not different between strains on NS. For rats on HS, the difference in flow-induced NO between strains was enhanced (SR rats: 44 ± 10 vs. SS: 9 ± 2 AU/min, P < 0.005). Tempol decreased the difference in flow-induced NO between strains by about 37% (P < 0.012). l-NAME did not significantly reduce flow-induced superoxide in either strain. HS increased NOS3 expression in TALs from SR rats but not in TALs from SS rats (P < 0.003). We conclude that 1) on NS, flow-induced NO is diminished in TALs from SS rats mainly due to NOS3 uncoupling such that it produces superoxide and 2) on HS, the difference is enhanced due to failure of TALs from SS rats to increase NOS3 expression.NEW & NOTEWORTHY The Dahl rat has been used extensively to study the causes and effects of salt-sensitive hypertension. Our study suggests that more complex processes other than simple scavenging of nitric oxide (NO) by superoxide lead to less NO production in thick ascending limbs of the Dahl salt-sensitive rat. The predominant mechanism involved depends on dietary salt. Impaired flow-induced NO production in thick ascending limbs most likely contributes to the Na+ retention associated with salt-sensitive hypertension.

Optical Metabolic Imaging of Mitochondrial Dysfunction on HADH Mutant Newborn Rat Hearts

BACKGROUND

Mitochondrial [Formula: see text]-oxidation of fatty acids is the primary energy source for the heart and carried out by Hydroxy Acyl-CoA Dehydrogenase (HADH) encoded trifunctional protein. Mutations in the genes encoding mitochondrial proteins result in functionally defective protein complexes that contribute to energy deficiencies, excessive reactive oxygen species (ROS) production, and accumulation of damaged mitochondria. We hypothesize that a dramatic alternation in redox state and associated mitochondrial dysfunction is the underlying cause of Fatty Acid Oxidation (FAO) deficiency mutant, resulting in heart failure. Mitochondrial co-enzymes, NADH and FAD, are autofluorescent metabolic indices of cells when imaged, yield a quantitative assessment of the cells' redox status and, in turn, that of the tissue and organ.

METHOD

We utilized an optical cryo-imager to quantitively evaluate the three-dimensional distribution of mitochondrial redox state in newborn rats' hearts and kidneys. Redox ratio (RR) assessment shows that mitochondrial dysfunction is extreme and could contribute to severe heart problems and eventual heart failure in the mutants.

RESULTS

Three-dimensional redox ratio (NADH/FAD) rendering, and the volumetric mean value calculations confirmed significantly decreased cardiac RR in mutants by 31.90% and 12.32%, in renal mitochondrial RR compared to wild-type control. Further, histological assessment of newborn heart myocardial tissue indicated no significant difference in myocardial tissue architecture in both control and severe (HADHAe4-/-) conditions.

CONCLUSION

These results demonstrate that optical imaging can accurately estimate the redox state changes in newborn rat organs. It is also apparent that the FAO mutant's heart tissue with a low redox ratio is probably more vulnerable to cumulative damages than kidneys and fails prematurely, contributing to sudden death.

3D Optical Cryo-Imaging Method: A Novel Approach to Quantify Renal Mitochondrial Bioenergetics Dysfunction

Mitochondrial dysfunction contributes to various injuries and diseases. A mechanistic understanding of how dysfunctional mitochondria modulates metabolism is of paramount importance. Three-dimensional (3D) optical cryo-imager is a custom-designed device that can quantify the volumetric bioenergetics of organs in small animal models. The instrument captures the autofluorescence of bioenergetics indices (NADH and FAD) from tissues at cryogenic temperature. The quantified redox ratio (NADH/FAD) is used as an optical indicator of mitochondrial redox state.

The early effects of uninephrectomy on rat kidney metabolic state using optical imaging

Uninephrectomy (UNX) is known to result in structural and metabolic changes to the remaining kidney, although it is uncertain if this alters the mitochondrial redox state and how soon such changes may occur. A custom-designed fluorescence cryo-imaging technique was used to quantitatively assess the effect of UNX by measuring the levels of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) in the remaining kidney. Kidneys were snap-frozen 3 days following UNX, and the intrinsic fluorescence of NADH and FAD were optically acquired. The 3D images were created to characterize the NADH/FAD redox ratios (RR) of the right kidneys, which underwent UNX and the remaining kidneys 3 days following UNX. Both the NADPH-oxidases (Nox2 and Nox4) and the mitochondria are the main sources of reactive oxygen species (ROS) production in tubular epithelial cells. Responses to the UNX were obtained in kidneys of normal Sprague Dawley (SD) rats, Dahl salt-sensitive (SS) rats and SS rats in which NADPH-oxidase isoform 4 (Nox4) was knocked out (SS^Nox4-/- ). The results found that each of the strains exhibited similar increase in kidney weights averaging 17% after 3 days of UNX. SD and SS^Nox4-/- rats both exhibited global reductions of the RR (P < .05) with a similar tendency observed in SS rats (P < .08), indicating increased ROS production. The unexpected reduction of the RR in the remnant kidneys of SS^Nox4-/- rats indicates that mechanisms independent of H₂ O₂ produced from Nox4 may be responsible for this global increase of ROS. We propose that the reduced RR was largely a consequence of enhanced mitochondrial bioenergetics due to increased tubular workload of the remaining kidney. The data indicate that mitochondria become the dominant source of increased ROS following UNX and could represent an important hypertrophic signaling mechanism.

Renal Glomerular Mitochondria Function in Salt-Sensitive Hypertension

Salt-sensitive (SS) hypertension is accompanied with an early onset of proteinuria, which results from the loss of glomerular podocytes. Here, we hypothesized that glomerular damage in the SS hypertension occurs in part due to mitochondria dysfunction, and we used a unique model of freshly isolated glomeruli to test this hypothesis. In order to mimic SS hypertension, we used Dahl SS rats, an established animal model. Animals were fed a 0.4% NaCl (normal salt, NS) diet or challenged with a high salt (HS) 4% NaCl diet for 21 days to induce an increase in blood pressure (BP). Similar to previous studies, we found that HS diet caused renal hypertrophy, increased BP, glomerulosclerosis, and renal lesions such as fibrosis and protein casts. We did not observe changes in mitochondrial biogenesis in the renal cortex or isolated glomeruli fractions. However, Seahorse assay performed on freshly isolated glomeruli revealed that basal mitochondrial respiration, maximal respiration, and spare respiratory capacity were lower in the HS compared to the NS group. Using confocal imaging and staining for mitochondrial H₂O₂ using mitoPY1, we detected an intensified response to an acute H₂O₂ application in the podocytes of the glomeruli isolated from the HS diet fed group. TEM analysis showed that glomerular mitochondria from the HS diet fed group have structural abnormalities (swelling, enlargement, less defined cristae). Therefore, we report that glomerular mitochondria in SS hypertension are functionally and structurally defective, and this impairment could eventually lead to loss of podocytes and proteinuria. Thus, the glomerular–mitochondria axis can be targeted in novel treatment strategies for hypertensive glomerulosclerosis.

Associations of NADPH oxidase-related genes with blood pressure changes and incident hypertension: The GenSalt Study

Previous studies have indicated that reactive oxygen species produced by NADPH oxidase (Nox) are important risk factors of hypertension. The current study aims to examine the associations of Nox-related genes with longitudinal blood pressure (BP) changes and the risk of incident hypertension in the Genetic Epidemiology Network of Salt Sensitivity (GenSalt) follow-up study. A total of 1,768 participants from 633 families were included in our analysis. Nine BP measurements were obtained in the morning at baseline and during two follow-up visits. The mixed-effect models were used to investigate the associations of 52 tagged single-nucleotide polymorphisms in 11 Nox-related genes with BP changes and incident hypertension. Gene-based analyses were performed by truncated product method (TPM) and Versatile Gene-based Association Study (VEGAS). Over the 7.2 years of follow-up, systolic BP (SBP) and diastolic BP (DBP) increased, and 32.1% (512) of participants developed hypertension. SNPs rs12094228, rs16861188 and rs12066019 in NCF2 were significantly associated with longitudinal change in SBP (P_interaction = 1.1 × 10^-3, 2.8 × 10^-3 and 1.2 × 10^-3, respectively). Gene-based analyses revealed that NCF2 was significantly associated with SBP (P_TPM = 1.00 × 10^-6, P_VEGAS = 1.26 × 10^-4) and DBP changes (P_TPM = 5.84 × 10^-4, P_VEGAS = 1.04 × 10^-3). These findings suggested that NCF2 may play an important role in BP changes over time in the Han Chinese population.

Optical metabolic imaging of irradiated rat heart exposed to ischemia-reperfusion injury

Whole thoracic irradiation (WTI) is known to cause deterioration in cardiac function. Whether irradiation predisposes the heart to further ischemia and reperfusion (IR) injury is not well known. The aim of this study is to examine the susceptibility of rat hearts to IR injury following a single fraction of 15 Gy WTI and to investigate the role of mitochondrial metabolism in the differential susceptibility to IR injury. After day 35 of irradiation, ex vivo hearts from irradiated and nonirradiated rats (controls) were exposed to 25-min global ischemia followed by 60-min IR, or hearts were perfused without IR for the same protocol duration [time controls (TC)]. Online fluorometry of metabolic indices [redox state: reduced nicotinamide adenine dinucleotide (NADH), oxidized flavin adenine dinucleotide (FAD), and NADH/FAD redox ratio] and functional variables [systolic left ventricular pressure (LVP), diastolic LVP (diaLVP), coronary flow (CF), and heart rate were recorded in the beating heart; developed LVP (dLVP) and rate pressure product (RPP)] were derived. At the end of each experimental protocol, hearts were immediately snap frozen in liquid N2 for later three-dimensional imaging of the mitochondrial redox state using optical cryoimaging. Irradiation caused a delay in recovery of dLVP and RPP after IR when compared to nonirradiated hearts but recovered to the same level at the end of reperfusion. CF in the irradiated hearts recovered better than the control hearts after IR injury. Both fluorometry and 3-D cryoimaging showed that in WTI and control hearts, the redox ratio increased during ischemia (reduced) and decreased on reperfusion (oxidized) when compared to their respective TCs; however, there was no significant difference in the redox state between WTI and controls. In conclusion, our results show that although irradiation of rat hearts compromised baseline cardiovascular function, it did not alter cardiac mitochondrial redox state and induce greater susceptibility of these hearts to IR injury.

Mitochondrial Dysfunction and Altered Renal Metabolism in Dahl Salt-Sensitive Rats

BACKGROUND/AIMS

The kidney plays a critical role in the control of blood pressure and its elevation in salt-induced hypertension. Mitochondrial dysfunction, especially in energy metabolism, has been associated with hypertension. Here, we aimed to investigate mitochondrial function and metabolic features in renal mitochondria of Dahl salt-sensitive (SS) rats to gain further insight into the relationship between mitochondrial metabolism and predisposition to hypertension.

METHODS

In this study, SS rats fed low-salt (LS) or high-salt (HS) diets were used to investigate mitochondrial function and metabolism including mitochondrial enzyme activities, pyridine nucleotides, metabolites, and oxidative stress by biochemical analysis and gas chromatography-mass spectrometer (GC-MS).

RESULTS

Significantly lower activity levels of fumarase, isocitrate dehydrogenase and succinyl-CoA synthetase were observed in renal mitochondria of SS rats compared with SS.13BN control rats fed LS diets. Intra-mitochondrial pyridine nucleotide content and mitochondrial metabolism were adversely affected in SS rats. In accordance with this, reduced ATP production, Δψm, and superoxide dismutase (SOD) activity were also observed in mitochondria of the renal medulla and cortex of SS rats. Moreover, ATP production was further impaired and oxidative stress was increased, confirming that the mitochondria of SS rats fed HS diets were dysfunctional compared to those of rats fed LS diets.

CONCLUSIONS

Our data demonstrated that the renal mitochondria of SS rats exhibited complicated metabolic alteration and dysfunction in low-salt diets, and high-salt diets aggravated these dysfunctions. Thus, these results may be associated with renal dysfunction, which, in turn, would help in understanding the development of salt-sensitive hypertension.

Involvement of ENaC in the development of salt-sensitive hypertension

Salt-sensitive hypertension is associated with renal and vascular dysfunctions, which lead to impaired fluid excretion, increased cardiac output, and total peripheral resistance. It is commonly accepted that increased renal sodium handling and plasma volume expansion are necessary factors for the development of salt-induced hypertension. The epithelial sodium channel (ENaC) is a trimeric ion channel expressed in the distal nephron that plays a critical role in the regulation of sodium reabsorption in both normal and pathological conditions. In this mini-review, we summarize recent studies investigating the role of ENaC in the development of salt-sensitive hypertension. On the basis of experimental data obtained from the Dahl salt-sensitive rats, we and others have demonstrated that abnormal ENaC activation in response to a dietary NaCl load contributes to the development of high blood pressure in this model. The role of different humoral factors, such as the components of the renin-angiotensin-aldosterone system, members of the epidermal growth factors family, arginine vasopressin, and oxidative stress mediating the effects of dietary salt on ENaC are discussed in this review to highlight future research directions and to determine potential molecular targets for drug development.

Optical Cryoimaging Reveals a Heterogeneous Distribution of Mitochondrial Redox State in ex vivo Guinea Pig Hearts and Its Alteration During Ischemia and Reperfusion

Oxidation of substrates to generate ATP in mitochondria is mediated by redox reactions of NADH and FADH2. Cardiac ischemia and reperfusion (IR) injury compromises mitochondrial oxidative phosphorylation. We hypothesize that IR alters the metabolic heterogeneity of mitochondrial redox state of the heart that is only evident in the 3-D optical cryoimaging of the perfused heart before, during, and after IR. The study involved four groups of hearts: time control (TC: heart perfusion without IR), global ischemia (Isch), global ischemia followed by reperfusion (IR) and TC with PCP (a mitochondrial uncoupler) perfusion. Mitochondrial NADH and FAD autofluorescence signals were recorded spectrofluorometrically online in guinea pig ex vivo-perfused hearts in the Langendorff mode. At the end of each specified protocol, hearts were rapidly removed and snap frozen in liquid N2 for later 3-D optical cryoimaging of the mitochondrial NADH, FAD, and NADH/FAD redox ratio (RR). The TC hearts revealed a heterogeneous spatial distribution of NADH, FAD, and RR. Ischemia and IR altered the spatial distribution and caused an overall increase and decrease in the RR by 55% and 64%, respectively. Uncoupling with PCP resulted in the lowest level of the RR (73% oxidation) compared with TC. The 3-D optical cryoimaging of the heart provides novel insights into the heterogeneous distribution of mitochondrial NADH, FAD, RR, and metabolism from the base to the apex during ischemia and IR. This 3-D information of the mitochondrial redox state in the normal and ischemic heart was not apparent in the dynamic spectrofluorometric data.

Role of Nox4 and p67phox subunit of Nox2 in ROS production in response to increased tubular flow in the mTAL of Dahl salt-sensitive rats

Nox4 and Nox2 are the most abundant NADPH oxidases (Nox) in the kidney and have been shown to contribute to hypertension, renal oxidative stress, and injury in Dahl salt-sensitive (SS) hypertensive rats. The present study focused on the role of Nox4 and p67phox/Nox2 in the generation of H2O2 and O2 (·-) in the renal medullary thick ascending limb of Henle (mTAL) of SS rats in response to increasing luminal flow (from 5 to 20 nl/min). Nox4 and p67phox/Nox2 genes were found to be expressed in the mTAL of SS rats. Responses of SS rats were compared with those of SS rats with knockout of Nox4 (SS(Nox4-/-)) or functional mutation of p67phox (SS(p67phox-/-)). Nox4 was the dominant source of increased intracellular H2O2 production in response to increased luminal flow as determined using the fluorescent dye peroxyfluor 6-AM (PF6-AM). The rate of mitochondrial H2O2 production [as determined by mitochondria peroxy yellow 1 (mitoPY1)] was also significantly reduced in SS(Nox4-/-) compared with SS rats, but not in SS(p67phox-/-) rats. In contrast, intracellular superoxide (O2 (·-)) production (the ratio of ethidium to dihydroethidium) in the mTAL of SS(Nox4-/-) rats was nearly identical to that of SS rats in response to luminal flow, indicating that Nox4 made no measurable contribution. mTAL O2 (·-) production was reduced in SS(p67phox-/-) compared with SS rats at the lower luminal flow of 5 nl/min and progressively increased when perfusion was changed to 20 nl/min. We conclude that increased mTAL luminal flow results in increases in intracellular and mitochondrial H2O2, which are dependent on the presence of Nox4, and that p67phox/Nox2 accounts solely for increases in O2 (·-) production.

The impact of aging, hearing loss, and body weight on mouse hippocampal redox state, measured in brain slices using fluorescence imaging

The relationships between oxidative stress in the hippocampus and other aging-related changes such as hearing loss, cortical thinning, or changes in body weight are not yet known. We measured the redox ratio in a number of neural structures in brain slices taken from young and aged mice. Hearing thresholds, body weight, and cortical thickness were also measured. We found striking aging-related increases in the redox ratio that were isolated to the stratum pyramidale, while such changes were not observed in thalamus or cortex. These changes were driven primarily by changes in flavin adenine dinucleotide, not nicotinamide adenine dinucleotide hydride. Multiple regression analysis suggested that neither hearing threshold nor cortical thickness independently contributed to this change in hippocampal redox ratio. However, body weight did independently contribute to predicted changes in hippocampal redox ratio. These data suggest that aging-related changes in hippocampal redox ratio are not a general reflection of overall brain oxidative state but are highly localized, while still being related to at least one marker of late aging, weight loss at the end of life.

Evidence of the Importance of Nox4 in Production of Hypertension in Dahl Salt-Sensitive Rats

This study reports the consequences of knocking out NADPH (nicotinamide adenine dinucleotide phosphate) oxidase 4 (Nox4) on the development of hypertension and kidney injury in the Dahl salt-sensitive (SS) rat. Zinc finger nuclease injection of single-cell SS embryos was used to create an 8 base-pair frame-shift deletion of Nox4, resulting in a loss of the ≈68 kDa band in Western blot analysis of renal cortical tissue of the knock out of Nox4 in the SS rat (SS(Nox4-/-)) rats. SS(Nox4-/-) rats exhibited a significant reduction of salt-induced hypertension compared with SS rats after 21 days of 4.0% NaCl diet (134±5 versus 151±3 mm Hg in SS) and a significant reduction of albuminuria, tubular casts, and glomerular injury. Optical fluorescence 3-dimensional cryoimaging revealed significantly higher redox ratios (NADH/FAD [reduced nicotinamide adenine dinucleotide/flavin adenine dinucleotide]) in the kidneys of SS(Nox4-/-) rats even when fed the 0.4% NaCl diet, indicating greater levels of mitochondrial electron transport chain metabolic activity and reduced oxidative stress compared with SS rats. Before the development of hypertension, RNA expression levels of Nox subunits Nox2, p67(phox), and p22(phox) were found to be significantly lower (P<0.05) in SS(Nox4-/-) compared with SS rats in the renal cortex. Thus, the mutation of Nox4 seems to modify transcription of several genes in ways that contribute to the protective effects observed in the SS(Nox4-/-) rats. We conclude that the reduced renal injury and attenuated blood pressure response to high salt in the SS(Nox4-/-) rat could be the result of multiple pathways, including gene transcription, mitochondrial energetics, oxidative stress, and protein matrix production impacted by the knock out of Nox4.