Transcriptomic changes in glomeruli in response to a high salt challenge in the Dahl SS rat

Salt sensitivity impacts a significant portion of the population and is an important contributor to the development of chronic kidney disease. One of the significant early predictors of salt-induced damage is albuminuria, which reflects the deterioration of the renal filtration barrier: the glomerulus. Despite significant research efforts, there is still a gap in knowledge regarding the molecular mechanisms and signaling networks contributing to instigating and/or perpetuating salt-induced glomerular injury. To address this gap, we used 8-wk-old male Dahl salt-sensitive rats fed a normal-salt diet (0.4% NaCl) or challenged with a high-salt diet (4% NaCl) for 3 wk. At the end of the protocol, a pure fraction of renal glomeruli obtained by differential sieving was used for next-generation RNA sequencing and comprehensive semi-automatic transcriptomic data analyses, which revealed 149 differentially expressed genes (107 and 42 genes were downregulated and upregulated, respectively). Furthermore, a combination of predictive gene correlation networks and computational bioinformatic analyses revealed pathways impacted by a high salt dietary challenge, including renal metabolism, mitochondrial function, apoptotic signaling and fibrosis, cell cycle, inflammatory and immune responses, circadian clock, cytoskeletal organization, G protein-coupled receptor signaling, and calcium transport. In conclusion, we report here novel transcriptomic interactions and corresponding predicted pathways affecting glomeruli under salt-induced stress.NEW & NOTEWORTHY Our study demonstrated novel pathways affecting glomeruli under stress induced by dietary salt. Predictive gene correlation networks and bioinformatic semi-automatic analysis revealed changes in the pathways relevant to mitochondrial function, inflammatory, apoptotic/fibrotic processes, and cell calcium transport.

Renal histaminergic system and acute effects of histamine receptor 2 blockade on renal damage in the Dahl salt-sensitive rat

Histamine is involved in the regulation of immune response, vasodilation, neurotransmission, and gastric acid secretion. Although elevated histamine levels and increased expression of histamine metabolizing enzymes have been reported in renal disease, there is a gap in knowledge regarding the mechanisms of histamine-related pathways in the kidney. We report here that all four histamine receptors as well as enzymes responsible for the metabolism of histamine are expressed in human and rat kidney tissues. In this study, we hypothesized that the histaminergic system plays a role in salt-induced kidney damage in the Dahl salt-sensitive (DSS) rat, a model characterized with inflammation-driven renal lesions. To induce renal damage related to salt sensitivity, DSS rats were challenged with 21 days of a high-salt diet (4% NaCl); normal-salt diet (0.4% NaCl)-fed rats were used as a control. We observed lower histamine decarboxylase and higher histamine N-methyltransferase levels in high-salt diet-fed rats, indicative of a shift in histaminergic tone; metabolomics showed higher histamine and histidine levels in the kidneys of high-salt diet-fed rats, whereas plasma levels for both compounds were lower. Acute systemic inhibition of histamine receptor 2 in the DSS rat revealed that it lowered vasopressin receptor 2 in the kidney. In summary, we established here the existence of the local histaminergic system, revealed a shift in the renal histamine balance during salt-induced kidney damage, and provided evidence that blockage of histamine receptor 2 in the DSS rat affects water balance and urine concentrating mechanisms.NEW & NOTEWORTHY Histamine is a nitrogenous compound crucial for the inflammatory response. The knowledge regarding the renal effects of histamine is very limited. We showed that renal epithelia exhibit expression of the components of the histaminergic system. Furthermore, we revealed that there was a shift in the histaminergic tone in salt-sensitive rats when they were challenged with a high-salt diet. These data support the notion that histamine plays a role in renal epithelial physiological and pathophysiological functions.

Functional role of histamine receptors in the renal cortical collecting duct cells

Histamine is an important immunomodulator, as well as a regulator of allergic inflammation, gastric acid secretion, and neurotransmission. Although substantial histamine level has been reported in the kidney, renal pathological and physiological effects of this compound have not been clearly defined. The goal of this study was to provide insight into the role of histamine-related pathways in the kidney, with emphasis on the collecting duct (CD), a distal part of the nephron important for the regulation of blood pressure. We report that all four histamine receptors (HRs) as well as enzymes responsible for histamine metabolism and synthesis are expressed in cultured mouse mpkCCD_cl4 cells, and histamine evokes a dose-dependent transient increase in intracellular Ca²⁺ in these cells. Furthermore, we observed a dose-dependent increase in cAMP in the CD cells in response to histamine. Short-circuit current studies aimed at measuring Na⁺ reabsorption via ENaC (epithelial Na⁺ channel) demonstrated inhibition of ENaC-mediated currents by histamine after a 4-hr incubation, and single-channel patch-clamp analysis revealed similar ENaC open probability before and after acute histamine application. The long-term (4 hr) effect on ENaC was corroborated in immunocytochemistry and qPCR, which showed a decrease in protein and gene expression for αENaC upon histamine treatment. In summary, our data highlight the functional importance of HRs in the CD cells and suggest potential implications of histamine in inflammation-related renal conditions. Further research is required to discern the molecular pathways downstream of HRs and assess the role of specific receptors in renal pathophysiology.

Abstract MP06: Insights Into The Role Of Anp In Hfpef During Ss Hypertension

Background: Atrial natriuretic peptide ( ANP) is a hormone produced primarily in the heart, which induces vasodilation and natriuresis. We showed previously that a knockout of ANP in Dahl SS rat results in hypertension and HFpEF phenotype. There is a gap in knowledge regarding the role of inflammation and mitochondrial function in HFpEF that accompanies SSH. We hypothesize that the ANP deficiency in SSH leads to pressure overload, which affects cardiac inflammation and mitochondrial function.

Methods: At 8 weeks of age, male wild type (SS WT ) and Nppa knockout (ANP knockout, SS Nppa-/- ) Dahl SS rats were placed on normal salt ( NS , 0.4% NaCl) or a high salt ( HS , 4% NaCl) for 21 days. Tissues were harvested for histological and Western analysis. TEM was used to obtain electron micrographs. EPR spectroscopy was performed in snap-frozen tissues post injection of POBN. 2-way ANOVA was employed for statistical comparison.

Results: Heart-to-body-weight ratio was higher in SS Nppa-/- rats (p<0.001), on NS and HS diet ( p <0.001). SS Nppa-/- rats displayed increases in cardiac fibrosis following HS diet ( p <0.01). EPR demonstrated that SS Nppa-/- hearts exhibit a trend for increased lipid peroxidation compared to SS WT . Cardiac cytokine profile demonstrated an increase in IL-1α, IL-1β, L-6, IL-17, TNF-α, sICAM-1, MIP-1α, MIP-3α, in the SS WT rats fed a HS diet vs NS diet, and lower levels of these in the HS-diet fed SS Nppa-/- rats vs SS WT . HS diet was sufficient to induce increases in mitochondrial area ( p <0.001), and decreases in density ( p 0.001), and overall health score ( p <0.001) in both SS WT and SS Nppa-/- rats, but no genotype-specific differences were reported. We observed changes in fission/fusion proteins: Opa1 expression was elevated in SS Nppa-/- vs SS WT ( p <0.01), and Parkin expression was lower in all rats on a HS diet ( p <0.05).

Conclusions: Although SS Nppa-/- rats develop a HFpEF phenotype, exacerbated cardiac fibrosis and higher lipid peroxidation vs SS WT rats, inflammation and overall mitochondrial damage are diminished in the hearts of these animals at day 21 of the HS diet. Further studies are needed to elucidate the mechanisms by which inflammation may affect the development of HFpEF during SS hypertension in a cell-specific and time-dependent manner.

Inhibition of neprilysin with sacubitril without RAS blockage aggravates renal disease in Dahl SS rats

Salt-sensitive (SS) hypertension is accompanied with severe cardiorenal complications. In this condition, elevated blood pressure (BP) resulting from salt retention is associated with counterintuitively lower levels of atrial natriuretic peptide (ANP). In plasma, ANP is degraded by the neprilysin; therefore, pharmacological inhibition of this metalloprotease (i.e., with sacubitril) can be employed to increase ANP level. We have shown earlier that sacubitril in combination with valsartan (75 μg/day each) had beneficial effects on renal function in Dahl SS rats. The goal of this study was to evaluate the effects of a higher dose of sacubitril on renal damage in this model. To induce hypertension, male Dahl SS rats were fed a 4% NaCl diet (HS) for 21 days, and were administered sacubitril (125 μg/day) or vehicle via s.c. osmotic pumps. At the end of the HS challenge, both groups exhibited similar outcomes for GFR, heart weight, plasma electrolytes, BUN, and creatinine. Sacubitril exacerbated kidney hypertrophy, but did not affect levels of renal fibrosis. We also observed aggravated glomerular lesions and increased formation of protein casts in the sacubitril-treated animals compared to controls. Thus, in Dahl SS rats, administration of sacubitril without renin-angiotensin-system blockage had adverse effects on renal disease progression, particularly in regards to glomerular damage and protein cast formation. We can speculate that while ANP levels are increased because of neprilysin inhibition, there are off-target effects of sacubitril, which are detrimental to renal function in the SS hypertensive state.

Abstract P019: Renal Mitochondrial Bioenergetics In Salt-sensitive Hypertension Is Affected By ANP

There are clinical data suggesting that low levels of Atrial Natriuretic Peptide (ANP) aggravate susceptibility to salt-sensitive (SS) hypertension. ANP is known to affect mitochondria in many tissues, however, little is known about the effects of ANP on renal mitochondrial function. According to our earlier studies, Dahl SS rats lacking ANP exhibit increased blood pressure and pronounced kidney injury. We hypothesized that in SS hypertension ANP deficiency affects renal mitochondrial bioenergetics and contributes to renal function impairment. SS hypertension was induced in male SS NPPA-/- ( Nppa knockout in Dahl SS background, KO) and SS WT (wild type, WT) rats by a high salt (HS) 4% NaCl diet administered for 21 days. Age-matched control animals were fed a normal (NS) 0.4% NaCl salt diet. A combination of in vivo studies, molecular biology and tests of mitochondria isolated from renal cortex (seahorse respiration and spectrofluorimetry assays using TMRM, Amplex Red and MCLA) were used to probe the role ANP in mitochondrial function. Data was analyzed with ANOVA followed by Holm-Sidak post hoc. We report a significant decrease in mitochondrial membrane potential in the SS NPPA-/- rats vs SS WT (25 ± 4% decrease in KO vs WT on NS, and a 16 ± 5% decrease in KO on HS). Furthermore, mitochondrial H 2 O 2 (57.7 ± 7.4 (WT) vs 57.6 ± 1.3 au (KO), p<0.0001) and superoxide (36.6± 1.8 au (WT) vs 67.9 ± 5.2 au (KO), p<0.0001) levels were increased in the KO on a HS diet. Next, we antioxidant capacity was elevated in the SS NPPA-/- rats on HS diet in comparison with SS WT , and increased SOD2 levels were observed in the KO animals. Furthermore, SS NPPA-/- rats exhibit higher MCU (mitochondrial calcium uniporter) activity than SS WT , implying an ANP-dependent effect on mitochondrial calcium influx. Seahorse assay revealed dramatic elevation of the basal, ATP-linked, maximal and spare oxygen consumption rate (OCR) in the cortical mitochondria of the knockout rats on HS, while in HS fed SS WT rats these OCR parameters were, as expected, reduced (vs NS). Therefore, deficiency of circulating ANP leads to significant changes of renal mitochondrial bioenergetics, potentially via effects on mitochondrial calcium uptake, which in turn affects respiratory chain activity leading to oxidative stress.

Abstract MP12: Increasing Circulating Anp Levels With Sacubitril Without Ras Blockage Aggravates Renal Disease In Dahl Ss Rats

Salt-sensitive (SS) hypertension is accompanied with severe cardiorenal complications. In this condition, elevated blood pressure (BP) resulting from high salt intake can be partially associated with a lower level of the atrial natriuretic peptide (ANP). In plasma, ANP is quickly degraded by neprilysin; therefore, pharmacological inhibition of this metalloprotease (f.i. with sacubitril) can increase ANP level. We have shown earlier that lower dose of sacubitril in combination with valsartan (75 μg/day) has beneficial effects on renal function in SS hypertension. The goal of this study was to evaluate the effects of sacubitril administered at a higher dose on renal damage of Dahl SS rats. Sacubitril (125 μg/day) or vehicle were administered to male Dahl SS rats (Charles River) via s.c. osmotic pumps. To induce hypertension, both groups were fed a purified 4% NaCl diet (HS, Dyets Inc) for 21 days. Glomerular filtration rate (GFR, FITC-inulin) and BP (tail cuff) values were obtained before and after the HS challenge. Data was analyzed with 1-way ANOVA. When compared to vehicle treated rats, 125 μg/day of sacubitril insignificantly increased systolic BP measured at the end of the 21-day HS challenge; no changes in GFR, heart weight, plasma electrolytes, BUN and creatinine were observed. However, sacubitril caused kidney hypertrophy (two kidneys to body weight: 8.8±0.4 vs 10.4±0.4 mg/g, p=0.04,), but did not affect renal medullary of cortical fibrosis. We also observed aggravated glomerular lesions in the sacubitril-treated animals compared to controls (glomerular injury score: 1.6±0.02 vs 0.9±0.08 au, p=0.034), and increased formation of protein casts (1.8±0.3 vs 1.3±0.2 au, p=0.034). Thus, in Dahl SS rats, administration of sacubitril at 125 μg/day had adverse effects on renal disease progression, especially glomerular damage and protein casts formation. In our prior study we showed that the combination of sacubitril at 75 μg/day with the angiotensin receptor blocker (ARB), valsartan, is effective to alleviate these outcomes. It is likely that the inhibition of neprilysin by sacubitril results in accumulation of Ang II and/or bradykinin and/or RAAS activation, which masks the beneficial effects of ANP level increase.

Differential effects of low-dose sacubitril and/or valsartan on renal disease in salt-sensitive hypertension

Diuretics and renin-angiotensin system blockers are often insufficient to control the blood pressure (BP) in salt-sensitive (SS) subjects. Abundant data support the proposal that the level of atrial natriuretic peptide may correlate with the pathogenesis of SS hypertension. We hypothesized here that increasing atrial natriuretic peptide levels with sacubitril, combined with renin-angiotensin system blockage by valsartan, can be beneficial for alleviation of renal damage in a model of SS hypertension, the Dahl SS rat. To induce a BP increase, rats were challenged with a high-salt 4% NaCl diet for 21 days, and chronic administration of vehicle or low-dose sacubitril and/or valsartan (75 μg/day each) was performed. Urine flow, Na⁺ excretion, and water consumption were increased on the high-salt diet compared with the starting point (0.4% NaCl) in all groups but remained similar among the groups at the end of the protocol. Upon salt challenge, we observed a mild decrease in systolic BP and urinary neutrophil gelatinase-associated lipocalin levels (indicative of alleviated tubular damage) in the valsartan-treated groups. Sacubitril, as well as sacubitril/valsartan, attenuated the glomerular filtration rate decline induced by salt. Alleviation of protein cast formation and lower renal medullary fibrosis were observed in the sacubitril/valsartan- and valsartan-treated groups, but not when sacubitril alone was administered. Interestingly, proteinuria was mildly mitigated only in rats that received sacubitril/valsartan. Further studies of the effects of sacubitril/valsartan in the setting of SS hypertension, perhaps involving a higher dose of the drug, are warranted to determine if it can interfere with the progression of the disease.

Comprehensive assessment of mitochondrial respiratory function in freshly isolated nephron segments

Changes in mitochondrial function are central to many forms of kidney disease, including acute injury, diabetic nephropathy, hypertension, and chronic kidney diseases. As such, there is an increasing need for reliable and fast methods for assessing mitochondrial respiratory function in renal cells. Despite being indispensable for many mechanistic studies, cultured cells or isolated mitochondria, however, often do not recapitulate in vivo or close to in vivo situations. Cultured and/or immortalized cells often change their bioenergetic profile and phenotype compared with in vivo or ex vivo situations, and isolated mitochondria are simply removed from their cellular milieu. This is especially important for extremely complex organs such as the kidney. Here, we report the development and validation of a new approach for the rapid assessment of mitochondrial oxygen consumption on freshly isolated glomeruli or proximal tubular fragments using Agilent SeaHorse XFe24 and XF96 Extracellular Flux Analyzers. We validated the technique in several healthy and diseased rodent models: the C57BL/6J mouse, the diabetic db/db mouse and matching db/+ control mouse, and the Dahl salt-sensitive rat. We compared the data to respiration from isolated mitochondria. The method can be adapted and used for the rapid assessment of mitochondrial oxygen consumption from any rodent model of the investigator's choice. The isolation methods presented here ensure viable and functional proximal tubular fragments and glomeruli, with a preserved cellular environment for studying mitochondrial function within the context of their surroundings and interactions.

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.

Regulation of mitochondria function by natriuretic peptides

Natriuretic peptides (NPs) are well known to promote renal Na⁺ excretion, counteracting the effects of the renin-angiotensin-aldosterone system. Thus, NPs serve as a key component in the maintenance of blood pressure, influencing fluid retention capabilities via osmoregulation. Recently, NPs have been shown to affect lipolysis and enhance lipid oxidation and mitochondrial respiration. Here, we provide an overview of current knowledge about the relationship between NPs and mitochondria-mediated processes such as reactive oxygen species production, Ca²⁺ signaling, and apoptosis. Establishing a clear physiological and mechanistic connection between NPs and mitochondria in the cardiovascular system will open new avenues of research aimed at understanding and potentially using it as a therapeutic target from a completely new angle.

Abstract P2049: Mitochondrial Respiration And Biogenesis In The Glomeruli Of Dahl Ss Rats

The mechanisms of salt-sensitive (SS) hypertension are complex and far from clear. Available medications are insufficient to control blood pressure (BP) in the SS subjects, and there is a need for the development of novel effective therapies. SS hypertension is accompanied with an early onset of proteinuria, which results from the loss of podocytes. Many reports highlight that in this disease ultrastructural abnormalities and deficient metabolism in the renal mitochondria may precede histological injury. We hypothesized that podocyte damage occurs in part due to mitochondria dysfunction.

Dahl SS rats were used here as an established animal model of SS hypertension. All rats were fed a 0.4% NaCl (normal salt, NS) diet until 8 weeks of age when they were challenged with a high salt (HS) 4% NaCl diet for 21 days. At the end of the protocol BP was measured, kidneys were cleared from blood, tissues were collected, and cortical glomeruli were isolated and subjected to imaging, WB and other applications.

Systolic BP was elevated in the HS diet fed rats (171±9 vs 148±6 mmHg in HS vs NS diet fed rats). HS diet fed rats exhibited renal lesions, kidney hypertrophy, decreased GFR and exacerbated glomeruli damage compared to the NS group. We did not observe changes in the renal cortical expression of PGC1α; cortical mtDNA content (ND1 and ND6 gene expression vs genomic DNA) was not affected by a HS diet. Seahorse assay performed on freshly isolated glomeruli revealed that basal mitochondria respiration, maximal respiration, and spare respiratory capacity were lower in the HS group (28 ± 14%, 47 ± 15% and 37 ± 9% lower, respectively, vs NS group). No difference was observed in ATP production, or mitochondrial Complex V (ATPase) activity (expressed as oxidation rate of NADH and measured in glomerular pellets). Using confocal imaging and mitoPY1 staining we detected higher baseline H 2 O 2 level, lower antioxidant capacity, and fragmented mitochondria in the podocytes of the glomeruli isolated from HS diet fed group. EM analysis showed that HS mitochondria have structural abnormalities (swelling, enlargement, less defined cristae, and reduced number). We conclude that glomerular mitochondria in SS hypertension are functionally and structurally impaired.

CD8+ T-cells negatively regulate inflammation post-myocardial infarction

The adaptive immune response is key for cardiac wound healing post-myocardial infarction (MI) despite low T-cell numbers. We hypothesized that CD8⁺ T-cells regulate the inflammatory response, leading to decreased survival and cardiac function post-MI. We performed permanent occlusion of the left anterior descending coronary artery on C57BL/6J and CD8a^tm1mak mice (deficient in functional CD8⁺ T-cells). CD8a^tm1mak mice had increased survival at 7 days post-MI compared with that of the wild-type (WT) and improved cardiac physiology at day 7 post-MI. Despite having less mortality, 100% of the CD8a^tm1mak group died because of cardiac rupture compared with only 33% of the WT. Picrosirius red staining and collagen immunoblotting indicated an acceleration of fibrosis in the infarct area as well as remote area in the CD8a^tm1mak mice; however, this increase was due to elevated soluble collagen implicating poor scar formation. Plasma and tissue inflammation were exacerbated as indicated by higher levels of Cxcl1, Ccl11, matrix metalloproteinase (MMP)-2, and MMP-9. Immunohistochemistry and flow cytometry indicated that the CD8a^tm1mak group had augmented numbers of neutrophils and macrophages at post-MI day 3 and increased mast cell markers at post-MI day 7. Cleavage of tyrosine-protein kinase MER was increased in the CD8a^tm1mak mice, resulting in delayed removal of necrotic tissue. In conclusion, despite having improved cardiac physiology and overall survival, CD8a^tm1mak mice had increased innate inflammation and poor scar formation, leading to higher incidence of cardiac rupture. Our data suggest that the role of CD8⁺ T-cells in post-MI recovery may be both beneficial and detrimental to cardiac remodeling and is mediated via a cell-specific mechanism.NEW & NOTEWORTHY We identified new mechanisms implicating CD8⁺ T-cells as regulators of the post-myocardial infarction (MI) wound healing process. Mice without functional CD8⁺ T-cells had improved cardiac physiology and less mortality 7 days post MI compared with wild-type animals. Despite having better overall survival, animals lacking functional CD8⁺ T-cells had delayed removal of necrotic tissue, leading to poor scar formation and increased cardiac rupture, suggesting that CD8⁺ T-cells play a dual role in the cardiac remodeling process.