The effects of the atrial natriuretic peptide deficiency on renal cortical mitochondrial bioenergetics in the Dahl SS rat.

Atrial Natriuretic Peptide (ANP) plays an important role in blood pressure regulation. Low levels of ANP correlate with the development of salt-sensitive hypertension (SS-HTN). Our previous studies indicated that ANP deficiency exacerbated renal function decline in SS-HTN. In the heart and fat tissue, ANP was reported to affect lipid peroxidation and mitochondrial bioenergetics but the effects of ANP on mitochondrial function in the kidney are unexplored. We hypothesized that ANP deficiency in SS-HTN causes renal bioenergetic shift, leading to disruption of mitochondrial network and oxidative stress. To address the hypothesis, we placed Dahl SS wild-type (SSWT) and ANP knockout (SSNPPA-/-) rats on 4% NaCl high salt (HS) diet to induce HTN or maintained them on 0.4% NaCl normal salt (NS) diet and assessed mitochondrial bioenergetics and dynamics using spectrofluorimetry, Seahorse assay, electron paramagnetic resonance (EPR) spectroscopy, Western blotting, electron microscopy, PCR and cytokine assays. We report that under high salt conditions, associated with hypertension and renal damage, the SSNPPA-/- rats exhibit a decrease in mitochondrial membrane potential and elevation in mitochondrial ROS levels compared to SSWT. The redox shift is also evident by the presence of more pronounced medullar lipid peroxidation in the SSNPPA-/- strain. We also revealed fragmented, more damaged mitochondria in the SSNPPA-/- rats, accompanied by increased turnover and biogenesis. Overall, our data indicate that ANP deficiency causes disruptions in mitochondrial bioenergetics and dynamics which likely contributes to aggravation of the renal damage and hypertension in the Dahl SS rat; the major pathological effects are evident in the groups subjected to a combined salt and ANP deficiency-induced mitochondrial stress.

Cilia-deficient renal tubule cells are primed for injury with mitochondrial defects and aberrant tryptophan metabolism.

The exocyst and Ift88 are necessary for primary ciliogenesis. Overexpression of Exoc5 (OE), a central exocyst component, resulted in longer cilia and enhanced injury recovery. Mitochondria are involved in acute kidney injury (AKI). To investigate cilia and mitochondria, basal respiration and mitochondrial maximal and spare respiratory capacity were measured in Exoc5 OE, Exoc5 knockdown (KD), Exoc5 ciliary targeting sequence mutant (CTS-mut), control Madin-Darby canine kidney (MDCK), Ift88 knockout (KO), and Ift88 rescue cells. In Exoc5 KD, Exoc5 CTS-mut, and Ift88 KO cells, these parameters were decreased. In Exoc5 OE and Ift88 rescue cells they were increased. Reactive oxygen species were higher in Exoc5 KD, Exoc5 CTS-mut, and Ift88 KO cells compared with Exoc5 OE, control, and Ift88 rescue cells. By electron microscopy, mitochondria appeared abnormal in Exoc5 KD, Exoc5 CTS-mut, and Ift88 KO cells. A metabolomics screen of control, Exoc5 KD, Exoc5 CTS-mut, Exoc5 OE, Ift88 KO, and Ift88 rescue cells showed a marked increase in tryptophan levels in Exoc5 CTS-mut (113-fold) and Exoc5 KD (58-fold) compared with control cells. A 21% increase was seen in Ift88 KO compared with rescue cells. In Exoc5 OE compared with control cells, tryptophan was decreased 59%. To determine the effects of ciliary loss on AKI, we generated proximal tubule-specific Exoc5 and Ift88 KO mice. These mice had loss of primary cilia, decreased mitochondrial ATP synthase, and increased tryptophan in proximal tubules with greater injury following ischemia-reperfusion. These data indicate that cilia-deficient renal tubule cells are primed for injury with mitochondrial defects in tryptophan metabolism.NEW & NOTEWORTHY Mitochondria are centrally involved in acute kidney injury (AKI). Here, we show that cilia-deficient renal tubule cells both in vitro in cell culture and in vivo in mice are primed for injury with mitochondrial defects and aberrant tryptophan metabolism. These data suggest therapeutic strategies such as enhancing ciliogenesis or improving mitochondrial function to protect patients at risk for AKI.

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.