Evidence for Sex-dependent Bmal1 Control of Mitochondrial Dynamics in Rat Kidney

Abstract

The circadian clock regulates Na+ transport with a necessary diurnal variation for optimal health. The clock gene, Bmal1, is known to regulate mitochondrial O2 consumption, but whether or how this occurs in the kidney has not been established. An estimated 95% of O2 consumption in the kidney is linked to sodium transport. We developed and used a novel Bmal1 knockout rat on a Sprague Dawley background. Male Bmal1 -/- rats do not have the typical night-day difference in Na+ excretion, while female rats maintain this pattern. We recently observed that ADP-dependent O2 consumption in permeabilized kidney tissue was significantly higher in both sexes of Bmal1 -/- rats but only during the active/dark period. We also observed a significant increase in cytochrome c oxidase activity in Bmal1 -/- rats during the dark vs. light period (164 ± 40 vs. 306 ± 40 pmol/s·mg; p=0.0031, t-test). Since these effects were observed in both male and female rats, these changes in mitochondrial activity cannot explain the difference in diurnal sodium excretion. Since mitochondrial morphology can impact oxidative phosphorylation and energy availability, we hypothesized that Bmal1 regulates mitochondrial morphology in the kidney. Male and female global Bmal1 +/+ and Bmal1 -/- rats at 12-14 weeks of age were maintained in regular 12:12 LD cycles. Outer renal medullary tissue was dissected and prepared for mRNA expression and transmission electron microscopy (EM). For EM, tissues were obtained at light (ZT2-4) or dark (ZT14-16) periods to correspond with the minimum and maximum whole-body energy consumption and peak and trough Bmal1 protein expression. Digital droplet PCR mRNA expression was conducted from tissue collected at 4hr intervals over 24hrs. EM images appear to show an elongation of mitochondrial morphology in females but not male Bmal1 -/- during the active time of day. Mitofusion1 (Mfn1) and fission 1 (Fis1) are genes that regulate mitochondrial structural dynamics. Mfn1 and Fis1 mRNA expression over 24hrs showed a significant interaction between genotype and time of day in male rats (p=0.004 and p=0.020, respectively, 2way ANOVA). In contrast, there were no significant differences in overall Mfn1 or Fis1 expression between female Bmal1 +/+ and Bmal1 -/- rats. Our findings demonstrate that Bmal1 is crucial in maintaining mitochondrial respiration coupling and ATP generation in the kidney. Furthermore, we suggest that female Bmal1 -/- rats can better compensate for impaired respiration by mitochondrial morphological changes when the molecular clock is dysfunctional. We further propose that the rhythmicity of Na+ excretion in females, but not male, Bmal1 -/- rats is due to adjustments in mitochondrial morphology. T32 HL007457, P01HL136267, AHA 908953 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.