Sexual dimorphism of the aging kidney: role of nitric oxide deficiency

Guidelines for sex-specific considerations to improve rigor in renal research and how we got there

Biological sex significantly influences disease presentation, progression, and therapeutic outcomes in chronic kidney disease and acute kidney injury. Sex hormones, including estrogen and testosterone, modulate key renal functions, including renal blood flow, glomerular filtration, and electrolyte transport, thereby affecting disease trajectory in a sex-specific manner. It is critical for researchers to understand why and how to integrate sex as a biological variable in data collection, analysis, and reporting. Integrating a sex-based perspective in kidney research will lead to more personalized and efficacious treatment strategies, optimizing therapeutic interventions for each sex. If addressed properly, the incorporation of sex as a biological variable (SABV) in renal research not only enhances the mechanistic understanding of renal disease, but also paves the way for precision medicine, promising improved clinical outcomes, and tailored treatment protocols for all patients. This paper is designed to serve as a guideline for researchers interested in rigorously incorporating sex as a biological variable in their studies.

Unraveling Sex Differences in Kidney Health and CKD: A Review of the Effect of Sex Hormones

Sexual dimorphism plays an important role in the pathogenesis and progression of CKD. Men with CKD often exhibit faster kidney function decline, leading to higher rates of kidney failure and mortality compared with women. Studies suggest that sex hormones may influence this apparent dimorphism, although the mechanisms underlying these influences remain poorly understood. In this review, we first summarize recent findings on sex differences in the prevalence and progression of CKD. Subsequently, we will focus on ( 1 ) the role of sex hormones in these sex differences, ( 2 ) kidney structural and hemodynamic differences between men and women, ( 3 ) the influence of sex hormones on pathophysiological processes leading to kidney disease, including glomerular hyperfiltration and key pathways involved in kidney inflammation and fibrosis, and finally, focus on the consequences of the underrepresentation of women in clinical trials. Understanding these sex differences is critical for advancing precision medicine and improving outcomes for both men and women with CKD.

Self-controlled Case Series Study for Acute Kidney Injury after Starting Proton Pump Inhibitors or Potassium-Competitive Acid Blocker in Patients with Cancer Using a Large Claims Database

To investigate the risk of acute kidney injury (AKI) in patients with cancer following the initiation of proton pump inhibitors (PPIs) and potassium-competitive acid blocker (PCAB), considering sex and anti-cancer drug use. We conducted a self-controlled case-series study using the Japan Medical Data Center claims data from 12422 patients with cancer who were prescribed PPIs or PCAB between January 2017 and December 2019. Considering the timing of PPI or PCAB, control period (days -120 to -1), risk period 1 (days 0 to +30), and risk period 2 (days +31 to +365) were defined. To assess the incidence rate ratio (IRR) and 95% confidence interval (CI) as the risk ratio, we adjusted for anti-cancer drugs to assess the risk of AKI. Additionally, we also examined sex differences to identify the risk of AKI. AKI was observed in risk period 1 [2.05 (1.12-3.72), p = 0.0192], but a slight reduction was noted in risk period 2 [0.60 (0.36-1.00), p = 0.0481]. A sex-specific increase in the risk of AKI was observed only in males during risk period 1 [2.18 (1.10-4.32), p = 0.0260], with a reduction in risk period 2 [0.48 (0.26-0.89), p = 0.0200]. We identified an increased risk of AKI in patients with cancer starting PPIs or PCAB particularly in males within 30 d after PPI or PCAB initiation, emphasizing the need for vigilant monitoring and management of AKI in this patient population.

Multiomics Analyses Reveal Sex Differences in Mouse Renal Proximal Subsegments

SIGNIFICANCE STATEMENT

Sex-dependent differences in kidney function are recognized but the underlying molecular mechanisms are largely unexplored. Advances in genomics and proteomic technologies now allow extensive characterization of differences between the same cell types of males and females. Multiomics integrating RNA-seq, ATAC-seq, and proteomics data to investigate differences in gene expression, chromatin accessibility, and protein expression in proximal tubules of male and female mice identified many sex-biased genes and proteins associated with kidney functions, including metabolic and transport processes. Sex differences may also arise from variations of the interaction between transcription factors and accessible chromatin regions. A comprehensive web resource is provided to advance understanding of sex differences in cells of the proximal tubule.

BACKGROUND

Sex differences have been increasingly recognized as important in kidney physiology and pathophysiology, but limited resources are available for comprehensive interrogation of sex differences.

METHODS

RNA-seq and ATAC-seq of microdissected mouse proximal tubules and protein mass spectrometry of homogenized perfused mouse kidneys reveal differences in proximal tubule cells of males and females.

RESULTS

The transcriptomic data indicated that the major differences in the proximal tubules between the sexes were in the S2/S3 segments, and most of the sex-biased transcripts mapped to autosomes rather than to the sex chromosomes. Many of the transcripts exhibiting sex-biased expression are involved in monocarboxylic acid metabolic processes, organic anion transport, and organic acid transport. The ATAC-seq method on microdissected tubules captured chromatin accessibility. Many of the more than 7000 differentially accessible DNA regions identified were in distal regions. Motif analyses revealed a lack of direct involvement of estrogen receptors or the androgen receptor (absence of canonical hormone response elements), suggesting an indirect regulatory role of sex hormones. Instead, analyses identified several transcription factors (TFs) ( Tead1 , Nfia/b , and Pou3f3 ) whose interplay with proximal tubule-specific TFs ( e.g. , Hnf1b , Hnf4a ) may contribute to sex differences. Finally, the whole-kidney proteome was correlated with the transcriptome, and many sex-biased proteins ( e.g. , Cyp2e1, Acsm2/3) were identified.

CONCLUSIONS

Sex-dependent cis-regulatory elements interact with TFs in ways that lead to sex-biased gene expression in proximal tubule cells. These data are provided as a user-friendly web page at https://esbl.nhlbi.nih.gov/MRECA/PT/ .

Gender Differences in the Association between Physical Activity and Mortality in Chronic Kidney Disease: Results from the National Health and Nutrition Examination Survey (2011-2018)

BACKGROUND

Physical activity is indispensable in the management of chronic kidney disease (CKD). The aim of this study was to investigate gender difference in the association of physical activity with mortality among the CKD population.

METHODS

In total, 3701 participants with CKD from the 2011 to 2018 NHANES with linked mortality data were classified into different groups based on the intensity of self-reported physical activity. Multivariable-adjusted Cox proportional hazards models were used to examine the associations between physical activity and mortality.

RESULTS

During the median follow-up of 53.7 months, 694 all-cause deaths and 226 cardiovascular deaths were recorded. Patients were categorized into extremely highly active (>1500 MET-min/week), highly active (>600, ≤1500 MET-min/week), low-active (>0, ≤600 MET-min/week), or inactive (0 MET-min/week) groups. Among males, the multivariable Cox regression showed that the low-active group (HR, 0.67; 95% CI, 0.48-0.93) and highly active group (HR, 0.60; 95% CI, 0.41-0.88) were independently associated with lower risks for all-cause mortality, compared to the inactive group. The risks of all-cause mortality did not further decrease once physical activity surpassed 1500 MET-min/week, indicating a U-shaped association in males. In females, only the extremely highly active group (>1500 MET-min/week) was significantly associated with a mortality risk compared to inactivity (HR, 0.59; 95% CI, 0.39-0.89).

CONCLUSIONS

Any amount of physical activity is associated with reduced all-cause mortality in male CKD participants, while in female patients, only the extremely highly active group shows the significant association.

Sex Differences in Age-Related Loss of Kidney Function

BACKGROUND

CKD is more prevalent in women, but more men receive kidney replacement therapy for kidney failure. This apparent contradiction is not well understood.

METHODS

We investigated sex differences in the loss of kidney function and whether any sex disparities could be explained by comorbidity or CKD risk factors. In the Renal Iohexol Clearance Survey (RENIS) in northern Europe, we recruited 1837 persons (53% women, aged 50-62 years) representative of the general population and without self-reported diabetes, CKD, or cardiovascular disease. Participants' GFR was measured by plasma iohexol clearance in 2007-2009 (n=1627), 2013-2015 (n=1324), and 2018-2020 (n=1384). At each study visit, healthy persons were defined as having no major chronic diseases or risk factors for CKD. We used generalized additive mixed models to assess age- and sex-specific GFR decline rates.

RESULTS

Women had a lower GFR than men at baseline (mean [SD], 90.0 [14.0] versus 98.0 [13.7] ml/min per 1.73 m2; P<0.001). The mean GFR change rate was -0.96 (95% confidence interval [CI], -0.88 to -1.04) ml/min per 1.73 m2 per year in women and -1.20 (95% confidence interval [CI], -1.12 to -1.28) in men. Although the relationship between age and GFR was very close to linear in women, it was curvilinear in men, with steeper GFR slopes at older ages (nonlinear effect; P<0.001). Healthy persons had a slower GFR decline, but health status did not explain the sex difference in the GFR decline.

CONCLUSION

Among middle-aged and elderly individuals in the general population, decline in the mean GFR in women was slower than in men, independent of health status.

Sex and the kidneys: current understanding and research opportunities

Concerns regarding sex differences are increasingly pertinent in scientific and societal arenas. Although biological sex and socio-cultural gender are increasingly recognized as important modulators of renal function under physiological and pathophysiological conditions, gaps remain in our understanding of the mechanisms underlying sex differences in renal pathophysiology, disease development, progression and management. In this Perspectives article, we discuss specific opportunities for future research aimed at addressing these knowledge gaps. Such opportunities include the development of standardized core data elements and outcomes related to sex for use in clinical studies to establish a connection between sex hormones and renal disease development or progression, development of a knowledge portal to promote fundamental understanding of physiological differences between male and female kidneys in animal models and in humans, and the creation of new or the development of existing resources and datasets to make them more readily available for interrogation of sex differences. These ideas are intended to stimulate thought and interest among the renal research community as they consider sex as a biological variable in future research projects.

Epochs in the depressor/pressor balance of the renin-angiotensin system

The renin-angiotensin system (RAS) plays a commanding role in the regulation of extracellular fluid homoeostasis. Tigerstadt and Bergman first identified the RAS more than two centuries ago. By the 1980s a voyage of research and discovery into the mechanisms and actions of this system led to the development of drugs that block the RAS, which have become the mainstay for the treatment of cardiovascular and renal disease. In the last 25 years new components of the RAS have come to light, including the angiotensin type 2 receptor (AT2R) and the angiotensin-converting enzyme 2 (ACE2)/angiotensin-(1-7) [Ang(1-7)]/Mas receptor (MasR) axis. These have been shown to counter the classical actions of angiotensin II (AngII) at the predominant angiotensin type 1 receptor (AT1R). Our studies, and those of others, have demonstrated that targeting these depressor RAS pathways may be therapeutically beneficial. It is apparent that the evolution of both the pressor and depressor RAS pathways is distinct throughout life and that the depressor/pressor balance of the RAS vary between the sexes. These temporal patterns of expression suggest that therapies targeting the RAS could be optimized for discrete epochs in life.

Progression of hypertension and kidney disease in aging fawn-hooded rats is mediated by enhanced influence of renin-angiotensin system and suppression of nitric oxide system and epoxyeicosanoids

The fawn-hooded hypertensive (FHH) rat serves as a genetic model of spontaneous hypertension associated with glomerular hyperfiltration and proteinuria. However, the knowledge of the natural course of hypertension and kidney disease in FHH rats remains fragmentary and the underlying pathophysiological mechanisms are unclear. In this study, over the animals' lifetime, we followed the survival rate, blood pressure (telemetry), indices of kidney damage, the activity of renin-angiotensin (RAS) and nitric oxide (NO) systems, and CYP450-epoxygenase products (EETs). Compared to normotensive controls, no elevation of plasma and renal RAS was observed in prehypertensive and hypertensive FHH rats; however, RAS inhibition significantly reduced systolic blood pressure (137 ± 9 to 116 ± 8, and 159 ± 8 to 126 ± 4 mmHg, respectively) and proteinuria (62 ± 2 to 37 ± 3, and 132 ± 8 to 87 ± 5 mg/day, respectively). Moreover, pharmacological RAS inhibition reduced angiotensin (ANG) II and increased ANG 1-7 in the kidney and thereby may have delayed the progression of kidney disease. Furthermore, renal NO and EETs declined in the aging FHH rats but not in the control strain. The present results, especially the demonstration of exaggerated vascular responsiveness to ANG II, indicate that RAS may contribute to the development of hypertension and kidney disease in FHH rats. The activity of factors opposing the development of hypertension and protecting the kidney declined with age in this model. Therefore, therapeutic enhancement of this activity besides RAS inhibition could be attempted in the therapy of human hypertension associated with kidney disease.

Antioxidant diet and sex interact to regulate NOS isoform expression and glomerular mesangium proliferation in Zucker diabetic rat kidney

Oxidative stress contributes substantially to the pathophysiology of diabetic nephropathy (DN). Consumption of an antioxidant-fortified (AO) diet from an early age prevents or delays later development of DN in the Zucker rat female with type 2 diabetes. We hypothesize this is due to effects on mesangial matrix and renal nitric oxide synthase (NOS) distribution and to sex-specific differences in NOS responses in the diabetic kidney. Total glomerular tuft area (GTA) and PAS-positive tuft area (PTA), endothelial (e), neuronal (n) and inducible (i) NOS were quantified in males and females on AO or regular (REG) diet at 6 and 20 weeks of age. eNOS was observed in glomeruli and tubules. nNOS predominantly localized to tubular epithelium in both cortex and medulla. iNOS was expressed in proximal and distal tubules and collecting ducts. Sex, diabetes duration and AO diet affected the distribution of the three isoforms. GTA and PTA increased with duration of hyperglycemia and showed a negative correlation with renal levels of all NOS isoforms. AO diet in both genders was associated with less PAS-positive staining and less mesangial expansion than the REG diet, an early increase in cortical iNOS in males, and sex-specific changes in cortical eNOS at 20 weeks. These effects of AO diet may contribute to sex-specific preservation of renal function in females.

Ovarian hormones and prolactin increase renal NaCl cotransporter phosphorylation

Unique situations in female physiology require volume retention. Accordingly, a dimorphic regulation of the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) has been reported, with a higher activity in females than in males. However, little is known about the hormones and mechanisms involved. Here, we present evidence that estrogens, progesterone, and prolactin stimulate NCC expression and phosphorylation. The sex difference in NCC abundance, however, is species dependent. In rats, NCC phosphorylation is higher in females than in males, while in mice both NCC expression and phosphorylation is higher in females, and this is associated with increased expression and phosphorylation of full-length STE-20 proline-alanine-rich kinase (SPAK). Higher expression/phosphorylation of NCC was corroborated in humans by urinary exosome analysis. Ovariectomy in rats resulted in decreased expression and phosphorylation of the cotransporter and promoted the shift of SPAK isoforms toward the short inhibitory variant SPAK2. Conversely, estradiol or progesterone administration to ovariectomized rats restored NCC phosphorylation levels and shifted SPAK expression and phosphorylation towards the full-length isoform. Estradiol administration to male rats induced a significant increase in NCC phosphorylation. NCC is also modulated by prolactin. Administration of this peptide hormone to male rats induced increased phosphorylation of NCC, an effect that was observed even using the ex vivo kidney perfusion strategy. Our results indicate that estradiol, progesterone, and prolactin, the hormones that are involved in sexual cycle, pregnancy and lactation, upregulate the activity of NCC.

Loss of a kidney during fetal life: long-term consequences and lessons learned

Epidemiological studies reveal that children born with a solitary functioning kidney (SFK) have a greater predisposition to develop renal insufficiency and hypertension in early adulthood. A congenital SFK is present in patients with unilateral renal agenesis or unilateral multicystic kidney dysplasia, leading to both structural and functional adaptations in the remaining kidney, which act to mitigate the reductions in glomerular filtration rate and sodium excretion that would otherwise ensue. To understand the mechanisms underlying the early development of renal insufficiency in children born with a SFK, we established a model of fetal uninephrectomy (uni-x) in sheep, a species that similar to humans complete nephrogenesis before birth. This model results in a 30% reduction in nephron number rather than 50%, due to compensatory nephrogenesis in the remaining kidney. Similar to children with a congenital SFK, uni-x sheep demonstrate a progressive increase in arterial pressure and a loss of renal function with aging. This review summarizes the compensatory changes in renal hemodynamics and tubular sodium handling that drive impairments in renal function and highlights the existence of sex differences in the functional adaptations following the loss of a kidney during fetal life.

Comparison of oxidative stress biomarkers in renal tissues of D-galactose induced, naturally aged and young rats

Ageing of kidneys is a clinical health issue of the society. Age-related renal insufficiency has important implications due to impaired redox homeostasis. We examined protein, DNA and lipid oxidation biomarkers as well as protein-bound sialic acid (SA) in the kidney tissues of D-galactose induced ageing rats, naturally aged rats and their corresponding young control group. Intraperitoneal injection of D-galactose (60 mg/kg/day) for 6 weeks to young male Sprague-Dawley rats (20-week-old) was used to establish mimetic ageing model. In this study, we investigated the levels of protein carbonyl groups (PCO), various thiol fractions such as total thiol groups (T-SH), protein (P-SH) and non-protein thiol groups (NP-SH), lipid oxidation parameters such as lipid hydroperoxides (LHP) and malondialdehyde (MDA), SA and 8-hydroxy-2'deoxyguanosine (8-OHdG) parameters for comparison of naturally aged, induced aged and young rats. In D-galactose induced aged group, PCO, LHP, MDA, and 8-OHdG concentrations were significantly higher than young control group, whereas T-SH, P-SH levels were significantly lower than the young rats. In addition, NP-SH and SA concentrations were similar between the mimetic ageing and young control groups. In naturally ageing rats, PCO and MDA levels were significantly higher, whereas T-SH, P-SH, NP-SH concentrations were low compared to young controls. On the other hand, SA and 8-OHdG levels were not different between the naturally ageing group and the young control group. Our results demonstrated that the rats in the mimetic ageing group, have significant similarities with the naturally aged rats in terms of impaired redox homeostasis and can be used as a reliable animal model for renal ageing.

Prenatal betamethasone exposure alters renal function in immature sheep: sex differences in effects

Synthetic glucocorticoids are commonly given to pregnant women when premature delivery threatens. Antenatal administration of clinically relevant doses of betamethasone to pregnant sheep causes sex-specific compromises of renal function and increases in blood pressure in adult offspring. However, it is unclear whether such effects are present in immature lambs. Therefore, the aims of the present study were to determine whether antenatal betamethasone at 80-81 days of gestation increases blood pressure and adversely impacts renal function in adolescent ewes and rams. Prenatal steroid exposure increased blood pressure significantly in the young male (84 +/- 2 vs. 74 +/- 3 mmHg) and female sheep (88 +/- 5 vs. 79 +/- 4), but it did not alter basal glomerular filtration rate, renal blood flow (RBF), or sodium excretion in either sex. However, antenatal betamethasone exposure blocked increases in RBF (P = 0.001), and enhanced excretion of an acute Na load (P < 0.05) in response to systemic infusions of angiotensin (ANG)-(1-7) at 10 pmol.kg(-1).min(-1) in males. In females, the natriuretic response to combined ANG-(1-7), and Na load was significantly altered by prenatal betamethasone exposure. These findings indicate that blood pressure is increased in immature animals in response to antenatal steroid exposure and that sex-specific effects on renal function also exist. These changes may reflect greater risk for further loss of renal function with age.

Sexual dimorphism in the aging kidney: differences in the nitric oxide system

Females-both rats and women-are substantially protected against the age-dependent decrease in renal function that occurs in males of the species. In part, this finding reflects the cardioprotective and renoprotective effects of estrogens, but estrogen has multiple actions, not all of which are beneficial. In addition, the low androgen level in women might be protective against a decline in renal function, but animal and clinical data on possible adverse effects of androgens are controversial. Androgens also have multiple actions, one of which-aromatization to estrogen-is likely to be protective. Sex steroids clearly have many complex actions, which explains the conflicting information on their relative benefits and dangers. Endothelial nitric oxide (NO) deficiency contributes importantly to cardiovascular risk and intrarenal NO deficiency is clearly linked to chronic kidney disease progression in animal models. Endothelial dysfunction develops with increasing age but is delayed in females, correlating with a delayed rise in asymmetric dimethylarginine level. There is no clear link between aging and arginine (the NO synthase substrate) deficiency. Animal data suggest that the aging kidney develops NO deficiency as a result of changes in neuronal NO synthase. The increased oxidative stress that occurs with aging affects multiple stages of the NO biosynthetic pathway and results in decreased production and/or action of NO. NO production is better preserved in females than in males, partly as a result of the actions of estrogens.

Ageing mouse kidney--not always the SAME old story

BACKGROUND

As interest in the ageing kidney grows rapidly, more experimental ageing studies are conducted in the field. One of the main obstacles that researchers have to face is that studies in old animals are often less reproducible than in young animals. We have observed that the aged animal's provenance can be an overlooked factor accounting for such experimental heterogeneity.

METHODS

Male C57BL/6J mice aged 19-22 months were purchased from four different suppliers. Baseline renal parameters were evaluated by measuring serum urea, serum creatinine and proteinuria. Renal morphology was analysed by quantifying glomerulosclerosis, interstitial fibrosis and amyloid deposits on paraffin sections stained with PAS, Masson trichrome, Sirius red and Congo red.

RESULTS

We found normal renal ageing in mice from three sources, but an unexpected renal pathology in mice from one major European supplier. Mice from this supplier had significantly elevated serum urea, creatinine values and an increased urinary protein excretion. Corresponding kidneys displayed massive glomerulosclerosis with evidence of amyloid deposits and increased interstitial fibrosis.

CONCLUSIONS

Supplier-dependent differences, such as observed here, can explain irreproducibility of experimental results in renal ageing research. This can be avoided by careful baseline analysis prior to in vivo experiments.

Contribution of endothelin A receptors in endothelin 1-dependent natriuresis in female rats

Renal medullary endothelin B receptors contribute to blood pressure regulation by facilitating salt excretion. Premenopausal females have relatively less hypertension than males; therefore, we examined whether there is a sex difference in the natriuretic response to renal medullary infusion of endothelin peptides in the rat. All of the experiments were conducted in anesthetized wild-type (wt) or endothelin B-deficient (sl/sl) rats. Infusion of endothelin 1 (ET-1) significantly increased sodium excretion (U(Na)V) in female, but not male, wt rats (Delta U(Na)V: 0.41+/-0.07 versus -0.04+/-0.06 micromol/min, respectively). The endothelin B receptor agonist sarafotoxin 6c produced similar increases in U(Na)V in both male (Delta 0.58+/-0.15 micromol/min) and female (Delta 0.67+/-0.18 micromol/min) wt rats. Surprisingly, ET-1 markedly increased U(Na)V in female (Delta 0.70+/-0.11 micromol/min) but not male sl/sl rats (Delta 0.00+/-0.05 micromol/min). ET-1 had no effect on medullary blood flow in females, although medullary blood flow was significantly reduced to a similar extent in males of both strains. These results suggest that the lack of a natriuretic response to ET-1 in male rats is because of reductions in medullary blood flow. Treatment with ABT-627, an endothelin A receptor antagonist, or N(G)-propyl-L-arginine, an NO synthase 1 inhibitor, prevented the increase in U(Na)V observed in female rats. Gonadectomy eliminated the sex difference in the U(Na)V and medullary blood flow response to ET-1. These findings demonstrate that there is no sex difference in endothelin B-dependent natriuresis, and the endothelin A receptor contributes to ET-1-dependent natriuresis in female rats, an effect that requires NO synthase 1. These findings provide a possible mechanism for why premenopausal women are more resistant to salt-dependent hypertension.