A roadmap for the genetic analysis of renal aging

Multi-omics reveals the role of MCM2 and hnRNP K phosphorylation in mouse renal aging through genomic instability

The process of aging is characterized by structural degeneration and functional decline, as well as diminished adaptability and resistance. The aging kidney exhibits a variety of structural and functional impairments. In aging mice, thinning and graying of fur were observed, along with a significant increase in kidney indices compared to young mice. Biochemical indicators revealed elevated levels of creatinine, urea nitrogen and serum uric acid, suggesting impaired kidney function. Histological analysis unveiled glomerular enlargement and sclerosis, severe hyaline degeneration, capillary occlusion, lymphocyte infiltration, tubular and glomerular fibrosis, and increased collagen deposition. Observations under electron microscopy showed thickened basement membranes, altered foot processes, and increased mesangium and mesangial matrix. Molecular marker analysis indicated upregulation of aging-related β-galactosidase, p16-INK4A, and the DNA damage marker γH2AX in the kidneys of aged mice. In metabolomics, a total of 62 significantly different metabolites were identified, and 10 pathways were enriched. We propose that citrulline, dopamine, and indoxyl sulfate have the potential to serve as markers of kidney damage related to aging in the future. Phosphoproteomics analysis identified 6656 phosphosites across 1555 proteins, annotated to 62 pathways, and indicated increased phosphorylation at the Ser27 site of Minichromosome maintenance complex component 2 (Mcm2) and decreased at the Ser284 site of heterogeneous nuclear ribonucleoprotein K (hnRNP K), with these modifications being confirmed by western blotting. The phosphorylation changes in these molecules may contribute to aging by affecting genome stability. Eleven common pathways were detected in both omics, including arginine biosynthesis, purine metabolism and biosynthesis of unsaturated fatty acids, etc., which are closely associated with aging and renal insufficiency.

Aging and urinary control: Alterations in the brain-bladder axis

Age-associated alterations in bladder control affect millions of older adults, with a heavy burden added to families both economically and in quality of life. Therapeutic options are limited with poor efficacy in older adults, lending to a growing need to address the gaps in our current understanding of urinary tract aging. This review summarizes the current knowledge of age-associated alterations in the structure and function of the brain-bladder axis and identifies important gaps in the field that have yet to be addressed. Urinary aging is associated with decreased tissue responsiveness, decreased control over the voiding reflex, signaling dysfunction along the brain-bladder axis, and structural changes within the bladder wall. Studies are needed to improve our understanding of how age affects the brain-bladder axis and identify genetic targets that correlate with functional outcomes.

Mitochondrial homeostasis: a potential target for delaying renal aging

Mitochondria, which are the energy factories of the cell, participate in many life activities, and the kidney is a high metabolic organ that contains abundant mitochondria. Renal aging is a degenerative process associated with the accumulation of harmful processes. Increasing attention has been given to the role of abnormal mitochondrial homeostasis in renal aging. However, the role of mitochondrial homeostasis in renal aging has not been reviewed in detail. Here, we summarize the current biochemical markers associated with aging and review the changes in renal structure and function during aging. Moreover, we also review in detail the role of mitochondrial homeostasis abnormalities, including mitochondrial function, mitophagy and mitochondria-mediated oxidative stress and inflammation, in renal aging. Finally, we describe some of the current antiaging compounds that target mitochondria and note that maintaining mitochondrial homeostasis is a potential strategy against renal aging.

Genetic background determines renal response to chronic lithium treatment in female mice

Background Chronic lithium treatment for bipolar disease causes mainly side effects in the kidney. A subset of lithium users develops nephrogenic diabetes insipidus (NDI), a urinary concentrating disorder, and chronic kidney disease (CKD). Age, lithium dose and duration of treatment are important risk factors, while genetic background might also play an important role. Methods In order to investigate the role of genetics, female mice of 29 different inbred strains were treated for one year with control or lithium chow and urine, blood and kidneys were analysed. Results Chronic lithium treatment increased urine production and/or reduced urine osmolality in 21 strains. Renal histology showed that lithium increased interstitial fibrosis and/or tubular atrophy in eight strains, while in none of the strains glomerular injury was induced. Interestingly, lithium did not elevate urinary albumin-creatinine ratio (ACR) in any strain, while eight strains even demonstrated a lowered ACR. The protective effect on ACR coincided with a similar decrease in urinary IgG levels, a marker of glomerular function, while the adverse effect of lithium on interstitial fibrosis/tubular atrophy coincided with a severe increase in urinary β2-microglobulin (B2M) levels, an indicator of proximal tubule damage. Conclusion Genetic background plays an important role in the development of lithium-induced NDI and chronic renal pathology in female mice. The strong correlation of renal pathology with urinary B2M levels indicates B2M as a promising biomarker for chronic renal damage induced by lithium.

Kidney and hypertension in older adults

Individuals over 65years are the fastest expanding population throughout the world, due to the increase in human life expectancy. This growing geriatric population, with increasingly associated chronic diseases, has relevant medical, social, and economic impact. Aging is characterized by progressive structural and functional changes in the kidney and in the cardiovascular system, leading to decline in renal function and hypertension. The purpose of this review is to describe the aging-related renal changes and blood pressure regulation during the aging process, their impact with regards to morbidity and mortality in these patients, and to summarize the most appropriate therapeutic approaches in these situations based on the particularities of this population group.

Frailty and the risk of kidney function decline in the elderly population: the Rugao Longevity and Ageing Study

BACKGROUND

The diverse risk factors for kidney impairments suggest that kidney function decline is more likely to occur in individuals with a broadly constituted health deficit. Here we conducted a longitudinal cohort study to evaluate the association of baseline frailty status with the risk of estimated glomerular filtration rate (eGFR) decline.

METHODS

Overall, 1269 participants aged 70-84 years from Rugao Longevity and Ageing cohort with 3-year follow-up were included. Frailty was measured using a modified Fried frailty assessment. GFR was estimated using the Chronic Kidney Disease Epidemiology Collaboration equation. Associations between baseline frailty status and rapid eGFR decline were examined by multinomial logistic analysis. A linear mixed-effect model was used to determine eGFR decline in mL/min/1.73 m2 over the study period comparing those with frail or prefrail at baseline versus those with robust status.

RESULTS

The mean (± standard deviation) age of participants was 75.1 ± 3.8 years. A total of 144 (11%) participants had rapid eGFR decline by at least 10% during the 3-year follow-up. Compared with robust status, baseline frail status was associated with a 2.48-fold [95% confidence interval (CI) 1.24-4.95] increased risk of rapid eGFR decline after multiple adjustments. In multivariate linear mixed model analysis, subjects with frail status but not prefrail status at baseline had a significant coefficient of -1.70 (95% CI -3.35 to -0.04) for the frail × visit term, which indicates an accelerated eGFR decline compared with robust subjects over the study period (P = 0.044).

CONCLUSIONS

Frailty may serve as an independent biomarker to predict the decline of kidney function.

Global transcriptomic changes occur in aged mouse podocytes

Glomerular podocytes undergo structural and functional changes with advanced age, that increase susceptibility of aging kidneys to worse outcomes following superimposed glomerular diseases. To delineate transcriptional changes in podocytes in aged mice, RNA-seq was performed on isolated populations of reporter-labeled (tdTomato) podocytes from multiple young (two to three months) and advanced aged mice (22 to 24 months, equivalent to 70 plus year old humans). Of the 2,494 differentially expressed genes, 1,219 were higher and 1,275 were lower in aged podocytes. Pathway enrichment showed that major biological processes increased in aged podocytes included immune responses, non-coding RNA metabolism, gene silencing and MAP kinase signaling. Conversely, aged podocytes showed downregulation of developmental, morphogenesis and metabolic processes. Canonical podocyte marker gene expression decreased in aged podocytes, with increases in apoptotic and senescence genes providing a mechanism for the progressive loss of podocytes seen with aging. In addition, we revealed aberrations in the podocyte autocrine signaling network, identified the top transcription factors perturbed in aged podocytes, and uncovered candidate gene modulations that might promote healthy aging in podocytes. The transcriptional signature of aging is distinct from other kidney diseases. Thus, our study provides insights into biomarker discovery and molecular targeting of the aging process itself within podocytes.

Identification of proteins potentially associated with renal aging in rats

We established a young (Y)-old (O) rat kidney transplantation model. With this model, we detected no age-related differences in renal structure between Y→Y and Y→O kidneys or O→O and O→Y kidneys. However, we did detect differences in levels of the senescence markers β-gal and p16 as well as the inflammatory cytokines TNF-α and IL-1β. Using proteomics analysis we detected 66 proteins associated with suppression of aging and 73 proteins associated with enhancement of aging. After construction of a protein-protein interaction network, a total of 73 nodes and 99 edges were analyzed using MCODE, and three significant modules were selected. GO and KEGG analyses showed that these proteins were mainly located in mitochondria and were largely related to oxidative stress. Among them, SOD1 expression was lower in Y→O than Y→Y kidneys and higher in O→Y than O→O kidneys. Acetylated (Ac)-NF-κB showed the opposite expression profile. In addition, SOD1 expression was higher in primary tubular epithelial cells from young rats than old rats, and SOD1 knockdown led to increased Ac-NF-κB expression. These findings suggest the local renal environment, particularly oxidative stress/mitochondrial function, affects renal aging.

Serum Stem Cell Factor Level Predicts Decline in Kidney Function in Healthy Aging Adults

BACKGROUND AND OBJECTIVES

Stem cell factor (SCF), the ligand of the c-kit receptor, actively participates in the organ reconstruction and fibrosis associated with various diseases, including kidney disease. However, it remains unclear whether SCF plays a role in kidney aging.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

In the present study, we measured the serum SCF level, estimated glomerular filtration rate (eGFR), and other biological parameters in a Chinese Han group of 892 subjects, and explored the relationship between SCF level and renal function during aging; we sought to define novel biomarkers of kidney aging.

RESULTS

Multiple linear regression was used to select potential indicators of decline in renal function. Only age, SCF level, and 25% maximum expiratory flow (25% MEF) were significant predictors after redundancy analysis (|r| > 0.70 and P < 0.05). Multiple linear regression showed that the relationship among eGFR, SCF level, and age could be described as follows: eGFR = 154.486 - (0.846 × age) - (0.011 × SCF level).

CONCLUSIONS

We found no between-gender difference in the effect of SCF on kidney aging. In conclusion, the SCF level is an ideal biomarker of renal aging and may help to predict changes in eGFR during aging.

Genetic risk score of common genetic variants for impaired fasting glucose and newly diagnosed type 2 diabetes influences oxidative stress

We tested the hypothesis that the cumulative effects of common genetic variants related to elevated fasting glucose are collectively associated with oxidative stress. Using 25 single nucleotide polymorphisms (SNPs), a weighted genetic risk score (wGRS) was constructed by summing nine risk alleles based on nominal significance and a consistent effect direction in 1,395 controls and 718 patients with impaired fasting glucose (IFG) or newly diagnosed type 2 diabetes. All the participants were divided into the following three groups: low-wGRS, middle-wGRS, and high-wGRS groups. Among the nine SNPs, five SNPs were significantly associated with IFG and type 2 diabetes in this Korean population. wGRS was significantly associated with increased IFG and newly diagnosed type 2 diabetes (p = 6.83 × 10⁻¹⁴, odds ratio = 1.839) after adjusting for confounding factors. Among the IFG and type 2 diabetes patients, the fasting serum glucose and HbA_1c levels were significantly higher in the high-wGRS group than in the other groups. The urinary 8-epi-PGF_2α and malondialdehyde concentrations were significantly higher in the high-wGRS group than in the other groups. Moreover, general population-level instrumental variable estimation (using wGRS as an instrument) strengthened the causal effect regarding the largely adverse influence of high levels of fasting serum glucose on markers of oxidative stress in the Korean population. Thus, the combination of common genetic variants with small effects on IFG and newly diagnosed type 2 diabetes are significantly associated with oxidative stress.

Renal Aging: Causes and Consequences

Individuals age >65 years old are the fastest expanding population demographic throughout the developed world. Consequently, more aged patients than before are receiving diagnoses of impaired renal function and nephrosclerosis-age-associated histologic changes in the kidneys. Recent studies have shown that the aged kidney undergoes a range of structural changes and has altered transcriptomic, hemodynamic, and physiologic behavior at rest and in response to renal insults. These changes impair the ability of the kidney to withstand and recover from injury, contributing to the high susceptibility of the aged population to AKI and their increased propensity to develop subsequent progressive CKD. In this review, we examine these features of the aged kidney and explore the various validated and putative pathways contributing to the changes observed with aging in both experimental animal models and humans. We also discuss the potential for additional study to increase understanding of the aged kidney and lead to novel therapeutic strategies.