The implications of anatomical and functional changes of the aging kidney: with an emphasis on the glomeruli

Age and association of kidney measures with mortality and end-stage renal disease

CONTEXT

Chronic kidney disease (CKD) is prevalent in older individuals, but the risk implications of low estimated glomerular filtration rate (eGFR) and high albuminuria across the full age range are controversial.

OBJECTIVE

To evaluate possible effect modification (interaction) by age of the association of eGFR and albuminuria with clinical risk, examining both relative and absolute risks.

DESIGN, SETTING, AND PARTICIPANTS

Individual-level meta-analysis including 2,051,244 participants from 33 general population or high-risk (of vascular disease) cohorts and 13 CKD cohorts from Asia, Australasia, Europe, and North/South America, conducted in 1972-2011 with a mean follow-up time of 5.8 years (range, 0-31 years).

MAIN OUTCOME MEASURES

Hazard ratios (HRs) of mortality and end-stage renal disease (ESRD) according to eGFR and albuminuria were meta-analyzed across age categories after adjusting for sex, race, cardiovascular disease, diabetes, systolic blood pressure, cholesterol, body mass index, and smoking. Absolute risks were estimated using HRs and average incidence rates.

RESULTS

Mortality (112,325 deaths) and ESRD (8411 events) risks were higher at lower eGFR and higher albuminuria in every age category. In general and high-risk cohorts, relative mortality risk for reduced eGFR decreased with increasing age; eg, adjusted HRs at an eGFR of 45 mL/min/1.73 m2 vs 80 mL/min/1.73 m2 were 3.50 (95% CI, 2.55-4.81), 2.21 (95% CI, 2.02-2.41), 1.59 (95% CI, 1.42-1.77), and 1.35 (95% CI, 1.23-1.48) in age categories 18-54, 55-64, 65-74, and ≥75 years, respectively (P <.05 for age interaction). Absolute risk differences for the same comparisons were higher at older age (9.0 [95% CI, 6.0-12.8], 12.2 [95% CI, 10.3-14.3], 13.3 [95% CI, 9.0-18.6], and 27.2 [95% CI, 13.5-45.5] excess deaths per 1000 person-years, respectively). For increased albuminuria, reduction of relative risk with increasing age was less evident, while differences in absolute risk were higher in older age categories (7.5 [95% CI, 4.3-11.9], 12.2 [95% CI, 7.9-17.6], 22.7 [95% CI, 15.3-31.6], and 34.3 [95% CI, 19.5-52.4] excess deaths per 1000 person-years, respectively by age category, at an albumin-creatinine ratio of 300 mg/g vs 10 mg/g). In CKD cohorts, adjusted relative hazards of mortality did not decrease with age. In all cohorts, ESRD relative risks and absolute risk differences at lower eGFR or higher albuminuria were comparable across age categories.

CONCLUSIONS

Both low eGFR and high albuminuria were independently associated with mortality and ESRD regardless of age across a wide range of populations. Mortality showed lower relative risk but higher absolute risk differences at older age.

Molecular events in matrix protein metabolism in the aging kidney

We explored molecular events associated with aging-induced matrix changes in the kidney. C57BL6 mice were studied in youth, middle age, and old age. Albuminuria and serum cystatin C level (an index of glomerular filtration) increased with aging. Renal hypertrophy was evident in middle-aged and old mice and was associated with glomerulomegaly and increase in mesangial fraction occupied by extracellular matrix. Content of collagen types I and III and fibronectin was increased with aging; increment in their mRNA varied with the phase of aging. The content of ZEB1 and ZEB2, collagen type I transcription inhibitors, and their binding to the collagen type Iα2 promoter by ChIP assay also showed age-phase-specific changes. Lack of increase in mRNA and data from polysome assay suggested decreased degradation as a potential mechanism for kidney collagen type I accumulation in the middle-aged mice. These changes occurred with increment in TGFβ mRNA and protein and activation of its SMAD3 pathway; SMAD3 binding to the collagen type Iα2 promoter was also increased. TGFβ-regulated microRNAs (miRs) exhibited selective regulation. The renal cortical content of miR-21 and miR-200c, but not miR-192, miR-200a, or miR-200b, was increased with aging. Increased miR-21 and miR-200c contents were associated with reduced expression of their targets, Sprouty-1 and ZEB2, respectively. These data show that aging is associated with complex molecular events in the kidney that are already evident in the middle age and progress to old age. Age-phase-specific regulation of matrix protein synthesis occurs and involves matrix protein-specific transcriptional and post-transcriptional mechanisms.

Accuracy of the MDRD (Modification of Diet in Renal Disease) study and CKD-EPI (CKD Epidemiology Collaboration) equations for estimation of GFR in the elderly

BACKGROUND

Glomerular filtration rate (GFR) is a measure of kidney function, commonly estimated using equations that adjust serum creatinine concentration for age, race, and sex. The Modification of Diet in Renal Disease (MDRD) Study equation is widely used, but underestimates GFR at higher levels. The serum creatinine-based Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI(cr)) equation generally provides more accurate estimation at GFR >60 mL/min/1.73 m(2). Newer equations have been reported using cystatin C concentration either alone (CKD-EPI(cys)) or in combination with creatinine concentration (CKD-EPI(cr-cys)). None of these equations has been well validated in older people. We tested the accuracy of these equations in people 74 years or older compared with GFR measured by a reference method.

STUDY DESIGN

Diagnostic test evaluation in a prospective cohort.

SETTING & PARTICIPANTS

Participants (n = 394; median age, 80 [range, 74-97] years) recruited from nephrology clinics and the community.

INDEX TEST

GFR estimated using the MDRD Study, CKD-EPI(cr), CKD-EPI(cys) and CKD-EPI(cr-cys) equations.

REFERENCE TEST

GFR measured using an iohexol clearance method.

RESULTS

Median measured GFR was 53.4 (range, 7.2-100.9) mL/min/1.73 m(2). MDRD Study-, CKD-EPI(cr)-, and CKD-EPI(cr-cys)-estimated GFRs overestimated GFR (median differences of 3.5 [P< 0.001], 1.7 [P < 0.001], and 0.8 [P = 0.02] mL/min/1.73 m(2), respectively); the CKD-EPI(cys) equation was unbiased. Accuracy (percentage of estimates within 30% of measured GFR [P(30)]) was 81%, 83%, 86%, and 86% for the MDRD Study, CKD-EPI(cr), CKD-EPI(cys), and CKD-EPI(cr-cys) equations, respectively. Accuracy of the MDRD Study equation was inferior (P = 0.004) to the CKD-EPI(cr) equation at GFR >60 mL/min/1.73 m(2).

LIMITATIONS

Those of non-European ancestry were not included. For practical reasons, only a 4-hour sampling protocol was used for iohexol clearance.

CONCLUSIONS

The CKD-EPI(cr) equation appeared less biased and was more accurate than the MDRD Study equation. No equation achieved an ideal P(30) in the overall population. Our data suggest that GFR estimation is as satisfactory in older people of European ancestry as it has been reported to be in younger individuals.

Proteinuria in the elderly: evaluation and management

While aging is accompanied by many age-related changes in renal physiology and function, proteinuria should not be considered to be a part of "normal aging". There are many age-prevalent illnesses that predispose one to developing proteinuria and early recognition, and treatment may help retard disease progression or offer an early cure. The presence of proteinuria warrants further evaluation and follow-up if one has any hope of avoiding its progression and delaying the initiation of treatment. This review article will discuss the anatomy and physiology of the aging kidney, the pathophysiology and etiology of proteinuria during later life, methods to evaluate proteinuria, and ways to monitor and manage this problem.

Aging in the glomerulus

Kidney function declines with age in the majority of the population. Although very few older people progress to end stage, the consequences of doing so are burdensome for the patient and very expensive for the society. Although some of the observed decline is likely due to changes in the vasculature, much is associated with the development of age-associated glomerulosclerosis. This article will review the well-established structural and functional changes in the glomerulus with age. The role of calorie restriction in modifying age-related pathology will be discussed. The importance of the podocyte as a critical cell in the aging process is considered using animal models and human biopsy material. Newer data on changes in gene expression driven by nuclear factor kappa beta (NFkB) and possible changes in biology in the glomerulus are discussed. The relationship between pathways involved in aging and the decline in kidney function is reviewed. There is speculation on the significance of these changes in relation to normal and pathological aging.