Senile nephrosclerosis--does it explain the decline in glomerular filtration rate with aging?

Cellular senescence and kidney aging

Life expectancy is increasing worldwide, and by 2050 the proportion of the world's population over 65 years of age is estimated to surpass 1.5 billion. Kidney aging is associated with molecular and physiological changes that cause a loss of renal function and of regenerative potential. As the aging population grows, it is crucial to understand the mechanisms underlying these changes, as they increase the susceptibility to developing acute kidney injury (AKI) and chronic kidney disease (CKD). Various cellular processes and molecular pathways take part in the complex process of kidney aging. In this review, we will focus on the phenomenon of cellular senescence as one of the involved mechanisms at the crossroad of kidney aging, age-related disease, and CKD. We will highlight experimental and clinical findings about the role of cellular senescence in kidney aging and CKD. In addition, we will review challenges in senescence research and emerging therapeutic aspects. We will highlight the great potential of senolytic strategies for the elimination of harmful senescent cells to promote healthy kidney aging and to avoid age-related disease and CKD. This review aims to give insight into recent discoveries and future developments, providing a comprehensive overview of current knowledge on cellular senescence and anti-senescent therapies in the kidney field.

Dietary Contributions to Metabolic Acidosis

Eating a net acid-producing diet can produce an "acid stress" of severity proportional to the diet net acid load, as indexed by the steady-state renal net acid excretion rate. Depending on how much acid or base is ingested or produced from endogenous metabolic processes and how well our homeostatic mechanisms can buffer or eliminate the additional acids or bases, we can alter our systemic acid-base balance. With increasing age, the kidney's ability to excrete daily net acid loads declines (a condition similar to that of mild CKD), invoking increased utilization of potential base stores (eg, bone, skeletal muscle) on a daily basis to mitigate the acid accumulation, thereby contributing to development of osteoporosis, loss of muscle mass, and age-related renal insufficiency. Patients suffering from more advanced CKD often present with more severe acid stress or metabolic acidosis, as the kidney can no longer excrete the entire acid load. Alkaline diets based on fruits and vegetables may have a positive effect on long-term preservation of renal function while maintaining nutritional status. This chapter discusses the biochemistry of dietary precursors that affect acid or base production.

Central fibrous area in the glomerular vascular pole consists of fibrous collagens and is associated with advanced age: a cross-sectional study

Background

For the optimal management of patients with both allograft kidneys and native kidney diseases, the recognition of the histological features associated with older age is important. This is because most pathological findings are non-specific. Central fibrous areas (CFAs) have recently been proposed to be age-related. However, the components of CFAs and whether CFAs are observed in various kidney diseases remain undetermined. This cross-sectional study was undertaken to clarify the histological features, epidemiology, and clinicopathological features of CFAs.

Methods

One hundred and one consecutive kidney needle biopsy specimens were retrospectively collected from seven facilities in the Hokuriku region and diagnosed at the Kanazawa University Hospital in 2015. First, the components of CFAs were analyzed using normal histostaining, immunostaining, and electron microscopy. Second, the patients were divided into two groups (CFA [+] or CFA [−]) according to the presence of CFA in the obtained samples. Clinical and histological features were compared between the two groups, and factors associated with CFA formation were determined using univariate and multivariate analyses. The number of CFAs per specimen was counted in the CFA (+) group. Third, the presence of myofibroblasts in CFA was examined by immunostaining.

Results

CFAs were observed in 56 of 101 patients (55.4%) with various kidney diseases. CFAs consist of fibrillar collagens (collagen I and III) in addition to non-fibrillar collagens (collagen IV and VI), as confirmed by electron microscopy. Clinically, the CFA (+) group was older and had a significantly higher prevalence of hypertension and hyperlipidemia than the CFA (−) group. Histologically, elastofibrosis of the interlobular artery, arteriolar hyalinosis, and membranous nephropathy were significantly more evident in the CFA (+) group than in the CFA (−) group. Multivariate analysis revealed that older age was the sole factor associated with CFA formation. Finally, 27 of 58 (46.6%) CFA-containing glomeruli in 26 cases included alpha-smooth muscle actin-positive cells in or adjacent to the CFA.

Conclusions

CFAs consist of fibrous collagens in addition to matrix collagens. CFA formation is associated with older age and was observed in various kidney diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-022-02835-2.

The Aging Kidney-As Influenced by Heavy Metal Exposure and Selenium Supplementation

The aging process in the kidneys has been well studied. It is known that the glomerular filtration rate (GFR) declines with age in subjects older than 50–60 years. However, there is still insufficient knowledge regarding the response of the aged kidney to environmental toxicants such as mercury, cadmium, and lead. Here, we present a review on the functional decline and proposed mechanisms in the aging kidney as influenced by metal pollutants. Due to the prevalence of these toxicants in the environment, human exposure is nearly unavoidable. Further, it is well known that acute and chronic exposures to toxic metals may be detrimental to kidneys of normal adults, thus it may be hypothesized that exposure of individuals with reduced GFR will result in additional reductions in renal function. Individuals with compromised renal function, either from aging or from a combination of aging and disease, may be particularly susceptible to environmental toxicants. The available data appear to show an association between exposure to mercury, cadmium and/or lead and an increase in incidence and severity of renal disease in elderly individuals. Furthermore, some physiological thiols, as well as adequate selenium status, appear to exert a protective action. Further studies providing improved insight into the mechanisms by which nephrotoxic metals are handled by aging kidneys, as well as possibilities of therapeutic protection, are of utmost importance.

[Very-old deceased donors in kidney transplantation: How far can we go?]

In order to increase the pool of organ donors, kidney transplantation from very old-donors, notably aged more than 70, is increasing. Compared to the United States, where the use of these grafts does not reach 5%, in France it reaches over 20%. Kidney aging is determined by a progressive glomerusclerosis, interstitial fibrosis, and nephrosclerosis, responsible of a linear decrease of glomerular filtration rate with time. Aging in kidney transplantation goes along also with an increased immunogenicity and risk of ischemia-reperfusion injuries. Hence, the prognosis of these transplantations is worse than those from younger donors, even though it remains better than dialysis. Data is lacking on risk factors of graft loss in this specific population. Hypothermic perfusion machine, pre-implantation kidney biopsy, dual kidney transplantation and immunosuppressive strategies have been evaluated to improve the long-term prognosis of these grafts.

Age modifies the risk factor profiles for acute kidney injury among recently diagnosed type 2 diabetic patients: a population-based study

The incidence of acute kidney injury (AKI) rises with age and is associated with multiple risk factors. Here, we compared the risk factors for AKI between younger and older incident diabetic patients to examine the trends in risk alteration for individual factors across different age groups. Between 2007 and 2013, we selected all incident type 2 diabetic adults from the Taiwan National Health Insurance registry, stratified based on age: young (< 65 years), old (≥ 65 but < 75 years), and older-old (≥ 75 years). All factors with potential renal influence (e.g., comorbidities, medications, and diagnostics/procedures) were recorded during the study period, with a nested case-controlled approach utilized to identify independent risk factors for AKI in each age group. Totally, 930,709 type 2 diabetic patients were categorized as young (68.7%), old (17.7%), or older-old (13.6%). Older-old patients showed a significantly higher incidence of AKI than the old and the young groups. Cardiovascular morbidities (hypertension, atrial fibrillation, acute coronary syndrome, and cerebrovascular disease) were shown to increase the risk of AKI, although the risk declined with increasing age. Chronic obstructive pulmonary disease and receiving cardiac catheterization elevated the risk of AKI preferentially in the older-old/old and older-old group, respectively, while the administration of angiotensin-converting enzyme/α-blocker and angiotensin receptor blocker/calcium channel blocker reduced the risk of AKI preferentially in the older-old and older-old/old group, respectively. In conclusion, our findings highlight the importance of devising age-specific risk factor panels for AKI in patients with recently diagnosed type 2 diabetes.

Graded Association Between Kidney Function and Impaired Orthostatic Blood Pressure Stabilization in Older Adults

Background

Impaired orthostatic blood pressure (BP) stabilization is highly prevalent in older adults and is a predictor of end‐organ injury, falls, and mortality. We sought to characterize the relationship between postural BP responses and the kidney.

Methods and Results

We performed a cross‐sectional analysis of 4204 participants from The Irish Longitudinal Study on Ageing, a national cohort of community‐dwelling adults aged ≥50 years. Beat‐to‐beat systolic and diastolic BP were measured during a 2‐minute active stand test. The primary predictor was cystatin C estimated glomerular filtration rate (eGFR) categorized as follows (mL/min per 1.73 m²): ≥90 (reference, n=1414); 75 to 89 (n=1379); 60 to 74 (n=942); 45 to 59 (n=337); <45 (n=132). We examined the association between eGFR categories and (1) sustained orthostatic hypotension, defined as a BP drop exceeding consensus thresholds (systolic BP drop ≥20 mm Hg±diastolic BP drop ≥10 mm Hg) at each 10‐second interval from 60 to 110 seconds inclusive; (2) pattern of BP stabilization, characterized as the difference from baseline in mean systolic BP/diastolic BP at 10‐second intervals. The mean age of subjects was 61.6 years; 47% of subjects were male, and the median eGFR was 82 mL/min per 1.73 m². After multivariable adjustment, participants with eGFR <60 mL/min per 1.73 m² were approximately twice as likely to have sustained orthostatic hypotension (P=0.008 for trend across eGFR categories). We observed a graded association between eGFR categories and impaired orthostatic BP stabilization, particularly within the first minute of standing.

Conclusions

We report a novel, graded relationship between diminished eGFR and impaired orthostatic BP stabilization. Mapping the postural BP response merits further study in kidney disease as a potential means of identifying those at risk of hypotension‐related events.

Progression of kidney disease in elderly stage 3 and 4 chronic kidney disease patients

PURPOSE

The prevalence of chronic kidney disease (CKD) has been rising steadily in the elderly population. We studied the rate of progression of CKD in this population and the factors associated with progression of CKD to better identify patients who are likely to progress to ESRD.

METHODS

This was an observational study including 4562 patients older than 65 years with two outpatient estimated glomerular filtration rates (eGFRs) of <60 ml/min/1.73 m², at least 90 days apart with no intervening eGFR >60 ml/min/1.73 m² (March 1, 2001, and March 31, 2008) at VA healthcare facilities. Patients with eGFR <15 ml/min/1.73 m² were excluded. Annual rate of decline of eGFR was studied and categorized as <1 ml/min/1.73 m², 1-4 ml/min/1.73 m², and >4 ml/min/1.73 m².

RESULTS

Mean age of the study participants was 77.2 years. 24.3% were diabetics. 4.3% had proteinuria. In univariate comparison of different rates of progression, 54.2% patients had an annual rate of progression of <1 ml/min/1.73 m². Multivariable mixed model analyses revealed that increasing age, body mass index, presence of cardiovascular disease, diabetes mellitus, and proteinuria were associated with significantly increased rate of progression of CKD. Serum albumin and hemoglobin level were inversely associated with progression of CKD.

CONCLUSIONS

CKD progresses at a slower rate in the elderly population. We have identified risk factors associated with an increased risk of progression of CKD in the elderly. This may help to improve health care planning and resource utilization.

The aging kidney and the nephrotoxic effects of mercury

Owing to advances in modern medicine, life expectancies are lengthening and leading to an increase in the population of older individuals. The aging process leads to significant alterations in many organ systems, with the kidney being particularly susceptible to age-related changes. Within the kidney, aging leads to ultrastructural changes such as glomerular and tubular hypertrophy, glomerulosclerosis, and tubulointerstitial fibrosis, which may compromise renal plasma flow (RPF) and glomerular filtration rate (GFR). These alterations may reduce the functional reserve of the kidneys, making them more susceptible to pathological events when challenged or stressed, such as following exposure to nephrotoxicants. An important and prevalent environmental toxicant that induces nephrotoxic effects is mercury (Hg). Since exposure of normal kidneys to mercuric ions might induce glomerular and tubular injury, aged kidneys, which may not be functioning at full capacity, may be more sensitive to the effects of Hg than normal kidneys. Age-related renal changes and the effects of Hg in the kidney have been characterized separately. However, little is known regarding the influence of nephrotoxicants, such as Hg, on aged kidneys. The purpose of this review was to summarize known findings related to exposure of aged and diseased kidneys to the environmentally relevant nephrotoxicant Hg.

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.

An Age-Calibrated Definition of Chronic Kidney Disease: Rationale and Benefits

Defining chronic kidney disease (CKD) is the subject of intense debate in the current nephrology literature. The debate concerns the threshold value of estimated glomerular filtration rate (eGFR) used to make the diagnosis of CKD. Current recommendations argue that a universal threshold of 60 mL/min/1.73m(2) should be used. This threshold has been defended by epidemiological studies showing that the risk of mortality or end-stage renal disease increases with an eGFR below 60 mL/min/1.73m(2). However, a universal threshold does not take into account the physiologic decline in GFR with ageing nor does it account for the risk of mortality and end-stage renal disease being trivial with isolated eGFR levels just below 60 mL/min/1.73m(2) in older subjects and significantly increased with eGFR levels just above 60 mL/min/1.73m(2) among younger patients. Overestimation of the CKD prevalence in the elderly (medicalisation of senescence) and underestimation of CKD (potentially from treatable primary nephrologic diseases) in younger patients is of primary concern. An age-calibrated definition of CKD has been proposed to distinguish age-related from disease-related changes in eGFR. For patients younger than 40 years, CKD is defined by eGFR below 75 mL/min/1.73m(2). For patients with ages between 40 and 65 years, CKD is defined by 60 mL/min/1.73m(2). For subjects older than 65 years without albuminuria or proteinuria, CKD is defined by eGFR below 45 mL/min/1.73m(2).

Non-neoplastic parenchymal changes in kidney cancer and post-partial nephrectomy recovery of renal function

OBJECTIVE

To explore the association of non-neoplastic parenchymal changes (nNPC) with patients' health and renal function recovery after partial nephrectomy (PN).

MATERIALS AND METHODS

This retrospective review identified 800 pT1a patients who underwent PN at Memorial Sloan Kettering Cancer Center from 2007 to 2012. Pathology reports were reviewed for nNPC graded as mild or severe: vascular sclerosis (VS), glomerulosclerosis (GS), and fibrosis/scarring. Correlations between nNPC and known preoperative predictors of renal function [age, sex, African-American race, estimated glomerular filtration rate (eGFR), American Society of Anesthesiologists (ASA) score, body mass index, coronary artery disease, and hypertension (HTN)] were assessed using Spearman's rank correlation (ρ). Multivariable linear regression, adjusted for the described known preoperative risk predictors, was performed to evaluate whether the parenchymal features were able to predict 6-month postoperative eGFR.

RESULTS

In this study, 46 % of tumors had benign surrounding parenchyma. We noted statistically significant yet weak associations of VS with age (ρ = 0.19; p < 0.001), ASA (ρ = 0.09; p < 0.001), preoperative eGFR (ρ = -0.14; p < 0.001), and HTN (ρ = 0.14; p < 0.001). GS also significantly correlated with HTN, but the correlation was again small (ρ = 0.12; p < 0.001). After adjusting for known risk predictors, only GS was a significant predictor of 6-month postoperative eGFR. When compared with no GS, mild and severe GS were negatively associated with a decrease of 4.9 and 10.8 mL/min/1.73 m(2) in 6-month postoperative eGFR, respectively.

CONCLUSIONS

Presence of VS and GS correlated with patients' baseline health, and presence of GS predicted postoperative renal function recovery.

Anatomic and physiologic changes of the aging kidney

Aging is associated with structural and functional changes in the kidney. Structural changes include glomerulosclerosis, thickening of the basement membrane, increase in mesangial matrix, tubulointerstitial fibrosis and arteriosclerosis. Glomerular filtration rate is maintained until the fourth decade of life, after which it declines. Parallel reductions in renal blood flow occur with redistribution of blood flow from the cortex to the medulla. Other functional changes include an increase in glomerular basement permeability and decreased ability to dilute or concentrate urine.

Combined application of eGFR and albuminuria for the precise diagnosis of stage 2 and 3a CKD in the elderly

BACKGROUND

Estimated glomerular filtration rate (eGFR) is the sole diagnostic criterion for stage 3a chronic kidney disease (CKD). Because eGFR decreases with age, its prognostic utility in the elderly is controversial. Albuminuria is an important prognostic factor. To confirm that eGFR use may lead to the overdiagnosis of CKD and to examine the utility of eGFR combined with albuminuria for diagnosing stage 3a CKD in the elderly.

METHODS

This study included 365 elderly patients (age ≥ 65 years) who were diagnosed with stage 2 or 3a CKD. All patients had 3 years of consecutive data at our hospitals from 2000 to 2012. For each eGFR level, patients were divided into normalbuminuria (NOR, urinary albumin excretion rate [UAER] < 30 mg/24 h), microalbuminuria (30 ≥ UAER < 299 mg/24 h), and macroalbuminuria groups (UAER ≥ 300 mg/24 h).

RESULTS

Albuminuria was associated with eGFR loss but not baseline eGFR level. When stage 2 NOR was used as a reference, the multivariable adjusted odds ratio (OR) for rapid kidney function decline (RKFD) of stage 3a NOR was 1.329 (95 % confidence interval (CI): 0.334-5.281, P = 0.686). ORs for other groups were significantly higher. In stage 3a NOR, higher ORs for RKFD were associated with younger age groups.

CONCLUSIONS

Lot of elderly patients with stage 3a CKD and normal albuminuria levels may be over-diagnosed. Albuminuria may be combined with eGFR for improved diagnosis and treatment of stage 3 CKD in the elderly.

Physiology and its importance for reference intervals

Reference intervals are ideally defined on apparently healthy individuals and should be distinguished from clinical decision limits that are derived from known diseased patients. Knowledge of physiological changes is a prerequisite for understanding and developing reference intervals. Reference intervals may differ for various subpopulations because of differences in their physiology, most obviously between men and women, but also in childhood, pregnancy and the elderly. Changes in laboratory measurements may be due to various physiological factors starting at birth including weaning, the active toddler, immunological learning, puberty, pregnancy, menopause and ageing. The need to partition reference intervals is required when there are significant physiological changes that need to be recognised. It is important that laboratorians are aware of these changes otherwise reference intervals that attempt to cover a widened inter-individual variability may lose their usefulness. It is virtually impossible for any laboratory to directly develop reference intervals for each of the physiological changes that are currently known, however indirect techniques can be used to develop or validate reference intervals in some difficult situations such as those for children. Physiology describes our life's journey, and it is only when we are familiar with that journey that we can appreciate a pathological departure.

Hypertension, glomerular hypertrophy and nephrosclerosis: the effect of race

Background

African Americans have more severe hypertensive nephrosclerosis than white Americans, possibly at similar levels of blood pressure. Glomerular volume is increased in African Americans relative to whites, but it is uncertain how this relates to nephrosclerosis and whether it contributes to or compensates for glomerulosclerosis.

Methods

Stereological disector/fractionator estimates of glomerular number (N_glom) and average glomerular volume (V_glom) were obtained on autopsy kidneys of 171 African Americans and 131 whites. Eighty-eight African Americans and 49 whites were identified as hypertensive. Nephrosclerosis was measured morphometrically as the percentage of glomerulosclerosis, proportion of cortical fibrosis and interlobular artery intimal thickness, and analyzed with V_glom by age, race, gender, body mass index (BMI) and blood pressure.

Results

African Americans were more frequently hypertensive (58.5%) than whites (35.8%) and when hypertensive had higher levels of blood pressure (P = 0.02). N_glom was significantly lower in hypertensive compared with non-hypertensive subjects among white women (P = 0.02) but not white males (P = 0.34) or African American females (P = 0.10) or males (P = 0.41). For each race and gender, glomerulosclerosis, cortical fibrosis and arterial intimal thickening were statistically correlated with age (P < 0.001) and hypertension (P < 0.001) and increased V_glom with hypertension (P < 0.001) and BMI (P < 0.001). In multivariate analysis, African American race was associated with increased V_glom (P = 0.01) and arterial intimal thickening (P < 0.01), while interactions between race and blood pressure indicated that the severity of nephrosclerosis including increased V_glom was linked most directly to hypertension without significant contributions from race. The hypertension-associated enlargement of V_glom was present with mild degrees of glomerulosclerosis and changed little as the severity of glomerulosclerosis increased.

Conclusions

Glomerular hypertrophy was identified as an integral feature of hypertensive nephropathy and appeared to precede rather than compensate for glomerulosclerosis. An effect of race on V_glom and arterial intimal thickening seemed to be related to the more frequent and more severe hypertension among African Americans.

Glomerular filtration rate in prospective living kidney donors

PURPOSE

Measurements of glomerular filtration rate (GFR) are frequently interpreted assuming a linear variation with age. Nonlinear relationships may give a better representation of the changes associated with normal ageing.

METHODS

This was a retrospective study of 904 subjects (468 women, 436 men; age range 18-84 years) undergoing assessment as prospective living kidney donors. GFR was evaluated from (51)Cr-EDTA plasma clearance using blood samples taken at 2, 3 and 4 h. The slope-intercept GFR was corrected for body surface area (BSA) using the Haycock formula and for the fast exponential using the Brochner-Mortensen equation. The relationship between age, gender and GFR was examined using best-fit curve analysis. Nonlinear relationships with age were explored using fractional polynomials.

RESULTS

There was no gender difference in BSA-corrected GFR over five decades of age (P = 0.40). However, female donors with a body mass index >30 kg/m(2) had a statistically significantly lower GFR than nonobese women (P < 0.01). The best-fit relationship between age and GFR was nonlinear and described using a fractional polynomial model of degree 1 (GFR = 103.9-0.0061 × Age(2) mL/min/1.73 m(2)) with a root mean standard error of 12.9 mL/min/1.73 m(2). The residual variance for this model was significantly smaller than for the best-fit linear model (P = 0.006).

CONCLUSIONS

GFR measurements in prospective living kidney donors are best corrected for age using a nonlinear relationship.

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

Aging is both a natural and inevitable biological process. With advancing age, the kidneys undergo anatomical and physiological changes that are not only the consequences of normal organ senescence but also of specific diseases (such as atherosclerosis or diabetes) that occur with greater frequency in older individuals. Disentangling these two processes, one pathologic and the other physiologic, is difficult. In this review we concentrate on the glomerular structural and functional alterations that accompany natural aging. We also analyze how these changes affect the identification of individuals of advancing age as having chronic kidney disease (CKD) and how these changes can influence prognosis for adverse outcomes, including all-cause mortality, end-stage renal disease, cardiovascular events and mortality, and acute kidney injury. This review describes important shortcomings and deficiencies with our current approach and understanding of CKD in the older and elderly adult.