Effects of aging on glomerular function and number in living kidney donors

Renal outcome of living kidney donors aged more than 70 years

BACKGROUND

The number of marginal living kidney donors has increased. Medically complex donors who have hypertension, older age, or low estimated glomerular filtration rate (eGFR) have been more likely to be used.

METHODS

We conducted a retrospective cohort study of living kidney donors at a single center. We analyzed 309 living donors and divided them into three groups: group with older donors (aged ≥70 years) (n = 41), middle-aged (aged 46-69 years) (n = 239), and young donors (aged <46 years) (N = 29). Donor factors associated with chronic kidney disease (CKD) stage 3b or worse within 5 years post-donation were investigated.

RESULTS

Of the 309 live donors, 86 (27.8%) developed CKD stage3b or worse within 5 years post-donation. The incidence of CKD stage3b or worse within 5 years post-donation was significantly higher in older donor (p < 0.01). Cox regression models revealed that older donor ages and lower eGFR were significantly related to the development of CKD stage3b or worse, independent of comorbidities such as obesity and hypertension [hazard ratio (95% CI); 4.59 (1.02-20.6), p = 047, 0.95 (0.94-0.96), p ≤ 0.01, respectively]. However, recovery of eGFR 4-5 years after donation was noted in the middle-aged and older donor groups, whereas the level of eGFR remained unchanged in the young group.

CONCLUSIONS

Older donors tend to develop CKD stage3b within 5 years post-donation but with the potential of recovery. Healthy older people (aged ≥70 years) could be candidates for living donors under careful monitoring of kidney function after donation.

Age and Pre-operative HbA1c levels affect renal function compensation in living kidney donors

OBJECTIVES

About 90% of Japanese kidney transplantations are conducted from living donors, and their safety and the maintenance of their renal function are critical. This study aims to identify factors that affect the compensation of renal function in living kidney donors after donor nephrectomy.

METHOD

In a retrospective cohort study, we reviewed data from 120 patients who underwent nephrectomy as living kidney transplant donors in our department from 2012 to 2021. Univariable and multivariable linear regression analyses were performed for donor factors affecting renal function after donor nephrectomy.

RESULT

The multivariable linear regression model revealed that the donor's age (p = 0.025), preoperative estimated Glomerular Filtration Rate (eGFR) (p < 0.001), and hemoglobin A1c (HbA1c) (p = 0.043) were independent risk factors for eGFR at six months after nephrectomy. The eGFR deterioration was more strongly associated with age in females than in males, whereas higher HbA1c values were more strongly associated with eGFR deterioration in males. Higher donor age and higher HbA1c each enhance the deterioration of eGFR six months after living donor nephrectomy. The data suggest that old age in especially female donors and preoperative higher HbA1c in male donors have a harmful impact on their renal function compensation.

Chronic Kidney Disease in the Older Adult Patient with Diabetes

Diabetes mellitus (DM) and chronic kidney disease (CKD) are common in middle aged and older adult individuals. DM may accelerate the aging process, and the age-related declines in the estimated glomerular filtration rate (eGFR) can pose a challenge to diagnosing diabetic kidney disease (DKD) using standard diagnostic criteria especially with the absence of severe albuminuria among older adults. In the presence of CKD and DM, older adult patients may need multidisciplinary care due to susceptibility to various health issues, e.g., cognitive decline, auditory or visual impairment, various comorbidities, complex medical regimens, and increased sensitivity to medication adverse effects. As a result, it can be challenging to apply recent therapeutic advancements for the general population to older adults. We review the evidence that the benefits from these newer therapies apply equally to older and younger patients with CKD and diabetes type 2 and propose a comprehensive management. This framework will address nonpharmacological measures and pharmacological management with renin angiotensin system inhibitors (RASi), sodium glucose co-transporter 2 inhibitors (SGLT2i), non-steroidal mineralocorticoids receptor antagonists (MRAs), and glucagon like peptide 1 receptor agonists (GLP1-RAs).

Renal function in very old critically ill patients

PURPOSE OF REVIEW

Current demographic change leads to higher number of elderly patients admitted to an ICU. Among other organs also the kidneys show age-related changes, which are associated with a decline in various aspects of renal function. The purpose of this review is to provide an overview of structural and functional changes in elderly and also to specifically address the increased risk of acute kidney injury (AKI) in this population.

RECENT FINDINGS

Ageing in the kidneys is affected by many different factors, such as low grade chronic inflammation, called inflammageing, and various comorbidities. Nevertheless, a decrease of glomerular filtration rate (GFR) occurs independent of the presence of comorbidities and a steady decline of GFR has been reported in both healthy men and women. Pharmacodynamic of many drugs is altered by these changes. Additionally the rate of diuretic resistance appears to be increased. The cause of AKI occurrence in older age is, multifactorial and includes preventable triggers (hypovolemia, hypotension, nephrotoxins) as well as changes associated with aging.

SUMMARY

Age-related alterations of the kidneys were found at microscopic and macroscopic levels of the cell. These changes lead to a reduced renal reserve and subsequently to an increased vulnerability of aged kidneys when an additional stressor is added. Age is an independent risk factor for developing AKI. Physicians should take into account the altered renal function in elderly patients and take renal protective measures at an early stage.

Podocyte Senescence and Aging

As the population in many industrial countries is aging, the risk, incidence, and prevalence of CKD increases. In the kidney, advancing age results in a progressive decrease in nephron number and an increase in glomerulosclerosis. In this review, we focus on the effect of aging on glomerular podocytes, the post-mitotic epithelial cells critical for the normal integrity and function of the glomerular filtration barrier. The podocytes undergo senescence and transition to a senescence-associated secretory phenotype typified by the production and secretion of inflammatory cytokines that can influence neighboring glomerular cells by paracrine signaling. In addition to senescence, the aging podocyte phenotype is characterized by ultrastructural and functional changes; hypertrophy; cellular, oxidative, and endoplasmic reticulum stress; reduced autophagy; and increased expression of aging genes. This results in a reduced podocyte health span and a shortened life span. Importantly, these changes in the pathways/processes characteristic of healthy podocyte aging are also often similar to pathways in the disease-induced injured podocyte. Finally, the better understanding of podocyte aging and senescence opens therapeutic options to slow the rate of podocyte aging and promote kidney health.

The diagnostic value of DMSA scan in differentiating functional pseudo-tumors from malignancies in scarred kidneys: case series and literature review

BACKGROUND

The terms "renal regenerating nodule" and "nodular compensatory hypertrophy" are used in the literature to describe functioning pseudo-tumors (FPT) in the setting of an extensively scarred kidney. FPTs are usually discovered incidentally during routine renal imaging. Differentiating these FPTs from renal neoplasms is critical but can be challenging in the setting of chronic kidney disease (CKD) given the limitations related to using contrast-based imaging.

CASE SUMMARIES

We report a pediatric case series of 5 CKD patients, with history of urinary tract infections, in which tumor-like lesions evolved in scarred kidneys and were incidentally discovered on routine renal imaging. These were diagnosed as FPT by utilizing dimercaptosuccinic acid (DMSA) imaging and showed stable size and appearance upon follow-up with ultrasound and MRI.

CONCLUSION

FPTs can be picked up on routine imaging of pediatric patients with CKD. Although larger cohort studies are needed to confirm these conclusions, our case series supports the evidence that DMSA scan showing uptake at the site of the mass can be a useful tool to suggest the diagnosis of FPTs in children with kidney scarring, and that SPECT DMSA scan adds more precision in picking up and accurately localizing FPTs compared to planar DMSA.

Renal function in aged C57BL/6J mice is impaired by deposition of age-related apolipoprotein A-II amyloid independent of kidney aging

Spontaneous and age-related amyloidosis has been reported in C57BL/6J mice; however, the biochemical characteristics of age-related amyloidosis remain unclear. Therefore, we herein investigated the age-related prevalence of amyloidosis, the types of amyloid fibril proteins, and the effects of amyloid deposition on renal function in C57BL/6J mice. The results obtained revealed a high incidence of amyloidosis in C57BL/6J mice originating from the Jackson laboratory as well as the deposition of large amounts of amyloid in the glomeruli of aged mice. We identified the amyloid fibril protein in C57BL/6J mice as wild-type apolipoprotein A-II. We induced renal amyloid deposition in 40-week-old mice, equivalent to that of spontaneous development in 80-week-old mice, to rule out the effects of aging, and revealed subsequent damage to kidney function by amyloid deposits. Furthermore, amyloid deposition in the mesangial region decreased podocyte density, compromised foot processes, and led to the accumulation of fibroblast growth factor 2 (FGF2) in glomeruli. Collectively, these results suggest that AApoAII deposition is a general pathology in aged C57BL/6J mice and is dependent on supplier colonies. Therefore, the effects of age-related amyloid deposition need to be considered in research on aging in mice.

Pathological and clinical characteristics of late-onset oligomeganephronia based on a histomorphometric study

BACKGROUND

Late-onset oligomeganephronia (OMN) is a rare chronic kidney disease and has no quantitative criteria for diagnosis yet. The current study aimed to explore its clinicopathological features by histomorphometric analysis.

METHODS

We retrospectively re-reviewed all patients with enlarged and sparse glomeruli by light microscopy at Peking University First Hospital from 2012 to 2021, excluding those with any factor known to contribute to similar changes. Age- and sex-matched patients with thin basement membrane nephropathy were selected as control to establish the cut-off values for glomerulomegaly and rarity. Late-onset OMN cases were then confirmed and the clinicopathological characteristics were summarized.

RESULTS

Mean diameter and density of cortical glomeruli in control was 156.53 ± 27.50 μm and 4.07 ± 0.63 /mm², giving a lower limit of 211.53 μm for glomerulomegaly and an upper of 2.81 /mm² for rarity. Seven adults of three females and four males were finally diagnosed as late-onset OMN with a mean age of 26.57 years. They showed mild to moderate proteinuria and/or renal dysfunction at biopsy with the mean proteinuria, serum creatinine (Scr) level, and estimated glomerular filtration rate of 0.50 g/d (0.10-0.95 g/d), 140.9 µmol/L (95.1-227.1 µmol/L), and 58.7 mL/min/1.73m² (21.3-98.0 mL/min/1.73m²), respectively. Four patients (57.1%) had normal Scr at diagnosis. Six patients with available data showed renal tubular injury with increased urinary microalbumin in all, elevated N-acetyl-β-glucosaminidase in two, and elevated α1 microglobulin in five. Kidney size was normal or slightly reduced. The mean density and glomerular diameter of the seven cases was 0.86 mm² (0.55-1.41 /mm²) and 229.73 μm (211.88-260.66 μm). Segmental glomerular sclerosis was observed in six (85.7%) with four (66.7%) of perihilar type. Proximal tubule dilation was observed in all, focal to diffuse, lining with enlarged epithelial cells. The mean foot process width was 634.02 nm, wider than 472.54 nm of the control (P = 0.0002).

CONCLUSION

Late-onset OMN should be considered a special entity with relatively slow clinical progress characterized by hypertrophy of the sparsely distributed nephron.

Prediction of Renal Function in Living Kidney Donors and Recipients of Living Donor Kidneys Using Quantitative Histology

BACKGROUND

Living kidney donors (LKDs) are at increased risk of chronic kidney disease, whereas transplant recipients experience progressive reduction of graft function. We examined the predictive value of quantitative stereology on renal function in LKDs and recipients of living donor kidneys, based on perioperative biopsies from the donated kidney.

METHODS

Cortex volume of both donor kidneys was determined by contrast-enhanced computed tomography and single-kidney glomerular filtration rate (GFR) by 51 chrome-EDTA clearance together with renography. Glomerular density was used to estimate total glomeruli number in addition to glomerular volume, glomerular sclerosis, kidney fibrosis, and arteriole dimensions. GFR measurements were repeated 1 y after transplantation in both LKDs and recipients. Associations between GFR at follow-up and cortex volume and histomorphometric parameters after adjustment of age, gender, body mass index, smoking status, 24-h blood pressure, and single-kidney GFR were examined.

RESULTS

We included 49 LKDs (age, 51 ± 12 y) and 51 recipients (age, 44 ± 13 y). At follow-up, GFR was 71 ± 16 mL/min in LKDs and 61 ± 18 mL/min in recipients with hyperfiltration being more prominent in LKDs (30.4%) as compared to recipients (16.4%; P < 0.05). One-year GFR in donors correlated to cortex volume ( P < 0.001) but not to any histological parameters, whereas GFR in recipients correlated to the amount of interstitial fibrosis ( P < 0.01) but not to other histological parameters or cortex volume.

CONCLUSIONS

Kidney cortex volume, but not renal histology parameters, predicts 1-y renal outcome in LKDs. In contrast, the amount of interstitial fibrosis, but not cortex volume, predicts 1-y graft function in recipients.

Renal inflamm-aging provokes intra-graft inflammation following experimental kidney transplantation

Donor age is a major risk factor for allograft outcome in kidney transplantation. The underlying cellular mechanisms and the recipient's immune response within an aged allograft have yet not been analyzed. A comprehensive immunophenotyping of naïve and transplanted young versus aged kidneys revealed that naïve aged murine kidneys harbor significantly higher frequencies of effector/memory T cells, whereas regulatory T cells were reduced. Aged kidney-derived CD8⁺ T cells produced more IFNγ than their young counterparts. Senescent renal CD8⁺ T and NK cells upregulated the cytotoxicity receptor NKG2D and the enrichment of memory-like CD49a⁺ CXCR6⁺ NK cells was documented in aged naïve kidneys. In the C57BL/6 to BALB/c kidney transplantation model, recipient-derived T cells infiltrating an aged graft produced significantly more IFNγ, granzyme B and perforin on day 7 post-transplantation, indicating an enhanced inflammatory, cytotoxic response towards the graft. Pre-treatment of aged kidney donors with the senolytic drug ABT-263 changed the recipient-derived effector molecule profile to significantly reduced levels of IFNγ and IL-10 compared to controls. Graft function after ABT-263 pre-treatment was significantly improved 28 days post kidney transplantation. In conclusion, renal senescence also occurs at the immunological level (inflamm-aging) and aged organs provoke an altered recipient-dominated immune response in the graft.

Surgical and functional outcomes of robot-assisted laparoscopic partial nephrectomy for renal cell carcinoma in adolescents and young adults: a propensity score matching study

BACKGROUND

Cancer development in adolescents and young adults (AYAs) has elicited recent interest. We investigated the surgical and functional outcomes of robot-assisted laparoscopic partial nephrectomy (RAPN) for renal cell carcinoma (RCC) in AYAs.

METHODS

We retrospectively reviewed the medical records of 1023 patients with clinical stage I RCC who underwent RAPN before January 2021. Patients were divided into two groups: AYAs (aged 18-39 years) and non-AYAs (aged 40-89 years). The trifecta criteria, defined as a negative surgical margin, no perioperative complications (Clavien-Dindo grade > 2), and preserved postoperative renal function (1-year postoperative estimated glomerular filtration rate > 90% of baseline), were used to compare outcomes. We performed 1:1 propensity-score matching on the patient cohort.

RESULTS

There were initially 125 and 898 patients in the AYAs and non-AYAs groups, respectively, and 108 patients were included in each group after propensity score matching. There were no significant differences in surgical factors (operation time, clamping ischemia time, estimated blood loss, length of hospital stay, surgical complication rate) or renal function in the early postoperative period. The mean postoperative renal function was better (p = 0.0200) and the decrease in estimated glomerular filtration rate was lower (p = 0.0026) in AYAs than in non-AYAs 12 months postoperatively. The trifecta achievement rates in the AYAs and non-AYAs groups were significantly different (67.6% and 53.7%, respectively, p = 0.0220).

CONCLUSION

Although there was no difference in surgical burden between the groups, the estimated glomerular filtration rate was better preserved in AYAs than in non-AYAs at 6 and 12 months post-RAPN.

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.

Impact of Size Matching Based on Donor-Recipient Height on Kidney Transplant Outcomes

Transplantation of kidneys from shorter donors into taller recipients may lead to suboptimal allograft survival. The effect of discrepancy in donor and recipient heights (ΔHeight) on long term transplant outcomes is not known. Adult patients ≥18 years undergoing living or deceased donor (LD or DD) kidney transplants alone from donors ≥18 years between 2000 and 2016 in the United States were included in this observational study. The cohort was divided into three groups based on ΔHeight of 5 inches as 1) Recipient &lt; Donor (DD: 31,688, LD: 12,384), 2) Recipient = Donor (DD: 84,711, LD: 54,709), and 3) Recipient &gt; Donor (DD: 21,741, LD: 18,753). Univariate analysis showed a higher risk of DCGL and mortality in both DD and LD (p &lt; 0.001 for both). The absolute difference in graft and patient survival between the two extremes of ΔHeight was 5.7% and 5.7% for DD, and 0.4% and 1.4% for LD. On multivariate analysis, the HR of DCGL for Recipient &lt; Donor and Recipient &gt; Donor was 0.95 (p = 0.05) and 1.07 (p = 0.01) in DD and 0.98 (p = 0.55) and 1.14 (p &lt; 0.001) in LD. Similarly, the corresponding HR of mortality were 0.97 (p = 0.07) and 1.07 (p = 0.003) for DD and 1.01 (p &lt; 0.001) and 1.05 (p = 0.13) for LD. For DGF, the HR were 1.04 (p = 0.1) and 1.01 (p = 0.7) for DD and 1.07 (p = 0.45) and 0.89 (p = 0.13) for LD. Height mismatch between the donor and recipient influences kidney transplant outcomes.

Emerging Roles for G Protein-Coupled Estrogen Receptor 1 in Cardio-Renal Health: Implications for Aging

Cardiovascular (CV) and renal diseases are increasingly prevalent in the United States and globally. CV-related mortality is the leading cause of death in the United States, while renal-related mortality is the 8th. Despite advanced therapeutics, both diseases persist, warranting continued exploration of disease mechanisms to develop novel therapeutics and advance clinical outcomes for cardio-renal health. CV and renal diseases increase with age, and there are sex differences evident in both the prevalence and progression of CV and renal disease. These age and sex differences seen in cardio-renal health implicate sex hormones as potentially important regulators to be studied. One such regulator is G protein-coupled estrogen receptor 1 (GPER1). GPER1 has been implicated in estrogen signaling and is expressed in a variety of tissues including the heart, vasculature, and kidney. GPER1 has been shown to be protective against CV and renal diseases in different experimental animal models. GPER1 actions involve multiple signaling pathways: interaction with aldosterone and endothelin-1 signaling, stimulation of the release of nitric oxide, and reduction in oxidative stress, inflammation, and immune infiltration. This review will discuss the current literature regarding GPER1 and cardio-renal health, particularly in the context of aging. Improving our understanding of GPER1-evoked mechanisms may reveal novel therapeutics aimed at improving cardio-renal health and clinical outcomes in the elderly.

Renal Aging Resembles a Continuum Between Normal and Diseased Kidneys That Potentiates Inflammatory Response to Injury

Renal aging is a natural process that can lead to structural changes and functional decline in the kidneys. These age-related changes are considered irreversible physiological processes, but resemble diseased kidneys. To enhance understanding of the molecular nature of renal aging, we first compared whole-kidney RNA sequencing between naturally aging mice (24-month-old) and adenine-induced nephropathy in young mice (2-month-old). Young mice (2-month-old) without intervention were used as the control group to investigate transcription alteration with aging or by adenine-enriched diet. Next, we compared the functional and structural renal consequences of aging and adenine-induced nephropathy between young (2-month-old) and old mice (18- to 22-month-old). C57BL/6 male mice were used in all experimental studies. Both aging kidneys and adenine-induced nephropathy showed similar transcriptional profiles characterized by upregulation in innate and adaptive immune system activation and inflammation, although these alterations were generally less significant in the aging kidneys. In contrast to aging kidneys, adenine-induced nephropathy showed prominent expression of the genes related to cytokines, T-cell activation, and fibrosis and decreased expression of the genes implicated in transporter activity and metabolism. The subclinical immunological micromilieu in aging kidneys potentially causes augmented kidney damage in response to injurious stimulus. When mice were fed with adenine-enriched diet, aging kidneys showed more extensive tubular injury and fibrosis with stronger inflammatory response than young kidneys. Taken together, our results suggest that renal aging may lie on a continuum between normal kidneys and diseased kidneys in the context of immune system upregulation that can worsen kidney damage upon injury.

Assessment of nephron number and single-nephron glomerular filtration rate in a clinical setting

Total nephron counts vary widely between individuals and may affect susceptibility to certain diseases, including hypertension and chronic kidney disease. Detailed analyses of whole kidneys collected from autopsy patients remain the only method for accurately counting nephrons in humans, with no equivalent option in living subjects. Current technological advances have enabled estimations of nephron numbers in vivo, particularly the use of total nephron number and whole-kidney glomerular filtration rate to estimate the mean single-nephron glomerular filtration rate. The use of this method would allow physicians to detect dynamic changes in filtration function at the single-nephron level rather than to simply count the number of nephrons that appear to be functioning. Currently available methods for estimating total nephron number in clinical practice have the potential to overcome limitations associated with autopsy analyses and may therefore pave the way for new therapeutic interventions and improved clinical outcomes.

Nephron number and its determinants: a 2020 update

Studies of human nephron number have been conducted for well over a century and have uncovered a large variability in nephron number. However, the mechanisms influencing nephron endowment and loss, along with the etiology for the wide range among individuals are largely unknown. Advances in imaging technology have allowed investigators to revisit the principles of renal structure and physiology and their roles in the progression of kidney disease. Here, we will review the latest data on the influences impacting nephron number, innovations made over the last 6 years to understand and integrate renal structure and function, and new developments in the tools used to count nephrons in vivo.

Factors Associated With Residual Kidney Function and Proteinuria After Living Kidney Donation in the United States

BACKGROUND

Living kidney donors are carefully screened, but despite overall good health, long-term donor outcomes have been shown to vary by predonation demographics. Since 2013, the United Network for Organ Sharing has mandated 2-year postdonation follow-up with measurements of kidney function and proteinuria.

METHODS

Using data from the Scientific Registry of Transplant Recipients, we sought to analyze donor factors associated with the percent change of kidney function from baseline (predonation) to 2-year postdonation, along with incidence of proteinuria reported within the same follow-up period.

RESULTS

Older donor age, male gender, black race, and body mass index >25 kg/m2 were independently associated with a greater percent decline in estimated glomerular filtration rate (eGFR). Male gender, black race, and higher body mass index were also independently associated with incident proteinuria. In contrast, younger donor age was associated with proteinuria, but proteinuria did not correlate with greater decline in eGFR in the overall cohort.

CONCLUSIONS

Donor factors associated with lower eGFR at 2-year postdonation were similar to those previously found to be associated with long-term risk for end-stage renal disease. Early postdonation assessment of kidney function and proteinuria may help to identify donors who are at greater risk of end-stage renal disease and who may benefit from more intense long-term monitoring.

Multiscale three-dimensional imaging of intact human organs down to the cellular scale using hierarchical phase-contrast tomography

Human organs are complex, three-dimensional and multiscale systems. Spatially mapping the human body down through its hierarchy, from entire organs to their individual functional units and specialised cells, is a major obstacle to fully understanding health and disease. To meet this challenge, we developed hierarchical phase-contrast tomography (HiP-CT), an X-ray phase propagation technique utilising the European Synchrotron Radiation Facility's Extremely Brilliant Source: the world's first high-energy 4 th generation X-ray source. HiP-CT enabled three-dimensional and non-destructive imaging at near-micron resolution in soft tissues at one hundred thousand times the voxel size whilst maintaining the organ's structure. We applied HiP-CT to image five intact human parenchymal organs: brain, lung, heart, kidney and spleen. These were hierarchically assessed with HiP-CT, providing a structural overview of the whole organ alongside detail of the organ's individual functional units and cells. The potential applications of HiP-CT were demonstrated through quantification and morphometry of glomeruli in an intact human kidney, and identification of regional changes to the architecture of the air-tissue interface and alveolar morphology in the lung of a deceased COVID-19 patient. Overall, we show that HiP-CT is a powerful tool which can provide a comprehensive picture of structural information for whole intact human organs, encompassing precise details on functional units and their constituent cells to better understand human health and disease.

Kidney Transplantation and "Sex Mismatch": A 10-Year Single-Center Analysis

Background

The effect of a relative disproportion in the size of a transplanted kidney (KT) on graft function and survival is well documented. However, the importance of the H-Y antigen (male donor and female recipient) has not been unambiguously confirmed.

Material/Methods

Our retrospective analysis consists of 230 deceased donor/recipient pairs. The aim of the study was to determine the effect of sex mismatch between donors and recipients on the function of the graft and the graft and patient survival.

Results

In the group of male donors, a statistically significantly lower value of the eGFR (estimated glomerular filtration rate) was recorded for female recipients in the fifth year after the KT (=0.0047). The male donor/female recipient group was an independent risk factor for: eGFR (<60 ml/min (CKD-EPI, Chronic Kidney Disease Epidemiology Collaboration) in the third year after KT [HR 0.1618; (P=0.0004)], acute rejection in the first year after KT [HR 1.8992; (P=0.0387)], and the 5-year graft survival was significantly worse in this group. By adjusting the results for age and induction, this group was at significantly higher risk for decreased graft function (eGFR <30 ml/min) if the age of the donor was ≤50 years old and the recipient was >45 years old in the fifth year [HR 11.1676; (P=0.0139)], the age of the donor was ≤50 years old/recipient was ≤45 years old in the third year [HR 1.2500; (P=0.0050)], and also in the fifth year after KT [HR 8.1993; (P=0.0183)].

Conclusions

Based on our analysis, the differences in the incidence of acute rejection episodes as well as in graft survival among the different groups of patients were confirmed. The group with the highest risk, in cases of an acute rejection episode, is a male donor/female recipient.

Rethinking toxicity testing: Influence of aging on the outcome of long-term toxicity testing and possible remediation

Traditionally, toxicity testing is conducted at fixed dose rates (i.e., mg/kg/day) without considering life-changing events, e.g., stress, sickness, aging- and/or pregnancy-related changes in physical, physiological and biochemical parameters. In humans, life-changing events may cause systemic dose non-proportionality requiring modulation of drug dosage; similar changes occur in animals altering systemic dose during chronic/carcinogenic testing leading to "late-occurring" effects in some studies. For example, propylene monomethyl ether, an industrial chemical, initially induced sedation in rats and mice with recovery upon induction of hepatic CYPs after ~1 week. Sedation reappeared in rats but not in mice after ~12 months of exposure due to decreased CYP activity in rats, elderly mice were able to maintain slightly higher CYP activity avoiding recurrence of sedation. The systemic dose of two pharmaceuticals (doxazosin and brimonidine tartrate) increased up to 6-fold in ≥12-month old rats with no toxicity. In a rat reproductive toxicity study, systemic dose of 2,4-D, an herbicide, rapidly increased due to increased consumption of 2,4-D-fortified diet during pregnancy, lactation and neonatal growth, requiring adjustment to maintain the targeted systemic dose. Ideally, toxicological studies should be based on systemic dose with the option of modulating external dose rates to maintain the targeted systemic dose. Systemic dose can easily be monitored in selected core study animals at desired intervals considering recent developments in sampling and analysis at a fraction of the overall cost of a study.

In-vivo techniques for determining nephron number

PURPOSE OF REVIEW

Many studies have suggested low nephron endowment at birth contributes to the risk of developing hypertension and chronic kidney disease (CKD) later in life. Loss of nephrons with age and disease is largely a subclinical process. New technologies are needed to count nephrons as glomerular filtration rate (GFR) is a poor surrogate for nephron number.

RECENT FINDINGS

Cortical volume, glomerular density, and percent globally sclerotic glomeruli are imperfect surrogates for nephron number. The disector-fractionator method is the most accurate method to count nephrons but is limited to autopsy settings. Glomerular density combined with kidney imaging and ultrafiltration coefficient-based methods require a kidney biopsy, and have been applied in living humans (kidney donors). Low nephron number predicts a higher postdonation urine albumin. Contrast-enhanced MRI has detected glomeruli without a biopsy, but so far, not in living humans.

SUMMARY

Currently, there is no accurate and well tolerated method for determining nephron number in living humans. A clinically useful method may allow GFR to be replaced by its more relevant determinants: nephron number and single nephron GFR. This could revolutionize nephrology by separating the measurement of chronic disease (nephron loss) from more reversible hemodynamic effects (nephron hyperfiltration/hypofiltration).

Clinical consequences of developmental programming of low nephron number

Nephron number in humans varies up to 13-fold, likely reflecting the impact of multiple factors on kidney development, including inherited body size and ethnicity, as well as maternal health and nutrition, fetal exposure to gestational diabetes or preeclampsia and other environmental factors, which may potentially be modifiable. Such conditions predispose to low or high offspring birth weight, growth restriction or preterm birth, which have all been associated with increased risks of higher blood pressures and/or kidney dysfunction in later life. Low birth weight, preterm birth, and intrauterine growth restriction are associated with reduced nephron numbers. Humans with hypertension and chronic kidney disease tend to have fewer nephrons than their counterparts with normal blood pressures or kidney function. A developmentally programmed reduction in nephron number therefore enhances an individual's susceptibility to hypertension and kidney disease in later life. A low nephron number at birth may not lead to kidney dysfunction alone except when severe, but in the face of superimposed acute or chronic kidney injury, a kidney endowed with fewer nephrons may be less able to adapt, and overt kidney disease may develop. Given that millions of babies are born either too small, too big or too soon each year, the population impact of altered renal programming is likely to be significant. Many gestational exposures are modifiable, therefore urgent attention is required to implement public health measures to optimize maternal, fetal, and child health, to prevent or mitigate the consequences of developmental programming, to improve the health future generations.

CKD: A Call for an Age-Adapted Definition

Current criteria for the diagnosis of CKD in adults include persistent signs of kidney damage, such as increased urine albumin-to-creatinine ratio or a GFR below the threshold of 60 ml/min per 1.73 m2 This threshold has important caveats because it does not separate kidney disease from kidney aging, and therefore does not hold for all ages. In an extensive review of the literature, we found that GFR declines with healthy aging without any overt signs of compensation (such as elevated single-nephron GFR) or kidney damage. Older living kidney donors, who are carefully selected based on good health, have a lower predonation GFR compared with younger donors. Furthermore, the results from the large meta-analyses conducted by the CKD Prognosis Consortium and from numerous other studies indicate that the GFR threshold above which the risk of mortality is increased is not consistent across all ages. Among younger persons, mortality is increased at GFR <75 ml/min per 1.73 m2, whereas in elderly people it is increased at levels <45 ml/min per 1.73 m2 Therefore, we suggest that amending the CKD definition to include age-specific thresholds for GFR. The implications of an updated definition are far reaching. Having fewer healthy elderly individuals diagnosed with CKD could help reduce inappropriate care and its associated adverse effects. Global prevalence estimates for CKD would be substantially reduced. Also, using an age-specific threshold for younger persons might lead to earlier identification of CKD onset for such individuals, at a point when progressive kidney damage may still be preventable.

Glomerular Volume and Glomerulosclerosis at Different Depths within the Human Kidney

BACKGROUND

Age, CKD risk factors, and kidney function are associated with larger glomerular volume and a higher percentage of globally sclerotic glomeruli. Knowledge of how these associations may differ by cortical depth is limited.

METHODS

To investigate glomerular volume and glomerulosclerosis across different depths of cortex, we studied wedge sections of the renal parenchyma from 812 patients who underwent a radical nephrectomy (for a tumor), separately characterizing glomeruli in the superficial (subcapsular), middle, and deep (juxtamedullary) regions. We compared the association of mean nonsclerotic glomerular volume and of glomerulosclerosis (measured as the percentage of globally sclerotic glomeruli) with age, obesity, diabetes, smoking, kidney function, and structural pathology in the superficial, middle, and deep regions.

RESULTS

The superficial, middle, and deep regions showed significant differences in glomerular volume (0.0025, 0.0031, and 0.0028 µm³, respectively) and in glomerulosclerosis (18%, 7%, and 11%, respectively). There was a marked increase in glomerulosclerosis with age in the superficial region, but larger glomerular volume was not associated with age at any cortical depth. Glomerulosclerosis associated more strongly with arteriosclerosis and ischemic-appearing glomeruli in the superficial region. Hypertension, lower eGFR, and interstitial fibrosis associated with glomerulosclerosis and glomerular volume to a similar extent at any depth. Diabetes and proteinuria more strongly associated with glomerulosclerosis in the deep and middle regions, respectively, but neither associated with glomerular volume differently by depth. Obesity associated more strongly with glomerular volume in the superficial cortex.

CONCLUSIONS

Most clinical characteristic show similar associations with glomerulosclerosis and glomerulomegaly at different cortical depths. Exceptions include age-related glomerulosclerosis, which appears to be an ischemic process and is more predominant in the superficial region.

Compensatory renal hypertrophy following nephrectomy: When and how?

Following surgical removal of one kidney, the other enlarges and increases its function. The mechanism for the sensing of this change and the growth is incompletely understood but begins within days and compensatory renal hypertrophy (CRH) is the dominant contributor to the growth. In many individuals undergoing nephrectomy for cancer or kidney donation this produces a substantial and helpful increase in renal function. Two main mechanisms have been proposed, one in which increased activity by the remaining kidney leads to hypertrophy, the second in which there is release of a kidney specific factor in response to a unilateral nephrectomy that initiates CRH. Whilst multiple growth factors and pathways such as the mTORC pathway have been implicated in experimental studies, their roles and the precise mechanism of CRH are not defined. Unrestrained hypoxia inducible factor activation in renal cancer promotes growth and may play an important role in driving CRH.

Hyperfiltration-mediated Injury in the Remaining Kidney of a Transplant Donor

Kidney donors face a small but definite risk of end-stage renal disease 15 to 30 years postdonation. The development of proteinuria, hypertension with gradual decrease in kidney function in the donor after surgical resection of 1 kidney, has been attributed to hyperfiltration. Genetic variations, physiological adaptations, and comorbidities exacerbate the hyperfiltration-induced loss of kidney function in the years after donation. A focus on glomerular hemodynamics and capillary pressure has led to the development of drugs that target the renin-angiotensin-aldosterone system (RAAS), but these agents yield mixed results in transplant recipients and donors. Recent work on glomerular biomechanical forces highlights the differential effects of tensile stress and fluid flow shear stress (FFSS) from hyperfiltration. Capillary wall stretch due to glomerular capillary pressure increases tensile stress on podocyte foot processes that cover the capillary. In parallel, increased flow of the ultrafiltrate due to single-nephron glomerular filtration rate elevates FFSS on the podocyte cell body. Although tensile stress invokes the RAAS, FFSS predominantly activates the cyclooxygenase 2-prostaglandin E2-EP2 receptor axis. Distinguishing these 2 mechanisms is critical, as current therapeutic approaches focus on the RAAS system. A better understanding of the biomechanical forces can lead to novel therapeutic agents to target FFSS through the cyclooxygenase 2-prostaglandin E2-EP2 receptor axis in hyperfiltration-mediated injury. We present an overview of several aspects of the risk to transplant donors and discuss the relevance of FFSS in podocyte injury, loss of glomerular barrier function leading to albuminuria and gradual loss of renal function, and potential therapeutic strategies to mitigate hyperfiltration-mediated injury to the remaining kidney.

Pre-donation BMI and preserved kidney volume can predict the cohort with unfavorable renal functional compensation at 1-year after kidney donation

BACKGROUND

The magnitude of renal function recovery after kidney donation differs in donors with a heterogeneous background. Preoperative assessment of candidates with potentially unfavorable renal functional compensation is critical when baseline kidney function is marginal. We explored the significance of preserved kidney volume (PKV) and known preoperative risk factors for the prediction of unfavorable renal function compensation.

METHODS

We enrolled 101 living donors for whom a 1-mm sliced enhanced computed tomography scan was performed preoperatively and clinical data could be collected up to 1 year after donation. The donors whose estimated glomerular filtration rate (eGFR) at 1 year after donation was 70% or higher of baseline eGFR were assigned to the "favorable renal compensation" group and the others to the "unfavorable renal compensation" group.

RESULTS

Age, sex, and preoperative serum uric acid level were not significant predictors for "unfavorable renal compensation." Multivariable logistic regression analysis revealed that body mass index (BMI) and body surface area (BSA)-adjusted PKV were independent preoperative risk factors for "unfavorable renal compensation" (adjusted odds ratio, 1.342 and 0.929, respectively). Hypertension and preoperative eGFR were not independent predictors when adjusted with BMI and BSA-adjusted PKV. Receiver operative characteristic analysis revealed that the predictive equation with the two independent predictors yielded a good accuracy to detect donor candidates with unfavorable renal functional compensation (area under the curve = 0.803), and the optimal cut-off values were identified as 23.4 kg/m² for BMI and 107.3 cm³/m² for BSA-adjusted PKV.

CONCLUSIONS

BMI and BSA-adjusted PKV may be useful to select candidates with potentially unfavorable renal function compensation before kidney donation.

Biopsy-based estimation of total nephron number in Japanese living kidney donors

BACKGROUND

Increasing evidence suggests that individuals with low nephron number have an increased lifetime risk of renal insufficiency, thereby emphasizing the importance of evaluating total nephron number in each individual. In recent years, new methods have been described for estimating human total nephron number using a combination of image analysis and renal biopsy, though the reproducibility and accuracy of these methods remain uncertain. This study estimated total nephron number in healthy Japanese subjects using such a method.

METHODS

Implantation biopsies from 44 living kidney donors were analyzed. Using pre-donation contrast CT angiograms, transplantation donor kidneys were three-dimensionally reconstructed, and total renal cortical volume was estimated. Total nephron number was estimated based on glomerular density in biopsy specimens and total renal cortical volume. The obtained results were analyzed in relation to clinical variables and compared with those of a previously reported Japanese autopsy study.

RESULTS

The estimated non-sclerotic and total numbers of glomeruli in this cohort were 650,000 ± 220,000 and 710,000 ± 220,000 (mean ± SD) per kidney. Non-sclerotic glomerular number ranged from 280,000 to 1,220,000 per kidney (4.4-fold) and correlated directly with eGFR (r = 0.328, p = 0.030) and inversely with age (r = - 0.355, p = 0.018).

CONCLUSION

The estimated total nephron number obtained in the present study was 25% less than that reported in American living kidney donors obtained using the same procedure and similar to that obtained in a previous Japanese autopsy study using the disector/fractionator method. These results confirm the feasibility of a combined CT angiography and biopsy-based method to estimate total nephron number in humans.

Pre- and postdonation kidney function in donors of a kidney paired donation with unique criteria for donor glomerular filtration rate - a longitudinal cohort analysis

Baseline predonation estimated GFR (eGFR) appears to predict the risk of postdonation chronic kidney disease in live donors. New KIDGO guidelines recommend an eGFR ≥90 ml/min/1.73 m² as an acceptable level of glomerular filtration rate (GFR) for kidney donation. In the Australian Paired Kidney Exchange (AKX) program, all donors with a raw measured GFR (mGFR) ≥80 ml/min are deemed suitable for donation, but the significance of this selection indicator is unclear. We analysed the first 129 live donors in the AKX program with at least 1-year follow-up linking records in the AKX database and ANZDATA. There were 73 male and 56 female donors; mean (±SD) age was 53 ± 11 years. Predonation eGFR was 94 ± 13 ml/min/1.73 m² , mGFR 99 ± 17 ml/min/1.73 m² and raw mGFR 108 ± 18 ml/min. Baseline eGFR was <80 ml/min/1.73 m² in 19 donors, and <90 ml/min/1.73 m² in 42 donors. At 1 year postdonation eGFR was 68 ± 15 ml/min/1.73 m² and the predicted eGFR at 30 years postdonation was on average 50 (29-83) ml/min/1.73 m² . The hypothetical mean age at end-stage kidney disease was estimated to be 145 (95% CI 120-263) years. Over 30% of AKX live donors would have been excluded from donation using KDIGO guidelines. Using AKX donor guidelines, the majority of donors with predicted eGFR <30 ml/min/1.73 m² 30-year postdonation were aged ≥50 years. Long-term outcome data on AKX donors with low eGFR will need careful monitoring.

The expression of two isoforms of matrix metalloproteinase-2 in aged mouse models of diabetes mellitus and chronic kidney disease

Background

This study was undertaken to explore the effects of aging on the kidneys in mouse models of diabetes and chronic kidney disease (CKD), and to compare the expression of two isoforms of matrix metalloproteinase-2 (MMP-2)–secretory full-length MMP-2 and intracellular N-terminal truncated MMP-2 (NTT-MMP-2)–in these models.

Methods

Two experimental ICR mouse models were used: a streptozotocin (STZ)-induced type 1 diabetes mellitus model and a 5/6 nephrectomized (5/6Nx) CKD model. The abundance of each isoform of MMP-2 was determined by quantitative polymerase chain reaction (qPCR), and functional analyses were conducted. Moreover, the protein levels of the two MMP-2 isoforms were determined semi-quantitatively by immunohistochemical staining, and their association with tissue damage was assessed.

Results

Both isoforms of MMP-2 were upregulated in the kidney tissues of STZ-induced diabetic mice and 5/6Nx mice, irrespective of age. Characteristically, NTT-MMP-2 protein expression was elevated in old control mice, in line with the qPCR results. NTT-MMP-2 expression was limited to the renal cortex, and to the tubulointerstitial area rather than the glomerular area. In terms of tissue damage, tubulointerstitial fibrosis was more severe in old 5/6Nx mice than in their young counterparts, whereas glomerulosclerosis was comparable in old and young 5/6Nx mice.

Conclusion

The intracellular isoform of MMP-2 was induced by ageing, irrespective of the presence of diabetes or CKD, and its induction may be related to tubulointerstitial fibrosis in chronic kidney disease.

Effect of unilateral nephrectomy on urinary angiotensinogen levels in living kidney donors: 1 year follow-up study

Background:

Urinary angiotensinogen (uAGT) has recently been proposed as a marker of kidney injury and activated intrarenal renin–angiotensin system. We investigated the effects of living donor nephrectomy on uAGT levels, blood pressure, estimated glomerular filtration rate, proteinuria and compensatory hypertrophy in the remaining kidney of living kidney donors.

Methods:

Twenty living kidney donors were included in the study and followed for 1 year. uAGT levels were measured with enzyme-linked immunosorbent assay preoperatively and postoperatively at the 15th day, 1, 6 and 12 months.

Results:

Four donors were excluded from the study due to lack of data. The mean baseline estimated glomerular filtration rate was 98 ± 15 ml/min/1.73 m². Serum creatinine, uAGT/creatinine, uAGT/protein levels were higher and estimated glomerular filtration rate was lower than baseline values at all time periods. Urinary protein/creatinine levels increased after donor nephrectomy, but after 6 months they returned to baseline values. Renal volume increased after nephrectomy, but these changes did not show any correlation with uAGT/creatinine, uAGT/protein, estimated glomerular filtration rate or systolic/diastolic blood pressures. uAGT/creatinine at 6 months and urinary protein/creatinine ratio at 12 months showed a positive correlation (P=0.008, r=0.639).

Conclusion:

After donor nephrectomy, increasing uAGT levels can be the result of activation of the intrarenal renin–angiotensin system affecting the compensatory changes in the remaining kidney. The long-term effects of increased uAGT levels on the remaining kidney should be examined more closely in future studies.

White blood cell fractions correlate with lesions of diabetic kidney disease and predict loss of kidney function in Type 2 diabetes

BACKGROUND

Inflammation linked to diabetic kidney disease (DKD) may affect white blood cell (WBC) counts and differentials. We examined the cross-sectional associations of total WBC count and WBC fractions with structural lesions of DKD in 108 Pima Indians with Type 2 diabetes who underwent research kidney biopsies. We also examined the longitudinal association of these WBC variables with renal function loss (RFL) in 941 Europeans with Type 2 diabetes from the SURDIAGENE study.

METHODS

Associations of WBC variables with morphometric parameters were assessed by linear regression. RFL was defined as≥40% loss of estimated glomerular filtration rate from baseline. Associations with RFL were evaluated by Cox regression. Hazard ratios (HRs) were reported per standard deviation increment of each WBC variable.

RESULTS

After multivariable adjustment, lymphocyte (r  = -0.20, P = 0.043) and eosinophil (r = 0.21, P = 0.032) fractions in the Pima Indians correlated with glomerular basement membrane width. Eosinophil fraction also correlated with glomerular filtration surface density (r  = -0.21, P = 0.031). Lymphocyte fraction (r = 0.25, P = 0.013), neutrophil fraction (r  = -0.23, P = 0.021) and the neutrophil:lymphocyte ratio (r  = -0.22, P = 0.024) correlated with percentage of normally fenestrated endothelial cells. During median follow-up of 4.5 years, 321 SURDIAGENE participants developed RFL. Lower lymphocyte fraction [HR = 0.67, 95% confidence interval (95% CI) 0.60-0.76] and higher neutrophil fraction (HR = 1.35, 95% CI 1.20-1.52), total WBC count (HR = 1.20, 95% CI 1.08-1.35) and neutrophil:lymphocyte ratio (HR = 1.44, 95% CI 1.28-1.62) each predicted RFL in this cohort.

CONCLUSIONS

WBC fractions associate with morphometric lesions of DKD and predict RFL in individuals with Type 2 diabetes.

Clinical and Pathology Findings Associate Consistently with Larger Glomerular Volume

Background Glomerular volume increases when demand exceeds nephron supply, which may lead to glomerulosclerosis. It is unclear if determinants of glomerular volume are consistent between populations that differ by severity of comorbidities.Methods We studied kidney biopsy specimens from living kidney donors (n=2453) and patients who underwent radical nephrectomy for a renal tumor (n=780). We scanned specimen sections into high-resolution digital images, manually traced glomerular profiles, and calculated mean glomerular volumes using the Weibel-Gomez stereologic formula (separately for nonsclerosed glomeruli and globally sclerosed glomeruli). We then assessed the relationship of glomerular volume with age, clinical characteristics, and nephrosclerosis on biopsy specimen.Results Compared with kidney donors, patients with tumors were older and more frequently men, obese, diabetic, or hypertensive, had more glomerulosclerosis and interstitial fibrosis on biopsy specimen, and had 12% larger nonsclerosed glomeruli (P<0.001). In both populations, male sex, taller height, obesity, hypertension, and proteinuria associated with larger nonsclerosed glomeruli to a similar extent. In patients with tumors, diabetes, glomerulosclerosis >25%, and interstitial fibrosis >25% also associated with larger nonsclerosed glomeruli. Independent clinical predictors of larger nonsclerotic glomeruli were family history of ESRD, male sex, taller height, obesity, diabetes, and proteinuria. After adjustment for these characteristics, nonsclerotic glomerular volume did not differ between populations and was stable up to age 75 years, after which it decreased with age. Many of these findings were also evident with globally sclerotic glomerular volume.Conclusions Characteristics associated with glomerular volume are consistent between patient populations with low and high levels of comorbidity.

Opportunities for organoids as new models of aging

The biology of aging is challenging to study, particularly in humans. As a result, model organisms are used to approximate the physiological context of aging in humans. However, the best model organisms remain expensive and time-consuming to use. More importantly, they may not reflect directly on the process of aging in people. Human cell culture provides an alternative, but many functional signs of aging occur at the level of tissues rather than cells and are therefore not readily apparent in traditional cell culture models. Organoids have the potential to effectively balance between the strengths and weaknesses of traditional models of aging. They have sufficient complexity to capture relevant signs of aging at the molecular, cellular, and tissue levels, while presenting an experimentally tractable alternative to animal studies. Organoid systems have been developed to model many human tissues and diseases. Here we provide a perspective on the potential for organoids to serve as models for aging and describe how current organoid techniques could be applied to aging research.

Structural and Functional Changes in Human Kidneys with Healthy Aging

Aging is associated with significant changes in structure and function of the kidney, even in the absence of age-related comorbidities. On the macrostructural level, kidney cortical volume decreases, surface roughness increases, and the number and size of simple renal cysts increase with age. On the microstructural level, the histologic signs of nephrosclerosis (arteriosclerosis/arteriolosclerosis, global glomerulosclerosis, interstitial fibrosis, and tubular atrophy) all increase with age. The decline of nephron number is accompanied by a comparable reduction in measured whole-kidney GFR. However, single-nephron GFR remains relatively constant with healthy aging as does glomerular volume. Only when glomerulosclerosis and arteriosclerosis exceed that expected for age is there an increase in single-nephron GFR. In the absence of albuminuria, age-related reduction in GFR with the corresponding increase in CKD (defined by an eGFR<60 ml/min per 1.73 m²) has been shown to associate with a very modest to no increase in age-standardized mortality risk or ESRD. These findings raise the question of whether disease labeling of an age-related decline in GFR is appropriate. These findings also emphasize the need for a different management approach for many elderly individuals considered to have CKD by current criteria.

Single-Nephron Glomerular Filtration Rate in Healthy Adults

BACKGROUND

The glomerular filtration rate (GFR) assesses the function of all nephrons, and the single-nephron GFR assesses the function of individual nephrons. How the single-nephron GFR relates to demographic and clinical characteristics and kidney-biopsy findings in humans is unknown.

METHODS

We identified 1388 living kidney donors at the Mayo Clinic and the Cleveland Clinic who underwent a computed tomographic (CT) scan of the kidney with the use of contrast material and an iothalamate-based measurement of the GFR during donor evaluation and who underwent a kidney biopsy at donation. The mean single-nephron GFR was calculated as the GFR divided by the number of nephrons (calculated as the cortical volume of both kidneys as assessed on CT times the biopsy-determined glomerular density). Demographic and clinical characteristics and biopsy findings were correlated with the single-nephron GFR.

RESULTS

A total of 58% of the donors were women, and the mean (±SD) age of the donors was 44±12 years. The mean GFR was 115±24 ml per minute, the mean number of nephrons was 860,000±370,000 per kidney, and the mean single-nephron GFR was 80±40 nl per minute. The single-nephron GFR did not vary significantly according to age (among donors <70 years of age), sex, or height (among donors ≤190 cm tall). A higher single-nephron GFR was independently associated with larger nephrons on biopsy and more glomerulosclerosis and arteriosclerosis than would be expected for age. A higher single-nephron GFR was associated with a height of more than 190 cm, obesity, and a family history of end-stage renal disease.

CONCLUSIONS

Among healthy adult kidney donors, the single-nephron GFR was fairly constant with regard to age, sex, and height (if ≤190 cm). A higher single-nephron GFR was associated with certain risk factors for chronic kidney disease and certain kidney-biopsy findings. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.).

The conundrums of chronic kidney disease and aging

Chronic kidney disease (CKD), as presently defined, is a common disorder. Aging is a nearly universal phenomenon that can affect renal anatomy and function, but at variable rates in individuals. Loss of nephrons and a decline in glomerular filtration rate (GFR) is a characteristic of normal aging, called renal senescence. Using fixed and absolute thresholds for defining CKD on the basis of GFR for all ages may lead to diagnostic uncertainty (a conundrum) in both young and older subjects. This brief review will consider the physiological and anatomical changes of the kidney occurring in the process of normal renal senescence focusing on GFR and will examine the relevance of these observation for the diagnosis of CKD using GFR as the distinguishing parameter. Once a better understanding of the pathobiology underlying renal senescence is obtained, specific interventions may become available to slow the process.

Age-Related Renal Microvascular Changes: Evaluation by Three-Dimensional Digital Imaging of the Human Renal Microcirculation Using Virtual Microscopy

The renal microvasculature is targeted during aging, sometimes producing chronic kidney disease (CKD). Overdiagnosis of CKD in older persons is concerning. To prevent it, a new concept of “healthy aging” is arising from a healthy renal donor study. We investigated the renal microcirculatory changes of three older persons and compared them with that of one patient with nephrosclerosis using a three-dimensional (3D) reconstruction technique that we previously developed. This method uses a virtual slide system and paraffin-embedded serial sections of surgical material that was double-immunostained by anti-CD34 and anti-α smooth muscle actin (SMA) antibodies for detecting endothelial cells and medial smooth muscle cells, respectively. In all cases, the 3D images proved that arteriosclerotic changes in large proximal interlobular arteries did not directly induce distal arterial change or glomerulosclerosis. The nephrosclerotic patient showed severe hyalinosis with luminal narrowing of small arteries directly inducing glomerulosclerosis. We also visualized an atubular glomerulus and intraglomerular dilatation of an afferent arteriole during healthy aging on the 3D image and showed that microcirculatory changes were responsible for them. Thus, we successfully visualized healthy aged kidneys on 3D images and confirmed the underlying pathology. This method has the ability to investigate renal microcirculatory damage during healthy aging.

Structural and Functional Changes With the Aging Kidney

Senescence or normal physiologic aging portrays the expected age-related changes in the kidney as compared to a disease that occurs in some but not all individuals. The microanatomical structural changes of the kidney with older age include a decreased number of functional glomeruli from an increased prevalence of nephrosclerosis (arteriosclerosis, glomerulosclerosis, and tubular atrophy with interstitial fibrosis), and to some extent, compensatory hypertrophy of remaining nephrons. Among the macroanatomical structural changes, older age associates with smaller cortical volume, larger medullary volume until middle age, and larger and more numerous kidney cysts. Among carefully screened healthy kidney donors, glomerular filtration rate (GFR) declines at a rate of 6.3 mL/min/1.73 m(2) per decade. There is reason to be concerned that the elderly are being misdiagnosed with CKD. Besides this expected kidney function decline, the lowest risk of mortality is at a GFR of ≥75 mL/min/1.73 m(2) for age <55 years but at a lower GFR of 45 to 104 mL/min/1.73 m(2) for age ≥65 years. Changes with normal aging are still of clinical significance. The elderly have less kidney functional reserve when they do actually develop CKD, and they are at higher risk for acute kidney injury.

The Substantial Loss of Nephrons in Healthy Human Kidneys with Aging

Nephron number may be an important determinant of kidney health but has been difficult to study in living humans. We evaluated 1638 living kidney donors at Mayo Clinic (MN and AZ sites) and Cleveland Clinic. We obtained cortical volumes of both kidneys from predonation computed tomography scans. At the time of kidney transplant, we obtained and analyzed the sections of a biopsy specimen of the cortex to determine the density of both nonsclerotic and globally sclerotic glomeruli; the total number of glomeruli was estimated from cortical volume×glomerular density. Donors 18-29 years old had a mean 990,661 nonsclerotic glomeruli and 16,614 globally sclerotic glomeruli per kidney, which progressively decreased to 520,410 nonsclerotic glomeruli per kidney and increased to 141,714 globally sclerotic glomeruli per kidney in donors 70-75 years old. Between the youngest and oldest age groups, the number of nonsclerotic glomeruli decreased by 48%, whereas cortical volume decreased by only 16% and the proportion of globally sclerotic glomeruli on biopsy increased by only 15%. Clinical characteristics that independently associated with fewer nonsclerotic glomeruli were older age, shorter height, family history of ESRD, higher serum uric acid level, and lower measured GFR. The incomplete representation of nephron loss with aging by either increased glomerulosclerosis or by cortical volume decline is consistent with atrophy and reabsorption of globally sclerotic glomeruli and hypertrophy of remaining nephrons. In conclusion, lower nephron number in healthy adults associates with characteristics reflective of both lower nephron endowment at birth and subsequent loss of nephrons.

The Substantial Loss of Nephrons in Healthy Human Kidneys with Aging

Nephron number may be an important determinant of kidney health but has been difficult to study in living humans. We evaluated 1638 living kidney donors at Mayo Clinic (MN and AZ sites) and Cleveland Clinic. We obtained cortical volumes of both kidneys from predonation computed tomography scans. At the time of kidney transplant, we obtained and analyzed the sections of a biopsy specimen of the cortex to determine the density of both nonsclerotic and globally sclerotic glomeruli; the total number of glomeruli was estimated from cortical volume×glomerular density. Donors 18-29 years old had a mean 990,661 nonsclerotic glomeruli and 16,614 globally sclerotic glomeruli per kidney, which progressively decreased to 520,410 nonsclerotic glomeruli per kidney and increased to 141,714 globally sclerotic glomeruli per kidney in donors 70-75 years old. Between the youngest and oldest age groups, the number of nonsclerotic glomeruli decreased by 48%, whereas cortical volume decreased by only 16% and the proportion of globally sclerotic glomeruli on biopsy increased by only 15%. Clinical characteristics that independently associated with fewer nonsclerotic glomeruli were older age, shorter height, family history of ESRD, higher serum uric acid level, and lower measured GFR. The incomplete representation of nephron loss with aging by either increased glomerulosclerosis or by cortical volume decline is consistent with atrophy and reabsorption of globally sclerotic glomeruli and hypertrophy of remaining nephrons. In conclusion, lower nephron number in healthy adults associates with characteristics reflective of both lower nephron endowment at birth and subsequent loss of nephrons.

Cardiovascular autonomic neuropathy associates with nephropathy lesions in American Indians with type 2 diabetes

AIMS

Cardiovascular autonomic neuropathy (CAN) predicts clinical diabetic nephropathy (DN). We investigated the relationship between DN structural lesions and CAN.

METHODS

Sixty three Pima Indians with type 2 diabetes underwent kidney biopsies following a 6-year clinical trial testing the renoprotective efficacy of losartan vs. placebo. CAN was assessed a median 9.2years later. CAN variables included expiration/inspiration ratio (E/I), standard deviation of the normal R-R interval (sdNN), and low and high frequency signal power and their ratio (LF, HF, LF/HF); lower values reflect more severe neuropathy. Associations of CAN with renal structural variables were assessed by linear regression adjusted for age, sex, diabetes duration, blood pressure, HbA1c, glomerular filtration rate, and treatment assignment during the trial.

RESULTS

Global glomerular sclerosis was negatively associated with sdNN (partial r=-0.35, p=0.01) and LF (r=-0.32, p=0.02); glomerular basement membrane width was negatively associated with all measures of CAN except for LF/HF (r=-0.28 to -0.42, p<0.05); filtration surface density was positively associated with sdNN, LF, and HF (r=0.31 to 0.38, p<0.05); and cortical interstitial fractional volume was negatively associated with HF (r=-0.27, p=0.04).

CONCLUSIONS

CAN associates with DN lesions.

Aging and the Kidneys: Anatomy, Physiology and Consequences for Defining Chronic Kidney Disease

The varied functions of the kidneys are influenced by the complex process of aging. The glomerular filtration rate (GFR) steadily declines with normal aging, and the progress of this process can be influenced by superimposed diseases. Microscopically, nephron numbers decrease as global glomerulosclerosis becomes more evident. The precise mechanisms underlying nephron loss with aging are not well understood, but derangements in podocyte biology appear to be involved. Classifications of chronic kidney disease (CKD) incorporate GFR values and attendant risk of adverse events. Arbitrary and fixed thresholds of GFR for defining CKD have led to an overdiagnosis of CKD in the elderly. An age-sensitive definition of CKD could offer a solution to this problem and more meaningfully capture the prognostic implications of CKD.

Glomerular Function and Structure in Living Donors: Lessons from Single Nephron Studies

One third of the kidney transplants performed in the USA come from living kidney donors. The long-term outcome of healthy individuals who donate kidneys is mostly excellent, although recent studies have suggested that living donation is associated with a small absolute increase in the risk of end stage renal failure. Much of our understanding about the progression of kidney disease comes from experimental models of nephron loss. For this reason, living kidney donation has long been of great interest to renal physiologists. This review will summarize the determinants of glomerular filtration and the physiology that underlies post-donation hyperfiltration. We describe the 'remnant kidney' model of kidney disease and the reasons why such progressive kidney disease very rarely ensues in healthy humans following uninephrectomy. We also review some of the methods used to determine glomerular number and size and outline their associations.

Chronic Kidney Disease in Aged Cats: Clinical Features, Morphology, and Proposed Pathogeneses

Chronic kidney disease (CKD) is the most common metabolic disease of domesticated cats, with most affected cats being geriatric (>12 years of age). The prevalence of CKD in cats exceeds that observed in dogs, and the frequency of the diagnosis of CKD in cats has increased in recent decades. Typical histologic features include interstitial inflammation, tubular atrophy, and fibrosis with secondary glomerulosclerosis. In contrast to people and dogs, primary glomerulopathies with marked proteinuria are remarkably rare findings in cats. Although a variety of primary renal diseases have been implicated, the disease is idiopathic in most cats. Tubulointerstitial changes, including fibrosis, are present in the early stages of feline CKD and become more severe in advanced disease. A variety of factors-including aging, ischemia, comorbid conditions, phosphorus overload, and routine vaccinations-have been implicated as factors that could contribute to the initiation of this disease in affected cats. Factors that are related to progression of established CKD, which occurs in some but not all cats, include dietary phosphorus intake, magnitude of proteinuria, and anemia. Renal fibrosis, a common histologic feature of aged feline kidneys, interferes with the normal relationship between peritubular capillaries and renal tubules. Experimentally, renal ischemia results in morphologic changes similar to those observed in spontaneous CKD. Renal hypoxia, perhaps episodic, may play a role in the initiation and progression of this disease.

Tumor necrosis factor receptors 1 and 2 are associated with early glomerular lesions in type 2 diabetes

Elevated serum tumor necrosis factor receptor 1 (TNFR1) and 2 (TNFR2) concentrations are strongly associated with increased risk of end-stage renal disease in type 2 diabetes. However, little is known about the early glomerular structural lesions that develop in patients when these markers are elevated. Here we examined the relationships between TNFRs and glomerular structure in 83 American Indians with type 2 diabetes. Serum TNFRs and glomerular filtration rates (GFR, iothalamate) were measured during a research exam performed within a median of 0.9 months from a percutaneous kidney biopsy. Associations of TNFRs with glomerular structural variables were quantified by Spearman's correlations and by multivariable linear regression after adjustment for age, gender, diabetes duration, hemoglobin A1c, body mass index, and mean arterial pressure. The baseline mean age was 46 years, median GFR 130 ml/min, median albumin/creatinine ratio 26 mg/g, median TNFR1 1500 pg/ml, and median TNFR2 3284 pg/ml. After multivariable adjustment, TNFR1 and TNFR2 significantly correlated inversely with the percentage of endothelial cell fenestration and the total filtration surface per glomerulus. There were significant positive correlations with mesangial fractional volume glomerular basement membrane width, podocyte foot process width, and percent of global glomerular sclerosis. Thus, TNFRs may be involved in the pathogenesis of early glomerular lesions in diabetic nephropathy.