Podometrics in Japanese Living Donor Kidneys: Associations with Nephron Number, Age, and Hypertension

Advancing the application of the analytical renal pathology system in allograft IgA nephropathy patients

BACKGROUND

The analytical renal pathology system (ARPS) based on convolutional neural networks has been used successfully in native IgA nephropathy (IgAN) patients. Considering the similarity of pathologic features, we aim to evaluate the performance of the ARPS in allograft IgAN patients and broaden its implementation.

METHODS

Biopsy-proven allograft IgAN patients from two different centers were enrolled for internal and external validation. We implemented the ARPS to identify glomerular lesions and intrinsic glomerular cells, and then evaluated its performance. Consistency between the ARPS and pathologists was assessed using intraclass correlation coefficients. The association of digital pathological features with clinical and pathological data was measured. Kaplan-Meier survival curve and cox proportional hazards model were applied to investigate prognosis prediction.

RESULTS

A total of 56 biopsy-proven allograft IgAN patients from the internal center and 17 biopsy-proven allograft IgAN patients from the external center were enrolled in this study. The ARPS was successfully applied to identify the glomerular lesions (F1-score, 0.696-0.959) and quantify intrinsic glomerular cells (F1-score, 0.888-0.968) in allograft IgAN patients rapidly and precisely. Furthermore, the mesangial hypercellularity score was positively correlated with all mesangial metrics provided by ARPS [Spearman's correlation coefficient (r), 0.439-0.472, and all p values < 0.001]. Besides, a higher allograft survival was noticed among patients in the high-level groups of the maximum and ratio of endothelial cells, as well as the maximum and density of podocytes.

CONCLUSION

We propose that the ARPS could be implemented in future clinical practice with outstanding capability.

Podocytes from hypertensive and obese mice acquire an inflammatory, senescent, and aged phenotype

Patients with hypertension or obesity can develop glomerular dysfunction characterized by injury and depletion of podocytes. To better understand the molecular processes involved, young mice were treated with either deoxycorticosterone acetate (DOCA) or fed a high-fat diet (HFD) to induce hypertension or obesity, respectively. The transcriptional changes associated with these phenotypes were measured by unbiased bulk mRNA sequencing of isolated podocytes from experimental models and their respective controls. Key findings were validated by immunostaining. In addition to a decrease in canonical proteins and reduced podocyte number, podocytes from both hypertensive and obese mice exhibited a sterile inflammatory phenotype characterized by increases in NLR family pyrin domain containing 3 (NLRP3) inflammasome, protein cell death-1, and Toll-like receptor pathways. Finally, although the mice were young, podocytes in both models exhibited increased expression of senescence and aging genes, including genes consistent with a senescence-associated secretory phenotype. However, there were differences between the hypertension- and obesity-associated senescence phenotypes. Both show stress-induced podocyte senescence characterized by increased p21 and p53. Moreover, in hypertensive mice, this is superimposed upon age-associated podocyte senescence characterized by increased p16 and p19. These results suggest that senescence, aging, and inflammation are critical aspects of the podocyte phenotype in experimental hypertension and obesity in mice.NEW & NOTEWORTHY Hypertension and obesity can lead to glomerular dysfunction in patients, causing podocyte injury and depletion. Here, young mice given deoxycorticosterone acetate or a high-fat diet to induce hypertension or obesity, respectively. mRNA sequencing of isolated podocytes showed transcriptional changes consistent with senescence, a senescent-associated secretory phenotype, and aging, which was confirmed by immunostaining. Ongoing studies are determining the mechanistic roles of the accelerated aging podocyte phenotype in experimental hypertension and obesity.

Podocyte number and glomerulosclerosis indices are associated with the response to therapy for primary focal segmental glomerulosclerosis

Corticosteroid therapy, often in combination with inhibition of the renin-angiotensin system, is first-line therapy for primary focal and segmental glomerulosclerosis (FSGS) with nephrotic-range proteinuria. However, the response to treatment is variable, and therefore new approaches to indicate the response to therapy are required. Podocyte depletion is a hallmark of early FSGS, and here we investigated whether podocyte number, density and/or size in diagnostic biopsies and/or the degree of glomerulosclerosis could indicate the clinical response to first-line therapy. In this retrospective single center cohort study, 19 participants (13 responders, 6 non-responders) were included. Biopsies obtained at diagnosis were prepared for analysis of podocyte number, density and size using design-based stereology. Renal function and proteinuria were assessed 6 months after therapy commenced. Responders and non-responders had similar levels of proteinuria at the time of biopsy and similar kidney function. Patients who did not respond to treatment at 6 months had a significantly higher percentage of glomeruli with global sclerosis than responders (p < 0.05) and glomerulosclerotic index (p < 0.05). Podocyte number per glomerulus in responders was 279 (203-507; median, IQR), 50% greater than that of non-responders (186, 118-310; p < 0.05). These findings suggest that primary FSGS patients with higher podocyte number per glomerulus and less advanced glomerulosclerosis are more likely to respond to first-line therapy at 6 months. A podocyte number less than approximately 216 per glomerulus, a GSI greater than 1 and percentage global sclerosis greater than approximately 20% are associated with a lack of response to therapy. Larger, prospective studies are warranted to confirm whether these parameters may help inform therapeutic decision making at the time of diagnosis of primary FSGS.

Morphometric analysis of chronicity on kidney biopsy: a useful prognostic exercise

Chronic changes on kidney biopsy specimens include increasing amounts of arteriosclerosis, glomerulosclerosis, interstitial fibrosis and tubular atrophy, enlarged nephron size, and reduced nephron number. These chronic changes are difficult to accurately assess by visual inspection but are reasonably quantified using morphometry. This review describes the various patient populations that have undergone morphometric analysis of kidney biopsies. The common approaches to morphometric analysis are described. The chronic kidney disease outcomes associated with various chronic changes by morphometry are also summarized. Morphometry enriches the characterization of chronicity on a kidney biopsy and this can supplement the pathologist's diagnosis. Artificial intelligence image processing tools are needed to automate the annotations needed for practical morphometric analysis of kidney biopsy specimens in routine clinical care.

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.

Spatial Heterogeneity of Glomerular Phenotypes Affects Kidney Biopsy Findings

Key Points

Background

Detection of rare glomerular phenotypes can affect diagnosis in indication kidney biopsies and in kidney tissue used for research studies. Nephropathologists are aware of potential sampling error when assessing needle biopsy cores, but quantitative data are lacking.

Methods

Kidney tissue from patients undergoing total nephrectomy enrolled in an observational, cross-sectional cohort study was used to characterize glomeruli as typical or atypical, which included globally sclerotic glomeruli (GSGs), segmentally sclerotic glomeruli, ischemic-like, and imploding. A 2D map of the glomerular annotations was generated. Spatial centrality of atypical glomeruli using the L2 metric and differences in pairwise distances between typical or atypical glomeruli were calculated. To determine how the yield of capturing atypical glomerular phenotype was affected by biopsy depth (i.e., not including the renal capsule), simulated kidney biopsies were generated from the 2D map.

Results

The mean number of glomeruli in a nephrectomy specimen was 209 (SD 143), and GSGs were the most common type of atypical glomeruli (median: 13% [interquartile range: 5,31]). Typical glomeruli were more likely to be surrounded by other glomeruli (i.e., centrally located in the kidney cortex) than GSGs, segmentally sclerosed glomeruli, ischemic-like glomeruli, and imploding glomeruli. Atypical glomeruli were 7.3% (95% confidence interval, 4.1 to 10.4) closer together than typical glomeruli and were more likely to be closer together in older patients or those with hypertension. In simulated kidney biopsies, failure to capture the capsule was associated with underdetection of GSGs, ischemic-like glomeruli, and imploding glomeruli.

Conclusions

Spatial analysis of large sections of kidney tissue provided quantitative evidence of spatial heterogeneity of glomerular phenotypes including clustering of atypical glomeruli in individuals with hypertension or older age. Most importantly, deep kidney biopsies that lack subcapsular area underdetect atypical glomerular phenotypes, suggesting that capturing the renal capsule is an important quality control measure for kidney biopsies.

Comparative evaluation of glomerular morphometric techniques reveals differential technical artifacts between focal segmental glomerulosclerosis and normal glomeruli

Morphometric estimates of mean or individual glomerular volume (MGV, IGV) have biological implications, over and above qualitative histologic data. However, morphometry is time-consuming and requires expertise limiting its utility in clinical cases. We evaluated MGV and IGV using plastic- and paraffin-embedded tissue from 10 control and 10 focal segmental glomerulosclerosis (FSGS) mice (aging and 5/6th nephrectomy models) using the gold standard Cavalieri (Cav) method versus the 2-profile and Weibel-Gomez (WG) methods and a novel 3-profile method. We compared accuracy, bias and precision, and quantified results obtained when sampling differing numbers of glomeruli. In both FSGS and controls, we identified an acceptable precision for MGV of 10-glomerular sampling versus 20-glomerular sampling using the Cav method, while 5-glomerular sampling was less precise. In plastic tissue, 2- or 3-profile MGVs showed greater concordance with MGV when using Cav, versus MGV with WG. IGV comparisons using the same glomeruli reported a consistent underestimation bias with both 2- or 3-profile methods versus the Cav method. FSGS glomeruli showed wider variations in bias estimation than controls. Our 3-profile method offered incremental benefit to the 2-profile method in both IGV and MGV estimation (improved correlation coefficient, Lin's concordance and reduced bias). In our control animals, we quantified a shrinkage artifact of 52% from tissue processed for paraffin-embedded versus plastic-embedded tissue. FSGS glomeruli showed overall reduced shrinkage albeit with variable artifact signifying periglomerular/glomerular fibrosis. A novel 3-profile method offers slightly improved concordance with reduced bias versus 2-profile. Our findings have implications for future studies using glomerular morphometry.

Nephrons, podocytes and chronic kidney disease: Strategic antihypertensive therapy for renoprotection

Chronic kidney disease (CKD) is one of the strongest risk factors for hypertension, and hypertension can exacerbate the progression of CKD. Thus, the management of CKD and antihypertensive therapy are inextricably linked. Research over the past decades has shown that the human kidney is more diverse than initially thought. Subjects with low nephron endowment are at increased risk of developing CKD and hypertension, which is consistent with the theory of the developmental origins of health and disease. Combined with other lifetime risks of CKD, hypertension may lead to a vicious cycle consisting of podocyte injury, glomerulosclerosis and further loss of nephrons. Of note, recent studies have shown that the number of nephrons correlates well with the number of podocytes, suggesting that these two components are intrinsically linked and may influence each other. Both nephrons and podocytes have no or very limited regenerative capacity and are destined to decrease throughout life. Therefore, one of the best strategies to slow the progression of CKD is to maintain the "numbers" of these essential components necessary to preserve renal function. To this end, both the achievement of an optimal blood pressure and a maximum reduction in urinary protein excretion are essential. Lifestyle modifications and antihypertensive drug therapy must be carefully individualized to address the potential diversity of the kidneys.

Associations between nephron number and podometrics in human kidneys

Podocyte loss and resultant nephron loss are common processes in the development of glomerulosclerosis and chronic kidney disease. While the cortical distribution of glomerulosclerosis is known to be non-uniform, the relationship between the numbers of non-sclerotic glomeruli (NSG), podometrics and zonal differences in podometrics remain incompletely understood. To help define this, we studied autopsy kidneys from 50 adults with median age 68 years and median eGFR 73.5 mL/min/1.73m² without apparent glomerular disease in a cross-sectional analysis. The number of NSG per kidney was estimated using the physical dissector/fractionator combination, while podometrics were estimated using model-based stereology. The number of NSG per kidney was directly correlated with podocyte number per tuft and podocyte density. Each additional 100,000 NSG per kidney was associated with 26 more podocytes per glomerulus and 16 podocytes per 10⁶ μm³ increase in podocyte density. These associations were independent of clinical factors and cortical zone. While podocyte number per glomerulus was similar in the three zones, superficial glomeruli were the smallest and had the highest podocyte density but smallest podocytes. Increasing age and hypertension were associated with lower podocyte number, with age mostly affecting superficial glomeruli, and hypertension mostly affecting juxtamedullary glomeruli. Thus, in this first study to report a direct correlation between the number of NSG and podometrics, we suggest that podocyte number is decreasing in NSG of individuals losing nephrons. However, another possible interpretation may be that more nephrons might protect against further podocyte loss.

Imaging the Kidney with an Unconventional Scanning Electron Microscopy Technique: Analysis of the Subpodocyte Space in Diabetic Mice

Transmission electron microscopy (TEM) remains the gold standard for renal histopathological diagnoses, given its higher resolving power, compared with light microscopy. However, it imposes several limitations on pathologists, including longer sample preparation time and a small observation area. To overcome these, we introduced a scanning electron microscopy (SEM) technique for imaging resin-embedded semi-thin sections of renal tissue. We developed a rapid tissue preparation protocol for experimental models and human biopsies which, alongside SEM digital imaging acquisition of secondary electrons (SE–SEM), enables fast electron microscopy examination, with a resolution similar to that achieved by TEM. We used this unconventional SEM imaging approach to investigate the subpodocyte space (SPS) in BTBR ob/ob mice with type 2 diabetes. Analysis of semi-thin sections with secondary electrons revealed that the SPS had expanded in volume and covered large areas of the glomerular basement membrane, forming wide spaces between the podocyte body and the underlying filtering membrane. Our results show that SE–SEM is a valuable tool for imaging the kidney at the ultrastructural level, filling the magnification gap between light microscopy and TEM, and reveal that in diabetic mice, the SPS is larger than in normal controls, which is associated with podocyte damage and impaired kidney function.

The developmental origins of health and chronic kidney disease: Current status and practices in Japan

The concept of the developmental origins of health and disease (DOHaD) views unfavorable perinatal circumstances as contributing to the development of diseases in later life. It is well known that such unfavorable circumstances play an important role as a risk factor for chronic kidney disease (CKD) in infants born with prematurity. Low birthweight (LBW) is believed to be a potential contributor to CKD in adulthood. Preterm and/or LBW infants are born with incomplete nephrogenesis. As a result, the number of nephrons is low. The poor intrauterine environment also causes epigenetic changes that adversely affect postnatal renal function. After birth, hyperfiltration of individual nephrons due to low nephron numbers causes proteinuria and secondary glomerulosclerosis. Furthermore, the risk of CKD increases as renal damage takes a second hit from exposure to nephrotoxic substances and acquired insults such as acute kidney injury after birth among infants in neonatal intensive care. Meanwhile, unfortunately, recent studies have shown that the number of nephrons in healthy Japanese individuals is approximately two-thirds lower than that in previous reports. This means that Japanese premature infants are clearly at a high risk of developing CKD in later life. Recently, several DOHaD-related CKD studies from Japanese researchers have been reported. Here, we summarize the relevance of CKD in conjunction with DOHaD and review recent studies that have examined the impact of the upward LBW trend in Japan on renal health.

Critical timing of ACEi initiation prevents compensatory glomerular hypertrophy in the remaining single kidney

Increasing evidence suggests that single in kidney states (e.g., kidney transplantation and living donation) progressive glomerulosclerosis limits kidney lifespan. Modeling shows that post-nephrectomy compensatory glomerular volume (GV) increase drives podocyte depletion and hypertrophic stress resulting in proteinuria and glomerulosclerosis, implying that GV increase could serve as a therapeutic target to prevent progression. In this report we examine how Angiotensin Converting Enzyme inhibition (ACEi), started before uninephrectomy can reduce compensatory GV increase in wild-type Fischer344 rats. An unbiased computer-assisted method was used for morphometric analysis. Urine Insulin-like growth factor-1 (IGF-1), the major diver of body and kidney growth, was used as a readout. In long-term (40-week) studies of uni-nephrectomized versus sham-nephrectomized rats a 2.2-fold increase in GV was associated with reduced podocyte density, increased proteinuria and glomerulosclerosis. Compensatory GV increase was largely prevented by ACEi started a week before but not after uni-nephrectomy with no measurable impact on long-term eGFR. Similarly, in short-term (14-day) studies, ACEi started a week before uni-nephrectomy reduced both GV increase and urine IGF-1 excretion. Thus, timing of ACEi in relation to uni-nephrectomy had significant impact on post-nephrectomy "compensatory" glomerular growth and outcomes that could potentially be used to improve kidney transplantation and live kidney donation outcomes.