Podocyte hypertrophy, "adaptation," and "decompensation" associated with glomerular enlargement and glomerulosclerosis in the aging rat: prevention by calorie restriction

Evidence from the large VALIGA cohort validates the subclassification of focal segmental glomerulosclerosis in IgA nephropathy

Evidence from the Oxford IgA nephropathy (IgAN) cohort supports the clinical value of subclassifying focal segmental glomerulosclerosis lesions (S1). Using the larger Validation in IgA (VALIGA) study cohort, we investigated the association between podocytopathic changes and higher proteinuria, kidney outcome and response to immunosuppressive therapy. All biopsies were evaluated for glomeruli with segmental capillary occlusion by matrix ("not otherwise specified", NOS lesion), simple capsular adhesion without capillary occlusion (Adh), tip lesions, and podocyte hypertrophy (PH). S1 required a NOS lesion and/or Adh. A Chi-Squared Automatic Interaction Detection method was used to identify subgroups of FSGS lesions associated with distinctive proteinuria at biopsy. We assessed survival from a combined event (kidney failure or 50% decline in estimated glomerular filtration rate). Finally, we evaluated within each subgroup if immunosuppression was associated with a favorable outcome using propensity analysis. In 1147 patients, S1 was found in 70% of biopsies. Subclassification found NOS lesions in 44%, Adh in 59%, PH in 13%, and tip lesions in 3%, with much overlap. Four subgroups were identified with progressively higher proteinuria: from lowest, S1 without NOS, S1 with NOS but without Adh/PH, to highest, S1 with NOS and Adh but without PH, and S1 with NOS and PH. These four subgroups showed progressively worse kidney survival. Immunosuppression was associated with a better outcome only in the two highest proteinuria subgroups. Propensity analysis in these two groups, adjusted for clinical and pathological findings, found a significantly reduced time-dependent hazard of combined outcome with corticosteroids. Podocyte hypertrophy and glomeruli with simple adhesions appeared to reflect active lesions associated with a response to corticosteroids, while other S1 lesions defined chronicity. Thus, our findings support subclassifying S1 lesions in IgAN.

SGLT2 inhibition to target kidney aging

Anti-aging therapy is the latest frontier in the world of medical science, especially for widespread diseases such as chronic kidney disease (CKD). Both renal aging and CKD are characterized by increased cellular senescence, inflammation and oxidative stress. A variety of cellular signalling mechanisms are involved in these processes, which provide new potential targets for therapeutic strategies aimed at counteracting the onset and progression of CKD. At the same time, sodium-glucose co-transporter 2 inhibitors (SGLT2is) continuously demonstrate large beneficial effects at all stages of the cardiorenal metabolic continuum. The broad-spectrum benefits of SGLT2is have led to changes in several treatment guidelines and to growing scientific interest in the underlying working principles. Multiple mechanisms have been studied to explain these great renal benefits, but many things remain to be solved. With this in mind, we provide an overview of the experimental evidence for the effects of SGLT2is on the molecular pathway's ability to modulate senescence, aging and parenchymal damage, especially at the kidney level. We propose to shed some light on the role of SGLT2is in kidney care by focusing on their potential to reduce the progression of kidney disease across the spectrum of aging and dysregulation of senescence.

Deletion of podocyte Rho-associated, coiled-coil-containing protein kinase 2 protects mice from focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) shares podocyte damage as an essential pathological finding. Several mechanisms underlying podocyte injury have been proposed, but many important questions remain. Rho-associated, coiled-coil-containing protein kinase 2 (ROCK2) is a serine/threonine kinase responsible for a wide array of cellular functions. We found that ROCK2 is activated in podocytes of adriamycin (ADR)-induced FSGS mice and cultured podocytes stimulated with ADR. Conditional knockout mice in which the ROCK2 gene was selectively disrupted in podocytes (PR2KO) were resistant to albuminuria, glomerular sclerosis, and podocyte damage induced by ADR injection. In addition, pharmacological intervention for ROCK2 significantly ameliorated podocyte loss and kidney sclerosis in a murine model of FSGS by abrogating profibrotic factors. RNA sequencing of podocytes treated with a ROCK2 inhibitor proved that ROCK2 is a cyclic nucleotide signaling pathway regulator. Our study highlights the potential utility of ROCK2 inhibition as a therapeutic option for FSGS.

From inflammation to renal fibrosis: A one-way road in autoimmunity?

Renal fibrosis is now recognized as a main determinant of renal pathology to include chronic kidney disease. Deposition of pathological matrix in the walls of glomerular capillaries, the interstitial space, and around arterioles predicts and contributes to the functional demise of the nephron and its surrounding vasculature. The recent identification of the major cell populations of fibroblast precursors in the kidney interstitium such as pericytes and tissue-resident mesenchymal stem cells, or bone-marrow-derived macrophages, and in the glomerulus such as podocytes, parietal epithelial and mesangial cells, has enabled the study of the fibrogenic process thought the lens of involved immunological pathways. Besides, a growing body of evidence is supporting the role of the lymphatic system in modulating the immunological response potentially leading to inflammation and ultimately renal damage. These notions have moved our understanding of renal fibrosis to be recognized as a clinical entity and new main player in autoimmunity, impacting directly the management of patients.

Obesity-related glomerulopathy is associated with elevated WT1 expression in podocytes

BACKGROUND

The prevalence of obesity is increasing worldwide at an alarming rate. In addition to the increased incidence of cardiovascular and metabolic diseases, obesity is the most potent risk factor for developing chronic kidney disease (CKD). Although systemic events such as hemodynamic factors, metabolic effects, and lipotoxicity were implicated in the pathophysiology of obesity-related glomerulopathy (ORG) and kidney dysfunction, the precise mechanisms underlying the association between obesity and CKD remain unexplored.

METHODS

In this study, we employed spontaneous WNIN/Ob rats to investigate the molecular events that promote ORG. Further, we fed a high-fat diet to mice and analyzed the incidence of ORG. Kidney functional parameters, micro-anatomical manifestations, and podocyte morphology were investigated in both experimental animal models. Gene expression analysis in the rodents was compared with human subjects by data mining using Nephroseq and Kidney Precision Medicine Project database.

RESULTS

WNIN/Ob rats were presented with proteinuria and several glomerular deformities, such as adaptive glomerulosclerosis, decreased expression of podocyte-specific markers, and effacement of podocyte foot process. Similarly, high-fat-fed mice also showed glomerular injury and proteinuria. Both experimental animal models showed increased expression of podocyte-specific transcription factor WT1. The altered expression of putative targets of WT1 such as E-cadherin, podocin (reduced), and α-SMA (increased) suggests elevated expression of WT1 in podocytes elicits mesenchymal phenotype. Curated data from CKD patients revealed increased expression of WT1 in the podocytes and its precursors, parietal epithelial cells.

CONCLUSION

WT1 is crucial during nephron development and has minimal expression in adult podocytes. Our study discovered elevated expression of WT1 in podocytes in obesity settings. Our analysis suggests a novel function for WT1 in the pathogenesis of ORG; however, the precise mechanism of WT1 induction and its involvement in podocyte pathobiology needs further investigation.

Caloric restriction reduces proteinuria in male rats with established nephropathy

Reducing proteinuria is a crucial approach in preventing kidney function loss. Previous preclinical studies indicated that caloric restriction (CR) imposed at a young age protects against age-related proteinuria. However, these studies have not explored CR in established renal disease. Therefore, this study aimed to investigate the impact of CR on established proteinuria. Rats, aged 12 ± 2 weeks, were administered 2.1 mg/kg of Adriamycin. Six weeks after injection, protein excretion was measured, and a [13 N]ammonia positron emission tomography (PET) scan was conducted to assess kidney perfusion. After 7 weeks rats were divided into four groups: ad libitum (AL) and CR groups fed either a 12% or a 20% protein diet. All groups were treated for 12 weeks. Blood pressure was measured and a second PET scan was acquired at the end of the study. The animals subjected to CR exhibited a 20.3% decrease in protein excretion (p = 0.003) compared to those in the AL groups. Additionally, blood pressure in the CR group was 21.2% lower (p < 0.001) than in the AL groups. While kidney function declined over time in all groups, the 20% CR group demonstrated the smallest decline. Thus CR effectively reduces urinary protein excretion and lowers blood pressure in rats with established proteinuria.

The Impact of Early-Stage Chronic Kidney Disease on Weight Loss Outcomes After Gastric Bypass

PURPOSE

Weight loss achieved through bariatric metabolic surgery was demonstrated to be effective at reversing chronic kidney dysfunction associated with obesity-related glomerulopathy. However, robust data on how pre-operative kidney status impacts on bariatric metabolic surgery weight loss outcomes is still lacking. The aim of this study was to evaluate the impact of kidney dysfunction on weight loss outcomes after bariatric metabolic surgery.

METHODS

Patients with obesity to be submitted to gastric bypass surgery underwent a pre-operative evaluation of creatinine clearance, estimated glomerular filtration rate (eGFR), proteinuria, and albuminuria in 24-hour urine. Body mass index (BMI), % total weight loss (%TWL), and % excess BMI loss (%EBMIL) were assessed at 6 and 12 months after surgery.

RESULTS

Before surgery, patients (N=127) had a mean BMI of 39.6 ± 3.0 kg/m2, and 56.7% (n=72) had a creatinine clearance > 130 mL/min, 23.6% (n= 30) presented proteinuria > 150 mg/24h, and 15.0% (n= 19) presented albuminuria > 30 mg/24h. After surgery, the mean BMI was 27.7 kg/m2 and 25.0 kg/m2 at 6 and 12 months, respectively (p<0.0001). The %TWL was lower in patients with pre-operative eGFR < percentile 25 (34.4 ± 5.8% vs 39.4 ± 4.9%, p=0.0007, at 12 months). There were no significant correlations between weight loss metrics and pre-operative creatinine clearance rate, proteinuria, or albuminuria.

CONCLUSION

Early-stage chronic kidney disease (G2) has a negative impact on short-term weight loss outcomes after bariatric metabolic surgery, albeit in a magnitude inferior to the clinically relevant threshold.

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.

Why is chronic kidney disease progressive? Evolutionary adaptations and maladaptations

Despite significant advances in renal physiology, the global prevalence of chronic kidney disease (CKD) continues to increase. The emergence of multicellular organisms gave rise to increasing complexity of life resulting in trade-offs reflecting ancestral adaptations to changing environments. Three evolutionary traits shape CKD over the lifespan: 1) variation in nephron number at birth, 2) progressive nephron loss with aging, and 3) adaptive kidney growth in response to decreased nephron number. Although providing plasticity in adaptation to changing environments, the cell cycle must function within constraints dictated by available energy. Prioritized allocation of energy available through the placenta can restrict fetal nephrogenesis, a risk factor for CKD. Moreover, nephron loss with aging is a consequence of cell senescence, a pathway accelerated by adaptive nephron hypertrophy that maintains metabolic homeostasis at the expense of increased vulnerability to stressors. Driven by reproductive fitness, natural selection operates in early life but diminishes thereafter, leading to an exponential increase in CKD with aging, a product of antagonistic pleiotropy. A deeper understanding of the evolutionary constraints on the cell cycle may lead to manipulation of the balance between progenitor cell renewal and differentiation, regulation of cell senescence, and modulation of the balance between cell proliferation and hypertrophy. Application of an evolutionary perspective may enhance understanding of adaptation and maladaptation by nephrons in the progression of CKD, leading to new therapeutic advances.

Caloric Restriction Mitigates Kidney Fibrosis in an Aged and Obese Rat Model

Caloric restriction is an effective intervention to protract healthspan and lifespan in several animal models from yeast to primates, including humans. Caloric restriction has been found to induce cardiometabolic adaptations associated with improved health and to delay the onset and progression of kidney disease in different species, particularly in rodent models. In both aging and obesity, fibrosis is a hallmark of kidney disease, and epithelial-mesenchymal transition is a key process that leads to fibrosis and renal dysfunction during aging. In this study, we used an aged and obese rat model to evaluate the effect of long-term (6 months) caloric restriction (-40%) on renal damage both from a structural and functional point of view. Renal interstitial fibrosis was analyzed by histological techniques, whereas effects on mesenchymal (N-cadherin, Vimentin, Desmin and α-SMA), antioxidant (SOD1, SOD2, Catalase and GSTP1) inflammatory (YM1 and iNOS) markers and apoptotic/cell cycle (BAX, BCL2, pJNK, Caspase 3 and p27) pathways were investigated using Western blot analysis. Our results clearly showed that caloric restriction promotes cell cycle division and reduces apoptotic injury and fibrosis phenotype through inflammation attenuation and leukocyte infiltration. In conclusion, we highlight the beneficial effects of caloric restriction to preserve elderly kidney function.

Sex differences in long-term kidney fibrosis following neonatal nephron loss during ongoing nephrogenesis

BACKGROUND

Clinical studies suggest that female sex plays a protective role in the development and progression of kidney disease. Recent experimental studies indicate that in male rats early nephron loss under ongoing nephrogenesis is accompanied by severe long-term sequelae. In humans, nephron formation occurs mainly in the third trimester, ceasing with 36 weeks of gestation. Due to perinatal complications, preterm infants delivered during this vulnerable period may undergo acute nephron loss. In rats nephrogenesis persists until postnatal day 10, reflecting the situation of human preterms with persisting nephrogenesis. In our animal model of neonatal uninephrectomy, female and male rats were uninephrectomized at day 1 of life. Hypothesizing sex-dependent differences, long-term renal outcome was assessed after 1 year.

RESULTS

In both sexes, neonatal uninephrectomy was not followed by arterial hypertension at 1 year of age. Compensatory weight gain and glomerular hypertrophy of the remaining kidney occurred in uninephrectomized female and male animals. Selected markers of interstitial inflammation and fibrosis were regulated sex-dependently. The expression of monocyte chemoattractant protein-1 was increased in females, while tubulointerstitial infiltration by M1 macrophages was significantly higher in males after neonatal uninephrectomy. Neonatally uninephrectomized male rats had more glomerulosclerosis and podocyte damage compared to females, which was assessed by a semiquantitative score and desmin staining. RT-PCR revealed that after neonatal uninephrectomy in the remaining contralateral kidney of female rats the expression of candidate genes of renal development and function, i.e., wt-1, nephrin, synaptopodin, gdnf, and itga8 was higher than in males.

CONCLUSIONS

Based on these observations we conclude that female sex is protective in the long-term response of the kidney to acute nephron loss under active nephrogenesis.

Inhibiting NLRP3 signaling in aging podocytes improves their life- and health-span

The decrease in the podocyte's lifespan and health-span that typify healthy kidney aging cause a decrease in their normal structure, physiology and function. The ability to halt and even reverse these changes becomes clinically relevant when disease is superimposed on an aged kidney. RNA-sequencing of podocytes from middle-aged mice showed an inflammatory phenotype with increases in the NLRP3 inflammasome, signaling for IL2/Stat5, IL6 and TNF, interferon gamma response, allograft rejection and complement, consistent with inflammaging. Furthermore, injury-induced NLRP3 signaling in podocytes was further augmented in aged mice compared to young ones. The NLRP3 inflammasome (NLRP3, Caspase-1, IL1β IL-18) was also increased in podocytes of middle-aged humans. Higher transcript expression for NLRP3 in human glomeruli was accompanied by reduced podocyte density and increased global glomerulosclerosis and glomerular volume. Pharmacological inhibition of NLRP3 with MCC950, or gene deletion, reduced podocyte senescence and the genes typifying aging in middle-aged mice, which was accompanied by an improved podocyte lifespan and health-span. Moreover, modeling the injury-dependent increase in NLRP3 signaling in human kidney organoids confirmed the anti-senescence effect of MC9950. Finally, NLRP3 also impacted liver aging. Together, these results suggest a critical role for the NLRP3 inflammasome in podocyte and liver aging.

PTEN-induced kinase 1 is associated with renal aging, via the cGAS-STING pathway

Mitochondrial dysfunction is considered to be an important mediator of the pro-aging process in chronic kidney disease, which is continuously increasing worldwide. Although PTEN-induced kinase 1 (PINK1) regulates mitochondrial function, its role in renal aging remains unclear. We investigated the association between PINK1 and renal aging, especially through the cGAS-STING pathway, which is known to result in an inflammatory phenotype. Pink1 knockout (Pink1-/- ) C57BL/6 mice and senescence-induced renal tubular epithelial cells (HKC-8) treated with H2 O2 were used as the renal aging models. Extensive analyses at transcriptomic-metabolic levels have explored changes in mitochondrial function in PINK1 deficiency. To investigate whether PINK1 deficiency affects renal aging through the cGAS-STING pathway, we explored their expression levels in PINK1 knockout mice and senescence-induced HKC-8 cells. PINK1 deficiency enhances kidney fibrosis and tubular injury, and increases senescence and the senescence-associated secretory phenotype (SASP). These phenomena were most apparent in the 24-month-old Pink1-/- mice and HKC-8 cells treated with PINK1 siRNA and H2 O2 . Gene expression analysis using RNA sequencing showed that PINK1 deficiency is associated with increased inflammatory responses, and transcriptomic and metabolomic analyses suggested that PINK1 deficiency is related to mitochondrial metabolic dysregulation. Activation of cGAS-STING was prominent in the 24-month-old Pink1-/- mice. The expression of SASPs was most noticeable in senescence-induced HKC-8 cells and was attenuated by the STING inhibitor, H151. PINK1 is associated with renal aging, and mitochondrial dysregulation by PINK1 deficiency might stimulate the cGAS-STING pathway, eventually leading to senescence-related inflammatory responses.

ABCA1 deficiency contributes to podocyte pyroptosis priming via the APE1/IRF1 axis in diabetic kidney disease

Decreased ATP Binding Cassette Transporter A1 (ABCA1) expression and caspase-4-mediated noncanonical inflammasome contribution have been described in podocytes in diabetic kidney disease (DKD). To investigate a link between these pathways, we evaluated pyroptosis-related mediators in human podocytes with stable knockdown of ABCA1 (siABCA1) and found that mRNA levels of IRF1, caspase-4, GSDMD, caspase-1 and IL1β were significantly increased in siABCA1 compared to control podocytes and that protein levels of caspase-4, GSDMD and IL1β were equally increased. IRF1 knockdown in siABCA1 podocytes prevented increases in caspase-4, GSDMD and IL1β. Whereas TLR4 inhibition did not decrease mRNA levels of IRF1 and caspase-4, APE1 protein expression increased in siABCA1 podocytes and an APE1 redox inhibitor abrogated siABCA1-induced expression of IRF1 and caspase-4. RELA knockdown also offset the pyroptosis priming, but ChIP did not demonstrate increased binding of NFκB to IRF1 promoter in siABCA1 podocytes. Finally, the APE1/IRF1/Casp1 axis was investigated in vivo. APE1 IF staining and mRNA levels of IRF1 and caspase 11 were increased in glomeruli of BTBR ob/ob compared to wildtype. In conclusion, ABCA1 deficiency in podocytes caused APE1 accumulation, which reduces transcription factors to increase the expression of IRF1 and IRF1 target inflammasome-related genes, leading to pyroptosispriming.

Roles of extracellular vesicles in ageing-related chronic kidney disease: demon or angel

Ageing is a universal and unavoidable phenomenon that significantly increases the risk of developing chronic kidney disease (CKD). It has been reported that ageing is associated with functional disruption and structural damage to the kidneys. Extracellular vesicles (EVs), which are nanoscale membranous vesicles containing lipids, proteins, and nucleic acids, are secreted by cells into the extracellular spaces. They have diverse functions such as repairing and regenerating different forms of ageing-related CKD and playing a crucial role in intercellular communication. This paper reviews the etiology of ageing in CKD, with particular attention paid to the roles of EVs as carriers of ageing signals and anti-ageing therapeutic strategies in CKD. In this regard, the double-edged role of EVs in ageing-related CKD is examined, along with the potential for their application in clinical settings.

Possible protective effect of zinc administration on renal and cognitive changes occurring in uninephrectomized adult male Wistar rats

NEW FINDINGS

What is the central question of this study? Are renal changes occurring post-nephrectomy accompanied by cognitive changes, and does early administration of zinc supplements such as ZnSO4 to uninephrectomized rats ameliorate the renal and cognitive changes if present? What is the main finding and its importance? Uninephrectomy-induced renal changes were accompanied by species-atypical behaviour in rats in both Morris water maze and T maze tests, together with hypozincaemia and hippocampal inflammatory and oxidative changes. Early zinc administration to uninephrectomized rats ameliorated the renal, behavioural, hippocampal and serum zinc changes.

ABSTRACT

Cognitive impairment is increasingly recognized as an important consequence of kidney disease in humans. Kidney donation is a safe procedure but is known to increase the long-term risk of cardiovascular and kidney disease. Whether kidney donation impairs cognitive function is not known. In the present study, we examined whether the renal changes occurring post-nephrectomy were accompanied by cognitive changes as well, and whether early administration of zinc supplements such as ZnSO4 to uninephrectomized (UNX) rats could ameliorate the renal and cognitive changes if present. The present study included 30 adult male Wistar rats that were randomly assigned to three groups (n = 10 per group): sham-operated rats, UNX and UNX treated with ZnSO4 for 20 weeks. Before termination, rats were subjected to 24-h urine collection and behavioural testing with the Morris water maze and T maze tests. UNX induced significant proteinuria, renal functional, fibrotic and oxidative changes, as well as increased renal desmin expression. UNX rats also showed significant behavioural changes indicating spatial learning and memory affection, together with decreased hippocampal brain derived neurotrophic factor (BDNF) and antioxidant capacity, and increased glial fibrillary acidic protein (GFAP), nitric oxide and malondialdehyde. In addition, UNX induced significant hyperglycaemia and dyslipidaemia, as well as significant reduction in serum zinc, copper and selenium. Early administration of ZnSO4 starting 1 week post-nephrectomy significantly ameliorated renal and behavioural changes, as well as hippocampal oxidative, BDNF and GFAP changes. Additionally, Zn recovered serum changes of triglycerides, cholesterol, zinc and copper. Therefore, early administration of zinc to humans undergoing nephrectomy may be of benefit and should be considered in human trials.

Nephron deficit and low podocyte density increase risk of albuminuria and glomerulosclerosis in a model of diabetes

Podocytes are terminally differentiated epithelial cells in glomeruli. Podocyte injury and loss are features of many diseases leading to chronic kidney disease (CKD). The developmental origins of health and disease hypothesis propose an adverse intrauterine environment can lead to CKD later in life, especially when a second postnatal challenge is experienced. The aim of this study was to examine whether a suboptimal maternal environment would result in reduced podocyte endowment, increasing susceptibility to diabetes-induced renal injury. Female C57BL/6 mice were fed a low protein diet (LPD) to induce growth restriction or a normal protein diet (NPD) from 3 weeks before mating until weaning (postnatal Day 21, P21) when nephron and podocyte endowment were assessed in one male and one female offspring per litter. Littermates were administered streptozotocin or vehicle at 6 weeks of age. Urinary albumin excretion, glomerular size, and podometrics were assessed following 18 weeks of hyperglycemia. LPD offspring were growth restricted and had lower nephron and podocyte number at P21. However, by 24 weeks the podocyte deficit was no longer evident and despite low nephron endowment neither albuminuria nor glomerulosclerosis were observed. Podocyte number was unaffected by 18 weeks of hyperglycemia in NPD and LPD offspring. Diabetes increased glomerular volume reducing podocyte density, with more pronounced effects in LPD offspring. LPD and NPD diabetic offspring developed mild albuminuria with LPD demonstrating an earlier onset. LPD offspring also developed glomerular pathology. These findings indicate that growth-restricted LPD offspring with low nephron number and normalized podocyte endowment were more susceptible to alterations in glomerular volume and podocyte density leading to more rapid onset of albuminuria and renal injury than NPD offspring.

Obesity and chronic kidney disease

The prevalence of obesity has increased dramatically during the past decades, which has been a major health problem. Since 1975, the number of people with obesity worldwide has nearly tripled. An increasing number of studies find obesity as a driver of chronic kidney disease (CKD) progression, and the mechanisms are complex and include hemodynamic changes, inflammation, oxidative stress, and activation of the renin-angiotensin-aldosterone system (RAAS). Obesity-related kidney disease is characterized by glomerulomegaly, which is often accompanied by localized and segmental glomerulosclerosis lesions. In these patients, the early symptoms are atypical, with microproteinuria being the main clinical manifestation and nephrotic syndrome being rare. Weight loss and RAAS blockers have a protective effect on obesity-related CKD, but even so, a significant proportion of patients eventually progress to end-stage renal disease despite treatment. Thus, it is critical to comprehend the mechanisms underlying obesity-related CKD to create new tactics for slowing or stopping disease progression. In this review, we summarize current knowledge on the mechanisms of obesity-related kidney disease, its pathological changes, and future perspectives on its treatment.

Podocyte-specific Transcription Factors: Could MafB Become a Therapeutic Target for Kidney Disease?

The increasing number of patients with chronic kidney disease (CKD) is being recognized as an emerging global health problem. Recently, it has become clear that injury and loss of glomerular visceral epithelial cells, known as podocytes, is a common early event in many forms of CKD. Podocytes are highly specialized epithelial cells that cover the outer layer of the glomerular basement membrane. They serve as the final barrier to urinary protein loss through the formation and maintenance of specialized foot-processes and an interposed slit-diaphragm. We previously reported that the transcription factor MafB regulates the podocyte slit diaphragm protein production and transcription factor Tcf21. We showed that the forced expression of MafB was able to prevent CKD. In this review, we discuss recent advances and offer an updated overview of the functions of podocyte-specific transcription factors in kidney biology, aiming to present new perspectives on the progression of CKD and respective therapeutic strategies.

Age-related glomerular histogenesis in inbred indigenous rabbit (Oryctolagus cuniculus): A morphological, morphometrical, and immunohistochemical study with emphasis on Lgr5-positive cells

Although the regeneration of renal glomeruli and nephrons after injuries especially in adult mammals is not possible, understanding normal glomerular histogenesis is important. Here, we sought to study the morphometrical and histological development of the normal renal glomeruli of rabbits from birth until postnatal day 40. Moreover, we immunohistochemically evaluated the extent and rate of the Lgr5 expression in the immature renal stem/progenitor cells. The untreated, clinically healthy inbred indigenous rabbits (from Duhok city of Iraqi Kurdistan) were sacrificed at postnatal days 1, 10, 15, 30, and 40. After being processed and embedded in paraffin, rabbit anti-human Lgr5 as a primary antibody and rabbit ImmunoCruz LSAB as a staining kit were used for the immunohistochemical detection of Lgr5^+ve cells. For normal histology, hematoxylin and eosin were used. The peak generation and regression of renal corpuscles were at postnatal days 10, and 40, respectively, with 50% decrease. The glomeruli diameter significantly increased (1.3-fold, p = 0.001), whereas the Bowman's space diameter decreased (50%, p < 0.0001) from postnatal day 1-40. The immature nephrons were seen only in one-day postnatal rabbits. While the superficial glomeruli were compact and small, the juxtamedullary glomeruli were larger and segmented. The formation and development of the juxtaglomerular apparatus were documented at postnatal days 30 and 40 only. Our data revealed highly expressed Lgr5 protein at postnatal day one, and the expression level decreased gradually with advancing age. It was moderately expressed on day 10 and mildly expressed on day 15, whereas no expression was recorded on days 30 and 40 postnatally. Our study provides evidence that the Lgr5 gene, within multipotent stem cells and their lineage progeny, was activated within newly formed glomeruli throughout the early postnatal stages of nephrogenesis.

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.

Molecular Mechanisms of Cellular Injury and Role of Toxic Heavy Metals in Chronic Kidney Disease

Chronic kidney disease (CKD) is a progressive disease that affects millions of adults every year. Major risk factors include diabetes, hypertension, and obesity, which affect millions of adults worldwide. CKD is characterized by cellular injury followed by permanent loss of functional nephrons. As injured cells die and nephrons become sclerotic, remaining healthy nephrons attempt to compensate by undergoing various structural, molecular, and functional changes. While these changes are designed to maintain appropriate renal function, they may lead to additional cellular injury and progression of disease. As CKD progresses and filtration decreases, the ability to eliminate metabolic wastes and environmental toxicants declines. The inability to eliminate environmental toxicants such as arsenic, cadmium, and mercury may contribute to cellular injury and enhance the progression of CKD. The present review describes major molecular alterations that contribute to the pathogenesis of CKD and the effects of arsenic, cadmium, and mercury on the progression of CKD.

Glomerular filtration in the aging population

In the last decades, improvements in the average life expectancy in the world population have been associated with a significant increase in the proportion of elderly people, in parallel with a higher prevalence of non-communicable diseases, such as hypertension and diabetes. As the kidney is a common target organ of a variety of diseases, an adequate evaluation of renal function in the approach of this population is of special relevance. It is also known that the kidneys undergo aging-related changes expressed by a decline in the glomerular filtration rate (GFR), reflecting the loss of kidney function, either by a natural senescence process associated with healthy aging or by the length of exposure to diseases with potential kidney damage. Accurate assessment of renal function in the older population is of particular importance to evaluate the degree of kidney function loss, enabling tailored therapeutic interventions. The present review addresses a relevant topic, which is the effects of aging on renal function. In order to do that, we analyze and discuss age-related structural and functional changes. The text also examines the different options for evaluating GFR, from the use of direct methods to the implementation of several estimating equations. Finally, this manuscript supports clinicians in the interpretation of GFR changes associated with age and the management of the older patients with decreased kidney function.

Upregulated PD-1 signaling antagonizes glomerular health in aged kidneys and disease

With an aging population, kidney health becomes an important medical and socioeconomic factor. Kidney aging mechanisms are not well understood. We previously showed that podocytes isolated from aged mice exhibit increased expression of programmed cell death protein 1 (PD-1) surface receptor and its 2 ligands (PD-L1 and PD-L2). PDCD1 transcript increased with age in microdissected human glomeruli, which correlated with lower estimated glomerular filtration rate and higher segmental glomerulosclerosis and vascular arterial intima-to-lumen ratio. In vitro studies in podocytes demonstrated a critical role for PD-1 signaling in cell survival and in the induction of a senescence-associated secretory phenotype. To prove PD-1 signaling was critical to podocyte aging, aged mice were injected with anti-PD-1 antibody. Treatment significantly improved the aging phenotype in both kidney and liver. In the glomerulus, it increased the life span of podocytes, but not that of parietal epithelial, mesangial, or endothelial cells. Transcriptomic and immunohistochemistry studies demonstrated that anti-PD-1 antibody treatment improved the health span of podocytes. Administering the same anti-PD-1 antibody to young mice with experimental focal segmental glomerulosclerosis (FSGS) lowered proteinuria and improved podocyte number. These results suggest a critical contribution of increased PD-1 signaling toward both kidney and liver aging and in FSGS.

A protective role for &lt;i&gt;Drosophila&lt;/i&gt; Filamin in nephrocytes via Yorkie mediated hypertrophy

Podocytes are specialized epithelial cells of the kidney glomerulus and are an essential part of the filtration barrier. Because of their position, they are exposed to constant biomechanical forces such as shear stress and hydrostatic pressure. These forces increase during disease, resulting in podocyte injury. It is likely podocytes have adaptative responses to help buffer against deleterious mechanical force and thus reduce injury. However, these responses remain largely unknown. Here, using the <i>Drosophila</i> model, we show the mechanosensor Cheerio (dFilamin) provides a key protective role in nephrocytes. We found expression of an activated mechanosensitive variant of Cheerio rescued filtration function and induced compensatory and hypertrophic growth in nephrocytes depleted of the nephrocyte diaphragm proteins Sns or Duf. Delineating the protective pathway downstream of Cheerio we found repression of the Hippo pathway induces nephrocyte hypertrophy, whereas Hippo activation reversed the Cheerio-mediated hypertrophy. Furthermore, we find Yorkie was activated upon expression of active Cheerio. Taken together, our data suggest that Cheerio acts via the Hippo pathway to induce hypertrophic growth, as a protective response in abnormal nephrocytes.

Blocking ribosomal protein S6 phosphorylation inhibits podocyte hypertrophy and focal segmental glomerulosclerosis

Ribosomal protein S6 (rpS6) phosphorylation mediates the hypertrophic growth of kidney proximal tubule cells. However, the role of rpS6 phosphorylation in podocyte hypertrophy and podocyte loss during the pathogenesis of focal segmental glomerulosclerosis (FSGS) remains undefined. Here, we examined rpS6 phosphorylation levels in kidney biopsy specimens from patients with FSGS and in podocytes from mouse kidneys with Adriamycin-induced FSGS. Using genetic and pharmacologic approaches in the mouse model of FSGS, we investigated the role of rpS6 phosphorylation in podocyte hypertrophy and loss during development and progression of FSGS. Phosphorylated rpS6 was found to be markedly increased in the podocytes of patients with FSGS and Adriamycin-induced FSGS mice. Genetic deletion of the Tuberous sclerosis 1 gene in kidney glomerular podocytes activated mammalian target of rapamycin complex 1 signaling to rpS6 phosphorylation, resulting in podocyte hypertrophy and pathologic features similar to those of patients with FSGS including podocyte loss, leading to segmental glomerulosclerosis. Since protein phosphatase 1 is known to negatively regulate rpS6 phosphorylation, treatment with an inhibitor increased phospho-rpS6 levels, promoted podocyte hypertrophy and exacerbated formation of FSGS lesions. Importantly, blocking rpS6 phosphorylation (either by generating congenic rpS6 knock-in mice expressing non-phosphorylatable rpS6 or by inhibiting ribosomal protein S6 kinase 1-mediated rpS6 phosphorylation with an inhibitor) significantly blunted podocyte hypertrophy, inhibited podocyte loss, and attenuated formation of FSGS lesions. Thus, our study provides genetic and pharmacologic evidence indicating that specifically targeting rpS6 phosphorylation can attenuate the development of FSGS lesions by inhibiting podocyte hypertrophy and associated podocyte depletion.

Clinicopathological characteristics of high-altitude polycythemia-related kidney disease in Tibetan inhabitants

High-altitude polycythemia (HAPC) is a clinical syndrome that occurs in native inhabitants or long-term residents living at altitude. The kidney is one of the most affected organs. However, the clinical and kidney histopathological profiles of HAPC-related kidney disease have rarely been reported. Here, we report kidney biopsy-based clinicopathological study on this disease. HAPC was defined as excessive erythrocytosis [females, hemoglobin 190 g/L or more; males, 210 g/L or more] in patients living above an altitude of 2500 m for more than ten years. A total of 416 Tibetan patients underwent kidney biopsy between January 1, 2016, and November 31, 2020. Of these patients 17 met the diagnostic criteria for HAPC-related kidney disease. Clinically, these patients had a median urinary protein level of 2.5 g/24-hour (range 1.81-6.85). Twelve patients had hyperuricemia, nine had hypertension, and three had kidney insufficiency. On histopathology, glomerular hypertrophy, glomerular basement membrane thickening, podocyte foot process effacement, segmental glomerulosclerosis and global glomerulosclerosis were the main features. Extraglomerular arterial/arteriolar lesions were common, presenting as intimal fibrosis, hyalinosis and endothelial cell swelling/subintimal edema. Expansion of the arterial/arteriolar medial wall area characterized by smooth muscle cell proliferation was clearly observed, potentially indicating vascular remodeling. Hypoxia-inducible factor 2α was expressed in the kidney tissues of these patients. Thus, the pathological changes of HAPC-related kidney disease encompassed both glomerular and extraglomerular vascular lesions, suggesting a key role of both chronic hypoxia itself and secondary hemodynamic changes in the pathogenesis of this disease.

The ability of remaining glomerular podocytes to adapt to the loss of their neighbours decreases with age

Progressive podocyte loss is a feature of healthy ageing. While previous studies have reported age-related changes in podocyte number, density and size and associations with proteinuria and glomerulosclerosis, few studies have examined how the response of remaining podocytes to podocyte depletion changes with age. Mild podocyte depletion was induced in PodCreiDTR mice aged 1, 6, 12 and 18 months via intraperitoneal administration of diphtheria toxin. Control mice received intraperitoneal vehicle. Podometrics, proteinuria and glomerular pathology were assessed, together with podocyte expression of p-rp-S6, a phosphorylation target that represents activity of the mammalian target of rapamycin (mTOR). Podocyte number per glomerulus did not change in control mice in the 18-month time period examined. However, control mice at 18 months had the largest podocytes and the lowest podocyte density. Podocyte depletion at 1, 6 and 12 months resulted in mild albuminuria but no glomerulosclerosis, whereas similar levels of podocyte depletion at 18 months resulted in both albuminuria and glomerulosclerosis. Following podocyte depletion at 6 and 12 months, the number of p-rp-S6 positive podocytes increased significantly, and this was associated with an adaptive increase in podocyte volume. However, at 18 months of age, remaining podocytes were unable to further elevate mTOR expression or undergo hypertrophic adaptation in response to mild podocyte depletion, resulting in marked glomerular pathology. These findings demonstrate the importance of mTORC1-mediated podocyte hypertrophy in both physiological (ageing) and adaptive settings, highlighting a functional limit to podocyte hypertrophy reached under physiological conditions.

Healthy and unhealthy aging on kidney structure and function: human studies

PURPOSE OF REVIEW

This review is intended to provide an up-to-date analysis of the structural and functional alterations of the kidneys that accompany healthy and unhealthy aging in humans. Macro- and micro- structural changes and glomerular filtration rate (whole kidney and single nephron) accompanying aging will be stressed.

RECENT FINDINGS

Comparative findings concerning distribution of anatomic changes of the kidney healthy and unhealthy aging are reviewed. Challenges concerning definition of chronic kidney disease (CKD) in otherwise healthy aging patients are discussed. The complex interactions of CKD and aging are discussed. The role of podocyte dysbiosis in kidney aging is reviewed.

SUMMARY

Kidney aging is a complex phenomenon often difficult to distinguish from CKD. Nonetheless, phenotypes of healthy and unhealthy aging are evident. Much more information concerning the molecular characteristics of normal kidney aging and its relevance to chronic kidney disease is needed.

Age-related GSK3β overexpression drives podocyte senescence and glomerular aging

As life expectancy continues to increase, clinicians are challenged by age-related renal impairment that involves podocyte senescence and glomerulosclerosis. There is now compelling evidence that lithium has a potent antiaging activity that ameliorates brain aging and increases longevity in Drosophila and Caenorhabditis elegans. As the major molecular target of lithium action and a multitasking protein kinase recently implicated in a variety of renal diseases, glycogen synthase kinase 3β (GSK3β) is overexpressed and hyperactive with age in glomerular podocytes, correlating with functional and histological signs of kidney aging. Moreover, podocyte-specific ablation of GSK3β substantially attenuated podocyte senescence and glomerular aging in mice. Mechanistically, key mediators of senescence signaling, such as p16^INK4A and p53, contain high numbers of GSK3β consensus motifs, physically interact with GSK3β, and act as its putative substrates. In addition, therapeutic targeting of GSK3β by microdose lithium later in life reduced senescence signaling and delayed kidney aging in mice. Furthermore, in psychiatric patients, lithium carbonate therapy inhibited GSK3β activity and mitigated senescence signaling in urinary exfoliated podocytes and was associated with preservation of kidney function. Thus, GSK3β appears to play a key role in podocyte senescence by modulating senescence signaling and may be an actionable senostatic target to delay kidney aging.

Podocyte Aging: Why and How Getting Old Matters

The effects of healthy aging on the kidney, and how these effects intersect with superimposed diseases, are highly relevant in the context of the population's increasing longevity. Age-associated changes to podocytes, which are terminally differentiated glomerular epithelial cells, adversely affect kidney health. This review discusses the molecular and cellular mechanisms underlying podocyte aging, how these mechanisms might be augmented by disease in the aged kidney, and approaches to mitigate progressive damage to podocytes. Furthermore, we address how biologic pathways such as those associated with cellular growth confound aging in humans and rodents.

Increasing urinary podocyte mRNA excretion and progressive podocyte loss in kidney contribute to the high risk of long-term renal disease caused by preterm birth

Podocyte abnormalities are common mechanism driving the progression of glomerular diseases, which account for most chronic kidney diseases (CKDs). However, the role of podocyte in the mechanism of high-risk long-term CKD caused by prematurity has not been well clarified. In present study, urine samples of 86 preterm infants and 32 full-term infants were collected, and podocyte-specific podocin mRNA levels in urine pellet were applied to indicate urinary podocyte mRNA excretion. In addition, in a preterm animal rat model, preterm rats were identified by delivery 2 days early. From the age of 3 weeks-12 months, urine samples were collected to examine podocyte mRNA excretion by measuring podocyte-specific podocin mRNA levels. Kidney samples at the age of 3 weeks, 2 months, and 12 months were collected from 8, 5 and 6 preterm rats and 9, 6 and 8 full-term rats, respectively, to examine podocyte density and podocyte area by measuring the podocyte specific nuclear marker WT-1 and the podocyte specific marker synaptopodin. As results, a more than threefold increase of urinary podocyte-specific podocin mRNA excretion rate was found in preterm infants compared with full-term infants. In addition, there was negative correlation between gestational age at birth and urinary podocin mRNA excretion. In preterm rats, a reduction in the total number of differentiated podocytes in glomeruli and an increased podocyte podocin mRNA excretion rate in urine were detected at the end of kidney differentiation. Moreover, long-term follow-up data in preterm rats showed there was an increased the risk of renal disease indicated by persistent podocyte mRNA loss, proteinuria, and enlarged glomeruli. In conclusion, increasing podocyte mRNA excretion in urine and podocyte loss in kidney led by prematurity drive the progression of long-term abnormal kidney function and could potentially explain the high risk of long-term CKD in preterm infants.

Effect of disease progression on the podocyte cell cycle in Alport Syndrome

Progression of glomerulosclerosis is associated with loss of podocytes with subsequent glomerular tuft instability. It is thought that a diminished number of podocytes may be able to preserve tuft stability through cell hypertrophy associated with cell cycle reentry. At the same time, reentry into the cell cycle risks podocyte detachment if podocytes cross the G1/S checkpoint and undergo abortive cytokinesis. In order to study cell cycle dynamics during chronic kidney disease (CKD) development, we used a FUCCI model (fluorescence ubiquitination-based cell cycle indicator) of mice with X-linked Alport Syndrome. This model exhibits progressive CKD and expresses fluorescent reporters of cell cycle stage exclusively in podocytes. With the development of CKD, an increasing fraction of podocytes in vivo were found to be in G1 or later cell cycle stages. Podocytes in G1 and G2 were hypertrophic. Heterozygous female mice, with milder manifestations of CKD, showed G1 fraction numbers intermediate between wild-type and male Alport mice. Proteomic analysis of podocytes in different cell cycle phases showed differences in cytoskeleton reorganization and metabolic processes between G0 and G1 in disease. Additionally, in vitro experiments confirmed that damaged podocytes reentered the cell cycle comparable to podocytes in vivo. Importantly, we confirmed the upregulation of PDlim2, a highly expressed protein in podocytes in G1, in a patient with Alport Syndrome, confirming our proteomics data in the human setting. Thus, our data showed that in the Alport model of progressive CKD, podocyte cell cycle distribution is altered, suggesting that cell cycle manipulation approaches may have a role in the treatment of various progressive glomerular diseases characterized by podocytopenia.

Podocyte endowment and the impact of adult body size on kidney health

Low birth weight is a risk factor for chronic kidney disease, whereas adult podocyte depletion is a key event in the pathogenesis of glomerulosclerosis. However, whether low birth weight due to poor maternal nutrition is associated with low podocyte endowment and glomerulosclerosis in later life is not known. Female Sprague-Dawley rats were fed a normal-protein diet (NPD; 20%) or low-protein diet (LPD; 8%), to induce low birth weight, from 3 wk before mating until postnatal day 21 (PN21), when kidneys from some male offspring were taken for quantitation of podocyte number and density in whole glomeruli using immunolabeling, tissue clearing, and confocal microscopy. The remaining offspring were fed a normal- or high-fat diet until 6 mo to induce catch-up growth and excessive weight gain, respectively. At PN21, podocyte number per glomerulus was 15% lower in low birth weight (LPD) than normal birth weight (NPD) offspring, with this deficit greater in outer glomeruli. Surprisingly, podocyte number in LPD offspring increased in outer glomeruli between PN21 and 6 mo, although an overall 9% podocyte deficit persisted. Postnatal fat feeding to LPD offspring did not alter podometric indexes or result in glomerular pathology at 6 mo, whereas fat feeding in NPD offspring was associated with far greater body and fat mass as well as podocyte loss, reduced podocyte density, albuminuria, and glomerulosclerosis. This is the first report that maternal diet can influence podocyte endowment. Our findings provide new insights into the impact of low birth weight, podocyte endowment, and postnatal weight on podometrics and kidney health in adulthood.NEW & NOTEWORTHY The present study shows, for the first time, that low birth weight as a result of maternal nutrition is associated with low podocyte endowment. However, a mild podocyte deficit at birth did not result in glomerular pathology in adulthood. In contrast, postnatal podocyte loss in combination with excessive body weight led to albuminuria and glomerulosclerosis. Taken together, these findings provide new insights into the associations between birth weight, podocyte indexes, postnatal weight, and glomerular pathology.

Urine marker analysis identifies evidence for persistent glomerular podocyte injury across allograft lifespan

Long-term kidney transplant (KT) survival has remained relatively stagnant. Protocol biopsy studies suggest that glomerulosclerosis is a significant contributor to long-term graft failure. We previously demonstrated that podocyte loss in the 1^st year post-transplantation predicted long-term allograft survival. However, whether increased podocyte loss continues over the lifespan of a KT remains unclear. We performed a cross-sectional analysis of 1,182 urine samples from 260 KT recipients up to 19-years after transplantation. Urine pellet mRNAs were assayed for podocyte (NPHS2/podocin and nephrin/NPHS1), distal tubule (aquaporin2), and profibrotic cytokine (TGFbeta1). Multivariable generalized estimating equations were used to obtain "population-averaged" effects for these markers over time post-KT. Consistent with early stresses both podocyte and tubular markers increased immediately post-KT. However, only podocyte markers continued to increase long-term. A role for hypertrophic stresses in driving podocyte loss over time is implied by their association with donor BMI, recipient BMI and donor-recipient BMI mismatch at transplantation. Furthermore, urine pellet podocin mRNA was associated with urine TGFbeta1, proteinuria and reduced eGFR, thereby linking podocyte injury to allograft fibrosis and survival. In conclusion we observed that podocyte loss continues long-term post-KT suggesting an important role in driving late graft loss. This article is protected by copyright. All rights reserved.

Caloric restriction inhibits renal artery ageing by reducing endothelin-1 expression

Background

The renal artery plays a central role in renal perfusion and is critical for proper renal function. Ageing is an independent risk factor for both impaired renal function and vascular disorders, and associated with an increase in the expression of the vasoconstrictor endothelin-1 (ET-1), and caloric restriction (CR) without malnutrition has been shown to be an effective inhibitor of renal dysfunction induced by ageing. The objective of this study was to determine whether CR-mediated alleviation of renal dysfunction is mediated by ET-1 expression.

Methods

The young (2 months, 2 M) and old (12 months, 12 M) Sprague-Dawley male rats were used and fed ad libitum. The 12-month-old rats were further divided into 12 M and 12 M-caloric restriction (CR) (30% calorie restriction). After 8 weeks, the renal tissues were showed by PAS staining, and age-related metabolic parameters and renal functions were detected in each group of rats. The inflammatory cytokines of interleukin (IL)-6, IL-1β, tumor necrosis factor alpha (TNF-α), and transforming growth factor beta 1 (TGF-β1) were analyzed using ELISA. The mRNA and protein expression in the renal artery were analysis by qRT-PCR and Immunoblot analysis.

Results

Ageing was associated with significant increases in 24 h urine protein content and serum triglyceride and cholesterol in 12 M rats, both of which were significantly inhibited in 12 M-CR. The mRNA expression and the secretion of IL-6, IL-1β, TNF-α, and TGF-β1 in the renal artery was significantly increased with ageing and inhibited by CR. CR also inhibited ageing-induced Edn1 (encoding ET-1) mRNA and protein expression in the renal artery. In addition, CR could regulate ET-1 expression by inhibiting the activation of NF-κB signaling and activation and induction in the expression of NF-E2-related factor 2 (Nrf2) and histone deacetylase and gene repressor sirtuin 1 (SIRT1), both of which play a central role in mitigating oxidative stress in young rats.

Conclusions

Moderate CR can reverse the ageing related kidney dysfunction by reducing the ET-1 expression. CR might be used as an alternative to prevent the ageing induced renal artery dysfunction.

Targeting Premature Renal Aging: from Molecular Mechanisms of Cellular Senescence to Senolytic Trials

The biological process of renal aging is characterized by progressive structural and functional deterioration of the kidney leading to end-stage renal disease, requiring renal replacement therapy. Since the discovery of pivotal mechanisms of senescence such as cell cycle arrest, apoptosis inhibition, and the development of a senescence-associated secretory phenotype (SASP), efforts in the understanding of how senescent cells participate in renal physiological and pathological aging have grown exponentially. This has been encouraged by both preclinical studies in animal models with senescent cell clearance or genetic depletion as well as due to evidence coming from the clinical oncologic experience. This review considers the molecular mechanism and pathways that trigger premature renal aging from mitochondrial dysfunction, epigenetic modifications to autophagy, DNA damage repair (DDR), and the involvement of extracellular vesicles. We also discuss the different pharmaceutical approaches to selectively target senescent cells (namely, senolytics) or the development of systemic SASP (called senomorphics) in basic models of CKD and clinical trials. Finally, an overview will be provided on the potential opportunities for their use in renal transplantation during ex vivo machine perfusion to improve the quality of the graft.

Effects of TRPC6 Inactivation on Glomerulosclerosis and Renal Fibrosis in Aging Rats

Canonical transient receptor potential 6 (TRPC6) channels have been implicated in familial and acquired forms of focal and segmental glomerulosclerosis (FSGS) in patients and animal models, as well as in renal fibrosis following ureteral obstruction in mice. Aging also evokes declines in renal function owing to effects on almost every renal compartment in humans and rodents. Here, we have examined the role of TRPC6 in driving inflammation and fibrosis during aging in Sprague-Dawley rats. This was assessed in rats with non-functional TRPC6 channels owing to CRISPR-Cas9 deletion of a portion of the ankyrin repeat domain required for the assembly of functional TRPC6 channels (Trpc6^del/del rats). Wild-type littermates (Trpc6^wt/wt rats) were used as controls. Animals were evaluated at 2 months and 12 months of age. There was no sign of kidney disease at 2 months of age, regardless of genotype. However, by 12 months of age, all rats examined showed declines in renal function associated with albuminuria, azotemia and increased urine excretion of β2-microglobulin, a marker for proximal tubule pathology. These changes were equally severe in Trpc6^wt/wt and Trpc6^del/del rats. We also observed age-related increases in renal cortical expression of markers of fibrosis (α-smooth muscle actin and vimentin) and inflammation (NLRP3 and pro-IL-1β), and there was no detectable protective effect of TRPC6 inactivation. Tubulointerstitial fibrosis assessed from histology also appeared equally severe in Trpc6^wt/wt and Trpc6^del/del rats. By contrast, glomerular pathology, blindly scored from histological sections, suggested a significant protective effect of TRPC6 inactivation, but only within the glomerular compartment.

Morphologic Analysis of Urinary Podocytes in Focal Segmental Glomerulosclerosis

BACKGROUND

The development of glomerulosclerosis in FSGS is associated with a reduction in podocyte number in the glomerular capillary tufts. Although it has been reported that the number of urinary podocytes in FSGS exceeds that of minimal-change nephrotic syndrome, the nature of events that promote podocyte detachment in FSGS remains elusive.

METHODS

In this study, we provide detailed, morphologic analysis of the urinary podocytes found in FSGS by examining the size of the urinary podocytes from patients with FSGS, minimal-change nephrotic syndrome, and GN. In addition, in urinary podocytes from patients with FSGS and minimal-change nephrotic syndrome, we analyzed podocyte hypertrophy and mitotic catastrophe using immunostaining of p21 and phospho-ribosomal protein S6.

RESULTS

The size of the urinary podocytes was strikingly larger in samples obtained from patients with FSGS compared with those with minimal-change nephrotic syndrome and GN (P=0.008). Urinary podocytes from patients with FSGS had a higher frequency of positive immunostaining for p21 (P<0.001) and phospho-ribosomal protein S6 (P=0.02) than those from patients with minimal-change nephrotic syndrome. Characteristic features of mitotic catastrophe were more commonly observed in FSGS than in minimal-change nephrotic syndrome urinary samples (P=0.001).

CONCLUSIONS

We posit that the significant increase in the size of urinary podocytes in FSGS, compared with those in minimal-change nephrotic syndrome, may be explained by hypertrophy and mitotic catastrophe.

Indirect podocyte injury manifested in a partial podocytectomy mouse model

In progressive glomerular diseases, segmental podocyte injury often expands, leading to global glomerulosclerosis by unclear mechanisms. To study the expansion of podocyte injury, we established a new mosaic mouse model in which a fraction of podocytes express human (h)CD25 and can be injured by the immunotoxin LMB2. hCD25⁺ and hCD25^- podocytes were designed to express tdTomato and enhanced green fluorescent protein (EGFP), respectively, which enabled cell sorting analysis of podocytes. After the injection of LMB2, mosaic mice developed proteinuria and glomerulosclerosis. Not only tdTomato⁺ podocytes but also EGFP⁺ podocytes were decreased in number and showed damage, as evidenced by a decrease in nephrin and an increase in desmin at both protein and RNA levels. Transcriptomics analysis found a decrease in the glucocorticoid-induced transcript 1 gene and an increase in the thrombospondin 4, heparin-binding EGF-like growth factor, and transforming growth factor-β genes in EGFP⁺ podocytes; these genes may be candidate mediators of secondary podocyte damage. Pathway analysis suggested that focal adhesion, integrin-mediated cell adhesion, and focal adhesion-phosphatidylinositol 3-kinase-Akt-mammalian target of rapamycin signaling are involved in secondary podocyte injury. Finally, treatment of mosaic mice with angiotensin II receptor blocker markedly ameliorated secondary podocyte injury. This mosaic podocyte injury model has distinctly demonstrated that damaged podocytes cause secondary podocyte damage, which may be a promising therapeutic target in progressive kidney diseases.NEW & NOTEWORTHY This novel mosaic model has demonstrated that when a fraction of podocytes is injured, other podocytes are subjected to secondary injury. This spreading of injury may occur ubiquitously irrespective of the primary cause of podocyte injury, leading to end-stage renal failure. Understanding the molecular mechanism of secondary podocyte injury and its prevention is important for the treatment of progressive kidney diseases. This model will be a powerful tool for studying the indirect podocyte injury.

Increased dishevelled associated activator of morphogenesis 2, a new podocyte-associated protein, in diabetic nephropathy

BACKGROUND

Previously, by using proteomic analysis and RNA sequencing in isolated glomeruli, we identified several novel differentially expressed proteins in human and mouse diabetic nephropathy (DN) versus controls, including dishevelled associated activator of morphogenesis 2 (DAAM2). DAAM2 binds the Wnt effector Dvl. We aimed to study possible contributions of DAAM2 to DN.

METHODS

We assessed DAAM2 by immunostaining in non-cancer regions of human nephrectomy (Nx), DN and normal donor kidney tissues. We also examined DAAM2 in DN mice (db/db eNOS-/-) and Nx mice. DN mice treated with angiotensin-converting enzyme inhibitor (ACEI), dipeptidyl peptidase 4 inhibitor (DPP4I) or vehicle were compared. DAAM2 was knocked down in primary cultured podocytes by small interfering RNA to study its effects on cell function.

RESULTS

In normal human glomeruli, DAAM2 was expressed only on podocytes. DAAM2 expression was increased in both Nx and DN versus normal donors. Podocyte DAAM2 expression was increased in DN and Nx mouse models. Glomerular DAAM2 expression correlated with glomerular size and was decreased significantly by ACEI while DPP4I only numerically reduced DAAM2. In primary cultured podocytes, knockdown of DAAM2 enhanced adhesion, slowed migration, activated Wnt-β-catenin signaling and downregulated mammalian target of rapamycin complex 1 (mTORC1) and Rho activity.

CONCLUSIONS

Podocyte DAAM2 is upregulated in both Nx and DN, which could be contributed to by glomerular hypertrophy. We hypothesize that DAAM2 regulates podocyte function through the mTORC1, Wnt/β-catenin and Rho signaling pathways.

Control of Podocyte and Glomerular Capillary Wall Structure and Elasticity by WNK1 Kinase

Cytoskeletal structure and its regulation are essential for maintenance of the differentiated state of specific types of cells and their adaptation to physiologic and pathophysiologic conditions. Renal glomerular capillaries, composed of podocytes, endothelial cells, and the glomerular basement membrane, have distinct structural and biophysical properties and are the site of injury in many glomerular diseases. Calcineurin inhibitors, immunosuppressant drugs used for organ transplantation and auto-immune diseases, can protect podocytes and glomerular capillaries from injury by preserving podocyte cytoskeletal structure. These drugs cause complications including hypertension and hyperkalemia which are mediated by WNK (With No Lysine) kinases as well as vasculopathy with glomerulopathy. WNK kinases and their target kinases oxidative stress-responsive kinase 1 (OSR1) and SPS1-related proline/alanine-rich kinase (SPAK) have fundamental roles in angiogenesis and are activated by calcineurin inhibitors, but the actions of these agents on kidney vasculature, and glomerular capillaries are not fully understood. We investigated WNK1 expression in cultured podocytes and isolated mouse glomerular capillaries to determine if WNK1 contributes to calcineurin inhibitor-induced preservation of podocyte and glomerular structure. WNK1 and OSR1/SPAK are expressed in podocytes, and in a pattern similar to podocyte synaptopodin in glomerular capillaries. Calcineurin inhibitors increased active OSR1/SPAK in glomerular capillaries, the Young’s modulus (E) of glomeruli, and the F/G actin ratio, effects all blocked by WNK inhibition. In glomeruli, WNK inhibition caused reduced and irregular synaptopodin-staining, abnormal capillary and foot process structures, and increased deformability. In cultured podocytes, FK506 activated OSR1/SPAK, increased lamellipodia, accelerated cell migration, and promoted traction force. These actions of FK506 were reduced by depletion of WNK1. Collectively, these results demonstrate the importance of WNK1 in regulation of the podocyte actin cytoskeleton, biophysical properties of glomerular capillaries, and slit diaphragm structure, all of which are essential to normal kidney function.

Roles of Hydrogen Sulfide Donors in Common Kidney Diseases

Hydrogen sulfide (H₂S) plays a key role in the regulation of physiological processes in mammals. The decline in H₂S level has been reported in numerous renal disorders. In animal models of renal disorders, treatment with H₂S donors could restore H₂S levels and improve renal functions. H₂S donors suppress renal dysfunction by regulating autophagy, apoptosis, oxidative stress, and inflammation through multiple signaling pathways, such as TRL4/NLRP3, AMP-activated protein kinase/mammalian target of rapamycin, transforming growth factor-β1/Smad3, extracellular signal-regulated protein kinases 1/2, mitogen-activated protein kinase, and nuclear factor kappa B. In this review, we summarize recent developments in the effects of H₂S donors on the treatment of common renal diseases, including acute/chronic kidney disease, renal fibrosis, unilateral ureteral obstruction, glomerulosclerosis, diabetic nephropathy, hyperhomocysteinemia, drug-induced nephrotoxicity, metal-induced nephrotoxicity, and urolithiasis. Novel H₂S donors can be designed and applied in the treatment of common renal diseases.

The implications of focal segmental glomerulosclerosis in children with IgA nephropathy

Focal segmental glomerular sclerotic lesions in IgA nephropathy (IgAN), considered for years a chronic histologic feature related to proteinuria in remnant nephrons without any active role in the pathogenesis and progression of glomerular damage of IgAN, have been recently reconsidered. The Oxford classification of IgAN reported it as the "S" score and found it to be an independent risk factor for progression of IgAN. Its prognostic value was confirmed also in children. The identification of some histologic subvariants of the S lesion has produced interesting insights into different pathogenetic mechanisms of glomerular damage in IgAN. Tip lesion and podocyte hypertrophy are considered secondary to active podocytopathy and are correlated with higher levels of proteinuria and a faster decline in glomerular filtration rate. Moreover, endocapillary and mesangial hypercellularity might contribute in children with IgAN to formation and progression of S lesions. Considering the pathophysiology of these processes, children with some S features may benefit not only from nephroprotective measures but also from immunosuppression.

The ageing kidney: Molecular mechanisms and clinical implications

As human life expectancy keeps increasing, ageing populations present a growing challenge for clinical practices. Human ageing is associated with molecular, structural, and functional changes in a variety of organ systems, including the kidney. During the ageing process, the kidney experiences progressive functional decline as well as macroscopic and microscopic histological alterations, which are accentuated by systemic comorbidities like hypertension and diabetes mellitus, or by preexisting or underlying kidney diseases. Although ageing per se does not cause kidney injury, physiologic changes associated with normal ageing processes are likely to impair the reparative capacity of the kidney and thus predispose older people to acute kidney disease, chronic kidney disease and other renal diseases. Mechanistically, cell senescence plays a key role in renal ageing, involving a number of cellular signaling mechanisms, many of which may be harnessed as international targets for slowing or even reversing kidney ageing. This review summarizes the clinical characteristics of renal ageing, highlights the latest progresses in deciphering the role of cell senescence in renal ageing, and envisages potential interventional strategies and novel therapeutic targets for preventing or improving renal ageing in the hope of maintaining long-term kidney health and function across the life course.

Global transcriptomic changes occur in aged mouse podocytes

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

Podocyte stress and detachment measured in urine are related to mean arterial pressure in healthy humans

Hypertension-associated progressive glomerulosclerosis is a significant driver of both de novo and all-cause chronic kidney disease leading to end-stage kidney failure. The progression of glomerular disease proceeds via continuing depletion of podocytes from the glomeruli into the ultrafiltrate. To non-invasively assess injury patterns associated with mean arterial pressure (MAP), we conducted an observational study of 87 healthy normotensive individuals who were cleared for living kidney donation. Urine pellet podocin and aquaporin2 mRNAs normalized to the urine creatinine concentration (UPod:Creat ratio and UAqp2:Creat ratio) were used as markers of podocyte detachment and tubular injury, respectively. The ratio of two podocyte mRNA markers, podocin to nephrin (UPod:Neph) as well as the ratio of podocin to the tubular marker aquaporin2 (UPod:Aqp2) estimated the relative rates of podocyte stress and glomerular vs. tubular injury. The MAP was positively correlated with the UPod:Neph and UPod:Aqp2, thereby confirming the relationship of MAP with podocyte stress and the preferential targeting of the glomerulus by higher MAP. In multivariable linear regression analysis, both UPod:Neph and UPod:Creat, but not UAqp2:Creat or proteinuria, were both significantly related to a range of normal MAP (70 to 110 mm Hg). Systolic, as opposed to diastolic or pulse pressure was associated with UPod:Creat. Thus, higher podocyte stress and detachment into the urine are associated with MAP even in a relatively "normal" range of MAP. Hence, urine pellet mRNA monitoring can potentially identify progression risk before the onset of overt hypertension, proteinuria or chronic kidney disease.

Parietal epithelial cell differentiation to a podocyte fate in the aged mouse kidney

Healthy aging is typified by a progressive and absolute loss of podocytes over the lifespan of animals and humans. To test the hypothesis that a subset of glomerular parietal epithelial cell (PEC) progenitors transition to a podocyte fate with aging, dual reporter PEC-rtTA|LC1|tdTomato|Nphs1-FLPo|FRT-EGFP mice were generated. PECs were inducibly labeled with a tdTomato reporter, and podocytes were constitutively labeled with an EGFP reporter. With advancing age (14 and 24 months) glomeruli in the juxta-medullary cortex (JMC) were more severely injured than those in the outer cortex (OC). In aged mice (24m), injured glomeruli with lower podocyte number (41% decrease), showed more PEC migration and differentiation to a podocyte fate than mildly injured or healthy glomeruli. PECs differentiated to a podocyte fate had ultrastructural features of podocytes and co-expressed the podocyte markers podocin, nephrin, p57 and VEGF164, but not markers of mesangial (Perlecan) or endothelial (ERG) cells. PECs differentiated to a podocyte fate did not express CD44, a marker of PEC activation. Taken together, we demonstrate that a subpopulation of PECs differentiate to a podocyte fate predominantly in injured glomeruli in mice of advanced age.