High Phosphate Induces and Klotho Attenuates Kidney Epithelial Senescence and Fibrosis

Targeting senescence to prevent diabetic kidney disease: Exploring molecular mechanisms and potential therapeutic targets for disease management

BACKGROUND/AIMS

As a microvascular complication, diabetic kidney disease is the leading cause of chronic kidney disease and end-stage renal disease worldwide. While the underlying pathophysiology driving transition of diabetic kidney disease to renal failure is yet to be fully understood, recent studies suggest that cellular senescence is central in disease development and progression. Consequently, understanding the molecular mechanisms which initiate and drive senescence in response to the diabetic milieu is crucial in developing targeted therapies that halt progression of renal disease.

METHODS

To understand the mechanistic pathways underpinning cellular senescence in the context of diabetic kidney disease, we reviewed the literature using PubMed for English language articles that contained key words related to senescence, inflammation, fibrosis, senescence-associated secretory phenotype (SASP), autophagy, and diabetes.

RESULTS

Aberrant accumulation of metabolically active senescent cells is a notable event in the progression of diabetic kidney disease. Through autocrine- and paracrine-mediated mechanisms, resident senescent cells potentiate inflammation and fibrosis through increased expression and secretion of pro-inflammatory cytokines, chemoattractants, recruitment of immune cells, myofibroblast activation, and extracellular matrix remodelling. Compounds that eliminate senescent cells and/or target the SASP - including senolytic and senomorphics drugs - demonstrate promising results in reducing the senescent cell burden and associated pro-inflammatory effect.

CONCLUSIONS

Here we evidence the link between senescence and diabetic kidney disease and highlight underlying molecular mechanisms and potential therapeutic targets that could be exploited to delay disease progression and improve outcomes for individuals with the disease. Trials are now required to translate their therapeutic potential to a clinical setting.

Women With a History of Preeclampsia Exhibit Accelerated Aging and Unfavorable Profiles of Senescence Markers

BACKGROUND

Senescence, a mechanism of cellular aging, which is characterized by irreversible proliferation arrest and a proinflammatory secretory phenotype, has been documented in women with preeclampsia. As cellular senescence can persist and progress, we postulated that it is associated with accelerated aging phenotype and accumulation of comorbidities in women with a history of preeclampsia.

METHODS

We included a cohort of women with a history of preeclampsia (n=40) age- and parity-matched to a group of referent women with normotensive pregnancies (n=40). Women with prior major cardiovascular events, neurological, or autoimmune conditions were excluded. We collected urine and blood samples to study markers of aging, data on multimorbidity at the time of enrollment, and prospectively followed them for events over the course of 6 years, on average.

RESULTS

Women with a history of preeclampsia exhibited unfavorable aging profiles compared with referent women, including decreased urinary α-Klotho (P=0.018); increased leptin (P=0.016) and leptin/adiponectin ratio (P=0.027), and increased extracellular vesicles positive for tissue factor (P=0.025). Women with a history of preeclampsia likewise had a higher rate of comorbidities at the time of enrollment (P=0.003) and had a 4× higher risk of developing major cardiovascular events compared with referent women (P=0.003).

CONCLUSIONS

Our data suggest that a history of preeclampsia is associated with accelerated aging as indicated by senescence marker differences and the accumulation of multimorbidity later in life. Targeting cellular senescence may offer novel, mechanism-based approaches for the diagnosis and treatment of adverse health outcomes in women with a history of preeclampsia.

Cellular senescence and kidney aging

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

Upstream and downstream regulators of Klotho expression in chronic kidney disease

Klotho is a critical protein that protects the kidney. Klotho is severely downregulated in chronic kidney disease (CKD), and its deficiency is implicated in the pathogenesis and progression of CKD. Conversely, an increase in Klotho levels results in improved kidney function and delays CKD progression, supporting the notion that modulating Klotho levels could represent a possible therapeutic strategy for CKD treatment. Nevertheless, the regulatory mechanisms responsible for the loss of Klotho remain elusive. Previous studies have demonstrated that oxidative stress, inflammation, and epigenetic modifications can modulate Klotho levels. These mechanisms result in a decrease in Klotho mRNA transcript levels and reduced translation, thus can be grouped together as upstream regulatory mechanisms. However, therapeutic strategies that aim to rescue Klotho levels by targeting these upstream mechanisms do not always result in increased Klotho, indicating the involvement of other regulatory mechanisms. Emerging evidence has shown that endoplasmic reticulum (ER) stress, the unfolded protein response, and ER-associated degradation also affect the modification, translocation, and degradation of Klotho, and thus are proposed to be downstream regulatory mechanisms. Here, we discuss the current understanding of upstream and downstream regulatory mechanisms of Klotho and examine potential therapeutic strategies to upregulate Klotho expression for CKD treatment.

Updates in the chronic kidney disease-mineral bone disorder show the role of osteocytic proteins, a potential mechanism of the bone-Vascular paradox, a therapeutic target, and a biomarker

The chronic kidney disease-mineral bone disorder (CKD-MBD) is a complex multi-component syndrome occurring during kidney disease and its progression. Here, we update progress in the components of the syndrome, and synthesize recent investigations, which suggest a potential mechanism of the bone-vascular paradox. The discovery that calcified arteries in chronic kidney disease inhibit bone remodeling lead to the identification of factors produced by the vasculature that inhibit the skeleton, thus providing a potential explanation for the bone-vascular paradox. Among the factors produced by calcifying arteries, sclerostin secretion is especially enlightening. Sclerostin is a potent inhibitor of bone remodeling and an osteocyte specific protein. Its production by the vasculature in chronic kidney disease identifies the key role of vascular cell osteoblastic/osteocytic transdifferentiation in vascular calcification and renal osteodystrophy. Subsequent studies showing that inhibition of sclerostin activity by a monoclonal antibody improved bone remodeling as expected, but stimulated vascular calcification, demonstrate that vascular sclerostin functions to brake the Wnt stimulation of the calcification milieu. Thus, the target of therapy in the chronic kidney disease-mineral bone disorder is not inhibition of sclerostin function, which would intensify vascular calcification. Rather, decreasing sclerostin production by decreasing the vascular osteoblastic/osteocytic transdifferentiation is the goal. This might decrease vascular calcification, decrease vascular stiffness, decrease cardiac hypertrophy, decrease sclerostin production, reduce serum sclerostin and improve skeletal remodeling. Thus, the therapeutic target of the chronic kidney disease-mineral bone disorder may be vascular osteoblastic transdifferentiation, and sclerostin levels may be a useful biomarker for the diagnosis of the chronic kidney disease-mineral bone disorder and the progress of its therapy.

VEGFR2 insufficiency enhances phosphotoxicity and undermines Klotho's protection against peritubular capillary rarefaction and kidney fibrosis

Vascular endothelial growth factor (VEGF) and its cognate receptor (VEGFR2) system are crucial for cell functions associated with angiogenesis and vasculogenesis. Klotho contributes to vascular health maintenance in the kidney and other organs in mammals, but it is unknown whether renoprotection by Klotho is dependent on VEGF/VEGFR2 signaling. We used heterozygous VEGFR2-haploinsufficient (VEGFR2+/-) mice resulting from heterozygous knockin of green fluorescent protein in the locus of fetal liver kinase 1 encoding VEGFR2 to test the interplay of Klotho, phosphate, and VEGFR2 in kidney function, the vasculature, and fibrosis. VEGFR2+/- mice displayed downregulated VEGF/VEGFR2 signaling in the kidney, lower density of peritubular capillaries, and accelerated kidney fibrosis, all of which were also found in the homozygous Klotho hypomorphic mice. High dietary phosphate induced higher plasma phosphate, greater peritubular capillary rarefaction, and more kidney fibrosis in VEGFR2+/- mice compared with wild-type mice. Genetic overexpression of Klotho significantly attenuated the elevated plasma phosphate, kidney dysfunction, peritubular capillary rarefaction, and kidney fibrosis induced by a high-phosphate diet in wild-type mice but only modestly ameliorated these changes in the VEGFR2+/- background. In cultured endothelial cells, VEGFR2 inhibition reduced free VEGFR2 but enhanced its costaining of an endothelial marker (CD31) and exacerbated phosphotoxicity. Klotho protein maintained VEGFR2 expression and attenuated high phosphate-induced cell injury, which was reduced by VEGFR2 inhibition. In conclusion, normal VEGFR2 function is required for vascular integrity and for Klotho to exert vascular protective and antifibrotic actions in the kidney partially through the regulation of VEGFR2 function.NEW & NOTEWORTHY This research paper studied the interplay of vascular endothelial growth factor receptor type 2 (VEGFR2), high dietary phosphate, and Klotho, an antiaging protein, in peritubular structure and kidney fibrosis. Klotho protein was shown to maintain VEGFR2 expression in the kidney and reduce high phosphate-induced cell injury. However, Klotho cytoprotection was attenuated by VEGFR2 inhibition. Thus, normal VEGFR2 function is required for vascular integrity and Klotho to exert vascular protective and antifibrotic actions in the kidney.

Fibroblast growth factor 23 and kidney function in patients with type 1 diabetes

Diabetic kidney disease (DKD) is a key determinant of morbidity and mortality in patients with type 1 diabetes (T1D). Identifying factors associated with early glomerular filtration rate (GFR) decline in T1D is important in prevention or early intervention for DKD. This study investigated whether phosphate metabolism, including fibroblast growth factor 23 (FGF23) is associated with the kidney function of patients with T1D. We randomly recruited 118 patients with T1D with a normal or mildly impaired kidney function [chronic kidney disease (CKD) stages of G1/G2, A1/A2], and measured their serum FGF23 levels. Serum FGF23 was significantly negatively associated with the estimated GFR (eGFR) (r = -0.292, P = 0.0016), but not urinary albumin creatinine ratio (UACR), and positively associated with serum phosphate (Pi; r = 0.273, P = 0.0027). Serum FGF23 increased with decreasing eGFR quartiles (P for linear trend = 0.0371), while FGF23 was modestly higher in the higher quartiles of UACR (not statistically significant). The multiple linear regression analysis also showed a significant inverse association between FGF23 and eGFR (Model 1: β = -0.149, P = 0.0429; Model 2: β = -0.141, P = 0.0370). The association remained significant after adjustment for Pi. We identified that FGF23 was inversely associated with the eGFR in T1D patients with a normal or mildly impaired kidney function.

The long non-coding RNA HOTAIR controls the self-renewal, cell senescence, and secretion of antiaging protein α-Klotho in human adult renal progenitor cells

The long non-coding RNAs (lncRNA) play an important role in several biological processes including some renal diseases. Nevertheless, little is known on lncRNA that are expressed in healthy kidney and involved in renal cell homeostasis and development, and even less is known about lncRNA involved in the maintenance of human adult renal stem/progenitor cells (ARPCs) that have been shown to be very important for renal homeostasis and repair processes. Through a whole genome transcriptome screening, we found that the HOTAIR lncRNA is highly expressed in renal progenitors and potentially involved in cell cycle and senescence biological processes. By CRISPR/Cas9 genome editing, we generated HOTAIR knock-out ARPC lines and established a key role of this lncRNA in ARPC self-renewal properties by sustaining their proliferative capacity and limiting the apoptotic process. Intriguingly, the HOTAIR knock-out led to the ARPC senescence and to a significant decrease of the CD133 stem cell marker expression, that is an inverse marker of ARPC senescence and can regulate renal tubular repair after the damage. Furthermore, we found that ARPCs expressed high levels of the α-Klotho anti-aging protein and especially 2.6-fold higher levels compared to that secreted by renal proximal tubular cells (RPTECs). Finally, we showed that HOTAIR exerts its function through the epigenetic silencing of the cell cycle inhibitor p15 inducing the trimethylation of the histone H3K27. Altogether, these results shed new light on the mechanisms of regulation of these important renal cells and may support the future development of precision therapies for kidney diseases.

Klotho Modulates Pro-Fibrotic Activities in Human Atrial Fibroblasts through Inhibition of Phospholipase C Signaling and Suppression of Store-Operated Calcium Entry

Background: Atrial fibroblasts activation causes atrial fibrosis, which is one major pathophysiological contributor to atrial fibrillation (AF) genesis. Klotho is a pleiotropic protein with remarkable cardiovascular effects, including anti-inflammatory, anti-oxidative, and anti-apoptotic effects. This study investigated whether Klotho can modulate the activity of human atrial fibroblasts and provides an anti-fibrotic effect. Methods: Cell migration assay and proliferation assay were used to investigate fibrogenesis activities in single human atrial fibroblasts with or without treatment of Klotho (10 and 100 pM, 48 h). Calcium fluorescence imaging, the whole-cell patch-clamp, and Western blotting were performed in human atrial fibroblasts treated with and without Klotho (100 pM, 48 h) to evaluate the store-operated calcium entry (SOCE), transient receptor potential (TRP) currents, and downstream signaling. Results: High dose of Klotho (100 pM, 48 h) significantly reduced the migration of human atrial fibroblasts without alternating their proliferation; in addition, treatment of Klotho (100 pM, 48 h) also decreased SOCE and TRP currents. In the presence of BI-749327 (a selective canonical TRP 6 channel inhibitor, 1 μM, 48 h), Klotho (100 pM, 48 h) could not inhibit fibroblast migration nor suppress the TRP currents. Klotho-treated fibroblasts (100 pM, 48 h) had lower phosphorylated phospholipase C (PLC) (p-PLCβ3 Ser537) expression than the control. The PLC inhibitor, U73122 (1 μM, 48 h), reduced the migration, decreased SOCE and TRP currents, and lowered p-PLCβ3 in atrial fibroblasts, similar to Klotho. In the presence of the U73122 (1 μM, 48 h), Klotho (100 pM, 48 h) could not further modulate the migration and collagen synthesis nor suppress the TRP currents in human atrial fibroblasts. Conclusions: Klotho inhibited pro-fibrotic activities and SOCE by inhibiting the PLC signaling and suppressing the TRP currents, which may provide a novel insight into atrial fibrosis and arrhythmogenesis.

Ageing, cellular senescence and chronic kidney disease: experimental evidence

PURPOSE OF REVIEW

Chronic kidney disease (CKD) is often viewed as an accelerated and premature ageing of the kidney, as they share common pathological features characterized by cellular senescence. In this review, we summarize the experimental evidence linking cellular senescence to the pathobiology of kidney ageing and CKD, and discuss the strategies for targeting senescent cells in developing therapeutics for ageing-related kidney disorders.

RECENT FINDINGS

Kidney ageing and CKD are featured with increased cellular senescence, an irreversible state of cell cycle arrest and the cessation of cell division. Senescent cells secrete a diverse array of proinflammatory and profibrotic factors known as senescence-associated secretory phenotype (SASP). Secondary senescence can be induced by primary senescent cells via a mechanism involving direct contact or the SASP. Various senolytic therapies aiming to selectively remove senescent cells in vivo have been developed. Senostatic approaches to suppress senescence or inhibit SASP, as well as nutrient signalling regulators are also validated in animal models of ageing.

SUMMARY

These recent studies provide experimental evidence supporting the notion that accumulation of senescent cells and their associated SASP is a main driver leading to structural and functional organ degeneration in CKD and other ageing-related disorder.

Cigarette Smoke Exposure Increases Glucose-6-phosphate Dehydrogenase, Autophagy, Fibrosis, and Senescence in Kidney Cells In Vitro and In Vivo

Cigarette smoke (CS) is a risk factor for chronic obstructive pulmonary disease. We attempted to investigate fully the possible effects of CS on kidney cells. We found that the viability of a human kidney proximal tubular epithelial cell line (HK-2 cells) was decreased after treatment with CS extract (CSE). In particular, the effects of CSE at low concentrations did not change the expression of apoptosis and necrosis. Furthermore, CSE increased autophagy- and fibrosis-related proteins in HK-2 cells. Senescence-related proteins and the senescence-associated secretory phenotype (SASP) increased after HK-2 cells were treated with CSE. In addition, both RNA sequencing and gene set enrichment analysis data revealed that glucose-6-phosphate dehydrogenase (G6PD) in the reactive oxygen species (ROS) pathway is responsible for the changes in CSE-treated HK-2 cells. CSE increased G6PD expression and its activity. Moreover, the inhibition of G6PD activity increased senescence in HK-2 cells. The inhibition of autophagy reinforced senescence in the CSE-treated cells. In a mouse model of CS exposure, CS caused kidney damage, including tubular injury and glomerulosclerosis. CS increased fibrosis, autophagy, and G6PD expression in kidney tissue sections. In conclusion, CS induced G6PD expression, autophagy, fibrosis, and senescence in kidney cells. G6PD has a protective role in CS-induced nephrotoxicity.

CCL5 Suppresses Klotho Expression via p-STAT3/DNA Methyltransferase1-Mediated Promoter Hypermethylation

Background

Enhanced inflammation and reduced Klotho are common features in chronic kidney disease (CKD). Inflammation induces DNA hypermethylation. This study assessed the performance of inflammatory marker C-C motif chemokine 5 (CCL5) in epigenetic regulation of Klotho expression.

Methods

Fifty CKD patients and 25 matched controls were enrolled, and serum CCL5 level, sKlotho level, and DNA methylation were evaluated in these subjects. A renal interstitial fibrosis (RIF) model with CKD was induced in mice via unilateral ureteral obstruction (UUO) in vivo and human proximal tubular epithelial (HK-2) cells treated with CCL5 in vitro. 5-aza-2′-deoxycytidine (5-Aza), a DNA methyltransferase inhibitor was given to UUO mice. Hematoxylin and eosin (HE) and Masson trichrome staining were adopted to evaluate renal pathological changes. Methylation-specific PCR was performed to assess DNA methylation of Klotho promoter in the peripheral blood leucocytes (PBLs) from CKD patients and obstructive kidney from UUO mice. CCL5, Klotho, and DNA methyltransferases (DNMTs) were determined by ELISAs, immunofluorescence, or western blotting. HK-2 cells were exposed to CCL5 with or without 5-Aza and stattic, a p-signal transducer and activator of transcription 3 (STAT3) inhibitor, and expressions of p-STAT3, DNMT1, and Klotho were determined by western blotting.

Results

CCL5 upregulation concomitant with Klotho downregulation in serum and global DNA methylation in PBLs were observed in CKD samples. UUO contributed to severe renal interstitial fibrosis and enhanced expressions of fibrotic markers. Moreover, UUO increased the CCL5 level, induced Klotho promoter methylation, suppressed Klotho level, activated p-STAT3 signaling, and upregulated DNMT1 level. A similar observation was made in HK-2 cells treated with CCL5. More importantly, 5-Aza inhibited UUO-induced Klotho hypermethylation, reversed Klotho, downregulated p-STAT3 expressions, and ameliorated RIF in vivo. The consistent findings in vitro were also obtained in HK-2 cells exposed to 5-Aza and stattic.

Conclusion

The CCL5/p-STAT3/DNMT1 axis is implicated in epigenetic regulation of Klotho expression in CKD. This study provides novel therapeutic possibilities for reversal of Klotho suppression by CKD.

Orally-active, clinically-translatable senolytics restore α-Klotho in mice and humans

BACKGROUND

α-Klotho is a geroprotective protein that can attenuate or alleviate deleterious changes with ageing and disease. Declines in α-Klotho play a role in the pathophysiology of multiple diseases and age-related phenotypes. Pre-clinical evidence suggests that boosting α-Klotho holds therapeutic potential. However, readily clinically-translatable, practical strategies for increasing α-Klotho are not at hand. Here, we report that orally-active, clinically-translatable senolytics can increase α-Klotho in mice and humans.

METHODS

We examined α-Klotho expression in three different human primary cell types co-cultured with conditioned medium (CM) from senescent or non-senescent cells with or without neutralizing antibodies. We assessed α-Klotho expression in aged, obese, and senescent cell-transplanted mice treated with vehicle or senolytics. We assayed urinary α-Klotho in patients with idiopathic pulmonary fibrosis (IPF) who were treated with the senolytic drug combination, Dasatinib plus Quercetin (D+Q).

FINDINGS

We found exposure to the senescent cell secretome reduces α-Klotho in multiple nonsenescent human cell types. This was partially prevented by neutralizing antibodies against the senescence-associated secretory phenotype (SASP) factors, activin A and Interleukin 1α (IL-1α). Consistent with senescent cells' being a cause of decreased α-Klotho, transplanting senescent cells into younger mice reduced brain and urine α-Klotho. Selectively removing senescent cells genetically or pharmacologically increased α-Klotho in urine, kidney, and brain of mice with increased senescent cell burden, including naturally-aged, diet-induced obese (DIO), or senescent cell-transplanted mice. D+Q increased α-Klotho in urine of patients with IPF, a disease linked to cellular senescence.

INTERPRETATION

Senescent cells cause reduced α-Klotho, partially due to their production of activin A and IL-1α. Targeting senescent cells boosts α-Klotho in mice and humans. Thus, clearing senescent cells restores α-Klotho, potentially opening a novel, translationally-feasible avenue for developing orally-active small molecule, α-Klotho-enhancing clinical interventions. Furthermore, urinary α-Klotho may prove to be a useful test for following treatments in senolytic clinical trials.

FUNDING

This work was supported by National Institute of Health grants AG013925 (J.L.K.), AG062413 (J.L.K., S.K.), AG044271 (N.M.), AG013319 (N.M.), and the Translational Geroscience Network (AG061456: J.L.K., T.T., N.M., S.B.K., S.K.), Robert and Arlene Kogod (J.L.K.), the Connor Group (J.L.K.), Robert J. and Theresa W. Ryan (J.L.K.), and the Noaber Foundation (J.L.K.). The previous IPF clinical trial was supported by the Claude D. Pepper Older Americans Independence Centers at WFSM (AG021332: J.N.J., S.B.K.), UTHSCA (AG044271: A.M.N.), and the Translational Geroscience Network.

NRF2 Pathway Activation Attenuates Aging-Related Renal Phenotypes due to α-Klotho Deficiency

Oxidative stress is one of the major causes of the age-related functional decline in cells and tissues. The KEAP1-NRF2 system plays a central role in the regulation of redox balance, and NRF2 activation exerts antiaging effects by controlling oxidative stress in aged tissues. α-Klotho was identified as an aging suppressor protein based on the premature aging phenotypes of its mutant mice, and its expression is known to gradually decrease during aging. Because α-Klotho has been shown to possess antioxidant function, aging-related phenotypes of α-Klotho mutant mice seem to be attributable to increased oxidative stress at least in part. To examine whether NRF2 activation antagonizes aging-related phenotypes caused by α-Klotho deficiency, we crossed α-Klotho-deficient (Kl–/–) mice with a Keap1-knockdown background, in which the NRF2 pathway is constitutively activated in the whole body. NRF2 pathway activation in Kl–/– mice extended the lifespan and dramatically improved aging-related renal phenotypes. With elevated expression of antioxidant genes accompanied by an oxidative stress decrease, the antioxidant effects of NRF2 seem to make a major contribution to the attenuation of aging-related renal phenotypes of Kl–/– mice. Thus, NRF2 is expected to exert an antiaging function by partly compensating for the functional decline of α-Klotho during physiological aging.

Oxidative Stress and Cellular Senescence Are Involved in the Aging Kidney

Chronic kidney disease (CKD) can be considered as a clinical model for premature aging. However, non-invasive biomarkers to detect early kidney damage and the onset of a senescent phenotype are lacking. Most of the preclinical senescence studies in aging have been done in very old mice. Furthermore, the precise characterization and over-time development of age-related senescence in the kidney remain unclear. To address these limitations, the age-related activation of cellular senescence-associated mechanisms and their correlation with early structural changes in the kidney were investigated in 3- to 18-month-old C57BL6 mice. Inflammatory cell infiltration was observed by 12 months, whereas tubular damage and collagen accumulation occurred later. Early activation of cellular-senescence-associated mechanisms was found in 12-month-old mice, characterized by activation of the DNA-damage-response (DDR), mainly in tubular cells; activation of the antioxidant NRF2 pathway; and klotho downregulation. However, induction of tubular-cell-cycle-arrest (CCA) and overexpression of renal senescent-associated secretory phenotype (SASP) components was only found in 18-month-old mice. In aging mice, both inflammation and oxidative stress (marked by elevated lipid peroxidation and NRF2 inactivation) remained increased. These findings support the hypothesis that prolonged DDR and CCA, loss of nephroprotective factors (klotho), and dysfunctional redox regulatory mechanisms (NRF2/antioxidant defense) can be early drivers of age-related kidney-damage progression.

Phosphate and Cellular Senescence

Cellular senescence is one type of permeant arrest of cell growth and one of increasingly recognized contributor to aging and age-associated disease. High phosphate and low Klotho individually and synergistically lead to age-related degeneration in multiple organs. Substantial evidence supports the causality of high phosphate in cellular senescence, and potential contribution to human aging, cancer, cardiovascular, kidney, neurodegenerative, and musculoskeletal diseases. Phosphate can induce cellular senescence both by direct phosphotoxicity, and indirectly through downregulation of Klotho and upregulation of plasminogen activator inhibitor-1. Restriction of dietary phosphate intake and blockage of intestinal absorption of phosphate help suppress cellular senescence. Supplementation of Klotho protein, cellular senescence inhibitor, and removal of senescent cells with senolytic agents are potential novel strategies to attenuate phosphate-induced cellular senescence, retard aging, and ameliorate age-associated, and phosphate-induced disorders.

TGF-β1 is involved in senescence-related pathways in glomerular endothelial cells via p16 translocation and p21 induction

p16 inhibits cyclin-dependent kinases and regulates senescence-mediated arrest as well as p21. Nuclear p16 promotes G1 cell cycle arrest and cellular senescence. In various glomerular diseases, nuclear p16 expression is associated with disease progression. Therefore, the location of p16 is important. However, the mechanism of p16 trafficking between the nucleus and cytoplasm is yet to be fully investigated. TGF-β1, a major cytokine involved in the development of kidney diseases, can upregulate p21 expression. However, the relationship between TGF-β1 and p16 is poorly understood. Here, we report the role of podocyte TGF-β1 in regulating the p16 behavior in glomerular endothelial cells. We analyzed podocyte-specific TGF-β1 overexpression mice. Although p16 was found in the nuclei of glomerular endothelial cells and led to endothelial cellular senescence, the expression of p16 did not increase in glomeruli. In cultured endothelial cells, TGF-β1 induced nuclear translocation of p16 without increasing its expression. Among human glomerular diseases, p16 was detected in the nuclei of glomerular endothelial cells. In summary, we demonstrated the novel role of podocyte TGF-β1 in managing p16 behavior and cellular senescence in glomeruli, which has clinical relevance for the progression of human glomerular diseases.

Constitutive transgenic alpha-Klotho overexpression enhances resilience to and recovery from murine acute lung injury

AIMS

Normal lungs do not express alpha-Klotho (Klotho) protein but derive cytoprotection from circulating soluble Klotho. It is unclear whether chronic supranormal Klotho levels confer additional benefit. To address this, we tested the age-related effects of Klotho overexpression on acute lung injury (ALI) and recovery.

METHODS

Transgenic Klotho-overexpressing (Tg-Kl) and wild-type (WT) mice (2 and 6 months old) were exposed to hyperoxia (95% O₂; 72 h) then returned to normoxia (21% O₂; 24 h) (Hx-R). Control mice were kept in normoxia. Renal and serum Klotho, lung histology, and bronchoalveolar lavage fluid oxidative damage markers were assessed. Effects of hyperoxia were tested in human embryonic kidney cells stably expressing Klotho. A549 lung epithelial cells transfected with Klotho cDNA or vector were exposed to cigarette smoke; lactate dehydrogenase and double-strand DNA breaks were measured.

RESULTS

Serum Klotho decreased with age. Hyperoxia suppressed renal Klotho at both ages and serum Klotho at 2-months of age. Tg-Kl mice at both ages and 2-months-old WT mice survived Hx-R; 6-months-old Tg-Kl mice showed lower lung damage than age-matched WT mice. Hyperoxia directly inhibited Klotho expression and release in vitro; Klotho transfection attenuated cigarette smoke-induced cytotoxicity and DNA double-strand breaks in lung epithelial cells.

CONCLUSIONS

Young animals with chronic high baseline Klotho expression are more resistant to ALI. Chronic constitutive Klotho overexpression in older Tg-Kl animals attenuates hyperoxia-induced lung damage and improves survival and short-term recovery despite an acute reduction in serum Klotho level during injury. We conclude that chronic enhancement of Klotho expression increases resilience to ALI.

The emerging role of phosphorus in human health

Phosphorus, an essential nutrient, performs vital functions in skeletal and non-skeletal tissues and is pivotal for energy production. The last two decades of research on the physiological importance of phosphorus have provided several novel insights about its dynamic nature as a nutrient performing functions as a phosphate ion. Phosphorous also acts as a signaling molecule and induces complex physiological responses. It is recognized that phosphorus homeostasis is critical for health. The intake of phosphorus by the general population world-wide is almost double the amount required to maintain health. This increase is attributed to the incorporation of phosphate containing food additives in processed foods purchased by consumers. Research findings assessed the impact of excessive phosphorus intake on cells' and organs' responses, and highlighted the potential pathogenic consequences. Research also identified a new class of bioactive phosphates composed of polymers of phosphate molecules varying in chain length. These polymers are involved in metabolic responses including hemostasis, brain and bone health, via complex mechanism(s) with positive or negative health effects, depending on their chain length. It is amazing, that phosphorus, a simple element, is capable of exerting multiple and powerful effects. The role of phosphorus and its polymers in the renal and cardiovascular system as well as on brain health appear to be important and promising future research directions.

High Dietary Phosphate Exacerbates and Acts Independently of Low Autophagy Activity in Pathological Cardiac Remodeling and Dysfunction

High phosphate contributes to uremic cardiomyopathy. Abnormal autophagy is associated with the development and progression of heart disease. What is unknown is the effects of phosphate on autophagy and whether the ill effects of phosphate on cardiomyocytes are mediated by low autophagy. High (2.0% w/w)-phosphate diet reduced LC3 puncta in cardiomyocytes and ratio of LC3 II/I and increased p62 protein, indicating that autophagy activity was suppressed. Mice with cardiomyocyte-specific deletion of autophagy-related protein 5 (H-atg5-/-) had reduced autophagy only in the heart, developed cardiac dysfunction with hypertrophy and fibrosis, and had a short lifespan. When H-atg5-/- mice were fed a high-phosphate diet, they developed more apoptosis in cardiomyocytes, more severe cardiac remodeling, and shorter lifespan than normal phosphate-fed H-atg5-/- mice, indicating that cardiac phosphotoxicity is imparted independently of atg5. In conclusion, although high phosphate suppresses autophagy, high phosphate and low autophagy independently trigger and additionally amplify cardiac remodeling and dysfunction.

Bovine serum albumin aggravates macrophage M1 activation and kidney injury in heterozygous Klotho-deficient mice via the gut microbiota-immune axis

Klotho expression abnormalities induces kidney injury and chronic kidney disease, however, the underlying mechanism remains unclear. Here, Klotho^+/- mice and wild-type mice were treated with low-dose bovine serum albumin (BSA). Pathological examination demonstrated that the area of glomerular collagen deposition and fibrosis in BSA-Kl^-/+ mice was significantly larger than that in BSA-WT mice. The serum levels of superoxide dismutase, malondialdehyde, creatinine, and urea in BSA-Kl^-/+ mice were significantly increased. Sequencing of gut microbiota 16S rRNA v3-v4 region indicated that BSA-Kl^-/+ mice showed a significantly higher relative abundance of the genera Dubosiella, Akkermansia, Alloprevotella, and Lachnospiraceae and a significantly lower relative abundance of the genera Allobaculum and Muribaculaceae than BSA-WT mice. KEGG analysis revealed that the metabolic pathways of signal transduction, xenobiotic biodegradation and metabolism, and lipid metabolism increased significantly in BSA-Kl^-/+ mice. Flow cytometry showed that the proportion of CD68⁺/CD11b⁺ cells in the peripheral blood was significantly higher in BSA-KL^-/+ mice than that in BSA-WT mice. qPCR and western blot suggested that Klotho and Nrf2 expression in MΦ1 cells of BSA-KL^-/+ mice was significantly decreased. Thus, the findings suggest during the immune activation and chronic inflammation induced by the gut microbiota imbalance in Klotho-deficient mice treated to BSA, disrupted expression of proteins in the Nrf2/NF-κB signaling pathway in monocyte-derived macrophage M1 cells leads to the aggravation of inflammation and kidney injury.

Cellular Senescence in Kidney Fibrosis: Pathologic Significance and Therapeutic Strategies

Age-related disorders such as chronic kidney disease (CKD) are increasingly prevalent globally and pose unprecedented challenges. In many aspects, CKD can be viewed as a state of accelerated and premature aging. Aging kidney and CKD share many common characteristic features with increased cellular senescence, a conserved program characterized by an irreversible cell cycle arrest with altered transcriptome and secretome. While developmental senescence and acute senescence may positively contribute to the fine-tuning of embryogenesis and injury repair, chronic senescence, when unresolved promptly, plays a crucial role in kidney fibrogenesis and CKD progression. Senescent cells elicit their fibrogenic actions primarily by secreting an assortment of inflammatory and profibrotic factors known as the senescence-associated secretory phenotype (SASP). Increasing evidence indicates that senescent cells could be a promising new target for therapeutic intervention known as senotherapy, which includes depleting senescent cells, modulating SASP and restoration of senescence inhibitors. In this review, we discuss current understanding of the role and mechanism of cellular senescence in kidney fibrosis. We also highlight potential options of targeting senescent cells for the treatment of CKD.