Cyclin G1 induces maladaptive proximal tubule cell dedifferentiation and renal fibrosis through CDK5 activation

DEAD-box protein 21 promotes renal fibrosis via p21-dependent cell cycle arrest in proximal tubular epithelial cells

Renal interstitial fibrosis is the final common outcome of various chronic kidney diseases (CKD). Renal tubular epithelial cells (TECs) G2/M cell cycle arrest play a pivotal role in renal fibrosis. Although RNA-binding proteins (RBPs) are implicated in organ fibrosis, the underlying mechanisms remain poorly understood. Here, we identify DEAD-box protein 21 (DDX21), a representative RBP, as highly expressed in fibrotic renal tissues, especially in TECs. Moreover, DDX21 expression is positively correlated with renal function decline in CKD patients, underscoring its role in disease progression. TECs-specific deletion of Ddx21 alleviates cell cycle arrest in G2/M, and attenuates fibrotic responses. Mechanistically, silencing DDX21 reduces p21 expression at both the mRNA and protein levels and decreases cell apoptosis, indicating that DDX21 promotes G2/M cell cycle arrest by regulating the p21 signaling pathway. This study suggests that DDX21 may serve as a promising therapeutic target for kidney fibrosis.

Crosstalk between glomeruli and tubules

Models of kidney injury have classically concentrated on glomeruli as the primary site of injury leading to glomerulosclerosis or on tubules as the primary site of injury leading to tubulointerstitial fibrosis. However, current evidence on the mechanisms of progression of chronic kidney disease indicates that a complex interplay between glomeruli and tubules underlies progressive kidney injury. Primary glomerular injury can clearly lead to subsequent tubule injury. For example, damage to the glomerular filtration barrier can expose tubular cells to serum proteins, including complement and cytokines, that would not be present in physiological conditions and can promote the development of tubulointerstitial fibrosis and progressive decline in kidney function. In addition, although less well-studied, increasing evidence suggests that tubule injury, whether primary or secondary, can also promote glomerular damage. This feedback from the tubule to the glomerulus might be mediated by changes in the reabsorptive capacity of the tubule, which can affect the glomerular filtration rate, or by mediators released by injured proximal tubular cells that can induce damage in both podocytes and parietal epithelial cells. Examining the crosstalk between the various compartments of the kidney is important for understanding the mechanisms underlying kidney pathology and identifying potential therapeutic interventions.

GALNT3 in Ischemia-Reperfusion Injury of the Kidney

Key Points

Background

Damages to subcellular organelles, such as mitochondria and endoplasmic reticulum, are well recognized in tubular cell injury and death in AKI. However, the changes and involvement of Golgi apparatus are much less known. In this study, we report the regulation and role of N-acetylgalactosaminyltransferase-3 (GALNT3), a key enzyme for protein glycosylation in Golgi apparatus, in AKI.

Methods

AKI was induced in mice by renal ischemia–reperfusion injury or cisplatin. In vitro, rat kidney proximal tubular cells were subjected to hypoxia/reoxygenation (H/R) injury. To determine the role of GALNT3, its specific inhibitor T3inh-1 was tested in mice, and the effects of GALNT3 overexpression as well as knockdown were examined in the rat renal proximal tubular cells. EGF receptor (EGFR) activation was induced by recombinant EGF or by overexpressing EGFR.

Results

GALNT3 was significantly decreased in both in vivo and in vitro models of AKI induced by renal ischemia–reperfusion injury and cisplatin. T3Inh-1, a specific GALNT3 inhibitor, exacerbated ischemic AKI and suppressed tubular cell proliferation in mice. Moreover, knockdown of GALNT3 increased apoptosis during H/R treatment in rat renal proximal tubular cells, whereas overexpression of GALNT3 attenuated H/R-induced apoptosis, further supporting a protective role of GALNT3. Mechanistically, GALNT3 contributed to O-glycosylation of EGFR and associated EGFR signaling. Activation or overexpression of EGFR suppressed the proapoptotic effect of GALNT3 knockdown in H/R-treated rat renal proximal tubular cells.

Conclusions

GALNT3 protected kidney tubular cells in AKI at least partially through O-glycosylation of EGFR.

SIRT2 inhibition attenuates myofibroblast transition through autophagy-mediated ciliogenesis in renal epithelial cells

Myofibroblast transition plays a crucial role in both fibrotic diseases and wound healing. Although SIRT2 regulates fibrosis, its mechanisms of action remain poorly understood. This study aimed to investigate the effects of SIRT2 inhibition on myofibroblast transition in human renal cells under quiescent conditions. HK-2 kidney proximal tubular epithelial cells were starved of serum, resulting in the formation of primary cilia. Transforming growth factor-β (TGF-β) stimulation reduced both the number of ciliated cells and ciliary length. The ciliary defects resulted from a failure in autophagy termination, leading to the accumulation of OFD1, a negative regulator of ciliogenesis, at centriolar satellites. This phenomenon was correlated with the upregulation of fibrosis-related proteins. To elucidate the role of SIRT2 in the autophagy-ciliogenesis-fibrosis axis, cells were treated with AGK2, a specific inhibitor of SIRT2. AGK2 treatment promoted the formation of both autophagosomes and autolysosomes and facilitated OFD1 degradation at the centriolar satellites, resulting in the lengthening of primary cilia. Restoration of primary cilia by AGK2 was associated with the suppression of myofibroblast transition. In conclusion, SIRT2 inhibition attenuates TGF-β-induced fibrosis by promoting autophagy-mediated ciliogenesis. This study highlights SIRT2 as a potential therapeutic target for fibrotic diseases.

The Role and Mechanisms of Aurora Kinases in Kidney Diseases

Aurora kinases are a family of serine/threonine kinases that includes Aurora kinase A, Aurora kinase B, and Aurora kinase C. These kinases play crucial roles in mitotic spindle formation and cell proliferation. Over the past several decades, extensive research has elucidated the multifaceted roles of Aurora kinases in cancer development and progression. Recent studies have also highlighted the significant involvement of Aurora kinases in various kidney diseases, such as renal cell carcinoma, diabetic nephropathy, chronic kidney disease, and polycystic kidney disease. The mechanisms by which Aurora kinases contribute to renal diseases are complex and influenced by both specific pathological conditions and environmental factors. In this review, we comprehensively summarize the role and mechanisms through which Aurora kinases operate in kidney diseases and discuss the efficacy and application of existing inhibitors targeting these kinases in managing renal disorders in animal models.

Biology of the proximal tubule in body homeostasis and kidney disease

The proximal tubule (PT) is known as the workhorse of the kidney, for both the range and magnitude of the functions that it performs. It is not only responsible for reabsorbing most solutes and proteins filtered by glomeruli, but also for secreting non-filtered substances including drugs and uremic toxins. The PT therefore plays a pivotal role in kidney physiology and body homeostasis. Moreover, it is the major site of damage in acute kidney injury and nephrotoxicity. In this review, we will provide an introduction to the cell biology of the PT and explore how it is adapted to the execution of a myriad of different functions and how these can differ between males and females. We will then discuss how the PT regulates phosphate, glucose and acid-base balance, and the consequences of alterations in PT function for bone and cardiovascular health. Finally, we explore why the PT is vulnerable to ischemic and toxic insults, and how acute injury in the PT can lead to maladaptive repair, chronic damage and kidney fibrosis. In summary, we will demonstrate that knowledge of the basic cell biology of the PT is critical for understanding kidney disease phenotypes and their associated systemic complications, and for developing new therapeutic strategies to prevent these.

Cyclin-dependent kinase 5 as a potential therapeutic target to alleviate high glucose-induced podocyte apoptosis and hyperglycemia-induced renal injury in mice

BACKGROUND

Hyperglycemia is a risk factor for impaired renal function, including cellular metabolic disturbance, apoptosis, inflammation, and histologic lesion. This study aims to investigate the potential therapeutic targeting of cyclin-dependent kinase 5 (Cdk5) in hyperglycemia-induced podocyte dysfunction and renal damage.

METHODS

Cell viability and apoptosis of podocytes were assessed through CCK-8 and TUNEL staining, respectively, following exposure to normal glucose (NG; 5 mM), high glucose (HG; 30 mM), or treatment with Cdk5 inhibitors (trans-resveratrol, myricetin, salvianolic acid A, and BML-259). Diabetic mice were established by intraperitoneal injection of freshly streptozotocin (STZ), which was given at a dose of 35 mg/kg in five successive injections. Additionally, histochemical staining was employed to evaluate the morphologic lesion of the kidney.

RESULTS

Cdk5 was found to be activated by HG stimulation both in vitro and in vivo. Notably, the inhibition of Cdk5 effectively mitigated the podocyte dysfunction induced by HG, including growth inhibition, membrane damage, and apoptosis. The compounds Trans-resveratrol, myricetin, salvianolic acid A, and BML-259 exhibited low binding energy values of -8.032 kcal/mol, -8.693 kcal/mol, -8.743 kcal/mol, and -10.952 kcal/mol, respectively, indicating strong and stable binding affinity between these candidates and Cdk5. The results of in vivo experimental analysis demonstrate that Cdk5 inhibitors, namely trans-resveratrol, myricetin, salvianolic acid A, and BML-259, confer protection against tubular and glomerular lesions induced by hyperglycemia.

CONCLUSION

Both myricetin and BML-259 exhibit comparable protective effects on renal injury by inhibiting Cdk5.

A Newly Identified Protective Role of C5a Receptor 1 in Kidney Tubules against Toxin-Induced Acute Kidney Injury

Acute kidney injury (AKI) remains a major reason for hospitalization with limited therapeutic options. Although complement activation is implicated in AKI, the role of C5a receptor 1 (C5aR1) in kidney tubular cells is unclear. Herein, aristolochic acid nephropathy (AAN) and folic acid nephropathy (FAN) models were used to establish the role of C5aR1 in kidney tubules during AKI in germline C5ar1-/-, myeloid cell-specific, and kidney tubule-specific C5ar1 knockout mice. After aristolochic acid and folic acid injection, C5ar1-/- mice had increased AKI severity and a higher degree of tubular injury. Macrophage depletion in C5ar1-/- mice or myeloid cell-specific C5ar1 deletion did not affect the outcomes of aristolochic acid-induced AKI. RNA-sequencing data from renal tubular epithelial cells (RTECs) showed that C5ar1 deletion was associated with the down-regulation of mitochondrial metabolism and ATP production transcriptional pathways. Metabolic studies confirmed reduced mitochondrial membrane potential at baseline and increased mitochondrial oxidative stress after injury in C5ar1-/- RTECs. Moreover, C5ar1-/- RTECs had enhanced glycolysis, glucose uptake, and lactate production on injury, corroborated by metabolomics analysis of kidneys from AAN mice. Kidney tubule-specific C5ar1 knockout mice recapitulated exacerbated AKI observed in C5ar1-/- mice in AAN and FAN. These data indicate that C5aR1 signaling in kidney tubules exerts renoprotective effects against toxin-induced AKI by limiting overt glycolysis and maintaining mitochondrial function, thereby revealing a novel link between the complement system and tubular cell metabolism.

5-Hydroxytryptamine 1F Receptor Agonist Lasmiditan Differentially Regulates Successful Repair and Failed Repair Genes in a Mouse Model of Acute Kidney Injury

Increasing evidence substantiates the role of mitochondrial dysfunction, inflammation, fibrosis, and cell senescence in the onset and progression of acute kidney injury (AKI) to chronic kidney disease . The underlying governing cellular and transcriptional events, however, are not fully understood. Recently, the key factors that regulate successful and failed repair states in the proximal tubule have been identified at a single-cell resolution following bilateral ischemia-reperfusion (I/R) in a mouse model of AKI. Previously, our group showed that treatment with the FDA-approved selective 5-hydroxytryptamine receptor 1F agonist lasmiditan following AKI induces mitochondrial biogenesis , restores renal mitochondrial function, and increases renal and vascular recovery in vivo. Here, we assessed the effect of lasmiditan on transcriptional and translational changes that are responsible for successful repair, injury, and failed repair states in the renal cortex following I/R-induced AKI. Increased levels of successful repair genes such as acyl-coA synthase medium-chain family member 2a, low-density lipoprotein receptor-related protein 2, solute carrier family 5 member 12, and hepatocyte nuclear factor 4 alpha were observed with 6 and 12 days of lasmiditan treatment following AKI compared to vehicle control. While 6 days of lasmiditan treatment had no effect on failed repair genes, the administration of lasmiditan for 12 days decreased the levels of vascular cell adhesion protein 1, tumor necrosis factor α, and interleukin-1β, which drive maladaptive repair. These data reveal that lasmiditan treatment post-AKI differentially regulates successful and failed repair gene expression in the renal cortex, likely contributing to the restoration of renal function and providing a potential targeted therapeutic pathway for the treatment of AKI.

HSPA12A stimulates "Smurf1-Hif1α-aerobic glycolysis" axis to promote proliferation of renal tubular epithelial cells after hypoxia/reoxygenation injury

Proliferation of renal tubular epithelial cells (TECs) is critical for the recovery after kidney ischemia/reperfusion (KI/R). However, there is still a lack of ideal therapies for promoting TEC proliferation. Heat shock protein A12A (HSPA12A) shows abundant expression in kidney in our previous studies. To investigate the role of HSPA12A in TEC proliferation after KI/R, an in vitro KI/R model was simulated by hypoxia (12 h) and reoxygenation (12 h) in human kidney tubular epithelial HK-2 cells. We found that, when hypoxia/reoxygenation (H/R) triggered HK-2 cell injury, HSPA12A expression was downregulated, and extracellular lactate, the readout of glycolysis, was also decreased. Loss and gain of functional studies showed that HSPA12A did not change cell viability after hypoxia but increased cell proliferation as well as glycolytic flux of HK-2 cells after H/R. When blocking glycolysis by 2-deoxy-D-glucose or oxamate, the HSPA12A promoted HK-2 cell proliferation was also abolished. Further analysis revealed that HSPA12A overexpression increased hypoxia-inducible factor 1α (Hif1α) protein expression and nuclear localization in HK-2 cells in response to H/R, whereas HSPA12A knockdown showed the opposite effects. Notably, pharmacological inhibition of Hif1α with YC-1 reversed the HSPA12A-induced increases of both glycolytic flux and proliferation of H/R HK-2 cells. Moreover, the HSPA12A increased Hif1α protein expression was not via upregulating its transcription but through increasing its protein stability in a Smurf1-dependent manner. The findings indicate that HSPA12A might serve as a promising target for TEC proliferation to help recovery after KI/R.

PBMC therapy reduces cell death and tissue fibrosis after acute kidney injury by modulating the pattern of monocyte/macrophage survival in tissue

In this study, we investigated if the therapeutic potential of peripheral blood mononuclear cell (PBMC) therapy in a murine model of ischemic AKI is related with the survival pattern of monocyte/macrophages in tissue. CD-1 mice were subjected to bilateral renal ischemia followed by reperfusion to induce AKI. M2-polarized PBMCs isolated from CD-1 mice were administered intravenously at different time points post-injury. Our results demonstrate that early administration of PBMC therapy attenuates renal tissue damage, reduces tissue cell death and prevents fibrosis development. Reduction of tissue pyroptosis was observed by reduction on NLRP3 inflammasome activation and decreasing IL-1beta and Caspase-1 expression in the kidney. Furthermore, the therapy was shown to mitigate ferroptosis by inducing GPX4 overexpression. Early administration of PBMCs increased the survival pattern of renal tissue-macrophages, promoting a "pro-survival phenotype" resulting in decreased pyroptotic marker NLRP3, IL-1beta and Caspase 1 and increased anti-ferroptotic gene GPX4. Conversely, delayed administration of PBMC therapy exhibits diminished efficacy in preventing cell death and fibrosis in tissue and provoked a decrease in the pro-survival phenotype of both monocyte /macrophages in tissue. Our findings highlight the therapeutic potential of PBMC therapy in mitigating AKI and preventing CKD progression by modulating tissue-resident macrophage survival and reducing their cell death pathways. The fact that the effectiveness of the therapy depends on the time of administration after the injury underscores the importance of early intervention in AKI management.

Gene regulation in regeneration after acute kidney injury

Acute kidney injury (AKI) is a common condition associated with significant morbidity, mortality, and cost. Injured kidney tissue can regenerate after many forms of AKI. However, there are no treatments in routine clinical practice to encourage recovery. In part, this shortcoming is due to an incomplete understanding of the genetic mechanisms that orchestrate kidney recovery. The advent of high-throughput sequencing technologies and genetic mouse models has opened an unprecedented window into the transcriptional dynamics that accompany both successful and maladaptive repair. AKI recovery shares similar cell-state transformations with kidney development, which can suggest common mechanisms of gene regulation. Several powerful bioinformatic strategies have been developed to infer the activity of gene regulatory networks by combining multiple forms of sequencing data at single-cell resolution. These studies highlight not only shared stress responses but also key changes in gene regulatory networks controlling metabolism. Furthermore, chromatin immunoprecipitation studies in injured kidneys have revealed dynamic epigenetic modifications at enhancer elements near target genes. This review will highlight how these studies have enhanced our understanding of gene regulation in injury response and regeneration.

Epithelial cell states associated with kidney and allograft injury

The kidney epithelium, with its intricate arrangement of highly specialized cell types, constitutes the functional core of the organ. Loss of kidney epithelium is linked to the loss of functional nephrons and a subsequent decline in kidney function. In kidney transplantation, epithelial injury signatures observed during post-transplantation surveillance are strong predictors of adverse kidney allograft outcomes. However, epithelial injury is currently neither monitored clinically nor addressed therapeutically after kidney transplantation. Several factors can contribute to allograft epithelial injury, including allograft rejection, drug toxicity, recurrent infections and postrenal obstruction. The injury mechanisms that underlie allograft injury overlap partially with those associated with acute kidney injury (AKI) and chronic kidney disease (CKD) in the native kidney. Studies using advanced transcriptomic analyses of single cells from kidney or urine have identified a role for kidney injury-induced epithelial cell states in exacerbating and sustaining damage in AKI and CKD. These epithelial cell states and their associated expression signatures are also observed in transplanted kidney allografts, suggesting that the identification and characterization of transcriptomic epithelial cell states in kidney allografts may have potential clinical implications for diagnosis and therapy.

Variants in tubule epithelial regulatory elements mediate most heritable differences in human kidney function

Kidney disease is highly heritable; however, the causal genetic variants, the cell types in which these variants function, and the molecular mechanisms underlying kidney disease remain largely unknown. To identify genetic loci affecting kidney function, we performed a GWAS using multiple kidney function biomarkers and identified 462 loci. To begin to investigate how these loci affect kidney function, we generated single-cell chromatin accessibility (scATAC-seq) maps of the human kidney and identified candidate cis-regulatory elements (cCREs) for kidney podocytes, tubule epithelial cells, and kidney endothelial, stromal, and immune cells. Kidney tubule epithelial cCREs explained 58% of kidney function SNP-heritability and kidney podocyte cCREs explained an additional 6.5% of SNP-heritability. In contrast, little kidney function heritability was explained by kidney endothelial, stromal, or immune cell-specific cCREs. Through functionally informed fine-mapping, we identified putative causal kidney function variants and their corresponding cCREs. Using kidney scATAC-seq data, we created a deep learning model (which we named ChromKid) to predict kidney cell type-specific chromatin accessibility from sequence. ChromKid and allele specific kidney scATAC-seq revealed that many fine-mapped kidney function variants locally change chromatin accessibility in tubule epithelial cells. Enhancer assays confirmed that fine-mapped kidney function variants alter tubule epithelial regulatory element function. To map the genes which these regulatory elements control, we used CRISPR interference (CRISPRi) to target these regulatory elements in tubule epithelial cells and assessed changes in gene expression. CRISPRi of enhancers harboring kidney function variants regulated NDRG1 and RBPMS expression. Thus, inherited differences in tubule epithelial NDRG1 and RBPMS expression may predispose to kidney disease in humans. We conclude that genetic variants affecting tubule epithelial regulatory element function account for most SNP-heritability of human kidney function. This work provides an experimental approach to identify the variants, regulatory elements, and genes involved in polygenic disease.

RNA-RNA interactions between respiratory syncytial virus and miR-26 and miR-27 are associated with regulation of cell cycle and antiviral immunity

microRNAs (miRNAs) regulate nearly all physiological processes but our understanding of exactly how they function remains incomplete, particularly in the context of viral infections. Here, we adapt a biochemical method (CLEAR-CLIP) and analysis pipeline to identify targets of miRNAs in lung cells infected with Respiratory syncytial virus (RSV). We show that RSV binds directly to miR-26 and miR-27 through seed pairing and demonstrate that these miRNAs target distinct gene networks associated with cell cycle and metabolism (miR-27) and antiviral immunity (miR-26). Many of the targets are de-repressed upon infection and we show that the miR-27 targets most sensitive to miRNA inhibition are those associated with cell cycle. Finally, we demonstrate that high confidence chimeras map to long noncoding RNAs (lncRNAs) and pseudogenes in transcriptional regulatory regions. We validate that a proportion of miR-27 and Argonaute 2 (AGO2) is nuclear and identify a long non-coding RNA (lncRNA) as a miR-27 target that is linked to transcriptional regulation of nearby genes. This work expands the target networks of miR-26 and miR-27 to include direct interactions with RSV and lncRNAs and implicate these miRNAs in regulation of key genes that impact the viral life cycle associated with cell cycle, metabolism, and antiviral immunity.

Autophagy activates EGR1 via MAPK/ERK to induce FGF2 in renal tubular cells for fibroblast activation and fibrosis during maladaptive kidney repair

Macroautophagy/autophagy contributes to maladaptive kidney repair by inducing pro-fibrotic factors such as FGF2 (fibroblast growth factor 2), but the underlying mechanism remains elusive. Here, we show that EGR1 (early growth response 1) was induced in injured proximal tubules after ischemic acute kidney injury (AKI) and this induction was suppressed by autophagy deficiency in inducible, renal tubule-specific atg7 (autophagy related 7) knockout (iRT-atg7 KO) mice. In cultured proximal tubular cells, TGFB1 (transforming growth factor beta 1) induced EGR1 and this induction was also autophagy dependent. Egr1 knockdown in tubular cells reduced FGF2 expression during TGFB1 treatment, leading to less FGF2 secretion and decreased paracrine effects on fibroblasts. ChIP assay detected an increased binding of EGR1 to the Fgf2 gene promoter in TGFB1-treated tubular cells. Both Fgf2 and Egr1 transcription was inhibited by FGF2 neutralizing antibody, suggesting a positive feedback for EGR1-mediated FGF2 autoregulation. This feedback was confirmed using fgf2-deficient tubular cells and fgf2-deficient mice. Upstream of EGR1, autophagy deficiency in mice suppressed MAPK/ERK (mitogen-activated protein kinase) activation in post-ischemic renal tubules. This inhibition correlated with SQSTM1/p62 (sequestosome 1) aggregation and its sequestration of MAPK/ERK. SQSTM1/p62 interacted with MAPK/ERK and blocked its activation during TGFB1 treatment in autophagy-deficient tubular cells. Inhibition of MAPK/ERK suppressed EGR1 and FGF2 expression in maladaptive tubules, leading to the amelioration of renal fibrosis and improvement of renal function. These results suggest that autophagy activates MAPK/ERK in renal tubular cells, which induces EGR1 to transactivate FGF2. FGF2 is then secreted into the interstitium to stimulate fibroblasts for fibrogenesis.Abbreviation: 3-MA: 3-methyladenine; ACTA2/α-SMA: actin alpha 2, smooth muscle, aorta; ACTB/β-actin: actin, beta; AKI: acute kidney injury; aa: amino acid; ATG/Atg: autophagy related; BUN: blood urea nitrogen; ChIP: chromatin immunoprecipitation; CKD: chronic kidney disease; CM: conditioned medium; COL1A1: collagen, type I, alpha 1; COL4A1: collagen, type IV, alpha 1; CQ: chloroquine; DBA: dolichos biflorus agglutinin; EGR1: early growth response 1; ELK1: ELK1, member of ETS oncogene family; FGF2: fibroblast growth factor 2; FN1: fibronectin 1; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; HAVCR1/KIM-1: hepatitis A virus cellular receptor 1; IP: immunoprecipitation; LIR: LC3-interacting region; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MAP2K/MEK: mitogen-activated protein kinase kinase; MAPK: mitogen-activated protein kinase; NFKB: nuclear factor kappa B; PB1: Phox and Bem1; PFT: pifithrin α; PPIB/cyclophilin B: peptidylprolyl isomerase B; RT-qPCR: real time-quantitative PCR; SQSTM1/p62: sequestosome 1; TGFB1/TGF-β1: transforming growth factor beta 1; VIM: vimentin.

The secreted micropeptide C4orf48 enhances renal fibrosis via an RNA-binding mechanism

Renal interstitial fibrosis is an important mechanism in the progression of chronic kidney disease (CKD) to end-stage kidney disease. However, we lack specific treatments to slow or halt renal fibrosis. Ribosome profiling identified upregulation of a secreted micropeptide, C4orf48 (Cf48), in mouse diabetic nephropathy. Cf48 RNA and protein levels were upregulated in tubular epithelial cells in human and experimental CKD. Serum Cf48 levels were increased in human CKD and correlated with loss of kidney function, increasing CKD stage, and the degree of active interstitial fibrosis. Cf48 overexpression in mice accelerated renal fibrosis, while Cf48 gene deletion or knockdown by antisense oligonucleotides significantly reduced renal fibrosis in CKD models. In vitro, recombinant Cf48 (rCf48) enhanced TGF-β1-induced fibrotic responses in renal fibroblasts and epithelial cells independently of Smad3 phosphorylation. Cellular uptake of Cf48 and its profibrotic response in fibroblasts operated via the transferrin receptor. RNA immunoprecipitation-sequencing identified Cf48 binding to mRNA of genes involved in the fibrotic response, including Serpine1, Acta2, Ccn2, and Col4a1. rCf48 binds to the 3'UTR of Serpine1 and increases mRNA half-life. We identify the secreted Cf48 micropeptide as a potential enhancer of renal fibrosis that operates as an RNA-binding peptide to promote the production of extracellular matrix.

Machine-learning model for predicting oliguria in critically ill patients

This retrospective cohort study aimed to develop and evaluate a machine-learning algorithm for predicting oliguria, a sign of acute kidney injury (AKI). To this end, electronic health record data from consecutive patients admitted to the intensive care unit (ICU) between 2010 and 2019 were used and oliguria was defined as a urine output of less than 0.5 mL/kg/h. Furthermore, a light-gradient boosting machine was used for model development. Among the 9,241 patients who participated in the study, the proportions of patients with urine output < 0.5 mL/kg/h for 6 h and with AKI during the ICU stay were 27.4% and 30.2%, respectively. The area under the curve (AUC) values provided by the prediction algorithm for the onset of oliguria at 6 h and 72 h using 28 clinically relevant variables were 0.964 (a 95% confidence interval (CI) of 0.963-0.965) and 0.916 (a 95% CI of 0.914-0.918), respectively. The Shapley additive explanation analysis for predicting oliguria at 6 h identified urine values, severity scores, serum creatinine, oxygen partial pressure, fibrinogen/fibrin degradation products, interleukin-6, and peripheral temperature as important variables. Thus, this study demonstrates that a machine-learning algorithm can accurately predict oliguria onset in ICU patients, suggesting the importance of oliguria in the early diagnosis and optimal management of AKI.

Polyploid tubular cells initiate a TGF-β1 controlled loop that sustains polyploidization and fibrosis after acute kidney injury

Polyploidization of tubular cells (TC) is triggered by acute kidney injury (AKI) to allow survival in the early phase after AKI, but in the long run promotes fibrosis and AKI-chronic kidney disease (CKD) transition. The molecular mechanism governing the link between polyploid TC and kidney fibrosis remains to be clarified. In this study, we demonstrate that immediately after AKI, expression of cell cycle markers mostly identifies a population of DNA-damaged polyploid TC. Using transgenic mouse models and single-cell RNA sequencing we show that, unlike diploid TC, polyploid TC accumulate DNA damage and survive, eventually resting in the G1 phase of the cell cycle. In vivo and in vitro single-cell RNA sequencing along with sorting of polyploid TC shows that these cells acquire a profibrotic phenotype culminating in transforming growth factor (TGF)-β1 expression and that TGF-β1 directly promotes polyploidization. This demonstrates that TC polyploidization is a self-sustained mechanism. Interactome analysis by single-cell RNA sequencing revealed that TGF-β1 signaling fosters a reciprocal activation loop among polyploid TC, macrophages, and fibroblasts to sustain kidney fibrosis and promote CKD progression. Collectively, this study contributes to the ongoing revision of the paradigm of kidney tubule response to AKI, supporting the existence of a tubulointerstitial cross talk mediated by TGF-β1 signaling produced by polyploid TC following DNA damage.NEW & NOTEWORTHY Polyploidization in tubular epithelial cells has been neglected until recently. Here, we showed that polyploidization is a self-sustained mechanism that plays an important role during chronic kidney disease development, proving the existence of a cross talk between infiltrating cells and polyploid tubular cells. This study contributes to the ongoing revision of kidney adaptation to injury, posing polyploid tubular cells at the center of the process.

Inhibition of ACSL4 ameliorates tubular ferroptotic cell death and protects against fibrotic kidney disease

Ferroptosis is a recently recognized form of regulated cell death, characterized by iron-dependent accumulation of lipid peroxidation. Ample evidence has depicted that ferroptosis plays an essential role in the cause or consequence of human diseases, including cancer, neurodegenerative disease and acute kidney injury. However, the exact role and underlying mechanism of ferroptosis in fibrotic kidney remain unknown. Acyl-CoA synthetase long-chain family member 4 (ACSL4) has been demonstrated as an essential component in ferroptosis execution by shaping lipid composition. In this study, we aim to discuss the potential role and underlying mechanism of ACSL4-mediated ferroptosis of tubular epithelial cells (TECs) during renal fibrosis. The unbiased gene expression studies showed that ACSL4 expression was tightly associated with decreased renal function and the progression of renal fibrosis. To explore the role of ACSL4 in fibrotic kidney, ACSL4 specific inhibitor rosiglitazone (ROSI) was used to disturb the high expression of ACSL4 in TECs induced by TGF-β, unilateral ureteral obstruction (UUO) and fatty acid (FA)-modeled mice in vivo, and ACSL4 siRNA was used to knockdown ACSL4 in TGF-β-induced HK2 cells in vitro. The results demonstrated that inhibition and knockdown of ACSL4 effectively attenuated the occurrence of ferroptosis in TECs and alleviated the interstitial fibrotic response. In addition, the expression of various profibrotic cytokines all decreased after ROSI-treated in vivo and in vitro. Further investigation showed that inhibition of ACSL4 obviously attenuates the progression of renal fibrosis by reducing the proferroptotic precursors arachidonic acid- and adrenic acid- containing phosphatidylethanolamine (AA-PE and AdA-PE). In conclusion, these results suggest ACSL4 is essential for tubular ferroptotic death during kidney fibrosis development and ACSL4 inhibition is a viable therapeutic approach to preventing fibrotic kidney diseases.

Fetal Reprogramming of Nutrient Surplus Signaling, O-GlcNAcylation, and the Evolution of CKD

ABSTRACT

Fetal kidney development is characterized by increased uptake of glucose, ATP production by glycolysis, and upregulation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1 alpha (HIF-1 α ), which (acting in concert) promote nephrogenesis in a hypoxic low-tubular-workload environment. By contrast, the healthy adult kidney is characterized by upregulation of sirtuin-1 and adenosine monophosphate-activated protein kinase, which enhances ATP production through fatty acid oxidation to fulfill the needs of a normoxic high-tubular-workload environment. During stress or injury, the kidney reverts to a fetal signaling program, which is adaptive in the short term, but is deleterious if sustained for prolonged periods when both oxygen tension and tubular workload are heightened. Prolonged increases in glucose uptake in glomerular and proximal tubular cells lead to enhanced flux through the hexosamine biosynthesis pathway; its end product-uridine diphosphate N -acetylglucosamine-drives the rapid and reversible O-GlcNAcylation of thousands of intracellular proteins, typically those that are not membrane-bound or secreted. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated only by O-GlcNAc transferase and O-GlcNAcase, which adds or removes N-acetylglucosamine, respectively, from target proteins. Diabetic and nondiabetic CKD is characterized by fetal reprogramming (with upregulation of mTOR and HIF-1 α ) and increased O-GlcNAcylation, both experimentally and clinically. Augmentation of O-GlcNAcylation in the adult kidney enhances oxidative stress, cell cycle entry, apoptosis, and activation of proinflammatory and profibrotic pathways, and it inhibits megalin-mediated albumin endocytosis in glomerular mesangial and proximal tubular cells-effects that can be aggravated and attenuated by augmentation and muting of O-GlcNAcylation, respectively. In addition, drugs with known nephroprotective effects-angiotensin receptor blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter 2 inhibitors-are accompanied by diminished O-GlcNAcylation in the kidney, although the role of such suppression in mediating their benefits has not been explored. The available evidence supports further work on the role of uridine diphosphate N -acetylglucosamine as a critical nutrient surplus sensor (acting in concert with upregulated mTOR and HIF-1 α signaling) in the development of diabetic and nondiabetic CKD.

Comparative analysis of mortality and progression to end-stage renal disease between surgically induced and medical chronic kidney disease: A study using the National Health Insurance customized database

PURPOSE

We aimed to compare the mortality rate and the risk for progression to end-stage renal disease (ESRD) and cardiovascular disease (CVD) between patients who underwent surgery for localized renal cell carcinoma (RCC) and those with chronic kidney disease (CKD) without surgery by investigating the National Health Insurance Service.

MATERIALS AND METHODS

The surgical group (CKD-S) included patients who underwent radical or partial nephrectomy for RCC from 2007 to 2009. Grades of surgical CKD were classified according to the estimated glomerular filtration rate (eGFR) measured at a health screening within 2 years after surgery. The nonsurgical group (CKD-M) was graded according to the eGFR in the 2009-2010 health screenings. We performed 1:5 propensity score matching for age, gender, diabetes, hypertension, Charlson comorbidity index, smoking, alcohol consumption, baseline eGFR, and body mass index.

RESULTS

A total of 8,698 patients (CKD-S, n=1,521; CKD-M, n=7,177) were analyzed. The CKD-M group was at higher risk for progression to ESRD (hazard ratio [HR] 1.90, 95% confidence interval [CI] 1.04-3.44, p=0.036) and CVD (HR 1.17, 95% CI 1.06-1.29, p=0.002) than the CKD-S group. In the group of patients with grade 3 disease or higher, the CKD-M group was at significantly higher risk for progression to ESRD (HR 2.21, 95% CI 1.47-3.31, p<0.001), CVD (HR 1.32, 95% CI 1.20-1.45, p<0.001), and overall mortality (HR 1.50, 95% CI 1.21-1.86, p<0.001).

CONCLUSIONS

The risk for progression to ESRD, CVD, or mortality in patients with CKD-S may be lower than in patients with CKD-M.

Proximal tubule responses to injury: interrogation by single-cell transcriptomics

PURPOSE OF REVIEW

Acute kidney injury (AKI) occurs in approximately 10-15% of patients admitted to hospital and is associated with adverse clinical outcomes. Despite recent advances, management of patients with AKI is still mainly supportive, including the avoidance of nephrotoxins, volume and haemodynamic management and renal replacement therapy. A better understanding of the renal response to injury is the prerequisite to overcome current limitations in AKI diagnostics and therapy.

RECENT FINDINGS

Single-cell technologies provided new opportunities to study the complexity of the kidney and have been instrumental for rapid advancements in the understanding of the cellular and molecular mechanisms of AKI.

SUMMARY

We provide an update on single-cell technologies and we summarize the recent discoveries on the cellular response to injury in proximal tubule cells from the early response in AKI, to the mechanisms of tubule repair and the relevance of maladaptive tubule repair in the transition to chronic kidney disease.

Decoupling dedifferentiation and G2/M arrest in kidney fibrosis

Understanding the cellular mechanisms underlying chronic kidney disease (CKD) progression is required to develop effective therapeutic approaches. In this issue of the JCI, Taguchi, Elias, et al. explore the relationship between cyclin G1 (CG1), an atypical cyclin that induces G2/M proximal tubule cell cycle arrest, and epithelial dedifferentiation during fibrogenesis. While CG1-knockout mice were protected from fibrosis and had reduced G2/M arrest, protection was unexpectedly independent of induction of G2/M arrest. Rather, CG1 drove fibrosis by regulating maladaptive dedifferentiation in a CDK5-dependent mechanism. These findings highlight the importance of maladaptive epithelial dedifferentiation in kidney fibrogenesis and identify CG1/CDK5 signaling as a therapeutic target in CKD progression.