Mechanisms and Models of Kidney Tubular Necrosis and Nephron Loss

Identification of featured necroptosis-related genes and imbalanced immune infiltration in sepsis via machine learning

Background: The precise diagnostic and prognostic biological markers were needed in immunotherapy for sepsis. Considering the role of necroptosis and immune cell infiltration in sepsis, differentially expressed necroptosis-related genes (DE-NRGs) were identified, and the relationship between DE-NRGs and the immune microenvironment in sepsis was analyzed.

Methods: Machine learning algorithms were applied for screening hub genes related to necroptosis in the training cohort. CIBERSORT algorithms were employed for immune infiltration landscape analysis. Then, the diagnostic value of these hub genes was verified by the receiver operating characteristic (ROC) curve and nomogram. In addition, consensus clustering was applied to divide the septic patients into different subgroups, and quantitative real-time PCR was used to detect the mRNA levels of the hub genes between septic patients (SP) (n = 30) and healthy controls (HC) (n = 15). Finally, a multivariate prediction model based on heart rate, temperature, white blood count and 4 hub genes was established.

Results: A total of 47 DE-NRGs were identified between SP and HC and 4 hub genes (BACH2, GATA3, LEF1, and BCL2) relevant to necroptosis were screened out via multiple machine learning algorithms. The high diagnostic value of these hub genes was validated by the ROC curve and Nomogram model. Besides, the immune scores, correlation analysis and immune cell infiltrations suggested an immunosuppressive microenvironment in sepsis. Septic patients were divided into 2 clusters based on the expressions of hub genes using consensus clustering, and the immune microenvironment landscapes and immune function between the 2 clusters were significantly different. The mRNA levels of the 4 hub genes significantly decreased in SP as compared with HC. The area under the curve (AUC) was better in the multivariate prediction model than in other indicators.

Conclusion: This study indicated that these necroptosis hub genes might have great potential in prognosis prediction and personalized immunotherapy for sepsis.

Pathogenesis of intrinsic acute kidney injury

PURPOSE OF REVIEW

This review focuses on the pathogenesis of intrinsic acute kidney injury (AKI), emphasizing recent advances that hold therapeutic promise.

RECENT FINDINGS

Enhanced endothelin and reduced endothelium-derived nitric oxide release in AKI can be blocked using endothelin receptor antagonists or nitric oxide supplementation. Vasodilatory agents such as theophylline and caffeine may prevent AKI. Free labile iron is a potent factor in the generation of reactive oxygen species and tubule damage in AKI. Apoptosis via induction of p53 is an important mechanism of cell death in AKI, which can be blocked using small interfering RNA. The AKI-driven reduction in nicotinamide adenine dinucleotide can be countered using oral supplements. Surviving tubule cells regenerate after AKI, by upregulating genes encoding growth factors, such as hepatocyte growth factor. Pro-angiogenic agents (statins and erythropoietin) that can mobilize endothelial progenitor cells after AKI are currently being tested. The inflammatory response in AKI, including activation of C5a, can be therapeutically targeted. Contemporary single cell profiling technologies have identified novel genes with altered expression, new signalling pathways and drug targets in AKI.

SUMMARY

Recent advances in the pathogenesis of intrinsic AKI have provided a better understanding of the clinical continuum and the rational deployment of promising therapeutics.

Regulated cell death pathways in kidney disease

Disorders of cell number that result from an imbalance between the death of parenchymal cells and the proliferation or recruitment of maladaptive cells contributes to the pathogenesis of kidney disease. Acute kidney injury can result from an acute loss of kidney epithelial cells. In chronic kidney disease, loss of kidney epithelial cells leads to glomerulosclerosis and tubular atrophy, whereas interstitial inflammation and fibrosis result from an excess of leukocytes and myofibroblasts. Other conditions, such as acquired cystic disease and kidney cancer, are characterized by excess numbers of cyst wall and malignant cells, respectively. Cell death modalities act to clear unwanted cells, but disproportionate responses can contribute to the detrimental loss of kidney cells. Indeed, pathways of regulated cell death - including apoptosis and necrosis - have emerged as central events in the pathogenesis of various kidney diseases that may be amenable to therapeutic intervention. Modes of regulated necrosis, such as ferroptosis, necroptosis and pyroptosis may cause kidney injury directly or through the recruitment of immune cells and stimulation of inflammatory responses. Importantly, multiple layers of interconnections exist between different modalities of regulated cell death, including shared triggers, molecular components and protective mechanisms.

Analysis of factors associated with postoperative acute kidney injury in patients with colorectal cancer and the development of a risk prediction model: a retrospective study

BACKGROUND

To investigate the factors associated with acute kidney injury (AKI) in postoperative colorectal cancer (CRC) patients and develop a risk prediction model.

METHODS

The clinical data of 389 CRC patients were retrospectively analyzed. The patients were divided into AKI (n = 30) and non-AKI groups (n = 359) according to KDIGO diagnostic criteria. Demographic data, the presence of underlying diseases, perioperative conditions and related examination results were compared between the two groups. Binary logistic regression was used to analyze the independent risk factors for postoperative AKI, and a risk prediction model was established. And a verification group (94 patients) was used to verify the model.

RESULTS

30 patients (7.71%) with CRC had postoperative AKI. Binary logistic regression analysis showed that preoperative combined hypertension, preoperative anemia, inadequate intraoperative crystalloid infusion, low intraoperative minimum mean arterial pressure (MAP) and moderate to severe postoperative decline in hemoglobin (Hb) levels were independent risk factors. The risk prediction model developed was expressed as Logit P = - 0.853 + 1.228 * preoperative combined hypertension + 1.275 *preoperative anemia - 0.002 * intraoperative crystalloid infusion (ml) - 0.091 * intraoperative minimum MAP (mmHg) + 1.482 * moderate to severe postoperative decline in Hb levels. In Hosmer-Lemeshow test, χ² = 8.157, P = 0.718 showed that the fitting effect was good. The area under ROC curve was 0.776 (95% CI 0.682-0.871, P < 0.001), with a prediction threshold of 1.570, a sensitivity of 63.3% and a specificity of 88.9%. The sensitivity and specificity of the verification group were 65.8% and 86.1%.

CONCLUSIONS

Preoperative combined hypertension, preoperative anemia, inadequate intraoperative crystalloid infusion, low intraoperative minimum MAP, and moderate to severe postoperative decline in Hb levels were independent risk factors for AKI development in CRC patients. The prediction model can effectively predict the occurrence of postoperative AKI in patients with CRC.

Human umbilical cord mesenchymal stem cell exosome-derived miR-874-3p targeting RIPK1/PGAM5 attenuates kidney tubular epithelial cell damage

BACKGROUND

Kidney insults due to various pathogenic factors, such as trauma, infection, and inflammation, can cause tubular epithelial cell injury and death, leading to acute kidney injury and the transformation of acute kidney injury to chronic kidney disease. There is no definitive treatment available. In previous studies, human umbilical cord mesenchymal stem cells have been shown to promote kidney injury. In this preclinical study, we investigate the role and mechanism of human umbilical cord mesenchymal stem cell exosomes (HucMSC-Exos) on the repair of renal tubular epithelial cells after injury.

METHODS

C57BL/6 mice underwent unilateral ureteral obstruction, and epithelial cell injury was induced in HK-2 cells by cisplatin. HucMSC-Exos were assessed in vivo and in vitro. The extent of renal cell injury, activation of necroptosis pathway, and mitochondrial quality-control-related factors were determined in different groups. We also analyzed the possible regulatory effector molecules in HucMSC-Exos by transcriptomics.

RESULTS

HucMSC-Exo inhibited necroptosis after renal tubular epithelial cell injury and promoted the dephosphorylation of the S637 site of the Drp1 gene by reducing the expression of PGAM5. This subsequently inhibited mitochondrial fission and maintained mitochondrial functional homeostasis, mitigating renal injury and promoting repair. In addition, HucMSC-Exo displayed a regulatory role by targeting RIPK1 through miR-874-3p.

CONCLUSION

The collective findings of the present study demonstrate that HucMSC-Exos can regulate necroptosis through miR-874-3p to attenuate renal tubular epithelial cell injury and enhance repair, providing new therapeutic modalities and ideas for the treatment of AKI and the process of AKI to CKD transformation to mitigate renal damage.

Minnelide combined with Angptl3 knockout completely protects mice with adriamycin nephropathy via suppression of TGF-β1-Smad2 and p53 pathways

Minimal change disease (MCD) is the common type of nephrotic syndrome in children. There is an urgent need to explore new treatment methods as current treatments have many drawbacks and cause significant side effects. Our group found that Angiopoietin-like protein 3 (Angptl3) is closely related to renal disease and Angptl3 knockout significantly alleviated proteinuria in mice with adriamycin nephropathy (AN), however, some proteinuria was still present. Minnelide is a water-soluble prodrug of triptolide which has been used for the treatment of glomerular diseases. Therefore, this study aimed to investigate whether minnelide, combined with Angptl3 knockout, could completely protect mice with AN and its mechanism. AN was induced in B6;129S5 female mice by tail vein injection of 25 mg/kg of Adriamycin (ADR), and treatment with 200 ug/kg/d of minnelide. The results showed that minnelide combined with Angptl3 knockout completely reduced proteinuria and restored the foot processes in mice with AN. Moreover, in Angptl3 knockout mice with AN, minnelide restored the distribution of nephrin, podocin and cd2ap and reduced inflammatory factors (Tumor necrosis factor alpha (TNF-α), Interleukin-6 (IL-6) and Interleukin-1β (IL-1β)). Through RNA sequencing and related experiments, we found minnelide could ameliorate fibrosis and apoptosis by inhibiting TGF-β1-Smad2 and p53 pathways in Angptl3 knockout mice with AN, respectively. In Angptl3 knockout primary podocytes, triptolide alleviates ADR-induced decreases in nephrin, podocin and cd2ap, upregulation of Bax and downregulation of Bcl-2. Overall, our study shows that minnelide combined with Angptl3 knockout completely protects mice with AN by inhibiting the TGF-β1-smad2 and p53 pathways.

Autophagy in acute kidney injury and maladaptive kidney repair

Acute kidney injury (AKI) is a major renal disease characterized by a sudden decrease in kidney function. After AKI, the kidney has the ability to repair, but if the initial injury is severe the repair may be incomplete or maladaptive and result in chronic kidney problems. Autophagy is a highly conserved pathway to deliver intracellular contents to lysosomes for degradation. Autophagy plays an important role in maintaining renal function and is involved in the pathogenesis of renal diseases. Autophagy is activated in various forms of AKI and acts as a defense mechanism against kidney cell injury and death. After AKI, autophagy is maintained at a relatively high level in kidney tubule cells during maladaptive kidney repair but the role of autophagy in maladaptive kidney repair has been controversial. Nonetheless, recent studies have demonstrated that autophagy may contribute to maladaptive kidney repair after AKI by inducing tubular degeneration and promoting a profibrotic phenotype in renal tubule cells. In this review, we analyze the role and regulation of autophagy in kidney injury and repair and discuss the therapeutic strategies by targeting autophagy.

Ferroptosis related gene signature in T cell-mediated rejection after kidney transplantation

BACKGROUND

T cell-mediated rejection is an important factor affecting early transplant kidney survival. Ferroptosis has been shown to play a pathogenic role in a variety of diseases, which was not reported in TCMR. Here we developed a model for assessing activation of ferroptosis-related genes in TCMR to find a better screening method and explore the contribution of ferroptosis in TCMR.

METHODS

We performed unsupervised consensus clustering according to expression of ferroptosis-related genes based on RNA-seq data from kidney transplant biopsies, and developed an assessment model characterized by ferroptosis gene expression through PCA, which was evaluated in multiple external datasets as well as blood and urine samples. Pathway enrichment and immune cell infiltration analysis were used to explore the possible targets and pathways involved in ferroptosis and TCMR.

RESULTS

A ferroptosis gene expression scoring model was established. The diagnostic specificity and sensitivity of TCMR in renal biopsy samples were both over 80%, AUC = 0.843, and AUC was around 0.8 in multi-dataset validation, and was also close to 0.7 in blood and urine samples, while in predicting of graft survival at 3 years, scoring model had a good prognostic effect as well. Pathway enrichment and PPI network speculated that TLR4, CD44, IFNG, etc. may be the key genes of ferroptosis in TCMR.

CONCLUSIONS

Ferroptosis scoring model could better diagnose TCMR and predict graft loss, and could be used as a potential screening method in blood and urine samples. We speculate that ferroptosis plays an important role in TCMR.

Role of Necroptosis in Central Nervous System Diseases

Necroptosis is a type of precisely regulated necrotic cell death activated in caspase-deficient conditions. Multiple factors initiate the necroptotic signaling pathway, including toll-like receptor 3/4, tumor necrosis factor (TNF), dsRNA viruses, and T cell receptors. Presently, TNF-induced necroptosis via the phosphorylation of three key proteins, receptor-interacting protein kinase 1, receptor-interacting protein kinase 3, and mixed lineage kinase domain-like protein, is the best-characterized process. Necroptosis induced by Z-DNA-binding protein 1 (ZBP-1) and toll/interleukin-1 receptor (TIR)-domain-containing adapter-inducing interferon (TRIF) plays a significant role in infectious diseases, such as influenza A virus, Zika virus, and herpesvirus infection. An increasing number of studies have demonstrated the close association of necroptosis with multiple diseases, and disrupting necroptosis has been confirmed to be effective for treating (or managing) these diseases. The central nervous system (CNS) exhibits unique physiological structures and immune characteristics. Necroptosis may occur without the sequential activation of signal proteins, and the necroptosis of supporting cells has more important implications in disease development. Additionally, necroptotic signals can be activated in the absence of necroptosis. Here, we summarize the role of necroptosis and its signal proteins in CNS diseases and characterize typical necroptosis regulators to provide a basis for the further development of therapeutic strategies for treating such diseases. In the present review, relevant information has been consolidated from recent studies (from 2010 until the present), excluding the patents in this field.

MFG-E8 has guiding significance for the prognosis and treatment of sepsis

Sepsis remains a significant clinical challenge. Ferroptosis is involved in the pathogenesis of sepsis. Ferroptosis is associated with oxidative stress, and excessive oxidative stress is suppressed by milk fat globule epidermal growth factor 8 (MFG-E8) under various conditions. However, the role of MFG-E8 in sepsis-induced ferroptosis and oxidative stress is still unclear. First, we collected blood samples from patients with sepsis and detected the expression of serum MFG-E8. Then, the relationship between serum concentrations of MFG-E8 and disease severity was detected. Finally, the effects of MFG-E8 treatment on ferroptosis and oxidative stress in the livers of septic mice were determined. The expression of serum MFG-E8 in healthy subjects was notably higher than that in septic patients. In addition, when nonsurvivors and survivors of sepsis were compared, MFG-E8 levels were considerably lower in the former. The ROC curve for MFG-E8 was also generated. The area under the curve for MFG-E8 was 0.768 (95% confidence interval [CI] 0.627-0.909, p = 0.003). The patients were separated into two groups based on the MFG-E8 cut-off value of 3.86 ng/mL. According to the Kaplan‒Meier survival analysis, patients with low MFG-E8 levels had a significantly decreased 28-day survival rate compared with patients with high MFG-E8 levels. High MFG-E8 levels were substantially related to a decreased risk of death, as demonstrated by the Cox proportional hazard model that we utilized. In addition, compared with sham mice, septic mice exhibited liver and kidney damage, and MFG-E8 may have protective effects. The survival study indicated that MFG-E8 could effectively improve the survival rate of septic mice. Treatment with MFG-E8 suppresses oxidative stress and ferroptosis in the livers of septic mice. Serum MFG-E8 levels are lower in septic patients and are negatively related to disease severity. Treatment with MFG-E8 suppresses oxidative stress and ferroptosis in the livers of septic mice, contributing to significantly improved survival in septic mice. These findings showed that MFG-E8 could be a new sepsis predictive biomarker. MFG-E8 may have therapeutic potential in the treatment of sepsis.

Primary cilia suppress Ripk3-mediated necroptosis

Cilia are sensory organelles that project from the surface of almost all cells. Nephronophthisis (NPH) and NPH-related ciliopathies are degenerative genetic diseases caused by mutation of cilia-associated genes. These kidney disorders are characterized by progressive loss of functional tubular epithelial cells which is associated with inflammation, progressive fibrosis, and cyst formation, ultimately leading to end-stage renal disease. However, disease mechanisms remain poorly understood. Here, we show that targeted deletion of cilia in renal epithelial cells enhanced susceptibility to necroptotic cell death under inflammatory conditions. Treatment of non-ciliated cells with tumor necrosis factor (TNF) α and the SMAC mimetic birinapant resulted in Ripk1-dependent cell death, while viability of ciliated cells was almost not affected. Cell death could be enhanced and shifted toward necroptosis by the caspase inhibitor emricasan, which could be blocked by inhibitors of Ripk1 and Ripk3. Moreover, combined treatment of ciliated and non-ciliated cells with TNFα and cycloheximide induced a cell death response that could be partially rescued with emricasan in ciliated cells. In contrast, non-ciliated cells responded with pronounced cell death that was blocked by necroptosis inhibitors. Consistently, combined treatment with interferon-γ and emricasan induced cell death only in non-ciliated cells. Mechanistically, enhanced necroptosis induced by loss of cilia could be explained by induction of Ripk3 and increased abundance of autophagy components, including p62 and LC3 associated with the Ripk1/Ripk3 necrosome. Genetic ablation of cilia in renal tubular epithelial cells in mice resulted in TUNEL positivity and increased expression of Ripk3 in kidney tissue. Moreover, loss of Nphp1, the most frequent cause of NPH, further increased susceptibility to necroptosis in non-ciliated epithelial cells, suggesting that necroptosis might contribute to the pathogenesis of the disease. Together, these data provide a link between cilia-related signaling and cell death responses and shed new light on the disease pathogenesis of NPH-related ciliopathies.

Implication of platelets and complement C3 as link between innate immunity and tubulointerstitial injury in renal vasculitis with MPO-ANCA seropositivity

Introduction

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a potentially life-threatening systemic small-vessel vasculitis that is characterized by pauci-immune glomerulonephritis, depicting in turn a major denominator of AAV mortality. It is well established that AAV patients feature an increased risk of developing thrombotic events, and platelets are activated in AAV patients being triggered by the alternative complement pathway. Platelets guard vessels integrity and initiate thrombus formation in response to endothelial damage, further constituting a triangular interconnection with the activation of neutrophils and the complement system. We here aimed to systematically assess the relevance of platelet counts and systemic complement system activation regarding distinct histopathological lesions in ANCA-associated renal vasculitis.

Methods

A cohort of 53 biopsy-proven cases of ANCA-associated renal vasculitis were retrospectively enrolled in a single-center observational study. Univariate and multivariate regression analysis was performed to identify parameters associated with platelet counts in ANCA-associated renal vasculitis compared to disease controls. Finally, the relevance of platelets for disease course and recovery was assessed by survival analysis.

Results

Lower platelet counts correlated with markers of kidney injury including eGFR loss (p=0.0004) and lower complement C3 levels (p=0.0037). Multivariate and subgroup analysis revealed that this association was only present in the subgroup with MPO-ANCA seropositivity (eGFR loss: p=0.0009, lower C3: p=0.0032). While lower platelet counts correlated with kidney injury in the PR3-ANCA subgroup (eGFR loss: p=0.0272), we did not observe an independent association with complement C3 levels (p=0.4497). Independent of any glomerular lesion, lower platelet counts correlated with interstitial fibrosis (p=0.0313), tubular atrophy (p=0.0073), and tubulitis in areas of interstitial fibrosis and tubular atrophy (p=0.0033). Finally, we observed significant differences with increased requirement of kidney replacement therapy (KRT) or death in the subgroup below median platelet counts (HR: 4.1, 95% CI: 1.6-10, p=0.0047), associated with a lower probability of discharge and prolonged hospitalization in this subgroup (HR: 0.5, 95% CI: 0.3-0.9, p=0.0113).

Conclusion

Based on our observation that an association between platelets and complement system activation is only observed in the MPO-ANCA subgroup, this could implicate that platelets and complement C3 link innate immunity to tubulointerstitial injury in the presence of MPO-ANCA autoantibodies.

Sex differences in resilience to ferroptosis underlie sexual dimorphism in kidney injury and repair

In both humans and mice, repair of acute kidney injury is worse in males than in females. Here, we provide evidence that this sexual dimorphism results from sex differences in ferroptosis, an iron-dependent, lipid-peroxidation-driven regulated cell death. Using genetic and single-cell transcriptomic approaches in mice, we report that female sex confers striking protection against ferroptosis, which was experimentally induced in proximal tubular (PT) cells by deleting glutathione peroxidase 4 (Gpx4). Single-cell transcriptomic analyses further identify the NFE2-related factor 2 (NRF2) antioxidant protective pathway as a female resilience mechanism against ferroptosis. Genetic inhibition and pharmacological activation studies show that NRF2 controls PT cell fate and plasticity by regulating ferroptosis. Importantly, pharmacological NRF2 activation protects male PT cells from ferroptosis and improves cellular plasticity as in females. Our data highlight NRF2 as a potential therapeutic target to prevent failed renal repair after acute kidney injury in both sexes by modulating cellular plasticity.

Ginsenoside Rg1 ameliorates sepsis-induced acute kidney injury by inhibiting ferroptosis in renal tubular epithelial cells

Acute kidney injury (AKI) represents a prevailing complication of sepsis, and its onset involves ferroptosis. Ginsenoside Rg1 exerts a positive effect on kidney diseases. This study explored the action of ginsenoside Rg1 in sepsis-induced AKI (SI-AKI) by regulating ferroptosis in renal tubular epithelial cells (TECs). Sepsis rat models were established using cecal ligation and puncture (CLP) and cell models were established by treating human renal TECs (HK-2) with LPS to induce ferroptosis. Serum creatinine (SCr) and blood urea nitrogen (BUN) and urine KIM1 contents in rats were determined by ELISA kits. Kidney tissues were subjected to immunohistochemical and H&E stainings. Iron concentration, malondialdehyde (MDA), glutathione (GSH), and ferroptosis-related protein (ferritin light chain [FTL], ferritin heavy chain [FTH], GSH peroxidase 4 [GPX4], and Ferroptosis suppressor protein 1 [FSP1]) levels in kidney tissues and HK-2 cells were measured using ELISA kits and Western blotting. HK-2 cell viability was detected by cell counting kit-8, and cell death was observed via propidium iodide staining. Reactive oxygen species accumulation in cells was detected using C11 BODIPY 581/591 as a molecular probe. In CLP rats, ginsenoside Rg1 reduced SCr, BUN, KIM1, and NGAL levels, thus palliating SI-AKI. Additionally, ginsenoside Rg1 decreased iron content, FTL, FTH, and MDA levels, and elevated GPX4, FSP1, and GSH levels, thereby inhibiting lipid peroxidation and ferroptosis. Moreover, FSP1 knockdown annulled the inhibition of ginsenoside Rg1 on ferroptosis. In vitro experiments, ginsenoside Rg1 raised HK-2 cell viability and lowered iron accumulation and lipid peroxidation during ferroptosis, and its antiferroptosis activity was dependent on FSP1. Ginsenoside Rg1 alleviates SI-AKI, possibly resulting from inhibition of ferroptosis in renal TECs through FSP1.

Physiological Effects of Ferroptosis on Organ Fibrosis

Ferroptosis is a new type of programmed cell death with unique morphological, biochemical, and genetic features. From the initial study of histomorphology to the exploration of subcellular organelles and even molecular mechanisms, a net connecting ferroptosis and fibrosis is being woven and formed. Inflammation may be the bridge between both processes. In this review, we will discuss the ferroptosis theory and process and the physiological functions of ferroptosis, followed by a description of the pathological effects and the underlying mechanisms of ferroptosis in the pathogenesis of tumorigenesis, ischemic damage, degenerative lesions, autoimmune diseases, and necroinflammation. We then focus on the role of ferroptosis in the fibrosis process in the liver, lung, kidney, heart, and other organs. Although the molecular mechanism of ferroptosis has been explored extensively in the past few years, many challenges remain to be resolved to translate this information into antifibrotic practice, which is becoming a promising new direction in the field of fibrotic disease prevention and treatment.

Mechanisms of ferroptosis in chronic kidney disease

Ferroptosis is a newly identified form of regulated cell death characterized by iron accumulation and lipid peroxidation. Ferroptosis plays an essential role in the pathology of numerous diseases and has emerged as a key area of focus in studies of chronic kidney disease (CKD). CKD is a major public health problem with high incidence and mortality that is characterized by a gradual loss of kidney function over time. The severity and complexity of CKD combined with the limited knowledge of its underlying molecular mechanism(s) have led to increased interest in this disease area. Here, we summarize recent advances in our understanding of the regulatory mechanism(s) of ferroptosis and highlight recent studies describing its role in the pathogenesis and progression of CKD. We further discuss the potential therapeutic benefits of targeting ferroptosis for the treatment of CKD and the major hurdles to overcome for the translation of in vitro studies into the clinic.

Impaired fatty acid metabolism perpetuates lipotoxicity along the transition to chronic kidney injury

Energy metabolism failure in proximal tubule cells (PTC) is a hallmark of chronic kidney injury. We combined transcriptomic, metabolomic and lipidomic approaches in experimental models and patient cohorts to investigate the molecular bases of the progression to chronic kidney allograft injury initiated by ischemia-reperfusion injury (IRI). The urinary metabolome of kidney transplant recipients with chronic allograft injury and who experienced severe IRI was significantly enriched with long chain fatty acids (FA). We identified a renal FA-related gene signature with low levels of Cpt2 and Acsm5 and high levels of Acsl4 and Acsm5 associated with IRI, transition to chronic injury, and established CKD in mouse models and kidney transplant recipients. The findings were consistent with the presence of Cpt2-, Acsl4+, Acsl5+, Acsm5- PTC failing to recover from IRI as identified by snRNAseq. In vitro experiments indicated that endoplasmic reticulum (ER) stress contributes to CPT2 repression, which, in turn, promotes lipids accumulation, drives profibrogenic epithelial phenotypic changes, and activates the unfolded protein response. ER stress through CPT2 inhibition and lipid accumulation, engages an auto-amplification loop leading to lipotoxicity and self-sustained cellular stress. Thus, IRI imprints a persistent FA metabolism disturbance in the proximal tubule sustaining the progression to chronic kidney allograft injury.

GSDMD-dependent neutrophil extracellular traps promote macrophage-to-myofibroblast transition and renal fibrosis in obstructive nephropathy

Renal fibrosis is a common consequence of various progressive nephropathies, including obstructive nephropathy, and ultimately leads to kidney failure. Infiltration of inflammatory cells is a prominent feature of renal injury after draining blockages from the kidney, and correlates closely with the development of renal fibrosis. However, the underlying molecular mechanism behind the promotion of renal fibrosis by inflammatory cells remains unclear. Herein, we showed that unilateral ureteral obstruction (UUO) induced Gasdermin D (GSDMD) activation in neutrophils, abundant neutrophil extracellular traps (NETs) formation and macrophage-to-myofibroblast transition (MMT) characterized by α-smooth muscle actin (α-SMA) expression in macrophages. Gsdmd deletion significantly reduced infiltration of inflammatory cells in the kidneys and inhibited NETs formation, MMT and thereby renal fibrosis. Chimera studies confirmed that Gsdmd deletion in bone marrow-derived cells, instead of renal parenchymal cells, provided protection against renal fibrosis. Further, specific deletion of Gsdmd in neutrophils instead of macrophages protected the kidney from undergoing fibrosis after UUO. Single-cell RNA sequencing identified robust crosstalk between neutrophils and macrophages. In vitro, GSDMD-dependent NETs triggered p65 translocation to the nucleus, which boosted the production of inflammatory cytokines and α-SMA expression in macrophages by activating TGF-β1/Smad pathway. In addition, we demonstrated that caspase-11, that could cleave GSDMD, was required for NETs formation and renal fibrosis after UUO. Collectively, our findings demonstrate that caspase-11/GSDMD-dependent NETs promote renal fibrosis by facilitating inflammation and MMT, therefore highlighting the role and mechanisms of NETs in renal fibrosis.

COVID-19 and Diabetic Nephropathy

Diabetic nephropathy is the most common condition that requires a chronic renal replacement therapy, such as hemodialysis, peritoneal dialysis, kidney transplantation, or simultaneous kidney-pancreas transplantation. Chronic kidney disease progression, that is the loss of nephrons, which causes the continuous decline of the eGFR, underlies the pathogenesis of diabetic nephropathy. During the COVID-19 pandemic, it became clear that diabetic nephropathy is amongst the independent risk factors that predicts unfavourable outcome upon SARS-CoV2 infection. While we still lack conclusive mechanistic insights into how nephrons are rapidly lost upon SARS-CoV2 infection and why patients with diabetic nephropathy are more susceptible to severe outcomes upon SARS-CoV2 infection, here, we discuss several aspects of the interface of COVID-19 with diabetic nephropathy. We identify the shortage of reliable rodent models of diabetic nephropathy, limited treatment options for human diabetic nephropathy and the lack of knowledge about virus-induced signalling pathways of regulated necrosis, such as necroptosis, as key factors that explain our failure to understand this system. Finally, we focus on immunosuppressed patients and discuss vaccination efficacy in these and diabetic patients. We conclude that more basic science and mechanistic understanding will be required both in diabetic nephropathy as well as in host immune responses to the SARS-CoV2 virus if novel therapeutic strategies are desired.

Tubular Mitochondrial Dysfunction, Oxidative Stress, and Progression of Chronic Kidney Disease

Acute kidney injury (AKI) and chronic kidney disease (CKD) are interconnected conditions, and CKD is projected to become the fifth leading global cause of death by 2040. New therapeutic approaches are needed. Mitochondrial dysfunction and oxidative stress have emerged as drivers of kidney injury in acute and chronic settings, promoting the AKI-to-CKD transition. In this work, we review the role of mitochondrial dysfunction and oxidative stress in AKI and CKD progression and discuss novel therapeutic approaches. Specifically, evidence for mitochondrial dysfunction in diverse models of AKI (nephrotoxicity, cytokine storm, and ischemia-reperfusion injury) and CKD (diabetic kidney disease, glomerulopathies) is discussed; the clinical implications of novel information on the key role of mitochondria-related transcriptional regulators peroxisome proliferator-activated receptor gamma coactivator 1-alpha, transcription factor EB (PGC-1α, TFEB), and carnitine palmitoyl-transferase 1A (CPT1A) in kidney disease are addressed; the current status of the clinical development of therapeutic approaches targeting mitochondria are updated; and barriers to the clinical development of mitochondria-targeted interventions are discussed, including the lack of clinical diagnostic tests that allow us to categorize the baseline renal mitochondrial dysfunction/mitochondrial oxidative stress and to monitor its response to therapeutic intervention. Finally, key milestones for further research are proposed.

Programmed Cell Death in Sepsis Associated Acute Kidney Injury

Sepsis-associated acute kidney injury (SA-AKI) is common in patients with severe sepsis, and has a high incidence rate and high mortality rate in ICU patients. Most patients progress to AKI before drug treatment is initiated. Early studies suggest that the main mechanism of SA-AKI is that sepsis leads to vasodilation, hypotension and shock, resulting in insufficient renal blood perfusion, finally leading to renal tubular cell ischemia and necrosis. Research results in recent years have shown that programmed cell death such as apoptosis, necroptosis, pyroptosis and autophagy play important roles. In the early stage of sepsis-related AKI, autophagy bodies form and inhibit various types of programmed cell death. With the progress of disease, programmed cell death begins. Apoptosis promoter represents caspase-8-induced apoptosis and apoptosis effector represents caspase-3-induced apoptosis, however, caspase-11 and caspase-1 regulate gasdermin D-mediated pyroptosis. Caspase-8 and receptor interacting kinase 1 bodies mediate necroptosis. This review focuses on the pathophysiological mechanisms of various programmed cell death in sepsis-related AKI.

Metabolic mechanisms of acute proximal tubular injury

Damage to the proximal tubule (PT) is the most frequent cause of acute kidney injury (AKI) in humans. Diagnostic and treatment options for AKI are currently limited, and a deeper understanding of pathogenic mechanisms at a cellular level is required to rectify this situation. Metabolism in the PT is complex and closely coupled to solute transport function. Recent studies have shown that major changes in PT metabolism occur during AKI and have highlighted some potential targets for intervention. However, translating these insights into effective new therapies still represents a substantial challenge. In this article, in addition to providing a brief overview of the current state of the field, we will highlight three emerging areas that we feel are worthy of greater attention. First, we will discuss the role of axial heterogeneity in cellular function along the PT in determining baseline susceptibility to different metabolic hits. Second, we will emphasize that elucidating insult specific pathogenic mechanisms will likely be critical in devising more personalized treatments for AKI. Finally, we will argue that uncovering links between tubular metabolism and whole-body homeostasis will identify new strategies to try to reduce the considerable morbidity and mortality associated with AKI. These concepts will be illustrated by examples of recent studies emanating from the authors' laboratories and performed under the auspices of the Swiss National Competence Center for Kidney Research (NCCR Kidney.ch).