Takeshi Yamamoto

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the progressive growth of renal cysts and loss of renal function. Here, it was demonstrated that transcription factor EB (TFEB) activation in cyst-lining epithelial cells enhances their susceptibility to apoptosis via endoplasmic reticulum (ER) stress-induced signaling, ultimately restricting cyst expansion. Single-cell RNA sequencing in Pkd1 -deficient mouse models identified G-protein coupled receptor family C group 5 member A (GPRC5A)-positive cyst-lining cell clusters originating from the loop of Henle and proximal tubules. These cells exhibited dysregulated Hippo signaling and enhanced unfolded protein response (UPR). With this change in UPR, nuclear localization of TFEB and C/EBP homologous protein (CHOP) was observed, and expression of pro-apoptotic target genes of CHOP, such as Ppp1r15 and Pmaip1 , increased. TFEB deficiency attenuated apoptosis, resulting in accelerated cyst growth. Human ADPKD kidney samples mirrored these findings, showing nuclear TFEB and CHOP accumulation in cyst-lining cells. These results indicated that TFEB is a key modulator of ER stress-induced apoptosis in cyst-lining cells, suggesting that TFEB activation paradoxically limits cyst expansion through apoptosis in ADPKD.

Autophagy is a cellular process that maintains kidney physiology by recycling intracellular components to preserve homeostasis. In the kidney, autophagy supports energy metabolism and integrity across multiple cell types. Its regulation is tightly governed by nutrient availability, hormonal cues, and oxygen levels, primarily through signaling pathways such as mechanistic target of rapamycin kinase (mTOR), AMP-activated protein kinase (AMPK), and transcription factor EB (TFEB). Under physiological conditions, autophagy is dynamically regulated to meet metabolic demands. However, aging, obesity, and metabolic stress impair lysosomal function, leading to a pathological state termed autophagic stagnation, in which autophagosomes accumulate but degradative flux is compromised. Rather than being uniformly protective, this stagnation promotes cellular damage and contributes to kidney disease progression. Notably, autophagic stagnation in proximal tubular epithelial cells (PTECs) contributes to acute kidney injury (AKI)-to-chronic kidney disease (CKD) transition and exacerbates lipotoxicity in obesity-related kidney disease. Recent studies highlight the importance of transcriptional regulators – including TFEB and MondoA – in maintaining autophagic activity and mitochondrial homeostasis. Therapeutic strategies aimed at restoring autophagic flux – pharmacologically or through lifestyle interventions such as caloric restriction – hold promise for preserving kidney function. Deeper understanding of cell type – specific autophagy regulation will be critical for developing targeted and context-specific therapies.

Aims/hypothesis Ferroptosis, a regulated form of cell death characterised by excessive lipid peroxidation, plays a critical role in acute kidney injury (AKI). Individuals with diabetes have an elevated risk of developing AKI. However, the contribution of ferroptosis to the heightened susceptibility to AKI in diabetic kidney disease (DKD) remains unclear. This study aimed to investigate whether DKD influences ferroptosis susceptibility in proximal tubular epithelial cells (PTECs), focusing on autophagy and AMP-activated protein kinase (AMPK) signalling. Methods We examined the association between ferroptotic signatures and autophagy/AMPK pathways in human kidney biopsy specimens. To explore the roles of autophagy and AMPK in modulating ferroptosis in PTECs during DKD, we subjected streptozocin (STZ)-induced type 1 diabetic mice and type 2 diabetic db/db mice to ischaemia–reperfusion injury. Primary Atg5-deficient and wild-type PTECs were used to further investigate the underlying cellular mechanisms. Results Analysis of human kidney biopsy specimens revealed an increased ferroptotic signature (4-hydroxynonenal immunostaining), impaired autophagy (SQSTM1 accumulation) and AMPK inactivation (reduced p-AMPK) in PTECs of individuals with DKD. In STZ-treated Atg5 knockout (Atg5KO) mice, experiments combining ischaemia–reperfusion injury with ferrostatin-1 treatment showed that autophagy suppressed ferroptotic susceptibility. Additionally, susceptibility to ferroptosis was heightened in db/db mice following ischaemia–reperfusion injury; however, this effect was mitigated by enhancing autophagy through rapamycin treatment. In primary PTECs isolated from Atg5KO mice, ferroptotic cell death and lipid peroxidation were significantly increased, together with elevated mitochondrial reactive oxygen species. Mitochondrial DNA/RNA depletion substantially abolished ferroptotic effects in Atg5KO cells. Furthermore, high-glucose treatment inactivated AMPK and promoted ferroptosis, whereas treatment with the AMPK activator 5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR) attenuated ferroptosis in vitro and reduced vulnerability to AKI in DKD models. Conclusions/interpretation These findings demonstrate that impaired autophagy and inactivated AMPK heighten susceptibility to ferroptosis in DKD, suggesting that therapeutic strategies targeting autophagy and AMPK activation may reduce ferroptosis-associated kidney injury in individuals with diabetes. Graphical Abstract

Autophagy, a critical intracellular degradation and recycling pathway mediated by lysosomes, is essential for maintaining cellular homeostasis through the quality control of proteins and organelles. Our research focused on the role of proximal tubular autophagy in the pathophysiology of aging, obesity, and diabetes. Using a novel method to monitor autophagic flux in kidney tissue, we revealed that age-associated high basal autophagy supports mitochondrial quality control and delays kidney aging. However, an impaired ability to upregulate autophagy under additional stress accelerates kidney aging. In obesity induced by a high-fat diet, lysosomal dysfunction disrupts autophagy, leading to renal lipotoxicity. Although autophagy is initially activated to repair organelle membranes and maintain proximal tubular cell integrity, this demand overwhelms lysosomes, resulting in “autophagic stagnation” characterized by phospholipid accumulation. Similar lysosomal phospholipid accumulation was observed in renal biopsies from elderly and obese patients. We identified TFEB-mediated lysosomal exocytosis as a mechanism to alleviate lipotoxicity by expelling accumulated phospholipids. Therapeutically, interventions such as the SGLT2 inhibitor empagliflozin and eicosapentaenoic acid restore lysosomal function and autophagic activity. Based on these findings, we propose a novel disease concept, “Obesity-Related Proximal Tubulopathy.” This study underscores autophagic stagnation as a key driver of kidney disease progression in aging and obesity, offering insights into the pathophysiology of kidney diseases and providing a foundation for targeted therapeutic strategies.

With the aging of society, the incidence of chronic kidney disease (CKD), a common cause of death, has been increasing. Transcription factor EB (TFEB), the master transcriptional regulator of the autophagy/lysosomal pathway, is regarded as a promising candidate for preventing various age-related diseases. However, whether TFEB in the proximal tubules plays a significant role in elderly patients with CKD remains unknown. First, we found that nuclear TFEB localization in proximal tubular epithelial cells (PTECs) declined with age in both mice and humans. Next, we generated PTEC-specific Tfeb -deficient mice and bred them for up to 24 months. We found that TFEB deficiency in the proximal tubules caused metabolic disorders and occasionally led to apolipoprotein A4 (APOA4) amyloidosis. Supporting this result, we identified markedly decreased nuclear TFEB localization in the proximal tubules of elderly patients with APOA4 amyloidosis. The metabolic disturbances were accompanied by mitochondrial dysfunction due to transcriptional changes involved in fatty acid oxidation and oxidative phosphorylation pathways, as well as decreased mitochondrial clearance. This decreased clearance was reflected by the accumulation of mitochondria–lysosome-related organelles, which depended on lysosomal function. These results shed light on the presumptive mechanisms of APOA4 amyloidosis pathogenesis and provide a therapeutic strategy for CKD-related metabolic disorders and APOA4 amyloidosis.

The renoprotective effects of SLC5A2/SGLT2 (solute carrier 5 (sodium/glucose cotransporter), member 2) inhibitors have recently been demonstrated in non-diabetic chronic kidney disease (CKD), even without overt albuminuria. However, the mechanism underlying this renoprotection is largely unclear. We investigated the renoprotective mechanisms of the SLC5A2 inhibitor empagliflozin with a focus on ALB (albumin) reabsorption and macroautophagy/autophagy in proximal tubules using wild-type or drug-inducible lrp2/Megalin or atg5 knockout mice with high-fat diet (HFD)-induced obesity or 5/6 nephrectomy that elevated intraglomerular pressure without overt albuminuria. Empagliflozin treatment of HFD-fed mice reduced several hallmarks of lipotoxicity in the proximal tubules, such as phospholipid accumulation in the lysosome, inflammation and fibrosis. Empagliflozin, which decreases intraglomerular pressure, not only reduced the HFD-induced increase in ALB reabsorption via LRP2 in the proximal tubules (i.e. total nephron ALB filtration), as assessed by urinary ALB excretion caused by genetic ablation of Lrp2, but also ameliorated the HFD-induced imbalance in circulating ALB-bound fatty acids. Empagliflozin alleviated the HFD-induced increase in autophagic demand and successfully prevented autophagic stagnation in the proximal tubules. Similarly, empagliflozin decreased ALB exposure and autophagic demand in 5/6 nephrectomized mice. Finally, empagliflozin reduced HFD-induced vulnerability to ischemia-reperfusion injury, whereas LRP2 blockade and atg5 ablation separately diminished this effect. Our findings indicate that empagliflozin reduces ALB exposure and prevents autophagic stagnation in the proximal tubules even without overt albuminuria. Autophagy improvement may be critical for the renoprotection mediated by SLC5A2 inhibition.

This study revealed the activation of peroxisome proliferator-activated receptor-α and fatty acid β-oxidation in proximal tubular epithelial cells as an endogenous mechanism to protect the kidney from phosphate toxicity. These findings highlight noncanonical metabolic reprogramming as a potential target for suppressing phosphate toxicity in the kidneys.

The kidney is a metabolically active organ that requires energy to drive processes such as tubular reabsorption and secretion, and shows a decline in function with advancing age. Various molecular mechanisms, including genomic instability, telomere attrition, inflammation, autophagy, mitochondrial function, and changes to the sirtuin and Klotho signalling pathways, are recognized regulators of individual lifespan and pivotal factors that govern kidney ageing. Thus, mechanisms that contribute to ageing not only dictate renal outcomes but also exert a substantial influence over life expectancy. Conversely, kidney dysfunction, in the context of chronic kidney disease (CKD), precipitates an expedited ageing trajectory in individuals, leading to premature ageing and a disconnect between biological and chronological age. As CKD advances, age-related manifestations such as frailty become increasingly conspicuous. Hence, the pursuit of healthy ageing necessitates not only the management of age-related complications but also a comprehensive understanding of the processes and markers that underlie systemic ageing. Here, we examine the hallmarks of ageing, focusing on the mechanisms by which they affect kidney health and contribute to premature organ ageing. We also review diagnostic methodologies and interventions for premature ageing, with special consideration given to the potential of emerging therapeutic avenues to target age-related kidney diseases.

Key Points: The expression of MondoA was decreased in the renal tubules of patients with CKD. Genetic ablation of MondoA in proximal tubules inhibited autophagy and increased vulnerability to AKI through increased expression of Rubicon. MondoA ablation during the recovery phase after ischemia-reperfusion aggravated kidney injury through downregulation of the transcription factor EB-peroxisome proliferator-activated receptor-γ coactivator-1α axis. Background: Elderly individuals and patients with CKD are at a higher risk of AKI. The transcription factor MondoA is downregulated in the kidneys of aged individuals or patients with AKI; however, its roles in AKI development and the AKI-to-CKD transition remain unknown. Methods: We investigated the expression of MondoA in human kidney biopsy samples, ischemia-reperfusion–injured (IRI) mouse kidneys, and cultured proximal tubular epithelial cells under hypoxia/reoxygenation. The role of MondoA during the initial and recovery phases after IRI was evaluated using proximal tubule–specific MondoA knockout mice and MondoA-deficient proximal tubular epithelial cells. Furthermore, we explored the involvement of Rubicon and transcription factor EB (TFEB), both of which are downstream factors of MondoA. Results: MONDOA expression was decreased in the renal tubules of patients with CKD. In mouse kidneys, MondoA expression was decreased under ischemia, whereas its expression was increased during reperfusion. Genetic ablation of MondoA in proximal tubular epithelial cells inhibited autophagy and increased vulnerability to AKI through increased expression of Rubicon. Ablation of Rubicon in MondoA-deficient IRI kidneys activated autophagy and protected mitochondrial function. MondoA ablation during the recovery phase after ischemia-reperfusion aggravated kidney injury through downregulation of the TFEB-peroxisome proliferator-activated receptor-γ coactivator-1α axis. Pharmacological upregulation of TFEB contributed to maintaining mitochondrial biogenesis and increased peroxisome proliferator-activated receptor-γ coactivator-1α transcription. Conclusions: Our findings demonstrate that MondoA protected against vulnerability to AKI by maintaining autophagy and subsequently supporting mitochondrial function to prevent progression to CKD.