Lysosome Depletion-Triggered Autophagy Impairment in Progressive Kidney Injury

Tubular toxicity of proteinuria and the progression of chronic kidney disease

Proteinuria is a well-established biomarker of chronic kidney disease (CKD) and a risk predictor of associated disease outcomes. Proteinuria is also a driver of CKD progression toward end-stage kidney disease. Toxic effects of filtered proteins on proximal tubular epithelial cells enhance tubular atrophy and interstitial fibrosis. The extent of protein toxicity and the underlying molecular mechanisms responsible for tubular injury during proteinuria remain unclear. Nevertheless, albumin elicits its toxic effects when degraded and reabsorbed by proximal tubular epithelial cells. Overall, healthy kidneys excrete over 1000 individual proteins, which may be potentially harmful to proximal tubular epithelial cells when filtered and/or reabsorbed in excess. Proteinuria can cause kidney damage, inflammation and fibrosis by increasing reactive oxygen species, autophagy dysfunction, lysosomal membrane permeabilization, endoplasmic reticulum stress and complement activation. Here we summarize toxic proteins reported in proteinuria and the current understanding of molecular mechanisms of toxicity of proteins on proximal tubular epithelial cells leading to CKD progression.

Defective autophagic flux aggravates cadmium-induced Sertoli cell apoptosis

The male reproductive dysfunction accounts for 50% of infertile couples in the world. Cadmium (Cd) is one of the most harmful heavy metals to both the environment and inhabitants. Accumulating data suggest that Cd could cause male infertility. Sertoli cell (SC) is a somatic cell of testis and a key regulator of spermatogenesis by providing physical and nutritional support for developing sperm. Many studies showed that Cd induced dysfunction of SCs was directly related to male reproductive damage. However, the mechanism of SCs injury caused by Cd remains to be clarified. We found that Cd treatment caused a significant increase of apoptosis in SCs cells, accompanied by a marked increase in the production of ROS. These results were associated with the formation of mitochondria-containing autophagosomes and increased expression of LC3-II in vitro. Interestingly, our results showed that Cd did not promote but inhibited the fusion of mitochondria-containing autophagosomes with lysosomes by reducing the function of lysosomes. Together, this study provides insight into the negative effects of Cd, which interferes with autophagic flux and induces the apoptosis of SCs.

PRKAA2, MTOR, and TFEB in the regulation of lysosomal damage response and autophagy

Lysosomes function as critical signaling hubs that govern essential enzyme complexes. LGALS proteins (LGALS3, LGALS8, and LGALS9) are integral to the endomembrane damage response. If ESCRT fails to rectify damage, LGALS-mediated ubiquitination occurs, recruiting autophagy receptors (CALCOCO2, TRIM16, and SQSTM1) and VCP/p97 complex containing UBXN6, PLAA, and YOD1, initiating selective autophagy. Lysosome replenishment through biogenesis is regulated by TFEB. LGALS3 interacts with TFRC and TRIM16, aiding ESCRT-mediated repair and autophagy-mediated removal of damaged lysosomes. LGALS8 inhibits MTOR and activates TFEB for ATG and lysosomal gene transcription. LGALS9 inhibits USP9X, activates PRKAA2, MAP3K7, ubiquitination, and autophagy. Conjugation of ATG8 to single membranes (CASM) initiates damage repair mediated by ATP6V1A, ATG16L1, ATG12, ATG5, ATG3, and TECPR1. ATG8ylation or CASM activates the MERIT system (ESCRT-mediated repair, autophagy-mediated clearance, MCOLN1 activation, Ca2+ release, RRAG-GTPase regulation, MTOR modulation, TFEB activation, and activation of GTPase IRGM). Annexins ANAX1 and ANAX2 aid damage repair. Stress granules stabilize damaged membranes, recruiting FLCN-FNIP1/2, G3BP1, and NUFIP1 to inhibit MTOR and activate TFEB. Lysosomes coordinate the synergistic response to endomembrane damage and are vital for innate and adaptive immunity. Future research should unveil the collaborative actions of ATG proteins, LGALSs, TRIMs, autophagy receptors, and lysosomal proteins in lysosomal damage response.

Asiatic acid alleviates cisplatin-induced renal fibrosis in tumor-bearing mice by improving the TFEB-mediated autophagy-lysosome pathway

Nephrotoxicity is a major side effect of cisplatin treatment of solid tumors in the clinical setting. Long-term low-dose cisplatin administration causes renal fibrosis and inflammation. However, few specific medicines with clinical application value have been developed to reduce or treat the nephrotoxic side effects of cisplatin without affecting its tumor-killing effect. The present study analyzed the potential reno-protective effect and mechanism of asiatic acid (AA) in long-term cisplatin-treated nude mice suffering from tumors. AA treatment significantly attenuated renal injury, inflammation, and fibrosis induced by long-term cisplatin injection in tumor-bearing mice. AA administration notably suppressed tubular necroptosis and improved the autophagy-lysosome pathway disruption caused by chronic cisplatin treatment in tumor-transplanted nude mice and HK-2 cells. AA promoted transcription factor EB (TFEB)-mediated lysosome biogenesis and reduced the accumulation of damaged lysosomes, resulting in enhanced autophagy flux. Mechanistically, AA increased TFEB expression by rebalancing Smad7/Smad3, whereas siRNA inhibition of Smad7 or TFEB abolished the effect of AA on autophagy flux in HK-2 cells. In addition, AA treatment did not weaken, but actually enhanced the anti-tumor effect of cisplatin, as evidenced by the promoted tumor apoptosis and inhibited proliferation in nude mice. In summary, AA alleviates cisplatin-induced renal fibrosis in tumor-bearing mice by improving the TFEB-mediated autophagy-lysosome pathway.

Plk1 promotes renal tubulointerstitial fibrosis by targeting autophagy/lysosome axis

The prevalence of chronic kidney disease (CKD) has been increasing over the past decades. However, no effective therapies are available for delaying or curing CKD. Progressive fibrosis is the major pathological feature of CKD, which leads to end-stage renal disease (ESRD). The present study showed that Polo-like kinase 1 (Plk1) was upregulated in the kidneys of CKD patients and mice subjected to unilateral ureteral obstruction (UUO) with location in proximal tubules and tubulointerstitial fibroblasts. Pharmacological inhibition, genetic silencing or knockout of Plk1 attenuated obstructive nephropathy due to suppressed fibroblast activation mediated by reduced autophagic flux. We found Plk1 plays a critical role in maintaining intralysosomal pH by regulating ATP6V1A phosphorylation, and inhibition of Plk1 impaired lysosomal function leading to blockade of autophagic flux. In addition, Plk1 also prevented partial epithelial-mesenchymal transition (pEMT) of tubular epithelial cells via autophagy pathway. In conclusion, this study demonstrated that Plk1 plays a pathogenic role in renal tubulointerstitial fibrosis by regulating autophagy/lysosome axis. Thus, targeting Plk1 could be a promising strategy for CKD treatment.

Autophagy and neurodegeneration: Unraveling the role of C9ORF72 in the regulation of autophagy and its relationship to ALS-FTD pathology

The proper functioning of the cell clearance machinery is critical for neuronal health within the central nervous system (CNS). In normal physiological conditions, the cell clearance machinery is actively involved in the elimination of misfolded and toxic proteins throughout the lifetime of an organism. The highly conserved and regulated pathway of autophagy is one of the important processes involved in preventing and neutralizing pathogenic buildup of toxic proteins that could eventually lead to the development of neurodegenerative diseases (NDs) such as Alzheimer’s disease or Amyotrophic lateral sclerosis (ALS). The most common genetic cause of ALS and frontotemporal dementia (FTD) is a hexanucleotide expansion consisting of GGGGCC (G4C2) repeats in the chromosome 9 open reading frame 72 gene (C9ORF72). These abnormally expanded repeats have been implicated in leading to three main modes of disease pathology: loss of function of the C9ORF72 protein, the generation of RNA foci, and the production of dipeptide repeat proteins (DPRs). In this review, we discuss the normal physiological role of C9ORF72 in the autophagy-lysosome pathway (ALP), and present recent research deciphering how dysfunction of the ALP synergizes with C9ORF72 haploinsufficiency, which together with the gain of toxic mechanisms involving hexanucleotide repeat expansions and DPRs, drive the disease process. This review delves further into the interactions of C9ORF72 with RAB proteins involved in endosomal/lysosomal trafficking, and their role in regulating various steps in autophagy and lysosomal pathways. Lastly, the review aims to provide a framework for further investigations of neuronal autophagy in C9ORF72-linked ALS-FTD as well as other neurodegenerative diseases.

Autophagy impairment in patients with obstructive sleep apnea modulates intermittent hypoxia-induced oxidative stress and cell apoptosis via hypermethylation of the ATG5 gene promoter region

BACKGROUND

Autophagy is a catabolic process that recycles damaged organelles and acts as a pro-survival mechanism, but little is known about autophagy dysfunction and epigenetic regulation in patients with obstructive sleep apnea (OSA).

METHODS

Protein/gene expressions and DNA methylation levels of the autophagy-related genes (ATG) were examined in blood leukocytes from 64 patients with treatment-naïve OSA and 24 subjects with primary snoring (PS).

RESULTS

LC3B protein expression of blood monocytes, and ATG5 protein expression of blood neutrophils were decreased in OSA patients versus PS subjects, while p62 protein expression of cytotoxic T cell was increased, particularly in those with nocturia. ATG5, ULK1, and BECN1 gene expressions of peripheral blood mononuclear cells were decreased in OSA patients versus PS subjects. LC3B gene promoter regions were hypermethylated in OSA patients, particularly in those with excessive daytime sleepiness, while ATG5 gene promoter regions were hypermethylated in those with morning headache or memory impairment. LC3B protein expression of blood monocytes and DNA methylation levels of the LC3B gene promoter region were negatively and positively correlated with apnea hyponea index, respectively. In vitro intermittent hypoxia with re-oxygenation exposure to human THP-1/HUVEC cell lines resulted in LC3B/ATG5/ULK1/BECN1 down-regulations and p62 up-regulation along with increased apoptosis and oxidative stress, while rapamycin and umbilical cord-mesenchymal stem cell treatment reversed these abnormalities through de-methylation of the ATG5 gene promoter.

CONCLUSIONS

Impaired autophagy activity in OSA patients was regulated by aberrant DNA methylation, correlated with clinical phenotypes, and contributed to increased cell apoptosis and oxidative stress. Autophagy enhancers may be novel therapeutics for OSA-related neurocognitive dysfunction.

Critical Reanalysis of the Mechanisms Underlying the Cardiorenal Benefits of SGLT2 Inhibitors and Reaffirmation of the Nutrient Deprivation Signaling/Autophagy Hypothesis

SGLT2 (sodium-glucose cotransporter 2) inhibitors produce a distinctive pattern of benefits on the evolution and progression of cardiomyopathy and nephropathy, which is characterized by a reduction in oxidative and endoplasmic reticulum stress, restoration of mitochondrial health and enhanced mitochondrial biogenesis, a decrease in proinflammatory and profibrotic pathways, and preservation of cellular and organ integrity and viability. A substantial body of evidence indicates that this characteristic pattern of responses can be explained by the action of SGLT2 inhibitors to promote cellular housekeeping by enhancing autophagic flux, an effect that may be related to the action of these drugs to produce simultaneous upregulation of nutrient deprivation signaling and downregulation of nutrient surplus signaling, as manifested by an increase in the expression and activity of AMPK (adenosine monophosphate-activated protein kinase), SIRT1 (sirtuin 1), SIRT3 (sirtuin 3), SIRT6 (sirtuin 6), and PGC1-α (peroxisome proliferator-activated receptor γ coactivator 1-α) and decreased activation of mTOR (mammalian target of rapamycin). The distinctive pattern of cardioprotective and renoprotective effects of SGLT2 inhibitors is abolished by specific inhibition or knockdown of autophagy, AMPK, and sirtuins. In the clinical setting, the pattern of differentially increased proteins identified in proteomics analyses of blood collected in randomized trials is consistent with these findings. Clinical studies have also shown that SGLT2 inhibitors promote gluconeogenesis, ketogenesis, and erythrocytosis and reduce uricemia, the hallmarks of nutrient deprivation signaling and the principal statistical mediators of the ability of SGLT2 inhibitors to reduce the risk of heart failure and serious renal events. The action of SGLT2 inhibitors to augment autophagic flux is seen in isolated cells and tissues that do not express SGLT2 and are not exposed to changes in environmental glucose or ketones and may be related to an ability of these drugs to bind directly to sirtuins or mTOR. Changes in renal or cardiovascular physiology or metabolism cannot explain the benefits of SGLT2 inhibitors either experimentally or clinically. The direct molecular effects of SGLT2 inhibitors in isolated cells are consistent with the concept that SGLT2 acts as a nutrient surplus sensor, and thus, its inhibition causes enhanced nutrient deprivation signaling and its attendant cytoprotective effects, which can be abolished by specific inhibition or knockdown of AMPK, sirtuins, and autophagic flux.

The endosomal sorting complex required for transport repairs the membrane to delay cell death

The endosomal sorting complex required for transport (ESCRT) machinery plays a key role in the repair of damaged plasma membranes with puncta form and removes pores from the plasma membrane in regulated cell death, apoptosis, necroptosis, pyroptosis, ferroptosis, and autophagy. ESCRT-I overexpression and ESCRT-III-associated charged multivesicular body protein (CHMP) 4B participate in apoptosis, and the ESCRT-1 protein TSG 101 maintains low levels of ALIX and ALG-2 and prevents predisposition to apoptosis. The ESCRT-III components CHMP2A and CHMP4B are recruited to broken membrane bubble sites with the requirement of extracellular Ca2+, remove membrane vesicles from cells, and delay the time required for active MLKL to mediate necroptosis, thus preserving cell survival. CHMP4B disturbed pyroptosis by recruiting around the plasma membrane neck to remove the GSDMD pores and preserve plasma membrane integrity depending on Ca2+ influx. The accumulation of the ESCRT-III subunits CHMP5 and CHMP6 in the plasma membrane is increased by the classical ferroptosis activators erastin-1 and ras-selective lethal small molecule 3 (RSL3) upon cytosolic calcium influx and repairs the ferroptotic plasma membrane. ESCRT-III- and VPS4-induced macroautophagy, ESCRT-0-initiated microautophagy. ESCRT-I, ESCRT-II, ESCRT-III, ALIX, and VPS4A are recruited to damaged lysosomes and precede lysophagy, indicating that ESCRT is a potential target to overcome drug resistance during tumor therapy.

Role of olmesartan in ameliorating diabetic nephropathy in rats by targeting the AGE/PKC, TLR4/P38-MAPK and SIRT-1 autophagic signaling pathways

Diabetic nephropathy (DN) is one of the most serious consequences of diabetes and the most common reason for end-stage renal disease. The current study was set out to investigate the ability of olmesartan medoxomil (OM) to treat DN by evaluating the reno-protective effects of this drug on fat/fructose/streptozotocin (F/Fr/STZ)-induced diabetic rat model. This model was induced by feeding rats high F/Fr diet for 7 weeks followed by injection of a single sub-diabetogenic dose of STZ (35mg/kg; i.p). The F/Fr/STZ-induced diabetic rats were orally treated with either OM (10 mg/kg) or pioglitazone (10 mg/kg); as a standard drug daily for four consecutive weeks. F/Fr/STZ-induced diabetic rats propagated inflammatory, oxidative, and fibrotic events. OM was able to oppose the injurious effects of diabetes; it significantly reduced the elevated levels of advanced glycated end products (AGEs) and downregulated PKC gene expression, therefore, indicating its antioxidant capacity evidenced by mitigation in GSH, MDA renal content. Moreover, OM impaired the inflammatory cascade by suppressing the elevated level of renal TLR4 as well as diminished the inflammatory profibrotic cytokine TGF-β1. Additionally, OM was able to turn off the MAPK cascade mediated by an upsurge in renal angiotensin 1-7 content and decrease the level of renal tubular injury marker, KIM-1. Furthermore, OM enhanced the autophagic activity pathway by upregulating of gene expression of SIRT-1. The histopathological examination confirmed these results. Finally, OM protected against type 2 diabetes-related nephropathy complications by altering inflammatory pathways, oxidative, fibrotic, and autophagic processes triggered by renal glucose overload. This study shows that OM has a reno-protective effect against DN in rats by inhibiting the AGE/PKC, TLR4/P38-MAPK, and SIRT-1 autophagic signaling pathways.

Targeting ferroptosis in acute kidney injury

Acute kidney injury (AKI) is a major public health problem with high incidence and mortality. As a form of programmed cell death (PCD), ferroptosis could be considered as a process of iron accumulation and enhanced lipid peroxidation. Recently, the fundamental roles of ferroptosis in AKI have attracted much attention. The network mechanism of ferroptosis in AKI and its roles in the AKI to chronic kidney disease (CKD) transition is complicated and multifactorial. Strategies targeting ferroptosis show great potential. Here, we review the research progress on ferroptosis and its participation in AKI. We hope that this work will provide clues for further studies of ferroptosis in AKI.

GSDMD-mediated pyroptosis: a critical mechanism of diabetic nephropathy

Pyroptosis is a recently identified mechanism of programmed cell death related to Caspase-1 that triggers a series of inflammatory reactions by releasing several proinflammatory factors such as IL-1β and IL-18. The process is characterised by the rupture of cell membranes and the release of cell contents through the mediation of gasdermin (GSDM) proteins. GSDMD is an important member of the GSDM family and plays a critical role in the two pathways of pyroptosis. Diabetic nephropathy (DN) is a microvascular complication of diabetes and a major cause of end-stage renal disease. Recently, it was revealed that GSDMD-mediated pyroptosis plays an important role in the occurrence and development of DN. In this review, we focus on two types of kidney cells, tubular epithelial cells and renal podocytes, to illustrate the mechanism of pyroptosis in DN and provide new ideas for the prevention, early diagnosis and molecular therapy of DN.

Surface Nanosieving Polyether Sulfone Particles with Graphene Oxide Encapsulation for the Negative Isolation toward Extracellular Vesicles

Extracellular vesicles (EVs) contain specific biomarkers for disease diagnosis. Current EV isolation methods are hampered in important biological applications due to their low recovery and purity. Herein, we first present a novel EV negative isolation strategy based on surface nanosieving polyether sulfone particles with graphene oxide encapsulation (SNAPs) by which the coexisting proteins are irreversibly adsorbed by graphene oxide (GO) inside the particles, while EVs with large sizes are excluded from the outside due to the well-defined surface pore sizes (10-40 nm). By this method, the purity of the isolated EVs from urine could be achieved 4.91 ± 1.01e¹⁰ particles/μg, 40.9-234 times higher than those obtained by the ultracentrifugation (UC), size-exclusion chromatography (SEC), and PEG-based precipitation. In addition, recovery ranging from 90.4 to 93.8% could be obtained with excellent reproducibility (RSD < 6%). This was 1.8-4.3 times higher than those obtained via SEC and UC, comparable to that obtained by PEG-based precipitation. Taking advantage of this strategy, we further isolated urinary EVs from IgA nephropathy (IgAN) patients and healthy donors for comparative proteome analysis, by which significantly regulated EV proteins were found to distinguish IgAN patients from healthy donors. All of the results indicated that our strategy would provide a new avenue for highly efficient EV isolation to enable many important clinical applications.