Ectodomain shedding of PLA2R1 is mediated by the metalloproteases ADAM10 and ADAM17.

Phospholipase A2 receptor 1 (PLA2R1) is a 180-kDa transmembrane protein that plays a role in inflammation and cancer and is the major autoantigen in membranous nephropathy, a rare but severe autoimmune kidney disease. A soluble form of PLA2R1 has been detected in mouse and human serum. It is likely produced by proteolytic shedding of membrane-bound PLA2R1 but the mechanism is unknown. Here, we show that human PLA2R1 is cleaved by A Disintegrin And Metalloprotease 10 (ADAM10) and ADAM17 in HEK293 cells, mouse embryonic fibroblasts, and human podocytes. By combining site-directed mutagenesis and sequencing, we determined the exact cleavage site within the extracellular juxtamembrane stalk of human PLA2R1. Orthologs and paralogs of PLA2R1 are also shed. By using pharmacological inhibitors and genetic approaches with RNA interference and knock-out cellular models, we identified a major role of ADAM10 in the constitutive shedding of PLA2R1 and a dual role of ADAM10 and ADAM17 in the stimulated shedding. We did not observe evidence for cleavage by ß- or γ-secretase, suggesting that PLA2R1 may not be a substrate for regulated intramembrane proteolysis. PLA2R1 shedding occurs constitutively and can be triggered by the calcium ionophore ionomycin, the protein kinase C activator PMA, cytokines, and lipopolysaccharides, in vitro and in vivo. Altogether, our results show that PLA2R1 is a novel substrate for ADAM10 and ADAM17, producing a soluble form that is increased in inflammatory conditions and likely exerts various functions in physiological and pathophysiological conditions including inflammation, cancer, and membranous nephropathy.

Bicellular Localization of Tricellular Junctional Protein Angulin-3/ILDR2 Allows Detection of Podocyte Injury.

Podocytes serve as part of the renal filtration unit with slit diaphragms. Although the structure of slit diaphragms between two cells is well characterized, how the tricellular contact of podocytes is organized and how it changes in injured podocytes remains unknown. This study focused on a tricellular junction protein, angulin-3, and its localization in healthy podocytes, in developmental stages, and in pathologic conditions, using a newly established monoclonal antibody. Angulin-3 was confined at tricellular junctions of primordial podocytes, then transiently localized at bicellular junctions as foot process interdigitation developed and the intercellular junctions rearranged into slit diaphragm, and eventually distributed in a sparse punctate pattern on the foot processes of adult podocytes. In the rodent podocyte injury models, angulin-3 showed bicellular localization between the foot processes, and the localization turned from punctate to dashed linear pattern along the effaced foot processes with the progression of podocyte injury. Angulin-3 also accumulated between foot processes in a linear pattern in kidney biopsy samples of human nephrotic syndrome. Additionally, the line length of angulin-3 staining signal correlated with risk of relapse under glucocorticoid therapy in patients with minimal change nephrotic syndrome. This study proposes an image program to score the linearity of the accumulation pattern of angulin-3 to evaluate the relapse risk of patients with minimal change nephrotic syndrome.

miR-4645-3p attenuates podocyte injury and mitochondrial dysfunction in diabetic kidney disease by targeting Cdk5.

Podocyte injury plays a critical role in the progression of diabetic kidney disease (DKD), but the underlying cellular and molecular mechanisms remain poorly understanding. MicroRNAs (miRNAs) can disrupt gene expression by inducing translation inhibition and mRNA degradation, and recent evidence has shown that miRNAs may play a key role in many kidney diseases. In this study, we identified miR-4645-3p by global transcriptome expression profiling as one of the major downregulated miRNAs in high glucose-cultured podocytes. Moreover, whether DKD patients or STZ-induced diabetic mice, expression of miR-4645-3p was also significantly decreased in kidney. In the podocytes cultured by normal glucose, inhibition of miR-4645-3p expression promoted mitochondrial damage and podocyte apoptosis. In the podocytes cultured by high glucose (30 mM glucose), overexpression of miR-4645-3p significantly attenuated mitochondrial dysfunction and podocyte apoptosis induced by high glucose. Furthermore, we found that miR-4645-3p exerted protective roles by targeting Cdk5 inhibition. In vitro, miR-4645-3p obviously antagonized podocyte injury by inhibiting overexpression of Cdk5. In vivo of diabetic mice, podocyte injury, proteinuria, and impaired renal function were all effectively ameliorated by treatment with exogenous miR-4645-3p. Collectively, these findings demonstrate that miR-4645-3p can attenuate podocyte injury and mitochondrial dysfunction in DKD by targeting Cdk5. Sustaining the expression of miR-4645-3p in podocytes may be a novel strategy to treat DKD.

c-Cbl induced podocin ubiquitination contributes to the podocytes injury in diabetic nephropathy.

The ubiquitination function in diabetic nephropathy (DN) has attracted much attention, but there is a lack of information on its ubiquitylome profile. To examine the differences in protein content and ubiquitination in the kidney between db/db mice and db/m mice, we deployed liquid chromatography-mass spectrometry (LC-MS/MS) to conduct analysis. We determined 145 sites in 86 upregulated modified proteins and 66 sites in 49 downregulated modified proteins at the ubiquitinated level. Moreover, 347 sites among the 319 modified proteins were present only in the db/db mouse kidneys, while 213 sites among the 199 modified proteins were present only in the db/m mouse kidneys. The subcellular localization study indicated that the cytoplasm had the highest proportion of ubiquitinated proteins (31.87%), followed by the nucleus (30.24%) and the plasma membrane (20.33%). The enrichment analysis revealed that the ubiquitinated proteins are mostly linked to tight junctions, oxidative phosphorylation, and thermogenesis. Podocin, as a typical protein of slit diaphragm, whose loss is a crucial cause of proteinuria in DN. Consistent with the results of ubiquitination omics, the K261R mutant of podocin induced the weakest ubiquitination compared with the K301R and K370R mutants. As an E3 ligase, c-Cbl binds to podocin, and the regulation of c-Cbl can impact the ubiquitination of podocin. In conclusion, in DN, podocin ubiquitination contributes to podocyte injury, and K261R is the most significant site. c-Cbl participates in podocin ubiquitination and may be a direct target for preserving the integrity of the slit diaphragm structure, hence reducing proteinuria in DN.

Ramipril therapy in integrin α1-null, autosomal recessive Alport mice triples lifespan: mechanistic clues from RNA-seq analysis.

The standard of care for patients with Alport syndrome (AS) is angiotensin-converting enzyme (ACE) inhibitors. In autosomal recessive Alport (ARAS) mice, ACE inhibitors double lifespan. We previously showed that deletion of Itga1 in Alport mice [double-knockout (DKO) mice] increased lifespan by 50%. This effect seemed dependent on the prevention of laminin 211-mediated podocyte injury. Here, we treated DKO mice with vehicle or ramipril starting at 4 weeks of age. Proteinuria and glomerular filtration rates were measured at 5-week intervals. Glomeruli were analyzed for laminin 211 deposition in the glomerular basement membrane (GBM) and GBM ultrastructure was analyzed using transmission electron microscopy (TEM). RNA sequencing (RNA-seq) was performed on isolated glomeruli at all time points and the results were compared with cultured podocytes overlaid (or not) with recombinant laminin 211. Glomerular filtration rate declined in ramipril-treated DKO mice between 30 and 35 weeks. Proteinuria followed these same patterns with normalization of foot process architecture in ramipril-treated DKO mice. RNA-seq revealed a decline in the expression of Foxc2, nephrin (Nphs1), and podocin (Nphs2) mRNAs, which was delayed in the ramipril-treated DKO mice. GBM accumulation of laminin 211 was delayed in ramipril-treated DKO mice, likely due to a role for α1ß1 integrin in CDC42 activation in Alport mesangial cells, which is required for mesangial filopodial invasion of the subendothelial spaces of the glomerular capillary loops. Ramipril synergized with Itga1 knockout, tripling lifespan compared with untreated ARAS mice. © 2023 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

C5a-C5aR1 axis controls mitochondrial fission to promote podocyte injury in lupus nephritis.

Podocytes are essential to maintaining the integrity of the glomerular filtration barrier, but they are frequently affected in lupus nephritis (LN). Here, we show that the significant upregulation of Drp1S616 phosphorylation in podocytes promotes mitochondrial fission, leading to mitochondrial dysfunction and podocyte injury in LN. Inhibition or knockdown of Drp1 promotes mitochondrial fusion and protects podocytes from injury induced by LN serum. In vivo, pharmacological inhibition of Drp1 reduces the phosphorylation of Drp1S616 in podocytes in lupus-prone mice. Podocyte injury is reversed when Drp1 is inhibited, resulting in the alleviation of proteinuria. Mechanistically, complement component C5a (C5a) upregulates the phosphorylation of Drp1S616 and promotes mitochondrial fission in podocytes. Moreover, the expression of C5a receptor 1 (C5aR1) is notably upregulated in podocytes in LN. C5a-C5aR1 axis-controlled phosphorylation of Drp1S616 and mitochondrial fission are substantially suppressed when C5aR1 is knocked down by siRNA. Moreover, lupus-prone mice treated with C5aR inhibitor show reduced phosphorylation of Drp1S616 in podocytes, resulting in significantly less podocyte damage. Together, this study uncovers a novel mechanism by which the C5a-C5aR1 axis promotes podocyte injury by enhancing Drp1-mediated mitochondrial fission, which could have significant implications for the treatment of LN.

Phospholipid scramblase 1: an essential component of the nephrocyte slit diaphragm.

Blood ultrafiltration in nephrons critically depends on specialized intercellular junctions between podocytes, named slit diaphragms (SDs). Here, by studying a homologous structure found in Drosophila nephrocytes, we identify the phospholipid scramblase Scramb1 as an essential component of the SD, uncovering a novel link between membrane dynamics and SD formation. In scramb1 mutants, SDs fail to form. Instead, the SD components Sticks and stones/nephrin, Polychaetoid/ZO-1, and the Src-kinase Src64B/Fyn associate in cortical foci lacking the key SD protein Dumbfounded/NEPH1. Scramb1 interaction with Polychaetoid/ZO-1 and Flotillin2, the presence of essential putative palmitoylation sites and its capacity to oligomerize, suggest a function in promoting SD assembly within lipid raft microdomains. Furthermore, Scramb1 interactors as well as its functional sensitivity to temperature, suggest an active involvement in membrane remodeling processes during SD assembly. Remarkably, putative Ca2+-binding sites in Scramb1 are essential for its activity raising the possibility that Ca2+ signaling may control the assembly of SDs by impacting on Scramb1 activity.

INF2 formin variants linked to human inherited kidney disease reprogram the transcriptome, causing mitotic chaos and cell death.

Mutations in the human INF2 gene cause autosomal dominant focal segmental glomerulosclerosis (FSGS)-a condition characterized by podocyte loss, scarring, and subsequent kidney degeneration. To understand INF2-linked pathogenicity, we examined the effect of pathogenic INF2 on renal epithelial cell lines and human primary podocytes. Our study revealed an increased incidence of mitotic cells with surplus microtubule-organizing centers fostering multipolar spindle assembly, leading to nuclear abnormalities, particularly multi-micronucleation. The levels of expression of exogenous pathogenic INF2 were similar to those of endogenous INF2. The aberrant nuclear phenotypes were observed regardless of the expression method used (retrovirus infection or plasmid transfection) or the promoter (LTR or CMV) used, and were absent with exogenous wild type INF2 expression. This indicates that the effect of pathogenic INF2 is not due to overexpression or experimental cell manipulation, but instead to the intrinsic properties of pathogenic INF2. Inactivation of the INF2 catalytic domain prevented aberrant nuclei formation. Pathogenic INF2 triggered the translocation of the transcriptional cofactor MRTF into the nucleus. RNA sequencing revealed a profound alteration in the transcriptome that could be primarily attributed to the sustained activation of the MRTF-SRF transcriptional complex. Cells eventually underwent mitotic catastrophe and death. Reducing MRTF-SRF activation mitigated multi-micronucleation, reducing the extent of cell death. Our results, if validated in animal models, could provide insights into the mechanism driving glomerular degeneration in INF2-linked FSGS and may suggest potential therapeutic strategies for impeding FSGS progression.

Cuproptosis associated cytoskeletal destruction contributes to podocyte injury in chronic kidney disease.

The podocyte cytoskeleton determines the stability of podocyte structure and function, and their imbalance plays a pathogenic role in podocyte diseases. However, the underlying mechanism of podocyte cytoskeleton damage is not fully understood. Here, we investigate the specific role of cuproptosis in inducing podocyte cytoskeleton injury. In in vitro and in vivo studies, exposure to high levels of copper and adriamycin (ADR) caused significant increases in copper concentration in intracellular and renal tissue. Moreover, excessive accumulation of copper induced cuproptosis, resulting in the destruction of the podocyte cytoskeleton. However, inhibition of copper accumulation to reduce cuproptosis also significantly alleviated the damage of podocyte cytoskeleton. In addition, inhibition of cuproptosis mitigated ADR-induced mitochondrial damage as well as the production of reactive oxygen species and depolarization of mitochondrial membrane potential, and restored adenosine triphosphate (ATP) synthesis. Among the transcriptome sequencing data, the difference of CXCL5 (C-X-C motif chemokine ligand 5) was the most significant. Both high copper and ADR exposure can cause upregulation of CXCL5, and CXCL5 deletion inhibits the occurrence of cuproptosis, thereby alleviating the podocyte cytoskeleton damage. This suggests that CXCL5 may act upstream of cuproptosis that mediates podocyte cytoskeleton damage. In conclusion, cuproptosis induced by excessive copper accumulation may induce podocyte cytoskeleton damage by promoting mitochondrial dysfunction, thereby causing podocyte injury. This indicates that cuproptosis plays an important role in the pathogenesis of podocyte injury and provides a basis for seeking potential targets for the treatment of chronic kidney disease.NEW & NOTEWORTHY Cuproptosis induced by excessive copper accumulation leads to podocyte cytoskeleton damage by promoting mitochondrial dysfunction, and CXCL5 acts as an upstream signal mediating the occurrence of cuproptosis.

Proteomic Analysis of Idiopathic Nephrotic Syndrome Triggered by Primary Podocytopathies in Adults: Regulatory Mechanisms and Diagnostic Implications.

Idiopathic nephrotic syndrome (NS) is a heterogeneous group of glomerular disorders which includes two major phenotypes: minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS). MCD and FSGS are classic types of primary podocytopathies. We aimed to explore the molecular mechanisms in NS triggered by primary podocytopathies and evaluate diagnostic value of the selected proteomic signatures by analyzing blood proteome profiling. Totally, we recruited 90 participants in two cohorts. The first cohort was analyzed using label-free quantitative (LFQ) proteomics to discover differential expressed proteins and identify enriched biological process in NS which were further studied in relation to clinical markers of kidney injury. The second cohort was analyzed using parallel reaction monitoring-based quantitative proteomics to verify the data of LFQ proteomics and assess the diagnostic performance of the selected proteins using receiver-operating characteristic curve analysis. Several biological processes (such as immune response, cell adhesion, and response to hypoxia) were found to be associated with kidney injury during MCD and FSGS. Moreover, three proteins (CSF1, APOC3, and LDLR) had over 90% sensitivity and specificity in detecting adult NS triggered by primary podocytopathies. The identified biological processes may play a crucial role in MCD and FSGS pathogenesis. The three blood protein markers are promising for diagnosing adult NS triggered by primary podocytopathies.

Central role of podocytes in mediating cellular cross talk in glomerular health and disease.

Podocytes are highly specialized epithelial cells that surround the capillaries of the glomeruli in the kidney. Together with the glomerular endothelial cells, these postmitotic cells are responsible for regulating filtrate from the circulating blood with their organized network of interdigitating foot processes that wrap around the glomerular basement membrane. Although podocyte injury and subsequent loss is the hallmark of many glomerular diseases, recent evidence suggests that the cell-cell communication between podocytes and other glomerular and nonglomerular cells is critical for the development and progression of kidney disease. In this review, we highlight these key cellular pathways of communication and how they might be a potential target for therapy in glomerular disease. We also postulate that podocytes might serve as a central hub for communication in the kidney under basal conditions and in response to cellular stress, which may have implications for the development and progression of glomerular diseases.

Quantitative phenotyping of Nphs1 knockout mice as a prerequisite for gene replacement studies.

Steroid-resistant nephrotic syndrome (SRNS) is the second most frequent cause of chronic kidney disease before the age of 25 yr. Nephrin, encoded by NPHS1, localizes to the slit diaphragm of glomerular podocytes and is the predominant structural component of the glomerular filtration barrier. Biallelic variants in NPHS1 can cause congenital nephrotic syndrome of the Finnish type, for which, to date, no causative therapy is available. Recently, adeno-associated virus (AAV) vectors targeting the glomerular podocyte have been assessed as a means for gene replacement therapy. Here, we established quantitative and reproducible phenotyping of a published, conditional Nphs1 knockout mouse model (Nphs1tm1.1Pgarg/J and Nphs2-Cre+) in preparation for a gene replacement study using AAV vectors. Nphs1 knockout mice (Nphs1fl/fl Nphs2-Cre+) exhibited 1) a median survival rate of 18 days (range: from 9 to 43 days; males: 16.5 days and females: 20 days); 2) an average foot process (FP) density of 1.0 FP/µm compared with 2.0 FP/µm in controls and a mean filtration slit density of 2.64 µm/µm2 compared with 4.36 µm/µm2 in controls; 3) a high number of proximal tubular microcysts; 4) the development of proteinuria within the first week of life as evidenced by urine albumin-to-creatinine ratios; and 5) significantly reduced levels of serum albumin and elevated blood urea nitrogen and creatinine levels. For none of these phenotypes, significant differences between sexes in Nphs1 knockout mice were observed. We quantitatively characterized five different phenotypic features of congenital nephrotic syndrome in Nphs1fl/fl Nphs2-Cre+ mice. Our results will facilitate future gene replacement therapy projects by allowing for sensitive detection of even subtle molecular effects.NEW & NOTEWORTHY To evaluate potential, even subtle molecular, therapeutic effects of gene replacement therapy (GRT) in a mouse model, prior rigorous quantifiable and reproducible disease phenotyping is necessary. Here, we, therefore, describe such a phenotyping effort in nephrin (Nphs1) knockout mice to establish the basis for GRT for congenital nephrotic syndrome. We believe that our findings set an important basis for upcoming/ongoing gene therapy approaches in the field of nephrology, especially for monogenic nephrotic syndrome.

Mechanisms and implications of podocyte autophagy in chronic kidney disease.

Autophagy is a protective mechanism through which cells degrade and recycle proteins and organelles to maintain cellular homeostasis and integrity. An accumulating body of evidence underscores the significant impact of dysregulated autophagy on podocyte injury in chronic kidney disease (CKD). In this review, we provide a comprehensive overview of the diverse types of autophagy and their regulation in cellular homeostasis, with a specific emphasis on podocytes. Furthermore, we discuss recent findings that focus on the functional role of different types of autophagy during podocyte injury in chronic kidney disease. The intricate interplay between different types of autophagy and podocyte health requires further research, which is critical for understanding the pathogenesis of CKD and developing targeted therapeutic interventions.

Non-voltage-gated Ca2+ channel signaling in glomerular cells in kidney health and disease.

Positioned at the head of the nephron, the renal corpuscle generates a plasma ultrafiltrate to initiate urine formation. Three major cell types within the renal corpuscle, the glomerular mesangial cells, podocytes, and glomerular capillary endothelial cells, communicate via endocrine- and paracrine-signaling mechanisms to maintain the structure and function of the glomerular capillary network and filtration barrier. Ca2+ signaling mediated by several distinct plasma membrane Ca2+ channels impacts the functions of all three cell types. The past two decades have witnessed pivotal advances in understanding of non-voltage-gated Ca2+ channel function and regulation in the renal corpuscle in health and renal disease. This review summarizes the current knowledge of the physiological and pathological impact of non-voltage-gated Ca2+ channel signaling in mesangial cells, podocytes and glomerular capillary endothelium. The main focus is on transient receptor potential and store-operated Ca2+ channels, but ionotropic N-methyl-d-aspartate receptors and purinergic receptors also are discussed. This update of Ca2+ channel functions and their cellular signaling cascades in the renal corpuscle is intended to inform the development of therapeutic strategies targeting these channels to treat kidney diseases, particularly diabetic nephropathy.

Mice with a Pax2 missense variant display impaired glomerular repair.

PAX2 regulates kidney development, and its expression persists in parietal epithelial cells (PECs), potentially serving as a podocyte reserve. We hypothesized that mice with a Pax2 pathogenic missense variant (Pax2A220G/+) have impaired PEC-mediated podocyte regeneration. Embryonic wild-type mouse kidneys showed overlapping expression of PAX2/Wilms' tumor-1 (WT-1) until PEC and podocyte differentiation, reflecting a close lineage relationship. Embryonic and adult Pax2A220G/+ mice have reduced nephron number but demonstrated no glomerular disease under baseline conditions. Pax2A220G/+ mice compared with wild-type mice were more susceptible to glomerular disease after adriamycin (ADR)-induced podocyte injury, as demonstrated by worsened glomerular scarring, increased podocyte foot process effacement, and podocyte loss. There was a decrease in PAX2-expressing PECs in wild-type mice after adriamycin injury accompanied by the occurrence of PAX2/WT-1-coexpressing glomerular tuft cells. In contrast, Pax2A220G/+ mice showed no changes in the numbers of PAX2-expressing PECs after adriamycin injury, associated with fewer PAX2/WT-1-coexpressing glomerular tuft cells compared with injured wild-type mice. A subset of PAX2-expressing glomerular tuft cells after adriamycin injury was increased in Pax2A220G/+ mice, suggesting a pathological process given the worse outcomes observed in this group. Finally, Pax2A220G/+ mice have increased numbers of glomerular tuft cells expressing Ki-67 and cleaved caspase-3 compared with wild-type mice after adriamycin injury, consistent with maladaptive responses to podocyte loss. Collectively, our results suggest that decreased glomerular numbers in Pax2A220G/+ mice are likely compounded with the inability of their mutated PECs to regenerate podocyte loss, and together these two mechanisms drive the worsened focal segmental glomerular sclerosis phenotype in these mice.NEW & NOTEWORTHY Congenital anomalies of the kidney and urinary tract comprise some of the leading causes of kidney failure in children, but our previous study showed that one of its genetic causes, PAX2, is also associated with adult-onset focal segmental glomerular sclerosis. Using a clinically relevant model, our present study demonstrated that after podocyte injury, parietal epithelial cells expressing PAX2 are deployed into the glomerular tuft to assist in repair in wild-type mice, but this mechanism is impaired in Pax2A220G/+ mice.

Podocytes from hypertensive and obese mice acquire an inflammatory, senescent, and aged phenotype.

Patients with hypertension or obesity can develop glomerular dysfunction characterized by injury and depletion of podocytes. To better understand the molecular processes involved, young mice were treated with either deoxycorticosterone acetate (DOCA) or fed a high-fat diet (HFD) to induce hypertension or obesity, respectively. The transcriptional changes associated with these phenotypes were measured by unbiased bulk mRNA sequencing of isolated podocytes from experimental models and their respective controls. Key findings were validated by immunostaining. In addition to a decrease in canonical proteins and reduced podocyte number, podocytes from both hypertensive and obese mice exhibited a sterile inflammatory phenotype characterized by increases in NLR family pyrin domain containing 3 (NLRP3) inflammasome, protein cell death-1, and Toll-like receptor pathways. Finally, although the mice were young, podocytes in both models exhibited increased expression of senescence and aging genes, including genes consistent with a senescence-associated secretory phenotype. However, there were differences between the hypertension- and obesity-associated senescence phenotypes. Both show stress-induced podocyte senescence characterized by increased p21 and p53. Moreover, in hypertensive mice, this is superimposed upon age-associated podocyte senescence characterized by increased p16 and p19. These results suggest that senescence, aging, and inflammation are critical aspects of the podocyte phenotype in experimental hypertension and obesity in mice.NEW & NOTEWORTHY Hypertension and obesity can lead to glomerular dysfunction in patients, causing podocyte injury and depletion. Here, young mice given deoxycorticosterone acetate or a high-fat diet to induce hypertension or obesity, respectively. mRNA sequencing of isolated podocytes showed transcriptional changes consistent with senescence, a senescent-associated secretory phenotype, and aging, which was confirmed by immunostaining. Ongoing studies are determining the mechanistic roles of the accelerated aging podocyte phenotype in experimental hypertension and obesity.

Oxysterol-binding protein-like 7 deficiency leads to ER stress-mediated apoptosis in podocytes and proteinuria.

Chronic kidney disease (CKD) is associated with renal lipid dysmetabolism among a variety of other pathways. We recently demonstrated that oxysterol-binding protein-like 7 (OSBPL7) modulates the expression and function of ATP-binding cassette subfamily A member 1 (ABCA1) in podocytes, a specialized type of cell essential for kidney filtration. Drugs that target OSBPL7 lead to improved renal outcomes in several experimental models of CKD. However, the role of OSBPL7 in podocyte injury remains unclear. Using mouse models and cellular assays, we investigated the influence of OSBPL7 deficiency on podocytes. We demonstrated that reduced renal OSBPL7 levels as observed in two different models of experimental CKD are linked to increased podocyte apoptosis, primarily mediated by heightened endoplasmic reticulum (ER) stress. Although as expected, the absence of OSBPL7 also resulted in lipid dysregulation (increased lipid droplets and triglycerides content), OSBPL7 deficiency-related lipid dysmetabolism did not contribute to podocyte injury. Similarly, we demonstrated that the decreased autophagic flux we observed in OSBPL7-deficient podocytes was not the mechanistic link between OSBPL7 deficiency and apoptosis. In a complementary zebrafish model, osbpl7 knockdown was sufficient to induce proteinuria and morphological damage to the glomerulus, underscoring its physiological relevance. Our study sheds new light on the mechanistic link between OSBPL7 deficiency and podocyte injury in glomerular diseases associated with CKD, and it strengthens the role of OSBPL7 as a novel therapeutic target.NEW & NOTEWORTHY OSBPL7 and ER stress comprise a central mechanism in glomerular injury. This study highlights a crucial link between OSBPL7 deficiency and ER stress in CKD. OSBPL7 deficiency causes ER stress, leading to podocyte apoptosis. There is a selective effect on lipid homeostasis in that OSBPL7 deficiency affects lipid homeostasis, altering cellular triglyceride but not cholesterol content. The interaction of ER stress and apoptosis supports that ER stress, not reduced autophagy, is the main driver of apoptosis in OSBPL7-deficient podocytes.

Podocyte-specific silencing of acid sphingomyelinase gene to abrogate hyperhomocysteinemia-induced NLRP3 inflammasome activation and glomerular inflammation.

Acid sphingomyelinase (ASM) has been reported to increase tissue ceramide and thereby mediate hyperhomocysteinemia (hHcy)-induced glomerular nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome activation, inflammation, and sclerosis. In the present study, we tested whether somatic podocyte-specific silencing of Smpd1 gene (mouse ASM gene code) attenuates hHcy-induced NLRP3 inflammasome activation and associated extracellular vesicle (EV) release in podocytes and thereby suppresses glomerular inflammatory response and injury. In vivo, somatic podocyte-specific Smpd1 gene silencing almost blocked hHcy-induced glomerular NLRP3 inflammasome activation in Podocre (podocyte-specific expression of cre recombinase) mice compared with control littermates. By nanoparticle tracking analysis (NTA), floxed Smpd1 shRNA transfection was found to abrogate hHcy-induced elevation of urinary EV excretion in Podocre mice. In addition, Smpd1 gene silencing in podocytes prevented hHcy-induced immune cell infiltration into glomeruli, proteinuria, and glomerular sclerosis in Podocre mice. Such protective effects of podocyte-specific Smpd1 gene silencing were mimicked by global knockout of Smpd1 gene in Smpd1-/- mice. On the contrary, podocyte-specific Smpd1 gene overexpression exaggerated hHcy-induced glomerular pathological changes in Smpd1trg/Podocre (podocyte-specific Smpd1 gene overexpression) mice, which were significantly attenuated by transfection of floxed Smpd1 shRNA. In cell studies, we also confirmed that Smpd1 gene knockout or silencing prevented homocysteine (Hcy)-induced elevation of EV release in the primary cultures of podocyte isolated from Smpd1-/- mice or podocytes of Podocre mice transfected with floxed Smpd1 shRNA compared with WT/WT podocytes. Smpd1 gene overexpression amplified Hcy-induced EV secretion from podocytes of Smpd1trg/Podocre mice, which was remarkably attenuated by transfection of floxed Smpd1 shRNA. Mechanistically, Hcy-induced elevation of EV release from podocytes was blocked by ASM inhibitor (amitriptyline, AMI), but not by NLRP3 inflammasome inhibitors (MCC950 and glycyrrhizin, GLY). Super-resolution microscopy also showed that ASM inhibitor, but not NLRP3 inflammasome inhibitors, prevented the inhibition of lysosome-multivesicular body interaction by Hcy in podocytes. Moreover, we found that podocyte-derived inflammatory EVs (released from podocytes treated with Hcy) induced podocyte injury, which was exaggerated by T cell coculture. Interstitial infusion of inflammatory EVs into renal cortex induced glomerular injury and immune cell infiltration. In conclusion, our findings suggest that ASM in podocytes plays a crucial role in the control of NLRP3 inflammasome activation and inflammatory EV release during hHcy and that the development of podocyte-specific ASM inhibition or Smpd1 gene silencing may be a novel therapeutic strategy for treatment of hHcy-induced glomerular disease with minimized side effect.NEW & NOTEWORTHY In the present study, we tested whether podocyte-specific silencing of Smpd1 gene attenuates hyperhomocysteinemia (hHcy)-induced nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome activation and associated inflammatory extracellular vesicle (EV) release in podocytes and thereby suppresses glomerular inflammatory response and injury. Our findings suggest that acid sphingomyelinase (ASM) in podocytes plays a crucial role in the control of NLRP3 inflammasome activation and inflammatory EV release during hHcy. Based on our findings, it is anticipated that the development of podocyte-specific ASM inhibition or Smpd1 gene silencing may be a novel therapeutic strategy for treatment of hHcy-induced glomerular disease with minimized side effects.

Investigating FSGS-like injury in zebrafish larvae by nifurpirinol: efficacy and molecular insight.

Identifying effective drugs for focal segmental glomerulosclerosis (FSGS) treatment holds significant importance. Our high-content drug screening on zebrafish larvae relies on nitroreductase/metronidazole (NTR/MTZ)-induced podocyte ablation to generate FSGS-like injury. A crucial factor for successful drug screenings is minimizing variability in injury induction. For this, we introduce nifurpirinol (NFP) as a more reliable prodrug for targeted podocyte depletion. NFP showed a 2.3-fold increase in efficiency at concentrations 1,600-fold lower compared with MTZ-mediated injury induction. Integration into the screening workflow validated its suitability for the high-content drug screening. The presence of crucial FSGS hallmarks, such as podocyte foot process effacement, proteinuria, and activation of parietal epithelial cells, was observed. After the isolation of the glomeruli from the larvae, we identified essential pathways by proteomic analysis. This study shows that NFP serves as a highly effective prodrug to induce the FSGS-like disease in zebrafish larvae and is well-suited for a high-content drug screening to identify new candidates for the treatment of FSGS.NEW & NOTEWORTHY This research investigated the use of nifurpirinol in nanomolar amounts as a prodrug to reliably induce focal segmental glomerulosclerosis (FSGS)-like damage in transgenic zebrafish larvae. Through proteomic analysis of isolated zebrafish glomeruli, we were further able to identify proteins that are significantly regulated after the manifestation of FSGS. These results are expected to expand our knowledge of the pathomechanism of FSGS.

Disrupting circadian control of autophagy induces podocyte injury and proteinuria.

The circadian clock influences a wide range of biological process and controls numerous aspects of physiology to adapt to the daily environmental changes caused by Earth's rotation. The kidney clock plays an important role in maintaining tubular function, but its effect on podocytes remains unclear. Here, we found that podocytes expressed CLOCK proteins, and that 2666 glomerular gene transcripts (13.4%), including autophagy related genes, had 24-hour circadian rhythms. Deletion of Clock in podocytes resulted in 1666 gene transcripts with the loss of circadian rhythm including autophagy genes. Podocyte-specific Clock knockout mice at age three and eight months showed deficient autophagy, loss of podocytes and increased albuminuria. Chromatin immunoprecipitation (ChIP) sequence analysis indicated autophagy related genes were targets of CLOCK in podocytes. ChIP-PCR further confirmed Clock binding to the promoter regions of Becn1 and Atg12, two autophagy related genes. Furthermore, the association of CLOCK regulated autophagy with chronic sleep fragmentation and diabetic kidney disease was analyzed. Chronic sleep fragmentation resulted in the loss of glomerular Clock rhythm, inhibition of podocyte autophagy, and proteinuria. Rhythmic oscillations of Clock also disappeared in high glucose treated podocytes and in glomeruli from diabetic mice. Finally, circadian differences in podocyte autophagy were also abolished in diabetic mice. Deletion Clock in podocytes aggravated podocyte injury and proteinuria in diabetic mice. Thus, our findings demonstrate that clock-dependent regulation of autophagy may be essential for podocyte survival. Hence. loss of circadian controlled autophagy may play an important role in podocyte injury and proteinuria.