Glycogen synthase kinase 3β dictates podocyte motility and focal adhesion turnover by modulating paxillin activity: implications for the protective effect of low-dose lithium in podocytopathy

The redox-sensitive GSK3β is a key regulator of glomerular podocyte injury in type 2 diabetic kidney disease

Emerging evidence suggests that GSK3β, a redox-sensitive transducer downstream of insulin signaling, acts as a convergent point for myriad pathways implicated in kidney injury, repair, and regeneration. However, its role in diabetic kidney disease remains controversial. In cultured glomerular podocytes, exposure to a milieu of type 2 diabetes elicited prominent signs of podocyte injury and degeneration, marked by loss of homeostatic marker proteins like synaptopodin, actin cytoskeleton disruption, oxidative stress, apoptosis, and stress-induced premature senescence, as shown by increased staining for senescence-associated β-galactosidase activity, amplified formation of γH2AX foci, and elevated expression of mediators of senescence signaling, like p21 and p16INK4A. These degenerative changes coincided with GSK3β hyperactivity, as evidenced by GSK3β overexpression and reduced inhibitory phosphorylation of GSK3β, and were averted by tideglusib, a highly-selective small molecule inhibitor of GSK3β. In agreement, post-hoc analysis of a publicly-available glomerular transcriptomics dataset from patients with type 2 diabetic nephropathy revealed that the curated diabetic nephropathy-related gene set was enriched in high GSK3β expression group. Mechanistically, GSK3β-modulated nuclear factor Nrf2 signaling is involved in diabetic podocytopathy, because GSK3β knockdown reinforced Nrf2 antioxidant response and suppressed oxidative stress, resulting in an improvement in podocyte injury and senescence. Conversely, ectopic expression of the constitutively active mutant of GSK3β impaired Nrf2 antioxidant response and augmented oxidative stress, culminating in an exacerbated diabetic podocyte injury and senescence. Moreover, IRS-1 was found to be a cognate substrate of GSK3β for phosphorylation at IRS-1S332, which negatively regulates IRS-1 activity. GSK3β hyperactivity promoted IRS-1 phosphorylation, denoting a desensitized insulin signaling. Consistently, in vivo in db/db mice with diabetic nephropathy, GSK3β was hyperactive in glomerular podocytes, associated with IRS-1 hyperphosphorylation, impaired Nrf2 response and premature senescence. Our finding suggests that GSK3β is likely a novel therapeutic target for treating type 2 diabetic glomerular injury.

Drug-induced glomerular diseases

Drug-induced kidney diseases represent a wide range of diseases that are responsible for a significant proportion of all acute kidney injuries and chronic kidney diseases. In the present review, we focused on drug-induced glomerular diseases, more precisely podocytopathies - minimal change diseases (MCD), focal segmental glomerulosclerosis (FSGS) - and membranous nephropathies (MN), from a physiological and a pharmacological point of view. The glomerular filtration barrier is composed of podocytes that form foot processes tightly connected and directly in contact with the basal membrane and surrounding capillaries. The common clinical feature of these diseases is represented by the loss of the ability of the filtration barrier to retain large proteins, leading to massive proteinuria and nephrotic syndrome. Drugs such as non-steroidal anti-inflammatory drugs (NSAIDs), D-penicillamine, tiopronin, trace elements, bisphosphonate, and interferons have been historically associated with the occurrence of MCD, FSGS, and MN. In the last ten years, the development of new anti-cancer agents, including tyrosine kinase inhibitors and immune checkpoint inhibitors, and research into their renal adverse effects highlighted these issues and have improved our comprehension of these diseases.

GSK3β: A ray of hope for the treatment of diabetic kidney disease

Diabetic kidney disease (DKD), a major microvascular complication of diabetes, is characterized by its complex pathogenesis, high risk of chronic renal failure, and lack of effective diagnosis and treatment methods. GSK3β (glycogen synthase kinase 3β), a highly conserved threonine/serine kinase, was found to activate glycogen synthase. As a key molecule of the glucose metabolism pathway, GSK3β participates in a variety of cellular activities and plays a pivotal role in multiple diseases. However, these effects are not only mediated by affecting glucose metabolism. This review elaborates on the role of GSK3β in DKD and its damage mechanism in different intrinsic renal cells. GSK3β is also a biomarker indicating the progression of DKD. Finally, the protective effects of GSK3β inhibitors on DKD are also discussed.

Research Progress of Antioxidants in Oxidative Stress Therapy after Spinal Cord Injury

Spinal cord injury (SCI) is a serious problem in the central nervous system resulting in high disability and mortality with complex pathophysiological mechanisms. Oxidative stress is one of the main secondary reactions of SCI, and its main pathophysiological marker is the production of excess reactive oxygen species. The overproduction of reactive oxygen species and insufficient antioxidant capacity lead to the occurrence of oxidative stress and neuroinflammation, and the dysregulation of oxidative stress and neuroinflammation leads to further aggravation of damage. Oxidative stress can initiate a variety of inflammatory and apoptotic pathways, and targeted antioxidant therapy can greatly reduce oxidative stress and reduce neuroinflammation, which has a certain positive effect on rehabilitation and prognosis in SCI. This article reviewed the research on different types of antioxidants and related treatments in SCI, focusing on the mechanisms of oxidative stress.

Intrarenal and Urinary Glycogen Synthase Kinase-3 Beta Levels in Diabetic and Nondiabetic Chronic Kidney Disease

BACKGROUND

Renal glycogen synthase kinase-3 beta (GSK3β) overactivity has been associated with a diverse range of kidney diseases. GSK3β activity in urinary exfoliated cells was reported to predict the progression of diabetic kidney disease (DKD). We compared the prognostic value of urinary and intrarenal GSK3β levels in DKD and nondiabetic chronic kidney disease (CKD).

METHODS

We recruited 118 consecutive biopsy-proved DKD patients and 115 nondiabetic CKD patients. Their urinary and intrarenal GSK3β levels were measured. They were then followed for dialysis-free survival and rate of renal function decline.

RESULTS

DKD group had higher intrarenal and urinary GSK3β levels than nondiabetic CKD (p < 0.0001 for both), but their urinary GSK3β mRNA levels were similar. Urinary p-GSK3β level is statistically significantly correlated with the baseline estimated glomerular filtration rate (eGFR), but urinary GSK3β level by ELISA, its mRNA level, the p-GSK3β level, or the p-GSK3β/GSK3β ratio had no association with dialysis-free survival or the slope of eGFR decline. In contrast, the intrarenal pY216-GSK3β/total GSK3β ratio significantly correlated with the slope of eGFR decline (r = -0.335, p = 0.006) and remained an independent predictor after adjusting for other clinical factors.

CONCLUSION

Intrarenal and urinary GSK3β levels were increased in DKD. The intrarenal pY216-GSK3β/total GSK3β ratio was associated with the rate of progression of DKD. The pathophysiological roles of GSK3β in kidney diseases deserve further studies.

Traditional Chinese Medicine in Treating Primary Podocytosis: From Fundamental Science to Clinical Research

Podocytes form a key component of the glomerular filtration barrier. Damage to podocytes is referred to as “podocyte disease.” There are many causes of podocyte injury, including primary injury, secondary injury, and gene mutations. Primary podocytosis mostly manifests as nephrotic syndrome. At present, first-line treatment is based on glucocorticoid administration combined with immunosuppressive therapy, but some patients still progress to end-stage renal disease. In Asia, especially in China, traditional Chinese medicine (TCM) still plays an important role in the treatment of kidney diseases. This study summarizes the potential mechanism of TCM and its active components in protecting podocytes, such as repairing podocyte injury, inhibiting podocyte proliferation, reducing podocyte apoptosis and excretion, maintaining podocyte skeleton structure, and upregulating podocyte-related protein expression. At the same time, the clinical efficacy of TCM in the treatment of primary podocytosis (including idiopathic membranous nephropathy, minimal change disease, and focal segmental glomerulosclerosis) is summarized to support the development of new treatment strategies for primary podocytosis.

Therapeutic Targeting of GSK3β-Regulated Nrf2 and NFκB Signaling Pathways by Salvianolic Acid A Ameliorates Peritoneal Fibrosis

Peritoneal fibrosis is a devastating complication in patients undergoing peritoneal dialysis, with no definite therapy yet available. Salvia miltiorrhiza and its major active component Salvianolic acid A (Sal A) have demonstrated a beneficial effect in myriad diseases. However, their effect on peritoneal fibrosis is unknown. In murine models of peritoneal dialysis, daily Sal A treatment substantially improved the peritoneal dialysis fluid (PDF) elicited peritoneal fibrosis, marked by thickening of the submesothelial compact zone, accumulation of extracellular matrix and increased expression of vimentin and PAI-1, concomitant with attenuation of GSK3β hyperactivity. This coincided with diminished nitrotyrosine in peritoneal tissues and increased Nrf2 nuclear translocation, entailing a lessened oxidative injury and reinforced Nrf2 antioxidant response. Meanwhile, inflammatory infiltration and maladaptive angiogenesis in peritoneal tissues provoked by PDF injury were also mitigated by Sal A, associated with a suppressed NFκB activation. Mechanistically, ectopic expression of the constitutively active GSK3β blunted the NFκB-suppressing and Nrf2-activating efficacy of Sal A in peritoneal mesothelial cells exposed to hypertonic dextrose, suggesting that GSK3β inhibition mediates the protective effect of Sal A. Collectively, our findings may open the avenue for developing a novel therapy based on Sal A for preventing peritoneal fibrosis in peritoneal dialysis.

Age-related GSK3β overexpression drives podocyte senescence and glomerular aging

As life expectancy continues to increase, clinicians are challenged by age-related renal impairment that involves podocyte senescence and glomerulosclerosis. There is now compelling evidence that lithium has a potent antiaging activity that ameliorates brain aging and increases longevity in Drosophila and Caenorhabditis elegans. As the major molecular target of lithium action and a multitasking protein kinase recently implicated in a variety of renal diseases, glycogen synthase kinase 3β (GSK3β) is overexpressed and hyperactive with age in glomerular podocytes, correlating with functional and histological signs of kidney aging. Moreover, podocyte-specific ablation of GSK3β substantially attenuated podocyte senescence and glomerular aging in mice. Mechanistically, key mediators of senescence signaling, such as p16^INK4A and p53, contain high numbers of GSK3β consensus motifs, physically interact with GSK3β, and act as its putative substrates. In addition, therapeutic targeting of GSK3β by microdose lithium later in life reduced senescence signaling and delayed kidney aging in mice. Furthermore, in psychiatric patients, lithium carbonate therapy inhibited GSK3β activity and mitigated senescence signaling in urinary exfoliated podocytes and was associated with preservation of kidney function. Thus, GSK3β appears to play a key role in podocyte senescence by modulating senescence signaling and may be an actionable senostatic target to delay kidney aging.

Serum and Glucocorticoid-Inducible Kinase 3/Nedd4-2 Signaling Pathway Participates in Podocyte Injury by Regulating the Stability of Nephrin

Serum and glucocorticoid-inducible kinase 3 (SGK3) is involved in maintaining podocyte function by regulating the protein levels of podocin and CD2-associated protein. Nephrin is also one of the slit diaphragm proteins of podocytes, but whether SGK3 participates in podocyte injury by regulating the levels of nephrin remains unclear. In this study, we focused on whether SGK3 affects nephrin levels and the mechanisms involved in the same. In the kidneys of adriamycin (ADR)-induced podocyte injury mouse model, the protein levels of SGK3 and nephrin were significantly decreased. Furthermore, the expression of SGK3 was negatively correlated with the output of proteinuria, and positively correlated with the levels of nephrin. In ADR-treated conditionally immortalized mouse podocyte cells (MPCs), the protein levels of nephrin and SGK3 were inhibited, while the constitutive expression of SGK3 reversed the ADR-induced decline in nephrin protein levels. Furthermore, ADR treatment or SGK3 inactivation enhanced the ubiquitin-proteasome degradation of nephrin in MPCs, and dramatically activated downstream effector proteins of SGK3, neural precursor cells expressing developmentally downregulated protein 4 subtype 2 (Nedd4-2) and glycogen synthase kinase-3 β (GSK3β). Similarly, Nedd4-2 or GSK3β overexpression resulted in increased activity of Nedd4-2 or GSK3β, and significantly downregulated nephrin levels. Interestingly, ubiquitin-mediated protein degradation of nephrin was regulated by Nedd4-2, rather than by GSK3β. In summary, SGK3 inactivation downregulated the levels of nephrin by increasing Nedd4-2 and GSK3β activity in ADR-induced podocyte injury model; in particular, the SGK3/Nedd4-2 signaling pathway was found to be involved in ubiquitin-mediated proteasome degradation of nephrin.

Cooperation of cell adhesion and autophagy in the brain: Functional roles in development and neurodegenerative disease

Cellular adhesive connections directed by the extracellular matrix (ECM) and maintenance of cellular homeostasis by autophagy are seemingly disparate functions that are molecularly intertwined, each regulating the other. This is an emerging field in the brain where the interplay between adhesion and autophagy functions at the intersection of neuroprotection and neurodegeneration. The ECM and adhesion proteins regulate autophagic responses to direct protein clearance and guide regenerative programs that go awry in brain disorders. Concomitantly, autophagic flux acts to regulate adhesion dynamics to mediate neurite outgrowth and synaptic plasticity with functional disruption contributed by neurodegenerative disease. This review highlights the cooperative exchange between cellular adhesion and autophagy in the brain during health and disease. As the mechanistic alliance between adhesion and autophagy has been leveraged therapeutically for metastatic disease, understanding overlapping molecular functions that direct the interplay between adhesion and autophagy might uncover therapeutic strategies to correct or compensate for neurodegeneration.

Lithium promotes recovery after spinal cord injury

Lithium is associated with oxidative stress and apoptosis, but the mechanism by which lithium protects against spinal cord injury remains poorly understood. In this study, we found that intraperitoneal administration of lithium chloride (LiCl) in a rat model of spinal cord injury alleviated pathological spinal cord injury and inhibited expression of tumor necrosis factor α, interleukin-6, and interleukin 1 β. Lithium inhibited pyroptosis and reduced inflammation by inhibiting Caspase-1 expression, reducing the oxidative stress response, and inhibiting activation of the Nod-like receptor protein 3 inflammasome. We also investigated the neuroprotective effects of lithium intervention on oxygen/glucose-deprived PC12 cells. We found that lithium reduced inflammation, oxidative damage, apoptosis, and necrosis and up-regulated nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase-1 in PC12 cells. All-trans retinoic acid, an Nrf2 inhibitor, reversed the effects of lithium. These results suggest that lithium exerts anti-inflammatory, anti-oxidant, and anti-pyroptotic effects through the Nrf2/heme oxygenase-1 pathway to promote recovery after spinal cord injury. This study was approved by the Animal Ethics Committee of Xi’an Jiaotong University (approval No. 2018-2053) on October 23, 2018.

Safety and Efficacy of Combined Low-Dose Lithium and Low-Dose Aspirin: A Pharmacological and Behavioral Proof-of-Concept Study in Rats

Despite established efficacy in bipolar disorder patients, lithium (Li) therapy has serious side effects, particularly chronic kidney disease. We examined the safety and behavioral effects of combined chronic low-dose aspirin plus low-dose Li in rats to explore the toxicity and therapeutic potential of this treatment. Rats were fed regular or Li-containing food (0.1% [low-dose, LLD-Li] or 0.2% [standard-dose, STD-Li]) for six weeks. Low-dose aspirin (1 mg/kg) was administered alone or together with Li. Renal function and gastric mucosal integrity were assessed. The effects of the combination treatment were evaluated in depression-like and anxiety-like behavioral models. Co-treatment with aspirin did not alter plasma Li levels. Chronic STD-Li treatment resulted in significant polyuria and polydipsia, elevated blood levels of creatinine and cystatin C, and increased levels of kidney nephrin and podocin—all suggestive of impaired renal function. Aspirin co-treatment significantly damped STD-Li-induced impairments in kidney parameters. There were no gastric ulcers or blood loss in any treatment group. Combined aspirin and LLD-Li resulted in a significant increase in sucrose consumption, and in the time spent in the open arms of an elevated plus-maze compared with the LLD-Li only group, suggestive of antidepressant-like and anxiolytic-like effects, respectively. Thus, we demonstrate that low-dose aspirin mitigated the typical renal side effects of STD-Li dose and enhanced the beneficial behavioral effects of LLD-Li therapy without aggravating its toxicity.

Lithium can mildly increase health during ageing but not lifespan in mice

Lithium is a nutritional trace element, used clinically as an anti‐depressant. Preclinically, lithium has neuroprotective effects in invertebrates and mice, and it can also extend lifespan in fission yeast, C. elegans and Drosophila. An inverse correlation of human mortality with the concentration of lithium in tap water suggests a possible, evolutionarily conserved mechanism mediating longevity. Here, we assessed the effects of lithium treatment on lifespan and ageing parameters in mice. Lithium has a narrow therapeutic dose range, and overdosing can severely affect organ health. Within the tolerable dosing range, we saw some mildly positive effects of lithium on health span but not on lifespan.

The protective effect of beta-hydroxybutyric acid on renal glomerular epithelial cells in adriamycin-induced injury

Beta-hydroxybutyric acid (BHB) exerts a protective effect in experimental of kidney disease models. However, the mechanisms underlying this activity are not well defined. BHB stands out for its ability to inhibit the Nε-lysine acetylation of histone and non-histone proteins, which may affect cellular processes and protein functions. In adriamycin-injured murine glomerular podocytes, BHB ameliorates podocyte damage and preserves actin cytoskeleton integrity, reminiscent of the effect of MS275, a highly selective inhibitor of lysine deacetylase. Further research found that adriamycin causes the reduced acetylation of nephrin, WT-1, and GSK3β. This process is abrogated by the lysine deacetylase inhibitor or BHB, suggesting that the acetylation of these molecules regulates their activity. In contrast, anacardic acid, a selective inhibitor of acetyltransferase, decreases the acetylation of nephrin, WT-1, and GSK3β and mitigates the podocyte protective effects of BHB. Taken together, BHB attenuates adriamycin-elicited glomerular epithelial cell injury, at least in part, by inhibiting the deacetylation of the key molecules implicated in glomerular injury.

GSK3 as a Regulator of Cytoskeleton Architecture: Consequences for Health and Disease

Glycogen synthase kinase 3 (GSK3) was initially isolated as a critical protein in energy metabolism. However, subsequent studies indicate that GSK-3 is a multi-tasking kinase that links numerous signaling pathways in a cell and plays a vital role in the regulation of many aspects of cellular physiology. As a regulator of actin and tubulin cytoskeleton, GSK3 influences processes of cell polarization, interaction with the extracellular matrix, and directional migration of cells and their organelles during the growth and development of an animal organism. In this review, the roles of GSK3–cytoskeleton interactions in brain development and pathology, migration of healthy and cancer cells, and in cellular trafficking of mitochondria will be discussed.

Sarsasapogenin restores podocyte autophagy in diabetic nephropathy by targeting GSK3β signaling pathway

Podocyte injury following abnormal podocyte autophagy plays an indispensable role in diabetic nephropathy (DN), therefore, restoration of podocyte autophagy is considered as a feasible strategy for the treatment of DN. Here, we investigated the preventive effects of sarsasapogenin (Sar), the main active ingredient in Anemarrhena asphodeloides Bunge, on the podocyte injury in diabetic rats, and tried to illustrate the mechanisms underlying the effects in high glucose (HG, 40 mM)-treated podocytes (MPs). Diabetes model was established in rats with single streptozocin (60 mg· kg^-1) intraperitoneal administration. The rats were then treated with Sar (20, 60 mg· kg^-1· d^-1, i.g.) or a positive control drug insulin (INS) (40 U· kg^-1· d^-1, i.h.) for 10 weeks. Our results showed that both Sar and insulin precluded the decreases of autophagy-related proteins (ATG5, Beclin1 and LC3B) and podocyte marker proteins (podocin, nephrin and synaptopodin) in the diabetic kidney. Furthermore, network pharmacology was utilized to assess GSK3β as the potential target involved in the action of Sar on DN and were substantiated by significant changes of GSK3β signaling in the diabetic kidney. The underlying protection mechanisms of Sar were explored in HG-treated MPs. Sar (20, 40 μM) or insulin (50 mU/L) significantly increased the expression of autophagy- related proteins and podocyte marker proteins in HG-treated MPs. Furthermore, Sar or insulin treatment efficiently regulatedphosphorylation at activation and inhibition sites of GSK3β. To sum up, this study certifies that Sar meliorates experimental DN through targeting GSK3β signaling pathway and restoring podocyte autophagy.

The ketone body β-hydroxybutyrate mitigates the senescence response of glomerular podocytes to diabetic insults

Diabetic kidney disease (DKD) is one of the most common complications of diabetes and clinically featured by progressive albuminuria, consequent to glomerular destruction that involves podocyte senescence. Burgeoning evidence suggests that ketosis, in particular β-hydroxybutyrate, exerts a beneficial effect on aging and on myriad metabolic or chronic diseases, including obesity, diabetes and chronic kidney diseases. Its effect on DKD is largely unknown. In vitro in podocytes exposed to a diabetic milieu, β-hydroxybutyrate treatment substantially mitigated cellular senescence and injury, as evidenced by reduced formation of γH2AX foci, reduced staining for senescence-associated-β-galactosidase activity, diminished expression of key mediators of senescence signaling like p16INK4A and p21, and preserved expression of synaptopodin. This beneficial action of β-hydroxybutyrate coincided with a reinforced transcription factor Nrf2 antioxidant response. Mechanistically, β-hydroxybutyrate inhibition of glycogen synthase kinase 3β (GSK3β), a convergent point for myriad signaling pathways regulating Nrf2 activity, seems to contribute. Indeed, trigonelline, a selective inhibitor of Nrf2, or ectopic expression of constitutively active mutant GSK3β abolished, whereas selective activation of Nrf2 was sufficient for the anti-senescent and podocyte protective effects of β-hydroxybutyrate. Moreover, molecular modeling and docking analysis revealed that β-hydroxybutyrate is able to directly target the ATP-binding pocket of GSK3β and thereby block its kinase activity. In murine models of streptozotocin-elicited DKD, β-hydroxybutyrate therapy inhibited GSK3β and reinforced Nrf2 activation in glomerular podocytes, resulting in lessened podocyte senescence and injury and improved diabetic glomerulopathy and albuminuria. Thus, our findings may pave the way for developing a β-hydroxybutyrate-based novel approach of therapeutic ketosis for treating DKD.

Lithium toxicity and the kidney with special focus on nephrotic syndrome associated with the acute kidney injury: A case-based systematic analysis

Despite the progress made in treating bipolar and unipolar affective disorders, lithium carbonate is still a common drug in psychiatric practice. Lithium-related renal side effects include nephrogenic diabetes insipidus, chronic tubulointerstitial nephropathy, and acute kidney injury (AKI). Nephrotic syndrome (NS) is an uncommon but severe complication of lithium treatment. We present a 49-year-old female treated with lithium carbonate due to a recurrent depressive disorder who developed NS during this therapy. NS spontaneously remitted after the drug withdrawal. Since her lithium serum levels were within the recommended values, we performed a retrospective analysis of lithium-induced NS cases trying to determine causes predisposing to the NS development, underlying histopathology, and preservation or irreversible loss of kidney function. This analysis revealed that in lithium-induced NS with AKI, lithium serum level was the key determinant of AKI development (the β coefficient = 0.8499 with a confidence interval ranging from 0.7452 to 0.9546 and p value < 0.0001). In these cases, the underlying pathology was mainly minimal change disease (MCD), which was quickly reversible upon the drug withdrawal. The development of chronic kidney disease (CKD) seemed to be associated with lithium therapy duration. However, the multiple regression analysis for CKD as the dependent variable showed that the decisive factor was focal segmental glomerulosclerosis (FSGS) as the underlying pathology (the β coefficient = 0.7866 with a confidence interval ranging from 0.600 to 0.9704 and the p value < 0.0001). Thus, we conclude that in lithium-induced NS/AKI, serum lithium levels contribute to these complications, while FSGS lesions are responsible for CKD's disease progression.

Dysregulated levels of glycogen synthase kinase-3β (GSK-3β) and miR-135 in peripheral blood samples of cases with nephrotic syndrome

Background

Glycogen synthase kinase-3 (GSK-3β) is a serine/threonine kinase with multifunctions in various physiological procedures. Aberrant level of GSK-3β in kidney cells has a harmful role in podocyte injury.

Methods

In this article, the expression levels of GSK-3β and one of its upstream regulators, miR-135a-5p, were measured in peripheral blood mononuclear cells (PBMCs) of cases with the most common types of nephrotic syndrome (NS); focal segmental glomerulosclerosis (FSGS) and membranous glomerulonephritis (MGN). In so doing, fifty-two cases along with twenty-four healthy controls were included based on the strict criteria.

Results

Levels of GSK-3β mRNA and miR-135 were measured with quantitative real-time PCR. There were statistically significant increases in GSK-3β expression level in NS (P = 0.001), MGN (P = 0.002), and FSGS (P = 0.015) groups compared to the control group. Dysregulated levels of miR-135a-5p in PBMCs was not significant between the studied groups. Moreover, a significant decrease was observed in the expression level of miR-135a-5p in the plasma of patients with NS (P = 0.020), MGN (P = 0.040), and FSGS (P = 0.046) compared to the control group. ROC curve analysis approved a diagnostic power of GSK-3β in discriminating patients from healthy controls (AUC: 0.72, P = 0.002) with high sensitivity and specificity.

Conclusions

Dysregulated levels of GSK-3β and its regulator miR-135a may participate in the pathogenesis of NS with different etiology. Therefore, more research is needed for understanding the relationship between them.

The ageing kidney: Molecular mechanisms and clinical implications

As human life expectancy keeps increasing, ageing populations present a growing challenge for clinical practices. Human ageing is associated with molecular, structural, and functional changes in a variety of organ systems, including the kidney. During the ageing process, the kidney experiences progressive functional decline as well as macroscopic and microscopic histological alterations, which are accentuated by systemic comorbidities like hypertension and diabetes mellitus, or by preexisting or underlying kidney diseases. Although ageing per se does not cause kidney injury, physiologic changes associated with normal ageing processes are likely to impair the reparative capacity of the kidney and thus predispose older people to acute kidney disease, chronic kidney disease and other renal diseases. Mechanistically, cell senescence plays a key role in renal ageing, involving a number of cellular signaling mechanisms, many of which may be harnessed as international targets for slowing or even reversing kidney ageing. This review summarizes the clinical characteristics of renal ageing, highlights the latest progresses in deciphering the role of cell senescence in renal ageing, and envisages potential interventional strategies and novel therapeutic targets for preventing or improving renal ageing in the hope of maintaining long-term kidney health and function across the life course.

Podocytes present antigen to activate specific T cell immune responses in inflammatory renal disease

Infiltration of activated T cells into renal tissue plays an essential role in inflammatory nephropathy. However, the mechanism enabling the renal recruitment and activation of T cells remains elusive. Here we report that inflammatory cytokine-promoted antigen presentation by podocytes is a key for recruiting and activating specific T cells. Our results showed that diabetes-associated inflammatory cytokines IFNγ and IL-17 all upregulated expression of MHC-I, MHC-II, CD80 and CD86 on the podocyte surface. Both IFNγ and IL-17 stimulated the uptake and processing of ovalbumin (OVA) by mouse podocytes, resulting in presentation of OVA antigen peptide on the cell surface. OVA antigen presentation by podocytes was also validated using human podocytes. Furthermore, OVA antigen-presenting mouse podocytes were able to activate OT-I mouse T cell proliferation and inflammatory cytokine secretion, which in turn caused podocyte injury and apoptosis. Finally, OT-I mice subjected to direct renal injection of OVA plus IFNγ/IL-17 but not OVA alone exhibited OVA antigen presentation by podocytes and developed nephropathy in 4 weeks. In conclusion, antigen presentation by podocytes under inflammatory conditions plays an important role in activating T cell immune responses and facilitating immune-mediated glomerular disease development. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Podocytopathies

Podocytopathies are kidney diseases in which direct or indirect podocyte injury drives proteinuria or nephrotic syndrome. In children and young adults, genetic variants in >50 podocyte-expressed genes, syndromal non-podocyte-specific genes and phenocopies with other underlying genetic abnormalities cause podocytopathies associated with steroid-resistant nephrotic syndrome or severe proteinuria. A variety of genetic variants likely contribute to disease development. Among genes with non-Mendelian inheritance, variants in APOL1 have the largest effect size. In addition to genetic variants, environmental triggers such as immune-related, infection-related, toxic and haemodynamic factors and obesity are also important causes of podocyte injury and frequently combine to cause various degrees of proteinuria in children and adults. Typical manifestations on kidney biopsy are minimal change lesions and focal segmental glomerulosclerosis lesions. Standard treatment for primary podocytopathies manifesting with focal segmental glomerulosclerosis lesions includes glucocorticoids and other immunosuppressive drugs; individuals not responding with a resolution of proteinuria have a poor renal prognosis. Renin-angiotensin system antagonists help to control proteinuria and slow the progression of fibrosis. Symptomatic management may include the use of diuretics, statins, infection prophylaxis and anticoagulation. This Primer discusses a shift in paradigm from patient stratification based on kidney biopsy findings towards personalized management based on clinical, morphological and genetic data as well as pathophysiological understanding.

Drug Testing for Residual Progression of Diabetic Kidney Disease in Mice Beyond Therapy with Metformin, Ramipril, and Empagliflozin

BACKGROUND

Progression of CKD in type 2 diabetes, despite dual inhibition of sodium-glucose transporter-2 and the renin-angiotensin system, remains a concern. Bromoindirubin-3'-oxime (BIO), previously reported to promote podocyte survival and regeneration, is a candidate additional drug to elicit renoprotective effects beyond therapy with metformin, ramipril, and empagliflozin (MRE). Evaluating a drug with standard therapeutics more closely mimics the clinical setting than evaluating the drug alone.

METHODS

Uninephrectomized BKS-Lepr-/- (db/db) mice treated with or without MRE served as a model of progressive CKD in type 2 diabetes. Mice on or off MRE were randomized to only 4 weeks of add-on BIO or vehicle. The primary end point was slope of GFR (ΔGFR).

RESULTS

Four weeks of MRE treatment alone did not affect ΔGFR, but significantly attenuated hyperglycemia, albuminuria, and glomerulosclerosis and increased podocyte filtration slit density, as assessed by STED super-resolution microscopy upon tissue clearing. BIO alone improved albuminuria, podocyte density in superficial and juxtamedullary nephrons, and podocyte filtration slit density. MRE+BIO combination therapy had additive protective effects on ΔGFR, glomerulosclerosis, podocyte density in juxtamedullary nephrons, and filtration slit density.

CONCLUSIONS

Add-on treatment with BIO for only 4 weeks attenuates progression of CKD beyond MRE therapy in mice with type 2 diabetes. Additional drug combinations may help to further delay ESKD in type 2 diabetes.

Crk1/2 and CrkL play critical roles in maintaining podocyte morphology and function

Podocytes are actin-rich epithelial cells whose effacement and detachment are the main cause of glomerular disease. Crk family proteins: Crk1/2 and CrkL are reported to be important intracellular signaling proteins that are involved in many biological processes. However, the roles of them in maintaining podocyte morphology and function remain poorly understood. In this study, specific knocking down of Crk1/2 and CrkL in podocytes caused abnormal cell morphology, actin cytoskeleton rearrangement and dysfunction in cell adhesion, spreading, migration, and viability. The p130Cas, focal adhesion kinase, phosphatidylinositol 3-kinase/Akt, p38 and JNK signaling pathways involved in these alterations. Furthermore, knocking down CrkL alone conferred a more modest phenotype than did the Crk1/2 knockdown and the double knockdown. Kidney biopsy specimens from patients with focal segmental glomerulosclerosis and minimal change nephropathy showed downregulation of Crk1/2 and CrkL in glomeruli. In zebrafish embryos, Crk1/2 and CrkL knockdown compromised the morphology and caused abnormal glomerular development. Thus, our results suggest that Crk1/2 and CrkL expression are important in podocytes; loss of either will cause podocyte dysfunction, leading to foot process effacement and podocyte detachment.

Melanocortin therapy ameliorates podocytopathy and proteinuria in experimental focal segmental glomerulosclerosis involving a podocyte specific non-MC1R-mediated melanocortinergic signaling

The clinical effectiveness of adrenocorticotropin in inducing remission of steroid-resistant nephrotic syndrome points to a steroidogenic-independent anti-proteinuric activity of melanocortins. However, which melanocortin receptors (MCR) convey this beneficial effect and if systemic or podocyte-specific mechanisms are involved remain uncertain. In vivo, wild-type (WT) mice developed heavy proteinuria and kidney dysfunction following Adriamycin insult, concomitant with focal segmental glomerulosclerosis (FSGS) and podocytopathy, marked by loss of podocin and synaptopodin, podocytopenia and extensive foot process effacement on electron microscopy. All these pathologic findings were prominently attenuated by NDP-MSH, a potent non-steroidogenic pan-MCR agonist. Surprisingly, MC1R deficiency in MC1R-null mice barely affected the severity of Adriamycin-elicited injury. Moreover, the beneficial effect of NDP-MSH was completely preserved in MC1R-null mice, suggesting that MC1R is likely non-essential for the protective action. A direct podocyte effect seems to contribute to the beneficial effect of NDP-MSH, because Adriamycin-inflicted cytopathic signs in primary podocytes prepared from WT mice were all mitigated by NDP-MSH, including apoptosis, loss of podocyte markers, de novo expression of the podocyte injury marker desmin, actin cytoskeleton derangement and podocyte hypermotility. Consistent with in vivo findings, the podoprotective activity of NDP-MSH was fully preserved in MC1R-null podocytes. Mechanistically, MC1R expression was predominantly distributed to glomerular endothelial cells in glomeruli but negligibly noted in podocytes in vivo and in vitro, suggesting that MC1R signaling is unlikely involved in direct podocyte protection. Ergo, melanocortin therapy protects against podocyte injury and ameliorates proteinuria and glomerulopathy in experimental FSGS, at least in part, via a podocyte-specific non-MC1R-mediated melanocortinergic signaling.

Triptolide potentiates the cytoskeleton-stabilizing activity of cyclosporine A in glomerular podocytes via a GSK3β dependent mechanism

Tripterygium wilfordii Hook F. (TwHF) is a traditional Chinese herb and has a broad spectrum of biological functions including immunosuppression and anti-inflammatory effects. When used in combination with other standard of care medications, such as glucocorticoids and calcineurin inhibitors like cyclosporine A, for treating glomerular diseases, TwHF demonstrates a remarkable dose-sparing effect, the molecular mechanism for which remains largely unknown. In an in vitro model of podocytopathy elicited by a diabetic milieu, triptolide, the major active component of TwHF, at low doses, potentiated the beneficial effect of cyclosporine A, and protected podocytes against diabetic milieu-elicited injury, mitigated cytoskeleton derangement, and preserved podocyte filtration barrier function, entailing a synergistic cytoskeleton-preserving and podocyte protective effect of triptolide and cyclosporine A. Mechanistically, inhibitory phosphorylation of GSK3β, a key molecule recently implicated as a convergence point of podocytopathic pathways, is likely required for the synergistic effect of triptolide and cyclosporine A on podocyte protection, because the synergistic effect was largely blunted in cells expressing the constitutively active GSK3β. Ergo, a synergistic podocyte cytoskeleton-stabilizing mechanism seems to underlie the cyclosporine A-sparing effect of triptolide in glomerulopathies. Combined triptolide and cyclosporine A therapy at reduced doses may be an invaluable regimen for treating diabetic nephropathy.

The Use of High-Throughput Transcriptomics to Identify Pathways with Therapeutic Significance in Podocytes

Podocytes have a unique structure that supports glomerular filtration function, and many glomerular diseases result in loss of this structure, leading to podocyte dysfunction and ESRD (end stage renal disease). These structural and functional changes involve a complex set of molecular and cellular mechanisms that remain poorly understood. To understand the molecular signature of podocyte injury, we performed transcriptome analysis of cultured human podocytes injured either with PAN (puromycin aminonucleoside) or doxorubicin/adriamycin (ADR). The pathway analysis through DE (differential expression) and gene-enrichment analysis of the injured podocytes showed Tumor protein p53 (P53) as one of the major signaling pathways that was significantly upregulated upon podocyte injury. Accordingly, P53 expression was also up-regulated in the glomeruli of nephrotoxic serum (NTS) and ADR-injured mice. To further confirm these observations, cultured podocytes were treated with the P53 inhibitor pifithrin-α, which showed significant protection from ADR-induced actin cytoskeleton damage. In conclusion, signaling pathways that are involved in podocyte pathogenesis and can be therapeutically targeted were identified by high-throughput transcriptomic analysis of injured podocytes.

Glycogen synthase kinase 3β hyperactivity in urinary exfoliated cells predicts progression of diabetic kidney disease

Burgeoning evidence points to glycogen synthase kinase (GSK)3β as a key player in diverse kidney diseases. However, as a pivotal transducer of the insulin signaling pathway, the role of GSK3β in diabetic kidney disease remains uncertain. In db/db mice, renal expression of total and activated GSK3β was increasingly elevated. This preceded the development of diabetic kidney disease, and correlated with the progression of signs of diabetic kidney injury, including albuminuria and extracellular matrix accumulation in glomeruli and tubulointerstitia. In vitro, exposure of glomerular podocytes, mesangial cells, and renal tubular cells to a diabetic milieu induced GSK3β overexpression and hyperactivity, which seem essential and sufficient for eliciting diabetic cellular damages in kidney cells, because the cytopathic effect of the diabetic milieu was mitigated by GSK3β knockdown, but was mimicked by ectopic expression of constitutively active GSK3β even in the normal milieu. In consistency, kidney biopsy specimens procured from patients with varying stages of diabetic nephropathy revealed an amplified expression of total and activated GSK3β in glomeruli and renal tubules, associated with the severity of diabetic nephropathy. Moreover, in retrospective cohorts of type 2 diabetic patients that were followed for over five years, the relative activity of GSK3β in banked urinary exfoliated cells represented an independent risk factor for development or progression of renal impairment. Furthermore, receiver operating characteristic curve analysis demonstrated that GSK3β activity in urinary exfoliated cells provided much better power than albuminuria in discriminating diabetic patients with progressive renal impairment from those with stable kidney function. Thus, renal expression and activity of GSK3β are amplified in experimental and clinical diabetic nephropathy. Hence, GSK3β in urinary exfoliated cells may serve as a novel biomarker for predicting diabetic kidney disease progression.

Eucalyptol Ameliorates Dysfunction of Actin Cytoskeleton Formation and Focal Adhesion Assembly in Glucose-Loaded Podocytes and Diabetic Kidney

SCOPE

Podocytes are a component of glomerular filtration barrier with interdigitating foot processes. The podocyte function depends on the dynamics of actin cytoskeletal and focal adhesion crucial for foot process structure. This study investigates the renoprotective effects of eucalyptol on the F-actin cytoskeleton formation and focal adhesion assembly in glucose-loaded podocytes and diabetic kidneys.

METHODS AND RESULTS

Eucalyptol at 1-20 µm reverses the reduction of cellular level of F-actin, ezrin, cortactin, and Arp2/3 in 33 mm glucose-loaded mouse podocytes, and oral administration of 10 mg kg^-1 eucalyptol elevates tissue levels of actin cytoskeletal proteins reduced in db/db mouse kidneys. Eucalyptol inhibits podocyte morphological changes, showing F-actin cytoskeleton formation in cortical regions and agminated F-actin along the cell periphery. Eucalyptol induces focal adhesion proteins of paxillin, vinculin, talin1, FAK, and Src in glucose-exposed podocytes and diabetic kidneys. Additionally, GTP-binding Rac1, Cdc42, Rho A, and ROCK are upregulated in glucose-stimulated podocytes and diabetic kidneys, which is attenuated by supplying eucalyptol. Rho A gene depletion partially diminishes GSK3β induction of podocytes by glucose.

CONCLUSION

Eucalyptol ameliorates F-actin cytoskeleton formation and focal adhesion assembly through blockade of the Rho signaling pathway, entailing partial involvement of GSK3β, which may inhibit barrier dysfunction of podocytes and resultant proteinuria.

GSK3β-mediated Keap1-independent regulation of Nrf2 antioxidant response: A molecular rheostat of acute kidney injury to chronic kidney disease transition

Transition of acute kidney injury (AKI) to chronic kidney disease (CKD) represents an important cause of kidney failure. However, how AKI is transformed into CKD remains elusive. Following folic acid injury, mice developed AKI with ensuing CKD transition, featured by variable degrees of interstitial fibrosis and tubular cell atrophy and growth arrest. This lingering injury of renal tubules was associated with sustained oxidative stress that was concomitant with an impaired Nrf2 antioxidant defense, marked by mitigated Nrf2 nuclear accumulation and blunted induction of its target antioxidant enzymes, like heme oxygenase (HO)-1. Activation of the canonical Keap1/Nrf2 signaling, nevertheless, seems intact during CKD transition because Nrf2 in injured tubules remained activated and elevated in cytoplasm. Moreover, oxidative thiol modification and activation of Keap1, the cytoplasmic repressor of Nrf2, was barely associated with CKD transition. In contrast, glycogen synthase kinase (GSK)3β, a key modulator of the Keap1-independent Nrf2 regulation, was persistently overexpressed and hyperactive in injured tubules. Likewise, in patients who developed CKD following AKI due to diverse etiologies, like volume depletion and exposure to radiocontrast agents or aristolochic acid, sustained GSK3β overexpression was evident in renal tubules and coincided with oxidative damages, impaired Nrf2 nuclear accumulation and mitigated induction of antioxidant gene expression. Mechanistically, the Nrf2 response against oxidative insult was sabotaged in renal tubular cells expressing a constitutively active mutant of GSK3β, but reinforced by ectopic expression of dominant negative GSK3β in a Keap1-independent manner. In vivo in folic acid-injured mice, targeting GSK3β in renal tubules via conditional knockout or by weekly microdose lithium treatment reinstated Nrf2 antioxidant response in the kidney and hindered AKI to CKD transition. Ergo, our findings suggest that GSK3β-mediated Keap1-independent regulation of Nrf2 may serve as an actionable therapeutic target for modifying the long-term sequelae of AKI.

•

AKI to CKD transition involves sustained GSK3β overactivation and impaired Nrf2 response in injured renal tubules.

•

Microdose lithium rectifies GSK3β overactivity in the kidney, reinstates Nrf2 response and hinders AKI to CKD transition.

•

GSK3β-mediated Keap1-independent regulation of Nrf2 is a novel therapeutic target for modifying long-term sequelae of AKI.

Podocyte GSK3α is important for autophagy and its loss detrimental for glomerular function

Podocytes are key cells in maintaining the integrity of the glomerular filtration barrier and preventing albuminuria. Glycogen synthase kinase 3 (GSK3) is a multi-functional serine/threonine kinase existing as two distinct but related isoforms (α and β). In the podocyte it has previously been reported that inhibition of the β isoform is beneficial in attenuating a variety of glomerular disease models but loss of both isoforms is catastrophic. However, it is not known what the role of GSK3α is in these cells. We now show that GSK3α is present and dynamically modulated in podocytes. When GSK3α is transgenically knocked down specifically in the podocytes of mice it causes mild but significant albuminuria by 6-weeks of life. Its loss also does not protect in models of diabetic or Adriamycin-induced nephropathy. In vitro deletion of podocyte GSK3α causes cell death and impaired autophagic flux suggesting it is important for this key cellular process. Collectively this work shows that GSK3α is important for podocyte health and that augmenting its function may be beneficial in treating glomerular disease.

Lithium ameliorates tubule-interstitial injury through activation of the mTORC2/protein kinase B pathway

Tubule-interstitial injury (TII) is a critical step in the progression of renal disease. It has been proposed that changes in proximal tubule (PT) albumin endocytosis plays an important role in the development of TII. Some reports have shown protective effects of lithium on kidney injury animal models that was correlated to proteinuria. We tested the hypothesis that lithium treatment ameliorates the development of TII due to changes in albumin endocytosis. Two experimental models were used: (1) TII induced by albumin overload in an animal model; (2) LLC-PK1 cells, a PT cell line. Lithium treatment ameliorates TII induced by albumin overload measured by (1) proteinuria; (2) collagen deposition; (3) area of tubule-interstitial space, and (4) macrophage infiltration. Lithium treatment increased mTORC2 activity leading to the phosphorylation of protein kinase B (PKB) at Ser473 and its activation. This mechanism enhanced albumin endocytosis in PT cells, which decreased the proteinuria observed in TII induced by albumin overload. This effect did not involve changes in the expression of megalin, a PT albumin receptor. In addition, activation of this pathway decreased apoptosis in LLC-PK1 cells, a PT cell line, induced by higher albumin concentration, similar to that found in pathophysiologic conditions. Our results indicate that the protective role of lithium treatment on TII induced by albumin overload involves an increase in PT albumin endocytosis due to activation of the mTORC2/PKB pathway. These results open new possibilities in understanding the effects of lithium on the progression of renal disease.

Brain natriuretic peptide mitigates TIMP2 induction and reinstates extracellular matrix catabolic activity via GSK3β inhibition in glomerular podocytes exposed to a profibrogenic milieu

Brain natriuretic peptide (BNP) has a demonstrable anti-fibrotic effect on diverse organ systems, including the kidney. To understand the molecular mechanism underlying this renoprotective effect, the efficacy of BNP was examined in an in vitro model of glomerular sclerosis by exposing glomerular podocytes to transforming growth factor (TGF)β1-containing media that recapitulates the profibrogenic milieu in chronic glomerular disease. BNP mitigates extracellular matrix (ECM) accumulation in TGFβ1-treated podocytes, as evidenced by Sirius red assay and staining, concomitant with a restoration of the ECM catabolizing activity, as assessed by pulse chase analysis. This effect was in parallel with a mitigating effect on TGFβ1-elicited overexpression of tissue inhibitor of metalloproteinases (TIMP)2, a key inhibitor of a multitude of ECM-degrading metalloproteinases. Mechanistically, glycogen synthase kinase (GSK)3β, a key player in pathogenesis of podocyte injury and glomerulopathies, seems to be involved. BNP treatment considerably induced GSK3β inhibition, marked by inhibitory phosphorylation at the serine 9 residue, and this significantly correlated with the abrogated TIMP2 induction in TGFβ1-injured podocytes. Moreover, genetic knockout of GSK3β in podocytes is sufficient to attenuate the TGFβ1 induced TIMP2 expression and ECM deposition, reminiscent of the effect of BNP. Conversely, ectopic expression of a nonphosphorylatable GSK3β mutant abolished the inhibitory effect of BNP on TGFβ1-elicited TIMP2 overexpression and ECM accumulation, signifying an essential role of GSK3β inhibition in mediating the effect of BNP. Collectively, BNP possesses an anti-fibrotic activity in glomerular epithelial cells. This finding, if validated in vivo, may open a new avenue to the treatment of glomerulosclerosis.

Activation of mineralocorticoid receptor by ecdysone, an adaptogenic and anabolic ecdysteroid, promotes glomerular injury and proteinuria involving overactive GSK3β pathway signaling

Ecdysone is an arthropod molting hormone and has been marketed as a non-androgenic natural anabolic and adaptogen. However, the safety profile of ecdysone is largely undetermined. After ecdysone treatment for 2 weeks, mice developed albuminuria with histologic signs of glomerular injury, including hypertrophy, mesangial expansion, mild glomerulosclerosis and podocyte injury. A direct glomerulopathic activity of ecdysone seems to contribute, since addition of ecdysone to cultured glomerular cells induced cytopathic changes, including apoptosis, activation of mesangial cells, podocyte shape changes and a decreased expression of podocyte markers. To explore the molecular target responsible for the pathogenic actions, we employed an in silico modeling system of compound-protein interaction and identified mineralocorticoid receptor (MR) as one of the top-ranking proteins with putative interactions with ecdysone. The molecular structure of ecdysone was highly homologous to mineralocorticoids, like aldosterone. Moreover, ecdysone was capable of both inducing and activating MR, as evidenced by MR nuclear accumulation in glomerular cells both in vitro and in vivo following ecdysone treatment. Mechanistically, glycogen synthase kinase (GSK) 3β, which has been recently implicated in pathogenesis of glomerular injury and proteinuria, was hyperactivated in glomeruli in ecdysone-treated mice, concomitant with diverse glomerulopathic changes. In contrast, spironolactone, a selective blockade of MR, largely abolished the cytopathic effect of ecdysone in vitro and attenuated albuminuria and glomerular lesions in ecdysone treated mice, associated with a mitigated GSK3β overactivity in glomeruli. Altogether, ecdysone seems able to activate MR and thereby promote glomerular injury and proteinuria involving overactive GSK3β pathway signaling.

hnRNP K plays a protective role in TNF-α-induced apoptosis in podocytes

Apoptosis of podocytes contributes to proteinuria in many chronic kidney diseases. The cytokine, tumor necrosis factor-α (TNF-α) is thought to be involved in podocyte apoptosis, but the underlying mechanism is not understood. In our study, we established a model of TNF-α-induced apoptosis by isolating primary podocytes from mice. After exposing cells to TNF-α, we determined the expression levels of heterogeneous nuclear ribonucleoprotein K (hnRNP K) and cellular FLICE-inhibitory protein (c-FLIP) and the phosphorylation levels of glycogen synthase kinase β (GSK3β) and extracellular signal-regulated kinase (ERK). We then knocked down or overexpressed the levels of hnRNP K and observed its effects on the expressions of c-FLIP, caspase-8, caspase-3, and the phosphorylation of GSK3β and ERK. In addition, we examined the percentage of cells undergoing apoptosis and studied cell cycle distribution. We found that TNF-α induced apoptosis in podocytes and that the expressions of hnRNP K and c-FLIP were significantly decreased, whereas the phosphorylations of GSK3β and ERK were significantly increased. Both gene knockdown and overexpression of hnRPN K resulted in varied expressions/phosphorylations of c-FLIP, GSK3β, and ERK. Moreover, decreased hnRPN K expression contributed to increased levels of caspase-8 and capase-3, as well as an increase in cell apoptosis and G0/G1 arrest. In conclusion, down-regulated expression of hnRNP K by TNF-α resulted in a decrease in the expression of c-FLIP as well as increases in phosphorylated GSK3β, ERK, caspase-8, and caspase-3, and then critically contributed to the podocyte apoptosis.

High-content screening assay-based discovery of paullones as novel podocyte-protective agents

Podocyte dysfunction and loss is an early event and a hallmark of proteinuric kidney diseases. A podocyte's normal function is maintained via its unique cellular architecture that relies on an intracellular network of filaments, including filamentous actin (F-actin) and microtubules, that provides mechanical support. Damage to this filamentous network leads to changes in cellular morphology and results in podocyte injury, dysfunction, and death. Conversely, stabilization of this network protects podocytes and ameliorates proteinuria. This suggests that stabilization of podocyte architecture via its filamentous network could be a key therapeutic strategy for proteinuric kidney diseases. However, development of podocyte-directed therapeutics, especially those that target the cell's filamentous network, is still lacking, partly because of unavailability of appropriate cellular assays for use in a drug discovery environment. Here, we describe a new high-content screening-based methodology and its implementation on podocytes to identify paullone derivatives as a novel group of podocyte-protective compounds. We find that three compounds, i.e., kenpaullone, 1-azakenpaullone, and alsterpaullone, dose dependently protect podocytes from puromycin aminonucleoside (PAN)-mediated injury in vitro by reducing PAN-induced changes in both the filamentous actin and microtubules, with alsterpaullone providing maximal protection. Mechanistic studies further show that alsterpaullone suppressed PAN-induced activation of signaling downstream of GSK3β and p38 mitogen-activated protein kinase. In vivo it reduced ADR-induced glomerular injury in a zebrafish model. Together, these results identify paullone derivatives as novel podocyte-protective agents for future therapeutic development.

Therapeutic Targeting of TAZ and YAP by Dimethyl Fumarate in Systemic Sclerosis Fibrosis

Systemic sclerosis (scleroderma, SSc) is a devastating fibrotic disease with few treatment options. Fumaric acid esters, including dimethyl fumarate (DMF, Tecfidera; Biogen, Cambridge, MA), have shown therapeutic effects in several disease models, prompting us to determine whether DMF is effective as a treatment for SSc dermal fibrosis. We found that DMF blocks the profibrotic effects of transforming growth factor-β (TGFβ) in SSc skin fibroblasts. Mechanistically, we found that DMF treatment reduced nuclear localization of transcriptional coactivator with PDZ binding motif (TAZ) and Yes-associated protein (YAP) proteins via inhibition of the phosphatidylinositol 3 kinase/protein kinase B (Akt) pathway. In addition, DMF abrogated TGFβ/Akt1 mediated inhibitory phosphorylation of glycogen kinase 3β (GSK3β) and a subsequent β-transducin repeat-containing proteins (βTRCP) mediated proteasomal degradation of TAZ, as well as a corresponding decrease of TAZ/YAP transcriptional targets. Depletion of TAZ/YAP recapitulated the antifibrotic effects of DMF. We also confirmed the increase of TAZ/YAP in skin biopsies from patients with diffuse SSc. We further showed that DMF significantly diminished nuclear TAZ/YAP localization in fibroblasts cultured on a stiff surface. Importantly, DMF prevented bleomycin-induced skin fibrosis in mice. Together, our work demonstrates a mechanism of the antifibrotic effect of DMF via inhibition of Akt1/GSK3β/TAZ/YAP signaling and confirms a critical role of TAZ/YAP in mediating the profibrotic responses in dermal fibroblasts. This study supports the use of DMF as a treatment for SSc dermal fibrosis.

Valproate hampers podocyte acquisition of immune phenotypes via intercepting the GSK3β facilitated NFkB activation

Glomerular podocytes are able to transdifferentiate under disease conditions, acquire de novo immune phenotypes and behave as immunocompetent cells, like phagocytes or antigen-presenting cells. Upon stimulation with lipopolysaccharide (LPS), a prototypical pathogen-associated molecular pattern, podocytes demonstrated de novo expression of a variety of NFkB-dependent immune molecules that are pivotal for immune response, including major histocompatibility complex (MHC) class II, costimulatory molecule CD80, lysosomal protease cathepsin L as well as CC chemokine ligand 2 and 5, ultimately resulting in podocyte dysfunction, characterized by cellular shrinkage, a spindle-like or asterlike cell shape and impairment of actin cytoskeleton integrity. The LPS-elicited podocyte phenotypic changes were concurrent with nuclear factor (NF) kB phosphorylation, which was associated with glycogen synthase kinase (GSK) 3β overactivity, marked by a diminished inhibitory phosphorylation of GSK3β. In contrast, valproate, an anticonvulsant and mood stabilizer, offset GSK3β overactivity in LPS-injured podocytes and mitigated NFkB activation and podocyte acquisition of immune phenotypes as well as the ensuing cytopathic changes, podocyte cytoskeleton disorganization and dysfunction. The protective effect of valproate was strikingly blunted in podocytes expressing the constitutively active GSK3β, suggesting an essential role of inhibitory phosphorylation of GSK3β. In vivo in LPS-injured mice, valproate therapy abolished GSK3β overactivity in glomeruli and attenuated podocyte injury and albuminuria, concomitant with a lessened NFkB activation and diminished induction of diverse podocytopathic immune molecules in podocytes and glomeruli. Taken together, valproate directly protects against podocyte injury and hampers podocyte acquisition of de novo immune phenotypes via intercepting the GSK3β facilitated NFkB activation.

The redox sensitive glycogen synthase kinase 3β suppresses the self-protective antioxidant response in podocytes upon oxidative glomerular injury

The redox sensitive glycogen synthase kinase (GSK) 3 has been recently implicated in the pathogenesis of proteinuric glomerulopathy. However, prior studies are less conclusive because they relied solely on chemical inhibitors of GSK3, which provide poor discrimination between the isoforms of GSK3 apart from potential off target activities. In murine kidneys, the β rather than the α isoform of GSK3 was predominantly expressed in glomeruli and distributed intensely in podocytes. By employing the doxycycline-activated Cre-loxP site specific gene targeting system, GSK3β was successfully knocked out (KO) selectively in podocytes in adult mice, resulting in a phenotype no different from control littermates. Electron microscopy of glomeruli in KO mice demonstrated more glycogen accumulation in podocytes but otherwise normal ultrastructures. Upon oxidative glomerular injury induced by protein overload, KO mice excreted significantly less albuminuria and had much attenuated podocytopathy and glomerular damage. The anti-proteinuric and glomerular protective effect was concomitant with diminished accumulation of reactive oxygen species in glomeruli in KO mice, which was likely secondary to a reinforced Nrf2 antioxidant response in podocytes. Collectively, our data suggest that GSK3β is dispensable for glomerular function and histology under normal circumstances but may serve as a therapeutic target for protecting from oxidative glomerular injuries.

Podocyte-actin dynamics in health and disease

Genetic studies of hereditary forms of nephrotic syndrome have identified several proteins that are involved in regulating the permselective properties of the glomerular filtration system. Further extensive research has elucidated the complex molecular basis of the glomerular filtration barrier and clearly established the pivotal role of podocytes in the pathophysiology of glomerular diseases. Podocyte architecture is centred on focal adhesions and slit diaphragms - multiprotein signalling hubs that regulate cell morphology and function. A highly interconnected actin cytoskeleton enables podocytes to adapt in order to accommodate environmental changes and maintain an intact glomerular filtration barrier. Actin-based endocytosis has now emerged as a regulator of podocyte integrity, providing an impetus for understanding the precise mechanisms that underlie the steady-state control of focal adhesion and slit diaphragm components. This Review outlines the role of actin dynamics and endocytosis in podocyte biology, and discusses how molecular heterogeneity in glomerular disorders could be exploited to deliver more rational therapeutic interventions, paving the way for targeted medicine in nephrology.

MC1R is dispensable for the proteinuria reducing and glomerular protective effect of melanocortin therapy

Melanocortin therapy by using adrenocorticotropic hormone (ACTH) or non-steroidogenic melanocortin peptides attenuates proteinuria and glomerular injury in experimental glomerular diseases and induces remission of nephrotic syndrome in patients with diverse glomerulopathies, even those resistant to steroids. The underlying mechanism remains elusive, but the role of melanocortin 1 receptor (MC1R) has been implicated and was examined here. Four patients with congenital red hair color and nephrotic syndrome caused by idiopathic membranous nephropathy or focal segmental glomerulosclerosis were confirmed by gene sequencing to bear dominant-negative MC1R mutations. Despite prior corticosteroid resistance, all patients responded to ACTH monotherapy and ultimately achieved clinical remission, inferring a steroidogenic-independent and MC1R-dispensable anti-proteinuric effect of melanocortin signaling. In confirmatory animal studies, the protective effect of [Nle⁴, D-Phe⁷]-α-melanocyte stimulating hormone (NDP-MSH), a potent non-steroidogenic pan-melanocortin receptor agonist, on the lipopolysaccharide elicited podocytopathy was completely preserved in MC1R-null mice, marked by reduced albuminuria and diminished histologic signs of podocyte injury. Moreover, in complementary in vitro studies, NDP-MSH attenuated the lipopolysaccharide elicited apoptosis, hypermotility and impairment of filtration barrier function equally in primary podocytes derived from MC1R-null and wild-type mice. Collectively, our findings suggest that melanocortin therapy confers a proteinuria reducing and podoprotective effect in proteinuric glomerulopathies via MC1R-independent mechanisms.

What we need to know about the effect of lithium on the kidney

Lithium has been a valuable treatment for bipolar affective disorders for decades. Clinical use of lithium, however, has been problematic due to its narrow therapeutic index and concerns for its toxicity in various organ systems. Renal side effects associated with lithium include polyuria, nephrogenic diabetes insipidus, proteinuria, distal renal tubular acidosis, and reduction in glomerular filtration rate. Histologically, chronic lithium nephrotoxicity is characterized by interstitial nephritis with microcyst formation and occasional focal segmental glomerulosclerosis. Nevertheless, this type of toxicity is uncommon, with the strongest risk factors being high serum levels of lithium and longer time on lithium therapy. In contrast, in experimental models of acute kidney injury and glomerular disease, lithium has antiproteinuric, kidney protective, and reparative effects. This paradox may be partially explained by lower lithium doses and short duration of therapy. While long-term exposure to higher psychiatric doses of lithium may be nephrotoxic, short-term low dose of lithium may be beneficial and ameliorate kidney and podocyte injury. Mechanistically, lithium targets glycogen synthase kinase-3β, a ubiquitously expressed serine/threonine protein kinase implicated in the processes of tissue injury, repair, and regeneration in multiple organ systems, including the kidney. Future studies are warranted to discover the exact "kidney-protective dose" of lithium and test the effects of low-dose lithium on acute and chronic kidney disease in humans.

The β isoform of GSK3 mediates podocyte autonomous injury in proteinuric glomerulopathy

Converging evidence points to glycogen synthase kinase (GSK) 3 as a key player in the pathogenesis of podocytopathy and proteinuria. However, it remains unclear if GSK3 is involved in podocyte autonomous injury in glomerular disease. In normal kidneys, the β isoform of GSK3 was found to be the major GSK3 expressed in glomeruli and intensely stained in podocytes. GSK3β expression in podocytes was markedly elevated in experimental or human proteinuric glomerulopathy. Podocyte-specific somatic ablation of GSK3β in adult mice attenuated proteinuria and ameliorated podocyte injury and glomerular damage in experimental adriamycin (ADR) nephropathy. Mechanistically, actin cytoskeleton integrity in podocytes was largely preserved in GSK3β knockout mice following ADR insult, concomitant with a correction of podocyte hypermotility and lessened phosphorylation and activation of paxillin, a focal adhesion-associated adaptor protein. In addition, GSK3β knockout diminished ADR-induced NFκB RelA/p65 phosphorylation selectively at serine 467; suppressed de novo expression by podocytes of NFκB-dependent podocytopathic mediators, including B7-1, cathepsin L, and MCP-1; but barely affected the induction of NFκB target pro-survival factors, such as Bcl-xL. Moreover, the ADR-elicited podocytopenia and podocyte death were significantly attenuated in GSK3β knockout mice, associated with protection against podocyte mitochondrial damage and reduced phosphorylation and activation of cyclophilin F, a structural component of mitochondria permeability transition pores. Overall, our findings suggest that the β isoform of GSK3 mediates autonomous podocyte injury in glomerulopathy by integrating multiple podocytopathic signalling pathways.

Targeting the podocyte cytoskeleton: from pathogenesis to therapy in proteinuric kidney disease

Glomerular injury often incites a progression to chronic kidney disease, which affects millions of patients worldwide. Despite our current understanding of this disease's pathogenesis, there is still a lack of therapy available to curtail its progression. However, exciting new data strongly suggest the podocyte-an actin-rich, terminally differentiated epithelial cell that lines the outside of the glomerular filtration barrier-as a therapeutic target. The importance of podocytes in the pathogenesis of human nephrotic syndrome is best characterized by identification of genetic mutations, many of which regulate the actin cytoskeleton. The intricate regulation of the podocyte actin cytoskeleton is fundamental in preserving an intact glomerular filtration barrier, and this knowledge has inspired new research targeting actin-regulating proteins in these cells. This review will shed light on recent findings, which have furthered our understanding of the molecular mechanisms regulating podocyte actin dynamics, as well as discoveries that have therapeutic implications in the treatment of proteinuric kidney disease.

Genetic and Pharmacologic Targeting of Glycogen Synthase Kinase 3β Reinforces the Nrf2 Antioxidant Defense against Podocytopathy

Evidence suggests that the glycogen synthase kinase 3 (GSK3)-dictated nuclear exclusion and degradation of Nrf2 is pivotal in switching off the self-protective antioxidant stress response after injury. Here, we examined the mechanisms underlying this regulation in glomerular disease. In primary podocytes, doxorubicin elicited cell death and actin cytoskeleton disorganization, concomitant with overactivation of GSK3β (the predominant GSK3 isoform expressed in glomerular podocytes) and minimal Nrf2 activation. SB216763, a highly selective small molecule inhibitor of GSK3, exerted a protective effect that depended on the potentiated Nrf2 antioxidant response, marked by increased Nrf2 expression and nuclear accumulation and augmented production of the Nrf2 target heme oxygenase-1. Ectopic expression of the kinase-dead mutant of GSK3β in cultured podocytes reinforced the doxorubicin-induced Nrf2 activation and prevented podocyte injury. Conversely, a constitutively active GSK3β mutant blunted the doxorubicin-induced Nrf2 response and exacerbated podocyte injury, which could be abolished by treatment with SB216763. In murine models of doxorubicin nephropathy or nephrotoxic serum nephritis, genetic targeting of GSK3β by doxycycline-inducible podocyte-specific knockout or pharmacologic targeting by SB216763 significantly attenuated albuminuria and ameliorated histologic signs of podocyte injury, including podocytopenia, loss of podocyte markers, podocyte de novo expression of desmin, and ultrastructural lesions of podocytopathy (such as foot process effacement). This beneficial outcome was likely attributable to an enhanced Nrf2 antioxidant response in glomerular podocytes because the selective Nrf2 antagonist trigonelline abolished the proteinuria-reducing and podocyte-protective effect. Collectively, our results suggest the GSK3β-regulated Nrf2 antioxidant response as a novel therapeutic target for protecting podocytes and treating proteinuric glomerulopathies.

Lithium in the Kidney: Friend and Foe?

Trace amounts of lithium are essential for our physical and mental health, and administration of lithium has improved the quality of life of millions of patients with bipolar disorder for >60 years. However, in a substantial number of patients with bipolar disorder, long-term lithium therapy comes at the cost of severe renal side effects, including nephrogenic diabetes insipidus and rarely, ESRD. Although the mechanisms underlying the lithium-induced renal pathologies are becoming clearer, several recent animal studies revealed that short-term administration of lower amounts of lithium prevents different forms of experimental AKI. In this review, we discuss the knowledge of the pathologic and therapeutic effects of lithium in the kidney. Furthermore, we discuss the underlying mechanisms of these seemingly paradoxical effects of lithium, in which fine-tuned regulation of glycogen synthase kinase type 3, a prime target for lithium, seems to be key. The new discoveries regarding the protective effect of lithium against AKI in rodents call for follow-up studies in humans and suggest that long-term therapy with low lithium concentrations could be beneficial in CKD.

Fine-tuning of NFκB by glycogen synthase kinase 3β directs the fate of glomerular podocytes upon injury

NFκB is regulated by a myriad of signaling cascades including glycogen synthase kinase (GSK) 3β and plays a Janus role in podocyte injury. In vitro, lipopolysaccharide or adriamycin elicited podocyte injury and cytoskeletal disruption, associated with NFκB activation and induced expression of NFκB target molecules, including pro-survival Bcl-xL and podocytopathic mediators like MCP-1, cathepsin L and B7-1. Broad range inhibition of NFκB diminished the expression of all NFκB target genes, restored cytoskeleton integrity, but potentiated apoptosis. In contrast, blockade of GSK3β by lithium or TDZD-8, mitigated the expression of podocytopathic mediators, ameliorated podocyte injury, but barely affected Bcl-xL expression or sensitized apoptosis. Mechanistically, GSK3β was sufficient and essential for RelA/p65 phosphorylation specifically at serine 467, which specifies the expression of selective NFκB target molecules, including podocytopathic mediators, but not Bcl-xL. In vivo, lithium or TDZD-8 therapy improved podocyte injury and proteinuria in mice treated with lipopolysaccharide or adriamycin, concomitant with suppression of podocytopathic mediators but retained Bcl-xL in glomerulus. Broad range inhibition of NFκB conferred similar but much weakened antiproteinuric and podoprotective effects accompanied with a blunted glomerular expression of Bcl-xL and marked podocyte apoptosis. Thus, the GSK3β dictated fine-tuning of NFκB may serve as a novel therapeutic target for podocytopathy.