Nestin protects mouse podocytes against high glucose-induced apoptosis by a Cdk5-dependent mechanism

Minimal Change Disease: Pathogenetic Insights from Glomerular Proteomics

The mechanism underlying podocyte dysfunction in minimal change disease (MCD) remains unknown. This study aimed to shed light on the potential pathophysiology of MCD using glomerular proteomic analysis. Shotgun proteomics using label-free quantitative mass spectrometry was performed on formalin-fixed, paraffin-embedded (FFPE) renal biopsies from two groups of samples: control (CTR) and MCD. Glomeruli were excised from FFPE renal biopsies using laser capture microdissection (LCM), and a single-pot solid-phase-enhanced sample preparation (SP3) digestion method was used to improve yield and protein identifications. Principal component analysis (PCA) revealed a distinct separation between the CTR and MCD groups. Forty-eight proteins with different abundance between the two groups (p-value ≤ 0.05 and |FC| ≥ 1.5) were identified. These may represent differences in podocyte structure, as well as changes in endothelial or mesangial cells and extracellular matrix, and some were indeed found in several of these structures. However, most differentially expressed proteins were linked to the podocyte cytoskeleton and its dynamics. Some of these proteins are known to be involved in focal adhesion (NID1 and ITGA3) or slit diaphragm signaling (ANXA2, TJP1 and MYO1C), while others are structural components of the actin and microtubule cytoskeleton of podocytes (ACTR3 and NES). This study suggests the potential of mass spectrometry-based shotgun proteomic analysis with LCM glomeruli to yield valuable insights into the pathogenesis of podocytopathies like MCD. The most significantly dysregulated proteins in MCD could be attributable to cytoskeleton dysfunction or may be a compensatory response to cytoskeleton malfunction caused by various triggers.

TRPC6 mediates high glucose-induced mitochondrial fission through activation of CDK5 in cultured human podocytes

Mitochondrial abnormalities contribute to the development of diabetic nephropathy (DN). However, the precise mechanisms of mitochondrial dysfunction in DN remain unclear. Transient receptor potential canonical channel-6 (TRPC6), a non-selective cation channel permeable to Ca²⁺, has been shown to regulate mitochondrial dynamics. This study was therefore aimed to explore the regulatory role and mechanisms of TRPC6 in high glucose (HG)-induced mitochondrial dysfunction in podocytes. Here we found that TRPC6 expression and TRPC6-induced Ca²⁺ influx were increased in HG-treated podocytes. Furthermore, the TRPC6 inhibitor and TRPC6 siRNA ameliorated mitochondrial dysfunction and apoptosis in HG-treated podocytes. BAPTA-AM, an intracellular calcium chelating agent, attenuated mitochondrial fission under HG conditions as well. Then, we found the activity of calpain and cyclin-dependent kinase 5 (CDK5) was markedly enhanced in HG-treated podocytes, which can be blocked by pretreatment with the TRPC6 inhibitor. Calpain-1 inhibition by calpeptin or by calpain-1 siRNA transfection not only attenuated HG-induced mitochondrial fission but also reduced the activity of CDK5. Additionally, the CDK5 inhibitor and its siRNA decreased mitochondrial fragmentation in HG-treated podocytes. Collectively, we revealed the essential role of TRPC6 in regulating HG-induced mitochondrial fission and apoptosis through the calpain-1/CDK5 pathway in human podocytes, which may provide new insights into the pathogenesis of DN.

Nephrinuria and podocytopathies

The discovery of nephrin in 1998 has launched a new era in glomerular diseases research, emphasizing its crucial role in the structure and function of the glomerular filtration barrier. In the past 20 years, substantial advances have been made in understanding podocyte structure and function as well as the discovery of several podocyte-related proteins including nephrin. The glomerular filtration barrier is comprised of podocytes, the glomerular basement membrane and endothelial cells. Podocytes, with their specialized slit diaphragm, form the essential backbone of the glomerular filtration barrier. Nephrin is a crucial structural and functional feature of the slit diaphragm that prevents plasma protein, blood cell and macromolecule leakage into the urine. Podocyte damage results in nephrin release. Podocytopathies are kidney diseases in which podocyte damage drives proteinuria, i.e., nephrotic syndrome. Many kidney diseases involve podocytopathy including congenital nephrotic syndrome of Finnish type, diffuse mesangial sclerosis, minimal change disease, focal segmental glomerulosclerosis, collapsing glomerulonephropathy, diabetic nephropathy, lupus nephropathy, hypertensive nephropathy and preeclampsia. Recently, urinary nephrin measurement has become important in the early detection of podocytopathies. In this chapter, we elaborate the main structural and functional features of nephrin as a podocyte-specific protein, pathomechanisms of podocytopathies which result in nephrinuria, highlight the most commonly used methods for detecting urinary nephrin and investigate the diagnostic, prognostic and potential therapeutic relevance of urinary nephrin in primary and secondary proteinuric kidney diseases.

Rapamycin attenuated podocyte apoptosis via upregulation of nestin in Ang II-induced podocyte injury

Background

Angiotensin II (Ang II) contributes to the progression of glomerulosclerosis, mainly by inducing podocyte injury. Convincing evidence indicates that the mTOR inhibitor rapamycin could play a fundamental role in protection against podocyte injury. Nestin, a major cytoskeletal protein, is stably expressed in podocytes and correlates with podocyte damage. The purpose of this study was to investigate the effect of rapamycin on podocyte injury induced by Ang II and to clarify the role and mechanism of nestin in the protective effect of rapamycin of podocyte injury.

Methods and results

We established an Ang II perfusion animal model, and the effects of rapamycin treatment on podocytes were investigated in vivo. In vitro, podocytes were stimulated with Ang II and rapamycin to observe podocyte injury, and nestin-siRNA was transfected to investigate the underlying mechanisms. We observed that Ang II induced podocyte injury both in vivo and in vitro, whereas rapamycin treatment relieved Ang II-induced podocyte injury. We further found that nestin co-localized with p-mTOR in glomeruli, and the protective effect of rapamycin was reduced by nestin-siRNA in podocytes. Moreover, co-IP indicated the interaction between nestin and p-mTOR, and nestin could affect podocyte injury via the mTOR/P70S6K signaling pathway.

Conclusion

We demonstrated that rapamycin attenuated podocyte apoptosis via upregulation of nestin expression through the mTOR/P70S6K signaling pathway in an Ang II-induced podocyte injury.

The Atypical Cyclin-Dependent Kinase 5 (Cdk5) Guards Podocytes from Apoptosis in Glomerular Disease While Being Dispensable for Podocyte Development

Cyclin-dependent kinase 5 (Cdk5) is expressed in terminally differentiated cells, where it drives development, morphogenesis, and survival. Temporal and spatial kinase activity is regulated by specific activators of Cdk5, dependent on the cell type and environmental factors. In the kidney, Cdk5 is exclusively expressed in terminally differentiated glomerular epithelial cells called podocytes. In glomerular disease, signaling mechanisms via Cdk5 have been addressed by single or combined conventional knockout of known specific activators of Cdk5. A protective, anti-apoptotic role has been ascribed to Cdk5 but not a developmental phenotype, as in terminally differentiated neurons. The effector kinase itself has never been addressed in animal models of glomerular disease. In the present study, conditional and inducible knockout models of Cdk5 were analyzed to investigate the role of Cdk5 in podocyte development and glomerular disease. While mice with podocyte-specific knockout of Cdk5 had no developmental defects and regular lifespan, loss of Cdk5 in podocytes increased susceptibility to glomerular damage in the nephrotoxic nephritis model. Glomerular damage was associated with reduced anti-apoptotic signals in Cdk5-deficient mice. In summary, Cdk5 acts primarily as master regulator of podocyte survival during glomerular disease and—in contrast to neurons—does not impact on glomerular development or maintenance.

Inhibition of NLRP3 inflammasome ameliorates podocyte damage by suppressing lipid accumulation in diabetic nephropathy

BACKGROUND

Nucleotide leukin-rich polypeptide 3 (NLRP3) inflammasome is documented as a potent target for treating metabolic diseases and inflammatory disorders. Our recent work demonstrated that inhibition of NLRP3 inflammasome activation inhibits renal inflammation and fibrosis in diabetic nephropathy. This study was to investigate the effect of NLRP3 inflammasome on podocyte injury and the underlying mechanism in diabetic nephropathy.

METHODS

In vivo, db/db mice were treated with MCC950, a NLRP3 inflammasome specific inhibitor. NLRP3 knockout (NKO) mice were induced to diabetes by intraperitoneal injections of streptozotocin (STZ). We assessed renal function, albuminuria, podocyte injury and glomerular lipid accumulation in diabetic mice. In vitro, apoptosis, cytoskeleton change, lipid accumulation, NF-κB p65 activation and reactive oxygen species (ROS) generation were evaluated in podocytes interfered with NLRP3 siRNA or MCC950 under high glucose (HG) conditions. In addition, the effect and mechanism of IL-1β on lipid accumulation was explored in podocytes exposed to normal glucose (NG) or HG.

RESULTS

MCC950 treatment improved renal function, attenuated albuminuria, mesangial expansion, podocyte loss, as well as glomerular lipid accumulation in db/db mice. The diabetes-induced podocyte loss and glomerular lipid accumulation were reversed in NLRP3 knockout mice. The increased expression of sterol regulatory element-binding protein1 (SREBP1) and SREBP2, and decreased expression of ATP-binding cassette A1 (ABCA1) in podocytes were reversed by MCC950 treatment or NLRP3 knockout in diabetic mice. In vitro, NLRP3 siRNA or MCC950 treatment markedly inhibited HG-induced apoptosis, cytoskeleton change, lipid accumulation, NF-κB p65 activation, and mitochondrial ROS production in cultured podocytes. In addition, BAY11-7082 or tempol treatment inhibited HG-induced lipid accumulation in podocytes. Moreover, exposure of IL-1β to podocytes induced lipid accumulation, NF-κB p65 activation and mitochondrial ROS generation.

CONCLUSION

Inhibition of NLRP3 inflammasome protects against podocyte damage through suppression of lipid accumulation in diabetic nephropathy. IL-1β/ROS/NF-κB p65 mediates diabetes-associated lipid accumulation in podocytes. The suppression of NLRP3 inflammasome activation may be an effective therapeutic approach to diabetic nephropathy.

Cdk5-Mediated Phosphorylation of Sirt1 Contributes to Podocyte Mitochondrial Dysfunction in Diabetic Nephropathy

Aims: Mitochondrial dysfunction contributes to podocyte injury, which is the leading cause of proteinuria in diabetic nephropathy (DN). In this study, we explored the role of cyclin-dependent kinase 5 (Cdk5) in mitochondrial dysfunction of podocytes under diabetic conditions. Results: Our results showed that the expression and activity of Cdk5 were significantly upregulated in vivo and in vitro under diabetic conditions, accompanied by the downregulation of synaptopodin and nephrin, as well as structural and functional mitochondrial dysfunction. Inhibition of Cdk5 with roscovitine or dominant-negative Cdk5 led to the attenuation of podocyte injury by upregulating synaptopodin and nephrin. The inhibition of Cdk5 also ameliorated mitochondrial dysfunction by decreasing reactive oxygen species levels and cytochrome c release, while increasing adenosine triphosphate production. Sirt1, an NAD⁺-dependent deacetylase, was decreased in podocytes with high glucose (HG) treatment; however, its phosphorylation level at S47 was significantly upregulated. We demonstrated that HG levels cause overactive Cdk5 to phosphorylate Sirt1 at S47. Suppression of Cdk5 reduced Sirt1 phosphorylation levels and mutation of S47 to nonphosphorable alanine (S47A), significantly attenuated podocyte injury and mitochondrial dysfunction in diabetic condition in vivo and in vitro. Innovation and Conclusion: Our study has demonstrated the role of Cdk5 in regulating mitochondrial function through Sirt1 phosphorylation and thus can potentially be a new therapeutic target for DN treatment. IRB number: 20190040. Antioxid. Redox Signal. 34, 171-190.

Carbohydrate response element-binding protein regulates lipid metabolism via mTOR complex1 in diabetic nephropathy

Lipid deposition caused by the disorder of renal lipid metabolism is involved in diabetic nephropathy (DN). Carbohydrate response element-binding protein (ChREBP) is a key transcription factor in high glucose-induced cellular fat synthesis. At present, the regulation and mechanism of ChREBP on fat metabolism in diabetic kidneys are still unclear. In this study, we showed that lack of ChREBP significantly improved renal injury, inhibited oxidative stress, lipid deposition, fatty acid synthase (FASN), acetyl-CoA carboxylase (ACC) and thioredoxin-interacting protein (TXNIP) expression, as well as the activity of mammalian target of rapamycin complex 1 (mTORC1) in diabetic kidneys. Meanwhile, ChREBP deficiency upregulated the expression of peroxisome proliferator-activated receptor-α (PPARα), carnitine palmitoyltransferaser 1A (CPT1A) and acyl-coenzyme A oxidase 1 (ACOX1) in diabetic kidneys. In vitro, knockdown of ChREBP attenuated lipid deposition, mTORC1 activation, and expression of FASN and ACC, increased PPARα, CPT1A, and ACOX1 expression in HK-2 cells and podocytes under high glucose (HG) conditions. Moreover, HG-induced lipid deposition, increased expression of FASN and ACC and decreased expression of PPARα, CPT1A, and ACOX1 were reversed by rapamycin, a specific inhibitor of mTORC1, in HK-2 cells. These results indicate that ChREBP deficiency alleviates diabetes-associated renal lipid accumulation by inhibiting mTORC1 activity and suggest that reduction of ChREBP is a potential therapeutic strategy to treat DN.

Tmem63c is a potential pro-survival factor in angiotensin II-treated human podocytes

AIMS

Human podocytes (hPC) play an important role in the pathogenesis of renal diseases. In this context, angiotensin II (Ang II) and nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) play a crucial role in podocyte injury. Recently, transmembrane protein (Tmem) 63c, a member of the Tmem-family was found to be expressed in kidney and associated with podocyte function. In this study, we analysed the expression regulation and functional impact of Tmem63c on cell viability and apoptosis in hPC in the context of Ang II activation.

MATERIALS AND METHODS

Expression of Tmem63c in response to Ang II and the NFκB inhibitor Bay 11-7082 was analysed by Real-Time PCR and Western blotting. Cellular functions were determined by functional assays.

KEY FINDINGS

We found Ang II to induce Tmem63c expression in hPC in a concentration-dependent manner. Inhibition of NFκB by Bay 11-7082 reduced basal as well as Ang II-induced Tmem63c expression. SiRNA-mediated down-regulation of Tmem63c diminished cell viability and protein kinase B (Akt) signaling and increased cell apoptosis of resting as well as Ang II-activated hPC.

SIGNIFICANCE

These data show that Ang II induced the expression of Tmem63c in hPC, possibly via NFκB-dependent mechanisms. Moreover, down-regulation of Tmem63c was associated with reduced cell viability, indicating Tmem63c to be a potential pro-survival factor in hPC.

Nestin protects podocyte from injury in lupus nephritis by mitophagy and oxidative stress

Podocyte injury is the main cause of proteinuria in lupus nephritis (LN). Nestin, an important cytoskeleton protein, is expressed stably in podocytes and is associated with podocyte injury. However, the role of nestin in the pathogenesis of proteinuria in LN remains unclear. The correlations among nestin, nephrin and proteinuria were analyzed in LN patients and MRL/lpr lupus-prone mice. The expression of nestin in mouse podocyte lines (MPCs) and MRL/lpr mice was knocked down to determine the role of nestin in podocyte injury. Inhibitors and RNAi method were used to explore the role of mitophagy and oxidative stress in nestin protection of podocyte from damage. There was a significantly negative correlation between nestin and proteinuria both in LN patients and MRL/lpr mice, whereas the expression of nephrin was positively correlated with nestin. Knockdown of nestin resulted in not only the decrease of nephrin, p-nephrin (Y1217) and mitophagy-associated proteins in cultured podocytes and the podocytes of MRL/lpr mice, but also mitochondrial dysfunction in podocytes stimulated with LN plasma. The expression and phosphorylation of nephrin was significantly decreased by reducing the level of mitophagy or production of reactive oxygen species (ROS) in cultured podocytes. Our findings suggested that nestin regulated the expression of nephrin through mitophagy and oxidative stress to protect the podocytes from injury in LN.

Cytoskeletal Proteins in Cancer and Intracellular Stress: A Therapeutic Perspective

Cytoskeletal proteins, which consist of different sub-families of proteins including microtubules, actin and intermediate filaments, are essential for survival and cellular processes in both normal as well as cancer cells. However, in cancer cells, these mechanisms can be altered to promote tumour development and progression, whereby the functions of cytoskeletal proteins are co-opted to facilitate increased migrative and invasive capabilities, proliferation, as well as resistance to cellular and environmental stresses. Herein, we discuss the cytoskeletal responses to important intracellular stresses (such as mitochondrial, endoplasmic reticulum and oxidative stresses), and delineate the consequences of these responses, including effects on oncogenic signalling. In addition, we elaborate how the cytoskeleton and its associated molecules present themselves as therapeutic targets. The potential and limitations of targeting new classes of cytoskeletal proteins are also explored, in the context of developing novel strategies that impact cancer progression.

Research Progress on the Pathological Mechanisms of Podocytes in Diabetic Nephropathy

Diabetic nephropathy (DN) is not only an important microvascular complication of diabetes but also the main cause of end-stage renal disease. Studies have shown that the occurrence and development of DN are closely related to morphological and functional changes in podocytes. A series of morphological changes after podocyte injury in DN mainly include podocyte hypertrophy, podocyte epithelial-mesenchymal transdifferentiation, podocyte detachment, and podocyte apoptosis; functional changes mainly involve podocyte autophagy. More and more studies have shown that multiple signaling pathways play important roles in the progression of podocyte injury in DN. Here, we review research progress on the pathological mechanism of morphological and functional changes in podocytes associated with DN, to provide a new target for delaying the occurrence and development of this disorder.

Role of Podocyte Injury in Glomerulosclerosis

Finding new therapeutic targets of glomerulosclerosis treatment is an ongoing quest. Due to a living environment of various stresses and pathological stimuli, podocytes are prone to injuries; moreover, as a cell without proliferative potential, loss of podocytes is vital in the pathogenesis of glomerulosclerosis. Thus, sufficient understanding of factors and underlying mechanisms of podocyte injury facilitates the advancement of treating and prevention of glomerulosclerosis. The clinical symptom of podocyte injury is proteinuria, sometimes with loss of kidney functions progressing to glomerulosclerosis. Injury-induced changes in podocyte physiology and function are actually not a simple passive process, but a complex interaction of proteins that comprise the anatomical structure of podocytes at molecular levels. This chapter lists several aspects of podocyte injuries along with potential mechanisms, including glucose and lipid metabolism disorder, hypertension, RAS activation, micro-inflammation, immune disorder, and other factors. These aspects are not technically separated items, but intertwined with each other in the pathogenesis of podocyte injuries.

A systems-approach reveals human nestin is an endothelial-enriched, angiogenesis-independent intermediate filament protein

The intermediate filament protein nestin is expressed during embryonic development, but considered largely restricted to areas of regeneration in the adult. Here, we perform a body-wide transcriptome and protein-profiling analysis to reveal that nestin is constitutively, and highly-selectively, expressed in adult human endothelial cells (EC), independent of proliferative status. Correspondingly, we demonstrate that it is not a marker for tumour EC in multiple malignancy types. Imaging of EC from different vascular beds reveals nestin subcellular distribution is shear-modulated. siRNA inhibition of nestin increases EC proliferation, and nestin expression is reduced in atherosclerotic plaque neovessels. eQTL analysis reveals an association between SNPs linked to cardiovascular disease and reduced aortic EC nestin mRNA expression. Our study challenges the dogma that nestin is a marker of proliferation, and provides insight into its regulation and function in EC. Furthermore, our systems-based approach can be applied to investigate body-wide expression profiles of any candidate protein.

Nestin Improves Preeclampsia-Like Symptoms by Inhibiting Activity of Cyclin-Dependent Kinase 5

BACKGROUND/AIMS

Preeclampsia (PE) is a pregnancy-specific hypertensive disorder that is characterised by a high incidence of hypertension and proteinuria. Podocytes are involved in the formation of a split membrane, which is the last barrier preventing the leakage of protein into the urine. Nestin, a cytoskeleton protein, is expressed stably in podocytes. However, the association between the Nestin concentration in urine and the progression of PE and the role of Nestin in PE remains unclear.

METHODS

In the present study, a mouse podocyte cell line, PE-like animal model and PE patients' urine samples were used. Eilsa kits were used to detect the levels of proteins expression in urine samples from patients and animal models. Western Blotting and immunofluorescence were used to detect proteins expression levels in cell samples and animal tissue samples. Flow cytometry was used to detect the level of apoptosis in cells. Tunel assay was used to detect the levels of apoptosis in animal tissue samples.

RESULTS

Nestin levels were significantly increased in PE patients than in hypertensive patients and healthy subjects, and positively correlated with proteinuria and podocalyxin. Ang II treatment decreased the expression of Nestin and Podocin in a time- and dose- dependent manner in podocytes. Restoration of the Nestin levels could reverse Ang II-induced F-actin degradation and attenuate Ang II-mediated podocyte apoptosis, while knockdown of the Nestin level exhibited the opposite. Moreover, the protective role of Nestin on podocytes is mediated by inhibition of the kinase activity of CDK5. In PE-like animal model induced by L-NAME injection, restoration of Nestin lowered the pressure and proteinuria concentration, attenuated the loss of podocytes, and decreased the expression of p35, p53 and the activity of CDK5 kinase, as compared with the control.

CONCLUSIONS

Our findings suggest that Nestin could improve preeclampsia-like symptoms by inhibiting the activity of CDK5, and Nestin may become a new prognostic factor and a potential therapy target for PE.

CDK5 promotes renal tubulointerstitial fibrosis in diabetic nephropathy via ERK1/2/PPARγ pathway

Cyclin-dependent kinase 5 (CDK5) has been documented in podocyte injuries in diabetic nephropathy (DN), however its role in renal tubular epithelial cells has not been elucidated. We report here that CDK5 is detrimental and promotes tubulointerstitial fibrosis (TIF) via the extracellular signal-regulated kinase 1/2 (ERK1/2)/peroxisome proliferator-activated receptor gamma (PPRAγ) pathway in DN. In high glucose cultured NRK52E cells, blocking CDK5 activity inhibited epithelial-to-mesenchymal transition (EMT) and fibrosis via ERK1/2/PPARγ pathway. In diabetic rats, CDK5 inhibitor roscovitine decreased renal fibrosis and improved renal function as demonstrated by a decrease in levels of blood urine nitrogen (BUN), serum creatinine and β2-microglobulin. Further studies revealed that improved renal fibrosis and function in diabetic rats were associated with inactivation of ERK1/2 and PPARγ signaling pathways. In late staged DN patients, the upregulation of CDK5 and p35 activated phosphorylated ERK1/2 and PPARγ, leading to decreased levels of E-cadherin but increased Vimentin and Collagen IV. Accordingly, renal fibrosis and function were worsened as revealed by decreased estimated glomerular filtration rate (eGFR) and increased serum BUN, creatinine, β2-microglobulin, 24-hour proteinuria and urine albumin to creatinine ratio (UACR). These findings demonstrate a novel mechanism that CDK5 increases tubulointerstitial fibrosis by activating the ERK1/2/PPARγ pathway and EMT in DN. CDK5 might have therapeutic potential in diabetic nephropathy.

Diabetes as risk factor for pancreatic cancer: Hyperglycemia promotes epithelial-mesenchymal-transition and stem cell properties in pancreatic ductal epithelial cells

Type 2 diabetes mellitus (T2DM) is associated with hyperglycemia and a risk to develop pancreatic ductal adenocarcinoma (PDAC), one of the most fatal malignancies. Cancer stem cells (CSC) are essential for initiation and maintenance of tumors, and acquisition of CSC-features is linked to epithelial-mesenchymal-transition (EMT). The present study investigated whether hyperglycemia promotes EMT and CSC-features in premalignant and malignant pancreatic ductal epithelial cells (PDEC). Under normoglycemia (5 mM d-glucose), Panc1 PDAC cells but not premalignant H6c7-kras cells exhibited a mesenchymal phenotype along with pronounced colony formation. While hyperglycemia (25 mM d-glucose) did not impact the mesenchymal phenotype of Panc1 cells, CSC-properties were aggravated exemplified by increased Nanog expression and Nanog-dependent formation of holo- and meroclones. In H6c7-kras cells, high glucose increased secretion of Transforming-Growth-Factor-beta1 (TGF-β1) as well as TGF-β1 signaling, and in a TGF-β1-dependent manner reduced E-cadherin expression, increased Nestin expression and number of meroclones. Finally, reduced E-cadherin expression was detected in pancreatic ducts of hyperglycemic but not normoglycemic mice. These data suggest that hyperglycemia promotes the acquisition of mesenchymal and CSC-properties in PDEC by activating TGF-β signaling and might explain how T2DM facilitates pancreatic tumorigenesis.

The Evolving Complexity of the Podocyte Cytoskeleton

Podocytes exhibit a unique cytoskeletal architecture that is fundamentally linked to their function in maintaining the kidney filtration barrier. The cytoskeleton regulates podocyte shape, structure, stability, slit diaphragm insertion, adhesion, plasticity, and dynamic response to environmental stimuli. Genetic mutations demonstrate that even slight impairment of the podocyte cytoskeletal apparatus results in proteinuria and glomerular disease. Moreover, mechanisms underpinning all acquired glomerular pathologies converge on disruption of the cytoskeleton, suggesting that this subcellular structure could be targeted for therapeutic purposes. This review summarizes our current understanding of the function of the cytoskeleton in podocytes and the associated implications for pathophysiology.

Intermediate Filaments and the Regulation of Cell Motility during Regeneration and Wound Healing

SUMMARYIntermediate filaments (IFs) comprise a diverse group of flexible cytoskeletal structures, the assembly, dynamics, and functions of which are regulated by posttranslational modifications. Characteristically, the expression of IF proteins is specific for tissues, differentiation stages, cell types, and functional contexts. Recent research has rapidly expanded the knowledge of IF protein functions. From being regarded as primarily structural proteins, it is now well established that IFs act as powerful modulators of cell motility and migration, playing crucial roles in wound healing and tissue regeneration, as well as inflammatory and immune responses. Although many of these IF-associated functions are essential for tissue repair, the involvement of IF proteins has been established in many additional facets of tissue healing and regeneration. Here, we review the recent progress in understanding the multiple functions of cytoplasmic IFs that relate to cell motility in the context of wound healing, taking examples from studies on keratin, vimentin, and nestin. Wound healing and regeneration include orchestration of a broad range of cellular processes, including regulation of cell attachment and migration, proliferation, differentiation, immune responses, angiogenesis, and remodeling of the extracellular matrix. In this respect, IF proteins now emerge as multifactorial and tissue-specific integrators of tissue regeneration, thereby acting as essential guardian biopolymers at the interface between health and disease, the failing of which contributes to a diverse range of pathologies.

Research Progress on Mechanism of Podocyte Depletion in Diabetic Nephropathy

Diabetic nephropathy (DN) together with glomerular hyperfiltration has been implicated in the development of diabetic microangiopathy in the initial stage of diabetic diseases. Increased amounts of urinary protein in DN may be associated with functional and morphological alterations of podocyte, mainly including podocyte hypertrophy, epithelial-mesenchymal transdifferentiation (EMT), podocyte detachment, and podocyte apoptosis. Accumulating studies have revealed that disruption in multiple renal signaling pathways had been critical in the progression of these pathological damages, such as adenosine monophosphate-activated kinase signaling pathways (AMPK), wnt/β-catenin signaling pathways, endoplasmic reticulum stress-related signaling pathways, mammalian target of rapamycin (mTOR)/autophagy pathway, and Rho GTPases. In this review, we highlight new molecular insights underlying podocyte injury in the progression of DN, which offer new therapeutic targets to develop important renoprotective treatments for DN over the next decade.

Cyclin-dependent kinase 5 contributes to endoplasmic reticulum stress induced podocyte apoptosis via promoting MEKK1 phosphorylation at Ser280 in diabetic nephropathy

Endoplasmic reticulum (ER) stress has been reported to be associated with podocyte apoptosis in diabetic nephropathy, but the mechanism of ER signaling in podocyte apoptosis hasn't been fully understood. Our previous studies have demonstrated that Cyclin-dependent kinase 5 (Cdk5) was associated with podocyte apoptosis in diabetic nephropathy. The present study was designed to examine whether and how Cdk5 activity plays a role in ER stress induced podocyte apoptosis in diabetic nephropathy. The results showed that along with induction of Cdk5 and apoptosis, GRP78 and its two sensors as well as CHOP and cleaved caspase-12 were induced in high glucose treated podocytes. These responses were attenuated by treated salubrinal. The ER stress inducer, tunicamycin, also up-regulated the kinase activity and protein expression of Cdk5 in podocytes accompanied with the increasing of GRP78. On the other hand, Cdk5 phosphorylates MEKK1 at Ser280 in tunicamycin treated podocytes, and together, they increase the JNK phosphorylation. Moreover, disruption of this pathway can decrease the podocyte apoptosis induced by tunicamycin. Therefore, our study proved that Cdk5 may play an important role in ER stress induced podocyte apoptosis through MEKK1/JNK pathway in diabetic nephropathy.

PTEN Inhibits High Glucose-Induced Phenotypic Transition in Podocytes

Accumulating evidence has suggested that podocytes undergo epithelial-mesenchymal transition (EMT) in diabetic nephropathy (DN). However, the underlying mechanisms of EMT in podocyte are not well understood. PI3K/Akt pathway is involved in the progression of DN. In the present study, we demonstrated that PI3K/Akt pathway was activated in podocytes exposed to high glucose conditions, accompanied by down-regulation of the podocalyxin (PCX) and nephrin expression and up-regulation of the desmin and α-smooth muscle actin (α-SMA) expression. Inhibition of PI3K/Akt pathway by chemical LY294002 or Phosphase and tensin homology deleted on chromosome ten (PTEN) prevented the phenotypic transition. These findings indicate that PTEN/PI3K/Akt pathway mediates high glucose-induced phenotypic transition in podocytes.

The Expression of miR-192 and Its Significance in Diabetic Nephropathy Patients with Different Urine Albumin Creatinine Ratio

OBJECTIVE

To investigate the expression of miR-192 and its significance in diabetic nephropathy (DN) patients.

METHODS

464 patients with type 2 diabetes mellitus (T2DM) were divided into normal albuminuria group (NA, n = 157), microalbuminuria group (MA, n = 159), and large amount of albuminuria group (LA, n = 148). 127 healthy persons were selected as the control group (NC, n = 127). The serum miR-192 levels were detected by Real-Time PCR and transforming growth factor-β1 (TGF-β1) and fibronectin (FN) were detected by enzyme-linked immunosorbent assay. The relationships among these parameters were analyzed by Pearson correlation analysis and multiple linear regression analysis.

RESULTS

The miR-192 in the LA group was significantly lower than other groups, which was lower in the MA group than in the NA group (P < 0.01). The TGF-β1 and FN in the LA group were significantly higher than other groups, which were higher in the MA group than in the NA group (P < 0.01). The expression of miR-192 was negatively correlated with TGF-β1, FN, and Ln (UACR) and miR-192, TGF-β1, and FN were independent relevant factors affecting Ln (UACR) in T2DM (P < 0.01).

CONCLUSIONS

These findings indicate that the levels of miR-192 were lower accompanied by the decrease of urine albumin creatinine ratio (UACR) and the association between miR-192 and nephritic fibrosis in DN.

Bestrophin-3 is differently expressed in normal and injured mouse glomerular podocytes

AIM

Bestrophins are putative calcium-activated chloride channels. Recently, cell-protective functions for Bestrophin-3 (Best3) were proposed. Best3 exists in different splice variants. We have here examined expression, alternative splicing and localization of Best3 in mouse podocytes under normal conditions and during endoplasmic reticulum (ER) stress.

METHODS

Best3 expression was determined on the mRNA level using quantitative PCR and on the protein level by immunohistochemistry and Western blotting.

RESULTS

Staining for Best3 was pronounced in glomeruli and was detected in cultured mouse podocytes. Best3 did not co-localize with markers for endothelial cells (CD31), podocyte foot processes (synaptopodin) or microtubules (actin). However, immunogold-based electron microscopy and co-localization with nestin showed Best3 presence in podocyte primary processes and cell bodies. Only two splice variants of Best3 mRNA (both lacking exons 2 and 3, and one also lacking exon 6), but no full-length variant, were detected. ER stress induced by lipopolysaccharides in vivo transiently elevated mRNA levels of total Best3 and its two splice variants with different time courses. In cultured podocytes under ER stress induced by thapsigargin, the expression of total Best3, its splice variants and nestin transiently increased with similar time courses. The ER stress marker C/EBP homologous protein (CHOP) and nestin mRNA increased during ER stress in vivo and in vitro.

CONCLUSIONS

Best3 is localized intracellularly in cell bodies and primary processes of mouse podocytes and is co-localized with nestin. Two splice variants of Best3 are expressed in glomeruli and in cultured podocytes, and their expression is differentially regulated in ER stress.

The Role of Cdk5 in Alzheimer's Disease

Alzheimer's disease (AD) is known as the most fatal chronic neurodegenerative disease in adults along with progressive loss of memory and other cognitive function disorders. Cyclin-dependent kinase 5 (Cdk5), a unique member of the cyclin-dependent kinases (Cdks), is reported to intimately associate with the process of the pathogenesis of AD. Cdk5 is of vital importance in the development of CNS and neuron movements such as neuronal migration and differentiation, synaptic functions, and memory consolidation. However, when neurons suffer from pathological stimuli, Cdk5 activity becomes hyperactive and causes aberrant hyperphosphorylation of various substrates of Cdk5 like amyloid precursor protein (APP), tau and neurofilament, resulting in neurodegenerative diseases like AD. Deregulation of Cdk5 contributes to an array of pathological events in AD, ranging from formation of senile plaques and neurofibrillary tangles, synaptic damage, mitochondrial dysfunction to cell cycle reactivation as well as neuronal cell apoptosis. More importantly, an inhibition of Cdk5 activity with inhibitors such as RNA inference (RNAi) could protect from memory decline and neuronal cell loss through suppressing β-amyloid (Aβ)-induced neurotoxicity and tauopathies. This review will briefly describe the above-mentioned possible roles of Cdk5 in the physiological and pathological mechanisms of AD, further discussing recent advances and challenges in Cdk5 as a therapeutic target.

Nestin depletion induces melanoma matrix metalloproteinases and invasion

Matrix metalloproteinases (MMPs) are key biological mediators of processes as diverse as wound healing, embryogenesis, and cancer progression. Although MMPs may be induced through multiple signaling pathways, the precise mechanisms for their regulation in cancer are incompletely understood. Because cytoskeletal changes are known to accompany MMP expression, we sought to examine the potential role of the poorly understood cytoskeletal protein, nestin, in modulating melanoma MMPs. Nestin knockdown (KD) upregulated the expression of specific MMPs and MMP-dependent invasion both through extracellular matrix barriers in vitro and in peritumoral connective tissue of xenografts in vivo. The development of three-dimensional melanospheres that in vitro partially recapitulate noninvasive tumorigenic melanoma growth was inhibited by nestin KD, although ECM invasion by aberrant melanospheres that did form was enhanced. Mechanistically, nestin KD-dependent melanoma invasion was associated with intracellular redistribution of phosphorylated focal adhesion kinase and increased melanoma cell responsiveness to transforming growth factor-beta, both implicated in pathways of melanoma invasion. The results suggest that the heretofore poorly understood intermediate filament, nestin, may serve as a novel mediator of MMPs critical to melanoma virulence.

Exploiting the cytoskeletal filaments of neoplastic cells to potentiate a novel therapeutic approach

Although cytoskeletal-directed agents have been a mainstay in chemotherapeutic protocols due to their ability to readily interfere with the rapid mitotic progression of neoplastic cells, they are all microtubule-based drugs, and there has yet to be any microfilament- or intermediate filament-directed agents approved for clinical use. There are many inherent differences between the cytoskeletal networks of malignant and normal cells, providing an ideal target to attain preferential damage. Further, numerous microfilament-directed agents, and an intermediate filament-directed agent of particular interest (withaferin A) have demonstrated in vitro and in vivo efficacy, suggesting that cytoskeletal filaments may be exploited to supplement chemotherapeutic approaches currently used in the clinical setting. Therefore, this review is intended to expose academics and clinicians to the tremendous variety of cytoskeletal filament-directed agents that are currently available for further chemotherapeutic evaluation. The mechanisms by which microfilament directed- and intermediate filament-directed agents damage malignant cells are discussed in detail in order to establish how the drugs can be used in combination with each other, or with currently approved chemotherapeutic agents to generate a substantial synergistic attack, potentially establishing a new paradigm of chemotherapeutic agents.

DBA/2J mice are susceptible to diabetic nephropathy and diabetic exacerbation of IOP elevation

Some pathological manifestations of diabetes in the eye include retinopathy, cataracts and elevated intraocular pressure (IOP). Loss of retinal ganglion cells (RGCs) in non-proliferative stages of diabetic retinopathy and small increases in IOP in diabetic patients has raised the possibility that diabetes affects the development and progression of ocular hypertension and glaucoma. The Ins2Akita mutation is known to cause diabetes and retinopathy on a C57BL/6J (B6) background by as early as 3 months of age. Here, the impact of the Akita mutation on glaucoma was assessed using DBA/2J (D2) mice, a widely used mouse model of ocular hypertension induced glaucoma. In D2.Ins2Akita/+ mice, the contribution of diabetes to vascular permeability, IOP elevation, RGC loss, and glaucoma development was assessed. D2.Ins2Akita/+ mice developed a severe diabetic nephropathy and early mortality between 6-8 months of age. This agrees with previous reports showing that the D2 background is more susceptible to diabetes than the B6 background. In addition, D2.Ins2Akita/+ mice had vascular leakage, astrocyte reactivity and a significant increase in IOP. However no RGC loss and no anterograde axonal transport dysfunction were found at 8.5 months of age. Therefore, our data show that despite severe diabetes and an increased IOP compared to controls, RGCs do not lose axon transport or degenerate. This may be due to a DBA/2J-specific genetic modifier(s) that could provide novel and important avenues for developing new therapies for diabetic retinopathy and possibly glaucoma.

Nestin downregulation in rat vascular smooth muscle cells represents an early marker of vascular disease in experimental type I diabetes

Background

Nestin was reported to directly contribute to cell proliferation and the intermediate filament protein was detected in vascular smooth muscle cells. In experimental type I diabetes, nestin downregulation in the heart was identified as an incipient pathophysiological event. The following study tested the hypothesis that dysregulation of nestin expression in vascular smooth muscle cells represented an early event of vascular disease in experimental type I diabetes.

Methods/Results

In the carotid artery and aorta of adult male Sprague-Dawley rats, a subpopulation of vascular smooth muscle cells co-expressed nestin and was actively involved in the cell cycle as reflected by the co-staining of nuclear phosphohistone-3. The infection of aortic vascular smooth muscle cells with a lentivirus containing a shRNAmir directed against nestin significantly reduced protein expression and concomitantly attenuated basal DNA synthesis. Two weeks following injection of adult male Sprague-Dawley rats with streptozotocin, the endothelial response of aortic rings to acetylcholine, vascular morphology and the total density of vascular smooth muscle cells in the vasculature of type I diabetic rats were similar to normal rats. By contrast, nestin protein levels and the density of nestin⁽⁺⁾/phosphohistone-3⁽⁺⁾-vascular smooth muscle cells were significantly reduced in type I diabetic rats. The in vivo observations were recapitulated in vitro as exposure of vascular smooth muscle cells to 30 mM D-glucose inhibited DNA synthesis and concomitantly reduced nestin protein expression.

Conclusions

Hyperglycaemia-mediated nestin downregulation and the concomitant reduction of cycling vascular smooth muscle cells represent early markers of vascular disease in experimental type I diabetes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-014-0119-6) contains supplementary material, which is available to authorized users.

Nestin expression in mesenchymal stromal cells: regulation by hypoxia and osteogenesis

Background

The intermediate filament protein nestin is used as a marker for neural stem cells, and its expression is inversely correlated with cellular differentiation. More recently, nestin expression has also been described in other cell types including multipotential mesenchymal stromal cells (MSCs). In this study, we examined the expression of nestin in equine, canine and human bone marrow-derived MSCs undergoing osteogenic differentiation, to determine whether nestin levels were attenuated as the cells acquired a more mature phenotype. In addition, the expression of nestin may be under the influence of cellular hypoxia, as nestin expression is known to increase in areas of ischemic tissue damage. Therefore, we also examined the effects of hypoxia on expression of nestin in human MSCs and examined a role for hypoxia inducible factor 1-alpha (HIF-1α) and vascular endothelial growth factor (VEGF) in the response. Additionally, we quantified the temporal expression of nestin in the fracture callus during bone regeneration, a site that has been characterized as hypoxic.

Results

There were no significant changes in nestin expression in MSCs during osteogenic differentiation. There was a significant increase in expression of nestin mRNA and protein in human MSCs in response to hypoxia (1% O₂) or the chemical hypoxia mimetic desferroxamine. This may be due to upregulation of VEGF under hypoxia, as treatment of cells with the VEGF receptor antagonist CPO-P11 attenuated hypoxia-induced nestin expression. A significant increase in nestin mRNA expression was observed in the fracture callus of mice three and seven days post fracture.

Conclusions

Nestin was not a selective marker for MSCs, as its expression was maintained during osteogenic differentiation, in all species examined. Furthermore our data suggest that nestin expression can be induced by hypoxia, and that this increase in nestin is partially regulated by HIF-1α and VEGF. Interestingly, nestin levels were significantly upregulated at the fracture site. Further studies are required to understand the role of nestin in bone cell biology and ultimately bone regeneration.

MicroRNA-29a promotion of nephrin acetylation ameliorates hyperglycemia-induced podocyte dysfunction

Podocyte dysfunction is a detrimental feature in diabetic nephropathy, with loss of nephrin integrity contributing to diabetic podocytopathy. MicroRNAs (miRs) reportedly modulate the hyperglycemia-induced perturbation of renal tissue homeostasis. This study investigated whether regulation of histone deacetylase (HDAC) actions and nephrin acetylation by miR-29 contributes to podocyte homeostasis and renal function in diabetic kidneys. Hyperglycemia accelerated podocyte injury and reduced nephrin, acetylated nephrin, and miR-29a levels in primary renal glomeruli from streptozotocin-induced diabetic mice. Diabetic miR-29a transgenic mice had better nephrin levels, podocyte viability, and renal function and less glomerular fibrosis and inflammation reaction compared with diabetic wild-type mice. Overexpression of miR-29a attenuated the promotion of HDAC4 signaling, nephrin ubiquitination, and urinary nephrin excretion associated with diabetes and restored nephrin acetylation. Knockdown of miR-29a by antisense oligonucleotides promoted HDAC4 action, nephrin loss, podocyte apoptosis, and proteinuria in nondiabetic mice. In vitro, interruption of HDAC4 signaling alleviated the high glucose-induced apoptosis and inhibition of nephrin acetylation in podocyte cultures. Furthermore, HDAC4 interference increased the acetylation status of histone H3 at lysine 9 (H3K9Ac), the enrichment of H3K9Ac in miR-29a proximal promoter, and miR-29a transcription in high glucose-stressed podocytes. In conclusion, hyperglycemia impairs miR-29a signaling to intensify HDAC4 actions that contribute to podocyte protein deacetylation and degradation as well as renal dysfunction. HDAC4, via epigenetic H3K9 hypoacetylation, reduces miR-29a transcription. The renoprotective effects of miR-29a in diabetes-induced loss of podocyte integrity and renal homeostasis highlights the importance of post-translational acetylation reactions in podocyte microenvironments. Increasing miR-29a action may protect against diabetic podocytopathy.

Interplay between the Notch and PI3K/Akt pathways in high glucose-induced podocyte apoptosis

Podocyte apoptosis contributes to the pathogenesis of diabetic nephropathy (DN). However, the mechanisms that mediate high glucose (HG)-induced podocyte apoptosis remain poorly understood. Conditionally immortalized mouse podocytes were cultured in HG medium. A chemical inhibitor or a specific short-hairpin RNA (shRNA) vector was used to inhibit the activation of the Notch pathway and the PI3K/Akt pathway in HG-treated podocytes. Western blotting and real-time PCR were used to evaluate the levels of Notch, PI3K/Akt, and apoptotic pathway signaling. The apoptosis rate of HG-treated podocytes was assessed by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling and annexin V/propidium iodide staining. In HG-treated podocytes, PI3K/Akt pathway activation prevented podocyte apoptosis in the early stage of HG stimulation and Notch pathway-induced podocyte apoptosis in the late stage of HG stimulation. The inhibition of the Notch pathway or the activation of the PI3K/Akt pathway prevented cell apoptosis in HG-treated podocytes. These findings suggest that the Notch and PI3K/Akt pathways may mediate HG-induced podocyte apoptosis.

Notch pathway is involved in high glucose-induced apoptosis in podocytes via Bcl-2 and p53 pathways

Recent studies have shown that Notch pathway plays a key role in the pathogenesis of diabetic nephropathy (DN), however, the exact mechanisms remain elusive. Here we demonstrated that high glucose (HG) upregulated Notch pathway in podocytes accompanied with the alteration of Bcl-2 and p53 pathways, subsequently leading to podocytes apoptosis. Inhibition of Notch pathway by chemical inhibitor or specific short hairpin RNA (shRNA) vector in podocytes prevented Bcl-2- and p53-dependent cell apoptosis. These findings suggest that Notch pathway mediates HG-induced podocytes apoptosis via Bcl-2 and p53 pathways.

Identification and cytoprotective function of a novel nestin isoform, Nes-S, in dorsal root ganglia neurons

In this study, the first nestin isoform, Nes-S, was identified in neurons of dorsal root ganglia (DRG) of adult rats. Nes-S cannot form filaments by itself in cytoplasmic intermediate filament-free SW13 cells. Instead, it co-assembles into filaments with vimentin when transfected into vimentin(+) SW13 cells, and with peripherin and neurofilament proteins when transfected into N2a cells. In primary DRG neurons, endogenous Nes-S co-assembles with peripherin and neurofilament proteins. The expression of Nes-S first appears in DRG at postnatal day 5 and persists to adulthood. Among the adult tissues we examined, the expression of Nes-S is restricted to the sensory and motor neurons. Finally, exogenous Nes-S enhances viability when transfected into N2a cells, and knockdown of endogenous Nes-S impairs the survival of DRG neurons in primary cultures. Taken together, Nes-S is a new neuronal intermediate filament protein that exerts a cytoprotective function in mature sensory and motor neurons.