The Role of Tubule-Interstitial Crosstalk in Renal Injury and Recovery

Claudin-2 Mediates the Proximal Tubular Epithelial Cell-Fibroblast Crosstalk via Paracrine CTGF

Purpose

Proximal tubular epithelial cell (PTEC) is vulnerable to injury in diabetic kidney disease (DKD) due to high energy expenditure. The injured PTECs-derived profibrotic factors are thought to be driving forces in tubulointerstitial fibrosis (TIF) as they activate surrounding fibroblasts. However, the mechanisms remain unclear.

Methods

The diabetes with uninephrectomy (DKD) rats were used to evaluated renal histological changes and the expression of Claudin-2 by immunofluorescence staining. Then, Claudin-2 expression in PTECs were modulated and subsequently determined the proliferation and activation of fibroblasts by building a transwell co-culture system in normal glucose (NG)or high glucose (HG) condition.

Results

Decreased expression of Claudin-2 in PTECs accompanied by tight junction disruption and increased interstitial fibrosis, were detected in DKD rats. In vitro, downregulated Claudin-2 in PTECs promoted proliferation and activation of fibroblasts, which coincided with elevated expression of profibrotic connective tissue growth factor (CTGF) in PTECs. Silenced CTGF inhibited the profibrotic of PTECs via Claudin-2 inhibition. Fibroblasts co-cultured with PTECs transitioned more to myofibroblasts and generated extracellular matrix (ECM) significantly in response to high glucose (HG) stimulation whereas overexpression of Claudin-2 in PTECs reversed the above results. Upregulating CTGF disrupted the beneficial anti-fibrosis effects obtained by overexpression of Claudin-2 in HG condition.

Conclusion

Our study suggested that Claudin-2 in PTECs, a key mediator of paracellular cation and water transport, promotes the activation and proliferation of surrounding fibroblasts significantly via CTGF in a paracrine manner.

Longitudinal tracking of acute kidney injury reveals injury propagation along the nephron

Acute kidney injury (AKI) is an important risk factor for chronic kidney disease (CKD), but the underlying mechanisms of failed tubule repair and AKI-CKD transition are incompletely understood. In this study, we aimed for dynamic tracking of tubule injury and remodeling to understand if focal injury upon AKI may spread over time. Here, we present a model of AKI, in which we rendered only half of the kidney ischemic. Using serial intravital 2-photon microscopy and genetic identification of cycling cells, we tracked dynamic tissue remodeling in post- and non-ischemic kidney regions simultaneously and over 3 weeks. Spatial and temporal analysis of cycling cells relative to initial necrotic cell death demonstrated pronounced injury propagation and expansion into non-necrotic tissue regions, which predicted tubule atrophy with epithelial VCAM1 expression. In summary, our longitudinal analyses of tubule injury, remodeling, and fate provide important insights into AKI pathology.

The role of the SGK3/TOPK signaling pathway in the transition from acute kidney injury to chronic kidney disease

Introduction: Profibrotic phenotype of renal tubular epithelial cells (TECs) featured with epithelial to mesenchymal transition (EMT) and profibrotic factors secretion, and aberrant accumulation of CD206⁺ M2 macrophages are the key points in the transition from acute kidney injury (AKI) to chronic kidney disease (CKD). Nevertheless, the underlying mechanisms involved remain incompletely understood. Serum and glucocorticoid-inducible kinase (SGK) is a serine/threonine protein kinase, required for intestinal nutrient transport and ion channels modulation. T-LAK-cell-originated protein kinase (TOPK) is a member of the mitogen activated protein kinase family, linked to cell cycle regulation. However, little is known about their roles in AKI-CKD transition. Methods: In this study, three models were constructed in C57BL/6 mice: low dose and multiple intraperitoneal injection of cisplatin, 5/6 nephrectomy and unilateral ureteral obstruction model. Rat renal tubular epithelial cells (NRK-52E) were dealt with cisplatin to induce profibrotic phenotype, while a mouse monocytic cell line (RAW264.7) were cultured with cisplatin or TGF-β1 to induce M1 or M2 macrophage polarization respectively. And co-cultured NRK-52E and RAW264.7 through transwell plate to explore the interaction between them. The expression of SGK3 and TOPK phosphorylation were detected by immunohistochemistry, immunofluorescence and western blot analysis. Results: In vivo, the expression of SGK3 and p-TOPK were gradually inhibited in TECs, but enhanced in CD206⁺ M2 macrophages. In vitro, SGK3 inhibition aggravated epithelial to mesenchymal transition through reducing the phosphorylation state of TOPK, and controlling TGF-β1 synthesis and secretion in TECs. However, SGK3/TOPK axis activation promoted CD206⁺ M2 macrophage polarization, which caused kidney fibrosis by mediating macrophage to myofibroblast transition (MMT). When co-cultured, the TGF-β1 from profibrotic TECs evoked CD206⁺ M2 macrophage polarization and MMT, which could be attenuated by SGK3/TOPK axis inhibition in macrophages. Conversely, SGK3/TOPK signaling pathway activation in TECs could reverse CD206⁺ M2 macrophages aggravated EMT. Discussion: We revealed for the first time that SGK3 regulated TOPK phosphorylation to mediate TECs profibrotic phenotype, macrophage plasticity and the crosstalk between TECs and macrophages during AKI-CKD transition. Our results demonstrated the inverse effect of SGK3/TOPK signaling pathway in profibrotic TECs and CD206⁺ M2 macrophages polarization during the AKI-CKD transition.

Epigenetic memory contributing to the pathogenesis of AKI-to-CKD transition

Epigenetic memory, which refers to the ability of cells to retain and transmit epigenetic marks to their daughter cells, maintains unique gene expression patterns. Establishing programmed epigenetic memory at each stage of development is required for cell differentiation. Moreover, accumulating evidence shows that epigenetic memory acquired in response to environmental stimuli may be associated with diverse diseases. In the field of kidney diseases, the “memory” of acute kidney injury (AKI) leads to progression to chronic kidney disease (CKD); epidemiological studies show that patients who recover from AKI are at high risk of developing CKD. The underlying pathological processes include nephron loss, maladaptive epithelial repair, inflammation, and endothelial injury with vascular rarefaction. Further, epigenetic alterations may contribute as well to the pathophysiology of this AKI-to-CKD transition. Epigenetic changes induced by AKI, which can be recorded in cells, exert long-term effects as epigenetic memory. Considering the latest findings on the molecular basis of epigenetic memory and the pathophysiology of AKI-to-CKD transition, we propose here that epigenetic memory contributing to AKI-to-CKD transition can be classified according to the presence or absence of persistent changes in the associated regulation of gene expression, which we designate “driving” memory and “priming” memory, respectively. “Driving” memory, which persistently alters the regulation of gene expression, may contribute to disease progression by activating fibrogenic genes or inhibiting renoprotective genes. This process may be involved in generating the proinflammatory and profibrotic phenotypes of maladaptively repaired tubular cells after kidney injury. “Priming” memory is stored in seemingly successfully repaired tubular cells in the absence of detectable persistent phenotypic changes, which may enhance a subsequent transcriptional response to the second stimulus. This type of memory may contribute to AKI-to-CKD transition through the cumulative effects of enhanced expression of profibrotic genes required for wound repair after recurrent AKI. Further understanding of epigenetic memory will identify therapeutic targets of future epigenetic intervention to prevent AKI-to-CKD transition.

The Extracellular Matrix Environment of Clear Cell Renal Cell Carcinoma

Simple Summary

The extracellular matrix (ECM) controls fundamental properties of tumors, including growth, blood vessel investment, and invasion. The ECM defines rigidity of tumor tissue and individual ECM proteins have distinct biological effects on tumor cells. This article reviews the composition and biological functions of the ECM of clear cell renal cell carcinoma (ccRCC). The most frequent initiating genetic mutation in ccRCC inactivates the VHL gene, which plays a direct role in organizing the ECM. This is predicted to result in local ECM modification, which promotes the growth of tumor cells and the invasion of blood vessels. Later in tumor growth, connective tissue cells are recruited, which are predicted to produce large amounts of ECM, affecting the growth and invasive behaviors of tumor cells. Strategies to therapeutically control the ECM are under active investigation and a better understanding of the ccRCC ECM will determine the applicability of ECM-modifying drugs to this type of cancer.

Abstract

The extracellular matrix (ECM) of tumors is a complex mix of components characteristic of the tissue of origin. In the majority of clear cell renal cell carcinomas (ccRCCs), the tumor suppressor VHL is inactivated. VHL controls matrix organization and its loss promotes a loosely organized and angiogenic matrix, predicted to be an early step in tumor formation. During tumor evolution, cancer-associated fibroblasts (CAFs) accumulate, and they are predicted to produce abundant ECM. The ccRCC ECM composition qualitatively resembles that of the healthy kidney cortex in which the tumor arises, but there are important differences. One is the quantitative difference between a healthy cortex ECM and a tumor ECM; a tumor ECM contains a higher proportion of interstitial matrix components and a lower proportion of basement membrane components. Another is the breakdown of tissue compartments in the tumor with mixing of ECM components that are physically separated in healthy kidney cortex. Numerous studies reviewed in this work reveal effects of specific ECM components on the growth and invasive behaviors of ccRCCs, and extrapolation from other work suggests an important role for ECM in controlling ccRCC tumor rigidity, which is predicted to be a key determinant of invasive behavior.

Mesenchymal Stem Cells Alleviate Renal Fibrosis and Inhibit Autophagy via Exosome Transfer of miRNA-122a

Exosomes derived from mesenchymal stem cell (MSC) alleviate kidney damage through autophagy. This study determined whether MSCs relieve renal fibrosis and inhibit autophagy by exosome transfer of miRNA-122a. The gene expression involved in the mTOR signaling pathway and autophagy was assessed in TGF-β1-treated human renal tubular epithelial cells (HK-2) and unilateral ureteral obstruction (UUO) mice before and after MSC-derived exosomes and miRNA-122a mimic treatment. Small RNA (sRNA) next-generation sequencing was also performed on TGF-β1-treated HK-2 cells. MSC-derived exosomes relieve fibrosis caused by TGFβ in HK-2 via regulation of the mTOR signaling pathway and downstream autophagy. Furthermore, we found that MSC-derived exosomes mediate miRNA-122a to relieve renal fibrosis in HK-2 cells in response to TGF-β1 through the regulation of mTOR signaling and autophagy. In the UUO mouse model, miRNA-122a mimic-transfected MSC treatment and its combination with 3-MA both recapitulated the same results as the in vitro experiments, along with reduced expansion of renal tubule, interstitial expansion, and preservation of kidney architecture. The antifibrotic activity of MSC-derived exosomes after renal fibrosis occurs partially by autophagy suppression via excreted exosomes containing mainly miRNA-122a. These findings indicate that the export of miRNA-122a via MSC-derived exosomes represents a novel strategy to alleviate renal fibrosis.

Mechanistic Pathogenesis of Endothelial Dysfunction in Diabetic Nephropathy and Retinopathy

Diabetic nephropathy (DN) and diabetic retinopathy (DR) are microvascular complications of diabetes. Microvascular endothelial cells are thought to be the major targets of hyperglycemic injury. In diabetic microvasculature, the intracellular hyperglycemia causes damages to the vascular endothelium, via multiple pathophysiological process consist of inflammation, endothelial cell crosstalk with podocytes/pericytes and exosomes. In addition, DN and DR diseases development are involved in several critical regulators including the cell adhesion molecules (CAMs), the vascular endothelial growth factor (VEGF) family and the Notch signal. The present review attempts to gain a deeper understanding of the pathogenesis complexities underlying the endothelial dysfunction in diabetes diabetic and retinopathy, contributing to the development of new mechanistic therapeutic strategies against diabetes-induced microvascular endothelial dysfunction.

Tackling the effects of extracellular vesicles in fibrosis

Fibrosis is a physiological process of tissue repair that turns into pathological when becomes chronic, damaging the functional structure of the tissue. In this review we outline the current status of extracellular vesicles as modulators of the fibrotic process at different levels. In adipose tissue, extracellular vesicles mediate the intercellular communication not only between adipocytes, but also between adipocytes and other cells of the stromal vascular fraction. Thus, they could be altering essential processes for the functionality of adipose tissue, such as adipocyte hypertrophy/hyperplasia, tissue plasticity, adipogenesis and/or inflammation, and ultimately trigger fibrosis. This process is particularly important in obesity, and may eventually, influence the development of obesity-associated alterations. In this regard, obesity is now recognized as an independent risk factor for the development of chronic kidney disease, although the role of extracellular vesicles in this connection has not been explored so far. Nonetheless, the role of extracellular vesicles in the onset and progression of renal fibrosis has been highlighted due to the critical role of fibrosis as a common feature of kidney diseases. In fact, the content of extracellular vesicles disturbs cellular signaling cascades involved in fibrosis in virtually all types of renal cells. What is certain is that the study of extracellular vesicles is complex, as their isolation and manipulation is still difficult to reproduce, which complicates the overview of their physiopathological effects. Nevertheless, new strategies have been developed to exploit the potential of extracellular vesicles and their cargo, both as biomarkers and as therapeutic tools to prevent the progression of fibrosis towards an irreversible event.

In vivo evidence for therapeutic applications of beclin 1 to promote recovery and inhibit fibrosis after acute kidney injury

Autophagy regulator beclin 1 activity determines the severity of kidney damage induced by ischemia reperfusion injury, but its role in kidney recovery and fibrosis are unknown and its therapeutic potentials have not been tested. Here, we explored beclin 1 effects on kidney fibrosis in three models of acute kidney injury (AKI)-ischemia reperfusion injury, cisplatin kidney toxicity, and unilateral ureteric obstruction in mouse strains with three levels of beclin 1 function: normal (wild type), low (heterozygous global deletion of beclin 1, Becn1^+/-), and high beclin 1 activity (knockin gain-of-function mutant Becn1, Becn1^FA). Fourteen days after AKI induction, heterozygous mice had more, but knockin mice had less kidney fibrosis than wild-type mice did. One day after ischemia reperfusion injury, heterozygous pan-kidney tubular Becn1 null mice had more severe kidney damage than homozygous distal tubular Becn1 null mice did, which was similar to the wild-type mice, implying that proximal tubular beclin 1 protects the kidney against ischemia reperfusion injury. By 14 days, both pan-kidney heterozygous Becn1 null and distal tubular homozygous Becn1 null mice had poorer kidney recovery than wild-type mice did. Injection of beclin 1 peptides increased cell proliferation in kidney tubules in normal mice. Beclin 1 peptides injection either before or after (2-5 days) ischemia reperfusion injury protected the kidney from injury and suppressed kidney fibrosis. Thus, both endogenous beclin 1 protein expression in kidney tubules and exogenous beclin 1 peptides are kidney protective via attenuation of acute kidney damage, promotion of cell proliferation, and inhibition of kidney fibrosis, consequently improving kidney recovery post-AKI. Hence, exogenous beclin 1 peptide may be a potential new therapy for AKI.

Identification of YAP1 as a novel downstream effector of the FGF2/STAT3 pathway in the pathogenesis of renal tubulointerstitial fibrosis

Chronic kidney disease is a global health problem and eventually develops into an end-stage renal disease (ESRD). It is now widely believed that renal tubulointerstitial fibrosis (TIF) plays an important role in the progression of ESRD. Renal tubular epithelial-mesenchymal transition (EMT) is an important cause of TIF. Studies have shown that FGF2 is highly expressed in fibrotic renal tissue, although the mechanism remains unclear. We found that FGF2 can activate STAT3 and induce EMT in renal tubular epithelial cells. STAT3, an important transcription factor, was predicted by the JASPAR biological database to bind to the promoter region of YAP1. In this study, STAT3 was shown to promote the expression of the downstream target gene YAP1 through transcription, promote EMT of renal tubular epithelial cells, and mediate the occurrence of renal TIF. This study provides a theoretical basis for the involvement of the FGF2/STAT3/YAP1 signaling pathway in the process of renal interstitial fibrosis and provides a potential target for the treatment of renal fibrosis.

Impairment in renal medulla development underlies salt wasting in Clc-k2 channel deficiency

The prevailing view is that the ClC-Ka chloride channel (mouse Clc-k1) functions in the thin ascending limb to control urine concentration, whereas the ClC-Kb channel (mouse Clc-k2) functions in the thick ascending limb (TAL) to control salt reabsorption. Mutations of ClC-Kb cause classic Bartter syndrome, characterized by renal salt wasting, with perinatal to adolescent onset. We studied the roles of Clc-k channels in perinatal mouse kidneys using constitutive or inducible kidney-specific gene ablation and 2D and advanced 3D imaging of optically cleared kidneys. We show that Clc-k1 and Clc-k2 were broadly expressed and colocalized in perinatal kidneys. Deletion of Clc-k1 and Clc-k2 revealed that both participated in NKCC2- and NCC-mediated NaCl reabsorption in neonatal kidneys. Embryonic deletion of Clc-k2 caused tubular injury and impaired renal medulla and TAL development. Inducible deletion of Clc-k2 beginning after medulla maturation produced mild salt wasting resulting from reduced NCC activity. Thus, both Clc-k1 and Clc-k2 contributed to salt reabsorption in TAL and distal convoluted tubule (DCT) in neonates, potentially explaining the less-severe phenotypes in classic Bartter syndrome. As opposed to the current understanding that salt wasting in adult patients with Bartter syndrome is due to Clc-k2 deficiency in adult TAL, our results suggest that it originates mainly from defects occurring in the medulla and TAL during development.

A Novel Standardized Method of Renal Biopsy in Mice

Introduction

In animal models, it can be difficult to confirm therapeutic effects due to technical inconsistencies and other reasons. Although renal biopsy is widely used in clinical diagnosis, it is rarely used in animal experimental models, especially in mice, because the problems of surgery-induced renal injury and bleeding have not been solved.

Methods

We developed an easy-to-use method of renal biopsy in mice and evaluated whether 3 consecutive renal biopsies can be performed in the same kidney in a standardized manner. This method was verified using 2 mouse models, a healthy mouse model and a unilateral ureteral obstruction (UUO) mouse model, and evaluated based on renal function and histological changes.

Results

There were no perioperative complications in any of the model mice. There was no significant difference in serum Cr, 24-h urine protein, or kidney/body weight ratio between the biopsy and control groups. Each biopsy sample contained sufficient renal tissue. The damage to the operated tissue was limited to the tissue near the biopsy site. Compared with renal tissues in the corresponding control group, the renal tissues obtained from the 3 biopsies (healthy model days 0, 4, and 7 and UUO model days 3, 7, and 10) and the remnant renal tissues after the biopsy showed no significant difference in the glomerular sclerosis index, degree of renal tubular damage, inflammatory response and renal fibrosis in these 2 models.

Conclusions

Our new standardized method of renal biopsy in mice is a safe and cost-saving approach that allows repeated renal biopsies and ensures minimal trauma and sufficient sample size to quality in experimental disease models.

Sox9 promotes renal tubular epithelial‑mesenchymal transition and extracellular matrix aggregation via the PI3K/AKT signaling pathway

Sox9 is important for multiple aspects of development, such as testis, pancreas and heart development. Previous studies have reported that Sox9 induced epithelial‑mesenchymal transition (EMT) and extracellular matrix (ECM) production in organ fibrosis and associated diseases, such as vascular calcification. However, to the best of our knowledge, the role and underlying mechanism of action of Sox9 in renal fibrogenesis remains unknown. The results of the present study revealed that Sox9 expression levels were upregulated in the tubular epithelial cells of a rat model of obstructive nephropathy. Furthermore, the overexpression of Sox9 in NRK‑52E cells was discovered to promote renal tubular EMT and ECM aggregation, and these fibrogenic actions were potentiated by TGF‑β1. Notably, RNA‑sequencing analysis indicated the possible regulatory role of the PI3K/AKT signaling pathway in Sox9‑mediated renal tubular EMT and ECM aggregation. It was further demonstrated that the expression levels of phosphorylated AKT were upregulated in NRK‑52E cells overexpressing Sox9, while the PI3K inhibitors, LY29002 and wortmannin, inhibited the renal tubular EMT and ECM aggregation induced by the overexpression of Sox9 in NEK‑52E cells. In conclusion, the findings of the present study suggested that Sox9 may serve a profibrotic role in the development of renal tubular EMT and ECM aggregation via the PI3K/AKT signaling pathway. Therefore, Sox9 may be considered as a promising target for treating renal fibrosis.

A Porthole over AKI: Cell Dynamics in Damage and Repair

Acute kidney injury (AKI) is associated with an increased risk of CKD. Injury-induced multifaceted renal cell-to-cell crosstalk can either lead to successful self-repair or chronic fibrosis and inflammation. In this mini-review, we will discuss critical renal cell types acting as victims or executioners in AKI pathology and introduce intravital imaging as a powerful technique to further dissect these cell-to-cell interactions.