Dopamine D2 receptor modulates Wnt expression and control of cell proliferation

The Wnt/β-catenin pathway is one of the most conserved signaling pathways across species with essential roles in development, cell proliferation, and disease. Wnt signaling occurs at the protein level and via β-catenin-mediated transcription of target genes. However, little is known about the underlying mechanisms regulating the expression of the key Wnt ligand Wnt3a or the modulation of its activity. Here, we provide evidence that there is significant cross-talk between the dopamine D2 receptor (D2R) and Wnt/β-catenin signaling pathways. Our data suggest that D2R-dependent cross-talk modulates Wnt3a expression via an evolutionarily-conserved TCF/LEF site within the WNT3A promoter. Moreover, D2R signaling also modulates cell proliferation and modifies the pathology in a renal ischemia/reperfusion-injury disease model, via its effects on Wnt/β-catenin signaling. Together, our results suggest that D2R is a transcriptional modulator of Wnt/β-catenin signal transduction with broad implications for health and development of new therapeutics.

Proximal tubule ATR regulates DNA repair to prevent maladaptive renal injury responses

Maladaptive proximal tubule (PT) repair has been implicated in kidney fibrosis through induction of cell-cycle arrest at G2/M. We explored the relative importance of the PT DNA damage response (DDR) in kidney fibrosis by genetically inactivating ataxia telangiectasia and Rad3-related (ATR), which is a sensor and upstream initiator of the DDR. In human chronic kidney disease, ATR expression inversely correlates with DNA damage. ATR was upregulated in approximately 70% of Lotus tetragonolobus lectin-positive (LTL+) PT cells in cisplatin-exposed human kidney organoids. Inhibition of ATR resulted in greater PT cell injury in organoids and cultured PT cells. PT-specific Atr-knockout (ATRRPTC-/-) mice exhibited greater kidney function impairment, DNA damage, and fibrosis than did WT mice in response to kidney injury induced by either cisplatin, bilateral ischemia-reperfusion, or unilateral ureteral obstruction. ATRRPTC-/- mice had more cells in the G2/M phase after injury than did WT mice after similar treatments. In conclusion, PT ATR activation is a key component of the DDR, which confers a protective effect mitigating the maladaptive repair and consequent fibrosis that follow kidney injury.

Inactivation of Tsc2 in Abcg2 lineage-derived cells drives the appearance of polycystic lesions and fibrosis in the adult kidney

Tuberous sclerosis complex 2 (TSC2), or tuberin, is a pivotal regulator of the mechanistic target of rapamycin signaling pathway that controls cell survival, proliferation, growth, and migration. Loss of Tsc2 function manifests in organ-specific consequences, the mechanisms of which remain incompletely understood. Recent single cell analysis of the kidney has identified ATP-binding cassette G2 (Abcg2) expression in renal proximal tubules of adult mice as well as a in a novel cell population. The impact in adult kidney of Tsc2 knockdown in the Abcg2-expressing lineage has not been evaluated. We engineered an inducible system in which expression of truncated Tsc2, lacking exons 36-37 with an intact 3' region and polycystin 1, is driven by Abcg2. Here, we demonstrate that selective expression of Tsc2fl36-37 in the Abcg2pos lineage drives recombination in proximal tubule epithelial and rare perivascular mesenchymal cells, which results in progressive proximal tubule injury, impaired kidney function, formation of cystic lesions, and fibrosis in adult mice. These data illustrate the critical importance of Tsc2 function in the Abcg2-expressing proximal tubule epithelium and mesenchyme during the development of cystic lesions and remodeling of kidney parenchyma.

The Extracellular Matrix Receptor Discoidin Domain Receptor 1 Regulates Collagen Transcription by Translocating to the Nucleus

BACKGROUND

The discoidin domain receptor 1 (DDR1) is activated by collagens, upregulated in injured and fibrotic kidneys, and contributes to fibrosis by regulating extracellular matrix production, but how DDR1 controls fibrosis is poorly understood. DDR1 is a receptor tyrosine kinase (RTK). RTKs can translocate to the nucleus via a nuclear localization sequence (NLS) present on the receptor itself or a ligand it is bound to. In the nucleus, RTKs regulate gene expression by binding chromatin directly or by interacting with transcription factors.

METHODS

To determine whether DDR1 translocates to the nucleus and whether this event is mediated by collagen-induced DDR1 activation, we generated renal cells expressing wild-type or mutant forms of DDR1 no longer able to bind collagen. Then, we determined the location of the DDR1 upon collagen stimulation. Using both biochemical assays and immunofluorescence, we analyzed the steps involved in DDR1 nuclear translocation.

RESULTS

We show that although DDR1 and its natural ligand, collagen, lack an NLS, DDR1 is present in the nucleus of injured human and mouse kidney proximal tubules. We show that DDR1 nuclear translocation requires collagen-mediated receptor activation and interaction of DDR1 with SEC61B, a component of the Sec61 translocon, and nonmuscle myosin IIA and β-actin. Once in the nucleus, DDR1 binds to chromatin to increase the transcription of collagen IV, a major collagen upregulated in fibrosis.

CONCLUSIONS

These findings reveal a novel mechanism whereby activated DDR1 translates to the nucleus to regulate synthesis of profibrotic molecules.

Dynamic signature of lymphangiogenesis during acute kidney injury and chronic kidney disease

Acute kidney injury (AKI) and chronic kidney disease (CKD) are interconnected syndromes with significant attributable morbidity and mortality. The disturbing trend of increasing incidence and prevalence of these clinical disorders highlights the urgent need for better understanding of the underlying mechanisms that are involved in pathogenesis of these conditions. Lymphangiogenesis and its involvement in various inflammatory conditions is increasingly recognized while its role in AKI and CKD remains to be fully elucidated. Here, we studied lymphangiogenesis in three models of kidney injury. Our results demonstrate that the main ligands for lymphangiogenesis, VEGF-C and VEGF-D, are abundantly present in tubules at baseline conditions and the expression pattern of these ligands is significantly altered following injury. In addition, we show that both of these ligands increase in serum and urine post-injury and suggest that such increment may serve as novel urinary biomarkers of AKI as well as in progression of kidney disease. We also provide evidence that irrespective of the nature of initial insult, lymphangiogenic pathways are rapidly and robustly induced as evidenced by higher expression of lymphatic markers within the kidney.

Role of carbonic anhydrase in acute recovery following renal ischemia reperfusion injury

Ischemia reperfusion (IR) injury can cause acute kidney injury. It has previously been reported that kidney oxygen consumption (QO2) in relation to glomerular filtration rate (GFR), and thus tubular sodium load, is markedly increased following IR injury, indicating reduced electrolyte transport efficiency. Since proximal tubular sodium reabsorption (TNa) is a major contributor to overall kidney QO2, we investigated whether inhibition of proximal tubular sodium transport through carbonic anhydrase (CA) inhibition would improve renal oxygenation following ischemia reperfusion. Anesthetized adult male Sprague Dawley rats were administered the CA inhibitor acetazolamide (50 mg/kg bolus iv), or volume-matched vehicle, and kidney function, hemodynamics and QO2 were estimated before and after 45 minutes of unilateral complete warm renal ischemia. CA inhibition per se reduced GFR (-20%) and TNa (-22%), while it increased urine flow and urinary sodium excretion (36-fold). Renal blood flow was reduced (-31%) due to increased renal vascular resistance (+37%) without affecting QO2. IR per se resulted in similar decrease in GFR and TNa, independently of CA activity. However, the QO2/TNa ratio following ischemia-reperfusion was profoundly increased in the group receiving CA inhibition, indicating a significant contribution of basal oxygen metabolism to the total kidney QO2 following inhibition of proximal tubular function after IR injury. Ischemia increased urinary excretion of kidney injury molecule-1, an effect that was unaffected by CA. In conclusion, this study demonstrates that CA inhibition further impairs renal oxygenation and does not protect tubular function in the acute phase following IR injury. Furthermore, these results indicate a major role of the proximal tubule in the acute recovery from an ischemic insult.

lncRNA MALAT1 mediated high glucose-induced HK-2 cell epithelial-to-mesenchymal transition and injury

Epithelial-to-mesenchymal transition (EMT) and injury of tubular cells play critical roles in the pathogenesis of diabetic nephropathy (DN). lncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) has been shown to be involved in DN progression. However, whether MALAT1 induces EMT and injury in tubular cells is unclear. Here, we investigated the effects of MALAT1 on human proximal tubular cells (HK-2 cells) and the underlying mechanism. We performed qPCR to detect MALAT1, E-cadherin, α-smooth muscle actin (α-SMA), kidney injury molecule 1 (KIM-1), and neutrophil gelatinase-associated lipocalin (NGAL). Additionally, we conducted Western blot analyses to measure E-cadherin, α-SMA, cyclin D1, c-Myc, and β-catenin in HK-2 cells cultured with normal glucose and high glucose (HG) and in transfected cells or cells treated with LiCl and DKK-1. The β-catenin localization was observed using immunofluorescence, and the protein levels of NGAL and KIM-1 were evaluated by ELISA. We found that HG-upregulated MALAT1 decreased E-cadherin and increased α-SMA, KIM-1, NGAL, cyclin D1, c-Myc, and β-catenin in HK-2 cells. LiCl exposure increased the expression of α-SMA but decreased that of E-cadherin on the base of knocking down MALAT1, and decreased NGAL and KIM-1 expression. DKK-1 showed the opposite effects. Our results suggested that upregulated MALAT1 induced EMT in HG-treated HK-2 cells through activating the Wnt/β-catenin pathway. However, MALAT1-mediated injury in HK-2 cells was not mediated by activation of the Wnt/β-catenin pathway. Our results indicate that MALAT1 might serve as a novel therapeutic target for suppressing the progression of DN.

Activation of the N-methyl-d-aspartate receptor is involved in glyphosate-induced renal proximal tubule cell apoptosis

Glyphosate-based herbicides have been used worldwide for decades and have been suggested to induce nephrotoxicity, but the underlying mechanism is not yet clear. In this study, we treated a human renal proximal tubule cell line (HK-2) with glyphosate for 24 hours at concentrations of 0, 20, 40 and 60 μm. Glyphosate was found to reduce cell viability and induce apoptosis and oxidative stress in a dose-dependent manner. Because the chemical structures of glyphosate and those of its metabolite AMPA are similar to glycine and glutamate, which are agonists of the N-methyl-d-aspartate receptor (NMDAR), we investigated the potential role of the NMDAR pathway in mediating the proapoptotic effect of glyphosate on proximal tubule cells. We found that NMDAR1 expression, as well as intracellular Ca2+ ([Ca2+ ]i ) and reactive oxygen species (ROS) levels, increased after glyphosate treatment. Blocking NMDAR attenuated glyphosate-induced upregulation of [Ca2+ ]i and ROS levels as well as apoptosis. Meanwhile, inhibition of [Ca2+ ]i reduced glyphosate-induced ROS and apoptosis, and inhibition of ROS alleviated glyphosate-induced apoptosis. In mice exposed to 400 mg/kg glyphosate, the urine low molecular weight protein levels started to increase from day 7. Upregulation of apoptosis and NMDAR1 expression in renal proximal tubule epithelium and an imbalance of oxidant and antioxidative products were observed. These results strongly suggest that activation of the NMDAR1 pathway, together with its downstream [Ca2+ ]i and oxidative stress, is involved in glyphosate-induced renal proximal tubule epithelium apoptosis.

Interaction between galectin-3 and cystinosin uncovers a pathogenic role of inflammation in kidney involvement of cystinosis

Inflammation is involved in the pathogenesis of many disorders. However, the underlying mechanisms are often unknown. Here, we test whether cystinosin, the protein involved in cystinosis, is a critical regulator of galectin-3, a member of the β-galactosidase binding protein family, during inflammation. Cystinosis is a lysosomal storage disorder and, despite ubiquitous expression of cystinosin, the kidney is the primary organ impacted by the disease. Cystinosin was found to enhance lysosomal localization and degradation of galectin-3. In Ctns-/- mice, a mouse model of cystinosis, galectin-3 is overexpressed in the kidney. The absence of galectin-3 in cystinotic mice ameliorates pathologic renal function and structure and decreases macrophage/monocyte infiltration in the kidney of the Ctns-/-Gal3-/- mice compared to Ctns-/- mice. These data strongly suggest that galectin-3 mediates inflammation involved in kidney disease progression in cystinosis. Furthermore, galectin-3 was found to interact with the pro-inflammatory cytokine Monocyte Chemoattractant Protein-1, which stimulates the recruitment of monocytes/macrophages, and proved to be significantly increased in the serum of Ctns-/- mice and also patients with cystinosis. Thus, our findings highlight a new role for cystinosin and galectin-3 interaction in inflammation and provide an additional mechanistic explanation for the kidney disease of cystinosis. This may lead to the identification of new drug targets to delay cystinosis progression.

Proximal Tubule-Specific Deletion of the NHE3 (Na+/H+ Exchanger 3) in the Kidney Attenuates Ang II (Angiotensin II)-Induced Hypertension in Mice

The present study directly tested the hypothesis that the NHE3 (Na⁺/H⁺ exchanger 3) in the proximal tubules of the kidney is required for the development of Ang II (angiotensin II)-induced hypertension using PT-Nhe3^-/- (proximal tubule-specific NHE3 knockout) mice. Specifically, PT-Nhe3^-/- mice were generated using the SGLT2-Cre/Nhe3^loxlox approach, whereas Ang II-induced hypertension was studied in 12 groups (n=5-12 per group) of adult male and female wild-type (WT) and PT-Nhe3^-/- mice. Under basal conditions, systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure were significantly lower in male and female PT-Nhe3^-/- than WT mice (P<0.01). A high pressor, 1.5 mg/kg per day, intraperitoneal or a slow pressor dose of Ang II, 0.5 mg/kg per day, intraperitoneal for 2 weeks significantly increased systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure in male and female WT mice (P<0.01), but the hypertensive response to Ang II was markedly attenuated in male and female PT-Nhe3^-/- mice (P<0.01). Ang II impaired the pressure-natriuresis response in WT mice, whereas proximal tubule-specific deletion of NHE3 improved the pressure-natriuresis response in Ang II-infused PT-Nhe3^-/- mice (P<0.01). AT₁ receptor blocker losartan completely blocked Ang II-induced hypertension in both WT and PT-Nhe3^-/- mice (P<0.01). However, inhibition of nitric oxide synthase with L-N^G-Nitroarginine methyl ester had no effect on Ang II-induced hypertension in WT or PT-Nhe3^-/- mice (not significant). Furthermore, Ang II-induced hypertension was significantly attenuated by an orally absorbable NHE3 inhibitor AVE0657. In conclusion, NHE3 in the proximal tubules of the kidney may be a therapeutical target in hypertension induced by Ang II or with increased NHE3 expression in the proximal tubules.

A virus-induced kidney disease model based on organ-on-a-chip: Pathogenesis exploration of virus-related renal dysfunctions

Renal dysfunctions usually happen in viral infections and many viruses specially infect distal renal tubules, however the pathogenesis remains unknown. Here, in order to explore the pathogenesis of virus-related renal dysfunctions, a Pseudorabies Virus (PrV) induced kidney disease model was built on a distal tubule-on-a-chip (DTC), for the first time. The barrier structure and Na reabsorption of distal renal tubules were successfully reconstituted in DTCs. After PrV infection, results showed electrolyte regulation dysfunction in Na reabsorption for the disordered Na transporters, the broken reabsorption barrier, and the transformed microvilli. And it would lead to virus induced serum electrolyte abnormalities. This work brought us a new cognition about the advantages of organ-on-a-chip (OOC) in virus research, for it had given us a better insight into the pathogenesis of virus induced dysfunctions, based on its unique ability in function reproduction.

Anti-Apoptotic Effect of G-Protein-Coupled Receptor 40 Activation on Tumor Necrosis Factor-α-Induced Injury of Rat Proximal Tubular Cells

G-protein-coupled receptor 40 (GPR40) has an anti-apoptotic effect in pancreatic β-cells. However, its role in renal tubular cell apoptosis remains unclear. To explore the role of GPR40 in renal tubular apoptosis, a two-week unilateral ureteral obstruction (UUO) mouse model was used. The protein expression of GPR40 was decreased, while the Bax/Bcl-2 protein expression ratio, the expression of tumor necrosis factor (TNF)-α mRNA, and angiotensin II type 1 receptor (AT1R) protein were increased in mice with UUO. In vitro, pretreatment of rat proximal tubular (NRK52E) cells with GW9508, a GPR40 agonist, attenuated the decreased cell viability, increased the Bax/Bcl-2 protein expression ratio, increased protein expression of cleaved caspase-3 and activated the nuclear translocation of nuclear factor-κB (NF-κB) p65 subunit induced by TNF-α treatment. TNF-α treatment significantly increased the expression of AT1R protein and the generation of reactive oxygen species (ROS), whereas GW9508 treatment markedly reversed these effects. Pretreatment with GW1100, a GPR40 antagonist, or silencing of GPR40 in NRK52E cells promoted the increased expression of the cleaved caspase-3 protein by TNF-α treatment. Our results demonstrate that decreased expression of GPR40 is associated with apoptosis via TNF-α and AT1R in the ureteral obstructed kidney. The activation of GPR40 attenuates TNF-α-induced apoptosis by inhibiting AT1R expression and ROS generation through regulation of the NF-κB signaling pathway.

Gene deletion of the Na+-glucose cotransporter SGLT1 ameliorates kidney recovery in a murine model of acute kidney injury induced by ischemia-reperfusion

Renal Na+-glucose cotransporter SGLT1 mediates glucose reabsorption in the late proximal tubule, a hypoxia-sensitive tubular segment that enters the outer medulla. Gene deletion in mice (Sglt1-/-) was used to determine the role of the cotransporter in acute kidney injury induced by ischemia-reperfusion (IR), including the initial injury and subsequent recovery phase. On days 1 and 16 after IR, absolute and fractional urinary glucose excretion remained greater in Sglt1-/- mice versus wild-type (WT) littermates, consistent with a sustained contribution of SGLT1 to tubular glucose reabsorption in WT mice. Absence of SGLT1 did not affect the initial kidney impairment versus WT mice, as indicated by similar increases on day 1 in plasma concentrations of creatinine and urinary excretion of the tubular injury marker kidney injury molecule-1 as well as a similar rise in plasma osmolality and fall in urine osmolality as indicators of impaired urine concentration. Recovery of kidney function on days 14/16, however, was improved in Sglt1-/- versus WT mice, as indicated by lower plasma creatinine, higher glomerula filtration rate (by FITC-sinistrin in awake mice), and more completely restored urine and plasma osmolality. This was associated with a reduced tubular injury score in the cortex and outer medulla, better preserved renal mRNA expression of tubular transporters (Sglt2 and Na+-K+-2Cl- cotransporter Nkcc2), and a lesser rise in renal mRNA expression of markers of injury, inflammation, and fibrosis [kidney injury molecule-1, chemokine (C-C motif) ligand 2, fibronectin 1, and collagen type I-α1] in Sglt1-/- versus WT mice. These results suggest that SGLT1 activity in the late proximal tubule may have deleterious effects during recovery of IR-induced acute kidney injury and identify SGLT1 as a potential therapeutic target.

Novel kidney dissociation protocol and image-based flow cytometry facilitate improved analysis of injured proximal tubules

Flow cytometry studies on injured kidney tubules are complicated by the low yield of nucleated single cells. Furthermore, cell-specific responses such as cell cycle dynamics in vivo have conventionally relied on indirect immunohistochemistry and proximal tubule markers that may be downregulated in injury. Here, we report a new tissue dissociation protocol for the kidney with an early fixation step that greatly enhances the yield of single cells. Genetic labeling of the proximal tubule with either mT/mG "tomato" or R26Fucci2aR (Fucci) cell cycle reporter mice allows us to follow proximal tubule-specific changes in cell cycle after renal injury. Image-based flow cytometry (FlowSight) enables gating of the cell cycle and concurrent visualization of the cells with bright field and fluorescence. We used the Fucci mouse in conjunction with FlowSight to identify a discrete polyploid population in proximal tubules after aristolochic acid injury. The tissue dissociation protocol in conjunction with genetic labeling and image-based flow cytometry is a tool that can improve our understanding of any discrete cell population after injury.

Lack of Cathepsin D in the Renal Proximal Tubular Cells Resulted in Increased Sensitivity against Renal Ischemia/Reperfusion Injury

Cathepsin D is one of the major lysosomal aspartic proteases that is essential for the normal functioning of the autophagy-lysosomal system. In the kidney, cathepsin D is enriched in renal proximal tubular epithelial cells, and its levels increase during acute kidney injury. To investigate how cathepsin D-deficiency impacts renal proximal tubular cells, we employed a conditional knockout CtsD^flox/−; Spink3^Cre mouse. Immunohistochemical analyses using anti-cathepsin D antibody revealed that cathepsin D was significantly decreased in tubular epithelial cells of the cortico-medullary region, mainly in renal proximal tubular cells of this mouse. Cathepsin D-deficient renal proximal tubular cells showed an increase of microtubule-associated protein light chain 3 (LC3; a marker for autophagosome/autolysosome)-signals and an accumulation of abnormal autophagic structures. Renal ischemia/reperfusion injury resulted in an increase of early kidney injury marker, Kidney injury molecule 1 (Kim-1), in the cathepsin D-deficient renal tubular epithelial cells of the CtsD^flox/−; Spink3^Cre mouse. Inflammation marker was also increased in the cortico-medullary region of the CtsD^flox/−; Spink3^Cre mouse. Our results indicated that lack of cathepsin D in the renal tubular epithelial cells led to an increase of sensitivity against ischemia/reperfusion injury.

Effect of long non-coding RNA growth arrest-specific 5 on apoptosis in renal ischaemia/reperfusion injury

AIM

Long non-coding RNA (lncRNAs) have been shown to play a critical role in a variety of pathophysiological processes, such as cell proliferation, apoptosis and migration. However, there were few studies addressing the function of lncRNAs in renal ischaemia/reperfusion (I/R) injury. Apoptosis is an important pathogenesis during I/R injury. Here, we identified the effect of hypoxia-responsive lncRNA growth arrest-specific 5 (GAS5) on apoptosis in renal I/R injury.

METHODS

Ischaemia/reperfusion injury in mice or hypoxia/re-oxygenation (H/R) in human proximal renal tubular epithelial cells (HK-2) was practiced to induce apoptosis. The kidneys and blood were collected at 24 h after reperfusion. The GAS5 messenger RNA (mRNA) expression and apoptosis-related gene mRNA and protein levels, including p53, cellular inhibitor of apoptosis protein 2 (cIAP2) and thrombospondin-1 (TSP-1), were analysed. GAS5 small-interfering RNA was transfected with H/R induced cells. Over-expression of GAS5 was performed by plasmid transfection.

RESULTS

Apoptotic cells significantly increased in I/R-injured kidneys. GAS5 could be up-regulated in kidneys at 24 h after reperfusion and 3 h after re-oxygenation, combined with increased expression of its downstream apoptosis-related proteins p53 and cIAP2. GAS5 small-interfering RNA treatment down-regulated the mRNA and protein levels of p53 and TSP-1, and attenuated apoptosis induced by H/R in HK-2 cells. Conversely, over-expression of GAS5 up-regulated the mRNA and protein levels of p53 and TSP-1, and promoted apoptosis in HK-2 cells.

CONCLUSION

Long non-coding RNA GAS5 induced by I/R injury could promote apoptosis in kidney. TSP-1 might be one of the downstream effectors of GAS5, which will be explored in the future.

Proximal Tubular Cell-Specific Ablation of Carnitine Acetyltransferase Causes Tubular Disease and Secondary Glomerulosclerosis

Proximal tubular epithelial cells are highly energy demanding. Their energy need is covered mostly from mitochondrial fatty acid oxidation. Whether derailments in fatty acid metabolism and mitochondrial dysfunction are forerunners of tubular damage has been suggested but is not entirely clear. Here we modeled mitochondrial overload by creating mice lacking the enzyme carnitine acetyltransferase (CrAT) in the proximal tubules, thus limiting a primary mechanism to export carbons under conditions of substrate excess. Mice developed tubular disease and, interestingly, secondary glomerulosclerosis. This was accompanied by increased levels of apoptosis regulator and fibrosis markers, increased oxidative stress, and abnormal profiles of acylcarnitines and organic acids suggesting profound impairments in all major forms of nutrient metabolism. When mice with CrAT deletion were fed a high-fat diet, kidney disease was more severe and developed faster. Primary proximal tubular cells isolated from the knockout mice displayed energy deficit and impaired respiration before the onset of pathology, suggesting mitochondrial respiratory abnormalities as a potential underlying mechanism. Our findings support the hypothesis that derailments of mitochondrial energy metabolism may be causative to chronic kidney disease. Our results also suggest that tubular injury may be a primary event followed by secondary glomerulosclerosis, raising the possibility that focusing on normalizing tubular cell mitochondrial function and energy balance could be an important preventative strategy.

Proximal Tubule β 2-Adrenergic Receptor Mediates Formoterol-Induced Recovery of Mitochondrial and Renal Function after Ischemia-Reperfusion Injury

Acute kidney injury (AKI) is the rapid loss of renal function after an insult, and renal proximal tubule cells (RPTCs) are central to the pathogenesis of AKI. The β 2-adrenergic receptor (β 2AR) agonist formoterol accelerates the recovery of renal function in mice after ischemia-reperfusion injury (IRI) with associated rescue of mitochondrial proteins; however, the cell type responsible for this recovery remains unknown. The role of RPTCs in formoterol-induced recovery of renal function was assessed in a proximal tubule-specific knockout of the β 2AR (γGT-Cre:ADRB2Flox/Flox). These mice and wild-type controls (ADRB2Flox/Flox) were subjected to renal IRI, followed by once-daily dosing of formoterol beginning 24 hours post-IRI and euthanized at 144 hours. Compared with ADRB2Flox/Flox mice, γGT-Cre:ADRB2Flox/Flox mice had decreased renal cortical mRNA expression of the β 2AR. After IRI, formoterol treatment restored renal function in ADRB2Flox/Flox but not γGT-Cre:ADRB2Flox/Flox mice as measured by serum creatinine, histopathology, and expression of kidney injury marker-1 (KIM-1). Formoterol-treated ADRB2Flox/Flox mice exhibited recovery of mitochondrial proteins and DNA copy number, whereas γGT-Cre:ADRB2Flox/Flox mice treated with formoterol did not. Analysis of mitochondrial morphology by transmission electron microscopy demonstrated that formoterol increased mitochondrial number and density in ADRB2Flox/Flox mice but not in γGT-Cre:ADRB2Flox/Flox mice. These data demonstrate that proximal tubule β 2AR regulates renal mitochondrial homeostasis. Formoterol accelerates the recovery of renal function after AKI by activating proximal tubule β 2AR to induce mitochondrial biogenesis and demonstrates the overall requirement of RPTCs in renal recovery.

Canagliflozin Prevents Intrarenal Angiotensinogen Augmentation and Mitigates Kidney Injury and Hypertension in Mouse Model of Type 2 Diabetes Mellitus

BACKGROUND

Hypertension and renal injury are common complications of type 2 diabetes mellitus (T2DM). Hyperglycemia stimulates renal proximal tubular angiotensinogen (AGT) expression via elevated oxidative stress contributing to the development of high blood pressure and diabetic nephropathy. The sodium glucose cotransporter 2 (SGLT2) in proximal tubules is responsible for the majority of glucose reabsorption by renal tubules. We tested the hypothesis that SGLT2 inhibition with canagliflozin (CANA) prevents intrarenal AGT augmentation and ameliorates kidney injury and hypertension in T2DM.

METHODS

We induced T2DM in New Zealand obese mice with a high fat diet (DM, 30% fat) with control mice receiving regular fat diet (ND, 4% fat). When DM mice exhibited > 350 mg/dL blood glucose levels, both DM- and ND-fed mice were treated with 10 mg/kg/day CANA or vehicle by oral gavage for 6 weeks. We evaluated intrarenal AGT, blood pressure, and the development of kidney injury.

RESULTS

Systolic blood pressure in DM mice (133.9 ± 2.0 mm Hg) was normalized by CANA (113.9 ± 4.0 mm Hg). CANA treatment ameliorated hyperglycemia-associated augmentation of renal AGT mRNA (148 ± 21 copies/ng RNA in DM, and 90 ± 16 copies/ng RNA in DM + CANA) and protein levels as well as elevation of urinary 8-isoprostane levels. Tubular fibrosis in DM mice (3.4 ± 0.9-fold, fibrotic score, ratio to ND) was suppressed by CANA (0.9 ± 0.3-fold). Furthermore, CANA attenuated DM associated increased macrophage infiltration and cell proliferation in kidneys of DM mice.

CONCLUSIONS

CANA prevents intrarenal AGT upregulation and oxidative stress and which may mitigate high blood pressure, renal tubular fibrosis, and renal inflammation in T2DM.

Megalin - a facultative marker of obesity-related glomerulopathy in children

Obesity-related glomerulopathy (ORG) is an increasingly detected syndrome present in children with obesity. Megalin, a constitutive proximal tubule cell protein, when present in urine, can be considered as a biomarker indicating renal injury in these children.

Myofibroblasts acquire retinoic acid-producing ability during fibroblast-to-myofibroblast transition following kidney injury

Tubular injury and interstitial fibrosis are the hallmarks of chronic kidney disease. While recent studies have verified that proximal tubular injury triggers interstitial fibrosis, the impact of fibrosis on tubular injury and regeneration remains poorly understood. We generated a novel mouse model expressing diphtheria toxin receptor on renal fibroblasts to allow for the selective disruption of renal fibroblast function. Administration of diphtheria toxin induced upregulation of the tubular injury marker Ngal and caused tubular proliferation in healthy kidneys, whereas administration of diphtheria toxin attenuated tubular regeneration in fibrotic kidneys. Microarray analysis revealed down-regulation of the retinol biosynthesis pathway in diphtheria toxin-treated kidneys. Healthy proximal tubules expressed retinaldehyde dehydrogenase 2 (RALDH2), a rate-limiting enzyme in retinoic acid biosynthesis. After injury, proximal tubules lost RALDH2 expression, whereas renal fibroblasts acquired strong expression of RALDH2 during the transition to myofibroblasts in several models of kidney injury. The retinoic acid receptor (RAR) RARγ was expressed in proximal tubules both with and without injury, and αB-crystallin, the product of an RAR target gene, was strongly expressed in proximal tubules after injury. Furthermore, BMS493, an inverse agonist of RARs, significantly attenuated tubular proliferation in vitro. In human biopsy tissue from patients with IgA nephropathy, detection of RALDH2 in the interstitium correlated with older age and lower kidney function. These results suggest a role of retinoic acid signaling and cross-talk between fibroblasts and tubular epithelial cells during tubular injury and regeneration, and may suggest a beneficial effect of fibrosis in the early response to injury.

Glutathione deficiency alters the vitamin D-metabolizing enzymes CYP27B1 and CYP24A1 in human renal proximal tubule epithelial cells and kidney of HFD-fed mice

Chronic kidney disease (CKD) is a worldwide public health problem with an estimated prevalence of 8.2%. This study reports glutathione deficiency, excess oxidative stress, and altered vitamin D metabolism in the kidney of mice fed a high-fat diet (HFD). The levels of GCLC and GCLM gene expression were significantly downregulated and the protein carbonylation level, a hallmark of oxidative damage, was significantly increased in the kidney of HFD-fed mice. While the levels of VD-regulatory genes 1-alpha-hydroxylase (CYP27B1), VDR, and RXRα were significantly downregulated in the kidney of mice fed a HFD, those of 24-hydroxylase (CYP24A1) were significantly elevated. In vitro, GSH deficiency per se causes excess oxidative damage (protein carbonylation), and significantly decreases the levels of VD-regulatory genes (CYP27B1, VDR, and RXRα), but increases levels of CYP24A1 in human renal proximal tubule epithelial cells (RPTEC), similar to findings in the kidney of HFD-fed diabetic mice. L-cysteine supplementation restores GSH and prevents oxidative damage in RPTEC. These studies suggest a potential role of GSH precursor in reducing excess oxidative stress and renal injury that commonly accompanies obesity/diabetes.

Cyclin G1 and TASCC regulate kidney epithelial cell G2-M arrest and fibrotic maladaptive repair

Fibrosis contributes to the progression of chronic kidney disease (CKD). Severe acute kidney injury can lead to CKD through proximal tubular cell (PTC) cycle arrest in the G₂-M phase, with secretion of profibrotic factors. Here, we show that epithelial cells in the G₂-M phase form target of rapamycin (TOR)-autophagy spatial coupling compartments (TASCCs), which promote profibrotic secretion similar to the senescence-associated secretory phenotype. Cyclin G1 (CG1), an atypical cyclin, promoted G₂-M arrest in PTCs and up-regulated TASCC formation. PTC TASCC formation was also present in humans with CKD. Prevention of TASCC formation in cultured PTCs blocked secretion of profibrotic factors. PTC-specific knockout of a key TASCC component reduced the rate of kidney fibrosis progression in mice with CKD. CG1 induction and TASCC formation also occur in liver fibrosis. Deletion of CG1 reduced G₂-M phase cells and TASCC formation in vivo. This study provides mechanistic evidence supporting how profibrotic G₂-M arrest is induced in kidney injury and how G₂-M-arrested PTCs promote fibrosis, identifying new therapeutic targets to mitigate kidney fibrosis.

ATF6α downregulation of PPARα promotes lipotoxicity-induced tubulointerstitial fibrosis

Tubulointerstitial fibrosis is a strong predictor of progression in patients with chronic kidney disease, and is often accompanied by lipid accumulation in renal tubules. However, the molecular mechanisms modulating the relationship between lipotoxicity and tubulointerstitial fibrosis remain obscure. ATF6α, a transcription factor of the unfolded protein response, is reported to be an upstream regulator of fatty acid metabolism. Owing to their high energy demand, proximal tubular cells (PTCs) use fatty acids as their main energy source. We therefore hypothesized that ATF6α regulates PTC fatty acid metabolism, contributing to lipotoxicity-induced tubulointerstitial fibrosis. Overexpression of activated ATF6α transcriptionally downregulated peroxisome proliferator-activated receptor-α (PPARα), the master regulator of lipid metabolism, leading to reduced activity of fatty acid β-oxidation and cytosolic accumulation of lipid droplets in a human PTC line (HK-2). ATF6α-induced lipid accumulation caused mitochondrial dysfunction, enhanced apoptosis, and increased expression of connective tissue growth factor (CTGF), as well as reduced cell viability. Atf6α-/- mice had sustained expression of PPARα and less tubular lipid accumulation following unilateral ischemia-reperfusion injury (uIRI), resulting in the amelioration of apoptosis; reduced expression of CTGF, α-smooth muscle actin, and collagen I; and less tubulointerstitial fibrosis. Administration of fenofibrate, a PPARα agonist, reduced lipid accumulation and tubulointerstitial fibrosis in the uIRI model. Taken together, these findings suggest that ATF6α deranges fatty acid metabolism in PTCs, which leads to lipotoxicity-mediated apoptosis and CTGF upregulation, both of which promote tubulointerstitial fibrosis.