Modelling and Prevention of Acute Kidney Injury through Ischemia and Reperfusion in a Combined Human Renal Proximal Tubule/Blood Vessel-on-a-Chip

Background

Renal ischemia/reperfusion injury (rIRI) is one of the major causes of AKI. Although animal models are suitable for investigating systemic symptoms of AKI, they are limited in translatability. Human in vitro models are crucial in giving mechanistic insights into rIRI; however, they miss out on crucial aspects such as reperfusion injury and the multitissue aspect of AKI.

Methods

We advanced the current renal proximal tubule-on-a-chip model to a coculture model with a perfused endothelial vessel separated by an extracellular matrix. The coculture was characterized for its three-dimensional structure, protein expression, and response to nephrotoxins. Then, rIRI was captured through control of oxygen levels, nutrient availability, and perfusion flow settings. Injury was quantified through morphologic assessment, caspase-3/7 activation, and cell viability.

Results

The combination of low oxygen, reduced glucose, and interrupted flow was potent to disturb the proximal tubules. This effect was strongly amplified upon reperfusion. Endothelial vessels were less sensitive to the ischemia-reperfusion parameters. Adenosine treatment showed a protective effect on the disruption of the epithelium and on the caspase-3/7 activation.

Conclusions

A human in vitro rIRI model was developed using a coculture of a proximal tubule and blood vessel on-a-chip, which was used to characterize the renoprotective effect of adenosine. The robustness of the model and assays in combination with the throughput of the platform make it ideal to advance pathophysiological research and enable the development of novel therapeutic modalities.

Role of mTOR Signaling for Tubular Function and Disease

The mechanistic target of rapamycin (mTOR) forms two distinct intracellular multiprotein complexes that control a multitude of intracellular processes linked to metabolism, proliferation, actin cytoskeleton, and survival. Recent studies have identified the importance of these complexes for transport regulation of ions and nutrients along the entire nephron. First reports could link altered activity of these complexes to certain disease entities, i.e. diabetic nephropathy, acute kidney injury or hyperkalemia.

Evidence For and Against Direct Kidney Infection by SARS-CoV-2 in Patients with COVID-19

Despite evidence of multiorgan tropism of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in patients with coronavirus disease 2019 (COVID-19), direct viral kidney invasion has been difficult to demonstrate. The question of whether SARS-CoV2 can directly infect the kidney is relevant to the understanding of pathogenesis of AKI and collapsing glomerulopathy in patients with COVID-19. Methodologies to document SARS-CoV-2 infection that have been used include immunohistochemistry, immunofluorescence, RT-PCR, in situ hybridization, and electron microscopy. In our review of studies to date, we found that SARS-CoV-2 in the kidneys of patients with COVID-19 was detected in 18 of 94 (19%) by immunohistochemistry, 71 of 144 (49%) by RT-PCR, and 11 of 84 (13%) by in situ hybridization. In a smaller number of patients with COVID-19 examined by immunofluorescence, SARS-CoV-2 was detected in 10 of 13 (77%). In total, in kidneys from 102 of 235 patients (43%), the presence of SARS-CoV-2 was suggested by at least one of the methods used. Despite these positive findings, caution is needed because many other studies have been negative for SARS-CoV-2 and it should be noted that when detected, it was only in kidneys obtained at autopsy. There is a clear need for studies from kidney biopsies, including those performed at early stages of the COVID-19-associated kidney disease. Development of tests to detect kidney viral infection in urine samples would be more practical as a noninvasive way to evaluate SARS-CoV-2 infection during the evolution of COVID-19-associated kidney disease.

Cadmium-Induced Kidney Injury: Oxidative Damage as a Unifying Mechanism

Cadmium is a nonessential metal that has heavily polluted the environment due to human activities. It can be absorbed into the human body via the gastrointestinal tract, respiratory tract, and the skin, and can cause chronic damage to the kidneys. The main site where cadmium accumulates and causes damage within the nephrons is the proximal tubule. This accumulation can induce dysfunction of the mitochondrial electron transport chain, leading to electron leakage and production of reactive oxygen species (ROS). Cadmium may also impair the function of NADPH oxidase, resulting in another source of ROS. These ROS together can cause oxidative damage to DNA, proteins, and lipids, triggering epithelial cell death and a decline in kidney function. In this article, we also reviewed evidence that the antioxidant power of plant extracts, herbal medicines, and pharmacological agents could ameliorate cadmium-induced kidney injury. Finally, a model of cadmium-induced kidney injury, centering on the notion that oxidative damage is a unifying mechanism of cadmium renal toxicity, is also presented. Given that cadmium exposure is inevitable, further studies using animal models are warranted for a detailed understanding of the mechanism underlying cadmium induced ROS production, and for the identification of more therapeutic targets.

Proximal Tubule p53 in Cold Storage/Transplantation-Associated Kidney Injury and Renal Graft Dysfunction

Kidney injury associated with cold storage/transplantation is a primary factor for delayed graft function and poor outcome of renal transplants. p53 contributes to both ischemic and nephrotoxic kidney injury, but its involvement in kidney cold storage/transplantation is unclear. Here, we report that p53 in kidney proximal tubules plays a critical role in cold storage/transplantation kidney injury and inhibition of p53 can effectively improve the histology and function of transplanted kidneys. In a mouse kidney cold storage/transplantation model, we detected p53 accumulation in proximal tubules in a cold storage time-dependent manner, which correlated with tubular injury and cell death. Pifithrin-α, a pharmacologic p53 inhibitor, could reduce acute tubular injury, apoptosis and inflammation at 24 h after cold storage/transplantation. Similar effects were shown by the ablation of p53 from proximal tubule cells. Notably, pifithrin-α also ameliorated kidney injury and improved the function of transplanted kidneys in 6 days when it became the sole life-supporting kidney in recipient mice. in vitro, cold storage followed by rewarming induced cell death in cultured proximal tubule cells, which was accompanied by p53 activation and suppressed by pifithrin-α and dominant-negative p53. Together, these results support a pathogenic role of p53 in cold storage/transplantation kidney injury and demonstrate the therapeutic potential of p53 inhibitors.

Iron Is Filtered by the Kidney and Is Reabsorbed by the Proximal Tubule

The aim of this study was to determine the iron (Fe) concentration profile within the lumen of the S2 renal proximal convoluted tubule (PCT) and to resolve whether this nephron segment transported Fe. To do this, we performed in vivo renal micropuncture on Wistar rats, collected PCT tubular fluid from superficial nephrons, and measured Fe concentration. The Fe concentration profile along the S2 PCT suggested significant Fe reabsorption. Proximal tubules were also microperfused in vivo with physiological solutions containing Fe and Zn, Cu, Mn, or Cd. PCTs perfused with 12μmol.l^{−1 55}FeCl₃ reabsorbed 105.2±12.7 fmol.mm⁻¹.min⁻¹ Fe, 435±52pmol.mm^-1.min⁻¹ Na, and 2.7±0.2nl.mm⁻¹.min⁻¹ water (mean ± SEM; n=19). Addition of ascorbate (1mmol.l⁻¹) to the perfusate did not significantly alter Fe, Na, or water reabsorption. Supplementing the control perfusate with 60μmol.l⁻¹ FeSO₄ significantly decreased ⁵⁵Fe uptake. Recalculating for the altered molar activity following addition of unlabeled Fe revealed a three-fold increase in Fe flux. Addition to the perfusate 12μmol.l⁻¹ CuSO₄, MnSO_4, CdSO_4, or ZnSO₄ did not affect Fe, Na, or water flux. In conclusion, (1) in vivo, S2 PCTs of rat reabsorb Fe and (2) Fe is reabsorbed along the PCT via a pathway that is insensitive to Cu, Mn, Cd, or Zn. Together, these data demonstrate for the first time the hitherto speculated process of renal Fe filtration and subsequent tubular Fe reabsorption in a living mammal.

Renal Biology Driven Macro- and Microscale Design Strategies for Creating an Artificial Proximal Tubule Using Fiber-Based Technologies

Chronic kidney disease affects one in six people worldwide. Due to the scarcity of donor kidneys and the complications associated with hemodialysis (HD), a cell-based bioartificial kidney (BAK) device is desired. One of the shortcomings of HD is the lack of active transport of solutes that would normally be performed by membrane transporters in kidney epithelial cells. Specifically, proximal tubule (PT) epithelial cells play a major role in the active transport of metabolic waste products. Therefore, a BAK containing an artificial PT to actively transport solutes between the blood and the filtrate could provide major therapeutic advances. Creating such an artificial PT requires a biocompatible tubular structure which supports the adhesion and function of PT-specific epithelial cells. Ideally, this scaffold should structurally replicate the natural PT basement membrane which consists mainly of collagen fibers. Fiber-based technologies such as electrospinning are therefore especially promising for PT scaffold manufacturing. This review discusses the use of electrospinning technologies to generate an artificial PT scaffold for ex vivo/in vivo cellularization. We offer a comparison of currently available electrospinning technologies and outline the desired scaffold properties required to serve as a PT scaffold. Discussed also are the potential technologies that may converge in the future, enabling the effective and biomimetic incorporation of synthetic PTs in to BAK devices and beyond.

Hyperglycemia-induced oxidative stress in isolated proximal tubules of mouse: the in vitro effects of myricitrin and its solid lipid nanoparticle

CONTEXT

The hyperglycemia (Hyper) induces oxidative stress in kidney tubular cells. Myricitrin (Myr) has an antioxidant effect along with low bioavailability.

OBJECTIVE

The present research investigated the effects of Myr and its solid lipid nanoparticles (SLN) on isolated proximal tubules exposed to the hyperglycemic condition.

MATERIALS AND METHODS

In this experimental study, the proximal tubules of mice were dissected by the microdissection method and the tubules were prepared for experimental or Real Time-PCR measurement.

RESULTS

The malondialdehyde level, transforming growth factor-β, nuclear factor kappa B and Bax genes expression increased in Hyper and decreased in Hyper + Myr and its SLN-treated groups compared to Hyper. Superoxide dismutase, total antioxidant capacity, the viability of proximal tubules and Bcl-2 gene expression decreased in untreated Hyper and increased in all treatment groups compared to Hyper.

CONCLUSION

The hyperglycemia-induced oxidative disorder, inflammation and apoptosis in proximal tubules and that administrating Myr and its SLN improved them.

Proximal tubule cyclophilin D mediates kidney fibrogenesis in obstructive nephropathy

The proximal tubule (PT) is highly vulnerable to acute injury, including ischemic insult and nephrotoxins, and chronic kidney injury. It has been established that PT injury is a primary cause of the development of chronic kidney disease, but the underlying molecular mechanism remains to be defined. Here, we tested whether PT cyclophilin D (CypD), a mitochondrial matrix protein, is a critical factor to cause kidney fibrosis progression. To define the role of CypD in kidney fibrosis, we used an established mouse model for kidney fibrosis: the unilateral ureteral obstruction (UUO) model in global and PT-specific CypD knockout (KO). Global CypD KO blunted kidney fibrosis progression with inhibition of myofibroblast activation and fibrosis. UUO-induced tubular atrophy was suppressed in kidneys of global CypD KO but not tubular dilation or apoptotic cell death. PT cell cycle arrest was highly increased in wild-type UUO kidneys but was markedly attenuated in global CypD KO UUO kidneys. The number of macrophages and neutrophils was less in UUO kidneys of global CypD KO than those of wild-type kidneys. Proinflammatory and profibrotic factors were all inhibited in global CypD KO. In line with those of global CypD KO, PT-specific CypD KO also blunted kidney fibrosis progression, along with less tubular atrophy, renal parenchymal loss, cell cycle arrest in PT, and inflammation, indicating a critical role for PT CypD in fibrogenesis. Collectively, our data demonstrate that CypD in the PT is a critical factor contributing to kidney fibrosis in UUO, providing a new paradigm for mitochondria-targeted therapeutics of fibrotic diseases.NEW & NOTEWORTHY It has been established that renal proximal tubule (PT) injury is a primary cause of the development of chronic kidney disease, but the underlying molecular mechanism remains to be defined. Here, we show that cyclophilin D, a mitochondrial matrix protein, in the PT causes kidney fibrogenesis in obstructive nephropathy. Our data suggest that targeting PT cyclophilin D could be beneficial to prevent fibrosis progression.

Single-Nucleus RNA Sequencing Identifies New Classes of Proximal Tubular Epithelial Cells in Kidney Fibrosis

BACKGROUND

Proximal tubular cells (PTCs) are the most abundant cell type in the kidney. PTCs are central to normal kidney function and to regeneration versus organ fibrosis following injury. This study used single-nucleus RNA sequencing (snRNAseq) to describe the phenotype of PTCs in renal fibrosis.

METHODS

Kidneys were harvested from naïve mice and from mice with renal fibrosis induced by chronic aristolochic acid administration. Nuclei were isolated using Nuclei EZ Lysis buffer. Libraries were prepared on the 10× platform, and snRNAseq was completed using the Illumina NextSeq 550 System. Genome mapping was carried out with high-performance computing.

RESULTS

A total of 23,885 nuclei were analyzed. PTCs were found in five abundant clusters, mapping to S1, S1-S2, S2, S2-cortical S3, and medullary S3 segments. Additional cell clusters ("new PTC clusters") were at low abundance in normal kidney and in increased number in kidneys undergoing regeneration/fibrosis following injury. These clusters exhibited clear molecular phenotypes, permitting labeling as proliferating, New-PT1, New-PT2, and (present only following injury) New-PT3. Each cluster exhibited a unique gene expression signature, including multiple genes previously associated with renal injury response and fibrosis progression. Comprehensive pathway analyses revealed metabolic reprogramming, enrichment of cellular communication and cell motility, and various immune activations in new PTC clusters. In ligand-receptor analysis, new PTC clusters promoted fibrotic signaling to fibroblasts and inflammatory activation to macrophages.

CONCLUSIONS

These data identify unrecognized PTC phenotype heterogeneity and reveal novel PTCs associated with kidney fibrosis.

Hypophosphatemia is an independent risk factor for AKI among hospitalized patients with COVID-19 infection

BACKGROUND

This study sought to investigate incidence and risk factors for acute kidney injury (AKI) in hospitalized COVID-19.

METHODS

In this retrospective study, we enrolled 823 COVID-19 patients with at least two evaluations of renal function during hospitalization from four hospitals in Wuhan, China between February 2020 and April 2020. Clinical and laboratory parameters at the time of admission and follow-up data were recorded. Systemic renal tubular dysfunction was evaluated via 24-h urine collections in a subgroup of 55 patients.

RESULTS

In total, 823 patients were enrolled (50.5% male) with a mean age of 60.9 ± 14.9 years. AKI occurred in 38 (40.9%) ICU cases but only 6 (0.8%) non-ICU cases. Using forward stepwise Cox regression analysis, we found eight independent risk factors for AKI including decreased platelet level, lower albumin level, lower phosphorus level, higher level of lactate dehydrogenase (LDH), procalcitonin, C-reactive protein (CRP), urea, and prothrombin time (PT) on admission. For every 0.1 mmol/L decreases in serum phosphorus level, patients had a 1.34-fold (95% CI 1.14-1.58) increased risk of AKI. Patients with hypophosphatemia were likely to be older and with lower lymphocyte count, lower serum albumin level, lower uric acid, higher LDH, and higher CRP. Furthermore, serum phosphorus level was positively correlated with phosphate tubular maximum per volume of filtrate (TmP/GFR) (Pearson r = 0.66, p < .001) in subgroup analysis, indicating renal phosphate loss via proximal renal tubular dysfunction.

CONCLUSION

The AKI incidence was very low in non-ICU patients as compared to ICU patients. Hypophosphatemia is an independent risk factor for AKI in patients hospitalized for COVID-19 infection.

Intratubular, Intracellular, and Mitochondrial Angiotensin II/AT1 (AT1a) Receptor/NHE3 Signaling Plays a Critical Role in Angiotensin II-Induced Hypertension and Kidney Injury

Hypertension is well recognized to be the most important risk factor for cardiovascular diseases, stroke, and end-stage kidney failure. A quarter of the world’s adult populations and 46% of the US adults develop hypertension and currently require antihypertensive treatments. Only 50% of hypertensive patients are responsive to current antihypertensive drugs, whereas remaining patients may continue to develop cardiovascular, stroke, and kidney diseases. The mechanisms underlying the poorly controlled hypertension remain incompletely understood. Recently, we have focused our efforts to uncover additional renal mechanisms, pathways, and therapeutic targets of poorly controlled hypertension and target organ injury using novel animal models or innovative experimental approaches. Specifically, we studied and elucidated the important roles of intratubular, intracellular, and mitochondrial angiotensin II (Ang II) system in the development of Ang II-dependent hypertension. The objectives of this invited article are to review and discuss our recent findings that (a) circulating and intratubular Ang II is taken up by the proximal tubules via the (AT₁) AT_1a receptor-dependent mechanism, (b) intracellular administration of Ang II in proximal tubule cells or adenovirus-mediated overexpression of an intracellular Ang II fusion protein selectively in the mitochonria of the proximal tubules induces blood pressure responses, and (c) genetic deletion of AT₁ (AT_1a) receptors or the Na⁺/H⁺ exchanger 3 selectively in the proximal tubules decreases basal blood pressure and attenuates Ang II-induced hypertension. These studies provide a new perspective into the important roles of the intratubular, intracellular, and mitochondrial angiotensin II/AT₁ (AT_1a) receptor signaling in Ang II-dependent hypertensive kidney diseases.

A validation study of UCSD-Mayo risk score in predicting hospital-acquired acute kidney injury in COVID-19 patients

Introduction

Acute kidney injury (AKI) in coronavirus disease 2019 (COVID-19) patients is associated with poor prognosis. Early prediction and intervention of AKI are vital for improving clinical outcome of COVID-19 patients. As lack of tools for early AKI detection in COVID-19 patients, this study aimed to validate the USCD-Mayo risk score in predicting hospital-acquired AKI in an extended multi-center COVID-19 cohort.

Methods

Five hundred seventy-two COVID-19 patients from Wuhan Tongji Hospital Guanggu Branch, Wuhan Leishenshan Hospital, and Wuhan No. Ninth Hospital was enrolled for this study. Patients who developed AKI or reached an outcome of recovery or death during the study period were included. Predictors were evaluated according to data extracted from medical records.

Results

Of all patients, a total of 44 (8%) developed AKI. The UCSD-Mayo risk score achieved excellent discrimination in predicting AKI with the C-statistic of 0.88 (95%CI: 0.84–0.91). Next, we determined the UCSD-Mayo risk score had good overall performance (Nagelkerke R² = 0.32) and calibration in our cohort. Further analysis showed that the UCSD-Mayo risk score performed well in subgroups defined by gender, age, and several chronic comorbidities. However, the discrimination of the UCSD-Mayo risk score in ICU patients and patients with mechanical ventilation was not good which might be resulted from different risk factors of these patients.

Conclusions

We validated the performance of UCSD-Mayo risk score in predicting hospital-acquired AKI in COVID-19 patients was excellent except for patients from ICU or patients with mechanical ventilation.

High Doses of Essential Oil of Croton Zehntneri Induces Renal Tubular Damage

The essential oil of Croton zehntneri (EOCZ) and its major compounds are known to have several biological activities. However, some evidence shows potential toxic effects of high doses of EOCZ (>300 mg/kg) in amphibian and human kidneys. The aim of the present work was to investigate the effects on renal function of EOCZ at 300 mg/kg/day in healthy Swiss mice and a subclinical acute kidney injury (subAKI) animal model, which presents tubule-interstitial injury (TII). Four experimental groups were generated: (1) CONT group (control); (2) EOCZ, mice treated with EOCZ; (3) subAKI; (4) subAKI+EOCZ, subAKI treated simultaneously with EOCZ. EOCZ treatment induced TII measured by increases in (1) proteinuria; (2) cortical tubule-interstitial space; (3) macrophage infiltration; (4) collagen deposition. A decrease in tubular sodium reabsorption was also observed. These results were similar and nonadditive to those observed in the subAKI group. These data suggest that treatment with EOCZ at higher concentrations induces TII in mice, which could be mediated by protein overload in the proximal tubule.

Ultrastructural identification of developing proximal tubules based on three-dimensional reconstruction

BACKGROUND

The cellular mechanisms involved in the development of proximal tubules are not only associated with morphogenesis in fetal life, but also with restoration of damaged tubules in adulthood. Knowledge about morphological features of cell differentiation and proliferation along the developing tubule is insufficient, which hinders identification of the cellular origin.

OBJECTIVES

This study aimed to investigate ultrastructures of the proximal tubule at different stages of nephrogenesis.

METHODS

Electron microscopy was used and guided by computer-assisted tubular tracing to identify the cellular structures.

RESULTS

Renal vesicles and S-shaped bodies revealed more proliferative features, such as densely-packed fusiform-shaped cells with numerous protein-producing organelles than membrane specializations typical for mature tubules. At the capillary-loop stage the proximal tubules demonstrated all characteristics of the mature tubules, but not as developed, including shorter but densely packed microvilli, fewer lateral processes with cell-cell contacts, lower basal membrane infoldings, and lower mitochondrial volume density. However, they exhibited an elaborated endocytic system above the nucleus, indicating a membrane transport is being established. Abundant free- and endoplasmic reticulum-adhered ribosomes and Golgi complexes reflected active protein synthesis for cell growth and proliferation. Interestingly, electron dense cells were occasionally intermixed with electron lucent cells characterized by various organelles in less cytosol and a larger nucleus with abundant euchromatin, which is a feature of active proliferation.

CONCLUSIONS

These ultrastructures indicate that the morphogenesis of the developing proximal tubule corresponds to the gradually established physiological activities. The two different cellular electron densities may suggest distinctive differentiation of the cells along the tubule.

An efficient method to generate kidney organoids at the air-liquid interface

The prevalence of kidney dysfunction continues to increase worldwide, driving the need to develop transplantable renal tissues. The kidney develops from four major renal progenitor populations: nephron epithelial, ureteric epithelial, interstitial and endothelial progenitors. Methods have been developed to generate kidney organoids but few or dispersed tubular clusters within the organoids hamper its use in regenerative applications. Here, we describe a detailed protocol of asynchronous mixing of kidney progenitors using organotypic culture conditions to generate kidney organoids tightly packed with tubular clusters and major renal structures including endothelial network and functional proximal tubules. This protocol provides guidance in the culture of human embryonic stem cells from a National Institute of Health-approved line and their directed differentiation into kidney organoids. Our 18-day protocol provides a rapid method to generate kidney organoids that facilitate the study of different nephrological events including in vitro tissue development, disease modeling and chemical screening. However, further studies are required to optimize the protocol to generate additional renal-specific cell types, interconnected nephron segments and physiologically functional renal tissues.

Single-nuclear transcriptomics reveals diversity of proximal tubule cell states in a dynamic response to acute kidney injury

A single acute kidney injury event increases the risk of progression to chronic kidney disease (CKD). Combining single-nucleus RNA sequencing with genetic tracing of injured proximal tubule cells identified a spatially dynamic, evolving injury response following ischemia–reperfusion injury. Failed proximal tubule repair leads to the persistence of a profibrotic, proinflammatory Vcam1⁺/Ccl2⁺ cell type exhibiting a senescence-associated secretory phenotype and a marked transcriptional activation of NF-κB and AP-1 pathway signatures, but no signs of G₂/M cell cycle arrest. Insights from this study can inform strategies to improve renal repair and prevent CKD progression.

Acute kidney injury (AKI), commonly caused by ischemia, sepsis, or nephrotoxic insult, is associated with increased mortality and a heightened risk of chronic kidney disease (CKD). AKI results in the dysfunction or death of proximal tubule cells (PTCs), triggering a poorly understood autologous cellular repair program. Defective repair associates with a long-term transition to CKD. We performed a mild-to-moderate ischemia–reperfusion injury (IRI) to model injury responses reflective of kidney injury in a variety of clinical settings, including kidney transplant surgery. Single-nucleus RNA sequencing of genetically labeled injured PTCs at 7-d (“early”) and 28-d (“late”) time points post-IRI identified specific gene and pathway activity in the injury–repair transition. In particular, we identified Vcam1⁺/Ccl2⁺ PTCs at a late injury stage distinguished by marked activation of NF-κB–, TNF-, and AP-1–signaling pathways. This population of PTCs showed features of a senescence-associated secretory phenotype but did not exhibit G₂/M cell cycle arrest, distinct from other reports of maladaptive PTCs following kidney injury. Fate-mapping experiments identified spatially and temporally distinct origins for these cells. At the cortico-medullary boundary (CMB), where injury initiates, the majority of Vcam1⁺/Ccl2⁺ PTCs arose from early replicating PTCs. In contrast, in cortical regions, only a subset of Vcam1⁺/Ccl2⁺ PTCs could be traced to early repairing cells, suggesting late-arising sites of secondary PTC injury. Together, these data indicate even moderate IRI is associated with a lasting injury, which spreads from the CMB to cortical regions. Remaining failed-repair PTCs are likely triggers for chronic disease progression.

Cadmium toxicity mediated by the inhibition of SLC2A4 expression in human proximal Tubule cells

Cadmium (Cd) is an environmental contaminant that causes renal toxicity. We have previously demonstrated that Cd induces renal toxicity by altering transcriptional activities. In this study, we show that Cd markedly inhibited the activity of transcription factor MEF2A in HK-2 human proximal tubule cells, which generated significant cytotoxicity in the cells. This reduction in the nuclear levels of MEF2A protein may be involved in the Cd-induced inhibition of MEF2A activity. We also demonstrate that one of the glucose transporters, GLUT4, was downregulated not only by Cd treatment but also by MEF2A knockdown. Knockdown of SLC2A4, encoding GLUT4, eliminated both cell viability and Cd toxicity. Cd treatment or SLC2A4 deficiency reduced the cellular concentration of glucose. Therefore, the suppression of SLC2A4 expression, which mediates the reduction in cellular glucose, is involved in Cd toxicity. The Cd toxicity induced by the reduction in GLUT4 may be associated with a reduction of cellular ATP levels in HK-2 cells. The levels of Slc2a4 mRNA in the kidney of mice exposed to Cd for 6 or 12 months were significantly lower than those in the control group. These results demonstrate that Cd exerts its cytotoxicity through the suppression in SLC2A4 expression and the subsequent inhibition of MEF2A transcriptional activity. Cd-induced suppression of SLC2A4 expression also reduces cellular ATP levels, partly by reducing glucose levels. This study suggests that the glucose transporter plays an important role in the renal toxicity of Cd, and provides a crucial breakthrough in our understanding of the mechanism of Cd toxicity.

Krüppel-like factor 6-mediated loss of BCAA catabolism contributes to kidney injury in mice and humans

Altered cellular metabolism in kidney proximal tubule (PT) cells plays a critical role in acute kidney injury (AKI). The transcription factor Krüppel-like factor 6 (KLF6) is rapidly and robustly induced early in the PT after AKI. We found that PT-specific Klf6 knockdown (Klf6 ^PTKD) is protective against AKI and kidney fibrosis in mice. Combined RNA and chromatin immunoprecipitation sequencing analysis demonstrated that expression of genes encoding branched-chain amino acid (BCAA) catabolic enzymes was preserved in Klf6 ^PTKD mice, with KLF6 occupying the promoter region of these genes. Conversely, inducible KLF6 overexpression suppressed expression of BCAA genes and exacerbated kidney injury and fibrosis in mice. In vitro, injured cells overexpressing KLF6 had similar decreases in BCAA catabolic gene expression and were less able to utilize BCAA. Furthermore, knockdown of BCKDHB, which encodes one subunit of the rate-limiting enzyme in BCAA catabolism, resulted in reduced ATP production, while treatment with BCAA catabolism enhancer BT2 increased metabolism. Analysis of kidney function, KLF6, and BCAA gene expression in human chronic kidney disease patients showed significant inverse correlations between KLF6 and both kidney function and BCAA expression. Thus, targeting KLF6-mediated suppression of BCAA catabolism may serve as a key therapeutic target in AKI and kidney fibrosis.

Defects in KCNJ16 Cause a Novel Tubulopathy with Hypokalemia, Salt Wasting, Disturbed Acid-Base Homeostasis, and Sensorineural Deafness

BACKGROUND

The transepithelial transport of electrolytes, solutes, and water in the kidney is a well-orchestrated process involving numerous membrane transport systems. Basolateral potassium channels in tubular cells not only mediate potassium recycling for proper Na⁺,K⁺-ATPase function but are also involved in potassium and pH sensing. Genetic defects in KCNJ10 cause EAST/SeSAME syndrome, characterized by renal salt wasting with hypokalemic alkalosis associated with epilepsy, ataxia, and sensorineural deafness.

METHODS

A candidate gene approach and whole-exome sequencing determined the underlying genetic defect in eight patients with a novel disease phenotype comprising a hypokalemic tubulopathy with renal salt wasting, disturbed acid-base homeostasis, and sensorineural deafness. Electrophysiologic studies and surface expression experiments investigated the functional consequences of newly identified gene variants.

RESULTS

We identified mutations in the KCNJ16 gene encoding KCNJ16, which along with KCNJ15 and KCNJ10, constitutes the major basolateral potassium channel of the proximal and distal tubules, respectively. Coexpression of mutant KCNJ16 together with KCNJ15 or KCNJ10 in Xenopus oocytes significantly reduced currents.

CONCLUSIONS

Biallelic variants in KCNJ16 were identified in patients with a novel disease phenotype comprising a variable proximal and distal tubulopathy associated with deafness. Variants affect the function of heteromeric potassium channels, disturbing proximal tubular bicarbonate handling as well as distal tubular salt reabsorption.

Local and downstream actions of proximal tubule angiotensin II signaling on Na+ transporters in the mouse nephron

The renal nephron consists of a series of distinct cell types that function in concert to maintain fluid and electrolyte balance and blood pressure. The renin-angiotensin system (RAS) is central to Na⁺ and volume balance. We aimed to determine how loss of angiotensin II signaling in the proximal tubule (PT), which reabsorbs the bulk of filtered Na⁺ and volume, impacts solute transport throughout the nephron. We hypothesized that PT renin-angiotensin system disruption would not only depress PT Na⁺ transporters but also impact downstream Na⁺ transporters. Using a mouse model in which the angiotensin type 1a receptor (AT_1aR) is deleted specifically within the PT (AT_1aR PTKO), we profiled the abundance of Na⁺ transporters, channels, and claudins along the nephron. Absence of PT AT_1aR signaling was associated with lower abundance of PT transporters (Na⁺/H⁺ exchanger isoform 3, electrogenic Na⁺-bicarbonate cotransporter 1, and claudin 2) as well as lower abundance of downstream transporters (total and phosphorylated Na⁺-K⁺-2Cl^- cotransporter, medullary Na⁺-K⁺-ATPase, phosphorylated NaCl cotransporter, and claudin 7) versus controls. However, transport activities of Na⁺-K⁺-2Cl^- cotransporter and NaCl cotransporter (assessed with diuretics) were similar between groups in order to maintain electrolyte balance. Together, these results demonstrate the primary impact of angiotensin II regulation on Na⁺ reabsorption in the PT at baseline and the associated influence on downstream Na⁺ transporters, highlighting the ability of the nephron to integrate Na⁺ transport along the nephron to maintain homeostasis.NEW & NOTEWORTHY Our study defines a novel role for proximal tubule angiotensin receptors in regulating the abundance of Na⁺ transporters throughout the nephron, thereby contributing to the integrated control of fluid balance in vivo.

Sugar or Fat? Renal Tubular Metabolism Reviewed in Health and Disease

The kidney is a highly metabolically active organ that relies on specialized epithelial cells comprising the renal tubules to reabsorb most of the filtered water and solutes. Most of this reabsorption is mediated by the proximal tubules, and high amounts of energy are needed to facilitate solute movement. Thus, proximal tubules use fatty acid oxidation, which generates more adenosine triphosphate (ATP) than glucose metabolism, as its preferred metabolic pathway. After kidney injury, metabolism is altered, leading to decreased fatty acid oxidation and increased lactic acid generation. This review discusses how metabolism differs between the proximal and more distal tubular segments of the healthy nephron. In addition, metabolic changes in acute kidney injury and chronic kidney disease are discussed, as well as how these changes in metabolism may impact tubule repair and chronic kidney disease progression.

KIM-1 mediates fatty acid uptake by renal tubular cells to promote progressive diabetic kidney disease

Tubulointerstitial abnormalities are predictive of the progression of diabetic kidney disease (DKD), and their targeting may be an effective means for prevention. Proximal tubular (PT) expression of kidney injury molecule (KIM)-1, as well as blood and urinary levels, are increased early in human diabetes and can predict the rate of disease progression. Here, we report that KIM-1 mediates PT uptake of palmitic acid (PA)-bound albumin, leading to enhanced tubule injury with DNA damage, PT cell-cycle arrest, interstitial inflammation and fibrosis, and secondary glomerulosclerosis. Such injury can be ameliorated by genetic ablation of the KIM-1 mucin domain in a high-fat-fed streptozotocin mouse model of DKD. We also identified TW-37 as a small molecule inhibitor of KIM-1-mediated PA-albumin uptake and showed in vivo in a kidney injury model in mice that it ameliorates renal inflammation and fibrosis. Together, our findings support KIM-1 as a new therapeutic target for DKD.

Pathophysiology of AKI to CKD progression

Acute kidney injury (AKI), defined as a rapid decrease in glomerular filtration rate, is a common and devastating pathologic condition. AKI is associated with significant morbidity and subsequent chronic kidney disease (CKD) development. Regardless of the initial insult, CKD progression after AKI involves multiple types of cells, including proximal tubular cells, fibroblasts, and immune cells. Although the mechanisms underlying this AKI to CKD progression have been investigated extensively over the past decade, therapeutic strategies still are lacking. One of the reasons for this stems from the fact that AKI and its progression toward CKD is multifactorial and variable because it is dependent on patient background. In this review, we describe the current understanding of AKI and its maladaptive repair with a focus on proximal tubules and resident fibroblasts. Subsequently, we discuss the unique pathophysiology of AKI in the elderly, highlighting our recent finding of age-dependent tertiary lymphoid tissues.

The Proximal Tubule Toxicity of Immunoglobulin Light Chains

Plasma and B cells dyscrasias that overproduce monoclonal immunoglobulin free light chains (FLCs) affect the kidney frequently in various ways. The hematologic dyscrasia responsible for the production of FLCs may or may not meet the criteria for cancer, such as multiple myeloma (MM) or lymphoma, or may remain subclinical. If there is overt malignancy, the accompanying kidney disorder is called myeloma- or lymphoma-associated. If the dyscrasia is subclinical, the associated kidney disorders are grouped as monoclonal gammopathy of renal significance. Glomeruli and tubules may both be involved. The proximal tubule disorders comprise a spectrum of interesting syndromes, which range in severity. This review focuses on the recent insights gained into the patterns and the mechanisms of proximal tubule toxicity of FLCs, including subtle transport disorders, such as proximal tubule acidosis, partial or complete Fanconi syndrome, or severe acute or chronic renal failure. Histologically, there may be crystal deposition in the proximal tubule cells, acute tubule injury, interstitial inflammation, fibrosis, and tubule atrophy. Specific structural alterations in the V domain of FLCs caused by somatic hypermutations are responsible for crystal formation as well as partial or complete Fanconi syndrome. Besides crystal formation, tubulointerstitial inflammation and proximal tubulopathy can be mediated by direct activation of inflammatory pathways through cytokines and Toll-like receptors due to cell stress responses induced by excessive FLC endocytosis into the proximal tubule cells. Therapy directed against the clonal source of the toxic light chain can prevent progression to more severe lesions and may help preserve kidney function.

Renal Sodium Handling in Relation to Environmental and Genetic Factors in Untreated Chinese

BACKGROUND

We investigated proximal and distal renal tubular sodium handling, as assessed by fractional excretion of lithium (FELi) and fractional distal reabsorption rate of sodium (FDRNa), in relation to environmental and genetic factors in untreated patients.

METHODS

Our study participants were suspected hypertensive patients being off antihypertensive medication for ≥2 weeks and referred for 24-hour ambulatory blood pressure monitoring. We collected serum and 24-hour urine for measurement of sodium, creatinine, and lithium concentration, and calculated FELi and FDRNa. We genotyped 19 single-nucleotide polymorphisms associated with renal sodium handling or blood pressure using the ABI SNapShot method.

RESULTS

The 1,409 participants (664 men, 47.1%) had a mean (±SD) age of 51.0 ± 10.5 years. After adjustment for host factors, both FELi and FDRNa were significantly (P ≤ 0.01) associated with season and humidity, explaining ~1.3% and ~3.5% of the variance, respectively. FELi was highest in autumn and lowest in summer and intermediate in spring and winter (P = 0.007). FDRNa was also highest in autumn but lowest in winter and intermediate in spring and summer (P < 0.001). Neither FELi nor FDRNa was associated with outdoor temperature or atmospheric pressure (P ≥ 0.13). After adjustment for host and environmental factors and Bonferroni multiple testing, among the 19 studied genetic variants, only rs12513375 was significantly associated with FELi and FDRNa (P ≤ 0.004) and explained about 1.7% of the variance.

CONCLUSIONS

Renal sodium handling as measured by endogenous lithium clearance was sensitive to major environmental and genetic factors. Our finding is toward the use of these indexes for the definition of renal tubular dysfunction.

Urine Metabolite Levels, Adverse Kidney Outcomes, and Mortality in CKD Patients: A Metabolome-wide Association Study

RATIONALE & OBJECTIVE

Mechanisms underlying the variable course of disease progression in patients with chronic kidney disease (CKD) are incompletely understood. The aim of this study was to identify novel biomarkers of adverse kidney outcomes and overall mortality, which may offer insights into pathophysiologic mechanisms.

STUDY DESIGN

Metabolome-wide association study.

SETTING & PARTICIPANTS

5,087 patients with CKD enrolled in the observational German Chronic Kidney Disease Study.

EXPOSURES

Measurements of 1,487 metabolites in urine.

OUTCOMES

End points of interest were time to kidney failure (KF), a combined end point of KF and acute kidney injury (KF+AKI), and overall mortality.

ANALYTICAL APPROACH

Statistical analysis was based on a discovery-replication design (ratio 2:1) and multivariable-adjusted Cox regression models.

RESULTS

After a median follow-up of 4 years, 362 patients died, 241 experienced KF, and 382 experienced KF+AKI. Overall, we identified 55 urine metabolites whose levels were significantly associated with adverse kidney outcomes and/or mortality. Higher levels of C-glycosyltryptophan were consistently associated with all 3 main end points (hazard ratios of 1.43 [95% CI, 1.27-1.61] for KF, 1.40 [95% CI, 1.27-1.55] for KF+AKI, and 1.47 [95% CI, 1.33-1.63] for death). Metabolites belonging to the phosphatidylcholine pathway showed significant enrichment. Members of this pathway contributed to the improvement of the prediction performance for KF observed when multiple metabolites were added to the well-established Kidney Failure Risk Equation.

LIMITATIONS

Findings among patients of European ancestry with CKD may not be generalizable to the general population.

CONCLUSIONS

Our comprehensive screen of the association between urine metabolite levels and adverse kidney outcomes and mortality identifies metabolites that predict KF and represents a valuable resource for future studies of biomarkers of CKD progression.

Proteinuria and Clinical Outcomes in Hospitalized COVID-19 Patients: A Retrospective Single-Center Study

BACKGROUND AND OBJECTIVES

Kidney involvement is frequent among patients with coronavirus disease 2019 (COVID-19), and occurrence of AKI is associated with higher mortality in this population. The objective of this study was to describe occurrence and significance of proteinuria in this setting.

DESIGN , SETTING, PARTICIPANTS MEASUREMENTS

We conducted a single-center retrospective study to describe the characteristic features of proteinuria measured within 48 hours following admission among patients with COVID-19 admitted in a tertiary care hospital in France, and to evaluate its association with initiation of dialysis, intensive care unit admission, and death.

RESULTS

Among 200 patients with available data, urine protein-creatinine ratio at admission was ≥1 g/g for 84 (42%), although kidney function was normal in most patients, with a median serum creatinine of 0.94 mg/dl (interquartile range, 0.75-1.21). Median urine albumin-creatinine ratio was 110 mg/g (interquartile range, 50-410), with a urine albumin-protein ratio <50% in 92% of patients. Urine retinol binding protein concentrations, available for 85 patients, were ≥0.03 mg/mmol in 62% of patients. Urine protein-creatinine ratio ≥1 g/g was associated with initiation of dialysis (odds ratio, 4.87; 95% confidence interval, 2.03 to 13.0; P <0.001), admission to the intensive care unit (odds ratio, 3.55; 95% confidence interval, 1.93 to 6.71; P <0.001), and death (odds ratio, 3.56; 95% confidence interval, 1.90 to 6.54; P <0.001).

CONCLUSIONS

Proteinuria is very frequent among patients admitted for COVID-19 and may precede AKI. Low levels of albuminuria suggest a predominant tubular origin, confirmed by the elevated levels of urine retinol binding protein. Urine protein-creatinine ratio ≥1 g/g at admission is strongly associated with poor kidney and patient outcome.

Enhanced MCP-1 Release in Early Autosomal Dominant Polycystic Kidney Disease

Introduction

Autosomal dominant polycystic kidney disease (ADPKD) causes kidney failure typically in adulthood, but the disease starts in utero. Copeptin, epidermal growth factor (EGF), and monocyte chemoattractant protein-1 (MCP-1) are associated with severity and hold prognostic value in adults but remain unstudied in the early disease stage. Kidneys from adults with ADPKD exhibit macrophage infiltration, and a prominent role of MCP-1 secretion by tubular epithelial cells is suggested from rodent models.

Methods

In a cross-sectional study, plasma copeptin, urinary EGF, and urinary MCP-1 were evaluated in a pediatric ADPKD cohort and compared with age-, sex-, and body mass index (BMI)-matched healthy controls. MCP-1 was studied in mouse collecting duct cells, human proximal tubular cells, and fetal kidney tissue.

Results

Fifty-three genotyped ADPKD patients and 53 controls were included. The mean (SD) age was 10.4 (5.9) versus 10.5 (6.1) years (P = 0.543), and the estimated glomerular filtration rate (eGFR) was 122.7 (39.8) versus 114.5 (23.1) ml/min per 1.73 m² (P = 0.177) in patients versus controls, respectively. Plasma copeptin and EGF secretion were comparable between groups. The median (interquartile range) urinary MCP-1 (pg/mg creatinine) was significantly higher in ADPKD patients (185.4 [213.8]) compared with controls (154.7 [98.0], P = 0.010). Human proximal tubular cells with a heterozygous PKD1 mutation and mouse collecting duct cells with a PKD1 knockout exhibited increased MCP-1 secretion. Human fetal ADPKD kidneys displayed prominent MCP-1 immunoreactivity and M2 macrophage infiltration.

Conclusion

An increase in tubular MCP-1 secretion is an early event in ADPKD. MCP-1 is an early disease severity marker and a potential treatment target.

A key role for the transporter OAT1 in systemic lipid metabolism

Organic anion transporter 1 (OAT1/SLC22A6) is a drug transporter with numerous xenobiotic and endogenous substrates. The Remote Sensing and Signaling Theory suggests that drug transporters with compatible ligand preferences can play a role in “organ crosstalk,” mediating overall organismal communication. Other drug transporters are well known to transport lipids, but surprisingly little is known about the role of OAT1 in lipid metabolism. To explore this subject, we constructed a genome-scale metabolic model using omics data from the Oat1 knockout mouse. The model implicated OAT1 in the regulation of many classes of lipids, including fatty acids, bile acids, and prostaglandins. Accordingly, serum metabolomics of Oat1 knockout mice revealed increased polyunsaturated fatty acids, diacylglycerols, and long-chain fatty acids and decreased ceramides and bile acids when compared with wildtype controls. Some aged knockout mice also displayed increased lipid droplets in the liver when compared with wildtype mice. Chemoinformatics and machine learning analyses of these altered lipids defined molecular properties that form the structural basis for lipid-transporter interactions, including the number of rings, positive charge/volume, and complexity of the lipids. Finally, we obtained targeted serum metabolomics data after short-term treatment of rodents with the OAT-inhibiting drug probenecid to identify potential drug–metabolite interactions. The treatment resulted in alterations in eicosanoids and fatty acids, further supporting our metabolic reconstruction predictions. Consistent with the Remote Sensing and Signaling Theory, the data support a role of OAT1 in systemic lipid metabolism.

Imaging the kidney: from light to super-resolution microscopy

The important achievements in kidney physiological and pathophysiological mechanisms can largely be ascribed to progress in the technology of microscopy. Much of what we know about the architecture of the kidney is based on the fundamental descriptions of anatomic microscopists using light microscopy and later by ultrastructural analysis provided by electron microscopy. These two techniques were used for the first classification systems of kidney diseases and for their constant updates. More recently, a series of novel imaging techniques added the analysis in further dimensions of time and space. Confocal microscopy allowed us to sequentially visualize optical sections along the z-axis and the availability of specific analysis software provided a three-dimensional rendering of thicker tissue specimens. Multiphoton microscopy permitted us to simultaneously investigate kidney function and structure in real time. Fluorescence-lifetime imaging microscopy allowed to study the spatial distribution of metabolites. Super-resolution microscopy increased sensitivity and resolution up to nanoscale levels. With cryo-electron microscopy, researchers could visualize the individual biomolecules at atomic levels directly in the tissues and understand their interaction at subcellular levels. Finally, matrix-assisted laser desorption/ionization imaging mass spectrometry permitted the measuring of hundreds of different molecules at the same time on tissue sections at high resolution. This review provides an overview of available kidney imaging strategies, with a focus on the possible impact of the most recent technical improvements.

Modulation of Tubular pH by Acetazolamide in a Ca2+ Transport Deficient Mice Facilitates Calcium Nephrolithiasis

Proximal tubular (PT) acidosis, which alkalinizes the urinary filtrate, together with Ca²⁺ supersaturation in PT can induce luminal calcium phosphate (CaP) crystal formation. While such CaP crystals are known to act as a nidus for CaP/calcium oxalate (CaOx) mixed stone formation, the regulation of PT luminal Ca²⁺ concentration ([Ca²⁺]) under elevated pH and/or high [Ca²⁺] conditions are unknown. Since we found that transient receptor potential canonical 3 (TRPC3) knockout (KO; -/-) mice could produce mild hypercalciuria with CaP urine crystals, we alkalinized the tubular pH in TRPC3-/- mice by oral acetazolamide (0.08%) to develop mixed urinary crystals akin to clinical signs of calcium nephrolithiasis (CaNL). Our ratiometric (λ340/380) intracellular [Ca²⁺] measurements reveal that such alkalization not only upsurges Ca²⁺ influx into PT cells, but the mode of Ca²⁺ entry switches from receptor-operated to store-operated pathway. Electrophysiological experiments show enhanced bicarbonate related current activity in treated PT cells which may determine the stone-forming phenotypes (CaP or CaP/CaOx). Moreover, such alkalization promotes reactive oxygen species generation, and upregulation of calcification, inflammation, fibrosis, and apoptosis in PT cells, which were exacerbated in absence of TRPC3. Altogether, the pH-induced alteration of the Ca²⁺ signaling signature in PT cells from TRPC3 ablated mice exacerbated the pathophysiology of mixed urinary stone formation, which may aid in uncovering the downstream mechanism of CaNL.

Long noncoding RNAs may impact podocytes and proximal tubule function through modulating miRNAs expression in Early Diabetic Kidney Disease of Type 2 Diabetes Mellitus patients

Aims: Long noncoding RNAs (lncRNAs) play key roles in the pathophysiology of DKD involving actions of microRNAs (miRNAs). The aims of the study were to establish the involvement of selected lncRNAs in the epigenetic mechanisms of podocyte damage and tubular injury in DKD of type 2 diabetes mellitus (DM) patients in relation to a particular miRNAs profile. Methods: A total of 136 patients with type 2 DM and 25 healthy subjects were assessed in a cross-sectional study concerning urinary albumin: creatinine ratio (UACR), eGFR, biomarkers of podocyte damage (synaptopodin, podocalyxin) and of proximal tubule (PT) dysfunction (Kidney injury molecule-1-KIM-1, N-acetyl-D-glucosaminidase-NAG), urinary lncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1), nuclear-enriched abundant transcript 1 (NEAT1), myocardial infarction-associated transcript (MIAT), taurine-upregulated gene 1 (TUG1), urinary miRNA21, 124, 93, 29a. Results: Multivariable regression analysis showed that urinary lncMALAT1 correlated directly with urinary synaptopodin, podocalyxin, KIM-1, NAG, miRNA21, 124, UACR, and negatively with eGFR, miRNA93, 29a (p<0.0001; R2=0.727); urinary lncNEAT1 correlated directly with synaptopodin, KIM-1, NAG, miRNA21, 124, and negatively with eGFR, miRNA93, 29a (p<0.0001; R2=0.702); urinary lncMIAT correlated directly with miRNA93 and 29a, eGFR (p<0.0001; R2=0.671) and negatively with synaptopodin, KIM-1, NAG, UACR, miRNA21, 124 (p<0.0001; R2=0.654); urinary lncTUG1 correlated directly with eGFR, miRNA93, 29a, and negatively with synaptopodin, podocalyxin, NAG, miRNA21, 124 (p<0.0001; R2=0.748). Conclusions: In patients with type 2 DM lncRNAs exert either deleterious or protective functions within glomeruli and PT. LncRNAs may contribute to DKD through modulating miRNAs expression and activities. This observation holds true independently of albuminuria and DKD stage.

Aligned Silk Sponge Fabrication and Perfusion Culture for Scalable Proximal Tubule Tissue Engineering

Chronic kidney disease and kidney failure are on the rise globally, yet there has not been a corresponding improvement in available therapies. A key challenge in a biological approach to developing kidney tissue is to identify scaffolding materials that support cell growth both in vitro and in vivo to facilitate translational goals. Scaffolds composed of silk fibroin protein possess the biocompatibility, mechanical robustness, and stability required for tissue engineering. Here, we use a silk sponge system to support kidney cells in a perfused bioreactor system. Silk fibroin protein underwent directional freezing to form parallel porous structures that mimic the native kidney structure of aligned tubules and are able to support more cells than nonaligned silk sponges. Adult immortalized renal proximal tubule epithelial cells were seeded into the sponges and cultured under static conditions for 1 week, then grown statically or with perfusion with culture media flowing through the sponge to enhance cell alignment and maturation. The sponges were imaged with confocal and scanning electron microscopies to analyze and quantify cell attachment, alignment, and expression of proteins important to proximal tubule differentiation and function. The perfused tissue constructs showed higher number of cells that are more evenly distributed through the construct and increased gene expression of several key markers of proximal tubule epithelial cell function compared to sponges grown under static conditions. These perfused tissue constructs represent a step toward a scalable approach to engineering proximal tubule structures with the potential to be used as in vitro models or as in vivo implantable tissues to supplement or replace impaired kidney function.

Urinary DPP4 correlates with renal dysfunction, and DPP4 inhibition protects against the reduction in megalin and podocin expression in experimental CKD

This study investigated the molecular mechanisms underlying the antiproteinuric effect of DPP4 inhibition in 5/6 renal ablation rats and tested the hypothesis that the urinary activity of DPP4 correlates with chronic kidney disease (CKD) progression. Experiments were conducted in male Wistar rats who underwent 5/6 nephrectomy (Nx) or sham operation followed by 8 wk of treatment with the DPP4 inhibitor (DPP4i) sitagliptin or vehicle. Proteinuria increased progressively in Nx rats throughout the observation period. This increase was remarkably mitigated by sitagliptin. Higher levels of proteinuria in Nx rats compared to control rats were accompanied by higher urinary excretion of retinol-binding protein 4, a marker of tubular proteinuria, as well as higher urinary levels of podocin, a marker of glomerular proteinuria. Retinol-binding protein 4 and podocin were not detected in the urine of Nx + DPP4i rats. Tubular and glomerular proteinuria was associated with the reduced expression of megalin and podocin in the renal cortex of Nx rats. Sitagliptin treatment partially prevented this decrease. Besides, the angiotensin II renal content was significantly reduced in the Nx rats that received sitagliptin compared to vehicle-treated Nx rats. Interestingly, both urinary DPP4 activity and abundance increased progressively in Nx rats. Additionally, urinary DPP4 activity correlated positively with serum creatinine levels, proteinuria, and blood pressure. Collectively, these results suggest that DPP4 inhibition ameliorated both tubular and glomerular proteinuria and prevented the reduction of megalin and podocin expression in CKD rats. Furthermore, these findings suggest that urinary DPP4 activity may serve as a biomarker of renal disease and progression.

Changes in NAD and Lipid Metabolism Drive Acidosis-Induced Acute Kidney Injury

BACKGROUND

The kidney plays an important role in maintaining normal blood pH. Metabolic acidosis (MA) upregulates the pathway that mitochondria in the proximal tubule (PT) use to produce ammonia and bicarbonate from glutamine, and is associated with AKI. However, the extent to which MA causes AKI, and thus whether treating MA would be beneficial, is unclear.

METHODS

Gavage with ammonium chloride induced acute MA. Multiphoton imaging of mitochondria (NADH/membrane potential) and transport function (dextran/albumin uptake), oxygen consumption rate (OCR) measurements in isolated tubules, histologic analysis, and electron microscopy in fixed tissue, and urinary biomarkers (KIM-1/clara cell 16) assessed tubular cell structure and function in mouse kidney cortex.

RESULTS

MA induces an acute change in NAD redox state (toward oxidation) in PT mitochondria, without changing the mitochondrial energization state. This change is associated with a switch toward complex I activity and decreased maximal OCR, and a major alteration in normal lipid metabolism, resulting in marked lipid accumulation in PTs and the formation of large multilamellar bodies. These changes, in turn, lead to acute tubular damage and a severe defect in solute uptake. Increasing blood pH with intravenous bicarbonate substantially improves tubular function, whereas preinjection with the NAD precursor nicotinamide (NAM) is highly protective.

CONCLUSIONS

MA induces AKI via changes in PT NAD and lipid metabolism, which can be reversed or prevented by treatment strategies that are viable in humans. These findings might also help to explain why MA accelerates decline in function in CKD.

Interferon-regulated genetic programs and JAK/STAT pathway activate the intronic promoter of the short ACE2 isoform in renal proximal tubules

Recently, a short, interferon-inducible isoform of Angiotensin-Converting Enzyme 2 (ACE2), dACE2 was identified. ACE2 is a SARS-Cov-2 receptor and changes in its renal expression have been linked to several human nephropathies. These changes were never analyzed in context of dACE2, as its expression was not investigated in the kidney. We used Human Primary Proximal Tubule (HPPT) cells to show genome-wide gene expression patterns after cytokine stimulation, with emphasis on the ACE2/dACE2 locus. Putative regulatory elements controlling dACE2 expression were identified using ChIP-seq and RNA-seq. qRT-PCR differentiating between ACE2 and dACE2 revealed 300- and 600-fold upregulation of dACE2 by IFNα and IFNβ, respectively, while full length ACE2 expression was almost unchanged. JAK inhibitor ruxolitinib ablated STAT1 and dACE2 expression after interferon treatment. Finally, with RNA-seq, we identified a set of genes, largely immune-related, induced by cytokine treatment. These gene expression profiles provide new insights into cytokine response of proximal tubule cells.

Association Between Kidney Clearance of Secretory Solutes and Cardiovascular Events: The Chronic Renal Insufficiency Cohort (CRIC) Study

RATIONALE & OBJECTIVE

The clearance of protein-bound solutes by the proximal tubules is an innate kidney mechanism for removing putative uremic toxins that could exert cardiovascular toxicity in humans. However, potential associations between impaired kidney clearances of secretory solutes and cardiovascular events among patients with chronic kidney disease (CKD) remains uncertain.

STUDY DESIGN

A multicenter, prospective, cohort study.

SETTING & PARTICIPANTS

We evaluated 3,407 participants from the Chronic Renal Insufficiency Cohort (CRIC) study.

EXPOSURES

Baseline kidney clearances of 8 secretory solutes. We measured concentrations of secretory solutes in plasma and paired 24-hour urine specimens using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

OUTCOMES

Incident heart failure, myocardial infarction, and stroke events.

ANALYTICAL APPROACH

We used Cox regression to evaluate associations of baseline secretory solute clearances with incident study outcomes adjusting for estimated GFR (eGFR) and other confounders.

RESULTS

Participants had a mean age of 56 years; 45% were women; 41% were Black; and the median estimated glomerular filtration rate (eGFR) was 43 mL/min/1.73 m². Lower 24-hour kidney clearance of secretory solutes were associated with incident heart failure and myocardial infarction but not incident stroke over long-term follow-up after controlling for demographics and traditional risk factors. However, these associations were attenuated and not statistically significant after adjustment for eGFR.

LIMITATIONS

Exclusion of patients with severely reduced eGFR at baseline; measurement variability in secretory solutes clearances.

CONCLUSIONS

In a national cohort study of CKD, no clinically or statistically relevant associations were observed between the kidney clearances of endogenous secretory solutes and incident heart failure, myocardial infarction, or stroke after adjustment for eGFR. These findings suggest that tubular secretory clearance provides little additional information about the development of cardiovascular disease events beyond glomerular measures of GFR and albuminuria among patients with mild-to-moderate CKD.

Mechanism of how carbamylation reduces albumin binding to FcRn contributing to increased vascular clearance

Chronic kidney disease results in high serum urea concentrations leading to excessive protein carbamylation, primarily albumin. This is associated with increased cardiovascular disease and mortality. Multiple methods were used to address whether carbamylation alters albumin metabolism. Intravital two-photon imaging of the Munich Wistar Frömter (MWF) rat kidney and liver allowed us to characterize filtration and proximal tubule uptake and liver uptake. Microscale thermophoresis enabled quantification of cubilin (CUB7,8 domain) and FcRn binding. Finally, multiple biophysical methods including dynamic light scattering, small-angle X-ray scattering, LC-MS/MS and in silico analyses were used to identify the critical structural alterations and amino acid modifications of rat albumin. Carbamylation of albumin reduced binding to CUB7,8 and FcRn in a dose-dependent fashion. Carbamylation markedly increased vascular clearance of carbamylated rat serum albumin (cRSA) and altered distribution of cRSA in both the kidney and liver at 16 h post intravenous injection. By evaluating the time course of carbamylation and associated charge, size, shape, and binding parameters in combination with in silico analysis and mass spectrometry, the critical binding interaction impacting carbamylated albumin's reduced FcRn binding was identified as K524. Carbamylation of RSA had no effect on glomerular filtration or proximal tubule uptake. These data indicate urea-mediated time-dependent carbamylation of albumin lysine K524 resulted in reduced binding to CUB7,8 and FcRn that contribute to altered albumin transport, leading to increased vascular clearance and increased liver and endothelial tissue accumulation.

Sex differences in renal ammonia metabolism

Sexual dimorphic variations are present in many aspects of biology and involve the structure and/or function of nearly every organ system. Acid-base homeostasis is critical for optimal health, and renal ammonia metabolism has a major role in the maintenance of acid-base homeostasis. Recent studies have shown sex-dependent differences in renal ammonia metabolism with regard to both basal ammonia excretion and the response to an exogenous acid load. These sexual dimorphisms are associated with structural changes in the proximal tubule and the collecting duct and variations in the expression of multiple proteins involved in ammonia metabolism and transport. Studies using orchiectomy-induced testosterone deficiency and physiological testosterone replacement have shown that testosterone underlies much of the sex-dependent differences in the proximal tubule. This parallels the finding that the canonical testosterone target receptor, androgen receptor (AR), is present exclusively in the proximal tubule. Thus testosterone, possibly acting through AR activation, regulates multiple components of renal structure and ammonia metabolism. The lack of detectable AR in the remainder of the nephron and collecting duct suggests that some dimorphisms in renal structure and ammonia transporter expression are mediated through mechanisms other than direct testosterone-dependent AR activation. A better understanding of the mechanism and biological implications of sex's effect on renal structure and ammonia metabolism is critical for optimizing our ability to care for both men and women with acid-base disturbances.

Transcriptomes of Major Proximal Tubule Cell Culture Models

BACKGROUND

Cultured cell lines are widely used for research in the physiology, pathophysiology, toxicology, and pharmacology of the renal proximal tubule. The lines that are most appropriate for a given use depend upon the genes expressed. New tools for transcriptomic profiling using RNA sequencing (RNA-Seq) make it possible to catalog expressed genes in each cell line.

METHODS

Fourteen different proximal tubule cell lines, representing six species, were grown on permeable supports under conditions specific for the respective lines. RNA-Seq followed standard procedures.

RESULTS

Transcripts expressed in cell lines variably matched transcripts selectively expressed in native proximal tubule. Opossum kidney (OK) cells displayed the highest percentage match (45% of proximal marker genes [TPM threshold =15]), with pig kidney cells (LLC-PK1) close behind (39%). Lower-percentage matches were seen for various human lines, including HK-2 (26%), and lines from rodent kidneys, such as NRK-52E (23%). Nominally, identical OK cells from different sources differed substantially in expression of proximal tubule markers. Mapping cell line transcriptomes to gene sets for various proximal tubule functions (sodium and water transport, protein transport, metabolic functions, endocrine functions) showed that different lines may be optimal for experimentally modeling each function. An online resource (https://esbl.nhlbi.nih.gov/JBrowse/KCT/) has been created to interrogate cell line transcriptome data. Proteomic analysis of NRK-52E cells confirmed low expression of many proximal tubule marker proteins.

CONCLUSIONS

No cell line fully matched the transcriptome of native proximal tubule cells. However, some of the lines tested are suitable for the study of particular metabolic and transport processes seen in the proximal tubule.

A well-developed endolysosomal system reflects protein reabsorption in segment 1 and 2 of rat proximal tubules

Proteinuria is a well-established marker and predictor of kidney disease. The receptors megalin and cubilin reabsorb filtered proteins and thereby proteinuria is avoided. It is unknown if all segments of the proximal tubule are involved in clearing the filtrate or if there exists a reserve capacity in case of increased glomerular protein filtration. To determine this, we performed serial sectioning of rat kidney and used stereology to quantify the endolysosomal system of the three segments of cortical and juxtamedullary nephrons by electron microscopy. Immunohistochemistry was applied to analyze the adaptor protein Dab2, which assists in megalin mediated endocytosis, megalin, and endocytic uptake of two endogenous megalin ligands; retinol binding protein and β2-microglobulin at exact tubular positions. Proteinuric rats (puromycin-treated) and mice (podocin knock-out) were analyzed to clarify the response of the tubule to increased protein filtration. We found that the endolysosomal system was most prominent in segment 1 and 2, whereas segment 3 was less developed. The depth of ligand uptake varied among nephrons, but it descended into segment 2 although uptake was lower than in segment 1 and it was never observed in segment 3. This was supported by prominent expression of Dab2 in segment 1 and 2. When protein filtration increased, segment 3 was included in the reabsorption process in proteinuric animals. Thus, segment 1 and 2 are responsible for clearing the filtrate for protein during normal physiological conditions, but the tubule exhibits plasticity and is able to include segment 3 under proteinuric stress.

Novel Fanconi renotubular syndromes provide insights in proximal tubule pathophysiology

The various forms of Fanconi renotubular syndromes (FRTS) offer significant challenges for clinicians and present unique opportunities for scientists who study proximal tubule physiology. This review will describe the clinical characteristics, genetic underpinnings, and underlying pathophysiology of the major forms of FRST. Although the classic forms of FRTS will be presented (e.g., Dent disease or Lowe syndrome), particular attention will be paid to five of the most recently discovered FRTS subtypes caused by mutations in the genes encoding for L-arginine:glycine amidinotransferase (GATM), solute carrier family 34 (type Ii sodium/phosphate cotransporter), member 1 (SLC34A1), enoyl-CoAhydratase/3-hydroxyacyl CoA dehydrogenase (EHHADH), hepatocyte nuclear factor 4A (HNF4A), or NADH dehydrogenase complex I, assembly factor 6 (NDUFAF6). We will explore how mutations in these genes revealed unexpected mechanisms that led to compromised proximal tubule functions. We will also describe the inherent challenges associated with gene discovery studies based on findings derived from small, single-family studies by focusing the story of FRTS type 2 (SLC34A1). Finally, we will explain how extensive alternative splicing of HNF4A has resulted in confusion with mutation nomenclature for FRTS type 4.

Matrigel-Free Laminin-Entactin Matrix to Induce Human Renal Proximal Tubule Structure Formation In Vitro

A successful in vitro tissue model must recapitulate the native tissue features while also being reproducible. Currently, Matrigel is the principal biomaterial used to induce the formation of proximal convoluted tubules (PCTs) in vitro, because of its similar composition and structure with the kidney tubular basement membrane and the presence of critical growth factors. However, Matrigel is not well-defined, and batch-to-batch variability is a significant issue. Here, we define a Matrigel-free method, using a laminin-entactin (L-E) matrix to support the formation of proximal tubular-like structures in vitro using immortalized human renal epithelial cells (RPTEC/TERT1) cocultured with murine fibroblast stromal cells (FOXD1^lacZ+). The matrix supports the presence of specific components of the tubular basement membrane (laminin, entactin/nidogen, and heparan sulfate proteoglycan) in addition to fibroblast growth factor 8a (FGF-8a). The matrix also induces tubulogenesis, leading to the formation of PCTs based on several key markers, including E-cadherin, aquaporin-1, and Na⁺/K⁺ ATPase. Moreover, these PCT structures displayed cell polarity and a well-defined lumen after 18 days in culture. This laminin-entactin (L-E) matrix constitutes a defined and consistent biomaterial that can be used in kidney tissue engineering for understanding in vitro proximal tubule development and for nephrotoxicity studies.

Transcriptional Programs Driving Shear Stress-Induced Differentiation of Kidney Proximal Tubule Cells in Culture

Cultured cell models are an essential complement to dissecting kidney proximal tubule (PT) function in health and disease but do not fully recapitulate key features of this nephron segment. We recently determined that culture of opossum kidney (OK) cells under continuous orbital shear stress (OSS) significantly augments their morphological and functional resemblance to PTs in vivo. Here we used RNASeq to identify temporal transcriptional changes upon cell culture under static or shear stress conditions. Comparison of gene expression in cells cultured under static or OSS conditions with a database of rat nephron segment gene expression confirms that OK cells cultured under OSS are more similar to the PT in vivo compared with cells maintained under static conditions. Both improved oxygenation and mechanosensitive stimuli contribute to the enhanced differentiation in these cells, and we identified temporal changes in gene expression of known mechanosensitive targets. We observed changes in mRNA and protein levels of membrane trafficking components that may contribute to the enhanced endocytic capacity of cells cultured under OSS. Our data reveal pathways that may be critical for PT differentiation in vivo and validate the utility of this improved cell culture model as a tool to study PT function.

Critical Role for AMPK in Metabolic Disease-Induced Chronic Kidney Disease

Chronic kidney disease (CKD) is prevalent in 9.1% of the global population and is a significant public health problem associated with increased morbidity and mortality. CKD is associated with highly prevalent physiological and metabolic disturbances such as hypertension, obesity, insulin resistance, cardiovascular disease, and aging, which are also risk factors for CKD pathogenesis and progression. Podocytes and proximal tubular cells of the kidney strongly express AMP-activated protein kinase (AMPK). AMPK plays essential roles in glucose and lipid metabolism, cell survival, growth, and inflammation. Thus, metabolic disease-induced renal diseases like obesity-related and diabetic chronic kidney disease demonstrate dysregulated AMPK in the kidney. Activating AMPK ameliorates the pathological and phenotypical features of both diseases. As a metabolic sensor, AMPK regulates active tubular transport and helps renal cells to survive low energy states. AMPK also exerts a key role in mitochondrial homeostasis and is known to regulate autophagy in mammalian cells. While the nutrient-sensing role of AMPK is critical in determining the fate of renal cells, the role of AMPK in kidney autophagy and mitochondrial quality control leading to pathology in metabolic disease-related CKD is not very clear and needs further investigation. This review highlights the crucial role of AMPK in renal cell dysfunction associated with metabolic diseases and aims to expand therapeutic strategies by understanding the molecular and cellular processes underlying CKD.

Beyond the tubule: pathological variants of LRP2, encoding the megalin receptor, result in glomerular loss and early progressive chronic kidney disease

Pathogenic variants in the LRP2 gene, encoding the multiligand receptor megalin, cause a rare autosomal recessive syndrome: Donnai-Barrow/Facio-Oculo-Acoustico-Renal (DB/FOAR) syndrome. Because of the rarity of the syndrome, the long-term consequences of the tubulopathy on human renal health have been difficult to ascertain, and the human clinical condition has hitherto been characterized as a benign tubular condition with asymptomatic low-molecular-weight proteinuria. We investigated renal function and morphology in a murine model of DB/FOAR syndrome and in patients with DB/FOAR. We analyzed glomerular filtration rate in mice by FITC-inulin clearance and clinically characterized six families, including nine patients with DB/FOAR and nine family members. Urine samples from patients were analyzed by Western blot analysis and biopsy materials were analyzed by histology. In the mouse model, we used histological methods to assess nephrogenesis and postnatal renal structure and contrast-enhanced magnetic resonance imaging to assess glomerular number. In megalin-deficient mice, we found a lower glomerular filtration rate and an increase in the abundance of injury markers, such as kidney injury molecule-1 and N-acetyl-β-d-glucosaminidase. Renal injury was validated in patients, who presented with increased urinary kidney injury molecule-1, classical markers of chronic kidney disease, and glomerular proteinuria early in life. Megalin-deficient mice had normal nephrogenesis, but they had 19% fewer nephrons in early adulthood and an increased fraction of nephrons with disconnected glomerulotubular junction. In conclusion, megalin dysfunction, as present in DB/FOAR syndrome, confers an increased risk of progression into chronic kidney disease.

Nephromegaly due to Disruption of Nephrons in the Green Iguana (Iguana iguana)

We describe a fatal kidney disease in green iguanas (Iguana iguana), associated with severe nephromegaly. Affected animals have enlarged kidneys, which fill the pelvic cavity, leading to compression of adjacent organs, obstipation and, ultimately, death. The pathological features of this disease have been poorly described and its aetiology is unknown. We performed detailed gross and histological analyses of 17 green iguanas with a clinical diagnosis of nephromegaly, and compared the results with those of eight healthy controls. Grossly, the kidneys of all 17 individuals were markedly enlarged and the distal colons were distended and overfilled with faecal contents. Histopathological examination revealed that these enlarged kidneys consisted mainly of tubular hyperplasia, resembling poorly developed proximal segments. The nephrogenic zones were either poorly developed or absent. There was a reduction in the density of glomeruli and the distal segments were reduced in number. There was no histological evidence of an aetiology for the nephron disruption and nephromegaly.

Does SARS-CoV-2 Infect the Kidney?

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The phosphoinositide 3-kinase inhibitor alpelisib restores actin organization and improves proximal tubule dysfunction in vitro and in a mouse model of Lowe syndrome and Dent disease

Loss-of-function mutations in the OCRL gene, which encodes the phosphatidylinositol [PI] 4,5-bisphosphate [PI(4,5)P2] 5-phosphatase OCRL, cause defective endocytosis and proximal tubule dysfunction in Lowe syndrome and Dent disease 2. The defect is due to increased levels of PI(4,5)P2 and aberrant actin polymerization, blocking endosomal trafficking. PI 3-phosphate [PI(3)P] has been recently identified as a coactivator with PI(4,5)P2 in the actin pathway. Here, we tested the hypothesis that phosphoinositide 3-kinase (PI3K) inhibitors may rescue the endocytic defect imparted by OCRL loss, by rebalancing phosphoinositide signals to the actin machinery. The broad-range PI3K inhibitor copanlisib and class IA p110α PI3K inhibitor alpelisib reduced aberrant actin polymerization in OCRL-deficient human kidney cells in vitro. Levels of PI 3,4,5-trisphosphate, PI(4,5)P2 and PI(3)P were all reduced with alpelisib treatment, and siRNA knockdown of the PI3K catalytic subunit p110α phenocopied the actin phenotype. In a humanized OcrlY/- mouse model, alpelisib reduced endosomal actin staining while restoring stress fiber architecture and levels of megalin at the plasma membrane of proximal tubule cells, reflected by improved endocytic uptake of low molecular weight proteins in vivo. Thus, our findings support the link between phosphoinositide lipids, actin polymerization and endocytic trafficking in the proximal tubule and represent a proof-of-concept for repurposing alpelisib in Lowe syndrome/Dent disease 2.