Role of Arginase-II in Podocyte Injury under Hypoxic Conditions

Hypoxia plays a crucial role in acute and chronic renal injury, which is attributable to renal tubular and glomerular cell damage. Some studies provide evidence that hypoxia-dependent upregulation of the mitochondrial enzyme arginase type-II (Arg-II) in tubular cells promotes renal tubular injury. It is, however, not known whether Arg-II is also expressed in glomerular cells, particularly podocytes under hypoxic conditions, contributing to hypoxia-induced podocyte injury. The effects of hypoxia on human podocyte cells (AB8/13) in cultures and on isolated kidneys from wild-type (wt) and arg-ii gene-deficient (arg-ii−/−) mice ex vivo, as well as on mice of the two genotypes in vivo, were investigated, respectively. We found that the Arg-II levels were enhanced in cultured podocytes in a time-dependent manner over 48 h, which was dependent on the stabilization of hypoxia-inducible factor 1α (HIF1α). Moreover, a hypoxia-induced derangement of cellular actin cytoskeletal fibers, a decrease in podocin, and an increase in mitochondrial ROS (mtROS) generation—as measured by MitoSOX—were inhibited by adenoviral-mediated arg-ii gene silencing. These effects of hypoxia on podocyte injury were mimicked by the HIFα stabilizing drug DMOG, which inhibits prolyl hydroxylases (PHD), the enzymes involved in HIFα degradation. The silencing of arg-ii prevented the detrimental effects of DMOG on podocytes. Furthermore, the inhibition of mtROS generation by rotenone—the inhibitor of respiration chain complex-I—recapitulated the protective effects of arg-ii silencing on podocytes under hypoxic conditions. Moreover, the ex vivo experiments with isolated kidney tissues and the in vivo experiments with mice exposed to hypoxic conditions showed increased Arg-II levels in podocytes and decreased podocyte markers regarding synaptopodin in wt mice but not in arg-ii−/− mice. While age-associated albuminuria was reduced in the arg-ii−/− mice, the hypoxia-induced increase in albuminuria was, however, not significantly affected in the arg-ii−/−. Our study demonstrates that Arg-II in podocytes promotes cell injury. Arg-ii ablation seems insufficient to protect mice in vivo against a hypoxia-induced increase in albuminuria, but it does reduce albuminuria in aging.

Thyroid hormone increases oxygen metabolism causing intrarenal tissue hypoxia; a pathway to kidney disease

The proposed mechanisms for the development of nephropathy are many, complex and often overlapping. Although recent literature strongly supports a role of kidney hypoxia as an independent pathway to nephropathy, the evidence remains inconclusive since the role of hypoxia is difficult to differentiate from confounding factors such as hyperglycemia, hypertension and oxidative stress. By increasing kidney oxygen consumption using triiodothyronine (T₃) and, thus, avoiding these confounding factors, the aim of the present study was to investigate renal hypoxia per se as a causal pathway for the development of nephropathy. Healthy Sprague-Dawley rats were treated with T₃ (10 μg/kg/day) and the angiotensin II AT₁-receptor antagonist candesartan (1 mg/kg in drinking water) to eliminate effects of T₃-induced renin release; and compared to a candesartan treated control group. After 7 weeks of treatment in vivo kidney function, oxygen metabolism and mitochondrial function were evaluated. T₃ did not affect glomerular filtration rate or renal blood flow, but increased total kidney oxygen consumption resulting in cortical hypoxia. Nephropathy, demonstrated as albuminuria and tubulointerstitial fibrosis, developed in T₃-treated animals. Mitochondria uncoupling mediated by uncoupling protein 2 and the adenosine nucleotide transporter was demonstrated as a mechanism causing the increased kidney oxygen consumption. Importantly, blood glucose levels, mean arterial blood pressure and oxidative stress levels were not affected by T₃. In conclusion, the present study provides further evidence for increased kidney oxygen consumption causing intrarenal tissue hypoxia, as a causal pathway for development of nephropathy.

Chronic hypoxia exacerbates diabetic glomerulosclerosis through mesangiolysis and podocyte injury in db/db mice

BACKGROUND

Chronic hypoxia may play a pivotal role in the development of diabetic nephropathy (DN). However, the precise mechanisms underlying progressive hypoxia-induced glomerular injury remain unclear.

METHODS

We housed db/db mice in a hypoxia chamber (12% O2) for up to 16 weeks beginning at 8 weeks of age. Various urine, serum and kidney abnormalities and glomerular messenger RNA (mRNA) expression were compared with those in age-matched db/db mice housed under normoxia.

RESULTS

Levels of urinary albumin and podocalyxin (PCX) were significantly higher in hypoxic mice early during hypoxia. Ultracentrifugation of urine samples revealed that podocytes in the hypoxic mice shed PCX-positive microparticles into the urine. After 16 weeks of hypoxia, the mice also had higher hematocrits with lower serum glucose and various degrees of mesangiolytic glomerulosclerosis with microaneurysms and the infrequent occurrence of nodular lesions. Immunohistologically, hypoxic mice showed significantly decreased endothelial cell densities early during hypoxia and decreased podocyte densities later. In both hypoxic and normoxic mice, glomerular macrophage and transforming growth factor-β1 (TGF-β1) staining significantly increased with aging, without changes in vascular endothelial growth factor or endothelial nitric oxide synthase (eNOS). Glomerular mRNA expression of monocyte chemoattractant protein-1, eNOS and TGF-β1 was significantly enhanced in the hypoxic mice.

CONCLUSIONS

These results indicate that chronic hypoxia induces advanced glomerulosclerosis with accelerated albuminuria triggered by mesangiolysis and podocyte injury in a murine model of DN.

Role of Renal Hypoxia in the Progression From Acute Kidney Injury to Chronic Kidney Disease

Over the past 20 years, there has been an increased appreciation of the long-term sequelae of acute kidney injury (AKI) and the potential development of chronic kidney disease (CKD). Several pathophysiologic features have been proposed to mediate AKI to CKD progression including maladaptive alterations in tubular, interstitial, inflammatory, and vascular cells. These alterations likely interact to culminate in the progression to CKD. In this article we focus primarily on evidence of vascular rarefaction secondary to AKI, and the potential mechanisms by which rarefaction occurs in relation to other alterations in tubular and interstitial compartments. We further focus on the potential that rarefaction contributes to renal hypoxia. Consideration of the role of hypoxia in AKI to CKD transition focuses on experimental evidence of persistent renal hypoxia after AKI and experimental maneuvers to evaluate the influence of hypoxia, per se, in progressive disease. Finally, consideration of methods to evaluate hypoxia in patients is provided with the suggestion that noninvasive measurement of renal hypoxia may provide insight into progression in post-AKI patients.

Branch retinal artery occlusion secondary to high-altitude exposure and diabetic retinopathy: a case report

Background

To report a case of branch retinal artery occlusion (BRAO) secondary to high-altitude exposure and diabetic retinopathy (DR), and to characterize the retinal changes before and after hyperbaric oxygen (HBO) treatment.

Case presentation

We present a case of a 42-year-old man with DR who travelled to Tibet (in China, 3800 m/12467 ft. above mean sea level). The day after the end of his journey, the patient presented with acute, painless visual loss and visual field loss in his left eye. He was then diagnosed with BRAO, which is an acute blockage of blood flow. After HBO treatment, visual acuity and visual field were improved.

Conclusions

High-altitude exposure and DR may be considered as relevant risk factors for BRAO. The ophthalmologist should be aware that the BRAO might be a rare presenting sign of high-altitude retinopathy (HAR), particularly in patients with DR. HBO treatment can be considered as a choice for ophthalmologists on treatment against BRAO.

Deletion of Uncoupling Protein-2 reduces renal mitochondrial leak respiration, intrarenal hypoxia and proteinuria in a mouse model of type 1 diabetes

AIM

Uncoupling protein-2 (UCP-2) can induce mitochondrial uncoupling in the diabetic kidney. Although mitochondrial uncoupling reduces oxidative stress originating from the mitochondria and can be regarded as a protective mechanism, the increased oxygen consumption occurring secondarily to increased mitochondria uncoupling, that is leak respiration, may contribute to kidney tissue hypoxia. Using UCP-2^-/- mice, we tested the hypothesis that UCP-2-mediated leak respiration is important for the development of diabetes-induced intrarenal hypoxia and proteinuria.

METHODS

Kidney function, in vivo oxygen metabolism, urinary protein leakage and mitochondrial function were determined in wild-type and UCP-2^-/- mice during normoglycaemia and 2 weeks after diabetes induction.

RESULTS

Diabetic wild-type mice displayed mitochondrial leak respiration, pronounced intrarenal hypoxia, proteinuria and increased urinary KIM-1 excretion. However, diabetic UCP-2^-/- mice did not develop increased mitochondrial leak respiration and presented with normal intrarenal oxygen levels, urinary protein and KIM-1 excretion.

CONCLUSION

Although functioning as an antioxidant system, mitochondria uncoupling is always in co-occurrence with increased oxygen consumption, that is leak respiration; a potentially detrimental side effect as it can result in kidney tissue hypoxia; an acknowledged unifying pathway to nephropathy. Indeed, this study demonstrates a novel mechanism in which UCP-2-mediated mitochondrial leak respiration is necessary for the development of diabetes-induced intrarenal tissue hypoxia and proteinuria.

Clinicopathological features of pediatric renal biopsies in the plateau regions of China

Objective

This study aimed to analyze the clinicopathological features of pediatric renal biopsies from plateau regions of China.

Methods

Clinicopathological features of pediatric renal biopsies were compared between plateau and non-plateau regions in patients who were admitted to West China Second University Hospital, Sichuan University between April 2001 and March 2017. Patients were children younger than 18 years.

Results

The proportion of primary glomerular disease in the plateau group was lower than that in the non-plateau group (45.56% vs 62.09%, respectively). In the plateau group, IgA nephropathy (IgAN) was the major primary glomerulonephritis (GN) pathology. IgAN accounted for 36.54% and 21.63% of GN cases with nephrotic syndrome and hematuria, respectively. Henoch-Schönlein purpura nephritis was the most common secondary GN in both groups. The proportion of hepatitis B virus-associated GN was higher and that of lupus nephritis was lower in the plateau group than in the non-plateau group.

Conclusions

There are differences in renal pathological types between children in plateau regions and those in non-plateau regions. Among children in plateau regions, IgAN and Henoch-Schönlein purpura nephritis were the most common kidney diseases.

High hemoglobin levels are associated with decreased risk of diabetic retinopathy in Korean type 2 diabetes

Anemia is an independent risk factor for the development of diabetic retinopathy (DR) in patients with type 2 diabetes mellitus (DM). Hemoglobin levels may also be associated with DR. We investigated the association between hemoglobin levels and DR risk. This cross-sectional, population-based study utilized data from 2,123 type 2 DM patients aged ≥30 years who participated in the Korea National Health and Nutrition Examination Survey from 2008 to 2012. Participants underwent an ophthalmic examination, including fundus photographs. A multiple logistic regression analysis was performed to evaluate the relationship between hemoglobin levels and DR risk. The mean hemoglobin levels in patients with and without DR were 13.76 ± 0.12 and 14.33 ± 0.05 g/dL, respectively, with anemia observed in 16.2 (2.4)% and 7.8 (0.8)%, respectively. A 19% decrease in DR risk was found with a 1.0-g/dL increase in hemoglobin level. DR risk exhibited a decreasing trend with increasing hemoglobin levels (P for trend <0.0001). The adjusted odds ratio of DR was significantly lower in the highest hemoglobin quartile. Our findings indicate that high hemoglobin levels are significantly related to a decreased DR risk in Korean type 2 diabetes.

Mitochondrial Reactive Oxygen Species and Kidney Hypoxia in the Development of Diabetic Nephropathy

The underlying mechanisms in the development of diabetic nephropathy are currently unclear and likely consist of a series of dynamic events from the early to late stages of the disease. Diabetic nephropathy is currently without curative treatments and it is acknowledged that even the earliest clinical manifestation of nephropathy is preceded by an established morphological renal injury that is in turn preceded by functional and metabolic alterations. An early manifestation of the diabetic kidney is the development of kidney hypoxia that has been acknowledged as a common pathway to nephropathy. There have been reports of altered mitochondrial function in the diabetic kidney such as altered mitophagy, mitochondrial dynamics, uncoupling, and cellular signaling through hypoxia inducible factors and AMP-kinase. These factors are also likely to be intertwined in a complex manner. In this review, we discuss how these pathways are connected to mitochondrial production of reactive oxygen species (ROS) and how they may relate to the development of kidney hypoxia in diabetic nephropathy. From available literature, it is evident that early correction and/or prevention of mitochondrial dysfunction may be pivotal in the prevention and treatment of diabetic nephropathy.

Renal transplantation induces mitochondrial uncoupling, increased kidney oxygen consumption, and decreased kidney oxygen tension

Hypoxia is an acknowledged pathway to renal injury and ischemia-reperfusion (I/R) and is known to reduce renal oxygen tension (Po2). We hypothesized that renal I/R increases oxidative damage and induces mitochondrial uncoupling, resulting in increased oxygen consumption and hence kidney hypoxia. Lewis rats underwent syngenic renal transplantation (TX) and contralateral nephrectomy. Controls were uninephrectomized (1K-CON) or left untreated (2K-CON). After 7 days, urinary excretion of protein and thiobarbituric acid-reactive substances were measured, and after 14 days glomerular filtration rate (GFR), renal blood flow, whole kidney Qo2, cortical Po2, kidney cortex mitochondrial uncoupling, renal oxidative damage, and tubulointerstitial injury were assessed. TX, compared with 1K-CON, resulted in mitochondrial uncoupling mediated via uncoupling protein-2 (16 ± 3.3 vs. 0.9 ± 0.4 pmol O2 · s(-1)· mg protein(-1), P < 0.05) and increased whole kidney Qo2 (55 ± 16 vs. 33 ± 10 μmol O2/min, P < 0.05). Corticomedullary Po2 was lower in TX compared with 1K-CON (30 ± 13 vs. 47 ± 4 μM, P < 0.05) whereas no significant difference was observed between 2K-CON and 1K-CON rats. Proteinuria, oxidative damage, and the tubulointerstitial injury score were not significantly different in 1K-CON and TX. Treatment of donors for 5 days with mito-TEMPO reduced mitochondrial uncoupling but did not affect renal hemodynamics, Qo2, Po2, or injury. Collectively, our results demonstrate increased mitochondrial uncoupling as an early event after experimental renal transplantation associated with increased oxygen consumption and kidney hypoxia in the absence of increases in markers of damage.

Physical activity at altitude: challenges for people with diabetes: a review

BACKGROUND

A growing number of subjects with diabetes take part in physical activities at altitude such as skiing, climbing, and trekking. Exercise under conditions of hypobaric hypoxia poses some unique challenges on subjects with diabetes, and the presence of diabetes can complicate safe and successful participation in mountain activities. Among others, altitude can alter glucoregulation. Furthermore, cold temperatures and altitude can complicate accurate reading of glucose monitoring equipment and storage of insulin. These factors potentially lead to dangerous hyperglycemia or hypoglycemia. Over the last years, more information has become available on this subject.

PURPOSE

To provide an up-to-date overview of the pathophysiological changes during physical activity at altitude and the potential problems related to diabetes, including the use of (continuous) blood glucose monitors and insulin pumps. To propose practical recommendations for preparations and travel to altitude for subjects with diabetes.

DATA SOURCES AND SYNTHESIS

We researched PubMed, medical textbooks, and related Internet sites, and extracted human studies and data based on relevance for diabetes, exercise, and altitude.

LIMITATIONS

Given the paucity of controlled trials regarding diabetes and altitude, we composed a narrative review and filled in areas lacking diabetes-specific studies with data obtained from nondiabetic subjects.

CONCLUSIONS

Subjects with diabetes can take part in activities at high, and even extreme, altitude. However, careful assessment of diabetes-related complications, optimal preparation, and adequate knowledge of glycemic regulation at altitude and altitude-related complications is needed.

High altitude may alter oxygen availability and renal metabolism in diabetics as measured by hyperpolarized [1-(13)C]pyruvate magnetic resonance imaging

The kidneys account for about 10% of the whole body oxygen consumption, whereas only 0.5% of the total body mass. It is known that intrarenal hypoxia is present in several diseases associated with development of kidney disease, including diabetes, and when renal blood flow is unaffected. The importance of deranged oxygen metabolism is further supported by deterioration of kidney function in patients with diabetes living at high altitude. Thus, we argue that reduced oxygen availability alters renal energy metabolism. Here, we introduce a novel magnetic resonance imaging (MRI) approach to monitor metabolic changes associated with diabetes and oxygen availability. Streptozotocin diabetic and control rats were given reduced, normal, or increased inspired oxygen in order to alter tissue oxygenation. The effects on kidney oxygen metabolism were studied using hyperpolarized [1-(13)C]pyruvate MRI. Reduced inspired oxygen did not alter renal metabolism in the control group. Reduced oxygen availability in the diabetic kidney altered energy metabolism by increasing lactate and alanine formation by 23% and 34%, respectively, whereas the bicarbonate flux was unchanged. Thus, the increased prevalence and severity of nephropathy in patients with diabetes at high altitudes may originate from the increased sensitivity toward inspired oxygen. This increased lactate production shifts the metabolic routs toward hypoxic pathways.

Improved glycemic control in moderate altitude type II diabetic residents

Exposure to altitude hypoxia may elicit changes in glucose homeostasis. This work is designated to investigate the glycemic control in type II diabetic patients (DMII) permanently resident at moderately high altitude (2400 m), and to investigate whether the arterial oxygen-related parameters are different in DMII patients living at altitude compared to those living at low altitude. Blood glucose, HbA1c, hemoglobin concentration, and hematocrit (HCT) were measured in moderate altitude type II diabetics and compared with both altitude nondiabetics and diabetic patients living at normoxic and normobaric conditions. The data revealed that fasting blood glucose was lower in altitude diabetic patients compared to diabetics living at low altitude (157±33 mg/dL and 176.81±15.98 mg/dL, respectively, p<0.01). Also, glycemic control was improved in altitude diabetic patients, where their HbA1c was lower than the corresponding low altitude diabetic patients (8.68±0.79% and 9.30±1.02% respectively, p<0.01). Low oxygen tension at altitude was compensated in both diabetics and nondiabetics by a significant increase in both hemoglobin and HCT (17.33±0.72 mg/dL and 50.7±2.20%, respectively) compared to the corresponding groups living at low levels (15.53±0.55 mg/dL and 45.8±1.64%, respectively). The underlying disease neither affected the arterial oxygen tension (PaO2) nor oxygen saturation (SaO2), where insignificant correlations were observed between glucose, PaO2 (r=-0.06) and SaO2 (r=-0.2). These results suggest that moderate altitude may improve the glycemic control in type II diabetic patients compared to diabetics living at sea level.

Kidney hypoxia, attributable to increased oxygen consumption, induces nephropathy independently of hyperglycemia and oxidative stress

Diabetic nephropathy is strongly associated with both increased oxidative stress and kidney tissue hypoxia. The increased oxidative stress causes increased kidney oxygen consumption resulting in kidney tissue hypoxia. To date, it has been difficult to determine the role of kidney hypoxia, per se, for the development of nephropathy. We tested the hypothesis that kidney hypoxia, without confounding factors such as hyperglycemia or elevated oxidative stress, results in nephropathy. To induce kidney hypoxia, dinitrophenol (30 mg per day per kg bodyweight by gavage), a mitochondrial uncoupler that increases oxygen consumption and causes kidney hypoxia, was administered for 30 consecutive days to rats. Thereafter, glomerular filtration rate, renal blood flow, kidney oxygen consumption, kidney oxygen tension, kidney concentrations of glucose and glycogen, markers of oxidative stress, urinary protein excretion, and histological findings were determined and compared with vehicle-treated controls. Dinitrophenol did not affect arterial blood pressure, renal blood flow, glomerular filtration rate, blood glucose, or markers of oxidative stress but increased kidney oxygen consumption, and reduced cortical and medullary concentrations of glucose and glycogen, and resulted in intrarenal tissue hypoxia. Furthermore, dinitrophenol treatment increased urinary protein excretion, kidney vimentin expression, and infiltration of inflammatory cells. In conclusion, increased mitochondrial oxygen consumption results in kidney hypoxia and subsequent nephropathy. Importantly, these results demonstrate that kidney tissue hypoxia, per se, without confounding hyperglycemia or oxidative stress, may be sufficient to initiate the development of nephropathy and therefore demonstrate a new interventional target for treating kidney disease.

Factors that render the kidney susceptible to tissue hypoxia in hypoxemia

To better understand what makes the kidney susceptible to tissue hypoxia, we compared, in the rabbit kidney and hindlimb, the ability of feedback mechanisms governing oxygen consumption (V̇o2) and oxygen delivery (Do2) to attenuate tissue hypoxia during hypoxemia. In the kidney (cortex and medulla) and hindlimb (biceps femoris muscle), we determined responses of whole organ blood flow and V̇o2, and local perfusion and tissue Po2, to reductions in Do2 mediated by graded systemic hypoxemia. Progressive hypoxemia reduced tissue Po2 similarly in the renal cortex, renal medulla, and biceps femoris. Falls in tissue Po2 could be detected when arterial oxygen content was reduced by as little as 4–8%. V̇o2 remained stable during progressive hypoxemia, only tending to fall once arterial oxygen content was reduced by 55% for the kidney or 42% for the hindlimb. Even then, the fall in renal V̇o2 could be accounted for by reduced oxygen demand for sodium transport rather than limited oxygen availability. Hindlimb blood flow and local biceps femoris perfusion increased progressively during graded hypoxia. In contrast, neither total renal blood flow nor cortical or medullary perfusion was altered by hypoxemia. Our data suggest that the absence in the kidney of hyperemic responses to hypoxia, and the insensitivity of renal V̇o2 to limited oxygen availability, contribute to kidney hypoxia during hypoxemia. The susceptibility of the kidney to tissue hypoxia, even in relatively mild hypoxemia, may have important implications for the progression of kidney disease, particularly in patients at high altitude or with chronic obstructive pulmonary disease.

Chronic kidney disease at high altitude

With a prevalence of 10 to 11% in the general population, it is likely that many patients with chronic kidney disease will visit or reside in mountainous areas. Little is known, however, about whether short- or long-duration, high-altitude exposure poses a risk in this patient population. Given that many areas of the kidney are marginally oxygenated even at sea level and that kidney disease may result in further renal hypoxia and hypoxia-associated renal injury, there is concern that high altitude may accelerate the progression of chronic kidney disease. In this review, we address how chronic kidney disease and its management is affected at high altitude. We postulate that arterial hypoxemia at high altitude poses a risk of faster disease progression in those with preexisting kidney disease. In addition, we consider the risks of developing acute altitude illness in patients with chronic kidney disease and the appropriate use of medications for the prevention and treatment of these problems.

Glomerular filtration rate estimates decrease during high altitude expedition but increase with Lake Louise acute mountain sickness scores

AIM

Acute mountain sickness (AMS) can result in pulmonary and cerebral oedema with overperfusion of microvascular beds, elevated hydrostatic capillary pressure, capillary leakage and consequent oedema as pathogenetic mechanisms. Data on changes in glomerular filtration rate (GFR) at altitudes above 5000 m are very limited.

METHODS

Thirty-four healthy mountaineers, who were randomized to two acclimatization protocols, undertook an expedition on Muztagh Ata Mountain (7549 m) in China. Tests were performed at five altitudes: Zurich pre-expedition (PE, 450 m), base camp (BC, 4497 m), Camp 1 (C1, 5533 m), Camp 2 (C2, 6265 m) and Camp 3 (C3, 6865 m). Cystatin C- and creatinine-based (Mayo Clinic quadratic equation) GFR estimates (eGFR) were assessed together with Lake Louise AMS score and other tests.

RESULTS

eGFR significantly decreased from PE to BC (P < 0.01). However, when analysing at changes between BC and C3, only cystatin C-based estimates indicated a significant decrease in GFR (P = 0.02). There was a linear decrease in eGFR from PE to C3, with a decrease of approx. 3.1 mL min(-1) 1.73 m(-2) per 1000 m increase in altitude. No differences between eGFR of the two groups with different acclimatization protocols could be observed. There was a significant association between eGFR and haematocrit (P = 0.01), whereas no significant association between eGFR and aldosterone, renin and brain natriuretic peptide could be observed. Finally, higher AMS scores were significantly associated with higher eGFR (P = 0.01).

CONCLUSIONS

Renal function declines when ascending from low to high altitude. Cystatin C-based eGFR decreases during ascent in high altitude expedition but increases with AMS scores. For individuals with eGFR <40 mL min(-1) 1.73 m(-2), caution may be necessary when planning trips to high altitude above 4500 m above sea level.