Chronic Metabolic Acidosis in Chronic Kidney Disease

BACKGROUND

Metabolic acidosis may be diagnosed as chronic (cMA) if it persists for at least 5 days, although an exact definition has not been provided by any guidelines yet. The most common cause is CKD; numerous less-known diseases can also account for cMA.

SUMMARY

In recent years, CKD-associated cMA has been proposed to induce several clinical complications. The aim of the article was to assess the current clinical evidence for complications and the respective management of CKD-associated cMA. In summary, cMA in CKD most likely promotes protein degradation and loss of bone mineral density. It aggravates CKD progression as indicated by experimental and (partly) clinical data. Therefore, cMA control must be recommended. Besides oral bicarbonate, dietary interventions potentially offer an alternative. Veverimer is a future option for cMA control; further systematic data are needed.

CONCLUSIONS

The most common cause of cMA is CKD. CKD-associated cMA most likely induces a negative protein balance; the exact role on bone metabolism remains uncertain. It presumably aggravates CKD progression. cMA control is recommendable; the serum bicarbonate target level should range around 24 mEq/L. Veverimer may be established as future option for cMA control; further systematic data are needed.

Constitutive Atg5 overexpression in mouse bone marrow endothelial progenitor cells improves experimental acute kidney injury

Background

Endothelial Progenitor Cells have been shown as effective tool in experimental AKI. Several pharmacological strategies for improving EPC-mediated AKI protection were identified in recent years. Aim of the current study was to analyze consequences of constitutive Atg5 activation in murine EPCs, utilized for AKI therapy.

Methods

Ischemic AKI was induced in male C57/Bl6N mice. Cultured murine EPCs were systemically injected post-ischemia, either natively or after Atg5 transfection (Adenovirus-based approach). Mice were analyzed 48 h and 6 weeks later.

Results

Both, native and transfected EPCs (EPCs^Atg5) improved persisting kidney dysfunction at week 6, such effects were more pronounced after injecting EPCs^Atg5. While matrix deposition and mesenchymal transdifferentiation of endothelial cells remained unaffected by cell therapy, EPCs, particularly EPCs^Atg5 completely prevented the post-ischemic loss of peritubular capillaries. The cells finally augmented the augophagocytic flux in endothelial cells.

Conclusions

Constitutive Atg5 activation augments AKI-protective effects of murine EPCs. The exact clinical consequences need to be determined.

Oxidation of HDL in patients with coronary artery disease

 

The function of high-density lipoprotein (HDL) cholesterol may play a more important role in the prevention of cardiovascular disease compared to the concentration of the HDL. The aim of the present study is to assess a novel cell-free test to quantify oxidation of HDL and its association to coronary artery disease (CAD).

We performed a prospective trial by including patients undergoing elective cardiac catheterization and healthy controls. A total of 895 subjects were included. In 483 patients CAD was confirmed (CAD group) and in 241 patients CAD was ruled-out (no CAD). Control patients (n=171) had no known CAD, had no diabetes and were not smokers. HDL function was measured in serum samples by determining its HDL-lipid peroxidation by a novel fluorometric cell-free biochemical assay. HDL lipid peroxide content (HDLox) is adjusted for the HDL cholesterol and has no units.

Patients with confirmed CAD had higher levels of HDLox (0.92±0.58, no units) compared to patients with no CAD (0.8±0.46, no units) and controls (0.78±0.41, no units, p=0.003). HDL-Cholesterol was lower in the CAD group (50.7±17.7mg/dl) compared to no CAD (58.4±17.6mg/dl) and healthy subjects (59.1±15.9mg/dl, p<0.001). HDLox was a strong predictor of coronary artery disease status (odds ratio for coronary disease 1.69; 95% confidence interval [CI], 1.24 to 2.38; P=0.001).

Oxidation of HDL, a metric of HDL function, measured by a cell-free biochemical assay, is increased in patients with CAD and is a predictor of the disease.

Logistic Regression analysis

Funding Acknowledgement

Type of funding source: Public Institution(s). Main funding source(s): Medical School of Brandenburg

Etiology and Management of Acute Metabolic Acidosis: An Update

BACKGROUND

The etiology of acute metabolic acidosis (aMA) is heterogeneous, and the consequences are potentially life-threatening. The aim of this article was to summarize the causes and management of aMA from a clinician's perspective.

SUMMARY

We performed a systematic search on PubMed, applying the following search terms: "acute metabolic acidosis," "lactic acidosis," "metformin" AND "acidosis," "unbalanced solutions" AND "acidosis," "bicarbonate" AND "acidosis" AND "outcome," "acute metabolic acidosis" AND "management," and "acute metabolic acidosis" AND "renal replacement therapy (RRT)/dialysis." The literature search did not consider diabetic ketoacidosis at all. Lactic acidosis evolves from various conditions, either with or without systemic hypoxia. The incidence of metformin-associated aMA is actually quite low. Unbalanced electrolyte preparations can induce hyperchloremic aMA. The latter potentially worsens kidney-related outcome parameters. Nevertheless, prospective and controlled data are missing at the moment. Recently, bicarbonate has been shown to improve clinically relevant endpoints in the critically ill, even if higher pH values (>7.3) are targeted. New therapeutics for aMA control are under development, since bicarbonate treatment can induce serious side effects. Key Messages: aMA is a frequent and potentially life-threatening complication of various conditions. Lactic acidosis might occur even in the absence of systemic hypoxia. The incidence of metformin-associated aMA is comparably low. Unbalanced electrolyte solutions induce hyperchloremic aMA, which most likely worsens the renal prognosis of critically ill patients. Bicarbonate, although potentially deleterious due to increased carbon dioxide production with subsequent intracellular acidosis, improves kidney-related endpoints in the critically ill.

P0535ATG5 OVEREXPRESSION IN PACS SIGNIFICANTLY IMPROVES CELL-MEDIATED AKI PROTECTION

Background and Aims

Acute Kidney Injury (AKI) significantly worsens the prognosis of hospitalized patients. Cell-based strategies have been established as reliable option for improving AKI outcomes in experimental AKI. Own studies performed in recent years utilized Pro-angiogenic Cells (PACs). Autophagy (AP) is commonly regarded as process of en-dogenous self-defense. The AP cascade, which may be stimulated by either substrate deprivation or certain exogenous / endogenous stressors, involves the activation of numerous proteins, the so-called Autophagy-related proteins (Atg proteins). Among these, Atg5 has been suggested to play a key role in augmenting AP. The current study evaluated whether selective Atg5 activation in syngeneic murine PACs may result in improved cell-mediated AKI protection.

Method

Cultured murine PACs were selectively transfected for Atg5. Successful transfection was verified by detecting red fluorescing cells. AKI was induced in male C57/Bl6N mice (8-12 weeks) by bilateral renal ischemia (IRI - 40 minutes). Transfected cells were i.v. injected post-ischemia. Mice were analyzed 48 hours and 6 weeks later.

Results

IRI induced significant kidney excretory dysfunction as reflected by higher serum cystatin C levels (48 hours and 6 weeks). Cell administration (either native or after transfection) did not improve AKI outcomes at 48 hours. At 6 weeks, injection of native cells resulted in lower serum cystatin C, this effect was even more pronounced if transcfected cells were applied.

Conclusion

Together, our data show that selective Atg5 overexpression in murine PACs sub-stantially augments cell-mediated AKI protection in the long-term. Thus, a new strategy for improving AKI protective effects of PACs has been identified.

P0548PAC-MEDIATED AKI PROTECTION IS CRITICALLY MEDIATED BUT DOES NOT EXCLUSIVELY DEPEND ON CELL-DERIVED MICROVESICLES

Background and Aims

Acute Kidney Injury (AKI) significantly worsens the prognosis of hospitalized patients. In recent years, cell-based strategies have been established as reliable option for improving AKI outcomes in experimental AKI. Own studies focused on so-called Proangiogenic Cells (PACs). Mechanisms that contribute to PAC-mediated AKI protection include production / secretion of extracellular vesicles (EV). In addition, the cells most likely act by paracrinic processes (secretome). The current study evaluated whether AKI may be preventable by the administration of either PAC-derived EV and / or the secretome alone.

Method

AKI was induced in male C57/Bl6N mice (8-12 weeks) by bilateral renal ischemia (IRI - 40 minutes). Syngeneic murine PACs were stimulated with either melatonine, Angiopoietin-1 or -2, or with Bone Morphogenetic Protein-5 (BMP-5) for one hour, respectively. The four mediators were chosen since previous own studies showed improved PAC-mediated AKI protection after cell preconditioning with these substances. PAC-derived EV and the vesicle-depleted supernatant were subsequently collected and i.v. injected post-ischemia. Mice were analyzed 48 hours later.

Results

IRI induced significant kidney excretory dysfunction as reflected by higher serum cystatin C levels. The only measure that improved AKI was the injection of EV, collected from native PACs. The following conditions worsened post-ischemic renal function even further: EV+Ang-1, EV+BMP-5, EV+melatonin, and EV+secretome+Ang-1.

Conclusion

Together, our data show that PAC-mediated AKI protection substantially depends on the availability of cell-derived EV. The secretome, either collected from native or preconditioned cells does not prevent mice from ischemia-induced dysfunction. However, since previous data showed improved AKI-protection by PACs after cell preconditioning with certain mediators (Ang-1 and -2, melatonine, BMP-5), other than exclusively vesicle-dependent mechanisms must be involved in PAC-mediated AKI protection. We suggest, that the mere presence of intact cells in the post-ischemic tissue is necessary for improving functional and structural outcome parameters under certain conditions.

Late left atrial appendage closure device displacement and massive thrombus formation: a case report

Background 

Left atrial appendage (LAA) closure with the WATCHMAN device is an alternative to anticoagulation therapy for the prevention of stroke in selected patients with atrial fibrillation (AF). Infrequently, left atrial (LA) device-related thrombus formation occurs and it is poorly understood. Thrombus formation due to incomplete covering of the LAA is even rarer and may occur within the first few months after device implantation.

Case summary 

Here, we present a case of a 68-year-old male patient with permanent AF, drug- and hepatitis induced liver cirrhosis (CILD Score B), and prior aortic valve replacement. The patient had a history of percutaneous LAA closure using a WATCHMAN device. He developed massive peri-device leak and thrombus arising from the space between the device and appendage cleft 2 years after implantation. Because of the high bleeding risk with a HAS-BLED score of 5 points, surgery was chosen as the therapy of choice instead of long-term anticoagulation. The patient was discharged in good clinical condition and has been scheduled for a yearly follow-up.

Discussion 

This case emphasizes the importance of choosing appropriately sized LAA occluder devices and planning for regular post-interventional follow-ups to minimize the risk of per-device leaks and thrombi.

Acute Kidney Injury-Associated Systemic Inflammation Is Aggravated in Insulin-Dependent Diabetes Mellitus

Background

Acute kidney injury (AKI) significantly worsens the prognosis of hospitalized patients. Diabetes mellitus (DM) affects a growing number of individuals in the western world. DM subjects are at a higher risk for acquiring AKI during the stay at the hospital. The current study intended to quantify serum levels of specific immunomodulatory cytokines in diabetic mice suffering from AKI.

Methods

DM was induced in male C57/Bl6N mice by systemic injections of beta cell-toxic streptozotocin. Animals underwent bilateral renal ischemia (45 min) 6 weeks later.

Results

Post-ischemic diabetic mice showed significantly differing serum concentrations of the majority of all analytes as compared to untreated controls and non-diabetic (post-ischemic) animals.

Conclusions

Together, our data suggest DM-associated immune activation in AKI. One may suppose that inadequate stimulation of the humoral/cellular immune response potentially contributes to the higher ischemia susceptibility of the organ in DM.

Loop Diuretics in Acute Kidney Injury Prevention, Therapy, and Risk Stratification

BACKGROUND

Loop diuretics (LD) are widely used in emergency and intensive care medicine.

SUMMARY

The substances increase the clearance of electrolytes and water; thus, they allow us to control hypervolemia and to prevent patients from pulmonary edema. LD are also frequently applied to patients with an acute decrease in glomerular filtration rate, namely, acute kidney injury (AKI). Nevertheless, volume depletion may be associated with reduced renal perfusion and possibly slower restitution or even aggravation of kidney dysfunction. Several trials on the preventive or therapeutic efficacy of LD have been published since the early 1970s. Our review article is intended to summarize the most important references related to this topic. In addition, we discuss the diagnostic value of the so-called furosemide stress test. The currently available data indicate that LD may act in a beneficial manner as long as euvolemia is maintained (matched hydration). Key Massages: LD are not beneficial for AKI treatment if kidney-related endpoints are considered. In certain situations, AKI prevention with LD can be associated with favorable outcomes as long as euvolemia is maintained. LD can help to identify AKI subjects at a higher risk of AKI progression, but the exact clinical consequences need to be determined.

The impact of aerobic and isometric exercise on different measures of dysfunctional high-density lipoprotein in patients with hypertension

BACKGROUND

Exercise training increases high-density lipoprotein (HDL) cholesterol, but its effect on HDL function is unclear. In hypertensives, exercise improves endothelial dysfunction, which is related to HDL function. In the present study, we assess for the first time the effects of different exercise modalities on two cell-free assays of HDL function.

DESIGN

The study was conducted as a prospective randomized controlled trial in 75 hypertensive patients.

METHODS

Patients were randomized in three groups: (a) handgrip isometric training five times weekly; (b) placebo-handgrip; and (c) aerobic exercise training at least three times per week. HDL function was assessed in serum samples at baseline and after 12 weeks of training by two independent assays that determine the proinflammatory phenotype (haptoglobin content) of a specific amount of HDL (Haptoglobin-HDL [HPHDL]) and oxidized HDL (HDLox) as a measure of reduced antioxidant function of HDL. HDL function measures were normalized by the measures of a pooled control of sera from healthy participants and by HDL-C levels (normalized ratio, no units).

RESULTS

Aerobic exercise led to significant reduction of the HDLox from 0.99 ± 0.27 to 0.90 ± 0.29 (no units, p = 0.03). The HPHDL did not change in any training group. Changes of HDLox correlated with reduction of the systolic blood pressure only after aerobic exercise (R = 0.64, p = 0.03).

CONCLUSIONS

Aerobic but not isometric exercise improves the antioxidant function of HDL in patients with hypertension. This improvement correlates positively with reductions of blood pressure.

Description of a novel RyR2 mutation in a juvenile patient with symptomatic catecholaminergic polymorphic ventricular tachycardia in sleep and during exercise: a case report

Background

Catecholaminergic polymorphic ventricular tachycardia is an inherited disease presenting with arrhythmic events during physical exercise or emotional stress. If untreated, catecholaminergic polymorphic ventricular tachycardia is a highly lethal condition: About 80% of affected individuals experience recurrent syncope, and 30% experience cardiac arrest. Catecholaminergic polymorphic ventricular tachycardia is caused by mutations in genes encoding ryanodine receptor type 2 (RyR2) and cardiac calsequestrin (CASQ2). In cases of sympathoadrenergic activation, both mutations result in a spontaneous Ca²⁺ release in cardiac cells, facilitating ventricular arrhythmias.

Case presentation

We present a case of a 17-year-old Caucasian boy who survived sudden cardiac death caused by ventricular fibrillation while performing running exercise in a fitness center. The diagnostic workup included blood tests, coronary angiography, electrophysiological testing, and cardiac magnetic resonance imaging, but all results were normal. Because the patient’s medical history included recurrent syncope during physical and emotional stress, we strongly suspected catecholaminergic polymorphic ventricular tachycardia as the underlying disease. Genetic screening was performed and confirmed the diagnosis, revealing a new heterozygous point mutation in the gene for RyR2, c.12520T>A (p.F4174 l, exon 90, RyR2 gene). The patient was discharged from our hospital after undergoing implantation of an implantable cardioverter defibrillator for secondary prevention. Shortly after implantation, the implantable cardioverter defibrillator terminated a sustaining ventricular tachycardia episode by antitachycardic pacing. This episode occurred early in the morning while the patient was asleep.

Conclusions

We present a case of catecholaminergic polymorphic ventricular tachycardia associated with a novel single point mutation in the RyR2 gene, which, to the best of our knowledge, has not been described in the literature so far. Our patient experienced arrhythmic events under both resting conditions and physical activity, an uncommon finding in patients with catecholaminergic polymorphic ventricular tachycardia. This novel mutation may cause arrhythmias independent of sympathoadrenergic stimulation, but further evidence is needed to prove causality.

Size matters: An observational study investigating estimated height as a reference size for calculating tidal volumes if low tidal volume ventilation is required

Purpose

Acute lung injury is a life threatening condition often requiring mechanical ventilation. Lung-protective ventilation with tidal volumes of 6 mL/kg predicted body weight (PBW, calculated on the basis of a patient’s sex and height), is part of current recommended ventilation strategy. Hence, an exact height is necessary to provide optimal mechanical ventilation. However, it is a common practice to visually estimate the body height of mechanically ventilated patients and use these estimates as a reference size for ventilator settings. We aimed to determine if the common practice of estimating visual height to define tidal volume reduces the possibility of receiving lung-protective ventilation.

Methods

In this prospective observational study, 28 mechanically ventilated patients had their heights visually estimated by 20 nurses and 20 physicians. All medical professionals calculated the PBW and a corresponding tidal volume with 6 ml/kg/PBW on the basis of their visual estimation. The patients’ true heights were measured and the true PBW with a corresponding tidal volume was calculated. Finally, estimates and measurements were compared.

Results

1033 estimations were undertaken by 153 medical professionals. The majority of the estimates were imprecise and resulting data comprised taller body heights, higher PBW and higher tidal volumes (all p≤0.01). When estimates of patients´ heights are used as a reference for tidal-volume definition, patients are exposed to mean tidal volumes of 6.5 ± 0.4 ml/kg/PBW. 526 estimation-based tidal volumes (51.1%) did not provide lung-protective ventilation. Shorter subjects (<175cm) were a specific risk group with an increased risk of not receiving lung protective ventilation (OR 6.6; 95%CI 1.2–35.4; p = 0.02), while taller subjects had a smaller risk of being exposed to inadequately high tidal volumes (OR 0.15; 95%CI 0.02–0.8; p = 0.02). Furthermore, we found an increased risk of overestimating if the assessor was a female (OR 1.74; 95%CI 1.14–2.65; p = 0.01).

Conclusion

The common practice of visually estimating body height and using these estimates for ventilator settings is imprecise and potentially harmful because it reduces the chance of receiving lung-protective ventilation. Avoiding this practice increases the patient safety. Instead, height should be measured as a standard procedure.

Autophagy activation in circulating proangiogenic cells aggravates AKI in type I diabetes mellitus

Acute kidney injury (AKI) occurs frequently in hospitals worldwide, but the therapeutic options are limited. Diabetes mellitus (DM) affects more and more people around the globe. The disease worsens the prognosis of AKI even further. In recent years, cell-based therapies have increasingly been applied in experimental AKI. The aim of the study was to utilize two established autophagy inducers for pharmacological preconditioning of so-called proangiogenic cells (PACs) in PAC treatment of diabetic AKI. Insulin-dependent DM was induced in male C57/Bl6N mice by intraperitoneal injections of streptozotocine. Six weeks later, animals underwent bilateral renal ischemia for 45 min, followed by intravenous injections of either native or zVAD (benzyloxycarbonyl-Val-Ala-Asp-fluoro-methylketone)- or Z-Leu-Leu-Leu-al (MG132)-pretreated syngeneic murine PACs. Mice were analyzed 48 h (short term) and 6 wk (long term) later, respectively. DM worsened postischemic AKI, and PAC preconditioning with zVAD and MG132 resulted in a further decline of excretory kidney function. Injection of native PACs reduced fibrosis in nondiabetic mice, but cell preconditioning promoted interstitial matrix accumulation significantly. Both substances aggravated endothelial-to-mesenchymal transition (EndoMT) under diabetic conditions; these effects occurred either exclusively in the short (zVAD) or in the short and long term (MG132). Preconditioned cells stimulated the autophagocytic flux in intrarenal endothelial cells, and all experimental groups displayed increased endothelial abundances of senescence-associated β-galactosidase, a marker of premature cell senescence. Pharmacological autophagy activation may not serve as an effective strategy for improving PAC competence in diabetic AKI in general. On the contrary, several outcome parameters (excretory function, fibrosis, EndoMT) may even be worsened.

Cell-Based Therapies in Acute Kidney Injury (AKI)

Acute kidney injury frequently occurs in hospitalized patients all over the world. The prognosis remains poor since specific therapies for promoting kidney regeneration/repair are still missing. In recent years cell-based strategies have improved AKI outcomes under experimental circumstances. Four groups of cells, each of them displaying certain biological and functional characteristics have been evaluated in AKI, induced Pluripotent Stem Cells (iPSCs), Spermatagonial Stem Cells (SSCs), Proangiogenic Cells (PACs) and Endothelial Colony Forming Cells (ECFCs), and Mesenchymal Stem Cells (MSCs). All of these have been documented to stabilize either parameters of kidney excretory dysfunction and/or certain morphological parameters. The mechanisms responsible for AKI protection include direct (cell incorporation) and indirect processes, the latter being mediated by humoral factors and particularly by the production of so-called extracellular vesicles. Cell-derived vesicular organelles have been shown to carry pro-regenerative micro-RNA molecules which stabilize the vascular and tubular function. The first trials in humans have been initiated, the majority of such trials employs MSCs. However, any transfer of cell-based strategies in the clinical practice is potentially associated with significant difficulties. These include cell availability, tolerance and competence. The article intends to summarize essential informations about all of the four populations mentioned above and to discuss implications for the management of human AKI.

Stroke-induced chronic systolic dysfunction driven by sympathetic overactivity

Objective

Cardiac diseases are established risk factors for ischemic stroke incidence and severity. Conversely, there is increasing evidence that brain ischemia can cause cardiac dysfunction. The mechanisms underlying this neurogenic heart disease are incompletely understood. Although it is established that ischemic stroke is associated with cardiac arrhythmias, myocardial damage, elevated cardiac enzymes, and plasma catecholamines in the acute phase, nothing is known about the delayed consequences of ischemic stroke on cardiovascular function.

Methods

To determine the long‐term cardiac consequences of a focal cerebral ischemia, we subjected young and aged mice to a 30‐minute transient middle cerebral artery occlusion and analyzed cardiac function by serial transthoracic echocardiography and hemodynamic measurements up to week 8 after surgery. Finally, animals were treated with metoprolol to evaluate a pharmacologic treatment option to prevent the development of heart failure.

Results

Focal cerebral ischemia induced a long‐term cardiac dysfunction with a reduction in left ventricular ejection fraction and an increase in left ventricular volumes; this development was associated with higher peripheral sympathetic activity. Metoprolol treatment prevented the development of chronic cardiac dysfunction by decelerating extracellular cardiac remodeling and inhibiting sympathetic signaling relevant to chronic autonomic dysfunction.

Interpretation

Focal cerebral ischemia in mice leads to the development of chronic systolic dysfunction driven by increased sympathetic activity. If these results can be confirmed in a clinical setting, treating physicians should be attentive to clinical signs of heart failure in every patient after ischemic stroke. Therapeutically, the successful β‐blockade with metoprolol in mice could also have future clinical implications. Ann Neurol 2017;82:729–743

TRPC4α and TRPC4β Similarly Affect Neonatal Cardiomyocyte Survival during Chronic GPCR Stimulation

The Transient Receptor Potential Channel Subunit 4 (TRPC4) has been considered as a crucial Ca²⁺ component in cardiomyocytes promoting structural and functional remodeling in the course of pathological cardiac hypertrophy. TRPC4 assembles as homo or hetero-tetramer in the plasma membrane, allowing a non-selective Na⁺ and Ca²⁺ influx. Gαq protein-coupled receptor (GPCR) stimulation is known to increase TRPC4 channel activity and a TRPC4-mediated Ca²⁺ influx which has been regarded as ideal Ca²⁺ source for calcineurin and subsequent nuclear factor of activated T-cells (NFAT) activation. Functional properties of TRPC4 are also based on the expression of the TRPC4 splice variants TRPC4α and TRPC4β. Aim of the present study was to analyze cytosolic Ca²⁺ signals, signaling, hypertrophy and vitality of cardiomyocytes in dependence on the expression level of either TRPC4α or TRPC4β. The analysis of Ca²⁺ transients in neonatal rat cardiomyocytes (NRCs) showed that TRPC4α and TRPC4β affected Ca²⁺ cycling in beating cardiomyocytes with both splice variants inducing an elevation of the Ca²⁺ transient amplitude at baseline and TRPC4β increasing the Ca²⁺ peak during angiotensin II (Ang II) stimulation. NRCs infected with TRPC4β (Ad-C4β) also responded with a sustained Ca²⁺ influx when treated with Ang II under non-pacing conditions. Consistent with the Ca²⁺ data, NRCs infected with TRPC4α (Ad-C4α) showed an elevated calcineurin/NFAT activity and a baseline hypertrophic phenotype but did not further develop hypertrophy during chronic Ang II/phenylephrine stimulation. Down-regulation of endogenous TRPC4α reversed these effects, resulting in less hypertrophy of NRCs at baseline but a markedly increased hypertrophic enlargement after chronic agonist stimulation. Ad-C4β NRCs did not exhibit baseline calcineurin/NFAT activity or hypertrophy but responded with an increased calcineurin/NFAT activity after GPCR stimulation. However, this effect was not translated into an increased propensity towards hypertrophy but rather less hypertrophy during GPCR stimulation. Further analyses revealed that, although hypertrophy was preserved in Ad-C4α NRCs and even attenuated in Ad-C4β NRCs, cardiomyocytes had an increased apoptosis rate and thus were less viable after chronic GPCR stimulation. These findings suggest that TRPC4α and TRPC4β differentially affect Ca²⁺ signals, calcineurin/NFAT signaling and hypertrophy but similarly impair cardiomyocyte viability during GPCR stimulation.

Sumoylation-independent activation of Calcineurin-NFAT-signaling via SUMO2 mediates cardiomyocyte hypertrophy

The objective of this study was to identify unknown modulators of Calcineurin (Cn)-NFAT signaling. Measurement of NFAT reporter driven luciferase activity was therefore utilized to screen a human cardiac cDNA-library (~10⁷ primary clones) in C2C12 cells through serial dilutions until single clones could be identified. This extensive screening strategy culminated in the identification of SUMO2 as a most efficient Cn-NFAT activator. SUMO2-mediated activation of Cn-NFAT signaling in cardiomyocytes translated into a hypertrophic phenotype. Prohypertrophic effects were also observed in mice expressing SUMO2 in the heart using AAV9 (Adeno-associated virus), complementing the in vitro findings. In addition, increased SUMO2-mediated sumoylation in human cardiomyopathy patients and in mouse models of cardiomyopathy were observed. To decipher the underlying mechanism, we generated a sumoylation-deficient SUMO2 mutant (ΔGG). Surprisingly, ΔGG replicated Cn-NFAT-activation and the prohypertrophic effects of native SUMO2, both in vitro and in vivo, suggesting a sumoylation-independent mechanism. Finally, we discerned a direct interaction between SUMO2 and CnA, which promotes CnA nuclear localization. In conclusion, we identified SUMO2 as a novel activator of Cn-NFAT signaling in cardiomyocytes. In broader terms, these findings reveal an unexpected role for SUMO2 in cardiac hypertrophy and cardiomyopathy, which may open the possibility for therapeutic manipulation of this pathway.

Inositol 1,4,5-trisphosphate-mediated sarcoplasmic reticulum-mitochondrial crosstalk influences adenosine triphosphate production via mitochondrial Ca2+ uptake through the mitochondrial ryanodine receptor in cardiac myocytes

AIMS

Elevated levels of inositol 1,4,5-trisphosphate (IP3) in adult cardiac myocytes are typically associated with the development of cardiac hypertrophy, arrhythmias, and heart failure. IP3 enhances intracellular Ca(2+ )release via IP3 receptors (IP3Rs) located at the sarcoplasmic reticulum (SR). We aimed to determine whether IP3-induced Ca(2+ )release affects mitochondrial function and determine the underlying mechanisms.

METHODS AND RESULTS

We compared the effects of IP3Rs- and ryanodine receptors (RyRs)-mediated cytosolic Ca(2+ )elevation achieved by endothelin-1 (ET-1) and isoproterenol (ISO) stimulation, respectively, on mitochondrial Ca(2+ )uptake and adenosine triphosphate (ATP) generation. Both ET-1 and isoproterenol induced an increase in mitochondrial Ca(2+ )(Ca(2 +) m) but only ET-1 led to an increase in ATP concentration. ET-1-induced effects were prevented by cell treatment with the IP3 antagonist 2-aminoethoxydiphenyl borate and absent in myocytes from transgenic mice expressing an IP3 chelating protein (IP3 sponge). Furthermore, ET-1-induced mitochondrial Ca(2+) uptake was insensitive to the mitochondrial Ca(2+ )uniporter inhibitor Ru360, however was attenuated by RyRs type 1 inhibitor dantrolene. Using real-time polymerase chain reaction, we detected the presence of all three isoforms of IP3Rs and RyRs in murine ventricular myocytes with a dominant presence of type 2 isoform for both receptors.

CONCLUSIONS

Stimulation of IP3Rs with ET-1 induces Ca(2+ )release from the SR which is tunnelled to mitochondria via mitochondrial RyR leading to stimulation of mitochondrial ATP production.