Calpain-1 induces apoptosis in pulmonary microvascular endothelial cells under septic conditions

Lactate induces vascular permeability via disruption of VE-cadherin in endothelial cells during sepsis

Circulating lactate levels are a critical biomarker for sepsis and are positively correlated with sepsis-associated mortality. We investigated whether lactate plays a biological role in causing endothelial barrier dysfunction in sepsis. We showed that lactate causes vascular permeability and worsens organ dysfunction in CLP sepsis. Mechanistically, lactate induces ERK-dependent activation of calpain1/2 for VE-cadherin proteolytic cleavage, leading to the enhanced endocytosis of VE-cadherin in endothelial cells. In addition, we found that ERK2 interacts with VE-cadherin and stabilizes VE-cadherin complex in resting endothelial cells. Lactate-induced ERK2 phosphorylation promotes ERK2 disassociation from VE-cadherin. In vivo suppression of lactate production or genetic depletion of lactate receptor GPR81 mitigates vascular permeability and multiple organ injury and improves survival outcome in polymicrobial sepsis. Our study reveals that metabolic cross-talk between glycolysis-derived lactate and the endothelium plays a critical role in the pathophysiology of sepsis.

Targeted inhibition of calpain in mitochondria alleviates oxidative stress-induced myocardial injury

The protein levels and activities of calpain-1 and calpain-2 are increased in cardiac mitochondria under pathological conditions including ischemia, diabetes, and sepsis, and transgenic overexpression of mitochondrial-targeted calpain-1 induces dilated heart failure, which underscores an important role of increased calpain in mitochondria in mediating myocardial injury. However, it remains to be determined whether selective inhibition of calpain in mitochondria protects the heart under pathological conditions. In this study, we generated transgenic mice overexpressing mitochondrial-targeted calpastatin in cardiomyocytes. Their hearts were isolated and subjected to global ischemia/reperfusion. Hyperglycemia was induced in the transgenic mice by injections of STZ. We showed that transgenic calpastatin was expressed exclusively in mitochondria isolated from their hearts but not from other organs including skeletal muscle and lung tissues. Transgenic overexpression of mitochondrial-targeted calpastatin significantly attenuated mitochondrial oxidative stress and cell death induced by global ischemia/reperfusion in isolated hearts, and ameliorated mitochondrial oxidative stress, cell death, myocardial remodeling and dysfunction in STZ-treated transgenic mice. The protective effects of mitochondrial-targeted calpastatin were correlated with increased ATP5A1 protein expression and ATP synthase activity in isolated hearts subjected to global ischemia/reperfusion and hearts of STZ-treated transgenic mice. In cultured rat myoblast H9c2 cells, overexpression of mitochondrial-targeted calpastatin maintained the protein levels of ATP5A1 and ATP synthase activity, prevented mitochondrial ROS production and decreased cell death following hypoxia/reoxygenation, whereas upregulation of ATP5A1 or scavenging of mitochondrial ROS by mito-TEMPO abrogated mitochondrial ROS production and decreased cell death. These results confirm the role of calpain in myocardial injury, suggesting that selective inhibition of calpain in myocardial mitochondria by mitochondrial-targeted calpastatin is an effective strategy for alleviating myocardial injury and dysfunction in cardiac pathologies.

Protective role of endothelial calpain knockout in lipopolysaccharide-induced acute kidney injury via attenuation of the p38-iNOS pathway and NO/ROS production

To explore the role of calpain and its signaling pathway in lipopolysaccharide (LPS)-induced acute kidney injury (AKI), animal models of endotoxemia were established by administration of LPS to mice with endothelial-specific Capn4 knockout (TEK/Capn4^−/−), mice with calpastatin (an endogenous calpain inhibitor) overexpression (Tg-CAST) and mice with myeloid-specific Capn4 knockout (LYZ/Capn4^−/−). Mouse pulmonary microvascular endothelial cells (PMECs) were used as a model of the microvascular endothelium and were stimulated with LPS. Renal function, renal inducible nitric oxide synthase (iNOS) and endothelial NOS (eNOS) expression, cellular apoptosis, plasma and renal levels of NO and reactive oxygen species (ROS), and phosphorylation of mitogen-activated protein kinase (MAPK) family members (p38, ERK1/2, and JNK1/2) were examined. Moreover, a calpain inhibitor, calpastatin overexpression adenoviruses and MAPK inhibitors were used. Significant renal dysfunction was induced by LPS stimulation, and recovery was observed in TEK/Capn4^−/− and Tg-CAST mice but not in LYZ/Capn4^−/− mice. Endothelial Capn4 knockout also abrogated the LPS-induced increases in renal iNOS expression, caspase-3 activity and apoptosis and plasma and renal NO and ROS levels but did not obviously affect renal eNOS expression. Moreover, LPS increased both calpain and caspase-3 activity, and only the expression of iNOS in PMECs was accompanied by increased phosphorylation of p38 and JNK. Inhibiting calpain activity or p38 phosphorylation alleviated the increased iNOS expression, NO/ROS production, and cellular apoptosis induced by LPS. These results suggest that endothelial calpain plays a protective role in LPS-induced AKI by inhibiting p38 phosphorylation, thus attenuating iNOS expression and further decreasing NO and ROS overproduction-induced endothelial apoptosis.

Therapies that inhibit the enzyme calpain could alleviate the effects of acute kidney injury according to researchers in China and Canada. Acute kidney injury is induced by endotoxemia, in which changes in the permeability of the intestine allow lipopolysaccharides (LPS) to pass from gut bacteria into the bloodstream. Calpain is known to be active during this process. Zhifeng Liu at the General Hospital of Guangzhou Military Command and co-workers induced endotoxemia in various mouse models by injecting them with LPS. The LPS induced significant kidney dysfunction and cell death, but these were alleviated in mice that were genetically modified to block calpain activity in the blood vessel lining, and in mice that overexpressed calpastatin, a calpain inhibitor. Blocking calpain reduces the expression of nitric oxide synthases that damage endothelial cells.

Redox-Mediated Post-Translational Modifications of Proteolytic Enzymes and Their Role in Protease Functioning

Proteolytic enzymes play a crucial role in metabolic processes, providing the cell with amino acids through the hydrolysis of multiple endogenous and exogenous proteins. In addition to this function, proteases are involved in numerous protein cascades to maintain cellular and extracellular homeostasis. The redox regulation of proteolysis provides a flexible dose-dependent mechanism for proteolytic activity control. The excessive reactive oxygen species (ROS) and reactive nitrogen species (RNS) in living organisms indicate pathological conditions, so redox-sensitive proteases can swiftly induce pro-survival responses or regulated cell death (RCD). At the same time, severe protein oxidation can lead to the dysregulation of proteolysis, which induces either protein aggregation or superfluous protein hydrolysis. Therefore, oxidative stress contributes to the onset of age-related dysfunction. In the present review, we consider the post-translational modifications (PTMs) of proteolytic enzymes and their impact on homeostasis.

Calpain inhibition improves erectile function in diabetic mice via upregulating endothelial nitric oxide synthase expression and reducing apoptosis

Calpain activation contributes to hyperglycemia-induced endothelial dysfunction and apoptosis. This study was designed to investigate the role of calpain inhibition in improving diabetic erectile dysfunction (ED) in mice. Thirty-eight-week-old male C57BL/6J mice were divided into three groups: (1) nondiabetic control group, (2) diabetic mice + vehicle group, and (3) diabetic mice + MDL28170 (an inhibitor of calpain) group. Type 1 diabetes was induced by intraperitoneal injection of streptozotocin at 60 mg kg⁻¹ body weight for 5 consecutive days. Thirteen weeks later, diabetic mice were treated with MDL28170 or vehicle for 4 weeks. The erectile function was assessed by electrical stimulation of the cavernous nerve. Penile tissues were collected for measurement of calpain activity and the endothelial nitric oxide synthase (eNOS)-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway. Terminal deoxynucleotidyl transferase 2'-deoxyuridine 5'-triphosphate nick end labeling (TUNEL) staining was used to evaluate apoptosis. Caspase-3 expression and activity were also measured to determine apoptosis. Our results showed that erectile function was enhanced by MDL28170 treatment in diabetic mice compared with the vehicle diabetic group. No differences in calpain-1 and calpain-2 expressions were observed among the three groups. However, calpain activity was increased in the diabetic group and reduced by MDL28170. The eNOS-NO-cGMP pathway was upregulated by MDL28170 treatment in diabetic mice. Additionally, MDL28170 could attenuate apoptosis and increase the endothelium and smooth muscle levels in corpus cavernosum. Inhibition of calpain could improve erectile function, probably by upregulating the eNOS-NO-cGMP pathway and reducing apoptosis.

Redox Regulation of Calpains: Consequences on Vascular Function

SIGNIFICANCE

Calpains (CAPNs) are a family of calcium-activated cysteine proteases. The ubiquitous isoforms CAPN1 and CAPN2 have been involved in the maintenance of vascular integrity, but uncontrolled CAPN activation plays a role in the pathogenesis of vascular diseases. Recent Advances: It is well accepted that chronic and acute overproduction of reactive oxygen species (ROS) is associated with the development of vascular diseases. There is increasing evidence that ROS can also affect the CAPN activity, suggesting CAPN as a potential link between oxidative stress and vascular disease.

CRITICAL ISSUES

The physiopathological relevance of ROS in regulating the CAPN activity is not fully understood but seems to involve direct effects on CAPNs, redox modifications of CAPN substrates, as well as indirect effect on CAPNs via changes in Ca²⁺ levels. Finally, CAPNs can also stimulate ROS production; however, data showing in which context ROS are the causes or the consequences of CAPN activation are missing.

FUTURE DIRECTIONS

Detailed characterization of the molecular mechanisms underlying the regulation of the different members of the CAPN system by specific ROS would help understanding the pathophysiological role of CAPN in the modulation of the vascular function. Moreover, given that CAPNs have been found in different cellular compartments such as mitochondria and nucleus as well as in the extracellular space, identification of new CAPN targets as well as their functional consequences would add new insights in the function of these enigmatic proteases.

Myocardial caspase-3 and NF-κB activation promotes calpain-induced septic apoptosis: The role of Akt/eNOS/NO pathway

AIMS

To explore the potential mechanism that the role of the Akt/eNOS/NO pathway in calpain-induced caspase-3 and NF-κB activation during septic apoptosis.

MAIN METHODS

Septic rats were stimulated by LPS (8 mg/kg, i.p.). Myocardial calpain, caspase-3, NO, TNF-α and IL-1β levels were detected by ELISA. The levels of Akt/p-Akt, eNOS/p-eNOS, iNOS proteins and number of apoptotic cells were evaluated by immunohistochemistry, western blot and TUNEL method.

KEY FINDINGS

Compared with sham, LPS treatment resulted in 4.1-fold and 1.8-fold increases in myocardial calpain activity and caspase-3 activation, respectively, and a significant increase (6.8-fold) in apoptotic cardiomyocytes was observed. The administration of calpain inhibitors (calpain inhibitor-IV, PD150606 and PD151746) showed that p-Akt and p-eNOS protein levels were correlated with the levels of LPS-induced myocardial calpain and caspase-3 activity. In addition, the quantity of p-Akt protein and NO content were markedly attenuated by wortmannin, a phosphoinositide 3-kinase (PI3K) inhibitor. Pretreatment with L-NAME, an NOS inhibitor, induced a decrease in p-eNOS proteins and apoptosis in myocardial tissues, while iNOS proteins were strongly increased in septic rats.

SIGNIFICANCE

This study suggests that the Akt/eNOS/NO pathway might lead to a novel pharmacological therapy for cardiomyocytes apoptosis in sepsis.

Differential Mechanisms of Septic Human Pulmonary Microvascular Endothelial Cell Barrier Dysfunction Depending on the Presence of Neutrophils

Sepsis is characterized by injury of pulmonary microvascular endothelial cells (PMVEC) leading to barrier dysfunction. Multiple mechanisms promote septic PMVEC barrier dysfunction, including interaction with circulating leukocytes and PMVEC apoptotic death. Our previous work demonstrated a strong correlation between septic neutrophil (PMN)-dependent PMVEC apoptosis and pulmonary microvascular albumin leak in septic mice in vivo; however, this remains uncertain in human PMVEC. Thus, we hypothesize that human PMVEC apoptosis is required for loss of PMVEC barrier function under septic conditions in vitro. To assess this hypothesis, human PMVECs cultured alone or in coculture with PMN were stimulated with PBS or cytomix (equimolar interferon γ, tumor necrosis factor α, and interleukin 1β) in the absence or presence of a pan-caspase inhibitor, Q-VD, or specific caspase inhibitors. PMVEC barrier function was assessed by transendothelial electrical resistance (TEER), as well as fluoroisothiocyanate-labeled dextran and Evans blue-labeled albumin flux across PMVEC monolayers. PMVEC apoptosis was identified by (1) loss of cell membrane polarity (Annexin V), (2) caspase activation (FLICA), and (3) DNA fragmentation [terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)]. Septic stimulation of human PMVECs cultured alone resulted in loss of barrier function (decreased TEER and increased macromolecular flux) associated with increased apoptosis (increased Annexin V, FLICA, and TUNEL staining). In addition, treatment of septic PMVEC cultured alone with Q-VD decreased PMVEC apoptosis and prevented septic PMVEC barrier dysfunction. In septic PMN-PMVEC cocultures, there was greater trans-PMVEC macromolecular flux (both dextran and albumin) vs. PMVEC cultured alone. PMN presence also augmented septic PMVEC caspase activation (FLICA staining) vs. PMVEC cultured alone but did not affect septic PMVEC apoptosis. Importantly, pan-caspase inhibition (Q-VD treatment) completely attenuated septic PMN-dependent PMVEC barrier dysfunction. Moreover, inhibition of caspase 3, 8, or 9 in PMN-PMVEC cocultures also reduced septic PMVEC barrier dysfunction whereas inhibition of caspase 1 had no effect. Our data demonstrate that human PMVEC barrier dysfunction under septic conditions in vitro (cytomix stimulation) is clearly caspase-dependent, but the mechanism differs depending on the presence of PMN. In isolated PMVEC, apoptosis contributes to septic barrier dysfunction, whereas PMN presence enhances caspase-dependent septic PMVEC barrier dysfunction independently of PMVEC apoptosis.

Heat stress prevents lipopolysaccharide-induced apoptosis in pulmonary microvascular endothelial cells by blocking calpain/p38 MAPK signalling

Pulmonary microvascular endothelial cells (PMECs) injury including apoptosis plays an important role in the pathogenesis of acute lung injury during sepsis. Our recent study has demonstrated that calpain activation contributes to apoptosis in PMECs under septic conditions. This study investigated how calpain activation mediated apoptosis and whether heat stress regulated calpain activation in lipopolysaccharides (LPS)-stimulated PMECs. In cultured mouse primary PMECs, incubation with LPS (1 μg/ml, 24 h) increased active caspase-3 fragments and DNA fragmentation, indicative of apoptosis. These effects of LPS were abrogated by pre-treatment with heat stress (43 °C for 2 h). LPS also induced calpain activation and increased phosphorylation of p38 MAPK. Inhibition of calpain and p38 MAPK prevented apoptosis induced by LPS. Furthermore, inhibition of calpain blocked p38 MAPK phosphorylation in LPS-stimulated PMECs. Notably, heat stress decreased the protein levels of calpain-1/2 and calpain activities, and blocked p38 MAPK phosphorylation in response to LPS. Additionally, forced up-regulation of calpain-1 or calpain-2 sufficiently induced p38 MAPK phosphorylation and apoptosis in PMECs, both of which were inhibited by heat stress. In conclusion, heat stress prevents LPS-induced apoptosis in PMECs. This effect of heat stress is associated with down-regulation of calpain expression and activation, and subsequent blockage of p38 MAPK activation in response to LPS. Thus, blocking calpain/p38 MAPK pathway may be a novel mechanism underlying heat stress-mediated inhibition of apoptosis in LPS-stimulated endothelial cells.

Association between myocardial cell apoptosis and calpain-1/caspase-3 expression in rats with hypoxic-ischemic brain damage

The present study aimed to investigate the association between myocardial cell apoptosis and calpain-1/caspase-3 expression in a rat model of hypoxic-ischemic brain damage (HIBD). A total of 64 newborn rats were divided into control (n=8; sacrificed on day 7) and HIBD groups (n=56). HIBD group rats were sacrificed 2, 12 or 24 h, or 2, 3, 5 or 7 days following HIBD (n=8/group). A terminal deoxynucleotidyl transferase dUTP nick-end labeling assay was performed to detect myocardial apoptotic cells and calculate the apoptosis index (AI), reverse transcription-polymerase chain reaction was performed to detect myocardial calpain-1/caspase-3 mRNA expression levels and a western blot analysis was conducted to detect calpain‑1 protein expression levels. The correlations between calpain‑1 and caspase‑3 expression levels and AI were analyzed. The results demonstrated that apoptotic myocardial cells in the HIBD groups were markedly increased compared with the control group, with AI peaking in the day 3 group. Caspase‑3 and calpain‑1 mRNA expression levels were increased from 2 and 12 h following HIBD, respectively, with the most elevated levels in the day 2 group. Compared with the control group, calpain‑1 protein expression levels were increased from 2 h, with the greatest expression levels in the day 3 group (P<0.05). Calpain‑1 mRNA and protein (76/80 kDa) expression levels demonstrated positive linear correlations with AI (r=0.786, P=0.001; and r=0.853, P=0.001, respectively) Caspase-3 mRNA expression levels were positively correlated with AI (r=0.894; P=0.001). In conclusion, the present study demonstrated that in rats with HIBD, there is a positive correlation between increased apoptosis of myocardial cells and expression levels of calpain-1 and caspase-3.

Attenuation of Blood-Brain Barrier Breakdown and Hyperpermeability by Calpain Inhibition

Blood-brain barrier (BBB) breakdown and the associated microvascular hyperpermeability followed by brain edema are hallmark features of several brain pathologies, including traumatic brain injuries (TBI). Recent studies indicate that pro-inflammatory cytokine interleukin-1β (IL-1β) that is up-regulated following traumatic injuries also promotes BBB dysfunction and hyperpermeability, but the underlying mechanisms are not clearly known. The objective of this study was to determine the role of calpains in mediating BBB dysfunction and hyperpermeability and to test the effect of calpain inhibition on the BBB following traumatic insults to the brain. In these studies, rat brain microvascular endothelial cell monolayers exposed to calpain inhibitors (calpain inhibitor III and calpastatin) or transfected with calpain-1 siRNA demonstrated attenuation of IL-1β-induced monolayer hyperpermeability. Calpain inhibition led to protection against IL-1β-induced loss of zonula occludens-1 (ZO-1) at the tight junctions and alterations in F-actin cytoskeletal assembly. IL-1β treatment had no effect on ZO-1 gene (tjp1) or protein expression. Calpain inhibition via calpain inhibitor III and calpastatin decreased IL-1β-induced calpain activity significantly (p < 0.05). IL-1β had no detectable effect on intracellular calcium mobilization or endothelial cell viability. Furthermore, calpain inhibition preserved BBB integrity/permeability in a mouse controlled cortical impact model of TBI when studied using Evans blue assay and intravital microscopy. These studies demonstrate that calpain-1 acts as a mediator of IL-1β-induced loss of BBB integrity and permeability by altering tight junction integrity, promoting the displacement of ZO-1, and disorganization of cytoskeletal assembly. IL-1β-mediated alterations in permeability are neither due to the changes in ZO-1 expression nor cell viability. Calpain inhibition has beneficial effects against TBI-induced BBB hyperpermeability.

Critical role of calpain in inflammation

Calpains are a family of cysteine proteases, implicated in a wide range of cellular calcium-regulated functions. Evidence from previous studies using an inhibitor of calpain indicates that calpain activation is involved in the process of numerous inflammation-associated diseases. As a result of in-depth studies, calpains have been proposed to influence the process of inflammation via a variety of mechanisms. The aim of the present study is to provide an overview of recent reports regarding the role of calpain in the process of inflammation, including regulation of immune cell migration, modulation of the activation of inflammatory mediators, degradation of certain associated proteins and induction of cell apoptosis. Understanding these mechanisms may contribute to the investigation of novel therapeutic targets for inflammation-associated diseases.

Heat stress pretreatment decreases lipopolysaccharide-induced apoptosis via the p38 signaling pathway in human umbilical vein endothelial cells

The present study aimed to investigate vascular endothelial apoptosis, and the regulatory molecules involved in the condition of heatstroke caused by direct hyperthermia due to high core temperature and gut‑derived endotoxemia. Human umbilical vascular endothelial cells (HUVECs) were isolated and treated with heat stress (43˚C for 1 h), lipopolysaccharide (LPS; 1 µg/ml), or a combination of heat stress pretreatment followed by LPS. Caspase‑3 activity, DNA fragmentation, and cell viability, determined using a 3‑(4, 5‑dimethyl thiazol‑2‑yl)‑2,5‑diphenyl tetrazolium bromide assay, were measured to examine cellular apoptosis. Changes in the expression levels of heat shock protein (HSP) 27, HSP90 and B‑cell lymphoma 2 (Bcl‑2), and the phosphorylation of p38 were detected using Western blot assays. The specific inhibitor of p38, SB203580, was also used. LPS induced endothelial apoptosis, as indicated by increased caspase‑3 activity, a high level of DNA fragmentation and low cell viability. LPS also increased p38 phosphorylation and decreased the expression levels of HSP27, HSP90 and Bcl‑2. Heat stress pretreatment inhibited LPS‑induced cellular apoptosis, increased the phosphorylation of p38, and increased the expression levels of HSP27, HSP90 and Bcl‑2. Pretreatment with SB203580 had effects similar to those of heat stress in the amelioration of LPS‑induced effects. These findings demonstrated that heat stress pretreatment decreased LPS‑induced Bcl‑2‑associated apoptosis in HUVECs by attenuating p38 activation, thereby increasing the expression levels of HSP27 and HSP90.

Role of pulmonary microvascular endothelial cell apoptosis in murine sepsis-induced lung injury in vivo

Background

Sepsis remains a common and serious condition with significant morbidity and mortality due to multiple organ dysfunction, especially acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Sepsis-induced ALI is characterized by injury and dysfunction of the pulmonary microvasculature and pulmonary microvascular endothelial cells (PMVEC), resulting in enhanced pulmonary microvascular sequestration and pulmonary infiltration of polymorphonuclear leukocytes (PMN) as well as disruption of the normal alveolo-capillary permeability barrier with leak of albumin-rich edema fluid into pulmonary interstitium and alveoli. The role of PMVEC death and specifically apoptosis in septic pulmonary microvascular dysfunction in vivo has not been established.

Methods

In a murine cecal ligation/perforation (CLP) model of sepsis, we quantified and correlated time-dependent changes in pulmonary microvascular Evans blue (EB)-labeled albumin permeability with (1) PMVEC death (propidium iodide [PI]-staining) by both fluorescent intravital videomicroscopy (IVVM) and histology, and (2) PMVEC apoptosis using histologic fluorescent microscopic assessment of a panel of 3 markers: cell surface phosphatidylserine (detected by Annexin V binding), caspase activation (detected by FLIVO labeling), and DNA fragmentation (TUNEL labeling).

Results

Compared to sham mice, CLP-sepsis resulted in pulmonary microvascular barrier dysfunction, quantified by increased EB-albumin leak, and PMVEC death (PI+ staining) as early as 2 h and more marked by 4 h after CLP. Septic PMVEC also exhibited increased presence of all 3 markers of apoptosis (Annexin V+, FLIVO+, TUNEL+) as early as 30 mins – 1 h after CLP-sepsis, which all similarly increased markedly until 4 h. The time-dependent changes in septic pulmonary microvascular albumin-permeability barrier dysfunction were highly correlated with PMVEC death (PI+; r = 0.976, p < 0.01) and PMVEC apoptosis (FLIVO+; r = 0.991, p < 0.01). Treatment with the pan-caspase inhibitor Q-VD prior to CLP reduced PMVEC death/apoptosis and attenuated septic pulmonary microvascular dysfunction, including both albumin-permeability barrier dysfunction and pulmonary microvascular PMN sequestration (p < 0.05). Septic PMVEC apoptosis and pulmonary microvascular dysfunction were also abrogated following CLP-sepsis in mice deficient in iNOS (Nos2^−/−) or NADPH oxidase (p47^phox−/− or gp91^phox−/−) and in wild-type mice treated with the NADPH oxidase inhibitor, apocynin.

Conclusions

Septic murine pulmonary microvascular dysfunction in vivo is due to PMVEC death, which is mediated through caspase-dependent apoptosis and iNOS/NADPH-oxidase dependent signaling.

PD150606 protects against ischemia/reperfusion injury by preventing μ-calpain-induced mitochondrial apoptosis

Calpain plays an important role in myocardial ischemia/reperfusion (I/R) injury. PD150606, a nonpeptide, cell-permeable and noncompetitive calpain inhibitor, has been shown to have protective properties in ischemic disease. The aims of the present study were to investigate whether PD150606 could alleviate myocardial I/R injury and to examine the possible mechanisms involved. The I/R model was established in vivo in C57BL/6 mice and in vitro using neonatal mouse cardiomyocytes, respectively. To evaluate the protective effects of PD150606 on I/R injury, we measured the myocardial infarct area, apoptosis, and expression of cleaved caspase-3. We also investigated the underlying mechanisms by examining mitochondrial function as reflected by the ATP concentration, translocation of cytochrome c, dynamics of mPTP opening, and membrane potential (ΔΨm), coupled with calpain activity. Pretreatment with PD150606 significantly reduced the infarct area and apoptosis caused by I/R. PD150606 pretreatment also reduced mitochondrial dysfunction by inhibiting calpain activation. Moreover, we found that μ-calpain is the main contributor to I/R-induced calpain activation. Knockdown of μ-calpain with siRNA significantly reversed calpain activation, mitochondrial dysfunction, and cardiomyocyte apoptosis caused by I/R in vitro. Our results suggest that PD150606 may protect against I/R injury via preventing μ-calpain-induced mitochondrial apoptosis.

Hyperhomocysteinemia and hyperglycemia induce and potentiate endothelial dysfunction via μ-calpain activation

Plasma homocysteine (Hcy) levels are positively correlated with cardiovascular mortality in diabetes. However, the joint effect of hyperhomocysteinemia (HHcy) and hyperglycemia (HG) on endothelial dysfunction (ED) and the underlying mechanisms have not been studied. Mild (22 µmol/L) and moderate (88 µmol/L) HHcy were induced in cystathionine β-synthase wild-type (Cbs(+/+)) and heterozygous-deficient (Cbs(-/+)) mice by a high-methionine (HM) diet. HG was induced by consecutive injection of streptozotocin. We found that HG worsened HHcy and elevated Hcy levels to 53 and 173 µmol/L in Cbs(+/+) and Cbs(-/+) mice fed an HM diet, respectively. Both mild and moderate HHcy aggravated HG-impaired endothelium-dependent vascular relaxation to acetylcholine, which was completely abolished by endothelial nitric oxide synthase (eNOS) inhibitor N(G)-nitro-L-arginine methyl ester. HHcy potentiated HG-induced calpain activation in aortic endothelial cells isolated from Cbs mice. Calpain inhibitors rescued HHcy- and HHcy/HG-induced ED in vivo and ex vivo. Moderate HHcy- and HG-induced μ-calpain activation was potentiated by a combination of HHcy and HG in the mouse aorta. μ-Calpain small interfering RNA (μ-calpsiRNA) prevented HHcy/HG-induced ED in the mouse aorta and calpain activation in human aortic endothelial cells (HAECs) treated with DL-Hcy (500 µmol/L) and d-glucose (25 mmol) for 48 h. In addition, HHcy accelerated HG-induced superoxide production as determined by dihydroethidium and 3-nitrotyrosin staining and urinary 8-isoprostane/creatinine assay. Antioxidants rescued HHcy/HG-induced ED in mouse aortas and calpain activation in cultured HAECs. Finally, HHcy potentiated HG-suppressed nitric oxide production and eNOS activity in HAECs, which were prevented by calpain inhibitors or μ-calpsiRNA. HHcy aggravated HG-increased phosphorylation of eNOS at threonine 497/495 (eNOS-pThr497/495) in the mouse aorta and HAECs. HHcy/HG-induced eNOS-pThr497/495 was reversed by µ-calpsiRNA and adenoviral transduced dominant negative protein kinase C (PKC)β2 in HAECs. HHcy and HG induced ED, which was potentiated by the combination of HHcy and HG via μ-calpain/PKCβ2 activation-induced eNOS-pThr497/495 and eNOS inactivation.

Pulmonary microvascular albumin leak is associated with endothelial cell death in murine sepsis-induced lung injury in vivo

Sepsis is a systemic inflammatory response that targets multiple components of the cardiovascular system including the microvasculature. Microvascular endothelial cells (MVEC) are central to normal microvascular function, including maintenance of the microvascular permeability barrier. Microvascular/MVEC dysfunction during sepsis is associated with barrier dysfunction, resulting in the leak of protein-rich edema fluid into organs, especially the lung. The specific role of MVEC apoptosis in septic microvascular/MVEC dysfunction in vivo remains to be determined. To examine pulmonary MVEC death in vivo under septic conditions, we used a murine cecal ligation/perforation (CLP) model of sepsis and identified non-viable pulmonary cells with propidium iodide (PI) by intravital videomicroscopy (IVVM), and confirmed this by histology. Septic pulmonary microvascular Evans blue (EB)-labeled albumin leak was associated with an increased number of PI-positive cells, which were confirmed to be predominantly MVEC based on specific labeling with three markers, anti-CD31 (PECAM), anti-CD34, and lectin binding. Furthermore, this septic death of pulmonary MVEC was markedly attenuated by cyclophosphamide-mediated depletion of neutrophils (PMN) or use of an anti-CD18 antibody developed for immunohistochemistry but shown to block CD18-dependent signaling. Additionally, septic pulmonary MVEC death was iNOS-dependent as mice lacking iNOS had markedly fewer PI-positive MVEC. Septic PI-positive pulmonary cell death was confirmed to be due to apoptosis by three independent markers: caspase activation by FLIVO, translocation of phosphatidylserine to the cell surface by Annexin V binding, and DNA fragmentation by TUNEL. Collectively, these findings indicate that septic pulmonary MVEC death, putatively apoptosis, is a result of leukocyte activation and iNOS-dependent signaling, and in turn, may contribute to pulmonary microvascular barrier dysfunction and albumin hyper-permeability during sepsis.

Cleavage of IκBα by calpain induces myocardial NF-κB activation, TNF-α expression, and cardiac dysfunction in septic mice

Recent studies in septic models have shown that myocardial calpain activity and TNF-α expression increase during sepsis and that inhibition of calpain activation downregulates myocardial TNF-α expression and improves cardiac dysfunction. However, the mechanism underlying this pathological process is unclear. Thus, in the present study, we aimed to explore whether IκBα/NF-κB signaling linked myocardial calpain activity and TNF-α expression in septic mice. Adult male mice were injected with LPS (4 mg/kg ip) to induce sepsis. Myocardial calpain activity, IκBα/NF-κB signaling activity, and TNF-α expression were assessed, and myocardial function was evaluated using the Langendorff system. In septic mice, myocardial calpain activity and TNF-α expression were increased and IκBα protein was degraded. Furthermore, NF-κB was activated, as indicated by increased NF-κB p65 phosphorylation, cleavage of p105 into p50, and its nuclear translocation. Administration of the calpain inhibitors calpain inhibitor Ш and PD-150606 prevented the LPS-induced degradation of myocardial IκBα, NF-κB activation, and TNF-α expression and ultimately improved myocardial function. In calpastatin transgenic mice, an endogenous calpain inhibitor and cultured neonatal mouse cardiomyocytes overexpressing calpastatin also inhibited calpain activity, IκBα protein degradation, and NF-κB activation after LPS treatment. In conclusion, myocardial calpain activity was increased in septic mice. Calpain induced myocardial NF-κB activation, TNF-α expression, and myocardial dysfunction in septic mice through IκBα protein cleavage.

Calpains promote neutrophil recruitment and bacterial clearance in an acute bacterial peritonitis model

Activation of the innate immune system is critical for clearance of bacterial pathogens to limit systemic infections and host tissue damage. Here, we report a key role for calpain proteases in bacterial clearance in mice with acute peritonitis. Using transgenic mice expressing Cre recombinase primarily in innate immune cells (fes-Cre), we generated conditional capns1 knockout mice. Consistent with capns1 being essential for stability and function of the ubiquitous calpains (calpain-1, calpain-2), peritoneal cells from these mice had reduced levels of calpain-2/capns1, and reduced proteolysis of their substrate selenoprotein K. Using an acute bacterial peritonitis model, we observed impaired bacterial killing within the peritoneum and development of bacteremia in calpain knockout mice. These defects correlated with significant reductions in IL-1α release, neutrophil recruitment, and generation of reactive oxygen species in calpain knockout mice with acute bacterial peritonitis. Peritoneal macrophages from calpain knockout mice infected with enterobacteria ex vivo, were competent in phagocytosis of bacteria, but showed impaired clearance of intracellular bacteria compared with control macrophages. Together, these results implicate calpains as key mediators of effective innate immune responses to acute bacterial infections, to prevent systemic dissemination of bacteria that can lead to sepsis.

Over-expression of calpastatin aggravates cardiotoxicity induced by doxorubicin

AIMS

Doxorubicin causes damage to the heart, which may present as cardiomyopathy. However, the mechanisms by which doxorubicin induces cardiotoxicity remain not fully understood and no effective prevention for doxorubicin cardiomyopathy is available. Calpains, a family of calcium-dependent thiol-proteases, have been implicated in cardiovascular diseases. Their activities are tightly controlled by calpastatin. This study employed transgenic mice over-expressing calpastatin to investigate the role of calpain in doxorubicin-induced cardiotoxicity.

METHODS AND RESULTS

Doxorubicin treatment decreased calpain activities in cultured neonatal mouse cardiomyocytes and in vivo mouse hearts, which correlated with down-regulation of calpain-1 and calpain-2 proteins. Over-expression of calpastatin or incubation with pharmacological calpain inhibitors enhanced apoptosis in neonatal and adult cardiomyocytes induced by doxorubicin. In contrast, over-expression of calpain-2 but not calpain-1 attenuated doxorubicin-induced apoptosis in cardiomyocytes. The pro-apoptotic effects of calpain inhibition were associated with down-regulation of protein kinase B (AKT) protein and mRNA expression, and a concomitant reduction in glycogen synthase kinase-3beta (GSK-3β) phosphorylation (Ser9) in doxorubicin-treated cardiomyocytes. Blocking AKT further increased doxorubicin-induced cardiac injuries, suggesting the effects of calpain inhibition may be mediated by inactivating the AKT signalling. In an in vivo model of doxorubicin-induced cardiotoxicity, over-expression of calpastatin exacerbated myocardial dysfunction as assessed by echocardiography and haemodynamic measurement in transgenic mice 5 days after doxorubicin injection. The 5-day mortality was higher in transgenic mice (29.16%) compared with their wild-type littermates (8%) after doxorubicin treatment.

CONCLUSION

Over-expression of calpastatin enhances doxorubicin-induced cardiac injuries through calpain inhibition and thus, calpains may protect cardiomyocytes against doxorubicin-induced cardiotoxicity.

The role of the Hsp90/Akt pathway in myocardial calpain-induced caspase-3 activation and apoptosis during sepsis

Background

Recent studies have demonstrated that myocardial calpain triggers caspase-3 activation and myocardial apoptosis in models of sepsis, whereas the inhibition of calpain activity down-regulates myocardial caspase-3 activation and apoptosis. However, the mechanism underlying this pathological process is unclear. Therefore, in this study, our aim was to explore whether the Hsp90/Akt signaling pathway plays a role in the induction of myocardial calpain activity, caspase-3 activation and apoptosis in the septic mice.

Methods

Adult male C57 mice were injected with lipopolysaccharide (LPS, 4 mg/kg, i.p.) to induce sepsis. Next, myocardial caspase-3 activity and the levels of Hsp90/p-Akt (phospho-Akt) proteins were detected, and apoptotic cells were assessed by performing the TUNEL assay.

Results

In the septic mice, there was an increase in myocardial calpain and caspase-3 activity in addition to an increase in the number of apoptotic cells; however, there was a time-dependent decrease in myocardial Hsp90/p-Akt protein levels. The administration of calpain inhibitors (calpain inhibitor-Ш or PD150606) prevented the LPS-induced degradation of myocardial Hsp90/p-Akt protein and its expression in cardiomyocytes in addition to inhibiting myocardial caspase-3 activation and apoptosis. The inhibition of Hsp90 by pretreatment with 17-AAG induced p-Akt degradation, and the inhibition of Akt activity by pretreatment with wortmannin resulted in caspase-3 activation in wildtype C57 murine heart tissues.

Conclusions

Myocardial calpain induces myocardial caspase-3 activation and apoptosis in septic mice via the activation of the Hsp90/Akt pathway.

Bench-to-bedside review: Immunoglobulin therapy for sepsis - biological plausibility from a critical care perspective

Sepsis represents a dysregulated host response to infection, the extent of which determines the severity of organ dysfunction and subsequent outcome. All trialled immunomodulatory strategies to date have resulted in either outright failure or inconsistent degrees of success. Intravenous immunoglobulin (IVIg) therapy falls into the latter category with opinion still divided as to its utility. This article provides a narrative review of the biological rationale for using IVIg in sepsis. A literature search was conducted using the PubMed database (1966 to February 2011). The strategy included the following text terms and combinations of these: IVIg, intravenous immune globulin, intravenous immunoglobulin, immunoglobulin, immunoglobulin therapy, pentaglobin, sepsis, inflammation, immune modulation, apoptosis. Preclinical and extrapolated clinical data of IVIg therapy in sepsis suggests improved bacterial clearance, inhibitory effects upon upstream mediators of the host response (for example, the nuclear factor kappa B (NF-κB) transcription factor), scavenging of downstream inflammatory mediators (for example, cytokines), direct anti-inflammatory effects mediated via Fcγ receptors, and a potential ability to attenuate lymphocyte apoptosis and thus sepsis-related immunosuppression. Characterizing the trajectory of change in immunoglobulin levels during sepsis, understanding mechanisms contributing to these changes, and undertaking IVIg dose-finding studies should be performed prior to further large-scale interventional trials to enhance the likelihood of a successful outcome.

Lycopene protects against hypoxia/reoxygenation-induced apoptosis by preventing mitochondrial dysfunction in primary neonatal mouse cardiomyocytes

BACKGROUND

Hypoxia/reoxygenation(H/R)-induced apoptosis of cardiomyocytes plays an important role in myocardial injury. Lycopene is a potent antioxidant carotenoid that has been shown to have protective properties on cardiovascular system. The aim of the present study is to investigate the potential for lycopene to protect the cardiomyocytes exposed to H/R. Moreover, the effect on mitochondrial function upon lycopene exposure was assessed.

METHODS AND FINDINGS

Primary cardiomyocytes were isolated from neonatal mouse and established an in vitro model of H/R which resembles ischemia/reperfusion in vivo. The pretreatment of cardiomyocytes with 5 µM lycopene significantly reduced the extent of apoptosis detected by TUNEL assays. To further study the mechanism underlying the benefits of lycopene, interactions between lycopene and the process of mitochondria-mediated apoptosis were examined. Lycopene pretreatment of cardiomyocytes suppressed the activation of the mitochondrial permeability transition pore (mPTP) by reducing the intracellular reactive oxygen species (ROS) levels and inhibiting the increase of malondialdehyde (MDA) levels caused by H/R. Moreover, the loss of mitochondrial membrane potential, a decline in cellular ATP levels, a reduction in the amount of cytochrome c translocated to the cytoplasm and caspase-3 activation were observed in lycopene-treated cultures.

CONCLUSION

The present results suggested that lycopene possesses great pharmacological potential in protecting against H/R-induced apoptosis. Importantly, the protective effects of lycopene may be attributed to its roles in improving mitochondrial function in H/R-treated cardiomyocytes.

Aging exacerbates microvascular endothelial damage induced by circulating factors present in the serum of septic patients

The elderly patients show a significantly elevated mortality rate during sepsis than younger patients, due to their higher propensity to microvascular dysfunction and consequential multiorgan failure. We tested whether aging renders vascular endothelial cells more susceptible to damage induced by inflammatory factors present in the circulation during sepsis. Primary microvascular endothelial cells derived from young (3 months) and aged (24 months) Fischer 344 × Brown Norway rats were treated with sera obtained from sepsis patients and healthy controls. Oxidative stress (MitoSox fluorescence), death receptor activation (caspase 8 activity), and apoptotic cell death (caspase 3 activity) induced by treatment with septic sera were exacerbated in aged endothelial cells as compared with responses obtained in young cells. Induction of heme oxygenase-1 and thrombomodulin in response to treatment with septic sera was impaired in aged endothelial cells. Treatment with septic sera elicited greater increases in tumor necrosis factor-α expression in aged endothelial cells, as compared with young cells, whereas induction of inducible nitric oxide synthase, intercellular adhesion molecule-1, and vascular cell adhesion molecule did not differ between the two groups. Collectively, aging increases sensitivity of microvascular endothelial cells (MVECs) to oxidative stress and cellular damage induced by inflammatory factors present in the circulation during septicemia. We hypothesize that these responses may contribute to the increased vulnerability of elderly patients to multiorgan failure associated with sepsis.

Cardiovascular management of septic shock in 2012

Septic shock is a major cause of morbidity and mortality throughout the world. Source control, antimicrobial therapy, early goal-directed fluid resuscitation, and infusion of vasoactive pharmaceuticals remain the cornerstones of treatment. However, the cardiovascular management of septic shock is evolving. Basic science and clinical researchers have identified novel drug targets and are testing the efficacy of new therapeutic agents. For example, prevention of microvascular leak during septic shock is the focus of active investigations and may soon provide considerable benefit to patients. Among the important topics that will be discussed in this review are the following: the role of vascular endothelial dysfunction in microvascular leak, the impact of cytokines upon structural and functional proteins within the endothelial barrier and within the heart, and the ability of selective vasopressin 1a receptor agonists to minimize tissue edema and improve hemodynamic status.

Particulate matter air pollution disrupts endothelial cell barrier via calpain-mediated tight junction protein degradation

Background

Exposure to particulate matter (PM) is a significant risk factor for increased cardiopulmonary morbidity and mortality. The mechanism of PM-mediated pathophysiology remains unknown. However, PM is proinflammatory to the endothelium and increases vascular permeability in vitro and in vivo via ROS generation.

Objectives

We explored the role of tight junction proteins as targets for PM-induced loss of lung endothelial cell (EC) barrier integrity and enhanced cardiopulmonary dysfunction.

Methods

Changes in human lung EC monolayer permeability were assessed by Transendothelial Electrical Resistance (TER) in response to PM challenge (collected from Ft. McHenry Tunnel, Baltimore, MD, particle size >0.1 μm). Biochemical assessment of ROS generation and Ca²⁺ mobilization were also measured.

Results

PM exposure induced tight junction protein Zona occludens-1 (ZO-1) relocation from the cell periphery, which was accompanied by significant reductions in ZO-1 protein levels but not in adherens junction proteins (VE-cadherin and β-catenin). N-acetyl-cysteine (NAC, 5 mM) reduced PM-induced ROS generation in ECs, which further prevented TER decreases and atteneuated ZO-1 degradation. PM also mediated intracellular calcium mobilization via the transient receptor potential cation channel M2 (TRPM2), in a ROS-dependent manner with subsequent activation of the Ca²⁺-dependent protease calpain. PM-activated calpain is responsible for ZO-1 degradation and EC barrier disruption. Overexpression of ZO-1 attenuated PM-induced endothelial barrier disruption and vascular hyperpermeability in vivo and in vitro.

Conclusions

These results demonstrate that PM induces marked increases in vascular permeability via ROS-mediated calcium leakage via activated TRPM2, and via ZO-1 degradation by activated calpain. These findings support a novel mechanism for PM-induced lung damage and adverse cardiovascular outcomes.

Pulmonary endothelial cell activation during experimental acute kidney injury

Acute kidney injury (AKI) leads to increased lung microvascular permeability, leukocyte infiltration, and upregulation of soluble inflammatory proteins in rodents. Most work investigating connections between AKI and pulmonary dysfunction, however, has focused on characterizing whole lung tissue changes associated with AKI. Studies at the cellular level are essential to understanding the molecular basis of lung changes during AKI. Given that the pulmonary microvascular barrier is functionally abnormal during AKI, we hypothesized that AKI induces a specific proinflammatory and proapoptotic lung endothelial cell (EC) response. Four and 24 h after kidney ischemia/reperfusion injury or bilateral nephrectomy, murine pulmonary ECs were isolated via tissue digestion followed by magnetic bead sorting. Purified lung ECs were analyzed for changes in mRNA expression using real-time SuperArray polymerase chain reaction analysis of genes related to EC function. In parallel experiments, confluent rat pulmonary microvascular ECs were treated with AKI or control serum to evaluate functional cellular alterations. Immunocytochemistry and FACS analysis of Annexin V/propidium iodide staining were used to evaluate cytoskeletal changes and promotion of apoptosis. Isolated murine pulmonary ECs exhibited significant changes in the expression of gene products related to inflammation, vascular reactivity, and programmed cell death. Further experiments using an in vitro rat pulmonary microvascular EC system revealed that AKI serum induced functional cellular changes related to apoptosis, including structural actin alterations and phosphatidylserine translocation. Analysis and segregation of both upregulated and downregulated genes into functional roles suggest that these transcriptional events likely participate in the transition to an activated proinflammatory and proapoptotic EC phenotype during AKI. Further mechanistic analysis of EC-specific events in the lung during AKI might reveal potential novel therapeutic targets for the deleterious kidney-lung crosstalk in the critically ill patient.

Activation of calpains mediates early lung neutrophilic inflammation in ventilator-induced lung injury

Lung inflammatory responses in the absence of infection are considered to be one of primary mechanisms of ventilator-induced lung injury. Here, we determined the role of calpain in the pathogenesis of lung inflammation attributable to mechanical ventilation. Male C57BL/6J mice were subjected to high (28 ml/kg) tidal volume ventilation for 2 h in the absence and presence of calpain inhibitor I (10 mg/kg). To address the isoform-specific functions of calpain 1 and calpain 2 during mechanical ventilation, we utilized a liposome-based delivery system to introduce small interfering RNAs targeting each isoform in pulmonary vasculature in vivo. Mechanical ventilation with high tidal volume induced rapid (within minutes) and persistent calpain activation and lung inflammation as evidenced by neutrophil recruitment, production of TNF-α and IL-6, pulmonary vascular hyperpermeability, and lung edema formation. Pharmaceutical calpain inhibition significantly attenuated these inflammatory responses caused by lung hyperinflation. Depletion of calpain 1 or calpain 2 had a protective effect against ventilator-induced lung inflammatory responses. Inhibition of calpain activity by means of siRNA silencing or pharmacological inhibition also reduced endothelial nitric oxide (NO) synthase (NOS-3)-mediated NO production and subsequent ICAM-1 phosphorylation following high tidal volume ventilation. These results suggest that calpain activation mediates early lung inflammation during ventilator-induced lung injury via NOS-3/NO-dependent ICAM-1 phosphorylation and neutrophil recruitment. Inhibition of calpain activation may therefore provide a novel and promising strategy for the prevention and treatment of ventilator-induced lung injury.

Endothelial activation, dysfunction and permeability during severe infections

PURPOSE OF REVIEW

Over the last few years, there have been major advances in our understanding of the role of the microvascular endothelium in the pathogenesis of severe, systemic infections.

RECENT FINDINGS

Endothelial activation and dysfunction contribute directly to the morbidity and mortality of sepsis and other, severe systemic infections. The end-result of diffuse endothelial activation and dysfunction may be the loss of microvascular barrier integrity, leading to tissue edema, shock and multiple organ failure. Endothelial activation also leads to an increase in angiopoietin-2, which is known to destabilize barrier function and promote inflammation.

SUMMARY

The ratio of the secreted endothelial growth factors, angiopoietin-2 and angiopoietin-1 appears to be a useful prognostic tool during severe infections. Finally, agents that enhance endothelial barrier integrity may prove useful as therapies for sepsis.

Calpain-catalyzed proteolysis of human dUTPase specifically removes the nuclear localization signal peptide

Background

Calpain proteases drive intracellular signal transduction via specific proteolysis of multiple substrates upon Ca²⁺-induced activation. Recently, dUTPase, an enzyme essential to maintain genomic integrity, was identified as a physiological calpain substrate in Drosophila cells. Here we investigate the potential structural/functional significance of calpain-activated proteolysis of human dUTPase.

Methodology/Principal Findings

Limited proteolysis of human dUTPase by mammalian m-calpain was investigated in the presence and absence of cognate ligands of either calpain or dUTPase. Significant proteolysis was observed only in the presence of Ca(II) ions, inducing calpain action. The presence or absence of the dUTP-analogue α,β-imido-dUTP did not show any effect on Ca²⁺-calpain-induced cleavage of human dUTPase. The catalytic rate constant of dUTPase was unaffected by calpain cleavage. Gel electrophoretic analysis showed that Ca²⁺-calpain-induced cleavage of human dUTPase resulted in several distinctly observable dUTPase fragments. Mass spectrometric identification of the calpain-cleaved fragments identified three calpain cleavage sites (between residues ⁴SE⁵; ⁷TP⁸; and ³¹LS³²). The cleavage between the ³¹LS³² peptide bond specifically removes the flexible N-terminal nuclear localization signal, indispensable for cognate localization.

Conclusions/Significance

Results argue for a mechanism where Ca²⁺-calpain may regulate nuclear availability and degradation of dUTPase.