Slowed relaxation of diaphragm in septic rats is associated with reduced expression of sarco-endoplasmic reticulum CA2+-ATPase genesSERCA1andSERCA2

Heart Dysfunction in Sepsis

Cardiac involvement during sepsis frequently occurs. A series of molecules induces a set of changes at the cellular level that result in the malfunction of the myocardium. The understanding of these molecular alterations has simultaneously promoted the implementation of diagnostic strategies that are much more precise and allowed the advance of the therapeutics. The heart is a vital organ for survival. Its well-being ensures the adequate supply of essential elements for organs and tissues.

Toll-Like Receptor-Mediated Cardiac Injury during Experimental Sepsis

Sepsis is associated with global cardiac dysfunction and with high mortality rate. The development of septic cardiomyopathy is due to complex interactions of damage-associated molecular patters, cytokines, and complement activation products. The aim of this study was to define the effects of sepsis on cardiac structure, gap junction, and tight junction (TJ) proteins. Sepsis was induced by cecal ligation and puncture in male C57BL/6 mice. After a period of 24 h, the expression of cardiac structure, gap junction, and TJ proteins was determined. Murine HL-1 cells were stimulated with LPS, and mRNA expression of cardiac structure and gap junction proteins, intracellular reactive oxygen species, and troponin I release was analyzed. Furthermore, pyrogenic receptor subtype 7 (P2X7) expression and troponin I release of human cardiomyocytes (iPS) were determined after LPS exposure. In vivo, protein expression of connexin43 and α-actinin was decreased after the onset of polymicrobial sepsis, whereas in HL-1 cells, mRNA expression of connexin43, α-actinin, and desmin was increased in the presence of LPS. Expression of TJ proteins was not affected in vivo during sepsis. Although the presence of LPS and nigericin resulted in a significant troponin I release from HL-1 cells. Sepsis affected cardiac structure and gap junction proteins in mice, potentially contributing to compromised cardiac function.

Does the perigestational exposure to chlorpyrifos and/or high-fat diet affect respiratory parameters and diaphragmatic muscle contractility in young rats?

The perinatal period is characterized by developmental stages with high sensitivity to environmental factors. Among the risk factors, maternal High-Fat Diet (HFD) consumption and early-life pesticide exposure can induce metabolic disorders at adulthood. We established the effects of perigestational exposure to Chlorpyrifos (CPF) and/or HFD on respiratory parameters, sleep apnea and diaphragm contractility in adult rats. Four groups of female rats were exposed starting from 4 months before gestation till the end of lactation period to CPF (1 mg/kg/day vs. vehicle) with or without HFD. Sleep apnea and respiratory parameters were measured by whole-body plethysmography in male offspring at postnatal day 60. Then diaphragm strips were dissected for the measurement of contractility, acetylcholinesterase (AChE) activity, and gene expression. The perigestational exposure to CPF and/or HFD increased the sleep apnea index but decreased the respiratory frequency. The twitch tension and the fatigability index were also increased, associated with reduced AChE activity and elevated mRNA expression of AChE, ryanodine receptor, and myosin heavy chain isoforms. Therefore, the perigestational exposure to either CPF and/or HFD could program the risks for altered ventilatory parameters and diaphragm contractility in young adult offspring despite the lack of direct contact to CPF and/or HFD.

Neuregulin-1β attenuates sepsis-induced diaphragm atrophy by activating the PI3K/Akt signaling pathway

The aim of this study was to investigate the protective effects of neuregulin-1β (NRG-1β) on sepsis-induced diaphragm atrophy and the possible underlying mechanisms. Sprague-Dawley rats were randomly divided into sham, sepsis and NRG groups. Sepsis was induced by cecal ligation and puncture (CLP). In the NRG group, rats received tail vein injections of NRG-1β (10 μg/kg) every 12 h for 72 h after CLP. At 3 days after surgery, diaphragm contractile forces were measured by determining the force-frequency curve and muscle fiber areas by hematoxylin-eosin staining. Moreover, the NRG-1 expression level in the diaphragm was detected by Western blotting. Furthermore, the proteins in the PI3K/Akt signaling pathway and its downstream Akt-mTOR and Akt-FOXO axes were detected by Western blotting analysis. In L6 myotubes treated with lipopolysaccharide (LPS) and NRG-1β, PI3K/Akt signaling pathway-related protein expression was further determined using the PI3K inhibitor LY294002. Exogenous NRG-1β could compensate for sepsis-induced diminished NRG-1 in the diaphragm and attenuate the reduction in diaphragm contractile forces and muscle fiber areas during sepsis. Moreover, NRG-1β treatment could activate the PI3K/Akt signaling pathway in the diaphragm during sepsis. The inhibition of p70S6K and 4E-BP1 on the Akt-mTOR axis and the increased expression of Murf1 on the Akt-FOXO axis were reversed after NRG-1 treatment. In addition, NRG-1β could activate the PI3K/Akt signaling pathway in L6 myotubes treated with LPS, while the PI3K inhibitor LY294002 blocked the effects of NRG-1β. NRG-1 expression in the diaphragm was reduced during sepsis, and exogenously administered recombinant human NRG-1β could attenuate sepsis-induced diaphragm atrophy by activating the PI3K/Akt signaling pathway.

Role of complement C5a and histones in septic cardiomyopathy

Polymicrobial sepsis (after cecal ligation and puncture, CLP) causes robust complement activation with release of C5a. Many adverse events develop thereafter and will be discussed in this review article. Activation of complement system results in generation of C5a which interacts with its receptors (C5aR1, C5aR2). This leads to a series of harmful events, some of which are connected to the cardiomyopathy of sepsis, resulting in defective action potentials in cardiomyocytes (CMs), activation of the NLRP3 inflammasome in CMs and the appearance of extracellular histones, likely arising from activated neutrophils which form neutrophil extracellular traps (NETs). These events are associated with activation of mitogen-activated protein kinases (MAPKs) in CMs. The ensuing release of histones results in defective action potentials in CMs and reduced levels of [Ca²⁺]i-regulatory enzymes including sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2) and Na⁺/Ca²⁺ exchanger (NCX) as well as Na⁺/K⁺-ATPase in CMs. There is also evidence that CLP causes release of IL-1β via activation of the NLRP3 inflammasome in CMs of septic hearts or in CMs incubated in vitro with C5a. Many of these events occur after in vivo or in vitro contact of CMs with histones. Together, these data emphasize the role of complement (C5a) and C5a receptors (C5aR1, C5aR2), as well as extracellular histones in events that lead to cardiac dysfunction of sepsis (septic cardiomyopathy).

Muscle metabolism

PURPOSE OF REVIEW

To review the recent findings on metabolic derangements leading to loss of muscle mass and function.

RECENT FINDINGS

Several recent studies investigated methods to assess muscle mass and function and its clinical relevance. These are also included. A few studies confirm that a low muscle mass is related to a worse outcome but also a compromised muscle function at discharge is related to long-term survival. A low quality of muscle assessed by the density of muscle from a computed tomography scan is related to mortality. For the metabolic derangements, a compromised handling of calcium is present in muscle of animal models and might be causing a decreased muscle function in patients. Transcriptomics analyses of muscle post-ICU indicated an upregulation of regenerative pathways, but still muscle mass is not recovering in most patients. This could be due to an impairment regenerative capacity due to satellite cells dysfunction.

SUMMARY

Muscle mass and function are related to outcome. New finding show that not only muscle mass but also muscle quality is important, that a compromised handling of calcium might be involved in muscle weakness and that regaining muscle could be compromised due to a defective regenerative capacity of satellite cells.

SERCA1 attenuates diaphragm relaxation and uptake rate of SERCA in rats with acute sepsis

The present study aimed to investigate the effects of acute sepsis on diaphragm contractility and relaxation, via examining the Ca2+‑uptake function of sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA), and the protein levels of SERCA1, SERCA2 and the ryanodine receptor (RyR) of the sarcoplasmic reticulum (SR). A sepsis rat model was established through cecal ligation and puncture (CLP). A total of 6 and 12 h following CLP, the isometric contractile and relaxation parameters of the diaphragm were measured. In addition, Ca2+ uptake and release from the SR, and the protein expression levels of SERCA1, SERCA2 and RyR in diaphragm muscle tissue were investigated. At 6 and 12 h post‑CLP, the diaphragm half‑relaxation time was prolonged and the maximum rate of tension decline was decreased and the Ca2+‑uptake function of SERCA was markedly reduced. The maximum rate of twitch force development, the maximal twitch and tetanic tension, and the release function of SR were decreased at 12 h post‑CLP. A total of 12 h following CLP, the protein expression levels of SERCA1 were significantly downregulated, and its activity was significantly reduced; conversely, the protein levels of SERCA2 remained unaltered. The present findings indicated that at the acute stage of sepsis induced by CLP the contractile and relaxation functions of the diaphragm were significantly compromised. The impairments in relaxation may be a result of the impaired uptake function of the SR and the downregulation in SERCA1 protein expression. Conversely, the compromised contractility may be a result of the impaired release function of the SR and the downregulation in RyR protein levels. This could provide some new insights into the treatment of sepsis. In acute stages of sepsis, the improvement of SERCA function could reduce the disequilibrium of calcium homeostasis to improve the critical illness myopathy and respiratory failure.

Decreased intracellular [Ca2+ ] coincides with reduced expression of Dhprα1s, RyR1, and diaphragmatic dysfunction in a rat model of sepsis

INTRODUCTION

Sepsis can cause decreased diaphragmatic contractility. Intracellular calcium as a second messenger is central to diaphragmatic contractility. However, changes in intracellular calcium concentration ([Ca²⁺ ]) and the distribution and co-localization of relevant calcium channels [dihydropyridine receptors, (DHPRα1s) and ryanodine receptors (RyR1)] remain unclear during sepsis. In this study we investigated the effect of changed intracellular [Ca²⁺ ] and expression and distribution of DHPRα1s and RyR1 on diaphragm function during sepsis.

METHODS

We measured diaphragm contractility and isolated diaphragm muscle cells in a rat model of sepsis. The distribution and co-localization of DHPRα1s and RyR1 were determined using immunohistochemistry and immunofluorescence, whereas intracellular [Ca²⁺ ] was measured by confocal microscopy and fluorescence spectrophotometry.

RESULTS

Septic rat diaphragm contractility, expression of DHPRα1s and RyR1, and intracellular [Ca²⁺ ] were significantly decreased in the rat sepsis model compared with controls.

DISCUSSION

Decreased intracellular [Ca²⁺ ] coincides with diaphragmatic contractility and decreased expression of DHPRα1s and RyR1 in sepsis. Muscle Nerve 56: 1128-1136, 2017.

Complement and sepsis-induced heart dysfunction

It is well known that cardiac dysfunction develops during sepsis in both humans and in rodents (rats, mice). These defects appear to be reversible, since after "recovery" from sepsis, cardiac dysfunction disappears and the heart returns to its function that was present before the onset of sepsis. Our studies, using in vivo and in vitro models, have demonstrated that C5a and its receptors (C5aR1 and C5aR2) play key roles in cardiac dysfunction developing during sepsis. Use of a neutralizing antibody to C5a largely attenuates cardiac dysfunction and other adverse events developing during sepsis. The molecular basis for cardiac dysfunctions is linked to generation of C5a and its interaction with C5a receptors present on surfaces of cardiomyocytes (CMs). It is established that C5a interactions with C5a receptors leads to significant reductions involving faulty contractility and relaxation in CMs. In addition, C5a interactions with C5a receptors on CMs results in reductions in Na⁺/K⁺-ATPase in CMs. This ATPase is essential for intact action potentials in CMs. The enzymatic activity and protein for this ATPase were strikingly reduced in CMs during sepsis by unknown mechanisms. In addition, C5a interactions with C5aRs also caused reductions in CM homeostatic proteins that regulate cytosolic [Ca²⁺]i in CMs: sarco/endoplasmic reticulum Ca²⁺-ATPase2 (SERCA2) and Na⁺/Ca²⁺ exchanger (NCX). In the absence of C5a receptors, defects in SERCA2 and NCX in CMs after sepsis are strikingly attenuated. These observations suggest new strategies to protect the heart from dysfunction developing during sepsis.