© 2005 American Thoracic Society doi: 10.1164/rccm.2507005
Respiratory Failure During Critical IllnessAre Mitochondria to Blame?The Ohio State University Medical Center, Columbus, Ohio The inability of critically ill patients to wean from ventilator support has serious implications, including predisposition to life-threatening infections and increased costs of care attendant to prolonged intensive care unit (ICU) stays. Recent investigations suggest that the cause of ventilator dependency in the ICU setting is multifactorial and that muscle weakness, particularly diaphragmatic, plays an important role (1). The cause of diaphragmatic weakness in the ICU setting remains unknown, but recent evidence suggests that muscle injury is a manifestation of systemic activation of inflammatory pathways, such as occurs during severe sepsis. The investigative team of Callahan and Supinsky has contributed greatly to the current understanding of diaphragmatic weakness in the context of critical illness. Using an animal model of sepsis (endotoxemia), they have previously shown that the diaphragm exhibits abnormal oxygen use and is susceptible to mitochondrial dysfunction, including impaired ATP production (2). From a mechanistic standpoint, mitochondria isolated from diaphragms of endotoxemic animals produce reactive oxygen species (ROS) at accelerated rates (3), which presumably contributes to oxidative modification of various diaphragmatic components, including proteins participating in glycolysis, ATP production, and muscle contraction (4). Oxidant stress, in turn, results in diaphragmatic contractile dysfunction (5). Considered together, it is reasonable to conjecture that mitochondrial dysfunction, including impaired high-energy phosphate production and accelerated ROS production, contributes significantly to the pathogenesis of diaphragmatic contractile dysfunction in critically ill patients.
In this issue of the Journal (pp. 861–868), Callahan and Supinski (6) sought to determine how changes in mitochondrial protein expression relate to altered mitochondrial functional in endotoxin-treated rats. The study design was unique in that intact mitochondrial respiratory complexes were isolated in a single dimension using Blue Native Polyacrylamide Gel Electrophoresis (BN PAGE) followed by separation of mitochondrial complex proteins in a second dimension using a denaturing gel, which provides for quantitative comparisons of mitochondrial complex constituents and the subsequent identification of specific proteins employing tryptic digest/MALDI-TOF techniques. Using this approach, they determined that seven proteins integral to mitochondrial electron transport at complexes I, III, and IV ( Although previous studies have shown that mitochondria are functionally compromised in the diaphragm in the context of sepsis (2), the current study significantly advances our understanding of the process. As noted by the authors, impaired mitochondrial protein gene transcription due to mitochondrial DNA damage, such as is observed in the liver of endotoxemic rats (7), is an unlikely explanation for the pattern of mitochondrial protein depletion observed in the diaphragm, as only two of the seven depleted proteins are encoded by mitochondrial DNA. It is interesting to note that five of the seven depleted proteins contain iron–sulfur complexes, which are highly susceptible to damage by oxidant stress (8). In light of previous investigations showing that endotoxemia induces accelerated mitochondrial ROS production in the diaphragm (3), and others demonstrating that oxidized proteins are targeted for proteolytic cleavage (9), it is logical to conclude that oxidative modification and subsequent clearance of susceptible mitochondrial proteins accounts for the observed selectivity of mitochondrial protein depletion. It is interesting to speculate that mitochondrial protection may be one mechanism by which antioxidant treatments effectively preserve diaphragmatic function in animal models of sepsis (10). Beyond the inherent limitations of employing animal models to mimic human sepsis, several potentially confounding factors should be considered while interpreting the results of these investigations. As noted by the authors, mitochondrial isolation by differential centrifugation selects for a subpopulation of mitochondria with uniform characteristics (e.g., similar size and density). As such, it is likely that relatively low-speed centrifugation (5,000 x g) excludes the most damaged (e.g., fragmented) mitochondria from analysis, and thereby underestimates the severity and extent of mitochondrial damage in the endotoxemia group. Furthermore, damaged mitochondria are targeted for lysosomal clearance (autophagy) (11), and this mechanism could result in reduced mitochondrial mass in the diaphragm, such as has been observed in heart muscle during sepsis (12). This possibility was not considered in the current study. Finally, altered mitochondrial protein expression is unlikely to be the sole determinant of impaired mitochondrial function during endotoxemia. For instance, the oxidative stress attendant to endotoxemia is likely to be associated with post-translational protein modifications and consequent mitochondrial dysfunction (13). Despite these limitations, the study by Callahan and Supinski makes important progress toward understanding the complex mechanisms by which systemic inflammation causes mitochondrial dysfunction by integrating the time-dependency of molecular events with functional outcomes in a relevant animal model. The clinical implications of mitochondrial dysfunction in the diaphragm remain unclear. In this context, mitochondrial respiratory rates are highly regulated in vivo such that significant respiratory reserve is available, at least under normal physiologic conditions (14). Thus, it is possible that mitochondrial dysfunction does not influence diaphragmatic contractile function, at least at lower workloads. Certainly, a 50% reduction in oxidative capacity, such as observed in diaphragm mitochondria during endotoxemia (6), predicts that maximal diaphragm work (contractility) would be limited, and this could have important implications in the context of acute lung injury, wherein the work of breathing is often increased. This being said, it is also important to consider that the mechanisms of mitochondrial damage and the severity of mitochondrial dysfunction may be organ-specific. Thus, caution should be exercised when attempting to extrapolate the results of these investigations to other organ systems. To the extent that mitochondrial dysfunction contributes to impaired organ function, new therapeutic opportunities should be considered. For example, antioxidant molecules that are generally targeted to mitochondria (15), and manipulation of proteins acting on mitochondrial membrane pores (16) are demonstrated to preserve mitochondrial integrity during conditions associated with oxidative stress, such as sepsis. Additional investigations detailing the mechanisms and timing of mitochondrial injury during critical illness are necessary before it is possible to develop highly effective mitochondrial protection strategies for application in the clinical setting. FOOTNOTES Conflict of Interest Statement: E.D.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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10% of all proteins comprising the electron transport chain) were depleted within 24 h of endotoxin treatment relative to controls. The temporal association between mitochondrial protein depletion and impaired mitochondrial respiration supports the authors' conclusion that selective depletion of proteins integrally involved in electron transport is causally related to impaired electron transport and reduced ATP production in the diaphragm of endotoxemic animals.