Rationale: Acute lung injury (ALI) is a debilitating condition associated with severe skeletal muscle weakness that persists in humans long after lung injury has resolved. The molecular mechanisms underlying this condition are unknown. Objectives: To identify the muscle-specific molecular mechanisms responsible for muscle wasting in a mouse model of ALI. Methods: Changes in skeletal muscle weight, fiber size, in vivo contractile performance, and expression of mRNAs and proteins encoding muscle atrophy-associated genes for muscle ring finger-1 (MuRF1) and atrogin1 were measured. Genetic inactivation of MuRF1 or electroporationmediated transduction of miRNA-based short hairpin RNAs targeting either MuRF1 or atrogin1 were used to identify their role in ALIassociated skeletal muscle wasting. Measurements and Main Results: Mice with ALI developed profound muscle atrophy and preferential loss of muscle contractile proteins associated with reduced muscle function in vivo. Although mRNA expression of the muscle-specific ubiquitin ligases, MuRF1 and atrogin1, was increased in ALI mice, only MuRF1 protein levels were up-regulated. Consistent with these changes, suppression of MuRF1 by genetic or biochemical approaches prevented muscle fiber atrophy, whereas suppression of atrogin1 expression was without effect. Despite resolution of lung injury and down-regulation of MuRF1 and atrogin1, force generation in ALI mice remained suppressed. Conclusions: These data show that MuRF1 is responsible for mediating muscle atrophy that occurs during the period of active lung injury in ALI mice and that, as in humans, skeletal muscle dysfunction persists despite resolution of lung injury.Keywords: skeletal muscle atrophy; intensive care unit-acquired weakness; critical illness myopathy; muscle atrophy genes; proteasomal-mediated protein degradation Acute lung injury (ALI) is a syndrome characterized by the acute onset of pulmonary infiltrates and respiratory failure often leading to the need for mechanical ventilation (1). Approximately 200,000 people per year develop ALI in the United States, with mortality high at 30-40% (2). A common complication associated with ALI is skeletal muscle weakness. Weakness in these patients results in decreased long-term mobility and functional status. Skeletal muscle weakness is initiated early in the course of ALI, and has been shown to persist in a large percentage of patients for up to 5 years after resolution of lung injury and hospital discharge (3-6). Although multiple factors may contribute to ALI-induced muscle atrophy including reduced nutrition, inactivity caused by bed rest, and systemic inflammation, the etiology of ALI-associated skeletal muscle atrophy remains incompletely understood.Skeletal muscle weakness is a common finding not only among patients with ALI, but also in patients with other critical illnesses. Clinically apparent weakness is present in 20-50% of patients with critical illness and has been shown to be an independent risk factor for mortality in these patients (3,5,7,8)....
Flow cytometry is a powerful tool capable of simultaneously analyzing multiple parameters on a cell-by-cell basis. Lung tissue preparation for flow cytometry requires creation of a single-cell suspension, which often employs enzymatic and mechanical dissociation techniques. These practices may damage cells and cause cell death that is unrelated to the experimental conditions under study. We tested methods of lung tissue dissociation and sought to minimize cell death in the epithelial, endothelial, and hematopoietic lineage cellular compartments. A protocol that involved flushing the pulmonary circulation and inflating the lung with Dispase, a bacillus-derived neutral metalloprotease, at the time of tissue harvest followed by mincing, digestion in a DNase and collagenase solution, and filtration before staining with fluorescent reagents concurrently maximized viable yields of epithelial, endothelial, and hematopoietic lineage cells compared with a standard method that did not use enzymes at the time of tissue harvest. Flow cytometry identified each population-epithelial (CD326(+)CD31(-)CD45(-)), endothelial (CD326(-)CD31(+)CD45(-)), and hematopoietic lineage (CD326(-)CD31(-)CD45(+))-and measured cellular viability by 7-aminoactinomycin D (7-AAD) staining. The Dispase method permitted discrimination of epithelial vs. endothelial cell death in a systemic lipopolysaccharide model of increased pulmonary vascular permeability. We conclude that application of a dissociative enzyme solution directly to the cellular compartments of interest at the time of tissue harvest maximized viable cellular yields of those compartments. Investigators could employ this dissociation method to simultaneously harvest epithelial, endothelial, and hematopoietic lineage and other lineage-negative cells for flow-cytometric analysis.
Apoptosis is a key pathologic feature in acute lung injury. Animal studies have demonstrated that pathways regulating apoptosis are necessary in the development of acute lung injury, and that activation of p38 mitogen-activated protein kinase (MAPK) is linked to the initiation of the apoptotic cascade. In this study, we assessed the role of the MAPK-activated protein kinase (MK) 2, one of p38 MAPK's immediate downstream effectors, in the development of apoptosis in an animal model of LPS-induced pulmonary vascular permeability. Our results indicate that wild-type (WT) mice exposed to LPS demonstrate increased apoptosis, as evidenced by cleavage of caspase 3 and poly (ADP-ribose) polymerase 1 and increased deoxynucleotidyl transferase-mediated dUDP nick-end labeling staining, which is accompanied by increases in markers of vascular permeability. In contrast, MK2 2/2 mice are protected from pulmonary vascular permeability and apoptosis in response to LPS. Although there was no difference in activation of caspase 3 in MK2 2/2 compared with WT mice, interestingly, cleaved caspase 3 translocated to the nucleus in WT mice while it remained in the cytosol of MK2 2/2 mice in response to LPS. In separate experiments, LPS-induced apoptosis in human lung microvascular endothelial cells was also associated with nuclear translocation of cleaved caspase 3 and apoptosis, which were both prevented by MK2 silencing. In conclusion, our data suggest that MK2 plays a critical role in the development of apoptosis and pulmonary vascular permeability, and its effects on apoptosis are in part related to its ability to regulate nuclear translocation of cleaved caspase 3.
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