Ventilator-induced diaphragmatic atrophy: role of FoxO signaling

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DESCRIPTION (provided by applicant): Mechanical ventilation (MV) is a clinical tool to sustain pulmonary gas exchange in patients that are incapable of maintaining adequate alveolar ventilation. Although MV is often a life-saving intervention, prolonged MV promotes problems in "weaning" patients from the ventilator. While several factors can contribute to difficult weaning, weak inspiratory muscles are a major factor. In this regard, MV results in diaphragmatic inactivity that promotes the rapid onset of inspiratory muscle weakness due to diaphragmatic atrophy and contractile dysfunction (known as ventilator-induced diaphragm dysfunction (VIDD)). Although MV-induced diaphragmatic atrophy occurs due to both increased protein breakdown and decreased protein synthesis, the mechanism(s) that regulate these processes are poorly understood and thus, no clinical therapy exists. Hence, our long-term goal is to identify biological targets that will assist in the development of a therapeutic strategy to preven VIDD and therefore, protect against weaning difficulties. Although numerous signaling events can contribute to VIDD, we predict that the forkhead boxO (FoxO) family of transcription factors are important because they regulate the expression of genes involved in both the ubiquitin-proteasome and the autophagy pathway of proteolysis. Moreover, gene targets of FoxO may also contribute to the depression of protein synthesis. HYPOTHESIS: We will test the hypothesis that activation of FoxO-dependent transcription is required for the decreased protein synthesis, increased expression of proteasome/autophagy genes, and fiber atrophy that occurs in the diaphragm during MV. APPROACH: Our hypothesis will be tested in a well-established rat model of MV. Cause and effect will be determined by using an adeno-associated virus vector for gene delivery to express a dominant negative FoxO to prevent activation of FoxO target genes in the diaphragm during prolonged MV. SPECIFIC AIMS: Aim 1 will establish if FoxO-dependent transcription is essential for the rapid decrease in diaphragmatic protein synthesis that is observed during prolonged MV. Aim 2 will determine if FoxO-dependent transcription is required for the MV-induced increase in the expression of proteasome and autophagy genes along with diaphragmatic atrophy. SIGNIFICANCE: Collectively, this work can provide the foundation for new therapeutic strategies in the prevention of VIDD, a major contributor to the inability to wean patients from the ventilator.