SUMMARY Acute respiratory distress syndrome (ARDS) is a life-threatening inflammatory lung disease due to significant airspace (alveolar) flooding, also known as pulmonary edema. Chronic alcohol abuse significantly increases the severity and risk of developing ARDS. In the normal lung, sodium-driven fluid clearance and epithelial barrier integrity maintain the airspace fluid balance. The alveoli, where gas exchange occurs, is surrounded by a fluid- filled interstitium and capillaries. In response to chronic alcohol use, fluid in the airspace increases due to paracellular leak from the interstitium and capillary blood vessels, rendering the lung more susceptible to developing ARDS. Patients with a history of chronic alcohol abuse resolve increased extravascular lung fluid slower. Alcohol abuse is also a significant risk factor for critical illnesses, such as pneumonia, that require rehydration or blood pressure stabilization, treatments that increase pressure from fluid in the interstitium surrounding the alveoli. I will first use a novel approach to measure fluid pressure-induced leak in the alcoholic lung and define the mechanisms that cause differential permeability by measuring the effect of increased fluid pressure on pulmonary edema in alcoholic lung syndrome at baseline and in pneumonia. To accomplish this, I will measure the threshold at which alveolar flooding occurs in normal and alcohol-fed mice at baseline and in response to bacterial pneumonia; at the same intravascular injection volume, we hypothesize that lungs from alcohol-fed mice are more susceptible to alveolar flooding than a healthy lung. Identifying a lowered leak threshold could impact the clinical management of patients with a history of alcohol abuse. In addition to leak threshold, the molecular mechanisms behind alcoholic lung syndrome need to be further investigated. Tight junction (TJ) proteins regulate paracellular solute passage and prevent fluid leakage into the airspace. Chronic alcohol abuse impairs the alveolar epithelial barrier by increasing paracellular diffusion and changing the protein composition of claudins, a family of TJ proteins that regulate fluid and solute homeostasis. Specifically, alcohol decreases the barrier-protective TJ protein Claudin-4 and increases the barrier-disruptive TJ protein Claudin-5 in cultured alveolar epithelial cells. In the second aim, I will measure the effect of altered Claudin-4 and/or Claudin-5 expression on lung permeability in vivo. I will manipulate levels of Claudin-4 and Claudin-5 in the lower airway and measure changes in baseline lung permeability using a modified Evans Blue permeability assay. In this project, I hypothesize that chronic alcoholism decreases Claudin-4 and increases Claudin-5 which impair alveolar epithelial barrier function and increases the sensitivity of the lung to develop alveolar flooding. A long-term goal for this translational project is to provide a foundation to inform future clinical studies by evaluating novel therapeutically viable targets to improve lung barrier function at the molecular level. By understanding how clinical parameters such as fluid pressure influence lung barrier function in response to alcohol, we hope to provide a novel evidence-based approach that can be applied to the treatment of ARDS.