The alveolar epithelial glycocalyx at the blood-gas barrier of the lung

The alveolar epithelial glycocalyx is a hydrogel consisting of proteoglycans, glycoproteins and glycosaminoglycans that covers the entire 120 m² of alveolar surface area in the human lung. As such, the alveolar epithelial glycocalyx is an essential part of the first line of defense of the blood-gas barrier, an ultrathin structure between the airspaces and the blood vessels of the lung that allows for gas exchange of O₂ and CO₂, while contemporaneously restricting leakage of fluids and proteins into the airspace, or entry of inhaled pathogens into the circulation. Despite its relevance in lung barrier function and pathogen defense, the alveolar epithelial glycocalyx is so far virtually unexplored. Importantly, the alveolar epithelial glycocalyx differs from other glycocalices at cell or organ surfaces with respect to its unique physicochemical microenvironment within the alveolar space where it is a) constantly exposed to cyclic biaxial stretch as a result of alveolar inflation and deflation over the respiratory cycle, and b) co-localizes, and presumably interacts, with alveolar surfactant, a surface tension lowering mix of amphiphilic lipids and surfactant proteins secreted by alveolar epithelial type II cells.

In the first funding period, we employed multiple electron (EM) and fluorescence microscopy techniques to visualize the AEGlx, including 3D electron tomography (ET) of thorium-dioxide stained samples to visualize the AEGlx and metabolic glyco-engineering (MGE) labeling to discern the localization and regulation of sialic acid (SA) in the AEGlx (C05N|Hackenberger/Lauster). We developed novel molecular tools to interrogate sialylated proteins (A04|Seitz/Block) and identified expression of glyco-RNA on the epithelial surface. Mechanical stretch as experienced in mechanical ventilation increased sialylation of distinct surface glycoproteins, while GAGs including hyaluronic acid (HA) and heparan sulfate (HS) were found to be shed from lungs of acute respiratory distress syndrome (ARDS) patients. We further showed that lung collectins bind to SA and GAGs, thereby altering the AEGlx ultrastructure (C05N|Hackenberger/Lauster).