Identification of the core exocytic machinery for surfactant secretion in type II alveolar epithelial cells

• Applicant: Professor Dr. Manfred Frick
• Subject Area: Anatomy and Physiology ; Cell Biology
• Term: since 2010
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 175083951

Secretion of surfactant in the alveoli of the lung is essential to maintain lung function. Dysregulated secretion of surfactant results in severe diseases. Selective targeting of surfactant secretion therefore offers great potential for treating lung diseases. However, selective targeting requires a detailed understanding of the underlying secretory mechanisms. Surfactant is secreted via exocytosis of intracellular storage granules, the lamellar bodies (LBs). Exocytosis is a conserved mechanism in eukaryotic cells comprising several stages. These include tethering and docking of exocytic vesicles to the plasma membrane during the prefusion phase until vecicles fuse with the plasma membrane and a fusion pore opens. Following fusion (during the postfusion phase) vesicle contents are released. Secretion can be regulated at all stages. Within recent years, we have unraveled a complex regulatory machinery that is required for efficient surfactant secretion during the postfusion phase. Now, we want to extend these efforts and decipher the core exocytic machinery that regulate LB fusion with the plasma membrane during the prefusion phase. These factors determine the number and kinetics of LB fusion events and hence directly control the amount of surfactant secretion. SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are ultimately responsible for catalyzing membrane fusion in secretory cells. The SNARE complex is a four-helix bundle comprised of three helices attached to the target membrane (t-SNAREs, i.e. syntaxins and SNAP proteins) and one attached to the vesicle membrane (v-SNARE, i.e. VAMP proteins). Specific binding of these helices confers accuracy and directionality on the fusion reaction. And can be viewed as the core exocytic machinery. Full coiling provides the energy to fuse the lipid membranes. Additionally, regulatory factors, including synaptotagmin and complexin, mediate docking and Ca2+-dependent fusion. We have recently demonstrated how a detailed understanding of the core secretion machinery enables the rationale development of a therapeutic compound. We selectively inhibited excess mucin secretion in the airways without impairing homeostatically required secretion. However, the core exocytic machinery of surfactant secretion, in particular it´s functional characterization, is mostly enigmatic. Within this project, we aim at unraveling this missing link in our understanding of the mechanisms regulating surfactant secretion. We want to elucidate the immediate regulatory machinery necessary for LB fusion with the PM, i.e. delineating the core exocytic machinery regulating LB exocytosis, similar to what has been achieved for mucus secretion already and decipher a potential regulatory role for Ca2+ in baseline / stimulated secretion.

• DFG Programme: Research Grants