Organelle-to-Organelle Crosstalk in Mast Cells: How does Endoplasmic Reticulum Secretory Granule Interaction impact the MC's Immune Responses?

Objective

Mast cells (MCs) are tissue resident immune cells best known for their role in allergy. However, as part of the adaptive and innate immune system they engage in various pathologies including cancer and neurodegeneration. When MCs exert their immune response, special lysosome-related organelles, called secretory granules (SG), in which protective and proinflammatory mediators are prestored, undergo exocytosis. The recent discovery that during this process also mitochondrial and endoplasmic reticulum (ER) proteins are secreted, suggests that SGs may have, in addition to their extracellular immune, an intracellular metabolic function, such as in mito- or ER-phagy. In preliminary work using ultra expansion microscopy (uExM) we indeed found that the ER-phagy receptor SEC62 also localizes to the SG lumen. Strikingly, we further discovered that the exocytosis of SGs is accompanied by drastic ER-remodelling, particularly peripheral ER-sheet formation. However, the biological purpose as well as the underlying mechanism of this process and whether ER-phagy may be involved is currently unclear. Further, it remains to be elucidated which of the three ER-phagic pathways (macro-, micro-ER-phagy, LC3-dependent vesicular transport) connects to the SGs and whether the fate of the resulting vesicle represents a way of unconventional protein secretion. Due to the size of ER membranes, the sub-vesicular/ER-phagic structures and the densely filled MC cytoplasm, the investigation of the assembly and evolution of SG-ER contacts demands a look beyond the diffraction limit of light. Therefore, I will employ different super-resolution methods such as uExM and live cell Stimulated Emission Depletion (STED) microscopy in combination with automated image analysis, to answer these questions. This work will advance our understanding of the unique cell biology of MCs and the formation and release of their SGs giving rise to new drug targets to modulate MC immune responses in health and disease.