Rapid formation of ER‐PM contacts during early stages of phagocytosis

Abstract

Membrane contact sites (MCS) are specialized structures where the endoplasmic reticulum (ER) and other organelle form come in close proximity (~10–30 nm). Functionally, these regions are thought to serve as hubs for cellular processes such as inter‐organellar exchange of lipids, calcium homeostasis, and the access of phosphatases to substrates. Consequently, the spatial‐temporal organization of these structures will impact numerous cellular functions. The formation of ER plasma membrane (PM) contact sites are controlled by a variety of membrane tether proteins as well as the morphology of the ER. We and others have found that the sub‐plasmalemmal cortical actin cytoskeleton, a mesh‐like network that occupies 100–200 nm below the PM, can also limit the formation of ER‐PM contact sites. Yet, how ER‐PM junctions are regulated in circumstances where cells have extensive cortical actin remodeling is not understood. An example of such cellular process involving dynamic cortical actin re‐arrangement is phagocytosis. Phagocytosis is an actin‐dependent processes used to internalize particulate material (≥0.5 μm) that serves both antimicrobial and homeostatic functions. The role of the ER in phagocytosis has been a matter of much controversy. Electron micrographs of macrophages undergoing phagocytosis revealed the presence of ER in extensive, close contact with the forming phagosome. It is speculated that the role for ER in this context is to form ER‐PM contact sites with the PM. My studies have revealed that during phagocytosis, the disassembly of F‐actin from the base of the phagocytic cup allows for the formation of new ER‐PM contact sites. ER‐PM contacts formed promptly and specifically where the polymerized actin has been cleared. As such I have found that the spatial occupancy of ER‐PM junction increased ~3 fold during phagocytosis. Finally, I identified PTP1B, a tyrosine phosphatase, as one of the ER‐PM contact proteins that may be functionally important during phagocytosis.Support or Funding InformationNSERC PGS‐DCIHR Project Grant