Macrophages bite off pieces of cells containing protein aggregates, guided by a signal on the cell surface: a preprint from immunologist Ruslan Medzhitov’s group at Yale University, based on experiments in mouse cells
Macrophages bite off pieces of cells containing protein aggregates, guided by a signal on the cell surface: a preprint from immunologist Ruslan Medzhitov’s group at Yale University, based on experiments in mouse cells
On October 19, Medzhitov’s group posted a study on bioRxiv, a server for manuscripts that have yet to undergo peer review. In mouse cell cultures, immune cells called macrophages pinched off pieces of cells with impaired lysosomes, taking away aggregates, or clumps of misfolded proteins. The signal was a membrane-bound form of the macrophage growth factor CSF1, which cells display on their surface in response to lysosomal stress.
Protein aggregates interfere with lysosomes, the compartments in which cells normally break down these aggregates, and accumulate in neurodegenerative diseases and with age. In 1997, Medzhitov and Charles Janeway described the first human Toll-like receptor, one of the sensors that immune cells use to detect microbes. His group proposed that when a stressed cell’s own responses are insufficient, it may call for help from cells with a high capacity to break down proteins, such as macrophages.
Mouse fibroblasts (connective tissue cells) were treated for 48 hours with bafilomycin A1, which prevents lysosomal acidification and impairs lysosomal function, causing aggregates to accumulate. When macrophages were added, they took small particles of labeled protein from the fibroblasts; a dye that detects protein aggregates stained more than 90 % of these particles. Separating the cells with a porous membrane almost eliminated uptake, indicating that the macrophage needs to touch the cell. This process, in which one cell bites off a piece of another, is called trogocytosis.
The researchers tested possible mechanisms by which macrophages recognize these cells, one at a time. The “eat me” signal displayed by dying cells and several established engulfment pathways proved unnecessary. Uptake decreased when they blocked CSF1R, the receptor for the growth factor CSF1. CSF1 has two forms: soluble and membrane-bound (tmCSF1). Because uptake required contact, the authors tested the membrane-bound form. In cells engineered to express one of the two forms, only the membrane-bound form triggered trogocytosis, and macrophages left neighboring cells without it untouched. In the presence of macrophages, fibroblasts cleared aggregates and survived better. Fibroblasts from mice lacking the membrane-bound form cleared aggregates less effectively and showed no improvement in survival. The same receptor, CSF1R, supports macrophage proliferation when it binds soluble CSF1 and triggers trogocytosis when it encounters the membrane-bound form.
Lysosomal stress shifts CSF1 production toward the membrane-bound form. According to the authors, aggregates are packaged into vesicles, and the motor protein KIF1A transports them along microtubules toward the cell surface. There, the scaffold protein MPP1 links the cargo to the intracellular tail of tmCSF1, so that a macrophage recognizing tmCSF1 takes up the aggregates specifically. Without MPP1, fewer aggregates reach the vicinity of the cell surface, and clearance is less effective.
In cells engineered to express membrane-bound CSF1, macrophages used the same signal and receptor pair to remove aggregates of mutant huntingtin, the protein associated with Huntington’s disease. These aggregates were produced by overexpression without bafilomycin. Fibroblast aggregates contain many cytoskeletal proteins, and the authors note that this composition resembles that of aggregates in tissues of aging fish, specifically turquoise killifish.
The authors suggest that trogocytosis “becomes increasingly important as the cell’s own protective mechanisms fail under chronic stress.” They propose that senescent cells, which have stopped dividing, may also need help from macrophages because their lysosomes function less effectively and misfolded proteins accumulate. Understanding how trogocytosis is regulated could, they suggest, lead to new treatments for diseases involving protein aggregates, particularly at advanced stages, “when most diagnoses are made.”