Endocytosis is the process by which cells take in molecules from their surroundings. Clathrin-dependent endocytosis requires the recruitment of clathrin to help curve the plasma membrane into the vesicle which absorbs the molecules. More recently, it was discovered that other, non-clathrin-dependent pathways existed. Macropinocytosis is one such pathway, creating larger vesicles without the use of clathrin.
Initiation
Macropinocytosis was first described by Warren Lewis in 1931. He observed that the cell membrane would create a wave budding out from the surface, which then folded back on the membrane and thus created a vesicle. While Lewis initially called the process pinocytosis, it was later renamed macropinocytosis to distinguish from smaller, clathrin-dependent, or uncoated vesicles.
The membrane folding and vesicle formation were later discovered to be dependent on actin, a component of the cytoskeleton, and Rac1, a signaling protein. Macropinocytosis is a non-selective endocytosis pathway, facilitating the uptake of several nutrients and antigens.
Macropinocytosis is a signal-dependent pathway, meaning it needs to be induced by certain ligands. Usually it is induced by growth factors such as macrophage colony-stimulating factor-1 (CSF-1), epidermal growth factor (EGF), and tumor promoting factor. Downstream of this, macropinocytosis is dependent on Rac1, an intracellular signaling protein. This protein can in turn be activated by PAK1, which is also implicated in cytoskeletal remodeling.
Cholesterol also has a role in macropinocytosis, as it is vital for the recruitment of Rac1 to membrane ruffling sites.
Once stimulated by signaling, the membrane is ruffled. This is done by actin, which forms the cellular cytoskeleton. The cytoskeletal rearrangements by actin give rise to membrane ruffles. These ruffles made by actin are also called lamellipodia. Although many of the lamellipodia disappear back into the membrane, some will rise and then fold back on themselves, creating a vesicle.
While folding, the lamellipodia trap solutes or molecules inside the newly formed vesicles. These vesicles are called macropinosomes.
Macropinosomes
The macropinosomes have no distinct membrane structure and can therefore vary in shape and size. However, they are larger than the vesicles produced by clathrin-dependent endocytosis. After formation, the macropinosomes mature inside the cell. This maturation process is likely to vary between cells.
In macrophages originating from bone marrow, macropinocytosis occurs due to CSF-1 signaling. The formed macropinosomes move towards the lysosome, in doing so they become smaller and go through changes of endocytic protein markers. Specifically, they first acquire transferrin receptors.
These are then lost, while Rab7 and lysosomal glycoprotein A are gained. Rab7 is lost, while lysosomal glycoprotein A remains on the macropinosome. The final step has the macropinosome merge with the tubular lysosomal compartments in the cell.
In contrast to this, macropinosomes in A341 cells do not fuse with the lysosome complex. When macropinocytosis is induced using EGF, the macropinosomes acquire transferrin receptors and endosomal antigen 1 (EEA1). They do not appear to mature beyond this stage. This is because EEA1 facilitates fusion of several macropinosomes together. The fused macropinosomes eventually recycle back to the plasma membrane, releasing their contents. How these different macropinosome fates are based in different cell types is not fully understood.
Regulation
The regulation of macropinocytosis is not well understood. Given the breadth of responses cells can have to it, only some processes have been discovered. The formation of the vesicle itself seems to involve actin-regulating proteins. Coronin is one protein involved in actin dynamics. It was shown that while coronin is present in the lamellipodia and the subsequent vesicle, coronin was lost from the macropinosome after one minute of internalization. It appears a similar process occurs for the actin itself, as it also disassociates from the macropinosome after formation. Therefore, it seems the actin association with the macropinosome decreases with maturation.
The process of macropinocytosis itself requires a functional cell plasma membrane. Using amiloride, a blocker of the Na+/H+ exchanger pump in the membrane, macropinocytosis can be blocked. Amiloride does not block clathrin-dependent endocytosis.
It is hypothesized that the macropinocytosis inhibition is due to acidification as excess H+ enters the cell at the membrane. While this does not affect receptors, it can have effects on downstream proteins, such as Rac1, which are critical for actin remodeling.
Further Reading