MadSci Network: Cell Biology |
Pinocytosis involves the fluid-phase takeup of extracellular media by cells undergoing membrane invagination. As such, it is a form of endocytosis. I will limit the next two paragraphs to mammalian cells, although it is likely that all eukaryotic cells can undergo some form of pinocytosis. Known examples of this process in non-mammalian cells include the parasitic protozoan Entamoeba histolytica (causes amoebic dysentery) and Dictyostelium, a slime mold studied often in cell biology to determine cellular components affecting locomotion. Requirements for pinocytosis in these unicellular organisms, including plasma membrane cholesterol and lipid rafts, mirror that of mammalian cells. Membrane flexibility and integrity seem to be universal requirements for unicellular and multicellular life. Neutrophils and macrophages are the two cell types that have been most commonly used to study pinocytosis. These phagocytic cells are the vanguard of the immune response, and often rush into inflamed sites to engulf microorganisms, damaged cells and other debris. Although these latter functions featuring the specific takeup of particles are more likely to involve directed, receptor dependent endocytosis, pinocytosis can also play an important role. Pinocytosis can aid the clearance of bacterial toxins, or the dampening of the immune response by the clearance of soluble activating factors. Apart from the absorption of the trapped volume of liquid that is internalized with each pinocytosed vesicle, the process also allows the recycling of membrane components that would otherwise build up during degranulation as secretory vesicles are delivered to the membrane to release their cargo to the outside environment. The surprising thing about pinocytosis is that unactivated macrophages have a very low amount of background pinocytosis. Hence, the primary event that stimulates pinocytosis is the activation of the cell. This can occur through numerous molecular pathways, such as kinase networks, and involve release of intracellular ions such as Ca2+. There is a long list of substances that can activate macrophages, such as bacterial lipopolysaccharides, CpG motifs in DNA, low density lipoproteins, and antibodies via Fc receptors. Hence all of these diverse substances can activate pinocytosis. It's like keeping track of daily stimuli that can arouse anger in a person, and trying to find a commonality. It would be very difficult to catalogue an extensive list and such a collection would always be context dependent. Nonetheless, we can be fairly certain that once angry, people will undergo predictable changes in behavior and physiology. As far as positively charged ions, it has been known for some time that small peptides containing positively charged amino acids (such as Lys and Arg) can be taken up non-specifically by mammalian cells, through an unknown mechanism that likely shares features with pinocytosis. However, the cells still need to be activated by other means. These peptides likely bind to the phosphate head groups of lipids on the outer leaflet of the plasma membrane, which are negatively charged. One must be careful to discriminate between agents that cause a process from ones that merely participate in it.
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