Receptor-Mediated Endocytosis

After docking, there is a secondary influx of calcium at the active zone, which causes the fusion of the vesicle to the membrane. This forms a temporary ion channel. As mentioned, this pore opens because of the binding of synaptophysin and physophilin. It requires ATP energy in order to do this. If calcium doesn’t influx, no fusion pore can happen.

Synaptotagmin binds to calcium; if it does not bind to calcium, it acts like a clamp in order to inhibit the fusion of the vesicle. It activates fusion when calcium is present by pulling the vesicle closer to the presynaptic membrane. After the pore happens, it dilates quickly so that exocytosis can occur.

This excess membrane made by fusion does not keep enlarging the membrane. Instead, it forms a pit that is coated with clathrin. This clathrin-coted pit eventually pinches off the vesicle again so that an empty synaptic vesicle is made again.

RECEPTOR-MEDIATED ENDOCYTOSIS

Endocytosis involves the essential opposite of exocytosis. Substances are brought into the cell, forming a vesicle. There are two types of endocytosis: pinocytosis, which is also referred to as cell-drinking, and phagocytosis, which is also referred to as cell-eating. There is also receptor-mediated endocytosis, and caveolae, which will be discussed.

Clathrin-mediated endocytosis is the same thing as receptor-mediated endocytosis. It makes small vesicles about 100 nanometers across, coated with clathrin. They are formed from clathrin-coated pits that start the entire process of endocytosis. There are different receptors that participate in this type of endocytosis.

Caveolae are not related to clathrin. There is a cholesterol-binding protein called caveolin that participates in making small pits in the membrane. They are seen in smooth muscle cells, adipocytes, endothelial cells, type 1 pneumocytes, and fibroblasts in high concentrations. There are receptors in these caveolae that mediate their formation. Caveolae participate in potocytosis, which is the uptake of molecules that are released into the cytosol rather than into other organelles.

With pinocytosis, there is a pocket that pinches of to make a vesicle that is filled with a lot of extracellular fluid. It fuses with other vesicles like endosomes or with lysosomes. Phagocytosis involves taking in large particles, sometimes as large as parts of apoptotic cells (dead cells). The big difference is that a lot of extracellular debris is taken up by the cell.

There is an endocytic pathway that starts at the plasma membrane and ends at the lysosomes. Early endosomes are seen in the cell periphery. They sort the vesicles near the cell membrane. Late endosomes will take material that is on the way to the lysosomes. They are acidic vesicles that help also to sort the contents of that which is endocytosed into the cell. Lysosomes are the end of the endocytic pathway. They break down the products of cellular waste into much simpler compounds. There are forty different hydrolytic enzymes that participate in the process. The pH is extremely low at about 4.8.

Clathrin-mediated endocytosis is the best understood and is a major route for cellular endocytosis. Clathrin can form pits on the inside of the plasma membrane in order to form a coated vesicle inside the cell. The coat needs to be shed before the vesicle can continue on to the endosomes down the endocytic pathway.

Endocytosis with clathrin usually means that a receptor has been bound by something that triggers clathrin to form the pit on the inside of the cell. There are many ligands that can bind to a receptor. The choices include attacking viruses that bind to the cell membrane, a nutrient molecule that needs to get inside, and any other molecule (such as LDL-cholesterol) that needs to be taken up by the cell. Transferrin is a serum protein that causes iron to be taken up by the cell. Some of the ligands get broken down, while others are exocytosed in order to be recycled.

Once these vesicles get taken up by an endosome, the contents get acidified, which can affect the characteristics of the ligand. In the case of LDL, the LDL disconnects from the LDL receptor gets packaged up again so it can be recycled. LDL goes into a different vesicle to be transported elsewhere in the cell.

The lysosome is highly acidic and has hydrolytic enzymes that work in the acidic environment but are inactivated in the environment of the cytosol. This means that, if

the lysosome breaks, it doesn’t allow enzymes to digest any part of the rest of the cell. There are proton pumps on the lysosome that lead to the high acidity of the environment.

Lysosomal enzymes are tagged with mannose-6-phosphate, which is added in the cisGolgi apparatus. This is how these enzymes get recognized as being lysosomal enzymes. They get sorted out in the trans-Golgi apparatus, where they are sent to the lysosomes.

When some of these acid hydrolases in the lysosome don’t work properly, the end result is an incomplete digestion of lysosomal contents. Inclusions can build up, leading to one of several lysosomal storage diseases. There are genetic diseases like Hurler’s disease and Hunter’s disease that are lysosomal storage diseases because of accumulation of certain things. Hurler’s disease, for example, is a buildup of glycosaminoglycans. Gaucher’s disease is also a lysosomal storage disease that affects the brain. Tay-Sachs and Niemann-Pick diseases are also lysosomal storage diseases. Mucolipidosis type II is the most severe disease of this type because none of the lysosomal enzymes are present.

Iron is transported into the cell via receptor-mediated endocytosis. Iron is bound to apo-transferrin, which binds two iron ions. It is then called transferrin, recognized by transferrin receptors on the cell. It starts clathrin to form a pit and a vesicle is made. A lysosome is not involved in this process, however. In the early endosome, iron is released from the transferrin. The iron then leaves the enzyme through the protein called DMT1. The receptor and the apo-transferrin together get recycled back to the cell surface. Apo-transferrin breaks off after this to travel around to bind more iron ions.