Molecular and Cellular Medicine

Week 4: Nucleotide and Protein Structure and Function

Endocytosis and Lysosomes

Judith White, PhD

1. List the four major functions of endocytosis.

Uptake of nutrients (cholesterol, folate, etc); general housekeeping; attenuating signaling responses (from growth factor receptors, etc); immunity and host defense.

2. List the major organelles along the pathway of clathrin-mediated endocytosis.

LDL receptor, coated vesicle, uncoated vesicle, early endosome, late endosome, lysosome.

3. Delineate the fates of low density lipoprotein (LDL) and its receptor (the LDL receptor) during clathrin-mediated endocytosis.

The LDL gets broken down into free cholesterol and proteins, while its receptor gets recycled to the plasma membrane via an endosome.

4. Describe how a mutation in the cytoplasmic tail of the LDL-receptor leads to a genetic disease.

The cytoplasmic tail interacts with adaptin, which interacts with clathrin to create a vesicle and bring the receptor and ligand into the cell. A mutation in this tail will result in the cell being unable to bring the receptor and ligand into the cell for degradation. As a result, the blood levels of LDL will rise.

5. Using transferrin and its receptor and epidermal growth factor and its receptor, describe two variations on the basic pathway of clathrin-mediated endocytosis (i.e. different fates of receptor and/or ligand than in the case of LDL and its receptor).

Iron binds to transferrin, which then binds to the transferrin receptor and the complex is taken into the cell by receptor-mediated endocytosis. The iron dissociated at a low pH in the endosome and is transported into the cytoplasm. The apotransferrin-transferrin receptor complex recycles to the cell surface, BUT apotransferrin dissociates to bind more iron in the blood.

Alternatively, EGF binds and stimulates endocytosis of EGF receptor, but both are degraded in the lysosomes, rather than the receptor being recycled to the surface of the cell.

6. Describe the consequences of lysosomal storage diseases.

Lysosomal storage diseases include any diseases that result from the malfunction of the enzymes in the lysosomes. Children with these diseases usually die at a young age. When a specific enzyme malfunctions, it cannot properly degrade its substrates, so that these substrates build up and the lysosomes enlarge. Examples include Tay-Sachs, Pompe's, Hurler's, and Gaucher's. Many organs can be affected, and the diseases are generally severe, progressive, and life limiting. The major therapeutic strategy is enzyme replacement—it works for Gaucher's disease, but is very expensive.

7. List two classes of professional phagocytic cells and describe how they phagocytose a bacterium.

Neutrophils and macrophages are professional phagocytic cells in the immune system. They extend their cell membrane as a pseudopod around bacteria to create new vesicles. These vesicles then fuse with the lysosomes and the bacteria are destroyed.

7.1 Ways in which phagocytosis differs from clathrin-mediated endocytosis. (not a learning objective, just random notes)

The size of phagocytosis is much greater than that of endocytosis—which can't take up bacteria. Phagocytosis involves a massive change in the cell membrane. Endocytosis occurs in most all cells, while phagocytosis mostly only happens in professional phagocytic cells.

8. Describe the major classes of pathogens that can utilize an endocytic pathway to enter cells.

Viruses (enveloped and non-enveloped), bacteria, and bacterial toxins can utilitze the endocytic pathway to enter cells.

9. Describe how influenza virus exploits the pathway of clathrin-mediated endocytosis in order to infect a cell.

Influenze fuses its membrane with an endosome membrane in response to low endosomal pH, allowing for the release of the viral RNA and proteins.