Question 1

How do multivesicular bodies form from early endosomes?

Accepted Answer

Multivesicular bodies form during early endosome maturation as intraluminal vesicles accumulate inside the endosome. ESCRT proteins translocate ubiquitinated receptors into these intraluminal vesicles, creating multiple vesicles within the maturing endosome. Once sufficient intraluminal vesicles form, the early endosome transforms into a multivesicular body ready for fusion with lysosomes.

Question 2

What role does ubiquitination play in receptor downregulation?

Accepted Answer

Ubiquitination marks receptors like EGFR for degradation by attaching ubiquitin chains to the receptor-ligand complex. This ubiquitin tag signals ESCRT proteins to sort the complex into intraluminal vesicles within multivesicular bodies. Without ubiquitination, receptors cannot be properly recognized and internalized for degradation, preventing effective downregulation of signaling pathways.

Question 3

Why is receptor downregulation important for preventing cancer?

Accepted Answer

Receptor downregulation through MVB-mediated degradation prevents excessive growth factor signaling that can lead to uncontrolled cell proliferation and cancer. By degrading activated receptors like EGFR, cells switch off proliferation signals and maintain normal growth control. Without this regulatory mechanism, sustained receptor activation promotes abnormal cell division and tumor formation.

Question 4

How does the EGF-EGFR complex get internalized into cells?

Accepted Answer

When epidermal growth factor binds to its receptor, the EGF-EGFR complex undergoes receptor-mediated endocytosis, a process triggered by ubiquitination of the dimerized complex. The complex is then internalized via clathrin-coated pits and delivered to the early endosome, where it begins its journey toward degradation in multivesicular bodies.

Question 5

What happens when MVBs fuse with lysosomes?

Accepted Answer

When multivesicular bodies fuse with lysosomes, lysosomal hydrolases are released into the MVB lumen, degrading the receptor-ligand complexes contained within intraluminal vesicles. This degradation process permanently removes signaling receptors from the cell, effectively terminating growth factor signaling and preventing excessive cell proliferation.

Question 6

How do defects in MVB-mediated degradation contribute to inflammatory diseases?

Accepted Answer

Defects in ubiquitination, endocytosis, or MVB-mediated degradation of cytokine receptors prevent proper downregulation of pro-inflammatory signaling. When receptor-cytokine complexes like TNFα or IL-1 are not degraded, signaling remains sustained or enhanced, leading to chronic inflammation and diseases such as Crohn's disease or rheumatoid arthritis.

Question 7

How do cytokine receptors undergo downregulation similar to EGFR?

Accepted Answer

Cytokine receptors are downregulated through the same pathway as EGFR: ubiquitinated receptor-cytokine complexes are endocytosed and delivered to multivesicular bodies for degradation. This process weakens cytokine signaling by removing activated receptors from the cell surface, preventing prolonged inflammatory responses and maintaining cellular homeostasis.

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