Lecture 5: Endo- and Exocytosis

Question 1

Learning Objectives

Answer 1

• What is the basic mechanism of endocytosis?
• What are some of the functions of endocytosis?
• What is the clinical relevance of knowing more about endocytosis?
• What is the basic mechanism of exocytosis?
• What are some of the functions of exocytosis?
• What is the clinical relevance of knowing more about exocytosis?

Question 2

What are the three types of endocytosis?

Answer 2

• Pinocytosis = ‘cell drinking’, takes in small amount of material, usually fluids and solutes. It it used when the cell is under mechanical stress, can repair plasma membrane (e.g. in blood vessel cells)
• Phagocytosis = ‘cell eating’, takes in larger material, can be used to remodel cytoskeleton
• Receptor-mediated (R-M) endocytosis - signalled process, ligand must bind to receptor for this to occur

Question 3

What is endocytosis?

Answer 3

When material is enclosed by a small portion of the plasma membrane, which invaginates and pinches off.

Endocytosis moves material from outside the cell to the lysosomes.

Question 4

What is pinocytosis?

Answer 4

Pinocytosis is ‘cell drinking’. It is the uptake of small amounts of fluids and solutes into small pinocytic vesicles. It is a default activity, so no signalling is required and it is continuous in eukaryotic cells.

There are two types of pinocytic vesicles:

• clathrin-coated vesicles, which take in extracellular fluid and take it to the early endosome. Clathrin-coated pits.
• caveolae, which are used to transport molecules across endothelial cells. They are present in the plasma membrane of most cells and are seen as invaginated flasks. They form from lipid rafts and localise proteins. They are composed of structural proteins called caveolins and cavin proteins. Caveolins and integral membrane proteins which insert hydrophobic loop inside cytosolic side of pm but not TM.
  Caveolae pinch off from the plasma membrane using dynamic, which wraps around neck of vesicle to help bud off).
  Caveolae deliver contents to endosome or plasma membrane of opposite side of polarised in transcytosis (process of material crossing a whole cell layer, the way some viruses enter cells)
  The calveolin coat is not shed. Calveolins are intermembrane proteins, so they don’t dissociate form vesicles after endocytosis.

Question 5

What is phagocytosis?

Answer 5

Phagocytosis is the endocytosis of larger material, e.g. microorganisms and dead cells. Cells use phagosomes (endocytic vesicles).
Some cells are specialised for phagocytosis called phagocytes and their main task is engulfing material. Examples of phagocytes include macrophages and neutrophils.
Macrophages recognise ‘eat me’ signals on cells, can be used to remodel tissue and heal wounds, engulf cells which have died by apoptosis.
Phagosomes, vesicles in a phagocyte, fuse with lysosomes, which are full of hydrolytic enzymes and digest material. Indigestable material remains in the lysosome as a residual body, which can be excreted from the cell by exocytosis.

Phagocytosis is a trigged process, so a signal is required. Phagocytosis requires activation of receptors which transmit signals to cell interior to initiate response:
- macrophages and neutrophils have FC receptors, which bind to the FC tail (constant region) of the antibody of organism. This activates phagocytosis and the phagocyte remodels its cytoskeleton for engulfing.

Phagocytosis of dead cells:

• in apoptosis, asymmetry of lipids lost by dead cell (sign cell is undergoing apoptosis)
• phagocytes recognise these ‘eat me’ signals
• phagocytosis depends on balance between positive signals (activate process) and negative signals (inhibit process)
• apoptotic cells gain more ‘eat me’ signals and lose ‘don’t eat me’ signals, which makes phagocytosis of apoptotic cells rapid.

Question 6

What is receptor-mediated endocytosis?

Answer 6

Macromolecules bind to complementary transmembrane receptor proteins, accumulate in coated pits (clathrin coated), enter cell as receptor-macromolecule complexes in clathrin coated vesicles. They are selective to specific macromolecules. This mechanism can concentrate a ligand 1000-fold without taking up lots of extracellular fluid.

Receptor-mediated endocytosed proteins end up either:

• recycled from endosome to pm
• moved to a different part of pm by transcytosis
• degraded in lysosomes and the materials are then recycled

Receptor proteins can be hidden to stop signals