Chapter 13: Vesicle Part 2

Lysosome

Membrane-enclosed organelles filled with soluble hydrolytic enzymes that digest macromolecules

• Lysosomes digest and recycle cellular materials, including:

  • Damaged organelles and proteins.

  • Materials brought into the cell through endocytosis or phagocytosis.

  • -Macromolecules such as proteins, lipids, carbohydrates, and nucleic acids.

How is the acidic environment of the lysozyme maintained?

A V-type H⁺-ATPase (proton pump) uses energy from ATP hydrolysis to pump hydrogen ions (H⁺) from the cytosol into the lysosome.

This increases the H⁺ concentration inside the lysosome, maintaining its low pH.

acid hydrolases

this is the part of the lysozyme that actually breaks down the macromolecules

Lysosome = the organelle that carries out digestion and recycling. Think of it as the recycling center.

Acid hydrolases = the enzymes that actually break down the materials. Think of them as the workers doing the recycling.

Why do we call lysosomes heterogeneous?

they vary in size, shape, and internal content, reflecting their dynamic role in multiple degradation pathways and their fusion with different vesicle types

Endolysosomes

formed when lysosomes fuse with late endosomes

When do Endolysosomes mature into lysosomes?

 once hydrolases complete digestion

Endocytosis

• cells take up PM components, fluid, solutes, macromolecules

• Material ingested is enclosed by small portion of PM, which first invaginates and then pinches off, forming an endocytic vesicle

• Cell regulates its membrane composition in response  to changing extracellular conditions

Micropinocytosis

cells internalize small amounts of extracellular fluid and dissolved molecules through tiny vesicles formed by membrane invagination.

Phagocytosis

• How the cell engules a large particle

Autophagy

• Self eating

• Important in growth and development (helps restructure differentiating cells

• Unlike proteasome, can digest large object

Nonselective Autophagy

bulk portion of cytoplasm is sequestered in autophagosomes. 

Selective Autophagy

autophagosomes tightly enclose specific cargo and mostly exclude the surrounding cytosol

Sorting and Transport of lysosomal hydrolases from the trans-golgi network to lysosomes using the mannose-6-phosphate

1. Tagging: Lysosomal hydrolases receive a mannose-6-phosphate (M6P) tag in the Golgi.

2. Recognition: M6P receptors in the trans-Golgi network recognize and bind the tagged hydrolases.

3. Transport: Clathrin-coated vesicles carry the hydrolases to an endosome.

4. Release: The acidic environment of the endosome causes hydrolases to dissociate from M6P receptors.

5. Receptor recycling: M6P receptors return to the Golgi to be reused.

6. Endosome maturation: The early endosome matures into a late endosome, becoming more acidic and accumulating active hydrolases. The early endosome primarily sorts cargo and recycles materials; the late endosome receives cargo destined for degradation.

7. Lysosome fusion: The late endosome fuses with a lysosome to form an endolysosome, where hydrolases digest the contents.

What does Mannose-6-phosphate do?

M6P labels the enzymes, the receptor recognizes the label, and the endosome releases and sorts the cargo

Early endosome

• Binds endocytic vesicles near PM

• Sorts incoming cargo

• Receiving compartment for endocytic vesicles

• major site of recyling

• very low hydrolase activity, high pH (less acidic)

• fuse with incoming endocytic vesicles

• more intralunina vesicles

Late endosome

• Smaller, no tubular domains

• farther from PM, movement (deeper in the cell)

• directs cargo towards degradation

• little to no recycling

• Increased acidity and hydrolase activity (more acidic)

• binds lysosome

• fuses w lysosomes to form endolysosomes

• more intraluminal vesicals

 I-cell disease mutation

• Almost all hydrolytic enzymes are missing from lysosomes of many cell types. Undigested material accumulates in lysosomes, formation of large inclusions in cells

• In I-Cell disease, most of the hydrolases are missing due to defect in GlcNAc phosphotransferase.

  • No M6-P tag

  • M6-P receptors fail to sort and target hydrolases lacking tag

  • Hydrolases are secreted (found in blood)

What do most endocytic vesicles fuse w near the cell periphery?

early endosomes

Endosomal maturation

early endosomes sort incoming cargo; some is recycled to the plasma membrane, while other cargo is sent for degradation. As early endosomes mature into late endosomes, they become more acidic, accumulate hydrolases and intraluminal vesicles, and lose recycling tubules. Late endosomes then fuse with lysosomes to form endolysosomes, where cargo is digested.

receptor-mediated endocytosis of LDL

• Receptors bind specific macromolecules in extracellular space

• Clathrin coated pits form and uptake to the early endosome

• LDL carries cholesterol into cells this way

Exocytosis

Fusion of the vesicles with plasma membrane:

1. From trans golgi to the cell exterior
   Transport vesicles destined for PM leave Trans Golgi network in steady stream as irregularly shaped tubules

2. Membrane proteins and lipids inside vesicles provide new components for PM. Soluble proteins are secreted to the extracellular space

constitutive secretory pathway

• In unpolarized cells (cells w no defined orientation of specialized regions; likely a single-cell organism, could be certain differentiated liver cells

• a constitutive secretory pathway delivers proteins with no special features to the cell surface.

 What kinds of cells use the regulated secretory pathway?

1. Convenient for secreting products rapidly on demand—such as with hormones, neurotransmitters, or digestive enzymes. 

2. Operates continuously in all cells

3. release their contents in response to a specific stimulus, often an increase in intracellular Ca²⁺ concentration. 

3 pathways of protein sorting in trans Golgi network

1. Signal-mediated diversion to lysosomes (via endosomes) 

2. Signal-mediated to secretory vesicles (fore regulated secretion)

3. Constitutive secretory pathway

Secretory vesicle:  Synaptic vesicle

Synaptic vesicle stores neurotransmitters

1. When an action potential arrives ar a presynaptic site, depolarization of the membrane opens thevolatge gared Ca 2+ channels

2. Ca2+ influx triggers release of neurotransmitters, which are

3. stored in synaptic vesicles and released by exocytosis

Steps of exocytosis of synaptic vesicles

1. Vesicle docks at presynatptc plasma membrane 

2. SNAREs partly paired, helices not fully wound. Complexins freeze the SNARE complex, prevents fusion to membrane

3. Action potential arrives at nerve terminal, influx of Calcium ions through voltage-gated Ca2+ channels

4. Rise in calcium: triggers binding of Synaptotagmin (contains calcium binding domains) to phospholipids and SNAREs, displacing complexins.

5. Fusion pore opens and neurotransmitters are released

How are lysosomal hydrolases transported to endosomes?

They are tagged with mannose-6-phosphate in the Golgi, recognized by specialized receptors, and transported in clathrin coated vesicles to endosomes

Complexin

has to be removed in order to complete the membrane fusion of the synaptic vesicle and presynaptic membrane in the regulated secretion of neurotransmitters

Synaptotagmin

acts as a calcium sensor that triggers rapid neurotransmitter release by mediating synaptic vesicle fusion with the presynaptic membrane.