BSCI 1510 Vesicular Transport II Lecture

Vesicle transport involves the movement of materials between organelles, particularly between the nucleus, endosome, and mitochondria.
It is essential for delivering proteins synthesized in the endoplasmic reticulum (ER) across membranes into the cytosol and other organelles, facilitating both intracellular communication and metabolic processes.

Big Questions:

What is the key difference between endocytosis and exocytosis, and how do these processes influence cellular homeostasis?
What are the various types of vesicles produced in the trans Golgi network, and what roles do they play in secretion and cellular transport?
How do constitutive and regulated exocytosis differ in terms of mechanisms and physiological contexts?
In what ways is the regulation of endocytosis essential for nutrient uptake and receptor recycling?
What are the structure and functions of lysosomes, and how do they contribute to cellular degradation processes?
What are the different pathways that target substances to lysosomes, and how do these processes maintain cellular health?

Exocytosis:

Involves the fusion of secretory vesicles with the plasma membrane, facilitating release of materials into the extracellular space.
Types of vesicles produced in the trans Golgi network:
- Regulated secretory vesicles: Release their contents in response to specific signals, such as hormones or neurotransmitters.
- Constitutive secretory vesicles: Continuously release substances, maintaining the basic functions of the cell.
- Clathrin-coated vesicles: Specialized vesicles targeted to endosomes for further processing and sorting.

Regulated vs. Constitutive Pathways:
- The regulated pathway diverges in the trans Golgi network for specific secretion, allowing cells to respond to internal and external signals rapidly.
- Regulated exocytic vesicles aggregate proteins under low pH and high Ca2+ conditions, achieving a higher concentration of cargo than the constitutive pathway.

SNARE proteins are essential for mediating different types of exocytosis:
- Regulated vesicles: Require calcium influx and the protein synaptotagmin to promote SNARE-dependent fusion with the plasma membrane.
- Constitutive vesicles: Utilize SNAREs but do not require calcium for the fusion process, allowing for constant cargo release.

Endocytosis Types:

Pinocytosis: The uptake of extracellular fluid and small molecules; the materials are often delivered to early and late endosomes, where sorting of cargo occurs based on cellular needs.
Receptor-mediated endocytosis: A selective process that concentrates specific substances through receptor activity. An example includes LDL transport, where the acidic environment in endosomes triggers the dissociation of LDL from its receptors, facilitating cellular uptake.

Lysosomes: These small spherical organelles, often referred to as the cell’s "digestive system," are critical for the degradation and recycling of macromolecules and cellular debris.
Function largely revolves around intracellular digestion, breaking down complex biomolecules into simpler components that can be reused by the cell.

Routes:

Biosynthetic Pathway: Newly synthesized lysosomal enzymes are tagged with mannose 6-phosphate and sorted from the trans Golgi network to lysosomes.
Endocytic Pathway: Involves the formation of clathrin-coated vesicles that merge with early endosomes, subsequently delivering cargo to lysosomes.
Autophagy: This process facilitates the removal of dysfunctional organelles via lysosomes, supporting cellular quality control and promoting cellular health (mitophagy is a specific form targeting mitochondria).

These are a group of rare genetic disorders caused by defects in lysosomal function, often characterized by specific enzyme deficiencies leading to the accumulation of substrates that the lysosomes cannot degrade.
Examples:
- Tay-Sachs Disease: Caused by a deficiency in the enzyme Hexosaminidase A, leading to neurodegenerative symptoms because of the accumulation of GM2 gangliosides in nerve cells.
- Gaucher’s Disease: Caused by a deficiency of the enzyme glucocerebrosidase, resulting in the accumulation of glucocerebrosides that can lead to organ enlargement, bone issues, and other systemic complications.

Vesicle transport is foundational for cellular functions, facilitating essential processes like nutrient uptake through endocytosis and waste disposal through exocytosis. A thorough understanding of lysosomal functions, enzyme pathways involved, and the consequences of lysosomal storage diseases is vital for appreciating the complexities of cellular metabolism and physiology.