🦠Cell Biology Unit 17 – Vesicular Trafficking and Secretion

Key Concepts and Terminology

• Vesicular trafficking involves the movement of molecules between cellular compartments via membrane-bound vesicles
• Secretion releases molecules from the cell into the extracellular space through exocytosis (neurotransmitters, hormones)
• Endocytosis brings molecules into the cell from the extracellular environment (nutrients, signaling molecules)
  • Phagocytosis engulfs large particles or microorganisms (immune cells)
  • Pinocytosis takes up fluid and dissolved solutes (intestinal epithelial cells)
• Coat proteins (clathrin, COPI, COPII) assist in vesicle formation and cargo selection
• SNARE proteins (v-SNAREs, t-SNAREs) mediate vesicle fusion with target membranes
• Rab GTPases regulate vesicle transport and targeting to specific compartments (Rab5, Rab7)

Cellular Compartments and Organelles

• Endoplasmic reticulum (ER) synthesizes proteins and lipids for transport to other organelles or secretion
  • Rough ER studded with ribosomes for protein synthesis
  • Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification
• Golgi apparatus modifies, sorts, and packages proteins and lipids for transport to specific destinations
  • Cis-Golgi receives vesicles from ER, trans-Golgi sends vesicles to plasma membrane or other organelles
• Endosomes sort and process internalized molecules from the plasma membrane
  • Early endosomes receive endocytic vesicles and sort cargo for recycling or degradation
  • Late endosomes fuse with lysosomes for degradation of cargo
• Lysosomes contain hydrolytic enzymes that break down macromolecules and cellular debris
• Peroxisomes detoxify harmful substances and break down fatty acids (liver cells)

Types of Vesicles and Their Functions

• Clathrin-coated vesicles mediate endocytosis and transport between Golgi and endosomes
  • Clathrin forms a cage-like structure around the vesicle
  • Adaptor proteins (AP-2) link clathrin to cargo and membrane
• COPI-coated vesicles transport proteins from Golgi back to ER for retrieval or recycling
• COPII-coated vesicles transport newly synthesized proteins from ER to Golgi
• Secretory vesicles store and release molecules for secretion (insulin granules in pancreatic beta cells)
• Synaptic vesicles store and release neurotransmitters at synapses for neurotransmission
• Multivesicular bodies (MVBs) contain intraluminal vesicles that can be released as exosomes for intercellular communication

Mechanisms of Vesicle Formation

• Coat proteins (clathrin, COPI, COPII) assemble on the donor membrane and induce curvature
• Adaptor proteins recruit specific cargo molecules into the forming vesicle
• Dynamin GTPase forms a spiral around the vesicle neck and constricts it, leading to scission
  • Dynamin recruitment is mediated by BAR domain-containing proteins (amphiphysin)
• Arf GTPases (Arf1, Sar1) initiate coat assembly and vesicle formation
• Phosphoinositides (PI(4,5)P2) play a role in recruiting coat proteins and adaptors to specific membranes

Vesicle Transport and Targeting

• Motor proteins (kinesins, dyneins) transport vesicles along cytoskeletal tracks (microtubules)
  • Kinesins move towards the plus end of microtubules (anterograde transport)
  • Dyneins move towards the minus end of microtubules (retrograde transport)
• Rab GTPases cycle between active (GTP-bound) and inactive (GDP-bound) states to regulate vesicle transport
  • Rab effector proteins mediate vesicle tethering and docking to target membranes
• Tethering factors (p115, EEA1) bring vesicles in close proximity to target membranes
• SNARE proteins on vesicles (v-SNAREs) and target membranes (t-SNAREs) form complexes to drive membrane fusion
  • Synaptobrevin (VAMP) is a v-SNARE, syntaxin and SNAP-25 are t-SNAREs

Membrane Fusion and Secretion

• SNARE complex formation brings vesicle and target membranes together, overcoming the energy barrier for fusion
  • Zippering of SNARE proteins from N-terminus to C-terminus drives membrane fusion
• Synaptotagmin acts as a calcium sensor and triggers fast exocytosis of synaptic vesicles upon calcium influx
• NSF (N-ethylmaleimide-sensitive factor) and SNAP (soluble NSF attachment protein) disassemble SNARE complexes after fusion
• Kiss-and-run exocytosis allows partial release of vesicle contents without full fusion (chromaffin cells)
• Constitutive secretion continuously releases molecules without regulation (extracellular matrix proteins)
• Regulated secretion requires a specific stimulus for vesicle fusion and release (neurotransmitters, hormones)

Regulatory Mechanisms and Signaling

• Phosphorylation by kinases (PKA, PKC) modulates the activity of proteins involved in vesicular trafficking
• Calcium signaling triggers exocytosis in regulated secretion (synaptic transmission, hormone release)
  • Voltage-gated calcium channels open upon depolarization, allowing calcium influx
• G protein-coupled receptors (GPCRs) activate signaling cascades that regulate vesicle formation and transport
• Small GTPases (Rab, Arf) act as molecular switches and recruit effector proteins to control vesicle trafficking
• Phosphoinositides serve as membrane markers and binding sites for trafficking proteins (PI(4,5)P2, PI(3)P)
• Retromer complex mediates retrograde transport from endosomes to Golgi or plasma membrane (Vps26, Vps29, Vps35)

Clinical and Research Applications

• Neurodegenerative diseases (Alzheimer's, Parkinson's) involve disruptions in vesicular trafficking and protein aggregation
  • Accumulation of amyloid-beta peptide in Alzheimer's disease due to impaired endosomal sorting
  • Alpha-synuclein aggregation in Parkinson's disease affects synaptic vesicle recycling
• Lysosomal storage disorders result from defects in lysosomal enzymes or trafficking (Tay-Sachs disease, Niemann-Pick disease)
• Bacterial and viral pathogens hijack host cell vesicular trafficking pathways for entry and replication (HIV, Legionella)
• Exosomes released by cancer cells can promote metastasis and modulate the tumor microenvironment
• Targeted drug delivery using engineered vesicles or exosomes for specific tissue or cell type uptake
• Optogenetic tools (channelrhodopsin) allow precise control of synaptic vesicle release and neuronal activity
• High-resolution imaging techniques (super-resolution microscopy, cryo-electron tomography) provide detailed insights into vesicle structure and dynamics