Lecture 67: Bulk Transport (Physiology)
MAJOR TYPE 1
ENDOCYTOSIS 💥
1) Phagocytosis ♻
2) Pinocytosis 🚩
2A) Receptor-Mediated Endocytosis ⚠
coated vesicles- proteins coating intracellular surface of endocytosing vesicles
cell eating
Receptor-mediated
internalize large particles
-bacteria
-dead or damaged cells
-asbestos fibers, glass beads, etc.
cell drinking
constitutive (always occurring)
internalize extracellular fluid
constitutive (always occurring)
induced (receptor-mediated)
CLINICAL RELEVANCE ⭐
(Phagocytosis)
2B)MACROPINOCYTOSIS
("large cell drinking") 🍾
MAJOR TYPE 2
EXOCYTOSIS 🏴
SNAREs 🏴
Tethering vs. Fusion 🏴
Regulation 🏴
(Examples)
Possible Disease Connections 🏴 ‼
MAJOR TYPE 3
TRANSCYTOSIS 🚩
Energy-dependent
Vesicles <250 nm diameter
Mammalian phagocytic cells
macrophages
dendritic cells
neutrophils
Induced process (initiated by ligand binding to receptors on phagocytic cells)
Actin cytoskeleton required for internalization!
Phagosomes fuse w/lysosomes to degrade contents/kill bacteria!
Live animal cells
"do not eat me!" signals
membrane proteins that bind to inhibitory receptors on phagocytic cells
early line of defense against bacteria!
Some bacteria have protection mechanisms that allow to escape degradation through phagocytosis!
Salmonella enterica
(food poisoning)
Mycobacteria
(TB)
blocks uncoating of coat proteins that's requires for lysosomal fusion!
secretes protein that blocks fusion of phagosome w/lysosomes -> blocks degradation!
NO degradation of phagocytosed bacteria
live in protected intracellular vesicular environment
Types of Pinocytosis
- Clathrin-mediated endocytosis ⚠
- Caveolin-Coated Vesicle-mediated Endocytosis ⚠
- Non-clathrin/non-caveolin-mediated endocytosis 🚫
large membrane ruffles fold over to engulf fluid & contents
Actin cytoskeleton involved
blocked by P21-activated kinase-1 (regulates actin dynamics & block/inhibits actin polymerization!)
Pinosomes fuse w/early endosomal compt.
Some viruses use this to enter cell!
vaccinia virus
HIV1
Echovirus type 1
Adenovirus 3
Energy- DEPENDENT!
Constitutive internalization of coated vesicles from plasma membrane
ligand binding to cell surface receptor accelerates int. rate
Coat proteins
"coat" cytoplasmic membrane surface
Types
-clathrin
-caveolin
-others
Basal internalization rate VARIES by cell type!
Macrophages internalize ~3% of membrane SA/minute
Fibroblasts ~1%/minute
accelerated by GF/hormone binding
Total plasma membrane SA NOT changed, so equivalent amounts of plasma membrane are being internalized & replaced
MANY viruses use this pathway for mammalian cell entry!
Examples
-Hepatitic C virus
-Dengue virus
-Reovirus
distinct membrane lipid & protein composition (cholesterol & sphingolipid-rich)
form signaling membrane subdomains
dynamin-2-dependent (pinching off similar to clathrin-coated vesicles)
regulated by src Family Kinases
MULTIPLE TYPES! Caveolin 1,2, 3
Caveolin 1 & 2
(expressed by most cells)
Caveolin 3= Unique! ❗
-only expressed in skeletal/smooth mm., glial cells, early post-natal peripheral nn.
Pathologies
Viruses using caveolin-coated vesicle entry pathway
SV40 virus
papillomarvirus
diseases assoc. w/Caveolin 3 mutations
limb girdle musc. dystrophy
rippling muscle disease
distal myopathy
hyperCKemia
LDL Receptor Recycling & Disease 🔥
Specific plasma membrane LDL (low-density lipoprotein) receptors
LDL receptors w/bound LDL cluster in clathrin-coated pits
LDL interalized more SLOWLY in the absence of LDL receptors
receptors w/bound LDL internalized FASTER (than those without)
LDL internalization downregulate cholesterol synthesis ❗
Hypercholesterolemia ‼
caused by:
-LDL receptor deficiency
-LDL receptor mutation inhibiting localization to clathrin-coated pits
Notice: xanthomas arise from deposition of plasma LDL-derived cholesterol into macrophages of skin
coat protein(s)-if any- & mechanism is UNKNOWN!
Viruses using pathway:
-Rotavirus
-Influenza virus
-Lymphocytic choriomeningitis virus
energy-dependent! (ATP)
"priming" of membrane site for vesicle fusion to occur
exocyst complex
secures secretory vesicle to plasma membrane
Types of Processes! ‼
Constitutive
Regulated
continuous
Ca2+ dependent!
respond to signal (ligand binding to membrane receptor, ex: NT release)
Ca2+ dependent!
Soluble NSF (NEM-Sensitive Factor) Activating Protein Receptor Protein
at least 39 different SNAREs identified!
2 Basic Types ‼
1) v-SNARE
2) t-SNARE
vesicle-SNARE
target-SNARE
mediate vesicle fusion
Ca2+ dependent self-assembly forms conducting channels spanning 2 lipid bilayers -> brings them VERY CLOSE together!
Exocyst complex
Tethering
v-SNARE/t-SNARE
Fusion! ⭐
different v/t-SNARE combo's impact specificity of vesicle fusion events!
Tethering Vesicles to target membrane mediated by "tethers" (Rab effector proteins)- small G-protein ❗
Homotypic Tethering
Heterotypic Tethering
2 intracellular vesicles fuse!
fusion of 1 intracellular vesicle + plasma membrane
Fusion of exocytic vesicle w/plasma membrane (mediated by SNAREs)
Secretion of Growth factor(s) like NT's from neurons
Insertion of membrane transporters (regulate transport activity)
1) Synaptic Exocytosis Defects 🏁
2) Type 2 Diabetes 🏁
3) Cystic Fibrosis (CF) 🏁
may play role in multiple neurodegenerative diseases
some types assoc. w/diminished depolarization-induced insulin exocytosis
some CFTR mutations impede movement by vesicles from ER -> plasma membrane
Endocytosis of fluid & solutes along with receptor-bound ligands from 1 solution (apical or basolateral)
movement within vesicles through cell
exocytosis of vesicle contents at other membrane surface
COMBO of endocytosis (1 side) and EXO (other side)!
Examples:
-Endothelia & capillaries
-Intestinal epithelia
-Lung
-Thyroid
-BBB