Endocytosis, etc. Flashcards
Define endocytosis
Same thing as pinocytosis
- uptake of macromolecules
- particulate substances and fluids
- other cells
- downregulation of receptors
- clathrin-dependent and independent pathways
How do types of endocyosis differ?
In size of endocytotic vesicle formed
Define phagocytosis
Uptake of large particles (such as whole bacterium) carried out by professional or non-professional cells, and requires actin polymerization
- uptake particles >0.5 microns
- removal of apoptotic cells by neighboring cells
- opsonization (identifying and removing invading particle)
- results in oxidative burst from destruction of microorganism
Define autophagy
engulfment of cellular structures and their digestion, having both positive and negative impacts on the cell
What is the primary method used by the cell to remove free microorganisms in the blood and tissue?
phagocytosis
What types of cells in the immune system engulf microorganisms via phagocytosis?
neutrophils, macrophages, dendritic cells, B lymphocytes
List the five phases of phagocytosis
- attraction
- adherence
- ingestion
- digestion or killing
- elimination
Describe the attraction step of phagocytosis
- phagocyte attracted to microbe by chemotaxis
- microorganism or damaged host cell put off compounds that are attractive
- cytokines from leukocytes can also attract phagocytes to infection site
Describe the adherence step of phagocytosis
- if microbe is first coated by an opsonin protein, the phagocyte more readily adheres to it
- this coating is called opsonization
Describe the ingestion step of phagocytosis
-microbe is surrounded by a sac called a phagosome
Describe the digestion or killing step of phagocytosis
- phagosomes fuse with lysosomes and macromolecules of microorganism are hydrolyzed
- hydrolyzing enzymes include lysozymes, hydrogen peroxide, superoxide radicals, and hydroxyl radicals
- residual body of indigested material is left and eliminated
Describe the elimination step of phagocytosis.
-residual body, or digested phagosome, is moved to plasma membrane and discharged by exocytosis
What are eat me signals, and list some.
Signals revealed on the surface of proteins/molecules/structures under certain conditions like stress.
- phosphatidylserine (normally on inner leaflet, but stress causes it to bind certain interactors that initiate phagocytosis
- some therapies are trying to mimic eat me signals on cancer cells
what are dont eat me signals, and how can they be lost?
Cells can expose these signals to block their phagocytosis (e.g. sialic acid residues). They can lose these signals when they are stressed or aged.
What two systems in macrophages take care of killing microorganisms inside phagolysosome?
- oxygen-dependent
- oxygen-independent
List the ROS involved in the oxygen-dependent killing system of phagocytosis.
These all cause macromolecular damage, especially to DNA of the invading microorganism
enzymatically made:
- superoxide
- hydrogen peroxide
- nitric oxide
non-enzymatic:
- hypochlorous acid
What is a respiratory burst and what causes it?
Rapid release of reactive oxygen species when a phagocyte engulfs an invading organism.
List the proteins involved in oxygen-independent killing of phagocytosis.
- anti-microbial proteins (elastase, lysozyme)
- anti-microbial peptides (defensins)
- binding proteins (lactoferrins)
- hydrogen ion transport: hydrogen ions acidify the lysosome
What are the two proposed mechanisms for phagocytosis induction?
- zipper mechanism: bacterial uptake is promoted from the outside driven by interaction between two membranes
- trigger mechanism: bacterial uptake is triggered from the inside by injection of bacterial effector molecules into the host cell
Describe how listeria can subvert phagocytosis to gain entry into cells.
- listeria is taken up into phagosome by the zipper method
- listeria then secretes listeriolysin within the phagosome to degrade the phagosome
- bacteria then replicates inside the host
What major process is authophagy involved in and thereby makes autophagy highly conserved?
Development
On what organelle does autophagy rely?
lysosome
Describe the similarities between phagocytosis and autophagy.
- engulfment
- vesicles
- membrane
- destruction
Describe the general process of autophagy
cytoplasmic contents are enclosed by an isolation membrane that elongates to form an autophagosome with a double membrane, and eventually fuses with the lysosome to degrade the enclosed contents
What is macroautophagy?
delivery of cytoplasmic cargo to the lysosome through the intermediary of a double membrane-bound vesicle. This is called an autophagosome and fuses with lysosome to form autolysosome
What are the roles of macroautophagy?
- housekeeping roles (removing misfolded or aggregate proteins)
- host-defense mechanism
- embryonic development
- cellular integrated stress response
What is the core autophagy protein?
Atg8; activated through a cascade of events culminating in its covalent linkage to phosphatidylethanolamine (PE) of the autophagosomal membrane
How is autophagy coordinated?
It is coordinated through the hierarchical targeting of six assemblies to the pre-autophagosomal structure (PAS)
-begins with PAS -> isolation membrane forms around cytoplasmic materials –> forms autophagosome
Describe initiation of autophagy
initiation of autophagy process is mediated by ULK1 complex, which is linked to mTORC1. Under starvation, mTORC1 inhibits ULK1 kinase to allow ULK1 complex to form.
Describe initiation and elongation of autophagophore.
requires PI3K complex. intracellular components are engulfed. Other autophagy (Atg) proteins mediate elongation of the autophagophore to form autophagosome.
Describe selective autophagy.
Ubiquitination of certain proteins can initiate autophagy.
What is an example of selective autophagy?
mitophagy (parkin), ubiquitinated proteins, invading bacteria
What are the five steps in the clathrin-coated vesicle cycle?
- nucleation
- cargo selection
- clathrin coat assembly
- vesicle scission
- uncoating and clathrin component recycling
Describe nucleation step of endocytosis.
- creation of nucleation module at plasma membrane
- includes intersectin and FCHO proteins that bind to slight membrane curvatures and increase membrane curvature
- uses PIP adaptor molecules
Describe the cargo selection step of endocytosis (pinocytosis)
- proteins of nucleation module recruit AP-2 along with cargo-specific adaptor proteins
- Ap-2 protein has head, ears, and hinge, and binds both to cargo, cargo adaptor proteins, and PIP
Describe clathrin-coat assembly step of endocytosis.
- clathrin triskelia are recruited from cytoplasm via interactions with AP-2 and cargo-specific adaptor proteins.
- polymerization of clathrin stabilizes the curvature of the forming vesicle
- BAR domain proteins recognize the curvature and bind to the forming “neck” region at the base of the vesicle, recruiting dynamin (only region where clathrin does not coat the vesicle)
Describe the vesicle scission step of endocytosis.
- vesicle scission is facilitated by dynamin
- involves conformational changes induced by the GTP to GDP hydrolysis of dynamin GTP enzyme
Describe the uncoating and clathrin recycling step of endocytosis.
- Once detached from the membrane, the clathrin coat is dissembled by Hsp70 and its cofactor (auxilin in neurons, GAK in non-neurons)
- dissembly process begins at the neck where there is no clathrin
At what step in endocytosis are vesicles able to participate with Rab proteins?
Only once the vesicle has become uncoated.
List the physiological functions of clathrin-mediated endocytosis.
- internalization of receptors, both constitutive (LDLR) and induced (GPCR)
- regulation of signaling (removal of surface receptors has large effect on signal transduction)
- synaptic vesicle recycling
How can clathrin-mediated endocytosis be subverted by pathogens? example?
toxins, bacteria, viruses (HIV) can co-opt this pathway to gain entry into cells (like listeria does with phagocytosis)
