Signal Transduction Flashcards
List five types of cell-cell communication and describe them.
Endocrine
- endocrine cell secretes hormone into bloodstream to signal target cell
Paracrine
- signalling cell neighbors target cells and sends signal (local mediator) to target cells
Neuronal
- …It’s a neuron.
Contact-Dependent
- Signalling cell has a membrane-bound signal molecule. Must make contact with target cell.
Autocrine
- Cell signals itself (cancer cells and other normal cells)
Define Signal Transduction.
Conversion of extracellular signals into intracellular events.
Describe the basic pathway of Signal Transduction.
- Receptor binds a ligand (primary messenger)
- Signal transduction via intracellular signalling molecules, second messengers, amplification.
- Cell responds
- Gene expression changes.
What are the two types of Intracellular Signalling Proteins?
Kinases
GTP Binding Proteins (G-Proteins)
What is Signal Amplification?
A small signal produces a large cellular response. The amplification can result from second messenger production/kinase activation.
What are the kinds of Receptors involved in Cell Signalling?
Surface Receptors:
- G-protein coupled Receptors (GPCR)
- Receptor Tyrosine Kinases (RTK)
- Serine/Theonine Kinase Receptors
- Receptors without kinase activity
- Ion Channel-linked Receptors
Intracellular Receptors
- Steroid Hormone Receptors
Not super high-yield
Name the 4 classes of membrane receptors and describe their action upon receiving a signal.
Ligand-Gated Channel (open/closes channel) Receptor-Enzyme (activates bound intracell enzyme) G-Protein Coupled Receptor (open ion channel or alter enzyme activity) Integrin Receptor (alters the cytoskeleton)
Which subunit of a heterotrimeric G-Protein typically has the GDP/GTP bound to it?
Alpha subunit.
Walk through the mechanism of GPCR activation.
- Ligand binds receptor (G-protein attaches to receptor if it wasn’t already), altering conformation to activate G-protein.
- GDP on alpha subunit replaced with GTP
- GTP causes G-protein to break into alpha and Beta-Gamma groups, exposing sites to bind to effectors
- alpha or Beta-Gamma subunits interact with an effector to modulate its activity
- alpha subunit has GTPase activity (hydrolyzes GTP to GDP)
- GDP on alpha reassociates G-protein, leaving effector and ending signal.
What are the two major pathways that use GPCR and G-Proteins?
cAMP 2nd messenger pathway
DAG and IP3 2nd messenger pathway
Describe the cAMP pathway.
- Signal binds to GPCR, activating G-Protein
- G-Protein activates adenylyl cyclase (amplifier enzyme)
- Adenylyl cyclase converts ATP to cAMP
- cAMP activates protein kinase A (pKA)
- pKA phosphorylates other proteins, leading to a cellular response
- pKA also enters nuclease and phosphorylate (activate) transcription factors (CREB) to bind to CRE (cAMP Responsive Element) in promoter of cAMP-responsive genes.
- cAMP phosphodiesterase catalyzes cAMP into 5’AMP, ending response.
What activity can the alpha subunit have?
Inhibitory (Gi) or Stimulatory (Gs). Both involve activation by a bound GDP converting to GTP.
What toxins cause an increase of cytosolic cAMP and signalling? Describe their different mechanisms.
Cholera (lock Gs into active state by blocking GTPase activity
E.Coli toxin (same as Cholera)
Pertussis toxin (Locks Gi into GDP-bound inactive state
Describe the DAG and IP3 pathway.
- Gq activates G-Protein (Phospholipase C)
- PLC breaks down inositol phospholipid into second messengers IP3 and DAG
- IP3 induces Ca2+ release from sER
- Both DAG and IP3 activate PKC, which activates transcription factors.
What is the Pathology of Retinitis Pigmentosa 4?
Defect: mutated Rhodopsin (rod pigment)
- results in degeneration of photoreceptor cells)
Symptoms: loss/diminished night and peripheral vision.
What is the essential step required for Receptor Tyrosine Kinases (RTK) to activate?
Dimerisation. This causes the two RTKs to cross auto-activate.
Briefly describe the signal pathway of RTK activation.
- Ligand binds (i.e Growth Factor)
- RTKs dimerise.
- Tyrosines are phosphorylated
- Kinases activated
- Kinases phosphorylate effector proteins for action.
What are the primary 3 RTKs that get activated by this process?
- Phospholipase C (PLC)
- PI3K (Phosphatidyl-inositol 3-kinase)
- GRB2
Note: PLC can be activated either by GPCR or RTK
What is the PI3K pathway responsible for? Describe the pathway.
Cell Survival (aka inhibition of Apoptosis)
- RTK phosphorylates PI3K, which activates it
- PI3K activates PKB/Akt
- PKB/Akt phosphorylates BAD, which is a Pro-Apoptotic bcl-2 protein. This TURNS IT OFF, INHIBITING APOPTOSIS.
Detail exactly how PKB/Akt promotes cell survival.
BAD normally inhibits Bcl-2, which is an anti-apoptotic protein that also inhibits BAX, a pore-forming protein.
PKB/Akt inhibits BAD, which means Bcl-2 is able to function normally and inhibit BAX as well.
What is the Ras-MAPK pathway?
A kinase cascade with the following steps:
- Ligand binds and dimerises the Receptor
- Receptors will cross auto-phosphorylate.
- Auto-phosphorylation will generate binding sites for signalling proteins (pTyr)
- GRB2 (adaptor protein) will bind to pTyr (via SH2 domain), and brings Sos (via SH3) to its substrate, Ras.
- binding Ras will initiate the Kinase Cascade: Ras -> Mek -> Erk
- Erk will perform various functions involving changes in protein activity and gene expression.
How do Insulin receptors reach the PM?
- Insulin acts as a ligand and binds to RTKs.
- Ras-MAPK pathway will stimulate transcription of GLUT4 receptors
- Insuiln signalling in the cytoplasm will also cause vesicles with GLUT4 to fuse with PM to increase glucose uptake.
What is NIDDM? (Hint: type of Diabetes)
Non Insulin-Dependent Diabetes Mellitus
- downregulation of insulin receptor decreases receptor phosphorylation and TK activity
- Results in Insulin resistance (there are no receptors to bind insulin!!) in skeletal muscle, liver, adipose tissue.
What is Pancreatic Beta Cell Dysfunction?
Insulin deficiency, which means no matter how many receptors you have for Insulin, there is no ligand to bind to the receptors.
What do NIDDM and Pancreatic B-Cell Dysfunction result in?
Hyperglycemia:
- causes glycosylation of many different structures in organs, causing disruption/hardening
Affects:
- Blood Vessels
- Kidneys
- Eyes
- Nerves
What does Cancer do to the RTK pathway? What are receptors that are commonly affected in cancers?
Causes TK signal overactivation (becomes a constitutive process) via various causes.
VEGF
- COlorectal cancer
- NSCLC
EGFR (HER1)
- Breast Cancer
- Colorectal cancer
EGFR2 (HER2)
- Breast Cancer
Describe one kind of receptor without TK activity
Cytokine receptors.
- Mechanisms are similar to RTKs, except without the TKs.
- Ligand binding (ILs, G-CSF, GF) activates Kinase domains on JAK proteins that are associated with them.
- JAK proteins will initiate JAK/STAT pathway, which transduce signals to initiate TX)
Describe another kind of receptor without TK activity (not Cytokine receptor).
Integrin Receptors.
- Ligand binding causes clustering of receptors which recruit and activate Kinases: Integrin-linked Kinase (ILF) and Focal Adhesion Kinase (FAK)
Integrin and RTK signalling work together to control migration, proliferation, growth, differentiation, survival, and apoptosis.
Describe the SIMPLE mechanism of an Ion Channel-Linked Receptor.
- Convert chemical signal (NT) into electrical signal
- Ligand binds, causing a conformational change that opens the gate
- Ions (usually) flood through the gate (usually Ca2+)
You would find this in the Post-synaptic cell of a neural synapse.
Describe Steroid Hormone Signalling!
It’s super easy!
- Lipid soluble ligand just chills its way through the membrane
- Lipid will bind to a Receptor-Chaperone complex
- This causes the receptor to change shape and release the Chaperone
- Receptor can now enter nucleus and do its thing.
What is the function of the Steroid Hormone receptor?
It is a nuclear receptor.
It binds to HREs (Hormone Responsive Elements) in DNA, acting as a TX Factor
What other element do Nuclear Receptors bind?
Zn2+; it is essential for DNA binding.
