Week 2 Signal Transduction 2 Lecutures Flashcards
what is signal transduction
the process by which extracellular signals effect a change in the internal working of a cell
why are there generally so many steps in signal transduction pathways? (3 potential reasons)
signals can be modified at each step, signals can be amplified, signals can be distributed to several parallel processes
how are signaling cascades relayed (3)?
enzyme activity, protein-protein interactions, second messengers
what are the three major classes of second messengers?
cyclic nucleotides (cAMP, cGMP), IP3/DAG, Calcium ions
why wont a cell survive in saline?
cells depend on signaling from their micro-environment of growth and survival. without these signals the cell dies
what explains why different cells respond differently to the same signal?
difference in receptor structure, or difference in internal signaling pathways
How do the following cell types respond to ACh? heart muscle cell, salivary gland, skeletal muscle cell
heart: decrease rate and force of contraction
salivary: increase secretion
skeletal: contraction
how are signals withdrawn?
the signaling molecule can be down-regulated (degraded) or the recepotr can be down-regulated (inactivated, reduced synthesis, internalized)
What is acetylcholinesterase?
enzyme normally found in the synapses between cholinergic neurons and muscle that degraged ACh
What causes Myasthenia Gravis? how can it be treated
body produces antibodies against ACh receptors. Treatment is an acetlycholinesterase inhibitor that increases the concentraiton of ACh to compensate for decreased receptors
What are some examples of immediate modifications to cells in response to signaling?
changes in cell shape or activity
what are some examples of long-term changes to cells in response to signlaing?
changes in growth rate or differentiation (things that require gene expression and protein synthesis)
what are the three types of extracellular signaling pathways
Cell-surface receptors, intracellular receptors, gases
describe the structure of GPCRs
extracellular ligand-binding domain, 7 transmembrane domain, intracellular domain that binds G-proteins
G-proteins are active when? inactive?
active when GTP bound, inactive when bound to GDP
What is the structure of lipid linked G-proteins?
trimeric: alpha, beta and gamma subunits
what are the steps involved in the GPCR pathway
- ligand binds GPCR causing confirmational change in the alpha subunit
- alpha subunit of G protein exchanges GDP for GTP (GTP is now attached)
- the alpha and beta/gamma subunits dissociate and move on to activate or inhibit target proteins
- GTP bound to alpha subunit is eventually hydrolyzed to GDP causing reassembly of alpha/beta/gamma
For each g-protein give the active unit and function:
Gs
Gi
Go
Gs: alpha, activates adenyl cyclase
Gi: alpha, inhibits adenyl cyclase
Go: beta/gamma, activates K channels; also phopholipase C
in general, enzyme linked receptors (RTKs) are involved with_____ that produce ______ effects on cells
GFs: cell growth, proliferation, differentiation, survival
long-term
what is the best example of an enzyme-linked receptor?
RTKs
what is a kinase?
an enzyme that post-translationally phosphorylates a target molecule
Tyrosine kinase phosphorylate which residue?
Tyrosine
Ser/Thr kinases phosphorlyate which residue?
Ser/Thr
what percent of human proteins are phosphorylated? what charge does phosphorylation add to a protein?
30%, negative charge
what is a phosphatase?
dephosphorylates molecules
describe the structure of RTKs
single pass transmembrane proteins with an extracellular receptor domain and an intracellular tyrosine kinase domain
Describe the steps in RTK signaling
- ligand binds causing alpha and beta RTK subunits to dimerize
- the subunits autophosphorylate each others Tyr residues
- activated tyrosine kinases bind SH2 domain-containing proteins
- various pathways proceed
where do we see ion-linked receptors? how are they opened/closed
transduction of neural signals. neurotransmitters bind to receptors that transiently open/close ion channels
how do intracellular receptors work?
small, hydrophobic molecules diffuse across plasma membrane and are capture by the intracellular receptors. the intracellular receptor then enters the nucleus where they can affect gene expression
how do small hydrophobic (intracellular receptor ligands) exist in plasma?
associated with a carrier protein becuase they cant exist in an aq environment by themselves
what is an example of a ligand for an intracellular receptor?
steroid hormones derived from cholesterol
How does NO act as a signal molecule?
NO binds to and activates guanylyl cyclase which produces second messenger cGMP
increased cGMP levels does what?
relaxes smooth muscle cells (NO indirectly causes vasodilation)
how is NO synthesized?
from Arginine by the enzyme NO synthase
Vasodilation has what impact on BP
Lowers it nogga
how do erectile dysfunctions drugs work?
inhibits cGMP breakdown and cause continuous vasodilation. this ensures blood goes to genitals
how is cAMP made? where is this enzyme located?
synthesized from ATP by adenylyl cyclase. located in the membrane
how is cAMP degraded?
cAMP phosphodiesterase this enzyme is always active
the actions of adenylyl cyclase (to produce cAMP) is stimulated by? inhibited by?
stimulated by Gs (GPCR, alpha subunit)
inhibited by Gi (GPCR, alpha subunit)
what is the function of cAMP
cAMP can activate protein kinase A
what is the fxn (2) of PKA?
PKA can phosphorylate protieins int eh cytosol or PKA can travel into the nucleus where it activates other proteins that regulate gene transcription
E and NE bind which G-protein linked class of receptors?
adrenergic receptors
what does the stimulation of heart adrenergic receptors do?
increase heart rate
what are beta blockers? what are they used for?
antagonist of some adrenergic receptors that are used to treat hypertension
how does Cholera toxin work?
cholera toxin adds ADP-ribose to the alpha subunit of Gs and prevent the hydrolysis of GTP to GDP. this causes excess cAMP and a massive flow of water into intestinal lumen.
how is cGMP produced? degraded?
produced: guanlyly cyclase
broken down: cGMP phosphodiesterase
Role of cGMP
important in smooth muscle relaxation and vasodilation
what is PIP2
phoshpatidylinositol (a phospholipid on the inner leaflet of PM) with 2 phosphate groups attached
what is the role of phospholipase C? how is it activated?
PLC breaks down PIP2 into DAG (remains in membrane) and IP3 free to roam cytosol. activated by a GPCR
the products of PIP2 cleavage via phospholipase C are DAG and IP2, what are their roles
DAG: activates PKC which plays a role in phosphorylation/activation of other proteins
IP3: triggers opening of Ca channels in ER
When talking about RTKs what are adaptors?
a protein that contains an SH2 domain and binds to RTK but only acts as a scaffold for other proteins to attach
what is Ras, how is it activated?
a monomeric G-protein, activated downstream of RTK (RTK–>Adaptor protein–>Ras-activating protein–>Ras)
What pathway does Ras activate?
the MAP-kinase cascade (MAP-kinase kinase kinase–>MAP kinase kinase–>MAP kinase). plays a role in cell proliferation. some MAP kinase pathways are activated by cell stress.
mutations in Ras are seen in what % of all human cancer?
30%, Ras is an ocogene
what is the role of PI-3 Kinase? how is PI-3K activated?
phosphorylates PIP2 to make PIP3. PI-3K is activated by Ras
what are two enzyme-linked receptors that do not require long cascaded of signal proteins?
JAK-STAT pathway and TGF-Beta pathway
what are cytokine recepotrs?
the collective name for JAK-STAT receptors, important in immunity and inflammation
what is unique about the JAK-STAT pathway
the receptor does not have intrinsic kinase ability so it bind JAK, a cytoplasmic kinase
describe the steps in the JAK-STAT pathway
cytokine binds receptor–> cytokine receptors dimerize and associated JAKs cross phosphorlyate each other–>activated receptors recruit and phosphorylate STAT–>STAT enter nucleus and affect gene expression
What type of enzymatic activity do TFG-beta receptors have
Ser/The kinase
what is the signal cascade for TGF-Beta receptors?
TGF-Beta binds TFG-Beta receptor–>receptor phosphorylates itself and becomes active–>activated TGF-beta activated intracellular SMAD–>SMAD travel to nucleus and regulate gene expression
what % of our genes encode protein kinases
2%! wow thats alot
do all signaling pathways act independtly of each other?
no there is extensive cross talk among signaling pathways
Beta-adrenergic receptors are what type of receptor? what is their ligand?
GPCR. E/NE
what is the role of B-adrenergic receptors?
play a predominant role in regulating contractility of heart muscle. there are atleast nine adrenergic receptors in the heart, but B-AR are most prominant in regulating heart fxn
what type of G-protein do B-ARs associate with?
Gs. Recall: Gs activates adenyl cyclase and the PKA pathway. PKA phophorylates cytoplasmic targets that regulate contractility, relxation, and Heart Rate
what is desensitization? which type of receptor is it seen in?
the minimization of receptor activity due to continous agonist binding. GPCRs
Why is desensitization helpful?
there are conditions such as chronic stress and disease in which high levels of E/NE are expressed. Desensit. prevents continual B-AR activation
what are the three mechanisms in which GPCR can be desensitized?
down regulation (decrease receptor synthesis or degrade), sequestration (internalization of receptor), inactivation (phosphorylation)
What is the role of GRKs? when is the only time GRKs work?
phosphorylate GPCRs and inactivate them. GRKs can only phosphorylate GPCRs that have been activated by ligand binding
what occurs after GRKs phosphorylate GPCRs?
Arrestin binds to GPCR rather than a G-protein (arrestin is involved in other signaling pathways
what effect does heart failure have on beta-AR signaling? how can you treat heart failure?
damage to heart causes an increase in Sympa NS and produces chronically high levels of E/NE. This desensitizes beta-ARs due to up regulation of GRKs. this further weakens the heart. paradoxically treating heart failure patients with beta blockers improves beta-AR signaling
insulin receptors are what type of receptor?
RTK
what is the main fxn of the insulin pathway?
triggers vesicles with GLUT4 in their membrane to fuse with the plasma membrane thus increasing glucose uptake in cells
what is the role of PTEN
PTEN is a phosphatase that converts PIP3 to PIP2
the activated insulin receptor activates what two pathways?
MAP kinase signaling pathway and PI-3K signaling pathway
much of the effects of the PI-3K pathway is due to downstream activating of….what are these effects?
AKT. cell cycle, apoptosis, cell survival, glucose metabolism, immune response, neural development
