Signal Transduction (Lecture 14-19) Flashcards
Examples of cell behaviour controlled by signals
- Growth
- Differentiation and dvelopment
- Metabolism
Name some signals for bacteria
- pH
- Osmotic strength
- Food
- Oxygen
- Light
What do bacteria do when they recieve a signal?
Signal → receptor → response
Define signaling
Information from beyond the plasma membrane
Define a receptor
Information detector
Define amplification
Small signals are usually amplified within the cell to give a large response
Define response
Chemical changes and/or changes in gene expression
Describe direct contact
- A protein (ligand) on the signalling cell binds a protein (receptor) on the target cell
- Target cell responds
- Common in tissue development
Describe gap junction
Exchange small signalling molecules and ions, coordinating metabolic reactions between cells
Examples of gap junction
- Gap junctions are made and broken during embryo development
- Electrical synapse use gap junctions between neurons for rapid electrical transmission
What enables the passage of electrical currents?
Clusters of gap junctions which connect the interior of 2 adjacent neurons
Describe autocrine signalling
- Ligand induces a response only in the signalling cell
- Autocrine ligands are typically rapidly degraded in the EC medium
- Used to enforce developmental decisions
Describe eicosanoids in terms of autocrine signalling
- Autocrine ligands derived from fatty acids
- Exert complex control
- Aggregation of platelets in the immune system
- Integration of pain and inflammatory responses
- Aspirin (antagonist)
- Contraction of smooth muscle
- Common feature of cancers
- Auto-production of growth hormones stimualtes cell proliferation
Describe paracrine signalling
- Ligand induces a response in target cells close to signaling cell
- Diffusion of the ligand is limited
- Destroyed by EC enzymes
- Internalized by adjacent cells
Example of paracrine signalling
Neuromuscular junctions
Describe the process of neuro-muscular junctions and paracrine signalling
- Nerve impulse stimulates the movement of synaptic vesicles which fuse w the cell membrane
- This releases acetylcholine
- Acetylcholine stimulates channel opening, allowing for ion exchange
- The muscle twitches and acetylcholinesterase degrades the acetylcholine
Describe endocrine signaling
- The ligand is produced by endocrine cells and is carried in the blood
- This induces a response in distant target cells
- Ligands are often called hormones
Explain cell-type specific expression
- Certain receptors are only present on certain cells
- TRH triggers pituitary responses but not liver responses
- Molecules downstream of the receptor are only present in some cells
- Epinephrine (adrenaline) alters glycogen metabolism in hepatocytes but not in erythrocytes
Explain high affinity interactions
- Precise molecular complementarity between ligand and receptor
- Mediated by non-covalent forces
How are signals amplified by enzyme cascades?
- Receptor/enzyme associated w receptor is activated
- Catalyzes activation of second enzyme → activate multiple molecules of a third molecule
Examples of desensitization in signaling
- Walking from bright light to dark room
- Visual transduction system has become desensitized
- Noxious smells
Describe how EGFR signalling occurs
- Highly specific, high affinity interaction
- Differential EGFR expression
- Epithelial cells +
- Hematopoetic cells -
- Amplification by the MAPK enzyme cascade
- Desensitization by dephosphorylation of EGFR
- Cross-talk n integration w other signalling pathways
Following translation, the IR subunuits:
- Enter the ER membrane
- Associate into dimers
- Exported to the cell surface via the Golgi complex
- During intracellular transport, the proteins are processed by z proteolytic cleavage, each into an alpha n beta subunit
- At the plasma membrane, they are displayed as trans-membrane proteins
Describe how insulin activates the IR at the cell surface
- Insulin binds to IR and stimulates change (allosteric change)
- This brings cytosolic domains close to each other
- Each of these domains are a kinase so activation leads to auto transphosphorylation
- This results in activation of IR
TERM: Ligand
EC substance (e.g. epinephrine, serotonin) that binds to a cell surface receptor n initiates signal transduction that results in a change in IC activity
TERM: Receptor
Protein that binds n responds to the first messenger
Receptor may be either displayed at the cell-surface (e.g. IR, EGFR, GPCRs) or may be intracellular
Why is IRS-1 bifunctional?
- Recruits n activates PI-3K
- Binds to phosphorylated tyrosine residues on the receptor through PTB domain
TERM: second messenger
Small metabolically unique molecule (not protein) whose concentrations can change rapidly
Relay signals from receptors to target molecules in the cytoplasm or nucleus
Describe glucose regulation
- Activation of the receptor
- Recruitment of a kinase that can phosphorylate a membrane lipid
- Amplifies sequence (second messenger)
- Lipid allows recruitment n activation of PKB
- PKB responsible for setting in motion events that reduce glucose lvl in the blood
Explain why insulin is a growth factor
- Phosphorylation of IRS-1 amplifies the signal
- Adaptors recruit n activate Ras
- Signal transduction via an amplifying MAPK cascade
Explain why insulin is a blood glucose regulator
- Phosphorylation of IRS-1 amplifies the signal
- Signal propagation n amplification via conversion of membrane lipids
- Amplification via lipid dependent kinase n activation of PKB
Describe the cellular responses to insulin within minutes
- Increased uptake of glucose into muscle cells n adipocytes
- Altered glucose metabolism by modulation of enzyme activities
Describe the cellular responses to insulin within hours
- Increased expression of:
- Liver enzymes → synthesize glycogen
- Adipocyte enzymes → synthesize triacyclglycerols
- Genes involved in some cell lines
How is the insulin signaling system normally turned off?
- PTEN removes phosphate at the 3 position of PIP3, converting it into PIP2
- PDKI n PKB can no longer be recruited to plasma membrane, shutting off signaling thru PKB
How do we deal w blood sugar?
- As glucose is removed from the blood, activation of PTEN occurs which shuts the system down
- Activated PKB stimulates movement of storage vesicles to cell surface
- In these vesicles hv GLUT4 transporter in their membrane
- GLUT4 is now expressed at the cell surface
- Substrate for GLUT4 is glucose
- Binding causes allosteric change allowing glucose to enter the cell
- Reduction of blood glucose lvl in muscle n fat tissue
- PKB regulates conversion of any excess glucose that cannot be used via glycolysis into glycogen n triacylglycerols
- If you can’t make or respond to insulin then you hv high blood glucose levels
Describe the basic structure of GCPR
- EC domains
- E1 n loops E2-4
- Trans-membrane domains
- TM1-7
- Cytosolic domains
- Loops C1-C3
- C4 tail
- C4 has a lipid anchor
What is the heterotrimeric G-protein made of?
Trimer of alpha, beta n gamma subunits
What happens after Gα activation?
- G-protein dissociates from receptor
- Yields a Gα-GTP monomer n a tightly interacting Gβγ-dimer
- They now modulate the activity of other IC proteins
How do heterotrimeric G proteins cycle b/w on n off states?
- Gα has slow GTP hydrolysis activity
- Regenerates inactive form of α-subunit (Gα-GDP)
- Allows reassociation w Gβγ-dimer to form “resting G protein”
- Regenerates inactive form of α-subunit (Gα-GDP)
- Resting G protein can bind to GPCR n await activation
What does Gs do?
- Stimulates adenylate cyclase
- Signals glucagon n epinephrine
What does Gi do?
- Inhibits adenylate cyclase
- Signals for adenosine, prostaglandin
What is the effect of cortisol on blood sugar levels and the immune system?
- Increases blood sugar thru glucogenesis
- Suppresses the immune system
What are adrenergic receptors?
G protein coupled receptors that bind to the hormones epinephrine n norepinephrine
What are the effects of binding to alpha-adrenergic receptors?
- Inhibits insulin secretion by the pancreas
- Stimulates glycogenolysis in the liver and muscle
- Stimulates glycolysis in muscle.
What are the effects of binding to beta-adrenergic receptors?
- Triggers glucagon secretion in the pancreas
- Increased lipolysis by adipose tissue
- Leads to increased blood glucose n fatty acids for energy production
Describe what happens when epinephrine binds to β-adrenergic GPCR receptor
- Gαs is activated
- Stimulates adenylate cyclase
- RESULT: increase in cAMP levels in the cell
Why is cAMP a second messenger?
- Signaling molecule in all cells
- Activates a variety of proteins
What are PKA’s targets?
- Transcription factors
- Ion channels
- Other enzymes
Role of cAMP in cells
Activates a variety of target proteins
When epinephrine binds to a β-adrenergic GPCR receptor coupled w a Gs heterotrimeric G protein?
- Activates Gas → stimulates adenylate cyclase
- Gβs subunits inhibits adenylate cyclase
When epinephrine binds to a α-adrenergic GPCR receptor coupled w a Gi heterotrimeric G protein?
- Gαi activated → inhibits adenylate cyclase
- Gβγi subunits activate a MAPK cascade
How does CTx traffic to the ER of target cells?
- CTx binds to cell surface of ganglioside lipid GM1 on target intestinal epithelial cells
- CTx undergoes retrograde trafficking via endosomes n Golgi complex to ER
- Disulfide bond b/w CTXA1 n CTxA2 is broken by PDI → helps CTx become unfolded n makes it easier to move across the ER membrane
- PDI: protein disulfide isomerase (ER resident protein)
- BiP keeps CTxA1 soluble until it dislocates across ER membrane in an unfolded form
- BiP (binding protein)
- CTxA1 refolds in cytosol
Function of CTxA
- ADP-ribosylase
- Transfers a ribose group onto a specific arginine on Gas
- Locks Gas in active state permanently → cannot degrade GTP
What is the consequence of CTx locking Gas in its active state?
Adenylate cyclase turned on permanently → cellular cAMP levels rise to over 100 fold abv normal
What is the consequence of increased cAMP levels caused by CTx?
- Activates CFTR membrane channels → increased efflux of Na+ and water into the intestine
- Causes massive secretory diarrhea → death from dehydration
How does light reception work in the vertebrate eye?
- Light passes neural layer thru cell bodies of light receptor cells (rods n cones)
- Acts as a signal in the photoreceptive membrane disc in the “outer segment” of the retina
What are the primary cilium extensions from the surface of vertebrate cells?
Extensions from the surface of most vertebrate cells that act as signalling organelles.
How do rod cells differ from cone cells in their response to light intensity?
- Rod cells → non-colour vision at low light intensity
- Cone cells → colour vision at high light intensity
What is the structure of the outer segment of a rod cell?
- ~1000 discs that are not connected to plasma membrane
- Each disc in the outer segment of a rod cell is a closed sac of membrane with embedded photosensitive rhodopsin molecules.
What are rhodopsin molecules responsible for?
Detecting light n initiating signal transduction cascade in rod cells
What is rhodopsin made of?
- GPCR
- Opsin (GPCR protein component)
- Linked to 11 cis retinal (prosthetic group that is the chromophore / light absorbing group)
How is light energy converted into atomic motion?
- Alternating single n double bonds form polyene w a long unsaturated network of electrons that can absorb light energy
- Light absorption causes cis-trans isomerization around C12 n C13 bond
- N of key lysine moves 5A
Describe the relationship light absorption n GPCR
- Light absorption by retinal → conformation of GPCR
- Inactive rhodopsin becomes activated metarhodopsin II
- Metarhodopsin stimulates nucleotide exchange on the α-subunit of a specific heterotrimeric G protein called transducin (Gt)
How does transducin Gαt activates cGMP phosphodiesterase?
- Light activates rhodopsin, which activates the Gt transducin (Gαt, Gβt, Gγt)
- GTP stimulates cGMP phosphodieseterase (GcMP PDE) which removes cGMP from cGMP-gated ion channels
What happens when the cGMP-gated ion channels close?
- Membrane becomes hyperpolarized
- Thus, light stimulus has been converted to a change in electrical potential across membrane
How does light affect the concentration of calcium ions in rod cells?
- Light closes cGMP gated ion channels → reduces influx of Ca++
- Ca++ is extruded by Na+/Ca++ antiporters → Ca++ concentrations in cell fall
- Low Ca++ activates guanylate cyclase
- cGMP levels rise
- Channels re-open → read to be closed again by light
How does the phosphorylation of rhodopsin affect the activation of transducin?
- Reduces the activation of transducin
- There are 7 phosphorylation sites
- Higher light intensity, more sites are phosphorylated
- Higher phosphorylation → lower ability to activate transducin
Describe how very high light intensity reduces the activation of transducin
- Light activates rhodopsin
- Light-activated rhodopsin can be phosphorylated by rhodopsin kinase
- Arrestin binds to fully phosphorylated rhodopsin → stops activation of transducin
What are the 3 mechanisms that make rods insensitive to high light?
- Prolonged cGMP-gated channel closure
- Phosphorylation of opsin reduces transducin activation
- Arrestin binding to phosphorylated opsin stops transducin activation
Describe the structure of the photoreceptor
- Opsin (modified GPCR)
- 11-cis-retinal (chromophore)
- Different transducin
How do amino acid differences in the modified GPCR affect color tuning?
- Alter the electronic environment that surrounds the 11-cis-retinal chromophore
- Chromophore responds (cis-trans isomerization) to different frequencies of light
What eye adaptations do cephalopods have to help them judge color?
- Cephalopods have wide pupils that accentuate chromatic aberration → detect and focus on narrow bands of wavelengths
- Change the depth of their eyeball → alter the distance between the lens and the retina
- Move the pupil around to change its off-axis location → adjust the amount of chromatic blur and further refine their perception of color.
How do cephalopods judge color?
Bringing specific wavelengths to a focus on their retina
Achieve this by using eye adaptations to adjust the amt of chromatic blur n refine perception of colour
What is chromatic aberration, and how do cephalopods use it to detect color?
- Different wavelengths of light are refracted differently by a lens, causing them to focus at slightly different points
- Cephalopods hv wide pupils that accentuate chromatic abberrtion → allows to detect n focus on narrow bands of wavelengths → used to perceive colour
How does nitric oxide signal?
- Nitric oxide diffuses across plasma membrane
- Binds n activates to its receptor
- Activated receptor (GC) converts GTP into cGMP
- cGMP is a 2nd messenger that alters the activity of target proteins
How do blood vessels normally dilate?
- Response to high BP
- Increases vessel volume → lowers BP
Describe how NO* production in vivo is stimulated by high BP
- Autonomous nerves in vessel wall respond to high BP n release Ach
- Acetylcholine binds to AchR (receptors) on plasma membrane of endothelial cells
- Stimulation by acetylcholine increases endothelial cell cytosolic [Ca++]
How does Ca++ act as a second messenger?
- High Ca++ activates nitric oxide synthase
- NOS catalyzes conversion of arginine to citrulline n nitric oxide
How does NO* act as a paracrine signal to smooth muscle?
- NO* activates soluble guanylate cyclase
- Binds to haem group → causes conformational chan
- GC converts GTP to cGMP
- cGMP: second messenger
What is the role of PKG in smooth muscle relaxation and blood pressure regulation?
- PKG is a cGMP-dependent protein kinase
- Phosphorylates myosin light chain
- Muscle cells w phosphorylated myosin light chain relax
- Smooth muscle relaxation → dilation of blood vessel
- Dilation increases volume of vessel n lowers BP
Describe sidenafil’s mechanism of action
- Sidenafil citrate is a cGMP mimic
- Potent inhibitor of cGMP phosphodieseterases
- Most active against phosphodieseterase 5
What are the different domains of the ER?
- N-terminal transactivation domain
- DNA binding domain
- Hormone binding domain
- Can bind to estrogen
How is the ER maintained in a soluble state?
The ER is stored in the cytosol in complex with Hsp90 (dimeric chaperone protein), which binds near the ligand-binding site and maintains the ER in a soluble state.
Why can’t the Hsp90:ER complex enter the nucleus?
- The Hsp90:ER complex is too large to enter the nucleus
- ER needs to dissociate from Hsp90 before it can translocate into the nucleus to exert its function as a transcription factor.
Describe how oestrogen-bound ER is a transcription factor
- Estrogen diffuses across PM n binds to ER
- ER is released from Hsp90
- ER-estrogen complex enters nucleus n binds estrogen response elements (EREs) as a dimer
- Estrogen-responsive genes are transcribed
What is the function of the ER in response to estrogen?
- ER is the receptor for estrogen and, upon activation by estrogen, binds to DNA and directs transcription of estrogen-response genes.
- This means that the ER is both the receptor and effector in this signaling pathway, and there are no amplification steps via protein cascades or second messengers.
What is sildenafil most active against?
Phosphodiesterase type 5 (PDE-5)
