Section 6: Signal Transduction Flashcards
What is signal transduction
Extracellular signals that eventually lead to a response inside the cell
Signal transduction - pathway
Signal --> Reception --> Transduction --> Amplification --> Response(s)
Signal transduction: Pathway - signal
Initiates the pathway
Signal transduction: Pathway - reception
Where the signal is received
Signal transduction: Pathway - transduction
Inside the cell
Signal transduction: Pathway - amplification
For a small amount of signal, you’re able to create a large response in the cell
How do cells communicate
Via chemical signals, which rely on hormones
Hormones
Extracellular signals secreted by cells that then diffuse or circulate to specific target cells
Why is cell signalling important
Helps maintain homeostasis
Involved in multiple systems in body
Many medicines control cell signalling events via receptors
Origins of a signal: Endocrine signalling
Endocrine hormone is released from a gland and travels through the blood to act upon a distant target organ
Origins of a signal: Endocrine signalling - example
Insulin, glucagon
A hormone is an example of a(n)…
Extracellular signal
Origins of a signal: Paracrine signalling
Released from cells to act upon adjacent cells
Origins of a signal: Paracrine signalling - example
Release of ACh at neuromuscular junction
Origins of a signal: Autocrine signalling
Act upon the same cell type they are released from
Origins of a signal: Autocrine signalling - example
Growth factors
Origins of a signal: Signalling by PM-attached proteins
Cell-cell signalling may also occur
Origins of a signal: Signalling by PM-attached proteins - example
T-cell activation by proteins on surface of antigen-presenting cells in immune system
How do hormones and other extracellular signals initiate a chain of events
By activating receptors
Receptor
A molecule on the surface of within a cell that recognises/binds to specific molecules
Produces a specific effect
Lock and key analogy
Describes how each hormone has its own specific receptor
Only when the hormone/ligand engages with the correct receptor, can it activate the receptor and trigger intracellular signalling –> response
Receptor - conformational change
A receptor is a protein (flexible), so when a ligand binds it leads to a change in shape of inside of receptor –> allows substrates in receptor to bind to activated receptor
Receptor - gatekeeper
Receptor is a gate-keeper of cellular activity
Controls hormone activity at cell surface
Signalling can occur with/without the hormone passing through the membrane?
Without
Receptor: Hormone and affinities
Binding of hormone changes the chemical affinities of receptor –> changes shape
Lock and key mechanism: Drugs
Can create molecules that mimic endogenous hormones, e.g. asthma
Or, can design molecules that fit in receptor pocket but leads to no signal
Types of ligands: Agonists
Produce the maximal response for a given tissue
Types of ligands: Partial agonists
Produce a response which is below the max for that tissue
Types of ligands: Antagonists
Produce no visible response and block effects of agonists
Receptor types/classes
G-protein coupled receptor (GPCR)
Receptor tyrosine kinases (RTK)
Ligand-gated ion channels (LGIC)
Receptor types: GPCR
7 transmembrane domains
Ligand binding site on extracellular side
Ligand binds –> change in shape that allows a G-protein to bind on intracellular side
Signals via G-proteins and second messnegers
Receptor types: Receptor tyrosine kinases (RTK)
Enzyme-linked receptor
Signals by phosphorylation
Receptor types: Ligand-gated ion channels
Directly allows ions through
Transduction
Cascades of molecular interactions that relay signals from receptors to target molecules in cell
Signal transduction - why are there multistep pathways
Can greatly amplify signal
Provides more opportunities for coordination and regulation of response
Signal transduction - mechanisms
Second messengers
Phosphorylation
Signal transduction: Second messengers - when are they produced
Produced following receptor activation
Signal transduction: Second messengers - what are they
Chemical signals that are often not embedded in the membrane - can diffuse intracellularly to pass on message
Signal transduction: Second messengers - how they work
They change in conc in response to environmental signals, and this change in conc conveys info inside the cell
i.e. are dose-dependent
Signal transduction: Second messengers - first messenger
The hormone/ligand that activates the receptor
Common second messengers
cAMP, cGMP
IP3
Calcium
Diacylglycerol (DAG)
A second messenger can work on…
Multiple substrates
Signal transduction: Second messengers - types of responses
- Pathway leads to a single response
- Pathway branches –> 2 responses
- Cross-talk between 2 pathways (response can be controlled by diff pathway
- Diff receptor leads to diff response
Signal transduction: Phosphorylation and dephosphorylation act like…
A molecular switch - turns protein activity on/off or up/down as required
Signal transduction: Phosphorylation regulates…
Protein activity
Signal transduction: Phosphorylation - protein kinases
Transfer phosphates from ATP to protein (phosphorylation)
Signal transduction: Phosphorylation - relay molecules
Many relay molecules are protein kinases –> creates a phosphorylation cascade
Signal transduction: Phosphorylation - protein phosphatases
Rapidly remove phosphates from proteins - dephosphorylation
Signal transduction: Phosphorylation - amino acids commonly phosphorylated
Tyrosine
Serine
Threonine
Signal transduction: Does phosphorylation always turn things on
No, it can turn it off
Signal transduction: Phosphorylation - allows you to control your response…
Quite tightly
Amplification - what does it mean
Only a v small amount of initial hormone is needed, and few receptors need to be activated, to produce a response
Response
The changes in chemicals result in activation or inhibition of proteins
Termination of signal
After the cell has completed its response to a signal, the process must be terminated so the cell can respond to new signals
Failure of termination of signalling processes
Can have highly undesirable consequences
Receptors - function examples
Vision Taste Smell Neurotransmission Cell growth Development Control of heart rate
Receptor types (classes)
GPCRs - work with help of a G protein
Receptor tyrosine kinases (RTKs) - attach phosphates to tyrosines to signal
Ligand-gated ion channel receptors - signal molecule binds as a ligand to the receptor –> opens receptor gate –> allows ions to pass
GPCR signalling - signal
Endocrine
Epinephrine
Binds to receptor
GPCR signalling - receptor
GPCR
Beta-adrenergic receptor
GPCR signalling - transduction
G-protein - αβγ, binds to –>
1° effector protein - adenylate cyclase, which makes –>
2nd messenger - cAMP, which activates –>
2° effector protein - protein kinase A, which causes –>
Phosphorylation - cascade
Effector proteins
Molecules (often enzymes) in a cell that respond to a stimulus and can be activated and further transduce a signal
Effector protein for G-proteins
Adenylate cyclase
Effector protein for cAMP
Protein kinase A
Where are GPCRs found
They exist in a range of organisms and express at the cell surface to respond to diverse extracellular signals
Structural features of a GPCR
7 transmembrane alpha helices
3 intracellular loops
3 extracellular loops
N-terminus on extracellular side, C-terminus on intracellular side
Receptor - ‘gatekeeper’
Controls hormone activity at cell surface
Conformational change of GPCR results in _____ affinity for G protein
Higher
GPCR - mutation on extracellular vs intracellular side of receptor
Extracellular - affects how it binds the ligand
Intracellular - affects how it binds a G-protein
Structure of a G-protein
Trimeric - 3 diff subunits (α, β, γ)
If α subunit binds GTP, it dissociates into 2 parts; the α subunit (acts on its own) and the βγ subunit (acts elsewhere)
What does G-protein stand for
Guanosine-binding protein
G-protein cycle
- Off position: GDP-bound - remains as a trimer and is inactive
- Ligand binds GPCR so receptor is attracted to G-protein –> conformational change –> G-α subunit releases GDP and binds GTP –> conformational change –> G-βγ dissociates
G-α is now active (‘on’ position) so can act on effector enzymes downstream, e.g. adenylate cyclase - G-α subunit hydrolyses GTP –> GDP, which reassociates with βγ –> off position
G-protein cycle - where can the system be shut off
At the point where G-α subunit is active because it has an intrinsic enzyme activity for GTPase
Types of G proteins
Gs
Gi
Gq
Types of G proteins: Gs
Stimulatory G protein
Activates adenylate cyclase by making it more catalytically active
Types of G proteins: Gi
Inhibitory G protein
Inactivates adenylate cyclase by making it less catalytically active
Types of G proteins: Gq
Activates a diff effector, phospholipase
Types of G proteins: Gs - steps
Binds ligand –> conformational change –> attracts Gαs stimulatory protein which is activated –> binds to adenylate cyclase
Types of G proteins: Gs - what does it result in
More ATP –> cAMP
More cAMP in cell
Types of G proteins: Gi - steps
Ligand binds –> conformational change –> attracts Gαi which binds to adenylate cyclase
Types of G proteins: Gi - what does it result in
Less ATP –> cAMP
Less cAMP in cell
Types of G proteins: Gs and Gi is an example of…
Cross talk (2 diff pathways that can affect each other
cAMP activates or inactivates….
Protein kinase A
Adenylate cyclase is known as a(n)…
Effector protein
cAMP is made by the enzyme…
Adenylate cyclase
cAMP stands for..
Cyclic adenosine monophosphate
Types of G proteins: Gq - steps
Ligand binds –> conformational change –> attracts Gq protein (now active) –> acts on phospholipase C (PLC), which converts PIP2 into DAG and IP3
DAG activates protein kinase C (PKC) but also need Ca2+ to activate PKC
IP3 causes release of Ca2+ from cell’s stores, so tgt they activate PKC
5 sensory perceptions and what they rely on as receptors
Hearing and touch generally rely on ion channels
Vision, smell and taste generally rely on GPCR
Neurons - normal RMP
About -70mV
Depolarisation, repolarisation, hyperpolarisation
Depolarisation = inside of cell becomes less -ve (influx of Na+) Repolarisation = inside of cell goes back to normal (efflux of K+) Hyperpolarisation = inside of cell becomes more -ve than RMP
What is vision based on
Absorption of light by photoreceptor cells in the eye
Vision: Types of photoreceptor cells
Rod and cone cells
Photoreceptor: Rod cells
Responsible for vision in low light and peripheral vision
This is why at low light, you can see but things appear grey because rod cells encode vision but not colour
Photoreceptor: Cone cells
Responsible for vision in bright light
Gives info about colour
Vision - signal
Light (photon)
Vision - receptor
GPCR (rhodopsin)
What is the photoreceptor molecule in rod cells
Rhodopsin
Rhodopsin - what is it made of
Consists of protein opsin, linked to 11-cis-retinal (prosthetic group)
Opsin
A 7 transmembrane protein that determines wavelength of light absorption
11-cis-retinal
A light-absorbing group (chromophore)
What happens when light hits 11-cis-retinal
It causes it to isomerase from 11-cis-retinal to all-trans-retinal
Undergoes a 5-angstrom twist
Vision: Where are cone receptors located
In cone cells
What is colour vision mediated by
3 cone receptors
Vision: Photoreceptor proteins (numbers)
In humans there are 3 distinct photoreceptor proteins with absorption maxima at 426 (blue), 530 (green) and 560 nm (red)
i.e. blue-opsin, green-opsin, red-opsin
Mechanism of action: Rod vs cone cells
Cone cells work under same principle as rod cells, except cone cells’ opsins are diff –> absorb light at diff wavelengths
Cone cells: Photoreceptor opsins - homology
Green and red photoreceptor opsins are 95% identical in amino acid sequences - theory is the green opsin mutated over time and became a red opsin
Blue and green photoreceptor opsins are 20% similar in amino acid sequence
Vision: In the dark, what happens
Photoreceptor cells are depolarised with continual influx of Na+ and Ca2+ through cGMP-gated ion channels
Vision - G-protein
Transducin
Vision - primary effector
Phosphodiesterase
Cleaves cGMP into GMP
Vision - second messenger
cGMP
Vision: Photoreceptor - special neuron?
Constantly has leakage of +ve ions into cell, so RMP is more +ve (about -40mV)
Vision: What happens when light hits rhodopsin (receptor)
Phosphodiesterase converts cGMP to GMP –> hyperpolarisation, which is sensed by next neuron
How is vision pathway different from other neuron pathways
It works by hyperpolarisation
Vision: Retinosa pigmentosa
A group of inherited diseases that affect photoreceptor (mainly rod) cells where they progressively deteriorate
Vision: Retinosa pigmentosa - symptoms
Initially may just not see in low light
Overtime can lead to:
Tunnel vision (because rod cells are responsible for peripheral vision)
Blindness
Vision: Retinosa pigmentosa - age
Can happen to people as young as 40 y/o
Vision: Retinosa pigmentosa - what stage does something go wrong
Reception
Vision: Colour blindness - how does it happen
Since red and green opsins sit on same chromosome v close tgt, during repro, 2 things could’ve gone wrong:
- Recombination between genes –> individual won’t have protein to receive either green light, or red light
- Recombination within genes –> individual has protein that can absorb neither green or red light
Vision: Colour blindness - who is more likely to get it
Since it lies on X chromosome, males are more likely to get it
Vision: Colour blindness - where in the pathway does it go wrong
Reception
Vision: Colour blindness - what type of cell
Cone cells
Why is taste perception important
Nutritious vs poisonous
Commercial value
Health intervention
Taste: Where is the signal initiated
Papillae contain taste buds which are made up of taste cells, which contain taste receptors
Taste: Papilla
Bumps on tongue
Creates trench around tongue to collect saliva
Taste - signal
Food (tastant)
Major taste sensations - receptors
GPCRs:
Sweetness
Umami
Bitterness
Ion channels:
Salty
Sour
Major taste sensations - ligands
Sweetness: sugars, sweeteners Umami: amino acids Bitterness: quinine and others Salty: Na+ Sour: H+
What is umami
Taste of savoury-ness
e.g. meat
Taste: Taste receptors - families
T1 and T2 family
Taste: Taste receptors (GPCRs) - subunits
Each family has subunits;
T1R: 1, 2, 3 = sweetness and umami
T2R: 1-65 = bitterness
Taste: G-protein-coupled taste receptor subunits - structure
7 transmembrane domains
Extracellular N-terminus
Taste: T1R
Each subunit has an additional venus fly-trap domain on extracellular side
Heterodimer required to be functional (i.e. requires 2 subunits)
Couples to / activates G protein
Taste: T1R - heterodimers
T1R2 + T1R3 = sweet
T1R1 + T1R3 = umami
Taste: T2R
Functions as monomer
Couples to / activates G protein
Have many of these to detect poisonous things
Taste transduction pathway - tastant / ligand
Sweet, bitter, umami
Taste transduction pathway - G protein
Gustducin
Activates phospholipase C, which cleaves PIP2 –> DAG and IP3 (second messengers)
Taste transduction pathway - Ca2+
IP3 releases Ca2+ which activates V-gated ion channels
Na+ comes into cell (depolarisation) –> APs
Taste disorders
Phantom taste sensation
Hypogeusia (lowered taste sensation) or ageusia (no taste)
Dygeusia (taste perceived isn’t what you ate)
Blood glucose - always kept between…
4-8 mM of blood
Hypoglycaemia
Too little glucose in blood
Can result in:
Coma or death
Brain damage
Hyperglycaemia
Too much glucose in blood
Can result in:
Diabetes
Ulcers (since blood is thicker)
Nerve damage –> blindness
Insulin
When too much glucose in blood, leads to uptake of glucose into muscle cells, and liver uses glucose to makes glycogen
Brings blood glucose back down
Causes of low blood glucose
Fasting –> glucagon is released –> makes glucose and break down of glycogen stores –> brings glucose back up
Under stress –> release epinephrine –> more glucose
Low blood glucose: Signal
Glucagon or epinephrine
What is glucagon
A large peptide
Glucagon and epinephrine: Receptor
GCGR (glucagon receptor)
AR (adrenoreceptor)
Glucagon and epinephrine: Transduction
Activation of G-protein: Gαs
Primary effector: AC
2nd messenger: cAMP
PKA phosphorylates B kinase –> phosphorylates D into a –> converts glycogen into glucose
Glucagon: Glycogen synthase
PKA is able to phosphorylate glycogen synthase –> inactivates it –> glycogen is not made
Avoids futile cycles
What does glucagon result in
Increased gluconeogenesis
Increased glycogen breakdown
Decreased glycolysis
Increased blood glucose
Where is glucagon recognised
By receptor cells on liver
Gluconeogenesis
Making of glucose from non-carbohydrate molecules
Glucagon - speed
Within 5 minutes, it causes glucose levels to rise, so works fairly quickly
What does epinephrine result in
Increased glycogen breakdown (glycogenolysis)
Increased glycolysis
Increased blood glucose
Where is epinephrine released from
Adrenal glands
What is epinephrine recognised by
Liver cells
Also in muscle cells; increases glycogen breakdown and glycolysis
Epinephrine: Liver vs muscle
Liver makes energy for body
Muscle makes energy for itself
How is glycogen degradation turned off
Hormone that stimulates glycogen breakdown is removed - pancreas no longer secretes glucagon
At G protein, GTPase hydrolyses GTP –> GDP (inactive)
At 2nd messenger, cAMP –> AMP by phosphodiesterase
Protein phosphatases remove phosphate groups from phosphorylase –> inactivates enzymes
High blood glucose: Signal
Insulin
Insulin: Receptor
Insulin receptor
Not a GPCR, but a tyrosine kinase
Insulin receptor - structure
V different to GPCRs
Dimer of two monomer - α and β subunits bound by disulphide bond
Tyrosine kinase - kinase is part of receptor
Insulin: Transduction pathway
Insulin binds
Receptor monomers become close tgt from extracellular side
Drags intracellular domains close tgt –> kinase enzymes cross-phosphorylate
Active kinase phosphorylates downstream molecules –> cascade
GLUT4 transporters inserted into muscle cells
What does insulin result in
Increased glucose uptake in muscles
Decreased blood glucose
Insulin - target tissue
Muscle
Insulin-glucagon regulation - complexities
Paracrine effects between β-islet cells can directly inhibit secretion of glucagon in α-islet cells
Insulin deficiency
Type I diabetes
Defect in signal
Insulin - drug
Used as a diabetes drug to mimic natural actions of this hormone at insulin receptor
GPCR gene mutations / diseases
Defect in reception
Cone opsins - colour blindness
Rhodopsin - retinitis pigmentosa
MC4R - extreme obesity
Cholera - what part of the pathway is affected
Transduction
Cholera toxin
Stops Gα subunit from being able to hydrolyse GTP –> Gs always active –> increase AC –> increase cAMP –> increase PKA –> increase phosphorylation –> more extrusion of Cl- –> huge loss of water –> diarrhoea –> dehydration
Affinity
A measure of how tightly a ligand binds to the receptor
Can generally be measured using dissociation constant Kd
Kd
Ligand conc where receptor is 50% saturated with ligand
Kd vs Ki
Ki = Kd if inhibitor
Kd and affinity
Lower Kd = higher affinity
What is the problem in asthma
Bronchoconstriction
What signalling pathway might affect asthma
Epinephrine pathway
Asthma: What kind of ligand-receptor action is desired
β-agonist –> relaxes airways (dilation)
Asthma: What chemical structure is desired
β-agonist is structurally similar to epinephrine
Asthma: How well does ligand/drug bind to receptor
β-agonist has low affinity, so need to keep taking it
Asthma: How does β-agonist work
Activates G-protein –> activates AC –> cAMP –> PKA –> phosphorylated myosin light chain kinase –> muscle relaxation
β-adrenoceptor agonists and asthma - function
Reverse bronchoconstriction associated with asthma
Asthma: Salbutamol vs salmeterol
Salbutamol: low affinity - acute symptoms
Salmeterol: high affinity - long-acting
Types of pain
Pain receptor pain (receives signal/stimulus) Neuropathic pain (nerves sensing pain regardless of presence of stimulus)
Nociceptors
Senses pain
FNEs that respond to diff stimuli
Nociceptors - process
Cells around cut release cytokines –> inflammation –> release prostaglandins which is received by nociceptor –> AP of neuron releases neurotransmitter, which relays signal to thalamus and is sensed as pain
Pain: Interneurons
In spinal cord
Modulate what you feel
Strategies to manage pain
Change initiator of pain
Change CNS modulation of pain
Strategies to manage pain: Change initiator
Reduce inflammation –> reduce prostaglandin
e.g. paracetamol, NSAIDs, aspirin
Strategies to manage pain: Change CNS modulation
All these drugs are opioids:
e.g. codeine, morphine, fentanyl, tramadol, oxycodone
WHO’s pain relief ladder
First give non-opioid drugs, but for moderate pain onwards, start giving opioids because much more effective
Opioids signal transduction pathway
Ligand + receptor: Endogenous opioids + m, k, d opioid receptors (GPCRs)
G-protein: Gαi/o
Effector: AC
2nd messenger: Decreased cAMP –> decreased Ca2+ influx and increased K+ efflux
Response: Less depolarisation
What are opioids released by
Interneurons
What do opioids bind to
GPCRs
Pain relief - normal pathway
- AP arrives –> V-gated Ca2+ channels open –> Ca2+ influx
- Release of glutamate binds to LGIC –> post-sympathetic neuron depolarisation
- Neurons repolarise by K+ efflux
Pain relief - opioid pathway
- Opioids bind to GPCR
- Goes through Gαi signal cascade
- -> inhibits Ca2+ channel
- -> no glutamate release into post-synaptic neron - Increased K+ efflux –> harder for further depolarisation of neurons
Drawbacks of opioid use - side effects
Since receptors also found in other areas:
- bowel and anal sphincter
- areas of brain responsible for respiration
Side effects include:
- constipation
- respiratory depression –> death
Drawbacks of opioid use - tolerance
Receptor and effectors adapt (densensitised, down-regulated) and is no longer inhibited at same dose
Need higher and higher doses
Drawbacks of opioid use - addiction
Apart form reducing pain, also causes release of dopamine that causes feeling of reward/pleasure
What is the main factor that limits how much opioids you can take
Respiratory depression
Opioid crisis
An exceptionally high mortality rate and harm linked to use/misuse of opioid drugs
Key factors leading to opioid crisis
1990s: well-intentioned push for doctors to treat pain in patients
Mistaken info that addiction was rare
Increased prescriptions –> increased misuse –> increased overdose deaths
Move to shut down prescription drug misuse –> increased heroin use –> increased use of fentanyl and synthetic opioids –> more deaths due to high affinity and potency
Opioid crisis: Solutions - antidotes
e.g. Naloxone
Competitive antagonist - itself doesn’t lead to pain relief, tolerance or addiction
Has higher affinity than opioids, so will preferentially bind to receptors –> reverse opioid overdose if administered in time
BUT this is not a solution to opioid crisis
Opioid crisis: Solutions - change in policies
Use of prescription drug monitoring programs
Increase access to drug abuse treatment services
Enforce rules for drug makers
Opioid crisis: Solutions - change doctors’ pain management practices
Opioids should be reserved as second or later line of pain management
Opioid crisis: Solutions - develop better pain medication
Better alternatives with low side effects, tolerance and addiction risk should be investigated
Opioid crisis: Solutions - develop better pain medication - strategies
Design opioid-like drugs that bind to receptor but doesn’t lead to adaptation of receptors or effectors
Design entirely novel drugs that use other mechanisms (non-opioid) that are efficient at reducing pain
