Section 6: Signal Transduction Flashcards
1
Q
What is signal transduction
A
Extracellular signals that eventually lead to a response inside the cell
2
Q
Signal transduction - pathway
A
Signal --> Reception --> Transduction --> Amplification --> Response(s)
3
Q
Signal transduction: Pathway - signal
A
Initiates the pathway
4
Q
Signal transduction: Pathway - reception
A
Where the signal is received
5
Q
Signal transduction: Pathway - transduction
A
Inside the cell
6
Q
Signal transduction: Pathway - amplification
A
For a small amount of signal, you’re able to create a large response in the cell
7
Q
How do cells communicate
A
Via chemical signals, which rely on hormones
8
Q
Hormones
A
Extracellular signals secreted by cells that then diffuse or circulate to specific target cells
9
Q
Why is cell signalling important
A
Helps maintain homeostasis
Involved in multiple systems in body
Many medicines control cell signalling events via receptors
10
Q
Origins of a signal: Endocrine signalling
A
Endocrine hormone is released from a gland and travels through the blood to act upon a distant target organ
11
Q
Origins of a signal: Endocrine signalling - example
A
Insulin, glucagon
12
Q
A hormone is an example of a(n)…
A
Extracellular signal
13
Q
Origins of a signal: Paracrine signalling
A
Released from cells to act upon adjacent cells
14
Q
Origins of a signal: Paracrine signalling - example
A
Release of ACh at neuromuscular junction
15
Q
Origins of a signal: Autocrine signalling
A
Act upon the same cell type they are released from
16
Q
Origins of a signal: Autocrine signalling - example
A
Growth factors
17
Q
Origins of a signal: Signalling by PM-attached proteins
A
Cell-cell signalling may also occur
18
Q
Origins of a signal: Signalling by PM-attached proteins - example
A
T-cell activation by proteins on surface of antigen-presenting cells in immune system
19
Q
How do hormones and other extracellular signals initiate a chain of events
A
By activating receptors
20
Q
Receptor
A
A molecule on the surface of within a cell that recognises/binds to specific molecules
Produces a specific effect
21
Q
Lock and key analogy
A
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
22
Q
Receptor - conformational change
A
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
23
Q
Receptor - gatekeeper
A
Receptor is a gate-keeper of cellular activity
Controls hormone activity at cell surface
24
Q
Signalling can occur with/without the hormone passing through the membrane?
A
Without
25
Receptor: Hormone and affinities
Binding of hormone changes the chemical affinities of receptor --> changes shape
26
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
27
Types of ligands: Agonists
Produce the maximal response for a given tissue
28
Types of ligands: Partial agonists
Produce a response which is below the max for that tissue
29
Types of ligands: Antagonists
Produce no visible response and block effects of agonists
30
Receptor types/classes
G-protein coupled receptor (GPCR)
Receptor tyrosine kinases (RTK)
Ligand-gated ion channels (LGIC)
31
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
32
Receptor types: Receptor tyrosine kinases (RTK)
Enzyme-linked receptor
| Signals by phosphorylation
33
Receptor types: Ligand-gated ion channels
Directly allows ions through
34
Transduction
Cascades of molecular interactions that relay signals from receptors to target molecules in cell
35
Signal transduction - why are there multistep pathways
Can greatly amplify signal
| Provides more opportunities for coordination and regulation of response
36
Signal transduction - mechanisms
Second messengers
| Phosphorylation
37
Signal transduction: Second messengers - when are they produced
Produced following receptor activation
38
Signal transduction: Second messengers - what are they
Chemical signals that are often not embedded in the membrane - can diffuse intracellularly to pass on message
39
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
40
Signal transduction: Second messengers - first messenger
The hormone/ligand that activates the receptor
41
Common second messengers
cAMP, cGMP
IP3
Calcium
Diacylglycerol (DAG)
42
A second messenger can work on...
Multiple substrates
43
Signal transduction: Second messengers - types of responses
1. Pathway leads to a single response
2. Pathway branches --> 2 responses
3. Cross-talk between 2 pathways (response can be controlled by diff pathway
4. Diff receptor leads to diff response
44
Signal transduction: Phosphorylation and dephosphorylation act like...
A molecular switch - turns protein activity on/off or up/down as required
45
Signal transduction: Phosphorylation regulates...
Protein activity
46
Signal transduction: Phosphorylation - protein kinases
Transfer phosphates from ATP to protein (phosphorylation)
47
Signal transduction: Phosphorylation - relay molecules
Many relay molecules are protein kinases --> creates a phosphorylation cascade
48
Signal transduction: Phosphorylation - protein phosphatases
Rapidly remove phosphates from proteins - dephosphorylation
49
Signal transduction: Phosphorylation - amino acids commonly phosphorylated
Tyrosine
Serine
Threonine
50
Signal transduction: Does phosphorylation always turn things on
No, it can turn it off
51
Signal transduction: Phosphorylation - allows you to control your response...
Quite tightly
52
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
53
Response
The changes in chemicals result in activation or inhibition of proteins
54
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
55
Failure of termination of signalling processes
Can have highly undesirable consequences
56
Receptors - function examples
```
Vision
Taste
Smell
Neurotransmission
Cell growth
Development
Control of heart rate
```
57
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
58
GPCR signalling - signal
Endocrine
Epinephrine
Binds to receptor
59
GPCR signalling - receptor
GPCR
| Beta-adrenergic receptor
60
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
61
Effector proteins
Molecules (often enzymes) in a cell that respond to a stimulus and can be activated and further transduce a signal
62
Effector protein for G-proteins
Adenylate cyclase
63
Effector protein for cAMP
Protein kinase A
64
Where are GPCRs found
They exist in a range of organisms and express at the cell surface to respond to diverse extracellular signals
65
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
66
Receptor - 'gatekeeper'
Controls hormone activity at cell surface
67
Conformational change of GPCR results in _____ affinity for G protein
Higher
68
GPCR - mutation on extracellular vs intracellular side of receptor
Extracellular - affects how it binds the ligand
| Intracellular - affects how it binds a G-protein
69
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)
70
What does G-protein stand for
Guanosine-binding protein
71
G-protein cycle
1. Off position: GDP-bound - remains as a trimer and is inactive
2. 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
3. G-α subunit hydrolyses GTP --> GDP, which reassociates with βγ --> off position
72
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
73
Types of G proteins
Gs
Gi
Gq
74
Types of G proteins: Gs
Stimulatory G protein
| Activates adenylate cyclase by making it more catalytically active
75
Types of G proteins: Gi
Inhibitory G protein
| Inactivates adenylate cyclase by making it less catalytically active
76
Types of G proteins: Gq
Activates a diff effector, phospholipase
77
Types of G proteins: Gs - steps
Binds ligand --> conformational change --> attracts Gαs stimulatory protein which is activated --> binds to adenylate cyclase
78
Types of G proteins: Gs - what does it result in
More ATP --> cAMP
| More cAMP in cell
79
Types of G proteins: Gi - steps
Ligand binds --> conformational change --> attracts Gαi which binds to adenylate cyclase
80
Types of G proteins: Gi - what does it result in
Less ATP --> cAMP
| Less cAMP in cell
81
Types of G proteins: Gs and Gi is an example of...
Cross talk (2 diff pathways that can affect each other
82
cAMP activates or inactivates....
Protein kinase A
83
Adenylate cyclase is known as a(n)...
Effector protein
84
cAMP is made by the enzyme...
Adenylate cyclase
85
cAMP stands for..
Cyclic adenosine monophosphate
86
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
87
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
88
Neurons - normal RMP
About -70mV
89
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
```
90
What is vision based on
Absorption of light by photoreceptor cells in the eye
91
Vision: Types of photoreceptor cells
Rod and cone cells
92
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
93
Photoreceptor: Cone cells
Responsible for vision in bright light
| Gives info about colour
94
Vision - signal
Light (photon)
95
Vision - receptor
GPCR (rhodopsin)
96
What is the photoreceptor molecule in rod cells
Rhodopsin
97
Rhodopsin - what is it made of
Consists of protein opsin, linked to 11-cis-retinal (prosthetic group)
98
Opsin
A 7 transmembrane protein that determines wavelength of light absorption
99
11-cis-retinal
A light-absorbing group (chromophore)
100
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
101
Vision: Where are cone receptors located
In cone cells
102
What is colour vision mediated by
3 cone receptors
103
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
104
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
105
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
106
Vision: In the dark, what happens
Photoreceptor cells are depolarised with continual influx of Na+ and Ca2+ through cGMP-gated ion channels
107
Vision - G-protein
Transducin
108
Vision - primary effector
Phosphodiesterase
| Cleaves cGMP into GMP
109
Vision - second messenger
cGMP
110
Vision: Photoreceptor - special neuron?
Constantly has leakage of +ve ions into cell, so RMP is more +ve (about -40mV)
111
Vision: What happens when light hits rhodopsin (receptor)
Phosphodiesterase converts cGMP to GMP --> hyperpolarisation, which is sensed by next neuron
112
How is vision pathway different from other neuron pathways
It works by hyperpolarisation
113
Vision: Retinosa pigmentosa
A group of inherited diseases that affect photoreceptor (mainly rod) cells where they progressively deteriorate
114
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
115
Vision: Retinosa pigmentosa - age
Can happen to people as young as 40 y/o
116
Vision: Retinosa pigmentosa - what stage does something go wrong
Reception
117
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
118
Vision: Colour blindness - who is more likely to get it
Since it lies on X chromosome, males are more likely to get it
119
Vision: Colour blindness - where in the pathway does it go wrong
Reception
120
Vision: Colour blindness - what type of cell
Cone cells
121
Why is taste perception important
Nutritious vs poisonous
Commercial value
Health intervention
122
Taste: Where is the signal initiated
Papillae contain taste buds which are made up of taste cells, which contain taste receptors
123
Taste: Papilla
Bumps on tongue
| Creates trench around tongue to collect saliva
124
Taste - signal
Food (tastant)
125
Major taste sensations - receptors
GPCRs:
Sweetness
Umami
Bitterness
Ion channels:
Salty
Sour
126
Major taste sensations - ligands
```
Sweetness: sugars, sweeteners
Umami: amino acids
Bitterness: quinine and others
Salty: Na+
Sour: H+
```
127
What is umami
Taste of savoury-ness
| e.g. meat
128
Taste: Taste receptors - families
T1 and T2 family
129
Taste: Taste receptors (GPCRs) - subunits
Each family has subunits;
T1R: 1, 2, 3 = sweetness and umami
T2R: 1-65 = bitterness
130
Taste: G-protein-coupled taste receptor subunits - structure
7 transmembrane domains
| Extracellular N-terminus
131
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
132
Taste: T1R - heterodimers
T1R2 + T1R3 = sweet
| T1R1 + T1R3 = umami
133
Taste: T2R
Functions as monomer
Couples to / activates G protein
Have many of these to detect poisonous things
134
Taste transduction pathway - tastant / ligand
Sweet, bitter, umami
135
Taste transduction pathway - G protein
Gustducin
| Activates phospholipase C, which cleaves PIP2 --> DAG and IP3 (second messengers)
136
Taste transduction pathway - Ca2+
IP3 releases Ca2+ which activates V-gated ion channels
| Na+ comes into cell (depolarisation) --> APs
137
Taste disorders
Phantom taste sensation
Hypogeusia (lowered taste sensation) or ageusia (no taste)
Dygeusia (taste perceived isn't what you ate)
138
Blood glucose - always kept between...
4-8 mM of blood
139
Hypoglycaemia
Too little glucose in blood
Can result in:
Coma or death
Brain damage
140
Hyperglycaemia
Too much glucose in blood
Can result in:
Diabetes
Ulcers (since blood is thicker)
Nerve damage --> blindness
141
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
142
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
143
Low blood glucose: Signal
Glucagon or epinephrine
144
What is glucagon
A large peptide
145
Glucagon and epinephrine: Receptor
GCGR (glucagon receptor)
| AR (adrenoreceptor)
146
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
147
Glucagon: Glycogen synthase
PKA is able to phosphorylate glycogen synthase --> inactivates it --> glycogen is not made
Avoids futile cycles
148
What does glucagon result in
Increased gluconeogenesis
Increased glycogen breakdown
Decreased glycolysis
Increased blood glucose
149
Where is glucagon recognised
By receptor cells on liver
150
Gluconeogenesis
Making of glucose from non-carbohydrate molecules
151
Glucagon - speed
Within 5 minutes, it causes glucose levels to rise, so works fairly quickly
152
What does epinephrine result in
Increased glycogen breakdown (glycogenolysis)
Increased glycolysis
Increased blood glucose
153
Where is epinephrine released from
Adrenal glands
154
What is epinephrine recognised by
Liver cells
| Also in muscle cells; increases glycogen breakdown and glycolysis
155
Epinephrine: Liver vs muscle
Liver makes energy for body
| Muscle makes energy for itself
156
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
157
High blood glucose: Signal
Insulin
158
Insulin: Receptor
Insulin receptor
| Not a GPCR, but a tyrosine kinase
159
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
160
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
161
What does insulin result in
Increased glucose uptake in muscles
Decreased blood glucose
162
Insulin - target tissue
Muscle
163
Insulin-glucagon regulation - complexities
Paracrine effects between β-islet cells can directly inhibit secretion of glucagon in α-islet cells
164
Insulin deficiency
Type I diabetes
| Defect in signal
165
Insulin - drug
Used as a diabetes drug to mimic natural actions of this hormone at insulin receptor
166
GPCR gene mutations / diseases
Defect in reception
Cone opsins - colour blindness
Rhodopsin - retinitis pigmentosa
MC4R - extreme obesity
167
Cholera - what part of the pathway is affected
Transduction
168
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
169
Affinity
A measure of how tightly a ligand binds to the receptor
| Can generally be measured using dissociation constant Kd
170
Kd
Ligand conc where receptor is 50% saturated with ligand
171
Kd vs Ki
Ki = Kd if inhibitor
172
Kd and affinity
Lower Kd = higher affinity
173
What is the problem in asthma
Bronchoconstriction
174
What signalling pathway might affect asthma
Epinephrine pathway
175
Asthma: What kind of ligand-receptor action is desired
β-agonist --> relaxes airways (dilation)
176
Asthma: What chemical structure is desired
β-agonist is structurally similar to epinephrine
177
Asthma: How well does ligand/drug bind to receptor
β-agonist has low affinity, so need to keep taking it
178
Asthma: How does β-agonist work
Activates G-protein --> activates AC --> cAMP --> PKA --> phosphorylated myosin light chain kinase --> muscle relaxation
179
β-adrenoceptor agonists and asthma - function
Reverse bronchoconstriction associated with asthma
180
Asthma: Salbutamol vs salmeterol
Salbutamol: low affinity - acute symptoms
Salmeterol: high affinity - long-acting
181
Types of pain
```
Pain receptor pain (receives signal/stimulus)
Neuropathic pain (nerves sensing pain regardless of presence of stimulus)
```
182
Nociceptors
Senses pain
| FNEs that respond to diff stimuli
183
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
184
Pain: Interneurons
In spinal cord
| Modulate what you feel
185
Strategies to manage pain
Change initiator of pain
| Change CNS modulation of pain
186
Strategies to manage pain: Change initiator
Reduce inflammation --> reduce prostaglandin
| e.g. paracetamol, NSAIDs, aspirin
187
Strategies to manage pain: Change CNS modulation
All these drugs are opioids:
| e.g. codeine, morphine, fentanyl, tramadol, oxycodone
188
WHO's pain relief ladder
First give non-opioid drugs, but for moderate pain onwards, start giving opioids because much more effective
189
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
190
What are opioids released by
Interneurons
191
What do opioids bind to
GPCRs
192
Pain relief - normal pathway
1. AP arrives --> V-gated Ca2+ channels open --> Ca2+ influx
2. Release of glutamate binds to LGIC --> post-sympathetic neuron depolarisation
3. Neurons repolarise by K+ efflux
193
Pain relief - opioid pathway
1. Opioids bind to GPCR
2. Goes through Gαi signal cascade
- -> inhibits Ca2+ channel
- -> no glutamate release into post-synaptic neron
3. Increased K+ efflux --> harder for further depolarisation of neurons
194
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
195
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
196
Drawbacks of opioid use - addiction
Apart form reducing pain, also causes release of dopamine that causes feeling of reward/pleasure
197
What is the main factor that limits how much opioids you can take
Respiratory depression
198
Opioid crisis
An exceptionally high mortality rate and harm linked to use/misuse of opioid drugs
199
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
200
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
201
Opioid crisis: Solutions - change in policies
Use of prescription drug monitoring programs
Increase access to drug abuse treatment services
Enforce rules for drug makers
202
Opioid crisis: Solutions - change doctors' pain management practices
Opioids should be reserved as second or later line of pain management
203
Opioid crisis: Solutions - develop better pain medication
Better alternatives with low side effects, tolerance and addiction risk should be investigated
204
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
