Cell signalling and pharmacology Flashcards
1
Q
What are the first messengers in cell signalling?
A
Extracellular signalling molecules (ligands) that initiate communication between cells
2
Q
What are the 2 broad classes of extracellular signalling molecules?
A
Large/hydrophilic → Bind to cell surface receptors
Small/hydrophobic → Enter cells and bind to intracellular receptors
3
Q
Why is understanding cell signalling important in medicine?
A
Abnormal signalling underpins most disease processes; signalling molecules and receptors are major drug targets.
4
Q
How do pathogenic organisms exploit host cell signalling?
A
They modify host signalling pathways to their advantage, e.g., cholera, TB, dysentery.
5
Q
Why are cell surface receptors more common in signalling?
A
Most signalling molecules are hydrophilic and cannot cross the plasma membrane.
6
Q
Define paracrine signalling and give an example.
A
Signals affect nearby cells; example: histamine or nitric oxide.
7
Q
Define autocrine signalling and give an example.
A
A cell signals itself; common in development and some growth factors.
8
Q
Define endocrine signalling and give examples.
A
Hormones travel via bloodstream to distant cells; e.g., insulin, oestrogen, adrenaline.
9
Q
What is synaptic signalling?
A
Neurons transmit electrical signals that trigger neurotransmitter release at synapses.
10
Q
Define juxtacrine signalling.
A
Contact-dependent signalling between adjacent cells, e.g., via integrins.
11
Q
What is signal transduction?
A
Conversion of an extracellular signal into an intracellular response through cascades.
12
Q
What is a second messenger?
A
An intracellular molecule that relays and amplifies signals from receptors to targets.
13
Q
What initiates intracellular signalling?
A
Activation of a cell surface receptor by a ligand.
14
Q
What types of protein modifications relay signals?
A
Shape changes, phosphorylation/dephosphorylation, or binding with other molecules.
15
Q
What is the main purpose of a signal transduction cascade?
A
Amplify, integrate, and distribute the original signal.
16
Q
How can the same signal produce different responses?
A
Via different receptor isoforms or distinct intracellular pathways in different cell types.
17
Q
How fast can signal-induced responses occur?
A
Responses can be fast (e.g., movement) or slow (e.g., gene expression).
18
Q
What role do scaffold proteins play in signalling?
A
They organize signalling components to enhance efficiency and specificity.
19
Q
Summarize the key steps in cell signalling.
A
- Ligand binds receptor, 2. Signal transduced, 3. Messenger cascade, 4. Effector response.
20
Q
What is the clinical significance of understanding cell signalling?
A
Provides therapeutic targets for diseases like cancer, diabetes, and infections.
21
Q
What is the central theme of intracellular signal relay?
A
Altering signalling molecules in a sequence to change their functionality.
22
Q
Name three ways in which signal molecules can undergo alteration.
A
Binding with each other, phosphorylation/dephosphorylation, binding to a phosphate group on another molecule.
23
Q
List the five main types of intracellular signalling molecules.
A
Proteins, lipids, small chemical mediators, ions, gases.
24
Q
Give three examples of lipid signalling molecules.
A
Phospholipids, ceramides, diacylglycerol (DAG).
25
Name three small chemical mediators involved in signalling.
cAMP, cGMP, inositol triphosphate (IP3).
26
Which ions are commonly involved in signal transduction?
Ca²⁺, Zn²⁺.
27
What gas is known to function as a signalling molecule?
Nitric oxide (NO).
28
What is a molecular switch?
A protein that toggles between active and inactive states during signal transduction.
29
What activates or deactivates molecular switches?
Binding to GTP/GDP or phosphorylation.
30
What are G proteins?
Molecular switches regulated by guanine nucleotides (GDP/GTP).
31
When is a G protein inactive?
When bound to GDP.
32
When is a G protein active?
When bound to GTP.
33
What intrinsic activity do G proteins possess?
GTPase activity – they hydrolyse GTP to GDP to deactivate themselves.
34
In what two forms can G proteins exist?
Trimeric complexes (GPCRs) and monomeric proteins.
35
What proteins regulate GDP/GTP exchange and GTP hydrolysis in monomeric G proteins?
GEFs (guanine exchange factors) and GAPs (GTPase-activating proteins).
36
Name four key monomeric G proteins and their roles.
Ras (cell division), Rab (vesicle transport), Rac & Rho (cytoskeleton & migration).
37
What do protein kinases do?
Add phosphate groups from ATP to specific amino acids on target proteins.
38
What amino acid residues are targeted by tyrosine and serine/threonine kinases?
Tyrosine for TKs; serine/threonine for STKs.
39
What reverses protein phosphorylation?
Protein phosphatases.
40
Are protein kinases themselves regulated by phosphorylation?
Yes, they are often switch proteins activated or deactivated by phosphorylation.
41
What does a signal transduction cascade of protein kinases look like?
A sequence where one kinase activates the next through phosphorylation.
42
What is cAMP and how is it produced?
A small molecule produced from ATP by adenylyl cyclase.
43
What enzyme degrades cAMP and why is this important?
cAMP phosphodiesterase; it regulates signal strength and duration.
44
What is the structure of inactive PKA (protein kinase A)?
Two regulatory subunits bound to two catalytic kinase subunits.
45
How does cAMP activate PKA?
It binds to regulatory subunits, releasing active catalytic subunits.
46
Aside from PKA activation, what else can cAMP do?
Directly activate certain ion channels.
47
List three key signalling pathways involving lipid conversion.
PIP2 to PIP3 (via PI3-Kinase), PIP2 to DAG & IP3 (via Phospholipase C), cAMP (via adenylyl cyclase).
48
What does PI3-kinase do?
Converts PIP2 into PIP3, triggering downstream signals including activation of PKB (Akt).
49
What is the role of Ca²⁺ and calmodulin in signalling?
Calcium binds to calmodulin, forming a complex that activates other enzymes.
50
What is PIP2 and where is it located?
A phospholipid (Phosphatidylinositol 4,5-bisphosphate) found in the inner leaflet of the cell membrane.
51
What are the structural components of PIP2?
Hydrophobic diacylglycerol (DAG) tail and hydrophilic inositol head group.
52
Which enzyme phosphorylates PIP2 and what is the product?
PI3-Kinase phosphorylates PIP2 to form PIP3.
53
What is the role of PTEN in the PI3K signalling pathway?
PTEN dephosphorylates PIP3 back to PIP2, terminating PKB signalling.
54
What kinase is activated downstream of PIP3?
PKB (also known as Akt).
55
Which enzyme breaks down PIP2 into DAG and IP3?
Phospholipase C (PLC).
56
What triggers the activation of phospholipase C?
Activation of a receptor on the cell surface.
57
What is the function of DAG in cell signalling?
Activates protein kinase C (PKC), important in cell growth.
58
What is the role of IP3 in signalling?
Triggers the release of Ca²⁺ from intracellular stores.
59
Name the two main outcomes of PIP2 cleavage.
Activation of PKC (via DAG) and Ca²⁺ release (via IP3).
60
Where does cytosolic Ca²⁺ come from during signalling events?
Influx through plasma membrane channels and release from ER, SR, or mitochondria.
61
How is cytosolic Ca²⁺ concentration reduced after signalling?
By Ca²⁺ ATPase pumps that sequester Ca²⁺ back into organelles or pump it out of the cell.
62
What is calmodulin and how is it activated?
A protein with 4 Ca²⁺ binding sites, activated when [Ca²⁺]i > 500nM.
63
What happens after calmodulin binds Ca²⁺?
It binds to and regulates activity of target proteins.
64
List four general ways to terminate cell signalling.
1) Receptor internalisation, 2) Signal molecule degradation, 3) Deactivation of signalling molecules, 4) Dephosphorylation by phosphatases.
65
What happens to internalised receptors?
Either degraded or recycled back to the cell surface.
66
What are the two fates of the signalling molecule after receptor internalisation?
Destroyed along with receptor or separated from receptor and destroyed.
67
How are extracellular signalling molecules removed?
By enzymatic degradation.
68
How can a cell modify its sensitivity to signalling molecules?
By changing receptor number or expressing different receptor isoforms.
69
What is an agonist?
A molecule that binds and activates a receptor, inducing a biological response.
70
What is the difference between full and partial agonists?
Full agonists fully activate the receptor; partial agonists produce only a partial response.
71
What is an antagonist?
A molecule that binds to a receptor but does not activate it, blocking signalling.
72
Name three main types of cell surface receptors.
Ion channel-linked receptors, G protein-coupled receptors (GPCRs), enzyme-linked receptors.
73
What are ion channel-linked receptors also called?
Ionotropic receptors.
74
What are G protein-coupled receptors also known as?
Metabotropic receptors.
75
How do ion channel-linked receptors work?
Ligand binding causes a conformational change that opens the ion channel.
76
What are two pharmacological modifiers of ion channels?
Channel blockers and channel modulators.
77
What ends synaptic transmission?
Neurotransmitter reuptake or enzymatic degradation.
78
Summarize PIP2’s dual roles in signalling.
Phosphorylation of PIP2 leads to PKB activation; cleavage of PIP2 drives Ca²⁺-mediated responses.
79
What is the structural hallmark of GPCRs?
Seven transmembrane alpha-helices with extracellular ligand-binding and intracellular G protein-binding domains.
80
How many GPCRs exist in humans?
Approximately 800.
81
Which G protein subunit binds GDP/GTP and has intrinsic GTPase activity?
Gα (alpha) subunit.
82
What happens when a GPCR binds its ligand?
Conformational change enables Gα to exchange GDP for GTP and dissociate from Gβγ.
83
What happens after GTP binds to Gα?
Gα becomes active, dissociates from Gβγ, and interacts with an effector molecule.
84
How is Gα turned off?
It hydrolyzes GTP to GDP, rejoining the Gβγ complex to form the inactive trimer.
85
What is the role of RGS proteins?
They accelerate GTP hydrolysis by Gα, aiding in signal termination.
86
Name the three common classes of Gα proteins.
Gαs, Gαi, and Gαq.
87
What does Gαs stimulate?
Adenylyl cyclase, increasing cAMP production.
88
What does Gαi inhibit?
Adenylyl cyclase, reducing cAMP production.
89
What does Gαq activate?
Phospholipase C, which produces IP3 and DAG.
90
What is an example of Gαq-mediated signalling?
Oxytocin receptor signalling in the brain via Ca²⁺-dependent pathways.
91
What physiological process is influenced by GPCR activation of PKA in the nucleus?
Long-term gene expression responses.
92
What disease is caused by Gαs being stuck in its active (GTP-bound) state?
Cholera.
93
How does cholera toxin affect GPCR signalling?
It locks Gαs in active form, overstimulating adenylyl cyclase and causing excessive ion and water loss.
94
What are the three main types of cell surface receptors?
Ion channel-linked receptors, GPCRs, enzyme-linked receptors.
95
What type of enzyme activity is found in receptor tyrosine kinases (RTKs)?
Intrinsic tyrosine kinase activity.
96
What must happen for RTKs to become active?
Two receptor monomers must dimerize and undergo transphosphorylation.
97
What happens after RTK phosphorylation?
Adaptor and effector proteins bind to phosphorylated tyrosines, initiating signalling.
98
Name three adaptor proteins involved in RTK signalling.
Grb2, Shc, IRS-1.
99
Name two effector proteins recruited by RTKs.
PI3-kinase and phospholipase C.
100
Which small GTPase is commonly activated by RTKs?
Ras.
101
How is Ras activated by RTKs?
RTK recruits a GEF, which facilitates GDP-GTP exchange on Ras.
102
What role does insulin signalling play in glucose uptake?
Activates IRS-1/PI3K/PKB pathway to move GLUT-4 transporters to the membrane.
103
What other effects does insulin signalling have?
Decreases glycogen metabolism and promotes glycolysis.
104
How do cytokine receptors differ from RTKs?
They lack intrinsic kinase activity and recruit JAKs.
105
What does JAK stand for?
Janus kinase.
106
Describe the cytokine receptor signalling mechanism.
Ligand binds, receptors dimerize, JAKs are recruited and activated, phosphorylating the receptor and STAT proteins.
107
What is STAT?
A transcription factor activated by phosphorylation in cytokine signalling.
108
What are the three core components of GPCR signalling?
GPCR itself, its associated G protein, and the effector molecule.
109
Why are GPCRs major drug targets?
They regulate a wide variety of physiological processes.
110
What activates the insulin signalling pathway leading to glucose uptake?
Binding of insulin to its receptor, triggering downstream signalling.
111
What is the final effect of GLUT-4 translocation in insulin signalling?
Increased glucose uptake into muscle and fat cells.
112
What are intracellular receptors also called?
Nuclear receptors.
113
Where are intracellular receptors located in the absence of ligand?
In the nucleus or cytoplasm.
114
What type of molecules bind to intracellular receptors?
Lipid-soluble molecules like steroid hormones.
115
What is the effect of ligand binding on cytoplasmic nuclear receptors?
They translocate to the nucleus and act as transcription factors.
116
What domains do nuclear receptors contain?
Ligand-binding domain, DNA-binding region, and a variable N-terminal region.
117
What is the primary function of nuclear receptors?
Ligand-activated transcription factors that regulate gene expression.
118
How does cortisol signal through its receptor?
Cortisol binds to a cytoplasmic receptor, translocates to the nucleus, and activates gene transcription.
119
How do some nuclear receptors behave in absence of ligand?
They remain bound to DNA and repress transcription until the ligand binds.
120
What is oestradiol and what does it bind to?
An estrogen that binds to the estrogen receptor (ER) in the nucleus.
121
What happens after oestradiol binds to ER?
Receptor dimerizes, binds co-activators, and activates estrogen response elements for gene transcription.
122
What are three types of plant cell-cell signalling based on distance?
Endocrine (long), paracrine (short), autocrine (same cell).
123
How do signalling molecules travel in plants?
Through active transport, diffusion, and via plasmodesmata.
124
What is juxtacrine signalling in plants?
Communication through plasmodesmata linking adjacent cells.
125
What plant structure is involved in electrical signalling like action potentials?
Plasmodesmata and ion channels in the Venus flytrap.
126
How does the Venus flytrap use electrical signalling?
Mechano-sensitive channels activate action potentials that trigger leaf closure.
127
What signalling molecules and mechanisms do plants and animals share?
Membrane receptors, intracellular receptors, kinases, and secondary messengers like Ca²⁺.
128
What kinase types are most used in plants?
Mainly serine/threonine kinases and some histidine kinases.
129
What is the primary mode of gene regulation in plant signalling?
Inactivation of transcriptional repressors.
130
What is auxin and where is it produced?
A plant hormone (IAA) produced in the seed embryo, apical buds, and young leaves.
131
List functions of auxin.
Stem elongation, root growth, apical dominance, fruit development.
132
How is auxin transported in plants?
Actively from cell to cell, locally and long-distance.
133
How does auxin mediate phototropism?
Redistributed to the shaded side of a shoot, promoting cell elongation and bending toward light.
134
How does auxin trigger gene transcription?
By binding to nuclear receptors which ubiquitinate and degrade transcriptional repressors.
135
What is ethylene and what does it regulate?
A gaseous hormone regulating fruit ripening and leaf abscission.
136
Where are ethylene receptors found?
In the ER and Golgi membranes.
137
What is the ethylene signalling mechanism?
Absence of ethylene activates a kinase that degrades transcription regulators; presence of ethylene stops degradation, allowing transcription.
138
What are the two major classes of plant photoreceptors?
Blue-light receptors and phytochromes (red/far-red light).
139
What three pigments are associated with blue-light receptors?
Cryptochromes, phototropins, and zeaxanthin.
140
What are phytochromes and how do they function?
Red/far-red light receptors that toggle between Pr and Pfr states and influence gene transcription.
141
How do phytochromes affect transcription?
Pfr translocates to the nucleus to activate or phosphorylate transcription factors.
142
What is phototropism and which receptor mediates it?
Growth towards light, mediated by phototropins.
143
What is de-etiolation and what receptor regulates it?
‘Greening’ of seedlings, regulated by cryptochromes.
144
What is photoperiodism and what regulates it?
Response to day/night cycle, regulated by phytochromes.
145
What are the two conformational states of phytochromes?
Pr (inactive, far-red light) and Pfr (active, red light).
146
What is the key difference between animal and plant nuclear signalling?
Plants rely more heavily on removing repression rather than activating new transcription.
147
What is apoptosis?
A form of controlled cell suicide seen in multicellular organisms.
148
Why is apoptosis important in development?
Removes redundant structures, e.g. sculpts limbs during embryogenesis.
149
How many cells die daily via apoptosis in an adult human?
About 60 billion.
150
What happens when apoptosis is dysregulated?
It can lead to diseases such as cancer (too little) or degenerative diseases (too much).
151
List three cellular triggers that can induce apoptosis.
DNA damage, protein misfolding (ER stress), viral infection.
152
What are the key morphological features of apoptosis?
Cell shrinkage, chromatin condensation, membrane blebbing, apoptotic bodies.
153
How is apoptosis different from necrosis?
Apoptosis maintains membrane integrity; necrosis involves membrane rupture and inflammation.
154
What are caspases?
A family of cysteine proteases that cleave proteins at aspartic acid residues.
155
How are caspases activated?
By proteolytic cleavage of inactive procaspases.
156
What are initiator caspases?
Caspases that auto-activate and activate effector caspases.
157
What are effector caspases?
They cleave cellular proteins and execute cell death.
158
What hallmark DNA event occurs in apoptosis?
Caspases activate endonucleases, leading to DNA fragmentation into 180-200 bp ladders.
159
What is the ‘eat me’ signal on apoptotic cells?
Phosphatidylserine (PS) exposure on the outer membrane leaflet.
160
Which enzyme flips PS to the outer membrane leaflet?
Xkr8 scramblase, activated by caspases.
161
What are the two main apoptosis pathways?
Intrinsic (mitochondrial) and extrinsic (death receptor-mediated).
162
What triggers the intrinsic pathway?
Withdrawal of growth factors, DNA damage, protein misfolding.
163
What mitochondrial molecule initiates intrinsic apoptosis?
Cytochrome c.
164
Which proteins regulate cytochrome c release?
BCL-2 (anti-apoptotic), BAX and BAK (pro-apoptotic).
165
How does BCL-2 prevent apoptosis?
It blocks BAX/BAK from forming channels in the mitochondrial membrane.
166
What happens when BAX > BCL-2?
Cytochrome c is released and apoptosis proceeds.
167
What is the role of protein kinase B (Akt) in survival?
It phosphorylates and inactivates pro-apoptotic proteins.
168
What initiates the extrinsic apoptosis pathway?
Binding of death ligands (e.g. FasL) to death receptors (e.g. Fas) on target cells.
169
Which immune cells use extrinsic apoptosis to kill targets?
T-lymphocytes.
170
What adaptor protein links Fas to procaspase 8?
FADD (Fas-associated death domain protein).
171
Which caspase is the initiator in the extrinsic pathway?
Procaspase 8.
172
What are apoptotic bodies?
Membrane-bound cell fragments containing cytoplasmic content and organelles.
173
How are apoptotic cells removed?
Phagocytosis by macrophages or neutrophils.
174
What happens to nuclear DNA during apoptosis?
It is cleaved into nucleosomal fragments by endonucleases.
175
Why is apoptosis considered beneficial to organisms?
It maintains homeostasis, prevents propagation of damaged cells, and is critical for development.
176
What key biochemical marker can be seen on an agarose gel for apoptotic cells?
DNA laddering pattern of 180-200 bp fragments.
177
What enzyme cleaves PIP2?
Phospholipase C
178
What does cleavage of PIP2 create?
IP3 and DAG
179
What is PIP3 produced by?
Phosphorylation of PIP2 by PI3-kinase
180
What does diacylglycerol do? DAG
Activates Protein Kinase C (PKC), an enzyme that:
Phosphorylates proteins inside the cell
Changes their function (e.g., enzyme activity, cell movement, secretion)
181
What does IP3 do?
Binds to IP3 receptors on the endoplasmic reticulum (ER), causing the ER to release calcium ions (Ca²⁺) into the cytoplasm
182
What does PIP3 do?
Acts as a docking site in the membrane for proteins like Akt/PKB and PDK1.
These proteins:
Get activated
Start signaling cascades that promote cell survival, growth, proliferation, and metabolism
183
What is calmodulin? Describe the structure
A small, highly conserved protein present in almost all eukaryotic cells.
It has four binding sites for Ca²⁺ ions (two on each "hand" of the molecule).
On its own, it's inactive. But when Ca²⁺ binds → it transforms into a super-activator
184
What does the calcium calmodulin complex activate?
1. CaM Kinase (Ca²⁺/Calmodulin-dependent protein kinase)
Phosphorylates other proteins
Regulates gene expression, memory formation, cell growth, etc.
2. Myosin Light Chain Kinase (MLCK)
Triggers smooth muscle contraction
3. Calcineurin (a phosphatase)
Dephosphorylates proteins
Important in immune cell activation (especially in T-cells)
