Module 21 - Cell Signalling Flashcards
Describe the two major receptor types and characteristics of the signalling molecules for each
The two major receptor types are :
- Cell surface (membrane): ligand hydrophilic (can’t cross membrane)
- Cytoplasmic/nuclear receptors (intracellular): ligand hydrophobic/lipophilic
Describe the major modes of signalling.
Short distance:
- Contact-dependent: cell in close contact, membrane to membrane
- Paracrine: extracellular release of signal that acts only locally on neighbouring cells.
- Autocrine: extracellular release of signal that acts on the cell itself
Long Distance:
- Synaptic (Neurons): electrical signal along axon (long distance) -> release of neurotransmitter across synapse (short distance)
- Endocrine: release of hormone into bloodstream, acts widely throughout body
Explain how cellular responses can be cell context dependent
Cells receive multiple signals (depending on the receptors at the cell surface). Different combinations of these signals may lead to different responses (combinatorial signalling).
However, the same signal may elicit different responses depending on the cell type as they may have different receptor types or different intracellular mediators.
Describe the mechanism of action of steroid hormones/steroid receptors.
Steroid hormones are transported in the blood by carrier proteins (since they are hydrophobic). They are able to cross the plasma membrane and bind intracellular receptors that have DNA-binding domains (receptors are homo/heterodimers). Its effect either activate or repress expression
Mention the 3 major classes of cell surface receptors.
- Ion channel-coupled receptors
- G-protein-coupled receptors
- Enzyme-coupled receptors: binding of extracellular ligand causes enzymatic activity on the intracellular side
Describe the structural characteristics of RTKs
RTKs are usually single TM domains with a highly variable extracellular domain region. Intracellular domain are similar (as they are tyrosine kinase domains)
Explain the mechanism of Eph receptor signalling process.
Ephrins bind to the Eph receptors, which leads to the receptor dimerisation (self-phosphorylation on the Tyr). One of the dimer receptors recruits and activate a tyrosine kinase (through the phospho-tyrosine), while the other recruits an ephexin (Guanine Exchange Factor) which binds to the phospho-tyrosine on the receptor.
The kinase phosphorylates ephexin and activates it, which in turn activates RhoA, which is responsible for myosin-actin interaction and growth cone collapse (axons).
Describe the mechanism of action of receptor tyrosine kinases (RTK)
Ligand (a dimer or multimer) binds to the RTK causing the receptor to dimerize (auto-phosphorylation). This activates the receptor which binds to other intracellular proteins via phospho-tyrosines of the receptor. This then can relay the signal downstream.
Describe how docking/binding proteins bind to activated RTKs
Binding proteins have homologous phospho-tyrosine binding domains (Src Homology Domains)
- SH2: binds activated phospho-tyrosines on receptor
- SH3: binds domains in other intracellular proteins (proline region rich)
Describe how Ras is activated.
An activated RTK binds to the SH2 domain of Grb-2. Grb-2 acts as a docking protein (via its SH3 domain) for GEFs (Guanine Exchange Factors). In turn, SOS (a type of GEF) activates Ras by exchanging GDP for GTP.
Describe how Ras (and other small GTPases) can function as a switch.
Ras is a superfamily of monomeric GTPases. In its inactive state, it binds GDP. When activated by GEFs, it exchanges GDP with GTP. GAPs (GTPase Activating Proteins) then cause the hydrolysis of the GTP, making it inactive again.
Summary: Ras functions as an ON/OFF switch, with GEF activating it and GAP inactivating it.
Mention an example of a membrane-bound ligand and its function.
Ephrins is an example of a membrane-bound ligand. It can function in bidirectional signalling (binding may lead to self-signalling as well) as well as in cell migration and axon guidance.
Describe how activation of RTKs can lead to activation of the mitogen-activated protein kinase (MAPK) pathway.
Activated RTKs lead to the activation of Ras by GEFs. Ras then activates the MAPK pathway by first activating Raf (MAPKKK), which phosphorylates (and activate) Ser/Thr residue in Mek (MAPKK), which in turns activates Erk (MAPK).
Active Erk enters the nucleus and activates multiple gene regulatory proteins.
Describe one mechanism by which the MAPK pathway can induce a cell to enter cell division (G1-S transition)
When Ras activates the MAPK cascade, MAPK (Erk) goes to the nucleus, leading to the expression of intermediate early response (IER) genes. One of these genes is Myc, which activates expression of delayed response gene, including cyclin proteins that act in G1 of the cell cycle. D cyclins then bind and activate G1-Cdk proteins. The active proteins phosphorylate and inactivate Rb protein, which normally inactivates E2F proteins. Active E2F activates transcription of cell cycle genes (S-cyclins -> S-Cdk), inducing a cell to enter cell division
Describe immediate early genes and delayed response genes.
Immediate early genes (IEGs) are genes which are activated transiently and transcribed rapidly in response to a wide variety of cellular stimuli.
Delayed response genes are only activated later, following the synthesis of IEG products (such as Myc)
How do CDK/cyclin complexes regulate the cell cycle?
G1-Cdk activates E2F by inactivating Rb protein, which initiates S-phase gene transcription (necessary for S-phase transition)
S phase cyclins cause DNA synthesis and entry into S phase. Since this phase is the most vulnerable time for mutations, S-cyclin is expressed longer than the actual S-phase period to ensure DNA synthesis occurs correctly.
Both S-Cdk and G1/S-Cdk has positive feedback on the activation of E2F, self reinforcing the S-phase (completely and rapidly)
Describe the role of Rb and how it acts as a brake on proliferation. Describe the role of E2F.
Rb normally binds and keeps E2F protein inactive. It acts as a brake on proliferation as an active ERF activates transcription of S phase cyclins and initiates G1-S transition
Describe a mechanism for arresting the cell cycle at G1 in response to DNA damage..
DNA damage will initiate ATM/ATR kinase activation and in turn the CHK1/2 (arrest at G2/M checkpoint) kinase activation, which phosphorylates p53.
In its unphosphorylated form, p53 binds with Mdm2 and is ubiquitylated to be degraded by proteasomes. When phosphorylated, it’s no longer able to bind with Mdm2 and becomes active.
Active p53 activates transcription of p21 protein, which prevents activation of G1 Cdk/cyclin complex
Describe how members of the TGFβ family activate cell surface receptors.
TGFβ ligands are usually a dimer and when it binds to the receptor induces tetramerisation, where the Ser/Thr kinase domain of Type II receptors phosphorylate Type I (activation). Smad proteins are then recruited to the phosphorylated Type I receptor.
Activated TGFβ receptors are then endocytosed via clathrin-coated pits (receptor-mediated endocytosis). Most of the signalling occurs in early endosomes.
Describe how Smads transmit TGFβ or BMP signals and how they function.
Receptor Smads (2,3 for TGFβ and 1,5,8 for BMP) after phosphorylation and activation by Type I receptors binds with Smad 4 (Common Smad) and activate transcription 9forming transcription regulatory complex)
R-Smads can also be inhibited by binding with Smad 6, 7 (Inhibitory Smad)
Describe common responses of the TGFβ superfamily pathways.
- Anti-proliferation (G1 Arrest)
- Differentiation
- ECM production
- EMT/Fibrosis: represses epithelial genes, activates mesenchymal genes
- Celll Type dependent (transcription factors)
Describe the mechanism of action of ion channel-coupled receptors
Ligand binds directly to ion channel receptors, no second messengers involved.
Describe the structure and mechanism of activation of G-protein coupled receptors.
G-protein coupled receptors consist of 7 transmembrane domain protein. When the signal molecule binds with the receptor, it induces a conformational change in the receptor allowing the binding and activation of a nearby G protein transiently where it would then activate an enzyme (acting as a secondary messenger).
GPCR acts as a GEF as it substitutes GDP for GTP when activated.
Describe the structure of a G-protein. Why is a G-protein like a molecular switch?
Guanine nucleotide-binding proteins (G proteins) consists of three subunits α, β, γ (α, γ membrane-tethered).
G proteins act as a molecular switch as when they are bound to GTP (through α subunit), they are activated, while when bound to GDP, they are inactivated. The regulation of this ‘switch’ is regulated by factors that control their ability to bind and hydrolyze GTP to GDP.
Describe how an activated G-protein can regulate gene expression and ion flux across the membrane.
Activation of G protein leads to dissociation of α and βγ subunits. The α subunit would activate an enzyme, such as adenylyl cyclase, which catalyses ATP to cAMP. This would activate PKA, which would activate gene transcription in the nucleus by phosphorylating a transcription factor (CREB).
βγ subunits, also bind to a target protein and activate them. In heart muscle, the subunit binds to a K+ channel and opens it, regulating ion flux across the membrane
Binding (to target protein or RGS) activates the GTPase activity of the α subunit and the subunits associates back, reforming the inactive state.
Describe how one ligand (Eg. acetylcholine) can regulate different responses in different cells?
Ach activates 5 different GPCR, each with a different signalling pathway downstream, with different intracellular mediators. Hence, it can elicit different responses depending on the cell type
Describe how light activates rhodopsin (GPCR) signalling in the photoreceptor?
Rhodopsin (G-coupled light receptor) is linked to cis-retinal. Light stimuli induce cis-trans isomerisation which causes a conformational change in rhodopsin. Activated transducin (G protein) α subunit activates cGMP phosphodiesterase, causing a drop in cGMP levels.
This closes cGMP-gated Na+ channels (hyperpolarization), causing calcium channels to close and low calcium reduces glutamate release.
How do you switch off light activates rhodopsin (GPCR) signalling in the photoreceptor?
To switch off light signal:
- RGS protein hydrolyses GTP to GDP in transducin
- Rhodopsin kinase phosphorylate rhodopsin (inactivation)
- Low calcium stimulate cGMP production
How does the release of glutamate in the photoreceptors transmit light information signals?
In the dark, photoreceptors release glutamate which inhibits ON bipolar cells and excites OFF bipolar cells.
In response to light, the reduced release of glutamate (due to photoreceptor hyperpolarization) stops inhibition of ON and inactivates OFF bipolar cells.
Activated bipolar cells synapse and transmit signals to the brain.
Describe how the canonical Wnt/β-catenin signaling pathway functions
It relies on regulating the degradation of β-catenin protein.
Without Wnt signal. β-catenin is phosphorylated by a protein complex (due to inactivation of Dishevelled protein) and the subsequent ubiquitylation leads to the degradation of β-catenin. Without it, Wnt target genes are turned off.
With the Wnt signal, the complex (dissociated by Dishevelled) is unable to phosphorylate β-catenin which then migrate to the nucleus, where it displaces Groucho and associate with coactivators (activate the transcription of Wnt target genes).
Describe the complex of proteins that regulates β-catenin degradation.
Complex of cytoplasmic complex of proteins target β-catenin for ubiquitylation and degradation by proteasomal enzymes, made up of:
- Axin
- Glycogen Synthase Kinase 3β (GSK3β)
- Casein Kinase 1 (CK1)
- Adenomatous polyposis coli (APC)
CK1 and GSK3β phosphorylate β-catenin on phospho Ser/Thr residues, which are targets for E3 ubiquitin ligase complex
Identify oncogenes and tumour suppressors in the β-catenin degradation pathway.
Oncogenes: Myc, β-catenin
TSG: APC, p53
Describe the cellular processes that Wnt/β-catenin signaling regulates
- Proliferation: APC is a TSG, it controls cell cycle by controlling β-catenin
- Cell Death: due to activation of p53 due to excessive Myc expression (over-active Wnt)
Analyse the consequences of modulating Wnt/β-catenin signaling pathway in tissues (skin, lens, gut)
Skin: Wnt pathway inhibition leads to stem cells adopting epidermal fate instead of follicle fate -> no hair follicle form
Lens: loss of wnt signals -> decreased epithelial cells in lens. Gain of wnt signals -> overproliferation of epithelial cells
Gut: APC mutation lead to build up of beta catenin, it activates target genes and cause increase proliferation as well as failure to differentiate -> polyps form
Describe how mutations in Wnt/β-catenin signaling lead to cancer
Mutations in Wnt-pathway (Eg. APC) – associated with colon polyps.
Apc LOH or oncogenic β-catenin: → increased proliferation (stem cells) → Failure to differentiate (fate switch) to form polyps
