L14-17: Cell Signalling Flashcards
Why is cell signalling important?
As it allows transfer of information from cell to cell and from cells to the environment
What are the components of a simple cell signalling pathway?
Signal: extracellular activator, chemical/ physical
Reception: detection by receptor protein
Response: change in cellular process (gene expression etc)
What are the important components of signalling pathways?
Signal: primary messenger/ ligand
Reception: receptors
Transduction: transducers
Amplification: 2nd messengers and signalling cascades
Response: effector proteins
How can the activity of proteins in a signalling pathway be altered?
Changing level of protein
Changing activity of fixed amount of protein
How does changing activity of proteins alter signalling pathways?
Via a conformational change (active/inactive) or covalent modification (phosphorylation)
How much of the genome of euk cells codes for signalling molecules?
10-15%
What are some examples of the responses to signals?
Pheromones, local hormones, hormones, NTs and cell surface molecules (physical & chemical)
How are gap junctions used in cell signalling?
Allows small molecules to pass directly from cell to cell (ions, metabolites) NOT MACROMOLECULES
How are gap junctions modulated?
By post-translation modification
How are contact dependent cells used in cell signalling?
Not secreted, signalling molecule on the cells surface that interacts with receptor on recipient cell
When are contact dependent cells used?
For immune signalling during development
What is paracrine signalling?
Uses local mediators and acts on different cell types in close proximity
When is paracrine signalling important?
During development
What is autocrine signalling?
“Self” signalling (binds to signal it secretes) other cells can also bind to the signal
What type of cells use autocrine signalling?
Cancer cells for more uncontrollable cell division
Where can receptors be located?
On the cell or inside the cell
Where do the different types of molecules bind to receptors?
Small hydrophobic molecules - intracellular receptors
Hydrophilic molecules - cell-surface receptors
What is an example of intracellular receptors?
Nuclear hormone receptors:
conformational change when ligand binds, complex regulates transcription target of genes
What are examples of hydrophobic molecules that bind to intracellular receptors?
Cortisol, estradiol, testosterone, vitamin D3, thyroxine and retinoic acid
What is an example of there being no receptor no response?
Androgen receptors:
Androgen steroids determine male secondary sexual characteristics
In the absence of signalling there is female pattern development
What happens during androgen insensitivity?
Deficiency of androgen receptors, normal levels of testosterone produced but not detected so genetically male but phenotypically female
What is a second example of intracellular receptors?
Nitric oxide receptors:
Conformational change when ligand binds, producing secondary messenger
What are the 3 classes of cell surface receptors?
Ion channel coupled
G-protein coupled
Enzyme coupled
How do ion channel coupled receptors work?
They convert chemical signal to electrical signal in nerve synapses, ligand binds causing a conformational change in receptor
What is an example of an ion channel coupled receptor?
Nicotinic acetylcholine receptor on skeletal muscle cell
How do nicotinic acetylcholine receptors work?
Acetylcholine binds opening the Na+ ligand gated channel so Na+ diffuses in causing Ca2+ to leave the sarcoplasmic reticulum so the muscle contracts
What is an example of a disease when the ion channel coupled receptor is blocked?
Myasthenia Gravis:
Autoimmune response to nicotinic acetylcholine receptor, auto-ABs block receptor
What are examples of processes G-protein linked receptors are involved in?
Vision, smell, neurotransmitters, immune recognition, autonomic nervous system
What can G-protein linked receptors also be known as?
Serpentine receptors
What are the 2 different types of G-proteins?
Trimeric and monomeric
How is the action of G-protein linked receptors mediated?
By trimeric G-proteins
What are G-proteins known as?
Transducers
What do G-proteins bind to?
GTP or GDP
What are the 3 subunits G-proteins are made up of?
Alpha, beta and gamma
What is the function of the alpha subunit in the G-protein?
It binds to GDP in its resting state as it is a GTPase
When are G-proteins active?
When the receptor is active
How do G-proteins turn themselves off?
GTP-hydrolysis
What are the 2 different 2nd messengers in a G-protein signalling pathway?
Adenylyl cyclase -> cyclic AMP
Phospholipase C -> Inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG)
How does cAMP pass information once its converted from adenylyl cyclase?
By activating cAMP dependent protein kinase (PKA)
What happens once cAMP binds to PKA?
A conformational change takes place which causes the release and activation of the catalytic subunits
How do PKA subunits pass information down the signalling pathway?
By phosphorylation of other proteins
How can phosphorylation proteins impact them?
By switching them on or off
How is glucose released in fight or flight?
Using G-protein coupled receptors as epinephrine provides the phosphorylation of the enzyme that hydrolyses glycogen
How is a cascade produced when adenylyl cyclase is used as a second messenger?
Signal molecule binds to receptor, conformational change takes place, G-protein is activated, adenylyl cyclase is activated which changes ATP to cAMP, cAMP activates PKA which phosphorylates other proteins :)
How is a cascade produced when Phospholipase C (PLC) is used as a second messenger?
Signal molecule binds to receptor, conformational change takes place, G-protein is activated, PLC converts DAG and IP3 using phospholipids from the membrane DAG activates PKC which alters Pi and protein to form phosphorylated protein and IP3 causes the release of Ca2+ which leads to a cellular response :)
How are IP3 and DAG produced?
By phospholipase C cleaves phosphoinositol 4,5- bisphosphate (PIP2)
Where is Ca2+ released from in PLC second messenger model?
Endoplasmic Reticulum
What is another commonly used second messenger?
Ca2+
What mediates Ca2+ dependent effects?
Calmodulin
How does calmodulin work?
Binds to 4 Ca2+ ions and allows the complex to wrap around the target proteins
What are enzyme linked receptors?
Single-span transmembrane proteins that have intrinsic enzyme activity or are associated with an enzyme
What is a common type of enzyme linked receptor?
Receptor Tyrosine kinases (RTK)
What are receptor tyrosine kinases used for?
As insulin receptors and growth factor receptors (cell proliferation)
How is cancer associated with receptor tyrosine kinases?
Due to problems in growth factor signalling
What type of activity does the cytoplasmic domain have?
Kinase activity
How do the enzyme linked receptors work?
Binding of the ligand to the receptors leads to cross-linking of receptor chains, oligomerisation of receptor chains allows cross-phosphorylation
How do insulin receptors differ to growth factor receptors?
They are tetramers which causes realignment of the polypeptide chains activating cross-phosphorylation
What is the function of the tyrosine residues on the receptor?
They provide docking sites for other signalling proteins
What is the main signalling pathway for growth factors?
Ras - small monomeric G-protein
What is the difference between trimeric and monomeric G-protein receptors?
Trimeric:
Bind directly to receptor
Receptor activated GDP release
GTP hydrolysis by intrinsic GTPase activity alone
Monomeric (eg Ras):
Not directly linked
GDP release activated by GEF (guanine nucleotide exchange factor)
Weak intrinsic GTPase activity - needs GAP (GTPase activating protein)
How does the RAS pathway begin?
RAS and GEF do not bind directly to RTK, binding mediated by adapter protein Grb-2
What is the phosphorylation cascade RAS activated?
The active Ras protein activated MAP kinase kinase kinase (Raf) then activated MAP kinase kinase (Mek) which then activates MAP kinase (Erk) which then leads to either changes in protein activity or changes in gene expression
What is a MAP kinase and what does it do?
Mitagen activated protein kinase - stimulates growth
Where does amplification of the Ras-MAPK pathway take place?
Between the EGF receptor- Sos complex and the activated Ras and at each stage of the phosphorylation cascade
How is Ras signalling pathway used in cancer?
Ras - proto-oncogene
Mutations are found in 20-30% of cancers
Most mutations reduce GTP hydrolysis
GTP stays bound longer so its constantly activated and leads to cell proliferation
What do inhibitors target in growth hormone receptors?
Ras signalling pathway
How do cells respond differently to the same signal?
Using different receptors
Activating different intracellular machinery
What is an example of the same signal being used but different receptors bound?
Nicotinic Ach receptor (ion-channel linked receptor) - induces contraction of skeletal muscle
Muscarinic Ach receptor (G-protein linked receptor) - reduces rate of heart contraction
How do Nitric Oxide receptors work?
They undergo a conformational change when a ligand (Ach) binds which produces a secondary messenger (cGMP) as Nitric Oxide binds to guanylyl cyclase which causes relaxation of smooth muscle
What is an example of different intracellular machinery being activated but same signal being used?
Pancreatic Acinar cell:
Ach binds then G-protein activates using PLC, once Ca2+ is released protein kinase allows secretory vesicles to fuse to the membrane to release digestive enzymes
Endothelial cell:
Ach binds then G-protein activates using PLC, once Ca2+ is released NO synthase is secreted inducing relaxation of smooth muscle
What is an example of signals combining to alter the activity of a signalling protein?
Glycogen metabolism in muscle cells - epinephrine produces cAMP which activates PKA inhibiting glycogen synthase (GS) so glycogen synthesis decreases and PKA activates GPK so more glycogen phosphorylase (GP) leading to increased glycogen degradation, both allow the production of energy
What is an example of signals combining to alter the level of active signalling protein?
Glycogen metabolism in muscle cells - Ach produces Ca2+ which allows for muscle contraction and also allows activation of GPK so more GP so more glycogen degradation and epinephrine produces cAMP so PKA activates which activates GPK and inhibits GS so less glycogen synthesis
How is there overlap between the signalling pathways?
2nd messengers are shared by many pathways
Signalling proteins are shared by different pathways
What are the advantages and disadvantages of overlap between signalling pathways?
Pros:
Allows fine-tuning response (controls levels or activity)
Cons:
Risk of signal producing the wrong response
What are the 3 types of signalling complex?
1) Stable: components linked to scaffolding protein
2) Transient: complex assembles after receptor is activated
3) Transient: modification of plasma phospholipid molecules
How can signals be switched off?
Removal/inactivation of signal
Removal/inactivation of receptor
Inactivation of activated signalling protein
Degradation/removal of second messengers
How is the signal switched off by being removed?
Degraded - eg hydrolysis
Recycling - blocks uptake of molecules eg serotonin and prozac
Sequestration - hiding signalling molecule away by using other proteins
How is the signal switched off by removal of the receptors?
Allows the cell to become adapted to a constant signal (desensitisation)
A common mechanism is ligand-dependent receptor-mediated endocytosis
How is the signal switched off by inactivating the activated signalling proteins?
Dephosphorylation/ phosphorylation
GTP hydrolysis
Allosterically
What is an example of interference of inactivating a signalling molecule to switch a signal off?
Cholera toxin:
ADP-ribosylation of the alpha G-protein subunit which prevents GTP hydrolysis so the G-protein is locked in an active state so adenylyl cyclase remains activated which increases cAMP so water and Cl- is lost into the intestinal lumen causing diarrhoea
What is an example of allosteric inactivated to switch off a signal?
Dissociation from activator or association with inhibitor eg the G-protein alpha subunit being associated/disassociated with adenylyl cyclase/ PLC
What is an example of the signal being switched off by degrading/removing the second messengers?
Removal of cAMP and cGMP by hydrolysis being catalysed by phosphodiesterase
How is viagra used in to inhibit a cell signalling pathway?
Nitric oxide binding to guanylyl cyclase which allows the conversion from GTP to cGMP, viagra inhibits the degradation of cGMP using phosphodiesterase so keeps the blood vessels relaxed
