Signal Transduction and Second Messengers Flashcards
Define autocrine
The cell producing the messenger expresses receptors on its surface that can respond to that messenger
Define paracrine
Messenger molecules travel short distances through the extracellular space to cells in close proximity to the cell generating the message (often unstable/easily degraded), e.g. acetlycholine
Define endocrine
Messenger molecules reach target cells via the blood stream (aka hormones), e.g. steroid hormones
How can cells communicate?
- Chemical messengers
- Directo contact
- Electrically
What are signalling molecules?
Molecules of the right shape dock using non covalent bonds to their receptors with great specificity
Primary signal molecules/first messengers/receptor ligands
What can signalling molecules be classified as?
- Neurotransmitters
- Hormones
- Growth factors and cytokines
- Vitamin A and D derivatives
- Nitric oxide
- Regardless of classification, all binds cellular receptors and elicit a response
What are neurotransmitters?
- Small amino acids (5-35) or their derivatives
- Released by neurons at synaptic junctions in response to electrical stimulus (action potential)
- Quickly degraded and small to prepare neuron for next impulse
Give two examples of neurotransmitters and their actions
- Acetylcholine at neuromuscular junction- botulinum toxin prevents release
- GABA- primary inhibitory neurotransmitter in brain, anti-stress/anti-anxiety effects, important in aiding control of all convulsive disorders like epilepsy
What are eicosanoids?
- Derived from arachidonic acid in plasma membrane
- Contains 4 double bonds
- Common fatty acid in phospholipids
- Makes local hormones
- Involved in all aspects of inflammation- blocking leukotriene receptors can play a role in management of inflammatory diseases
- Anti-Inflame drugs act by cyclooxyrgenase and therefore prostanoid synthesis
What is arachidonic acid?
20-carbon polyunsaturated fatty acid ‘Eicosa’
What must a cell have in order to respond to an extracellular chemical signal?
- Cell must express receptors that specifically recognise and bind to a particular messenger molecule
What two ways can a signal molecule bind?
- Via plasma membrane, polar
- Via intracellular receptors (cytosolic receptors), most are gene-specific transcription factors, steroid hormones (e.g. cortisol, oestrogen), thyroid hormones, vitamins A and D
Give examples of classical steroid ligands
- Sex steroid (e.g. oestrogen)
- Glucocorticoid (e.g. cortisol)
- Mineralocorticoid (e.g. aldosterone)
- Vitamins A and D
Why are signalling molecules able to easily cross membranes?
They are hydrophobic
Where can receptors be located?
- Cytoplasm (e.g. cortisol)
- Or nucleus (e.g. oestrogen)
What happens when signalling molecules bind to a receptor?
Steroid hormone-receptor complex receptors bind to the hormone response element (HRE) of the gene, activating promoter and initiating gene transcription
What is GTPase?
Large family of hydrolase enzymes that bind to nucleotide guanosine triphosphate (GTP) and hydrolyse it to guanosine diphosphate (GDP)- cleavage of high energy bond
What is phosphodiesterase (PDE)?
Enzyme that breaks the phosphodiester bond
What is the cyclic nucleotide phosphodiesterase group?
Group of enzymes that degrade bond in 2nd messenger molecules cAMP (cyclic adenosine monophosphate) and cGMP (cyclic guanosine monophosphate)
What is kinase?
Catalyse transfer of phosphate group from donor, such ATP, to an acceptor (i.e. switch on)
Kinase kinase and kinase kinase kinases activate kinases
What are phosphatases?
An enzyme that catalyses the hydrolysis of organic phosphates in a specific (acid or alkaline) environment
Describe nitric oxide as a messenger
- Formed from amino acid arginine and oxygen by NO synthase
- Acts as a second messenger in smooth muscle relaxation pathway and as a primary signal molecule
How does nitric oxide act?
- Locally by diffusing through cell membrane (lipophilic)
- Binding and activating an intracellular receptor (guanylyl cyclase) which converts GTP to second messenger cGMP
- cGMP activates kinase that phosphorylates target proteins to elicit smooth muscle relaxation- vasodilation
What is phosphodiesterase 5 (PDE5)?
An intracellular that degrades cGMP
What occurs when PDE5 is inhibited?
Permits the cyclic nucleotide (cGMP) to remain active as a second messenger, leading to physiological outcome
What are plasma membrane receptors?
- Extracellular domain (binds to chemical messenger with high affinity)
- Membrane spanning domain (alpha helices)
- Intercellular domain (initiates signal transduction)
What are the major effects produced by plasma membrane receptors?
- Rapid and immediate effects on cellular ion levels or activation/inhibition of enzymes
- Slower changes in rate of gene expression for a specific set of proteins
- Often signal transduction pathways can diverge to produce both effects
What are the major classes of plasma membrane receptors?
- Ion channel receptors (e.g. acetylcholine)
- G-protein coupled receptors (or heptahelical receptors)
- Receptors tyrosine kinases
Describe the structure of G-protein coupled receptors
- 7 alpha helices that span the membrane, domains and binds a G protein (GTP activity)
- G proteins are heterotrimers, they have alpha beta and gamma subunits
What are the features of G-protein coupled receptor?
- Extracellular binding domain is specific
- No kinase/enzymatic activity, signal is transducer through G protein (accessory protein)
- Stimulatory (GS) or inhibitory (GI)
Describe the action of G protein coupled receptors
- Primary signalling molecule binds to its specific receptor
- Upon ligand binding (active), associated G protein exchanges GDP for GTP
- Time for hydrolysis depends on length of signal
- Activated G protein stimulates effector molecule, e.g. adenylyl cyclase
What occurs with second messengers with G protein coupled receptors?
- Second messengers, e.g. cAMP created, binds to further downstream proteins (i.e. kinase A activated by cAMP)
- Requires energy
- Inactive state, G protein had GDP associated with it
- Alpha unit that can be stimulatory/inhibitory
Describe cAMP as a second messenger
- Activated by adenylyl cyclase (AC) and converts ATP to cAMP
- Transfers signals of hormones such as glucagon and adrenalin
- Where 1st messenger can bind to single receptor, cAMP can stimulate many activities
- Elicit cellular response via PKA into 2 ways
In what ways does cAMP elicit a cellular response via protein kinase A?
- Phosphorylates a large number of metabolic downstream enzymes in the cytosol
- Alters gene transcription in the nucleus
How is signal amplification achieved?
- G proteins use a chain of other messengers to amplify relayed signal
- One receptor-ligand complex can activate many G proteins
- Each G protein can activate many adenylyl cyclases
- Each adenylyl cyclase does ATP-> cAMP
- cAMP activates protein kinases that activate several molecules of target enzyme
How is signal amplification switched off?
- Enzyme degradation of cAMP by phosphodiesterases
How do G proteins activate phospholipase C?
- GTP bound G protein activates effectors other then adenylyl cyclase, e.g. phospholipase C (releases IP3)
- IP3 diffuses through cytosol to ER and binds to receptor opening a channel for calcium ions
- DAG and calcium activate protein kinase C, cascades signal through series of phosphorylation reactions
- DAG, IP3 and calcium ions are all second messengers
What is phospholipase C?
An enzyme that degrades cell membrane phosphatidyl inositol (PTI) releasing inositol triphosphate (IP3) and leaving diacyl glycerol (DAG)
Describe adrenaline-receptor action
- One ligand
- Multiple effects
What is the link between G proteins and disease?
- Sometimes G protein receptor is defective- e.g. GPCR rhodopsin causes retinitis pigments
- Sometimes associated G protein is effective
What are receptor tyrosine kinases?
- Exist as inactive monomers
- Activated by growth and differentiation factors (e.g. EGF) or by metabolic regulators like insulin
- Tyrosine residues on the intracellular domain
Describe the action of receptor tyrosine kinases
- Primary signalling molecule ligand binds to TKRs, causing them to dimerise
- This activates tyrosine kinase portion of protein when kinase domain switched on, autophosphorylation occurs at residues
- Able to activate adaptor proteins- RAS G protein
- RAS activates MAPKKK (Raf)- autophosphorylation
- MAPKKK activates MAPKK (MEKK)
- Activated MAPK (ERK)
Activated relay proteins can then pass on signal within cell to elicit. a response
What do all kinase reactions require?
ATP
What is Trastuzumab (Herceptin)
- Herceptin monoclonal antibody targets HER2 tyrosine kinase receptor found on 25% of breast cancer cells
- Binding of Herceptin to HER2 blocks binding of natural ligand, epidermal growth factor (EGF) to HER2 and subsequent downstream signalling cell to grow and proliferate
- Target this to prevent
What do uncontrolled signalling pathways lead to?
Cancer
Describe uncontrolled signalling pathways
- BCR_ABL fusion gene
- Translocation in which long arm of chrome. 22 attaches to long arm of chrome. 9
- Acquired mutation
- Contains TK domain, transmits a cell proliferation signal, constitutively switched on
- BCR-ABL (inappropriately fused) oncogene instructs constant cell division, prevents apoptosis for leukaemia
- Causes chronic myelegnous leukaemia
What is Imatnib (Gleevec)?
Successful anti-cancer drug, binds to TK domain and deactivated erroneous signal
