Cell Signalling and Receptors Flashcards
Name the different signalling methods cells can use.
- Autocrine signalling- cell releases a chemical substance that acts upon a specific receptor on that same cell.
- Intracrine when a hormone is synthesized inside a cel and doesn’t leave cell–> binds to receptors inside the cell immediate signalling + response
- Paracrine signalling- secretory cell releases a substance into interstitium that acts local mediator binding to adjacent cells
- Endocrine signalling- cell secretes substance into the bloodstream, and travels to a distant target cell.
- Neuroendocrine (mainly hypothalamus to anterior pituitary) – electrical stimulus in a neuron causes synthesis and release of a hormone directly from that nerve cell via neurosecretion
Whats the difference between and endocrine and exocrine gland
exocrine glands not part of endocrine system secretions go into ducts
Endocrine glands lack ducts secrete chemical substances directly into the blood stream
What is Signal Transduction
Process of intercellular signalling by which a signalling molecule binds to a specific receptors causing single transduction and an intracellular response
Whats a first messenger and a secondary messenger. Whats the difference?
First messengers ligands that bind to and initially activate the receptor
Secondary messengers are produced upon receptor activation and subsequent cascades
Name the main families of receptor
Ligand Gated ion channels
G protein coupled receptors
Tyrosine Kinase Receptors
Cytokine Receptors
Nuclear Receptors
Explain how ligand gated ion channels (ionotropic receptors) work
- 1st messenger binds to allosteric binding site on receptor (cooperatively/ induced fit theory)
- Causes a conformational change in the receptor
- This allows the ion to flow through
Give an example of an ionotropic receptor pathway
Nicotinic Ach Receptor
Nicotine + Ach agonist
Curare Antagonist
4-5 subunit Quaternary protein 2ACh binding sites
- Hydrophobic interactions on non-polar R groups cause the channel to be closed
- ACh binds to receptor
- Conformational change (rotation of membrane spanning a helices resulting in polar/ hydrophilic R groups being exposed
- Results in the channel opening
- Na+ can diffuse through
Defining structural features of GPCR’s
7 transmembrane a helices
2 binding sites one for ligand and one for associated G-protein
What are G proteins ?structure?
- Transducing proteins
- Mediate the signal from the receptor to the effector (intracellular transduction pathway)
heterotrimeric have three subunits an a, B and Gamma subunit.
o The alpha subunit is attached to the plasma membrane via a lipid anchor. The a subunit hydrolyses GTP to GDP
o In its inactive state the a subunit of the G-protein contains GDP.
o The B and Gamma form a tight non-covalent BG dimer. The gamma ppc is also linked to the plasma membrane via a lipid anchor.
How do GPCR’s Work?
- Receptor initially inactive (GDP bound to a subunit of G-protein)
- Ligand cooperatively binds with 7Tm receptor (1st messenger)
- Conformation change in GPCR
- G-protein binds to GPCR if not already bound and is activated
- Results in a conformational change and the release of GDP allowing GTP to bind to binding site on a subunit
- This in turn results in further conformational change and dissociation G-protein:
o a subunit (containing GTP) dissociate and bind to and effector thus stimulating it
o BG subunits stay together and can also signal and activate effectors - Stimulation of effector -> production of 2nd messengers –> Transduction of signal –>cellular response
How are G protein signalling pathways terminated?
intrinsic GTP-ase activity of a subunit hydrolyses GTP–> GDP +Pi
Deactivates a subunit
a subunit now reassociates with BG subunits heterotrimeric inactive G-protein reformed
Ga/s G protein what does it do?
Cholera toxin impacts?
Ga/s activates adenylyl cyclase –> cAMP production (2nd messenger) –> PKA –> kinase cascade –> intracellular response
Cholera toxin inhibits GTP-ase activity of subunit
no hydrolysis of GTP continuous activation of AC elevated cAMP –> excess water secretion diarrhoea
Gai?
inhibits adenylyl cyclase decreasing cAMP production inactivating cascade
cAMP broken down to AMP by phosphodiesterase terminating signal
Gaq
G protein Activates phospholipase C (effector):
o Acts on a lipid in membrane PIP2 (Phosphatidylinositol 4,5-bisphosphate)
o PLC hydrolyses PIP2 to diacylglycerol (DAG) and Inositol 1,4,5-trisphosphate (InsP3)
o DAG activates membrane bound protein kinase C (also needs Ca2+ ions to function) –>phosphorylation–>cellular response
o InsP3 binds to intracellular receptors–>release of Ca2+ intracellular stores (ER)–>protein phosphorylation–>cellular response
G beta/gamma
can activate GIRK channels
G protein coupled inwardly rectifying potassium channels
Examples of regulation by G protein pathways?
Glycogen metabolism
Regulation of smooth muscle tone
Glycogen metabolism G protein pathway?
- Glucagon/adrenaline ligand binds to and stimulates GPCR
- Activates a Gs G-protein stimulating AC–>production of secondary messenger cAMP.
- cAMP activates Protein Kinase A
- Protein Kinase A phosphorylates Glycogen synthase to P-Glycogen synthase halting glycogen synthesis (deactivation)
- Protein Kinase A also triggers a cascade. PKA phosphorylates an inactive enzyme Phosphorylase kinase to Phosphorylase kinase-P. (active)
- Phosphorylase kinase -P activates the inactive enzyme glycogen phosphorylase b to Glycogen phosphorylase a which catalyses conversion of glycogen to glucose.
Smooth muscle tone regulation contraction and relaxation
- Important regulator is Ca2+
- Gq coupled receptors lead to release of Ca2+from ER through PLC and InsP3
- Ca2+ bind to calmodulin leads to phosphorylation of MLC to MLCK (active) SM contraction
- inc Ca2+=MLCK=contraction
- Gi coupled receptor decreases cAMP levels = contraction
Relaxation:
cAMP inhibits activity of MLCK:
• Gs coupled receptor inc cAMP–> inhibits activity of MLCK via Protein Kinase A SM relaxation
• Nitrous oxide (NO) production of cGMP which activates phosphatase–> MLCK converted to MLC–> relaxation
Mutations in GPCR’s
Activating mutations: tumor development and hyperfunction
Deactivating mutations: lead to hormone resistance
Disease examples due to faulty GPCR’s
TSH receptor is a GPCR linked to Gs G protein. Via Protein kinase A increases replication of thyroid cells + synthesis and release thyroid hormones from follicle cells.
Autonomous thyroid adenoma (ATA):
• Benign tumors in the thyroid gland
• Results in excessive production of thyroid hormones (Hyperthyroidism)
mutations in receptor and Gs protein
TSH resistance:
• Cells not response to TSH (Hypothyroidism)
How do tyrosine kinase receptors work?
respond to growth factors important in cell proliferation
• Binding of 1st messenger
• Causes dimerisation of two units
• Activates intrinsic enzyme (tyrosine Kinase)
• Autophosphorylation allows the binding and activation of subsequent plasma proteins.
How do cytokine receptors work?
Very similar to TKR’s except don’t have intrinsic kinase activity instead associated strongly with JAK kinase
• Cytokine ligand binds to receptor–>dimerisation–>JAK’s brought close together–>phosphorylate each other–>triggers JAK-STAT signalling pathway–>ultimately leads to gene augmentation or suppression
how do Nuclear Receptors work
located in the nucleoplasm (type II) and the cytosol (type I)
• In cytosol receptors bound to HSP (heat shock protein) steroid ligand binds to receptor–> release of HSP–>receptor ligand complex into nucleus via pore–>binds to regulatory regions on DNA augment or suppress expression of particular genes.
• Type II work the same however there is no HSP.
