Cell Signalling and Receptors

Question 1

Name the different signalling methods cells can use.

Answer 1

• 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

Question 2

Whats the difference between and endocrine and exocrine gland

Answer 2

exocrine glands not part of endocrine system secretions go into ducts

Endocrine glands lack ducts secrete chemical substances directly into the blood stream

Question 3

What is Signal Transduction

Answer 3

Process of intercellular signalling by which a signalling molecule binds to a specific receptors causing single transduction and an intracellular response

Question 4

Whats a first messenger and a secondary messenger. Whats the difference?

Answer 4

First messengers ligands that bind to and initially activate the receptor

Secondary messengers are produced upon receptor activation and subsequent cascades

Question 5

Name the main families of receptor

Answer 5

Ligand Gated ion channels

G protein coupled receptors

Tyrosine Kinase Receptors

Cytokine Receptors

Nuclear Receptors

Question 6

Explain how ligand gated ion channels (ionotropic receptors) work

Answer 6

• 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

Question 7

Give an example of an ionotropic receptor pathway

Answer 7

Nicotinic Ach Receptor
Nicotine + Ach agonist
Curare Antagonist

4-5 subunit Quaternary protein 2ACh binding sites

1. Hydrophobic interactions on non-polar R groups cause the channel to be closed
2. ACh binds to receptor
3. Conformational change (rotation of membrane spanning a helices resulting in polar/ hydrophilic R groups being exposed
4. Results in the channel opening
5. Na+ can diffuse through

Question 8

Defining structural features of GPCR’s

Answer 8

7 transmembrane a helices

2 binding sites one for ligand and one for associated G-protein

Question 9

What are G proteins ?structure?

Answer 9

• 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.

Question 10

How do GPCR’s Work?

Answer 10

1. Receptor initially inactive (GDP bound to a subunit of G-protein)
2. Ligand cooperatively binds with 7Tm receptor (1st messenger)
3. Conformation change in GPCR
4. G-protein binds to GPCR if not already bound and is activated
5. Results in a conformational change and the release of GDP allowing GTP to bind to binding site on a subunit
6. 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
7. Stimulation of effector -> production of 2nd messengers –> Transduction of signal –>cellular response

Question 11

How are G protein signalling pathways terminated?

Answer 11

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

Question 12

Ga/s G protein what does it do?

Cholera toxin impacts?

Answer 12

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

Question 13

Gai?

Answer 13

inhibits adenylyl cyclase decreasing cAMP production inactivating cascade

cAMP broken down to AMP by phosphodiesterase terminating signal

Question 14

Gaq

Answer 14

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

Question 15

G beta/gamma

Answer 15

can activate GIRK channels

G protein coupled inwardly rectifying potassium channels

Question 16

Examples of regulation by G protein pathways?

Answer 16

Glycogen metabolism

Regulation of smooth muscle tone

Question 17

Glycogen metabolism G protein pathway?

Answer 17

• 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.

Question 18

Smooth muscle tone regulation contraction and relaxation

Answer 18

• 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

Question 19

Mutations in GPCR’s

Answer 19

Activating mutations: tumor development and hyperfunction

Deactivating mutations: lead to hormone resistance

Question 20

Disease examples due to faulty GPCR’s

Answer 20

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)

Question 21

How do tyrosine kinase receptors work?

Answer 21

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.

Question 22

How do cytokine receptors work?

Answer 22

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

Question 23

how do Nuclear Receptors work

Answer 23

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.