Lecture 6: Receptors and Signal Transduction Mechanisms

Question 1

Endocrine signaling

Answer 1

Long distance communication. A hormone carried to the blood to a distinct target cell where it has effect.

Question 2

Paracrine signalling

Answer 2

A secretory cell releases signalling molecules to an adjacent target cell, activating receptors on this cell. Could be across a synapse. Must have a neighbour for this type of signalling

Question 3

Autocrine signalling

Answer 3

A cell releases a signalling ligand that feeds back to the cell that released it and binds to receptors.

Question 4

Signalling by plasma membrane attached proteins

Answer 4

The ligand is attached to one cell and binds to a receptor on an adjacent cell. The cells are conjoined by this arrangement.

Question 5

For a drug to have selective action on a cell it muster interact with:

Answer 5

A discrete target (receptor) on that cell

Question 6

Receptors (what they are made of, where they are located)

Answer 6

Made of protein

Typically at the cell surface but also nuclear membrane proteins and even enzymes can act as receptors.

Question 7

Binding of drugs to receptors results in (3):

Answer 7

• Conformational changes in receptor.
• Transduction of the signal via alterations in cytosolic metabolism
• Changes in cell function and gene transcription

Question 8

Orthosteric site

Answer 8

Where endogenous ligands bind a receptor. Most drugs mimic endogenous ligands, so they will also bind here. The receptor site that binds the neurotransmitter.

Question 9

Allosteric site

Answer 9

A distinct site on a receptor that the endogenous ligand doesn’t bind to. Can cause a change in the orthosteric site.

Question 10

Ligand

Answer 10

Any endogenous neurotransmitter. Something that is recognized by and binds to a receptor.

Question 11

The 5 ways in which drugs can intervene in neuronal receptor signalling (neuronal modulation)

Answer 11

1. Synthesis of transmitter in presynaptic nerve terminal
2. Release of transmitter and transport to the target cell
3. Detection of the signal by a specific receptor protein
4. Change in cellular metabolism triggered by the receptor signalling molecule complex
5. Removal of the signal, terminating the cellular response.

Question 12

Receptor types

Answer 12

• Ligand gated ion channel (LGIC)
• GPCR
• Receptor tyrosine kinases (RTK)
• Nuclear receptors

Question 13

Another term for LGIC

Answer 13

Ionotropic receptors

Question 14

LGICs

Answer 14

• Present on cell membrane
• Composed of 3-5 subunits and a central aqueous channel
• Central pore requires binding of a ligand to change the conformation of the channel so that ions can pass through.
• Involved in fast neurotransmission.
• If the charge across the membrane is a favourable gradient, the ions will flow. NOT Voltage gated

Question 15

Things that pass through LGICs

Answer 15

Na+, Ca2+, Cl-

Question 16

Examples of LGICs

Answer 16

• Nicotinic ACh (non-selective for sodium and calcium)
• GABAᴬ: Inhibitory
• 5-HT₃:

Question 17

How do LGIC and voltage gated ion channels differ?

Answer 17

While LGIC need favourable membrane potential for ions to flow through, they are gated by the binding of a ligand. VGICs don’t need this.

Question 18

Nicotinic Receptor Signalling

Answer 18

• ACh is released and binds to its receptor
• Receptor is able to pass sodium ions into the cell with the gradient
• Small depolarizations in the muscle sum and cause action potentials and the muscle contracts.

Question 19

Acetylcholine Esterase

Answer 19

• Membrane bound enzyme

- Hydrolyzes released ACh, producing choline and acetate that are recycled.

Question 20

Where are LGICs that respond to ACh normally found?

Answer 20

-Nerve synapses at neuromuscular junctions.

Question 21

Most drugs are targeted toward

Answer 21

GPCRs (at least 50%)

Question 22

GPCR Superfamily

Answer 22

• Family A: Rhodopsin-like
• Family B: Glucagon/ VIP/Calcitonin
• Family C: Metabotropic glutamate and chemosensor

Question 23

GPCR features

Answer 23

• 7 transmembrane domains
• N terminal that is frequently extracellular
• Intracellular carboxy terminal

Question 24

Function of IL3 loop

Answer 24

Slightly longer than others; has a function in binding downstream signalling molecules i.e. G-proteins

Question 25

C-terminus

Answer 25

Important in decreasing association of the receptor with the G-protein

Question 26

The G protein is inactive when bound to

Answer 26

GDP

Question 27

When an agonist that stabilizes the signalling form of the receptor binds, it allows:

Answer 27

Association with the G protein and the removal of GDP, so that it can bind GTP. The heterotrimeric g protein breaks apart, and the G-alpha subunit will signal downstream proteins.

Question 28

Effectors commonly associated with GPCRs (5)

Answer 28

• Adenylyl cyclase
• Phospholipases C & A2
• cGMP Phosphodiesterase
• Potassium channels
• Calcium channels

Question 29

Orphan receptors and ligands

Answer 29

Things we see in research but haven’t been able to put them into a class because we dont know how they function.

Question 30

Steps of GPCR activation

Answer 30

• Ligand binds orthosteric binding site. GDP is exchanged for GTP and the G protein associates with the receptor.
• Dissociation of the heterotrimeric G protein. alpha-GTP goes to effector and activates it
• Activation of effector is stopped when the alpha subunit hydrolyzes GTP. The heterotrimeric subunits come back together.

Question 31

Gs G-protein effector, intracellular signal, and signal change

Answer 31

Effector: adenylate cyclase
Intra Signal: Increase in cAMP
Signal change: Increase

Question 32

Gi G-protein effector, intracellular signal, and signal change

Answer 32

Effector: adenylate cyclase
Intra Signal: Decrease in cAMP
Signal Change: Can increase or decrease signalling, depending on the cell

Question 33

Gq G-protein effector, intracellular signal, and signal change

Answer 33

Effector: Phospholipid C
Intra Signal: DG, IP3, and Ca2+
Signal change: Increase in signalling. Involved in contraction

Question 34

cAMP and adenylyl cyclase signalling steps

Answer 34

• Agonist binds receptor
• alpha-GTP binds adenylyl cyclase
• Production of cAMP (uses ATP)
• Production of PKA enzyme
• Protein phosphorylation by PKA.

Question 35

If a cell expresses both beta and alpha adrenergic receptors, what happens in the presence of adrenaline?

Answer 35

• When bound to the beta receptor, adrenaline increases cAMP production
• When bound to the alpha-2 receptor, there is coupling to Gi which inhibits adenylyl cyclase and decreases cAMP.

NET RESULT: Depends on which receptor is activated first, how long it is activated for, and how many of the receptors are present in the cell.

Question 36

Muscarinic receptor

Answer 36

GPCRs that respond with an IP3 signalling pathway

Question 37

Activation steps after acetylcholine binds to a Gq receptor

Answer 37

• Activation of PLC enzyme
• Hydrolyzing of membrane phospholipids, to make IP3 and DAG.
• IP3 binds receptors on endoplasmic reticulum, dumping calcium into the cell.
• DAG activates PKC, involved in protein phosphorylation

Question 38

Desensitization

Answer 38

Typically when you persistently expose a receptor to an agonist, you will not see signalling. Can involve down regulation of the receptor and can lead to drug tolerance.

Question 39

GPCR Desensitization

Answer 39

When agonist is present and binds the G protein, you get various enzymes (GRKs/PKA). These kinases target tails with serine residues. They add phosphates to the tails. Beta arrestin then moves in and associates with the GPCR, enabling GPCR internalization in clatherin pits. The GPCRs are in endosomes where the agonist is cleaved off, beta-arrestin dissociates and there is degradation (with long term exposure) or recycling.

Question 40

What is the main step that initiates GPCR desensitization?

Answer 40

Phosphorylation of the carboxy tail by GRKs and PKAs

Question 41

Cholera

Answer 41

• Bacteria produces toxin
• Toxin changes Gs protein activation, locking it in the on position (by transferring a sugar onto the G protein)
• cAMP is needed for chloride conductance out of the cell by CFTR. When ions move, water must follow so there is a huge efflux of water into the gut, leading to watery diarrhea.