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Cell Biology > GPCR Signalling > Flashcards

GPCR Signalling Flashcards

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1
Q

What does ‘GPCRs’ stand for?

A

G-protein coupled receptor(s)

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2
Q

What is another name for GPCRs?

A

7TM Receptors (seven transmembrane receptors)

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3
Q

Fun fact!

A

Around 500 different GCPRs in humans

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4
Q

Fun fact!

A

50% of all pharmacological agents act on GPCRs!

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5
Q

What is the purpose of intracellular signalling cascades?

A

To amplify the extracellular signal across the membrane to the target, with strict control and cross-talk with other signals.

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6
Q

What are the two most important families of cell surface receptors?

A

GPCRs and Enzyme-Linked Receptors

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7
Q

Describe how conformational flexibility of the beta-adrenergic receptor changes upon ligand binding.

A

Adrenaline closes the space at the extracellular side between TM domains 3, 5 and 6, thereby forcing them apart at the cytosolic side. This induces a conformational change in the 5/6 loop promoting signal transduction.

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8
Q

Explain how the same signal can exert different effects in specific cells.

A

The receptor determines how a signal is interpreted, a different GPCR means a different outcome. Allows target cells to activate physiological responses that are appropriate for a given tissue while utilising the same common signalling components.

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9
Q

Which G protein is recruited to the GPCR when adrenaline binds to the alpha-2-adrenergic receptor in platelets?

A

G-alpha-i

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10
Q

Which G protein is recruited to the GPCR when adrenaline binds to the beta-2-adrenergic receptor in muscle cells?

A

G-alpha-s

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11
Q

What are G proteins?

A

Guanine Nucleotide Binding Proteins with a roll in signal transduction belonging to the larger group of GTPases that are recruited upon activation of a GPCR by its ligand.

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12
Q

Describe the G-protein cycle.

A

G-proteins bind tightly to their nucleotide cofactor GDP. GEFs catalyse the release of GDP from the G-protein.
GTP spontaneously binds to G-proteins.
GAPs enables the hydrolysis of GTP by the G-protein to GDP.
The cycle then continues.

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13
Q

What does GAP stand for?

A

GTPase Activating Protein

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14
Q

What does GEF stand for?

A

Guanine Exchange Factor

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15
Q

Why is a GAP needed in the G-protein cycle?

A

G-proteins are GTPases with extremely low activity. A GAP is needed for productive GTP hydrolysis.

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16
Q

GPCRs are able to act as GEFs. True or False?

A

True. Because of the conformational change due to ligand binding - movement of a helix - the GCPR mediates the removal of GDP on the intracellular G-protein

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17
Q

What is the role of the target of activated G-proteins?

A

Activated G-proteins target effector proteins which can act as GAPs to inactivate the G-protein-effector complex, while also mediating synthesis of another messenger.

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18
Q

Give an example of an effector protein and state its function.

A 
Adenylyl Cyclase (AC) can be bound by G-alpha-s (stimulatory) or G-alpha-i (inhibitory) to regulate synthesis of cAMP - a second messenger. 
AC meanwhile acts as a GAP hydrolysing GTP so that the G-protein no longer binds AC and AC is inactivated.
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19
Q

What are second messengers?

A

Small molecules produced/released upon activation of a signalling pathway. They generally amplify a signalling cascade.

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20
Q

How can second messengers ensure a localised target response?

A

Localised production and constant destruction ensures they are released when and exactly where they are required.

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21
Q

What is ADP-ribosylation?

A

The addition of one or more ADP-ribose moieties to a protein. It is a reversible post-translational modification that is involved in many cellular processes

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22
Q

Explain how the cholera toxin leads to the presentation of cholera symptoms.

A

CTA1 fragment of Cholera Toxin catalyses the covalent ADP-ribosylation of G-alpha-s at an arginine residue at the active site.

This inhibits GTPase activity of G-alpha-s maintaining it in its active state.

This leads to overactive AC and excess cAMP production, and overactive PKA.

PKA then phosphorylate the CFTR chloride channel proteins, leading to ATP-mediated efflux of Cl-,

This causes the secretion of Water, Na+, K+ and HCO3- into the intestinal lumen.

Results in severe dehydration from rapid fluid loss

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23
Q

How does the Pertussis toxin cause excess cAMP production?

A

Pertussis toxin ADP-ribosylates the G-alpha-i subunit at a cysteine residue preventing exchange of GDP for GTP.
G-alpha-i is permanently inactive and AC can never be inhibited. This leads to excessive cAMP synthesis.

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24
Q

Give some examples of second messengers.

A 
1,2-Diacylglycerol (DAG)
3’,5’-cAMP (cAMP)
Inositol-1,4,5-triphosphate (IP3)
3’,5’-cGMP (cGMP)
Ca2+ ions 
NO-radicals
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25
Q

Which protein kinase 1,2-Diacylglycerol activate?

A

PKC

26
Q

Which protein kinase does cAMP activate?

A

PKA

27
Q

What is a function of inositol-1,4,5-triphosphate

A

Open Ca2+ channels in the ER

28
Q

Which protein kinase does cGMP activate?

A

PKG

-can also open cation channels in Rod cells

29
Q

Outline the method used by Earl W. Sutherland in 1971 in discovering second messengers

A
  1. Liver tissue is homogenized and separated into plasma membrane and cytoplasmic fractions.
  2. Membranes contain adrenaline receptors. Cytoplasm contains inactive glycogen phosphorylase.
  3. Hormone adrenaline added to membranes and allowed to incubate.
  4. Membranes removed by centrifugation, leaving only their incubated solution.
  5. Drops of membrane-free solution are added to the cytoplasm and activate glycogen phosphorylase.
30
Q

Describe how cAMP activates PKA

A

cAMP binding to PKA’s regulatory subunits causing their dissociation, revealing PKA’s catalytic subunit allowing it to phosphorylate targets, e.g. some nuclear receptors, CREBs, Ca2+ channels in the ER.. etc.

31
Q

How many isoforms are there of the catalytic subunits and regulatory subunits of PKA?

A

3 isoforms of catalytic subunits

4 isoforms of regulatory subunits

32
Q

What is the role of glycogen phosphorylase?

A

Glycogenolysis; to cleave glucose units from glycogen polymers

33
Q

What is the role of glycogen synthase?

A

Glycogen synthesis; to add glucose units onto glycogen chains

34
Q

What is the result if glycogen breakdown and glycogen synthesis without regulation?

A

Glycogen is degraded and synthesised spontaneously and an equilibrium is reached

35
Q

What does GP stand for?

A

Glycogen Phosphorylase

36
Q

What does GPK stand for?

A

Glycogen phosphorylase kinase

37
Q

What does PP stand for?

A

Phosphoprotein phosphatase

38
Q

What does IP stand for?

A

Inhibitor of phosphoprotein phosphatase

39
Q

When PKA is activated by cAMP it phosphorylates GPK to GPK-P. What is the role of GPK-P?

A

GPK-P is active which phosphorylates GP to GP-P

40
Q

When is GP in its active state?

A

When it is phosphorylated to GP-P by PKA

41
Q

What is the role of GP-P?

A

To cleave glucose units from

42
Q

When is IP in its active state?

A

When it is phosphorylated to IP-P by PKA

43
Q

What is the role of IP-P?

A

To bind to PP forming the IP-PP complex, which inactivates PP

44
Q

What is the role of active PP?

A

To inactivate GP-P

45
Q

What is the role of the IP-PP complex

A

To attenuate the inhibition of glycogen phosphorylase by PP

46
Q

Explain the effects of PKA being activated by cAMP on glycogen degredation

A

cAMP activates PKA, which catalyses GPK (inactive) -> GPK-P (active), which catalyses GP (inactive) -> GP-P (active) which actively cleaves glucose units.

Active PKA also catalyses IP (inactive) -> IP-P (active) which forms a complex with PP, IP-PP, inactivating PP. Inactive PP means no inhibition of GP-P as active PP functions to inactivate GP.
This attenuates the inhibition of glycogen breakdown, ultimately allowing blood glucose levels to rise.

47
Q

Explain the effects on active PKA by cAMP on glycogen synthesis.

A

cAMP activates PKA, which catalyses GS (active) -> GS-P (inactive). Inactive GS-P means glycogen is not synthesised, and blood glucose levels remain stable, until to rise from glycogenolysis.

48
Q

What is the effect of an inactive PKA on blood glucose levels?

A

Inactive PKA leads to deactivation of GP-P to GP as PP is not inhibited by IP (which it would be if PKA was active). Inactive GP means glycogen breakdown does not occur.
Inactive PKA also means no phosphorylation of GS, remaining active and so glycogen is synthesised from glucose.
This results in a decrease in blood glucose levels.

49
Q

Explain the overall effect of adrenalin on blood glucose levels

A

Adrenaline triggers degradation of glycogen and thus rapid release of glucose from the liver into the bloodstream.
This is achieved by adrenaline binding to adrenergic receptors, the activation of PKA, and therefore the inhibition of glycogen synthesis and simultaneously the promotion of glycogen degradation.

50
Q

What is/are the cellular response(s) to cAMP in ADIPOSE tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormones: Adrenaline, ACTH, Glucagon

Cellular responses: Increase in triglyceride hydrolysis, decrease in amino acid uptake

51
Q

What is/are the cellular response(s) to cAMP in LIVER tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormones: adrenaline, noradrenaline, glucagon
Cellular responses: Increase in glycogenolysis, inhibition of glycogen synthesis, increase in amino acid uptake, increase in gluconeogenesis

52
Q

What is/are the cellular response(s) to cAMP in OVARIAN FOLLICLE tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormones: FSH, LH

Cellular response: Increase of oestrogen and progesterone synthesis

53
Q

What is/are the cellular response(s) to cAMP in ADRENAL CORTEX tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: ACTH

Cellular response: Increase in aldosterone and cortisol synthesis

54
Q

What is/are the cellular response(s) to cAMP in CARDIAC MUSCLE tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: Adrenaline

Cellular response: Increase in contraction rate

55
Q

What is/are the cellular response(s) to cAMP in THYROID GLAND tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: TSH

Cellular response: Thyroxine secretion

56
Q

What is/are the cellular response(s) to cAMP in BONE tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: Parathyroid Hormone

Cellular response: Increase in reabsorption of calcium from bone.

57
Q

What is/are the cellular response(s) to cAMP in SKELETAL MUSCLE tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: Adrenaline

Cellular response: Conversion of glycogen to glucose

58
Q

What is/are the cellular response(s) to cAMP in INTESTINAL tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: Adrenaline

Cellular response: Fluid secretion

59
Q

What is/are the cellular response(s) to cAMP in RENAL tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: Vasopressin

Cellular response: Reabsorption of water

60
Q

What is/are the cellular response(s) to cAMP in PLATELET tissue? Include the hormone that would induce cAMP synthesis in this case.

A

Hormone: Prostaglandin I

Cellular response: inhibition of aggregation

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