III. Signal transduction and cell cycle | 47. Role of AMPK in the regulation of the metabolism and autophagy

Question 1

I. Basics
1. What is AMP kinase (5’ adenosine monophosphate-activated protein kinase)?

Answer 1

AMP kinase (5’ adenosine monophosphate-activated protein kinase) is an enzyme that plays a role in the cellular energy homeostasis.
=> In the eukaryotic cell, it maintains a balance between ATP production and consumption.

Question 2

I. Basics
2. During physiological status, when there is a high ATP:ADP ratio
=> What will happen?

Answer 2

During physiological status, when there is a high ATP:ADP ratio, the reaction will be catalyzed by adenylate kinase which will shift to ADP synthesis.
=> This will result in a low AMP:ATP ratio -> AMP kinase is inactive.

Question 3

I. Basics
3. What happen to AMP:ATP ratio when there is metabolic stress and/or ATP consumption processes?

Answer 3

When there is metabolic stress and/or ATP consumption processes (e.g. during hypoxia, glucose deprivation), the AMP:ATP ratio will increase.
=> This will activate AMP kinase, which will restore the energy homeostasis by promoting catabolism (breakdown of molecules) and inhibition of ATP consumption.

Question 4

II. Structure and activation of AMP kinase
1. What is AMP kinase?

Answer 4

• AMP kinase is a serine/threonine protein kinase, usually found both in the nucleus and the cytoplasm.
• It is a heterotrimeric protein, consisting of 1 catalytic subunit (α-subunit) and 2 regulatory subunits (β-subunits).

Question 5

II. Structure and activation of AMP kinase
2. How To activate AMP kinase?

Answer 5

1) The Threonine172 on α-subunit needs to be phosphorylated
2) γ-subunit needs to be allosterically activated – by having an AMP bound to it

Question 6

II. Structure and activation of AMP kinase
3. What is The role of ATP in AMP kinase activity?

Answer 6

1. The phosphorylation of α-subunit is inhibited if ATP is bound on the γ-subunit -> in the case of already phosphorylated α-subunit, AMP will increase the AMP kinase activation allosterically
2. The phosphorylation of α-subunit is inhibited if ATP is bound on γ-subunit
   => Need high AMP levels & low ATP levels!

Question 7

III. Regulation of AMP kinase activity
1. How will AMPK get activated?

Answer 7

LKB1 (liver kinase B1), CaMKKβ (Ca2+/calmodulin-dependent kinase kinase β) and TAK1 (TGFβ-activated kinase 1) which phosphorylate Thr172 residue

Question 8

III. Regulation of AMP kinase activity
2. How will AMPK get inactivated?

Answer 8

PP2A + PP2C (protein phosphatases), PKA and PKC which dephosphorylate Thr172 residue

Question 9

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
1. What is autophagy?

Answer 9

Autophagy is a self-digestive mechanism inside the cell, where the cell divides defective proteins and unnecessary cellular organelles into their constituents.

Question 10

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
2. What are the 3 types of autophagy?

Answer 10

1. Chaperone-mediated autophagy
2. Microautophagy
3. Macroautophagy

Question 11

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
3. What are the features of Chaperone-mediated autophagy?

Answer 11

• Chaperone-mediated autophagy: breaks down proteins only.
• Chaperone proteins recognize damaged proteins, bind them and the complex will be linked to lysosomal receptors.
• The proteins are then transferred into the lysosomes and decomposed
  => The degrading products (e.g. amino acids) can be used in protein synthesis, energy production, gluconeogenesis etc.

Question 12

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
4. What are the features of Microautophagy?

Answer 12

Microautophagy: cytoplasmic regions or cellular organs will be surrounded by the lysosomal membrane, there will be a direct intake and then degradation in the lysosomal membrane
=> The degrading products (e.g. amino acids) can be used in protein synthesis, energy production, gluconeogenesis etc.

Question 13

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
5. What are the features of Macroautophagy?

Answer 13

• Macroautophagy: cell forms autophagosome around a portion of cytoplasm to eliminate organelles or unused proteins.
• The autophagosome fuses with lysosome forming an autolysosome -> degradation
  => The degrading products (e.g. amino acids) can be used in protein synthesis, energy production, gluconeogenesis etc.

Question 14

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
7. What happen if autophagy get activated?

Answer 14

Autophagy will be activated, when Ulk1/2 forms a complex with Atg13, Atg101 and Fip200.

Question 15

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
8. What is the relationship between AMP kinase and mTOR regarding autophagy in the PRESENCE of nutrients?

Answer 15

In the presence of nutrients:
- mTORC1 is active
- AMPK is inactive
=> Autophagy inactive: the complex is inhibited by the phosphorylation of Ulk1/2 and Atg13 by mTORC1 = no assembly of the complex

Question 16

IV. Relationship between AMP kinase and mTOR in regulation of autophagy
8. What is the relationship between AMP kinase and mTOR regarding autophagy in the ABSENCE of nutrients?

Answer 16

In the absence of nutrients:
- mTORC1 is inactive
- AMPK is active (starvation = high levels of AMP!)
=> Autophagy is active: AMPK inhibits mTORC1 and activates Ulk1/2. Active Ulk1/2 can phosphorylate Atg13 and Fip200 = assembly of complex occurs