Purine Signaling NEJM

Question 1

What is the concentration of ATP in mammalian cells?

Answer 1

5 - 8 mM

Question 2

What are the major mechanisms by which ATP metabolites are released from the cell?

Answer 2

Apoptosis: Pannexin hemichannels and Gasdermin D pores enable ATP to diffuse outside of the cell.

Inflammatory Signaling: Activated endothelial cells and inflammatory cells (especially PMNs) may secrete ATP through connexin hemichannels.

Granule release: Activated platelets contain granules which store ADP and may release upon degranulation.

Question 3

P2X Receptors

Answer 3

Family of ATP receptors which are ligand-gated ion channel.

Question 4

P2Y Receptors

Answer 4

Family of ATP receptors which are G-protein-coupled receptors.

Question 5

Conversion of ATP and AMP to adenosine in the extracellular space.

Answer 5

ATP and ADP in the extracellular space are rapidly metabolized first to AMP by CD39 (ectonucleoside triphosphate dephosphorylase) and then to adenosine by CD73 (ecto-5’-nucleotidase).

Question 6

P1 Receptors

Answer 6

Family of G-protein-coupled receptors containing A1, A2a, A2b, and A3.

Question 7

A2a Receptors

Answer 7

Member of the P1 Adenosine Receptor family.

Highly expressed on neutrophils and lymphocytes. Binding by adenosine triggers anti-inflammatory signaling.

Question 8

A2b Receptors

Answer 8

Member of the P1 Adenosine Receptor family.

Highly expressed on the vascular endothelium. Binding by adenosine triggers adaptation to inflammation and ischemia-reperfusion.

Question 9

ENT1 and ENT2

Answer 9

Equilibriative nucleoside transporter 1 and 2.

Uptake adenosine from the extracellular space and import it into the cell.

Question 10

Intracellular adenosine metabolism

Answer 10

Adenosine imported by ENT1/2 is rapidly converted either to AMP (via adenosine kinase, or AK) or to inosine (via adenosine deaminase, or ADA).

Question 11

ADA Defficiency

Answer 11

Patients with a loss-of-function or deletion in ADA suffer severe combined immunodeficiency (SCID) due to the cytotoxic effects of adenosine metabolites on lymphocytes.

Bone marrow transplantation with ADA-competent marrow

Question 12

Thrombosis

Answer 12

Formation of a blood clot within a blood vessel, preventing the flow of blood. Often associated with inflamation.

Question 13

ADP-mediated platelet aggregation

Answer 13

Activation of P2Y1 activates phospholipase C, inducing conformational changes downstream. Activation of P2Y12 inhibits adenylyl cyclase activity as well as the activity of other anti-coagulation factors.

Question 14

Ticlopidine and Clopidogrel

Answer 14

Anticoagulants/Antithrombotics.

Pharmaceutical antagonists of the P2Y12 receptor that prevent receptor-ligand binding and downstream coagulation.

Question 15

P2Y12 Loss-of-Function Mutations

Answer 15

Result in congenital hemophilia.

Question 16

P2Y12 Gain-of-Function Mutations

Answer 16

Result in hyperactive platelet aggregation in response to extracellular ADP. Risk factor for arteriosclerosis.

Question 17

Intestinal inflammation often results in _____ due to an accompanying shift in metabolic activity.

Answer 17

hypoxia of the inflamed mucosa

Question 18

Link between chronic IBD and ATP signaling

Answer 18

Chronic ATP signaling in the intestinal mucosa can result in dysmotility and enteric-nerve injury.

Specifically, the proteins P2X7, pannexin-1, the Asc adapter protein, and caspases mediate the ATP signaling pathways that drive nerve injury in the gut.

Question 19

Adora2a and Adora2b

Answer 19

Adenosine receptors which serve an anti-inflammatory function, attenuating the activity of the P2 receptors.

Question 20

Difference between anti-inflammatory and pro-resolution treatments in the context of ATP signaling in IBD.

Answer 20

Anti-inflammatory signaling can be achieved with P2 receptor antagonists, which block the pro-inflammatory effects of ATP and ADP. However, this does not initiate pro-resolution signaling. A pro-resolution treatment would be the prescription of methotrexate or sulfasazine, which promote the conversion of ATP and ADP to adenosine.

Question 21

CD39+CD73+ Th17

Answer 21

A subset of Th17 cells express CD39 and CD73 ectopically, facilitating conversion of extracellular ATP to adenosine. This regulates other T-cell compartments which express the Adora receptors, leading to immunosupressive signaling.

Question 22

Dipyridamole

Answer 22

Antagonist of ENT1/2. Increased extracellular adenosine levels due to decreased adenosine uptake.