ADENOSINE RECEPTOR

Adenosine 1. coupling of cellular metabolism to energy supply. 2. Suppresses neuronal firing and increases blood flow. 3. four types : A1, A2A, A2B, A3

Adenosine in the brain 1. physiological neuromodulator 2. extracellular adenosine rises from nmol to  mol under seizures, ischaemia and hypoxia 3. Function: a. neuroprotective effect mainly by A1 receptors.

b. in neurons: inhibits the release of excitatory neurotransmitters  hyperpolarization. c. stimulation of glial adenosine receptors  synthesis of various neuroprotective substances. d. adenosine A1 receptor stimulation in astrocytes  release of nerve growth factor and S100B protein. e. stimulation of adenosine A2B receptors in astrocytes induces synthesis and release of interleukin-6 (IL-6).

Selective adenosine receptor agonists: a. CPA (A1) and CGS (A2A) b. NECA: agonist, A3 receptors c. 8PT: antagonist, high affinity for A1, A2A, intermediate affinity for A2B, very low affinity for A3 adenosine receptors

Adenosine production a. S-adenosylhomocysteine (SAH) by SAH hydrolase to l-homocysteine and adenosine b. hydrolysis of AMP by 5'-nucleotidase, predominates during ischemic or hypoxic conditions.

Potential signaling pathways for adenosine in modulating cardiomyocyte hypertrophy. a. Stimulator of Gq-coupled receptors a)norepinephrine phenylephrine angiotensin II endothelin-1 b) pathways: activates a Gq-PLC/PLD (phospholipase C, D) signaling pathway b. stimulator of Gs-coupled receptors a) ß 1 -adrenergic receptors (isoproterenol) b) pathway: activates the Gs-cAMP signaling pathway.

c. Activation of Gq and Gs results: activation of Ca 2+ and cAMP signaling → contractility  and energy demands  and results in hypertrophy d. Activation of the Gi-coupled adenosine A 1 receptor results: → inhibits Gs and Gq signaling and protects the myocytes from hypertrophy

Adenosine A 1 receptor a. overexpression → increased myocardial resistance to ischemia b. Adenosine inhibits norepinephrine release from presynaptic vesicles→ attenuates the renin- angiotensin system, decreases endothelin-1 release, and exerts antiinflammatory effects

Adenosine A 1 and A 3: contribute to myocardial preconditioning Adenosine A 2A receptors: a. vascular system → vasodilation. b. also found in cardiac myocytes → coupling to cAMP ( reported in rat but not in porcine) c. suggests: many adenosine effects have the potential to influence the cardiac response to stress

Adenosine: attenuate myocardial hypertrophy a. CAD (2-chloroadenosine ): a stable analogue of adenosine→ inhibited the hypertrophic response to phenylephrine, endothelin-1, angiotensin II, or isoproterenol. b. adenosine A 1 agonist mimick (N-cyclopentyl adenosine, CPA) c. A 2 or A 3 agonists: did not

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Molecular genetic analysis of the calcineurin signaling pathways

1. calcineurin ： Ca2 and calmodulin-dependent protein phosphatase (type 2B) 2. serine:threonine-specific protein phosphatases 3. target of the immunosuppressant drugreceptor 4. Inhibitor: cyclosporin A (CsA)-cyclophilin and tacrolimus (FK506)-FKBP 5. Structure: heterodimer a. catalytic (calcineurin A) b. regulatory (calcineurin B) (fig. 1)

Fig. 1

1. Molecular cloning studies identified 3 distinct genes encoding the , ,  isoforms of calcineurin A 2.  and  isoforms serve different roles in neuronal signaling 3.  isoform is expressed in the testis 4. calcineurin-mediated dephosphorylation and nuclear translocation is a central event in signal transduction, which responses to Ca2-mobilizing stimuli.

T cell activation 1. Inhibitors: CsA and FK506 for treat graft rejection 2. Pathway: T cell receptor (TCR)-activated signal transduction pathway 3. Procedure: Antigen + TCR → Ca2↑ → calmodulin + calcineurin B → bind to Ca2 → moveaway Cn A from the catalytic active site of calcineurin → Cn activated

4. Cn→ dephosphorylates NF-AT (nuclear factor of activated T cells) → DNA recognition → bind with activator protein-1 (AP-1, transcription factor ) (fig. 2) Activated calcineurin 5. Cn → dephosphorylates NF-AT → into nucleus → transcription of the T cell gene↑→ IL-2↑

FIG.2

INHIBITOR: immunosuppressants 1. CsA → bound to cyclophilin (receptor) 2. FK506 → bound to FKBP 3. The complexes → inhibit calcineurin →dephosphorylation↓ → activation of NF- AT↓ → suppression of the TCR-activated signal transduction pathway by CsA and FK506

FIG. 3

NF-AT kinases (fig. 1) counteracts calcineurin 1. c-Jun amino-terminal kinase (JNK): a. function: phosphorylate NF-AT4 b. JNK activation → nuclear exclusion of NF-AT4 2. Casein kinase Ia (CKIa): binds and phosphorylates NF-AT4→ inhibition of NF-AT4 nuclear translocation.

3. Mitogen-activated protein kinase:extracellular signal-regulated kinase kinase 1 (MEKK1) →stabilizing the interaction between NF- AT4 and CKI  → suppresses NF-AT4 nuclear import 4. Glycogen synthase kinase-3 (GSK-3) : phosphorylation and translocation of NFAT

Muscle hypertrophy 1. cardiac hypertrophy: calcineurin→ NF-AT3 interacts with the cardiac zinc finger transcription factor GATA-4 → synergistic activation of cardiac transcription (fig. 2) 2. Immunosuppressants prevented hypertrophic cardiomyopathy 3. CsA: similar effect, suggesting similar pathway of T cell activation

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