GRK and B arrestins Flashcards
What are GRKS
Family of 7 serine/threonine kinases which recognise and phosphorylate agonist-activated GPCRs
This causes:
- Translocation and binding of B-arrestin - This mechanism is used, for example, with rhodopsin in retinal cells to compensate for exposure to bright light
- Bound B-arrestin acts as an adaptor to clathrin-mediated endocytosis,
- This re-sensitises and recycles receptors back to the plasma membrane and inhibits further GPCR activation
o GRKS also promotes agonist + GPCR internalisation
o These are then:
♣ Recycled back to plasma membrane
♣ Taken to lysosomes for degradation
Why is excess GRK bad
- While initially adaptive to compensate for increased catecholamine stimulation
- Over time, the excess GRK2 causes dysregulation of the βAR system, leading to a loss of inotropic reserve and contributing to HF.
- GRK2 is increased 3-4 fold in the myocardium in HF
Lacarrino et al 2005
o In myocardial biopsies from explanted failing human hearts
o GRK2 up-regulation appears to be one of the first molecular alterations in the myocyte after cardiac injury/stress
o May have novel diagnostic and prognostic value as its levels in the heart are mirrored by levels in white blood cells and so it can be measured peripherally
Raake et al 2008
o Cardiac-specific GRK2 KO mice where GRK2 is ablated after birth
o Improved cardiac function and prevention of HF development after a myocardial infarction
o Moreover, when KO of GRK2 is induced in the cardiomyocyte after HF development
♣ Active reverse remodelling and improved cardiac function
Koch et al
o Currently no suitable pharmacological small-molecule GRK2 inhibitor for targeted GRK2 inhibition in HF
o Engineered a mini protein inhibitor βARKct that blocks the membrane translocation and activation of GRK2
Raake et al 2013
o Used adeno-associated virus serotype 6 (AAV6) to express βARKct in a porcine model of HF
o Long-term βARKct expression induced a significant amelioration of left ventricular haemodynamics and contractile function in pigs with HF compared to controls, which showed an impaired cardiac function.
• Limitations
o Limited amount of DNA that AAVs can carry
o High inflammatory response of the ADs, which limits the time of expression of the transgene.
Schumaker et al 2015
o Paroxetine, the selective serotonin reuptake inhibitor (SSRI), can inhibit GRK2 activity
o This is based on work by Thal and colleagues
♣ Binds the active site of GRK2 stabilizing the kinase domain in a novel conformation
♣ Both in vitro in isolated cardiomyocytes and in vivo in mice, pre-treatment with paroxetine potentiates isoproterenol effects on βAR-mediated contractility
o In wild-type mice with myocardial infarction, paroxetine significantly improves cardiac function
o A major limitation for the use of this drug is the very high dosage at which it is effective to inhibit the kinase. Indeed, the effective doses exceed those approved for the use of paroxetine in humans, making unavoidable effects on the central nervous system
Cyclic peptides
o Cyclic compounds have been designed, which are modelled on the conformation of a part of the GRK structure
o Cyclic peptide 7, can inhibit GRK2 activity and is more active than its linear precursor.
o In cultured cells, this peptide confirms its potentiality and specificity as a GRK2 inhibitor
o Thus, this cyclic peptide has a great potentiality to be translated to clinical trials.
Conclusion
• Several approaches have been developed to inhibit GRK2 activity based on different molecular mechanisms.
• Most of them are far from clinical applications, but they will be helpful for the development of novel inhibitors
• Most feasible approach, which could easily be translated to clinical trials, seems to be the peptide-based drug, and cyclic peptides are the most promising.
However, advancements in the knowledge of the multiple roles of GRK2 within the cell in HF will allow the identification of more selective inhibitors.
