Constitutively Active Receptors and Inverse Agonists Flashcards
The two-state model of receptor activation
Traditionally, a receptor is thought to be present in two
conformational states, ‘resting’ (Ri) and ‘active’ (Ra), which exist in equilibrium.
Normally, when no ligand is present, the equilibrium lies mainly to the ‘resting’ state.
Agonist (e.g. drug, hormone, transmitter) binding results in the equilibrium from the resting state (Ri) shifted to the active (Ra) state, and stabilises the active state; hence produces a response.
The two-state model of receptor activation -
constitutively active receptors
Many GPCRs and ion channels exhibit constitutive activity, producing spontaneous regulation of effectors in the absence of activation by agonists. The term for this is constitutive activity.
Constitutively active receptors and inverse agonists
Ligands that can reduce or abolish receptor spontaneous, agonist-independent activity are termed inverse agonists.
Inverse agonists bind to constitutively active receptors and shift the equilibrium to the inactive
conformational state; thus reverse receptor activity.
Constitutively active GPCR
Constitutively active receptors are thought to be coupled to second messenger pathways in the absence of agonists
- when inverse agonist is added, second messenger unbinds, inactivating the constitutively active receptor
Inverse agonist - efficacy
Efficacy
Agonist - positive efficacy
Antagonist - zero efficacy
Inverse agonist - negative efficacy
Inverse agonists are effective
against receptors which have
constitutive activity
see diagram
Types of inverse agonists
Type 1. Orthosteric inverse agonist - it binds to the same receptor binding sites as an agonist for that receptor but exerts the opposite effect.
Type 2. Allosteric inverse agonist – it binds to an allosteric site and affects the receptor signalling.
Type 3. Pseudo inverse agonist – it also binds to an allosteric site and affect orthosteric agonist binding pocket with no effect on signalling
The spectrum of drug activity
Full agonist (orthosteric, allosteric) (alpha=1) > partial agonist (orthosteric, allosteric) > neutral antagonist (competitive, non-competitive) (alpha=0) > partial inverse agonist (orthosteric, allosteric) > full inverse agonist (orthosteric, allosteric) (alpha=-1)
Ranked in order of efficacy (alpha)
It is not easy to tell whether a given compound behaves as an inverse agonist or as a neutral antagonist or a modulator
Two factors determine whether a given compound behaves as an inverse agonist or not:
The GPCR that the compound interacts with is constitutively present in a confirmation that is already coupled to G proteins, also in the absence of any other ligand.
The compound must have a high affinity for the “Resting” conformations of the GPCR .
Constitutively active receptors – experimental evidence in transgenic animals
Transgenic mouse (TG) overexpressing the human β1-adrenergic receptor (β1AR)
A significantly higher spontaneous frequency of isolated right atria was seen compared with WT –constitutively active β1AR in TG
CGP20712A caused a 25% decline of the frequency in TG, but only a small decline in WT. The effect of CGP20712A could be prevented by propranolol
– GP20712A is an inverse agonist in TG
The H2 antagonists, cimetidine and ranitidine, are in fact inverse agonists!
● The histamine H2 receptor has constitutive activity
● Cimetidine and ranitidine act as inverse agonists to inhibit H2 receptor activity
● On the other hand, burimamide acts as a neutral antagonist
Design an experiment to test if Drug X is an M3 receptor inverse agonist or an antagonist using the isolated guinea pig ileum setup
Add drug X
- no change in baseline - true antagonist
- decrease in baseline - inverse agonist
You can also consider measuring the production of the second
messenger IP3, which is associated with M3
receptor activation.
- Drug X reduced IP3 formation - might be an inverse agonist - need to do something else to confirm this
- Apply antagonist, if antagonist can block drug X induced reduction of IP3 then that proves that drug X is M3 receptor inverse agonist
Constitutively active receptors and diseases
The concept of a constitutively active receptor may be of help to explain some pathophysiological conditions.
For example, if the process of disease induced the expression of a constitutively active α1-receptor, the
receptor would no longer be under the normal influence of the sympathetic nervous system. This could occur in hypertension.
In this scenario, drugs with inverse agonist properties
could prove to be safe, rational therapeutics.
β2-AR and asthma
Selective β2AR agonists have been the drug of choice for relief of life-threatening bronchospasm experienced by asthmatics for 40 years.
Short and long lasting β2AR agonists have also been used
extensively for the prophylactic management of asthma.
Long-term use of β2-AR agonists can actually increase
mortality of asthma patients.
It was thought to be caused by β-agonists induced
desensitisation of airway β2AR.
However, a recent study by Nguyen et al. 2009 PNAS has
suggested….
β2AR and asthma –study by Nguyen et al in mice
Alp:.alprenolol , a β2 antagonist did not reduce S/C antigen challenge induced
mucous metaplasia.
The inverse agonist nadolol inhibited the inflammation responses
It is the constitutively active β2-AR signalling that is responsible for the development of airway inflammation.
β2AR inverse agonism in the asthmatic airway, a potential therapeutic model
Although direct effects of βagonists relax ASM to
prevent/reverse
bronchoconstriction, airflow
conductance is affected by the
pro-contractile effects of
inflammatory mediators on ASM, and by the obstruction caused by airway mucus.
This is because β2AR is turned on (constitutively active). Under this condition, endogenous catecholamines, or inhaled β-agonists, have a permissive effect on airway inflammation.
Inflammatory mediators cause airway smooth muscle (ASM) contraction and promote airway mucus secretion that increases airway resistance
●β2AR inverse agonists can switch β2AR “off” and inhibit all β2AR activities.
●This will result in the inhibition of allergic inflammation with a minor impact on the inhibition of the relaxing effect of β2AR.
●ASM contractile state and airflow are effectively managed by the anti inflammatory effects and inhibition of mucus production.
