Biochemical basis for therapy: receptors and signalling

Receptors can be categorised into 4 classes:-

1. Ligand gated ion channel (ionotropic receptors)
2. G protein coupled receptors (metabotropic)
3. Kinase linked receptors
4. Nuclear receptors

The agonist (could be a neurotransmitter, drug, hormone) binds to the ligand gated ion channel receptor which opens the pore of the channel by moving the protein subunits to widen the gap. This allows the ions to go in or out of the cell. The response occurs in milliseconds.

Nicotinic acetylcholine receptors are membrane bound ligand gated ion channels. The acetylcholine neurotransmitter is bound in vesicles which activate the nicotinic acetylcholine receptors on the post synaptic receptors which allow the Na+ to go in and cause depolarisation and generate action potential (excitation).

1. Acetylcholine- full agonist at nAChR- useful in cataract surgery
2. Nicotine- Full agonist at nAChR- delivery of nicotine via controlled release is useful for smoking cessation
3. Varenicline- inhibits the binding of nicotine to the brain nAChR and exerts partial agonist activity at the receptor. Useful in easing nicotine withdrawal symptoms.

G proteins coupled receptors have 7 transmembrane alpha helices. They are bound to G proteins which are found in a heterotrimeric form (alpha, beta and gamma subunits). In its inactive form, it is bound to a GDP (guanosine diphosphate) molecule.

1. The ligand binds to the GPCR
2. The GPCR undergoes a conformational change
3. The alpha subunit exchanges the GDP molecule for the GTP (guanosine triphosphate).
4. The alpha subunit dissociates and moves away from the beta gamma dimer and regulates target proteins.
5. Once the alpha subunit activates the target protein, it relays a signal via a 2nd messenger.
6. To return to normal, the GTP gets hydrolysed to GDP.

Adrenaline binds to adrenoreceptors using the G protein coupled receptors mechanism.

1. The adrenaline molecule binds with the adrenoreceptor. (which is a GPCR)
2. The adrenoreceptor undergoes a conformational change.
3. The GDP molecule bound to the G protein trimer alpha subunit gets exchanged for a GTP molecule.
4. The G alpha subunit dissociates from the beta gamma dimer and associates with the adenylyl cyclase protein.
5. The adenylyl cyclase turns the ATP into cAMP (cyclic monophosphate) which is the 2nd messenger.
6. The cAMP induces the effects like blood vessel dilation, increases heart rate and converts glycogen to glucose.

The adrenaline leads to the activation of the phospholipase C protein which converts PIP2 and IP3 causing the increase in intracellular calcium ions. This causes excitation and stimulation of MLCK activity. This causes vasoconstriction.

The adrenaline leads to the inhibition of the protein adenylyl cylclase and inhibits the formation of cAMP. It also leads to a reduced amount of intracellular calcium, which decreases neurotransmitter release and vasodilation.

The activation of these receptors leads to presynaptic inhibition of noradrenaline in the CNS and relaxation of the GI tract.

Blood vessels with α1 receptors are present in the skin, the sphincters of gastrointestinal system, kidney (renal artery) and brain. These tissues are not important in a fight or flight response.

Activation of alpha 2 adrenoreceptors and inhibition of adenylyl cyclase causes the presynaptic inhibition of noradrenaline in the CNS which causes a relaxation of the GI tract.

Increased heart rate and cardiac muscle contraction.

Dilation of the bronchi, increased heart rate, cardiac muscle contraction (lesser extent to beta 1)

Thermogenesis in skeletal muscles, lipolysis

Due to receptor downregulation and prolonged contact with an agonist (salbutamol), a drug tolerance is built up. Thus, another signalling pathway must be identified for treatment for chronic asthma.

Since salbutamol cannot be used due to receptor down regulation in the case of chronic asthma, another enzyme called phosphodiesterase must be identified. Phosphodiesterase is an enzyme that works to hydrolyse the GTP into GDP and terminates the action of the cAMP to stop the action. In order to maintain the bronchodilator for a longer time (in a chronic condition), the cAMP can be activated for a long time.

Theophylline. It is indicated for conditions like chronic asthma in COPD

Seconds

Kinase linked receptors consists of the ligand binding domain (extracellular) and the functional enzymatic domain.

1. The extracellular signal binding with the ligand binding site induces the two nearby Receptor Tyrosine Kinases to associate to form a cross linked dimer.
2. The tyrosine molecules bound to the RTK cause the ATP to become ADP and pick up the Phosphorus. Both RTKs work together to perform cross-phosphorylation.
3. The enzymatic section of RTKs allow different proteins to dock on the phosphorylated tyrosines.
4. This induces signal transduction which regulation of gene transcription.
5. It can cause several different and parallel signalling cascades.

1. The insulin molecule which is an extracellular signal binds with the ligand binding domain.
2. This brings the two receptor tyrosine kinases to associate.
3. The ATP convert into ADP and the tyrosine molecules on the receptor together perform cross phosphorylation.
4. The phosphorylated tyrosine can now act as docking regions for proteins to associate with and induce cellular responses.
5. This is going to activate different and parallel signalling cascades.

1. Recruitment of glucose transporters
2. Increased formation of glycogen
3. Increased formation of fat
4. Changes in gene expression
5. Increased formation of protein
6. Decreased formation of glucose from glycogen.

A few hours

There are 3 domains in the nuclear receptors:-

1. Ligand binding domain
2. DNA binding domain
3. Transactivation domain- this brings about changes in the DNA conformation to initiate transcription.

1. The binding of a ligand with the ligand binding domain activates two receptors in a pair and they enter the nucleus.
2. The pair binds with a coregulator (coactivator or corepressor)
3. The complex attaches to a specific gene (like Glucocorticoid Responsive element)
4. This binding facilitates or suppresses gene expression. If it is facilitative, mRNA is produced which goes to the cytoplasm and produces proteins.

1. Receptors
2. Enzymes
3. Ion channels
4. Carrier proteins