FRCA Notes

Receptors

  • A receptor is a (glyco)protein containing a region to which a natural ligand binds specifically, in order to bring about a response
    • Natural ligands may bind to more than one receptor and have different mechanisms of action at each
    • For example, GABA binds to both GABAA (ionotropic) and GABAB (metabotropic) receptors

  • Receptors may be located in different areas, e.g.:
    • Associated with the cell membrane
    • Span or be integral to the cell membrane
    • Present in the membranes of intracellular organelles e.g. endoplasmic reticulum
    • Found in the cytoplasm or nucleus

  • Receptors may be classified as being:
    1. Ionotropic - ion channels which alter ion permeability
    2. Metabotropic - linked to intermediate-messengers which propagate signals
    3. Gene transcription regulators
Receptor Subtype Location Effector Speed Examples
Ligand-gated ion channel Cell membrane Ions Milliseconds nAChR, GABAA
G-protein coupled Cell membrane G-proteins Seconds mAChR, adrenoreceptors
Tyrosine kinase Cell membrane Tyrosine kinase Minutes Insulin
Intracellular Cytoplasm, nucleus Gene transcription Hours - days Steroid, thyroid

  • Ionotropic receptors are part of a membrane-spanning complex of protein subunits that have the potential to form a channel through the membrane
  • Ligand binding causes a conformational change in the protein, allowing a channel to open
  • Once opened, they allow passage of ions down concentration and/or electrical gradients
  • Important examples of ionotropic receptors include:
    • Pentameric
    • Ionotropic glutamate
    • Ionotropic purinergic

Pentameric

  • As their name suggests, pentameric ionotropic receptors have five membrane-spanning subunits
  • Key receptors of this subtype include:
    • Nicotinic AChR - binding of two acetylcholine molecules to the two ɑ-subunits opens an ion channel that permits passage of Na+, causing membrane depolarisation
    • GABAA receptor - GABA is the natural ligand and opens an ion channel that permits passage of Cl-
    • 5-HT3 receptor - the only serotonin receptor to act through ion-channel opening

Ionotropic glutamate

  • Glutamate is an excitatory neurotransmitter and acts at several receptor types
  • The NMDA, AMPA and Kainate receptors are all ligand-gated ion channels

  • The NMDA receptor contains two subunits
    • NR1 → pore-forming
    • NR2 → regulatory, binds the co-activator glycine
  • It has high permeability to Ca2+, although does have permeability to Na+ and K+

Iontropic purinergic

  • Includes the PX1 and PX2 receptors
  • Form cationic channels that are permeable to Na+ and K+ (and to a lesser extent Ca2+)

  • These receptors are membrane-bound systems that transduce a ligand-generated signal into an intracellular signal
  • Metabotropic receptors can be classified as:
    1. G-protein coupled receptors
    2. Tyrosine kinase linked receptors
    3. Guanyl cyclase systems

G-protein coupled receptors

  • G-protein coupled receptors are membrane-bound, serpentine proteins consisting of seven helical regions
  • They are associated with G-proteins; heterotrimeric proteins which act as universal transducers in bringing about intracellular change from extracellular stimulus
  • In addition to transducing the signal, the G-protein system amplifies the signal
    • One receptor can stimulate multiple G-proteins (ratio thought to be 1:100)
    • One G-protein can activate several intermediate messengers

    G-protein coupled receptor Mechanism of action
    • An extracellular ligand binds the receptor, inducing conformational change
    • This increases the chance of coupling with G-proteins
    • G-proteins are bound by GDP at rest, but once coupled (activated) by the receptor the ɑ-subunit binds GTP
    • The ɑ-GTP complex then acts on effector proteins or ion channels, for example:
      • Activation of adenylyl cyclase - Gs ɑ-subunits (e.g. β-adrenoreceptors)
      • Inhibition of adenylyl cyclase - Gi ɑ-subunits (e.g. ɑi - adrenoreceptors)
      • Activation of phospholipase C - Gq ɑ-subunits (e.g. ɑ1 - adrenoreceptors, M1/3/5 receptors, AT2 receptors)
      • Inhibition of N-type Ca2+ channels - morphine acting on opioid receptors
    • The ɑ-subunit acts as a GTPase by performing these actions, forming GDP and combining with the β/Ɣ subunits to reform an inactive G-protein

    Adenylyl cyclase Adenylyl cyclase
    • Adenylyl cyclase is a membrane-bound enzyme, although is also found in the cytoplasm
    • It catalyses the formation of cAMP from ATP

    Cyclic AMP (cAMP)
    • cAMP activates protein kinase A, which is responsible for subsequent biochemical effects
    • cAMP is broken down by phosphodiesterases (PDE, of which there are five subtypes)
    • Inhibition of PDE can therefore increase intracellular cAMP and exert biochemical effects
    • Examples of PDE inhibition include:
      • Non-specific inhibition e.g. theophylline
      • PDE-III specific inhibition e.g. milrinone
      • PDE-V specific inhibition e.g. sildenafil

    Phospholipase C
    • G-protein coupled receptors associated with Gq ɑ-subunits do not act via the adenylyl cyclase pathway
    • They instead activate the enzyme phospholipase C (PLC), which cleaves the phospholipid PIP2 into two messengers:
      1. Inositol triphosphate (IP3), which causes calcium release from the endoplasmic reticulum
      2. Diacylglycerol (DAG), which activates protein kinase C

Tyrosine kinase

  • Examples of tyrosine kinase receptors include the insulin receptor and the growth factor receptor
  • The insulin receptor has two alpha and two beta subunits
    • Insulin binds to the alpha subunits
    • The beta subunits are membrane-spanning
    • When insulin binds, intracellular tyrosine residues on its beta-subunits are phosphorylated and become active tyrosine kinases

Guanyl cyclase

  • Atrial natriuretic peptide exerts its action via membrane-bound receptors that have intrinsic guanylyl cyclase activity
  • This increases cGMP levels and acts as a second messenger by phosphorylation of intracellular messengers

  • Nitric oxide (NO) exerts its effects by stimulating cytosolic guanylyl cyclase, thus increasing intracellular cGMP

  • Steroid hormones and thyroid hormones act through intracellular receptors to alter RNA and DNA expression
  • They indirectly alter production of cellular proteins, and therefore have a much slower effect
  • Their receptors are cytoplasmic ligand-regulated transcription factors
  • Binding of the ligand (hormone) induces a conformational change, activating the receptor and permitting translocation to the nucleolus

  • Examples of receptors which act as regulators of gene transcription include:
    • Steroid hormone receptors
    • Thyroid hormone receptors
    • Nuclear peroxisome proliferator-activated receptor (to which pioglitazone binds)

Adrenosteroid hormones receptors

  • Glucocorticoid receptor (GR) - widespread in cells

  • Mineralocorticoid receptor (MR) - restricted to epithelial tissue in the renal tubules and the colon
    • These cells also contain GR
    • Selective MR receptor activation occurs due to the action of 11-β-hydroxysteroid dehydrogenase
    • This enzyme converts cortisol to cortisone - cortisone is inactive at the GR receptor