Receptors

Question 1

Which families are receptors categorized in?

Answer 1

Receptors have been divided into four major superfamilies:
- G protein-coupled receptors
- Ligand-gated ion channels
- Tyrosine kinase receptors
- Nuclear receptors

Question 2

How are G protein-coupled receptors subdivided?

Answer 2

Subdivision based on their amino-acid sequence homology and orthosteric binding site:

• Class A:
  Binding in the TM region
  (acetylcholine, histamine, dopamine, serotonin, opioid and cannabinoid GPCRs)
• Class B:
  Binding in both the extracellular loops and amino-terminal domain
  (glucagon and GLP-1 GPCRs)
• Class C:
  Binding exclusively in the extracellular amino-terminal domain
  (glutamate and GABA GPCRs)

Question 3

Describe the structure of G protein-coupled receptors

Answer 3

• GPCRs have seven α-helical transmembrane segments, and thus often referred to as the seven transmembrane (7TM) receptors.
• Intracellular loops of GPCRs interact with G proteins.
• The G protein is trimeric consisting of G α, G β, and G γ subunits.
  ___________________________________
• Recent evidence has shown that some if not all class C GPCRs exist as dimeric or even oligomeric complexes – meaning they form complexes with other GPCRs.
  o Dimeric and oligomeric complexes can be either homo- or heteromeric.
   GABAB receptors are formed by heterodimerization of GABAB1 and GABAB2 receptor subunits.
   mGluRs homodimerize.

Question 4

What is the activation process / mechanism of action for GPCRs?

Answer 4

1) Receptor activation upon binding causes an interaction of the receptor with the trimeric G protein, catalyzing an exchange of GDP for GTP in the G α subunit whereupon the G protein disassociates into activated G α and G βγ subunits.
2) Both activated G α and G βγ subunits will activate effector molecules such as adenylate cyclase (AC) or ion channels.

3) Most GPCRs desensitize quickly upon activation via phosphorylation of specific serine/ threonine residues in the intracellular loops and/or C-terminal by kinases such as G protein-coupled receptor kinases (GRKs).

4) Once phosphorylated, β-arrestin molecules will bind to the receptor and cause arrest of the G protein-mediated signaling and induce internalization.

Question 5

How are Ligand-Gated Ion Channel Receptors subdivided?

Answer 5

Ligand-Gated ion channels receptors can be divided into three major groups:

• Cys-loop
  Either excitatory (serotonin and nicotinic acetylcholine receptors) by influx of Na+/Ca2+ or inhibitory (glycine and GABA receptors) by influx of Cl-.
• Ionotropic glutamate receptors
  Excitatory cation-permeable
• Purinergic P2X families
  Excitatory cation-permeable

Question 6

Describe the general structure of Cys-Loop channels

Answer 6

• Comprises two α1 subunits and three other subunits (β1, γ and δ), forming a pentameric pore in the cell membrane.
• Agonist binding site is located in subunit interfaces in the extracellular domain.
• The pore is lined with five α-helices (M2), one from each of the five receptor subunits.
  o The M2 helices contain hydrophobic leucine and valine residues, forming a “gate” that regulates the passage of hydrophilic ions.

Question 7

What is the activation process / mechanism of action for Cys-Loop channels?

Answer 7

• Agonist binding to the α-subunits in the extracellular domain triggers conformational changes.
• These conformational changes are transmitted through the receptor subunits.
• The process results in the tilting of the pore-lining Μ2 helices.
• This tilt causes the removal of the hydrophobic gate-lock and ultimately leads to channel opening, allowing the flow of ions.

Question 8

What receptors does the Ionotropic Glutamate Receptor Family consist of?

Answer 8

Comprises 16 known receptors, including NMDA (GluN), AMPA (GluA), and kainic acid receptors (GluK), as well as two orphan receptors termed δ1–2 with unknown functions.

– Notably, some receptors in this family, such as GluN1 and GluN3A-B, are activated by glycine, despite the family name mentioning glutamate.

Question 9

Describe the general structure of the Ionotropic Glutamate Receptor Family

Answer 9

All 18 receptor subunits share a common overall structure, consisting of four main domains:
- (ATD) amino-terminal domain,
- (LBD) ligand-binding domain,
- (TMD) transmembrane domain consisting of three transmembrane segments and a re-entry loop,
- (CTD) C-terminal domain

The ATD/LBD of glutamate receptor subunits is structurally similar to the amino-terminal domain of mGluRs and potassium channels and the TMD of glutamate receptors is structurally similar to the transmembrane domain of potassium channels.

Question 10

How are the Ionotropic Glutamate Receptor Family functionally assembled?

Answer 10

• Functional ionotropic glutamate receptors are tetramers, meaning they are comprised of four subunits.
• These subunits assemble around the ion channel, which is lined by M3 helices from each subunit.
• NMDA receptors are heteromeric assemblies, consisting of GluN1 together with either GluN2 and/or GluN3 subunits.
• AMPA and kainic acid receptors can be either homo- (identical subunits) or heteromeric (different subunits) assemblies.

Question 11

What is the activation process / mechanism of action for Ionotropic Glutamate Receptor Family?

Answer 11

Activation of ionotropic glutamate receptors is initiated by the closure of the LBD around the ligand (glutamate or glycine).

This closure leads to an outward pull of the M3 gating helices, causing an opening of the channel pore.