Modulation of histamine H(3) receptor function by monovalent ions

Area of Science:

• Biochemistry
• Pharmacology
• Molecular Biology

Background:

• Monovalent ions differentially modulate G protein-coupled receptor (GPCR) ligand binding through poorly understood mechanisms.
• Sodium chloride (NaCl) typically decreases agonist affinity while increasing antagonist affinity at GPCRs.
• The human histamine H(3) receptor (hH(3)R) is a key target for neurological and psychiatric disorders.

Purpose of the Study:

• To investigate the impact of various monovalent ions on the human histamine H(3) receptor (hH(3)R).
• To elucidate the role of specific residues, such as Asp80, in hH(3)R signaling and G protein activation.
• To understand the mechanisms by which monovalent ions affect hH(3)R function.

Main Methods:

• Co-expression of hH(3)R with different mammalian G proteins (Galpha(i1), Galpha(i2), Galpha(i3), Galpha(o1), beta(1)gamma(2)) in Sf9 insect cell membranes.
• Measurement of agonist binding using [(3)H]N(alpha)-methylhistamine ([(3)H]NAMH).
• Assessment of G protein activation via steady-state GTPase assays and site-directed mutagenesis (D2.50N-hH(3)R).

Main Results:

• NaCl (100mM) did not affect [(3)H]NAMH binding affinity but altered agonist and inverse agonist potency in GTPase assays.
• Monovalent ions inhibited hH(3)R signaling efficacy in the order Li(+) ≈ Na(+) ≈ K(+) < Cl(-) < Br(-) < I(-).
• The mutation D2.50N-hH(3)R abolished Galpha(i3) activation, highlighting Asp80 as critical for hH(3)R/Galpha(i3) coupling.

Conclusions:

• Monovalent ions and the D2.50N mutation reduce hH(3)R interaction with G proteins.
• Monovalent anions may indirectly affect hH(3)R signaling by modulating G protein GDP affinity.
• Asp80 is essential for hH(3)R/Galpha(i3) activation, suggesting distinct receptor-G protein interfaces even among related subunits.