CB1 receptor coupling to extracellular regulated kinase via multiple Gαi/o isoforms

Area of Science:

• Molecular Pharmacology
• Cell Signaling
• Neuroscience

Background:

• Cannabinoid type 1 receptors (CB1Rs) are critical for numerous physiological processes, including neurotransmission and cell survival.
• CB1R signaling involves pertussis toxin (PTX)-sensitive Gαi/o proteins, leading to extracellular signal-regulated kinase (ERK) pathway activation.
• The specific Gαi/o isoforms mediating CB1R-induced ERK phosphorylation remain largely uncharacterized.

Purpose of the Study:

• To elucidate which specific Gαi/o isoforms are responsible for coupling CB1 receptors to ERK phosphorylation.
• To investigate the differential roles of Gαi/o isoforms in mediating responses to CB1R agonists with varying efficacy.

Main Methods:

• HEK293 cells expressing CB1Rs were utilized.
• Cells were transfected with PTX-insensitive mutants of Gαo, Gαi1, Gαi2, or Gαi3.
• ERK phosphorylation was assessed via Western blotting following treatment with vehicle, CP55940 (full agonist), or delta-9-tetrahydrocannabinol (∆9-THC; partial agonist) after PTX-induced inactivation of endogenous Gαi/o proteins.

Main Results:

• CP55940-induced ERK phosphorylation was abolished by PTX but rescued by expressing PTX-insensitive Gαo, Gαi1, Gαi2, or Gαi3, demonstrating CB1R coupling to all tested isoforms.
• ∆9-THC induced modest ERK phosphorylation, which was statistically significant only in cells expressing PTX-insensitive Gαi3.
• These findings indicate that CB1R can activate ERK through multiple Gαi/o isoforms, with Gαi3 potentially playing a key role in partial agonist signaling.

Conclusions:

• Cannabinoid type 1 receptors couple to ERK phosphorylation via Gαo, Gαi1, Gαi2, and Gαi3 when stimulated by a full agonist (CP55940).
• Partial agonist (∆9-THC)-mediated ERK activation may necessitate substantial expression of the Gαi3 protein.
• This study clarifies the specific G protein-mediated signaling pathways downstream of CB1R activation.