Modulation of soluble guanylate cyclase activity by phosphorylation

Area of Science:

• Gastroenterology
• Molecular Biology
• Biochemistry

Background:

• Cyclic guanosine monophosphate (cGMP) levels in gut smooth muscle are regulated by soluble guanylate cyclase (sGC) synthesis and phosphodiesterase 5 (PDE5) breakdown.
• Soluble guanylate cyclase (sGC) is a heme-containing enzyme activated by nitric oxide (NO), crucial for cGMP generation.

Purpose of the Study:

• To investigate the regulatory mechanisms of protein kinases on soluble guanylate cyclase (sGC) in gastric smooth muscle.
• To elucidate the role of cGMP-dependent protein kinase (PKG) in modulating sGC activity and cGMP levels.

Main Methods:

• Utilized sodium nitroprusside (SNP) as a nitric oxide (NO) donor to stimulate sGC activity and cGMP production.
• Examined sGC phosphorylation in response to SNP using selective PKG inhibitors (Rp-cGMPS, KT-5823) and a cAMP-dependent protein kinase activator (Sp-5, 6-DCl-cBiMPS).
• Investigated the role of phosphatases using purified phosphatase 1 and okadaic acid in vitro and in gastric smooth muscle cells.

Main Results:

• SNP stimulated sGC activity and increased cGMP levels, inducing sGC phosphorylation.
• PKG inhibitors enhanced SNP-stimulated sGC activity and cGMP levels, while abolishing SNP-induced phosphorylation, indicating PKG-mediated inhibition.
• Inhibition of phosphatase activity increased sGC phosphorylation, which in turn inhibited sGC activity and cGMP formation, confirming a feedback loop.

Conclusions:

• PKG-dependent phosphorylation of soluble guanylate cyclase (sGC) acts as a feedback mechanism to inhibit its activity.
• This phosphorylation reduces cyclic guanosine monophosphate (cGMP) formation in gastric smooth muscle.
• The findings highlight a novel regulatory pathway involving protein kinases and phosphatases in controlling sGC function and cGMP signaling.