PPP6C Negatively Regulates STING-Dependent Innate Immune Responses

Area of Science:

• Immunology
• Molecular Biology
• Virology

Background:

• The cyclic GMP-AMP synthase (cGAS)-Stimulator of interferon genes (STING) pathway is crucial for detecting cytosolic DNA and initiating innate immune responses against pathogens.
• STING protein activity is tightly regulated by posttranslational modifications, particularly phosphorylation, but the precise roles of kinases and phosphatases remain incompletely understood.

Purpose of the Study:

• To elucidate the role of protein phosphatase 6 catalytic subunit (PPP6C) in regulating the cGAS-STING pathway.
• To investigate the interaction between PPP6C and STING and its impact on STING phosphorylation and activation.
• To understand the implications of PPP6C-mediated regulation of STING in antiviral immunity and autoinflammatory diseases.

Main Methods:

• Identification of PPP6C as a binding partner of Kaposi's sarcoma-associated herpesvirus (KSHV) ORF48, a negative regulator of the cGAS-STING pathway.
• Assessment of innate immune responses upon PPP6C depletion using double-stranded DNA (dsDNA) and double-stranded RNA (dsRNA) stimuli.
• Analysis of viral replication (HSV-1, VSV, KSHV) and type I interferon production in PPP6C-deficient cells.
• Co-immunoprecipitation assays to confirm the interaction between PPP6C and STING and to evaluate STING phosphorylation levels.

Main Results:

• Depletion of PPP6C enhanced dsDNA- and 5'ppp dsRNA-induced innate immune responses, specifically IRF3 activation, but not NF-κB activation.
• PPP6C deficiency significantly inhibited the replication of herpes simplex virus 1 (HSV-1), vesicular stomatitis virus (VSV), and KSHV reactivation, correlating with increased type I interferon production.
• PPP6C was shown to interact with STING, and its absence led to enhanced STING phosphorylation, indicating PPP6C acts as a negative regulator by dephosphorylating STING.

Conclusions:

• PPP6C negatively regulates the cGAS-STING pathway by dephosphorylating STING, thereby controlling innate immune activation.
• This regulatory mechanism is important for limiting host responses to viral infections and preventing excessive inflammation.
• Understanding PPP6C's role in STING dephosphorylation offers insights into potential therapeutic strategies for autoinflammatory disorders linked to chronic STING activation.