MDA5 ISGylation is crucial for immune signaling to control viral replication and pathogenesis

Area of Science:

• Immunology
• Virology
• Molecular Biology

Background:

• Posttranslational modifications (PTMs) like ISGylation regulate innate immune sensors.
• Melanoma differentiation-associated protein 5 (MDA5) signaling, crucial for antiviral responses, is modulated by ISGylation within its CARD domain.
• The in vivo significance of MDA5 ISGylation for antiviral immunity remained unclear.

Purpose of the Study:

• To investigate the role of MDA5 ISGylation in antiviral immunity using a genetically modified mouse model.
• To elucidate the molecular mechanisms linking MDA5 ISGylation to its signaling and antiviral function.
• To identify the E3 ligases responsible for MDA5 ISGylation.

Main Methods:

• Generation of knock-in mice (MDA5^K23R/K43R) with mutated ISGylation sites (K23, K43).
• Analysis of primary cells and whole mice infected with RNA viruses (EMCV, WNV).
• Assessment of MDA5 oligomerization, cytokine responses, viral load, and mortality.
• Identification of E3 ligases using molecular approaches.

Main Results:

• MDA5^K23R/K43R cells showed abrogated ISGylation and impaired MDA5 assembly, resulting in blunted cytokine responses.
• MDA5^K23R/K43R mice infected with EMCV exhibited increased mortality, viral titers, and myocardial injury, similar to MDA5^-/- mice.
• HERC5 and HERC6 were identified as the primary E3 ligases mediating MDA5 ISGylation.

Conclusions:

• CARD ISGylation is essential for MDA5-mediated restriction of RNA viruses.
• This study establishes MDA5 ISGylation as a critical component of the innate antiviral immune response.
• Findings suggest potential for immunomodulatory drug development targeting MDA5 ISGylation for antiviral therapies.