In a study published online in PNAS on April 7, a research team led by Prof. ZHANG Haibing from the Shanghai Institute of Nutrition and Health (SINH) of the Chinese Academy of Sciences uncovered a novel mechanism by which receptor-interacting protein kinase 1 (RIPK1) ubiquitination regulates both cell death and inflammation.

RIPK1 is a key molecular hub that integrates cell death and inflammatory signaling. Its activity is tightly controlled by post-translational modifications. Among them, K63-linked ubiquitination at lysine 376 (K376) has been shown to suppress apoptosis and necroptosis. However, its role in regulating inflammation has remained unclear.

Human genetic studies have indicated that dysregulation of RIPK1 is closely associated with autoinflammatory diseases: loss-of-function mutations cause immunodeficiency, cleavage-resistant variants lead to dominant autoinflammatory syndromes, and recent identification of ubiquitination-defective variants (K377E/R390G) suggests a direct link between impaired ubiquitination and systemic inflammation.

In this study, researchers generated multiple genetically engineered mouse models. Based on their previous findings which showed that K376 ubiquitination is essential for normal development, they introduced a kinase-inactive mutation (D138N) into Ripk1^K376R mice.

Researchers found that loss of K376 ubiquitination led to aberrant RIPK1 activation, causing excessive cell death and embryonic lethality via a kinase-dependent mechanism. Notably, inactivation of RIPK1 kinase activity rescued embryonic lethality but resulted in progressive systemic inflammation in adult mice, including skin and liver inflammation, splenomegaly, and immune dysregulation.

Mechanistically, researchers demonstrated that the RIPK1 K376R mutation not only affected its kinase activity but also promoted inflammation through its scaffold function. Specifically, this mutation triggered intrinsic activation of the NLRP3 inflammasome, leading to increased IL-1β secretion.

Genetic analyses revealed that deletion of Caspase-1/11 significantly alleviated inflammation, while TRIF deficiency had no effect, highlighting the central role of inflammasome signaling. Moreover, this inflammatory process was found to be independent of MLKL-mediated necroptosis but dependent on RIPK3 and Caspase-1/11, revealing a noncanonical inflammatory pathway.

This work elucidates how RIPK1 ubiquitination governs the balance between cell death and inflammation, and uncovers a dual regulatory mechanism involving kinase-dependent cell death and kinase-independent inflammatory signaling. It provides mechanistic insights into how RIPK1 signaling drives autoinflammation, and offers potential diagnostic markers and therapeutic targets for systemic autoinflammatory diseases.