The Biochemical Role of Protein Phosphatase Pp4 in Regulating Immune Response during Sepsis

The host innate immune system serves as the first line of defense against invading pathogens, such as bacteria and virus, by recognizing pathogen-associated molecular patterns (PAMPs), which activate a family of Toll-like receptors (TLRs). Many of the TLRs signal through TRAF6 to activate downstream kinases and eventually transcription of immune mediators. According to our previous discovery, somatic Nuclear Autoantigenic Sperm Protein (sNASP) is novel checkpoint for TRAF6 signaling which is a critical control of TLR and is required for maintaining homeostasis of the innate immune response. In unstimulated macrophages, sNASP binds to TRAF6 to prevent TRAF6 auto-activation in the cytosol. Following LPS stimulation, a complex consisting of sNASP, TRAF6, IRAK4, and casein kinase 2 (CK2) is formed. The sNASP is phosphorylated by CK2 and released from TRAF6 to allow downstream NF-κB signaling and IL6/TNF cytokines production for against bacterial invasion. Using a dominant-negative sNASP knockin mouse model, we showed that the inability to release sNASP from TRAF6 prevented mice from mounting an effective immune response during sepsis. Therefore, phosphorylated sNASP is a key factor for the activation of the TLR-TRAF6 signaling pathway. TLR signaling is tightly regulated to maintain immune homeostasis because hyperactivation or hypoactivation of TLR signaling causes human diseases. However, the mechanism of phosphatase regulates phosphorylated sNASP to avoid excessive activation of the TLR-TRAF6 signaling pathway is still unclear. According to preliminary experimental results, protein phosphatase PP4 can decrease sNASP phosphorylation through directly binding to phosphorylated sNASP. In addition, IL6 production was found diminished when PP4 overexpression, indicating that PP4 is involved in the regulation of innate immune response through sNASP. In this project, we would like to further demonstrate the mechanism and physiologic effects of PP4 on the innate immunity. The specific aims of the proposal are:Aim 1: To explore the mechanism of Protein Phosphatase PP4 in innate immune system Aim 2: To explore the role of Protein Phosphatase PP4 in innate immune responsesAim 3: To study physiological immune response in myelomonoytic cell -specific pp4 knockout mouse model following by sepsis challenge