ABSTRACT
TRADD is an adaptor for TNFR1-induced apoptosis and NFκB activation. However, TRADD-deficient mice undergo normal development and contain normal lymphoid populations, which contrasts with an embryonic defect in mice lacking FADD, the shared adaptor mediating apoptosis. Recent studies indicate FADD suppresses embryonic necroptosis mediated by RIPK1. TRADD was suggested to also mediate necroptosis. Here we report that targeting TRADD fails to rescue Fadd-/- embryos from necroptosis, and ablation of TRADD rescues Ripk1-/- mice from perinatal lethality when RIPK3-mediated necroptosis is disabled. The resulting Ripk1-/-Ripk3-/-Tradd-/- mice survive until early adulthood, but die thereafter. A single allele of Tradd is optimal for survival of Ripk1-/-Ripk3-/-Tradd+/- mice. We show that TRADD plays a more dominating role in NFκB-signaling than RIPK1. While RIPK1 protects thymocytes from TNFα-induced apoptosis, TRADD promotes this process. The data demonstrate that TRADD is critical in perinatal and adult mice lacking RIPK1 and RIPK3, which has not been appreciated in prior studies.
Subject(s)
Receptor-Interacting Protein Serine-Threonine Kinases/metabolism , TNF Receptor-Associated Death Domain Protein/metabolism , Animals , Apoptosis/drug effects , Caspase 8/genetics , Caspase 8/metabolism , Cell Death , Cell Proliferation/drug effects , Fas-Associated Death Domain Protein/metabolism , Fibroblasts , Gene Deletion , Gene Expression Regulation , Intestines/pathology , Mice , Mice, Inbred C57BL , Mice, Knockout , NF-kappa B , Necrosis , Receptor-Interacting Protein Serine-Threonine Kinases/genetics , Receptor-Interacting Protein Serine-Threonine Kinases/pharmacology , Signal Transduction , Survival Analysis , T-Lymphocytes/drug effects , T-Lymphocytes/metabolism , TNF Receptor-Associated Death Domain Protein/genetics , TNF Receptor-Associated Death Domain Protein/pharmacology , Thymocytes/drug effects , Transcriptome , Tumor Necrosis Factor-alphaABSTRACT
While a number of studies have documented the neurotropism of Japanese encephalitis virus (JEV), little is known regarding the molecular mechanism of neuronal death following viral infection. The tumor necrosis factor receptor (TNFR)-associated death domain (TRADD) has been suggested to be the crucial signal adaptor that mediates all intracellular responses from TNFR-1. Using mouse (Neuro2a) and human (SK-N-SH) neuroblastoma cell lines, we have shown that the altered expression of TNFR-1 and TRADD following JEV infection regulates the downstream apoptotic cascades. Activation of TRADD led to mitochondria-mediated neuronal apoptosis. As TRADD-knockout animals or deficient cell lines are unavailable, it has been difficult to definitively address the physiological role of TRADD in diseases pathology following JEV infection. We circumvented this problem by silencing TRADD expression with small-interfering RNA (siRNA) and have found that TRADD is required for TNFR-1-initiated neuronal apoptosis following in vitro infection with JEV. Interestingly, siRNA against TRADD also decreased the viral load in Neuro2a cells. Furthermore, siRNA against TRADD increased the survival of JEV-infected mice by altering the expression of pro apoptotic versus antiapoptotic molecules. These studies show that the engagement of TNFR-1 and TRADD following JEV infection plays a crucial role in neuronal apoptosis.
