Necroptotic Cell Death Signaling and Execution Pathway: Lessons from Knockout Mice

Table 2

Summary of phenotypes in double- and triple-knockout mice ablated for genes that regulate apoptosis, necroptosis, and inflammation.

Crossed; mice	ripk1−/−; die postnatal; day 1	ripk1−/−; tnfr−/−; die postnatal; day 1	sharpin−/−; viable; die postnatal days 10–14; inflammation	a20−/−; viable; die postnatal; days 7–14; inflammation	cyld−/−; viable; reach adulthood; inflammation
ripk3−/−; casp8−/−; viable	Viable	Viable?	Embryonic; lethal	Embryonic; lethal	?
ripk3−/−; fadd−/−; viable	Viable	Viable?	Embryonic; lethal	Embryonic; lethal	?
ripk3−/−; tnfr−/−; viable	Reach adulthood	Viable?	Prevent; inflammation; skin dermatitis	Prevent; inflammation; Cachexia	?
ifnar−/−; viable	Die postnatal; day 1	Die later than ripk1−/−; tnfr1−/−	?	?	?
trif−/−; viable	Die postnatal; day 1	Die later than ripk1−/−; tnfr1−/−	?	?	?
ripk3−/−; mlkl−/−; die postnatal day 4	Die postnatal; day 4	Die later than; ripk1−/−; tnfr1−/−	Prevent; systemic; inflammation	?	?

Summary of phenotypes for double- and triple-knockout mice models of apoptosis, necroptosis, and inflammation genes. Knockout mice for RIPK1, Ripk1−/−, die at birth of systemic inflammation whereas Ripk3−/− mice are normal but are resistant to proinflammatory stimuli. Mlkl−/− mice are anatomically normal, viable, and fertile. Triple-knockout mice Fadd−/− Flip−/− Ripk3−/− have a normal cell-death pathway and develop to normal birth because of absence of necrosis and apoptosis which are modulated by caspase-8. Casp8−/− Mlkl−/− double knockout mice are normal and resistant to TNF-induced necroptosis. Deletion of Tnfr gene provides protection from Ripk1−/− perinatal lethality and double KO mice Ripk1−/− tnfr−/− can be partially protected from lethality from systemic inflammation by mating these mice with ifnar−/− or trif−/− mice. This indicated that both proteins can engage RIPK3-MLKL interaction independent of RIPK1. A20 and CYLD target similar molecular substrates including TRAF2, TRAF6, NF-κB essential modulator (NEMO), and RIPK1. The deubiquitinase CYLD removes the K63-Ub of RIPK1, and A20 promotes the removal of K63-linked ubiquitin chains to terminate signaling induced NF-κB activation. The ablation of A20 and SHARPIN genes is potentially lethal. A20/TNFAIP3, zinc finger and ubiquitin editing protein, CYLD, cylindromatosis, deubiquitylating enzyme; SHARPIN, a protein that together with HOIL-1 and HOIP forms the LUBAC, the heterotrimeric linear ubiquitin chain assembly complex; both are involved in the TNF signaling pathways; IFNAR, the type I IFN receptor. The question mark indicates possible or still unknown phenotype.