Apoptosis and necroptosis are complementary pathways controlled by common signaling adaptors, kinases and proteases; among these, caspase-8 (Casp8) is critical for death receptor (DR)-induced apoptosis. This caspase has also been implicated in nonapoptotic pathways that regulate Fas-associated via death domain (FADD)-dependent signaling and other less defined biological processes as diverse as innate immune signaling and myeloid or lymphoid differentiation patterns 1. Casp8 suppresses RIP3/RIP1 kinase complex-dependent 2–4 necroptosis 5 that follows DR-activation as well as a RIP3-dependent, RIP1-independent necrotic pathway that has emerged as a host defense mechanism against murine cytomegalovirus (MCMV) 6. Disruption of Casp8 expression leads to embryonic lethality in mice between E10.5 and E11.5 7. Thus, Casp8 may naturally hold alternative RIP3-dependent death pathways in check in addition to its role promoting apoptosis. We find that RIP3 is responsible for the midgestational death of Casp8-deficient embryos. Remarkably, Casp8−/−Rip3−/− double mutant mice are viable and mature into fertile adults with a full immune complement of myeloid and lymphoid cell types. These mice appear immunocompetent but develop lymphadenopathy by four months of age marked by accumulation of abnormal T cells in the periphery, a phenotype reminiscent of mice with Fas-deficiency (lpr/lpr). Casp8 contributes to homeostatic control in the adult immune system; however, RIP3 and Casp8 are together completely dispensable for mammalian development.

Several studies have linked cilia and Hedgehog signaling, but the precise roles of ciliary proteins in signal transduction remain enigmatic. Here we describe a mouse mutation, hennin (hnn), that causes coupled defects in cilia structure and Sonic hedgehog (Shh) signaling. The hnn mutant cilia are short with a specific defect in the structure of the ciliary axoneme, and the hnn neural tube shows a Shh-independent expansion of the domain of motor neuron progenitors. The hnn mutation is a null allele of Arl13b, a small GTPase of the Arf/Arl family, and the Arl13b protein is localized to cilia. Double mutant analysis indicates that Gli3 repressor activity is normal in hnn embryos, but Gli activators are constitutively active at low levels. Thus, normal structure of the ciliary axoneme is required for the cell to translate different levels of Shh ligand into differential regulation of the Gli transcription factors that implement Hedgehog signals.

Joubert syndrome (JS) and related disorders are a group of autosomal-recessive conditions sharing the "molar tooth sign" on axial brain MRI, together with cerebellar vermis hypoplasia, ataxia, and psychomotor delay. JS is suggested to be a disorder of cilia function and is part of a spectrum of disorders involving retinal, renal, digital, oral, hepatic, and cerebral organs. We identified mutations in ARL13B in two families with the classical form of JS. ARL13B belongs to the Ras GTPase family, and in other species is required for ciliogenesis, body axis formation, and renal function. The encoded Arl13b protein was expressed in developing murine cerebellum and localized to the cilia in primary neurons. Overexpression of human wild-type but not patient mutant ARL13B rescued the Arl13b scorpion zebrafish mutant. Thus, ARL13B has an evolutionarily conserved role mediating cilia function in multiple organs.

We show Arl13b is localized to the ciliary membrane and regulates tubulin modifications and ciliary length in vitro. Significantly, we found that Smoothened is enriched in Arl13b null fibroblasts, even without Sonic hedgehog stimulation, but that Glis are not similarly enriched.