Flexible Usage and Interconnectivity of Diverse Cell Death Pathways Protect against Intracellular Infection.

Programmed cell death contributes to host defense against pathogens. To investigate the relative importance of pyroptosis, necroptosis, and apoptosis during Salmonella infection, we infected mice and macrophages deficient for diverse combinations of caspases-1, -11, -12, and -8 and receptor interacting serine/threonine kinase 3 (RIPK3). Loss of pyroptosis, caspase-8-driven apoptosis, or necroptosis had minor impact on Salmonella control. However, combined deficiency of these cell death pathways caused loss of bacterial control in mice and their macrophages, demonstrating that host defense can employ varying components of several cell death pathways to limit intracellular infections. This flexible use of distinct cell death pathways involved extensive cross-talk between initiators and effectors of pyroptosis and apoptosis, where initiator caspases-1 and -8 also functioned as executioners when all known effectors of cell death were absent. These findings uncover a highly coordinated and flexible cell death system with in-built fail-safe processes that protect the host from intracellular infections.

The Role of Caspase-12 in Retinal Bystander Cell Death and Innate Immune Responses against MCMV Retinitis.

(1) Background: caspase-12 is activated during cytomegalovirus retinitis, although its role is presently unclear. (2) Methods: caspase-12-/- (KO) or caspase-12+/+ (WT) mice were immunosup eyes were analyzed by plaque assay, TUNEL assay, immunohistochemical staining, western blotting, and real-time PCR. (3) Results: increased retinitis and a more extensive virus spread were detected in the retina of infected eyes of KO mice compared to WT mice at day 14 p.i. Compared to MCMV injected WT eyes, mRNA levels of interferons α, ß and γ were significantly reduced in the neural retina of MCMV-infected KO eyes at day 14 p.i. Although similar numbers of MCMV infected cells, similar virus titers and similar numbers of TUNEL-staining cells were detected in injected eyes of both KO and WT mice at days 7 and 10 p.i., significantly lower amounts of cleaved caspase-3 and p53 protein were detected in infected eyes of KO mice at both time points. (4) Conclusions: caspase-12 contributes to caspase-3-dependent and independent retinal bystander cell death during MCMV retinitis and may also play an important role in innate immunity against virus infection of the retina.

Interleukin-18 in intestinal inflammation: friend and foe?

Genetic studies and other experimental data have linked inflammatory bowel diseases with inflammasome activation. In this issue of Immunity, Zaki et al. (2010) and Dupaul-Chicoine et al. (2010) provide a detailed characterization of the regulatory task of the inflammasome in intestinal epithelial cells.

Control of intestinal homeostasis, colitis, and colitis-associated colorectal cancer by the inflammatory caspases.

Inflammatory caspases are essential effectors of inflammation and cell death. Here, we investigated their roles in colitis and colorectal cancer and report a bimodal regulation of intestinal homeostasis, inflammation and tumorigenesis by caspases-1 and -12. Casp1(-/-) mice exhibited defects in mucosal tissue repair and succumbed rapidly after dextran sulfate sodium administration. This phenotype was rescued by administration of exogenous interleukin-18 and was partially reproduced in mice deficient in the inflammasome adaptor ASC. Casp12(-/-) mice, in which the inflammasome is derepressed, were resistant to acute colitis and showed signs of enhanced repair. Together with their increased inflammatory response, the enhanced repair response of Casp12(-/-) mice rendered them more susceptible to colorectal cancer induced by azoxymethane (AOM)+DSS. Taken together, our results indicate that the inflammatory caspases are critical in the induction of inflammation in the gut after injury, which is necessary for tissue repair and maintenance of immune tolerance.

Caspase-12 deficiency enhances cytokine responses but does not protect against lethal Plasmodium yoelii 17XL infection.

To investigate the effect of caspase-12 deficiency on IFN-γ- independent control of blood-stage malaria, we compared lethal Plasmodium yoelii 17XL infection in wild-type C57BL / 6J and caspase-12-/-mice. Infected caspase-12-/- mice exhibited higher parasitaemia than WT mice on days 8 and 9 post-inoculation, but all WT and caspase-12-/- mice succumbed by day 10. In addition, infected caspase-12-/-mice had significantly elevated levels of IFN-γ, TNF, IL-18,and IL-10 in sera compared to infected WT mice. At the terminal stage of disease, there were no differences in cytokine levels in the tissues of infected WT and caspase-12-/- mice. However, liver pathology was more severe in infected caspase-12-/- mice compared to infected WT mice. Together, these findings indicate that although caspase-12 deficiency results in enhanced pro-inflammatory and immunoregulatory cytokine levels in sera during P. yoelii 17XL infection, these responses are not essential for protection against lethal malaria infection.

Characterisation of mice lacking the inflammatory caspases-1/11/12 reveals no contribution of caspase-12 to cell death and sepsis.

Caspases exert critical functions in diverse cell death pathways, including apoptosis and pyroptosis, but some caspases also have roles in the processing of cytokines into their functional forms during inflammation. The roles of many caspases have been unravelled by the generation of knockout mice, but still very little is known about the overlapping functions of caspases as only a few studies report on double or triple caspase knockout mice. For example, the functions of caspase-12 in cell death and inflammation, on its own or overlapping with the functions of caspase-1 and caspase-11, are only poorly understood. Therefore, we generated a novel mutant mouse strain lacking all three inflammatory caspases, caspases-1, -11 and -12. Analysis under steady state conditions showed no obvious differences between caspase-1/11/12-/- and wildtype (WT) mice. Since caspases-1 and -11 are involved in endotoxic shock, we analysed the response of caspase-1/11/12-/- mice to high-dose LPS injection. Interestingly, we could not detect any differences in responses between caspase-1/11/12-/- mice vs. caspase-1/11 double knockout mice. Furthermore, cell lines generated from caspase-1/11/12-/- mice showed no differences in their apoptotic or necroptotic responses to a diverse set of cytotoxic drugs in vitro when compared to WT cells. Importantly, these drugs also included ER stress-inducing agents, such as thapsigargin and tunicamycin, a form of cell death for which a critical pro-apoptotic function of caspase-12 has previously been reported. Additionally, we found no differences between caspase-1/11/12-/- and WT mice in their in vivo responses to the ER stress-inducing agent, tunicamycin. Collectively, these findings reveal that caspase-12 does not have readily recognisable overlapping roles with caspases-1 and -11 in the inflammatory response induced by LPS and in necroptosis and apoptosis induced by diverse cytotoxic agents, including the ones that elicit ER stress.

Caspase-12 mediates carbon tetrachloride-induced hepatocyte apoptosis in mice.

AIM: To investigate the role of caspase-12 and its downstream targets in carbon tetrachloride (CCl4)-induced hepatocyte apoptosis. METHODS: The role of caspase-12 was determined by using caspase-12 knock-out ((-/-)) mice. CCl4 (300 µL/kg body weight) or vehicle (corn oil) was administered to caspase-12(+/+) or caspase-12(-/-) mice as a single intraperitoneal injection. The animals were sacrificed 24 h after the CCl4 treatment. Blood was collected to evaluate liver function by the measurement of the activity of alanine aminotransferase. Liver samples were used for the measurements of reactive oxygen species using plasma malondialdehyde as biomarker, hepatocyte apoptosis was evaluated via terminal transferase-mediated dUTP nick-end labeling and controlled by morphologic study, and cytochrome C release and caspase activations were measured by Western blotting. RESULTS: Administration of a low dose of CCl4 resulted in hepatocyte apoptosis and acute liver injury in wild-type mice. CCl4 also induced the generation of reactive oxygen species and induction of endoplasmic reticulum stress in the liver followed by activations of caspase-12, -9 and -3 as well as release of small amounts of cytochrome C. However, in the CCl4-treated caspase-12(-/-) mice, activation of caspase-9 and -3 were significantly attenuated (P < 0.05); no effect was seen in cytochrome C release. CCl4-induced apoptosis and liver damage was markedly reduced in caspase-12(-/-) mice compared to caspase-12(+/+) mice (P < 0.05). The active form of caspase-8 was not detected in either caspase-12(+/+) or caspase-12(-/-) mice. There was no significant different in the formation of reactive oxygen species in the livers of caspase-12(+/+) and caspase-12(-/-) mice treated with CCl4. CONCLUSION: Caspase-12 plays a pivotal role in CCl4-induced hepatic apoptosis through the activation of the downstream effector caspase-3 directly and/or indirectly via caspase-9 activation.

Caspase-12 modulates NOD signaling and regulates antimicrobial peptide production and mucosal immunity.

Bacterial sensing by intracellular Nod proteins and other Nod-like receptors (NLRs) activates signaling pathways that mediate inflammation and pathogen clearance. Nod1 and Nod2 associate with the kinase Rip2 to stimulate NF-kappaB signaling. Other cytosolic NLRs assemble caspase-1-activating multiprotein complexes termed inflammasomes. Caspase-12 modulates the caspase-1 inflammasome, but unlike other NLRs, Nod1 and Nod2 have not been linked to caspases, and mechanisms regulating the Nod-Rip2 complex are less clear. We report that caspase-12 dampens mucosal immunity to bacterial infection independent of its effects on caspase-1. Caspase-12 deficiency enhances production of antimicrobial peptides, cytokines, and chemokines to entric pathogens, an effect dependent on bacterial type III secretion and the Nod pathway. Mechanistically, caspase-12 binds to Rip2, displacing Traf6 from the signaling complex, inhibiting its ubiquitin ligase activity, and blunting NF-kappaB activation. Nod activation and resulting antimicrobial peptide production constitute an early innate defense mechanism, and caspase-12 inhibits this mucosal antimicrobial response.

Minimal role for caspase 12 in the unfolded protein response in oligodendrocytes in vivo.

The unfolded protein response (UPR) is implicated in many neurodegenerative disorders including Alzheimer, Parkinson and prion diseases, and the leukodystrophy, Pelizaeus-Merzbacher disease (PMD). Critical features of degeneration in several of these diseases involve activation of cell death pathways in various neural cell populations, and the initiator caspase 12 has been proposed to play a central role. Accordingly, pharmacological strategies to inhibit caspase 12 activity have received remarkable attention in anticipation of effecting disease amelioration. Our investigation in animal models of PMD demonstrates that caspase 12 is activated following accumulation of mutant proteins in oligodendrocytes; however, eliminating caspase 12 activity does not alter pathophysiology with respect to levels of apoptosis, oligodendrocyte function, disease severity or life span. We conclude that caspase 12 activation by UPR signaling is an epiphenomenon that plays little discernable role in the loss of oligodendrocytes in vivo and may portend the inconsequence of caspase 12 to the pathophysiology of other protein conformational diseases.