Novel Caspase-8 Roles Revealed by Knockouts

Caspase-8 is the prototypical apoptosis initiator Caspase downstream of TNF super- family death receptors. With substrates that include apoptosis-related effector Caspases and pro-apoptotic Bcl-2 family members, active Caspase-8 is capable of unleashing cascades of cellular events that result in apoptosis.¹ Indeed, Caspase-8-deficient cells are resistant to death receptor-mediated cell death both in vitro and in vivo.^2-4 While recent knockout studies support the role of Caspase-8 in apoptosis, they also reveal several other putative functions (Figure 1).

Figure 1. Knockouts suggest both apoptosis-related and apoptosis-independent roles for Caspase-8. Caspase-8-deficient cells are insensitive to death receptor ligand-induced apoptosis. The embryonic lethal phenotype observed is likely due to degeneration of the extra-embryonic yolk sac vasculature and abnormal cardiac development. Knockouts in cells of the hematopoietic lineage reveal putative roles for Caspase-8 in lymphoid and myeloid development, the proliferation and activation of T cells, and the ability to generate an effective anti-viral immune response.

Caspase-8 knockout is embryonic lethal at approximately day 11.5.³ The first abnormality, observed at approximately day 10.5, is a regression of the extra-embryonic yolk sac vasculature followed by abdominal hemorrhage due to cardiac rupture.^3,5 Apoptosis is observed in cells surrounding the yolk sac vessels and in the cardiac myocytes themselves.^4,6 Also accompanying Caspase-8 knockout are defects in neural tube morphogenesis.⁵ Interestingly, cardiac tissue and neural tube abnormalities are rescued by ex vivo culture, suggesting that the Caspase-8^-/- phenotype is not intrinsic to these tissues specifically.⁵ These phenotypes may be due to Caspase-8 deficiency in endothelial cells or their precursors. When Caspase-8 is targeted for knockout in endothelial cells using a Cre/loxP recombination system (Tie-1/Cre), transgenic mice exhibit cardiovascular, neural tube, and embryonic lethal phenotypes similar to the full knockout.⁴

Certain aspects of lymphoid cell development and activity appear to be affected by Caspase-8 knockout. For instance, Caspase-8-deficient bone marrow (BM) hematopoietic progenitors are unable to repopulate B cells and T cells in the BM and lymphoid organs of sub-lethally irradiated mice.⁴ When Caspase-8 is knocked out in the T cell lineage using Cre/loxP recombination (Lck/Cre), transgenic mice are viable, but appear to have deficiencies in both T cell expansion and activation.6 Caspase-8 deletion might also affect T cell development at early stages of differentiation, but its role may decrease in importance as the cells mature to, and beyond, the late double negative stage.^4,6 In addition, Lck/Cre transgenic mice are unable to develop an effective immune response to viral infection.⁶ Caspase-8 may have a role in myeloid development as well. When Caspase-8 knockout is targeted to the myeloid lineage using Cre/loxP recombination (LysM/Cre), myeloid progenitors do not differentiate into macrophages in response to M-CSF treatment.⁴

Recent knockout studies appear to reveal several potential roles for Caspase-8 that are independent of apoptosis. In fact, apoptosis appears to be a consequence of Caspase-8 knockout in several instances suggesting a pro-survival function.^4,5 Caspase-8 knockouts exhibit deficiencies in leukocyte differentiation, proliferation, and the immune response further indicating Caspase-8 may be a multifunctional enzyme.^4,6 The mechanisms underlying these activities are poorly understood. It will be of interest to monitor future studies that identify thesubstrates involved, and further define the role of Caspase-8 in both apoptosis and in apoptosis-independent activities.

References

1. Lawen, A. (2003) BioEssays 25:888.
2. Juo, P. et al. (1998) Curr. Biol. 8:1001.
3. Varfalomeev, E.E. et al. (1998) Immunity 9:267.
4. Kang, T.-B. et al. (2004) J. Immunol. 173:2976.
5. Sakamaki, K. et al. (2002) Cell Death Differ. 9:1196.
6. Salmena, L. et al. (2003) Genes Dev. 17:883.