Non-apoptotic functions of caspase-2 in cell division and genomic stability

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PROJECT SUMMARY/ABSTRACT Several groups have shown a strong association between caspase-2 deficiency and accelerated tumorigenesis in murine models of lymphoma, breast, and lung cancer. Such phenotypes are often accompanied by enhanced cell proliferation and increased genomic instability with minimal measurable apoptotic defects. Therefore caspase-2 appears to play a crucial role in maintaining genomic stability and may do so independent of its role in apoptosis. To test this, this proposal will study the mechanisms of caspase-2 is activation during the DNA damage response with a focus on the upstream caspase-2 regulator PIDD. The central hypothesis is that DNA damage induces two distinct caspase-2 activation platforms – a cytoplasmic platform that is PIDD independent and a nucleolar platform that requires PIDD – each providing access to distinct substrates that regulate genomic instability by both pro-apoptotic and non-apoptotic mechanisms. This hypothesis has been formulated based on published and preliminary data produced in the applicant’s laboratory showing that caspase-2 is activated in the cytoplasm and in the nucleolus and that the nucleolar activation is dependent on the nucleolar phosphoprotein nucleophosmin (NPM1) for both assembly and function. This hypothesis will be tested by pursuing two specific aims: 1) Determine the mechanisms of caspase-2 activation in the nucleolus versus the cytosol; and 2) Identify the relative contributions of the nucleolar and cytoplasmic complexes to downstream caspase-2 functions and how these protect from genomic instability. Under the first aim, an already proven imaging-based method for measuring caspase-2 activation, will be used to investigate the distinct mechanisms of differential caspase-2 activation in the nucleolus and in the cytosol. These experiments will specifically probe the roles of PIDD and NPM1. Under the second aim, based on preliminary data that shows that caspase-2-deficient cells, proliferate faster and accumulate more DNA damage following replication stress, the requirement of the nucleolar and cytoplasmic complexes for apoptotic and non-apoptotic caspase-2 functions will be explored in the context of apoptosis, cell cycle regulation, substrate cleavage and DNA repair mechanisms. The approach is innovative because it utilizes novel imaging-based techniques that are designed to assess caspase-2 activation in single cells so that the relationship between the localization of caspase-2 activation with apoptosis, DNA damage, and cell division will be directly explored on a per cell basis. It also provides a new paradigm of caspase activation in the nucleolus. The proposed research is significant because it is proposed that site-specific activation of caspase-2 in the cytosol and nucleolus governs distinct functions of this protease that cooperate to protect from genomic instability. These mechanisms may underlie the known physiological roles of caspase-2 in tumor suppression and in protecting against accelerated aging. Ultimately, such knowledge has the potential to inform how diseases where caspase pathways are disrupted can be treated or prevented.