Identification of regulatory and catalytic domains in the apoptosis nuclease DFF40/CAD

Solution structure of the CIDE-N domain of CIDE-B and a model for CIDE-N/CIDE-N interactions in the DNA fragmentation pathway of apoptosis

Apoptotic DNA fragmentation and chromatin condensation are mediated by the caspase-activated DFF40/ CAD nuclease, which is chaperoned and inhibited by DFF45/ICAD. CIDE proteins share a homologous regulatory CIDE-N domain with DFF40/CAD and DFF45/ ICAD. Here we report the solution structure of CIDE-N of human CIDE-B. We show that the CIDE-N of CIDE-B interacts with CIDE-N domains of both DFF40 and DFF45. The binding epitopes are similar and map to a highly charged bipolar surface region of CIDE-B. Furthermore, we demonstrate that the CIDE-N of CIDE-B regulates enzymatic activity of the DFF40/ DFF45 complex in vitro. Based on these results and mutagenesis data, we propose a model for the CIDE-N/ CIDE-N complex and discuss the role of this novel bipolar interaction in mediating downstream events of apoptosis.

Multiple domains of DFF45 bind synergistically to DFF40: roles of caspase cleavage and sequestration of activator domain of DFF40

CAD/DFF40, the nuclease responsible for DNA fragmentation during apoptosis, exists as a heterodimeric complex with DFF45/ICAD. The study presented here augments the accompanying inhibition and chaperone study with an analysis of specific binding strengths and locations of DFF45 binding sites within DFF40. This allows us to show that DFF40/45 interaction is mediated by binding of three functional domains (D1, D2, and D3) of DFF45 to two domains (activator and catalytic) of DFF40. D1 binds exclusively to the activator domain and D2 binds to the catalytic domain of DFF40. Inhibition of DFF40 nuclease activity arises independently from D1 functional sequestration of the activator domain and D2 blockage of the catalytic domain of DFF40. The mechanism of caspase activation of DFF40 is the disruption of the synergistic binding activity of DFF45 domains to DFF40 after caspase recognition and cleavage of DFF45 in the context of a DFF45/40 complex.

Recent advances on neuronal caspases in development and neurodegeneration

In view of a large and growing literature, this overview emphasizes recent advances in neuronal caspases and their role in cell death. To provide historical perspective, morphology and methods are surveyed with emphasis on early studies on interleukin converting enzyme (ICE) as a prototype for identifying zymogen subunits. The unexpected homology of ICE (caspase-1) to Caenorhabditis elegans death gene CED-3 provided early clues linking caspases to programmed cell death, and led later to discovery of bcl-2 proteins (CED-9 homologs) and 'apoptosis associated factors' (Apafs). Availability of substrates, inhibitors, and cDNAs led to identification of up to 16 caspases as a new superfamily of unique cysteine proteinases targeting Asp groups. Those acting as putative death effectors dismantle neurons by catabolism of proteins essential for survival. Caspases degrade amyloid precursor protein (APP), presenilins (PS1, PS2), tau, and huntingtin, raising questions on their role in neurodegeneration. Brain contains 'inhibitors of apoptosis proteins' (IAPs) survivin and NAIP associated also with some neuronal disorders. Apoptotic stress in neurons initiates a chain of events leading to activation of distal caspases by pathways that remain to be fully mapped. Neuronal caspases play multiple roles for initiation and execution of cell death, for morphogenesis, and in non-mitotic neurons for homeostasis. Recent studies focus on cytochrome c as pivotal in mediating conversion of procaspase-9 as a major initiator for apoptosis. Identifying signaling pathways and related events paves the way to design useful therapeutic remedies to prevent neuronal loss in disease or aging.