The regulation of DNAse activities in subcellular compartments of activated thymocytes

Measurement of two caspase activities simultaneously in living cells by a novel dual FRET fluorescent indicator probe

BACKGROUND

A number of fluorescent caspase substrates and FRET-based indicators have been developed to study the in vivo activation of caspases, a conserved family of proteases critical in inflammatory, and apoptosis signaling pathways. To date, all substrates have measured only one caspase activity. Here, we describe a FRET-based probe for simultaneously measuring two distinct caspase activities in living cells.

METHODS

This probe consists of a CFP-YFP-mRFP fusion protein containing a caspase-3-cleavage motif, DEVD, between CFP and YFP, and a caspase-6-cleavage site, VEID, between YFP and mRFP. DEVDase and VEIDase activities could be assessed simultaneously by monitoring diminished FRET mediated by cleavage of either or both of these protease cleavage sites using flow cytometry.

RESULTS

DEVDase and VEIDase activities were completely inhibited by the pan-caspase inhibitor z-VAD-fmk and enhanced by DNA-damaging drugs or by anti-Fas stimulation. DEVD and VEID cleavage specificities were validated by using caspase-3-deficient MCF7-Fas cells and a caspase-6-specific inhibitor. Kinetic analysis with the FRET probe revealed that caspase-3 activation consistently preceded caspase-6 by approximately 30 min following induction of apoptosis.

CONCLUSIONS

We have developed a novel FRET-based probe for simultaneous detection of two caspase activities in living cells using flow cytometry. Simultaneous detection of two caspase activities using this probe has clearly provided information of the ordering of caspase-3 and -6 in the apoptotic pathway.

Mechanisms of apoptosis

The understanding of the apoptotic program has grown exponentially over the past decade. Numerous human diseases have been directly linked to genetic defects in the apoptotic pathways, including cancer, neurodegenerative disorders, and autoimmune diseases. Caspases initiate and amplify various death signals, allowing for selective and ordered cellular demolition. The fine balance between pro- and antiapoptotic Bcl-2 family members regulates the cell fate in response to many (but not all) stress or signaling pathways. Recent discoveries highlight the complex integration of signals from various organelles that determine cell fate and the multiple functions of central players in the apoptotic process. It is likely that the knowledge obtained in a relatively time will translate into better diagnostics and therapies to enhance or retard cell death in the appropriate clinical circumstances.

Heat shock protein 70 binds caspase-activated DNase and enhances its activity in TCR-stimulated T cells

DNA fragmentation is a hallmark of cells undergoing apoptosis and is mediated mainly by the caspase-activated DNase (CAD or DNA-fragmentation factor 40 [DFF40]), which is activated when released from its inhibitor protein (ICAD or DFF45) upon apoptosis signals. Here we analyzed the effect of heat shock protein 70 (Hsp70) on CAD activity in T-cell receptor (TCR)-induced apoptosis using a T-cell line (TAg-Jurkat). Overexpression of Hsp70 significantly augmented the apoptotic cell death as well as DNA fragmentation in CD3/CD28- or staurosporine-stimulated cells. Following stimulation of cells with CD3/CD28 or staurosporine, Hsp70 was coprecipitated with free CAD, but not with CAD associated with ICAD. Furthermore, the purified Hsp70 dose-dependently augmented DNA-fragmentation activity of caspase-3-activated CAD in a cell-free system. Peptide-binding domain-deleted Hsp70 could neither bind nor augment its activity, while adenosine triphosphate (ATP)-binding domain-deleted Hsp70 or the peptide-binding domain itself bound CAD and augmented its activity. These results indicate that the the binding of Hsp70 to the activated CAD via the peptide-binding domain augments its activity. Although CAD lost its activity in an hour after being released from ICAD in vitro, its activity was retained after an hour of incubation in the presence of Hsp70, suggesting that Hsp70 may be involved in stabilization of CAD activity. Finally, CAD that had been coprecipitated with Hsp70 from the cell lysate of staurosporine-activated 293T cells induced chromatin DNA fragmentation and its activity was not inhibited by ICAD. These results suggest that Hsp70 binds free CAD in TCR-stimulated T cells to stabilize and augment its activity.

Degradation of chromosomal DNA during apoptosis

Apoptosis is often accompanied by degradation of chromosomal DNA. CAD, caspase-activated DNase, was identified in 1998 as a DNase that is responsible for this process. In the last several years, mice deficient in the CAD system have been generated. Studies with these mice indicated that apoptotic DNA degradation occurs in two different systems. In one, the DNA fragmentation is carried out by CAD in the dying cells and in the other, by lysosomal DNase II after the dying cells are phagocytosed. Several other endonucleases have also been suggested as candidate effectors for the apoptotic degradation of chromosomal DNA. In this review, we will discuss the mechanism and role of DNA degradation during apoptosis.