Page 14 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

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Encyclopedia of Physical Science and Technology EN002G-90 May 17, 2001 20:42

552 Cell Death (Apoptosis)

show no chromatin condensation in nuclei after apop- genic mice die as efficient by as those from wild-type mice
totic stimulation. On the other hand, significantly, dying following exposure to γ -rays. Why is there such a discrep-
cells overexpressing caspase-resistant ICAD show apop- ancy even though there is no CAD function in thymocytes
totic chromatin condensation without DNA fragmenta- in both cases? There is a possibility that either endoge-
tion. These results suggest that apoptotic chromatin con- neous ICAD cleaved by caspase-3 or CAD/ICAD-Sdm
densation is caused by CAD in particular tissues such as complex may contribute to some killing activity in thy-
thymocytes and that another factor(s) is involved in this mocytes from ICAD-Sdm transgenic mice. ICAD might
process in some situations. We also cannot rule out the be required for amplification of death signal in some con-
possibility that CAD has dual functions, DNA fragmen- ditions, or the modification of anti-apoptotic proteins in
tation and condensation activities, because of complete cells.IthasrecentlybeenreportedthatICAD-nullmiceex-
denatured structure of CAD in ICAD-null cells (CAD is hibit enhanced spatial learning and memory accompanied
present as an insoluble form) but not transformants ex- by the increase (∼8%) of granule cells in the hippocampal
pressing caspase-resistant ICAD (endogenous CAD and dentate gyrus region compared to wild-type mice. Detail
ICAD are still present as a soluble form), or that cleaved analyses would be elucidated whether or not CAD/ICAD
ICAD cooperatively work with cellular factor(s) to con- system is actually involved in the induction of cell death.
dense chromatin although cleaved ICAD themselves have It has been thought that phagocytes recognize the apop-
no chromatin condensation activity. Recently, it has been totic cells undergoing DNA fragmentation and ingest and
reported that a protein other than CAD, called Acinus, recycle the dead cells, and that cleaved DNA is utilized
is responsible for apoptotic chromatin condensation how- as a marker for the recognition of apoptotic cells by
ever this remains to be confirmed. phagocytes. However, phagocytosis of dying thymocytes
from ICAD-Sdm transgenic mice, in which DNA degra-
dation is not seen, occurs as efficiently in the case of
VIII. PHYSIOLOGICAL DNA wild-type mice, indicating that DNA fragmentation is dis-
FRAGMENTATION AND pensable for phagocytosis. Moreover, it has recently been
PHAGOCYTOSIS OF observed that TUNEL-positive cells are still present in
APOPTOTIC CELLS various tissues from ICAD-Sdm transgenic mice at simi-
lar levels as in wild-type mice after γ -irradiation. When
Extensive DNA fragmentation and chromatin condensa- dyingcellsoverexpressingICAD-Sdmarecoculturedwith
tion are hallmarks of apoptosis and are tightly regulated macrophages,theirintactDNAsarequicklyprocessedinto
by the CAD/ICAD system. It is thought that these pro- nucleosomal units. Furthermore, reagents that block the
cesses play an important role in cell killing. To reveal acidification of lysosomes inhibit DNA degradation in-
physiological functions of the CAD/ICAD system in vivo, duced by macrophages, suggesting that lysosomal DNase
Zhang et al. have established ICAD-deficient mice. These in engulfing cells is involved in the generation of TUNEL-
mice develop normally compared to wild-type mice and positive cells in various tissues in vivo. A candidate for
do not show any pathological signs. However, thymocytes the responsible DNase during phagocytic DNA fragmen-
from ICAD-null mice are obviously lacking CAD activity tation is DNase II existing in lysosomes as an acidic
and show neither DNA fragmentation nor chromatin con- DNase. The recent molecular cloning of nuc-1, which
densation following exposure to apoptotic stimuli. Con- is responsible for DNA degradation during programmed
sistent with the observations described above, no func- cell death in C. elegans, has revealed that the gene prod-
tional CAD is expressed, at least in thymocytes, in the uct is a homologue of mammalian DNase II. This report
absence of ICAD. Moreover, transgenic mice expressing proposes a model for apoptotic DNA degradation com-
caspase-resistant ICAD-S (ICAD-Sdm; it is ubiquitously posed of three distinct steps (Fig. 6). The first step ap-
expressed under the control of elongation factor 1 α pro- pears to involve an unidentified endonuclease to produce
moter) have been established. ICAD-Sdm is expressed TUNEL-positive DNA. The enzyme working in this step
in the major organs of the transgenic mice, and isolated would be a CAD-like DNase. In the second step NUC-1
thymocytes from the transgenic mice undergo apoptosis converts TUNEL-positive to TUNEL-negative DNA. In
without DNA fragmentation after irradiation with γ -rays. the final step, “cell-corpse DNA” is completely digested
However, these transgenic mice have no phenotypic ab- by engulfment-dependent nuclease(s). DNase II creates
normality during development, like ICAD-deficient mice. DNA fragments having 3 -phosphate ends rather than the

The thymocytes from ICAD-null mice, but not from wild- 3 -hydroxyl ends that are required as primers for termi-
type mice, are partially resistant to several apoptotic nal deoxynucleotidyl transferase. In the mammalian sys-
stimuli including dexamethasone, etoposide, and stau- tem, after cleavage of DNA by lysosomal DNase II, the
rosporine, whereas thymocytes from ICAD-Sdm trans- 3 -phosphate groups are presumably removed by acid