has suggested that programmed cell death evolved during establishment of
endosymbiosis between bacteria and the ancestors of the present-day eukaryote cell.
Possibly some of the proteins involved in controlling apoptosis, such as bcl2 and AIF,
are related to molecules involved in establishing the original endosymbiosis.
3.2
Programmed cell death in TetrahymenaConjugating Tetrahymena cells have been shown to carry out an unusual form of
nuclear death that bears some resemblance to nuclear degradation during apoptosis.
Conjugation in ciliates is unusually dynamic because ciliates possess two nuclei, a
germ-line micronucleus and a somatic macronucleus. The micronucleus is
transcriptionally inactive and serves to propagate the genome from one generation to
the next. The macronucleus, by comparison, is transcriptionally active and contains
amplified copies of chromosome fragments. During sexual reproduction
(conjugation), the micronucleus undergoes meiotic cell divisions to yield four haploid
micronuclei, three of which degenerate. The remaining micronucleus replicates, and
one daughter nucleus is exchanged between conjugation partners. A new zygotic
nucleus is then formed in each conjugating cell, and from this zygotic nucleus new
macro—and micronuclei are generated. At this point, the old macronucleus is
degraded.
This process of macronuclear degradation is especially intriguing because one
nucleus in a cell survives while the other is degraded. Careful histologic observations
have shown that the degradation process is similar to nuclear degradation in apoptotic
cells. Nuclei become highly condensed and TUNEL positive. Nucleosome-sized
DNA fragments are formed (Davis et al., 1992; Mpoke and Wolfe, 1996). The dying
nucleus is surrounded by lysosomes, and nuclear acidification can be observed with
the metachromatic dye acridine orange (Mpoke and Wolfe, 1997). The lysosomal
marker acid phosphatase is enriched in the dying macronucleus, suggesting that final
elimination of the old macronucleus is achieved by autophagy (Lu and Wolfe, 2001).
Recent work has shown that caspase inhibitors prevent degradation of the
macronucleus in Tetrahymena, and caspase enzyme activity has been demonstrated
in extracts of Tetrahymena cells (Ejercito and Wolfe, personal communication).
Despite these similarities to apoptosis, caution is needed in drawing conclusions, since
the caspase activity appears to be located in vacuoles, not in the cytoplasm, and to
occur before nuclear degradation. Indeed, the observations are more similar to those
for stalk-cell differentiation in Dictyostelium (see above), in which AIF, and not
caspase, appears to the central regulator of the cell-death phenotype.
In addition to the nuclear death described above, there is evidence for cell density-
controlled cell death in cultures of Tetrahymena. Cell death can be induced by
cultivating cells at low cell density (Christensen et al., 1998) or by treatment with
the protein kinase-inhibitor staurosporine, which presumably interferes with signal
transduction (Straarup et al., 1997; Christensen et al., 2001). These results suggest
that extracellular signals regulate cell survival in Tetrahymena. One such signal
158 GENETICS OF APOPTOSIS