Growth, Differentiation and Sexuality

168 B.C.K. Lu
(Esser et al. 1980; Kück et al. 1981). It is a part of
theintronofthecytochromecoxidasegenethat
contains a 48-bp autonomous replication sequence
(Osiewacz and Esser 1984). The accumulation of
plDNA in mitochondria leads to mitochondrial instability
and death. Interestingly, senescence can be
cured by ethidium bromide that eliminates plDNA,
also known as senDNA (Koll et al. 1984; see reviews
in Griffiths 1992; Osiewacz 1995). In recent years,
PCD is gaining recognition in fungi, which have
been shown to share this vital cellular program (albeit
prototypal) with all eukaryotes. In this chapter,
I review some of the key findings in this field.
Some important works may regrettably not have
been included because of space constraints, and
because they are reviewed elsewhere in this volume
(Chaps. 7, 8 and 10). More details may be seen
in recent reviews (Madeo et al. 2004; Ramsdale, The
Mycota, Vol. XIII, Chap. 7).
II. Do Fungi Have Apoptosis, the Type I
Programmed Cell Death?
From model systems such as C. elegans and
mammalian cells, a number of proteins (and their
genes) that regulate apoptosis have been identified.
These are CED-3, CED-4, and CED-9 of C. elegans,
and the Bcl-2 (β-cell lymphocytic-leukemia
protooncoprotein-2) family proteins of mammals
(see reviews in Ellis et al. 1991; Green and Reed
1998; Mignotte and Vayssiere 1998; Kuwana and
Newmeyer 2003). No homologs of these genes are
foundinthegenomeofSaccharomyces cerevisiae,
Schizosaccharomyces pombe (Koonin and Aravind
2002) and Neurospora crassa (Glass and Kaneko
2003). A question has been raised whether fungi
have apoptosis. In recent years, evidence has
been accumulating that processes of apoptosis do
indeed exist in fungi.
Before I examine cases in fungi, let me take
a brief look at what is known in the higher
eukaryotes. Bcl-2 family proteins are key cell-death
regulatory proteins in mammalian cells, and
they all share one or more Bcl-2 homology (BH)
domains (Zha et al. 1996; Harris and Thompson
2000). There are two opposing groups, proapoptotic
and antiapoptotic. The proapoptotic group
contains two subgroups: Bax and Bak contain
BH1, BH2, and BH3, and Bid and Bad (also others)
contain a BH3-only domain. The antiapoptotic
Bcl-2 and Bcl-xL contain all four (BH1-4) domains.
All Bcl-2 family proteins can be classified as mitochondrial
membrane proteins, as they are destined
to mitochondria for their actions (see review in
Kuwana and Newmeyer 2003). It appears that BH3
is essential for the cell-death process (Huang and
Strasser 2000). When a death stimulus is received,
the BH3-only Bid activates Bax/Bak while Bad
inactivates Bcl-2/Bcl-xL. Bax is then translocated
to the mitochondrial outer membrane to trigger
apoptosis. Bax is suggested to form membrane
pores from which cytochrome c and other apoptogenic
molecules can leak from mitochondrial
intermembrane space into the cytosol to activate
the caspases (Kuwana et al. 2002; see also references
in Kuwana and Newmeyer 2003). Recently, Bax has
been shown, by electron microscopy, to localize
at the constricted mitochondrial fission sites to
effect mitochondrial fragmentation in apoptotic
Cos-7 (and HeLa) cells (Karbowski et al. 2002).
In C. elegans, four genes have been identified,
ced9, ced4, ced3 (for cell death-defective), and egl1
(for egg laying-defective). CED-9 is equivalent to
Bcl-2, CED-4 is equivalent to mammalian Apaf1
(apoptosis protease activating factor 1), CED-3
is a caspase (a cysteine protease), and EGL-1
is a BH3-only dynamin-interacting protein that
interacts with Drp-1 (dynamin-related protein
1) to mediate mitochondrial fragmentation (Jagasla
et al. 2005). Other regulators of apoptosis
are caspase-1/caspase-3, effectors of apoptosis,
AIF (apoptosis-inducing factor), Htr-A (hightemperature
resistance A), a nuclear mediator of
apoptosis, BI (Bax inhibitor) and IAP (inhibitor of
apoptosis protein). More details are documented
below in this chapter.
A. Apoptosis Is Induced by Expression
of Heterologous Proapoptotic Genes
Expression of proapoptotic genes from both mammals
and C. elegans in the budding and fission
yeasts brought about symptoms consistent with
apoptotic cell death (Sato et al. 1994; Ink et al. 1997;
James et al. 1997; Ligr et al. 1998; Fröhlich and
Madeo 2000). For example, when Bax is expressed
in the budding yeast, or when human Bak is
expressed in the fission yeast, cell death is induced
in these hosts. The cell killing can be prevented
when the antiapoptotic gene bcl-2, bcl-xL,ormcl-1
is co-transformed (Sato et al. 1994; Greenhalf et al.
1996; Ink et al. 1997; Tao et al. 1997; Fröhlich and
Madeo 2000). Likewise, when the mammalian