Growth, Differentiation and Sexuality

192 H.D. Osiewacz and A. Hamann
endogenous generation of toxic products in form
of ROS during normal energy transduction at the
respiratory chain, (2) scavenging systems which
reduce ROS levels (e.g. superoxide dismutase), and
(3) repair of damaged biomolecules (e.g. protein
turnover), which depends on protein biosynthesis
and a set of nuclear and mitochondrial genes. Any
imbalance of these processes leads to accelerated
aging. In P. anserina wild-type cultures, it is
evident that protein turnover must be impaired
in aged cultures, since the DNA rearrangements
result in an almost quantitative disappearance of
complete mitochondrial genomes.
In the last decades of research, a number of
long-lived mutant strains of P. anserina have been
identified and characterized which have provided
important data towards the understanding of parts
of the molecular network governing life span in
this organism. Some of them are extrachromosomal
mutants, others carry nuclear mutations.
AL2-1 is an extrachromosomal long-lived mutant
in which the mtDNA rearrangements leading
to the plDNA amplification are delayed. This
strain contains a linear plasmid, pAL2-1, encoding
an RNA and DNA polymerase, which was demonstrated
to interfere with the process of mtDNA
reorganization, leading ultimately to a stabilization
of the mitochondrial genome which remains
available for protein biosynthesis and, thus, protein
turnover over a long period of growth (Osiewacz
et al. 1989; Hermanns and Osiewacz 1992, 1996;
Hermanns et al. 1994). Interestingly, a recent publication
demonstrates an influence of this plasmid
on the effect of caloric restriction, an experimental
regime demonstrated to be effective in increasing
life span virtually in any system analysed so far.
Although the life span of wild-type strains of P.
anserina is also increased significantly by caloric
restriction (e.g. a 100-fold reduced glucose content
ofculturemediumcomparedtothenormalculture
medium), this effect is rather low in cultures
carrying the mitochondrial pAL2-1 plasmid (Maas
et al. 2004). The molecular basis of this type of
interference is not clear at the moment but may
provide new clues into the mechanisms of aging in
P. anserina (Maas et al. 2004).
Another way of escaping senescence is represented
by extrachromosomal mutants in which
parts of the mtDNA are lacking. Such mutants have
been isolated as partial outgrowth of senescent cultures
(Tudzynski et al. 1982; Vierny et al. 1982; Kück
et al. 1985b; Belcour and Vierny 1986; Schulte et al.
1988). Deletion of the intron 1 of CoxI in the mex
mutants (Vierny et al. 1982; Belcour and Vierny
1986) as well as complete deletion of CoxI in the
ex mutants (Kück et al. 1985b; Schulte et al. 1988)
result in longevity. This clearly stresses the importance
of this part of the mtDNA which, in wild-type
strains, efficiently contributes to the observed agespecific
mtDNA reorganization processes. In the
mutant lacking the corresponding DNA region, the
mt genome is stabilized.
D. TheRetrogradeResponse
The characterization of different long-lived P. anserina
mutants demonstrated that severe mitochondrial
dysfunction can be compensated by a “retrograde
response”. This pathway, leading to the induction
of certain genes which under normal conditions
are not or only partly expressed, has first
been reported in yeast (Liao et al. 1991; Liao and Butow
1993; Sekito et al. 2000) and was found to lead
to an increase in life span of yeast cells (Kirchman
et al. 1999; Jazwinski 2000, 2004; Kim et al. 2004).
The retrograde response in yeast is a signalling
pathway of interorganelle communication (Butow
2002), which is induced by mitochondrial dysfunction.
It results in the induction of numerous nuclear
genes coding for metabolic enzymes and stress proteins.
The retrograde response leads to a remodelling
of the cell metabolism increasing yeast life
span (reviewed in Jazwinski 2004, 2005).
In P. anserina, it was found that different types
of mutations leading to severe mitochondrial dysfunction
result in the induction of the nuclearencoded
PaAox gene, resembling a retrograde response.
One mutant of this type is the grisea mutant
(Esser and Keller 1976; Prillinger and Esser
1977). In this mutant, the age-related, wild-typespecific
rearrangements do not occur, due to impairments
of the homologous recombination between
repeated mtDNA (Borghouts et al. 2000).
This type of impairment can be overcome by the
supplementation of copper (Marbach et al. 1994) to
the medium, indicating that the process of homologous
recombination directly or indirectly depends
on the availability of copper (Borghouts et al. 2000).
The relevant mutation in the grisea mutant
is a loss of function of the transcription factor
GRISEA. This transcription factor is involved in the
tight control of cellular copper levels (Osiewacz and
Nuber 1996; Borghouts et al. 1997; Borghouts and
Osiewacz 1998). At high copper concentrations, it
is repressed.