Senescence comes of age

monitor

TIG - - June 1986

One of the most string chamc. tefisticsoflivingorganisms is that they age and eventually die. The molecular basis of this phenomenon has always intrigued scienfists (even they get old) and a number of provocative theories have been put forward. One of these, the so called 'error catastrophy' hypothesis of Orgelt caught some attention a few yearsago by its eloquent presentation". In essence, the hypothesis states that cells age because of the accumulation of mutations. Particulady Imrmful are mutations in genes whose prodacts perform essentialtaslm in processes such as replication, transcription or translatio~ A mutated DNA polymerase, lacking editing control, could for instance be itself responsible for the introduction of further mutations. Errors would quickly multiply according to such a scenado, finally cascading into catastrophy and cell death. This hypothesis should be verifiable, since it impliesthat the number of mutations in cellularproteins and RNAs would largely increase with cell age. So far, however, systematic analysis of 'old' gene(product)s has not been carded out. Certain fdamentousfungiseem to be idealorganisms for studying the phenomenon of senescence. Cultures of Podospora ansenna, Nmmspova intmwdia, A s ~ -

Senescence comesof age R. Benneand H. F. Tabak Sedion/orMolemlarB ioloo, Labomto~ o/B iochemislry, Univegs~ofAmsterdam, Kmislaan318,1098 SMAms~dam, TheNetheda~Is.

Inearlyexperimentsitwas estabfished that senescence shows strain-dependent characteristics (early or late loss of ~fmbilityetc. ) and that such characteristics can be transferred in sexual crosses. Transmission of the genetic factors involvedshows aspects of maternal inheritance, and grafting of old h~hae to young ones coders a dominant senescent phenotype to the mixed mycefia. From this, a linkto the mitochondrial genetic system can be quickly inferred: indeed, dramatic changes take placeinmtDNA during s e ~ c e s. Results obtained by various groups over the past few years have resulted in a we~,h of specific data and it is interesting to try to put themin perspective. In general, substantialalterations are noticed in mitochondrial genomesisolated fromsenescent cells. These may range from deletions, as observed inthe well known 'poky' mutant of Nmrospova crassa4 to large insertions 8illusamstdodamii, etc.,losevi- offoreignDNAas foundincertain abilityunder continuous cultiva- mtural isolates of Neurosp~a tion. Hyphal tips swell, accum- i n ~ from the Haw'~;mn ulate pigments and finallyburst. islandss's. Since N e u ~ a is

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an obligate aerobe, total loss d mitochondrial genes cannot be tolerated as it can in Saccharo. myca cem~/ae, and functional mtDNA must remain present. Strains linger between life and death, probablydependingon the ratio of functional and nonfunctional mtDNAs. Podospora amev/v4is the most remarkable of the senescenceprone fungi:when it is kept under conditions of continuous vegetative growth, an irreversible sequenceofeventsis started that leads to senescence and, eventually, '~:ocelldeath. In senescent cultures, certain regions of the mtDNAare exdsed and continue to replicate as independent plasraids (senI)NA). ,Severalregions c~n suffer this fate, but the most frequent and interesting event is the accumulation of a specific region, a-senDNA. Detailed sequence analysis of this DNA and the mtDNA region from whichit originated, reveals that o~senDNA constitutes a correctly excised group II intron from the split gone codingfor cytochrome oxidase subunit I, (cod; see Fig. 1)7. How do the excision

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Fig. 1. lntron DNA excisionin senescence.resistantand senescence-pronestrains o[ Podospomansefina.Part of the mitockondrialgene coeb'ng[ov ~yto@romeoxidmesubanitI is n~qvesentedwithcmss-hatc~ blockssymhol£~gexom. Sandwic~,dbe~een On~mistl~intronu~ththeopenredan~ ~ting

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events fit the assumption that senescence is brought about by genomic mutations? The insertion of non-mtDNA into the nfitochondrialgenome, an event associated with senescence of certain Neuros~a intermedia strains (see above), causes fewer conceptual prob. lems of this type. In ~ s case, insertion takes place within a mitochondrialprotein gene. This gene, situated in an intron of the large rRNA, codes for a n ~ somal protein, and its disruption willevidently have a deleterious effect on mitochondrial protein synthesis in general, resulting in diminishedcytochrome spectra. What, then, canbe the set-hackin Podospom, considering the fact that the coxI gene is left intact after o~-senDNAexdsionT'S?Optionalintronsare commonplacein the mitochondrial genomes of S. cenv/s~ strains. Loss of one of them does not seem to be crucial, especially since the function~ ofmost of tie products of intronic unidentified reading frames (URF) merely serves bureaucratic purposes: they catalyse the excisionof their own codingsequences fromprecursor RNA(i.e. they act as'maturases' or 'spligases'). It is possible, though, that some of these URF products in P odospoveare transacting, a situationreminiscent of that described for the product of intron b4 of the S. cends/ae cytochrome b gene (cob-box effect). The nmturase encoded in one intron contributes not only to its ownexcision,but alsoto that of another intron9. If the Podospova cod intron had a similar transacting activity, certain other mitochondrialprecursor RNA(s) might become unspiiceable, resuiting in a situation comparable to gene loss. Belcour and Viemy'° have taken the problema step further. TheyisolatedPodos~ramutants resistant to senescence and showed that such strata possess an =-senDNA that has been excised differently,lea~nga nonfunctionalcod gene behind (Fig. 1). How strains with a nonfunctionalmitochondrialgenecan escape senescence is a complete mystery. It certainlyshows that a non.functionalmaturase derived from the coxl intron (if required at all) is not sufficientto induce senescence. Instead, t.hese resuits pointto a possiblerole of the sen-plasmids themselves in the onset of senescence, since the strains with 'abnormal' plasmids resume growth. In order to be 147

monitor able to do that, the mutants appear to resort to an alternafive, SHAM (salicylhydroxamic acid)-sensifive,respiratory pathwa~,. The theoretical poss~ility, therefore, exists that the mere absenceofcytocb~meoxidaseis sufficientto prevent senescence. Mutants lackingnucle~-enc~ed cox.subunits that are still prone to senescence, should allow us to discard this possibility. They have not been found so far. In summary, the study of senescence is getting to a stage at which new ways of phrasing old questions are being discovered. Increasingknowledge, however, has not yet elucidated the mol-

148

TIG --]ume 1986

ecular events that cause cellular ageing, nor has the issue of whetherthe observed alterations in mtDNA are the cause or the consequenceof senescence been completely settled. Nevertheless, in a broad sense Orgers error catastrophy theory seems applicableto fungal senescence: certain key enzymes (or RNAs) involved in mitochondrial gene expression go berserk and create havoc (and sen-plasmids) in the mitodmndfialgenetic systent In thislightit is not surprish~gthat the nu'tochondrialgenetic system is the first to show signs of senescent degeneration, considering its high mutation rate when

compared to that ofnuclearDNA. 3 Sager, R. (1972) Cytop/mm/c Genesand Organeiles, Academic It also appears likelythat a price Press must be paid for having split C. A., Geewert. R. It. genes and complex modes of 4 Manella, and Lambowitz, A.M. (1979) gene expressio~ Mammalian Cell 18, 1197-1207 mtDNAs, however, lack split 5 Griflith,A.J.F. andBertrand,H. genes, and therefore ageing of (1984) Cuw. Gemet.8, 387-398 cells in higher organisms willnot 6 Bertrand,H., Chan,B. S. S. and Gfiflith,A. J. F. (1985)Cell41, proceed along the same lines as 877-884 that in fungLFuture research will show how rea~ this apparent 70siewacz, H. D. and EsseX, K. (1984) Cuw. Ge~et. 8, 299-305 distinction is. s c ~ , D. j. and Wr~ht, IL M. (1983)Nud~Adds Ra. 11, 2111-2119 9 Dujardi~ G., Jacq, C. and References Slonimski, P. P. (1982) Natum~ 10rgel, L. E. (1963)Proc Na~l 298, 628-632 Acad. Sd. USA 49, 517-521 10 Belcour,L. andViemy,C. (1986) 2 News and Views (1969)Nat~ ~,vIBO]. 5, 609-614 221, 1191