Sci. Aging Knowl. Environ., 7 January 2004
Vol. 2004, Issue 1, p. pe1
[DOI: 10.1126/sageke.2004.1.pe1]

PERSPECTIVES

There's a Problem in the Furnace

John Tower

The author is in the Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089, USA. E-mail: jtower{at}usc.edu

http://sageke.sciencemag.org/cgi/content/full/2004/1/pe1

Key Words: superoxide dismutase • oxidative damage • reactive oxygen species

A long-sought-after mutation in the manganese superoxide dismutase gene (MnSOD) has been isolated in Drosophila melanogaster, increasing the utility of the fly for studies of oxidative damage and aging. It is somewhat surprising that no such mutant was obtained before now, because apparently there have been several independent efforts to isolate one. The difficulty led to speculation that the MnSOD gene might be haplolethal: so critical for life that the fly needs both copies intact. However, thanks to the awesome power of the P element, the hunt has ended, and the MnSOD null mutation has been found to be a plain old recessive lethal. The kicker is the timing of death: post-eclosion (when the adult comes out of the pupal case), which is as close to neonatal lethality as a fly gets.

Mitochondria produce the majority of cellular reactive oxygen species (ROS) in the form of superoxide, which is dealt with by the antioxidant enzyme superoxide dismutase (SOD). In Drosophila, there are two forms of SOD: MnSOD, which resides in the inner mitochondrial space, and Cu/ZnSOD, which is housed in the cytoplasm and the outer mitochondrial space. Mammals have an extra SOD in the extracellular space. A Cu/ZnSOD null mutation in Drosophila starts killing flies during pupation, and none make it past early adulthood (1). In contrast, the mouse with a Cu/ZnSOD null mutation is only mildly inconvenienced (2), and it has been speculated that this difference might be due to compensation by the extracellular SOD. The MnSOD mutant mouse is less lucky and dies as a neonate with damage to the heart, brain, and muscle (3, 4). These tissues presumably take the hit because they have the largest energy demand and the most active mitochondria, which act as the agents of their own destruction by generating ROS. Being so close to the ROS action, the mouse MnSOD appears to be the most critical of the three SODs. The phenotype of the fly MnSOD null reported by Duttaroy is so exciting because it is exactly what was hoped for: a mighty model for the mammals.

So how did they get that mutation? Through the awesome power of the P element. As part of the Drosophila genome project, an extensive collection of Drosophila strains has been curated, in which each contains a single insertion of the P transposable element at a different defined location in the genome (5). Duttaroy et al. (6) spotted a P element insertion with no phenotype that was very close to the MnSOD gene. When a source of P element transposase is introduced into the same fly, the P element is excised. At a moderate frequency (~10% of the time or so), when the P element is excised, it takes with it a chunk of the flanking DNA, an event known as an imprecise excision (7). This imprecision provides a way to make small, targeted deletions in the genome, and the authors used this approach to create a specific deletion of the MnSOD gene. The result was a perfect null mutation: Fly preparations contained no enzyme activity and no protein when analyzed by Western blot, and these phenotypes were rescued by transforming the MnSOD null flies with a genomic fragment that contained an intact MnSOD gene.

Heterozygous flies, which contain only one good copy of the MnSOD gene, have half the normal amount of MnSOD activity. As expected, these flies were more sensitive to oxidative stress caused by the drug paraquat than were wild-type flies with two good copies of MnSOD (6). In fact, the same phenotypes of paraquat sensitivity and post-eclosion lethality had already been observed when MnSOD activity was reduced using a small inhibitory RNA (RNAi) strategy (8). Of course, up until now it wasn't certain whether the RNAi phenotype represented a complete or partial loss of MnSOD activity. The answer is that the RNAi phenotype is not quite a null, as the flies live a few days longer than the MnSOD null flies.

All this brings us to one of the hottest areas in research on aging: In mice, MnSOD heterozygotes are more sensitive to paraquat and have greater amounts of oxidatively damaged products that accumulate with age than do wild-type mice (9) (see "Is Less Enough?"). Surprisingly, however, the mice thus far have exhibited normal life spans, a result that is tricky to reconcile with the oxidative damage theory of aging. If oxidative damage causes aging and you've made oxidative damage worse, shouldn't the life span of the mice be shorter than normal? The leading possible explanations for the unexpected observation are (i) the mice compensate somehow with redundant functions; (ii) paraquat is not as good a model for the type of oxidative stress that causes aging as we had thought; and (iii) goodbye oxidative damage theory--at least for the mouse (Fig. 1).



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Fig. 1. Testing the oxidative damage theory of aging. Both mouse and fly MnSOD heterozygotes accumulate oxidative damage. The mouse lives to a ripe old age. The fly lives ??? Stay tuned.

 
Turning to the fly, overexpression of MnSOD increases life span, as does overexpression of Cu/ZnSOD, thereby providing strong support for the idea that the oxidative damage theory of aging does hold, at least in flies (10-12). The fly is notoriously unable to compensate for the absence of one copy of a gene, so MnSOD null heterozygotes should be shorter lived than wild-type flies. Alas, the answer will have to wait, as Duttoroy et al. aren't telling--yet.


January 7, 2004
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Citation: J. Tower, There's a Problem in the Furnace. Sci. Aging Knowl. Environ. 2004 (1), pe1 (2004).








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