Sci. Aging Knowl. Environ., 8 October 2003
Vol. 2003, Issue 40, p. pe28
[DOI: 10.1126/sageke.2003.40.pe28]

PERSPECTIVES

Reducing Integrins Improves the Quality of Fly Life

Catherine N. Torgler, and Nicholas H. Brown

The authors are in the Wellcome Trust/Cancer Research UK Institute and Department of Anatomy, University of Cambridge, Cambridge CB2 1QR, UK. E-mail: cnt20{at}mole.bio.cam.ac.uk

http://sageke.sciencemag.org/cgi/content/full/sageke;2003/40/pe28

Key Words: integrin • mobility • Drosophila • stress

"I've only ever got one wrinkle, and I'm sitting on it," said Jeanne Calment (1875-1997), the longest-lived person on record, when she turned 120 (1). With these words, she reminded people of their dream to look and stay young while aging. What was the secret of her longevity? Did she have a particularly healthy lifestyle? Or did she inherit longevity genes from her mother and father, who died at the ages of 86 and 94, respectively?

Recent experimental data have shown that genes that influence life span are conserved among a variety of species [reviewed in (2)]. Many scientists now use simple invertebrate model organisms as a means to discover conserved longevity genes, with the hope that their results will be transferable to higher organisms [reviewed in (3)]. For example, the fruit fly Drosophila melanogaster has been used by researchers to study the aging process. Like humans, flies become less mobile and less fertile and show signs of physical decline with age (Fig. 1). Now, new work by Grotewiel and colleagues (4), published in the journal Aging Cell, has shown that flies with reduced levels of integrins, which are cell adhesion receptors, are more mobile and less prone to death during the first half of their life span as compared with wild-type flies.



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Fig. 1. Phenotypic changes associated with aging in fruit flies. From left to right: young male 2 hours after eclosion, a 1-week-old male, and an old male (>1 month of age; shows reduced body size and mass).

 
Cells within tissues require two types of adhesion: direct cell-cell adhesion, primarily mediated by the cadherins; and cell adhesion to the extracellular matrix, primarily mediated by integrins. Integrins also function as signal transducers, and their signals are integrated with other signaling pathways to regulate cell migration, differentiation, growth, and survival [reviewed in (5) and (6)].

Goddeeris et al. (4) noticed that mutant flies having one wild-type and one mutated copy of the gene for the integrin {beta}PS subunit (flies heterozygous for the myospheroid gene) were more energetic than wild-type flies of the same age. {beta}PS is the major integrin {beta} subunit in flies, and although its functions during fly development have been well studied, its functions in the physiology and behavior of the adult fly have not been well characterized. To test the hypothesis that flies with decreased levels of integrin {beta}PS are fitter or active longer than are wild-type flies, the authors used a climbing assay that relies on the instinctive behavior of a fly to walk upward when put in a tube (Fig. 2) or in a vertical maze. A wild-type fly's performance of this task normally decreases with age. The reason for the decline is not known and may be a combination of physical changes, with the muscles becoming weaker or less active, and behavioral changes, with the flies becoming less inclined to move.



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Fig. 2. Geotaxis in Drosophila. (A) The cylinder containing fruit flies was tapped to cause their fall to the bottom of the cylinder. (B) Picture of the same cylinder after an interval of 10 s. Most of the flies instinctively climb upward.

 
After tapping flies to the bottom of the tube, Goddeeris et al. measured the distance that the flies walked and found that heterozygous integrin mutant flies maintained their ability to walk rapidly up the tube as they aged, relative to wild-type flies. This suggests that flies that express fewer integrins are more mobile than wild-type flies. In addition, the mean life span of the mutant flies was 20% greater than that of wild-type flies. Therefore, the chance that an integrin mutant fly would be alive at midlife was greater than that of a wild-type fly. However, the mutant flies did not outlive the longest-lived control flies, and all were dead after 100 days. Therefore, judging by the increased activity in late middle age, reducing the quantity of integrins resulted in an improved quality of life rather than an increase in the maximal attainable age.

In order to assess the cause of this increased midlife activity in the integrin heterozygotes, the authors investigated various biological conditions known to be associated with increased longevity, such as increased resistance to stress (7). Alleviating oxidative stress by overexpressing the superoxide dismutase 1 (SOD1) or SOD2 genes, which encode enzymes required to metabolize reactive oxygen species, extends life span in Drosophila (8). To test whether the novel phenotype of integrin heterozygotes was caused by increased resistance to stress, the authors measured the survival of these flies under diverse stressful conditions. First, they tested survival in response to treatment with paraquat, an herbicide known to induce oxidative damage in cells and tissues. Resistance to paraquat was not observed in the integrin mutant flies. In previous studies, flies that had been selected for their resistance to starvation and desiccation displayed an increased life span (7), but again, integrin heterozygotes were not found to be resistant to these environmental changes.

Previous experiments showed that transgenic fly lines that overexpress SOD activity, which exhibited an increase in mean life span, were also more resistant to hyperoxia (9). In addition, longevity mutants such as methuselah show an enhanced resistance to stress, including heat (10). For these reasons, the authors tested the integrin heterozygous mutants for resistance to hyperoxia and heat stress, but no increased resistance was observed. In addition, because survival has previously been linked to reduced body mass and fecundity (11, 12), they tested these two parameters in mutant and wild-type flies but did not find any significant differences. To summarize, the increased fitness of the integrin heterozygotes does not result from an improved resistance to stress nor to a decrease in energy-consuming activities such as laying eggs. As the authors conclude, these findings suggest that integrins are required for processes that lead to the decline in mobility and survival that occurs with age. We do not yet know whether the maintenance of an active lifestyle can by itself account for the increase in survival at midlife; at present, all we know is that both are consequences of a reduction in integrin levels.

It should be noted that the adult flies examined in the Goddeeris et al. study that show increased mobility have one normal and one mutant copy of the integrin gene. Because integrins are heterodimers, this genetic change could cause a relatively small reduction in the amount of integrin produced on the cell surface. Integrin {beta} subunits must form a heterodimer with an integrin {alpha} subunit to be transported out of the endoplasmic reticulum. Only if the {beta} subunit is limiting will this mutant condition result in a 50% decrease in integrins on the cell surface; if the {alpha} subunit is limiting, then the amount of integrin on the surface of cells will be much closer to wild-type levels. Also, this reduction in integrin activity was present throughout the development of the fly. Therefore, at present, we cannot distinguish between a role for integrins in adult flies as they age and a role for integrins during development that results in adult flies that are more resistant to the aging process. Flies that have null mutations in both copies of the {beta}PS gene die as embryos, and the study of these embryos has led to our understanding of how integrins contribute to developmental processes. From the current knowledge about integrin function in flies, we can speculate on how reducing integrin function might lead to an apparent increase in the quality of fly life. We can envision three possibilities: (i) integrin function in the muscles contributes to a normal decline in muscle function, (ii) loss of integrins results in an increase in activity of the muscles by stimulating muscle innervation or a change in slothful behavior, or (iii) integrins have a role in a novel aging-related pathway that is completely independent of their role in cell-extracellular matrix adhesion.

One of the key functions of integrins during embryonic development is the attachment of the ends of muscles to the tendon matrix [reviewed in (5)]. In the complete absence of integrins in flies, the muscles detach and round up during development, a defect that gave the gene encoding the integrin {beta}PS subunit the name myospheroid. In addition, integrins are thought to play a role in assembly of the muscle sarcomeric structure, which gives rise to the contractile function of muscles. It is possible that flies heterozygous for the {beta}PS integrin mutation have mild defects in these functions. Because the adult flies remain active, it seems unlikely that there is a defect in muscle contractility; however, it is possible that a reduction of muscle attachment to the tendon matrix results in the joints remaining more limber.

During the growth of the fly larvae, synapses between the innervating nerves and the muscles (neuromuscular junctions) expand in size as the muscles get larger. The size of the junction is also regulated by the activity of the junction, so if the larva is very active, the junction enlarges. Integrins play a role in the formation of neuromuscular junctions (13). Weak mutations in the myospheroid gene have been identified that permit survival to larval stages, and these homozygous mutant larvae have been found to have either an abnormal reduction or expansion of the neuromuscular junction, depending on the allele examined. This suggests that integrins play a role in communication between the nerve and the muscle that results in the regulation of the size of the synapse. Therefore, another feasible scenario is that the reduction in integrin levels causes an expansion of the neuromuscular junctions in the adult, which counteracts a possible age-related reduction in their function.

An integrin-dependent modulation of synaptic function, as seen at the neuromuscular junction, may also account for the function of integrins in short-term memory (14). The instinctive behavior of a fly is to walk upward. However, flies kept in a vial may remember that there is no advantage (such as access to food or freedom) to walking up the walls of the vial and may resist this instinctive behavior as they age. Flies heterozygous for a mutation in a particular integrin {alpha} subunit have short-term memory defects (14). Heterozygous {beta}PS integrin mutants may also have memory defects and thus may not remember that they gain no advantage by running up to the top of the vial. And so they continue to engage in this instinctive behavior. Therefore, the geotaxis behavior in the mutant flies may be similar to a situation in which a person looks nervously for his keys in the morning, checking every possible place several times, not remembering where he has already looked, whereas a less forgetful person (which corresponds to the wild-type flies) would remember that the keys are on the table where he left them.

Finally, as cell surface receptors, integrins could transduce a signal that modulates an as-yet-unknown pathway that promotes a decline in activity with age.

There are ways in which the genetic tools of Drosophila can be used to distinguish between the various possibilities. One is to increase or decrease integrin function specifically in the muscles or nervous system, thus identifying the essential focus of integrin activity. A second would be to restore integrin function during adult stages, to assay the time of function of the integrins in the aging process. A third would be to examine other proteins known to be required for integrin adhesion, such as the cytoskeletal linker protein talin (15), to determine whether loss of such a protein has a similar affect on aging. If it does not, this result would support the novel signaling pathway possibility.

Will the finding by Goddeeris et al. initiate a race to identify a treatment for age-related loss of mobility in humans? Clearly, eliminating all integrin function would not be a desirable approach. Recessive human hereditary diseases caused by mutations in integrin genes have been identified that result in skin blistering, the inability to clot blood, and the inability to fight bacterial infections [reviewed in (16)]. Mutations have not yet been identified in the human {beta}1 integrin subunit, which is the ortholog of the fly {beta}PS subunit, most likely because the loss of this widely expressed subunit would cause lethality in the embryo, as found for mice and Drosophila. Nonetheless, it would be interesting to assess whether being heterozygous for mutations in the other integrin subunits (for example, the parents of affected individuals) have any tendency toward a more active lifestyle or an increased mean life span. It might be possible to modestly reduce integrin function with pharmacological treatment. However, the desirability of this treatment depends on how integrins contribute to increased activity and survival; more limber joints sound great, but forgetting where you parked the car and having to walk home would be less of an advantage!


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Citation: C. N. Torgler, N. H. Brown, Reducing Integrins Improves the Quality of Fly Life. Sci. SAGE KE 2003 (40), pe28 (2003).








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