Study helps understand at the cellular level how physical activity contributes to fitness during aging

Regular exercise can guarantee quality of life and longevity, specialists agree. However, exactly how exercise influences the functioning of muscle cells is poorly understood. A study conducted at the University of São Paulo's Biomedical Sciences Institute (ICB-USP) helps understand at the cellular level how physical activity contributes to fitness during the aging process.

The study was supported by FAPESP. An article describing it is published in the Proceedings of the National Academy of Sciences (PNAS).

According to the authors, the answer lies in mitochondria, membrane-bound organelles that generate most of the chemical energy needed to power the cell's biochemical reactions. Mitochondria are constantly being remodeled in processes that include fission and fusion. Mitochondrial dynamics regulate the distribution and functioning of hundreds or thousands of mitochondria in muscle cells.

The researchers investigated the role of mitochondrial dynamics during exercise in the model organism Caenorhabditis elegans, a simple, well-studied microscopic nematode worm species. The results pointed to a buildup of fragmented and hence dysfunctional mitochondria in muscle cells during aging. Regular exercise throughout the subject's life increases the frequency of mitochondrial fusion, benefiting both the mitochondrial metabolism and cellular functioning sufficiently to maintain muscle physiology even into old age.

In the study, we demonstrated that a single exercise session rapidly induced mitochondrial fission in muscles followed after a period of recovery by mitochondrial fusion. Daily sessions throughout life favor the appearance of connected mitochondria, delaying fragmentation and the decline in physical fitness seen during aging. In sum, we confirmed that exercise and mitochondrial dynamics are significantly associated with the maintenance of muscle function in senescence. This was the proof of concept we were looking for."

Julio Cesar Batista Ferreira, last author of the article and professor at ICB-USP

Previous research by the group showed that aerobic exercise protects the heart by facilitating the removal of dysfunctional mitochondria and promoting mitochondrial fusion in cardiac cells (read more at: agencia.fapesp.br/25890).

The next step was to find out how exercise affects aging in healthy organisms. To do this, the researchers decided to use C. elegans, an experimental model organism frequently used in metabolic and aging research (read more at: agencia.fapesp.br/38425).

"Studies of aging that follow people or rodents for years are laborious and costly. The advantage of C. elegans is that it has several similarities with human beings but has a life cycle lasting only 25 days, so that we were able to show for the first time what happens to an organism that takes exercise throughout its life, identifying the critical cellular events in the process," Ferreira explained.

Mitochondrial quantity and quality, and hence good muscle functioning, are profoundly influenced by mitochondrial dynamics, he added. Fission and fusion are assisted by proteins known as GTPases that cleave and splice mitochondria.

"Under stressful conditions, they remove non-functional parts of mitochondria for destruction and join up the functional parts with other mitochondria. Mitochondrial segregation and adequate cell functioning result from these fission and fusion dynamics," he said.

The results of the study showed that mitochondrial connectivity and the fission-fusion cycle are essential to maintain physical fitness and the capacity to respond to exercise while aging.

Training protocol

In designing the study, the researchers developed an exercise training protocol for the worms. "These organisms mostly live in a solid medium [such as soil in nature or gelatin in the laboratory]. When we move them to a liquid medium, we find that they increase the frequency of their bending or undulatory motion, expending more energy as they do so, just as humans do when exercising," Ferreira said.

Daily exposure of the worms to the liquid medium resulted in physiological and biochemical adaptations similar to those observed in humans and rodents after exercise training. "When they exercised throughout their lives, mitochondrial fission and fusion remained integral in senescence, whereas fragmented and dysfunctional mitochondria accumulated in sedentary worms at around ten days of life when they're considered senile. Regular exercise improves well-being measured in terms of indicators such as muscle function, mobility, food intake and resistance to different kinds of stress. All these indicators were better in the physically active worms," he said.

According to Ferreira, the indicators for worms that swam regularly until adulthood but became sedentary when senile were also better than for worms that were always sedentary. "The reason is the cellular memory created by the daily stimulus of physical activity from mitochondrial fission and fusion. This protects the organism during aging," he said.

Accelerated aging

Genetic engineering techniques were used to "switch off" the main genes involved in mitochondrial fission and fusion. This genetic modification led to accelerated aging, and exercise became toxic for these worms since remodeling, segregation and removal of dysfunctional mitochondria did not occur. "This confirms the importance of mitochondrial dynamics both to senescence and to the benefits of regular exercise," Ferreira said.

In a second part of the study, the researchers investigated whether an increase in longevity was accompanied by an improvement in physical fitness, conducting experiments with worm lineages capable of living for up to 40 days thanks to alterations to parts of their genome. Surprisingly, exercise had a toxic effect on four of the five long-lived genotypes tested.

"We wanted to find out whether the increase in lifespan was associated with an improvement in physical fitness and responsiveness to exercise throughout their lives. This is a crucial matter, given the worldwide increase in human longevity. The study showed, however, that longevity doesn't necessarily correlate with quality of life. It should be borne in mind that nothing in humans corresponds to these genetically modified worms, which live almost twice as long as wild-type worms," Ferreira said.

Metabolic sensor

Only one long-lived lineage (out of the five included in the study) displayed an improvement in fitness during its entire life. This lineage expressed more AMP-activated protein kinase (AMP standing for adenosine monophosphate), or AMPK, a constitutively active enzyme that acts as a metabolic sensor in cells, regulating energy and mitochondrial metabolism. Production of AMPK tends to decline with aging.

"In this experiment, only worms with lifelong active AMPK [thanks to mutations produced in the laboratory] lived and swam better for longer. Moreover, when we genetically removed the proteins that regulate mitochondrial dynamics, the effects of AMPK were abolished. In this case, the worms exhibited reduced physical fitness and consequently declining muscle function in old age," Ferreira said.

The experiments with AMPK suggest activation of the enzyme may mimic some benefits of exercise via regulation of mitochondrial dynamics. "Mechanisms that optimize mitochondrial fission and fusion, as well as AMPK activation, could be part of promising healthy aging strategies, improving muscle contractile and biochemical functions," Ferreira said.

Regular exercise contributes to healthy aging by regulating mitochondrial dynamics and the other systems on which good cellular functioning is based. "As we know, however, only a small proportion of the population exercise regularly. Science-based public policy is needed to stimulate the habit. Furthermore, we shouldn't forget that pharmacological interventions can control the processes involved and potentially treat several aging-related diseases," he said.

In recent years, the group led by Ferreira at ICB-USP has developed a molecule called "SAMβA" that facilitates mitochondrial fusion and improves the quality of life for animals with heart failure (read more at: agencia.fapesp.br/29842). The compound is currently undergoing preclinical trials to assure safety and efficacy.