AGE CHANGES IN SIZE AND NUMBER OF THE GIANT MITOCHONDRIA IN THE FLIGHT MUSCLE OF THE COMMON HOUSEFLY (MUSCA DOMESTICA L.)

Regulation of muscle and mitochondrial health by the mitochondrial fission protein Drp1 in aged mice

KEY POINTS

The maintenance of mitochondrial integrity is critical for skeletal muscle health. Mitochondrial dynamics play key roles in mitochondrial quality control; however, the exact role that mitochondrial fission plays in the muscle ageing process remains unclear. Here we report that both Drp1 knockdown and Drp1 overexpression late in life in mice is detrimental to skeletal muscle function and mitochondrial health. Drp1 knockdown in 18-month-old mice resulted in severe skeletal muscle atrophy, mitochondrial dysfunction, muscle degeneration/regeneration, oxidative stress and impaired autophagy. Overexpressing Drp1 in 18-month-old mice resulted in mild skeletal muscle atrophy and decreased mitochondrial quality. Our data indicate that silencing or overexpressing Drp1 late in life is detrimental to skeletal muscle integrity. We conclude that modulating Drp1 expression is unlikely to be a viable approach to counter the muscle ageing process.

ABSTRACT

Sarcopenia, the ageing-related loss of skeletal muscle mass and function, is a debilitating process negatively impacting the quality of life of afflicted individuals. Although the mechanisms underlying sarcopenia are still only partly understood, impairments in mitochondrial dynamics, and specifically mitochondrial fission, have been proposed as an underlying mechanism. Importantly, conflicting data exist in the field and both excessive and insufficient mitochondrial fission were proposed to contribute to sarcopenia. In Drosophila melanogaster, enhancing mitochondrial fission in midlife through overexpression of dynamin-1-like protein (Drp1) extended lifespan and attenuated several key hallmarks of muscle ageing. Whether a similar outcome of Drp1 overexpression is observed in mammalian muscles remains unknown. In this study, we investigated the impact of knocking down and overexpressing Drp1 protein for 4 months in skeletal muscles of late middle-aged (18 months) mice using intra-muscular injections of adeno-associated viruses expressing shRNA targeting Drp1 or full Drp1 cDNA. We report that knocking down Drp1 expression late in life triggers severe muscle atrophy, mitochondrial dysfunctions, degeneration/regeneration, oxidative stress and impaired autophagy. Drp1 overexpression late in life triggered mild muscle atrophy and decreased mitochondrial quality. Taken altogether, our results indicate that both overexpression and silencing of Drp1 in late middle-aged mice negatively impact skeletal muscle mass and mitochondrial health. These data suggest that Drp1 content must remain within a narrow physiological range to preserve muscle and mitochondrial integrity during ageing. Altering Drp1 expression is therefore unlikely to be a viable target to counter sarcopenia.

The Interplay between Mitochondrial Morphology and Myomitokines in Aging Sarcopenia

Sarcopenia is a chronic disease characterized by the progressive loss of skeletal muscle mass, force, and function during aging. It is an emerging public problem associated with poor quality of life, disability, frailty, and high mortality. A decline in mitochondria quality control pathways constitutes a major mechanism driving aging sarcopenia, causing abnormal organelle accumulation over a lifetime. The resulting mitochondrial dysfunction in sarcopenic muscles feedbacks systemically by releasing the myomitokines fibroblast growth factor 21 (FGF21) and growth and differentiation factor 15 (GDF15), influencing the whole-body homeostasis and dictating healthy or unhealthy aging. This review describes the principal pathways controlling mitochondrial quality, many of which are potential therapeutic targets against muscle aging, and the connection between mitochondrial dysfunction and the myomitokines FGF21 and GDF15 in the pathogenesis of aging sarcopenia.

Mitochondrial Quality Control and Muscle Mass Maintenance

Loss of muscle mass and force occurs in many diseases such as disuse/inactivity, diabetes, cancer, renal, and cardiac failure and in aging-sarcopenia. In these catabolic conditions the mitochondrial content, morphology and function are greatly affected. The changes of mitochondrial network influence the production of reactive oxygen species (ROS) that play an important role in muscle function. Moreover, dysfunctional mitochondria trigger catabolic signaling pathways which feed-forward to the nucleus to promote the activation of muscle atrophy. Exercise, on the other hand, improves mitochondrial function by activating mitochondrial biogenesis and mitophagy, possibly playing an important part in the beneficial effects of physical activity in several diseases. Optimized mitochondrial function is strictly maintained by the coordinated activation of different mitochondrial quality control pathways. In this review we outline the current knowledge linking mitochondria-dependent signaling pathways to muscle homeostasis in aging and disease and the resulting implications for the development of novel therapeutic approaches to prevent muscle loss.

Giant mitochondria do not fuse and exchange their contents with normal mitochondria

Giant mitochondria accumulate within aged or diseased postmitotic cells as a consequence of insufficient autophagy, which is normally responsible for mitochondrial degradation. We report that giant mitochondria accumulating in cultured rat myoblasts due to inhibition of autophagy have low inner membrane potential and do not fuse with each other or with normal mitochondria. In addition to the low inner mitochondrial membrane potential in giant mitochondria, the quantity of the OPA1 mitochondrial fusion protein in these mitochondria was low, but the abundance of mitofusin-2 (Mfn2) remained unchanged. The combination of these factors may explain the lack of mitochondrial fusion in giant mitochondria and imply that the dysfunctional giant mitochondria cannot restore their function by fusing and exchanging their contents with fully functional mitochondria. These findings have important implications for understanding the mechanisms of accumulation of age-related mitochondrial damage in postmitotic cells.

Nature, origin and fates of membranous lamellae in the lung of the neonate rat

This study explores the basic nature and formation of lamellar accumulations in the vertebrate lung, and the problematical interrelationship of the lamellae with mitochondria. Autolysosomes are a constant feature of the type II alveolar pneumonocyte of the 2-day-old rat. They are characterized by a single boundary membrane, enclosing a heterogeneous collection of vesicles and membraneous lamellae. The autolysosomes result from repeated episodes of glycogen catabolism, and eventually transform into osmiophilic lamellated bodies. Membranous lamellae within autolysosomes and lamellated bodies represent isolating membranes of cellular autophagy, emptied of their digested contents. Proliferated isolating membranes themselves undergo lysis, providing recycled constituents for the differentiating or dividing cell. Mitochondria of the type II alveolar cell often display invaginations occupied by membranous masses; these masses are demonstrated by high magnification electron micrographs to be continuous with and derived from lamellated bodies, a new finding. Inner mitochondrial membranes lining the invaginations are thinned and crista-free. It is concluded that undergraded or partially lysed isolating membranes follow either of two cellular pathways: they may be eliminated from the cell into the alveolar cavity, or may fill mitochondrial indentations.

The fine structural morphology of the midgut of aged Drosophila: a morphometric analysis

The midguts of 1-day and 72 day-old fruitflies were examined morphometrically at the electron microscopic level. The major alterations noted were that the number of supranuclear mitochondria decreased by approximately 50%, while the volume of individual mitochondria doubled as a function of age. Moreover, approximately 29% of the nuclear volume of old flies, was occupied by inclusion bodies as was 19% of the supranuclear cytoplasmic volume. Additionally, the surface density of rough endoplasmic reticulum was reduced to more than half that of young flies. It is suggested that the functional capability of the parenchymal cells become debilitated due to the presence of these inclusion bodies, and that the cell's ability to manufacture proteins and produce energy are seriously hindered by the mitochondrial alterations and reduction in the surface density of the rough endoplasmic reticulum.

Age-related chromatin condensation in flight muscle nuclei of the adult male housefly, Musca domestica

Computer analysis of indirect flight muscle nuclei from the adult male housefly, M. domestica, has shown significant change with age in the chromatin condensation pattern. The pattern was analyzed by examining low, medium, and high density chromatin components (LDC, MDC, HDC). No significant change occurred in HDC with age, and the amount and distribution of LDC and MDC remain relatively stable between Day 1 and Day 4 post-eclosion. However, the analysis showed a significant increase in the amount of MDC with a corresponding decrease in the amount of LDC between Day 4 and Day 14. This exchange was accompanied by a significant redistribution of both components. These results are discussed with reference to the biochemical and ultrastructural profile of the flight muscle with age, and to age-related changes in the condensation pattern of specific brain and Malpighian tubule nuclei described earlier.

Freeze-fracture analysis of the effects of age and 2,4-dinitrophenol on the morphology of flight muscle mitochondria of Musca domestica L

Morphometric comparison of freeze-fractured mitochondria in flight muscles of adult (37-day-old) and old (68-day-old) houseflies revealed a 28% decrease of cristae in the old flies. The major membrane change with age was an increase in the 90-120-A particles in the inner membrane external face concomitant with a loss of particle clusters associated with the openings of the cristae on to the inner membrane. In vitro treatment of flight muscle with 2,4-dinitrophenol, and uncoupler of mitochondrial respiration, did not produce this change but resulted in the formation of smooth particle-free vesicular swellings in the mitochondria. Such swelling were infrequent in the old muscle. The cause for the aging change is not clear, but a reduction in the ability of the intramembranous particles to aggregate, either through modification of altered synthesis, is indicated.

Mitochondrial changes in flight muscles of normal and flightless Drosophila melanogaster with age

Fine structural changes in mitochondrial morphology pertaining to size, number and growth were examined in flight muscles of normal and experimentally dewinged male Drosophila melanogaster ranging up to 26 days of age. In the normal winged flies, the number of mitochondria decreases during the first week of adult life whereas the size of individual mitochondrial profile increases significantly. Changes in mitochondrial size and number are due to the fusion of mitochondria. Fused mitochondria are are extremely large in size and irregular in shape. In 26-day old normal flies, the number of mitochondria increases while the mitochondrial size is recuced indicating mitochondrial division. In comparison to the normal flies, dewinged flies exhibit a similar degree of mitochondrial fusion and growth during the first week of life. However, the extent of mitochondrial fission in 26-day old dewinged flies is greater than in the normal flies of this age. Structural mechanisms of mitochondrial fusion and fission are described. The objective of this study was to examine the relative effects of age and flight activity on the mitochondria.

Migration Strategies of Insects

Physiological and ecological results from a variety of species are consistent with what seem to be valid general statements concerning insect migration. These are as follows: (i)During migration locomotory functions are enhanced and vegetative functions such as feeding and reproduction are suppressed. (ii) Migration usually occurs prereproductively in the life of the adult insect (the oogenesis-flight syndrome). (iii)Since migrant individuals are usually prereproductive, their reproductive values, and hence colonizing abilities, are at or near maximum. (iv) Migrants usually reside in temporary habitats. (v)Migrants have a high potential for population increase, r, which is also advantageous for colonizers. (vi)Both the physiological and ecological parameters of migration are modifiable by environmental factors (that is, phenotypically modifiable)to suit the prevailing conditions. Taken together, these criteria establish a comprehensive theory and adumbrate the basic strategy for migrant insects. This basic strategy is modified to suit the ecological requirements of individual species. Comparative studies of these modifications are of considerable theoretical and practical interest, the more so since most economically important insects are migrants. No satisfactory general statements can as yet be made with respect to the genotype and migration. Certainly we expect colonizing populiations to possess genotypes favoring a high r, but genotypic variation in r depends on the heritabilities of life table statistics, and such measurements are yet to be made (10, 53). The fact that flight duration can be increased by appropriate selection in Oncopeltus fasciatus, and the demonstration of additive genetic variance for this trait in Lygaeus kalmii, suggest that heritability studies of migratory behavior would also be worth pursuing. Most interesting of course, will be possible genetic correlations between migration and life history parameters. Also, migration often transports genotypes across long distances with considerable mixing of populations. An understanding of its operation therefore carries with it implications for population genetics, zoogeography, and evolutionary theory. Finally, at least parts of the above general theory would seem to be applicable to forms other than insects. Bird and insect migrations, for example, are in many respects ecologically and physiologically similar. Birds, like insects, emphasize locomotory. as opposed to vegetative functions during long-distance flight; the well-known Zugenruhe or migratory restlessness is a case in point. Further, many birds migrateat nigt at a time when they would ordinarily roost(vegetative activity). Because their life spans exceed single seasons, bird migrants are not prereproductive in the same sense that insect migrants are, and hence reproductive values do not have the same meaning(but note that some insects are also interreproductive migrants). The situaion is complicated further by the fact that in many birds adult survivorship is virtually independent of age so that colonizing ability tends to be also (10, 54). Nevertheless, birds arrive on their nesting grounds in reproductive condition with the result that migration is a colonizing episode. It is also phenotypically modifiable by environmental factors, some of which, for example, photoperiod, influence insects as well (55). The similarities between birds and insects thus seem sufficient to indicate, at least provisionally, that the theory developed for insects applies also to birds with appropriate modifications for longer life span and more complex social behavior; comparisons between insects and fish (56) lead to the same conclusion. In birds especially, and also in other forms, various functions accessory to migration such as reproductive endocrinology, energy budgets, and orientation mechanisms have been studied extensively (55, 56). But there is need in vertebrates for more data andtheoy on the ecology and physiology of migratory behavior per se in order tobetter understand its evolution and its role in ecosystem function (5, 57). Migration in any animal cannot be understood until viewed in its entirety as a physiological, behavioral, and ecological syndrome.