The alpha-glycerophosphate cycle in Drosophila melanogaster. IV. Metabolic, ultrastructural, and adaptive consequences of alphaGpdh-l "null" mutations

"Null" mutations previously isolated at the alphaGpdh-1 locus of Drosophila melanogaster, because of disruption of the energy-producing alpha-glycerophosphate cycle, severely restrict the flight ability and relative viability of affected individuals. Two "null" alleles, alphaGpdh-1(BO-1-4), and alphaGpdh-1(BO-1-5,) when made hemizygous with a deficiency of the alphaGpdh-1 locus, Df(2L)GdhA, were rendered homozygous by recombination with and selective elimination of the Df(2L)GdhA chromosome. After over 25 generations, a homozygous alphaGpdh-1(BO-1-4) stock regained the ability to fly despite the continued absence of measurable alphaGPDH activity. Inter se heterozygotes of three noncomplementing alphaGpdh-1 "null" alleles and the "adapted" alphaGpdh-1(BO-1-4) homozygotes were examined for metabolic enzymatic activities related to the energy-producing and pyridine nucleotide-regulating functions of the alpha-glycerophosphate cycle in Drosophila. The enzyme functions tested included glyceraldehyde-3-phosphate dehydrogenase, cytoplasmic and soluble malate dehydrogenase, lactate dehydrogenase, mitochondrial NADH oxidation, oxidative phosphorylation, and respiratory control with the substrates alpha-glycerophosphate, succinate, and pyruvate. These activities in any of the mutant genotypes in early adult life were indistinguishable from those in the wild type. There was, however, a premature deterioration and atrophy of the ultrastructural integrity of flight muscle sarcosomes observed by electron microscopy in the "null" mutants. These observations were correlated with a decrease in state 3 mitochondrial oxidation with alpha-glycerophosphate, succinate, and pyruvate, as well as with loss of respiratory control in adults as early as 2 wk after eclosion. Such observations, which normally are seen in aged dipterans, were accompanied by premature mortality of the mutant heterozygotes. The adapted alphaGpdh-1(BO-1-4) was identical with wild type in each of the aging characters with the single exception of lowered rates of mitochondrial oxidative phosphorylation.

Degenerative changes in the mitochondria of flight muscle from aging blowflies

Mitochondria from flight muscle of aging blowflies, Phormia regina, were examined morphologically and biochemically with the electron microscope. An age-dependent degeneration of the mitochondria that is characterized, in part, by the reorganization of the inner membrane into myelin-like whorls has been found. The concentric rings increase in size and number, eventually replacing the normal cristal conformation. Glycogen rosettes are frequently seen in the center of the whorl and may represent the intrusion into the mitochondria of the glycogen in the cytoplasmic matrix of the muscle. The degenerating mitochondria are not associated with lysosomal activity, as indicated by the absence of acid phosphatase. An intense acid phosphatase activity is noted, however, in the dyad, comprising elements of the T system and sarcoplasmic reticulurn. Cytochrome oxidase is active in the ultrastructurally intact portion of the mitochondrion but activity is not evident in that part of the mitochondrion that has undergone morphological change. Thus, the ultrastructural degradation of the mitochondria is correlated with a decrease in biochemical function. This suggests a correspondence between a decrease in the bioenergetic capacity of the flight muscle and a decline in the ability of the aged insect to fly.

Electron microscope observations on the fusion of chick myoblasts in vitro

The process of myoblast fusion was observed in embryonic chick skeletal muscle cells grown in monolayer cultures at the fine structural level. At the first step, the sarcolemmas of cells destined to fuse are closely applied to each other. They are linked in some places by fasciae adherentes; in other places, engulfment of small processes of one cell by another is seen. At a somewhat more advanced stage of myogenesis, vesicles and tubules are formed between the adjacent cytoplasms; presumably, the apposed membranes have opened at several points and their edges have fused to each other. Finally, remnants of cell membranes (vesicles and tubules) disappear completely, and the confluent cytoplasm is formed. The cytoplasmic contents of the multinucleated cells are often poorly admixed, giving the cytoplasm a mosaic appearance in which different zones can be designated as arising from separate cells. This observation suggests, however, that there is slow diffusion of myoblast contents (ribosomes and, possibly, other materials) into the myotube. In agreement with the previous works at the light microscopic level, the present study suggests the occurence of fusion between mononucleated cells, between mononucleated cells and multinucleated myotubes, and between nascent multinucleated myotubes.

Formation of neuromuscular junctions in embryonic cell cultures

The formation of nerve-muscle junctions in monolayer cultures of embryonic muscle and spinal cord cells is described. Muscle-forming cells (myoblasts) from leg muscles of 12-day chick embryos were separated with trypsin and cultured on a collagen substrate for two days. Suspensions of ventral spinal cord cells from six-day chick embryos were then plated over the differentiating muscle cells. The cultures were subsequently examined by electron microscopy and in silver-stained light microscopic preparations. After 10-12 days in culture, irregular thickenings were observed along the nerve cell processes in contact with the muscle fibers which by that time had undergone advanced differentiation (myogenesis). By electron microscopy it was demonstrated that many of the nerve-muscle contacts had the characteristics of a synapse. Some aspects of the fine structure of these junctions are described. The possibilities raised by these findings with respect to innervation mechanisms and specificities in nerve-muscle interactions are briefly discussed.

The fine structure of embryonic chick skeletal muscle cells differentiated in vitro

Dissociated myoblasts from 12-day chick embryos were cultured in monolayer, and the differentiation of skeletal muscle cells was studied by electron microscopy. The results have revealed a striking ultrastructural similarity between the in vivo and the in vitro developing muscle, particularly with respect to the myofibrils and sarcoplasmic reticulum. This study demonstrates that all the characteristic organelles of mature skeletal muscle can develop in vitro in the absence of nerves.