Glycogen synthetase and phosphorylase in developing chick glycogen body

Expression of lactate dehydrogenase-A and -B messenger ribonucleic acids in chick glycogen body

Glucose is a main metabolic substrate in the central nervous system (CNS). Recently, lactate has attracted renewed attention because it plays an important role in the CNS as a metabolic substrate between neurons and astrocytes. Lactate and lactate dehydrogenase (LDH), a key enzyme for lactate and pyruvate interconversion, have been investigated. The glycogen body (GB) is a gelatinous tissue in the dorsal area of the lumbosacral spinal cord in birds. It is composed of uniform cells with high glycogen storage and is thought to be in the astroglia lineage. In this article, we investigated the LDH enzyme activity in embryo GB (embryonic d 12, 14, 16, 17, and 18) and chickens. To detect the subtype of the LDH, mRNA expressions in GB were investigated and compared with those of cerebral cortex and spinal cord. By histochemical observations, LDH enzyme activity was detected in the cytoplasm of GB cells of all embryonic periods after embryonic d 12. In biochemical analysis, the enzymatic activities of the GB were higher than in the cerebral cortex. In the GB the subtype of LDH mRNA was LDM-B only, and in the cerebral cortex and spinal cord, the subtype of LDH mRNA was predominantly LDH-B, and a small amount of LDH-A was found in the chicken and embryo. The LDH-B mRNA expression in GB was detected during all periods of the study (after embryonic d 12). These observations suggest that GB expressed only LDH-B and that GB cells are not lactate-producing cells, like astrocyte, but rather are lactate-consuming cells, similar to neurons in CNS.

An hypothesis of function for the avian glycogen body: a novel role for glycogen in the central nervous system

Our own and other recent data have led us to hypothesize that the glycogen body, heretofore generally considered to be metabolically inert, may be functionally geared to support the process of myelin formation in the avian central nervous system (CNS). We envision that the abundant glycogen stores in this tissue, unlike those in the liver or in skeletal muscle, can serve as a recyclable substrate for the ultimate production of reducing equivalents that would be available for the synthesis of myelin lipid cholesterol. In addition, it is suggested that such glycogen also may serve as a source of organic acids which might provide alternate substrates to the CNS under conditions of metabolic stress.

Glycogen metabolism in the developing accessory lobes of Lachi in the nerve cord of the chick: metabolic correlations with the avian glycogen body

Glycogen synthase, glycogen phosphorylase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase were determined for the first time in the necessary lobes of Lachi from late embryonic chicks. The activities of these enzymes were compared with those found in other glycogen-metabolizing tissues, specifically the glycogen body, liver, and skeletal muscle, obtained from the same embryos. The data show that, as in the glycogen body, the accessory lobes of Lachi lack glucose-6-phosphatase, but contain relatively high activity levels of glycogen synthase I, total and active glycogen phosphorylase, and the dehydrogenases of glucose-6-phosphate and 6-phosphogluconate. The percent of glycogen synthase I activity in the Lachi lobes is from two- to 20-fold greater than observed in the glycogen body, liver, or muscle, whereas the percent of glycogen phosphorylase a activity is comparable to that of the liver, but greater than that in the glycogen body or muscle. The activity of each dehydrogenase of the pentose phosphate cycle in the Lachi lobes is similar to that noted in the glycogen body, but is over two- or fivefold greater than that activity found in muscle or liver. Our data, together with other recent evidence, suggest that the role of glycogen in these functionally enigmatic tissues may be to support the precocious process of myelin synthesis in the developing bird, as well as possibly to provide alternate sources of energy for the avian central nervous system.

An ultrastructural study of the craniocaudal continuation of the glycogen body

An ultrastructural analysis of the chicken glycogen body and its craniocaudal continuation areas shows a continuum of astroglial cell types. Characteristic glycogen body astroglia are confined to the classically defined body located in the chicken lumbosacral spinal cord. These are large cells which have an eccentric dark nucleus surrounded by a rim of dense cytoplasm which contains the usual complement of organelles. The remainder of the cell volume is occupied by alpha and beta glycogen particles interspersed with a flocculo-granular material continuous with the main cytoplasmic mass. Astroglial cells of continuation areas usually have a light cytoplasm and a centrally placed nucleus. They contain beta glycogen particles of varying sizes, but like the glycogen body cells, may have beta particles as large as 48 nm. Such particles, which resemble four leaf clovers in shape, are suggestive of an ordered substructure. Gliofilaments are not always conspicuous in astroglial perikarya, but large numbers of them are present in the processes. Although the continuation areas are mostly confined to gray matter regions, the contained astroglial processes exhibit circular, triangular, or cylindrical shapes and form an unpatterned mosaic. Astrocytic processes forming the glia limitans on the anterior and posterior margins of the cord often contain conspicuous amounts of glycogen. The ultrastructural identification of such large amounts of glycogen within the chicken nervous system suggests that it plays a major role in avian neural metabolism.

Ultrastructural characterization of the accessory lobes of Lachi (Hofmann's nuclei) in the nerve cord of the chick. I. Axoglial synapses

Light microscopic observations of the accessory lobes of Lachi of one-day-old chicks show that this tissue contains abundant amounts of glycogen and consists of cells which are similar in appearance to those of the glycogen body. Ultrastructural studies reported here for the first time confirm the presence of glycogen-rich cells in the accessory lobes and reveal that these cells are intimately associated with nerve axons. The finding of synaptic complexes and other junctional specializations between nerves and accessory lobe cells suggests that they may have a functional relationship with the nervous system. It is felt that the accessory lobe cells may be neuroglia, possibly of the astrocytic type, which have an innate capacity for glycogen storage. While the functional significance of such glycogen remains obscure, the close morphological association between neurons and the accessory lobe cells enhances the hypothesis put forth by us regarding the glycogen body, namely that neural glycogen is involved in myelin synthesis in the avian nervous system.

Glycogen metabloism in the developing chick glycogen body: functional significance of the direct oxidative pathway

Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase were quantitatively determined for the first time in glycogen body tissue from late embryonic and neonatal chicks. For comparative purposes, the activities of these enzymes were examined also in liver and skeletal muscle from pre- and post-hatched chicks. The present data show that both the embryonic and neonatal glycogen body lack glucose-6-phosphatase, but contain relatively high levels of glucose-6-phosphate dehydrogenase. The activity of each dehydrogenase in either embryonic or neonatal glycogen body tissue is two- to five-fold greater than that found in muscle or liver from pre- or post-hatched chicks. The relatively high activities observed for both dehydrogenases in the glycogen body, together with the absence of glucose-6-phosphatase activity in that tissue, suggest that the direct oxidative pathway (pentose phosphate cycle) of glucose metabolism is a functionally significant route for glycogen utilization in the glycogen body. It is hypothesized that the glycogen body is metabolically linked to lipid synthesis and myelin formation in the central nervous system of the avian embryo.