Stimulatory effects of growth hormone and thyroxine on the concentration of gangliosides in the Snell dwarf cerebrum

Specific ganglioside changes in extraneural tissues of adult rats with hypothyroidism

Adults rats with hypothyroidism were prepared by administration of 6-propyl-2-thiouracil (PTU) or methimazole, and the tissues were examined for their gangliosides through methods including glycolipid-overlay techniques. Normal thyroid tissue contained GM3, GD3, and GD1a as the major gangliosides, with GM1, GD1b, GT1b, and GQ1b in lesser amounts. The goitrous tissue of PTU-induced hypothyroid rats had higher concentrations of GM1 and GD1a with a concomitant decrease of GM3. The amount of GT3 in thyroid tissue was increased in hypothyroid animals. While normal liver tissue had a complex ganglioside pattern with a- and b-series gangliosides, the PTU-induced hypothyroid tissue showed a simpler ganglioside profile that consisted mainly of a-series gangliosides with almost undetectable amounts of b-series gangliosides. The expression of c-series gangliosides was suppressed in the hypothyroid liver tissue. Heart tissue had higher contents of GM3 and GT3 than control. No apparent change was observed in the compositions of major and c-series gangliosides in other extraneural tissues (i.e., kidney, lung, spleen, thymus, pancreas, testis, skeletal muscle, and eye lenses), and neural tissues (i.e., cerebrum and cerebellum) from PTU-induced hypothyroid rats. The ganglioside changes of thyroid, liver, and heart tissues were reproduced in corresponding tissues of methimazole-induced hypothyroid rats. These results suggest that hypothyroid conditions affect the biosynthesis and expression of gangliosides in specific tissue and cell types.

Cerebellar microfolia and other abnormalities of neuronal growth, migration, and lamination in the Pit1dw-J homozygote mutant mouse

The Snell dwarf mouse (Pit1dw-J homozygote) has a mutation in the Pit1 gene that prevents the normal formation of the anterior pituitary. In neonates and adults there is almost complete absence of growth hormone (GH), prolactin (PRL), thyroxin (T4), and thyroid-stimulating hormone (TSH). Since these hormones have been suggested to play a role in normal development of the central nervous system (CNS), we have investigated the effects of the Pit1dw-J mutation on the cerebellum and hippocampal formation. In the cerebellum, there were abnormalities of both foliation and lamination. The major foliation anomalies were 1) changes in the relative size of specific folia and also the proportional sizes of the anterior vs posterior cerebellum; and 2) the presence of between one and three microfolia per half cerebellum. The microfolia were all in the medial portion of the hemisphere in the caudal part of the cerebellum. Each microfolium was just rostral to a normal fissure and interposed between the fissure and a normal gyrus. Lamination abnormalities included an increase in the number of single ectopic granule cells in the molecular layer in both cerebellar vermis (86%) and hemisphere (40%) in comparison with the wild-type mouse. In the hippocampus of the Pit1dw-J homozygote mouse, the number of pyramidal cells was decreased, although the width of the pyramidal cell layer throughout areas CA1-CA3 appeared to be normal, but less densely populated than in the wild-type mouse. Moreover, the number of granule cells that form the granule cell layer was decreased from the wild-type mouse and some ectopic granule cells (occurring both as single cells and as small clusters) were observed in the innermost portion of the molecular layer. The abnormalities observed in the Pit1dw-J homozygote mouse seem to be caused by both direct and indirect effects of the deficiency of TSH (or T4), PRL, or GH rather than by a direct effect of the deletion of Pit1.

Hippocalcin expression in the brain of the Snell dwarf mutant mouse

To determine factors contributing to the expression of the brain-derived protein, hippocalcin, we mapped its distribution in the brain of Snell pituitary dwarf mutant mice (dw) by immunohistochemical and immunoblot methods. Our findings are as follows. (1) In the hippocampus, hippocalcin immunoreactivity was found in the cell body and dendrites of pyramidal neurons of the normal controls and dw mice, although the intensity of immunoreactivity in the dw mice was lower. (2) In the cerebellum, hippocalcin immunoreactivity was strongly expressed in the Purkinje cell body of both the control and dw mice. However, the Purkinje cell dendrites were found to be more intensely stained in the dw mice than in the normal controls. (3) In the dw cerebral cortex, the pyramidal neurons of layers II to VI strongly expressed hippocalcin, whereas its expression in the controls was weak. (4) The amount of hippocalcin in the dw hippocampus was less than in the normal controls, whereas the amount in the dw cerebral cortex and cerebellum was greater. These results indicate that the developmental expression of hippocalcin in the dw brain is affected by the retarded maturation of the neuronal network due to the deficient hormonal state (the lack of growth and thyroid hormones).

Stimulatory effects of exogenous thyroid hormone and growth hormone on sn-glycerol-3-phosphate dehydrogenase activity in the genetic-hypothyroid mutant mouse cerebellum: restricted to the second 20 days of postnatal life

We recently reported that by postnatal day 40 the activity of sn-glycerol-3-phosphate dehydrogenase (GPDH) was significantly depressed in the cerebellum of genetic-hypothyroid mutant mice. This mutant mouse-GPDH combination was used in the present study to define the critical time period during which thyroid hormone (T4) and growth hormone (GH) are essential for maturation of Bergmann glial cells. Our findings are that (a) induction of GPDH activity in the Bergmann glial cell is dependent on T4, (b) T4 is most effective when administered during the second 20 days of postnatal life, (c) the effect of GH on GPDH activity is complementary to or synergistic with that of T4, and (d) Bergmann glial cells and radial glial fibers of the mutant mice contain immunoreactive GPDH following various hormonal treatments. These results suggest that T4 is indispensable for the maturation of Bergmann glial cells.

Actions of insulin-like growth factor-I on the B104 neuronal cell line: effects on cell replication, receptor characteristics, and influence of secreted binding protein on ligand binding

Several peptide growth factors influence the growth and differentiation of neural cells. To investigate further the growth-promoting effects of the somatomedins on cells of neural origin, the authors characterized the binding and mitogenic effects of insulin-like growth factor-I (IGF-I) on a functionally differentiated rat neuronal cell line (B104). Specific, high-affinity (Kd approximately equal to 10(-9) M) receptors for IGF-I were abundant (approximately 124,000 binding sites/B104 cell). These IGF-I receptors were similar to those of non-neural tissue in that they contained 135,000 dalton binding subunits (demonstrated by affinity labeling and autoradiography) and recognized insulin at high concentrations. IGF-I was more potent than insulin at stimulating B104 cell replication in serum-free medium and, at an initial concentration of 100 ng/ml, was the only exogenous growth factor needed to maintain growth through several cell divisions. Furthermore, cells of later passage were found to secrete specific IGF binding proteins that produced an unusual, biphasic binding curve in radioligand displacement studies. These binding proteins apparently sequester IGF-I, limiting its access to the cell. Experiments with B104 cells may provide useful information about the role of IGFs and their binding proteins as potential regulators of growth and differentiation of the primitive neuroblast.

Factors contributing to cerebral hypomyelination in the growth hormone-deficient little mouse

We attempted to delineate the events leading to hypomyelination in the brain of the little mouse, a promising murine model of isolated growth hormone deficiency. At 20 days of age, the mutant mouse brain weighed less than its normal counterpart, and this difference in brain weight persisted. Increase in CNPase activity was found to be suppressed in the cerebrum throughout the developmental stage, but not in the other parts of the brain. Differences in cerebral DNA content between the little and normal mice first became apparent on the 10th day of age. Thereafter, the rate of increase in the little brain consistently lagged behind the normal. [3H]Thymidine incorporation into the DNA fraction in vivo on the 7th day of age, when glial cell proliferation in the normal cerebrum is most active, was approximately half that of the controls in all parts of the little brain. These findings indicate that the hypomyelination of the mutant cerebrum might result from reduced oligodendroglial proliferation due to growth hormone deficiency.

Alteration of cerebral ganglioside metabolism in developing Snell dwarf mice

The ganglioside metabolism of the Snell dwarf cerebrum was examined on postnatal days 15 and 20, by monitoring the rate of incorporation of radioactivity into each ganglioside species from tritiated N-acetyl-D-mannosamine. It was found that the turnover rate of the GM3 ganglioside was reduced throughout the entire period of development, resulting in retardation of A pathway metabolism which becomes abundant during late cerebral development. In addition, the turnover rate of the GM4 species, which is considered related to myelin formation, was also found to be reduced throughout the entire period of cerebral development.