Cerebral cortical ganglioside and glycoprotein metabolism in immature hypothyroidism

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.

Comparisons of capillary maturation in control and hypothyroid rat spinal cord: an ultrastructural study

Quantitative and qualitative features of capillary maturation were examined in the ventral horn of the lumbar spinal cord of control and neonatally induced hypothyroid rats from birth to 6 weeks, using light (LM) and electron microscopy (EM). Quantification of the capillary densities by LM in the control animals and their hypothyroid litter mates have shown three- and twofold increases, respectively, from birth to Postnatal (Pn) Day 21. The following features were observed in the control animals at the EM level: (a) The newborn animals showed varying degrees of capillary maturation; (b) The majority of capillaries possessed mature characteristics by Pn Day 21; (c) By Pn Day 42, mature characteristics were found in nearly all capillary profiles. The hypothyroid animals demonstrated: (1) reactive perivascular cells and astrocytes; (2) delayed appearance of glycogen in the early Pn period and its persistence in extensive amounts during the latter part (3-6 weeks) of development; (3) cytoplasmic extensions of endothelial cells and perivascular cells, and (4) the presence of mitotic endothelial cells and perivascular cells even during the latter period of development. The observations suggest that the peak period of vasculogenesis in the lumbar spinal cord of the normal rat occurs during the second and third Pn weeks. The results from the hypothyroid rats point toward a delay in development and maturation of capillaries resulting in a hypoplastic vascular bed of the ventral horn. The reactive cells and the accumulation of glycogen particles could be morphological expressions of biochemical changes in hypothyroidism during the critical period of CNS development.

Impairment of human brain development: glycoconjugate and lipid changes in congenital athyroidism

The brain glycoconjugates, glycosidases and lipids have been studied in a case of human congenital athyroidism never treated with hormonal replacement. Increase of membrane-bound mucopolysaccharides has been found in the grey matter. Total lipids in the white matter, and phospholipids, and total glycolipids and cholesterol in both white and grey matter were reduced. Sialic acid (NeuNAc) and hexosamines in the cytosol grey matter were significantly raised, while they were reduced in the membrane-bound grey fraction. These biochemical findings confirm impairment of the myelination process in human hypothyroidism as is well established in experimental hypothyroidism. The significant changes in the carbohydrate moieties of the brain glycoproteins focus attention on complex alterations of structure and function of the cell plasma membranes.

Neonatal hypothyroidism and early undernutrition in the rat: defective maturation of structural membrane components in the central nervous system

The action of neonatal hypothyroidism and early undernutrition on the lipid and protein composition, as well as, on the activity of 2'3' cyclic nucleotide 3' phosphohydrolase was studied in different subcellular fractions isolated from 20 day old hypothyroid and undernourished rats. Based on protein content, a marked decrease (70%) in the recovery of myelin was observed in both experimental conditions. The lipid composition of myelin in both groups was, however, different; while cholesterol, total phospholipids, and total galactolipids decreased in a similar fashion in the two situations; sulfatides and plasmalogens were much more affected in hypothyroid rats.

Thyroid hormone state and the incorporation of (14C) leucine by brain microsomes in developing rats

Cell-free incorporation of (14C) leucine into protein was 38% greater for cerebral cortical microsomes from 22-day old neonatally thyroidectomized rats compared to littermate controls. In contrast, incorporation by liver microsomes of hypothyroid rats was 33% lower compared to controls, confirming their deficient hormonal state. Incubation of cerebral microsomes from either hypothyroid or euthyroid rats with both homologous and heterologous pH 5 enzyme fractions clearly implicated the pH 5 fraction as the source of the apparent increase in protein synthetic capacity in the hypothyroid brain. Daily administration of L-thyroxine (20 microgram/100 g body wt) to hypothyroid animals between 22 and 25 days of age produced an additional increase in (14C) leucine incorporation into protein by cerebral microsomes, whereas the cell-free protein synthesis rate of euthyroid rats was unaffected by similar hormonal treatment. Liver preparations from both hypothyroid and euthyroid rats exhibited the expected increase in cell-free protein synthesis following thyroxine administration. The results support the hypothesis that the young hypothyroid brain exhibits delayed maturation and that thyroid hormones play a regulatory role in cerebral protein synthesis during a defined developmental period.