Degrees of cooperativity between triiodothyronine and hydrocortisone in their regulation of the expression of myelin basic protein and proteolipid protein during brain development

Expression of thyroid hormone receptor isoforms in the oligodendrocyte lineage

Thyroid hormone (T3) regulates brain development and function and in particular ensures normal myelination. Animal models and in vitro systems have been employed to demonstrate the effects of T3, which acts via nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression, such as myelin basic protein (MBP), in a hormone-dependent or -independent fashion. Two distinct genes, TR alpha and TR beta, encode several receptor isoforms with specific functions. This overview summarizes current knowledge on the cellular expression and the role of these isoforms and also examines the action of T3 on oligodendrocyte lineage cell types at defined developmental stages. Re-expression of TRs and also that of other transcription factors in oligodendrocytes may constitute some of the metabolic changes required for succesfull remyelination in the adult central nervous system after demyelinating lesions.

Transient stimulation of myelin basic protein gene expression in differentiating cultured oligodendrocytes: a model for 3,5,3'-triiodothyronine-induced brain development

We compared the regulation of myelin basic protein (MBP) gene expression by T3 in differentiating oligodendrocytes in culture with that previously observed by us in the neonatal rat brain. As in intact brain, expression of the T3R alpha gene preceded that of the T3R beta gene. Although the absence of T3 retarded the rate of accumulation of MBP messenger RNA, the level ultimately attained was similar to that reached in the presence of T3. This relationship mirrored the pattern observed in the neonatal brain. Transient transfection experiments showed that T3 regulates MBP expression at the transcriptional level, but only for a limited period during differentiation. These observations imply that the early rise of MBP messenger RNA is T3 dependent, whereas the terminal levels are maintained independently of T3. Both the T3-dependent and, surprisingly, the T3-independent expression of MBP require the presence of an intact T3 response element. T3 receptor may regulate MBP expression in a ligand-independent manner, or a nuclear factor other than T3 receptor may bind to the T3 response element of MBP to regulate terminal gene expression. These findings support the use of differentiating oligodendrocytes as a model of T3-induced brain development.

Complexity analysis of oligodendroglial processes expressing myelin-associated glycoprotein

Oligodendroglia synthesize myelin in the mammalian central nervous system. Mature oligodendroglia have been identified in culture by two criteria; the expression of molecules characteristic of myelin, such as galactocerebroside (galC) and myelin-associated glycoprotein (MAG), and the elaboration of complex processes. Myelin gene expression can be documented by the binding of specific antibodies and antisera to the myelin-specific molecules; process complexity can be described by the fractal dimension, D. In this study, anti-MAG antisera was used to document MAG expression in the processes of oligodendroglia. Eighty percent of the galC+ oligodendroglia bound anti-MAG antiserum. With time in culture, MAG immunoreactivity seemed to extend from the cell soma into the oligodendroglial processes. To quantify this observation, fractal dimensions were calculated using either galC or MAG immunoreactivity to visualize oligodendroglial processes. A fractal dimension of 1.5 was calculated for O1+ processes by day 4 of culture; this value for D remained constant over the course of 1 month in culture. The fractal dimension calculated for MAG+ processes increased from 1.2 to 1.5 over the course of 28 days in culture. This change in fractal dimension confirms our visual impression that galC-containing processes acquire MAG slowly over the course of several weeks in culture.

Limited postnatal ethanol exposure permanently alters the expression of mRNAS encoding myelin basic protein and myelin-associated glycoprotein in cerebellum

Experiments were designed to test the hypothesis that ethanol exposure during development can selectively affect the expression of specific isoforms of myelin protein gene expression in the rat cerebellum. We focused on myelin basic protein (MBP) and myelin-associated glycoprotein (MAG) gene expression. Both of these genes are alternatively spliced to yield 4 (MBP) or 2 (MAG) mRNA isoforms. Prenatal ethanol exposure, delivered to the dams in a liquid diet, did not significantly alter the expression of MBP or MAG gene expression in the cerebellums of 15-day-old pups, as measured by quantitative in situ hybridization using specific oligodeoxynucleotide probes. In contrast, postnatal ethanol exposure delivered directly to the pups over a 6-day period by gastrostomy tube (PN days 4-10) reduced the expression of specific MBP and MAG isoforms in the cerebellum of animals in adulthood. These data demonstrate that ethanol exposure, especially during the period of rapid myelination, has selective effects on mRNA isoforms encoding specific MBPs and MAG.

Serum factors and hydrocortisone influence the synthesis of myelin basic proteins in mouse brain primary cultures

Mouse brain primary cultures were used to study the influence of serum factors and hydrocortisone on myelin basic protein (MBP) gene expression. Serum factors increased the synthesis of the MBP isoforms in 18-day and older cultures. Synthesis of the 17 and 18.5 kDa MBPs steadily increased from 14 to 26 DIV (days in vitro). Synthesis of the 14 kDa MBP reached a peak at 22 days, after which it fell off rapidly. Addition of serum to the medium also stimulated MBP mRNA expression. However, in the presence of serum, maximal stimulation of MBP mRNA expression occurred at 16-20 days, significantly earlier than maximal stimulation of 17 and 18.5 kDa MBP synthesis by serum. These observations suggest that serum influences both transcriptional and post-transcriptional steps in the expression of the MBP genes in primary cultures. Hydrocortisone increased the synthesis of the 14, 18.5 and 17 kDa MBP isoforms by two- to three-fold. This effect was seen in cultures older than 15 DIV. This effect of hydrocortisone on the synthesis of the MBPs may be responsible for the increase in the steady state levels of MBP in the presence of hydrocortisone. These studies suggest a role for serum factors and hydrocortisone in the transcriptional and post-transcriptional steps of MBP gene expression. They also suggest that this role may be developmentally regulated.

Structural features of myelin basic protein mRNAs influence their translational efficiencies

The myelin basic protein (MBP) gene expresses several alternatively spliced products with the same 5' and 3' untranslated regions (UTRs). It has been reported that its expression may be regulated not only at the transcriptional level but also at the translational level during development. We engineered several MBP mRNA deletion mutants with 5' (-48, -37, -27, -22, and -10) and 3' UTRs of differing lengths and examined the translational efficiencies of these constructs in cell-free systems. The translational efficiencies of the constructs differed significantly over a range of almost 10-fold. A deletion of 11 nucleotides from the 5' end of the natural (i.e., -48) MBP mRNA resulted in an approximate fourfold reduction in translational efficiency. Further truncation of the 5' UTR increased the translational efficiencies of the constructs as has been observed with many RNAs. These results suggest that there may be a positive control element between -48 and -37 nucleotides in the 5' UTR of MBP mRNA. The effects of modifying the lengths of the 5' UTR on the translational efficiency of mRNAs encoding the 21.5-kDa and 14-kDa MBPs were the same, suggesting that the effect observed was not unique to the 21.5-kDa MBP mRNA. Truncating the 3' UTR of four different alternatively spliced MBP mRNAs also altered their translational efficiencies. Thus, the 5' and 3' UTRs of MBP mRNAs appear to influence the translation of these mRNAs, and such factors may be involved in the translational regulation of MBP gene expression.

Myelination in cerebellar slice cultures: development of a system amenable to biochemical analysis

Myelin deposition and maintenance are critical to proper function of the mammalian nervous system. Previous investigations of myelination in the central nervous system (CNS) were hampered by the lack of an in vitro system that can faithfully reproduce in vivo events yet is amenable to biochemical investigation. We have developed a procedure, based on organotypic cultures, which permits efficient preparation of large numbers of cerebellar slice cultures that can be easily manipulated. Cultures have been examined to document myelination biochemically (by incorporation of [35S]sulfate into sulfolipids), immunohistochemically (by labeling the myelin components myelin basic protein and galactocerebroside), and morphologically (by both light and electron microscopy). We tested the effects of biologically active peptides and antibodies on myelination in the thin slices. The results indicate that the cultures provide an in vitro system that can be used to examine specific cellular events that occur during CNS myelination.

Myocardial contractile protein ATPase activities in adrenalectomized and thyroidectomized rats

This report compares the effects of adrenalectomy and thyroidectomy, with and without hormone replacement, on loss of contractile protein ATPase activities. The rationale for this study was derived from the similarities in their intracellular receptors, mechanisms of action, and the large number of proteins regulated by both hormones. Rats were adrenalectomized, thyroidectomized, or both, and were subsequently treated for 6 weeks with hydrocortisone, triiodothyronine, or saline. Sham-operated rats were given saline for the same period of time. Six weeks of adrenal insufficiency resulted in diminished enzymatic activity of myofibrillar, Ca(2+)-activated myosin ATPase, and actin-activated myosin ATPase fractions. Treatment with hydrocortisone prevented the decline in enzymatic activity due to adrenalectomy. Likewise, thyroidectomy caused a loss of enzymatic activity which was prevented by treatment with triiodothyronine. The full deleterious effect of combined ablation could be partially prevented by treatment with either hydrocortisone or triiodothyronine, but the latter was most effective. The results suggest that hydrocortisone and triiodothyronine each had significant positive effects in the presence of the other, but not in its absence, on the activity of myofibrillar Ca(2+)-dependent Mg-ATPase and Ca(2+)-activated myosin ATPase. The effects of these two hormones on actin-activated myosin ATPase activity were more independent of each other. We conclude that the actions of thyroid and glucocorticoid hormones on the heart are interrelated and that optimum myocardial function results from their combined action.