Evidence for a direct effect of thyroid hormone on maturation of nervous tissue grown

An in vivo accelerated developmental myelination model for testing promyelinating therapeutics

Background

Therapeutic agents stimulating the process of myelination could be beneficial for the treatment of demyelinating diseases, such as multiple sclerosis. The efficient translation of compounds promoting myelination in vitro to efficacy in vivo is inherently time-consuming and expensive. Thyroid hormones accelerate the differentiation and maturation of oligodendrocytes, thereby promoting myelination. Systemic administration of the thyroid hormone thyroxine (T4) accelerates brain maturation, including myelination, during early postnatal development. The objective of this study was to validate an animal model for rapid testing of promyelinating therapeutic candidates for their effects on early postnatal development by using T4 as a reference compound.

Methods

Daily subcutaneous injections of T4 were given to Sprague Dawley rat pups from postnatal day (PND) 2 to PND10. Changes in white matter were determined at PND10 using diffusion tensor magnetic resonance imaging (DTI). Temporal changes in myelination from PND3 to PND11 were also assessed by quantifying myelin basic protein (MBP) expression levels in the brain using the resonance Raman spectroscopy/enzyme-linked immunosorbent assay (RRS-ELISA) and quantitative immunohistochemistry.

Results

DTI of white matter tracts showed significantly higher fractional anisotropy in the internal capsule of T4-treated rat pups. The distribution of total FA values in the forebrain was significantly shifted towards higher values in the T4-treated group, suggesting increased myelination. In vivo imaging data were supported by in vitro observations, as T4 administration significantly potentiated the developmental increase in MBP levels in brain lysates starting from PND8. MBP levels in the brain of animals that received treatment for 9 days correlated with the FA metric determined in the same pups in vivo a day earlier. Furthermore, accelerated developmental myelination following T4 administration was confirmed by immunohistochemical staining for MBP in coronal brain sections of treated rat pups.

Conclusions

T4-treated rat pups had increased MBP expression levels and higher MRI fractional anisotropy values, both indications of accelerated myelination. This simple developmental myelination model affords a rapid test of promyelinating activity in vivo within several days, which could facilitate in vivo prescreening of candidate therapeutic compounds for developmental hypomyelinating diseases. Further research will be necessary to assess the utility of this platform for screening promyelination compounds in more complex demyelination disease models, such us multiple sclerosis.

Supplementary information

The online version contains supplementary material available at 10.1186/s12868-022-00714-y.

Local administration of thyroid hormones in silicone chamber increases regeneration of rat transected sciatic nerve

Conflicting actions of the exogenous thyroid hormone on regenerating peripheral nerve have been reported. These contradictory results were probably due to daily intraperitoneal injections which induce a high concentration of thyroid hormone after administration. In our present study we adapted a technique which allows a local administration of thyroid hormones in a closed system. The effect of a single and local treatment with triiodothyronine (T3) on axonal growth across a gap between sectioned ends of sciatic nerve within silicone chambers was examined in Wistar rats. After nerve transection and surgical implantation, silicone chambers were filled with either a neutral pH solution of triiodothyronine dissolved in NaOH or with sterile solvent as control. Regeneration of the nerves was examined 2 to 8 weeks following the surgery. Early regeneration (4 weeks) was studied by morphological analysis of nerves which showed a significant difference between T3-treated and control groups. Morphometric analysis revealed: (1) a significant difference in the mean diameter of myelinated axons between T3-treated nerve (phi 3.80 +/- 0.22 microns) and control (phi 3.07 +/- 0.44 microns); (2) that T3 increased significantly (1.4-fold) the number of myelinated axons that grew into the middle and distal ends of regeneration chambers; (3) that ultrastructural analysis showed significantly higher percentage of myelinated axons per total axon population in T3-treated groups (38.8 +/- 5.9%) as compared to control (16.0 +/- 2.3%); and (4) that the myelinated axons had thicker myelin sheaths. The beneficial effects of T3 on regeneration, observed at 4 weeks, were sustained over a prolonged period of time. Thus, at 8 weeks of regeneration, the number, the mean diameter of myelinated axons, and the thickness of myelin sheaths remained significantly greater in T3-treated groups. Therefore, a single and local administration of thyroid hormone at the level of the transected sciatic nerve is sufficient to rapidly set off several mechanisms which, in turn, produce a stimulating and lasting effect on peripheral nerve regeneration. The beneficial effects of T3 upon injured peripheral nerve may have considerable therapeutic potential.

Effects of triiodothyronine on the synthesis of sulfolipids by oligodendrocyte-enriched glial cultures

Glial cultures were obtained from the brains of 1-week-old rats and were grown in a chemically defined, serum-free medium. We investigated the development of oligodendrocytes in these cultures and the synthesis of sulfolipids in the presence and absence of triiodothyronine (T3) in the medium: (1) In the presence of T3, the incorporation of [35S]sulfate into sulfolipids exhibited a developmental profile which is comparable to that found in the developing brain in vivo. A sharp peak of sulfolipid synthesis was observed at day 5 in vitro, which is equivalent to day 12 after birth. As observed in vivo, the percentage of label incorporated into sulfogalactosyldiradylglycerols decreased with time in culture. (2) Addition of T3 to the medium stimulated sulfolipid synthesis by oligodendrocytes in a dose-related manner (optimal T3 concentration, 30 nM). The hormone also enhanced the rates of cholesterogenesis and lipogenesis but to a lesser extent than sulfolipid synthesis. (3) The temporary omission of T3 from the medium resulted in lower rates of sulfolipid synthesis that could not be restored by readdition of T3. This inhibitory effect was most pronounced if the hormone was omitted from the medium on days 2 and 3 in culture. (4) Omission of T3 also resulted in the development of fewer oligodendrocytes in the cultures. Our results show that T3 is essential for the development of oligodendrocytes in our neurone-free culture system. They also indicate that the stimulation of myelination by thyroid hormones can, at least partially, be explained as a direct effect of T3 on oligodendrocytes, independent of an effect of T3 on neuronal growth.

Cerebroside and sulfatide biosynthesis in the brain of Snell dwarf mouse: effects of thyroxine and growth hormone in the early postnatal period

Snell dwarf mice (dw/dw) and normal mice (+/?) were injected with thyroxine (T4) (1 microgram/animal, four injections) and growth hormone (GH) (20 micrograms/animal, four injections) from the 5th to the 15th day of life. In the untreated dw/dw mouse brain, the specific activities of UDP-galactose:ceramide galactosyltransferase (CGalT), PAPS:cerebroside sulfotransferase (CST), and 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNP) were decreased by 28, 25, and 37%, respectively, compared with the control untreated +/? mice. The major effect of T4 was an increase of the brain CNP in the +/? mice (+40%) and dw/dw mice (+111%). The treatment with T4 also brought to normal the level of CGalT in dw/dw brain; a somewhat less marked effect on CST was observed. The treatment with GH had a great stimulatory effect on CNP: the specific activity of this enzyme increased by 40 and 69% in +/? and dw/dw mouse brain, respectively. On the contrary, no effect of GH on the CGalT activity was observed in this study. Our results suggest that T4 and GH may have both independent and complementary actions on the myelin-associated enzymes during the early postnatal period of brain development.

Altered developmental changes of neuromuscular junction in hypo- and hyperthyroid rats

1. Effects of thyroid hormone on the development of neuromuscular junctions (n.m.j.s.) were investigated electrophysiologically in the diaphragms (sternal region) of normal, hypo- and hyperthyroid rats from the age of birth (day 0) to day 35.2. Hypothyroidism in new-born rats was induced either by daily administration of propylthiouracil to mothers or by subcutaneous injection of 150 muCi (131)I on day 1. Hyperthyroidism was induced by daily injection of thyroxine.3. In normal rats up to day 10, muscle fibres were innervated polyneuronally. By day 20, multiple innervation was eliminated and muscle fibres received only a single input. In hypothyroid rats elimination of polyneuronal innervation was retarded by 5-8 days, while in hyperthyroid rats the elimination was accelerated by 2-3 days.4. The frequency of miniature end-plate potentials (m.e.p.p.s) in normal rats increased from one per 40 sec on days 0-5 to 1/sec on days 25-35. The m.e.p.p. frequency in hypothyroid rats was 25-65% of that in normal rats of the same age. In hyperthyroid rats the m.e.p.p. frequency was normal up to day 18 but subnormal afterwards. The duration of m.e.p.p. measured on day 22-23 was slower in hypothyroid rats and faster in hyperthyroid rats, relative to m.e.p.ps in normal rats.5. The sensitivity to acetylcholine (ACh) at extrajunctional regions in normal rats was about 100 mV/nC at birth and declined to 1 mV/nC by day 26. In hypothyroid rats, the ACh sensitivity was as high as 30 mV/nC on day 26; in hyperthyroid rats, ACh sensitivity on day 26 was undetectable.6. With pairs of nerve stimuli (applied at a 50 msec interval), the second end-plate potential was facilitated until day 10 and depressed after day 16 in normal rats. This shift from facilitation to depression during development was not altered in either hypo-or hyperthyroid rats.7. It is concluded that the lack and excess of thyroid hormone retards and facilitates the development of n.m.j.s. respectively. Possible mechanisms for this altered development are discussed.

Influence of thyroid hormones on myelin proteins in the developing rat brain

This communication describes developmental changes in myelin proteins prepared from control, hypothyroid, and hyperthyroid rat brain. In the 10- to 37-day postnatal period studied, total myelin protein was found to double, and this change mainly reflected the increase in proteolipid and basic protein constituents. Thyroid states affect differentially the various myelin proteins. Hypothyroidism decreases the proteolipid and slow-moving basic protein, but has no effect on the fast basic or minor proteins. In hyperthyroidism, an increase was observed in proteolipid as well as both slow- and fast-moving proteins. The protein alterations were correlated to the changes previously found in lipid composition of myelin consequent upon hypo- and hyperthyroidism, and the role of thyroid hormones in brain development.