Thyroid hormone administration enhances remyelination in chronic demyelinating inflammatory disease

Abstract

Chronic disabilities in multiple sclerosis are believed to be due to neuron damage and degeneration, which follow remyelination failure. Due to the presence of numerous oligodendrocyte precursors inside demyelination plaques, one reason for demyelination failure could be the inability of oligodendrocyte precursor cells to turn into myelinating oligodendrocytes. In this study, we show that thyroid hormone enhances and accelerates remyelination in an experimental model of chronic demyelination, i.e., experimental allergic encephalomyelitis in congenic female Dark Agouti rats immunized with complete guinea pig spinal cord. Thyroid hormone, when administered during the acute phase of the disease, increases expression of platelet-derived growth factor alpha receptor, restores normal levels of myelin basic protein mRNA and protein, and allows an early and morphologically competent reassembly of myelin sheaths. Moreover, thyroid hormone exerts a neuroprotective effect with respect to axonal pathology.

Fig. 1.

Clinical score of EAE in Lewis (Upper) and DA (Lower) rats. T4 treatment is effective in reducing the severity of the relapse in Lewis, but not DA, rats. Statistical analysis was performed with ANOVA and Dunnett's test. ^*, P < 0.05.

Fig. 2.

T4 significantly increases PDGFαR mRNA expression in the spinal cord of EAE-DA rats, as measured by semiquantitative RT-PCR 18 dpi. Data are from six rats per group. Statistical analysis was performed with Student's t test. ^*, P < 0.05.

Fig. 3.

Effect of T4 treatment on oligodendrocytes in the spinal cord of DA-EAE rats. T4 treatment accelerates recovery of RIP immunostaining in both DA and Lewis EAE rats. (A) Confocal microscopy of RIP immunostaining in DA control rat, showing the sample area for semiquantitative analysis. (B-D) Color-coded confocal images of RIP immunoreactivity in the different experimental groups in DA strain.

Fig. 4.

Effect of T4 treatment on myelin sheath. (A-D) Confocal images of RIP immunoreactivity in control (A), EAE 35 dpi (B), EAE 55 dpi (C), and EAE 55 dpi + T4 (D) rats. (E) Confocal image of MBP (green) and β-tubulin (red) double-stained section, illustrating the sampling strategy. Areas of the entire fiber (area_ex) and of the axon (area_in) are measured. Form factor is calculated on the base of area_ex values, assuming 1 as circle (F), axonal diameter as square root of area_in/π (H), with myelin thickness according to the formula square root of area_ex/π- square root of area_in/π (G). Data were obtained from three to five rats per group. EAE caused a severe shift from 1 of the form factor value (F), myelin sheath is thinner (G), and axonal diameter is reduced (H). T4 treatment significantly improved all these indices. Statistical analysis was performed with Student's t test. ^*, P < 0.05; ^**, P < 0.01; ^***, P < 0.001. (Bars, 5 μm.)

Fig. 5.

Effect of T4 administration on MBP expression during EAE in DA rats. Low (A and D) and high (C and F, coronal; B and E, longitudinal) confocal images of MBP immunoreactivity in the ventral funiculus of the lumbar spinal cord of untreated EAE-DA rats (A-C) and EAE-DA rats treated with T4 (D-F), at dpi 55. Images show a clear decline in staining intensity and myelin sheet disaggregation in EAE rats and substantial recovery in EAE rats treated with T4. Red spots indicate the extensive infiltration of BrdUrd-positive cells. Graphs show the quantification of MBP mRNA and protein expression, indicating both the decline in EAE rats compared with control rats and recovery due to T4 treatment. Data were obtained from six rats per group. R.O.D., relative OD.

Fig. 6.

Effect of different schedules of T4 administration on MBP protein content in the optic nerve of EAE-DA rats. The bar legends indicate the phase of the disease at which T4 was administered. Samples all were analyzed at dpi 55. Data were obtained from six rats per group. Statistical analysis was performed with ANOVA and Dunnett's test. ^*, P < 0.05. R.O.D., relative OD.