Transient expression of 3,5,3'-triiodothyronine nuclear receptors in rat oligodendrocytes: in vivo and in vitro immunocytochemical studies

A selective thyroid hormone β receptor agonist enhances human and rodent oligodendrocyte differentiation

Nerve conduction within the mammalian central nervous system is made efficient by oligodendrocyte-derived myelin. Historically, thyroid hormones have a well described role in regulating oligodendrocyte differentiation and myelination during development; however, it remains unclear which thyroid hormone receptors are required to drive these effects. This is a question with clinical relevance since nonspecific thyroid receptor stimulation can produce deleterious side-effects. Here we report that GC-1, a thyromimetic with selective thyroid receptor β action and a potentially limited side-effect profile, promotes in vitro oligodendrogenesis from both rodent and human oligodendrocyte progenitor cells. In addition, we used in vivo genetic fate tracing of oligodendrocyte progenitor cells via PDGFαR-CreER;Rosa26-eYFP double-transgenic mice to examine the effect of GC-1 on cellular fate and find that treatment with GC-1 during developmental myelination promotes oligodendrogenesis within the corpus callosum, occipital cortex and optic nerve. GC-1 was also observed to enhance the expression of the myelin proteins MBP, CNP and MAG within the same regions. These results indicate that a β receptor selective thyromimetic can enhance oligodendrocyte differentiation in vitro and during developmental myelination in vivo and warrants further study as a therapeutic agent for demyelinating models.

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.

Recovery of adult oligodendrocytes is preceded by a "lag period" accompanied by upregulation of transcription factors expressed in developing young cells

Cell cultures prepared from oligodendrocytes directly obtained from adult rat brain are composed of mature cells that lose their cell processes and myelin membrane during their isolation and therefore represent a very useful model to investigate the factors that could stimulate their recovery. We have observed that mature oligodendrocytes isolated from adult animals remain as round cells that lack processes for the first 3-4 days in culture. At the end of this lag period, however, the majority of the adult oligodendrocytes show a remarkable recovery, rapidly growing complex and extensive cell processes. Interestingly, the end of this lag period is accompanied by a dramatic upregulation in the expression of thyroid hormone (T(3)) receptor (TR). The functional importance of this increase in TR levels is supported by the observation that the majority of the cells cultured in the presence of T(3) show significantly more extensive and complex process outgrowth than the control cells in cultures lacking this hormone. In addition, this reactivation of the adult cells was also preceded by an increased expression of glucocorticoid receptor (GR) and cyclic AMP-response element binding protein (CREB), two transcription factors that together with TR appear to play important roles in the control of neonatal oligodendrocyte development. Thus, it is possible to hypothesize that upregulation of these proteins may be part of the metabolic changes that occur during the lag period required for recovery of the adult oligodendrocytes. These observations raise the question of whether these transcription factors may play any significant role during remyelination after demyelinating lesions of adult CNS.

Coexpression of thyroid hormone receptor isoforms in mouse oligodendrocytes

Double and triple immunocytochemistry with stage-specific markers and specific antireceptor antibodies was used to study expression of nuclear thyroid hormone receptor (TR) isoforms in cultured mouse oligodendrocytes. To evaluate the coexpression of each TR isoform, antibodies were raised in rabbits and mice against specific regions of alpha1-TR and alpha2-TR common to both alpha isoforms and beta1-TR. Their specificities were assessed by Western blotting and by immunocytochemistry on rat hepatocytes. Oligodendrocyte subpopulations were found to coexpress the alpha- and beta1-TR epitopes at defined developmental stages. Both alpha- and beta1-TR isoforms are colocalized in oligodendrocytes during an early stage identified by the marker OL-1, before 2',3'-cyclic nucleotide 3'-phosphohydrolase is expressed. Expression of beta1-TR varies during maturation, and that of alpha-TR decreases during terminal maturation.

Selective effect of hypothyroidism on expression of myelin markers during development

Thyroid hormones are critical for maturation of the central nervous system. In a previous study, we showed a change in the pattern of mature myelinated nerve fibers by 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in developing hypothyroid animals, which suggests a possible role for thyroid hormones in myelin compaction. The classical myelin markers myelin basic protein (MBP) and proteolipidic protein (PLP) are expressed later in oligodendroglial development, when myelin sheath formation is in progress. A myelin constituent designated myelin-associated/oligodendrocytic basic protein (MOBP) has been identified and related to myelin compaction. We assessed the developmental sequence of appearance of CNPase, MBP, MOPB, and PLP proteins in cerebellum (Cb) and corpus callosum (cc) in an experimental hypothyroidism model. The appearance of both MOBP isoforms occurred at postnatal day (P)25 and P30 in cc and Cb, respectively, followed by an increase with age in the control group. However, all the MOBP isoforms were weakly detectable in both regions at P30 from the hypothyroid (H) group, and the higher molecular weight isoform remains decreased in cc, even at P90. The developmental pattern of expression of CNPase, MBP, and PLP proteins was also delayed in the H group. CNPase and MBP expression was recovered in cc and Cb, whereas PLP remained below control levels at P90 in cc. Our data show that the experimental hypothyroidism affects the developmental pattern of the oligodendrocytic/myelin markers. Furthermore, thyroid hormone may modulate specific genes, as demonstrated by permanent down-regulation of MOBP and PLP expression in adulthood.

2'3'Cyclic nucleotide 3'phosphodiesterase immunohistochemistry shows an impairment on myelin compaction in hypothyroid rats

The effects of hypothyroidism on oligodendroglial differentiation and myelination are for the first time studied by immunohistochemical localization of an early oligodendroglial marker, the 2'3'cyclic nucleotide 3'phosphodiesterase (E.C. 3.1.4.37-CNPase), in developing rats. Two groups received methimazol; one during gestation (H) and another postnatally (PN). One H sub-group received thyroxine after birth (T). We observed a delay in CNPase expression followed by a decrease in the number of CNPase immunoreactive fibers in both H and PN groups. The T sub-group was not different from controls. Furthermore, the immunoreactive fibers, in mature hypothyroid animals, showed a continuous pattern of staining in contrast with a discontinuous one in controls. Myelinogenesis is a highly regulated timed event. CNPase links myelin related proteins to the cytoskeleton also interacting with membrane lipids during extension and wrapping of the oligodendroglial process around the axon (ensheathment phase). In mature myelinated fiber the CNPase is absent from compact myelin sheath, being located only in the inner and outer loops and in paranodal loops. Thus, our data suggest a disorder in myelin compaction and point once more to the post-natal period as critical for the mechanisms that are thyroid hormone regulated in myelinogenesis.

Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3)

The present study investigates the presence of vitamin D receptor (VDR) in cells of the rat oligodendrocyte (OL) lineage. VDR transcripts were detected by in situ hybridization in a fraction of rat OL in secondary cultures. The VDR protein was shown to be co-localized in cells that are also recognized by an anti-myelin basic protein (MBP) antibody. Likewise, in vivo, VDR-positive cells were found in the brain white matter, such as the internal capsule of the striatum or the corpus callosum but also in the spinal cord. At least part of these positive cells in vivo correspond to OL, since they were co-stained by an anti-carbonic anhydrase II antiserum. Northern blot analyses of the CG-4 OL cell line demonstrated that the VDR transcripts are already found in the O-2A precursors. There was a two-fold increase in the relative abundance of these transcripts in differentiated OL or in type-2 astrocytes. 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] increased the pool of transcripts encoding its own receptor, the VDR. The hormone also enhanced the abundance of the mRNA of the nerve growth factor (NGF) and of its low-affinity receptor, the p75(NTR) protein. By contrast, the hormone had no effect on the levels of MBP or proteolipid protein (PLP) mRNA. This finding suggests that unlike retinoic acid (RA) or thyroid hormone, 1,25-(OH)(2)D(3) has no regulatory action on the synthesis of myelin proteins.

Thyroid hormone receptor isoforms are sequentially expressed in oligodendrocyte lineage cells during rat cerebral development

In the mammalian brain, thyroid hormones regulate myelination. Their actions are mediated by interactions with nuclear receptors that function as ligand-regulated transcription factors. Two genes, alpha and beta, encode different isoforms, of which only the beta and alpha1 isoforms are authentic nuclear triiodothyronine (T3)-receptors (NT3R). In agreement with the important role of T3 on myelination and oligodendrocyte generation, the presence of NT3Rs has been reported in oligodendrocytes and their precursors. We and others have shown that both progenitors and oligodendrocytes in vitro express the alpha1 and alpha2 isoforms, but the expression of the beta1 isoform is confined to differentiated oligodendrocytes, suggesting that they have different functions. To establish if this is the case during development in vivo, we have studied NT3R isoform expression in glial cells isolated by density gradient centrifugation from rat brains of various ages. We report the presence of the alpha1 NT3R and its variant alpha2, but not that of the beta1 isoform, in newborn rat glial progenitors. The pattern of expression of beta1, both at the level of mRNA and protein, parallels the increase in the number of oligodendrocytes. We found a significant change in the kinetic parameters of [125I]-T3 binding to NT3Rs in these cells during the first month of life, consisting of an increase in the binding capacity that peaks with myelination, and a significative decrease in Kd that coincides with the switch from the alpha to the beta1 isoform. Thus, the expression of NT3R isoforms in the rat oligodendrocyte lineage changes radically from the alpha to the beta1 isoform during the period when oligodendrocytes differentiate from progenitors.

Posttranscriptional regulation of oligodendroglial thyroid hormone (T3) receptor beta 1 by T3

3,5,3'-triiodo-L-thyronine interacts with the genome by binding and activating nuclear 3,5,3'-triiodo-L-thyronine receptors. To determine how in secondary oligodendrocyte cultures, exogenous 3,5,3'-triiodo-L-thyronine influences the expression of different 3,5,3'-triiodo-L-thyronine receptor isoforms, we studied the regulation of alpha 1, alpha 2 and beta 1 3,5,3'-triiodo-L-thyronine receptor mRNAs. In culture, we find that beta 1, 3,5,3'-triiodo-L-thyronine receptor mRNA, but not alpha 1 and alpha 2 3,5,3'-triiodo-L-thyronine receptor mRNAs, is up-regulated by 3,5,3'-triiodo-L-thyronine in a time and dose dependent manner. In addition, we present evidence indicating that beta 1 3,5,3'-triiodo-L-thyronine receptor expression is posttranscriptionally regulated by 3,5,3'-triiodo-L-thyronine. Previous studies from our laboratory and others have shown that in the rat oligodendrocyte lineage, 3,5,3'-triiodo-L-thyronine receptors alpha 1 and alpha 2 were expressed in both early progenitor cells and mature oligodendrocytes. In contrast, beta 1 3,5,3'-triiodo-L-thyronine receptor was found to be expressed only in mature oligodendrocytes. This suggests that thyroid hormone may influence oligodendrocyte differentiation and maturation via 3,5,3'-triiodo-L-thyronine receptor beta 1, which is expressed only in oligodendrocytes and not in progenitor cells. We therefore show that this effect is indirect and is mediated by 3,5,3'-triiodo-L-thyronine which acts posttranscriptionally on the 3,5,3'-triiodo-L-thyronine receptor beta 1 gene.

Growth and fate of PSA-NCAM+ precursors of the postnatal brain

Oligodendrocyte-type 2 astrocyte (O-2A) lineage cells are derived from multipotential stem cells of the developing CNS. Precursors of O-2A progenitors express the polysialylated (PSA) form of the neural cell adhesion molecule (NCAM) and are detected in neonatal rat brain glial cultures. It is unclear how such PSA-NCAM+ "pre-progenitors" are related to neural stem cells and whether they still have the potential to differentiate along several neural lineages. Here we isolated PSA-NCAM+ pre-progenitor cells from glial cultures by immunopanning and found that most of these cells expressed nestin and PDGF-receptor-alpha but not O-2A antigens. PSA-NCAM+ cells synthesized transcripts for fibroblast growth factor (FGF) receptors 1, 2, and 3 and responded to FGF2 by survival and proliferation, growing into large clusters resembling neural spheres. FGF2-induced proliferation of PSA-NCAM+ pre-progenitors was significantly enhanced by thyroid hormone (T3), which on its own did not increase cell survival or mitosis. After adhesion and withdrawal of the mitogen, spheres generated mostly oligodendrocytes and astrocytes but very rarely neurons. PSA-NCAM immunopanned cells grown in epidermal growth factor (EGF) also adopted a mostly glial fate after differentiation. In contrast, PSA-NCAM-negative cells and striatal neonatal stem cells, grown in EGF or FGF2, generated the three CNS cell types. Like neural stem cells, PSA-negative cells generated more oligodendrocytes and fewer neurons when expanded in FGF2 and T3. Thus emergence of PSA-NCAM at the surface of neonatal brain precursors coincides with their restriction to a glial fate. T3 modulates these events by enhancing PSA-NCAM+ pre-progenitor growth in FGF2 and favoring an oligodendrocyte fate.

Oligodendrocyte maturation and progenitor cell proliferation are independently regulated by thyroid hormone

The development of oligodendrocyte progenitor cells is regulated by epigenetic factors which control their proliferation and differentiation. When oligodendrocyte progenitor cells, purified on a Percoll centrifugation gradient from neonate rat brain, are cultured in serum-free medium in the presence of platelet-derived-growth factor (PDGF), they divide and their differentiation is delayed. Triiodothyronine (T3) treatment of progenitor cells blocks their proliferation and induces their differentiation into oligodendrocytes. T3 also induces morphological differentiation of oligodendrocytes as indicated by the marked increase in the length of oligodendrocyte processes. To determine whether the effects of T3 on progenitor cell proliferation and oligodendrocyte maturation are causally related, or instead, are independent, we examined the influence of T3 on secondary cultures of postmitotic oligodendrocytes. We show that T3 increases morphological and functional maturation of postmitotic oligodendrocytes as indicated by a well developed network of branched processes and by the expression of myelin/oligodendrocyte glycoprotein (MOG) and glutamine synthetase (GS). T3 increases glutamine synthetase activity and its message level after a lag period of 24-48 h, and these levels increase through a posttranscriptional event. In contrast, no effect of T3 was observed on myelin basic protein (MBP) gene expression as determined by Northern blot analysis. Our results indicate that thyroid hormones participate in the control of the progenitor cell proliferation and differentiation as well as in oligodendrocyte maturation and that these two T3-regulated events are independent.

High gelsolin content of developing oligodendrocytes

The actin-binding protein gelsolin that severs and caps the actin microfilaments under the control of the cytoplasmic free calcium and the membranous phosphatidylinositol 4,5-bisphosphate, is essentially restricted to the oligodendroglia in the central nervous system. Immunocytochemistry showed that gelsolin is an early marker of oligodendrocytes, both in vivo, in the rat cerebellum, and in vitro, in oligodendrocyte culture. We report the early appearance of gelsolin in A2B5-positive precursor oligodendrocyte cells and the specific expression of gelsolin in OL-1-, GC-, and MBP-positive oligodendrocytes in culture. The protein was distributed throughout the cell body and in the branched cell processes of cultured oligodendrocytes, but not in the MBP-positive membrane sheets. Gelsolin is thus cytosolic and not a myelin component. The quantitative study demonstrated that that the cerebellar gelsolin content changes significantly with age, with the maximal value at the age of 21 days, confirming that large amounts of gelsolin are transiently synthesized during development, especially from the first events of myelinogenesis. The results are consistent with gelsolin being involved, through its effects on the actin cytoskeleton, in the motile events occurring during the growth of the oligodendroglial processes towards the axons and the wrapping of the myelin sheaths around the axons.

Expression of thyroid hormone receptor isoforms in rat oligodendrocyte cultures. Effect of 3,5,3'-triiodo-L-thyronine

3,5,3'-Triiodo-L-thyronine (T3) acts at the genomic level by interacting with nuclear T3 receptors (T3Rs). We have used double immunostaining to follow the expression of T3Rs and oligodendrocytes (OL) lineage markers in rat secondary cultures consisting of 85-90% OL. Using antibodies against different synthetic peptides of T3Rs (alpha common: alpha 1 + alpha 2 and beta 1) we find that alpha-T3R is expressed in both O-2A progenitors and in mature OL, while beta 1-T3R is expressed only in mature OL. In cultured OL, beta 1-T3R mRNA is upregulated the most by T3. OL exhibit more numerous and longer processes when treated by T3.