Thyroid hormone induces glial lineage of primary neurospheres derived from non-pathological and pathological rat brain: implications for remyelination-enhancing therapies

Engineered PDGFA-ligand-modified exosomes delivery T3 for demyelinating disease targeted therapy

Demyelination is a proper syndrome in plenty of central nervous system (CNS) diseases, which is the main obstacle to recovery and still lacks an effective treatment. To overcome the limitations of the brain-blood barrier on drug permeability, we modified an exosome secreted by neural stem cells (NSCs), which had transfected with lentivirus armed with platelet-derived growth factors A (PDGFA)-ligand. Through the in vivo and in vitro exosomes targeting test, the migration ability to the lesion areas and OPCs significantly improved after ligand modification. Furthermore, the targeted exosomes loaded with 3,5, 30-L-triiodothyronine (T3) have a critical myelination ability in CNS development, administrated to the cuprizone animal model treatment. The data shows that the novel drug vector loaded with T3 significantly promotes remyelination compared with T3 alone. At the same time, it improved the CNS microenvironment by reducing astrogliosis, inhibiting pro-inflammatory microglia, and alleviating axon damage. This investigation provides a straightforward strategy to produce a targeting exosome and indicates a possible therapeutic manner for demyelinating disease.

The Effects of Cuprizone on Murine Subventricular Zone-Derived Neural Stem Cells and Progenitor Cells Grown as Neurospheres

Despite the extensive use of the cuprizone (CPZ) demyelination animal model, there is little evidence regarding the effects of CPZ on a cellular level. Initial studies have suggested that oligodendrocytes (OL) are the main cell targets for CPZ toxicity. However, recent data have revealed additional effects on neural stem cells and progenitor cells (NSC/NPC), which constitute a reservoir for OL regeneration during brain remyelination. We cultured NSC/NPC as neurospheres to investigate CPZ effects on cell mechanisms which are thought to be involved in demyelination and remyelination processes in vivo. Proliferating NSC/NPC cultures exposed to CPZ showed overproduction of intracellular reactive oxygen species and increased progenitor migration at the expense of a significant inhibition of cell proliferation. Although NSC/NPC survival was not affected by CPZ in proliferative conditions, we found that CPZ-treated cultures undergoing cell differentiation were more prone to cell death than controls. The commitment and cell differentiation towards neural lineages did not seem to be affected by CPZ, as shown by the conserved proportions of OL, astrocytes, and neurons. Nevertheless, when CPZ treatment was performed after cell differentiation, we detected a significant reduction in the number and the morphological complexity of OL, astrogliosis, and neuronal damage. We conclude that, in addition to damaging mature OL, CPZ also reduces NSC/NPC proliferation and activates progenitor migration. These results shed light on CPZ direct effects on NSC proliferation and the progression of in vitro differentiation.

Docosahexaenoic Acid and Melatonin Prevent Impaired Oligodendrogenesis Induced by Intrauterine Growth Restriction (IUGR)

In this study, our aims were to characterize oligodendrogenesis alterations in fetuses with intrauterine growth restriction (IUGR) and to find therapeutic strategies to prevent/treat them using a novel rabbit in vitro neurosphere culture. IUGR was surgically induced in one uterine horn of pregnant rabbits, while the contralateral horn served as a control. Neural progenitor cells (NPCs) were obtained from pup’s whole brain and cultured as neurospheres mimicking the basic processes of brain development including migration and cell differentiation. Five substances, chosen based on evidence provided in the literature, were screened in vitro in neurospheres from untreated rabbits: Docosahexaenoic acid (DHA), melatonin (MEL), zinc, 3,3′,5-Triiodo-L-thyronine (T3), and lactoferrin (LF) or its metabolite sialic acid (SA). DHA, MEL and LF were further selected for in vivo administration and subsequent evaluation in the Neurosphere Assay. In the IUGR culture, we observed a significantly reduced percentage of oligodendrocytes (OLs) which correlated with clinical findings indicating white matter injury in IUGR infants. We identified DHA and MEL as the most effective therapies. In all cases, our in vitro rabbit neurosphere assay predicted the outcome of the in vivo administration of the therapies and confirmed the reliability of the model, making it a powerful and consistent tool to select new neuroprotective therapies.

Hormones in experimental autoimmune encephalomyelitis (EAE) animal models

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) in which activated immune cells attack the CNS and cause inflammation and demyelination. While the etiology of MS is still largely unknown, the interaction between hormones and the immune system plays a role in disease progression, but the mechanisms by which this occurs are incompletely understood. Several in vitro and in vivo experimental, but also clinical studies, have addressed the possible role of the endocrine system in susceptibility and severity of autoimmune diseases. Although there are several demyelinating models, experimental autoimmune encephalomyelitis (EAE) is the oldest and most commonly used model for MS in laboratory animals which enables researchers to translate their findings from EAE into human. Evidences imply that there is great heterogeneity in the susceptibility to the induction, the method of induction, and the response to various immunological or pharmacological interventions, which led to conflicting results on the role of specific hormones in the EAE model. In this review, we address the role of endocrine system in EAE model to provide a comprehensive view and a better understanding of the interactions between the endocrine and the immune systems in various models of EAE, to open up a ground for further detailed studies in this field by considering and comparing the results and models used in previous studies.

Hormonal Regulation of Oligodendrogenesis II: Implications for Myelin Repair

Alterations in myelin, the protective and insulating sheath surrounding axons, affect brain function, as is evident in demyelinating diseases where the loss of myelin leads to cognitive and motor dysfunction. Recent evidence suggests that changes in myelination, including both hyper- and hypo-myelination, may also play a role in numerous neurological and psychiatric diseases. Protecting myelin and promoting remyelination is thus crucial for a wide range of disorders. Oligodendrocytes (OLs) are the cells that generate myelin, and oligodendrogenesis, the creation of new OLs, continues throughout life and is necessary for myelin plasticity and remyelination. Understanding the regulation of oligodendrogenesis and myelin plasticity within disease contexts is, therefore, critical for the development of novel therapeutic targets. In our companion manuscript, we review literature demonstrating that multiple hormone classes are involved in the regulation of oligodendrogenesis under physiological conditions. The majority of hormones enhance oligodendrogenesis, increasing oligodendrocyte precursor cell differentiation and inducing maturation and myelin production in OLs. Thus, hormonal treatments present a promising route to promote remyelination. Here, we review the literature on hormonal regulation of oligodendrogenesis within the context of disorders. We focus on steroid hormones, including glucocorticoids and sex hormones, peptide hormones such as insulin-like growth factor 1, and thyroid hormones. For each hormone, we describe whether they aid in OL survival, differentiation, or remyelination, and we discuss their mechanisms of action, if known. Several of these hormones have yielded promising results in both animal models and in human conditions; however, a better understanding of hormonal effects, interactions, and their mechanisms will ultimately lead to more targeted therapeutics for myelin repair.

Hormonal Regulation of Oligodendrogenesis I: Effects across the Lifespan

The brain’s capacity to respond to changing environments via hormonal signaling is critical to fine-tuned function. An emerging body of literature highlights a role for myelin plasticity as a prominent type of experience-dependent plasticity in the adult brain. Myelin plasticity is driven by oligodendrocytes (OLs) and their precursor cells (OPCs). OPC differentiation regulates the trajectory of myelin production throughout development, and importantly, OPCs maintain the ability to proliferate and generate new OLs throughout adulthood. The process of oligodendrogenesis, the creation of new OLs, can be dramatically influenced during early development and in adulthood by internal and environmental conditions such as hormones. Here, we review the current literature describing hormonal regulation of oligodendrogenesis within physiological conditions, focusing on several classes of hormones: steroid, peptide, and thyroid hormones. We discuss hormonal regulation at each stage of oligodendrogenesis and describe mechanisms of action, where known. Overall, the majority of hormones enhance oligodendrogenesis, increasing OPC differentiation and inducing maturation and myelin production in OLs. The mechanisms underlying these processes vary for each hormone but may ultimately converge upon common signaling pathways, mediated by specific receptors expressed across the OL lineage. However, not all of the mechanisms have been fully elucidated, and here, we note the remaining gaps in the literature, including the complex interactions between hormonal systems and with the immune system. In the companion manuscript in this issue, we discuss the implications of hormonal regulation of oligodendrogenesis for neurological and psychiatric disorders characterized by white matter loss. Ultimately, a better understanding of the fundamental mechanisms of hormonal regulation of oligodendrogenesis across the entire lifespan, especially in vivo, will progress both basic and translational research.

Thyroid Hormone Signaling in Embryonic Stem Cells: Crosstalk with the Retinoic Acid Pathway

While the role of thyroid hormones (THs) during fetal and postnatal life is well-established, their role at preimplantation and during blastocyst development remains unclear. In this study, we used an embryonic stem cell line isolated from rat (RESC) to study the effects of THs and retinoic acid (RA) on early embryonic development during the pre-implantation stage. The results showed that THs play an important role in the differentiation/maturation processes of cells obtained from embryoid bodies (EB), with thyroid hormone nuclear receptors (TR) (TRα and TRβ), metabolic enzymes (deiodinases 1, 2, 3) and membrane transporters (Monocarboxylate transporters -MCT- 8 and 10) being expressed throughout in vitro differentiation until the Embryoid body (EB) stage. Moreover, thyroid hormone receptor antagonist TR (1-850) impaired RA-induced neuroectodermal lineage specification. This effect was significantly higher when cells were treated with retinoic acid (RA) to induce neuroectodermal lineage, studied through the gene and protein expression of nestin, an undifferentiated progenitor marker from the neuroectoderm lineage, as established by nestin mRNA and protein regulation. These results demonstrate the contribution of the two nuclear receptors, TR and RA, to the process of neuroectoderm maturation of the in vitro model embryonic stem cells obtained from rat.

Thyroid Hormone and Neural Stem Cells: Repair Potential Following Brain and Spinal Cord Injury

Neurodegenerative diseases are characterized by chronic neuronal and/or glial cell loss, while traumatic injury is often accompanied by the acute loss of both. Multipotent neural stem cells (NSCs) in the adult mammalian brain spontaneously proliferate, forming neuronal and glial progenitors that migrate toward lesion sites upon injury. However, they fail to replace neurons and glial cells due to molecular inhibition and the lack of pro-regenerative cues. A major challenge in regenerative biology therefore is to unveil signaling pathways that could override molecular brakes and boost endogenous repair. In physiological conditions, thyroid hormone (TH) acts on NSC commitment in the subventricular zone, and the subgranular zone, the two largest NSC niches in mammals, including humans. Here, we discuss whether TH could have beneficial actions in various pathological contexts too, by evaluating recent data obtained in mammalian models of multiple sclerosis (MS; loss of oligodendroglial cells), Alzheimer’s disease (loss of neuronal cells), stroke and spinal cord injury (neuroglial cell loss). So far, TH has shown promising effects as a stimulator of remyelination in MS models, while its role in NSC-mediated repair in other diseases remains elusive. Disentangling the spatiotemporal aspects of the injury-driven repair response as well as the molecular and cellular mechanisms by which TH acts, could unveil new ways to further exploit its pro-regenerative potential, while TH (ant)agonists with cell type-specific action could provide safer and more target-directed approaches that translate easier to clinical settings.

Possible Strategies to Optimize a Biomarker Discovery Approach to Correlate with Neurological Outcome in Patients with Spinal Cord Injury: A Pilot Study

The lack of reliable diagnostic and prognostic markers for spinal cord injured (SCI) patients is a severe obstacle in development and testing of new therapies, and it also impairs appropriate rehabilitation care. The sparse available data on the biochemical composition of cerebrospinal fluid (CSF) during the acute and/or chronic phase of the lesion provide, up until now, inconsistent results. In this pilot study, we then explored the possibility of combining a multi-parametric and bioinformatic analysis of CSF for its biological properties tested on different cells types, suitable for investigating inflammation and re-myelination. The patient enrollment was based on stringent inclusion criteria; that is, cervical and thoracic SCI trauma, CSF collection within 24 h of trauma, type of surgical approach for spine stabilization, and absence of steroid therapy before CSF collection. Eleven SCI patients and four healthy controls were included, and in three patients, CSF was also collected at 3 months after lesion. We identified 19 proteins among the 60 investigated cytokines, chemokines, growth factors, and structural biomarkers, which are transiently regulated 24 h after SCI. A bioinformatic analysis indicated that interleukin (IL)-6 and IL-10 are in the core of the interconnected net of activated proteins. Cell-based experiments indicate that CSF from SCI patients stimulates astroglia derivation from neural precursor cells, and an inverse correlation between IL-8 CSF level and oligodendrocyte precursor cells generated from neural stem cells was also observed. Results from this pilot study suggest that using a combined bioanalytic and biological approach to analyze SCI CSF at different times after injury could be a useful approach for identifying reliable diagnostic and prognostic markers in SCI.

Local delivery of thyroid hormone enhances oligodendrogenesis and myelination after spinal cord injury

OBJECTIVE

Traumatic spinal cord injury (SCI) causes apoptosis of myelin-forming oligodendrocytes (OLs) and demyelination of surviving axons, resulting in conduction failure. Remyelination of surviving denuded axons provides a promising therapeutic target for spinal cord repair. While cell transplantation has demonstrated efficacy in promoting remyelination and functional recovery, the lack of ideal cell sources presents a major obstacle to clinical application. The adult spinal cord contains oligodendrocyte precursor cells and multipotent neural stem/progenitor cells that have the capacity to differentiate into mature, myelinating OLs. However, endogenous oligodendrogenesis and remyelination processes are limited by the upregulation of remyelination-inhibitory molecules in the post-injury microenvironment. Multiple growth factors/molecules have been shown to promote OL differentiation and myelination.

APPROACH

In this study we screened these therapeutics and found that 3, 3', 5-triiodothyronine (T3) is the most effective in promoting oligodendrogenesis and OL maturation in vitro. However, systemic administration of T3 to achieve therapeutic doses in the injured spinal cord is likely to induce hyperthyroidism, resulting in serious side effects.

MAIN RESULTS

In this study we developed a novel hydrogel-based drug delivery system for local delivery of T3 to the injury site without eliciting systemic toxicity.

SIGNIFICANCE

Using a clinically relevant cervical contusion injury model, we demonstrate that local delivery of T3 at doses comparable to safe human doses promoted new mature OL formation and myelination after SCI.

Thyroid Hormone Signaling in Oligodendrocytes: from Extracellular Transport to Intracellular Signal

Thyroid hormone plays an important role in central nervous system (CNS) development, including the myelination of variable axonal calibers. It is well-established that thyroid hormone is required for the terminal differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes by inducing rapid cell-cycle arrest and constant transcription of pro-differentiation genes. This is well supported by the hypomyelinating phenotypes exhibited by patients with congenital hypothyroidism, cretinism. During development, myelinating oligodendrocytes only appear after the formation of neural circuits, indicating that the timing of oligodendrocyte differentiation is important. Since fetal and post-natal serum thyroid hormone levels peak at the stage of active myelination, it is suspected that the timing of oligodendrocyte development is finely controlled by thyroid hormone. The essential machinery for thyroid hormone signaling such as deiodinase activity (utilized by cells to auto-regulate the level of thyroid hormone), and nuclear thyroid hormone receptors (for gene transcription) are expressed on oligodendrocytes. In this review, we discuss the known and potential thyroid hormone signaling pathways that may regulate oligodendrocyte development and CNS myelination. Moreover, we evaluate the potential of targeting thyroid hormone signaling for white matter injury or disease.

Inflammation severely alters thyroid hormone signaling in the central nervous system during experimental allergic encephalomyelitis in rat: Direct impact on OPCs differentiation failure

Differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes is severely impaired by inflammatory cytokines and this could lead to remyelination failure in inflammatory/demyelinating diseases. Due to the role of thyroid hormone in the maturation of OPCs and developmental myelination, in this study we investigated (i) the possible occurrence of dysregulation of thyroid hormone signaling in the CNS tissue during experimental neuroinflammation; (ii) the possible impact of inflammatory cytokines on thyroid hormone signaling and OPCs differentiation in vitro. The disease model is the experimental allergic encephalomyelitis in female Dark-Agouti rats, whereas in vitro experiments were carried out in OPCs derived from neural stem cells. The main results are the following: (i) a strong upregulation of cytokine mRNA expression level was found in the spinal cord during experimental allergic encephalomyelitis; (ii) thyroid hormone signaling in the spinal cord (thyroid hormone receptors; deiodinase; thyroid hormone membrane transporter) is substantially downregulated, due to the upregulation of the thyroid hormone inactivating enzyme deiodinase 3 and the downregulation of thyroid hormone receptors, as investigated at mRNA expression level; (iii) when exposed to inflammatory cytokines, deiodinase 3 is upregulated in OPCs as well, and OPCs differentiation is blocked; (iv) deiodinase 3 inhibition by iopanoic acid recovers OPCs differentiation in the presence on inflammatory cytokines. These data suggest that cellular hypothyroidism occurs during experimental allergic encephalomyelitis, possibly impacting on thyroid hormone-dependent cellular processes, including maturation of OPCs into myelinating oligodendrocytes. GLIA 2016;64:1573-1589.

Transplantation of Neural Stem Cells Cotreated with Thyroid Hormone and GDNF Gene Induces Neuroprotection in Rats of Chronic Experimental Allergic Encephalomyelitis

The present study investigates whether transplantation of NSCs treated with T3 alone (T3/NSCs), or in conjunction with GDNF gene (GDNF-T3/NSCs), provides a better therapeutic effect than NSCs for chronic EAE. EAE rats were, respectively, injected with NSCs, T3/NSCs, GDNF-T3/NSCs, and saline at 10 days and sacrificed at 60 days after EAE immunization. The three cell grafted groups showed a significant reduction in clinical scores, inflammatory infiltration, and demyelination compared with the saline-injected group, and among the cell grafted groups, the reduction in GDNF-T3/NSCs group was the most notable, followed by T3/NSCs group. Grafted T3/NSCs and GDNF-T3/NSCs acquired more MAP2, GalC, and less GFAP in brain compared with grafted NSCs, and grafted GDNF-T3/NSCs acquired most MAP2 and least GalC among the cell grafted groups. Furthermore, T3/NSCs and GDNF-T3/NSCs grafting increased the expression of mRNA for PDGFαR, GalC, and MBP in lesion areas of brain compared with NSCs grafting, and the expression of mRNA for GalC and MBP in GDNF-T3/NSCs group was higher than that in T3/NSCs group. In conclusion, T3/NSCs grafting, especially GDNF-T3/NSCs grafting, provides a better neuroprotective effect for EAE than NSCs transplantation.

Understanding How the Subcommissural Organ and Other Periventricular Secretory Structures Contribute via the Cerebrospinal Fluid to Neurogenesis

The dynamic and molecular composition of the cerebrospinal fluid (CSF) and, consequently, the CSF physiology is much more complex and fascinating than the simplistic view held for decades. Signal molecules either transported from blood to CSF or secreted into the CSF by circumventricular organs and CSF-contacting neurons, use the CSF to reach their targets in the brain, including the pre- and postnatal neurogenic niche. The subcommissural organ (SCO), a highly conserved brain gland present throughout the vertebrate phylum, is one of the sources for signals, as well as the choroid plexus, tanycytes and CSF-contacting neurons. The SCO secretes into the fetal and adult CSF SCO-spondin, transthyretin, and basic fibroblast growth factor. These proteins participate in certain aspects of neurogenesis, such as cell cycle of neural stem cells, neuronal differentiation, and axon pathfinding. Through the CSF, the SCO-secretory proteins may reach virtually any target in the embryonic and adult central nervous system. Since the SCO continues to secrete throughout life span, it seems likely that the neurogenetic property of the SCO compounds would be targeted to the niches where neurogenesis continues in adulthood. This review is aimed to bring into discussion early and new evidence concerning the role(s) of the SCO, and the probable mechanisms by which SCO compounds can readily reach the neurogenic niche of the subventricular zone flowing with the CSF to participate in the regulation of the neurogenic niche. As we unfold the multiples trans-fluid talks between discrete brain domains we will have more tools to influence such talks.

Thyroid hormones: Possible roles in epilepsy pathology

Thyroid hormones (THs) L-thyroxine and L-triiodothyronine, primarily known as metabolism regulators, are tyrosine-derived hormones produced by the thyroid gland. They play an essential role in normal central nervous system development and physiological function. By binding to nuclear receptors and modulating gene expression, THs influence neuronal migration, differentiation, myelination, synaptogenesis and neurogenesis in developing and adult brains. Any uncorrected THs supply deficiency in early life may result in irreversible neurological and motor deficits. The development and function of GABAergic neurons as well as glutamatergic transmission are also affected by THs. Though the underlying molecular mechanisms still remain unknown, the effects of THs on inhibitory and excitatory neurons may affect brain seizure activity. The enduring predisposition of the brain to generate epileptic seizures leads to a complex chronic brain disorder known as epilepsy. Pathologically, epilepsy may be accompanied by mitochondrial dysfunction, oxidative stress and eventually dysregulation of excitatory glutamatergic and inhibitory GABAergic neurotransmission. Based on the latest evidence on the association between THs and epilepsy, we hypothesize that THs abnormalities may contribute to the pathogenesis of epilepsy. We also review gender differences and the presumed underlying mechanisms through which TH abnormalities may affect epilepsy here.

Neural stem cells isolated from amyloid precursor protein-mutated mice for drug discovery

AIM: To develop an in vitro model based on neural stem cells derived from transgenic animals, to be used in the study of pathological mechanisms of Alzheimer’s disease and for testing new molecules.

METHODS: Neural stem cells (NSCs) were isolated from the subventricular zone of Wild type (Wt) and Tg2576 mice. Primary and secondary neurosphere generation was studied, analysing population doubling and the cell yield per animal. Secondary neurospheres were dissociated and plated on MCM Gel Cultrex 2D and after 6 d in vitro (DIVs) in mitogen withdrawal conditions, spontaneous differentiation was studied using specific neural markers (MAP2 and TuJ-1 for neurons, GFAP for astroglial cells and CNPase for oligodendrocytes). Gene expression pathways were analysed in secondary neurospheres, using the QIAGEN PCR array for neurogenesis, comparing the Tg2576 derived cell expression with the Wt cells. Proteins encoded by the altered genes were clustered using STRING web software.

RESULTS: As revealed by 6E10 positive staining, all Tg2576 derived cells retain the expression of the human transgenic Amyloid Precursor Protein. Tg2576 derived primary neurospheres show a decrease in population doubling. Morphological analysis of differentiated NSCs reveals a decrease in MAP2- and an increase in GFAP-positive cells in Tg2576 derived cells. Analysing the branching of TuJ-1 positive cells, a clear decrease in neurite number and length is observed in Tg2576 cells. The gene expression neurogenesis pathway revealed 11 altered genes in Tg2576 NSCs compared to Wt.

CONCLUSION: Tg2576 NSCs represent an appropriate AD in vitro model resembling some cellular alterations observed in vivo, both as stem and differentiated cells.

Functional and molecular evidence of myelin- and neuroprotection by thyroid hormone administration in experimental allergic encephalomyelitis

AIMS

Recent data in mouse and rat demyelination models indicate that administration of thyroid hormone (TH) has a positive effect on the demyelination/remyelination balance. As axonal pathology has been recognized as an early neuropathological event in multiple sclerosis, and remyelination is considered a pre-eminent neuroprotective strategy, in this study we investigated whether TH administration improves nerve impulse propagation and protects axons.

METHODS

We followed up the somatosensory evoked potentials (SEPs) in triiodothyronine (T3)-treated and untreated experimental allergic encephalomyelitis (EAE) Dark-Agouti female rats during the electrical stimulation of the tail nerve. T3 treatment started on the 10th day post immunization (DPI) and a pulse administration was continued until the end of the study (33 DPI). SEPs were recorded at baseline (8 DPI) and the day after each hormone/ vehicle administration.

RESULTS

T3 treatment was associated with better outcome of clinical and neurophysiological parameters. SEPs latencies of the two groups behaved differently, being briefer and closer to control values (=faster impulse propagation) in T3-treated animals. The effect was evident on 24 DPI. In the same groups of animals, we also investigated axonal proteins, showing that T3 administration normalizes neurofilament immunoreactivity in the fasciculus gracilis and tau hyperphosphorylation in the lumbar spinal cord of EAE animals. No sign of plasma hyperthyroidism was found; moreover, the dysregulation of TH nuclear receptor expression observed in the spinal cord of EAE animals was corrected by T3 treatment.

CONCLUSIONS

T3 supplementation results in myelin sheath protection, nerve conduction preservation and axon protection in this animal model of multiple sclerosis.

Thyroid hormone accelerates the differentiation of adult hippocampal progenitors

Disrupted thyroid hormone function evokes severe physiological consequences in the immature brain. In adulthood, although clinical reports document an effect of thyroid hormone status on mood and cognition, the molecular and cellular changes underlying these behavioural effects are poorly understood. More recently, the subtle effects of thyroid hormone on structural plasticity in the mature brain, in particular on adult hippocampal neurogenesis, have come to be appreciated. However, the specific stages of adult hippocampal progenitor development that are sensitive to thyroid hormone are not defined. Using nestin-green fluorescent protein reporter mice, we demonstrate that thyroid hormone mediates its effects on hippocampal neurogenesis by influencing Type 2b and Type 3 progenitors, although it does not alter proliferation of either the Type 1 quiescent progenitor or the Type 2a amplifying neural progenitor. Thyroid hormone increases the number of doublecortin (DCX)-positive Type 3 progenitors, and accelerates neuronal differentiation into both DCX-positive immature neurones and neuronal nuclei-positive granule cell neurones. Furthermore, we show that this increase in neuronal differentiation is accompanied by a significant induction of specific transcription factors involved in hippocampal progenitor differentiation. In vitro studies using the neurosphere assay support a direct effect of thyroid hormone on progenitor development because neurospheres treated with thyroid hormone are shifted to a more differentiated state. Taken together, our results indicate that thyroid hormone mediates its neurogenic effects via targeting Type 2b and Type 3 hippocampal progenitors, and suggests a role for proneural transcription factors in contributing to the effects of thyroid hormone on neuronal differentiation of adult hippocampal progenitors.

Thyroid hormone promotes neuronal differentiation of embryonic neural stem cells by inhibiting STAT3 signaling through TRα1

A deficiency of maternal thyroid hormones (THs) during pregnancy may have severe impacts on fetal brain development. However, the cellular targets of THs and their underlying mechanisms are still unclear. In this study, we found that maternal hypothyroidism during pregnancy in mice inhibited neurogenesis in the embryonic telencephalon and caused learning and memory impairment in the offspring. To explore the underlying mechanisms, we treated cultured mouse embryonic neural stem cells (eNSCs) with a physiological level of 3, 5, 3'-triiodo-L-thyronine (T3). We found that T3 promoted the neuronal differentiation of eNSCs, while inhibiting astrocytic differentiation. In addition, the proliferation and maintenance of eNSCs were inhibited by T3. Furthermore, the TH receptor alpha 1 (TRα1) was detected in the eNSCs both in vivo and in vitro. Silencing TRα1 protein expression with specific siRNA eliminated the effects of T3 on eNSCs. We also found that T3 decreased STAT3 phosphorylation and STAT3-DNA binding activity through TRα1. The over expression of STAT3 attenuated the promotive effects of T3 on neuronal differentiation of eNSCs. Taken together, these results suggest that T3 promotes the neuronal differentiation of eNSCs by inhibiting STAT3 signaling activity through TRα1 and contributes to early neurogenesis in the embryonic telencephalon. Our studies reveal the physiological effects of TH in regulating eNSCs differentiation and suggest that eNSCs are one of the major cellular targets in the central nervous system by which TH influences early brain development. These findings also provide new insights into the mechanisms of neurological deficits caused by TH deficiency during embryogenesis.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Cuprizone-induced demyelination in the rat cerebral cortex and thyroid hormone effects on cortical remyelination

Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the Central Nervous System which is characterized by multifocal demyelinated lesions dispersed throughout the brain. Although white matter lesions have been the most extensively studied, cortical demyelinaton lesions are also detected in MS brains. Cuprizone (CPZ)-induced demyelination in rodents has been widely used as a model for MS. Most of these studies focus on oligodendrocyte-rich structures, such as the corpus callosum (CC) and the cerebellar peduncles. However, it has been recently described that CPZ administration in mice also produces cortical demyelination, resembling some of the lesions found in MS patients. In this work we used CPZ-demyelinating model in Wistar rats to study demyelination in cortical forebrain areas. At the ultrastructural level, demyelination in the cortex was observed before detectable myelin loss in the subcortical white matter. During the course of CPZ intoxication Myelin Basic Protein immunodetection was decreased in cortical layers I-III due to a reduction in the number of cortical oligodendrocytes (OL). Oligodendroglial loss in CPZ-intoxicated rats correlated with an increase in the number of Glial Fibrillary Acidic Protein positive astrocytes and a shift in the location of Carbonic Anhydrase II from OL to astrocytes. After removal of CPZ from the diet, we evaluate intranasal Thyroid hormone (TH) effects on the progression of cortical lesions. As previously reported in the CC, TH treatment also accelerates remyelination rate in the cortex compared to rats undergoing spontaneous remyelination. Our results suggest that manipulation of TH levels could be considered as a strategy to promote remyelination process in the cortex and to prevent neuronal irreversible damage in patients suffering from MS.

Functional identification of neural stem cell-derived oligodendrocytes

Directing neural stem cells (NSCs) differentiation towards oligodendroglial cell lineage is a crucial step in the endeavor of developing cell replacement-based therapies for demyelinating diseases. Evaluation of NSCs differentiation is mostly performed by methodologies that use fixed cells, like immunocytochemistry, or lysates, like Western blot. On the other hand, electrophysiology allows differentiation studies on living cells, but it is highly time-consuming and endowed with important limitations concerning population studies. Herein, we describe a functional method, based on single cell calcium imaging, which accurately and rapidly distinguishes cell types among NSCs progeny, in living cultures prepared from the major reservoir of NSCs in the postnatal mouse brain, the subventricular zone (SVZ). Indeed, by applying a rational sequence of three stimuli-KCl, histamine, and thrombin-to the heterogeneous SVZ cell population, one can identify each cell phenotype according to its unique calcium signature. Mature oligodendrocytes, the myelin-forming cells of the central nervous system, are the thrombin-responsive cells in SVZ cell culture and display no intracellular calcium increase upon KCl or histamine perfusion. On the other hand, KCl and histamine stimulate neurons and immature cells, respectively. The method described in this chapter is a valuable tool to identify novel pro-oligodendrogenic compounds, which may play an important role in the design of future treatments for demyelinating disorders such as multiple sclerosis.

Triiodothyronine administration ameliorates the demyelination/remyelination ratio in a non-human primate model of multiple sclerosis by correcting tissue hypothyroidism

Remyelination failure is a key landmark in chronic progression of multiple sclerosis (MS), the most diffuse demyelinating disease in human, but the reasons for this are still unknown. It has been shown that thyroid hormone administration in the rodent models of acute and chronic demyelinating diseases improved their clinical course, pathology and remyelination. In the present study, we translated this therapeutic attempt to experimental allergic encephalomyelitis (EAE) in the non-human primate Callithrix Jacchus (marmoset). We report that short protocols of triiodothyronine treatment shifts the demyelination/remyelination balance toward remyelination, as assessed by morphology, immunohistochemistry and molecular biology, and improves the clinical course of the disease. We also found that severely ill animals display hypothyroidism and severe alteration of deiodinase and thyroid hormone receptor mRNAs expression in the spinal cord, which was completely corrected by thyroid hormone treatment. We therefore suggest that thyroid hormone treatment improves myelin sheath morphology in marmoset EAE, by correcting the dysfunction of thyroid hormone cellular effectors.

Tissue transglutaminase activity is involved in the differentiation of oligodendrocyte precursor cells into myelin-forming oligodendrocytes during CNS remyelination

During normal brain development, axons are myelinated by mature oligodendrocytes (OLGs). Under pathological, demyelinating conditions within the central nervous system (CNS), axonal remyelination is only partially successful because oligodendrocyte precursor cells (OPCs) largely remain in an undifferentiated state resulting in a failure to generate myelinating OLGs. Tissue Transglutaminase (TG2) is a multifunctional enzyme, which amongst other functions, is involved in cell differentiation. Therefore, we hypothesized that TG2 contributes to differentiation of OPCs into OLGs and thereby stimulates remyelination. In vivo studies, using the cuprizone model for de- and remyelination in TG2(-/-) and wild-type mice, showed that during remyelination expression of proteolipid protein mRNA, as a marker for remyelination, in the corpus callosum lags behind in TG2(-/-) mice resulting in less myelin formation and, moreover, impaired recovery of motor behavior. Subsequent in vitro studies showed that rat OPCs express TG2 protein and activity which reduces when the cells have matured into OLGs. Furthermore, when TG2 activity is pharmacologically inhibited, the differentiation of OPCs into myelin-forming OLGs is dramatically reduced. We conclude that TG2 plays a prominent role in remyelination of the CNS, probably through stimulating OPC differentiation into myelin-forming OLGs. Therefore, manipulating TG2 activity may represent an interesting new target for remyelination in demyelinating diseases.

Ex vivo study of dentate gyrus neurogenesis in human pharmacoresistant temporal lobe epilepsy

AIMS

Neurogenesis in adult humans occurs in at least two areas of the brain, the subventricular zone of the telencephalon and the subgranular layer of the dentate gyrus in the hippocampal formation. We studied dentate gyrus subgranular layer neurogenesis in patients subjected to tailored antero-mesial temporal resection including amygdalohippocampectomy due to pharmacoresistant temporal lobe epilepsy (TLE) using the in vitro neurosphere assay.

METHODS

Sixteen patients were enrolled in the study; mesial temporal sclerosis (MTS) was present in eight patients. Neurogenesis was investigated by ex vivo neurosphere expansion in the presence of mitogens (epidermal growth factor + basic fibroblast growth factor) and spontaneous differentiation after mitogen withdrawal. Growth factor synthesis was investigated by qRT-PCR in neurospheres.

RESULTS

We demonstrate that in vitro proliferation of cells derived from dentate gyrus of TLE patients is dependent on disease duration. Moreover, the presence of MTS impairs proliferation. As long as in vitro proliferation occurs, neurogenesis is maintained, and cells expressing a mature neurone phenotype (TuJ1, MAP2, GAD) are spontaneously formed after mitogen withdrawal. Finally, formed neurospheres express mRNAs encoding for growth (vascular endothelial growth factor) as well as neurotrophic factors (brain-derived neurotrophic factor, ciliary neurotrophic factor, glial-derived neurotrophic factor, nerve growth factor).

CONCLUSION

We demonstrated that residual neurogenesis in the subgranular layer of the dentate gyrus in TLE is dependent on diseases duration and absent in MTS.

Excess thyroid hormone inhibits embryonic neural stem/progenitor cells proliferation and maintenance through STAT3 signalling pathway

Hyperthyroidism is prevalent during pregnancy, but little is known about the effects of excess thyroid hormone on the development of embryonic neural stem/progenitor cells (NSCs), and the mechanisms underlying these effects. Previous studies indicate that STAT3 plays a crucial role in determining NSC fate during neurodevelopment. In this study, we investigated the effects of a supraphysiological dose of 3,5,3'-L-triiodothyronine (T3) on the proliferation and maintenance of NSCs derived from embryonic day 13.5 mouse neocortex, and the involvement of STAT3 in this process. Our results suggest that excess T3 treatment inhibits NSC proliferation and maintenance. T3 decreased tyrosine phosphorylation of JAK1, JAK2 and STAT3, and subsequently inhibited STAT3-DNA binding activity. Furthermore, proliferation and maintenance of NSCs were decreased by inhibitors of JAKs and STAT3, indicating that the STAT3 signalling pathway is involved in the process of NSC proliferation and maintenance. Taken together, these results suggest that the STAT3 signalling pathway is involved in the process of T3-induced inhibition of embryonic NSC proliferation and maintenance. These findings provide data for understanding the effects of hyperthyroidism during pregnancy on fetal brain development, and the mechanisms underlying these effects.

Inflammation induced neurological handicap processes in multiple sclerosis: new insights from preclinical studies

Multiple sclerosis (MS) is described as originating from incompletely explained neuroinflammatory processes, dysfunction of neuronal repair mechanisms and chronicity of inflammation events. Blood-borne immune cell infiltration and microglia activation are causing both neuronal destruction and myelin loss, which are responsible for progressive motor deficiencies, organic and cognitive dysfunctions. MRI as a non-invasive imaging method offers various ways to visualise de- and remyelination, neuronal loss, leukocyte infiltration, blood-brain barrier modification and new sensors are emerging to detect inflammatory lesions at an early stage. We describe studies performed on experimental autoimmune encephalomyelitis (EAE) animal models of MS that shed new light on mechanisms of functional impairments to understand the neurological handicap in MS. We focus on examples of neuroinflammation-mediated inhibition of CNS repair involving adult neurogenesis in the sub-ventricular zone and hippocampus and such experimentally observed inhibitions could reflect deficient plasticity and activation of compensatory mechanisms in MS. In parallel with cognitive decline, organic deficits such as bladder dysfunction are described in most of MS patients. Neuropharmacological interventions, electrical stimulation of nerves, MRI and histopathology follow-up studies helped in understanding the operating events to remodel the neurological networks and to compensate the inflammatory lesions both in spinal cord and in cortical regions. At the molecular level, the local production of reactive products is a well-described phenomenon: oxidative species disturb cellular physiology and generate new molecular epitopes that could further promote immune reactions. The translational research from EAE animal models to MS patient cohorts helps in understanding the mechanisms of the neurological handicap and in development of new therapeutic concepts in MS.

Functional identification of neural stem cell-derived oligodendrocytes by means of calcium transients elicited by thrombin

Current immunosuppressive treatments for central nervous system demyelinating diseases fail to prevent long-term motor and cognitive decline in patients. Excitingly, glial cell transplantation arises as a promising complementary strategy to challenge oligodendrocytes loss occurring in myelination disorders. A potential source of new oligodendrocytes is the subventricular zone (SVZ) pool of multipotent neural stem cells. However, this approach has been handicapped by the lack of functional methods for identification and pharmacological analysis of differentiating oligodendrocytes, prior to transplantation. In this study, we questioned whether SVZ-derived oligodendrocytes could be functionally discriminated due to intracellular calcium level ([Ca(2+)](i)) variations following KCl, histamine, and thrombin stimulations. Previously, we have shown that SVZ-derived neurons and immature cells can be discriminated on the basis of their selective [Ca(2+)](i) rise upon KCl and histamine stimulation, respectively. Herein, we demonstrate that O4+ and proteolipid protein-positive (PLP+) oligodendrocytes do not respond to these stimuli, but display a robust [Ca(2+)](i) rise following thrombin stimulation, whereas other cell types are thrombin-insensitive. Thrombin-induced Ca(2+) increase in oligodendrocytes is mediated by protease-activated receptor-1 (PAR-1) activation and downstream signaling through G(q/11) and phospholipase C (PLC), resulting in Ca(2+) recruitment from intracellular compartments. This method allows the analysis of functional properties of oligodendrocytes in living SVZ cultures, which is of major interest for the development of effective grafting strategies in the demyelinated brain. Additionally, it opens new perspectives for the search of new pro-oligodendrogenic factors to be used prior grafting.

Robust generation of oligodendrocyte progenitors from human neural stem cells and engraftment in experimental demyelination models in mice

BACKGROUND

Cell-based therapy holds great promises for demyelinating diseases. Human-derived fetal and adult oligodendrocyte progenitors (OPC) gave encouraging results in experimental models of dysmyelination but their limited proliferation in vitro and their potential immunogenicity might restrict their use in clinical applications. Virtually unlimited numbers of oligodendroglial cells could be generated from long-term self-renewing human (h)-derived neural stem cells (hNSC). However, robust oligodendrocyte production from hNSC has not been reported so far, indicating the need for improved understanding of the molecular and environmental signals controlling hNSC progression through the oligodendroglial lineage. The aim of this work was to obtain enriched and renewable cultures of hNSC-derived oligodendroglial cells by means of epigenetic manipulation.

METHODOLOGY/PRINCIPAL FINDINGS

We report here the generation of large numbers of hNSC-derived oligodendroglial cells by concurrent/sequential in vitro exposure to combinations of growth factors (FGF2, PDGF-AA), neurotrophins (NT3) and hormones (T3). In particular, the combination FGF2+NT3+PDGF-AA resulted in the maintenance and enrichment of an oligodendroglial cell population displaying immature phenotype (i.e., proliferation capacity and expression of PDGFRalpha, Olig1 and Sox10), limited self-renewal and increased migratory activity in vitro. These cells generate large numbers of oligodendroglial progeny at the early stages of maturation, both in vitro and after transplantation in models of CNS demyelination.

CONCLUSIONS/SIGNIFICANCE

We describe a reliable method to generate large numbers of oligodendrocytes from a renewable source of somatic, non-immortalized NSC from the human foetal brain. We also provide insights on the mechanisms underlying the pro-oligodendrogenic effect of the treatments in vitro and discuss potential issues responsible for the limited myelinating capacity shown by hNSC-derived oligodendrocytes in vivo.