Signalling pathways that inhibit the capacity of precursor cells for myelin repair

The Role of Herpes Simplex Virus Type 1 Infection in Demyelination of the Central Nervous System

Herpes simplex type 1 (HSV-1) is a neurotropic virus that infects the peripheral and central nervous systems. After primary infection in epithelial cells, HSV-1 spreads retrogradely to the peripheral nervous system (PNS), where it establishes a latent infection in the trigeminal ganglia (TG). The virus can reactivate from the latent state, traveling anterogradely along the axon and replicating in the local surrounding tissue. Occasionally, HSV-1 may spread trans-synaptically from the TG to the brainstem, from where it may disseminate to higher areas of the central nervous system (CNS). It is not completely understood how HSV-1 reaches the CNS, although the most accepted idea is retrograde transport through the trigeminal or olfactory tracts. Once in the CNS, HSV-1 may induce demyelination, either as a direct trigger or as a risk factor, modulating processes such as remyelination, regulation of endogenous retroviruses, or molecular mimicry. In this review, we describe the current knowledge about the involvement of HSV-1 in demyelination, describing the pathways used by this herpesvirus to spread throughout the CNS and discussing the data that suggest its implication in demyelinating processes.

Co-Ultramicronized Palmitoylethanolamide/Luteolin Facilitates the Development of Differentiating and Undifferentiated Rat Oligodendrocyte Progenitor Cells

Oligodendrocytes, the myelin-producing cells of the central nervous system (CNS), have limited capability to bring about repair in chronic CNS neuroinflammatory demyelinating disorders such as multiple sclerosis (MS). MS lesions are characterized by a compromised pool of undifferentiated oligodendrocyte progenitor cells (OPCs) unable to mature into myelin-producing oligodendrocytes. An attractive strategy may be to replace lost OLs and/or promote their maturation. N-palmitoylethanolamine (PEA) is an endogenous fatty acid amide signaling molecule with anti-inflammatory and neuroprotective actions. Recent studies show a co-ultramicronized composite of PEA and the flavonoid luteolin (co-ultraPEALut) to be more efficacious than PEA in improving outcome in CNS injury models. Here, we examined the effects of co-ultraPEALut on development of OPCs from newborn rat cortex cultured under conditions favoring either differentiation (Sato medium) or proliferation (fibroblast growth factor-2 and platelet-derived growth factor (PDGF)-AA-supplemented serum-free medium ("SFM")). OPCs in SFM displayed high expression of PDGF receptor alpha gene and the proliferation marker Ki-67. In Sato medium, in contrast, OPCs showed rapid decreases in PDGF receptor alpha and Ki-67 expression with a concomitant rise in myelin basic protein (MBP) expression. In these conditions, co-ultraPEALut (10 μM) enhanced OPC morphological complexity and expression of MBP and the transcription factor TCF7l2. Surprisingly, co-ultraPEALut also up-regulated MBP mRNA expression in OPCs in SFM. MBP expression in all cases was sensitive to inhibition of mammalian target of rapamycin. Within the context of strategies to promote endogenous remyelination in MS which focus on enhancing long-term survival of OPCs and stimulating their differentiation into remyelinating oligodendrocytes, co-ultraPEALut may represent a novel pharmacological approach.

9-cis-Retinoic Acid and 1,25-dihydroxy Vitamin D3 Improve the Differentiation of Neural Stem Cells into Oligodendrocytes through the Inhibition of the Notch and Wnt Signaling Pathways

BACKGROUND

Differentiating oligodendrocyte precursor cells (OPCs) into oligodendrocytes could be improved by inhibiting signaling pathways such as Wnt and Notch. 9-cis-retinoic acid (9-cis-RA) and 1,25-dihydroxyvitamin D₃ (1,25[OH]₂D₃) can ameliorate oligodendrogenesis. We investigated whether they could increase oligodendrogenesis by inhibiting the Wnt and Notch signaling pathways.

METHODS

Cortical neural stem cells were isolated from 14-day-old rat embryos and cultured using the neurosphere assay. The cells were treated in 4 different conditions for 1 week: the negative control group received only the basic fibroblast growth factor, the positive control group received only T₃ without growth factors, the RA group was treated with 9-cis-RA, and the Vit D₃ group was treated with 1,25(OH)₂D₃. The effects of 9-cis-RA and 1,25(OH)₂D₃ on the level of the myelin basic protein (MBP) and the gene expression of the SOX10, MBP gene, HES5, and LRP6 were studied using flow cytometry and real-time PCR. The data were analyzed using one-way ANOVA with GraphPad Prism. A P value less than 0.05 was considered significant.

RESULTS

The mRNA expressions of the SOX10, MBP, and MBP gene were significantly increased in the treated groups compared with the negative control group; the increase was similar in the 9-cis-RA group and the positive control group. Furthermore, 9-cis-RA significantly decreased the expression of the HES5 gene, a Notch signaling pathway transcription factor, and 1,25(OH)₂D₃ significantly reduced the expression of the LRP6 gene, a Wnt signaling pathway co-receptor.

CONCLUSION

It seems that 9-cis-RA and 1,25(OH)₂D₃ are good candidates to improve the differentiation of OPCs into oligodendrocytes.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

Signaling by the transforming growth factor β (TGF-β) family is necessary for proper neural development and function throughout life. Sequential waves of activation, inhibition, and reactivation of TGF-β family members regulate numerous elements of the nervous system from the earliest stages of embryogenesis through adulthood. This review discusses the expression, regulation, and function of TGF-β family members in the central nervous system at various developmental stages, beginning with induction and patterning of the nervous system to their importance in the adult as modulators of inflammatory response and involvement in degenerative diseases.

Transcription factor 7 like 2 promotes oligodendrocyte differentiation and remyelination

Transcription factor 7 like 2 (TCF7L2, also termed TCF4), is a Wnt effector induced transiently in the oligodendroglial lineage. The current well accepted hypothesis is that TCF7L2 inhibits oligodendrocyte differentiation and remyelination through canonical Wnt/β‑catenin signaling. However, recent studies indicated that TCF7L2 activity is required during oligodendrocyte differentiation and remyelination. In order to clarify this, in situ hybridization, immunofluorescence and western blot analysis using in vivo TCF7L2 conditional knockout mice, were performed and it was found that TCF7L2 promotes oligodendrocyte differentiation during myelin formation and remyelination. Furthermore, it was established that TCF7L2 does not affect oligodendrocyte precursor cells during remyelination. These data are of important clinical significance to develop novel therapeutic targets to overcome multiple sclerosis and other demyelinating diseases.

Ameliorating the Effect of Pioglitazone on LPS-Induced Inflammation of Human Oligodendrocyte Progenitor Cells

Oligodendrocyte progenitor cells (OPCs) are appropriate model cells for studying the progress of neurodegenerative disorders and evaluation of pharmacological efficacies of small molecules for treatment of these disorders. Here, we focused on the therapeutic role of Pioglitazone, which is a selective agonist of peroxisome proliferator-activated receptor gamma (PPARγ), a respective nuclear receptor in inflammatory responses. Human embryonic stem cell-derived OPCs were pretreated by Pioglitazone at differing concentrations. Pretreated OPCs were further examined after induction of inflammation by LPS. Interestingly, Pioglitazone reversed the inflammatory conditions and enhanced OPC viability. Data showed that Pioglitazone reduced Nitric Oxide (NO) production. Moreover, Pioglitazone enhanced cell viability through distinct mechanisms including reduction of apoptosis and regulation of cell cycle markers. This study demonstrated that NO induces apoptosis through FOXO1 and degradation of β-catenin, while the presence of Pioglitazone inhibited these effects in rescuing human OPCs from apoptosis. Also, Pioglitazone did not show a significant influence on mRNA levels of TLR2, TRL4, and TNFα. Furthermore, simultaneous treatment of Pioglitazone with CHIR, a GSKβ inhibitor, facilitated anti-apoptotic responses of OPCs. Taken together, therapy with Pioglitazone represents a novel potential drug in alleviating the loss of OPCs in neurodegenerative conditions.

Hypomyelination following deletion of Tsc2 in oligodendrocyte precursors

Objective

While abnormalities in myelin in tuberous sclerosis complex (TSC) have been known for some time, recent imaging‐based data suggest myelin abnormalities may be independent of the pathognomonic cortical lesions (“tubers”). Multiple mouse models of TSC exhibit myelination deficits, though the cell types responsible and the mechanisms underlying the myelin abnormalities remain unclear.

Methods

To determine the role of alterations in mTOR signaling in myelination, we generated a conditional knockout (CKO) mouse model using Cre‐recombinase and the Olig2 promoter to inactivate the Tsc2 gene in oligodendrocyte precursor cells.

Results

Characterization of myelin and myelin constituent proteins demonstrated a marked hypomyelination phenotype. Diffusion‐based magnetic resonance imaging studies were likewise consistent with hypomyelination. Hypomyelination was due in part to decreased myelinated axon density and myelin thickness as well as decreased oligodendrocyte numbers. Coincident with hypomyelination, an extensive gliosis was seen in both the cortex and white matter tracks, suggesting alterations in cell fate due to changes in mTOR activity in oligodendrocyte precursors. Despite a high‐frequency appendicular tremor and altered gait in CKO mice, no significant changes in activity, vocalizations, or anxiety‐like phenotypes were seen.

Interpretation

Our findings support a known role of mTOR signaling in regulation of myelination and demonstrate that increased mTORC1 activity early in development within oligodendrocytes results in hypomyelination and not hypermyelination. Our data further support a dissociation between decreased Akt activity and increased mTORC1 activity toward hypomyelination. Thus, therapies promoting activation of Akt‐dependent pathways while reducing mTORC1 activity may prove beneficial in treatment of human disease.

The Wnt effector transcription factor 7-like 2 positively regulates oligodendrocyte differentiation in a manner independent of Wnt/β-catenin signaling

Genetic or pharmacological activation of canonical Wnt/β-catenin signaling inhibits oligodendrocyte differentiation. Transcription factor 7-like 2 (TCF7l2), also known as TCF4, is a Wnt effector induced transiently in the oligodendroglial lineage. A well accepted dogma is that TCF7l2 inhibits oligodendrocyte differentiation through activation of Wnt/β-catenin signaling. We report that TCF7l2 is upregulated transiently in postmitotic, newly differentiated oligodendrocytes. Using in vivo gene conditional ablation, we found surprisingly that TCF7l2 positively regulates neonatal and postnatal mouse oligodendrocyte differentiation during developmental myelination and remyelination in a manner independent of the Wnt/β-catenin signaling pathway. We also reveal a novel role of TCF7l2 in repressing a bone morphogenetic protein signaling pathway that is known to inhibit oligodendrocyte differentiation. Thus, our study provides novel data justifying therapeutic attempts to enhance, rather than inhibit, TCF7l2 signaling to overcome arrested oligodendroglial differentiation in multiple sclerosis and other demyelinating diseases.

MRI phenotypes based on cerebral lesions and atrophy in patients with multiple sclerosis

BACKGROUND

While disease categories (i.e. clinical phenotypes) of multiple sclerosis (MS) are established, there remains MRI heterogeneity among patients within those definitions. MRI-defined lesions and atrophy show only moderate inter-correlations, suggesting that they represent partly different processes in MS. We assessed the ability of MRI-based categorization of cerebral lesions and atrophy in individual patients to identify distinct phenotypes.

METHODS

We studied 175 patients with MS [age (mean ± SD) 42.7 ± 9.1 years, 124 (71%) women, Expanded Disability Status (EDSS) score 2.5 ± 2.3, n = 18 (10%) clinically isolated demyelinating syndrome (CIS), n=115 (66%) relapsing-remitting (RR), and n = 42 (24%) secondary progressive (SP)]. Brain MRI measures included T2 hyperintense lesion volume (T2LV) and brain parenchymal fraction (to assess whole brain atrophy). Medians were used to create bins for each parameter, with patients assigned a low or high severity score.

RESULTS

Four MRI phenotype categories emerged: Type I = low T2LV/mild atrophy [n = 67 (38%); CIS = 14, RR = 47, SP = 6]; Type II = high T2LV/mild atrophy [n = 21 (12%); RR = 19, SP = 2]; Type III = low T2LV/high atrophy [n = 21 (12%); CIS = 4, RR = 16, SP = 1]; and Type IV = high T2LV/high atrophy [n = 66 (38%); RR = 33, S P = 33]. Type IV was the most disabled and was the only group showing a correlation between T2LV vs. BPF and MRI vs. EDSS score (all p < 0.05).

CONCLUSIONS

We described MRI-categorization based on the relationship between lesions and atrophy in individual patients to identify four phenotypes in MS. Most patients have congruent extremes related to the degree of lesions and atrophy. However, many have a dissociation. Longitudinal studies will help define the stability of these patterns and their role in risk stratification.

Disease-modifying treatments for progressive multiple sclerosis

The last 20 years have seen major progress in the treatment of relapsing-remitting multiple sclerosis (RRMS) using a variety of drugs targeting immune dysfunction. In contrast, all clinical trials of such agents in primary progressive multiple sclerosis (PPMS) have failed and there is limited evidence of their efficacy in secondary progressive disease. Evolving concepts of the complex interplay between inflammatory and neurodegenerative processes across the course of multiple sclerosis (MS) may explain this discrepancy. This paper will provide an up-to-date overview of the rationale and results of the published clinical trials that have sought to alter the trajectory of both primary and secondary MS, considering studies involving drugs with a primary immune target and also those aiming for neuroprotection. Future areas of study will be discussed, building on these results combined with the experience of treating RRMS and new concepts emerging from laboratory science and animal models.

BMP signaling in telencephalic neural cell specification and maturation

Bone morphogenetic proteins (BMPs) make up a family of morphogens that are critical for patterning, development, and function of the central and peripheral nervous system. Their effects on neural cells are pleiotropic and highly dynamic depending on the stage of development and the local niche. Neural cells display a broad expression profile of BMP ligands, receptors, and transducer molecules. Moreover, interactions of BMP signaling with other incoming morphogens and signaling pathways are crucial for most of these processes. The key role of BMP signaling suggests that it includes many regulatory mechanisms that restrict BMP activity both temporally and spatially. BMPs affect neural cell fate specification in a dynamic fashion. Initially they inhibit proliferation of neural precursors and promote the first steps in neuronal differentiation. Later on, BMP signaling effects switch from neuronal induction to promotion of astroglial identity and inhibition of neuronal or oligodendroglial lineage commitment. Furthermore, in postmitotic cells, BMPs regulate cell survival and death, to modulate neuronal subtype specification, promote dendritic and axonal growth and induce synapse formation and stabilization. In this review, we examine the canonical and non-canonical mechanisms of BMP signal transduction. Moreover, we focus on the specific role of BMPs in the nervous system including their ability to regulate neural stem cell proliferation, self-renewal, lineage specification, and neuronal function.

Multiple sclerosis: from molecules to treatment

The treatment of multiple sclerosis has been radically transformed over the past 20 years and this special issue of IJMS, focusing on the molecular aspects of the disease, highlights the growing conformity of the various investigative approaches. It is a very exciting time to be involved in the research of this disease.