The Shh receptor Boc is important for myelin formation and repair

Activation of Shh/Smo is sufficient to maintain oligodendrocyte precursor cells in an undifferentiated state and is not necessary for myelin formation and (re)myelination

Myelination is the terminal step in a complex and precisely timed program that orchestrates the proliferation, migration and differentiation of oligodendroglial cells. It is thought that Sonic Hedgehog (Shh) acting on Smoothened (Smo) participates in regulating this process, but that these effects are highly context dependent. Here, we investigate oligodendroglial development and remyelination from three specific transgenic lines: NG2-CreERT2 (control), Smofl/fl/NG2-CreERT2 (loss of function), and SmoM2/NG2-CreERT2 (gain of function), as well as pharmacological manipulation that enhance or inhibit the Smo pathway (Smoothened Agonist (SAG) or cyclopamine treatment, respectively). To explore the effects of Shh/Smo on differentiation and myelination in vivo, we developed a highly quantifiable model by transplanting oligodendrocyte precursor cells (OPCs) in the retina. We find that myelination is greatly enhanced upon cyclopamine treatment and hypothesize that Shh/Smo could promote OPC proliferation to subsequently inhibit differentiation. Consistent with this hypothesis, we find that the genetic activation of Smo significantly increased numbers of OPCs and decreased oligodendrocyte differentiation when we examined the corpus callosum during development and after cuprizone demyelination and remyelination. However, upon loss of function with the conditional ablation of Smo, myelination in the same scenarios are unchanged. Taken together, our present findings suggest that the Shh pathway is sufficient to maintain OPCs in an undifferentiated state, but is not necessary for myelination and remyelination.

Hedgehog signaling regulates Wolffian duct development through the primary cilium†

Primary cilia play pivotal roles in embryonic patterning and organogenesis through transduction of the Hedgehog signaling pathway (Hh). Although mutations in Hh morphogens impair the development of the gonads and trigger male infertility, the contribution of Hh and primary cilia in the development of male reproductive ductules, including the epididymis, remains unknown. From a Pax2Cre; IFT88fl/fl knock-out mouse model, we found that primary cilia deletion is associated with imbalanced Hh signaling and morphometric changes in the Wolffian duct (WD), the embryonic precursor of the epididymis. Similar effects were observed following pharmacological blockade of primary cilia formation and Hh modulation on WD organotypic cultures. The expression of genes involved in extracellular matrix, mesenchymal-epithelial transition, canonical Hh and WD development was significantly altered after treatments. Altogether, we identified the primary cilia-dependent Hh signaling as a master regulator of genes involved in WD development. This provides new insights regarding the etiology of sexual differentiation and male infertility issues.

Sonic Hedgehog Signaling Pathway: A Role in Pain Processing

Pain, as one of the most prevalent clinical symptoms, is a complex physiological and psychological activity. Long-term severe pain can become unbearable to the body. However, existing treatments do not provide satisfactory results. Therefore, new mechanisms and therapeutic targets need to be urgently explored for pain management. The Sonic hedgehog (Shh) signaling pathway is crucial in embryonic development, cell differentiation and proliferation, and nervous system regulation. Here, we review the recent studies on the Shh signaling pathway and its action in multiple pain-related diseases. The Shh signaling pathway is dysregulated under various pain conditions, such as pancreatic cancer pain, bone cancer pain, chronic post-thoracotomy pain, pain caused by degenerative lumbar disc disease, and toothache. Further studies on the Shh signaling pathway may provide new therapeutic options for pain patients.

Shaking up the silence: consequences of HMGN1 antagonizing PRC2 in the Down syndrome brain

Intellectual disability is a well-known hallmark of Down Syndrome (DS) that results from the triplication of the critical region of human chromosome 21 (HSA21). Major studies were conducted in recent years to gain an understanding about the contribution of individual triplicated genes to DS-related brain pathology. Global transcriptomic alterations and widespread changes in the establishment of neural lineages, as well as their differentiation and functional maturity, suggest genome-wide chromatin organization alterations in trisomy. High Mobility Group Nucleosome Binding Domain 1 (HMGN1), expressed from HSA21, is a chromatin remodeling protein that facilitates chromatin decompaction and is associated with acetylated lysine 27 on histone H3 (H3K27ac), a mark correlated with active transcription. Recent studies causatively linked overexpression of HMGN1 in trisomy and the development of DS-associated B cell acute lymphoblastic leukemia (B-ALL). HMGN1 has been shown to antagonize the activity of the Polycomb Repressive Complex 2 (PRC2) and prevent the deposition of histone H3 lysine 27 trimethylation mark (H3K27me3), which is associated with transcriptional repression and gene silencing. However, the possible ramifications of the increased levels of HMGN1 through the derepression of PRC2 target genes on brain cell pathology have not gained attention. In this review, we discuss the functional significance of HMGN1 in brain development and summarize accumulating reports about the essential role of PRC2 in the development of the neural system. Mechanistic understanding of how overexpression of HMGN1 may contribute to aberrant brain cell phenotypes in DS, such as altered proliferation of neural progenitors, abnormal cortical architecture, diminished myelination, neurodegeneration, and Alzheimer's disease-related pathology in trisomy 21, will facilitate the development of DS therapeutic approaches targeting chromatin.

Hedgehog Signaling in CNS Remyelination

Remyelination is a fundamental repair process in the central nervous system (CNS) that is triggered by demyelinating events. In demyelinating diseases, oligodendrocytes (OLs) are targeted, leading to myelin loss, axonal damage, and severe functional impairment. While spontaneous remyelination often fails in the progression of demyelinating diseases, increased understanding of the mechanisms and identification of targets that regulate myelin regeneration becomes crucial. To date, several signaling pathways have been implicated in the remyelination process, including the Hedgehog (Hh) signaling pathway. This review summarizes the current data concerning the complicated roles of the Hh signaling pathway in the context of remyelination. We will highlight the open issues that have to be clarified prior to bringing molecules targeting the Hh signaling to demyelinating therapy.

A Glance at the Molecules That Regulate Oligodendrocyte Myelination

Oligodendrocyte (OL) myelination is a critical process for the neuronal axon function in the central nervous system. After demyelination occurs because of pathophysiology, remyelination makes repairs similar to myelination. Proliferation and differentiation are the two main stages in OL myelination, and most factors commonly play converse roles in these two stages, except for a few factors and signaling pathways, such as OLIG2 (Oligodendrocyte transcription factor 2). Moreover, some OL maturation gene mutations induce hypomyelination or hypermyelination without an obvious function in proliferation and differentiation. Herein, three types of factors regulating myelination are reviewed in sequence.

The Shh/Gli1 signaling pathway regulates regeneration via transcription factor Olig1 expression after focal cerebral ischemia in rats

OBJECTIVE

Ischemic stroke is a major cause of death in the global population, with a high disability and mortality rate. Lack of regenerative ability is considered to be the fundamental cause. This study aims to determine the effect of Shh pathway, which mediates regenerative signaling in response to CNS injury, on myelin repair and Olig1 expression in focal ischemic lesions in the rat.

METHODS

A model of middle cerebral artery occlusion (MCAO) was established using the intraluminal suture method where the middle cerebral artery (MCA) was restricted for 120 min. Cyclopamine, a specific inhibitor of Shh, or saline was administered 12 h after MCAO surgery and lasted for 7 days. After MCA occlusion, male Sprague-Dawley rats were randomly allocated to cyclopamine- or saline-treated groups. A group of no-injection animals after MCAO were used as controls. The Shh signaling pathway, myelinogenesis-related factor MBP and Olig1 were testedby immunohistochemistry and RT-PCR assay.

RESULTS

The levels of Shh and its component Gli1 were elevated from 1 d up to 14 d following ischemia, indicating that the Shh-Gli1 axis was broadly reactivated. Treatment with cyclopamine can partially block the Shh signaling pathway, prevent myelin repair, and decrease the Olig1 expression following ischemic stroke.

CONCLUSION

That blockade of Shh signaling concurrently with the creation of a lesion aggravated ischemic myelin damage, probably via its downstream effects on Olig1 transcription. Shh plays a contributory role during regeneration in the CNS, thereby providing promising new therapeutic strategies to assist in recovery from ischemic stroke.

Zebrafish spinal cord oligodendrocyte formation requires boc function

The axis of the vertebrate neural tube is patterned, in part, by a ventral to dorsal gradient of Shh signaling. In the ventral spinal cord, Shh induces concentration-dependent expression of transcription factors, subdividing neural progenitors into distinct domains that subsequently produce distinct neuronal and glial subtypes. In particular, progenitors of the pMN domain express the bHLH transcription factor Olig2 and produce motor neurons followed by oligodendrocytes, the myelinating glial cell type of the central nervous system. In addition to its role in patterning ventral progenitors, Shh signaling must be maintained through development to specify pMN progenitors for oligodendrocyte fate. Using a forward genetic screen in zebrafish for mutations that disrupt development of oligodendrocytes, we identified a new mutant allele of boc, which encodes a type I transmembrane protein that functions as a coreceptor for Shh. Embryos homozygous for the bocco25 allele, which creates a missense mutation in a Fibronectin type III domain that binds Shh, have normally patterned spinal cords but fail to maintain pMN progenitors, resulting in a deficit of oligodendrocytes. Using a sensitive fluorescent detection method for in situ RNA hybridization, we found that spinal cord cells express boc in a graded fashion that is inverse to the gradient of Shh signaling activity and that boc function is necessary to maintain pMN progenitors by shaping the Shh signaling gradient.

X chromosome-dependent disruption of placental regulatory networks in hybrid dwarf hamsters

Embryonic development in mammals is highly sensitive to changes in gene expression within the placenta. The placenta is also highly enriched for genes showing parent-of-origin or imprinted expression, which is predicted to evolve rapidly in response to parental conflict. However, little is known about the evolution of placental gene expression, or if divergence of placental gene expression plays an important role in mammalian speciation. We used crosses between two species of dwarf hamsters (Phodopus sungorus and Phodopus campbelli) to examine the genetic and regulatory underpinnings of severe placental overgrowth in their hybrids. Using quantitative genetic mapping and mitochondrial substitution lines, we show that overgrowth of hybrid placentas was primarily caused by genetic differences on the maternally inherited P. sungorus X chromosome. Mitochondrial interactions did not contribute to abnormal hybrid placental development, and there was only weak correspondence between placental disruption and embryonic growth. Genome-wide analyses of placental transcriptomes from the parental species and first- and second-generation hybrids revealed a central group of co-expressed X-linked and autosomal genes that were highly enriched for maternally biased expression. Expression of this gene network was strongly correlated with placental size and showed widespread misexpression dependent on epistatic interactions with X-linked hybrid incompatibilities. Collectively, our results indicate that the X chromosome is likely to play a prominent role in the evolution of placental gene expression and the accumulation of hybrid developmental barriers between mammalian species.

Functional cooperation of the hedgehog and androgen signaling pathways during developmental and repairing myelination

Hedgehog morphogens control fundamental cellular processes during tissue development and regeneration. In the central nervous system (CNS), Hedgehog signaling has been implicated in oligodendrocyte and myelin production, where it functions in a concerted manner with other pathways. Since androgen receptor (AR) plays a key role in establishing the sexual phenotype of myelin during development and is required for spontaneous myelin regeneration in the adult CNS, we hypothesized the existence of a possible coordination between Hedgehog and androgen signals in oligodendrocyte and myelin production. Here, we report complementary activities of both pathways during early postnatal oligodendrogenesis further revealing that persistent Hedgehog signaling activation impedes myelin production. The data also uncover prominent pro-myelinating activity of testosterone and involvement of AR in the control of neural stem cell commitment toward the oligodendroglial lineage. In the context of CNS demyelination, we provide evidence for the functional cooperation of the pathways leading to acceleration of myelin regeneration that might be related to their respective role on microglial and astroglial responses, higher preservation of axonal integrity, lower neuroinflammation, and functional improvement of animals in an immune model of CNS demyelination. Strong decreases of deleterious cytokines in the CNS (GM-CSF, TNF-α, IL-17A) and spleen (IL-2, IFN-γ) stand as unique features of the combined drugs while the potent therapeutic activity of testosterone on peripheral immune cells contributes to increase tolerogenic CD11c⁺ dendritic cells, reduce the clonal expansion of conventional CD4⁺ T cells and increase CD4⁺ Foxp3⁺ regulatory T cells. Altogether, these data might open promising perspectives for demyelinating diseases.

Low-Field Magnetic Stimulation Accelerates the Differentiation of Oligodendrocyte Precursor Cells via Non-canonical TGF-β Signaling Pathways

Demyelination and oligodendrocyte loss are characteristic changes in demyelinating disorders. Low-field magnetic stimulation (LFMS) is a novel transcranial neuromodulation technology that has shown promising therapeutic potential for a variety of neuropsychiatric conditions. The cellular and molecular mechanisms of magnetic stimulation remain unclear. Previous studies mainly focused on the effects of magnetic stimulation on neuronal cells. Here we aimed to examine the effects of a gamma frequency LFMS on the glial progenitor cells. We used rat central glia-4 (CG4) cell line as an in vitro model. CG4 is a bipotential glial progenitor cell line that can differentiate into either oligodendrocyte or type 2-astrocyte. The cells cultured in a defined differentiation media were exposed to a 40-Hz LFMS 20 min daily for five consecutive days. We found that LFMS transiently elevated the level of TGF-β1 in the culture media in the first 24 h after the treatment. In correlation with the TGF-β1 levels, the percentage of cells possessing complex branches and expressing the late oligodendrocyte progenitor marker O4 was increased, indicating the accelerated differentiation of CG4 cells towards oligodendrocyte in LFMS-treated cultures. LFMS increased phosphorylation of Akt and Erk1/2 proteins, but not SMAD2/3. TGF-β1 receptor I specific inhibitor LY 364947 partially suppressed the effects of LFMS on differentiation and on levels of pAkt and pErk1/2, indicating that LFMS enhances the differentiation of oligodendrocyte progenitor cells via activation of non-canonical TGF-β-Akt and TGF-β-Erk1/2 pathways but not the canonical SMAD pathway. The data from this study reveal a novel mechanism of magnetic stimulation as a potential therapy for demyelination disorders.

Mature oligodendrocytes bordering lesions limit demyelination and favor myelin repair via heparan sulfate production

Myelin destruction is followed by resident glia activation and mobilization of endogenous progenitors (OPC) which participate in myelin repair. Here we show that in response to demyelination, mature oligodendrocytes (OLG) bordering the lesion express Ndst1, a key enzyme for heparan sulfates (HS) synthesis. Ndst1+ OLG form a belt that demarcates lesioned from intact white matter. Mice with selective inactivation of Ndst1 in the OLG lineage display increased lesion size, sustained microglia and OPC reactivity. HS production around the lesion allows Sonic hedgehog (Shh) binding and favors the local enrichment of this morphogen involved in myelin regeneration. In MS patients, Ndst1 is also found overexpressed in oligodendroglia and the number of Ndst1-expressing oligodendroglia is inversely correlated with lesion size and positively correlated with remyelination potential. Our study suggests that mature OLG surrounding demyelinated lesions are not passive witnesses but contribute to protection and regeneration by producing HS.

Modulation of Stem Cells as Therapeutics for Severe Mental Disorders and Cognitive Impairments

Severe mental illnesses (SMI) such as schizophrenia and bipolar disorder affect 2-4% of the world population. Current medications and diagnostic methods for mental illnesses are not satisfying. In animal studies, stem cell therapy is promising for some neuropsychiatric disorders and cognitive/social deficits, not only treating during development (targeting modulation and balancing) but also following neurodegeneration (cell replacement and regenerating support). We believe that novel interventions such as modulation of particular cell populations to develop cell-based treatment can improve cognitive and social functions in SMI. With pathological synaptic/myelin damage, oligodendrocytes seem to play a role. In this review, we have summarized oligodendrogenesis mechanisms and some related calcium signals in neural cells and stem/progenitor cells. The related benefits from endogenous stem/progenitor cells within the brain and exogenous stem cells, including multipotent mesenchymal-derived stromal cells (MSC), fetal neural stem cells (NSC), pluripotent stem cells (PSC), and differentiated progenitors, are discussed. These also include stimulating mechanisms of oligodendrocyte proliferation, maturation, and myelination, responsive to the regenerative effects by both endogenous stem cells and transplanted cells. Among the mechanisms, calcium signaling regulates the neuronal/glial progenitor cell (NPC/GPC)/oligodendrocyte precursor cell (OPC) proliferation, migration, and differentiation, dendrite development, and synaptic plasticity, which are involved in many neuropsychiatric diseases in human. On the basis of numerous protein annotation and protein-protein interaction databases, a total of 119 calcium-dependent/activated proteins that are related to neuropsychiatry in human are summarized in this investigation. One of the advanced methods, the calcium/cation-channel-optogenetics-based stimulation of stem cells and transplanted cells, can take advantage of calcium signaling regulations. Intranasal-to-brain delivery of drugs and stem cells or local delivery with the guidance of brain imaging techniques may provide a unique new approach for treating psychiatric disorders. It is also expected that preconditioning stem cell therapy following precise brain imaging as pathological confirmation has high potential if translated to cell clinic use. Generally, modulable cell transplantation followed by stimulations should provide paracrine protection, synaptic modulation, and myelin repair for the brain in SMI.

A discrete subtype of neural progenitor crucial for cortical folding in the gyrencephalic mammalian brain

An increase in the diversity of neural progenitor subtypes and folding of the cerebral cortex are characteristic features which appeared during the evolution of the mammalian brain. Here, we show that the expansion of a specific subtype of neural progenitor is crucial for cortical folding. We found that outer radial glial (oRG) cells can be subdivided by HOPX expression in the gyrencephalic cerebral cortex of ferrets. Compared with HOPX-negative oRG cells, HOPX-positive oRG cells had high self-renewal activity and were accumulated in prospective gyral regions. Using our in vivo genetic manipulation technique for ferrets, we found that the number of HOPX-positive oRG cells and their self-renewal activity were regulated by sonic hedgehog (Shh) signaling. Importantly, suppressing Shh signaling reduced HOPX-positive oRG cells and cortical folding, while enhancing it had opposing effects. Our results reveal a novel subtype of neural progenitor important for cortical folding in gyrencephalic mammalian cerebral cortex.

Astrocytes and Microglia as Major Players of Myelin Production in Normal and Pathological Conditions

Myelination is an essential process that consists of the ensheathment of axons by myelin. In the central nervous system (CNS), myelin is synthesized by oligodendrocytes. The proliferation, migration, and differentiation of oligodendrocyte precursor cells constitute a prerequisite before mature oligodendrocytes extend their processes around the axons and progressively generate a multilamellar lipidic sheath. Although myelination is predominately driven by oligodendrocytes, the other glial cells including astrocytes and microglia, also contribute to this process. The present review is an update of the most recent emerging mechanisms involving astrocyte and microglia in myelin production. The contribution of these cells will be first described during developmental myelination that occurs in the early postnatal period and is critical for the proper development of cognition and behavior. Then, we will report the novel findings regarding the beneficial or deleterious effects of astroglia and microglia, which respectively promote or impair the endogenous capacity of oligodendrocyte progenitor cells (OPCs) to induce spontaneous remyelination after myelin loss. Acute delineation of astrocyte and microglia activities and cross-talk should uncover the way towards novel therapeutic perspectives aimed at recovering proper myelination during development or at breaking down the barriers impeding the regeneration of the damaged myelin that occurs in CNS demyelinating diseases.