Relative Levels of Gli1 and Gli2 Determine the Response of Ventral Neural Stem Cells to Demyelination

Remyelinating Drugs at a Crossroad: How to Improve Clinical Efficacy and Drug Screenings

Axons wrapped around the myelin sheath enable fast transmission of neuronal signals in the Central Nervous System (CNS). Unfortunately, myelin can be damaged by injury, viral infection, and inflammatory and neurodegenerative diseases. Remyelination is a spontaneous process that can restore nerve conductivity and thus movement and cognition after a demyelination event. Cumulative evidence indicates that remyelination can be pharmacologically stimulated, either by targeting natural inhibitors of Oligodendrocyte Precursor Cells (OPCs) differentiation or by reactivating quiescent Neural Stem Cells (qNSCs) proliferation and differentiation in myelinating Oligodendrocytes (OLs). Although promising results were obtained in animal models for demyelination diseases, none of the compounds identified have passed all the clinical stages. The significant number of patients who could benefit from remyelination therapies reinforces the urgent need to reassess drug selection approaches and develop strategies that effectively promote remyelination. Integrating Artificial Intelligence (AI)-driven technologies with patient-derived cell-based assays and organoid models is expected to lead to novel strategies and drug screening pipelines to achieve this goal. In this review, we explore the current literature on these technologies and their potential to enhance the identification of more effective drugs for clinical use in CNS remyelination therapies.

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.

Immunofluorescence assay for demyelination, remyelination, and proliferation in an acute cuprizone mouse model

Here, we present a protocol to assess demyelination in the corpus callosum of an acute cuprizone mouse model, which is routinely used to induce demyelination for studying myelin regeneration in the rodent brain. We describe the tracing of neural stem cells via intraperitoneal injection of tamoxifen into adult Gli1CreERT2;Ai9 mice and the induction of demyelination with cuprizone diet. We also detail EdU administration, cryosectioning of the mouse brain, EdU labeling, and immunofluorescence staining to examine proliferation and myelination. For complete details on the use and execution of this protocol, please refer to Radecki et al. (2020).1.

Cell-based experimental strategies for myelin repair in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune demyelinating disorder of the central nervous system (CNS), diagnosed at a mean age of 32 years. CNS glia are crucial players in the onset of MS, primarily involving astrocytes and microglia that can cause/allow massive oligodendroglial cells death, without immune cell infiltration. Current therapeutic approaches are aimed at modulating inflammatory reactions during relapsing episodes, but lack the ability to induce very significant repair mechanisms. In this review article, different experimental approaches based mainly on the application of different cell types as therapeutic strategies applied for the induction of myelin repair and/or the amelioration of the disease are discussed. Regarding this issue, different cell sources were applied in various experimental models of MS, with different results, both in significant improvements in remyelination and the reduction of neuroinflammation and glial activation, or in neuroprotection. All cell types tested have advantages and disadvantages, which makes it difficult to choose a better option for therapeutic application in MS. New strategies combining cell-based treatment with other applications would result in further improvements and would be good candidates for MS cell therapy and myelin repair.

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.

Endogenous Neural Stem Cell Mediated Oligodendrogenesis in the Adult Mammalian Brain

Oligodendrogenesis is essential for replacing worn-out oligodendrocytes, promoting myelin plasticity, and for myelin repair following a demyelinating injury in the adult mammalian brain. Neural stem cells are an important source of oligodendrocytes in the adult brain; however, there are considerable differences in oligodendrogenesis from neural stem cells residing in different areas of the adult brain. Amongst the distinct niches containing neural stem cells, the subventricular zone lining the lateral ventricles and the subgranular zone in the dentate gyrus of the hippocampus are considered the principle areas of adult neurogenesis. In addition to these areas, radial glia-like cells, which are the precursors of neural stem cells, are found in the lining of the third ventricle, where they are called tanycytes, and in the cerebellum, where they are called Bergmann glia. In this review, we will describe the contribution and regulation of each of these niches in adult oligodendrogenesis.

Isolation and culture of neural stem cells from adult mouse subventricular zone for genetic and pharmacological treatments with proliferation analysis

Neural stem cells (NSCs) from the subventricular zone (SVZ) of the mouse brain can be expanded in vitro and grown as neurospheres, which can be stored long-term in liquid nitrogen. Here, we present a protocol for isolation and culture of NSCs from the adult mouse SVZ. We describe how to grow and expand primary NSCs to neurospheres, followed by differentiation and nucleofection/pharmacological treatments. Finally, we describe RNA extraction, EdU labeling of the cells, and immunofluorescent analysis to examine their proliferation. For complete details on the use and execution of this protocol, please refer to Radecki et al. (2020).

GLI3 Is Required for OLIG2+ Progeny Production in Adult Dorsal Neural Stem Cells

The ventricular-subventricular zone (V-SVZ) is a postnatal germinal niche. It holds a large population of neural stem cells (NSCs) that generate neurons and oligodendrocytes for the olfactory bulb and (primarily) the corpus callosum, respectively. These NSCs are heterogeneous and generate different types of neurons depending on their location. Positional identity among NSCs is thought to be controlled in part by intrinsic pathways. However, extrinsic cell signaling through the secreted ligand Sonic hedgehog (Shh) is essential for neurogenesis in both the dorsal and ventral V-SVZ. Here we used a genetic approach to investigate the role of the transcription factors GLI2 and GLI3 in the proliferation and cell fate of dorsal and ventral V-SVZ NSCs. We find that while GLI3 is expressed in stem cell cultures from both dorsal and ventral V-SVZ, the repressor form of GLI3 is more abundant in dorsal V-SVZ. Despite this high dorsal expression and the requirement for other Shh pathway members, GLI3 loss affects the generation of ventrally-, but not dorsally-derived olfactory interneurons in vivo and does not affect trilineage differentiation in vitro. However, loss of GLI3 in the adult dorsal V-SVZ in vivo results in decreased numbers of OLIG2-expressing progeny, indicating a role in gliogenesis.

Smoothened/AMP-Activated Protein Kinase Signaling in Oligodendroglial Cell Maturation

The regeneration of myelin is known to restore axonal conduction velocity after a demyelinating event. Remyelination failure in the central nervous system contributes to the severity and progression of demyelinating diseases such as multiple sclerosis. Remyelination is controlled by many signaling pathways, such as the Sonic hedgehog (Shh) pathway, as shown by the canonical activation of its key effector Smoothened (Smo), which increases the proliferation of oligodendrocyte precursor cells via the upregulation of the transcription factor Gli1. On the other hand, the inhibition of Gli1 was also found to promote the recruitment of a subset of adult neural stem cells and their subsequent differentiation into oligodendrocytes. Since Smo is also able to transduce Shh signals via various non-canonical pathways such as the blockade of Gli1, we addressed the potential of non-canonical Smo signaling to contribute to oligodendroglial cell maturation in myelinating cells using the non-canonical Smo agonist GSA-10, which downregulates Gli1. Using the Oli-neuM cell line, we show that GSA-10 promotes Gli2 upregulation, MBP and MAL/OPALIN expression via Smo/AMP-activated Protein Kinase (AMPK) signaling, and efficiently increases the number of axonal contact/ensheathment for each oligodendroglial cell. Moreover, GSA-10 promotes the recruitment and differentiation of oligodendroglial progenitors into the demyelinated corpus callosum in vivo. Altogether, our data indicate that non-canonical signaling involving Smo/AMPK modulation and Gli1 downregulation promotes oligodendroglia maturation until axon engagement. Thus, GSA-10, by activation of this signaling pathway, represents a novel potential remyelinating agent.

Transcriptomic analysis of loss of Gli1 in neural stem cells responding to demyelination in the mouse brain

In the adult mammalian brain, Gli1 expressing neural stem cells reside in the subventricular zone and their progeny are recruited to sites of demyelination in the white matter where they generate new oligodendrocytes, the myelin forming cells. Remarkably, genetic loss or pharmacologic inhibition of Gli1 enhances the efficacy of remyelination by these neural stem cells. To understand the molecular mechanisms involved, we performed a transcriptomic analysis of this Gli1-pool of neural stem cells. We compared murine NSCs with either intact or deficient Gli1 expression from adult mice on a control diet or on a cuprizone diet which induces widespread demyelination. These data will be a valuable resource for identifying therapeutic targets for enhancing remyelination in demyelinating diseases like multiple sclerosis.

Histological Studies of the Ventricular-Subventricular Zone as Neural Stem Cell and Glioma Stem Cell Niche

The neural stem cell niche of the ventricular-subventricular zone supports the persistence of stem and progenitor cells in the mature brain. This niche has many notable cytoarchitectural features that affect the activity of stem cells and may also support the survival and growth of invading tumor cells. Histochemical studies of the niche have revealed many proteins that, in combination, can help to reveal stem-like cells in the normal or cancer context, although many caveats persist in the quest to consistently identify these cells in the human brain. Here, we explore the complex relationship between the persistent proliferative capacity of the neural stem cell niche and the malignant proliferation of brain tumors, with a special focus on histochemical identification of stem cells and stem-like tumor cells and an eye toward the potential application of high-dimensional imaging approaches to the field.