Oligodendrocytes, GRPs and MNOPs

Potential of Nano-Engineered Stem Cells in the Treatment of Multiple Sclerosis: A Comprehensive Review

Multiple sclerosis (MS) is a chronic and degrading autoimmune disorder mainly targeting the central nervous system, leading to progressive neurodegeneration, demyelination, and axonal damage. Current treatment options for MS are limited in efficacy, generally linked to adverse side effects, and do not offer a cure. Stem cell therapies have emerged as a promising therapeutic strategy for MS, potentially promoting remyelination, exerting immunomodulatory effects and protecting against neurodegeneration. Therefore, this review article focussed on the potential of nano-engineering in stem cells as a therapeutic approach for MS, focusing on the synergistic effects of combining stem cell biology with nanotechnology to stimulate the proliferation of oligodendrocytes (OLs) from neural stem cells and OL precursor cells, by manipulating neural signalling pathways-PDGF, BMP, Wnt, Notch and their essential genes such as Sox, bHLH, Nkx. Here we discuss the pathophysiology of MS, the use of various types of stem cells in MS treatment and their mechanisms of action. In the context of nanotechnology, we present an overview of its applications in the medical and research field and discuss different methods and materials used to nano-engineer stem cells, including surface modification, biomaterials and scaffolds, and nanoparticle-based delivery systems. We further elaborate on nano-engineered stem cell techniques, such as nano script, nano-exosome hybrid, nano-topography and their potentials in MS. The article also highlights enhanced homing, engraftment, and survival of nano-engineered stem cells, targeted and controlled release of therapeutic agents, and immunomodulatory and tissue repair effects with their challenges and limitations. This visual illustration depicts the process of utilizing nano-engineering in stem cells and exosomes for the purpose of delivering more accurate and improved treatments for Multiple Sclerosis (MS). This approach targets specifically the creation of oligodendrocytes, the breakdown of which is the primary pathological factor in MS.

From signalling to form: the coordination of neural tube patterning

The development of the vertebrate spinal cord involves the formation of the neural tube and the generation of multiple distinct cell types. The process starts during gastrulation, combining axial elongation with specification of neural cells and the formation of the neuroepithelium. Tissue movements produce the neural tube which is then exposed to signals that provide patterning information to neural progenitors. The intracellular response to these signals, via a gene regulatory network, governs the spatial and temporal differentiation of progenitors into specific cell types, facilitating the assembly of functional neuronal circuits. The interplay between the gene regulatory network, cell movement, and tissue mechanics generates the conserved neural tube pattern observed across species. In this review we offer an overview of the molecular and cellular processes governing the formation and patterning of the neural tube, highlighting how the remarkable complexity and precision of vertebrate nervous system arises. We argue that a multidisciplinary and multiscale understanding of the neural tube development, paired with the study of species-specific strategies, will be crucial to tackle the open questions.

Motor neurons and oligodendrocytes arise from distinct cell lineages by progenitor recruitment

During spinal cord development, ventral neural progenitor cells that express the transcription factors Olig1 and Olig2, called pMN progenitors, produce motor neurons and then oligodendrocytes. Whether motor neurons and oligodendrocytes arise from common or distinct progenitors in vivo is not known. Using zebrafish, we found that motor neurons and oligodendrocytes are produced sequentially by distinct progenitors that have distinct origins. When olig2(+) cells were tracked during the peak period of motor neuron formation, most differentiated as motor neurons without further cell division. Using time-lapse imaging, we found that, as motor neurons differentiated, more dorsally positioned neuroepithelial progenitors descended to the pMN domain and initiated olig2 expression. Inhibition of Hedgehog signaling during motor neuron differentiation blocked the ventral movement of progenitors, the progressive initiation of olig2 expression, and oligodendrocyte formation. We therefore propose that the motor neuron-to-oligodendrocyte switch results from Hedgehog-mediated recruitment of glial-fated progenitors to the pMN domain subsequent to neurogenesis.

Leukemia inhibitory factor is essential for subventricular zone neural stem cell and progenitor homeostasis as revealed by a novel flow cytometric analysis

Stem cells rely on extracellular signals produced by the niche, which dictate their ability to self-renew, expand and differentiate. It is essential to have sensitive and reproducible methods of either quantifying or isolating these stem cells and progenitors to understand their intrinsic properties and how extrinsic signals regulate their development. However, stem cells are difficult to distinguish from multipotential progenitors, which may look and act like them. Here we define a 4-color flow cytometry panel using CD133, LeX, CD140a, NG2 to define a neural stem cell (NSC) as well as 4 classes of multipotential progenitors and 3 classes of bipotential progenitors, several of which have not been described previously. We performed gain and loss of function studies for leukemia inhibitory factor (LIF) and showed a depletion of NSCs, a subset of multipotential neural precursors and immature oligodendrocytes in LIF null mice. Gain of function studies showed that LIF increased the abundance of these precursors. Our studies also show that these NPs have differential requirements for LIF and ciliary neurotrophic factor (CNTF) and for epidermal growth factor (EGF), fibroblast growth factor (FGF-2) and platelet-derived growth factor (PDGF) for their propagation in vitro. Surprisingly, the related cytokine, CNTF, was less potent than LIF in increasing the NSCs and more potent than LIF in increasing the PDGF responsive multipotential precursors. Finally, we show that LIF increases the expression of the core transcription factors: Klf4, Fbx15, Nanog, Sox2 and c-Myc. Altogether our FACS (fluorescence-activated cell sorter) analyses reveal that the neonatal subventricular zone is far more heterogeneous than previously suspected and our studies provide new insights into the signals and mechanisms that regulate their self-renewal and proliferation.

Dynamic signaling for neural stem cell fate determination

Central nervous system (CNS) development starts from neural stem cells (NSCs) which ultimately give rise to the three major cell types (neurons, oligodendrocytes and astrocytes) of the CNS. NSCs are specified in space- and time-related fashions, becoming spatially heterogeneous and generating a progressively restricted repertoire of cell types. Mammalian NSCs produce different cell types at different time points during development under the influence of multiple signaling pathways. These pathways act in a dynamic web mode to determine the fate of NSCs via modulating the expression and activity of distinct set of transcription factors which in turn trigger the transcription of neural fate-associated genes. This review thus introduces the major signal pathways, transcription factors and their cross-talks and coordinative interactions in NSC fate determination.

Epigenetic background of neuronal fate determination

The development of the central nervous system (CNS) starts from neural stem cells (NSCs). During this process, NSCs are specified in space- and time-related fashions, becoming spatially heterogeneous and generating a progressively restricted repertoire of cell types: neurons, astrocytes and oligodendrocytes. The processes of neurodevelopment are determined reciprocally by intrinsic and external factors which interface to program and re-program the profiling of fate-determination gene expression. Multiple signaling pathways act in a dynamic web mode to determine the fate of NSCs through modulating the activity of a distinct set of transcription factors which in turn trigger the transcription of neural fate-determination genes. Accumulating evidence reveals that during CNS development, multiple epigenetic factors regulate the activities of extracellular signaling and corresponding transcription factors in a coordinative manner, leading to the formation of a system with sophisticated structure and magic functions. This review aims to introduce recent advances in the epigenetic background of neural cell fate determination.

Stage-dependent Olig2 expression in motor neurons and oligodendrocytes differentiated from embryonic stem cells

Although embryonic stem (ES) cells are capable of forming any cell type in the body, the mechanisms that control cell type-specific differentiation are largely unknown. In the present study, we examined the process of differentiation to motor neurons and oligodendrocytes from mouse (Olig2GFP) ES cells. Mouse ES cells undergo a sequential process of differentiation over a 3-week period to generate motor neurons and oligodendrocytes. At day 7 of differentiation, Olig2-expressing cells are biased to a neuronal lineage. However, further differentiation (day 32) resulted in the majority of Olig2-expressing cells exhibiting an oligodendrocyte phenotype as well as a reduced ability to make motor neurons. Exposure of human ES cells to Sonic hedgehog (Shh) likewise resulted in enhanced motor neuron differentiation. Our results establish the requirements for directing ES cells to become motor neurons and oligodendrocytes and show that ES cell-derived Olig2 + cells can give rise to both motor neurons and oligodendrocytes, depending on the time at which differentiation is initiated.

Motoneurons and oligodendrocytes are sequentially generated from neural stem cells but do not appear to share common lineage-restricted progenitors in vivo

Olig gene expression is proposed to mark the common progenitors of motoneurons and oligodendrocytes. In an attempt to further dissect the in vivo lineage relationships between motoneurons and oligodendrocytes, we used a conditional cell-ablation approach to kill Olig-expressing cells. Although differentiated motoneurons and oligodendrocytes were eliminated, our ablation study revealed a continuous generation and subsequent death of their precursors. Most remarkably, a normal number of oligodendrocyte precursors are formed at day 12 of mouse development, after all motoneuron precursors have been killed. The data presented herein supports a sequential model in which motoneuron and oligodendrocyte precursors are sequentially generated in vivo from neuroepithelial stem cells, but do not share a common lineage-restricted progenitor.

Oligodendrocyte wars

Oligodendrocyte precursors first arise in a restricted ventral part of the embryonic spinal cord and migrate laterally and dorsally from there. Later, secondary sources develop in the dorsal cord. Normally, the ventrally-derived precursors compete with and suppress their dorsal counterparts. There are also ventral and dorsal sources in the forebrain, but here the more dorsal precursors prevail and the ventral-most lineage is eliminated during postnatal life. How do the different populations compete and what is the outcome of the competition? Do different embryonic origins signify different functional subgroups of oligodendrocyte?

Isolation of a novel platelet-derived growth factor-responsive precursor from the embryonic ventral forebrain

Oligodendrocyte progenitor cells express platelet-derived growth factor (PDGF) receptor-alpha and, when expanded in PDGF only, have been shown to generate oligodendrocytes and astrocytes but never neurons. Recent evidence suggests that oligodendrocytes are generated by a common progenitor that also generates neurons but not astrocytes. We used the neurosphere culture system to isolate embryonic ventral forebrain, PDGF-responsive precursors (PRPs). We report that the medial ganglionic eminence is the source of PRP-generated neurospheres and that the progeny can differentiate into parvalbumin-positive interneurons, oligodendrocytes, and astrocytes. Thyroid hormone and bone morphogenetic protein-2 (BMP-2) promote the mutually exclusive differentiation of oligodendrocytes and neurons, respectively, whereas ciliary neurotrophic factor acts with BMP-2 to suppress OLIG2 expression and promote astroglial differentiation. PRPs require fibroblast growth factor-2 together with PDGF to maintain self-renewal, which is dependent on sonic hedgehog signaling. We present evidence for forebrain oligodendrocytes and parvalbumin-positive interneurons being generated by a common precursor and elucidate signals regulating the multiple differentiation routes of the progeny of this precursor.

CD44 expression identifies astrocyte-restricted precursor cells

The precise lineage between neural stem cells and mature astrocytes remains poorly defined. To examine astrocyte development, we have characterized glial precursors from neural tissue derived from early embryonic ages. We show that CD44 identifies an astrocyte-restricted precursor cell (ARP) that is committed to generating astrocytes in vitro and in vivo in both rodent and human tissue. CD44+ cells arise later in development than neuronal-restricted precursors (NRPs) or tripotential glial-restricted precursors (GRPs). ARPs are distinguished from GRP and NRP cells by their antigenic profile and differentiation ability. ARPs can be generated from GRP cells in mass or clonal cultures and in vivo after transplantation, suggesting a sequential differentiation of neuroepithelial stem cells (NEPs) to GRPs to ARPs and then to astrocytes. The properties of ARPs are different from other astrocyte precursors described previously in their expression of CD44 and S-100beta and absence of other lineage markers. Using a CD44 misexpression transgenic mouse model (CNP-CD44 mouse), we show that CD44 overexpression in vivo and in vitro decreases the number of mature glia and increases the number of O4+/GFAP+ cells tenfold. Misexpression of CD44 in culture inhibits oligodendrocytes and arrests cells at the precursor state. In summary, our data provide strong evidence for the existence of a CD44+ ARP in the developing nervous system.

Increased expression of Nkx2.2 and Olig2 identifies reactive oligodendrocyte progenitor cells responding to demyelination in the adult CNS

Within the adult CNS, a quiescent population of oligodendrocyte progenitor cells (OPCs) become activated in response to demyelination and give rise to remyelinating oligodendrocytes. During development, OPC differentiation is controlled by several transcription factors including Olig1 and Olig2, and Nkx2.2. We hypothesized that these genes may serve similar functions in activated adult OPCs allowing them to become remyelinating oligodendrocytes and tested this hypothesis by examining their expression during the remyelination of a toxin-induced rodent model of demyelination. During the acute phase of demyelination, OPCs within the lesion increased their expression of Nkx2.2 and Olig2, two transcription factors that in combination are critical for oligodendrocyte differentiation during developmental myelination. This activation was not associated with increases in Sonic hedgehog (Shh) expression, which does not appear essential for CNS remyelination. Consistent with a role in the activation and differentiation of OPCs, these increases were delayed in old adult animals where the rate of remyelination is slowed. Our data suggest the hypothesis that increased expression of Nkx2.2 and Olig2 plays a critically important role in the differentiation of adult OPCs into remyelinating oligodendrocytes and that these genes may present novel targets for therapeutic manipulation in cases where remyelination is impaired.

Clonal identification of multipotent precursors from adult mouse pancreas that generate neural and pancreatic lineages

The clonal isolation of putative adult pancreatic precursors has been an elusive goal of researchers seeking to develop cell replacement strategies for diabetes. We report the clonal identification of multipotent precursor cells from the adult mouse pancreas. The application of a serum-free, colony-forming assay to pancreatic cells enabled the identification of precursors from pancreatic islet and ductal populations. These cells proliferate in vitro to form clonal colonies that coexpress neural and pancreatic precursor markers. Upon differentiation, individual clonal colonies produce distinct populations of neurons and glial cells, pancreatic endocrine beta-, alpha- and delta-cells, and pancreatic exocrine and stellate cells. Moreover, the newly generated beta-like cells demonstrate glucose-dependent Ca(2+) responsiveness and insulin release. Pancreas colonies do not express markers of embryonic stem cells, nor genes suggestive of mesodermal or neural crest origins. These cells represent a previously unidentified adult intrinsic pancreatic precursor population and are a promising candidate for cell-based therapeutic strategies.

Lost in Space

In a widely held view of vertebrate CNS development, neurons, astrocytes, and oligodendrocytes arise from a common tripotent progenitor cell. However, tripotent progenitors have never been detected in developing embryos. In this issue of Neuron, Gabay et al. show that tripotent progenitors can be created in vitro by deregulation of normal dorsoventral positional cues.