Bone morphogenetic proteins induce astroglial differentiation of oligodendroglial-astroglial progenitor cells

A New Acquaintance of Oligodendrocyte Precursor Cells in the Central Nervous System

Oligodendrocyte precursor cells (OPCs) are a heterogeneous multipotent population in the central nervous system (CNS) that appear during embryogenesis and persist as resident cells in the adult brain parenchyma. OPCs could generate oligodendrocytes to participate in myelination. Recent advances have renewed our knowledge of OPC biology by discovering novel markers of oligodendroglial cells, the myelin-independent roles of OPCs, and the regulatory mechanism of OPC development. In this review, we will explore the updated knowledge on OPC identity, their multifaceted roles in the CNS in health and diseases, as well as the regulatory mechanisms that are involved in their developmental stages, which hopefully would contribute to a further understanding of OPCs and attract attention in the field of OPC biology.

Human Pluripotent Stem Cell-Derived Astrocyte Functionality Compares Favorably with Primary Rat Astrocytes

Astrocytes are essential for the formation and maintenance of neural networks. However, a major technical challenge for investigating astrocyte function and disease-related pathophysiology has been the limited ability to obtain functional human astrocytes. Despite recent advances in human pluripotent stem cell (hPSC) techniques, primary rodent astrocytes remain the gold standard in coculture with human neurons. We demonstrate that a combination of leukemia inhibitory factor (LIF) and bone morphogenetic protein-4 (BMP4) directs hPSC-derived neural precursor cells to a highly pure population of astroglia in 28 d. Using single-cell RNA sequencing, we confirm the astroglial identity of these cells and highlight profound transcriptional adaptations in cocultured hPSC-derived astrocytes and neurons, consistent with their further maturation. In coculture with human neurons, multielectrode array recordings revealed robust network activity of human neurons in a coculture with hPSC-derived or rat astrocytes [3.63 ± 0.44 min-1 (hPSC-derived), 2.86 ± 0.64 min-1 (rat); p = 0.19]. In comparison, we found increased spike frequency within network bursts of human neurons cocultured with hPSC-derived astrocytes [56.31 ± 8.56 Hz (hPSC-derived), 24.77 ± 4.04 Hz (rat); p < 0.01], and whole-cell patch-clamp recordings revealed an increase of postsynaptic currents [2.76 ± 0.39 Hz (hPSC-derived), 1.07 ± 0.14 Hz (rat); p < 0.001], consistent with a corresponding increase in synapse density [14.90 ± 1.27/100 μm2 (hPSC-derived), 8.39 ± 0.63/100 μm2 (rat); p < 0.001]. Taken together, we show that hPSC-derived astrocytes compare favorably with rat astrocytes in supporting human neural network activity and maturation, providing a fully human platform for investigating astrocyte function and neuronal-glial interactions.

Tools to study neural and glioma stem cell quiescence

Quiescence is a prolonged but reversible state of cell-cycle arrest that is an adaptive feature of most adult stem cell populations. In the brain, quiescence helps to protect adult neural stem cells from stress and supports lifelong neurogenesis. Unfortunately however, entry into a quiescent or a slow-cycling state is also a malignant feature of brain cancer stem cells. In glioblastoma, where the process has been best characterised, quiescent glioma stem cells preferentially survive chemoradiation, and after therapy, reactivate to regrow the tumour and drive recurrence. In this Review, we discuss the in vitro and in vivo models that have been developed for studying neural stem cell quiescence and how these tools may be used to deepen biological understanding and to develop novel therapies targeting quiescent glioma stem cells.

A Novel Role for the Histone Demethylase JMJD3 in Mediating Heroin-Induced Relapse-Like Behaviors

BACKGROUND

Epigenetic changes that lead to long-term neuroadaptations following opioid exposure are not well understood. We examined how histone demethylase JMJD3 in the nucleus accumbens (NAc) influences heroin seeking after abstinence from self-administration.

METHODS

Male Sprague Dawley rats were trained to self-administer heroin. Western blotting and quantitative polymerase chain reaction were performed to quantify JMJD3 and bone morphogenetic protein (BMP) pathway expression in the NAc (n = 7-11/group). Pharmacological inhibitors or viral expression vectors were microinfused into the NAc to manipulate JMJD3 or the BMP pathway member SMAD1 (n = 9-11/group). The RiboTag capture method (n = 3-5/group) and viral vectors (n = 7-8/group) were used in male transgenic rats to identify the contributions of D1- and D2-expressing medium spiny neurons in the NAc. Drug seeking was tested by cue-induced response previously paired with drug infusion.

RESULTS

Levels of JMJD3 and phosphorylated SMAD1/5 in the NAc were increased after 14 days of abstinence from heroin self-administration. Pharmacological and virus-mediated inhibition of JMJD3 or the BMP pathway attenuated cue-induced seeking. Pharmacological inhibition of BMP signaling reduced JMJD3 expression and H3K27me3 levels. JMJD3 bidirectionally affected seeking: expression of the wild-type increased cue-induced seeking whereas expression of a catalytic dead mutant decreased it. JMJD3 expression was increased in D2+ but not D1+ medium spiny neurons. Expression of the mutant JMJD3 in D2+ neurons was sufficient to decrease cue-induced heroin seeking.

CONCLUSIONS

JMJD3 mediates persistent cellular and behavioral adaptations that underlie heroin relapse, and this activity is regulated by the BMP pathway.

Changes in the Expression of TGF-Beta Regulatory Pathway Genes Induced by Vitamin D in Patients with Relapsing-Remitting Multiple Sclerosis

Vitamin D is an environmental factor related to multiple sclerosis that plays a significant role in immune regulation. TGF-β is a superfamily of cytokines with an important dual effect on the immune system. TGF-β inhibits the Th1 response while facilitating the preservation of regulatory T cells (FOXP3+) in an immunoregulatory capacity. However, when IL-6 is present, it stimulates the Th17 response. Our aim was to analyze the regulatory effect of vitamin D on the in vivo TGF-β signaling pathway in patients with relapsing-remitting multiple sclerosis (RRMS). A total of 21 patients with vitamin D levels < 30 ng/mL were recruited and supplemented with oral vitamin D. All patients were receiving disease-modifying therapy, with the majority being on natalizumab. Expression of SMAD7, ERK1, ZMIZ1, BMP2, BMPRII, BMP4, and BMP5 was measured in CD4+ lymphocytes isolated from peripheral blood at baseline and one and six months after supplementation. SMAD7 was overexpressed at six months with respect to baseline and month one. ERK1 was overexpressed at six months with respect to month one of treatment. No significant differences in expression were observed for the remaining genes. No direct correlation was found with serum vitamin D levels. BMPRII expression changed differentially in non-natalizumab- versus natalizumab-treated patients. Changes were observed in the expression of ERK1, BMP2, and BMP5 based on disease activity measured using the Rio-Score, BMP2 in patients who had relapses, and BMP5 in those whose EDSS worsened. Our results suggest indirect regulation of vitamin D in TGF-β pathway genes in patients with RRMS.

The cortical NG2-glia response to traumatic brain injury

Traumatic brain injury (TBI) is a significant worldwide cause of morbidity and mortality. A chronic neurologic disease bearing the moniker of "the silent epidemic," TBI currently has no targeted therapies to ameliorate cellular loss or enhance functional recovery. Compared with those of astrocytes, microglia, and peripheral immune cells, the functions and mechanisms of NG2-glia following TBI are far less understood, despite NG2-glia comprising the largest population of regenerative cells in the mature cortex. Here, we synthesize the results from multiple rodent models of TBI, with a focus on cortical NG2-glia proliferation and lineage potential, and propose future avenues for glia researchers to address this unique cell type in TBI. As the molecular mechanisms that regulate NG2-glia regenerative potential are uncovered, we posit that future therapeutic strategies may exploit cortical NG2-glia to augment local cellular recovery following TBI.

Glial progenitor cells of the adult human white and grey matter are contextually distinct

Genomic analyses have revealed heterogeneity among glial progenitor cells (GPCs), but the compartment selectivity of human GPCs (hGPCs) is unclear. Here, we asked if GPCs of human grey and white brain matter are distinct in their architecture and associated gene expression. RNA profiling of NG2-defined hGPCs derived from adult human neocortex and white matter differed in their expression of genes involved in Wnt, NOTCH, BMP and TGFβ signaling, suggesting compartment-selective biases in fate and self-renewal. White matter hGPCs over-expressed the BMP antagonists BAMBI and CHRDL1, suggesting their tonic suppression of astrocytic fate relative to cortical hGPCs, whose relative enrichment of cytoskeletal genes presaged their greater morphological complexity. In human glial chimeric mice, cortical hGPCs assumed larger and more complex morphologies than white matter hGPCs, and both were more complex than their mouse counterparts. These findings suggest that human grey and white matter GPCs comprise context-specific pools with distinct functional biases.

Retinal ganglion cell expression of cytokine enhances occupancy of NG2 cell-derived astrocytes at the nerve injury site: Implication for axon regeneration

Following injury in the central nervous system, a population of astrocytes occupy the lesion site, form glial bridges and facilitate axon regeneration. These astrocytes originate primarily from resident astrocytes or NG2+ oligodendrocyte progenitor cells. However, the extent to which these cell types give rise to the lesion-filling astrocytes, and whether the astrocytes derived from different cell types contribute similarly to optic nerve regeneration remain unclear. Here we examine the distribution of astrocytes and NG2+ cells in an optic nerve crush model. We show that optic nerve astrocytes partially fill the injury site over time after a crush injury. Viral mediated expression of a growth-promoting factor, ciliary neurotrophic factor (CNTF), in retinal ganglion cells (RGCs) promotes axon regeneration without altering the lesion size or the degree of lesion-filling GFAP+ cells. Strikingly, using inducible NG2CreER driver mice, we found that CNTF overexpression in RGCs increases the occupancy of NG2+ cell-derived astrocytes in the optic nerve lesion. An EdU pulse-chase experiment shows that the increase in NG2 cell-derived astrocytes is not due to an increase in cell proliferation. Lastly, we performed RNA-sequencing on the injured optic nerve and reveal that CNTF overexpression in RGCs results in significant changes in the expression of distinct genes, including those that encode chemokines, growth factor receptors, and immune cell modulators. Even though CNTF-induced axon regeneration has long been recognized, this is the first evidence of this procedure affecting glial cell fate at the optic nerve crush site. We discuss possible implication of these results for axon regeneration.

A TCF7L2-responsive suppression of both homeostatic and compensatory remyelination in Huntington disease mice

Huntington's disease (HD) is characterized by defective oligodendroglial differentiation and white matter disease. Here, we investigate the role of oligodendrocyte progenitor cell (OPC) dysfunction in adult myelin maintenance in HD. We first note a progressive, age-related loss of myelin in both R6/2 and zQ175 HD mice compared with wild-type controls. Adult R6/2 mice then manifest a significant delay in remyelination following cuprizone demyelination. RNA-sequencing and proteomic analysis of callosal white matter and OPCs isolated from both R6/2 and zQ175 mice reveals a systematic downregulation of genes associated with oligodendrocyte differentiation and myelinogenesis. Gene co-expression and network analysis predicts repressed Tcf7l2 signaling as a major driver of this expression pattern. In vivo Tcf7l2 overexpression restores both myelin gene expression and remyelination in demyelinated R6/2 mice. These data causally link impaired TCF7L2-dependent transcription to the poor development and homeostatic retention of myelin in HD and provide a mechanism for its therapeutic restoration.

Astrogenesis in the murine dentate gyrus is a life-long and dynamic process

Astrocytes are highly abundant in the mammalian brain, and their functions are of vital importance for all aspects of development, adaption, and aging of the central nervous system (CNS). Mounting evidence indicates the important contributions of astrocytes to a wide range of neuropathies. Still, our understanding of astrocyte development significantly lags behind that of other CNS cells. We here combine immunohistochemical approaches with genetic fate-mapping, behavioral paradigms, single-cell transcriptomics, and in vivo two-photon imaging, to comprehensively assess the generation and the proliferation of astrocytes in the dentate gyrus (DG) across the life span of a mouse. Astrogenesis in the DG is initiated by radial glia-like neural stem cells giving rise to locally dividing astrocytes that enlarge the astrocyte compartment in an outside-in-pattern. Also in the adult DG, the vast majority of astrogenesis is mediated through the proliferation of local astrocytes. Interestingly, locally dividing astrocytes were able to adapt their proliferation to environmental and behavioral stimuli revealing an unexpected plasticity. Our study establishes astrocytes as enduring plastic elements in DG circuits, implicating a vital contribution of astrocyte dynamics to hippocampal plasticity.

Molecular diversity of diencephalic astrocytes reveals adult astrogenesis regulated by Smad4

Astrocytes regulate brain‐wide functions and also show region‐specific differences, but little is known about how general and region‐specific functions are aligned at the single‐cell level. To explore this, we isolated adult mouse diencephalic astrocytes by ACSA‐2‐mediated magnetic‐activated cell sorting (MACS). Single‐cell RNA‐seq revealed 7 gene expression clusters of astrocytes, with 4 forming a supercluster. Within the supercluster, cells differed by gene expression related to ion homeostasis or metabolism, with the former sharing gene expression with other regions and the latter being restricted to specific regions. All clusters showed expression of proliferation‐related genes, and proliferation of diencephalic astrocytes was confirmed by immunostaining. Clonal analysis demonstrated low level of astrogenesis in the adult diencephalon, but not in cerebral cortex grey matter. This led to the identification of Smad4 as a key regulator of diencephalic astrocyte in vivo proliferation and in vitro neurosphere formation. Thus, astrocytes show diverse gene expression states related to distinct functions with some subsets being more widespread while others are more regionally restricted. However, all share low‐level proliferation revealing the novel concept of adult astrogenesis in the diencephalon.

Elezanumab, a clinical stage human monoclonal antibody that selectively targets repulsive guidance molecule A to promote neuroregeneration and neuroprotection in neuronal injury and demyelination models

Repulsive guidance molecule A (RGMa) is a potent inhibitor of axonal growth and a regulator of neuronal cell death. It is up-regulated following neuronal injury and accumulates in chronic neurodegenerative diseases. Neutralizing RGMa has the potential to promote neuroregeneration and neuroprotection. Previously we reported that a rat anti-N terminal RGMa (N-RGMa) antibody r5F9 and its humanized version h5F9 (ABT-207) promote neuroprotection and neuroregeneration in preclinical neurodegenerative disease models. However, due to its cross-reactivity to RGMc/hemojuvelin, ABT-207 causes iron accumulation in vivo, which could present a safety liability. Here we report the generation and characterization of a novel RGMa-selective anti-N-RGMa antibody elezanumab, which is currently under Phase 2 clinical evaluation in multiple disease indications. Elezanumab, a human monoclonal antibody generated by in vitro PROfusion mRNA display technology, competes with ABT-207 in binding to N-RGMa but lacks RGMc cross-reactivity with no impact on iron metabolism. It neutralizes repulsive activity of soluble RGMa in vitro and blocks membrane RGMa mediated BMP signaling. In the optic nerve crush and optic neuritis models, elezanumab promotes axonal regeneration and prevents retinal nerve fiber layer degeneration. In the spinal targeted experimental autoimmune encephalomyelitis (EAE) model, elezanumab promotes axonal regeneration and remyelination, decreases inflammatory lesion area and improves functional recovery. Finally, in the mouse cuprizone model, elezanumab reduces demyelination, which is consistent with its inhibitory effect on BMP signaling. Taken together, these preclinical data demonstrate that elezanumab has neuroregenerative and neuroprotective activities without impact on iron metabolism, thus providing a compelling rationale for its clinical development in neurodegenerative diseases.

LDL receptor-related protein 1 and its interacting partners in tissue homeostasis

PURPOSE OF REVIEW

LDL receptor-related protein 1 (LRP1) is a multifunctional protein with endocytic and signal transduction properties due to its interaction with numerous extracellular ligands and intracellular proteins. This brief review highlights key developments in identifying novel functions of LRP1 in liver, lung, and the central nervous system in disease pathogenesis.

RECENT FINDINGS

In hepatocytes, LRP1 complexes with phosphatidylinositol 4-phosphate 5-kinase-1 and its related protein to maintain intracellular levels of phosphatidylinositol (4,5) bisphosphate and preserve lysosome and mitochondria integrity. In contrast, in smooth muscle cells, macrophages, and endothelial cells, LRP1 interacts with various different extracellular ligands and intracellular proteins in a tissue-dependent and microenvironment-dependent manner to either enhance or suppress inflammation, disease progression or resolution. Similarly, LRP1 expression in astrocytes and oligodendrocyte progenitor cells regulates cell differentiation and maturation in a developmental-dependent manner to modulate neurogenesis, gliogenesis, and white matter repair after injury.

SUMMARY

LRP1 modulates metabolic disease manifestation, inflammation, and differentiation in a cell-dependent, time-dependent, and tissue-dependent manner. Whether LRP1 expression is protective or pathogenic is dependent on its interaction with specific ligands and intracellular proteins, which in turn is dependent on the cell type and the microenvironment where these cells reside.

BMP receptor blockade overcomes extrinsic inhibition of remyelination and restores neurovascular homeostasis

Extrinsic inhibitors at sites of blood–brain barrier disruption and neurovascular damage contribute to remyelination failure in neurological diseases. However, therapies to overcome the extrinsic inhibition of remyelination are not widely available and the dynamics of glial progenitor niche remodelling at sites of neurovascular dysfunction are largely unknown. By integrating in vivo two-photon imaging co-registered with electron microscopy and transcriptomics in chronic neuroinflammatory lesions, we found that oligodendrocyte precursor cells clustered perivascularly at sites of limited remyelination with deposition of fibrinogen, a blood coagulation factor abundantly deposited in multiple sclerosis lesions. By developing a screen (OPC-X-screen) to identify compounds that promote remyelination in the presence of extrinsic inhibitors, we showed that known promyelinating drugs did not rescue the extrinsic inhibition of remyelination by fibrinogen. In contrast, bone morphogenetic protein type I receptor blockade rescued the inhibitory fibrinogen effects and restored a promyelinating progenitor niche by promoting myelinating oligodendrocytes, while suppressing astrocyte cell fate, with potent therapeutic effects in chronic models of multiple sclerosis. Thus, abortive oligodendrocyte precursor cell differentiation by fibrinogen is refractory to known promyelinating compounds, suggesting that blockade of the bone morphogenetic protein signalling pathway may enhance remyelinating efficacy by overcoming extrinsic inhibition in neuroinflammatory lesions with vascular damage.

Overexpression of bone morphogenetic protein 7 reduces oligodendrocytes loss and promotes functional recovery after spinal cord injury

Spinal cord injury (SCI), as a severe disease with no effective therapeutic measures, has always been a hot topic for scientists. Bone morphogenetic protein 7 (BMP7), as a multifunctional cytokine, has been reported to exert protective effects on the nervous system. The present study aimed to investigate the neuroprotective effect and the potential mechanisms of BMP7 on rats that suffered SCI. Rat models of SCI were established by the modified Allen^'s method. Adeno‐associated virus (AAV) was injected at T9 immediately before SCI to overexpress BMP7. Results showed that the expression of BMP7 decreased in the injured spinal cords that were at the same time demyelinated. AAV‐BMP7 partly reversed oligodendrocyte (OL) loss, and it was beneficial to maintain the normal structure of myelin. The intervention group showed an increase in the number of axons and Basso‐Beattie‐Bresnahan scores. Moreover, double‐labelled immunofluorescence images indicated p‐Smad1/5/9 and p‐STAT3 in OLs induced by BMP7 might be involved in the protective effects of BMP7. These findings suggest that BMP7 may be a feasible therapy for SCI to reduce demyelination and promote functional recovery.

Specific Blockade of Bone Morphogenetic Protein-2/4 Induces Oligodendrogenesis and Remyelination in Demyelinating Disorders

Oligodendrocyte precursor cells (OPCs) are present in demyelinated lesions of multiple sclerosis (MS) patients. However, their differentiation into functional oligodendrocytes is insufficient, and most lesions evolve into nonfunctional astroglial scars. Blockade of bone morphogenetic protein (BMP) signaling induces differentiation of OPCs into myelin-producing oligodendrocytes. We studied the effect of specific blockade of BMP-2/4 signaling, by intravenous (IV) treatment with anti-BMP-2/4 neutralizing mAb in both the inflammatory model of relapsing experimental autoimmune encephalomyelitis (R-EAE) and the cuprizone-toxic model of demyelination in mice. Administration of anti-BMP-2/4 to R-EAE-induced mice, on day 9 post-immunization (p.i.), ameliorated R-EAE signs, diminished the expression of phospho-SMAD1/5/8, primarily within the astrocytic lineage, increased the numbers of de novo immature and mature oligodendrocytes, and reduced the numbers of newly generated astrocytes within the spinal cord as early as day 18 p.i. This effect was accompanied with elevated remyelination, manifested by increased density of remyelinating axons (0.8 < g-ratios < 1), and reduced fully demyelinated and demyelinating axons, in the anti-BMP-2/4-treated R-EAE mice, studied by electron microscopy. No significant immunosuppressive effect was observed in the CNS and in the periphery, during the peak of the first attack, or at the end of the experiment. Moreover, IV treatment with anti-BMP-2/4 mAb in the cuprizone-challenged mice augmented the numbers of mature oligodendrocytes and remyelination in the corpus callosum during the recovery phase of the disease. Based on our findings, the specific blockade of BMP-2/4 has a therapeutic potential in demyelinating disorders such as MS, by inducing early oligodendrogenesis-mediated remyelination in the affected tissue.

BMP9 expression in the adult rat brain

Bone morphogenetic protein 9 (BMP9), also known as growth differentiation factor 2 (GDF2), is a member of the transforming growth factor β (TGF β) superfamily. Although BMP9 plays pivotal roles during development, including angiogenesis, hematopoiesis, hepatogenesis, osteogenesis, and glucose metabolism, little information is available for BMP9 expression in the central nervous system (CNS). We, thus, investigated BMP9 expression in the adult rat CNS using immunohistochemistry. BMP9 was intensely expressed in most neurons and their axons. Furthermore, we found that oligodendrocytes and ependymal cells also express BMP9 protein. These data indicate that BMP9 is widely expressed throughout the adult CNS, and this abundant expression strongly supports the idea that BMP9 also plays important roles in the adult brain.

Schwann cell remyelination of the central nervous system: why does it happen and what are the benefits?

Myelin sheaths, by supporting axonal integrity and allowing rapid saltatory impulse conduction, are of fundamental importance for neuronal function. In response to demyelinating injuries in the central nervous system (CNS), oligodendrocyte progenitor cells (OPCs) migrate to the lesion area, proliferate and differentiate into new oligodendrocytes that make new myelin sheaths. This process is termed remyelination. Under specific conditions, demyelinated axons in the CNS can also be remyelinated by Schwann cells (SCs), the myelinating cell of the peripheral nervous system. OPCs can be a major source of these CNS-resident SCs-a surprising finding given the distinct embryonic origins, and physiological compartmentalization of the peripheral and central nervous system. Although the mechanisms and cues governing OPC-to-SC differentiation remain largely undiscovered, it might nevertheless be an attractive target for promoting endogenous remyelination. This article will (i) review current knowledge on the origins of SCs in the CNS, with a particular focus on OPC to SC differentiation, (ii) discuss the necessary criteria for SC myelination in the CNS and (iii) highlight the potential of using SCs for myelin regeneration in the CNS.

Inhibition of the BMP Signaling Pathway Ameliorated Established Clinical Symptoms of Experimental Autoimmune Encephalomyelitis

Bone morphogenetic proteins (BMPs) are secreted growth factors that belong to the transforming growth factor beta superfamily. BMPs have been implicated in physiological processes, but they are also involved in many pathological conditions. Multiple sclerosis (MS) is an immune-mediated disease of the central nervous system (CNS); however, its etiology remains elusive. Some evidence points to BMPs as important players in the pathogenesis of inflammatory and autoimmune disorders. In the present work, we studied the expression of BMP2, BMP4, BMP5, BMP6, BMP7, BMP type II receptor, and noggin in the immune system during different phases of experimental autoimmune encephalomyelitis (EAE). Major changes in the expression of BMPs took place in the initial phases of EAE. Indeed, those changes mainly affected BMP6 (whose expression was abrogated), BMP2, and BMP7 (whose expression was increased). In addition, we showed that in vivo inhibition of the BMP signaling pathway with small molecules ameliorated the already established clinical symptoms of EAE, as well as the CNS histopathological features. At the immune level, we observed an expansion of plasmacytoid dendritic cells (pDCs) in mice treated with small molecules that inhibit the BMP signaling pathway. pDCs could play an important role in promoting the expansion of antigen-specific regulatory T cells. Altogether, our data suggest a role for BMPs in early immune events that take place in myelin oligodendrocyte glycoprotein (MOG)-induced EAE. In addition, the clinical outcome of the disease was improved when the BMP signaling pathway was inhibited in mice that presented established EAE symptoms.

Inhibitors of Myelination and Remyelination, Bone Morphogenetic Proteins, are Upregulated in Human Neurological Disease

During demyelinating disease such as multiple sclerosis and stroke, myelin is destroyed and along with it, the oligodendrocytes that synthesize the myelin. Thus, recovery is limited due to both interruptions in neuronal transmission as well as lack of support for neurons. Although oligodendrocyte progenitor cells remain abundant in the central nervous system, they rarely mature and form new functional myelin in the diseased CNS. In cell culture and in experimental models of demyelinating disease, inhibitory signaling factors decrease myelination and remyelination. One of the most potent of these are the bone morphogenetic proteins (BMPs), a family of proteins that strongly inhibits oligodendrocyte progenitor differentiation and myelination in culture. BMPs are highly expressed in the dorsal CNS during pre-natal development and serve to regulate dorsal ventral patterning. Their expression decreases after birth but is significantly increased in rodent demyelination models such as experimental autoimmune encephalomyelitis, cuprizone ingestion and spinal cord injury. However, until recently, evidence for BMP upregulation in human disease has been scarce. This review discusses new human studies showing that in multiple sclerosis and other demyelinating diseases, BMPs are expressed by immune cells invading the CNS as well as resident CNS cell types, mostly astrocytes and microglia. Expression of endogenous BMP antagonists is also regulated. Identification of BMPs in the CNS is correlated with areas of demyelination and inflammation. These studies further support BMP as a potential therapeutic target.

Regulation of Synaptic Development by Astrocyte Signaling Factors and Their Emerging Roles in Substance Abuse

Astrocytes have critical functions throughout the central nervous system (CNS) and have emerged as regulators of synaptic development and function. With their highly complex morphologies, they are able to interact with thousands of synapses via peripheral astrocytic processes (PAPs), ensheathing neuronal axons and dendrites to form the tripartite synapse. In this way, astrocytes engage in crosstalk with neurons to mediate a variety of CNS processes including the regulation of extracellular matrix protein signaling, formation and maintenance of the blood-brain barrier (BBB), axon growth and guidance, homeostasis of the synaptic microenvironment, synaptogenesis, and the promotion of synaptic diversity. In this review, we discuss several key astrocyte signaling factors (thrombospondins, netrins, apolipoproteins, neuregulins, bone morphogenetic proteins, and neuroligins) in the maintenance and regulation of synapse formation. We also explore how these astrocyte signaling factors are impacted by and contribute to substance abuse, particularly alcohol and cocaine use.

Paired Related Homeobox Protein 1 Regulates Quiescence in Human Oligodendrocyte Progenitors

Human oligodendrocyte progenitor cells (hOPCs) persist into adulthood as an abundant precursor population capable of division and differentiation. The transcriptional mechanisms that regulate hOPC homeostasis remain poorly defined. Herein, we identify paired related homeobox protein 1 (PRRX1) in primary PDGFαR⁺ hOPCs. We show that enforced PRRX1 expression results in reversible G_1/0 arrest. While both PRRX1 splice variants reduce hOPC proliferation, only PRRX1a abrogates migration. hOPC engraftment into hypomyelinated shiverer/rag2 mouse brain is severely impaired by PRRX1a, characterized by reduced cell proliferation and migration. PRRX1 induces a gene expression signature characteristic of stem cell quiescence. Both IFN-γ and BMP signaling upregulate PRRX1 and induce quiescence. PRRX1 knockdown modulates IFN-γ-induced quiescence. In mouse brain, PRRX1 mRNA was detected in non-dividing OPCs and is upregulated in OPCs following demyelination. Together, these data identify PRRX1 as a regulator of quiescence in hOPCs and as a potential regulator of pathological quiescence.

Galectin-3 modulates postnatal subventricular zone gliogenesis

Postnatal subventricular zone (SVZ) neural stem cells generate forebrain glia, namely astrocytes and oligodendrocytes. The cues necessary for this process are unclear, despite this phase of brain development being pivotal in forebrain gliogenesis. Galectin‐3 (Gal‐3) is increased in multiple brain pathologies and thereby regulates astrocyte proliferation and inflammation in injury. To study the function of Gal‐3 in inflammation and gliogenesis, we carried out functional studies in mouse. We overexpressed Gal‐3 with electroporation and using immunohistochemistry surprisingly found no inflammation in the healthy postnatal SVZ. This allowed investigation of inflammation‐independent effects of Gal‐3 on gliogenesis. Loss of Gal‐3 function via knockdown or conditional knockout reduced gliogenesis, whereas Gal‐3 overexpression increased it. Gal‐3 overexpression also increased the percentage of striatal astrocytes generated by the SVZ but decreased the percentage of oligodendrocytes. These novel findings were further elaborated with multiple analyses demonstrating that Gal‐3 binds to the bone morphogenetic protein receptor one alpha (BMPR1α) and increases bone morphogenetic protein (BMP) signaling. Conditional knockout of BMPR1α abolished the effect of Gal‐3 overexpression on gliogenesis. Gain‐of‐function of Gal‐3 is relevant in pathological conditions involving the human forebrain, which is particularly vulnerable to hypoxia/ischemia during perinatal gliogenesis. Hypoxic/ischemic injury induces astrogliosis, inflammation and cell death. We show that Gal‐3 immunoreactivity was increased in the perinatal human SVZ and striatum after hypoxia/ischemia. Our findings thus show a novel inflammation‐independent function for Gal‐3; it is necessary for gliogenesis and when increased in expression can induce astrogenesis via BMP signaling.

Myelin status and oligodendrocyte lineage cells over time after spinal cord injury: What do we know and what still needs to be unwrapped?

Spinal cord injury (SCI) affects over 17,000 individuals in the United States per year, resulting in sudden motor, sensory and autonomic impairments below the level of injury. These deficits may be due at least in part to the loss of oligodendrocytes and demyelination of spared axons as it leads to slowed or blocked conduction through the lesion site. It has long been accepted that progenitor cells form new oligodendrocytes after SCI, resulting in the acute formation of new myelin on demyelinated axons. However, the chronicity of demyelination and the functional significance of remyelination remain contentious. Here we review work examining demyelination and remyelination after SCI as well as the current understanding of oligodendrocyte lineage cell responses to spinal trauma, including the surprisingly long-lasting response of NG2+ oligodendrocyte progenitor cells (OPCs) to proliferate and differentiate into new myelinating oligodendrocytes for months after SCI. OPCs are highly sensitive to microenvironmental changes, and therefore respond to the ever-changing post-SCI milieu, including influx of blood, monocytes and neutrophils; activation of microglia and macrophages; changes in cytokines, chemokines and growth factors such as ciliary neurotrophic factor and fibroblast growth factor-2; glutamate excitotoxicity; and axon degeneration and sprouting. We discuss how these changes relate to spontaneous oligodendrogenesis and remyelination, the evidence for and against demyelination being an important clinical problem and if remyelination contributes to motor recovery.

Transforming growth factor-beta renders ageing microglia inhibitory to oligodendrocyte generation by CNS progenitors

It is now well-established that the macrophage and microglial response to CNS demyelination influences remyelination by removing myelin debris and secreting a variety of signaling molecules that influence the behaviour of oligodendrocyte progenitor cells (OPCs). Previous studies have shown that changes in microglia contribute to the age-related decline in the efficiency of remyelination. In this study, we show that microglia increase their expression of the proteoglycan NG2 with age, and that this is associated with an altered micro-niche generated by aged, but not young, microglia that can divert the differentiation OPCs from oligodendrocytes into astrocytes in vitro. We further show that these changes in ageing microglia are generated by exposure to high levels of TGFβ. Thus, our findings suggest that the rising levels of circulating TGFβ known to occur with ageing contribute to the age-related decline in remyelination by impairing the ability of microglia to promote oligodendrocyte differentiation from OPCs, and therefore could be a potential therapeutic target to promote remyelination.

Could the inhibitor of DNA binding 2 and 4 play a role in white matter injury?

White matter injury (WMI) prevents the normal development of myelination, leading to central nervous system myelination disorders and the production of chronic sequelae associated with WMI, such as chronic dyskinesia, cognitive impairment and cerebral palsy. This results in a large emotional and socioeconomic burden. Decreased myelination in preterm infant WMI is associated with the delayed development or destruction of oligodendrocyte (OL) lineage cells, particularly oligodendrocyte precursor cells (OPCs). The development of cells from the OL lineage involves the migration, proliferation and different stages of OL differentiation, finally leading to myelination. A series of complex intrinsic, extrinsic and epigenetic factors regulate the OPC cell cycle withdrawal, OL lineage progression and myelination. We focus on the inhibitor of DNA binding 2 (ID2), because it is widely involved in the different stages of OL differentiation and genesis. ID2 is a key transcription factor for the normal development of OL lineage cells, and the pathogenesis of WMI is closely linked with OL developmental disorders. ID4, another family member of the IDs protein, also plays a similar role in OL differentiation and genesis. ID2 and ID4 belong to the helix-loop-helix family; they lack the DNA-binding sequences and inhibit oligodendrogenesis and OPC differentiation. In this review, we mainly discuss the roles of ID2 in OL development, especially during OPC differentiation, and summarize the ID2-mediated intracellular and extracellular signaling pathways that regulate these processes. We also discuss ID4 in relation to bone morphogenetic protein signaling and oligodendrogenesis. It is likely that these developmental mechanisms are also involved in the myelin repair or remyelination in human neurological diseases.

Reduced levels of Coco in sera of multiple sclerosis patients: A potential role in neuro-regeneration failure

Demyelination, axonal loss and failure of tissue repair characterize MS lesions. Bone morphogenetic proteins (BMPs) signaling is associated with remyelination failure. Coco is one of the BMP antagonists. We found reduced Coco serum levels in relapsing-remitting MS (RR-MS) and primary progressive MS (PP-MS) patients compared to matched healthy controls (HC) and patients with rheumatoid arthritis. Exposure of P19 cells, in the presence of retinoic acid, BMP-2, or BMP-4 to Coco, at average sera level of MS patients failed to induce neuronal phenotype, in contrast to the average sera level of HC. Coco may be a player in the BMP dysregulation and the tissue repair failure in MS.

Myelination in Multiple Sclerosis Lesions Is Associated with Regulation of Bone Morphogenetic Protein 4 and Its Antagonist Noggin

Remyelination is a central aspect of new multiple sclerosis (MS) therapies, in which one aims to alleviate disease symptoms by improving axonal protection. However, a central problem is mediators expressed in MS lesions that prevent effective remyelination. Bone morphogenetic protein4 (BMP4) inhibits the development of mature oligodendrocytes in cell culture and also blocks the expression of myelin proteins. Additionally, numerous studies have shown that Noggin (SYM1)—among other physiological antagonists of BMP4—plays a prominent role in myelin formation in the developing but also the adult central nervous system. Nonetheless, neither BMP4 nor Noggin have been systematically studied in human MS lesions. In this study, we demonstrated by transcript analysis and immunohistochemistry that BMP4 is expressed by astrocytes and microglia/macrophages in association with inflammatory infiltrates in MS lesions, and that astrocytes also express BMP4 in chronic inactive lesions that failed to remyelinate. Furthermore, the demonstration of an increased expression of Noggin in so-called shadow plaques (i.e., remyelinated lesions with thinner myelin sheaths) in comparison to chronically inactive demyelinated lesions implies that antagonizing BMP4 is associated with successful remyelination in MS plaques in humans. However, although BMP4 is strongly overexpressed in inflammatory lesion areas, its levels are also elevated in remyelinated lesion areas, which raises the possibility that BMP4 signaling itself may be required for remyelination. Therefore, remyelination might be influenced by a small number of key factors. Manipulating these molecules, i.e., BMP4 and Noggin, could be a promising therapeutic approach for effective remyelination.

Expression of Bone Morphogenetic Proteins in Multiple Sclerosis Lesions

Bone morphogenetic proteins (BMPs) are secreted proteins that belong to the transforming growth factor-β superfamily. In the adult brain, they modulate neurogenesis, favor astrogliogenesis, and inhibit oligodendrogenesis. Because BMPs may be involved in the failure of remyelination in multiple sclerosis (MS), we characterized the expression of BMP-2, BMP-4, BMP-5, and BMP-7; BMP type II receptor (BMPRII); and phosphorylated SMAD (pSMAD) 1/5/8 in lesions of MS and other demyelinating diseases. A total of 42 MS lesions, 12 acute ischemic lesions, 8 progressive multifocal leukoencephalopathy lesions, and 10 central nervous system areas from four nonneuropathological patients were included. Lesions were histologically classified according to the inflammatory activity. The expression of BMP-2, BMP-4, BMP-5, BMP-7, BMPRII, and pSMAD1/5/8 was quantified by immunostaining, and colocalization studies were performed. In MS lesions, astrocytes, microglia/macrophages, and neurons expressed BMP-2, BMP-4, BMP-5, and BMP-7; BMPRII; and pSMAD1/5/8. Oligodendrocytes expressed BMP-2 and BMP-7 and pSMAD1/5/8. The percentage of cells that expressed BMPs, BMPRII, and pSMAD1/5/8 correlated with the inflammatory activity of MS lesions, and changes in the percentage of positive cells were more relevant in MS than in other white matter-damaging diseases. These data indicate that BMPs are increased in active MS lesions, suggesting a possible role in MS pathogenesis.

BMSCs promote the differentiation of NSCs into oligodendrocytes via mediating Id2 and Olig expression through BMP/Smad signaling pathway

Neural stem cells (NSCs) have emerged as a promising treatment for spinal cord injuries. However, the increasing expression of bone morphogenetic proteins (BMPs) in spinal cord injury lesion sites seems to have contributed to the limited oligodendroglial differentiation and the majority of the astroglial differentiation of NSCs. In the present study, we demonstrate that BMPs promote NSCs differentiation toward astrocytes and prevent them from differentiating into oligodendrocytes. This effect is accompanied by the increasing expression of Id2 and the reduction in Oilg1/2 expression. Treatment with bone marrow stromal cells (BMSCs) can enhance the development of oligodendrocytes in the presence of BMPs. The analysis of Id2, as well as Olig1 and Olig2 gene expression, reveals that the effect of BMPs on these gene expressions is reversed with the addition of BMSCs. In sum, these data strongly suggest that BMSCs can promote the differentiation of NSCs into oligodendrocytes through mediating Id2 and Olig1/2 expression by blocking the BMP/Smad signaling pathway.

Injury-induced perivascular niche supports alternative differentiation of adult rodent CNS progenitor cells

Following CNS demyelination, oligodendrocyte progenitor cells (OPCs) are able to differentiate into either remyelinating oligodendrocytes (OLs) or remyelinating Schwann cells (SCs). However, the signals that determine which type of remyelinating cell is generated and the underlying mechanisms involved have not been identified. Here, we show that distinctive microenvironments created in discrete niches within demyelinated white matter determine fate decisions of adult OPCs. By comparative transcriptome profiling we demonstrate that an ectopic, injury-induced perivascular niche is enriched with secreted ligands of the BMP and Wnt signalling pathways, produced by activated OPCs and endothelium, whereas reactive astrocyte within non-vascular area express the dual BMP/Wnt antagonist Sostdc1. The balance of BMP/Wnt signalling network is instructive for OPCs to undertake fate decision shortly after their activation: disruption of the OPCs homeostasis during demyelination results in BMP4 upregulation, which, in the absence of Socstdc1, favours SCs differentiation.

A transcriptome comparison of time-matched developing human, mouse and rat neural progenitor cells reveals human uniqueness

It is widely accepted that human brain development has unique features that cannot be represented by rodents. Obvious reasons are the evolutionary distance and divergent physiology. This might lead to false predictions when rodents are used for safety or pharmacological efficacy studies. For a better translation of animal-based research to the human situation, human in vitro systems might be useful. In this study, we characterize developing neural progenitor cells from prenatal human and time-matched rat and mouse brains by analyzing the changes in their transcriptome profile during neural differentiation. Moreover, we identify hub molecules that regulate neurodevelopmental processes like migration and differentiation. Consequences of modulation of three of those hubs on these processes were studied in a species-specific context. We found that although the gene expression profiles of the three species largely differ qualitatively and quantitatively, they cluster in similar GO terms like cell migration, gliogenesis, neurogenesis or development of multicellular organism. Pharmacological modulation of the identified hub molecules triggered species-specific cellular responses. This study underlines the importance of understanding species differences on the molecular level and advocates the use of human based in vitro models for pharmacological and toxicological research.

Bone Morphogenetic Protein-Based Therapeutic Approaches

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor (TGF)-β family of ligands and exert most of their effects through the canonical effectors Smad1, 5, and 8. Appropriate regulation of BMP signaling is critical for the development and homeostasis of numerous human organ systems. Aberrations in BMP pathways or their regulation are increasingly associated with diverse human pathologies, and there is an urgent and growing need to develop effective approaches to modulate BMP signaling in the clinic. In this review, we provide a wide perspective on diseases and/or conditions associated with dysregulated BMP signal transduction, outline the current strategies available to modulate BMP pathways, highlight emerging second-generation technologies, and postulate prospective avenues for future investigation.

Bone morphogenetic proteins in multiple sclerosis: Role in neuroinflammation

Bone morphogenetic proteins (BMPs) are growth factors that represent the largest subgroup of signalling ligands of the transforming growth factor beta (TGF-β) superfamily. Their participation in the proliferation, survival and cell fate of several cell types and their involvement in many pathological conditions are now well known. BMP expression is altered in multiple sclerosis (MS) patients, suggesting that BMPs have a role in the pathogenesis of this disease. MS is a demyelinating and neurodegenerative autoimmune disorder of the central nervous system (CNS). MS is a complex pathological condition in which genetic, epigenetic and environmental factors converge, although its aetiology remains elusive. Multifunctional molecules, such as BMPs, are extremely interesting in the field of MS because they are involved in the regulation of several adult tissues, including the CNS and the immune system. In this review, we discuss the extensive data available regarding the role of BMP signalling in neuronal progenitor/stem cell fate and focus on the participation and expression of BMPs in CNS demyelination. Additionally, we provide an overview of the involvement of BMPs as modulators of the immune system, as this subject has not been thoroughly explored even though it is of great interest in autoimmune disorders. Moreover, we describe the data on BMP signalling in autoimmunity and inflammatory diseases, including MS and its experimental models. Thus, we aim to provide an integrated view of the putative role of BMPs in MS pathogenesis and to open the field for the further development of alternative therapeutic strategies for MS patients.

f-scLVM: scalable and versatile factor analysis for single-cell RNA-seq

Single-cell RNA-sequencing (scRNA-seq) allows studying heterogeneity in gene expression in large cell populations. Such heterogeneity can arise due to technical or biological factors, making decomposing sources of variation difficult. We here describe f-scLVM (factorial single-cell latent variable model), a method based on factor analysis that uses pathway annotations to guide the inference of interpretable factors underpinning the heterogeneity. Our model jointly estimates the relevance of individual factors, refines gene set annotations, and infers factors without annotation. In applications to multiple scRNA-seq datasets, we find that f-scLVM robustly decomposes scRNA-seq datasets into interpretable components, thereby facilitating the identification of novel subpopulations.

Fibrinogen Activates BMP Signaling in Oligodendrocyte Progenitor Cells and Inhibits Remyelination after Vascular Damage

Blood-brain barrier (BBB) disruption alters the composition of the brain microenvironment by allowing blood proteins into the CNS. However, whether blood-derived molecules serve as extrinsic inhibitors of remyelination is unknown. Here we show that the coagulation factor fibrinogen activates the bone morphogenetic protein (BMP) signaling pathway in oligodendrocyte progenitor cells (OPCs) and suppresses remyelination. Fibrinogen induces phosphorylation of Smad 1/5/8 and inhibits OPC differentiation into myelinating oligodendrocytes (OLs) while promoting an astrocytic fate in vitro. Fibrinogen effects are rescued by BMP type I receptor inhibition using dorsomorphin homolog 1 (DMH1) or CRISPR/Cas9 activin A receptor type I (ACVR1) knockout in OPCs. Fibrinogen and the BMP target Id2 are increased in demyelinated multiple sclerosis (MS) lesions. Therapeutic depletion of fibrinogen decreases BMP signaling and enhances remyelination in vivo. Targeting fibrinogen may be an upstream therapeutic strategy to promote the regenerative potential of CNS progenitors in diseases with remyelination failure.

Differentiation of Oligodendrocyte Precursor Cells from Sox10-Venus Mice to Oligodendrocytes and Astrocytes

Oligodendrocytes are well known as myelin-forming cells in the central nervous system (CNS). However, detailed mechanisms of oligodendrocyte differentiation and myelination are poorly understood, particularly due to the difficulty of the purification of murine oligodendrocyte precursor cells (OPCs). We have recently established a transgenic mouse line that expresses a fluorescent protein Venus under the promoter of Sox10, whose expression is restricted to OPCs and oligodendrocytes in the CNS. Here, we have characterized Venus-positive cells from the Sox10-Venus mouse brain for analyzing oligodendrocyte differentiation. We first purified Venus-positive cells from the postnatal day 0-2 brain by flow cytometry. Most of the Venus-positive cells expressed NG2, an OPC marker. After induction of differentiation, an increased population of galactocerebroside-positive oligodendrocytes and decrease of OPCs were observed in the Venus-positive culture. Furthermore, a time-lapse analysis showed that Venus-positive oligodendrocytes dynamically changed their morphology with highly branched cell processes during differentiation. In addition, we found that Venus-positive OPCs were able to differentiate to type II astrocytes. In vivo, OPCs and oligodendrocytes express Venus, and some of astrocytes were positive for Venus in the ventral cortex. Taken together, the Sox10-Venus mouse system is useful for analyzing differentiation and multipotency of murine OPCs.

Reduced white matter MRI transverse relaxation rate in cognitively normal H63D-HFE human carriers and H67D-HFE mice

Mutations within the HFE protein gene sequence have been associated with increased risk of developing a number of neurodegenerative disorders. To this effect, an animal model has been created which incorporates the mouse homologue to the human H63D-HFE mutation: the H67D-HFE knock-in mouse. These mice exhibit alterations in iron management proteins, have increased neuronal oxidative stress, and a disruption in cholesterol regulation. However, it remains undetermined how these differences translate to human H63D carriers in regards to white matter (WM) integrity. To this endeavor, MRI transverse relaxation rate (R₂) parametrics were employed to test the hypothesis that WM alterations are present in H63D human carriers and are recapitulated in the H67D mice. H63D carriers exhibit widespread reductions in brain R₂ compared to non-carriers within white matter association fibers in the brain. Similar R₂ decreases within white matter tracts were observed in the H67D mouse brain. Additionally, an exacerbation of age-related R₂ decrease is found in the H67D animal model in white matter regions of interest. The decrease in R₂ within white matter tracts of both species is speculated to be multifaceted. The R₂ changes are hypothesized to be due to alterations in axonal biochemical tissue composition. The R₂ changes observed in both the human-H63D and mouse-H67D data suggest that modified white matter myelination is occurring in subjects with HFE mutations, potentially increasing vulnerability to neurodegenerative disorders.

Pericyte-derived bone morphogenetic protein 4 underlies white matter damage after chronic hypoperfusion

Subcortical small vessel disease (SVD) is characterized by white matter damage resulting from arteriolosclerosis and chronic hypoperfusion. Transforming growth factor beta 1 (TGFB1) is dysregulated in the hereditary SVD, CARASIL (cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy). However, very little is known about the role of the largest group in the TGFB superfamily - the bone morphogenetic proteins (BMPs) - in SVD pathogenesis. The aim of this study was to characterize signaling abnormalities of BMPs in sporadic SVD. We examined immunostaining of TGFB1 and BMPs (BMP2/BMP4/BMP6/BMP7/BMP9) in a total of 19 post-mortem human brain samples as follows: 7 SVD patients (4 males, 76-90 years old); 6 Alzheimer's disease (AD) patients (2 males, 67-93 years old) and 6 age-matched disease controls (3 males, 68-78 years old). We subsequently investigated the effects of oxygen-glucose deprivation and BMP4 addition on cultured cells. Furthermore, adult mice were subjected to chronic cerebral hypoperfusion using bilateral common carotid artery stenosis, followed by continuous intracerebroventricular infusion of the BMP antagonist, noggin. In the SVD cases, BMP4 was highly expressed in white matter pericytes. Oxygen-glucose deprivation induced BMP4 expression in cultured pericytes in vitro. Recombinant BMP4 increased the number of cultured endothelial cells and pericytes and converted oligodendrocyte precursor cells into astrocytes. Chronic cerebral hypoperfusion in vivo also upregulated BMP4 with concomitant white matter astrogliogenesis and reduced oligodendrocyte lineage cells, both of which were suppressed by intracerebroventricular noggin infusion. Our findings suggest ischemic white matter damage evolves in parallel with BMP4 upregulation in pericytes. BMP4 promotes angiogenesis, but induces astrogliogenesis at the expense of oligodendrocyte precursor cell proliferation and maturation, thereby aggravating white matter damage. This may explain white matter vulnerability to chronic hypoperfusion. The regulation of BMP4 signaling is a potential therapeutic strategy for treating SVD.

Immunocytochemical characterisation of neural stem-progenitor cells from green terror cichlid Aequidens rivulatus

In this study, cultures of neural stem-progenitor cells (NSPC) from the brain of green terror cichlid Aequidens rivulatus were established and various NSPCs were demonstrated using immunocytochemistry. All of the NSPCs expressed brain lipid-binding protein, dopamine- and cAMP-regulated neuronal phosphoprotein 32 (DARPP-32), oligodendrocyte transcription factor 2, paired box 6 and sex determining region Y-box 2. The intensity and localisation of these proteins, however, varied among the different NSPCs. Despite being intermediate cells, NSPCs can be divided into radial glial cells, oligodendrocyte progenitor cells (OPC) and neuroblasts by expressing the astrocyte marker glial fibrillary acidic protein (GFAP), OPC marker A2B5 and neuronal markers, including acetyl-tubulin, βIII-tubulin, microtubule-associated protein 2 and neurofilament protein. Nevertheless, astrocytes were polymorphic and were the most dominant cells in the NSPC cultures. By using Matrigel, radial glia exhibiting a long GFAP⁺ or DARPP-32⁺ fibre and neurons exhibiting a significant acetyl-tubulin⁺ process were obtained. The results confirmed that NSPCs obtained from A. rivulatus brains can proliferate and differentiate into neurons in vitro. Clonal culture can be useful for further studying the distinct NSPCs.

Understanding the NG2 Glial Scar after Spinal Cord Injury

NG2 cells, also known as oligodendrocyte progenitor cells, are located throughout the central nervous system and serve as a pool of progenitors to differentiate into oligodendrocytes. In response to spinal cord injury (SCI), NG2 cells increase their proliferation and differentiation into remyelinating oligodendrocytes. While astrocytes are typically associated with being the major cell type in the glial scar, many NG2 cells also accumulate within the glial scar but their function remains poorly understood. Similar to astrocytes, these cells hypertrophy, upregulate expression of chondroitin sulfate proteoglycans, inhibit axon regeneration, contribute to the glial-fibrotic scar border, and some even differentiate into astrocytes. Whether NG2 cells also have a role in other astrocyte functions, such as preventing the spread of infiltrating leukocytes and expression of inflammatory cytokines, is not yet known. Thus, NG2 cells are not only important for remyelination after SCI but are also a major component of the glial scar with functions that overlap with astrocytes in this region. In this review, we describe the signaling pathways important for the proliferation and differentiation of NG2 cells, as well as the role of NG2 cells in scar formation and tissue repair.

Extracellular cues influencing oligodendrocyte differentiation and (re)myelination

There is an increasing number of neurologic disorders found to be associated with loss and/or dysfunction of the CNS myelin sheath, ranging from the classic demyelinating disease, multiple sclerosis, through CNS injury, to neuropsychiatric diseases. The disabling burden of these diseases has sparked a growing interest in gaining a better understanding of the molecular mechanisms regulating the differentiation of the myelinating cells of the CNS, oligodendrocytes (OLGs), and the process of (re)myelination. In this context, the importance of the extracellular milieu is becoming increasingly recognized. Under pathological conditions, changes in inhibitory as well as permissive/promotional cues are thought to lead to an overall extracellular environment that is obstructive for the regeneration of the myelin sheath. Given the general view that remyelination is, even though limited in human, a natural response to demyelination, targeting pathologically 'dysregulated' extracellular cues and their downstream pathways is regarded as a promising approach toward the enhancement of remyelination by endogenous (or if necessary transplanted) OLG progenitor cells. In this review, we will introduce the extracellular cues that have been implicated in the modulation of (re)myelination. These cues can be soluble, part of the extracellular matrix (ECM) or mediators of cell-cell interactions. Their inhibitory and permissive/promotional roles with regard to remyelination as well as their potential for therapeutic intervention will be discussed.

How to make an oligodendrocyte

Oligodendrocytes produce myelin, an insulating sheath required for the saltatory conduction of electrical impulses along axons. Oligodendrocyte loss results in demyelination, which leads to impaired neurological function in a broad array of diseases ranging from pediatric leukodystrophies and cerebral palsy, to multiple sclerosis and white matter stroke. Accordingly, replacing lost oligodendrocytes, whether by transplanting oligodendrocyte progenitor cells (OPCs) or by mobilizing endogenous progenitors, holds great promise as a therapeutic strategy for the diseases of central white matter. In this Primer, we describe the molecular events regulating oligodendrocyte development and how our understanding of this process has led to the establishment of methods for producing OPCs and oligodendrocytes from embryonic stem cells and induced pluripotent stem cells, as well as directly from somatic cells. In addition, we will discuss the safety of engrafted stem cell-derived OPCs, as well as approaches by which to modulate their differentiation and myelinogenesis in vivo following transplantation.

Oligodendrocyte progenitor programming and reprogramming: Toward myelin regeneration

Demyelinating diseases such as multiple sclerosis (MS) are among the most disabling and cost-intensive neurological disorders. The loss of myelin in the central nervous system, produced by oligodendrocytes (OLs), impairs saltatory nerve conduction, leading to motor and cognitive deficits. Immunosuppression therapy has a limited efficacy in MS patients, arguing for a paradigm shift to strategies that target OL lineage cells to achieve myelin repair. The inhibitory microenvironment in MS lesions abrogates the expansion and differentiation of resident OL precursor cells (OPCs) into mature myelin-forming OLs. Recent studies indicate that OPCs display a highly plastic ability to differentiate into alternative cell lineages under certain circumstances. Thus, understanding the mechanisms that maintain and control OPC fate and differentiation into mature OLs in a hostile, non-permissive lesion environment may open new opportunities for regenerative therapies. In this review, we will focus on 1) the plasticity of OPCs in terms of their developmental origins, distribution, and differentiation potentials in the normal and injured brain; 2) recent discoveries of extrinsic and intrinsic factors and small molecule compounds that control OPC specification and differentiation; and 3) therapeutic potential for motivation of neural progenitor cells and reprogramming of differentiated cells into OPCs and their likely impacts on remyelination. OL-based therapies through activating regenerative potentials of OPCs or cell replacement offer exciting opportunities for innovative strategies to promote remyelination and neuroprotection in devastating demyelinating diseases like MS. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

Oligodendrocyte generation during mouse development

Oligodendrocytes (OLs) are glial cells, which generate myelin in the central nervous system. Their interesting developmental features attract many neurobiologists eager to study cell differentiation, gene expression regulation, or dynamic morphogenesis. Their primary role in protecting the axons has major impacts in the medical research field: in multiple sclerosis, a demyelinating disease in which remyelination is blocked. Oligodendrogenesis is involved in higher brain function including motor skill learning and cognitive function. Here, we review advances in the research on OL development and highlight areas where questions remain to be answered in both developmental biology and neurobiology related aspects.

NG2-glia and their functions in the central nervous system

In the central nervous system, NG2-glia represent a neural cell population that is distinct from neurons, astrocytes, and oligodendrocytes. While in the past the main role ascribed to these cells was that of progenitors for oligodendrocytes, in the last years it has become more obvious that they have further functions in the brain. Here, we will discuss some of the most current and highly debated issues regarding NG2-glia: Do these cells represent a heterogeneous population? Can they give rise to different progenies, and does this change under pathological conditions? How do they respond to injury or pathology? What is the role of neurotransmitter signaling between neurons and NG2-glia? We will first give an overview on the developmental origin of NG2-glia, and then discuss whether their distinct properties in different brain regions are the result of environmental influences, or due to intrinsic differences. We will then review and discuss their in vitro differentiation potential and in vivo lineage under physiological and pathological conditions, together with their electrophysiological properties in distinct brain regions and at different developmental stages. Finally, we will focus on their potential to be used as therapeutic targets in demyelinating and neurodegenerative diseases. Therefore, this review article will highlight the importance of NG2-glia not only in the healthy, but also in the diseased brain.

A transient wave of BMP signaling in the retina is necessary for Müller glial differentiation

The primary glial cells in the retina, the Müller glia, differentiate from retinal progenitors in the first postnatal week. CNTF/LIF/STAT3 signaling has been shown to promote their differentiation; however, another key glial differentiation signal, BMP, has not been examined during this period of Müller glial differentiation. In the course of our analysis of the BMP signaling pathway, we observed a transient wave of Smad1/5/8 signaling in the inner nuclear layer at the end of the first postnatal week, from postnatal day (P) 5 to P9, after the end of neurogenesis. To determine the function of this transient wave, we blocked BMP signaling during this period in vitro or in vivo, using either a BMP receptor antagonist or noggin (Nog). Either treatment leads to a reduction in expression of the Müller glia-specific genes Rlbp1 and Glul, and the failure of many of the Müller glia to repress the bipolar/photoreceptor gene Otx2. These changes in normal Müller glial differentiation result in permanent disruption of the retina, including defects in the outer limiting membrane, rosette formation and a reduction in functional acuity. Our results thus show that Müller glia require a transient BMP signal at the end of neurogenesis to fully repress the neural gene expression program and to promote glial gene expression.

Summary: BMP signalling is transiently activated in the postnatal mouse retina to terminate the neurogenic program and promote the expression of glial-specific genes.

Redox biology in normal cells and cancer: restoring function of the redox/Fyn/c-Cbl pathway in cancer cells offers new approaches to cancer treatment

This review discusses a unique discovery path starting with novel findings on redox regulation of precursor cell and signaling pathway function and identification of a new mechanism by which relatively small changes in redox status can control entire signaling networks that regulate self-renewal, differentiation, and survival. The pathway central to this work, the redox/Fyn/c-Cbl (RFC) pathway, converts small increases in oxidative status to pan-activation of the c-Cbl ubiquitin ligase, which controls multiple receptors and other proteins of central importance in precursor cell and cancer cell function. Integration of work on the RFC pathway with attempts to understand how treatment with systemic chemotherapy causes neurological problems led to the discovery that glioblastomas (GBMs) and basal-like breast cancers (BLBCs) inhibit c-Cbl function through altered utilization of the cytoskeletal regulators Cool-1/βpix and Cdc42, respectively. Inhibition of these proteins to restore normal c-Cbl function suppresses cancer cell division, increases sensitivity to chemotherapy, disrupts tumor-initiating cell (TIC) activity in GBMs and BLBCs, controls multiple critical TIC regulators, and also allows targeting of non-TICs. Moreover, these manipulations do not increase chemosensitivity or suppress division of nontransformed cells. Restoration of normal c-Cbl function also allows more effective harnessing of estrogen receptor-α (ERα)-independent activities of tamoxifen to activate the RFC pathway and target ERα-negative cancer cells. Our work thus provides a discovery strategy that reveals mechanisms and therapeutic targets that cannot be deduced by standard genetics analyses, which fail to reveal the metabolic information, isoform shifts, protein activation, protein complexes, and protein degradation critical to our discoveries.