Mechanisms of glial development

How mechanisms of stem cell polarity shape the human cerebral cortex

Apical-basal progenitor cell polarity establishes key features of the radial and laminar architecture of the developing human cortex. The unique diversity of cortical stem cell populations and an expansion of progenitor population size in the human cortex have been mirrored by an increase in the complexity of cellular processes that regulate stem cell morphology and behaviour, including their polarity. The study of human cells in primary tissue samples and human stem cell-derived model systems (such as cortical organoids) has provided insight into these processes, revealing that protein complexes regulate progenitor polarity by controlling cell membrane adherence within appropriate cortical niches and are themselves regulated by cytoskeletal proteins, signalling molecules and receptors, and cellular organelles. Studies exploring how cortical stem cell polarity is established and maintained are key for understanding the features of human brain development and have implications for neurological dysfunction.

Dynamic behaviour of human neuroepithelial cells in the developing forebrain

To understand how diverse progenitor cells contribute to human neocortex development, we examined forebrain progenitor behaviour using timelapse imaging. Here we find that cell cycle dynamics of human neuroepithelial (NE) cells differ from radial glial (RG) cells in both primary tissue and in stem cell-derived organoids. NE cells undergoing proliferative, symmetric divisions retract their basal processes, and both daughter cells regrow a new process following cytokinesis. The mitotic retraction of the basal process is recapitulated by NE cells in cerebral organoids generated from human-induced pluripotent stem cells. In contrast, RG cells undergoing vertical cleavage retain their basal fibres throughout mitosis, both in primary tissue and in older organoids. Our findings highlight developmentally regulated changes in mitotic behaviour that may relate to the role of RG cells to provide a stable scaffold for neuronal migration, and suggest that the transition in mitotic dynamics can be studied in organoid models.

The dynamics of progenitor cells in human neocortex development has not been studied directly. Here, the authors timelapse image human neuroepithelial (NE) and radial glial (RG) cells in embryonic brain slices and find properties of NE cells and RG that are mimicked in cerebral organoids.

Fyn is an intermediate kinase that BDNF utilizes to promote oligodendrocyte myelination

Fyn, a member of the Src family of nonreceptor tyrosine kinases, promotes central nervous system myelination during development; however the mechanisms mediating this effect remain unknown. Here we show that Fyn phosphorylation is modulated by BDNF in vivo. Concordant with this, we find that BDNF stimulates Fyn phosphorylation in myelinating cocultures, an effect dependent on oligodendroglial expression of TrkB. Importantly, PP2, a pharmacological inhibitor of Src family kinases, not only abrogated the promyelinating influence of BDNF in vitro, but also attenuated BDNF-induced phosphorylation of Erk1/2 in oligodendrocytes. Over-expression of Fyn in oligodendrocytes significantly promotes phosphorylation of Erk1/2, and promotes myelination to the extent that exogenous BDNF exerts no additive effect in vitro. In contrast, expression of a kinase-dead mutant of Fyn in oligodendrocytes significantly inhibited BDNF-induced activation of Erk1/2 and abrogated the promyelinating effect of BDNF. Analysis of white matter tracts in vivo revealed that phosphorylated Fyn primarily colocalized with mature oligodendrocytes, and was rarely observed in oligodendrocyte progenitor cells, a profile that closely parallels the detection of phosphorylated Erk1/2 in the developing central nervous system. Taken together, these data identify that Fyn kinase exerts a key role in mediating the promyelinating influence of BDNF. Here we identify a pathway in which BDNF activation of oligodendroglial TrkB receptors stimulates the phosphorylation of Fyn, a necessary step required to potentiate the phosphorylation of Erk1/2, which in turn regulates oligodendrocyte myelination.

Axon-glia interaction and membrane traffic in myelin formation

In vertebrate nervous systems myelination of neuronal axons has evolved to increase conduction velocity of electrical impulses with minimal space and energy requirements. Myelin is formed by specialized glial cells which ensheath axons with a lipid-rich insulating membrane. Myelination is a multi-step process initiated by axon-glia recognition triggering glial polarization followed by targeted myelin membrane expansion and compaction. Thereby, a myelin sheath of complex subdomain structure is established. Continuous communication between neurons and glial cells is essential for myelin maintenance and axonal integrity. A diverse group of diseases, from multiple sclerosis to schizophrenia, have been linked to malfunction of myelinating cells reflecting the physiological importance of the axon-glial unit. This review describes the mechanisms of axonal signal integration by oligodendrocytes emphasizing the central role of the Src-family kinase Fyn during central nervous system (CNS) myelination. Furthermore, we discuss myelin membrane trafficking with particular focus on endocytic recycling and the control of proteolipid protein (PLP) transport by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Finally, PLP mistrafficking is considered in the context of myelin diseases.

Phospholipase D is a target for inhibition of astroglial proliferation by ethanol

The proliferation of astrocytes during early brain development is driven by growth factors and is accompanied by the activation of phospholipase D (PLD). Ethanol disrupts PLD signaling in astrocytes, a process which may contribute to delayed brain growth of fetuses exposed to alcohol during pregnancy. We here report that insulin-like growth factor 1 (IGF-1) is a strong mitogen for rat astrocytes (EC50 0.2 μg/ml) and a strong stimulator of astroglial PLD activity; both effects are inhibited by ethanol and 1-butanol, but not t-butanol, suggesting participation of PLD. Downregulation of PLD1 and exposure to the PLD1 inhibitor VU0359595 attenuated PLD activity and strongly reduced the mitogenic activity of serum and IGF-1. The PLD2 inhibitor VU0285655-1 also reduced PLD activity but had lesser effects on IGF-1-driven proliferation. PLD2 down-regulation affected serum - but not IGF-1-induced proliferation. In separate experiments, alcohol treatment of murine astrocytes taken from PLD-deficient animals revealed an insensitivity of PLD1(-/-) cells to 1-butanol whereas PLD2(-/-) cells were not affected. We conclude that astroglial proliferation induced by IGF-1 is critically dependent on the PLD signaling pathway, with a stronger contribution from PLD1 than PLD2. The teratogenic effects of ethanol may be explained, at least in part, by disruption of the IGF1-PLD signaling pathway.

Nucleus-localized 21.5-kDa myelin basic protein promotes oligodendrocyte proliferation and enhances neurite outgrowth in coculture, unlike the plasma membrane-associated 18.5-kDa isoform

The classic myelin basic protein (MBP) family of central nervous system (CNS) myelin arises from transcription start site 3 of the Golli (gene of oligodendrocyte lineage) complex and comprises splice isoforms ranging in nominal molecular mass from 14 kDa to (full-length) 21.5 kDa. We have determined here a number of distinct functional differences between the major 18.5-kDa and minor 21.5-kDa isoforms of classic MBP with respect to oligodendrocyte (OLG) proliferation. We have found that, in contrast to 18.5-kDa MBP, 21.5-kDa MBP increases proliferation of early developmental immortalized N19-OLGs by elevating the levels of phosphorylated ERK1/2 and Akt1 kinases and of ribosomal protein S6. Coculture of N2a neuronal cells with N19-OLGs transfected with the 21.5-kDa isoform (or conditioned medium from), but not the 18.5-kDa isoform, caused the N2a cells to have increased neurite outgrowth and process branching complexity. These roles were dependent on subcellular localization of 21.5-kDa MBP to the nucleus and on the exon II-encoded segment, suggesting that the nuclear localization of early minor isoforms of MBP may play a crucial role in regulating and/or initiating myelin and neuronal development in the mammalian CNS.

Multi-compartment neuron-glia co-culture platform for localized CNS axon-glia interaction study

Formation of myelin sheaths by oligodendrocytes (OLs) in the central nervous system (CNS) is essential for rapid nerve impulse conduction. Reciprocal signaling between axons and OLs orchestrates myelinogenesis but remains largely elusive. In this study, we present a multi-compartment CNS neuron-glia microfluidic co-culture platform. The platform is capable of conducting parallel localized drug and biomolecule treatments while carrying out multiple co-culture conditions in a single device for studying axon-glia interactions at a higher throughput. The "micro-macro hybrid soft-lithography master fabrication" (MMHSM) technique enables a large number of precisely replicated PDMS devices incorporating both millimeter and micrometer scale structures to be rapidly fabricated without any manual reservoir punching processes. Axons grown from the neuronal somata were physically and fluidically isolated inside the six satellite axon/glia compartments for localized treatments. Astrocytes, when seeded and co-cultured after the establishment of the isolated axons in the satellite axon/glia compartments, were found to physically damage the established axonal layer, as they tend to grow underneath the axons. In contrast, oligodendrocyte progenitor cells (OPCs) could be co-cultured successfully with the isolated axons and differentiated into mature myelin basic protein-expressing OLs with processes aligning to neighboring axons. OPCs inside the six axon/glia compartments were treated with a high concentration of ceramide (150 μM) to confirm the fluidic isolation among the satellite compartments. In addition, isolated axons were treated with varying concentrations of chondroitin sulfate proteoglycan (CSPG, 0-25 μg ml(-1)) within a single device to demonstrate the parallel localized biomolecular treatment capability of the device. These results indicate that the proposed platform can be used as a powerful tool to study CNS axonal biology and axon-glia interactions with the capacity for localized biomolecular treatments.

Drosophila melanogaster as a model system for human brain cancers

Glioblastomas (GBM), the most common primary brain tumors, infiltrate the brain, grow rapidly, and are refractory to current therapies. Signature genetic lesions in glioblastomas include mutation of the epidermal growth factor receptor tyrosine kinase (EGFR) receptor tyrosine kinase and activating mutations in components of the PI-3 kinase (PI3K) pathway. Despite years of study, how these pathways specifically regulate glial pathogenesis is unclear. To address the genetic and cellular origins of this disease, a novel Drosophila GBM model has been developed in which glial progenitor cells give rise to proliferative and invasive neoplastic cells that create transplantable tumors in response to constitutive co-activation of the EGFR-Ras and PI3K pathways. Standing with a rich literature demonstrating the direct relevance of Drosophila to studies on human cancer, neurological disease, and neurodevelopment, this model represents a robust cell-type specific Drosophila neurological disease model in which malignant cells are created by mutations in genetic pathways thought to be driving forces in a homologous human disease. Using lineage analysis and cell-type specific markers, neoplastic glial cells were found to originate from committed glial progenitor cells, rather than from multipotent neuroblasts. Genetic analyses demonstrated that EGFR-Ras and PI3K induce fly glial neoplasia through activation of a combinatorial genetic network composed, in part, of other genetic pathways also commonly mutated in human glioblastomas. In the future, large-scale forward genetic screens with this model may reveal new insights into the origins and treatments of human glioblastoma.

Tyrosine phosphatases Shp1 and Shp2 have unique and opposing roles in oligodendrocyte development

Oligodendrocyte progenitor cells first proliferate to generate sufficient cell numbers and then differentiate into myelin-producing oligodendrocytes. The signal transduction mediators that underlie these events, however, remain poorly understood. The tyrosine phosphatase Shp1 has been linked to oligodendrocyte differentiation as Shp1-deficient mice show hypomyelination. The Shp1 homolog, Shp2, has recently been shown to regulate astrogliogenesis, but its role in oligodendrocyte development remains unknown. Here, we report that Shp2 protein levels were developmentally regulated in oligodendrocytes, with Shp2 phosphorylation being promoted by oligodendroglial mitogens but suppressed by laminin, an extracellular matrix protein that promotes oligodendroglial differentiation. In contrast, oligodendrocyte progenitors were found to be unresponsive to mitogens following Shp2, but not Shp1, depletion. In agreement with previous studies, Shp1 depletion led to decreased levels of myelin basic protein in differentiating oligodendrocytes, as well as reduced outgrowth of myelin membrane sheets. Shp2 depletion in contrast did not prevent oligodendrocyte differentiation but promoted expanded myelin membrane outgrowth. Taken together these data suggest that Shp1 and Shp2 have distinct functions in oligodendrocyte development: Shp2 regulates oligodendrocyte progenitor proliferation and Shp1 regulates oligodendrocyte differentiation. Adhesion to laminin may additionally provide extrinsic regulation of Shp2 activity and thus promote the transition from progenitor to differentiating oligodendrocyte.

Glial-neuronal interactions--implications for plasticity and drug addiction

Among neuroscientists, astrocytes have for long played Cinderella to their neuron stepsisters. While the importance of glia in regulating brain activity was predicted by Ramon y Cajal more than a century ago (Garcia-Marin et al., Trends. Neurosci. 30:479-787, 2007), these cells, until recently, have been thought to play mainly a passive part in synaptic signaling. Results obtained over the last decade have begun to suggest otherwise. Experiments carried out in a number of labs have shown that glial cells, especially astrocytes, directly participate in synaptic signaling and potentially regulate synaptic plasticity and network excitability. The presence of signaling pathways on astrocytes that are analogous to those at presynaptic terminals suggests a role for these cells in network plasticity. Findings that the same signaling pathways can be activated by receptors for drugs of abuse present on astrocytes suggest a role for these cells in the addictive process. In this review, we summarize current understanding of astrocytic role in synaptic signaling and suggest that a complete understanding of the process of addiction requires a better understanding of the functional role of these cells.

Wrapping it up: the cell biology of myelination

During nervous system development, oligodendroglia in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) synthesise large amounts of specific proteins and lipids to generate myelin, a specialised membrane that spirally ensheathes axons and facilitates fast conduction of the action potential. Myelination is initiated after glial processes have attached to the axon and polarisation of the plasma membrane has been triggered. Myelin assembly is a multi-step process that occurs in spatially distinct regions of the cell. We propose that assembly of myelin proteins and lipids starts during their transport through the biosynthetic pathway and continues at the plasma membrane aided by myelin-basic protein (MBP). These sequential processes create the special lipid and protein composition necessary for myelin to perform its insulating function during nerve conduction.

Basement membrane attachment is dispensable for radial glial cell fate and for proliferation, but affects positioning of neuronal subtypes

Radial glial cells have been shown to act as neuronal precursors in the developing cortex and to maintain their radial processes attached to the basement membrane (BM) during cell division. Here, we examined a potential role of direct signalling from the BM to radial glial cells in three mouse mutants where radial glia attachment to the BM is disrupted. This is the case if the nidogen-binding site of the laminin gamma1 chain is mutated, in the absence of alpha6 integrin or of perlecan, an essential BM component. Surprisingly, cortical radial glial cells lacking contact to the BM were not affected in their proliferation, interkinetic nuclear migration, orientation of cell division and neurogenesis. Only a small subset of precursors was located ectopically within the cortical parenchyma. Notably, however, neuronal subtype composition was severely disturbed at late developmental stages (E18) in the cortex of the laminin gamma1III4-/- mice. Thus, although BM attachment seems dispensable for precursor cells, an intact BM is required for adequate neuronal composition of the cerebral cortex.

Microglia: a newly discovered role in visceral hypersensitivity?

Given the growing body of evidence for a role of glia in pain modulation, it is plausible that the exaggerated visceral pain in chronic conditions might be regulated by glial activation. In this study, we have investigated a possible role for microglia in rats with chronic visceral hypersensitivity and previously documented altered neuronal function. Experiments were performed on adult male Sprague-Dawley rats pre-treated with neonatal colon irritation (CI) and on control rats. Effects of fractalkine (FKN, a chemokine involved in neuron-to-microglia signaling) and of minocycline (an inhibitor of microglia) on visceral sensitivity were examined. Visceral sensitivity was assessed by recording the electromyographic (EMG) responses to graded colorectal distension (CRD) in mildly sedated rats. Responses to CRD were recorded before and after injection of FKN, minocycline or vehicle. Somatic thermal hyperalgesia was measured by latency of paw withdrawal to radiant heat. The pattern and intensity of microglial distribution at L6-S2 in the spinal cord was also compared in rats with CI and controls by fluorescence microscopy using OX-42. Results show that: (1) FKN significantly facilitated EMG responses to noxious CRD by >52% in control rats. FKN also induced thermal hyperalgesia in control rats, consistent with previous reports; (2) minocycline significantly inhibited EMG responses to noxious CRD by >70% in rats with CI compared to controls 60 min after injection. The anti-nociceptive effect of minocycline lasted for 180 min in rats with CI, reaching peak values 60 min after injection. Our results show that FKN enhances visceral and somatic nociception, whereas minocycline inhibits visceral hypersensitivity in chronically sensitized rats, which indicates a role for microglia in visceral hypersensitivity.

Basal lamina: Schwann cells wrap to the rhythm of space-time

Schwann cells form myelin in the peripheral nervous system. All Schwann cells are surrounded by a basal lamina. Extracellular matrix molecules in the basal lamina, such as laminin, regulate key aspects of Schwann cell development including the formation, architecture and function of myelin. Recent genetic and cell biological experiments suggest that Schwann cells regulate the basal lamina and its receptors in both time and space, resulting in differential functions. These findings have important implications for diseases resulting from laminin dysfunction, such as congenital muscular dystrophy 1A.

Fc receptor-positive cells in remyelinating multiple sclerosis lesions

The capacity for spontaneous remyelination in cases of multiple sclerosis (MS) is limited and lesions are not fully repaired. Recent evidence has shown that oligodendrocyte precursor cells and immature oligodendrocytes (OPC/iOligs) are preserved in MS lesions. Induced differentiation of these cells into myelinating cells may ultimately lead to a novel remyelination therapy. A previous study showed that the gamma chain of immunoglobulin Fc receptors (FcRgamma), expressed in OPC/iOligs, is essential for their differentiation. Whether FcRgamma is expressed in preserved OPC/iOligs within MS lesions, however, remains uncertain. In the present study, we examined 10 autopsy cases of MS for the expression of FcRgamma both in remyelinating areas and demyelinated plaques. The expression of FcRgamma was confirmed in both OPC/iOligs and microglia in MS lesions. Statistical analysis showed that the density of FcRgamma-positive OPC/iOligs was approximately 3 times greater in remyelinating areas compared with demyelinated plaques; the opposite was true of FcRgamma-positive microglia. The distribution of FcRgamma-negative OPC/iOligs did not differ between the 2 types of lesions. Thus, an increase in FcRgamma-positive OPC/iOligs and a decrease in FcRgamma-positive microglia, but not in FcRgamma-negative OPC/iOligs, are associated with spontaneous remyelination in MS brains, suggesting a possible role for FcRgamma in the induction of remyelination.

Mechanisms of axon ensheathment and myelin growth

The evolution of complex nervous systems in vertebrates has been accompanied by, and probably dependent on, the acquisition of the myelin sheath. Although there has been substantial progress in our understanding of the factors that determine glial cell fate, much less is known about the cellular mechanisms that determine how the myelin sheath is extended and stabilized around axons. This review highlights four crucial stages of myelination, namely, the selection of axons and initiation of cell-cell interactions between them and glial cells, the establishment of stable intercellular contact and assembly of the nodes of Ranvier, regulation of myelin thickness and, finally, longitudinal extension of myelin segments in response to the lengthening of axons during postnatal growth.

Involvement of CD45 in central nervous system myelination

Myelin is a multi-layered membranous lipid insulator surrounding axons that allows the rapid conduction of neuronal impulses. In the central nervous system (CNS), myelin is produced by oligodendrocytes. During development, morphologically immature oligodendrocyte precursor cells (OPCs) arise from neural stem cells before differentiating into myelinating oligodendrocytes shortly after birth. Fyn tyrosine kinase (Fyn) has been shown to play a central role during OPC differentiation, including inducing morphological changes in the cells and initiating the expression of myelin basic protein (MBP), a major structural protein required for the compaction of myelin sheaths. Recently, we have shown that signaling via the gamma chain of immunoglobulin Fc receptors (FcRgamma) induces the Fyn-MBP cascade and promotes the morphological differentiation of OPCs. The protein tyrosine phosphatases that are responsible for the positive regulation of Fyn tyrosine kinase activity during this cascade, however, remained unknown. Here we report that a protein tyrosine phosphatase, CD45, is involved in this process. Fyn co-immunoprecipitated with CD45 from differentiating wild-type OPCs in vitro, while CD45-deficient OPCs failed to differentiate. Additionally, dysmyelination was observed in CD45-deficient mice in vivo. Our findings suggest that CD45 is a key phosphatase involved in OPC differentiation and provide a preliminary explanation for the previously reported CD45 mutations observed in some multiple sclerosis (MS) patients.

Protection of p27Kip1 mRNA by quaking RNA binding proteins promotes oligodendrocyte differentiation

The quaking (Qk) locus expresses a family of RNA binding proteins, and the expression of several alternatively spliced isoforms coincides with the development of oligodendrocytes and the onset of myelination. Quaking viable (Qk(v)) mice harboring an autosomal recessive mutation in this locus have uncompacted myelin in the central nervous system owing to the inability of oligodendrocytes to properly mature. Here we show that the expression of two QKI isoforms, absent from oligodendrocytes of Qk(v) mice, induces cell cycle arrest of primary rat oligodendrocyte progenitor cells and differentiation into oligodendrocytes. Injection of retroviruses expressing QKI into the telencephalon of mouse embryos induced differentiation and migration of multipotential neural progenitor cells into mature oligodendrocytes localized in the corpus callosum. The mRNA encoding the cyclin-dependent kinase (CDK)-inhibitor p27(Kip1) was bound and stabilized by QKI, leading to an increased accumulation of p27(Kip1) protein in oligodendrocytes. Our findings demonstrate that QKI is upstream of p27(Kip1) during oligodendrocyte differentiation.