O-2A progenitors of the mouse optic nerve exhibit a developmental pattern of antigen expression different from the rat

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.

Adipose tissue-derived stromal cells (ADSC) express oligodendrocyte and myelin markers, but they do not function as oligodendrocytes

Mesenchymal cells cultured from the vasculo-stromal fraction of adipose tissue (ADSC) show adult stem cell characteristics and several groups have claimed generating neural cells from them. However, we have observed that many markers commonly used for the identification of neural cells are spontaneously expressed by ADSC in culture. In the present study, we have examined the expression of characteristic oligodendrocyte molecules in cultured ADSC, aiming to test if myelinating cells could be generated from accessible non-neural adult tissues. In basal growth conditions, rat ADSC spontaneously expressed CNPase, MBP, MOG, protein zero, GAP43, Sox10, and Olig2, as shown by immunocytrochemistry and western blot. A small population of cultured ADSC expressed membrane galactocerebroside (O1 antibody), but no cell stained with O4 antibody. RT-PCR analyses showed the expression of CNPase, MBP, DM20, and low levels of Olig2, Sox10, and Sox2 mRNA by rat ADSC. When rat ADSC were treated with combinations of factors commonly used in neural-inducing media (retinoic acid, dbcAMP, EGF, basic FGF, NT3, and/or PDGF), the number of O1-positive cells changed, but in no case, mRNA expression of Sox10 and Olig2 transcription factors approached CNS oligodendrocyte levels. In co-culture with rat dorsal root ganglion neurons, no sign of axonal myelination by rat ADSC was observed. These studies show that the expression of oligodendrocyte traits by cultured ADSC is not a proof of functional competence as oligodendroglia and suggest that in culture conditions, ADSC acquire intermediate, uncommitted phenotypes.

Oligodendrocyte Development and Plasticity

Oligodendrocyte precursor cells (OPCs) originate in the ventricular zones (VZs) of the brain and spinal cord and migrate throughout the developing central nervous system (CNS) before differentiating into myelinating oligodendrocytes (OLs). It is not known whether OPCs or OLs from different parts of the VZ are functionally distinct. OPCs persist in the postnatal CNS, where they continue to divide and generate myelinating OLs at a decreasing rate throughout adult life in rodents. Adult OPCs respond to injury or disease by accelerating their cell cycle and increasing production of OLs to replace lost myelin. They also form synapses with unmyelinated axons and respond to electrical activity in those axons by generating more OLs and myelin locally. This experience-dependent "adaptive" myelination is important in some forms of plasticity and learning, for example, motor learning. We review the control of OL lineage development, including OL population dynamics and adaptive myelination in the adult CNS.

Finding degrees of separation: experimental approaches for astroglial and oligodendroglial cell isolation and genetic targeting

The study of CNS glial cell function requires experimental methods to detect, purify, and manipulate each cell population with fidelity and specificity. With the identification and cloning of cell- and stage-specific markers, glial cell analysis techniques have grown beyond physical methods of tissue dissociation and cell culture, and become highly specific with immunoselection of cell cultures in vitro and genetic targeting in vivo. The unique plasticity of glial cells offers the potential for cell replacement therapies in neurological disease that utilize neural cells derived from transplanted neural stem and progenitor cells. In this mini-review, we outline general physical and genetic approaches for macroglial cell generation. We summarize cell culture methods to obtain astrocytes and oligodendrocytes and their precursors, from developing and adult tissue, as well as approaches to obtain human neural progenitor cells through the establishment of stem cells. We discuss popular targeting rodent strains designed for cell-specific detection, selection and manipulation of neuroglial cell progenitors and their committed progeny. Based on shared markers between astrocytes and stem cells, we discuss genetically modified mouse strains with overlapping expression, and highlight SOX-expressing strains available for targeting of stem and progenitor cell populations. We also include recently established mouse strains for detection, and tag-assisted RNA and miRNA analysis. This discussion aims to provide a brief overview of the rapidly expanding collection of experimental approaches and genetic resources for the isolation and targeting of macroglial cells, their sources, progeny and gene products to facilitate our understanding of their properties and potential application in pathology.

Phenotype, differentiation, and function differ in rat and mouse neocortical astrocytes cultured under the same conditions

The study of slowly progressing brain diseases in which glial cells play a pathogenic role requires astrocytes that have been cultured for several weeks. We characterized neocortical astrocytes, grown for up to 42 days in vitro (DIV), from newborn rats and mice by indirect immunofluorescence technique, Western blot, and real-time RT-PCR analyses. We obtained highly enriched rat and mouse astrocyte cultures, where most cells were positively stained for the astrocyte markers GFAP, vimentin, and S100β, whereas neuronal and oligodendrocyte markers were undetectable. The protein and mRNA levels of GFAP, vimentin, and nestin were higher in rat than in mouse astrocytes. From 28 to 42 DIV, the levels of vimentin and nestin, but not of GFAP, decreased in both species, with an increase in the vimentin-GFAP ratio of 1.7 for rat, and of 0.9 for mouse astrocytes suggesting that the rat cultures were more differentiated than the mouse cultures, although both remained partially immature. The protoplasmic appearance of the cells, the negative A2B5 immunoreactivity, and the expression of the glutamate transporters GLAST and GLT-1 indicate that the rat and mouse cultures contained mainly type I astrocytes. The protein levels of GLAST and GLT-1 decreased from 28 to 42 DIV in the mouse, but not in the rat astrocytes, suggesting that the rat cultures are suitable for functional studies. Thus, under the same culture conditions, astrocyte cultures from rats and mice differ in phenotype, differentiation, and functionality. This finding should be taken into account when long-lasting glial reaction patterns are being studied.

Immunopanning selection of A2B5-positive cells increased the differentiation efficiency of induced pluripotent stem cells into oligodendrocytes

Oligodendrocytes are the myelinating cells of the central nervous system (CNS), and defects in these cells can result in CNS dysfunction. Although oligodendrocyte precursor cell (OPC) transplantation therapy is an effective cure for several disorders, there is no readily available source of these cells. Recent studies have described the generation of induced pluripotent stem cell (iPSC) from somatic cells, leading to speculation that this technique might become a novel therapeutic tool in regenerative medicine. In a previous study, we were able to produce O4 positive (O4(+)) oligodendrocytes from mouse iPSC in vitro. Unfortunately, the efficiency of differentiation achieved was relatively low (2.3%). In the current study, we improved the differentiation efficiency using a mouse monoclonal antibody (A2B5) to select cells of oligodendrocyte lineage. During in vitro differentiation, we purified A2B5-positive (A2B5(+)) cells by immunopanning from a mixed culture of iPSC-derived cells. This procedure increased the differentiation efficiency of O4(+) oligodendrocytes to 43.5%. We also examined the expression of myelin basic protein (MBP), a marker of mature oligodendrocytes. After 21 days of terminal differentiation, 62.3% of iPSC-derived O4(+) oligodendrocytes expressed MBP.

Differing in vitro survival dependency of mouse and rat NG2+ oligodendroglial progenitor cells

NG2 chondroitin sulfate proteoglycan is a surface marker of oligodendroglial progenitor cells (OPCs) in various species. In contrast to well-studied rat OPCs, however, we found that purified mouse NG2 surface positive cells (NG2(+) cells) require additional activation of cyclic AMP (cAMP) signaling for survival in a medium containing 30% B104 neuroblastoma conditioned medium supplemented with fibroblast growth factor-2 (B104CM+FGF2), whereas B104CM+FGF2 alone is sufficient for survival and selective proliferation of rat OPCs. After induction of in vitro differentiation, more than 90% of mouse NG2(+) cells became O4-positive, and a majority expressed myelin basic protein by 5 day of differentiation, which confirmed the identity of isolated mouse NG2(+) cells as OPCs. In comparison to rat OPCs, mouse OPCs in B104CM+FGF2 were less motile, and demonstrated lower basal phosphorylation levels of ERK1/2 and cAMP response element-binding protein (CREB) and a higher incidence of apoptosis mediated by the intrinsic pathway. Transient up-regulation of cAMP-CREB signaling partially inhibited apoptosis of mouse OPCs independently of the ERK pathway. This study demonstrates a difference in trophic requirements between mouse and rat OPCs, with an essential role for cAMP signaling to preserve viability of mouse OPCs.

Theiler's murine encephalomyelitis virus preferentially infects immature stages of the murine oligodendrocyte precursor cell line BO-1 and blocks oligodendrocytic differentiation in vitro

Theiler's murine encephalomyelitis virus (TMEV)-induced demyelination is an important animal model for multiple sclerosis. The presence of oligodendrocyte precursor cells (OPCs) within demyelinated lesions together with the limited extent of remyelination has raised the question of how OPCs are affected by TMEV. It is well established that oligodendrocytes, astrocytes and microglia are targets during the chronic phase of the disease. However, whether TMEV infection interferes with the capacity of OPCs to generate oligodendrocytes has remained unclear. In the present study, a bipotential murine OPC cell line termed BO-1 was used to determine the antigenic phenotype susceptible to TMEV and the impact of TMEV infection upon cell differentiation. We show here that retinoic acid increased oligodendrocytic differentiation and decreased proliferation and TMEV infection rates. TMEV under serum-free conditions infected about 75% and 60% of early OPCs (NG2(+) and A2B5(+)) and immature oligodendrocytes (CNPase(+)), respectively, but only approximately 18% of mature oligodendrocytes (MBP(+)). Infection with TMEV prior to application of retinoic acid significantly reduced the percentage of MBP(+) BO-1 cells. These data demonstrate that TMEV preferentially infects early stages of the oligodendrocytic lineage and blocks oligodendrocyte maturation. The first demonstration of TMEV-mediated effects on OPC differentiation may shed new light on the pathogenesis of TMEV-induced demyelination and offers an explanation for the limited remyelination observed in vivo.

Maintenance of the relative proportion of oligodendrocytes to axons even in the absence of BAX and BAK

Highly purified oligodendroglial lineage cells from mice lacking functional bax and bak genes were resistant to apoptosis after in-vitro differentiation, indicating an essential role of the intrinsic apoptotic pathway in apoptosis of oligodendrocytes in the absence of neurons (axons) and other glial cells. These mice therefore provide a valuable tool with which to evaluate the significance of the intrinsic apoptotic pathway in regulating the population sizes of oligodendrocytes and oligodendroglial progenitor cells. Quantitative analysis of the optic nerves and the dorsal columns of the spinal cord revealed that the absolute numbers of mature oligodendrocytes immunolabeled for aspartoacylase and adult glial progenitor cells expressing NG2 chondroitin sulfate proteoglycan were increased in both white matter tracts of adult bax/bak-deficient mice and, to a lesser extent, bax-deficient mice, except that there was no increase in NG2-positive progenitor cells in the dorsal columns of these strains of mutant mice. These increases in mature oligodendrocytes and progenitor cells in bax/bak-deficient mice were unexpectedly proportional to increases in numbers of axons in these white matter tracts, thus retaining the oligodendroglial lineage to axon ratios of at most 1.3-fold of the physiological numbers. This is in contrast to the prominent expansion in numbers of neural precursor cells in the subventricular zones of these adult mutant mice. Our study indicates that homeostatic control of cell number is different for progenitors of the oligodendroglial and neuronal lineages. Furthermore, regulatory mechanism(s) operating in addition to apoptotic elimination through the intrinsic pathway, appear to prevent the overproduction of highly mitotic oligodendroglial progenitor cells.

Protein-tyrosine phosphatase alpha acts as an upstream regulator of Fyn signaling to promote oligodendrocyte differentiation and myelination

The tyrosine kinase Fyn plays a key role in oligodendrocyte differentiation and myelination in the central nervous system, but the molecules responsible for regulating Fyn activation in these processes remain poorly defined. Here we show that receptor-like protein-tyrosine phosphatase alpha (PTPalpha) is an important positive regulator of Fyn activation and signaling that is required for the differentiation of oligodendrocyte progenitor cells (OPCs). PTPalpha is expressed in OPCs and is up-regulated during differentiation. We used two model systems to investigate the role of PTPalpha in OPC differentiation: the rat CG4 cell line where PTPalpha expression was silenced by small interfering RNA, and oligosphere-derived primary OPCs isolated from wild-type and PTPalpha-null mouse embryos. In both cell systems, the ablation of PTPalpha inhibited differentiation and morphological changes that accompany this process. Although Fyn was activated upon induction of differentiation, the level of activation was severely reduced in cells lacking PTPalpha, as was the activation of Fyn effector molecules focal adhesion kinase, Rac1, and Cdc42, and inactivation of Rho. Interestingly, another downstream effector of Fyn, p190RhoGAP, which is responsible for Rho inactivation during differentiation, was not affected by PTPalpha ablation. In vivo studies revealed defective myelination in the PTPalpha(-/-) mouse brain. Together, our findings demonstrate that PTPalpha is a critical regulator of Fyn activation and of specific Fyn signaling events during differentiation, and is essential for promoting OPC differentiation and central nervous system myelination.

A novel role for anosmin-1 in the adhesion and migration of oligodendrocyte precursors

At embryonic stages of development, oligodendrocyte precursors (OPCs) generated in the preoptic area colonize the entire optic nerve (ON). Different factors controlling migration of ON OPCs have been identified, including secreted growth factors, morphogens and guidance cues, as well as cell adhesion molecules. We have shown previously that the soluble form of the extracellular matrix (ECM) protein anosmin-1, impairs OPC migration induced by FGF-2. In the present work, we show that anosmin-1 is expressed by both migrating OPCs and axons of the retinal ganglion cells in the embryonic ON. In vitro, we observe that OPC migration is strongly impaired by contact with anosmin-1 when used as a substrate and, in contrast to previous results, this effect is independent of FGF-2/FGFR1 signaling. We also show that OPCs preferentially adhere to anosmin-1 when compared with other ECM molecules used as substrates, and that when the endogenous anosmin-1 expressed by OPCs is blocked, OPC adhesion to all the different substrates (including anosmin-1), is significantly reduced. This novel effect of anosmin-1 on cell adhesion is also independent of FGF-2/FGFR1. We finally demonstrate that the blockade of the endogenous anosmin-1 expressed by OPCs impairs their migration. Our data suggest that the endogenous anosmin-1 expressed by OPCs is necessary for the correct adhesion of these cells to the different components of the ECM (including anosmin-1 itself), contributing to the migration of these cells.

In vitro characterization of a murine oligodendrocyte precursor cell line (BO-1) following spontaneous immortalization

The understanding of oligodendrocyte differentiation is crucial for designing therapies of demyelinating diseases. Oligodendrocyte precursor cells are of particular interest in this context, because of their remyelinating potential. Permanent cell lines, which are a versatile tool for studying oligodendrocyte physiology, have been so far mainly established from the rat CNS. In the present study, we describe a novel murine oligodendrocyte precursor cell line (BO-1) established by spontaneous immortalization using light microscopy, immunocytochemical phenotyping and genetic analysis. BO-1 cells displayed a bi- to multipolar morphology and expressed early oligodendrocytic lineage markers, such as A2B5 and NG-2. Expression of pre-oligodendrocyte (O4, CNPase) and mature oligodendrocyte markers (e.g. myelin basic protein) was found in about 30% and 1.5% of the cells, respectively. Addition of serum, known to promote type-2 astrocyte differentiation, significantly increased the number of GFAP-positive cells, while thyroid hormones, (T3/T4) known to foster oligodendrocyte differentiation, did not substantially alter the antigenic and gene expression of myelin markers. This deficiency might be related to the high intrinsic proliferation rate of BO-1 cells that was unaltered upon removal of mitogenic factors. Expression of O4 and CNPase in BO-1 cells could be significantly increased by co-culture with primary astrocytes suggesting that the differentiating potential of BO-1 cells was influenced by environmental factors and may have to be fully explored in future studies. In summary, the novel murine BO-1 cell line shares several characteristics with oligodendrocyte precursor cells but displays a restricted differentiation into mature oligodendrocytes.

Sonic hedgehog promotes the migration and proliferation of optic nerve oligodendrocyte precursors

Optic nerve (ON) oligodendrocyte precursors (OPCs) are generated under the influence of the Sonic hedgehog (Shh) in the preoptic area from where they migrate to colonise the entire nerve. The molecular events that control this migration are still poorly understood. Recent studies suggested that Shh is often used by the same cell population to control different processes, including cell proliferation and migration, raising the possibility that Shh could contribute to these aspects of OPC development. In support of this idea, we show here that Shh induces the proliferation of OPCs derived from embryonic mouse ON explants and acts as a chemoattractant for their migration. In ovo injections of hybridomas secreting Shh-specific blocking antibody decreases the number of OPCs present in chick ONs, particularly in the retinal portion of the nerve. Altogether these data indicate that Shh contributes to OPC proliferation and distribution along the ON, in addition to their specification.

Isolation and culture of rat and mouse oligodendrocyte precursor cells

The ability to isolate oligodendroglial precursor cells (OPCs) provides a powerful means to characterize their differentiation, properties and potential for myelin repair. Although much knowledge is available for isolation of OPCs from the rat central nervous system, preparation and maintenance of mouse OPCs has been until recently a challenge owing to difficulties in obtaining a sufficient quantity of purified OPCs. Here, we describe protocols to prepare highly enriched rat OPCs and nearly homogenous mouse OPCs. The mouse method generates predominantly OPCs from cortical neural progenitor cells as clonal aggregates called "oligospheres" by taking advantage of molecular genetic tools. Isolated OPCs can be further differentiated into oligodendrocytes. Collectively, we describe simple and efficient methods for the preparation and in vitro maintenance of enriched OPCs from rats and mice. Isolation and culture of a large, homogenous population of rodent OPCs should significantly facilitate studies on OPC lineage progression and their utility in myelin repair after injury.

Downregulation of platelet-derived growth factor-alpha receptor-mediated tyrosine kinase activity as a cellular mechanism for K+-channel regulation during oligodendrocyte development in situ

Oligodendrocyte maturation has been defined based on expression of developmentally regulated antigens. However, transitions at early stages of the lineage have not been functionally characterized fully in situ. Combining 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP)-promoter driven enhanced green fluorescent protein expression and whole-cell capacitance measurements permitted a reliable distinction between subcortical white matter NG2+ oligodendrocyte progenitors (OPs) and O4+ preoligodendrocytes (pre-OLs) in situ. We focused on K+ channels because their expression has been associated previously with the proliferation and differentiation potential of OPs. Using whole-cell patch clamp, we observed a downregulation of the delayed outward-rectifying current (IKDR) between the NG2+ and O4+ stages but no significant changes in transient K+-channel current (IKA) amplitude. Tyrosine kinase inhibition in NG2+ cells reduced IKDR amplitude with no effect on IKA, which mimicked the endogenous changes observed between OPs and pre-OLs. Tyrosine kinase inhibition also reduced the proliferative capacity of NG2+ OPs in slice cultures. Conversely, acute platelet-derived growth factor receptor-alpha (PDGFR-alpha) activation caused an increase of IKDR in NG2+ but not in O4+ cells. Consistent with this finding, PDGFR-alpha immunoreactivity was confined to NG2+ cells with undetectable levels in O4+ cells, suggesting that PDGFR-alpha signaling is absent in pre-OLs in situ. Importantly, the PDGF-induced increase of IKDR in NG2+ cells was prevented by tyrosine kinase inhibition. Together, these data indicate that PDGFR-alpha and tyrosine kinase activity act via a common pathway that influences functional expression of K+ channels and proliferative capacity of OPs in situ.

Directional guidance of oligodendroglial migration by class 3 semaphorins and netrin-1

Oligodendrocytes, the myelin-forming cells of the CNS, are generated from multiple foci distributed along the developing neural tube. Little is known about the endogenous guidance cues controlling the migration of oligodendrocyte precursor cells (OPCs) from their site of emergence toward their final destination, mainly the future white matter tracts. During embryonic development, the optic nerve is populated by OPCs originating in the diencephalon that migrate from the chiasm toward the retina. Here we show that OPCs migrating into the embryonic optic nerve express the semaphorin receptors neuropilin-1 and -2, as well as deleted in colorectal cancer (DCC) and, to a lesser extend unc5H1, two of the netrin-1 receptors. Using a functional migration assay, we provide evidence that Sema 3A and netrin-1 exert opposite chemotactic effects, repulsive or attractive, respectively, on embryonic OPCs. In addition, we show that Sema 3F has a dual effect, chemoattractive and mitogenic on embryonic OPCs. The localization of cells expressing Sema 3A, Sema 3F, and netrin-1 is consistent with a role for these ligands in the migration of OPCs in the embryonic optic nerve. Altogether, our results suggest that the migration of OPCs in the embryonic optic nerve is modulated by a balance of effects mediated by members of the semaphorin and netrin families.

Negative regulation of oligodendrocyte differentiation by galactosphingolipids

Galactocerebroside and sulfatide, major galactosphingolipid components of oligodendrocyte plasma membranes and myelin, are first expressed at a critical point, when progenitors cease to proliferate and commence terminal differentiation. We showed previously that an antibody to galactocerebroside/sulfatide arrested terminal differentiation, suggesting a role for these galactolipids in oligodendrocyte differentiation. We have now investigated the differentiation of oligodendrocytes (1) in response to other anti-galactolipid antibodies, showing that anti-sulfatide O4 but not anti-galactocerebroside O1 blocks terminal differentiation, perhaps by mimicking an endogenous ligand, and (2) in a transgenic mouse unable to synthesize these lipids because of mutation of the gene for ceramide galactosyltransferase, a key enzyme for galactosphingolipid synthesis. We find that galactosyltransferase mRNA expression begins at the late progenitor [pro-oligodendroblast (Pro-OL)] stage of the lineage and that the late progenitor marker pro-oligodendroblast antigen is not synthesized in the absence of galactosyltransferase. The principal outcome of the elimination of these galactolipids is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo. Because the general pattern of differentiation and the level of progenitor proliferation and survival appear to be unaltered in the mutant cultures, we conclude that the increased number of oligodendrocytes is caused by an increased rate and probability of differentiation. In agreement with these two experimental approaches, we present a model in which galactosphingolipids (in particular galactocerebroside and/or sulfatide) act as sensors and/or transmitters of environmental information, interacting with endogenous ligands to function as negative regulators of oligodendrocyte differentiation, monitoring the timely progress of Pro-OLs into terminally differentiating, myelin-producing oligodendrocytes.

Heterogeneous expression of voltage-gated potassium channels of the shaker family (Kv1) in oligodendrocyte progenitors

Outwardly rectifying K(+) channels determine the membrane conductance and influence the proliferation rate of glial progenitor cells. To analyze the molecular identity and the functional role of K(+) channels in glial progenitors of mouse brain, expression of shaker-type Kv1 genes was studied at three levels: (1) presence of Kv1 mRNAs, (2) biosynthesis of channel proteins and (3) electrophysiological and pharmacological properties of K(+) currents. mRNA expression of Kv1.1 to Kv1.6 genes was studied by single-cell reverse transcription-mediated polymerase chain reaction (RT-PCR) using degenerate primers to amplify the six Kv1 transcripts. Most cells expressed several mRNA combinations simultaneously. In more than half of the cells, messages for Kv1.2, Kv1.5 and Kv1.6 were found, while Kv1.1, Kv1.3 and Kv1.4 were detected in only a minority of cells. In contrast, at the level of protein expression - employing immunocytochemistry with subtype-specific antibodies - Kv1. 2 and Kv1.3 were undetectable (<2%), while almost all cells expressed Kv1.4 (85%), Kv1.5 (99%) and Kv1.6 (99%). Kv1.1 was present in a minor cell population (10%). Functional contribution of Kv1 proteins to progenitor membrane conductance was determined by analyzing the voltage-dependence of K(+) current activation and inactivation as well as their current sensitivities to the subtype-preferring blockers and toxins tetraethylammonium (TEA), 4-aminopyridine (4-AP), charybdotoxin (CTX), alpha-dendrotoxin (DTX) and mast-cell degranulating peptide (MCDP). From these results, it is concluded: first, glial progenitor cells can express all transcripts of the six Kv1 genes, but do not express all proteins; second, Kv1.4, Kv1.5 and Kv1.6 proteins are most abundant and were found in the majority of cells; and third, K(+) currents flow predominantly either through heteromeric channel complexes or through homomeric Kv1.5 ion pores, but not through homomeric Kv1.4 or Kv1.6 channels.

Atypical glial cells in demyelinated and hypomyelinated mouse brains

Bis-cyclohexanone oxalyldihydrazone (cuprizone) was administered to young adult mice in order to investigate the hypothesis that the differentiation of oligodendrocyte progenitors or precursors, or of immature oligodendrocytes, might be interrupted during cuprizone intoxication. Vibratome sections were prepared from brains from control mice, mice that were fed cuprizone for 27 days and mice that were fed cuprizone for 25 days, followed by normal diet for 2 days, and the sections were immunostained with monoclonal antibodies: MAbO4, which is directed against galactocerebroside sulfate (sulfatide); and RMAb, which is directed against galactocerebroside (GC). Process-bearing RMAb+/O4- cells were abundant in the brains of mice that had consumed cuprizone for 27 days, and the numbers of O4-positive cells were subnormal. Two days after refeeding the normal diet the RMAb+/O4- structures were less abundant and O4-positive cells more numerous. Moreover, the numbers of O4-positive cells were only approximately 20% of normal in the brains of hypomyelinating jimpy mutant mice, while the numbers of RMAb-positive cells were approximately 80% of normal, and the processes of the latter were associated with axons. It is suggested that RMAb+/O4- cells in the affected brains may be process-bearing oligodendrocyte precursors containing unsulfated GC or a toxic galactolipid.

Voltage-activated K+ channels and membrane depolarization regulate accumulation of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(CIP1) in glial progenitor cells

Neural cell development is regulated by membrane ion channel activity. We have previously demonstrated that cell membrane depolarization with veratridine or blockage of K+ channels with tetraethylammonium (TEA) inhibit oligodendrocyte progenitor (OP) proliferation and differentiation (); however the molecular events involved are largely unknown. Here we show that forskolin (FSK) and its derivative dideoxyforskolin (DFSK) block K+ channels in OPs and inhibit cell proliferation. The antiproliferative effects of TEA, FSK, DFSK, and veratridine were attributable to OP cell cycle arrest in G1 phase. In fact, (1) cyclin D accumulation in synchronized OP cells was not affected by K+ channel blockers or veratridine; (2) these agents prevented OP cell proliferation only if present during G1 phase; and (3) G1 blockers, such as rapamycin and deferoxamine, mimicked the anti-proliferative effects of K+ channel blockers. DFSK also prevented OP differentiation, whereas FSK had no effect. Blockage of K+ channels and membrane depolarization also caused accumulation of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(CIP1) in OP cells. The antiproliferative effects of K+ channel blockers and veratridine were still present in OP cells isolated from INK4a-/- mice, lacking the cyclin-dependent kinase inhibitors p16(INK4a) and p19(ARF). Our results demonstrate that blockage of K+ channels and cell depolarization induce G1 arrest in the OP cell cycle through a mechanism that may involve p27(Kip1) and p21(CIP1) and further support the conclusion that OP cell cycle arrest and differentiation are two uncoupled events.

Insulin-like growth factor I protects oligodendrocytes from tumor necrosis factor-alpha-induced injury

Tumor necrosis factor-alpha (TNF-alpha) has been causally implicated in several demyelinating disorders, including multiple sclerosis. Because insulin-like growth factor I (IGF-I) is a potent stimulator of myelination, we investigated whether it can protect oligodendrocytes and myelination from TNF-alpha-induced damage using mouse glial cultures as a model. Compared with controls, TNF-alpha decreased oligodendrocyte number by approximately 40% and doubled the number of apoptotic oligodendrocytes and their precursors. Addition of Boc-aspartyl(Ome)-fluoromethyl ketone (BAF), an inhibitor of interleukin-1beta converting enzyme (ICE)/caspase proteases, blocked TNF-alpha-induced reductions in oligodendrocytes, indicating that the TNF-alpha-induced reduction in oligodendrocytes is, at least in part, due to apoptosis, and that ICE/caspases are one of TNF-alpha action mediators. Simultaneous addition of IGF-I to TNF-alpha-treated cultures negated these TNF-alpha effects nearly completely. Furthermore, IGF-I promoted oligodendrocyte precursor proliferation and/or differentiation in TNF-alpha-treated cultures. To analyze TNF-alpha and IGF-I actions on oligodendrocyte function, we measured the abundance of messenger RNAs (mRNAs) for two major myelin-specific proteins, myelin basic protein (MBP) and proteolipid protein (PLP). While TNF-alpha decreased MBP and PLP mRNA abundance by 5- to 6-fold, IGF-I abrogated TNF-alpha-induced reductions in a dose- and time-dependent manner. The changes in MBP and PLP mRNA abundance could not be completely explained by the changes in oligodendrocyte number, indicating that myelin protein gene expression is regulated by both TNF-alpha and IGF-I. These data support the hypothesis that TNF-alpha can mediate oligodendrocyte and myelin damage, and indicate that IGF-I protects oligodendrocytes from TNF-alpha insults by blocking TNF-alpha-induced apoptosis, and by promoting oligodendrocyte and precursor proliferation/differentiation and myelin protein gene expression.

Neurotransmitter receptor activation triggers p27(Kip1)and p21(CIP1) accumulation and G1 cell cycle arrest in oligodendrocyte progenitors

We examined the pathways that link neurotransmitter receptor activation and cell cycle arrest in oligodendrocyte progenitors. We had previously demonstrated that glutamate receptor activation inhibits oligodendrocyte progenitor proliferation and lineage progression. Here, using purified oligodendrocyte progenitors and cerebellar slice cultures, we show that norepinephrine and the beta-adrenergic receptor agonist isoproterenol also inhibited the proliferation, but in contrast to glutamate, isoproterenol stimulated progenitor lineage progression, as determined by O4 and O1 antibody staining. This antiproliferative effect was specifically attributable to a beta-adrenoceptor-mediated increase in cyclic adenosine monophosphate, since analogs of this cyclic nucleotide mimicked the effects of isoproterenol on oligodendrocyte progenitor proliferation, while alpha-adrenoceptor agonists were ineffective. Despite the opposite effects on lineage progression, both isoproterenol and the glutamate receptor agonist kainate caused accumulation of the cyclin-dependent kinase inhibitors p27(Kip1)and p21(CIP1), and G1 arrest. Studies with oligodendrocyte progenitor cells from INK4a−/− mice indicated that the G1 cyclin kinase inhibitor p16(INK4a) as well as p19(ARF)were not required for agonist-stimulated proliferation arrest. Our results demonstrate that beta-adrenergic and glutamatergic receptor activation inhibit oligodendrocyte progenitor proliferation through a mechanism that may involve p27(Kip1) and p21(CIP1); but while neurotransmitter-induced accumulation of p27(Kip1) is associated with cell cycle arrest, it does not by itself promote oligodendrocyte progenitor differentiation.

Isolation and characterization of immature oligodendrocyte lineage cells

Using in vitro systems, the proliferation, migration, differentiation, and survival of immature oligodendrocyte lineage cells can be examined to elucidate the cellular and molecular interactions that regulate this lineage. The ability to monitor progressive stages of differentiation within the lineage by immunophenotyping and to manipulate the cellular responses with growth factors makes these cultures advantageous as both a method for studying the cell biology of myelination and as a model system for lineage analysis in the mammalian central nervous system. In addition, cultured oligodendrocytes carry out the normal in vivo sequence of expression of a set of cell type-specific genes, some of which are extremely highly expressed, and so provide advantages for analysis of gene regulation. This paper describes commonly used methods for the preparation of mixed glial cell cultures from perinatal rodent brain. Although these cultures are most commonly derived from perinatal rat brain, a protocol for preparation from mouse brain is also provided because of the increasing number of studies that use mice to facilitate molecular biological techniques. Methods to prepare secondary cultures of different stages of oligodendrocyte lineage cells are detailed. As examples of methods to use for the characterization of these cells, immunophenotypes of each stage of the oligodendrocyte lineage are illustrated, incorporation of [3H]thymidine for analysis of cell proliferation is illustrated, and detailed methods are provided for analysis of migration in a microchemotaxis chamber.

p27Kip1 alters the response of cells to mitogen and is part of a cell-intrinsic timer that arrests the cell cycle and initiates differentiation

BACKGROUND

In many vertebrate cell lineages, precursor cells divide a limited number of times before they arrest and terminally differentiate into postmitotic cells. It is not known what causes them to stop dividing. We have been studying the 'stopping' mechanism in the proliferating precursor cells that give rise to oligodendrocytes, the cells that make myelin in the central nervous system. We showed previously that the cyclin-dependent kinase inhibitor p27Kip1 (p27) progressively accumulates in cultured precursor cells as they proliferate and that the time course of the increase is consistent with the possibility that p27 accumulation is part of a cell-intrinsic timer that arrests the cell cycle and initiates differentiation at the appropriate time.

RESULTS

We now provide direct evidence that p27 is part of the intrinsic timer. We show that although p27-/- precursor cells stop dividing and differentiate almost as fast as wild-type cells when deprived of mitogen, when stimulated by saturating amounts of mitogen they have a normal cell-cycle time but tend to go through one or two more divisions than wild-type cells before they stop and differentiate. Cells that are p27+/- behave in an intermediate way, going through at most one extra division, indicating that the levels of p27 matter in the way the timer works. We also show that p27-/- precursor cells are more sensitive than wild-type cells to the mitogenic effect of platelet-derived growth factor.

CONCLUSIONS

These findings demonstrate that p27 is part of the normal timer that determines when oligodendrocyte precursor cells stop dividing and differentiate, at least in vitro. It seems likely that p27 plays a similar role in many other cell lineages, which could explain the phenotypes of the p27-/- and p27+/- mice.

Cytoskeletal and nuclear localization of myelin oligodendrocytic basic protein isoforms

The recently described single copy myelin-associated oligodendrocytic basic protein (Mobp) gene is expressed exclusively in the central nervous system (CNS). The gene encodes a family of small highly basic polypeptides with predicted amino acid lengths of 69, 71, 81, 99 and 170, all of which share a 68 residue amino terminal. Here we report on the subcellular distribution of two of these polypeptides termed MOBP81 and MOBP170 in transiently transfected Cos7 cells using an antibody raised against a region common to all isoforms of MOBP. Additionally, we describe MOBP trafficking in cultured mouse spinal cord oligodendrocytes. Immunostaining for MOBP81 is intense in the perinuclear region and extends throughout the cytoplasm colocalizing with the microtubular cytoskeletal network. Consistent with this we demonstrate that MOBP partitions with the cytoskeletal fraction prepared from myelin. In contrast, although MOBP170 is present in the cytoplasm it does not colocalize with the cytoskeleton and displays a greater variation in distribution. In the majority of transfectants immunostaining is present throughout the karyoplasm but with increased intensity around the nucleolus. Within mouse primary oligodendrocytes endogenous MOBP is present in the cell body and processes colocalizing with the microtubular network. Immunoreactivity is not detectable in the nucleus in these mature oligodendrocytes. These significant differences in MOBP81 and MOBP170 protein kinesis coupled to different expression profiles of their respective message populations may be indicative of both myelin structural and cellular/regulatory functions, respectively, for these polypeptides.

OlP-1, a novel protein that distinguishes early oligodendrocyte precursors

Oligodendrocyte development may be divided into three distinct stages: I) commitment of neuroectoderm cells to the oligodendrocyte lineage, II) migration of precursors into the surrounding parenchyma concomitant with increased proliferation, and III) cessation of migration and proliferation and initiation of myelination. Stage II of development has remained enigmatic because of the paucity of known molecules that distinguish these immature migratory cells. We describe a novel surface protein, termed OlP-1, which is restricted in expression to this developmental stage in the mouse. Cytofluorographic comparisons with known developmental markers showed OlP-1 to be expressed primarily by stage II precursors in vitro. Histologic analyses supported this conclusion by showing co-localization of OlP-1 with stage II molecules in vivo. Two conclusions were drawn from these results. First, OlP-1 was a novel protein expressed by murine oligodendrocyte precursors at a point in development that suggested a role in migration or proliferation. Second, dispersal of OlP-1-positive cells throughout the developing brain did not correlate with the location of myelination which, observed days later, progressed in a caudal to rostral manner. These data supported the concept that the final steps of maturation and myelin gene expression may be dependent upon extrinsic factors located predominantly within white matter tracts.

Characterization of a putative novel type of oligodendrocyte in cultures from rat spinal cord

Oligodendrocytes originate in different neural tube domains, within boundaries of expression of a series of patterning genes which condition the diverse morphogenetic programme of each area. Although neuronal and astrocyte heterogeneity are widely accepted, and despite accumulating evidence for oligodendrocyte heterogeneity in vivo, oligodendrocytes in vitro are currently considered as a homogeneous cell population. The present investigation demonstrates that oligodendrocyte diversity can be detected in vitro and characterizes a novel morphological class of O4-positive oligodendrocyte which is consistently identifiable in rat central nervous system cultures. These cells have a very characteristic epithelioid, unbranched and often lobulated morphology which enables their identification within 2 h of plating. Immunostaining shows that this morphological type is sometimes positive for GD3, A2B5 and vimentin, and most of the time positive for Ranscht antibody, O1 and Rip but negative for glial fibrillary acidic protein, OX-42, neuron-specific enolase, nestin and erbB2. The apparent levels and/or distributions of (i) microtubules, (ii) surface glycolipids recognized by O4, O1 and Ranscht antibody, and (iii) the less specific marker carbonic anhydrase II, typically differ from those of nearby classical, branched oligodendrocytes. Cells with this epithelioid morphology also express myelin basic protein and O10 (a proteolipid protein epitope), both of which are markers for mature oligodendrocytes. Conversely, O4+/O1- cells with this membranous appearance were also seen. Although these atypical oligodendrocytes were most abundant in spinal cord cultures (representing >10% of the O4+ population), they were not exclusive to this region and occurred at a low frequency in neonatal optic nerve cultures.

Oligodendrocytes direct glycosyl phosphatidylinositol-anchored proteins to the myelin sheath in glycosphingolipid-rich complexes

The myelin sheath synthesized by oligodendrocytes insulates central nervous system axons and is a specialized subdomain of the plasma membrane, containing a restricted pattern of proteins and lipids. Myelin is enriched in glycosphingolipids and cholesterol, a lipid environment favored by glycosylphosphatidylinositol (GPI)-anchored proteins, which associate with these lipids in detergent-insoluble complexes in many cell types. Since proteins regulating oligodendroglia-neuron interaction are largely unknown and GPI-anchored proteins are often involved in cell-cell interactions, we examined oligodendrocytes and myelin for their expression of these proteins. Oligodendrocyte precursors and maturing oligodendrocytes express a similar pattern of GPI-anchored proteins, which unlike the majority of oligodendrocyte plasma membrane proteins, accumulate in myelin. To elucidate mechanisms underlying the expression of GPI-anchored proteins in myelin, we analyzed detergent-insoluble complexes from cells and myelin using TX-100 extraction and sucrose density gradients. In precursor cells, the GPI-anchored proteins are not incorporated in detergent-insoluble complexes. In contrast, GPI-anchored proteins from maturing oligodendrocytes and from myelin were isolated as complexes associated with glycosphingolipids and cholesterol. These results show a specific association of GPI-anchored proteins with glycosphingolipids and cholesterol during oligodendrocyte maturation and suggest sorting of these macromolecular complexes to myelin.

Normal temporal and spatial distribution of oligodendrocyte progenitors in the myelin-deficient (md) rat

A point mutation in exon 3 of the proteolipid protein (PLP) gene of the myelin-deficient (md) rat leads to a failure of oligodendrocyte maturation and early death of oligodendrocytes, resulting in dysmyelination. It has been suggested that an alternative-splice isoform of PLP, known as DM-20, might be expressed in oligodendrocyte progenitors in the embryonic central nervous system (CNS), raising the possibility that early development of the oligodendrocyte lineage might also be affected in the md rat. To test this suggestion, we visualized oligodendrocyte progenitors in the embryonic md rat spinal cord and brain by in situ hybridization with a probe to the platelet-derived growth factor alpha receptor (PDGFR). We could detect no abnormalities in the time of first appearance of oligodendrocyte precursors, nor in their subsequent proliferation and dispersal throughout the CNS. These data strongly suggest that the PLP mutation in the md rat primarily or exclusively affects the later stages of oligodendrocyte lineage.

Evidence that some oligodendrocyte progenitors in the developing optic pathway express the plp gene

DM-20, a product of the proteolipid protein (plp) gene, has been demonstrated in the spinal cord of the mouse embryo as early as embryonic day 12 (E12) in certain cells, some of which are identifiable as oligodendrocyte progenitors. The present work uses optic pathways of rat and mouse as well-characterized systems for the study of gliogenesis. plp gene expression was monitored with a combination of reverse transcriptase polymerase chain reaction, in situ hybridization, and immunostaining with antibodies to different PLP peptide sequences, combined with O-2A lineage markers. In tissue sections, hybridizing cells were detected initially in the proximal optic tracts between E18 and birth and thereafter progressively in the chiasm and optic nerves. Small unbranched cells expressing DM-20 but not myelin basic protein (MBP) and probably representing progenitors were detectable by immunostaining in similar locations. With increasing postnatal ages, cells representing maturing oligodendrocytes which co-label for PLP and MBP are present in the optic pathways. In vitro analysis of freshly dissociated cells from premyelinated optic nerve demonstrated that the plp gene is expressed in some O-2A progenitor cells as well as mature oligodendrocytes. We also present evidence that increase in expression of the plp gene along the O-2A lineage differentiation is not progressive but that downregulation at the proligodendroblast (O4+/O1-) stage probably occurs. We suggest that progenitors express the dm-20 isoform while oligodendrocytes express predominantly the plp isoform. Not all progenitors express the plp gene at the times studied, indicating that the presence of DM-20 is either transitory in individual cells or that only a sub-population is involved. The function of DM-20 at this early stage of the oligodendrocyte lineage has yet to be determined.

Expression of glial antigens in mouse astrocytes: species differences and regulation in vitro

Expression of developmentally regulated antigens was used to characterize glial cells in cultures from embryonic mouse cerebral cortex. Over 90% of the cells had a flat morphology, and about 50% of these flat cells also expressed the ganglioside GD3. Up to 40% of all the GD3 expressing cells also expressed A2B5 antigen. Flat cells expressing either glial fibrillary acidic protein (GFAP), or GD3 or both were present at all times in vitro. These three populations of flat cells could not be further distinguished on the basis of NG2 or fibronectin expression, or with respect to their responses to the mitogens FGF-2, PDGF, or EGF. The glial cultures also contain a small number (approximately 5%) of process bearing cells with the morphological and immunocytochemical characteristics of oligodendrocyte precursors. The expression of GD3 by flat cells changed with time in culture as the fraction of flat cells expressing only GD3 declined and the fraction of cells expressing GFAP (with or without GD3) increased. The data are consistent with those flat cells expressing only GD3 being astrocyte precursors. Furthermore, between 1 and 3 weeks in vitro GD3/GFAP cells lose GD3 while retaining GFAP. Cells expressing only GFAP could be induced to express GD3 and A2B5 by treatment with FGF-2. The widespread and regulated expression of GD3 and A2B5 by murine glia is different from the restricted pattern of expression previously reported for these antigens in rat brain cell cultures. These results demonstrate that expression of GD3 and A2B5 by murine astrocytes depends on both culture age and extracellular signals and that these gangliosides are not markers for cell lineage in the mouse.