Identification and characterization of early glial progenitors using a transgenic selection strategy

In vitro segregation and isolation of human pluripotent stem cell-derived neural crest cells

The neural crest (NC) is a transient embryonic cell population with remarkable characteristics. After delaminating from the neural tube, NC cells (NCCs) migrate extensively, populate nearly every tissue of the body and differentiate into highly diverse cell types such as peripheral neurons and glia, but also mesenchymal cells including chondrocytes, osteocytes, and adipocytes. While the NC has been extensively studied in several animal models, little is known about human NC development. A number of methods have been established to derive NCCs in vitro from human pluripotent stem cells (hPSC). Typically, these protocols comprise several cell culture steps to enrich for NCCs in the neural derivatives of the differentiating hPSCs. Here we report on a remarkable and hitherto unnoticed in vitro segregation phenomenon that enables direct extraction of virtually pure NCCs during the earliest stages of hPSC differentiation. Upon aggregation to embryoid bodies (EB) and replating, differentiating hPSCs give rise to a population of NCCs, which spontaneously segregate from the EB outgrowth to form conspicuous, macroscopically visible atoll-shaped clusters in the periphery of the EB outgrowth. Isolation of these NC clusters yields p75NTR(+)/SOXE(+) NCCs, which differentiate to peripheral neurons and glia as well as mesenchymal derivatives. Our data indicate that differentiating hPSC cultures recapitulate, in a simplified manner, the physical segregation of central nervous system (CNS) tissue and NCCs. This phenomenon may be exploited for NCC purification and for studying segregation and differentiation processes observed during early human NC development in vitro.

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.

Ras-driven transcriptome analysis identifies aurora kinase A as a potential malignant peripheral nerve sheath tumor therapeutic target

PURPOSE

Patients with neurofibromatosis type 1 (NF1) develop malignant peripheral nerve sheath tumors (MPNST), which are often inoperable and do not respond well to current chemotherapies or radiation. The goal of this study was to use comprehensive gene expression analysis to identify novel therapeutic targets.

EXPERIMENTAL DESIGN

Nerve Schwann cells and/or their precursors are the tumorigenic cell types in MPNST because of the loss of the NF1 gene, which encodes the RasGAP protein neurofibromin. Therefore, we created a transgenic mouse model, CNP-HRas12V, expressing constitutively active HRas in Schwann cells and defined a Ras-induced gene expression signature to drive a Bayesian factor regression model analysis of differentially expressed genes in mouse and human neurofibromas and MPNSTs. We tested functional significance of Aurora kinase overexpression in MPNST in vitro and in vivo using Aurora kinase short hairpin RNAs (shRNA) and compounds that inhibit Aurora kinase.

RESULTS

We identified 2,000 genes with probability of linkage to nerve Ras signaling of which 339 were significantly differentially expressed in mouse and human NF1-related tumor samples relative to normal nerves, including Aurora kinase A (AURKA). AURKA was dramatically overexpressed and genomically amplified in MPNSTs but not neurofibromas. Aurora kinase shRNAs and Aurora kinase inhibitors blocked MPNST cell growth in vitro. Furthermore, an AURKA selective inhibitor, MLN8237, stabilized tumor volume and significantly increased survival of mice with MPNST xenografts.

CONCLUSION

Integrative cross-species transcriptome analyses combined with preclinical testing has provided an effective method for identifying candidates for molecular-targeted therapeutics. Blocking Aurora kinases may be a viable treatment platform for MPNST.

An efficient and economical culture approach for the enrichment of purified oligodendrocyte progenitor cells

Oligodendrocyte progenitor cell (OPC) culture has provided a powerful approach to mechanistically investigate the proliferation and differentiation of oligodendroglia. However, existing culture methods (including the traditional shake-off method) have limitations, particularly their low productivities. Therefore, we developed a simplified and highly efficient method to produce a large yield of OPCs with low expense by using specialised modified media, in which B104-conditioned medium (B104-CM) instead of growth factors was used as a mitogenic source for OPC propagation, while a modified OPC isolation-medium was applied to improve the isolation of OPCs. First, we withdrew foetal bovine serum when primary mixed glial cultures were 65-75% confluent and substituted with modified oligodendrocyte growth medium to enrich OPCs. Second, we employed a chemical-based method to isolate and purify OPCs from mixed glial cultures using a modified oligodendrocyte isolation medium. As a result, our approach produced a high yield of purified OPCs, approximately 90-fold higher than that produced via the traditional shake-off method. Importantly, the purified OPCs produced via our modified approach maintained typical capacities of proliferation and differentiation observed in oligodendrocyte lineage cells. Together, our modified method provides a highly efficient approach to OPC culture for oligodendrocyte research.

Genetic approaches to study glial cells in the rodent brain

The development, function, and pathology of the brain depend on interactions of neurons and different types of glial cells, namely astrocytes, oligodendrocytes, microglia, and ependymal cells. Understanding neuron-glia interactions in vivo requires dedicated experimental approaches to manipulate each cell type independently. In this review, we first summarize techniques that allow for cell-specific gene modification including targeted mutagenesis and viral transduction. In the second part, we describe the genetic models that allow to target the main glial cell types in the central nervous system. The existing arsenal of approaches to study glial cells in vivo and its expansion in the future are key to understand neuron-glia interactions under normal and pathologic conditions.

Oligodendrocyte progenitor cells can act as cell of origin for experimental glioma

Gliomas are primary brain tumors mainly affecting adults. The cellular origin is unknown. The recent identification of tumor-initiating cells in glioma, which share many similarities with normal neural stem cells, has suggested the cell of origin to be a transformed neural stem cell. In previous studies, using the RCAS/tv-a mouse model, platelet-derived growth factor B (PDGF-B)-induced gliomas have been generated from nestin or glial fibrillary acidic protein-expressing cells, markers of neural stem cells. To investigate if committed glial progenitor cells could be the cell of origin for glioma, we generated the Ctv-a mouse where tumor induction would be restricted to myelinating oligodendrocyte progenitor cells (OPCs) expressing 2',3'-cyclic nucleotide 3'-phosphodiesterase. We showed that PDGF-B transfer to OPCs could induce gliomas with an incidence of 33%. The majority of tumors resembled human WHO grade II oligodendroglioma based on close similarities in histopathology and expression of cellular markers. Thus, with the Ctv-a mouse we have showed that the cell of origin for glioma may be a committed glial progenitor cell.

Glial cells: old cells with new twists

Based on their characteristics and function--migration, neural protection, proliferation, axonal guidance and trophic effects--glial cells may be regarded as probably the most versatile cells in our body. For many years, these cells were considered as simply support cells for neurons. Recently, it has been shown that they are more versatile than previously believed--as true stem cells in the nervous system--and are important players in neural function and development. There are several glial cell types in the nervous system: the two most abundant are oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Although both of these cells are responsible for myelination, their developmental origins are quite different. Oligodendrocytes originate from small niche populations from different regions of the central nervous system, while Schwann cells develop from a stem cell population (the neural crest) that gives rise to many cell derivatives besides glia and which is a highly migratory group of cells.

Loss of erbB signaling in oligodendrocytes alters myelin and dopaminergic function, a potential mechanism for neuropsychiatric disorders

Several psychiatric disorders are associated with white matter defects, suggesting that oligodendrocyte (OL) abnormalities underlie some aspects of these diseases. Neuregulin 1 (NRG1) and its receptor, erbB4, are genetically linked with susceptibility to schizophrenia and bipolar disorder. In vitro studies suggest that NRG1-erbB signaling is important for OL development. To test whether erbB signaling contributes to psychiatric disorders by regulating the structure or function of OLs, we analyzed transgenic mice in which erbB signaling is blocked in OLs in vivo. Here we show that loss of erbB signaling leads to changes in OL number and morphology, reduced myelin thickness, and slower conduction velocity in CNS axons. Furthermore, these transgenic mice have increased levels of dopamine receptors and transporters and behavioral alterations consistent with neuropsychiatric disorders. These results indicate that defects in white matter can cause alterations in dopaminergic function and behavior relevant to neuropsychiatric disorders.

Novel role for aspartoacylase in regulation of BDNF and timing of postnatal oligodendrogenesis

Neuronal growth factors are thought to exert a significant degree of control over postnatal oligodendrogenesis, but mechanisms by which these factors coordinateoligodendrocyte development with the maturation of neural networks are poorly characterized. We present here a developmental analysis of aspartoacylase (Aspa)-null tremor rats and show a potential role for this hydrolytic enzyme in the regulation of a postnatal neurotrophic stimulus that impacts on early stages of oligodendrocyte differentiation. Abnormally high levels of brain-derived neurotrophic factor (BDNF) expression in the Aspa-null Tremor brain are associated with dysregulated oligodendrogenesis at a stage in development normally characterized by high levels of Aspa expression. BDNF promotes the survival of proliferating cells during the early stages of oligodendrocyte maturation in vitro, but seems to compromise the ability of these cells to populate the cortex in vivo. Aspartoacylase activity in oligodendrocytes is shown to provide for the negative regulation of BDNF in neurons, thereby determining the availability of a developmental stimulus via a mechanism that links oligodendroglial differentiation with neuronal maturation.

Nab proteins are essential for peripheral nervous system myelination

Mutations that disrupt Egr2 transcriptional activity cause severe demyelinating peripheral neuropathies. Here we provide evidence that Nab1 and Nab2 proteins are critical transcriptional modulators of Egr2 in myelinating Schwann cells. Like Egr2, these proteins are essential for Schwann cell differentiation into the myelinating state. Mice lacking both Nab1 and Nab2 show severe congenital hypomyelination of peripheral nerves, with Schwann cell development arresting at the promyelinating stage, despite elevated Egr2 expression. As observed for Egr2, Nab proteins are necessary for Schwann cells to exit the cell cycle, downregulate suppressed cAMP-inducible protein (SCIP) expression and upregulate expression of critical myelination genes. The mRNA expression signature of Schwann cells deficient in both Nab1 and Nab2 is highly similar to that of Egr2-deficient Schwann cells, further indicating that the Egr2/Nab protein complex is a key regulator of the Schwann cell myelination program and that disruption of this transcriptional complex is likely to result in Schwann cell dysfunction in patients with Egr2 mutations.

Apoptosis and proliferation of oligodendrocyte progenitor cells in the irradiated rodent spinal cord

PURPOSE

Oligodendrocytes undergo early apoptosis after irradiation. The aim of this study was to determine the relationship between oligodendroglial apoptosis and proliferation of oligodendrocyte progenitor cells (OPC) in the irradiated central nervous system.

METHODS AND MATERIALS

Adult rats and p53 transgenic mice were given single doses of 2 Gy, 8 Gy, or 22 Gy to the cervical spinal cord. Apoptosis was assessed using TUNEL (Tdt-mediated dUTP terminal nick-end labeling) staining or by examining nuclear morphology. Oligodendrocyte progenitor cells were identified with an NG2 antibody or by in situ hybridization for platelet-derived growth factor receptor alpha. Proliferation of OPC was assessed by in vivo bromodeoxyuridine (BrdU) labeling and subsequent immunohistochemistry. Because radiation-induced apoptosis of oligodendroglial cells is p53 dependent, p53 transgenic mice were used to study the relationship between apoptosis and cell proliferation.

RESULTS

Oligodendrocyte progenitor cells underwent apoptosis within 24 h of irradiation in the rat. That did not result in a change in OPC density at 24 h. Oligodendrocyte progenitor cell density was significantly reduced by 2-4 weeks, but showed recovery by 6 weeks after irradiation. An increase in BrdU-labeled cells was observed at 2 weeks after 8 Gy or 22 Gy, and proliferating cells in the rat spinal cord were immunoreactive for NG2. The mouse spinal cord showed a similar early cell proliferation after irradiation. No difference was observed in the proliferation response in the spinal cord of p53 -/- mice compared with wild type animals.

CONCLUSIONS

Oligodendroglial cells undergo early apoptosis and OPC undergo early proliferation after ionizing radiation. However, apoptosis is not likely to be the trigger for early proliferation of OPC in the irradiated central nervous system.

Expression of a catalytically inactive transmembrane protein tyrosine phosphatase epsilon (tm-PTP epsilon) delays optic nerve myelination

Reversible tyrosine phosphorylation is integral to the process of oligodendrocyte differentiation. To interfere with the subset of the phosphorylation cycle overseen by protein tyrosine phosphatase epsilon (PTP epsilon) in oligodendrocytes, we applied a substrate-trapping approach in the development of transgenic mice overexpressing a catalytically inactive, transmembrane PTP epsilon-hemaglutinin (tm-PTP epsilon-HA) from the dual promoter element of the gene encoding the myelin protein 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). Transgene expression peaked during the active myelinating period, at 2-3 weeks postnatal. Two tyrosine phosphoproteins, alpha-enolase and beta-actin, were phosphorylated to a greater degree in transgenic mice. Despite a high degree of tm-PTP epsilon-HA expression, myelin was grossly normal in nearly all axonal tracts. Phenotypic abnormalities were limited to optic nerve, where a decrease in the degree of myelination was reflected by reduced levels of myelin proteins on postnatal day 21 (PND21), as well as a decrease in the density of differentiated oligodendrocytes. The optic chiasm was reduced in thickness in transgenic mice; optic nerves similarly exhibited a reduction in transverse width. Further analyses of the optic pathway demonstrated that transgenic protein was unexpectedly present in retinal ganglion cells, whose axons are the targets of myelination by optic nerve oligodendrocytes. On PND28, transgenic protein declined dramatically in both oligodendrocytes and retinal ganglion cells contributing to the recovery of optic nerve myelination. Thus, delayed myelination arises only when tm-PTP epsilon-HA is simultaneously expressed in myelin-forming glia and their neuronal targets. While tm-PTP epsilon related signaling pathways may figure in axon-glial interactions, interfering with tm-PTP epsilon activity does not perceptibly affect the development or myelinating capacity of most oligodendrocytes.

Identification of oligodendrocytes in experimental disease models

The ability to identify oligodendrocytes in culture, in fixed tissue, and in vivo using unique markers is a requisite step to understanding their responses in any damage, recovery, or developmental process. Their nuclei are readily seen in histological preparations of healthy white and gray matter, and their cell bodies can be reliably identified with a variety of immunocytochemical markers. However, there is little consensus regarding optimal methods to assess oligodendrocyte survival or morphology under experimental injury conditions. We review common approaches for histological and immunocytochemical identification of these cells. Transgenic and viral methods for cell type-selective transfer of genes encoding fluorescent proteins offer promising new approaches for manipulating and visualizing oligodendrocytes in models of health and disease.

Role for the epidermal growth factor receptor in neurofibromatosis-related peripheral nerve tumorigenesis

Benign neurofibromas and malignant peripheral nerve sheath tumors are serious complications of neurofibromatosis type 1. The epidermal growth factor receptor is not expressed by normal Schwann cells, yet is overexpressed in subpopulations of Nf1 mutant Schwann cells. We evaluated the role of EGFR in Schwann cell tumorigenesis. Expression of EGFR in transgenic mouse Schwann cells elicited features of neurofibromas: Schwann cell hyperplasia, excess collagen, mast cell accumulation, and progressive dissociation of non-myelin-forming Schwann cells from axons. Mating EGFR transgenic mice to Nf1 hemizygotes did not enhance this phenotype. Genetic reduction of EGFR in Nf1(+/-);p53(+/-) mice that develop sarcomas significantly improved survival. Thus, gain- and loss-of-function experiments support the relevance of EGFR to peripheral nerve tumor formation.

CD44 expression identifies astrocyte-restricted precursor cells

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

Generation of purified oligodendrocyte progenitors from embryonic stem cells

Demyelination is a key component in the pathogenesis of many neurological disorders. Transplantation of myelinating cells may offer a therapeutic approach to restore neurological function in these diseases. Recent findings suggest that pluripotent embryonic stem (ES) cells can serve as an unlimited donor source for neural transplantation. The clinical application of ES cells for myelin repair will depend critically on the ability to enrich oligodendroglial progenitors in high purity. Combining controlled differentiation in the presence of growth factors and genetic lineage selection, we devised a cell culture protocol yielding highly purified oligodendrocyte progenitors. Murine ES cell clones stably transfected with a construct encoding the beta-galactosidase-neomycine phosphotransferase fusion protein (beta(geo)) under control of the 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter were differentiated into bipotential glial precursors. Subsequent induction of a CNP-positive stage and selection in neomycine resulted in a homogenous cell population with a pre-oligodendrocyte phenotype. The selected cells continued to proliferate in the presence of FGF-2 and PDGF and, upon growth factor withdrawal, differentiated into mature galactocerebroside (GalC)-positive oligodendrocytes. Transplantation studies in myelin-deficient (md) rats indicate that ES cell-derived oligodendrocyte progenitors generated with this method may serve as an attractive donor source for myelin repair.

Cell depletion due to diphtheria toxin fragment A after Cre-mediated recombination

Abnormal cell loss is the common cause of a large number of developmental and degenerative diseases. To model such diseases in transgenic animals, we have developed a line of mice that allows the efficient depletion of virtually any cell type in vivo following somatic Cre-mediated gene recombination. By introducing the diphtheria toxin fragment A (DT-A) gene as a conditional expression construct (floxed lacZ-DT-A) into the ubiquitously expressed ROSA26 locus, we produced a line of mice that would permit cell-specific activation of the toxin gene. Following Cre-mediated recombination under the control of cell-type-specific promoters, lacZ gene expression was efficiently replaced by de novo transcription of the Cre-recombined DT-A gene. We provide proof of this principle, initially for cells of the central nervous system (pyramidal neurons and oligodendrocytes), the immune system (B cells), and liver tissue (hepatocytes), that the conditional expression of DT-A is functional in vivo, resulting in the generation of novel degenerative disease models.

Insertion of the beta Geo promoter trap into the Fem1c gene of ROSA3 mice

ROSA3 mice were developed by retroviral insertion of the beta Geo gene trap vector. Adult ROSA3 mice exhibit widespread expression of the trap gene in epithelial cells found in most organs. In the central nervous system the highest expression of beta Geo is found in CA1 pyramidal cells of the hippocampus, Purkinje cells of the cerebellum, and ganglion cells of the retina. Characterization of the genomic insertion site for beta Geo in ROSA3 mice shows that the trap vector is located in the first intron of Fem1c, a gene homologous to the sex-determining gene fem-1 of Caenorhabditis elegans. Transcription of the Rosa3 allele (R3) yields a spliced message that includes the first exon of Fem1c and the beta Geo coding region. Although normal processing of the Fem1c transcript is disrupted in homozygous Rosa3 (Fem1c(R3/R3)) mice, some tissues show low levels of a partially processed transcript containing exons 2 and 3. Since the entire coding region of Fem1c is located in these two exons, Fem1c(R3/R3) mice may still be able to express a putative FEM1C protein. To this extent, Fem1c(R3/R3) mice show no adverse effects in their sexual development or fertility or in the attenuation of neuronal cell death, another function that has been attributed to both fem-1 and a second mouse homolog, Fem1b. Examination of beta Geo expression in ganglion cells after exposure to damaging stimuli indicates that protein levels are rapidly depleted prior to cell death, making the beta Geo reporter gene a potentially useful marker to study early molecular events in damaged neurons.

Myelin transcription factor 1 (Myt1) modulates the proliferation and differentiation of oligodendrocyte lineage cells

Myelin transcription factor 1 (Myt1) is a zinc finger DNA-binding protein that is expressed in neural progenitors and oligodendrocyte lineage cells. This study examines the role of Myt1 in oligodendrocyte lineage cells by overexpressing putative functional domains, a four-zinc finger DNA-binding region (4FMyt1) or a central protein-protein interaction domain (CDMyt1), without the predicted transcriptional activation domain. In the presence of mitogens, overexpression of 4FMyt1 inhibited proliferation of oligodendrocyte progenitors, but not cell types (astrocytes and NIH3T3 cells) lacking endogenous Myt1. Expression of 4FMyt1 inhibited the differentiation of oligodendrocyte progenitors into oligodendrocytes as assessed by morphology, immunostaining, and myelin gene expression. Progenitor differentiation was similarly inhibited by expression of CDMyt1 but only partially suppressed by overexpression of the intact Myt1. These data indicate that Myt1 may regulate a critical transition point in oligodendrocyte lineage development by modulating oligodendrocyte progenitor proliferation relative to terminal differentiation and up-regulation of myelin gene transcription.

Changes in oligodendrocytes and myelin gene expression after radiation in the rodent spinal cord

PURPOSE

The aim of this study was to assess changes in oligodendrocytes (OL), myelin gene expression, and their relationships with late demyelination after irradiation.

METHODS AND MATERIALS

Adult rats were given single doses of 8 or 22 Gy to the cervical spinal cord. Immunohistochemistry for APC or GST-pi was used to identify OL. Changes in myelin gene expression were assessed using RT-PCR for proteolipid protein (PLP). Luxol fast blue staining was used to assess demyelination. CNP-(beta)geo transgenic mice were used to confirm some of the results of the rat model. Cells of the oligodendroglial lineage in these animals express beta-galactosidase (beta-gal).

RESULTS

Early apoptosis of APC, GST-pi, and beta-galactosidase positive cells was observed in the spinal cord of rats and CNP-(beta)geo mice. At 24 h after 22 Gy, there was a significant decrease in OL density. Cell density continued to decline thereafter after both 8 and 22 Gy, and a reduction in PLP expression was observed at 4-5 weeks. A further decrease in PLP expression was seen beginning at 18 weeks after 22 Gy only. Demyelination was observed at 19 weeks after 22 Gy.

CONCLUSIONS

Apoptosis of OL and changes in OL density and PLP gene expression were observed early after both 8 and 22 Gy. This suggests that these early changes are unlikely to be directly related to the late demyelination observed.

Expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in the developing olfactory bulb and subventricular zone rostral extension

The olfactory bulb (OB) presents a unique pattern of permanent acquisition of primary afferents and interneurons, but not much detail is known about the differentiation of its oligodendroglial cells. We studied the expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a protein related to axonal ensheathment by myelinating cells. Expression of CNPase in OB follows a general caudorostral gradient, with the exception of the glomerular layer (GL). At postnatal day 5-6 (P5-P6), the first CNPase(+) profiles appeared in the dorsal lateral olfactory tract adjacent to the accessory OB (AOB), followed by rare cell bodies and processes in AOB internal plexiform layer at P7. At P9, the main OB (MOB) granular cell layer (GrCL) already showed intensely stained CNPase(+) processes. From P5 to P12, small numbers of CNPase(+) cells were found in the subventricular zone (SVZ), throughout its rostral extension (SVZ-RE), and in the intrabulbar subependymal layer. The appearance of CNPase(+) profiles delimiting glomeruli started in the GL rostralmost region at P12, extending to all GL levels, but glomeruli remained open caudally at P15. At P18, oligodendroglial glomeruli were evident throughout OB, but the adult pattern was established only after P30. There was no age-related loss of CNPase immunoreactivity in glial cell bodies, possibly indicating de novo ensheathment of neurites. Our results show an earlier onset of oligodendroglial differentiation in OB than previously reported and a rostrocaudal gradient of formation of oligodendroglial glomeruli. They also raise the possibility that a minor fraction of OB oligodendrocytes might derive from the SVZ-RE.

Brain lipid binding protein in axon-Schwann cell interactions and peripheral nerve tumorigenesis

Loss of axonal contact characterizes Schwann cells in benign and malignant peripheral nerve sheath tumors (MPNST) from neurofibromatosis type 1 (NF1) patients. Tumor Schwann cells demonstrate NF1 mutations, elevated Ras activity, and aberrant epidermal growth factor receptor (EGFR) expression. Using cDNA microarrays, we found that brain lipid binding protein (BLBP) is elevated in an EGFR-positive subpopulation of Nf1 mutant mouse Schwann cells (Nf1(-/-) TXF) that grows away from axons; BLBP expression was not affected by farnesyltransferase inhibitor, an inhibitor of H-Ras. BLBP was also detected in EGFR-positive cell lines derived from Nf1:p53 double mutant mice and human MPNST. BLBP expression was induced in normal Schwann cells following transfection with EGFR but not H-Ras12V. Furthermore, EGFR-mediated BLBP expression was not inhibited by dominant-negative H-Ras, indicating that BLBP expression is downstream of Ras-independent EGFR signaling. BLBP-blocking antibodies enabled process outgrowth from Nf1(-/-) TXF cells and restored interaction with axons, without affecting cell proliferation or migration. Following injury, BLBP expression was induced in normal sciatic nerves when nonmyelinating Schwann cells remodeled their processes. These data suggest that BLBP, stimulated by Ras-independent pathways, regulates Schwann cell-axon interactions in normal peripheral nerve and peripheral nerve tumors.

Disruption of Cnp1 uncouples oligodendroglial functions in axonal support and myelination

Myelination of axons by oligodendrocytes enables rapid impulse propagation in the central nervous system. But long-term interactions between axons and their myelin sheaths are poorly understood. Here we show that Cnp1, which encodes 2',3'-cyclic nucleotide phosphodiesterase in oligodendrocytes, is essential for axonal survival but not for myelin assembly. In the absence of glial cyclic nucleotide phosphodiesterase, mice developed axonal swellings and neurodegeneration throughout the brain, leading to hydrocephalus and premature death. But, in contrast to previously studied myelin mutants, the ultrastructure, periodicity and physical stability of myelin were not altered in these mice. Genetically, the chief function of glia in supporting axonal integrity can thus be completely uncoupled from its function in maintaining compact myelin. Oligodendrocyte dysfunction, such as that in multiple sclerosis lesions, may suffice to cause secondary axonal loss.

Appearance of neurons and glia with respect to the wavefront during colonization of the avian gut by neural crest cells

The enteric nervous system is formed by neural crest cells that migrate, proliferate, and differentiate into neurons and glia distributed in ganglia along the gastrointestinal tract. In the developing embryo some enteric crest cells cease their caudal movements, whereas others continue to migrate. Subsequently, the enteric neurons form a reticular network of ganglia interconnected by axonal projections. We studied the developing avian gut to characterize the pattern of migration of the crest cells, and the relationship between migration and differentiation. Crest cells at the leading edge of the migratory front appear as strands of cells; isolated individual crest cells are rarely seen. In the foregut and midgut, these strands are located immediately beneath the serosa. In contrast, crest cells entering the colon appear first in the deeper submucosal mesenchyme and later beneath the serosa. As the neural crest wavefront passes caudally, the crest cell cords become highly branched, forming a reticular lattice that presages the mature organization of the enteric nervous system. Neurons and glia first appear within the strands at the advancing wavefront. Later neurons are consistently located at the nodes where branches of the lattice intersect. In the most rostral foregut and in the colon, some neurons initially appear in close association with extrinsic nerve fibers from the vagus and Remak's nerve, respectively. We conclude that crest cells colonize the gut as chains of cells and that, within these chains, both neurons and glia appear close to the wavefront.

Expression of the green fluorescent protein in the oligodendrocyte lineage: a transgenic mouse for developmental and physiological studies

We generated a transgenic mouse expressing the enhanced green fluorescent protein (EGFP) under the control of the 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter. EGFP(+) cells were visualized in live tissue throughout embryonic and postnatal development. Immunohistochemical analysis in brain tissue and in sciatic nerve demonstrated that EGFP expression was restricted to cells of the oligodendrocyte and Schwann cell lineages. EGFP was also strongly expressed in "adult" oligodendrocyte progenitors (OPs) and in gray matter oligodendrocytes. Fluorescence-activated cell sorting allowed high-yield purification of EGFP(+) oligodendrocyte-lineage cells from transgenic brains. Electrophysiological patch clamp recordings of EGFP(+) cells in situ demonstrated that OP cells displayed large outward tetraethylammonium (TEA)-sensitive K(+) currents and very small inward currents, whereas mature oligodendrocytes were characterized by expression of large inward currents and small outward K(+) currents. The proliferation rate of EGFP(+) cells in developing white matter decreased with the age of the animals and was strongly inhibited by TEA. Oligodendrocyte development and physiology can be studied in live tissue of CNP-EGFP transgenic mice, which represent a source of pure EGFP(+) oligodendrocyte-lineage cells throughout development.

Notch1 control of oligodendrocyte differentiation in the spinal cord

We have selectively inhibited Notch1 signaling in oligodendrocyte precursors (OPCs) using the Cre/loxP system in transgenic mice to investigate the role of Notch1 in oligodendrocyte (OL) development and differentiation. Early development of OPCs appeared normal in the spinal cord. However, at embryonic day 17.5, premature OL differentiation was observed and ectopic immature OLs were present in the gray matter. At birth, OL apoptosis was strongly increased in Notch1 mutant animals. Premature OL differentiation was also observed in the cerebrum, indicating that Notch1 is required for the correct spatial and temporal regulation of OL differentiation in various regions of the central nervous system. These findings establish a widespread function of Notch1 in the late steps of mammalian OPC development in vivo.

Culture of oligodendrocyte precursor cells (NG2(+)/O1(-)) and oligodendrocytes (NG2(-)/O1(+)) from embryonic rat cerebrum

I have established a novel culture technique of oligodendrocyte precursor cells (OPC, NG2(+)/O1(-)) and oligodendrocytes (OL, NG2(-)/O1(+)) from embryonic day 16 (E16) rat cerebrum distinguished by morphological and immunocytochemical analyses. This novel protocol does not require immunopanning techniques using specific antibodies. The OPC were isolated by two passages every 7 days and culturing them on Petri dishes in different culture medium at each step to eliminate neurons and astrocytes. The yield of pure OPC and OL was relatively large compared with immunopanning techniques. In addition, to examine myelination processes in vitro, the OL from different stages were co-cultured with primary neurons from E17 rat cerebrum on a feeder layer of rat cerebrum astrocytes. This co-culture system resulted in successful formation of myelin in the presence or absence of astrocytes. This culture system was useful for studying OL lineage and initiation of myelination.

Deciphering peripheral nerve myelination by using Schwann cell expression profiling

Although mutations in multiple genes are associated with inherited demyelinating neuropathies, the molecular components and pathways crucial for myelination remain largely unknown. To approach this question, we performed genome-wide expression analysis in several paradigms where the status of peripheral nerve myelination is dynamically changing. Anchor gene correlation analysis, a form of microarray analysis that integrates functional information, using correlation-based clustering, with a statistically rigorous test, the Westfall and Young step-down algorithm, was applied to this data set. Biological pathways active in myelination, genes encoding proteins involved in myelin synthesis, and genes whose mutation results in myelination defects were identified. Many known genes and previously uncharacterized ESTs not heretofore associated with myelination were also identified. One of these ESTs, MASR (myelin-associated SUR4 protein), encodes a member of the SUR4 family of fatty acid desaturases, enzymes involved in elongation of very long chain fatty acids. Its specific localization in myelinating Schwann cells indicates a crucial role for MASR in normal myelin lipid synthesis.

Identification of the regulatory region of the peripheral myelin protein 22 (PMP22) gene that directs temporal and spatial expression in development and regeneration of peripheral nerves

Minor changes in PMP22 gene dosage have profound effects on the development and maintenance of peripheral nerves. This is evident from the genetic disease mechanisms in Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) as well as transgenic animals with altered PMP22 gene dosage. Thus, regulation of PMP22 is a crucial aspect in understanding the function of this protein in health and disease. In this study, we have generated transgenic mice containing 10 kb of the 5'-flanking region of the PMP22 gene, including the two previously identified alternative promoters, fused to a lacZ reporter gene. We show that this part of the PMP22 gene contains the necessary information to mirror the endogenous expression pattern in peripheral nerves during development and regeneration and in mouse models of demyelination due to genetic lesions. Transgene expression is strongly regulated during myelination, demyelination, and remyelination in Schwann cells, demonstrating the crucial influence of neuron-Schwann cell interactions in the regulation of PMP22. In addition, the region of the PMP22 gene present on this transgene confers also neuronal expression in sensory and motor neurons. These results provide the crucial basis for further dissection of the elements that direct the temporal and spatial regulation of the PMP22 gene and to elucidate the molecular basis of the master program regulating peripheral nerve myelination.

Expression of the Krüppel-type zinc finger protein rKr2 in the developing nervous system

Zinc finger transcription factors of the Krüppel-class figure prominently in cell fate specification and differentiation in the nervous system. One of the Krüppel-type genes that was originally cloned from an oligodendrocyte library by virtue of its homology with the prototypic Krüppel motif is the rat rKr2 gene (Pott et al., 1995). In primary cultures of rat glial cells, the rKr2 protein was only present in the oligodendrocyte lineage, predominantly in progenitors. Ninety percent of A2B5(+) oligodendrocyte progenitors displayed rKr2 immunoreactivity, while most MBP(+) oligodendrocytes lacked detectable rKr2. A similar pattern was found in vivo, in which the peak expression of rKr2 in the oligodendrocyte lineage of rats coincided with the wave of progenitor proliferation in early postnatal life. The subventricular zone, a source of neuronal and glial progenitors, displayed intense staining for rKr2 at late embryonic and postnatal stages. In the adult, cells within the remnants of this germinal zone continued to express rKr2 protein strongly. Some populations of mature neurons also displayed rKr2 immunostaining. Astrocytes and microglia were not labeled with the polyclonal anti-rKr2 antibody in vitro or in vivo. At all developmental stages, the rKr2 protein was localized to the nucleus. The stage-specific expression pattern and the subcellular localization of rKr2 recommend a role for this Krüppel-type gene in the progression of neural stem cells and in the early development of the oligodendrocyte lineage.

Tracking oligodendrocytes during development and regeneration

Over the past decade, advances in strategies to tag cells have opened new avenues for examining the development of myelin-forming glial cells and for monitoring transplanted cells in animal models of myelin insufficiency. The strategies for labelling glial cells have encompassed a range of genetic modifications as well as methods for directly attaching labels to cells. Genetically modified oligodendrocytes have been engineered to express enzymatic (e.g., beta-galactosidase, alkaline phosphatase), naturally fluorescent (e.g., green fluorescent protein), and antibiotic resistance (e.g., neomycin, zeomycin) reporters. Genes have been introduced in vivo and in vitro with viral or plasmid vectors to somatically label glial cells. To generate germ-line transmission of tagged oligodendrocytes, transgenic mice have been created both by direct injection into mouse fertilized eggs and by "knock-in" of reporters targetted to myelin gene loci in embryonic stem cells. Each experimental approach has advantages and limitations that need to be considered for individual applications. The availability of tagged glial cells has expanded our basic understanding of how oligodendrocytes are specified from stem cells and should continue to fill in the gaps in our understanding of how oligodendrocytes differentiate, myelinate, and maintain their myelin sheaths. Moreover, the ability to select oligodendrocytes by virtue of their acquired antibiotic resistance has provided an important new tool for isolating and purifying oligodendrocytes. Tagged glial cells have also been invaluable in evaluating cell transplant therapies in the nervous system. The tracking technologies that have driven these advances in glial cell biology are continuing to evolve and present new opportunities for examining oligodendrocytes in living systems. Microsc. Res. Tech. 52:766-777, 2001. Published 2001 Wiley-Liss, Inc.

Turning blood into brain: cells bearing neuronal antigens generated in vivo from bone marrow

Bone marrow stem cells give rise to a variety of hematopoietic lineages and repopulate the blood throughout adult life. We show that, in a strain of mice incapable of developing cells of the myeloid and lymphoid lineages, transplanted adult bone marrow cells migrated into the brain and differentiated into cells that expressed neuron-specific antigens. These findings raise the possibility that bone marrow-derived cells may provide an alternative source of neurons in patients with neurodegenerative diseases or central nervous system injury.

Transcriptional regulation of 2',3'-cyclic nucleotide 3'-phosphodiesterase gene expression by cyclic AMP in C6 cells

It was recently shown that the two transcripts encoding the isoforms of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP1 and CNP2) are differentially regulated during the process of oligodendrocyte maturation. In oligodendrocyte precursors, only CNP2 mRNA is present, whereas in differentiating oligodendrocytes, both CNP1 and CNP2 mRNAs are expressed. This pattern of CNP expression is likely due to stage-specific transcriptional regulation of the two CNP promoters during the process of oligodendrocyte differentiation. Here, we report the influence of increased intracellular cyclic AMP (cAMP) levels on the transcription of both CNP1 and CNP2 mRNAs in rat C6 glioma cells. We found that the transcription of CNP1 mRNA was significantly increased in comparison with that of CNP2 mRNA in cells treated with cAMP analogues to elevate intracellular cAMP levels. This up-regulation of CNP1 expression (a) is due to an increase of transcription, (b) requires de novo protein synthesis, and (c) requires the activity of protein kinase A. These results are physiologically significant and support the idea that a cAMP-mediated pathway is part of the molecular mechanisms regulating the expression of CNP1 in oligodendrocytes. The regulation of CNP1 promoter activity by cAMP was then investigated in stably transfected C6 cell lines containing various deletions of the CNP promoter directing the bacterial chloramphenicol acetyltransferase gene. We showed that the sequence between nucleotides -126 and -102 was essential for the cAMP-dependent induction of CNP1 expression. Gel retardation analysis showed that two protein-DNA complexes are formed between this sequence and nuclear factors from C6 cells treated or not treated with cAMP. This suggests that the induction of CNP1 mRNA transcription is not mediated by changes in binding of nuclear factors that interact directly with the -126/-102 sequence. Sequence analysis of this region revealed the presence of a putative activator protein-2 (AP-2) binding site. It is interesting that mutagenesis of this region resulted in a significant reduction in transcriptional responses to cAMP, implying a possible role for the AP-2 factor in the expression of CNP1. In addition, we have shown that putative binding sites for activator protein-4 and nuclear factor-1 adjacent to the AP-2 site are required for efficient induction of CNP1 expression by cAMP. Taken together, our results show that the cAMP-dependent accumulation of CNP1 mRNA appears to depend on the synergistic interaction of several regulatory elements.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.

Identification, isolation, and promoter-defined separation of mitotic oligodendrocyte progenitor cells from the adult human subcortical white matter

Previous studies have suggested the persistence of oligodendrocyte progenitor cells in the adult mammalian subcortical white matter. To identify oligodendrocyte progenitors in the adult human subcortical white matter, we transfected dissociates of capsular white matter with plasmid DNA bearing the gene for green fluorescence protein (hGFP), placed under the control of the human early promoter (P2) for the oligodendrocytic protein cyclic nucleotide phosphodiesterase (P/hCNP2). Within 4 d after transfection with P/hCNP2:hGFP, a discrete population of small, bipolar cells were noted to express GFP. These cells were A2B5-positive (A2B5(+)), incorporated bromodeoxyuridine in vitro, and constituted <0.5% of all cells. Using fluorescence-activated cell sorting (FACS), the P/hCNP2-driven GFP(+) cells were then isolated and enriched to near-purity. In the weeks after FACS, most P/hCNP2:hGFP-sorted cells matured as morphologically and antigenically characteristic oligodendrocytes. Thus, the human subcortical white matter harbors mitotically competent progenitor cells, which give rise primarily to oligodendrocytes in vitro. By using fluorescent transgenes of GFP expressed under the control of an early oligodendrocytic promoter, these oligodendrocyte progenitor cells may be extracted and purified from adult human white matter in sufficient numbers for implantation and cell-based therapy.

From neural stem cells to myelinating oligodendrocytes

The potential to generate oligodendrocytes progenitors (OP) from neural stem cells (NSCs) exists throughout the developing CNS. Yet, in the embryonic spinal cord, the oligodendrocyte phenotype is induced by sonic hedgehog in a restricted anterior region. In addition, neuregulins are emerging as potent regulators of early and late OP development. The ability to isolate and grow NSCs as well as glial-restricted progenitors has revealed that FGF2 and thyroid hormone favor an oligodendrocyte fate. Analysis of genetically modified mice showed that PDGF controls the migration and production of oligodendrocytes in vivo. Interplay between mitogens, thyroid hormone, and neurotransmitters may maintain the undifferentiated stage or result in OP growth arrest. Notch signaling by axons inhibits oligodendrocyte differentiation until neuronal signals--linked to electrical activity-trigger initiation of myelination. To repair myelin in adult CNS, multipotential neural precursors, rather than slowly cycling OP, appear the cells of choice to rapidly generate myelin-forming cells.