Distribution and morphological characteristics of oligodendrocytes in the rat hippocampus in situ and in vitro: an immunocytochemical study with the monoclonal Rip antibody

Neuroimaging hippocampal subfields in schizophrenia and bipolar disorder: A systematic review and meta-analysis

The hippocampus is a complex structure consisting of subregions with specialized cytoarchitecture and functions. Magnetic resonance imaging (MRI) studies in psychotic disorders show hippocampal subfield abnormalities, but affected regions differ between studies. We here present an overview of hippocampal anatomy and function relevant to psychosis, and the first systematic review and meta-analysis of MRI studies of hippocampal subfield morphology in schizophrenia and bipolar disorder. Twenty-one MRI studies assessing hippocampal subfield volumes or shape in schizophrenia or bipolar disorder were included (n 15-887 subjects). Nine volumetric group comparison studies (total n = 2593) were included in random effects meta-analyses of group differences. The review showed mixed results, with volume reductions reported in most subfields in schizophrenia and bipolar disorder. Volumetric studies using ex-vivo based image analysis templates corresponded best with the shape studies, with CA1 as the most affected region. The meta-analyses showed volume reductions in all subfields in schizophrenia and bipolar disorder compared to healthy controls (all p < .005; schizophrenia: d = 0.28-0.49, bipolar disorder: d = 0.20-0.35), and smaller left CA2/3 and right subiculum in schizophrenia than bipolar disorder. In conclusion, the hippocampal subfields appear to be differently affected in psychotic disorders. However, due to the lack of control for putative confounders such as medication, alcohol and illicit substance use, and illness stage, the results from the meta-analysis should be interpreted with caution. Methodological subfield segmentation weaknesses should be addressed in future studies.

Intrinsic Inflammation Is a Potential Anti-Epileptogenic Target in the Organotypic Hippocampal Slice Model

Understanding the mechanisms of epileptogenesis is essential to develop novel drugs that could prevent or modify the disease. Neuroinflammation has been proposed as a promising target for therapeutic interventions to inhibit the epileptogenic process that evolves from traumatic brain injury. However, it remains unclear whether cytokine-related pathways, particularly TNFα signaling, have a critical role in the development of epilepsy. In this study, we investigated the role of innate inflammation in an in vitro model of post-traumatic epileptogenesis. We combined organotypic hippocampal slice cultures, representing an in vitro model of post-traumatic epilepsy, with multi-electrode array recordings to directly monitor the development of epileptiform activity and to examine the concomitant changes in cytokine release, cell death, and glial cell activation. We report that synchronized ictal- and interictal-like activities spontaneously evolve in this culture. Dynamic changes in the release of the pro-inflammatory cytokines IL-1β, TNFα, and IL-6 were observed throughout the culture period (3 to 21 days in vitro) with persistent activation of microglia and astrocytes. We found that neutralizing TNFα with a polyclonal antibody significantly reduced ictal discharges, and this effect lasted for 1 week after antibody washout. Neither phenytoin nor an anti-IL-6 polyclonal antibody was efficacious in inhibiting the development of epileptiform activity. Our data show a sustained effect of the anti-TNFα antibody on the ictal progression in organotypic hippocampal slice cultures supporting the critical role of inflammatory mediators in epilepsy and establishing a proof-of-principle evidence for the utility of this preparation to test the therapeutic effects of anti-inflammatory treatments.

Intrinsic Inflammation Is a Potential Anti-Epileptogenic Target in the Organotypic Hippocampal Slice Model

Understanding the mechanisms of epileptogenesis is essential to develop novel drugs that could prevent or modify the disease. Neuroinflammation has been proposed as a promising target for therapeutic interventions to inhibit the epileptogenic process that evolves from traumatic brain injury. However, it remains unclear whether cytokine-related pathways, particularly TNFα signaling, have a critical role in the development of epilepsy. In this study, we investigated the role of innate inflammation in an in vitro model of post-traumatic epileptogenesis. We combined organotypic hippocampal slice cultures, representing an in vitro model of post-traumatic epilepsy, with multi-electrode array recordings to directly monitor the development of epileptiform activity and to examine the concomitant changes in cytokine release, cell death, and glial cell activation. We report that synchronized ictal- and interictal-like activities spontaneously evolve in this culture. Dynamic changes in the release of the pro-inflammatory cytokines IL-1β, TNFα, and IL-6 were observed throughout the culture period (3 to 21 days in vitro) with persistent activation of microglia and astrocytes. We found that neutralizing TNFα with a polyclonal antibody significantly reduced ictal discharges, and this effect lasted for 1 week after antibody washout. Neither phenytoin nor an anti-IL-6 polyclonal antibody was efficacious in inhibiting the development of epileptiform activity. Our data show a sustained effect of the anti-TNFα antibody on the ictal progression in organotypic hippocampal slice cultures supporting the critical role of inflammatory mediators in epilepsy and establishing a proof-of-principle evidence for the utility of this preparation to test the therapeutic effects of anti-inflammatory treatments.

Organotypic Hippocampal Slice Cultures As a Model to Study Neuroprotection and Invasiveness of Tumor Cells

In organotypic hippocampal slice cultures (OHSC), the morphological and functional characteristics of both neurons and glial cells are well preserved. This model is suitable for addressing different research questions that involve studies on neuroprotection, electrophysiological experiments on neurons, neuronal networks or tumor invasion. The hippocampal architecture and neuronal activity in multisynaptic circuits are well conserved in OHSC, even though the slicing procedure itself initially lesions and leads to formation of a glial scar. The scar formation alters presumably the mechanical properties and diffusive behavior of small molecules, etc. Slices allow the monitoring of time dependent processes after brain injury without animal surgery, and studies on interactions between various brain-derived cell types, namely astrocytes, microglia and neurons under both physiological and pathological conditions. An ambivalent aspect of this model is the absence of blood flow and immune blood cells. During the progression of the neuronal injury, migrating immune cells from the blood play an important role. As those cells are missing in slices, the intrinsic processes in the culture may be observed without external interference. Moreover, in OHSC the composition of the medium-external environment is precisely controlled. A further advantage of this method is the lower number of sacrificed animals compared to standard preparations. Several OHSC can be obtained from one animal making simultaneous studies with multiple treatments in one animal possible. For these reasons, OHSC are well suited to analyze the effects of new protective therapeutics after tissue damage or during tumor invasion. The protocol presented here describes a preparation method of OHSC that allows generating highly reproducible, well preserved slices that can be used for a variety of experimental research, like neuroprotection or tumor invasion studies.

Rediscovering area CA2: unique properties and functions

Hippocampal area CA2 has several features that distinguish it from CA1 and CA3, including a unique gene expression profile, failure to display long-term potentiation and relative resistance to cell death. A recent increase in interest in the CA2 region, combined with the development of new methods to define and manipulate its neurons, has led to some exciting new discoveries on the properties of CA2 neurons and their role in behaviour. Here, we review these findings and call attention to the idea that the definition of area CA2 ought to be revised in light of gene expression data.

Impact of environmental enrichment on neurogenesis in the dentate gyrus during the early postnatal period

Accumulating evidence shows that environmental enrichment increases neurogenesis in the adult hippocampal dentate gyrus. The goal of the current study was to examine the effect of environmental enrichment on hippocampal neurogenesis during early life stages. We used as an animal model the guinea pig, a precocious rodent that is early independent from maternal care. Animals were assigned to either a standard (control) or an enriched environment a few days after birth (P5-P6). Between P14 and P17 animals received one daily bromodeoxyuridine (BrdU) injection, to label dividing cells, and were sacrificed either on P18, to evaluate cell proliferation or on P45, to evaluate cell survival and differentiation. In 18-day old enriched animals, there was a larger number of BrdU-positive cells compared to that found in controls. At P45, enriched animals had more surviving cells and more cells with a neuronal phenotype than controls. Unbiased stereology revealed that enriched animals had more granule cells (+37% at P18 and +31% at P45). Results show that environmental enrichment in the early postnatal period notably increases cell proliferation and survival, with a large increase in the number of neurons forming the granule cell layer. The impact of environmental enrichment in the early postnatal period emphasizes the relevance of extrinsic factors in the modulation of neurogenesis during critical time windows of hippocampal development.

Defining the morphological phenotype: 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) is a novel marker for in situ detection of canine but not rat olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are the non-myelinating glial cells of the olfactory nerves and bulb. The fragmentary characterization of OECs in situ during normal development may be due to their small size requiring intricate ultrastructural analysis and to the fact that available markers for in situ detection are either expressed only by OEC subpopulations or lost during development. In the present study, we searched for markers with stable expression in OECs and investigated the spatiotemporal distribution of CNPase, an early oligodendrocyte/Schwann cell marker, in comparison with the prototype marker p75(NTR). Anti-CNPase antibodies labeled canine but not rat OECs in situ, while Schwann cells and oligodendrocytes were positive in both species. CNPase immunoreactivity in the dog was confined to all OECs throughout the postnatal development and associated with the entire cell body, including its finest processes, while p75(NTR) was mainly detected in perineural cells and only in some neonatal OECs. Adult olfactory bulb slices displayed CNPase expression after 4 and 10 days, while p75(NTR) was detectable only after 10 days in vitro. Finally, treatment of purified adult canine OECs with fibroblast growth factor-2 significantly reduced CNPase expression at the protein and mRNA level. Taken together, we conclude that CNPase but not p75(NTR) is a stable marker suitable for in situ visualization of OECs that will facilitate their light-microscopic characterization and challenge our general view of OEC marker expression in situ. The fact that canine but not rat OECs expressed CNPase supports the idea that glia from large animals differs substantially from rodents.

Oligodendrocytes: biology and pathology

Oligodendrocytes are the myelinating cells of the central nervous system (CNS). They are the end product of a cell lineage which has to undergo a complex and precisely timed program of proliferation, migration, differentiation, and myelination to finally produce the insulating sheath of axons. Due to this complex differentiation program, and due to their unique metabolism/physiology, oligodendrocytes count among the most vulnerable cells of the CNS. In this review, we first describe the different steps eventually culminating in the formation of mature oligodendrocytes and myelin sheaths, as they were revealed by studies in rodents. We will then show differences and similarities of human oligodendrocyte development. Finally, we will lay out the different pathways leading to oligodendrocyte and myelin loss in human CNS diseases, and we will reveal the different principles leading to the restoration of myelin sheaths or to a failure to do so.

Enhanced microglial clearance of myelin debris in T cell-infiltrated central nervous system

Acute multiple sclerosis lesions are characterized by accumulation of T cells and macrophages, destruction of myelin and oligodendrocytes, and axonal damage. There is, however, limited information on neuroimmune interactions distal to sites of axonal damage in the T cell-infiltrated central nervous system. We investigated T-cell infiltration, myelin clearance, microglial activation, and phagocytic activity distal to sites of axonal transection through analysis of the perforant pathway deafferented dentate gyrus in SJL mice that had received T cells specific for myelin basic protein (TMBP) or ovalbumin (TOVA). The axonal lesion of TMBP-recipient mice resulted in lesion-specific recruitment of large numbers of T cells in contrast to very limited T-cell infiltration in TOVA-recipient and -naïve perforant pathway-deafferented mice. By double immunofluorescence and confocal microscopy, infiltration with TMBP but not TOVA enhanced the microglial response to axonal transection and microglial phagocytosis of myelin debris associated with the degenerating axons. Because myelin antigen-specific immune responses may provoke protective immunity, increased phagocytosis of myelin debris might enhance regeneration after a neural antigen-specific T cell-mediated immune response in multiple sclerosis.

Morphometric analysis of oligodendrocytes in the adult mouse frontal cortex

Oligodendrocytes (OLs), the myelinating cells of the central nervous system, have specialized morphologies that subserve their function. Numerous qualitative studies suggest that OLs in different brain regions can differ in their morphological characteristics, including number of branches and internodes, internode length, etc. However, progress in identifying and characterizing the diverse types of OLs and their distribution in the brain has been made difficult by several technical constraints. Here we report a new strategy to analyze OL morphology with a high degree of quantitative power and throughput. We used confocal microscopy and three-dimensional cell tracing software to study OLs in the frontal cortex of mice expressing enhanced green fluorescent protein (eGFP) under the control of the proteolipid protein (Plp) gene promoter. Three-dimensional reconstructions were then used to analyze and quantify cell morphology, including total process length, total process surface area, total internode length, number of primary processes, number of branch points, and number of internodes. In addition, these reconstructions were subjected to Sholl analysis, which allows for the quantitative measure of OL arbor complexity. By using this approach, we identified and characterized a previously undescribed population of small OLs with a compact but complex morphology that includes numerous branching processes and a large number of short internodes. Our data suggest that other populations of OLs remain to be identified and characterized and that the tools we have developed could help in the process of characterizing them.

Cell cycle alteration and decreased cell proliferation in the hippocampal dentate gyrus and in the neocortical germinal matrix of fetuses with Down syndrome and in Ts65Dn mice

Down syndrome (DS), the leading genetic cause of mental retardation, is characterized by reduced number of cortical neurons and brain size. The occurrence of these defects starting from early life stages points at altered developmental neurogenesis as their major determinant. The goal of our study was to obtain comparative evidence for impaired neurogenesis in the hippocampal dentate gyrus (DG) of DS fetuses and Ts65Dn mice, an animal model for DS. Cell proliferation in human fetuses was evaluated with Ki-67 (a marker of cells in S + G(2) + M phases of cell cycle) and cyclin A (a marker of cells in S phase) immunohistochemistry. We found that in the DG of DS fetuses the number of proliferating cells was notably reduced when compared with controls. A similar reduction was observed in the germinal matrix of the lateral ventricle. In both structures, DS fetuses showed a reduced ratio between cyclin A- and Ki-67-positive cells when compared with controls, indicating that they had a reduced number of cycling cells in S phase. In the DG of P2 Ts65Dn mice cell proliferation, assessed 2 h after an injection of bromodeoxyuridine (BrdU), was notably reduced, similarly to DS fetuses. After 28 days, Ts65Dn mice had still less BrdU-positive cells than controls. Phenotypic analysis of the surviving cells showed that Ts65Dn mice had a percent number of cells with astrocytic phenotype larger than controls. Using phospho-histone H3 immunohistochemistry we found that both DS fetuses and P2 Ts65Dn mice had a higher number of proliferating cells in G(2) and a smaller number of cells in M phase of cell cycle. Results provide novel evidence for proliferation impairment in the hippocampal DG of the DS fetal brain, comparable to that of the P2 mouse model, and suggest that cell cycle alterations may be critical determinants of the reduced proliferation potency.

Synaptic plasticity (and the lack thereof) in hippocampal CA2 neurons

The hippocampus is critical for some forms of memory and spatial navigation, but previous research has mostly neglected the CA2, a unique region situated between CA3 and CA1. Here, we show that CA2 pyramidal neurons have distinctive physiological characteristics that include an unprecedented synaptic stability. Although basal synaptic currents in CA1 and CA2 are quite similar, synaptic plasticity including long-term potentiation and long-term depression is absent or less likely to be induced with conventional methods of stimulation in CA2. We also find that CA2 neurons have larger leak currents and more negative resting membrane potentials than CA1 neurons, and consequently, more current is required for action potential generation in CA2 neurons. These data suggest that the molecular "conspiracy against plasticity" in CA2 makes it functionally distinct from the other hippocampal CA regions. This work provides critical insight into hippocampal function and may lead to an understanding of the resistance of CA2 to damage from disease, trauma, and hypoxia.

Differential RIP antigen (CNPase) expression in peripheral ensheathing glia

The RIP monoclonal antibody is commonly used to identify oligodendrocytes. Recently, the RIP antigen was identified as 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a known non-compact myelin protein [Watanabe, M., Sakurai, Y., Ichinose, T., Aikawa, Y., Kotani, M., Itoh, K., 2006. Monoclonal antibody Rip specifically recognizes 2',3'-cyclic nucleotide 3'-phosphodiesterase in oligodendrocytes. J. Neurosci. Res. 84, 525-533]. In the present study we characterize normal and axotomy-induced changes in RIP immunoreactivity in peripheral glia. In myelinating Schwann cells, RIP demarcated paranodal regions of myelinated axons and clearly defined Schmidt-Lantermann incisures. Surprisingly, RIP immunoreactivity was not confined to myelinating glia. Robust RIP immunoreactivity was present in Remak bundles in mixed nerves and in sympathetic ganglia and grey rami. Following peripheral nerve injury, RIP immunoreactivity was redistributed diffusely throughout de-differentiating Schwann cell cytoplasm. In uninjured rats, low levels of RIP immunoreactivity were detectable in satellite cells surrounding dorsal root ganglion (DRG) neurons and in terminal Schwann cells at neuromuscular junctions. This pattern suggested a correlation between RIP immunoreactivity and the amount of axon-glial contact. We therefore injured the L5 spinal nerve to induce sympathetic sprouting and pericellular basket formation in the DRG, and asked whether relatively RIP-negative satellite glia, which normally contact only neuronal somata, would upregulate the RIP antigen upon contact with sprouting sympathetic axons. All perineuronal sympathetic sprouts infiltrated heavily RIP-immunoreactive satellite cell sheaths. RIP immunoreactivity was absent from placode-derived olfactory ensheathing glia, indicating that the relationship between axon-glial contact and RIP-immunoreactivity is restricted to peripheral ensheathing glia of the neural crest-derived Schwann cell lineage.

Monoclonal antibody Rip specifically recognizes 2',3'-cyclic nucleotide 3'-phosphodiesterase in oligodendrocytes

The antigen recognized with monoclonal antibody (mAb) Rip (Rip-antigen) has been long used as a marker of oligodendrocytes and myelin sheaths. However, the identity of Rip-antigen has yet to be elucidated. We herein identified the Rip-antigen. No signal recognized by mAb-Rip was detected by immunoblot analyses in the rat brain, cultured rat oligodendrocytes, or the oligodendrocyte cell line CG-4. As this antibody worked very well on immunocytochemistry and immunohistochemistry, Rip-antigen was immunopurified with mAb-Rip from the differentiated CG-4 cells. Eight strong-intensity bands thus appeared on 5-20% SDS-PAGE with SYPRO ruby fluorescence staining. To identify these molecules, each band extracted from the gel was analyzed by MALDI-QIT/TOF mass spectrometry. We found an interesting molecule in the oligodendrocytes from an approximately 44-kDa band as 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). To test whether CNP was recognized by mAb-Rip, double-immunofluorescence staining was performed by using Alexa Fluor 488-conjugated mAb-Rip and Alexa Fluor 568-conjugated mAb-CNP in the rat cerebellum, mouse cerebellum, cultured rat oligodendrocytes, and CG-4 cells. The Rip-antigen was colocalized with CNP in these cells and tissues. To provide direct evidence that CNP was recognized by mAb-Rip, rat Cnp1-transfected HEK293T cells were used for double-immunofluorescence staining with mAb-Rip and mAb-CNP. The Rip-antigen was colocalized with CNP in rat Cnp1-transfected HEK293T cells, but the antigen was not detected by mAb-Rip and mAb-CNP in mock-transfected HEK293T cells. Overall, we have demonstrated that the antigen labeled with mAb-Rip is CNP in the oligodendrocytes.

Requirement for Id1 in opioid-induced oligodendrogenesis in cultured adult rat hippocampal progenitors

Growth factors and peptides playing important roles during early development of the central nervous system have also been shown to maintain their regulation of cell genesis in the adult brain. We have previously described that endogenous opioids, expressed in the developing hippocampus, regulate proliferation and differentiation in the adult rat hippocampus. The aim of this study was to investigate the effects of the opioid beta-endorphin on gene expression and glial differentiation in cultures of adult rat hippocampal progenitors (AHPs). Changes in gene expression after stimulation of AHPs with beta-endorphin for 48 h were investigated using cDNA arrays. Confirmation experiments verified that stimulation with beta-endorphin increased the mRNA levels of myelin basic protein, glutathione S-transferase pi, c-junD and rab16 (P < 0.05), genes that are associated with oligodendrogenesis. Furthermore, beta-endorphin increased the levels of Id1, but not Id3, mRNA on the arrays. Incubation of AHPs with beta-endorphin resulted in a threefold increase in oligodendrogenesis (P < 0.01) but no significant change in astrogliogenesis. No effect on oligodendrogenesis was observed in the presence of the opioid antagonist naloxone. Coincubation of beta-endorphin with Id1 antisense oligonucleotides for 10 days also entirely blocked the induced oligodendrogenesis in our AHP cultures. Moreover, a subpopulation of AHPs (25%) showed nuclear expression of the proneural transcriptional activator Mash1 that was reduced to approximately 5% of the cells when exposed to beta-endorphin. We suggest a requirement for Id1 in opioid-induced oligodendrogenesis in cultured AHPs possibly acting on opioid-responsive AHPs expressing the proneural transcriptional activator Mash1.

Comparative postnatal development of spinal, trigeminal and vagal sensory root entry zones

Somatic and visceral sensory information enters the central nervous system (CNS) via root entry zones where sensory axons span an environment consisting of Schwann cells in the peripheral nervous system (PNS) and astrocytes and oligodendrocytes in the CNS. While the embryonic extension of these sensory axons into the CNS has been well-characterized, little is known about the subsequent, largely postnatal development of the glial elements of the root entry zones. Here we sought to establish a comparative developmental timecourse of the glial elements in the postnatal (P0, P3, P7, P14) and adult rat of three root entry zones: the spinal nerve dorsal root entry zone, the trigeminal root entry zone, and the vagal dorsal root entry zone. We compared entry zone development based on the expression of antigens known to be expressed in astrocytes, oligodendrocytes, oligodendrocyte precursor cells, Schwann cells, radial glial fibres and the PNS extracellular matrix. These studies revealed an unexpected distribution among glial cells of several antigens. In particular, antibodies used to label mature oligodendrocytes (RIP) transiently labelled immature Schwann cell cytoplasm, and a radial glial antigen (recognized by the 3CB2 antibody) initially decreased, and then increased in postnatal astrocytes. While all three root entry zones had reached morphological and antigenic maturity by P14, the glial elements comprising the PNS-CNS interface of cranial root entry zones (the trigeminal root entry zone and the vagal dorsal root entry zone) matured earlier than those of the spinal nerve dorsal root entry zone.

Chronological distribution of Rip immunoreactivity in the gerbil hippocampus during normal aging

Age-dependent studies on oligodendrocytes, which are the myelinating cells in the central nervous system, have been relatively less investigated. We examined age-dependent changes in Rip immunoreactivity and its protein level in the gerbil hippocampus during normal aging using immunohistochemistry and Western blot analysis with Rip antibody, an oligodendrocyte marker. Rip immunoreactivity and its protein level in the hippocampal CA1 region significantly increased at postnatal month 3 (PM 3). Thereafter, they decreased in the hippocampal CA1 region with age. At PM 24, Rip immunoreactive processes in the hippocampal CA1 region markedly decreased in the stratum radiatum. In the hippocampal CA2/3 region and dentate gyrus, the pattern of changes in Rip immunoreactivity and its protein level was similar to those in the hippocampal CA1 region; however, no significant changes were found in the CA2/3 region and dentate gyrus at various age stages. These results indicate that Rip immunoreactivity and protein level in the hippocampal CA1 region decreases significantly at PM 24 compared to the CA2/3 region and dentate gyrus.

Astrocytic factors deactivate antigen presenting cells that invade the central nervous system

We hypothesized that CNS tissue has the potential to deactivate invading monocytes/macrophages in order to maintain the immune privilege of the brain, and furthermore, that astrocytes are the cells that initiate monocyte/macrophage deactivation. To test this hypothesis, fluorescent prelabeled rat spleen macrophages with typical amoeboid morphology were transferred into organotypic hippocampal slice cultures (OHSCs), where they gradually developed a ramified morphology similar to the appearance of resting microglial cells. This morphological transformation also occurred if macrophages or monocytes were co-cultured with mixed glial cultures or with astrocytoma cells, and ramification was accompanied by reduced expression of adhesion molecules leukocyte function antigen (LFA)-1, intercellular adhesion molecule (ICAM)-1, and major histocompatibility complex (MHC)-class-II molecules. Moreover, treatment of macrophages with astrocyte culture supernatant effectively down-regulated the LPS-induced expression of adhesion- and MHC-class-II-molecules. Astrocyte supernatant-induced inhibition of adhesion and MHC-class-II-molecule expression was mimicked by transforming growth factor (TGF)-beta1, furthermore, this inhibitory effect was diminished by simultaneous treatment with neutralizing anti-TGF-beta-antibodies. In conclusion, our results suggest that astrocyte-derived, soluble factors that are present in the CNS microenvironment deactivate invading macrophages, thus contributing to the maintenance of CNS immune-privilege following impairment of blood-brain-barrier (BBB) integrity.

Rip immunoreactivity significantly decreases in the stratum oriens of hippocampal CA1 region after transient forebrain ischemia in gerbils

In the present study, we observed ischemia-related changes in Rip recognizing the promyelinating and myelinating oligodendrocytes in the hippocampus proper after 5 min of transient forebrain ischemia in gerbils. Rip immunoreactivity was significantly altered in the hippocampal CA1 region but not in the CA2/3 region after ischemic insult. In the sham-operated group, Rip immunoreactivity was shown in the cell bodies and processes of oligodendrocytes in all layers of the hippocampus proper. From 15 min to 2 days after ischemic insult, Rip immunoreactivity was similar to that of sham-operated group. Three days after ischemic insult, Rip-immunoreactive processes were tangled in the stratum oriens of the CA1 region, and Rip protein level decreased from this time after ischemia. Thereafter, Rip immunoreactivity was decreased time dependently in the CA1 region. Seven days after ischemic insult, Rip-immunoreactive processes were tangled and densely detected in the stratum oriens adjacent to the stratum pyramidale. In brief, these results indicate that the significant decrease of Rip immunoreactivity in processes in the stratum oriens of the hippocampal CA1 region occurs at late time after ischemia, and this decrease in Rip immunoreactivity may be associated with delayed neuronal death of CA1 pyramidal cells.

Progesterone increases oligodendroglial cell proliferation in rat cerebellar slice cultures

We have previously demonstrated that progesterone significantly increases the rate of myelination in organotypic slice cultures of 7-day-old rat and mouse cerebellum. Here, we show that progesterone (20microM) stimulates the proliferation of oligodendrocyte precursors in cultured cerebellar slices of 7-day-old rats. The steroid increased the number of pre-oligodendrocytes (NG2(+), O4(+)) and to some extent of oligodendrocyte precursors, corresponding to an earlier developmental stage (nestin(+), PDGFalphaR(+), NG2(+), O4(-)). Progesterone stimulated the proliferation of both NG2(+) and O4(+) cells as shown by increased double-immunolabeling with the cell proliferation marker Ki67. The mitogenic effect of progesterone was inhibited by the progesterone receptor antagonist mifepristone (10microM) and could not be mimicked by its GABA-active metabolite 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone), even at the high concentration of 50microM. Results indicate that progesterone first strongly and transiently stimulates the proliferation of oligodendrocyte precursors, and that it may thereafter accelerate their maturation into myelinating oligodendrocytes. Although oligodendrocyte precursors may be a direct target for the actions of progesterone, their number may also be indirectly influenced by the effects of the steroid on neurons and microglial cells, since treatment of the cerebellar slices with progesterone enhanced staining of the neuronal cytoskeleton marker microtubule-associated protein-2 and increased the number of OX-42(+) microglia. A small percentage (about 0.1%) of the NG2(+) cells transiently became OX-42(+) in response to progesterone. These results point to novel mechanisms by which progesterone may promote myelination in the CNS, specifically by stimulating the proliferation and maturation of oligodendrocyte precursors into myelinating oligodendrocytes.

Expression of the transcription factor Pax 6 in the adult rat dentate gyrus

The transcription factor Pax 6 is expressed in precursor cells during embryonic CNS development, and it plays an important role in the regulation of cell proliferation and neuronal fate determination. Pax 6-expressing cells are also present in the adult hippocampal dentate gyrus and subventricular zone/rostral migratory stream, regions in which neuronal precursors exist during adult life. In the adult dentate gyrus, precursor cells are located in the innermost portion of the granule cell layer, and Pax 6-expressing nuclei are most abundant in this region. To examine the putative role of Pax 6 in adult hippocampal neurogenesis, we have studied the proliferative activity, distribution, and phenotype of Pax 6-expressing cells by using immunohistochemistry. Our results indicate that Pax 6 is intensely expressed in proliferating precursors of the adult dentate gyrus. Pax 6 is also expressed in nonproliferating cells, which may correspond to resting progenitor cells and to granule neurons in their very early developmental stages, because this transcription factor is strongly down-regulated during granule neuron differentiation. However, a small subpopulation of hilar mature neurons and certain astrocytes of the adult hippocampus also express Pax 6. Although the precise roles of this transcription factor in the adult brain remain to be determined, our findings support the idea that its function in the control of cell proliferation and neuronal fate determination during embryogenesis is also operative in the adult hippocampus. However, the expression of Pax 6 in astrocytes and certain mature neurons may indicate the existence of other roles for this transcription factor in this telencephalic region.

Neurogenesis in hippocampal slice cultures

A major challenge in studying neurogenesis in the adult brain is gaining access to neural stem cells for experimental manipulation. We developed an approach utilizing mouse hippocampal organotypic cultures to characterize neurogenesis under controlled conditions. After 2 weeks in culture, double immunostaining using the mitotic marker BrdU and cell type-specific markers revealed persistent proliferation of various cell types. The birth of new neurons was restricted to a third subgranular germinal zone as shown by analysis of the expression pattern of the proneural transcription factor neurogenin-2 and colocalization of BrdU with neuronal phenotypic markers. The regional distribution of newly born neurons closely resembled that observed in vivo in the adult hippocampus. Furthermore, neurogenesis was increased by chronic application of epidermal growth factor (EGF) and abolished by adding serum to the culture medium. Our study therefore establishes the hippocampal slice culture as a promising ex vivo model for investigating neurogenesis.

Guanosine promotes myelination and functional recovery in chronic spinal injury

Functional loss after spinal cord injury (SCI) is caused, in part, by demyelination of axons surviving the trauma. Administration of guanosine (8 mg/kg/day, i.p.) for 7 consecutive days, starting 5 weeks after moderate SCI in rats, improved locomotor function and spinal cord remyelination. Myelinogenesis was associated with an increase in the number of mature oligodendrocytes detected in guanosine-treated spinal cord sections in comparison with controls. These data indicate that guanosine-induced remyelination resulted, at least in part, from activation of endogenous oligodendrocyte lineage cells. These findings may have significant implications for chronic demyelinating diseases.

Efficacy of doublecortin as a marker to analyse the absolute number anddendritic growth of newly generated neurons in the adult dentate gyrus

Doublecortin (DCX), a microtubule-associated phosphoprotein, has been recently utilized as a marker of newly born neurons in the adult dentate gyrus (DG). Nonetheless, it is unknown whether DCX exclusively labels newly formed neurons, as certain granule cells with the phenotype of differentiated neurons express DCX. We addressed the authenticity of DCX as a marker of new neurons in the adult DG by quantifying cells that are positive for 5'-bromodeoxyuridine (BrdU), DCX and both BrdU and DCX in hippocampal tissues of adult rats treated with daily injections of BrdU for 12 consecutive days. We provide new evidence that neurons visualized with DCX immunostaining in the adult rat DG are new neurons that are predominantly born during the 12 days before euthanasia. This is confirmed by the robust expression of BrdU in 90% of DCX-positive neurons in the DG of animals injected with BrdU for 12 days. Furthermore, DCX expression is specific to newly generated healthy neurons, as virtually all DCX-positive cells express early neuronal antigens but lack antigens specific to glia, undifferentiated cells or apoptotic cells. As DCX expression is also robust in the dendrites, DCX immunocytochemistry of thicker sections facilitates quantification of the dendritic growth in newly born neurons. Thus, both absolute number and dendritic growth of new neurons that are generated in the adult DG over a 12-day period can be quantified reliably with DCX immunostaining. This could be particularly useful for analysing changes in dentate neurogenesis in human hippocampal tissues as a function of ageing or neurodegenerative diseases.

Parallel activation of field CA2 and dentate gyrus by synaptically elicited perforant path volleys

Previous studies showed that dorsal psalterium (PSD) volleys to the entorhinal cortex (ENT) activated in layer II perforant path neurons projecting to the dentate gyrus. The discharge of layer II neurons was followed by the sequential activation of the dentate gyrus (DG), field CA3, field CA1. The aim of the present study was to ascertain whether in this experimental model field, CA2, a largely ignored sector, is activated either directly by perforant path volleys and/or indirectly by recurrent hippocampal projections. Field potentials evoked by single-shock PSD stimulation were recorded in anesthetized guinea pigs from ENT, DG, fields CA2, CA1, and CA3. Current source-density (CSD) analysis was used to localize the input/s to field CA2. The results showed the presence in field CA2 of an early population spike superimposed on a slow wave (early response) and of a late and smaller population spike, superimposed on a slow wave (late response). CSD analysis during the early CA2 response showed a current sink in stratum lacunosum-moleculare, followed by a sink moving from stratum radiatum to stratum pyramidale, suggesting that this response represented the activation and discharge of CA2 pyramidal neurons, mediated by perforant path fibers to this field. CSD analysis during the late response showed a current sink in middle stratum radiatum of CA2 followed by a sink moving from inner stratum radiatum to stratum pyramidale, suggesting that this response was mediated by Schaffer collaterals from field CA3. No early population spike was evoked in CA3. However, an early current sink of small magnitude was evoked in stratum lacunosum-moleculare of CA3, suggesting the presence of synaptic currents mediated by perforant path fibers to this field. The results provide novel information about the perforant path system, by showing that dorsal psalterium volleys to the entorhinal cortex activate perforant path neurons that evoke the parallel discharge of granule cells and CA2 pyramidal neurons and depolarization, but no discharge of CA3 pyramidal neurons. Consequently, field CA2 may mediate the direct transfer of ENT signals to hippocampal and extrahippocampal structures in parallel with the DG-CA3-CA1 system and may provide a security factor in situations in which the latter is disrupted.

Effects of early environment on field CA2 pyramidal neurons in the guinea-pig

Previous work showed that isolation rearing produces remarkable changes in the dendritic pattern and soma of the principal neurons in the dentate gyrus and hippocampal fields CA3 and CA1 of the guinea-pig. The aim of the present study was to obtain information about the effects of early postnatal isolation on neuron morphology in field CA2, the "resistant sector" of the hippocampal formation. Male and female guinea-pigs were assigned at 6-7 days of age to either a control (social) or an isolated environment where they remained for 80-90 days. The apical and basal dendritic trees and the soma of CA2 pyramidal neurons were analyzed and quantified in Golgi-stained brains. The results showed that in both males and females early isolation caused no effects on the length and dendritic branching density of the apical tree of field CA2 pyramidal neurons. In males but not in females isolation caused a spine density reduction in the inner apical tree. Isolation notably influenced the morphology of the basal tree, but in males only. Isolated males exhibited a significant reduction in the length of the basal tree and number of dendritic branches accompanied by a reduction in spine density. The comparison of animals reared in the same environment showed that in the control environment males had more apical and basal dendritic branches and a larger neuron soma than females. In the isolated environment the sex differences in the apical tree disappeared and those in the basal tree changed direction.The results demonstrate structural changes in field CA2 pyramidal neurons following neonatal isolation, with a specific reactivity to environment of the basal tree of males. The dendritic atrophy in field CA2 of isolated males is in line with previous evidence that males react to isolation mainly with dendritic atrophy, though field CA2 neurons appear to be less damaged than those of the other hippocampal fields. This is in line with the resistance of this field to neurodegeneration. The absence of structural changes in field CA2 of isolated females confirms, once again, that males are more liable to be endangered by early isolation than females.

Functional network integration of embryonic stem cell-derived astrocytes in hippocampal slice cultures

Embryonic stem (ES) cells provide attractive prospects for neural transplantation. So far, grafting strategies in the CNS have focused mainly on neuronal replacement. Employing a slice culture model, we found that ES cell-derived glial precursors (ESGPs) possess a remarkable capacity to integrate into the host glial network. Following deposition on the surface of hippocampal slices, ESGPs actively migrate into the recipient tissue and establish extensive cell-cell contacts with recipient glia. Gap junction-mediated coupling between donor and host astrocytes permits widespread delivery of dye from single donor cells. During maturation,engrafted donor cells display morphological, immunochemical and electrophysiological properties that are characteristic of differentiating native glia. Our findings provide the first evidence of functional integration of grafted astrocytes, and depict glial network integration as a potential route for widespread transcellular delivery of small molecules to the CNS.

Semliki Forest virus A7(74) transduces hippocampal neurons and glial cells in a temperature-dependent dual manner

In central nervous system (CNS) tissue preparations, wild-type Semliki Forest virus (SFV) mainly infects neurons, and in vivo it causes lethal encephalitis in neonatal and adult rodents. The SFV strain A7(74), by contrast, is avirulent in adult rodents, triggering only limited CNS infection. To examine A7(74) infection in hippocampal tissue, the authors constructed a replicon, termed SFV(A774nsP)-GFP, expressing green fluorescent protein. The results were compared to replication-proficient recombinant A7(74) encoding GFP, named VA7-EGFP. As nonstructural gene mutations can confer temperature sensitivity, the authors also tested whether infection was temperature-dependent. Indeed, at 31 degrees C both viral recombinants transduced significantly more baby hamster kidney cells than at 37 degrees C. When rat hippocampal slices and dissociated cells were incubated at 37 degrees C, SFV(A774nsP)-GFP transduced glial cells but virtually no neurons-the opposite of conventional SFV. For VA7-EGFP at 37 degrees C, the preferred GFP-positive cells in hippocampal slices were also non-neuronal cells. At 31 degrees C, however, a more wild-type phenotype was found, with 33% and 94% of the GFP-positive cells being neurons for SFV(A774nsP)-GFP in slices and dissociated cells, respectively, and 94% neurons for VA7-EGFP in slices. Immunochemical and electrophysiological analyses confirmed that at 37 degrees C virtually all cells transduced by SFV(A774nsP)-GFP in slices were astrocytes, while at 31 degrees C they also contained neurons. These results show that in addition to the developmental age, the temperature determines which cell type becomes infected by A7(74). Our data suggest that A7(74) is avirulent in adult animals because it does not readily replicate in mature neurons at body temperature, whereas it still does so at lower temperatures.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Widespread expression of rat collapsin response-mediated protein 4 in the telencephalon and other areas of the adult rat central nervous system

The rat collapsin response-mediated protein 4 (rCRMP-4) is a member of a family of proteins that are involved in axonal growth. It is found transiently in postmitotic neurons, such as those that are generated in the adult hippocampus. The authors used immunocytochemistry to investigate whether areas of the rat central nervous system (CNS) that retain postnatal neurogenesis express this protein. They found pronounced rCRMP-4 immunoreactivity in recently generated cells in the dentate granular layer, the subventricular zone, the olfactory bulbs, and the rostral migratory stream, four areas in which the production or migration of neurons occurs in adulthood. However, rCRMP-4 immunoreactivity also is expressed in many other regions of the rat brain in which there is no record of adult neurogenesis or neuronal migration, e.g., in the olfactory glomeruli and in neurons of the cerebral cortex. In the hypothalamus, intensely rCRMP-4-labeled neurons populated the supraoptic, paraventricular, and periventricular nuclei as well as the median eminence and the arcuate nucleus. Immunoreactivity for rCRMP-4 also was present in certain neurons of the interpeduncular nucleus, median raphe, superior colliculus, and scattered granule cerebellar neurons. Many of these regions are known to display axonal outgrowth and/or synaptic rearrangement in adulthood and to coexpress the polysialylated form of the neural cell adhesion molecule. Thus, the results of this study suggest that rCRMP-4 expression in the CNS is associated with cells that are migrating or are undergoing axonal growth. Nevertheless, small, rCRMP-4-immunoreactive cells were seen throughout the brain. These cells did not express neuronal, astroglial, or microglial markers, although some of them also were immunoreactive for rip antibody, suggesting an oligodendroglial lineage.

The role of oligodendrocytes and oligodendrocyte progenitors in CNS remyelination

Remyelination enables restoration of saltatory conduction and a return of normal function lost during demyelination. Unfortunately, remyelination is often incomplete in the adult human central nervous system (CNS) and this failure of remyelination is one of the main reasons for clinical deficits in demyelinating disease. An understanding of the failure of remyelination in demyelinating diseases such as Multiple Sclerosis depends upon the elucidation of cellular events underlying successful remyelination. Although the potential for remyelination of the adult CNS has been well established, there is still some dispute regarding the origin of the remyelinating cell population. The literature variously reports that remyelinating oligodendrocytes arise from dedifferentiation and/or proliferation of mature oligodendrocytes, or are generated solely from proliferation and differentiation of glial progenitor cells. This review focuses on studies carried out on remyelinating lesions in the adult rat spinal cord produced by injection of antibodies to galactocerebroside plus serum complement that demonstrate: 1) oligodendrocytes which survive within an area of demyelination do not contribute to remyelination, 2) remyelination is carried out by oligodendrocyte progenitor cells, 3) recruitment of oligodendrocyte progenitors to an area of demyelination is a local response, and 4) division of oligodendrocyte progenitors is symmetrical and results in chronic depletion of the oligodendrocyte progenitor population in the normal white matter around an area of remyelination. These results suggest that failure of remyelination may be contributed to by a depletion of oligodendrocyte progenitors especially following repeated episodes of demyelination. Remyelination allows the return of saltatory conduction (Smith et al., 1979) and the functional recovery of demyelination-induced deficits (Jeffery et al., 1997). Findings such as these have encouraged research aimed at enhancing the limited remyelination found in Multiple Sclerosis (MS) lesions, evidenced by a rim of thin myelin sheaths around the edges of a lesion, or, in a minority of acute foci, throughout the entire lesion (Prineas et al., 1989; Raine et al., 1981). It must be said, however, that although remyelination is clearly a prerequisite to sustained functional recovery, other factors such as the state of the inflammatory response and degree of axonal survival within the demyelinated region contribute to the extent of functional recovery that may be possible following therapeutic intervention aimed at halting disease progression. It is not yet clear whether the progression of functional deficits in MS is primarily the result of an increasing load of demyelination, or axon loss, or a combination of the two processes. However, given the increasing recognition that myelin sheaths play a role in protecting axons from degeneration, the success or failure of remyelination has functional consequences for the patient. To understand why remyelination should fail in demyelinating disease and develop strategies to enhance remyelination requires an understanding of the biology of successful remyelination. Firstly, what is the origin of the remyelinating cell population in the adult CNS? Secondly, what are the dynamics of the cellular response of this population during demyelination and remyelination? And thirdly, what are the consequences to the tissue of an episode of demyelination? This review will focus on studies that address these issues, and discuss the implications of the results of these experiments for our understanding of MS and the development of therapeutic interventions aimed at enhancing remyelination.

Organotypic culture system of chicken retina

Analysis of developmental mechanisms during neuroembryogenesis, evaluation of toxicological effects and testing of neuroprotheses rely to an increasing extent on in vivo-like in vitro models. We have developed a novel organotypic culture system of the chick retina. Tissue slices of embryonic retinae were immobilized on glass coverslips by a fibrin clot and permanently rotated between the gas and medium phase, resulting in regular formation and the maintenance of the retinal cytoarchitecture. Selection of embryonic stage, slice thickness and specimen processing were optimized for culturing. Scanning electron microscopy revealed degradation during increasing culture periods of the fibrin clot, which was used for initial immobilization of explants on glass coverslips. Simultaneously, retinal cells became exposed on the tissue surface. Even after several weeks in vitro, formation and maintenance of plexiform and nuclear layers was evident as revealed by two specific monoclonal antibodies. Immunocytochemistry employing two additional photoreceptor- and radial Müller-antibodies indicated differentiation of neuronal and glial cells specific for the retina. The organotypic culture system promises to facilitate developmental studies of retinal development. Quantitative evaluation of Na(+)-channel blocker mexiletine impact on the histogenesis of retinal explants proved the organotypic culture system to be a valuable tool also for neurotoxicological investigations.

Pyramidal neuron types in field CA2 of the guinea pig

The aim of the present study was to obtain information about the pyramidal neuron types in hippocampal field CA2 of the guinea pig. The apical and basal dendritic trees and the somata of CA2 pyramidal neurons were analyzed and quantified in Golgi-stained brains of adult guinea pigs of both sexes. Most of field CA2 pyramidal neurons (92%) had a single apical shaft (monoapical neurons) and very few neurons (8%) had two apical shafts (biapical neurons). The monoapical neurons were subdivided into three classes on the basis of the pattern of their apical tree. Morphometric analysis showed differences among the different neuron classes in the number of apical and basal dendritic branches, in the spread of the apical tree, in shaft dimensions and in soma diameters. Several morphometric sex differences were also found for two of the three monoapical neuron classes and for the biapical neurons. The results demonstrate that field CA2 of the guinea pig contains four different types of pyramidal neurons and that also this small hippocampal field is characterized, like the other hippocampal fields, by the presence of a heterogeneous population of pyramidal neurons.

Microglial reactivity correlates to the density and the myelination of the anterogradely degenerating axons and terminals following perforant path denervation of the mouse fascia dentata

Transection of the entorhino-dentate perforant path is a well known model for lesion-induced axonal sprouting and glial reactions in the rat. In this study, we have characterized the microglial reaction in the dentate molecular layer of the SJL/J and C57Bl/6 mouse. The morphological transformation of the microglial cells and their densitometrically measured Mac-1 immunoreactivity were correlated with the density of silver-impregnated axonal and terminal degeneration and the myelination of the degenerating medial and lateral perforant pathways. Anterograde axonal and terminal degeneration leads to: (i) altered myelin basic protein immunoreactivity with the appearance of discrete myelin deposits preferentially in the denervated medial and significantly less so in the lateral perforant path zone from day 2 after lesioning; (ii) an increase in number and Mac-1 immunoreactivity of morphologically-changed microglial cells in the denervated perforant path zones with more pronounced morphological transformation of microglia in the medial than in the lateral perforant path zones at day 2 but not day 5 after lesioning; and (iii) a linear correlation between the density of microglial Mac-1 reactivity and axonal degeneration in the medial but not in the lateral perforant path zone at two days postlesion, and a linear correlation in both zones at five days postlesion. We propose that the differentiated microglial response is due to the different densities of axonal and terminal degeneration, as observed in the individual cases. The finding of a potentiated or accelerated microglial activation in the medial as compared to the lateral perforant path zone suggests different kinetics of microglial activation in areas with degenerating myelinated and unmyelinated fibers.

The origin of remyelinating cells in the central nervous system

A clear understanding of the cellular events underlying successful remyelination of demyelinating lesions is a necessary prerequisite for an understanding of the failure of remyelination in multiple sclerosis (MS). The potential for remyelination of the adult central nervous system (CNS) has been well-established. However, there is still some dispute whether remyelinating oligodendrocytes arise from dedifferentiation and/or proliferation of mature oligodendrocytes, or are generated solely from proliferation and differentiation of glial progenitor cells. This review focuses on studies carried out on remyelinating lesions in the adult rat spinal cord produced by injection of antibodies to galactocerebroside and serum complement that show: (1) oligodendrocytes which survive within an area of demyelination do not contribute to remyelination, (2) remyelination is carried out by oligodendrocyte progenitor cells, (3) recruitment of oligodendrocyte progenitors to an area of demyelination is a local response, and (4) division of oligodendrocyte progenitors is symmetrical, resulting in chronic depletion of the oligodendrocyte progenitor population in the normal white matter around an area of remyelination. Such results suggest that repeated episodes of demyelination could lead to a failure of remyelination due to a depletion of oligodendrocyte progenitors.

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.

Oligodendrocyte-specific gene expression in mouse brain: use of a myelin-forming cell type-specific promoter in an adeno-associated virus

To explore the feasibility of cell type-specific gene expression in oligodendrocytes as a possible therapeutic approach for demyelinating diseases, the cell specificity, tissue specificity, and duration of gene expression were investigated using recombinant adeno-associated viral vectors (rAAV) carrying a green fluorescence protein (GFP) gene. Recombinant AAV vectors carrying either the myelin basic protein (MBP) promoter (rAAV-MBP-GFP) or the cytomegalovirus (CMV) immediate early promoter (rAAV-CMV-GFP) were semistereotactically injected into the brain of C57BL/6J mice. Injection of the rAAV-MBP-GFP vector into or near the corpus callosum resulted in high levels of GFP expression in white matter regions. Double immunostaining with cell- specific markers proved that these GFP-expressing cells were oligodendrocytes. Injection of the rAAV- MBP-GFP vector into gray matter rarely produced GFP expression. In contrast, injection of the rAAV-CMV-GFP vector resulted in few GFP-expressing cells in the white matter, with most of the GFP-expressing cells being neurons located in the cerebral cortex along the needle track. The expression of the GFP driven by the MBP promoter persisted for at least 3 months.

Radial astrocytes: toxic effects induced by antiepileptic drug in the developing rat hippocampus in vitro

Neuron-glia relationships are crucial for differentiation of both glial and neuronal cells. Interference with these intricate cell interactions could affect regular neuroembryogenesis. In order to analyse potential developmental neurotoxic effects of therapeutically administered antiepileptics such as valproate, we employed organotypic cultures of the rat hippocampus. In these cultures thin tissue slices were continuously rotated between the gas and medium phases, which greatly improved oxygen and nutrient accessibility. This resulted in long-term preservation of the native cytoarchitecture. Exposure of organotypically cultured hippocampi to valproate hampered, in a dose-dependent manner, regular formation of the pyramidal cell layer. Most interestingly, radial astrocytes, which comprise a transient cell population during distinct developmental periods, were selectively affected even by low doses of valproate, but not by structurally related non-teratogenic isomer 2-ethyl-4-methyl-pentanoic acid. The xenobiotic effect did not represent a general gliotoxic insult, since neither the glutathione quotient as determined by HPLC, nor the DNA content, nor the total amount of glial fibrillary acidic protein evaluated by ELISA were significantly altered. Instead, the morphology of astrocytes proved to be the most sensitive index of intoxication with the orientation of radial astrocytes being most affected as revealed by immunofluorescence. In contrast to radial astrocytes, other astrocytic populations proved to be fairly resistent. The data indicate that developmentally regulated cell polarity of astrocytes is a target of therapeutically relevant xenobiotics. This could in turn disturb neuronal differentiation and normal histogenesis.

A role for glutamate and its receptors in the regulation of oligodendrocyte development in cerebellar tissue slices

We tested the hypothesis that the neurotransmitter glutamate would influence glial proliferation and differentiation in a cytoarchitecturally intact system. Postnatal day 6 cerebellar slices were maintained in organotypic culture and treated with glutamate receptor agonists or antagonists. After dissociation, cells were stained with antibodies for different oligodendrocyte developmentally regulated antigens. Treatment of the slices with the glutamate receptor agonists kainate or alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid significantly decreased the percentage of LB1(+), NG2(+) and O4(+) cells, and their bromodeoxyuridine labeling index. The non-N-methyl-D-aspartate glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione increased the percentage and bromodeoxyuridine labeling of LB1(+), NG2(+) and O4(+) cells. In intact slices, RNA levels of the oligodendrocyte gene for 2′,3′-cyclic nucleotide 3′-phosphodiesterase were decreased by kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and increased by 6,7-dinitroquinoxaline-2,3-dione. The percentage of astrocytes was not modified by kainate, alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or 6, 7-dinitroquinoxaline-2,3-dione. Treatment with the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid did not alter the percentage of O4(+) cells, nor their proliferation. Incubation with the gamma-aminobutyric acid receptor antagonist bicuculline did not modify the percentage of LB1(+), A2B5(+) and O4(+) cells. In purified cerebellar oligodendrocyte progenitor cells, glutamate receptor agonists blocked K+ currents, and inhibited cell proliferation and lineage progression. The K+ channel blocker tetraethylammonium also inhibited oligodendrocyte progenitor cell proliferation. These findings indicate that in rat cerebellar tissue slices: (i) glutamate specifically modulates oligodendrocyte but not astrocyte development through selective activation of alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, and (ii) cell depolarization and blockage of voltage-dependent K+ channels is likely to be the triggering mechanism.

Unique responses of differentiating neuronal growth cones to inhibitory cues presented by oligodendrocytes

During central nervous system development, neurons differentiate distinct axonal and dendritic processes whose outgrowth is influenced by environmental cues. Given the known intrinsic differences between axons and dendrites and that little is known about the response of dendrites to inhibitory cues, we tested the hypothesis that outgrowth of differentiating axons and dendrites of hippocampal neurons is differentially influenced by inhibitory environmental cues. A sensitive growth cone behavior assay was used to assess responses of differentiating axonal and dendritic growth cones to oligodendrocytes and oligodendrocyte- derived, myelin-associated glycoprotein (MAG). We report that >90% of axonal growth cones collapsed after contact with oligodendrocytes. None of the encounters between differentiating, MAP-2 positive dendritic growth cones and oligodendrocytes resulted in growth cone collapse. The insensitivity of differentiating dendritic growth cones appears to be acquired since they develop from minor processes whose growth cones are inhibited (nearly 70% collapse) by contact with oligodendrocytes. Recombinant MAG(rMAG)-coated beads caused collapse of 72% of axonal growth cones but only 29% of differentiating dendritic growth cones. Unlike their response to contact with oligodendrocytes, few growth cones of minor processes were inhibited by rMAG-coated beads (20% collapsed). These results reveal the capability of differentiating growth cones of the same neuron to partition the complex molecular terrain they navigate by generating unique responses to particular inhibitory environmental cues.

Nonspecific labeling of myelin with secondary antisera and high concentrations of Triton X-100

Triton X-100 is used in immunohistochemistry to make tissue permeable, to present certain antigens to antisera, and to prevent certain nonspecific interactions. This detergent is routinely dissolved in buffers at concentrations of 0.01-0.2%. Using high concentrations of Triton X-100 (0.2-2%) and anti-immunoglobulins G (anti-IgGs), labeling of myelin and microglia was detected in fixed brain tissue by indirect fluorescence and avidin-biotin-immunoperoxidase techniques. Differences were found between the species studied (mouse and rat), the type of anti-IgG (anti-mouse, anti-rabbit, anti-sheep, anti-rat, or anti-guinea pig), the detergent concentration, and whether Triton X-100 was included in the incubation media or applied as a pretreatment. Mouse brain displayed strong myelin labeling with all anti-IgGs but rat brain only with anti-rabbit or anti-sheep IgGs. Staining of ramified microglia occurred only in mouse tissue when anti-mouse IgG was used. Nonspecific staining of myelin was also intense in paraffin-embedded tissue and in human brain frozen sections. These results are significant for the prevention of undesirable staining in routine immunolabeling and they also provide a comparatively inexpensive, easy to perform strong labeling of myelin. In addition, the double marker signal (peroxidase and fluorescence) is useful for double labeling studies. (J Histochem Cytochem 46:109-117, 1998)