Expression of two neuronal markers, growth-associated protein 43 and neuron-specific enolase, in rat glial cells

Recent studies have revealed that proteins such as growth-associated protein 43 (GAP-43) and neuron-specific enolase (NSE), believed for many years to be expressed exclusively in neurons, are also present in glial cells under some circumstances. Here we present an overview of these observations. GAP-43 is expressed both in vitro and in vivo transiently in immature rat oligodendroglial cells of the central nervous system, in Schwann cell precursors, and in non-myelin-forming Schwann cells of the peripheral nervous system. GAP-43 mRNA is also present in oligodendroglial cells and Schwann cells, indicating that GAP-43 is synthesized in these cells. GAP-43 is also expressed in type 2 astrocytes (stellate-shaped astrocytes) and in some reactive astrocytes but not in type 1 astrocytes (flat protoplasmic astrocytes). These results suggest that GAP-43 plays a more general role in neural plasticity during development of the central and peripheral nervous systems. NSE enzymatic activity and protein and mRNA have been detected in rat cultured oligodendrocytes at levels comparable to those of cultured neurons. NSE expression increases during the differentiation of oligodendrocyte precursors into oligodendrocytes. In vivo, NSE protein is expressed in differentiating oligodendrocytes and is repressed in fully mature adult cells. The upregulation of NSE in differentiating oligodendrocytes coincides with the formation of large amounts of membrane structures and of protoplasmic processes. Similarly, NSE becomes detectable in glial neoplasms and reactive glial cells at the time when these cells undergo morphological changes. The expression of the glycolytic isozyme NSE in these cells, which do not normally contain it, could reflect a response to higher energy demands. This expression may also be related to the neurotrophic and neuroprotective properties demonstrated for this enolase isoform. NSE activity and protein and mRNA have also been found in cultured rat type 1-like astrocytes but at much lower levels than in neurons and oligodendrocytes. Thus GAP-43 and NSE should be used with caution as neuron-specific markers in studies of normal and pathological neural development.     

Development and regeneration of the nervous system: a role for neurosteroids

Several steroids, termed 'neurosteroids', are synthesized from cholesterol within both the central and peripheral nervous systems. These include pregnenolone and its sulfate ester, progesterone and its 5 alpha-reduced metabolites. Dehydroepiandrosterone, mainly in its sulfated form, also remains present in the brain long after removal of the steroidogenic endocrine glands. Its biosynthesis in brain remains an open possibility, but the pathways involved are unknown. Little information is available concerning the role of neurosteroids during the maturation of the nervous system, although they are already synthesized by glial cells and by some populations of neurons during embryonic life. Cell culture experiments suggest that neurosteroids may increase the survival and differentiation of both neurons and glial cells. In the adult nervous system, neurosteroids modulate neurotransmission by acting directly on the neuronal membrane and also produce structural changes in neurons and in astrocytes. Studies of neurosteroid levels are currently conducted to examine their possible role during aging. We have recently reported that progesterone, synthesized by Schwann cells, promotes the formation of new myelin sheaths after lesion of the mouse sciatic nerve. Thus, neurosteroids may also play an important role during regeneration of the nervous system.     

Experimental jaw-muscle pain does not change heteronymous H-reflexes in the human temporalis muscle

The role of growth factors in controlling the development of glial cells in both the peripheral and central nervous systems has been investigated for a number of years. The recent discovery of a new family of growth factors termed the neuregulins (NRGs) has led to an explosion of information concerning the putative role of these growth factors in the development of Schwann cells (SC), oligodendrocytes (OLG), and astrocytes. Many of these previous studies have focused on the effects of exogenous NRGs on glial cell development and differentiation. We now review the evidence that these glial cells themselves produce NRGs and discuss the major implications of these findings with respect to glial cell development and diseases which affect glial cell function. We also discuss the potential role of endogenous NRGs following neural injury.     

Calcium waves in retinal glial cells

Calcium signals were recorded from glial cells in acutely isolated rat retina to determine whether Ca2+ waves occur in glial cells of intact central nervous system tissue. Chemical (adenosine triphosphate), electrical, and mechanical stimulation of astrocytes initiated increases in the intracellular concentration of Ca2+ that propagated at approximately 23 micrometers per second through astrocytes and Müller cells as intercellular waves. The Ca2+ waves persisted in the absence of extracellular Ca2+ but were largely abolished by thapsigargin and intracellular heparin, indicating that Ca2+ was released from intracellular stores. The waves did not evoke changes in cell membrane potential but traveled synchronously in astrocytes and Müller cells, suggesting a functional linkage between these two types of glial cells. Such glial Ca2+ waves may constitute an extraneuronal signaling pathway in the central nervous system.     

Effects of axonal injury on astrocyte proliferation and morphology in vitro: implications for astrogliosis

Mechanisms inducing gliosis following injury in the central nervous sy stem are poorly understood. We evaluated the effect of axonal injury on astrocyte and Schwann cell proliferation and morphology in vitro. Purified rat dorsal root ganglion neurons grown on monolayers of rat neonatal cortical astrocytes (N-ASneonatal cultures) or sciatic nerve-derived Schwann cells (N-SC cultures) were mechanically injured. Non-injured cultures served as controls. Cell proliferation near lesions was monitored by autoradiography 1,2,4, and 8 days postinjury. Axonal injury caused a significant transient increase in astrocyte proliferation immediately proximal and distal to the lesion. The lesion did not induce marked changes in the intensity of glial fibrillary acidic protein (GFAP) immunoreactivity. However, processes from GFAP-positive cells usually arranged in random fashion in noninjured cultures were aligned perpendicularly to the cut distal to lesions. Ultrastructural analysis in lesioned N-ASneonatal cultures indicated that proximal to the lesion filament-filled astrocytes were intermingled with axons. Distal to the lesion astrocyte processes formed layers, between which an increased amount of collagen-like material appeared with time postlesion. Axons distal to the lesion degenerated by 2 days, coinciding with the early disappearance of neurofilament immunoreactivity. In noninjured and proximally in injured N-SC cultures, Schwann cells extended processes, engulfing some axons. Distal to the lesion, Schwann cells appeared more rounded and neurites remained until 4 days postinjury. Media conditioned by injured or non-injured N-ASneonatal cultures did not affect neuron-induced Schwann cell proliferation. These findings demonstrate that axonal injury and degeneration cause a transient increase in astrocyte proliferation and induce morphological changes in astrocytes consistent with the onset of gliosis.     

Establishment and neurite outgrowth properties of neonatal and adult rat olfactory bulb glial cell lines

Two glial cell types surround olfactory axons and glomeruli in the olfactory bulb (OB) and may influence synapse development and regeneration. OB astrocytes resemble type-1 astrocytes, and OB ensheathing cells resemble non-myelinating Schwann cells. We have produced clonal OB astrocyte and ensheathing cell lines from rat neonatal and adult OB cultures by SV40 large T antigen transduction. These cell lines have been characterized by morphology, growth characteristics, immunophenotype, and ability to promote neurite outgrowth in vitro. Neonatal and adult ensheathing cell lines were found to support higher neurite outgrowth than OB astrocyte lines. Neonatal OB astrocyte lines were of two types, high and low outgrowth support. The low support astrocyte lines express J1 and a chondroitin sulfate-containing proteoglycan as do astrocytes encircling the neonatal glomeruli in vivo. The adult OB astrocyte cell lines supported lower levels of outgrowth than adult ensheathing cell lines. These results are consistent with a positive role for ensheathing cells in OB synapse regeneration, in vivo. Further, based on our results, we hypothesize that ensheathing cells and high-outgrowth astrocytes facilitate axon growth in vivo, while low outgrowth astrocytes inhibit axon growth and may facilitate glomerulus formation.     

Distribution of glial fibrillary acidic protein in central nervous system lesions of tuberous sclerosis

The distribution of glial fibrillary acidic protein (GFAP) in the central nervous system (CNS) lesions of tuberous sclerosis (TS) was examined using antiserum against GFAP and the peroxidase antiperoxidase method of Sternberger. In cortical tubers there were islands of gemistocytic astrocytes staining intensely for GFAP and occasional giant cells having some cytoplasmic staining. The majority of the cortical giant cells had no GFAP. The islands were separated by areas devoid of astrocytes with perikaryal staining. A faintly staining fibrous network was found between these islands. The majority of cells in the subependymal nodules stained. The retinal phakoma stained but not as intensely as the subependymal nodules. There was no staining whatsoever in the giant cell subependymal tumors. Absence of GFAP staining in the subependymal giant cell tumors makes their classification as astrocytomas less certain.     

Neural tissue engineering: from polymer to biohybrid organs

This investigation reports on the immobilization of neuronal and glial cells (Schwann cells and astrocytes) within N-(2-hydroxypropyl) methacrylamide (HPMA) polymer hydrogels for the production of cell-based polymer hybrid devices. Cells were included within HPMA polymer networks by gel-entrapment, and these biogels were maintained in vitro for up to 6 days. Cell viability and differentiation were studied using immunocytochemical methods and image analysis techniques. The polymer structure and its relationships with cells were examined by scanning electron microscopy. A proportion of the cell population was viable, expressing its own antigenic profile throughout the period of gel incubation, as cells do in conventional culture conditions, and some cells exhibited behaviour such as spreading or process outgrowth and secretion of laminin. The result of the present study allows us to envisage tissue replacement in the central nervous system by means of such cell-based polymer constructs.     

Steroid 5alpha-reductase type 1 immunolocalized in the rat peripheral nervous system and paraganglia

Steroid 5alpha-reductase is an enzyme that converts a number of steroids with a C-4, 5 double bond and C-3 ketone to 5alpha-reduced metabolites. This enzyme has been suggested to play a role in brain development and myelination in the rat nervous system. In the present study, we examined the cellular and subcellular localization of the enzyme immunocytochemically in the rat peripheral nervous system and paraganglia using a polyclonal antibody against rat 5alpha-reductase type 1. Light and electron microscopical studies localized 5alpha-reductase in the Schwann cells of myelinated and unmyelinated nerve fibres, the satellite cells of the ganglia, the enteric glial cells and the supporting/sustentacular cells of the paraganglia. In the myelinated nerve fibres, immunoreactivity was observed in the outer loops, the nodes of Ranvier and the Schmidt-Lanterman incisures. Subcellularly, the immunoreactivity was localized in the cytoplasm of various glial cells. No immunoreactivity was observed in the myelin membrane, the axon or the neuronal perikaryon. These findings suggest that 5alpha-reductase is widely distributed in glial cells, and that, in addition to myelination, 5alpha-reduced steroids play a role in some glial functions in the peripheral nervous system.     

Calcium binding protein calcyphosine in dog central astrocytes and ependymal cells and in peripheral neurons

Calcyphosine is a calcium binding protein discovered in the dog thyroid in 1979. Calcyphosine mRNA and immunoreactivity were detected using Western and Northern blotting in the cerebral cortex, cerebral white matter and cerebellum. Using immunohistochemistry and in situ hybridization, both are present in ependymal cells, choroid plexus cells and several types of astrocytes of the subependymal cerebral layer, the cerebellar Bergmann layer, the retinal ganglion cell layer, the optic nerve and the posterior pituitary. Both are also present in neurons of nasal olfactory mucosa, enteric Auerbach and Meissner plexuses, orthosympathic and spinal cord ganglia as well as in endocrine cells of neural crest origin in the adrenal medulla. Calcyphosine immunoreactive astrocytes were also present mainly in hemispheric cerebral gray and white matter, hemispheric subcortical structures, brain stem and spinal cord. These results show that calcyphosine is a characteristic calcium binding protein of astrocytes and ependymal cells in the central nervous system and of neurons in the peripheral nervous system. This is of interest in view of the importance of calcium regulation in these cells, and since calcyphosine a calcium binding protein phosphorylated by cAMP dependent process, may be an intermediate between cAMP and inositol phosphate cascades.     

The morphogenesis of mouse vallate gustatory epithelium and taste buds requires BDNF-dependent taste neurons

Heterotypic coupling, defined as gap-junctional coupling between cells of different classes, may be common among the different types of non-neuronal cells in the central nervous system. Since gap junctions provide a route for the intercellular exchange of signaling molecules, heterotypic coupling may serve to coordinate the activities of many types of "support cells" in the brain. The evidence for heterotypic coupling between astrocytes and oligodendrocytes, astrocytes and retinal Müller glial cells, and astrocytes and ependymal cells is reviewed. The finding that some heterotypic gap junctions are chemically rectifying implies that there is asymmetry between the two sides of these gap junctions, and the connexin composition of heterotypic gap junctions is discussed. Finally, I speculate about the functions of heterotypic gap junctions, including their proposed roles in K+ spatial buffering around axons and in the propagation of intercellular Ca2+ waves between astrocytes and other glial cells.     

An increased number of follicles containing activated CD69+ helper T cells and proliferating CD71+ B cells are found in H. pylori-infected gastric mucosa

Multiple sclerosis is characterized by myelin destruction and oligodendrocyte loss. The neuropathological hallmark of the disease is the presence of demyelinated plaques in the central nervous system. We have recently found a gliotoxic factor in MS cerebrospinal fluid which induces programmed cell death in vitro, in glial cells. Here we show DNA fragmentation and glial cell death in biopsy samples, obtained from a patient who underwent surgery with suspicion of tumor, and whose disease record, including brain autopsy, demonstrated an active multiple sclerosis. We used the in situ TUNEL technique, a method which sensitively detects the DNA fragmentation accompanying programmed cell death in tissue sections, and compatible with classical fixation techniques. We found intense DNA fragmentation in nuclei of glial cells at-or very near-to the site of demyelination. A double labeling technique showed that glial fibrillary associated protein positive astrocytes may undergo programmed cell death in multiple sclerosis.     

The relation between rhinitis and allergic asthma. The action of antihistaminics on bronchial hyperreactivity

The intermediate filament nestin is highly expressed in multipotential stem cells of the developing central nervous system (CNS). During neuro- and gliogenesis, nestin is replaced by cell type-specific intermediate filaments, e.g. neurofilaments and glial fibrillary acidic protein (GFAP). In this study, we demonstrate that nestin expression is re-induced in reactive astrocytes in the lesioned adult brain. Following ischaemic and mechanical lesioning, a strong and sustained expression of nestin was noted in GFAP-positive cells surrounding the lesion site. Lesion experiments in transgenic mice carrying the lacZ gene under control of regulatory sequences from the nestin gene suggested that the upregulation of nestin in reactive astrocytes is mediated via the same sequences that control nestin expression during CNS development. These observations and recent data on the co-expression of glial and neuronal marker antigens in reactive astrocytes point to a close relationship between proliferating astrocytes and neuroepithelial precursor cells.     

Expression of the brain creatine kinase gene in rat RT4 peripheral neurotumor cell lines and its modulation by cell confluence

Creatine kinases (CK) catalyze the reversible transfer of a high energy phosphate group between creatine phosphate and ADP to regenerate ATP in cell types where the requirements for ATP are extensive and/or sudden. Previously, we have shown in primary rat brain cell cultures that brain CK (CKB) mRNA levels are highest in astrocytes and oligodendrocytes and much lower in neuronal cells. However, little is known of the factors which regulate CKB expression in the central nervous system and peripheral nervous system. To begin to investigate these factors, we asked in this report (1) if this pattern of CKB expression was also characteristic of some established glial and neuronal cell lines derived from the PNS; (2) whether CKB expression could be rapidly modulated by culture conditions, and (3) if CKB is expressed in cells with characteristics of glial cell progenitors. In subconfluent cells, CKB mRNA and enzyme activity were found to be high in both the rat RT4 peripheral neurotumor stem cell RT4-AC36A and its glial cell derivative RT4-D6. Conversely, CKB mRNA and activity were 5- and 8-fold lower, respectively, in the neuronal derivative RT4-E5 and, more dramatically, CKB was undetectable in neuronal RT4-B8 cells. Maintaining RT4-D6 glial cells at confluence rapidly increased CKB enzyme activity by 7-fold, such that D6 cells contained about 25% of the CKB level in lysates prepared from either whole adult rat brain or primary cultures of rat brain astrocytes. The levels of CKB mRNA and immunoreactive protein were also correspondingly increased in confluent D6 cells. These confluence-mediated increases in CKB appeared to be due to cell-cell contact and not the depletion of serum growth factors or an increase in intracellular cAMP. This study indicates that CKB expression is highest in cells displaying glial properties and can be rapidly modulated by appropriate culture conditions. The results are discussed in relation to the factors which may regulate CKB expression in vivo.     

A culture model of reactive astrocytes: increased nerve growth factor synthesis and reexpression of cytokine responsiveness

Reactive gliosis, which occurs in response to damage to the central nervous system, has been recognized for years but is not yet understood. We describe here a tissue culture model of reactive astrocytes used to characterize their properties. Cultures are prepared 1 week following 6-hydroxydopamine (6-OHDA) lesion of rat substantia nigra and compared with astrocytes cultured from normal adult rats or rats injected with saline only. Astrocytes from the 6-OHDA-lesioned side contained elevated levels of glial fibrillary acidic protein (GFAP) and GFAP mRNA and were intensely immunoreactive for GFAP, vimentin, and two epitopes that in vivo are found only on reactive astrocytes. The basal content of nerve growth factor (NGF) mRNA and NGF in astrocytes from 6-OHDA-lesioned rats was significantly higher relative to control astrocytes. Two inflammatory cytokines, interleukin-1beta and interferon-gamma, increased synthesis of NGF up to 20-fold in the reactive cells, whereas there was no response in the normal adult astrocytes. Astrocytes from postnatal day 2 rats shared many of the properties of the reactive adult astrocytes. These cultures offer the possibility to characterize the cellular and molecular properties of reactive astrocytes and to determine the factors responsible for activation of astrocytes.     

Purification of the insecticidal toxin in crystals of Bacillus thuringiensis

Newly transected or denervated segments of isogeneic rat tibial nerve were implanted into the rat midbrain and sampled at weekly intervals up to 6 weeks post-operation. By 3 weeks, the peripheral nervous system (PNS) grafts were well-vascularized and contained Schwann cells, axons associated with Schwann cell processes, and macrophages. From 3 to 6 weeks, many axons within both the fresh and predegenerated grafts were myelinated by Schwann cells. The nerve fiber arrangement within the implant was similar to that of regenerating peripheral nerve in situ. The central nervous system (CNS) border of the implant was clearly demarcated by a rim of astrocytes behind which was a layer of regenerating oligodendrocytes and axons. Extending from the CNS margin were radial bridges of astroglial tissue which apprarently guided regenerating axons into the implant. Between the CNS and the PNS implant, abundant collagen deposition was present. The findings suggest that regenerating CNS axons grow via astroglial bridges into transplanted PNS tissue and are capable of stimulating the implanted Schwann cells to form myelin. Even Schwann cells deprived of axonal contact for prolonged periods were still capable of PNS myelin formation.     

Neurosteroids: a novel function of the brain

Neurosteroids are synthetized in the central and peripheral nervous system, particularly but not exclusively in myelinating glial cells, from cholesterol or steroidal precursors imported from peripheral sources. They include 3-hydroxy-delta 5-compounds, such as pregnenolone (PREG) and dehydroepiandrosterone (DHEA), their sulfates, and reduced metabolites such as the tetrahydroderivative of progesterone 3 alpha-hydroxy-5 alpha-pregnane-20-one (3 alpha, 5 alpha-TH PROG). These compounds can act as allosteric modulators of neurotransmitter receptors, such as GABAA, NMDA and sigma receptors. Progesterone (PROG) is also a neurosteroid, and a progesterone receptor (PROG-R) has been identified in peripheral and central glial cells. At different places in the brain, neurosteroid concentrations vary according to environmental and behavioral circumstances, such as stress, sex recognition and aggressiveness. A physiological function of neurosteroids in the central nervous system is strongly suggested by the role of hippocampal PREGS with respect to memory, observed in aging rats. In the peripheral nervous system, a role for PROG synthesized in Schwann cells has been demonstrated in the repair of myelin after cryolesion of the sciatic nerve in vivo and in cultures of dorsal root ganglia neurites. It may be important to study the effect of abnormal neurosteroid concentrations/metabolism with a view to the possible treatment of functional and trophic disturbances of the nervous system.     

The use of transplanted glial cells to reconstruct glial environments in the CNS

Transplantation studies have demonstrated that glia-depleted areas of the CNS can be reconstituted by the introduction of cultured cells. Thus, the influx of Schwann cells into glia-free areas of demyelination in the spinal cord can be prevented by the combined introduction of astrocytes and cells of the O-2A lineage. Although Schwann cell invasion of areas of demyelination is associated with destruction of astrocytes, the transplantation of rat tissue culture astrocytes ("type-1") alone cannot suppress this invasion, indicating a role for cells of the O-2A lineage in reconstruction of glial environments. By transplanting different glial cell preparations and manipulating lesions so as to prevent meningeal cell and Schwann cell proliferation it is possible to demonstrate that the behaviour of tissue culture astrocytes ("type-1") and astrocytes derived from O-2A progenitor cells ("type-2") is different. In the presence of meningeal cells, tissue culture astrocytes clump together to form cords of cells. In contrast, type-2 astrocytes spread throughout glia-free areas in a manner unaffected by the presence of meningeal cells or Schwann cells. Thus, progenitor-derived astrocytes show a greater ability to fill glia-free areas than tissue culture astrocytes. Similarly, when introduced into infarcted white matter in the spinal cord, progenitor-derived astrocytes fill the malacic area more effectively than tissue culture astrocytes, although axons do not regenerate into the reconstituted area.