Glial fibrillary acidic protein (GFAP) from goldfish: its localisation in visual pathway

First study on response of astrocytes in alevines of red-bellied pacu (Piaractus brachypomus) to subchronic exposure to chlorpyrifos and trichlorfon

Background and Aim: Organophosphate pesticides (OPs) used in agricultural production pose environmental and public health risks whenever non-target organisms are exposed to them. Oxon-type OPs, such as trichlorfon (TCF) and chlorpyrifos (CPF), are frequently used in Colombia and have been detected in water bodies in the vicinity of croplands; however, their effect on aquatic organisms, especially fish, is largely unknown. The neurotoxicity of OPs includes inhibition of esterase enzymes, neuronal damage, and increased glial reactivity. This study aimed to assess the astrocytic response in the brain tissue of juvenile red-bellied pacu (Piaractus brachypomus) exposed to TCF and CPF. Materials and Methods: A 25-day subchronic assay was conducted in which juvenile red-bellied pacu were exposed to CPF and TCF. After 25 days of exposure, the fish were killed and brain samples were collected and processed for immunohistochemistry to assess the morphology and reactivity of astrocytes; glial acidic fibrillary protein was used as a biomarker. Results: The brain samples from animals under subchronic exposure to OPs for 25 days showed higher cellular density as well as changes in astrocyte phenotype characterized by shortening of cytoplasmic projections, hypertrophy, and ameboid morphology compared to those from nonexposed animals. Similarly, astrocyte hyperreactivity was detected in the optic tectum and medial longitudinal fasciculus of the exposed group. Conclusion: Immunoreactivity of brain glial cells under subchronic exposure to OPs measured through immunohistochemical tests as well as OPs-induced neuropathology may be useful as a biomarker for monitoring environmental pollution. The results also indicate that P. brachypomus is a suitable biomonitoring model for studying neurotoxicological and neurodegenerative diseases.

Evolutionary Modifications Are Moderate in the Astroglial System of Actinopterygii as Revealed by GFAP Immunohistochemistry

The present paper is the first comparative study on the astroglia of several actinopterygian species at different phylogenetical positions, teleosts (16 species), and non-teleosts (3 species), based on the immunohistochemical staining of GFAP (glial fibrillary acidic protein), the characteristic cytoskeletal intermediary filament protein, and immunohistochemical marker of astroglia. The question was, how the astroglial architecture reflexes the high diversity of this largest vertebrate group. The actinopterygian telencephalon has a so-called ‘eversive’ development in contrast to the ‘evagination’ found in sarcopterygii (including tetrapods). Several brain parts either have no equivalents in tetrapod vertebrates (e.g., torus longitudinalis, lobus inferior, lobus nervi vagi), or have rather different shapes (e.g., the cerebellum). GFAP was visualized applying DAKO polyclonal anti-GFAP serum. The study was focused mainly on the telencephalon (eversion), tectum (visual orientation), and cerebellum (motor coordination) where the evolutionary changes were most expected, but the other areas were also investigated. The predominant astroglial elements were tanycytes (long, thin, fiber-like cells). In the teleost telencephala a ‘fan-shape’ re-arrangement of radial glia reflects the eversion. In bichir, starlet, and gar, in which the eversion is less pronounced, the ‘fan-shape’ re-arrangement did not form. In the tectum the radial glial processes were immunostained, but in Ostariophysi and Euteleostei it did not extend into their deep segments. In the cerebellum Bergmann-like glia was found in each group, including non-teleosts, except for Cyprinidae. The vagal lobe was uniquely enlarged and layered in Cyprininae, and had a corresponding layered astroglial system, which left almost free of GFAP the zones of sensory and motor neurons. In conclusion, despite the diversity and evolutionary alterations of Actinopterygii brains, the diversity of the astroglial architecture is moderate. In contrast to Chondrichthyes and Amniotes; in Actinopterygii true astrocytes (stellate-shaped extraependymal cells) did not appear during evolution, and the expansion of GFAP-free areas was limited.

Identification of Radial Glia Progenitors in the Developing and Adult Retina of Sharks

Neural stem cells give rise to transient progenitors termed neuroepithelial cells (NECs) and radial glial cells (RGCs). RGCs represent the major source of neurons, glia and adult stem cells in several regions of the central nervous system (CNS). RGCs are mostly transient in mammals, but they are widely maintained in the adult CNS of fishes, where they continue to be morphologically similar to RGCs in the mammalian brain and fulfill similar roles as progenitors and guide for migrating neurons. The retina of fishes offers an exceptional model to approach the study of adult neurogenesis because of the presence of constitutive proliferation from the ciliary marginal zone (CMZ), containing NECs, and from adult glial cells with radial morphology (the Müller glia). However, the cellular hierarchies and precise contribution of different types of progenitors to adult neurogenesis remain unsolved. We have analyzed the transition from NECs to RGCs and RGC differentiation in the retina of the cartilaginous fish Scyliorhinus canicula, which offers a particularly good spatial and temporal frame to investigate this process. We have characterized progenitor and adult RGCs by immunohistochemical detection of glial markers as glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). We have compared the emergence and localization of glial markers with that of proliferating cell nuclear antigen (PCNA, a proliferation maker) and Doublecortin (DCX, which increases at early stages of neuronal differentiation). During retinal development, GFAP-immunoreactive NECs located in the most peripheral CMZ (CMZp) codistribute with DCX-immunonegative cells. GFAP-immunoreactive RGCs and Müller cells are located in successive more central parts of the retina and codistribute with DCX- and DCX/GS-immunoreactive cells, respectively. The same types of progenitors are found in juveniles, suggesting that the contribution of the CMZ to adult neurogenesis implies a transition through the radial glia (RG) state.

Frizzled-3a and Wnt-8b genetically interact during forebrain commissural formation in embryonic zebrafish

The commissural plate forms the rostral surface of the embryonic vertebrate forebrain and provides a cellular substrate for forebrain commissural axons. We have previously reported that the Wnt receptor frizzled-3a (fzd3a) restricts the expression of the chemorepulsive guidance ligand slit2 to a discrete domain of neuroepithelial cells in the commissural plate of embryonic zebrafish. Loss of Fzd3a function perturbed slit2 expression and disrupted the formation of glial bridges which guide the formation of forebrain commissures. We now show that Wnt8b is also necessary for anterior commissural formation as well as for patterning of slit2 expression at the midline. Knock down of Wnt8b produced the same phenotype as loss of Fzd3a which suggested that these genes were acting together to regulate axon guidance. Simultaneous sub-threshold knock down of both Fzd3a and Wnt8b led to a greater than additive increase in the penetrance of the mutant phenotype which indicated that these two genes were indeed interacting. We have shown here that Fzd3a/Wnt8b signaling is essential for normal patterning of the commissural plate and that loss-of-function in either receptor or ligand causes Slit2-dependent defects in glial bridge morphology which indirectly attenuated axon midline crossing in the embryonic vertebrate forebrain.

Frizzled-3a and slit2 genetically interact to modulate midline axon crossing in the telencephalon

The anterior commissure forms the first axon connections between the two sides of the embryonic telencephalon. We investigated the role of the transmembrane receptor Frizzled-3a in the development of this commissure using zebrafish as an experimental model. Knock down of Frizzled-3a resulted in complete loss of the anterior commissure. This defect was accompanied by a loss of the glial bridge, expansion of the slit2 expression domain and perturbation of the midline telencephalic-diencephalic boundary. Blocking Slit2 activity following knock down of Frizzled-3a effectively rescued the anterior commissure defect which suggested that Frizzled-3a was indirectly controlling the growth of axons across the rostral midline. We have shown here that Frizzled-3a is essential for normal development of the commissural plate and that loss-of-function causes Slit2-dependent defects in axon midline crossing in the embryonic vertebrate forebrain. These data supports a model whereby Wnt signaling through Frizzled-3a attenuates expression of Slit2 in the rostral midline of the forebrain. The absence of Slit2 facilitates the formation of a midline bridge of glial cells which is used as a substrate for commissural axons. In the absence of this platform of glia, commissural axons fail to cross the rostral midline of the forebrain.

Essential genes for astroglial development and axon pathfinding during zebrafish embryogenesis

The formation of the central nervous system depends on the coordinated development of neural and glial cell types that arise from a common precursor. Using an existing group of zebrafish mutants generated by viral insertion, we performed a "shelf-screen" to identify genes necessary for astroglial development and axon scaffold formation. We screened 274 of 315 viral insertion lines using antibodies that label axons (anti-Acetylated Tubulin) and astroglia (anti-Gfap) and identified 25 mutants with defects in gliogenesis, glial patterning, neurogenesis, and axon guidance. We also identified a novel class of mutants affecting radial glial cell numbers. Defects in astroglial patterning were always associated with axon defects, supporting an important role for axon-glial interactions during axon scaffold development. The genes disrupted in these viral lines have all been identified, providing a powerful new resource for the study of axon guidance, glio- and neurogenesis, and neuron-glial interactions during development of the vertebrate CNS.

Neural and synaptic defects in slytherin, a zebrafish model for human congenital disorders of glycosylation

Congenital disorder of glycosylation type IIc (CDG IIc) is characterized by mental retardation, slowed growth and severe immunodeficiency, attributed to the lack of fucosylated glycoproteins. While impaired Notch signaling has been implicated in some aspects of CDG IIc pathogenesis, the molecular and cellular mechanisms remain poorly understood. We have identified a zebrafish mutant slytherin (srn), which harbors a missense point mutation in GDP-mannose 4,6 dehydratase (GMDS), the rate-limiting enzyme in protein fucosylation, including that of Notch. Here we report that some of the mechanisms underlying the neural phenotypes in srn and in CGD IIc are Notch-dependent, while others are Notch-independent. We show, for the first time in a vertebrate in vivo, that defects in protein fucosylation leads to defects in neuronal differentiation, maintenance, axon branching, and synapse formation. Srn is thus a useful and important vertebrate model for human CDG IIc that has provided new insights into the neural phenotypes that are hallmarks of the human disorder and has also highlighted the role of protein fucosylation in neural development.

Macroglial cells of the teleost central nervous system: a survey of the main types

Following our previous review of teleost microglia, we focus here on the morphological and histochemical features of the three principal macroglia types in the teleost central nervous system (ependymal cells, astrocyte-like cells/radial glia and oligodendrocytes). This review is concerned with recent literature and not only provides insights into the various individual aspects of the different types of macroglial cells plus a comparison with mammalian glia, but also indicates the several potentials that the neural tissue of teleosts exhibits in neurobiological research. Indeed, some areas of the teleost brain are particularly suitable in terms of the establishment of a "simple" but complete research model (i.e. the visual pathway complex and the supramedullary neuron cluster in puffer fish). The relationships between neurons and glial cells are considered in fish, with the aim of providing an integrated picture of the complex ways in which neurons and glia communicate and collaborate in normal and injured neural tissues. The recent setting up of successful protocols for fish glia and mixed neuron-glia cultures, together with the molecular facilities offered by the knowledge of some teleost genomes, should allow consistent input towards the achievement of this aim.

Contactin1a expression is associated with oligodendrocyte differentiation and axonal regeneration in the central nervous system of zebrafish

Contactin1a (Cntn1a) is a zebrafish homolog of contactin1 (F3/F11/contactin) in mammals, an immunoglobulin superfamily recognition molecule of neurons and oligodendrocytes. We describe conspicuous Cntn1a mRNA expression in oligodendrocytes in the developing optic pathway of zebrafish. In adults, this expression is only retained in glial cells in the intraretinal optic fiber layer, which contains 'loose' myelin. After optic nerve lesion, oligodendrocytes re-express Cntn1a mRNA independently of the presence of regenerating axons and retinal ganglion cells upregulate Cntn1a expression to levels that are significantly higher than those during development. After spinal cord lesion, expression of Cntn1a mRNA is similarly increased in axotomized brainstem neurons and white matter glial cells in the spinal cord. In addition, reduced mRNA expression in the trigeminal/anterior lateral line ganglion in erbb3-deficient mutant larvae implies Cntn1a in Schwann cell differentiation. These complex regulation patterns suggest roles for Cntn1a in myelinating cells and neurons particularly in successful CNS regeneration.

belladonna/(Ihx2) is required for neural patterning and midline axon guidance in the zebrafish forebrain

Some of the earliest axon pathways to form in the vertebrate forebrain are established as commissural and retinal axons cross the midline of the diencephalon and telencephalon. To better understand axon guidance in the forebrain, we characterized the zebrafish belladonna (bel) mutation, which disrupts commissural and retinal axon guidance in the forebrain. Using a positional cloning strategy, we determined that the bel locus encodes zebrafish Lhx2, a lim-homeodomain transcription factor expressed in the brain, eye and fin buds. We show that bel(Ihx2) function is required for patterning in the ventral forebrain and eye, and that loss of bel function leads to alterations in regulatory gene expression, perturbations in axon guidance factors, and the absence of an optic chiasm and forebrain commissures. Our analysis reveals new roles for Ihx2 in midline axon guidance, forebrain patterning and eye morphogenesis.

Hedgehog regulated Slit expression determines commissure and glial cell position in the zebrafish forebrain

Three major axon pathways cross the midline of the vertebrate forebrain early in embryonic development: the postoptic commissure (POC), the anterior commissure (AC) and the optic nerve. We show that a small population of Gfap+ astroglia spans the midline of the zebrafish forebrain in the position of, and prior to, commissural and retinal axon crossing. These glial ;bridges' form in regions devoid of the guidance molecules slit2 and slit3, although a subset of these glial cells express slit1a. We show that Hh signaling is required for commissure formation, glial bridge formation, and the restricted expression of the guidance molecules slit1a, slit2, slit3 and sema3d, but that Hh does not appear to play a direct role in commissural and retinal axon guidance. Reducing Slit2 and/or Slit3 function expanded the glial bridges and caused defasciculation of the POC, consistent with a ;channeling' role for these repellent molecules. By contrast, reducing Slit1a function led to reduced midline axon crossing, suggesting a distinct role for Slit1a in midline axon guidance. Blocking Slit2 and Slit3, but not Slit1a, function in the Hh pathway mutant yot (gli2DR) dramatically rescued POC axon crossing and glial bridge formation at the midline, indicating that expanded Slit2 and Slit3 repellent function is largely responsible for the lack of midline crossing in these mutants. This analysis shows that Hh signaling helps to pattern the expression of Slit guidance molecules that then help to regulate glial cell position and axon guidance across the midline of the forebrain.

Histone deacetylase 1 is required to repress Notch target gene expression during zebrafish neurogenesis and to maintain the production of motoneurones in response to hedgehog signalling

Histone deacetylases (Hdacs) are widely implicated as key components of transcriptional silencing mechanisms. Here, I show that hdac1 is specifically required in the zebrafish embryonic CNS to maintain neurogenesis. In hdac1 mutant embryos, the Notch-responsive E(spl)-related neurogenic gene her6 is ectopically expressed at distinct sites within the developing CNS and proneural gene expression is correspondingly reduced or eliminated. Using an hdac1-specific morpholino, I show that this requirement for hdac1 is epistatic to the requirement for Notch signalling. Consequently, hdac1-deficient embryos exhibit several defects of neuronal specification and patterning, including a dramatic deficit of hedgehog-dependent branchiomotor neurones that is refractory to elevated levels of hedgehog signalling. Thus, in the zebrafish embryo, hdac1 is an essential component of the transcriptional silencing machinery that supports the formation and subsequent differentiation of neuronal precursors.

Development of vimentin and glial fibrillary acidic protein immunoreactivities in the brain of gray mullet (Chelon labrosus), an advanced teleost

Previous studies in teleosts have revealed the presence of the intermediate filaments vimentin (Vim) and glial fibrillary acidic protein (GFAP) in glial cells of the spinal cord and/or some brain regions, but there is no comprehensive study of their distribution and developmental changes in fishes. Here, the distribution of Vim and GFAP immunoreactivities was studied in the brain of larvae, juveniles, and adults of an advanced teleost, the gray mullet (Chelon labrosus). A different sequence of appearance was observed for expression of these proteins: Vim levels decreased with age, whereas GFAP increased. In general, both immunoreactivities were expressed early in perikarya and endfeet of ependymocytes (tanycytes), whereas expression in radial processes appeared later. In large larvae, the similar expression patterns of Vim and GFAP suggest that some of these glial cells contain both proteins. Subependymal radial glia cells were observed mainly in the optic tectum, exhibiting Vim and GFAP immunoreactivity. The only immunoreactive cells with astrocyte-like morphology were observed in the optic chiasm of the adult, and they were positive for both GFAP and Vim. The perivascular processes of glial cells showed a different distribution of Vim and GFAP during development and had a caudorostral sequence of appearance of immunoreactivities similar to that observed for ependymal and radial glia cells. Several circumventricular organs (the organon vasculosum hypothalami, saccus vasculosus, and area postrema) exhibited highly specialized Vim- and/or GFAP-expressing glial cells. The glial cells of the midline septa of several brain regions were also Vim and/or GFAP immunoreactive. In the adult brain, tanycytes retain Vim expression in several brain regions. As in other vertebrates, the regions with Vim-immunoreactive ventricular and midline glia may represent areas with the capability of plasticity and regeneration in adult brain.

Monitoring neural progenitor fate through multiple rounds of division in an intact vertebrate brain

The behaviour of neural progenitors in the intact vertebrate brain and spinal cord is poorly understood, chiefly because of the inaccessibility and poor optical qualities inherent in many model systems. To overcome these problems we have studied the optically superior brain of the zebrafish embryo and have monitored the in vivo behaviour of fluorescently labelled neural progenitors and their daughter cells throughout a substantial period of hindbrain development. We find the majority (84%) of hindbrain neurons are born from progenitor divisions that generate two neurons and 68% of reconstructed lineage trees contained no asymmetric stem cell-like divisions. No progenitors divided in the manner expected of a classic stem cell; i.e. one that repeatedly self-renews and generates a differentiated cell type by asymmetric division. We also analysed the orientation of progenitor divisions relative to the plane of the ventricular zone (VZ) and find that this does not correlate with the fate of the daughter cells. Our results suggest that in this vertebrate system the molecular determinants that control whether a cell will become a neuron are usually not linked to a mechanism that generates asymmetric divisions.

Expression of protein zero is increased in lesioned axon pathways in the central nervous system of adult zebrafish

The immunoglobulin superfamily molecule protein zero (P0) is important for myelin formation and may also play a role in adult axon regeneration, since it promotes neurite outgrowth in vitro. Moreover, it is expressed in the regenerating central nervous system (CNS) of fish, but not in the nonregenerating CNS of mammals. We identified a P0 homolog in zebrafish. Cell type-specific expression of P0 begins in the ventromedial hindbrain and the optic chiasm at 3-5 days of development. Later (at 4 weeks) expression has spread throughout the optic system and spinal cord. This is consistent with a role for P0 in CNS myelination during development. In the adult CNS, glial cells constitutively express P0 mRNA. After an optic nerve crush, expression is increased within 2 days in the entire optic pathway. Expression peaks at 1 to 2 months and remains elevated for at least 6 months postlesion. After enucleation, P0 mRNA expression is also upregulated but fails to reach the high levels observed in crush-lesioned animals at 4 weeks postlesion. Spinal cord transection leads to increased expression of P0 mRNA in the spinal cord caudal to the lesion site. The glial upregulation of P0 mRNA expression after a lesion of the adult zebrafish CNS suggests roles for P0 in promoting axon regeneration and remyelination after injury.

The glial design of a teleost optic nerve head supporting continuous growth

This study demonstrates the peculiarities of the glial organization of the optic nerve head (ONH) of a fish, the tench (Tinca tinca), by using immunohistochemistry and electron microscopy. We employed antibodies specific for the macroglial cells: glutamine synthetase (GS), glial fibrillary acidic protein (GFAP), and S100. We also used the N518 antibody to label the new ganglion cells' axons, which are continuously added to the fish retina, and the anti-proliferating cell nuclear antigen (PCNA) antibody to specifically locate dividing cells. We demonstrate a specific regional adaptation of the GS-S100-positive Müller cells' vitreal processes around the optic disc, strongly labeled with the anti-GFAP antibody. In direct contact with these Müller cells' vitreal processes, there are S100-positive astrocytes and S100-negative cells ultrastructurally identified as microglial cells. Moreover, a population of PCNA-positive cells, characterized as glioblasts, forms the limit between the retina and the optic nerve in a region homologous to the Kuhnt intermediary tissue of mammals. Finally, in the intraocular portion of the optic nerve there are differentiating oligodendrocytes arranged in rows. Both the glioblasts and the rows of developing cells could serve as a pool of glial elements for the continuous growth of the visual system.

Distribution of GFAP immunoreactive structures in the rhombencephalon of the sterlet (Acipenser ruthenus) and its evolutionary implication

Previous studies have revealed that although the brains of cypriniform teleosts (iberian barb, Barbus comiza; carp, Cyprinus carpio; goldfish, Carassius auratus) are rich in glial fibrillary acidic protein (GFAP), they have, however, areas devoid of GFAP immunoreactivity. The largest ones of these are in the rhombencephalon, e.g., the zones of the sensory and motor neurons in the vagal lobe. Our studies in amniotes suggested that the GFAP immunonegative areas could be characteristic of the more advanced brains (avian and mammalian), whereas no similar areas were found in reptiles. A similar tendency was found in the Chondrichthyes, i.e., GFAP immunonegative areas appeared as brain complexity progressed. The question arose whether the GFAP immunonegative brain areas in the Teleostei were also the result of such a tendency. Within the radiation of ray-finned fishes (Actinopterygii), Chondrostei represent a less advanced level as compared to the Teleostei. Therefore, the distribution of GFAP immunoreactivity was investigated in the rhombencephalon of the sterlet (Acipenser ruthenus) as a representative of Chondrostei, and in the carp. Serial vibratome sections were processed according to the avidin-biotinylated horseradish peroxidase method.Several comparable GFAP immunoreactive structures were found in the two species: the dense periventricular ependymoglial plexus, the midsagittal glial septum, the small glial septa separating the nerve fiber bundles, and the wide glial endfeet lining the meningeal surface. In the vagal lobe in the zones adjacent to the meningeal and ventricular surfaces, the glial structures also proved to be similar. In contrast to the carp, however, no areas were found devoid of GFAP immunoreactivity in the sterlet.The results suggest that this trend of glial evolution, i.e., GFAP immunonegative areas appearing as brain complexity progressed, is a common feature shared by Actinopterygii, Amniota, and Chondrichthyes, despite their separate evolutionary histories. J. Exp. Zool. 293:395-406, 2002.

Effects of axotomy on the expression of NADPH-diaphorase in the visual pathway of the tench

The distribution of NADPH-diaphorase (ND) positive elements was analyzed throughout the visual pathway of the tench in normal conditions and after optic nerve transection. In the control retina, ND-labeled elements were observed in the photoreceptor, inner nuclear, outer nuclear and ganglion cell layers. In the optic nerve of control animals, small and numerous ND-positive glial cells that were identified as presumably astrocyte-like cells were observed. In the optic tracts and optic tectum, a different type of ND-positive glial cell was detected. Axotomy induced severe changes in the ND staining pattern in the visual pathway. A decrease in the number of ND-stained cells was detected in the retina. In the optic nerve of lesioned animals, the number of small cells gradually decreased, whereas the number of large cells did not change. Two new ND-positive cell populations were observed after the lesion: microglial-like cells appeared close to the lesioned area from 24 h to 7 days after transection, and astrocyte-like cells were found throughout the optic nerve from 14 days up to at least 120 days. The total number of ND-stained glial cells increased at 30 and 60 days and returned to control parameters at 120 days. In addition, the number of ND-positive cells increased at the same survival times in the optic tracts and in the retinorecipient strata of the optic tectum with respect to control animals. Thus, degenerative/regenerative processes in the fish visual pathway are accompanied by an increase in the number of ND-positive cells. Synthesis of nitric oxide is elicited in microglial-like cells as a response to axon injury, whereas the expression in astrocyte-like cells seems to be associated with both normal processes under physiological conditions and with the regenerative phase after the lesion.

Detailed immunohistology of Pax6 protein and tyrosine hydroxylase in the early zebrafish brain suggests role of Pax6 gene in development of dopaminergic diencephalic neurons

Spatiotemporal developmental dynamics of Pax6 protein containing (i.e., Pax6) cells were investigated immunohistochemically in embryonic and postembryonic zebrafish brain sections (especially at 2 and 5 day), allowing for a neuroanatomically detailed resolution previously only reported for the mouse. Besides strikingly close correspondences of early Pax6 domains - including many spatiotemporal changes - in mouse and zebrafish brains, some critical differences were noted. There is no pallial (i.e., cortical) Pax6 expression domain in the ventricular proliferative layer in the zebrafish as in the mouse. The main pallial Pax6 domain in the zebrafish is comparable to the migrating stream of Pax6 cells at the pallial-subpallial boundary. This indicates that some developmental functions of Pax6 (i.e., inhibition of subpallial cell migration into pallium by Pax6 migrating stream) might be shared with the mouse and maybe all vertebrates, while others (i.e., control of intrapallial neuronal radial migration via Pax6 expressing radial glia cells) may be special for mammals. Another prominent feature in the early zebrafish forebrain is that the alar plate ventral thalamic Pax6 domain extends far caudolaterally into the periphery of the basal plate posterior tuberculum and hypothalamic inferior lobe. This indicates that the alar plate ventral thalamus invades the forebrain basal plate and contributes to the development of basal forebrain structures. The close spatiotemporal association of Pax6 cells and TH cells of the ventral thalamus indicates a local role of Pax6 in the development of ventral thalamic (as recently demonstrated in the mouse) and, maybe, posterior tubercular TH cells. However, our confocal microscopical analysis of zebrafish brain sections double-immunostained for Pax6 and TH did not reveal cells double-labeled for these two proteins in this location, but rather indicates an inductive interaction of Pax6 cells onto TH cells.

Lesions do not provoke GFAP-expression in the GFAP-immunonegative areas of the teleost brain

In the mammalian and avian brains the predominant astroglial elements are astrocytes, and the distribution of GFAP-immunopositivity is rather uneven, some large brain areas being almost devoid of GFAP-immunopositivity. In these areas however, an intense GFAP-immunopositivity appears following injury. In the teleost brain most of the areas are GFAP-immunopositive and ependymoglia predominates. However, a large area, the layer of the sensory neurons in the vagal lobe, is devoid of GFAP. The question arises, whether the lack of GFAP-immunopositivity in the teleost brain is also due to the repression of the GFAP-production, as in birds and mammals, or due to the lack of cells capable of expressing GFAP. To answer this question, stab wounds were made in the vagal lobe of goldfish, as well as in the tectum, in which moderately dense but highly organized GFAP-immunopositive glia has been detected in intact animals. In the layer of the sensory neurons in the vagal lobe no GFAP-immunopositivity appeared even after lesions had been introduced. In the tectum, a rather slight increase of the intensity of the immunostaining was observed in the glial fibers near the lesions but no typical reactive glia similar to that found in mammals or birds, was observed. The results suggest that a lesion does not provoke GFAP-expression in GFAP-immunonegative brain areas in teleosts, in contrast to what is observed in mammals and birds.

Voltage-gated sodium and potassium channels in radial glial cells of trout optic tectum studied by patch clamp analysis and single cell RT-PCR

Radial glial cells in the visual center of trout were analyzed immunocytochemically and with the whole cell mode of the patch-clamp technique in combination with RT-PCR. By immunostaining with anti-GFAP antibodies radially oriented cell processes spanning the entire width of the tectum were brightly labeled, while with anti-S-100 antiserum the cell bodies residing in a discrete layer close to the ventricular border became most clearly visible. Virtually all radial glial cells examined in brain slices exhibited voltage-gated sodium inward currents that were activated above -40 mV, blocked by micromolar concentrations of TTX and totally eliminated if sodium was substituted for Tris in the bath solution. In contrast with adjacent nerve cells of the same slices radial glial cells did not exhibit spontaneous electrical activity and could not be stimulated to generate action potentials by depolarizing current injections. Two types of voltage-gated potassium outward currents were elicited by depolarizing voltage steps: a sustained current with delayed rectifier properties and a superimposed transient "A"-type current, both being activated at a threshold potential of -40 mV. In cultured radial glial cells subtle differences were noticed regarding current density, inactivation kinetics, and TEA-sensitivity of the potassium currents. Inwardly rectifying potassium currents activating at hyperpolarized voltages were not observed. By single cell RT-PCR the transcripts of two shaker-related potassium channel genes (termed tsha1-a fish homologue to Kv1.2- and tsha3) were amplified, while transcripts for tsha 2 and tsha 4 were not detected.

Response of microglial cells after a cryolesion in the peripheral proliferative retina of tench

We studied the glial response after inducing a lesion in the zone of the peripheral retina of tench, where there is proliferative neuroepithelium. In the retina and optic nerve, the microglial response was analysed with tomato lectin and the macroglial response with antibodies against GFAP and S-100. In lesioned retinas, there was a temporal-spatial distribution pattern of microglia. One day after lesion, primitive ramified cells appeared in the nerve fibre layer. These cells appeared progressively from the vitreal to the scleral layers until day 7 when cells appeared in all layers, with the exception of the outer plexiform layer. From this point, labelling decreased. In the optic nerve, 3 days after lesion, an increase in the number of microglial cells was observed, first in the nerve folds and from day 15 in specific areas of the optic nerve. In the central retina, in the optic nerve head and within the optic nerve itself, the appearance of microglial cells, after the lesion, near the blood vessels, could indicate a vascular origin of microglia, as has been proposed by many authors. However, we cannot discount the idea that some of the reactive microglial cells arise by proliferation of the microglia existing in the normal state. Using GFAP and S-100 antibodies, no important changes in the retina were observed, however in the optic nerve there was response to the lesion. Thus, the macroglial cells appeared to be involved in reorganisation of the optic nerve axons after lesion.

Molecular structure and expression of shaker type potassium channels in glial cells of trout CNS

Two shaker-related potassium channel transcripts (tsha1, tsha2) were identified in glial cells of trout central nervous system (CNS) by polymerase chain reaction (PCR)-cloning and sequencing. While tsha1 was highly similar to the mammalian Kv1.2 subtype of potassium channels, tsha2 did not show a preferential sequence homology with a particular subtype of shaker, but exhibited uniform similarity with mammalian Kv1.1, Kv1.2, and Kv1.3, respectively. Transcripts for the shaw and shal subfamilies of voltage-gated potassium channels were detected in whole brain tissue only but not in freshly dissociated glial cells. Using mRNA extracted from different glial cell types in combination with sequence specific PCR primers, tshal was found restricted to oligodendrocytes and their progenitor cells, while tsha2 was common to astrocytes, as well.

S-100-positive glial cells are involved in the regeneration of the visual pathway of teleosts

Glial cells in the normal and regenerating visual pathways of Tinca tinca (Cyprinid, Teleost) were studied by labelling with anti-S-100 antibody. In normal fish, S-100-positive bipolar cells were found in the optic nerve, optic tract, and in the diencephalic visual pathways. After crushing the left optic nerve, the distribution and the number of S-100-immunoreactive cells were modified. In the injured nerve, 7 to 15 days after crushing no immunoreactive cell bodies were found in the crushed area, but a greater number of S-100-positive cells were found on both sides of the injured area. Sixty days after crushing, positive cells penetrating the crushed area were observed; the normal pattern was almost restored 200 days after crushing. In the diencephalon, 25 days after crushing, the number of S-100-positive cells increased remarkably and the most intense immunostaining of glial processes was observed 60 days after crushing. The density of S-100-labelled cells decreased after 4 months postcrushing. However, in the optic tectum no changes were observed. The increase of glial cells in the lesioned visual pathway suggests that they could play an important role in axonal regeneration after crushing.

Distribution of S100 immunoreactivity in the retina and optic nerve head of the teleost Tinca tinca L

The distribution of S100 immunoreactivity within the normal and regenerating retina and optic nerve head of the teleost Tinca tinca L. has been investigated using the avidin-biotin complex (ABC) method and a polyclonal antibody against S100. Astrocytes and Müller cells were labeled with this antibody. This represents the first description of astrocytes localized in the optic nerve head and in the nerve fiber layer of the fish retina displaying a typical bipolar morphology. Horizontal cells in the inner nuclear layer were immunolabeled; we also observed species-specific S100 labeling of horizontal cells of the H1 subtype. No significant changes were seen in the S100 immunoreactive Müller cells, astrocytes, or horizontal cells in the tench retina after optic nerve crushing and during regeneration. These results might help to understand the function of glial cells in the normal and experimentally induced regenerating fish visual system.

Glial repair at the lesion site in regenerating goldfish spinal cord: an immunohistochemical study using species-specific antibodies

We have used fish-specific antibodies to show that repair in regenerating goldfish spinal cord is accompanied by the recovery of the astrocytic environment and restoration of the central canal. Astrocyte processes trailed the regenerated axons bridging the new cord, suggesting that they are not needed for axonal regrowth.

Fibroblasts at the transection site of the injured goldfish optic nerve and their potential role during retinal axonal regeneration

The region at and around the site of optic nerve transection (ONS) in goldfish, topologically the equivalent of the glial scar in mammals, is reported to remain free of astrocytes over weeks, but its cellular constituents are unknown. To learn what type of cell occupies the site of injury and thus provides support for the rapidly regenerating retinal growth cones, immunostaining experiments at the light microscopic level and electron microscopic examinations were undertaken. Between 2 and 30 days after ONS, an area up to 150 micrograms wide at the transection site exhibits intense anti-fibronectin immunoreactivity. This site contained cells and processes with ultrastructural characteristics of fibroblasts and abundant collagen fibrils. Moreover, on fibroblast cultures derived from regenerating optic nerves, retinal axons grew to considerable density in vitro. Since fibroblasts are constituents of the interfascicular spaces and outer nerve sheath of the normal goldfish optic nerve, the present data imply that fibroblasts of either source migrate into the lesion. Judging form fibronectin immunostaining they remain there during the passage of regenerating axons, and thus may provide physical and perhaps molecular support for axon growth. The fibroblasts are again restricted to interfascicular spaces after restoration of the astrocytic glia limitans around regenerated fascicles.

Changes in ion channel expression during in vitro differentiation of trout oligodendrocyte precursor cells

Voltage-gated ionic currents were studied in cultured trout oligodendrocyte precursor cells derived from larval trout brain with the whole-cell mode of the patch-clamp technique. These bipolar cells which carry the ganglioside epitope A2B5 on their surface differentiated in vitro into immature multipolar oligodendrocytes expressing the myelin glycoprotein IP2, which signifies the initial step of oligodendroglial development in fish CNS. Depolarization above -40 mV activated a fast transient sodium inward current that was eliminated by substituting Na+ for choline and blocked in the presence of 1 microM TTX. The kinetics and the voltage-dependence of inactivation (half-maximal inactivation at -68 mV) resembled those of sodium currents described in mammalian oligodendrocyte precursor cells and CNS neurons. The expression of Na+ channels was developmentally regulated, since high amplitudes were measured only in A2B5+ cells with a characteristic bipolar morphology of glial progenitors. Depolarizing voltage steps, additionally elicited outward potassium currents that were sensitive to external 4-AP. In a subpopulation of cells this outward current consisted of a sustained and a transient component. The amplitude of both components were dependent on the prepulse potential.

Expression of glial fibrillary acidic protein and its relation to tract formation in embryonic zebrafish (Danio rerio)

To address possible roles of glial cells during axon outgrowth in the vertebrate central nervous system, we investigated the appearance and distribution of the glial-specific intermediate filament, glial fibrillary acidic protein (GFAP), during early embryogenesis of the zebrafish (Danio rerio). Immunopositive cells first appear at 15 hours, which is at the time of, or slightly before, the first axon outgrowth in the brain. Immunopositive processes are not initially present in a pattern that prefigures the location of the first tracts but rather are distributed widely as endfeet adjacent to the pia, overlying most of the surface of the brain with the exception of the dorsal and ventral midline. The first evidence for a specific association of immunopositive cells with the developing tracts is observed at 24 hours in the hindbrain, where immunopositive processes border axons in the medial longitudinal fasciculus. By 48 hours, immunopositive processes have disappeared from most of the subpial lamina and are found exclusively in association with tracts and commissures in three forms: endfeet, radially oriented processes, and tangentially oriented processes parallel to axons. This last form is particularly prominent in the transverse plane of the hindbrain, where they define the boundaries between rhombomeres. These results suggest that glial cells contribute to the development and organization of the central nervous system by supporting early axon outgrowth in the subpial lamina and by forming boundaries around tracts and between neuromeres. The results are discussed in relation to previous results on neuron-glia interactions and possible roles of glial cells in axonal guidance.

Glutamate-activated ionic currents in cultured astrocytes from trout: evidence for the occurrence of non-N-methyl-D-aspartate receptors

Glutamate-induced currents were recorded from cultured trout astrocytes with the whole-cell variation of the patch-clamp technique. Ninety percent of the tested cells were directly depolarized by the amino acid neurotransmitter in a concentration-dependent manner. The depolarizing effect was due to an inward current that reversed near 0 mV and was accompanied by a noise increase, indicating the opening of an ion channel. Ion substitution experiments revealed that the glutamate-induced current was mainly carried by sodium ions but not chloride or calcium ions. The glutamate-induced response could be mimicked by the neuronal glutamate receptor subtype agonists kainate and quisqualate, while N-methyl-D-aspartate was without detectable effect.

Behaviour of macroglial cells, as identified by their intermediate filament complement, during optic nerve regeneration of Xenopus tadpole

Assessment of glial cell behaviour during optic nerve (ON) regeneration in Xenopus tadpoles is hampered by the lack of classical cellular markers that distinguish different glial cells in mammals. We thus have characterized the intermediate filament (IF) complement of tadpole glial cells and used it to follow the fate of glial cell subsets during the first 10 days after ON crush. Glial cells synthesize a restricted number of cytokeratin (CK) species and vimentin. This pattern remains essentially unchanged during metamorphosis and regeneration. However, vimentin turnover is specifically enhanced after injury. The expression of CKs and vimentin has been followed immunocytochemically in situ and in isolated cells recovered from dissociated ON segments. In the normal nerve, 79% of ramified glial cells express both CK and vimentin, 1% CK and 4% vimentin only, whereas 16% express neither IF protein. We tentatively classified CK expressing cells as mature astrocytes and those without IF proteins as oligodendrocytes. In the regenerating ON, the relative number of oligodendrocytes is decreased, while the astrocytic subset becomes accordingly larger but is decreased by day 10 already in favour of cells expressing vimentin only. Astrocytes invade the lesion site soon after crush, arrange into a central core within the distal nerve segment and establish a peripheral scaffold that is readily crossed by axons. Unlike mammalian astrocytes that remain absent from the lesion site but form a scar at some distance to it, amphibian astrocytes appear to provide active guidance to axons growing through the lesion site.

Glial cell types, lineages, and response to injury in rat and fish: implications for regeneration

Axons of the mammalian central nervous system do not regenerate spontaneously after axonal injury, unlike the central nervous system axons of fish and amphibians and the peripheral nervous system of mammals, which possess a good regenerative ability (Grafstein: The Retina: A Model for Cell Biology Studies, Part II, 1986; Kiernan: Biol Rev 54:155-197, 1979; Murray: J Comp Neurol 168:175-196, 1976; Ramón y Cajal: Degeneration and Regeneration of the Nervous System, 1928; Reier and Webster: J Neurocytol 3:591-618, 1974; Sperry: Physiol Zool 23:351-361, 1948). It was previously believed that intrinsic differences between the central nervous system neurons of mammals and fish account for their differences in regenerative ability. The past decade, however, has seen an accumulation of evidence, indicating that mammalian central nervous system neurons are able to regenerate injured axons, at least to some extent. This was first demonstrated by Aguayo and colleagues (David and Aguayo: Science 214:931-933, 1981; Kierstead et al: Science 246:255-257, 1989), who showed that injured mammalian central nervous system axons can grow for a considerable distance into an autograft of a peripheral nerve. It was also demonstrated that injured rabbit optic axons can regenerate into their own environment (i.e., into the distal part of the injured optic nerve), if the injured nerve is treated so as to make it conducive for growth (Lavie et al: J Comp Neurol 298:293-314, 1990; Eitan et al: Science 264:1764-1768, 1994).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunolocalization of exoglycoproteins ("ependymins") in the goldfish brain

Exoglycoproteins (X-GPs) are a group of very abundant soluble glycoproteins in the goldfish brain. Immunostaining with polyclonal antisera to X-GPs revealed consistent perinuclear staining in the cells of the inner and intermediate layers of the leptomeninx, which is homologous to the pia-arachnoid. Immunolabelling was also prominent in the outer wall of capillaries, and in a variable population of 10-12 microns granular cells that appeared mainly near the ventricles and occasionally within the ventricles or under the meninges. In some cases, small and medium-sized lymphocytes were immunostained. Lymphocytes were sometimes associated with the granular cells, which may be hematogenous cells in transit toward the ventricles. The choroid plexus, saccus dorsalis, the roof of the third ventricle and Reissner's fiber showed strong immunostaining. The localization of the X-GPs suggests that they may contribute to maintenance of the blood-brain barrier or to regulation of immune function within the brain.

An immunocytochemical investigation of glial morphology in the Pacific hagfish: radial and astrocyte-like glia have the same phylogenetic age

This study attempts to reconstruct the early phylogenetic history of macroglial cells among craniates. Since glia does not fossilize, such a reconstruction must be based on a cladistic comparison of glial characters in the Recent craniate taxa (hagfishes, lampreys, and gnathostomes); however, there are only few data on glial morphology and none on glial immunocytochemistry in hagfishes. Therefore, we investigated the presence and localization of various macroglia-specific epitopes in the brain and spinal cord of the Pacific hagfish, Eptatretus stouti (Myxinoidea) by means of immunocytochemistry. Antibodies directed against S100-protein and vimentin showed no cross reactivity. Antibodies directed against glial fibrillary acidic protein and glutamine synthetase labelled various glial structures. Glial fibrillary acidic protein-like immunoreactivity was observed in ependymal cells with radially oriented processes in some regions. However, throughout the entire CNS, labelling of non-ependymal cells and their processes prevailed. The processes of these cells made occasional vascular contacts and they also made contacts with neuronal perikarya. Glutamine synthetase-like immunoreactivity was also found in some processes with radial orientation and in ependymal cells; but the antibody stained mainly non-ependymal cells which gave rise to a felt-like meshwork of interdigitating fine and very fine processes penetrating the neuropil of the entire brain. Additionally, there was labelling in the walls of blood vessels and in processes enwrapping individual neurons. The occurrence of glial fibrillary acidic protein- and glutamine synthetase-like immunoreactivity in non-ependymal glial elements in the brain of hagfishes and the relative scarcity of labelling in radial glial elements necessitates a re-interpretation of the evolutionary history of glial cells. Non-ependymal macroglia with immunocytochemical and morphological characters resembling typical (mammalian) astrocytes appears to be as primitive as the various forms of radial ependymal glia.

Similarities and differences between fish oligodendrocytes and Schwann cells in vitro

In light of the striking differences between oligodendrocytes of the optic nerve/tract of adult goldfish and their mammalian counterparts, a further characterization of goldfish oligodendrocytes was performed. A comparison with Schwann cells was included because fish optic nerve/tract-derived oligodendrocytes bear remarkable similarities to this type of glial cell. Fish optic nerve/tract-derived oligodendrocytes that had differentiated into 04 and 6D2-positive cells and thus expressed early myelin marker molecules were found to incorporate BrdU and to divide in vitro over a period of weeks. For the induction of more advanced markers of myelinogenesis such as the CNS-specific myelin protein 36K, oligodendrocytes required extensive contact with axons. Other agents, such as fetal calf or carp serum, substrate components, or forscolin failed, however, to induce 36K expression. 04/6D2-positive oligodendrocytes could be distinguished from fish 6D2-positive Schwann cells derived from cranial nerves by their antigenic phenotype: Schwann cells but not oligodendrocytes exhibited the low affinity NGF receptor. While both cell types carry the cell adhesion molecules NCAM, E 587 antigen, and the L2/HNK-1 epitope, only Schwann cells possess a further adhesion molecule, Neurolin.

Ependyma: phylogenetic evolution of glial fibrillary acidic protein (GFAP) and vimentin expression in vertebrate spinal cord

The phylogenetic evolution was studied of both glial fibrillary acidic protein (GFAP) and vimentin expression in the ependyma of the adult vertebrate spinal cord. Eleven species from different vertebrate groups were examined using different fixatives and fixation procedures to demonstrate any differences in immunoreactivity. GFAP expression in the ependymal cells showed a clear inverse relation with phylogenetic evolution because it was more elevated in lower than in higher vertebrates. GFAP positive cells can be ependymocytes and tanycytes, although depending on their structural characteristics and distribution, the scarce GFAP positive ependymal cells in higher vertebrates may be tanycytes. Ependymal vimentin expression showed a species-dependent pattern instead of a phylogenetic pattern of expression. Vimentin positive ependymal cells were only found in fish and rats; in fish, they were tanycytes and were quite scarce, with only one or two cells per section being immunostained. However, in the rat spinal cord, all the ependymocytes showed positive immunostaining for vimentin. The importance of the immunohistochemical procedure, the cellular nature of GFAP positive ependymal cells and the relationship between tanycytes and ependymocytes are discussed, as well as GFAP and vimentin expression.

Voltage-dependent sodium and potassium currents in cultured trout astrocytes

Voltage-gated ionic currents were recorded from cultured trout astrocytes with the whole-cell variation of the patch-clamp technique. In a subpopulation of astrocytes depolarizations above -40 mV activated a fast transient inward current that was identified as a sodium current by ion substitution experiments, its current reversal potential, and its TTX-sensitivity. Regarding threshold of activation, peak current voltage, and amplitude this current closely resembled those previously described for mammalian astrocytes. Voltage-dependence of inactivation and kinetics, however, markedly differed from the "glial-like" sodium current occurring in mammalian hippocampal or optic nerve astrocytes, since the sodium current of trout astrocytes exhibited a faster time course of activation and decay and a more depolarized steady-state inactivation curve with midpoints close to -60 mV. During a period of 2 weeks in culture the biophysical properties of the sodium current did not change significantly, albeit a continuous decrease in current density was observed. At depolarizing voltage steps positive to -40 mV, additionally voltage-gated potassium outward currents were evoked, which could be separated into a steady-state current with delayed rectifier properties and an inactivating component resembling the A-type current. Moreover, in a subpopulation of astrocytes an inward potassium current was elicited at hyperpolarizing potentials, which exhibited biophysical features consistent with the potassium inward rectifier of mammalian astrocytes.

Myelin repair by Schwann cells in the regenerating goldfish visual pathway: regional patterns revealed by X-irradiation

In the regenerating goldfish optic nerves, Schwann cells of unknown origin reliably infiltrate the lesion site forming a band of peripheral-type myelinating tissue by 1-2 months, sharply demarcated from the adjacent new CNS myelin. To investigate this effect, we have interfered with cell proliferation by locally X-irradiating the fish visual pathway 24h after the lesion. As assayed by immunohistochemistry and EM, irradiation retards until 6 months formation of new myelin by Schwann cells at the lesion site, and virtually abolishes oligodendrocyte myelination distally, but has little or no effect on nerve fibre regrowth. Optic nerve astrocyte processes normally fail to re-infiltrate the lesion, but re-occupy it after irradiation, suggesting that they are normally excluded by early cell proliferation at this site. Moreover, scattered myelinating Schwann cells also appear in the oligodendrocyte-depleted distal optic nerve after irradiation, although only as far as the optic tract. Optic nerve reticular astrocytes differ in various ways from radial glia elsewhere in the fish CNS, and our observations suggest that they may be more permissive to Schwann cell invasion of CNS tissue.

Differential expression of keratins in goldfish optic nerve during regeneration

The goldfish visual pathway, unlike the visual pathway of higher vertebrates, retains continuous growth and development throughout life and is capable of functional regeneration. The structure and expression of proteins that support the physiological attributes of this system are of interest. Glial cells in this pathway express keratins as the predominant intermediate filament proteins rather than the expected glial fibrillary acidic protein. Previously we identified and characterized cDNA clones representing two type I keratins from the goldfish optic nerve, GK48 and GK49. The GK48 protein is the type I keratin partner to the type II keratin ON3, while the GK49 protein is expressed in a different cell type. Here, we extend our studies on the expression of mRNA for the GK48, GK49, and ON3 proteins at the early stages of optic nerve regeneration. RNase protection assays show that at 10 days post-crush, there is no overall change in levels of mRNA for these proteins as compared to uncrushed control nerves and nerves from unoperated fish. In addition, we show by in situ hybridization that the GK49 protein shows no changes in its distribution of mRNA in the optic nerve after crush. In contrast, the levels of GK48 and ON3 mRNA are greatly reduced within the crush zone. However, these two mRNAs are differentially expressed at different time points during regeneration, with GK48 mRNA appearing in the crush zone before ON3. These results indicate that the mRNA for the GK48 and ON3 proteins are differentially regulated during regeneration and that these two proteins are expressed in a different cell type from the GK49 protein.

Vimentin immunoreactive glial cells in the fish optic nerve: implications for regeneration

The poor regenerative ability of neurons of the central nervous system in mammals, as compared with their counterpart in fish or amphibians, is thought to stem from differences in their immediate nonneuronal environment and its response to axonal injury. We describe one aspect of the environmental response to axonal injury in a spontaneously regenerating system--the fish optic nerve. The aspect under investigation was the reaction of glial cells at the injury site. This was examined by the use of antibodies that specifically recognize vimentin in fish glial cells. In the present study, affinity-purified vimentin antibodies were raised against a nonconserved N-terminal 14-amino acid peptide, which was predicted from the nucleotide sequence of vimentin. These antibodies were found to react specifically with glial cells in vitro. Moreover, the antivimentin antibodies stained both the optic nerve and the optic tract, but with different patterns. Specificity of the antibodies was verified by protein immunoblotting, tissue distribution, and labeling patterns. After injury, vimentin immunoreactivity initially disappeared from the site of the lesion due to cell death. Early signs of glial cell migration toward the injury site were evident a few days later. It is suggested that the reappearance of vimentin-positive glial cells at the site of injury is associated with axonal elongation across it, and that they contribute to the regenerative ability of the fish optic nerve.

Astrocytes play a major role in the control of neuronal proliferation in vitro

The elements that control neuronal proliferation are largely unknown. Proliferating neurons in cultures of goldfish brain were studied in an attempt to identify the cell types involved. Neuronal proliferation was found to occur only when the neuronal stem cells were in direct contact with astrocytes, and never directly on the substrate. The regulation of neuronal proliferation thus appears to be mediated, at least in part, by contact with astrocytes. In addition, neurite extension was inhibited by medium conditioned by fish astrocytes. Since neurite extension and neuronal proliferation are mutually exclusive processes, inhibition of neurite extension by soluble substances derived from the astrocytes is probably one of the mechanisms controlling neuronal proliferation. The complex reciprocal relationship between neurons and astrocytes is also demonstrated by an observed inhibition of astrocytic proliferation by medium conditioned by differentiating fish neurons. This inhibition of astrocytic proliferation might be part of a mechanism through which interference with neuronal differentiation by astrocytes is avoided. The results of this study thus suggest that astrocytes, in addition to their known roles in controlling neuronal migration, neuronal differentiation and neurite elongation, may also play a role in the control of neuronal proliferation.

Cloning and characteristics of fish glial fibrillary acidic protein: implications for optic nerve regeneration

Mammalian central nervous system neurons do not regenerate after axonal injury, unlike their counterparts in fish and amphibians. After axonal injury, glial cells in mammals do not support regrowth of axons, while in fish they support the regeneration process. Controversy exists as to whether or not the intact fish optic nerve expresses glial fibrillary acidic protein, a well-known marker for mature astrocytes, and thus whether its astrocytes differ in this respect from those of the brain and spinal cord, as well as from optic nerve astrocytes of other species. In an attempt to resolve this question we cloned fish glial fibrillary acidic protein. Two different complementary DNA clones were isolated from a carp brain complementary DNA library, each encoding a different form of glial fibrillary acidic protein apparently originating from different genes. Monospecific polyclonal antibodies were raised against a peptide synthesized according to the predicted amino acid sequence, and used to identify and localize the fish glial fibrillary acidic protein. Two glial fibrillary acidic proteins (of 49 kDa and 51 kDa) were identified by the antibodies in all tested fish central nervous system tissues. The antibodies were then used to examine glial fibrillary acidic protein immunoreactivity in sections taken from uninjured and injured optic nerves of goldfish. Injury was followed by an elevation in glial fibrillary acidic protein immunoreactivity along the whole length of the nerve, except at the site of the injury, where--as in the case of vimentin--no immunoreactivity was detectable. However, in contrast to vimentin-positive glial cells, which repopulate the site of the injury soon after the optic nerve is injured, glial fibrillary acidic protein-positive glial cells remained outside the injury site for as long as 6 weeks after the injury. Despite the injury-induced changes in glial fibrillary acidic protein immunoreactivity, no change was observed in the level of transcript encoding glial fibrillary acidic protein after injury, while there was an increase in the amount of glial fibrillary acidic protein associated with the cytoskeleton and a reduction in the soluble form. These results suggest that the injury-induced changes in immunoreactivity on sections involve changes not in transcription or translation of glial fibrillary acidic protein, but in glial fibrillary acidic protein compartmentalization.

Axon dependent glial changes during optic fiber regeneration in the goldfish

During optic fiber regeneration in the goldfish, astrocytes in the visual system undergo a number of changes. These include hypertrophy of cell processes, increased reactivity with anti-intermediate filament antisera, and expression of cytoskeletal antigens not usually seen in these cells. In the present study, I have asked how much of this response might be due to interactions of glial cells with regenerating optic axons. Animals with and without a retina (regenerating and nonregenerating animals, respectively) had their optic nerve crushed and were then examined at various postoperative times with immunohistochemical methods. Three major differences between these two groups of animals were observed. First, in nonregenerating animals the crush lesion is not repopulated by immunoreactive glial cells while in regenerating animals it is. Second, the nature of the glial hypertrophy in the optic nerve is different in regenerating and nonregenerating animals. Finally, there is marked submeningeal swelling in regenerating nerves that is absent from nonregenerating nerves. Thus, these three aspects of the cellular response to optic nerve crush in the goldfish--wound healing, optic nerve gliosis, and non-neural cellular responses--appear to depend on interactions between regenerating optic axons and astrocytes or other non-neuronal cells of the visual paths for their expression.