The olfactory nerve contains two populations of glia, identified both in vivo and in vitro

In vitro glial responses to nerve growth factor

Nerve growth factor (NGF) stimulates expression of the low affinity neurotrophin receptor p75NGFR mRNA in primary cultures of neonatal rat cortical type I astrocytes. Nerve growth factor treatment altered glial morphology in glial fibrillary acidic protein positive (GFAP+) cell cultures derived from newborn (P0) and 3-day-old (P3) rat pups. When P0- or P3-derived primary glial cultures were serum-deprived, in the presence of 200 pM NGF for 5 days, the flat polygonal glia present in culture assumed a fibrous morphology, an effect not seen in the untreated serum-deprived controls. The NGF effect on astrocytic morphology was blocked by continuous serum treatment. Nerve growth factor did not stimulate astrocytic proliferation under these culture conditions, as assayed by cell cycle analysis using 3H thymidine autoradiography. P0-derived primary glial cultures expressed the signal transducing neurotrophin receptors p145trkB and p140trkA as determined by reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR products were identified by sequencing or restriction enzyme analysis. Astrocytes internalized 125I-NGF at 37 degrees C but not at 4 degrees C, consistent with energy requirements for internalization. Also, internalization of 125I-NGF was abolished by the addition of a 300-1,000-fold excess of unlabeled NGF. Thus, astroglial cells in culture internalize NGF through a specific receptor-mediated process, express trkA and full-length trkB mRNAs at low levels, and respond to exogenous NGF by expressing a fibrous morphology under serum-free culture conditions.

Differential distribution of six calcium-binding proteins in the rat olfactory epithelium during postnatal development and adulthood

Odorant stimulation of receptor cells results in a calcium influx that activates the transduction pathway. Ca2+ acceptors, such as calmodulin, may mediate between the change in intracellular calcium and the conductance mechanism underlying the initial electrical event. Ca2+ acceptors also may participate in subsequent processing of olfactory information. The identification and characterization of these molecules, therefore, should provide important information about the complex signal transduction pathway involving calcium in olfaction as well as other sensory systems. The present study describes the distribution of six calcium-binding proteins in the rat main olfactory epithelium during postnatal development to determine when different Ca2+ acceptors can be detected and whether they segregate into different layers or portions of the epithelium. Calmodulin, calretinin, calbindin-D28k, neurocalcin, and recoverin were detected immunohistochemically in olfactory receptors but not in basal cells. S-100 immunoreactivity was restricted to glial cells primarily around the cribriform plate. During postnatal development (from P1 to P20), calmodulin, calretinin, calbindin-D28k, and neurocalcin formed a gradient of immunoreactivity descending from the central to the lateral areas in the nasal cavity, whereas recoverin was expressed only in sporadic, mature receptors in the proximal region of the mucosa. At P20, the immunoreactivity pattern for each calcium-binding protein was identical to the adult profile, indicating that the olfactory epithelium had reached maturity by this stage. Olfactory nerve fiber bundles displayed a differential staining pattern from P1 until adulthood for calbindin-D28k and calretinin (internal portions of bundles). Differential calmodulin immunoreactivity of olfactory nerves (large external portions of bundles) appeared at P10. The immunoreactivity of the nerve fiber bundles may reflect a further degree of organization relevant to odor discrimination.

Spontaneous equine pulmonary granular cell tumors: morphologic, histochemical, and immunohistochemical characterization

Spontaneous equine pulmonary granular cell tumors were diagnosed in six mature horses at slaughter. These tumors were grossly recognized as multiple (5/6) or single (1/6) creamy white, firm nodules. The tumors, located adjacent to bronchi and bronchioles, often invaded airways, resulting in partial to complete occlusion of the lumina. Neoplastic cells were rounded to polyhedral with numerous eosinophilic cytoplasmic granules that reacted uniformly positive with S-100 and neuron-specific enolase antibodies and multifocally with glial fibrillary acidic protein antibodies. These cells were negative for muscle-specific actin, lysozyme, cytokeratin, chromogranin A, and myelin basic protein antigens and did not stain with silver by the Grimelius technique. Uniformly blue-green and scattered pink intracytoplasmic granules were evident with luxol fast blue and periodic acid-Schiff counterstain for myelin and myelin breakdown products. Histochemical and immunohistochemical staining results of these tumors suggest that they are composed primarily of myelinating Schwann cells with lesser numbers of scattered nonmyelinating Schwann cells. The morphologic features of the equine pulmonary granular cell tumors are strikingly similar to those of endobronchial granular cell tumors of human beings.

Elevated intracellular levels of cAMP induce olfactory ensheathing cells to express GAL-C and GFAP but not MBP

The primary olfactory pathway contains non-myelinating glial cells, called ensheathing cells, that exhibit a variety of phenotypes depending on their immediate environment. In vivo, these cells normally possess a mixture of astrocyte- and Schwann cell-specific phenotypic features. When co-cultured with dorsal root ganglion neurons, their phenotype can become more like that of a myelinating Schwann cell. The objective of this study was to determine whether ensheathing cells would express a myelinating phenotype in culture in the absence of neurons but in the presence of cAMP analogues that are known to induce the expression of myelin associated molecules in Schwann cell cultures. The ensheathing cell cultures were initiated using the nerve fiber layers of Theiler stage 23 rat olfactory bulb primordia and were fed for 1 day to 3 weeks with serum containing (1% or 10% FBS) or serum-free media to which was added different concentrations of dBcAMP (0.1 to 1 mM) or forskolin (10 microM). These cultures were double-labelled with a rabbit polyclonal antibody to S100 in combination with mouse anti-GAL-C (O1 and BRD1 hybridomas) or anti-MBP monoclonal antibodies. The remaining cultures were double-labeled with a rabbit polyclonal antibody to GFAP in combination with the BRD1 antibody. Treatment with dBcAMP or forskolin failed to induce ensheathing cells to express MBP regardless of the concentration. On the other hand, the treatment induced approximately one tenth of the cells to express GAL-C, and virtually all of the cells to express GFAP. These results indicate that although ensheathing cells can synthesize myelin associated molecules, the cAMP second messenger system appears to play a lesser role in controlling the expression of a myelinating phenotype in ensheathing cells than it does in Schwann cells.

The expression pattern of a novel gene encoding brain-fatty acid binding protein correlates with neuronal and glial cell development

Fatty acid binding proteins (FABPs) are a multigene family of small intracellular proteins that bind hydrophobic ligands. In this report we describe the cloning and expression pattern of a novel member of this gene family that is specifically expressed in the developing and adult nervous system and thus was designated brain (B)-FABP. B-FABP is closely related to heart (H)-FABP with 67% amino acid identity. B-FABP expression was first detected at mouse embryonic day 10 in neuroepithelial cells and its pattern correlates with early neuronal differentiation. Upon further development, B-FABP was confined to radial glial cells and immature astrocytes. B-FABP mRNA and protein were found in glial cells of the peripheral nervous system such as satellite cells of spinal and cranial ganglia and ensheathing cells of the olfactory nerve layer from as early as embryonic day 11 until adulthood. In the adult mouse brain, B-FABP was found in the glia limitans, in radial glial cells of the hippocampal dentate gyrus and Bergman glial cells. These findings suggest a function of B-FABP during neurogenesis or neuronal migration in the developing nervous system. The partially overlapping expression pattern with that of cellular retinoid binding proteins suggests that B-FABP is involved in the metabolism of a so far unknown hydrophobic ligand with potential morphogenic activity during CNS development.

Astrocyte-Schwann cell interactions in culture

After injury, either as a result of trauma or degenerating/demyelinating diseases, axons of the central nervous system (CNS) normally fail to regenerate. Transplantation of glial cells, particularly Schwann cells, into areas of injury or demyelination has been considered a promising approach to promote recovery. However, the extent of Schwann cell interaction with CNS axons is greatly influenced by the presence of astrocytes which redefine the CNS-PNS (peripheral nervous system) boundary in a lesioned CNS, thereby preventing invasion of Schwann cells. The molecular basis for this restrictive effect of astrocytes on Schwann cells is not known. In the present study, we have cocultured astrocytes and Schwann cells to develop an in vitro model to characterize this interaction. Astrocytes in contact with Schwann cells appeared hypertrophied and showed increased staining for glial fibrillary acidic protein (GFAP). In cocultures maintained for 2-3 weeks, segregation of the two cell types was observed, Schwann cells appeared in groups, and each group was surrounded and separated from one another by astrocytic processes. Since the behavior of these two cell types observed in culture is very similar to their interaction seen in vivo, this coculture model may be useful in further studying the relationship between astrocytes and Schwann cells.

Localization and regulation of low affinity nerve growth factor receptor expression in the rat olfactory system during development and regeneration

Nerve growth factor (NGF), a classic neurotrophic factor, promotes neuronal survival, maintenance, regeneration and differentiation in the peripheral nervous system and parts of the central nervous system. NGF activity is mediated by cell surface bound receptors including the low affinity NGF receptor (LNGFr) which is expressed by some peripheral and central neurons and is present on peripheral nerve Schwann cells during development and regeneration. The olfactory system is a useful model for the study of the role of LNGFr in neuronal development and regeneration. The growth of olfactory axons into the brain begins in the embryo and continues through the first few postnatal weeks. In mature animals there is persistent turnover and generation of olfactory receptor neurons (ORNs) and continuous growth of new axons into the olfactory bulb. These new axons grow along the preexisting olfactory pathway. In the mature olfactory system, LNGFr has been observed in the glomerular layer of the olfactory bulb, the target of ORNs. However, neither the cellular localization nor the developmental expression of LNGFr has been characterized. Here, we tested the hypothesis that LNGFr expression is developmentally regulated in the olfactory nerve and is reinduced following injury to the mature olfactory nerve. LNGFr-immunoreactivity (IR) was first observed in the olfactory mucosa at embryonic day (E)13 and in the olfactory nerve at E14. LNGFr-IR increased in the nerve during embryonic development, began to decrease at around postnatal day (P)5 and was scarcely detectable in normal adults. The staining pattern suggests that LNGFr is located on the olfactory nerve Schwann cells. Streaks of LNGFr-IR were present in the adult olfactory nerve. We reasoned that these streaks might represent transient reexpression of LNGFr associated with normal olfactory neuron turnover and replacement. Consistent with this hypothesis, LNGFr was robustly reexpressed in the adult olfactory nerve following lesion of the olfactory epithelium. Starting late in development (E21) and in the adult, LNGFr-IR was also observed on fibers in deep layers of the olfactory bulb. LNGFr-IR was also observed in neurons of the nucleus of the diagonal band (NDB) in the basal forebrain. NDB is the sole source of cholinergic afferents of the olfactory bulb. Thus, we tested the hypothesis that LNGFr in the deep layers of the olfactory bulb is located on NDB axons by making lesions of NDB. Following the lesion, LNGFr-IR disappeared in the deep layers of the olfactory bulb but remained in the glomerular layer.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide tyrosine is expressed in ensheathing cells around the olfactory nerves in the rat olfactory bulb

The olfactory bulbs of young and adult normal rats and of colchicine-treated rats and of some other species were analysed for the presence of neuropeptide Y and neuropeptide Y messenger RNA, using immunohistochemistry at the light- and electron-microscopic levels and with in situ hybridization. In the rat and mouse, but not in monkey and guinea-pig, neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA were observed in ensheathing cells in the olfactory nerve layer of the olfactory bulb and within nerve bundles in the olfactory mucosa. Double staining experiments revealed that neuropeptide Y-like immunoreactivity was often present in a restricted compartment, mainly the Golgi apparatus, of S-100 protein-positive ensheathing cells. After colchicine treatment a different distribution of neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA was observed. Thus, in the outer olfactory nerve layer both neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA disappeared, whereas in the inner part messenger RNA levels remained high and neuropeptide Y-like immunoreactivity was observed in many granule-like structures distributed diffusely in the cytoplasm. The present findings suggest that neuropeptide Y may be involved in the control of regeneration, growth and/or guiding of the axons of the olfactory sensory neurons, the only mammalian neurons known to have a continuous renewal and growth during adult life.

Expression of the tight junction protein ZO-1 in the olfactory system: presence of ZO-1 on olfactory sensory neurons and glial cells

The olfactory system is a unique part of the central nervous system since it retains neuronal turnover and regenerative capacities in adulthood. Thus it provides an ideal model to study plasticity of membrane moities involved in cell-cell interactions. One structure particularly involved in cell-cell interaction is the tight junction, which establishes polarization of epithelial cells and creates diffusion barriers to paracellular passages. ZO-1 is a phosphoprotein peripherally associated with tight junctions. We have studied expression of ZO-1 protein in the developing and adult olfactory system of the mouse in order to get information about the localization and developmental expression of this tight junction component. ZO-1 expression has also been determined in cell cultures of olfactory bulbs. ZO-1 was present in the olfactory placode prior to formation of tight junctions. ZO-1 was localized in the developing and mature olfactory epithelium at heterotypic contacts between supporting cells and olfactory neurons as well as at homotypic contacts between both these cell types. Confocal microscopy showed quantitative differences in the ZO-1 expression among different olfactory dendrites. In the olfactory nerves ZO-1 immunolabeling was detectable between olfactory ensheathing cells. From the seventh postnatal day ZO-1 immunolabeling was detected at the mitral cell layer of the bulb on cells tentatively identified as oligodendrocytes. Myelinated tracts of the bulb were ZO-1 negative. Cell cultures of olfactory bulbs showed ZO-1 immunoreaction, mostly localized on glial fibrillary acidic protein (GFAP)-positive cells. Our results provide further evidence that ZO-1 serves functions unrelated to the tight junction complex and indicate molecular heterogeneity of these cell-cell contacts.

On the formation of neuromata in the primary olfactory projection

Olfactory axons have been shown to grow aberrantly and form dense collections of axons, termed neuromas, in the olfactory epithelium of rats in which the olfactory bulb was ablated. Likewise, in human olfactory mucosa, collections of neurites have been noted in a variety of disease states, including Alzheimer's disease. We report here an immunohistochemical and electron microscopic analysis of aberrant axonal growth in the rat olfactory mucosa induced by experimental lesion. In particular, we have used the monoclonal antibody 2G12, which binds to the phosphorylated form of GAP-43, as an extremely sensitive marker for neuromatous axons, because it does not label neuronal cell bodies. In unilaterally bulbectomized rats, neuromas form in posterior olfactory epithelium on the operated side. Several lines of evidence, including serial section reconstruction, indicate that olfactory axons are induced to grow back into the epithelium at a distance from their point of origin as a consequence of bulbectomy, and are accompanied by glial cells from the olfactory nerve. Avulsion of a part of the olfactory nerve has similar effects as destruction of the olfactory bulb. Intraepithelial neuromas also develop in the olfactory mucosa of rats simultaneously exposed to methyl bromide gas and injected with 3-methyl indole; this treatment severely damages the olfactory epithelium directly. Exposure to methyl bromide alone causes milder damage, and the neuromas that form are transient. The evidence indicates that neuromas form after the epithelium is directly damaged because axons are trapped in the epithelium. Both of the mechanisms identified here should be taken into account when considering the findings in the human olfactory mucosa.

In vitro enfolding of olfactory neurites by p75 NGF receptor positive ensheathing cells from adult rat olfactory bulb

Secondary cultures of adult rat olfactory bulb (OB) contained three different types of cell: (i) process-bearing cells; (ii) macrophage-like cells and (iii) fusiform cells. The immunohistochemical properties of process-bearing cells closely corresponded to those described for ensheathing glia in vivo. The most distinctive feature of these cells was their immunoreactivity for low affinity nerve growth factor receptor (NGFR). Process-bearing cells also shared the ultrastructural properties of ensheathing glia in vivo, as well as the ability to ensheath olfactory axons. In contrast, macrophage-like cells had the immunostaining properties of microglia, and fusiform cells were likely capillary endothelial cells. Neurites outgrowing from olfactory epithelium explants, when co-cultured with adult OB cells, grew preferentially over NGFR positive cells. Olfactory neurites exhibited NGFR immunoreactivity and were enfolded by NGFR positive cells. After ensheathment, this immunoreactivity decreased from the neurite and disappeared from the glial membrane in contact with the neurite. However, NGFR immunoreactivity was maintained in the portion of the glial membrane not involved in ensheathing. In summary, ensheathing cells in vitro retained both the ultrastructure shown in vivo and the ability to ensheath olfactory neurites. The Schwann cell-like properties of ensheathing glia, could partially explain the permissibility of adult OB to axonal growth.

Glial progenitor cells of the nerve fiber layer of the olfactory bulb: effect of astrocyte growth media

There are two morphologically distinct types of glial cells (i.e., ensheathing cells and astrocytes) in the nerve fiber layer (NFL) of the adult mammalian olfactory bulb. Ensheathing cells provide ensheathment for olfactory axons, whereas astrocytes occupy the interfascicular spaces of the olfactory NFL. During embryonic development, however, only one type of glial cell is found in this layer of the olfactory bulb, namely, the ensheathing cell. Even though ensheathing cells take up residence within the CNS, they are actually derived from the olfactory placode. Far less is known about the developmental origin of interfascicular astrocytes, which arise either from the glial progenitor cells that give rise to ensheathing cells or from astrocyte precursor cells that migrate into the NFL from deeper layers of the bulb primordium. In the present study, enriched populations of ensheathing cells were grown in vitro in media known to promote the growth and differentiation of astrocytes to determine whether ensheathing cell progenitors could differentiate into astrocytes. These media failed to induce the appearance of astrocytes in the ensheathing cell cultures. It was concluded that the astrocytes of the NFL most likely arise from progenitor cells that migrate into this layer from deeper parts of the developing bulb.

Elaboration of glial cell processes in the rat olfactory bulb associated with early learning

Odor preference training early in life induces anatomical changes in focal areas of the glomerular layer of the rat main olfactory bulb. We examined the associated focal changes in glial cell morphology using immunohistochemistry for glial fibrillary acidic protein (GFAP) and found that the density of immunoreactive processes was higher in glomeruli responsive to an odor for which pups had developed a preference. The increase in process density in trained pups was specific to focal responsive regions of the bulb, revealed with [14C]2-deoxyglucose autoradiography. There was no change in the number of GFAP-immunoreactive cells between trained and control pups.

2-Mercaptoethanol is a survival factor for olfactory, cortical and hippocampal neurons in short-term dissociated cell culture

A medium originally designed for lymphocyte growth promoted robust survival of olfactory receptor neurons (ORNs) in short-term (4-day), dissociated cell culture. The key ingredient for survival of neurons in both serum and serum-free conditions was 2-mercaptoethanol (2-ME). Enhancement of survival may be thiol-mediated because two other thiol compounds, 2-mercaptoethylamine and monothioglycerol, also increased ORN survival. Addition of 2-ME also significantly increased survival of embryonic cortical and hippocampal neurons in a serum-free medium, and embryonic cortical neurons in a serum-containing medium. After plating and growth in a serum-free medium containing 2-ME, survival of all three types of neurons was equivalent to, or greater than, survival in serum-containing media. Thus, thiols such as 2-ME promote the survival of multiple types of neurons in short-term cell culture.

Astrocyte subtypes in the rat olfactory bulb: morphological heterogeneity and differential laminar distribution

Despite increased recognition of the importance and heterogeneity of astrocyte functions throughout the central nervous system (CNS) relatively little attention has been paid to morphological diversity among astrocytes. Recent studies have indicated that subsets of astrocytes are involved in glial-axonal interactions critical to both development and reinnervation of the rat olfactory bulb. Here, we have characterized the morphologies and distribution of astrocytes within anatomically and functionally distinct layers of the adult main olfactory bulb (MOB). Using a known immunohistochemical marker for astrocytes, glial fibrillary acidic protein (GFAP), and the classic gold sublimate method, we identified six astrocyte subtypes based on their morphology and distribution: (1) unipolar, (2) irregular, (3) wedge-shape, (4) circular, (5) semicircular, and (6) elongate. Unipolar, irregular and wedge-shape astrocytes have not been previously described in the CNS. The unipolar and irregular types are located exclusively in the olfactory nerve layer. Wedge-shape astrocytes are unique to, and are the major subtype in, the glomerular layer. These three morphologically unique astrocyte subtypes may correspond to olfactory nerve layer (ONL) and glomerular layer (GL) astrocytes, which express molecules that regulate axonal growth or synaptogenesis during development and/or regeneration of the olfactory nerve. In the glomerular layer, astrocytes are highly organized with respect to the glomeruli. Individual astrocytes are loyal to a single glomerulus. In the external plexiform layer, astrocytes are spaced relatively uniformly. In the granule cell layer, astrocytes appear to compartmentalize granule cell aggregates, recently shown to be coupled by tight junctions. The distribution and patterns of astrocyte processes and the density of GFAP immunoreactivity are distinctive for each of the layers of the olfactory bulb. The spacing of astrocytes and the organization of their processes may be important to compartmentalization of neuronal functions. High levels of GFAP immunoreactivity correlated with layers of high neuronal plasticity. The morphological diversity and differential distribution of astrocytes in the olfactory bulb reported here support growing evidence for functional diversity of astrocytes and important interactions among specific astrocyte and neuron subtypes. It is reasonable to hypothesize, therefore, that as for neurons, morphologically distinctive astrocyte subtypes may correspond to functionally specific classes.

Glial cells in the nerve fiber layer of the main olfactory bulb of embryonic and adult mammals

This article provides a detailed description of the glial cell types in the nerve fiber layer of the main olfactory bulb during embryonic development, in adult mammals, and at the nerve entry zone of the first cranial nerve. In adult mammals, the glial cell types of the olfactory nerve fiber layer include intrafascicular ensheathing cells, which have the exclusive role of ensheathing the olfactory axons in both the PNS and CNS, and interfascicular astrocytes, which occupy the spaces between adjacent olfactory fascicles. The ensheathing cells are particularly interesting because they possess a mixture of Schwann cell and astrocytic phenotypic features, are more likely to be of placodal than of CNS origin, and have the exclusive role of forming the glia limitans at the PNS-CNS transitional zone. It is proposed that one important function of ensheathing cells is to modulate the growth of olfactory axons within the CNS; this modulation is probably mediated by selective cell adhesion molecules, extracellular matrix molecules, and chemotropic agents.

Development of olfactory nerve glia defined by a monoclonal antibody specific for Schwann cells

Although there is considerable interest in the possible role of olfactory glia in the pathfinding abilities of olfactory nerve axons, the complete development of these glia in vivo has not been described. Using a specific Schwann cell marker, the 1E8 antibody, we have found that olfactory nerve glia can be identified throughout development. These glia appear to originate in the olfactory placode and migrate initially into the periphery of the olfactory nerve, and later into the center of the nerve. Olfactory nerve glia enter the presumptive olfactory bulb with the olfactory receptor neuron axons and distribute themselves along the edge of the olfactory nerve layer. The fact that olfactory nerve glia are specifically immunostained by the 1E8 monoclonal antibody, which recognizes the Schwann cell-specific protein P0, suggests that these cells more closely resemble Schwann cells than astrocytes or enteric glia. These results support and extend previous findings suggesting that olfactory nerve glia have distinctive developmental and anatomical features which may be important to the regenerative capacity of the olfactory system.

CNS glial cells support in vitro survival, division, and differentiation of dissociated olfactory neuronal progenitor cells

Olfactory receptor neurons (ORNs) are replaced and differentiate in adult animals, but differentiation in dissociated cell culture has not been demonstrated. To test whether contact with the CNS regulates maturation, neonatal rat olfactory cells were grown on a culture substrate or on CNS astrocytes. Mature ORNs, immunopositive for olfactory marker protein (OMP), disappeared rapidly from both systems. Neurons positive for neuron-specific tubulin (immature and mature) disappeared from substrate-only cultures, but remained abundant in the cocultures. OMP-positive neurons reappeared after 10 days in vitro. Pulse labeling with [3H]thymidine showed extensive neurogenesis of both immature and mature olfactory neurons. This demonstrates, in vitro, both division and differentiation of olfactory progenitor cells.