Axonal contact regulates expression of alpha2 and beta2 isoforms of Na+, K+-ATPase in Schwann cells: adhesion molecules and nerve regeneration

Three isoforms of catalytic alpha subunits and two isoforms of beta subunits of Na+,K+-ATPase were detected in rat sciatic nerves by western blotting. Unlike the enzyme in brain, sciatic nerve Na+,K+-ATPase was highly resistant to ouabain. The ouabain-resistant alpha1 isoform was demonstrated to be the predominant form in rat intact sciatic nerve by quantitative densitometric analysis and is mainly responsible for sciatic nerve Na+,K+-ATPase activity. After sciatic nerve injury, the alpha3 and beta1 isoforms completely disappeared from the distal segment owing to Wallerian degeneration. In contrast, alpha2 and beta2 isoform expression and Na+,K+-ATPase activity sensitive to pyrithiamine (a specific inhibitor of the alpha2 isoform) were markedly increased in Schwann cells in the distal segment of the injured sciatic nerve. These latter levels returned to baseline with nerve regeneration. Our results suggest that alpha3 and beta1 isoforms are exclusive for the axon and alpha2 and beta2 isoforms are exclusive for the Schwann cell, although axonal contact regulates alpha2 and beta2 isoform expressions. Because the beta2 isoform of Na+,K+-ATPase is known as an adhesion molecule on glia (AMOG), increased expression of AMOG/beta2 on Schwann cells in the segment distal to sciatic nerve injury suggests that AMOG/beta2 may act as an adhesion molecule in peripheral nerve regeneration.

Upregulation of the endosomal-lysosomal pathway in the trembler-J neuropathy

A nonconservative leucine to proline mutation in peripheral myelin protein 22 (PMP22) causes the Trembler-J (TrJ) neuropathy in mice and humans. The expression levels and localization of the PMP22 protein in the TrJ mouse have not been previously determined. The aim of our studies was to reevaluate the extent of myelin deficit in genotyped heterozygous and homozygous animals and to examine how the TrJ mutation alters the normal in vivo post-translational processing of PMP22. Morphological studies show evidence for primary dysmyelination and myelin instability in affected animals. As expected, Western blot analysis indicates that in adult heterozygous TrJ animals, the level of PMP22 is markedly decreased, similar to myelin basic protein and protein zero, whereas myelin-associated glycoprotein is largely unaffected. The decrease in myelin protein expression is associated with an increase in lysosomal biogenesis, suggestive of augmented endocytosis or autophagy. Double-immunolabeling experiments show the accumulation of PMP22 in endosomal/lysosomal structures of TrJ Schwann cells, and chloroquine treatment of nerve segments indicates that the degradation of protein zero, PMP22, and myelin basic protein is augmented in TrJ nerves. These studies suggest that the TrJ mutation alters myelin stability and that the mutant protein is likely degraded via the lysosomal pathway.

Occurrence of myelin-associated glycoprotein (MAG)-like immunoreactivity in some nervous, endocrine, and immune-related cells of the rat. An immunohistochemical study

The occurrence and distribution of myelin-associated glycoprotein (MAG)-like immunoreactivity was investigated in the rat using a polyclonal antibody to MAG purified from rat brain. In the nervous system, MAG immunoreactivity was found in the periaxonal portion of the myelinated fibers and in a small number of oligodendroglia in the cortex, hippocampus, and the spinal cord. The sheath of Schwann cells in unmyelinated fibers and satellite cells in the spinal ganglia were also immunoreactive for MAG. In the endocrine system, the noradrenaline-containing cells in the adrenal medulla and some endocrine cells in the duodenum showed MAG immunoreactivity. In the immune system, numerous reticular cells with slender cytoplasmic processes, which formed a dense network, were immunopositive for MAG within the germinal center in the lymph nodes and spleen. In the thymus, a number of epithelial reticular cells within the medulla showed variation in staining intensity. These findings provide new information on the wide distribution of MAG immunoreactivity in the nervous, endocrine, and immune systems, and may contribute to the further understanding of the biological roles of this protein.

Insulin-like growth factor-I (IGF-I) and IGF binding protein-5 in Schwann cell differentiation

Schwann cells (SCs) are the myelin producing cells of the peripheral nervous system. During development, SCs cease proliferation and differentiate into either a myelin-forming or non-myelin forming mature phenotype. We are interested in the role of insulin-like growth factor-I (IGF-I) in SC development. We have shown previously SCs proliferate in response to IGF-I in vitro. In the current study, we investigated the role of IGF-I in SC differentiation. SC differentiation was determined by morphological criteria and expression of myelin proteins. Addition of 1 mM 8-bromo cyclic AMP (cAMP) or growth on Matrigel matrix decreased proliferation and induced differentiation of SCs. IGF-I enhanced both cAMP and Matrigel matrix-induced SC differentiation, as assessed by both morphological criteria and myelin gene expression. Cultured SCs also express IGF binding protein-5 (IGFBP-5), which can modulate the actions of IGF-I. We examined the expression of IGFBP-5 during SC differentiation. Both cAMP and Matrigel matrix treatment enhanced IGFBP-5 protein expression and cAMP increased IGFBP-5 gene expression five fold. These findings suggest IGF-I potentiates SC differentiation. The concomitant up-regulation of IGFBP-5 may play a role in targeting IGF-I to SCs and thus increase local IGF-I bioavailability.

Axonal neuregulin signals cells of the oligodendrocyte lineage through activation of HER4 and Schwann cells through HER2 and HER3

We are interested in the signaling between axons and glia that leads to myelination and maintenance of the myelin internode, and we have focused on the role of neuregulins and their receptors. Neuregulins are a family of ligands that includes heregulin, neu differentiation factor, glial growth factor, and the acetylcholine receptor-inducing activity. Three signal transducing transmembrane receptors for neuregulins, which bear significant homology to the EGF receptor, are currently known: HER2 (erbB2), HER3 (erbB3), and HER4 (erbB4). We have found that oligodendrocite-type II astrocyte (O2A) progenitor cells and mature oligodendrocytes express HER2 and HER4 but no HER3. Schwann cells express HER2 and HER3 but little HER4. In O2A progenitor cells and oligodendrocytes, recombinant neuregulin induces the rapid tyrosine phosphorylation of only HER4. HER2 is not phosphorylated in cells of the oligodendrocyte lineage, but a physical interaction between HER2 and HER4 was detected in coimmunoprecipitation experiments. In Schwann cells, neuregulin induces the phosphorylation of both HER2 and HER3. Coimmunoprecipitation experiments indicate that receptor activation in Schwann cells results in the formation of HER2:HER3 heterodimers. Neuregulin localized immunocytochemically was present on neurites of cultured dorsal root ganglion neurons, and it was released into the medium in a form that promoted receptor tyrosine phosphorylation. Neuregulins therefore meet important criteria expected of molecules involved in axonal-glial signaling. The use of unique neuregulin receptor combinations in oligodendrocytes and Schwann cells likely results in recruitment of different signaling pathways and thus provides a basis for different biological responses.

Studies on the effects of altered PMP22 expression during myelination in vitro

Severe inherited dysmyelinating diseases of the peripheral nervous system, the Charcot-Marie-Tooth type1A disease (CMT1A) and the hereditary neuropathy with liability to pressure palsies (HNPP) are associated with a large DNA duplication or deletion of a chromosomal region containing the peripheral myelin protein 22 (PMP22) gene. It has been suggested that a gene dosage effect involving PMP22 is responsible for the pathological phenotype. We investigated if altered PMP22 expression affects the onset of myelin formation and the ultrastructure of myelin. Rat Schwann cell cultures were stably infected with recombinant retrovirus vectors harboring the rat PMP22 cDNA in sense or antisense orientation. Schwann cells over- or underexpressing PMP22 were cocultured with purified DRG neurons under conditions that promote myelination. We examined PMP22 expression and localization in the myelin forming cultures by RT-PCR, immunohistochemistry and confocal microscopy, and we analyzed myelin ultrastructure by electron microscopy. Our results demonstrate that abnormal levels of PMP22 expression do not impair the early stages of myelination and membrane compaction and do not interfere with the expression of other myelin genes. Our observations further indicate that PMP22 is involved more in controlling myelin thickness and stability than in the events determining the initial steps of myelin formation.

Neural targeting of Mycobacterium leprae mediated by the G domain of the laminin-alpha2 chain

We report that the molecular basis of the neural tropism of Mycobacterium leprae is attributable to the specific binding of M. leprae to the laminin-alpha2 (LN-alpha2) chain on Schwann cell-axon units. Using recombinant fragments of LN-alpha2 (rLN-alpha2), the M. leprae-binding site was localized to the G domain. rLN-alpha2G mediated M. leprae binding to cell lines and to sciatic nerves of dystrophic dy/dy mice lacking LN-alpha2, but expressing laminin receptors. Anti-beta4 integrin antibody attenuated rLN-alpha2G-mediated M. leprae adherence, suggesting that M. leprae interacts with cells by binding to beta4 integrin via an LN-alpha2G bridge. Our results indicate a novel role for the G domain of LN-2 in infection and reveal a model in which a host-derived bridging molecule determines nerve tropism of a pathogen.

Ciliary neurotrophic factor immunoreactivity in rat intramuscular nerve during reinnervation through a silicone tube after severing of the rat sciatic nerve

The immunoreactivity of ciliary neurotrophic factor (CNTF) and S100 was studied in the degenerating and regenerating intramuscular nerves after the sciatic nerve was severed. The sciatic nerves of male Wistar rats were transected at the midpoint of the thigh, and silicone tubing was used to obtain effective reinnervation. The strong immunoreactivity of CNTF and S100 was observed in the Schwann cell cytoplasm of intramuscular nerves (IMN) and at the neuromuscular junction (NMJ) on the control sections. The CNTF immunoreactivity gradually became weak and indistinct in the Schwann cell cytoplasm after the operation. However, it was recognized again in the IMN at 4 weeks after the operation. On the other hand, the S100 immunoreactivity was continuously observed except at the NMJ through the denervating and reinnervating period. At 12 weeks after the operation, the strong immunoreactivity of both CNTF and S100 was observed again. These findings suggest that the amount of CNTF protein decreased in Schwann cells of the IMN and NMJ during the denervating period and increased during the reinnervating period in proportion to the number of remyelinated Schwann cells after severing of the sciatic nerve. They also suggest that CNTF was more highly correlated than the S100 protein with the reinnervation activity of Schwann cells.

Association of the tetraspan protein CD9 with integrins on the surface of S-16 Schwann cells

The cell surface glycoprotein CD9 is a member of a group of proteins known as the "tetraspan" or "transmembrane 4 superfamily." Previous work with non-neural cells has shown that CD9 associates in cis with integrins and small GTP-binding proteins on the cell surface. To extend our recent findings showing that perturbation of CD9 alters Schwann cell adhesion, proliferation, and migration, as well as neurite outgrowth in sympathetic neurons, we have searched for CD9-associated proteins in S-16 Schwann cells. We demonstrate here that CD9 is specifically coprecipitated from S-16 cell extracts by antibodies against integrins alpha 3, alpha 6, and beta 1. In addition, double immunofluorescence labeling and co-capping experiments indicate that CD9 is specifically co-localized with these integrins on the cell membrane of S-16 Schwann cells.

Connexin43 is another gap junction protein in the peripheral nervous system

That many cells express more than one connexin (Cx) led us to examine whether Cxs other than Cx32 are expressed in the PNS. In addition to Cx32 mRNA, Cx43 and Cx26 mRNAs were detected in rat sciatic nerve by northern blot analysis. Cx43 mRNA, but not Cx26 mRNA, was expressed in both the primary Schwann cell culture and immortalized Schwann cell line (T93). The steady-state levels of the Cx43 mRNA in the primary Schwann cell culture increased 2.0-fold with 100 microM forskolin, whereas that of Po increased 7.0-fold. Immunoreactivity to Cx43 was detected on western blots of cultured Schwann cells, T93 cells, and sciatic nerves but not on blots of PNS myelin. Immunohistochemical study using human peripheral nerves revealed that anti-Cx43 antibody stained cytoplasm around nucleus of Schwann cells but not myelin, confirming western blot results. Although Po expression was markedly decreased by crush injury of the sciatic nerves, Cx43 expression showed no apparent change. Developmental profiles showed that Cx43 expression in the sciatic nerve increased rapidly after birth, peaked at about postnatal day 6, and then decreased gradually to a low level. In adult rats, the Cx43 mRNA value was much lower than that of Cx32. These findings suggest that Cx43 is localized in Schwann cell bodies and that, compared with Po, its expression is less influenced by axonal contact and cyclic AMP levels. The high expression on postnatal day 6 indicates that Cx43 may be related to PNS myelination. Cx43 is another gap junction, but its function appears to differ from that of Cx32, as judged by the differences in their localization and developmental profiles.

N-acetylated alpha-linked acidic dipeptidase may be involved in axon-Schwann cell signalling

N-Acetylated alpha-linked acidic dipeptidase is a membrane-bound peptidase that cleaves the neuropeptide N-acetyl-aspartyl-glutamate to N-acetyl-aspartate and glutamate. Previously, we have shown that in adult rat this enzyme is expressed by the non-myelinating Schwann cells in peripheral nerve. In the present study, we have determined the expression pattern of this peptidase in rat sciatic nerve during late embryonal and early postnatal development, using double-label immunofluorescence, enzyme assays and immunoblotting. We demonstrate that N-acetylated alpha-linked acidic dipeptidase is expressed by all Schwann cell precursor cells on embryonal day 14/15 and by all undifferentiated Schwann cells on embryonal days 16/17 and 20/21 and postnatal day 1. Moreover, we show that during the first postnatal week, the peptidase expression is down-regulated in the myelinating Schwann cells while the total enzyme activity levels and the enzyme amounts present in the nerve are transiently increased. To determine whether Schwann cell peptidase expression is dependent on axonal contact, we performed immunofluorescence experiments in cultured Schwann cells. These in vitro experiments demonstrate that the expression of this enzyme is maintained in culture for several weeks without axonal contact. Furthermore, they confirm previous suggestions that this peptidase is expressed on the extracellular side of the Schwann cell membrane. These findings support the notion that N-acetylated alpha-linked acidic dipeptidase takes part in signaling between peripheral axons and Schwann cells. The temporary increase in peptidase activity during the first postnatal week strongly implicates a role for this enzyme in the process of axon ensheathment and/or axon myelination.

Role of ploidy, chromosome 1p, and Schwann cells in the maturation of neuroblastoma

BACKGROUND

Neuroblastoma is a heterogeneous disease, with manifestations ranging from spontaneous regression to lethal spread. Sometimes the tumor spontaneously differentiates toward a benign ganglioneuroma (maturing neuroblastoma). The prognosis is frequently related to ploidy, deletions in the short arm of chromosome 1, and amplifications of the N-myc oncogene. Maturing neuroblastomas consist of both neuronal cells and Schwann cells. We investigated the genetic composition of both cell types in maturing neuroblastomas, to determine the relation between genetic abnormalities and maturation.

METHODS

We studied 20 maturing and mature neuroblastomas by in situ hybridization to count the chromosomes and evaluate possible deletions in the short arm of chromosome 1 in neuronal and Schwann cells. The DNA content of the cells was measured by flow cytometry.

RESULTS

Neuroblastic and ganglionic cells showed aberrations in the number of chromosomes. In situ hybridization and flow cytometry demonstrated near-trip-loidy in 18 of 19 tumors and pentaploidy in the remaining tumor. The Schwann cells in all 20 neuroblastomas contained normal numbers of chromosomes. In 18 tumors studied, there were no chromosome 1 deletions in either type of cell.

CONCLUSIONS

The Schwann cells in maturing neuroblastomas differ genetically from the neuronal cells. The normal number of chromosomes in Schwann cells and the abnormal number in neuroblastic ganglionic cells suggests that Schwann cells are a reactive population of normal cells that invade the neuroblastoma. Near-trip-loidy of neuroblastoma cells and intact chromosome 1 are presumably genetic prerequisites for spontaneous organoid maturation, because we found no diploidy or chromosome 1 depletions in the neuronal cells of spontaneously maturing neuroblastomas.

The sensory innervation of the mouse spinal cord may be patterned by differential expression of and differential responsiveness to semaphorins

To better understand the regulatory processes underlying axonal pathfinding we analyzed the embryonic expression of seven murine semaphorin genes by in situ hybridization In the spinal cord, transcripts of all seven semaphorin genes were detected from Embryonic Day 11.5 (E11.5) onward and restricted to distinct regions at E15.5. Interestingly, semE, F, and G mRNAs were in addition differentially expressed in the ventricular zone of the telencephalon. In order to correlate these expression patterns to the behavior of different types of sensory afferents, we tested their response to recombinant semaphorin proteins. Specific subpopulations of dorsal root ganglion sensory neurons displayed a developmentally regulated differential response to Sem D. Whereas extension of both NGF- and NT-3-dependent neurites was inhibited by Sem D at E12.5, only neurites formed in the presence of NGF responded at E14.5. This suggests that Sem D may be involved in preventing an early penetration of the spinal cord by sensory afferents and subsequently shaping their lamina-specific termination.

Demonstration of progesterone receptors in rat Schwann cells

We have recently shown that progesterone promotes myelin formation in peripheral nerves of rodents. In this study, we demonstrate the presence of progesterone receptors (PR) in primary cultures of rat Schwann cells, the glial cells of the PNS, prepared from sciatic nerves of 4-5 days old rats. After 3 weeks of culture, the presence of PR was measured by whole cell assay after incubating living cells for 1 h at 37 degrees C with [3H]-Organon 2058 as a ligand, and about 5000 specific binding sites per cell were found. In contrast to the PR of rat glial cells from the central nervous system (CNS), which is induced by estrogens, treatment of Schwann cells with estradiol did not increase the PR-binding, even after exposure of cells to high doses of estrogen under various culture conditions. Progesterone receptors were also visualized in Schwann cells by indirect immunofluorescence staining with a monoclonal anti-PR antibody. Again, treatment of the cells with estradiol did not increase the immunofluorescence staining of the PR. Specific PR binding was also measured in sciatic nerves of adult female rats. Cytosol was prepared and labeled with [3H]-Organon 2058 for 15 h at 2 degrees C. After treatment with dextran-coated charcoal, specific ligand binding was about 30 fmol/mg cytosolic protein. When castrated adult female rats were treated with estradiol (20 micrograms EB/day for 3 days), no PR-induction was observed in the cytosol of sciatic nerves. In contrast, PR-binding sites in cytosols prepared from pituitary gland and uteri of the same animals were significantly increased by the estrogen.

Cultured rat neuronal and glial cells entrapped within hydrogel polymer matrices: a potential tool for neural tissue replacement

Cultured Schwann cells, neonatal astrocytes or cells dissociated from embryonic cerebral hemispheres were dispersed within poly-[N-(2-hydroxypropyl)-methacrylamide]-based hydrogel matrices by gel entrapment and maintained in vitro for 1-6 days. Glial cells were pre-labelled with Hoechst 33342. Cell differentiation and viability were studied by immunocytochemistry. Up to 15% of Schwann cells initially entrapped within the polymer matrices were immunopositive for the low affinity nerve growth factor receptor, S100, glial fibrillary acidic protein (GFAP) and laminin; up to 10% of pre-labelled astrocytes were immunopositive for GFAP and laminin. Embryonic neurons displayed immunostaining for neurofilaments. Hydrogels containing entrapped Schwann cells were implanted into the rat neocortex. These polymers supported cellular and axonal ingrowth within parts of the polymer matrix.

Cloning of a sodium channel alpha subunit from rabbit Schwann cells

Overlapping cDNA clones spanning the entire coding region of a Na-channel alpha subunit were isolated from cultured Schwann cells from rabbits. The coding region predicts a polypeptide (Nas) of 1984 amino acids exhibiting several features characteristic of Na-channel alpha subunits isolated from other tissues. Sequence comparisons showed that the Nas alpha subunit resembles most the family of Na channels isolated from brain (approximately 80% amino acid identity) and is least similar (approximately 55% amino acid identity) to the atypical Na channel expressed in human heart and the partial rat cDNA, NaG. As for the brain II and III isoforms, two variants of Nas exist that appear to arise by alternative splicing. The results of reverse transcriptase-polymerase chain reaction experiments suggest that expression of Nas transcripts is restricted to cells in the peripheral and central nervous systems. Expression was detected in cultured Schwann cells, sciatic nerve, brain, and spinal cord but not in skeletal or cardiac muscle, liver, kidney, or lung.

Single-cell PCR performed with neurofibroma Schwann cells reveals the presence of both alleles of the neurofibromatosis type 1 (NF1) gene

It is commonly held that Schwann cells (SC) are the progenitor cells of benign neurofibromas. To test for loss of heterozygosity (LOH) at the neurofibromatosis 1 (NF1) gene locus, three intragenic polymorphic markers were analyzed after polymerase chain reaction amplification, starting from 98 single SC isolated from primary cultures of neurofibromas, of five informative NF1 patients. The patterns obtained did not provide evidence for LOH at the NF1 gene. LOH by nondisjunction, large deletions, or somatic recombination in SC seems not to be the mechanism of generation of neurofibromas.

Immunohistochemical distribution of S-100 protein and type IV collagen in human embryonic and fetal sympathetic neuroblasts

The expression and distribution of S-100 protein and type IV collagen was studied immunohistochemically in sympathetic neuroblasts from the paravertebral region to the adrenal glands in human embryos and fetuses ranging from 7 to 12 weeks gestational age. From 7 weeks gestational age, S-100 protein was detected in round or oval cells mingling with sympathetic neuroblasts, and in spindle-shaped cells forming a continuous layer around them. The latter S-100 protein-positive cells were found in contact with the Schwann cells of nerve fibres entering the groups of sympathetic neuroblasts. Staining for type IV collagen showed that all groups of sympathetic neuroblasts were surrounded by a continuous basement membrane. By examining serial sections stained for type IV collagen and S-100 protein, a continuous basement membrane was found along the distribution pattern of the peripheral S-100 protein-positive spindle cells. The morphology of these cells, and their relationships with Schwann cells and with the basement membrane of the sympathetic neuroblasts, indicated that they were Schwann-like cells probably capable of synthesizing a continuous basement membrane separating the neuroblasts from the adjacent tissues. In contrast, the round or oval S-100 protein-positive cells, in contact with the sympathetic neuroblasts and not associated with nerve fibres, were considered as sustentacular or sustentacular precursor cells. At week 7 gestational age, the peri-adrenal sympathetic neuroblasts and their sustentacular and Schwann-like cells started to invade the adrenal glands and mingled with the adrenal cortical cells. These findings suggest the extra-adrenal origin of the sustentacular cells in embryonic and fetal adrenal glands.

An endogenous lectin and its glycoprotein ligands are triggering basal and axon-induced Schwann cell proliferation

The proliferation of Schwann cells (the myelinating cells of the peripheral nervous system) is stimulated by the contact with axonal membranes. It is suggested that the endogenous carbohydrate-binding protein (lectin) cerebellar soluble lectin (CSL) bound to ligands at the surface of axonal preparations is mitogenic for Schwann cells. Both autocrine and axon-stimulated Schwann cell proliferations seem to be dependent on the presence of CSL and its ligands at the Schwann cell surface, as suggested by the effects of N-glycosylation inhibitors and anti-CSL Fab fragments. These data suggest that CSL regulates Schwann cell proliferation by clustering of a few glycoprotein ligands at the cell surface, consequently modulating phosphorylations.

Chondroitin sulfate proteoglycan staining in astrocyte-Schwann cell co-cultures

Transplantation of Schwann cells (SCs) in the central nervous system (CNS) for remyelination in pathological situations has been considered a promising approach. However, numerous studies have indicated that astrocytes have a restrictive effect on SC migration within the CNS. We have previously established an in vitro model which demonstrates the restrictive effect of astrocytes on SCs (Ghirnikar and Eng, Glia 4:367-377, 1994). Using this culture model, in the present study, we have characterized the molecular basis underlying astrocyte-SC interaction and demonstrated chondroitin sulfate proteoglycan (CSP) staining in the co-cultures. Following 1-2 weeks of incubation, CSP staining was specifically associated with SCs co-cultured with astrocytes. Staining with antibodies specific for the different chondroitin sulfate isomers revealed the presence of both, chondroitin-4- and 6-sulfates in SCs. In contrast, SCs when cultured alone, or in the presence of astrocytes conditioned medium did not show CSP staining. These data suggest that CSP staining is associated with SCs following co-culture with astrocytes and mediated by cell to cell contact. We hypothesize that the CSP, alone or in combination with other molecules expressed by astrocytes and/or SCs, may be involved in the restrictive effects of astrocytes on SCs. Identification of molecules involved in the unfavorable interaction between astrocytes and SCs will have an important bearing on efforts to remyelinate demyelinated axons by SC transplantation within the damaged CNS.

Cellular and subcellular localization of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system

The cellular and subcellular distribution of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system was examined using antibodies against peptides corresponding to the C-terminal portions of AMPA receptor subunits: GluR1, GluR2/R3 and GluR4. The light and electron microscopic immunocytochemical studies were carried out on Vibratome sections of rat and guinea pig vestibular sensory epithelial and ganglia. In the epithelium, GluR1 subunit immunoreactivity appeared as accumulations of patches outlining the baso-lateral periphery of the type I sensory cells. The GluR1-immunoreactive microareas were postsynaptically distributed on the membranes of calyceal afferent fibers. GluR2/R3 immunoreactivity was present in the sensory cells. GluR4 was not detected. In the vestibular ganglion, the neurons were densely stained with antibodies to GluR2/R3 and GluR4. The fibroblasts and the Schwann cells were also intensely stained with antibodies to GluR2/R3 and GluR4. In the sensory cells, the AMPA receptors, GluR2/R3, may function as (1) autoreceptors controlling afferent neurotransmitter release or (2) 'postsynaptic' receptors activated by the neurotransmitter release of the afferent calyx. The detection of GluR1 at postsynaptic sites in the afferent fibers provides anatomical evidence for the role of glutamate as a neurotransmitter of sensory cells. In the ganglion neurons, GluR2/R3 and GluR4 may represent reserve intracytoplasmic pools of receptor subunits in transit to the postsynaptic sites. In the Schwann cells, GluR2/R3 and GluR4 may be involved in neuronal-glial signalling at the nodes of Ranvier.

Nuclear and cytoplasmic triiodothyronine-binding sites in primary sensory neurons and Schwann cells: radioautographic study during development

The effects of the thyroid hormones on target cells are mediated through nuclear T3 receptors. In the peripheral nervous system, nuclear T3 receptors were previously detected with the monoclonal antibody 2B3 mAb in all the primary sensory neurons throughout neuronal life and in peripheral glia at the perinatal period only (Eur. J. Neurosci. 5, 319, 1993). To determine whether these nuclear T3 receptors correspond to functional ones able to bind T3, cryostat sections and in vitro cell cultures of dorsal root ganglion (DRG) or sciatic nerve were incubated with 0.1 nM [125I]-labeled T3, either alone to visualize the total T3-binding sites or added with a 10(3) fold excess of unlabeled T3 to estimate the part due to the non-specific T3-binding. After glutaraldehyde fixation, radioautography showed that the specific T3-binding sites were largely prevalent. The T3-binding capacity of peripheral glia in DRG and sciatic nerve was restricted to the perinatal period in vivo and to Schwann cells cultured in vitro. In all the primary sensory neurons, specific T3-binding sites were disclosed in foetal as well as adult rats. The detection of the T3-binding sites in the nucleus indicated that the nuclear T3 receptors are functional. Moreover the concomitant presence of both T3-binding sites and T3 receptors alpha isoforms in the perikaryon of DRG neurons infers that: 1) [125I]-labeled T3 can be retained on the T3-binding 'E' domain of nascent alpha 1 isoform molecules newly-synthesized on the perikaryal ribosomes; 2) the alpha isoforms translocated to the nucleus are modified by posttranslational changes and finally recognized by 2B3 mAb as nuclear T3 receptor. In conclusion, the radioautographic visualization of the T3-binding sites in peripheral neurons and glia confirms that the nuclear T3 receptors are functional and contributes to clarify the discordant intracellular localization provided by the immunocytochemical detection of nuclear T3 receptors and T3 receptor alpha isoforms.

Cryo-immunogold ultrastructural localization of laminin in adult rat peripheral nerve

Elucidation of the functional roles of the extracellular matrix component laminin in adult peripheral nerve has been hindered by differing accounts of its ultrastructural localization. This is the first report applying the advantages of the cryo-immunogold technique to laminin localization in peripheral nerve. Laminin labelling was found over the basal lamina and possibly over the immediately subjacent Schwann cell plasma membrane, but specific labelling appeared to be absent from other membranes (including those of non-myelinated axon/Schwann cell clusters) and from endoneurial collagen fibrils. It would appear that the functional roles played by laminin in normal adult peripheral nerve are likely to be mediated via its localization in the basal lamina, rather than through a more widespread distribution within the endoneurium.

N-acetylated alpha-linked acidic dipeptidase is expressed by non-myelinating Schwann cells in the peripheral nervous system

N-acetylated alpha-linked acidic dipeptidase is a membrane-bound brain peptidase which cleaves the neuropeptide N-acetyl-aspartyl-glutamate to N-acetyl-aspartate and glutamate. In the present study, we have determined the localization of N-acetylated alpha-linked acidic dipeptidase in the peripheral nervous system. Using enzyme assays and immunoblotting, we demonstrate that sciatic nerve, phrenic nerve, cervical dorsal root ganglion and superior cervical ganglion contain N-acetylated alpha-linked acidic dipeptidase activity as well as an N-acetylated alpha-linked acidic dipeptidase-like protein. Furthermore, we show that N-acetylated alpha-linked acidic dipeptidase-like immunoreactivity is extensively co-localized in peripheral nerves with immunoreactivity for glial fibrillary acidic protein, a known marker for non-myelinating Schwann cells. Using electron microscopy, we demonstrate N-acetylated alpha-linked acidic dipeptidase-like immunoreactivity in cell membranes of non-myelinating Schwann cells in the superior cervical ganglion. These results show that N-acetylated alpha-linked acidic dipeptidase is expressed in the peripheral nervous system by non-myelinating Schwann cells. This cellular localization suggests that N-acetylated alpha-linked acidic dipeptidase may be involved in the signalling between axons and Schwann cells, for example during development or regeneration.