Spatial mapping of juxtacrine axo-glial interactions identifies novel molecules in peripheral myelination

Cell–cell interactions promote juxtacrine signals in specific subcellular domains, which are difficult to capture in the complexity of the nervous system. For example, contact between axons and Schwann cells triggers signals required for radial sorting and myelination. Failure in this interaction causes dysmyelination and axonal degeneration. Despite its importance, few molecules at the axo-glial surface are known. To identify novel molecules in axo-glial interactions, we modified the ‘pseudopodia' sub-fractionation system and isolated the projections that glia extend when they receive juxtacrine signals from axons. By proteomics we identified the signalling networks present at the glial-leading edge, and novel proteins, including members of the Prohibitin family. Glial-specific deletion of Prohibitin-2 in mice impairs axo-glial interactions and myelination. We thus validate a novel method to model morphogenesis and juxtacrine signalling, provide insights into the molecular organization of the axo-glial contact, and identify a novel class of molecules in myelination.

Neuron–glia interactions are critical in the nervous system, where they result in the extension of glial pseudopodia. Poitelon et al. isolate these protrusions using an in vitro assay, and, by characterising their proteomes, identify Prohibitin-2 as a regulator of myelination.

Multipotential neural precursors transplanted into the metachromatic leukodystrophy brain fail to generate oligodendrocytes but contribute to limit brain dysfunction

Neural stem cells appear to be best suited for regenerative therapy in neurological diseases. However, the effects of high levels of potentially toxic substances such as sulfatides--which accumulate in metachromatic leukodystrophy (MLD)--on this regenerative ability are still largely unclear. To start addressing this question, in vitro and in vivo experiments were used to examine the behavior of multipotential neural precursors exposed to abnormally high levels of sulfatides. Following transplantation of dissociated neurospheres into the brain of presymptomatic MLD pups, the majority of donor-derived cells were distributed in a caudal to rostral direction, with higher numbers in the cortex. Most if not all of the donor cells acquired an astroglial phenotype. We found no evidence of oligodendrocyte or neuronal commitment of transplanted cells in long-term-treated MLD mice (e.g. up to 1.5 years of age). This was in line with our in vitro findings of sulfatides blocking oligodendrocyte formation after induction of differentiation in sulfatide-treated epidermal growth factor/fibroblast growth factor responsive neurospheres. Transplanted MLD mice showed an improved arylsulfatase A (ARSA) activity and a significant amelioration of sulfatide metabolism, neurodegeneration and motor-learning/memory deficits. Furthermore, transplanted cells were shown to act as a source of ARSA enzyme that accumulated in endogenous brain cells, indicating the occurrence of enzyme cross-correction between transplanted and host cells. These results provide a first insight into the effect of sulfatides on the stemness properties of neural stem cells and on the effects of the MLD environment on the in vivo expectations of using neural stem cells in cell therapy.

Schwann cells synthesize alpha7beta1 integrin which is dispensable for peripheral nerve development and myelination

Defects in laminins or laminin receptors are responsible for various neuromuscular disorders, including peripheral neuropathies. Interactions between Schwann cells and their basal lamina are fundamental to peripheral nerve development and successful myelination. Selected laminins are expressed in the endoneurium, and their receptors are developmentally regulated during peripheral nerve formation. Loss-of-function mutations have confirmed the importance and the role of some of these molecules. Here we show for the first time that another laminin receptor, alpha7beta1 integrin, previously described only in neurons, is also expressed in Schwann cells. The expression of alpha7 appears postnatally, such that alpha7beta1 is the last laminin receptor expressed by differentiating Schwann cells. Genetic inactivation of the alpha7 subunit in mice does not affect peripheral nerve formation or the expression of other laminin receptors. Of note, alpha7beta1 is not necessary for basal lamina formation and myelination. Nonetheless, these data taken together with the previous demonstration of impaired axonal regrowth in alpha7-null mice suggest a possible Schwann cell-autonomous role for alpha7 in nerve regeneration.

Do Schwann cells stop, DR(o)P2, and roll?

Dystrophin-dystroglycan complexes in Schwann cells may play a role in both signaling and structural interactions between the extracellular matrix and the cytoskeleton. Sherman et al. (2001) show that a new complex containing dystrophin-related protein 2 (DRP2) and periaxin plays a role in Schwann cell-basal lamina interactions and PNS myelination.

Role of integrins in the peripheral nervous system

Integrins, a subgroup of adhesion receptors, are transmembrane glycoproteins that mediate interactions between cytoplasm and the extracellular environment. These interactions influence, among others, events such as cell migration, proliferation, and differentiation. Differential expression of integrins is developmentally regulated in the peripheral nervous system (PNS) and is associated with crucial events in both physiological and pathological processes. Preliminary studies suggest that integrin expression influences neural crest cell migration, axonal outgrowth, and Schwann cell differentiation. Similarly, the abnormal expression of integrins or their ligands, is associated with degenerative, inflammatory, and malignant disorders of the PNS. Finally, integrins participate in the complex interactions that promote repair of the PNS. A better comprehension of the role of integrins in the PNS, their protein interactions and transducing signals is being achieved by selected biochemical and genetic experiments. Here we review a large bias of evidence suggesting the key functions for integrins in the PNS.

Human IgM anti-GM1 autoantibodies modulate intracellular calcium homeostasis in neuroblastoma cells

Increased titers of IgM anti-GM1 antibodies are present in some patients with Lower Motor Neuron Disease (LMND) or Motor Neuropathy (MN), but their pathogenic role and the mechanism of action are unclear. Previous studies have shown that the B subunit of Cholera Toxin (CT), which binds and crosslinks ganglioside GM1, modulate intracellular calcium in murine neuroblastoma cells via the activation of L-type voltage-dependent calcium channels (VGCC). Therefore, using a fluorimetric approach, we have examined the hypothesis that the pentameric IgM anti-GM1 antibodies, could similarly alter calcium concentration in N18 neuroblastoma cells. Sera with human IgM anti-GM1 antibodies were obtained from 5 patients with LMND and 2 patients with MN. Human IgG anti-GM1, IgM anti-Myelin Associated Glycoprotein (MAG), IgM anti-sulfatide antibodies and lectin peanut agglutinin (PNA), that recognizes specifically the Gal(betal-3)GalNAc epitope, were used as control sera. Direct application of either human IgM anti-GM1 antibodies or the B subunit of CT to N18 neuroblastoma cells induced a sustained influx of manganese ions, as indicated by a quench of the intracellular fura-2 fluorescence. Furthermore, the dihydropyridine L-type channel antagonists completely inhibited the manganese influx, suggesting that it is due to activation of an L-type VGCC. The magnitude of the influx was correlated with antibody titers. None of human IgG anti-GM1, IgM anti-MAG, IgM anti-sulfatide antibodies or PNA induce an ion influx, pointing to the selective participation of the pentameric IgM isotype of anti-GM1 in the modulation of L-type calcium channels opening. Given that L-type calcium channels are present on motor neurons, the modulation of L-type calcium channels by IgM GM1 antisera may have important implications in diseases such as LMND and MN.

Epitope-tagged P(0) glycoprotein causes Charcot-Marie-Tooth-like neuropathy in transgenic mice

In peripheral nerve myelin, the intraperiod line results from compaction of the extracellular space due to homophilic adhesion between extracellular domains (ECD) of the protein zero (P(0)) glycoprotein. Point mutations in this region of P(0) cause human hereditary demyelinating neuropathies such as Charcot-Marie-Tooth. We describe transgenic mice expressing a full-length P(0) modified in the ECD with a myc epitope tag. The presence of the myc sequence caused a dysmyelinating peripheral neuropathy similar to two distinct subtypes of Charcot-Marie-Tooth, with hypomyelination, altered intraperiod lines, and tomacula (thickened myelin). The tagged protein was incorporated into myelin and was associated with the morphological abnormalities. In vivo and in vitro experiments showed that P(0)myc retained partial adhesive function, and suggested that the transgene inhibits P(0)-mediated adhesion in a dominant-negative fashion. These mice suggest new mechanisms underlying both the pathogenesis of P(0) ECD mutants and the normal interactions of P(0) in the myelin sheath.

Schwann cell myelination requires timely and precise targeting of P(0) protein

This report investigated mechanisms responsible for failed Schwann cell myelination in mice that overexpress P(0) (P(0)(tg)), the major structural protein of PNS myelin. Quantitative ultrastructural immunocytochemistry established that P(0) protein was mistargeted to abaxonal, periaxonal, and mesaxon membranes in P(0)(tg) Schwann cells with arrested myelination. The extracellular leaflets of P(0)-containing mesaxon membranes were closely apposed with periodicities of compact myelin. The myelin-associated glycoprotein was appropriately sorted in the Golgi apparatus and targeted to periaxonal membranes. In adult mice, occasional Schwann cells myelinated axons possibly with the aid of endocytic removal of mistargeted P(0). These results indicate that P(0) gene multiplication causes P(0) mistargeting to mesaxon membranes, and through obligate P(0) homophilic adhesion, renders these dynamic membranes inert and halts myelination.

P(0) glycoprotein overexpression causes congenital hypomyelination of peripheral nerves

We show that normal peripheral nerve myelination depends on strict dosage of the most abundantly expressed myelin gene, myelin protein zero (Mpz). Transgenic mice containing extra copies of Mpz manifested a dose-dependent, dysmyelinating neuropathy, ranging from transient perinatal hypomyelination to arrested myelination and impaired sorting of axons by Schwann cells. Myelination was restored by breeding the transgene into the Mpz-null background, demonstrating that dysmyelination does not result from a structural alteration or Schwann cell-extrinsic effect of the transgenic P(0) glycoprotein. Mpz mRNA overexpression ranged from 30-700%, whereas an increased level of P(0) protein was detected only in nerves of low copy-number animals. Breeding experiments placed the threshold for dysmyelination between 30 and 80% Mpz overexpression. These data reveal new points in nerve development at which Schwann cells are susceptible to increased gene dosage, and suggest a novel basis for hereditary neuropathy.

P0-Cre transgenic mice for inactivation of adhesion molecules in Schwann cells

Normal peripheral nerve myelination depends on Schwann cell-basal lamina interactions. An important component of Schwann cell basal lamina is laminin--predominantly laminins 2 and 4. Mutations in the alpha 2 chain common to these two isoforms are associated with dysmyelination in mouse (dy) and man (congenital muscular dystrophy). Thus, laminin 2 and 4 receptors are also likely to be important for myelin formation. Several laminin 2/4 receptors are detected at the basal lamina surface of myelin-forming Schwann cells, namely, alpha 6 beta 4 and alpha 6 beta 1 integrins and dystroglycan. The evidence linking these receptors to myelination is suggestive, but not conclusive. Genetic studies have not yet confirmed a role for these molecules in myelin formation. Natural or targeted inactivation of alpha 6, beta 4, and beta 1 integrins and of dystroglycan have profound effects on other tissues causing embryonic or perinatal death before myelination. Therefore, to conditionally inactivate these receptors specifically in myelin-forming Schwann cells, we have constructed and initially characterized a P0-Cre transgene that activates Cre-mediated recombination of loxP-containing genes in peripheral nerve.

Peripheral nerve dysmyelination due to P0 glycoprotein overexpression is dose-dependent

We have previously shown that increased dosage of the mouse protein zero gene (Mpz) causes a dysmyelinating neuropathy in transgenic (Tg80) mice. To ask whether the dysmyelination is dose dependent, we inbred one of the Tg80 lines and compared the resulting phenotype in homozygous and heterozygous mice. Whereas heterozygous mice (30% overexpression) have only transient peripheral nerve hypomyelination at two weeks after birth and normal myelin at four weeks after birth, homozygous mice demonstrated more severely hypomyelinated nerves. In the latter, many Schwann cells had achieved a one-to-one relationship with large axons but formed no myelin at four weeks after birth. Expression analysis confirmed a doubling of Mpz overexpression in the sciatic nerves of the homozygous mice. Thus, a threshold exists for Mpz overexpression, above which dysmyelination results. These data have important implications for replacement therapy in Charcot-Marie-Tooth 1B neuropathies due to loss of P0 function.

A novel P0 glycoprotein transgene activates expression of lacZ in myelin-forming Schwann cells

P0 glycoprotein, the most abundant protein in peripheral nerve, is expressed specifically in the Schwann cell lineage. Upstream of the rat P0 gene 1.1 kb of DNA can activate expression of cDNAs specifically in Schwann cells in transgenic mice. However, the expression of P0 promoter-based transgenes has been inconsistent. As much as 9 kb of 5' flanking sequence fused to lacZ never yielded detectable levels of beta-galactosidase in multiple lines of mice. We describe transgenic mice that express lacZ in peripheral nerve, using the complete mouse P0 gene, including 6 kb of 5' flanking sequence, all exons and introns, and the natural polyadenylation signal. This vector activated lacZ expression specifically in cultured Schwann cells, and myelin-forming Schwann cells in four out of six transgenic lines. Transgene expression paralleled that of the endogenous P0 gene, both during development and after Wallerian degeneration. lacZ expression was lower than endogenous P0 expression, and was not detected in neural crest or Schwann cell precursors, where low levels of P0 mRNA are present. However, when the same vector contained a small myc tag instead of the 3.2-kb lacZ insert, the resulting transgenic mRNA was expressed at levels comparable to endogenous P0 mRNA. These data suggest that intragenic or 3' flanking sequences are necessary to generate the remarkable levels of endogenous P0 gene expression.

Laminin receptor alpha6beta4 integrin is highly expressed in ENU-induced glioma in rat

Laminins and their receptors influence neoplastic growth and invasiveness. We recently reported the abnormal expression of a laminin receptor, alpha6beta4 integrin, in human astrocytomas. To further investigate the role of alpha6beta4 in gliomas, we produced an experimental model of glioma in rat by transplacental ethylnitrosourea (ENU) administration. This animal model allowed us to study the timing of alpha6beta4 expression during tumor development and the topography of expression in the tumor and the surrounding tissue. Immunohistochemistry, in situ hybridization, and immunoprecipitation studies demonstrated that alpha6beta4 heterodimer forms in experimental gliomas, and confirmed that alpha6beta4 is expressed diffusely in neoplastic cells and reactive astrocytes, but not in normal glia surrounding the tumors. Interestingly, alpha6beta4 was expressed from the early phases of tumor development, and more highly expressed by cells in the proliferative centers of the tumors. Both neoplastic cells and reactive astrocytes also expressed the glial growth factor (neuregulin) receptors, Erb-B2 and Erb-B3. Finally, alpha6beta4 expression was reduced in a subset of tumor blood vessels. Thus, this study suggests a potential role for alpha6beta4 in the pathogenesis of gliomas. Furthermore, this is the first description of altered integrin expression in experimental gliomas; transplacental ENU-induced gliomas in rat will provide a useful model to study the role of altered adhesion in the pathogenesis of human gliomas.

MEBA derepresses the proximal myelin basic protein promoter in oligodendrocytes

The central nervous system expression of myelin basic protein (MBP) is restricted to oligodendrocytes and is developmentally regulated; these regulatory features are transcriptionally mediated. We have previously shown that the proximal 149 nucleotides of the MBP promoter were both necessary and sufficient to activate the transcription of MBP in cultured oligodendrocytes, but not in other cell types. Sequences within the distal portion of this promoter, which contains a nuclear factor 1 (NF1) binding site, repressed activation of the MBP promoter in Cos-7 cells, but not in oligodendrocytes. We now describe a sequence upstream of and partially overlapping the NF1 site that activates the MBP promoter in oligodendrocytes, but not in Cos-7 cells. A protein complex binds to this site, designated MEBA (myelinating glia-enriched DNA binding activity), and is enriched in nuclear extracts prepared from the brain, oligodendrocytes, and Schwann cells. The amount of MEBA parallels MBP expression and myelinogenesis in the developing brain and parallels new MBP expression as purified oligodendrocytes differentiate. Mutational analyses of binding and function distinguish MEBA, an activator, from NF1, a repressor of MBP transcription, and suggest that MEBA consists of at least two proteins. Because the binding sites of MEBA and NF1 overlap, we suggest that MEBA may either compete with or modify NF1 binding, thereby activating the MBP promoter in oligodendrocytes.

Transduced fibroblasts and metachromatic leukodystrophy lymphocytes transfer arylsulfatase A to myelinating glia and deficient cells in vitro

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease, caused by deficiency of arylsulfatase A (ASA), that manifests primarily in the white matter of the nervous system. Currently, no specific treatment exists that will reverse its fatal outcome. Replacement therapy has been hampered by the blood-brain barrier (BBB). To circumvent this problem we designed an ex vivo gene therapy strategy that includes the retrovirus-mediated ASA transduction of cells, such as activated lymphocytes, that are able to traverse the BBB or other membranes of the CNS. For this purpose, two recombinant retroviruses based on the pLXSN vector were produced, containing the wild-type ASA cDNA or a pseudodeficiency ASA cDNA, which encodes a smaller enzyme with normal activity. After transduction, ASA activity increased more than 100-fold in fibroblasts from an MLD patient. Furthermore, ASA-transduced MLD PBLs expressed 30 times higher ASA activity when compared with control PBLs. Moreover, cell culture experiments demonstrated that transduced fibroblasts could efficiently transfer ASA to deficient cells across a transwell barrier, whereas transduced MLD lymphocytes could transfer ASA to deficient fibroblasts only by direct cell-to-cell contact. Finally, ASA was taken up by normal oligodendrocytes and Schwann cells, the target myelinating glial cells for therapy in MLD. These data suggest possible short-term strategies for transfer of ASA into the CNS via transduced autologous cells while long-term strategies, related to autologous transduced bone marrow transplant, take effect in patients.

A minimal human MBP promoter-lacZ transgene is appropriately regulated in developing brain and after optic enucleation, but not in shiverer mutant mice

Previous studies, both in vitro and in vivo, suggest that small portions of the mouse myelin basic protein (MBP) promoter are sufficient to activate regulated expression of MBP. To confirm our previous in vitro studies, we prepared transgenic mice with short regions of the human MBP promoter fused to the lacZ reporter gene. We found that 750 nucleotides of the proximal human MBP promoter is sufficient to activate oligodendrocyte-specific, developmentally regulated expression of lacZ in three independent lines. This promoter, however, does not activate expression of lacZ in Schwann cells in peripheral nerve or in adult mouse brain. The relative levels of beta-galactosidase specific activity, mRNA, and transcription parallel those of MBP mRNA during myelinogenesis. Thus, we exploited this transgene as a quantitative tool to evaluate the response to stimuli known to affect myelination. Transgene expression is reduced 75 % after optic enucleation, as previously reported for levels of MBP mRNA, indicating that axons signal to this portion of the proximal MBP promoter to fully activate MBP expression during myelinogenesis. Instead, in adult shiverer mice, another setting in which MBP transcription is modulated, transgene expression is not increased, in contrast to the increased transcriptional activation of MBP previously reported in these mice. These data suggest that the regulatory region that mediates transcriptional activation of the MBP gene is modular, since discrete subregions are required for activation in Schwann cells, during myelinogenesis in oligodendrocytes, during maintenance myelination in adult brain, and in the dysmyelinating mutant shiverer mouse.

Axonal neuropathy associated with interferon-alpha treatment for hepatitis C: HLA-DR immunoreactivity in Schwann cells

A 44-year-old man developed a peripheral neuropathy during treatment with interferon-alpha for chronic hepatitis C. The onset was insidious, beginning symmetrically in the hands with paresthesia. Neurophysiological investigation revealed a predominantly sensory axonal neuropathy. A sural nerve biopsy confirmed primary axonal damage. Immunofluorescence studies showed increased expression of HLA-DR molecules prevalently on Schwann cells of non-myelin-forming type.

Atypical hereditary neuropathy with liability to pressure palsies (HNPP): the value of direct DNA diagnosis

We report two patients with suspected hereditary liability to pressure palsies. Neurophysiological studies showed a mixed axonal-demyelinating sensory-motor polyneuropathy with focal slowing of conduction velocities at the common sites of entrapment. Morphological studies on sural nerve biopsy from the proband showed active axonal regeneration without typical tomacula. Molecular analysis confirmed the presence of a deletion of chromosome 17p11.2 in both patients. Our observation confirms the heterogeneity of hereditary liability to pressure palsies and the relevance of DNA testing for the diagnosis of this hereditary neuropathy.

Toward a transgenic mouse model of remyelination

The molecular mechanisms necessary for remyelination by oligodendrocytes remain unexplored. We previously characterized a myelin basic protein promoter-lacZ (MBP-lacZ) transgene whose expression is regulated uniquely during development, and also in pathological situations, suggesting that it may be a useful reporter of molecular mechanisms during remyelination. As a first step toward creating a transgenic mouse model of remyelination, we cultured oligodendrocytes from these transgenic mice and showed that expression of MBP-lacZ appeared in parallel with a marker of oligodendrocyte maturation, galactocerebroside (GC). In addition, basic fibroblast growth factor blocked the expression of both MBP-lacZ and GC in these cells. Therefore, expression of MBP-lacZ reflects not only the developmental stage of oligodendrocytes, but also extrinsic influences on oligodendrocytes. These data suggest that MBP-lacZ may be a useful marker in transgenic mouse models of remyelination.

Cloning and sequence of the cDNA encoding the beta 4 integrin subunit in rat peripheral nerve

beta 4 and alpha 6 integrin subunits dimerize to form an adhesion receptor that is necessary to nucleate hemidesmosomes and to anchor epithelial cells to their basal laminae. beta 4 is also expressed in Schwann cell (which do not contain hemidesmosomes) in peripheral nerve, where it may function in the formation or maintenance of myelin. The cDNA for beta 4 integrin has been cloned from epithelia-derived human and mouse tissues. We cloned cDNAs encoding beta 4 integrin from libraries derived from rat peripheral nerve, and determined the complete nucleotide sequence encoding the signal peptide and mature protein. Comparison of the deduced amino acid (aa) sequence revealed 95.1% and 87.5% identity with the mouse and human epithelia-derived sequences, respectively. The amino acid sequence of postulated signal transduction domains in beta 4 was 100% identical among rat, mouse, and human. Our cDNA clones included two of the four postulated alternatively spliced variants previously described in epithelial clones. Despite the potentially diverse functions of beta 4 integrin in Schwann cells and keratinocytes, the cDNAs for nerve-derived beta 4 integrin are highly similar to those cloned from epithelia.

Beta 4 integrin and other Schwann cell markers in axonal neuropathy

Schwann cell gene expression is dynamically regulated after peripheral nerve injury and during regeneration. We hypothesized that the changes in protein expression described after rat peripheral nerve injury could be used to identify single Schwann cell-axon units in human axonal neuropathy. Therefore, we performed immuno-fluorescence staining on sections of injured rat sciatic nerves compared with sections of neuropathic human sural nerves. We chose the markers beta 4 integrin, P0 glycoprotein, and glial fibrillary acidic protein (GFAP) to characterize Schwann cells, and neurofilament-heavy (NF-H) to recognize axons. Normal rat or human myelin-forming units demonstrated a sharp ring of beta 4 staining at their outer surface, P0 staining in the myelin sheath, and NF-H staining in the axon. Acutely denervated rat units transited from broken rings of beta 4 and P0 staining, to diffuse beta 4 and absent P0 and NF-H staining. Chronically denervated rat Schwann cells re-expressed beta 4 more highly, but in a diffuse, non-polarized pattern. In contrast, regenerating units re-expressed beta 4, P0, and NF-H; beta 4 staining was polarized to the outer surface of Schwann cells. Finally, GFAP staining increased progressively after injury and decreased during regeneration in the distal nerve stump. In neuropathic human sural nerves, we identified units exhibiting each of these beta 4, P0, and NF-H staining patterns; the proportion of each pattern correlated best with the extent and chronicity of axonal injury. Thus, synchronous injury of rat sciatic nerve predicts patterns of Schwann cell marker expression in human axonal neuropathy. In addition, the unique changes in the polarity of beta 4 integrin expression, in combination with changes in P0 and NF-H expression, may distinguish normal from denervated or reinnervated myelin-forming Schwann cells in human sural nerve biopsies.

Alpha6 beta4 and alpha6 beta1 integrins in astrocytomas and other CNS tumors

Laminin may alter the biological behavior of gliomas. Therefore, we investigated the expression of two laminin receptors, alpha6 beta1 and alpha6 beta4 integrins in normal brain, astrogliotic brain, and astrocytomas as compared to other central nervous system (CNS) tumors. In most CNS tumors, the expression of these integrins was unchanged in neoplastic as compared to normal counterpart cells. In contrast, increased numbers of reactive and neoplastic astrocytes expressed beta4 integrin as compared to normal astrocytes, whereas alpha6 and beta1 integrin expression did not change. Conversely, lower numbers of astrocytoma blood vessels expressed beta4, whereas all blood vessels in normal brain expressed beta4. These data suggest that the profile of laminin receptors changes in neoplastic astrocytes and in astrocytoma blood vessels; this change may play an important role in astrocytoma pathogenesis.

Axon-Schwann cell interactions regulate the expression of c-jun in Schwann cells

The transcription factor c-jun is selectively expressed by non-myelinating Schwann cells in normal peripheral nerve, and be "denervated," previously myelinatng Schwann cells, after axotomy. When axons regenerate into the distal nerve-stump, the expression of c-jun declines as Schwann cells remyelinate axons. Treating cultured Schwann cells with forskolin, a drug that mimics many of the effects of axon-Schwann cell interactions, decreases the expression of myelin-specific genes. Overexpressing c-jun in cultured Schwann cells, however, does not decrease the expression of a myelin basic protein promoter-reporter construct, indicating that c-jun expression may not directly regulate myelin-specific gene expression. These data suggest that c-jun is invloved in regulating the phenotype of non-myelinating and denervated Schwann cells.

Regulation of neurofibromin expression in rat sciatic nerve and cultured Schwann cells

Loss of function mutations at the NF1 locus may act intrinsically in Schwann cells to cause the formation of benign Schwann cell tumors (neurofibromas) in patients with type 1 neurofibromatosis. To identify contexts in Schwann cells in which such mutations may play an important role, we measured the levels of NF1 mRNA and neurofibromin in rat sciatic nerve during development, after axotomy, and in cultured rat Schwann cells. NF1 mRNA was present in developing sciatic nerve throughout the period of active Schwann cell proliferation and myelination. After nerve transection, no alteration in NF1 message level was detected, but neurofibromin levels increased, as assessed by immunohistochemistry and Western blotting, suggesting that, in vivo, neurofibromin expression in Schwann cells is post-transcriptionally induced during Wallerian degeneration. Cultured rat Schwann cells constitutively expressed NF1 mRNA and neurofibromin. Schwann cell proliferation induced by exposure to serum and forskolin was not associated with changes in NF1 mRNA or neurofibromin expression, whereas Schwann cell proliferation induced by extracts of embryonic brain membranes was associated with increased NF1 message and neurofibromin expression. Thus, Schwann cells, both in vivo and in vitro, express NF1 mRNA constitutively; the expression of NF1 mRNA and neurofibromin is modulated by only some mitogenic stimuli in Schwann cells.

Expression of growth-associated protein-43 kD in Schwann cells is regulated by axon-Schwann cell interactions and cAMP

We have examined the regulation of growth-associated protein 43 kD (GAP-43) in rat Schwann cells. In unlesioned adult nerves, GAP-43-immunoreactivity was restricted to non-myelinating Schwann cells and unmyelinated axons. When adult nerves were transected to cause permanent axotomy, previously myelinating Schwann cells expressed progressively more GAP-43-immunoreactivity over 3 weeks, and GAP-43 mRNA levels increased over a similar time course. The peak level of GAP-43 mRNA occurred at least 2 weeks later than that of nerve growth factor receptor, another marker of denervated Schwann cells. In contrast, after nerve-crush, which allows axonal regeneration, many fewer Schwann cells had GAP-43-immunoreactivity, and the amount of GAP-43 mRNA was markedly lower than in transected nerves. Forskolin, a drug that activates adenylate cyclase and mimics many effects of axon-Schwann cell interactions, markedly reduced GAP-43-immunoreactivity and mRNA expression in cultured Schwann cells, whereas interleukin-1 had no effect. These data demonstrate that axon-Schwann cell interactions inhibit the expression of GAP-43 in Schwann cells and that this effect is mimicked by forskolin.

A novel cyclic AMP response element, CACTTGATC, mediates forskolin induction of the myelin basic protein promoter in the rat Schwannoma line, D6P2T

The rat Schwannoma cell line D6P2T constitutively expresses the mRNA encoding the major myelin protein, P0, but only expresses the mRNA encoding myelin basic protein (MBP) after exposure to forskolin or other substances that raise the levels of intracellular cyclic AMP. In this study we have investigated the molecular basis for forskolin induction of MBP transcription in D6P2T cells. We have found that a 9-bp sequence element, CACTTGATC, located between nucleotides -85 and -77 in the MBP promoter, is necessary for forskolin induction of chloramphenicol acetyltransferase (CAT) expression after transient transfection of MBP promoter-CAT fusion constructs into D6P2T cells. Although similar DNase I footprints, one of which is located within the above 9-bp sequence element, are produced by nuclear extracts prepared from both forskolin-treated and untreated cells, this same sequence can be shown to interact with a forskolin-inducible protein complex using an electrophoretic mobility shift assay. In addition, mutation of this 9-bp sequence abolishes both formation of this new protein--DNA complex and forskolin-inducible CAT expression from the heterologous SV40 promoter. Finally, we have shown that the appearance of this forskolin-inducible protein--DNA complex precedes that of MBP mRNA. Taken together, these data strongly support the notion that the induction of MBP transcription by forskolin in D6P2T cells is mediated by the binding of a forskolin-inducible protein complex to the MBP promoter sequence CACTTGATC.

Hypomyelinating peripheral neuropathies and schwannomas in transgenic mice expressing SV40 T-antigen

We have prepared transgenic mice carrying a temperature-sensitive mutant of the SV40 oncogene (tsA-1609) under the control of 5' flanking sequences from the Schwann cell-specific P0 gene. Four of six founder mice showed moderate to severe hypomyelination in peripheral nerves of tail biopsies, with only rare myelinated fibers. Offspring were obtained from three of these founders. Northern blot and immunohistochemical analyses showed that expression of T-antigen was restricted to the PNS. Mice expressing the highest levels of T-antigen exhibited the most severe hypomyelination. Mice expressing lower levels developed transient mild hypomyelination, but after long latencies developed sporadic schwannomas. An immortalized cell line exhibiting properties of Schwann cells at an arrested stage of differentiation, termed "SCT-1," was derived from one of these tumors.

Beta 4 integrin expression in myelinating Schwann cells is polarized, developmentally regulated and axonally dependent

In developing and regenerating peripheral nerve, Schwann cells interact with axons and extracellular matrix in order to ensheath and myelinate axons. Both of these interactions are likely to be mediated by adhesion molecules, including integrins, which mediate cell-cell and cell-extracellular matrix interactions. Recently, the beta 4 integrin subunit was reported to be expressed by Schwann cells in peripheral nerve. We have examined the expression of beta 4, beta 1 and their common heterodimeric partner, the alpha 6 integrin subunit, in developing and regenerating rat peripheral nerve. beta 4 and alpha 6 are enriched in peripheral nerve and they co-localize at the abaxonal surface of myelinating Schwann cells, opposite the Schwann cell basal lamina, which contains possible ligands of alpha 6 beta 4. In contrast, beta 4 and alpha 6 are expressed in a different pattern in non-myelinating Schwann cells. The level of beta 4, but not alpha 6 or beta 1 mRNAs, increases progressively in developing nerves, reaching a peak in adult nerves well after the peak of the myelin-specific mRNAs. After axotomy, the expression of beta 4 mRNA and protein, but not alpha 6 or beta 1 mRNAs, fall rapidly but subsequently are reinduced by regenerating axons. Similarly, in cultured Schwann cells, the expression of beta 4 mRNA, but not alpha 6 mRNA, is significantly modulated by forskolin, a drug that elevates cAMP and mimics some of the effects of axonal contact. beta 4 integrin expression in Schwann cells, therefore, is regulated by Schwann cell-axon interactions, which are known to be critical in determining the Schwann cell phenotype. Furthermore, the polarized expression of alpha 6 beta 4 to the abaxonal surface of myelinating Schwann cells suggests that alpha 6 beta 4 may mediate in part the morphological changes required of Schwann cells in the process of myelination in the peripheral nervous system.

Axons regulate Schwann cell expression of the POU transcription factor SCIP

SCIP (suppressed cAMP-inducible POU) is a POU domain transcription factor expressed by Schwann cells. Drugs that elevate intracellular cAMP, such as forskolin, increase the expression of SCIP and partially mimic the inductive effects of axons on Schwann cell gene expression. Thus, SCIP may be involved in a differentiation pathway in Schwann cells that is activated by axons. We have examined this issue by studying SCIP expression in developing, degenerating, and regenerating rat peripheral nerves, and in Schwann cell-neuron cocultures. High levels of SCIP mRNA were detected in developing and regenerating nerves, and axotomy at these times caused the level of SCIP mRNA to plummet. Similarly, there were many SCIP-immunoreactive Schwann cell nuclei in developing and regenerating nerves, and their number fell sharply after axotomy. SCIP-immunoreactive Schwann cells were associated with axons in developing and regenerating nerves, and in Schwann cell-neuron cocultures. These data demonstrate that axons upregulate the expression of SCIP in Schwann cells, and that SCIP is expressed in Schwann cells that ensheathe axons. Thus, SCIP may mediate some of the changes in Schwann cell gene expression that accompany axonal ensheathment.

Cerebellar ataxia, dystonia, and tremor within a family: variable phenotypes of a single genetic disorder?

We report a non-Jewish, Anglo-Saxon, American family, in which one sibling has dystonia, a second has cerebellar ataxia, and a third has a combination of dystonia and ataxia. All three siblings have pyramidal signs. Their mother and maternal uncle have tremor, and their maternal grandmother may have had a neurodegenerative disorder. Although the inheritance pattern is uncertain, this may represent phenotypic variability resulting from a single gene mutation. The multiple phenotypes within this family do not fit any known inherited neurodegenerative or metabolic disorder.

Analysis of the human MBP promoter in primary cultures of oligodendrocytes: positive and negative cis-acting elements in the proximal MBP promoter mediate oligodendrocyte-specific expression of MBP

Since the regulation of myelin basic protein expression depends primarily on the initiation of transcription, we analyzed the 5' flanking region of the human myelin basic protein gene in transient transfection studies in primary cultures of developing oligodendrocytes. We demonstrated that 149 base pairs 5' of the initiation of transcription was sufficient to direct oligodendrocyte-specific expression of myelin basic protein. The capsite of the fusion transcript was identical with that of the endogenous myelin basic protein transcript, and chloramphenicol acetyl transferase reporter gene expression was restricted to oligodendrocytes in these cultures. Within this 149 base pair region, one distal, negative cis-acting segment, containing a consensus nuclear factor I site, and one proximal, positive cis-acting segment were identified. The distal segment behaved more negatively in Cos-7 cells than in oligodendrocytes, reducing expression to background levels. Furthermore, these functionally important cis-acting segments bound oligodendrocyte nuclear proteins in a pattern differing from other cells, including Cos-7 cells. Interestingly, the distal segment increased heterologous SV40 promoter activity in oligodendrocytes but had no effect on the SV40 promoter in Cos-7 cells. We conclude that the functionally negative distal segment may mediate oligodendrocyte-specific expression of MBP by restricting its expression in other cells. These experiments strongly support using primary cultures of oligodendrocytes for analyzing the myelin-specific promoters.

Fluorescent approaches to diagnosis of Lesch-Nyhan syndrome and quantitative analysis of carrier status

Lesch-Nyhan syndrome is an X-linked recessive disorder caused by molecular defects within the HPRT gene. Deletional forms of this syndrome, most of which are inherited, account for 15% of the cases. In addition, a large percentage of cases are due to de novo point mutations. We have used complementary fluorescence-based PCR assays to analyse disease-causing mutations in three unrelated families: (1) inheritance of dye-labelled PCR products of linked polymorphic loci mapping within and flanking the HPRT gene; (2) dye-labelled exon dosage analysis and (3) automated fluorescence-based DNA sequence analysis. Our results using fluorescent, dye-tagged PCR products show that inheritance of two polymorphic small tandem repeats, HPRTB [AGAT]n, mapping within intron 3 of the HPRT gene, and the CA-repeat at DXS294 can be used to establish linkage to the disease. In addition, we modified a previously described PCR protocol to use fluorescent dye-labelled oligoprimers and an ABI Gene Scanner in order to rapidly quantitate deletional forms of Lesch-Nyhan syndrome. Quantitative PCR analysis of individual exons followed by dosage analysis confirmed a deletion encompassing exon 9. A similar approach was used to confirm a previously described HPRT gene duplication involving exons 2 and 3. In this analysis, we co-amplified the HPRTB [AGAT]n and HUMARA [AGC]n repeats and confirmed increased exon dosage in carriers for the duplication. DNA sequence analysis remains the method of choice for delineating new disease-causing mutations, most of which are non-deletional forms of Lesch-Nyhan syndrome. We have also used a cycle-sequencing strategy employing dye-labelled dideoxy terminators and a laser-activated, fluorescence-emission DNA sequencer in order to define carrier status in 10 family members at risk for Lesch-Nyhan syndrome due to a splice donor mutation in intron 7. Our DNA sequence analyses corroborate small tandem repeat (STR) inheritance patterns in this family. Multiple fluorescence-based strategies should facilitate rapid diagnosis of the various Lesch-Nyhan disease-causing mutations.

Oligodendrocyte maturation and myelin gene expression in PDGF-treated cultures from rat cerebral white matter

Myelination in the CNS is accompanied by the differentiation of oligodendrocytes as well as the coordinate expression of a group of myelin-specific genes, including those encoding proteolipid protein and myelin basic protein. In order to compare the timing of the onset of myelin gene expression with the known sequence of oligodendrocyte maturation, we analyzed cerebral white matter cultures grown in the presence of platelet-derived growth factor for expression of the mRNAs encoding these myelin proteins, as well as for the numbers of oligodendrocytes and their precursors. Platelet-derived growth factor treatment increased the rate of oligodendrocyte precursor cell proliferation and the number of mature oligodendrocytes. Platelet-derived growth factor also produced a significant increase in oligodendrocyte precursors prior to an increase in their proliferation rate, suggesting that platelet-derived growth factor may also have an effect on oligodendrocyte precursor survival. Furthermore, steady-state levels of proteolipid protein and myelin basic protein mRNAs increased within 24 of the addition of platelet-derived growth factor, before any significant change in the numbers of oligodendrocytes or their precursors, demonstrating that platelet-derived growth factor also regulates myelin gene expression. At later times after platelet-derived growth factor addition, however, when the number of oligodendrocytes and their precursors was rapidly increasing, the increase in proteolipid protein and myelin basic protein mRNA levels was proportionally much greater than the increase in oligodendroglial lineage cells, suggesting that platelet-derived growth factor also increased the number of proteolipid protein and myelin basic protein transcripts per cell; this interpretation was confirmed by in situ hybridization analysis. Finally, by examining the co-expression of galactocerebroside using the epitopes recognized by the Ranscht monoclonal antibody and proteolipid protein mRNA in individual cells by a combination of in situ hybridization and immunohistochemistry, we demonstrated that oligodendrocytes express proteolipid protein and myelin basic protein mRNA. Oligodendrocyte maturation, as measured by surface galactocerebroside expression, is thus contemporaneous with the activation of myelin-specific gene expression.