axon-Schwann Cell Research Articles

Disruption of Laminin-γ1 Expression Induces Rapid Decrease In Spleen Size.

AbstractAbstract 2596Laminins are heterotrimeric proteins composed of an alpha, beta, and gamma subunit, and are expressed in a majority of organ systems, including hematopoietic and lymphoid tissues. Specific laminin subunits have been identified in prognostic stromal gene expression profiles from diffuse large B-cell lymphomas (Lenz et al. 2008). Laminin g1 is the gamma-subunit present in a majority of physiologically expressed laminin heterotrimers. Global laminin-g1 deficiency in mice is embryologically lethal at E5.5 and results in part from failure of the primordial basal lamina to form (Smyth et al. 1998). Conditional knockout mouse lines have therefore been developed to examine the role of laminin-g1 in specific tissues. Disruption of Schwann cell-specific expression of laminin-g1 prevents basal lamina formation within peripheral nerves, decreases glial cell proliferation, and increases glial cell apoptosis leading to a severe dysmyelinated phenotype (Chen and Strickland 2003; Yu et al. 2005).Here we present data that global disruption of laminin- g1 expression in adult animals induces involution of the spleen and suggests a fundamental role for laminins in maintenance of spleen integrity. Mice homozygous for the LoxP flanked laminin-g1 allele (floxed allele) were crossed with mice expressing a global Cre-ERT2 transgene. Mice homozygous or heterozygous for the floxed laminin-g1 allele that carry the inducible Cre transgene are hereafter referred to as homozygous and heterozygous mutant mice, respectively. Adult heterozygous and homozygous mutant animals were treated with either tamoxifen (TM) or corn oil (control), and three weeks afterwards, tissues were harvested for analysis. Treatment of homozygous mutant mice with TM, but not corn oil, induced laminin-g1 gene recombination. There was a significant decrease in spleen size in TM-treated but not control-treated homozygous mutant animals. Neither TM nor control treatment induced changes in the spleen size of heterozygous mutant animals. Similarly, TM treatment did not alter spleen size of animals that expressed Cre-transgene but were wild type with respect to the floxed laminin-g1 gene, or which were homozygous for the floxed laminin-g1 gene but lacked the Cre-transgene (Figure 1). [Display omitted] This is the first animal model to show that continued expression of laminins are critical for maintenance of the normal spleen and suggests a potential new target for disorders of lymphoid or splenic hyperproliferation.Chen ZL, Strickland S. 2003. Laminin gamma1 is critical for Schwann cell differentiation, axon myelination, and regeneration in the peripheral nerve. J Cell Biol 163(4):889-99.Lenz G, Wright G, Dave SS, Xiao W, Powell J, Zhao H, Xu W, Tan B, Goldschmidt N, Iqbal J and others. 2008. Stromal gene signatures in large-B-cell lymphomas. N Engl J Med 359(22):2313–23.Smyth N, Vatansever HS, Meyer M, Frie C, Paulsson M, Edgar D. 1998. The targeted deletion of the LAMC1 gene. Ann N Y Acad Sci 857:283–6.Yu WM, Feltri ML, Wrabetz L, Strickland S, Chen ZL. 2005. Schwann cell-specific ablation of laminin gamma1 causes apoptosis and prevents proliferation. J Neurosci 25(18):4463–72. Disclosures:No relevant conflicts of interest to declare

Schwann cells roll the dice

Dicer plays a key part in Schwann cell maturation and axon myelination.

CNS-Resident Glial Progenitor/Stem Cells Produce Schwann Cells as well as Oligodendrocytes during Repair of CNS Demyelination

After central nervous system (CNS) demyelination-such as occurs during multiple sclerosis-there is often spontaneous regeneration of myelin sheaths, mainly by oligodendrocytes but also by Schwann cells. The origins of the remyelinating cells have not previously been established. We have used Cre-lox fate mapping in transgenic mice to show that PDGFRA/NG2-expressing glia, a distributed population of stem/progenitor cells in the adult CNS, produce the remyelinating oligodendrocytes and almost all of the Schwann cells in chemically induced demyelinated lesions. In contrast, the great majority of reactive astrocytes in the vicinity of the lesions are derived from preexisting FGFR3-expressing cells, likely to be astrocytes. These data resolve a long-running debate about the origins of the main players in CNS remyelination and reveal a surprising capacity of CNS precursors to generate Schwann cells, which normally develop from the embryonic neural crest and are restricted to the peripheral nervous system.

Alcoholic neuropathy and hepatopathy in mice. An experimental study.

Seventeen young adult C57BL male mice were given 15% (w/v) solution of ethanol as their sole drinking fluid for seven to nine months. C57BL male mice given regular drinking water were used as controls. After decapitation samples from the livers and the sciatic nerves of the mice were processed for electron microscopy and morphometry. The ultrastructural analysis revealed slight alterations in the nerves of the ethanol-exposed mice. The changes were mostly in the Schwann cells. Pathological Schwann cell-axon relationships were also more abundant in the ethanol-exposed mice. The thickness distribution of the myelinated nerve fibers was similar in the exposed and the control mice. The transverse-sectional area of hepatocytes was greater in the mice on ethanol than in the controls. The volume density of lipid vacuoles of the liver cells was significantly increased in the mice treated with ethanol (p less than 0.001). The hepatic changes indicate a significant effect of ethanol on the liver. The findings suggest that, in mice, chronic peroral treatment with ethanol can produce slight changes suggesting peripheral neuropathy together with fatty metamorphosis of the liver.

Are BMPs Involved in Normal Nerve and Following Transection?: A Pilot Study

Bone morphogenic proteins (BMPs) may have neurotrophic functions but there is limited evidence of these functions in the peripheral nervous system. We therefore investigated the expression of BMPs and BMP receptors (BMPRs) in normal and injured peripheral nerves. In 10 of 15 Sprague-Dawley rats, a 3-mm segment of sciatic nerve was resected at the trifurcation in the thigh. One day (n = 5) and 7 days (n = 5) after transection, proximal and distal stumps were removed and immunohistochemically analyzed for BMP-2, -7, BMPR-1A, -1B, and -2. The other five animals served as normal controls. In normal nerves, BMP-2 expression was localized at Ranvier's node, and BMP-7 and BMPR-1B were expressed in several axon-Schwann cell units, whereas other receptors were not expressed. After nerve transection, BMP-7 expression was upregulated at both proximal and distal stumps along with Schwann cell columns during Wallerian degeneration. BMPRs were also upregulated compared with the normal nerve. The upregulation in BMP expression after nerve transection suggests that BMPs may play a role in the healing response of the peripheral nerve.

Maximum-entropy network analysis reveals a role for tumor necrosis factor in peripheral nerve development and function

Gene regulatory interactions that shape developmental processes can often can be inferred from microarray analysis of gene expression, but most computational methods used require extensive datasets that can be difficult to generate. Here, we show that maximum-entropy network analysis allows extraction of genetic interactions from limited microarray datasets. Maximum-entropy networks indicated that the inflammatory cytokine TNF-alpha plays a pivotal role in Schwann cell-axon interactions, and these data suggested that TNF mediates its effects by orchestrating cytoplasmic movement and axon guidance. In vivo and in vitro experiments confirmed these predictions, showing that Schwann cells in TNF(-/-) peripheral sensory bundles fail to envelop axons efficiently, and that recombinant TNF can partially correct these defects. These data demonstrate the power of maximum-entropy network-based methods for analysis of microarray data, and they indicate that TNF-alpha plays a direct role in Schwann cell-axon communication.

Integrative genomic analyses of neurofibromatosis tumours identify SOX9 as a biomarker and survival gene

Understanding the biological pathways critical for common neurofibromatosis type 1 (NF1) peripheral nerve tumours is essential, as there is a lack of tumour biomarkers, prognostic factors and therapeutics. We used gene expression profiling to define transcriptional changes between primary normal Schwann cells (n = 10), NF1-derived primary benign neurofibroma Schwann cells (NFSCs) (n = 22), malignant peripheral nerve sheath tumour (MPNST) cell lines (n = 13), benign neurofibromas (NF) (n = 26) and MPNST (n = 6). Dermal and plexiform NFs were indistinguishable. A prominent theme in the analysis was aberrant differentiation. NFs repressed gene programs normally active in Schwann cell precursors and immature Schwann cells. MPNST signatures strongly differed; genes up-regulated in sarcomas were significantly enriched for genes activated in neural crest cells. We validated the differential expression of 82 genes including the neural crest transcription factor SOX9 and SOX9 predicted targets. SOX9 immunoreactivity was robust in NF and MPSNT tissue sections and targeting SOX9 – strongly expressed in NF1-related tumours – caused MPNST cell death. SOX9 is a biomarker of NF and MPNST, and possibly a therapeutic target in NF1.

Disruption of Fibroblast Growth Factor Receptor Signaling in Nonmyelinating Schwann Cells Causes Sensory Axonal Neuropathy and Impairment of Thermal Pain Sensitivity

Axon-glial interactions are critical for normal functioning of peripheral nerves, and their disruption leads to peripheral neuropathies. Fibroblast growth factors (FGFs) are key players in peripheral nerve regeneration after injury. We investigated the role of FGF receptor (Fgfr) signaling in Schwann cells and the consequent regulation of normal Schwann cell-axon interactions. Fgfr1 and Fgfr2 were conditionally inactivated, either singly or in combination, in myelinating and nonmyelinating Schwann cells (NMSCs) of transgenic mice. The double mutant mice displayed significant loss of thermal sensitivity accompanied by marked neuropathy of unmyelinated nociceptive sensory axons terminating in the dorsal horn of spinal cords, the primary site for integrating pain and temperature inputs. Neuropathy, although to a lesser extent, was also observed in the nociceptive C-fibers in the Remak bundles of sciatic nerves; however, there was no loss of NMSCs that ensheathe these axons. Furthermore, axons wrapped by myelinating Schwann cells and associated myelin sheaths appeared to be unaffected. Relative to the double mutants, axonal neuropathy developed much later in the Fgfr1 but not Fgfr2 single mutant, indicating a difference in signaling potential of the two receptors, with Fgfr1 being more robust than Fgfr2. These findings emphasize the importance of Fgfr1 and Fgfr2 signaling as potential mediators of axon-glial interaction in the peripheral sensory pain pathway primarily via influencing NMSC function, which in turn modulates the structure and function of unmyelinated sensory axons. This study provides a novel molecular mechanism for nociception with possible implications for pain sensitivity in peripheral sensory neuropathies.

Differential cell‐specific location of Cav‐1 and Ca2+‐ATPase in terminal Schwann cells and mechanoreceptive Ruffini endings in the periodontal ligament of the rat incisor

Caveolae are involved in clathrin-independent endocytosis, transcytosis, signal transduction, and tumor suppression - all of which depend on their main constituent protein caveolin families. The periodontal Ruffini ending has been reported to develop a caveola-like structure on the cell membrane of both the axon terminals and Schwann sheaths, suggesting the existence of an axon-Schwann cell interaction in the periodontal Ruffini endings. However, little information is available concerning the functional significance of these caveolae. The present study was undertaken to examine the immunolocalization of caveolin-1, -3 (Cav-1, Cav-3) and Ca(2+)-ATPase in the periodontal Ruffini endings of the rat incisor. Decalcified sections of the upper jaws were processed for immunocytochemistry at the levels of light and electron microscopy. Some immunostained sections were treated with histochemistry for nonspecific cholinesterase (nChE) activity. Observations showed the periodontal Ruffini endings were immunopositive for Cav-1, but not Cav-3. Immunoreactive products for Cav-1 were confined to caveola-like structures in the cell membranes of the cytoplasmic extensions and cell bodies of the terminal Schwann cells associated with the periodontal Ruffini endings. However, the axonal membranes of the terminals did not express any Cav-1 immunoreaction. Double staining with Ca(2+)-ATPase and either protein gene product 9.5 (PGP 9.5) or S-100 protein disclosed the co-localization of immunoreactions in the axonal branches of the periodontal Ruffini endings, but not in the terminal Schwann cells. As Ca(2+) plays an important role in mechanotransduction, these characteristic immunolocalizations show Cav-1/Ca(2+)-ATPase might be involved in the quick elimination of intracellular Ca(2+) in mechanotransduction.

A novel method for isolating Schwann cells using the extracellular domain of Necl1

Myelinating cocultures of Schwann cells and dorsal root ganglion neurons are a powerful experimental system for probing the molecular mechanisms of axon-Schwann cell interaction. The isolation of a pure population of myelination-competent Schwann cells is a prerequisite for this experimental system. We describe here a protocol for a FACS-based isolation of Schwann cells utilizing a specific affinity reagent (Necl1-Fc) and the use of these isolated cells in myelinating cocultures. An advantage of the myelinating coculture system is that Schwann cells and the neurons can be genetically manipulated before they are cocultured. We further show that our method allows the isolation of virally transduced Schwann cells in a single purification step. This protocol for the FACS-based isolation of myelination-competent Schwann cells by Necl1-Fc and the use of these cells in myelinating cocultures should significantly facilitate future studies aimed at delineation of the molecular mechanisms of axon-Schwann cell interactions and myelination.

Protein kinase A-induced phosphorylation of the p65 subunit of nuclear factor-kappaB promotes Schwann cell differentiation into a myelinating phenotype.

Axon-Schwann cell interactions are critical for myelin formation during peripheral nerve development and regeneration. Axonal contact promotes Schwann cell precursors to differentiate into a myelinating phenotype, and cAMP-elevating agents can mimic this; however, the mechanisms underlying this differentiation are poorly understood. We demonstrated previously that the transcription factor nuclear factor-kappaB (NF-kappaB) is required for myelin formation by Schwann cells (Nickols et al., 2003), although how it is activated during this process remained to be determined. Here, we report that culturing Schwann cells with sensory neurons results in the activation of cAMP-dependent protein kinase (PKA), and this kinase phosphorylates the p65 subunit of NF-kappaB at S276. The phosphorylation was also induced in cultured Schwann cells by treatment with forskolin, dibutyryl-cAMP, or by overexpression of a catalytic subunit of PKA, and this increased the transcriptional activity of NF-kappaB. In developing perinatal rat sciatic nerve, the kinetics of p65 phosphorylation at S276 paralleled that of PKA and NF-kappaB activation. To elucidate the role of p65 phosphorylation in myelin formation, we overexpressed an S276A mutant of p65 in cultured Schwann cells, which blocked PKA-mediated transcriptional activation of NF-kappaB. When the Schwann cells expressing the mutant were cocultured with sensory neurons, there was a 45% reduction in the number of myelinated fibers relative to controls, demonstrating a requirement for p65 phosphorylation by PKA during myelin formation.

Nonmyelinating Schwann Cell Involvement With Well-Preserved Unmyelinated Axons in Charcot-Marie-Tooth Disease Type 1A

Electron microscopic examination was performed to compare morphologic changes of nonmyelinating Schwann cells and unmyelinated axons in patients with Charcot-Marie-Tooth disease type 1A (CMT1A) with peripheral myelin protein 22 duplication (n = 27) and normal control individuals (n = 14). Complete transverse sural nerve cross-sections were obtained in 16 patients and the total number of axons and Schwann cells in each cross-section was estimated. In patients with CMT1A, the number of myelinated axons was significantly decreased, whereas unmyelinated axons were well-preserved and did not show any marked changes. The numbers of nuclei, subunits, and profiles of nonmyelinating Schwann cells were all increased significantly in patients with CMT1A, whereas the numbers of axons per unmyelinated axon-containing subunit were significantly decreased. Schwann cell subunits consisted of layers of flattened cytoplasmic profiles wrapped around unmyelinated axons in the patient with CMT1A. The numbers of nonmyelinating Schwann cell profiles were increased and the numbers of axons per unmyelinated axon-containing subunit were reduced even in young patients with well-preserved myelinated fibers. In conclusion, there is marked alteration of the population and morphology of nonmyelinating Schwann cells, and axon-Schwann cell interactions seem to be regulated differently between myelinated and unmyelinated fibers in CMT1A.

Nectin-like proteins mediate axon–Schwann cell interactions along the internode and are essential for myelination

Axon–glial interactions are critical for the induction of myelination and the domain organization of myelinated fibers. Although molecular complexes that mediate these interactions in the nodal region are known, their counterparts along the internode are poorly defined. We report that neurons and Schwann cells express distinct sets of nectin-like (Necl) proteins: axons highly express Necl-1 and -2, whereas Schwann cells express Necl-4 and lower amounts of Necl-2. These proteins are strikingly localized to the internode, where Necl-1 and -2 on the axon are directly apposed by Necl-4 on the Schwann cell; all three proteins are also enriched at Schmidt-Lanterman incisures. Binding experiments demonstrate that the Necl proteins preferentially mediate heterophilic rather than homophilic interactions. In particular, Necl-1 on axons binds specifically to Necl-4 on Schwann cells. Knockdown of Necl-4 by short hairpin RNA inhibits Schwann cell differentiation and subsequent myelination in cocultures. These results demonstrate a key role for Necl-4 in initiating peripheral nervous system myelination and implicate the Necl proteins as mediators of axo–glial interactions along the internode.

Mast cells can contribute to axon–glial dissociation and fibrosis in peripheral nerve

Expression of the human epidermal growth factor receptor (EGFR) in murine Schwann cells results in loss of axon-Schwann cell interactions and collagen deposition, modeling peripheral nerve response to injury and tumorigenesis. Mast cells infiltrate nerves in all three situations. We show that mast cells are present in normal mouse peripheral nerve beginning at 4 weeks of age, and that the number of mast-cells in EGFR(+) nerves increases abruptly at 5-6 weeks of age as axons and Schwann cells dissociate. The increase in mast cell number is preceded and accompanied by elevated levels of mRNAs encoding the mast-cell chemoattractants Rantes, SCF and VEGF. Genetic ablation of mast cells and bone marrow reconstitution in W(41) x EGFR(+) mice indicate a role for mast cells in loss of axon-Schwann cell interactions and collagen deposition. Pharmacological stabilization of mast cells by disodium cromoglycate administration to EGFR(+) mice also diminished loss of axon-Schwann cell interaction. Together these three lines of evidence support the hypothesis that mast cells can contribute to alterations in peripheral nerves.

Sodium channel Nav1.6 accumulates at the site of infraorbital nerve injury

BackgroundSodium channel (NaCh) expressions change following nerve and inflammatory lesions and this change may contribute to the activation of pain pathways. In a previous study we found a dramatic increase in the size and density of NaCh accumulations, and a remodeling of NaChs at intact and altered myelinated sites at a location just proximal to a combined partial axotomy and chromic suture lesion of the rat infraorbital nerve (ION) with the use of an antibody that identifies all NaCh isoforms. Here we evaluate the contribution of the major nodal NaCh isoform, Nav1.6, to this remodeling of NaChs following the same lesion. Sections of the ION from normal and ION lesioned subjects were double-stained with antibodies against Nav1.6 and caspr (contactin-associated protein; a paranodal protein to identify nodes of Ranvier) and then z-series of optically sectioned images were captured with a confocal microscope. ImageJ (NIH) software was used to quantify the average size and density of Nav1.6 accumulations, while additional single fiber analyses measured the axial length of the nodal gap, and the immunofluorescence intensity of Nav1.6 in nodes and of caspr in the paranodal region.ResultsThe findings showed a significant increase in the average size and density of Nav1.6 accumulations in lesioned IONs when compared to normal IONs. The results of the single fiber analyses in caspr-identified typical nodes showed an increased axial length of the nodal gap, an increased immunofluorescence intensity of nodal Nav1.6 and a decreased immunofluorescence intensity of paranodal caspr in lesioned IONs when compared to normal IONs. In the lesioned IONs, Nav1.6 accumulations were also seen in association with altered caspr-relationships, such as heminodes.ConclusionThe results of the present study identify Nav1.6 as one isoform involved in the augmentation and remodeling of NaChs at nodal sites following a combined partial axotomy and chromic suture ION lesion. The augmentation of Nav1.6 may result from an alteration in axon-Schwann cell signaling mechanisms as suggested by changes in caspr expression. The changes identified in this study suggest that the participation of Nav1.6 should be considered when examining changes in the excitability of myelinated axons in neuropathic pain models.

A central role for Necl4 (SynCAM4) in Schwann cell–axon interaction and myelination

Myelination in the peripheral nervous system requires close contact between Schwann cells and the axon, but the underlying molecular basis remains largely unknown. Here we show that cell adhesion molecules (CAMs) of the nectin-like (Necl, also known as SynCAM or Cadm) family mediate Schwann cell-axon interaction during myelination. Necl4 is the main Necl expressed by myelinating Schwann cells and is located along the internodes in direct apposition to Necl1, which is localized on axons. Necl4 serves as the glial binding partner for axonal Necl1, and the interaction between these two CAMs mediates Schwann cell adhesion. The disruption of the interaction between Necl1 and Necl4 by their soluble extracellular domains, or the expression of a dominant-negative Necl4 in Schwann cells, inhibits myelination. These results suggest that Necl proteins are important for mediating axon-glia contact during myelination in peripheral nerves.

Immunolocalization of aquaporin-1 in the mechanoreceptive Ruffini endings in the periodontal ligament

Previous ultrastructural studies have suggested an axon–Schwann cell interaction in the periodontal Ruffini ending, a primary mechanoreceptor. However, no information is available on the transport mechanism between them. The present study examined the immunolocalization of aquaporin-1 (AQP1) and -4 (AQP4), a member of the water-selective channel, in the periodontal Ruffini endings of the rat incisors and trigeminal ganglion. In addition, the expression of mRNA for AQP1 and 4 was detected in the trigeminal ganglion by a RT-PCR technique. A single PCR product of the sizes anticipated for AQP1 and 4 was detectable in a reverse transcripted cDNA sample from the trigeminal ganglion, whose neurons innervate the periodontal Ruffini endings. An AQP1 immunoreaction was recognizable in the axon terminals of the periodontal Ruffini endings as well as their associated terminal Schwann cells, as confirmed with a double staining with AQP1 and either PGP9.5 or S-100 protein. However, no immunoreaction for AQP4 was found in periodontal Ruffini endings. Although the AQP4 immunoreaction was localized in some satellite cells – but never in neurons – of the trigeminal ganglion, 16.1% trigeminal neurons showed the AQP1 immunoreaction. Furthermore, the AQP1 immunoreaction was found in certain satellite cells which surrounded AQP1-positive or -negative neurons. An analysis of a cross-sectional area of these positive neurons demonstrated that approximately 66.9% of the positive neurons were 400–1000 μm 2 (671.4 ± 172.4 μm 2), indicating that they could be categorized as medium-sized neurons which mediate mechanotransduction. These findings suggest that AQP1 controls water transport in the periodontal Ruffini endings.

Role of N‐cadherin in Schwann cell precursors of growing nerves

In the present paper, we determine the localization and developmental regulation of N-cadherin in embryonic rat nerves and examine the role of N-cadherin in this system. We also identify a major transition in the architecture of embryonic nerves and relating it to N-cadherin expression. We find that in early embryonic nerves, N-cadherin is primarily expressed in Schwann cell precursors. Pronounced expression is seen at distal nerve fronts where these cells associate with growth cones, and the proximal nerve ends, in boundary cap cells. Unexpectedly, N-cadherin is downregulated as precursors generate Schwann cells, coinciding with the time at which most axons make target connections. Therefore, glial N-cadherin expression is essentially restricted to the period of axon outgrowth. We also provide evidence that N-cadherin supports the formation of contacts between Schwann cell precursors and show that these cells are a favorable substrate for axon growth, unlike N-cadherin-negative Schwann cells. Induction of N-cadherin expression in Schwann cells by neuregulin-1 restores their ability to form contacts and support axon growth. Finally, we show that the loss of glial N-cadherin during embryonic nerve development is accompanied by a transformation of nerve architecture, involving the appearance of endoneurial connective tissue space, fibroblasts, Schwann cell basal lamina, and blood vessels. Because N-cadherin is likely to promote the extensive glial contacts typical of the compact embryonic nerve, we suggest that N-cadherin loss at the time of Schwann cell generation allows endoneurial space to appear between the glial cells, a development that eventually permits the extensive interactions between connective tissue and individual axon-Schwann cell units necessary for myelination.

Reelin is transiently expressed in the peripheral nerve during development and is upregulated following nerve crush

Reelin is an extracellular matrix protein which is critical for the positioning of migrating post-mitotic neurons and the laminar organization of several brain structures during development. We investigated the expression and localization of Reelin in the rodent peripheral nerve during postnatal development and following crush injury in the adult stage. As shown with Western blotting, immunocytochemistry and RT-PCR, Schwann cells in the developing peripheral nerve and in primary cultures from neonatal nerves produce and secrete Reelin. While Reelin levels are downregulated in adult stages, they are again induced following sciatic nerve injury. A morphometric analysis of sciatic nerve sections of reeler mice suggests that Reelin is not essential for axonal ensheathment by Schwann cells, however, it influences the caliber of myelinated axons and the absolute number of fibers per unit area. This indicates that Reelin may play a role in peripheral nervous system development and repair by regulating Schwann cell–axon interactions.

Temporal Control of Rac in Schwann Cell–Axon Interaction Is Disrupted inNF2-Mutant Schwannoma Cells

Schwann cell-axon interaction is the hallmark feature of peripheral nerves, yet the intracellular signals underlying this interaction are unknown. Schwann cells extend processes and migrate on developing axons before differentiation, requiring coordinated regulation of the Schwann cell cytoskeleton. Small GTPases of the Rho family, including Rho, Rac, and cell division cycle 42, regulate the actin cytoskeleton. The neurofibromatosis type 2 (NF2) gene is commonly mutated in schwannomas, Schwann cell tumors that contain cells lacking axon interaction. NF2 is involved in suppression of Rac signaling, and cultured schwannoma cells contain elevated, GTP-bound, active Rac. Despite these previous studies, a causal relationship between Rac activation and the abnormal cellular morphology of schwannoma is unknown. We used fluorescence resonance energy transfer to follow Rac activity in normal human Schwann cells and schwannoma cells during interaction with neurons. Normal Schwann cells elongated processes along neurites under low Rac activity. Schwannoma cells showed high Rac activity at distal regions of the cells and failed to align processes with neurites. Application of a Rac-specific inhibitor, the chemical compound NSC23766, to schwannoma cells restored neuronal interaction. The data support the significance of regulated Rac signaling in mediating Schwann cell-axon interaction and suggest that controlling Rac activity as a possible therapy for schwannomas.