Activation of mTORC1 and c-Jun by Prohibitin1 loss in Schwann cells may link mitochondrial dysfunction to demyelination

Schwann cell (SC) mitochondria are quickly emerging as an important regulator of myelin maintenance in the peripheral nervous system (PNS). However, the mechanisms underlying demyelination in the context of mitochondrial dysfunction in the PNS are incompletely understood. We recently showed that conditional ablation of the mitochondrial protein Prohibitin 1 (PHB1) in SCs causes a severe and fast progressing demyelinating peripheral neuropathy in mice, but the mechanism that causes failure of myelin maintenance remained unknown. Here, we report that mTORC1 and c-Jun are continuously activated in the absence of Phb1, likely as part of the SC response to mitochondrial damage. Moreover, we demonstrate that these pathways are involved in the demyelination process, and that inhibition of mTORC1 using rapamycin partially rescues the demyelinating pathology. Therefore, we propose that mTORC1 and c-Jun may play a critical role as executioners of demyelination in the context of perturbations to SC mitochondria.

TXNIP, a novel key factor to cause Schwann cell dysfunction in diabetic peripheral neuropathy, under the regulation of PI3K/Akt pathway inhibition-induced DNMT1 and DNMT3a overexpression

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes mellitus (DM) and the dysfunction of Schwann cells plays an important role in the pathogenesis of DPN. Thioredoxin-interacting protein (TXNIP) is known as an inhibitor of thioredoxin and associated with oxidative stress and inflammation. However, whether TXNIP is involved in dysfunction of Schwann cells of DPN and the exact mechanism is still not known. In this study, we first reported that TXNIP expression was significantly increased in the sciatic nerves of diabetic mice, accompanied by abnormal electrophysiological indexes and myelin sheath structure. Similarly, in vitro cultured Schwann cells TXNIP was evidently enhanced by high glucose stimulation. Again, the function experiment found that knockdown of TXNIP in high glucose-treated RSC96 cells led to a 4.12 times increase of LC3-II/LC3-I ratio and a 25.94% decrease of cleaved caspase 3/total caspase 3 ratio. Then, DNA methyltransferase (DNMT) inhibitor 5-Aza has been reported to benefit Schwann cell in DPN, and here 5-Aza treatment reduced TXNIP protein expression, improved autophagy and inhibited apoptosis in high glucose-treated RSC96 cells and the sciatic nerves of diabetic mice. Furthermore, DNMT1 and DNMT3a upregulation were found to be involved in TXNIP overexpression in high glucose-stimulated RSC96 cells. Silencing of DNMT1 and DNMT3a effectively reversed high glucose-enhanced TXNIP. Moreover, high glucose-inhibited PI3K/Akt pathway led to DNMT1, DNMT3a, and TXNIP upregulation in RSC96 cells. Knockdown of DNMT1 and DNMT3a prevented PI3K/Akt pathway inhibition-caused TXNIP upregulation in RSC96 cells. Finally, in vivo knockout of TXNIP improved nerve conduction function, increased autophagosome and LC3 expression, and decreased cleaved Caspase 3 and Bax expression in diabetic mice. Taken together, PI3K/Akt pathway inhibition mediated high glucose-induced DNMT1 and DNMT3a overexpression, leading to cell autophagy inhibition and apoptosis via TXNIP protein upregulation in Schwann cells of DPN.

Macrophages Expressing GALC Improve Peripheral Krabbe Disease by a Mechanism Independent of Cross-Correction

Many therapies for lysosomal storage disorders rely on cross-correction of lysosomal enzymes. In globoid cell leukodystrophy (GLD), mutations in GALC cause psychosine accumulation, inducing demyelination, a neuroinflammatory "globoid" reaction and neurodegeneration. The efficiency of GALC cross-correction in vivo, the role of the GALC substrate galactosylceramide, and the origin of psychosine are poorly understood. Using a novel GLD model, we show that cross-correction does not occur efficiently in vivo and that Galc-deficient Schwann cells autonomously produce psychosine. Furthermore, macrophages require GALC to degrade myelin, as Galc-deficient macrophages are transformed into globoid cells by exposure to galactosylceramide and produce a more severe GLD phenotype. Finally, hematopoietic stem cell transplantation in patients reduces globoid cells in nerves, suggesting that the phagocytic response of healthy macrophages, rather than cross-correction, contributes to the therapeutic effect. Thus, GLD may be caused by at least two mechanisms: psychosine-induced demyelination and secondary neuroinflammation from galactosylceramide storage in macrophages.

HDAC3 Regulates the Transition to the Homeostatic Myelinating Schwann Cell State

The formation of myelinating Schwann cells (mSCs) involves the remarkable biogenic process, which rapidly generates the myelin sheath. Once formed, the mSC transitions to a stable homeostatic state, with loss of this stability associated with neuropathies. The histone deacetylases histone deacetylase 1 (HDAC1) and HDAC2 are required for the myelination transcriptional program. Here, we show a distinct role for HDAC3, in that, while dispensable for the formation of mSCs, it is essential for the stability of the myelin sheath once formed-with loss resulting in progressive severe neuropathy in adulthood. This is associated with the prior failure to downregulate the biogenic program upon entering the homeostatic state leading to hypertrophy and hypermyelination of the mSCs, progressing to the development of severe myelination defects. Our results highlight distinct roles of HDAC1/2 and HDAC3 in controlling the differentiation and homeostatic states of a cell with broad implications for the understanding of this important cell-state transition.

Insulin-induced upregulation of lipoprotein lipase in Schwann cells during diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is one of the major complications associated with diabetes. It is characterized by the degeneration of the myelin sheath around axons, referred to as demyelination. Such demyelinations are often caused by reduced lipid component of the myelin sheath. Since, lipoprotein lipase (LPL) provides the lipid for myelin sheath by hydrolysing the triglyceride rich lipoproteins, and also helps in the uptake of lipids by the Schwann cells (SCs) for its utilization, LPL is considered as the important factor in the regeneration of myelin sheath during diabetic neuropathy. Earlier reports from our laboratory have provided the insights of insulin and its receptor in SCs during diabetic neuropathy. In order to evaluate the long term effect of insulin on lipid metabolism during diabetic neuropathy, in this study, we analyzed the expression of LPL in SCs under normal, high glucose and insulin treated conditions. A decrease in the expression of LPL was observed in SCs of high glucose condition and it was reversed upon insulin treatment. Histochemical observations of sciatic nerve of insulin treated neuropathy subjects showed the improved nerve morphology, signifying the importance of insulin in restoring the pathophysiology of diabetic neuropathy.

Hyperglycemia-induced Bcl-2/Bax-mediated apoptosis of Schwann cells via mTORC1/S6K1 inhibition in diabetic peripheral neuropathy

Schwann cell apoptosis is one of the characteristics of diabetic peripheral neuropathy (DPN). The mammalian target of rapamycin (mTOR) is a multifunctional signaling pathway that regulates cell apoptosis in various types of tissues and cells. To investigate whether the mTOR pathway is involved in cell apoptosis in the Schwann cells of DPN, diabetic mice and rat Schwann cells (RSC96) were chosen to detect phospho-mTOR (Ser 2448), phospho-S6K1 (Thr 389), phospho-4EBP1 (Thr 37/46), Bcl-2, Bax and cleaved caspase-3 by diverse pathological and biological techniques. The results showed that phospho-mTOR (Ser 2448) was decreased in the sciatic nerves of diabetic mice, concomitant with decreased Bcl-2, increased Bax, cleaved caspase-3 and cell apoptosis. In addition, high glucose treatment for 72 h caused a 35.95% decrease in the phospho-mTOR (Ser 2448)/mTOR ratio, a 65.50% decrease in the phospho-S6K1 (Thr 389)/S6K1 ratio, a 3.67-fold increase in the Bax/Bcl-2 ratio and a 1.47-fold increase in the cleaved caspase-3/caspase-3 ratio. Furthermore, mTORC1 inhibition, rather than mTORC2 inhibition, resulted in mitochondrial controlled apoptosis in RSC96 cells by silencing RAPTOR or RICTOR. Again, suppression of the mTORC1 pathway by a chemical inhibitor led to mitochondrial controlled apoptosis in cultured RSC96 cells in vitro. By contrast, activation of the mTORC1 pathway with MHY1485 prevented decreased phospho-S6K1 (Thr 389) levels caused by high glucose and cell apoptosis. Additionally, constitutive activation of S6K1 avoided high glucose-induced cell apoptosis in RSC96 cells. In summary, our findings suggest that activating mTORC1/S6K1 signaling in Schwann cells may be a promising strategy for the prevention and treatment of DPN.

Proliferation-enhancing effects of gastrodin on RSC96 Schwann cells by regulating ERK1/2 and PI3K signaling pathways

The proliferation and migration of Schwann cells (SCs) are essential in the process of peripheral nerve repair. A large amount of studies focused on the promotion of the growth of SCs for cell based therapy. Gastrodin (GAS), the main constituent of a Chinese traditional herbal medicine named Gastrodia elata Blume, has been reported to be associated with neuroprotective properties. Besides, GAS activated MAPK and PI3K signaling pathways which are often involved in growth of nerve cells were also reported. Based on the hypothesis that GAS may have an effect on SCs growth, we studied the effect of GAS on rat RSC96 Schwann cells (SCs) and further explored the underlying mechanism. Various concentration of GAS (0μM, 50μM, 100μM, and 200μM) was used for treatment of RSC96 SCs, with the cell proliferation and gene expression of several neurotrophic factors to be detected. Regulation of MAPK and PI3K signaling pathways were assayed by detecting phosphorylation of ERK1/2 and Akt. The results showed that GAS could effectively promote proliferation of RSC96 SCs in a dose- and time-dependent manner. The best performance was obtained at the concentration of 200μM. Exploration of the underlying mechanism showed that GAS probably affects SCs metabolism through inhibiting ERK1/2 phosphorylation and activating Akt phosphorylation in RSC96 SCs. This study may provide reference for its application in treatment of peripheral nerve injuries.

High glucose upregulates CYP24A1 expression which attenuates the ability of 1,25(OH)2D3 to increase NGF secretion in a rat Schwann cell line RSC96

Vitamin D deficiency or insufficiency is an independent risk factor for diabetic peripheral neuropathy (DPN), but the relationship between 1,25(OH)2D3 and DPN remains unknown. We found that 1,25(OH)2D3 stimulated the secretion of nerve growth factor (NGF) in rat Schwann cell line RSC96, but ability of 1,25(OH)2D3 to increase NGF protein was impaired under high glucose conditions. High glucose upregulated the expression of CYP24A1 protein, which catalyzes the conversion of 1,25(OH)2D3 into inactive products, further impairing the ability of 1,25(OH)2D3 to upregulate NGF secretion in Schwann cells. Inhibition of CYP24A1 protein expression ameliorated the secretion of NGF in response to 1,25(OH)2D3. The findings of this study suggest that CYP24A1 protein plays an important role in the relationship between DPN and 1,25(OH)2D3.

Plastic fantastic: Schwann cells and repair of the peripheral nervous system

Repair in the peripheral nervous system (PNS) depends upon the plasticity of the myelinating cells, Schwann cells, and their ability to dedifferentiate, direct axonal regrowth, remyelinate, and allow functional recovery. The ability of such an exquisitely specialized myelinating cell to revert to an immature dedifferentiated cell that can direct repair is remarkable, making Schwann cells one of the very few regenerative cell types in our bodies. However, the idea that the PNS always repairs after injury, in contrast to the central nervous system, is not true. Repair in patients after nerve trauma can be incredibly variable, depending on the site and type of injury, and only a relatively small number of axons may fully regrow and reinnervate their targets. Recent research has shown that it is an active process that drives Schwann cells back to an immature state after injury and that this requires activity of the p38 and extracellular-regulated kinase 1/2 mitogen-activated protein kinases, as well as the transcription factor cJun. Analysis of the events after peripheral nerve transection has shown how signaling from nerve fibroblasts forms Schwann cells into cords in the newly generated nerve bridge, via Sox2 induction, to allow the regenerating axons to cross the gap. Understanding these pathways and identifying additional mechanisms involved in these processes raises the possibility of both boosting repair after PNS trauma and even, possibly, blocking the inappropriate demyelination seen in some disorders of the peripheral nervous system.

Caspase-2 is upregulated after sciatic nerve transection and its inhibition protects dorsal root ganglion neurons from apoptosis after serum withdrawal

Sciatic nerve (SN) transection-induced apoptosis of dorsal root ganglion neurons (DRGN) is one factor determining the efficacy of peripheral axonal regeneration and the return of sensation. Here, we tested the hypothesis that caspase-2 (CASP2) orchestrates apoptosis of axotomised DRGN both in vivo and in vitro by disrupting the local neurotrophic supply to DRGN. We observed significantly elevated levels of cleaved CASP2 (C-CASP2), compared to cleaved caspase-3 (C-CASP3), within TUNEL+DRGN and DRG glia (satellite and Schwann cells) after SN transection. A serum withdrawal cell culture model, which induced 40% apoptotic death in DRGN and 60% in glia, was used to model DRGN loss after neurotrophic factor withdrawal. Elevated C-CASP2 and TUNEL were observed in both DRGN and DRG glia, with C-CASP2 localisation shifting from the cytosol to the nucleus, a required step for induction of direct CASP2-mediated apoptosis. Furthermore, siRNA-mediated downregulation of CASP2 protected 50% of DRGN from apoptosis after serum withdrawal, while downregulation of CASP3 had no effect on DRGN or DRG glia survival. We conclude that CASP2 orchestrates the death of SN-axotomised DRGN directly and also indirectly through loss of DRG glia and their local neurotrophic factor support. Accordingly, inhibiting CASP2 expression is a potential therapy for improving both the SN regeneration response and peripheral sensory recovery.

Lck tyrosine kinase mediates β1-integrin signalling to regulate Schwann cell migration and myelination

The interaction between laminin and β1-integrin on the surface of Schwann cells regulates Schwann cell proliferation, maturation and differentiation. However, the signalling mediators that fine-tune these outcomes are not fully elucidated. Here we show that lymphoid cell kinase is the crucial effector of β1-integrin signalling in Schwann cells. Lymphoid cell kinase is activated after laminin treatment of Schwann cells, while downregulation of β1-integrin with short interfering RNAs inhibits lymphoid cell kinase phosphorylation. Treatment of Schwann cells with a selective lymphoid cell kinase inhibitor reveals a pathway that involves paxillin and CrkII, which ultimately elevates Rac-GTP levels to induce radial lamellipodia formation. Inhibition of lymphoid cell kinase in Schwann cell-dorsal root ganglion cocultures and dorsal root ganglions from Lck(-/-) mice show a reduction of Schwann cell longitudinal migration, reduced myelin formation and internode length. Finally, Lck(-/-) mice exhibit delays in myelination, thinner myelin with abnormal g-ratios and aberrant myelin outfoldings. Our data implicate lymphoid cell kinase as a major regulator of cytoskeletal dynamics, migration and myelination in the peripheral nervous system.

Genetic interaction between MTMR2 and FIG4 phospholipid phosphatases involved in Charcot-Marie-Tooth neuropathies

We previously reported that autosomal recessive demyelinating Charcot-Marie-Tooth (CMT) type 4B1 neuropathy with myelin outfoldings is caused by loss of MTMR2 (Myotubularin-related 2) in humans, and we created a faithful mouse model of the disease. MTMR2 dephosphorylates both PtdIns3P and PtdIns(3,5)P(2), thereby regulating membrane trafficking. However, the function of MTMR2 and the role of the MTMR2 phospholipid phosphatase activity in vivo in the nerve still remain to be assessed. Mutations in FIG4 are associated with CMT4J neuropathy characterized by both axonal and myelin damage in peripheral nerve. Loss of Fig4 function in the plt (pale tremor) mouse produces spongiform degeneration of the brain and peripheral neuropathy. Since FIG4 has a role in generation of PtdIns(3,5)P(2) and MTMR2 catalyzes its dephosphorylation, these two phosphatases might be expected to have opposite effects in the control of PtdIns(3,5)P(2) homeostasis and their mutations might have compensatory effects in vivo. To explore the role of the MTMR2 phospholipid phosphatase activity in vivo, we generated and characterized the Mtmr2/Fig4 double null mutant mice. Here we provide strong evidence that Mtmr2 and Fig4 functionally interact in both Schwann cells and neurons, and we reveal for the first time a role of Mtmr2 in neurons in vivo. Our results also suggest that imbalance of PtdIns(3,5)P(2) is at the basis of altered longitudinal myelin growth and of myelin outfolding formation. Reduction of Fig4 by null heterozygosity and downregulation of PIKfyve both rescue Mtmr2-null myelin outfoldings in vivo and in vitro.

Inducible nitric oxide synthase is present in motor neuron mitochondria and Schwann cells and contributes to disease mechanisms in ALS mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of motor neurons (MNs). The molecular pathogenesis of ALS is not understood, thus effective therapies for this disease are lacking. Some forms of ALS are inherited by mutations in the superoxide dismutase-1 (SOD1) gene. Transgenic mice expressing human Gly93 --> Ala (G93A) mutant SOD1 (mSOD1) develop severe MN disease, oxidative and nitrative damage, and mitochondrial pathology that appears to involve nitric oxide-mediated mechanisms. We used G93A-mSOD1 mice to test the hypothesis that the degeneration of MNs is associated with an aberrant up-regulation of the inducible form of nitric oxide synthase (iNOS or NOS2) activity within MNs. Western blotting and immunoprecipitation showed that iNOS protein levels in mitochondrial-enriched membrane fractions of spinal cord are increased significantly in mSOD1 mice at pre-symptomatic stages of disease. The catalytic activity of iNOS was also increased significantly in mitochondrial-enriched membrane fractions of mSOD1 mouse spinal cord at pre-symptomatic stages of disease. Reverse transcription-PCR showed that iNOS mRNA was present in the spinal cord and brainstem MN regions in mice and was increased in pre-symptomatic and early symptomatic mice. Immunohistochemistry showed that iNOS immunoreactivty was up-regulated first in spinal cord and brainstem MNs in pre-symptomatic and early symptomatic mice and then later in the course of disease in numerous microglia and few astrocytes. iNOS accumulated in the mitochondria in mSOD1 mouse MNs. iNOS immunoreactivity was also up-regulated in Schwann cells of peripheral nerves and was enriched particularly at the paranodal regions of the nodes of Ranvier. Drug inhibitors of iNOS delayed disease onset and significantly extended the lifespan of G93A-mSOD1 mice. This work identifies two new potential early mechanisms for MN degeneration in mouse ALS involving iNOS at MN mitochondria and Schwann cells and suggests that therapies targeting iNOS might be beneficial in treating human ALS.

Regulatory role of cytochrome P450scc and pregnenolone in myelination by rat Schwann cells

To investigate the production of steroid hormones by Schwann cells and to examine the regulation of steroid hormone production during myelination, cultures of rat Schwann cells were differentiated into their myelinating phenotype in the absence of neurons with dibutyryl cAMP (db-cAMP). During this process, the expression of P450scc (involved in steroid biosynthesis) was elevated at both the mRNA and protein levels as evident in RT-PCR, Western blots, and immunostaining. Labeling of the cells with [14C] acetate revealed enhanced production of pregnenolone during differentiation into the myelinating phenotype. Disruption of P450scc's activity with an inhibitor diminished the extent of differentiation into the myelinating phenotype as levels of mRNA and protein expression of myelin protein zero (P0) declined. However, the effect was reversed with the addition of pregnenolone. Furthermore, when the differentiating cultures were treated with pregnenolone, mRNA expression of P0 was upregulated, suggesting the stimulation of the differentiation process. Together, these results provide evidence for Schwann cells as a major producer of steroid hormones and pregnenolone production by P450scc as an important regulatory step during myelination.

The Na, K-ATPase alpha3-isoform specifically localizes in the Schmidt-Lanterman incisures of human nerve

INTRODUCTION

To our knowledge, there is little reference in literature with regards to alpha3-isoform of Na+,K+-ATPase in human peripheral nerves. The aim of this study was to determine the expression of the neuronal alpha3-isoform of Na+,K+-ATPase in human sural nerves from patients with a permanent medullary central nervous system injury.

MATERIALS AND METHODS

We studied the immunolocalization of alpha3-isoform of Na+,K+-ATPase using a polyclonal antibody against the amino sequence near the phosphorylation site of the alpha3-isoforms of Na+,K+-ATPase using immunohistochemistry and confocal laser scanning microscopy. An antibody specific for alpha2-isoform of Na+,K+-ATPase was used to label the Schwann cells.

RESULTS

Morphometric analysis of longitudinal section of human sural nerves showed that the alpha3-isoform of Na+,K+-ATPase was distributed along the length of axolemma. The myelin sheath of the Schwann cells showed clearly a distribution of alpha3- but not alpha2-isoforms of Na+,K+-ATPase at the level of Schmidt-Lanterman incisures.

CONCLUSION

The human sural nerve shows a specific localization of the Na+,K+-ATPase alpha3-isoform in the Schmidt-Lanterman incisures of Schwann cells in addition to its localization in axonal membranes.

Rho kinase inhibitor improves motor dysfunction and hypoalgesia in a rat model of lumbar spinal canal stenosis

STUDY DESIGN

Immunohistochemical and behavioral study using a rat cauda equina compression model.

OBJECTIVE

To investigate, after cauda equina compression by spinal canal stenosis (SCS), Rho activation in the spinal cord and cauda equina, and the effect of intrathecal administration of a Rho kinase inhibitor on hypoalgesia and motor dysfunction.

SUMMARY OF BACKGROUND DATA

Compression of the cauda equina caused by SCS is a common clinical disorder associated with sensory disturbance and intermittent claudication. Cauda equina compression is thought to reduce blood flow and result in nerve degeneration caused by various cytokines. Rho, a member of the small GTPases, is a signal transmitter. It promotes Wallerian degeneration, decreases blood flow in the spinal cord and brain, and increases expression of several cytokines. Currently, Rho kinase inhibitor is used clinically to treat progressive nerve damage due to cerebrovascular disorders. However, its effect for SCS has not been evaluated.

METHODS

Forty-two 6-week-old male Sprague-Dawley rats (200-250 g) were used. For the SCS model (n = 27), a small piece of silicon was placed under the lamina of the fourth lumbar vertebra. In the sham-operated group, laminectomies were performed at L5 only (n = 15). We examined mechanical sensitivity and motor function using von Frey hairs and a treadmill, and immunohistochemically localized Rho in the spinal ventral neurons, axons, and Schwann cells in the cauda equina. We also examined the effects of intrathecally administered Rho kinase inhibitor for hypoalgesia or motor dysfunction caused by SCS.

RESULTS

We observed motor dysfunction and hypoalgesia and activated Rho-immunoreactive cells in spinal ventral neuroreported to induce neurite and axonal outgrowth in the spinal cord and brain after nervous system injury. In addition, 1 report showed that Rho kinase was involved in Wallerian degeneration that was rescued by Rho kinase inhibitor. Furthermore, it is thought that Rho is involved in TNF-alpha and interleukin (IL) production in the central nervous system, and the production was inhibited by administering Rho kinase inhibitor in the central nervous system. Regardns, axons, and Schwann cells in the cauda equina. Intrathecal administration of Rho kinase inhibitor improved mechanical hypoalgesia and motor dysfunction caused by SCS.

CONCLUSION

Activated Rho may play an important role in nerve damage in the cauda equina in SCS. Rho kinase inhibitor may be a useful tool in determining the pathomechanism of cauda equina syndrome caused by SCS.

Activation of extracellular-signal-regulated kinase-1/2 precedes and is required for injury-induced Schwann cell proliferation

Activation of extracellular-signal-regulated kinase-1/2 (Erk1/2) by phosphorylation to p-Erk1/2, and proliferation of Schwann cells were investigated in the rat sciatic nerve by immunohistochemistry. Axotomy in vivo and culturing of nerve segments in vitro resulted in a rapid (30 min) increase of p-Erk1/2 in Schwann cells with peaks at 2 and 24 h. Proliferation measured by bromodeoxy uridine incorporation and immunostaining in vivo and in vitro 48 h after axotomy showed an increase in Schwann cell proliferation at the sites of Erk1/2 activation. The Erk1/2 inhibitor U0126 inhibited both the increase in p-Erk1/2 and the bromodeoxy uridine incorporation. We suggest that an increase in p-Erk1/2 is required for nerve injury-induced proliferation of Schwann cells.

Mutations in FGD4 encoding the Rho GDP/GTP exchange factor FRABIN cause autosomal recessive Charcot-Marie-Tooth type 4H

Charcot-Marie-Tooth (CMT) disorders are a clinically and genetically heterogeneous group of hereditary motor and sensory neuropathies characterized by muscle weakness and wasting, foot and hand deformities, and electrophysiological changes. The CMT4H subtype is an autosomal recessive demyelinating form of CMT that was recently mapped to a 15.8-Mb region at chromosome 12p11.21-q13.11, in two consanguineous families of Mediterranean origin, by homozygosity mapping. We report here the identification of mutations in FGD4, encoding FGD4 or FRABIN (FGD1-related F-actin binding protein), in both families. FRABIN is a GDP/GTP nucleotide exchange factor (GEF), specific to Cdc42, a member of the Rho family of small guanosine triphosphate (GTP)-binding proteins (Rho GTPases). Rho GTPases play a key role in regulating signal-transduction pathways in eukaryotes. In particular, they have a pivotal role in mediating actin cytoskeleton changes during cell migration, morphogenesis, polarization, and division. Consistent with these reported functions, expression of truncated FRABIN mutants in rat primary motoneurons and rat Schwann cells induced significantly fewer microspikes than expression of wild-type FRABIN. To our knowledge, this is the first report of mutations in a Rho GEF protein being involved in CMT.

Peripheral nerve demyelination caused by a mutant Rho GTPase guanine nucleotide exchange factor, frabin/FGD4

GTPases of the Rho subfamily are widely involved in the myelination of the vertebrate nervous system. Rho GTPase activity is temporally and spatially regulated by a set of specific guanine nucleotide exchange factors (GEFs). Here, we report that disruption of frabin/FGD4, a GEF for the Rho GTPase cell-division cycle 42 (Cdc42), causes peripheral nerve demyelination in patients with autosomal recessive Charcot-Marie-Tooth (CMT) neuropathy. These data, together with the ability of frabin to induce Cdc42-mediated cell-shape changes in transfected Schwann cells, suggest that Rho GTPase signaling is essential for proper myelination of the peripheral nervous system.

Double-stranded RNA induces iNOS gene expression in Schwann cells, sensory neuronal death, and peripheral nerve demyelination

Inflammation in the peripheral nervous system (PNS) is one of the characteristics of virus-induced peripheral neuropathy. In this inflammatory response, Schwann cells are actively involved. Previously, toll-like receptor 3 (TLR3) was reported as a receptor for double-stranded RNA (dsRNA) that induces antiviral and inflammatory responses in cells of the innate immune system. In this study, we investigated the expression and putative role of TLR3 in Schwann cells. TLR3 was constitutively expressed in Schwann cells. Stimulation with polyinosinic-polycytidylic acid, a synthetic dsRNA analogue, induced the expression of inducible nitric oxide synthase (iNOS) gene in Schwann cells. Studies on the intracellular signal transduction pathways using iSC, an immortalized Schwann cell line, revealed that dsRNA induces the activation of NF-kappaB, p38, and c-Jun N-terminal kinase (JNK). The activation of NF-kappaB, p38, JNK, and dsRNA-dependent protein kinase is required for dsRNA-mediated iNOS gene expression. However, the activation of PI3 kinase and GSK-3beta inhibited iNOS gene induction, a process mediated by their inhibitory effects on NF-kappaB and p38 activation. dsRNA-induced NO production caused neuronal cell death in cultured dorsal root ganglion. Finally, the introduction of dsRNA into the rat sciatic nerve induced iNOS gene expression and peripheral nerve demyelination in vivo. Taken together, these data suggest that viral RNA may induce inflammatory Schwann cell activation via TLR3 and peripheral nerve damage in the PNS.

Mice lacking protease nexin-1 show delayed structural and functional recovery after sciatic nerve crush

Multiple molecular mechanisms influence nerve regeneration. Because serine proteases were shown to affect peripheral nerve regeneration, we performed nerve crush experiments to study synapse reinnervation in adult mice lacking the serpin protease nexin-1 (PN-1). PN-1 is a potent endogenous inhibitor of thrombin, trypsin, tissue plasminogen activators (tPAs), and urokinase plasminogen activators. Compared with the wild type, a significant delay in synapse reinnervation was detected in PN-1 knock-out (KO) animals, which was associated with both reduced proliferation and increased apoptosis of Schwann cells. Various factors known to affect Schwann cells were also altered. Fibrin deposits, tPA activity, mature BDNF, and the low-affinity p75 neurotrophin receptor were increased in injured sciatic nerves of mutant mice. To test whether the absence of PN-1 in Schwann cells or in the axon caused delay in reinnervation, PN-1 was overexpressed exclusively in the nerves of PN-1 KO mice. Neuronal PN-1 expression did not rescue the delayed reinnervation. The results suggest that Schwann cell-derived PN-1 is crucial for proper reinnervation through its contribution to the autocrine control of proliferation and survival. Thus, the precise balance between distinct proteases and serpins such as PN-1 can modulate the overall impact on the kinetics of recovery.

Novel sites of aldose reductase immunolocalization in normal and streptozotocin-diabetic rats

Glucose metabolism by aldose reductase (AR) is implicated in the pathogenesis of many diabetic complications, including neuropathy. We have re-evaluated the distribution of AR in the sciatic nerve and dorsal root ganglion (DRG) of normal rats, expanded these observations to describe the location of AR in the spinal cord and footpad skin, and investigated whether diabetes alters the distribution of AR. In sciatic nerve, AR was restricted to cytoplasm of myelinated Schwann cells and endothelial cells of epineurial, but not endoneurial, blood vessels. AR immunoreactivity (IR) was present in satellite cells in the DRG. In skin, AR-IR was detected in vascular endothelial cells, Schwann cells of myelinated fibers, and axons of perivascular sympathetic nerves. AR was localized selectively to oligodendrocytes of the white matter of spinal cord. The distribution of AR-IR in sciatic nerve, DRG, skin, and spinal cord was not altered by up to 12 weeks of streptozotocin-induced diabetes. Identification of perineuronal satellite cells, oligodendrocytes, and perivascular sympathetic nerves as AR-expressing cells reveals them as cellular sites with the potential to contribute to diabetic neuropathy.

Glial phosphorylated p38 MAP kinase mediates pain in a rat model of lumbar disc herniation and induces motor dysfunction in a rat model of lumbar spinal canal stenosis

STUDY DESIGN

Immunohistochemical and behavioral study using rat models of lumbar disc herniation and cauda equina syndrome.

OBJECTIVE

To investigate the expression of activated p38 mitogen-activated protein kinases (p38 MAP kinase; p38) in the spinal cord and to determine the effect of intrathecal administration of a specific p38 inhibitor on pain in a lumbar disc herniation model and on motor function and hypoalgesia in a spinal canal stenosis (SCS) model.

SUMMARY OF BACKGROUND DATA

In pathologic lumbar disc herniation-induced neuropathic pain and compression of cauda equina-induced motor dysfunction and hypoalgesia caused by SCS, glia are activated and produce certain cytokines, including tumor necrosis factor-alpha (TNF-alpha) and interleukins, which play a crucial role in the pathogenesis of nerve degeneration. p38 is phosphorylated by these cytokines, suggesting that it may play an important role in pain transmission and nerve degeneration. Here we have examined the role of p38 in rat models of lumbar disc herniation and SCS.

METHODS

Six-week-old male Sprague-Dawley rats were used. For the disc herniation model, autologous nucleus pulposus was applied to L5 nerve roots, which were then crushed. For the SCS model, a piece of silicon was placed under the lamina of the fourth lumbar vertebra. We assessed mechanical allodynia, hypoalgesia, and motor function using von Frey hairs, treadmill tests, and immunohistochemical localization of phosphorylated p38 (P-p38) in the cauda equina, dorsal root ganglion (DRG), and spinal cord, which were also double-stained with NeuN (neuronal marker), GFAP (astrocyte/Schwann cell marker), or isolectin B4 (IB4; microglia marker). We also examined the effects of intrathecal administration of a specific p38 inhibitor, FR167653, on nucleus pulposus-induced pain, hypoalgesia, and motor dysfunction following SCS.

RESULTS

We demonstrated that activated P-p38-immunoreactive cells in the spinal cord and cauda equina were not observed before nerve injury but appeared in the cauda equina, DRG, and spinal dorsal horn in the disc herniation and SCS models. Double-labeling revealed that most P-p38-immunoreactive cells were isolectin B4-labeled microglia and GFAP-immunoreactive Schwann cells. Intrathecal administration of the p38 inhibitor FR167653 decreased mechanical allodynia in the disc herniation model and improved hypoalgesia and intermittent motor dysfunction in the SCS model.

CONCLUSIONS

Our findings suggest that activated p38 may play an important role in the involvement of microglia in the pathophysiology of pain following lumbar disc herniation and mechanical hypoalgesia, and motor nerve dysfunction of cauda equina following SCS.

Activation of matrix metalloproteinase-3 is altered at the frog neuromuscular junction following changes in synaptic activity

The extracellular matrix surrounding the neuromuscular junction is a highly specialized and dynamic structure. Matrix Metalloproteinases are enzymes that sculpt the extracellular matrix. Since synaptic activity is critical to the structure and function of this synapse, we investigated whether changes in synaptic activity levels could alter the activity of Matrix Metalloproteinases at the neuromuscular junction. In particular, we focused on Matrix Metalloproteinase 3 (MMP3), since antibodies to MMP3 recognize molecules at the frog neuromuscular junction, and MMP3 cleaves a number of synaptic basal lamina molecules, including agrin. Here we show that the fluorogenic compound (M2300) can be used to perform in vivo proteolytic imaging of the frog neuromuscular junction to directly measure the activity state of MMP3. Application of this compound reveals that active MMP3 is concentrated at the normal frog neuromuscular junction, and is tightly associated with the terminal Schwann cell. Blocking presynaptic activity via denervation, or TTX nerve blockade, results in a decreased level of active MMP3 at the neuromuscular junction. The loss of active MMP3 at the neuromuscular junction in denervated muscles can result from decreased activation of pro-MMP3, or it could result from increased inhibition of MMP3. These results support the hypothesis that changes in synaptic activity can alter the level of active MMP3 at the neuromuscular junction.

Plexiform-like neurofibromas develop in the mouse by intraneural xenograft of an NF1 tumor-derived Schwann cell line

Plexiform neurofibromas are peripheral nerve sheath tumors that arise frequently in neurofibromatosis type 1 (NF1) and have a risk of malignant progression. Past efforts to establish xenograft models for neurofibroma involved the implantation of tumor fragments or heterogeneous primary cultures, which rarely achieved significant tumor growth. We report a practical and reproducible animal model of plexiform-like neurofibroma by xenograft of an immortal human NF1 tumor-derived Schwann cell line into the peripheral nerve of scid mice. The S100 and p75 positive sNF94.3 cell line was shown to possess a normal karyotype and have apparent full-length neurofibromin by Western blot. These cells were shown to have a constitutional NF1 microdeletion and elevated Ras-GTP activity, however, suggesting loss of normal neurofibromin function. Localized intraneural injection of the cell line sNF94.3 produced consistent and slow growing tumors that infiltrated and disrupted the host nerve. The xenograft tumors resembled plexiform neurofibromas with a low rate of proliferation, abundant extracellular matrix (hypocellularity), basal laminae, high vascularity, and mast cell infiltration. The histologic features of the developed tumors were particularly consistent with those of human plexiform neurofibroma as well. Intraneural xenograft of sNF94.3 cells enables the precise initiation of intraneural, plexiform-like tumors and provides a highly reproducible model for the study of plexiform neurofibroma tumorigenesis. This model facilitates testing of potential therapeutic interventions, including angiogenesis inhibitors, in a relevant cellular environment.

Differential RIP antigen (CNPase) expression in peripheral ensheathing glia

The RIP monoclonal antibody is commonly used to identify oligodendrocytes. Recently, the RIP antigen was identified as 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a known non-compact myelin protein [Watanabe, M., Sakurai, Y., Ichinose, T., Aikawa, Y., Kotani, M., Itoh, K., 2006. Monoclonal antibody Rip specifically recognizes 2',3'-cyclic nucleotide 3'-phosphodiesterase in oligodendrocytes. J. Neurosci. Res. 84, 525-533]. In the present study we characterize normal and axotomy-induced changes in RIP immunoreactivity in peripheral glia. In myelinating Schwann cells, RIP demarcated paranodal regions of myelinated axons and clearly defined Schmidt-Lantermann incisures. Surprisingly, RIP immunoreactivity was not confined to myelinating glia. Robust RIP immunoreactivity was present in Remak bundles in mixed nerves and in sympathetic ganglia and grey rami. Following peripheral nerve injury, RIP immunoreactivity was redistributed diffusely throughout de-differentiating Schwann cell cytoplasm. In uninjured rats, low levels of RIP immunoreactivity were detectable in satellite cells surrounding dorsal root ganglion (DRG) neurons and in terminal Schwann cells at neuromuscular junctions. This pattern suggested a correlation between RIP immunoreactivity and the amount of axon-glial contact. We therefore injured the L5 spinal nerve to induce sympathetic sprouting and pericellular basket formation in the DRG, and asked whether relatively RIP-negative satellite glia, which normally contact only neuronal somata, would upregulate the RIP antigen upon contact with sprouting sympathetic axons. All perineuronal sympathetic sprouts infiltrated heavily RIP-immunoreactive satellite cell sheaths. RIP immunoreactivity was absent from placode-derived olfactory ensheathing glia, indicating that the relationship between axon-glial contact and RIP-immunoreactivity is restricted to peripheral ensheathing glia of the neural crest-derived Schwann cell lineage.

Rewiring enervated: thinking LARGEr than myodystrophy

LARGE is a glycosyltransferase known to glycosylate alpha-dystroglycan, a component of the dystrophin-associated glycoprotein complex. Spontaneous deletions in the Large gene (Large(myd) and Large(vls)) result in muscular dystrophy accompanied by heart, brain, and eye defects. Another Large mouse mutant, enervated (Large(enr)), is the result of a transgene integration event that disrupts Large gene expression. In addition to myodystrophy, enr mice have been shown to display peripheral nerve abnormalities, including altered axonal sorting resulting from Schwann cell defects, poor regeneration after nerve injury, and abnormal neuromuscular junctions. These data have provided new insight into our understanding of the function of LARGE and have suggested the possibility of involvement of substrates in addition to alpha-dystroglycan in the generation of the LARGE phenotype. The Large mutants are excellent models for addressing the importance of glycosylation in neuromuscular disease.

ErbB2 receptor tyrosine kinase signaling mediates early demyelination induced by leprosy bacilli

Demyelination is a common pathologic feature in many neurodegenerative diseases including infection with leprosy-causing Mycobacterium leprae. Because of the long incubation time and highly complex disease pathogenesis, the management of nerve damage in leprosy, as in other demyelinating diseases, is extremely difficult. Therefore, an important challenge in therapeutic interventions is to identify the molecular events that occur in the early phase before the progression of the disease. Here we provide evidence that M. leprae-induced demyelination is a result of direct bacterial ligation to and activation of ErbB2 receptor tyrosine kinase (RTK) signaling without ErbB2-ErbB3 heterodimerization, a previously unknown mechanism that bypasses the neuregulin-ErbB3-mediated ErbB2 phosphorylation. MEK-dependent Erk1 and Erk2 (hereafter referred to as Erk1/2) signaling is identified as a downstream target of M. leprae-induced ErbB2 activation that mediates demyelination. Herceptin (trastuzumab), a therapeutic humanized ErbB2-specific antibody, inhibits M. leprae binding to and activation of ErbB2 and Erk1/2 in human primary Schwann cells, and the blockade of ErbB2 activity by the small molecule dual ErbB1-ErbB2 kinase inhibitor PKI-166 (ref. 11) effectively abrogates M. leprae-induced myelin damage in in vitro and in vivo models. These results may have implications for the design of ErbB2 RTK-based therapies for both leprosy nerve damage and other demyelinating neurodegenerative diseases.

Cathepsin D deficiency is associated with a human neurodegenerative disorder

Cathepsin D is a ubiquitously expressed lysosomal protease that is involved in proteolytic degradation, cell invasion, and apoptosis. In mice and sheep, cathepsin D deficiency is known to cause a fatal neurodegenerative disease. Here, we report a novel disorder in a child with early blindness and progressive psychomotor disability. Two missense mutations in the CTSD gene, F229I and W383C, were identified and were found to cause markedly reduced proteolytic activity and a diminished amount of cathepsin D in patient fibroblasts. Expression of cathepsin D mutants in cathepsin D(-/-) mouse fibroblasts revealed disturbed posttranslational processing and intracellular targeting for W383C and diminished maximal enzyme velocity for F229I. The structural effects of cathepsin D mutants were estimated by computer modeling, which suggested larger structural alterations for W383C than for F229I. Our studies broaden the group of human neurodegenerative disorders and add new insight into the cellular functions of human cathepsin D.

Cyclic AMP synergistically enhances neuregulin-dependent ERK and Akt activation and cell cycle progression in Schwann cells

The elevation of intracellular cAMP synergistically enhances the neuregulin-dependent proliferation of cultured Schwann cells (SCs); however, the mechanism by which this occurs has not been completely defined. To better understand this mechanism, we investigated the effect of cAMP on the activation of the extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3-K)-Akt (PKB) pathways by heregulin, a member of the neuregulin family. Using primary cultures of adult SCs, we demonstrated that the adenylyl cyclase activator, forskolin, enhanced heregulin-dependent SC proliferation by reducing the time required for S-phase entry. When cAMP levels were increased, using either forskolin or a cell permeable analogue of cAMP, the heregulin-induced phosphorylation of ERK was converted from transient to sustained and the heregulin-induced phosphorylation of Akt was synergistically increased. Consistent with these observations, studies in which inhibitors of MEK, the upstream stimulating ERK kinase, and PI3-K were administered at different times following the onset of stimulation indicated that sustained high levels of both MEK/ERK and PI3-K/Akt activity before S-phase initiation were essential for S-phase entry. Overall, these novel results indicate that in neuregulin-stimulated SCs the activation of cAMP-mediated pathways accelerates G1-S progression by prolonging ERK activation and concurrently enhancing Akt activation.

Loss of Mtmr2 phosphatase in Schwann cells but not in motor neurons causes Charcot-Marie-Tooth type 4B1 neuropathy with myelin outfoldings

Mutations in MTMR2, the myotubularin-related 2 gene, cause autosomal recessive Charcot-Marie-Tooth type 4B1 (CMT4B1). This disorder is characterized by childhood onset of weakness and sensory loss, severely decreased nerve conduction velocity, demyelination in the nerve with myelin outfoldings, and severe functional impairment of affected patients, mainly resulting from loss of myelinated fibers in the nerve. We recently generated Mtmr2-null(neo) mice, which show a dysmyelinating neuropathy with myelin outfoldings, thus reproducing human CMT4B1. Mtmr2 is detected in both Schwann cells and neurons, in which it interacts with discs large 1/synapse-associated protein 97 and neurofilament light chain, respectively. Here, we specifically ablated Mtmr2 in either Schwann cells or motor neurons. Disruption of Mtmr2 in Schwann cells produced a dysmyelinating phenotype very similar to that of the Mtmr2-null(neo) mouse. Disruption of Mtmr2 in motor neurons does not provoke myelin outfoldings nor axonal defects. We propose that loss of Mtmr2 in Schwann cells, but not in motor neurons, is both sufficient and necessary to cause CMT4B1 neuropathy. Thus, therapeutical approaches might be designed in the future to specifically deliver the Mtmr2 phospholipid phosphatase to Schwann cells in affected nerves.

Hijacking the ERK signaling pathway: Mycobacterium leprae shuns MEK to drive the proliferation of infected Schwann cells

Schwann cells are the target of Mycobacterium leprae, the pathogen responsible for leprosy. Once inside the cell, M. leprae activates the host's proliferative machinery, thereby increasing the number of cells susceptible to infection. This astonishing manipulation of the mammalian cell cycle is the subject of recent work by Tapinos and Rambukkana, who show that M. leprae drives proliferation through a novel route to extracellular signal-regulated kinase (ERK). In this Perspective, we discuss this important piece of work and highlight the noncanonical pathway used by M. leprae to induce proliferation.

Methylglyoxal induces apoptosis through oxidative stress-mediated activation of p38 mitogen-activated protein kinase in rat Schwann cells

Although recent studies have suggested the potential involvement of apoptotic cell death in the development of diabetic neuropathy, the precise mechanism remains to be elucidated. On the other hand, it is known that the formation of methylglyoxal (MG), a highly reactive dicarbonyl compound, is accelerated under diabetic conditions through several glucose-related metabolisms including the glycation reaction. We found that MG was capable of inducing apoptosis in peripheral nerve-derived Schwann cells (SCs) in a time- and dose-dependent manner, accompanied by a reduction of intracellular glutathione content. Furthermore, MG induced phosphorylation of MKK3/MKK6, an upstream molecule in the p38 MAPK pathway. N-acetyl-L-cysteine, an antioxidant, successfully suppressed the activity of the p38 MAPK signaling pathway along with the inhibition of apoptosis, indicating the involvement of oxidative stress in the MG-induced apoptosis via the p38 MAPK pathway. These results suggest a possible contribution of glucose-derived MG to the development of diabetic neuropathy by injuring the cellular constituent of the peripheral nerve system, such as SCs, in the hyperglycemic milieu.

Osteoactivin upregulates expression of MMP-3 and MMP-9 in fibroblasts infiltrated into denervated skeletal muscle in mice

In this study, we examined pathophysiological roles of osteoactivin, a functionally unknown type I membrane glycoprotein, in mouse skeletal muscle atrophied by denervation (sciatic neurectomy). Denervation increased the amounts of osteoactivin, vimentin, matrix metalloproteinase-3 (MMP-3), and MMP-9 in mouse gastrocnemius muscle. Interestingly, immunohistochemical analysis revealed that vimentin, MMP-3, and MMP-9 were mainly present in fibroblast-like cells infiltrated into denervated mouse gastrocnemius muscle, whereas osteoactivin was expressed in the sarcolemma of myofibers adjacent to the fibroblast-like cells. On the basis of these findings, we reasoned that osteoactivin in myocytes was involved in activation of the infiltrated fibroblasts. To address this issue, we examined effects of osteoactivin on expression of MMPs in fibroblasts in vitro and in vivo. Overexpression of osteoactivin in NIH-3T3 fibroblasts induced expression of MMP-3, but not in mouse C(2)C(12) myoblasts, indicating that osteoactivin might functionally target fibroblasts. Treatment with recombinant mouse osteoactivin increased the amounts of collagen type I, MMP-3, and MMP-9 in mouse NIH-3T3 fibroblasts. The upregulated expression of these fibroblast marker proteins was significantly inhibited by heparin, but not by an integrin inhibitor, indicating that a heparin-binding motif in the extracellular domain might be an active site of osteoactivin. In osteoactivin-transgenic mice, denervation further enhanced expression of MMP-3 and MMP-9 in fibroblasts infiltrated into gastrocnemius muscle, compared with wild-type mice. Our present results suggest that osteoactivin might function as an activator for fibroblasts infiltrated into denervated skeletal muscles and play an important role in regulating degeneration/regeneration of extracellular matrix.

Establishment of immortalized Schwann cells from Sandhoff mice and corrective effect of recombinant human β-hexosaminidase A on the accumulated GM2 ganglioside

We have established spontaneously immortalized Schwann cell lines from dorsal root ganglia and peripheral nerves of Sandhoff mice. One of the cell lines exhibited genetically and biochemically distinct features of Sandhoff Schwann cells. The enzyme activities toward 4-methylumbelliferyl N-acetyl-beta-D-glucosamine (beta-hexosaminidases A, B, and S) and 4-methylumbelliferyl N-acetyl-beta-D-glucosamine-6-sulfate (beta-hexosaminidases A and S) were decreased, and GM2 ganglioside accumulated in lysosomes of the cells. Incorporation of recombinant human beta-hexosaminidase isozymes expressed in Chinese hamster ovary cells into the cultured Sandhoff Schwann cells via cation-independent mannose 6-phosphate receptors was found, and the incorporated beta-hexosaminidase A degraded the accumulated GM2 ganglioside. The established Sandhoff Schwann cell line is useful for investigation and development of therapies for Sandhoff disease.

Insights into regulation of human Schwann cell proliferation by Erk1/2 via a MEK-independent and p56Lck-dependent pathway from leprosy bacilli

Activation of extracellular signal-regulated kinase (Erk) 1/2, which plays a critical role in diverse cellular processes, including cell proliferation, is known to be mediated by the canonical Raf-mitogen-activated protein kinase kinase (MEK) kinase cascade. Alternative MEK-independent signaling pathways for Erk1/2 activation in mammalian cells are not known. During our studies of human primary Schwann cell response to long-term infection of Mycobacterium leprae, the causative organism of leprosy, we identified that intracellular M. leprae activated Erk1/2 directly by lymphoid cell kinase (p56Lck), a Src family member, by means of a PKCepsilon-dependent and MEK-independent signaling pathway. Activation of this signaling induced nuclear accumulation of cyclin D1, G1/S-phase progression, and continuous proliferation, but without transformation. Thus, our data reveal a previously unknown signaling mechanism of glial cell proliferation, which might play a role in dedifferentiation as well as nerve regeneration and degeneration. Our findings may also provide a potential mechanism by which an obligate intracellular bacterial pathogen like M. leprae subverts nervous system signaling to propagate its cellular niche for colonization and long-term bacterial survival.

Opposing extracellular signal-regulated kinase and Akt pathways control Schwann cell myelination

Schwann cells are the myelinating glia of the peripheral nervous system, and their development is regulated by various growth factors, such as neuregulin, platelet-derived growth factor (PDGF), and insulin-like growth factor-I (IGF-I). However, the mechanism of intracellular signaling pathways following these ligand stimuli in Schwann cell differentiation remains elusive. Here, we demonstrate that in cultured Schwann cells, neuregulin and PDGF suppressed the expression of myelin-associated protein markers, whereas IGF-I promoted it. Although these ligands activated common downstream signaling pathways [i.e., extracellular signal-regulated kinase (Erk) and phosphatidylinositol-3-kinase (PI3K)-Akt pathways], the profiles of activation varied among ligands. To elucidate the function of these pathways and the mechanisms underlying Schwann cell differentiation, we used adenoviral vectors to selectively activate or inactivate these pathways. We found that the selective activation of Erk pathways suppressed Schwann cell differentiation, whereas that of PI3K pathways promoted it. Furthermore, lithium chloride, a modulator of glycogen synthase kinase-3beta (GSK-3beta) promoted Schwann cell differentiation, suggesting the involvement of GSK-3beta as a downstream molecule of PI3K-Akt pathways. Selective activation of PI3K pathways in Schwann cells by gene transfer also demonstrated increased myelination in in vitro Schwann cell-DRG neuron cocultures and in vivo allogenic nerve graft experiments. We conclude that signals mediated by PI3K-Akt are crucial for initiation of myelination and that the effects of growth factors are primarily dependent on the balance between Erk and PI3K-Akt activation. Our results also propose the possibility of augmenting Schwann cell functions by modulating intracellular signals in light of future cell therapies.

Schwann cell and epineural fibroblast expression of serine racemase

D-serine appears to be a natural agonist at the "glycine site" of the NMDA receptor and is created by conversion from L-serine by serine racemase. This racemase has been localized to protoplasmic astrocytes that ensheath synapses and modulate neuronal activity in the CNS, but serine racemase expression in the PNS has not been reported. Immunofluorescence indicated that Schwann cells and other endoneurial components of rat spinal nerve contain serine racemase, and western blot analysis detected the enzyme in lysates of sciatic nerve. Cultures from sciatic nerve contained Schwann cells and fibroblasts, and both cell types showed serine racemase expression by immunofluorescence and Western blot; the quantities per unit of total protein appeared slightly lower than that expressed in cultured astrocytes. Cultures enriched for each cell type were subjected to reverse transcriptase polymerase chain reaction, further confirming serine racemase mRNA in Schwann cells and fibroblasts. Finally, immunodetection of D-serine itself was observed in cultured Schwann cells and fibroblasts. These expression patterns of serine racemase may indicate roles for D-serine in peripheral nerve transduction.

Selective expression of L-serine synthetic enzyme 3PGDH in schwann cells, perineuronal glia, and endoneurial fibroblasts along rat sciatic nerves and its upregulation after crush injury

Non-essential amino acid L-serine functions as a highly potent, glia-derived neurotrophic factor, because it is a precursor for syntheses of proteins, other amino acids, membrane lipids, and nucleotides, and also because its biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH) is preferentially expressed in particular glial cells within the brain. Here we pursued 3PGDH expression in peripheral nerves and its change after crush injury. In the pathway of rat sciatic nerves, 3PGDH was selectively expressed in non-neuronal elements: Schwann sheaths and endoneurial fibroblasts in sciatic nerves, satellite cells in dorsal root ganglia, and astrocytes and oligodendrocytes in the spinal ventral horn. In contrast, 3PGDH was immunonegative in axons, somata of spinal motoneurons and ganglion cells, and endoneurial macrophages. One week after crush injury, 3PGDH was upregulated in the distal segment of injured nerves, where 3PGDH was intensified in activated Schwann cells and fibroblasts. 3PGDH was still negative in activated macrophages, which were instead associated or surrounded by activated Schwann cells with intensified 3PGDH. These results suggest that in the peripheral nervous system, these non-neuronal cells synthesize and may supply L-serine to satisfy metabolic demands for maintenance and regeneration of peripheral nerves and for proliferation and activation of macrophages upon nerve injury.

Cyclooxygenase-2 expression in Schwann cells and macrophages in the sciatic nerve after single spinal nerve injury in rats

Recent evidence suggested that cyclooxygenase-2 (COX-2) expression in the peripheral nerve early after nerve injury might be involved in the development of neuropathic pain. Although previous investigators have demonstrated that COX-2 is expressed in peripheral nerve at a late phase (2-4 weeks) after nerve injury, COX-2 up-regulation at an early phase after nerve injury has not been determined. Using immunohistochemistry, we observed biphasic increases of COX-2 expression after L5 single spinal nerve injury. The first increment of COX-2 positive cells was noted 1 day after nerve injury and these cells co-expressed the Schwann cell marker S-100. A second increment was noted after 7-14 days and these cells co-expressed the macrophage marker ED-1. These results suggested that the cellular sources of COX-2 expression might be different between the early and late phases after nerve injury.

Association of the ecto-ATPase NTPDase2 with glial cells of the peripheral nervous system

Cellular signaling via extracellular nucleotides appears to play a major role in the functioning of the peripheral nervous system. Information regarding the functional characterization of nucleotide P2 receptors or their expression pattern has been accumulating rapidly; however, very little is known regarding the distribution of ecto-nucleotidases in the periphery. The extracellular level of nucleotides is controlled by ecto-nucleotidases, whereby the three membrane-bound members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family are of special functional importance. Using enzyme histochemistry and immunostaining, we demonstrate that NTPDase2 is associated with nonmyelinating Schwann cells of the rat sciatic nerve, whereas NTPDase1 is restricted to blood vessel walls. NTPDase2 immunoreactivity was detected from embryonic day E18 onward, suggesting that immature Schwann cells express the enzyme. With the onset of myelination, NTPDase2 immunoreactivity remained associated solely with nonmyelinating Schwann cells. NTPDase2 was absent from perisynaptic Schwann cells but was associated with fibroblasts covering the endplate at some distance. In addition, NTPDase2 immunoreactivity was associated with the satellite glial cells in dorsal root ganglia and sympathetic ganglia, and with the enteric glia surrounding the cell bodies of ganglionic neurons of the myenteric and the submucous plexus. In contrast to NTPDase1, NTPDase2 preferentially hydrolyzes nucleoside triphosphates over nucleoside diphosphates and thus can act either in inactivating or in producing P2 receptor ligands. Our results suggest that NTPDase2 plays an important role in the control of nucleotide-mediated activation of peripheral neurons or glia and in the dialogue between these two cell types.

Proteomic mapping provides powerful insights into functional myelin biology

Myelin is a dynamic, functionally active membrane necessary for rapid action potential conduction, axon survival, and cytoarchitecture. The number of debilitating neurological disorders that occur when myelin is disrupted emphasizes its importance. Using high-resolution 2D gel electrophoresis, mass spectrometry, and immunoblotting, we have developed an extensive proteomic map of proteins present in myelin, identifying 98 proteins corresponding to at least 130 of the approximately 200 spots on the map. This proteomic map has been applied to analyses of the localization and function of selected proteins, providing a powerful tool to investigate the diverse functions of myelin.

Involvement of cyclooxygenase-1 and -2 in the sciatic nerve of rats with experimental autoimmune neuritis

The expression of both cyclooxygenase (COX)-1 and COX-2, which are representative enzymes in prostaglandin synthesis, was evaluated in the sciatic nerve of rats with experimental autoimmune neuritis (EAN). Western blot analysis showed that both COX-1 and COX-2 were significantly increased in the sciatic nerve at the peak stage of EAN and declined during the recovery stage. Vascular endothelial cells in normal sciatic nerves immunostained for both COX-1 and COX-2. COX-1 was mainly detected in macrophages, and not in other cell types, while COX-2 was detected in Schwann cells and axons as well as inflammatory macrophages in EAN lesions. This suggests that COXs are involved in the pathogenesis of peripheral demyelinating disease, including EAN, and the major cellular source of both COXs in EAN lesions is inflammatory macrophages. Furthermore, COX-2 is enhanced in some Schwann cells and neural elements, possibly mediating peripheral nervous system inflammation.

Transgenic mice overexpressing aldose reductase in Schwann cells show more severe nerve conduction velocity deficit and oxidative stress under hyperglycemic stress

To further understand the role of aldose reductase (AR) in the etiology of diabetic neuropathy, we generated transgenic mice that overexpress AR specifically in the Schwann cells under the control of the rat myelin protein zero (P0) promoter. One of the transgenic mouse lines, which has overexpression of AR mRNA in the Schwann cell only and higher AR activity in the sciatic nerve, was used to examine the relationship between increased AR activity and motor nerve conduction velocity (MNCV) deficit under diabetic and galactosemic conditions. Under these conditions, nontransgenic mice showed a slight reduction in MNCV compared to those of controls. However, transgenic mice exhibited a significantly greater reduction in MNCV under these conditions, particularly under galactosemic condition, indicating that a Schwann cell-specific increase in aldose reductase activity is sufficient to produce the phenotype. Interestingly, under galactosemic condition where the difference in MNCV deficit between transgenic and nontransgenic mice was most pronounced, there was no significant difference in accumulated galactitol levels in the sciatic nerve between these mice. These results indicate that increase in AR activity leads to greater reduction of MNCV under galactosemic and diabetic conditions, but galactitol and sorbitol levels may not be good indicators of the severity of neuropathy. On the other hand, the level of reduced glutathione (GSH) in the sciatic nerve was found to be correlated with the severity of MNCV deficit under the diabetic condition. Diabetic AR transgenic mice showed significant reduction of GSH in their sciatic nerve, whereas the diabetic nontransgenic mice showed no reduction in GSH level compared to the nondiabetic control, suggesting that AR is a key contributor to oxidative stress under diabetic condition.

Overexpression of beta-1,4-galactosyltransferase I in rat Schwann cells promotes the growth of co-cultured dorsal root ganglia

The cell surface beta-1,4-galactosyltransferase I (beta-1,4-GalT-I) functions as one of the receptors of laminin during the neurite outgrowth on basal lamina by binding to N-linked oligosaccharides in the laminin E8 domain. In this study, we demonstrated that the purified rat Schwann cells transfected with the expression plasmid of beta-1,4-GalT-I cDNA transiently promoted outgrowth and elongation of the neurites from co-cultured rat dorsal root ganglia, while those transfected with the antisense expression plasmid of beta-1,4-GalT-I had the opposite effects. These results suggested that the expression of beta-1,4-GalT-I in Schwann cells of peripheral nerve might promote both growth of developmental neuron and regeneration of injured nerve.

Metalloproteinase-dependent predegeneration in vitro enhances axonal regeneration within acellular peripheral nerve grafts

Injury to peripheral nerve initiates a degenerative process that converts the denervated nerve from a suppressive environment to one that promotes axonal regeneration. We investigated the role of matrix metalloproteinases (MMPs) in this degenerative process and whether effective predegenerated nerve grafts could be produced in vitro. Rat peripheral nerve explants were cultured for 1-7 d in various media, and their neurite-promoting activity was assessed by cryoculture assay, in which neurons are grown directly on nerve sections. The neurite-promoting activity of cultured nerves increased rapidly and, compared with uncultured nerve, a maximum increase of 72% resulted by 2 d of culture in the presence of serum. Remarkably, the neurite-promoting activity of short-term cultured nerves was also significantly better than nerves degenerated in vivo. We examined whether in vitro degeneration is MMP dependent and found that the MMP inhibitor N-[(2R)-2(hydroxamidocarbonylmethyl)-4-methylpantanoyl]-l-tryptophan methylamide primarily blocked the degenerative increase in neurite-promoting activity. In the absence of hematogenic macrophages, MMP-9 was trivial, whereas elevated MMP-2 expression and activation paralleled the increase in neurite-promoting activity. MMP-2 immunoreactivity localized to Schwann cells and the endoneurium and colocalized with gelatinolytic activity as demonstrated by in situ zymography. Finally, in vitro predegenerated nerves were tested as acellular grafts and, compared with normal acellular nerve grafts, axonal ingress in vivo was approximately doubled. We conclude that Schwann cell expression of MMP-2 plays a principal role in the degenerative process that enhances the regeneration-promoting properties of denervated nerve. Combined with their low immunogenicity, acellular nerve grafts activated by in vitro predegeneration may be a significant advancement for clinical nerve allografting.

Hyperglycemia triggers abnormal signaling and proliferative responses in Schwann cells

Peripheral neuropathy is a serious diabetic complication. Delayed nerve regeneration in diabetic animal models suggests abnormalities in proliferation/differentiation of Schwann cells (SC). We recently reported that endothelins (ETs) regulate proliferation and phenotype in primary and immortalized SC (iSC). We now investigated changes in the effects of ETs on SC proliferation and signaling in nerve segments from streptozotocin-induced diabetic rats and in iSC exposed to high glucose. Cultured explants from diabetic rats displayed a delay in the time-course of [3H]-thymidine incorporation as well as enhanced sensitivity to endothelin-1 (ET-1) or insulin. iSC cultured in high (25 mM) glucose-containing media also exhibited higher [3H]-thymidine incorporation, along with an enhanced activation of p38 mitogen-activated protein kinase and phospholipase C in response to ET-1 or platelet-derived growth factor as compared to controls (5.5 mM glucose). These studies support an extra-vascular role of ETs in peripheral nerves and SC. The increased sensitivity to ET-1 in nerves and iSC exposed to high glucose may contribute to abnormal SC proliferation characterizing diabetic neuropathy.

Increased expression of phospholipase D1 in the sciatic nerve of rats with experimental autoimmune neuritis

Phospholipase D1 (PLD1) expression in the sciatic nerve was studied in induced experimental autoimmune neuritis (EAN) in Lewis rats. PLD1 immunoreactivity was seen in some Schwann cells in the sciatic nerves of normal rats. In parallel with the progression of EAN, PLD1-positive Schwann cells significantly increased in number and showed intense immunoreactivity. PLD1 was also detected in some ED1+ macrophages in EAN lesions. These results suggest that PLD1 in macrophages and Schwann cells plays an important role in the activation of these cells in the pathogenesis of EAN, an animal model of human peripheral demyelinating disease.

Expression of beta-1,4-galactosyltransferase II and V in rat injured sciatic nerves

Beta-1,4-Galactosyltransferases II and V (beta-1,4-GalT II and V) are involved in the biosynthesis of N-linked oligosaccharides as beta-1,4-GalT I which plays important roles in promoting neuron outgrowth. In the present paper, it was illustrated that beta-1,4-GalT II and V were localized mainly in Schwann cells of lesion sciatic nerves by in situ hybridization. Northern blot showed that the expression of beta-1,4-GalT II increased gradually at both stumps of injured nerves, while that of beta-1,4-GalT V decreased at proximal stumps but increased and reached its peak on the third day post-operation at distal stumps, before it declined. The different expression of beta-1,4-GalT II and V in Schwann cells suggested that they would affect the different galactosylation of glycoproteins in injured nerves regeneration.