Regulation of the myelin gene periaxin provides evidence for Krox-20-independent myelin-related signalling in Schwann cells

Anti-LGI4 Antibody Is a Novel Juxtaparanodal Autoantibody for Chronic Inflammatory Demyelinating Polyneuropathy

BACKGROUND AND OBJECTIVES

The objective of this study was to discover novel nodal autoantibodies in chronic inflammatory demyelinating polyneuropathy (CIDP).

METHODS

We screened for autoantibodies that bind to mouse sciatic nerves and dorsal root ganglia (DRG) using indirect immunofluorescence (IFA) assays with sera from 113 patients with CIDP seronegative for anti-neurofascin 155 and anticontactin-1 antibodies and 127 controls. Western blotting, IFA assays using HEK293T cells transfected with relevant antigen expression plasmids, and cell-based RNA interference assays were used to identify target antigens. Krox20 and Periaxin expression, both of which independently control peripheral nerve myelination, was assessed by quantitative real-time PCR after application of patient and control sera to Schwann cells.

RESULTS

Sera from 4 patients with CIDP, but not control sera, selectively bound to the nodal regions of sciatic nerves and DRG satellite glia (p = 0.048). The main immunoglobulin G (IgG) subtype was IgG4. IgG from these 4 patients stained a 60-kDa band on Western blots of mouse DRG and sciatic nerve lysates. These features indicated leucine-rich repeat LGI family member 4 (LGI4) as a candidate antigen. A commercial anti-LGI4 antibody and IgG from all 4 seropositive patients with CIDP showed the same immunostaining patterns of DRG and cultured rat Schwann cells and bound to the 60-kDa protein in Western blots of LGI4 overexpression lysates. IgG from 3 seropositive patients, but none from controls, bound to cells cotransfected with plasmids containing LGI4 and a disintegrin and metalloprotease domain-containing protein 22 (ADAM22), an LGI4 receptor. In cultured rat Schwann and human melanoma cells constitutively expressing LGI4, LGI4 siRNA effectively downregulated LGI4 and reduced patients' IgG binding compared with scrambled siRNA. Application of serum from a positive patient to Schwann cells expressing ADAM22 significantly reduced the expression of Krox20, but not Periaxin. Anti-LGI4 antibody-positive patients had a relatively old age at onset (mean age 58 years), motor weakness, deep and superficial sensory impairment with Romberg sign, and extremely high levels of CSF protein. Three patients showed subacute CIDP onset resembling Guillain-Barré syndrome.

DISCUSSION

IgG4 anti-LGI4 antibodies are found in some elderly patients with CIDP who present subacute sensory impairment and motor weakness and are worth measuring, particularly in patients with symptoms resembling Guillain-Barré syndrome.

Schwann Cell Cultures: Biology, Technology and Therapeutics

Schwann cell (SC) cultures from experimental animals and human donors can be prepared using nearly any type of nerve at any stage of maturation to render stage- and patient-specific populations. Methods to isolate, purify, expand in number, and differentiate SCs from adult, postnatal and embryonic sources are efficient and reproducible as these have resulted from accumulated refinements introduced over many decades of work. Albeit some exceptions, SCs can be passaged extensively while maintaining their normal proliferation and differentiation controls. Due to their lineage commitment and strong resistance to tumorigenic transformation, SCs are safe for use in therapeutic approaches in the peripheral and central nervous systems. This review summarizes the evolution of work that led to the robust technologies used today in SC culturing along with the main features of the primary and expanded SCs that make them irreplaceable models to understand SC biology in health and disease. Traditional and emerging approaches in SC culture are discussed in light of their prospective applications. Lastly, some basic assumptions in vitro SC models are identified in an attempt to uncover the combined value of old and new trends in culture protocols and the cellular products that are derived.

Sox2 expression in Schwann cells inhibits myelination in vivo and induces influx of macrophages to the nerve

Correct myelination is crucial for the function of the peripheral nervous system. Both positive and negative regulators within the axon and Schwann cell function to ensure the correct onset and progression of myelination during both development and following peripheral nerve injury and repair. The Sox2 transcription factor is well known for its roles in the development and maintenance of progenitor and stem cell populations, but has also been proposed in vitro as a negative regulator of myelination in Schwann cells. We wished to test fully whether Sox2 regulates myelination in vivo and show here that, in mice, sustained Sox2 expression in vivo blocks myelination in the peripheral nerves and maintains Schwann cells in a proliferative non-differentiated state, which is also associated with increased inflammation within the nerve. The plasticity of Schwann cells allows them to re-myelinate regenerated axons following injury and we show that re-myelination is also blocked by Sox2 expression in Schwann cells. These findings identify Sox2 as a physiological regulator of Schwann cell myelination in vivo and its potential to play a role in disorders of myelination in the peripheral nervous system.

Merlin controls the repair capacity of Schwann cells after injury by regulating Hippo/YAP activity

The regenerative capacity of Schwann cells in the PNS underlies functional repair after injury. In this study, Mindos et al. show a new function for the tumor suppressor Merlin and Hippo/YAP signaling in the generation of repair-competent Schwann cells after injury.

Loss of the Merlin tumor suppressor and activation of the Hippo signaling pathway play major roles in the control of cell proliferation and tumorigenesis. We have identified completely novel roles for Merlin and the Hippo pathway effector Yes-associated protein (YAP) in the control of Schwann cell (SC) plasticity and peripheral nerve repair after injury. Injury to the peripheral nervous system (PNS) causes a dramatic shift in SC molecular phenotype and the generation of repair-competent SCs, which direct functional repair. We find that loss of Merlin in these cells causes a catastrophic failure of axonal regeneration and remyelination in the PNS. This effect is mediated by activation of YAP expression in Merlin-null SCs, and loss of YAP restores axonal regrowth and functional repair. This work identifies new mechanisms that control the regenerative potential of SCs and gives new insight into understanding the correct control of functional nerve repair in the PNS.

The role of p38alpha in Schwann cells in regulating peripheral nerve myelination and repair

Myelination in the peripheral nervous system (PNS) is controlled by both positive and negative regulators within Schwann cells to ensure timely onset and correct myelin thickness for saltatory conduction by neurons. Transcription factors such as Sox10, octamer-binding transcription factor 6 (Oct6) and Krox20 form a positive regulatory network, whereas negative regulators such as cJun and Sox2 oppose myelination in Schwann cells. The role of the p38 MAPK pathway has been studied in PNS myelination, but its precise function remains unclear, with both positive and negative effects of p38 activity reported upon both myelination and processes of nerve repair. To clarify the role of p38 MAPK in the PNS, we have analysed mice with a Schwann cell-specific ablation of the major p38 isoform, p38alpha. In line with previous findings of an inhibitory role for p38 MAPK, we observe acceleration of post-natal myelination in p38alpha null nerves, a delay in myelin down-regulation following injury, together with a small increase in levels of re-myelination following injury. Finally we explored roles for p38alpha in controlling axonal regeneration and functional repair following PNS injury and observe that loss of p38alpha function in Schwann cells does not appear to affect these processes as previously reported. These studies therefore provide further proof for a role of p38 MAPK signalling in the control of myelination by Schwann cells in the PNS, but do not show an apparent role for signalling by this MAP kinase in Schwann cells controlling other elements of Wallerian degeneration and functional repair following injury. Cover Image for this issue: doi: 10.1111/jnc.13793.

Microprocessor complex subunit DiGeorge syndrome critical region gene 8 (Dgcr8) is required for schwann cell myelination and myelin maintenance

We investigated the role of a key component of the Microprocessor complex, DGCR8, in the regulation of myelin formation and maintenance. We found that conditionally ablating Dgcr8 in Schwann cells (SCs) during development results in an arrest of SC differentiation. Dgcr8 conditional knock-out (cKO) SCs fail to form 1:1 relationships with axons or, having achieved this, fail to form myelin sheaths. The expression of genes normally found in immature SCs, such as sex-determining region Y-box 2 (Sox2), is increased in Dgcr8 cKO SCs, whereas the expression of myelin-related genes, including the master regulatory transcription factor early growth response 2 (Egr2), is decreased. Additionally, expression of a novel gene expression program involving sonic hedgehog (Shh), activated de novo in injured nerves, is elevated in Dgcr8 cKOs but not in Egr2 null mice, a model of SC differentiation arrest, suggesting that the injury-related gene expression program in Dgcr8 cKOs cannot be attributed to differentiation arrest. Inducible ablation of Dgcr8 in adult SCs results in gene expression changes similar to those found in cKOs, including an increase in the expression of Sox2 and Shh. Analyses of these nerves mainly reveal normal myelin thickness and axon size distribution but some dedifferentiated SCs and increased macrophage infiltration. Together our data suggest that Dgcr8 is responsible for modulation of gene expression programs underlying myelin formation and maintenance as well as suppression of an injury-related gene expression program.

New insights on Schwann cell development

In the peripheral nervous system, Schwann cells are glial cells that are in intimate contact with axons throughout development. Schwann cells generate the insulating myelin sheath and provide vital trophic support to the neurons that they ensheathe. Schwann cell precursors arise from neural crest progenitor cells, and a highly ordered developmental sequence controls the progression of these cells to become mature myelinating or nonmyelinating Schwann cells. Here, we discuss both seminal discoveries and recent advances in our understanding of the molecular mechanisms that drive Schwann cell development and myelination with a focus on cell-cell and cell-matrix signaling events.

FAK is required for Schwann cell spreading on immature basal lamina to coordinate the radial sorting of peripheral axons with myelination

Without Focal Adhesion Kinase (FAK), developing murine Schwann cells (SCs) proliferate poorly, sort axons inefficiently, and cannot myelinate peripheral nerves. Here we show that FAK is required for the development of SCs when their basal lamina (BL) is fragmentary, but not when it is mature in vivo. Mutant SCs fail to spread on fragmentary BL during development in vivo, and this is phenocopied by SCs lacking functional FAK on low laminin (LN) in vitro. Furthermore, SCs without functional FAK initiate differentiation prematurely, both in vivo and in vitro. In contrast to their behavior on high levels of LN, SCs lacking functional FAK grown on low LN display reduced spreading, proliferation, and indicators of contractility (i.e., stress fibers, arcs, and focal adhesions) and are primed to differentiate. Growth of SCs lacking functional FAK on increasing LN concentrations in vitro revealed that differentiation is not regulated by G1 arrest but rather by cell spreading and the level of contractile actomyosin. The importance of FAK as a critical regulator of the specific response of developing SCs to fragmentary BL was supported by the ability of adult FAK mutant SCs to remyelinate demyelinated adult nerves on mature BL in vivo. We conclude that FAK promotes the spreading and actomyosin contractility of immature SCs on fragmentary BL, thus maintaining their proliferation, and preventing differentiation until they reach high density, thereby promoting radial sorting. Hence, FAK has a critical role in the response of SCs to limiting BL by promoting proliferation and preventing premature SC differentiation.

MAL overexpression leads to disturbed expression of genes that influence cytoskeletal organization and differentiation of Schwann cells

In the developing peripheral nervous system, a coordinated reciprocal signaling between Schwann cells and axons is crucial for accurate myelination. The myelin and lymphocyte protein MAL is a component of lipid rafts that is important for targeting proteins and lipids to distinct domains. MAL overexpression impedes peripheral myelinogenesis, which is evident by a delayed onset of myelination and reduced expression of the myelin protein zero (Mpz/P0) and the low-affinity neurotrophin receptor p75(NTR). This study shows that MAL overexpression leads to a significant reduction of Mpz and p75(NTR) expression in primary mouse Schwann cell cultures, which was already evident before differentiation, implicating an effect of MAL in early Schwann cell development. Their transcription was robustly reduced, despite normal expression of essential transcription factors and receptors. Further, the cAMP response element-binding protein (CREB) and phosphoinositide 3-kinase signaling pathways important for Schwann cell differentiation were correctly induced, highlighting that other so far unknown rate limiting factors do exist. We identified novel genes expressed by Schwann cells in a MAL-dependent manner in vivo and in vitro. A number of those, including S100a4, RhoU and Krt23, are implicated in cytoskeletal organization and plasma membrane dynamics. We showed that S100a4 is predominantly expressed by nonmyelinating Schwann cells, whereas RhoU was localized within myelin membranes, and Krt23 was detected in nonmyelinating as well as in myelinating Schwann cells. Their differential expression during early peripheral nerve development further underlines their possible role in influencing Schwann cell differentiation and myelination.

Transcriptional regulation induced by cAMP elevation in mouse Schwann cells

In peripheral nerves, Schwann cell development is regulated by a variety of signals. Some of the aspects of Schwann cell differentiation can be reproduced in vitro in response to forskolin, an adenylyl cyclase activator elevating intracellular cAMP levels. Herein, the effect of forskolin treatment was investigated by a comprehensive genome-wide expression study on primary mouse Schwann cell cultures. Additional to myelin-related genes, many so far unconsidered genes were ascertained to be modulated by forskolin. One of the strongest differentially regulated gene transcripts was the transcription factor Olig1 (oligodendrocyte transcription factor 1), whose mRNA expression levels were reduced in treated Schwann cells. Olig1 protein was localized in myelinating and nonmyelinating Schwann cells within the sciatic nerve as well as in primary Schwann cells, proposing it as a novel transcription factor of the Schwann cell lineage. Data analysis further revealed that a number of differentially expressed genes in forskolin-treated Schwann cells were associated with the ECM (extracellular matrix), underlining its importance during Schwann cell differentiation in vitro. Comparison of samples derived from postnatal sciatic nerves and from both treated and untreated Schwann cell cultures showed considerable differences in gene expression between in vivo and in vitro, allowing us to separate Schwann cell autonomous from tissue-related changes. The whole data set of the cell culture microarray study is provided to offer an interactive search tool for genes of interest.

Opposing roles of PKA and EPAC in the cAMP-dependent regulation of schwann cell proliferation and differentiation [corrected]

In Schwann cells (SCs), cyclic adenosine monophosphate (cAMP) not only induces differentiation into a myelinating SC-related phenotype, but also synergistically enhances the mitogenic action of growth factors such as neuregulin. To better understand the molecular mechanism by which cAMP exerts these apparently contradictory functions, we investigated the role of the two main effectors of cAMP, protein kinase A (PKA) and the exchange protein activated by cAMP (EPAC), on the proliferation and differentiation of both isolated and axon-related SCs. For these studies, a variety of PKA and EPAC agonists and antagonists were used, including pathway-selective analogs of cAMP and pharmacological inhibitors. Our studies indicated that the activity of PKA rather than EPAC was required for the adjuvant effect of cAMP on S-phase entry, whereas the activity of EPAC rather than PKA was required for SC differentiation and myelin formation. Even though selective EPAC activation had an overall anti-proliferative effect in SCs, it failed to drive the expression of Krox-20, a master regulator of myelination, and that of myelin-specific proteins and lipids, suggesting that EPAC activation was insufficient to drive a full differentiating response. Interestingly, inhibition of EPAC activity resulted in a drastic impairment of SC differentiation and myelin formation but not Krox-20 expression, which indicates an independent mechanism of Krox-20 regulation in response to cAMP. In conclusion, our data supports the idea that the outcome of cAMP signaling in SCs depends on the particular set of effectors activated. Whereas the mitogenic action of cAMP relies exclusively on PKA activity, the differentiating action of cAMP requires a PKA-independent (non-canonical) cAMP-specific pathway that is partially transduced by EPAC.

Loss of SOX10 function contributes to the phenotype of human Merlin-null schwannoma cells

Loss of the Merlin tumour suppressor causes abnormal de-differentiation and proliferation of Schwann cells and formation of schwannoma tumours in patients with neurofibromatosis type 2. Within the mature peripheral nerve the normal development, differentiation and maintenance of myelinating and non-myelinating Schwann cells is regulated by a network of transcription factors that include SOX10, OCT6 (now known as POU3F1), NFATC4 and KROX20 (also known as Egr2). We have examined for the first time how their regulation of Schwann cell development is disrupted in primary human schwannoma cells. We find that induction of both KROX20 and OCT6 is impaired, whereas enforced expression of KROX20 drives both myelin gene expression and cell cycle arrest in Merlin-null cells. Importantly, we show that human schwannoma cells have reduced expression of SOX10 protein and messenger RNA. Analysis of mouse SOX10-null Schwann cells shows they display many of the characteristics of human schwannoma cells, including increased expression of platelet derived growth factor receptor beta (PDGFRB) messenger RNA and protein, enhanced proliferation, increased focal adhesions and schwannoma-like morphology. Correspondingly, reintroduction of SOX10 into human Merlin-null cells restores the ability of these cells to induce KROX20 and myelin protein zero (MPZ), localizes NFATC4 to the nucleus, reduces cell proliferation and suppresses PDGFRB expression. Thus, we propose that loss of the SOX10 protein, which is vital for normal Schwann cell development, is also key to the pathology of Merlin-null schwannoma tumours.

Recessively transmitted predominantly motor neuropathies

Recessively transmitted predominantly motor neuropathies are rare and show a severe phenotype. They are frequently observed in populations with a high rate of consanguineous marriages. At least 15 genes and six loci have been found to be associated with autosomal recessive CMT (AR-CMT) and X-linked CMT (AR-CMTX) and also distal hereditary motor neuronopathy (AR-dHMN). These disorders are genetically heterogeneous but the clinical phenotype is relatively homogeneous. Distal muscle weakness and atrophy predominating in the lower extremities, diminished or absent deep tendon reflexes, distal sensory loss, and pes cavus are the main clinical features of this disorder with occasional cranial nerve involvement. Although genetic diagnosis of some of subtypes of AR-CMT are now available, rapid advances in the molecular genetics and cell biology show a great complexity. Animal models for the most common subtypes of human AR-CMT disease provide clues for understanding the pathogenesis of CMT and also help to reveal possible treatment strategies of inherited neuropathies. This chapter highlights the clinical features and the recent genetic and biological findings in these disorders based on the current classification.

Calcineurin-nuclear factor of activated T cells regulation of Krox-20 expression in Schwann cells requires elevation of intracellular cyclic AMP

The transcription factor Krox-20 (Egr2) is a master regulator of Schwann cell myelination. In mice from which calcineurin B had been excised in cells of the neural crest lineage, calcineurin-nuclear factor of activated T cells (NFAT) signaling was required for neuregulin-related Schwann cell myelination (Kao et al. [2009] Immunity 12:359-372). Whether NFAT signaling required simultaneous elevation of intracellular cAMP levels was not explored. In vivo, Krox-20 expression requires continuous axon-Schwann cell signaling that in Schwann cell cultures can be mimicked by elevation of intracellular cAMP. We have investigated the role of the calcineurin-NFAT pathway in Krox-20 induction in purified rat Schwann cell cultures. Activation of this pathway requires elevation of intracellular Ca(2+) levels. The calcium ionophore A23187 or ionomycin was used to increase intracellular Ca(2+) levels in Schwann cell cultures that had been treated with dibutyryl cAMP to induce Krox-20. Increase in Ca(2+) levels significantly potentiated Krox-20 induction, determined by Krox-20 immunolabeling of individual cells and Western blotting. Levels of the myelin proteins periaxin and P(0) were also elevated. The potentiating effect was blocked by cyclosporin A, a specific blocker of the calcineurin-NFAT pathway. We found that, in the absence of cAMP elevation, treatment with A23187 alone failed to induce Krox-20 expression, indicating that NFAT upregulation of Krox-20 requires elevation of cAMP levels in Schwann cells. P-VIVIT, another specific inhibitor of calcineurin-NFAT interaction, blocked Krox-20 induction in response to dibutyryl cAMP and ionophore. HA-NFAT1 (1-460)-GFP translocated to the nucleus on treatment with dibutyryl cAMP with or without added ionophore. NFAT isoforms 1-4 were detected in purified Schwann cells by quantitative RT-PCR.

Epineurial adipocytes are dispensable for Schwann cell myelination

Previous clinical observations and data from mouse models with defects in lipid metabolism suggested that epineurial adipocytes may play a role in peripheral nervous system myelination. We have used adipocyte-specific Lpin1 knockout mice to characterize the consequences of the presence of impaired epineurial adipocytes on the myelinating peripheral nerve. Our data revealed that the capacity of Schwann cells to establish myelin, and the functional properties of peripheral nerves, were not affected by compromised epineurial adipocytes in adipocyte-specific Lpin1 knockout mice. To evaluate the possibility that Lpin1-negative adipocytes are still able to support endoneurial Schwann cells, we also characterized sciatic nerves from mice carrying epiblast-specific deletion of peroxisome proliferator-activated receptor gamma, which develop general lipoatrophy. Interestingly, even the complete loss of adipocytes in the epineurium of peroxisome proliferator-activated receptor gamma knockout mice did not lead to detectable defects in Schwann cell myelination. However, probably as a consequence of their hyperglycemia, these mice have reduced nerve conduction velocity, thus mimicking the phenotype observed under diabetic condition. Together, our data indicate that while adipocytes, as regulators of lipid and glucose homeostasis, play a role in nerve function, their presence in epineurium is not essential for establishment or maintenance of proper myelin.

Regulation of Schwann cell differentiation and proliferation by the Pax-3 transcription factor

Pax-3 is a paired domain transcription factor that plays many roles during vertebrate development. In the Schwann cell lineage, Pax-3 is expressed at an early stage in Schwann cells precursors of the embryonic nerve, is maintained in the nonmyelinating cells of the adult nerve, and is upregulated in Schwann cells after peripheral nerve injury. Consistent with this expression pattern, Pax-3 has previously been shown to play a role in repressing the expression of the myelin basic protein gene in Schwann cells. We have studied the role of Pax-3 in Schwann cells and have found that it controls not only the regulation of cell differentiation but also the survival and proliferation of Schwann cells. Pax-3 expression blocks both the induction of Oct-6 and Krox-20 (K20) by cyclic AMP and completely inhibits the ability of K20, the physiological regulator of myelination in the peripheral nervous system, to induce myelin gene expression in Schwann cells. In contrast to other inhibitors of myelination, we find that Pax-3 represses myelin gene expression in a c-Jun-independent manner. In addition to this, we find that Pax-3 expression alone is sufficient to inhibit the induction of apoptosis by TGFβ1 in Schwann cells. Expression of Pax-3 is also sufficient to induce the proliferation of Schwann cells in the absence of added growth factors and to reverse K20-induced exit from the cell cycle. These findings indicate new roles for the Pax-3 transcription factor in controlling the differentiation and proliferation of Schwann cells during development and after peripheral nerve injury.

Mouse schwann cells need both NRG1 and cyclic AMP to myelinate

Genetically modified mice have been a major source of information about the molecular control of Schwann-cell myelin formation, and the role of β-neuregulin 1 (NRG1) in this process in vivo. In vitro, on the other hand, Schwann cells from rats have been used in most analyses of the signaling pathways involved in myelination. To correlate more effectively in vivo and in vitro data, we used purified cultures of mouse Schwann cells in addition to rat Schwann cells to examine two important myelin-related signals, cyclic adenosine monophosphate (cAMP), and NRG1 and to determine whether they interact to control myelin differentiation. We find that in mouse Schwann cells, neither cAMP nor NRG1, when used separately, induced markers of myelin differentiation. When combined, however, they induced strong protein expression of the myelin markers, Krox-20 and P(0) . Importantly, the level of cAMP signaling was crucial in switching NRG1 from a proliferative signal to a myelin differentiation signal. Also in cultured rat Schwann cells, NRG1 promoted cAMP-induced Krox-20 and P(0) expression. Finally, we found that cAMP/NRG1-induced Schwann-cell differentiation required the activity of the cAMP response element binding family of transcription factors in both mouse and rat cells. These observations reconcile observations in vivo and on neuron-Schwann-cell cultures with studies on purified Schwann cells. They demonstrate unambiguously the promyelin effects of NRG1 in purified cells, and they show that the cAMP pathway determines whether NRG1 drives proliferation or induces myelin differentiation.

Sodium-dependent vitamin C transporter 2 (SVCT2) is necessary for the uptake of L-ascorbic acid into Schwann cells

Ascorbic acid has been shown to be an essential component for in vitro myelination and to improve the clinical and pathological phenotype of a mouse model of Charcot-Marie-tooth disease 1A. The mechanism of ascorbic acid uptake into peripheral nerves, however, has not been addressed so far. Hence, we studied the expression and activity of sodium-dependent vitamin C transporters 1 and 2 (SVCT1 and 2) in the peripheral nervous system. Using immunohistochemistry, immunoblotting, and reverse transcription PCR, we could show that SVCT1 and 2 were differentially expressed in myelinated peripheral nerve fibers and Schwann cell (SC) cultures. SVCT1 was expressed at very low levels confined to the axons, whereas SVCT2 was highly expressed both in the axons and in the SCs. SVCT2 was localized particularly in SC compartments of uncompacted myelin. Uptake assays using (14)C-labeled ascorbic acid showed transport of ascorbic acid into SC cultures. Ascorbic acid transport was dependent on the concentration of sodium, magnesium, and calcium in the extracellular medium. Treatment with the flavonoid phloretin, a known inhibitor of SVCT1 and 2, and specific RNA interference with SVCT2 caused significant reductions in ascorbic acid uptake into SCs. Phloretin-inhibited uptake of ascorbic acid was further shown in freshly dissected, cell-culture-naïve rat sciatic nerves. These results provide evidence for the first time that uptake of ascorbic acid in the peripheral nervous system is crucially dependent on the expression and activity of SVCT2.

Notch controls embryonic Schwann cell differentiation, postnatal myelination and adult plasticity

Notch signaling is central to vertebrate development, and analysis of Notch has provided important insights into pathogenetic mechanisms in the CNS and many other tissues. However, surprisingly little is known about the role of Notch in the development and pathology of Schwann cells and peripheral nerves. Using transgenic mice and cell cultures, we found that Notch has complex and extensive regulatory functions in Schwann cells. Notch promoted the generation of Schwann cells from Schwann cell precursors and regulated the size of the Schwann cell pool by controlling proliferation. Notch inhibited myelination, establishing that myelination is subject to negative transcriptional regulation that opposes forward drives such as Krox20. Notably, in the adult, Notch dysregulation resulted in demyelination; this finding identifies a signaling pathway that induces myelin breakdown in vivo. These findings are relevant for understanding the molecular mechanisms that control Schwann cell plasticity and underlie nerve pathology, including demyelinating neuropathies and tumorigenesis.

The molecular machinery of myelin gene transcription in Schwann cells

During late fetal life, Schwann cells in the peripheral nerves singled out by the larger axons will transit through a promyelinating stage before exiting the cell cycle and initiating myelin formation. A network of extra- and intracellular signaling pathways, regulating a transcriptional program of cell differentiation, governs this progression of cellular changes, culminating in a highly differentiated cell. In this review, we focus on the roles of a number of transcription factors not only in myelination, during normal development, but also in demyelination, following nerve trauma. These factors include specification factors involved in early development of Schwann cells from neural crest (Sox10) as well as factors specifically required for transitions into the promyelinating and myelinating stages (Oct6/Scip and Krox20/Egr2). From this description, we can glean the first, still very incomplete, contours of a gene regulatory network that governs myelination and demyelination during development and regeneration.

Active gene repression by the Egr2.NAB complex during peripheral nerve myelination

The Egr2/Krox20 transactivator is required for activation of many myelin-associated genes during peripheral nerve myelination by Schwann cells. However, recent work has indicated that Egr2 not only activates genes required for peripheral nerve myelination but may also be involved in gene repression. The NAB (NGFI-A/Egr-binding) corepressors interact with Egr2 and are required for proper coordination of myelin formation. Therefore, NAB proteins could mediate repression of some Egr2 target genes, although direct repression by Egr2 or NAB proteins during myelination has not been demonstrated. To define the physiological role of NAB corepression in gene repression by Egr2, we tested whether the Egr2.NAB complex directly repressed specific target genes. A screen for NAB-regulated genes identified several (including Id2, Id4, and Rad) that declined during the course of peripheral nerve myelination. In vivo chromatin immunoprecipitation analysis of the myelinating sciatic nerve was used to show developmental association of both Egr2 and NAB2 on the Id2, Id4, and Rad promoters as they were repressed during the myelination process. In addition, NAB2 represses transcription by interaction with the chromodomain helicase DNA-binding protein 4 (CHD4) subunit of the nucleosome remodeling and deacetylase chromatin remodeling complex, and we demonstrate that CHD4 occupies NAB-repressed promoters in a developmentally regulated manner in vivo. These results illustrate a novel aspect of genetic regulation of peripheral nerve myelination by showing that Egr2 directly represses genes during myelination in conjunction with NAB corepressors. Furthermore, repression of Id2 was found to augment activation of Mpz (myelin protein zero) expression.

c-Jun is a negative regulator of myelination

Schwann cell myelination depends on Krox-20/Egr2 and other promyelin transcription factors that are activated by axonal signals and control the generation of myelin-forming cells. Myelin-forming cells remain remarkably plastic and can revert to the immature phenotype, a process which is seen in injured nerves and demyelinating neuropathies. We report that c-Jun is an important regulator of this plasticity. At physiological levels, c-Jun inhibits myelin gene activation by Krox-20 or cyclic adenosine monophosphate. c-Jun also drives myelinating cells back to the immature state in transected nerves in vivo. Enforced c-Jun expression inhibits myelination in cocultures. Furthermore, c-Jun and Krox-20 show a cross-antagonistic functional relationship. c-Jun therefore negatively regulates the myelinating Schwann cell phenotype, representing a signal that functionally stands in opposition to the promyelin transcription factors. Negative regulation of myelination is likely to have significant implications for three areas of Schwann cell biology: the molecular analysis of plasticity, demyelinating pathologies, and the response of peripheral nerves to injury.

Chronic treatment with aripiprazole induces differential gene expression in the rat frontal cortex

Chronic treatment of antipsychotic drugs can modulate gene expression in the brain, which may underscore their clinical efficacy. Aripiprazole is the first approved antipsychotic drug of the class of dopamine D2 receptor partial agonist, which has been shown to have similar efficacy and favourable side-effects profile compared to other antipsychotic drugs. This study aimed to identify differential gene expression induced by chronic treatment of aripiprazole. We used microarray-based gene expression profiling technology, real-time quantitative PCR and Western blot analysis to identify differentially expressed genes in the frontal cortex of rats under 4 wk treatment of aripiprazole (10 mg/kg). We were able to detect ten up-regulated genes, including early growth response gene 1, 2, 4 (Egr1, Egr2, Egr4), chromobox homolog 7 (Cbx7), cannabinoid receptor (Cnr1), catechol-O-methyltransferase (Comt), protein phosphatase 2c, magnesium dependent (Ppm2c), tachykinin receptor 3 (Tacr3), Wiscott-Aldrich syndrome-like gene (Wasl) and DNA methyltransferase 3a (Dnmt3a). Our data indicate that chronic administration of aripiprazole can induce differential expression of genes involved in transcriptional regulation and chromatin remodelling and genes implicated in the pathogenesis of psychosis.

Interactions of Sox10 and Egr2 in myelin gene regulation

Myelination in the PNS is accompanied by a large induction of the myelin protein zero (Mpz) gene to produce the most abundant component in peripheral myelin. Analyses of knockout mice have shown that the EGR2/Krox20 and SOX10 transcription factors are required for Mpz expression. Our recent work has shown that the dominant EGR2 mutations associated with human peripheral neuropathies cause disruption of EGR2/SOX10 synergy at specific sites, including a conserved enhancer element in the first intron of the Mpz gene. Further investigation of Egr2/Sox10 interactions reveals that activation of the Mpz intron element by Egr2 requires both Sox10-binding sites. In addition, both Egr1 and Egr3 cooperate with Sox10 to activate this element, which indicates that this capacity is conserved among Egr family members. Finally, a conserved composite structure of Egr2/Sox10-binding sites in the genes encoding Mpz, myelin-associated glycoprotein and myelin basic protein genes was used to screen for similar modules in other myelin genes, revealing a potential regulatory element in the periaxin gene. Overall, these results elucidate a working model for developmental regulation of Mpz expression, several facets of which extend to regulation of other peripheral myelin genes.

Neuropathy-associated Egr2 mutants disrupt cooperative activation of myelin protein zero by Egr2 and Sox10

Dominant mutations in the early growth response 2 (Egr2/Krox20) transactivator, a critical regulator of peripheral myelin development, have been associated with peripheral myelinopathies. These dominant mutants interfere with the expression of genes required for myelination by Schwann cells, including that for the most abundant peripheral myelin protein, Myelin protein zero (Mpz). In this study, we show that Egr2 mutants specifically affect an Egr2-responsive element within the Mpz first intron that also contains binding sites for the transcription factor Sox10. Furthermore, Egr2 activation through this element is impaired by mutation of the Sox10 binding sites. Using chromatin immunoprecipitation assays, we found that Egr2 and Sox10 bind to this element in myelinating sciatic nerve and that a dominant Egr2 mutant does not perturb Egr2 binding but rather attenuates binding of Sox10 to the Mpz intron element. Sox10 binding at other sites of Egr2/Sox10 synergy, including a novel site in the Myelin-associated glycoprotein (Mag) gene, is also reduced by the dominant Egr2 mutant. These results provide the first demonstration of binding of Egr2/Sox10 to adjacent sites in vivo and also demonstrate that neuropathy-associated Egr2 mutants antagonize binding of Sox10 at specific sites, thereby disrupting genetic control of the myelination program.

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.

In vivo detection of Egr2 binding to target genes during peripheral nerve myelination

Egr2/Krox20 is a zinc finger transactivator that regulates a diverse array of genes required for peripheral nerve myelination. Although several studies have elucidated the Egr2-regulated gene network, it is not clear if Egr2 regulates its target genes directly or indirectly through induction of other transactivators. Moreover, very few Egr2 binding sites have been identified in regulatory elements of myelin genes. To address this issue, we have successfully adapted chromatin immunoprecipitation assays to test if Egr2 binds directly to target genes in myelinating rat sciatic nerve. These experiments demonstrate direct binding of Egr2 to previously described binding sites within the Schwann cell enhancer of the myelin basic protein gene. Furthermore, we show Egr2 binding to a conserved site within the myelin-associated glycoprotein gene. Finally, our experiments provide the first evidence that Egr2 directly regulates expression of desert hedgehog, which is critically involved in development, maintenance and regeneration of multiple nerve elements including myelinated fibers. Surprisingly, this analysis has identified an apparent preponderance of Egr2 binding sites within conserved intron sequences of several myelin genes. Application of chromatin immunoprecipitation analysis to myelination in vivo will prove to be a valuable asset in assaying transcription factor binding and chromatin modifications during activation of myelin genes.

TGFbeta type II receptor signaling controls Schwann cell death and proliferation in developing nerves

During development, Schwann cell numbers are precisely adjusted to match the number of axons. It is essentially unknown which growth factors or receptors carry out this important control in vivo. Here, we tested whether the type II transforming growth factor (TGF) beta receptor has a role in this process. We generated a conditional knock-out mouse in which the type II TGFbeta receptor is specifically ablated only in Schwann cells. Inactivation of the receptor, evident at least from embryonic day 18, resulted in suppressed Schwann cell death in normally developing and injured nerves. Notably, the mutants also showed a strong reduction in Schwann cell proliferation. Consequently, Schwann cell numbers in wild-type and mutant nerves remained similar. Lack of TGFbeta signaling did not appear to affect other processes in which TGFbeta had been implicated previously, including myelination and response of adult nerves to injury. This is the first in vivo evidence for a growth factor receptor involved in promoting Schwann cell division during development and the first genetic evidence for a receptor that controls normal developmental Schwann cell death.

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.

Gene profiling and bioinformatic analysis of Schwann cell embryonic development and myelination

To elucidate the molecular mechanisms involved in Schwann cell development, we profiled gene expression in the developing and injured rat sciatic nerve. The genes that showed significant changes in expression in developing and dedifferentiated nerve were validated with RT-PCR, in situ hybridisation, Western blot and immunofluorescence. A comprehensive approach to annotating micro-array probes and their associated transcripts was performed using Biopendium, a database of sequence and structural annotation. This approach significantly increased the number of genes for which a functional insight could be found. The analysis implicates agrin and two members of the collapsin response-mediated protein (CRMP) family in the switch from precursors to Schwann cells, and synuclein-1 and alphaB-crystallin in peripheral nerve myelination. We also identified a group of genes typically related to chondrogenesis and cartilage/bone development, including type II collagen, that were expressed in a manner similar to that of myelin-associated genes. The comprehensive function annotation also identified, among the genes regulated during nerve development or after nerve injury, proteins belonging to high-interest families, such as cytokines and kinases, and should therefore provide a uniquely valuable resource for future research.

Functional organization of a Schwann cell enhancer

Myelin basic protein (MBP) gene expression is conferred in oligodendrocytes and Schwann cells by different upstream enhancers. In Schwann cells, expression is controlled by a 422 bp enhancer lying -9 kb from the gene. We show here that it contains 22 mammalian conserved motifs > or =6 bp. To investigate their functional significance, different combinations of wild-type or mutated motifs were introduced into reporter constructs that were inserted in single copy at a common hypoxanthine phosphoribosyltransferase docking site in embryonic stem cells. Lines of transgenic mice were derived, and the subsequent qualitative and quantitative expression phenotypes were compared at different stages of maturation. In the enhancer core, seven contiguous motifs cooperate to confer Schwann cell specificity while different combinations of flanking motifs engage, at different stages of Schwann cell maturation, to modulate expression level. Mutation of a Krox-20 binding site reduces the level of reporter expression, whereas mutation of a potential Sox element silences reporter expression. This potential Sox motif was also found conserved in other Schwann cell enhancers, suggesting that it contributes widely to regulatory function. These results demonstrate a close relationship between phylogenetic footprints and regulatory function and suggest a general model of enhancer organization. Finally, this investigation demonstrates that in vivo functional analysis, supported by controlled transgenesis, can be a robust complement to molecular and bioinformatics approaches to regulatory mechanisms.

Direct regulation of myelin protein zero expression by the Egr2 transactivator

During myelination of the peripheral nervous system, the myelin protein zero (Mpz) gene is induced to produce the most abundant protein component (P(0)) of mature myelin. Although the basal embryonic expression of Mpz in Schwann cells has been attributed to regulation by Sox10, the molecular mechanism for the profound up-regulation of this gene during myelination has not been established. In this study, we have identified a highly conserved element within the first intron of the Mpz gene, which contains binding sites for the early growth response 2 (Egr2/Krox20) transcription factor, a critical regulator of peripheral nerve myelination. Egr2 can transactivate the intron element, and the induction is blocked by two known repressors of Egr2 activity. Using chromatin immunoprecipitation assays, we find that Egr2 binds in vivo to the intron element, but not to the Mpz promoter. Known inducers of Mpz expression such as forskolin and insulin-like growth factor-1 also activate the element in an Egr2-dependent manner. In addition, we found that Egr2 can act synergistically with Sox10 to activate this intron element, suggesting a model in which cooperative interactions between Egr2 and Sox10 mediate a large increase in Mpz expression to the high levels found in myelinating Schwann cells.

Gene expression profiling of experimental traumatic spinal cord injury as a function of distance from impact site and injury severity

Changes in gene expression contribute to pathophysiological alterations following spinal cord injury (SCI). We examined gene expression over time (4 h, 24 h, 7 days) at the impact site, as well as rostral and caudal regions, following mild, moderate, or severe contusion SCI in rats. High-density oligonucleotide microarrays were used that included approximately 27,000 genes/ESTs (Affymetrix RG-U34; A, B and C arrays), together with multiple analyses (MAS 5.0, dChip). Alterations after mild injury were relatively rapid (4 and 24 h), whereas they were delayed and prolonged after severe injury (24 h and 7 days). The number and magnitude of gene expression changes were greatest at the injury site after moderate injury and increased in rostral and caudal regions as a function of injury severity. Sham surgery resulted in expression changes that were similar to mild injury, suggesting the importance of using time-linked surgical controls as well as naive animals for these kinds of studies. Expression of many genes and ESTs was altered; these were classified functionally based on ontology. Overall representation of these functional classes varied with distance from the site of injury and injury severity, as did the individual genes that contributed to each functional class. Different clustering approaches were used to identify changes in neuronal-specific genes and several transcription factors that have not previously been associated with SCI. This study represents the most comprehensive evaluation of gene expression changes after SCI to date. The results underscore the power of microarray approaches to reveal global genomic responses as well as changes in particular gene clusters and/or families that may be important in the secondary injury cascade.

Mycobacterium leprae-induced demyelination: a model for early nerve degeneration

The molecular events that occur at the early phase of many demyelinating neurodegenerative diseases are unknown. A recent demonstration of rapid demyelination and axonal injury induced by Mycobacterium leprae provides a model for elucidating the molecular events of early nerve degeneration which might be common to neurodegenerative diseases of both infectious origin and unknown etiology. The identification of the M. leprae-targeted Schwann cell receptor, dystroglycan, and its associated molecules in myelination, demyelination and axonal functions suggests a role for these molecules in early nerve degeneration.

Krox-20 inhibits Jun-NH2-terminal kinase/c-Jun to control Schwann cell proliferation and death

The transcription factor Krox-20 controls Schwann cell myelination. Schwann cells in Krox-20 null mice fail to myelinate, and unlike myelinating Schwann cells, continue to proliferate and are susceptible to death. We find that enforced Krox-20 expression in Schwann cells cell-autonomously inactivates the proliferative response of Schwann cells to the major axonal mitogen β–neuregulin-1 and the death response to TGFβ or serum deprivation. Even in 3T3 fibroblasts, Krox-20 not only blocks proliferation and death but also activates the myelin genes periaxin and protein zero, showing properties in common with master regulatory genes in other cell types. Significantly, a major function of Krox-20 is to suppress the c-Jun NH₂-terminal protein kinase (JNK)–c-Jun pathway, activation of which is required for both proliferation and death. Thus, Krox-20 can coordinately control suppression of mitogenic and death responses. Krox-20 also up-regulates the scaffold protein JNK-interacting protein 1 (JIP-1). We propose this as a possible component of the mechanism by which Krox-20 regulates JNK activity during Schwann cell development.

Cell autonomy of the mouse claw paw mutation

Mice homozygous for the autosomal recessive mutation claw paw (clp) are characterized by limb posture abnormalities and congenital hypomyelination, with delayed onset of myelination of the peripheral nervous system but not the central nervous system. Although this combination of limb and peripheral nerve abnormalities in clp/clp mice might suggest a common neurogenic origin of the syndrome, it is not clear whether the clp gene acts primarily in the neurone, the Schwann cell or both. In the work described here, we address this question of cell autonomy of the clp mutation through reciprocal nerve grafting experiments between wild-type and clp/clp animals. Our results demonstrate that the clp mutation affects the Schwann cell compartment and possibly also the neuronal compartment. These data suggest that the clp gene product is expressed in Schwann cells as well as neurones and is likely to be involved in direct axon--Schwann cell interactions. Within the Schwann cell, clp affects a myelin-related signaling pathway that regulates periaxin and Krox-20 expression, but not Oct-6.

Implication of gamma-secretase in neuregulin-induced maturation of oligodendrocytes

Increasing evidences suggest that, after neuregulin (NRG) stimulation, ErbB4 undergoes a series of proteolysis, including gamma-secretase cleavage. The released ErbB4 intracellular domain (EICD) is translocated into nucleus and has a transcriptional function. Although NRG-ErbB4 signaling mediates maturation of oligodendrocytes (OLs), the role of EICD and gamma-secretase in this process remains elusive. Here, we showed that NRG-ErbB4 interaction accumulated EICD in the nucleus and promoted the expression of myelin basic protein expression in OLs. Conversely, inhibitor of ErbB4 or gamma-secretase blocked the capacity of NRG. Nuclear accumulation of EICD did not influence maturation of neurons and astrocytes and early development of OLs. We also found that EICD translocation accorded a temporal pattern, consistent with the developmental gradient of hippocampus. Our data suggest that gamma-secretase activation and EICD nuclear translocation are required for OL maturation induced by NRG, and ErbB4 acts as a functional receptor depending on a new signaling cascade.