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

The TGR5 Agonist INT-777 Promotes Peripheral Nerve Regeneration by Activating cAMP-dependent Protein Kinase A in Schwann Cells

Schwann cell (SC) myelination is a pivotal event in the normal physiological functioning of the peripheral nervous system (PNS), where myelination is finely controlled by a series of factors within SCs to ensure timely onset and correct myelin thickness for saltatory conduction. Among these, cyclic AMP (cAMP) is a promising factor for driving myelin gene expression in SCs. It has been shown that TGR5 activation is often associated with increased production of cAMP. Therefore, we speculated that the G-protein-coupled receptor (TGR5) might be involved in the PNS myelination. To test this hypothesis, sciatic nerve crush-injured mice were treated with INT-777, a specific agonist of TGR5, which significantly improved remyelination and functional recovery. Furthermore, rats that underwent sciatic nerve transection were treated with INT-777, which also promoted nerve regeneration and functional recovery. In primary SCs, the stimulatory effect of INT-777 on myelin gene expression was largely counteracted by H89, a potent inhibitor of cAMP-dependent protein kinase A (PKA). Additionally, INT-777 stimulated cell migration was blunted in the presence of H89. Overall, these data indicate that INT-777 is capable of promoting peripheral nerve regeneration and functional recovery after injury, and these benefits are likely due to the activation of the TGR5/cAMP/PKA axis. As such, INT-777, together with other TGR5 agonists, may hold great therapeutic potential for treating peripheral nerve injury.

Review: Myelin clearance is critical for regeneration after peripheral nerve injury

Traumatic peripheral nerve injury occurs frequently and is a major clinical and public health problem that can lead to functional impairment and permanent disability. Despite the availability of modern diagnostic procedures and advanced microsurgical techniques, active recovery after peripheral nerve repair is often unsatisfactory. Peripheral nerve regeneration involves several critical events, including the recreation of the microenvironment and remyelination. Results from previous studies suggest that the peripheral nervous system (PNS) has a greater capacity for repair than the central nervous system. Thus, it will be important to understand myelin and myelination specifically in the PNS. This review provides an update on myelin biology and myelination in the PNS and discusses the mechanisms that promote myelin clearance after injury. The roles of Schwann cells and macrophages are considered at length, together with the possibility of exogenous intervention.

Cyclic-AMP Increases Nuclear Actin Monomer Which Promotes Proteasomal Degradation of RelA/p65 Leading to Anti-Inflammatory Effects

The second messenger, cAMP has potent immunosuppressive and anti-inflammatory actions. These have been attributed, in part, to the ability of cAMP-induced signals to interfere with the function of the proinflammatory transcription factor Nuclear Factor-kappa B (NF-κB). However, the mechanisms underlying the modulation of NF-κB activity by cAMP remain unclear. Here we demonstrate an important role for cAMP-mediated increase in nuclear actin monomer levels in inhibiting NF-κB activity. Elevated cAMP or forced expression of a nuclear localised polymerisation defective actin mutant (NLS-Actin_R62D) inhibited basal and TNFα induced mRNA levels of NF-κB-dependent genes and NF-κB-dependent reporter gene activity. Elevated cAMP or NLS-Actin_R62D did not affect NF-κB nuclear translocation but did reduce total cellular and nuclear RelA/p65 levels. Preventing the cAMP-induced increase in nuclear actin monomer, either by expressing a nuclear localised active mutant of the actin polymerising protein mDIA, silencing components of the nuclear actin import complex IPO9 and CFL1 or overexpressing the nuclear export complex XPO6, rescued RelA/p65 levels and NF-κB reporter gene activity in forskolin-stimulated cells. Elevated cAMP or NLS-Actin_R62D reduced the half-life of RelA/p65, which was reversed by the proteasome inhibitor MG132. Accordingly, forskolin stimulated association of RelA/p65 with ubiquitin affinity beads, indicating increased ubiquitination of RelA/p65 or associated proteins. Taken together, our data demonstrate a novel mechanism underlying the anti-inflammatory effects of cAMP and highlight the important role played by nuclear actin in the regulation of inflammation.

Melatonin Induced Schwann Cell Proliferation and Dedifferentiation Through NF-ĸB, FAK-Dependent but Src-Independent Pathways

BACKGROUND

Peripheral nerve injury (PNI) is a common condition that compromises motor and sensory functions. Peripheral nerves are known to have regenerative capability and the pineal hormone, melatonin, is known to aid nerve regeneration. However, the role of Schwann cells and the pathways involved remain unclear. Thus, the aim of this study is to identify the effects of melatonin on Schwann cell proliferation, dedifferentiation, and the involvement of nuclear factor kappa light chain enhancer of activated B cells (NF-ĸB), focal adhesion kinase (FAK) and proto-oncogene tyrosine-protein kinase, Src pathways in this process.

METHODS

Schwann cells was treated with melatonin and its proliferation and dedifferentiation were identified using MTT assay and immunofluorescence staining for SRY (sex determining region Y)-box 2 (SOX2). Next, the protein expressions of NF-ĸB, FAK and Src pathways were identified by Western blot.

RESULTS

MTT results confirmed increased proliferation of Schwann cells with melatonin treatment, and it was highest at 10 μM melatonin. Immunofluorescent staining revealed an increase in the green fluorescence staining for SOX2 in melatonin-treated cells, showing enhanced dedifferentiation. Western blot assay revealed melatonin increased phospho-NF-ĸB (PNF-ĸB), IKK-α, FAK (D2R2E), phospho-FAK (Tyr 576/577 and Tyr 397) protein expressions as compared with control. However, Src (32G6), Lyn (C13F9), Fyn, Csk (C74C1) protein expressions were not increased as compared with control.

CONCLUSION

Melatonin promotes Schwann cell proliferation and dedifferentiation via NF-ĸB, FAK-dependent but Src-independent pathways.

PKACα negatively regulates TAK1/IRF7 signaling in black carp Mylopharyngodon piceus

Protein Kinase A catalytic subunit α (PKACα), plays an important role in the PKA and NF-κB signaling pathway in mammals. However, the function of PKACα in teleost fish remains largely unknown. In this study, PKACα from black carp (bcPKACα) has been cloned and its role in the innate immune antiviral signaling pathway was investigated. The open reading frame of bcPKACα gene contains 1056 nucleotides and the immunofluorescence assay verified that PKACα was mainly distributed in the cytoplasm. The reporter assay showed that bcPKACα expression and co-expression of bcPKACα and black carp TAK1 (bcTAK1) could activate the transcription of NF-κB. However, bcTAK1/bcIRF7-mediated IFN transcription was inhibited by bcPKACα. Knockdown of bcPKACα showed slightly enhanced antiviral activity against spring viremia of carp virus (SVCV) compared with control group. Accordingly, the antiviral activity against SVCV and grass carp reovirus (GCRV) of EPC cells co-expressing bcPKACα, bcTAK1 and bcIRF7 was obviously lower than that of EPC cells co-expressing bcTAK1 and bcIRF7. The similar subcellular distribution and interaction between bcPKACα and bcTAK1 were detected by immunofluorescent staining and co-immunoprecipitation assay separately. The data generated in this study demonstrates that bcPKACα associates with bcTAK1 and positively regulates NF-κB signaling, however, negatively regulates TAK1/IRF7 signaling pathway.

TAS2R16 Activation Suppresses LPS-Induced Cytokine Expression in Human Gingival Fibroblasts

Sustained and non-resolved inflammation is a characteristic of periodontitis. Upon acute inflammation, gingival fibroblasts release cytokines to recruit immune cells to counter environmental stimuli. The intricate regulation of pro-inflammatory signaling pathways, such as NF-κB, is necessary to maintain periodontal homeostasis. Nonetheless, how inflammation is resolved has not yet been elucidated. In this study, 22 subtypes of taste receptor family 2 (TAS2Rs), as well as the downstream machineries of Gα-gustducin and phospholipase C-β2 (PLCβ2), were identified in human gingival fibroblasts (HGFs). Various bitter agonists could induce an intensive cytosolic Ca²⁺ response in HGFs. More importantly, TAS2R16 was expressed at a relatively high level, and its agonist, salicin, showed robust Ca²⁺ evocative effects in HGFs. Activation of TAS2R16 signaling by salicin inhibited the release of lipopolysaccharide (LPS)-induced pro-inflammatory cytokines, at least in part, by repressing LPS-induced intracellular cAMP elevation and NF-κB p65 nuclear translocation in HGFs. These findings indicate that TAS2Rs activation in HGFs may mediate endogenous pro-inflammation resolution by antagonizing NF-κB signaling, providing a novel paradigm and treatment target for the better management of periodontitis.

Involvement of the miR-363-5p/P2RX4 Axis in Regulating Schwann Cell Phenotype after Nerve Injury

Although microRNAs (miRNAs or miRs) have been studied in the peripheral nervous system, their function in Schwann cells remains elusive. In this study, we performed a microRNA array analysis of cyclic adenosine monophosphate (cAMP)-induced differentiated primary Schwann cells. KEGG pathway enrichment analysis of the target genes showed that upregulated miRNAs (mR212-5p, miR335, miR20b-5p, miR146b-3p, and miR363-5p) were related to the calcium signaling pathway, regulation of actin cytoskeleton, retrograde endocannabinoid signaling, and central carbon metabolism in cancer. Several key factors, such as purinergic receptors (P2X), guanine nucleotide-binding protein G(olf) subunit alpha (GNAL), P2RX5, P2RX3, platelet-derived growth factor receptor alpha (PDGFRA), and inositol 1,4,5-trisphosphate receptor type 2 (ITPR2; calcium signaling pathway) are potential targets of miRNAs regulating cAMP. Our analysis revealed that miRNAs were differentially expressed in cAMP-treated Schwann cells; miRNA363-5p was upregulated and directly targeted the P2X purinoceptor 4 (P2RX4)-UTR, reducing the luciferase activity of P2RX4. The expression of miRNA363-5p was inhibited and the expression of P2RX4 was upregulated in sciatic nerve injury. In contrast, miRNA363-5p expression was upregulated and P2RX4 expression was downregulated during postnatal development. Of note, a P2RX4 antagonist counteracted myelin degradation after nerve injury and increased pERK and c-Jun expression. Interestingly, a P2RX4 antagonist increased the levels of miRNA363-5p. This study suggests that a double-negative feedback loop between miRNA363-5p and P2RX4 contributes to the dedifferentiation and migration of Schwann cells after nerve injury.

Schwann cell interactions during the development of the peripheral nervous system

Schwann cells play a critical role in the development of the peripheral nervous system (PNS), establishing important relationships both with the extracellular milieu and other cell types, particularly neurons. In this review, we discuss various Schwann cell interactions integral to the proper establishment, spatial arrangement, and function of the PNS. We include signals that cascade onto Schwann cells from axons and from the extracellular matrix, bidirectional signals that help to establish the axo-glial relationship and how Schwann cells in turn support the axon. Further, we speculate on how Schwann cell interactions with other components of the developing PNS ultimately promote the complete construction of the peripheral nerve.

Tumor-neuroglia interaction promotes pancreatic cancer metastasis

Rationale: The peripheral nervous system (PNS) plays an important role in tumor growth and progression. Schwann cells (SCs), the main glia cells of the PNS, augment cancer metastasis in contact-dependent or contact-independent manner in various malignancies. In the present study, we aimed to determine whether interplay between pancreatic cancer cells and SCs via paracrine signaling contributes to cancer progression.

Methods: Immunofluorescence analysis was performed to reveal the distribution of SCs in PDAC tissues and to determine the prognostic value and clinicopathological relevance of the level of intra‑tumoral SC markers for patients diagnosed with PDAC. Transwell assays and wound healing assays were carried out to investigate the influence of SC conditioned medium (SCM), SC co‑culture, or co-cultured CM on the migratory and invasive abilities of pancreatic cancer cells. The mechanism of SCs induced cancer cells migration and invasion was confirmed using quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR), enzyme-linked immunosorbent assays (ELISAs), western blotting, immunofluorescence, immunohistochemistry, siRNA-mediated gene interference, and an in vivo mouse model.

Results: Immunofluorescence analysis of tissue samples revealed that there were two different types of SCs distributed in the tumor microenvironment, the presence of which correlated with several clinicopathological characteristics and overall survival for patients with PDAC. Although SCM had no impact on the motility and invasiveness of tumor cells, both co-cultivation with SCs and co‑cultured CM enhanced pancreatic cancer cell migration and invasion. Mechanistically, SC‑derived Interleukin 6 (IL6), which was induced by co-culture with pancreatic cancer cells, augmented cancer cell migration and invasion by activating STAT3 signaling in cancer cells, while IL6 neutralization or STAT3 downregulation abrogated these effects. Furthermore, Interleukin 1β (IL1β), secreted by tumor cells, activated the nuclear actor (NF)-kappa B pathway in SCs, resulting in increased cytokines production, including IL6, while inhibiting the IL1β-IL1R1 axis led to inactivation of NF-kappa B signaling and downregulated cytokines expression in SCs. Interfering with tumor-neuroglia crosstalk impeded cancer cell dissemination in vivo.

Conclusion: Schwann cells were extensively distributed in the PDAC tumor microenvironment and high level of intra-tumoral SC markers could serve as an independent prognostic factor for poor survival of patients with PDAC. The tumor-neuroglia interaction is indispensable for SCs to acquire a tumor-facilitating phenotype. Targeting the tumor-neuroglia interplay might be a promising strategy to treat PDAC.

miRNA-1246 induces pro-inflammatory responses in mesenchymal stem/stromal cells by regulating PKA and PP2A

The tumor microenvironment (TME) has an impact on breast cancer progression by creating a pro-inflammatory milieu within the tumor. However, little is known about the roles of miRNAs in cells of the TME during this process. We identified six putative oncomiRs in a breast cancer dataset, all strongly correlating with poor overall patient survival. Out of the six candidates, miR-1246 was upregulated in aggressive breast cancer subtypes and expressed at highest levels in mesenchymal stem/stroma cells (MSCs). Functionally, miR-1246 led to a p65-dependent increase in transcription and release of pro-inflammatory mediators IL-6, CCL2 and CCL5 in MSCs, and increased NF-κB activity. The pro-inflammatory phenotype of miR-1246 in MSCs was independent of TNFα stimulations and mediated by direct targeting of the tumor-suppressors PRKAR1A and PPP2CB. In vitro recapitulation of the TME revealed increased Stat3 phosphorylation in breast epithelial (MCF10A) and cancer cells (SK-BR-3, MCF7, T47D) upon incubation with conditioned medium (CM) of MSCs overexpressing miR-1246. Additionally, this stimulation enhanced proliferation of MCF10A cells, increased migration of MDA-MB-231 cells and induced attraction of THP-1 monocytic cells. Our data shows that miR-1246 acts as both key-enhancer of pro-inflammatory responses in MSCs and putative oncomiR in breast cancer, suggesting its influence on cancer-related inflammation and breast cancer progression.

miR-221-3p Inhibits Schwann Cell Myelination

Following peripheral nerve injury, Schwann Cells (SCs) undergo dedifferentiation, proliferation, migration, and remyelination. Recent works demonstrated the importance of the short non-coding RNA (miRNAs) in SC dedifferentiation and remyelination after nerve injury. Previously, we found some miRNAs like miR-9, miR-221, miR-222 and miR-182 could regulate the proliferation and migration of SCs. Therefore, it is imperative to ask whether these miRNAs could regulate the myelination of SCs. Here we demonstrated that miR-221-3p could inhibit the myelination of SCs when co-cultured with dorsal root ganglion cells in vitro. In addition, NGF1-A binding protein 1 (Nab1) which was essential for SCs myelination could be downregulated by miR-221-3p. Suppressing the expression of Nab1 could reverse the promotion of miR-221-3p antagomir on SC myelination. The effects of miR-221-3p on SC myelination might be used to improve peripheral nerve regeneration, thus offering a new approach to peripheral nerve repair.

Uridine-5'-Triphosphate Partially Blocks Differentiation Signals and Favors a more Repair State in Cultured rat Schwann Cells

Schwann cells (SCs) play a key role in peripheral nerve regeneration. After damage, they respond acquiring a repair phenotype that allows them to proliferate, migrate and redirect axonal growth. Previous studies have shown that Uridine-5'-Triphosphate (UTP) and its purinergic receptors participate in several pathophysiological responses in the nervous system. Our group has previously described how UTP induces the migration of a Schwannoma cell line and promotes wound healing. These data suggest that UTP participates in the signaling involved in the regeneration process. In the present study we evaluated UTP effects in isolated rat SCs and cocultures of SCs and dorsal root ganglia neurons. UTP reduced cAMP-dependent Krox-20 induction in SCs. UTP also reduced the N-cadherin re-expression that occurs when SCs and axons make contact. In myelinating cocultures, a non-significant tendency to a lower expression of P0 and MAG proteins in presence of UTP was observed. We also demonstrated that UTP induced SC migration without affecting cell proliferation. Interestingly, UTP was found to block neuregulin-induced phosphorylation of the ErbB3 receptor, a pathway involved in the regeneration process. These results indicate that UTP could acts as a brake to the differentiation signals, promoting a more migratory state in the repair-SCs.

Laminin 211 inhibits protein kinase A in Schwann cells to modulate neuregulin 1 type III-driven myelination

Myelin is required for proper nervous system function. Schwann cells in developing nerves depend on extrinsic signals from the axon and from the extracellular matrix to first sort and ensheathe a single axon and then myelinate it. Neuregulin 1 type III (Nrg1III) and laminin α2β1γ1 (Lm211) are the key axonal and matrix signals, respectively, but how their signaling is integrated and if each molecule controls both axonal sorting and myelination is unclear. Here, we use a series of epistasis experiments to show that Lm211 modulates neuregulin signaling to ensure the correct timing and amount of myelination. Lm211 can inhibit Nrg1III by limiting protein kinase A (PKA) activation, which is required to initiate myelination. We provide evidence that excessive PKA activation amplifies promyelinating signals downstream of neuregulin, including direct activation of the neuregulin receptor ErbB2 and its effector Grb2-Associated Binder-1 (Gab1), thereby elevating the expression of the key transcription factors Oct6 and early growth response protein 2 (Egr2). The inhibitory effect of Lm211 is seen only in fibers of small caliber. These data may explain why hereditary neuropathies associated with decreased laminin function are characterized by focally thick and redundant myelin.

TNFa/TNFR2 signaling is required for glial ensheathment at the dorsal root entry zone

Somatosensory information from the periphery is routed to the spinal cord through centrally-projecting sensory axons that cross into the central nervous system (CNS) via the dorsal root entry zone (DREZ). The glial cells that ensheath these axons ensure rapid propagation of this information. Despite the importance of this glial-axon arrangement, how this afferent nerve is assembled during development is unknown. Using in vivo, time-lapse imaging we show that as centrally-projecting pioneer axons from dorsal root ganglia (DRG) enter the spinal cord, they initiate expression of the cytokine TNFalpha. This induction coincides with ensheathment of these axons by associated glia via a TNF receptor 2 (TNFR2)-mediated process. This work identifies a signaling cascade that mediates peripheral glial-axon interactions and it functions to ensure that DRG afferent projections are ensheathed after pioneer axons complete their navigation, which promotes efficient somatosensory neural function.

Molecular Mechanisms Involved in Schwann Cell Plasticity

Schwann cell incredible plasticity is a hallmark of the utmost importance following nerve damage or in demyelinating neuropathies. After injury, Schwann cells undergo dedifferentiation before redifferentiating to promote nerve regeneration and complete functional recovery. This review updates and discusses the molecular mechanisms involved in the negative regulation of myelination as well as in the reprogramming of Schwann cells taking place early following nerve lesion to support repair. Significant advance has been made on signaling pathways and molecular components that regulate SC regenerative properties. These include for instance transcriptional regulators such as c-Jun or Notch, the MAPK and the Nrg1/ErbB2/3 pathways. This comprehensive overview ends with some therapeutical applications targeting factors that control Schwann cell plasticity and highlights the need to carefully modulate and balance this capacity to drive nerve repair.

Glutamate signals through mGluR2 to control Schwann cell differentiation and proliferation

Rapid saltatory nerve conduction is facilitated by myelin structure, which is produced by Schwann cells (SC) in the peripheral nervous system (PNS). Proper development and degeneration/regeneration after injury requires regulated phenotypic changes of SC. We have previously shown that glutamate can induce SC proliferation in culture. Here we show that glutamate signals through metabotropic glutamate receptor 2 (mGluR2) to induce Erk phosphorylation in SC. mGluR2-elicited Erk phosphorylation requires ErbB2/3 receptor tyrosine kinase phosphorylation to limit the signaling cascade that promotes phosphorylation of Erk, but not Akt. We found that Gβγ and Src are involved in subcellular signaling downstream of mGluR2. We also found that glutamate can transform myelinating SC to proliferating SC, while inhibition of mGluR2 signaling can inhibit demyelination of injured nerves in vivo. These data suggest pathophysiological significance of mGluR2 signaling in PNS and its possible therapeutic importance to combat demyelinating disorders including Charcot-Marie-Tooth disease.

Stimulating the proliferation, migration and lamellipodia of Schwann cells using low-dose curcumin

Transplantation of peripheral glia is being trialled for neural repair therapies, and identification of compounds that enhance the activity of glia is therefore of therapeutic interest. We have previously shown that curcumin potently stimulates the activity of olfactory glia. We have now examined the effect of curcumin on Schwann cell (SC) activities including proliferation, migration and the expression of protein markers. SCs were treated with control media and with different concentrations of curcumin (0.02-20 μM). Cell proliferation was determined by MTS assay and migration changes were determined by single live cell migration tracking. We found that small doses of curcumin (40 nM) dramatically increased the proliferation and migration in SCs within just one day. When compared with olfactory glia, curcumin stimulated SC proliferation more rapidly and at lower concentrations. Curcumin significantly increased the migration of SCs, and also increased the dynamic activity of lamellipodial waves which are essential for SC migration. Expression of the activated form of the MAP kinase p38 (p-p38) was significantly decreased in curcumin-treated SCs. These results show that curcumin's effects on SCs differ remarkably to its effects on olfactory glia, suggesting that subtypes of closely related glia can be differentially stimulated by curcumin. Overall these results demonstrate that the therapeutically beneficial activities of glia can be differentially enhanced by curcumin which could be used to improve outcomes of neural repair therapies.

Schwann cells and their transcriptional network: Evolution of key regulators of peripheral myelination

As derivatives of the neural crest, Schwann cells represent a vertebrate invention. Their development and differentiation is under control of a newly constructed, vertebrate-specific regulatory network that contains Sox10, Oct6 and Krox20 as cornerstones and central regulators of peripheral myelination. In this review, we discuss the function and relationship of these transcription factors among each other and in the context of their regulatory network, and present ideas of how neofunctionalization may have helped to recruit them to their novel task in Schwann cells. This article is part of a Special Issue entitled SI: Myelin Evolution.

The Activation of RAGE and NF-KB in Nerve Biopsies of Patients with Axonal and Vasculitic Neuropathy

INTRODUCTION

The receptor for advanced glycation end products (RAGE) is a pattern recognition receptor expressed in tissues and cells, which plays a role in immunity. The activation of RAGE results in the translocation of nuclear factor kappa B (NF-κB) to the nucleus for expression of proinflammatory molecules. The role of the RAGE pathway in the pathogenesis of diabetic complications is well determined. We aimed to investigate the role of the RAGE pathway in axonal and vasculitic neuropathy.

METHODS

We immunoreacted nerve biopsy samples from 17 axonal neuropathy (AN), 11 vasculitic neuropathy (VN) and 12 hereditary neuropathy (as a control group) with liability to pressure palsy (HNPP) patients with antibodies to NF-κB and RAGE. Subsequently, we performed double staining with the antibodies to NF-κB or RAGE and T cells, macrophages and Schwann cells.

RESULTS

RAGE and NF-κB immunoreactivities were higher in the perivascular cuff and in endoneurial cells in VN than in AN and HNPP. Although there is no significant difference, nerve biopsies with AN showed higher NFκB and RAGE immunoreactivities than HNPP. The colocalization study showed that most of the NFκB- and RAGE-positive cells were CD8 (+) T cells in VN. In AN, all NFκB- and RAGE-positive cells were macrophages, whereas all NFκB- and RAGE-positive cells were Schwann cells in HNPP.

CONCLUSION

The activation of the RAGE pathway predominant in CD8 (+) T cells underscores its role in VN. In AN patients, the immunoreactivity to NFκB and RAGE in macrophages may support their role in axonal degeneration without inflammatory milieu.

Sox4 participates in the modulation of Schwann cell myelination

In order to identify new regulators of Schwann cell myelination potentially playing a role in peripheral nervous system (PNS) pathologies, we analysed gene expression profiling data from three mouse models of demyelinating neuropathies and from the developing PNS. This analysis revealed that Sox4, which encodes a member of the Sry-related high-mobility group box protein family, was consistently upregulated in all three analysed models of neuropathy. Moreover, Sox4 showed a peak in its expression during development that corresponded with the onset of myelination. To gain further insights into the role of Sox4 in PNS development, we generated a transgenic mouse that specifically overexpresses Sox4 in Schwann cells. Sox4 overexpression led to a temporary delay in PNS myelination without affecting axonal sorting. Importantly, we observed that, whereas Sox4 mRNA could be efficiently overexpressed, Sox4 protein expression in Schwann cells was strictly regulated. Finally, our data showed that enforced expression of Sox4 in the mouse model for Charcot-Marie-Tooth 4C aggravated its neuropathic phenotype. Together, these observations reveal that Sox4 contributes to the regulation of Schwann cell myelination, and also indicates its involvement in the pathophysiology of peripheral neuropathies.

Involvement of nuclear factor kappa B in the maintenance of persistent inflammatory hypernociception

The pathophysiology of chronic inflammatory pain remains poorly understood. In this context, we developed an experimental model in which successive daily injection of prostaglandin E2 (PGE2) for 14days into rat hind paws produces a persistent state of hypernociception (i.e. decrease in mechanical nociceptive threshold). This state persists for more than 30days after discontinuing PGE2 injection. In the present study, we investigated the participation of nuclear factor kappa B (NF-κB), in the maintenance of this process. Mechanical hypernociception was evaluated using the electronic von Frey test. Activation of NF-κB signaling was measured through the determination of NF-κB p65 subunit translocation to the nucleus of dorsal root ganglion neurons (DRG) by immunofluorescence and western blotting. Herein, we detected an increase in NF-κB p65 subunit translocation to the nucleus of DRG neurons along with persistent inflammatory hypernociception compared with controls. Intrathecal treatment with either dexamethasone or PDTC (NF-κB activation inhibitor) after ending of the induction phase of the persistent inflammatory hypernociception, curtailed the hypernociception period as well as reducing NF-κB p65 subunit translocation. Treatment with antisense oligonucleotides against the NF-κB p65 subunit for 5 consecutive days also reduced persistent inflammatory hypernociception. Inhibition of PKA and PKCε reduced persistent inflammatory hypernociception, which was associated with inhibition of NF-κB p65 subunit translocation. Together these results suggest that peripheral activation of NF-κB by PKA and PKC in primary sensory neurons plays an important role in maintaining persistent inflammatory pain.

Genetic markers of white matter integrity in schizophrenia revealed by parallel ICA

It is becoming a consensus that white matter integrity is compromised in schizophrenia (SZ), however the underlying genetics remains elusive. Evidence suggests a polygenic basis of the disorder, which involves various genetic variants with modest individual effect sizes. In this work, we used a multivariate approach, parallel independent component analysis (P-ICA), to explore the genetic underpinnings of white matter abnormalities in SZ. A pre-filtering step was first applied to locate 6527 single nucleotide polymorphisms (SNPs) discriminating patients from controls with a nominal uncorrected p-value of 0.01. These potential susceptibility loci were then investigated for associations with fractional anisotropy (FA) images in a cohort consisting of 73 SZ patients and 87 healthy controls (HC). A significant correlation (r = −0.37, p = 1.25 × 10⁻⁶) was identified between one genetic factor and one FA component after controlling for scanning site, ethnicity, age, and sex. The identified FA-SNP association remained stable in a 10-fold validation. A 5000-run permutation test yielded a p-value of 2.00 × 10⁻⁴. The FA component reflected decreased white matter integrity in the forceps major for SZ patients. The SNP component was overrepresented in genes whose products are involved in corpus callosum morphology (e.g., CNTNAP2, NPAS3, and NFIB) as well as canonical pathways of synaptic long term depression and protein kinase A signaling. Taken together, our finding delineates a part of genetic architecture underlying SZ-related FA reduction, emphasizing the important role of genetic variants involved in neural development.

β2-Adrenergic receptors in immunity and inflammation: stressing NF-κB

β2-Adrenergic receptors (β2-ARs) transduce the effects of (nor)epinephrine on a variety of cell types and act as key mediators of the body's reaction to stress. β2-ARs are also expressed on immune cells and there is ample evidence for their role in immunomodulation. A key regulator of the immune response and a target for regulation by stress-induced signals is the transcription factor Nuclear Factor-kappaB (NF-κB). NF-κB shapes the course of both innate and adaptive immune responses and plays an important role in susceptibility to disease. In this review, we summarise the literature that has been accumulated in the past 20years on adrenergic modulation of NF-κB function. We here focus on the molecular basis of the reported interactions and show that both physiological and pharmacological triggers of β2-ARs intersect with the NF-κB signalling cascade at different levels. Importantly, the action of β2-AR-derived signals on NF-κB activity appears to be highly cell type specific and gene selective, providing opportunities for the development of selective NF-κB modulators.

Requirement of cAMP signaling for Schwann cell differentiation restricts the onset of myelination

Isolated Schwann cells (SCs) respond to cAMP elevation by adopting a differentiated post-mitotic state that exhibits high levels of Krox-20, a transcriptional enhancer of myelination, and mature SC markers such as the myelin lipid galactocerebroside (O1). To address how cAMP controls myelination, we performed a series of cell culture experiments which compared the differentiating responses of isolated and axon-related SCs to cAMP analogs and ascorbate, a known inducer of axon ensheathment, basal lamina formation and myelination. In axon-related SCs, cAMP induced the expression of Krox-20 and O1 without a concomitant increase in the expression of myelin basic protein (MBP) and without promoting axon ensheathment, collagen synthesis or basal lamina assembly. When cAMP was provided together with ascorbate, a dramatic enhancement of MBP expression occurred, indicating that cAMP primes SCs to form myelin only under conditions supportive of basal lamina formation. Experiments using a combination of cell permeable cAMP analogs and type-selective adenylyl cyclase (AC) agonists and antagonists revealed that selective transmembrane AC (tmAC) activation with forskolin was not sufficient for full SC differentiation and that the attainment of an O1 positive state also relied on the activity of the soluble AC (sAC), a bicarbonate sensor that is insensitive to forskolin and GPCR activation. Pharmacological and immunological evidence indicated that SCs expressed sAC and that sAC activity was required for morphological differentiation and the expression of myelin markers such as O1 and protein zero. To conclude, our data indicates that cAMP did not directly drive myelination but rather the transition into an O1 positive state, which is perhaps the most critical cAMP-dependent rate limiting step for the onset of myelination. The temporally restricted role of cAMP in inducing differentiation independently of basal lamina formation provides a clear example of the uncoupling of signals controlling differentiation and myelination in SCs.

p75NTR is highly expressed in vestibular schwannomas and promotes cell survival by activating nuclear transcription factor κB

Vestibular schwannomas (VSs) arise from Schwann cells (SCs) and result from the loss of function of merlin, the protein product of the NF2 tumor suppressor gene. In contrast to non-neoplastic SCs, VS cells survive long-term in the absence of axons. We find that p75(NTR) is overexpressed in VSs compared with normal nerves, both at the transcript and protein level, similar to the response of non-neoplastic SCs following axotomy. Despite elevated p75(NTR) expression, VS cells are resistant to apoptosis due to treatment with proNGF, a high affinity ligand for p75(NTR) . Furthermore, treatment with proNGF protects VS cells from apoptosis due to c-Jun N-terminal kinase (JNK) inhibition indicating that p75(NTR) promotes VS cell survival. Treatment of VS cells with proNGF activated NF-κB while inhibition of JNK with SP600125 or siRNA-mediated knockdown reduced NF-κB activity. Significantly, proNGF also activated NF-κB in cultures treated with JNK inhibitors. Thus, JNK activity appears to be required for basal levels of NF-κB activity but not for proNGF-induced NF-κB activity. To confirm that the increase in NF-κB activity contributes to the prosurvival effect of proNGF, we infected VS cultures with Ad.IκB.SerS32/36A virus, which inhibits NF-κB activation. Compared with control virus, Ad.IκB.SerS32/36A significantly increased apoptosis including in VS cells treated with proNGF. Thus, in contrast to non-neoplastic SCs, p75(NTR) signaling provides a prosurvival response in VS cells by activating NF-κB independent of JNK. Such differences may contribute to the ability of VS cells to survive long-term in the absence of axons.

Phosphorylation of LKB1/Par-4 establishes Schwann cell polarity to initiate and control myelin extent

The Schwann cell (SC)-axon interface represents a membrane specialization that integrates axonal signals to coordinate cytoskeletal dynamics resulting in myelination. Here we show that LKB1/Par-4 is asymmetrically localized to the SC-axon interface and colocalizes with the polarity protein Par-3. Using purified SCs and myelinating cocultures, we demonstrate that localization is dependent on the phosphorylation of LKB1 at serine-431. SC-specific deletion of LKB1 significantly attenuates developmental myelination, delaying the initiation and altering the myelin extent into adulthood, resulting in a 30% reduction in the conduction velocity along adult sciatic nerves. Phosphorylation of LKB1 by protein kinase A is essential to establish the asymmetric localization of LKB1 and Par-3 and rescues the delay in myelination observed in the SC-specific knockout of LKB1. Our findings suggest that SC polarity may coordinate multiple signaling complexes that couple SC-axon contact to the redistribution of specific membrane components necessary to initiate and control myelin extent.

Relationships between cobalamin, epidermal growth factor, and normal prions in the myelin maintenance of central nervous system

Cobalamin (Cbl), epidermal growth factor (EGF), and prions (PrPs) are key molecules for myelin maintenance in the central and peripheral nervous systems. Cbl and EGF increase normal prion (PrP(C)) synthesis and PrP(C) levels in rat spinal cord (SC) and elsewhere. Cbl deficiency increases PrP(C) levels in rat SC and cerebrospinal fluid (CSF), and decreases PrP(C)-mRNA levels in rat SC. The administration of anti-octapeptide repeat PrP(C) region antibodies (Abs) to Cbl-deficient (Cbl-D) rats prevents SC myelin lesions and a local increase in tumor necrosis factor (TNF)-α levels, whereas anti-TNF-α Abs prevent SC myelin lesions and the increase in SC and CSF PrP(C) levels. As it is known that both Cbl and EGF regulate SC PrP(C) synthesis independently, and that Cbl regulates SC EGF synthesis, EGF may play both Cbl-independent and Cbl-dependent roles. When Cbl-D rats undergo Cbl replacement therapy, SC PrP(C) levels are similar to those observed in Cbl-D rats. In rat frontal cortex (which is marginally affected by Cbl deficiency in histological terms), Cbl deficiency decreases PrP(C) levels and the increase induced by Cbl replacement leads to their normalization. Increased nerve PrP(C) levels are detected in the myelin lesions of the peripheral neuropathy of Cbl-D rats, and CSF PrP(C) levels are also increased in Cbl-D patients (but not in patients with Cbl-unrelated neurological diseases). Various common steps in the downstream signaling pathway of Cbl, EGF, and PrP(C) underlines the close relationship between the three molecules in keeping myelin normal.

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.

The protein kinase A regulatory subunit R1A (Prkar1a) plays critical roles in peripheral nerve development

Signaling through cAMP has been implicated in Schwann cell (SC) proliferation and myelination, but the signaling pathway components downstream of cAMP required for SC function remain unknown. Protein kinase A (PKA) is a potential downstream effector of cAMP. Here, we induced loss of Prkar1a, the gene encoding the type 1A regulatory subunit of PKA, in SC to study its role in nerve development; loss of Prkar1a is predicted to elevate PKA activity. Conditional Prkar1a knock-out in mouse SC (Prkar1a-SCKO) resulted in a dramatic and persistent axonal sorting defect, and unexpectedly decreased SC proliferation in Prkar1a-SCKO nerves in vivo. Effects were cell autonomous as they were recapitulated in vitro in Prkar1a-SCKO SC, which showed elevated PKA activity. In the few SCs sorted into 1:1 relationships with axons in vivo, SC myelination was premature in Prkar1a-SCKO nerves, correlating with global increase in the cAMP-regulated transcription factor Oct-6 and expression of myelin basic protein. These data reveal a previously unknown role of PKA in axon sorting, an unexpected inhibitory role of PKA on SC cell proliferation in vivo and define the importance of Prkar1a in peripheral nerve development.

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.

Signals regulating myelination in peripheral nerves and the Schwann cell response to injury

In peripheral nerves, Schwann cells form myelin, which facilitates the rapid conduction of action potentials along axons in the vertebrate nervous system. Myelinating Schwann cells are derived from neural crest progenitors in a step-wise process that is regulated by extracellular signals and transcription factors. In addition to forming the myelin sheath, Schwann cells orchestrate much of the regenerative response that occurs after injury to peripheral nerves. In response to injury, myelinating Schwann cells dedifferentiate into repair cells that are essential for axonal regeneration, and then redifferentiate into myelinating Schwann cells to restore nerve function. Although this remarkable plasticity has long been recognized, many questions remain unanswered regarding the signaling pathways regulating both myelination and the Schwann cell response to injury.

Diabetic Schwann cells suffer from nerve growth factor and neurotrophin-3 underproduction and poor associability with axons

Schwann cells (SCs) are integral to peripheral nerve biology, contributing to saltatory conduction along axons, nerve and axon development, and axonal regeneration. SCs also provide a microenvironment favoring neural regeneration partially due to production of several neurotrophic factors. Dysfunction of SCs may also play an important role in the pathogenesis of peripheral nerve diseases such as diabetic peripheral neuropathy where hyperglycemia is often considered pathogenic. In order to study the impact of diabetes mellitus (DM) upon the regenerative capacity of adult SCs, we investigated the differential production of the neurotrophic factors nerve growth factor (NGF) and neurotrophin-3 (NT3) by SCs harvested from the sciatic nerves of murine models of type 1 DM (streptozotocin treated C57BL/6J mice) and type 2 DM (LepR(-/-) or db/db mice) or non-diabetic cohorts. In vitro, SCs from diabetic and control mice were maintained under similar hyperglycemic and euglycemic conditions respectively. Mature SCs from diabetic mice produced lower levels of NGF and NT3 under hyperglycemic conditions when compared to SCs in euglycemia. In addition, SCs from both DM and non-DM mice appear to be incapable of insulin production, but responded to exogenous insulin with greater proliferation and heightened myelination potentiation. Moreover, SCs from diabetic animals showed poorer association with co-cultured axons. Hyperglycemia had significant impact upon SCs, potentially contributing to the pathogenesis of diabetic peripheral neuropathy.

Low-density lipoprotein receptor related protein-1 (LRP1)-dependent cell signaling promotes neurotrophic activity in embryonic sensory neurons

Developing sensory neurons require neurotrophic support for survival, neurite outgrowth and myelination. The low-density lipoprotein receptor-related protein-1 (LRP1) transactivates Trk receptors and thereby functions as a putative neurotrophin. Herein, we show that LRP1 is abundantly expressed in developing dorsal root ganglia (DRG) and that LRP1-dependent cell signaling supports survival, neurite extension and receptivity to Schwann cells even in the absence of neurotrophins. Cultured embryonic DRG neurons (E15) were treated with previously characterized LRP1 ligands, LRP1-receptor binding domain of α2-macroglobulin (RBD), hemopexin domain of MMP-9 (PEX) or controls (GST) for two weeks. These structurally diverse LRP1 ligands significantly activated and sustained extracellular signal-regulated kinases (ERK1/2) 5-fold (p<0.05), increased expression of growth-associated protein-43(GAP43) 15-fold (P<0.01), and increased neurite outgrowth 20-fold (P<0.01). Primary sensory neurons treated with LRP1 ligands survived > 2 weeks in vitro, to an extent equaling NGF, a finding associated with canonical signaling mechanisms and blockade of caspase-3 cleavage. LRP1 ligand-induced survival and sprouting were blocked by co-incubation with the LRP1 antagonist, receptor associated protein (RAP), whereas RAP had no effect on NGF-induced activity. Site directed mutagenesis of the LRP1 ligand, RBD, in which Lys¹³⁷⁰ and Lys¹³⁷⁴ are converted to alanine to preclude LRP1 binding, were ineffective in promoting cell signaling, survival or inducing neurite extension in primary sensory neurons, confirming LRP1 specificity. Furthermore, LRP1-induced neurite sprouting was mediated by Src-family kinase (SFK) activation, suggesting transactivation of Trk receptors. Co-cultures of primary embryonic neurons and Schwann cells showed that LRP1 agonists promoted axonal receptivity to myelination to Schwann cells. Collectively, these findings identify LRP1 as a novel and perhaps essential trophic molecule for sensory neuronal survival and development.

Activation of NF-κB in Schwann cells is dispensable for myelination in vivo

Peripheral myelination is a dynamic process orchestrated by axons and Schwann cells. Although the signaling mechanisms governing myelination are not fully understood, NF-κB activation in Schwann cells has been implicated as a key regulator in vitro. Using a mouse model, we show that nuclear factor κB activation in Schwann cells is not required for myelination in vivo.

Analysis of Gpr126 function defines distinct mechanisms controlling the initiation and maturation of myelin

In peripheral nerves, Schwann cells form the myelin sheath, which allows the efficient propagation of action potentials along axons. The transcription factor Krox20 regulates the initiation of myelination in Schwann cells and is also required to maintain mature myelin. The adhesion G protein-coupled receptor (GPCR) Gpr126 is essential for Schwann cells to initiate myelination, but previous studies have not addressed the role of Gpr126 signaling in myelin maturation and maintenance. Through analysis of Gpr126 in zebrafish, we define two distinct mechanisms controlling the initiation and maturation of myelin. We show that gpr126 mutant Schwann cells elaborate mature myelin sheaths and maintain krox20 expression for months, provided that the early signaling defect is bypassed by transient elevation of cAMP. At the onset of myelination, Gpr126 and protein kinase A (PKA) function as a switch that allows Schwann cells to initiate krox20 expression and myelination. After myelination is initiated, krox20 expression is maintained and myelin maturation proceeds independently of Gpr126 signaling. Transgenic analysis indicates that the Krox20 cis-regulatory myelinating Schwann cell element (MSE) becomes active at the onset of myelination and that this activity is dependent on Gpr126 signaling. Activity of the MSE declines after initiation, suggesting that other elements are responsible for maintaining krox20 expression in mature nerves. We also show that elevated cAMP does not initiate myelination in the absence of functional Neuregulin 1 (Nrg1) signaling. These results indicate that the mechanisms regulating the initiation of myelination are distinct from those mediating the maturation and maintenance of myelin.

Isolated dorsal root ganglion neurones inhibit receptor-dependent adenylyl cyclase activity in associated glial cells

BACKGROUND AND PURPOSE

Hyper-nociceptive PGE(2) EP(4) receptors and prostacyclin (IP) receptors are present in adult rat dorsal root ganglion (DRG) neurones and glial cells in culture. The present study has investigated the cell-specific expression of two other G(s) -protein coupled hyper-nociceptive receptor systems: β-adrenoceptors and calcitonin gene-related peptide (CGRP) receptors in isolated DRG cells and has examined the influence of neurone-glial cell interactions in regulating adenylyl cyclase (AC) activity.

EXPERIMENTAL APPROACH

Agonist-stimulated AC activity was determined in mixed DRG cell cultures from adult rats and compared with activity in DRG neurone-enriched cell cultures and pure DRG glial cell cultures.

KEY RESULTS

Pharmacological analysis showed the presence of G(s) -coupled β(2) -adrenoceptors and CGRP receptors, but not β(1) -adrenoceptors, in all three DRG cell preparations. Agonist-stimulated AC activity was weakest in DRG neurone-enriched cell cultures. DRG neurones inhibited IP receptor-stimulated glial cell AC activity by a process dependent on both cell-cell contact and neurone-derived soluble factors, but this is unlikely to involve purine or glutamine receptor activation.

CONCLUSIONS AND IMPLICATIONS

G(s) -coupled hyper-nociceptive receptors are readily expressed on DRG glial cells in isolated cell cultures and the activity of CGRP, EP(4) and IP receptors, but not β(2) -adrenoceptors, in glial cells is inhibited by DRG neurones. Studies using isolated DRG cells should be aware that hyper-nociceptive ligands may stimulate receptors on glial cells in addition to neurones, and that variable numbers of neurones and glial cells will influence absolute measures of AC activity and affect downstream functional responses.

NF-κB forms a complex with the chromatin remodeler BRG1 to regulate Schwann cell differentiation

In the developing peripheral nervous system, axon-derived signals stimulate Schwann cells to undergo a global genetic reprogramming involving the cessation of cellular division and the upregulation of myelin genes. How such a comprehensive change in gene transcription is regulated is poorly understood. Here we report that BRG1/SMARCA4, the central helicase of the mammalian SWI/SNF-related chromatin remodeling complex, is required for Schwann cells to differentiate and form myelin, both in vitro and in vivo, in the mouse. BRG1 was highly activated in Schwann cells at early stages of myelination, and loss of the enzyme inhibited their differentiation and completely prevented myelin formation. Furthermore, we identify NF-κB as a key transcription factor that associates with the BRG1 complex in response to neuregulin 1 type III. During myelination, BRG1 was activated through the formation of a complex with NF-κB, and both proteins bound to the promoter region of Sox10, an inducer of myelination. These findings delineate a novel mechanism whereby axonal signals promote myelination through the remodeling of chromatin structure.

Rac1 controls Schwann cell myelination through cAMP and NF2/merlin

During peripheral nervous system development, Schwann cells (SCs) surrounding single large axons differentiate into myelinating SCs. Previous studies implicate RhoGTPases in SC myelination, but the mechanisms involved in RhoGTPase regulation of SC myelination are unknown. Here, we show that SC myelination is arrested in Rac1 conditional knock-out (Rac1-CKO) mice. Rac1 knock-out abrogated phosphorylation of the effector p21-activated kinase and decreased NF2/merlin phosphorylation. Mutation of NF2/merlin rescued the myelin deficit in Rac1-CKO mice in vivo and the shortened processes in cultured Rac1-CKO SCs in vitro. Mechanistically, cAMP levels and E-cadherin expression were decreased in the absence of Rac1, and both were restored by mutation of NF2/merlin. Reduced cAMP is a cause of the myelin deficiency in Rac1-CKO mice, because elevation of cAMP by rolipram in Rac1-CKO mice in vivo allowed myelin formation. Thus, NF2/merlin and cAMP function downstream of Rac1 signaling in SC myelination, and cAMP levels control Rac1-regulated SC myelination.

Thyroid peroxidase gene expression is induced by lipopolysaccharide involving nuclear factor (NF)-κB p65 subunit phosphorylation

Thyroid peroxidase (TPO), a tissue-specific enzyme expressed in differentiated thyroid follicular cells, is a major antigen that has been linked to autoimmune thyroid disease. We have previously reported the functional expression of the lipopolysaccharide (LPS) receptor Toll-like receptor 4 on thyroid follicular cells. Here we investigated the effect of LPS in TPO expression and analyzed the mechanisms involved. We found a dose-dependent enhancement of TSH-induced TPO expression in response to LPS stimulation. EMSAs demonstrated that LPS treatment increased thyroid transcription factor-1 and -2 binding to the B and Z regions of TPO promoter, respectively. Moreover, LPS increased TSH-stimulated TPO promoter activity. Using bioinformatic analysis, we identified a conserved binding site for transcription nuclear factor-κB (NF-κB) in the TPO promoter. Chemical inhibition of NF-κB signaling and site-directed mutagenesis of the identified κB-cis-acting element abolished LPS stimulation. Furthermore, chromatin immunoprecipitation assays confirmed that TPO constitutes a novel NF-κB p65 subunit target gene in response to LPS. Additionally, our results indicate that p65 phosphorylation of serine 536 constitutes an essential step in the p65-dependent, LPS-induced transcriptional expression of TPO. In conclusion, here we demonstrated that LPS increases TPO expression, suggesting a novel mechanism involved in the regulation of a major thyroid autoantigen. Our results provide new insights into the potential effects of infectious processes on thyroid homeostasis.

Curcumin derivatives promote Schwann cell differentiation and improve neuropathy in R98C CMT1B mice

Charcot-Marie-Tooth disease type 1B is caused by mutations in myelin protein zero. R98C mice, an authentic model of early onset Charcot-Marie-Tooth disease type 1B, develop neuropathy in part because the misfolded mutant myelin protein zero is retained in the endoplasmic reticulum where it activates the unfolded protein response. Because oral curcumin, a component of the spice turmeric, has been shown to relieve endoplasmic reticulum stress and decrease the activation of the unfolded protein response, we treated R98C mutant mice with daily gastric lavage of curcumin or curcumin derivatives starting at 4 days of age and analysed them for clinical disability, electrophysiological parameters and peripheral nerve morphology. Heterozygous R98C mice treated with curcumin dissolved in sesame oil or phosphatidylcholine curcumin performed as well as wild-type littermates on a rotarod test and had increased numbers of large-diameter axons in their sciatic nerves. Treatment with the latter two compounds also increased compound muscle action potential amplitudes and the innervation of neuromuscular junctions in both heterozygous and homozygous R98C animals, but it did not improve nerve conduction velocity, myelin thickness, G-ratios or myelin period. The expression of c-Jun and suppressed cAMP-inducible POU (SCIP)-transcription factors that inhibit myelination when overexpressed-was also decreased by treatment. Consistent with its role in reducing endoplasmic reticulum stress, treatment with curcumin dissolved in sesame oil or phosphatidylcholine curcumin was associated with decreased X-box binding protein (XBP1) splicing. Taken together, these data demonstrate that treatment with curcumin dissolved in sesame oil or phosphatidylcholine curcumin improves the peripheral neuropathy of R98C mice by alleviating endoplasmic reticulum stress, by reducing the activation of unfolded protein response and by promoting Schwann cell differentiation.

The role of C-terminal binding protein 2 in Schwann cell differentiation after sciatic nerve crush

C-terminal binding protein 2 (CtBP2), as a transcriptional repressor, plays an essential role in development and tumorigenesis. However, its distribution and function in peripheral system lesion and repair are still unknown. Here, we investigated the spatiotemporal expression of CtBP2 in rat sciatic nerve crush model. Western blot analysis revealed that CtBP2 was expressed in normal sciatic nerve. It gradually decreased, reached minimal levels at 7 days after crush, and then returned to the normal level at 4 weeks. We observed that CtBP2 is mainly expressed in Schwann cells (SCs). In vitro, we induced SC differentiation via cyclic adenosine monophosphate (cAMP) and found that CtBP2 expression was downregulated during the process of differentiation. CtBP2-specific siRNA inhibited the cAMP-induced expression of the immature SC marker P75(NTR), and exogenous CtBP2 expression upregulated the expression of P75(NTR). Taken together, we hypothesized that peripheral nerve crush-induced downregulation of CtBP2 in the sciatic nerve was associated with SC differentiation, and CtBP2 likely played an important role in peripheral nerve injury and regeneration.

NF-κB signaling pathways: role in nervous system physiology and pathology

Intracellular pathways related to cell survival regulate neuronal physiology during development and neurodegenerative disorders. One of the pathways that have recently emerged with an important role in these processes is nuclear factor-κB (NF-κB). The activity of this pathway leads to the nuclear translocation of the NF-κB transcription factors and the regulation of anti-apoptotic gene expression. Different stimuli can activate the pathway through different intracellular cascades (canonical, non-canonical, and atypical), contributing to the translocation of specific dimers of the NF-κB transcription factors, and each of these dimers can regulate the transcription of different genes. Recent studies have shown that the activation of this pathway regulates opposite responses such as cell survival or neuronal degeneration. These apparent contradictory effects depend on conditions such as the pathway stimuli, the origin of the cells, or the cellular context. In the present review, the authors summarize these findings and discuss their significance with respect to survival or death in the nervous system.

Iron and holotransferrin induce cAMP-dependent differentiation of Schwann cells

The differentiation of myelin-forming Schwann cells (SC) is completed with the appearance of myelin proteins MBP and P(0) and a concomitant downregulation of markers GFAP and p75NTR, which are expressed by immature and adult non-myelin-forming SC. We have previously demonstrated that holotransferrin (hTf) can prevent SC dedifferentiation in culture (Salis et al., 2002), while apotransferrin (aTf) cannot. As a consequence, we used pure cultured SC and submitted them to serum deprivation in order to promote dedifferentiation and evaluate the prodifferentiating ability of ferric ammonium citrate (FAC) through the expression of MBP, P(0), p75NTR and c-myc. The levels of cAMP, CREB and p-CREB were also measured. Results show that Fe(3+), either in its free form or as hTf, can prevent the dedifferentiation promoted by serum withdrawal. Both FAC and hTf were proven to promote differentiation, probably through the increase in cAMP levels and CREB phosphorylation, as well as levels of reactive oxygen species. This effect was inhibited by deferroxamine (Dfx, an iron chelator), H9 (a cAMP-PKA antagonist) and N-acetylcysteine (NAC, a powerful antioxidant).

Synthetic curcumin analog EF31 inhibits the growth of head and neck squamous cell carcinoma xenografts

Objectives are to examine the efficacy, pharmacokinetics, and toxicology of a synthetic curcumin analog EF31 in head and neck squamous cell carcinoma. The synthesis of EF31 was described for the first time. Solubility of EF24 and EF31 was compared using nephelometric analysis. Human head and neck squamous cell carcinoma Tu212 xenograft tumors were established in athymic nude mice and treated with EF31 i.p. once daily five days a week for about 5-6 weeks. The long term effect of EF31 on the NF-κB signaling system in the tumors was examined by Western blot analysis. EF31 at 25 mg kg(-1), i.p. inhibited tumor growth almost completely. Solubilities of EF24 and EF31 are <10 and 13 μg mL(-1) or <32 and 47 μM, respectively. The serum chemistry profiles of treated mice were within the limits of normal, they revealed a linear increase of C(max). EF31 decreased the level of phosphorylation of NF-κB p65. In conclusion, the novel synthetic curcumin analog EF31 is efficacious in inhibiting the growth of Tu212 xenograft tumors and may be useful for treating head and neck squamous cell carcinoma. The long term EF31 treatment inhibited NF-κB p65 phosphorylation in xenografts, implicating downregulation of cancer promoting transcription factors such as angiogenesis and metastasis.

Dynamic change of Numbl expression after sciatic nerve crush and its role in Schwann cell differentiation

Numbl, as a conserved homolog of Drosophila Numb, has been implicated in early development of the nervous system, but its expression and roles in nervous system lesion and repair remained unknown. Here, we performed an acute sciatic nerve injury model in adult rats and studied the dynamic changes of Numbl expression in the sciatic nerve. Temporally, Numbl expression was sharply decreased after sciatic nerve crush and reached a valley at day 7. Spatially, Numbl was widely expressed in the normal sciatic nerve, including axons and Schwann cells, whereas, after injury, Numbl expression was decreased predominantly in Schwann cells. In vitro, we induced Schwann cell differentiation with cAMP and found that Numbl expression was decreased in the differentiated process. Depletion of Numbl could promote Schwann cell differentiation. In addition, we demonstrated that in vitro myelination was suppressed by overexpression of Numbl in Schwann cells. Collectively, we hypothesized peripheral nerve injury induced a downregulation of Numbl in the sciatic nerve, which was associated with Schwann cell differentiation.

Nuclear factor-κB activation in Schwann cells regulates regeneration and remyelination

Schwann cells (SCs) are crucial for peripheral nerve development and regeneration; however, the intrinsic regulatory mechanisms governing postinjury responses are poorly understood. Activation and deacetylation of nuclear factor-κB (NF- κB) in SCs have been implicated as prerequisites for peripheral nerve myelination. Using GFAP-IκBα-dn mice in which NF- κB transcriptional activation is inhibited in SCs, we found no discernable differences in the quantity or structure of myelinated axons in adult facial nerves. Following crush injury, axonal regeneration was impaired at 31 days and significantly enhanced at 65 days in transgenic animals. Compact remyelination and Remak bundle organization were significantly compromised at 31 days and restored by 65 days post injury. Together, these data indicate that inhibition of NF-κB activation in SCs transiently delays axonal regeneration and compact remyelination. Manipulating the temporal activation of nuclear factor-κB in Schwann cells may offer new therapeutic avenues for PNS and CNS regeneration.

Cyclic AMP: a selective modulator of NF-κB action

It has been known for several decades that cyclic AMP (cAMP), a prototypical second messenger, transducing the action of a variety of G-protein-coupled receptor ligands, has potent immunosuppressive and anti-inflammatory actions. These actions have been attributed in part to the ability of cAMP-induced signals to interfere with the function of the proinflammatory transcription factor Nuclear Factor-kappaB (NF-κB). NF-κB plays a crucial role in switching on the gene expression of a plethora of inflammatory and immune mediators, and as such is one of the master regulators of the immune response and a key target for anti-inflammatory drug design. A number of fundamental molecular mechanisms, contributing to the overall inhibitory actions of cAMP on NF-κB function, are well established. Paradoxically, recent reports indicate that cAMP, via its main effector, the protein kinase A (PKA), also promotes NF-κB activity. Indeed, cAMP actions appear to be highly cell type- and context-dependent. Importantly, several novel players in the cAMP/NF-κB connection, which selectively direct cAMP action, have been recently identified. These findings not only open up exciting new research avenues but also reveal novel opportunities for the design of more selective, NF-κB-targeting, anti-inflammatory drugs.

Brain-derived neurotrophic factor (BDNF) induces polarized signaling of small GTPase (Rac1) protein at the onset of Schwann cell myelination through partitioning-defective 3 (Par3) protein

Brain-derived neurotrophic factor (BDNF) was shown to play a role in Schwann cell myelination by recruiting Par3 to the axon-glial interface, but the underlying mechanism has remained unclear. Here we report that Par3 regulates Rac1 activation by BDNF but not by NRG1-Type III in Schwann cells, although both ligands activate Rac1 in vivo. During development, active Rac1 signaling is localized to the axon-glial interface in Schwann cells by a Par3-dependent polarization mechanism. Knockdown of p75 and Par3 individually inhibits Rac1 activation, whereas constitutive activation of Rac1 disturbs the polarized activation of Rac1 in vivo. Polarized Rac1 activation is necessary for myelination as Par3 knockdown attenuates myelination in mouse sciatic nerves as well as in zebrafish. Specifically, Par3 knockdown in zebrafish disrupts proper alignment between the axon and Schwann cells without perturbing Schwann cell migration, suggesting that localized Rac1 activation at the axon-glial interface helps identify the initial wrapping sites. We therefore conclude that polarization of Rac1 activation is critical for myelination.