Axon degeneration: make the Schwann cell great again

Axonal degeneration is a pivotal feature of many neurodegenerative conditions and substantially accounts for neurological morbidity. A widely used experimental model to study the mechanisms of axonal degeneration is Wallerian degeneration(WD), which occurs after acute axonal injury. In the peripheral nervous system(PNS), WD is characterized by swift dismantling and clearance of injured axons with their myelin sheaths. This is a prerequisite for successful axonal regeneration. In the central nervous system(CNS), WD is much slower, which significantly contributes to failed axonal regeneration. Although it is well-documented that Schwann cells(SCs) have a critical role in the regenerative potential of the PNS, to date we have only scarce knowledge as to how SCs ‘sense' axonal injury and immediately respond to it. In this regard, it remains unknown as to whether SCs play the role of a passive bystander or an active director during the execution of the highly orchestrated disintegration program of axons. Older reports, together with more recent studies, suggest that SCs mount dynamic injury responses minutes after axonal injury, long before axonal breakdown occurs. The swift SC response to axonal injury could play either a pro-degenerative role, or alternatively a supportive role, to the integrity of distressed axons that have not yet committed to degenerate. Indeed, supporting the latter concept, recent findings in a chronic PNS neurodegeneration model indicate that deactivation of a key molecule promoting SC injury responses exacerbates axonal loss. If this holds true in a broader spectrum of conditions, it may provide the grounds for the development of new glia-centric therapeutic approaches to counteract axonal loss.     

Exosomes as mediators of neuron-glia communication in neuroinflammation

In recent years,a type of extracellular vesicles named exosomes has emerged that play an important role in intercellular communication under physiological and pathological conditions.These nanovesicles (30–150 nm) contain proteins,RNAs and lipids,and their internalization by bystander cells could alter their normal functions.This review focuses on recent knowledge about exosomes as messengers of neuron-glia communication and their participation in the physiological and pathological functions in the central nervous system.Special emphasis is placed on the role of exosomes under toxic or pathological stimuli within the brain,in which the glial exosomes containing inflammatory molecules are able to communicate with neurons and contribute to the pathogenesis of neuroinflammation and neurodegenerative disorders.Given the small size and characteristics of exosomes,they can cross the blood-brain barrier and be used as biomarkers and diagnosis for brain disorders and neuropathologies.Finally,although the application potential of exosome is still limited,current studies indicate that exosomes represent a promising strategy to gain pathogenic information to identify therapeutically targets and biomarkers for neurological disorders and neuroinflammation.     

Schwann cell‐derived exosomes enhance axonal regeneration in the peripheral nervous system

Axonal regeneration in the peripheral nervous system is greatly supported by Schwann cells (SCs). After nerve injury, SCs dedifferentiate to a progenitor‐like state and efficiently guide axons to their original target tissues. Contact and soluble factors participate in the crosstalk between SCs and axons during axonal regeneration. Here we show that dedifferentiated SCs secrete nano‐vesicles known as exosomes which are specifically internalized by axons. Surprisingly, SC‐derived exosomes markedly increase axonal regeneration in vitro and enhance regeneration after sciatic nerve injury in vivo. Exosomes shift the growth cone morphology to a pro‐regenerating phenotype and decrease the activity of the GTPase RhoA, involved in growth cone collapse and axon retraction. Altogether, our work identifies a novel mechanism by which SCs communicate with neighboring axons during regenerative processes. We propose that SC exosomes represent an important mechanism by which these cells locally support axonal maintenance and regeneration after nerve damage. GLIA 2013;61:1795–1806     

LIPUS-SCs-Exo promotes peripheral nerve regeneration in cavernous nerve crush injury-induced ED rats via PI3K/Akt/FoxO signaling pathway

Objective

Clinical treatment of erectile dysfunction (ED) caused by cavernous nerve (CN) injury during pelvic surgery is difficult. Low-intensity pulsed ultrasound (LIPUS) can be a potential strategy for neurogenic ED (NED). However, whether Schwann cells (SCs) can respond to LIPUS stimulation signals is unclear. This study aims to elucidate the signal transmission between SCs paracrine exosome (Exo) and neurons stimulated by LIPUS, as well as to analyze the role and potential mechanisms of exosomes in CN repair after injury.

Methods

The major pelvic ganglion (MPG) neurons and MPG/CN explants were stimulated with LIPUS of different energy intensities to explore the appropriate LIPUS energy intensity. The exosomes were isolated and purified from LIPUS-stimulated SCs (LIPUS-SCs-Exo) and non-stimulated SCs (SCs-Exo). The effects of LIPUS-SCs-Exo on neurite outgrowth, erectile function, and cavernous penis histology were identified in bilateral cavernous nerve crush injury (BCNI)-induced ED rats.

Results

LIPUS-SCs-Exo group can enhance the axon elongation of MPG/CN and MPG neurons compared to SCs-Exo group in vitro. Then, the LIPUS-SCs-Exo group showed a stronger ability to promote the injured CN regeneration and SCs proliferation compared to the SCs-Exo group in vivo. Furthermore, the LIPUS-SCs-Exo group increased the Max intracavernous pressure (ICP)/mean arterial pressure (MAP), lumen to parenchyma and smooth muscle to collagen ratios compared to the SCs-Exo group in vivo. Additionally, high-throughput sequencing combined with bioinformatics analysis revealed the differential expression of 1689 miRNAs between the SCs-Exo group and the LIPUS-SCs-Exo group. After LIPUS-SCs-Exo treatment, the phosphorylated levels of Phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt) and forkhead box O (FoxO) in MPG neurons increased significantly compared to negative control (NC) and SCs-Exo groups.

Conclusion

Our study revealed that LIPUS stimulation could regulate the gene of MPG neurons by changing miRNAs derived from SCs-Exo, then activating the PI3K-Akt-FoxO signal pathway to enhance nerve regeneration and restore erectile function. This study had important theoretical and practical significance for improving the NED treatment.     

原代培养许旺细胞的神经营养性作用受神经细胞的调节

许旺细胞（Ｓｃｈｗａｎｎ ｃｅｌｌｓ，ＳＣｓ）是外周神经的重要组成部分，在周围神经系统发育和再生中起着重要作用。为了研究许旺细胞的神经营养性作用及其产生的条件，我们将新生大鼠许旺细胞与１４ｄ胚龄的大鼠腹侧中脑细胞或皮质细胞联合培养，收集许旺细胞条件液并观察它们对培养的腹侧中脑多巴胺能神经元的支持作用。结果显示：与腹侧中脑细胞接触或不接触联合培养的许旺细胞条件液以及与皮质细胞不接触联合培养的许旺细胞     

Dedifferentiated Schwann cells secrete progranulin that enhances the survival and axon growth of motor neurons

Schwann cells (SCs), the primary glia in the peripheral nervous system (PNS), display remarkable plasticity in that fully mature SCs undergo dedifferentiation and convert to repair SCs upon nerve injury. Dedifferentiated SCs provide essential support for PNS regeneration by producing signals that enhance the survival and axon regrowth of damaged neurons, but the identities of neurotrophic factors remain incompletely understood. Here we show that SCs express and secrete progranulin (PGRN), depending on the differentiation status of SCs. PGRN expression and secretion markedly increased as primary SCs underwent dedifferentiation, while PGRN secretion was prevented by administration of cAMP, which induced SC differentiation. We also found that sciatic nerve injury, a physiological trigger of SC dedifferentiation, induced PGRN expression in SCs in vivo. These results suggest that dedifferentiated SCs express and secrete PGRN that functions as a paracrine factor to support the survival and axon growth of neighboring neurons after injury.     

Proteinase-activated receptors: novel signals for peripheral nerves

The discovery of proteinase-activated receptors (PARs) in the nervous system has led to new insights about the potential physiological functions of these enzymes, which were traditionally considered merely as degradative molecules. This review summarizes evidence that proteinases, through activation of PARs, interact with the peripheral nervous system (PNS), playing roles in neurogenic inflammation, pain perception, secretory and motor functions, as well as in the response to nerve injuries. Activation of PARs interferes with numerous physiological events that are under tight neural control, in addition to modulating nerve survival. New potential roles are suggested for members of the PAR family, highlighting proteinases and their receptors as potential therapeutic targets for diseases associated with PNS activation.     

Exosome Drug Delivery through the Blood–Brain Barrier: Experimental Approaches and Potential Applications

This review discusses the current experimental data on targeted drug delivery through the blood–brain barrier (BBB). We focus on exosome drug delivery and examine the unique biological structure of exosomes, which allows them to pack, store, and deliver various molecules to the brain. Various approaches to modification and engineering of exosomes are reviewed with an analysis of experimental studies of exosome drug delivery through the BBB. The problems associated with potential clinical uses of exosomes and their possible solutions are discussed.     

Differential expression of interleukin-10 mRNA in Wallerian degeneration and immune-mediated inflammation of the rat peripheral nervous system

Interleukin-10 (IL-10) is a potent immunosuppressant cytokine which downregulates MHC class II antigen expression and inflammatory cytokine production. In this study we localized mRNA for IL-10 in the rat peripheral nervous system (PNS) by nonradioactive in situ hybridization using a digoxygenin-labeled riboprobe specific for rat IL-10. IL-10 mRNA was expressed by some Schwann cells (SCs) in the normal sciatic nerve. During Wallerian degeneration, SCs strongly expressed IL-10 mRNA between days 2 and 4 after transection. By day 14 only occasional cells were positive for IL-10 mRNA. The vast majority of ED1-positive macrophages were IL-10 negative after axotomy. Contrastingly, infiltrating macrophages expressed IL-10 mRNA coincident with beginning clinical recovery in experimental autoimmune neuritis (EAN), the rat model of human Guillain-Barré syndrome. Our data suggest that SCs provide a constitutive immunosuppressant system in the PNS. In EAN additional macrophage-derived IL-10 may be important for the resolution of the T cell-mediated immune response. © 1996 Wiley-Liss, Inc.     

The therapeutic potential of natural killer cells in neuropathic pain

Novel disease-modifying treatments for neuropathic pain are urgently required. The cellular immune response to nerve injury represents a promising target for therapeutic development. Recently, the role of natural killer (NK) cells in both CNS and PNS disease has been the subject of growing interest. In this opinion article, we set out the case for NK cell-based intervention as a promising avenue for development in the management of neuropathic pain. We explore the potential cellular and molecular targets of NK cells in the PNS by contrasting with their reported functional roles in CNS diseases, and we suggest strategies for using the beneficial functions of NK cells and immune-based therapeutics in the context of neuropathic pain.     

Temporal expression pattern of peripheral myelin protein 22 during in vivo and in vitro myelination

Peripheral myelin protein 22 (PMP22) was initially described as a minor component of peripheral myelin. Mutations affecting the PMP22 gene cause demyelinating neuropathies, supporting a role for the protein in PNS myelination. Furthermore, PMP22 carries the L2/HNK-1 carbohydrate epitope suggesting an adhesion/recognition function. Despite advances in characterizing the PMP22 gene, the specific role(s) of the protein in myelin remains unknown. In this study we determined the temporal expression pattern of PMP22 in comparison to galactocerebroside (GalC) and myelin associated glycoprotein (MAG), early constituents of PNS myelin, and to protein zero (P0) and myelin basic protein (MBP), late components of myelin. In sciatic nerve lysates, PMP22 was detected at postnatal day 3, after MAG, but before MBP expression. The same results were obtained in cocultures of dorsal root ganglion neurons and Schwann cells (SCs). Low levels of PMP22 were found in early, anti-MAG and anti-GalC immunoreactive, myelinating cocultures. However, PMP22 could only be detected in the SC plasma membrane after basal lamina formation. In long-term myelinating cocultures PMP22 levels continued to increase and the protein was found in anti-P0 and anti-MBP immunoreactive myelin segments. Furthermore, PMP22, MBP, and P0 protein levels were greatly enhanced by progesterone treatment of the cocultures. The highest levels of PMP22 expression were associated with late stages of myelination; however the presence of the protein in nonmyelinating SCs and in SCs commencing myelination supports multiple roles for PMP22 in peripheral nerve biology.     

β-1,4-galactosyltransferase I promotes tumor necrosis factor-α autocrine via the activation of MAP kinase signal pathways in Schwann cells

Recent studies have demonstrated that aberrant galactosylation is associated with some inflammation diseases. β-1,4-Galactosyltransferase-I (β-1,4-GalT I), which transferred galactose to the terminal N-acetylglucosamine of N- and O-linked glycans in a β-1,4-linkage, was considered to be the major galactosyltransferase among the seven members of the subfamily responsible for β4 galactosylation. To elucidate the expression and possible function of β-1,4-GalT I in the peripheral nervous system (PNS) inflammatory diseases, we performed a tumor necrosis factor-alpha (TNF-α) autocrine inflammatory model in Schwann cells (SCs). In this study, we found that silencing of β-1,4-GalT I suppressed TNF-α autocrine, while overexpression of β-1,4-GalT I promoted TNF-α autocrine in TNF-α-treated SCs. Meanwhile, anti-TNFR1 antibody suppressed the expression of β-1,4-GalT I, and TNF-α autocrine. β-1,4-GalT I conferred its effect by promoting ERK, JNK, and P38 MAP kinase signal pathways activation in TNF-α-induced SCs. Thus, the present data shows that during SCs activation, β-1,4-GalT I may play an important role in the release of inflammatory mediators.     

Peroxisome proliferator-activated receptor-γ agonists suppress iNOS expression induced by LPS in rat primary Schwann cells

In bacterial-induced peripheral nervous system (PNS) inflammation, Schwann cells (SCs) are activated, producing inducible nitric oxide synthase (iNOS), contributed to the pathogenesis of demyelinating disease, such as multiple sclerosis. Peroxisome proliferator-activated receptor-gamma (PPAR-γ) has been shown to play a protective role in cellular inflammatory responses. Here we showed that LPS-induced iNOS biosynthesis was in a concentration and time-dependent manner. In LPS-treated primary SCs, retreatment with PPAR-γ agonist remitted the increase of iNOS, p38 phosphorylation and TLR4, MyD88, augmented the expression of PPAR-γ and localization in nuclear. Coadministration of GW 9662 reversed the effect of PPAR-γ agonists. These results suggest that PPAR-γ agonists, 15d-PGJ₂ and pioglitazone, had the anti-inflammatory effects.     

Voltage-gated sodium channels in neurological disorders

Voltage-gated sodium channels play an essential biophysical role in many excitable cells such as neurons. They transmit electrical signals through action potential (AP) generation and propagation in the peripheral (PNS) and central nervous systems (CNS). Each sodium channel is formed by one alpha-subunit and one or more beta-subunits. There is growing evidence indicating that mutations, changes in expression, or inappropriate modulation of these channels can lead to electrical instability of the cell membrane and inappropriate spontaneous activity observed during pathological states. This review describes the biochemical, biophysical and pharmacological properties of neuronal voltage-gated sodium channels (VGSC) and their implication in several neurological disorders.     

Schwann cells differentiated from spheroid-forming cells of rat subcutaneous fat tissue myelinate axons in the spinal cord injury

In the present study, we found that nestin-expressing spheroid cells derived from multipotent adipose stem cells of subcutaneous fat tissue could efficiently differentiate into Schwann cells (SCs) in vitro based on expression of SC markers such as A2B5, GFAP, O4, p75, S100, Sox10, Krox-20, and L1. The induced SC is engrafted to spinal cord injury lesions and formed a peripheral nervous system (PNS)-type myelin sheath on central nervous system (CNS) axons. PNS-type myelin sheath formation in repaired tissue was confirmed by transplantation of both induced PKH26-labeled SC and induced EGFP-expressing SC generated from EGFP transgenic rats. In addition to direct participation as myelin sheath-forming SC in repaired tissue, the induced SC also expressed several neurotrophic factors, as did native SC, which may suggest an additional role for induced SC in stimulation of endogenous healing responses. Thus, spheroid-forming cells from subcutaneous fat tissue demonstrated rapid and efficient induction into SC, and such cells show therapeutic promise for repair of damage to the CNS and PNS.