Exogenous tissue plasminogen activator enhances peripheral nerve regeneration and functional recovery after injury in mice

Fibrin(ogen) in human disease: both friend and foe

Fibrinogen is an abundant protein synthesized in the liver, present in human blood plasma at concentrations ranging from 1.5-4 g/L in healthy individuals with a normal half-life of 3-5 days. With fibrin, produced by thrombin-mediated cleavage, fibrinogen plays important roles in many physiological processes. Indeed, the formation of a stable blood clot, containing polymerized and cross-linked fibrin, is crucial to prevent blood loss and drive wound healing upon vascular injury. A balance between clotting, notably the conversion of fibrinogen to fibrin, and fibrinolysis, the proteolytic degradation of the fibrin mesh, is essential. Disruption of this equilibrium can cause disease in distinct manners. While some pathological conditions are the consequence of altered levels of fibrinogen, others are related to structural properties of the molecule. The source of fibrinogen expression and the localization of fibrin(ogen) protein also have clinical implications. Low levels of fibrinogen expression have been detected in extra-hepatic tissues, including carcinomas, potentially contributing to disease. Fibrin(ogen) deposits at aberrant sites including the central nervous system or kidney, can also be pathological. In this review, we discuss disorders in which fibrinogen and fibrin are implicated, highlighting mechanisms that may contribute to disease.

An Intimate Role for Adult Dorsal Root Ganglia Resident Cycling Cells in the Generation of Local Macrophages and Satellite Glial Cells

The intricate interactions between neurons, glial, and inflammatory cells within peripheral ganglia are physiologically important, but not well explored. Here, we show that adult dorsal root ganglia (DRG) contain populations of self-renewing cells, collectively referred as DRG resident cycling cells (DRCCs), that are active not only in "quiescent" ganglia but also accelerate their turnover in response to distal axotomy. An unexpected proportion of DRCCs were resident macrophages. These cells closely accompanied, and aligned with recycling satellite glial cells (SGCs) that were juxtaposed to sensory neurons and possessed stem cell-like properties. Selective inhibition of colony stimulating factor 1 receptor prevented the local proliferation of macrophages. Interestingly, DRCC turnover was accompanied by apoptosis at later intervals indicating a balanced cellular milieu in the DRGs. These findings identify a complex interactive multicellular DRG microenvironment supporting self-renewal of both macrophages and SGCs and its potential implications in the overall response of adult DRGs to injury.

Bottom up proteomics reveals novel differentiation proteins in neuroblastoma cells treated with 13-cis retinoic acid

Neuroblastoma, a cancer of the sympathetic nervous system, is the second most common pediatric cancer. A unique feature of neuroblastoma is remission in some patients due to spontaneous differentiation of metastatic tumors. 13-cis retinoic acid (13-cis RA) is currently used in the clinic to treat neuroblastoma due to its differentiation inducing effects. In this study, we used shotgun proteomics to identify proteins affected by 13-cis RA treatment in neuroblastoma SK-N-SH cells. Our results showed that 13-cis RA reduced proteins involved in extracellular matrix synthesis and organization and increased proteins involved in cell adhesion and neurofilament formation. These changes indicate that 13-cis RA induces tumor cell differentiation by decreasing extracellular matrix rigidity and increasing neurite overgrowth. Differentially-affected proteins identified in this study may be novel biomarkers of drug efficacy in the treatment of neuroblastoma. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of novel treatments. 13-cis retinoic acid is currently used in the clinic as a differentiation inducer. Here we have established a proteome map of SK-N-SH cells treated with 13-cis retinoic acid. Bioinformatic analysis revealed the involvement of development, differentiation, extracellular matrix assembly, collagen biosynthesis, and neurofilament bundle association. This proteome map provides information as to which proteins are important for differentiation and identifies networks that can be targeted by drugs to treat neuroblastoma [1].

The Complex Work of Proteases and Secretases in Wallerian Degeneration: Beyond Neuregulin-1

After damage, axons in the peripheral nervous system (PNS) regenerate and regrow following a process termed Wallerian degeneration, but the regenerative process is often incomplete and usually the system does not reach full recovery. Key steps to the creation of a permissive environment for axonal regrowth are the trans-differentiation of Schwann cells and the remodeling of the extracellular matrix (ECM). In this review article, we will discuss how proteases and secretases promote effective regeneration and remyelination. We will detail how they control neuregulin-1 (NRG-1) activity at the post-translational level, as the concerted action of alpha, beta and gamma secretases cooperates to balance activating and inhibitory signals necessary for physiological myelination and remyelination. In addition, we will discuss the role of other proteases in nerve repair, among which A Disintegrin And Metalloproteinases (ADAMs) and gamma-secretases substrates. Moreover, we will present how matrix metalloproteinases (MMPs) and proteases of the blood coagulation cascade participate in forming newly synthetized myelin and in regulating axonal regeneration. Overall, we will highlight how a deeper comprehension of secretases and proteases mechanism of action in Wallerian degeneration might be useful to develop new therapies with the potential of readily and efficiently improve the regenerative process.

L1cam-mediated developmental processes of the nervous system are differentially regulated by proteolytic processing

Normal brain development depends on tight temporal and spatial regulation of connections between cells. Mutations in L1cam, a member of the immunoglobulin (Ig) superfamily that mediate cell-cell contacts through homo- and heterophilic interactions, are associated with several developmental abnormalities of the nervous system, including mental retardation, limb spasticity, hydrocephalus, and corpus callosum aplasia. L1cam has been reported to be shed from the cell surface, but the significance of this during different phases of brain development is unknown. We here show that ADAM10-mediated shedding of L1cam is regulated by its fibronectin type III (FNIII) domains. Specifically, the third FNIII domain is important for maintaining a conformation where access to a membrane proximal cleavage site is restricted. To define the role of ADAM10/17/BACE1-mediated shedding of L1cam during brain development, we used a zebrafish model system. Knockdown of the zebrafish, l1camb, caused hydrocephalus, defects in axonal outgrowth, and myelination abnormalities. Rescue experiments with proteinase-resistant and soluble L1cam variants showed that proteolytic cleavage is not required for normal axonal outgrowth and development of the ventricular system. In contrast, metalloproteinase-mediated shedding is required for efficient myelination, and only specific fragments are able to mediate this stimulatory function of the shedded L1cam.

Tissue plasminogen activator and neuropathy open the blood-nerve barrier with upregulation of microRNA-155-5p in male rats

The blood-nerve barrier (BNB) consisting of the perineurium and endoneurial vessels is sealed by tight junction proteins. BNB alterations are a crucial factor in the pathogenesis of peripheral neuropathies. However, barrier opening, e.g. by tissue plasminogen activator (tPA), can also facilitate topical application of analgesics. Here, we examined tPA both in the pathophysiology of neuropathy-induced BNB opening or via exogenous application and its effect on the cytoplasmatic tight junction protein anchoring protein, zona occludens-1 (ZO-1), the adherens molecule JAM-C and microRNA(miR)-155-5p. Specifically, we investigated whether tPA alone and barrier opening lead to pain behavioral changes, i.e. hyperalgesia, or whether these effects require further factors. Male Wistar rats underwent chronic constriction injury (CCI) or were treated by a single perisciatic application of recombinant (r)tPA. CCI elicited mechanical allodynia, tPA mRNA upregulation, macrophage invasion, BNB leakage for large molecule tracers, downregulation of ZO-1 and JAM-C mRNA/protein, and a loss of immunoreactivity of both in perineurium and endoneurial cells. Similarly, after perisciatic rtPA injection, ZO-1 and JAM-C mRNA as well as cytosolic/membrane protein and ZO-1 immunoreactivity were downregulated, and the BNB was opened. Neither mechanical hypersensitivity nor macrophage infiltration was observed after rtPA in contrast to CCI. Mechanistically, miR-155-5p, which is known to destabilize barriers and tight junction proteins like claudin-1 and ZO-1, was increased in CCI and to lesser extent after rtPA application. In summary, tPA transiently opens the BNB possibly via miR-155-5p. However, tPA does not provoke allodynia in the absence of a neuropathic stimulus like a ligation or inflammation.

Secretome analysis of nerve repair mediating Schwann cells reveals Smad-dependent trophism

Schwann cells promote nerve regeneration by adaptation of a regenerative phenotype referred to as repair mediating Schwann cell. Down-regulation of myelin proteins, myelin clearance, formation of Bungner's bands, and secretion of trophic factors characterize this cell type. We have previously shown that the sphingosine-1-phosphate receptor agonist Fingolimod/FTY720P promotes the generation of this particular Schwann cell phenotype by activation of dedifferentiation markers and concomitant release of trophic factors resulting in enhanced neurite growth of dorsal root ganglion neurons. Despite its biomedical relevance, a detailed characterization of the corresponding Schwann cell secretome is lacking, and the impact of FTY720P on enhancing neurite growth is not defined. Here, we applied a label-free quantitative mass spectrometry approach to characterize the secretomes derived from primary neonatal and adult rat Schwann cells in response to FTY720P. We identified a large proportion of secreted proteins with a high overlap between the neonatal and adult Schwann cells, which can be associated with biologic processes such as development, axon growth, and regeneration. Moreover, FTY720P-treated Schwann cells release proteins downstream of Smad signaling known to support neurite growth. Our results therefore uncover a network of trophic factors involved in glial-mediated repair of the peripheral nervous system.-Schira, J., Heinen, A., Poschmann, G., Ziegler, B., Hartung, H.-P., Stühler, K., Küry, P. Secretome analysis of nerve repair mediating Schwann cells reveals Smad-dependent trophism.

Tissue plasminogen activator promotes white matter integrity and functional recovery in a murine model of traumatic brain injury

Recombinant tissue plasminogen activator (tPA) is a Food and Drug Administration-approved thrombolytic treatment for ischemic stroke. tPA is also naturally expressed in glial and neuronal cells of the brain, where it promotes axon outgrowth and synaptic plasticity. However, there are conflicting reports of harmful versus neuroprotective effects of tPA in acute brain injury models. Furthermore, its impact on white matter integrity in preclinical traumatic brain injury (TBI) has not been thoroughly explored, although white matter disruption is a better predictor of long-term clinical outcomes than focal lesion volumes. Here we show that the absence of endogenous tPA in knockout mice impedes long-term recovery of white matter and neurological function after TBI. tPA-knockout mice exhibited greater asymmetries in forepaw use, poorer sensorimotor balance and coordination, and inferior spatial learning and memory up to 35 d after TBI. White matter damage was also more prominent in tPA knockouts, as shown by diffusion tensor imaging, histological criteria, and electrophysiological assessments of axon conduction properties. Replenishment of tPA through intranasal application of the recombinant protein in tPA-knockout mice enhanced neurological function, the structural and functional integrity of white matter, and postinjury compensatory sprouting in corticofugal projections. tPA also promoted neurite outgrowth in vitro, partly through the epidermal growth factor receptor. Both endogenous and exogenous tPA protected against white matter injury after TBI without increasing intracerebral hemorrhage volumes. These results unveil a previously unappreciated role for tPA in the protection and/or repair of white matter and long-term functional recovery after TBI.

Characterization of Tissue Plasminogen Activator Expression and Trafficking in the Adult Murine Brain

Tissue plasminogen activator (tPA) is an immediate-early gene important for regulating physiological processes like synaptic plasticity and neurovascular coupling. It has also been implicated in several pathological processes including blood-brain barrier (BBB) permeability, seizure progression, and stroke. These varied reports suggest that tPA is a pleiotropic mediator whose actions are highly compartmentalized in space and time. The specific localization of tPA, therefore, can provide useful information about its function. Accordingly, the goal of this study was to provide a detailed characterization of tPA's regional, cellular, and subcellular localization in the brain. To achieve this, two new transgenic mouse lines were utilized: (1) a PlatβGAL reporter mouse, which houses the β-galactosidase gene in the tPA locus and (2) a tPABAC-Cerulean mouse, which has a cerulean-fluorescent protein fused in-frame to the tPA C-terminus. Using these two transgenic reporters, we show that while tPA is expressed throughout most regions of the adult murine brain, it appears to be preferentially targeted to fiber tracts in the limbic system. In the hippocampus, confocal microscopy revealed tPA-Cerulean (tPA-Cer) puncta localized to giant mossy fiber boutons (MFBs) and astrocytes in stratum lucidum. With amplification of the tPA-Cer signal, somatically localized tPA was also observed in the stratum oriens (SO)/alveus layer of both CA1 and CA3 subfields. Coimmunostaining of tPA-Cer and interneuronal markers indicates that these tPA-positive cell bodies belong to a subclass of somatostatin (SST)/oriens-lacunosum moleculare (O-LM) interneurons. Together, these data imply that tPA's localization is differentially regulated, suggesting that its neuromodulatory effects may be compartmentalized and specialized to cell type.

Effect of chitosan combined with hyaluronate on promoting the recovery of postoperative facial nerve regeneration and function in rabbits

To determine better solutions for postoperative nerve functional recovery, the effects of chitosan and hyaluronate on perineural scar formation and neural function recovery were investigated in 40 rabbits. Rabbits were randomized into 4 groups: A (chitosan), B (chitosan + hyaluronate), C (hyaluronate) and D (control). The rabbits underwent the same parotidectomy surgery, but different materials were used to cover the operated nerves. By evaluating specific indicators, including vibrissae motion tests, neural electrophysiological examinations and extraneural examinations, it was revealed that the amplitude of vibrissae motion of all groups had increased 6 weeks after surgery. The recovery of Group B was superior compared with all other groups at 4 and 12 weeks post-surgery; however no significant differences were detected. Group B exhibited a great number of nerve fibers, thicker myelin sheath and greater nerve conduction velocity. In summary, the use of a chitosan conduit combined with sodium hyaluronate gel may prevent perineural scar formation in facial nerves and promote nerve functional recovery.

Fibrinogen in neurological diseases: mechanisms, imaging and therapeutics

The blood coagulation protein fibrinogen is deposited in the brain in a wide range of neurological diseases and traumatic injuries with blood-brain barrier (BBB) disruption. Recent research has uncovered pleiotropic roles for fibrinogen in the activation of CNS inflammation, induction of scar formation in the brain, promotion of cognitive decline and inhibition of repair. Such diverse roles are possible in part because of the unique structure of fibrinogen, which contains multiple binding sites for cellular receptors and proteins expressed in the nervous system. The cellular and molecular mechanisms underlying the actions of fibrinogen are beginning to be elucidated, providing insight into its involvement in neurological diseases, such as multiple sclerosis, Alzheimer disease and traumatic CNS injury. Selective drug targeting to suppress the damaging functions of fibrinogen in the nervous system without affecting its beneficial effects in haemostasis opens a new fibrinogen therapeutics pipeline for neurological disease.

Schwann-Cell Autophagy, Functional Recovery, and Scar Reduction After Peripheral Nerve Repair

The functional outcome after peripheral nerve repair is often unpredictable for many reasons, e.g., the severity of neuronal death and scarring. Axonal degeneration significantly affects outcomes. Post-injury axonal degeneration in peripheral nerves is accompanied by myelin degradation initiated by Schwann cells (SCs), which activate autophagy, a ubiquitous cytoprotective process essential for degrading and recycling cellular constituents. Scar formation occurs concomitantly with nerve insult and axonal degeneration. The association between SC autophagy and the mechanisms of nerve scar formation is still unknown. A rat model of peripheral nerve lesions induced by sciatic nerve transection injuries was used to examine the function of autophagy in fibrosis reduction during the early phase of nerve repair. Rats were treated with rapamycin (autophagy inducer) or 3-methyladenine (autophagy inhibitor). One week after the nerve damage, fibrosis was potently inhibited in rapamycin-treated rats and, based on gait analysis, yielded a better functional outcome. Immunohistochemistry showed that the autophagic activity of SCs and the accumulation of neurofilaments were upregulated in rapamycin-treated rats. A deficiency of SC autophagic activity might be an early event in nerve scar formation, and modulating autophagy might be a powerful pharmacological approach for improving functional outcomes.

Ischemia-Reperfusion Injury of Sciatic Nerve in Rats: Protective Role of Combination of Vitamin C with E and Tissue Plasminogen Activator

An ischemia/reperfusion injury of rat's sciatic nerve was experimentally developed. In this model, we measured the in vivo production of superoxide radical, as a marker of oxidative stress and the occludin expression as an indicator of blood-nerve barrier function and we examined potential protective innervations against these abnormalities. Right sciatic nerves of the animals underwent 3 h of ischemia followed by 7 days of reperfusion and were divided into three groups: ischemic, pretreated with vitamin C in conjunction with vitamin E and treated with tissue plasminogen activator. Compared to measurements from left sciatic nerves used as sham, the ischemic group showed significantly increased superoxide radical and reduced expression of occludin in western blot and immunohistochemistry. No such differences were detected between sham and nerves in the vitamin or tissue plasminogen activator groups. It is suggested that the experimental ischemia/reperfusion model was suitable for studying the relationship between oxidative state and blood-nerve barrier. The reversion of abnormalities by the applied neuroprotective agents might prove to be a clinically important finding in view of the implication of vascular supply derangement in various neuropathies in humans.

MiR-340 Regulates Fibrinolysis and Axon Regrowth Following Sciatic Nerve Injury

After peripheral nerve injury, the degenerative debris and inflammatory alterations at the injury site may block the elongation of regenerating axons to reach target organs. Tissue plasminogen activator (tPA), a serine protease, has a capability of degrading matrix molecules and cell adhesions. In this study, we found that either tPA or miR-340 was differentially expressed in the injured nerve after sciatic nerve injury, and that the expressions of tPA and miR-340 were negatively correlated to each other. Moreover, miR-340 and tPA were co-localized in sciatic nerve. miR-340 regulated tPA through direct targeting of the 3'-UTR of tPA. Functionally, over- or under-expression of miR-340 reduced or augmented the fibrinolytic activity and migration ability of cultured Schwann cells as well as tPA secretion from the cells, respectively. In rats with sciatic nerve crush injury, dysregulation of the miR-340 expression in the injury site affected the cell debris removal and axonal regrowth. Obviously, unlike many previous studies that investigate the functional impact of miRNAs on peripheral nerve regeneration in the perspective of miRNA regulation of neural cell behaviors, the present study focused on miRNA regulation of debris clearance, thus updating our understanding of the regulatory roles of miRNAs in peripheral nerve regeneration.

Physiological and pathological roles of tissue plasminogen activator and its inhibitor neuroserpin in the nervous system

Although its roles in the vascular space are most well-known, tissue plasminogen activator (tPA) is widely expressed in the developing and adult nervous system, where its activity is believed to be regulated by neuroserpin, a predominantly brain-specific member of the serpin family of protease inhibitors. In the normal physiological state, tPA has been shown to play roles in the development and plasticity of the nervous system. Ischemic damage, however, may lead to excess tPA activity in the brain and this is believed to contribute to neurodegeneration. In this article, we briefly review the physiological and pathological roles of tPA in the nervous system, which includes neuronal migration, axonal growth, synaptic plasticity, neuroprotection and neurodegeneration, as well as a contribution to neurological disease. We summarize tPA's multiple mechanisms of action and also highlight the contributions of the inhibitor neuroserpin to these processes.

Chinese tuina downregulates the elevated levels of tissue plasminogen activator in sciatic nerve injured Sprague-Dawley rats

OBJECTIVE

To elucidate the mechanism of Chinese tuina in treating sciatic nerve crush injury, and to detect the levels of tissue plasminogen activator (tPA) and plasminogen activator inhibitor-1 (PAI-1), which is thought to play an important role in nerve regeneration.

METHODS

Thirty-two adult male Sprague-Dawley rats were subjected to sciatic nerve crush injury and 16 rats (sham-operated group) went through a sham operation. Control group was given no treatment while tuina group received tuina therapy since day 7 post-surgery. Tuina treatment was performed once a day and lasted for 20 days. The sciatic functional index was examined every 5 days during the treatment session. The rats' gastrocnemius muscles were evaluated for changes in mass and immunohistochemistry techniques were performed to detect the levels of tPA and PAI-1.

RESULTS

Tuina therapy improved the motor function of sciatic nerve injured rats (P<0.05), however, it did not increase muscle volume (P<0.05). Tuina downregulated the levels of tPA and PAI-1 (P<0.05).

CONCLUSIONS

The present study implies that tuina treatment could accelerate rehabilitation of peripheral nerve injury.

Inhibition of plasmin protects against colitis in mice by suppressing matrix metalloproteinase 9-mediated cytokine release from myeloid cells

BACKGROUND & AIMS

Activated proteases such as plasmin and matrix metalloproteinases (MMPs) are activated in intestinal tissues of patients with active inflammatory bowel diseases. We investigated the effect of plasmin on the progression of acute colitis.

METHODS

Colitis was induced in Mmp9(-/-), Plg(-/-), and C57BL/6 (control) mice by the administration of dextran sulfate sodium, trinitrobenzene sulfonic acid, or CD40 antibody. Plasmin was inhibited in control mice by intraperitoneal injection of YO-2, which blocks its active site. Mucosal and blood samples were collected and analyzed by reverse-transcription polymerase chain reaction and immunohistochemical analyses, as well as for mucosal inflammation and levels of cytokines and chemokines.

RESULTS

Circulating levels of plasmin were increased in mice with colitis, compared with controls. Colitis did not develop in control mice injected with YO-2 or in Plg(-/-) mice. Colons from these mice had reduced infiltration of Gr1+ neutrophils and F4/80+ macrophages, and reduced levels of inflammatory cytokines and chemokines. Colonic inflammation and colitis induction required activation of endogenous MMP9. After colitis induction, mice given YO-2, Plg(-/-) mice, and Mmp9(-/-) mice had reduced serum levels of tumor necrosis factor and C-X-C motif chemokine ligand 5, compared with control mice.

CONCLUSIONS

In mice, plasmin induces a feedback mechanism in which activation of the fibrinolytic system promotes the development of colitis via activation of MMP9 or proteolytic enzymes. The proteolytic environment stimulates the influx of myeloid cells into the colonic epithelium and the production of tumor necrosis factor and C-X-C motif chemokine ligand 5. In turn, myeloid CD11b+ cells release the urokinase plasminogen activator, which accelerates plasmin production. Disruption of the plasmin-induced chronic inflammatory circuit therefore might be a strategy for colitis treatment.

PACAP interacts with PAC1 receptors to induce tissue plasminogen activator (tPA) expression and activity in schwann cell-like cultures

Regeneration of peripheral nerves depends on the abilities of rejuvenating axons to migrate at the injury site through cellular debris and altered extracellular matrix, and then grow along the residual distal nerve sheath conduit and reinnervate synaptic targets. Considerable evidence suggest that glial cells participate in this process, although the mechanisms remain to be clarified. In cell culture, regenerating neurites secrete PACAP, a peptide shown to induce the expression of the protease tissue plasminogen activator (tPA) in neural cell types. In the present studies, we tested the hypothesis that PACAP can stimulate peripheral glial cells to produce tPA. More specifically, we addressed whether or not PACAP promoted the expression and activity of tPA in the Schwann cell line RT4-D6P2T, which shares biochemical and physical properties with Schwann cells. We found that PACAP dose- and time-dependently stimulated tPA expression both at the mRNA and protein level. Such effect was mimicked by maxadilan, a potent PAC1 receptor agonist, but not by the PACAP-related homolog VIP, suggesting a PAC1-mediated function. These actions appeared to be mediated at least in part by the Akt/CREB signaling cascade because wortmannin, a PI3K inhibitor, prevented peptide-driven CREB phosphorylation and tPA increase. Interestingly, treatment with BDNF mimicked PACAP actions on tPA, but acted through both the Akt and MAPK signaling pathways, while causing a robust increase in PACAP and PAC1 expression. PACAP6-38 totally blocked PACAP-driven tPA expression and in part hampered BDNF-mediated effects. We conclude that PACAP, acting through PAC1 receptors, stimulates tPA expression and activity in a Akt/CREB-dependent manner to promote proteolytic activity in Schwann-cell like cultures.

Genome wide expression profiling during spinal cord regeneration identifies comprehensive cellular responses in zebrafish

BACKGROUND

Among the vertebrates, teleost and urodele amphibians are capable of regenerating their central nervous system. We have used zebrafish as a model to study spinal cord injury and regeneration. Relatively little is known about the molecular mechanisms underlying spinal cord regeneration and information based on high density oligonucleotide microarray was not available. We have used a high density microarray to profile the temporal transcriptome dynamics during the entire phenomenon.

RESULTS

A total of 3842 genes expressed differentially with significant fold changes during spinal cord regeneration. Cluster analysis revealed event specific dynamic expression of genes related to inflammation, cell death, cell migration, cell proliferation, neurogenesis, neural patterning and axonal regrowth. Spatio-temporal analysis of stat3 expression suggested its possible function in controlling inflammation and cell proliferation. Genes involved in neurogenesis and their dorso-ventral patterning (sox2 and dbx2) are differentially expressed. Injury induced cell proliferation is controlled by many cell cycle regulators and some are commonly expressed in regenerating fin, heart and retina. Expression pattern of certain pathway genes are identified for the first time during regeneration of spinal cord. Several genes involved in PNS regeneration in mammals like stat3, socs3, atf3, mmp9 and sox11 are upregulated in zebrafish SCI thus creating PNS like environment after injury.

CONCLUSION

Our study provides a comprehensive genetic blue print of diverse cellular response(s) during regeneration of zebrafish spinal cord. The data highlights the importance of different event specific gene expression that could be better understood and manipulated further to induce successful regeneration in mammals.

Cleavage of the NR2B subunit amino terminus of N-methyl-D-aspartate (NMDA) receptor by tissue plasminogen activator: identification of the cleavage site and characterization of ifenprodil and glycine affinities on truncated NMDA receptor

Thrombolysis using tissue plasminogen activator (tPA) has been the key treatment for patients with acute ischemic stroke for the past decade. Recent studies, however, suggest that this clot-busting protease also plays various roles in brain physiological and pathophysiological glutamatergic-dependent processes, such as synaptic plasticity and neurodegeneration. In addition, increasing evidence implicates tPA as an important neuromodulator of the N-methyl-d-aspartate (NMDA) receptors. Here, we demonstrate that recombinant human tPA cleaves the NR2B subunit of NMDA receptor. Analysis of NR2B in rat brain lysates and cortical neurons treated with tPA revealed concentration- and time-dependent degradation of NR2B proteins. Peptide sequencing studies performed on the cleaved-off products obtained from the tPA treatment on a recombinant fusion protein of the amino-terminal domain of NR2B revealed that tPA-mediated cleavage occurred at arginine 67 (Arg(67)). This cleavage is tPA-specific, plasmin-independent, and removes a predicted ~4-kDa fragment (Arg(27)-Arg(67)) from the amino-terminal domain of the NR2B protein. Site-directed mutagenesis of putative cleavage site Arg(67) to Ala(67) impeded tPA-mediated degradation of recombinant protein. This analysis revealed that NR2B is a novel substrate of tPA and suggested that an Arg(27)-Arg(67)-truncated NR2B-containing NMDA receptor could be formed. Heterologous expression of NR2B with Gln(29)-Arg(67) deleted is functional but exhibits reduced ifenprodil inhibition and increased glycine EC(50) with no change in glutamate EC(50). Our results confirmed NR2B as a novel proteolytic substrate of tPA, where tPA may directly interact with NR2B subunits leading to a change in pharmacological properties of NR2B-containing NMDA receptors.

Subacute intranasal administration of tissue plasminogen activator increases functional recovery and axonal remodeling after stroke in rats

As a thrombolytic agent, application of recombinant tissue plasminogen activator (tPA) to ischemic stroke is limited by the narrow time window and side effects on brain edema and hemorrhage. This study examined whether tPA, administered by intranasal delivery directly targeting the brain and spinal cord, provides therapeutic benefit during the subacute phase after stroke. Adult male Wistar rats were subjected to permanent right middle cerebral artery occlusion (MCAo). Animals were treated intranasally with saline, 60 μg or 600 μg recombinant human tPA at 7 and 14days after MCAo (n=8/group), respectively. An adhesive-removal test and a foot-fault test were used to monitor functional recovery. Biotinylated dextran amine (BDA) was injected into the left motor cortex to anterogradely label the corticorubral tract (CRT) and the corticospinal tract (CST). Naive rats (n=6) were employed as normal control. Animals were euthanized 8 weeks after stroke. Compared with saline treated animals, significant functional improvements were evident in rats treated with 600 μg tPA (p<0.05), but not in 60 μg tPA treated rats. Furthermore, 600 μg tPA treatment significantly enhanced both CRT and CST sprouting originating from the contralesional cortex into the denervated side of the red nucleus and cervical gray matter compared with control group (p<0.01), respectively. The behavioral outcomes were highly correlated with CRT and CST axonal remodeling. Our data suggest that delayed tPA intranasal treatment provides therapeutic benefits for neurological recovery after stroke by, at least in part, promoting neuronal remodeling in the brain and spinal cord.

Histomorphometric changes in repaired mouse sciatic nerves are unaffected by the application of a scar-reducing agent

Microsurgical repair of transected peripheral nerves is compromised by the formation of scar tissue and the development of a neuroma, thereby limiting the success of regeneration. The aim of this study was to quantify histomorphometrically the structural changes in neural tissue that result from repair, and determine the effect of mannose-6-phosphate (M6P), a scar-reducing agent previously shown to enhance regeneration. In anaesthetised C57-black-6 mice, the left sciatic nerve was sectioned and repaired using four epineurial sutures. Either 100 μL of 600 mm M6P (five animals) or 100 μL of phosphate-buffered saline (placebo controls, five animals) was injected into and around the nerve repair site. A further group acted as sham-operated controls. After recovery for 6 weeks, the nerve was harvested for analysis using light and electron microscopy. Analysis revealed that when compared with sham controls, myelinated axons had smaller diameters both proximal and distal to the repair. Myelinated axon counts, axonal density and size all decreased across the repair site. There were normal numbers and densities of non-myelinated axons both proximal and distal to the repair. However, there were more Remak bundles distal to the repair site, and fewer non-myelinated axons per Remak bundle. Application of M6P did not affect any of these parameters.

Treatment of TBI with collagen scaffolds and human marrow stromal cells increases the expression of tissue plasminogen activator

This study examines the effects of combination therapy of collagen scaffolds and human marrow stromal cells (hMSCs) on the expression of tissue plasminogen activator (tPA) after traumatic brain injury (TBI) in rats. Adult male Wistar rats (n=48) were injured with controlled cortical impact and treated either with scaffolds suffused with hMSCs (3×10(6)) or hMSCs (3×10(6)) alone transplanted into the lesion cavity 1 week after TBI. A control group was treated with saline. Neurological function was assessed using the Morris Water Maze test (MWM) and modified Neurological Severity Scores (mNSS). The rats were sacrificed 14 days after TBI and brain samples were processed for immunohistochemical analysis and quantitative Western blot and quantitative real-time polymerase chain reaction (qRT-PCR) studies. Enhanced functional improvement was observed on both the mNSS and MWM tests in the scaffold+hMSC-treated group compared to the other two groups. Immunostaining with anti-human mitochondrial antibody (E5204) showed more hMSCs in the injury zone of the scaffold+hMSC group compared to the hMSC-alone group. Triple staining showed that more neurons were tPA-positive in the scaffold+hMSC group compared to the other two groups (p<0.05). Western blot analysis and qRT-PCR showed that scaffold+hMSC and hMSC-alone treatment enhanced the expression of tPA compared to controls (p<0.05), but tPA expression was significantly greater in the scaffold+hMSC group. The induction of tPA by hMSCs after TBI may be one of the mechanisms involved in promoting functional improvement after TBI.

The effect of Mannose-6-Phosphate on recovery after sciatic nerve repair

We have determined the effect of applying Mannose-6-Phosphate (M6P), a scar reducing agent, to a site of sciatic nerve repair. In anaesthetised C57-Black-6 mice, the left sciatic nerve was sectioned and repaired using 4 epineurial sutures. Either 100 μl of 600 mM Mannose-6-Phosphate (29 animals), or 100 μl of phosphate buffered saline as a placebo control (29 animals), was injected into and around the nerve repair site. A further group acted as sham-operated controls. After 6 or 12 weeks of recovery the extent of regeneration was assessed electrophysiologically and the percentage area of collagen staining at the repair site was analysed using picrosirius red and image analysis. Gait analysis was undertaken pre-operatively and at 1, 3, 6, 9 and 12 weeks postoperatively, to assess functional recovery. At 6 weeks the compound action potentials recorded from the regenerated nerves in the M6P group were significantly larger than in the placebo controls (P=0.015), and the conduction velocities were significantly faster (P=0.005), but there were no significant differences between these groups at 12 weeks. Gait analysis suggested better early functional recovery in the M6P group. In both repair groups there was a significant reduction in collagen staining between 6 and 12 weeks, suggestive of scar remodelling. We conclude that the normal scar remodelling process aids long term recovery in repaired nerves. Administration of 600 mM M6P to the nerve repair site enhances nerve regeneration and functional recovery in the early stages, and may lead to improved outcomes.

SMN deficiency reduces cellular ability to form stress granules, sensitizing cells to stress

Spinal Muscular Atrophy (SMA) is a neurodegenerative disease that is caused by deletion of the SMN (Survival of Motor Neuron) gene. The SMN protein is essential for cell survival and co-localized with TIA-1/R and G3BP, two characteristic markers of stress granules (SGs). To further study the SMN function in stress granules and in response to stress, we generated stable cell lines with SMN knockdown. Our data indicate that suppression of SMN drastically reduces cellular ability to form stress granules in response to stress treatment. In addition, we show that SMN deficiency sensitizes cells to sodium arsenite and H(2)O(2), two well-known stress inducers, leading to cell death at a much lower concentration of inducers in SMN knockdown cells than in control cells. Interestingly, the cell death is correlated with formation of stress granules, suggesting that involvement of SMN in formation of stress granules may play an important role in cell survival. Furthermore, rescue of SGs formation by overexpression of G3BP can reverse the defective formation of stress granules and results in partial abrogation of cell death against SMN deficiency. We deduce that modulation of stress response may be useful for potential SMN treatment.

Non-surgical therapies for peripheral nerve injury

BACKGROUND

Non-surgical approaches have been developed to enhance nerve recovery, which are complementary to surgery and are an adjunct to the reinnervation process.

SOURCES OF DATA

A search of PubMed, Medline, CINAHL, DH data and Embase databases was performed using the keywords 'peripheral nerve injury' and 'treatment'.

AREAS OF CONTROVERSY

Most of the conservative therapies are focused to control neuropathic pain after nerve tissue damage. Only physical therapy modalities have been studied in humans and their effectiveness is not proved.

GROWING POINTS

Many modalities have been experimented with to promote nerve healing and restore function in animal models and in vitro studies. Despite this, none have been actually translated into clinical practice.

AREAS TIMELY FOR DEVELOPING RESEARCH

The hypotheses proved in animals and in vitro should be translated to human clinical practice.

Dietary supplement with fermented soybeans, natto, improved the neurobehavioral deficits after sciatic nerve injury in rats

Clearance of fibrin and associated inflammatory cytokines by tissue-type plasminogen activator (t-PA) is related to improved regeneration in neurological disorder. The biological activity of fermented soybean (natto) is very similar to that of t-PA. We investigated the effect of the dietary supplement of natto on peripheral nerve regeneration. The peripheral nerve injury was produced by crushing the left sciatic nerve with a vessel clamp in Sprague-Dawley rats. The injured animals were fed orally either with saline or natto (16 mg/day) for seven consecutive days after injury. Increased functional outcome such as sciatic nerve functional index, angle of ankle, compound muscle action potential and conduction latency were observed in natto-treated group. Histological examination demonstrated that natto treatment improved injury-induced vacuole formation, S-100 and vessel immunoreactivities and axon loss. Oral intake of natto prolonged prothrombin time and reduced fibrinogen but did not change activated partial thromboplastin time and bleeding time. Furthermore, natto decreased injury-induced fibrin deposition, indicating a tolerant fibrinolytic activity. The treatment of natto significantly improved injury-induced disruption of blood-nerve barrier and loss of matrix component such as laminin and fibronectin. Sciatic nerve crush injury induced elevation of tumor necrosis factor alpha (TNF-alpha) production and caused apoptosis. The increased production of TNF-alpha and apoptosis were attenuated by natto treatment. These findings indicate that oral intake of natto has the potential to augment regeneration in peripheral nerve injury, possibly mediated by the clearance of fibrin and decreased production of TNF-alpha.

Cell-to-cell signaling at the neuromuscular junction: the dynamic role of the extracellular matrix

The extracellular matrix at the neuromuscular junction plays many roles. The matrix plays a structural role in that it maintains the spatial relationship between the muscle cell, Schwann cell, and presynaptic motor neuron. The matrix also plays a role in cell-to-cell signaling. The most studied member of this group is the heparan sulfate proteoglycan, agrin. Agrin is an integral member of the synaptic matrix, and it plays the pivotal role of instructing the muscle cell to aggregate acetylcholine receptors (AChRs) to the synapse. Agrin is released by the motor neuron, where it binds stably to the extracellular matrix. Agrin interacts with the muscle-specific tyrosine kinase (MuSK). Mice that lack agrin, or MuSK, fail to form neuromuscular junctions. Thus, the extracellular matrix is critical to both the structure and function of the neuromuscular junction. Remodeling of the extracellular matrix at the neuromuscular junction is needed to maintain stability, to allow growth, or to destabilize and remove synapses. Matrix metalloproteinases are key regulators of the extracellular matrix. In particular, matrix metalloproteinase 3 (MMP3) has been implicated in regulation of synaptic structure. MMP3 cleaves agrin. Antibodies to MMP3 recognize molecules concentrated at the synapses of frog neuromuscular junctions. Neuromuscular junctions in MMP3 null mutant mice have increased junctional folds, and AChR aggregates. Changes in synaptic activity will alter the activity of MMP3 at the synapse. Thus, the extracellular matrix is critical to the formation of the synapse, and synaptic activity controls the structure and function of the molecules in the extracellular matrix.

The plasminogen fibrinolytic pathway is required for hematopoietic regeneration

Hematopoietic stem cells within the bone marrow exist in a quiescent state. They can differentiate and proliferate in response to hematopoietic stress (e.g., myelosuppression), thereby ensuring a well-regulated supply of mature and immature hematopoietic cells within the circulation. However, little is known about how this stress response is coordinated. Here, we show that plasminogen (Plg), a classical fibrinolytic factor, is a key player in controlling this stress response. Deletion of Plg in mice prevented hematopoietic stem cells from entering the cell cycle and undergoing multilineage differentiation after myelosuppression, leading to the death of the mice. Activation of Plg by administration of tissue-type plasminogen activator promoted matrix metalloproteinase-mediated release of Kit ligand from stromal cells, thereby promoting hematopoietic progenitor cell proliferation and differentiation. Thus, activation of the fibrinolytic cascade is a critical step in regulating the hematopoietic stress response.

Interleukin-10 reduces scarring and enhances regeneration at a site of sciatic nerve repair

Axonal regeneration at a site of peripheral nerve repair can be impeded by the formation of scar tissue, which creates a mechanical barrier and initiates the development of multiple branched axonal sprouts that form a neuroma. We have investigated the hypothesis that the application of a scar-reducing agent to the nerve repair site would permit better axonal regeneration. In anaesthetised C57 Black-6 mice, the left sciatic nerve was sectioned and immediately re-approximated using four epineurial sutures. In five groups of eight mice, we injected transforming growth factor-beta3 (50 or 500 ng), interleukin-10 (IL-10) (125 or 500 ng), or saline into and around the repair site, both before and after the nerve section. Another group of eight animals acted as sham-operated controls. After 6 weeks, the outcome was assessed by recording compound action potentials (CAPs), measuring collagen levels using picrosirius red staining, and counting the number of myelinated axons proximal and distal to the repair. CAPs evoked by electrical stimulation distal to the repair were significantly smaller in all repair groups except for the low-dose IL-10 group, where they were not significantly different from that in controls. The area of staining for collagen had significantly increased in all repair groups except for the low-dose IL-10 group, which was not significantly different from that in controls. The myelinated fibre counts were always higher distal to the repair site, but there were no significant differences between groups. We conclude that administration of a low-dose of IL-10 to a site of sciatic nerve repair reduces scar formation and permits better regeneration of the damaged axons.

Comparison of blood-nerve barrier disruption and matrix metalloprotease-9 expression in injured central and peripheral nerves in mice

To investigate the involvement of blood-born factors and extracellular proteases in axonal degeneration and regeneration in both PNS and CNS, we directly compared the differences of blood-nerve barrier (BNB) disruption and matrix metalloprotease-9 (MMP-9) induction between the sciatic nerve and optic nerve after crush injury in the same animal. In sciatic nerve, BNB disruption, fibrin(ogen) deposition and MMP-9 expression were observed only in the first week following injury. Neurofilament (NF) immunoreactivity dramatically decreased in the first 2 days, gradually recovered to the normal levels by day 28. In contrast, the immunoglobulin G deposits spanned from 4 h to 28 days in crushed optic nerves. Fibrin(ogen) deposition was only observed in the first 2 days, while MMP-9 induction did not occur until a week after injury but lasted for 3 weeks in the crushed optic nerves. The NF immunoreactivity did not change much until day 7 and almost completely disappeared on day 28. The decrease of NF immunoreactivity coincided with the induction of MMP-9 after optic nerve crush. These results show that BNB disruption and MMP-9 induction are differentially regulated in the PNS and CNS after injuries, and they may contribute to the different regeneration capacities of the two systems.

Peripheral regeneration

Whereas the central nervous system (CNS) usually cannot regenerate, peripheral nerves regenerate spontaneously after injury because of a permissive environment and activation of the intrinsic growth capacity of neurons. Functional regeneration requires axon regrowth and remyelination of the regenerated axons by Schwann cells. Multiple factors including neurotrophic factors, extracellular matrix (ECM) proteins, and hormones participate in Schwann cell dedifferentiation, proliferation, and remyelination. We describe the current understanding of peripheral axon regeneration and focus on the molecules and potential mechanisms involved in remyelination.

Localization of annexin II in the paranodal regions and Schmidt-Lanterman incisures in the peripheral nervous system

Annexin II (AX II) is a member of the family of calcium-dependent actin- and phospholipid-binding proteins implicated in numerous intracellular functions such as signal transduction, membrane trafficking, and mRNA transport, as well as in the regulation of membrane/cytoskeleton contacts and extracellular functions. AX II is expressed in the central nervous system (CNS) and is upregulated in some pathological conditions. However, expression and localization of this protein in the peripheral nervous system (PNS) is still uncertain. In the present study, we examined the expression and distribution of AX II in the PNS. By western blot analysis, we found that a higher level of AX II was present in sciatic nerve homogenates than in brain homogenates. RT-PCR of total RNA from rat sciatic nerves revealed that AX II was synthesized within the nerves. Immunohistological analysis showed the characteristic distribution of AX II in Schmidt-Lanterman incisures (SLI) as well as in the paranodal regions. Localization of AX II in the PNS was examined in two mutant mouse models, shiverer and cerebroside sulfotransferase knockout mice, both of which show increased numbers of SLI. The paranodal axo-glial junction is also disrupted in the latter. Interestingly, the staining intensities of AX II in these regions were increased markedly in both mutants, suggesting that not only the numbers but also AX II content in each incisure and paranodal loop were affected. From its characteristic distribution and molecular features, AX II may be important for myelin function in the PNS.

The effect of antibodies to TGF-beta1 and TGF-beta2 at a site of sciatic nerve repair

Scar formation at a site of nerve injury can cause a mechanical barrier to axonal regeneration and lead to the development of multiple axonal sprouts to form a neuroma. We have investigated the hypothesis that the application of a scar-preventing agent to a nerve repair site would enhance regeneration of the nerve and reduce neuroma formation. The left sciatic nerve was exposed under general anaesthesia in 18 adult Sprague-Dawley rats. In 12 animals, the nerve was sectioned and immediately re-approximated using four epineurial sutures, and in 6 of these animals neutralising antibodies to transforming growth factor (TGF)-beta1 and TGF-beta2 were injected into and around the repair site. The six other animals acted as controls. After 7 weeks, the outcome was assessed by recording compound action potential (CAP) ratios, measuring collagen levels using picrosirius red staining, and counting the number of myelinated axons proximal and distal to the repair. After repair alone, the mean percentage of area of staining (PAS) for collagen within the nerve had significantly increased. However, after repair with the administration of antibodies, the PAS was not significantly different from that in the sham controls. After administration of antibodies, the CAP ratios were significantly smaller than in controls but not after repair alone. In both nerve injury groups, the myelinated fibre counts were significantly increased distal to the injury site, but there was no difference between these two groups. We conclude that administration of antibodies to TGF-beta1 and TGF-beta2 reduced scar formation at the repair site but did not enhance regeneration of the nerve or reduce the development of multiple axonal sprouts.

Tissue type plasminogen activator induced in rat dorsal horn astrocytes contributes to mechanical hypersensitivity following dorsal root injury

Dorsal root injury is known to induce alteration of the extracellular environment in the spinal cord and synaptic reorganization with degradation of injured primary afferent and sprouting of spared terminal. These changes affect behavioral sensitivity and sometimes lead to neuropathic pain. We have hypothesized that changes in extracellular proteolysis in the dorsal horn is involved in neuroplastic changes in the dorsal horn after nerve injury. Tissue type plasminogen activator (tPA) is a well-known extracellular serine protease and is involved in the modification of the extracellular matrix, which leads to neuroplastic changes such as long-term potentiation in the hippocampus. In the present study, we found a marked induction of tPA in activated astrocytes following L4/5 root injury and a resultant increase of proteolytic enzymatic activity in the dorsal horn. We also examined the involvement of tPA activity on mechanical hypersensitivity using a root ligation model which has been used for investigating radiculopathy pain behavior. Intrathecal and continuous administration of tPA inhibitor, tPA-STOP, suppressed root ligation-induced mechanical allodynia in a dose-dependent manner during an early stage of injury (0-4 days). In contrast, the delayed administration of tPA-STOP during the chronic stage of injury (10 days) did not affect pain behavior. These data suggest an important contribution of astrocytes in the dorsal horn to the pathophysiology of radiculopathy pain, and astrocyte-derived tPA and the proteolytic activity in the dorsal horn may be one of the essential factors involved in pain following root injury.

An inhibitor of serine proteases, neuroserpin, acts as a neuroprotective agent in a mouse model of neurodegenerative disease

Various studies suggest that proteolytic activity may be involved in a number of neurodegenerative disorders, including stroke and seizure. In this report, we examined the role of tryptic serine proteases, plasminogen activators (PAs), in the evolution of a neurodegenerative disease. Transgenic mice overexpressing an axonally secreted inhibitor of serine proteases (neuroserpin) were crossed with mice characterized by a "dying-back" motor neuron disease [progressive motor neuronopathy (pmn/pmn)]. Compared with pmn/pmn mice that showed an increase in PA activity, double mutant mice had decreased PA activity in sciatic nerves and spinal cord; their lifespan was increased by 50%, their motor behavior was stabilized, and histological analysis revealed increased numbers of myelinated axons and rescue of motoneuron number and size. This is the first report showing that a class of serine proteases (PAs) may be involved in the pathogenesis of a motor neuron disease and more specifically in axonal degeneration. Inhibiting serine proteases could offer a new strategy for delaying these disorders.