Targeted expression of a multifunctional chimeric neurotrophin in the lesioned sciatic nerve accelerates regeneration of sensory and motor axons

Lessons from Injury: How Nerve Injury Studies Reveal Basic Biological Mechanisms and Therapeutic Opportunities for Peripheral Nerve Diseases

Since Waller and Cajal in the nineteenth and early twentieth centuries, laboratory traumatic peripheral nerve injury studies have provided great insight into cellular and molecular mechanisms governing axon degeneration and the responses of Schwann cells, the major glial cell type of peripheral nerves. It is now evident that pathways underlying injury-induced axon degeneration and the Schwann cell injury-specific state, the repair Schwann cell, are relevant to many inherited and acquired disorders of peripheral nerves. This review provides a timely update on the molecular understanding of axon degeneration and formation of the repair Schwann cell. We discuss how nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile alpha TIR motif containing protein 1 (SARM1) are required for axon survival and degeneration, respectively, how transcription factor c-JUN is essential for the Schwann cell response to nerve injury and what each tells us about disease mechanisms and potential therapies. Human genetic association with NMNAT2 and SARM1 strongly suggests aberrant activation of programmed axon death in polyneuropathies and motor neuron disorders, respectively, and animal studies suggest wider involvement including in chemotherapy-induced and diabetic neuropathies. In repair Schwann cells, cJUN is aberrantly expressed in a wide variety of human acquired and inherited neuropathies. Animal models suggest it limits axon loss in both genetic and traumatic neuropathies, whereas in contrast, Schwann cell secreted Neuregulin-1 type 1 drives onion bulb pathology in CMT1A. Finally, we discuss opportunities for drug-based and gene therapies to prevent axon loss or manipulate the repair Schwann cell state to treat acquired and inherited neuropathies and neuronopathies.

BDNF is required for taste axon regeneration following unilateral chorda tympani nerve section

Taste nerves readily regenerate to reinnervate denervated taste buds; however, factors required for regeneration have not yet been identified. When the chorda tympani nerve is sectioned, expression of brain-derived neurotrophic factor (BDNF) remains high in the geniculate ganglion and lingual epithelium, despite the loss of taste buds. These observations suggest that BDNF is present in the taste system after nerve section and may support taste nerve regeneration. To test this hypothesis, we inducibly deleted Bdnf during adulthood in mice. Shortly after Bdnf gene recombination, the chorda tympani nerve was unilaterally sectioned causing a loss of both taste buds and neurons, irrespective of BDNF levels. Eight weeks after nerve section, however, regeneration was differentially affected by Bdnf deletion. In control mice, there was regeneration of the chorda tympani nerve and taste buds reappeared with innervation. In contrast, few taste buds were reinnervated in mice lacking normal Bdnf expression such that taste bud number remained low. In all genotypes, taste buds that were reinnervated were normal-sized, but non-innervated taste buds remained small and atrophic. On the side of the tongue contralateral to the nerve section, taste buds for some genotypes became larger and all taste buds remained innervated. Our findings suggest that BDNF is required for nerve regeneration following gustatory nerve section.

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.

Peripheral nerve injury modulates neurotrophin signaling in the peripheral and central nervous system

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75(NTR) and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.

The pros and cons of growth factors and cytokines in peripheral axon regeneration

Injury to a peripheral nerve induces a complex cellular and molecular response required for successful axon regeneration. Proliferating Schwann cells organize into chains of cells bridging the lesion site, which is invaded by macrophages. Approximately half of the injured neuron population sends out axons that enter the glial guidance channels in response to secreted neurotrophic factors and neuropoietic cytokines. These lesion-associated polypeptides create an environment that is highly supportive for axon regrowth, particularly after acute injury, and ensure that the vast majority of regenerating axons are directed toward the distal nerve stump. Unfortunately, most neurotrophic factors and neuropoietic cytokines are also strong stimulators of axonal sprouting. Although some of the axonal branches will withdraw at later stages, the sprouting effect contributes to the misdirection of reinnervation that results in the lack of functional recovery observed in many patients with peripheral nerve injuries. Here, we critically review the role of neuronal growth factors and cytokines during axon regeneration in the peripheral nervous system. Their differential effects on axon elongation and sprouting were elucidated in various studies on intraneuronal signaling mechanisms following nerve lesion. The present data define a goal for future therapeutic strategies, namely, to selectively stimulate a Ras/Raf/ERK-mediated axon elongation program over an intrinsic PI3K-dependent axonal sprouting program in lesioned motor and sensory neurons. Instead of modulating growth factor or cytokine levels at the lesion site, targeting specific intraneuronal molecules, such as the negative feedback inhibitors of ERK signaling, has been shown to promote long-distance regeneration while avoiding sprouting of regenerating axons until they have reached their target areas.

Neural and molecular features on Charcot-Marie-Tooth disease plasticity and therapy

In the peripheral nervous system disorders plasticity is related to changes on the axon and Schwann cell biology, and the synaptic formations and connections, which could be also a focus for therapeutic research. Charcot-Marie-Tooth disease (CMT) represents a large group of inherited peripheral neuropathies that involve mainly both motor and sensory nerves and induce muscular atrophy and weakness. Genetic analysis has identified several pathways and molecular mechanisms involving myelin structure and proper nerve myelination, transcriptional regulation, protein turnover, vesicle trafficking, axonal transport and mitochondrial dynamics. These pathogenic mechanisms affect the continuous signaling and dialogue between the Schwann cell and the axon, having as final result the loss of myelin and nerve maintenance; however, some late onset axonal CMT neuropathies are a consequence of Schwann cell specific changes not affecting myelin. Comprehension of molecular pathways involved in Schwann cell-axonal interactions is likely not only to increase the understanding of nerve biology but also to identify the molecular targets and cell pathways to design novel therapeutic approaches for inherited neuropathies but also for most common peripheral neuropathies. These approaches should improve the plasticity of the synaptic connections at the neuromuscular junction and regenerate cell viability based on improving myelin and axon interaction.

BAC transgenic mice reveal distal cis-regulatory elements governing BDNF gene expression

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family of neurotrophic factors, has important functions in the peripheral and central nervous system of vertebrates. We have generated bacterial artificial chromosome (BAC) transgenic mice harboring 207 kb of the rat BDNF (rBDNF) locus containing the gene, 13 kb of genomic sequences upstream of BDNF exon I, and 144 kb downstream of protein encoding exon IX, in which protein coding region was replaced with the lacZ reporter gene. This BDNF-BAC drove transgene expression in the brain, heart, and lung, recapitulating endogenous BDNF expression to a larger extent than shorter rat BDNF transgenes employed previously. Moreover, kainic acid induced the expression of the transgenic BDNF mRNA in the cerebral cortex and hippocampus through preferential activation of promoters I and IV, thus recapitulating neuronal activity-dependent transcription of the endogenous BDNF gene. genesis 48:214–219, 2010. © 2010 Wiley-Liss, Inc.

Acetyl-l-carnitine increases nerve regeneration and target organ reinnervation - a morphological study

Peripheral nerve injury frequently results in functional morbidity since standard management fails to adequately address many of the neurobiological hurdles to optimal regeneration. Neuronal survival and regeneration are neurotrophin dependent and require increased aerobic capacity. Acetyl-l-carnitine (ALCAR) facilitates this need and prevents neuronal loss. ALCAR is clinically safe and is shown here to significantly improve nerve regeneration and target organ reinnervation. Two groups of five rats underwent sciatic nerve division followed by immediate repair. One group received parenteral ALCAR (50mg/kg/day) from time of operation until termination at 12 weeks. A 'sham treatment' group received normal saline. A third group was left unoperated and did not receive any treatment. A segment of nerve was harvested between 5mm proximal and 10mm distal to the repair in operated groups, and at the corresponding level in the unoperated group. Mean axonal count in normal, non-axotomised nerve was 14,720 (SD 2378). That of the saline group (17,217 SD 1808) was not significantly different from normal nerve (P=0.0985). Mean number of myelinated axons in the ALCAR group (24,460 SD 3750) was significantly greater than both sham group (P<0.01) and normal nerve (P=0.0012). Mean myelin thickness in the saline treated group (0.408 microm SD 0.067 microm) was less than normal nerve (0.770 microm SD 0.143 microm) (P<0.001). Mean myelin thickness in the ALCAR group (0.627 microm SD 0.052 microm) was greater than the sham (saline) group (P<0.01) and not statistically different from normal nerve (P=0.07). ALCAR increased dermal PGP9.5 staining by 210% compared to sham treatment (P<0.0001) and significantly reduced the mean percentage weight loss in gastrocnemius muscle (ALCAR group 0.203% vs. 0.312% in sham group P=0.015). ALCAR not only increases the number of regenerating nerve fibres but also morphologically improves the quality of regeneration and target organ reinnervation. Adjuvant ALCAR treatment may improve both sensory and motor outcomes and merits further investigation.

Tryptophan 2,3-dioxygenase is a key modulator of physiological neurogenesis and anxiety-related behavior in mice

Although nutrients, including amino acids and their metabolites such as serotonin (5-HT), are strong modulators of anxiety-related behavior, the metabolic pathway(s) responsible for this physiological modulation is not fully understood. Regarding tryptophan (Trp), the initial rate-limiting enzymes for the kynurenine pathway of tryptophan metabolism are tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO). Here, we generated mice deficient for tdo (Tdo^-/-). Compared with wild-type littermates, Tdo^-/- mice showed increased plasma levels of Trp and its metabolites 5-hydroxyindoleacetic acid (5-HIAA) and kynurenine, as well as increased levels of Trp, 5-HT and 5-HIAA in the hippocampus and midbrain. These mice also showed anxiolytic modulation in the elevated plus maze and open field tests, and increased adult neurogenesis, as evidenced by double staining of BrdU and neural progenitor/neuronal markers. These findings demonstrate a direct molecular link between Trp metabolism and neurogenesis and anxiety-related behavior under physiological conditions.

Impaired hypoglossal nerve regeneration in mutant mice lacking complex gangliosides: down-regulation of neurotrophic factors and receptors as possible mechanisms

Gangliosides, sialic acid-containing glycosphingolipids, have been considered to play roles as neurotrophic factors. Exogenous gangliosides added to the culture medium of neuronal cells or injected in artificially injured sites of nerve tissues actually showed neurotrophic factor-like effects such as neurite extension and alleviation of nerve tissue deterioration. In this study, neuroregeneration in the mutant mice lacking complex gangliosides was examined. To determine whether the nervous system maintains regenerative activity in the long-term absence of complex gangliosides, we analyzed hypoglossal nerve regeneration after axotomy in the mutant mice of GM2/GD2 synthase. These mice exhibited marked impairment of regenerative activity both in the number of surviving neurons and in the number of peroxidase-positive neurons. Moreover, reduced levels of gene expression of neurotrophic factors and their receptors including CNTF, p75 NTR, TrkB, and others in hypoglossal neurons were observed in real-time reverse transcription-polymerase chain reaction combined with laser capture microdissection, suggesting that these molecules are, at least partly, involved in the regeneration of lesioned nerves and that their expression levels are precisely controlled in the presence of intact expression of complex gangliosides.

Genetic deletion of BACE1 in mice affects remyelination of sciatic nerves

BACE1 is a promising therapeutic and preventive target for Alzheimer's disease because it is essential for amyloid deposition. However, the recent demonstration of BACE1 in modulating developmental myelination in both peripheral and central nervous systems raises a concern of its effect on myelin maintenance or remyelination, and inhibition of these processes will potentially be detrimental to the BACE1 inhibitor users who are susceptible to myelination diseases such as adult peripheral nerve injury or multiple sclerosis. In this report, we investigated the role of BACE1 during peripheral nerve remyelination in wild-type (WT) and BACE1-null mice. We show here that genetic deletion of BACE1 affects sciatic nerve remyelination. The impaired remyelination appears to stem from the loss of neuregulin-1 cleavage by BACE1. To demonstrate a direct cleavage of neuregulin-1 by BACE1, we have identified a BACE1 cleavage site that turns out be highly conserved among neuregulin-1 paralogues. Moreover, we show that neuregulin-1 family member neuregulin-3 is also cleavable by BACE1. We hypothesize that the BACE1-cleaved extracellular domain of axonal neuregulin-1, perhaps neuregulin-3 as well, binds to Schwann cell ErbB receptors, which in turn regulate remyelination. Pharmacological inhibition of BACE1 should be carefully monitored to avoid alteration of signaling pathway that regulates remyelination.

Hepatocyte growth factor (HGF) attenuates gliosis and motoneuronal degeneration in the brainstem motor nuclei of a transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive loss of brainstem and spinal motoneurons. Although prevention of motoneuronal degeneration has been postulated as the primary target for a cure, accumulating evidence suggests that microglial accumulation contributes to disease progression. This study was designed to assess the ability of HGF to modulate microglial accumulation and motoneuronal degeneration in brainstem motor nuclei, using double transgenic mice overexpressing mutated SOD1(G93A) and HGF (G93A/HGF). Histological and immunohistochemical analyses of the tissues of G93A/HGF mice revealed a marked decrease in the number of microglia and reactive astrocytes and an attenuation of the loss of motoneurons in facial and hypoglossal nuclei compared with G93A mice. HGF overexpression attenuated monocyte chemoattractant protein-1 (MCP-1) induction, predominantly in astrocytes; suppressed activation of caspase-1, -3 and -9; and, increased X chromosome-linked inhibition of apoptosis protein (XIAP) in the motoneurons of G93A mice. The implication is that HGF reduces microglial accumulation by suppressing MCP-1 induction and prevents motoneuronal death through inhibition of pro-apoptotic protein activation. These findings suggest that, in addition to direct neurotrophic activity on motoneurons, HGF-suppression of gliosis may retard disease progression, making HGF a potential therapeutic agent for the treatment of ALS patients.

Identification and characterization of cholest-4-en-3-one, oxime (TRO19622), a novel drug candidate for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive death of cortical and spinal motor neurons, for which there is no effective treatment. Using a cell-based assay for compounds capable of preventing motor neuron cell death in vitro, a collection of approximately 40,000 low-molecular-weight compounds was screened to identify potential small-molecule therapeutics. We report the identification of cholest-4-en-3-one, oxime (TRO19622) as a potential drug candidate for the treatment of ALS. In vitro, TRO19622 promoted motor neuron survival in the absence of trophic support in a dose-dependent manner. In vivo, TRO19622 rescued motor neurons from axotomy-induced cell death in neonatal rats and promoted nerve regeneration following sciatic nerve crush in mice. In SOD1(G93A) transgenic mice, a model of familial ALS, TRO19622 treatment improved motor performance, delayed the onset of the clinical disease, and extended survival. TRO19622 bound directly to two components of the mitochondrial permeability transition pore: the voltage-dependent anion channel and the translocator protein 18 kDa (or peripheral benzodiazepine receptor), suggesting a potential mechanism for its neuroprotective activity. TRO19622 may have therapeutic potential for ALS and other motor neuron and neurodegenerative diseases.

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

Tissue plasminogen activator (tPA) is an essential component of the proteolytic cascade that lyses blood clots. Various studies also suggest that tPA plays important roles in the nervous system. We show that exogenous tPA or tPA/plasminogen (plg) promotes axonal regeneration, remyelination, and functional recovery after sciatic nerve injury in the mouse. Local application of tPA or tPA/plg 7 days after sciatic nerve crush significantly increased the total number of axons and myelinated axons, which is accompanied by enhanced expression of neurofilament. Treatment with tPA or tPA/plg reduced the deposition of fibrin(ogen) after nerve injury. Moreover, tPA or tPA/plg increased the number of macrophages and induced MMP-9 expression at the injury site, coincident with reduced collagen scar formation and accelerated clearance of myelin and lipid debris after treatment. Consequently, tPA or tPA/plg treatment protected muscles from atrophy after nerve injury, indicating better functional recovery. These results suggest that administration of exogenous tPA or tPA/plg promotes axonal regeneration and remyelination through removal of fibrin deposition and activation of MMP-9-positive macrophages, which may be responsible for myelin debris clearance and preventing collagen scar formation. Therefore, tPA may be useful for treatment of peripheral nerve injury.

Muscle-nerve-muscle neurotization for the reinnervation of denervated somatic muscle

Muscle-Nerve-Muscle (MNM) is the reinnervation of a denervated (recipient) muscle via a nerve graft inserted into the belly of an innervated (donor) muscle. MNM is studied for the reinnervation of intrinsic denervated somatic skeletal muscle by evaluating both restored muscle contractile ability and innervation state. In a rat model, muscle function is tested following MNM neurotization from an innervated (donor), extensor digitorum longus muscle to a denervated (recipient), peroneus digit quinti (PDQ) muscle. PDQ muscle cross-sections labeled for neural cell adhesion molecule protein (NCAM), a marker for fiber denervation. MNM neurotization results in the recovery of PDQ muscle force generating capacity (58% of Normal-control) and a significantly lower percentage of residual muscle fiber denervation (38% denervated) compared with the Denervated-control (79% denervated) group. MNM neurotization reinnervates 62% of the previously denervated muscle fibers in the PDQ muscle. No decrement in force capacity is observed in the donor EDL muscle. Nerve grafting for MNM neurotization may restore modest contractile function to denervated muscle and reinnervate relatively more denervated muscle fibers than the Denervated-control.

Response of motoneurons to neonatal sciatic nerve axotomy in Bax-knockout mice

Neonatal motoneurons (MNs) die rapidly after axotomy, a response that is mediated by the pro-apoptotic gene Bax and is followed by a mitochondria-mediated apoptotic cascade. Although motoneurons in neonatal Bax-deficient mice fail to degenerate following axotomy, it has not been previously examined whether the rescued MNs can regenerate following injury. We report here that although spinal MNs in Bax-knockout (Bax-KO) mice survive indefinitely, they undergo severe atrophy by 14 days after axotomy. By 1 month following axotomy, MN regeneration was observed and cellular atrophy was partially reversed. Interestingly, we observed that all MNs, including those previously rescued from normal developmental cell death in the embryo by Bax deletion, exhibit a regenerative response to peripheral nerve injury. The regenerative response may be mediated by specific trophic factors because the expression of glial cell line-derived neurotrophic factor (GDNF) was greatly increased in the proximal stump of injured nerves and application of a GDNF-blocking antibody greatly reduced regeneration/regrowth of rescued MNs in Bax-KO mice. These results indicate that MNs rescued from developmental or injury-induced cell death by Bax deletion have the potential to regenerate or regrow in response to nerve-derived signals following neonatal axotomy.

Repair of peripheral nerve transections with fibrin sealant containing neurotrophic factors

Peripheral nerve injury is often followed by incomplete recovery of function and sometimes associated with neuropathic pain. There is, therefore, need for therapies which improve the speed of recovery and the final functional outcome after peripheral nerve injuries. In addition, neuropathic pain is not easily dealt with clinically and should preferably be eliminated. Neurotrophic factors have well-documented abilities to support neuron survival and stimulate neurite outgrowth, making them excellent candidates for use in repairing injured nerves. We investigated the possible beneficial effects of repairing the transected rat sciatic nerve by local application of a fibrin sealant containing nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), or acidic fibroblast growth factor (aFGF). Fibrin sealant was used in conjunction with sutures. Evaluation of motor and sensory function, autotomy, and histological parameters was carried out from 1 to 12 weeks after injury. We demonstrate that NGF cotreatment decreased the occurance of autotomy, suggesting a reduction of neuropathic pain, and improved the performance in motor and sensory tests. In addition, the number of regenerating motoneurons was significantly increased after NGF administration. GDNF increased the speed of sensory recovery, but also markedly increased autotomy, indicating an increased degree of neuropathic pain. aFGF did not alter the outcome of the motor or sensory tests. Fibrin sealant could easily be used in conjunction with sutures to deliver neurotrophic substances locally to the damaged nerve and to enhance recovery of nerve function.

Neurotrophic factors and their receptors in axonal regeneration and functional recovery after peripheral nerve injury

Over a half a century of research has confirmed that neurotrophic factors promote the survival and process outgrowth of isolated neurons in vitro. The mechanisms by which neurotrophic factors mediate these survival-promoting effects have also been well characterized. In vivo, peripheral neurons are critically dependent on limited amounts of neurotrophic factors during development. After peripheral nerve injury, the adult mammalian peripheral nervous system responds by making neurotrophic factors once again available, either by autocrine or paracrine sources. Three families of neurotrophic factors were compared, the neurotrophins, the GDNF family of neurotrophic factors, and the neuropoetic cytokines. Following a general overview of the mechanisms by which these neurotrophic factors mediate their effects, we reviewed the temporal pattern of expression of the neurotrophic factors and their receptors by axotomized motoneurons as well as in the distal nerve stump after peripheral nerve injury. We discussed recent experiments from our lab and others which have examined the role of neurotrophic factors in peripheral nerve injury. Although our understanding of the mechanisms by which neurotrophic factors mediate their effects in vivo are poorly understood, evidence is beginning to emerge that similar phenomena observed in vitro also apply to nerve regeneration in vivo.

Overexpression of HGF retards disease progression and prolongs life span in a transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by a progressive loss of motoneurons and degeneration of motor axons. We show that overexpression of hepatocyte growth factor (HGF) in the nervous system attenuates motoneuron death and axonal degeneration and prolongs the life span of transgenic mice overexpressing mutated Cu2+/Zn2+ superoxide dismutase 1. HGF prevented induction of caspase-1 and inducible nitric oxide synthase (iNOS) in motoneurons and retained the levels of the glial-specific glutamate transporter (excitatory amino acid transporter 2/glutamate transporter 1) in reactive astrocytes. We propose that HGF may be the first example of an endogenous growth factor that can alleviate the symptoms of ALS by direct neurotrophic activities on motoneurons and indirect activities on glial cells, presumably favoring a reduction in glutamatergic neurotoxicity.

Identification of Gas6, a putative ligand for Sky and Axl receptor tyrosine kinases, as a novel neurotrophic factor for hippocampal neurons

Sky (also known as "Tyro3" and "Rse") is a member of the Axl/Sky/Mer receptor tyrosine kinase family and has two immunoglobulin-like repeats and two fibronectin type III-like repeats in the extracellular domain. Gas6 is a ligand for all members of the Axl family, each of which (Axl, Sky, and c-Mer) has different affinities to Gas6. Physiological functions of Sky and Gas6 in the nervous system are not well understood, despite their importance, which is suggested by Sky structural features and its predominant expression in the brain. We found in the RNase protection assays that gas6 and sky mRNAs are expressed in the adult rat hippocampus and are similarly regulated during development. Expression levels were low during embryonic stages but gradually increased during development and reached the highest level in adulthood. Sky, but not Axl, immunoreactivity was observed in the adult hippocampus. Recombinant rat Gas6 attenuated hippocampal neuronal cell death that was caused by serum starvation in vitro, indicating that Gas6 is a novel neurotrophic factor for hippocampal neurons. Gas6 showed regulated expression in the sciatic nerve after nerve transection. Therefore, Sky and Gas6 have neurotrophic roles in the nervous system.

Analysis of gene expression following sciatic nerve crush and spinal cord hemisection in the mouse by microarray expression profiling

1. The responses of periphery (PNS) and central nervous systems (CNS) towards nerve injury are different: while injured mammalian periphery nerons can successfully undergo regeneration, axons in the central nervous system are usually not able to regenerate. 2. In the present study, the genes which were differentially expressed in the PNS and CNS following nerve injury were identified and compared by microarray profiling techniques. 3. Sciatic nerve crush and hemisection of the spinal cord of adult mice were used as the models for nerve injury in PNS and CNS respectivey. 4. It was found that of all the genes examined, 14% (80/588) showed changes in expression following either PNS or CNS injury, and only 3% (18/588) showed changes in both types of injuries. 5. Among all the differentially expressed genes, only 8% (6/80) exhibited similar changes in gene expression (either up- or down-regulation) following injury in both PNS and CNS nerve injuries. 6. Our results indicated that microarray expression profiling is an efficient and useful method to identify genes that are involved in the regeneration process following nerve injuries, and several genes which are differentially expressed in the PNS and/or CNS following nerve injuries were identified in the present study.

Targeted disruption of the galanin gene reduces the number of sensory neurons and their regenerative capacity

The neuropeptide galanin is expressed developmentally in the dorsal root ganglion (DRG) and is rapidly up-regulated 120-fold after peripheral nerve section in the adult. Here we report that adult mice carrying a loss-of-function mutation in the galanin gene have a 13% reduction in the number of cells in the DRG associated with a 24% decrease in the percentage of neurons that express substance P. These deficits are associated with a 2.8- and 2.6-fold increase in the number of apoptotic cells in the DRG at postnatal days 3 and 4, respectively. After crush injury to the sciatic nerve, the rate of peripheral nerve regeneration is reduced by 35% with associated long-term functional deficits. Cultured DRG neurons from adult mutant mice demonstrate similar deficits in neurite number and length. These results identify a critical role for galanin in the development and regeneration of sensory neurons.

R, Silva CFD, Lainetti RD. Estudo experimental comparativo da ação das neurocinas cardiotrofina-1 e oncostatina-m na regeneração nervosa periférica

Os avanços das técnicas microcirúrgicas e o conhecimento detalhado do microambiente da regeneração podem contribuir significativamente na melhoria dos resultados das reparações nervosas periféricas. Nos últimos anos vários autores têm utilizado uma série de tecidos e substâncias interpostos entre os cotos de um nervo periférico seccionado, buscando estimular o crescimento axonal no local da lesão. Através da técnica de tubulização, os autores estudam o efeito de duas neurocinas, a cardiotrofina-1 (CT-1) e a oncostatina-M (OsM), no crescimento axonal e na sobrevida dos neurônios sensitivos nos gânglios da raiz dorsal de L5, após a lesão de nervos ciáticos em camundongos C57BL/6J. Utilizam 3 grupos de 7 animais que tiveram seus nervos seccionados e tubulizados com próteses de polietileno preenchidas com cardiotrofina-1, oncostatina-M e citocromo-C, associadas a um extrato de colágeno. Um quarto grupo de 3 animais, não operados, foi considerado por nós como grupo controle de normalidade. Após 4 semanas da cirurgia, os camundongos foram sacrificados, e realizada a contagem das fibras mielínicas nos cabos de regeneração retirados. Os gânglios das raizes dorsais de L5 também foram dissecados possibilitando a contagem dos neurônios sensitivos. Os dados foram analisados estatisticamente, permitindo concluir que as duas substâncias, utilizadas por nós, foram efetivas no estímulo ao brotamento axonal, porém, as mesmas não conseguiram impedir a morte dos neurônios sensitivos no gânglio da raiz dorsal de L5.

Tissue plasminogen activator-mediated fibrinolysis protects against axonal degeneration and demyelination after sciatic nerve injury

Tissue plasminogen activator (tPA) is a serine protease that converts plasminogen to plasmin and can trigger the degradation of extracellular matrix proteins. In the nervous system, under noninflammatory conditions, tPA contributes to excitotoxic neuronal death, probably through degradation of laminin. To evaluate the contribution of extracellular proteolysis in inflammatory neuronal degeneration, we performed sciatic nerve injury in mice. Proteolytic activity was increased in the nerve after injury, and this activity was primarily because of Schwann cell-produced tPA. To identify whether tPA release after nerve damage played a beneficial or deleterious role, we crushed the sciatic nerve of mice deficient for tPA. Axonal demyelination was exacerbated in the absence of tPA or plasminogen, indicating that tPA has a protective role in nerve injury, and that this protective effect is due to its proteolytic action on plasminogen. Axonal damage was correlated with increased fibrin(ogen) deposition, suggesting that this protein might play a role in neuronal injury. Consistent with this idea, the increased axonal degeneration phenotype in tPA- or plasminogen-deficient mice was ameliorated by genetic or pharmacological depletion of fibrinogen, identifying fibrin as the plasmin substrate in the nervous system under inflammatory axonal damage. This study shows that fibrin deposition exacerbates axonal injury, and that induction of an extracellular proteolytic cascade is a beneficial response of the tissue to remove fibrin. tPA/plasmin-mediated fibrinolysis may be a widespread protective mechanism in neuroinflammatory pathologies.

Upregulation of GFRalpha-1 and c-ret in primary sensory neurons and spinal motoneurons of aged rats

Aging is associated with a decline in neuromuscular and somatosensory functions. Senile muscle atrophy, considered to be of neurogenic origin, is prevalent, and sensory thresholds increase with age. However, the loss of motoneurons and primary sensory neurons is small, while sensory and motor innervation appears disturbed due to aging-related axon lesions. One mechanism which may play a role in this process is altered trophin signaling. We here report that the glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha-1 mRNA and GFRalpha-1 protein-like immunoreactivity are upregulated in spinal motoneurons, and in dorsal root ganglion neurons of 30-month-old rats. The established signaling mechanism for the GDNF/GFRalpha-1 complex is through binding to the tyrosine kinase receptor encoded by the c-ret proto-oncogene, and we also show here that c-ret mRNA is upregulated in both motoneurons and primary sensory neurons of aged rats. The findings reported here, combined with evidence presented in other studies of changes in p75(NTR) and trk receptor expressions in aging primary sensory neurons and motoneurons, point at marked alterations in trophin signaling in senescence.

Receptor recognition sites of cytokines are organized as exchangeable modules. Transfer of the leukemia inhibitory factor receptor-binding site from ciliary neurotrophic factor to interleukin-6

Interleukin-6 (IL-6) and ciliary neurotrophic factor (CNTF) are "4-helical bundle" cytokines of the IL-6 type family of neuropoietic and hematopoietic cytokines. IL-6 signals by induction of a gp130 homodimer (e.g. IL-6), whereas CNTF and leukemia inhibitory factor (LIF) signal via a heterodimer of gp130 and LIF receptor (LIFR). Despite binding to the same receptor component (gp130) and a similar protein structure, IL-6 and CNTF share only 6% sequence identity. Using molecular modeling we defined a putative LIFR binding epitope on CNTF that consists of three distinct regions (C-terminal A-helix/N-terminal AB loop, BC loop, C-terminal CD-loop/N-terminal D-helix). A corresponding gp130-binding site on IL-6 was exchanged with this epitope. The resulting IL-6/CNTF chimera lost the capacity to signal via gp130 on cells without LIFR, but acquired the ability to signal via the gp130/LIFR heterodimer and STAT3 on responsive cells. Besides identifying a specific LIFR binding epitope on CNTF, our results suggest that receptor recognition sites of cytokines are organized as modules that are exchangeable even between cytokines with limited sequence homology.

Emerging themes in structural biology of neurotrophic factors

Neurotrophic factors control the survival, differentiation and maintenance of neurons in the peripheral and central nervous systems. Their discovery and characterization have been instrumental to our understanding of a wide range of phenomena in the development, plasticity and repair of the nervous system. Their potential importance in the development of therapeutic agents against neurodegenerative disorders and nerve injury has led to a flurry of activity towards understanding their structure, function and signaling mechanisms. This knowledge has increased dramatically in recent years, in particular due to the elucidation of three-dimensional structures, the discovery of families of structurally related neurotrophic factors and the characterization of receptors and downstream signaling components. Common themes are emerging from these recent studies that allow us to make new insights and predictions as to the function and possible clinical utility of these molecules.