Arginase I and polyamines act downstream from cyclic AMP in overcoming inhibition of axonal growth MAG and myelin in vitro

Cyclic AMP and Polyamines Overcome Inhibition by Myelin-Associated Glycoprotein through eIF5A-Mediated Increases in p35 Expression and Activation of Cdk5

Inhibitory molecules associated with CNS myelin, such as myelin-associated glycoprotein (MAG), represent major obstacles to axonal regeneration following CNS injury. Our laboratory has shown that elevating levels of intracellular cAMP, via application of the nonhydrolyzable analog dibutyryl cAMP (dbcAMP), can block the inhibitory effects of MAG and myelin. We have also shown that elevation of cAMP results in upregulation of arginase I and increased polyamine synthesis. Treatment with putrescine or spermidine blocks myelin-mediated inhibition of neurite outgrowth, but the mechanism underlying this effect has not yet been elucidated. Here we show that cyclin-dependent kinase 5 (Cdk5) is required for dbcAMP and putrescine to overcome MAG-mediated inhibition. The ability of dbcAMP and putrescine to overcome inhibition by MAG is abolished in the presence of roscovitine, a Cdk inhibitor that has greater selectivity for Cdk5, and expression of dominant negative Cdk5 abolishes the ability of dbcAMP or putrescine to enhance neurite outgrowth in the presence of MAG. Importantly, dbcAMP and putrescine increase expression of p35, the neuron-specific activator of Cdk5, and rat DRG neurons transduced with HSV overexpressing p35 can overcome inhibition by MAG. The upregulation of p35 by putrescine is also reflected in increased localization of p35 to neurites and growth cones. Last, we show that putrescine upregulates p35 expression by serving as a substrate for hypusine modification of eIF5A, and that this hypusination is necessary for putrescine's ability to overcome inhibition by MAG. Our findings reveal a previously unknown mechanism by which polyamines may encourage regeneration after CNS injury.

Protein Prenylation Constitutes an Endogenous Brake on Axonal Growth

Suboptimal axonal regeneration contributes to the consequences of nervous system trauma and neurodegenerative disease, but the intrinsic mechanisms that regulate axon growth remain unclear. We screened 50,400 small molecules for their ability to promote axon outgrowth on inhibitory substrata. The most potent hits were the statins, which stimulated growth of all mouse- and human-patient-derived neurons tested, both in vitro and in vivo, as did combined inhibition of the protein prenylation enzymes farnesyltransferase (PFT) and geranylgeranyl transferase I (PGGT-1). Compensatory sprouting of motor axons may delay clinical onset of amyotrophic lateral sclerosis (ALS). Accordingly, elevated levels of PGGT1B, which would be predicted to reduce sprouting, were found in motor neurons of early- versus late-onset ALS patients postmortem. The mevalonate-prenylation pathway therefore constitutes an endogenous brake on axonal growth, and its inhibition provides a potential therapeutic approach to accelerate neuronal regeneration in humans.

BDNF promotes the axonal regrowth after sciatic nerve crush through intrinsic neuronal capability upregulation and distal portion protection

Nowadays peripheral nerve injurie occurs more common, the outcome is often poor because of the ineffective treatment. Recent researches revealed the duration of BDNF administration acts a positive role during the nerve regeneration, but its potential mechanisms beneath the behavioral recovery and axonal regrowth after peripheral nerve injury are still controversial. To observe the potential mechanisms we established sciatic nerve injury model and detected the expression of several axonal regeneration and function related genes. The results showed that, BDNF promotes axonal regrowth through increasing the activation of neuronal intrinsic growth capacity and strengthening the deference effects against distal portion atrophy. To further study, we determined the expression of protein associated to neuronal intrinsic growth capacity and investigated the ultrastructure of the distal portion of the injured nerve were analyzed. These data revealed that BDNF triggers multiple effects including neuronal intrinsic growth capacity improvement and distal portion atrophy protection to promote behavioral recovery following sciatic nerve crush injury in mouse.

Canadian Association of Neuroscience Review: Axonal Regeneration in the Peripheral and Central Nervous Systems – Current Issues and Advances

Injured nerves regenerate their axons in the peripheral (PNS) but not the central nervous system (CNS). The contrasting capacities have been attributed to the growth permissive Schwann cells in the PNS and the growth inhibitory environment of the oligodendrocytes in the CNS. In the current review, we first contrast the robust regenerative response of injured PNS neurons with the weak response of the CNS neurons, and the capacity of Schwann cells and not the oligodendrocytes to support axonal regeneration. We then consider the factors that limit axonal regeneration in both the PNS and CNS. Limiting factors in the PNS include slow regeneration of axons across the injury site, progressive decline in the regenerative capacity of axotomized neurons (chronic axotomy) and progressive failure of denervated Schwann cells to support axonal regeneration (chronic denervation). In the CNS on the other hand, it is the poor regenerative response of neurons, the inhibitory proteins that are expressed by oligodendrocytes and act via a common receptor on CNS neurons, and the formation of the glial scar that prevent axonal regeneration in the CNS. Strategies to overcome these limitations in the PNS are considered in detail and contrasted with strategies in the CNS.

Sustained Arginase 1 Expression Modulates Pathological Tau Deposits in a Mouse Model of Tauopathy

Tau accumulation remains one of the closest correlates of neuronal loss in Alzheimer's disease. In addition, tau associates with several other neurodegenerative diseases, collectively known as tauopathies, in which clinical phenotypes manifest as cognitive impairment, behavioral disturbances, and motor impairment. Polyamines act as bivalent regulators of cellular function and are involved in numerous biological processes. The regulation of the polyamines system can become dysfunctional during disease states. Arginase 1 (Arg1) and nitric oxide synthases compete for l-arginine to produce either polyamines or nitric oxide, respectively. Herein, we show that overexpression of Arg1 using adeno-associated virus (AAV) in the CNS of rTg4510 tau transgenic mice significantly reduced phospho-tau species and tangle pathology. Sustained Arg1 overexpression decreased several kinases capable of phosphorylating tau, decreased inflammation, and modulated changes in the mammalian target of rapamycin and related proteins, suggesting activation of autophagy. Arg1 overexpression also mitigated hippocampal atrophy in tau transgenic mice. Conversely, conditional deletion of Arg1 in myeloid cells resulted in increased tau accumulation relative to Arg1-sufficient mice after transduction with a recombinant AAV-tau construct. These data suggest that Arg1 and the polyamine pathway may offer novel therapeutic targets for tauopathies.

Neuro-immune Interactions Drive Tissue Programming in Intestinal Macrophages

Proper adaptation to environmental perturbations is essential for tissue homeostasis. In the intestine, diverse environmental cues can be sensed by immune cells, which must balance resistance to microorganisms with tolerance, avoiding excess tissue damage. By applying imaging and transcriptional profiling tools, we interrogated how distinct microenvironments in the gut regulate resident macrophages. We discovered that macrophages exhibit a high degree of gene-expression specialization dependent on their proximity to the gut lumen. Lamina propria macrophages (LpMs) preferentially expressed a pro-inflammatory phenotype when compared to muscularis macrophages (MMs), which displayed a tissue-protective phenotype. Upon luminal bacterial infection, MMs further enhanced tissue-protective programs, and this was attributed to swift activation of extrinsic sympathetic neurons innervating the gut muscularis and norepinephrine signaling to β2 adrenergic receptors on MMs. Our results reveal unique intra-tissue macrophage specialization and identify neuro-immune communication between enteric neurons and macrophages that induces rapid tissue-protective responses to distal perturbations.

Purines in neurite growth and astroglia activation

The mammalian nervous system is a complex, functional network of neurons, consisting of local and long-range connections. Neuronal growth is highly coordinated by a variety of extracellular and intracellular signaling molecules. Purines turned out to be an essential component of these processes. Here, we review the current knowledge about the involvement of purinergic signaling in the regulation of neuronal development. We particularly focus on its role in neuritogenesis: the formation and extension of neurites. In the course of maturation mammals generally lose their ability to regenerate the central nervous system (CNS) e.g. after traumatic brain injury; although, spontaneous regeneration still occurs in the peripheral nervous system (PNS). Thus, it is crucial to translate the knowledge about CNS development and PNS regeneration into novel approaches to enable neurons of the mature CNS to regenerate. In this context we give a general overview of growth-inhibitory and growth-stimulatory factors and mechanisms involved in neurite growth. With regard to neuronal growth, astrocytes are an important cell population. They provide structural and metabolic support to neurons and actively participate in brain signaling. Astrocytes respond to injury with beneficial or detrimental reactions with regard to axonal growth. In this review we present the current knowledge of purines in these glial functions. Moreover, we discuss organotypic brain slice co-cultures as a model which retains neuron-glia interactions, and further presents at once a model for CNS development and regeneration. In summary, the purinergic system is a pivotal factor in neuronal development and in the response to injury. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Axon Regeneration Is Regulated by Ets-C/EBP Transcription Complexes Generated by Activation of the cAMP/Ca2+ Signaling Pathways

The ability of specific neurons to regenerate their axons after injury is governed by cell-intrinsic regeneration pathways. In Caenorhabditis elegans, the JNK and p38 MAPK pathways are important for axon regeneration. Axonal injury induces expression of the svh-2 gene encoding a receptor tyrosine kinase, stimulation of which by the SVH-1 growth factor leads to activation of the JNK pathway. Here, we identify ETS-4 and CEBP-1, related to mammalian Ets and C/EBP, respectively, as transcriptional activators of svh-2 expression following axon injury. ETS-4 and CEBP-1 function downstream of the cAMP and Ca2+-p38 MAPK pathways, respectively. We show that PKA-dependent phosphorylation of ETS-4 promotes its complex formation with CEBP-1. Furthermore, activation of both cAMP and Ca2+ signaling is required for activation of svh-2 expression. Thus, the cAMP/Ca2+ signaling pathways cooperatively activate the JNK pathway, which then promotes axon regeneration.

Acute Putrescine Supplementation with Schwann Cell Implantation Improves Sensory and Serotonergic Axon Growth and Functional Recovery in Spinal Cord Injured Rats

Schwann cell (SC) transplantation exhibits significant potential for spinal cord injury (SCI) repair and its use as a therapeutic modality has now progressed to clinical trials for subacute and chronic human SCI. Although SC implants provide a receptive environment for axonal regrowth and support functional recovery in a number of experimental SCI models, axonal regeneration is largely limited to local systems and the behavioral improvements are modest without additional combinatory approaches. In the current study we investigated whether the concurrent delivery of the polyamine putrescine, started either 30 min or 1 week after SCI, could enhance the efficacy of SCs when implanted subacutely (1 week after injury) into the contused rat spinal cord. Polyamines are ubiquitous organic cations that play an important role in the regulation of the cell cycle, cell division, cytoskeletal organization, and cell differentiation. We show that the combination of putrescine with SCs provides a significant increase in implant size, an enhancement in axonal (sensory and serotonergic) sparing and/or growth, and improved open field locomotion after SCI, as compared to SC implantation alone. These findings demonstrate that polyamine supplementation can augment the effectiveness of SCs when used as a therapeutic approach for subacute SCI repair.

miR-142-3p is a Potential Therapeutic Target for Sensory Function Recovery of Spinal Cord Injury

Spinal cord injury (SCI), which is a leading cause of disability in modern society, commonly results from trauma. It has been reported that application of sciatic nerve conditioning injury plays a positive role in repairing the injury of the ascending spinal sensory pathway in laboratory animals. Because of the complexity of SCI and related ethics challenges, sciatic nerve conditioning injury cannot be applied in clinical therapy. Accordingly, it is extremely important to study its mechanism and develop replacement therapy. Based on empirical study and clinical trials, this article suggests that miR-142-3p is the key therapeutic target for repairing sensory function, based on the following evidence. Firstly, studies have reported that endogenous cAMP is the upstream regulator of 3 signal pathways that are partially involved in the mechanisms of sciatic nerve conditioning injury, promoting neurite growth. The regulated miR-142-3p can induce cAMP elevation via adenylyl cyclase 9 (AC9), which is abundant in dorsal root ganglia (DRG). Secondly, compared with gene expression regulation in the injured spinal cord, inhibition of microRNA (miRNA) in DRG is less likely to cause trauma and infection. Thirdly, evidence of miRNAs as biomarkers and therapeutic targets in many diseases has been reported. In this article we suggest, for the first time, imitating sciatic nerve conditioning injury, thereby enhancing central regeneration of primary sensory neurons via interfering with the congenerous upstream regulator AC9 of the 3 above-mentioned signal pathways. We hope to provide a new clinical treatment strategy for the recovery of sensory function in SCI patients.

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

What makes a RAG regeneration associated?

Regenerative failure remains a significant barrier for functional recovery after central nervous system (CNS) injury. As such, understanding the physiological processes that regulate axon regeneration is a central focus of regenerative medicine. Studying the gene transcription responses to axon injury of regeneration competent neurons, such as those of the peripheral nervous system (PNS), has provided insight into the genes associated with regeneration. Though several individual “regeneration-associated genes” (RAGs) have been identified from these studies, the response to injury likely regulates the expression of functionally coordinated and complementary gene groups. For instance, successful regeneration would require the induction of genes that drive the intrinsic growth capacity of neurons, while simultaneously downregulating the genes that convey environmental inhibitory cues. Thus, this view emphasizes the transcriptional regulation of gene “programs” that contribute to the overall goal of axonal regeneration. Here, we review the known RAGs, focusing on how their transcriptional regulation can reveal the underlying gene programs that drive a regenerative phenotype. Finally, we will discuss paradigms under which we can determine whether these genes are injury-associated, or indeed necessary for regeneration.

Flipping the transcriptional switch from myelin inhibition to axon growth in the CNS

Poor regeneration of severed axons in the central nervous system (CNS) limits functional recovery. Regeneration failure involves interplay of inhibitory environmental elements and the growth state of the neuron. To find internal changes in gene expression that might overcome inhibitory environmental cues, we compared several paradigms that allow growth in the inhibitory environment. Conditions that allow axon growth by axotomized and cultured dorsal root ganglion (DRG) neurons on CNS myelin include immaturity (the first few postnatal days), high levels of cyclic adenosine mono phosphate (cAMP), and conditioning with a peripheral nerve lesion before explant. This shift from inhibition to growth depends on transcription. Seeking to understand the transcriptome changes that allow axon growth in the CNS, we collaborated with the Marie Filbin laboratory to identify several mRNAs that are functionally relevant, as determined by gain- and loss-of-function studies. In this Perspective, we review evidence from these experiments and discuss the merits of comparing multiple regenerative paradigms to identify a core transcriptional program for CNS axon regeneration.

Lack of galectin-3 improves the functional outcome and tissue sparing by modulating inflammatory response after a compressive spinal cord injury

Spinal cord injury (SCI) is a traumatic event that results in motor, sensitive or autonomic function disturbances, which have direct impact on the life quality of the affected individual. Recent studies have shown that attenuation of the inflammatory response after SCI plays a key role in the reestablishment of motor function. Galectin-3 is a pleiotropic molecule belonging to the carbohydrate-ligand lectin family, which is expressed by different cells in different tissues. Studies have shown that galectin-3 induces the recruitment and activation of neutrophils, monocytes/macrophages, lymphocytes and microglia. Thus, the aim of this study was to evaluate the effects of the lack of galectin-3 on the functional outcome, cellular recruitment and morphological changes in tissue, after SCI. C57BL/6 wild-type and galectin-3 knockout mice were used in this study. A vascular clip was used for 1 min to generate a compressive SCI. By BMS we detected that the Gal-3(-/-) presented a better functional outcome during the studied period. This finding is related to a decrease in the injury length and a higher volume of spared white matter at 7 and 42 days post injury (dpi). Moreover, Gal-3(-/-) mice showed a higher number of spared fibers at 28 dpi. Because of the importance of the inflammatory response after SCI and the role that galectin-3 plays in it, we investigated possible differences in the inflammatory response between the analyzed groups. No differences in neutrophils were observed 24h after injury. However, at 3 dpi, the Gal-3(-/-) mice showed more neutrophils infiltrated into the spinal tissue when compared with the WT mice. At this same time point, no differences in the percentage of the CD11b/Arginase1 positive cells were observed. Remarkably, Gal-3(-/-) mice displayed a decrease in CD11b staining at 7 dpi, compared with the WT mice. At the same time, Gal-3(-/-) mice presented a more prominent Arginase1 stained area, suggesting an anti-inflammatory cell phenotype. Taken together, these results demonstrated that the lack of galectin-3 plays a key role in the inflammatory process triggered by SCI, leading to better and early recovery of locomotor function.

The Acquisition of Target Dependence by Developing Rat Retinal Ganglion Cells

Similar to neurons in the peripheral nervous system, immature CNS-derived RGCs become dependent on target-derived neurotrophic support as their axons reach termination sites in the brain. To study the factors that influence this developmental transition we took advantage of the fact that rat RGCs are born, and target innervation occurs, over a protracted period of time. Early-born RGCs have axons in the SC by birth (P0), whereas axons from late-born RGCs do not innervate the SC until P4-P5. Birth dating RGCs using EdU allowed us to identify RGCs (1) with axons still growing toward targets, (2) transitioning to target dependence, and (3) entirely dependent on target-derived support. Using laser-capture microdissection we isolated ∼34,000 EdU⁺ RGCs and analyzed transcript expression by custom qPCR array. Statistical analyses revealed a difference in gene expression profiles in actively growing RGCs compared with target-dependent RGCs, as well as in transitional versus target-dependent RGCs. Prior to innervation RGCs expressed high levels of BDNF and CNTFR α but lower levels of neurexin 1 mRNA. Analysis also revealed greater expression of transcripts for signaling molecules such as MAPK, Akt, CREB, and STAT. In a supporting in vitro study, purified birth-dated P1 RGCs were cultured for 24-48 h with or without BDNF; lack of BDNF resulted in significant loss of early-born but not late-born RGCs. In summary, we identified several important changes in RGC signaling that may form the basis for the switch from target independence to dependence.

Looking downstream: the role of cyclic AMP-regulated genes in axonal regeneration

Elevation of intracellular cyclic AMP (cAMP) levels has proven to be one of the most effective means of overcoming inhibition of axonal regeneration by myelin-associated inhibitors such as myelin-associated glycoprotein (MAG), Nogo, and oligodendrocyte myelin glycoprotein. Pharmacological manipulation of cAMP through the administration of dibutyryl cAMP or rolipram leads to enhanced axonal growth both in vivo and in vitro, and importantly, upregulation of cAMP within dorsal root ganglion neurons is responsible for the conditioning lesion effect, which indicates that cAMP plays a significant role in the endogenous mechanisms that promote axonal regeneration. The effects of cAMP are transcription-dependent and are mediated through the activation of protein kinase A (PKA) and the transcription factor cyclic AMP response element binding protein (CREB). This leads to the induction of a variety of genes, several of which have been shown to overcome myelin-mediated inhibition in their own right. In this review, we will highlight the pro-regenerative effects of arginase I (ArgI), interleukin (IL)-6, secretory leukocyte protease inhibitor (SLPI), and metallothionein (MT)-I/II, and discuss their potential for therapeutic use in spinal cord injury.

Metallothionein-I/II Promotes Axonal Regeneration in the Central Nervous System

The adult CNS does not spontaneously regenerate after injury, due in large part to myelin-associated inhibitors such as myelin-associated glycoprotein (MAG), Nogo-A, and oligodendrocyte-myelin glycoprotein. All three inhibitors can interact with either the Nogo receptor complex or paired immunoglobulin-like receptor B. A conditioning lesion of the sciatic nerve allows the central processes of dorsal root ganglion (DRG) neurons to spontaneously regenerate in vivo after a dorsal column lesion. After a conditioning lesion, DRG neurons are no longer inhibited by myelin, and this effect is cyclic AMP (cAMP)- and transcription-dependent. Using a microarray analysis, we identified several genes that are up-regulated both in adult DRGs after a conditioning lesion and in DRG neurons treated with cAMP analogues. One gene that was up-regulated under both conditions is metallothionein (MT)-I. We show here that treatment with two closely related isoforms of MT (MT-I/II) can overcome the inhibitory effects of both myelin and MAG for cortical, hippocampal, and DRG neurons. Intrathecal delivery of MT-I/II to adult DRGs also promotes neurite outgrowth in the presence of MAG. Adult DRGs from MT-I/II-deficient mice extend significantly shorter processes on MAG compared with wild-type DRG neurons, and regeneration of dorsal column axons does not occur after a conditioning lesion in MT-I/II-deficient mice. Furthermore, a single intravitreal injection of MT-I/II after optic nerve crush promotes axonal regeneration. Mechanistically, MT-I/II ability to overcome MAG-mediated inhibition is transcription-dependent, and MT-I/II can block the proteolytic activity of α-secretase and the activation of PKC and Rho in response to soluble MAG.

cAMP-responsive element-binding protein (CREB) and cAMP co-regulate activator protein 1 (AP1)-dependent regeneration-associated gene expression and neurite growth

To regenerate damaged axons, neurons must express a cassette of regeneration-associated genes (RAGs) that increases intrinsic growth capacity and confers resistance to extrinsic inhibitory cues. Here we show that dibutyrl-cAMP or forskolin combined with constitutive-active CREB are superior to either agent alone in driving neurite growth on permissive and inhibitory substrates. Of the RAGs examined, only arginase 1 (Arg1) expression correlated with the increased neurite growth induced by the cAMP/CREB combination, both of which were AP1-dependent. This suggests that cAMP-induced AP1 activity is necessary and interacts with CREB to drive expression of RAGs relevant for regeneration and demonstrates that combining a small molecule (cAMP) with an activated transcription factor (CREB) stimulates the gene expression necessary to enhance axonal regeneration.

Cyclic AMP signaling: a molecular determinant of peripheral nerve regeneration

Disruption of axonal integrity during injury to the peripheral nerve system (PNS) sets into motion a cascade of responses that includes inflammation, Schwann cell mobilization, and the degeneration of the nerve fibers distal to the injury site. Yet, the injured PNS differentiates itself from the injured central nervous system (CNS) in its remarkable capacity for self-recovery, which, depending upon the length and type of nerve injury, involves a series of molecular events in both the injured neuron and associated Schwann cells that leads to axon regeneration, remyelination repair, and functional restitution. Herein we discuss the essential function of the second messenger, cyclic adenosine monophosphate (cyclic AMP), in the PNS repair process, highlighting the important role the conditioning lesion paradigm has played in understanding the mechanism(s) by which cyclic AMP exerts its proregenerative action. Furthermore, we review the studies that have therapeutically targeted cyclic AMP to enhance endogenous nerve repair.

Mechanisms of axon regeneration and its inhibition: roles of sulfated glycans

Axons in the peripheral nervous system can regenerate after injury, whereas axons in the central nervous system (CNS) do not readily regenerate. Intrinsic regenerating capacity and emerging inhibitors could explain these contrasting phenotypes. Among the inhibitors, sulfated sugar chains including chondroitin sulfate and keratan sulfate have recently attracted attention, since these sugar chains strongly inhibit axon regeneration and also induce dystrophic endball formation, a hallmark of injured axons in the adult mammalian CNS. In addition, chondroitin sulfate is a negative regulator of synaptic plasticity. To overcome the inability of CNS axons to regenerate, a comprehensive understanding of both the positive and negative regulations of axon regeneration is required. These may include signaling waves from the injury site to the nucleus, intracellular signals for growth cone formation and axon regeneration, intracellular signals for the inhibition of axon regeneration, and extracellular inhibitory signals and their receptors. This review addresses these issues, with a focus on the roles of chondroitin sulfate and keratan sulfate.

Neuroinflammation and M2 microglia: the good, the bad, and the inflamed

The concept of multiple macrophage activation states is not new. However, extending this idea to resident tissue macrophages, like microglia, has gained increased interest in recent years. Unfortunately, the research on peripheral macrophage polarization does not necessarily translate accurately to their central nervous system (CNS) counterparts. Even though pro- and anti-inflammatory cytokines can polarize microglia to distinct activation states, the specific functions of these states is still an area of intense debate. This review examines the multiple possible activation states microglia can be polarized to. This is followed by a detailed description of microglial polarization and the functional relevance of this process in both acute and chronic CNS disease models described in the literature. Particular attention is given to utilizing M2 microglial polarization as a potential therapeutic option in treating diseases.

Cyclic AMP and the regeneration of retinal ganglion cell axons

In this paper we present a brief review of studies that have reported therapeutic benefits of elevated cAMP on plasticity and regeneration after injury to the central nervous system (CNS). We also provide new data on the cellular mechanisms by which elevation of cyclic adenosine monophosphate (cAMP) promotes cytokine driven regeneration of adult CNS axons, using the visual system as the experimental model. cAMP is a second messenger for many intracellular signalling pathways. Elevation of cAMP in the eye by intravitreal injection of the cell permeant analogue (8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphate; CPT-cAMP), when added to recombinant ciliary neurotrophic factor (rCNTF), significantly enhances rCNTF-induced regeneration of adult rat retinal ganglion cell (RGC) axons into peripheral nerve (PN) grafted onto transected optic nerve. This effect is mediated to some extent by protein kinase A (PKA) signalling, but CPT-cAMP also acts via PI3K/Akt signalling to reduce suppressor of cytokine signalling protein 3 (SOCS3) activity in RGCs. Another target for cAMP is the exchange protein activated by cAMP (Epac), which can also mediate cAMP-induced axonal growth. Here we describe some novel results and discuss to what extent the pro-regenerative effects of CPT-cAMP on adult RGCs are mediated via Epac as well as via PKA-dependent pathways. We used the established PN-optic nerve graft model and quantified the survival and regenerative growth of adult rat RGCs after intravitreal injection of rCNTF in combination with a selective activator of PKA and/or a specific activator of Epac. Viable RGCs were identified by βIII-tubulin immunohistochemistry and regenerating RGCs retrogradely labelled and quantified after an injection of fluorogold into the distal end of the PN grafts, 4 weeks post-transplantation. The specific agonists of either PKA or Epac were both effective in enhancing the effects of rCNTF on RGC axonal regeneration, but interestingly, injections that combined rCNTF with both agonists were significantly less effective. The results are discussed in relation to previous CPT-cAMP studies on RGCs, and we also consider the need to modulate cAMP levels in order to obtain the most functionally effective regenerative response after CNS trauma. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

Awakening the stalled axon - surprises in CSPG gradients

The remarkably poor regeneration of axons seen after injury of the brain and spinal cord can result in permanent loss of neural function. This failure of meaningful regeneration has been attributed to both a low intrinsic growth potential of CNS neurons and extrinsic factors that actively block axon growth in the adult CNS. Injury exacerbates this situation by increasing the expression of and exposure to proteins that actively block axonal growth in the CNS. Much experimental efforts have been aimed at overcoming the extrinsic growth inhibitory environment of the injured brain and spinal cord. A recent publication in Experimental Neurology from Kuboyama and colleagues shows that activation of protein kinase A signaling is responsible for the stalling of axon growth in gradients of CNS inhibitory molecules. This observation is unexpected given the role of cAMP signaling in supporting intrinsic growth mechanisms, emphasizing the need to consider spatial and temporal aspects of intracellular signaling in future strategies for neural repair.

Regenerative medicine for the treatment of spinal cord injury: more than just promises?

Spinal cord injury triggers a complex set of events that lead to tissue healing without the restoration of normal function due to the poor regenerative capacity of the spinal cord. Nevertheless, current knowledge about the intrinsic regenerative ability of central nervous system axons, when in a supportive environment, has made the prospect of treating spinal cord injury a reality. Among the range of strategies under investigation, cell-based therapies offer the most promising results, due to the multifactorial roles that these cells can fulfil. However, the best cell source is still a matter of debate, as are clinical issues that include the optimal cell dose as well as the timing and route of administration. In this context, the role of biomaterials is gaining importance. These can not only act as vehicles for the administered cells but also, in the case of chronic lesions, can be used to fill the permanent cyst, thus creating a more favourable and conducive environment for axonal regeneration in addition to serving as local delivery systems of therapeutic agents to improve the regenerative milieu. Some of the candidate molecules for the future are discussed in view of the knowledge derived from studying the mechanisms that facilitate the intrinsic regenerative capacity of central nervous system neurons. The future challenge for the multidisciplinary teams working in the field is to translate the knowledge acquired in basic research into effective combinatorial therapies to be applied in the clinic.

Study of the effects of oral zinc supplementation on peroxynitrite levels, arginase activity and NO synthase activity in seminal plasma of Iraqi asthenospermic patients

BACKGROUND

Low concentrations of nitric oxide (NO) are necessary for the biology and physiology of spermatozoa, but high levels of NO are toxic and have negative effects on sperm functions. Although several studies have considered the relationship between infertility and semen NO concentrations, no study on the effects of asthenospermia treatments such as oral zinc supplementation on concentrations of NO, which are important in fertility, has been reported. Studies have shown that oral zinc supplementation develops sperm count, motility and the physical characteristics of sperm in animals and in some groups of infertile men. The present study was conducted to study the effect of zinc supplementation on the quantitative and qualitative characteristics of semen, along with enzymes of the NO pathway in the seminal plasma of asthenospermic patients.

METHODS

Semen samples were obtained from 60 fertile and 60 asthenozoospermic infertile men of matched age. The subfertile group was treated with zinc sulfate; each participant took two capsules (220 mg per capsule) per day for 3 months. Semen samples were obtained (before and after zinc sulfate supplementation). After liquefaction of the seminal fluid at room temperature, routine semen analyses were performed. The stable metabolites of NO (nitrite) in seminal plasma were measured by nitrophenol assay. Arginase activity and NO synthase activity were measured spectrophotometrically.

RESULTS

Peroxynitrite levels, arginase activity, NO synthase activity and various sperm parameters were compared among fertile controls and infertile patients (before and after treatment with zinc sulfate). Peroxynitrite levels and NO synthase activity were significantly higher in the infertile patients compared to the fertile group. Conversely, arginase activity was significantly higher in the fertile group than the infertile patients. Peroxynitrite levels, arginase activity and NO synthase activity of the infertile patient were restored to normal values after treatment with zinc sulfate. Volume of semen, progressive sperm motility percentage and total normal sperm count were increased after zinc supplementation.

CONCLUSIONS

Treatment of asthenospermic patients with zinc supplementation leads to restored peroxynitrite levels, arginase activity and NO synthase activity to normal values and gives a statistically significant improvement of semen parameters compared with controls.

Contribution of macrophages to enhanced regenerative capacity of dorsal root ganglia sensory neurons by conditioning injury

Although the central branches of the dorsal root ganglion (DRG) sensory neurons do not spontaneously regenerate, a conditioning peripheral injury can promote their regeneration. A potential role of macrophages in axonal regeneration was proposed, but it has not been critically addressed whether macrophages play an essential role in the conditioning injury model. After sciatic nerve injury (SNI) in rats, the number of macrophages in DRGs gradually increased by day 7. The increase persisted up to 28 d and was accompanied by upregulation of inflammatory mediators, including oncomodulin. A macrophage deactivator, minocycline, reduced the macrophage number and expressions of the inflammatory mediators. Molecular signatures of conditioning effects were abrogated by minocycline, and enhanced regenerative capacity was substantially attenuated both in vitro and in vivo. Delayed minocycline infusion abrogated the SNI-induced long-lasting heightened neurite outgrowth potential, indicating a role for macrophages in the maintenance of regenerative capacity. Intraganglionic cAMP injection also resulted in an increase in macrophages, and minocycline abolished the cAMP effect on neurite outgrowth. However, conditioned media (CM) from macrophages treated with cAMP did not exhibit neurite growth-promoting activity. In contrast, CM from neuron-macrophage cocultures treated with cAMP promoted neurite outgrowth greatly, highlighting a requirement for neuron-macrophage interactions for the induction of a proregenerative macrophage phenotype. The growth-promoting activity in the CM was profoundly attenuated by an oncomodulin neutralizing antibody. These results suggest that the neuron-macrophage interactions involved in eliciting a proregenerative phenotype in macrophages may be a novel target to induce long-lasting regenerative processes after axonal injuries in the CNS.

Sustained delivery of dbcAMP by poly(propylene carbonate) micron fibers promotes axonal regenerative sprouting and functional recovery after spinal cord hemisection

This study describes the use of poly(propylene carbonate) (PPC) electrospun fibers as vehicle for the sustained delivery of dibutyryl cyclic adenosine monophosphate (dbcAMP) to the hemisected spinal cord. The dbcAMP and PPC were uniformly mixed with acetonitrile; then, electrospinning was used to generate micron fibers. The release of dbcAMP was assessed by ELISA in vitro. Our results showed that the encapsulation of dbcAMP in the fibers led to stable and prolonged release in vitro. The PPC micron fibers containing dbcAMP and the PPC micron fibers without dbcAMP were then implanted into the hemisected thoracic spinal cord, followed by testing of the functional recovery and immunohistochemistry. Compared with the control group, sustained delivery of dbcAMP promoted axonal regenerative sprouting and functional recovery and reduced glial scar formation, and the PPC micron fibers without dbcAMP did not have these effects. Our findings demonstrated the feasibility of using PPC electrospun fibers containing dbcAMP for spinal cord injury. The approach described here also will provide a platform for the potential delivery of other axon-growth-promoting or scar-inhibiting agents.

SnoN facilitates axonal regeneration after spinal cord injury

Adult CNS neurons exhibit a reduced capacity for growth compared to developing neurons, due in part to downregulation of growth-associated genes as development is completed. We tested the hypothesis that SnoN, an embryonically regulated transcription factor that specifies growth of the axonal compartment, can enhance growth in injured adult neurons. In vitro, SnoN overexpression in dissociated adult DRG neuronal cultures significantly enhanced neurite outgrowth. Moreover, TGF-β1, a negative regulator of SnoN, inhibited neurite outgrowth, and SnoN over-expression overcame this inhibition. We then examined whether SnoN influenced axonal regeneration in vivo: indeed, expression of a mutant form of SnoN resistant to degradation significantly enhanced axonal regeneration following cervical spinal cord injury, despite peri-lesional upregulation of TGF-β1. Thus, a developmental mechanism that specifies extension of the axonal compartment also promotes axonal regeneration after adult CNS injury.

Transglutaminase is a therapeutic target for oxidative stress, excitotoxicity and stroke: a new epigenetic kid on the CNS block

Transglutaminases (TGs) are multifunctional, calcium-dependent enzymes that have been recently implicated in stroke pathophysiology. Classically, these enzymes are thought to participate in cell injury and death in chronic neurodegenerative conditions via their ability to catalyze covalent, nondegradable crosslinks between proteins or to incorporate polyamines into protein substrates. Accumulating lines of inquiry indicate that specific TG isoforms can shuttle into the nucleus when they sense pathologic changes in calcium or oxidative stress, bind to chromatin and thereby transduce these changes into transcriptional repression of genes involved in metabolic or oxidant adaptation. Here, we review the evidence that supports principally a role for one isoform of this family, TG2, in cell injury and death associated with hemorrhagic or ischemic stroke. We also outline an evolving model in which TG2 is a critical mediator between pathologic signaling and epigenetic modifications that lead to gene repression. Accordingly, the salutary effects of TG inhibitors in stroke may derive from their ability to restore homeostasis by removing inappropriate deactivation of adaptive genetic programs by oxidative stress or extrasynaptic glutamate receptor signaling.

Secretory leukocyte protease inhibitor reverses inhibition by CNS myelin, promotes regeneration in the optic nerve, and suppresses expression of the transforming growth factor-β signaling protein Smad2

After CNS injury, axonal regeneration is limited by myelin-associated inhibitors; however, this can be overcome through elevation of intracellular cyclic AMP (cAMP), as occurs with conditioning lesions of the sciatic nerve. This study reports that expression of secretory leukocyte protease inhibitor (SLPI) is strongly upregulated in response to elevation of cAMP. We also show that SLPI can overcome inhibition by CNS myelin and significantly enhance regeneration of transected retinal ganglion cell axons in rats. Furthermore, regeneration of dorsal column axons does not occur after a conditioning lesion in SLPI null mutant mice, indicating that expression of SLPI is required for the conditioning lesion effect. Mechanistically, we demonstrate that SLPI localizes to the nuclei of neurons, binds to the Smad2 promoter, and reduces levels of Smad2 protein. Adenoviral overexpression of Smad2 also blocked SLPI-induced axonal regeneration. SLPI and Smad2 may therefore represent new targets for therapeutic intervention in CNS injury.

Waking up the sleepers: shared transcriptional pathways in axonal regeneration and neurogenesis

In the last several years, relevant progress has been made in our understanding of the transcriptional machinery regulating CNS repair after acute injury, such as following trauma or stroke. In order to survive and functionally reconnect to the synaptic network, injured neurons activate an intrinsic rescue program aimed to increase their plasticity. Perhaps, in the attempt to switch back to a plastic and growth-competent state, post-mitotic neurons wake up and re-express a set of transcription factors that are also critical for the regulation of their younger brothers, the neural stem cells. Here, we review and discuss the transcriptional pathways regulating both axonal regeneration and neurogenesis highlighting the connection between the two. Clarification of their common molecular substrate may help simultaneous targeting of both neurogenesis and axonal regeneration with the hope to enhance functional recovery following CNS injury.

microRNA-222 targeting PTEN promotes neurite outgrowth from adult dorsal root ganglion neurons following sciatic nerve transection

Dorsal root ganglia (DRG) neurons spontaneously undergo neurite growth after nerve injury. MicroRNAs (miRNAs), as small, non-coding RNAs, negatively regulate gene expression in a variety of biological processes. The roles of miRNAs in the regulation of responses of DRG neurons to injury stimuli, however, are not fully understood. Here, microarray analysis was performed to profile the miRNAs in L4-L6 DRGs following rat sciatic nerve transection. The 26 known miRNAs were differentially expressed at 0, 1, 4, 7, 14 d post injury, and the potential targets of the miRNAs were involved in nerve regeneration, as analyzed by bioinformatics. Among the 26 miRNAs, microRNA-222 (miR-222) was our research focus because its increased expression promoted neurite outgrowth while it silencing by miR-222 inhibitor reduced neurite outgrowth. Knockdown experiments confirmed that phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a major inhibitor of nerve regeneration, was a direct target of miR-222 in DRG neurons. In addition, we found that miR-222 might regulate the phosphorylation of cAMP response element binding protein (CREB) through PTEN, and c-Jun activation might enhance the miR-222 expression. Collectively, our data suggest that miR-222 could regulate neurite outgrowth from DRG neurons by targeting PTEN.

Need for a paradigm shift in therapeutic approaches to CNS injury

Irreversible damage to the nervous system can result from many causes including trauma, disruption of blood supply, pathogen infection or neurodegenerative disease. Common features following CNS injury include a disruption of axons, neuron death and injury, local B-cell and microglial activation, and the synthesis of pathogenic autoantibodies. CNS injury results in a pervasive inhibitory microenvironment that hinders regeneration. Current approaches to eliminate the inhibitory environment have met with limited success. These results argue for a paradigm shift in therapeutic approaches to CNS injury. Targeting CNS cells (neurons, oligodendrocytes and astrocytes) themselves may drive CNS repair. For example, our group and others have demonstrated that autoreactive antibodies can participate in aspects of CNS regeneration, including remyelination. We have developed recombinant autoreactive natural human IgM antibodies with the therapeutic potential for CNS repair in several neurologic diseases.

Acute injury in the peripheral nervous system triggers an alternative macrophage response

BACKGROUND

The activation of the immune system in neurodegeneration has detrimental as well as beneficial effects. Which aspects of this immune response aggravate the neurodegenerative breakdown and which stimulate regeneration remains an open question. To unravel the neuroprotective aspects of the immune system we focused on a model of acute peripheral nerve injury, in which the immune system was shown to be protective.

METHODS

To determine the type of immune response triggered after axotomy of the sciatic nerve, a model for Wallerian degeneration in the peripheral nervous system, we evaluated markers representing the two extremes of a type I and type II immune response (classical vs. alternative) using real-time quantitative polymerase chain reaction (RT-qPCR), western blot, and immunohistochemistry.

RESULTS

Our results showed that acute peripheral nerve injury triggers an anti-inflammatory and immunosuppressive response, rather than a pro-inflammatory response. This was reflected by the complete absence of classical macrophage markers (iNOS, IFN γ, and IL12p40), and the strong up-regulation of tissue repair markers (arginase-1, Ym1, and Trem2). The signal favoring the alternative macrophage environment was induced immediately after nerve damage and appeared to be established within the nerve, well before the infiltration of macrophages. In addition, negative regulators of the innate immune response, as well as the anti-inflammatory cytokine IL-10 were induced. The strict regulation of the immune system dampens the potential tissue damaging effects of an over-activated response.

CONCLUSIONS

We here demonstrate that acute peripheral nerve injury triggers an inherent protective environment by inducing the M2 phenotype of macrophages and the expression of arginase-1. We believe that the M2 phenotype, associated with a sterile inflammatory response and tissue repair, might explain their neuroprotective capacity. As such, shifting the neurodegeneration-induced immune responses towards an M2/Th2 response could be an important therapeutic strategy.

Peripheral nerve graft with immunosuppression modifies gene expression in axotomized CNS neurons

Adult central nervous system (CNS) neurons do not regenerate severed axons unaided but may regenerate axons into apposed predegenerated peripheral nerve grafts (PNGs). We examined gene expression by using microarray technology in laser-dissected lateral vestibular (LV) neurons whose axons were severed by a lateral hemisection at C3 (HX) and in lateral vestibular nucleus (LVN) neurons that were hemisected at C3 and that received immunosuppression with cyclosporine A (CsA) and a predegenerated PNG (termed I-PNG) into the lesion site. The results provide an expression analysis of temporal changes that occur in LVN neurons in nonregenerative and potentially regenerative states and over a period of 42 days. Axotomy alone resulted in a prolonged change in regulation of probe sets, with more being upregulated than downregulated. Apposition of a PNG with immunosuppression muted gene expression overall. Axotomized neurons (HX) upregulated genes commonly associated with axonal growth, whereas axotomized neurons whose axons were apposed to the PNG showed diminished expression of many of these genes but greater expression of genes related to energy production. The results suggest that axotomized LVN neurons express many genes thought to be associated with regeneration to a greater extent than LVN neurons that are apposed to a PNG. Thus the LVN neurons remain in a regenerative state following axotomy but the conditions provided by the I-PNG appear to be neuroprotective, preserving or enhancing mitochondrial activity, which may provide required energy for regeneration. We speculate that the graft also enables sufficient axonal synthesis of cytoskeletal components to allow axonal growth without marked increase in expression of genes normally associated with regeneration.

Chemokines in CNS injury and repair

Recruitment of inflammatory cells is known to drive the secondary damage cascades that are common to injuries of the central nervous system (CNS). Cell activation and infiltration to the injury site is orchestrated by changes in the expression of chemokines, the chemoattractive cytokines. Reducing the numbers of recruited inflammatory cells by the blocking of the action of chemokines has turned out be a promising approach to diminish neuroinflammation and to improve tissue preservation and neovascularization. In addition, several chemokines have been shown to be essential for stem/progenitor cell attraction, their survival, differentiation and cytokine production. Thus, chemokines might indirectly participate in remyelination, neovascularization and neuroprotection, which are important prerequisites for CNS repair after trauma. Moreover, CXCL12 promotes neurite outgrowth in the presence of growth inhibitory CNS myelin and enhances axonal sprouting after spinal cord injury (SCI). Here, we review current knowledge about the exciting functions of chemokines in CNS trauma, including SCI, traumatic brain injury and stroke. We identify common principles of chemokine action and discuss the potentials and challenges of therapeutic interventions with chemokines.

The role of Nogo-A in axonal plasticity, regrowth and repair

Axonal damage leads to permanent deficits in the adult central nervous system (CNS) not only because of the weak intrinsic ability of adult neurons to activate their growth program but importantly also because of the presence of specific growth inhibitors in the CNS tissue and the environment of the damaged axons. The well-studied myelin-derived protein Nogo-A is involved in various cellular and molecular events contributing to the failure of CNS axons to regrow and reconnect after transection. Recent studies have shown that, by acting in a negative way on the cytoskeleton and on the growth program of axotomized neurons, Nogo-A exerts fast and chronic inhibitory effects on neurite outgrowth. On the other hand, the blockade of Nogo-A results in a marked enhancement of compensatory and regenerative axonal extension in vivo; this enhancement is often paralleled by significant functional recovery, for example, of locomotion or skilled forelimb reaching after spinal cord or stroke lesions in rats and monkeys. Surprisingly, the blockade of Nogo-A or its receptor NgR in the hippocampus has recently been demonstrated to enhance long-term potentiation. A role of Nogo-A in synaptic plasticity/stability might therefore represent an additional, new and important aspect of CNS circuit remodeling. Function-blocking anti-Nogo-A antibodies are currently being tested in a clinical trial for improved outcome after spinal cord injury.

Krüppel-like Factor 7 engineered for transcriptional activation promotes axon regeneration in the adult corticospinal tract

Axon regeneration in the central nervous system normally fails, in part because of a developmental decline in the intrinsic ability of CNS projection neurons to extend axons. Members of the KLF family of transcription factors regulate regenerative potential in developing CNS neurons. Expression of one family member, KLF7, is down-regulated developmentally, and overexpression of KLF7 in cortical neurons in vitro promotes axonal growth. To circumvent difficulties in achieving high neuronal expression of exogenous KLF7, we created a chimera with the VP16 transactivation domain, which displayed enhanced neuronal expression compared with the native protein while maintaining transcriptional activation and growth promotion in vitro. Overexpression of VP16-KLF7 overcame the developmental loss of regenerative ability in cortical slice cultures. Adult corticospinal tract (CST) neurons failed to up-regulate KLF7 in response to axon injury, and overexpression of VP16-KLF7 in vivo promoted both sprouting and regenerative axon growth in the CST of adult mice. These findings identify a unique means of promoting CST axon regeneration in vivo by reengineering a developmentally down-regulated, growth-promoting transcription factor.

IL-6 promotes regeneration and functional recovery after cortical spinal tract injury by reactivating intrinsic growth program of neurons and enhancing synapse formation

Most neurons in adult mammalian central nervous system (CNS) fail to regenerate their axons after injury. Peripherally conditioned primary sensory neurons have an increased capacity to regenerate their central processes. Recent studies demonstrate that a conditioning lesion increased intrinsic growth capability is associated with the up-regulation of a group of growth-associated genes, one of the most established is interleukin-6 (IL-6). However, the cellular and molecular mechanisms by which IL-6 exerts its beneficial effect on axonal regeneration and functional recovery remain to be elucidated. The purpose of this study is to further investigate the molecular mechanisms of IL-6 in promoting regeneration and functional recovery after spinal cord injury (SCI). Here, we demonstrate that in vitro administration of IL-6 enhances neurite outgrowth of neurons on an inhibitory substrate myelin proteins, accompanied by increased expression of growth-associated genes GAP-43, SPRR1A and Arginase I. In vivo, intrathecal delivery of IL-6 for 7 days after cortical spinal tract injury induces synaptic rearrangements of sprouting axons and increases the expression of mTOR in neurons surrounding the lesion site, accompanied by improved functional recovery. In conclusion, our results show that IL-6 increases the expression of growth-associated genes and induces the expression of mTOR in lesion adjacent neurons, resulting in reactivating the intrinsic growth program of neurons to promote axonal regrowth and functional recovery after SCI.

Metabolomics of cerebrospinal fluid reveals changes in the central nervous system metabolism in a rat model of multiple sclerosis

Experimental Autoimmune Encephalomyelitis (EAE) is the most commonly used animal model for Multiple Sclerosis (MScl). CSF metabolomics in an acute EAE rat model was investigated using targetted LC-MS and GC-MS. Acute EAE in Lewis rats was induced by co-injection of Myelin Basic Protein with Complete Freund's Adjuvant. CSF samples were collected at two time points: 10 days after inoculation, which was during the onset of the disease, and 14 days after inoculation, which was during the peak of the disease. The obtained metabolite profiles from the two time points of EAE development show profound differences between onset and the peak of the disease, suggesting significant changes in CNS metabolism over the course of MBP-induced neuroinflammation. Around the onset of EAE the metabolome profile shows significant decreases in arginine, alanine and branched amino acid levels, relative to controls. At the peak of the disease, significant increases in concentrations of multiple metabolites are observed, including glutamine, O-phosphoethanolamine, branched-chain amino acids and putrescine. Observed changes in metabolite levels suggest profound changes in CNS metabolism over the course of EAE. Affected pathways include nitric oxide synthesis, altered energy metabolism, polyamine synthesis and levels of endogenous antioxidants. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-011-0306-3) contains supplementary material, which is available to authorized users.

Blockade of interleukin-6 signaling inhibits the classic pathway and promotes an alternative pathway of macrophage activation after spinal cord injury in mice

Background

Recent in vivo and in vitro studies in non-neuronal and neuronal tissues have shown that different pathways of macrophage activation result in cells with different properties. Interleukin (IL)-6 triggers the classically activated inflammatory macrophages (M1 phenotype), whereas the alternatively activated macrophages (M2 phenotype) are anti-inflammatory. The objective of this study was to clarify the effects of a temporal blockade of IL-6/IL-6 receptor (IL-6R) engagement, using an anti-mouse IL-6R monoclonal antibody (MR16-1), on macrophage activation and the inflammatory response in the acute phase after spinal cord injury (SCI) in mice.

Methods

MR16-1 antibodies versus isotype control antibodies or saline alone were administered immediately after thoracic SCI in mice. SC tissue repair was compared between the two groups by Luxol fast blue (LFB) staining for myelination and immunoreactivity for the neuronal markers growth-associated protein (GAP)-43 and neurofilament heavy 200 kDa (NF-H) and for locomotor function. The expression of T helper (Th)1 cytokines (interferon (IFN)-γ and tumor necrosis factor-α) and Th2 cytokines (IL-4, IL-13) was determined by immunoblot analysis. The presence of M1 (inducible nitric oxide synthase (iNOS)-positive, CD16/32-positive) and M2 (arginase 1-positive, CD206-positive) macrophages was determined by immunohistology. Using flow cytometry, we also quantified IFN-γ and IL-4 levels in neutrophils, microglia, and macrophages, and Mac-2 (macrophage antigen-2) and Mac-3 in M2 macrophages and microglia.

Results

LFB-positive spared myelin was increased in the MR16-1-treated group compared with the controls, and this increase correlated with enhanced positivity for GAP-43 or NF-H, and improved locomotor Basso Mouse Scale scores. Immunoblot analysis of the MR16-1-treated samples identified downregulation of Th1 and upregulation of Th2 cytokines. Whereas iNOS-positive, CD16/32-positive M1 macrophages were the predominant phenotype in the injured SC of non-treated control mice, MR16-1 treatment promoted arginase 1-positive, CD206-positive M2 macrophages, with preferential localization of these cells at the injury site. MR16-1 treatment suppressed the number of IFN-γ-positive neutrophils, and increased the number of microglia present and their positivity for IL-4. Among the arginase 1-positive M2 macrophages, MR16-1 treatment increased positivity for Mac-2 and Mac-3, suggestive of increased phagocytic behavior.

Conclusion

The results suggest that temporal blockade of IL-6 signaling after SCI abrogates damaging inflammatory activity and promotes functional recovery by promoting the formation of alternatively activated M2 macrophages.

Toxicogenomic analysis of a sustained release local anesthetic delivery system

Concerns over neurotoxicity have impeded the development of sustained release formulations providing prolonged duration local anesthesia (PDLA) from a single injection, for which there is an urgent clinical need. Here, we have used toxicogenomics to investigate whether nerve injury occurred during week-long continuous sciatic nerve blockade by microspheres containing bupivacaine, tetrodotoxin, and dexamethasone (TBD). Animals treated with amitriptyline solution (our positive control for local anesthetic-associated nerve injury) developed irreversible nerve blockade, had severely abnormal nerve histology, and the expression of hundreds of genes was altered in the dorsal root ganglia at 4 and 7 days after injection. In marked contrast, TBD-treated nerves reverted to normal function, were normal histologically and there were changes in the expression of a small number of genes. Toxicogenomic studies have great potential in delineating patterns of gene expression associated with specific patterns of tissue injury (e.g. amitriptyline neurotoxicity), and in identifying related changes in gene expression upon exposure to a drug, biomaterial, or drug delivery system.