Actions of neuropoietic cytokines and cyclic AMP in regenerative conditioning of rat primary sensory neurons

An In Vitro Model for Conditioning Lesion Effect

Axons of a peripheral nerve grow faster after an axotomy if it attains a prior injury a few days earlier. This is called conditioning lesion effect (CLE) and very much valued since it may provide new insights into neuron biology and axonal regeneration. There are established in vivo experimental paradigms to study CLE, however, there is a need to have an in vitro conditioning technique where CLE occurs in a maximally controlled environment. Mouse primary sensory neurons were isolated from lumbar 4-5 dorsal root ganglia and incubated at 37 °C on a silicon-coated watch glass that prevents cell attachment. After this conditioning period they were transferred to laminin coated culture dishes. Similar cultures were set up with freshly isolated neurons from control animals and from the animals that received a sciatic nerve cut 3 days earlier. All preparations were placed on a live cell imaging microscopy providing physiological conditions and photographed for 48 h. Axonal regeneration and neuronal survival was assessed. During the conditioning incubation period neurons remained in suspended aggregates and did not grow axons. The regeneration rate of the in vitro conditioned neurons was much higher than the in vivo conditioned and control preparations during the first day of normal incubation. However, higher regeneration rates were compromised by progressive substantial neuronal death in both types of conditioned cultures but not in the control preparations. By using neutralizing antibodies, we demonstrated that activity of endogenous leukemia inhibitory factor is essential for induction of CLE in this model.

Bilateral activation of STAT3 by phosphorylation at the tyrosine-705 (Y705) and serine-727 (S727) positions and its nuclear translocation in primary sensory neurons following unilateral sciatic nerve injury

Unilateral sciatic nerve compression (SNC) or complete sciatic nerve transection (CSNT), both varying degrees of nerve injury, induced activation of STAT3 bilaterally in the dorsal root ganglia (DRG) neurons of lumbar (L4-L5) as well as cervical (C6-C8) spinal cord segments. STAT3 activation was by phosphorylation at the tyrosine-705 (Y705) and serine-727 (S727) positions and was followed by their nuclear translocation. This is the first evidence of STAT3(S727) activation together with the well-known activation of STAT3(Y705) in primary sensory neurons upon peripheral nerve injury. Bilateral activation of STAT3 in DRG neurons of spinal segments anatomically both associated as well as non-associated with the injured nerve indicates diffusion of STAT3 activation inducers along the spinal cord. Increased levels of IL-6 protein in the CSF following nerve injury as well as activation and nuclear translocation of STAT3 in DRG after intrathecal injection of IL-6 shows that this cytokine, released into the subarachnoid space can penetrate the DRG to activate STAT3. Previous results on increased bilateral IL-6 synthesis and the present manifestation of STAT3 activation in remote DRG following unilateral sciatic nerve injury may reflect a systemic reaction of the DRG neurons to nerve injury.

Crush injury to motor nerves in the G93A transgenic mouse model of amyotrophic lateral sclerosis promotes muscle reinnervation and survival of functionally intact nerve-muscle contacts

Selective survival of small motor nerve fibers and their neuromuscular contacts in the SOD1^G93A transgenic mouse model of amyotrophic lateral sclerosis (ALS) suggests that smaller regenerated nerve fibers are more able to sustain reformed nerve-muscle connections as functionally intact motor units (MUs). The sciatic nerve was crushed unilaterally in SOD1^G93A transgenic mice at 40 days of age and contractile forces of reinnervated muscles and their MUs were recorded at 90 days in order to determine the capacities of the nerves to regenerate and to form and retain functional neuromuscular connections. Reduced MU numbers in fast-twitch tibialis anterior, extensor digitorum longus and medial gastrocnemius muscles and the lesser reductions in slow-twitch soleus muscle of SOD1^G93A transgenic mice were reversed in reinnervated muscles: there were more reinnervated MUs and their contractile forces and the muscle forces and weights increased. In line with the contrasting ability of only small not large nerve fibers to sprout to form enlarged MUs in the SOD1^G93A transgenic mouse, the smaller regenerating nerve fibers formed enlarged MUs that were better able to survive. Because nerve fibers with and without muscle contacts were severed by the sciatic nerve crush injury, the conditioning lesion is untenable as the explanation for improved maintenance of reinnervated neuromuscular junctions. Elevated neurotrophic factor expression in axotomized motoneurons and/or denervated Schwann cells and the synapse withdrawal from axotomized motoneurons are other factors that, in addition to reduced size of nerve fibers reinnervating muscles, may account for increased survival and size of reinnervated MUs in ALS.

Promoting axonal regeneration following nerve surgery: a perspective on ultrasound treatment for nerve injuries

Nerve injury is often associated with limited axonal regeneration and thus leads to delayed or incomplete axonal reinnervation. As a consequence of slow nerve regeneration, target muscle function is often insufficient and leads to a lifelong burden. Recently, the diagnosis of nerve injuries has been improved and likewise surgical reconstruction has undergone significant developments. However, the problem of slow nerve regeneration has not been solved. In a recent meta-analysis, we have shown that the application of low-intensity ultrasound promotes nerve regeneration experimentally and thereby can improve functional outcomes. Here we want to demonstrate the experimental effect of low intensity ultrasound on nerve regeneration, the current state of investigations and its possible future clinical applications.

A Combinatorial Approach to Induce Sensory Axon Regeneration into the Dorsal Root Avulsed Spinal Cord

Spinal root injuries result in newly formed glial scar formation, which prevents regeneration of sensory axons causing permanent sensory loss. Previous studies showed that delivery of trophic factors or implantation of human neural progenitor cells supports sensory axon regeneration and partly restores sensory functions. In this study, we elucidate mechanisms underlying stem cell-mediated ingrowth of sensory axons after dorsal root avulsion (DRA). We show that human spinal cord neural stem/progenitor cells (hscNSPC), and also, mesoporous silica particles loaded with growth factor mimetics (MesoMIM), supported sensory axon regeneration. However, when hscNSPC and MesoMIM were combined, sensory axon regeneration failed. Morphological and tracing analysis showed that sensory axons grow through the newly established glial scar along "bridges" formed by migrating stem cells. Coimplantation of MesoMIM prevented stem cell migration, "bridges" were not formed, and sensory axons failed to enter the spinal cord. MesoMIM applied alone supported sensory axons ingrowth, but without affecting glial scar formation. In vitro, the presence of MesoMIM significantly impaired migration of hscNSPC without affecting their level of differentiation. Our data show that (1) the ability of stem cells to migrate into the spinal cord and organize cellular "bridges" in the newly formed interface is crucial for successful sensory axon regeneration, (2) trophic factor mimetics delivered by mesoporous silica may be a convenient alternative way to induce sensory axon regeneration, and (3) a combinatorial approach of individually beneficial components is not necessarily additive, but can be counterproductive for axonal growth.

The prostaglandin E2/EP4 receptor/cyclic AMP/T-type Ca(2+) channel pathway mediates neuritogenesis in sensory neuron-like ND7/23 cells

We investigated mechanisms for the neuritogenesis caused by prostaglandin E2 (PGE2) or intracellular cyclic AMP (cAMP) in sensory neuron-like ND7/23 cells. PGE2 caused neuritogenesis, an effect abolished by an EP4 receptor antagonist or inhibitors of adenylyl cyclase (AC) or protein kinase A (PKA) and mimicked by the AC activator forskolin, dibutyryl cAMP (db-cAMP), and selective activators of PKA or Epac. ND7/23 cells expressed both Cav3.1 and Cav3.2 T-type Ca(2+) channels (T-channels). The neuritogenesis induced by db-cAMP or PGE2 was abolished by T-channel blockers. T-channels were functionally upregulated by db-cAMP. The PGE2/EP4/cAMP/T-channel pathway thus appears to mediate neuritogenesis in sensory neurons.

Overexpression of ATF3 or the combination of ATF3, c-Jun, STAT3 and Smad1 promotes regeneration of the central axon branch of sensory neurons but without synergistic effects

Peripheral nerve injury results in the activation of a number of transcription factors (TFs) in injured neurons, some of which may be key regulators of the regeneration-associated gene (RAG) programme. Among known RAG TFs, ATF3, Smad1, STAT3 and c-Jun have all been linked to successful axonal regeneration and have known functional and physical interactions. We hypothesised that TF expression would promote regeneration of the central axon branch of DRG neurons in the absence of a peripheral nerve lesion and that simultaneous overexpression of multiple RAG TFs would lead to greater effects than delivery of a single TF. Using adeno-associated viral vectors, we overexpressed either the combination of ATF3, Smad1, STAT3 and c-Jun with farnesylated GFP (fGFP), ATF3 only with fGFP, or fGFP only, in DRG neurons and assessed axonal regeneration after dorsal root transection or dorsal column injury and functional improvement after dorsal root injury. ATF3 alone and the combination of TFs promoted faster regeneration in the injured dorsal root. Surprisingly, however, the combination did not perform better than ATF3 alone. Neither treatment was able to induce functional improvement on sensory tests after dorsal root injury or promote regeneration in a dorsal column injury model. The lack of synergistic effects among these factors indicates that while they do increase the speed of axon growth, there may be functional redundancy between these TFs. Because axon growth is considerably less than that seen after a conditioning lesion, it appears these TFs do not induce the full regeneration programme.

Spinal cord injury and the neuron-intrinsic regeneration-associated gene program

Spinal cord injury (SCI) affects millions of people worldwide and causes a significant physical, emotional, social and economic burden. The main clinical hallmark of SCI is the permanent loss of motor, sensory and autonomic function below the level of injury. In general, neurons of the central nervous system (CNS) are incapable of regeneration, whereas injury to the peripheral nervous system is followed by axonal regeneration and usually results in some degree of functional recovery. The weak neuron-intrinsic regeneration-associated gene (RAG) response upon injury is an important reason for the failure of neurons in the CNS to regenerate an axon. This response consists of the expression of many RAGs, including regeneration-associated transcription factors (TFs). Regeneration-associated TFs are potential key regulators of the RAG program. The function of some regeneration-associated TFs has been studied in transgenic and knock-out mice and by adeno-associated viral vector-mediated overexpression in injured neurons. Here, we review these studies and propose that AAV-mediated gene delivery of combinations of regeneration-associated TFs is a potential strategy to activate the RAG program in injured CNS neurons and achieve long-distance axon regeneration.

Diabetes impairs an interleukin-1β-dependent pathway that enhances neurite outgrowth through JAK/STAT3 modulation of mitochondrial bioenergetics in adult sensory neurons

Background

A luminex-based screen of cytokine expression in dorsal root ganglia (DRG) and nerve of type 1 diabetic rodents revealed interleukin-1 (IL-1α) and IL-1β to be significantly depressed. We, therefore, tested the hypothesis that impaired IL-1α and IL-1β expression in DRG may contribute to aberrant axon regeneration and plasticity seen in diabetic sensory neuropathy. In addition, we determined if these cytokines could optimize mitochondrial bioenergetics since mitochondrial dysfunction is a key etiological factor in diabetic neuropathy.

Results

Cytokines IL-1α and IL-1β were reduced 2-fold (p<0.05) in DRG and/or nerve of 2 and 5 month streptozotocin (STZ)-diabetic rats. IL-2 and IL-10 were unchanged. IL-1α and IL-1β induced similar 2 to 3-fold increases in neurite outgrowth in cultures derived from control or diabetic rats (p<0.05). STAT3 phosphorylation on Tyr705 or Ser727 was depressed in DRG from STZ-diabetic mice and treatment of cultures derived from STZ-diabetic rats with IL-1β for 30 min raised phosphorylation of STAT3 on Tyr705 and Ser727 by 1.5 to 2-fold (p<0.05). shRNA-based or AG490 inhibition of STAT3 activity or shRNA blockade of endogenous IL-1β expression completely blocked neurite outgrowth. Cultured neurons derived from STZ-diabetic mice were treated for 24 hr with IL-1β and maximal oxygen consumption rate and spare respiratory capacity, both key measures of bioenergetic fidelity that were depressed in diabetic compared with control neurons, were enhanced 2-fold. This effect was blocked by AG490.

Conclusions

Endogenous synthesis of IL-1β is diminished in nerve tissue in type 1 diabetes and we propose this defect triggers reduced STAT3 signaling and mitochondrial function leading to sup-optimal axonal regeneration and plasticity.

Neurotrophic factors in combinatorial approaches for spinal cord regeneration

Axonal regeneration is inhibited by a plethora of different mechanisms in the adult central nervous system (CNS). While neurotrophic factors have been shown to stimulate axonal growth in numerous animal models of nervous system injury, a lack of suitable growth substrates, an insufficient activation of neuron-intrinsic regenerative programs, and extracellular inhibitors of regeneration limit the efficacy of neurotrophic factor delivery for anatomical and functional recovery after spinal cord injury. Thus, growth-stimulating factors will likely have to be combined with other treatment approaches to tap into the full potential of growth factor therapy for axonal regeneration. In addition, the temporal and spatial distribution of growth factors have to be tightly controlled to achieve biologically active concentrations, to allow for the chemotropic guidance of axons, and to prevent adverse effects related to the widespread distribution of neurotrophic factors. Here, we will review the rationale for combinatorial treatments in axonal regeneration and summarize some recent progress in promoting axonal regeneration in the injured CNS using such approaches.

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.

Dorsal root ganglion - a potential new therapeutic target for neuropathic pain

A regional approach can protect our patients from often unacceptable adverse effects produced by systematically applied drugs. Regional therapeutic approaches, as well as interventions at the level of the peripheral nervous system and particularly the dorsal root ganglion (DRG), represent an alternative to the systemic application of therapeutic agents. This article provides an overview of DRG anatomical peculiarities, explains why the DRG is an important therapeutic target, and how animal models of targeted drug delivery can help us in the translation of basic research into clinical practice.

gp130 cytokines are positive signals triggering changes in gene expression and axon outgrowth in peripheral neurons following injury

Adult peripheral neurons, in contrast to adult central neurons, are capable of regeneration after axonal damage. Much attention has focused on the changes that accompany this regeneration in two places, the distal nerve segment (where phagocytosis of axonal debris, changes in the surface properties of Schwann cells, and induction of growth factors and cytokines occur) and the neuronal cell body (where dramatic changes in cell morphology and gene expression occur). The changes in the axotomized cell body are often referred to as the "cell body response." The focus of the current review is a family of cytokines, the glycoprotein 130 (gp130) cytokines, which produce their actions through a common gp130 signaling receptor and which function as injury signals for axotomized peripheral neurons, triggering changes in gene expression and in neurite outgrowth. These cytokines play important roles in the responses of sympathetic, sensory, and motor neurons to injury. The best studied of these cytokines in this context are leukemia inhibitory factor (LIF) and interleukin (IL)-6, but experiments with conditional gp130 knockout animals suggest that other members of this family, not yet determined, are also involved. The primary gp130 signaling pathway shown to be involved is the activation of Janus kinase (JAK) and the transcription factors Signal Transducers and Activators of Transcription (STAT), though other downstream pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) may also play a role. gp130 signaling may involve paracrine, retrograde, and autocrine actions of these cytokines. Recent studies suggest that manipulation of this cytokine system can also stimulate regeneration by injured central neurons.

Axonal transcription factors signal retrogradely in lesioned peripheral nerve

Retrograde axonal injury signalling stimulates cell body responses in lesioned peripheral neurons. The involvement of importins in retrograde transport suggests that transcription factors (TFs) might be directly involved in axonal injury signalling. Here, we show that multiple TFs are found in axons and associate with dynein in axoplasm from injured nerve. Biochemical and functional validation for one TF family establishes that axonal STAT3 is locally translated and activated upon injury, and is transported retrogradely with dynein and importin α5 to modulate survival of peripheral sensory neurons after injury. Hence, retrograde transport of TFs from axonal lesion sites provides a direct link between axon and nucleus.

Tuning the orchestra: transcriptional pathways controlling axon regeneration

Trauma in the adult mammalian central nervous system leads to irreversible structural and functional impairment due to failed regeneration attempts. In contrast, neurons in the peripheral nervous system exhibit a greater regenerative ability. It has been proposed that an orchestrated sequence of transcriptional events controlling the expression of specific sets of genes may be the underlying basis of an early cell-autonomous regenerative response. Understanding whether transcriptional fine tuning, in parallel with strategies aimed at counteracting extrinsic impediments promotes axon re-growth following central nervous system injuries represents an exciting challenge for future studies. Transcriptional pathways controlling axon regeneration are presented and discussed in this review.

Gene therapy, neurotrophic factors and spinal cord regeneration

Significant advances have been made in understanding the mechanisms that limit axon regeneration in the adult mammalian central nervous system and in addressing some of the obstacles for axon growth. Despite this progress numerous challenges remain to achieve regeneration of a large number of axons sufficient to mediate functional improvement. Given the complexity of injury-induced changes in axon, cell body, and parenchyma surrounding a spinal cord lesion, it seems likely that multiple factors both intrinsic and extrinsic to injured neurons have to be addressed to augment axon regeneration and useful reorganization of spared circuitry. Neurotrophic factors have been shown to be one potent means to increase the number and range of regenerating axons, to guide regenerating axons across a lesion site, and to augment regenerative cell body responses to injury. In this chapter we will review the potential and current limitations of neurotrophic factors and gene therapy, in combination with cellular transplants, for axon regeneration and sprouting in the injured spinal cord.

Transcribing the path to neurological recovery-From early signals through transcription factors to downstream effectors of successful regeneration

The peripheral nervous system is known to regenerate comparatively well and this ability is mirrored in the de novo expression or upregulation of a wide variety of molecules involved in axonal outgrowth starting with transcription factors, but also including growth-stimulating substances, guidance and cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions. Recent studies using pharmacological agents, and global as well as neuron-selective gene inactivation techniques have shed light on those endogenous molecules that play a non-redundant role in mediating regenerative axonal outgrowth in vivo. The aim of the current review is to sketch the sequence of molecular events from early sensors of injury to transcription factors to downstream effectors that cooperate in successful regeneration and functional recovery.

Gene therapy approaches to enhancing plasticity and regeneration after spinal cord injury

During the past decades, new insights into mechanisms that limit plasticity and functional recovery after spinal cord injury have spurred the development of novel approaches to enhance axonal regeneration and rearrangement of spared circuitry. Gene therapy may provide one means to address mechanisms that underlie the insufficient regenerative response of injured neurons and can also be used to identify factors important for axonal growth. Several genetic approaches aimed to modulate the environment of injured axons, for example by localized expression of growth factors, to enhance axonal sprouting and regeneration and to guide regenerating axons towards their target have been described. In addition, genetic modification of injured neurons via intraparenchymal injection, or via retrograde transport of viral vectors has been used to manipulate the intrinsic growth capacity of injured neurons. In this review we will summarize some of the progress and limitations of cell transplantation and gene therapy to enhance axonal bridging and regeneration across a lesion site, and to maximize the function, collateral sprouting and connectivity of spared axonal systems.

Injury-induced CRMP4 expression in adult sensory neurons; a possible target gene for ciliary neurotrophic factor

Neurotrophic cytokines, such as ciliary neurotrophic factor (CNTF) play an important role in the development and regeneration of the nervous system. In the present study, we screened gene expression induced by CNTF in adult dorsal root ganglion (DRG) neurons using the Illumina microarray. We found that the expression of both short and long forms of collapsin response-mediator protein 4 (CRMP4) was increased in cultured primary sensory neurons by CNTF. In addition, sciatic nerve injury induced the expression of CRMP4 mRNA and protein in DRG neurons. Finally, the increased CRMP4 protein was transported into peripheral axons following nerve injury. These findings indicate that CRMP4 may be a target gene for CNTF in the regenerative axon growth of DRG neurons after injury.

The conditioning lesion effect on sympathetic neurite outgrowth is dependent on gp130 cytokines

Sympathetic neurons, like sensory neurons, increase neurite outgrowth after a conditioning lesion. Studies in leukemia inhibitory factor (LIF) knockout animals showed that the conditioning lesion effect in sensory neurons is dependent in part on this cytokine; however, similar studies on sympathetic neurons revealed no such effect. Comparable studies with sensory neurons taken from mice lacking the related cytokine interleukin-6 (IL-6) have yielded conflicting results. LIF and IL-6 belong to a family of cytokines known as the gp130 family because they act on receptors containing the subunit gp130. In sympathetic ganglia, axotomy leads to increases in mRNA for four of these cytokines (LIF, IL-6, IL-11, and oncostatin M). To test the role of this family of cytokines as a whole in the conditioning lesion response in sympathetic neurons, mice in which gp130 was selectively eliminated in noradrenergic neurons were studied. The postganglionic axons of the SCG were transected, and 7days later the ganglia were removed and neurite outgrowth was measured in explant and dissociated cell cultures. In both systems, neurons from wild type animals showed enhanced growth after a conditioning lesion. In contrast, no enhancement occurred in neurons from mutant animals. This lack of stimulation of outgrowth occurred despite an increase in expression of activating transcription factor 3 (ATF3) in the mutant mice. These studies demonstrate that stimulation of enhanced growth of sympathetic neurons after a conditioning lesion is dependent on gp130 cytokine signaling and is blocked in the absence of signaling by these cytokines in spite of an increase in ATF3.

Lentiviral vector-mediated gene transfer and RNA silencing technology in neuronal dysfunctions

Lentiviral-mediated gene transfer in vivo or in cultured mammalian neurons can be used to address a wide variety of biological questions, to design animal models for specific neurodegenerative pathologies, or to test potential therapeutic approaches in a variety of brain disorders. Lentiviruses can infect nondividing cells, thereby allowing stable gene transfer in postmitotic cells such as mature neurons. An important contribution has been the use of inducible vectors: the same animal can thus be used repeatedly in the doxycycline-on or -off state, providing a powerful mean for assessing the function of a gene candidate in a disorder within a specific neuronal circuit. Furthermore, lentivirus vectors provide a unique tool to integrate siRNA expression constructs with the aim to locally knockdown expression of a specific gene, enabling to assess the function of a gene in a very specific neuronal pathway. Lentiviral vector-mediated delivery of short hairpin RNA results in persistent knockdown of gene expression in the brain. Therefore, the use of lentiviruses for stable expression of siRNA in brain is a powerful aid to probe gene functions in vivo and for gene therapy of diseases of the central nervous system. In this chapter, I review the applications of lentivirus-mediated gene transfer in the investigation of specific gene candidates involved in major brain disorders and neurodegenerative processes. Major applications have been in polyglutamine disorders, such as synucleinopathies and Parkinson's disease, or in investigating gene function in Huntington's disease, dystonia, or muscular dystrophy. Recently, lentivirus gene transfer has been an invaluable tool for evaluation of gene function in behavioral disorders such as drug addiction and attention-deficit hyperactivity disorder or in learning and cognition.

The role of glial cells in influencing neurite extension by dorsal root ganglion cells

When pretreated with pertussis toxin (PTX), the neurites of adult rat dorsal root ganglion (DRG) cells in mixed cell cultures retract over a period of 2 h following the initial stimulus of removal from the cell culture incubator for brief periods of observation. The purpose of this investigation was to determine whether this PTX-dependent response was specific to any one of the three subpopulations of DRG neurons. However, no neurite retraction response was observed in neuron-enriched populations of cells, or in cultures enriched in isolectin B4 (IB4)-positive neurons or in IB4-negative neurons. But, the addition of non-neuronal cells, and/or medium conditioned by non-neuronal cells, was sufficient to restore the PTX-dependent neurite retraction response, but only in large diameter IB4-negative neurons. In conclusion, we have identified a regulatory response, mediated by Gi/o-proteins, which prevents retraction of neurites in large diameter IB4-negative cells of adult rat DRG. The non-neuronal cells of adult rat DRG constitutively release factor/s that can stimulate neurite retraction of a subset of isolated DRG neurons, but this property of non-neuronal cells is only observed when the Gi/o-proteins of large diameter IB4-negative cells are inhibited.

Optic nerve and vitreal inflammation are both RGC neuroprotective but only the latter is RGC axogenic

Intravitreal inflammation, induced by either lens injury, or intravitreal injection of zymosan (IVZ), protects RGC from apoptosis and stimulates axon regeneration after optic nerve transection. Here, we investigate the differential effects of intra-optic nerve zymosan (ONZ) and IVZ injections on RGC neuroprotection and axogenesis. After both IVZ and ONZ injection, zymosan-induced inflammation promoted a similar 4-/5-fold enhancement in RGC survival, compared to optic nerve transected controls, but only IVZ promoted RGC axon regeneration. IVZ was the most effective in activating retinal astrocyte/Müller cells while regulated intramembraneous proteolysis (RIP) of p75(NTR) and inactivation of Rho (key components of the axon growth inhibitory signalling cascade) occurred in both ONZ and IVZ, but only in the latter did RGC axons regenerate. We suggest that neuroprotective factors may be transported to RGC somata by retrograde transport after ONZ and diffuse into the retina after IVZ injection, but an axogenic agent is required to initiate and maintain disinhibited RGC axon regeneration that may be an exclusive property of a Müller cell-derived factor released after IVZ.

RESPONSES OF THE NERVE CELL BODY TO AXOTOMY

OBJECTIVE

Peripheral nerve injury causes retrograde changes in the damaged neurons, which are beneficial to axonal regeneration. Better understanding of the mechanisms of induction and mediation of these conditioning responses would help to design strategies to invoke stronger regenerative responses in neurons in situations when these responses are inadequate.

METHODS

Relevant literature is reviewed.

RESULTS

Experimental preparations that measure the influence of peripheral axotomy on regeneration in the central axons of primary sensory neurons are useful to examine mechanisms of conditioning neurons. Despite 4 decades of speculation, the nature of the damage signals from injured nerves that initiate axonal signals to the nerve cell body remains elusive. Members of the family of neuropoietic cytokines are clearly implicated, but what induces them is unknown. Multiple changes in gene regulation in axotomized neurons have been described, and dozens of growth-associated genes have been identified: neurotrophic factors, transcription factors, molecules participating in axonal transport, and molecules active in the growth cone. The mechanisms of interaction of a few regeneration-associated molecules with the signaling cascades that lead to actin and tubulin remodeling at the growth cone are understood in some detail. In animals, viral gene therapy to deliver regeneration-associated genes to neurons or other local measures to induce these genes can improve regeneration. A few pharmacological agents, administered systemically, have small beneficial effects on axonal regeneration.

CONCLUSION

Advances in laboratory research have provided knowledge of cell body responses to axotomy with clinical relevance.

Macrophages promote axon regeneration with concurrent neurotoxicity

Activated macrophages can promote regeneration of CNS axons. However, macrophages also release factors that kill neurons. These opposing functions are likely induced simultaneously but are rarely considered together in the same experimental preparation. A goal of this study was to unequivocally document the concurrent neurotoxic and neuroregenerative potential of activated macrophages. To do so, we quantified the length and magnitude of axon growth from enhanced green fluorescent protein-expressing dorsal root ganglion (DRG) neurons transplanted into the spinal cord in relationship to discrete foci of activated macrophages. Macrophages were activated via intraspinal injections of zymosan, a potent inflammatory stimulus known to increase axon growth and cause neurotoxicity. Using this approach, a significant increase in axon growth up to macrophage foci was evident. Within and adjacent to macrophages, DRG and spinal cord axons were destroyed. Macrophage toxicity became more evident when zymosan was injected closer to DRG soma. Under these conditions, DRG neurons were killed or their ability to extend axons was dramatically impaired. The concurrent induction of pro-regenerative and neurotoxic functions in zymosan-activated macrophages (ZAMs) was confirmed in vitro using DRG and cortical neurons. Importantly, the ability of ZAMs to stimulate axon growth was transient; prolonged exposure to factors produced by ZAMs enhanced cell death and impaired axon growth in surviving neurons. Lipopolysaccharide, another potent macrophage activator, elicited a florid macrophage response, but without enhancing axon growth or notable toxicity. Together, these data show that a single mode of activation endows macrophages with the ability to simultaneously promote axon regeneration and cell killing.

Nerve injury signaling

Although neurons within the peripheral nervous system (PNS) have a remarkable ability to repair themselves after injury, neurons within the central nervous system (CNS) do not spontaneously regenerate. This problem has remained recalcitrant despite a century of research on the reaction of axons to injury. The balance between inhibitory cues present in the environment and the intrinsic growth capacity of the injured neuron determines the extent of axonal regeneration following injury. The cell body of an injured neuron must receive accurate and timely information about the site and extent of axonal damage in order to increase its intrinsic growth capacity and successfully regenerate. One of the mechanisms contributing to this process is retrograde transport of injury signals. For example, molecules activated at the injury site convey information to the cell body leading to the expression of regeneration-associated genes and increased growth capacity of the neuron. Here we discuss recent studies that have begun to dissect the injury-signaling pathways involved in stimulating the intrinsic growth capacity of injured neurons.

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.

Targeted delivery of pharmacological agents into rat dorsal root ganglion

We sought an optimal method for targeted delivery into dorsal root ganglia (DRGs) for experimental studies, in terms of precision of delivery and avoidance of behavioral disturbances. We examined three approaches for injection into rat DRGs: percutaneous injection without surgical exposure, injection after deep exposure, and injection following deep exposure and partial laminectomy. Coomassie blue and Fast Blue were injected into DRGs for validation. At necropsy, the spread of Coomassie blue and Fast Blue was investigated under stereomicroscope and fluorescent microscope, respectively. We found that percutaneous approach did not provide any successful DRG injections. Deep exposure prior to intraganglionic injection provided variable results, but intraganglionic injection after deep exposure plus partial laminectomy was successful in 100% of attempts. Our subsequent skeletal analysis showed that the anatomical location of DRG is not compatible with successful DRG injection without surgical exposure. Neither of the methods using surgical exposure caused behavioral disturbances. Based on these results we conclude that partial laminectomy offers the most precise method of injecting DRG and does not produce behavioral evidence of nerve damage. Intraganglionic injection after deep exposure alone is less predictable, while percutaneous approaches only allow injection in the peripheral nerve.

Electrical stimulation of intact peripheral sensory axons in rats promotes outgrowth of their central projections

A lesion of a peripheral nerve before a second injury (conditioning lesion, CL), enhances peripheral and central regeneration of dorsal root ganglion (DRG) axons. This effect is mediated by elevated neuronal cAMP. Here we wanted to investigate whether electrical stimulation (ES) of an intact nerve, which has been shown to accelerate peripheral axon outgrowth, is also effective in promoting axon regeneration of injured DRG axons in vitro and of the central DRG axons in vivo and, whether this effect is mediated by elevation of cAMP. For the in vitro assay, the intact sciatic nerve of adult rats was stimulated at 20 Hz for 1 h, 7 days before harvest and primary culture of DRG neurons on a growth permissive substrate. In the in vivo study, the central axons of the lumbosacral DRGs were cut in the Th8 dorsal column, and the sciatic nerve was either cut or left intact, and subjected to 1 h ES at 20 Hz or 200 Hz. In vitro, ES increased neurite outgrowth 4-fold as compared to non-stimulated DRG neurons. In vivo, ES at 20 Hz significantly increased axon outgrowth into the central lesion site as compared to the Sham control. The 20 Hz ES was as effective as the CL in increasing axon outgrowth into the lesion site but not in promoting axonal elongation even though 20 Hz ES increased intracellular cAMP levels in DRG neurons as effectively as the CL. Thus elevation of cAMP may account for the central axonal outgrowth after ES and a CL.

CNS injury, glial scars, and inflammation: Inhibitory extracellular matrices and regeneration failure

Spinal cord and brain injuries lead to complex cellular and molecular interactions within the central nervous system in an attempt to repair the initial tissue damage. Many studies have illustrated the importance of the glial cell response to injury, and the influences of inflammation and wound healing processes on the overall morbidity and permanent disability that result. The abortive attempts of neuronal regeneration after spinal cord injury are influenced by inflammatory cell activation, reactive astrogliosis and the production of both growth promoting and inhibitory extracellular molecules. Despite the historical perspective that the glial scar was a mechanical barrier to regeneration, inhibitory molecules in the forming scar and methods to overcome them have suggested molecular modification strategies to allow neuronal growth and functional regeneration. Unlike myelin associated inhibitory molecules, which remain at largely static levels before and after central nervous system trauma, inhibitory extracellular matrix molecules are dramatically upregulated during the inflammatory stages after injury providing a window of opportunity for the delivery of candidate therapeutic interventions. While high dose methylprednisolone steroid therapy alone has not proved to be the solution to this difficult clinical problem, other strategies for modulating inflammation and changing the make up of inhibitory molecules in the extracellular matrix are providing robust evidence that rehabilitation after spinal cord and brain injury has the potential to significantly change the outcome for what was once thought to be permanent disability.