ATF3 increases the intrinsic growth state of DRG neurons to enhance peripheral nerve regeneration

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.

Axon regeneration: electrical silencing is a condition for regrowth

A recent study in primary sensory neurons shows that electrical activity--mediated through L-type voltage-gated calcium channels--could suppress axon growth after injury.

Increased mitochondrial calcium sensitivity and abnormal expression of innate immunity genes precede dopaminergic defects in Pink1-deficient mice

Background

PTEN-induced kinase 1 (PINK1) is linked to recessive Parkinsonism (EOPD). Pink1 deletion results in impaired dopamine (DA) release and decreased mitochondrial respiration in the striatum of mice. To reveal additional mechanisms of Pink1-related dopaminergic dysfunction, we studied Ca²⁺ vulnerability of purified brain mitochondria, DA levels and metabolism and whether signaling pathways implicated in Parkinson's disease (PD) display altered activity in the nigrostriatal system of Pink1^−/− mice.

Methods and Findings

Purified brain mitochondria of Pink1^−/− mice showed impaired Ca²⁺ storage capacity, resulting in increased Ca²⁺ induced mitochondrial permeability transition (mPT) that was rescued by cyclosporine A. A subpopulation of neurons in the substantia nigra of Pink1^−/− mice accumulated phospho-c-Jun, showing that Jun N-terminal kinase (JNK) activity is increased. Pink1^−/− mice 6 months and older displayed reduced DA levels associated with increased DA turnover. Moreover, Pink1^−/− mice had increased levels of IL-1β, IL-12 and IL-10 in the striatum after peripheral challenge with lipopolysaccharide (LPS), and Pink1^−/− embryonic fibroblasts showed decreased basal and inflammatory cytokine-induced nuclear factor kappa-β (NF-κB) activity. Quantitative transcriptional profiling in the striatum revealed that Pink1^−/− mice differentially express genes that (i) are upregulated in animals with experimentally induced dopaminergic lesions, (ii) regulate innate immune responses and/or apoptosis and (iii) promote axonal regeneration and sprouting.

Conclusions

Increased mitochondrial Ca²⁺ sensitivity and JNK activity are early defects in Pink1^−/− mice that precede reduced DA levels and abnormal DA homeostasis and may contribute to neuronal dysfunction in familial PD. Differential gene expression in the nigrostriatal system of Pink1^−/− mice supports early dopaminergic dysfunction and shows that Pink1 deletion causes aberrant expression of genes that regulate innate immune responses. While some differentially expressed genes may mitigate neurodegeneration, increased LPS-induced brain cytokine expression and impaired cytokine-induced NF-κB activation may predispose neurons of Pink1^−/− mice to inflammation and injury-induced cell death.

Long descending cervical propriospinal neurons differ from thoracic propriospinal neurons in response to low thoracic spinal injury

BACKGROUND

Propriospinal neurons, with axonal projections intrinsic to the spinal cord, have shown a greater regenerative response than supraspinal neurons after axotomy due to spinal cord injury (SCI). Our previous work focused on the response of axotomized short thoracic propriospinal (TPS) neurons following a low thoracic SCI (T9 spinal transection or moderate spinal contusion injury) in the rat. The present investigation analyzes the intrinsic response of cervical propriospinal neurons having long descending axons which project into the lumbosacral enlargement, long descending propriospinal tract (LDPT) axons. These neurons also were axotomized by T9 spinal injury in the same animals used in our previous study.

RESULTS

Utilizing laser microdissection (LMD), qRT-PCR, and immunohistochemistry, we studied LDPT neurons (located in the C5-C6 spinal segments) between 3-days, and 1-month following a low thoracic (T9) spinal cord injury. We examined the response of 89 genes related to growth factors, cell surface receptors, apoptosis, axonal regeneration, and neuroprotection/cell survival. We found a strong and significant down-regulation of ~25% of the genes analyzed early after injury (3-days post-injury) with a sustained down-regulation in most instances. In the few genes that were up-regulated (Actb, Atf3, Frs2, Hspb1, Nrap, Stat1) post-axotomy, the expression for all but one was down-regulated by 2-weeks post-injury. We also compared the uninjured TPS control neurons to the uninjured LDPT neurons used in this experiment for phenotypic differences between these two subpopulations of propriospinal neurons. We found significant differences in expression in 37 of the 84 genes examined between these two subpopulations of propriospinal neurons with LDPT neurons exhibiting a significantly higher base line expression for all but 3 of these genes compared to TPS neurons.

CONCLUSIONS

Taken collectively these data indicate a broad overall down-regulation in the genes examined, including genes for neurotrophic/growth factor receptors as well as for several growth factors. There was a lack of a significant regenerative response, with the exception of an up-regulation of Atf3 and early up-regulation of Hspb1 (Hsp27), both involved in cell stress/neuroprotection as well as axonal regeneration. There was no indication of a cell death response over the first month post-injury. In addition, there appear to be significant phenotypic differences between uninjured TPS and LDPT neurons, which may partly account for the differences observed in their post-axotomy responses. The findings in this current study stand in stark contrast to the findings from our previous work on TPS neurons. This suggests that different approaches will be needed to enhance the capacity for each population of propriospinal neuron to survive and undergo successful axonal regeneration after SCI.

Transcriptional profiling of intrinsic PNS factors in the postnatal mouse

Neurons in the peripheral nervous system (PNS) display a higher capacity to regenerate after injury than those in the central nervous system, suggesting cell specific transcriptional modules underlying axon growth and inhibition. We report a systems biology based search for PNS specific transcription factors (TFs). Messenger RNAs enriched in dorsal root ganglion (DRG) neurons compared to cerebellar granule neurons (CGNs) were identified using subtractive hybridization and DNA microarray approaches. Network and transcription factor binding site enrichment analyses were used to further identify TFs that may be differentially active. Combining these techniques, we identified 32 TFs likely to be enriched and/or active in the PNS. Twenty-five of these TFs were then tested for an ability to promote CNS neurite outgrowth in an overexpression screen. Real-time PCR and immunohistochemical studies confirmed that one representative TF, STAT3, is intrinsic to PNS neurons, and that constitutively active STAT3 is sufficient to promote CGN neurite outgrowth.

Signaling to transcription networks in the neuronal retrograde injury response

Retrograde signaling from axon to soma activates intrinsic regeneration mechanisms in lesioned peripheral sensory neurons; however, the links between axonal injury signaling and the cell body response are not well understood. Here, we used phosphoproteomics and microarrays to implicate approximately 900 phosphoproteins in retrograde injury signaling in rat sciatic nerve axons in vivo and approximately 4500 transcripts in the in vivo response to injury in the dorsal root ganglia. Computational analyses of these data sets identified approximately 400 redundant axonal signaling networks connected to 39 transcription factors implicated in the sensory neuron response to axonal injury. Experimental perturbation of individual overrepresented signaling hub proteins, including Abl, AKT, p38, and protein kinase C, affected neurite outgrowth in sensory neurons. Paradoxically, however, combined perturbation of Abl together with other hub proteins had a reduced effect relative to perturbation of individual proteins. Our data indicate that nerve injury responses are controlled by multiple regulatory components, and suggest that network redundancies provide robustness to the injury response.

Demyelination increases axonal stationary mitochondrial size and the speed of axonal mitochondrial transport

Axonal degeneration contributes to permanent neurological disability in inherited and acquired diseases of myelin. Mitochondrial dysfunction has been proposed as a major contributor to this axonal degeneration. It remains to be determined, however, if myelination, demyelination, or remyelination alter the size and distribution of axonal mitochondrial stationary sites or the rates of axonal mitochondrial transport. Using live myelinated rat dorsal root ganglion (DRG) cultures, we investigated whether myelination and lysolecithin-induced demyelination affect axonal mitochondria. Myelination increased the size of axonal stationary mitochondrial sites by 2.3-fold. After demyelination, the size of axonal stationary mitochondrial sites was increased by an additional 2.2-fold and the transport velocity of motile mitochondria was increased by 47%. These measures returned to the levels of myelinated axons after remyelination. Demyelination induced activating transcription factor 3 (ATF3) in DRG neurons. Knockdown of neuronal ATF3 by short hairpin RNA abolished the demyelination-induced increase in axonal mitochondrial transport and increased nitrotyrosine immunoreactivity in axonal mitochondria, suggesting that neuronal ATF3 expression and increased mitochondrial transport protect demyelinated axons from oxidative damage. In response to insufficient ATP production, demyelinated axons increase the size of stationary mitochondrial sites and thereby balance ATP production with the increased energy needs of nerve conduction.

Intrinsic response of thoracic propriospinal neurons to axotomy

Background

Central nervous system axons lack a robust regenerative response following spinal cord injury (SCI) and regeneration is usually abortive. Supraspinal pathways, which are the most commonly studied for their regenerative potential, demonstrate a limited regenerative ability. On the other hand, propriospinal (PS) neurons, with axons intrinsic to the spinal cord, have shown a greater regenerative response than their supraspinal counterparts, but remain relatively understudied in regards to spinal cord injury.

Results

Utilizing laser microdissection, gene-microarray, qRT-PCR, and immunohistochemistry, we focused on the intrinsic post-axotomy response of specifically labelled thoracic propriospinal neurons at periods from 3-days to 1-month following T9 spinal cord injury. We found a strong and early (3-days post injury, p.i) upregulation in the expression of genes involved in the immune/inflammatory response that returned towards normal by 1-week p.i. In addition, several regeneration associated and cell survival/neuroprotective genes were significantly up-regulated at the earliest p.i. period studied. Significant upregulation of several growth factor receptor genes (GFRa1, Ret, Lifr) also occurred only during the initial period examined. The expression of a number of pro-apoptotic genes up-regulated at 3-days p.i. suggest that changes in gene expression after this period may have resulted from analyzing surviving TPS neurons after the cell death of the remainder of the axotomized TPS neuronal population.

Conclusions

Taken collectively these data demonstrate that thoracic propriospinal (TPS) neurons mount a very dynamic response following low thoracic axotomy that includes a strong regenerative response, but also results in the cell death of many axotomized TPS neurons in the first week after spinal cord injury. These data also suggest that the immune/inflammatory response may have an important role in mediating the early strong regenerative response, as well as the apoptotic response, since expression of all of three classes of gene are up-regulated only during the initial period examined, 3-days post-SCI. The up-regulation in the expression of genes for several growth factor receptors during the first week post-SCI also suggest that administration of these factors may protect TPS neurons from cell death and maintain a regenerative response, but only if given during the early period after injury.

Skin incision induces expression of axonal regeneration-related genes in adult rat spinal sensory neurons

Skin incision and nerve injury both induce painful conditions. Incisional and postsurgical pain is believed to arise primarily from inflammation of tissue and the subsequent sensitization of peripheral and central neurons. The role of axonal regeneration-related processes in development of pain has only been considered when there has been injury to the peripheral nerve itself, even though tissue damage likely induces injury of resident axons. We sought to determine if skin incision would affect expression of regeneration-related genes such as activating transcription factor 3 (ATF3) in dorsal root ganglion (DRG) neurons. ATF3 is absent from DRG neurons of the normal adult rodent, but is induced by injury of peripheral nerves and modulates the regenerative capacity of axons. Image analysis of immunolabeled DRG sections revealed that skin incision led to an increase in the number of DRG neurons expressing ATF3. RT-PCR indicated that other regeneration-associated genes (galanin, GAP-43, Gadd45a) were also increased, further suggesting an injury-like response in DRG neurons. Our finding that injury of skin can induce expression of neuronal injury/regeneration-associated genes may impact how clinical postsurgical pain is investigated and treated.

PERSPECTIVE

Tissue injury, even without direct nerve injury, may induce a state of enhanced growth capacity in sensory neurons. Axonal regeneration-associated processes should be considered alongside nerve signal conduction and inflammatory/sensitization processes as possible mechanisms contributing to pain, particularly the transition from acute to chronic pain.

Matrix metalloproteinase inhibition enhances the rate of nerve regeneration in vivo by promoting dedifferentiation and mitosis of supporting schwann cells

After peripheral nerve injury, Schwann cells (SCs) vigorously divide to survive and produce a sufficient number of cells to accompany regenerating axons. Matrix metalloproteinases (MMPs) have emerged as modulators of SC signaling and mitosis. Using a 5-bromo-2-deoxyuridine (BrdU) incorporation assay, we previously found that a broad-spectrum MMP inhibitor (MMPi), GM6001 (or ilomastat), enhanced division of cultured primary SCs. Here, we tested the hypothesis that the ability of MMPi to stimulate SC mitosis may advance nerve regeneration in vivo. GM6001 administration immediately after rat sciatic nerve crush and daily thereafter produced increased nerve regeneration as determined by nerve pinch test and growth-associated protein 43 expression. The MMPi promoted endoneurial BrdU incorporation relative to vehicle control. The dividing cells were mainly SCs and were associated with growth-associated protein 43-positive regenerating axons. After MMP inhibition, myelin basic protein mRNA expression (determined by Taqman real-time quantitative polymerase chain reaction) and active mitosis of myelin-forming SCs were reduced, indicating that MMPs may suppress their dedifferentiation preceding mitosis. Intrasciatic injection of mitomycin,the inhibitor of SC mitosis, suppressed nerve regrowth, which was reversed by MMPi, suggesting that its effect on axonal growth promotion depends on its promitogenic action in SCs. These studies establish novel roles for MMPs in peripheral nerve repair via control of SC mitosis, differentiation, and myelin protein mRNA expression.

Activation transcription factor-3 activation and the development of spinal cord degeneration in a rat model of amyotrophic lateral sclerosis

It has been reported that an early activation of glial fibrillary acid protein (GFAP) in astroglial cells occurs simultaneously in peripheral nerves and spinal cord from the G93A SOD1 mouse model of amyotrophic lateral sclerosis (ALS), an invariably fatal neurodegenerative disorder. In ALS, the contribute to the pathological process of different cell types varies according to the disease stage, with a florid immune response in spinal cord at end stage disease. In this study, we have mapped in different anatomical sites the process of disease-induced functional perturbation from a pre-symptomatic stage using a marker of cellular distress expressed in neurons and glial cells, the activating transcription factor 3 (ATF-3), and applied large-scale gene expression analysis to define the pattern or transcriptional changes occurring in spinal cord from the G93A SOD1 rat model of ALS in parallel with ATF-3 neuronal activation. From the disease onset onward, transgenic lumbar spinal cord displayed ATF-3 transcriptional regulation and motor cells immunostaining in association with the over-expression of genes promoting cell growth, the functional integrity of cell organelles and involved in the modulation of immune responses. While spinal cord from the pre-symptomatic rat showed no detectable ATF-3 transcriptional regulation, ATF-3 activation was appreciated in large size neurofilament-rich, small size non-peptidergic and parvalbumin-positive neurons within the dorsal root ganglia (DRG), and in ventral roots Schwann cells alongside macrophages infiltration. This pattern of peripheral ATF-3 activation remained detectable throughout the disease process. In the G93A SOD1 rat model of ALS, signs of roots and nerves subtle distress preceded overt clinical-pathological changes, involving both glial cells and neurons that function as receptors of peripheral sensory stimuli from the muscle. In addition, factors previously described to be linked to ATF-3 activation under various experimental conditions of stress, become switched on in spinal cord from the end-stage transgenic rat model of ALS.

Neuronal intrinsic barriers for axon regeneration in the adult CNS

A major reason for the devastating and permanent disabilities after spinal cord and other types of CNS injury is the failure of injured axons to regenerate and to re-build the functional circuits. Thus, a long-standing goal has been to develop strategies that could promote axon regeneration and restore functions. Recent studies revealed that simply removing extracellular inhibitory activities is insufficient for successful axon regeneration in the adult CNS. On the other side, evidence from different species and different models is accumulating to support the notion that diminished intrinsic regenerative ability of mature neurons is a major contributor to regeneration failure. This review will summarize the molecular mechanisms regulating intrinsic axon growth capacity in the adult CNS and discuss potential implications for therapeutic strategies.

Activating transcription factor 3 confers protection against ventilator-induced lung injury

RATIONALE

Ventilator-induced lung injury (VILI) significantly contributes to mortality in patients with acute respiratory distress syndrome, the most severe form of acute lung injury. Understanding the molecular basis for response to cyclic stretch (CS) and its derangement during high-volume ventilation is of high priority.

OBJECTIVES

To identify specific molecular regulators involved in the development of VILI.

METHODS

We undertook a comparative examination of cis-regulatory sequences involved in the coordinated expression of CS-responsive genes using microarray analysis. Analysis of stretched versus nonstretched cells identified significant enrichment for genes containing putative binding sites for the transcription factor activating transcription factor 3 (ATF3). To determine the role of ATF3 in vivo, we compared the response of ATF3 gene-deficient mice to wild-type mice in an in vivo model of VILI.

MEASUREMENTS AND MAIN RESULTS

ATF3 protein expression and nuclear translocation is increased in the lung after mechanical ventilation in wild-type mice. ATF3-deficient mice have greater sensitivity to mechanical ventilation alone or in conjunction with inhaled endotoxin, as demonstrated by increased cell infiltration and proinflammatory cytokines in the lung and bronchoalveolar lavage, and increased pulmonary edema and indices of tissue injury. The expression of stretch-responsive genes containing putative ATF3 cis-regulatory regions was significantly altered in ATF3-deficient mice.

CONCLUSIONS

ATF3 deficiency confers increased sensitivity to mechanical ventilation alone or in combination with inhaled endotoxin. We propose ATF3 acts to counterbalance CS and high volume-induced inflammation, dampening its ability to cause injury and consequently protecting animals from injurious CS.

PACAP expression in explant cultured mouse major pelvic ganglia

The major pelvic ganglia (MPG) contain both parasympathetic and sympathetic postganglionic neurons and provide much of the autonomic innervation to urogenital organs and components of the lower bowel. Whereas many parasympathetic neurons were found to express vasoactive intestinal polypeptide (VIP), no MPG neurons exhibited immunoreactivity for pituitary adenylate cyclase-activating polypeptide (PACAP). However, in 3-day cultured MPGs, numerous PACAP-IR cells and nerve fibers were present, and transcript levels for PACAP increased significantly. In 3-day cultured MPGs, PACAP immunoreactivity was seen in cells that were also immunoreactive for VIP or neuronal nitric oxide synthase, but not tyrosine hydroxylase, indicating that PACAP expression occurred preferentially in MPG parasympathetic postganglionic neurons. Transcript levels for the VPAC2, but not VPAC1 or PAC1 receptor, also increased significantly following 3 days in culture. Transcript levels of activating transcription factor 3 (ATF-3), a marker of cellular injury, were increased 64-fold in 3-day explants, and ATF-3-IR nuclei were evident in both TH-IR and nNOS-IR neurons as well as in non-neuronal cells. In sum, these results demonstrate that, although only the parasympathetic neurons in explant cultured MPGs increase expression of PACAP, both sympathetic and parasympathetic postganglionic neurons in the cultured MPG whole-mount increase expression of ATF-3.

Immediate anti-tumor necrosis factor-alpha (etanercept) therapy enhances axonal regeneration after sciatic nerve crush

Peripheral nerve regeneration begins immediately after injury. Understanding the mechanisms by which early modulators of axonal degeneration regulate neurite outgrowth may affect the development of new strategies to promote nerve repair. Tumor necrosis factor-alpha (TNF-alpha) plays a crucial role in the initiation of degenerative cascades after peripheral nerve injury. Here we demonstrate using real-time Taqman quantitative RT-PCR that, during the time course (days 1-60) of sciatic nerve crush, TNF-alpha mRNA expression is induced at 1 day and returned to baseline at 5 days after injury in nerve and the corresponding dorsal root ganglia (DRG). Immediate therapy with the TNF-alpha antagonist etanercept (fusion protein of TNFRII and human IgG), administered systemically (i.p.) and locally (epineurially) after nerve crush injury, enhanced the rate of axonal regeneration, as determined by nerve pinch test and increased number of characteristic clusters of regenerating nerve fibers distal to nerve crush segments. These fibers were immunoreactive for growth associated protein-43 (GAP-43) and etanercept, detected by anti-human IgG immunofluorescence. Increased GAP-43 expression was found in the injured nerve and in the corresponding DRG and ventral spinal cord after systemic etanercept compared with vehicle treatments. This study established that immediate therapy with TNF-alpha antagonist supports axonal regeneration after peripheral nerve injury.

Activating transcription factor 3 is a positive regulator of human IFNG gene expression

IL-12 and IL-18 are essential for Th1 differentiation, whereas the role of IFN-alpha in Th1 development is less understood. In this microarray-based study, we searched for genes that are regulated by IFN-alpha, IL-12, or the combination of IL-12 plus IL-18 during the early differentiation of human umbilical cord blood CD4(+) Th cells. Twenty-six genes were similarly regulated in response to treatment with IL-12, IFN-alpha, or the combination of IL-12 plus IL-18. These genes could therefore play a role in Th1 lineage decision. Transcription factor activating transcription factor (ATF) 3 was upregulated by these cytokines and selected for further study. Ectopic expression of ATF3 in CD4(+) T cells enhanced the production of IFN-gamma, the hallmark cytokine of Th1 cells, whereas small interfering RNA knockdown of ATF3 reduced IFN-gamma production. Furthermore, ATF3 formed an endogenous complex with JUN in CD4(+) T cells induced to Th1. Chromatin immunoprecipitation and luciferase reporter assays showed that both ATF3 and JUN are recruited to and transactivate the IFNG promoter during early Th1 differentiation. Collectively, these data indicate that ATF3 promotes human Th1 differentiation.

Peripheral facial nerve axotomy in mice causes sprouting of motor axons into perineuronal central white matter: time course and molecular characterization

Generation of new axonal sprouts plays an important role in neural repair. In the current study, we examined the appearance, composition and effects of gene deletions on intrabrainstem sprouts following peripheral facial nerve axotomy. Axotomy was followed by the appearance of galanin(+) and calcitonin gene-related peptide (CGRP)(+) sprouts peaking at day 14, matching both large, neuropeptide(+) subpopulations of axotomized facial motoneurons, but with CGRP(+) sprouts considerably rarer. Strong immunoreactivity for vesicular acetylcholine transporter (VAChT) and retrogradely transported MiniRuby following its application on freshly cut proximal facial nerve stump confirmed their axotomized motoneuron origin; the sprouts expressed CD44 and alpha7beta1 integrin adhesion molecules and grew apparently unhindered along neighboring central white matter tracts. Quantification of the galanin(+) sprouts revealed a stronger response following cut compared with crush (day 7-14) as well as enhanced sprouting after recut (day 8 + 6 vs. 14; 14 + 8 vs. 22), arguing against delayed appearance of sprouting being the result of the initial phase of reinnervation. Sprouting was strongly diminished in brain Jun-deficient mice but enhanced in alpha7 null animals that showed apparently compensatory up-regulation in beta1, suggesting important regulatory roles for transcription factors and the sprout-associated adhesion molecules. Analysis of inflammatory stimuli revealed a 50% reduction 12-48 hours following systemic endotoxin associated with neural inflammation and a tendency toward more sprouts in TNFR1/2 null mutants (P = 10%) with a reduced inflammatory response, indicating detrimental effects of excessive inflammation. Moreover, the study points to the usefulness of the facial axotomy model in exploring physiological and molecular stimuli regulating central sprouting.

A dual promoter lentiviral vector for the in vivo evaluation of gene therapeutic approaches to axon regeneration after spinal cord injury

The identification of axon growth-promoting genes, and overexpression of these genes in central nervous system (CNS) neurons projecting to the spinal cord, has emerged as one potential approach to enhancing CNS regeneration. Assessment of the regenerative potential of candidate genes usually requires axonal tracing of spinal projections, ideally limited to neurons that express the candidate gene. Alternatively, coexpression of a reporter gene such as enhanced green fluorescent protein (GFP) from an internal ribosomal entry site can be used to identify neurons expressing the candidate gene, but this strategy does not label corticospinal axons in the spinal cord. We therefore developed a dual promoter lentiviral vector in which a potentially therapeutic transgene is expressed from the cytomegalovirus-enhanced chicken beta-actin promoter and the fluorescent protein copGFP is expressed from the elongation factor-1alpha promoter. The vector was constructed to be compatible with the Gateway recombination system for efficient introduction of transgenes through entry shuttle vectors. We show both simultaneous expression of a candidate and reporter gene in corticospinal and red nucleus neurons, and efficient labeling of their axons after lesions in the cervical spinal cord. This expression system is therefore an accurate and efficient means of screening candidate genes in vivo for enhancement of axonal growth.

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.

NFIL3 and cAMP response element-binding protein form a transcriptional feedforward loop that controls neuronal regeneration-associated gene expression

Successful regeneration of damaged neurons depends on the coordinated expression of neuron-intrinsic genes. At present however, there is no comprehensive view of the transcriptional regulatory mechanisms underlying neuronal regeneration. We used high-content cellular screening to investigate the functional contribution of 62 transcription factors to regenerative neuron outgrowth. Ten transcription factors are identified that either increase or decrease neurite outgrowth. One of these, NFIL3, is specifically upregulated during successful regeneration in vivo. Paradoxically however, knockdown of NFIL3 and overexpression of dominant-negative NFIL3 both increase neurite outgrowth. Our data show that NFIL3, together with CREB, forms an incoherent feedforward transcriptional regulatory loop in which NFIL3 acts as a negative regulator of CREB-induced regeneration-associated genes.

Measuring nerve regeneration in the mouse

Genetic engineering of mice has become a major tool in understanding the roles of individual molecules in regeneration of nerves, and will play an increasing role in the future. Mice are in many ways well suited to assessment both of nerve regeneration after axotomy and of collateral sprouting of intact fibers into areas of denervation. However, mouse models present special challenges because of their small size, their inherent capacity for regeneration, and the potential strain effects. The most widely used model of regeneration, sciatic nerve injury, has its inherent limitations, and there is a need for other models of injury to long nerves. Measures of regeneration in the mouse can be divided into those that assess the latency to initiate growth, those sensitive to the rate of growth and the proportion of fibers growing at fast rates, those that assess the time to reinnervation of specific targets and the completeness of reinnervation, and those that assess specificity of reinnervation and functional recovery. The short length of nerve available in the mouse limits measures of the rates of outgrowth, and thus introduces a greater potential for "noise" of measurement than is seen in larger animals such as the rat. For both regeneration of interrupted fibers and collateral regeneration from intact fibers histological and physiological measures of "time to target" have the advantages of direct correlation with restoration of function, the ability to assess regeneration of different fiber types efficiently, and the fact that most of these measures are easier in the mouse than in the rat.

Introduction to special issue: Challenges and opportunities for regeneration in the peripheral nervous system

Regeneration in the peripheral nervous system offers unique opportunities and challenges to medicine. Compared to the central nervous system, peripheral axons can and do regenerate resulting in functional recovery, especially if the distance to target is short as in distal limb injuries. However, this regenerative capacity is often incomplete and functional recovery with proximal lesions is limited. Furthermore, regeneration of axons to the appropriate targets remains a challenge with inappropriate reinnervation being an impediment to full recovery. The reviews and selected original research papers in this Special Issue will address some of these challenges and highlight new opportunities for development of effective therapies for nerve regeneration.

Peripherin and ATF3 genes are differentially regulated in regenerating and non-regenerating primary sensory neurons

Peripheral nerve injury leads to deficient recovery of sensation and a causative factor may be that only 50-60% of primary sensory neurons succeed in regenerating axons after primary nerve repair. In this study, an in vivo rat sciatic nerve injury and regeneration model was combined with laser microdissection and quantitative real-time polymerase chain reaction with the aim of examining the gene expression of regenerative molecules in cutaneous and muscular sensory neurons. Recent studies have identified peripherin and ATF-3 molecules as crucial for neurite outgrowth propagation; our novel findings demonstrate a subpopulation of non-regenerating sensory neurons characterized by a failure to upregulate transcription of these molecules and that a greater peripherin mRNA expression in injured cutaneous neurons may potentiate this subpopulation to regenerate more axons than muscle afferent neurons following injury. The gene expression of the structural neurofilament NF-H is found to be significantly downregulated following injury in both sensory subpopulations.

Activating transcription factor 3 (ATF3) expression in the neural retina and optic nerve of zebrafish during optic nerve regeneration

Fish, unlike mammals, can regenerate axons in the optic nerve following optic nerve injury. We hypothesized that using microarray analysis to compare gene expression in fish which had experienced optic nerve lesion to fish which had undergone a similar operation but without optic nerve injury would reveal genes specifically involved in responding to optic nerve injury (including repair), reducing detection of genes involved in the general stress and inflammatory responses. We discovered 120 genes were significantly (minimally two-fold with a P-value < or = 0.05) differentially expressed (up or down) at one or more time point. Among these was ATF3, a member of the cAMP-response element binding protein family. Work by others has indicated that elevated cAMP could be important in axon regeneration. We investigated ATF3 expression further by qRT-PCR, in situ hybridization and immunohistochemistry and found ATF3 expression is significantly upregulated in the ganglion cell layer of the retina, the nerve fiber layer and the optic nerve of the injured eye. The upregulation in retina is detectable by qRT-PCR by 24 h after injury, at which time ATF-3 mRNA levels are 8-fold higher than in retinas from sham-operated fish. We conclude ATF3 may be an important mediator of optic nerve regeneration-promoting gene expression in fish, a role which merits further investigation.

Expression of stress-response ATF3 is mediated by Nrf2 in astrocytes

Activating Transcription Factor 3 (ATF3), a member of the ATF/CREB family, is induced rapidly by various stresses. Its induction mechanism and role in response to changes in cellular redox status, however, have not been elucidated. Here, we found that NF-E2-related factor 2 (Nrf2), a transcription factor known to bind to antioxidant response element (ARE) in promoters, transcriptionally upregulated ATF3 expression in astrocytes. Treatment with Nrf2 activators and oxidants provoked ATF3 induction in astrocytes, whereas its expression was reduced in Nrf2-depleted cells. We further demonstrated that the consensus ARE in the ATF3 promoter is critical for Nrf2-mediation by promoter analyses using an ATF3 promoter-driven luciferase construct and a chromatin immunoprecipitation assay. In addition, we found that Nrf2-dependent ATF3 induction contributed to the antioxidative and cytoprotective functions of Nrf2 in astrocytes. Taken together, our findings suggest that ATF3 is a new target for Nrf2 and has a cytoprotective function in astrocytes.

Neuropathic pain: a maladaptive response of the nervous system to damage

Neuropathic pain is triggered by lesions to the somatosensory nervous system that alter its structure and function so that pain occurs spontaneously and responses to noxious and innocuous stimuli are pathologically amplified. The pain is an expression of maladaptive plasticity within the nociceptive system, a series of changes that constitute a neural disease state. Multiple alterations distributed widely across the nervous system contribute to complex pain phenotypes. These alterations include ectopic generation of action potentials, facilitation and disinhibition of synaptic transmission, loss of synaptic connectivity and formation of new synaptic circuits, and neuroimmune interactions. Although neural lesions are necessary, they are not sufficient to generate neuropathic pain; genetic polymorphisms, gender, and age all influence the risk of developing persistent pain. Treatment needs to move from merely suppressing symptoms to a disease-modifying strategy aimed at both preventing maladaptive plasticity and reducing intrinsic risk.

Peripheral and central sensitization in remote spinal cord regions contribute to central neuropathic pain after spinal cord injury

Central neuropathic pain (CNP) developing after spinal cord injury (SCI) is described by the region affected: above-level, at-level and below-level pain occurs in dermatomes rostral, at/near, or below the SCI level, respectively. People with SCI and rodent models of SCI develop above-level pain characterized by mechanical allodynia and thermal hyperalgesia. Mechanisms underlying this pain are unknown and the goals of this study were to elucidate components contributing to the generation of above-level CNP. Following a thoracic (T10) contusion, forelimb nociceptors had enhanced spontaneous activity and were sensitized to mechanical and thermal stimulation of the forepaws 35 days post-injury. Cervical dorsal horn neurons showed enhanced responses to non-noxious and noxious mechanical stimulation as well as thermal stimulation of receptive fields. Immunostaining dorsal root ganglion (DRG) cells and cord segments with activating transcription factor 3 (ATF3, a marker for neuronal injury) ruled out neuronal damage as a cause for above-level sensitization since few C8 DRG cells expressed AFT3 and cervical cord segments had few to no ATF3-labeled cells. Finally, activated microglia and astrocytes were present in thoracic and cervical cord at 35 days post-SCI, indicating a rostral spread of glial activation from the injury site. Based on these data, we conclude that peripheral and central sensitization as well as reactive glia in the uninjured cervical cord contribute to CNP. We hypothesize that reactive glia in the cervical cord release pro-inflammatory substances which drive chronic CNP. Thus a complex cascade of events spanning many cord segments underlies above-level CNP.

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.

A conditioning lesion induces changes in gene expression and axonal transport that enhance regeneration by increasing the intrinsic growth state of axons

Injury of axons in the peripheral nervous system (PNS) induces transcription-dependent changes in gene expression and axonal transport that promote effective regeneration by increasing the intrinsic growth state of axons. Regeneration is enhanced in axons re-injured 1-2 weeks after the intrinsic growth state has been increased by such a prior conditioning lesion (CL). The intrinsic growth state does not increase after axons are injured in the mammalian central nervous system (CNS), where they lack the capacity for effective regeneration. Sensory neurons in the dorsal root ganglion (DRG) have two axonal branches that respond differently to injury. Peripheral branches, which are located entirely in the PNS, are capable of effective regeneration. Central branches regenerate in the PNS (i.e., in the dorsal root, which extends from the DRG to the spinal cord), but not in the CNS (i.e., the spinal cord). A CL of peripheral branches increases the intrinsic growth state of central branches in the dorsal columns of the spinal cord, enabling these axons to undergo lengthy regeneration in a segment of peripheral nerve transplanted into the spinal cord (i.e., a peripheral nerve graft). This regeneration does not occur in the absence of a CL. We will examine how changes in gene expression and axonal transport induced by a CL may promote this regeneration.

Overcoming macrophage-mediated axonal dieback following CNS injury

Trauma to the adult CNS initiates multiple processes including primary and secondary axotomy, inflammation, and glial scar formation that have devastating effects on neuronal regeneration. After spinal cord injury, the infiltration of phagocytic macrophages coincides with long-distance axonal retraction from the initial site of injury, a deleterious phenomenon known as axonal dieback. We have previously shown that activated macrophages directly induce long-distance retraction of dystrophic axons in an in vitro model of the glial scar. We hypothesized that treatments that are primarily thought to increase neuronal regeneration following spinal cord injury may in fact derive a portion of their beneficial effects from inhibition of macrophage-mediated axonal retraction. We analyzed the effects of protease inhibition, substrate modification, and neuronal preconditioning on macrophage-axon interactions using our established in vitro model. General inhibition of matrix metalloproteinases and specific inhibition of MMP-9 prevented macrophage-induced axonal retraction despite significant physical interactions between the two cell types, whereas inhibition of MMP-2 had no effect. Chondroitinase ABC-mediated digestion of the aggrecan substrate also prevented macrophage-induced axonal retraction in the presence of extensive macrophage-axon interactions. The use of a conditioning lesion to stimulate intrinsic neuronal growth potential in the absence of substrate modification likewise prevented macrophage-induced axonal retraction in vitro and in vivo following spinal cord injury. These data provide valuable insight into the cellular and molecular mechanisms underlying macrophage-mediated axonal retraction and demonstrate modifications that can alleviate the detrimental effects of this unfavorable phenomenon on the postlesion CNS.

Best1 is a gene regulated by nerve injury and required for Ca2+-activated Cl- current expression in axotomized sensory neurons

We investigated the molecular determinants of Ca(2+)-activated chloride current (CaCC) expressed in adult sensory neurons after a nerve injury. Dorsal root ganglia express the transcripts of three gene families known to induce CaCCs in heterologous systems: bestrophin, tweety, and TMEM16. We found with quantitative transcriptional analysis and in situ hybridization that nerve injury induced upregulation of solely bestrophin-1 transcripts in sensory neurons. Gene screening with RNA interference in single neurons demonstrated that mouse Best1 is required for the expression of CaCC in injured sensory neurons. Transfecting injured sensory neurons with bestrophin-1 mutants inhibited endogenous CaCC. Exogenous expression of the fusion protein green fluorescent protein-Bestrophin-1 in naive neurons demonstrated a plasma membrane localization of the protein that generates a CaCC with biophysical and pharmacological properties similar to endogenous CaCC. Our data suggest that Best1 belongs to a group of genes upregulated by nerve injury and supports functional CaCC expression in injured sensory neurons.

Increased synthesis of spermidine as a result of upregulation of arginase I promotes axonal regeneration in culture and in vivo

Adult spinal axons do not spontaneously regenerate after injury. However, if the peripheral branch of dorsal root ganglion neurons is lesioned before lesioning the central branch of the same neurons in the dorsal column, these central axons will regenerate and, if cultured, are not inhibited from extending neurites by myelin-associated inhibitors of regeneration such as myelin-associated glycoprotein (MAG). This effect can be mimicked by elevating cAMP and is transcription dependent. The ability of cAMP to overcome inhibition by MAG in culture involves the upregulation of the enzyme arginase I (Arg I) and subsequent increase in synthesis of polyamines such as putrescine. Now we show that a peripheral lesion also induces an increase in Arg I expression and synthesis of polyamines. We also show that the conditioning lesion effect in overcoming inhibition by MAG is initially dependent on ongoing polyamine synthesis but, with time after lesion, becomes independent of ongoing synthesis. However, if synthesis of polyamines is blocked in vivo the early phase of good growth after a conditioning lesion is completely blocked and the later phase of growth, when ongoing polyamine synthesis is not required during culture, is attenuated. We also show that putrescine must be converted to spermidine both in culture and in vivo to overcome inhibition by MAG and that spermidine can promote optic nerve regeneration in vivo. These results suggest that spermidine could be a useful tool in promoting CNS axon regeneration after injury.

ATF3 plays a protective role against toxicity by N-terminal fragment of mutant huntingtin in stable PC12 cell line

Huntington's disease is a progressive neurodegenerative disorder caused by a polyglutamine expansion near the N-terminus of huntingtin. The mechanisms of polyglutamine neurotoxicity, and cellular responses are not fully understood. We have studied gene expression profiles by short oligo array using an inducible PC12 cell model expressing an N-terminal huntingtin fragment with expanded polyglutamine (Htt-N63-148Q). Mutant huntingtin Htt-N63 induced cell death and increased the mRNA and protein levels of activating transcription factor 3 (ATF3). Mutant Htt-N63 also significantly enhanced ATF3 transcriptional activity by a promoter-based reporter assay. Overexpression of ATF3 protects against mutant Htt-N63 toxicity and knocking down ATF3 expression reduced Htt-N63 toxicity in a stable PC12 cell line. These results indicated that ATF3 plays a critical role in toxicity induced by mutant Htt-N63 and may lead to a useful therapeutic target.

Axotomy-induced Smad1 activation promotes axonal growth in adult sensory neurons

Mature neurons have diminished intrinsic regenerative capacity. Axotomy of the peripheral branch of adult dorsal root ganglia (a "conditioning" lesion) triggers a transcription-dependent axon growth program. Here, we show that this growth program requires the function of the transcription factor Smad1. After peripheral axotomy, neuronal Smad1 is upregulated, and phosphorylated Smad1 accumulates in the nucleus. Both events precede the onset of axonal extension. Reducing Smad1 by RNA interference in vitro impairs axonal growth, and the continued presence of Smad1 is required to maintain the growth program. Furthermore, intraganglionic injection of BMP2 or 4, which activates Smad1, markedly enhances axonal growth capacity, mimicking the effect of a conditioning lesion. Thus, activation of Smad1 by axotomy is a key component of the transcriptional switch that promotes an enhanced growth state of adult sensory neurons.

Collaborative models for translational neuroscience and rehabilitation research

Little formal research has been conducted on strategies to structure basic, preclinical, and clinical research to increase the likelihood of discovering efficacious interventions for patients with neurological diseases. How academic research is organized and funded by government agencies and foundations seems likely to affect the quality and rate of production of valued therapeutic agents. Few models for translational biomedical research, however, have been defined and no strategies have been compared. Given the narrow width of expertise and laboratory capacity of individual investigators, the complexity of identifying and manipulating mechanisms of disease components over time, and the demand for solutions from society, our continued reliance on funding therapeutic discovery through standalone investigators and projects seems counterproductive. Models are described for funding collaborations of basic and clinical scientists to work in iterative, adaptable, cross-disciplinary interactions around key progress-limiting questions. Problem-oriented collaborations require leadership, incentives, trust, ongoing assessment, and an efficient infrastructure that overcomes barriers. These models are as testable as the hypotheses that drive scientific research.

Chronically CNS-injured adult sensory neurons gain regenerative competence upon a lesion of their peripheral axon

Several experimental manipulations result in axonal regeneration in the central nervous system (CNS) when applied before or at the time of injury but not when initiated after a delay, which would be clinically more relevant. As centrally injured neurons show signs of atrophy and degeneration, it raises the question whether chronically injured neurons are able to regenerate. To address this question, we used adult rodent primary sensory neurons that regenerate their central axon when their peripheral axon is cut (called conditioning) beforehand but not afterwards. We found that primary sensory neurons express regeneration-associated genes and efficiently regrow their axon in cell culture two months after a central lesion upon conditioning. Moreover, conditioning enables central axons to regenerate through a fresh lesion independent of a previous central lesion. Using in vivo imaging we demonstrated that conditioned neurons rapidly regrow their axons through a fresh central lesion. Finally, when single sensory axons were cut with a two-photon laser, they robustly regenerate within days after attaining growth competence through conditioning. We conclude that sensory neurons can acquire the intrinsic potential to regenerate their axons months after a CNS lesion, which they implement in the absence of traumatic tissue.

Peripheral sensory axon growth: from receptor binding to cellular signaling

Regeneration following axonal injury of the adult peripheral sensory nervous system is heavily influenced by factors located in a neuron's extracellular environment. These factors include neurotrophins, such as Nerve Growth Factor (NGF) and the extracellular matrix, such as laminin. The presence of these molecules in the peripheral nervous system (PNS) is a major contributing factor for the dichotomy between regenerative capacities of central vs. peripheral neurons. Although PNS neurons are capable of spontaneous regeneration, this response is critically dependent on many different factors including the type, location and severity of the injury. In this article, we will focus on the plasticity of adult dorsal root ganglion (DRG) sensory neurons and how trophic factors and the extracellular environment stimulate the activation of intracellular signaling cascades that promote axonal growth in adult dorsal root ganglion neurons.

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.

Sox11 transcription factor modulates peripheral nerve regeneration in adult mice

The ability of adult peripheral sensory neurons to undergo functional and anatomical recovery following nerve injury is due in part to successful activation of transcriptional regulatory pathways. Previous in vitro evidence had suggested that the transcription factor Sox11, a HMG-domain containing protein that is highly expressed in developing sensory neurons, is an important component of this regenerative transcriptional control program. To further test the role of Sox11 in an in vivo system, we developed a new approach to specifically target small interfering RNAs (siRNAs) conjugated to the membrane permeable molecule Penetratin to injured sensory afferents. Injection of Sox11 siRNAs into the mouse saphenous nerve caused a transient knockdown of Sox11 mRNA that transiently inhibited in vivo regeneration. Electron microscopic level analysis of Sox11 RNAi-injected nerves showed that regeneration of myelinated and unmyelinated axons was inhibited. Nearly all neurons in ganglia of crushed nerves that were Sox11 immunopositive showed colabeling for the stress and injury-associated activating transcription factor 3 (ATF3). In addition, treatment with Sox11 siRNAs in vitro and in vivo caused a transcriptional and translational level reduction in ATF3 expression. These anatomical and expression data support an intrinsic role for Sox11 in events that underlie successful regeneration following peripheral nerve injury.

Expression of the regeneration-associated protein SPRR1A in primary sensory neurons and spinal cord of the adult mouse following peripheral and central injury

Small proline-rich repeat protein 1A (SPRR1A) is expressed in dorsal root ganglion (DRG) neurons following peripheral nerve injury but it is not known whether SPRR1A is differentially expressed following injury to peripheral versus central DRG projections and a detailed characterization of expression in sensory neuron subpopulations and spinal cord has not been performed. Here we use immunocytochemical techniques to characterize SPRR1A expression following sciatic nerve, dorsal root, and dorsal column injury in adult mice. SPRR1A was not detected in naïve spinal cord, DRG, or peripheral nerves and there was minimal expression following injury to the centrally projecting branches of DRG neurons. However, following peripheral (sciatic) nerve injury, intense SPRR1A immunoreactivity was observed in the dorsal horn and motoneurons of the spinal cord, in L4/5 DRG neurons, and in the injured nerve. A time-course study comparing expression following sciatic nerve crush and transection revealed maximum SPRR1A levels at day 7 in both models. However, while SPRR1A was downregulated to baseline by 30 days postlesion following crush injury, it remained elevated 30 days after transection. Cell-size and double-labeling studies revealed that SPRR1A was expressed by DRG cells of all sizes and colocalized with classical markers of DRG subpopulations and their primary afferent terminals. High coexpression of SPRR1A with activating transcription factor-3 and growth-associated protein-43 was observed, indicating that it is expressed by injured and regenerating neurons. This study supports the hypothesis that SPRR1A is a regeneration-associated gene and that SPRR1A provides a valuable marker to assess the regenerative potential of injured neurons.

Enhancement of axonal regeneration by in vitro conditioning and its inhibition by cyclopentenone prostaglandins

Axonal regeneration is enhanced by the prior ;conditioning' of peripheral nerve lesions. Here we show that Xenopus dorsal root ganglia (DRG) with attached peripheral nerves (PN-DRG) can be conditioned in vitro, thereafter showing enhanced neurotrophin-induced axonal growth similar to preparations conditioned by axotomy in vivo. Actinomycin D inhibits axonal outgrowth from freshly dissected PN-DRG, but not from conditioned preparations. Synthesis of mRNAs that encode proteins necessary for axonal elongation might therefore occur during the conditioning period, a suggestion that was confirmed by oligonucleotide microarray analysis. Culturing PN-DRG in a compartmentalized system showed that inhibition of protein synthesis (but not RNA synthesis) in the distal nerve impaired the conditioning response, suggesting that changes in gene expression in cultured DRG depend on the synthesis and retrograde transport of protein(s) in peripheral nerves. The culture system was also used to demonstrate retrograde axonal transport of several proteins, including thioredoxin (Trx). Cyclopentenone prostaglandins, which react with Trx, blocked the in vitro conditioning effect, whereas inhibition of other signalling pathways thought to be involved in axonal regeneration did not. This suggests that Trx and/or other targets of these electrophilic prostaglandins regulate axonal regeneration. Consistent with this hypothesis, morpholino-induced suppression of Trx expression in dissociated DRG neurons was associated with reduced neurite outgrowth.

Expression of ATF3 and axonal outgrowth are impaired after delayed nerve repair

BACKGROUND

A delay in surgical nerve repair results in impaired nerve function in humans, but mechanisms behind the weakened nerve regeneration are not known. Activating transcription factor 3 (ATF3) increases the intrinsic growth state of injured neurons early after injury, but the role of long-term changes and their relation to axonal outgrowth after a delayed nerve repair are not well understood. ATF3 expression was examined by immunohistochemistry in motor and sensory neurons and in Schwann cells in rat sciatic nerve and related to axonal outgrowth after transection and delayed nerve repair (repair 0, 30, 90 or 180 days post-injury). Expression of the neuronal cell adhesion molecule (NCAM), which is expressed in non-myelinating Schwann cells, was also examined.

RESULTS

The number of neurons and Schwann cells expressing ATF3 declined and the length of axonal outgrowth was impaired if the repair was delayed. The decline was more rapid in motor neurons than in sensory neurons and Schwann cells. Regeneration distances over time correlated to number of ATF3 stained neurons and Schwann cells. Many neurofilament stained axons grew along ATF3 stained Schwann cells. If nerve repair was delayed the majority of Schwann cells in the distal nerve segment stained for NCAM.

CONCLUSION

Delayed nerve repair impairs nerve regeneration and length of axonal outgrowth correlates to ATF3 expression in both neurons and Schwann cells. Mainly non-myelinating Schwann cells (NCAM stained) are present in distal nerve segments after delayed nerve repair. These data provide a neurobiological basis for the poor outcomes associated with delayed nerve repair. Nerve trunks should, if possible, be promptly repaired.

The genomic response of the ipsilateral and contralateral cortex to stroke in aged rats

Aged rats recover poorly after unilateral stroke, whereas young rats recover readily possibly with the help from the contralateral, healthy hemisphere. In this study we asked whether anomalous, age-related changes in the transcriptional activity in the brains of aged rats could be one underlying factor contributing to reduced functional recovery. We analysed gene expression in the periinfarct and contralateral areas of 3-month- and 18-month-old Sprague Dawley rats. Our experimental end-points were cDNA arrays containing genes related to hypoxia signalling, DNA damage and apoptosis, cellular response to injury, axonal damage and re-growth, cell lineage differentiation, dendritogenesis and neurogenesis. The major transcriptional events observed were: (i) Early up-regulation of DNA damage and down-regulation of anti-apoptosis-related genes in the periinfarct region of aged rats after stroke; (ii) Impaired neurogenesis in the periinfarct area, especially in aged rats; (iii) Impaired neurogenesis in the contralateral (unlesioned) hemisphere of both young and aged rats at all times after stroke and (iv) Marked up-regulation, in aged rats, of genes associated with inflammation and scar formation. These results were confirmed with quantitative real-time PCR. We conclude that reduced transcriptional activity in the healthy, contralateral hemisphere of aged rats in conjunction with an early up-regulation of DNA damage-related genes and pro-apoptotic genes and down-regulation of axono- and neurogenesis in the periinfarct area are likely to account for poor neurorehabilitation after stroke in old rats.