Retinoic acid receptor β2 promotes functional regeneration of sensory axons in the spinal cord

CXCL5 mediates UVB irradiation-induced pain

Many persistent pain states (pain lasting for hours, days, or longer) are poorly treated because of the limitations of existing therapies. Analgesics such as nonsteroidal anti-inflammatory drugs and opioids often provide incomplete pain relief and prolonged use results in the development of severe side effects. Identification of the key mediators of various types of pain could improve such therapies. Here, we tested the hypothesis that hitherto unrecognized cytokines and chemokines might act as mediators in inflammatory pain. We used ultraviolet B (UVB) irradiation to induce persistent, abnormal sensitivity to pain in humans and rats. The expression of more than 90 different inflammatory mediators was measured in treated skin at the peak of UVB-induced hypersensitivity with custom-made polymerase chain reaction arrays. There was a significant positive correlation in the overall expression profiles between the two species. The expression of several genes [interleukin-1β (IL-1β), IL-6, and cyclooxygenase-2 (COX-2)], previously shown to contribute to pain hypersensitivity, was significantly increased after UVB exposure, and there was dysregulation of several chemokines (CCL2, CCL3, CCL4, CCL7, CCL11, CXCL1, CXCL2, CXCL4, CXCL7, and CXCL8). Among the genes measured, CXCL5 was induced to the greatest extent by UVB treatment in human skin; when injected into the skin of rats, CXCL5 recapitulated the mechanical hypersensitivity caused by UVB irradiation. This hypersensitivity was associated with the infiltration of neutrophils and macrophages into the dermis, and neutralizing the effects of CXCL5 attenuated the abnormal pain-like behavior. Our findings demonstrate that the chemokine CXCL5 is a peripheral mediator of UVB-induced inflammatory pain, likely in humans as well as rats.

Neuritogenic activity of a genipin derivative in retinal ganglion cells is mediated by retinoic acid receptor β expression through nitric oxide/S-nitrosylation signaling

Genipin, a herbal iridoid, is known to have both neuroprotective and neuritogenic activity in neuronal cell lines. As it is structurally similar to tetrahydrobiopterin, its activity is believed to be nitric oxide (NO)-dependent. We previously proposed a novel neuroprotective activity of a genipin derivative, (1R)-isoPropyloxygenipin (IPRG001), whereby it reduces oxidative stress in RGC-5, a neuronal precursor cell line of retinal origin through protein S-nitrosylation. In the present study, we investigated another neuritogenic property of IPRG001 in RGC-5 cells and retinal explant culture where in we focused on the NO-cGMP-dependent and protein S-nitrosylation pathways. IPRG001 stimulated neurite outgrowth in RGC-5 cells and retinal explant culture through NO-dependent signaling, but not NO-dependent cGMP signaling. Neurite outgrowth with IPRG001 requires retinoic acid receptor β (RARβ) expression, which is suppressed by an RAR blocking agent and siRNA inhibition. Thereby, we hypothesized that RARβ expression is mediated by protein S-nitrosylation. S-nitrosylation of histone deacetylase 2 is a key mechanism in chromatin remodeling leading to transcriptional gene activation. We found a parallelism between S-nitrosylation of histone diacetylase 2 and the induction of RARβ expression with IPRG001 treatment. The both neuroprotective and neuritogenic activities of genipin could be a new target for the regeneration of retinal ganglion cells after glaucomatous conditions.

Neurotrophins: potential therapeutic tools for the treatment of spinal cord injury

Spinal cord injury permanently disrupts neuroanatomical circuitry and can result in severe functional deficits. These functional deficits, however, are not immutable and spontaneous recovery occurs in some patients. It is highly likely that this recovery is dependent upon spared tissue and the endogenous plasticity of the central nervous system. Neurotrophic factors are mediators of neuronal plasticity throughout development and into adulthood, affecting proliferation of neuronal precursors, neuronal survival, axonal growth, dendritic arborization and synapse formation. Neurotrophic factors are therefore excellent candidates for enhancing axonal plasticity and regeneration after spinal cord injury. Understanding growth factor effects on axonal growth and utilizing them to alter the intrinsic limitations on regenerative growth will provide potent tools for the development of translational therapeutic interventions for spinal cord injury.

Axotomy-induced miR-21 promotes axon growth in adult dorsal root ganglion neurons

Following injury, dorsal root ganglion (DRG) neurons undergo transcriptional changes so as to adopt phenotypic changes that promote cell survival and axonal regeneration. Here we used a microarray approach to profile changes in a population of small noncoding RNAs known as microRNAs (miRNAs) in the L4 and L5 DRG following sciatic nerve transection. Results showed that 20 miRNA transcripts displayed a significant change in expression levels, with 8 miRNAs transcripts being altered by more than 1.5-fold. Using quantitative reverse transcription PCR, we demonstrated that one of these miRNAs, miR-21, was upregulated by 7-fold in the DRG at 7 days post-axotomy. In dissociated adult rat DRG neurons lentiviral vector-mediated overexpression of miR-21 promoted neurite outgrowth on a reduced laminin substrate. miR-21 directly downregulated expression of Sprouty2 protein, as confirmed by Western blot analysis and 3' untranslated region (UTR) luciferase assays. Our data show that miR-21 is an axotomy-induced miRNA that enhances axon growth, and suggest that miRNAs are important players in regulating growth pathways following peripheral nerve injury.

Emerging roles for retinoids in regeneration and differentiation in normal and disease states

The vitamin A (retinol) metabolite, all-trans retinoic acid (RA), is a signaling molecule that plays key roles in the development of the body plan and induces the differentiation of many types of cells. In this review the physiological and pathophysiological roles of retinoids (retinol and related metabolites) in mature animals are discussed. Both in the developing embryo and in the adult, RA signaling via combinatorial Hox gene expression is important for cell positional memory. The genes that require RA for the maturation/differentiation of T cells are only beginning to be cataloged, but it is clear that retinoids play a major role in expression of key genes in the immune system. An exciting, recent publication in regeneration research shows that ALDH1a2 (RALDH2), which is the rate-limiting enzyme in the production of RA from retinaldehyde, is highly induced shortly after amputation in the regenerating heart, adult fin, and larval fin in zebrafish. Thus, local generation of RA presumably plays a key role in fin formation during both embryogenesis and in fin regeneration. HIV transgenic mice and human patients with HIV-associated kidney disease exhibit a profound reduction in the level of RARβ protein in the glomeruli, and HIV transgenic mice show reduced retinol dehydrogenase levels, concomitant with a greater than 3-fold reduction in endogenous RA levels in the glomeruli. Levels of endogenous retinoids (those synthesized from retinol within cells) are altered in many different diseases in the lung, kidney, and central nervous system, contributing to pathophysiology. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

RAR/RXR and PPAR/RXR Signaling in Spinal Cord Injury

The retinoid acid receptors (RAR) and peroxisome proliferator-activated receptors (PPAR) have been implicated in the regulation of inflammatory reactions. Both receptor families contain ligand-activated transcription factors which form heterodimers with retinoid X receptors (RXR). We review data that imply RAR/RXR and PPAR/RXR pathways in physiological reactions after spinal cord injury. Experiments show how RAR signaling may improve axonal regeneration and modulate reactions of glia cells. While anti-inflammatory properties of PPAR are well documented in the periphery, their possible roles in the central nervous system have only recently become evident. Due to its anti-inflammatory function this transcription factor family promises to be a useful target after spinal cord or brain lesions.

RA-RAR-β counteracts myelin-dependent inhibition of neurite outgrowth via Lingo-1 repression

Central nervous system injury results in the release of molecules that inhibit neuronal regeneration, but retinoic acid counteracts this effect by inhibiting Lingo-1.

After an acute central nervous system injury, axonal regeneration is limited as the result of a lack of neuronal intrinsic competence and the presence of extrinsic inhibitory signals. The injury fragments the myelin neuronal insulating layer, releasing extrinsic inhibitory molecules to signal through the neuronal membrane–bound Nogo receptor (NgR) complex. In this paper, we show that a neuronal transcriptional pathway can interfere with extrinsic inhibitory myelin-dependent signaling, thereby promoting neurite outgrowth. Specifically, retinoic acid (RA), acting through the RA receptor β (RAR-β), inhibited myelin-activated NgR signaling through the transcriptional repression of the NgR complex member Lingo-1. We show that suppression of Lingo-1 was required for RA–RAR-β to counteract extrinsic inhibition of neurite outgrowth. Furthermore, we confirm in vivo that RA treatment after a dorsal column overhemisection injury inhibited Lingo-1 expression, specifically through RAR-β. Our findings identify a novel link between RA–RAR-β–dependent proaxonal outgrowth and inhibitory NgR complex–dependent signaling, potentially allowing for the development of molecular strategies to enhance axonal regeneration after a central nervous system injury.

Midbrain dopaminergic neurons utilize nitric oxide/cyclic GMP signaling to recruit ERK that links retinoic acid receptor stimulation to up-regulation of BDNF

Stimulation of retinoic acid receptors (RARs) protects midbrain dopaminergic neurons, presumably via up-regulation of brain-derived neurotrophic factor (BDNF) expression. The present study was focused on unexplored signaling mechanisms linking RAR stimulation to BDNF expression. Rat midbrain slice cultures treated with an RAR agonist Am80 showed increased tissue levels of BDNF mRNA and protein as compared to cultures without treatment. Am80-induced increase in BDNF expression was observed in dopaminergic neurons, which was blocked by inhibition of extracellular signal-regulated kinase (ERK) activation. We also found that Am80 increased neuronal nitric oxide synthase expression in dopaminergic neurons even during ERK inhibition, and this increase was accompanied by 8-nitro-cyclic GMP formation. Notably, the effect of Am80 on BDNF expression was attenuated by inhibitors of nitric oxide synthase, soluble guanylyl cyclase and cyclic GMP-dependent protein kinase (PKG). Am80-induced ERK phosphorylation in dopaminergic neurons was also attenuated by inhibition of soluble guanylyl cyclase and PKG. Moreover, 8-Br-cyclic GMP induced ERK phosphorylation and BDNF expression in dopaminergic neurons. These results suggest that, by recruiting cyclic GMP and PKG, neuronal nitric oxide synthase-derived nitric oxide plays a novel and essential role in RAR signaling leading to ERK-dependent BDNF up-regulation in midbrain dopaminergic neurons.

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.

The pattern of retinoic acid receptor expression and subcellular, anatomic and functional area translocation during the postnatal development of the rat cerebral cortex and white matter

Retinoic acid (RA), which is an important modulator of brain development, neural cell proliferation, neurite outgrowth, and synaptic plasticity, is regulated via changes in RA receptors. The pattern of RA receptor changes in the rat cerebral cortex and white matter during postnatal development has not been extensively studied. Therefore, we studied the mRNA expression patterns of 6 RA receptors in the postnatal rat cerebral cortex and white matter at 1, 3, 7, 10, 14, 21, 28, and 35days. We found that RARβ, RXRα and RXRβ mRNA levels gradually increased during postnatal development. RARα presented a nearly unimodal trend, but RARγ and RXRγ were generally bimodal. RARα, RARγ, and RXRγ mRNA levels peaked at postnatal day 21 (P21). The pattern of RARα expression was consistent with that of its mRNA levels. RARα and RXRγ mRNA levels were the highest among those of all RA receptors during postnatal development. Interestingly, RARα expression was mainly located in the cytoplasm in the postnatal development apart from P3d. We further showed that RARα is expressed mainly in layers 2 and 3, partly in layers 1 and 4, and in a limited manner in layers 5 and 6 in the parietal cortex. Most RARα expression occurs in layers 2, 3, and 4 in the temporal lobe cortex. We realized that the M1 S2 region of RARα is highly expressed and that the position of RARα changes dynamically to meet the needs of different regions during development. These results support the idea that the RA receptor plays an important role in the cerebrum during postnatal development and implementation of these functions may be mainly dependent on the non-transcriptional or post- transcriptional regulation.

Retinoic acid receptor modulators: a perspective on recent advances and promises

IMPORTANCE OF THE FIELD

Retinoids are currently used in the clinic for the treatment of skin diseases and acute promielocytic leukemia and are known to contribute to early development and organogenesis in embryo and throughout life. Most of these activities are primarily due to the binding of the retinoid to the retinoic acid receptors (RARs, subtypes α, β and γ). Ligand modulates, via allosteric conformational changes, the ability of RARs to interact with different sets of co-regulators. Structure-based insights on the ligand-binding domain of the ligand-bound RARs have clearly linked retinoid function to co-activator (CoA) recruitment for agonists, CoA dissociation for antagonists and corepressor stabilization for inverse agonists.

AREAS COVERED IN THIS REVIEW

To help understand ligand-modulated RAR action as a consequence of its interaction with different sets of co-regulators, we present the chemical engineering of subsets of retinoid chemotypes (rexinoids, i.e., the ligands of the retinoid X receptors, α, β and γ, with impact in the treatment of cancer and metabolic diseases, are not covered) that display the whole range of ligand functions, including subtype- and isotype-selectivities.

WHAT THE READER WILL GAIN

An understanding of the correlation of retinoid ligand structure and function. Structural insights into ligand action and retinoid chemotypes. Potential for clinical application of retinoid receptor modulators.

TAKE HOME MESSAGE

Potential pharmacological/therapeutic applications of these chemical tools extend beyond cancer prevention and therapy to the treatment of autoimmune and neurodegenerative diseases.

Systemic blockade of P2X3 and P2X2/3 receptors attenuates bone cancer pain behaviour in rats

Pain remains an area of considerable unmet clinical need, and this is particularly true of pain associated with bone metastases, in part because existing analgesic drugs show only limited efficacy in many patients and in part because of the adverse side effects associated with these agents. An important issue is that the nature and roles of the algogens produced in bone that drive pain-signalling systems remain unknown. Here, we tested the hypothesis that adenosine triphosphate is one such key mediator through actions on P2X3 and P2X2/3 receptors, which are expressed selectively on primary afferent nocioceptors, including those innervating the bone. Using a well-established rat model of bone cancer pain, AF-353, a recently described potent and selective P2X3 and P2X2/3 receptor antagonist, was administered orally to rats and found to produce highly significant prevention and reversal of bone cancer pain behaviour. This attenuation occurred without apparent modification of the disease, since bone destruction induced by rat MRMT-1 carcinoma cells was not significantly altered by AF-353. Using in vivo electrophysiology, evidence for a central site of action was provided by dose-dependent reductions in electrical, mechanical and thermal stimuli-evoked dorsal horn neuronal hyperexcitability following direct AF-353 administration onto the spinal cord of bone cancer animals. A peripheral site of action was also suggested by studies on the extracellular release of adenosine triphosphate from MRMT-1 carcinoma cells. Moreover, elevated phosphorylated-extracellular signal-regulated kinase expression in dorsal root ganglion neurons, induced by co-cultured MRMT-1 carcinoma cells, was significantly reduced in the presence of AF-353. These data suggest that blockade of P2X3 and P2X2/3 receptors on both the peripheral and central terminals of nocioceptors contributes to analgesic efficacy in a model of bone cancer pain. Thus, systemic P2X3 and P2X2/3 receptor antagonists with central nervous system penetration may offer a promising therapeutic tool in treating bone cancer pain.

Perspectives on the treatment of the longitudinal spinal cord injury

The current technique for surgical treatment of the longitudinal spinal cord injury has proven to be successful for restoration of some motor function and alleviation of pain. This has been the first step in finding a cure for spinal cord injuries, but so far there are many obstacles still to be overcome. In this particular injury return of function from spinal cord surgery is only partial. Some of the main remaining problems are related to injury-induced neuronal death, direction and specificity of regeneration and muscle, and receptor disintegration from long time denervation. Currently this is a surgical procedure without any adjuvant treatments. In order to gain further functional improvement combinational therapies developed in basic science experiments are essential.

Cortical overexpression of neuronal calcium sensor-1 induces functional plasticity in spinal cord following unilateral pyramidal tract injury in rat

Overexpression of neuronal calcium sensor 1 in cortical neurons can help restore axonal plasticity and regeneration following axonal injury in adult rats, and can also improve behavioral function.

Following trauma of the adult brain or spinal cord the injured axons of central neurons fail to regenerate or if intact display only limited anatomical plasticity through sprouting. Adult cortical neurons forming the corticospinal tract (CST) normally have low levels of the neuronal calcium sensor-1 (NCS1) protein. In primary cultured adult cortical neurons, the lentivector-induced overexpression of NCS1 induces neurite sprouting associated with increased phospho-Akt levels. When the PI3K/Akt signalling pathway was pharmacologically inhibited the NCS1-induced neurite sprouting was abolished. The overexpression of NCS1 in uninjured corticospinal neurons exhibited axonal sprouting across the midline into the CST-denervated side of the spinal cord following unilateral pyramidotomy. Improved forelimb function was demonstrated behaviourally and electrophysiologically. In injured corticospinal neurons, overexpression of NCS1 induced axonal sprouting and regeneration and also neuroprotection. These findings demonstrate that increasing the levels of intracellular NCS1 in injured and uninjured central neurons enhances their intrinsic anatomical plasticity within the injured adult central nervous system.

Following trauma to the central nervous system (brain or spinal cord), neurons show very little capacity to re-grow their axons, which can lead to a permanent loss of function in those regions. In this study, we show that this failure of axon re-growth is associated with low intracellular levels of a small molecule called neuronal calcium sensor-1 (NCS1). We modified a non-replicating virus in two ways so as to increase the level of NCS1 in neurons while simultaneously labelling them with a green fluorescent protein, which allowed us to track neuronal growth. Using this virus to increase the level of NCS1 in a particular group of neurons that communicate between the brain and spinal cord, we showed that new axonal growth occurred in the spinal cord with or without injury to the neurons. Electrophysiological assessments demonstrated that these new processes formed functional connections in the spinal cord, and behavioural experiments revealed that this recovery also helped the animals move their limbs more effectively. Furthermore, an increase in NCS1 protected these neurons, such that fewer of them died after the injury. These findings demonstrate that increasing the intracellular levels of NCS1 in neurons can aid in the recovery from central nervous system injury, and can help improve behavioural function.

Inflammatory chemokine release of astrocytes in vitro is reduced by all-trans retinoic acid

The production of chemokines by astrocytes constitutes an important component of neuroinflammatory processes in the brain. As the transcriptional activator retinoic acid (RA), used for chemotherapy and dermatological applications, exerts anti-inflammatory effects on monocytes and lymphocytes, we have tested whether the physiologically occurring isomer, all-trans RA, affects chemokine expression by astrocytes. Under control conditions, primary cultures of murine cortical astrocytes expressed no or very low levels of CCL and CXCL chemokines. After treatment with bacterial lipopolysaccharides to simulate inflammation in vitro, we detected a strong increase in the release of CCL2 (to > 4 ng/mL in cell culture supernatant), CCL3 (> 20 ng/mL), CCL5 (> 25 ng/mL), CXCL1 (> 30 ng/mL) and CXCL2 (> 20 ng/mL). Although simultaneous exposure to RA did not significantly affect this response, 12 h pre-treatment with 0.1 microM all-trans RA strongly suppressed mRNA expression and protein release of all chemokines. The anti-inflammatory activity of RA engaged RA and retinoid X receptors and correlated with a decreased expression of the lipopolysaccharides co-receptor CD14. A minor reduction of nuclear NF-kappaB was observed but not significant, activation of Jun amino-terminal kinase, p38 and signal transducer and activator of transcription 3 were not altered by RA. The results suggest that retinoids should be further investigated as candidates for the treatment of neuroinflammation.

Endogenous purinergic control of bladder activity via presynaptic P2X3 and P2X2/3 receptors in the spinal cord

P2X(3) and P2X(2/3) receptors are localized on sensory afferents both peripherally and centrally and have been implicated in various sensory functions. However, the physiological role of these receptors expressed presynaptically in the spinal cord in regulating sensory transmission remains to be elucidated. Here, a novel selective P2X(3) and P2X(2/3) antagonist, AF-792 [5-(5-ethynyl-2-isopropyl-4-methoxy-phenoxy)-pyrimidine-2,4-diamine, previously known as RO-5], in addition to less selective purinoceptor ligands, was applied intrathecally in vivo. Cystometry recordings were made to assess changes in the micturition reflex contractions after drug treatments. We found that AF-792 inhibited micturition reflex activity significantly (300 nmol; from baseline contraction intervals of 1.18 +/- 0.07 to 9.33 +/- 2.50 min). Furthermore, inhibition of P2X(3) and P2X(2/3) receptors in the spinal cord significantly attenuated spinal activation of extracellular-signal regulated kinases induced by acute peripheral stimulation of the bladder with 1% acetic acid by 46.4 +/- 12.0% on average. Hence, the data suggest that afferent signals originating from the bladder are regulated by spinal P2X(3) and P2X(2/3) receptors and establish directly an endogenous central presynaptic purinergic mechanism to regulate visceral sensory transmission. Identification of this spinal purinergic control in visceral activities may help the development of P2X(3) and P2X(2/3) antagonist to treat urological dysfunction, such as overactive bladder, and possibly other debilitating sensory disorders, including chronic pain states.

Comparison of AAV serotypes for gene delivery to dorsal root ganglion neurons

For many experiments in the study of the peripheral nervous system, it would be useful to genetically manipulate primary sensory neurons. We have compared vectors based on adeno-associated virus (AAV) serotypes 1, 2, 3, 4, 5, 6, and 8, and lentivirus (LV), all expressing green fluorescent protein (GFP), for efficiency of transduction of sensory neurons, expression level, cellular tropism, and persistence of transgene expression following direct injection into the dorsal root ganglia (DRG), using histological quantification and qPCR. Two weeks after injection, AAV1, AAV5, and AAV6 had transduced the most neurons. The time course of GFP expression from these three vectors was studied from 1 to 12 weeks after injection. AAV5 was the most effective serotype overall, followed by AAV1. Both these serotypes showed increasing neuronal transduction rates at later time points, with some injections of AAV5 yielding over 90% of DRG neurons GFP(+) at 12 weeks. AAV6 performed well initially, but transduction rates declined dramatically between 4 and 12 weeks. AAV1 and AAV5 both transduced large-diameter neurons, IB4(+) neurons, and CGRP(+) neurons. In conclusion, AAV5 is a highly effective gene therapy vector for primary sensory neurons following direct injection into the DRG.

Conversion in ASIA impairment scale during the first year after traumatic spinal cord injury

The neurological severity of a spinal cord injury (SCI) is commonly classified according to the American Spinal Injury Association (ASIA) Impairment Scale (AIS). The aim of this study was to assess the course of the AIS following SCI, and to discern the nature of any changes in the classification that occur. Assessments were performed in a European cohort of SCI patients within 2 weeks and at 1, 3, 6, and 12 months after the initial injury. Overall, about 70% of the patients initially diagnosed as AIS A did not convert, as did 90% of the AIS D patients. When only evaluating patients with complete datasets, 68% did not convert, while the AIS category improved in 30% of patients and deteriorated in 2%. A change in the last sacral segments (40%), motor improvement (31%), sensory improvement (19%), and a change in the neurological level of the SCI (10%) contributed to or accompanied the AIS conversion. When the AIS remained unchanged between successive assessment points, there was no change in the number of muscles graded three or more (NMG3(+)) in 73% of the transitions. An improvement in AIS was associated with a gain in NMG3(+) in 49% of the transitions, while an aggravation in AIS was accompanied by a loss in NMG3(+) in 10% of the transitions. These results, documenting a substantial amount of spontaneous AIS conversions, should be taken into consideration when designing clinical trials to assess the effects of potential new treatments for SCI.

A retinoic acid receptor beta agonist (CD2019) overcomes inhibition of axonal outgrowth via phosphoinositide 3-kinase signalling in the injured adult spinal cord

After spinal cord injury in the adult mammal, axons do not normally regrow and this commonly leads to paralysis. Retinoic acid (RA) can stimulate neurite outgrowth in vitro of both the embryonic central and peripheral nervous system, via activation of the retinoic acid receptor (RAR) beta2. We show here that regions of the adult CNS, including the cerebellum and cerebral cortex, express RARbeta2. We show that when cerebellar neurons are grown in the presence of myelin-associated glycoprotein (MAG) which inhibits neurite outgrowth, RARbeta can be activated in a dose dependent manner by a RARbeta agonist (CD2019) and neurite outgrowth can occur via phosphoinositide 3-kinase (PI3K) signalling. In a model of spinal cord injury CD2019 also acts through PI3K signalling to induce axonal outgrowth of descending corticospinal fibres and promote functional recovery. Our data suggest that RARbeta agonists may be of therapeutic potential for human spinal cord injuries.

Rapid isolation and culture of primary microglia from adult mouse spinal cord

Microglia are important in homeostasis and widely considered to have roles in the pathogenesis of conditions such as neuropathic pain and multiple sclerosis. The need to study microglia from the adult spinal cord is essential to further understand the role of these cells in disease pathology. Primary microglia are often prepared from brain tissues obtained from embryonic or perinatal age rodents and the process can take over a week to complete. The protocol in this study provides rapid isolation of microglia from adult spinal cord, allowing immediate availability for experimentation of both ex vivo and in vitro within a few hours. A purity of 99% with little or no neuronal or astrocytic contamination can be achieved. Between 70% and 85% of these adult microglia were in a relatively non-activated state. Functionally, these microglia respond to lipopolysaccharide incubation with increases in both phospho-p38 MAPK and OX42 immunostaining, as well as release of ATP, as compared to un-stimulated microglia. This technique provides a protocol to achieve rapid and efficient extraction of high purity, quiescent and functionally active microglia from adult mouse spinal cord, allowing greater study of adult spinal microglia in physiological and pathophysiological states.

Towards retinoid therapy for Alzheimer's disease

Alzheimer's disease(AD) is associated with a variety of pathophysiological features, including amyloid plaques, inflammation, immunological changes, cell death and regeneration processes, altered neurotransmission, and age-related changes. Retinoic acid receptors (RARs) and retinoids are relevant to all of these. Here we review the pathology, pharmacology, and biochemistry of AD in relation to RARs and retinoids, and we suggest that retinoids are candidate drugs for treatment of AD.

Combination of microsurgery and gene therapy for spinal dorsal root injury repair

Brachial plexus injury is frequent after traffic accident in adults or shoulder dystocia in newborns. Whereas surgery can restore arm movements, therapeutic options are missing for sensory defects. Dorsal root (DR) ganglion neurons convey sensory information to the central nervous system (CNS) through a peripheral and a central axon. Central axons severed through DR section or avulsion during brachial plexus injury inefficiently regenerate and do not reenter the spinal cord. We show that a combination of microsurgery and gene therapy circumvented the functional barrier to axonal regrowth at the peripheral and CNS interface. After cervical DR section in rats, microsurgery restored anatomical continuity through a nerve graft that laterally connected the injured DR to an intact DR. Gene transfer to cells in the nerve graft induced the local release of neurotrophin-3 (NT-3) and glial cell line-derived neurotrophic factor (GDNF) and stimulated axonal regrowth. Central DR ganglion axons efficiently regenerated and invaded appropriate areas of the spinal cord dorsal horn, leading to partial recovery of nociception and proprioception. Microsurgery created conditions for functional restoration of DR ganglion central axons, which were improved in combination with gene therapy. This combination treatment provides means to reduce disability due to somatosensory defects after brachial plexus injury.

The liberation of fractalkine in the dorsal horn requires microglial cathepsin S

Understanding of the sequence and nature of the events that govern neuron-microglia communication is critical for the discovery of new mechanisms and targets for chronic pain treatment. The neuronal chemokine fractalkine (FKN) and its microglial receptor CX3CR1 may mediate such a function in the dorsal horn of the spinal cord after cleavage of the extracellular domain of this transmembrane chemokine by a protease. Here we report that in neuropathic rat dorsal horn, with dorsal root-attached preparations, soluble FKN (sFKN) contents are increased in the superfusates collected after noxious-like electrical stimulation of ipsilateral primary afferent fibers. The increase of sFKN is prevented by morpholinurea-leucine-homophenylalanine-vinyl sulfone-phenyl (LHVS), an irreversible inhibitor of cathepsin S (CatS) whose proteolytic activity is also increased in the superfusates. The source of CatS activity is microglial cells activated by the peripheral nerve injury and secreting the enzyme, as a result of primary afferent fiber stimulation. Indeed, the acute activation of dorsal horn microglia by lipopolysaccharide results in increased CatS activity in the superfusates, followed by increased sFKN contents. Consistent with these observations ex vivo, the levels of both sFKN and CatS activity in CSF samples increased significantly after peripheral nerve injury, associated with spinal microglial activation. Finally, because we found that both FKN immunoreactivity and mRNA are confined to dorsal horn neurons, we suggest that under neuropathic conditions, noxious stimulation of primary afferent fibers induces release of CatS from microglia, which liberates FKN from dorsal horn neurons, thereby contributing to the amplification and maintenance of chronic pain.

Induction of corticospinal regeneration by lentiviral trkB-induced Erk activation

Several experimental manipulations of the CNS environment successfully elicit regeneration of sensory and bulbospinal motor axons but fail to elicit regeneration of corticospinal axons, suggesting that cell-intrinsic mechanisms limit the regeneration of this critical class of motor neurons. We hypothesized that enhancement of intrinsic neuronal growth mechanisms would enable adult corticospinal motor axon regeneration. Lentiviral vectors were used to overexpress the BDNF receptor trkB in layer V corticospinal motor neurons. After subcortical axotomy, trkB transduction induced corticospinal axon regeneration into subcortical lesion sites expressing BDNF. In the absence of trkB overexpression, no regeneration occurred. Selective deletion of canonical, trkB-mediated neurite outgrowth signaling by mutation of the Shc/FRS-2 activation domain prohibited Erk activation and eliminated regeneration. These findings support the hypothesis that the refractory regenerative state of adult corticospinal axons can be attributed at least in part to neuron-intrinsic mechanisms, and that activation of ERK signaling can elicit corticospinal tract regeneration.

FARP1 promotes the dendritic growth of spinal motor neuron subtypes through transmembrane Semaphorin6A and PlexinA4 signaling

The dendritic morphology of neurons dictates their abilities to process and transmit information; however, the signaling pathways that regulate dendritic growth and complexity are poorly understood. Here, we show that retinoids induce the expression of the FERM Rho-GEF protein FARP1 in the developing spinal cord. FARP1 is expressed in subsets of motor neurons and is enriched in dendrites of lateral motor column (LMC) neurons that innervate the limb. FARP1 is necessary and sufficient to promote LMC dendritic growth but does not affect dendrite number or axonal morphology. We show that FARP1 serves as a specific effector of transmembrane Semaphorin6A and PlexinA4 signals to regulate LMC dendritic growth, and that its Rho-GEF domain is necessary for this function. These findings reveal that retinoid and Sema6A/PlexA4 signaling pathways intersect through FARP1 to control dendritic growth, and uncover the existence of subtype-specific signaling networks that control dendritic developmental programs in spinal motor neurons.

The effect of Am-80, a synthetic retinoid, on spinal cord injury-induced motor dysfunction in rats

The present study investigated the effect of 4[(5,6,7,8-tetrahydro-5,5,8,8,-tetramethyl-2-naphthalenyl)carbamoyl] benzoic acid (Am-80), a synthetic retinoid, on spinal cord injury (SCI) in rats. Treatment with Am-80 (orally and subcutaneously) significantly promoted recovery from SCI-induced motor dysfunction. On day 28 after injury, the lesion cavity was markedly reduced, while the expression of myelin basic protein (MBP; myelin), betaIIItubulin (neuron), and glial fibrillary acidic protein (GFAP; astrocyte) was increased, in comparison with SCI controls. Interestingly, expression of neurotrophin receptor, tyrosine kinase B (TrkB) was over 3-fold higher after Am-80 treatment than in SCI controls. A lot of TrkB-positive cells as well as brain-derived neurotrophic factor (BDNF)-positive ones were observed around the injured site. Am-80 (10 microM) combined with BDNF (100 ng/ml) promoted extensive neurite outgrowth and TrkB gene expression by cultured SH-SY5Y cells, as did all-trans retinoic acid (ATRA). Thymidine incorporation was dramatically suppressed, but there was little effect on cell viability. These findings suggest that Am-80 has the potential to be used for treating neurodegenerative disorders, including SCI. Its efficacy may be partly ascribed to promotion of cell viability and differentiation of neural stem cells through increased TrkB expression.

Involvement of retinoic acid signaling in goldfish optic nerve regeneration

Recently, we identified a retina-specific retinol-binding protein, purpurin, as a trigger molecule in the early stage of goldfish optic nerve regeneration. Purpurin protein was secreted by photoreceptors to injured ganglion cells, at 2-5 days after optic nerve injury. Purpurin bound to retinol induced neurite outgrowth in retinal explant cultures and retinoic acid (RA) had a comparable effect on neurite outgrowth. These results indicate that purpurin acts as a retinol transporter and facilitates conversion of retinol to RA. Intracellularly, RA is transported into the nucleus with cellular retinoic acid-binding protein IIb (CRABPIIb) and binds with retinoic acid receptor alpha (RARalpha) as a transcriptional regulator of target genes. Here, we investigated the RA signaling through RA synthesis to RARalpha in the goldfish retina during optic nerve regeneration by RT-PCR. Retinaldehyde dehydrogenase 2 (RALDH2; an RA synthetic enzyme) mRNA was increased by 2.7-fold in the retina at 7-10 days and then gradually decreased until 40 days after nerve injury. In contrast, cytochrome P450 26a1 (CYP26a1; an RA degradative enzyme) mRNA was decreased to less than half in the retina at 5-20 days and then gradually returned to the control level by 40 days after nerve injury. CRABPIIb mRNA was increased by 1.5-fold in the retina at 10 days after axotomy, RARalphaa mRNA was increased by 1.8-fold in the retina at 10 days after axotomy. The cellular changes in the RA signaling molecules after optic nerve injury were almost all located in the ganglion cells, as evaluated by in situ hybridization. The present data described for the first time that RA signaling through RALDH2 and CRABPIIb to RARalpha was serially upregulated in the ganglion cells at 7-10 days just after the purpurin induction. Therefore, we conclude that the triggering action of purpurin on optic nerve regeneration is mediated by RA signaling pathway.

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.

All-trans retinoic acid induces nerve regeneration and increases serum and nerve contents of neural growth factor in experimental diabetic neuropathy

Local diminution of the neural growth factor (NGF) contributes to the apparition of diabetic neuropathy. All-trans retinoic acid (RA) increases the expression of neural growth factor and its receptor participating in translation pathways. This study evaluates RA as a treatment of diabetic neuropathy: 120 mice were assigned randomly to 4 groups. Group A (n = 30) was taken as control; group B (n = 30) received 50 mg/kg intraperitoneal streptozotocin (STZ); group C (n = 30) received STZ, and after diabetic neuropathy developed, they were treated with subcutaneous RA 20 mg/kg daily during 60 days; and group D (n = 30) only received RA. Plasma glucose, thermosensitive tests, serum, and the nerve contents of NGF were measured in all animals. Evaluation by electron microscopy was performed in search of morphologic changes secondary to neuropathy and nerve regeneration. Diabetic mice had an increased threshold to pain. Treatment with RA in diabetic mice reverted changes in sensitivity as compared with diabetic mice that received placebo (P < 0.001). No differences in pain threshold among controls, RA, and diabetes mellitus (DM) + RA groups were found. Glucose levels were not affected by the treatment with RA. NGF diminished significantly in the sciatic nerve in diabetic mice as compared with controls and with the RA group. Animals with DM + RA had a significant increase of NGF in nerves as compared with the other groups. RA also regressed the ultrastructural changes induced by diabetes that showed increased neural regeneration. RA can revert functional and ultrastructural changes and induce neural regeneration after the establishment of diabetic neuropathy, possibly because of the increased of NGF concentrations in nerve terminals.

Retinoic acid in development: towards an integrated view

Retinoic acid (RA) has complex and pleiotropic functions during vertebrate development. Recent work in several species has increased our understanding of the roles of RA as a signalling molecule. These functions rely on a tight control of RA distribution within embryonic tissues through the combined action of synthesizing and metabolizing enzymes, possibly leading to diffusion gradients. Also important is the switching of nuclear receptors from a transcriptionally repressing state to an activating state. In addition, cross-talk with other key embryonic signals, especially fibroblast growth factors (FGFs) and sonic hedgehog (SHH), is being uncovered. Some of these functions could be maintained throughout the life of an organism to regulate cell-lineage decisions and/or the differentiation of stem cell populations, highlighting possibilities for regenerative medicine.

Persistent restoration of sensory function by immediate or delayed systemic artemin after dorsal root injury

Dorsal root injury results in substantial and often irreversible loss of sensory functions as a result of the limited regenerative capacity of sensory axons and the inhibitory barriers that prevent both axonal entry into and regeneration in the spinal cord. Here, we describe previously unknown effects of the growth factor artemin after crush injury of the dorsal spinal nerve roots in rats. Artemin not only promoted re-entry of multiple classes of sensory fibers into the spinal cord and re-establishment of synaptic function and simple behavior, but it also, surprisingly, promoted the recovery of complex behavior. These effects occurred after a 2-week schedule of intermittent, systemic administration of artemin and persisted for at least 6 months following treatment, suggesting a substantial translational advantage. Systemic artemin administration produced essentially complete and persistent restoration of nociceptive and sensorimotor functions, and could represent a promising therapy that may effectively promote sensory neuronal regeneration and functional recovery after injury.

Is the modulation of retinoid and retinoid-associated signaling a future therapeutic strategy in neurological trauma and neurodegeneration?

The complex molecular pathways that mediate the effects of vitamin A and its derivatives, are increasingly recognized as a component of the repair capacity that could be activated to induce protection and regeneration in the mature nervous tissue. Retinoid and retinoid-associated signaling plays an essential role in normal neurodevelopment and appears to remain active in the adult CNS. In this paper, we review evidence which supports the hypothesis of an activation of retinoid-associated signaling molecular pathways in the mature nervous tissue and its significance in the context of neurodegenerative, trauma-induced and psychiatric disorders, at spinal and supra-spinal levels. Finally, we summarize the potential therapeutic avenues based on the modulation of retinoid targets undergoing reactivation under conditions of acute injury and chronic degeneration in the central nervous system, and discuss some of the unresolved issues linked to this treatment strategy.

Retinoic acid in the development, regeneration and maintenance of the nervous system

Retinoic acid (RA) is involved in the induction of neural differentiation, motor axon outgrowth and neural patterning. Like other developmental molecules, RA continues to play a role after development has been completed. Elevated RA signalling in the adult triggers axon outgrowth and, consequently, nerve regeneration. RA is also involved in the maintenance of the differentiated state of adult neurons, and disruption of RA signalling in the adult leads to the degeneration of motor neurons (motor neuron disease), the development of Alzheimer's disease and, possibly, the development of Parkinson's disease. The data described here strongly suggest that RA could be used as a therapeutic molecule for the induction of axon regeneration and the treatment of neurodegeneration.

Retinoic acid enhances Erk phosphorylation in the chick retina

The transcriptional activator retinoic acid (RA) is a regulator of neural development and regeneration. Synergistic effects with brain-derived neurotrophic factor suggested that RA influences neurotrophin signaling. To test this hypothesis RA was administered systemically to E17 chick embryos, and retinas were prepared 12h and 24h later to measure mRNA or protein expression. While there was no significant influence on activation of Akt, CREB and STAT-3, RA-treatment caused elevated levels of Erk-phosphorylation, a kinase involved in Trk signaling. A small but significant increase in the expression of TrkB mRNA and protein was observed but no significant change in TrkA, TrkC and p75 expression.

Inhibition of spinal microglial cathepsin S for the reversal of neuropathic pain

A recent major conceptual advance has been the recognition of the importance of immune system-neuronal interactions in the modulation of brain function, one example of which is spinal pain processing in neuropathic states. Here, we report that in peripheral nerve-injured rats, the lysosomal cysteine protease cathepsin S (CatS) is critical for the maintenance of neuropathic pain and spinal microglia activation. After injury, CatS was exclusively expressed by activated microglia in the ipsilateral dorsal horn, where expression peaked at day 7, remaining high on day 14. Intrathecal delivery of an irreversible CatS inhibitor, morpholinurea-leucine-homophenylalanine-vinyl phenyl sulfone (LHVS), was antihyperalgesic and antiallodynic in neuropathic rats and attenuated spinal microglia activation. Consistent with a pronociceptive role of endogenous CatS, spinal intrathecal delivery of rat recombinant CatS (rrCatS) induced hyperalgesia and allodynia in naïve rats and activated p38 mitogen-activated protein kinase (MAPK) in spinal cord microglia. A bioinformatics approach revealed that the transmembrane chemokine fractalkine (FKN) is a potential substrate for CatS cleavage. We show that rrCatS incubation reduced the levels of cell-associated FKN in cultured sensory neurons and that a neutralizing antibody against FKN prevented both FKN- and CatS-induced allodynia, hyperalgesia, and p38 MAPK activation. Furthermore, rrCatS induced allodynia in wild-type but not CX3CR1-knockout mice. We suggest that under conditions of increased nociception, microglial CatS is responsible for the liberation of neuronal FKN, which stimulates p38 MAPK phosphorylation in microglia, thereby activating neurons via the release of pronociceptive mediators.

Identification of candidate transcriptional modulators involved in successful regeneration after nerve injury

Successful regeneration of injured neurons requires a complex molecular response that involves the expression, modification and transport of a large number of proteins. The identity of neuronal proteins responsible for the initiation of regenerative neurite outgrowth is largely unknown. Dorsal root ganglion (DRG) neurons display robust and successful regeneration following lesion of their peripheral neurite, whereas outgrowth of central neurites is weak and does not lead to functional recovery. We have utilized this differential response to gain insight in the early transcriptional events associated with successful regeneration. Surprisingly, our study shows that peripheral and central nerve crushes elicit very distinct transcriptional activation, revealing a large set of novel genes that are differentially regulated within the first 24 h after the lesion. Here we show that Ankrd1, a gene known to act as a transcriptional modulator, is involved in neurite outgrowth of a DRG neuron-derived cell line as well as in cultured adult DRG neurons. This gene, and others identified in this study, may be part of the transcriptional regulatory module that orchestrates the onset of successful regeneration.

Systemic bud induction and retinoic acid signaling underlie whole body regeneration in the urochordate Botrylloides leachi

Regeneration in adult chordates is confined to a few model cases and terminates in restoration of restricted tissues and organs. Here, we study the unique phenomenon of whole body regeneration (WBR) in the colonial urochordate Botrylloides leachi in which an entire adult zooid is restored from a miniscule blood vessel fragment. In contrast to all other documented cases, regeneration is induced systemically in blood vessels. Multiple buds appear simultaneously in newly established regeneration niches within vasculature fragments, stemming from composites of pluripotent blood cells and terminating in one functional zooid. We found that retinoic acid (RA) regulates diverse developmental aspects in WBR. The homologue of the RA receptor and a retinaldehyde dehydrogenase-related gene were expressed specifically in blood cells within regeneration niches and throughout bud development. The addition of RA inhibitors as well as RNA interference knockdown experiments resulted in WBR arrest and bud malformations. The administration of all-trans RA to blood vessel fragments resulted in doubly accelerated regeneration and multibud formation, leading to restored colonies with multiple zooids. The Botrylloides system differs from known regeneration model systems by several fundamental criteria, including epimorphosis without the formation of blastema and the induction of a "multifocal regeneration niche" system. This is also to our knowledge the first documented case of WBR from circulating blood cells that restores not only the soma, but also the germ line. This unique Botrylloides WBR process could serve as a new in vivo model system for regeneration, suggesting that RA signaling may have had ancestral roles in body restoration events.

The therapeutic potential of gene transfer for the treatment of peripheral neuropathies

Peripheral neuropathy is a common medical problem with numerous aetiologies. Unfortunately, for the majority of cases there is no available medical solution for the underlying cause, and the only option is to try to treat the resulting symptoms. Treatment options exist when neuropathy results in positive symptoms such as pain, but there is a significant lack of treatments for negative symptoms such as numbness and weakness. Systemic application of growth factor peptides has shown promise in protecting nerves from neuropathic insults in preclinical animal studies, but translation into human trials has been problematic and disappointing. Significant advancements have been made in the past few years in utilising gene therapy approaches to treat peripheral neuropathy by expressing neuroprotective gene products either systemically or in specific nervous tissues. For example, plasmids expressing vascular endothelial growth factor injected into muscle, or herpes-simplex-virus-based vectors expressing neurotrophin gene products delivered to dorsal root ganglion neurons, have been used to protect peripheral nerve function in animal models of diabetes-associated peripheral neuropathy. Many published studies support the feasibility of this approach, although several questions still need to be addressed as gene therapy to treat peripheral neuropathy moves out of the laboratory and into the clinic.

Effects of inflammatory cytokines IL-1beta, IL-6, and TNFalpha on the intracellular localization of retinoid receptors in Schwann cells

It was investigated whether retinoic acid (RA) and the proinflammatory cytokines IL-1beta, IL-6, and TNFalpha influence the intracellular distribution of retinoic acid receptors (RAR) and retinoid X receptors (RXR) in Schwann cells. This question arose because nuclear translocation of RARalpha, RXRalpha, and RXRbeta was observed after nerve injury, and because mutual interactions exist between the signal transduction pathways of RA and proinflammatory cytokines. Schwann cell primary cultures from the rat sciatic nerve were incubated with IL-1beta, IL-6, and TNFalpha, with all-trans RA and with a combination of IL-1beta and RA. After incubation periods ranging from 5 min to 5 h, the intracellular distributions of RARalpha, RARbeta, RXRalpha, and RXRbeta were analyzed. All three cytokines caused a shift of RARalpha from the cytosolic compartments into the cell nuclei. This was also observed with RA, and combining RA with IL-1beta produced an additive effect. IL-1beta and IL-6 also affected the distribution of RARbeta, although immunoreactivity of this receptor always remained stronger in the cytosol. No effect of the cytokines on RXRalpha or RXRbeta was observed, whereas RA treatment caused a stronger nuclear signal of both receptors. Effects on the subcellular localization of retinoid receptors may provide a link in a feedback loop between RA/RAR and cytokines.

Characterization of retinaldehyde dehydrogenase‐2 induction in NG2‐positive glia after spinal cord contusion injury

The transcriptional activator retinoic acid (RA) supports axonal regeneration of several neuronal cell populations in vitro, and it has been suggested that its receptor RARbeta2 may be used to support axonal regeneration in the adult mammalian spinal cord. We have previously shown that spinal cord injury induces activity of the RA synthesizing enzyme retinaldehyde dehydrogenase (RALDH)2 in NG2-positive cells. This report quantifies the increase of RALDH2 protein in the injured spinal cord and characterizes the RALDH2/NG2 expressing cells probably as a unique RA synthesizing subpopulation of activated oligodendrocyte precursors or "polydendrocytes". In the uninjured spinal cord low levels of RALDH2 are present in oligodendrocytes as well as in the meninges and in blood vessels. Following injury there is a significant increase in RALDH2 in these latter two tissues and, given that the RALDH2/NG2 positive cells are clustered in the same area, this implies that these are specific foci of RA synthesis. It is presumed that these cells release RA in a paracrine fashion in the region of the wound; however, the RALDH2/NG2-immunoreactive cells expressed the retinoid receptors RARalpha, RARbeta, RXRalpha and RXRbeta, suggesting that RA also serves an autocrine function.

A review of the functional role and of the expression profile of retinoid signaling and of nuclear receptors in human spinal cord

Spinal cord degenerative pathologies in humans cause extensive disability and require a broad range of specialist and palliative medical interventions. In amyotrophic lateral sclerosis (ALS), motor cell loss leads to extensive paralysis and to death from respiratory failure in 3-5 years form disease onset. A wide range of molecular changes forms the basis of spinal cord involvement in ALS, including the reactivation of molecular pathways with potentially neurorestorative properties. Central to this tissue repair mechanism is the differential regulation of components of the retinoid signaling (ReS), a molecular pathway encompassing a variety of proteins functioning as transporters, signaling factors and metabolizing enzymes for retinoic acid. In this paper, we review the strong body of experimental evidence supporting retinoid signaling's primary role in spinal cord embryonic differentiation and its likely survival-promoting function in ALS. We discuss the potential involvement in ALS pathogenesis of a subgroup of nuclear receptors (NRs) that act as functional partners of retinoid receptors in human spinal cord. We also provide a review of the expression profile of 25 ReS and NRs genes in human adult spinal cord and in motor neurons of healthy and ALS individuals, using data retrieved from independent datasets obtained through serial analysis of gene expression and array investigations. Based on published expression data, we outline a tentative expression profile of ReS and functionally synergic NR genes in human spinal cord that could guide further experiments to clarify the role of these molecules in mature nervous tissue and suggest potential treatment strategies that could have therapeutic potentials in ALS.

Lentiviral vector expressing retinoic acid receptor beta2 promotes recovery of function after corticospinal tract injury in the adult rat spinal cord

Spinal cord injury often results in permanent and devastating neurological deficits and disability. This is due to the limited regenerative capacity of neurones in the central nervous system (CNS). We recently demonstrated that a transcription factor retinoic acid receptor beta2 (RARbeta2) promoted axonal regeneration in adult sensory neurones located peripherally. However, it is not known if RARbeta2 can promote axonal regeneration in cortical neurones of the CNS. Here, we demonstrate that delivery of RARbeta2 via a lentiviral vector to adult dissociated cortical neurones significantly enhances neurite outgrowth on adult cortical cryosections, which normally provide an unfavourable substrate for growth. We also show that lentiviral-mediated transduction of corticospinal neurones resulted in robust transgene expression in layer V corticospinal neurones and their axonal projections in the corticospinal tract (CST) of the spinal cord. Expression of RARbeta2 in these neurones enhanced regeneration of the descending CST fibres after injury to these axons in the mid-cervical spinal cord. Furthermore, we observed functional recovery in sensory and locomotor behavioural tests in RARbeta2-treated animals. These results suggest that a direct and selective delivery of RARbeta2 to the corticospinal neurones promotes long-distance functional regeneration of axons in the spinal cord and may thus offer new therapeutic gene strategy for the treatment of human spinal cord injuries.