The role of retinoic acid receptors in neurite outgrowth from different populations of embryonic mouse dorsal root ganglia

Retinoic acid regulates murine enteric nervous system precursor proliferation, enhances neuronal precursor differentiation, and reduces neurite growth in vitro

Enteric nervous system (ENS) precursors undergo a complex process of cell migration, proliferation, and differentiation to form an integrated network of neurons and glia within the bowel wall. Although retinoids regulate ENS development, molecular and cellular mechanisms of retinoid effects on the ENS are not well understood. We hypothesized that retinoids might directly affect ENS precursor differentiation and proliferation, and tested that hypothesis using immunoselected fetal ENS precursors in primary culture. We now demonstrate that all retinoid receptors and many retinoid biosynthetic enzymes are present in the fetal bowel at about the time that migrating ENS precursors reach the distal bowel. We further demonstrate that retinoic acid (RA) enhances proliferation of subsets of ENS precursors in a time-dependent fashion and increases neuronal differentiation. Surprisingly, however, enteric neurons that develop in retinoid deficient media have dramatically longer neurites than those exposed to RA. This difference in neurite growth correlates with increased RhoA protein at the neurite tip, decreased Smurf1 (a protein that targets RhoA for degradation), and dramatically decreased Smurf1 mRNA in response to RA. Collectively these data demonstrate diverse effects of RA on ENS precursor development and suggest that altered fetal retinoid availability or metabolism could contribute to intestinal motility disorders.

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.

RAR/RXR and PPAR/RXR signaling in neurological and psychiatric diseases

Retinoids are important signals in brain development. They regulate gene transcription by binding to retinoic acid receptors (RAR) and, as was discovered recently, a peroxisome proliferator-activated receptor (PPAR). Traditional ligands of PPAR are best known for their functions in lipid metabolism and inflammation. RAR and PPAR are ligand-activated transcription factors, which share members of the retinoid X receptor (RXR) family as heterodimeric partners. Both signal transduction pathways have recently been implicated in the progression of neurodegenerative and psychiatric diseases. Since inflammatory processes contribute to various neurodegenerative diseases, the anti-inflammatory activity of retinoids and PPARgamma agonists recommends them as potential therapeutic targets. In addition, genetic linkage studies, transgenic mouse models and experiments with vitamin A deprivation provide evidence that retinoic acid signaling is directly involved in physiology and pathology of motoneurons, of the basal ganglia and of cognitive functions. The activation of PPAR/RXR and RAR/RXR transcription factors has therefore been proposed as a therapeutic strategy in disorders of the central nervous system.

The transcriptional corepressor TPA-inducible sequence 7 regulates adult axon growth through cellular retinoic acid binding protein II expression

TPA-inducible sequence 7 (TIS7) expression is regulated in epithelial cells and acts as a transcriptional corepressor. Using a TIS7 knock-out mouse we demonstrated that TIS7 is involved in the process of muscle regeneration. In this study, we analysed the role of TIS7 in axon regeneration, applying primary neurone cultures derived from adult dorsal root ganglia (DRGs) of TIS7+/+ and TIS7-/- mice. TIS7-/- DRG neurones exhibited a significant decrease in axon initiation and maximal axon extension. In contrast, nerve growth factor-induced axon initiation and branching were significantly enhanced in cultures obtained from TIS7-/- DRGs when compared with wildtype ganglia, suggesting an inhibitory effect of TIS7 on nerve growth factor-stimulated axon growth. TIS7 overexpression in TIS7-/- DRG neurones caused their morphological appearance to revert back to the wildtype phenotype. Furthermore, the expression of cellular retinoic acid binding protein II (CRABP II), previously identified by us as a TIS7 target gene, was up-regulated in adult DRG sensory neurones from TIS7-/- mice. Overexpression of CRABP II in TIS7+/+ neurones strongly increased the number of branch points, making them morphologically similar to TIS7-/- neurones. Based on these results we propose that TIS7 inhibits CRABP II expression during axonal regeneration, thereby modulating retinoic acid signalling. Hence, neurite initiation and branching are regulated by a negative feedback mechanism involving TIS7 and CRABP II.

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.

Analysis of the action of euxanthone, a plant-derived compound that stimulates neurite outgrowth

We have investigated the neurite growth-stimulating properties of euxanthone, a xanthone derivative isolated from the Chinese medicinal plant Polygala caudata. Euxanthone was shown to exert a marked stimulatory action on neurite outgrowth from chick embryo dorsal root ganglia explanted in collagen gels, in the absence of added neurotrophins. It was also shown to promote cell survival in explanted chick embryo ganglia, and to stimulate neurite outgrowth from isolated adult rat primary sensory neurons in vitro. The further finding that euxanthone stimulates neurite outgrowth from explants of chick embryo retina and ventral spinal cord suggests an action on signaling pathways downstream of neuronal receptors for specific neurotrophic factors. Consistent with this, euxanthone did not promote neurite outgrowth from non-transfected PC12 cells, or from PC12 cells transfected with TrkB or TrkC, under conditions in which these cells extended neurites in response to, respectively, the neurotrophins nerve growth factor, brain-derived neurotrophic factor and neurotrophin 3. Western blot analysis of euxanthone-stimulated dorsal root ganglion explants showed that expression of phospho-mitogen-activated protein (MAP) kinase was up-regulated after 1 h of euxanthone-treatment. Inhibition of the MAP kinase pathway using PD98059, a specific inhibitor of MAP kinase kinase, blocked all euxanthone-stimulated neurite outgrowth. However, analysis of phospho-Akt expression indicated that the phosphatidylinositol-3 kinase-Akt pathway, another major signaling pathway engaged by neurotrophins, is not significantly activated by euxanthone. These results suggest that euxanthone promotes neurite outgrowth by selectively activating the MAP kinase pathway.

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.

Hyperglycemia inhibits retinoic acid-induced activation of Rac1, prevents differentiation of cortical neurons, and causes oxidative stress in a rat model of diabetic pregnancy

Diabetes is a risk factor for neuronal dysfunction. Impairment in signaling mechanisms that regulate differentiation of neurons is hypothesized to be one of the main causes of neuronal dysfunction. Retinoic acid, a physiologically active retinoid synthesized from vitamin A, regulates neuronal differentiation during embryonic development and is required for maintenance of plasticity in differentiated neurons. To date, little is known about the molecular events underlying hyperglycemia-induced complications in the central nervous system (CNS). Here, we provide evidence, in a diabetes rat model, of hyperglycemia-induced oxidative stress along with apoptotic stress in developing cortical neurons isolated from 16-day-old rat embryos. We also demonstrate impaired retinoic acid signaling that is involved in neuronal differentiation. Retinoic acid-induced neurite outgrowth and expression of neuronal markers were reduced in this model. The activation of small-molecular weight G-protein, Rac1, that mediates these effects was also reduced. Retinoic acid applied at a physiological concentration significantly decreased hyperglycemia-induced oxidative stress and thus supported the antioxidant defense system. These results suggest that diabetes-induced neuronal complications during pregnancy might be due to impaired retinoic acid signaling, and exogenously administered retinoic acid may be useful against CNS complications associated with diabetes.

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.

Role of all-trans retinoic acid in neurite outgrowth and axonal elongation

The vitamin A metabolite, all-trans retinoic acid (atRA) plays essential roles in nervous system development, including neuronal patterning, survival, and neurite outgrowth. Our understanding of how the vitamin A acid functions in neurite outgrowth comes largely from cultured embryonic neurons and model neuronal cell systems including human neuroblastoma cells. Specifically, atRA has been shown to increase neurite outgrowth from embryonic DRG, sympathetic, spinal cord, and olfactory receptor neurons, as well as dissociated cerebra and retina explants. A role for atRA in axonal elongation is also supported by a limited number of studies in vivo, in which a deficiency in retinoid signaling produced either by dietary or genetic means has been shown to alter neurite outgrowth from the spinal cord and hindbrain regions. Human neuroblastoma cells also show enhanced numbers of neurites and longer processes in response to atRA. The mechanism whereby retinoids regulate neurite outgrowth includes, but is not limited to, the regulation of the transcription of neurotrophin receptors. More recent evidence supports a role for atRA in regulating components of other signaling pathways or candidate neurite-regulating factors. Some of these effects, such as that on neuron navigator 2 (NAV2), may be direct, whereas others may be secondary to other atRA-induced changes in the cell. This review focuses on what is currently known about neurite initiation and growth, with emphasis on the manner in which atRA may influence these events.

Overview of retinoid metabolism and function

Retinoids (vitamin A) are crucial for most forms of life. In chordates, they have important roles in the developing nervous system and notochord and many other embryonic structures, as well as in maintenance of epithelial surfaces, immune competence, and reproduction. The ability of all-trans retinoic acid to regulate expression of several hundred genes through binding to nuclear transcription factors is believed to mediate most of these functions. The role of all-trans retinoic may extend beyond the regulation of gene transcription because a large number of noncoding RNAs also are regulated by retinoic acid. Additionally, extra-nuclear mechanisms of action of retinoids are also being identified. In organisms ranging from prokaryotes to humans, retinal is covalently linked to G protein-coupled transmembrane receptors called opsins. These receptors function as light-driven ion pumps, mediators of phototaxis, or photosensory pigments. In vertebrates phototransduction is initiated by a photochemical reaction where opsin-bound 11-cis-retinal is isomerized to all-trans-retinal. The photosensitive receptor is restored via the retinoid visual cycle. Multiple genes encoding components of this cycle have been identified and linked to many human retinal diseases. Central aspects of vitamin A absorption, enzymatic oxidation of all-trans retinol to all-trans retinal and all-trans retinoic acid, and esterification of all-trans retinol have been clarified. Furthermore, specific binding proteins are involved in several of these enzymatic processes as well as in delivery of all-trans retinoic acid to nuclear receptors. Thus, substantial progress has been made in our understanding of retinoid metabolism and function. This insight has improved our view of retinoids as critical molecules in vision, normal embryonic development, and in control of cellular growth, differentiation, and death throughout life.

Interactions between retinoic acid, nerve growth factor and sonic hedgehog signalling pathways in neurite outgrowth

Many studies have shown a role of retinoid signalling in neurite outgrowth in vitro, and that the retinoic acid receptor (RAR) beta2 is critical for this process. We show here that RARbeta2 is expressed predominantly in dorsal root ganglia (DRG) neuronal subtypes that express neurofilament (NF) 200 and calcitonin gene-related peptide (CGRP), and that these neurons extend neurites in response to RA. We demonstrate that retinoid signalling has a role in neurite outgrowth in vivo, by showing that in a peripheral nerve crush model there is less neurite outgrowth from RARbeta null DRG compared to wild-type. We identify sonic hedgehog (Shh) as a downstream target of the RARbeta2 signalling pathway as it is expressed in the injured DRG of wild-type but not RARbeta null mice. This regulation is direct as when RARbeta2 is overexpressed in adult motoneurons Shh is induced in them. Finally we show that Shh alone cannot induce neurite outgrowth but potentiates RARbeta2 signalling in this process.

Retinoic acid induces neurite outgrowth and growth cone turning in invertebrate neurons

Identification of molecules involved in neurite outgrowth during development and/or regeneration is a major goal in the field of neuroscience. Retinoic acid (RA) is a biologically important metabolite of vitamin A that acts as a trophic factor and has been implicated in neurite outgrowth and regeneration in many vertebrate species. Although abundant in the CNS of many vertebrates, the precise role of RA in neural regeneration has yet to be determined. Moreover, very little information is available regarding the role of RA in invertebrate nervous systems. Here, we demonstrate for the first time that RA induces neurite outgrowth from invertebrate neurons. Using individually identified neurons isolated from the CNS of Lymnaea stagnalis, we demonstrated that a significantly greater proportion of cells produced neurite outgrowth in RA. RA also extended the duration of time that cells remained electrically excitable in vitro, and we showed that exogenously applied RA acted as a chemoattractive factor and induced growth cone turning toward the source of RA. This is the first demonstration that RA can induce turning of an individual growth cone. These data strongly suggest that the actions of RA on neurite outgrowth and cell survival are highly conserved across species.

CyclinB2 and BIRC5 genes as surrogate biomarkers for neurite outgrowth in SH-SY5Y subclonal cells

Neurite outgrowth plays a key role in neuronal development and regeneration, and is the hallmark assay for the effects of neurotrophic factors such as nerve growth factor (NGF). However, measuring neurite outgrowth is a slow and resource-intensive process. We therefore wanted to identify surrogate biomarkers for neurite outgrowth activity by gene expression analysis in SH-O10 cells, a subclone of the human SH-SY5Y neuroblastoma cell line but with much higher NGF-induced neurite outgrowth activity. Microarray analysis identified seven genes where mRNA levels were changed. NGF-induced decreases in levels of two genes, CyclinB2 and BIRC5, were confirmed by quantitative real-time RT-PCR. Levels of NGF-induced decreases in CyclinB2 and BIRC5 mRNA in several SH-SY5Y subclones with different neurite outgrowth responses correlated with their neurite outgrowth activities. Decreases in CyclinB2 and BIRC5 mRNA induced by FK506 or retinoic acid, both of which exert potentiation of NGF-induced neurite outgrowth effects but with different mechanisms, also correlated with their neurite outgrowth activities. In conclusion, decreasing levels of CyclinB2 and BIRC5 mRNA strongly correlate with neurite outgrowth activities in terms of NGF-related effect in SH-SY5Y subclonal cells, and have potential to become quantitative surrogate biomarkers for measuring NGF-related neurite outgrowth.

Retinoids have differing efficacies on alveolar regeneration in a dexamethasone-treated mouse

We have investigated the relative efficacy of a range of natural and synthetic retinoids on the induction of alveolar regeneration in a dexamethasone-treated mouse model. The aim was to explore the roles of the different retinoic acid receptors using receptor-selective agonists and to determine whether other natural retinoids in addition to all-trans-retinoic acid (tRA) were effective. Dexamethasone treatment of newborn pups led to a reduced lung surface area and increased mean chord length. Subsequently, tRA induced alveolar repair, improved mean chord length, and improved the lung surface area to volume ratio. We found that 4-oxo-RA and a retinoic acid receptor (RAR) alpha-selective compound were as effective as tRA at inducing alveolar regeneration, with neither showing a significantly better efficacy. An RARbeta-selective compound was also effective, whereas a RARgamma-selective compound was not. Other retinoids, such as 9-cis-RA, 13-cis-RA, retinol, and a pan retinoid X receptor (RXR) agonist, do not induce significant responses. Neither did granulocyte colony-stimulating factor. We also showed that an RARbeta-null mutant mouse line responded to dexamethasone by failing to develop alveoli appropriately and that tRA induced alveolar regeneration, suggesting that RARbeta was not required for the regenerative response.

Macrophages and neurons are targets of retinoic acid signaling after spinal cord contusion injury

The physiological reactions after spinal cord injury are accompanied by local synthesis of the transcriptional activator retinoic acid (RA). RA exerts its effects by binding to retinoic acid receptors (RAR) which heterodimerize with retinoid X receptors (RXR) and then act as ligand-activated transcription factors. To identify possible cellular targets of RA we investigated protein levels and cellular distribution of retinoid receptors in the rat spinal cord at 4, 7, 14 and 21 days after a contusion injury. In the nonlesioned spinal cord, immunoreactivity for RARalpha, RXRalpha, RXRbeta and RXRgamma was localized in the cytosol of neurons, that of RXRalpha and RXRbeta in astrocytes and that of RARalpha, RXRalpha and RXRgamma in some oligodendrocytes. After contusion injury RARalpha and all RXRs appeared in the cell nuclei of reactive microglia and macrophages. This nuclear staining began at 4 days, was most prominent at 7 and 14 days and had decreased at 21 days after injury. A similar nuclear translocation was also observed for the RARalpha, RXRalpha and RXRbeta staining in neurons situated around the border of the contusion. These observations suggest that RA participates as a signal for the physiological responses of microglia and neurons after CNS injury.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

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

The embryonic CNS readily undergoes regeneration, unlike the adult CNS, which has limited axonal repair after injury. Here we tested the hypothesis that retinoic acid receptor beta2 (RARbeta2), critical in development for neuronal growth, may enable adult neurons to grow in an inhibitory environment. Overexpression of RARbeta2 in adult rat dorsal root ganglion cultures increased intracellular levels of cyclic AMP and stimulated neurite outgrowth. Stable RARbeta2 expression in DRG neurons in vitro and in vivo enabled their axons to regenerate across the inhibitory dorsal root entry zone and project into the gray matter of the spinal cord. The regenerated neurons enhanced second-order neuronal activity in the spinal cord, and RARbeta2-treated rats showed highly significant improvement in sensorimotor tasks. These findings show that RARbeta2 induces axonal regeneration programs within injured neurons and may thus offer new therapeutic opportunities for CNS regeneration.

Identification of novel subtype selective RAR agonists

Drugs targeting retinoid receptors have been developed to treat a variety of therapeutic indications, but their success has been limited in part due to lack of selectivity. A novel functional cell-based assay R-SATtrade mark was employed to screen a small molecule chemical library and identify a variety of novel RAR agonists with various subtype selectivities, including RARbeta/gamma and RARgamma selective agonists. A novel class of synthetic compounds that distinguishes between the different RARbeta isoforms is described. This pharmacophore displays anti-proliferative activity and induces differentiation in a neuronal cell line, consistent with a classical retinoid mechanism of action while providing unique subtype selectivity. These novel subtype selective RAR agonists could serve as powerful tools to probe into subtype and isoform-specific retinoid function.

Retinoic acid-dependent attraction of adult spinal cord axons towards regenerating newt limb blastemas in vitro

Adult urodele amphibians possess the unique ability to regenerate amputated limbs and to re-innervate these regenerating structures; however, the factors involved in mediating this re-innervation are largely unknown. Here, we investigated the role of retinoic acid (RA) and one of its receptors, RARbeta, in the reciprocal neurotropic interactions between regenerating limb blastemas and spinal cord explants from the adult newt Notophthalmus viridescens. First, we showed that retinoic acid induced directed axonal outgrowth from cultured spinal cord tissue. This RA-induced outgrowth was significantly reduced when spinal cord explants were pre-treated with either the synthetic RAR pan antagonist, LE540, or the specific RARbeta antagonist, LE135. The role of RARbeta was also investigated using co-cultured regenerating limb blastemas and spinal cord explants. Blastemas induced significantly more axonal outgrowth from the near side of co-cultured explants, than from the far side (when cultured less than 1 mm apart). This blastema-induced directed outgrowth from co-cultured spinal cord explants was also abolished in the presence of the RARbeta antagonist, LE135. These data strongly suggest that endogenous retinoic acid is one of the tropic factors produced by the blastema and that it may be capable of guiding re-innervating axons to their targets. Moreover, this interaction is likely mediated by the retinoic acid beta nuclear receptor.

Sonic hedgehog and retinoic acid synergistically promote sensory fate specification from bone marrow-derived pluripotent stem cells

Recent studies demonstrated that stromal cells isolated from adult bone marrow have the competence of differentiating into neuronal cells in vitro and in vivo. However, the capacity of marrow stromal cells or mesenchymal stem cells (MSCs) to differentiate into diverse neuronal cell populations and the identity of molecular factors that confer marrow stromal cells with the competence of a neuronal subtype have yet to be elucidated. Here, we show that Sonic hedgehog (Shh) and retinoic acid (RA), signaling molecules secreted from tissues in the vicinity of peripheral sensory ganglia during embryogenesis, exert synergistic effects on neural-competent MSCs to express a comprehensive set of glutamatergic sensory neuron markers. Application of Shh or RA alone had little or no effect on the expression of these neuronal subtype markers. In addition, incubation of MSCs with embryonic hindbrain/somite/otocyst conditioned medium or prenatal cochlea explants promoted up-regulation of additional sensory neuron markers and process outgrowth. These results identify Shh and RA as sensory competence factors for adult pluripotent cells and establish the importance of interactions between adult pluripotent cells and the host microenvironment in neuronal subtype specification.

Timing of the retinoid-signalling pathway determines the expression of neuronal markers in neural progenitor cells

By culturing neural progenitor cells in the presence of retinoid receptor agonists, we have defined the components of the retinoid-signalling pathway that are important for the birth and maintenance of neuronal cells. We provide evidence that depending on the order and combination of retinoid receptors activated, different neuronal cells are obtained. Astrocytes and oligodendrocytes are predominantly formed in the presence of activated retinoic acid receptor (RAR) alpha, whereas motoneurons are formed when RARbeta is activated. We have looked at the regulation of two transcription factors islet-1/2 which are involved in neuronal development. We find that activated RARbeta up-regulates islet-1 expression, whereas activation of RARalpha can either act in combination with RARbeta signalling to maintain islet-1 expression or induce islet-2 expression in the absence of activated RARbeta. RARgamma cannot directly regulate islet-1/2 but can down-regulate RARbeta expression, which results in loss of islet-1 expression. We finally show that activated RARalpha is one of the final steps required for a mature motoneuron phenotype.

Retinoic acid signaling in the nervous system of adult vertebrates

The majority of the functions of vitamin A are carried out by its metabolite, retinoic acid (RA), a potent transcriptional activator acting through members of the nuclear receptor family of transcription factors. In the CNS, RA was first recognized to be essential for the control of patterning and differentiation in the developing embryo. It has recently come to light, however, that many of the same functions that RA directs in the embryo are involved in the regulation of plasticity and regeneration in the adult brain. The same intricate metabolic control system of synthetic and catabolic enzymes, combined with cytoplasmic binding proteins, is used in both embryo and adult to create regions of high and low RA to modulate gene transcription. This review summarizes some of the discoveries in the new field of retinoid neurobiology including its functions in neural plasticity and LTP in the hippocampus; its possible role in motor disorders such as Parkinson's disease, motoneuron disease, and Huntington's disease; its role in regeneration after sciatic nerve and spinal cord injury; and its possible involvement in psychiatric diseases such as depression.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.

CNS regeneration: A morphogen's tale

Tissue regeneration will soon become an avenue for repair of damaged or diseased tissues as stem cell niches have been found in almost every organ of the vertebrate body including the CNS. In addition, different animals display an array of regenerative capabilities that are currently being researched to dissect the molecular mechanisms involved. This review concentrates on the different ways in which CNS tissues such as brain, spinal cord and retina can regenerate or display neurogenic potential and how these abilities are modulated by morphogens.

Disruption of the retinoid signalling pathway causes a deposition of amyloid beta in the adult rat brain

We have disrupted the retinoid signalling pathway in adult rats by a dietary deficiency of vitamin A. After 1 year of this dietary deficiency, there was a deposition of amyloid beta in the cerebral blood vessels. There is a downregulation of retinoic acid receptor alpha in the forebrain neurons of the retinoid-deficient rats and a loss of choline acetyl transferase expression, which precedes amyloid beta deposition. In neocortex of pathology samples of patients with Alzheimer's disease, the same retinoic acid receptor alpha deficit in the surviving neurons was observed. We have identified the retinoid-synthesizing enzymes involved in this process, retinaldehyde dehydrogenase-2 and class IV alcohol dehydrogenase, only the former is downregulated in patients with Alzheimer's disease. This suggests that retinoids are important for the maintenance of the adult nervous system and their loss may in part play a role in Alzheimer's disease.

Rapid effects of retinoic acid on CREB and ERK phosphorylation in neuronal cells

Retinoic acid (RA) is a potent regulator of neuronal cell differentiation. RA normally activates gene expression by binding to nuclear receptors that interact with response elements (RAREs) in regulatory regions of target genes. We show here that in PC12 cell subclones in which the retinoid causes neurite extension, RA induces a rapid and sustained phosphorylation of CREB (cyclic AMP response element binding protein), compatible with a nongenomic effect. RA also causes a rapid increase of CREB phosphorylation in primary cultures of cerebrocortical cells and of dorsal root ganglia neurons from rat embryos. RA-mediated phosphorylation of CREB leads to a direct stimulation of CREB-dependent transcriptional activity and to activation of the expression of genes such as c-fos, which do not contain RAREs but contain cAMP response elements (CREs) in their promoters. CREB is a major target of extracellular signal regulated kinase ERK1/2 signaling in neuronal cells, and we demonstrate here that RA induces an early stimulation of ERK1/2, which is required both for CREB phosphorylation and transcriptional activity. These results demonstrate that RA, by a nongenomic mechanism, stimulates signaling pathways that lead to phosphorylation of transcription factors, which in turn activate the transcription of genes involved in neuronal differentiation.

Innervated human corneal equivalents as in vitro models for nerve‐target cell interactions

A sensory nerve supply is crucial for optimal tissue function. However, the mechanisms for successful innervation and the signaling pathways between nerves and their target tissue are not fully understood. Engineered tissue substitutes can provide controllable environments in which to study tissue innervation. We have therefore engineered human corneal substitutes that promote nerve in-growth in a pattern similar to in vivo re-innervation. We demonstrate that these nerves (a) are morphologically equivalent to natural corneal nerves; (b) make appropriate contact with target cells; (c) can generate action potentials; (d) respond to chemical and physical stimuli; and (e) play an important role in the overall functioning of the bioengineered tissue. This model can be used for studying the more general topics of nerve ingrowth or regeneration and the interaction between nerves and their target cells and, more specifically, the role of nerves in corneal function. This model could also be used as an in vitro alternative to animals for safety and efficacy testing of chemicals and drugs.

Ca2+-independent phospholipases A2 and production of arachidonic acid in nuclei of LA-N-1 cell cultures: a specific receptor activation mediated with retinoic acid

The LA-N-1 cell nucleus contains Ca2+-independent phospholipase A2 (PLA2) activity hydrolyzing plasmenylethanolamine (PlsEtn) and 1,2-diacyl-sn-glycero-3-phosphoethanolamine (PtdEtn). These enzymes hydrolyze glycerophospholipids to produce arachidonic acid and lysoglycerophospholipids. The treatment of LA-N-1 cell cultures with all-trans retinoic acid (atRA) results in time- and dose-dependent stimulation of PlsEtn-PLA2 and PtdEtn-PLA2 activities in the nuclear fraction. PLA2 activities in the non-nuclear fraction (microsomes) are not affected by atRA, whilst the pan retinoic acid receptor (RAR) antagonist, BMS493, blocks the PLA2 activities in the nuclear fraction. This indicates that the stimulation of PLA2 activities is a receptor-mediated process. Treatment of LA-N-1 cell cultures with cycloheximide has no effect on basal PLA2 activities. However, atRA-mediated stimulation of PLA2 activities in LA-N-1 cell nuclei is partially inhibited by cycloheximide indicating that this decrease in PLA2 activity is due to a general decreased protein synthesis. Our results also support earlier studies in which atRA induces morphologic differentiation through the stimulation of PLA2-generated second messengers such as arachidonic acid and eicosanoids.

Retinoic acid, a regeneration-inducing molecule

Retinoic acid (RA) is the biologically active metabolite of vitamin A. It is a low molecular weight, lipophilic molecule that acts on the nucleus to induce gene transcription. In amphibians and mammals, it induces the regeneration of several tissues and organs and these examples are reviewed here. RA induces the "super-regeneration" of organs that can already regenerate such as the urodele amphibian limb by respecifying positional information in the limb. In organs that cannot normally regenerate such as the adult mammalian lung, RA induces the complete regeneration of alveoli that have been destroyed by various noxious treatments. In the mammalian central nervous system (CNS), which is another tissue that cannot regenerate, RA does not induce neurite outgrowth as it does in the embryonic CNS, because one of the retinoic acid receptors, RAR beta 2, is not up-regulated. When RAR beta 2 is transfected into the adult spinal cord in vitro, then neurite outgrowth is stimulated. In all these cases, RA is required for the development of the organ, in the first place suggesting that the same gene pathways are likely to be used for both development and regeneration. This suggestion, therefore, might serve as a strategy for identifying potential tissue or organ targets that have the capacity to be stimulated to regenerate.

Absence of retinoids can induce motoneuron disease in the adult rat and a retinoid defect is present in motoneuron disease patients

We generated retinoid-deficient adult rats by the removal of retinoids from their diet. We show that their motoneurons undergo neurodegeneration and that there is an accumulation of neurofilaments and an increase in astrocytosis, which is associated with motoneuron disease. These effects are mediated through the retinoic acid receptor alpha. The same receptor deficit is found in motoneurons from patients suffering from spontaneous amyotrophic lateral sclerosis. Furthermore, we show that there is a loss of expression of the retinaldehyde dehydrogenase enzyme II in motoneurons. Therefore, we propose that a defect in the retinoid signalling pathway is in part be responsible for some types of motoneuron disease.

Retinoic acid receptor beta2 and neurite outgrowth in the adult mouse spinal cord in vitro

Retinoic acid, acting through the nuclear retinoic acid receptor beta2 (RARbeta2), stimulates neurite outgrowth from peripheral nervous system tissue that has the capacity to regenerate neurites, namely, embryonic and adult dorsal root ganglia. Similarly, in central nervous system tissue that can regenerate, namely, embryonic mouse spinal cord, retinoic acid also stimulates neurite outgrowth and RARbeta2 is upregulated. By contrast, in the adult mouse spinal cord, which cannot regenerate, no such upregulation of RARbeta2 by retinoic acid is observed and no neurites are extended in vitro. To test our hypothesis that the upregulation of RARbeta2 is crucial to neurite regeneration, we have transduced adult mouse or rat spinal cord in vitro with a minimal equine infectious anaemia virus vector expressing RARbeta2. After transduction, prolific neurite outgrowth occurs. Outgrowth does not occur when the cord is transduced with a different isoform of RARbeta nor does it occur following treatment with nerve growth factor. These data demonstrate that RARbeta2 is involved in neurite outgrowth, at least in vitro, and that this gene may in the future be of some therapeutic use.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Temporal/spatial expression of retinoid binding proteins and RAR isoforms in the postnatal lung

Endogenous retinoids have been implicated in alveologenesis in both the rat and the mouse, and exogenous retinoic acid (RA) can reverse or partially reverse experimental emphysema in adult rat and mouse models by an unknown mechanism. In this study, we examine the cellular and molecular biology of retinoid signaling during alveologenesis in the mouse. We describe the temporal and spatial expression of the retinoid binding proteins CRBP-I, CRBP-II, and CRABP-I using RT-PCR and immunohistochemistry. We identify the retinoic acid receptor isoforms RAR-alpha 1, RAR-beta 2, RAR-beta 4, and RAR-gamma 2 and describe their temporal and spatial expression using RT-PCR and in situ hybridization. We demonstrate that both retinoid binding proteins and RAR isoforms are temporally regulated and found within the alveolar septal regions during alveologenesis. These data support a role of dynamic endogenous RA signaling during alveolar formation.

Alveolar proliferation, retinoid synthesizing enzymes, and endogenous retinoids in the postnatal mouse lung. Different roles for Aldh-1 and Raldh-2

Alveoli are formed postnatally in the rat, mouse, and human. The molecular signals controlling the patterning of this developmental process are not well understood. Here we describe immunohistochemical studies that label proliferating alveolar wall cells which suggest two distinct patterns of alveologenesis: (1) a low grade, peripheral subpleural parenchymal process which occurs from P1 through to P15; and (2) a dramatic increase in central cell proliferation from P4 which is complete by P15, corresponding to the well described period of alveolar septation. We describe the temporal and spatial expression of the retinoid-synthesizing enzymes Aldh-1 and Raldh-2 in the postnatal mouse lung. Both enzymes are upregulated during the period of maximal alveolar wall cell proliferation. Aldh-1 is located in the bronchial epithelium and alveolar parenchyma, and Raldh-2 is restricted to the bronchial epithelium and pleural mesothelial cells. High-pressure liquid chromatography (HPLC) reveals that rapidly septating lungs have relatively simple chromatographic profiles; in contrast, the adult lungs have a complex profile that includes many novel retinoids. These data suggest two patterns of alveolar proliferation with temporal and spatial association of the enzymes Aldh-1 and Raldh-2 and a dynamic role for different retinoids in both the septating and adult mouse lung.