Retinoic acid-mediated increase in TrkA expression is sufficient to elicit NGF-dependent survival of sympathetic neurons

The Therapeutic Potential of Antioxidants in Chemotherapy-Induced Peripheral Neuropathy: Evidence from Preclinical and Clinical Studies

As cancer therapies advance and patient survival improves, there has been growing concern about the long-term adverse effects that patients may experience following treatment, and concerns have been raised about such persistent, progressive, and often irreversible adverse effects. Chemotherapy is a potentially life-extending treatment, and chemotherapy-induced peripheral neuropathy (CIPN) is one of its most common long-term toxicities. At present, strategies for the prevention and treatment of CIPN are still an open problem faced by medicine, and there has been a large amount of previous evidence that oxidative damage is involved in the process of CIPN. In this review, we focus on the lines of defense involving antioxidants that exert the effect of inhibiting CIPN. We also provide an update on the targets and clinical prospects of different antioxidants (melatonin, N-acetylcysteine, vitamins, α-lipoic acid, mineral elements, phytochemicals, nutritional antioxidants, cytoprotectants and synthetic compounds) in the treatment of CIPN with the help of preclinical and clinical studies, emphasizing the great potential of antioxidants as adjuvant strategies to mitigate CIPN.

11q Deletion or ALK Activity Curbs DLG2 Expression to Maintain an Undifferentiated State in Neuroblastoma

High-risk neuroblastomas typically display an undifferentiated or poorly differentiated morphology. It is therefore vital to understand molecular mechanisms that block the differentiation process. We identify an important role for oncogenic ALK-ERK1/2-SP1 signaling in the maintenance of undifferentiated neural crest-derived progenitors through the repression of DLG2, a candidate tumor suppressor gene in neuroblastoma. DLG2 is expressed in the murine "bridge signature" that represents the transcriptional transition state when neural crest cells or Schwann cell precursors differentiate to chromaffin cells of the adrenal gland. We show that the restoration of DLG2 expression spontaneously drives neuroblastoma cell differentiation, highlighting the importance of DLG2 in this process. These findings are supported by genetic analyses of high-risk 11q deletion neuroblastomas, which identified genetic lesions in the DLG2 gene. Our data also suggest that further exploration of other bridge genes may help elucidate the mechanisms underlying the differentiation of NC-derived progenitors and their contribution to neuroblastomas.

Human bone marrow-derived MSCs spontaneously express specific Schwann cell markers

In peripheral nerve injuries, Schwann cells (SC) play pivotal roles in regenerating damaged nerve. However, the use of SC in clinical cell-based therapy is hampered due to its limited availability. In this study, we aim to evaluate the effectiveness of using an established induction protocol for human bone marrow derived-MSC (hBM-MSCs) transdifferentiation into a SC lineage. A relatively homogenous culture of hBM-MSCs was first established after serial passaging (P3), with profiles conforming to the minimal criteria set by International Society for Cellular Therapy (ISCT). The cultures (n = 3) were then subjected to a series of induction media containing β-mercaptoethanol, retinoic acid, and growth factors. Quantitative RT-PCR, flow cytometry, and immunocytochemistry analyses were performed to quantify the expression of specific SC markers, that is, S100, GFAP, MPZ and p75 NGFR, in both undifferentiated and transdifferentiated hBM-MSCs. Based on these analyses, all markers were expressed in undifferentiated hBM-MSCs and MPZ expression (mRNA transcripts) was consistently detected before and after transdifferentiation across all samples. There was upregulation at the transcript level of more than twofolds for NGF, MPB, GDNF, p75 NGFR post-transdifferentiation. This study highlights the existence of spontaneous expression of specific SC markers in cultured hBM-MSCs, inter-donor variability and that MSC transdifferentiation is a heterogenous process. These findings strongly oppose the use of a single marker to indicate SC fate. The heterogenous nature of MSC may influence the efficiency of SC transdifferentiation protocols. Therefore, there is an urgent need to re-define the MSC subpopulations and revise the minimal criteria for MSC identification.

Exogenous Modulation of Retinoic Acid Signaling Affects Adult RGC Survival in the Frog Visual System after Optic Nerve Injury

After lesions to the mammalian optic nerve, the great majority of retinal ganglion cells (RGCs) die before their axons have even had a chance to regenerate. Frog RGCs, on the other hand, suffer only an approximately 50% cell loss, and we have previously investigated the mechanisms by which the application of growth factors can increase their survival rate. Retinoic acid (RA) is a vitamin A-derived lipophilic molecule that plays major roles during development of the nervous system. The RA signaling pathway is also present in parts of the adult nervous system, and components of it are upregulated after injury in peripheral nerves but not in the CNS. Here we investigate whether RA signaling affects long-term RGC survival at 6 weeks after axotomy. Intraocular injection of all-trans retinoic acid (ATRA), the retinoic acid receptor (RAR) type-α agonist AM80, the RARβ agonist CD2314, or the RARγ agonist CD1530, returned axotomized RGC numbers to almost normal levels. On the other hand, inhibition of RA synthesis with disulfiram, or of RAR receptors with the pan-RAR antagonist Ro-41-5253, or the RARβ antagonist LE135E, greatly reduced the survival of the axotomized neurons. Axotomy elicited a strong activation of the MAPK, STAT3 and AKT pathways; this activation was prevented by disulfiram or by RAR antagonists. Finally, addition of exogenous ATRA stimulated the activation of the first two of these pathways. Future experiments will investigate whether these strong survival-promoting effects of RA are mediated via the upregulation of neurotrophins.

Loss of the transcription factor Meis1 prevents sympathetic neurons target-field innervation and increases susceptibility to sudden cardiac death

Although cardio-vascular incidents and sudden cardiac death (SCD) are among the leading causes of premature death in the general population, the origins remain unidentified in many cases. Genome-wide association studies have identified Meis1 as a risk factor for SCD. We report that Meis1 inactivation in the mouse neural crest leads to an altered sympatho-vagal regulation of cardiac rhythmicity in adults characterized by a chronotropic incompetence and cardiac conduction defects, thus increasing the susceptibility to SCD. We demonstrated that Meis1 is a major regulator of sympathetic target-field innervation and that Meis1 deficient sympathetic neurons die by apoptosis from early embryonic stages to perinatal stages. In addition, we showed that Meis1 regulates the transcription of key molecules necessary for the endosomal machinery. Accordingly, the traffic of Rab5⁺ endosomes is severely altered in Meis1-inactivated sympathetic neurons. These results suggest that Meis1 interacts with various trophic factors signaling pathways during postmitotic neurons differentiation.

DOI:http://dx.doi.org/10.7554/eLife.11627.001

Nerve cells called sympathetic neurons can control the activity of almost all of our organs without any conscious thought on our part. For example, these nerve cells are responsible for accelerating the heart rate during exercise.

In a developing embryo, there are initially more of these neurons than are needed, and only those that develop correctly and form a connection with a target cell will survive. This is because the target cells provide the growing neurons with vital molecules called neurotrophins, which are trafficked back along the nerve fiber and into the main body of the nerve cell to ensure its survival. However, it is largely unknown which proteins or genes are also involved in this developmental process.

Now, Bouilloux, Thireau et al. show that if a gene called Meis1 is inactivated in mice, the sympathetic neurons start to develop and grow nerve fibers, but then fail to establish connections with their target cells and finally die. The Meis1 gene encodes a transcription factor, which is a protein that regulates gene activity. Therefore, Bouilloux, Thireau et al. looked for the genes that are regulated by this transcription factor in sympathetic neurons. This search uncovered several genes that are involved in the packaging and trafficking of molecules within cells.

Other experiments then revealed that the trafficking of molecules back along the nerve fiber was altered in mutant neurons in which the Meis1 gene had been inactivated. Furthermore, Meis1 mutant mice had problems with their heart rate, especially during exercise, and an increased risk of dying from a sudden cardiac arrest.

These findings reveal a transcription factor that helps to establish a connection between a neuron and its target, and that activates a pattern of gene expression that works alongside the neurotrophin-based signals. Since all neurons undergo similar processes during development, future work could ask if comparable patterns of gene expression exist in other types of neurons, and if problems with such processes contribute to some neurodegenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.11627.002

Mesenchymal stem cells as a source of Schwann cells: their anticipated use in peripheral nerve regeneration

Schwann cells form myelin, sustain axons and provide the microenvironment for nerve fibers, thereby playing a key role in the peripheral nervous system (PNS). Schwann cells also provide support for the damaged PNS by producing factors that strongly promote axonal regrowth and contribute to remyelination, which is crucial for the recovery of neural function. These advantages are not confined to the PNS and also apply to the central nervous system. Many diseases, including peripheral nerve injury, neuropathy, multiple sclerosis and spinal cord injury, are targets for Schwann cell therapy. The collection of Schwann cells, however, causes new damage to other peripheral nerve segments. Furthermore, the doubling time of Schwann cells is not very fast, and thus adequate amounts of Schwann cells for clinical use cannot be collected within a reasonable amount of time. Mesenchymal stem cells, which are highly proliferative, are easily accessible from various types of mesenchymal tissues, such as the bone marrow, umbilical cord and fat tissue. Because these cells have the ability to cross oligolineage boundaries between mesodermal to ectodermal lineages, they are capable of differentiating into Schwann cells with step-by-step cytokine stimulation. In this review, we summarize the properties of mesenchymal stem cell-derived Schwann cells, which are comparable to authentic Schwann cells, and discuss future perspectives.

Mammalian Glycerophosphodiester Phosphodiesterases

Bacterial glycerophosphodiester phosphodiesterases (GP-PDEs), GlpQ and UgpQ, are well-characterized periplasmic and cytosolic proteins that play critical roles in the hydrolysis of deacylated glycerophospholipids to glycerol phosphate and alcohol, which are utilized as major sources of carbon and phosphate. In contrast, two novel mammalian GP-PDEs, GDE1/MIR16 and GDE3, were recently identified, and were shown to be involved in several physiological functions. GDE1/MIR16 was identified as a membrane protein interacting with RGS16, a regulator of G protein signaling, and found to hydrolyze glycerophosphoinositol preferentially. We have found that expression of GDE3 is significantly up-regulated during osteoblast differentiation and is involved in morphological changes of cells. Furthermore, five mammalian GP-PDEs were virtually identified, and very recent studies indicate that retinoic acid-induced expression of GDE2 plays essential roles in neuronal differentiation and neurite outgrowth. Thus mammalian GP-PDEs are likely to be important in controlling numerous cellular events, indicating that the GP-PDE superfamily in mammals might be a pharmacological target in the future.

Chondroitin sulfate proteoglycans regulate astrocyte-dependent synaptogenesis and modulate synaptic activity in primary embryonic hippocampal neurons

It has been shown that astrocyte-derived extracellular matrix (ECM) is important for formation and maintenance of CNS synapses. In order to study the effects of glial-derived ECM on synaptogenesis, E18 rat hippocampal neurons and primary astrocytes were co-cultivated using a cell-insert system. Under these conditions, neurons differentiated under low density conditions (3500 cells/cm(2) ) in defined, serum-free medium and in the absence of direct, membrane-mediated neuron-astrocyte interactions. Astrocytes promoted the formation of structurally intact synapses, as documented by the co-localisation of bassoon- and ProSAP1/Shank2-positive puncta, markers of the pre- and postsynapse, respectively. The development of synapses was paralleled by the emergence of perineuronal net (PNN)-like structures that contained various ECM components such as hyaluronic acid, brevican and neurocan. In order to assess potential functions for synaptogenesis, the ECM was removed by treatment with hyaluronidase or chondroitinase ABC. Both enzymes significantly enhanced the number of synaptic puncta. Whole-cell voltage-clamp recordings of control and enzyme-treated hippocampal neurons revealed that chondroitinase ABC treatment led to a significant decrease in amplitude and a reduced charge of miniature excitatory postsynaptic currents, whereas inhibitory postsynaptic currents were not affected. When the response to the application of glutamate was measured, a reduced sensitivity could be detected and resulted in decreased currents in response to the excitatory neurotransmitter. These findings are consistent with the interpretation that the ECM partakes in the regulation of the density of glutamate receptors in subsynaptic sites.

Differentiation of neural stem cells into Schwann-like cells in vitro

Neural stem cells (NSCs) are multipotent stem cells that have the potential to differentiate into different cells of the neural lineage like neurons, astrocytes and oligodendrocytes. In the current work, we explored whether NSCs could be differentiated into functional Schwann-like cells, which has not been investigated up to date. NSCs were harvested from the hippocampus of rats and identified by single cell cloning and immunofluorescence staining. Then NSCs were treated with DMEM/F12 supplemented with forskolin, heregulin, bFGF, PDGF-AA and retinoic acid (RA). Differentiated NSCs (dNSCs) exhibited a spindle-like morphology similar to Schwann cells and expressed the glial markers p75 and S100. We also found that dNSCs could enhance neurite outgrowth when co-cultured with NG108-15 cells. These results indicated that NSCs, derived from hippocampus of rats, could differentiate into Schwann-like cells with morphological, phenotypic and functional similarities.

Human umbilical cord-derived mesenchymal stromal cells differentiate into functional Schwann cells that sustain peripheral nerve regeneration

Human umbilical cord-derived mesenchymal stromal cells (UC-MSCs) that are available from cell banks can be induced to differentiate into various cell types, thereby making them practical potential sources for cell-based therapies. In injured peripheral nerves, Schwann cells (SCs) contribute to functional recovery by supporting axonal regeneration and myelin reconstruction. Here, we first demonstrate a system to induce UC-MSCs to differentiate into cells with SC properties (UC-SCs) by treatment with β-mercaptoethanol followed by retinoic acid and a set of specific cytokines. The UC-SCs are morphologically similar to SCs and express SC markers, including P0, as assessed by immunocytochemistry and reverse transcription polymerase chain reaction. Transplantation of UC-SCs into transected sciatic nerves in adult rats enhanced nerve regeneration. The effectiveness of UC-SCs for axonal regeneration was comparable to that of authentic human SCs based on histological criteria and functional recovery. Immunohistochemistry and immunoelectron microscopy also demonstrated myelination of regenerated axons by UC-SCs. These findings indicate that cells with SC properties and with the ability to support axonal regeneration and reconstruct myelin can be successfully induced from UC-MSCs to promote functional recovery after peripheral nerve injury. This system may be applicable for the development of cell-based therapies.

Neurotrophin-3 improves retinoic acid-induced neural differentiation of skin-derived precursors through a p75NTR-dependent signaling pathway

Skin-derived precursors (SKPs) are derived from mesoblast and can differentiate into smooth muscle cells, adipocytes, and less neuronal phenotypes. This study demonstrates that retinoic acid (RA) improves SKPs exit from self-proliferation to neural differentiation through up-regulating of NeuroD and cell-cycle regulatory protein p21, meanwhile RA also induces p75 neurotrophin receptor (p75NTR) up-regulation and apoptosis of SKPs. When treated sequentially with neurotrophin-3 (NT-3) after RA induction, the survival and neural differentiation of SKPs were enhanced significantly, and cell apoptosis induced by RA was decreased. These effects could be reversed by p75NTR inhibitor Pep5 instead of Trk receptor inhibitor K252a. The results indicate that NT-3 improves the neural differentiation of SKPs induced by RA through a p75NTR-dependent signaling pathway.

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.

Neural Stem/Progenitor Cells Express 20 Tenascin C Isoforms That Are Differentially Regulated by Pax6

Tenascin C (Tnc) is an alternatively spliced, multimodular extracellular matrix glycoprotein present in the ventricular zone of the developing brain. Pax6-deficient small eye (sey) mouse mutants show an altered Tnc expression pattern. Here, we investigated the expression of Tnc isoforms in neural stem/progenitor cells and their regulation by the paired-box transcription factor Pax6. Neural stem/progenitor cells cultured as neurospheres strongly expressed Tnc on the protein level. The Tnc isoform expression in neural stem/progenitor cells was analyzed by reverse transcriptase-PCR and dot blot Southern hybridization. In total, 20 different Tnc isoforms were detected in neurospheres derived from embryonic fore-brain cell suspensions. The Tnc isoform containing the fibronectin type III domains A1A4BD is novel and might be neural stem/progenitor cell-specific. Transient overexpression of Pax6 in neurospheres of the medial ganglionic eminence did not alter the total Tnc mRNA expression level but showed a pronounced regulative effect on different Tnc isoforms. The larger Tnc isoforms containing four, five, and six additional alternatively spliced fibronectin type III domains were up-regulated, whereas the small Tnc isoforms without any or with one additional domain were down-regulated. Thus, Pax6 is a homeodomain protein that also modulates the splicing machinery. We conclude that the combinatorial code of Tnc isoform expression in the neural stem/progenitor cell is complex and regulated by Pax6. These findings suggest a functional significance for individual Tnc isoforms in neural stem/progenitor cells.

Involvement of membrane protein GDE2 in retinoic acid-induced neurite formation in Neuro2A cells

We show that a glycerophosphodiester phosphodiesterase homolog, GDE2, is widely expressed in brain tissues including primary neurons, and that the expression of GDE2 in neuroblastoma Neuro2A cells is significantly upregulated during neuronal differentiation by retinoic acid (RA) treatment. Stable expression of GDE2 resulted in neurite formation in the absence of RA, and GDE2 accumulated at the regions of perinuclear and growth cones in Neuro2A cells. Furthermore, a loss-of-function of GDE2 in Neuro2A cells by RNAi blocked RA-induced neurite formation. These results demonstrate that GDE2 expression during neuronal differentiation plays an important role for growing neurites.

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.

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.

Retinoic acid is required early during adult neurogenesis in the dentate gyrus

Retinoic acid (RA) is commonly used in vitro to differentiate stem cell populations including adult neural stem cells into neurons; however, the in vivo function of RA during adult neurogenesis remains largely unexplored. We found that depletion of RA in adult mice leads to significantly decreased neuronal differentiation within the granular cell layer of the dentate gyrus. RA contribution to neurogenesis occurs early, for RA deficiency also results in a decrease in newborn cells expressing an immature neuronal marker. Furthermore, although proliferation is unaffected during RA absence, cell survival is significantly reduced. Finally, a screen for retinoid-induced genes identifies metabolic targets including the lipid transporters, CD-36 and ABCA-1, the lipogenic master regulator SREBP1c as well as components of the Wnt signaling pathway. Our results reveal RA as a crucial contributor to early stages of adult neurogenesis and survival in vivo.

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 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.

Neurotrophin and GDNF family ligands promote survival and alter excitotoxic vulnerability of neurons derived from murine embryonic stem cells

Embryonic stem (ES) cells are genetically manipulable pluripotential cells that can be differentiated in vitro into neurons, oligodendrocytes, and astrocytes. Given their potential utility as a source of replacement cells for the injured nervous system and the likelihood that transplantation interventions might include co-application of growth factors, we examined the effects of neurotrophin and GDNF family ligands on the survival and excitotoxic vulnerability of ES cell-derived neurons (ES neurons) grown in vitro. ES cells were differentiated down a neural lineage in vitro using the 4-/4+ protocol (Bain et al., Dev Biol 168:342-57, 1995). RT-PCR demonstrated expression of receptors for neurotrophins and GDNF family ligands in ES neural lineage cells. Neuronal expression of GFRalpha1, GFRalpha2, and ret was confirmed by immunocytochemistry. Exposure to 30-100 ng/ml GDNF or neurturin (NRTN) resulted in activation of ret. Addition of NT-3 and GDNF did not increase cell division but did increase the number of neurons in the cultures 7 days after plating. Pretreatment with NT-3 enhanced the vulnerability of ES neurons to NMDA-induced death (100 microM NMDA for 10 min) and enhanced the NMDA-induced increase in neuronal [Ca2+]i, but did not alter expression of NMDA receptor subunits NR2A or NR2B. In contrast, pretreatment with GDNF reduced the vulnerability of ES neurons to NMDA-induced death while modestly enhancing the NMDA-induced increase in neuronal [Ca2+]i. These findings demonstrate that the response of ES-derived neurons to neurotrophins and GDNF family ligands is largely similar to that of other cultured central neurons.

Activation of retinoic acid signalling after sciatic nerve injury: up-regulation of cellular retinoid binding proteins

In mammalian peripheral nerves a crush lesion causes interactions between injured neurons, Schwann cells and haematogenous macrophages that can lead to successful axonal regeneration. We suggest that the transcriptional activator retinoic acid (RA), takes part in gene regulation after peripheral nerve injury and that RA signalling is activated via the cellular retinoic acid binding protein (CRABP)-II and cellular retinol binding protein (CRBP)-I. With RT-PCR and immunoblotting all necessary components of the RA signalling pathway were detected in the sciatic nerve of adult rats. These are retinoic acid receptors, retinoid X receptors, the retinoic acid synthesizing enzymes RALDH-1, RALDH-2, and RALDH-3, in addition, the cellular retinoid binding proteins CRBP-I, CRABP-I and CRABP-II. Enzyme activity of RALDH-2 was detectable in the nerve, and using a transgenic reporter mouse we found local activation of RA responsive elements in the regenerating nerve. Sciatic nerve crush as well as transection resulted in a more than 10-fold up-regulation of CRBP-I, which is thought to facilitate the synthesis of RA. Both kinds of injury also caused a 15-fold increase in transcript and protein concentration of CRABP-II, a possible mediator of RA transfer to its nuclear receptors.

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.

Sciatic nerve regeneration in rats induced by transplantation of in vitro differentiated bone-marrow stromal cells

Bone marrow stromal cells (MSCs) are multipotent stem cells that have the potential to differentiate into bone, cartilage, fat and muscle. We now demonstrate that MSCs can be induced to differentiate into cells with Schwann cell characteristics, capable of eliciting peripheral nervous system regeneration in adult rats. MSCs treated with beta-mercaptoethanol followed by retinoic acid and cultured in the presence of forskolin, basic-FGF, PDGF and heregulin, changed morphologically into cells resembling primary cultured Schwann cells and expressing p75, S-100, GFAP and O4. The MSCs were genetically engineered by transduction with retrovirus encoding green fluorescent protein (GFP), and then differentiated by treatment with factors described above. They were transplanted into the cut ends of sciatic nerves, which then responded with vigorous nerve fibre regeneration within 3 weeks of the operation. Myelination of regenerated fibers by GFP-expressing MSCs was recognized using confocal and immunoelectron microscopy. The results suggest that MSCs are able to differentiate into myelinating cells, capable of supporting nerve fibre re-growth, and they can therefore be applied to induce nerve regeneration.

Retinoic acid combined with neurotrophin-3 enhances the survival and neurite outgrowth of embryonic sympathetic neurons

Both nerve growth factor (NGF) and neurotrophin-3 (NT-3) are necessary for the survival of embryonic sympathetic neurons in vivo. All-trans retinoic acid (atRA) has been shown to promote neurite outgrowth and long-term survival of chick embryonic sympathetic neurons cultured in the presence of NGF. The present study shows that atRA can also potentiate the survival and neurite outgrowth-promoting activities of NT-3. This was accomplished by enhancing the survival of existing neurons, as cell proliferation was unaffected by exposure to atRA. atRA also enhanced neurite outgrowth of the NT-3-treated cells; however, the neurites appeared thicker and less branched than cells treated with atRA in combination with NGF. Using a quantitative PCR assay, trkA and p75(NTR) mRNAs, but not trkC mRNA, were increased ( approximately 1.5- to 2-fold) after 72 and 48 hr of exposure of the cultures to atRA, respectively. The atRA-induced increase in trkA mRNA may play a role in the enhanced survival of neurons cultured in the presence of either NGF or NT-3, as both neurotrophins have been shown to signal through this receptor. The time course of these mRNA changes would indicate that atRA does not regulate the neurotrophin receptor mRNA directly, rather, intervening gene transcription is required. Thus, during development, atRA may play a role in fine-tuning embryonic responsiveness to both NT-3 and NGF.

Sequential treatment of SH-SY5Y cells with retinoic acid and brain-derived neurotrophic factor gives rise to fully differentiated, neurotrophic factor-dependent, human neuron-like cells

A rapid and simple procedure is presented to obtain nearly pure populations of human neuron-like cells from the SH-SY5Y neuroblastoma cell line. Sequential exposure of SH-SY5Y cells to retinoic acid and brain-derived neurotrophic factor in serum-free medium yields homogeneous populations of cells with neuronal morphology, avoiding the presence of other neural crest derivatives that would normally arise from those cells. Cells are withdrawn from the cell cycle, as shown by 5-bromo-2'-deoxyuridine uptake and retinoblastoma hypophosphorylation. Cell survival is dependent on the continuous presence of brain-derived neurotrophic factor, and removal of this neurotrophin causes apoptotic cell death accompanied by an attempt to reenter the cell cycle. Differentiated cells express neuronal markers, including neurofilaments, neuron-specific enolase, and growth-associated protein-43 as well as neuronal polarity markers such as tau and microtubule-associated protein 2. Moreover, differentiated cultures do not contain glial cells, as could be evidenced after the negative staining for glial fibrillary acidic protein. In conclusion, the protocol presented herein yields homogeneous populations of human neuronal differentiated cells that present many of the characteristics of primary cultures of neurons. This model may be useful to perform large-scale biochemical and molecular studies due to its susceptibility to genetic manipulation and the availability of an unlimited amount of cells.

Retinoic acid potentiated the protective effect of NGF against staurosporine-induced apoptosis in cultured chick neurons by increasing the trkA protein expression

Nerve growth factor (NGF) has already been shown to protect neurons and PC12 cells from cell death induced by different stimuli. When chick embryonic neurons were exposed to staurosporine (200 nM, 24 hr), the percentage of apoptotic neurons increased from 15% in controls to 80%, but the treatment with NGF alone did not show any neuroprotection. In the presence of retinoic acid (RA, 5 microM), however, NGF (20 pg/ml) reduced staurosporine-induced damage to 42% apoptotic neurons compared to 58% in the presence of RA (5 icroM) alone. TrkA protein expression in chick neurons was markedly reduced by staurosporine, but was found to be increased in the presence of RA and NGF compared with the treatment with staurosporine alone. The antiapoptotic effect caused by RA and NGF was abolished by the tyrosine kinase inhibitor K-252a, as well as by anti-trkA antibodies and anti-NGF antibodies suggesting that the increase in trkA protein expression contributed to its mechanism of action. In addition, RA-enhanced 2.6-fold the NGF secretion from cultured rat cortical astrocytes and conditioned medium of RA-treated astrocytes reduced the percentage of apoptotic chick neurons after a 24 hr-incubation with staurosporine in the same manner as the external addition of RA and NGF. Increasing the endogenous synthesis of growth factors as well as the expression of their receptors by small, blood-brain barrier-permeable drugs was suggested as a promising concept for neuroprotection.

Brain-derived neurotrophic factor promotes survival and chemoprotection of human neuroblastoma cells

Brain-derived neurotrophic factor (BDNF) promotes neuronal survival and protection against neuronal damage. We addressed whether BDNF might promote survival and chemoprotection in neuroblastoma (NB) using a drug-sensitive human NB cell line. All-trans-retinoic acid (ATRA) induces a striking phenotypic differentiation of NB1643 cells, and exogenous BDNF treatment promotes survival of these differentiated cells. ATRA induces TRKB expression, and exogenous BDNF stimulates both autophosphorylation of TRKB and induction of the immediate early gene, FOS, in these cells. BDNF mRNA is expressed in NB1643 cells. Because the time course of TRKB induction closely parallels phenotypic differentiation of these cells, it seems probable that ATRA induces differentiation of NB1643 cells by establishing an autocrine loop involving BDNF and TRKB. Exogenous BDNF treatment resulted in a further increase in neurite outgrowth, which again suggests that an autocrine loop is involved in differentiation of NB1643 cells in response to ATRA. We then tested whether BDNF might afford drug resistance in NB and found that BDNF does indeed protect in this NB model against cisplatin, a DNA-damaging agent actually used in the treatment of NB.

Regulation of neurotrophin receptor expression by retinoic acid in mouse sympathetic neuroblasts

We have studied the effect of retinoic acid on the expression of the neurotrophin receptors trkA, trkC, and p75 by neuroblasts and neurons at different axial levels along the embryonic mouse paravertebral sympathetic chain. In dissociated cultures of sympathetic neuroblasts, retinoic acid inhibited the developmental increase in trkA mRNA expression and the developmental decrease in trkC mRNA expression that normally occurs in these cells but did not affect p75 mRNA expression. At higher concentrations, retinoic acid also increased the proliferation of sympathetic neuroblasts. After sympathetic neuroblasts became postmitotic, retinoic acid no longer affected receptor expression. Studies with retinoic acid receptor agonists and antagonists indicated that the effects of retinoic acid on neurotrophin receptor expression were mediated mainly by alpha retinoic acid receptors, not beta or gamma receptors. The observation that alpha-antagonists increased trkA mRNA expression in intact sympathetic ganglion explants suggests that endogenous retinoic acid is a physiological regulator of trkA receptor expression.

Segment-specific pattern of sympathetic preganglionic projections in the chicken embryo spinal cord is altered by retinoids

Sympathetic preganglionic neurons exhibit segment-specific projections. Preganglionic neurons located in rostral spinal segments project rostrally within the sympathetic chain, those located in caudal spinal segments project caudally, and those in midthoracic segments project either rostrally or caudally in segmentally graded proportions. Moreover, rostrally and caudally projecting preganglionic neurons are skewed toward the rostral and caudal regions, respectively, of each midthoracic segment. The mechanisms that establish these segment-specific projections are unknown. Here we show that experimental manipulation of retinoid signaling in the chicken embryo alters the segment-specific pattern of sympathetic preganglionic projections and that this effect is mediated by the somitic mesoderm. Application of exogenous retinoic acid to a single rostral thoracic somite decreases the number of rostrally projecting preganglionic neurons at that level. Conversely, disrupting endogenous synthesis of retinoic acid in a single caudal thoracic somite increases the number of rostrally projecting preganglionic neurons at that level. The number of caudally projecting neurons does not change in either case, indicating that the effect is specific for rostrally projecting preganglionic neurons. These results indicate that the sizes of the rostrally and caudally projecting populations may be independently regulated by different factors. Opposing gradients of such factors along the longitudinal axis of the thoracic region of the embryo could be sufficient, in combination, to determine the segment-specific identity of preganglionic projections.

Experimental cisplatin neuronopathy in rats and the effect of retinoic acid administration

Peripheral nervous system alterations were induced in adult rats by administration of cisplatin (CDDP) 2 mg/kg twice weekly for 4.5 weeks. Dorsal root ganglion neurons showed pathological changes. Morphometric determinations demonstrated a reduction in size of the somatic, nuclear and nucleolar area. The nucleoli were the most involved subcellular structures. Nerve conduction velocity and the tail-flick test for pain were both significantly altered in CDDP treated rats, whereas the rota-rod test for coordination revealed no changes in either treated or control rats. Analytical determinations demonstrated platinum accumulation in the DRG of CDDP treated rats. Spontaneous recovery, demonstrated by morphometric, electrophysiological, functional and analytical determinations, occurred after treatment discontinuation within about 7 weeks. A pilot study of the possible neuroprotective action of retinoic acid (RA) was also performed with this model of cisplatin neuronopathy. The rationale for using RA was based on its assumed antioxidant and neurotrophic effect. However, RA failed to prevent morphometric, electrophysiological, functional and analytical alterations induced by CDDP on DRG neurons. RA induced only a mild generalized protective effect.

Glycine receptors in cultured chick sympathetic neurons are excitatory and trigger neurotransmitter release

1. Total RNA isolated from embryonic chick paravertebral sympathetic ganglia was used in a reverse transcription-polymerase chain reaction (RT-PCR) assay with a pair of degenerate oligonucleotide primers deduced from conserved regions of mammalian glycine receptor alpha-subunits. Three classes of cDNA were identified which encode portions of the chicken homologues of the mammalian glycine receptor alpha 1, alpha 2 and alpha 3 subunits. 2. The presence of functional glycine receptors was investigated in the whole-cell configuration of the patch-clamp technique in neurons dissociated from the ganglia and kept in culture for 7-8 days. In cells voltage clamped to -70 mV, glycine consistently induced inward currents in a concentration-dependent manner and elicited half-maximal peak current amplitudes at 43 microM. 3. The steady-state current-voltage relation for glycine-induced currents was linear between +80 and -60 mV, but showed outward rectification at more hyperpolarized potentials. Reversal potentials of these currents shifted with changes in intracellular chloride concentrations and matched the calculated Nernst potentials for chloride. 4. beta-Alanine and taurine were significantly less potent than glycine in triggering inward currents, with half-maximal responses at 79 and 86 microM, respectively. At maximally active concentrations, beta-alanine-evoked currents were identical in amplitude to those induced by glycine. Taurine-evoked currents, in contrast, never reached the same amplitude as glycine-induced currents. 5. The classical glycine receptor antagonist strychnine reversibly reduced glycine-induced currents, with half-maximal inhibition occurring at 62 nM. Two more recently characterized glycine receptor antagonists, isonipecotic acid (half-maximal inhibition at 2 mM) and 7-trifluoromethyl-4-hydroxyquinoline-3-carboxylic acid (half-maximal inhibition at 67 microM), also blocked glycine-evoked currents in a reversible manner. The chloride channel blocker picrotoxin reduced glycine-evoked currents, with half-maximal effects at 348 microM. Inhibition by the glycine receptor channel blocker cyanotriphenylborate was half-maximal at 4 microM. 6. Apart from evoking inward currents, glycine occasionally triggered short (< 100 ms) spike-like currents which were abolished by hexamethonium and thus reflected synaptic release of endogenous acetylcholine. In addition, glycine caused Ca(2+)-dependent and tetrodotoxin-sensitive tritium overflow from neurons previously labelled with [3H]noradrenaline. This stimulatory action of glycine was reduced in the presence of strychnine and after treatment with the chloride uptake inhibitor furosemide (frusemide). 7. In 65% of neurons loaded with the Ca2+ indicator fura-2 acetoxymethyl ester, glycine increased the ratio of the fluorescence signal obtained with excitation wavelengths of 340 and 380 nm, respectively, which indicates a rise in intracellular Ca2+ concentration. 8. The results show that sympathetic neurons contain transcripts for different glycine receptor alpha-subunits and carry functional heteromeric glycine receptors which depolarize the majority of neurons to trigger transmitter release.

TrkA expression levels of sympathetic neurons correlate with NGF-dependent survival during development and after treatment with retinoic acid

Sympathetic neurons depend on the classical neurotrophin nerve growth factor (NGF) for survival by the time they innervate their targets, but not before. The acquisition of NGF responsiveness is thought to be controlled by environmental cues in sympathetic neurons. We have investigated the expression of the signal transducing NGF receptor trkA on mRNA and protein level during development of chick sympathetic neurons obtained from lumbosacral, paravertebral chain ganglia between embryonic days (E) 6.5 and 10. We demonstrate that trkA mRNA levels increase between E6.5 and E10, whereas the levels of trkC and p75 do not change. We also observed a similar increase in trkA protein during this time period. This increase correlates with the increase in NGF-dependent survival of sympathetic neurons from the corresponding stages in vitro. To define the correlation between trkA expression and NGF-mediated survival in more detail, trkA expression was adjusted to different levels by treatment with increasing concentrations of retinoic acid. We observed that small changes of trkA mRNA expression levels, below one order of magnitude, are decisive for the ability of immature sympathetic neurons to survive in the presence of NGF. A small and transient increase in trkA mRNA expression was also elicited in vivo by application of retinoids. These data provide evidence that sympathetic neurons upregulate the NGF receptor trkA and in this way acquire NGF-dependency.

Neurotrophin switching: where does it stand?

In vitro and in vivo studies suggest that certain populations of neurons switch their survival requirements from one neurotrophin to another during an early stage in their development. Although there is good evidence for neurotrophin switching in sensory neurons, the evidence for switching in sympathetic neurons has become more controversial, as has the identity of the factors that regulate their responsiveness to particular neurotrophins.