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

The Pleiotropic Role of Retinoic Acid/Retinoic Acid Receptors Signaling: From Vitamin A Metabolism to Gene Rearrangements in Acute Promyelocytic Leukemia

The family of retinoic acid receptors (RARs: RARα, -β, and -γ) has remarkable pleiotropy characteristics, since the retinoic acid/RARs pathway is involved in numerous biological processes not only during embryonic development, but also in the postnatal phase and during adulthood. In this review, we trace the roles of RA/RARs signaling in the immune system (where this pathway has both an immunosuppressive role or is involved in the inflammatory response), in hematopoiesis (enhancing hematopoietic stem cell self-renewal, progenitor cells differentiation or maintaining the bone marrow microenvironment homeostasis), and in bone remodeling (where this pathway seems to have controversial effects on bone formation or osteoclast activation). Moreover, in this review is shown the involvement of RAR genes in multiple chromosomal rearrangements generating different fusion genes in hematological neoplasms, with a particular focus on acute promyelocytic leukemia and its variant subtypes. The effect of different RARs fusion proteins on leukemic transformation, on patients’ outcome, and on therapy response is also discussed.

Genomic and non-genomic pathways are both crucial for peak induction of neurite outgrowth by retinoids

Retinoic acid (RA) is the active metabolite of vitamin A and essential for many physiological processes, particularly the induction of cell differentiation. In addition to regulating genomic transcriptional activity via RA receptors (RARs) and retinoid X receptors (RXRs), non-genomic mechanisms of RA have been described, including the regulation of ERK1/2 kinase phosphorylation, but are poorly characterised. In this study, we test the hypothesis that genomic and non-genomic mechanisms of RA are regulated independently with respect to the involvement of ligand-dependent RA receptors. A panel of 28 retinoids (compounds with vitamin A-like activity) showed a marked disparity in genomic (gene expression) versus non-genomic (ERK1/2 phosphorylation) assays. These results demonstrate that the capacity of a compound to activate gene transcription does not necessarily correlate with its ability to regulate a non-genomic activity such as ERK 1/2 phosphorylation. Furthermore, a neurite outgrowth assay indicated that retinoids that could only induce either genomic, or non-genomic activities, were not strong promoters of neurite outgrowth, and that activities with respect to both transcriptional regulation and ERK1/2 phosphorylation produced maximum neurite outgrowth. These results suggest that the development of effective retinoids for clinical use will depend on the selection of compounds which have maximal activity in non-genomic as well as genomic assays.

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

Non-classical Transcriptional Activity of Retinoic Acid

It has long been established that the transcriptional activity of retinoic acid (RA) is mediated by members of the nuclear receptor family of ligand-activated transcription factors termed RA receptors (RARs). More recent observations have established that RA also activates an additional nuclear receptor, PPARβ/δ. Partitioning RA between RARs and PPARβ/δ is governed by different intracellular lipid-binding proteins: cellular RA binding protein 2 (CRABP2) delivers RA to nuclear RARs and a fatty acid binding protein (FABP5) delivers the hormone from the cytosol to nuclear PPARβ/δ. Consequently, RA signals through RARs in CRABP2-expressing cells, but activates PPARβ/δ in cells that express a high level of FABP5. RA elicits different and sometimes opposing responses in cells that express different FABP5/CRABP2 ratios because PPARβ/δ and RARs regulate the expression of distinct sets of genes. An overview of the observations that led to the discovery of this non-classical activity of RA are presented here, along with a discussion of evidence demonstrating the involvement of the dual transcriptional activities of RA in regulating energy homeostasis, insulin responses, and adipocyte and neuron differentiation.

The orchestra of lipid-transfer proteins at the crossroads between metabolism and signaling

Within the eukaryotic cell, more than 1000 species of lipids define a series of membranes essential for cell function. Tightly controlled systems of lipid transport underlie the proper spatiotemporal distribution of membrane lipids, the coordination of spatially separated lipid metabolic pathways, and lipid signaling mediated by soluble proteins that may be localized some distance away from membranes. Alongside the well-established vesicular transport of lipids, non-vesicular transport mediated by a group of proteins referred to as lipid-transfer proteins (LTPs) is emerging as a key mechanism of lipid transport in a broad range of biological processes. More than a hundred LTPs exist in humans and these can be divided into at least ten protein families. LTPs are widely distributed in tissues, organelles and membrane contact sites (MCSs), as well as in the extracellular space. They all possess a soluble and globular domain that encapsulates a lipid monomer and they specifically bind and transport a wide range of lipids. Here, we present the most recent discoveries in the functions and physiological roles of LTPs, which have expanded the playground of lipids into the aqueous spaces of cells.

Curcumin restores sensitivity to retinoic acid in triple negative breast cancer cells

Background

A major obstacle in the use of retinoid therapy in cancer is the resistance to this agent in tumors. Retinoic acid facilitates the growth of mammary carcinoma cells which express high levels of fatty acid-binding protein 5 (FABP5). This protein delivers retinoic acid to peroxisome proliferator-activated receptor β/δ (PPARβ/δ) that targets genes involved in cell proliferation and survival. One approach to overcome resistance of mammary carcinoma cells to retinoic acid is to target and suppress the FABP5/ PPARβ/δ pathway. The objective of this research was to investigate the effect of curcumin, a polyphenol extract from the plant Curcuma longa, on the FABP5/ PPARβ/δ pathway in retinoic acid resistant triple negative breast cancer cells.

Methods

Cell viability and proliferation of triple negative breast cancer cell lines (MDA-MB-231 and MD-MB-468) treated with curcumin and/or retinoic was analyzed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and 5-bromo-2’-deoxyuridine (BrdU). Expression level of FABP5 and PPARβ/δ in these cells treated with curcumin was examined by Western Blotting analysis and Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR). Effect of curcumin and retinoic acid on PPARβ/δ target genes, PDK1and VEGF-A were also examined using qRT-PCR. Western Blotting was utilized to examine the protein expression level of the p65 subunit of NF-κB.

Results

Treatment of retinoic acid resistant triple negative breast cancer cells with curcumin sensitized these cells to retinoic acid mediated growth suppression, as well as suppressed incorporation of BrdU. Further studies demonstrated that curcumin showed a marked reduction in the expression level of FABP5 and PPARβ/δ. We provide evidence that curcumin suppresses p65, a transcription factor known to regulate FABP5. The combination of curcumin with retinoic acid suppressed PPARβ/δ target genes, VEGF-A and PDK1.

Conclusions

Curcumin suppresses the expression level of FABP5 and PPARβ/δ in triple negative mammary carcinoma cells. By targeting the FABP5/PPARβ/δ pathway, curcumin prevents the delivery of retinoic acid to PPARβ/δ and suppresses retinoic acid-induced PPARβ/δ target gene, VEGF-A. Our data demonstrates that suppression of the FABP5/ PPARβ/δ pathway by curcumin sensitizes retinoic acid resistant triple negative breast cancer cells to retinoic acid mediated growth suppression.

Retinoids: novel immunomodulators and tumour-suppressive agents?

Retinoids play important roles in the transcriptional activity of normal, degenerative and tumour cells. Retinoid analogues may be promising therapeutic agents for the treatment of immune disorders as different as type I diabetes and systemic lupus erythematosus. In addition, the use of retinoids in cancer treatment has progressed significantly in the last two decades; thus, numerous retinoid compounds have been synthesized and tested. In this paper, the actual or potential use of retinoids as immunomodulators or tumour-suppressive agents is discussed.

Quantification of retinoid concentrations in human serum and brain tumor tissues

Retinoic acid signaling is essential for central nervous system (CNS) differentiation and appears to be impaired in tumors. Thus far, there are no established methods to quantify relevant retinoids (all-trans-retinoic acid, 9-cis-retinoic acid, 13-cis retinoic acid, and retinol) in human brain tumors. We developed a single step extraction and quantification procedure for polar and apolar retinoids in normal tissue, lipid-rich brain tumor tissues, and serum. This quantification procedure is based on high performance liquid chromatography (HPLC) with diode-array detection (DAD) using all-trans-acitretin as an internal standard and extraction by liquid-liquid partition with ethyl acetate and borate buffer at pH 9. Recovery with this extraction procedure was higher than earlier (two-step) liquid-liquid extraction procedures based on hexane, NaOH, and HCl. The overall quantification procedure was validated according to Food and Drug Administration (FDA) guidelines and fulfilled all criteria of accuracy, precision, selectivity, recovery, and stability. The overall method accuracy varied between -5.6% and +5.4% for serum and -3.8% and +6.2% for tissues, and overall precision ranged from 3.1% to 6.9% for serum and 2.1% to 8.3% for tissues (%CV batch-to-batch). The lower limit of quantification for all compounds in tumor tissue (and serum) was 3.9 ng g(-1) (ng mL(-1)). Using this assay, photodegradation of the retinoids was evaluated and endogenous polar and apolar retinoids were quantified in sera and brain tumor tissues of patients and compared with serum and tonsil tissue concentrations of controls. It may thus serve as a suitable method for the characterization of retinoid uptake and metabolism in the respective compartments.

Retinoic acid-dependent signaling pathways and lineage events in the developing mouse spinal cord

Studies in avian models have demonstrated an involvement of retinoid signaling in early neural tube patterning. The roles of this signaling pathway at later stages of spinal cord development are only partly characterized. Here we use Raldh2-null mouse mutants rescued from early embryonic lethality to study the consequences of lack of endogenous retinoic acid (RA) in the differentiating spinal cord. Mid-gestation RA deficiency produces prominent structural and molecular deficiencies in dorsal regions of the spinal cord. While targets of Wnt signaling in the dorsal neuronal lineage are unaltered, reductions in Fibroblast Growth Factor (FGF) and Notch signaling are clearly observed. We further provide evidence that endogenous RA is capable of driving stem cell differentiation. Raldh2 deficiency results in a decreased number of spinal cord derived neurospheres, which exhibit a reduced differentiation potential. Raldh2-null neurospheres have a decreased number of cells expressing the neuronal marker β-III-tubulin, while the nestin-positive cell population is increased. Hence, in vivo retinoid deficiency impaired neural stem cell growth. We propose that RA has separable functions in the developing spinal cord to (i) maintain high levels of FGF and Notch signaling and (ii) drive stem cell differentiation, thus restricting both the numbers and the pluripotent character of neural stem cells.

Aberrant expression of retinoic acid signaling molecules influences patient survival in astrocytic gliomas

Undifferentiated cell populations may influence tumor growth in malignant glioma. We investigated potential disruptions in the retinoic acid (RA) differentiation pathway that could lead to a loss of differentiation capacity, influencing patient prognosis. Expression of key molecules belonging to the RA differentiation pathway was analyzed in 283 astrocytic gliomas and was correlated with tumor proliferation, tumor differentiation, and patient survival. In addition, in situ concentrations of retinoids were measured in tumors, and RA signaling events were studied in vitro. Unlike other tumors, in gliomas expression of most RA signaling molecules increased with malignancy and was associated with augmented intratumoral retinoid levels in high-grade gliomas. Aberrantly expressed RA signaling molecules included i) the retinol-binding protein CRBP1, which facilitates cellular retinoid uptake; ii) ALDH1A1, capable of activating RA precursors; iii) the RA-degrading enzyme CYP26B1; and iv) the RA-binding protein FABP5, which can inhibit RA-induced differentiation. In contrast, expression of the RA-binding protein CRABP2, which fosters differentiation, was decreased in high-grade tumors. Moreover, expression of CRBP1 correlated with tumor proliferation, and FABP5 expression correlated with an undifferentiated tumor phenotype. CRBP1 and ALDH1A1 were independent prognostic markers for adverse patient survival. Our data indicate a complex and clinically relevant deregulation of RA signaling, which seems to be a central event in glioma pathogenesis.

The MYCN oncogene and differentiation in neuroblastoma

Childhood neuroblastoma exhibits a heterogeneous clinical behavior ranging from low-risk tumors with the ability to spontaneously differentiate and regress, to high-risk tumors causing the highest number of cancer related deaths in infants. Amplification of the MYCN oncogene is one of the few prediction markers for adverse outcome. This gene encodes the MYCN transcriptional regulator predominantly expressed in the developing peripheral neural crest. MYCN is vital for proliferation, migration and stem cell homeostasis while decreased levels are associated with terminal neuronal differentiation. Interestingly, high-risk tumors without MYCN amplification frequently display increased c-MYC expression and/or activation of MYC signaling pathways. On the other hand, downregulation of MYCN leads to decreased proliferation and differentiation, emphasizing the importance of MYC signaling in neuroblastoma biology. Furthermore, expression of the neurotrophin receptor TrkA is associated with good prognosis, the ability to differentiate and spontaneous regression while expression of the related TrkB receptor is correlated with bad prognosis and MYCN amplification. Here we discuss the role of MYCN in neuroblastoma with a special focus on the contribution of elevated MYCN signaling for an aggressive and undifferentiated phenotype as well as the potential of using MYCN as a therapeutic target.

Vitamin A in reproduction and development

The requirement for vitamin A in reproduction was first recognized in the early 1900's, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.

Between death and survival: retinoic acid in regulation of apoptosis

The vitamin A metabolite all-trans-retinoic acid (RA) regulates multiple biological processes by virtue of its ability to regulate gene expression. It thus plays critical roles in embryonic development and is involved in regulating growth, remodeling, and metabolic responses in adult tissues. RA can also suppress carcinoma cell growth and is currently used in treatment of some cancers. Growth inhibition by RA may be exerted by induction of differentiation, cell cycle arrest, or apoptosis, or by a combination of these activities. Paradoxically, in the context of some cells, RA not only fails to inhibit growth but, instead, enhances proliferation and survival. This review focuses on the involvement of RA in regulating apoptotic responses. It includes brief overviews of transcriptional signaling by RA and of apoptotic pathways, and then addresses available information on the mechanisms by which RA induces apoptosis or, conversely, inhibits cell death and enhances survival.

Nav2 is necessary for cranial nerve development and blood pressure regulation

BACKGROUND

All-trans retinoic acid (atRA) is required for nervous system development, including the developing hindbrain region. Neuron navigator 2 (Nav2) was first identified as an atRA-responsive gene in human neuroblastoma cells (retinoic acid-induced in neuroblastoma 1, Rainb1), and is required for atRA-mediated neurite outgrowth. In this paper, we explore the importance of Nav2 in nervous system development and function in vivo.

RESULTS

Nav2 hypomorphic homozygous mutants show decreased survival starting at birth. Nav2 mutant embryos show an overall reduction in nerve fiber density, as well as specific defects in cranial nerves IX (glossopharyngeal) and X (vagus). Nav2 hypomorphic mutant adult mice also display a blunted baroreceptor response compared to wild-type controls.

CONCLUSIONS

Nav2 functions in mammalian nervous system development, and is required for normal cranial nerve development and blood pressure regulation in the adult.

Detection of endogenous retinoids in the molluscan CNS and characterization of the trophic and tropic actions of 9-cis retinoic acid on isolated neurons

Retinoic acid (RA) is an active metabolite of Vitamin A that plays an important role in the growth and differentiation of many cell types. All-trans RA (atRA) is the retinoic acid isomer that has been most widely studied in the nervous system, and can induce and direct neurite outgrowth from both vertebrate and invertebrate preparations. The presence and role of the 9-cis-RA isomer in the nervous system is far less well defined. Here, we used high-pressure liquid chromatography (HPLC) and mass spectrometry (MS) to show for the first time, the presence of both atRA and 9-cis-RA in the CNS of an invertebrate. We then demonstrated that 9-cis-RA was capable of exerting the same neurotrophic and chemotropic effects on cultured neurons as atRA. In this study, significantly more cells showed neurite outgrowth in 9-cis-RA versus the EtOH vehicle control, and 9-cis-RA significantly increased the number and length of neurites from identified neurons after 4 d in culture. 9-cis-RA also extended the duration of time that cells remained electrically excitable in culture. Furthermore, we showed for the first time in any species, that exogenous application of 9-cis-RA induced positive growth cone turning of cultured neurons. This study provides the first evidence for the presence of both atRA and 9-cis-RA in an invertebrate CNS and also provides the first direct evidence for a potential physiological role for 9-cis-RA in neuronal regeneration and axon pathfinding.

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.

All-trans retinoic acid arrests neuroblastoma cells in a dormant state. Subsequent nerve growth factor/brain-derived neurotrophic factor treatment adds modest benefit

BACKGROUND

Therapies aiming at inducing differentiation or apoptosis of neuroblastoma (NB) are an important research topic. Although retinoic acid showed promising antitumoral results, its effects against refractory disease are limited. Putative candidates for combination therapies are nerve growth factor (NGF; Tebu-Bio/Peprotech, Offenbach, Germany) and brain-derived neurotrophic factor (BDNF; Tebu-Bio/Peprotech, Offenbach, Germany) because their receptors are of prognostic clinical value in clinical neuroblastoma. Another clinical prognostic factor is the number of Schwann cells. Substances secreted by Schwann cells proved antitumoral capacities in vitro. The aim of the study was to analyze whether retinoic acid may offer an additional line of attack acting independent from Schwann cells and whether additive treatment with the neurotrophin-receptor ligands NGF/BDNF confers additional benefit.

METHODS

Human SHSY-5Y NB cells were cultured in vitro. After a 7-day all-trans retinoic acid (ATRA; Sigma-Aldrich Chemie, Taufkirchen, Germany) treatment (15 mumol/L of ATRA), NB proliferation was proportional to extinction in dimethyl-thiazol-diphenyltetrazoliumbromide (MTT) tests. Fluorescence-activated cell sorter (FACS) analysis for annexin and propidium iodide determined the degree of apoptosis and necrosis as well as the expression of the Schwann type cell marker S100. The S100 messenger RNA was assessed by reverse transcriptase polymerase chain reaction. In addition, the effect on NB proliferation was investigated when ATRA was combined with a 7-day treatment with NGF or BDNF (10, 50, 100 ng/mL) either before or after the 7-day ATRA treatment.

RESULTS

All-trans retinoic acid reduced proliferation (0.116 +/- 0.006 SEM vs 0.359 +/- 0.010 SEM in the untreated control group; P < .001). After ATRA treatment, 95% +/- 1.82% SEM were still viable, with only 2.61% +/- 1.17% SEM apoptotic and 2.38% +/- 0.69% SEM necrotic cells. All-trans retinoic acid induced a remarkable decrease in S100 expression in FACS (16.91% +/- 1.72% SEM vs 32.33% +/- 2.54% SEM in controls; P = .009). The S100 messenger RNA levels were not increased by ATRA (DeltaDeltaT values: 1.73, 2.77, and 1.43; n = 3). Both NGF and BDNF had only a modest synergistic effect when given after ATRA treatment. No effect was seen when they were administered before ATRA treatment.

CONCLUSIONS

All-trans retinoic proved to be a vigorous inhibitor of NB proliferation in vitro. However, because most NB cells remained viable combination therapies are required. Treatment with NGF and BDNF showed only a modest benefit and did not reflect the strong prognostic impact of tyrosine kinase receptors in clinical NB. The ATRA-induced proliferation arrest is not related to Schwann type subdifferentiation. This suggests that substances secreted by Schwann cells could be possible independent combination partners. We suggest studies using combinations of ATRA and substances secreted by Schwann cells.

Retinoic acid enhances Erk phosphorylation in the chick retina

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

Opposing effects of retinoic acid on cell growth result from alternate activation of two different nuclear receptors

Transcriptional activation of the nuclear receptor RAR by retinoic acid (RA) often leads to inhibition of cell growth. However, in some tissues, RA promotes cell survival and hyperplasia, activities that are unlikely to be mediated by RAR. Here, we show that, in addition to functioning through RAR, RA activates the "orphan" nuclear receptor PPARbeta/delta, which, in turn, induces the expression of prosurvival genes. Partitioning of RA between the two receptors is regulated by the intracellular lipid binding proteins CRABP-II and FABP5. These proteins specifically deliver RA from the cytosol to nuclear RAR and PPARbeta/delta, respectively, thereby selectively enhancing the transcriptional activity of their cognate receptors. Consequently, RA functions through RAR and is a proapoptotic agent in cells with high CRABP-II/FABP5 ratio, but it signals through PPARbeta/delta and promotes survival in cells that highly express FABP5. Opposing effects of RA on cell growth thus emanate from alternate activation of two different nuclear receptors.

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.

Regulation of retinoic acid receptors alpha, beta and retinoid X receptor alpha after sciatic nerve injury

Cell culture experiments indicated that activation of the retinoic acid signaling system is involved in axonal regeneration. This hypothesis was tested with sciatic nerve injury in the rat. Since the effect of retinoic acid is mediated via retinoic acid receptors and retinoid X receptors, we investigated mRNA and protein expression of these receptors during injury-induced degeneration and regeneration. Seven days after crush injury, transcript concentrations of all retinoic acid receptors and of retinoid X receptor alpha were significantly higher than in non-lesioned nerves. Protein levels of retinoic acid receptor alpha, retinoic acid receptor beta and retinoid X receptor alpha were upregulated 4, 7 and 14 days after injury. In degenerating nerves a significant increase of retinoic acid receptor alpha was detected 7 and 14 days, and of retinoic acid receptor beta 14 and 21 days after complete transection. Immunohistochemical staining of retinoid receptors revealed their expression in Schwann cells and macrophages. In addition, we observed that retinoic acid receptor alpha and retinoid X receptor alpha appeared in the cell nuclei of macrophages during the lesion-induced inflammatory reaction, and that retinoid X receptor alpha-staining co-localized with some regenerating axons. Experiments with Schwann cell primary cultures revealed an effect of retinoic acid on the expression of the neuregulin receptor ErbB3, suggesting that one function of retinoic acid consists in the regulation of neuroglial interactions after peripheral nerve injury.

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.

Retinoic acid receptor-dependent survival of olfactory sensory neurons in postnatal and adult mice

To address the hypothesis that retinoids produced by synthesizing enzymes present in the primary olfactory system influence the mouse olfactory sensory map, we expressed a dominant-negative retinoic acid receptor selectively in olfactory sensory neurons. We show that neurons deficient in nuclear retinoid signaling are responsive to odors and form correct odorant receptor-specific axonal projections to target neurons in the olfactory bulb of the brain. Subsequent to the formation of the map, the neurons die prematurely by retrograde-driven caspase-3 activation, which resembles the previously described mechanism of neural death after olfactory bulb ablation. This neurodegenerative event is initiated the second postnatal week and occurs in the adult animal without a compensatory increase of progenitor cell proliferation. In addition, we find that nuclear retinoid signaling is required for the expression of a retinoic acid-degrading enzyme, Cyp26B1, in a small fraction of mature neurons. Collectively, the results provide evidence for a role of locally regulated retinoid metabolism in neuroprotection and in determining population size of neurons at a late stage of neural circuit formation.

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

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.

Human scalp skin and hair follicles express neurotrophin-3 and its high-affinity receptor tyrosine kinase C, and show hair cycle-dependent alterations in expression

BACKGROUND

Neurotrophin (NT)-3 and its high-affinity receptor tyrosine kinase C (Trk C) are essential for nervous system development. These members of the NT family are also involved in murine hair morphogenesis and cycling. However, their role in human hair follicle (HF) biology remains to be elucidated.

OBJECTIVES

To explore the role of NTs in human skin and HF biology.

METHODS

The immunoreactivity (IR) of NT-3 and Trk C was studied in human scalp skin and HFs by immunofluorescent and light microscopic immunohistology. Skin biopsies were obtained from normal human scalp containing mainly anagen VI HFs from women (age 53-57 years) undergoing elective plastic surgery.

RESULTS

Both NT-3 and Trk C showed prominent, yet distinct, IR patterns in human scalp anagen HFs (anagen VI), whereas they were weakly expressed in catagen and increased again in telogen HFs. Within HF compartments, NT-3 IR was prominent in the outer root sheath, inner root sheath, dermal papilla and connective tissue sheath. Trk C IR was prominent in all HF epithelial and mesenchymal compartments. Outside the HF, both NT-3 and Trk C showed prominent IR in the epidermis, sebaceous glands and sweat glands.

CONCLUSIONS

These observations provide the first indication that NT-3 and Trk C are expressed in human scalp skin and HFs, and suggest that Trk C-mediated signalling is involved not only in murine but also in human HF biology. They may be useful in determining therapeutic strategies for the treatment of hair cycle and skin-related disorders.

Retinoic acid increases tissue and plasma contents of nerve growth factor and prevents neuropathy in diabetic mice

BACKGROUND

Decreased production of nerve growth factor (NGF) may contribute to diabetic neuropathy; however, exogenous administration of NGF induces only a modest benefit. Retinoic acid (RA) promotes the endogenous expression of nerve growth factor and its receptor. We studied the effects of RA on diabetic neuropathy in mice with streptozotocin-induced diabetes.

MATERIAL AND METHODS

One hundred and twenty National Institutes of Health (NIH) albino mice randomly separated into three groups (A, n = 30; B, n = 30; C, n = 60). Diabetes mellitus was induced with streptozotocin in groups A and B. Animals from group A received a subcutaneous injection of 25 microl of mineral oil daily for 90 days, while those from group B received a subcutaneous injection of 20 mg kg(-1) of all trans RA. Animals from group C were taken as controls. At the end of the experiment, blood glucose and NGF levels (both in serum and sciatic nerve) were measured. Two behavioural tests were conducted in a blind fashion to detect abnormalities of thermal and nociceptive thresholds.

RESULTS

Contents of NGF in healthy untreated mice were 1490 +/- 190 pg mg(-1) in nerve and 113 +/- 67 pg mg(-1) in serum; in diabetic untreated mice the values were 697 +/- 219 pg mL(-1) in nerve and 55 +/- 41 pg mL(-1) in serum; and in diabetic mice treated with RA the values were 2432 +/- 80 pg mL(-1) in nerve and 235 +/- 133 pg mg(-1) in serum (P < 0.002). Ultrastructural evidence of nerve regeneration and sensitivity tests improved in diabetic mice treated with RA as compared with nontreated diabetic mice.

CONCLUSION

Our findings indicate that administration of RA increases serum and nerve contents of NGF in diabetic mice and suggest a potential therapeutic role for retinoic acid in diabetic patients.

Retinoic acid synthesis by a population of NG2-positive cells in the injured spinal cord

Retinoic acid (RA) promotes growth and differentiation in many developing tissues but less is known about its influence on CNS regeneration. We investigated the possible involvement of RA in rat spinal cord injury (SCI) using the New York University (NYU) impactor to induce mild or moderate spinal cord contusion injury. Changes in RA at the lesion site were determined by measuring the activity of the enzymes for its synthesis, the retinaldehyde dehydrogenases (RALDHs). A marked increase in enzyme activity occurred by day 4 and peaked at days 8-14 following the injuries. RALDH2 was the only detectable RALDH present in the control or injured spinal cord. The cellular localization of RALDH2 was identified by immunostaining. In the noninjured spinal cord, RALDH2 was detected in oligodendroglia positive for the markers RIP and CNPase. Expression was also intense in the arachnoid membrane surrounding the spinal cord. After SCI the increase in RALDH2 was independent of the RIP- and CNPase-positive cells, which were severely depleted. Instead, RALDH2 was present in a cell type not previously identified as capable of synthesizing RA, that expressed NG2 and that was negative for markers of astrocytes, oligodendroglia, microglia, neurons, Schwann cells and immature lymphocytes. We postulate that the RALDH2- and NG2-positive cells migrate into the injured sites from the adjacent arachnoid membrane, where the RALDH2-positive cells proliferate substantially following SCI. These findings indicate that close correlations exist between RA synthesis and SCI and that RA may play a role in the secondary events that follow acute SCI.

Proteasome inhibition arrests neurite outgrowth and causes "dying-back" degeneration in primary culture

Proteasome inhibitors such as lactacystin were first isolated when assaying their ability to stimulate neurite outgrowth in neuronal-like cell lines; however, their effect on neurites in primary culture has been largely neglected. We report here that lactacystin causes immediate arrest of nerve growth factor (NGF)-stimulated neurite outgrowth in sympathetic and sensory explant cultures. This is followed by neurite degeneration that in sympathetic cultures has a distinctive "dying-back" morphology. Remarkably, this occurs even at concentrations below that required to induce neurite outgrowth in PC12 cells. Thus, lactacystin opposes rather than potentiates the effect of NGF on sympathetic neurite outgrowth and the role of the ubiquitin proteasome pathway in growth and long-term maintenance of axons and dendrites differs from that in neuritogenesis in neuronal-like cell lines. Retrograde degeneration caused by blocking of the ubiquitin proteasome pathway may mimic some aspects of gracile axonal dystrophy, a dying-back axonopathy in mice caused by ubiquitin hydrolase (Uch-l1) deficiency, and may be relevant to human neurodegenerative diseases involving ubiquitination or proteasome abnormalities.