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

RXRα Blocks Nerve Regeneration after Spinal Cord Injury by Targeting p66shc

Nerve regeneration after spinal cord injury is regulated by many factors. Studies have found that the expression of retinoid X receptor α (RXRα) does not change significantly after spinal cord injury but that the distribution of RXRα in cells changes significantly. In undamaged tissues, RXRα is distributed in motor neurons and the cytoplasm of glial cells. RXRα migrates to the nucleus of surviving neurons after injury, indicating that RXRα is involved in the regulation of gene expression after spinal cord injury. p66shc is an important protein that regulates cell senescence and oxidative stress. It can induce the apoptosis and necrosis of many cell types, promoting body aging. The absence of p66shc enhances the resistance of cells to reactive oxygen species (ROS) and thus prolongs life. It has been found that p66shc deletion can promote hippocampal neurogenesis and play a neuroprotective role in mice with multiple sclerosis. To verify the function of RXRα after spinal cord injury, we established a rat T9 spinal cord transection model. After RXRα agonist or antagonist administration, we found that RXRα agonists inhibited nerve regeneration after spinal cord injury, while RXRα antagonists promoted the regeneration of injured neurites and the recovery of motor function in rats. The results showed that RXRα played an impeding role in repair after spinal cord injury. Immunofluorescence staining showed that p66shc expression was upregulated in neurons after spinal cord injury (in vivo and in vitro) and colocalized with RXRα. RXRα overexpression in cultured neurons promoted the expression of p66shc, while RXRα interference inhibited the expression of p66shc. Using a luciferase assay, we found that RXRα could bind to the promoter region of p66shc and regulate the expression of p66shc, thereby regulating nerve regeneration after spinal cord injury. The above results showed that RXRα inhibited nerve regeneration after spinal cord injury by promoting p66shc expression, and interference with RXRα or p66shc promoted functional recovery after spinal cord injury.

Retinoic acid synthesis by a population of choroidal stromal cells

Choroidal all- trans -retinoic acid (atRA) may play a key role in the control of postnatal eye growth in a variety of vertebrates through modulation of scleral extracellular matrix synthesis and may therefore play a crucial role in the development of myopia. In the chick eye, choroidal atRA synthesis is exclusively regulated by its synthesizing enzyme, retinaldehyde dehydrogenase 2 (RALDH2). In chicks and humans, RALDH2 has been detected in a population of hitherto uncharacterized choroidal cells.Therefore, the aim of this study was to identify the RALDH2+ cell type(s) in the choroid and determine how these cells modulate atRA concentrations during periods of visually guided eye growth. Chicks wore translucent goggles on one eye for 10 days and choroids were analyzed for RALDH activity and RALDH2 protein expression at days 0, 1, 4, 7, 15 following removal of the goggle ("recovery"); choroids from contralateral eyes served as controls. The presence of RALDH2+ cells was assessed in chick choroid wholemounts using multiphoton microscopy. RALDH2 protein expression was measured by western blot and RALDH2 activity was assessed via HPLC quantification of atRA. Cell proliferation was assessed by BrdU-labelling in combination with RALDH2-immunohistochemistry. For characterization of RALDH2+ cells, immunohistochemistry for various tissue specific markers was applied in chicken (Ia antigen, CD5, Col1-propeptide, desmin, IgY, L-Cam, Cadherin1, MHC-II; Tcr-γδ, vimentin) and human donor tissue (α-smooth-muscle-actin, CD's 31/34/68/146, desmin, IBA1, LYVE-1, PGP9.5, vimentin) followed by confocal microscopy. In the chick and human choroid, RALDH2+ cells with variable morphology were present in the stroma and adjacent to choroidal blood vessels. In chick wholemounts, RALDH2+ cells were concentrated toward the choriocapillaris, and their number increased nearly linearly between 1 and 7 days of recovery and plateaued between 7 and 15 days compared to corresponding controls. A significant increase in choroidal RALDH2 protein concentration and atRA synthetic activity was observed by four days of recovery (↑107% and ↑120%) by western blot and HPLC, respectively. A 3-fold increase in RALDH2+/BrDU+ cells was observed following 4 days of recovery compared to controls (12.43 ± 0.73% of all RALDH2+ cells in recovering eyes as compared with 4.46 ± 0.63% in control eyes, p < 0.001). In chick choroids, the vast majority of RALDH2+ cells co-expressed Col1-propetide, but did not co-label with any other antibodies tested. In human choroid, some, but not all RALDH2+ cells colocalized with vimentin, but were negative for all other antibodies tested. RALDH2+ cells represent a novel cell type in the chick and human choroid. Our findings that some human RALDH2+ cells were positive for vimentin and all chick RALDH2+ cells were positive for Col1, suggest that RALDH2+ cells most closely resemble perivascular and stromal fibroblasts. The increased number of RALDH2+/BRDU+ cells following 4 days of recovery suggests that choroidal atRA concentrations are partially controlled by proliferation of RALDH2+ cells. The identification of this choroidal cell type will provide a broader understanding of the cellular events responsible for the regulation of postnatal ocular growth, and may provide new avenues for specifically targeted strategies for the treatment of myopia.

Retinoic acid signalling in fibro/adipogenic progenitors robustly enhances muscle regeneration

BACKGROUND

During muscle regeneration, excessive formation of adipogenic and fibrogenic tissues, from their respective fibro/adipogenic progenitors (FAPs), impairs functional recovery. Intrinsic mechanisms controlling the proliferation and differentiation of FAPs remain largely unexplored.

METHODS

Here, we investigated the role of retinoic acid (RA) signalling in regulating FAPs and the subsequent effects on muscle restoration from a cardiotoxin-induced injury. Blockage of retinoic acid receptor (RAR) signalling was achieved through dominant negative retinoic acid receptor α (RARα403) expression specific in PDGFRα+ FAPs in vivo and by BMS493 treatment in vitro. Effects of RAR-signalling on FAP cellularity and muscle regeneration were also investigated in a high-fat diet-induced obese mice model.

FINDINGS

Supplementation of RA increased the proliferation of FAPs during the early stages of regeneration while suppressing FAP differentiation and promoting apoptosis during the remodelling stage. Loss of RAR-signalling caused ectopic adipogenic differentiation of FAPs and impaired muscle regeneration. Furthermore, obesity disrupted the cellular transition of FAPs and attenuated muscle regeneration. Supplementation of RA to obese mice not only rescued impaired muscle fibre regeneration, but also inhibited infiltration of fat and fibrotic tissues during muscle repair. These beneficial effects were abolished after blocking RAR-signalling in FAPs of obese mice.

INTERPRETATION

These data suggest that RAR-signalling in FAPs is a critical therapeutic target for suppressing differentiation of FAPs and facilitating the regeneration of muscle and other tissues.

FUNDING

This study was supported by grants from the National Institutes of Health (R01-HD067449 and R21-AG049976) to M.D.

Retinoic Acid Is Required for Oligodendrocyte Precursor Cell Production and Differentiation in the Postnatal Mouse Corpus Callosum

Myelination of the CNS relies on the production and differentiation of oligodendrocyte (OL) precursor cells (OPCs) into mature OLs. During the first month of postnatal life, OPCs that populate the corpus callosum (CC) arise from neural stem cells (NSCs) in the subcallosal subventricular zone (SVZ), and then differentiate to generate myelinating OLs. However, the signals that regulate these processes are not fully understood.

Myelination of the CNS relies on the production and differentiation of oligodendrocyte (OL) precursor cells (OPCs) into mature OLs. During the first month of postnatal life, OPCs that populate the corpus callosum (CC) arise from neural stem cells (NSCs) in the subcallosal subventricular zone (SVZ), and then differentiate to generate myelinating OLs. However, the signals that regulate these processes are not fully understood. In this study, we show that endogenous expression of the retinoic acid (RA)-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2) is required for OPC generation and differentiation in the postnatal subcortical white matter. In male and female pups, conditional deletion of Raldh2 reduced OPC numbers and differentiation. Moreover, decreased OPC numbers coincided with reductions in NSC survival and expression of the sonic hedgehog (SHH) signaling effector protein Gli1 in the SVZ. Additionally, GFAP expression in the CC was decreased, and cortical neuron numbers were altered. Our work suggests a role for endogenous RALDH2-dependent RA synthesis in OPC production and differentiation in the CC, as well as in the development of other cell types derived from NSCs in the embryonic ventricular zone (VZ) and SVZ, as well as the postnatal subcallosal SVZ.

Decay in Retinoic Acid Signaling in Varied Models of Alzheimer's Disease and In-Vitro Test of Novel Retinoic Acid Receptor Ligands (RAR-Ms) to Regulate Protective Genes

Retinoic acid has been previously proposed in the treatment of Alzheimer's disease (AD). Here, five transgenic mouse models expressing AD and frontotemporal dementia risk genes (i.e., PLB2APP, PLB2TAU, PLB1Double, PLB1Triple, and PLB4) were used to investigate if consistent alterations exist in multiple elements of the retinoic acid signaling pathway in these models. Many steps of the retinoic acid signaling pathway including binding proteins and metabolic enzymes decline, while the previously reported increase in RBP4 was only consistent at late (6 months) but not early (3 month) ages. The retinoic acid receptors were exceptional in their consistent decline in mRNA and protein with transcript decline of retinoic acid receptors β and γ by 3 months, before significant pathology, suggesting involvement in early stages of disease. Decline in RBP1 transcript may also be an early but not late marker of disease. The decline in the retinoic acid signaling system may therefore be a therapeutic target for AD and frontotemporal dementia. Thus, novel stable retinoic acid receptor modulators (RAR-Ms) activating multiple genomic and non-genomic pathways were probed for therapeutic control of gene expression in rat primary hippocampal and cortical cultures. RAR-Ms promoted the non-amyloidogenic pathway, repressed lipopolysaccharide induced inflammatory genes and induced genes with neurotrophic action. RAR-Ms had diverse effects on gene expression allowing particular RAR-Ms to be selected for maximal therapeutic effect. Overall the results demonstrated the early decline of retinoic acid signaling in AD and frontotemporal dementia models and the activity of stable and potent alternatives to retinoic acid as potential therapeutics.

Nuclear Receptors and Neuroinflammation in Schizophrenia

Introduction

Several nuclear receptor family members have been associated with schizophrenia and inflammation. Vitamins A and D exert anti-inflammatory actions, but their receptors (mainly nuclear receptors) have not been extensively studied in either schizophrenia brains or in association with neuroinflammation. We examined the expression of vitamin A (RARs and RXRs) and vitamin D and protein disulphide-isomerase A3 (PDIA3) receptors, as well as nuclear orphan receptors (NR4As), in the context of elevated cytokine expression in the dorsolateral prefrontal cortex (DLPFC).

Methods

mRNA levels of nuclear receptors were measured in DLPFC tissues via RT-qPCR. ANCOVAs comparing high inflammation schizophrenia, low inflammation schizophrenia and low inflammation control groups were performed.

Results

RARG, RXRB, NR4A1 and NR4A3 transcripts showed significant differential expression across the three groups (ANCOVA p = 0.02-0.001). Post hoc testing revealed significant reductions in RARG expression in schizophrenia with low inflammation compared to schizophrenia with high inflammation and to controls, and RXRB mRNA was significantly reduced in schizophrenia with low inflammation compared to controls. NR4A1 and NR4A3 mRNAs were decreased in schizophrenia with high inflammation compared to schizophrenia with low inflammation, with NR4A1 also significantly different to controls.

Conclusion

In schizophrenia, changes in nuclear receptor mRNA levels involved with mediating actions of vitamin A derivatives vary according to the inflammatory state of brains.

Retinoic acid synthesis by NG2 expressing cells promotes a permissive environment for axonal outgrowth

Stimulation of retinoic acid (RA) mediated signalling pathways following neural injury leads to regeneration in the adult nervous system and numerous studies have shown that the specific activation of the retinoic acid receptor β (RARβ) is required for this process. Here we identify a novel mechanism by which neuronal RARβ activation results in the endogenous synthesis of RA which is released in association with exosomes and acts as a positive cue to axonal/neurite outgrowth. Using an established rodent model of RARβ induced axonal regeneration, we show that neuronal RARβ activation upregulates the enzymes involved in RA synthesis in a cell specific manner; alcohol dehydrogenase7 (ADH7) in neurons and aldehyde dehydrogenase 2 (Raldh2) in NG2 expressing cells (NG2 + cells). These release RA in association with exosomes providing a permissive substrate to neurite outgrowth. Conversely, deletion of Raldh2 in the NG2 + cells in our in vivo regeneration model is sufficient to compromise axonal outgrowth. This hitherto unidentified RA paracrine signalling is required for axonal/neurite outgrowth and is initiated by the activation of neuronal RARβ signalling.

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Raldh2, the enzyme for retinoic acid synthesis, is upregulated in NG2 + cells during axonal regeneration.

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Deletion of Raldh2 in NG2 + cells prevents regeneration.

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RA signalling modulates axonal pathfinding.

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Fine-tuned regulation of RA distribution via exosome transport

Effects of electroacupuncture and the retinoid X receptor (RXR) signalling pathway on oligodendrocyte differentiation in the demyelinated spinal cord of rats

Objectives

In spinal cord demyelination, some oligodendrocyte precursor cells (OPCs) remain in the demyelinated region but have a reduced capacity to differentiate into oligodendrocytes. This study investigated whether ‘Governor Vessel’ (GV) electroacupuncture (EA) would promote the differentiation of endogenous OPCs into oligodendrocytes by activating the retinoid X receptor γ (RXR-γ)-mediated signalling pathway.

Methods

Adult rats were microinjected with ethidium bromide (EB) into the T10 spinal cord to establish a model of spinal cord demyelination. EB-injected rats remained untreated (EB group, n=26) or received EA treatment (EB+EA group, n=26). A control group (n=26) was also included that underwent dural exposure without EB injection. After euthanasia at 7 days (n=5 per group), 15 days (n=8 per group) or 30 days (n=13 per group), protein expression of RXR-γ in the demyelinated spinal cord was evaluated by immunohistochemistry and Western blotting. In addition, OPCs derived from rat embryonic spinal cord were cultured in vitro, and exogenous 9-cis-RA (retinoic acid) and RXR-γ antagonist HX531 were administered to determine whether RA could activate RXR-γ and promote OPC differentiation.

Results

EA was found to increase the numbers of both OPCs and oligodendrocytes expressing RXR-γ and RALDH2, and promote remyelination in the remyelinated spinal cord. Exogenous 9-cis-RA enhanced the differentiation of OPCs into mature oligodendrocytes by activating RXR-γ.

Conclusions

The results suggest that EA may activate RXR signalling to promote the differentiation of OPCs into oligodendrocytes in spinal cord demyelination.

Col1a1+ perivascular cells in the brain are a source of retinoic acid following stroke

Background

Perivascular stromal cells (PSCs) are a recently identified cell type that comprises a small percentage of the platelet derived growth factor receptor-β+ cells within the CNS perivascular space. PSCs are activated following injury to the brain or spinal cord, expand in number and contribute to fibrotic scar formation within the injury site. Beyond fibrosis, their high density in the lesion core makes them a potential significant source of signals that act on neural cells adjacent to the lesion site.

Results

Our developmental analysis of PSCs, defined by expression of Collagen1a1 in the maturing brain, revealed that PSCs first appear postnatally and may originate from the meninges. PSCs express many of the same markers as meningeal fibroblasts, including expression of the retinoic acid (RA) synthesis proteins Raldh1 and Raldh2. Using a focal brain ischemia injury model to induce PSC activation and expansion, we show a substantial increase in Raldh1+/Raldh2+ PSCs and Raldh1+ activated macrophages in the lesion core. We find that RA levels are significantly elevated in the ischemic hemisphere and induce signaling in astrocytes and neurons in the peri-infarct region.

Conclusions

This study highlights a dual role for activated, non-neural cells where PSCs deposit fibrotic ECM proteins and, along with macrophages, act as a potentially important source of RA, a potent signaling molecule that could influence recovery events in a neuroprotective fashion following brain injury.

Human articular chondrocytes with higher aldehyde dehydrogenase activity have stronger expression of COL2A1 and SOX9

OBJECTIVE

To determine in human articular chondrocytes the activity of Aldehyde dehydrogenase (ALDH), which are reported as stem/progenitor cell marker in various adult tissues and evaluate gene expression of ALDH1A isoforms.

DESIGN

ALDH activity was evaluated by flow cytometry with Aldefluor™ assay in cells, isolated from human osteoarthritic (OA) cartilage. Its coexpression with surface markers was identified. Cells were sorted according to ALDH activity, and gene expression in sorted populations (ALDH(+) and ALDH(-)) was analyzed by RTq-PCR with Taqman(®) assay.

RESULTS

About 40% of freshly isolated chondrocytes demonstrated ALDH activity that remarkably declined during monolayer culture. Markers CD54 and CD55 were significantly stronger expressed, while CD47, CD140b, CD146 and CD166 were depleted in ALDH-expressing (ALDH(pos)) cells. Gene expression analysis revealed significantly higher expression of chondrocyte-specific genes COL2A1, SOX9 and SERPINA1 and lower expression of osteogenic markers RUNX2 and osteocalcin (BGLAP) in sorted ALDH(+) fraction. COL1A1, ACAN, ALPL and stem cell markers NANOG, OCT4, SOX2 and ABCG2 did not differ remarkably between the populations. Genes of isoenzymes ALDH1A2, ALDH1A3 and ALDH2 were strongly expressed, while ALDH1A1 was weakly expressed in chondrocytes. Only ALDH1A2 and ALDH1A3 were significantly enriched in ALDH(+) fraction.

CONCLUSIONS

We identified ALDH activity with significantly stronger expression of CD54 and CD55 in human articular chondrocytes. Gene expression of isotypes ALDH1A2, ALDH1A3 and ALDH2 was identified. Coexpression of ALDH activity with chondrogenic markers suggests its association with collagen II producing chondrocyte phenotype. Isotypes ALDH1A2 and ALDH1A3 can be associated with the ALDH activity in these cells.

Metabolic disturbances of the vitamin A pathway in human diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is a common life-threatening congenital anomaly resulting in high rates of perinatal death and neonatal respiratory distress. Some of the nonisolated forms are related to single-gene mutations or genomic rearrangements, but the genetics of the isolated forms (60% of cases) still remains a challenging issue. Retinoid signaling (RA) is critical for both diaphragm and lung development, and it has been hypothesized that subtle disruptions of this pathway could contribute to isolated CDH etiology. Here we used time series of normal and CDH lungs in humans, in nitrofen-exposed rats, and in surgically induced hernia in rabbits to perform a systematic transcriptional analysis of the RA pathway key components. The results point to CRPBP2, CY26B1, and ALDH1A2 as deregulated RA signaling genes in human CDH. Furthermore, the expression profile comparisons suggest that ALDH1A2 overexpression is not a primary event, but rather a consequence of the CDH-induced lung injury. Taken together, these data show that RA signaling disruption is part of CDH pathogenesis, and also that dysregulation of this pathway should be considered organ specifically.

Spinal cord regeneration

Three theories of regeneration dominate neuroscience today, all purporting to explain why the adult central nervous system (CNS) cannot regenerate. One theory proposes that Nogo, a molecule expressed by myelin, prevents axonal growth. The second theory emphasizes the role of glial scars. The third theory proposes that chondroitin sulfate proteoglycans (CSPGs) prevent axon growth. Blockade of Nogo, CSPG, and their receptors indeed can stop axon growth in vitro and improve functional recovery in animal spinal cord injury (SCI) models. These therapies also increase sprouting of surviving axons and plasticity. However, many investigators have reported regenerating spinal tracts without eliminating Nogo, glial scar, or CSPG. For example, many motor and sensory axons grow spontaneously in contused spinal cords, crossing gliotic tissue and white matter surrounding the injury site. Sensory axons grow long distances in injured dorsal columns after peripheral nerve lesions. Cell transplants and treatments that increase cAMP and neurotrophins stimulate motor and sensory axons to cross glial scars and to grow long distances in white matter. Genetic studies deleting all members of the Nogo family and even the Nogo receptor do not always improve regeneration in mice. A recent study reported that suppressing the phosphatase and tensin homolog (PTEN) gene promotes prolific corticospinal tract regeneration. These findings cannot be explained by the current theories proposing that Nogo and glial scars prevent regeneration. Spinal axons clearly can and will grow through glial scars and Nogo-expressing tissue under some circumstances. The observation that deleting PTEN allows corticospinal tract regeneration indicates that the PTEN/AKT/mTOR pathway regulates axonal growth. Finally, many other factors stimulate spinal axonal growth, including conditioning lesions, cAMP, glycogen synthetase kinase inhibition, and neurotrophins. To explain these disparate regenerative phenomena, I propose that the spinal cord has evolved regenerative mechanisms that are normally suppressed by multiple extrinsic and intrinsic factors but can be activated by injury, mediated by the PTEN/AKT/mTOR, cAMP, and GSK3b pathways, to stimulate neural growth and proliferation.

Spinal cord injury induced changes of nuclear receptors PPARα and LXRβ and modulation with oleic acid/albumin treatment

In previous studies with animal models of spinal cord injury (SCI) pharmacological activation of peroxisome proliferator activated receptors (PPAR) and liver X receptors (LXR) were used to reduce tissue damage and promote behavioral recovery in animal models. We have studied the endogenous expression of the transcription factors PPARα and LXRβ in the chronic stage after SCI in rats. The immunohistochemical investigation revealed a long lasting increase in the level of PPARα in white matter in the vicinity of the lesion site. The source of this signal was identified in a subpopulation of astrocytes outside of the glial scar area. Intrathecal injections of oleic acid/albumin reduced the lesion-induced PPARα immunoreactivity. In addition, ependymal cells displayed a prominent PPARα signal in the non-injured spinal cord, and continued to express the receptor as they proliferated and migrated within the damaged tissue. The nuclear receptor LXRβ was detected at similar levels after SCI as in sham operated animals. We found high levels of immunoreactivity in the gray matter, while in the white matter it was present in subpopulations of astrocytes and oligodendrocytes. Macrophages that had accumulated within the center of the lesion contained LXRβ in their cell nuclei. Possible endogenous functions of PPARα and LXRβ after SCI are discussed, specifically the control of fatty acid and cholesterol metabolism and the regulation of inflammatory reactions.

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

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

Physiological insights into all-trans-retinoic acid biosynthesis

All-trans-retinoic acid (atRA) provides essential support to diverse biological systems and physiological processes. Epithelial differentiation and its relationship to cancer, and embryogenesis have typified intense areas of interest into atRA function. Recently, however, interest in atRA action in the nervous system, the immune system, energy balance and obesity has increased considerably, especially concerning postnatal function. atRA action depends on atRA biosynthesis: defects in retinoid-dependent processes increasingly relate to defects in atRA biogenesis. Considerable evidence indicates that physiological atRA biosynthesis occurs via a regulated process, consisting of a complex interaction of retinoid binding-proteins and retinoid recognizing enzymes. An accrual of biochemical, physiological and genetic data have identified specific functional outcomes for the retinol dehydrogenases, RDH1, RDH10, and DHRS9, as physiological catalysts of the first step in atRA biosynthesis, and for the retinal dehydrogenases RALDH1, RALDH2, and RALDH3, as catalysts of the second and irreversible step. Each of these enzymes associates with explicit biological processes mediated by atRA. Redundancy occurs, but seems limited. Cumulative data support a model of interactions among these enzymes with retinoid binding-proteins, with feedback regulation and/or control by atRA via modulating gene expression of multiple participants. The ratio apo-CRBP1/holo-CRBP1 participates by influencing retinol flux into and out of storage as retinyl esters, thereby modulating substrate to support atRA biosynthesis. atRA biosynthesis requires the presence of both an RDH and an RALDH: conversely, absence of one isozyme of either step does not indicate lack of atRA biosynthesis at the site. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Oligodendrocyte fate after spinal cord injury

Oligodendrocytes (OLs) are particularly susceptible to the toxicity of the acute lesion environment after spinal cord injury (SCI). They undergo both necrosis and apoptosis acutely, with apoptosis continuing at chronic time points. Loss of OLs causes demyelination and impairs axon function and survival. In parallel, a rapid and protracted OL progenitor cell proliferative response occurs, especially at the lesion borders. Proliferating and migrating OL progenitor cells differentiate into myelinating OLs, which remyelinate demyelinated axons starting at 2 weeks post-injury. The progression of OL lineage cells into mature OLs in the adult after injury recapitulates development to some degree, owing to the plethora of factors within the injury milieu. Although robust, this endogenous oligogenic response is insufficient against OL loss and demyelination. First, in this review we analyze the major spatial-temporal mechanisms of OL loss, replacement, and myelination, with the purpose of highlighting potential areas of intervention after SCI. We then discuss studies on OL protection and replacement. Growth factors have been used both to boost the endogenous progenitor response, and in conjunction with progenitor transplantation to facilitate survival and OL fate. Considerable progress has been made with embryonic stem cell-derived cells and adult neural progenitor cells. For therapies targeting oligogenesis to be successful, endogenous responses and the effects of the acute and chronic lesion environment on OL lineage cells must be understood in detail, and in relation, the optimal therapeutic window for such strategies must also be determined.

Distribution of retinoic acid receptor-α immunoreactivity in the human hypothalamus

Retinoids, a family of molecules that is derived from vitamin A, are involved in a complex signaling pathway that regulates gene expression and controls neuronal differentiation in the central nervous system. The physiological actions of retinoids are mainly mediated by retinoic acid receptors. Here we describe the distribution of retinoic acid receptor α (RARα) in the human hypothalamus by immunohistochemistry. RARα immunoreactivity showed a widespread pattern throughout the hypothalamus, with high density in the suprachiasmatic nucleus (SCN), paraventricular nucleus (PVN), supraoptic nucleus (SON), infundibular nucleus and medial mamillary nucleus. No staining was observed in the sexually dimorphic nucleus of preoptic area, tuberomamillary nucleus and lateral hypothalamic area. RARα was co-localized with vasopressin (AVP) neurons in the SCN, PVN and SON, and co-localized with corticotropin releasing hormone (CRH) neurons in the PVN. These findings provide a neurobiological basis for the participation of retinoids in the regulation of various hypothalamic functions. As shown earlier, the co-localization of RARα in CRH neurons suggests that retinoids might directly modulate the hypothalamus-pituitary-adrenal axis in the PVN, which may have implications for the stress response and its involvement in mood disorders. Functional studies in the other sites of RARα localization have to follow in the future.

The involvement of retinoic acid receptor-alpha in corticotropin-releasing hormone gene expression and affective disorders

BACKGROUND

Corticotropin-releasing hormone (CRH) is considered the central driving force in the stress response and plays a key role in the pathogenesis of depression. Retinoic acid (RA) has been suggested by clinical studies to be associated with affective disorders.

METHODS

First, hypothalamic tissues of 12 patients with affective disorders and 12 matched control subjects were studied by double-label immunofluorescence to analyze the expression of CRH and retinoic acid receptor-alpha (RAR-alpha). Second, critical genes involved in the RA signaling pathways were analyzed in a rat model of depression. Finally, the regulatory effect of RAR-alpha on CRH gene expression was studied in vitro.

RESULTS

We found that the expression of RAR-alpha was colocalized with CRH neurons in human hypothalamic paraventricular nucleus (PVN). The density of RAR-alpha-immunoreactive neurons and CRH-RAR-alpha double-staining neurons was significantly increased in the PVN of patients with affective disorders. The ratio of the CRH-RAR-alpha double-staining neurons to the CRH-immunoreactive neurons in affective disorder patients was also increased. Recruitment of RAR-alpha by the CRH promoter was observed in the rat hypothalamus. A dysregulated RA metabolism and signaling was also found in the hypothalamus of a rat model for depression. Finally, in vitro studies demonstrated that RAR-alpha mediated an upregulation of CRH gene expression.

CONCLUSIONS

These results suggest that RAR-alpha might contribute to regulating the activity of CRH neurons in vivo, and the vulnerable character of the critical proteins in RA signaling pathways might provide novel targets for therapeutic strategies for depression.

Towards retinoid therapy for Alzheimer's disease

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

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

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

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

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