Photoperiodic expression of two RALDH enzymes and the regulation of cell proliferation by retinoic acid in the rat hypothalamus

Tanycytes from a bird's eye view: gene expression profiling of the tanycytic region under different seasonal states in the Svalbard ptarmigan

In mammals and birds, tanycytes are known to regulate thyroid hormone conversion, and this process is central to the control of seasonal reproduction. In mammals, this cell type is also implicated in retinoic acid signalling, neurogenesis, and nutritional gatekeeping, all of which have been linked to hypothalamic regulation of energy metabolism. Less is known about these potential wider roles of tanycytes in birds. To address this gap, we combined LASER capture microdissection and transcriptomics to profile the tanycytic region in male Svalbard ptarmigan, a High Arctic species with photoperiod-dependent seasonal rhythms in reproductive activation and body mass. Short photoperiod (SP) adapted birds were transferred to constant light (LL) to trigger breeding and body mass loss. After five months under LL, the development of photorefractoriness led to spontaneous re-emergence of the winter phenotype, marked by the termination of breeding and gain in body mass. The transfer from SP to LL initiated gene expression changes in both thyroid hormone and retinoic acid pathways, as described in seasonal mammals. Furthermore, transcriptomic signatures of cell differentiation and migration were observed. Comparison to data from Siberian hamsters demonstrated that a photoperiod-dependent re-organisation of the hypothalamic tanycytic region is likely a conserved feature. Conversely, the spontaneous development of photorefractoriness showed a surprisingly small number of genes that reverted in expression level, despite reversal of the reproductive and metabolic phenotype. Our data suggest general conservation of tanycyte biology between photoperiodic birds and mammals and raise questions about the mechanistic origins of the photorefractory state.

Control by the brain of vitamin A homeostasis

Vitamin A is a micronutrient essential for vertebrate animals maintained in homeostatic balance in the body; however, little is known about the control of this balance. This study investigated whether the hypothalamus, a key integrative brain region, regulates vitamin A levels in the liver and circulation. Vitamin A in the form of retinol or retinoic acid was stereotactically injected into the 3rd ventricle of the rat brain. Alternatively, retinoids in the mouse hypothalamus were altered through retinol-binding protein 4 (Rbp4) gene knockdown. This led to rapid change in the liver proteins controlling vitamin A homeostasis as well as vitamin A itself in liver and the circulation. Prolonged disruption of Rbp4 in the region of the arcuate nucleus of the mouse hypothalamus altered retinol levels in the liver. This supports the concept that the brain may sense retinoids and influence whole-body vitamin A homeostasis with a possible "vitaminostatic" role.

Seasonal remodeling of the progenitor pool and its distribution in the ewe mediobasal hypothalamus

Recent studies have reported the presence of adult neurogenesis in the arcuate nucleus periventricular space (pvARH) and in the median eminence (ME), two structures involved in reproductive function. In sheep, a seasonal mammal, decreasing daylight in autumn induces a higher neurogenic activity in these two structures. However, the different types of neural stem and progenitor cells (NSCs/NPCs) that populate the arcuate nucleus and median eminence, as well as their location, have not been evaluated. Here, using semi-automatic image analyzing processes, we identified and quantified the different populations of NSCs/NPCs, showing that, during short days, higher densities of [SOX2 +] cells are found in pvARH and ME. In the pvARH, higher densities of astrocytic and oligodendrocitic progenitors mainly contribute to these variations. The different populations of NSCs/NPCs were mapped according to their position relative to the third ventricle and their proximity to the vasculature. We showed that [SOX2 +] cells extended deeper into the hypothalamic parenchyma during short days. Similarly, [SOX2 +] cells were found further from the vasculature in the pvARH and the ME, at this time of year, indicating the existence of migratory signals. The expression levels of neuregulin transcripts (NRGs), whose proteins are known to stimulate proliferation and adult neurogenesis and to regulate progenitor migration, as well as the expression levels of ERBB mRNAs, cognate receptors for NRGs, were assessed. We showed that mRNA expression changed seasonally in pvARH and ME, suggesting that the ErbB-NRG system is potentially involved in the photoperiodic regulation of neurogenesis in seasonal adult mammals.

Thyroid hormone regulation of adult neural stem cell fate: A comparative analysis between rodents and primates

Thyroid hormone (TH) signaling, a highly conserved pathway across vertebrates, is crucial for brain development and function throughout life. In the adult mammalian brain, including that of humans, multipotent neural stem cells (NSCs) proliferate and generate neuronal and glial progenitors. The role of TH has been intensively investigated in the two main neurogenic niches of the adult mouse brain, the subventricular and the subgranular zone. A key finding is that T3, the biologically active form of THs, promotes NSC commitment toward a neuronal fate. In this review, we first discuss the roles of THs in the regulation of adult rodent neurogenesis, as well as how it relates to functional behavior, notably olfaction and cognition. Most research uncovering these roles of TH in adult neurogenesis was conducted in rodents, whose genetic background, brain structure and rate of neurogenesis are considerably different from that of humans. To bridge the phylogenetic gap, we also explore the similarities and divergences of TH-dependent adult neurogenesis in non-human primate models. Lastly, we examine how photoperiodic length changes TH homeostasis, and how that might affect adult neurogenesis in seasonal species to increase fitness. Several aspects by which TH acts on adult NSCs seem to be conserved among mammals, while we only start to uncover the molecular pathways, as well as how other in- and extrinsic factors are intertwined. A multispecies approach delivering more insights in the matter will pave the way for novel NSC-based therapies to combat neurological disorders.

Pleiotropic effects of proopiomelanocortin and VGF nerve growth factor inducible neuropeptides for the long-term regulation of energy balance

Seasonal rhythms in energy balance are well documented across temperate and equatorial zones animals. The long-term regulated changes in seasonal physiology consists of a rheostatic system that is essential to successful time annual cycles in reproduction, hibernation, torpor, and migration. Most animals use the annual change in photoperiod as a reliable and robust environmental cue to entrain endogenous (i.e. circannual) rhythms. Research over the past few decades has predominantly examined the role of first order neuroendocrine peptides for the rheostatic changes in energy balance. These anorexigenic and orexigenic neuropeptides in the arcuate nucleus include neuropeptide y (Npy), agouti-related peptide (Agrp), cocaine and amphetamine related transcript (Cart) and pro-opiomelanocortin (Pomc). Recent studies also indicate that VGF nerve growth factor inducible (Vgf) in the arcuate nucleus is involved in the seasonal regulation of energy balance. In situ hybridization, qPCR and RNA-sequencing studies have identified that Pomc expression across fish, avian and mammalian species, is a neuroendocrine marker that reflects seasonal energetic states. Here we highlight that long-term changes in arcuate Pomc and Vgf expression is conserved across species and may provide rheostatic regulation of seasonal energy balance.

Neurogenesis: A process ontogenically linked to brain cavities and their content, CSF

Neurogenesis is the process underlying the development of the highly evolved central nervous system (CNS) in vertebrates. Neurogenesis takes place by differentiation of specific Neural Precursor Cells in the neurogenic niche. The main objective of this review is to highlight the specific relationship between the brain cavities, and neurogenesis from neural precursors. Brain cavities and their content, Cerebrospinal Fluid (CSF), establish a key relation with the neurogenic "niche" because of the presence in this fluid of neurogenic signals able to control neural precursor cell behaviour, inducing precursor proliferation and neuronal differentiation. This influence seems to be ontogenically preserved, despite the temporal and spatial variations that occur throughout life. In order to better understand this concept, we consider three main life periods in the CSF-Neurogenesis interaction: The "Embryonic" period, which take place at the Neural Tube stage and extends from the isolation of the neural tube at the end of "neurulation" to the beginning of Choroid Plexus activity; the "Fetal" period, which includes the remaining developmental and the early postnatal stages; and the "Adult" period, which continues for the rest of adult life. Each period has specific characteristics in respect of CSF synthesis and composition, and the location, extension and neurogenic activity of the neurogenic niche. However, CSF interaction with the neurogenic niche is a common factor, which should be taken into account to better understand the ontogeny of neuron formation and replacement, as well as its potential role in the success or failure of therapies for the ageing, injured or diseased brain.

Photoperiodic and clock regulation of the vitamin A pathway in the brain mediates seasonal responsiveness in the monarch butterfly

Seasonal adaptation to changes in light:dark regimes (i.e., photoperiod) allows organisms living at temperate latitudes to anticipate environmental changes. In nearly all animals studied so far, the circadian system has been implicated in measurement and response to the photoperiod. In insects, genetic evidence further supports the involvement of several clock genes in photoperiodic responses. Yet, the key molecular pathways linking clock genes or the circadian clock to insect photoperiodic responses remain largely unknown. Here, we show that inactivating the clock in the North American monarch butterfly using loss-of-function mutants for the circadian activators CLOCK and BMAL1 and the circadian repressor CRYPTOCHROME 2 abolishes photoperiodic responses in reproductive output. Transcriptomic approaches in the brain of monarchs raised in long and short photoperiods, summer monarchs, and fall migrants revealed a molecular signature of seasonal-specific rhythmic gene expression that included several genes belonging to the vitamin A pathway. We found that the rhythmic expression of these genes was abolished in clock-deficient mutants, suggesting that the vitamin A pathway operates downstream of the circadian clock. Importantly, we showed that a CRISPR/Cas9-mediated loss-of-function mutation in the gene encoding the pathway's rate-limiting enzyme, ninaB1, abolished photoperiod responsiveness independently of visual function in the compound eye and without affecting circadian rhythms. Together, these results provide genetic evidence that the clock-controlled vitamin A pathway mediates photoperiod responsiveness in an insect. Given previously reported seasonal changes associated with this pathway in the mammalian brain, our findings suggest an evolutionarily conserved function of vitamin A in animal photoperiodism.

A unifying hypothesis for control of body weight and reproduction in seasonally breeding mammals

Animals have evolved diverse seasonal variations in physiology and reproduction to accommodate yearly changes in environmental and climatic conditions. These changes in physiology are initiated by changes in photoperiod (daylength) and are mediated through melatonin, which relays photoperiodic information to the pars tuberalis of the pituitary gland. Melatonin drives thyroid-stimulating hormone transcription and synthesis in the pars tuberalis, which, in turn, regulates thyroid hormone and retinoic acid synthesis in the tanycytes lining the third ventricle of the hypothalamus. Seasonal variation in central thyroid hormone signalling is conserved among photoperiodic animals. Despite this, different species adopt divergent phenotypes to cope with the same seasonal changes. A common response amongst different species is increased hypothalamic cell proliferation/neurogenesis in short photoperiod. That cell proliferation/neurogenesis may be important for seasonal timing is based on (i) the neurogenic potential of tanycytes; (ii) the fact that they are the locus of striking seasonal morphological changes; and (iii) the similarities to mechanisms involved in de novo neurogenesis of energy balance neurones. We propose that a decrease in hypothalamic thyroid hormone and retinoic acid signalling initiates localised neurodegeneration and apoptosis, which leads to a reduction in appetite and body weight. Neurodegeneration induces compensatory cell proliferation from the neurogenic niche in tanycytes and new cells are born under short photoperiod. Because these cells have the potential to differentiate into a number of different neuronal phenotypes, this could provide a mechanistic basis to explain the seasonal regulation of energy balance, as well as reproduction. This cycle can be achieved without changes in thyroid hormone/retinoic acid and explains recent data obtained from seasonal animals held in natural conditions. However, thyroid/retinoic acid signalling is required to synchronise the cycles of apoptosis, proliferation and differentiation. Thus, hypothalamic neurogenesis provides a framework to explain diverse photoperiodic responses.

Tanycyte Gene Expression Dynamics in the Regulation of Energy Homeostasis

Animal survival relies on a constant balance between energy supply and energy expenditure, which is controlled by several neuroendocrine functions that integrate metabolic information and adapt the response of the organism to physiological demands. Polarized ependymoglial cells lining the floor of the third ventricle and sending a single process within metabolic hypothalamic parenchyma, tanycytes are henceforth described as key components of the hypothalamic neural network controlling energy balance. Their strategic position and peculiar properties convey them diverse physiological functions ranging from blood/brain traffic controllers, metabolic modulators, and neural stem/progenitor cells. At the molecular level, these functions rely on an accurate regulation of gene expression. Indeed, tanycytes are characterized by their own molecular signature which is mostly associated to their diverse physiological functions, and the detection of variations in nutrient/hormone levels leads to an adequate modulation of genetic profile in order to ensure energy homeostasis. The aim of this review is to summarize recent knowledge on the nutritional control of tanycyte gene expression.

Nutrient Sensing by Hypothalamic Tanycytes

Nutritional signals have long been implicated in the control of cellular processes that take place in the hypothalamus. This includes food intake regulation and energy balance, inflammation, and most recently, neurogenesis. One of the main glial cells residing in the hypothalamus are tanycytes, radial glial-like cells, whose bodies are located in the lining of the third ventricle, with processes extending to the parenchyma and reaching neuronal nuclei. Their unique anatomical location makes them directly exposed to nutrients in the cerebrospinal fluid. Several research groups have shown that tanycytes can respond to nutritional signals by different mechanisms, such as calcium signaling, metabolic shift, and changes in proliferation/differentiation potential. Despite cumulative evidence showing tanycytes have the molecular components to participate in nutrient detection and response, there are no enough functional studies connecting tanycyte nutrient sensing with hypothalamic functions, nor that highlight the relevance of this process in physiological and pathological context. This review will summarize recent evidence that supports a nutrient sensor role for tanycytes in the hypothalamus, highlighting the need for more detailed analysis on the actual implications of tanycyte-nutrient sensing and how this process can be modulated, which might allow the discovery of new metabolic and signaling pathways as therapeutic targets, for the treatment of hypothalamic related diseases.

Rhythmic Diurnal Synthesis and Signaling of Retinoic Acid in the Rat Pineal Gland and Its Action to Rapidly Downregulate ERK Phosphorylation

Vitamin A is important for the circadian timing system; deficiency disrupts daily rhythms in activity and clock gene expression, and reduces the nocturnal peak in melatonin in the pineal gland. However, it is currently unknown how these effects are mediated. Vitamin A primarily acts via the active metabolite, retinoic acid (RA), a transcriptional regulator with emerging non-genomic activities. We investigated whether RA is subject to diurnal variation in synthesis and signaling in the rat pineal gland. Its involvement in two key molecular rhythms in this gland was also examined: kinase activation and induction of Aanat, which encodes the rhythm-generating melatonin synthetic enzyme. We found diurnal changes in expression of several genes required for RA signaling, including a RA receptor and synthetic enzymes. The RA-responsive gene Cyp26a1 was found to change between day and night, suggesting diurnal changes in RA activity. This corresponded to changes in RA synthesis, suggesting rhythmic production of RA. Long-term RA treatment in vitro upregulated Aanat transcription, while short-term treatment had no effect. RA was also found to rapidly downregulate extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, suggesting a rapid non-genomic action which may be involved in driving the molecular rhythm in ERK1/2 activation in this gland. These results demonstrate that there are diurnal changes in RA synthesis and activity in the rat pineal gland which are partially under circadian control. These may be key to the effects of vitamin A on circadian rhythms, therefore providing insight into the molecular link between this nutrient and the circadian system.

A seasonal switch in histone deacetylase gene expression in the hypothalamus and their capacity to modulate nuclear signaling pathways

Seasonal animals undergo changes in physiology and behavior between summer and winter conditions. These changes are in part driven by a switch in a series of hypothalamic genes under transcriptional control by hormones and, of recent interest, inflammatory factors. Crucial to the control of transcription are histone deacetylases (HDACs), generally acting to repress transcription by local histone modification. Seasonal changes in hypothalamic HDAC transcripts were investigated in photoperiod-sensitive F344 rats by altering the day-length (photoperiod). HDAC4, 6 and 9 were found to change in expression. The potential influence of HDACs on two hypothalamic signaling pathways that regulate transcription, inflammatory and nuclear receptor signaling, was investigated. For inflammatory signaling the focus was on NF-κB because of the novel finding made that its expression is seasonally regulated in the rat hypothalamus. For nuclear receptor signaling it was discovered that expression of retinoic acid receptor beta was regulated seasonally. HDAC modulation of NF-κB-induced pathways was examined in a hypothalamic neuronal cell line and primary hypothalamic tanycytes. HDAC4/5/6 inhibition altered the control of gene expression (Fos, Prkca, Prkcd and Ptp1b) by inducers of NF-κB that activate inflammation. These inhibitors also modified the action of nuclear receptor ligands thyroid hormone and retinoic acid. Thus seasonal changes in HDAC4 and 6 have the potential to epigenetically modify multiple gene regulatory pathways in the hypothalamus that could act to limit inflammatory pathways in the hypothalamus during long-day summer-like conditions.

Inhibition of brain retinoic acid catabolism: a mechanism for minocycline's pleiotropic actions?

OBJECTIVES

Minocycline is a tetracycline antibiotic increasingly recognized in psychiatry for its pleiotropic anti-inflammatory and neuroprotective potential. While underlying mechanisms are still incompletely understood, several lines of evidence suggest a relevant functional overlap with retinoic acid (RA), a highly potent small molecule exhibiting a great variety of anti-inflammatory and neuroprotective properties in the adult central nervous system (CNS). RA homeostasis in the adult CNS is tightly controlled through local RA synthesis and cytochrome P450 (CYP450)-mediated inactivation of RA. Here, we hypothesized that minocycline may directly affect RA homeostasis in the CNS via altering local RA degradation.

METHODS

We used in vitro RA metabolism assays with metabolically competent synaptosomal preparations from murine brain and human SH-SY5Y neuronal cells as well as viable human SH-SY5Y neuroblastoma cell cultures.

RESULTS

We revealed that minocycline potently blocks RA degradation as measured by reversed-phase high-performance liquid chromatography and in a viable RA reporter cell line, even at low micromolar levels of minocycline.

CONCLUSIONS

Our findings provide evidence for enhanced RA signalling to be involved in minocycline's pleiotropic mode of action in the CNS. This novel mode of action of minocycline may help in developing more specific and effective strategies in the treatment of neuroinflammatory or neurodegenerative disorders.

Seasonal Control of Mammalian Energy Balance: Recent Advances in the Understanding of Daily Torpor and Hibernation

Endothermic mammals and birds require intensive energy turnover to sustain high body temperatures and metabolic rates. To cope with the energetic bottlenecks associated with the change of seasons, and to minimise energy expenditure, complex mechanisms and strategies are used, such as daily torpor and hibernation. During torpor, metabolic depression and low body temperatures save energy. However, these bouts of torpor, lasting for hours to weeks, are interrupted by active 'euthermic' phases with high body temperatures. These dynamic transitions require precise communication between the brain and peripheral tissues to defend rheostasis in energetics, body mass and body temperature. The hypothalamus appears to be the major control centre in the brain, coordinating energy metabolism and body temperature. The sympathetic nervous system controls body temperature by adjustments of shivering and nonshivering thermogenesis, with the latter being primarily executed by brown adipose tissue. Over the last decade, comparative physiologists have put forward integrative studies on the ecophysiology, biochemistry and molecular regulation of energy balance in response to seasonal challenges, food availability and ambient temperature. Mammals coping with such environments comprise excellent model organisms for studying the dynamic regulation of energy metabolism. Beyond the understanding of how animals survive in nature, these studies also uncover general mechanisms of mammalian energy homeostasis. This research will benefit efforts of translational medicine aiming to combat emerging human metabolic disorders. The present review focuses on recent advances in the understanding of energy balance and its neuronal and endocrine control during the most extreme metabolic fluctuations in nature: daily torpor and hibernation.

A neuroendocrine role for chemerin in hypothalamic remodelling and photoperiodic control of energy balance

Long-term and reversible changes in body weight are typical of seasonal animals. Thyroid hormone (TH) and retinoic acid (RA) within the tanycytes and ependymal cells of the hypothalamus have been implicated in the photoperiodic response. We investigated signalling downstream of RA and how this links to the control of body weight and food intake in photoperiodic F344 rats. Chemerin, an inflammatory chemokine, with a known role in energy metabolism, was identified as a target of RA. Gene expression of chemerin (Rarres2) and its receptors were localised within the tanycytes and ependymal cells, with higher expression under long (LD) versus short (SD) photoperiod, pointing to a physiological role. The SD to LD transition (increased food intake) was mimicked by 2 weeks of ICV infusion of chemerin into rats. Chemerin also increased expression of the cytoskeletal protein vimentin, implicating hypothalamic remodelling in this response. By contrast, acute ICV bolus injection of chemerin on a 12 h:12 h photoperiod inhibited food intake and decreased body weight with associated changes in hypothalamic neuropeptides involved in growth and feeding after 24 hr. We describe the hypothalamic ventricular zone as a key site of neuroendocrine regulation, where the inflammatory signal, chemerin, links TH and RA signaling to hypothalamic remodeling.

Hypothalamic Wnt Signalling and its Role in Energy Balance Regulation

Wnt signalling and its downstream effectors are well known for their roles in embryogenesis and tumourigenesis, including the regulation of cell proliferation, survival and differentiation. In the nervous system, Wnt signalling has been described mainly during embryonic development, although accumulating evidence suggests that it also plays a major role in adult brain morphogenesis and function. Studies have predominantly concentrated on memory formation in the hippocampus, although recent data indicate that Wnt signalling is also critical for neuroendocrine control of the developed hypothalamus, a brain centre that is key in energy balance regulation and whose dysfunction is implicated in metabolic disorders such as type 2 diabetes and obesity. Based on scattered findings that report the presence of Wnt molecules in the tanycytes and ependymal cells lining the third ventricle and arcuate nucleus neurones of the hypothalamus, their potential importance in key regions of food intake and body weight regulation has been investigated in recent studies. The present review brings together current knowledge on Wnt signalling in the hypothalamus of adult animals and discusses the evidence suggesting a key role for members of the Wnt signalling family in glucose and energy balance regulation in the hypothalamus in diet-induced and genetically obese (leptin deficient) mice. Aspects of Wnt signalling in seasonal (photoperiod sensitive) rodents are also highlighted, given the recent evidence indicating that the Wnt pathway in the hypothalamus is not only regulated by diet and leptin, but also by photoperiod in seasonal animals, which is connected to natural adaptive changes in food intake and body weight. Thus, Wnt signalling appears to be critical as a modulator for normal functioning of the physiological state in the healthy adult brain, and is also crucial for normal glucose and energy homeostasis where its dysregulation can lead to a range of metabolic disorders.

Thyroid hormone activation of retinoic acid synthesis in hypothalamic tanycytes

Thyroid hormone (TH) is essential for adult brain function and its actions include several key roles in the hypothalamus. Although TH controls gene expression via specific TH receptors of the nuclear receptor class, surprisingly few genes have been demonstrated to be directly regulated by TH in the hypothalamus, or the adult brain as a whole. This study explored the rapid induction by TH of retinaldehyde dehydrogenase 1 (Raldh1), encoding a retinoic acid (RA)-synthesizing enzyme, as a gene specifically expressed in hypothalamic tanycytes, cells that mediate a number of actions of TH in the hypothalamus. The resulting increase in RA may then regulate gene expression via the RA receptors, also of the nuclear receptor class. In vivo exposure of the rat to TH led to a significant and rapid increase in hypothalamic Raldh1 within 4 hours. That this may lead to an in vivo increase in RA is suggested by the later induction by TH of the RA-responsive gene Cyp26b1. To explore the actions of RA in the hypothalamus as a potential mediator of TH control of gene regulation, an ex vivo hypothalamic rat slice culture method was developed in which the Raldh1-expressing tanycytes were maintained. These slice cultures confirmed that TH did not act on genes regulating energy balance but could induce Raldh1. RA has the potential to upregulate expression of genes involved in growth and appetite, Ghrh and Agrp. This regulation is acutely sensitive to epigenetic changes, as has been shown for TH action in vivo. These results indicate that sequential triggering of two nuclear receptor signalling systems has the capability to mediate some of the functions of TH in the hypothalamus.

Hypothalamic tanycytes-masters and servants of metabolic, neuroendocrine, and neurogenic functions

There is a resurgent interest in tanycytes, a radial glial-like cell population occupying the floor and ventro-lateral walls of the third ventricle (3V). Tanycytes reside in close proximity to hypothalamic neuronal nuclei that regulate appetite and energy expenditure, with a subset sending projections into these nuclei. Moreover, tanycytes are exposed to 3V cerebrospinal fluid and have privileged access to plasma metabolites and hormones, through fenestrated capillaries. Indeed, some tanycytes act as conduits for trafficking of these molecules into the brain parenchyma. Tanycytes can also act as neural stem/progenitor cells, supplying the postnatal and adult hypothalamus with new neurons. Collectively, these findings suggest that tanycytes regulate and integrate important trophic and metabolic processes and possibly endow functional malleability to neuronal circuits of the hypothalamus. Hence, manipulation of tanycyte biology could provide a valuable tool for modulating hypothalamic functions such as energy uptake and expenditure in order to tackle prevalent eating disorders such as obesity and anorexia.

Dual signal transduction pathways activated by TSH receptors in rat primary tanycyte cultures

Tanycytes play multiple roles in hypothalamic functions, including sensing peripheral nutrients and metabolic hormones, regulating neurosecretion and mediating seasonal cycles of reproduction and metabolic physiology. This last function reflects the expression of TSH receptors in tanycytes, which detect photoperiod-regulated changes in TSH secretion from the neighbouring pars tuberalis. The present overall aim was to determine the signal transduction pathway by which TSH signals in tanycytes. Expression of the TSH receptor in tanycytes of 10-day-old Sprague Dawley rats was observed by in situ hybridisation. Primary ependymal cell cultures prepared from 10-day-old rats were found by immunohistochemistry to express vimentin but not GFAP and by PCR to express mRNA for Dio2, Gpr50, Darpp-32 and Tsh receptors that are characteristic of tanycytes. Treatment of primary tanycyte/ependymal cultures with TSH (100  IU/l) increased cAMP as assessed by ELISA and induced a cAMP-independent increase in the phosphorylation of ERK1/2 as assessed by western blot analysis. Furthermore, TSH (100  IU/l) stimulated a 2.17-fold increase in Dio2 mRNA expression. We conclude that TSH signal transduction in cultured tanycytes signals via Gαs to increase cAMP and via an alternative G protein to increase phosphorylation of ERK1/2.

Direct inhibition of retinoic acid catabolism by fluoxetine

Recent evidence from animal and human studies suggests neuroprotective effects of the SSRI fluoxetine, e.g., in the aftermath of stroke. The underlying molecular mechanisms remain to be fully defined. Because of its effects on the cytochrome P450 system (CYP450), we hypothesized that neuroprotection by fluoxetine is related to altered metabolism of retinoic acid (RA), whose CYP450-mediated degradation in brain tissue constitutes an important step in the regulation of its site-specific auto- and paracrine actions. Using traditional pharmacological in vitro assays, the effects of fluoxetine on RA degradation were probed in crude synaptosomes from rat brain and human-derived SH-SY5Y cells, and in cultures of neuron-like SH-SY5Y cells. Furthermore, retinoid-dependent effects of fluoxetine on neuronal survival following glutamate exposure were investigated in rat primary neurons cells using specific retinoid receptor antagonists. Experiments revealed dose-dependent inhibition of synaptosomal RA degradation by fluoxetine along with dose-dependent increases in RA levels in cell cultures. Furthermore, fluoxetine's neuroprotective effects against glutamate excitotoxicity in rat primary neurons were demonstrated to partially depend on RA signaling. Taken together, these findings demonstrate for the first time that the potent, pleiotropic antidepressant fluoxetine directly interacts with RA homeostasis in brain tissue, thereby exerting its neuroprotective effects.

Long-distance retinoid signaling in the zebra finch brain

All-trans retinoic acid (ATRA), the main active metabolite of vitamin A, is a powerful signaling molecule that regulates large-scale morphogenetic processes during vertebrate embryonic development, but is also involved post-natally in regulating neural plasticity and cognition. In songbirds, it plays an important role in the maturation of learned song. The distribution of the ATRA-synthesizing enzyme, zRalDH, and of ATRA receptors (RARs) have been described, but information on the distribution of other components of the retinoid signaling pathway is still lacking. To address this gap, we have determined the expression patterns of two obligatory RAR co-receptors, the retinoid X receptors (RXR) α and γ, and of the three ATRA-degrading cytochromes CYP26A1, CYP26B1, and CYP26C1. We have also studied the distribution of zRalDH protein using immunohistochemistry, and generated a refined map of ATRA localization, using a modified reporter cell assay to examine entire brain sections. Our results show that (1) ATRA is more broadly distributed in the brain than previously predicted by the spatially restricted distribution of zRalDH transcripts. This could be due to long-range transport of zRalDH enzyme between different nuclei of the song system: Experimental lesions of putative zRalDH peptide source regions diminish ATRA-induced transcription in target regions. (2) Four telencephalic song nuclei express different and specific subsets of retinoid-related receptors and could be targets of retinoid regulation; in the case of the lateral magnocellular nucleus of the anterior nidopallium (lMAN), receptor expression is dynamically regulated in a circadian and age-dependent manner. (3) High-order auditory areas exhibit a complex distribution of transcripts representing ATRA synthesizing and degrading enzymes and could also be a target of retinoid signaling. Together, our survey across multiple connected song nuclei and auditory brain regions underscores the prominent role of retinoid signaling in modulating the circuitry that underlies the acquisition and production of learned vocalizations.

The rhythm of retinoids in the brain

The retinoids are a family of compounds that in nature are derived from vitamin A or pro-vitamin A carotenoids. An essential part of the diet for mammals, vitamin A has long been known to be essential for many organ systems in the adult. More recently, however, they have been shown to be necessary for function of the brain and new discoveries point to a central role in processes ranging from neuroplasticity to neurogenesis. Acting in several regions of the central nervous system including the eye, hippocampus and hypothalamus, one common factor in its action is control of biological rhythms. This review summarizes the role of vitamin A in the brain; its action through the metabolite retinoic acid via specific nuclear receptors, and the regulation of its concentration through controlled synthesis and catabolism. The action of retinoic acid to regulate several rhythms in the brain and body, from circadian to seasonal, is then discussed to finish with the importance of retinoic acid in the regular pattern of sleep. We review the role of vitamin A and retinoic acid (RA) as mediators of rhythm in the brain. In the suprachiasmatic nucleus and hippocampus they control expression of circadian clock genes while in the cortex retinoic acid is required for delta oscillations of sleep. Retinoic acid is also central to a second rhythm that keeps pace with the seasons, regulating function in the hypothalamus and pineal gland.

Retinoic X receptor subtypes exert differential effects on the regulation of Trh transcription

How Retinoid X receptors (RXR) and thyroid hormone receptors (TR) interact on negative TREs and whether RXR subtype specificity is determinant in such regulations is unknown. In a set of functional studies, we analyzed RXR subtype effects in T3-dependent repression of hypothalamic thyrotropin-releasing hormone (Trh). Two-hybrid screening of a hypothalamic paraventricular nucleus cDNA bank revealed specific, T3-dependent interaction of TRs with RXRβ. In vivo chromatin immuno-precipitation showed recruitment of RXRs to the TRE-site 4 region of the Trh promoter in the absence of T3. In vivo overexpression of RXRα in the mouse hypothalamus heightened T3-independent Trh transcription, whereas RXRβ overexpression abrogated this activity. Loss of function of RXRα and β by shRNAs induced inverse regulations. Thus, RXRα and RXRβ display specific roles in modulating T3-dependent regulation of Trh. These results provide insight into the actions of these different TR heterodimerization partners within the context of a negatively regulated gene.

Neuromedin U partly mimics thyroid-stimulating hormone and triggers Wnt/β-catenin signalling in the photoperiodic response of F344 rats

In seasonal animals, photoperiod exerts profound effects on physiology, such as growth, energy balance and reproduction, via changes in the neuroendocrine axes. A key element of the photoperiodic response is the thyroid hormone level in the hypothalamus, which is controlled via retrograde transport of thyroid-stimulating hormone (TSH) from the pars tuberalis of the pituitary. TSH regulates type II deiodinase (Dio2) expression, which transforms inactive thyroid hormone to its active form, via TSH receptors expressed in the ependymal cells of the hypothalamus. In the present study, we hypothesised that a second peptide hormone, neuromedin U (NMU), may play a role in the photoperiodic response alongside TSH because the gene for NMU is also expressed in a strongly photoperiod-dependent manner in the pars tuberalis and its receptor NMU2 is expressed in the ependymal layer of the third ventricle in photoperiod-sensitive F344 rats. Consistent with other studies conducted in nonseasonal mammals, we found that acute i.c.v. injections of NMU into the hypothalamus negatively regulated food intake and body weight and increased core body temperature in F344 rats. At the same time, NMU increased Dio2 mRNA expression in the ependymal region of the hypothalamus similar to the effects of TSH. These data suggest that NMU may affect acute and photoperiodically controlled energy balance through distinct pathways. We also showed that TSH inhibits the expression of type III deiodinase (Dio3) in F344 rats, a response not mimicked by NMU. Furthermore, NMU also increased the expression of genes from the Wnt/β-catenin pathway within the ependymal layer of the third ventricle. This effect was not influenced by TSH. These data indicate that, although NMU acts with some similarities to TSH, it also has completely distinct signalling functions that do not overlap. In summary, the present study of NMU signalling reveals the potential for a new player in the control of seasonal biology.

Astrocyte fatty acid binding protein-7 is a marker for neurogenic niches in the rat hippocampus

Recent research has determined that newborn neurons in the dentate gyrus of the hippocampus of the macaque are frequently adjacent to astrocytes immunoreactive for fatty acid binding protein-7 (FABP7). To investigate if a similar relationship between FABP7-positive (FABP7+) astrocytes and proliferating cells exists in the rodent brain, sections of brains from juvenile rats were stained by immunohistochemistry to demonstrate newborn cells (antibody to Ki67 protein) and FABP7+ astrocytes. In rat brains, FABP7+ astrocytes were particularly abundant in the dentate gyrus of the hippocampus and were frequently close to dividing cells immunoreactive for Ki67 protein. FABP7+ astrocytes were also present in the olfactory bulbs, arcuate nucleus of the hypothalamus, and in the dorsal medulla subjacent to the area postrema, sites where more modest numbers of newborn neurons can also be found. These data suggest that regional accumulations of FABP7+ astrocytes may represent reservoirs of cells having the potential for neurogenesis. Because FABP7+ astrocytes are particularly abundant in the hippocampus, and since the gene for FABP7 has been linked to Alzheimer's disease, age-related changes in FABP7+ astrocytes (mitochondrial degeneration) may be relevant to age-associated disorders of the hippocampus.

Hypothalamic tanycytes: potential roles in the control of feeding and energy balance

Tanycytes, glial-like cells that line the third ventricle, are emerging as components of the hypothalamic networks that control body weight and energy balance. They contact the cerebrospinal fluid (CSF) and send processes that come into close contact with neurons in the arcuate and ventromedial hypothalamic nuclei. Tanycytes are glucosensitive and are able to respond to transmitters associated with arousal and the drive to feed. At least some tanycytes are stem cells and, in the median eminence, may be stimulated by diet to generate new neurons. The quest is on to understand how tanycytes detect and respond to changes in energy balance and how they communicate with the rest of the nervous system to effect their functions.

A vitamin for the brain

In the central nervous system (CNS) the function of retinoic acid, the active metabolite of vitamin A, is best understood from its action in guiding embryonic development; as development comes to completion, retinoic acid signaling declines. However, it is increasingly recognized that this signaling mechanism does not disappear in the adult brain but becomes more regionally focused and takes on new roles. These functions are often tied to processes of neural plasticity whether in the hippocampus, through homeostatic neural plasticity, the olfactory bulb or the hypothalamus. The role of retinoic acid in the control of plastic processes has led to suggestions of its involvement in neural disorders, both degenerative and psychiatric. This review presents a snapshot of developments in these areas over recent years.