Hypothalamic ependymal-glial cells express the glucose transporter GLUT2, a protein involved in glucose sensing

Saturated fatty acids stimulate cytokine production in tanycytes via the PP2Ac-dependent signaling pathway

The hypothalamic tanycytes are crucial for free fatty acids (FFAs) detection, storage, and transport within the central nervous system. They have been shown to effectively respond to fluctuations in circulating FFAs, thereby regulating energy homeostasis. However, the precise molecular mechanisms by which tanycytes modulate lipid utilization remain unclear. Here, we report that the catalytic subunit of protein phosphatase 2 A (PP2Ac), a serine/threonine phosphatase, is expressed in tanycytes and its accumulation and activation occur in response to high-fat diet consumption. In vitro, tanycytic PP2Ac responds to palmitic acid (PA) exposure and accumulates and is activated at an early stage in an AMPK-dependent manner. Furthermore, activated PP2Ac boosts hypoxia-inducible factor-1α (HIF-1α) accumulation, resulting in upregulation of an array of cytokines. Pretreatment with a PP2Ac inhibitor, LB100, prevented the PA-induced elevation of vascular endothelial growth factor (VEGF), fibroblast growth factor 1 (FGF1), hepatocyte growth factor (HGF), and dipeptidyl peptidase IV (DPPIV or CD26). Our results disclose a mechanism of lipid metabolism in tanycytes that involves the activation of PP2Ac and highlight the physiological significance of PP2Ac in hypothalamic tanycytes in response to overnutrition and efficacious treatment of obesity.

Neurogenesis impairment with glial activation in the hippocampus-connected regions of intracerebroventricular streptozotocin-injected mice

Adult neurogenesis in the hippocampus and subventricular zone (SVZ) is impaired by intracerebroventricular administration of streptozotocin (icv-STZ) to rodents. Although neural cells in the several brain regions which connect with the hippocampus or SVZ is thought to be involved in the adult neurogenesis, few studies have investigated morphological alterations of glial cells in these areas. The present study revealed that icv-STZ induces reduction of neural progenitor cells and a dramatic increase in reactive astrocytes and microglia especially in the hippocampus and various hippocampus-connected brain areas. In contrast, there was no significant neuronal damage excluding demyelination of the stria medullaris. The results indicate the hippocampal neurogenesis impairment of this model might be occurred by activated glial cells in the hippocampus, or hippocampus-connected regions.

Current progress and challenges in the development of brain tissue models: How to grow up the changeable brain in vitro?

In vitro modeling of brain tissue is a promising but not yet resolved problem in modern neurobiology and neuropharmacology. Complexity of the brain structure and diversity of cell-to-cell communication in (patho)physiological conditions make this task almost unachievable. However, establishment of novel in vitro brain models would ultimately lead to better understanding of development-associated or experience-driven brain plasticity, designing efficient approaches to restore aberrant brain functioning. The main goal of this review is to summarize the available data on methodological approaches that are currently in use, and to identify the most prospective trends in development of neurovascular unit, blood-brain barrier, blood-cerebrospinal fluid barrier, and neurogenic niche in vitro models. The manuscript focuses on the regulation of adult neurogenesis, cerebral microcirculation and fluids dynamics that should be reproduced in the in vitro 4D models to mimic brain development and its alterations in brain pathology. We discuss approaches that are critical for studying brain plasticity, deciphering the individual person-specific trajectory of brain development and aging, and testing new drug candidates in the in vitro models.

Hyperglycemia increases SCO-spondin and Wnt5a secretion into the cerebrospinal fluid to regulate ependymal cell beating and glucose sensing

Hyperglycemia increases glucose concentrations in the cerebrospinal fluid (CSF), activating glucose-sensing mechanisms and feeding behavior in the hypothalamus. Here, we discuss how hyperglycemia temporarily modifies ependymal cell ciliary beating to increase hypothalamic glucose sensing. A high level of glucose in the rat CSF stimulates glucose transporter 2 (GLUT2)-positive subcommissural organ (SCO) cells to release SCO-spondin into the dorsal third ventricle. Genetic inactivation of mice GLUT2 decreases hyperglycemia-induced SCO-spondin secretion. In addition, SCO cells secrete Wnt5a-positive vesicles; thus, Wnt5a and SCO-spondin are found at the apex of dorsal ependymal cilia to regulate ciliary beating. Frizzled-2 and ROR2 receptors, as well as specific proteoglycans, such as glypican/testican (essential for the interaction of Wnt5a with its receptors) and Cx43 coupling, were also analyzed in ependymal cells. Finally, we propose that the SCO-spondin/Wnt5a/Frizzled-2/Cx43 axis in ependymal cells regulates ciliary beating, a cyclic and adaptive signaling mechanism to control glucose sensing.

The role of GLUT2 in glucose metabolism in multiple organs and tissues

The glucose transporter family has an important role in the initial stage of glucose metabolism; Glucose transporters 2 (GLUTs, encoded by the solute carrier family 2, SLC2A genes) is the major glucose transporter in β-cells of pancreatic islets and hepatocytes but is also expressed in the small intestine, kidneys, and central nervous system; GLUT2 has a relatively low affinity to glucose. Under physiological conditions, GLUT2 transports glucose into cells and allows the glucose concentration to reach balance on the bilateral sides of the cellular membrane; Variation of GLUT2 is associated with various endocrine and metabolic disorders; In this study, we discussed the role of GLUT2 in participating in glucose metabolism and regulation in multiple organs and tissues and its effects on maintaining glucose homeostasis.

Emerging roles of brain tanycytes in regulating blood-hypothalamus barrier plasticity and energy homeostasis

Seasonal changes in food intake and adiposity in many animal species are triggered by changes in the photoperiod. These latter changes are faithfully transduced into a biochemical signal by melatonin secreted by the pineal gland. Seasonal variations, encoded by melatonin, are integrated by third ventricular tanycytes of the mediobasal hypothalamus through the detection of the thyroid-stimulating hormone (TSH) released from the pars tuberalis. The mediobasal hypothalamus is a critical brain region that maintains energy homeostasis by acting as an interface between the neural networks of the central nervous system and the periphery to control metabolic functions, including ingestive behavior, energy homeostasis, and reproduction. Among the cells involved in the regulation of energy balance and the blood-hypothalamus barrier (BHB) plasticity are tanycytes. Increasing evidence suggests that anterior pituitary hormones, specifically TSH, traditionally considered to have unitary functions in targeting single endocrine sites, display actions on multiple somatic tissues and central neurons. Notably, modulation of tanycytic TSH receptors seems critical for BHB plasticity in relation to energy homeostasis, but this needs to be proven.

Functional importance of glucose transporters and chromatin epigenetic factors in Glioblastoma Multiforme (GBM): possible therapeutics

Glioblastoma Multiforme (GBM) is an aggressive brain cancer affecting glial cells and is chemo- and radio-resistant. Glucose is considered the most vital energy source for cancer cell proliferation. During metabolism, hexose molecules will be transported into the cells via transmembrane proteins known as glucose transporter (GLUT). Among them, GLUT-1 and GLUT-3 play pivotal roles in glucose transport in GBM. Knockdown studies have established the role of GLUT-1, and GLUT-3 mediated glucose transport in GBM cells, providing insight into GLUT-mediated cancer signaling and cancer aggressiveness. This review focussed on the vital role of GLUT-1 and GLUT-3 proteins, which regulate glucose transport. Recent studies have identified the role of GLUT inhibitors in effective cancer prevention. Several of them are in clinical trials. Understanding and functional approaches towards glucose-mediated cell metabolism and chromatin epigenetics will provide valuable insights into the mechanism of cancer aggressiveness, cancer stemness, and chemo-resistance in Glioblastoma Multiforme (GBM). This review summarizes the role of GLUT inhibitors, micro-RNAs, and long non-coding RNAs that aid in inhibiting glucose uptake by the GBM cells and other cancer cells leading to the identification of potential therapeutic, prognostic as well as diagnostic markers. Furthermore, the involvement of epigenetic factors, such as microRNAs, in regulating glycolytic genes was demonstrated.

Ontogeny of ependymoglial cells lining the third ventricle in mice

Introduction

During hypothalamic development, the germinative neuroepithelium gives birth to diverse neural cells that regulate numerous physiological functions in adulthood.

Methods

Here, we studied the ontogeny of ependymal cells in the mouse mediobasal hypothalamus using the BrdU approach and publicly available single-cell RNAseq datasets.

Results

We observed that while typical ependymal cells are mainly produced at E13, tanycyte birth depends on time and subtypes and lasts up to P8. Typical ependymocytes and β tanycytes are the first to arise at the top and bottom of the dorsoventral axis around E13, whereas α tanycytes emerge later in development, generating an outside-in dorsoventral gradient along the third ventricle. Additionally, α tanycyte generation displayed a rostral-to-caudal pattern. Finally, tanycytes mature progressively until they reach transcriptional maturity between P4 and P14.

Discussion

Altogether, this data shows that ependyma generation differs in time and distribution, highlighting the heterogeneity of the third ventricle.

GLUT2 expression by glial fibrillary acidic protein-positive tanycytes is required for promoting feeding-response to fasting

Feeding behavior is a complex process that depends on the ability of the brain to integrate hormonal and nutritional signals, such as glucose. One glucosensing mechanism relies on the glucose transporter 2 (GLUT2) in the hypothalamus, especially in radial glia-like cells called tanycytes. Here, we analyzed whether a GLUT2-dependent glucosensing mechanism is required for the normal regulation of feeding behavior in GFAP-positive tanycytes. Genetic inactivation of Glut2 in GFAP-expressing tanycytes was performed using Cre/Lox technology. The efficiency of GFAP-tanycyte targeting was analyzed in the anteroposterior and dorsoventral axes by evaluating GFP fluorescence. Feeding behavior, hormonal levels, neuronal activity using c-Fos, and neuropeptide expression were also analyzed in the fasting-to-refeeding transition. In basal conditions, Glut2-inactivated mice had normal food intake and meal patterns. Implementation of a preceeding fasting period led to decreased total food intake and a delay in meal initiation during refeeding. Additionally, Glut2 inactivation increased the number of c-Fos-positive cells in the ventromedial nucleus in response to fasting and a deregulation of Pomc expression in the fasting-to-refeeding transition. Thus, a GLUT2-dependent glucose-sensing mechanism in GFAP-tanycytes is required to control food consumption and promote meal initiation after a fasting period.

A Short-Term Sucrose Diet Impacts Cell Proliferation of Neural Precursors in the Adult Hypothalamus

Radial glia-like cells in the hypothalamus and dorsal vagal complex are neural precursors (NPs) located near subventricular organs: median eminence and area postrema, respectively. Their strategic position can detect blood-borne nutrients, hormones, and mitogenic signals. Hypothalamic NPs increase their proliferation with a mechanism that involves hemichannel (HC) activity. NPs can originate new neurons in response to a short-term high-fat diet as a compensatory mechanism. The effects of high carbohydrate Western diets on adult neurogenesis are unknown. Although sugars are usually consumed as sucrose, more free fructose is now incorporated into food items. Here, we studied the proliferation of both types of NPs in Sprague Dawley rats exposed to a short-term high sucrose diet (HSD) and a control diet. In tanycyte cultures, we evaluated the effects of glucose and fructose and a mix of both hexoses on HC activity. In rats fed an HSD, we observed an increase in the proliferative state of both precursors. Glucose, either in the presence or absence of fructose, but not fructose alone, induced in vitro HC activity. These results should broaden the understanding of the nutrient monitoring capacity of NPs in reacting to changes in feeding behavior, specifically to high sugar western diets.

The ventromedial hypothalamic nucleus: watchdog of whole-body glucose homeostasis

The brain, particularly the ventromedial hypothalamic nucleus (VMH), has been long known for its involvement in glucose sensing and whole-body glucose homeostasis. However, it is still not fully understood how the brain detects and responds to the changes in the circulating glucose levels, as well as brain-body coordinated control of glucose homeostasis. In this review, we address the growing evidence implicating the brain in glucose homeostasis, especially in the contexts of hypoglycemia and diabetes. In addition to neurons, we emphasize the potential roles played by non-neuronal cells, as well as extracellular matrix in the hypothalamus in whole-body glucose homeostasis. Further, we review the ionic mechanisms by which glucose-sensing neurons sense fluctuations of ambient glucose levels. We also introduce the significant implications of heterogeneous neurons in the VMH upon glucose sensing and whole-body glucose homeostasis, in which sex difference is also addressed. Meanwhile, research gaps have also been identified, which necessities further mechanistic studies in future.

Maternal Metabolic Programming of the Developing Central Nervous System: Unified Pathways to Metabolic and Psychiatric Disorders

The perinatal period presents a critical time in offspring development where environmental insults can have damaging impacts on the future health of the offspring. This can lead to sustained alterations in offspring development, metabolism, and predisposition to both metabolic and psychiatric diseases. The central nervous system is one of the most sensitive targets in response to maternal obesity and/or type 2 diabetes mellitus. While many of the effects of obesity on brain function in adults are known, we are only now beginning to understand the multitude of changes that occur in the brain during development on exposure to maternal overnutrition. Specifically, given recent links between maternal metabolic state and onset of neurodevelopmental diseases, the specific changes that are occurring in the offspring are even more relevant for the study of disease onset. It is therefore critical to understand the developmental effects of maternal obesity and/or type 2 diabetes mellitus and further to define the underlying cellular and molecular changes in the fetal brain. This review focuses on the current advancements in the study of maternal programming of brain development with particular emphasis on brain connectivity, specific regional effects, newly studied peripheral contributors, and key windows of interventions where maternal bodyweight and food intake may drive the most detrimental effects on the brain and associated metabolic and behavioral consequences.

Simultaneous exposure to electromagnetic field from mobile phone and unimpeded fructose drinking during pre-, peri-, and post-pubertal stages perturbs the hypothalamic and hepatic regulation of energy homeostasis by early adulthood: experimental evidence

The present-day children-adolescents ubiquitously use the mobile phones and unrestrictedly consume fructose-laden diet. Unfortunately, a rise in the incidence of insulin resistance and fatty liver syndrome in young adults has also been recorded. To delineate a possible correlate, the effect of exposure to electromagnetic field (EMF) from the mobile phone and unrestricted fructose intake during pre-, peri-, and post-pubertal stages of development on orexigenic and anorexigenic signals arising from the hypothalamus and liver of rats is investigated here. The study design included four arms, i.e., "Normal", "Exposure Only (ExpO)", "Fructose Only (FruO)", and "Exposure with Fructose (EF)", wherein weaned rats received either "normal chow and drinking water" or "normal chow and fructose (15%) drinking solution" in presence and absence of EMF exposure (2 h/day) for 8 weeks. The results indicate that the total calories consumed by the EF were higher by early adulthood than normal, possibly under the influence of the raised levels of the orexigenic hormone, i.e., ghrelin, and it reflected as raised rate of weight gain. At early adulthood, the EF recorded mitigated response and sensitivity of insulin. Despite EF being a "fed-state", both centrally and peripherally, the glycolysis was restrained, but the gluconeogenesis was raised. Additionally, the altered lipid profile and the glycogen levels indicate that the EF developed fatty liver. The energy homeostasis of the EF was compromised as evidenced by (a) reduced expression of the glucosensors-GLUT2 and glucokinase in the hypothalamus and liver and (b) reduced expression of the cellular energy regulator-AMPK, orexigenic peptide-NPY, and anorexigenic peptide-POMC in the hypothalamus. Taken together, the present study evidences that the exposure to EMFfrom the mobile phone and unrestricted fructose intake during childhood-adolescence impairs the central and peripheral pathways that mediate the glucosensing, glucoregulation, feeding, and satiety behavior by early adulthood.

Insulin signalling in tanycytes gates hypothalamic insulin uptake and regulation of AgRP neuron activity

Insulin acts on neurons and glial cells to regulate systemic glucose metabolism and feeding. However, the mechanisms of insulin access in discrete brain regions are incompletely defined. Here we show that insulin receptors in tanycytes, but not in brain endothelial cells, are required to regulate insulin access to the hypothalamic arcuate nucleus. Mice lacking insulin receptors in tanycytes (IR^∆Tan mice) exhibit systemic insulin resistance, while displaying normal food intake and energy expenditure. Tanycytic insulin receptors are also necessary for the orexigenic effects of ghrelin, but not for the anorexic effects of leptin. IR^∆Tan mice exhibit increased agouti-related peptide (AgRP) neuronal activity, while displaying blunted AgRP neuronal adaptations to feeding-related stimuli. Lastly, a highly palatable food decreases tanycytic and arcuate nucleus insulin signalling to levels comparable to those seen in IR^∆Tan mice. These changes are rooted in modifications of cellular stress responses and of mitochondrial protein quality control in tanycytes. Conclusively, we reveal a critical role of tanycyte insulin receptors in gating feeding-state-dependent regulation of AgRP neurons and systemic insulin sensitivity, and show that insulin resistance in tanycytes contributes to the pleiotropic manifestations of obesity-associated insulin resistance.

Fructose-rich diet and walnut supplementation differently regulate rat hypothalamic and hippocampal glucose transporters expression

BACKGROUND

Nutritional modulations may be considered a strategy to protect mental health. Neuronal homeostasis is highly dependent on the availability of glucose, which represents the primary energy source for the brain. In this study, we evaluated the effects of walnut intake and fructose-rich diet on the expression of glucose transporters (GLUTs) in two rat brain regions: hypothalamus and hippocampus.

RESULTS

Our results show that walnut supplementation of fructose-fed animals restored the hypothalamic content of GLUT1 and GLUT3 protein. Furthermore, walnut intake did not affect increased hypothalamic GLUT2 content upon fructose consumption. These effects were accompanied by distinctive alterations of hippocampal GLUTs levels. Specifically, walnut intake increased GLUT1 content, whereas GLUT2 protein was decreased within the rat hippocampus after both individual and combined treatments.

CONCLUSION

Overall, our study suggests that walnut supplementation exerted modulatory effects on the glucose transporters within specific brain regions in the presence of developed metabolic disorder. © 2021 Society of Chemical Industry.

Alterations in tanycytes and related cell populations of arcuate nucleus in streptozotocin-induced Alzheimer disease model

It is assumed that dysfunction of tanycytes could be one of the components of pathogenesis of both Alzheimer disease and type 2 diabetes mellitus. The study was aimed to assess alterations in the tanycyte morphology in the Alzheimer disease model. The 3 mg/kg streptozotocin dose was injected in the lateral ventricles of Wistar rats in order to model the Alzheimer disease. Alterations in hypothalamic tanycytes were assessed 2 weeks, 4 weeks, 3 months and 6 months after administration of the toxin. Immunohistochemistry was used to identify the protein markers of tanycytes (vimentin, nestin), astrocytes (GFAP, glutamine synthetase) and neurons (HuC/D), as well as to assess cell proliferation (with the use of Ki67 protein) and mitochondrial alterations (mitochondrial complex IV, PGC1a). Administration of streptozotocin lead to β-amyloid accumulation in hypothalamus and ventricular enlargement (p < 0.001). Streptozotocin damaged both α1/α2 tanycytes and β1 tanycytes. The intensity of vimentin staining in α1/α2 tanycytes decreased by week 4 (p = 0.003), and in β1 tanycytes it decreased in three months (p < 0.001). The same trend was observed for nestin. The number of Ki67+ nuclei decreased (p < 0.05), and the expression of proteins associated with mitochondria changed. The density of hypothalamic tanycytes decreased by week 4 after administration of the toxin. Moreover, astrocyte activation was revealed. However, no prominent damage to both astrocytes and neurons was observed within four weeks after administration of streptozotocin. The revealed high tanycyte vulnerability to streptozotocin is in line with the hypothesis of the role of damage to hypothalamic structures in both local and systemic metabolic disorders occurring in the Alzheimer disease models.

Anatomical Organization of the Rat Subfornical Organ

The subfornical organ (SFO) is a sensory circumventricular organ located along the anterodorsal wall of the third ventricle. SFO lacks a complete blood-brain barrier (BBB), and thus peripherally-circulating factors can penetrate the SFO parenchyma. These signals are detected by local neurons providing the brain with information from the periphery to mediate central responses to humoral signals and physiological stressors. Circumventricular organs are characterized by the presence of unique populations of non-neuronal cells, such as tanycytes and fenestrated endothelium. However, how these populations are organized within the SFO is not well understood. In this study, we used histological techniques to analyze the anatomical organization of the rat SFO and examined the distribution of neurons, fenestrated and non-fenestrated vasculature, tanycytes, ependymocytes, glia cells, and pericytes within its confines. Our data show that the shell of SFO contains non-fenestrated vasculature, while fenestrated capillaries are restricted to the medial-posterior core region of the SFO and associated with a higher BBB permeability. In contrast to non-fenestrated vessels, fenestrated capillaries are encased in a scaffold created by pericytes and embedded in a network of tanycytic processes. Analysis of c-Fos expression following systemic injections of angiotensin II or hypertonic NaCl reveals distinct neuronal populations responding to these stimuli. Hypertonic NaCl activates ∼13% of SFO neurons located in the shell. Angiotensin II-sensitive neurons represent ∼35% of SFO neurons and their location varies between sexes. Our study provides a comprehensive description of the organization of diverse cellular elements within the SFO, facilitating future investigations in this important brain area.

Brain Glucose Transporters: Role in Pathogenesis and Potential Targets for the Treatment of Alzheimer's Disease

The most common cause of dementia, especially in elderly people, is Alzheimer’s disease (AD), with aging as its main risk factor. AD is a multifactorial neurodegenerative disease. There are several factors increasing the risk of AD development. One of the main features of Alzheimer’s disease is impairment of brain energy. Hypometabolism caused by decreased glucose uptake is observed in specific areas of the AD-affected brain. Therefore, glucose hypometabolism and energy deficit are hallmarks of AD. There are several hypotheses that explain the role of glucose hypometabolism in AD, but data available on this subject are poor. Reduced transport of glucose into neurons may be related to decreased expression of glucose transporters in neurons and glia. On the other hand, glucose transporters may play a role as potential targets for the treatment of AD. Compounds such as antidiabetic drugs, agonists of SGLT1, insulin, siRNA and liposomes are suggested as therapeutics. Nevertheless, the suggested targets of therapy need further investigations.

Neural mechanisms underlying the role of fructose in overfeeding

Fructose consumption has been linked with metabolic syndrome and obesity. Fructose-based sweeteners like high fructose corn syrup taste sweeter, improve food palatability, and are increasingly prevalent in our diet. The increase in fructose consumption precedes the rise in obesity and is a contributing driver to the obesity epidemic worldwide. The role of dietary fructose in obesity can be multifactorial by promoting visceral adiposity, hypertension, and insulin resistance. Interestingly, one emergent finding from human and animal studies is that dietary fructose promotes overfeeding. As the brain is a critical regulator of food intake, we reviewed the evidence that fructose can act in the brain and elucidated the major brain systems underlying fructose-induced overfeeding. We found that fructose acts on multiple interdependent brain systems to increase orexigenic drive and the incentive salience of food while decreasing the latency between food bouts and reducing cognitive control to disinhibit feeding. We concluded that the collective actions of fructose may promote feeding behavior by producing a hunger-like state in the brain.

Hypothalamic Astrocytes as a Specialized and Responsive Cell Population in Obesity

Astrocytes are a type of glial cell anatomically and functionally integrated into the neuronal regulatory circuits for the neuroendocrine control of metabolism. Being functional integral compounds of synapses, astrocytes are actively involved in the physiological regulatory aspects of metabolic control, but also in the pathological processes that link neuronal dysfunction and obesity. Between brain areas, the hypothalamus harbors specialized functional circuits that seem selectively vulnerable to metabolic damage, undergoing early cellular rearrangements which are thought to be at the core of the pathogenesis of diet-induced obesity. Such changes in the hypothalamic brain region consist of a rise in proinflammatory cytokines, the presence of a reactive phenotype in astrocytes and microglia, alterations in the cytoarchitecture and synaptology of hypothalamic circuits, and angiogenesis, a phenomenon that cannot be found elsewhere in the brain. Increasing evidence points to the direct involvement of hypothalamic astrocytes in such early metabolic disturbances, thus moving the study of these glial cells to the forefront of obesity research. Here we provide a comprehensive review of the most relevant findings of molecular and pathophysiological mechanisms by which hypothalamic astrocytes might be involved in the pathogenesis of obesity.

Purinergic signaling in tanycytes and its contribution to nutritional sensing

Tanycytes are hypothalamic radial glial-like cells with an important role in the regulation of neuroendocrine axes and energy homeostasis. These cells have been implicated in glucose, amino acids, and fatty acid sensing in the hypothalamus of rodents, where they are strategically positioned. While their cell bodies contact the cerebrospinal fluid, their extensive processes contact neurons of the arcuate and ventromedial nuclei, protagonists in the regulation of food intake. A growing body of evidence has shown that purinergic signaling plays a relevant role in this homeostatic role of tanycytes, likely regulating the release of gliotransmitters that will modify the activity of satiety-controlling hypothalamic neurons. Connexin hemichannels have proven to be particularly relevant in these mechanisms since they are responsible for the release of ATP from tanycytes in response to nutritional signals. On the other hand, either ionotropic or metabotropic ATP receptors are involved in the generation of intracellular Ca²⁺ waves in response to hypothalamic nutrients, which can spread between glial cells and towards neighboring neurons. This review will summarize recent evidence that supports a nutrient sensor role for tanycytes, highlighting the participation of purinergic signaling in this process.

Astrocyte Clocks and Glucose Homeostasis

The endogenous timekeeping system evolved to anticipate the time of the day through the 24 hours cycle of the Earth's rotation. In mammals, the circadian clock governs rhythmic physiological and behavioral processes, including the daily oscillation in glucose metabolism, food intake, energy expenditure, and whole-body insulin sensitivity. The results from a series of studies have demonstrated that environmental or genetic alterations of the circadian cycle in humans and rodents are strongly associated with metabolic diseases such as obesity and type 2 diabetes. Emerging evidence suggests that astrocyte clocks have a crucial role in regulating molecular, physiological, and behavioral circadian rhythms such as glucose metabolism and insulin sensitivity. Given the concurrent high prevalence of type 2 diabetes and circadian disruption, understanding the mechanisms underlying glucose homeostasis regulation by the circadian clock and its dysregulation may improve glycemic control. In this review, we summarize the current knowledge on the tight interconnection between the timekeeping system, glucose homeostasis, and insulin sensitivity. We focus specifically on the involvement of astrocyte clocks, at the organism, cellular, and molecular levels, in the regulation of glucose metabolism.

Role of insulin in the neuroendocrine control of reproduction

Infertility associated with insulin resistance is characterised by abnormal hormone secretion by the hypothalamus, pituitary gland and gonads. These endocrine tissues can maintain insulin sensitivity even when tissues such as the muscle and liver become insulin-resistant, resulting in excessive insulin stimulation as hyperinsulinaemia develops. Experiments conducted to determine the role of neuronal insulin signalling in fertility were unable to recapitulate early findings of hypogonadotrophic hypogonadism in mice lacking insulin receptors throughout the brain. Rather, it was eventually shown that astrocytes critically mediate the effects of insulin on puberty timing and adult reproductive function. However, specific roles for neurones and gonadotrophs have been revealed under conditions of hyperinsulinaemia and by ablation of insulin and leptin receptors. The collective picture is one of multiple insulin-responsive inputs to gonadotrophin releasing hormone neurones, with astrocytes being the most important player.

Expression of glucose transporters in human neurodegenerative diseases

The central nervous system (CNS) plays an important role in the human body. It is involved in the receive, store and participation in information retrieval. It can use several substrates as a source of energy, however, the main source of energy is glucose. Cells of the central nervous system need a continuous supply of energy, therefore, transport of glucose into these cells is very important. There are three distinct families of glucose transporters: sodium-independent glucose transporters (GLUTs), sodium-dependent glucose cotransporters (SGLTs), and uniporter, SWEET protein. In the human brain only GLUTs and SGLTs were detected. In neurodegenerative diseases was observed hypometabolism of glucose due to decreased expression of glucose transporters, in particular GLUT1 and GLUT3. On the other hand, animal studies revealed, that increased levels of these glucose transporters, due to for example by the increased copy number of SLC2A genes, may have a beneficial effect and may be a targeted therapy in the treatment of patients with AD, HD and PD.

Tanycytes in the infundibular nucleus and median eminence and their role in the blood-brain barrier

The blood-brain barrier is generally attributed to endothelial cells. However, in circumventricular organs, such as the median eminence, tanycytes take over the barrier function. These ependymoglial cells form the wall of the third ventricle and send long extensions into the parenchyma to contact blood vessels and hypothalamic neurons. The shape and location of tanycytes put them in an ideal position to connect the periphery with central nervous compartments. In line with this, tanycytes control the transport of hormones and key metabolites in and out of the hypothalamus. They function as sensors of peripheral homeostasis for central regulatory networks. This chapter discusses current evidence that tanycytes play a key role in regulating glucose balance, food intake, endocrine axes, seasonal changes, reproductive function, and aging. The understanding of how tanycytes perform these diverse tasks is only just beginning to emerge and will probably lead to a more differentiated view of how the brain and the periphery interact.

Immunometabolic Changes in Glia - A Potential Role in the Pathophysiology of Obesity and Diabetes

Chronic low-grade inflammation is a feature of the pathophysiology of obesity and diabetes in the CNS as well as peripheral tissues. Glial cells are critical mediators of the response to inflammation in the brain. Key features of glia include their metabolic flexibility, sensitivity to changes in the CNS microenvironment, and ability to rapidly adapt their function accordingly. They are specialised cells which cooperate to promote and preserve neuronal health, playing important roles in regulating the activity of neuronal networks across the brain during different life stages. Increasing evidence points to a role of glia, most notably astrocytes and microglia, in the systemic regulation of energy and glucose homeostasis in the course of normal physiological control and during disease. Inflammation is an energetically expensive process that requires adaptive changes in cellular metabolism and, in turn, metabolic intermediates can also have immunomodulatory actions. Such "immunometabolic" changes in peripheral immune cells have been implicated in contributing to disease pathology in obesity and diabetes. This review will discuss the evidence for a role of immunometabolic changes in glial cells in the systemic regulation of energy and glucose homeostasis, and how this changes in the context of obesity and diabetes.

The FGF2-induced tanycyte proliferation involves a connexin 43 hemichannel/purinergic-dependent pathway

In the adult hypothalamus, the neuronal precursor role is attributed to the radial glia-like cells that line the third-ventricle (3V) wall called tanycytes. Under nutritional cues, including hypercaloric diets, tanycytes proliferate and differentiate into mature neurons that moderate body weight, suggesting that hypothalamic neurogenesis is an adaptive mechanism in response to metabolic changes. Previous studies have shown that the tanycyte glucosensing mechanism depends on connexin-43 hemichannels (Cx43 HCs), purine release, and increased intracellular free calcium ion concentration [(Ca²⁺ )_i ] mediated by purinergic P2Y receptors. Since, Fibroblast Growth Factor 2 (FGF2) causes similar purinergic events in other cell types, we hypothesize that this pathway can be also activated by FGF2 in tanycytes to promote their proliferation. Here, we used bromodeoxyuridine (BrdU) incorporation to evaluate if FGF2-induced tanycyte cell division is sensitive to Cx43 HC inhibition in vitro and in vivo. Immunocytochemical analyses showed that cultured tanycytes maintain the expression of in situ markers. After FGF2 exposure, tanycytic Cx43 HCs opened, enabling release of ATP to the extracellular milieu. Moreover, application of external ATP was enough to induce their cell division, which could be suppressed by Cx43 HC or P2Y1-receptor inhibitors. Similarly, in vivo experiments performed on rats by continuous infusion of FGF2 and a Cx43 HC inhibitor into the 3V, demonstrated that FGF2-induced β-tanycyte proliferation is sensitive to Cx43 HC blockade. Thus, FGF2 induced Cx43 HC opening, triggered purinergic signaling, and increased β-tanycytes proliferation, highlighting some of the molecular mechanisms involved in the cell division response of tanycyte. This article has an Editorial Highlight see https://doi.org/10.1111/jnc.15218.

Fanconi-Bickel Syndrome: A Review of the Mechanisms That Lead to Dysglycaemia

Accumulation of glycogen in the kidney and liver is the main feature of Fanconi-Bickel Syndrome (FBS), a rare disorder of carbohydrate metabolism inherited in an autosomal recessive manner due to SLC2A2 gene mutations. Missense, nonsense, frame-shift (fs), in-frame indels, splice site, and compound heterozygous variants have all been identified in SLC2A2 gene of FBS cases. Approximately 144 FBS cases with 70 different SLC2A2 gene variants have been reported so far. SLC2A2 encodes for glucose transporter 2 (GLUT2) a low affinity facilitative transporter of glucose mainly expressed in tissues playing important roles in glucose homeostasis, such as renal tubular cells, enterocytes, pancreatic β-cells, hepatocytes and discrete regions of the brain. Dysfunctional mutations and decreased GLUT2 expression leads to dysglycaemia (fasting hypoglycemia, postprandial hyperglycemia, glucose intolerance, and rarely diabetes mellitus), hepatomegaly, galactose intolerance, rickets, and poor growth. The molecular mechanisms of dysglycaemia in FBS are still not clearly understood. In this review, we discuss the physiological roles of GLUT2 and the pathophysiology of mutants, highlight all of the previously reported SLC2A2 mutations associated with dysglycaemia, and review the potential molecular mechanisms leading to dysglycaemia and diabetes mellitus in FBS patients.

Glucose transporters in brain in health and disease

Energy demand of neurons in brain that is covered by glucose supply from the blood is ensured by glucose transporters in capillaries and brain cells. In brain, the facilitative diffusion glucose transporters GLUT1-6 and GLUT8, and the Na⁺-d-glucose cotransporters SGLT1 are expressed. The glucose transporters mediate uptake of d-glucose across the blood-brain barrier and delivery of d-glucose to astrocytes and neurons. They are critically involved in regulatory adaptations to varying energy demands in response to differing neuronal activities and glucose supply. In this review, a comprehensive overview about verified and proposed roles of cerebral glucose transporters during health and diseases is presented. Our current knowledge is mainly based on experiments performed in rodents. First, the functional properties of human glucose transporters expressed in brain and their cerebral locations are described. Thereafter, proposed physiological functions of GLUT1, GLUT2, GLUT3, GLUT4, and SGLT1 for energy supply to neurons, glucose sensing, central regulation of glucohomeostasis, and feeding behavior are compiled, and their roles in learning and memory formation are discussed. In addition, diseases are described in which functional changes of cerebral glucose transporters are relevant. These are GLUT1 deficiency syndrome (GLUT1-SD), diabetes mellitus, Alzheimer’s disease (AD), stroke, and traumatic brain injury (TBI). GLUT1-SD is caused by defect mutations in GLUT1. Diabetes and AD are associated with changed expression of glucose transporters in brain, and transporter-related energy deficiency of neurons may contribute to pathogenesis of AD. Stroke and TBI are associated with changes of glucose transporter expression that influence clinical outcome.

Inhibition of complex I-III activity of brain mitochondria after intracerebroventricular administration of streptozotocin in rats is possibly related to loss of body weight

The effects of streptozotocin (STZ) on the brain after intracerebroventricular (ICV) administration in rodents have been suggested to mimic the pathogenesis of sporadic Alzheimer's disease (AD). Oxidative damage, decreased glucose utilization, mitochondrial bioenergetic changes, neuroinflammation and behavioral impairment have been reported in rodents after ICV-STZ administration. However, the molecular mechanisms of STZ effects on brain after ICV administration remain highly controversial. In this study we re-examined several bioenergetic parameters of rat brain mitochondria on day 15 following ICV-STZ treatment. We observed only a moderate but statistically significant decrease in complex I-III activity in brain mitochondria from streptozotocin-treated rats. There were no changes in complex II-III activity or phosphorylation capacity of brain mitochondria after streptozotocin treatment. More importantly, it was observed that ICV-STZ treatment caused variable degrees of body-weight loss in rats, and complex I-III activity was decreased only in those rats showing a significant (more than 10%-35%) loss in body-weights.

Nutritionally induced tanycytic plasticity in the hypothalamus of adult ewes

The blood-brain barrier regulates the transport of molecules that convey global energetic status to the feeding circuitry within the hypothalamus. Capillaries within the median eminence (ME) and tight junctions between tanycytes lining the third ventricle (3V) are critical components of this barrier. Herein, we tested the hypothesis that altering the plane of nutrition results in the structural reorganization of tanycytes, tight junctions, and capillary structure within the medial basal hypothalamus. Proopiomelanocortin (POMC) neuronal content within the arcuate nucleus of the hypothalamus (ARC) was also assessed to test whether reduced nutritional status improved access of nutrients to the ARC, while decreasing the access of nutrients of overfed animals. Multiparous, nongestating ewes were stratified by weight and randomly assigned to dietary treatments offered for 75 d: 200% of dietary recommendations (overfed), 100% of dietary recommendations (control), or 60% of dietary recommendations (underfed). The number of POMC-expressing neurons within the ARC was increased (P ≤ 0.002) in underfed ewes. Overfeeding increased (P ≤ 0.01) tanycyte cellular process penetration and density compared with control and underfeeding as assessed using vimentin immunostaining. Immunostaining of tight junctions along the wall of the 3V did not differ (P = 0.32) between treatments. No differences were observed in capillary density (P = 0.21) or classification (P ≥ 0.47) within the ME. These results implicate that changes within the satiety center and morphology of tanycytes within the ARC occur as an adaptation to nutrient availability.

Inhibition of Hypothalamic MCT4 and MCT1-MCT4 Expressions Affects Food Intake and Alters Orexigenic and Anorexigenic Neuropeptide Expressions

Feeding behavior regulation is a complex process, which depends on the central integration of different signals, such as glucose, leptin, and ghrelin. Recent studies have shown that glial cells known as tanycytes that border the basal third ventricle (3V) detect glucose and then use glucose-derived signaling to inform energy status to arcuate nucleus (ARC) neurons to regulate feeding behavior. Monocarboxylate transporters (MCT) 1 and MCT4 are localized in the cellular processes of tanycytes, which could facilitate monocarboxylate release to orexigenic and anorexigenic neurons. We hypothesize that MCT1 and MCT4 inhibitions could alter the metabolic communication between tanycytes and ARC neurons, affecting feeding behavior. We have previously shown that MCT1 knockdown rats eat more and exhibit altered satiety parameters. Here, we generate MCT4 knockdown rats and MCT1-MCT4 double knockdown rats using adenovirus-mediated transduction of a shRNA into the 3V. Feeding behavior was evaluated in MCT4 and double knockdown animals, and neuropeptide expression in response to intracerebroventricular glucose administration was measured. MCT4 inhibition produced a decrease in food intake, contrary to double knockdown. MCT4 inhibition was accompanied by a decrease in eating rate and mean meal size and an increase in mean meal duration, parameters that are not changed in the double knockdown animals with exception of eating rate. Finally, we observed a loss in glucose regulation of orexigenic neuropeptides and abnormal expression of anorexigenic neuropeptides in response to fasting when these transporters are inhibited. Taken together, these results indicate that MCT1 and MCT4 expressions in tanycytes play a role in feeding behavior regulation.

The median eminence as the hypothalamic area involved in rapid transfer of glucose to the brain: functional and cellular mechanisms

Our data proposes that glucose is transferred directly to the cerebrospinal fluid (CSF) of the hypothalamic ventricular cavity through a rapid "fast-track-type mechanism" that would efficiently stimulate the glucosensing areas. This mechanism would occur at the level of the median eminence (ME), a periventricular hypothalamic zone with no blood-brain barrier. This "fast-track" mechanism would involve specific glial cells of the ME known as β2 tanycytes that could function as "inverted enterocytes," expressing low-affinity glucose transporters GLUT2 and GLUT6 in order to rapidly transfer glucose to the CSF. Due to the large size of tanycytes, the presence of a high concentration of mitochondria and the expression of low-affinity glucose transporters, it would be expected that these cells accumulate glucose in the endoplasmic reticulum (ER) by sequestering glucose-6-phosphate (G-6-P), in a similar way to that recently demonstrated in astrocytes. Glucose could diffuse through the cells by micrometric distances to be released in the apical region of β2 tanycytes, towards the CSF. Through this mechanism, levels of glucose would increase inside the hypothalamus, stimulating glucosensing mechanisms quickly and efficiently. KEY MESSAGES: • Glucose diffuses through the median eminence cells (β2 tanycytes), towards the hypothalamic CSF. • Glucose is transferred through a rapid "fast-track-type mechanism" via GLUT2 and GLUT6. • Through this mechanism, hypothalamic glucose levels increase, stimulating glucosensing.

Dissecting the Brain/Islet Axis in Metabesity

The high prevalence of type 2 diabetes mellitus (T2DM), together with the fact that current treatments are only palliative and do not avoid major secondary complications, reveals the need for novel approaches to treat the cause of this disease. Efforts are currently underway to identify therapeutic targets implicated in either the regeneration or re-differentiation of a functional pancreatic islet β-cell mass to restore insulin levels and normoglycemia. However, T2DM is not only caused by failures in β-cells but also by dysfunctions in the central nervous system (CNS), especially in the hypothalamus and brainstem. Herein, we review the physiological contribution of hypothalamic neuronal and glial populations, particularly astrocytes, in the control of the systemic response that regulates blood glucose levels. The glucosensing capacity of hypothalamic astrocytes, together with their regulation by metabolic hormones, highlights the relevance of these cells in the control of glucose homeostasis. Moreover, the critical role of astrocytes in the response to inflammation, a process associated with obesity and T2DM, further emphasizes the importance of these cells as novel targets to stimulate the CNS in response to metabesity (over-nutrition-derived metabolic dysfunctions). We suggest that novel T2DM therapies should aim at stimulating the CNS astrocytic response, as well as recovering the functional pancreatic β-cell mass. Whether or not a common factor expressed in both cell types can be feasibly targeted is also discussed.

When a Little Bit More Makes the Difference: Expression Levels of GKRP Determines the Subcellular Localization of GK in Tanycytes

Glucose homeostasis is performed by specialized cells types that detect and respond to changes in systemic glucose concentration. Hepatocytes, β-cells and hypothalamic tanycytes are part of the glucosensor cell types, which express several proteins involved in the glucose sensing mechanism such as GLUT2, Glucokinase (GK) and Glucokinase regulatory protein (GKRP). GK catalyzes the phosphorylation of glucose to glucose-6-phosphate (G-6P), and its activity and subcellular localization are regulated by GKRP. In liver, when glucose concentration is low, GKRP binds to GK holding it in the nucleus, while the rise in glucose concentration induces a rapid export of GK from the nucleus to the cytoplasm. In contrast, hypothalamic tanycytes display inverse compartmentalization dynamic in response to glucose: a rise in the glucose concentration drives nuclear compartmentalization of GK. The underlying mechanism responsible for differential GK subcellular localization in tanycytes has not been described yet. However, it has been suggested that relative expression between GK and GKRP might play a role. To study the effects of GKRP expression levels in the subcellular localization of GK, we used insulinoma 832/13 cells and hypothalamic tanycytes to overexpress the tanycytic sequences of Gckr. By immunocytochemistry and Western blot analysis, we observed that overexpression of GKRP, independently of the cellular context, turns GK localization to a liver-like fashion, as GK is mainly localized in the nucleus in response to low glucose. Evaluating the expression levels of GKRP in relation to GK through RT-qPCR, suggest that excess of GKRP might influence the pattern of GK subcellular localization. In this sense, we propose that the low expression of GKRP (in relation to GK) observed in tanycytes is responsible, at least in part, for the compartmentalization pattern observed in this cell type. Since GKRP behaves as a GK inhibitor, the regulation of GKRP expression levels or activity in tanycytes could be used as a therapeutic target to regulate the glucosensing activity of these cells and consequently to regulate feeding behavior.

The methanolic extract of Thymus praecox subsp. skorpilii var. skorpilii restores glucose homeostasis, ameliorates insulin resistance and improves pancreatic β-cell function on streptozotocin/nicotinamide-induced type 2 diabetic rats

ETHNOPHARMACOLOGICAL RELEVANCE

Thymus praecox subsp. skorpilii var. skorpilii (syn. Thymus praecox subsp. jankae (Celak.) Jalas) is consumed as a Turkish folk medicine for the treatment of spasm, sore throat and shortness of breath, also having strong antioxidant activity and the leaves of the plant have been utilized for the treatment of diabetes as the decoction in Turkey.

AIM OF THE STUDY

In the present study, we aimed to investigate the potential mechanism of antidiabetic action of Thymus praecox subsp. skorpilii var. skorpilii methanolic extract (TPSE) on streptozotocin (STZ)/nicotinamide (NA)-induced type 2 diabetic rats.

MATERIALS AND METHODS

Sprague Dawley rats were randomly divided into four groups; control, diabetes, TPSE (100 mg/kg b.w, p.o.) and metformin group (400 mg/kg b.w, p.o.). Diabetes was established in all groups except control group by 55 mg/kg STZ (i.p.) for once 15 min after 100 mg/kg NA injection. 3 days after STZ/NA injection, treatments were administered for three weeks and then rats were decapitated; tissue and blood samples were obtained for measuring the level of glucose transporters (both GLUTs and sodium glucose co-transporters (SGLTs)), enzymes related to glucose (Hexokinase (HK), phosphoenolpyruvate carboxykinase (PEPCK), α-glucosidase) and lipid metabolism (Acetyl-coenzyme carboxylase (ACC)), AST, ALT, creatinine, insulin, anti-inflammatory (IL-10) and inflammatory (TNF-α, IL-1β, IL-6) cytokines, AMP-activated protein kinase (AMPK), peroxisome proliferator-activated receptor gamma (PPAR-γ) and glucagon like peptide-1 (GLP-1). Histopathological alterations of the pancreas were examined.

RESULTS

After three weeks of treatment, TPSE has exhibited a significant reduction of plasma levels of the proinflammatory cytokines. Besides, TPSE treatment elevated plasma insulin levels and normalized blood glucose levels. Moreover, it improved the values of AMPK in liver and GLP-1 in pancreas. Increased α-glucosidase, PEPCK, GLUT-2 and SGLTs levels with the induction of diabetes considerably lowered with TPSE treatment. Especially on SGLT-2, TPSE achieved a more prominent decrease. After the atrophy in Langerhans islets due to diabetes induction, treatment was found to prevent the damage of islets.

CONCLUSIONS

Based on the findings presented here, it has been concluded that TPSE has marked antidiabetic effects through various pathways on STZ/NA-induced diabetic rats and it may potentially be used as an effective treatment for type 2 diabetes mellitus (T2DM). Further research on isolation of the bioactive components is underway.

Glucose Increase DAGLα Levels in Tanycytes and Its Inhibition Alters Orexigenic and Anorexigenic Neuropeptides Expression in Response to Glucose

The endocannabinoid system (ECS) is composed of a group of Gi-coupled protein receptors and enzymes, producing and degrading the endocannabinoids, 2-arachidonoylglycerol (2-AG) and N-arachidonoyl-ethanolamine (AEA). Endocannabinoid-mediated signaling modulates brain functions, such as pain, mood, memory, and feeding behavior. The activation of the ECS is associated with overeating and obesity; however, the expression of components of this system has been only partially studied in the hypothalamus, a critical region implicated in feeding behavior. Within this brain region, anorexigenic, and orexigenic neurons of the arcuate nucleus (ARC) are in close contact with tanycytes, glial radial-like cells that line the lateral walls and floor of the third ventricle (3V). The specific function of tanycytes and the effects of metabolic signals generated by them on adjacent neurons is starting to be elucidated. We have proposed that the ECS within tanycytes modulates ARC neurons, thus modifying food intake. Here, we evaluated the expression and the loss of function of the 2-AG-producing enzyme, diacylglycerol lipase-alpha (DAGLα). Using Western blot and immunohistochemistry analyses in basal hypothalamus sections of adult rats under several glycemic conditions, we confirm that DAGLα is strongly expressed at the basal hypothalamus in glial and neuronal cells, increasing further in response to greater extracellular glucose levels. Using a DAGLα-inhibiting adenovirus (shRNA), suppression of DAGLα expression in tanycytes altered the usual response to intracerebroventricular glucose in terms of neuropeptides produced by neurons of the ARC. Thus, these results strongly suggest that the tanycytes could generate 2-AG, which modulates the function of anorexigenic and orexigenic neurons.

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.

Connexin-43 Gap Junctions Are Responsible for the Hypothalamic Tanycyte-Coupled Network

Tanycytes are hypothalamic radial glia-like cells that form the basal wall of the third ventricle (3V) where they sense glucose and modulate neighboring neuronal activity to control feeding behavior. This role requires the coupling of hypothalamic cells since transient decreased hypothalamic Cx43 expression inhibits the increase of brain glucose-induced insulin secretion. Tanycytes have been postulated as possible hypothalamic neuronal precursors due to their privileged position in the hypothalamus that allows them to detect mitogenic signals and because they share the markers and characteristics of neuronal precursors located in other neurogenic niches, including the formation of coupled networks through connexins. Using wild-type (WT), Cx30^−/– and Cx30^−/–, Cx43^fl/fl:glial fibrillary acidic protein (GFAP)-Cre (double knockout, dKO) mouse lines, we demonstrated that tanycytes are highly coupled to each other and also give rise to a panglial network specifically through Cx43. Using the human GFAP (hGFAP)-enhanced green fluorescent protein (EGFP) transgenic mouse line, we provided evidence that the main parenchymal-coupled cells were astrocytes. In addition, electrophysiological parameters, such as membrane resistance, were altered when Cx43 was genetically absent or pharmacologically inhibited. Finally, in the dKO mouse line, we detected a significant decrease in the number of hypothalamic proliferative parenchymal cells. Our results demonstrate the importance of Cx43 in tanycyte homotypic and panglial coupling and show that Cx43 function influences the proliferative potential of hypothalamic cells.

Food For Thought: Short-Term Fasting Upregulates Glucose Transporters in Neurons and Endothelial Cells, But Not in Astrocytes

Our group previously reported that 6-h fasting increased both insulin II mRNA expression and insulin level in rat hypothalamus. Given that insulin effects on central glucose metabolism are insufficiently understood, we wanted to examine if the centrally produced insulin affects expression and/or regional distribution of glucose transporters, and glycogen stores in the hypothalamus during short-term fasting. In addition to determining the amount of total and activated insulin receptor, glucose transporters, and glycogen, we also studied distribution of insulin receptors and glucose transporters within the hypothalamus. We found that short-term fasting did not affect the astrocytic 45 kDa GLUT1 isoform, but it significantly increased the amount of endothelial 55 kDa GLUT1, and neuronal GLUT3 in the membrane fractions of hypothalamic proteins. The level of GLUT2 whose presence was detected in neurons, ependymocytes and tanycytes was also elevated. Unlike hepatic glycogen which was decreased, hypothalamic glycogen content was not changed after 6-h fasting. Our findings suggest that neurons may be given a priority over astrocytes in terms of glucose supply even during the initial phase of metabolic response to fasting. Namely, increase in glucose influx into the brain extracellular fluid and neurons by increasing the translocation of GLUT1, and GLUT3 in the cell membrane may represent the first line of defense in times of scarcity. The absence of co-localization of these membrane transporters with the activated insulin receptor suggests this process takes place in an insulin-independent manner.

Role of hypothalamic tanycytes in nutrient sensing and energy balance

Animal models are valuable for the study of complex behaviours and physiology such as the control of appetite because genetic, pharmacological and surgical approaches allow the investigation of underlying mechanisms. However, the majority of such studies are carried out in just two species, laboratory mice and rats. These conventional laboratory species have been intensely selected for high growth rate and fecundity, and have a high metabolic rate and short lifespan. These aspects limit their translational relevance for human appetite control. This review will consider the value of studies carried out in a seasonal species, the Siberian hamster, which shows natural photoperiod-regulated annual cycles in appetite, growth and fattening. Such studies reveal that this long-term control is not simply an adjustment of the known hypothalamic neuronal systems that control hunger and satiety in the short term. Long-term cyclicity is probably driven by hypothalamic tanycytes, glial cells that line the ventricular walls of the hypothalamus. These unique cells sense nutrients and metabolic hormones, integrate seasonal signals and effect plasticity of surrounding neural circuits through their function as a stem cell niche in the adult. Studies of glial cell function in the hypothalamus offer new potential for identifying central targets for appetite and body weight control amenable to dietary or pharmacological manipulation.

Fructose and prostate cancer: toward an integrated view of cancer cell metabolism

Activation of glucose transporter-1 (Glut-1) gene expression is a molecular feature of cancer cells that increases glucose uptake and metabolism. Increased glucose uptake is the basis for the clinical localization of primary tumors using positron emission tomography (PET) and 2-deoxy-2-[18F]-fluoro-D-glucose (FDG) as a radiotracer. However, previous studies have demonstrated that a considerable number of cancers, which include prostate cancer (CaP), express low to undetectable levels of Glut-1 and that FDG-PET has limited clinical applicability in CaP. This observation could be explained by a low metabolic activity of CaP cells that may be overcome using different hexoses, such as fructose, as the preferred energy source. However, these hypotheses have not been examined critically in CaP. This review article summarizes what is currently known about transport and metabolism of hexoses, and more specifically fructose, in CaP and provides experimental evidences indicating that CaP cells may have increased capacity to transport and metabolize fructose in vitro and in vivo. Moreover, this review highlights recent findings that allow better understanding of how metabolism of fructose may regulate cancer cell proliferation and how fructose uptake and metabolism, through the de novo lipogenesis pathway, may provide new opportunities for CaP early diagnosis, staging, and treatment.

Cellular fate decisions in the developing female anteroventral periventricular nucleus are regulated by canonical Notch signaling

The hypothalamic anteroventral periventricular nucleus (AVPV) is the major regulator of reproductive function within the hypothalamic-pituitary-gonadal (HPG) axis. Despite an understanding of the function of neuronal subtypes within the AVPV, little is known about the molecular mechanisms regulating their development. Previous work from our laboratory has demonstrated that Notch signaling is required in progenitor cell maintenance and formation of kisspeptin neurons of the arcuate nucleus (ARC) while simultaneously restraining POMC neuron number. Based on these findings, we hypothesized that the Notch signaling pathway may act similarly in the AVPV by promoting development of kisspeptin neurons at the expense of other neuronal subtypes. To address this hypothesis, we utilized a genetic mouse model with a conditional loss of Rbpj in Nkx2.1 expressing cells (Rbpj cKO). We noted an increase in cellular proliferation, as marked by Ki-67, in the hypothalamic ventricular zone (HVZ) in Rbpj cKO mice at E13.5. This corresponded to an increase in general neurogenesis and more TH-positive neurons. Additionally, an increase in OLIG2-positive early oligodendrocytic precursor cells was observed at postnatal day 0 in Rbpj cKO mice. By 5 weeks of age in Rbpj cKO mice, TH-positive cells were readily detected in the AVPV but few kisspeptin neurons were present. To elucidate the direct effects of Notch signaling on neuron and glia differentiation, an in vitro primary hypothalamic neurosphere assay was employed. We demonstrated that treatment with the chemical Notch inhibitor DAPT increased mKi67 and Olig2 mRNA expression while decreasing astroglial Gfap expression, suggesting Notch signaling regulates both proliferation and early glial fate decisions. A modest increase in expression of TH in both the cell soma and neurite extensions was observed after extended culture, suggesting that inhibition of Notch signaling alone is enough to bias progenitors towards a dopaminergic fate. Together, these data suggest that Notch signaling restricts early cellular proliferation and differentiation of neurons and oligodendrocytes both in vivo and in vitro and acts as a fate selector of kisspeptin neurons.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

Are Tanycytes the Missing Link Between Type 2 Diabetes and Alzheimer's Disease?

Tanycytes are highly specialized bipolar ependymal cells that line the ventrolateral wall and the floor of the third ventricle in the brain and form a blood-cerebrospinal fluid barrier at the level of the median eminence. They play a pivotal role in regulating metabolic networks that control body weight and energy homeostasis. Due to the glucosensing function of tanycytes, they could be considered as a critical player in the pathogenesis of type 2 diabetes. Genetic fate mapping studies have established the role of tanycytes for the newly detected adult hypothalamic neurogenesis with important implications for metabolism as well as pathophysiology of various neurodegenerative diseases. We believe that a comprehensive understanding of the physiological mechanisms underlying their neuroplasticity, glucosensing, and cross talk with endothelial cells will enable us to achieve metabolic homeostasis in type 2 diabetes patients and possibly delay the progression of Alzheimer's disease and hopefully improve cognitive function.

Brain Glucose-Sensing Mechanism and Energy Homeostasis

The metabolic and energy state of the organism depends largely on the availability of substrates, such as glucose for ATP production, necessary for maintaining physiological functions. Deregulation in glucose levels leads to the appearance of pathological signs that result in failures in the cardiovascular system and various diseases, such as diabetes, obesity, nephropathy, and neuropathy. Particularly, the brain relies on glucose as fuel for the normal development of neuronal activity. Regions adjacent to the cerebral ventricles, such as the hypothalamus and brainstem, exercise central control in energy homeostasis. These centers house nuclei of neurons whose excitatory activity is sensitive to changes in glucose levels. Determining the different detection mechanisms, the phenotype of neurosecretion, and neural connections involving glucose-sensitive neurons is essential to understanding the response to hypoglycemia through modulation of food intake, thermogenesis, and activation of sympathetic and parasympathetic branches, inducing glucagon and epinephrine secretion and other hypothalamic-pituitary axis-dependent counterregulatory hormones, such as glucocorticoids and growth hormone. The aim of this review focuses on integrating the current understanding of various glucose-sensing mechanisms described in the brain, thereby establishing a relationship between neuroanatomy and control of physiological processes involved in both metabolic and energy balance. This will advance the understanding of increasingly prevalent diseases in the modern world, especially diabetes, and emphasize patterns that regulate and stimulate intake, thermogenesis, and the overall synergistic effect of the neuroendocrine system.