Tanycyte-like cells form a blood-cerebrospinal fluid barrier in the circumventricular organs of the mouse brain

Physiological and Pathological Factors Affecting Drug Delivery to the Brain by Nanoparticles

The prevalence of neurological/neurodegenerative diseases, such as Alzheimer's disease is known to be increasing due to an aging population and is anticipated to further grow in the decades ahead. The treatment of brain diseases is challenging partly due to the inaccessibility of therapeutic agents to the brain. An increasingly important observation is that the physiology of the brain alters during many brain diseases, and aging adds even more to the complexity of the disease. There is a notion that the permeability of the blood-brain barrier (BBB) increases with aging or disease, however, the body has a defense mechanism that still retains the separation of the brain from harmful chemicals in the blood. This makes drug delivery to the diseased brain, even more challenging and complex task. Here, the physiological changes to the diseased brain and aged brain are covered in the context of drug delivery to the brain using nanoparticles. Also, recent and novel approaches are discussed for the delivery of therapeutic agents to the diseased brain using nanoparticle based or magnetic resonance imaging guided systems. Furthermore, the complement activation, toxicity, and immunogenicity of brain targeting nanoparticles as well as novel in vitro BBB models are discussed.

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.

Biological insights from the direct measurement of purine release

Although purinergic signalling has been a well-established and accepted mechanism of chemical communication for many years, it remains important to measure the extracellular concentration of ATP and adenosine in real time. In this review I summarize the reasons why such measurements are still needed, how they provide additional mechanistic insight and give an overview of the techniques currently available to make spatially localised measurements of ATP and adenosine in real time. To illustrate the impact of direct real-time measurements, I explore CO2 and nutrient sensing in the medulla oblongata and hypothalamus. In both of these examples, the sensing involves hemichannel mediated ATP release from glial cells. For CO2 the hemichannels involved, connexin26, are directly CO2-sensitive. This mechanism contributes to the chemosensory control of breathing. In the hypothamalus, specialised glial cells, tanycytes, directly contact the cerebrospinal fluid in the 3rd ventricle and sense nutrients via sweet and umami taste receptors. Nutrient sensing by tanycytes is likely to contribute to the control of body weight as their selective stimulation alters food intake. To illustrate the importance of direct adenosine measurements, I consider the complex and multiple mechanisms of activity-dependent adenosine release in different brain regions. This activity dependent release of adenosine is likely to mediate important feedback regulation and may also be involved in controlling the sleep-wake state. I finish by briefly considering the potential of whole blood purine measurements in clinical practice.

Adult hypothalamic neurogenesis and sleep-wake dysfunction in aging

In the mammalian brain, adult neurogenesis has been extensively studied in the hippocampal sub-granular zone and the sub-ventricular zone of the anterolateral ventricles. However, growing evidence suggests that new cells are not only "born" constitutively in the adult hypothalamus, but many of these cells also differentiate into neurons and glia and serve specific functions. The preoptic-hypothalamic area plays a central role in the regulation of many critical functions, including sleep-wakefulness and circadian rhythms. While a role for adult hippocampal neurogenesis in regulating hippocampus-dependent functions, including cognition, has been extensively studied, adult hypothalamic neurogenic process and its contributions to various hypothalamic functions, including sleep-wake regulation are just beginning to unravel. This review is aimed at providing the current understanding of the hypothalamic adult neurogenic processes and the extent to which it affects hypothalamic functions, including sleep-wake regulation. We propose that hypothalamic neurogenic processes are vital for maintaining the proper functioning of the hypothalamic sleep-wake and circadian systems in the face of regulatory challenges. Sleep-wake disturbance is a frequent and challenging problem of aging and age-related neurodegenerative diseases. Aging is also associated with a decline in the neurogenic process. We discuss a hypothesis that a decrease in the hypothalamic neurogenic process underlies the aging of its sleep-wake and circadian systems and associated sleep-wake disturbance. We further discuss whether neuro-regenerative approaches, including pharmacological and non-pharmacological stimulation of endogenous neural stem and progenitor cells in hypothalamic neurogenic niches, can be used for mitigating sleep-wake and other hypothalamic dysfunctions in aging.

What is the physiological role of hypothalamic tanycytes in metabolism?

In vertebrates, the energy balance process is tightly controlled by complex neural circuits that sense metabolic signals and adjust food intake and energy expenditure in line with the physiological requirements of optimal conditions. Within neural networks controlling energy balance, tanycytes are peculiar ependymoglial cells that are nowadays recognized as multifunctional players in the metabolic hypothalamus. However, the physiological function of hypothalamic tanycytes remains unclear, creating a number of ambiguities in the field. Here, we review data accumulated over the years that demonstrate the physiological function of tanycytes in the maintenance of metabolic homeostasis, opening up new research avenues. The presumed involvement of tanycytes in the pathophysiology of metabolic disorders and age-related neurodegenerative diseases will be finally discussed.

A genetic map of the mouse dorsal vagal complex and its role in obesity

The brainstem dorsal vagal complex (DVC) is known to regulate energy balance and is the target of appetite-suppressing hormones, such as glucagon-like peptide 1 (GLP-1). Here we provide a comprehensive genetic map of the DVC and identify neuronal populations that control feeding. Combining bulk and single-nucleus gene expression and chromatin profiling of DVC cells, we reveal 25 neuronal populations with unique transcriptional and chromatin accessibility landscapes and peptide receptor expression profiles. GLP-1 receptor (GLP-1R) agonist administration induces gene expression alterations specific to two distinct sets of Glp1r neurons-one population in the area postrema and one in the nucleus of the solitary tract that also expresses calcitonin receptor (Calcr). Transcripts and regions of accessible chromatin near obesity-associated genetic variants are enriched in the area postrema and the nucleus of the solitary tract neurons that express Glp1r and/or Calcr, and activating several of these neuronal populations decreases feeding in rodents. Thus, DVC neuronal populations associated with obesity predisposition suppress feeding and may represent therapeutic targets for obesity.

Seasonal changes in adenosine kinase in tanycytes of the Arctic ground squirrel (Urocitellus parryii)

Hibernation is a seasonal strategy to conserve energy, characterized by modified thermoregulation, an increase in sleep pressure and drastic metabolic changes. Glial cells such as astrocytes and tanycytes are the brain metabolic sensors, but it remains unknown whether they contribute to seasonal expression of hibernation. The onset of hibernation is controlled by an undefined endogenous circannual rhythm in which adenosine plays a role through the activation of the A1 adenosine receptor (A1AR). Seasonal changes in brain levels of adenosine may contribute to an increase in A1AR sensitivity leading to the onset of hibernation. The primary regulator of extracellular adenosine concentration is adenosine kinase, which is located in astrocytes. Using immunohistochemistry to localize and quantify adenosine kinase in Arctic ground squirrels' brain collected during different seasons, we report lower expression of adenosine kinase in the third ventricle tanycytes in winter compared to summer; a similar change was not seen in astrocytes. Moreover, for the first time, we describe adenosine kinase expression in tanycyte cell bodies in the hypothalamus and in the area postrema, both brain regions involved in energy homeostasis. Next we describe seasonal changes in tanycyte morphology in the hypothalamus. Although still speculative, our findings contribute to a model whereby adenosine kinase in tanycytes regulates seasonal changes in extracellular concentration of adenosine underling the seasonal expression of hibernation.

Thyroid hormone and hypothalamic stem cells in seasonal functions

Seasonal rhythms are a pervasive feature of most living organisms, which underlie yearly timeliness in breeding, migration, hibernation or weight gain and loss. To achieve this, organisms have developed inner timing devices (circannual clocks) that endow them with the ability to predict then anticipate changes to come, usually using daylength as the proximate cue. In Vertebrates, daylength interpretation involves photoperiodic control of TSH production by the pars tuberalis (PT) of the pituitary, which governs a seasonal switch in thyroid hormone (TH) availability in the neighboring hypothalamus. Tanycytes, specialized glial cells lining the third ventricle (3V), are responsible for this TH output through the opposite, PT-TSH-driven, seasonal control of deiodinases 2/3 (Dio 2/3). Tanycytes comprise a photoperiod-sensitive stem cell niche and TH is known to play major roles in cell proliferation and differentiation, which suggests that seasonal control of tanycyte proliferation may be involved in the photoperiodic synchronization of seasonal rhythms. Here we review our current knowledge of the molecular and neuroendocrine pathway linking photoperiodic information to seasonal changes in physiological functions and discuss the potential implication of tanycytes, TH and cell proliferation in seasonal timing.

The Normal and Brain Tumor Vasculature: Morphological and Functional Characteristics and Therapeutic Targeting

Glioblastoma is among the most common tumor of the central nervous system in adults. Overall survival has not significantly improved over the last decade, even with optimizing standard therapeutic care including extent of resection and radio- and chemotherapy. In this article, we review features of the brain vasculature found in healthy cerebral tissue and in glioblastoma. Brain vessels are of various sizes and composed of several vascular cell types. Non-vascular cells such as astrocytes or microglia also interact with the vasculature and play important roles. We also discuss in vitro engineered artificial blood vessels which may represent useful models for better understanding the tumor-vessel interaction. Finally, we summarize results from clinical trials with anti-angiogenic therapy alone or in combination, and discuss the value of these approaches for targeting glioblastoma.

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.

Thermoregulation in hibernating mammals: The role of the "thyroid hormones system"

Hibernation is a unique evolutionary adaptation to conserve energy. During the pre-hibernation (i.e. fall) season, a progressive decline in core body temperature and further decrease in metabolism underlie a seasonal modulation in thermoregulation. The onset of hibernation requires marked changes in thermoregulatory attributes including adjustment in body temperature and tissue specific increases in thermogenic capacity. The hibernation season is characterized by a regulated suppression in thermogenesis allowing the onset of torpor interrupted by periodic activation of thermogenesis to sustain interbout arousals. Thyroid hormones are known to regulate both body temperature and metabolism, and for this reason, the hypothalamic-pituitary-thyroid axis and thyroid hormones have been investigated as modulators of thermogenesis in the phenomenon of hibernation, but the mechanisms remain poorly understood. In this review, we present an overview of what is known about the thermogenic roles of thyroid hormones in hibernating species across seasons and within the hibernating season (torpor-interbout arousal cycle). Overall, the hypothalamic-pituitary-thyroid axis and thyroid hormones play a role in the pre-hibernation season to enhance thermogenic capacity. During hibernation, thermogenesis is attenuated at the level of sympathetic premotor neurons within the raphe pallidus and by deiodinase expression in the hypothalamus. Further, as recent work highlights the direct effect of thyroid hormones within the central nervous system in activating thermogenesis, we speculate how similar mechanisms may occur in hibernating species to modulate thermogenesis across seasons and to sustain interbout arousals. However, further experiments are needed to elucidate the role of thyroid hormones in hibernation, moving towards the understanding that thyroid hormones metabolism, transport and availability within tissues may be the most telling indicator of thyroid status.

Neurogenesis in the adult hypothalamus: A distinct form of structural plasticity involved in metabolic and circadian regulation, with potential relevance for human pathophysiology

The adult brain harbors specific niches where stem cells undergo substantial plasticity and, in some regions, generate new neurons throughout life. This phenomenon is well known in the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampus and has recently also been described in the hypothalamus of several rodent and primate species. After a brief overview of preclinical studies illustrating the pathophysiologic significance of hypothalamic neurogenesis in the control of energy metabolism, reproduction, thermoregulation, sleep, and aging, we review current literature on the neurogenic niche of the human hypothalamus. A comparison of the organization of the niche between humans and rodents highlights some common features, but also substantial differences, e.g., in the distribution and extent of the hypothalamic neural stem cells. Exploring the full dynamics of hypothalamic neurogenesis in humans raises a formidable challenge however, given among others, inherent technical limitations. We close with discussing possible functional role(s) of the human hypothalamic niche, and how gaining more insights into this form of plasticity could be relevant for a better understanding of pathologies associated with disturbed hypothalamic function.

Neuroanatomical basis of the nerve growth factor ovulation-induction pathway in llamas†

The objective of the study was to characterize the anatomical framework and sites of action of the nerve growth factor (NGF)-mediated ovulation-inducing system of llamas. The expression patterns of NGF and its receptors in the hypothalamus of llamas (n = 5) were examined using single and double immunohistochemistry/immunofluorescence. We also compare the expression pattern of the P75 receptor in the hypothalamus of llama and a spontaneous ovulator species (sheep, n = 5). Both NGF receptors (TrkA and P75) were highly expressed in the medial septum and diagonal band of Broca, and populations of TrkA cells were observed in the periventricular and dorsal hypothalamus. Unexpectedly, we found NGF immunoreactive cell bodies with widespread distribution in the hypothalamus but not in areas endowed with NGF receptors. The organum vasculosum of the lamina terminalis (OVLT) and the median eminence displayed immunoreactivity for P75. Double immunofluorescence using vimentin, a marker of tanycytes, confirmed that tanycytes were immunoreactive to P75 in the median eminence and in the OVLT. Additionally, tanycytes were in close association with GnRH and kisspeptin in the arcuate nucleus and median eminence of llamas. The choroid plexus of llamas contained TrkA and NGF immunoreactivity but no P75 immunoreactivity. Results of the present study demonstrate sites of action of NGF in the llama hypothalamus, providing support for the hypothesis of a central effect of NGF in the ovulation-inducing mechanism in llamas.

Nitric oxide signalling in the brain and its control of bodily functions

Nitric oxide (NO) is a versatile molecule that plays key roles in the development and survival of mammalian species by endowing brain neuronal networks with the ability to make continual adjustments to function in response to moment-to-moment changes in physiological input. Here, we summarize the progress in the field and argue that NO-synthetizing neurons and NO signalling in the brain provide a core hub for integrating sensory- and homeostatic-related cues, control key bodily functions, and provide a potential target for new therapeutic opportunities against several neuroendocrine and behavioural abnormalities.

The blood-brain barrier in health and disease: Important unanswered questions

The blood vessels vascularizing the central nervous system exhibit a series of distinct properties that tightly control the movement of ions, molecules, and cells between the blood and the parenchyma. This "blood-brain barrier" is initiated during angiogenesis via signals from the surrounding neural environment, and its integrity remains vital for homeostasis and neural protection throughout life. Blood-brain barrier dysfunction contributes to pathology in a range of neurological conditions including multiple sclerosis, stroke, and epilepsy, and has also been implicated in neurodegenerative diseases such as Alzheimer's disease. This review will discuss current knowledge and key unanswered questions regarding the blood-brain barrier in health and disease.

Permeability of the windows of the brain: feasibility of dynamic contrast-enhanced MRI of the circumventricular organs

BACKGROUND

Circumventricular organs (CVOs) are small structures without a blood-brain barrier surrounding the brain ventricles that serve homeostasic functions and facilitate communication between the blood, cerebrospinal fluid and brain. Secretory CVOs release peptides and sensory CVOs regulate signal transmission. However, pathogens may enter the brain through the CVOs and trigger neuroinflammation and neurodegeneration. We investigated the feasibility of dynamic contrast-enhanced (DCE) MRI to assess the CVO permeability characteristics in vivo, and expected significant contrast uptake in these regions, due to blood-brain barrier absence.

METHODS

Twenty healthy, middle-aged to older males underwent brain DCE MRI. Pharmacokinetic modeling was applied to contrast concentration time-courses of CVOs, and in reference to white and gray matter. We investigated whether a significant and positive transfer from blood to brain could be measured in the CVOs, and whether this differed between secretory and sensory CVOs or from normal-appearing brain matter.

RESULTS

In both the secretory and sensory CVOs, the transfer constants were significantly positive, and all secretory CVOs had significantly higher transfer than each sensory CVO. The transfer constants in both the secretory and sensory CVOs were higher than in the white and gray matter.

CONCLUSIONS

Current measurements confirm the often-held assumption of highly permeable CVOs, of which the secretory types have the strongest blood-to-brain transfer. The current study suggests that DCE MRI could be a promising technique to further assess the function of the CVOs and how pathogens can potentially enter the brain via these structures.

TRIAL REGISTRATION

Netherlands Trial Register number: NL6358, date of registration: 2017-03-24.

Astrocytes in the regulation of cerebrovascular functions

Astrocytes are the most numerous type of neuroglia in the brain and have a predominant influence on the cerebrovascular system; they control perivascular homeostasis, the integrity of the blood-brain barrier, the dialogue with the peripheral immune system, the transfer of metabolites from the blood, and blood vessel contractility in response to neuronal activity. These regulatory processes occur in a specialized interface composed of perivascular astrocyte extensions that almost completely cover the cerebral blood vessels. Scientists have only recently started to study how this interface is formed and how it influences cerebrovascular functions. Here, we review the literature on the astrocytes' role in the regulation of the cerebrovascular system. We cover the anatomy and development of the gliovascular interface, the known gliovascular functions, and molecular factors, the latter's implication in certain pathophysiological situations, and recent cutting-edge experimental tools developed to examine the astrocytes' role at the vascular interface. Finally, we highlight some open questions in this field of research.

Targeting the Choroid Plexuses for Protein Drug Delivery

Delivery of therapeutic agents to the central nervous system is challenged by the barriers in place to regulate brain homeostasis. This is especially true for protein therapeutics. Targeting the barrier formed by the choroid plexuses at the interfaces of the systemic circulation and ventricular system may be a surrogate brain delivery strategy to circumvent the blood-brain barrier. Heterogenous cell populations located at the choroid plexuses provide diverse functions in regulating the exchange of material within the ventricular space. Receptor-mediated transcytosis may be a promising mechanism to deliver protein therapeutics across the tight junctions formed by choroid plexus epithelial cells. However, cerebrospinal fluid flow and other barriers formed by ependymal cells and perivascular spaces should also be considered for evaluation of protein therapeutic disposition. Various preclinical methods have been applied to delineate protein transport across the choroid plexuses, including imaging strategies, ventriculocisternal perfusions, and primary choroid plexus epithelial cell models. When used in combination with simultaneous measures of cerebrospinal fluid dynamics, they can yield important insight into pharmacokinetic properties within the brain. This review aims to provide an overview of the choroid plexuses and ventricular system to address their function as a barrier to pharmaceutical interventions and relevance for central nervous system drug delivery of protein therapeutics. Protein therapeutics targeting the ventricular system may provide new approaches in treating central nervous system diseases.

Sirtuin (SIRT)-1: At the crossroads of puberty and metabolism

In the arcuate nucleus (ARC) of the hypothalamus reside two neuronal systems in charge of regulating feeding control and reproductive development. The melanocortin system responds to metabolic fluctuations adjusting food intake, whereas kisspeptin neurons are in charge of the excitatory control of Gonadotropin Hormone Releasing Hormone (GnRH) neurons. While it is known that the melanocortin system regulates GnRH neuronal activity, it was recently demonstrated that kisspeptin neurons not only innervate melanocortin neurons, but also play an active role in the control of metabolism. These two neuronal systems are intricately interconnected forming loops of stimulation and inhibition according to metabolic status. Furthermore, intracellular and epigenetic pathways respond to external environmental signals by changing DNA conformation and gene expression. Here we review the role of Silent mating type Information Regulation 2 homologue 1 (Sirt1), a class III NAD+ dependent protein deacetylase, in the ARC control of pubertal development and feeding behavior.

Peripheral-to-central immune communication at the area postrema glial-barrier following bleomycin-induced sterile lung injury in adult rats

The pathways for peripheral-to-central immune communication (P → C I-comm) following sterile lung injury (SLI) are unknown. SLI evokes systemic and central inflammation, which alters central respiratory control and viscerosensory transmission in the nucleus tractus solitarii (nTS). These functional changes coincide with increased interleukin-1 beta (IL-1β) in the area postrema, a sensory circumventricular organ that connects P → C I-comm to brainstem circuits that control homeostasis. We hypothesize that IL-1β and its downstream transcriptional target, cyclooxygenase-2 (COX-2), mediate P → C I-comm in the nTS. In a rodent model of SLI induced by intratracheal bleomycin (Bleo), the sigh frequency and duration of post-sigh apnea increased in Bleo- compared to saline- treated rats one week after injury. This SLI-dependent change in respiratory control occurred concurrently with augmented IL-1β and COX-2 immunoreactivity (IR) in the funiculus separans (FS), a barrier between the AP and the brainstem. At this barrier, increases in IL-1β and COX-2 IR were confined to processes that stained for glial fibrillary acidic protein (GFAP) and that projected basolaterally to the nTS. Further, FS radial-glia did not express TNF-α or IL-6 following SLI. To test our hypothesis, we blocked central COX-1/2 activity by intracerebroventricular (ICV) infusion of Indomethacin (Ind). Continuous ICV Ind treatment prevented Bleo-dependent increases in GFAP + and IL-1β + IR, and restored characteristics of sighs that reset the rhythm. These data indicate that changes in sighs following SLI depend partially on activation of a central COX-dependent P → C I-comm via radial-glia of the FS.

Peculiar protrusions along tanycyte processes face diverse neural and nonneural cell types in the hypothalamic parenchyma

Tanycytes are highly specialized ependymal cells that line the bottom and the lateral walls of the third ventricle. In contact with the cerebrospinal fluid through their cell bodies, they send processes into the arcuate nucleus, the ventromedial nucleus, and the dorsomedial nucleus of the hypothalamus. In the present work, we combined transgenic and immunohistochemical approaches to investigate the neuroanatomical associations between tanycytes and neural cells present in the hypothalamic parenchyma, in particular in the arcuate nucleus. The specific expression of tdTomato in tanycytes first allowed the observation of peculiar subcellular protrusions along tanycyte processes and at their endfeet such as spines, swelling, en passant boutons, boutons, or claws. Interestingly, these protrusions contact different neural cells in the brain parenchyma including blood vessels and neurons, and in particular NPY and POMC neurons in the arcuate nucleus. Using both fluorescent and electron microscopy, we finally observed that these tanycyte protrusions contain ribosomes, mitochondria, diverse vesicles, and transporters, suggesting dense tanycyte/neuron and tanycyte/blood vessel communications. Altogether, our results lay the neuroanatomical basis for tanycyte/neural cell interactions, which will be useful to further understand cell-to-cell communications involved in the regulation of neuroendocrine functions.

The anatomy and immunology of vasculature in the central nervous system

Barriers between circulation and the central nervous system (CNS) play a key role in the development and modulation of CNS immune responses. Structural variations in the vasculature traversing different anatomical regions within the CNS strongly influence where and how CNS immune responses first develop. Here, we provide an overview of cerebrovascular anatomy, focusing on the blood-CNS interface and how anatomical variations influence steady-state immunology in the compartment. We then discuss how CNS vasculature is affected by and influences the development of different pathophysiological states, such as CNS autoimmune disease, cerebrovascular injury, cerebral ischemia, and infection.

Ciliary Neurotrophic Factor Acts on Distinctive Hypothalamic Arcuate Neurons and Promotes Leptin Entry Into and Action on the Mouse Hypothalamus

In humans and experimental animals, the administration of ciliary neurotrophic factor (CNTF) reduces food intake and body weight. To gain further insights into the mechanism(s) underlying its satiety effect, we: (i) evaluated the CNTF-dependent activation of the Janus kinase 2 (JAK2) and signal transducer and activator of transcription 3 (STAT3) pathway in mouse models where neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) neurons can be identified by green fluorescent protein (GFP); and (ii) assessed whether CNTF promotes leptin signaling in hypothalamic feeding centers. Immunohistochemical experiments enabled us to establish that intraperitoneal injection of mouse recombinant CNTF activated the JAK2-STAT3 pathway in a substantial proportion of arcuate nucleus (ARC) NPY neurons (18.68% ± 0.60 in 24-h fasted mice and 25.50% ± 1.17 in fed mice) but exerted a limited effect on POMC neurons (4.15% ± 0.33 in 24-h fasted mice and 2.84% ± 0.45 in fed mice). CNTF-responsive NPY neurons resided in the ventromedial ARC, facing the median eminence (ME), and were surrounded by albumin immunoreactivity, suggesting that they are located outside the blood-brain barrier (BBB). In both normally fed and high-fat diet (HFD) obese animals, CNTF activated extracellular signal-regulated kinase signaling in ME β1- and β2-tanycytes, an effect that has been linked to the promotion of leptin entry into the brain. Accordingly, compared to the animals treated with leptin, mice treated with leptin/CNTF showed: (i) a significantly greater leptin content in hypothalamic protein extracts; (ii) a significant increase in phospho-STAT3 (P-STAT3)-positive neurons in the ARC and the ventromedial hypothalamic nucleus of normally fed mice; and (iii) a significantly increased number of P-STAT3-positive neurons in the ARC and dorsomedial hypothalamic nucleus of HFD obese mice. Collectively, these data suggest that exogenously administered CNTF reduces food intake by exerting a leptin-like action on distinctive NPY ARC neurons and by promoting leptin signaling in hypothalamic feeding centers.

Timekeeping in the hindbrain: a multi-oscillatory circadian centre in the mouse dorsal vagal complex

Metabolic and cardiovascular processes controlled by the hindbrain exhibit 24 h rhythms, but the extent to which the hindbrain possesses endogenous circadian timekeeping is unresolved. Here we provide compelling evidence that genetic, neuronal, and vascular activities of the brainstem’s dorsal vagal complex are subject to intrinsic circadian control with a crucial role for the connection between its components in regulating their rhythmic properties. Robust 24 h variation in clock gene expression in vivo and neuronal firing ex vivo were observed in the area postrema (AP) and nucleus of the solitary tract (NTS), together with enhanced nocturnal responsiveness to metabolic cues. Unexpectedly, we also find functional and molecular evidence for increased penetration of blood borne molecules into the NTS at night. Our findings reveal that the hindbrain houses a local network complex of neuronal and non-neuronal autonomous circadian oscillators, with clear implications for understanding local temporal control of physiology in the brainstem.

Lukasz Chrobok, Rebecca Northeast et al. show circadian variation in clock gene expression and neuronal firing within the area postrema and the nucleus of the solitary tract in mice. These regions also exhibit variation in metabolic processes and blood-brain barrier permeability across the 24 hour cycle suggesting the presence of circadian oscillators within the dorsal vagal complex.

The Pierced Lasso Topology Leptin has a Bolt on Dynamic Domain Composed by the Disordered Loops I and III

Leptin is an important signaling hormone, mostly known for its role in energy expenditure and satiety. Furthermore, leptin plays a major role in other proteinopathies, such as cancer, marked hyperphagia, impaired immune function, and inflammation. In spite of its biological relevance in human health, there are no NMR resonance assignments of the human protein available, obscuring high-resolution characterization of the soluble protein and/or its conformational dynamics, suggested as being important for receptor interaction and biological activity. Here, we report the nearly complete backbone resonance assignments of human leptin. Chemical shift-based secondary structure prediction confirms that in solution leptin forms a four-helix bundle including a pierced lasso topology. The conformational dynamics, determined on several timescales, show that leptin is monomeric, has a rigid four-helix scaffold, and a dynamic domain, including a transiently formed helix. The dynamic domain is anchored to the helical scaffold by a secondary hydrophobic core, pinning down the long loops of leptin to the protein body, inducing motional restriction without a well-defined secondary or tertiary hydrogen bond stabilized structure. This dynamic region is well suited for and may be involved in functional allosteric dynamics upon receptor binding.

GABA neurons in the nucleus tractus solitarius express GLP-1 receptors and mediate anorectic effects of liraglutide in rats

The glucagon-like peptide-1 receptor (GLP-1R) agonist liraglutide is approved for the treatment of obesity; however, there is still much to be learned regarding the neuronal sites of action that underlie its suppressive effects on food intake and body weight. Peripherally administered liraglutide in rats acts in part through central GLP-1Rs in both the hypothalamus and the hindbrain. Here, we extend findings supporting a role for hindbrain GLP-1Rs in mediating the anorectic effects of liraglutide in male rats. To dissociate the contribution of GLP-1Rs in the area postrema (AP) and the nucleus tractus solitarius (NTS), we examined the effects of liraglutide in both NTS AAV-shRNA-driven Glp1r knockdown and AP-lesioned animals. Knockdown of NTS GLP-1Rs, but not surgical lesioning of the AP, attenuated the anorectic and body weight-reducing effects of acutely delivered liraglutide. In addition, NTS c-Fos responses were maintained in AP-lesioned animals. Moreover, NTS Glp1r knockdown was sufficient to attenuate the intake- and body weight-reducing effects of chronic daily administered liraglutide over 3 weeks. Development of improved obesity pharmacotherapies requires an understanding of the cellular phenotypes targeted by GLP-1R agonists. Fluorescence in situ hybridization identified Glp1r transcripts in NTS GABAergic neurons, which when inhibited using chemogenetics, attenuated the food intake- and body weight-reducing effects of liraglutide. This work demonstrates the contribution of NTS GLP-1Rs to the anorectic potential of liraglutide and highlights a phenotypically distinct (GABAergic) population of neurons within the NTS that express the GLP-1R and are involved in the mediation of liraglutide signaling.

Neural stem cell phenotype of tanycyte-like ependymal cells in the circumventricular organs and central canal of adult mouse brain

Tanycyte is a subtype of ependymal cells which extend long radial processes to brain parenchyma. The present study showed that tanycyte-like ependymal cells in the organum vasculosum of the lamina terminalis, subfornical organ and central canal (CC) expressed neural stem cell (NSC) marker nestin, glial fibrillar acidic protein and sex determining region Y. Proliferation of these tanycyte-like ependymal cells was promoted by continuous intracerebroventricular infusion of fibroblast growth factor-2 and epidermal growth factor. Tanycytes-like ependymal cells in the CC are able to form self-renewing neurospheres and give rise mostly to new astrocytes and oligodendrocytes. Collagenase-induced small medullary hemorrhage increased proliferation of tanycyte-like ependymal cells in the CC. These results demonstrate that these tanycyte-like ependymal cells of the adult mouse brain are NSCs and suggest that they serve as a source for providing new neuronal lineage cells upon brain damage in the medulla oblongata.

Structure and Function of the Blood-Brain Barrier (BBB)

The blood-brain barrier (BBB) protects the vertebrate central nervous system from harmful blood-borne, endogenous and exogenous substances to ensure proper neuronal function. The BBB describes a function that is established by endothelial cells of CNS vessels in conjunction with pericytes, astrocytes, neurons and microglia, together forming the neurovascular unit (NVU). Endothelial barrier function is crucially induced and maintained by the Wnt/β-catenin pathway and requires intact NVU for proper functionality. The BBB and the NVU are characterized by a specialized assortment of molecular specializations, providing the basis for tightening, transport and immune response functionality.The present chapter introduces state-of-the-art knowledge of BBB structure and function and highlights current research topics, aiming to understanding in more depth the cellular and molecular interactions at the NVU, determining functionality of the BBB in health and disease, and providing novel potential targets for therapeutic BBB modulation. Moreover, we highlight recent advances in understanding BBB and NVU heterogeneity within the CNS as well as their contribution to CNS physiology, such as neurovascular coupling, and pathophysiology, is discussed. Finally, we give an outlook onto new avenues of BBB research.

Targeting Tanycytes: Balance between Efficiency and Specificity

Tanycytes are peculiar ependymoglial cells lining the bottom and the lateral wall of the third ventricle. For a decade, the utilization of molecular genetic approaches allowed us to make important discoveries about their diverse physiological functions. Here, I review the current methods used to target tanycytes, focusing on their specificity, their efficiency, their limitations, as well as their potential future improvements.

Metal homeostasis disturbances in neurodegenerative disorders, with special emphasis on Creutzfeldt-Jakob disease - Potential pathogenetic mechanism and therapeutic implications

Creutzfeldt-Jakob disease (CJD) is characterized by a rapidly progressive dementia often accompanied by myoclonus and other signs of brain dysfunction, relying on the conversion of the normal cellular form of the prion protein (PrPC) to a misfolded form (PrPSc). The neuropathological changes include spongiform degeneration, neuronal loss, astrogliosis, and deposition of PrPSc. It is still unclear how these pathological changes correlate with the development of CJD symptoms because few patients survive beyond 2 years after diagnosis. Inasmuch as the symptoms of CJD overlap some of those observed in Alzheimer's, Parkinson's, and Huntington's diseases, there may be some underlying pathologic mechanisms associated with CJD that may contribute to the symptoms of non-prion neurodegenerative diseases as well. Data suggest that imbalance of metals, including copper, zinc, iron, and manganese, induces abnormalities in processing and degradation of prion proteins that are accompanied by self-propagation of PrPSc. These events appear to be responsible for glutamatergic synaptic dysfunctions, neuronal death, and PrPSc aggregation. Given that the prodromal symptoms of CJD such as sleep disturbances and mood disorders are associated with brain stem and limbic system dysfunction, the pathological changes may initially occur in these brain regions, then spread throughout the entire brain. Alterations in cerebrospinal fluid homeostasis, which may be linked to imbalance of these metals, seem to be more important than neuroinflammation in causing the cell death. It is proposed that metal dyshomeostasis could be responsible for the initiation and progression of the pathological changes associated with symptoms of CJD and other neurodegenerative disorders.

Brain angiotensin type-1 and type-2 receptors: cellular locations under normal and hypertensive conditions

Brain angiotensin-II (Ang-II) type-1 receptors (AT1Rs), which exert profound effects on normal cardiovascular, fluid, and metabolic homeostasis, are overactivated in and contribute to chronic sympathoexcitation and hypertension. Accumulating evidence indicates that the activation of Ang-II type-2 receptors (AT2Rs) in the brain exerts effects that are opposite to those of AT1Rs, lowering blood pressure, and reducing hypertension. Thus, it would be interesting to understand the relative cellular localization of AT1R and AT2R in the brain under normal conditions and whether this localization changes during hypertension. Here, we developed a novel AT1aR-tdTomato reporter mouse strain in which the location of brain AT1aR was largely consistent with that determined in the previous studies. This AT1aR-tdTomato reporter mouse strain was crossed with our previously described AT2R-eGFP reporter mouse strain to yield a novel dual AT1aR/AT2R reporter mouse strain, which allowed us to determine that AT1aR and AT2R are primarily localized to different populations of neurons in brain regions controlling cardiovascular, fluid, and metabolic homeostasis. Using the individual AT1aR-tdTomato reporter mice, we also demonstrated that during hypertension induced by the administration of deoxycorticosterone acetate-salt, there was no shift in the expression of AT1aR from neurons to microglia or astrocytes in the paraventricular nucleus, a brain area important for sympathetic regulation. Using AT2R-eGFP reporter mice under similar hypertensive conditions, we demonstrated that the same was true of AT2R expression in the nucleus of the solitary tract (NTS), an area critical for baroreflex control. Collectively, these findings provided a novel means to assess the colocalization of AT1R and AT2R in the brain and a novel view of their cellular localization in hypertension.

Three-plane description of astroglial populations of OVLT subdivisions in rat: Tanycyte connections to distant parts of third ventricle

This study demonstrates glial and gliovascular markers of organon vasculosum laminae terminalis (OVLT) in three planes. The distribution of glial markers displayed similarities to the subfornical organ. There was an inner part with vimentin- and nestin-immunopositive glia whereas GFAP and the water-channel aquaporin 4 were found at the periphery. This separation indicates different functions of the two regions. The presence of nestin may indicate stem cell-capabilities whereas aquaporin 4 has been reported to promote the osmoreceptor function. Glutamine synthetase immunoreactivity was sparse like in the area postrema and subfornical organ. The laminin and β-dystroglycan immunolabelings altered along the vessels such as in the subfornical organ indicating altering gliovascular relations. The different subdivisions of OVLT received glial processes of different origins. The posterior periventricular zone contained short vimentin-immunopositive processes from the ependyma of the adjacent surface of the third ventricle. The lateral periventricular zone received forceps-like process systems from the anterolateral part of the third ventricle. Most interestingly, the "dorsal cap" received a mixed group of long GFAP- and vimentin-immunopositive processes from a distant part of the third ventricle. The processes may have two functions: a guidance for newly produced cells like radial glia in immature brain and/or a connection between distant parts of the third ventricle and OVLT.

Keeping time in the lamina terminalis: Novel oscillator properties of forebrain sensory circumventricular organs

Drinking behavior and osmotic regulatory mechanisms exhibit clear daily variation which is necessary for achieving the homeostatic osmolality. In mammals, the master clock in the brain's suprachiasmatic nuclei has long been held as the main driver of circadian (24 h) rhythms in physiology and behavior. However, rhythmic clock gene expression in other brain sites raises the possibility of local circadian control of neural activity and function. The subfornical organ (SFO) and the organum vasculosum laminae terminalis (OVLT) are two sensory circumventricular organs (sCVOs) that play key roles in the central control of thirst and water homeostasis, but the extent to which they are subject to intrinsic circadian control remains undefined. Using a combination of ex vivo bioluminescence and in vivo gene expression, we report for the first time that the SFO contains an unexpectedly robust autonomous clock with unusual spatiotemporal characteristics in core and noncore clock gene expression. Furthermore, putative single-cell oscillators in the SFO and OVLT are strongly rhythmic and require action potential-dependent communication to maintain synchrony. Our results reveal that these thirst-controlling sCVOs possess intrinsic circadian timekeeping properties and raise the possibility that these contribute to daily regulation of drinking behavior.

Obesity, Neuroinflammation, and Reproductive Function

The increasing occurrence of obesity has become a significant public health concern. Individuals with obesity have higher prevalence of heart disease, stroke, osteoarthritis, diabetes, and reproductive disorders. Reproductive problems include menstrual irregularities, pregnancy complications, and infertility due to anovulation, in women, and lower testosterone and diminished sperm count, in men. In particular, women with obesity have reduced levels of both gonadotropin hormones, and, in obese men, lower testosterone is accompanied by diminished LH. Taken together, these findings indicate central dysregulation of the hypothalamic-pituitary-gonadal axis, specifically at the level of the GnRH neuron function, which is the final brain output for the regulation of reproduction. Obesity is a state of hyperinsulinemia, hyperlipidemia, hyperleptinemia, and chronic inflammation. Herein, we review recent advances in our understanding of how these metabolic and immune changes affect hypothalamic function and regulation of GnRH neurons. In the latter part, we focus on neuroinflammation as a major consequence of obesity and discuss findings that reveal that GnRH neurons are uniquely positioned to respond to inflammatory changes.

Opioid and Endocannabinoid System in Orofacial Pain

Orofacial pain disorders are frequent in the general population and their pharmacological treatment is difficult and controversial. Therefore, the search for novel, safe and efficient analgesics is an important but still elusive goal for contemporary medicine. In the recent years, the antinociceptive potential of endocannabinoids and opioids has been emphasized. However, concerns for the safety of their use limit their clinical applications. the possibility of modulating the activity of endocannabinoids by regulation of their synthesis and/or degradation offers an innovative approach to the treatment of pain. A rat model of trigeminal pain, utilizing tongue jerks evoked by electrical tooth pulp stimulation during perfusion of the cerebral ventricles with various neurotransmitter solutions can be used in the pharmacological studies of nociception in the orofacial area. The aim of this review is to present the effects of pharmacological activity of opioids and endocannabinoids affecting the transmission of the sensory information from the orofacial area on the example of trigemino-hypoglossal reflex in rats.

Hypothalamic Fatty Acids and Ketone Bodies Sensing and Role of FAT/CD36 in the Regulation of Food Intake

The obesity and type-2 diabetes epidemic is escalating and represents one of the costliest biomedical challenges confronting modern society. Moreover, the increasing consumption of high fat food is often correlated with an increase in body mass index. In people predisposed to be obese or already obese, the impaired ability of the brain to monitor and respond to alterations in fatty acid (FA) metabolism is increasingly recognized as playing a role in the pathophysiological development of these disorders. The brain senses and regulates metabolism using highly specialized nutrient-sensing neurons located mainly in the hypothalamus. The same neurons are able to detect variation in the extracellular levels of glucose, FA and ketone bodies as a way to monitor nutrient availability and to alter its own activity. In addition, glial cells such as astrocytes create major connections to neurons and form a tight relationship to closely regulate nutrient uptake and metabolism. This review will examine the different pathways by which neurons are able to detect free fatty acids (FFA) to alter its activity and how high fat diet (HFD)-astrocytes induced ketone bodies production interplays with neuronal FA sensing. The role of HFD-induced inflammation and how FA modulate the reward system will also be investigated here.

Brain accessibility delineates the central effects of circulating ghrelin

Ghrelin is a hormone produced in the gastrointestinal tract that acts via the growth hormone secretagogue receptor. In the central nervous system, ghrelin signalling is able to recruit different neuronal targets that regulate the behavioural, neuroendocrine, metabolic and autonomic effects of the hormone. Notably, several studies using radioactive or fluorescent variants of ghrelin have found that the accessibility of circulating ghrelin into the mouse brain is both strikingly low and restricted to some specific brain areas. A variety of studies addressing central effects of systemically injected ghrelin in mice have also provided indirect evidence that the accessibility of plasma ghrelin into the brain is limited. Here, we review these previous observations and discuss the putative pathways that would allow plasma ghrelin to gain access into the brain together with their physiological implications. Additionally, we discuss some potential features regarding the accessibility of plasma ghrelin into the human brain based on the observations reported by studies that investigate the consequences of ghrelin administration to humans.

Tanycyte-Like Cells Derived From Mouse Embryonic Stem Culture Show Hypothalamic Neural Stem/Progenitor Cell Functions

Tanycytes have recently been accepted as neural stem/progenitor cells in the postnatal hypothalamus. Persistent retina and anterior neural fold homeobox (Rax) expression is characteristic of tanycytes in contrast to its transient expression of whole hypothalamic precursors. In this study, we found that Rax+ residual cells in the maturation phase of hypothalamic differentiation in mouse embryonic stem cell (mESC) cultures had similar characteristics to ventral tanycytes. They expressed typical neural stem/progenitor cell markers, including Sox2, vimentin, and nestin, and differentiated into mature neurons and glial cells. Quantitative RT-PCR analysis showed that Rax+ residual cells expressed Fgf-10, Fgf-18, and Lhx2, which are expressed by ventral tanycytes. They highly expressed tanycyte-specific genes Dio2 and Gpr50 compared with Rax+ early hypothalamic progenitor cells. Therefore, Rax+ residual cells in the maturation phase of hypothalamic differentiation were considered to be more differentiated and similar to late progenitor cells and tanycytes. They self-renewed and formed neurospheres when cultured with exogenous FGF-2. Additionally, these Rax+ neurospheres differentiated into three neuronal lineages (neurons, astrocytes, and oligodendrocytes), including neuropeptide Y+ neuron, that are reported to be differentiated from ventral tanycytes toward the arcuate nuclei. Thus, Rax+ residual cells were multipotent neural stem/progenitor cells. Rax+ neurospheres were stably passaged and retained high Sox2 expression even after multiple passages. These results suggest the successful induction of Rax+ tanycyte-like cells from mESCs [induced tanycyte-like (iTan) cells]. These hypothalamic neural stem/progenitor cells may have potential in regenerative medicine and as a research tool.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

Low wnt/β-catenin signaling determines leaky vessels in the subfornical organ and affects water homeostasis in mice

The circumventricular organs (CVOs) in the central nervous system (CNS) lack a vascular blood-brain barrier (BBB), creating communication sites for sensory or secretory neurons, involved in body homeostasis. Wnt/β-catenin signaling is essential for BBB development and maintenance in endothelial cells (ECs) in most CNS vessels. Here we show that in mouse development, as well as in adult mouse and zebrafish, CVO ECs rendered Wnt-reporter negative, suggesting low level pathway activity. Characterization of the subfornical organ (SFO) vasculature revealed heterogenous claudin-5 (Cldn5) and Plvap/Meca32 expression indicative for tight and leaky vessels, respectively. Dominant, EC-specific β-catenin transcription in mice, converted phenotypically leaky into BBB-like vessels, by augmenting Cldn5⁺vessels, stabilizing junctions and by reducing Plvap/Meca32⁺ and fenestrated vessels, resulting in decreased tracer permeability. Endothelial tightening augmented neuronal activity in the SFO of water restricted mice. Hence, regulating the SFO vessel barrier may influence neuronal function in the context of water homeostasis.

Infections and diseases in the brain and spine can be very damaging and debilitating. Indeed, the central nervous system also needs a carefully controlled biochemical environment to survive. As such, all animals with a backbone have barriers and defenses to protect and preserve this key system. One of these is the blood-brain barrier, a physical barrier between the brain and the outside world. Where most blood vessels allow relatively free exchange of chemicals between the blood and surrounding cells, the blood-brain barrier controls what can move between the bloodstream and the brain.

Yet, there are gaps in the blood-brain barrier, specifically within structures in the brain called the circumventricular organs. These leaky vessels allow the brain cells in these regions to monitor the blood and respond to changes, for example, by triggering sensations such as hunger, thirst or nausea. It is not clear what stops the blood-brain barrier from forming in these regions and what effect the presence of a barrier would have on the brains activity, or the health and behavior of the animal.

Benz et al. have now used mice and zebrafish to examine the development and structure of the blood-brain barrier. The investigation revealed that the signals that induce the blood-brain barrier throughout the brain are absent in the circumventricular organs of both species. Next, by artificially activating a protein involved in cell-cell interactions in mice, Benz et al. created blood-brain barrier-like structures in circumventricular organs by converting the leaky vessels into tight ones. This change meant that the brain cells in these regions did not respond properly to water deprivation, which potentially may have affected the regulation of thirst in these mice.

Understanding the blood-brain barrier could have a variety of impacts on how we treat diseases in the central nervous system. This includes stroke, brain tumors and Alzheimers disease. These findings could particularly help scientists to better understand conditions that affect basic needs like thirst and hunger.

Tanycyte-Independent Control of Hypothalamic Leptin Signaling

Leptin is secreted by adipocytes to regulate appetite and body weight. Recent studies have reported that tanycytes actively transport circulating leptin across the brain barrier into the hypothalamus, and are required for normal levels of hypothalamic leptin signaling. However, direct evidence for leptin receptor (LepR) expression is lacking, and the effect of tanycyte-specific deletion of LepR has not been investigated. In this study, we analyze the expression and function of the tanycytic LepR in mice. Using single-molecule fluorescent in situ hybridization (smfISH), RT-qPCR, single-cell RNA sequencing (scRNA-Seq), and selective deletion of the LepR in tanycytes, we are unable to detect expression of LepR in the tanycytes. Tanycyte-specific deletion of LepR likewise did not affect leptin-induced pSTAT3 expression in hypothalamic neurons, regardless of whether leptin was delivered by intraperitoneal or intracerebroventricular injection. Finally, we use activity-regulated scRNA-Seq (act-Seq) to comprehensively profile leptin-induced changes in gene expression in all cell types in mediobasal hypothalamus. Clear evidence for leptin signaling is only seen in endothelial cells and subsets of neurons, although virtually all cell types show leptin-induced changes in gene expression. We thus conclude that LepR expression in tanycytes is either absent or undetectably low, that tanycytes do not directly regulate hypothalamic leptin signaling through a LepR-dependent mechanism, and that leptin regulates gene expression in diverse hypothalamic cell types through both direct and indirect mechanisms.

The autoimmune basis of hypopituitarism in traumatic brain injury: fiction or reality?

Post-traumatic hypopituitarism has remained as an obscured cause of worsening morbidity and mortality in head injury patients. Researchers have for decades been puzzled by the mechanism of pituitary dysfunction in these cases. Amongst other causes like direct injury, vascular injury etc, an immunological basis of hypopituitarism has been suggested in some animal studies as well as human research. In this article, we have reviewed the latest articles and compiled the evidence which suggests for or against the role of autoimmunity in post-traumatic hypopituitarism or which defines the strength to which autoimmunity has been established as a cause of head-injury induced pituitary dysfunction.

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.

The Discovery and Development of Liraglutide and Semaglutide

The discovery of glucagon-like peptide-1 (GLP-1), an incretin hormone with important effects on glycemic control and body weight regulation, led to efforts to extend its half-life and make it therapeutically effective in people with type 2 diabetes (T2D). The development of short- and then long-acting GLP-1 receptor agonists (GLP-1RAs) followed. Our article charts the discovery and development of the long-acting GLP-1 analogs liraglutide and, subsequently, semaglutide. We examine the chemistry employed in designing liraglutide and semaglutide, the human and non-human studies used to investigate their cellular targets and pharmacological effects, and ongoing investigations into new applications and formulations of these drugs. Reversible binding to albumin was used for the systemic protraction of liraglutide and semaglutide, with optimal fatty acid and linker combinations identified to maximize albumin binding while maintaining GLP-1 receptor (GLP-1R) potency. GLP-1RAs mediate their effects via this receptor, which is expressed in the pancreas, gastrointestinal tract, heart, lungs, kidneys, and brain. GLP-1Rs in the pancreas and brain have been shown to account for the respective improvements in glycemic control and body weight that are evident with liraglutide and semaglutide. Both liraglutide and semaglutide also positively affect cardiovascular (CV) outcomes in individuals with T2D, although the precise mechanism is still being explored. Significant weight loss, through an effect to reduce energy intake, led to the approval of liraglutide (3.0 mg) for the treatment of obesity, an indication currently under investigation with semaglutide. Other ongoing investigations with semaglutide include the treatment of non-alcoholic fatty liver disease (NASH) and its use in an oral formulation for the treatment of T2D. In summary, rational design has led to the development of two long-acting GLP-1 analogs, liraglutide and semaglutide, that have made a vast contribution to the management of T2D in terms of improvements in glycemic control, body weight, blood pressure, lipids, beta-cell function, and CV outcomes. Furthermore, the development of an oral formulation for semaglutide may provide individuals with additional benefits in relation to treatment adherence. In addition to T2D, liraglutide is used in the treatment of obesity, while semaglutide is currently under investigation for use in obesity and NASH.

Circumventricular Organs and Parasite Neurotropism: Neglected Gates to the Brain?

Circumventricular organs (CVOs), neural structures located around the third and fourth ventricles, harbor, similarly to the choroid plexus, vessels devoid of a blood-brain barrier (BBB). This enables them to sense immune-stimulatory molecules in the blood circulation, but may also increase chances of exposure to microbes. In spite of this, attacks to CVOs by microbes are rarely described. It is here highlighted that CVOs and choroid plexus can be infected by pathogens circulating in the bloodstream, providing a route for brain penetration, as shown by infections with the parasites Trypanosoma brucei. Immune responses elicited by pathogens or systemic infections in the choroid plexus and CVOs are briefly outlined. From the choroid plexus trypanosomes can seed into the ventricles and initiate accelerated infiltration of T cells and parasites in periventricular areas. The highly motile trypanosomes may also enter the brain parenchyma from the median eminence, a CVO located at the base of the third ventricle, by crossing the border into the BBB-protected hypothalamic arcuate nuclei. A gate may, thus, be provided for trypanosomes to move into brain areas connected to networks of regulation of circadian rhythms and sleep-wakefulness, to which other CVOs are also connected. Functional imbalances in these networks characterize human African trypanosomiasis, also called sleeping sickness. They are distinct from the sickness response to bacterial infections, but can occur in common neuropsychiatric diseases. Altogether the findings lead to the question: does the neglect in reporting microbe attacks to CVOs reflect lack of awareness in investigations or of gate-opening capability by microbes?

Neural Progenitor Cell Terminology

Since descriptions of neural precursor cells (NPCs) were published in the late 19th century, neuroanatomists have used a variety of terms to describe these cells, each term reflecting contemporary understanding of cellular characteristics and function. As the field gained knowledge through a combination of technical advance and individual insight, the terminology describing NPCs changed to incorporate new information. While there is a trend toward consensus and streamlining of terminology over time, to this day scientists use different terms for NPCs that reflect their field and perspective, i.e., terms arising from molecular, cellular, or anatomical sciences. Here we review past and current terminology used to refer to NPCs, including embryonic and adult precursor cells of the cerebral cortex and hippocampus.