Distribution of the tight junction-associated protein ZO-1 in circumventricular organs of the CNS

The diet-microbiota axis: a key regulator of intestinal permeability in human health and disease

The intestinal barrier orchestrates selective permeability to nutrients and metabolites while excluding noxious stimuli. Recent scientific advances establishing a causal role for the gut microbiota in human health outcomes have generated a resurgent interest toward intestinal permeability. Considering the well-established role of the gut barrier in protection against foreign antigens, there is mounting evidence for a causal link between gut permeability and the microbiome in regulating human health. However, an understanding of the dynamic host-microbiota interactions that govern intestinal barrier functions remains poorly defined. Furthermore, the system-level mechanisms by which microbiome-targeted therapies, such as probiotics and prebiotics, simultaneously promote intestinal barrier function and host health remain an area of active investigation. This review summarizes the recent advances in understanding the dynamics of intestinal permeability in human health and its integration with gut microbiota. We further summarize mechanisms by which probiotics/prebiotics influence the gut microbiota and intestinal barrier functions.

Adherens, tight, and gap junctions in ependymal cells: A systematic review of their contribution to CSF-brain barrier

Introduction

The movement of fluids and solutes across the ependymal barrier, and their changes in physiologic and disease states are poorly understood. This gap in knowledge contributes strongly to treatment failures and complications in various neurological disorders.

Methods

We systematically searched and reviewed original research articles treating ependymal intercellular junctions on PubMed. Reviews, opinion papers, and abstracts were excluded. Research conducted on tissue samples, cell lines, CSF, and animal models was considered.

Results

A total of 45 novel articles treating tight, adherens and gap junctions of the ependyma were included in our review, spanning from 1960 to 2022. The findings of this review point toward a central and not yet fully characterized role of the ependymal lining ultrastructure in fluid flow interactions in the brain. In particular, tight junctions circumferentially line the apical equator of ependymal cells, changing between embryonal and adult life in several rodent models, shaping fluid and solute transit in this location. Further, adherens and gap junctions appear to have a pivotal role in several forms of congenital hydrocephalus.

Conclusions

These findings may provide an opportunity for medical management of CSF disorders, potentially allowing for tuning of CSF secretion and absorption. Beyond hydrocephalus, stroke, trauma, this information has relevance for metabolite clearance and drug delivery, with potential to affect many patients with a variety of neurological disorders. This critical look at intercellular junctions in ependyma and the surrounding interstitial spaces is meant to inspire future research on a central and rather unknown component of the CSF-brain interface.

Historical and current perspectives on blood endothelial cell heterogeneity in the brain

Dynamic brain activity requires timely communications between the brain parenchyma and circulating blood. Brain-blood communication is facilitated by intricate networks of brain vasculature, which display striking heterogeneity in structure and function. This vascular cell heterogeneity in the brain is fundamental to mediating diverse brain functions and has long been recognized. However, the molecular basis of this biological phenomenon has only recently begun to be elucidated. Over the past century, various animal species and in vitro systems have contributed to the accumulation of our fundamental and phylogenetic knowledge about brain vasculature, collectively advancing this research field. Historically, dye tracer and microscopic observations have provided valuable insights into the anatomical and functional properties of vasculature across the brain, and these techniques remain an important approach. Additionally, recent advances in molecular genetics and omics technologies have revealed significant molecular heterogeneity within brain endothelial and perivascular cell types. The combination of these conventional and modern approaches has enabled us to identify phenotypic differences between healthy and abnormal conditions at the single-cell level. Accordingly, our understanding of brain vascular cell states during physiological, pathological, and aging processes has rapidly expanded. In this review, we summarize major historical advances and current knowledge on blood endothelial cell heterogeneity in the brain, and discuss important unsolved questions in the field.

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.

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.

Tight junctions in the blood-brain barrier promote edema formation and infarct size in stroke - Ambivalent effects of sealing proteins

The outcome of stroke is greatly influenced by the state of the blood-brain barrier (BBB). The BBB endothelium is sealed paracellularly by tight junction (TJ) proteins, i.e., claudins (Cldns) and the redox regulator occludin. Functions of Cldn3 and occludin at the BBB are largely unknown, particularly after stroke. We address the effects of Cldn3 deficiency and stress factors on the BBB and its TJs. Cldn3 tightened the BBB for small molecules and ions, limited endothelial endocytosis, strengthened the TJ structure and controlled Cldn1 expression. After middle cerebral artery occlusion (MCAO) and 3-h reperfusion or hypoxia of isolated brain capillaries, Cldn1, Cldn3 and occludin were downregulated. In Cldn3 knockout mice (C3KO), the reduction in Cldn1 was even greater and TJ ultrastructure was impaired; 48 h after MCAO of wt mice, infarct volumes were enlarged and edema developed, but endothelial TJs were preserved. In contrast, junctional localization of Cldn5 and occludin, TJ density, swelling and infarction size were reduced in affected brain areas of C3KO. Taken together, Cldn3 and occludin protect TJs in stroke, and this keeps the BBB intact. However, functional Cldn3, Cldn3-regulated TJ proteins and occludin promote edema and infarction, which suggests that TJ modulation could improve the outcome of stroke.

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.

Origin and development of circumventricular organs in living vertebrate

The circumventricular organs (CVOs) function by mediating chemical communication between blood and brain across the blood-brain barrier. Their origin and developmental mechanisms involved are not understood in enough detail due to a lack of molecular markers common for CVOs. These rather small and inconspicuous organs are found in close vicinity to the third and fourth brain ventricles suggestive of ancient evolutionary origin. Recently, an integrated approach based on analysis of CVOs development in the enhancer-trap transgenic zebrafish led to an idea that almost all of CVOs could be highlighted by GFP expression in this transgenic line. This in turn suggested that an enhancer along with a set of genes it regulates may illustrate the first common element of developmental regulation of CVOs. It seems to be related to a mechanism of suppression of the canonical Wnt/ β-catenin signaling that functions in development of fenestrated capillaries typical for CVOs. Based on that observation the common molecular elements of the putative developmental mechanism of CVOs will be discussed in this review.

The Gut Microbiota Links Dietary Polyphenols With Management of Psychiatric Mood Disorders

The pathophysiology of depression is multifactorial yet generally aggravated by stress and its associated physiological consequences. To effectively treat these diverse risk factors, a broad acting strategy is required and is has been suggested that gut-brain-axis signaling may play a pinnacle role in promoting resilience to several of these stress-induced changes including pathogenic load, inflammation, HPA-axis activation, oxidative stress and neurotransmitter imbalances. The gut microbiota also manages the bioaccessibility of phenolic metabolites from dietary polyphenols whose multiple beneficial properties have known therapeutic efficacy against depression. Although several potential therapeutic mechanisms of dietary polyphenols toward establishing cognitive resilience to neuropsychiatric disorders have been established, only a handful of studies have systematically identified how the interaction of the gut microbiota with dietary polyphenols can synergistically alleviate the biological signatures of depression. The current review investigates several of these potential mechanisms and how synbiotics, that combine probiotics with dietary polyphenols, may provide a novel therapeutic strategy for depression. In particular, synbiotics have the potential to alleviate neuroinflammation by modulating microglial and inflammasome activation, reduce oxidative stress and balance serotonin metabolism therefore simultaneously targeting several of the major pathological risk factors of depression. Overall, synbiotics may act as a novel therapeutic paradigm for neuropsychiatric disorders and further understanding the fundamental mechanisms of gut-brain-axis signaling will allow full utilization of the gut microbiota's as a therapeutic tool.

R40.76 binds to the α domain of ZO-1: role of ZO-1 (α+) in epithelial differentiation and mechano-sensing

The barrier function of epithelia and endothelia depends on tight junctions, which are formed by the polymerization of claudins on a scaffold of ZO proteins. Two differentially spliced isoforms of ZO-1 have been described, depending on the presence of the α domain, but the function of this domain is unclear. ZO-1 also contains a C-terminal ZU5 domain, which is involved in a mechano-sensitive intramolecular interaction with the central (ZPSG) region of ZO-1. Here we use immunoblotting and immunofluorescence to map the binding sites for commercially available monoclonal and polyclonal antibodies against ZO-1, and for a new polyclonal antibody (R3) that we developed against the ZO-1 C-terminus. We demonstrate that antibody R40.76 binds to the α domain, and the R3 antibody binds to the ZU5 domain. The (α+) isoform of ZO-1 shows higher expression in epithelial versus endothelial cells, and in differentiated versus undifferentiated primary keratinocytes, suggesting a link to epithelial differentiation and a potential molecular adaptation to junctions subjected to stronger mechanical forces. These results provide new tools and hypotheses to investigate the role of the α and ZU5 domains in ZO-1 mechano-sensing and dynamic interactions with the cytoskeleton and junctional ligands.

Fluorescent blood-brain barrier tracing shows intact leptin transport in obese mice

Background/objectives

Individuals carrying loss-of-function gene mutations for the adipocyte hormone leptin are morbidly obese, but respond favorably to replacement therapy. Recombinant leptin is however largely ineffective for the vast majority of obese individuals due to leptin resistance. One theory underlying leptin resistance is impaired leptin transport across the blood–brain-barrier (BBB). Here, we aim to gain new insights into the mechanisms of leptin BBB transport, and its role in leptin resistance.

Methods

We developed a novel tool for visualizing leptin transport using infrared fluorescently labeled leptin, combined with tissue clearing and light-sheet fluorescence microscopy. We corroborated these data using western blotting.

Results

Using 3D whole brain imaging, we display comparable leptin accumulation in circumventricular organs of lean and obese mice, predominantly in the choroid plexus (CP). Protein quantification revealed comparable leptin levels in microdissected mediobasal hypothalami (MBH) of lean and obese mice (p = 0.99). We further found increased leptin receptor expression in the CP (p = 0.025, p = 0.0002) and a trend toward elevated leptin protein levels in the MBH (p = 0.17, p = 0.078) of obese mice undergoing weight loss interventions by calorie restriction or exendin-4 treatment.

Conclusions

Overall, our findings suggest a crucial role for the CP in controlling the transport of leptin into the cerebrospinal fluid and from there to target areas such as the MBH, potentially mediated via the leptin receptor. Similar leptin levels in circumventricular organs and the MBH of lean and obese mice further suggest intact leptin BBB transport in leptin resistant mice.

Neuroimmune Axes of the Blood-Brain Barriers and Blood-Brain Interfaces: Bases for Physiological Regulation, Disease States, and Pharmacological Interventions

Central nervous system (CNS) barriers predominantly mediate the immune-privileged status of the brain, and are also important regulators of neuroimmune communication. It is increasingly appreciated that communication between the brain and immune system contributes to physiologic processes, adaptive responses, and disease states. In this review, we discuss the highly specialized features of brain barriers that regulate neuroimmune communication in health and disease. In section I, we discuss the concept of immune privilege, provide working definitions of brain barriers, and outline the historical work that contributed to the understanding of CNS barrier functions. In section II, we discuss the unique anatomic, cellular, and molecular characteristics of the vascular blood-brain barrier (BBB), blood-cerebrospinal fluid barrier, and tanycytic barriers that confer their functions as neuroimmune interfaces. In section III, we consider BBB-mediated neuroimmune functions and interactions categorized as five neuroimmune axes: disruption, responses to immune stimuli, uptake and transport of immunoactive substances, immune cell trafficking, and secretions of immunoactive substances. In section IV, we discuss neuroimmune functions of CNS barriers in physiologic and disease states, as well as pharmacological interventions for CNS diseases. Throughout this review, we highlight many recent advances that have contributed to the modern understanding of CNS barriers and their interface functions.

Dusp16 Deficiency Causes Congenital Obstructive Hydrocephalus and Brain Overgrowth by Expansion of the Neural Progenitor Pool

Hydrocephalus can occur in children alone or in combination with other neurodevelopmental disorders that are often associated with brain overgrowth. Despite the severity of these disorders, the molecular and cellular mechanisms underlying these pathologies and their comorbidity are poorly understood. Here, we studied the consequences of genetically inactivating in mice dual-specificity phosphatase 16 (Dusp16), which is known to negatively regulate mitogen-activated protein kinases (MAPKs) and which has never previously been implicated in brain development and disorders. Mouse mutants lacking a functional Dusp16 gene (Dusp16^−/−) developed fully-penetrant congenital obstructive hydrocephalus together with brain overgrowth. The midbrain aqueduct in Dusp16^−/− mutants was obstructed during mid-gestation by an expansion of neural progenitors, and during later gestational stages by neurons resulting in a blockage of cerebrospinal fluid (CSF) outflow. In contrast, the roof plate and ependymal cells developed normally. We identified a delayed cell cycle exit of neural progenitors in Dusp16^−/− mutants as a cause of progenitor overproliferation during mid-gestation. At later gestational stages, this expanded neural progenitor pool generated an increased number of neurons associated with enlarged brain volume. Taken together, we found that Dusp16 plays a critical role in neurogenesis by balancing neural progenitor cell proliferation and neural differentiation. Moreover our results suggest that a lack of functional Dusp16 could play a central role in the molecular mechanisms linking brain overgrowth and hydrocephalus.

Immunohistochemical and in situ hybridization study of urate transporters GLUT9/URATv1, ABCG2, and URAT1 in the murine brain

BACKGROUND

Uric acid (UA) is known to exert neuroprotective effects in the brain. However, the mechanism of UA regulation in the brain is not well characterized. In our previous study, we described that the mouse urate transporter URAT1 is localized to the cilia and apical surface of ventricular ependymal cells. To further strengthen the hypothesis that UA is transported transcellularly at the ependymal cells, we aimed to assess the distribution of other UA transporters in the murine brain.

METHODS

Immunostaining and highly-sensitive in situ hybridization was used to assess the distribution of UA transporters: GLUT9/URATv1, ABCG2, and URAT1.

RESULTS

Immunostaining for GLUT9 was observed in ependymal cells, neurons, and brain capillaries. Immunostaining for ABCG2 was observed in the choroid plexus epithelium and brain capillaries, but not in ependymal cells. These results were validated by in situ hybridization.

CONCLUSIONS

We propose that given their specific expression patterns in ependymal, choroid plexus epithelial, and brain capillary endothelial cells in this study, UA may be transported by these UA transporters in the murine brain. This may provide a novel strategy for targeted neuroprotection.

Heterogeneity of the blood-brain barrier

The brain microvascular network is comprised of capillaries, arterioles and venules, all of which retain - although to a different extent - blood-brain barrier (BBB) properties. Capillaries constitute the largest and tightest microvasculature. In contrast, venules have a looser junctional arrangement, while arterioles have a lower expression of P-gp. Development and maintenance of the BBB depends on the interaction of cerebral endothelial cells with pericytes and astrocytes, which are all heterogeneous in different regions of the central nervous system. At the level of circumventricular organs microvessels are permeable, containing fenestrations and discontinuous tight junctions. In addition, the blood-spinal cord barrier - where the number of pericytes is lower and expression of junctional proteins is reduced - is also more permeable than the BBB. However, much less is known about the cellular, molecular and functional differences among other regions of the brain. This review summarizes our current knowledge on the heterogeneity of the brain microvasculature.

tPA Deficiency in Mice Leads to Rearrangement in the Cerebrovascular Tree and Cerebroventricular Malformations

The serine protease tissue-type plasminogen activator (tPA) is used as a thrombolytic agent in the management of ischemic stroke, but concerns for hemorrhagic conversion greatly limits the number of patients that receive this treatment. It has been suggested that the bleeding complications associated with thrombolytic tPA may be due to unanticipated roles of tPA in the brain. Recent work has suggested tPA regulation of neurovascular barrier integrity, mediated via platelet derived growth factor (PDGF)-C/PDGF receptor-α (PDGFRα) signaling, as a possible molecular mechanism affecting the outcome of stroke. To better understand the role of tPA in neurovascular regulation we conducted a detailed analysis of the cerebrovasculature in brains from adult tPA deficient (tPA^−/−) mice. Our analysis demonstrates that life-long deficiency of tPA is associated with rearrangements in the cerebrovascular tree, including a reduction in the number of vascular smooth-muscle cell covered, large diameter, vessels and a decrease in vessel-associated PDGFRα expression as compared to wild-type (WT) littermate controls. In addition, we found that ablation of tPA results in an increased number of ERG-positive endothelial cells and increased junctional localization of the tight junction protein ZO1. This is intriguing since ERG is an endothelial transcription factor implicated in regulation of vascular integrity. Based on these results, we propose that the protection of barrier properties seen utilizing these tPA^−/− mice might be due, at least in part, to these cerebrovascular rearrangements. In addition, we found that tPA^−/− mice displayed mild cerebral ventricular malformations, a feature previously associated with ablation of PDGF-C, thereby providing an in vivo link between tPA and PDGF signaling in central nervous system (CNS) development. Taken together, the data presented here will advance our understanding of the role of tPA within the CNS and in regulation of cerebrovascular permeability.

New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

Heterogeneous vascular permeability and alternative diffusion barrier in sensory circumventricular organs of adult mouse brain

Fenestrated capillaries of the sensory circumventricular organs (CVOs), including the organum vasculosum of the lamina terminalis, the subfornical organ and the area postrema, lack completeness of the blood-brain barrier (BBB) to sense a variety of blood-derived molecules and to convey the information into other brain regions. We examine the vascular permeability of blood-derived molecules and the expression of tight-junction proteins in sensory CVOs. The present tracer assays revealed that blood-derived dextran 10 k (Dex10k) having a molecular weight (MW) of 10,000 remained in the perivascular space between the inner and outer basement membranes, but fluorescein isothiocyanate (FITC; MW: 389) and Dex3k (MW: 3000) diffused into the parenchyma. The vascular permeability of FITC was higher at central subdivisions than at distal subdivisions. Neither FITC nor Dex3k diffused beyond the dense network of glial fibrillar acidic protein (GFAP)-positive astrocytes/tanycytes. The expression of tight-junction proteins such as occludin, claudin-5 and zonula occludens-1 (ZO-1) was undetectable at the central subdivisions of the sensory CVOs but some was expressed at the distal subdivisions. Electron microscopic observation showed that capillaries were surrounded with numerous layers of astrocyte processes and dendrites. The expression of occludin and ZO-1 was also observed as puncta on GFAP-positive astrocytes/tanycytes of the sensory CVOs. Our study thus demonstrates the heterogeneity of vascular permeability and expression of tight-junction proteins and indicates that the outer basement membrane and dense astrocyte/tanycyte connection are possible alternative mechanisms for a diffusion barrier of blood-derived molecules, instead of the BBB.

[Role of tanycytes within the blood-hypothalamus interface]

Information exchanges between the brain and the periphery are key stages in the regulation of various physiological functions. The mediobasal hypothalamus, which ensures a large part of these functions, must be permanently informed about the physiological state of the body to guarantee the maintaining of homeostasis. For that purpose, it possesses a peculiar blood-brain interface due to the presence of specialized glial cells called tanycytes. This review describes the organization of the blood-hypothalamus interface and characterizes the peculiar place of tanycytes within it, as well as their striking capacity to remodel their own interface in order to ensure the regulation of various physiological functions.

Anatomical, molecular and pathological consideration of the circumventricular organs

BACKGROUND AND PURPOSE

Circumventricular organs (CVOs) are a diverse group of specialised structures characterized by peculiar vascular and position around the third and fourth ventricles of the brain. In humans, these organs are present during the fetal period and some become vestigial after birth. Some, such as the pineal gland (PG), subcommissural organ (SCO) and organum vasculosum of the lamina terminalis (OVLT), which are located around the third ventricle, might be the site of origin of periventricular tumours. In contrast to humans, CVOs are present in the adult rat and can be dissected by laser capture microdissection (LCM).

METHODS

In this study, we used LCM and microarrays to analyse the transcriptomes of three CVOs, the SCO, the subfornical organ (SFO) and the PG and the third ventricle ependyma of the adult rat, in order to better characterise these organs at the molecular level. Furthermore, an immunohistochemical study of Claudin-3 (CLDN3), a membrane protein involved in forming cellular tight junctions, was performed at the level of the SCO.

RESULTS

This study highlighted some potentially new or already described specific markers of these structures as Erbb2 and Col11a1 in ependyma, Epcam and CLDN3 in the SCO, Ren1 and Slc22a3 in the SFO and Tph, Anat and Asmt in the PG. Moreover, we found that CLDN3 expression was restricted to the apical pole of ependymocytes in the SCO.

Chronic functional bowel syndrome enhances gut-brain axis dysfunction, neuroinflammation, cognitive impairment, and vulnerability to dementia

The irritable bowel syndrome (IBS) is a common chronic functional gastrointestinal disorder world wide that lasts for decades. The human gut harbors a diverse population of microbial organisms which is symbiotic and important for well being. However, studies on conventional, germ-free, and obese animals have shown that alteration in normal commensal gut microbiota and an increase in pathogenic microbiota-termed "dysbiosis", impact gut function, homeostasis, and health. Diarrhea, constipation, visceral hypersensitivity, and abdominal pain arise in IBS from the gut-induced dysfunctional metabolic, immune, and neuro-immune communication. Dysbiosis in IBS is associated with gut inflammation. Gut-related inflammation is pivotal in promoting endotoxemia, systemic inflammation, and neuroinflammation. A significant proportion of IBS patients chronically consume alcohol, non-steroidal anti-inflammatories, and fatty diet; they may also suffer from co-morbid respiratory, neuromuscular, psychological, sleep, and neurological disorders. The above pathophysiological substrate is underpinned by dysbiosis, and dysfunctional bidirectional "Gut-Brain Axis" pathways. Pathogenic gut microbiota-related systemic inflammation (due to increased lipopolysaccharide and pro-inflammatory cytokines, and barrier dysfunction), may trigger neuroinflammation enhancing dysfunctional brain regions including hippocampus and cerebellum. These as well as dysfunctional vago-vagal gut-brain axis may promote cognitive impairment. Indeed, inflammation is characteristic of a broad spectrum of neurodegenerative diseases that manifest demntia. It is argued that an awareness of pathophysiological impact of IBS and implementation of appropriate therapeutic measures may prevent cognitive impairment and minimize vulnerability to dementia.

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

Tanycytes are highly specialized ependymal cells that form a blood-cerebrospinal fluid (CSF) barrier at the level of the median eminence (ME), a circumventricular organ (CVO) located in the tuberal region of the hypothalamus. This ependymal layer harbors well-organized tight junctions, a hallmark of central nervous system barriers that is lacking in the fenestrated portal vessels of the ME. The displacement of barrier properties from the vascular to the ventricular side allows the diffusion of blood-borne molecules into the parenchyma of the ME while tanycyte tight junctions control their diffusion into the CSF, thus maintaining brain homeostasis. In the present work, we combined immunohistochemical and permeability studies to investigate the presence of tanycyte barriers along the ventricular walls of other brain CVOs. Our data indicate that, unlike cuboidal ependymal cells, ependymal cells bordering the CVOs possess long processes that project into the parenchyma of the CVOs to reach the fenestrated capillary network. Remarkably, these tanycyte-like cells display well-organized tight junctions around their cell bodies. Consistent with these observations, permeability studies show that this ependymal layer acts as a diffusion barrier. Together, our results suggest that tanycytes are a characteristic feature of all CVOs and yield potential new insights into their involvement in regulating the exchange between the blood, the brain, and the CSF within these "brain windows."

Accessibility of low-molecular-mass molecules to the median eminence and arcuate hypothalamic nucleus of adult mouse

Blood-derived molecules are able to access to the median eminence (ME) and arcuate hypothalamic nucleus (Arc) due to the lack of the blood-brain barrier. In the present study, we examined the accessibility of low-molecular-mass (LMM) molecules into parenchyma in the ME and Arc of adult mice by administration of Dextran 3000 (Dex3k), Dex10k, Evans blue (EB) and fluorescein isothiocyanate (FITC). In the external zone of the ME, the fluorescence of Dex3k, EB and FITC tracers generated an intensity gradient from fenestrated capillary, but that of Dex10k was detected only between the inner and outer basement membrane of pericapillary space. The fluorescence of FITC in the external zone of the ME was closely associated with axonal terminals and surrounded by cellular processes of tanycytes-like cells and astrocytes. In the ependymal/internal zone of the ME and Arc, the fluorescence of all LMM tracers was seen at tanycytes-like cells and neurons. The fluorescence of EB and FITC in these regions was not detected when brains were fixed during or before the administration of tracers. The inhomogeneity of accessibility for fluorescent tracers depended on routes for tracer administration. Thus, the present study indicates that the accessibility of LMM blood-derived molecules to parenchyma depends on fenestration of the capillary in the external zone of the ME and active transport of ependymal cells in the ependymal/internal zone of the ME and Arc.

Molecular characterization of circumventricular organs and third ventricle ependyma in the rat: potential markers for periventricular tumors

Circumventricular organs (CVOs) are specialized ventricular structures around the third and fourth ventricles of the brain. In humans, these structures are present during the fetal period and some become vestigial after birth. Some of these organs, such as the pineal gland (PG), subcommissural organ (SCO), and organum vasculosum of the lamina terminalis, might be the sites of origin of periventricular tumors, notably pineal parenchymal tumors, papillary tumor of the pineal region and chordoid glioma. In contrast to the situation in humans, CVOs are present in the adult rat and can be dissected by laser capture microdissection (LCM). In this study, we used LCM and microarrays to analyze the transcriptomes of three CVOs, the SCO, the subfornical organ (SFO), and the PG and the third ventricle ependyma in the adult rat, in order to better characterize these organs at the molecular level. Several genes were expressed only, or mainly, in one of these structures, for example, Erbb2 and Col11a1 in the ependyma, Epcam and Claudin-3 (CLDN3) in the SCO, Ren1 and Slc22a3 in the SFO and Tph, Aanat and Asmt in the PG. The expression of these genes in periventricular tumors should be examined as evidence for a possible origin from the CVOs. Furthermore, we performed an immunohistochemical study of CLDN3, a membrane protein involved in forming cellular tight junctions and found that CLDN3 expression was restricted to the apical pole of ependymocytes in the SCO. This microarray study provides new evidence regarding the possible origin of some rare periventricular tumors.

An angiogenic inhibitor, cyclic RGDfV, attenuates MPTP-induced dopamine neuron toxicity

We previously demonstrated that several dopamine (DA) neurotoxins produced punctate areas of FITC-labeled albumin (FITC-LA) leakage in the substantia nigra and striatum suggesting blood brain barrier (BBB) dysfunction. Further, this leakage was co-localized with αvβ3 integrin up-regulation, a marker for angiogenesis. This suggested that the FITC-LA leakage might have been a result of angiogenesis. To assess the possible role of angiogenesis in DA neuron loss, we treated mice with 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) and on the following day treated with cyRGDfV, a cyclic peptide that binds to integrin αvβ3 and prevents angiogenesis. Post-treatment for 3 days (b.i.d.) with cyRGDfV blocked the MPTP-induced upregulation of integrin β3 immunoreactivity (a marker for angiogenesis), leakage of FITC-LA into brain parenchyma (a marker for BBB disruption) as well as the down regulation of Zona Occludin-1 (ZO-1; a marker for tight junction integrity). In addition, cyRGDfV also completely prevented tyrosine hydroxylase immunoreactive cell loss (a marker for DA neurons) and markedly attenuated the up-regulation of activated microglia (Iba1 cell counts and morphology). These data suggest that cyRGDfV, and perhaps other anti-angiogenic drugs, are neuroprotective following acute MPTP treatment and may suggest that compensatory angiogenesis and BBB dysfunction may contribute to inflammation and DA neuron loss.

Sensory circumventricular organs in health and disease

Circumventricular organs (CVOs) are specialized brain structures located around the third and fourth ventricles. They differ from the rest of the brain parenchyma in that they are highly vascularised areas that lack a blood-brain barrier. These neurohaemal organs are classified as "sensory", when they contain neurons that can receive chemical inputs from the bloodstream. This review focuses on the sensory CVOs to describe their unique structure, and their functional roles in the maintenance of body fluid homeostasis and cardiovascular regulation, and in the generation of central acute immune and febrile responses. In doing so, the main neural connections to visceral regulatory centres such as the hypothalamus, the medulla oblongata and the endocrine hypothalamic-pituitary axis, as well as some of the relevant chemical substances involved, are described. The CVOs are vulnerable to circulating pathogens and can be portals for their entry in the brain. This review highlights recent investigations that show that the CVOs and related structures are involved in pathological conditions such as sepsis, stress, trypanosomiasis, autoimmune encephalitis, systemic amyloidosis and prion infections, while detailed information on their role in other neurodegenerative diseases such as Alzheimer's disease or multiple sclerosis is lacking. It is concluded that studies of the CVOs and related structures may help in the early diagnosis and treatment of such disorders.

Immune surveillance of the normal human CNS takes place in dependence of the locoregional blood-brain barrier configuration and is mainly performed by CD3(+)/CD8(+) lymphocytes

Despite the blood-brain barrier (BBB) the human CNS is continuously screened by blood-derived immunological cells. In certain brain areas the local BBB configuration grants passage of large molecules, whereas others are better shielded. We investigated whether these regional BBB compositions are paralleled by differences in the degree of cellular immunosurveillance by investigating tissue from 23 normal human brains for several CD markers, FoxP3, granzyme B, and perforin. Our results provide evidence that immunosurveillance is associated with locoregional BBB configuration and is mainly performed by CD3(+)/CD8(+)/granzyme B(-)/perforin(-) lymphocytes.

Circulating signals as critical regulators of autonomic state--central roles for the subfornical organ

To maintain homeostasis autonomic control centers in the hypothalamus and medulla must respond appropriately to both external and internal stimuli. Although protected behind the blood-brain barrier, neurons in these autonomic control centers are known to be influenced by changing levels of important signaling molecules in the systemic circulation (e.g., osmolarity, glucose concentrations, and regulatory peptides). The subfornical organ belongs to a group of specialized central nervous system structures, the circumventricular organs, which are characterized by the lack of the normal blood-brain barrier, such that circulating lipophobic substances may act on neurons within this region and via well-documented efferent neural projections to hypothalamic autonomic control centers, influence autonomic function. This review focuses on the role of the subfornical organ in sensing peripheral signals and transmitting this information to autonomic control centers in the hypothalamus.

Differential distribution of tight junction proteins suggests a role for tanycytes in blood-hypothalamus barrier regulation in the adult mouse brain

The median eminence is one of the seven so-called circumventricular organs. It is located in the basal hypothalamus, ventral to the third ventricle and adjacent to the arcuate nucleus. This structure characteristically contains a rich capillary plexus and features a fenestrated endothelium, making it a direct target of blood-borne molecules. The median eminence also contains highly specialized ependymal cells called tanycytes, which line the floor of the third ventricle. It has been hypothesized that one of the functions of these cells is to create a barrier that prevents substances in the portal capillary spaces from entering the brain. In this paper, we utilize immunohistochemistry to study the expression of tight junction proteins in the cells that compose the median eminence in adult mice. Our results indicate that tanycytes of the median eminence express occludin, ZO-1, and claudin 1 and 5, but not claudin 3. Remarkably, these molecules are organized as a continuous belt around the cell bodies of the tanycytes that line the ventral part of the third ventricle. In contrast, the tanycytes at the periphery of the arcuate nucleus do not express claudin 1 and instead exhibit a disorganized expression pattern of occludin, ZO-1, and claudin 5. Consistent with these observations, permeability studies using peripheral or central injections of Evans blue dye show that only the tanycytes of the median eminence are joined at their apices by functional tight junctions, whereas tanycytes located at the level of the arcuate nucleus form a permeable layer. In conclusion, this study reveals a unique expression pattern of tight junction proteins in hypothalamic tanycytes, which yields new insights into their barrier properties.

The design of barriers in the hypothalamus allows the median eminence and the arcuate nucleus to enjoy private milieus: the former opens to the portal blood and the latter to the cerebrospinal fluid

The blood-brain barrier (BBB) is a single uninterrupted barrier that in the brain capillaries is located at the endothelial cells and in the circumventricular organs, such as the choroid plexuses (CP) and median eminence (ME), is displaced to specialized ependymal cells. How do hypothalamic hormones reach the portal circulation without making the BBB leaky? The ME milieu is open to the portal vessels, while it is closed to the cerebrospinal fluid (CSF) and to the arcuate nucleus. The cell body and most of the axons of neurons projecting to the ME are localized in areas protected by the BBB, while the axon terminals are localized in the BBB-free area of the ME. This design implies a complex organization of the intercellular space of the median basal hypothalamus. The privacy of the ME milieu implies that those neurons projecting to this area would not be under the influence of compounds leaking from the portal capillaries, unless receptors for such compounds are located at the axon terminal. Amazingly, the arcuate nucleus also has its private milieu that is closed to all adjacent neural structures and open to the infundibular recess. The absence of multiciliated cells in this recess should result in a slow CSF flow at this level. This whole arrangement should facilitate the arrival of CSF signal to the arcuate nucleus. This review will show how peripheral hormones can reach hypothalamic targets without making the BBB leaky.

Aquaporin-1 in blood vessels of rat circumventricular organs

Although the water channel protein aquaporin-1 (AQP1) is widely observed outside the rat brain in continuous, but not fenestrated, vascular endothelia, it has not previously been observed in any endothelia within the normal rat brain and only to a limited extent in the human brain. In this immunohistochemical study of rat brain, AQP1 has also been found in microvessel endothelia, probably of the fenestrated type, in all circumventricular organs (except the subcommissural organ and the vascular organ of the lamina terminalis): in the median eminence, pineal, subfornical organ, area postrema and choroid plexus. The majority of microvessels in the median eminence, pineal and choroid plexus, known to be exclusively fenestrated, are shown to be AQP1-immunoreactive. In the subfornical organ and area postrema in which many, but not all, microvessels are fenestrated, not all microvessels are AQP1-immunoreactive. In the AQP1-immunoreactive microvessels, the AQP1 probably facilitates water movement between blood and interstitium as one component of the normal fluxes that occur in these specialised sensory and secretory areas. AQP1-immunoreactive endothelia have also been seen in a small population of blood vessels in the cerebral parenchyma outside the circumventricular organs, similar to other observations in human brain. The proposed development of AQP1 modulators to treat various brain pathologies in which AQP1 plays a deleterious role will necessitate further work to determine the effect of such modulators on the normal function of the circumventricular organs.

Cellular transfer of macromolecules across the developing choroid plexus of Monodelphis domestica

Choroid plexus epithelial cells secrete cerebrospinal fluid (CSF) and transfer molecules from blood into CSF. Tight junctions between choroidal epithelial cells are functionally effective from early in development: the route of transfer is suggested to be transcellular. Routes of transfer for endogenous and exogenous plasma proteins and dextrans were studied in Monodelphis domestica (opossum). Pups at postnatal (P) days 1-65 and young adults were injected with biotinylated dextrans (3-70 kDa) and/or foetal protein fetuin. CSF, plasma and brain samples were collected from terminally anaesthetized animals. Choroid plexus cells containing plasma proteins were detected immunocytochemically. Numbers of plasma protein-positive epithelial cells increased to adult levels by P28, but their percentage of plexus cells declined. Numbers of cells positive for biotinylated probes increased with age, while their percentage remained constant. Colocalization studies showed specificity for individual proteins in some epithelial cells. Biotinylated probes and endogenous proteins colocalized in about 10% of cells in younger animals, increasing towards 100% by adulthood. Injections of markers into the ventricles demonstrated that protein is transferred only from blood into CSF, whereas dextrans pass in both directions. These results indicate that protein and lipid-insoluble markers are transferred by separate mechanisms present in choroid plexuses from the earliest stage of brain development, and transfer of proteins from plasma across choroid plexus epithelial cells contributes to the high protein concentration in CSF in the immature brain.

Gonadotropin-releasing hormone in third ventricular cerebrospinal fluid: endogenous distribution and exogenous uptake

GnRH is detectable in the cerebrospinal fluid (CSF), but its source remains unidentified. Previous studies have harvested CSF for GnRH analysis from the median eminence region, but it is unknown whether GnRH in CSF is restricted to this region. If CSF-GnRH plays a physiological role, through volume transmission, to communicate with brain regions that express GnRH receptors but are not evidently innervated by GnRH neurons, then it is essential to establish whether GnRH is more pervasive throughout the cerebroventricular system. Three cannulae were placed in the supraoptic, infundibular, and pineal recesses of the third ventricle. GnRH was undetectable in lateral ventricle CSF. GnRH pulses were detected in all ewes in infundibular recess CSF, but at sites more rostral (supraoptic) and caudal (pineal), GnRH pulse frequency and amplitude significantly (P<0.05) decreased. A GnRH surge was evident in CSF collected simultaneously from all cannulae, but the amplitude was greatest (P<0.05) at the infundibular recess. A final study established whether iv administered GnRH enters the CSF. A 250-ng GnRH dose did not affect CSF-GnRH concentrations (1.6+/-0.3 pg/ml), but 2.5 microg (2.7+/-0.2 pg/ml; P<0.001) and 1 mg (38.5+/-10.6 pg/ml; P<0.05) significantly increased CSF-GnRH concentrations. The present study shows: 1) the median eminence region is likely to be the major, if not only, source of GnRH entering the cerebroventricular system; and 2) exogenous GnRH crosses the blood-brain barrier, but extremely high doses are required to elevate CSF concentrations to physiological levels. Thus, CSF-GnRH may affect sites that are closer in proximity to the infundibular recess region than previously thought.

GlialCAM, an immunoglobulin-like cell adhesion molecule is expressed in glial cells of the central nervous system

Using structure based genome mining targeting vascular endothelial and platelet derived growth factor immunoglobulin (Ig) like folds, we have identified a sequence corresponding to a single transmembrane protein with two Ig domains, which we cloned from a human brain cDNA library. The cDNA is identical to hepatocyte cell adhesion molecule (hepaCAM), which was originally described as a tumor suppressor gene in liver. Here, we show that the protein is predominantly expressed in the mouse and human nervous system. In liver, the expression is very low in humans, and is not detected in mice. To identify the central nervous system (CNS) regions and cell types expressing the protein, we performed a LacZ reporter gene assay on heterozygous mice in which one copy of the gene encoding the novel protein had been replaced with beta-galactosidase. beta-galactosidase expression was prominent in white matter tracts of the CNS. Furthermore, expression was detected in ependymal cells of the brain ventricular zones and the central canal of the spinal cord. Double labeling experiments showed expression mainly in CNPase positive oligodendrocytes (OL). Since the protein is predominantly expressed in the CNS glial cells, we named the molecule glial cell adhesion molecule (GlialCAM). A potential role for GlialCAM in myelination was supported by its up-regulation during postnatal mouse brain development, where it was concomitantly expressed with myelin basic protein (MBP). In addition, in vitro, GlialCAM was observed in various developmental stages of OL and in astrocytes in processes and at cell contact sites. In A2B5 positive OL, GlialCAM colocalizes with GAP43 in OL growth cone like structures. Overall, the data presented here indicate a potential function for GlialCAM in glial cell biology.

Protein components of the blood-brain barrier (BBB) in the mediobasal hypothalamus

The blood-brain barrier (BBB) plays an important role in controlling the access of substances to the brain. Of the circumventricular organs (CVO), i.e. areas that lack a BBB, the median eminence and its close relationship with the hypothalamic arcuate nucleus plays an important role in controlling the entry of blood-borne substances to neurons of the mediobasal hypothalamus. In order to clarify the nature of the BBB in the median eminence-arcuate nucleus complex, we have used immunohistochemistry and antisera to protein components of the BBB-(1) tight junctions, claudin-5 and zona occludens-1 (ZO-1); (2) endothelial cells: (a) all endothelial cells: rat endothelial cell antigen-1 (RECA-1), (b) endothelial cells at BBB: endothelial barrier antigen (EBA), glucose transporter 1 (GLUT1) and transferrin receptor (TfR), and (c) endothelial cells at CVOs: dysferlin; (3) basal lamina: laminin; (4) vascular smooth muscle cells: smooth muscle actin (SMA); (5) pericytes: chondroitin sulfate proteoglycan (NG2); (6) glial cells: (a) astrocytes: glial fibrillary acidic protein (GFAP), (b) tanycytes: dopamine- and cAMP-regulated phosphoprotein of 32kDA (DARPP-32), (c) microglia: CD11b. Neuronal cell bodies located in the ventromedial aspect of the arcuate nucleus were visualized by antiserum to agouti-related protein (AgRP). The study provides a detailed analysis on the cellular localization of BBB components in the mediobasal hypothalamus. Some vessels in the ventromedial aspect of the arcuate nucleus lacked the BBB markers EBA and TfR, suggesting an absence of an intact BBB. These vessels may represent a route of entry for circulating substances to a subpopulation of arcuate nucleus neurons.

Regional analysis of the ependyma of the third ventricle of rat by light and electron microscopy

Ependymal lining of cerebral ventricles lies at the interface between the ventricular cavities and the brain parenchyma. Ependymal cells are involved in various functions within the brain and play a major role in the production of the chemical principals of the cerebrospinal fluid. Histological studies on the regional variation of the third ventricular ependyma and the subependyma of adult rats were carried out by light and electron microscopic methods. For light microscopic analysis, methacrylate sections were used. In addition to the routine haematoxylin and eosin (H and E) staining for histological studies, the sections were stained with toluidine blue, cresyl violet and periodic acid Schiff's reagent (PAS). A regional analysis of the ependyma of the third ventricle showed that in most regions the ependyma was monolayered. The sidewalls and floor of the ventral portion of the third ventricle showed a multilayered ependyma. For descriptive purposes at the light microscopic level, the ependymal cells were classified, based on the cell shape (flat, cuboidal or columnar), presence or absence of cilia and the number of cytoplasmic granules present in the cells. Studies of transmission electron microscope have shown that these granules represent the cell organelles of the ependyma. The subependyma also showed a regional morphological variation, and, in most instances, contained glial and neuronal elements. In regions of specific brain nuclei, neurons were the major cell type of the subependyma. PAS staining did not show any positive granules in the ependymal cytosol. Characteristic supraependymal elements were present at the ependymal surface of the third ventricle.

A diffusion barrier between the area postrema and nucleus tractus solitarius

The blood-brain barrier (BBB) is a structural and functional barrier that prevents free exchange of circulating substances with the brain, where the endothelial cells of microvessels are joined by tight junctions. The circumventricular organs (CVOs), by contrast, lack tight junctions and exhibit more direct communication with the circulating blood and cerebrospinal fluid. Despite many outstanding morphological studies at the electron microscopic level, there remain misconceptions that the CVOs provide direct passage of blood-borne substances to the rest of the brain. This study will show the structure of the anatomical borders of the dorsal vagal complex in the brainstem. A distinct diffusion barrier between the area postrema (AP, a CVO) and the nucleus tractus solitarius (NTS) was illustrated by immunohistochemistry at both the light and electron microscopic levels. The border zone between the AP and NTS was underlined by a continuous monolayer of columnar cells that were immunopositive for both the tight junction protein zona occludin-1 and the astrocyte marker glial fibrillary acidic protein. This observation of a diffusion barrier between the AP and NTS resolves a long-standing dispute about whether the NTS is a structural extension of the AP with a leaky BBB.

The area postrema: a brain monitor and integrator of systemic autonomic state

The area postrema is a medullary structure lying at the base of the fourth ventricle. The area postrema's privileged location outside of the blood-brain barrier make this sensory circumventricular organ a vital player in the control of autonomic functions by the central nervous system. By virtue of its lack of tight junctions between endothelial cells in this densely vascularized structure and the presence of fenestrated capillaries, peptide and other physiological signals borne in the blood have direct access to neurons that project to brain areas with important roles in the autonomic control of many physiological systems, including the cardiovascular system and systems controlling feeding and metabolism. However, the area postrema is not simply a conduit through which signals flow into the brain, but it is now being recognized as the initial site of integration for these signals as they enter the circuitry of the central nervous system.

Differential changes in junctional complex proteins suggest the ependymal lining as the main source of leukocyte infiltration into ventricles in murine neurocysticercosis

The blood brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCB) limit the influx of immune mediators and bloodstream compounds into the central nervous system (CNS). Upon injury or infection, the integrity of these barriers is compromised and leukocyte infiltration occurs. The BCB is located in the choroid plexuses (CPs) found within ventricles of the brain, and it is considered one of the main routes of cellular infiltration into the CNS into healthy individuals. Our group recently showed that in a murine model of neurocysticercosis (NCC), there is a moderate increase in infiltration of leukocytes into ventricles, but the BCB is hardly compromised. To elucidate the role played by CPs and surrounding ependyma in leukocyte infiltration at ventricular sites, we analyzed changes in the expression of junctional complex proteins in animals intracranially infected with Mesocestoides corti. The results indicate that infection does not change the expression pattern of junctional complex proteins in CPs, but structural alterations and disappearance of these proteins were evident in ependyma adjacent to the internal leptomeninges. The kinetics and magnitude of these changes directly correlated with the extent of leukocyte infiltration through ependyma and with the expression and activity of MMPs. The results of this study indicate that the anatomical elements of the BCB are minimally disrupted during the course of murine NCC. Thus, most of the leukocytes infiltrating ventricles appear to extravasate through pial vessels located in the internal leptomeninges juxtaposed to the ependyma layer and then traverse the ependyma cells. In addition, MMP activity seems to be involved in this process. These results provide evidence for a previously undescribed entry route for leukocytes into the CNS.

The circumventricular organs: an atlas of comparative anatomy and vascularization

The circumventricular organs are small sized structures lining the cavity of the third ventricle (neurohypophysis, vascular organ of the lamina terminalis, subfornical organ, pineal gland and subcommissural organ) and of the fourth ventricle (area postrema). Their particular location in relation to the ventricular cavities is to be noted: the subfornical organ, the subcommissural organ and the area postrema are situated at the confluence between ventricles while the neurohypophysis, the vascular organ of the lamina terminalis and the pineal gland line ventricular recesses. The main object of this work is to study the specific characteristics of the vascular architecture of these organs: their capillaries have a wall devoid of blood-brain barrier, as opposed to central capillaries. This particular arrangement allows direct exchange between the blood and the nervous tissue of these organs. This work is based on a unique set of histological preparations from 12 species of mammals and 5 species of birds, and is taking the form of an atlas.

Differential uptake of molecules from the circulation and CSF reveals regional and cellular specialisation in CNS detection of homeostatic signals

The uptake of hydroxystilbamidine (OHSt, FluoroGold equivalent) and wheat germ agglutinin (WGA), into the hypothalamus, two hours after injections into either the circulation or the cerebrospinal fluid, were compared in adult rats. Following intravenous injection, OHSt was found in astrocytes of the median eminence and medial part of the arcuate nucleus whereas WGA intensely labelled the blood vessels and ependymal cells throughout the hypothalamus. In complete contrast, intracerebroventricular (icv) injection into the lateral ventricle resulted in OHSt uptake by ependymocytes and astrocytes in the area adjacent to the third ventricle, with virtually no uptake in regions taking up this dye following systematic injections, i.e., the median eminence and medial arcuate. Following icv injection WGA labelling was intense in all parts of the ependymal layer of the third ventricle, including the alpha- and beta-tanycytes. Injections into the cisterna magna gave a different pattern of uptake with OHSt being found only in astrocytes in the ventral part of the hypothalamus lateral to the arcuate nucleus whilst WGA uptake was virtually absent. This highlights the regional and cellular specialisation for uptake of molecules from the circulation and CSF. The median eminence and medial arcuate take up molecules from the circulation, with different cell types taking up different molecules. As the CSF flows through the ventricular system, different cells lining the ventricular and subarachnoid spaces take up molecules differentially. Molecules in the CSF appear to be excluded from the median eminence and medial arcuate region.

Heterogeneous occurrence of aquaporin-4 in the ependyma and in the circumventricular organs in rat and chicken

Aquaporins are selective water channel proteins critical in volume homeostasis. In the CNS AQP4 predominates, localized mainly in the glia limitans, the perivascular endfeet and ependyma. The present immunofluorescent study reveals the distribution of aquaporin-4 in the circumventricular organs in rat and chicken brains. The ventricular ependyma (especially in the third one), the subfornical organ, the area postrema, the rat pineal body (in part), and the vascular organ of lamina terminalis were marked by intense immunopositivity. Several areas, however, proved to be immunonegative: the central canal, the subcommissural organ, the ependymal zone of the median eminence in rat but its whole thickness in chicken, the subtrochlear organ, and the paraventricular organ. The immunostaining of the lateral septal and subseptal organs were similar to their environment. Results on developing rats suggested that the aquaporin-4 immunonegativity is a secondary phenomenon. Surveying other structural and functional features, no clear explanation of the heterogeneous occurrence of aquaporin-4 was found. The absence of aquaporin-4 seems to correlate with some features of the "ependymal organs" (thickened, pseudostratified ependyma, presence of blood-brain barrier) and with the avoidance of GFAP. On the other hand, the organs rich in aquaporin-4 have features of the "hypendymal organs" (glial and vascular plexus but no blood-brain barrier). There are organs, however, which do not fit into either group completely, i.e. the lateral septal and subseptal organs. Presence of tight junctions coincides with the absence of aquaporin-4 in the ependyma of spinal cord, the subcommissural organ and the ependyma of median eminence.

Expression of zonula occludens-1 (ZO-1) and the transcription factor ZO-1-associated nucleic acid-binding protein (ZONAB)-MsY3 in glial cells and colocalization at oligodendrocyte and astrocyte gap junctions in mouse brain

The PDZ domain-containing protein zonula occludens-1 (ZO-1) interacts with several members of the connexin (Cx) family of gap junction-forming proteins and has been localized to gap junctions, including those containing Cx47 in oligodendrocytes. We now provide evidence for ZO-1 expression in astrocytes in vivo and association with astrocytic connexins by confocal immunofluorescence demonstration of ZO-1 colocalization with astrocytic Cx30 and Cx43, and by ZO-1 coimmunoprecipitation with Cx30 and Cx43. Evidence for direct interaction of Cx30 with ZO-1 was obtained by pull-down assays that indicated binding of Cx30 to the second of the three PDZ domains in ZO-1. Further, we investigated mouse Y-box transcription factor MsY3, the canine ortholog of which has been termed ZO-1-associated nucleic acid-binding protein (ZONAB) and previously reported to interact with ZO-1. By immunofluorescence using specific antimouse ZONAB antibody, ZONAB was found to be associated with oligodendrocytes throughout mouse brain and spinal cord, and to be colocalized with oligodendrocytic Cx47 and Cx32 as well as with astrocytic Cx43. Our results extend the CNS cell types that express the multifunctional protein ZO-1, demonstrate an additional connexin (Cx30) that directly interacts with ZO-1, and show for the first time the association of a transcription factor (ZONAB) with ZO-1 localized to oligodendrocyte and astrocyte gap junctions. Given previous observations that ZONAB and ZO-1 in combination regulate gene expression, our results suggest roles of glial gap junction-mediated anchoring of signalling molecules in a wide variety of glial homeostatic processes.

Role of thyroid hormone deiodination in the hypothalamus

Iodothyronine deiodinases (D1, D2, and D3) comprise a family of selenoproteins that are involved in the conversion of thyroxine (T(4)) to active triiodothyronine (T(3)), and also the inactivation of both thyroid hormones. The deiodinase enzymes are of critical importance for the normal development and function of the central nervous system. D1 is absent from the human brain, suggesting that D2 and D3 are the two main enzymes involved in the maintenance of thyroid hormone homeostasis in the central nervous system, D2 as the primary T(3)-producing enzyme, and D3 as the primary inactivating enzyme. While the coordinated action of D2 and D3 maintain constant T(3) levels in the cortex independently from the circulating thyroid hormone levels, the role of deiodinases in the hypothalamus may be more complex, as suggested by the regulation of D2 activity in the hypothalamus by infection, fasting and changes in photoperiod. Tanycytes, the primary source of D2 activity in the hypothalamus, integrate hormonal and probably neuronal signals, and under specific conditions, may influence neuroendocrine functions by altering local T(3) tissue concentrations. This function may be of particular importance in the regulation of the hypothalamic-pituitary-thyroid axis during fasting and infection, and in the regulation of appetite and reproductive function. Transient expression of D3 in the preoptic region during a critical time of development suggests a special role for this deiodinase in sexual differentiation of the brain.

Connexin47, connexin29 and connexin32 co-expression in oligodendrocytes and Cx47 association with zonula occludens-1 (ZO-1) in mouse brain

Gap junctions between glial cells in mammalian CNS are known to contain several connexins (Cx), including Cx26, Cx30 and Cx43 at astrocyte-to-astrocyte junctions, and Cx29 and Cx32 on the oligodendrocyte side of astrocyte-to-oligodendrocyte junctions. Recent reports indicating that oligodendrocytes also express Cx47 prompted the present studies of Cx47 localization and relationships to other glial connexins in mouse CNS. In view of the increasing number of connexins reported to interact directly with the scaffolding protein zonula occludens-1 (ZO-1), we investigated ZO-1 expression and Cx47/ZO-1 interaction capabilities in brain, spinal cord and Cx47-transfected HeLa cells. From counts of over 9000 oligodendrocytes labeled by immunofluorescence in various brain regions, virtually all of these cells were found to express Cx29, Cx32 and Cx47. Oligodendrocyte somata displayed robust Cx47-immunopositive puncta that were co-localized with punctate labeling for Cx32 and Cx43. By freeze-fracture replica immunogold labeling, Cx47 was abundant on the oligodendrocyte-side of oligodendrocyte/astrocyte gap junctions. By immunofluorescence, labeling for Cx47 along myelinated fibers was sparse in most brain regions, whereas Cx29 and Cx32 were previously found to be concentrated along these fibers. By immunogold labeling, Cx47 was found in numerous small gap junctions linking myelin to astrocytes, but not within deeper layers of myelin. Brain subcellular fractionation revealed a lack of Cx47 enrichment in myelin fractions, which nevertheless contained an enrichment of Cx32 and Cx29. Oligodendrocytes were immunopositive for ZO-1, and displayed almost total Cx47/ZO-1 co-localization. ZO-1 was found to co-immunoprecipitate with Cx47, and pull-down assays indicated binding of Cx47 to the second PDZ domain of ZO-1. Our results indicate widespread expression of Cx47 by oligodendrocytes, but with a distribution pattern in relative levels inverse to the abundance of Cx29 in myelin and paucity of Cx29 in oligodendrocyte somata. Further, our findings suggest a scaffolding and/or regulatory role of ZO-1 at the oligodendrocyte side of astrocyte-to-oligodendrocyte gap junctions.

Sensory circumventricular organs: central roles in integrated autonomic regulation

Circumventricular organs (CVO) play a critical role as transducers of information between the blood, neurons and the cerebral spinal fluid (CSF). They permit both the release and sensing of hormones without disrupting the blood-brain barrier (BBB) and as a consequence of such abilities the CVOs are now well established to have essential regulatory actions in diverse physiological functions. The sensory CVOs are essential signal transducers located at the blood-brain interface regulating autonomic function. They have a proven role in the control of cardiovascular function and body fluid regulation, and have significant involvement in central immune response, feeding behavior and reproduction, the extent of which is still to be determined. This review will attempt to summarize the research on these topics to date. The complexities associated with sensory CVO exploration are intense, but should continue to result in valuable contributions to our understanding of brain function.