Expanding the definition of the blood-brain barrier to protein

Underlying Mechanisms and Treatment of Cellular Senescence-Induced Biological Barrier Interruption and Related Diseases

Given its increasing prevalence, aging is of great concern to researchers worldwide. Cellular senescence is a physiological or pathological cellular state caused by aging and a prominent risk factor for the interruption of the integrity and functionality of human biological barriers. Health barriers play an important role in maintaining microenvironmental homeostasis within the body. The senescence of barrier cells leads to barrier dysfunction and age-related diseases. Cellular senescence has been reported to be a key target for the prevention of age-related barrier diseases, including Alzheimer's disease, Parkinson's disease, age-related macular degeneration, diabetic retinopathy, and preeclampsia. Drugs such as metformin, dasatinib, quercetin, BCL-2 inhibitors, and rapamycin have been shown to intervene in cellular senescence and age-related diseases. In this review, we conclude that cellular senescence is involved in age-related biological barrier impairment. We further outline the cellular pathways and mechanisms underlying barrier impairment caused by cellular senescence and describe age-related barrier diseases associated with senescent cells. Finally, we summarize the currently used anti-senescence pharmacological interventions and discuss their therapeutic potential for preventing age-related barrier diseases.

Connecting the Dots: The Cerebral Lymphatic System as a Bridge Between the Central Nervous System and Peripheral System in Health and Disease

As a recently discovered waste removal system in the brain, cerebral lymphatic system is thought to play an important role in regulating the homeostasis of the central nervous system. Currently, more and more attention is being focused on the cerebral lymphatic system. Further understanding of the structural and functional characteristics of cerebral lymphatic system is essential to better understand the pathogenesis of diseases and to explore therapeutic approaches. In this review, we summarize the structural components and functional characteristics of cerebral lymphatic system. More importantly, it is closely associated with peripheral system diseases in the gastrointestinal tract, liver, and kidney. However, there is still a gap in the study of the cerebral lymphatic system. However, we believe that it is a critical mediator of the interactions between the central nervous system and the peripheral system.

Vesicular Transport Machinery in Brain Endothelial Cells: What We Know and What We Do not

The vesicular transport machinery regulates numerous essential functions in cells such as cell polarity, signaling pathways, and the transport of receptors and their cargoes. From a pharmaceutical perspective, vesicular transport offers avenues to facilitate the uptake of therapeutic agents into cells and across cellular barriers. In order to improve receptor-mediated transcytosis of biologics across the blood-brain barrier and into the diseased brain, a detailed understanding of intracellular transport mechanisms is essential. The vesicular transport machinery is a highly complex network and involves an array of protein complexes, cytosolic adaptor proteins, and the subcellular structures of the endo-lysosomal system. The endo-lysosomal system includes several types of vesicular entities such as early, late, and recycling endosomes, exosomes, ectosomes, retromer-coated vesicles, lysosomes, trans-endothelial channels, and tubules. While extensive research has been done on the trafficking system in many cell types, little is known about vesicular trafficking in brain endothelial cells. Consequently, assumptions on the transport system in endothelial cells are based on findings in polarised epithelial cells, although recent studies have highlighted differences in the endothelial system. This review highlights aspects of the vesicular trafficking machinery in brain endothelial cells, including recent findings, limitations, and opportunities for further studies.

Blood-brain barrier integrity in the pathogenesis of Alzheimer's disease

The blood-brain barrier (BBB) tightly controls the molecular exchange between the brain parenchyma and blood. Accumulated evidence from transgenic animal Alzheimer's disease (AD) models and human AD patients have demonstrated that BBB dysfunction is a major player in AD pathology. In this review, we discuss the role of the BBB in maintaining brain integrity and how this is mediated by crosstalk between BBB-associated cells within the neurovascular unit (NVU). We then discuss the role of the NVU, in particular its endothelial cell, pericyte, and glial cell constituents, in AD pathogenesis. The effect of substances released by the neuroendocrine system in modulating BBB function and AD pathogenesis is also discussed. We perform a systematic review of currently available AD treatments specifically targeting pericytes and BBB glial cells. In summary, this review provides a comprehensive overview of BBB dysfunction in AD and a new perspective on the development of therapeutics for AD.

α-synuclein inclusions are abundant in non-neuronal cells in the anterior olfactory nucleus of the Parkinson's disease olfactory bulb

Reduced olfactory function (hyposmia) is one of the most common non-motor symptoms experienced by those living with Parkinson's disease (PD), however, the underlying pathology of the dysfunction is unclear. Recent evidence indicates that α-synuclein (α-syn) pathology accumulates in the anterior olfactory nucleus of the olfactory bulb years before the motor symptoms are present. It is well established that neuronal cells in the olfactory bulb are affected by α-syn, but the involvement of other non-neuronal cell types is unknown. The occurrence of intracellular α-syn inclusions were quantified in four non-neuronal cell types - microglia, pericytes, astrocytes and oligodendrocytes as well as neurons in the anterior olfactory nucleus of post-mortem human PD olfactory bulbs (n = 11) and normal olfactory bulbs (n = 11). In the anterior olfactory nucleus, α-syn inclusions were confirmed to be intracellular in three of the four non-neuronal cell types, where 7.78% of microglia, 3.14% of pericytes and 1.97% of astrocytes were affected. Neurons containing α-syn inclusions comprised 8.60% of the total neuron population. Oligodendrocytes did not contain α-syn. The data provides evidence that non-neuronal cells in the PD olfactory bulb contain α-syn inclusions, suggesting that they may play an important role in the progression of PD.

Development and Cell Biology of the Blood-Brain Barrier

The vertebrate vasculature displays high organotypic specialization, with the structure and function of blood vessels catering to the specific needs of each tissue. A unique feature of the central nervous system (CNS) vasculature is the blood-brain barrier (BBB). The BBB regulates substance influx and efflux to maintain a homeostatic environment for proper brain function. Here, we review the development and cell biology of the BBB, focusing on the cellular and molecular regulation of barrier formation and the maintenance of the BBB through adulthood. We summarize unique features of CNS endothelial cells and highlight recent progress in and general principles of barrier regulation. Finally, we illustrate why a mechanistic understanding of the development and maintenance of the BBB could provide novel therapeutic opportunities for CNS drug delivery.

Intracellular transport and regulation of transcytosis across the blood-brain barrier

The blood-brain barrier is a dynamic multicellular interface that regulates the transport of molecules between the blood circulation and the brain parenchyma. Proteins and peptides required for brain homeostasis cross the blood-brain barrier via transcellular transport, but the mechanisms that control this pathway are not well characterized. Here, we highlight recent studies on intracellular transport and transcytosis across the blood-brain barrier. Endothelial cells at the blood-brain barrier possess an intricate endosomal network that allows sorting to diverse cellular destinations. Internalization from the plasma membrane, endosomal sorting, and exocytosis all contribute to the regulation of transcytosis. Transmembrane receptors and blood-borne proteins utilize different pathways and mechanisms for transport across brain endothelial cells. Alterations to intracellular transport in brain endothelial cells during diseases of the central nervous system contribute to blood-brain barrier disruption and disease progression. Harnessing the intracellular sorting mechanisms at the blood-brain barrier can help improve delivery of biotherapeutics to the brain.

The role of non-endothelial cells on the penetration of nanoparticles through the blood brain barrier

The blood brain barrier (BBB) is a well-established cell-based membrane that circumvents the central nervous system (CNS), protecting it from harmful substances. Due to its robustness and cell integrity, it is also an outstanding opponent when it comes to the delivery of several therapeutic agents to the brain, which requires the crossing through its highly-organized structure. This regulation and cell-cell communications occur mostly between astrocytes, pericytes and endothelial cells. Therefore, alternative ways to deliver drugs to the CNS, overcoming the BBB are required, to improve the efficacy of brain target drugs. Nanoparticles emerge here as a promising drug delivery strategy, due to their ability of high drug loading and the capability to exploit specific delivery pathways that most drugs are unable to when administered freely, increasing their bioavailability in the CNS. Thus, further attempts to assess the possible influence of non-endothelial may have on the BBB translocation of nanoparticles are here revised. Furthermore, the use of macrophages and/or monocytes as nanoparticle delivery cells are also approached. Lastly, the temporarily disruption of the overall organization and normal structure of the BBB to promote the penetration of nanoparticles aimed at the CNS is described, as a synergistic path.

Trafficking of Endogenous Immunoglobulins by Endothelial Cells at the Blood-Brain Barrier

The Blood-Brain Barrier (BBB) restricts access of large molecules to the brain. The low endocytic activity of brain endothelial cells (BECs) is believed to limit delivery of immunoglobulins (IgG) to the brain parenchyma. Here, we report that endogenous mouse IgG are localized within intracellular vesicles at steady state in BECs in vivo. Using high-resolution quantitative microscopy, we found a fraction of endocytosed IgG in lysosomes. We observed that loss of pericytes (key components of the BBB) in pdgf-b^ret/ret mice affects the intracellular distribution of endogenous mouse IgG in BECs. In these mice, endogenous IgG was not detected within lysosomes but instead accumulate at the basement membrane and brain parenchyma. Such IgG accumulation could be due to reduced lysosomal clearance and increased sorting to the abluminal membrane of BECs. Our results suggest that, in addition to low uptake from circulation, IgG lysosomal degradation may be a downstream mechanism by which BECs further restrict IgG access to the brain.

Molecular pathophysiology of cerebral edema

Advancements in molecular biology have led to a greater understanding of the individual proteins responsible for generating cerebral edema. In large part, the study of cerebral edema is the study of maladaptive ion transport. Following acute CNS injury, cells of the neurovascular unit, particularly brain endothelial cells and astrocytes, undergo a program of pre- and post-transcriptional changes in the activity of ion channels and transporters. These changes can result in maladaptive ion transport and the generation of abnormal osmotic forces that, ultimately, manifest as cerebral edema. This review discusses past models and current knowledge regarding the molecular and cellular pathophysiology of cerebral edema.

Vascular dysfunction in the pathogenesis of Alzheimer's disease--A review of endothelium-mediated mechanisms and ensuing vicious circles

Late-onset dementia is a major health concern in the ageing population. Alzheimer's disease (AD) accounts for the largest proportion (65-70%) of dementia cases in the older population. Despite considerable research effort, the pathogenesis of late-onset AD remains unclear. Substantial evidence suggests that the neurodegenerative process is initiated by chronic cerebral hypoperfusion (CCH) caused by ageing and cardiovascular conditions. CCH causes reduced oxygen, glucose and other nutrient supply to the brain, with direct damage not only to the parenchymal cells, but also to the blood-brain barrier (BBB), a key mediator of cerebral homeostasis. BBB dysfunction mediates the indirect neurotoxic effects of CCH by promoting oxidative stress, inflammation, paracellular permeability, and dysregulation of nitric oxide, a key regulator of regional blood flow. As such, BBB dysfunction mediates a vicious circle in which cerebral perfusion is reduced further and the neurodegenerative process is accelerated. Endothelial interaction with pericytes and astrocytes could also play a role in the process. Reciprocal interactions between vascular dysfunction and neurodegeneration could further contribute to the development of the disease. A comprehensive overview of the complex scenario of interacting endothelium-mediated processes is currently lacking, and could prospectively contribute to the identification of adequate therapeutic interventions. This study reviews the current literature of in vitro and ex vivo studies on endothelium-mediated mechanisms underlying vascular dysfunction in AD pathogenesis, with the aim of presenting a comprehensive overview of the complex network of causative relationships. Particular emphasis is given to vicious circles which can accelerate the process of neurovascular degeneration.

Dissecting the Contribution of Vascular Alterations and Aging to Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive decline that afflicts as many as 45 % of individuals who survive past the age of 85. AD has been associated with neurovascular dysfunction and brain accumulation of amyloid-β peptide, as well as tau phosphorylation and neurodegeneration, but the pathogenesis of the disease is still somewhat unclear. According to the amyloid cascade hypothesis of AD, accumulation of amyloid-β peptide (Aβ) aggregates initiates a sequence of events leading to neuronal injury and loss, and dementia. Alternatively, the vascular hypothesis of AD incorporates the vascular contribution to the disease, stating that a primary insult to brain microcirculation (e.g., stroke) not only contributes to amyloidopathy but initiates a non-amyloidogenic pathway of vascular-mediated neuronal dysfunction and injury, which involves blood-brain barrier compromise, with increased permeability of blood vessels, leakage of blood-borne components into the brain, and, consequently, neurotoxicity. Vascular dysfunction also includes a diminished brain capillary flow, causing multiple focal ischemic or hypoxic microinjuries, diminished amyloid-β clearance, and formation of neurotoxic oligomers, which lead to neuronal dysfunction. Here we present and discuss relevant findings on the contribution of vascular alterations during aging to AD, with the hope that a better understanding of the players in the "orchestra" of neurodegeneration will be useful in developing therapies to modulate the "symphony".

Tau depletion prevents progressive blood-brain barrier damage in a mouse model of tauopathy

Introduction

The blood-brain barrier (BBB) is damaged in tauopathies, including progressive supranuclear palsy (PSP) and Alzheimer’s disease (AD), which is thought to contribute to pathogenesis later in the disease course. In AD, BBB dysfunction has been associated with amyloid beta (Aß) pathology, but the role of tau in this process is not well characterized. Since increased BBB permeability is found in tauopathies without Aß pathology, like PSP, we suspected that tau accumulation alone could not only be sufficient, but even more important than Aß for BBB damage.

Results

Longitudinal evaluation of brain tissue from the tetracycline-regulatable rTg4510 tau transgenic mouse model showed progressive IgG, T cell and red blood cell infiltration. The Evans blue (EB) dye that is excluded from the brain when the BBB is intact also permeated the brains of rTg4510 mice following peripheral administration, indicative of a bonafide BBB defect, but this was only evident later in life. Thus, despite the marked brain atrophy and inflammation that occurs earlier in this model, BBB integrity is maintained. Interestingly, BBB dysfunction emerged at the same time that perivascular tau emerged around major hippocampal blood vessels. However, when tau expression was suppressed using doxycycline, BBB integrity was preserved, suggesting that the BBB can be stabilized in a tauopathic brain by reducing tau levels.

Conclusions

For the first time, these data demonstrate that tau alone can initiate breakdown of the BBB, but the BBB is remarkably resilient, maintaining its integrity in the face of marked brain atrophy, neuroinflammation and toxic tau accumulation. Moreover, the BBB can recover integrity when tau levels are reduced. Thus, late stage interventions targeting tau may slow the vascular contributions to cognitive impairment and dementia that occur in tauopathies.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0186-2) contains supplementary material, which is available to authorized users.

Lysosomal enzymes may cross the blood-brain-barrier by pinocytosis: implications for enzyme replacement therapy

Here we hypothesized that the water-soluble lysosomal enzymes may cross the blood-brain-barrier and reach the brain using the mechanism of unspecific fluid-phase endocytosis. We also highlight studies that show that, at higher serum concentrations, a fraction of these proteins can reach the brain after intravenous injection, and we suggest some experiments to study this hypothesis. Finally we discuss the implications of this for treatments such as enzyme replacement of lysosomal storage disorders.

High Molecular Weight Polyethylene Glycol Cellular Distribution and PEG-associated Cytoplasmic Vacuolation Is Molecular Weight Dependent and Does Not Require Conjugation to Proteins

Conjugation of therapeutic proteins with high molecular weight polyethylene glycols (HMW PEGs) is used to extend the half-life of biologics. To evaluate the effects of HMW PEGs in animals, we used an immunohistochemical procedure to study the tissue distribution and toxicity of unconjugated HMW PEGs in rats given 100 mg/kg 10KPEG, 20KPEG, or 40KPEG intravenously. Both the PEG cellular distribution and the histology were different between groups. In 10KPEG and 20KPEG groups, PEG immunoreactivity was most prominent in the renal tubule epithelium and in alveolar macrophages and hepatic Kupffer cells and cellular vacuolation was absent. In contrast, rats given 40KPEG had strong PEG immunoreactivity in splenic subcapsular red pulp macrophages, renal interstitial macrophages, and choroid plexus epithelial cells that was frequently associated with cytoplasmic vacuolation. While the vacuolation appeared to be an adaptive response, there was focal renal tubular epithelial degeneration associated with strong PEG immunoreactivity in one rat given 40KPEG. These data indicate that both the tissue distribution and the vacuolation observed with unconjugated HMW PEGs are markedly influenced by the molecular weight of the PEG and that when vacuolation is observed it is likely an adaptive change that is associated with PEG cytoplasmic immunoreactivity.

Neurovascular pathways to neurodegeneration in Alzheimer's disease and other disorders

The neurovascular unit (NVU) comprises brain endothelial cells, pericytes or vascular smooth muscle cells, glia and neurons. The NVU controls blood-brain barrier (BBB) permeability and cerebral blood flow, and maintains the chemical composition of the neuronal 'milieu', which is required for proper functioning of neuronal circuits. Recent evidence indicates that BBB dysfunction is associated with the accumulation of several vasculotoxic and neurotoxic molecules within brain parenchyma, a reduction in cerebral blood flow, and hypoxia. Together, these vascular-derived insults might initiate and/or contribute to neuronal degeneration. This article examines mechanisms of BBB dysfunction in neurodegenerative disorders, notably Alzheimer's disease, and highlights therapeutic opportunities relating to these neurovascular deficits.

Endocytosis at the blood–brain barrier: From basic understanding to drug delivery strategies

The blood-brain barrier (BBB) protects the central nervous system (CNS) from potentially harmful xenobiotics and endogenous molecules. Anatomically, it comprises the brain microvasculature whose functionality is nevertheless influenced by associated astrocyte, pericyte and neuronal cells. The highly restrictive paracellular pathway within brain microvasculature restricts significant CNS penetration to only those drugs whose physicochemical properties afford ready penetration into hydrophobic cell membranes or are capable of exploiting endogenous active transport processes such as solute carriers or endocytosis pathways. Endocytosis at the BBB is an essential pathway by which the brain obtains its nutrients and affords communication with the periphery. The development of strategies to exploit these endocytic pathways for the purposes of drug delivery to the CNS is still an immature field although some impressive results have been documented with the targeting of particular receptors. This current article initially provides an overview of general endocytosis processes and pathways showing evidence of their functional existence within the BBB. Subsequent sections provide, in an entity-specific manner, comprehensive reviews on BBB transport investigations of endocytosis involving: transferrin and the targeting of the transferrin receptor; hormones; cytokines; cell penetrating peptides; microorganisms and toxins, and nanoparticles aimed at more effectively delivering drugs to the CNS.

Immune-to-brain signalling: the role of cerebral CD163-positive macrophages

Systemic inflammation induces cytokine synthesis within the central nervous system. This results in sickness behaviour and may exacerbate ongoing neuroinflammatory disease. The precise mechanisms underlying the relay of signal from the periphery to the central nervous system are not entirely understood. CD163-positive macrophages occupy a unique position at the blood-brain barrier and upregulate prostaglandin-synthesizing enzymes in response to systemic inflammation. This finding suggests that they might play a role in signalling inflammation to the central nervous system. However, here we demonstrate that de novo brain cytokine transcription during systemic endotoxaemia may be prostaglandin-independent. We therefore set out to interrogate more directly the role of CD163-positive macrophages in immune-to-brain signalling. Intracerebroventricular injections of clodronate liposomes were used to selectively deplete CD163-positive macrophages. We show that de novo brain cytokine synthesis during systemic endotoxaemia persists in the absence of CD163-positive macrophages. Cerebral endothelial cells outnumber CD163-positive macrophages and are arguably better situated to signal circulating inflammatory stimuli to the brain.

Barrier cells in gills of the alligator gar

Studies of barrier cells in fishes are few and have been restricted primarily to salmonids. Moreover, these studies revealed the presence of barrier cells only in hematopoietic and lymphoid organs. The alligator gar, Lepisosteus spatula, possesses similar tissues within the gills which are readily accessible; therefore, they were the organs of choice. An electron microscopical analysis of the gills was undertaken to determine if barrier cells exist and if they possess an ultrastructural design comparable to their counterparts in the brain and lungs of higher vertebrates and hematopoietic/lymphoid tissues in fishes. The present study revealed that barrier cells were found only within non-hematopoietic/lymphoid areas. Barrier cells surrounded endocrine components, nerves, and sinusoids rather than capillary endothelium or hematopoietic/lymphoid tissues. Barrier cells in the alligator gar displayed a complex envelopment more similar to those found in the blood-cerebrospinal fluid, blood-brain, and blood-gas barriers in higher vertebrates than in salmonids. The barrier in the alligator gar consisted of: (1) an endothelium whose cells displayed tight junctions; (2) a basement membrane; and (3) an outer adventitia composed of fibrocytic cells in syncytium.

(Peri)vascular production and action of pro-inflammatory cytokines in brain pathology

In response to tissue injury or infection, the peripheral tissue macrophage induces an inflammatory response through the release of IL-1β (interleukin-1β) and TNFα (tumour necrosis factor α). These cytokines stimulate macrophages and endothelial cells to express chemokines and adhesion molecules that attract leucocytes into the peripheral site of injury or infection. The aims of the present review are to (i) discuss the relevance of brain (peri)vascular cells and compartments to bacterial meningitis, HIV-1-associated dementia, multiple sclerosis, ischaemic and traumatic brain injury, and Alzheimer's disease, and (ii) to provide an overview of the production and action of pro-inflammatory cytokines by (peri)vascular cells in these pathologies of the CNS (central nervous system). The brain (peri)vascular compartments are highly relevant to pathologies affecting the CNS, as infections are almost exclusively blood-borne. Insults disrupt blood and energy flow to neurons, and active brain-to-blood transport mechanisms, which are the bottleneck in the clearance of unwanted molecules from the brain. Perivascular macrophages are the most reactive cell type and produce IL-1β and TNFα after infection or injury to the CNS. The main cellular target for IL-1β and TNFα produced in the brain (peri)vascular compartment is the endothelium, where these cytokines induce the expression of adhesion molecules and promote leucocyte infiltration. Whether this and other effects of IL-1 and TNF in the brain (peri)vascular compartments are detrimental or beneficial in neuropathology remains to be shown and requires a clear understanding of the role of these cytokines in both damaging and repair processes in the CNS.

Induction of blood-brain barrier properties in cultured brain capillary endothelial cells: comparison between primary glial cells and C6 cell line

The communication between glial cells and brain capillary endothelial cells is crucial for a well-differentiated blood-brain barrier (BBB). It has been suggested that in vitro primary glial cells (GCs) be replaced by the glial C6 cell line to standardise the model further. This study compares directly the structural and functional differentiation of bovine brain capillary endothelial cells (BBCECs) induced by co-culture with rat primary GCs or C6 cells, for the first time. Trans-endothelial electrical resistance (TEER) measurements showed that under no condition were C6 cells able to reproduce TEER values as high as in the presence of GCs. At the same time, permeability of the BBCECs to both radioactive sucrose and FITC-inulin was 2.5-fold higher when cells were co-cultured with C6 than with GCs. Furthermore, immunocytochemistry studies showed different cell morphology and less developed tight junction pattern of BBCECs co-cultured with C6 toward GCs. Additionally, studies on P-glycoprotein (P-gp) showed much lower P-gp presence and activity in BBCECs co-cultured with C6 than GCs. Both VEGF mRNA expression and protein content were dramatically increased when compared with GCs, suggesting that VEGF could be one of the factors responsible for higher permeability of BBB. Our results clearly indicate that, in the presence of the glial C6 cell line, BBCECs did not differentiate as well as in the co-culture with primary GCs at both structural and functional levels.

Hematopoietic origin of microglial and perivascular cells in brain

BACKGROUND

Bone marrow (BM)-derived cells differentiate into a wide variety of cell types. BM contains a heterogeneous population of stem and progenitor cells including hematopoietic stem cells, marrow stromal cells, and perhaps other progenitor cells. To establish unequivocally the transdifferentiation capability of a hematopoietic cell to a nonhematopoietic cell (endothelial cells, neurons, and glial cells), it is imperative to demonstrate that a single cell or clone of that single cell (clonal analysis) differentiates into cells comprising vessels or other cells in the brain.

METHODS

We generated mice that exhibited a high level of hematopoietic reconstitution from a single enhanced green fluorescent protein (EGFP) stem cell. To achieve this, we combined FACS sorting and cell culture to generate a population of cells derived from a single hematopoietic stem cell (Lin-, CD34-, c-kit+, and Sca-1+). Clonal populations of cells were then transplanted into lethally irradiated recipient mice. After 3-4 months of engraftment, some mice underwent middle cerebral artery (MCA) suture occlusion. EGFP immunocytochemistry and dual labeling was performed with cell-specific markers on tissue from various time points.

RESULTS

In all transplanted mice, EGFP+ highly ramified cells were seen in the brain parenchyma. These cells stained with RCA120 lectin and had the characteristics of parenchymal microglial cells. In brains without infarction and in uninfarcted brain regions of mice that underwent MCA occlusion, there were many EGFP+ cells in a perivascular distribution, associated with both small and larger blood vessels. The cells were tightly apposed to the vessel wall and some had long processes that enveloped the endothelial cells. After MCA occlusion, there was an influx of EGFP expressing cells in the ischemic tissue that colocalized with the "neovascularization." These EGFP+ cells were wrapped around endothelial cells in an albuminal location and did not coexpress von Willebrand Factor or CD31. We detected rare dual-labeled EGFP and NeuN-expressing cells. We detected two staining patterns. The more frequent pattern was phagocytosis of NeuN cells by EGFP expressing cells. However, we also detected rarer cells where the EGFP and NeuN appeared to be colocalized by confocal microscopy.

CONCLUSIONS

HSC differentiate into parenchymal microglial cells and perivascular cells in the brain. The numbers of these cells increase after cerebral ischemia. The HSC is therefore one source of parenchymal microglial cells and a source for perivascular cells. After a cerebral infarction, there are rare HSC-derived cells that stain with the neuronal marker, NeuN. However, the more common pattern appears to represent phagocytosis of damaged neurons by EGFP+ microglial cells.

[Morphology and physiology of the blood-brain barrier]

The blood-brain barrier (BBB) is a complex biological system that consists of endothelial cells, pericytes and astrocytes, which are involved in the induction and maintenance of its physiological and ultrastructural characteristics. The BBB plays a primordial role in isolating the cerebral parenchyma as well as in controlling brain homeostasis by its selective permeability to nutriments and other molecules flowing through the cerebral microcapillaries. A better knowledge of this system is crucial in order to improve the efficiency of brain penetration by drugs, and in order to prevent BBB opening, leading to brain edema, in physiopathological situations such as brain ischemia, trauma or inflammatory processes.

Intravenous infusion of cereport increases uptake and efficacy of acyclovir in herpes simplex virus-infected rat brains

The outcome of herpes simplex virus (HSV) infections manifesting as encephalitis in healthy or immunocompromised individuals is generally very poor with mortality rates of about 8 to 28% with treatment. The long-term prognosis of survivors is often problematic, posing the need for alternative treatments that may decrease the mortality and morbidity associated with herpes encephalitis. This study addresses one such approach that includes a temporary permeabilization of the blood-brain barrier during treatment with acyclovir (ACV). In these studies we utilized a synthetic bradykinin analog, Cereport (RMP-7), in conjunction with ACV to treat HSV infection of the brain in a rat model. Cereport, infused intravenously via the jugular vein, was shown to increase [(14)C]ACV uptake in both the HSV-1-infected and -uninfected rat brain by approximately two- to threefold, correlating with enhanced efficacy of ACV in various brain compartments. In another series of experiments to determine efficacy, various doses of unlabeled ACV were administered during infusion with RMP-7. The decrease in viral titers in the temporal regions of the brain after 5 days of treatment suggested that this approach enhanced the efficacy of ACV treatment. These data indicated that Cereport infused with ACV enhances both the penetration and efficacy of this drug in the treatment of an experimental HSV-1 infection of the rat brain.

Cellular mechanisms of CNS pericytes

Three major functional roles have been ascribed to pericytes associated with central nervous system microvasculature-contractility, regulation o f endothelial cell activity, and macrophage activity. A host of different cell factors and signalling agents appear to be involved with these cellular functions, some effecting the pericyte and others produced by this cell. These include neuromodulators, vasoactive peptides, metabolic factors, growth factors and cytokines. The specific compounds and their actions are collectively viewed in an effort to provide an overall picture of the regulation of pericyte functional activity. This small vascular cell is emerging as a significant entity in several physiological processes through the functions of above; these processes include control of blood flow, regulation of vascular development and immune responses. Defining the regulatory agents and their mechanisms is key to understanding the role that pericytes play in these processes. Because these cells have begun to receive increasing attention in neurobiological studies, an overview of signalling properties should be timely and beneficial.

Brain macrophages: on the role of pericytes and perivascular cells

Pericytes are a unique cell group intimately associated with the vasculature and that appear to be present in most tissues. Their presence is generally considered to be restricted to the microvessels - arterioles, venules and particularly capillaries, where there is little or no smooth muscle. Morphologically, the pericytes exhibit a small, oval cell body with multiple processes extending for some distance along the vessel axis; these primary processes then give rise to orthogonal secondary branches which encircle the vascular wall. Through this morphology and their close association with the vasculature, the contour of the cells conforms to that of the adjacent vascular element; also, they are usually enclosed within the basal lamina of the microvasculature. While many earlier studies suggested brain pericytes as a source of macrophage activity, recent results substantiate this functional role; these recent findings include the demonstration of macrophage markers, phagocytosis and antigen presentation. Coupled with current knowledge on the entry of lymphoblasts into brain tissue and perivascular areas as potentially being the primary site of cellular interactions for production of immune responses, this places the pericytes in a position to significantly contribute to central nervous system (CNS) immune mechanisms. They may in fact be the population of brain macrophages most instrumental in the initiation of an immune response. Although these cells constitutively express several macrophage properties, they are also capable of up-regulation to display the full range of macrophage functional activity. At least, some of the pericytic macrophages are located on the surface of the basal lamina as opposed to completely within it; however, their potential transformation into microglia of the parenchyma remains an open issue. In addition to their function as macrophages, pericytes appear to serve a host of other functional roles. They are contractile and seem to serve as a smooth muscle equivalent in the capillaries performing vasoconstriction; they regulate endothelial cell properties and contribute to the stability and maintenance of blood vessels; and they appear to directly participate in coagulation through the extrinsic pathway. Also, pericytes have been suggested to be pluripotential and serve as precursors for a variety of other cell types. From these functional roles, comes their involvement in various disease processes. In association with the macrophage function, they are involved in numerous autoimmune and infectious diseases. Through their vascular role, they are involved in diabetic retinopathy and inflammation. Also, the pericytes appear to have involvement in Alzheimer's as well as other diseases. Thus, these cells are presented not only as macrophages but as a group with broad functional activities and significant potential for contributing to disease states.

Role of the CNS microvascular pericyte in the blood-brain barrier

Pericytes are a very important cellular constituent of the blood-brain barrier. They play a regulatory role in brain angiogenesis, endothelial cell tight junction formation, blood-brain barrier differentiation, as well as contribute to the microvascular vasodynamic capacity and structural stability. Central nervous system pericytes express macrophage functions and are actively involved in the neuroimmune network operating at the blood-brain barrier. They exhibit unique functional characteristics critical for the pathogenesis of a number of cerebrovascular, neurodegenerative, and neuroimmune diseases.

Endothelin-1 as a mediator of endothelial cell-pericyte interactions in bovine brain capillaries

Endothelial cells and pericytes are closely associated in brain capillaries. Together with astrocytic foot processes, they form the blood-brain barrier. Capillaries were isolated from bovine brain cortex. Pure populations of endothelial cells and pericytes were isolated and cultured in vitro. Polarized monolayers of endothelial cells preferentially secreted immunoreactive endothelin-1 (Et-1) at their abluminal (brain-facing) membrane. They did not express receptors for Et-1. Pericytes expressed BQ-123-sensitive ETA receptors for endothelins as evidenced by 125I-Et-1 binding experiments. These receptors were coupled to phospholipase C as demonstrated by intracellular calcium measurements using indo-1-loaded cells. Addition of Et-1 to pericytes induced marked changes in the cell morphology that were associated with a reorganization of F-actin and intermediate filaments. It is concluded that Et-1 is a paracrine mediator at the bovine blood-brain barrier and that capillary pericytes are target cells for endothelium-derived Et-1.

The role of macrophage subpopulations in autoimmune disease of the central nervous system

In this review the role of various subpopulations of macrophages in the pathogenesis of experimental autoimmune encephalomyetitis is discussed. Immunohistochemistry with macrophage markers shows that in this disease different populations of macrophages (i.e. perivascular cells, microglia and infiltrating blood-borne macrophages) are present in the central nervous system. These subpopulations partially overlap in some functional activity while other activities seem to be restricted to a distinct subpopulation, indicating that these subpopulations have different roles in the pathogenesis of encephalomyelitis. The studies discussed in this review reveal that immunocytochemical and morphological studies, combined with new techniques such as in situ nick translation and experimental approaches like the use of bone marrow chimeras and macrophage depletion techniques, give valuable information about the types and functions of cells involved in central nervous system inflammation. The review is divided in three parts. In the first part the experimental autoimmune encephalomyelitis model is introduced. The second part gives an overview of the origin, morphology and functions of the various subpopulations. In the third part the role of these subpopulations is discussed in relation to the various stages (i.e. preclinical, clinical and recovery) of the experimental disease.

Evidence of persistent blood-brain barrier abnormalities in chronic-progressive multiple sclerosis

Brain capillaries were analyzed morphometrically for alterations in organelle distribution and density in biopsy samples of central nervous system tissue from seven patients diagnosed as having chronic progressive multiple sclerosis. Data were expressed as percentage of endothelial cytoplasm occupied by the respective organelles. The density of pinocytotic vesicles in endothelium ranged from 0.53% within normal-appearing parenchyma to 1.2% in gliotic areas. For mitochondria the values ranged from 10.87% in normal areas to 4.72% in the same respective samples. Thus, an inverse correlation between vesicular and mitochondrial content was observed. These findings suggest that endothelial cells in gliotic areas are similar to endothelial cells of the systemic circulation in their mitochondrial content and pinocytotic activity. Interendothelial junctions in capillaries of all areas examined appeared normal. Additional evidence for a continuous blood-brain barrier anomaly in multiple sclerosis was the accumulation of perivascular fibrin, suggesting an increase in microvascular permeability. Perivascular collagen deposits, degenerative changes in pericytes and astrocytic swelling were also indicators of an increase in blood-brain barrier permeability. Taken together with previous data from experimental autoimmune encephalomyelitis, the present findings in chronic silent multiple sclerosis lesions suggest that central nervous system endothelial cells show persistent abnormalities of the blood-brain barrier, even in the absence of active inflammation.

Pericyte-endothelial gap junctions in developing rat cerebral capillaries: a fine structural study

BACKGROUND

Fine structural study revealed the intercellular coupling between the pericyte and the endothelial cells via the gap junctions, in the capillaries of the basal forebrain of rat embryos.

RESULTS

Gap junctions were constructed by the adluminal plasmalemma of pericyte and the abluminal plasmalemma of endothelial cells.

CONCLUSIONS

Gap junctions are membranous channels that directly join the cytoplasms of the pericyte and endothelial cell and imply some substantial role for the pericyte on the endothelial proliferation. It is postulated that the function of the pericyte in the prenatal mammals are assigned to the regulation of the development of cerebral microcirculation.

The 140-kDa protein of blood-brain barrier-associated pericytes is identical to aminopeptidase N

Although ample evidence has been accumulated on the structure and functional role of endothelial cells in blood-brain barrier mechanisms, little is known about the contribution that cerebral pericytes provide to this phenomenon. We have reported recently on a monoclonal antibody specific for cerebral pericytes at blood-brain barrier sites. To confirm the pericytic localization of this antigen, and in order to elucidate its biochemical identity, we have performed immunocytochemical, biochemical, and molecular biological studies. By immunocytochemistry on the light microscopic as well as electron microscopic level, we provide definite evidence that the 140-kDa antigen recognized by this monoclonal antibody is confined to cerebral pericytes, whereas endothelial cells are devoid of this antigen. N-Terminal sequencing of the corresponding immunocrossreacting renal protein revealed that the protein detected by the monoclonal antibody is identical to aminopeptidase N. By means of the reverse transcriptase-polymerase chain reaction, the identity of the 140-kDa antigen as aminopeptidase N could also be verified for cerebral microvascular cells. Cerebral pericytic aminopeptidase N may be involved in neurotransmitter (enkephalin) metabolism at the blood-brain interface. By taking into account that brain pericytes have been found to express further plasma membrane-bound enzymes, these results strongly suggest the contribution of cerebral pericytes in the metabolic concert of the homeostatic balance regulated by the blood-brain barrier.

Blood to brain and brain to blood passage of native horseradish peroxidase, wheat germ agglutinin, and albumin: pharmacokinetic and morphological assessments

Native horseradish peroxidase (HRP) and the lectin wheat germ agglutinin (WGA) conjugated to HRP are protein probes represented in the blood-brain barrier (BBB) literature for elucidating morphological routes of passage between blood and brain. We report the application of established pharmacokinetic methods, e.g., multiple-time regression analysis and capillary depletion technique, to measure and compare bidirectional rates of passage between blood and brain for radioactive iodine-labeled HRP (I-HRP), WGA-HRP (I-WGA-HRP), and the serum protein albumin (I-ALB) following administration of the probes intravenously (i.v.) or by intracerebroventricular (i.c.v.) injection in mice. The pharmacokinetic data are supplemented with light and electron microscopic analyses of HRP and WGA-HRP delivered i.v. or by i.c.v. injection. The rates of bidirectional movement between blood and brain are the same for coinjected I-HRP and I-ALB. Blood-borne HRP, unlike WGA-HRP, has unimpeded access to the CNS extracellularly through sites deficient in a BBB, such as the circumventricular organs and subarachnoid space/pial surface. Nevertheless, blood-borne I-WGA-HRP enters the brain approximately 10 times more rapidly than I-HRP and I-ALB. Separation of blood vessels from the neocortical parenchyma confirms the entry of blood-borne I-WGA-HRP to the brain and sequestration of I-WGA-HRP by cerebral endothelial cells. Nearly half the I-WGA-HRP radioactivity associated with cortical vessels is judged to be subcellular. Light microscopic results suggest the extracellular pathways into the brain available to blood-borne native HRP do not represent predominant routes of entry for blood-borne WGA-HRP. Ultrastructural analysis further suggests WGA-HRP is likely to undergo adsorptive transcytosis through cerebral endothelia from blood to brain via specific subcellular compartments within the endothelium. Entry of blood-borne I-WGA-HRP, but not of I-ALB, is stimulated with coinjected unlabeled WGA-HRP, suggesting the latter may enhance the adsorptive endocytosis of blood-borne I-WGA-HRP. With i.c.v. coinjection of I-WGA-HRP and I-ALB, I-WGA-HRP exists the brain more slowly than I-ALB. The brain to blood passage of I-WGA-HRP is nil with inclusion of unlabeled WGA-HRP, which does not alter the exist of I-ALB. Adsorptive endocytosis of i.c.v. injected WGA-HRP appears restricted largely to cells lining the ventricular cavities, e.g., ependymal and choroid plexus epithelia. In summary, the data suggest that the bidirectional rates of passage between brain and blood for native HRP are comparable to those for albumin.

The neurotoxicity of alpha-chlorohydrin in rats and mice: I. Evolution of the cellular changes

Mice and rats are found to be equally susceptible to developing symmetrical brain stem lesions on exposure to alpha-chlorohydrin and in both species the earliest neurotoxic changes are strictly confined to glial cells, particularly astrocytes; haemorrhages are not found in either species. Minimal evidence of increased vascular leakage of horse-radish peroxidase (HRP) in rats is shown by increased HRP content of perivascular cells within the lesions. Later macrophage invasion and capillary proliferation is accompanied by rare focal leakiness of HRP. Gross astrocytic damage, therefore, does not necessarily impair integrity of the blood-brain barrier. While early in intoxication, astrocytes are severely distended with fluid and their organelles seriously disorganized, they do not die but rapidly regenerate their processes. They thus appear to undergo a process of 'clasmatodendrosis' from which they recover. Comparisons are made with the genesis of symmetrical brain stem lesions in other acute energy deprivation syndromes, including Wernicke's encephalopathy.

Differences in contrast enhancement between intra- and extra-axial lesions after two hours of iodine injection

In order to try to improve discrimination between the brain lesions that produce damage to the brain-blood barrier and those which are irrigated by extracerebral vessels, the behavior of contrast enhancement up to two hours after iodine injection was studied in two groups of patients. Eight patients had extra-axial lesions (7 meningiomas, one acoustic neuroma) and eleven had intra-axial lesions (6 of astrocytic type, 3 metastases, 2 inflammatory). In the first group, mean values of the lesioned areas in HU decreased progressively with time. However, in many cases in the second group the values at the end of the study were equal to or higher than those observed immediately after contrast medium injection. Highly significant differences were observed between the two groups. The contrast medium in the intra-axial lesions conform to the behavior of all other fluids in a damaged barrier, remaining for a very long time in the interstitial space. In the extra-axial lesions, the contrast moved freely and the return from the interstitial to the intravascular space was also unobstructed, since values decayed immediately after injection. For these reasons, the observation of scans after two hours of contrast injection might be useful for differential diagnosis in those cases where doubt exists.

Heterogeneity of microglial and perivascular cell populations: insights gained from the facial nucleus paradigm

We reflect here on the development of a neuroimmunological concept which has been formulated over the past 5 years through studying microglial cell responses in the facial nerve system. A simple axotomy of the adult rat facial nerve which causes regeneration of facial motor neurons and little, if any, cell death can activate microglial cells just as easily as a full-blown degeneration of the entire nucleus induced by toxic ricin. In both instances, the prompt microglial reaction is characterized by a series of structural and phenotypic changes which are in many ways similar to an immune response, e.g., there is cell proliferation and upregulation of MHC antigens. However, since white blood cells do not participate in the retrograde response of facial motor neurons, we have adopted a notion which views microglia as a CNS-wide network of immunocompetent cells whose morphological dissimilarities from leukocytes are a result of their unique adaptation to the CNS architecture. We have continued our in vivo investigations of the phagocytic and immunophenotypic properties of microglial and perivascular cells during the retrograde reaction of facial motor neurons by using intra-neural injections of fluorogold (FG) and ricin followed by lectin and immunostaining for microglia. Two new findings can be added to the microglial neuroimmune network: (1) Microglia take up FG only after motor neuron degeneration, whereas perivascular cells may take up FG under nondegenerating conditions. (2) Immunologically important molecules, such as MHC class II, CD4, and leukocyte common antigens, are expressed by different microglial subpopulations. Thus there is functional and phenotypic heterogeneity among immunocompetent cells of the CNS.

Blood-brain barrier pericytes are the main source of gamma-glutamyltranspeptidase activity in brain capillaries

Cerebral endothelial cells form the selective permeability barrier between brain and blood by virtue of their impermeable tight junctions and the presence of specific carrier systems. These specialized properties of brain capillaries are reflected in the presence of proteins that are not found in other capillaries of the body. gamma-Glutamyltranspeptidase (GGT) has been widely used as a marker for brain capillaries and differentiated properties of brain endothelial cells. By using histochemical and biochemical methods we have investigated the expression of GGT in isolated capillaries, cultured brain endothelial cells and pericytes, and cocultures of astrocytes and brain endothelial cells. It was surprising that the majority of GGT activity was associated with pericytes, but not endothelial cells, suggesting that GGT is a specific marker for brain pericytes. The remaining GGT activity that was associated with endothelial cells rapidly disappeared from cultured cells but was reinduced in cocultures with astrocytes. Our results emphasize the need for pure endothelial cells for the investigation of blood-brain barrier characteristics.

Chapter 17: Blood-brain barrier: a molecular approach to its structural and functional characterization

Our approach to analyze molecular components of the blood-brain barrier led to the identification of additional transcripts which can be regarded as "BBB markers". Other candidates are presently analyzed in order to find hitherto unknown cell type-specific transcripts. We investigated the expression of these marker-genes in cell culture and found all genes still being transcribed after 10 days in primary cultures, although at a lower level. This is surprising, since other authors report the disappearance of BBB characteristics under such conditions. Moreover, the BBB marker gamma-GT is found to be not only expressed in BMEC, but also in the closely associated pericytes. The hitherto unknown physiological function of the enzyme, especially the abundance in pericytes is still under investigation. Since the method of subtractive cloning has been proven as a fruitful approach, we consider to establish further subtractive cDNA libraries, using different subtraction parameters. The PCR method is applicable for amplification of subtracted cDNA (Timblin et al., 1990) and we expect to find additional clones, mainly of lower abundance which are of functional importance for the BBB phenomenon. The described characterization of cultured BMEC now allows to proceed to study BBB-specific gene expression with special regard to regulatory elements. We will perform these experiments by use of enhancer trap vectors transfected into BMEC. The isolation of the corresponding genomic DNA fragments of the BBB markers is in progress.

Pericytes of the brain microvasculature express gamma-glutamyl transpeptidase

The expression of gamma-glutamyl transpeptidase (GGT) is a specific property of the brain capillary endothelium that constitutes the blood-brain barrier. We report here the detection of GGT, not only in endothelial cells, but also in pericytes, demonstrating that a brain capillary-specific pericyte population exists. We raised antibodies to GGT using a porcine brain microvessel GGT-protein-A (staphylococcal protein A) fusion protein as antigen which was expressed in Escherichia coli. The immunohistochemical analysis of the subcapillary distribution of GGT in porcine brain cortex and cerebellum sections by both light and electron microscopy revealed the expression of GGT in the capillary-adjacent pericytes in addition to the GGT-positive endothelial layer. We confirmed these data for cultured porcine brain microvascular endothelial cells and pericytes. GGT immunofluorescence could be detected in both cell types in culture. Endothelial cells exhibited a weak staining, whereas pericytes were strongly positive for GGT. Due to the high phagocytotic activity of pericytes and their location on the abluminal surface of the microvessels, we propose a possible protective or detoxifying function of GGT in cerebrovascular pericytes.

Delivering peptides to the central nervous system: dilemmas and strategies

Peptides have been shown to cross the blood-brain barrier (BBB) as intact molecules so that they can influence the central nervous system. Peptides cross by saturable and nonsaturable mechanisms in the direction of both brain to blood and blood to brain. Passage of peptides, especially by saturable transport, has been shown to be influenced by pharmacological agents and physiological events. These findings support the view that peptides or their analogues could be useful as therapeutic agents for disorders of the central nervous system. They also suggest strategies in approaching therapeutic goals, including manipulating transport rates, targeting diseases due to altered BBB-peptide interactions, and designing analogues capable of taking advantage of such mechanisms of passage as paracellular transmembrane diffusion and brain-to-blood transport.

Japanese encephalitis virus (JEV): potentiation of lethality in mice by microwave radiation

The expression of Japanese Encephalitis Virus (JEV) lethality in mice requires entry of the virus into the central nervous system. This entry is presumably through the capillary endothelial cells (CEC), because entry between CECs is inhibited by bands of circumferential tight-junctions. A viremic stage occurs during the first 4 to 5 days after JEV administration in mice, and both microwave radiation (2.45-GHz, continuous wave, 10-min exposure) and hypercarbia were employed to increase CEC permeability to JEV. Exposure to microwaves at power densities of 10-50 mW/cm2 resulted in a dose-dependent increase in JEV-induced lethality. Mice did not become tolerant or sensitized to microwave potentiation of JEV-induced mortality because 4 daily exposures at 10 or 50 mW/cm2 (SARS, approximately 24-98 W/kg) did not alter the lethality pattern to subsequent microwave radiation of JEV-exposed animals. Similarly, hypercarbia (5, 10, and 20% CO2) was observed to produce a dose-dependent increase in JEV-induced lethality. Both microwave radiation and hypercarbia are thought to promote pinocytosis in CNS capillary endothelial cells. This may be one mechanism by which they enhance JEV-induced lethality in adult Swiss-Cox mice.

Structural and functional aspects of the interaction of inflammatory cells with the blood-brain barrier in experimental brain inflammation

Interaction between various subclasses of inflammatory cells (ICs) and endothelial cells (ECs) lining selective blood vessels of the mammalian blood-brain barrier (BBB) is an initial, important event during inflammatory conditions of the central nervous system (CNS). In this review, we will present a brief ultracytochemical and immunocytochemical assessment of our perspective on this intimate cellular interaction which has been described recently in conditions that involve immunological alterations of the BBB. We will discuss some morphological aspects of what is currently known about acute and chronic inflammatory BBB disorders that are involved in inflammatory processes. We will focus, in particular, on experimental allergic encephalomyelitis (EAE), an animal model for human multiple sclerosis (MS). Many of the past and more recent concepts found in the literature concerning IC attachment and diapedesis, as well as our own experimental efforts over more than two decades will be presented.