Immunochemical and immunocytochemical characterization of a novel monoclonal antibody recognizing a 140 kDa protein in cerebral pericytes of the rat

Angiogenesis of the blood-brain barrier in vitro and the function of cerebral pericytes

Cerebral pericytes constitute an essential component of the blood-brain barrier (BBB) and are involved in blood vessel assembly. Recently, we reported on the induction of a BBB-specific enzyme expressed by cerebral pericytes (pericytic aminopeptidase N/pAPN) in coculture with cerebral endothelial cells. We completed this in vitro BBB system by adding astrocytes to these mixed cultures of endothelial cells and pericytes. Under these triculture conditions, endothelial cells and pericytes reorganize into capillary-like structures (CLSs). Capillary formation can also be achieved by the application of transforming growth factor beta 1 (TGF-b1) in the culture medium of endothelial-pericyte cultures lacking astrocytes. In contrast to the effect achieved by astrocytes, pericytes did not assemble with endothelial cells. In both cases (application of astrocytes or TGF-b1), endothelial cells underwent apoptosis. However, endothelial cells that form CLSs in the presence of pericytes appeared to be resistant to induction of apoptosis. On the basis of these observations, we concluded that astrocytes have a profound influence on the morphogenetic events underlying the organization of the vessel wall; that the effect of TGF-b1 is different from the astrocytic effect because it lacks induction of endothelial-pericyte association; and that pericytes stabilize CLSs formed by endothelial cells in coculture with astrocytes.

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.

Brain parenchyma vessels and the angiotensin system

It is now recognized that the brain contains an autonomous angiotensin (AG) system, including the aminopeptidases A and N required for angiotensin metabolism. Using immunohistochemical techniques, we show that capillary pericytes and periendothelial cells of other vessels express aminopeptidase A (APA) and aminopeptidase N (APN) at their plasma membrane in adult mouse brain parenchyma. We therefore investigated the localization of angiotensin II(III), known as putative substrates for these enzymes, as well as that of their precursor angiotensin I. We report here the presence of immunoreactivity to angiotensin I and II(III) around most brain vessels. Angiotensins are present at the plasma membrane of brain parenchymal cells, presumably perivascular astrocytes which are also immunoreactive to AT1-receptor antibodies. The very close relationship between AGII(III) and their metabolizing enzymes APA and APN suggests a specific functional role for brain perivascular angiotensins.

Regulation of a blood-brain barrier-specific enzyme expressed by cerebral pericytes (pericytic aminopeptidase N/pAPN) under cell culture conditions

In this study we show that the aminopeptidase N of cerebral pericytes (pAPN) associated with the blood-brain barrier (BBB) is downregulated in pericytic cell cultures. This observation is in accordance with previous data describing comparable in vitro effects for BBB-specific enzymes of endothelial or pericytic origin, such as gamma-glutamyl transpeptidase or alkaline phosphatase. By polymerase chain reaction and in situ hybridization we were able to determine that the down-regulation of pAPN occurs at the posttranscriptional level. The mRNA of pAPN was found to be constitutively expressed even when the protein is no longer detectable. Culturing the pericytes in an endothelial cell-conditioned medium allowed pAPN to be reexpressed. However, the reexpression effect depended largely on the culturing conditions of the pericytes. Although purified pericytes deprived of endothelial cells did not reveal a reexpression effect, pericytes that were kept in contact with endothelial cells were able to acquire a pAPN-positive phenotype, indicating that endothelial cells constitute an essential requirement for the in vitro reexpression of pAPN. Astrocytes, however, were insufficient in exerting any reexpression effect.

Aminopeptidase a is a constituent of activated pericytes in angiogenesis

Monoclonal antibody (MAb) RC38 recognizes a human renal antigen of 160 kD recently identified as human aminopeptidase A (APA; EC 3.4.11.7). This ectoenzyme is able to hydrolyse selectively N-terminal glutamyl and aspartyl residues from oligopeptides. By enzyme histochemistry, APA activity has also been localized in the microvessels of all organs in animals and man. The purpose of this study was to investigate the distribution of human APA as recognized by MAb RC38 in the microvasculature of normal human tissues and pathological conditions associated with neovascularization. Unexpectedly, in normal tissues vascular staining with MAb RC38 was generally weak and often absent, while in tumours, granulation tissue, and chronic synovitis, marked microvascular staining was demonstrated. By immuno-electron microscopy, the antigen was found on the cell membrane of activated pericytes and their processes in the tumour vasculature. RC38 expression could not be detected on cultured human endothelial cells or pericytes. These observations suggest that pericyte expression of a subtype of APA (as recognized by MAb RC38) is markedly enhanced in the vasculature of tumours and wound healing tissue as compared with normal resting tissues. This provides further evidence of the altered state of pericytes in these conditions. Pericyte APA may be involved in the metabolism of biologically active oligopeptides during neovascularization, supporting a regulatory role of pericytes in this process. In addition, MAb RC38 may be useful as a marker of pericyte activation in tissue sections.

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.

The blood-brain barrier in vitro: the second decade

Ever since the discovery of Paul Ehrlich (1885 Das Sauerstoff-bedürfnis des Organismus: Hirschwald, Berlin) about the restricted material exchange, existing between the blood and the brain, the ultimate goal of subsequent studies has been mainly directed towards the elucidation of relative importance of different cellular compartments in the peculiar penetration barrier consisting the structural basis of the blood-brain barrier (BBB). It is now generally agreed that, in most vertebrates, the endothelial cells of the central nervous system (CNS) are responsible for the unique penetration barrier, which restricts the free passage of nutrients, hormones, immunologically relevant molecules and drugs to the brain. After an era of studying with endogenous or exogenous tracers the unique permeability properties of cerebral endothelial cells in vivo, the next generation, i.e. the in vitro blood-brain barrier model system was introduced in 1973. Recent advances in our knowledge of the BBB have in part been made by studying the properties and function of cerebral endothelial cells (CEC) with this in vitro approach. This review summarizes the results obtained on isolated brain microvessels in the second decade of its advent.

Ontogeny of the erythroid/HepG2-type glucose transporter (GLUT-1) in the rat nervous system

Central nervous system (CNS) microvessels of adult mammals have an unusually high density of the facilitative glucose transporter GLUT-1. Most systemic microvessels and those of the brain's circumventricular organs, which lack 'barrier' properties, do not express a high density of GLUT-1. Thus, a high GLUT-1 density is a marker of adult brain endothelium. To determine the stage at which CNS microvessels acquire GLUT-1, we studied by immunocytochemistry GLUT-1 ontogeny in the rat CNS from embryonic day (E) 11 to senescence. At E11, before blood vessels invaded the neuroectodermal tube, GLUT-1 immunoreactivity was already evident in the perineural plexus of vessels and in most of the vascular endothelium of the embryo. GLUT-1 immunoreactivity was also evident in the neuroectoderm. The neuroectoderm gradually lost GLUT-1 expression, and at about E16, GLUT-1 immunoreactivity was no longer detectable in most of the neuroectodermal epithelium, while CNS microvessels had increased their GLUT-1 immunoreactivity. By birth, GLUT-1 immunoreactivity in the CNS was restricted to the endothelium, the epithelium (but not the endothelium) of the choroid plexus, and tanycytes. This cellular distribution of GLUT-1 did not change much between birth and senescence despite considerable postnatal brain development and the increased brain capillary density. Our results suggest that while a CNS factor(s) may not have a role in the induction of the high expression of GLUT-1 in CNS endothelium, such a factor(s) is probably important in maintaining the high level of GLUT-1 in these endothelia.

Cerebral pericytes--a second line of defense in controlling blood-brain barrier peptide metabolism

The above data clearly indicate that cerebral pericytes are equipped with a specific aminopeptidase (AP-N). Functionally, this finding suggests that cerebral pericytes are involved in amino acid and peptide catabolism of the brain and that they constitute an essential carrier of the enzymatic blood-brain barrier.

Development of neural lobe-like neurovascular contact regions after intrahypothalamic transection of the hypothalamo-neurohypophysial tract

Fifteen days after bilateral transection of the hypothalamo-neurohypophysial tract at the level of the lateral retrochiasmatic area, neurovascular contact regions had developed proximal to 66% of the lesions. Contact regions developed in every case when neural lobe explants were placed into the lesions, and near approximately half of the lesions into which small pieces of sciatic or optic nerve were transplanted. Neurovascular contact regions were characterized by microvascular networks surrounded by dense neurophysin-immunoreactive plexuses. At the fine structural level, the organization of such regions resembled that of the neural lobe, with the single exception that capillaries were not fenestrated. Numerous neurosecretory axons were present, and palisades of neurosecretory axon terminals abutted perivascular basal laminae. Lamellopodia from glial cells partially ensheathed regenerating neurosecretory axons and often lay between terminals and the perivascular basal lamina. Terminals with many microvesicles and few neurosecretory granulated vesicles provided morphological evidence of hormone release.

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.

Pattern of glucose transporter (Glut 1) expression in embryonic brains is related to maturation of blood-brain barrier tightness

A constant supply of blood-borne glucose is vital to cerebral metabolism. Although transport of glucose into the nervous tissue, effectively separated from the blood by a functional barrier (the blood-brain barrier, BBB), is one of the essential properties of the cerebral endothelium, little is known about its metabolic regulation and developmental expression in the BBB. In this study we provide evidence by immunocytochemistry that the pattern of the brain endothelial glucose transporter in rat brains (BBB-GT), immunologically homologous with the human hepatoma (G2), human erythrocyte transporter (Glut 1), changes with BBB maturation. While the neuroepithelium at embryonic days 12 and 13 shows a high incidence of immuno-detectable BBB-GT, vascularisation of the cerebral anlage and subsequent development of vascular tightness, as evidenced by intravascularly applied horseradish peroxidase and fluorescinated dextrans, is accompanied by a significant reduction of BBB-GT expression in neuroepithelial cells and confinement of BBB-GT expression to the cerebral endothelium. Immunoblots and Northern blots of embryonic brain homogenates corroborate this change in BBB-GT expression in the brain anlage at the time of BBB maturation. However, low molecular weight glucose transporters, presumed to be of non-endothelial origin, are less dramatically reduced. The development of BBB tightness, therefore, seems to play a pivotal role in the pattern of BBB-GT expression during brain differentiation.

Molecular anatomy of the blood-brain barrier as defined by immunocytochemistry

This review outlines the recent developments and improvements of our knowledge concerning the molecular composition of the BBB as revealed by immunocytochemistry. Data have been accumulated which show that the BBB exhibits a specific collection of structural and metabolic properties which are also found in tight transporting epithelia. This conclusion is substantiated by (i) the implementation of antibodies which recognize proteins of non-BBB origin, to show that these biochemical markers and the functions that they represent are localized in the BBB endothelium; and (ii) the characterization of target molecules to which polyclonal or monoclonal antibodies which have been generated to epitopes of the BBB endothelium or brain homogenates. According to these data the protein assemblies comprising the phenotypical appearance of the BBB can therefore be defined by the particular selection as well as topological expression of common epithelial antigens, rather than the expression of BBB-unique molecular species. In this respect the immunocytochemical data corroborate the physiological assumption that the BBB possesses the character of a specific polarized epithelium. Attention is also given to the description of developmental expression of BBB-related immunomarkers. By collecting the data from different sources we introduce a classification of the BBB marker proteins according to their developmental appearance. Three groups of proteins are classified with respect to their sequential expression around the time of BBB closure: Phase E (early) markers which appear before BBB closure, phase I (intermediate) markers which are expressed at the time of BBB tightening, and phase L (late) markers which are detectable after the closure of the BBB. Such a scheme may to be useful in better defining the maturation process of BBB, which apparently is not a momentary event in brain development, but rather consists of a temporally sequenced process of hierarchically structured gene expression which finally define the molecular properties of the BBB. This process continues even after parturition, especially with regard to the achievement of immunological properties of the mature BBB. By examining the developmental spatio-temporal expression of different BBB markers we conclude that the mechanisms governing the pattern of BBB maturation are not limited to the interactions occurring between glial and endothelial cells. We therefore suggest a heuristic model in a triangular interrelationship that includes differentiation effects of neurons on glia and of glia cells on the BBB endothelium.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the blood-brain barrier

The microenvironment of the CNS is important for neuronal function, and the blood-brain barrier is involved in its maintenance. The barrier is present in a complex cellular system at the level of the tight junctions between endothelial cells. The unique properties of the endothelial cells in the CNS compared with those present in other organs are not predetermined by brain-specific endothelial precursors but are induced by the neural environment during the development of the vascular system. Astrocytes that tightly appose endfeet onto the abluminal side of brain capillaries seem to be important for the induction and maintenance of the endothelial barrier.