Astrocytes secrete a factor inducing the expression of HT7-protein and neurothelin in endothelial cells of chorioallantoic vessels

Endothelial Progenitor Cells Physiology and Metabolic Plasticity in Brain Angiogenesis and Blood-Brain Barrier Modeling

Currently, there is a considerable interest to the assessment of blood-brain barrier (BBB) development as a part of cerebral angiogenesis developmental program. Embryonic and adult angiogenesis in the brain is governed by the coordinated activity of endothelial progenitor cells, brain microvascular endothelial cells, and non-endothelial cells contributing to the establishment of the BBB (pericytes, astrocytes, neurons). Metabolic and functional plasticity of endothelial progenitor cells controls their timely recruitment, precise homing to the brain microvessels, and efficient support of brain angiogenesis. Deciphering endothelial progenitor cells physiology would provide novel engineering approaches to establish adequate microfluidically-supported BBB models and brain microphysiological systems for translational studies.

An immortalised astrocyte cell line maintains the in vivo phenotype of a primary porcine in vitro blood-brain barrier model

Whilst it is well documented that all components of the neurovascular unit contribute to the restrictive nature of the blood-brain barrier (BBB), astrocytes have been identified as the cellular component most likely to play an essential role in maintaining the barrier properties. The aim of this study was to examine the impact of the rat astrocyte cell line, CTX-TNA2, on the structural and functional characteristics of an in vitro BBB and determine the capacity of this astrocyte cell line to maintain the BBB phenotype. Co-culture of the CTX-TNA2 cells with primary porcine brain endothelial cells produced an in vitro BBB model which retains key features of the in vivo BBB. High transendothelial electrical resistances, comparable to those reported in vivo, were obtained. Ultrastructural analysis revealed distinct intercellular tight junction protein complexes and immunocytochemistry confirmed expression of the tight junction proteins ZO-1 and occludin. Western blotting and fluorescent tracer assays confirmed expression and functional activity of P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) efflux transporters. Studies employing Alexa-fluor 555-conjugated human transferrin revealed temperature-sensitive internalisation indicating the BBB model retains functional receptor-mediated transferrin uptake. The findings of this study indicate that a robust BBB model has been produced and this is the first report of the inductive capacity of the CTX-TNA2 cell line. Since this in vitro BBB model possesses many key characteristics of the BBB in vivo it has the potential to be a valuable tool for the study of biochemical and physiological processes associated with the BBB.

Strengthening tight junctions of retinal microvascular endothelial cells by pericytes under normoxia and hypoxia involving angiopoietin-1 signal way

PURPOSE

To determine the effects of pericytes and angiopoietin-1 on the expression of occludin and zonula occludens-1 (ZO-1) in retinal endothelial cells (ECs) under both normoxic and hypoxic conditions.

METHODS

Rat primary retinal microvascular ECs were cultured under normoxia or hypoxia in either absence or presence of pericytes conditioned medium (PCM). PCM was pretreated with or without angiopoietin-1 neutralizing antibody. Immuofluorescent staining, Western blot and RT-PCR were used to detect the alterations of occludin and ZO-1 expression.

RESULTS

Under normoxia, PCM strengthened occludin and ZO-1 immunofluorescent staining at cytomembrane as well as increased their expression at both protein and mRNA level. When pretreated with angiopoietin-1 neutralizing antibody, occludin upregulation induced by PCM was significantly blocked at protein level (62%) and mRNA level (34%). Under hypoxia, the continuity of occludin and ZO-1 staining at cell boundaries was disrupted consistent with a decrease of their protein level by 31 and 27%, respectively. Also occludin and ZO-1 mRNA level decreased by 46 and 57%, respectively. PCM was observed to partially increase expression of occludin at protein and mRNA level. Angiopoietin-1 antibody slightly inhibited (16%) PCM induced occludin mRNA increase under hypoxia.

CONCLUSION

Pericytes improved the integrity of endothelial barrier through inducing occludin and ZO-1 expression at protein and mRNA level under normoxia. Under hypoxia, pericytes could partially reverse occludin decrease. These protecting effects of pericytes on endothelial barrier were at least in part mediated by angiopoietin-1.

The critical component to establish in vitro BBB model: Pericyte

The blood-brain barrier (BBB), a highly regulated membranous barrier of brain capillaries, consists of an intricate network of tight junctions (TJs) that segregate the central nervous system (CNS) from systemic blood circulation and maintain a delicate homeostasis of the CNS environment. While endothelial cells (ECs) of brain capillaries are clearly the principal cellular element of BBB, the formation and regulation of intact BBB structure appear to require the interactions of endothelial cells with other cellular components. Astrocytes, one of the major non-neural cells in the brain, associate closely and interact with capillary endothelial cells during the angiogenesis and BBB development. Current in vitro cellular models for the study of BBB functions often incorporate astrocytes with endothelial cells. However, another foremost cell type, CNS pericyte, which intimately embraces brain capillary endothelium, attracts relatively little attention for its role in developing the in vitro BBB system. This review will analyze the critical functions of pericytes in angiogenesis in various systems and discuss the relevance of these functions in mediating the development, maintenance, and regulation of BBB. The author will also discuss the functional role of actin in both ECs and pericytes, and further elaborate the molecular mechanisms of BBB permeability regulation that involves the transduction pathway-mediated actin remodeling process. Finally, the rationale of incorporating pericytes for establishing a better in vitro BBB model will be emphasized.

Polyunsaturated fatty acids induce tight junctions to form in brain capillary endothelial cells

Tight junctions create a rate-limiting barrier to the diffusion of solutes between vertebrate epithelial cells and endothelial cells. They are also controlled within individual cells by a variety of physiologically relevant signals. We investigated the effects of polyunsaturated fatty acids on the formation of tight junctions in brain capillary endothelial cells, monitoring the transepithelial electrical resistance, and analyzed the expression of occludin messenger RNA. Brain-capillary endothelial cells were grown to confluence on filters and exposed to eicosapentaenoic acids, gamma linolenic acid and linoleic acid. Transepithelial electrical resistance was determined with voltage-measuring electrodes. The messenger RNA expression of occludin was quantitated by real-time quantitative reverse transcriptase-polymerase chain reaction. The basal resistance across monolayers of porcine brain capillary endothelial cells was 83+/-8.1 Omega cm(2). Cells cultured in eicosapentaenoic acids and gamma linolenic acid, but not linolenic acid, displayed a 2.7-fold increase in transepithelial electrical resistance at 10 microM in brain capillary endothelial cells. The expression level of occludin messenger RNA increased markedly immediately after the exposure to eicosapentaenoic acids or gamma linolenic acid. Following an 8 h exposure to exogenous eicosapentaenoic acids or gamma linolenic acid, occludin messenger RNA levels were significantly increased. In addition, the rise in transepithelial electrical resistance induced by eicosapentaenoic acids and gamma linolenic acid was markedly inhibited by the tyrosine kinase inhibitors genistein and PP2 and protein kinase C inhibitor, calphostin C. In contrast, the rise in transepithelial electrical resistance induced by eicosapentaenoic acids and gamma linolenic acid was not inhibited by the PI 3-kinase inhibitor, LY294002. We conclude that eicosapentaenoic acids and gamma linolenic acid increased the transepithelial electrical resistance and the expression of occludin messenger RNA in brain capillary endothelial cells. This gamma linolenic acid and eicosapentaenoic acid induced assembly of tight junction is likely to be regulated by protein kinase C and tyrosine kinase activity.

Cerebral microvascular pathology in aging and Alzheimer's disease

The aging of the central nervous system and the development of incapacitating neurological diseases like Alzheimer's disease (AD) are generally associated with a wide range of histological and pathophysiological changes eventually leading to a compromised cognitive status. Although the diverse triggers of the neurodegenerative processes and their interactions are still the topic of extensive debate, the possible contribution of cerebrovascular deficiencies has been vigorously promoted in recent years. Various forms of cerebrovascular insufficiency such as reduced blood supply to the brain or disrupted microvascular integrity in cortical regions may occupy an initiating or intermediate position in the chain of events ending with cognitive failure. When, for example, vasoconstriction takes over a dominating role in the cerebral vessels, the perfusion rate of the brain can considerably decrease causing directly or through structural vascular damage a drop in cerebral glucose utilization. Consequently, cerebral metabolism can suffer a setback leading to neuronal damage and a concomitant suboptimal cognitive capacity. The present review focuses on the microvascular aspects of neurodegenerative processes in aging and AD with special attention to cerebral blood flow, neural metabolic changes and the abnormalities in microvascular ultrastructure. In this context, a few of the specific triggers leading to the prominent cerebrovascular pathology, as well as the potential neurological outcome of the compromised cerebral microvascular system are also going to be touched upon to a certain extent, without aiming at total comprehensiveness. Finally, a set of animal models are going to be presented that are frequently used to uncover the functional relationship between cerebrovascular factors and the damage to neural networks.

Age-related changes in the blood-brain barrier

Aging of the cerebral microcirculation results in significant alteration in the blood-brain barrier (BBB). The barrier function appears to remain intact in older animals, although it may be more susceptible to disruption by external factors (hypertension) and drugs (haloperidol). While overall transport processes do not change with age, aging animals and humans have altered BBB function of select carrier mediated transport systems including the transport of choline, glucose, butyrate and triiodothyronine. These age-related changes are the result of either alteration in the carrier molecules or the physiochemical properties of the cerebral microvessels. At the present time, it is not known whether changes in the BBB contribute to the age-related neurodegenerative diseases or are merely epiphenomena of aging.

Immunohistochemical study of human optic nerve head astroglia

Immunocytochemical localization of glial fibrillary acidic protein (GFAP) has been used to study the distribution of astrocytes and their morphology in sections of the optic nerve (ON) of human eye. Although all ON regions presented GFAP immunoreactivity, immunostained tissue was most common in the posterior prelaminar region (PR) and least common in the laminar region (LR). Two shapes of astrocytes were distinguished: thick and thin bodied astrocytes. Astrocytes with thick cell bodies are located in the superficial nerve fiber layer (SNFL), PR, LR and retrolaminar region (RR). Astrocytes with thin cell bodies were found in the SNFL and anterior PR. Sometimes thin bodied astrocytes presented another shape with a long process running parallel to the axons and these were found in the PR and LR. In the SNFL the thin bodied astrocytes accompany the axons and contact the capillaries derived from the central retinal artery. In the anterior PR the thin bodied astrocytes with a stellate shape lie over the vessels forming a sieve through which the axons pass. In the posterior PR, the thick bodied astrocytes form glial tubes that direct axons towards the LR. These astrocytes form a layer in the LR that lines the pores of the lamina cribrosa and separates the connective septa from the axon bundles in the RR. The limiting glial membranes separate the ON tissues from the adjacent tissues and from the course of the central retinal artery and are composed of many thick bodied astrocytes.

Cloning of the rabbit homologue of mouse 'basigin' and rat 'OX-47': kidney cell type-specific expression, and regulation in collecting duct cells

Monoclonal antibody '4D4' was generated against a gel-purified 43-50 kDa fraction of rabbit erythrocyte (RBC) ghosts. Immunoblots of rabbit RBCs, skeletal muscle, and kidney, and of a rabbit cortical collecting duct cell line (RC.SV3) yielded broad bands of 30-70 kDa that migrated at approximately 31 kDa after deglycosylation. In kidney sections, 4D4 labeled the basal plasma membranes of the proximal tubule, medullary thick ascending limb of Henle, cortical, medullary, and papillary collecting ducts, and papillary surface epithelium, as well as the lateral membranes of alpha and beta-type intercalated cells. Antibody 4D4 was used to clone a full-length kidney cDNA, which predicted a 31 kDa immunoglobulin-like glycoprotein with high homology to mouse 'gp42' or 'basigin', human 'M6' or 'EMMPRIN', rat 'OX-47' or 'CE-9', and avian 'neurothelin', 'HT7', or '5A11'. When heterologously expressed in HeLa cells, glycosylated immunoreactive protein was expressed at the plasma membrane. In the case of the endogenous protein in RC.SV3 cells, interferon-gamma and A23187 decreased, and fetal calf serum increased, steady-state mRNA levels. Thus, this molecule exhibits a high degree of cell type-specific expression in the kidney and undergoes regulation by cytokines and serum in kidney epithelial cells.

CNS antigen presentation

Presentation of antigens for the CNS follows the same general rules as for other tissues. However, the presence of special CNS cells with immune functions plus the blood-brain barrier (BBB) suggests that differences in the way that the immune system functions in the CNS might help to explain why some autoimmune diseases are unique to the CNS. Irrespective of whether CNS antigen presentation takes place inside or outside the CNS (or both), the BBB clearly plays a major role in CNS immune function. The BBB governs the quantity and type of lymphocytes that enter the CNS by way of specific adhesion-molecule binding between lymphocytes and endothelium and possibly by selecting for antigen-specific lymphocytes in antigen-recognition events.

Dehydration-induced proliferation of identified pituicytes in fully adult rats

The mitotic activity astrocytes in the adult central nervous system (CNS) is of interest due to their roles in gliosis and tumorigenesis, and their potential in aiding recovery of function following injury or disease. The posterior pituitary offers a potentially powerful model to study proliferation in vivo, since its resident astrocytes, called pituicytes, have been reported to divide concurrently with hormone release from the neurosecretory terminals there. our aim in this study was to confirm and characterize this proliferative response during dehydration and rehydration in fully adult animals using contemporary techniques. Adult male rats were given 2% saline in substitution for water for 0-9 days. Proliferation of pituicytes was quantified in tissue sections triple-labeled with the proliferation marker, 5-bromodeoxyuridine (BrdU), the astrocyte marker glial fibrillary acidic protein (GFAP), and the DNA marker 4,6,diamidino-2-phenylindole, HCL(DAPI). A robust proliferative response began within three days of dehydration and continued at a constant rate thereafter. In animals allowed to rehydrate, this response continued. After 9 days of dehydration, approximately 35% of pituicytes had participated in mitosis. While cell density remained constant across conditions, a reversible increase in posterior pituitary area was seen, suggesting that some cell death also occurs simultaneously. A significant proportion of non-pituicytes also underwent similar changes. These results indicate that pituicytes in the adult posterior pituitary retain characteristics necessary for reentering the cell cycle in response to local factors present during neurosecretory activity. We hypothesize that this proliferative response is directly related to the morphological changes previously reported for these cells under activating conditions.

The blood-brain barrier: cellular basis

Perfusion experiments with horseradish peroxidase have established that the morphological substrate of the blood-brain barrier is represented by microvascular endothelial cells. They are characterized by complexly arranged tight junctions and a very low rate of transcytotic vesicular transport. They express transport enzymes, carrier systems and brain endothelial cell-specific molecules of unknown function not expressed by any other endothelial cell population. These blood-brain barrier properties are not intrinsic to these cells but are inducible by the surrounding brain tissue. Type I astrocytes injected into the anterior eye chamber of the rat or onto the chick chorioallantoic membrane are able to induce a host-derived angiogenesis and some blood-brain barrier properties in endothelial cells of non-neural origin. Recently we have shown that this cellular interaction is due to the secretion of a soluble astrocyte derived factor(s). Astrocytes are also implicated in the maintenance, functional regulation and the repair of the blood-brain barrier. Complex interactions between other constituents of the microenvironment surrounding the endothelial cells, such as the basement membrane, pericytes, nerve endings, microglial cells and the extracellular fluid, take place and are required for the proper functioning of the blood-brain barrier, which in addition is regionally different as reflected by endothelial cell heterogeneity.