Astrocytic orthogonal arrays of intramembranous particle assemblies are modulated by brain endothelial cells in vitro

WT1 expression in angiogenic tumours of the skin

AIMS

To determine the expression of WT1 in endothelial proliferations and tumours. Endothelial cells are derived from angioblasts which differentiate into bone marrow stem cells (BMSC). BMSC are characterized by the constitutive expression of the WT1 gene and we have postulated that its expression may be maintained during the differentiation of angioblasts to endothelial cells.

METHODS AND RESULTS

The expression of WT1 was studied in human umbilical vein-derived (HUVEC) and brain microvascular endothelial cells (HBME) as well as in a Kaposi sarcoma (KS) cell line in vitro. Forty-two human skin biopsy samples of endothelial proliferations and tumours were analysed for the protein expression of WT1 using the monoclonal antibodies for wt-WT1 (6F-H2) and its 17AA+ variant (2C12). WT1 expression was detectable in HUVEC and KS cells and all WT1 splice variants examined (17AA+/- KTS+/-) were detectable in KS cells, while the 17AA+/- and KTS- variants were present in HUVEC. Immunohistochemical analysis of the 42 human skin biopsy samples revealed cytoplasmic WT1 expression using wild-type specific antibody (6FH2) in microvessels, which is maintained during neoangiogenesis (inflammation, haemorrhage, peritumoral angiogenesis). Around one-third of haemangiomas (3/10) and non-HIV-Kaposi sarcomas (7/18) expressed the WT1 protein in the cytoplasm of tumour cells compared with its frequent expression in angiosarcomas (7/8) using the same antibody (6FH2). The nuclear 17AA+ isoform of WT1 was detectable at protein level in a small proportion of KS cases exclusively (3/7).

CONCLUSION

Our data suggest that WT1 protein expression is maintained during angiogenesis and malignant transformation of endothelial cells and can be considered as a new endothelial marker.

Expression of CD44v3 protein in human endothelial cells in vitro and in tumoral microvessels in vivo

The most universal angiogenic cytokines (VEGF, bFGF, HGF) are all heparin-binding proteins, the function of which is dependent on cell surface heparan sulfate proteoglycans (HSPG). Several proteoglycans have been demonstrated in endothelial cells, but only glypican-1 from the cell surface HSPG subfamily was documented at protein level. Here, we show that CD44v3 is expressed in human immortalized endothelial cells [anchorage-dependent human umbilical vein endothelial cells (HUVEC) and anchorage-independent Kaposi sarcoma (KS-Imm)] at mRNA and protein level, but is absent from the primary culture of human brain microvascular endothelial cells. We have shown that CD44v3 has a large cytoplasmic pool in endothelial cells, but a limited surface expression, mainly at filopodia, colocalized with MMP-2. Angiogenic factors like VEGF or bFGF did not affect surface detection of CD44v3 suggesting a constitutive expression. The putative functional role for endothelial cell surface CD44v3 was identified in chemotaxis assay when anti-CD44v3 antibody pretreatment proved to be inhibitory for HUVEC. Furthermore, we provided evidence for the CD44v3 protein expression in human endothelial cells in vivo in peritumoral microvessels of both human melanoma and glottic cancers, suggesting a role for this part-time heparan sulfate proteoglycan in tumor induced angiogenesis.

In search of the astrocytic factor(s) modulating blood-brain barrier functions in brain capillary endothelial cells in vitro

(1) The blood-brain barrier (BBB) is formed by brain capillary endothelial cells (ECs). There are various cell types, in particular astrocytes, but also pericytes and neurons, located in close vicinity to the capillary ECs which may influence formation and function of the BBB. Based on this consideration, this paper discusses various aspects of the influence of the surrounding cells on brain capillary ECs with special focus on the role of astrocytes. (2) Based on the morphology of the BBB, important aspects of brain EC functions are summarized, such as transport functions and maintenance of low paracellular permeability. Moreover, various facets are discussed with respect to the influence of astrocytes, pericytes, microglia, and neurons on the BBB. Data on the role of glial cells in the ontogenesis of the BBB are presented subsequently. The knowledge on this subject is far from being complete, however, these data imply that the neural/neuronal environment rather than glial cells may be of importance in the maturation of the barrier. (3) The role of glial cells in the induction and maintenance of the BBB is discussed under physiological as well as pathological conditions. Although the literature presents manifold evidence for a great variety of effects induced by astroglia, there are also many controversies, which may result from different cellular models and experimental conditions used in the respective studies. Numerous factors secreted by astrocytes have been shown to induce a BBB phenotype. On the molecular level, increased expression of barrier-relevant proteins (e.g., tight junction proteins) is documented in the presence of astrocyte-derived factors, and many studies demonstrate the improvement of physiological parameters, such as increased transendothelial resistance and decreased paracellular permeability, in different in vitro models of the BBB. Moreover, one has to take into account that the interaction of brain ECs and astrocytes is bi-directional, and that the other cell types surrounding the brain microvasculature also contribute to BBB function or dysfunction, respectively. (4) In conclusion, it is expected that the present and future research focused on molecular mechanisms and signaling pathways will produce new and exciting insights into the complex network of BBB regulation: the cornerstone is laid.

Human cerebral microvessel endothelial cell culture as a model system to study the blood-brain interface in ischemic/hypoxic conditions

(1) Cerebral ischemia and reperfusion induce several changes on the endothelial cells at the microcirculatory level. (2) Vasogenic brain edema due to compromised blood-brain barrier, transformation of the endothelial cell surface from an anticoagulant to a procoagulant surface are important factors in the pathogenesis of ischemic stroke. (3) Release of prostaglandins, endothelin-1, complement proteins, and matrix metalloproteinase-9 by microvascular endothelial cells are other components in the complex mechanism of brain ischemia/hypoxia. (4) Ultrastructural studies documented the opened paracellular avenues in the course of vasogenic edema in different experimental models (5) Tight junctions of endothelial cells have been characterized with freeze fracture electron microscopy, and the process of transvesiculation was analyzed using rapid freeze and freeze substitution procedure before electron microscopy studies (6) In endothelial cell-culture experiments, we used rodent and later human brains. (7) Endothelial cells co-cultured with astroglia resulted in an elaborate tight junctional complex. (8) This co-culture technique becomes the basis of in vitro blood-brain barrier studies On endothelial cells of human brain origin, different regulatory factors found to be responsible for the complex mechanism of ischemic stroke.

Effect of HGF-like basic hexapeptides on angiogenesis

The interaction of glycosaminoglycans (GAG) with peptides relies on noncovalent binding to basic amino acid sequences, for which a minimal requirement is a pentapeptide region in the protein and the sulfated and carboxyl region in the GAG. Since such sequences are present in the heparin-binding angiogenic cytokines, including hepatocyte growth factor (HGF), we have postulated that such small peptides may have biological activity. Two basic peptide regions of the beta chain of HGF (RYRNKH512-516, HHRGK645-649) exhibited significant antiangiogenic activity in vivo in the chorioallantoic membrane assay and showed some antiproliferative activity in vitro on normal human brain microvessel endothelial-but not on anchorage-independent endothelial-cells (Kaposi sarcoma). Basic HIV-TAT peptides and scrambled hexapeptides did not show similar activity, except for KRKRKR, indicating sequence specificity of the phenomena. An HGF-derived basic peptide, HHRGK, modulated tumor-induced angiogenesis in vivo by interfering with the morphogenic, but not with the proliferative, phase of the process. These observations suggest small basic peptides as a new class of angiogenesis modulators.

The blood-nerve barrier: enzymes, transporters and receptors--a comparison with the blood-brain barrier

The blood-brain barrier (BBB) has been much more extensively investigated than the blood-nerve barrier (BNB). Nevertheless it is clear that there are both similarities and differences in the molecular and morphophysiological characteristics of the two barrier systems. A number of enzymes, transporters and receptors have been investigated at both the BNB and BBB, as well as in the perineurium of peripheral nerves, which is also a metabolically active diffusion barrier. While there have been few systematic comparisons of the distribution of these molecules in both the BNB and BBB, it is apparent from the data available, reviewed in this article, that their distribution also supports the concept of the BNB and BBB having some features in common but also showing distinct identities. These similarities and differences cannot simply be accounted for by the presence of the inductive influences of astrocytes at the BBB and absence at the BNB. Whether the Schwann cell also has the capacity to induce some BNB properties remains to be determined.

Modes of regulation of laminin-5 production by rat astrocytes

Astrocytes have a prominent role in central nervous system-specific microvascular growth and function. Extracellular matrix molecules (ECMs) bind to specific adhesion molecules - integrins. These adhesion complexes are responsible for tissue integrity and cellular signalling. We have examined astrocyte-endothelial cell interactions in relation to expression of the ECM laminin-5. Astrocytes produce laminin-5 in cell culture. The production is enhanced by endothelial cells. Direct cellular contact possibly involving a beta1 integrin or an insoluble matrix compound detected by the astrocytes is responsible for the induction of astrocytic laminin production by endothelial cells. Hypoxia also influences ECM production. The described regulation goes in concert with the known effects of astrocyte-endothelial cell interaction and stresses its importance for microvascular integrity.

Astrocytes alter their polarity in organotypic slice cultures of rat visual cortex

The ultrastructure of astrocytes in an organotypic slice culture of the rat visual cortex was investigated using ultrathin sections and freeze-fracture replicas. After a culture period of 9-15 days, a glial scaffold formed that separated the bulk of the slice neuropil from the medium and the underlying plasma clot. However, the glial cells and processes did not build a dense barrier but allowed the outgrowth of neurites. A basal lamina covering the medium-oriented surface of the astrocytes was not found. In freeze-fracture replicas, orthogonal arrays of particles (OAP) were characteristic components of astrocytic membranes. The OAP density in membranes bordering the medium was 35 +/- 13 OAP/microns 2, corresponding to 2.5% of this membrane area; the OAP density in membranes within the slice neuropil was 22 +/- 12 OAP/microns 2, corresponding to 1.4% of this membrane area. Although the difference was significant, it was greatly reduced when comparing OAP densities in endfoot and non-endfoot membranes in vivo. Another node of polarity was recognized in astrocytes of the organotypic slice culture. In membranes of astrocytes bordering upon the medium, the density of non-OAP intramembranous particles (IMP) was clearly higher (1130 +/- 136 IMP/microns 2) than in membranes of astrocytes in the center of the slice (700 +/- 172 IMP/microns 2). This pronounced IMP-related polarity was observed neither in vivo nor in cultured astrocytes. The present study suggests, together with data from the literature, that the distribution of astrocytic OAP across the cell surface is influenced by the existence of a basal lamina and neuronal activity, and that astrocytes possess a more remarkable plasticity of membrane structure than previously suspected.

Upregulation of the low density lipoprotein receptor at the blood-brain barrier: intercommunications between brain capillary endothelial cells and astrocytes

In contrast to the endothelial cells in large vessels where LDL receptors are downregulated, brain capillary endothelial cells in vivo express an LDL receptor. Using a cell culture model of the blood-brain barrier consisting of a coculture of brain capillary endothelial cells and astrocytes, we observed that the capacity of endothelial cells to bind LDL is enhanced threefold when cocultured with astrocytes. We next investigated the ability of astrocytes to modulate endothelial cell LDL receptor expression. We have shown that the lipid requirement of astrocytes increases the expression of endothelial cell LDL receptors. Experiments with dialysis membranes of different pore size showed that this effect is mediated by a soluble factor(s) with relative molecular mass somewhere between 3,500 and 14,000. Substituting astrocytes with smooth muscle cells or brain endothelium with endothelium from the aorta or the adrenal cortex did not enhance the luminal LDL receptor expression on endothelial cells, demonstrating the specificity of the interactions. This factor(s) is exclusively secreted by astrocytes cocultured with brain capillary endothelial cells, but it also upregulates the LDL receptor on other cell types. This study confirms the notion that the final fine tuning of cell differentiation is under local control.

The blood-brain barrier: morphology, molecules, and neurothelin

The blood-brain barrier (BBB) is a complex structure formed by vascular endothelial cells, which serve to stabilize the homeostasic processes that are essential for neural functioning. The barrier relies on tight junctions between neighboring endothelial cells and a highly restricted passage of blood-borne components through the endothelial lining. Selective transport mechanisms guarantee the essential import and export of metabolites through the BBB into and out of the neural microenvironment. The dual functions of barrier and carrier depend on distinct proteins, some of which have been characterized in detail.

Astrocyte-endothelial interaction: physiology and pathology

The blood-brain barrier of higher vertebrates is formed by the layer of endothelial cells lining the brain microvessels. The close anatomical association between endothelial cells and perivascular astrocytic end feet suggests cooperation between these cell types in forming and maintaining the blood-brain barrier. This review considers evidence from grafting experiments, developmental studies and culture models of the brain endothelium, concerning the inductive influences acting on the endothelium, and from endothelial cells acting on perivascular astrocytes. Examples from pathology and neurotoxicology which may involve breakdown of induction are also considered.

Astroglial membrane structure is affected by agents that raise cyclic AMP and by phosphatidylcholine phospholipase C

The role of signal transduction mechanisms in the production of the characteristic orthogonal arrays of particle assemblies in the astroglial plasma membrane was investigated in vitro by freeze-fracture electron microscopy. Agents which raise cellular cAMP levels and subsequently activate protein kinase A, such as forskolin (50 microM), isoproterenol (10 microM) and 8-bromo-cAMP (1 mM), increased the density, the number of assemblies per unit area of cleaved cell membrane, and the frequency of astrocytes with assemblies. Agents that lead to the activation of protein kinase C, such as phorbol 12,13-myristate acetate (at 50 nM) and choline-dependent phospholipase C (at 0.01-0.1 U ml-1), did not affect the assembly concentration. Thus, protein kinase A but not protein kinase C appears to be involved in the production of assemblies or their insertion into the astroglial plasma membrane. Although choline-dependent phospholipase C did not affect the astroglial assemblies, it caused the non-assembly, background particles to aggregate. A choline-dependent phospholipase C from a different source (B. cereus) was also active though at a higher concentration. Phospholipases of different specificities, such as phospholipase A2, phospholipase D or inositol-dependent phospholipase C were inactive over a wide range of concentrations. Two other astroglia derived cells, Müller cells and cells of the C6 glioma cell line, were also similarly affected by choline-dependent phospholipase C, while six other cells types including neurons, endothelial cells and fibroblasts were unaffected. It appears that phosphatidylcholine plays a significant role in determining the membrane structure of astrocytes. In a search for a means of isolating the assemblies, the binding of three lectins: ConA, WGA and PNA, conjugated to gold, was tested by label-fracture to ascertain whether the assemblies have an external oligosaccharide component. None of the lectins bound specifically to assemblies.

The cerebral microvessels in culture, an update

Recent advances in our knowledge of the blood-brain barrier (BBB) have in part been made by studying the properties and function of cerebral endothelial cells in vitro. After an era of working with a fraction, enriched in cerebral microvessels by centrifugation, the next generation of in vitro BBB model systems was introduced, when the conditions for routinely culturing the endothelial cells were established. This review summarizes the results obtained from this rapidly growing field. It can be stated with certainty that, in addition to providing a better insight into the chemical composition of cerebral endothelial cells, much has been learned from these studies about the characteristics of transport processes and cell-to-cell interactions during the last 12 years. With the application of new technologies, the approach offers a new means of investigation, applicable not only to biochemistry and physiology but also to the drug research, and may improve the transport of substances through the BBB. The in vitro approach has been and should remain an excellent model of the BBB to help unravel the complex molecular interactions underlying and regulating the permeability of the cerebral endothelium.