Expression of the gap junction protein connexin43 in human telencephalon microvessels

The contribution of CXCL12-expressing radial glia cells to neuro-vascular patterning during human cerebral cortex development

This study was conducted on human developing brain by laser confocal and transmission electron microscopy (TEM) to make a detailed analysis of important features of blood-brain barrier (BBB) microvessels and possible control mechanisms of vessel growth and differentiation during cerebral cortex vascularization. The BBB status of cortex microvessels was examined at a defined stage of cortex development, at the end of neuroblast waves of migration, and before cortex lamination, with BBB-endothelial cell markers, namely tight junction (TJ) proteins (occludin and claudin-5) and influx and efflux transporters (Glut-1 and P-glycoprotein), the latter supporting evidence for functional effectiveness of the fetal BBB. According to the well-known roles of astroglia cells on microvessel growth and differentiation, the early composition of astroglia/endothelial cell relationships was analyzed by detecting the appropriate astroglia, endothelial, and pericyte markers. GFAP, chemokine CXCL12, and connexin 43 (Cx43) were utilized as markers of radial glia cells, CD105 (endoglin) as a marker of angiogenically activated endothelial cells (ECs), and proteoglycan NG2 as a marker of immature pericytes. Immunolabeling for CXCL12 showed the highest level of the ligand in radial glial (RG) fibers in contact with the growing cortex microvessels. These specialized contacts, recognizable on both perforating radial vessels and growing collaterals, appeared as CXCL12-reactive en passant, symmetrical and asymmetrical, vessel-specific RG fiber swellings. At the highest confocal resolution, these RG varicosities showed a CXCL12-reactive dot-like content whose microvesicular nature was confirmed by ultrastructural observations. A further analysis of RG varicosities reveals colocalization of CXCL12 with Cx43, which is possibly implicated in vessel-specific chemokine signaling.

Tyrosine-dependent basolateral targeting of human connexin43-eYFP in Madin-Darby canine kidney cells can be disrupted by the oculodentodigital dysplasia mutation L90V

Polarized membrane sorting of connexin 43 (Cx43) has not been well-characterized. Based on the presence of a putative sorting signal, YKLV(286-289), within its C-terminal cytoplasmic domain, we hypothesized that Cx43 is selectively expressed on the basolateral surface of Madin-Darby canine kidney (MDCK) cells in a tyrosine-dependent manner. We generated stable MDCK cell lines expressing human wild-type and mutant Cx43-eYFP, and analyzed the membrane localization of Cx43-eYFP within polarized monolayers using confocal microscopy and selective surface biotinylation. We found that wild-type Cx43-eYFP was selectively targeted to the basolateral membrane domain of MDCK cells. Substitution of alanine for Y286 disrupted basolateral targeting of Cx43-eYFP. Additionally, substitution of a sequence containing the transferrin receptor internalization signal, LSYTRF, for PGYKLV(284-289) also disrupted basolateral targeting. Taken together, these results indicate that Y286 in its native amino acid sequence is necessary for targeting Cx43-eYFP to the basolateral membrane domain of MDCK cells. To determine whether the F52dup or L90V oculodentodigital dysplasia-associated mutations could affect polarized sorting of Cx43-eYFP, we analyzed the expression of these Cx43-eYFP mutant constructs and found that the L90V mutation disrupted basolateral expression. These findings raise the possibility that some oculodentodigitial dysplasia-associated mutations contribute to disease by altering polarized targeting of Cx43.

Neuroimmunity and the blood-brain barrier: molecular regulation of leukocyte transmigration and viral entry into the nervous system with a focus on neuroAIDS

HIV infection of the central nervous system (CNS) can result in neurologic dysfunction with devastating consequences in a significant number of individuals with AIDS. Two main CNS complications in individuals with HIV are encephalitis and dementia, which are characterized by leukocyte infiltration into the CNS, microglia activation, aberrant chemokine expression, blood-brain barrier (BBB) disruption, and eventual damage and/or loss of neurons. One of the major mediators of NeuroAIDS is the transmigration of HIV-infected leukocytes across the BBB into the CNS. This review summarizes new key findings that support a critical role of the BBB in regulating leukocyte transmigration. In addition, we discuss studies on communication among cells of the immune system, BBB, and the CNS parenchyma, and suggest how these interactions contribute to the pathogenesis of NeuroAIDS. We also describe some of the animal models that have been used to study and characterize important mechanisms that have been proposed to be involved in HIV-induced CNS dysfunction. Finally, we review the pharmacologic interventions that address neuroinflammation, and the effect of substance abuse on HIV-1 related neuroimmunity.

Simian immunodeficiency virus disrupts extended lengths of the blood-brain barrier

It is known that there is disruption of the blood-brain barrier during terminal AIDS encephalitis in both human immunodeficiency virus (HIV)-infected humans and simian immunodeficiency virus (SIV)-infected rhesus macaques. Much, although by no means all, of the neuropathological findings of HIV and SIV infection involves accumulation of monocytes/macrophages that have likely crossed the blood-brain barrier (BBB). There is no convincing, rigorous, demonstration of HIV (or SIV) infecting endothelial cells in vivo. However, this is not to say that HIV infection would not have any effects on the physiology of microvascular brain endothelial cells. Because of the elaborate nature of cerebral microvessels, previous studies of cerebral endothelial cells have been constrained by sectioning artifacts. Examination of freshly isolated cerebral microvessels allows investigation of extended lengths of vessels (>150 mum) without sectioning artifacts. These studies determine the changes in the expression of the tight junction protein zo-1 protein on the endothelial cells of cerebral capillaries at terminal acquired immune deficiency syndrome, demonstrating that there is a decreased expression of zo-1 protein over extended lengths of microvessels.

Connexin expression and conducted vasodilation along arteriolar endothelium in mouse skeletal muscle

Functional hyperemia requires the coordination of smooth muscle cell relaxation along and between branches of the arteriolar network. Vasodilation is conducted from cell to cell along the arteriolar wall through gap junction channels composed of connexin protein subunits. Within skeletal muscle, it is unclear whether arteriolar endothelium, smooth muscle, or both cell layers provide the cellular pathway for conduction. Furthermore, the constitutive profile of connexin expression within the microcirculation is unknown. We tested the hypothesis that conducted vasodilation and connexin expression are intrinsic to the endothelium of arterioles (17 +/- 1 microm diameter) that supply the skeletal muscle fibers in the cremaster of anesthetized C57BL/6 mice. ACh delivered to an arteriole (500 ms, 1-microA pulse; 1-microm micropipette) produced local dilation of 17 +/- 1 microm; conducted vasodilation observed 1 mm upstream was 9 +/- 1 microm (n = 5). After light-dye treatment to selectively disrupt endothelium (250-microm segment centered 500 microm upstream, confirmed by loss of local response to ACh while constriction to phenylephrine and dilation to sodium nitroprusside remained intact), we found that conducted vasodilation was nearly abolished (2 +/- 1 microm; P < 0.05). Whole-mount immunohistochemistry for connexins revealed punctate labeling at borders of arteriolar endothelial cells, with connexin40 and connexin37 in all branches and connexin43 only in the largest branches. Immunoreactivity for connexins was not apparent in smooth muscle or in capillary or venular endothelium, despite robust immunolabeling for alpha-actin and platelet endothelial cell adhesion molecule-1, respectively. We conclude that vasodilation is conducted along the endothelium of mouse skeletal muscle arterioles and that connexin40 and connexin37 are the primary connexins forming gap junction channels between arteriolar endothelial cells.

Chemokine-dependent mechanisms of leukocyte trafficking across a model of the blood-brain barrier

Leukocyte transmigration across the blood-brain barrier (BBB) is a multistep process that can be mediated by chemokines. These low-molecular-weight chemoattractant proteins are secreted by cells within the central nervous system (CNS) in response to injury or on activation. Leukocytes transmigrate toward this chemokine gradient, crossing the BBB and gaining access to the CNS parenchyma. Depending on the chemokine, the nature of the insult, and the type of cell that transmigrates, the BBB integrity may be disrupted, leading to its increased permeability. Both the inflammation resulting from leukocyte transmigration and BBB perturbations contribute to CNS pathology. The mechanisms that mediate leukocyte transmigration and BBB disruption, as well as tissue culture models that are used to study leukocyte trafficking, are the focus of this review.

Differential expression of connexin43 in foetal, adult and tumour-associated human brain endothelial cells

Connexin43 (Cx43), the main protein constituting the gap junctions between astrocytes, has previously been demonstrated in endothelial cells of somatic vessels where the intercellular coupling that it provides plays a role in endothelial proliferation and migration. In this study, Cx43 expression was analysed in human brain microvascular endothelial cells of the cortical plate of 18-week foetal telencephalon, in adult cerebral cortex and glioma (astrocytomas). The study was carried out by immunocytochemistry utilizing a Cx43 monoclonal antibody and a polyclonal antibody anti-GLUT1 (glucose transporter isoform 1) to identify the endothelial cells and to localize Cx43. Endothelial Cx43 is differently expressed in the cortical plate, cerebral cortex and astrocytoma. Within the cortical plate and tumour, Cx43 is highly expressed in microvascular endothelial cells whereas it is virtually absent in the cerebral cortex microvessels. The high expression of the gap junction protein in developing brain, as well as in brain tumours, may be related to the growth status of the microvessels during brain and tumour angiogenesis. The lack of endothelial Cx43 in the cerebral cortex is in agreement with the characteristics of the mature brain endothelial cells that are sealed by tight junctions. In conclusion, the results indicate that endothelial Cx43 expression is developmentally regulated in the normal human brain and suggest that it is controlled by the microenvironment in both normal and tumour-related conditions.