Endothelial α6ß4 integrin protects during experimental autoimmune encephalomyelitis-induced neuroinflammation by maintaining vascular integrity and tight junction protein expression.

BACKGROUND: Extracellular matrix (ECM) proteins play critical functions regulating vascular formation and function. Laminin is a major component of the vascular basal lamina, and transgenic mice deficient in astrocyte or pericyte laminin show defective blood-brain barrier (BBB) integrity, indicating an important instructive role for laminin in cerebral blood vessels. As previous work shows that in the normal brain, vascular expression of the laminin receptor α6ß4 integrin is predominantly restricted to arterioles, but induced on all vessels during neuroinflammation, it is important to define the role of this integrin in the maintenance of BBB integrity. METHODS: α6ß4 integrin expression was analyzed using dual immunofluorescence (dual-IF) of brain sections taken from the mouse model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). To investigate the role of endothelial α6ß4 integrin, transgenic mice lacking ß4 integrin in endothelial cells (ß4-EC-KO) and wild-type (WT) littermates were subject to EAE, and clinical score and various neuropathological parameters were examined by immunofluorescence. In addition, ß4 integrin null brain endothelial cells (BECs) were examined in culture for expression of tight junction proteins using immunocytochemistry and flow cytometry. RESULTS: Cerebrovascular expression of ß4 integrin was markedly upregulated during EAE progression, such that by the acute stage of EAE (day 21), the vast majority of blood vessels expressed ß4 integrin. In the EAE model, while the ß4-EC-KO mice showed the same time of disease onset as the WT littermates, they developed significantly worse clinical disease over time, resulting in increased clinical score at the peak of disease and maintained elevated thereafter. Consistent with this, the ß4-EC-KO mice showed enhanced levels of leukocyte infiltration and BBB breakdown and also displayed increased loss of the endothelial tight junction proteins claudin-5 and ZO-1. Under pro-inflammatory conditions, primary cultures of ß4KO BECs also showed increased loss of claudin-5 and ZO-1 expression. CONCLUSIONS: Taken together, our data suggest that α6ß4 integrin upregulation is an inducible protective mechanism that stabilizes the BBB during neuroinflammatory conditions.

Derivation of microglia-free astrocyte cultures from neural stem cells.

Astrocytes play fundamental roles in the establishment and maintenance of tissue homeostasis in the central nervous system (CNS). To examine these different functions in vitro, it is important to be able to generate pure astrocyte cultures. While many "enriched" astrocyte cultures have been described, these are complicated by the presence of other contaminating cell types, such as microglia. In this chapter, we describe a method in which microglia-free astrocyte cultures are generated from neurospheres and also include an immunocytochemical approach to demonstrate the purity of these cultures.

Use of astrocyte-microglial cocultures to examine the regulatory influence of astrocytes on microglial activation.

Microglia are the principal immune effector cells of the central nervous system (CNS). Under normal conditions, they occupy a quiescent surveillance phenotype, but following stimulation by microorganisms or inflammatory cytokines, microglia transform into highly activated migratory, phagocytic cells producing inflammatory cytokines and chemokines. Significantly, several studies have demonstrated that astrocytes attenuate microglial activation, reducing microglial adhesion, production of interleukin-12 (IL-12) and reactive oxygen species (ROS), and expression of inducible nitric oxide synthase (iNOS). In this chapter, we describe an astrocyte-microglia coculture system that can be used to investigate interactions between these two cell types. We also describe a flow cytometry approach to quantify microglial activation state, as assessed by microglial expression of cellular activation markers, including MHC class I and the Mac-1 and α4 integrins.

Microglial activation state exerts a biphasic influence on brain endothelial cell proliferation by regulating the balance of TNF and TGF-ß1.

BACKGROUND: Studies of cerebral ischemia and other neuroinflammatory states have demonstrated a strong association between new vessel formation and microglial recruitment and activation, raising the possibility that microglia may be involved in promoting angiogenesis. As endothelial cell proliferation is a fundamental early step in angiogenesis, the aim of this study was to test this hypothesis by examining the influence of microglial secreted factors on brain endothelial cell (BEC) proliferation using BrdU incorporation. METHODS: Primary cultures of mouse BEC, microglia and astrocytes were used in this study. Proliferation of BEC was examined by BrdU incorporation. ELISA was used to quantify TNF and TGF-ß1 levels within cell culture supernatants. RESULTS: Microglia regulated BEC proliferation in a biphasic manner; microglia conditioned medium (MG-CM) from resting microglia inhibited, while that from activated microglia promoted BEC proliferation. A screen of microglial cytokines revealed that BEC proliferation was inhibited by TGF-ß1, but promoted by TNF. ELISA showed that TNF and TGF-ß1 were both present in MG-CM, and that while TGF-ß1 dominated in resting MG-CM, TNF levels were massively increased in activated MG-CM, shifting the balance in favor of TNF. Antibody-blocking studies revealed that the influence of MG-CM to inhibit or promote BEC proliferation was largely attributable to the cytokines TGF-ß1 and TNF, respectively. CONCLUSION: This data suggests that microglial activation state might be an important determinant of cerebral angiogenesis; inhibiting BEC proliferation and neovascularization in the normal central nervous system (CNS), but stimulating the growth of new capillaries under neuroinflammatory conditions.

In the hypoxic central nervous system, endothelial cell proliferation is followed by astrocyte activation, proliferation, and increased expression of the alpha 6 beta 4 integrin and dystroglycan.

Cerebral hypoxia induces a profound angiogenic response in the central nervous system (CNS). Using a mouse model of chronic cerebral hypoxia, we previously demonstrated that angiogenic vessels in the hypoxic CNS show marked upregulation of the extracellular matrix (ECM) protein fibronectin, along with increased expression of its major receptor, alpha 5 beta 1 integrin on brain endothelial cells (BEC). As cerebral hypoxia also leads to glial activation, the aim of the current study was to define the temporal relationship between BEC responses and glial cell activation in this model of cerebral hypoxia. This revealed that BEC fibronectin/alpha 5 beta 1 integrin expression and proliferation both reached maximal level after 4-day hypoxia. Interestingly, up to 4-day hypoxia, all dividing cells were BEC, but at later time-points proliferating astrocytes were also observed. GFAP staining revealed that hypoxia induced marked astrocyte activation that reached maximal level between 7- and 14-day hypoxia. As newly formed cerebral capillaries require ensheathment by astrocyte end-feet to acquire mature brain endothelium characteristics, we next examined how expression of astrocyte end-feet adhesion molecules is regulated by hypoxia. This showed that the astrocyte adhesion receptors alpha 6 beta 4 integrin and dystroglycan were both markedly upregulated, with a time-course that closely resembled astrocyte activation. Taken together, this evidence shows that cerebral hypoxia promotes first an endothelial response, in which fibronectin promotes BEC proliferation. This is then followed by an astrocyte response, involving astrocyte activation, proliferation, and reorganization of astrocyte end-feet, which correlates with increased expression of astrocyte end-feet adhesion molecules.

Absence of the alpha v beta 3 integrin dictates the time-course of angiogenesis in the hypoxic central nervous system: accelerated endothelial proliferation correlates with compensatory increases in alpha 5 beta 1 integrin expression.

Cerebral angiogenesis is an important adaptive response to hypoxia. As the alpha v beta 3 integrin is induced on angiogenic vessels in the ischemic central nervous system (CNS), and the suggested angiogenic role for this integrin in other systems, it is important to determine whether the alpha v beta 3 integrin is an important mediator of cerebral angiogenesis. alpha v beta 3 integrin expression was examined in a model of cerebral hypoxia, in which mice were subject to hypoxia (8% O(2)) for 0, 4, 7, or 14 days. Immunofluorescence and western blot analysis revealed that in the hypoxic CNS, alpha v beta 3 integrin was strongly induced on angiogenic brain endothelial cells (BEC), along with its ligand vitronectin. In the hypoxia model, beta 3 integrin-null mice showed no obvious defect in cerebral angiogenesis. However, early in the angiogenic process, BEC in these mice showed an increased mitotic index that correlated closely with increased alpha 5 integrin expression. In vitro experiments confirmed alpha 5 integrin upregulation on beta 3 integrin-null BEC, which also correlated with increased BEC proliferation on fibronectin. These studies confirm hypoxic induction of alpha v beta 3 integrin on angiogenic vessels, but suggest distinct roles for the BEC integrins alpha v beta 3 and alpha 5 beta 1 in cerebral angiogenesis, with alpha v beta 3 having a nonessential role, and alpha 5 beta 1 promoting BEC proliferation.

Myotendinous junction defects and reduced force transmission in mice that lack alpha7 integrin and utrophin.

The alpha7beta1 integrin, dystrophin, and utrophin glycoprotein complexes are the major laminin receptors in skeletal muscle. Loss of dystrophin causes Duchenne muscular dystrophy, a lethal muscle wasting disease. Duchenne muscular dystrophy-affected muscle exhibits increased expression of alpha7beta1 integrin and utrophin, which suggests that these laminin binding complexes may act as surrogates in the absence of dystrophin. Indeed, mice that lack dystrophin and alpha7 integrin (mdx/alpha7(-/-)), or dystrophin and utrophin (mdx/utr(-/-)), exhibit severe muscle pathology and die prematurely. To explore the contribution of the alpha7beta1 integrin and utrophin to muscle integrity and function, we generated mice lacking both alpha7 integrin and utrophin. Surprisingly, mice that lack both alpha7 integrin and utrophin (alpha7/utr(-/-)) were viable and fertile. However, these mice had partial embryonic lethality and mild muscle pathology, similar to alpha7 integrin-deficient mice. Dystrophin levels were increased 1.4-fold in alpha7/utr(-/-) skeletal muscle and were enriched at neuromuscular junctions. Ultrastructural analysis revealed abnormal myotendinous junctions, and functional tests showed a ninefold reduction in endurance and 1.6-fold decrease in muscle strength in these mice. The alpha7/utr(-/-) mouse, therefore, demonstrates the critical roles of alpha7 integrin and utrophin in maintaining myotendinous junction structure and enabling force transmission during muscle contraction. Together, these results indicate that the alpha7beta1 integrin, dystrophin, and utrophin complexes act in a concerted manner to maintain the structural and functional integrity of skeletal muscle.

Loss of the alpha7 integrin promotes extracellular signal-regulated kinase activation and altered vascular remodeling.

Vascular smooth muscle cell (VSMC) proliferation and migration are underlying factors in the development and progression of cardiovascular disease. Studies have shown that altered expression of vascular integrins and extracellular matrix proteins may contribute to the vascular remodeling observed after arterial injury and during disease. We have recently shown that loss of the alpha7beta1 integrin results in VSMC hyperplasia. To investigate the cellular mechanisms underlying this phenotype, we have examined changes in cell signaling pathways associated with VSMC proliferation. Several studies have demonstrated the mitogen-activated protein kinase signaling pathway is activated in response to vascular injury and disease. In this study, we show that loss of the alpha7 integrin in VSMCs results in activation of the extracellular signal-regulated kinase and translocation of the activated kinase to the nucleus. Forced expression of the alpha7 integrin or use of the mitogen-activated protein kinase kinase 1 inhibitor U0126 in alpha7 integrin-deficient VSMCs suppressed extracellular signal-regulated kinase activation and restored the differentiated phenotype to alpha7 integrin-null cells in a manner dependent on Ras signaling. Alpha7 integrin-null mice displayed profound vascular remodeling in response to injury with pronounced neointimal formation and reduced vascular compliance. These findings demonstrate that the alpha7beta1 integrin negatively regulates extracellular signal-regulated kinase activation and suggests an important role for this integrin as part of a signaling complex regulating VSMC phenotype switching.

Severe muscular dystrophy in mice that lack dystrophin and alpha7 integrin.

The dystrophin glycoprotein complex links laminin in the extracellular matrix to the cell cytoskeleton. Loss of dystrophin causes Duchenne muscular dystrophy, the most common human X-chromosome-linked genetic disease. The alpha7beta1 integrin is a second transmembrane laminin receptor expressed in skeletal muscle. Mutations in the alpha7 integrin gene cause congenital myopathy in humans and mice. The alpha7beta1 integrin is increased in the skeletal muscle of Duchenne muscular dystrophy patients and mdx mice. This observation has led to the suggestion that dystrophin and alpha7beta1 integrin have complementary functional and structural roles. To test this hypothesis, we generated mice lacking both dystrophin and alpha7 integrin (mdx/alpha7(-/-)). The mdx/alpha7(-/-) mice developed early-onset muscular dystrophy and died at 2-4 weeks of age. Muscle fibers from mdx/alpha7(-/-) mice exhibited extensive loss of membrane integrity, increased centrally located nuclei and inflammatory cell infiltrate, greater necrosis and increased muscle degeneration compared to mdx or alpha7-integrin null animals. In addition, loss of dystrophin and/or alpha7 integrin resulted in altered expression of laminin-alpha2 chain. These results point to complementary roles for dystrophin and alpha7beta1 integrin in maintaining the functional integrity of skeletal muscle.