哈尔滨医科大学附属第一医院神经外科

血脑屏障的是人体自然屏障之一。其主要作用是阻止有害物质通过颅内血管进入脑实质,并同时辅助排出脑内代谢物质等。对相当多的颅内恶性肿瘤术后患者,血脑屏障在一定程度上阻碍了化疗药物进入脑实质,从而影响化疗效果。因此近年来越来越多的学者将研究重点放在如何开放血脑屏障这个问题上。血脑屏障构成主要为毛细血管的内皮细胞、基膜周细胞和星状胶质细胞的足突,其中血管内皮细胞处于最重要的地位。原因归结于它自身的一个特殊结构--紧密连接。紧密连接是否完整,功能是否可以正常发挥关系到内皮细胞的完整性,因此对血脑屏障的开放有着举足轻重的作用。维持紧密连接结构中功能蛋白功能的能量物质为葡萄糖。脑血管中的葡萄糖进入脑实质需载体或通道,脑组织负责此过程的物质为葡萄糖转运蛋白1(GLUT1)。本文作者通过松胞菌素B抑制葡萄糖转运蛋白1,降低能量供应从而影响紧密连接功能,最终引起血脑屏障开放角度做一综述。     

Rho GAPs and GEFs

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.     

The role of pericytes in angiogenesis

Pericytes are branched cells embedded within the basement membrane of capillaries and post-capillary venules. They provide an incomplete investment to endothelial cells, thus reinforcing vascular structure and regulating microvascular blood flow. Pericytes exert an important role on endothelial cell proliferation, migration and stabilization. Endothelial cells, in turn, stimulate expansion and activation of the pericyte precursor cell population. The balance between the number of endothelial cells and pericytes is highly controlled by a series of signaling pathway mechanisms operating in an autocrine and/or paracrine manner. In this review, we will first examine the molecular aspects of the pericyte activating factors secreted by endothelial cells, such as platelet derived growth factor B (PDGF-B), vascular endothelial growth factor (VEGF), transforming growth factor beta (TGF-β) and angiopoietins (Angs), as well as signaling pathways involving Notch and ephrins. We will then consider the complex and multivarious contribution of pericytes to the different aspects of angiogenesis with particular emphasis on the potential role of these cells as targets in tumor therapy.     

Enhancing integrin function by VEGF/neuropilin signaling

This review advances the hypothesis that the ability of integrins to engage their extracellular matrix ligands and signal can be regulated in tumor cells by vascular endothelial growth factor (VEGF), a major angiogenic factor that also has direct effects on the function of tumor cells. More specifically, we will discuss how neuropilins (NRPs), a distinct class of VEGF receptors, enable the function of specific integrins that contribute to tumor initiation and progression.     

Enhancing integrin function by VEGF/neuropilin signaling: Implications for tumor biology

This review advances the hypothesis that the ability of integrins to engage their extracellular matrix ligands and signal can be regulated in tumor cells by vascular endothelial growth factor (VEGF), a major angiogenic factor that also has direct effects on the function of tumor cells. More specifically, we will discuss how neuropilins (NRPs), a distinct class of VEGF receptors, enable the function of specific integrins that contribute to tumor initiation and progression.     

Rho GAPs and GEFs: Controling switches in endothelial cell adhesion

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.     

Modulation of VEGF signalling output by the Notch pathway

The formation of blood vessels within the vascular system entails a variety of cellular processes, including proliferation, migration and differentiation. In many cases, these diverse processes need to be finely coordinated among neighbouring endothelial cells in order to establish a functional vascular network. For instance, during angiogenic sprouting specialized endothelial tip cells follow guidance cues and migrate extensively into avascular tissues while trailing stalk cells must stay connected to the patent blood vessel. The vascular endothelial growth factor (VEGF) and Notch signalling pathways have emerged as the major players in governing these different cellular behaviours. In particular, recent work indicates an important role for Notch signalling in determining how an endothelial cell responds to VEGF. In this review, we provide an overview of these biochemically distinct pathways and discuss how they may interact during endothelial cell differentiation and angiogenesis.     

Plasmalemmal vesicles are involved in transendothelial transport of albumin, lysosomal enzymes and mannose 6-phosphate receptor fragments in capillary endothelium

With the use of immunoelectron microscopy we have demonstrated the presence of lysosomal enzymes (acid alpha-glucosidase and glucocerebrosidase) and fragments of the 270 kDa receptor for mannose 6-phosphate and insulin-like growth factor II in blood plasma, plasmalemmal vesicles of endothelial cells and pericapillary spaces in human skeletal muscle tissue. At these locations, the three proteins colocalized with albumin known to be transported from the capillaries into the pericapillary spaces. Immunoblot analysis of plasma revealed the presence of relatively high molecular weight polypeptides in this material. These observations strongly suggest that high molecular weight species of lysosomal enzymes can pass the endothelial barrier in skeletal muscle tissue.     

Endothelial cell functions

Endothelial cells play a wide variety of critical roles in the control of vascular function. Indeed, since the early 1980s, the accumulating knowledge of the endothelial cell structure as well as of the functional properties of the endothelial cells shifted their role from a passive membrane or barrier to a complex tissue with complex functions adaptable to needs specific in time and location. Hence, it participates to all aspects of the vascular homeostasis but also to physiological or pathological processes like thrombosis, inflammation, or vascular wall remodeling. Some of the most important endothelial functions will be described in the following review and more specifically, their role in blood vessel formation, in coagulation and fibribolysis, in the regulation of vascular tone as well as their participation in inflammatory reactions and in tumor neoangiogenesis.     

Microvascular development: learning from pancreatic islets

Microvascular development is determined by the interplay between tissue cells and microvascular endothelial cells. Because the pancreatic islet is an organ composed mainly of endothelial and endocrine cells, it represents a good model tissue for studying microvascular development in the context of a tissue. In this review, we will describe the special morphology of islet capillaries and its role in the physiologic function of islets: secretion of insulin in response to blood glucose levels. We will speculate on how islet-secreted VEGF-A generates a permeable endothelium that allows insulin to pass quickly into the blood stream. In addition, we speculate on how endothelial cells might form a capillary lumen within the islets. At the end, we look at the islet microvasculature from a medical point of view, thus describing its critical role during type I diabetes and islet transplantation.     

Fatty acid transport into the brain: of fatty acid fables and lipid tails

The blood-brain barrier formed by the brain capillary endothelial cells provides a protective barrier between the systemic blood and the extracellular environment of the central nervous system. Brain capillaries are a continuous layer of endothelial cells with highly developed tight junctional complexes and a lack of fenestrations. The presence of these tight junctions in the cerebral microvessel endothelial cells aids in the restriction of movement of molecules and solutes into the brain. Fatty acids are important components of biological membranes, are precursors for the biosynthesis of phospholipids and sphingolipids and are utilized for mitochondrial β-oxidation. The brain is capable of synthesizing only a few fatty acids. Hence, most fatty acids must enter into the brain from the blood. Here we review current mechanisms of transport of free fatty acids into cells and describe how free fatty acids move from the blood into the brain. We discuss both diffusional as well as protein-mediated movement of fatty acids across biological membranes.     

The enigmatic role of angiopoietin-1 in tumor angiogenesis

A tumor vasculature is highly unstable and immature, characterized by a high proliferation rate of endothelial cells, hyper-permeability, and chaotic blood flow. The dysfunctional vasculature gives rise to continual plasma leakage and hypoxia in the tumor, resulting in constant on-sets of inflammation and angiogenesis. Tumors are thus likened to wounds that will not heal. The lack of functional mural cells, including pericytes and vascular smooth muscle cells, in tumor vascular structure contributes significantly to the abnormality of tumor vessels. Angiopoietin-1 (Ang 1) is aphysiological angiogenesis promoter during embryonic development. The function of Angl is essential to endothelial cell survival, vascular branching, and pericyte recruitment. However, an increasing amount of experimental data suggest that Angl-stimulated association of mural cells with endothelial cells lead to stabilization of newly formed blood vessels. This in turn may limit the otherwise continuous angiogenesis in the tumor, and consequently give riseto inhibition of tumor growth. We discuss the enigmatic role of Angl in tumor angiogenesis in this review.     

Connexin Channels at the Glio-Vascular Interface: Gatekeepers of the Brain

Neuronal survival, electrical signaling and synaptic activity require a well-balanced micro-environment in the central nervous system. This is achieved by the blood–brain barrier (BBB), an endothelial barrier situated in the brain capillaries, that controls near-to-all passage in and out of the brain. The endothelial barrier function is highly dependent on signaling interactions with surrounding glial, neuronal and vascular cells, together forming the neuro-glio-vascular unit. Within this functional unit, connexin (Cx) channels are of utmost importance for intercellular communication between the different cellular compartments. Connexins are best known as the building blocks of gap junction (GJ) channels that enable direct cell–cell transfer of metabolic, biochemical and electric signals. In addition, beyond their role in direct intercellular communication, Cxs also form unapposed, non-junctional hemichannels in the plasma membrane that allow the passage of several paracrine messengers, complementing direct GJ communication. Within the NGVU, Cxs are expressed in vascular endothelial cells, including those that form the BBB, and are eminent in astrocytes, especially at their endfoot processes that wrap around cerebral vessels. However, despite the density of Cx channels at this so-called gliovascular interface, it remains unclear as to how Cx-based signaling between astrocytes and BBB endothelial cells may converge control over BBB permeability in health and disease. In this review we describe available evidence that supports a role for astroglial as well as endothelial Cxs in the regulation of BBB permeability during development as well as in disease states.     

Activation of the coagulation mechanism on tumor necrosis factor-stimulated cultured endothelial cells and their extracellular matrix. The role of flow and factor IX/IXa

Infusion of tumor necrosis factor (TNF) into tumor-bearing mice led to intravascular clot formation with fibrin deposition in microvessels in the tumor bed in close association with the vessel wall, which could be prevented by active site-blocked factor IXa (IXai). This observation prompted us to examine the role of the intrinsic system in activation of the coagulation mechanism on TNF-stimulated human endothelial cell monolayers and endothelial-derived matrix during exposure to purified coagulation factors or flowing blood. Treatment of endothelial cells in intact monolayers with TNF induced expression of the procoagulant cofactor tissue factor (TF) in a dose-dependent manner, and after removal of the cells, TF was present in the matrix. TNF-treated endothelial cell monolayers exposed to blood anticoagulated with low molecular weight heparin induced activation of coagulation. Addition of IXai blocked the procoagulant response on TNF-treated endothelial cells, and consistent with this, the presence of factor IX/VIIIa enhanced endothelial TF/factor VII(a) factor X activation over a wide range of cytokine concentrations (0-600 pM). When TF-dependent factor X activation on endothelial cells was compared with preparations of subendothelium, the extracellular matrix was 10-20 times more effective. IXai blocked TF/factor VII(a) mediated activated coagulation on matrix, but only at lower concentration of TNF (less than 50 pM). Similarly, enhancement of factor Xa formation on matrix by factors IX/VIIIa was most evident at lower TNF concentrations. When anticoagulated whole blood flowing with a shear of 300 s-1 was exposed to matrices from TNF-treated endothelial cells, but not matrices from control cells, fibrinopeptide A (FPA) generation, fibrin deposition, and platelet aggregate formation were observed. FPA generation could be prevented by a blocking antibody to TF and by active site-blocked factor Xa (Xai) over a wide range of TNF concentrations (0-600 pM), whereas IXai only blocked FPA generation at lower TNF concentrations (less than 50 pM). Activation of coagulation on matrix from TNF-stimulated endothelial cells was dependent on the presence of platelets, indicating the important role of platelets in propagating the reactions leading to fibrin formation. These observations demonstrate the potential of cytokine-stimulated endothelium and their matrix to activate coagulation and suggest the importance of the intrinsic system in factor Xa formation on cellular surfaces.     

Lipid peroxidation and endothelial cell injury: implications in atherosclerosis

Vascular endothelial cells, which play an active role in the physiological processes of vessel tone regulation and vascular permeability, form a border separating deeper layers of the blood vessel wall and cellular interstitial space from the blood and circulating cells. Damage or dysfunction of endothelial cells may reduce the effectiveness of the endothelium to act as a selectively permeable barrier to plasma components, including cholesterol-rich lipoprotein remnants. This may be involved in the etiology of atherosclerosis. Experimental evidence indicates that free radical-mediated lipid peroxidation can induce endothelial cell injury/dysfunction. Reactive oxygen species, including peroxidized lipids capable of initiating cell injury, may be generated within endothelial cells, be present in plasma components, or be derived from neutrophils or other blood-borne cells. Lipid peroxidation could initiate or promote the process of atherosclerotic lesion formation by directly damaging endothelial cells, and by enhancing the adhesion and activation of neutrophils and the susceptibility of platelets to aggregate. Endothelial cell injury by lipid hydroperoxides also could increase the uptake of LDL into the vessel wall. These events and other cellular dysfunctions may individually or collectively initiate and/or help to sustain the development of atherosclerosis.     

Cells in focus: endothelial cell

The endothelial cell is thought to arise from the splanchnopleuric mesoderm. Endothelial cells form the inner lining of a blood vessel and provides an anticoagulant barrier between the vessel wall and blood. In addition to its role as a selective permeability barrier, the endothelial cell is a unique multifunctional cell with critical basal and inducible metabolic and synthetic functions. The endothelial cell reacts with physical and chemical stimuli within the circulation and regulates hemostasis, vasomotor tone, and immune and inflammatory responses. In addition, the endothelial cell is pivotal in angiogenesis and vasculogenesis. Endothelial cell injury, activation or dysfunction is a hallmark of many pathologic states including atherosclerosis, loss of semi-permeable membrane function, and thrombosis.

Cell facts: (1) Endothelium consists of approximately (1–6)×10¹³ endothelial cells forming an almost 1 kg organ. (2) They uniquely contain Weibel–Palade bodies, 0.1 μm wide, 3 μm long membrane-bound structures that represent the storage organelle for von Willebrand factor (vWF). (3) The endothelial cell is not only a permeability barrier but also a multifunctional paracrine and endocrine organ. It is involved in the immune response, coagulation, growth regulation, production of extracellular matrix components, and is a modulator of blood flow and blood vessel tone.