Dynamics of CNS Barriers: Evolution,Differentiation, and Modulation

Summary 1. Three main barrier layers at the interface between blood and tissue protect the central nervous system (CNS): the endothelium of brain capillaries, and the epithelia of the choroid plexus (CP) and the arachnoid. The classical work on these barriers in situ until the 1970s laid the foundations for modern understanding. Techniques for brain endothelial cell isolation and culture pioneered by Ferenc Joó in the 1970s opened up new fields of examination, enabling study of mechanisms at the cellular and molecular level.2. Astrocytic glial cells are closely associated with the brain endothelial barrier. During evolution the barrier appears to have shifted from the glial to the endothelial layer, in parallel with the increasing importance of the microvasculature and its regulation. Vestiges of the barrier potential of glia remain in the modern mammalian CNS.3. Evolutionary evidence suggests that the advantage derived from ionic homeostasis around central synapses was the major selective pressure leading to refinement of CNS barrier systems. This is one element of the modern multitasking barrier function.4. While epithelia are constitutively able to form barriers at appropriate interfaces, the default condition for endothelia is more leaky; inductive influences from associated cells especially astrocytes are important in generating the full blood–brain barrier (BBB) phenotype in brain capillaries. The underlying mechanisms are being elucidated at the molecular and genomics level.5. The barrier layers of the nervous system can be modulated by a number of receptor-mediated processes, involving several signal transduction pathways, both calcium dependent and independent. Some agents acting as inducers in the long term can act as modulators in the short-term, with some overlap of signaling pathways. Modulating agents may be derived both from the blood and from cells associated with cerebral vessels. Less is known about the modulation of the CP.6. The challenge for the next era of CNS barrier studies will be to apply new knowledge from proteomics and genomics to understanding the in vivo condition in physiology and pathology.This revised article was published online in May 2005 with a February 2005 cover date.     

Determinants of Passive Drug Entry into the Central Nervous System

1. The blood–brain barriers restrict the passive diffusion of many drugs into the brain and constitute a significant obstacle in the pharmacological treatment of central nervous system diseases and disorders. The degree of restriction they impose is variable, with some lipid-insoluble drugs effectively excluded from the brain, while many lipid-soluble drugs do not appear to be subject to any restriction.2. The ease with which any particular drug diffuses across the blood–brain barrier is determined largely by the number and strength of intermolecular forces holding it to surrounding water molecules. By quantifying the molecular features that contribute to these forces, it is possible to predict the in vivo blood–brain barrier permeability of a drug from its molecular structure. Dipolarity, polarizability, and hydrogen bonding ability are factors that appear to reduce permeability, whereas molecular volume (size) and molar refraction are associated with increased permeability.3. Increasing the passive entry of restricted drugs into the central nervous system can be achieved by disrupting the blood–brain barrier (increased paracellular diffusion) or by modifying the structure of restricted drugs to temporarily or permanently increase their lipid solubility (increased transcellular permeability).4. Competitive inhibition of outwardly directed active efflux mechanisms (P-glycoprotein and MRP, the multidrug resistance-related protein) can also significantly increase the accumulation of certain drugs within the central nervous system.     

Brain iron transport

Brain iron is a crucial participant and regulator of normal physiological activity. However, excess iron is involved in the formation of free radicals, and has been associated with oxidative damage to neuronal and other brain cells. Abnormally high brain iron levels have been observed in various neurodegenerative diseases, including neurodegeneration with brain iron accumulation, Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the key question of why iron levels increase in the relevant regions of the brain remains to be answered. A full understanding of the homeostatic mechanisms involved in brain iron transport and metabolism is therefore critical not only for elucidating the pathophysiological mechanisms responsible for excess iron accumulation in the brain but also for developing pharmacological interventions to disrupt the chain of pathological events occurring in these neurodegenerative diseases. Numerous studies have been conducted, but to date no effort to synthesize these studies and ideas into a systematic and coherent summary has been made, especially concerning iron transport across the luminal (apical) membrane of the capillary endothelium and the membranes of different brain cell types. Herein, we review key findings on brain iron transport, highlighting the mechanisms involved in iron transport across the luminal (apical) as well as the abluminal (basal) membrane of the blood–brain barrier, the blood–cerebrospinal fluid barrier, and iron uptake and release in neurons, oligodendrocytes, astrocytes and microglia within the brain. We offer suggestions for addressing the many important gaps in our understanding of this important topic, and provide new insights into the potential causes of abnormally increased iron levels in regions of the brain in neurodegenerative disorders.     

Transferrin and Transferrin Receptor Function in Brain Barrier Systems

1. Iron (Fe) is an essential component of virtually all types of cells and organisms. In plasma and interstitial fluids, Fe is carried by transferrin. Iron-containing transferrin has a high affinity for the transferrin receptor, which is present on all cells with a requirement for Fe. The degree of expression of transferrin receptors on most types of cells is determined by the level of Fe supply and their rate of proliferation.2. The brain, like other organs, requires Fe for metabolic processes and suffers from disturbed function when a Fe deficiency or excess occurs. Hence, the transport of Fe across brain barrier systems must be regulated. The interaction between transferrin and transferrin receptor appears to serve this function in the blood–brain, blood–CSF, and cellular–plasmalemma barriers. Transferrin is present in blood plasma and brain extracellular fluids, and the transferrin receptor is present on brain capillary endothelial cells, choroid plexus epithelial cells, neurons, and probably also glial cells.3. The rate of Fe transport from plasma to brain is developmentally regulated, peaking in the first few weeks of postnatal life in the rat, after which it decreases rapidly to low values. Two mechanisms for Fe transport across the blood–brain barrier have been proposed. One is that the Fe–transferrin complex is transported intact across the capillary wall by receptor-mediated transcytosis. In the second, Fe transport is the result of receptor-mediated endocytosis of Fe–transferrin by capillary endothelial cells, followed by release of Fe from transferrin within the cell, recycling of transferrin to the blood, and transport of Fe into the brain. Current evidence indicates that although some transcytosis of transferrin does occur, the amount is quantitatively insufficient to account for the rate of Fe transport, and the majority of Fe transport probably occurs by the second of the above mechanisms.4. An additional route of Fe and transferrin transport from the blood to the brain is via the blood–CSF barrier and from the CSF into the brain. Iron-containing transferrin is transported through the blood–CSF barrier by a mechanism that appears to be regulated by developmental stage and iron status. The transfer of transferrin from blood to CSF is higher than that of albumin, which may be due to the presence of transferrin receptors on choroid plexus epithelial cells so that transferrin can be transported across the cells by a receptor-mediated process as well as by nonselective mechanisms.5. Transferrin receptors have been detected in neurons in vivo and in cultured glial cells. Transferrin is present in the brain interstitial fluid, and it is generally assumed that Fe which transverses the blood–brain barrier is rapidly bound by brain transferrin and can then be taken up by receptor-mediated endocytosis in brain cells. The uptake of transferrin-bound Fe by neurons and glial cells is probably regulated by the number of transferrin receptors present on cells, which changes during development and in conditions with an altered iron status.6. This review focuses on the information available on the functions of transferrin and transferrin receptor with respect to Fe transport across the blood–brain and blood–CSF barriers and the cell membranes of neurons and glial cells.     

Amyloid‐β concentration and structure influences the transport and immunomodulatory effects of IVIG

Intravenous immunoglobulin (IVIG) contains anti‐amyloid‐β antibodies as well as antibodies providing immunomodulatory effects that may modify chronic inflammation in Alzheimer's disease. Answers to important questions about IVIG transport into the central nervous system and assessments of any impact amyloid‐β has on this transport can be provided by in vitro models of the blood–brain barrier. In this study, amyloid‐β_[1‐42] was pre‐aggregated into fibrillar or oligomeric structures, and various concentrations were incubated in the brain side of the blood–brain barrier model, followed by IVIG administration in the blood side at the therapeutically relevant concentrations of 5 and 20 mg/mL. IVIG accumulated in the brain side at physiologically relevant levels, with amyloid‐β pre‐incubation increasing IVIG accumulation. The increased transport effect was dependent on amyloid‐β structural form, amyloid‐β concentration, and IVIG dose. IVIG was found to decrease monocyte chemotactic protein‐1 levels 6.5–18% when low amyloid‐β levels were present and increase levels 4.2–23% when high amyloid‐β levels were present. Therefore, the presence, concentration, and structure of amyloid‐β plays an important role in the effect of IVIG therapy in the brain.

     

Platelet activating factor induces transient blood‐brain barrier opening to facilitate edaravone penetration into the brain

The blood–brain barrier (BBB) greatly limits the efficacy of many neuroprotective drugs' delivery to the brain, so improving drug penetration through the BBB has been an important focus of research. Here we report that platelet activating factor (PAF) transiently opened BBB and facilitated neuroprotectant edaravone penetration into the brain. Intravenous infusion with PAF induced a transient BBB opening in rats, reflected by increased Evans blue leakage and mild edema formation, which ceased within 6 h. Furthermore, rat regional cerebral blood flow (rCBF) declined acutely during PAF infusion, but recovered slowly. More importantly, this transient BBB opening significantly increased the penetration of edaravone into the brain, evidenced by increased edaravone concentrations in tissue interstitial fluid collected by microdialysis and analyzed by Ultra‐performance liquid chromatograph combined with a hybrid quadrupole time‐of‐flight mass spectrometer (UPLC‐MS/MS). Similarly, incubation of rat brain microvessel endothelial cells monolayer with 1 μM PAF for 1 h significantly increased monolayer permeability to ¹²⁵I‐albumin, which recovered 1 h after PAF elimination. However, PAF incubation with rat brain microvessel endothelial cells for 1 h did not cause detectable cytotoxicity, and did not regulate intercellular adhesion molecule‐1, matrix‐metalloproteinase‐9 and P‐glycoprotein expression. In conclusion, PAF could induce transient and reversible BBB opening through abrupt rCBF decline, which significantly improved edaravone penetration into the brain.

     

Frequent blood-brain barrier disruption in the human cerebral cortex

1. The blood–brain barrier (BBB) protects the brain from circulating xenobiotic agents. The pathophysiology, time span, spatial pattern, and pathophysiological consequences of BBB disruptions are not known.2. Here, we report the quantification of BBB disruption by measuring enhancement levels in computerized tomography brain images.3. Pathological diffuse enhancement associated with elevated albumin levels in the cerebrospinal fluid (CSF) was observed in the cerebral cortex of 28 out of 43 patients, but not in controls. Four patients displayed weeks-long focal BBB impairment. In 19 other patients, BBB disruption was significantly associated with elevated blood pressure, body temperature, serum cortisol, and stress-associated CSF readthrough acetylcholinesterase. Multielectrode electroencephalography revealed enhanced slow-wave activities in areas of focal BBB disruption. Thus, quantification of BBB disruption using minimally invasive procedures, demonstrated correlations with molecular, clinical, and physiological stress-associated indices.4. These sequelae accompany a wide range of neurological disorders, suggesting that persistent, detrimental BBB disruption is considerably more frequent than previously assumed.     

Neural Induction of the Blood–Brain Barrier: Still an Enigma

1. The study of the blood–brain barrier and its various realms offers a myriad of opportunities for scientific exploration. This review focuses on two of these areas in particular: the induction of the blood–brain barrier and the molecular mechanisms underlying this developmental process.2. The creation of the blood–brain barrier is considered a specific step in the differentiation of cerebral capillary endothelial cells, resulting in a number of biochemical and functional alterations. Although the specific endothelial properties which maintain the homeostasis in the central nervous system necessary for neuronal function have been well described, the inductive mechanisms which trigger blood–brain barrier establishment in capillary endothelial cells are unknown.3. The timetable of blood–brain barrier formation is still a matter of debate, caused largely by the use of varying experimental systems and by the general difficulty of quantitatively measuring the degree of blood–brain barrier tightness. However, there is a general consensus that a gradual formation of the blood–brain barrier starts shortly after intraneural neovascularization and that the neural microenvironment (neurons and/or astrocytes) plays a key role in inducing blood–brain barrier function in capillary endothelial cells. This view stems from numerous in vitro experiments using mostly cocultures of capillary endothelial cells and astrocytes and assays for easily measurable blood–brain barrier markers. In vivo, there are great difficulties in proving the inductive influence of the neuronal environment. Also dealt with in this article are brain tumors, the least understood in vivo systems, and the induction or noninduction of barrier function in the newly established tumor vascularization.4. Finally, this review tries to elucidate the question concerning the nature of the inductive signal eliciting blood–brain barrier formation in the cerebral microvasculature.     

Exogenous arachidonic acid mediates permeability of human brain microvessel endothelial cells through prostaglandin E2 activation of EP3 and EP4 receptors

The blood–brain barrier, formed by microvessel endothelial cells, is the restrictive barrier between the brain parenchyma and the circulating blood. Arachidonic acid (ARA; 5,8,11,14‐cis‐eicosatetraenoic acid) is a conditionally essential polyunsaturated fatty acid [20:4(n ? 6)] and is a major constituent of brain lipids. The current study examined the transport processes for ARA in confluent monolayers of human brain microvascular endothelial cells (HBMEC). Addition of radioactive ARA to the apical compartment of HBMEC cultured on Transwell^® inserts resulted in rapid incorporation of radioactivity into the basolateral medium. Knock down of fatty acid transport proteins did not alter ARA passage into the basolateral medium as a result of the rapid generation of prostaglandin E₂ (PGE₂), an eicosanoid known to facilitate opening of the blood–brain barrier. Permeability following ARA or PGE₂ exposure was confirmed by an increased movement of fluorescein‐labeled dextran from apical to basolateral medium. ARA‐mediated permeability was attenuated by specific cyclooxygenase‐2 inhibitors. EP₃ and EP₄ receptor antagonists attenuated the ARA‐mediated permeability of HBMEC. The results indicate that ARA increases permeability of HBMEC monolayers likely via increased production of PGE₂ which acts upon EP₃ and EP₄ receptors to mediate permeability. These observations may explain the rapid influx of ARA into the brain previously observed upon plasma infusion with ARA.

     

The Nature and Composition of the Internal Environment of the Developing Brain

1. The fetal brain develops within its own environment, which is protected from free exchange of most molecules among its extracellular fluid, blood plasma, and cerebrospinal fluid (CSF) by a set of mechanisms described collectively as brain barriers.2. There are high concentrations of proteins in fetal CSF, which are due not to immaturity of the blood–CSF barrier (tight junctions between the epithelial cells of the choroid plexus), but to a specialized transcellular mechanism that specifically transfers some proteins across choroid plexus epithelial cells in the immature brain.3. The proteins in CSF are excluded from the extracellular fluid of the immature brain by the presence of barriers at the CSF–brain interfaces on the inner and outer surfaces of the immature brain. These barriers are not present in the adult.4. Some plasma proteins are present within the cells of the developing brain. Their presence may be explained by a combination of specific uptake from the CSF and synthesis in situ. 5. Information about the composition of the CSF (electrolytes as well as proteins) in the developing brain is of importance for the culture conditions used for experiments with fetal brain tissue in vitro, as neurons in the developing brain are exposed to relatively high concentrations of proteins only when they have cell surface membrane contact with CSF.6. The developmental importance of high protein concentrations in CSF of the immature brain is not understood but may be involved in providing the physical force (colloid osmotic pressure) for expansion of the cerebral ventricles during brain development, as well as possibly having nutritive and specific cell development functions.     

Human Cerebral Microvessel Endothelial Cell Culture as a Model System to Study the Blood–Brain Interface in Ischemic/Hypoxic Conditions

Summary 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 property 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.This paper is dedicated to the memory of F. Joó, the good friend and pioneer in endothelial cell research.This revised article was published online in May 2005 with a February 2005 cover date.     

CHO cell expression,long‐term stability,and primate pharmacokinetics and brain uptake of an IgG–paroxonase‐1 fusion protein

Paraoxonase (PON)‐1 is the most potent human organophosphatase known, but recombinant forms of human PON1 have been difficult to produce owing to poor secretion by host cells. In the present investigation, human PON1 is re‐engineered as an IgG–PON1 fusion protein. The 355 amino acid human PON1 is fused to the carboxyl terminus of the heavy chain of a chimeric monoclonal antibody (MAb) against the human insulin receptor (HIR), and this fusion protein is designated HIRMAb–PON1. The HIRMAb part of the fusion protein enables brain penetration of the PON1, which was considered important, because organophosphate toxicity causes death via a central nervous system site of action. A high producing line of stably transfected Chinese hamster ovary (CHO) cells secreting the HIRMAb–PON1 fusion protein in the absence of serum or lipid acceptors was cloned. The bioreactor generated fusion protein was purified to homogeneity with low impurities by protein A affinity chromatography and anion exchange chromatography. The HIRMAb–PON1 fusion protein was stable as a sterile liquid formulation stored at 4°C for at least 1 year. The plasma pharmacokinetics (PK) of the HIRMAb–PON1 fusion protein was evaluated in Rhesus monkeys, which is the first PK evaluation of a recombinant PON1 protein. The fusion protein was rapidly removed from blood, primarily by the liver. The blood–brain barrier permeation of the HIRMAb–PON1 fusion protein was high and comparable to other HIRMAb fusion proteins. Re‐engineering human PON1 as the HIRMAb fusion protein allows for production of a stable, field‐deployable formulation of the enzyme that is brain‐penetrating. Biotechnol. Bioeng. 2011; 108:186–196. © 2010 Wiley Periodicals, Inc.     

Endothelial Vesicles in the Blood–Brain Barrier: Are They Related to Permeability?

1. Macromolecules cross capillary walls via large vascular pores that are thought to be formed by plasmalemmal vesicles. Early hypotheses suggested that vesicles transferred plasma constituents across the endothelial wall either by a shuttle mechanism or by fusing to form transient patent channels for diffusion. Recent evidence shows that the transcytotic pathway involves both movement of vesicles within the cell and a series of fusions and fissions of the vesicular and cellular membranes.2. The transfer of macromolecules across the capillary wall is highly specific and is mediated by receptors incorporated into specific membrane domains. Therefore, despite their morphological similarity, endothelial vesicles form heterogeneous populations in which the predominant receptor proteins incorporated in their membranes define the functions of individual vesicles.3. Blood–brain barrier capillaries have very low permeabilities to most hydrophilic molecules. Their low permeability to macromolecules has been presumed to be due to an inhibition of the transcytotic mechanism, resulting in a low density of endothelial vesicles.4. A comparison of vesicular densities and protein permeabilities in a number of vascular beds shows only a very weak correlation, therefore vesicle numbers alone cannot be used to predict permeability to macromolecules.5. Blood–brain barrier capillaries are fully capable of transcytosing specific proteins, for example, insulin and transferrin, although the details are still somewhat controversial.6. It has recently been shown that the albumin binding protein gp60 (also known as albondin), which facilitates the transcytosis of native albumin in other vascular beds, is virtually absent in brain capillaries.7. It seems likely that the low blood–brain barrier permeability to macromolecules may be due to a low level of expression of specific receptors, rather than to an inhibition of the transcytosis mechanism.     

Mitochondrial diseases

By convention, the term "mitochondrial diseases" refers to disorders of the mitochondrial respiratory chain, which is the only metabolic pathway in the cell that is under the dual control of the mitochondrial genome (mtDNA) and the nuclear genome (nDNA). Therefore, a genetic classification of the mitochondrial diseases distinguishes disorders due to mutations in mtDNA, which are governed by the relatively lax rules of mitochondrial genetics, and disorders due to mutations in nDNA, which are governed by the stricter rules of mendelian genetics. Mutations in mtDNA can be divided into those that impair mitochondrial protein synthesis in toto and those that affect any one of the 13 respiratory chain subunits encoded by mtDNA. Essential clinical features for each group of diseases are reviewed. Disorders due to mutations in nDNA are more abundant not only because most respiratory chain subunits are nucleus-encoded but also because correct assembly and functioning of the respiratory chain require numerous steps, all of which are under the control of nDNA. These steps (and related diseases) include: (i) synthesis of assembly proteins; (ii) intergenomic signaling; (iii) mitochondrial importation of nDNA-encoded proteins; (iv) synthesis of inner mitochondrial membrane phospholipids; (v) mitochondrial motility and fission.     

Laminins and their receptors in the CNS

Laminin, an extracellular matrix protein, is widely expressed in the central nervous system (CNS). By interacting with integrin and non‐integrin receptors, laminin exerts a large variety of important functions in the CNS in both physiological and pathological conditions. Due to the existence of many laminin isoforms and their differential expression in various cell types in the CNS, the exact functions of each individual laminin molecule in CNS development and homeostasis remain largely unclear. In this review, we first briefly introduce the structure and biochemistry of laminins and their receptors. Next, the dynamic expression of laminins and their receptors in the CNS during both development and in adulthood is summarized in a cell‐type‐specific manner, which allows appreciation of their functional redundancy/compensation. Furthermore, we discuss the biological functions of laminins and their receptors in CNS development, blood–brain barrier (BBB) maintenance, neurodegeneration, stroke, and neuroinflammation. Last, key challenges and potential future research directions are summarized and discussed. Our goals are to provide a synthetic review to stimulate future studies and promote the formation of new ideas/hypotheses and new lines of research in this field.     

Therapeutic effect of peripheral administration of an anti‐prion protein antibody on mice infected with prions

It is generally thought that effective treatments for prion diseases need to inhibit prion propagation, protect neuronal tissues and promote functional recovery of degenerated nerve tissues. In addition, such treatments should be effective even when given after clinical onset of the disease and administered via a peripheral route. In this study, the effect of peripheral administration of an anti‐PrP antibody on disease progression in prion‐infected mice was examined. mAb 31C6 was administered via the tail veins of prion‐infected mice at the time of clinical onset (120 days post‐inoculation with the Chandler prion strain) and the distribution of this mAb in the brain and its effect on mouse survival assessed. The antibody was distributed to the cerebellums and thalami of the infected mice and more than half these mice survived longer than mice that had been given a negative control mAb. The level of PrP^Sc in the mAb 31C6‐treated mice was lower than that in mice treated with the negative control mAb and progression of neuropathological lesions in the cerebellum, where the mAb 31C6 was well distributed, appeared to be mitigated. These results suggest that administration of an anti‐PrP mAb through a peripheral route is a candidate for the treatment of prion diseases.     

Comparative proteomics as a new tool for exploring human mitochondrial tRNA disorders.

More than 70 different point mutations in human mitochondrial tRNA genes are correlated with severe disorders, including fatal cardiopathies, encephalopathies, myopathies, and others. So far, investigation of the molecular impact(s) of mutations has focused on the affected tRNA itself by seeking structural and/or functional perturbations capable of interfering with synthesis of the 13 mitochondrion-encoded subunits of respiratory chain complexes. Here, a proteomic approach was used to investigate whether such mutations would affect the pattern of mitochondrial proteins at a broader level. Analysis of several hundred mitochondrial proteins from sibling cybrid cell lines by two-dimensional electrophoresis, an approach that takes into account all regulatory steps of mitochondrial and nuclear gene expression, indeed reveals a number of up- and downregulated proteins when healthy and single-point-mutation-carrying mitochondria representative of either MELAS or MERRF syndrome were compared. Assignment by mass spectrometry of the two proteins which exhibit obvious large quantitative decreases in the levels of both pathologic mitochondria identified nuclear-encoded subunits of cytochrome c oxidase, a respiratory chain complex. This clearly shows a linkage between the effects of mutations in mitochondrial tRNA genes and the steady-state level of nuclear-encoded proteins in mitochondria. It opens new routes toward a large-scale exploration of potential proteic partners involved in the genotype-phenotype correlation of mitochondrial disorders.     

线粒体呼吸链膜蛋白复合体的结构

线粒体作为真核细胞的重要“能量工厂”,是细胞进行呼吸作用的场所,呼吸作用包括柠檬酸循环和氧化磷酸化两个过程,其中氧化磷酸化过程的电子传递链（又称线粒体呼吸链）位于线粒体内膜上,由四个相对分子质量很大的跨膜蛋白复合体（Ⅰ、Ⅱ、Ⅲ、和Ⅳ）、介于Ⅰ／Ⅱ与Ⅲ之间的泛醌以及介于Ⅲ与Ⅳ之间的细胞色素c共同组成。线粒体呼吸链的功能是进行生物氧化,并与称之为复合物V的ATP合成酶（磷酸化过程）相偶联,共同完成氧化磷酸化过程,并生产能量分子ATP。线粒体呼吸链的结构生物学研究对于彻底了解电子传递和能量转化的机理是至关重要的,本文分别论述线粒体呼吸链复合体Ⅰ、Ⅱ、Ⅲ和Ⅳ的结构,并跟踪线粒体呼吸链超复合体的结构研究进展。     

Measurement of blood-brain barrier permeation in rats during exposure to 2450-MHz microwaves

Adult rats anesthesized with pentobarbital and injected intravenously with a mixture of [¹⁴C]sucrose and [³H]inulin were exposed for 30 min to an environment at an ambient temperature of 22, 30, or 40 °C, or were exposed at 22 °C to 2450-MHz CW microwave radiation at power densities of 0, 10, 20, or 30 mW/cm². Following exposure, the brain was perfused and sectioned into eight regions, and the radioactivity in each region was counted. The data were analyzed by two methods. First, the data for each of the eight regions and for each of the two radioactive tracers were analyzed by regression analysis for a total of 16 analyses and Bonferroni's Inequality was applied to prevent false positive results from numerous analyses. By this conservative test, no statistically significant increase in permeation was found for either tracer in any brain region of rats exposed to microwaves. Second, a profile analysis was used to test for a general change in tracer uptake across all brain regions. Using this statistical method, a significant increase in permeation was found for sucrose but not for inulin. A correction factor was then derived from the warm-air experiments to correct for the increase in permeation of the brain associated with change in body temperature. This correction factor was applied to the data for the irradiated animals. After correcting the data for thermal effects of the microwave radiation, no significant increase in permeation was found.     

Tight Junctions of the Blood–Brain Barrier

1. The blood–brain barrier is essential for the maintainance and regulation of the neural microenvironment. The blood–brain barrier endothelial cells comprise an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. The latter is realized by the tight junctions between the endothelial cells of the brain microvasculature, which are subject of this review. Morphologically, blood–brain barrier-tight junctions are more similar to epithelial tight junctions than to endothelial tight junctions in peripheral blood vessels.2. Although blood–brain barrier-tight junctions share many characteristics with epithelial tight junctions, there are also essential differences. However, in contrast to tight junctions in epithelial systems, structural and functional characteristics of tight junctions in endothelial cells are highly sensitive to ambient factors.3. Many ubiquitous molecular constituents of tight junctions have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin, and 7H6. Signaling pathways involved in tight junction regulation comprise, among others, G-proteins, serine, threonine, and tyrosine kinases, extra- and intracellular calcium levels, cAMP levels, proteases, and TNF. Common to most of these pathways is the modulation of cytoskeletal elements which may define blood–brain barrier characteristics. Additionally, cross-talk between components of the tight junction– and the cadherin–catenin system suggests a close functional interdependence of the two cell–cell contact systems.4. Recent studies were able to elucidate crucial aspects of the molecular basis of tight junction regulation. An integration of new results into previous morphological work is the central intention of this review.