Structural mechanisms of acute VEGF effect on microvessel permeability

Vascular Effects on Cerebrovascular Permeability and Neurodegeneration

Neurons and glial cells in the brain are protected by the blood brain barrier (BBB). The local regulation of blood flow is determined by neurons and signal conducting cells called astrocytes. Although alterations in neurons and glial cells affect the function of neurons, the majority of effects are coming from other cells and organs of the body. Although it seems obvious that effects beginning in brain vasculature would play an important role in the development of various neuroinflammatory and neurodegenerative pathologies, significant interest has only been directed to the possible mechanisms involved in the development of vascular cognitive impairment and dementia (VCID) for the last decade. Presently, the National Institute of Neurological Disorders and Stroke applies considerable attention toward research related to VCID and vascular impairments during Alzheimer's disease. Thus, any changes in cerebral vessels, such as in blood flow, thrombogenesis, permeability, or others, which affect the proper vasculo-neuronal connection and interaction and result in neuronal degeneration that leads to memory decline should be considered as a subject of investigation under the VCID category. Out of several vascular effects that can trigger neurodegeneration, changes in cerebrovascular permeability seem to result in the most devastating effects. The present review emphasizes the importance of changes in the BBB and possible mechanisms primarily involving fibrinogen in the development and/or progression of neuroinflammatory and neurodegenerative diseases resulting in memory decline.

Ultra-Wide-Field Fluorescein Angiography Assessment of Non-Perfusion in Patients with Diabetic Retinopathy Treated with Anti-Vascular Endothelial Growth Factor Therapy

Purpose: To follow the evolution of peripheral ischemia by fluorescein angiography (FA) on ultra-wide-field (UWF) images in diabetic patients treated with anti-vascular endothelial growth factor (anti-VEGF) for macular edema. Methods: Prospective, non-interventional cohort study analyzing UWF-FA images of 48 patients with diabetic retinopathy (48 eyes) treated for diabetic macular edema. UWF-FA was performed at baseline and after one year of anti-VEGF therapy (M12). The primary endpoint was the change in the non-perfusion index. Results: Of the 48 patients included in this study, 25 completed the one-year follow-up, and 20 had FA images of sufficient quality to be interpreted. The non-perfusion index did not significantly change from baseline after one year of anti-VEGF treatment (0.7% of the non-perfused area at baseline versus 0.5% at M12; p = 0.29). In contrast, the diabetic retinopathy severity score improved significantly between baseline and M12. Conclusions: Anti-VEGF treatment with aflibercept for diabetic macular edema had no impact on the retinal perfusion assessed by FA, but it allowed for artificially improving diabetic retinopathy severity scores.

Direct Current Stimulation Disrupts Endothelial Glycocalyx and Tight Junctions of the Blood-Brain Barrier in vitro

Transcranial direct current stimulation (tDCS) is a non-invasive physical therapy to treat many psychiatric disorders and to enhance memory and cognition in healthy individuals. Our recent studies showed that tDCS with the proper dosage and duration can transiently enhance the permeability (P) of the blood-brain barrier (BBB) in rat brain to various sized solutes. Based on the in vivo permeability data, a transport model for the paracellular pathway of the BBB also predicted that tDCS can transiently disrupt the endothelial glycocalyx (EG) and the tight junction between endothelial cells. To confirm these predictions and to investigate the structural mechanisms by which tDCS modulates P of the BBB, we directly quantified the EG and tight junctions of in vitro BBB models after DCS treatment. Human cerebral microvascular endothelial cells (hCMECs) and mouse brain microvascular endothelial cells (bEnd3) were cultured on the Transwell filter with 3 μm pores to generate in vitro BBBs. After confluence, 0.1–1 mA/cm² DCS was applied for 5 and 10 min. TEER and P to dextran-70k of the in vitro BBB were measured, HS (heparan sulfate) and hyaluronic acid (HA) of EG was immuno-stained and quantified, as well as the tight junction ZO-1. We found disrupted EG and ZO-1 when P to dextran-70k was increased and TEER was decreased by the DCS. To further investigate the cellular signaling mechanism of DCS on the BBB permeability, we pretreated the in vitro BBB with a nitric oxide synthase (NOS) inhibitor, L-NMMA. L-NMMA diminished the effect of DCS on the BBB permeability by protecting the EG and reinforcing tight junctions. These in vitro results conform to the in vivo observations and confirm the model prediction that DCS can disrupt the EG and tight junction of the BBB. Nevertheless, the in vivo effects of DCS are transient which backup its safety in the clinical application. In conclusion, our current study directly elucidates the structural and signaling mechanisms by which DCS modulates the BBB permeability.

The Effects of Fibrinogen's Interactions with Its Neuronal Receptors, Intercellular Adhesion Molecule-1 and Cellular Prion Protein

Neuroinflammatory diseases, such as Alzheimer's disease (AD) and traumatic brain injury (TBI), are associated with the extravascular deposition of the fibrinogen (Fg) derivative fibrin and are accompanied with memory impairment. We found that during the hyperfibrinogenemia that typically occurs during AD and TBI, extravasated Fg was associated with amyloid beta and astrocytic cellular prion protein (PrPC). These effects coincided with short-term memory (STM) reduction and neurodegeneration. However, the mechanisms of a direct Fg-neuron interaction and its functional role in neurodegeneration are still unclear. Cultured mouse brain neurons were treated with Fg in the presence or absence of function-blockers of its receptors, PrPC or intercellular adhesion molecule-1 (ICAM-1). Associations of Fg with neuronal PrPC and ICAM-1 were characterized. The expression of proinflammatory marker interleukin 6 (IL-6) and the generation of reactive oxygen species (ROS), mitochondrial superoxide, and nitrite in neurons were assessed. Fg-induced neuronal death was also evaluated. A strong association of Fg with neuronal PrPC and ICAM-1, accompanied with overexpression of IL-6 and enhanced generation of ROS, mitochondrial superoxide, and nitrite as well as the resulting neuronal death, was found. These effects were reduced by blocking the function of neuronal PrPC and ICAM-1, suggesting that the direct interaction of Fg with its neuronal receptors can induce overexpression of IL-6 and increase the generation of ROS, nitrite, and mitochondrial superoxide, ultimately leading to neuronal death. These effects can be a mechanism of neurodegeneration and the resultant memory reduction seen during TBI and AD.

Combined Influence of Nutrient Supply Level and Tissue Mechanical Properties on Benign Tumor Growth as Revealed by Mathematical Modeling

A continuous mathematical model of non-invasive avascular tumor growth in tissue is presented. The model considers tissue as a biphasic material, comprised of a solid matrix and interstitial fluid. The convective motion of tissue elements happens due to the gradients of stress, which change as a result of tumor cells proliferation and death. The model accounts for glucose as the crucial nutrient, supplied from the normal tissue, and can reproduce both diffusion-limited and stress-limited tumor growth. Approximate tumor growth curves are obtained semi-analytically in the limit of infinite tissue hydraulic conductivity, which implies instantaneous equalization of arising stress gradients. These growth curves correspond well to the numerical solutions and represent classical sigmoidal curves with a short initial exponential phase, subsequent almost linear growth phase and a phase with growth deceleration, in which tumor tends to reach its maximum volume. The influence of two model parameters on tumor growth curves is investigated: tissue hydraulic conductivity, which links the values of stress gradient and convective velocity of tissue phases, and tumor nutrient supply level, which corresponds to different permeability and surface area density of capillaries in the normal tissue that surrounds the tumor. In particular, it is demonstrated, that sufficiently low tissue hydraulic conductivity (intrinsic, e.g., to tumors arising from connective tissue) and sufficiently high nutrient supply can lead to formation of giant benign tumors, reaching tens of centimeters in diameter, which are indeed observed clinically.

Primaquine Diphosphate, a Known Antimalarial Drug, Blocks Vascular Leakage Acting Through Junction Stabilization

Endothelial barrier integrity is important for vascular homeostasis, and hyperpermeability participates in the progression of many pathological states, such as diabetic retinopathy, ischemic stroke, chronic bowel disease, and inflammatory disease. Here, using drug repositioning, we discovered that primaquine diphosphate (PD), previously known as an antimalarial drug, was a potential blocker of vascular leakage. PD inhibited the linear pattern of vascular endothelial growth factors (VEGF)-induced disruption at the cell boundaries, blocked the formation of VEGF-induced actin stress fibers, and stabilized the cortactin actin rings in endothelial cells. PD significantly reduced leakage in the Miles assay and mouse model of streptozotocin (STZ)-induced diabetic retinopathy. Targeted prediction programs and deubiquitinating enzyme activity assays identified a potential mechanism of action for PD and demonstrated that this operates via ubiquitin specific protease 1 (USP1). USP1 inhibition demonstrated a conserved barrier function by inhibiting VEGF-induced leakage in endothelial permeability assays. Taken together, these findings suggest that PD could be used as a novel drug for vascular leakage by maintaining endothelial integrity.

Modulation of solute diffusivity in brain tissue as a novel mechanism of transcranial direct current stimulation (tDCS)

The breadth of brain disorders and functions reported responsive to transcranial direct current stimulation (tDCS) suggests a generalizable mechanism of action. Prior efforts characterized its cellular targets including neuron, glia and endothelial cells. We propose tDCS also modulates the substance transport in brain tissue. High resolution multiphoton microscopy imaged the spread across rat brain tissue of fluorescently-labeled solutes injected through the carotid artery after tDCS. The effective solute diffusion coefficient of brain tissue (D_eff) was determined from the spatio-temporal solute concentration profiles using an unsteady diffusion transport model. 5–10 min post 20 min–1 mA tDCS, D_eff increased by ~ 10% for a small solute, sodium fluorescein, and ~ 120% for larger solutes, BSA and Dex-70k. All increases in D_eff returned to the control level 25–30 min post tDCS. A mathematical model for D_eff in the extracelluar space (ECS) further predicts that this dose of tDCS increases D_eff by transiently enhancing the brain ECS gap spacing by ~ 1.5-fold and accordingly reducing the extracellular matrix density. The cascades leading ECS modulation and its impact on excitability, synaptic function, plasticity, and brain clearance require further study. Modulation of solute diffusivity and ECS could explain diverse outcomes of tDCS and suggest novel therapeutic strategies.

Mathematical Modeling Shows That the Response of a Solid Tumor to Antiangiogenic Therapy Depends on the Type of Growth

It has been hypothesized that solid tumors with invasive type of growth should possess intrinsic resistance to antiangiogenic therapy, which is aimed at cessation of the formation of new blood vessels and subsequent shortage of nutrient inflow to the tumor. In order to investigate this effect, a continuous mathematical model of tumor growth is developed, which considers variables of tumor cells, necrotic tissue, capillaries, and glucose as the crucial nutrient. The model accounts for the intrinsic motility of tumor cells and for the convective motion, arising due to their proliferation, thus allowing considering two types of tumor growth—invasive and compact—as well as their combination. Analytical estimations of tumor growth speed are obtained for compact and invasive tumors. They suggest that antiangiogenic therapy may provide a several times decrease of compact tumor growth speed, but the decrease of growth speed for invasive tumors should be only modest. These estimations are confirmed by numerical simulations, which further allow evaluating the effect of antiangiogenic therapy on tumors with mixed growth type and highlight the non-additive character of the two types of growth.

In vitro and numerical simulation of blood removal from cerebrospinal fluid: comparison of lumbar drain to Neurapheresis therapy

BACKGROUND

Blood removal from cerebrospinal fluid (CSF) in post-subarachnoid hemorrhage patients may reduce the risk of related secondary brain injury. We formulated a computational fluid dynamics (CFD) model to investigate the impact of a dual-lumen catheter-based CSF filtration system, called Neurapheresis™ therapy, on blood removal from CSF compared to lumbar drain.

METHODS

A subject-specific multiphase CFD model of CSF system-wide solute transport was constructed based on MRI measurements. The Neurapheresis catheter geometry was added to the model within the spinal subarachnoid space (SAS). Neurapheresis flow aspiration and return rate was 2.0 and 1.8 mL/min, versus 0.2 mL/min drainage for lumbar drain. Blood was modeled as a bulk fluid phase within CSF with a 10% initial tracer concentration and identical viscosity and density as CSF. Subject-specific oscillatory CSF flow was applied at the model inlet. The dura and spinal cord geometry were considered to be stationary. Spatial-temporal tracer concentration was quantified based on time-average steady-streaming velocities throughout the domain under Neurapheresis therapy and lumbar drain. To help verify CFD results, an optically clear in vitro CSF model was constructed with fluorescein used as a blood surrogate. Quantitative comparison of numerical and in vitro results was performed by linear regression of spatial-temporal tracer concentration over 24-h.

RESULTS

After 24-h, tracer concentration was reduced to 4.9% under Neurapheresis therapy compared to 6.5% under lumbar drain. Tracer clearance was most rapid between the catheter aspiration and return ports. Neurapheresis therapy was found to have a greater impact on steady-streaming compared to lumbar drain. Steady-streaming in the cranial SAS was ~ 50× smaller than in the spinal SAS for both cases. CFD results were strongly correlated with the in vitro spatial-temporal tracer concentration under Neurapheresis therapy (R² = 0.89 with + 2.13% and - 1.93% tracer concentration confidence interval).

CONCLUSION

A subject-specific CFD model of CSF system-wide solute transport was used to investigate the impact of Neurapheresis therapy on tracer removal from CSF compared to lumbar drain over a 24-h period. Neurapheresis therapy was found to substantially increase tracer clearance compared to lumbar drain. The multiphase CFD results were verified by in vitro fluorescein tracer experiments.

Microvascular ion transport through endothelial glycocalyx layer: new mechanism and improved Starling principle

Ion transport through the endothelial glycocalyx layer is closely associated with many vascular diseases. Clarification of ion behaviors around the endothelial glycocalyx layer under varying circumstances will benefit pathologies related to cardiovascular and renal diseases. In this research, a series of large-scale molecular dynamics simulations are conducted to study the response of ion transport to the changing blood flow velocity and the shedding of endothelial glycocalyx sugar chains. Results indicate that blood flow promotes the outward Na+ transport from the near-membrane region to the lumen via the endothelial glycocalyx layer. Scrutiny of sugar-chain dynamics and their interactions with Na+ suggests that corner conformation of endothelial glycocalyx sugar chains confines the movement of the Na+, whereas stretching conformation facilitates the motion of Na+ ions. The flow impact on ion transport of Na+ is nonlinear. Based on the findings, the Starling principle and its revised version, which are prevailingly used to predict the ion transport of the endothelial glycocalyx layer, are further improved. An estimation based on the further revised Starling principle indicates that physiological flow changes the osmotic part of transendothelial water flux by 8% compared with the stationary situation.

NEW & NOTEWORTHY The biophysical roles of negatively charged oligosaccharides of the endothelial glycocalyx have gained increasing attention due to their importance in regulating microvascular fluid exchange. The Starling principle and its revisions are at the heart of the understanding of fluid homeostasis in the periphery. Here, the blood flow changes the conformations of glycocalyx sugar chains, thereby influencing availability of Na+ for transport. Based on the findings, the Starling principle and its revision are further improved.

Transient alleviation of tumor hypoxia during first days of antiangiogenic therapy as a result of therapy-induced alterations in nutrient supply and tumor metabolism - Analysis by mathematical modeling

A number of experiments on mouse tumor models, as well as certain clinical data, have demonstrated, that antiangiogenic therapy can lead to transient improvement in tumor oxygenation, that allows to increase efficiency of following radiotherapy. In the majority of works, this phenomenon has been explained by enhanced tumor perfusion due to normalization of capillaries' structure, that results in elevated oxygen inflow in tumor. However, changes in tumor perfusion often haven't been directly measured in relevant works, moreover, antiangiogenic therapy has been proven to have ambiguous effect on tumor perfusion both in mouse tumor models and in clinics. Herein, we suggest that elevation of blood perfusion may be not the only reason for transient alleviation of tumor hypoxia, and that it may manifest itself even under unchanged tumor blood flow. We propose that it may be as well caused by the decrease in tumor oxygen consumption rate (OCR) due to the reduction of tumor proliferation level, caused by nutrient shortage in result of antiangiogenic treatment. We provide detailed explanation of this hypothesis and visualize it using a specially developed mathematical model, which takes into account basic features of tumor growth and antiangiogenic therapy. We investigate the influence of the model parameters on oxygen dynamics; demonstrate, that transient alleviation of tumor hypoxia occurs in a fairly wide range of physiologically justified values of parameters; and point out the major factors, that determine oxygen dynamics during antiangiogenic therapy.

The importance of geometry in the corneal micropocket angiogenesis assay

The corneal micropocket angiogenesis assay is an experimental protocol for studying vessel network formation, or neovascularization, in vivo. The assay is attractive due to the ease with which the developing vessel network can be observed in the same animal over time. Measurements from the assay have been used in combination with mathematical modeling to gain insights into the mechanisms of angiogenesis. While previous modeling studies have adopted planar domains to represent the assay, the hemispherical shape of the cornea and asymmetric positioning of the angiogenic source can be seen to affect vascular patterning in experimental images. As such, we aim to better understand: i) how the geometry of the assay influences vessel network formation and ii) how to relate observations from planar domains to those in the hemispherical cornea. To do so, we develop a three-dimensional, off-lattice mathematical model of neovascularization in the cornea, using a spatially resolved representation of the assay for the first time. Relative to the detailed model, we predict that the adoption of planar geometries has a noticeable impact on vascular patterning, leading to increased vessel 'merging', or anastomosis, in particular when circular geometries are adopted. Significant differences in the dynamics of diffusible aniogenesis simulators are also predicted between different domains. In terms of comparing predictions across domains, the 'distance of the vascular front to the limbus' metric is found to have low sensitivity to domain choice, while metrics such as densities of tip cells and vessels and 'vascularized fraction' are sensitive to domain choice. Given the widespread adoption and attractive simplicity of planar tissue domains, both in silico and in vitro, the differences identified in the present study should prove useful in relating the results of previous and future theoretical studies of neovascularization to in vivo observations in the cornea.

Therapeutic angiogenesis by vascular endothelial growth factor supplementation for treatment of renal disease

PURPOSE OF REVIEW

Vascular endothelial growth factors (VEGFs) influence renal function through angiogenesis, with VEGF-A being the most potent inducer of vascular formation. In the normal glomerulus, tight homeostatic balance is maintained between the levels of VEGF-A isoforms produced by podocyte cells, and the VEGF receptors (VEGFRs) expressed by glomerular endothelial, mesangial, and podocyte cells. Renal disease occurs when this homeostatic balance is lost, manifesting in the abnormal autocrine and paracrine VEGF-A/VEGFR signaling, ultrastructural glomerular and tubular damage, and impaired filtration.

RECENT FINDINGS

Preclinical disease models of ischemic renal injury, including acute ischemia/reperfusion, thrombotic microangiopathy, and chronic renovascular disease, treated with exogenous VEGF supplementation demonstrated therapeutic efficacy. These results suggest a therapeutic VEGF-A paracrine effect on endothelial cells in the context of acute or chronic obstructive ischemia. Conversely, renal dysfunction in diabetic nephropathy appears to occur through an upregulated VEGF autocrine effect on podocyte cells, which is exacerbated by hyperglycemia. Therefore, VEGF supplementation therapy may be contraindicated for treatment of diabetic nephropathy, but specific results will depend on dose and on the specific site of VEGF delivery. A drug delivery system that demonstrates cell specificity for glomerular or peritubular capillaries could be employed to restore balance to VEGF-A/VEGFR2 signaling, and by doing so, prevent the progression to end-stage renal disease.

SUMMARY

The review discusses the preclinical data available for VEGF supplementation therapy in models of renal disease.

Reinforcing endothelial junctions prevents microvessel permeability increase and tumor cell adhesion in microvessels in vivo

Tumor cell adhesion to the microvessel wall is a critical step during tumor metastasis. Vascular endothelial growth factor (VEGF), a secretion of tumor cells, can increase microvessel permeability and tumor cell adhesion in the microvessel. To test the hypothesis that inhibiting permeability increase can reduce tumor cell adhesion, we used in vivo fluorescence microscopy to measure both microvessel permeability and adhesion rates of human mammary carcinoma MDA-MB-231 cells in post-capillary venules of rat mesentery under the treatment of VEGF and a cAMP analog, 8-bromo-cAMP, which can decrease microvessel permeability. By immunostaining adherens junction proteins between endothelial cells forming the microvessel wall, we further investigated the structural mechanism by which cAMP abolishes VEGF-induced increase in microvessel permeability and tumor cell adhesion. Our results demonstrate that 1) Pretreatment of microvessels with cAMP can abolish VEGF-enhanced microvessel permeability and tumor cell adhesion; 2) Tumor cells prefer to adhere to the endothelial cell junctions instead of cell bodies; 3) VEGF increases microvessel permeability and tumor cell adhesion by compromising endothelial junctions while cAMP abolishes these effects of VEGF by reinforcing the junctions. These results suggest that strengthening the microvessel wall integrity can be a potential approach to inhibiting hematogenous tumor metastasis.

Loss of the endothelial glycocalyx is associated with increased E-selectin mediated adhesion of lung tumour cells to the brain microvascular endothelium

Background

Arrest of metastasising lung cancer cells to the brain microvasculature maybe mediated by interactions between ligands on circulating tumour cells and endothelial E-selectin adhesion molecules; a process likely to be regulated by the endothelial glycocalyx. Using human cerebral microvascular endothelial cells and non-small cell lung cancer (NSCLC) cell lines, we describe how factors secreted by NSCLC cells i.e. cystatin C, cathepsin L, insulin-like growth factor-binding protein 7 (IGFBP7), vascular endothelial growth factor (VEGF) and tumour necrosis factor-alpha (TNF-α), damage the glycocalyx and enhance initial contacts between lung tumour and cerebral endothelial cells.

Methods

Endothelial cells were treated with tumour secreted-proteins or lung tumour conditioned medium (CM). Surface levels of E-selectin were quantified by ELISA. Adhesion of A549 and SK-MES-1 cells was examined under flow conditions (1 dyne/cm²). Alterations in the endothelial glycocalyx were quantified by binding of fluorescein isothiocyanate-linked wheat germ agglutinin (WGA-FITC).

Results

A549 and SK-MES-1 CM and secreted-proteins significantly enhanced endothelial surface E-selectin levels after 30 min and 4 h and tumour cell adhesion after 30 min, 4 and 24 h. Both coincided with significant glycocalyx degradation; A549 and SK-MES-1 CM removing 55 ± 12 % and 58 ± 18.7 % of WGA-FITC binding, respectively. Inhibition of E-selectin binding by monoclonal anti-E-selectin antibody completely attenuated tumour cell adhesion.

Conclusion

These data suggest that metastasising lung cancer cells facilitate their own adhesion to the brain endothelium by secreting factors that damage the endothelial glycocalyx, resulting in exposure of the previously shielded adhesion molecules and engagement of the E-selectin-mediated adhesion axis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-015-0223-9) contains supplementary material, which is available to authorized users.

Quantification of blood-brain barrier solute permeability and brain transport by multiphoton microscopy

Development of an optimal systemic drug delivery strategy to the brain will require noninvasive or minimally invasive methods to quantify the permeability of the cerebral microvessel wall or blood-brain barrier (BBB) to various therapeutic agents and to measure their transport in the brain tissue. To address this problem, we used laser-scanning multiphoton microscopy to determine BBB permeability to solutes (P) and effective solute diffusion coefficients (Deff) in rat brain tissue 100-250 μm below the pia mater. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a microgrinder. Sodium fluorescein, fluorescein isothiocyanate (FITC)-dextrans, or Alexa Fluor 488-immunoglobulin G (IgG) in 1% bovine serum albumin (BSA) mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery by a syringe pump at a constant rate of ∼3 ml/min. P and Deff were determined from the rate of tissue solute accumulation and the radial concentration gradient around individual microvessels in the brain tissue. The mean apparent permeability P values for sodium fluorescein (molecular weight (MW) 376 Da), dextran-4k, -20k, -40k, -70k, and IgG (MW ∼160 kDa) were 14.6, 6.2, 1.8, 1.4, 1.3, and 0.54 × 10-7 cm/s, respectively. These P values were not significantly different from those of rat pial microvessels for the same-sized solutes (Yuan et al., 2009, "Non-Invasive Measurement of Solute Permeability in Cerebral Microvessels of the Rat," Microvasc. Res., 77(2), pp. 166-73), except for the small solute sodium fluorescein, suggesting that pial microvessels can be a good model for studying BBB transport of relatively large solutes. The mean Deff values were 33.2, 4.4, 1.3, 0.89, 0.59, and 0.47 × 10-7 cm2/s, respectively, for sodium fluorescein, dextran-4k, -20k, -40k, -70k, and IgG. The corresponding mean ratio of Deff to the free diffusion coefficient Dfree, Deff/Dfree, were 0.46, 0.19, 0.12, 0.12, 0.11, and 0.11 for these solutes. While there is a significant difference in Deff/Dfree between small (e.g., sodium fluorescein) and larger solutes, there is no significant difference in Deff/Dfree between solutes with molecular weights from 20,000 to 160,000 Da, suggesting that the relative resistance of the brain tissue to macromolecular solutes is similar over a wide size range. The quantitative transport parameters measured from this study can be used to develop better strategies for brain drug delivery.

Quantification of transient increase of the blood-brain barrier permeability to macromolecules by optimized focused ultrasound combined with microbubbles

Radioimmunotherapy using a radiolabeled monoclonal antibody that targets tumor cells has been shown to be efficient for the treatment of many malignant cancers, with reduced side effects. However, the blood-brain barrier (BBB) inhibits the transport of intravenous antibodies to tumors in the brain. Recent studies have demonstrated that focused ultrasound (FUS) combined with microbubbles (MBs) is a promising method to transiently disrupt the BBB for the drug delivery to the central nervous system. To find the optimal FUS and MBs that can induce reversible increase in the BBB permeability, we employed minimally invasive multiphoton microscopy to quantify the BBB permeability to dextran-155 kDa with similar molecular weight to an antibody by applying different doses of FUS in the presence of MBs with an optimal size and concentration. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a micro-grinder. About 5 minutes after applying the FUS on the thinned skull in the presence of MBs for 1 minute, TRITC (tetramethylrhodamine isothiocyanate)-dextran-155 kDa in 1% bovine serum albumin in mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery by a syringe pump. Simultaneously, the temporal images were collected from the brain parenchyma ~100-200 μm below the pia mater. Permeability was determined from the rate of tissue solute accumulation around individual microvessels. After several trials, we found the optimal dose of FUS. At the optimal dose, permeability increased by ~14-fold after 5 minutes post-FUS, and permeability returned to the control level after 25 minutes. FUS without MBs or MBs injected without FUS did not change the permeability. Our method provides an accurate in vivo assessment for the transient BBB permeability change under the treatment of FUS. The optimal FUS dose found for the reversible BBB permeability increase without BBB disruption is reliable and can be applied to future clinical trials.

Temporal effects of vascular endothelial growth factor and 3,5-cyclic monophosphate on blood-brain barrier solute permeability in vivo

To test the hypothesis that vascular endothelial growth factor (VEGF) can transiently increase the blood-brain barrier permeability, P, as for peripheral microvessels and that the elevation of 3,5-cyclic monophosphate (cAMP) levels can inhibit the VEGF-induced acute hyperpermeability, we employed multiphoton microscopy to quantify the cerebral microvessel permeability P to various-sized solutes under VEGF and cAMP treatments. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a microgrinder. Fluorescein (MW 376Da), fluorescein isothioyanate-dextran-20k (FITC-Dex-20k), FITC-Dex-70k, or Alexa Fluor 488-IgG in 1% bovine serum albumin mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery with a syringe pump. Simultaneously, temporal images were collected from the brain parenchyma ∼100-200 μm below the pia mater. P was determined from the rate of tissue solute accumulation around individual microvessels. Exposure to 1 nM VEGF transiently increased P to 2.2, 10.5, 9.8, and 12.8 times control values, for fluorescein, Dex-20k, Dex-70k, and IgG, respectively, within 30 sec, and all returned to control levels within 2 min. After 20 min of pretreatment with 2 mM of the cAMP analog 8-bromo-cAMP, the initial increase by 1 nM VEGF was completely abolished in P of all solutes. The response pattern of P to VEGF and cAMP and the ratios of the peak to control values for rat cerebral microvessels are similar to those for rat mesenteric (peripheral) microvessels, except that the ratios are higher in P of cerebral microvessels for the intermediate and large solutes. These results imply a new approach for delivering large therapeutic agents to the brain.

Glycosaminoglycan regulation by VEGFA and VEGFC of the glomerular microvascular endothelial cell glycocalyx in vitro

Damage to endothelial glycocalyx impairs vascular barrier function and may contribute to progression of chronic vascular disease. An early indicator is microalbuminuria resulting from glomerular filtration barrier damage. We investigated the contributions of hyaluronic acid (HA) and chondroitin sulfate (CS) to glomerular microvascular endothelial cell (GEnC) glycocalyx and examined whether these are modified by vascular endothelial growth factors A and C (VEGFA and VEGFC). HA and CS were imaged on GEnCs and their resynthesis was examined. The effect of HA and CS on transendothelial electrical resistance (TEER) and labeled albumin flux across monolayers was assessed. Effects of VEGFA and VEGFC on production and charge characteristics of glycosaminoglycan (GAG) were examined via metabolic labeling and liquid chromatography. GAG shedding was quantified using Alcian Blue. NDST2 expression was examined using real-time PCR. GEnCs expressed HA and CS in the glycocalyx. CS contributed to the barrier to both ion (TEER) and protein flux across the monolayer; HA had only a limited effect. VEGFC promoted HA synthesis and increased the charge density of synthesized GAGs. In contrast, VEGFA induced shedding of charged GAGs. CS plays a role in restriction of macromolecular flux across GEnC monolayers, and VEGFA and VEGFC differentially regulate synthesis, charge, and shedding of GAGs in GEnCs. These observations have important implications for endothelial barrier regulation in glomerular and other microvascular beds.

Aquaporin-4 upregulated expression in glioma tissue is a reaction to glioma-associated edema induced by vascular endothelial growth factor

Glioma-associated edema contributes significantly to morbidity and death in patients with glioma. It has been suggested that vascular endothelial growth factor (VEGF) and aquaporin-4 (AQP4) play important roles in glioma-associated edema. However, the effect of VEGF on AQP4 expression is not clear. In this study, AQP4 expression was assayed in cultured glioma cells that express different amounts of VEGF, and AQP4 expression, vessel permeability and water content were assayed in glioma xenografts that express different amounts of VEGF. No difference in AQP4 expression was found between glioma cells expressing different VEGF amounts in vitro. However, AQP4 expression was increased in glioma tissue with increased VEGF, vessel permeability and water content. In conclusion, VEGF does not directly affect AQP-4 expression. The redistribution of AQP4 in glioblastoma cells is a reaction to vasogenic edema induced by VEGF for the purpose of facilitating reabsorption of excess fluid. The pattern of AQP4 expression in glioma provides new insights into the molecular changes occurring in glioma-associated edema and may help plan future therapeutic strategies.

Adhesion of malignant mammary tumor cells MDA-MB-231 to microvessel wall increases microvascular permeability via degradation of endothelial surface glycocalyx

To investigate the effect of tumor cell adhesion on microvascular permeability (P) in intact microvessels, we measured the adhesion rate of human mammary carcinoma MDA-MB-231, the hydraulic conductivity (L(p)), the P, and reflection coefficient (σ) to albumin of the microvessels at the initial tumor cell adhesion and after ∼45 min cell perfusion in the postcapillary venules of rat mesentery in vivo. Rats (Sprague-Dawley, 250-300 g) were anesthetized with pentobarbital sodium given subcutaneously. A midline incision was made in the abdominal wall, and the mesentery was gently taken out and arranged on the surface of a glass coverslip for the measurement. An individual postcapillary venule was perfused with cells at a rate of ∼1 mm/s, which is the mean blood flow velocity in this type of microvessels. At the initial tumor cell adhesion, which was defined as one adherent cell in ∼100- to 145-μm vessel segment, L(p) was 1.5-fold and P was 2.3-fold of their controls, and σ decreased from 0.92 to 0.64; after ∼45-min perfusion, the adhesion increased to ∼5 adherent cells in ∼100- to 145-μm vessel segment, while L(p) increased to 2.8-fold, P to 5.7-fold of their controls, and σ decreased from 0.92 to 0.42. Combining these measured data with the predictions from a mathematical model for the interendothelial transport suggests that tumor cell adhesion to the microvessel wall degrades the endothelial surface glycocalyx (ESG) layer. This suggestion was confirmed by immunostaining of heparan sulfate of the ESG on the microvessel wall. Preserving of the ESG by a plasma glycoprotein orosomucoid decreased the P to albumin and reduced the tumor cell adhesion.

A dual-tracer method for differentiating transendothelial transport from paracellular leakage in vivo and in vitro

Inflammation-induced impaired function of vascular endothelium may cause leakage of plasma proteins that can lead to edema. Proteins may leave the vascular lumen through two main paracellular and transcellular pathways. As the first involves endothelial cell (EC) junction proteins and the second caveolae formation, these two pathways are interconnected. Therefore, it is difficult to differentiate the prevailing role of one or the other pathway during pathology that causes inflammation. Here we present a newly developed dual-tracer probing method that allows differentiation of transcellular from paracellular transport during pathology. This fluorescence-based method can be used in vitro to test changes in EC layer permeability and in vivo in various animal vascular preparations. The method is based on comparison of low molecular weight molecule (LMWM) transport to that of high molecular weight molecule (HMWM) transport through the EC layer or the vascular wall during physiological and pathological conditions. Since the LMWM will leak through mainly the paracellular and HMWM will move through paracellular (when gaps between the ECs are wide enough) and transcellular pathways, the difference in transport rate (during normal conditions and pathology) of these molecules will indicate the prevailing transport pathway involved in overall protein crossing of vascular wall. Thus, the novel approach of assessing the transport kinetics of different size tracers in vivo by intravital microscopy can clarify questions related to identification of target pathways for drug delivery during various pathologies associated with elevated microvascular permeability.

Microvascular transport and tumor cell adhesion in the microcirculation

One critical step in tumor metastasis is tumor cell adhesion to the endothelium forming the microvessel wall. Understanding this step may lead to new therapeutic concepts for tumor metastasis. Vascular endothelium forming the microvessel wall and the glycocalyx layer at its surface are the principal barriers to, and regulators of the material exchange between circulating blood and body tissues. The cleft between adjacent ECs (interendothelial cleft) is the principal pathway for water and solutes transport through the microvessel wall in health. It is also suggested to be the pathway for high molecular weight plasma proteins, leukocytes and tumor cells across microvessel walls in disease. Thus the first part of the review introduced the mathematical models for water and solutes transport through the interendothelial cleft. These models, combined with the experimental results from in vivo animal studies and electron microscopic observations, are used to evaluate the role of the endothelial surface glycocalyx, the junction strand geometry in the interendothelial cleft, and the surrounding extracellular matrix and tissue cells, as the determinants of microvascular transport. The second part of the review demonstrated how the microvascular permeability, hydrodynamic factors, microvascular geometry and cell adhesion molecules affect tumor cell adhesion in the microcirculation.

Methods for forming human microvascular tubes in vitro and measuring their macromolecular permeability

This chapter describes a protocol for forming open endothelial tubes in vitro and quantifying their permeability to macromolecules. These tubes consist of confluent monolayers of human microvascular endothelial cells in perfused microfluidic collagen gels. The cylindrical geometry of the tubes mimics the shape of microvessels in vivo; it allows simultaneous and/or repeated measurements of permeability coefficients and detection of focal leaks. We have used these in vitro models to test the effects of agonists on microvascular permeability and are developing arrays of microvascular tubes to enable large-scale testing.

AQP-4 in peritumoral edematous tissue is correlated with the degree of glioma and with expression of VEGF and HIF-alpha

It is recognized that expression of AQP4 protein is much greater in gliomas than in normal tissue. The relationship between AQP4 and glioma-associated brain edema is affected by osmotic pressure and hypoxia. In this study, we detected changes of AQP4 expression in tumor and peritumoral edematous tissues to analyze the relationship between AQP4 protein and the edema index (EI). We also detected expression of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor-1α (HIF-1α) to investigate their relationship with AQP4 protein, and thus to uncover the molecular biological mechanisms of AQP4 expression in glioma-associated brain edema. Sixty-five patients with brain glioma were divided into tumor and peritumor groups. Fresh tumor specimens, including six cases of grade I glioma, 18 of grade II, 11 of grade III and 30 of grade IV, and peritumoral edematous tissue specimens (1 cm distant from the tumor) were resected from these patients, and AQP4 protein expression levels were detected by western blot. Different AQP4 expression in the tumor and peritumor groups were compared. The relationship between AQP4 expression levels and the degree of peritumoral edema, and expression differences in different grades, were analyzed. Immunofluorescence cytochemistry was used to detect positive expression of AQP4 protein, VEGF protein, and HIF-1α protein in tumor tissue, and differences between expression were analyzed. Western blot showed that AQP4 expression in the peritumor (0.7697 ± 0.0941) and tumor (0.6934 ± 0.0625) groups was higher than in the control group (0.6215 ± 0.0884), and was highest in the peritumor group (both P < 0.01). AQP4 expression level in the peritumor group was positively correlated with EI (r = 0.677, P < 0.001) whereas AQP4 expression level in the tumor group was not correlated with EI (r = 0.096, P > 0.05). AQP4 expression increased with higher tumor grades in the peritumor group, but differences were not significant in the tumor group. Immunofluorescence cytochemical staining revealed that AQP4 protein in normal brain tissue was mainly expressed in the cell membrane surface, and that cytoplasm and nuclear staining was shallow. In glioma cells, AQP4 was widely distributed in the cytoplasm, particularly in the edematous area around the tumor. AQP4 protein expression in the tumor was significantly positively correlated with both VEGF protein (r = 0.877, P < 0.001) and HIF-1α protein (r = 0.876, P < 0.001). AQP4 expression was higher in brain tumor, especially peritumor. The degree of peritumoral edema correlates with AQP4 protein expression only in peritumor, whereas AQP4 expression is in accordance with expression of VEGF and HIF-1α. In glioma-associated brain edema, AQP4 is coregulated by osmotic pressure and hypoxia, with predominance of osmotic regulation, and is redistributed in glioma cells, mainly in the cytoplasm, and its expression level increased with higher glioma grades.

Vascular endothelial growth factors and vascular permeability

Vascular endothelial growth factors (VEGFs) are key regulators of permeability. The principal evidence behind how they increase vascular permeability in vivo and in vitro and the consequences of that increase are addressed here. Detailed analysis of the published literature has shown that in vivo and in vitro VEGF-mediated permeability differs in its time course, but has common involvement of many specific signalling pathways, in particular VEGF receptor-2 activation, calcium influx through transient receptor potential channels, activation of phospholipase C gamma and downstream activation of nitric oxide synthase. Pathways downstream of endothelial nitric oxide synthase appear to involve the guanylyl cyclase-mediated activation of the Rho–Rac pathway and subsequent involvement of junctional signalling proteins such as vascular endothelial cadherin and the tight junctional proteins zona occludens and occludin linked to the actin cytoskeleton. The signalling appears to be co-ordinated through spatial organization of the cascade into a signalplex, and arguments for why this may be important are considered. Many proteins have been identified to be involved in the regulation of vascular permeability by VEGF, but still the mechanisms through which these are thought to interact to control permeability are dependent on the experimental system, and a synthesis of existing data reveals that in intact vessels the co-ordination of the pathways is still not understood.

Vascular endothelial growth factor enhances cancer cell adhesion to microvascular endothelium in vivo

To investigate whether vascular endothelial growth factor (VEGF) enhances cancer cell adhesion to normal microvessels, we used in vivo video microscopy to measure adhesion rates of MDA-MB-435s human breast cancer cells and ErbB2-transformed mouse mammary carcinomas in the postcapillary venules of rat mesentery. An individual postcapillary venule in the mesentery was injected via a glass micropipette with cancer cells either in a perfusate of mammalian Ringer solution containing 1% bovine serum albumin as a control, or with the addition of 1 nm VEGF for test measurements. Cell adhesion was measured as either the number of adherent cells or the fluorescence intensity of adherent cells in a vessel segment for 60 min. Our results showed that during both control and VEGF treatments, the number of adherent cells increased almost linearly with time over 60 min. The VEGF treatment increased the adhesion rates of human tumour cells and mouse carcinomas 1.9-fold and 1.8-fold, respectively, over those in control conditions. We also measured cancer cell adhesion after pretreatment of cells with an antibody blocking VEGF or an antibody blocking alpha 6 integrin, and pretreatment of the microvessel with VEGF receptor (KDR/Flk-1) inhibitor, SU1498, or anti-integrin extracellular matrix ligand antibody, anti-laminin-5. All antibodies and inhibitor significantly reduced adhesion, with anti-VEGF and SU1498 reducing it the most. Our results indicate that VEGF enhances cancer cell adhesion to the normal microvessel wall, and further suggest that VEGF and its receptor, KDR/Flk-1, as well as integrins of tumour cells and their ligands at the endothelium, contribute to mammary cancer cell adhesion to vascular endothelium in vivo.

Neutralizing endogenous VEGF following traumatic spinal cord injury modulates microvascular plasticity but not tissue sparing or functional recovery

Acute loss of spinal cord vascularity followed by an endogenous adaptive angiogenic response with concomitant microvascular dysfunction is a hallmark of traumatic spinal cord injury (SCI). Recently, the potent vasoactive factor vascular endothelial growth factor (VEGF) has received much attention as a putative therapeutic for the treatment of various neurodegenerative disorders, including SCI. Exogenous VEGF exerts both protective and destabilizing effects on microvascular elements and tissue following SCI but the role of endogenous VEGF is unclear. In the present study, we systemically applied a potent and well characterized soluble VEGF antagonist to adult C57Bl/6 mice post-SCI to elucidate the relative contribution of VEGF on the acute evolving microvascular response and its impact on functional recovery. While the VEGF Trap did not alter vascular density in the injury epicenter or penumbra, an overall increase in the number of Griffonia simplicifolia isolectin-B4 bound microvessels was observed, suggesting a VEGF-dependency to more subtle aspects of endothelial plasticity post-SCI. Neutralizing endogenous VEGF neither attenuated nor exacerbated chronic histopathology or functional recovery. These results support the idea that overall, endogenous VEGF is not neuroprotective or detrimental following traumatic SCI. Furthermore, they suggest that angiogenesis in traumatically injured spinal tissue is regulated by multiple effectors and is not limited by endogenous VEGF activation of affected spinal microvessels.

Modeling of growth factor-receptor systems from molecular-level protein interaction networks to whole-body compartment models

Most physiological processes are subjected to molecular regulation by growth factors, which are secreted proteins that activate chemical signal transduction pathways through binding of specific cell-surface receptors. One particular growth factor system involved in the in vivo regulation of blood vessel growth is called the vascular endothelial growth factor (VEGF) system. Computational and numerical techniques are well suited to handle the molecular complexity (the number of binding partners involved, including ligands, receptors, and inert binding sites) and multiscale nature (intratissue vs. intertissue transport and local vs. systemic effects within an organism) involved in modeling growth factor system interactions and effects. This chapter introduces a variety of in silico models that seek to recapitulate different aspects of VEGF system biology at various spatial and temporal scales: molecular-level kinetic models focus on VEGF ligand-receptor interactions at and near the endothelial cell surface; mesoscale single-tissue 3D models can simulate the effects of multicellular tissue architecture on the spatial variation in VEGF ligand production and receptor activation; compartmental modeling allows efficient prediction of average interstitial VEGF concentrations and cell-surface VEGF signaling intensities across multiple large tissue volumes, permitting the investigation of whole-body intertissue transport (e.g., vascular permeability and lymphatic drainage). The given examples will demonstrate the utility of computational models in aiding both basic science and clinical research on VEGF systems biology.

Vascular endothelial growth factor mediates the estrogen-induced breakdown of tight junctions between and increase in proliferation of microvessel endothelial cells in the baboon endometrium

To assess whether there is a link between estrogen, vascular endothelial growth factor (VEGF), and early aspects of uterine angiogenesis, an acute temporal study was conducted in which ovariectomized baboons were pretreated with VEGF Trap, which sequesters endogenous VEGF, and administered estradiol at time 0 h. Serum estradiol levels approximated 500 pg/ml 4-6 h after estradiol administration. VEGF mRNA levels in endometrial glandular epithelial and stromal cells were increased to values 6 h after estradiol that were 3.74 +/- 0.99-fold (mean +/- se) and 5.70 +/- 1.60-fold greater (P < 0.05), respectively, than at 0 h. Microvessel interendothelial cell tight junctions, which control paracellular permeability, were present in the endometrium at time 0 h, but not evident 6 h after estradiol administration. Thus, microvessel paracellular cleft width increased (P < 0.01, ANOVA) from 5.03 +/- 0.22 nm at 0 h to 7.27 +/- 0.48 nm 6 h after estrogen. In contrast, tight junctions remained intact, and paracellular cleft widths were unaltered in estradiol/VEGF Trap and vehicle-treated animals. Endometrial microvessel endothelial cell mitosis, i.e. percent Ki67+/Ki67- immunolabeled endothelial cells, increased (P < 0.05) from 2.9 +/- 0.3% at 0 h to 21.4 +/- 7.0% 6 h after estrogen treatment but was unchanged in estradiol/VEGF Trap and vehicle-treated animals. In summary, the estrogen-induced disruption of endometrial microvessel endothelial tight junctions and increase in endothelial cell proliferation were prevented by VEGF Trap. Therefore, we propose that VEGF mediates the estrogen-induced increase in microvessel permeability and endothelial cell proliferation as early steps in angiogenesis in the primate endometrium.

Non-invasive measurement of solute permeability in cerebral microvessels of the rat

To quantify the solute permeability of rat cerebral microvessels, we measured the apparent permeability (P) of pial microvessels to various sized solutes. The pial microcirculation was observed by a high numerical aperture objective lens through a section of the frontoparietal bone thinned with a micro-grinder (a revised method from Easton and Fraser, 1994, J Physiol. 475:147-157, 1994). Sodium fluorescein (MW 376) at concentration 0.1 mg/ml or FITC-dextrans (MW 4k, 10k, 20k, 40k, 70k) at concentration 1 mg/ml in 1% BSA mammalian Ringer, was introduced into the cerebral circulation via the ipsilateral carotid artery by a syringe pump at a constant rate of approximately 3 ml/min. P was determined using a highly sensitive quantitative fluorescence imaging and analyzing method. The mean P to sodium fluorescein was 2.71 (+/-0.76 SD, n=11)x10(-6) cm/s. The mean P to FITC-dextrans were 0.92 (+/-0.46 SD, n=10)x10(-6) cm/s for Dextran-4k, 0.31 (+/-0.13 SD, n=7)x10(-6) cm/s for Dextran-10k, 0.24 (+/-0.10 SD, n=6)x10(-6) cm/s for Dextran-20k, 0.19 (+/-0.11 SD, n=10)x10(-6) cm/s for Dextran-40k, and 0.15 (+/-0.05 SD, n=11)x10(-6) cm/s for Dextran-70k. These values were 1/10 to 1/6 of those of rat mesenteric microvessels for similar sized solutes (Fu, B.M., Shen, S., 2004. Acute VEGF effect on solute permeability of mammalian microvessels in vivo. Microvasc. Res. 68, 51-62.).

A compartment model of VEGF distribution in blood, healthy and diseased tissues

BACKGROUND

Angiogenesis is a process by which new capillaries are formed from pre-existing blood vessels in physiological (e.g., exercise, wound healing) or pathological (e.g., ischemic limb as in peripheral arterial disease, cancer) contexts. This neovascular mechanism is mediated by the vascular endothelial growth factor (VEGF) family of cytokines. Although VEGF is often targeted in anti-angiogenic therapies, there is little knowledge about how its concentration may vary between tissues and the vascular system. A compartment model is constructed to study the VEGF distribution in the tissue (including matrix-bound, cell surface receptor-bound and free VEGF isoforms) and in the blood. We analyze the sensitivity of this distribution to the secretion rate, clearance rate and vascular permeability of VEGF.

RESULTS

We find that, in a physiological context, VEGF concentration varies approximately linearly with the VEGF secretion rate. VEGF concentration in blood but not in tissue is dependent on the vascular permeability of healthy tissue. Model simulations suggest that relative VEGF increases are similar in blood and tissue during exercise and return to baseline within several hours. In a pathological context (tumor), we find that blood VEGF concentration is relatively insensitive to increased vascular permeability in tumors, to the secretion rate of VEGF by tumors and to the clearance. However, it is sensitive to the vascular permeability in the healthy tissue. Finally, the VEGF distribution profile in healthy tissue reveals that about half of the VEGF is complexed with the receptor tyrosine kinase VEGFR2 and the co-receptor Neuropilin-1. In diseased tissues, this binding can be reduced to 15% while VEGF bound to the extracellular matrix and basement membranes increases.

CONCLUSION

The results are of importance for physiological conditions (e.g., exercise) and pathological conditions (e.g., peripheral arterial disease, coronary artery disease, cancer). This mathematical model can serve as a tool for understanding the VEGF distribution in physiological and pathological contexts as well as a foundation to investigate pro- or anti-angiogenic strategies.

Proteomic analysis of proteins from bronchoalveolar lavage fluid reveals the action mechanism of ultrafine carbon black-induced lung injury in mice

Previous studies have shown that ultrafine carbon black (ufCB) could cause oxidative stress and lung injury, but the mechanisms have not been clearly demonstrated. In this study, 1-D gel electrophoresis coupled with LC/MS/MS (1-D geLC/MS/MS) was carried out with bronchoalveolar lavage fluid (BALF) to identify proteins associated with ufCB-induced lung injury. If required, Western blot was conducted additionally to validate proteins. Thirty-three proteins were identified, including leukemia inhibitory factor receptor (LIFR) and epidermal growth factor receptor (EGFR). Western blot analysis showed that ufCB exposure caused the increases of LIFR and EGFR in BALF and decreases of both receptors in lung tissues, suggesting the acceleration of epithelial shedding from the lung and increase of cell debris with membrane proteins EGFR and LIFR in BALF. There were strong correlations between vascular endothelial growth factor (VEGF) and albumin (p<0.01) or alpha2-macroglobulin (alpha2M) in BALF (p<0.05). Importantly, antioxidant ceruloplasmin (Cp) was shown to be produced from lung epithelial cells in response to ufCB exposure. This is the first study to apply 1-D ge LC/MS/MS and experimental studies to reveal the mechanisms involved in the pathogenesis of ufCB-induced lung injury.

ERKs activation and calcium signaling are both required for VEGF induction by vanadium in mouse epidermal Cl41 cells

The previous studies have demonstrated that vanadium exposure can cause a variety of biological effects. However, the mechanisms involved in the biological effects caused by vanadium are not well understood. Our previous studies have shown that exposure of mouse epidermal Cl 41 cells to vanadate stimulated the phosphorylation of both ERKs and p38K, and calcium signaling leading NFAT activation. In view of the evidence that ERKs and p38 kinase contribute to VEGF induction, we investigated in the present study the potential roles of ERKs, p38K, and calcium signaling in VEGF induction caused by vanadium exposure. Exposure of Cl 41 cells to vanadium led to VEGF induction in both time- and dose-dependent manners. Pre-treatment of Cl 41 cells with PD98059, an inhibitor of MEK1/2-ERKs pathway, but not SB202190, an inhibitor for p38K pathway, resulted in a dramatic inhibition of VEGF induction by vanadium. More interesting, pre-treatment of Cl 41 cells with intracellular calcium chelator, but not calcium channel blocker, resulted in a dramatic decrease in VEGF induction by vanadium. However, both PI-3K inhibitors and overexpression of Deltap85, a dominant negative PI-3K mutant, resulted in only a marginal decrease in VEGF induction by vanadium. Moreover, mTOR, as a downstream molecule of PI-3K, did not attribute to VEGF induction by vanadium because rapamycin pre-treatment did not show any inhibitory effect on VEGF induction. These results indicate that ERKs and intracellular stored calcium release play a critical role in VEGF induction by vanadium. PI-3K is partially involved in VEGF induction by vanadium, while p38K and mTOR are not involved. Those results will help us to understand the molecular mechanisms involved in vanadium-induced biological effects.

Placental growth factor-1 and epithelial haemato-retinal barrier breakdown: potential implication in the pathogenesis of diabetic retinopathy

AIMS/HYPOTHESIS

Disruption of the retinal pigment epithelial (RPE) barrier contributes to sub-retinal fluid and retinal oedema as observed in diabetic retinopathy. High placental growth factor (PLGF) vitreous levels have been found in diabetic patients. This work aimed to elucidate the influence of PLGF-1 on a human RPE cell line (ARPE-19) barrier in vitro and on normal rat eyes in vivo.

METHODS

ARPE-19 permeability was measured using transepithelial resistance and inulin flux under stimulation of PLGF-1, vascular endothelial growth factor (VEGF)-E and VEGF 165. Using RT-PCR, we evaluated the effect of hypoxic conditions or insulin on transepithelial resistance and on PLGF-1 and VEGF receptors. The involvement of mitogen-activated protein kinase (MEK, also known as MAPK)/extracellular signal-regulated kinase (ERK, also known as EPHB2) signalling pathways under PLGF-1 stimulation was evaluated by western blot analysis and specific inhibitors. The effect of PLGF-1 on the external haemato-retinal barrier was evaluated after intravitreous injection of PLGF-1 in the rat eye; evaluation was by semi-thin analysis and zonula occludens-1 immunolocalisation on flat-mounted RPE.

RESULTS

In vitro, PLGF-1 induced a reversible decrease of transepithelial resistance and enhanced tritiated inulin flux. These effects were specifically abolished by an antisense oligonucleotide directed at VEGF receptor 1. Exposure of ARPE-19 cells to hypoxic conditions or to insulin induced an upregulation of PLGF-1 expression along with increased transcellular permeability. The PLGF-1-induced RPE cell permeability involved the MEK signalling pathway. Injection of PLGF-1 in the rat eye vitreous induced an opening of the RPE tight junctions with subsequent sub-retinal fluid accumulation, retinal oedema and cytoplasm translocation of junction proteins.

CONCLUSIONS/INTERPRETATION

Our results indicate that PLGF-1 may be a potential regulation target for the control of diabetic retinal and macular oedema.

Früher antiexsudativer Effekt – OCT-Monitoring nach intravitrealer Bevacizumab-Applikation

VEGF is more potent than histamine by a factor of 50,000 for inducing increased vessel permeability. Already in the first few minutes, hydraulic conductivity and diffusive permeability are significantly increased, followed by a longer-lasting, marked leakage over 20 h. Specific inhibition of the angiogenic, vasoactive, and permeability-inducing protein VEGF is now possible by new drugs, one of which is the first available (off-label) treatment in Germany for routine clinical use (Avastin). Retinal edema is composed of increased outflow of water and low molecular substances in the interstitial environment and is an important determinate of functional development in different ocular diseases. First experiences with the anti-hyperpermeability effect show early response and high potential in pathologic leakage. Future examinations have to assess when a permanent benefit can be achieved in respect to the other antiproliferative capabilities of the drug.

The anti-angiogenic VEGF isoform VEGF165b transiently increases hydraulic conductivity, probably through VEGF receptor 1 in vivo

Vascular endothelial growth factor (VEGF) is the principal agent that increases microvascular permeability during physiological and pathological angiogenesis. VEGF is differentially spliced to form two families of isoforms: VEGF(xxx), and VEGF(xxx)b. Whereas VEGF(165) stimulates angiogenesis, VEGF(165)b is anti-angiogenic. To determine the effect of VEGF(165)b on permeability, hydraulic conductivity (L(p)) was measured in individually perfused microvessels in the mesentery of frogs and rats. As with VEGF(165), VEGF(165)b increased L(p) in amphibian (2.4 +/- 0.3-fold) and mammalian (1.9 +/- 0.2-fold) mesenteric microvessels. A dose-response relationship showed that VEGF(165)b (EC(50), 0.65 pm) was approximately 25 times more potent than VEGF(165) (EC(50), 16 pm) in amphibian microvessels. VEGF(165) has been shown to increase permeability through VEGF receptor 2 (VEGF-R2) signalling. However, VEGF(165)b increased L(p) of frog vessels to the same extent in the presence of the VEGF-R2 inhibitor ZM323881, indicating that it does not increase permeability via VEGF-R2 signalling, and was inhibited by the VEGF receptor inhibitor SU5416 at doses that are specific for VEGF receptor 1 (VEGF-R1). VEGF(165)b, in contrast to VEGF(165), did not result in a sustained chronic increase in L(p). These results show that although VEGF(165)b is anti-angiogenic in the mesentery, it does signal in endothelial cells in vivo resulting in a transient, but not sustained, increase in microvascular L(p), probably through VEGF-R1.

Vascular endothelial growth factor increases the ultrafiltration coefficient in isolated intact Wistar rat glomeruli

Vascular endothelial growth factor (VEGF) is expressed by the podocytes of renal glomeruli, and has profound influences on systemic microvascular permeability and haemodynamics. We describe an extensive refinement of a model that permits evaluation of the ultrafiltration coefficient (LpA) of isolated mammalian glomeruli, in the absence of circulating and haemodynamic influences, and tested the hypothesis that VEGF influences glomerular LpA via an effect on endothelial cells. Glomeruli were isolated by sieving Wistar rat renal cortical tissue, and individually loaded onto a suction micropipette. Flowing perifusate containing 1% bovine serum albumin (BSA) was rapidly switched to an oncopressive perifusate containing 8% BSA, eliciting transglomerular fluid efflux. The rate of the resultant reduction in glomerular volume was used to calculate glomerular LpA (1.07 +/- 0.53 nl min(-1) mmHg(-1) (mean +/-s.d.), n= 51), which compares favourably with those reported in the same rat strain using different techniques. A significant relationship between LpA and initial glomerular volume (Vi) (r= 0.72, n= 41, P < 0.0001) necessitated correction of LpA for Vi. The initial rate of change of glomerular volume, normalized for Vi, showed a strong positive correlation with applied oncotic gradient (Pearson r= 0.59, n= 28, P < 0.001), as predicted by Starling's law of filtration. A 60 min exposure of glomeruli to 1 nm VEGF increased glomerular LpA/Vi (1.19 +/- 0.19 (n= 10) to 2.23 +/- 0.33 (n= 9) min(-1) mmHg(-1) (mean +/-s.e.m.); P < 0.02). Time- and concentration-dependent relations between VEGF and LpA/Vi were observed. The VEGF-induced elevation of LpA/Vi was blocked by the selective VEGF-R2 inhibitor ZM323881. We suggest that glomerular VEGF contributes to the high physiological permeability of mammalian glomeruli to water through an action on endothelial cells.

An electrodiffusion-filtration model for effects of endothelial surface glycocalyx on microvessel permeability to macromolecules

Endothelial surface glycocalyx plays an important role in the regulation of microvessel permeability by possibly changing its charge and configuration. To investigate the mechanisms by which surface properties of the endothelial cells control the changes in microvessel permeability, we extended the electrodiffusion model developed by Fu et al. [Am. J. Physiol. 284, H1240-1250 (2003)], which is for the interendothelial cleft with a negatively charged surface glycocalyx layer, to include the filtration due to hydrostatic and oncotic pressures across the microvessel wall as well as the electrical potential across the glycocalyx layer On the basis of the hypotheses proposed by Curry [Microcirculation 1(1): 11-26 (1994)], the predictions from this electrodiffusion-filtration model provide a good agreement with experimental data for permeability of negatively charged a-lactalbumin summarized in Curry [Microcirculation 1(1), 11-26 (1994)] under various conditions. In addition, we applied this new model to describe the transport of negatively charged macromolecules, bovine serum albumin (BSA), across venular microvessels in frog mesentery. According to the model, the convective component of the albumin transport is greatly diminished by the presence of a negatively charged glycocalyx under both normal and increased permeability conditions.

Acute VEGF effect on solute permeability of mammalian microvessels in vivo

To investigate the effect of vascular endothelial growth factor (VEGF) on solute permeability of mammalian microvessels, we measured the apparent permeability (P) of various-sized solutes on the postcapillary venules of rat mesentery in vivo. Exposure to 1 nM VEGF transiently increased P from a mean of 1.4 (+/-0.11 SE, n = 17) to a peak of 2.8 (+/-0.28 SE) x 10(-5) cm/s, a 2.4-fold increase for small solute sodium fluorescein (Stokes radius 0.45 nm), from a mean of 0.44 (+/-0.05 SE, n = 16) to a peak of 1.5 (+/-0.19 SE) x 10(-)5 cm/s, a 3.6-fold increase for intermediate-sized solute alpha-lactalbumin (Stokes radius 2.01 nm), from a mean of 0.049 (+/-0.0032 SE, n = 16) to a peak of 0.36 (+/-0.032 SE) x 10(-5) cm/s, a 7.9-fold increase for large solute bovine serum albumin (Stokes radius 3.55 nm), within 30 s. In approximately 2 min, all increased P returned to the baseline values. The response pattern of P to VEGF and the ratios of the peak to control values for rat mesenteric microvessels are similar to those of frog mesenteric microvessels [Am. J. Physiol.: Heart Circ. Physiol. 284 (2003) H2124]. Instead of considerable heterogeneity in the frog mesenteric microvessels, the acute response to 1 nM VEGF is homogeneous in the rat mesenteric microvessels.

The Aqueous Outflow System as a Mechanical Pump

PURPOSE

To describe a new aqueous outflow model involving a mechanical pump.

MATERIALS AND METHODS

Laboratory materials include human and monkey eyes; methods include the dissecting microscope, light microscopy, scanning electron microscopy, transmission electron microscopy, and tracer studies. Clinical methods involve human subject slit lamp, gonioscopy, and operating microscope examination.

RESULTS

Laboratory evidence demonstrates that aqueous outflow tissues are responsive to intraocular pressure induced deformation. Deformation occurs in response to small pressure gradients. Laboratory evidence also demonstrates the presence of valves discharging aqueous to Schlemm's canal. The laboratory model predicts pulsatile aqueous discharge in vivo. Clinical in vivo evidence demonstrates pulsatile aqueous flow from the anterior chamber into Schlemm's canal, from Schlemm's canal into collector channels, and from Schlemm's canal into aqueous and episcleral veins, all synchronous with the ocular pulse.

CONCLUSIONS

Aqueous outflow tissue deformation caused by normal intraocular pressure transients induces pulsatile one-way discharge of aqueous to the vascular system. The model identifies biomechanical coupling of intraocular pressure with aqueous outflow tissue deformation and also sites of high flow capable of inducing shear stress. These mechanotransduction mechanisms, well characterized as a means of controlling pressure and flow in the vascular system, also provide a means of regulatory feedback to control intraocular pressure and aqueous flow.

Intracoronary administration of FGF-2: a computational model of myocardial deposition and retention

This study uses a computational model to characterize the myocardial deposition and retention of basic fibroblast growth factor (FGF-2) at the cellular level after intracoronary (IC) administration of exogenous FGF-2. The model is applied to the in situ conditions present within the myocardium of a dog for which the plasma pharmacokinetics resulting from IC injection of FGF-2 were recorded. Our estimates show that the processes involved in FGF-2 signaling are not diffusion limited; rather, the response time is determined by the reaction time of FGF-2 binding to cell surface receptors. Additionally, the processes of receptor secretion and internalization are found to play crucial roles in the FGF-2 dynamics; future experiments are required to quantify these processes. The model predictions obtained in this study suggest that IC administration of FGF-2 via either a single bolus or repetitive injections causes a transient increase (time scale of hours) in myocardial FGF-2 concentration if the endogenous level of free interstitial FGF-2 is low enough to allow permeation of FGF-2 molecules from the microvascular to the interstitial spaces. The model shows that the majority (64%) of the extracellular FGF-2 ligands are located within the interstitium, and similar fractions are found in the basement membrane and extracellular matrix. Among the FGF-2 molecules found within the interstitium, 2% are free and 98% are bound to interstitial heparan sulfate proteoglycans. These results support the theory of extracellular control of the bioavailability of FGF-2 via dynamic storage of FGF-2 within the basement membrane and extracellular matrix.

Vasodilator effect and mechanism of action of vascular endothelial growth factor in skin vasculature

Various laboratories have reported that local subcutaneous or subdermal injection of VEGF165 at the time of surgery effectively attenuated ischemic necrosis in rat skin flaps, but the mechanism was not studied and enhanced angiogenesis was implicated. In the present study, we used the clinically relevant isolated perfused 6 × 16-cm pig buttock skin flap model to 1) test our hypothesis that VEGF165 is a potent vasodilator and acute VEGF165 treatment increases skin perfusion; and 2) investigate the mechanism of VEGF165-induced skin vasorelaxation. We observed that VEGF165 (5 × 10–16–5 × 10–11 M) elicited a concentration-dependent decrease in perfusion pressure (i.e., vasorelaxation) in skin flaps preconstricted with a submaximal concentration of norepinephrine (NE), endothelin-1, or U-46619. The VEGF165-induced skin vasorelaxation was confirmed using a dermofluorometry technique for assessment of skin perfusion. The vasorelaxation potency of VEGF165 in NE-preconstricted skin flaps (pD2 = 13.57 ± 0.31) was higher ( P < 0.05) than that of acetylcholine (pD2 = 7.08 ± 0.24). Human placental factor, a specific VEGF receptor-1 agonist, did not elicit any vasorelaxation effect. However, a specific antibody to VEGF receptor-2 (1 μg/ml) or a specific VEGF receptor-2 inhibitor (5 × 10–6 M SU-1498) blocked the vasorelaxation effect of VEGF165 in NE-preconstricted skin flaps. These observations indicate that the potent vasorelaxation effect of VEGF165 in the skin vasculature is initiated by the activation of VEGF receptor-2. Furthermore, using pharmacological probes, we observed that the postreceptor signaling pathways of VEGF165-induced skin vasorelaxation involved activation of phospholipase C and protein kinase C, an increase in inositol 1,4,5-trisphosphate activity, release of the intra-cellular Ca2+ store, and synthesis/release of endothelial nitric oxide, which predominantly triggered the effector mechanism of VEGF165-induced vasorelaxation. This information provides, for the first time, an important insight into the mechanism of VEGF165 protein or gene therapy in the prevention/treatment of ischemia in skin flap surgery and skin ischemic diseases.

The Role of Vascular Endothelial Growth Factor in the Kidney in Health and Disease

Vascular endothelial growth factor (VEGF) is a potent endothelial cell mitogen, angiogenic factor and enhancer of vascular permeability. Expressed in the epithelial cells of the developing glomerulus and tubular epithelium, VEGF plays an important role in the development and maintenance of the early vasculature of the kidney. Here, we review the available literature regarding the expression and function of VEGF both in the developing and healthy adult kidney. Furthermore, we highlight how VEGF expression is altered in the diseased kidney and how this modulated expression may impact on and reflect underlying functional changes occurring during the disease process. As discussed, many controversial issues remain, particularly concerning the role of VEGF in the diseased kidney. That VEGF has been proposed as a potential future therapeutic target for the management of some renal diseases requires first that the precise role of VEGF in the normal kidney and various renal pathologies be further and more clearly defined.

Regulation of vascular permeability by vascular endothelial growth factors

Increased vascular permeability is one of the first stages in both physiological and pathological angiogenesis-the generation of new blood vessels from preexisting vasculature. Although this has been hypothesised to be true in physiological angiogenesis, it is clearly a mark of blood vessel growth in disease. Normal, healthy blood vessel growth (physiological angiogenesis) occurs throughout development as well as during tissue repair and growth in adult tissues. Angiogenesis is also seen in a wide variety of diseases, which include all the major causes of mortality in the West-heart disease, cancer, stroke, vascular disease, and diabetes. Much of this angiogenesis is significantly different from normal blood vessel growth and is termed pathological angiogenesis. Angiogenesis is regulated by vascular growth factors, the most notable being the vascular endothelial growth factor family of proteins (VEGF). These act on specific receptors in the vascular system to stimulate new vessel growth by a number of mechanisms. VEGFs also directly stimulate increased vascular permeability to water and large molecular weight proteins and vasodilatation. These two effects result in a large flux of water and macromolecules from the vasculature to the interstitium, often resulting in oedema. This review will outline the mechanisms by which VEGFs do this and discuss some of the difficulties in interpreting data from VEGF studies due to the conflicting and synergistic effects of these actions.