3D reconstruction of the glycocalyx structure in mammalian capillaries using electron tomography

Efficacy of laparoscopic low anterior resection for colorectal cancer patients with 3D-vascular reconstruction for left coronary artery preservation

BACKGROUND

Laparoscopic low anterior resection (LLAR) has become a mainstream surgical method for the treatment of colorectal cancer, which has shown many advantages in the aspects of surgical trauma and postoperative rehabilitation. However, the effect of surgery on patients' left coronary artery and its vascular reconstruction have not been deeply discussed. With the development of medical imaging technology, 3D vascular reconstruction has become an effective means to evaluate the curative effect of surgery.

AIM

To investigate the clinical value of preoperative 3D vascular reconstruction in LLAR of rectal cancer with the left colic artery (LCA) preserved.

METHODS

A retrospective cohort study was performed to analyze the clinical data of 146 patients who underwent LLAR for rectal cancer with LCA preservation from January to December 2023 in our hospital. All patients underwent LLAR of rectal cancer with the LCA preserved, and the intraoperative and postoperative data were complete. The patients were divided into a reconstruction group (72 patients) and a nonreconstruction group (74 patients) according to whether 3D vascular reconstruction was performed before surgery. The clinical features, operation conditions, complications, pathological results and postoperative recovery of the two groups were collected and compared.

RESULTS

A total of 146 patients with rectal cancer were included in the study, including 72 patients in the reconstruction group and 74 patients in the nonreconstruction group. There were 47 males and 25 females in the reconstruction group, aged (59.75 ± 6.2) years, with a body mass index (BMI) (24.1 ± 2.2) kg/m2, and 51 males and 23 females in the nonreconstruction group, aged (58.77 ± 6.1) years, with a BMI (23.6 ± 2.7) kg/m2. There was no significant difference in the baseline data between the two groups (P > 0.05). In the submesenteric artery reconstruction group, 35 patients were type I, 25 patients were type II, 11 patients were type III, and 1 patient was type IV. There were 37 type I patients, 24 type II patients, 12 type III patients, and 1 type IV patient in the nonreconstruction group. There was no significant difference in arterial typing between the two groups (P > 0.05). The operation time of the reconstruction group was 162.2 ± 10.8 min, and that of the nonreconstruction group was 197.9 ± 19.1 min. Compared with that of the reconstruction group, the operation time of the two groups was shorter, and the difference was statistically significant (t = 13.840, P < 0.05). The amount of intraoperative blood loss was 30.4 ± 20.0 mL in the reconstruction group and 61.2 ± 26.4 mL in the nonreconstruction group. The amount of blood loss in the reconstruction group was less than that in the control group, and the difference was statistically significant (t = -7.930, P < 0.05). The rates of anastomotic leakage (1.4% vs 1.4%, P = 0.984), anastomotic hemorrhage (2.8% vs 4.1%, P = 0.672), and postoperative hospital stay (6.8 ± 0.7 d vs 7.0 ± 0.7 d, P = 0.141) were not significantly different between the two groups.

CONCLUSION

Preoperative 3D vascular reconstruction technology can shorten the operation time and reduce the amount of intraoperative blood loss. Preoperative 3D vascular reconstruction is recommended to provide an intraoperative reference for laparoscopic low anterior resection with LCA preservation.

Reduction of Endothelial Glycocalyx on Peritubular Capillaries in Chronic Kidney Disease

In chronic kidney disease (CKD), peritubular capillaries undergo anatomic and functional alterations, such as rarefaction and increased permeability. The endothelial glycocalyx (EG) is a carbohydrate-rich gel-like mesh, which covers the luminal surface of endothelial cells. It is involved in many regulatory functions of the endothelium, including vascular permeability. Herein, we investigated ultrastructural alterations of the EG in different murine CKD models. Fluorescence staining using different lectins with high affinity to components of the renal glycocalyx revealed a reduced binding to the endothelium in CKD in the animal models, and there were similar finding in human kidney specimens. Lanthanum Dysprosium Glycosamino Glycan adhesion staining technique was used to visualize the ultrastructure of the glycocalyx in transmission electron microscopy. This also enabled quantitative analyses, showing a significant reduction of the EG thickness and density. In addition, mRNA expression of proteins involved in glycocalyx biology, synthesis, and turnover (ie, syndecan 1 and glypican 1), which are main components of the glycocalyx, and exostosin 2, involved in the synthesis of the glycocalyx, were significantly up-regulated in endothelial cells isolated from murine CKD models. Visualization of glycocalyx using specific transmission electron microscopy analyses allows qualitative and quantitative analyses and revealed significant pathologic alterations in peritubular capillaries in CKD.

Combined Electron Microscopy Approaches for Arterial Glycocalyx Visualization

Mainly constituted of glycosaminoglycans and proteoglycans, the glycocalyx is anchored in the plasma membrane, covering, in particular, the extracellular face of the arterial endothelium. Due to its complex three-dimensional (3D) architecture, the glycocalyx interacts with a wide variety of proteins, contributing to vascular permeability, the flow of mechanotransduction, and the modulation of local inflammatory processes. Alterations of glycocalyx structure mediate the endothelial dysfunction and contribute to the aggravation of peripheral vascular diseases. Therefore, the exploration of its ultrastructure becomes a priority to evaluate the degree of injury under physiopathological conditions and to assess the impact of therapeutic approaches. The objective of this study was to develop innovative approaches in electron microscopy to visualize the glycocalyx at the subcellular scale. Intravenous perfusion on rats with a fixing solution containing aldehyde fixatives enriched with lanthanum ions was performed to prepare arterial samples. The addition of lanthanum nitrate in the fixing solution allowed the enhancement of the staining of the glycocalyx for transmission electron microscopy (TEM) and to detect elastic and inelastic scattered electrons, providing complementary qualitative information. The strength of scanning electron microscopy (SEM) was used on resin-embedded serial sections, allowing rapid and efficient large field imaging and previous correlative TEM observations for ultrastructural fine details. To demonstrate the dynamic feature of the glycocalyx, 3D tomography was provided by dual-beam focus-ion-beam-SEM (FIB-SEM). These approaches allowed us to visualize and characterize the ultrastructure of the pulmonary artery glycocalyx under physiological conditions and in a rat pulmonary ischemia-reperfusion model, known to induce endothelial dysfunction. This study demonstrates the feasibility of combined SEM, TEM, and FIB-SEM tomography approaches on the same sample as the multiscale visualization and the identification of structural indicators of arterial endothelial glycocalyx integrity.

Transmission Electron Microscopy of Endothelium

Transmission electron microscopy using resin sections still remains an exceptionally useful tool in evaluating cellular ultrastucture within tissue. For the endothelium the best method for maintaining such structure is perfusion fixation: fixing the tissue under physiological pressure. Here the focus is on a method of maintaining the vascular wall structure including the endothelial glycocalyx and extending this with tilt series tomography. Shown are typical histological sections from multiple capillary beds including brain, heart and retina using a lanthanide staining technique (LaDy GAGa) to highlight that the differences in normo-physiology are substantial.It is hoped that users will find the notes useful in deciding which specific staining and imaging method would suit their needs so this technically challenging, and low throughput methodology, is used to its best effect.

A Reinterpretation of Evidence for the Endothelial Glycocalyx Filtration Structure

The endothelial glycocalyx (eGlx) is thought to be the primary macromolecular filter for fluid flux out of the vasculature. This filter maintains the higher protein concentration within the vessel lumen relative to the tissue. Whilst the arguments for the eGlx being the size filter are convincing the structural evidence has been limited to specialized stains of perfusion fixed tissue, which are further processed for resin embedding for transmission electron microscopy. The staining and processing of the delicate pore structure has left many researchers struggling to interpret the observed surface coat. Previous work has alluded to a 19.5 nm spacing between fibers; however, whilst repeatable it does not give an eGlx pore size consistent with known glycosaminoglycan (GAG) molecular structure due to the required fiber thickness of >10 nm. Here a new interpretation is proposed based on the likelihood that the electron micrographs of are often of collapsed eGlx. The 19.5 nm spacing measured may therefore be the core protein of the proteoglycans (PGs) with the GAGs wrapped up around them rather than in an expanded in vivo state. The concept is explored to determine that this is indeed consistent with experimental measurements of permeability if the syndecans are predominately dimerized. Further an alteration of core protein lattice from hexagonal packing to square packing dramatically changes the permeability which could be facilitated via known mechanisms such as transient actin binding.

The need to freeze-Dehydration during specimen preparation for electron microscopy collapses the endothelial glycocalyx regardless of fixation method

OBJECTIVE

The endothelial glycocalyx covers the luminal surface of the endothelium and plays key roles in vascular function. Despite its biological importance, ideal visualization techniques are lacking. The current study aimed to improve the preservation and subsequent imaging quality of the endothelial glycocalyx.

METHODS

In mice, the endothelial glycocalyx was contrasted with a mixture of lanthanum and dysprosium (LaDy). Standard chemical fixation was compared with high-pressure frozen specimens processed with freeze substitution. Also, isolated brain microvessels and cultured endothelial cells were high-pressure frozen and by transmission soft x-rays, imaged under cryogenic conditions.

RESULTS

The endothelial glycocalyx was in some tissues significantly more voluminous from chemically fixed specimens compared with high-pressure frozen specimens. LaDy labeling introduced excessive absorption contrast, which impeded glycocalyx measurements in isolated brain microvessels when using transmission soft x-rays. In non-contrasted vessels, the glycocalyx was not resolved. LaDy-contrasted, cultured brain endothelial cells allowed to assess glycocalyx volume in vitro.

CONCLUSIONS

Both chemical and cryogenic fixation followed by dehydration lead to substantial collapse of the glycocalyx. Cryogenic fixation without freeze substitution could be a way forward although transmission soft x-ray tomography based solely on amplitude contrast seems unsuitable.

The endothelial glycocalyx: Barrier functions versus red cell hemodynamics: A model of steady state ultrafiltration through a bi-layer formed by a porous outer layer and more selective membrane-associated inner layer

BACKGROUND

Ultrastructural investigations of the endothelial glycocalyx reveal a layer adjacent to the cell surface with a structure consistent with the primary  ultrafilter of vascular walls. Theory predicts this layer can be no greater than 200-300 nm thick, a result  to be reconciled with observations that red cells and large macromolecules are excluded  from a region 1 micrometer or more from the cell membrane.

OBJECTIVE

To determine whether this apparent inconsistency might be accounted for by a model of steady state water and protein transport through a glycocalyx bi-layer formed by a porous outer layer in series with a more selective inner layer.

METHODS

Expressions for coupled water and albumin fluxes through the two layers were used to describe steady state ultra-filtration though the bi-layer model.

RESULTS

Albumin accumulates at the interface between the porous layer and the selective inner layer. The osmotic pressure of accumulated albumin significantly modifies the observed permeability properties of the microvessel wall by an effective unstirred layer effect.

CONCLUSIONS

The model places significant constraints on the outer layer permeability properties . The only outer layer properties that are consistent with measured steady state filtration rates and models of red cell flux through microvessels are an albumin permeability coefficient and hydraulic conductivity more than an order of magnitude larger than the those of the inner layer.

Endothelial surface glycocalyx (ESG) components and ultra-structure revealed by stochastic optical reconstruction microscopy (STORM)

BACKGROUND

In order to play different roles in vascular functions as a mechanosensor to blood flows and as a barrier to transvascular exchange, the endothelial surface glycocalyx (ESG) should have an organized structure. Due to the limitations of optical and electron microscopy, the ultra-structure of ESG has not been revealed until the recent development of super-resolution optical microscopy, STORM.

OBJECTIVES

To investigate the ESG components and their organization on bEnd3 (mouse brain microvascular endothelial cells) monolayer.

METHODS

ESG was immunolabeled with anti-heparan sulfate (HS), followed by an ATTO488 conjugated goat anti-mouse IgG, and with biotinylated hyaluronic acid (HA) binding protein, followed by an AF647 conjugated anti-biotin. The ESG was then imaged by the STORM.

RESULTS

HA is a long molecule weaving into a network which covers the endothelial luminal surface. In contrast, HS is a shorter molecule, perpendicular to the cell surface. HA and HS are partially overlapped with each other at the endothelial luminal surface. We also quantified the length, diameter, orientation, and density of HS at the top, middle and bottom regions of the endothelial surface.

CONCLUSIONS

Our results suggest that HS plays a major role in mechanosensing and HA plays a major role in the molecular sieve.

Mechanotransmission of haemodynamic forces by the endothelial glycocalyx in a full-scale arterial model

The glycocalyx has been identified as a key mechano-sensor of the shear forces exerted by streaming blood onto the vascular endothelial lining. Although the biochemical reaction to the blood flow has been extensively studied, the mechanism of transmission of the haemodynamic shear forces to the endothelial transmembrane anchoring structures and, consequently, to the subcellular elements in the cytoskeleton, is still not fully understood. Here we apply a multiscale approach to elucidate how haemodynamic shear forces are transmitted to the transmembrane anchors of endothelial cells. Wall shear stress time histories, as obtained from image-based computational haemodynamics models of a carotid bifurcation, are used as a load and a continuum model is applied to obtain the mechanical response of the glycocalyx all along the cardiac cycle. The main findings of this in silico study are that: (1) the forces transmitted to the transmembrane anchors are in the range of 1-10 pN, which is in the order of magnitude reported for the different conformational states of transmembrane mechanotranductors; (2) locally, the forces transmitted to the anchors of the glycocalyx structure can be markedly different from the near-wall haemodynamic shear forces both in amplitude and frequency content. The findings of this in silico approach warrant future studies focusing on the actual forces transmitted to the transmembrane mechanotransductors, which might outperform haemodynamic descriptors of disturbed shear as localizing factors of vascular disease.

Investigation of Endothelial Surface Glycocalyx Components and Ultrastructure by Single Molecule Localization Microscopy: Stochastic Optical Reconstruction Microscopy (STORM)

On the luminal surface of our blood vessels, there is a thin layer called endothelial surface glycocalyx (ESG) which consists of proteoglycans, glycosaminoglycans (GAGs), and glycoproteins. The GAGs in the ESG are heparan sulfate (HS), hyaluronic acid (HA), chondroitin sulfate (CS), and sialic acid (SA). In order to play important roles in regulating vascular functions, such as being a mechanosensor and transducer for the endothelial cells (ECs) to sense the blood flow, a molecular sieve to maintain normal microvessel permeability and a barrier between the circulating cells and endothelial cells forming the vessel wall, the ESG should have an organized structure at the molecular level. Due to the limitations of conventional optical and electrical microscopy, the ultrastructure of ESG, in the order of 10 to 100 nanometers, has not been revealed until recent development of a super resolution fluorescence optical microscope, Stochastic Optical Reconstruction Microscope (STORM), which is one type of single molecule localization microscopy. This short review describes how the STORM can overcome the diffraction barrier in the conventional fluorescence microscopy to identify the chemical components of the ESG at a high spatial resolution. Examples of the organized ultrastructure of the ESG on the in vitro EC monolayer revealed by the Nikon-STORM system are given as well as how its components get lost during the onset of sepsis, a systemic inflammatory syndrome induced by bacterial infection, which demonstrate that this new technique can be applied to discover the structural and molecular mechanisms at nanometer scales in the native cellular environment for the cellular functions under normal and disease conditions.

The Lymphatic Fluid

This review will highlight our current understanding of the formation, circulation, and immunological role of lymphatic fluid. The formation of the extracellular fluid depends on the net balance between the hydrostatic and osmotic pressure gradients effective in the capillary beds. Lymph originates from the extracellular fluid and its composition combines the ultrafiltrated plasma proteins with the proteome generated by the metabolic activities of each parenchymal tissue. Several analyses have indicated how the lymph composition reflects the organs' physiological and pathological states. The collected lymphatic fluid moves from the capillaries into progressively larger collectors toward the draining lymph node aided by the lymphangion contractility and unidirectional valves, which prevent backflow. The proteomic composition of the lymphatic fluid is reflected in the MHC II peptidome presented by nodal antigen-presenting cells. Taken together, the past few years have generated new interest in the formation, transport, and immunological role of the lymphatic fluid.

Transport Across the Blood-Brain Barrier

The blood-brain barrier (BBB) is a dynamic barrier essential for maintaining the microenvironment of the brain. Although the special anatomical features of the BBB determine its protective role for the central nervous system (CNS) from blood-borne neurotoxins, however, the BBB extremely limits the therapeutic efficacy of drugs into the CNS, which greatly hinders the treatment of major brain diseases. This chapter summarized the unique structures of the BBB; described a variety of in vivo and in vitro experimental methods for determining the transport properties of the BBB and the permeability of the BBB to water, ions, and solutes including nutrients, therapeutic agents, and drug carriers; and presented recently developed mathematical models which quantitatively correlate the anatomical structures of the BBB with its barrier functions. Recent findings for modulation of the BBB permeability by chemical and physical stimuli were described. Finally, drug delivery strategies through specific trans-BBB routes were discussed.

The Role of Endothelial Surface Glycocalyx in Mechanosensing and Transduction

The endothelial cells (ECs) forming the inner wall of every blood vessel are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. A variety of mechanosensors and transducers, locating on the EC surface, intra- and trans-EC membrane, and within the EC cytoskeleton, have thus been identified to ensure proper functions of ECs. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins and is close to other prominent EC mechanosensors and transducers. This chapter summarizes the ESG composition, thickness, and structure observed by different labeling and visualization techniques and in different types of vessels. It also presents the literature in determining the ESG mechanical properties by atomic force microscopy and optical tweezers. The molecular mechanisms by which the ESG plays the role in EC mechanosensing and transduction are described as well as the ESG remodeling by shear stress, the actin cytoskeleton, the membrane rafts, the angiogenic factors, and the sphingosine-1-phosphate.

An Ichor-dependent apical extracellular matrix regulates seamless tube shape and integrity

During sprouting angiogenesis in the vertebrate vascular system, and primary branching in the Drosophila tracheal system, specialized tip cells direct branch outgrowth and network formation. When tip cells lumenize, they form subcellular (seamless) tubes. How these seamless tubes are made, shaped and maintained remains poorly understood. Here we characterize a Drosophila mutant called ichor (ich), and show that ich is essential for the integrity and shape of seamless tubes in tracheal terminal cells. We find that Ich regulates seamless tubulogenesis via its role in promoting the formation of a mature apical extracellular matrix (aECM) lining the lumen of the seamless tubes. We determined that ich encodes a zinc finger protein (CG11966) that acts, as a transcriptional activator required for the expression of multiple aECM factors, including a novel membrane-anchored trypsin protease (CG8213). Thus, the integrity and shape of seamless tubes are regulated by the aECM that lines their lumens.

Three-dimensional ultrastructure of capillary endothelial glycocalyx under normal and experimental endotoxemic conditions

BACKGROUND

Sugar-protein glycocalyx coats healthy endothelium, but its ultrastructure is not well described. Our aim was to determine the three-dimensional ultrastructure of capillary endothelial glycocalyx in the heart, kidney, and liver, where capillaries are, respectively, continuous, fenestrated, and sinusoidal.

METHODS

Tissue samples were processed with lanthanum-containing alkaline fixative, which preserves the structure of glycocalyx.

RESULTS

Scanning and transmission electron microscopy revealed that the endothelial glycocalyx layer in continuous and fenestrated capillaries was substantially thicker than in sinusoids. In the heart, the endothelial glycocalyx presented as moss- or broccoli-like and covered the entire luminal endothelial cell surface. In the kidney, the glycocalyx appeared to nearly occlude the endothelial pores of the fenestrated capillaries and was also present on the surface of the renal podocytes. In sinusoids of the liver, glycocalyx covered not only the luminal side but also the opposite side, facing the space of Disse. In a mouse lipopolysaccharide-induced experimental endotoxemia model, the capillary endothelial glycocalyx was severely disrupted; that is, it appeared to be peeling off the cells and clumping. Serum concentrations of syndecan-1, a marker of glycocalyx damage, were significantly increased 24 h after administration of lipopolysaccharide.

CONCLUSIONS

In the present study, we visualized the three-dimensional ultrastructure of endothelial glycocalyx in healthy continuous, fenestrated, and sinusoidal capillaries, and we also showed their disruption under experimental endotoxemic conditions. The latter may provide a morphological basis for the microvascular endothelial dysfunction associated with septic injury to organs.

Changes in ceftriaxone pharmacokinetics/pharmacodynamics during the early phase of sepsis: a prospective, experimental study in the rat

BACKGROUND

Sepsis is characterized by the loss of the perm-selectivity properties of the glomerular filtration barrier (GFB) with consequent albuminuria. We examined whether the pharmacokinetics-pharmacodynamics (PK/PD) of ceftriaxone (CTX), an extensively protein-bound 3rd generation cephalosporin, is altered during early sepsis and whether an increase in urinary loss of bound-CTX, due to GFB alteration, can occur in this condition.

METHODS

A prospective, experimental, randomized study was carried out in adult male Sprague-Dawley rats. Sepsis was induced by cecal ligation and puncture (CLP). Rats were divided into two groups: Sham-operated and CLP. CTX (100 mg i.p., equivalent to 1 g dose in humans) was administered in order to measure plasma and lung CTX concentrations at several time-points: baseline and 1, 2, 4 and 6 h after administration. CTX was measured by High Performance Liquid Chromatography (HPLC). The morphological status of the sialic components of the GFB barrier was assessed by lectin histo-chemistry. Monte Carlo simulation was performed to calculate the probability of target attainment (PTA >90%) for 80 and 100% of T_free > minimum inhibitory concentration (MIC) for 80 and 100% of dosing interval.

MEASUREMENTS AND MAIN RESULTS

After CLP, sepsis developed in rats as documented by the growth of polymicrobial flora in the peritoneal fluid (≤1 × 10¹ CFU in sham rats vs 5 × 10⁴-1 × 10⁵ CFU in CLP rats). CTX plasma concentrations were higher in CLP than in sham rats at 2 and 4 h after administration (difference at 2 h was 47.3, p = 0.012; difference at 4 h was 24.94, p = 0.004), while lung penetration tended to be lower. An increased urinary elimination of protein-bound CTX occurred (553 ± 689 vs 149 ± 128 mg/L, p < 0.05; % of bound/total CTX 22 ± 6 in septic rats vs 11 ± 4 in sham rats, p < 0.01) and it was associated with loss of the GFB sialic components. According to Monte Carlo simulation a PTA > 90% for 100% of the dosing interval was reached neither for sham nor CLP rats using MIC = 1 mg/L, the clinical breakpoint for Enterobacteriacee.

CONCLUSIONS

Sepsis causes changes in the PK of CTX and an alteration in the sialic components of the GFB, with consequent loss of protein-bound CTX. Among factors that can affect drug pharmacokinetics during the early phases of sepsis, urinary loss of both free and albumin-bound antimicrobials should be considered.

Albumen Transport to Bruch's Membrane and RPE by Choriocapillaris Caveolae

PURPOSE

The choriocapillaris (CC), the capillary network of the choroid, is positioned adjacent to Bruch's membrane (BM) and the RPE. The aim of this study was to clarify the mechanism(s) for transport of serum albumen from CC lumen to RPE.

METHODS

Alexa647 conjugated to BSA (BSA-A647) or PBS was administrated via the femoral vein to young and aged wild-type (WT; C57BL/6J) mice and Caveolin-1 knockout mice (Cav1(-/-)). Mice were perfused with PBS and killed at 30 minutes, 1 hour, and 4 hours after injection. Eyecups were cryopreserved, and cryosections were analyzed on a Zeiss 710 confocal microscope. Bovine serum albumin conjugated to gold nanoparticles (BSA-GNP) was administrated through the left common carotid artery. Mice were perfused with PBS and killed at 30 minutes after injection. Eyecups were embedded after fixation, and 70-nm-thick sections were analyzed on a Hitachi H7600 transmission electron microscope.

RESULTS

In eyes of WT young mice, BSA-A647 was transported to the RPE at 30 minutes and diffused to the photoreceptor layer by 1 hour. In contrast, most BSA-A647 was found in the CC in Cav1(-/-) eyes. The majority of BSA-GNP found in the CC of young WT mice was on the luminal side in caveolae at 30 minutes after injection. In aged WT mice, BSA-GNPs were found in defective tight junctions between endothelial cells and appeared trapped at the diaphragm of fenestrations.

CONCLUSIONS

Normally, CC carefully regulates transport system of BSA from lumen to BM by caveolae-mediated transcytosis; however, endothelium cells of aged control WT mice have leaky tight junctions and lacked regulated BSA transport.

Direct detection and measurement of wall shear stress using a filamentous bio-nanoparticle

The wall shear stress (WSS) that a moving fluid exerts on a surface affects many processes including those relating to vascular function. WSS plays an important role in normal physiology (e.g. angiogenesis) and affects the microvasculature's primary function of molecular transport. Points of fluctuating WSS show abnormalities in a number of diseases; however, there is no established technique for measuring WSS directly in physiological systems. All current methods rely on estimates obtained from measured velocity gradients in bulk flow data. In this work, we report a nanosensor that can directly measure WSS in microfluidic chambers with sub-micron spatial resolution by using a specific type of virus, the bacteriophage M13, which has been fluorescently labeled and anchored to a surface. It is demonstrated that the nanosensor can be calibrated and adapted for biological tissue, revealing WSS in micro-domains of cells that cannot be calculated accurately from bulk flow measurements. This method lends itself to a platform applicable to many applications in biology and microfluidics.

Surface-enhanced Raman spectroscopy of the endothelial cell membrane

We applied surface-enhanced Raman spectroscopy (SERS) to cationic gold-labeled endothelial cells to derive SERS-enhanced spectra of the bimolecular makeup of the plasma membrane. A two-step protocol with cationic charged gold nanoparticles followed by silver-intensification to generate silver nanoparticles on the cell surface was employed. This protocol of post-labelling silver-intensification facilitates the collection of SERS-enhanced spectra from the cell membrane without contribution from conjugated antibodies or other molecules. This approach generated a 100-fold SERS-enhancement of the spectral signal. The SERS spectra exhibited many vibrational peaks that can be assigned to components of the cell membrane. We were able to carry out spectral mapping using some of the enhanced wavenumbers. Significantly, the spectral maps suggest the distribution of some membrane components are was not evenly distributed over the cells plasma membrane. These results provide some possible evidence for the existence of lipid rafts in the plasma membrane and show that SERS has great potential for the study and characterization of cell surfaces.

Mechanosensing at the vascular interface

Mammals are endowed with a complex set of mechanisms that sense mechanical forces imparted by blood flow to endothelial cells (ECs), smooth muscle cells, and circulating blood cells to elicit biochemical responses through a process referred to as mechanotransduction. These biochemical responses are critical for a host of other responses, including regulation of blood pressure, control of vascular permeability for maintaining adequate perfusion of tissues, and control of leukocyte recruitment during immunosurveillance and inflammation. This review focuses on the role of the endothelial surface proteoglycan/glycoprotein layer-the glycocalyx (GCX)-that lines all blood vessel walls and is an agent in mechanotransduction and the modulation of blood cell interactions with the EC surface. We first discuss the biochemical composition and ultrastructure of the GCX, highlighting recent developments that reveal gaps in our understanding of the relationship between composition and spatial organization. We then consider the roles of the GCX in mechanotransduction and in vascular permeability control and review the prominent interaction of plasma-borne sphingosine-1 phosphate (S1P), which has been shown to regulate both the composition of the GCX and the endothelial junctions. Finally, we consider the association of GCX degradation with inflammation and vascular disease and end with a final section on future research directions.

The GPIHBP1-LPL complex is responsible for the margination of triglyceride-rich lipoproteins in capillaries

Triglyceride-rich lipoproteins (TRLs) undergo lipolysis by lipoprotein lipase (LPL), an enzyme that is transported to the capillary lumen by an endothelial cell protein, GPIHBP1. For LPL-mediated lipolysis to occur, TRLs must bind to the lumen of capillaries. This process is often assumed to involve heparan sulfate proteoglycans (HSPGs), but we suspected that TRL margination might instead require GPIHBP1. Indeed, TRLs marginate along the heart capillaries of wild-type but not Gpihbp1⁻/⁻ mice, as judged by fluorescence microscopy, quantitative assays with infrared-dye-labeled lipoproteins, and EM tomography. Both cell-culture and in vivo studies showed that TRL margination depends on LPL bound to GPIHBP1. Notably, the expression of LPL by endothelial cells in Gpihbp1⁻/⁻ mice did not restore defective TRL margination, implying that the binding of LPL to HSPGs is ineffective in promoting TRL margination. Our studies show that GPIHBP1-bound LPL is the main determinant of TRL margination.

Albumin modulates S1P delivery from red blood cells in perfused microvessels: mechanism of the protein effect

Removal of plasma proteins from perfusates increases vascular permeability. The common interpretation of the action of albumin is that it forms part of the permeability barrier by electrostatic binding to the endothelial glycocalyx. We tested the alternate hypothesis that removal of perfusate albumin in rat venular microvessels decreased the availability of sphingosine-1-phosphate (S1P), which is normally carried in plasma bound to albumin and lipoproteins and is required to maintain stable baseline endothelial barriers (Am J Physiol Heart Circ Physiol 303: H825-H834, 2012). Red blood cells (RBCs) are a primary source of S1P in the normal circulation. We compared apparent albumin permeability coefficients [solute permeability (Ps)] measured using perfusates containing albumin (10 mg/ml, control) and conditioned by 20-min exposure to rat RBCs with Ps when test perfusates were in RBC-conditioned protein-free Ringer solution. The control perfusate S1P concentration (439 ± 46 nM) was near the normal plasma value at 37 °C and established a stable baseline Ps (0.9 ± 0.4 × 10(-6) cm/s). Ringer solution perfusate contained 52 ± 8 nM S1P and increased Ps more than 10-fold (16.1 ± 3.9 × 10(-6) cm/s). Consistent with albumin-dependent transport of S1P from RBCs, S1P concentrations in RBC-conditioned solutions decreased as albumin concentration, hematocrit, and temperature decreased. Protein-free Ringer solution perfusates that used liposomes instead of RBCs as flow markers failed to maintain normal permeability, reproducing the "albumin effect" in these mammalian microvessels. We conclude that the albumin effect depends on the action of albumin to facilitate the release and transport of S1P from RBCs that normally provide a significant amount of S1P to the endothelium.

Resolution of the three dimensional structure of components of the glomerular filtration barrier

Background

The human glomerulus is the primary filtration unit of the kidney, and contains the Glomerular Filtration Barrier (GFB). The GFB had been thought to comprise 3 layers – the endothelium, the basement membrane and the podocyte foot processes. However, recent studies have suggested that at least two additional layers contribute to the function of the GFB, the endothelial glycocalyx on the vascular side, and the sub-podocyte space on the urinary side. To investigate the structure of these additional layers is difficult as it requires three-dimensional reconstruction of delicate sub-microscopic (<1 μm) cellular and extracellular elements.

Methods

Here we have combined three different advanced electron microscopic techniques that cover multiple orders of magnitude of volume sampled, with a novel staining methodology (Lanthanum Dysprosium Glycosaminoglycan adhesion, or LaDy GAGa), to determine the structural basis of these two additional layers. Serial Block Face Scanning Electron Microscopy (SBF-SEM) was used to generate a 3-D image stack with a volume of a 5.3 x 10⁵ μm³ volume of a whole kidney glomerulus (13% of glomerular volume). Secondly, Focused Ion Beam milling Scanning Electron Microscopy (FIB-SEM) was used to image a filtration region (48 μm³ volume). Lastly Transmission Electron Tomography (Tom-TEM) was performed on a 0.3 μm³ volume to identify the fine structure of the glycocalyx.

Results

Tom-TEM clearly showed 20 nm fibre spacing in the glycocalyx, within a limited field of view. FIB-SEM demonstrated, in a far greater field of view, how the glycocalyx structure related to fenestrations and the filtration slits, though without the resolution of TomTEM. SBF-SEM was able to determine the extent of the sub-podocyte space and glycocalyx coverage, without additional heavy metal staining. Neither SBF- nor FIB-SEM suffered the anisotropic shrinkage under the electron beam that is seen with Tom-TEM.

Conclusions

These images demonstrate that the three dimensional structure of the GFB can be imaged, and investigated from the whole glomerulus to the fine structure of the glycocalyx using three dimensional electron microscopy techniques. This should allow the identification of structural features regulating physiology, and their disruption in pathological states, aiding the understanding of kidney disease.

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.

Mechano-sensing and transduction by endothelial surface glycocalyx: composition, structure, and function

The endothelial cells (ECs) lining every blood vessel wall are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. To ensure proper EC mechano-sensing and transduction, there are a variety of mechano-sensors and transducers that have been identified on the EC surface, intra- and trans-EC membrane and within the EC cytoskeleton. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins, and is close to other prominent EC mechano-sensors and transducers. The ESG thickness was found to be in the order of 0.1-1 µm by different visualization techniques and in different types of vessels. Detailed analysis on the electron microscopy (EM) images of the microvascular ESG revealed a quasi-periodic substructure with the ESG fiber diameter of 10-12 and 20 nm spacing between adjacent fibers. Atomic force microscopy and optical tweezers were applied to investigate the mechanical properties of the ESG on the cultured EC monolayers and in solutions. Enzymatic degradation of specific ESG glycosaminoglycan components was used to directly elucidate the role of the ESG in EC mechano-sensing and transduction by measuring the shear-induced productions of nitric oxide and prostacyclin, two characteristic responses of the ECs to the flow. The unique location, composition, and structure of the ESG determine its role in EC mechano-sensing and transduction.

Electron tomography of vesicles

In this issue of Microcirculation, Wagner, Modla, Hossler and Czmmek [25] describe the use of electron tomography to visualize the three-dimensional arrangement of small endothelial vesicles and caveolae of muscle capillaries. Their images show the well-known clusters of fused vesicles communicating with caveolae at the luminal and abluminal surfaces. The advantages of electron tomography are shown by well resolved images of single cytoplasmic vesicles separate from fused vesicle clusters and also by occasional chains of fused vesicles forming trans-endothelial channels. Twenty five to thirty years ago the existence of both trans-endothelial channels and single unattached vesicles was disputed. Also, since some single vesicles and all of the trans-endothelial channels are labeled with a lanthanide tracer present in the perfusate at the time of fixation, this evidence once again raises the question of whether vesicles have a role in vascular permeability to macromolecules. This brief review describes the origin of the vesicle controversy, some of the more recent evidence for and against the participation of vesicles in macromolecular transport and considers some criticisms of ultra-structural evidence for vesicular transport that still require answers.