Caveolar and intercellular channels provide major transport pathways of macromolecules across vascular endothelial cells

Application of advances in endocytosis and membrane trafficking to drug delivery

Multidisciplinary research efforts in the field of drug delivery have led to the development of a variety of drug delivery systems (DDS) designed for site-specific delivery of diagnostic and therapeutic agents. Since efficient uptake of drug carriers into target cells is central to effective drug delivery, a comprehensive understanding of the biological pathways for cellular internalization of DDS can facilitate the development of DDS capable of precise tissue targeting and enhanced therapeutic outcomes. Diverse methods have been applied to study the internalization mechanisms responsible for endocytotic uptake of extracellular materials, which are also the principal pathways exploited by many DDS. Chemical inhibitors remain the most commonly used method to explore endocytotic internalization mechanisms, although genetic methods are increasingly accessible and may constitute more specific approaches. This review highlights the molecular basis of internalization pathways most relevant to internalization of DDS, and the principal methods used to study each route. This review also showcases examples of DDS that are internalized by each route, and reviews the general effects of biophysical properties of DDS on the internalization efficiency. Finally, options for intracellular trafficking and targeting of internalized DDS are briefly reviewed, representing an additional opportunity for multi-level targeting to achieve further specificity and therapeutic efficacy.

Understanding Inter-Individual Variability in Monoclonal Antibody Disposition

Monoclonal antibodies (mAbs) are currently the largest and most dominant class of therapeutic proteins. Inter-individual variability has been observed for several mAbs; however, an understanding of the underlying mechanisms and factors contributing to inter-subject differences in mAb disposition is still lacking. In this review, we analyze the mechanisms of antibody disposition and the putative mechanistic determinants of inter-individual variability. Results from in vitro, preclinical, and clinical studies were reviewed evaluate the role of the neonatal Fc receptor and Fc gamma receptors (expression and polymorphism), target properties (expression, shedding, turnover, internalization, heterogeneity, polymorphism), and the influence of anti-drug antibodies. Particular attention is given to the influence of co-administered drugs and disease, and to the physiological relevance of covariates identified by population pharmacokinetic modeling, as determinants of variability in mAb pharmacokinetics.

Caveolae: One Function or Many?

Caveolae are small, bulb-shaped plasma membrane invaginations. Mutations that ablate caveolae lead to diverse phenotypes in mice and humans, making it challenging to uncover their molecular mechanisms. Caveolae have been described to function in endocytosis and transcytosis (a specialized form of endocytosis) and in maintaining membrane lipid composition, as well as acting as signaling platforms. New data also support a model in which the central function of caveolae could be related to the protection of cells from mechanical stress within the plasma membrane. We present evidence for these diverse roles and consider in vitro and in vivo experiments confirming a mechanoprotective role. We conclude by highlighting current gaps in our knowledge of how mechanical signals may be transduced by caveolae.

In vitro microvessels for the study of angiogenesis and thrombosis

Microvascular networks support metabolic activity and define microenvironmental conditions within tissues in health and pathology. Recapitulation of functional microvascular structures in vitro could provide a platform for the study of complex vascular phenomena, including angiogenesis and thrombosis. We have engineered living microvascular networks in three-dimensional tissue scaffolds and demonstrated their biofunctionality in vitro. We describe the lithographic technique used to form endothelialized microfluidic vessels within a native collagen matrix; we characterize the morphology, mass transfer processes, and long-term stability of the endothelium; we elucidate the angiogenic activities of the endothelia and differential interactions with perivascular cells seeded in the collagen bulk; and we demonstrate the nonthrombotic nature of the vascular endothelium and its transition to a prothrombotic state during an inflammatory response. The success of these microvascular networks in recapitulating these phenomena points to the broad potential of this platform for the study of cardiovascular biology and pathophysiology.

Transvascular protein transport in mice lacking endothelial caveolae

Caveolae are Ω-shaped vesicular structures postulated to play a role in transvascular protein transport. Studies on mice lacking endothelial caveolae, caveolin-1 knockout (Cav-1-KO) mice, indicate increased macromolecular transport rates. This was postulated to be due to the appearance of an alternative pathway. The present study tested whether an alternative pathway had appeared in Cav-1-KO mice. Male Cav-1-KO ( n = 12) and male control mice ( n = 13) were intubated and anesthetized using 2% isoflurane.125I-labeled albumin,131I-labeled immunoglobulin M (IgM), and polydisperse FITC-Ficoll were administered intravenously. During tracer administration, a 90-min peritoneal dialysis dwell was performed. Clearance of tracers to dialysate and permeability-surface area product for glucose were assessed. Transvascular protein transport was higher in Cav-1-KO compared with control mice. Albumin clearance from plasma to peritoneum was 0.088 ± 0.008 μl/min in control and 0.179 ± 0.012 μl/min in Cav-1-KO ( P = 0.001) mice. IgM clearance was 0.049 ± 0.003 and 0.083 ± 0.010 μl/min in control and Cav-1-KO mice, respectively ( P = 0.016). Ficoll clearance was increased in Cav-1-KO mice. In conclusion, the lack of caveolae in Cav-1-KO mice resulted in a marked increase in macromolecular transport. A two-pore analysis of the Ficoll clearance data revealed that the higher transport rate in Cav-1-KO mice was not compatible with the appearance of an alternative pathway for macromolecular transport. In contrast, the higher transperitoneal protein and Ficoll clearance is consistent with passive porous transport through an unperturbed two-pore system, presumably at an elevated capillary hydraulic pressure. Alternatively, the data may be explained by reductions in the selectivity of the endothelial glycocalyx, leading to an increased capillary hydraulic conductivity and large solute filtration.

Selective Suppression of an Endothelin and Platelet-Derived Growth Factor Containing Vesicular System in Endothelium of Rat Saphenous Vein by Long-Term Orthostasis

Electron-dense vesicles were observed in rat vascular endothelium. The purpose of this study was to characterize their content(s), venous-arterial distribution and response to chronic orthostatic stress in extremity vessels. Saphenous and brachial vessels - saphenous vein (SV), saphenous artery (SA), brachial vein, brachial artery - were prepared for electron microscopy to quantitate the vesicle area within the endothelium following immunohistochemical and immunocytochemical identification. The effect of long-term orthostasis was assessed by exposure to head-up tilt for 2 weeks. The vesicular area in relation to the total cross-sectional area of the endothelial cells in the SV and SA of normal and confined control groups was 3.88 +/- 0.38 versus 0.89 +/- 0.06% (p < 0.05) and 4.92 +/- 0.25 versus 1.09 +/- 0.47% (p < 0.05), respectively. Head-up tilt suppressed the vesicle content of the SV to 2.26 +/- 0.39% (p < 0.05), but it remained low in the SA (1.29 +/- 0.45%), brachial vein (0.45 +/- 0.12%) and brachial artery (0.59 +/- 0.17%). Endothelin and platelet-derived growth factor, but not acidic phosphatase activity or lipid content, could be identified in the vesicles. Plasma endothelin levels were unchanged. We conclude that dense vesicles in the endothelium of extremity vessels are not cell degradation products. They may represent a vesicular secretory or storage system for endothelin and platelet-derived growth factor which participates in regional vascular adaptation to long-term orthostatic load.

Vacuole formation in the endothelium of rat extremity vessels depends on fixation techniques and vessel type

Applying immersion fixation for electron microscopy, huge clear endothelial membrane-bound vacuoles of 0.1-3 microm diameter were noted in the extremity veins of Sprague-Dawley rats. Histological and electron microscopic histochemical methods were applied to determine whether they were the product of programmed cell death or any other kind of cell damage. Image analyzer was used to measure the total area of the vacuoles in the endothelium cells. Neither lipid content nor acidic phosphatase activity could be identified in the vacuoles. In saphenous and brachial veins, the vacuoles occupied 20.6 +/- 2.21% and 18 +/- 2.45% of the endothelium, respectively. Venous endothelium of two different strains of rat also contained the vacuoles. No such structures appeared in extremity arteries. Long-term tilting did not influence vacuolization. Using in vivo whole body fixation, only pinocytotic and dense microvesicles, but no vacuoles were noted. In conclusion, the clear vacuolar structures represent neither lipid inclusions nor secondary lysosomes. The method of tissue fixation is critical when venous endothelial vesicles are investigated. It is presumed that the vacuoles originated from intra- or intercellular microstructures, and that in case of the collapsible vein segments, their size is increased under the pathological-hypoxic and low-pressure-conditions of in vitro fixation.

Transvascular Passage of Macromolecules into the Peritoneal Cavity of Normo- and Hypothermic Rats in vivo: Active or Passive Transport?

During the last decades there has been a debate regarding whether transvascular protein transport is an active (transcytosis) or a passive (porous) process. To separate cooling-sensitive transcytosis from passive transport processes between blood and peritoneal fluid, we induced hypothermia in rats in vivo, reducing their body temperature to 19 degrees C. Control rats were kept at 37 degrees C. Either human albumin, or IgG, or IgM, or LDL, radiolabeled with (125)I, was given intra-arterially together with (51)Cr-EDTA. During tracer administration, a 2-hour peritoneal dialysis dwell was performed. Clearance of the tracers to dialysate, and the permeability-surface area coefficient (PS) for (51)Cr-EDTA and glucose were assessed. During cooling, mean arterial blood pressure (MAP) was reduced to 40% of control and plasma viscosity increased by 48.5%, while peritoneal blood flow was reduced to 10%. At 19 degrees C, clearance of albumin to dialysate fell from 9.30 +/- 1.62 (SEM) to 3.13 +/- 0.28 microl/min (p < 0.05), clearance of IgG from 6.33 +/- 0.42 to 2.54 +/- 0.12 microl/min (p < 0.05), clearance of IgM from 3.65 +/- 0.33 to 1.10 +/- 0.12 microl/min (p < 0.05), and clearance of LDL from 3.54 +/- 0.20 to 0.73 +/- 0.06 microl/min (p < 0.05). The fall in PS for (51)Cr-EDTA was from 0.320 +/- 0.01 to 0.075 +/- 0.003 ml/min (p < 0.05), and that for glucose from 0.438 +/- 0.02 to 0.105 +/- 0.01 ml/min (p < 0.05). Tissue cooling reduced large solute transport largely in proportion to the cooling-induced reductions of MAP (to 40%), and the concomitant increase in viscosity (to 67%), i.e. to approximately 20-30% (0.40 x 0.67) of control, though LDL clearance was reduced further. The fall in small solute PS, in excess of the viscosity effect, mirrored the fall in peritoneal blood flow occurring during hypothermia. In conclusion, the good correlation of predicted to calculated changes suggests that the overall transendothelial macromolecular passage in vivo occurs passively, and not due to active processes.

Age-related endothelial dysfunction : potential implications for pharmacotherapy

Aging per se is associated with abnormalities of the vascular wall linked to both structural and functional changes that can take place at the level of the extracellular matrix, the vascular smooth muscle and the endothelium of blood vessels. Endothelial dysfunction is generally defined as a decrease in the capacity of the endothelium to dilate blood vessels in response to physical and chemical stimuli. It is one of the characteristic changes that occur with age, independently of other known cardiovascular risk factors. This may account in part for the increased incidence of cardiovascular events in elderly people that can be reversed by restoring endothelial function. A better understanding of the mechanisms involved and the aetiopathogenesis of this process will help in the search for new therapeutic agents.Age-dependent alteration of endothelium-dependent relaxation seems to be a widespread phenomenon both in conductance and resistance arteries from several species. In the course of aging, there is an alteration in the equilibrium between relaxing and contracting factors released by the endothelium. Hence, there is a progressive reduction in the participation of nitric oxide and endothelium-derived hyperpolarising factor associated with increased participation of oxygen-derived free radicals and cyclo-oxygenase-derived prostanoids. Also, the endothelin-1 and angiotensin II pathways may play a role in age-related endothelial dysfunction. The use of drugs acting at different levels of these signalling cascades, including antioxidant therapy, lipid-lowering drugs and estrogens, seems to be promising.

Endocytic pathway in mouse cardiac cells

Primary cultures of heart muscle cells provide powerful tools for cardiac cell biological research that permits both physiological and biochemical approaches. In the present study we analyzed the endocytosis of cardiac cells and presented morphological characterization of the endocytic machinery using markers, which enabled us to follow the fluid-phase, receptor-mediated endocytosis and the internalization of large particles. Our results demonstrated the route of the internalized cargo to early endosomes followed or not by its discharge in the late compartments. We also confirmed the ability of cardiac muscle cells to ingest large particles such as the mannosylated ligand zymosan A, and even internalize whole eukaryotic cells such as the protozoan parasite Trypanosoma cruzi. Since endocytosis is involved in many important cellular functions, the present work contributes to the knowledge of possible additional roles played by cardiac muscle cells besides their well known ability to act as physically energetic cells in the body, constantly contracting without tiring.

Transendothelial transport of low-density lipoprotein and albumin across the rat peritoneum in vivo: effects of the transcytosis inhibitors NEM and filipin

This study was performed to investigate the mechanisms responsible for the transport of albumin and low-density lipoprotein (LDL) across capillary walls in vivo. To separate transcytosis from passive, 'porous' transport, we tested the effects of the transcytosis inhibitors N-ethylmaleimide (NEM) and filipin given intraperitoneally on the peritoneal capillary clearance of LDL and albumin in anesthetized rats undergoing peritoneal dialysis. Radiolabeled human albumin or LDL was given intra-arterially, and (51)Cr-EDTA was infused intravenously. A 2-hour peritoneal dialysis dwell was performed using 16 ml of conventional 1.36% glucose-based dialysis fluid. The clearance of LDL and albumin to the dialysate and the peritoneal mass transfer coefficient for (51)Cr-EDTA were assessed. Following intraperitoneal NEM incubations (0.5-5 mM), there were marked increases in the peritoneal transport of albumin and LDL for NEM doses exceeding 1 mM. For lower NEM doses, there were no reductions in clearance. Filipin incubations (0.2-4 microg/ml) did not affect the clearance of either macromolecule. In conclusion, neither NEM nor filipin caused reductions in albumin or LDL clearance across the peritoneal capillaries. The present data clearly show that NEM and filipin are unsuitable as transcytosis inhibitors in vivo.