Transcellular vesicular transport in epithelial and endothelial cells: Challenges and opportunities

Vesicle‐mediated transcellular transport or simply “transcytosis” is a cellular process used to shuttle macromolecules such as lipoproteins, antibodies, and albumin from one surface of a polarized cell to the other. This mechanism is in contrast to the transit of small molecules such as anions, cations and amino acids that occur via uptake, diffusion through the cytosol and release and is also distinct from paracellular leak between cells. Importantly, transcytosis has evolved as a process to selectively move macromolecules between 2 neighboring yet unique microenvironments within a multicellular organism. Examples include the movement of lipoproteins out of the circulatory system and into tissues and the delivery of immunoglobulins to mucosal surfaces. Regardless of whether the transport is conducted by endothelial or epithelial cells, the process often involves receptor‐mediated uptake of a ligand into an endocytic vesicle, regulated transit of the carrier through the cytoplasm and release of the cargo via an exocytic event. While transcytosis has been examined in detail in epithelial cells, for both historical and technical reasons, the process is less understood in endothelial cells. Here, we spotlight aspects of epithelial transcytosis including recent findings and review the comparative dearth of knowledge regarding the process in endothelial cells highlighting the opportunity for further study.      

Insights into the secretory pathway and vesicular transport in plant cells

Summary— The vectorial transport of membrane and macromolecules within the cytoplasm of eukaryotic cells has been the subject of intense investigation over the last decade. In this paper we review some of the recent advances made in our understanding of vesicle transport and the secretory pathway in plant cells.     

A theoretical model for electron transport through chlorophyll-containing bileaflet membranes

Summary A model of electron transport through bileaflet membranes comprised of chlorophylls and carotenoids is proposed and can account for the photoresponses observed in chloroplast-extract membranes. Some features of the model are as follows: In the absence of any specific additive, such as phycocyanin, the energy barrier for electron transfer between chromophores in the membrane and a redox pair in the aqueous solution is significantly high, so that the only realistic mechanism for electron exchange through the interface is by quantum-mechanical tunneling. In an analogy to theories developed for the explanation of semiconductor electrode behavior, the essential requirement of energy overlap between occupied and unoccupied electronic states on either side of the membrane-water interface is explored. The extent of overlap can be controlled by the potential difference developed across the interface. The presence of the carotenoids can reduce the energy barrier for electron movement within she membrane so that excited electrons can pass from the chlorophyll chromophores on one tide of the membrane interface to those on the other side. It is suggested that electron tunneling is the mechanism by which reducing substances belonging to a redox pair of high oxidation-reduction potential deliver their electrons to vacant orbitals in the photosynthetic membranes.     

A theoretical model of localized heat and water vapor transport in the human respiratory tract

A steady-state, one-dimensional theoretical model of human respiratory heat and water vapor transport is developed. Local mass transfer coefficients measured in a cast replica of the upper respiratory tract are incorporated into the model along with heat transfer coefficients determined from the Chilton-Colburn analogy and from data in the literature. The model agrees well with reported experimental measurements and predicts that the two most important parameters of the human air-conditioning process are: the blood temperature distribution along the airway walls, and the total cross-sectional area and perimeter of the nasal cavity. The model also shows that the larynx and pharynx can actually gain water over a respiratory cycle and are the regions of the respiratory tract most subject to drying. With slight modification, the model can be used to investigate respiratory heat and water vapor transport in high stress environments, pollutant gas uptake in the respiratory tract, and the connection between respiratory air-conditioning and the function of the mucociliary escalator.     

Flavonoid transport by mammalian endothelial cells

Despite the ever-growing body of literature reporting the effects of flavonoids on animals at both the cellular and systemic levels, one of the most basic questions-"Are the effects of flavonoids on animal cells initiated through their interaction with extracellular targets or intracellular targets?"-has yet to be addressed. Because many effects of flavonoids on cells can be detected within minutes of flavonoid application and because flavonoids diffuse across lipid membranes slowly or not at all, intracellular mechanisms would necessitate a flavonoid transport system for rapid flavonoid uptake. The specific aims of this investigation were (1) to determine if endothelial cells contain a mechanism that mediates rapid flavonoid uptake and (2) to provide evidence for or against the hypothesis that rapid flavonoid effects on endothelial cell synthesis of prostacyclin and endothelin are initiated through the interaction of flavonoids with intracellular targets. Data show that bovine and human aortic endothelial cells possess a transport system that mediates rapid uptake of the flavonoid morin and suggest that the flavonoid uptake system utilizes a variety of oxygenated phenolic compounds as substrates. Further investigation into flavonoid transport should expedite future investigation into the mechanisms of flavonoid actions, because it may allow research to focus on the cellular locations where flavonoids are concentrated. Although endothelial cells contain a mechanism for the rapid uptake of morin, data reported herein suggest that morin initiates its rapid effects on endothelial cell synthesis of prostacyclin and endothelin through an interaction with extracellular targets.     

Taking vesicular transport to the cilium

Defects in protein trafficking within the cell body and cilia are thought to underlie the human disease Bardet-Biedl syndrome (BBS). In this issue, Nachury et al. (2007) reveal that a large complex of proteins implicated in BBS cooperates with Rabin8-the GTP exchange factor for the small GTPase Rab8-to promote cilia formation and presumably movement of membrane proteins from the cell into the cilium.     

Cation transport in vascular endothelial cells and aging

Summary To understand the generation and maintenance of Na and K gradients in cultured vascular endothelial cells, net Na and K movements were studied. Ouabain-sensitive (OS) net Na gain and K loss were estimated as the difference between the cation content in the presence of ouabain and that in the control. Ouabain-and furosemide-resistant (OFR) fluxes were determined in the presence of the two inhibitors. When the normal medium bicarbonate and phosphate buffers were replaced by N-2-hydroxyethylpiperazine-N'-2-ethane sulfonic acid both the OS ans OFR fluxes decreased more than 50%. Ouabain-sensitive and ouabain-and furosemide-resistant fluxes decreased with increasing cellular age (passage number) an effect not observed when the cation movements were studied in the absence of bicarbonate and phosphate. These results suggest that cultured vascular endothelial cells possess bicarbonate-and phosphate-dependent Na and K pathways which account for a significant portion of their passive movements. Furthermore, the behavior of cation permeabilities with passage number suggests that these modulations may be related to the cellular aging process.     

A new model of human aortic endothelial cells in vitro

Vascular endothelial cells play an important role in coagulation regulation of vascular tone and in a variety of synthetic and metabolic functions. Endothelial cells also have a pivotal role in immunological diseases atherogenesis and tumor angiogenesis. Endothelial cells are often used as system to study the pathophysiology of late complications in diabetes mellitus atherosclerotic damages and leukocyte adhesion in inflammatory diseases. Most of the studies have been performed on primary arterial and venous endothelial cell cultures with problems such as availability of autoptic material and reproducibility of cell cultures. We have isolated and characterized a novel system of proliferating long-term cultures of human aortic endothelial cells that maintain their differentiated characteristics for many generations in vitro. They produce antithrombotic and thrombotic factors such as t-PA and PAI-1 and respond to TNFalpha, an important factor correlated with the inflammatory process by modifying growth characteristics by producing cytokines such as GM-CSF by expressing ICAM-1 on the surface and by producing large amounts of nitric oxide and endothelin. This new system may be very useful to understand and study the molecular mechanisms involved in many vascular alteration pathologies and in the aging process.     

Targeting and fusion in vesicular transport

Vesicular transport of proteins and lipids between distinct subcellular compartments is directly responsible for generating and maintaining the structure of the organelles of the secretory and endocytic pathways in eukaryotic cells. Rapid advances in a variety of experimental systems have resulted in the identification of molecules involved in late steps of the transport process. This article presents a general paradigm for vesicular fusion and reviews the available experimental evidence.     

Small GTP-binding proteins in vesicular transport

Recent recognition of the abundance of small GTP-binding proteins in eukaryotic cells has sparked off a search for the possible function of these proteins. Evidence is accumulating that SAR1, ARF, SEC4 and YPT1 in yeast and the rab and arf family in mammalian cells play a central role in the regulation of vesicle transport and organelle function.     

Regulation of sterol transport in human microvascular endothelial cells

In cultured human dermal microvessel endothelial cells, the rate of efflux (about twofold greater than for fibroblasts under equivalent conditions) was coupled to an equivalent high rate of sterol net transport from the cells to the medium. This net transport was linked with esterification via lecithin:cholesterol acyltransferase. Since the use of free sterol by plasma transferase is constant, such increased net transport indicates that endothelial cells are highly efficient, in competition with plasma lipoproteins, in supplying free sterol for esterification. These results indicate the marked ability of endothelial cells to regulate and maintain their sterol balance in the face of high sterol levels to which these cells are uniquely exposed in human plasma.     

Studies of L-arginine transport in bovine aortic endothelial cells

We have previously demonstrated that p(1),p(4)-diadenosine 5'-tetraphosphate induces the release of NO and modulates the uptake of L-arginine by bovine aortic endothelial cells (BAEC) [Hilderman, R. H., and Christensen, E. F. (1998) FEBS Lett. 407, 320-324; Hilderman, R. H., Casey, T. E., and Pojoga, L. H. (2000) Arch. Biochem. Biophys. 375, 124-130]. In this communication we characterize the uptake of L-Arg by BAEC. L-Arg is transported into BAEC by at least two different transporter systems. One transporter system is protein synthesis dependent, and L-Arg transported by this system is incorporated into proteins. The second transporter system involved in L-Arg uptake is protein synthesis independent, and uptake occurs by facilitated diffusion. The L-Arg transported by facilitated diffusion is metabolized into L-argininosuccinate. Homologous and heterologous competition uptake studies were performed using a fixed concentration of radiolabeled L-Arg, L-lysine, and L-leucine with varying concentrations of competing nonradiolabeled amino acids. The results of these competition uptake studies are consistent with the protein-synthesis-dependent uptake of L-Arg taking place through a transporter system that is highly specific for L-Arg and with the facilitated diffusion uptake taking place through a transporter that is specific for L-Arg and L-Leu.