Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

Mechanisms of Endocytosis

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

The Caveolae Membrane System

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite "sensor" that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

https://www.physiology.org/doi/pdf/10.1152/physrev.00012.2005

Signaling Mechanisms Regulating Endothelial Permeability

https://www.jbc.org/content/273/10/5419.full.pdf

Caveolins, a Family of Scaffolding Proteins for Organizing “Preassembled Signaling Complexes” at the Plasma Membrane

https://www.cell.com/trends/neurosciences/pdf/S0166-2236(00)02004-X.pdf

Molecular physiology and pathophysiology of tight junctions in the blood–brain barrier

Albumin-binding proteins function in the receptor-mediated binding and transcytosis of albumin across cultured endothelial cells

Establishment of a Pure Vascular Endothelial Cell Line from Human Placenta

Ultrastructural Evidence of Differential Solubility in Triton X-100 of Endothelial Vesicles and Plasma Membrane

The endothelial cell (EC) dysfunction is a common characteristic of various pathologies that include atherosclerosis, hypertension, and Fabry's disease. Aware of the role of eNO and ACE in EC dysfunction, we questioned whether polymorphism of eNOS and/or ACE gene may be a common denominator in these pathologies. Patients with CHD (108), HT (109), Fabry's disease (37) and healthy subjects (control, 141) were genotyped for the eNOSG894T by RFLP-PCR technique and for eNOS4b/a, and ACEI/D polymorphisms by PCR amplification. The results of these studies were statistically evaluated. Compared to controls, the frequency of the eNOSG894T (T allele) was higher in CHD (P=0.03) and Fabry (P=0.01), while the eNOS4b/a (a allele) in CHD (P=0.01) and HT patients (P=0.01). The proportion of the ACEI/D was similar in all subjects. In CHD patients at "low risk" of atherogenic factors, the frequency of the T and a alleles of eNOS gene was high (P=0.03 and 0.02, respectively). Carriers of the T allele of eNOSG894T were over-represented (P=0.04) in Fabry subgroup with renal failure. Compared to women, the eNOS894T alleles were more frequent (P=0.03) in men with CHD and HT, whereas ACE I/D in men (P=0.03) with HT. These findings suggest: (i) the frequency of eNOSG894T and/or eNOS4b/a is significantly associated with coronary dysfunction; (ii) eNOS4b/a confers a relatively high risk of hypertension in subjects with atherogenic risk factors; (iii) the frequency of eNOSG894T is high in Fabry hemizygotes with renal complications. Therefore, eNOS gene polymorphism represent a frequent risk factor for vascular abnormalities in CHD, HT and Fabry's disease, afflictions which have in common, the endothelial dysfunction.

Relationship of eNOS gene variants to diseases that have in common an endothelial cell dysfunction.

The physiological activities of histamine on various systems including the heart and blood vessels'-9 are mediated by at least two different types of receptors.10-12 As defined on a pharmacological basis, the H1 receptors are specifically inhibited by classical antihistamines such as mepyramine, and have as agonists drugs such as 2-pyridylethylamine, The Hz receptors are blocked by antagonists such as cimetidine, and have as a specific agonist 4-methylhistamine.4~~~~~~~~~~~4 The identification of histamine receptors and their subclasses on diverse cells, including those of the cardiovascular system, has relied primarily on the target responses to histamine and its specific agonists and antagonists.15-26 It has been demonstrated that histamine receptors are located on the plasma membrane of several mammalian cells.27-33 Their presence was so far detected either indirectly, by the binding of various cells to histamine insolubilized on agarose beads,z7.34.35 or visualized at the light microscope level by coupling histamine to bovine serum albumin-fluoresceine isothiocyanatezz or to o-phthaldialdehyde.36 Another indirect way of identifying histamine receptors was by measuring the binding of the antagonist [3H]mepyramine.37 However, the direct detection and the topographical distribution of histamine receptors and their subclasses at the ultrastructural level have not been accomplished yet. We have carried out studies with the electron microscope to localize histamine receptors in various cells of the vessel wall. For this purpose, we have produced biologically active, electron-opaque conjugates of histamine and its agonists, which we have recently used to localize in situ the histamine receptors on the vascular endothelium.38-40

Constantina Heltianu

Papers

Albumin-binding proteins function in the receptor-mediated binding and transcytosis of albumin across cultured endothelial cells

Establishment of a Pure Vascular Endothelial Cell Line from Human Placenta

Ultrastructural Evidence of Differential Solubility in Triton X-100 of Endothelial Vesicles and Plasma Membrane

Relationship of eNOS gene variants to diseases that have in common an endothelial cell dysfunction.

Endothelial cell receptors for histamine