Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 516

Intracellular transport can begin with receptor binding. 80 Once bound to their
receptors, the molecules assemble in a patch that then bulges to form a vesicle. From
the starting membrane granulation to its fusion with target membrane, the vesicle
carries the molecules. Such types of shuttles also transport manufactured molecules
from the endoplasmic reticulum to the Golgi body, from which are launched vesicles
for immediate extracellular release, vesicles for signal-dependent secretion, and
digestive lysosomes.
The intracellular receptor-mediated transport is aimed at transfering molecules
either to feed endothelial cell ( endocytosis ) or across the cytoplasm to supply
wall tissues with nutrients and growth factors from the blood ( transcytosis ).
Endocytosis allows not only nutrient uptake, but also membrane maintenance. After
internalization, main routes lead to degradation, recycling, 81 transient sequestration
in endosomes, or transcytosis. Pinching off of vesicles requires dynamin. Vesicular
traffic is controlled.
Clathrin-mediated endocytosis, which involves actin, captures receptors, ligands
and extracellular fluid. Caveolae-mediated endocytosis 82 is involved in receptor-
mediated endocytosis and the transport of blood macromolecules. Caveosomes
transport albumin across the endothelium. Vesicles used for intracellular transport
are also involved in signal transduction. Caveolin of the caveolar membrane binds
to certain signaling molecules and can be involved in angiogenesis and apoptosis.
Actin and microtubules are implicated in raft-mediated endocytosis.

9.6.2.3 Caveolae in Transcytosis

Transcellular transport, or transcytosis, corresponds to endocytosis followed by
exocytosis at apical and basal plasma membranes of endothelial cells, respectively.
Caveolae correspond to a large fraction of the plasma membrane of endothelial
cells (10,000-30,000 per cell; size 50-100 nm) [ 854 ]. Caveolae trigger transport
of plasma proteins, fatty acids, hormones, and other signaling molecules.
Transcytosis involves a sequential series of events: budding, fission, transloca-
tion, docking, and fusion of caveolae to the abluminal membrane. Transcytosis is
initiated by interaction of plasma proteins with specific docking molecules in plas-
malemmal caveolae that are released upon scission. Caveola-mediated transcellular
route uses multiple caveola-associated regulators, such as dynamin and intersectin.

80 Glucose is a hydrophilic compound that requires specific carriers for its transport into the cytosol.
Glucose GLUT transporters are expressed in endothelial cells [ 952 ]. These carriers are intrinsic
transmembrane proteins with different tissue-specific isoforms. Cells have a continuous supply of
glucose to be used either as a precursor of bigger molecules or an energy source by generating ATP
through glycolysis.
81 Expression of plasmalemmal receptors is regulated by recycling.
82 Caveolae, invaginations of the plasma membrane that trap extracellular substances, abound in
endothelial cells. Caveolae are main locations of PDGF receptors at the platelet surface.