The role of scavenger receptor BI in hepatitis - eTheses Repository ...

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blood brain barrier is permeable only to relatively small molecules such as
sugars. This is achieved by tissue polarisation; the cells found within tissue
layers have distinct ‘top’ and ‘bottom’ surfaces, termed apical and basolateral
respectively. Each surface will have a specific protein expression profile suited
to the tasks it performs. The maintenance of tissue integrity and polarity is
largely mediated by tight junctions. These are lateral structures that form at
cell junctions, binding adjacent cell membranes together in a way that
prevents the free movement of solutes across the tissue layer. Tight junctions
are comprised of transmembrane proteins such as occludins, claudins,
junctional adhesion molecules and e-cadherin, these interact with the cell
interior via cytoplasmic partners, for instance ZO-1 and cingulin (27, 160).
Other types of cell junctions include, gap and adherence junctions which
mediate selective solute exchange and the attachment of adjacent cells,
respectively (74).
There are 24 members of the claudin family and they share a basic structure
with tetraspanins; 4 transmembrane domains anchoring 2 extracellular (EC)
loops. The larger of the loops, EC1, is responsible for sealing the tight
junctions and the formation of selective ion channels, whereas the other
mediates lateral and oppositional organisation of the protein (160).
Intracellular domains interact with adaptor molecules that tether the proteins
to the cytoskeleton (160). The liver is a composed of highly polarised units
called lobules and claudin-1 (CLDN1) is expressed in the tight junctions
between hepatocytes that maintain these structures (157, 160).