Insights into the Role of Cell–Cell Junctions in Physiology and Disease - International Review of Cell and Molecular Biology - page 189

2.4. Tight junctions
Tight junctions primarily establish a selective permeability barrier across epi-
thelial cell sheets ( Aijaz et al., 2006 ). Structurally, tight junctions establish a
diffusion barrier through a network of paired sealing strands which are
anchored to the actin cytoskeleton ( Furuse, 2010 ). The claudin family helps
define the selectivity of the tight junctions ( Schneeberger and Lynch, 2004;
Yamazaki et al., 2008 ). The associations among claudins and other tight
junctional proteins such as occludin, and the cytoplasmic plaque proteins
zona occludens (ZO), regulate tight junction formation, localization, and
function ( Furuse, 2010 ). Other proteins, such as Junctional Adhesion Mol-
ecule A (JAM-A), are also involved in tight junctions, although not to the
extent of claudins and occludens.
The barrier function of tight junctions is largely a consequence of their
complex molecular composition. Tight junctional proteins establish a diffu-
sion barrier by controlling selective paracellular permeability of solutes and
restricting apical-basolateral diffusion of membrane components, which
additionally maintains cell surface polarity ( Adamson et al., 1999 ). More-
over, recent evidence suggests tight junctions participate in coordinating
multiple cellular processes, such as regulating epithelial polarization and
intercellular junctional assembly during development ( Katsuno et al., 2008 ).

2.5. Gap junctions
Gap junctions are aggregates of intercellular channels that chemically and
electrically couple neighboring cells through direct cell-cell transfer of ions
and small molecules. The intercellular channels are formed by head-to-head
docking of two hemichannels, the connexons, which themselves are homo-
or hetero-hexamers of individual connexins ( Goodenough and Paul, 2009 ).
The connexons hold the interacting membranes at a fixed distance, or gap,
apart, likely lending the name of this type of junction. Gap junctions are
abundant in both epithelial and nonepithelial tissues and are structurally very
different from other cell junctions, relying on variations of connexins for
diversity instead of a variety of different protein classes. The permeability
of their individual channels varies depending on their tissues, reflecting dif-
ferences in the connexin proteins that form the junctions ( Kobielak and
Fuchs, 2006 ).
Gap junctions are notable for their roles in coordinating the activities of
electrically excitable cells, as their functions depend on the ability to direct
cell-cell transfer of ions and small molecules through low-resistance