Laboratory of Kidney and Electrolyte Metabolism, National Institutes of Health, Bethesda, Maryland 20892-1603
Since the initial report by Ussing and Windhager
(2) in 1964 of an extracellular pathway for ion
flow across frog skin, investigators have speculated about the nature
of this route. As the evidence accumulated for such "shunt" paths
in a wide variety of epithelia, it was apparent that all shunts were
not created equal. The original concept of the paracellular pathway as
a nonspecific leak across the tight junction has been replaced with the
realization that the selectivity and permeability of tight junctions
differ dramatically between tissues. In addition, it has become clear that the permeability of tight junctions can be regulated by a variety
of second messengers and physical forces. What remained a mystery until
the work of Colegio et al., the current article in focus (Ref.
1, see p. C142 in this issue), was
the molecular basis for the differences in ion selectivity of the
paracellular pathway. Their elegant work shows that the claudins are
the long-sought ionic gatekeepers of tight junctions.
Two fundamental factors limited previous investigations of the
permeability properties of the paracellular pathway in leaky epithelia:
1) transepithelial measurements are difficult to interpret because they represent an admixture of cellular and paracellular pathways confounded by tissue heterogeneity; and 2) none of
the identified molecular components of tight junctions appeared to be
directly involved in the determination of paracellular ionic selectivity, although tight junctional structure often was altered when
the expression levels of these proteins were manipulated. The first
issue, the complexity of interpreting transepithelial measurements, has
plagued physiologists for four decades. Measurements of transepithelial
electrical resistance in most fluid-transporting epithelia revealed a
very high conductance consistent with a substantial paracellular shunt.
Investigators observed that the transepithelial resistance could change
in either direction when the expression level of putative protein
constituents of the tight junction was experimentally increased in
cultured epithelia. They concluded that the extent of cell-to-cell
adhesion was most likely affected by the protein expression. However,
electrical resistance is the sum of all ionic conductances across the
entire epithelium, and changes in it do not give insight into the
mechanism or site of the change.
The transepithelial flux of a neutral tracer molecule such as a sugar
alcohol or dextran has been used for decades as a metric of
paracellular pore size with the assumption that it could not cross the
epithelium by the transcellular route. Size discrimination studies of
these neutral species led investigators to conclude that such large
holes perforate the tight junctions that discrimination between ionic
species seemed unlikely. In contrast, dilution and bi-ionic diffusion
potential measurements in many leaky epithelia clearly showed that they
were selective for cations or anions. Difficulties in reconciling these
apparently conflicting results spawned a variety of speculations about
tight-junctional structure and function all directed toward answering
the question, "What kind of structure could simultaneously be so
leaky and yet so selective?"
Colegio et al. (1) convincingly show us that ionic
selectivity resides in specific residues in the claudins and that this charge discrimination can be reversed in polarity by substitution of
three amino acids in the extracellular domain of claudin-15. They
postulate that combinations of the 20 known claudins serve as building
blocks in tight junctions and thereby confer tissue-specific ionic
permeability properties. Mutations in claudins-14 and -16 are already
known to cause two human diseases, and the prospects are bright that
other clinical disturbances are the result of altered paracellular selectivity.
Finally, this study elevates the tight junction from the realm of the
"black box" to that of a channel protein that can be experimentally
manipulated. The authors draw a parallel between the charge-selective
properties of the extracellular domain of the claudins and those of the
connexons of the gap junctions. While this analogy may be correct, we
are finally in a position to critically test the various hypotheses
about tight junction structure-function relationships at the molecular level.
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REFERENCES
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FOOTNOTES |
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Address for reprint requests and other correspondence: K. R. Spring, National Institutes of Health, Laboratory of Kidney and Electrolyte Metabolism, 10 Center Dr., MSC 1603, Bldg. 10, Rm. 6N260, Bethesda, MD 20892-1603 (E-mail: springk{at}nhlbi.nih.gov).
10.1152/ajpcell.00128.2002
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1.
Colegio, OR,
Van Itallie CM,
McCrea HJ,
Rahner C,
and
Anderson JM.
Claudins create charge-selective channels in the paracellular pathway between epithelial cells.
Am J Physiol Cell Physiol
283:
C142-C147,
2002
2.
Ussing, HH,
and
Windhager EE.
Nature of the shunt path and active sodium transport through frog skin epithelium.
Acta Physiol Scand
61:
484-504,
1964[ISI].