Table 4.

Putative ion permeability characteristics of claudin isoforms

Claudin	Reference Nos.
Cation-selective claudins
Predominantly pore-forming (↑permeability to cations)
Claudin-2	10, 96, 416
Claudin-10b	373
Claudin-15a	57, 159, 343, 364
(Claudin-16)b	119, 143, 152, 180
Predominantly barrier-forming (↓permeability to anions)
Claudin-7	6, 7
Claudin-19c	145
Anion-selective claudins
Predominantly pore-forming (↑permeability to anions)
Claudin-7	142
Claudin-10a	373
Claudin-17	196
Predominantly barrier-forming (↓permeability to cations)
Claudin-1	160, 238
Claudin-3	245
Claudin-4d	142, 146, 365
Claudin-5	395
Claudin-6	317
Claudin-8d	16, 418
Claudin-9	317
Claudin-11d	364
Claudin-14	30
Claudin-18-2	169

Based on in vitro overexpression or knockdown studies in cultured cell lines. We assume that permeability and selectivity are properties intrinsic to individual claudin isoforms and ignore the possible confounding effect of heteromeric interactions between isoforms. Pore-forming claudins refer to those that predominantly decrease transepithelial resistance (TER) or increase solute permeability, while barrier-forming claudins refer to those that predominantly increase TER or decrease solute permeability. The distinction is somewhat arbitrary since most claudins probably have some finite permeability to most solutes, and the observable phenotype is highly dependent on the properties of the host cell line (16, 364).

^aActs as a Cl⁻ barrier in MDCK II cells but as a Na channel in LLC-PK1, MDCK I cells and in mouse intestine.

^bConflicting data as discussed in detail in section VI.

^cConflicting data also exist suggesting claudin-19 can act as a Na⁺ barrier (14).

^dActs as a Na⁺ barrier in MDCK II cells but as a Cl⁻ pore in LLC-PK1 and/or collecting duct cells.