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Claudins and epithelial paracellular transport: the end of the beginning

Introduction The nephron reabsorbs and secretes solutes and water by vectorial transport across the tubule epithelium. Transport occurs via the transcellular or paracellular pathway (Fig. 1). Transcellular transport is very well understood, both at the physiological and molecular level (Table 1). This process involves sequential transport across the apical and basolateral epithelial membranes, which can be active or passive. In most cases (with the exception of rare solutes transported by transcytosis or lipid diffusion) these steps are mediated by transmembrane transport proteins falling into three classes: channels, carriers, and pumps. Many of these proteins are highly specific for their transported solute, and highly regulated. Consequently, there is considerable molecular heterogeneity in transmembrane transport proteins. For example, there are approximately 170 different human genes solely for transmembrane cation channels [1]. By contrast, the paracellular pathway is poorly understood (Table 1). It is often portrayed as a passive, constitutive and indiscriminate leak. The driving forces for paracellular transport are preexisting electrochemical or osmotic gradients, often established by transcellular transport processes.