Little is known regarding the quaternary structure of cationCl ؊ cotransporters (CCCs) except that the Na ؉ -dependent CCCs can exist as homooligomeric units. Given that each of the CCCs exhibits unique functional properties and that several of these carriers coexist in various cell types, it would be of interest to determine whether the four K ؉ -Cl ؊ cotransporter (KCC) isoforms and their splice variants can also assemble into such units and, more importantly, whether they can form heterooligomers by interacting with each other or with the secretory Na (1-3), and the KCCs exist as four isoforms and several splice variants (at least five for KCC3 and two for KCC1) (4 -11). All of these structures are predicted to contain 12 transmembrane domains flanked by cytoplasmic termini (1,(12)(13)(14)(15)(16).In mammals, NKCC1 as well as KCC1, -3, and -4 have been shown to exhibit wide tissue distributions, whereas KCC2 is apparently confined to the nervous system (4 -11, 16 -21). They have also been shown to coexist in certain cell types, such as erythrocytes or lens cells, where a number of isoforms/variants (KCC1, KCC3, KCC4) have been identified (10, 11). In certain tissues, localization studies have suggested a more differential distribution (9, 16 -21).Although very homologous to each other, the KCCs display variant affinities for each of the transported ions and for the drug furosemide. In addition, their transport capacity and response to various stimuli are not the same under controlled conditions. In Xenopus laevis oocytes, for example, heterologously expressed KCC2 displays higher K m values for Cl Ϫ but lower K m values for Rb ϩ compared with KCC1 and KCC3 (16,(22)(23)(24)(25). Along the same line, KCC4 is less active than KCC2 at low levels of intracellular Cl Ϫ but more sensitive to phorbol ester-triggered events (26). Not surprisingly, differences between KCCs and NKCCs are even more pronounced (4,16,27).Several lines of evidence suggest that the NKCCs exist as homooligomers in cells. They are as follows. 1) The size of NKCC1 and NKCC2 has been found to increase by a ϳ2-fold factor when membranes expressing either protein were treated with cross-linking agents (12, 28). 2) Through GST pull-down assays and yeast two-hybrid mapping analyses, the cytosolic carboxyl terminus (Ct) of the NKCCs was found to harbor two domains that are endowed with self-interacting properties, * This work was supported by grants from the Kidney Foundation of Canada and from the Canadian Institute of Health and Research (CIHR) through MOP-68949 and MOP-15405. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. □ S The on-line version of this article (available at http://www.jbc.org) contains supplemental

In animals, silicon is an abundant and differentially distributed trace element that is believed to play important biological functions. One would thus expect silicon concentrations in body fluids to be regulated by silicon transporters at the surface of many cell types. Curiously, however, and even though they exist in plants and algae, no such transporters have been identified to date in vertebrates. Here, we show for the first time that the human aquaglyceroporins, i.e., AQP3, AQP7, AQP9 and AQP10 can act as silicon transporters in both Xenopus laevis oocytes and HEK-293 cells. In particular, heterologously expressed AQP7, AQP9 and AQP10 are all able to induce robust, saturable, phloretin-sensitive silicon transport activity in the range that was observed for low silicon rice 1 (lsi1), a silicon transporter in plant. Furthermore, we show that the aquaglyceroporins appear as relevant silicon permeation pathways in both mice and humans based on 1) the kinetics of substrate transport, 2) their presence in tissues where silicon is presumed to play key roles and 3) their transcriptional responses to changes in dietary silicon. Taken together, our data provide new evidence that silicon is a potentially important biological element in animals and that its body distribution is regulated. They should open up original areas of investigations aimed at deciphering the true physiological role of silicon in vertebrates.

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