Recent molecular cloning of the epithelial sodium channel (ENaC) provides the opportunity to identify ENaC-associated proteins that function in regulating its cell surface expression and activity. We have examined whether ENaC is associated with Apx (apical protein Xenopus) and the spectrin-based membrane cytoskeleton in Xenopus A6 renal epithelial cells. We have also addressed whether Apx is required for the expression of amiloride-sensitive Na ؉ currents by cloned ENaC. Sucrose density gradient centrifugation of A6 cell detergent extracts showed co-sedimentation of xENaC, ␣-spectrin, and Apx. Immunoblot analysis of proteins co-immunoprecipitating under high stringency conditions from peak Xenopus ENaC/Apx-containing gradient fractions indicate that ENaC, Apx, and ␣-spectrin are associated in a macromolecular complex. To examine whether Apx is required for the functional expression of ENaC, ␣␤␥ mENaC cRNAs were coinjected into Xenopus oocytes with Apx sense or antisense oligodeoxynucleotides. The two-electrode voltage clamp technique showed there was a marked reduction in amiloride-sensitive current in oocytes coinjected with antisense oligonucleotides when to compared with oocytes coinjected with sense oligonucleotides. These studies indicate that ENaC is associated in a macromolecular complex with Apx and ␣-spectrin in A6 cells and suggest that Apx is required for the functional expression of ENaC in Xenopus epithelia.

Recently we reported that the ATP-binding cassette transporter Abcc10, also known as multidrug resistance protein 7 (Mrp7), is able to confer resistance to a variety of anticancer agents including taxanes. However, the in vivo functions of the pump have not been determined to any extent. Here we generated and analyzed Abcc10−/− mice in order to investigate the ability of Abcc10 to function as an endogenous resistance factor. Mouse embryo fibroblasts derived from Abcc10 −/− mice were hypersensitive to docetaxel, paclitaxel, vincristine and Ara-C and exhibited increased cellular drug accumulation, relative to wild type controls. Abcc10 null mice treated with paclitaxel exhibited increased lethality associated with neutropenia and marked bone marrow toxicity. Toxicity in spleen and thymus was also evident. These findings indicate that Abcc10 is dispensable for health and viability, and that it is an endogenous resistance factor for taxanes, other natural product agents and nucleoside analogs. This is the first demonstration that an ATP-binding cassette transporter other than P-glycoprotein can affect in vivo tissue sensitivity towards taxanes.