ENaC channels in oocytes from Xenopus laevis and their regulation by xShroom1 protein.

Shroom is a family of related proteins linked to the actin cytoskeleton. xShroom1 is constitutively expressed in X. oocytes and is required for the expression of amiloride sensitive sodium channels (ENaC). Oocytes were injected with α, β, and γ mENaC and xShroom1 sense or antisense oligonucleotides. We used voltage clamp techniques to study the amiloride-sensitive Na(+) currents (INa((amil))). We observed a marked reduction in INa((amil)) in oocytes co-injected with xShroom1 antisense. Oocytes expressing a DEG mutant β-mENaC subunit (β-S518K) with an open probability of 1 had enhanced INa((amil)) although these currents were also reduced when co-injected with xShroom1 antisense. Addition of low concentration (20 ng/ml) of trypsin which activates the membrane-resident ENaC channels led to a slow increase in INa((amil)) in oocytes with xShroom1 sense but had no effect on the currents in oocytes coinjected with ENaC and xShroom1 antisense. The same results were obtained with higher concentrations of trypsin (2 μg/ml) exposed during 2.5 min. In addition, fluorescence positive staining of plasma membrane in the oocytes expressing α, β and γ mENaC and xShroom1 sense were observed but not in oocytes coinjected with ENaC and xShroom1 antisense oligonucleotides. On this basis, we suggest that xShroom1-dependent ENaC inhibition may be through the number of channels inserted in the membrane.