Metanephrogenic mesenchyme-to-epithelium transition induces profound expression changes of ion channels

Abstract

The expression patterns of plasma membrane transporters that specify the epithelial cell type are acquired with ontogeny. To study this process during metanephrogenic mesenchyme-to-epithelium transition, branching ureteric buds with their adjacent mesenchymal blastema (mouse embryonic day E14) were dissected and explanted on a collagen matrix. In culture, induced mesenchymal cells condensed, aggregated, and converted to the comma- and S-shaped body. During in vitro condensation and aggregation, transcription factor Pax-2 protein was downregulated while the epithelial markers E-cadherin and beta-catenin proteins were upregulated. In addition, Wilms' tumor suppressor protein WT-1 was detectable upon condensation and downregulated in the S stage, where expression persisted in the long arm of the S. Patch-clamp, whole cell conductance (G, in nS/10 pF) of pre-epithelial condensed mesenchymal cells (n = 7) was compared with that of tubular proximal S-shaped-body epithelium (n = 6). Both stages expressed E-cadherin and WT-1 mRNA, as demonstrated by single-cell RT-PCR, testifying further to the epithelial as well as the nephrogenic commitment of the recorded cells. Mesenchymal cells exhibited whole cell currents (G = 6.7 +/- 1.3) with reversal potentials (V(rev), in mV) near equilibrium potential for Cl(-) (E(Cl)) (V(rev) = -40 +/- 7) suggestive of a high fractional Cl(-) conductance. Currents of the S-shaped-body cells (G = 4.0 +/- 1.1), in sharp contrast, had a V(rev) at E(K) (V(rev) = -82 +/- 6) indicating a high fractional K(+) conductance. Further, analysis of K(+)-selective whole cell tail currents and single-channel recording revealed a change in K(+) channel expression. Also, Kir6.1 K(+) channel mRNA and protein were downregulated between both stages, whereas K(v)LQT K(+) channel mRNA was abundant throughout. In conclusion, metanephrogenic mesenchyme-to-epithelium transition is accompanied by a profound reorganization of plasma membrane ion channel conductance.