Imaging nuclear pores of aldosterone-sensitive kidney cells by atomic force microscopy.

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Abstract
In nuclei of renal target cells, aldosterone enhances transcriptional activity followed by the translocation of specific RNA molecules across the nuclear envelope. Trafficking between cell nucleus and cytoplasm occurs via nuclear pore complexes (NPCs) located in the double-layered nuclear envelope. We investigated the nucleocytoplasmic transport route by structure-function analysis at subcellular level in quiescent and aldosterone-stimulated cells. With atomic-force microscopy (AFM) we imaged individual pores of the nuclear surface of cultured kidney cells and related the number of pores per micron2 to nuclear envelope conductance (Gn, per micron2) evaluated electrically by current injection into the isolated nucleus. NPCs were equally distributed resembling "donut-like" structures with outer diameters of 134 +/- 12 nm (n = 50), each equipped with a central channel. Six hours of aldosterone exposure (0.1 microM) increased the number of NPCs per micron 2 of nuclear surface from 7.4 +/- 0.4 to 9.8 +/- 0.4 (n = 12; P < 0.01). At the same time Gn rose from 6900 +/- 520 to 9600 +/- 610 pS/micron2 paralleled by an increase of the intranuclear electrical potential from -2.8 +/- 0.2 to -6.2 +/- 0.4 mV (n = 18; P < 0.01). Assuming that NPCs represent the sole conductive pathway in the nuclear envelope, we calculate a mean single NPC conductance of 932 and 980 pS, in the absence and presence of aldosterone, respectively. We conclude that aldosterone facilitates nucleocytoplasmic transport by increasing the number of NPCs but not by modifying their biophysical properties. Possibly, aldosterone controls similar transport mechanisms in both plasma membrane and nuclear envelope.