Initiation of transcription is an early step in steroid hormone action. We
investigated by means of atomic force microscopy (AFM) and fluorescence ima
ging the role of nuclear pore complexes (NPCs) in mediating signal transduc
tion of the mineralocorticoid hormone aldosterone from the extracellular sp
ace into the cell nucleus. With AFM, we imaged single NPCs of isolated nucl
ear envelopes under native conditions. We observed that individual NPCs con
tract in response to a Ca2+ signal, which is known to occur in seconds afte
r aldosterone exposure. In living kidney cells in culture (MDCK cells), ald
osterone led within seconds to the contraction of the whole nucleus measure
d by DNA-fluorescence imaging. Nuclear contraction was elicited at similar
time scale and to similar extent by bradykinin, a peptide hormone known to
mobilize Ca2+ from internal stores, and by ionomycin, a Ca2+ ionophore know
n to directly increase intracellular Ca2+. Nuclear contraction is explained
by the individual contraction of calcium-sensitive NPCs that occur in high
density in the nuclear envelope. We present a model in which nuclear pore
complexes play a key role as barrier molecules of high plasticity in the co
ntrol of aldosterone-induced gene expression. (C) 1999 Elsevier Science Inc
. All rights reserved.