The rise of nuclear and cytosolic Ca²⁺ can be uncoupledin HeLa cells

Abstract

 It has long been assumed that ionized Ca²⁺ is transmitted from the cytosol to the cell nucleus through the nuclear pore complexes to trigger many nuclear functions such as gene transcription after agonist stimulation. However, this hypothesis has been challenged recently. In the present study, we have investigated the interplay of Ca²⁺ in the cytosol and nucleus of HeLa cells upon histamine stimulation by using confocal microscopy. In resting cells, addition of histamine (50 µM) produced synchronous Ca²⁺ signals in the cytosol and nucleus with a stronger fluo-3 emission in the nucleus. Our results also demonstrate that the rise of cytosolic and nuclear free Ca²⁺ can be uncoupled after histamine stimulation. This assertion is supported by the following observations: (1) when heparin was injected into the cytosol, the increase in fluo-3 fluorescence in the cytosol mediated by histamine was almost eliminated while that in the nucleus still occurred; (2) when heparin was in the nucleus, the increase in the free nucleoplasmic Ca²⁺ concentration ([Ca²⁺]n) elicited by histamine was abolished while the rise of cytosolic Ca²⁺ level ([Ca²⁺]c) was seen; (3) when we injected inositol 1,4,5-trisphosphate (IP₃) directly into the nucleus, [Ca²⁺]n increased; and (4) in the cells given cytosolic injections of BAPTA-dextran (70 kDa), histamine evoked additional Ca²⁺ transients in the nucleus. These results suggest that the nucleus of the HeLa cell has its own IP₃-sensitive Ca²⁺ store and that the Ca²⁺ signals in the nucleus and cytosol can be uncoupled.