Nuclear Ca2+ concentration measured with specifically targeted recombinant aequorin

Activation of nuclear transcription factors, breakdown of nuclear envelope and apoptosis represent a group of nuclear events thought to be modulated by changes in nucleoplasmic Ca2+ concentration, [Ca2+](n). Direct evidence for, or against, this possibility has been, however, difficult to obtain because measurements of [Ca2+](n) are hampered by major technical problems. Here we describe a new approach for selectively monitoring Ca2+ concentrations inside the nucleus of living cells, which is based on the construction of a chimeric cDNA encoding a fusion protein composed of the photoprotein aequorin and a nuclear translocation signal derived from the rat glucocorticoid receptor. This modified aequorin (nuAEQ), stably expressed in HeLa cells, was largely confined to the nucleoplasm and thus utilized for monitoring [Ca2+](n) in intact cells. No significant differences were observed between [Ca2+](n) and cytosolic Ca2+ concentration ([Ca2+](i)) under resting conditions. Upon stimulation of surface receptors linked to inositol-1,4,5-trisphosphate (InsP3) generation, and thus to intracellular Ca2+ signalling, the kinetics of [Ca2+](i) and [Ca2+](n) increases were indistinguishable. However, for the same rise in [Ca2+](i), the amplitude of [Ca2+](n) increase was larger when evoked by Ca2+ mobilization from internal stores than when induced by Ca2+ influx across the plasma membrane. The functional significance of these transient nucleus-cytosol Ca2+ gradients is discussed.