Differential codes for free Ca2+–calmodulin signals in nucleus and cytosol (original) (raw)
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Dampening of Cytosolic Ca2+ Oscillations on Propagation to Nucleus
Journal of Biological Chemistry, 2002
Ca 2؉ signals may regulate gene expression. The increase of the cytosolic Ca 2؉ concentration ([Ca 2؉ ] c ) promotes activation and/or nuclear import of some transcription factors, but others require the increase of the nuclear Ca 2؉ concentration ([Ca 2؉ ] N ) for activation. Whether the nuclear envelope may act as a diffusion barrier for propagation of [Ca 2؉ ] c signals remains controversial. We have studied the spreading of Ca 2؉ from the cytosol to the nucleus by comparing the cytosolic and the nuclear Ca 2؉ signals reported by targeted aequorins in adrenal chromaffin, PC12, and GH 3 pituitary cells. Strong stimulation of either Ca 2؉ entry (by depolarization with high K ؉ or acethylcholine) or Ca 2؉ release from the intracellular Ca 2؉ stores (by stimulation with caffeine, UTP, bradykinin, or thyrotropin-releasing hormone (TRH)) produced similar Ca 2؉ signals in cytosol and nucleus. In contrast, both spontaneous and TRH-stimulated oscillations of cytosolic Ca 2؉ in single GH 3 cells were considerably dampened during propagation to the nucleus. These results are consistent with the existence of a kinetic barrier that filters high frequency physiological [Ca 2؉ ] c oscillations without disturbing sustained [Ca 2؉ ] c increases. Thus, encoding of the Ca 2؉ signal may allow differential control of Ca 2؉ -dependent mechanisms located at either the cytosol or the nucleus.
An update on nuclear calcium signalling
Journal of cell …, 2009
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Journal of Biological Chemistry, 2004
The interaction of calmodulin with its target proteins is known to affect the kinetics and affinity of Ca 2؉ binding to calmodulin. Based on thermodynamic principles, proteins that bind to Ca 2؉-calmodulin should increase the affinity of calmodulin for Ca 2؉ , while proteins that bind to apo-calmodulin should decrease its affinity for Ca 2؉. We quantified the effects on Ca 2؉-calmodulin interaction of two neuronal calmodulin targets: RC3, which binds both Ca 2؉-and apo-calmodulin, and ␣CaM kinase II, which binds selectively to Ca 2؉-calmodulin. RC3 was found to decrease the affinity of calmodulin for Ca 2؉ , whereas CaM kinase II increases the calmodulin affinity for Ca 2؉. Specifically, RC3 increases the rate of Ca 2؉ dissociation from the C-terminal sites of calmodulin up to 60-fold while having little effect on the rate of Ca 2؉ association. Conversely, CaM kinase II decreases the rates of dissociation of Ca 2؉ from both lobes of calmodulin and autophosphorylation of CaM kinase II at Thr 286 induces a further decrease in the rates of Ca 2؉ dissociation. RC3 dampens the effects of CaM kinase II on Ca 2؉ dissociation by increasing the rate of dissociation from the C-terminal lobe of calmodulin when in the presence of CaM kinase II. This effect is not seen with phosphorylated CaM kinase II. The results are interpreted according to a kinetic scheme in which there are competing pathways for dissociation of the Ca 2؉-calmodulin target complex. This work indicates that the Ca 2؉ binding properties of calmodulin are highly regulated and reveals a role for RC3 in accelerating the dissociation of Ca 2؉-calmodulin target complexes at the end of a Ca 2؉ signal. Calmodulin (CaM) 1 is a small (16.8 kDa) ubiquitous Ca 2ϩbinding protein that has been shown to play a central role in Ca 2ϩ signaling in a wide variety of cell types. Ca 2ϩ binding to CaM leads to a conformational change in the protein, allowing it to bind to and activate a large number of intracellular target proteins, including enzymes (1-3), ion channels (4-6), cytoskeletal elements (7, 8), and transcriptional and translational machinery (9, 10). In this way, CaM is poised as a critical intermediate in numerous cell processes. CaM binds four ions of Ca 2ϩ via EF-hand domains, two in the C-terminal lobe, and two in the N-terminal lobe. Stopped flow fluorescence studies and 43 Ca 2ϩ-exchange experiments showed that two binding sites bind Ca 2ϩ with high affinity and have slow off-rates (ϳ10 s Ϫ1), while the other two sites bind Ca 2ϩ with low affinity and have fast off-rates (Ͼ500 s Ϫ1) (11, 12). Using CaM fragments, the high affinity sites were mapped to the C-terminal lobe and the low affinity sites to the N-terminal lobe (12). These differences in Ca 2ϩ binding kinetics of the two lobes provide CaM with the potential for lobe-specific tuning of its interactions with target proteins in response to rises and falls in Ca 2ϩ levels. Ca 2ϩ levels in cells range from roughly 50 nM to tens of M (13), with temporal dynamics that range from slow oscillations (min to h) to very fast repetitive spikes (ms to s). A fundamental issue is how CaM can decode such a wide range of Ca 2ϩ signals to the multitude of CaM-dependent targets in order to provide a functionally integrated cellular response. Our work (14) and that of other groups (15-17) has shown that the Ca 2ϩ binding properties of CaM are modulated by its interaction with target proteins, leading to the idea that the unique features of each CaM binding partner tune CaM to adapt to the frequency and amplitude of different Ca 2ϩ signals. In the present work, we examine the effects of a member of two distinct classes of neuronal CaM targets, RC3, and CaM kinase II, on the interaction of Ca 2ϩ with CaM. RC3, also known as neurogranin, is a small neuronal IQ domain protein (SNIQ), found in the soma, dendrites, and dendritic spines of many neurons (18). Proteins of this family, which also includes GAP-43 and PEP-19 (19), contain an IQtype CaM binding domain and bind to both Ca 2ϩ-bound and Ca 2ϩ-free forms of CaM (20). RC3, like other SNIQs, is hypothesized to function as a regulator of the availability of CaM, either by sequestering CaM to limit its abundance in the cytoplasm, or by acting as a source of CaM, by keeping it localized in specific regions of the cytoplasm (20-22). However, the mechanism by which RC3 modulates CaM function is unclear. Our recent work has shown that interaction of PEP-19 with CaM increases the rates of association and dissociation of Ca 2ϩ from the C-terminal lobe of CaM (14). Therefore, we hypothesize that RC3 may act to alter the interaction of CaM with Ca 2ϩ. In this article, we investigate the effect of RC3 on the binding of Ca 2ϩ to CaM and describe a new potential role for RC3 in regulating CaM function at the end of a Ca 2ϩ transient.
Nuclear Calcium and Its Regulation
Calcium and Calmodulin Function in the Cell Nucleus, 1995
Ca 2 + CONTENT IN THE CELL NUCLEUS E lectron-probe X-ray microanalysis of unfixed rapidly frozen cells O. Bachs et al., Calcium and Calmodulin Function in the Cell Nucleus
Nuclear calcium signalling by individual cytoplasmic calcium puffs
The EMBO Journal, 1997
It is known that the nucleoplasmic ionised calcium concentration (Ca n ) controls nuclear functions such as transcription, although the source and nature of the signals which modulate Ca n are unclear. Using confocal imaging, we investigated the subcellular origin of Ca n signals in Fluo-3-loaded HeLa cells. Our data indicate that all signals which increased Ca n were of cytoplasmic origin. Ca n was elevated during the propagation of global Ca waves within cells. More strikingly, we found that individual cytoplasmic elementary release events e.g. Ca puffs, evoked by physiological levels of stimulation, caused transient Ca n increases. Significantly, Ͼ70% of all Ca puffs originated within a 2-3 µm perinuclear zone and propagated anisotropically across the entire nucleus. Due to the relatively slow relaxation of Ca n transients compared with those in the cytoplasm, repetitive perinuclear Ca puffs were integrated into a 'staircase' of increasing Ca n . Due to the effective diffusion of Ca in the nucleoplasm, the nucleus served as a 'Ca tunnel', distributing Ca to parts of the cytosol which were otherwise not within the cytoplasmic diffusion radii of Ca puffs. Given the close proximity of the majority of puff sites to the nucleus, it seems that the elementary Ca release system is designed to facilitate nuclear Ca signalling. Consequently, Cadependent regulation of nuclear function must be considered at the microscopic elementary level.
Decoding calcium signaling across the nucleus
Physiology (Bethesda, Md.), 2014
Calcium (Ca(2+)) is an important multifaceted second messenger that regulates a wide range of cellular events. A Ca(2+)-signaling toolkit has been shown to exist in the nucleus and to be capable of generating and modulating nucleoplasmic Ca(2+) transients. Within the nucleus, Ca(2+) controls cellular events that are different from those modulated by cytosolic Ca(2+). This review focuses on nuclear Ca(2+) signals and their role in regulating physiological and pathological processes.