Immunocytochemical localization of calmodulin in PC12 cells and its possible interaction with histones

Calmodulin-binding proteins in the model organism Dictyostelium: a complete & critical review

Calmodulin is an essential protein in the model organism Dictyostelium discoideum. As in other organisms, this small, calcium-regulated protein mediates a diversity of cellular events including chemotaxis, spore germination, and fertilization. Calmodulin works in a calcium-dependent or -independent manner by binding to and regulating the activity of target proteins called calmodulin-binding proteins. Profiling suggests that Dictyostelium has 60 or more calmodulin-binding proteins with specific subcellular localizations. In spite of the central importance of calmodulin, the study of these target proteins is still in its infancy. Here we critically review the history and state of the art of research into all of the identified and presumptive calmodulin-binding proteins of Dictyostelium detailing what is known about each one with suggestions for future research. Two individual calmodulin-binding proteins, the classic enzyme calcineurin A (CNA; protein phosphatase 2B) and the nuclear protein nucleomorphin (NumA), which is a regulator of nuclear number, have been particularly well studied. Research on the role of calmodulin in the function and regulation of the various myosins of Dictyostelium, especially during motility and chemotaxis, suggests that this is an area in which future active study would be particularly valuable. A general, hypothetical model for the role of calmodulin in myosin regulation is proposed.

Calmodulin spatial dynamics in RBL-2H3 mast cells

A line of rat basophilic leukaemia (RBL) cells, a model of mast cells, stably expressing EGFP-tagged calmodulin secreted normally in response to standard agonists. As reported for other cell types, calmodulin was concentrated in the mitotic spindle poles of dividing cells. In unstimulated interphase cells calmodulin was concentrated in the cell cortex and at a single central location. Disruption of cortical actin eliminated the concentration of calmodulin at the cortex while the central calmodulin concentration was associated with an enrichment of tubulin and is likely to represent the centrosome. Following stimulation with either an agonist that crosslinks Fc receptors or co-application of phorbol ester and a calcium ionophore the interior of the cells lost calmodulin while cortical fluorescence became more pronounced but also less uniform. After stimulation discrete bright puncta of calmodulin-EGFP (CaM-EGFP) appeared in the cell interior. Puncta colocalised with moving lysotracker-labelled granules, suggesting that calmodulin may play a role in organising their transport. Our results show that in interphase RBL cells a large fraction of the calmodulin pool is associated with targets in the actin cytoskeleton and demonstrate the utility of this model system for studying calmodulin biology.

Calmodulin priming: nuclear translocation of a calmodulin complex and the memory of prior neuronal activity

The neuronal nucleus plays a vital role in information processing, but whether it supports computational functions such as paired-pulse facilitation, comparable to synapses, is unclear. Ca(2+)-dependent movement of calmodulin (CaM) to the nucleus is highly responsive to Ca(2+) entry through L-type channels and promotes activation of the transcription factor CREB (cAMP-responsive element binding protein) through phosphorylation by CaM-sensitive kinases. We characterized key features of this CaM translocation and its possible role in facilitation of nuclear signaling. Nuclear CaM was elevated within 15 s of stimulus onset, preceding the first signs of CREB phosphorylation in hippocampal pyramidal neurons. Depolarization-induced elevation of nuclear CaM also was observed in cerebellar granule cells, neocortical neurons, and dentate gyrus granule cells. Nuclear translocation of CaM was not blocked by disruption of actin filaments or microtubules, or by emptying endoplasmic reticulum Ca(2+) stores with thapsigargin. Translocation of fluorescently tagged CaM was prevented by fusing it with the Ca(2+)/CaM binding peptide M13, suggesting that nuclear CaM accumulation depends on association with endogenous Ca(2+)/CaM binding proteins. To determine whether increased nuclear [CaM] might influence subsequent nuclear signal processing, we compared responses to two consecutive depolarizing stimuli. After a weak "priming" stimulus that caused CaM translocation, CREB phosphorylation caused by a subsequent stimulus was significantly faster, more sensitive to Ca(2+) elevation, and less specifically dependent on Ca(2+) influx through L-type channels. CaM translocation not only supports rapid signaling to the nucleus, but also could provide a "memory" for facilitatory effects of repeated neural activity, seen in altered phosphorylated CREB dynamics and Ca(2+) channel dependence.

Calmodulin enhances the stability of the estrogen receptor

The estrogen receptor mediates breast cell proliferation and is the principal target for chemotherapy of breast carcinoma. Previous studies have demonstrated that the estrogen receptor binds to calmodulin-Sepharose in vitro. However, the association of endogenous calmodulin with endogenous estrogen receptors in intact cells has not been reported, and the function of the interaction is obscure. Here we demonstrate by co-immunoprecipitation from MCF-7 human breast epithelial cells that endogenous estrogen receptors bind to endogenous calmodulin. Estradiol treatment of the cells had no significant effect on the interaction. However, incubation of the cells with tamoxifen enhanced by 5-10-fold the association of calmodulin with the estrogen receptor and increased the total cellular content of estrogen receptors by 1.5-2-fold. In contrast, the structurally distinct calmodulin antagonists trifluoperazine and CGS9343B attenuated the interaction between calmodulin and the estrogen receptor and dramatically reduced the number of estrogen receptors in the cell. Neither of these agents altered the amount of estrogen receptor mRNA, suggesting that calmodulin stabilizes the protein. This hypothesis is supported by the observation that, in the presence of Ca2+, calmodulin protected estrogen receptors from in vitro proteolysis by trypsin. Furthermore, overexpression of wild type calmodulin, but not a mutant calmodulin incapable of binding Ca2+, increased the concentration of estrogen receptors in MCF-7 cells, whereas transient expression of a calmodulin inhibitor peptide reduced the estrogen receptor concentration. These data demonstrate that calmodulin binds to the estrogen receptor in intact cells in a Ca2+-dependent, but estradiol-independent, manner, thereby modulating the stability and the steady state level of estrogen receptors.

Effect of antisense oligodeoxynucleotides directed to individual calmodulin gene transcripts on the proliferation and differentiation of PC12 cells

Calmodulin (CaM) is encoded by three different genes that collectively give rise to five transcripts. In the present study, we used antisense oligodeoxynucleotides targeted to unique sequences in the transcripts from the individual CaM genes to selectively block the expression of the different genes and to investigate the roles these individual genes play in the proliferation and nerve growth factor (NGF)-induced differentiation of PC12 cells. Culturing PC12 cells in the presence of oligodeoxynucleotide antisense to the transcripts from CaM genes I and II caused a significant decrease in the proliferation and a significant delay in the NGF-induced differentiation of PC12 cells when compared with untreated cells and with cells treated with the corresponding randomized oligodeoxynucleotides. However, an oligodeoxynucleotide antisense to CaM gene III did not significantly alter the proliferation or the NGF-induced differentiation of PC12 cells. The inhibition of cell proliferation could be reversed by washing out the antisense oligodeoxynucleotides. The levels of CaM in cells treated with oligodeoxynucleotides antisense to CaM genes I or II were reduced 52% or 63%, respectively, of the levels found in the control cells. However, the levels of CaM were not significantly reduced in PC12 cells treated with CaM gene III antisense oligodeoxynucleotide. None of the randomized oligodeoxynucleotides had any effect on the levels of CaM in PC12 cells. The reduced levels of CaM in PC12 cells treated with an oligodeoxynucleotide antisense to CaM gene I were accompanied by a reduction in the levels of the CaM gene I mRNAs, supporting a true antisense mechanism of action for these oligodeoxynucleotides. These results suggest that altering the level of CaM by using antisense oligodeoxynucleotides targeted to the dominant CaM transcripts in a particular cell type will specifically inhibit their proliferation and, in the case of neuronal cells, alter the course of their differentiation.

Reduced level of calmodulin in PC12 cells induced by stable expression of calmodulin antisense RNA inhibits cell proliferation and induces neurite outgrowth

The role calmodulin plays in the growth and differentiation of nerve cells was assessed by altering the levels of calmodulin in the PC12 rat pheochromocytoma cell line and determining the effects of altering these levels on cellular proliferation and differentiation. Calmodulin levels in the PC12 cells were increased or decreased by transfecting the cells with a mammalian expression vector into which the rat calmodulin gene I had been cloned in the sense or antisense orientation, respectively. The cells transfected with the calmodulin sense gene showed increased levels of calmodulin immunoreactivity and increased levels of calmodulin messenger RNA as ascertained by immunocytochemistry and slot-blot analysis, respectively. Cells transfected with the calmodulin antisense construct showed reduced levels of calmodulin immunoreactivity. Reducing the levels of calmodulin by expression of antisense calmodulin messenger RNA resulted in a marked inhibition of cell growth, whereas increasing the levels of calmodulin by overexpressing calmodulin messenger RNA resulted in an acceleration of cell growth. Transfected PC12 cells having reduced levels of calmodulin immunoreactivity exhibited spontaneous outgrowth of long, stable and highly branched neuritic processes. PC12 cells in which calmodulin was overexpressed showed no apparent changes in cell morphology, but did show an altered response to the addition of nerve growth factor. While nerve growth factor slowed cellular proliferation and induced extensive neurite outgrowth, in parental PC12 cells nerve growth factor induced little or no neurite outgrowth and little inhibition of cell proliferation in transfected cells overexpressing calmodulin. These results indicate that calmodulin is essential for the proliferation of nerve cells and for the morphological changes that nerve cells undergo during differentiation. The study also suggests the possibility that a calmodulin antisense approach may be used to inhibit the proliferation of neuronal tumors.