Cell cycle. Sharper than a needle

Gravitational force modulates G2/M phase exit in mechanically unloaded myoblasts

Prolonged spaceflight gives rise to muscle loss and reduced strength, a condition commonly referred to as space atrophy. During exposure to microgravity, skeletal muscle myoblasts are mechanically unloaded and respond with attenuated cell proliferation, slowed cell cycle progression, and modified protein expression. To elucidate the underlying mechanisms by which muscle mass declines in response to prolonged microgravity exposure, we grew C2C12 mouse muscle cells under conditions of simulated microgravity (SM) and analyzed their proliferative capacity, cell cycle progression, and cyclin B and D expression. We demonstrated that the retarded cell growth observed in SM was correlated with an approximate 16 h delay in G₂/M phase progression, where cells accumulated specifically between the G₂ checkpoint and the onset of anaphase, concomitantly with a positive expression for cyclin B. The effect was specific for gravitational mechanical unloading as cells grown under conditions of hypergravity (HG, 4 g) for similar durations of time exhibited normal proliferation and normal cell cycle progression. Our results show that SM and HG exert phenomenological distinct responses over cell cycle progression. The deficits of SM can be restored by terrestrial gravitational force, whereas the effects of HG are indistinguishable from the 1 g control. This suggests that the mechanotransduction apparatus of cells responds differently to mechanical unloading and loading.

Ca2+ signal blockers can inhibit M/A transition in mammalian cells by interfering with the spindle checkpoint

A key step in mitosis is the sister-chromatid separation at the metaphase-anaphase (M/A) transition. Several earlier studies had suggested that Ca(2+) signal is involved in regulating this process in somatic cells. The detailed mechanisms, however, are not yet well understood. In this study, we used the GFP-gene fusion method and a living-cell imaging technique to examine the effects of suppressing cytosolic Ca(2+) level on the mitotic process in HeLa and PtK2 cells. We observed that application of the Ca(2+) chelator BAPTA/AM can block or severely delay the M/A transition. This blockage was caused by a failure in activating the anaphase-promoting complex (APC), since both cyclin B and securin could not be degraded under this situation. Furthermore, using YFP-labeled tubulin, we found that the mitotic spindle structure in most of the BAPTA-treated cells gradually deformed with time. Other Ca(2+) signal blockers, such as heparin, also produced a similar effect. These results suggest that one pathway for the blockage of M/A transition by suppressing cytosolic Ca(2+) level is due to its interference with the mitotic spindle checkpoint.

Calcium and mitosis

Calcium signals often accompany mitosis. The most obvious example of calcium as a mitotic signal is at fertilization in vertebrate eggs, where the calcium transient induces anaphase onset. New imaging methods have demonstrated smaller calcium signals that control mitosis entry and mitosis exit in sea urchin embryos. Other experiments in mouse and frog embryos indicate that similar signals with similar function may play a part in these embryos, too. The links between these calcium control signals and mitotic kinase activation are adumbrated. It appears that calcium oscillations are a property of the mitotic state. A case is made that calcium may be a universal mitotic signal, with the possible exception of early meiotic events in oocytes.

The role of calcium in the cell cycle: facts and hypotheses

The regulation of cell cycle progression is a complex process which involves kinase cascades, protease action, production of second messengers and other operations. Increasing evidence now compellingly suggests that changes in the intracellular Ca2+ concentration may also have a crucial role. Ca2+ transients occur at the awakening from quiescence, at the G/S transition, during S-phase, and at the exit from mitosis. They may lead to the activation of Ca2+ binding proteins like S-100, but the key decoder of the Ca2+ signals in the cycle is calmodulin. Activation of calmodulin leads to the stimulation of protein kinases, i.e., CaM-kinase II, and of the CaM-dependent protein phosphatase calcineurin. Ample evidence now indicates the G/S transition, the progression from G2 to M, and the metaphase/anaphase transition as specific points of intervention of CaM-kinase II. Another attractive possibility for the role of Ca2+ in the cycle is through the activation of the Ca(2+)-dependent protease calpain: other proteases (e.g., the proteasome) have been suggested to be responsible for the degradation of some of cyclins, which is essential to the progression of the cycle. One of the cyclins, however, (D1) is instead degraded by calpain, which has been shown to promote both mitosis and meiosis when injected into somatic cells or oocytes.

An anaphase calcium signal controls chromosome disjunction in early sea urchin embryos

A transient increase in intracellular calcium concentration [Ca2+]i occurs throughout the cell as sea urchin embryos enter anaphase of the first cell cycle. The transient just precedes chromatid disjunction and spindle elongation. Microinjection of calcium chelators or heparin, an InsP3 receptor antagonist, blocks chromosome separation. Photorelease of calcium or InsP3 can reverse the block. Nuclear reformation is merely delayed by calcium antagonists at concentrations that block chromatid separation. Thus, the calcium signal triggers the separation of chromatids, while calcium-independent pathways can bring about the alterations in microtubule dynamics and nuclear events associated with anaphase progression. That calcium triggers chromosome disjunction alone is unexpected. It helps explain previous conflicting results and allows the prediction that calcium plays a similar role at anaphase in other cell types.

Changes in calpain during meiosis in the rat egg

Resumption of meiosis at fertilization is mediated by increased levels of calcium which activate several calcium-dependent enzymes. Calpain, a neutral calcium-activated thiol protease, is present in the cytoplasm of many cells. Its activation is associated with limited autolysis and relocalization in the cell. Calpain is thought to participate in the regulation of mitosis and resumption of meiosis in Xenopus oocytes. In this study we followed the activation and localization of calpain during maturation and fertilization in rat eggs using a polyclonal antibody raised against chicken muscle calpain. A band of 80 kDa was detected in GV oocytes and its level increased in unfertilized MII eggs. At the early stages of fertilization, we observed a transient decrease in the level of calpain which was regained at the pronuclear stage. Adding Ca2+ to lysate of MII eggs resulted in an additional band, representing the degraded fragment of the activated protein. In eggs activated by ionomycin, calpain level decreased, followed by an increase in a dynamic similar to that observed in fertilized eggs. Egg activation also led to changes in calpain localization. A homogenous distribution was observed in GV and in MII eggs, while in activated eggs it was localized predominantly overlying the metaphase plate. In the current study we demonstrate the presence of calpain in the rat egg. During maturation, calpain level increases; however, during egg activation, in response to [Ca2+]i changes, calpain undergoes autolysis, translocaton, and fluctuation in its level. We therefore suggest a correlation between calpain activation and fertilization.

Calcium and cell cycle progression: possible effects of external perturbations on cell proliferation

Exit from the phase of cellular division appears to be driven by a calcium signal that triggers a cascade of events leading to the completion of mitosis. Here we propose a model that relates the dynamics of cytosolic calcium to progression through mitosis, G1 and G2 phases of the cell cycle. To this end, the assumption has been made that the transient rise ir cytosolic calcium concentration during mitosis is induced by inositol(1,4,5)triphosphate (IP3), which in turn is released at high levels of mitosis-promoting factor (MPF). On this basis, a system of ordinary differential equations is proposed to simulate the evolution of ten cell-cycle-specific molecular species, including cyclins A and B, MPF, IP3, Ca2+, the CaMKII holoenzyme, and the ubiquitination complex. The influence on the cell proliferation capacity exerted by external perturbations, like calcium microinjections, depletion of intracellular calcium stores, electromagnetic fields, or stimulation/inhibition of different calcium currents through the plasma membrane, can be studied by appropriate modulation of the parameters involved in the signal transduction pathway.

Exit from mitosis induced by a calcium transient: the relation to the MPF and InsP3 dynamics

Cells divide only after passing through a control point in late G1. This passage is followed by the accumulation of the mitotic cyclin which binds to p34cdc2, allowing for the subsequent phosphorylation and dephosphorylation of the latter protein. It is the active MPF, i.e. the phosphorylated mitotic cyclin-p34cdc2 kinase complex that triggers entry into mitosis. MPF becomes increasingly active as the cell forwards to anaphase, when a sudden increase in InsP3 takes place. This in turn induces a large Ca2+ release in cytosol from internal calcium stores and a calcium-dependent positive feedback control on InsP3-induced calcium release enhances the effect on InsP3-mediated Ca2+ transient. Meanwhile, empty calcium stores signal to plasma membrane for a constant calcium influx at high InsP3 levels. The cytosolic calcium excess is assumed to activate the CaMKll holloenzyme which involves the production of the ubiquitination complex necessary for cyclin degradation and MPF inactivation. Accordingly, a mathematical model was proposed by means of an eight-dimensional dynamical system that yields the time dependence of the main cellular quantities in a picture of the mitosis specific events.