Calcium transported to isolated rat liver nuclei by nicotinamide adenine dinucleotide is insensitive to thapsigargin

Mechanism regulating nuclear calcium signalingThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

Although the outer nuclear membrane is continuous with the endoplasmic reticulum, it is possible to isolate nuclei both intact and free from endoplasmic reticulum contaminants. The outer and the inner nuclear membranes can be purified free from cross-contamination. Evidence in support of autonomous regulation of nuclear calcium signaling relies upon the investigations with isolated nuclei. Mechanisms for generating calcium signaling in the nucleus have been identified. Two calcium transporting systems, an ATP-dependant nuclear Ca2+-ATPase and an IP4-mediated inositol 1,3,4,5-tetrakisphosphate receptor, are located on the outer nuclear membrane. Thus, ATP and IP4, depending on external free calcium concentrations, are responsible for filling the nuclear envelope calcium pool. The inositol 1,4,5-trisphosphate receptor is located on the inner nuclear membrane with its ligand binding domain facing toward the nucleoplasm. Likewise, the ryanodine receptor is located on the inner nuclear membrane and its ligand cADP-ribose is generated within the nucleus. A 120 kDa protein fragment of nuclear PLC-γ1 is stimulated in vivo by epidermal growth factor nuclear signaling coincident with the time course of nuclear membrane epidermal growth factor receptor activation. Stimulated 120 kDa protein fragment interacts with PIKE, a nuclear GTPase, and together they form a complex with PI[3]kinase serving as a module for nuclear PI[3]K stimulation. Thus, the nucleus has its own IP3generating system.

Hypoxia modifies nuclear calcium uptake pathways in the cerebral cortex of the guinea-pig fetus

Nuclear Ca2+ signals are thought to play a critical role in the initiation and progression of programmed cell death. The present study tests the hypothesis that hypoxia alters nuclear Ca2+ transport pathways and leads to an increase in nuclear Ca(2+)-influx in cerebral cortical neuronal nuclei. To test this hypothesis the effect of tissue hypoxia on high affinity Ca(2+)-ATPase activity and the binding characteristics of inositol 1,4,5-triphosphate (IP3) and inositol 1,3,4,5-tetrakisphosphate (IP4) receptors were studied in neuronal nuclei from the cerebral cortex of guinea-pig fetuses. Results show increased high-affinity Ca(2+)-ATPase activity (nmol/mg protein/h) in the hypoxic group 969.7+/-79 as compared with 602.4+/-90.9 in the normoxic group, P<0.05. The number of IP3 receptors (Bmax, fmol/mg protein) increased from 61+/-21 in the normoxic group to 164+/-49 in the hypoxic group, P<0.05. K(d) values did not change following hypoxia. In contrast, IP4 receptor Bmax (fmol/mg protein) and K(d) (nM) values increased from 360+/-32 in the normoxic group to 626+/-136 in the hypoxic group (P<0.001) and, from 26+/-1 in the normoxic group to 61+/-9 in the hypoxic group (P<0.001), respectively. 45Ca(2+)-influx (pmol/mg protein) significantly increased from 6.3+/-1.9 in the normoxic group to 10.9+/-1.1 the hypoxic group (P<0.001). The data show that hypoxia modifies nuclear Ca2+ transport pathways and results in increased nuclear Ca(2+)-influx. We speculate that hypoxia increases nuclear Ca2+ uptake from the cytoplasm to the nucleoplasm, resulting in increased transcription of proapoptotic genes and subsequent activation of programmed cell death pathways.

In vivo nuclear Ca2+-ATPase phosphorylation triggers intermediate size molecular transport to the nucleus

Outer nuclear membrane is endowed with a SERCA type Ca(2+)-ATPase which pumps calcium into the nuclear envelope lumen and creates calcium stores. Variation in this calcium pool, among other things, regulates nuclear transport. The transport of Nuclear Localization Signal (NLS)-containing molecules into the nucleus is well established. Intermediate size molecules lacking an NLS translocate to the nucleus and its mechanism remains obscure. It is observed here that the treatment of HEK 293 cells in culture with dibutyryl cyclic AMP (db-cAMP) or forskolin (FK) triggered transport of Calcium Green 10 kDa dextran into the nucleus. Under similar conditions Fluo-3-AM accumulated around the nuclei. cAMP-dependent protein kinase phosphorylated 105 kDa nuclear Ca(2+)-ATPase (NCA) which served as a trigger for NLS-independent transport into the nucleus.

A new function for CD38/ADP-ribosyl cyclase in nuclear Ca2+ homeostasis

Nucleoplasmic calcium ions (Ca2+) influence nuclear functions as critical as gene transcription, apoptosis, DNA repair, topoisomerase activation and polymerase unfolding. Although both inositol trisphosphate receptors and ryanodine receptors, types of Ca2+ channel, are present in the nuclear membrane, their role in the homeostasis of nuclear Ca2+ remains unclear. Here we report the existence in the inner nuclear membrane of a functionally active CD38/ADP-ribosyl cyclase that has its catalytic site within the nucleoplasm. We propose that the enzyme catalyses the intranuclear cyclization of nicotinamide adenine dinucleotide to cyclic adenosine diphosphate ribose. The latter activates ryanodine receptors of the inner nuclear membrane to trigger nucleoplasmic Ca2+ release.

Cellular mechanisms of hypoxic injury in the developing brain

The susceptibility of the developing brain to hypoxia should depend on the lipid composition of the brain cell membrane; the rate of lipid peroxidation; the presence of antioxidant defenses; and the development and modulation of the excitatory neurotransmitter receptors such as the N-methyl-D-aspartate (NMDA) receptor, the intracellular Ca++ and intranuclear Ca++-dependent mechanisms. In addition to the developmental status of these cellular components, the response of these potential mechanisms to hypoxia determines the fate of the hypoxic brain cell in the developing brain. In the fetal guinea pig and newborn piglet models, studies have demonstrated that brain tissue hypoxia results in brain cell membrane damage as evidenced by increased membrane lipid peroxidation and decreased Na+,K+-ATPase activity. Using electron spin resonance spectroscopy of alpha-phenyl-N-tert-butyl-nitrone spin-adducts, studies from our laboratory have demonstrated that tissue hypoxia results in increased free radical generation in the cortex of fetal guinea pigs and newborn piglets. We have also shown that brain tissue hypoxia modifies the N-methyl-D-aspartate receptor-ion channel, recognition and modulatory sites. Furthermore, a higher increase in NMDA receptor agonist-dependent Ca++ in synaptosomes of hypoxic as compared to normoxic fetuses was demonstrated. The increase in intracellular Ca++ may activate several enzymatic pathways such as phospholipase A2 and metabolism of arachidonic acid by cyclooxygenase and lipoxygenase, conversion of xanthine dehydrogenase to xanthine oxidase by proteases and activation of nitric oxide synthase. Using specific inhibitors of each of these enzymes such as cyclooxygenase (indomethacin), lipoxygenase (nordihydroguaiaretic acid), xanthine oxidase (allopurinol) and nitric oxide synthase (N-nitro-L-arginine), studies have shown that these enzyme reactions result in oxygen free radical generation, membrane lipid peroxidation and cell membrane dysfunction in the hypoxic brain. We suggest that, during hypoxia, the increased intracellular Ca++ may lead to an increased intranuclear Ca++ concentration and alter nuclear events including transcription of specific genes responsible for programmed cell death. In view of the developmental studies presented, the susceptibility of the fetal brain to hypoxia appears to increase with brain development as gestation approaches term.