Presence of zinc and calcium permeant channels in the inner membrane of the nuclear envelope

Monovalent cationic channel activity in the inner membrane of nuclei from skeletal muscle fibers

Nuclear ion channels remain among the least studied and biophysically characterized channels. Although considerable progress has been made in characterizing calcium release channels in the nuclear membrane, very little is known regarding the properties of nuclear monovalent cationic channels. Here, we describe a method to isolate nuclei from adult skeletal muscle fibers that are suitable for electrophysiological experiments. Using this approach, we show for the first time, to our knowledge, that a nuclear monovalent cationic channel (NMCC) is prominently expressed in the inner membrane of nuclei isolated from flexor digitorum brevis skeletal muscle fibers of adult mice. In isotonic 140 mM KCl, the skeletal muscle NMCC exhibits a unitary conductance of ?160 pS and high, voltage-independent open probability. Based on single-channel reversal potential measurements, NMCCs are slightly more permeable to potassium ions over sodium (PK/PNa = 2.68 ± 0.21) and cesium (PK/PCs = 1.39 ± 0.03) ions. In addition, NMCCs do not permeate divalent cations, are inhibited by calcium ions, and demonstrate weak rectification in asymmetric Ca(2+)-containing solutions. Together, these studies characterize a voltage-independent NMCC in skeletal muscle, the properties of which are ideally suited to serve as a countercurrent mechanism during calcium release from the nuclear envelope.

Functional K(v)10.1 channels localize to the inner nuclear membrane

Ectopically expressed human K_V10.1 channels are relevant players in tumor biology. However, their function as ion channels at the plasma membrane does not totally explain their crucial role in tumors. Both in native and heterologous systems, it has been observed that a majority of K_V10.1 channels remain at intracellular locations. In this study we investigated the localization and possible roles of perinuclear K_V10.1. We show that K_V10.1 is expressed at the inner nuclear membrane in both human and rat models; it co-purifies with established inner nuclear membrane markers, shows resistance to detergent extraction and restricted mobility, all of them typical features of proteins at the inner nuclear membrane. K_V10.1 channels at the inner nuclear membrane are not all transported directly from the ER but rather have been exposed to the extracellular milieu. Patch clamp experiments on nuclei devoid of external nuclear membrane reveal the existence of channel activity compatible with K_V10.1. We hypothesize that K_V10.1 channels at the nuclear envelope might participate in the homeostasis of nuclear K⁺, or indirectly interact with heterochromatin, both factors known to affect gene expression.

Nuclear prostaglandin signaling system: biogenesis and actions via heptahelical receptors

Prostaglandins are ubiquitous lipid mediators that play pivotal roles in cardiovascular homeostasis, reproduction, and inflammation, as well as in many important cellular processes including gene expression and cell proliferation. The mechanism of action of these lipid messengers is thought to be primarily dependent on their interaction with specific cell surface receptors that belong to the heptahelical transmembrane spanning G protein-coupled receptor superfamily. Accumulating evidence suggests that these receptors may co-localize at the cell nucleus where they can modulate gene expression through a series of biochemical events. In this context, we have recently demonstrated that prostaglandin E2-EP3 receptors display an atypical nuclear compartmentalization in cerebral microvascular endothelial cells. Stimulation of these nuclear EP3 receptors leads to an increase of eNOS RNA in a cell-free isolated nuclear system. This review will emphasize these findings and describe how nuclear prostaglandin receptors, notably EP3 receptors, may affect gene expression, specifically of eNOS, by identifying putative transducing elements located within this organelle. The potential sources of lipid ligand activators for these intracellular sites will also be addressed. The expressional control of G-protein-coupled receptors located at the perinuclear envelope constitutes a novel and distinctive mode of gene regulation.

Intracellular zinc distribution and transport in C6 rat glioma cells

In mammalian cells, the intracellular availability of zinc influences numerous crucial processes. Its distribution has previously been visualized with several fluorescent probes, but it was unclear how these probes are compartmentalized within the cell. Here, we show that in C6 cells the zinc-specific probe Zinquin is evenly distributed. Thus, the significantly lower level of fluorescence in the nucleus and a punctuate vesicular staining are real differences in the concentrations of zinc. Chemical perturbation of the steady state by releasing intracellular protein-bound zinc with the sulfhydryl-reactive N-ethylmaleimide (NEM) resulted in a vanadate sensitive transport of zinc out of the nucleus and into zincosomes. If the zinc-release was performed with the histidine-reactive diethylpyrocarbonate, sequestration was reduced compared to treatment with NEM, indicating the importance of histidine within membrane zinc transporters. Another major factor regulating the zinc homeostasis is ion export. As determined by atomic absorption spectroscopy, up to 50% of the cellular zinc was exported by a mechanism sensitive to lanthanum ions. We conclude that different concentrations of labile zinc exist in different cellular compartments, which are maintained by export and intracellular transport of zinc.

Nuclear KATP channels trigger nuclear Ca(2+) transients that modulate nuclear function

Glucose, the principal regulator of endocrine pancreas, has several effects on pancreatic beta cells, including the regulation of insulin release, cell proliferation, apoptosis, differentiation, and gene expression. Although the sequence of events linking glycemia with insulin release is well described, the mechanism whereby glucose regulates nuclear function is still largely unknown. Here, we have shown that an ATP-sensitive K(+) channel (K(ATP)) with similar properties to that found on the plasma membrane is also present on the nuclear envelope of pancreatic beta cells. In isolated nuclei, blockade of the K(ATP) channel with tolbutamide or diadenosine polyphosphates triggers nuclear Ca(2+) transients and induces phosphorylation of the transcription factor cAMP response element binding protein. In whole cells, fluorescence in situ hybridization revealed that these Ca(2+) signals may trigger c-myc expression. These results demonstrate a functional K(ATP) channel in nuclei linking glucose metabolism, nuclear Ca(2+) signals, and nuclear function.

[Role of intracellular zinc in programmed cell death]

Apoptosis is a type of cell death involved in several biological events during tissue development, remodelling or involution. It could be induced by several extracellular or intracellular stimuli with an important role for metals like zinc or calcium. Cellular zinc is described as an inhibitor of apoptosis, while its depletion induces death in many cell lines. Using different chemical tools like specific zinc-chelators or ionophores, it is possible to study and understand the mechanisms of programmed cell death induction. The decrease in intracellular zinc concentration induces a characteristic apoptosis with apoptotic bodies formation and nuclear DNA condensation and fragmentation. This zinc depletion activates the caspases-3, -8 and -9, responsible for the proteolysis of several target proteins like poly(ADP-ribose) polymerase or transcription factors. Zinc addition in cell culture medium prevents the apparition of morphological and biochemical signs induced by intracellular zinc chelation, but also by other apoptosis inducers like etoposide or tumour necrosis factor alpha (TNF alpha). However, excess of zinc can also be cytotoxic. The balance between life and cell death is maintained by several zinc channels, controlling the intracellular zinc movements and the free amount of the metal.

Role of cellular zinc in programmed cell death: temporal relationship between zinc depletion, activation of caspases, and cleavage of Sp family transcription factors

Zinc is a potent inhibitor of apoptosis, whereas zinc depletion induces apoptosis in many cell lines. To investigate the mechanisms of zinc depletion-induced apoptosis, HeLa cells were treated with the membrane permeable metal ion chelator, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). TPEN decreased the intracellular level of zinc and induced apoptosis with a characteristic cellular pattern, i.e. cell shrinkage and formation of apoptotic bodies, with DNA fragmentation and formation of a typical DNA ladder pattern. Following TPEN treatment, caspases-3, -8, and -9 were activated and caspase target proteins, poly(ADP-ribose) polymerase, and Sp transcription factors were cleaved. These effects were inhibited by adding zinc to the medium. To assess the role of zinc in the activation of the caspase cascade, we compared zinc inhibition during tumor necrosis factor alpha/cycloheximide- and etoposide-induced apoptosis with that induced by TPEN. Zinc addition partially inhibited caspase-3 activation, but not caspase-8 and -9 cleavage in HeLa cells treated with tumor necrosis factor alpha or etoposide. These results suggest that caspase-3 is rapidly and directly activated by zinc chelation, without a requirement for an upstream event. Caspase-3 activation is therefore the main event leading to apoptosis after intracellular zinc chelation. Finally, we conclude that cellular zinc inhibits apoptosis by maintaining caspase-3 inactive.

The nuclear chloride ion channel NCC27 is involved in regulation of the cell cycle

NCC27 is a nuclear chloride ion channel, identified in the PMA-activated U937 human monocyte cell line. NCC27 mRNA is expressed in virtually all cells and tissues and the gene encoding NCC27 is also highly conserved. Because of these factors, we have examined the hypothesis that NCC27 is involved in cell cycle regulation. Electrophysiological studies in Chinese hamster ovary (CHO-K1) cells indicated that NCC27 chloride conductance varied according to the stage of the cell cycle, being expressed only on the plasma membrane of cells in G2/M phase. We also demonstrate that Cl- ion channel blockers known to block NCC27 led to arrest of CHO-K1 cells in the G2/M stage of the cell cycle, the same stage at which this ion channel is selectively expressed on the plasma membrane. These data strongly support the hypothesis that NCC27 is involved, in some as yet undetermined manner, in regulation of the cell cycle.

Patch-clamp study of liver nuclear ionic channels reconstituted into giant proteoliposomes

Nuclear ionic channels (NICs) represent ubiquitous structures of living cells, although little is known about their functional properties and encoding genes. To characterize NICs, liver nuclear membrane vesicles were reconstituted into either planar lipid bilayers or proteoliposomes. Reconstitution of nuclear envelope (NE) vesicles into planar lipid bilayer proceeded with low efficiency. NE vesicle reconstitution into proteoliposomes led to NIC observations by the patch-clamp technique. Large conductance, voltage-gated, K(+)-permeant and Cl(-)-permeant NICs were characterized. An 80-105-pS K(+)-permeant NIC with conducting sub-state was also recorded. Our data establish that NICs can be characterized upon reconstitution into giant proteoliposomes and retain biophysical properties consistent with those described for native NICs.

Sphingosylphosphorylcholine increases calcium concentration in isolated brain nuclei

Sphingosylphosphorylcholine (SPC) caused a rapid increase of Ca2+ concentration in isolated brain nuclei. This effect was prevented by nimodipine, an inhibitor of L-type Ca2+ channels, and by thapsigargin, an inhibitor of Ca(2+)-ATPase. Neither heparin nor U73122 modified this effect, suggesting that phospholipase C activation and inositol 1,4,5-trisphosphate (IP3) production are not involved. Results also indicated that SPC-induced increase in Ca2+ concentration is not protein kinase C-dependent.

A Ca2+- and voltage-dependent cation channel in the nuclear envelope of red beet

The patch-clamp technique was applied to study ion conductances in various configurations of the nuclear envelope of non-enzyme-treated red beet (Beta vulgaris L.) nuclei. With excised patches a non-selective cation channel was observed, that was activated by micromolar concentrations of Ca2+ on the nucleoplasmic side of the envelope. The channel activity was also voltage-dependent and the voltage threshold of channel activation changed with the nucleoplasmic Ca2+ concentration. The most prominent conductance level was 110+/-22 pS with 150 mM KCl in the bath and pipette. The channel was permeable to small cations: permeabilities relative to K+ were PK congruent with PNa=1, PCs=0.3, but PCl=0.09. Calcium ions also permeated the channel with PCa=0.43, estimated from reversal potential, or 0.14, estimated from conductance ratio. Zn2+ (1 mM) when applied to the cytoplasmic side of the envelope blocked the channel activity completely, while amiloride (2 mM) reduced the channel current by 86% from the nucleoplasmic side. The properties of the whole-nucleus current (voltage-, time- and Ca2+-dependence) paralleled those observed with excised patches. The channel may provide a Ca2+-regulated pathway for passive diffusion of cations across the nuclear envelope and thus may play an important role in Ca2+-dependent nuclear processes ranging from gene transcription to apoptosis.

Patch-clamp detection of macromolecular translocation along nuclear pores

The present paper reviews the application of patch-clamp principles to the detection and measurement of macromolecular translocation along the nuclear pores. We demonstrate that the tight-seal 'gigaseal' between the pipette tip and the nuclear membrane is possible in the presence of fully operational nuclear pores. We show that the ability to form a gigaseal in nucleus-attached configurations does not mean that only the activity of channels from the outer membrane of the nuclear envelope can be detected. Instead, we show that, in the presence of fully operational nuclear pores, it is likely that the large-conductance ion channel activity recorded derives from the nuclear pores. We conclude the technical section with the suggestion that the best way to demonstrate that the nuclear pores are responsible for ion channel activity is by showing with fluorescence microscopy the nuclear translocation of ions and small molecules and the exclusion of the same from the cisterna enclosed by the two membranes of the envelope. Since transcription factors and mRNAs, two major groups of nuclear macromolecules, use nuclear pores to enter and exit the nucleus and play essential roles in the control of gene activity and expression, this review should be useful to cell and molecular biologists interested in understanding how patch-clamp can be used to quantitate the translocation of such macromolecules into and out of the nucleus.