Evidence for Ca2+- and ATP-sensitive peripheral channels in nuclear pore complexes

Structural adaptation of the nuclear pore complex in stem cell-derived cardiomyocytes

Macromolecules are transported in and out of the nucleus through nuclear pores. It is poorly understood how these megadalton conduits support nucleocytoplasmic traffic during genetic reprogramming associated with cell commitment to a specific lineage. Murine embryonic stem cells were differentiated into cardiomyocytes within embryoid bodies, and contracting cells expressing myocardial-specific proteins were isolated from the mesodermal layer. Compared with postmitotic cardiac cells from heart muscle, these proliferative and differentiating stem cell-derived cardiomyocytes demonstrated a significantly lower density of nuclear pores. At nanoscale resolution, the pore channel was commonly unoccupied in heart muscle-isolated cardiac cells, yet a dense material, presumably the central transporter, protruded toward the cytosolic face of the nuclear pore complex in stem cell-derived cardiomyocytes. Stem cell-derived cardiac cells distributed the nuclear transport factor Ran in the nucleus, decreased the number of spare nuclear pore complexes from the cytosolic annulate lamellae reservoir, and expressed a set of nucleoporins, NUP214, NUP358, NUP153, and p62, involved in nuclear transport. Stem cell-derived cardiomyocytes secured transport of nuclear constitutive proteins, cardiogenic transcription factors, and cell cycle regulators, including the prototypic histone H1, myocyte enhancer binding factor 2, and p53. Thus, differentiating stem cell-derived cardiomyocytes undergo structural adaptation and mobilize nuclear transport regulators in support of nucleocytoplasmic communication during commitment to mature cardiac lineage.

Nucleoplasmic calcium regulation in rabbit aortic vascular smooth muscle cells

In this study, we investigated whether nucleoplasmic free Ca2+ in aortic vascular smooth muscle cells (VSMCs) might be independently regulated from cytosolic free Ca2+. Understanding mechanisms and pathways responsible for this regulation is especially relevant given the role of a numerous intranuclear Ca2+-sensitive proteins in transcriptional regulation, apoptosis and cell division. The question of an independent regulatory mechanism remains largely unsettled because the previous use of intensitometric fluorophores (e.g., Fluo-3) has been criticized on technical grounds. To circumvent the potential problem of fluorescence artifact, we utilized confocal laser scanning microscopy to image intracellular Ca2+ movements with the ratiometric fluorophore Indo-1. In cultured rabbit VSMCs, we found sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) pumps and ryanodine receptor (RyR) Ca2+ channel proteins to be discretely arranged within a perinuclear locus, as determined by fluorescent staining patterns of BODIPY FL thapsigargin and BODIPY FL-X Ry. When intracellular Ca2+ stores were mobilized by addition of thapsigargin (5 microM) and activatory concentrations of ryanodine (1 microM), Indo-1 ratiometric signals were largely restricted to the nucleoplasm. Cytosolic signals, by comparison, were relatively small and even then its spatial distribution was largely perinuclear rather homogeneous. These observations indicate perinuclear RyR and SERCA proteins are intimately involved in regulating VSMC nucleoplasmic Ca2+ concentrations. We also observed a similar pattern of largely nucleoplasmic Ca2+ mobilization upon exposure of cells to the immunosuppressant drug FK506 (tacrolimus), which binds to the RyR-associated immunophillin-binding proteins FKBP12 and FKBP12.6. However, initial FK506-induced nucleoplasmic Ca2+ mobilization was followed by marked reduction of Indo-1 signal intensity close to pretreatment levels. This suggested FK506 exerts both activatory and inhibitory effects upon RyR channels. The latter was reinforced by observed effects of FK506 to only reduce nucleoplasmic Indo-1 signal intensity when added following pretreatment with both activatory and inhibitory concentrations of ryanodine. These latter observations raise the possibility that VSMC nuclei represent an important sink of intracellular Ca2+ and may help explain vasodilatory actions of FK506 observed by others.

Real-time imaging of nuclear permeation by EGFP in single intact cells

The NPC is the portal for the exchange of proteins, mRNA, and ions between nucleus and cytoplasm. Many small molecules (<10 kDa) permeate the nucleus by simple diffusion through the pore, but molecules larger than 70 kDa require ATP and a nuclear localization sequence for their transport. In isolated Xenopus oocyte nuclei, diffusion of intermediate-sized molecules appears to be regulated by the NPC, dependent upon [Ca(2+)] in the nuclear envelope. We have applied real-time imaging and fluorescence recovery after photobleaching to examine the nuclear pore permeability of 27-kDa EGFP in single intact cells. We found that EGFP diffused bidirectionally via the NPC across the nuclear envelope. Although diffusion is slowed approximately 100-fold at the nuclear envelope boundary compared to diffusion within the nucleus or cytoplasm, this delay is expected for the reduced cross-sectional area of the NPCs. We found no evidence for significant nuclear pore gating or block of EGFP diffusion by depletion of perinuclear Ca(2+) stores, as assayed by a nuclear cisterna-targeted Ca(2+) indicator. We also found that EGFP exchange was not altered significantly during the cell cycle.

[Atomic force microscopy: from cellular imaging to molecular manipulation]

Using a sharp tip attached at the end of a soft cantilever as a probe, the atomic force microscope (AFM) explores the surface topography of biological samples bathed in physiological solutions. In the last few years, the AFM has gained popularity among biologists. This has been obtained through the improvement of the equipment and imaging techniques as well as through the development of new non-imaging applications. Biological imaging has to face a main difficulty that is the softness and the dynamics of most biological materials. Progress in understanding the AFM tip-biological samples interactions provided spectacular results in different biological fields. Recent examples of the possibilities offered by the AFM in the imaging of intact cells, isolated membranes, membrane model systems and single molecules at work are discussed in this review. Applications where the AFM tip is used as a nanotool to manipulate biomolecules and to determine intra- and intermolecular forces from single molecules are also presented.

Energetic communication between mitochondria and nucleus directed by catalyzed phosphotransfer

Exchange of information between the nucleus and cytosol depends on the metabolic state of the cell, yet the energy-supply pathways to the nuclear compartment are unknown. Here, the energetics of nucleocytoplasmic communication was determined by imaging import of a constitutive nuclear protein histone H1. Translocation of H1 through nuclear pores in cardiac cells relied on ATP supplied by mitochondrial oxidative phosphorylation, but not by glycolysis. Although mitochondria clustered around the nucleus, reducing the distance for energy transfer, simple nucleotide diffusion was insufficient to meet the energetic demands of nuclear transport. Rather, the integrated phosphotransfer network was required for delivery of high-energy phosphoryls from mitochondria to the nucleus. In neonatal cardiomyocytes with low creatine kinase activity, inhibition of adenylate kinase-catalyzed phosphotransfer abolished nuclear import. With deficient adenylate kinase, nucleoside diphosphate kinase, which secures phosphoryl exchange between ATP and GTP, was unable to sustain nuclear import. Up-regulation of creatine kinase phosphotransfer, to mimic metabolic conditions of adult cardiac cells, rescued H1 import, suggesting a developmental plasticity of the cellular energetic system. Thus, mitochondrial oxidative phosphorylation coupled with phosphotransfer relays provides an efficient energetic unit in support of nuclear transport.

Aldosterone signaling pathway across the nuclear envelope

We describe the route by which aldosterone-triggered macromolecules enter and exit the cell nucleus of Xenopus laevis oocyte. Oocytes were microinjected with 50 fmol aldosterone and then enucleated 2-30 min after injection. After isolation, nuclear envelope electrical resistance (NEER) was measured in the intact cell nuclei by using the nuclear hourglass technique. We observed three NEER stages: an early peak 2 min after injection, a sustained depression after 5-15 min, and a final late peak 20 min after injection. Because NEER reflects the passive electrical permeability of nuclear pores, we investigated with atomic force microscopy aldosterone-induced conformational changes of individual nuclear pore complexes (NPCs). At the early peak we observed small ( congruent with 100 kDa) molecules (flags) attached to the NPC surface. At the sustained depression NPCs were found free of flags. At the late peak large ( congruent with 800 kDa) molecules (plugs) were detected inside the central channels. Ribonuclease or actinomycin D treatment prevented the late NEER peak. Coinjection of aldosterone (50 fmol) and its competitive inhibitor spironolactone (500 fmol) eliminated the electrical changes as well as flag and plug formation. We conclude: (i) The genomic response of aldosterone can be electrically measured in intact oocyte nuclei. (ii) Flags represent aldosterone receptors on their way into the cell nucleus whereas plugs represent ribonucleoproteins carrying aldosterone-induced mRNA from the nucleoplasm into the cytoplasm. (iii) Because plugs can be mechanically harvested with the atomic force microscopy stylus, oocytes could serve as a bioassay system for identifying aldosterone-induced early genes.