Nuclear surface complex as observed with the high resolution scanning electron microscope. Visualization of the membrane surfaces of the neclear envelope and the nuclear cortex from Xenopus laevis oocytes

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.

Mechanics and deformation of the nucleus in micropipette aspiration experiment

Robust biomechanical models are essential for the study of nuclear mechanics and deformation and can help shed light on the underlying mechanisms of stress transition in nuclear elements. Here, we develop a computational model for an isolated nucleus undergoing micropipette aspiration. Our model includes distinct components representing the nucleoplasm and nuclear envelope. The nuclear envelope itself comprises three layers: inner and outer nuclear membranes and one thicker layer representing the nuclear lamina. The nucleoplasm is modeled as a viscoelastic Maxwell material with a single time constant, while a modified Maxwell model, equivalent to a spring and a dashpot in series and both in parallel with a spring, is adopted for the inner and outer nuclear membranes. The nuclear envelope layer is taken as a linear elastic material. The proposed computational model, validated using experimental observations of Guilak et al. [2000. Viscoelastic properties of the cell nucleus. Biochemical and Biophysical Research Communications 269, 781-786] and Deguchi et al. [2005, Flow-induced hardening of endothelial nucleus as an intracellular stress-bearing organelle. Journal of Biomechanics 38, 1751-1759], is employed to study nuclear mechanics and deformation in micropipette aspiration and to shed light on the contribution of individual nuclear components on the response. The results indicate that the overall response of an isolated nucleus in micropipette aspiration is highly sensitive to the apparent stiffness of the nuclear lamina. This observation suggests that micropipette aspiration is an effective technique for examining the influence of various kinds of alteration in the nuclear lamina, such as mutations in the gene encoding lamin A, and also structural remodeling due to mechanical perturbation.

The nuclear barrier is structurally and functionally highly responsive to glucocorticoids

Nuclear pore complexes mediate and control transport between the cytosol and the nucleus. They form a highly selective and, thus, tight nuclear barrier between these compartments. The nuclear barrier provides the cell with the opportunity to control access to its DNA, a defining feature of eukaryotes. The tightness of the nuclear barrier is therefore physiologically pivotal and any remarkable change in its structure and permeability can prove pathophysiological, e.g. as a result of viral attack. However, there is accumulating evidence that nuclear barrier structure and permeability are highly responsive to hydrophobic cargos of crucial physiological and therapeutic relevance, glucocorticoids (steroid hormones). The present review highlights the glucocorticoid-induced effects on the nuclear barrier structure and permeability concluding that they are physiologically essential to mediate glucocorticoid action.

Fluorescence photobleaching analysis of nuclear transport: dynamic evidence for auxiliary channels in detergent-treated nuclei

Nuclear transport experiments were performed on isolated rat liver nuclei to examine the permeability of membrane and detergent-free peripheral nuclear lamina. The transport of 64K molecular weight fluorescent-derivatized dextrans was measured by using the technique of fluorescence redistribution after photobleaching. Results of these experiments provide evidence for transport pathways that appear to be functionally distinct from nuclear pore complex channels. The suggestion is made that these supplemental pathways are embedded in the peripheral nuclear lamina and are normally masked by the inner nuclear membrane.

Preparation and characterization of nuclear-envelope vesicles from rat liver nuclei

We describe a procedure for the preparation of sealed nuclear-envelope vesicles from rat liver nuclei. These vesicles are strikingly similar in their polypeptide composition when compared with those of nuclear envelopes prepared conventionally using deoxyribonuclease I. Subfractionation analysis by means of extraction with high salt and urea show that the components of the nuclear envelope, e.g. the pore-complex/lamina fraction, are present. The residual DNA content is only 1.5%, and typical preparations consist of about 80% vesicles, with the vesicular character of these envelopes shown by microscopic and biochemical studies. The vesicles can be obtained in high yield, are tight and stable for at least two days and are enriched in a nucleoside triphosphatase thought to be involved in nucleocytoplasmic transport processes. Because the vesicles are largely free of components of the nuclear interior, but retain properties of intact nuclei, we believe that they are a valuable model system to study nucleocytoplasmic transport. Although in transport studies with isolated nuclei interference from intranuclear events has to be considered, the nuclear-envelope vesicles provide the possibility of studying translocation alone. Furthermore, the less complex nature of these vesicles compared with whole nuclei should facilitate investigation of the components involved in the regulation of nuclear transport processes.

Lateral diffusion in nuclear membranes

Chemical modification of rat liver nuclei with citraconic anhydride selectively removed outer nuclear membrane. This conclusion was based on (a) transmission electron microscopy, (b) lipid analysis, (c) lamin B as an inner membrane-associated marker, and (d) the demonstration of phospholipid lateral mobility on outer membrane-depleted nuclei as a criteria for inner membrane integrity. Addition of urea or N-ethylmaleimide resulted in the additional disruption of inner membrane. Fluorescence photobleaching was used to determine the long range (greater than 4 microns) lateral transport of lectin receptors and a phospholipid analog in both membranes. The diffusion coefficient for wheat germ agglutinin on whole nuclei was 3.9 X 10(-10) cm2/s whereas the diffusion coefficient for wheat germ agglutinin in outer membrane-depleted nuclei was less than or equal to 10(-12) cm2/s. Phospholipid mobilities were the same in whole and outer membrane-depleted nuclei (3.8 X 10(-9) cm2/s). The protein diffusion differences observed between whole and outer membrane-depleted nuclei may be interpreted in the context of two functionally different membrane systems that compose the double bilayer of the nucleus.

Protein loss during nuclear isolation

Cryomicrodissection makes possible the measurement of the entire in vivo protein content of the amphibian oocyte nucleus and provides a heretofore missing baseline for estimating protein loss during nuclear isolation by other methods. When oocyte nuclei are isolated into an aqueous medium, they lose 95% of their protein with a half-time of 250 s. This result implies an even more rapid loss of protein from aqueously isolated nuclei of ordinary-size cells.