Nuclear ion channel activity is regulated by actin filaments

Nuclear deformations, from signaling to perturbation and damage

During cell growth and motility in crowded tissues or interstitial spaces, cells must integrate multiple physical and biochemical environmental inputs. After a number of recent studies, the view of the nucleus as a passive object that cells have to drag along has become obsolete, placing the nucleus as a central player in sensing some of these inputs. In the present review, we will focus on changes in nuclear shape caused by external and internal forces. Depending on their magnitude, nuclear deformations can generate signaling events that modulate cell behavior and fate, or be a source of perturbations or even damage, having detrimental effects on cellular functions. On very large deformations, nuclear envelope rupture events become frequent, leading to uncontrolled nucleocytoplasmic mixing and DNA damage. We will also discuss the consequences of repeated compromised nuclear integrity, which can trigger DNA surveillance mechanisms, with critical consequences to cell fate and tissue homeostasis.

Signal transmission through elements of the cytoskeleton form an optimized information network in eukaryotic cells

Multiple prior empirical and theoretical studies have demonstrated wire-like flow of electrons and ions along elements of the cytoskeleton but this has never been linked to a biological function. Here we propose that eukaryotes use this mode of signal transmission to convey spatial and temporal environmental information from the cell membrane to the nucleus. The cell membrane, as the interface between intra- and extra-cellular environments, is the site at which much external information is received. Prior studies have demonstrated that transmembrane ion gradients permit information acquisition when an environmental signal interacts with specialized protein gates in membrane ion channels and producing specific ions to flow into or out of the cell along concentration gradients. The resulting localized change in cytoplasmic ion concentrations and charge density can alter location and enzymatic function of peripheral membrane proteins. This allows the cell to process the information and rapidly deploy a local response. Here we investigate transmission of information received and processed in and around the cell membrane by elements of the cytoskeleton to the nucleus to alter gene expression. We demonstrate signal transmission by ion flow along the cytoskeleton is highly optimized. In particular, microtubules, with diameters of about 30 nm, carry coarse-grained Shannon information to the centrosome adjacent to the nucleus with minimum loss of input source information. And, microfilaments, with diameters of about 4 nm, transmit maximum Fisher (fine-grained) information to protein complexes in the nuclear membrane. These previously unrecognized information dynamics allow continuous integration of spatial and temporal environmental signals with inherited information in the genome.

Mechanosensitive channels and their functions in stem cell differentiation

Stem cells continuously perceive and respond to various environmental signals during development, tissue homeostasis, and pathological conditions. Mechanical force, one of the fundamental signals in the physical world, plays a vital role in the regulation of multiple functions of stem cells. The importance of cell adhesion to the extracellular matrix (ECM), cell-cell junctions, and a mechanoresponsive cell cytoskeleton has been under intensive study in the fields of stem cell biology and mechanobiology. However, the involvement of mechanosensitive (MS) ion channels in the mechanical regulation of stem cell activity has just begun to be realized. Here, we review the diversity and importance of mechanosensitive channels (MSCs), and discuss recently discovered functions of MSCs in stem cell regulation, especially in the determination of cell fate.

Mechanotransduction: Relevance to Physical Therapist Practice-Understanding Our Ability to Affect Genetic Expression Through Mechanical Forces

Mechanotransduction, the mechanism by which mechanical perturbation influences genetic expression and cellular behavior, is an area of molecular biology undergoing rapid exploration and discovery. Cells are sensitive to forces such as shear, tension, and compression, and they respond accordingly through cellular proliferation, migration, tissue repair, altered metabolism, and even stem cell differentiation and maturation. The study of how cells sense and respond to mechanical stimulation is under robust expansion, with new scientific methods and technologies at our disposal. The application of these technologies to physical therapist practice may hold answers to some of our age-old questions while creating new avenues for our profession to optimize movement for societal health. Embracing this science as foundational to our profession will allow us to be valuable scientific collaborators with distinctive knowledge of the effects of loading. These partnerships will be key to augmenting the clinical utility of emerging therapies such as regenerative medicine, tissue engineering, and gene therapy. Collaboration with other scientific disciplines in these endeavors, along with the inclusion and application of these discoveries in our academic programs, will enhance the understanding of the impact of our practice on biologic and genetic processes. A basic understanding of mechanotransduction and its relevance to physical therapist practice is warranted to begin the conversation.

A Case for the Nuclear Membrane as a Mechanotransducer

The cell nucleus is becoming increasingly recognized as a mechanosensitive organelle. Most research on nuclear mechanosignaling focuses on the nuclear lamina and coupled actin structures. In this commentary, we discuss the possibility that the nuclear membrane senses and transduces mechanical signals similar to the plasma membrane. We briefly summarize possible (i) pathophysiological sources of nuclear membrane tension, (ii) features that render nuclear membranes particularly suited for mechanotransduction, and (iii) molecular sensing mechanisms.

Endogenous voltage gradients as mediators of cell-cell communication: strategies for investigating bioelectrical signals during pattern formation

Alongside the well-known chemical modes of cell-cell communication, we find an important and powerful system of bioelectrical signaling: changes in the resting voltage potential (Vmem) of the plasma membrane driven by ion channels, pumps and gap junctions. Slow Vmem changes in all cells serve as a highly conserved, information-bearing pathway that regulates cell proliferation, migration and differentiation. In embryonic and regenerative pattern formation and in the disorganization of neoplasia, bioelectrical cues serve as mediators of large-scale anatomical polarity, organ identity and positional information. Recent developments have resulted in tools that enable a high-resolution analysis of these biophysical signals and their linkage with upstream and downstream canonical genetic pathways. Here, we provide an overview for the study of bioelectric signaling, focusing on state-of-the-art approaches that use molecular physiology and developmental genetics to probe the roles of bioelectric events functionally. We highlight the logic, strategies and well-developed technologies that any group of researchers can employ to identify and dissect ionic signaling components in their own work and thus to help crack the bioelectric code. The dissection of bioelectric events as instructive signals enabling the orchestration of cell behaviors into large-scale coherent patterning programs will enrich on-going work in diverse areas of biology, as biophysical factors become incorporated into our systems-level understanding of cell interactions.

Mechanosensitive mechanisms in transcriptional regulation

Transcriptional regulation contributes to the maintenance of pluripotency, self-renewal and differentiation in embryonic cells and in stem cells. Therefore, control of gene expression at the level of transcription is crucial for embryonic development, as well as for organogenesis, functional adaptation, and regeneration in adult tissues and organs. In the past, most work has focused on how transcriptional regulation results from the complex interplay between chemical cues, adhesion signals, transcription factors and their co-regulators during development. However, chemical signaling alone is not sufficient to explain how three-dimensional (3D) tissues and organs are constructed and maintained through the spatiotemporal control of transcriptional activities. Accumulated evidence indicates that mechanical cues, which include physical forces (e.g. tension, compression or shear stress), alterations in extracellular matrix (ECM) mechanics and changes in cell shape, are transmitted to the nucleus directly or indirectly to orchestrate transcriptional activities that are crucial for embryogenesis and organogenesis. In this Commentary, we review how the mechanical control of gene transcription contributes to the maintenance of pluripotency, determination of cell fate, pattern formation and organogenesis, as well as how it is involved in the control of cell and tissue function throughout embryogenesis and adult life. A deeper understanding of these mechanosensitive transcriptional control mechanisms should lead to new approaches to tissue engineering and regenerative medicine.

Dynamics of mechanical signal transmission through prestressed stress fibers

Transmission of mechanical stimuli through the actin cytoskeleton has been proposed as a mechanism for rapid long-distance mechanotransduction in cells; however, a quantitative understanding of the dynamics of this transmission and the physical factors governing it remains lacking. Two key features of the actin cytoskeleton are its viscoelastic nature and the presence of prestress due to actomyosin motor activity. We develop a model of mechanical signal transmission through prestressed viscoelastic actin stress fibers that directly connect the cell surface to the nucleus. The analysis considers both temporally stationary and oscillatory mechanical signals and accounts for cytosolic drag on the stress fibers. To elucidate the physical parameters that govern mechanical signal transmission, we initially focus on the highly simplified case of a single stress fiber. The results demonstrate that the dynamics of mechanical signal transmission depend on whether the applied force leads to transverse or axial motion of the stress fiber. For transverse motion, mechanical signal transmission is dominated by prestress while fiber elasticity has a negligible effect. Conversely, signal transmission for axial motion is mediated uniquely by elasticity due to the absence of a prestress restoring force. Mechanical signal transmission is significantly delayed by stress fiber material viscosity, while cytosolic damping becomes important only for longer stress fibers. Only transverse motion yields the rapid and long-distance mechanical signal transmission dynamics observed experimentally. For simple networks of stress fibers, mechanical signals are transmitted rapidly to the nucleus when the fibers are oriented largely orthogonal to the applied force, whereas the presence of fibers parallel to the applied force slows down mechanical signal transmission significantly. The present results suggest that cytoskeletal prestress mediates rapid mechanical signal transmission and allows temporally oscillatory signals in the physiological frequency range to travel a long distance without significant decay due to material viscosity and/or cytosolic drag.

Mechanosensitive-mediated interaction, integration, and cardiac control

This review covers aspects of the cardiac mechanotransduction field at different levels, and advocates the possibility that mechanoelectro-chemical transduction forms part of a network of mechanically linked integration in heart-mechanically mediated integration (MMI). It assembles evidence and observations in the literature to promote this hypothesis. Mechanical components can provide the bond between interactions at molecular, cellular, and macro levels to enable the integration. Stretch-activated channels (SACs) exist in the heart, but stresses and strains can affect other membrane channels or receptors. A cellular mechanical change can thus promote several ionic or downstream changes. Cell signal cascades have been implicated and can affect membrane electrophysiology. MMI could shape intracellular and downstream signals using the cytoskeleton and intracellular Ca(2+). MMI also spans other regulatory systems and processes such as the autonomic nervous system (ANS) and operates throughout the whole heart as an integrative system. Finally, supporting the hypothesis, if elements of the normal integration become deranged it contributes to cardiovascular disease and, potentially, lethal arrhythmia.

Cytoskeletal stability and heat shock-mediated thermoprotection of central pattern generation in Locusta migratoria

Prior exposure to extreme temperatures can induce thermoprotection in migratory locusts, which is important for survival in their natural environment. An important motor activity that needs to be protected is ventilation. The mechanism underlying heat shock is not fully understood, and our goal was to test the idea that cytoskeletal stability is critical for such thermoprotection. Cytoskeletal stabilizers (concanavalin A) and destabilizers (colchicine) were bath-applied in semi-intact locust preparations in both control (C) and pre-treated heat-shocked (3 h, 45 degrees C) animals. We measured parameters of the ventilatory motor pattern during maintained high temperature (43 degrees C) and recorded the times taken for motor pattern generation to fail and then recover on returning to room temperature. We found that concanavalin A mimicked the effects of a prior heat stress in control animals by increasing time to failure and decreasing time to recovery of motor pattern generation. However, colchicine destroyed protection in heat-shocked animals by decreasing time to failure and increasing time to recovery. Our findings confirm that the cytoskeleton has a mechanistic role in preserving neural function at high temperatures, possibly through stabilizing ion channels and other integral membrane proteins (e.g. Na(+)/K(+) ATPase) and their interactions with heat shock proteins.

Actin polymerisation at the cytoplasmic face of eukaryotic nuclei

Background

There exists abundant molecular and ultra-structural evidence to suggest that cytoplasmic actin can physically interact with the nuclear envelope (NE) membrane system. However, this interaction has yet to be characterised in living interphase cells.

Results

Using a fluorescent conjugate of the actin binding drug cytochalasin D (CD-BODIPY) we provide evidence that polymerising actin accumulates in vicinity to the NE. In addition, both transiently expressed fluorescent actin and cytoplasmic micro-injection of fluorescent actin resulted in accumulation of actin at the NE-membrane. Consistent with the idea that the cytoplasmic phase of NE-membranes can support this novel pool of perinuclear actin polymerisation we show that isolated, intact, differentiated primary hepatocyte nuclei support actin polymerisation in vitro. Further this phenomenon was inhibited by treatments hindering steric access to outer-nuclear-membrane proteins (e.g. wheat germ agglutinin, anti-nesprin and anti-nucleoporin antibodies).

Conclusion

We conclude that actin polymerisation occurs around interphase nuclei of living cells at the cytoplasmic phase of NE-membranes.

Challenging accepted ion channel biology: p64 and the CLIC family of putative intracellular anion channel proteins (Review)

Parchorin, p64 and the related chloride intracellular channel (CLIC) proteins are widely expressed in multicellular organisms and have emerged as candidates for novel, auto-inserting, self-assembling intracellular anion channels involved in a wide variety of fundamental cellular events including regulated secretion, cell division and apoptosis. Although the mammalian phosphoproteins p64 and parchorin (49 and 65K, respectively) have only been indirectly implicated in anion channel activity, two CLIC proteins (CLIC1 and CLIC4, 27 and 29K, respectively) appear to be essential molecular components of anion channels, and CLIC1 can form anion channels in planar lipid bilayers in the absence of other cellular proteins. However, these putative ion channel proteins are controversial because they exist in both soluble and membrane forms, with at least one transmembrane domain. Even more surprisingly, soluble CLICs share the same glutaredoxin fold as soluble omega class glutathione-S-transferases. Working out how these ubiquitous, soluble proteins unfold, insert into membranes and then refold to form integral membrane proteins, and how cells control this potentially dangerous process and make use of the associated ion channels, are challenging prospects. Critical to this future work is the need for better characterization of membrane topology, careful functional analysis of reconstituted and native channels, including their conductances and selectivities, and detailed structure/function studies including targeted mutagenesis to investigate the structure of the putative pore, the role of protein phosphorylation and the role of conserved cysteine residues.

Cell cycling determines integrin-mediated adhesion in osteoblastic ROS 17/2.8 cells exposed to space-related conditions

Six days of microgravity (Bion10 mission) induced dramatic shape changes in ROS 17/2.8 osteoblasts (7). During the Foton 11 and 12 space flights, we studied the kinetics (0-4 days) of ROS 17/2.8 morphology and adhesion, the relationships between adhesion and cell cycle progression after 4 days in space, and osteoblastic growth and activity after 6 days in space. Quantitative analysis of high-resolution adhesion [focal adhesion area imaged by total interference reflection fluorescent microscopy (TIRFM)] and integrin-dependent adhesion (imaged on confocal microscope by vinculin and phosphotyrosine staining) as well as cell cycle phase classification [Ki-67 staining, S-G2, mitotic cells and G1 (postmitotic cells)] were performed using programs validated in parabolic flight and clinostat. We observed disorganization of the cytoskeleton associated with disassembling of vinculin spots and phosphorylated proteins within focal contacts with no major change in TIRFM adhesion after 2 and 4 days of microgravity. Postmitotic cells, alone, accounted for the differences observed in the whole population. They are characterized by immature peripheral contacts with complete loss of central spots and decreased spreading. Osteocalcin, P1CP and alkaline phosphatase, and proliferation were similar in flight cells and 1 g centrifuge and ground controls after 6 days. In conclusion, microgravity substantially affected osteoblastic integrin-mediated cell adhesion. ROS17/2.8 cells responded differently, whether or not they were cycling by reorganizing adhesion plaque topography or morphology. In ROS 17/2.8, this reorganization did not impair osteoblastic phenotype.

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

In eukaryotic cells the nuclear envelope (NE) serves as a functional barrier between cytosol and nucleoplasm perforated by nuclear pore complexes (NPCs). Both active and passive transport of ions and macromolecules are thought to be mediated by the centrally located large NPC channel. However, 3-dimensional imaging of NPCs based on electron microscopy indicates the existence of additional small channels of unknown function located in the NPC periphery. By means of the recently developed nuclear hourglass technique that measures NE electrical conductance, we evaluated passive electrically driven transport through NPCs. In isolated Xenopus laevis oocyte nuclei, we varied ambient Ca2+ and ATP in the cytosolic solution and/or chelated Ca2+ in the perinuclear stores in order to assess the role of Ca2+ in regulating passive ion transport. We noticed that NE electrical conductance is large under conditions where macromolecule permeability is known to be low. In addition, atomic force microscopy applied to native NPCs detects multiple small pores in the NPC periphery consistent with channel openings. Peripheral pores were detectable only in the presence of ATP. We conclude that NPC transport of ions and macromolecules occurs through different routes. We present a model in which NE ion flux does not occur through the central NPC channel but rather through Ca2+- and ATP-activated peripheral channels of individual NPCs.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

Cytoskeletal disruption accelerates caspase-3 activation and alters the intracellular membrane reorganization in DNA damage-induced apoptosis

In actinomycin D (AD)-induced apoptosis, caspase-3 activation and DNA cleavage in human megakaryoblastic leukemia CMK-7 cells were greatly accelerated by tubulin and actin polymerization inhibitors [e.g., colcemid (CL) and cytochalasin D (CD), respectively], but the acceleration was not found with Taxol or phalloidin. A decrease in mitochondrial transmembrane potential, release of cytochrome c into the cytosol, and cleavage of procaspase-9 to its active form preceded the activation of caspase-3 and, moreover, all of these events began earlier and/or proceeded faster in cells treated with AD plus CL or CD than in cells treated with AD only. These results suggest that cytoskeletal disruption in the apoptotic cells promotes damage of the mitochondrial membrane, resulting in the enhanced release of cytochrome c necessary for the activation of caspase-9 that initiates the caspase cascade. On the other hand, apoptotic bodies were rapidly formed from cells treated with AD and CL, but were suppressed when treated with AD and CD. Intracellular membranes and the actin system were reorganized to surround the nuclear fragments in the AD- and CL-treated cells, but such a membrane system was not formed in the presence of CD, implying that the apoptotic bodies are formed via reorganization of intracellular membranes under regulation by actin polymerization. Thus, the cytoskeletal change in CMK-7 cells has a strong effect on the early biochemical process as well as on the later morphologic process in AD-induced apoptosis.

Calcium, ATP and nuclear pore channel gating

Nuclear envelope (NE) cisternal Ca2+ and cytosolic ATP are required for nuclear-pore-complex-(NPC-) mediated transport of DNAs, RNAs, transcription factors and other large molecules. Isolated cardiomyocyte nuclei, capable of macromolecular transport (MMT), have intrinsic NPC ion channel behavior. The large ion conductance (gamma) activity of the NPC channel (NPCC) is blocked by the NPC monoclonal antibody mAb414, known to block MMT, and is also silenced during periods of MMT. In cardiomyocytes, neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. To test the role of Ca2+ and ATP in NPCC activity, we carried out the present patch-clamp study with the pipette attached to the outer NE membrane of nuclei isolated from cultured Dunning G prostate cancer cells. Our investigations demonstrate that in these isolated nuclei neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. However, when simultaneously applied to the bath and pipette, they transiently silence NPCC activity through stimulation of MMT by raising the Ca2+ concentration in the NE cisterna ([Ca2+]NE). Our fluorescence microscopy observations with nuclear-targeted macromolecular fluorochromes (B-phycoerythrin and plasmid for the enhanced green fluorescence protein EGFP, pEGFP-C1) and with FITC-labeled RNA support the view that channel silence accompanies MMT. Repeated Ca2+ loading of the NE with Ca2+ and ATP, after unloading with 1-5 microM inositol 1,4,5-trisphosphate (IP3), thapsigargin (TSG) or 5 mM BAPTA or EGTA, failed to affect channel gating. This result indicates that other factors are involved in this phenomenon and that they are exhausted during the first cycle of NE Ca2+ loading/unloading--in agreement with current theories of NPC-mediated MMT. The results explain how Ca2+ and IP3 waves may convert the NE into an effective Ca2+ barrier and, consequently, affect the regulation of gene activity and expression through their feedback on MMT and NPCC gating. Thus, [Ca2+]NE regulation by intracellular messengers is an effective mechanism for synchronizing gene activity and expression to the cellular rhythm.

Nuclear hourglass technique: an approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Nuclear pore complexes (NPCs) mediate both active transport and passive diffusion across the nuclear envelope (NE). Determination of NE electrical conductance, however, has been confounded by the lack of an appropriate technical approach. The nuclear patch clamp technique is restricted to preparations with electrically closed NPCs, and microelectrode techniques fail to resolve the extremely low input resistance of large oocyte nuclei. To address the problem, we have developed an approach for measuring the NE electrical conductance of Xenopus laevis oocyte nuclei. The method uses a tapered glass tube, which narrows in its middle part to 2/3 of the diameter of the nucleus. The isolated nucleus is sucked into the narrow part of the capillary by gentle fluid movement, while the resulting change in electrical resistance is monitored. NE electrical conductance was unexpectedly large (7.9 +/- 0.34 S/cm(2)). Evaluation of NPC density by atomic force microscopy showed that this conductance corresponded to 3.7 x 10(6) NPCs. In contrast to earlier conclusions drawn from nuclear patch clamp experiments, NPCs were in an electrically "open" state with a mean single NPC electrical conductance of 1.7 +/- 0.07 nS. Enabling or blocking of active NPC transport (accomplished by the addition of cytosolic extracts or gp62-directed antibodies) revealed this large NPC conductance to be independent of the activation state of the transport machinery located in the center of NPCs. We conclude that peripheral channels, which are presumed to reside in the NPC subunits, establish a high ionic permeability that is virtually independent of the active protein transport mechanism.

Distinct calcium signaling pathways regulate calmodulin gene expression in tobacco

Cold shock and wind stimuli initiate Ca(2+) transients in transgenic tobacco (Nicotiana plumbaginifolia) seedlings (named MAQ 2.4) containing cytoplasmic aequorin. To investigate whether these stimuli initiate Ca(2+) pathways that are spatially distinct, stress-induced nuclear and cytoplasmic Ca(2+) transients and the expression of a stress-induced calmodulin gene were compared. Tobacco seedlings were transformed with a construct that encodes a fusion protein between nucleoplasmin (a major oocyte nuclear protein) and aequorin. Immunocytochemical evidence indicated targeting of the fusion protein to the nucleus in these plants, which were named MAQ 7.11. Comparison between MAQ 7.11 and MAQ 2.4 seedlings confirmed that wind stimuli and cold shock invoke separate Ca(2+) signaling pathways. Partial cDNAs encoding two tobacco calmodulin genes, NpCaM-1 and NpCaM-2, were identified and shown to have distinct nucleotide sequences that encode identical polypeptides. Expression of NpCaM-1, but not NpCaM-2, responded to wind and cold shock stimulation. Comparison of the Ca(2+) dynamics with NpCaM-1 expression after stimulation suggested that wind-induced NpCaM-1 expression is regulated by a Ca(2+) signaling pathway operational predominantly in the nucleus. In contrast, expression of NpCaM-1 in response to cold shock is regulated by a pathway operational predominantly in the cytoplasm.

Changes of G-actin localisation in the mitotic spindle region or nucleus during mitosis and after heat shock: a histochemical study of G-actin in various cell lines with fluorescent labelled vitamin D-binding protein

The presence and localisation of G-actin in various cell lines was studied using the highly G-actin specific, fluorescence-labelled vitamin D-binding protein. In various cell-types, pig kidney-derived cells (LLC-PK1), Chinese hamster ovary (CHO) cells, SV-40 transformed African green monkey kidney (COS) cells and human hepatoma (HepG2) cells, G-actin was only visible in the cytoplasm of interphase cells. However, in mitotic cells, depending on the mitotic phase, intense G-actin staining was found associated with the mitotic spindle (early mitosis) or overlapping the DNA-staining pattern (late mitosis). Also after heat shock (60-180 min at 43 degrees C), an intense nuclear staining of G-actin was observed. In LLC-PK1 cells, the increase of nuclear G-actin staining disappeared again after 24 h at 37 degrees C, but in COS, CHO and HepG2 cells, it was still present in the nucleus after 24 h at 37 degrees C, indicating that the process was not rapidly reversible in these cells; the increased nuclear G-actin was not associated with cell division. Comparison of the amount of G-actin present in the nucleus and in the cytosol before and after heat shock using Western blotting demonstrated that the total amount of G-actin in both nucleus and cytosol was unchanged after heat shock. This indicates that the increased G-actin staining is not a result of import of G-actin into the nucleus. These observations suggest a rearrangement of G-actin in the nucleus during both mitosis and heat shock, which may be due to changes in interaction of G-actin with chromosomes.

Gating mechanism of the nuclear pore complex channel in isolated neonatal and adult mouse liver nuclei

Several types of ionic channels on the outer membrane of the nuclear envelope communicate with the nuclear cisternae. These are distinct from nucleocytoplasmic pathways, the nuclear pores that span the double membrane of the envelope and are the route for RNA and protein traffic in the nucleus. Recent data indicate that the nuclear pores may also function as ion channels. The most probable candidate for nucleocytoplasmic ion flux is a 300-400 pS pathway observed in many nuclear preparations. Morphological and functional studies of nuclear envelope suggest a tight relationship between the large conductance channel and the pore complex. However, there is no direct evidence for gating of the nuclear pore or its ability to open and close as a conventional channel. This study shows that in liver nuclei isolated from newborn mouse, there is a substantial correspondence between the number of pores and the number of channels recorded during patch-clamp. This is not the case for adult nuclei. Although pore density is comparable, some nuclear cytoskeletal components, such as actin and nonmuscle myosin, show a significant increase in the adult preparation. Previous studies demonstrate the presence of these two proteins in association with the pore complex. Here we show that by using actin filament disrupter, we were able to increase the number of active channels in adult isolated nuclei. We suggest that a functional interaction between actin filaments and the nuclear pore complex could regulate nucleocytoplasmic permeability.

beta-thymosin is required for axonal tract formation in developing zebrafish brain

beta-Thymosins are polypeptides that bind monomeric actin and thereby function as actin buffers in many cells. We show that during zebrafish development, β-thymosin expression is tightly correlated with neuronal growth and differentiation. It is transiently expressed in a subset of axon-extending neurons, essentially primary neurons that extend long axons, glia and muscle. Non-neuronal expression in the brain is restricted to a subset of glia surrounding newly forming axonal tracts. Skeletal muscle cells in somites, jaw and fin express beta-thymosin during differentiation, coinciding with the time of innervation. Injection of beta-thymosin antisense RNA into zebrafish embryos results in brain defects and impairment of the development of beta-thymosin-associated axon tracts. Furthermore, irregularities in somite formation can be seen in a subset of embryos. Compared to wild-type, antisense-injected embryos show slightly weaker and more diffuse engrailed staining at the midbrain-hindbrain boundary and a strong reduction of Isl-1 labeling in Rohon Beard and trigeminal neurons. The decreased expression is not based on a loss of neurons indicating that beta-thymosin may be involved in the maintenance of the expression of molecules necessary for neuronal differentiation. Taken together, our results strongly indicate that beta-thymosin is an important regulator of development.

Mechanosensitivity as an integrative system in heart: an audit

This review examines a manifold of apparently loosely linked observations and mechanisms, from membrane to man, and assembles them to support the notion that mechanoelectric transduction is an integrative regulatory system in the heart. For this, the assemblage has to satisfy, at least to some extent, criteria that apply to other integrative regulatory systems such as the endocrine and nervous systems. The integrative effectors in the endocrine system are chemical linkages, circulating hormones: in the nervous system the linkage is a network of cables, nerve conduction and neurotransmitters. Mechanical integration is would be effected through mechanical machinery, cardiac contractile and hydraulic function with attendant stress and strain transmitted via "tensegrity". This can, through the cytoskeleton, begin with membrane integrins and transmit intracellularly for example via F actins to reach the rest of the membranous integrins. Further transmission to the organ is via cell-to-cell adhesion complexes and the extracellular matrix. This tensegrity facilitates integration of force and strain changes from area to area. In consequence, and analogous to the neurendocrine system, mechanoelectric transduction should, and does (1) operate at the molecular or membrane level--this would be via mechanotransducers affecting transmembrane ionic flow; (2) operate in the cell--to influence electrophysiology; (3) have a multicellular expression--e.g. mechanical distortion of one cell can raise intracellular calcium of an adjacent cell; (4) express in the intact organ--e.g. an increase in venous return hydraulically distends the sinoatrial node, steepening its pacemaker potential, thus increasing heart rate. It should also (5) demonstrate elements of a feedback system--"mechanoelectric feedback", and (6) interact with other systems--the cytoskeleton incorporates cell signalling complexes intersecting with other signal cascades. Finally, (7) it can malfunction to produce clinical abnormality--it contributes electrophysiologically to lethal cardiac arrhythmia. This anatomical and functional behaviour of mechanoelectric transduction could sanction the prospect of viewing it as analogous to the other integrative physiological systems.

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.

Current evidence suggests independent regulation of nuclear calcium

We review and present current evidence supporting independent regulation of nuclear Ca2+ ([Ca2+]n). The nucleus and nuclear envelope contain proteins to both regulate and respond to changes in [Ca2+]n. However, this does not prove that [Ca2+]n is independently regulated from cytosolic Ca2+ ([Ca2+]c). Studies using fluorescent dyes suggested that changes in [Ca2+]n differed in magnitude from changes in [Ca2+]c. These studies have been criticised as the nuclear environment alters the fluorescent characteristics of these dyes. We have evaluated this question with aequorin targeted to the nucleus and cytoplasm and shown that the characteristics of the indicators are not altered in their respective environments. We have demonstrated that different stimuli induce changes in [Ca2+]n and [Ca2+]c that vary both temporally and in magnitude. The nucleus appeared to be shielded from increases in [Ca2+]c, either through a mechanism involving the nuclear envelope or by cytosolic buffering of localised increases in Ca2+. In addition, agonist stimulation resulted in an increase in [Ca2+]n, consistent with release from the perinuclear Ca2+ store. There was a stimulus dependence of the relation between [Ca2+]n and [Ca2+]c suggesting differential regulation of [Ca2+]n. These results have important implications for the role of Ca2+ as a specific regulator of nuclear events through Ca2+ binding proteins. In addition, they highlight the advantages of using targeted aequorin in intact cells to monitor changes in organelle [Ca2+].