Building distinct actin filament networks in a common cytoplasm

Eukaryotic cells generate a diversity of actin filament networks in a common cytoplasm to optimally perform functions such as cell motility, cell adhesion, endocytosis and cytokinesis. Each of these networks maintains precise mechanical and dynamic properties by autonomously controlling the composition of its interacting proteins and spatial organization of its actin filaments. In this review, we discuss the chemical and physical mechanisms that target distinct sets of actin-binding proteins to distinct actin filament populations after nucleation, resulting in the assembly of actin filament networks that are optimized for specific functions.     

Immunoreactivity to cell surface syndecans in cytoplasm and nucleus: tubulin-dependent rearrangements

Syndecans are transmembrane proteoglycans implicated in the regulation of cell growth and differentiation, by interacting with growth factors. Although syndecans play a major role in regulating cell morphology, little is known about their subcellular distribution and in vivo association with the cytoskeleton. To address this question, we investigated the subcellular distribution and dynamic rearrangement of syndecans-1, -2, and -4, using confocal laser microscopy. Furthermore, we monitored the spatial relation of syndecans to tubulin in both mitotic and interphase cells. Initially, the reactivity to syndecans was confined to the cytoplasm, staining of the cell membranes appearing later. Syndecan-1 also seems to translocate to the nucleus in a time-dependent manner. The mitotic spindle shows unexpectedly more syndecans than that found in interphase cells. After vinblastine treatment, both syndecan-1 and tubulin were recovered as paracrystalline occlusion bodies, and the nuclear reactivity to syndecan-1 disappeared, suggesting tubulin-mediated nuclear transport of this proteoglycan. Plasma membrane staining reappeared in the postmitotic cells. Nuclear translocation predominantly affected syndecan-1, whereas syndecan-2 and -4 remained in cytoplasm and cell membrane. This is the first report on regulated nuclear translocation and the presence of syndecan-1 in the mitotic spindle, where it may stabilize the mitotic machinery. The syndecan-1/tubulin complex may also act as a vehicle for the transport of protein growth factors to the cell nucleus.     

Reassembly of anAcetabularia mediterranea cell from the nucleus,cytoplasm, and cell wall

Summary A. mediterranea cells capable of full morphogenesis were reassembled from nuclei, cytoplasm, and cell wall fraction.Reassembly was performed stepwise with the recombination of cytoplasm and cell walls and finally a nucleus was implanted.Reassembly of anucleate cells was carried out by means of retransplantation of their own cytoplasm or transplantation of cytoplasm from another cell. Combinations between cytoplasm and cell walls of dark or light maintained cells were prepared. The nuclei were always transferred from light maintained cells.     

On the architecture of cell regulation networks

Background  

With the rapid development of high-throughput experiments, detecting functional modules has become increasingly important in analyzing biological networks. However, the growing size and complexity of these networks preclude structural breaking in terms of simplest units. We propose a novel graph theoretic decomposition scheme combined with dynamics consideration for probing the architecture of complex biological networks.     

Migration of the nucleus and cytoplasm along the pseudopodium of the differentiated neuroblastoma cell

Motility of neuroblastoma cells in the culture of cell line C-1300, clone N-18-A was investigated microcinematographically. In the course of morphological differentiation of the cells, after cytochalasin B treatment (1.8 mkg/ml for 24 hours), in some differentiated cells a special type of movement of the cytoplasmic mass together with the nucleus along elongated pseudopodia was detected. Such a type of movement has never been described. Sometimes, a shift in the nucleus position resulted in the complete change or reversion of cell polarity. The phenomenon of cell nucleus displacement relative to the cell configuration or reversion of the cell polarity can possibly play an important functional role for neural cells.     

Transport between the cytoplasm and the nucleus

Summary Active transport of proteins and RNAs across the nuclear-pore complex (NPC) is mediated by a family of related transport receptors which shuttle between the cytoplasm and the nucleoplasm. A number of import and export pathways have been described. Some transport substrates require adapters which mediate association with certain transporters. The transport receptors specifically bind to a recognition signal within the transport substrate or adapter, pass the NPC in one direction, and deliver their cargo to the other side of the nuclear envelope. The Ran GTPase is the crucial regulator of bidirectional transport. Ran-modulating proteins establish an asymmetric intracellular distribution of Ran. As a result, Ran is mainly bound to GTP in the nucleus and to GDP in the cytoplasm. Evidently, RanGTP regulates binding and release of the transport substrates by binding to the transport receptors in the nucleus as well as the transport direction across the NPC. However, little is known about the molecular mechanism of translocation through the NPC.     

Coupling of the nucleus and cytoplasm: role of the LINC complex

The nuclear envelope defines the barrier between the nucleus and cytoplasm and features inner and outer membranes separated by a perinuclear space (PNS). The inner nuclear membrane contains specific integral proteins that include Sun1 and Sun2. Although the outer nuclear membrane (ONM) is continuous with the endoplasmic reticulum, it is nevertheless enriched in several integral membrane proteins, including nesprin 2 Giant (nesp2G), an 800-kD protein featuring an NH(2)-terminal actin-binding domain. A recent study (Padmakumar, V.C., T. Libotte, W. Lu, H. Zaim, S. Abraham, A.A. Noegel, J. Gotzmann, R. Foisner, and I. Karakesisoglou. 2005. J. Cell Sci. 118:3419-3430) has shown that localization of nesp2G to the ONM is dependent upon an interaction with Sun1. In this study, we confirm and extend these results by demonstrating that both Sun1 and Sun2 contribute to nesp2G localization. Codepletion of both of these proteins in HeLa cells leads to the loss of ONM-associated nesp2G, as does overexpression of the Sun1 lumenal domain. Both treatments result in the expansion of the PNS. These data, together with those of Padmakumar et al. (2005), support a model in which Sun proteins tether nesprins in the ONM via interactions spanning the PNS. In this way, Sun proteins and nesprins form a complex that links the nucleoskeleton and cytoskeleton (the LINC complex).     

Shuttling of galectin-3 between the nucleus and cytoplasm

In previous studies, we documented that galectin-3 (M(r) approximately 30,000) is a pre-mRNA splicing factor. Recently, galectin-3 was identified as a component of a nuclear and cytoplasmic complex, the survival of motor neuron complex, through its interaction with Gemin4. To test the possibility that galectin-3 may shuttle between the nucleus and the cytoplasm, human fibroblasts (LG-1) were fused with mouse fibroblasts (3T3). The monoclonal antibody NCL-GAL3, which recognizes human galectin-3 but not the mouse homolog, was used to monitor the localization of human galectin-3 in heterodikaryons. Human galectin-3 localized to both nuclei of a large percentage of heterodikaryons. Addition of the antibiotic leptomycin B, which inhibits nuclear export of galectin-3, decreased the percentage of heterodikaryons showing human galectin-3 in both nuclei. In a parallel experiment, mouse 3T3 fibroblasts, which express galectin-3, were fused with fibroblasts derived from a mouse in which the galectin-3 gene was inactivated. Mouse galectin-3 localized to both nuclei of a large percentage of heterodikaryons. Again, addition of leptomycin B restricted the presence of galectin-3 to one nucleus of a heterodikaryon. The results from both heterodikaryon assays suggest that galectin-3 can exit one nucleus, travel through the cytoplasm, and enter the second nucleus, matching the definition of shuttling.