Expression of a wild-type CFTR maintains the integrity of the biosynthetic/secretory pathway in human cystic fibrosis pancreatic duct cells.

The structural integrity of the Golgi complex is essential to its functions in the maturation, sorting, and transport of plasma membrane proteins. Previously, we demonstrated that in pancreatic duct CFPAC-1 cells, which express DeltaF508 CFTR (cystic fibrosis transmembrane conductance regulator), the intracellular trafficking of carbonic anhydrase IV (CA IV), a membrane protein involved in HCO(3)(-) secretion, was impaired. To determine whether these abnormalities were related to changes in the Golgi complex, we examined the ultrastructure and distribution of Golgi compartments with regard to the microtubule cytoskeleton in CFPAC-1 cells transfected or not with the wild-type CFTR. Ultrastructural and immunocytochemical analysis showed that in polarized CFPAC-1 cells, Golgi stacks were disconnected from one another and scattered throughout the cytoplasm. The colocalization of CA IV with markers of Golgi compartments indicated the ability of stacks to transfer this enzyme. This Golgi dispersal was associated with abnormal microtubule distribution and multiplicity of the microtubule-organizing centers (MTOCs). In reverted cells, the normalization of Golgi structure, microtubule distribution, and MTOC number was observed. These observations suggest that the entire biosynthetic/secretory pathway is disrupted in CFPAC-1 cells, which might explain the abnormal intracellular transport of CA IV. Taken together, these results point to the fact that the expression of DeltaF508 CFTR affects the integrity of the secretory pathway.     

Intrastriatal injection of colchicine induces striatonigral degeneration in mice

Recent studies from environmental toxicology and molecular genetics demonstrate that midbrain dopamine (DA) neurons are particularly vulnerable to microtubule depolymerizing agents, indicating the involvement of microtubule dysfunction in the pathogenesis of Parkinson's disease. Here we show that intrastriatal injection of colchicine (COL), a well-known microtubule disruptor, induced degeneration of striatonigral pathway. Microtubule disruption caused by unilateral injection of COL blocked the retrograde axonal transport of fluorogold previously injected into striatum and induced substantial death of striatal and DA neurons in substantia nigra pars compacta. Furthermore, COL-induced pathologic changes were associated with robust glial reaction, which may be conducive to the degeneration of striatonigral pathway. We also found that intrastriatal injection of COL resulted in side bias in spontaneous turning activities and apomorphine-induced rotational behavior. Together, our results provide in vivo data lending support to the concept that microtubule dysfunction may play a significant role in the death of DA neurons, though glial reaction may be involved and contribute to the degenerative process. Moreover, intrastriatal COL may serve as another experimental model of striatonigral degeneration (Parkinson's variant of multiple system atrophy), given the concurrent loss of both striatal and DA neurons.     

Dynein and dynactin as organizers of the system of cell microtubules

A review of the role of the microtubule motor dynein and its cofactor dynactin in the formation of a radial system of microtubules in the interphase cells and of mitotic spindle. Deciphering of the structure, functions, and regulation of activity of dynein and dynactin promoted the understanding of mechanisms of cell and tissue morphogenesis, since it turned out that these cells help the cell in finding its center and organize microtubule-determined anisotropy of intracellular space. The structure of dynein and dynactin molecules has been considered, as well as possible pathways of regulation of the dynein activity and the role of dynein in transport of cell components along the microtubules. Attention has also been paid to the functions of dynein and dynactin not related directly to transport: their involvement in the formation of an interphase radial system of microtubules. This system can be formed by self-organization of microtubules and dynein-containing organelles or via organization of microtubules by the centrosome, whose functioning requires dynein. In addition, dynein and dynactin are responsible for cell polarization during its movement, as well as for the position of nucleus, centrosomes, and mitotic spindle in the cell.     

Ezrin tunes T‐cell activation by controlling Dlg1 and microtubule positioning at the immunological synapse

T‐cell receptor (TCR) signalling is triggered and tuned at immunological synapses by the generation of signalling complexes that associate into dynamic microclusters. Microcluster movement is necessary to tune TCR signalling, but the molecular mechanism involved remains poorly known. We show here that the membrane‐microfilament linker ezrin has an important function in microcluster dynamics and in TCR signalling through its ability to set the microtubule network organization at the immunological synapse. Importantly, ezrin and microtubules are important to down‐regulate signalling events leading to Erk1/2 activation. In addition, ezrin is required for appropriate NF‐AT activation through p38 MAP kinase. Our data strongly support the notion that ezrin regulates immune synapse architecture and T‐cell activation through its interaction with the scaffold protein Dlg1. These results uncover a crucial function for ezrin, Dlg1 and microtubules in the organization of the immune synapse and TCR signal down‐regulation. Moreover, they underscore the importance of ezrin and Dlg1 in the regulation of NF‐AT activation through p38.     

The effect of low temperature on the microtubules in root meristem cells of spring and winter cultivars of wheat <Emphasis Type="Italic">Triticum aestivum L.</Emphasis>

We have studied the response of interphase and mitotic microtubule arrays in root meristem cells of spring and winter cultivars of wheat Triticum aestivum L. (Moskovskaya 35 and Moskovskaya 39) to cold stress (1 h at 0°C) and acclimation to cold (3–48 h at 0°C). We show that, in general, interphase microtubules are more resistant to cold then mitotic arrays in both cultivars. During cold stress, no changes are detected in the microtubule system of interphase cells of spring wheat, whereas the density of endoplasmic microtubules increases in interphase cells of winter wheat. During mitosis, the density of the kinetochore fibers of the spindle decreases in the cells of both cultivars, but it is prevailing in the cells of spring cultivar of wheat. During acclimation to cold, the disorganization of the cortical microtubule bundles and the enhanced growth of the endoplasmic microtubule network, which is comprised of microtubule converging centers, are observed in cells of both cultivars. However, the mitotic microtubule systems of winter and spring cultivars respond differently to cold acclimation. During prophase, a diffuse tubulin “halo,”followed by the assembly of microtubule converging centers, accumulate at the perinuclear area in the cells of winter wheat. In cells of spring cultivar, the prophase spindle is only detected during initial stages of cold acclimation. During metaphase, aberrant mitotic spindles, abnormal metaphase plates, and the excessive appearance of microtubule converging centers are observed in cells of both cultivars. Acclimation induces the disorganization of the phragmoplast and the formation of multiple microtubule converging centers during telophase in the cells of both cultivars. Microtubule converging centers are detected at the perinuclear area of daughter cells in winter wheat and in the cortical cytoplasm in spring wheat. The excessive formation of microtubule converging centers suggests the activation of microtubule assembly during prolonged exposure to low temperature. Our data also demonstrates common pathways of microtubule response to cold treatment (0°C).     

Structural insight into the inhibition of tubulin by vinca domain peptide ligands

The tubulin vinca domain is the target of widely different microtubule inhibitors that interfere with the binding of vinblastine. Although all these ligands inhibit the hydrolysis of GTP, they affect nucleotide exchange to variable extents. The structures of two vinca domain antimitotic peptides--phomopsin A and soblidotin (a dolastatin 10 analogue)--bound to tubulin in a complex with a stathmin-like domain show that their sites partly overlap with that of vinblastine and extend the definition of the vinca domain. The structural data, together with the biochemical results from the ligands we studied, highlight two main contributors in nucleotide exchange: the flexibility of the tubulin subunits' arrangement at their interfaces and the residues in the carboxy-terminal part of the beta-tubulin H6-H7 loop. The structures also highlight common features of the mechanisms by which vinca domain ligands favour curved tubulin assemblies and destabilize microtubules.     

Situs: A package for docking crystal structures into low-resolution maps from electron microscopy

Three-dimensional image reconstructions of large-scale protein aggregates are routinely determined by electron microscopy (EM). We combine low-resolution EM data with high-resolution structures of proteins determined by x-ray crystallography. A set of visualization and analysis procedures, termed the Situs package, has been developed to provide an efficient and robust method for the localization of protein subunits in low-resolution data. Topology-representing neural networks are employed to vector-quantize and to correlate features within the structural data sets. Microtubules decorated with kinesin-related ncd motors are used as model aggregates to demonstrate the utility of this package of routines. The precision of the docking has allowed for the extraction of unique conformations of the macromolecules and is limited only by the reliability of the underlying structural data.     

Cytoplasmic organization of POXvirus DNA replication

Poxviruses, a family of large DNA viruses, are unique among DNA viruses, because they carry out DNA replication in the cytoplasm rather than the nucleus. This process does not occur randomly, but instead, these viruses create cytoplasmic 'mini-nuclei', distinct sites that are surrounded by membranes derived from the rough endoplasmic reticulum (ER) that support viral replication. This review summarizes how distinct steps preceding cytoplasmic DNA replication, as well as replication itself, operate in the host cell. The collective data point to an important role for both the rough ER and the microtubules and indicate that these cellular structures help to co-ordinate the virus life cycle to ensure that individual steps occur at the right time and place. In a broader sense, they emphasize how viruses have evolved sophisticated ways to use host cells to optimize their life cycles to ensure efficient production of infectious progeny.     

A microtubule associated protein (hNUDC) binds to the extracellular domain of thrombopoietin receptor (Mpl)

Human NUDC (hNUDC) was initially characterized as a nuclear migration protein based on the similarity of its C-terminus to that of fungal NUDC from Aspergillus nidulans. However, hNUDC is a 331 amino acid protein whereas fungal NUDC is 198 amino acids in length. The extra N-terminal portion of hNUDC has no known function or homology to other proteins. In this study, we report the binding of hNUDC to the extracellular domain of the thrombopoietin receptor (Mpl) as detected by the yeast two-hybrid system, GST pull-down, and co-immunoprecipitation. Our deletion analysis demonstrated that amino acids between positions 100 and 238 as the critical domain mediating the hNUDC and Mpl interactions as detected by the two-hybrid system and GST pull-down assay. Immunofluorescence staining of human megakaryocyte cells indicated that hNUDC and Mpl colocalized at all stages of megakaryocyte development. Substantial colocalization of hNUDC with microtubules was also detected around nuclei and elongated microtubular structures, especially in proplatelet extensions.