The actin-binding protein comitin (p24) is a component of the Golgi apparatus.

Comitin (p24) was first identified in Dictyostelium discoideum as a membrane-associated protein which binds in gel overlay assays to G and F actin. To analyze its actin-binding properties we used purified, bacterially expressed comitin and found that it binds to F actin in spin down experiments and increases the viscosity of F actin solutions even under high-salt conditions. Immunofluorescence studies, cell fractionation experiments and EM studies of vesicles precipitated with comitin-specific monoclonal antibodies showed that comitin was present in D. discoideum on: (a) a perinuclear structure with tubular or fibrillary extensions; and (b) on vesicles distributed throughout the cell. In immunofluorescence experiments using comitin antibodies NIH 3T3 fibroblasts showed a similar staining pattern as D. discoideum cells. Using bona fide Golgi markers the perinuclear structure was identified as the Golgi apparatus. The results were supported by an electron microscopic study using cryosections. Based on these data we propose that also in Dictyostelium the stained perinuclear structure is the Golgi apparatus. In vivo the perinuclear structure was found to be attached to the actin and the microtubule network. Alteration of the actin network or depolymerization of the microtubules led to its dispersal into vesicles distributed throughout the cell. These results suggest that the Golgi apparatus in D. discoideum is connected to the actin network by comitin. This protein seems also to be present in mammalian cells.

Effects of silibinin and of a synthetic analogue on isolated rat hepatic stellate cells and myofibroblasts.

Hepatic stellate cells and the derived myofibroblasts play a central pathogenic role in liver fibrogenesis. In order to identify the still unknown hepatoprotective properties of the flavonoid silibinin and the related pyridylchromone NH40 x HCl (2-(3-pyridyl)-4-H-1-benzopyran-4-one hydrochloride), their effects on isolated rat hepatic stellate cells and derived myofibroblasts were determined. Concentrations of 10(-4) mol/l silibinin reduced the proliferation of freshly isolated rat hepatic stellate cells by about 75%, but had no detectable effect on their viability, morphology and their cytoskeletal architecture. It reduced the transformation towards myofibroblasts and down-regulated the gene expression of extracellular matrix components and the profibrogenic transforming growth beta. Whereas silibinin concentrations higher than 10(-4) mol/l were toxic, lower concentrations had no effects on the proliferation and transformation behavior. Although 10(-4) mol/l NH40 x HCl reduced the proliferation rate by about 50%, this substance had no significant effect on the transformation process. The results indicate that one important aspect of the potential antifibrotic properties of silibinin might be the inhibition of hepatic stellate cell proliferation and transformation.

Gene expression of syndecans and betaglycan in isolated rat liver cells.

Membrane-bound heparan sulfate proteoglycans act as coreceptors for cytokines and are involved in cell-matrix or cell-cell adhesion. We have determined the gene expression of all four members of the syndecan-like integral membrane proteoglycans and of betaglycan, the transforming growth factor-beta type III receptor, in various types of isolated hepatic cells of the rat. Fat-storing cells express syndecan-1, -2, -3, -4, and betaglycan. During the transformation of fat-storing cells into myofibroblasts (the key process in the development of liver cirrhosis), the levels of mRNA for syndecan-1, -3, and -4 remain constant, whereas the amount of syndecan-2 mRNA increases and that for betaglycan decreases. Liver macrophages express syndecan-3 and -4, but only small amounts of syndecan-1. Freshly isolated hepatocytes express only syndecan-1, -2, and -4, but fail to express betaglycan. During cultivation, hepatocytes start to express betaglycan. Syndecan-3, -4, and betaglycan are transcribed into one mRNA population, whereas syndecan-1 and -2 are expressed in different-sized mRNA populations. The data show that the genes of all tested membrane heparan sulfate proteoglycans are expressed by hepatic cells, but that each cell type is characterized by its specific heparan sulfate proteoglycan mRNA profile.

Identification of a widespread nuclear actin binding protein.

The many different cellular functions so far shown to involve actin and to be regulated by specific actin binding proteins are located primarily, if not exclusively, in the cytoplasm. Actin is also found in the nucleus of various cells, but because of the problems of cell fractionation the significance of nuclear actin has remained unclear. The large amphibian oocyte nucleus (germinal vesicle), however, can be isolated manually with little cytoplasmic contamination. This nucleus contains high concentrations (4-6 mg ml-1) of mostly soluble, although polymerization-competent beta- and gamma-actin, which exists in a nucleocytoplasmic exchange pool. The findings that drastic effects on transcription and chromosome morphology are caused by the injection of actin antibodies or actin binding proteins into germinal vesicles, and that a factor required for accurate transcription by RNA polymerase II is actin, suggest that nuclear actin is involved in specific nuclear functions. We have recently identified two main components in Xenopus laevis oocytes with actin binding activities; one of these activities is Ca2+-dependent, is located predominantly, if not exclusively, in the cytoplasm and is attributable to gelsolin. Here we report that the second component, having a Ca2+-independent activity, is a heterodimeric acting binding protein; this protein is markedly enriched in the nuclei of oocytes and somatic cells of amphibia, but also occurs in nuclei of other vertebrate cells.

Rapid and reproducible quantification of hepatitis C virus cDNA by fluorescence correlation spectroscopy.

BACKGROUND/AIMS: Standard methods for hepatitis C virus (HCV) RNA quantification are time-consuming and often hampered by low sensitivity. Therefore, we aimed to test whether fluorescence correlation spectroscopy (FCS) could be used to read out HCV polymerase chain reactions (PCR). METHODS: A single-step reverse transcriptase (RT) PCR system was adjusted to the clinically relevant range of 1 x 10(3) to 5 x 10(6) HCV cDNA copies/ml serum. Unpurified amplification mixtures were analyzed by FCS and controlled by HPLC analysis. RESULTS: The outcome of HCV RNA quantitation was nearly identical no matter whether FCS or HPLC techniques were used. FCS-generated standard curves displayed sufficient linearity to allow reproducible determinations. The intraserial variation of cDNA quantification after PCR amplification was +/-3.2%, the interserial variation +/-4.3%. Repeated quantifications of HCV genotype 1b RNA from the sera of 8 patients revealed titers from 1 x 10(4)-5 x 10(6) genome equivalents/ml. The results correlated significantly (r = 0.755; p = 0.03) with a widely used commercially available assay. CONCLUSION: FCS may become a useful tool for rapid and reproducible HCV RNA quantification in the future.