SARS-coronavirus-2 replication in Vero E6 cells: replication kinetics, rapid adaptation and cytopathology.

The sudden emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at the end of 2019 from the Chinese province of Hubei and its subsequent pandemic spread highlight the importance of understanding the full molecular details of coronavirus infection and pathogenesis. Here, we compared a variety of replication features of SARS-CoV-2 and SARS-CoV and analysed the cytopathology caused by the two closely related viruses in the commonly used Vero E6 cell line. Compared to SARS-CoV, SARS-CoV-2 generated higher levels of intracellular viral RNA, but strikingly about 50-fold less infectious viral progeny was recovered from the culture medium. Immunofluorescence microscopy of SARS-CoV-2-infected cells established extensive cross-reactivity of antisera previously raised against a variety of non-structural proteins, membrane and nucleocapsid protein of SARS-CoV. Electron microscopy revealed that the ultrastructural changes induced by the two SARS viruses are very similar and occur within comparable time frames after infection. Furthermore, we determined that the sensitivity of the two viruses to three established inhibitors of coronavirus replication (remdesivir, alisporivir and chloroquine) is very similar, but that SARS-CoV-2 infection was substantially more sensitive to pre-treatment of cells with pegylated interferon alpha. An important difference between the two viruses is the fact that - upon passaging in Vero E6 cells - SARS-CoV-2 apparently is under strong selection pressure to acquire adaptive mutations in its spike protein gene. These mutations change or delete a putative furin-like cleavage site in the region connecting the S1 and S2 domains and result in a very prominent phenotypic change in plaque assays.

Ultrastructural analysis of mitotic Drosophila S2 cells identifies distinctive microtubule and intracellular membrane behaviors.

BACKGROUND: S2 cells are one of the most widely used Drosophila melanogaster cell lines. A series of studies has shown that they are particularly suitable for RNAi-based screens aimed at the dissection of cellular pathways, including those controlling cell shape and motility, cell metabolism, and host-pathogen interactions. In addition, RNAi in S2 cells has been successfully used to identify many new mitotic genes that are conserved in the higher eukaryotes, and for the analysis of several aspects of the mitotic process. However, no detailed and complete description of S2 cell mitosis at the ultrastructural level has been done. Here, we provide a detailed characterization of all phases of S2 cell mitosis visualized by transmission electron microscopy (TEM). RESULTS: We analyzed by TEM a random sample of 144 cells undergoing mitosis, focusing on intracellular membrane and microtubule (MT) behaviors. This unbiased approach provided a comprehensive ultrastructural view of the dividing cells, and allowed us to discover that S2 cells exhibit a previously uncharacterized behavior of intracellular membranes, involving the formation of a quadruple nuclear membrane in early prometaphase and its disassembly during late prometaphase. After nuclear envelope disassembly, the mitotic apparatus becomes encased by a discontinuous network of endoplasmic reticulum membranes, which associate with mitochondria, presumably to prevent their diffusion into the spindle area. We also observed a peculiar metaphase spindle organization. We found that kinetochores with attached k-fibers are almost invariably associated with lateral MT bundles that can be either interpolar bundles or k-fibers connected to a different kinetochore. This spindle organization is likely to favor chromosome alignment at metaphase and subsequent segregation during anaphase. CONCLUSIONS: We discovered several previously unknown features of membrane and MT organization during S2 cell mitosis. The genetic determinants of these mitotic features can now be investigated, for instance by using an RNAi-based approach, which is particularly easy and efficient in S2 cells.

Methylation of Wnt antagonist genes: a useful prognostic marker for myelodysplastic syndrome.

Activation of the Wnt signaling pathway has been implicated in the pathogenesis of many tumors as well as in leukemia. However, its role in myelodysplastic syndrome (MDS) is unknown. In this study, we employed methylation-specific PCR to examine the methylation status of six Wnt antagonist genes in 144 MDS patients and in the MDS cell line SKM-1. We also used real-time PCR to examine the expression of Wnt antagonist genes and Wnt pathway genes in the SKM-1 cell line after treatment with 5-aza-2'-deoxycytidine. We found that methylation of the gene promoters of each of the six genes were observed in MDS patients at the following methylation frequencies: 41 % for sFRP1, 89.6 % for sFRP2, 43.1 % for sFRP4, 50.7 % for sFRP5, 44.4 % for DKK-1, and 69.4 % for DKK-3. In the SKM-1 cell line, the gene promoters sFRP1, sFRP2, sFRP5, DKK-1, and DKK-3 were methylated, while sFRP4 was not methylated. Treatment of the SKM-1 cell line with 5-aza-2'-deoxycytidine induced re-expression of methylated Wnt antagonists and inactivation of the Wnt pathway. Survival analysis showed that methylation status of sFRP1, sFRP4, and sFRP5 was associated with worse survival in MDS and sFRP5 methylation also predicted a high risk of leukemia evolution (P = 0.018). Our results indicate that epigenetic regulation of the Wnt pathway in MDS cell line, and the methylation status of Wnt antagonists predicts prognoses of MDS patients.

The G2385R variant of leucine-rich repeat kinase 2 associated with Parkinson's disease is a partial loss-of-function mutation.

Autosomal-dominant missense mutations in LRRK2 (leucine-rich repeat kinase 2) are a common genetic cause of PD (Parkinson's disease). LRRK2 is a multidomain protein with kinase and GTPase activities. Dominant mutations are found in the domains that have these two enzyme activities, including the common G2019S mutation that increases kinase activity 2-3-fold. However, there is also a genetic variant in some populations, G2385R, that lies in a C-terminal WD40 domain of LRRK2 and acts as a risk factor for PD. In the present study we show that the G2385R mutation causes a partial loss of the kinase function of LRRK2 and deletion of the C-terminus completely abolishes kinase activity. This effect is strong enough to overcome the kinase-activating effects of the G2019S mutation in the kinase domain. Hsp90 (heat-shock protein of 90 kDa) has an increased affinity for the G2385R variant compared with WT (wild-type) LRRK2, and inhibition of the chaperone binding combined with proteasome inhibition leads to association of mutant LRRK2 with high molecular mass native fractions that probably represent proteasome degradation pathways. The loss-of-function of G2385R correlates with several cellular phenotypes that have been proposed to be kinase-dependent. These results suggest that the C-terminus of LRRK2 plays an important role in maintaining enzymatic function of the protein and that G2385R may be associated with PD in a way that is different from kinase-activating mutations. These results may be important in understanding the differing mechanism(s) by which mutations in LRRK2 act and may also have implications for therapeutic strategies for PD.

Association of coagulation factor XIII-A with Golgi proteins within monocyte-macrophages: implications for subcellular trafficking and secretion.

Factor XIII-A (FXIII-A) is present in the cytosol of platelets, megakaryocytes, monocytes, osteoblasts, and macrophages and may be released from cells by a nonclassical pathway. We observed that plasma FXIII-A levels were unchanged in thrombocytopenic mice (Bcl-x(Plt20/Plt20) and Mpl(-/-)), which implicates nonclassical secretion from nucleated cells as the source of plasma FXIII-A. We, therefore, examined the intracellular targeting of FXIII-A in the THP-1 (monocyte/macrophage) cell line and in human monocyte-derived macrophages. Metabolic labeling of THP-1 cells did not show release of (35)S-FXIII-A either under basal conditions or when interleukin 1-beta was released in response to cell stress. However, immunofluorescence of THP-1 cells and primary macrophages showed that FXIII-A associated with podosomes and other structures adjacent to the plasma membrane, which also contain trans-Golgi network protein-46 and Golgi matrix protein-130 (GM130) but not the endoplasmic reticulum luminal protein, protein disulphide isomerase. Further, FXIII-A was present in GM130-positive intracellular vesicles that could mediate its transport, and in other contexts GM130 and its binding partner GRASP have been implicated in the delivery of nonclassically secreted proteins to the plasma membrane. Hence, this mechanism may precede FXIII-A release into the extracellular matrix from macrophages and its release into plasma from the cell type of origin.

Characterization and virus susceptibility of a skin cell line from red-spotted grouper (Epinephelus akaara).

A red-spotted grouper Epinephelus akaara skin (RGS) cell line was established and characterized. RGS cells had a normal diploid chromosome number of 2n = 48, the morphology of which was fibroblastic-like in 3 days and epithelial-like over 5 after 16 passages. The cells multiplied well in Dulbecco's modified Eagle's medium supplemented with 10% of fetal bovine serum at 25°C. Susceptibilities of RGS and grass carp ovary (GCO) cells to two viruses were tested, and the results showed that the titer of an iridovirus Rana grylio virus (RGV) in RGS cells was 10³·5 TCID50 ml⁻¹, which was much higher than a rhabdovirus spring viremia of carp virus (SVCV) in the cells (10°·5 TCID50 ml⁻¹). The titers of RGV and SVCV in GCO were 106·° TCID50 ml⁻¹ and 108·° TCID50 ml⁻¹, respectively, which were higher than those in RGS cells. The data may imply that RGS cells could be selectively resistible to some viruses during infection. RT-PCR analysis of RGV-infected RGS cells showed that RGV could replicate in RGS cells. Further study of virus replications in RGS cells was conducted by electron microscopy and immunofluorescence microscopy has shown that virus particles scattered in the cytoplasm and virus protein appeared in both the cytoplasm and nucleus. The results suggested that RGS cells could be used as a potential in vitro model to study the cutaneous barrier function against virus infection.

[Comparative characteristics of new human embryonic stem cell lines SC5, SC6, SC7, and SC3a].

Numerous human embryonic stem cell lines with different genetic background are widely used as cell models for fundamental, biomedical and pharmacological research. New hES cell lines SC5, SC6, SC7, and SC3a are derived from the blastocysts and maintained on mitotically inactivated human feeder cells. All derived hES cell lines passed through more than 120 cell population doublings, retained normal diploid karyotype and ability of in vitro differentiation in the derivates of three germ layers. These lines express the markers of undifferentiated hES cells: Oct-4, Nanog, SSEA-4, TRA-1-60, and alkaline phosphatase. Moreover, undifferentiated cells of SC5, SC6, and SC7 lines expressed germ line specific genes DPPA3/STELLA and DAZL and did not express somatic lineages specific genes. In contrast, undifferentiated cells of SC3a line did not express DPPA3/STELLA and DAZL but expressed extra embryonic endoderm cell markers GATA4 and AFP. Double staining of SC5 and SC3a colonies by antibodies against transcription factors Oct-4 and GATA4 has demonstrated that most SC3a cells in colonies were positive for both factors. Furthermore, the cells of SC5, SC6, SC7 lines but not of SC3a line formed teratomas containing the derivates of the three germ layers. These results indicate that, in contrast to the other cell lines, the cells in the SC3a colonies represent an early committed cell population. Moreover, expression of the multidrug resistance transporter gene ABCG2 was detected in undifferentiated cells and differentiating embryonic bodies during 10 days of all lines by immunofluorescent and RT-PCR analyses, whereas RT-PCR analysis has revealed up-regulation of the ABCB1 transporter gene expression in differentiating embryoid bodies of SC5, SC6, and SC7 cells only. Thus, these findings demonstrate different characteristics and differentiation potential of SC5, SC6, SC7, and SC3a hES cell lines which were derived in different conditions.

A novel heart-cell line from brown-marbled grouper Epinephelus fuscoguttatus and its susceptibility to iridovirus.

A novel cell line (bmGH) was established from the heart of brown-marbled grouper Epinephelus fuscoguttatus and its viral susceptibility was evaluated. The bmGH cells have been subcultured to passage 65 in Dulbecco's modified eagle medium:Ham's nutrient mixture F-12 (1:1) medium (DMEM/F12) which was further supplemented with foetal bovine serum (FBS), carboxymethyl-chitosan, basic fibroblast growth factor (bFGF) and insulin-like growth factor-I (IGF-I) at 24 degrees C. The heart cells have a fibroblastic morphology and proliferated to confluence 14 days later. The cells grew at a steady rate during subsequent subculture and had a population doubling time of 40.3 h at passage 60. Karyotype analysis showed that these cells exhibited chromosomal aneuploidy with a modal chromosome number of 48. The results of viral susceptibility characterization revealed that cytopathic effects (CPE) of bmGH cells appeared after infection by two iridoviruses, turbot reddish body iridovirus (TRBIV) and lymphocystis disease virus (LCDV). A large number of TRBIV and LCDV particles were also observed in the infected bmGH cells by electron microscope examination. All of these facts indicate that the bmGH cell line established here may serve as a valuable tool for studies of cell-virus interactions and has potential applications in fish virus isolation, propagation and vaccine development.

Measurement and visualization of cell membrane surface charge in fixed cultured cells related with cell morphology.

The diagnosis of patients with malignancies relies on the results of a clinical cytological examination. To enhance the diagnostic qualities of cytological examinations, it is important to have a detailed analysis of the cell's characteristics. There is, therefore, a need for developing a new auxiliary method for cytological diagnosis. In this study, we focused on studying the charge of the cell membrane surface of fixed cells, which is one of important cell's characteristics. Although fixed cells lose membrane potential which is observed in living cells owing to ion dynamics, we hypothesized that fixed cells still have a cell membrane surface charge due to cell membrane components and structure. We used 5 cell lines in this study (ARO, C32TG, RT4, TK, UM-UC-14). After fixation with CytoRich Red, we measured the cell membrane surface charge of fixed cells in solution using zeta potential measurements and fixed cells on glass slides, visualizing it using antibody-labeled beads and positively-charged beads. Furthermore, we measured the cell membrane surface charge of fixed cells under different conditions, such as different solution of fixative, ion concentration, pH, and pepsin treatments. The zeta potential measurements and visualization using the beads indicated that the cell membrane surface of fixed cells was negatively charged, and also that the charge varied among fixed cells. The charge state was affected by the different treatments. Moreover, the number of cell-bound beads was small in interphase, anaphase, and apoptotic cells. We concluded that the negative cell membrane surface charge was influenced by the three-dimensional structure of proteins as well as the different types of amino acids and lipids on the cell membrane. Thus, cell surface charge visualization can be applied as a new auxiliary method for clinical cytological diagnosis. This is the first systematic report of the cell membrane surface charge of fixed cells.

Baseline mechanical characterization of J774 macrophages.

Macrophage cell lines like J774 cells are ideal model systems for establishing the biophysical foundations of autonomous deformation and motility of immune cells. To aid comparative studies on these and other types of motile cells, we report measurements of the cortical tension and cytoplasmic viscosity of J774 macrophages using micropipette aspiration. Passive J774 cells cultured in suspension exhibited a cortical resting tension of approximately 0.14 mN/m and a viscosity (at room temperature) of 0.93 kPa.s. Both values are about one order of magnitude higher than the respective values obtained for human neutrophils, lending support to the hypothesis that a tight balance between cortical tension and cytoplasmic viscosity is a physical prerequisite for eukaryotic cell motility. The relatively large stiffness of passive J774 cells contrasts with their capacity for a more than fivefold increase in apparent surface area during active deformation in phagocytosis. Scanning electron micrographs show how microscopic membrane wrinkles are smoothed out and recruited into the apparent surface area during phagocytosis of large targets.

Amyloid precursor protein promotes endoplasmic reticulum stress-induced cell death via C/EBP homologous protein-mediated pathway.

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) is known to activate the ER, which is termed ER stress. Here, we demonstrated that amyloid precursor protein (APP) is a novel mediator of ER stress-induced apoptosis through the C/EBP homologous protein (CHOP) pathway. Expression of APP mRNA was elevated by tunicamycin- or dithiothreitol-induced ER stress. The levels of C83 and APP intracellular domain (AICD) fragments, which are cleaved from APP, were significantly increased under ER stress, although the protein level of full-length APP was decreased. Cellular viability was reduced in APP-over-expressing cells, which was attenuated by treatment with a gamma-secretase inhibitor, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT). Cellular viability was also reduced in AICD-FLAG-over-expressing cells. The mRNA and protein levels of CHOP, an ER stress-responsive gene, were remarkably increased by APP over-expression, which was attenuated by treatment with DAPT. CHOP mRNA induction was also found in AICD-FLAG-over-expressing cells. Cell death and CHOP up-regulation by ER stress were attenuated by APP knockdown. Data obtained with a luciferase assay and chromatin immunoprecipitation assay indicated that AICD associates with the promoter region of the CHOP gene. In conclusion, ER stress-induced APP undergoes alpha- and gamma-secretase cleavage and subsequently induces CHOP-mediated cell death.

Inhibitory effects of armepavine against hepatic fibrosis in rats.

Activation of hepatic stellate cells (HSCs) plays a crucial role in liver fibrogenesis. armepavine (Arm, C19H23O3N), an active compound from Nelumbo nucifera, has been shown to exert immunosuppressive effects on T lymphocytes and on lupus nephritic mice. The aim of this study was to investigate whether Arm could exert anti-hepatic fibrogenic effects in vitro and in vivo. A cell line of rat HSCs (HSC-T6) was stimulated with tumor necrosis factor-alpha (TNF-alpha) or lipopolysaccharide (LPS) to evaluate the inhibitory effects of Arm. An in vivo therapeutic study was conducted in bile duct-ligated (BDL) rats. BDL rats were given Arm (3 or 10 mg/kg) by gavage twice daily for 3 weeks starting from the onset of BDL. Liver sections were taken for fibrosis scoring, immuno-fluorescence staining and quantitative real-time mRNA measurements. In vitro, Arm (1-10 microM) concentration-dependently attenuated TNF-alpha- and LPS-stimulated alpha-SMA protein expression and AP-1 activation by HSC-T6 cells without adverse cytotoxicity. Arm also suppressed TNF-alpha-induced collagen collagen deposition, NFkappaB activation and MAPK (p38, ERK1/2, and JNK) phosphorylations. In vivo, Arm treatment significantly reduced plasma AST and ALT levels, hepatic alpha-SMA expression and collagen contents, and fibrosis scores of BDL rats as compared with vehicle treatment. Moreover, Arm attenuated the mRNA expression levels of col 1alpha2, TGF-beta1, TIMP-1, ICAM-1, iNOS, and IL-6 genes, but up-regulated metallothionein genes. Our study results showed that Arm exerted both in vitro and in vivo antifibrotic effects in rats, possibly through anti-NF-kappaB activation pathways.

Mechanism of centrosome positioning during the wound response in BSC-1 cells.

Locomoting cells are characterized by a pronounced external and internal anterior-posterior polarity. One of the events associated with cell polarization at the onset of locomotion is a shift of the centrosome, or MTOC, ahead of the nucleus. This position is believed to be of strategic importance for directional cell movement and cell polarity. We have used BSC-1 cells at the edge of an in vitro wound to clarify the causal relationship between MTOC position and the initiation of cell polarization. We find that pronounced cell polarization (the extension of a lamellipod) can take place in the absence of MTOC repositioning or microtubules. Conversely, MTOCs will reposition even after lamellar extension and cell polarization have occurred. Repositioning requires microtubules that extend to the cell periphery and is independent of selective detyrosination of microtubules extending towards the cell front. Significantly, MTOCs maintain, or at least attempt to maintain, a position at the cell's centroid. This is most clearly demonstrated in wounded monolayers of enucleated cells where the MTOC closely follows the centroid position. We suggest that the primary response to the would is the biased extension of a lamellipod, which can occur in the absence of microtubules and MTOC repositioning. Lamellipod extension leads to a shift of the cell's centroid towards the wound. The MTOC, in an attempt to maintain a position near the cell center, will follow. This will automatically put the MTOC ahead of the nucleus in the vast majority of cells. The nucleus as a reference for MTOC position may not be as meaningful as previously thought.

Dynamics of the neuronal intermediate filaments.

We have analyzed the dynamics of neuronal intermediate filaments in living neurons by using the method of photobleaching of fluorescently-labeled neurofilament L protein and immunoelectron microscopy of incorporation sites of biotinylated neurofilament L protein. Low-light-level imaging and photobleaching of growing axons of mouse sensory neurons did not affect the rate of either axonal growth or the addition of intermediate filament structures at the axon terminal, suggesting that any perturbations caused by these optical methods would be minimal. After laser photobleaching, recovery of fluorescence did occur slowly with a recovery half-time of 40 min. Furthermore, we observed a more rapid fluorescence recovery in growing axons than in quiescent ones, indicating a growth-dependent regulation of the turnover rate. Incorporation sites of biotin-labeled neurofilament L protein were localized as numerous discrete sites along the axon, and they slowly elongated to become continuous arrays 24 h after injection. Collectively, these results indicate that neuronal intermediate filaments in growing axons turn over within the small area of the axoplasm possibly by the mechanism of lateral and segmental incorporation of new subunits.

The desmoplakin carboxyl terminus coaligns with and specifically disrupts intermediate filament networks when expressed in cultured cells.

Specific interactions between desmoplakins I and II (DP I and II) and other desmosomal or cytoskeletal molecules have been difficult to determine in part because of the complexity and insolubility of the desmosome and its constituents. We have used a molecular genetic approach to investigate the role that DP I and II may play in the association of the desmosomal plaque with cytoplasmic intermediate filaments (IF). A series of mammalian expression vectors encoding specific predicted domains of DP I were transiently expressed in cultured cells that form (COS-7) and do not form (NIH-3T3) desmosomes. Sequence encoding a small antigenic peptide was added to the 3' end of each mutant DP cDNA to facilitate immunolocalization of mutant DP protein. Light and electron microscopical observations revealed that DP polypeptides including the 90-kD carboxy-terminal globular domain of DP I specifically colocalized with and ultimately resulted in the complete disruption of IF in both cell lines. This effect was specific for IF as microtubule and microfilament networks were unaltered. This effect was also specific for the carboxyl terminus of DP, as the expression of the 95-kD rod domain of DP I did not visibly alter IF networks. Immunogold localization of COS-7 cells transfected with constructs including the carboxyl terminus of DP demonstrated an accumulation of mutant protein in perinuclear aggregates within which IF subunits were sequestered. These results suggest a role for the DP carboxyl terminus in the attachment of IF to the desmosome in either a direct or indirect manner.

alpha-2 Macroglobulin receptor/Ldl receptor-related protein(Lrp)-dependent internalization of the urokinase receptor.

The GPI-anchored urokinase plasminogen activator receptor (uPAR) does not internalize free urokinase (uPA). On the contrary, uPAR-bound complexes of uPA with its serpin inhibitors PAI-1 (plasminogen activator inhibitor type-1) or PN-1 (protease nexin-1) are readily internalized in several cell types. Here we address the question whether uPAR is internalized as well upon binding of uPA-serpin complexes. Both LB6 clone 19 cells, a mouse cell line transfected with the human uPAR cDNA, and the human U937 monocytic cell line, express in addition to uPAR also the endocytic alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP/alpha 2-MR) which is required to internalize uPAR-bound uPA-PAI-1 and uPA-PN-1 complexes. Downregulation of cell surface uPAR molecules in U937 cells was detected by cytofluorimetric analysis after uPA-PAI-1 and uPA-PN-1 incubation for 30 min at 37 degrees C; this effect was blocked by preincubation with the ligand of LRP/alpha 2-MR, RAP (LRP/alpha 2-MR-associated protein), known to block the binding of the uPA complexes to LRP/alpha 2-. MR. Downregulation correlated in time with the intracellular appearance of uPAR as assessed by confocal microscopy and immuno-electron microscopy. After 30 min incubation with uPA-PAI-1 or uPA-PN-1 (but not with free uPA), confocal microscopy showed that uPAR staining in permeabilized LB6 clone 19 cells moved from a mostly surface associated to a largely perinuclear position. This effect was inhibited by the LRP/alpha 2-MR RAP. Perinuclear uPAR did not represent newly synthesized nor a preexisting intracellular pool of uPAR, since this fluorescence pattern was not modified by treatment with the protein synthesis inhibitor cycloheximide, and since in LB6 clone 19 cells all of uPAR was expressed on the cell surface. Immuno-electron microscopy confirmed the plasma membrane to intracellular translocation of uPAR, and its dependence on LRP/alpha 2-MR in LB6 clone 19 cells only after binding to the uPA-PAI-1 complex. After 30 min incubation at 37 degrees C with uPA-PAI-1, 93% of the specific immunogold particles were present in cytoplasmic vacuoles vs 17.6% in the case of DFP-uPA. We conclude therefore that in the process of uPA-serpin internalization, uPAR itself is internalized, and that internalization requires the LRP/alpha 2-MR.

Multiple PEX genes are required for proper subcellular distribution and stability of Pex5p, the PTS1 receptor: evidence that PTS1 protein import is mediated by a cycling receptor.

PEX5 encodes the type-1 peroxisomal targeting signal (PTS1) receptor, one of at least 15 peroxins required for peroxisome biogenesis. Pex5p has a bimodal distribution within the cell, mostly cytosolic with a small amount bound to peroxisomes. This distribution indicates that Pex5p may function as a cycling receptor, a mode of action likely to require interaction with additional peroxins. Loss of peroxins required for protein translocation into the peroxisome (PEX2 or PEX12) resulted in accumulation of Pex5p at docking sites on the peroxisome surface. Pex5p also accumulated on peroxisomes in normal cells under conditions which inhibit protein translocation into peroxisomes (low temperature or ATP depletion), returned to the cytoplasm when translocation was restored, and reaccumulated on peroxisomes when translocation was again inhibited. Translocation inhibiting conditions did not result in Pex5p redistribution in cells that lack detectable peroxisomes. Thus, it appears that Pex5p can cycle repeatedly between the cytoplasm and peroxisome. Altered activity of the peroxin defective in CG7 cells leads to accumulation of Pex5p within the peroxisome, indicating that Pex5p may actually enter the peroxisome lumen at one point in its cycle. In addition, we found that the PTS1 receptor was extremely unstable in the peroxin-deficient CG1, CG4, and CG8 cells. Altered distribution or stability of the PTS1 receptor in all cells with a defect in PTS1 protein import implies that the genes mutated in these cell lines encode proteins with a direct role in peroxisomal protein import.

Hormonal regulation of caveolae internalization.

Caveolae undergo a cyclic transition from a flat segment of membrane to a vesicle that then returns to the cell surface. Here we present evidence that this cycle depends on a population of protein kinase C-alpha (PKC-alpha) molecules that reside in the caveolae membrane where they phosphorylate a 90-kD protein. This cycle can be interrupted by treatment of the cells with phorbol-12,13-dibutyrate or agents that raise the concentration of diacylglycerol in the cell. Each of these conditions displaces PKC-alpha from caveolae, inhibits the phosphorylation of the 90-kD protein, and prevents internalization. Caveolae also contain a protein phosphatase that dephosphorylates the 90-kD once PKC-alpha is gone. A similar dissociation of PKC-alpha from caveolae and inhibition of invagination was observed when cells were treated with histamine. This effect was blocked by pyrilamine but not cimetidine, indicating the involvement of histamine H1 receptors. These findings suggest that the caveolae internalization cycle is hormonally regulated.

Myosin II filament assemblies in the active lamella of fibroblasts: their morphogenesis and role in the formation of actin filament bundles.

The morphogenesis of myosin II structures in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetramethylrhodamine-myosin II, nascent myosin spots formed close to the active edge during periods of retraction and then elongated into wavy ribbons of uniform width. The spots and ribbons initially behaved as distinct structural entities but subsequently aligned with each other in a sarcomeric-like pattern. Electron microscopy established that the spots and ribbons consisted of bipolar minifilaments associated with each other at their head-containing ends and arranged in a single row in an "open" zig-zag conformation or as a "closed" parallel stack. Ribbons also contacted each other in a nonsarcomeric, network-like arrangement as described previously (Verkhovsky and Borisy, 1993. J. Cell Biol. 123:637-652). Myosin ribbons were particularly pronounced in REF 52 cells, but small ribbons and networks were found also in a range of other mammalian cells. At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filaments. Further from the edge, myosin filament alignment increased in parallel with the development of actin bundles. In actin bundles, the actin cross-linking protein, alpha-actinin, was excluded from sites of myosin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may initiate a pathway for the formation of actin filament bundles. We propose that zig-zag assemblies of myosin II filaments induce the formation of actin bundles by pulling on an actin filament network and that co-alignment of actin and myosin filaments proceeds via folding of myosin II filament assemblies in an accordion-like fashion.

In vivo analysis of functional domains from villin and gelsolin.

Transfected CV1 cells were used to compare the in vivo effects of various domains of villin and gelsolin. These two homologous actin modulating proteins both contain a duplicated severin-like sequence. Villin has in addition a carboxy-terminal domain, the headpiece, which accounts for its bundling activity. The effects of the villin-deleted mutants were compared with those of native villin. Our results show that essential domains of villin required to induce the growth of microvilli and F-actin redistribution are present in the first half of the core and in the headpiece. We also show that the second half of the villin core cannot be exchanged by its homolog in gelsolin. When expressed at high levels of CV1 cells, full length gelsolin completely disrupted stress fibers without change of the cell shape. Addition of the villin headpiece to gelsolin had no effect on the phenotype induced by gelsolin alone. Expression of the first half of gelsolin induced similar modifications as capping proteins and rapid cell mortality; this deleterious effect on the cell structure was also observed when the headpiece was linked to the first half of gelsolin. In cells expressing the second half of gelsolin, a dotted F-actin staining was often seen. Moreover elongated dorsal F-actin structures were observed when the headpiece was linked to the second gelsolin domain. These studies illustrate the patent in vivo severing activity of gelsolin as well as the distinct functional properties of villin core in contrast to gelsolin.