Regulation of actin filament cross-linking and bundle shape in Drosophila bristles

Previous studies demonstrate that in developing Drosophila bristles, two cross-linking proteins are required sequentially to bundle the actin filaments that support elongating bristle cells. The forked protein initiates the process and facilitates subsequent cross-linking by fascin. Using cross-linker-specific antibodies, mutants, and drugs we show that fascin and actin are present in excessive amounts throughout bundle elongation. In contrast, the forked cross-linker is limited throughout bundle formation, and accordingly, regulates bundle size and shape. We also show that regulation of cross-linking by phosphorylation can affect bundle size. Specifically, inhibition of phosphorylation by staurosporine results in a failure to form large bundles if added during bundle formation, and leads to a loss of cross-linking by fascin if added after the bundles form. Interestingly, inhibition of dephosphorylation by okadaic acid results in the separation of the actin bundles from the plasma membrane. We further show by thin section electron microscopy analysis of mutant and wild-type bristles that the amount of material that connects the actin bundles to the plasma membrane is also limited throughout bristle elongation. Therefore, overall bundle shape is determined by the number of actin filaments assembled onto the limited area provided by the connector material. We conclude that assembly of actin bundles in Drosophila bristles is controlled in part by the controlled availability of a single cross-linking protein, forked, and in part by controlled phosphorylation of cross-links and membrane actin connector proteins.     

Actin filament turnover regulated by cross-linking accounts for the size, shape, location, and number of actin bundles in Drosophila bristles

Drosophila bristle cells are shaped during growth by longitudinal bundles of cross-linked actin filaments attached to the plasma membrane. We used confocal and electron microscopy to examine actin bundle structure and found that during bristle elongation, snarls of uncross-linked actin filaments and small internal bundles also form in the shaft cytoplasm only to disappear within 4 min. Thus, formation and later removal of actin filaments are prominent features of growing bristles. These transient snarls and internal bundles can be stabilized by culturing elongating bristles with jasplakinolide, a membrane-permeant inhibitor of actin filament depolymerization, resulting in enormous numbers of internal bundles and uncross-linked filaments. Examination of bundle disassembly in mutant bristles shows that plasma membrane association and cross-bridging adjacent actin filaments together inhibits depolymerization. Thus, highly cross-bridged and membrane-bound actin filaments turn over slowly and persist, whereas poorly cross-linked filaments turnover more rapidly. We argue that the selection of stable bundles relative to poorly cross-bridged filaments can account for the size, shape, number, and location of the longitudinal actin bundles in bristles. As a result, filament turnover plays an important role in regulating cytoskeleton assembly and consequently cell shape.     

Adaptation of a nematode parasite to living within the mammalian epithelium

Trichuris muris is a large metazoan pathogen that has been proposed to live intracellularly within living host intestinal epithelial cells. We sought to determine how Trichuris bores its way through the mucosal epithelium and to elucidate the parasite strategies for taking advantage of this intracellular niche. Since the apical surface of the mucosal epithelium is stabilized by the actin cytoskeleton and cell junctions, it remains intact over the worm following its entry into cells. In contrast, non-stabilized lateral membranes of the host epithelial cells are ruptured and cells are killed to form an inert syncytial tunnel. The ventral surface of the nematode worm is studded by pores that overlie bacillary cells; these pores penetrate through the cuticle and are in direct contact with host cytoplasm. From scanning electron micrographs of isolated worms, we calculate that each adult contains approximately 50,000 bacillary cells. The apical surface of the bacillary cells is extensively folded into plicae 40 nm in diameter, thereby increasing the surface area many-fold. Bacillary cells lack organelles for enzyme synthesis and secretion and fail to export protons. However, by confocal light microscopy it was observed that fluorescent macromolecules in excess of 100,000 Da can penetrate into the pores. Taken together, we conclude that the bacillary cells are essential for living inside host epithelium and function predominantly in absorption of soluble molecules from the host mucosal cytoplasm, in essence behaving as an external gut epithelium that is protected from abrasion by the cuticle that surrounds the openings of the bacillary cells.     

<Emphasis Type="Italic">Chlamydia pneumoniae</Emphasis> induces aponecrosis in human aortic smooth muscle cells

Background  

The intracellular bacterium Chlamydia pneumoniae is suspected to play a role in formation and progression of atherosclerosis. Many studies investigated cell death initiation versus inhibition by Chlamydia pneumoniae in established cell lines but nothing is known in primary human aortic smooth muscle cells, a cell type among others known to be involved in the formation of the atherosclerotic plaque. Type of cell death was analyzed by various methods in primary aortic smooth muscle cells after infection with Chlamydia pneumoniae to investigate a possible pathogenic link in atherosclerosis.     

Actin Filament Cables in Drosophila Nurse Cells Are Composed of Modules That Slide Passively Past One Another during Dumping

At a late stage in Drosophila oogenesis, nurse cells rapidly expel their cytoplasm into the oocyte via intracellular bridges by a process called nurse cell dumping. Before dumping, numerous cables composed of actin filaments appear in the cytoplasm and extend inward from the plasma membrane toward the nucleus. This actin cage prevents the nucleus, which becomes highly lobed, from physically blocking the intracellular bridges during dumping. Each cable is composed of a linear series of modules composed of ~25 cross-linked actin filaments. Adjacent modules overlap in the cable like the units of an extension ladder. During cable formation, individual modules are nucleated from the cell surface as microvilli, released, and then cross-linked to an adjacent forming module. The filaments in all the modules in a cable are unidirectionally polarized. During dumping as the volume of the cytoplasm decreases, the nucleus to plasma membrane distance decreases, compressing the actin cables that shorten as adjacent modules slide passively past one another just as the elements of an extension ladder slide past one another for storage. In Drosophila, the modular construction of actin cytoskeletons seems to be a generalized strategy. The behavior of modular actin cytoskeletons has implications for other actin-based cytoskeletal systems, e.g., those involved in Listeria movement, in cell spreading, and in retrograde flow in growth cones and fibroblasts.     

Cloning and physical characterization of chromosomal conjugative elements in streptococci.

A transport system for polyamines was studied with both intact cells and membrane vesicles of an Escherichia coli polyamine-deficient mutant. Polyamine uptake by intact cells and membrane vesicles was inhibited by various protonophores, and polyamines accumulated in membrane vesicles when D-lactate was added as an energy source or when a membrane potential was imposed artificially by the addition of valinomycin to K+-loaded vesicles. These results show that the uptake was dependent on proton motive force. Transported [14C]putrescine and [14C]spermidine were not excreted by intact cells upon the addition either of carbonyl cyanide m-chlorophenylhydrazone, A23187, and Ca2+ or of an excess amount of nonlabeled polyamine. However, they were excreted by membrane vesicles, although the degree of spermidine efflux was much lower than that of putrescine efflux. These results suggest that the apparent unidirectionality in intact cells has arisen from polyamine binding to nucleic acids, thus giving rise to a negligible free intracellular concentration of polyamines. Polyamine uptake, especially putrescine uptake, was inhibited strongly by monovalent cations. The Mg2+ ion inhibited spermidine and spermine uptake but not putrescine uptake.     

Larval salivary gland secretion proteins in Drosophila structural analysis of the Sgs-5 gene

The structure of the Drosophila melanogaster salivary gland secretion gene Sgs-5 has been determined by DNA sequence analysis of cloned genomic DNA. This developmentally and tissue-specific gene is a member of the third instar intermolt gene set and is under control of the insect molting hormone ecdysterone. RNA protection experiments show that the RNA coding region of Sgs-5 contains 769 nucleotides and is divided into three exons by two small introns. The protein-coding region appears to begin after a short untranslated RNA leader (33 nucleotides) and to result in a protein of 163 amino acids. The first 18 amino acids give the amino-terminal end the highly hydrophobic nature characteristic of a signal peptide.