Dense fibrillar collagen matrices: a model to study myofibroblast behaviour during wound healing

Fibroblastic cells play an important part in wound healing. Human dermal fibroblasts seeded onto three-dimensional fibrillar collagen matrices migrate into the collagen network and differentiate into myofibroblasts. In order to evaluate the use of collagen matrices as model systems for studying myofibroblast phenotype during wound healing, myofibroblast behaviour migrating into dense or loose matrices was compared. The effect of collagen concentration on cell morphology, remodelling, proliferation and apoptosis of human myofibroblasts was evaluated. Myofibroblasts within dense collagen matrices (40 mg/ml) were spindle shaped, similar to cells observed during tissue repair. In contrast, cells within loose matrices (5mg/ml) were more rounded. Matrix hydrolysis activities (MT1-MMP and MMP2) did not differ between the two collagen concentrations. The myofibroblast proliferation rate was measured after 24h bromodeoxyuridine incorporation (BrdU). Cells in dense collagen matrices proliferated at a higher rate than cells in loose matrices at each culture time point tested. For example, 40% of cells in dense matrices were replicating compared to 10% of cells in loose matrices after 28 days in culture. Apoptotic cells were only detected in dense matrices from day 21 onwards when cells had already migrated into the collagen network. Taken together, these results show that a high collagen concentration has a stimulatory effect on myofibroblast proliferation and apoptosis, two important events in wound healing. Thus, dense matrices can be used to create controlled conditions to study myofibroblast phenotype.

A cylinder-shaped double ribbon structure formed by an amyloid hairpin peptide derived from the β-sheet of murine PrP: An X-ray and molecular dynamics simulation study

A structural model of the murine PrP small beta-sheet was obtained by synthesizing the RGYMLGSADPNGNQVYYRG peptide comprising the two beta-strands 127-133 and 159-164 linked by a four-residue sequence of high turn propensity. The DPNG turn sequence is a "short circuit" replacing the original protein sequence between the two strands. This 19-residue peptide spontaneously forms very long single fibrils as observed by electron microscopy. The X-ray diffraction patterns of a partially oriented sample reveals an average arrangement of the hairpin peptides into a structure which can be geometrically approximated by an empty-core cylinder. The hairpins are oriented perpendicular to the cylinder axis and a 130 A helix period is observed. Based on X-ray diffraction constraints and on more indirect general protein structure considerations, a precise and consistent fibril model was built. The structure consists of two beta-sheet ribbons wound around a cylinder and assembled into a single fibril with a hairpin orientation perpendicular to the fibril axis. Subsequent implicit and explicit solvent molecular dynamics simulations provided the final structure at atomic resolution and further insights into the stabilizing interactions. Particularly important are the zipper-like network of polar interactions between the edges of the two ribbons, including the partially buried water molecules. The hydrophobic core is not optimally compact explaining the low density of this region seen by X-ray diffraction. The present findings provide also a simple model for further investigating the sequence-stability relationship using a mutational approach with a quasi-independent consideration of the polar and apolar interactions.

Most of the structural elements of the globular domain of murine prion protein form fibrils with predominant beta-sheet structure

The conversion of the cellular prion protein into the beta-sheet-rich scrapie prion protein is thought to be the key step in the pathogenesis of prion diseases. To gain insight into this structural conversion, we analyzed the intrinsic structural propensity of the amino acid sequence of the murine prion C-terminal domain. For that purpose, this globular domain was dissected into its secondary structural elements and the structural propensity of the protein fragments was determined. Our results show that all these fragments, excepted that strictly encompassing helix 1, have a very high propensity to form structured aggregates with a dominant content of beta-sheet structures.

Three-dimensional structure of tubular networks, presumably Golgi in nature, in various yeast strains: a comparative study

BACKGROUND

In the yeast Saccharomyces cerevisiae, the Golgi apparatus consists of discrete units distributed throughout the cytoplasm. When such units are examined in three dimensions, in relatively thick sections prepared for the electron microscope, they usually appear as small tubular networks with a stained material accumulating in dilations located at the junctions of membranous tubules. To see whether such tubular networks are observed in other yeast species, the three-dimensional structure of organelles in eight additional yeast strains, endowed with diverse biological properties, are examined.

METHODS

Yeast strains were grown at 24 degrees C in YPD medium (2% Bactopeptone, 1% Bactoyeast extract, and 2% glucose). Cells that were examined by electron microscopy came from exponentially growing cultures grown in a shaking water bath and maintained at a OD 600 (optical density at 600 nm) of 0.5. Cells were fixed in a fixative containing 2% glutaraldehyde in 0.1 M cacodylate buffer pH 7.4 and 0.8 M sorbitol. They were then treated for 15 min in 1% sodium metaperiodate and postfixed for 1 hr in potassium ferrocyanide-osmic acid. They were preembedded in agarose prior to dehydration and finally embedded in Epon. In these conditions, the preservation of cell organelles was improved and the cytoplasmic retraction from the cell wall was minimized. Photographs of sections tilted at +/- 15 degrees from the 0 degrees position of the goniometric stage were used to prepare stereopairs from which the three-dimensional configuration of the organelles was visualized.

RESULTS

In all yeast strains, tubular networks appeared as separate elements or units disperse throughout the cytoplasm. Each unit consisted of anastomosed membranous tubules. In some strains such as Saccharomyces cerevisiae, Zygosaccharomyces rouxii, or Saccharomyces pombe, such units appeared mainly as polygonal networks of intensely stained membranous tubules. Along these networks, distensions filled with stained material were similar in size to nearby secretory granules, suggesting that the latter formed by fragmentation of the tubular networks. In Hansenula polymorpha, Pichia pastoris, and Debaryomyces hansenii, networks of anastomosed tubules were closely superposed to each other and formed parallel arrays reminiscent of the stacks of Golgi saccules seen in mammalian cells. However, in contrast to what is usually found in the latter, the layers making up the parallel arrays in yeasts, were clearly continuous to each other. In other strains, i.e., Kluyveromyces lactis, Candida albicans, and Candida parapsilosis, the situation was intermediate and their cytoplasm contained only arrays of small size with two or at most three superposed layers of membranous tubules. Small vesicles in the 30-50 nm range were rarely encountered in most yeast strains.

CONCLUSIONS

It is therefore concluded that tubular networks, presumably Golgi in nature, are present in all yeasts examined so far. Yet, in some strains, these tubular networks may be arranged in parallel arrays or stacks.

The operation and efficacy of cryosurgical, nitrous oxide-driven cryoprobe. I. Cryoprobe physical characteristics: their effects on cell cryodestruction

For specification of the requirements for efficient cell cryodestruction in tumors, we tested a N2O-driven cryoprobe on experimental models. The cryoprobe was a 3-mm-diameter type for operation via fiber optic bronchoscopes in respiratory medicine. The freezing process, namely the "ice-ball" formation around the cryoprobe tip, was monitored with an impedancemeter. Physical characteristics and formation kinetics of the ice-ball formation (volume, diameter, freezing rate) were studied under defined experimental conditions in various biological liquids, including saline, serum, whole blood, and tumor cell suspensions (rat ascitic hepatoma), either plain or supplemented with gelling agents to approximate solid tumor consistency. Cell destruction (i.e., cryotoxicity to cells) within the ice ball produced in rat ascitic hepatoma was assessed in two ways: the cells, collected after ice-ball thawing, were (1) seeded and cultured according to methods currently in use, or (2) injected into a rat to check for possible development of ascites. Both tests showed that cryotoxicity correlated with freezing rate within the ice ball, cell mortality was total next to the cryoprobe tip (i.e., site of highest freezing rate), while it was absent within the ice-ball periphery. In the area in between, mortality varied gradually. Together our experimental results show that cryotoxicity to cells may be improved by increasing the freezing rate (e.g., by brief precooling of the cryoprobe). Furthermore, for tumor cryosurgery, since cell mortality is maximal next to the cryoprobe, we point out that higher efficacy might be achieved by several overlapping short freezing spots in tumoral tissue, instead of one single prolonged freeze.

Pathway and kinetics of vitellogenin-gold internalization in the Xenopus oocyte

After in vitro incubation of Xenopus oocytes with vitellogenin (VTG)-gold conjugate, the gold particles are distributed on the whole plasma membrane. Their concentration in coated pits still occurs at 0 degrees C. At +20 degrees C the label quickly (30 sec) appears in multi-vesicular endosomes (MVE) which segregate together with primary endocytic vesicles into distinct clusters below the plasma membrane. From this step up to crystallization of the yolk platelets, the gold particles stay in the same compartment. During 5.5 h the label progressively increases along the MVE membrane, first (1.5 h) by fusion of primary endocytic vesicles with consecutively enlarging endosomes, then (4 h) by decreasing of the MVE membrane. As concerns the yolk platelet formation, concentration of primordial yolk platelets (PYP) occurs at 5.5 h from the incubation onset, the labeling of preexisting yolk platelets starts at 7 h, while crystallization of PYP begins only after 12-13 h. Our results indicate that VTG receptors are not preclustered in coated pits and their lateral translation is not inhibited at 0 degrees C. The yolk protein processing takes place within one compartment only. The VTG condensation begins with a long concentration phase of receptor-VTG complexes still integrated in the endosome membrane. It occurs in MVE by: i) a repeated fusion of primary endocytic vesicles; ii) removing part of the endosome membrane by internal vesiculation. Fusion between endosomes occurs only after VTG has dissociated from its receptors and VTG dissociates only when when the density of the VTG-receptor complexes in the endosome membrane is sufficient. Crystallization begins after a 7-8 h delay. The endosome migration into the oocyte is also controlled by the binding of VTG to its receptors. Our results also demonstrate that binding of VTG colloidal gold modifies neither the vitellogenic pathway nor the duration of the vitellogenin internalization. However when vitellogenin is bound to colloidal gold, dissociation of ligand-receptor complexes is delayed because the amount of ligand in the incubation medium is necessarily low.

Cytochalasin D suppresses beta-adrenergic induced protein discharge without inhibiting membrane fusion

Biochemical studies have shown that low doses of Cytochalasin D (CD) inhibit protein discharge evoked by isoproterenol, a beta-adrenergic inducer, in isolated lobules of Rat parotid gland. Ultrastructural and radioautographic studies reported here indicate that, when lobules are treated by CD before isoproterenol stimulation, the connection of secretory granules with the acinar lumen is indeed prevented, but fusion of membrane granules evoked by isoproterenol still occurs. This fusion takes place with vacuoles induced by CD and possibly with other granules.

Microtubules and protein secretion in rat lacrimal glands: localization of short-term effects of colchicine on the secretory process

The pathway and kinetics of the secretory protein transport in rat lacrimal exorbital gland have been established by an in vitro time-course radioautographic study of pulse-labeled protein secretion. The colchicine-sensitive steps have been localized by using the drug at various times with respect to the pulse labeling of proteins. Colchicine (10 microM) does not block any step of the secretory protein transport, but when introduced before the pulse it decreases the transfer of labeled proteins from the rough endoplasmic reticulum to the Golgi area, suppressing their temporary accumulation in the Golgi area before any alteration of this organelle is detectable. Moreover, colchicine inhibits protein release only from the secretory granules formed in its presence because the peroxidase discharge is diminished 1 h after colchicine addition, and the secretion of newly synthesized proteins is strongly inhibited only when colchicine is introduced before secretory granule formation. Morphometric studies show that there is a great increase of secondary lysosomes, related to crinophagy, as early as 40-50 min after colchicine is added. However, changes in lysosomal enzymatic activities remained biochemically undetectable. We conclude that: (a) the labile microtubular system does not seem indispensable for protein transport in the rough endoplasmic reticulum-Golgi area but may facilitate this step, perhaps by maintaining the spatial organization of this area; and (b) in the lacrimal gland, colchicine inhibits protein release not by acting on the steps of secretion following the secretory granule formation, but by acting chiefly on the steps preceding secretory granule formation, perhaps by making the secretory granules formed in its presence incapable of discharging their content.