Scanning electron microscopy and electron probe microanalysis of calcified alpha-elastin coacervates

Fibroblast involvement in soft connective tissue calcification

Soft connective tissue calcification is not a passive process, but the consequence of metabolic changes of local mesenchymal cells that, depending on both genetic and environmental factors, alter the balance between pro- and anti-calcifying pathways. While the role of smooth muscle cells and pericytes in ectopic calcifications has been widely investigated, the involvement of fibroblasts is still elusive. Fibroblasts isolated from the dermis of pseudoxanthoma elasticum (PXE) patients and of patients exhibiting PXE-like clinical and histopathological findings offer an attractive model to investigate the mechanisms leading to the precipitation of mineral deposits within elastic fibers and to explore the influence of the genetic background and of the extracellular environment on fibroblast-associated calcifications, thus improving the knowledge on the role of mesenchymal cells on pathologic mineralization.

Osteopontin is a constitutive component of normal elastic fibers in human skin and aorta

Osteopontin is an acidic matrix protein, mainly expressed in mineralized tissues, kidney and atherosclerotic vessels; its biological role is still largely undefined. In the present study, immunocytochemical approaches showed that osteopontin is localized within normal elastic fibers of human skin and aorta. Antibodies raised against human bone osteopontin (LF7) or against human osteopontin synthetic peptide (amino acids 1-10, LF19) recognized epitopes associated with the amorphous material within the elastic fibers. Elastic fiber-associated microfibrils were always negative. The positivity for osteopontin of the elastic fibers was independent of age and could be observed in fetal skin and aorta as well as in the same of children, young adults and old subjects. The altered elastic fibers in the skin of old individuals were only fairly positive for osteopontin. The presence of osteopontin within the elastic fibers suggests that it may play a role against the observed tendency of elastic fibers to favor mineral precipitation. A role of osteopontin in modulating crystal nucleation and growth in mineralizing tissues and, more generally, in conditions in which mineral precipitation should be controlled is also possible.

A synthetic polypentapeptide of elastin for initiating calcification

A polypentapeptide (PPP) of tropoelastin having a repeating amino acid sequence of (Val-Pro-Gly-Val-Gly)n was evaluated for its potential to initiate in vivo calcification and to enhance bone formation in nonhealing calvarial wounds (8.0 mm) in 396 adult Walter Reed rats. There were four configurations of the PPP (molecular weight range of 50-100K dalton) consisting of 1-dry PPP; 2-coacervate PPP; 3-gamma irradiated, cross-linked PPP; 4-calcified, gamma irradiated, cross-linked PPP. These four iterations plus a control group of animals constituted the five treatment classes that were evaluated at days 1, 3, 7, 21, 42, and 147. Seventy two rats were used for each treatment and 36 rats for the control. Following euthanatization, specimens were placed into 70% ethanol, embedded in polymethyl methacrylate, sectioned at 3.5 micrometers, and alternating sections were stained with Masson-Goldner trichrome and von Kossa stains. Histomorphometric analysis was accomplished using a Zeiss Universal microscope (250 X) and Videoplan Image Analysis System to evaluate five random histologic fields from margin to margin of the craniotomy. Trabecular bony volume and area of calcification islands were quantitated. A Student's t test for unpaired data to determine treatment differences (within the same temporal groups) revealed that there was no significant difference between treatments and control for trabecular bony volume; however, there was a significant difference between experimentals and control for calcification islands (P less than 0.05) such that calcifications islands for the experimentals were greater than the control. There was not a significant difference between experimental treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphopeptides of enamel matrix

Although the tripeptides Glu-O-Phosphoserine-Tyr and Glu-O-Phosphoserine-Leu have been identified in embryonic bovine enamel proteins, 1, 2 the issue of whether both sequences occur in each of the phosphopeptides, or whether certain sequences occur in specific peptides only, has recently been resolved by isolating homogeneous samples of E33 and E44. All three of the Ser residues of both peptides are phosphorylated. All three in E3 are in the sequence Glu-O-Phosphoserine-Leu, and all three in E4 are in the sequence Glu-O-Phosphoserine-Tyr. It was not possible to sequence either of the polypeptide chains directly by automatic peptide sequencing. However, a partial sequence of E4 was constructed from data derived from peptides isolated after cyanogen bromide, trypsin and chymotrypsin digestions. The presence of Glu, Tyr and Leu adjacent to and near the O-Phosphoserine [Ser(L)] residues and the 2 degrees, 3 degrees and higher ordered structures of the enamel phosphopeptides may be important in calcium binding and mineralization.

Conformational transitions of alpha-elastin

The alpha-elastin, the soluble product obtained from elastin by oxalic acid hydrolysis, has been studied from a conformational point of view in different conditions using circular dichroism spectroscopy. The spectral variations of the alpha-elastin dissolved in water and in different organic solvents/water mixtures have been examined in order to establish the effect of the solvents on the structural properties of the protein. The effect of calcium ions has also been studied on alpha-elastin dissolved either in water or in organic solvents. Finally, the effect of pH and temperature on the conformational properties of the alpha-elastin has been investigated.

Cross-linked polypentapeptide of tropoelastin: an insoluble, serum calcifiable matrix

The synthetic, cross-linked polypentapeptide of tropoelastin has been shown to calcify from serum alone even when separated from the serum medium by a dialysis membrane with a low-molecular-weight cut off. By microprobe analysis, it appeared that the only serum elements required for the calcification were calcium and phosphorus. Furthermore, thin sections of the calcified matrix showed the calcification to occur throughout the matrix, and thereby verified that it is a bulk property of the matrix and not an interfacial property. To our knowledge this is the first demonstration of an insoluble, synthetic polypeptide to function as a serum calcifiable matrix and by doing so it opens the door to potential medical applications.

Calcification of alpha-elastin coacervates: a bulk property of elastin

Scanning electron microscopy and electron probe microanalysis studies are reported on thin sections of calcified coacervates of alpha-elastin. It is found that the capacity of elastin coacervates to initiate calcification is a bulk property of the coacervate and not limited to the serum-coacervate interface, that the calcium phosphate deposits act to bind the protein units together and slow the dissolution and spreading of the coacervate as it floats on an airwater interface, and that, within the limits of detectability, there is no involvement of sulfur. As the charged groups of alpha-elastin had been blocked, the initiation of deposition is due to neutral sites in the protein which are tightly bound to the calcium phosphate deposits.

Optical diffraction of tropoelastin and alpha-elastin coacervates

Optical diffraction applied to micrographs of coacervated tropoelastin and alpha-elastin show an equatorial repeat around 50 A. This confirms a 50 A center-to-center distance of parallel aligned filaments to be a fundamental property of the tropoelastin and alpha-elastin coacervates. This periodicity is similar to that of mature cross-linked elastin. These results allow the conclusion that hydrophobic association is the predominant driving force for formation of filamentous elastin in vitro. It is suggested that the coacervate is a model for relaxed fibrous elastin.