Enhanced bioactivity of Si3N4 through trench-patterning and back-filling with Bioglass®

Using a simple and innovative sandblasting process, disks of monolithic biomedical silicon nitride (β-Si₃N₄) were texturized with a matrix of regular, discrete square trenches with a total depth in the range of hundreds of microns. The process consisted of sandblasting Si₃N₄ substrates through a stainless-steel wire-mesh (150 or 200 μm) using abrasive silicon carbide powders (α-SiC, ∼40 μm) under 1,034 kPa (150 psi) of gas pressure. The depth of the porosities could be controlled varying both the treatment time and the distance from the surface. Part of the samples were then filled with 45S5 Bioglass® powders to improve the osteointegration and stimulate the production of bone tissue. Due to the increased macroscopic and microscopic roughness, biological testing using human osteosarcoma cells (SaOS-2) showed improved cell proliferation and greater production of both mineral (hydroxyapatite) and organic (collagen) phases on the patterned surfaces compared to untreated β-Si₃N₄ or to the biomedical titanium control samples. Both of these effects were further enhanced when the porosities were filled with Bioglass®.

Bioglass functionalization of laser-patterned bioceramic surfaces and their enhanced bioactivity

The surfaces of silicon nitride (β-Si3N4) and zirconia toughened alumina (ZTA) were patterned using a high-energy laser source, which operated at a wavelength of 1064 nm. The patterning procedure yielded a series regular, cylindrical cavities 500 and 300 μm in diameter and depth, respectively. These cavities were subsequently filled with bioglass mixed with different fractions of Si3N4 powder (0, 5, and 10 mol.%) to obtain bioactive functionalized bioceramic surfaces. The laser-patterned samples were first characterized using several spectroscopic techniques before and after functionalization, and then tested in vitro with respect to their osteoconductivity using a human osteosarcoma cell line (SaOS-2). After in vitro testing, fluorescence microscopy was used to address the biological response and to estimate osteopontin and osteocalcin protein contents and distributions. The presence of bioglass greatly enhanced the biological response of both ceramic surfaces, but mainly induced production of inorganic apatite. On the other hand, the addition of minor fraction of Si3N4 into the bioglass-filled holes greatly enhanced bio-mineralization and stimulated the SaOS-2 cells to produce higher amounts of bone extracellular matrix (collagen and proteins), thus enhancing the osteopontin to osteocalcin ratio. It was also observed that the presence of a fraction of Si3N4 in the powder mixture filling the holes bestowed more uniform cell colonization on the otherwise bioinert ZTA surface.

In vitroantibacterial activity of oxide and non-oxide bioceramics for arthroplastic devices: II. Fourier transform infrared spectroscopy

The metabolic response of Gram-positiveStaphylococcus epidermidisbacteria to bioceramic substrates was probed by Fourier transform infrared spectroscopy.

In vitroantibacterial activity of oxide and non-oxide bioceramics for arthroplastic devices: I.In situtime-lapse Raman spectroscopy

Raman spectroscopy proved why the antibacterial response of non-oxide Si₃N₄bioceramic is superior to those of alumina-based oxide bioceramics.

Development of a SiYAlON glaze for improved osteoconductivity of implantable medical devices

The application of bioactive coatings onto orthopaedic appliances is commonly performed to compensate for the otherwise bioinert nature of medical devices and to improve their osseointegration. Calcium phosphates, hydroxyapatite (HAp), and bioglasses are commercially available for this purpose. Until recently, few other inorganic compounds have been identified with similar biofunctionality. However, silicon nitride (Si₃ N₄ ) has emerged as a new orthopaedic material whose unique surface chemistry also enhances osteoconductivity. Recent research has confirmed that its minority intergranular phase, consisting of silicon yttrium aluminum oxynitride (SiYAlON), is principally responsible for this improvement. As a result, it was hypothesized that SiYAlON itself might serve as an effective osteoconductive coating or glaze for medical devices. To test this hypothesis, a process inspired by traditional ceramic whiteware glazing was developed. A slurry containing ingredients similar to the intergranular SiYAlON composition was applied to a Si₃ N₄ surface, which was then subjected to a heat treatment to form a glaze. Various analytical tools were employed to assess its chemistry and morphology. It was found that the glaze was comprised predominately of Y₅ Si₃ O₁₂ N, a compound commonly referred to as N-apatite, which is isostructural to native HAp. Subsequent exposure of the glazed surface to acellular simulated body fluid led to increased deposition of biomimetic HAp-like crystals, while exposure to Saos-2 osteosarcoma cells in vitro resulted in greater HAp deposition relative to control samples. The observation that SiYAlON exhibits enhanced osteoconductivity portends its potential as a therapeutic aid in bone and tissue repair. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1084-1096, 2018.

Bioactive silicon nitride: A new therapeutic material for osteoarthropathy

While the reciprocity between bioceramics and living cells is complex, it is principally governed by the implant's surface chemistry. Consequently, a deeper understanding of the chemical interactions of bioceramics with living tissue could ultimately lead to new therapeutic strategies. However, the physical and chemical principles that govern these interactions remain unclear. The intricacies of this biological synergy are explored within this paper by examining the peculiar surface chemistry of a relatively new bioceramic, silicon nitride (Si3N4). Building upon prior research, this paper aims at obtaining new insights into the biological interactions between Si3N4 and living cells, as a consequence of the off-stoichiometric chemical nature of its surface at the nanometer scale. We show here yet unveiled details of surface chemistry and, based on these new data, formulate a model on how, ultimately, Si3N4 influences cellular signal transduction functions and differentiation mechanisms. In other words, we interpret its reciprocity with living cells in chemical terms. These new findings suggest that Si3N4 might provide unique new medicinal therapies and effective remedies for various bone or joint maladies and diseases.

Bacteriostatic behavior of surface modulated silicon nitride in comparison to polyetheretherketone and titanium

Perioperative and latent infections are leading causes of revision surgery for orthopaedic devices resulting in significant increased patient care, comorbidities, and attendant costs. Identifying biomaterial surfaces that inherently resist biofilm adhesion and bacterial expression is an important emerging strategy in addressing implant-related infections. This in vitro study was designed to compare biofilm formation on three biomaterials commonly employed in spinal fusion surgery-silicon nitride (Si₃ N₄ ), polyetheretherketone (PEEK), and a titanium alloy (Ti6Al4V-ELI) -using one gram-positive and one gram-negative bacterial species. Disc samples from various surface treated Si₃ N₄ , PEEK, and Ti6Al4V were inoculated with 10⁵ CFU/mm² Staphylococcus epidermidis (ATCC®14990™) or Escherichia coli (ATCC^® 25922™) and cultured in PBS, 7% glucose, and 10% human plasma for 24 and 48 h, followed by retrieval and rinsing. Vortexed solutions were diluted, plated, and incubated at 37 °C for 24 to 48 h. Colony forming units (CFU/mm² ) were determined using applicable dilution factors and surface areas. A two-tailed, heteroscedastic Student's t-test (95% confidence) was used to determine statistical significance. The various Si₃ N₄ samples showed the most favorable bacterial resistance for both bacilli tested. The mechanisms for the bacteriostatic behavior of Si₃ N₄ are likely due to multivariate surface effects including submicron-topography, negative charging, and chemical interactions which form peroxynitrite (an oxidative agent). Si₃ N₄ is a new biomaterial with the apparent potential to inhibit biofilm formation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1521-1534, 2017.

Silicon Nitride Bioceramics Induce Chemically Driven Lysis in Porphyromonas gingivalis

Organisms of Gram-negative phylum bacteroidetes, Porphyromonas gingivalis, underwent lysis on polished surfaces of silicon nitride (Si3N4) bioceramics. The antibacterial activity of Si3N4 was mainly the result of chemically driven principles. The lytic activity, although not osmotic in nature, was related to the peculiar pH-dependent surface chemistry of Si3N4. A buffering effect via the formation of ammonium ions (NH4(+)) (and their modifications) was experimentally observed by pH microscopy. Lysis was confirmed by conventional fluorescence spectroscopy, and the bacteria's metabolism was traced with the aid of in situ Raman microprobe spectroscopy. This latter technique revealed the formation of peroxynitrite within the bacterium itself. Degradation of the bacteria's nucleic acid, drastic reduction in phenilalanine, and reduction of lipid concentration were observed due to short-term exposure (6 days) to Si3N4. Altering the surface chemistry of Si3N4 by either chemical etching or thermal oxidation influenced peroxynitrite formation and affected bacteria metabolism in different ways. Exploiting the peculiar surface chemistry of Si3N4 bioceramics could be helpful in counteracting Porphyromonas gingivalis in an alkaline pH environment.

High level expression in Saccharomyces cerevisiae of an artificial gene encoding a repeated tripeptide aspartyl-phenylyalanyl-lysine

A chemically synthesized gene, which encodes a 64 or 128 times-repeated tripeptide, aspartyl-phenylalanyl-lysine, has been cloned onto the yeast expression vector pAM82 containing the PHO5 promoter. The artificial gene (LAP gene) contains the untranslated leader sequence of the E. coli lipoprotein gene (lpp) with its transcription terminator sequence. When yeast AH22 cells transformed by recombinant plasmid containing repeated tripeptide gene were derepressed in low phosphate medium, the artificial polypeptides were synthesized to the amounts of about 30% of the total cell protein. SDS-polyacrylamide gel electrophoresis and immunoblot analysis indicated that the artificial polypeptides synthesized in yeast have molecular weights ranging from about 30,000 and 60,000 and have immunoreactivity with the artificial polypeptides expressed in E. coli. The artificial popypeptides in whole cell extract were insoluble and seem to be synthesized as insoluble aggregates. Electron microscopy showed the presence of inclusion bodies in the cell. These polypeptides can be hydrolyzed to tripeptides with trypsin or chymotrypsin. These properties along with the high expression and easy separation may make the artificial polypeptides a potential raw material for the production of an artificial sweetener, Aspartame.

Antisuppressor mutation in Escherichia coli defective in biosynthesis of 5-methylaminomethyl-2-thiouridine

Mutations in three Escherichia coli K-12 genes were isolated that reduce the efficiency of the lysine-inserting nonsense suppressor supL. These antisuppressor mutations asuD, asuE, and asuF map at 61.9, 25.3, and 76.3 min, respectively, on the E. coli chromosome. Biochemical and genetic analysis of the mutant strains revealed the reason for the antisuppressor phenotype for two of these genes. The activity of lysyl-tRNA synthetase was reduced in strains with asuD mutations. The modification of 5-methylaminomethyl-2-thiouridine, the wobble base of tRNALys, was impaired in asuE mutant strains, presumably at the 2-thiolation step.

Isolation and characterization of antisuppressor mutations in Escherichia coli

Nonsense mutations in lacI have been shown to be useful as indicators of the efficiency of nonsense suppression. From strains containing supE and a lacI nonsense mutation, selection for LacI- mutants has resulted in the isolation of four antisuppressor mutations. Tn10 insertions linked to these mutations were isolated and used to group the four mutations into three loci. The asuA1 and asuA2 mutations are linked to trp, reduce suppression by supE approximately twofold, and affect a variety of suppressors. The asuB3 mutation was mapped by P1 cotransduction to rpsL but does not confer resistance to streptomycin. The asuC4 mutation reduced suppression by supE by 95% and was shown biochemically to result in the loss of two pseudouridine modifications from the 3' side of the anticodon stem and loop of tRNA2Gln. This mutation is linked to purF, suggesting that it is a new allele of hisT.

Changing postdoctoral career patterns for biomedical scientists

Between 1973 and 1977 the total number of Ph.D.'s holding postdoctoral appointments in the biomedical sciences increased at a rate of more than 550 individuals (12.5 percent) per year. During this same period the total number of doctorates awarded each year in these disciplines showed very little change. The postdoctoral growth can be attributed to substantial increases in both the numbers of recent graduates taking postdoctorals and the length of stay on these appointments. The lack of alternative employment opportunities has contributed heavily to the postdoctoral buildup. Continued growth is likely to have important consequences for biomedical research and research training.

Isopentenyladenosine deficient tRNA from an antisuppressor mutant of Saccharomyces cerevisiae

We have isolated a mutant of Saccharomyces cerevisiae that contains 1.5% of the normal tRNA complement of isopentenyladenosine (i6A). The mutant was characterized by the reduction in efficiency of a tyrosine inserting UAA nonsense suppressor. The chromatographic profiles of tRNATyr and tRNASer on benzoylated DEAE-cellulose are consistent with the loss of i6A by these species. Transfer RNA from the mutant exhibits 6.5% of the cytokinin biological activity expected for yeast tRNA. Transfer RNAs from the mutant that normally contain i6A accept the same levels of amino acids in vitro as the fully modified species. With the exception of i6A, the level of modified bases in unfractionated tRNA from the mutant appears to be normal. The loss of i6A apparently affects tRNA's role in protein synthesis at a step subsequent to aminoacylation.

Isolation and identification of cytokinins from Euglena gracilis transfer ribonucleic acid

Three ribonucleosides responsible for cytokinin activity in Euglena gracilis var Bacillaris tRNA have been isolated and identified as 6-(3-methyl-2-butenylamino)-9-beta-D-ribofuranosylpurine, 6-(4-hydroxy-3-methyl-cis-2-butenylamino)-9-beta-D-ribofuranosylpurine, and 6-(4-hydroxy-3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine. The structures of these compounds were assigned on the basis of their chromatographic properties and ultraviolet and mass spectra which were identical with those of the corresponding synthetic compounds. The elution profiles of cytokinin bioassay activity and of 35S radioactivity suggest the presence of a trace amount of 6-(3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine.

Subcellular localization of cytokinins in transfer ribonucleic Acid

Evidence on the localization of cytokinins in chloroplast tRNA was obtained by comparison of Euglena gracilis var. bacillaris light-grown and dark-grown wild type cultures and chloroplast-bleached mutant strains. The several cytokinins characteristic of tRNA were separated by Sephadex LH-20 column chromatography of the hydrolysates and were quantitatively determined by tobacco bioassays of the eluates. The results indicate that 6-(3-methyl-2-butenylamino)-9-beta-d-ribofuranosylpurine (i(6) A) is formed in both the cytoplasmic and chloroplast tRNA, whereas 6-(4-hydroxy-3-methyl-cis-2-butenylamino)-9-beta-d- ribofurano-sylpurine (c-io(6)A) is produced mainly in the cytoplasmic tRNA and 6-(4-hydroxy-3-methyl-2-butenylamino)-2-methylthio-9-beta-d- ribofurano-sylpurine (ms(2)io(6)A) is localized exclusively in chloroplast tRNA. The restriction of the methiolation reaction to the chloroplast is supported by results of radioisotope experiments showing that (35)S-labeled MgSO(4) is incorporated into ms(2)io(6)A in the wild type cultures, but not in the chloroplast-bleached mutant strains.

Preparation of Escherichia coli tRNAs terminating of modified nucleosides by the use of CTP(ATP):tRNA nucleotidyltransferase and polynucleotide phosphorylase

Two procedures were investigated for the modification of tRNAs at the 3'-terminal nucleoside. The first involved the incubation of an enzymatically abreviated tRNA (tRNA-C-COH) with appropriate nucleoside triphosphates in the presence of CTP(ATP):tRNA nucleotidyltransferase from Escherichia coli and yeast. The E. coli enzyme did not utilize 2'- or 3'-deoxyadenosine 5'-triphosphate as substrates, but affected incorporation of the 2'- and 3'-O-methyladenosine triphosphates onto tRNA-C-Cou to the extent of 30 and 37%, respectively. Although incorporation of the deoxynucleotides could not be effected using the E. coli enzyme, yeast CTP(ATP:tRNA nucleotidyltransferase produced the desired tRNAs in yields of 45-65%. The second modification procedure involved incubation of tRNA-C-COH with (appropriately blocked) nucleoside diphosphates in the presence of polynucleotide phosphorylase. This procedure afforded the tRNAs terminating in 2'- and 3'-deoxyadenosine in yields of 4% (and the yield of the former was increased to 36% when the incubation was carried out in the presence of 20% methanol). The yields of tRNAs terminating in 2'- and 3'-O-methyladenosing produced by this procedure were 55 and 17%, respectively. Because only single isomers of most of the tRNAs terminating in 2'- and 3'-deoxy- and O-methyladenosine are aminoacylated, attempts were made to obtain the other isomericaminoacyl-tRNA by enzymatic introduction of chemically preaminoacylated nucleotides onto tRNA-C-COH. Although incubation of tRNA-C-COH with three aminoacylated nucleoside 5'-triphosphates and E. coli CTP(ATP):tRNA nucleotidyltransferase did not result in production of the desired tRNAs to a detectable extent, incubation with 2'-deoxy-3'-O-L-phenylalanyladenosine 5'-diphosphate and polynucleotide phosphorylase afforded E. coli tRNA terminating with the corresponding aminoacylated deoxynucleoside.

Genetic analysis of a transposable suppressor gene in Saccharomyces cerevisiae

We have demonstrated in Saccharomyces cerevisiae the transposition of a gene coding for an efficient ochre (UAA) suppressor from a centromere-linked site on chromosome III to two new sites in the yeast genome. One site is on chromosome VI, very close to, if not allelic with, SUP11, one of eight genes coding for a tyrosine-inserting suppressor. The second site is on chromosome III, unlinked to the centromere and distal to the mating type locus. This site is very close to those mapped for the recessive lethal amber suppressors, SUP-RL1 and SUP61.

Indirect selection for auxotrophic mutants of Saccharomyces cerevisiae using the antibiotic netropsin

The small basic oligopeptide antibiotic, netropsin, can be successfully employed as an effective counterselecting agent in Saccharomyces cerevisiae. The use of the drug results in approximately a 35-fold enrichment of auxotrophic mutants in a mutagenized culture of yeast. The experimental procedure is quite simple and less time consuming than other presently used methods for indirect mutant selection in yeast.

Transfer ribonucleic acid biosynthesis. Substrate specificity of ribonuclease P

Bacteriophage T4 synthesizes proline and serine tRNA species which are derived from a common precursor RNA. The processing of this precursor RNA involves the replacement of a U-A-A terminus in serine tRNA by C-C-A prior to precursor cleavage. In the present work we have examined in detail the cleavage of T4 proline-serine precursor RNA by the previously identified ribonuclease P. Ribonuclease P accurately cleaves precursor RNA terminating in either C-C-A or U-A-A to generate the 5' termini characteristic of both mature tRNA species. These cleavages do not depend solely on the nucleotide sequence of the precursor RNA since isolated oligonucleotides spanning the cleavage sites are not substrates for the enzyme. Two types of experiments show that RNase P kinetically favors precursor RNA ending C-C-A over that ending U-A-A. Isolated preparations of precursor RNA containing the C-C-A sequence were cleaved more rapidly by RNase P than precursor RNA ending U-A-A. In addition, the serine tRNA generated by limited cleavage of a mixed population of precursor RNA ending C-C-A or U-A-A was enriched 3-fold in the C-A-A sequence relative to the starting material. Bacteriophage T4 proline-serine precursor RNA, in contrast to other tRNA precursors, accumulates in measurable amounts in wild type cells. This accumulation would appear to be a consequence of the requirement for the generation of the C-C-A sequence prior to RNase P cleavage. The enzymic specificity of RNase P in vitro therefore reflects the in vivo pathway for serine tRNA biosynthesis, where the C-C-A sequence is synthesized while the serine tRNA sequence is still a part of the large precursor RNA.