Alan G. Brownlee

Regulation of gene expression by pH of the growth medium in Aspergillus nidulans

SummaryIn the fungus Aspergillus nidulans the levels of a number of enzymes whose location is at least in part extracellular (e.g. acid phosphatase, alkaline phosphatase, phosphodiesterase) and of certain permeases (e.g. that for γ-amino-n-butyrate) are controlled by the pH of the growth medium. For example, at acidic pH, levels of acid phosphatase are high and those of alkaline phosphatase are low whereas at alkaline pH the reverse is true. Mutations in five genes, palA, B, C, E and F, mimic the effects of growth at acid pH whereas mutations in pacC mimic the effects of growth at alkaline pH. palA, B, C, E and F mutations result in an intracellular pH (pHin) which is more alkaline than that of the wild type whereas pacC mutations result in a pHin more acidic than that of the wild type. This indicates that these mutations exert their primary effects on the regulation of gene expression by pH rather than on the pH homeostatic mechanism but that the expression of at least some component(s) of the pH homeostatic mechanism is subject to the pH regulatory system. It is suggested that pacC might be a wide domain regulatory gene whose product acts positively in some cases (e.g. acid phosphatase) and negatively in others (e.g. alkaline phosphatase). The products of palA, B, C, E and F are proposed to be involved in a metabolic pathway leading to synthesis of an effector molecule able to prevent the (positive and negative) action of the pacC product.These genes are, to our knowledge, the first examples of genes involved in the regulation of extracellular enzyme and permease synthesis by the pH of the growth medium to be described in any organism.

A highly elastic tissue sealant based on photopolymerised gelatin

Gelatin is widely used as a medical biomaterial because it is readily available, cheap, biodegradable and demonstrates favourable biocompatibility. Many applications require stabilisation of the biomaterial by chemical crosslinking, and this often involves derivatisation of the protein or treatment with cytotoxic crosslinking agents. We have previously shown that a facile photochemical method, using blue light, a ruthenium catalyst and a persulphate oxidant, produces covalent di-tyrosine crosslinks in resilin and fibrinogen to form stable hydrogel biomaterials. Here we show that various gelatins can also be rapidly crosslinked to form highly elastic (extension to break >650%) and adhesive (stress at break >100 kPa) biomaterials. Although the method does not require derivatisation of the protein, we show that when the phenolic (tyrosine-like) content of gelatin is increased, the crosslinked material becomes resistant to swelling, yet retains considerable elasticity and high adhesive strength. The reagents are not cytotoxic at the concentration used in the photopolymerisation reaction. When tested in vivo in sheep lung, the photopolymerised gelatin effectively sealed a wound in lung tissue from blood and air leakage, was not cytotoxic and did not produce an inflammatory response. The elastic properties, thermal stability, speed of curing and high tissue adhesive strength of this photopolymerised gelatin, offer considerable improvement over current surgical tissue sealants.

The development of photochemically crosslinked native fibrinogen as a rapidly formed and mechanically strong surgical tissue sealant.

We recently reported the generation of a highly elastic, crosslinked protein biomaterial via a rapid photochemical process using visible light illumination. In light of these findings, we predicted that other unmodified, tyrosine-rich, self-associating proteins might also be susceptible to this covalent crosslinking method. Here we show that unmodified native fibrinogen can also be photochemically crosslinked into an elastic hydrogel biomaterial through the rapid formation of intermolecular dityrosine. Photochemically crosslinked fibrinogen forms tissue sealant bonds at least 5-fold stronger than commercial fibrin glue and is capable of producing maximum bond strength within 20s. In vitro studies showed that components of the photochemical crosslinking reaction are non-toxic to cells. This material will find useful application in various surgical procedures where rapid curing for high strength tissue sealing is required.

The 5S ribosomal RNA gene is linked to large and small subunit ribosomal RNA genes in the oomycetes, Phytophthora vignae, P. cinnamomi, P. megasperma f.sp. glycinea and Saprolegnia ferax

Southern hybridization and polymerase chain reaction data indicate that the 5S ribosomal RNA gene is linked to the ribosomal RNA gene repeat unit in the oomycetes, Phytophthora vignae, P. cinnamomi, P. megasperma f.sp. glycinea and Saprolegnia ferax, and is apparently transcribed in the same direction as the large and small subunit ribosomal RNA genes. The polymerase chain reaction has been used to amplify all components of the entire ribosomal RNA gene repeat unit for each of these oomycetes. The total size of all amplified products is identical to the size of the ribosomal RNA gene repeat unit, as determined by Southern analysis.

Nitrogen metabolite repression in Aspergillus nidulans: A farewell to tamA?

SummaryPrevious work has established that nitrogen metabolite repression in Aspergillus nidulans is mediated by the positive acting regulatory gene areA. Pateman and Kinghorn (1977) proposed that the gene tamA plays an equally important regulatory role in nitrogen metabolite repression as the result of work with “tamAr-50,” an “allele” leading to inability to utilise nitrogen sources other than ammonium, and “tamAd-1,” an “allele” leading to nitrogen metabolite derepression. Both “tamAr-50” and “tamAd-1” were subsequently lost. We have therefore attempted to reconstruct Pateman and Kinghorns work with tamA. We propose that “tamAr-50” was in fact a pyroB−tamA− double mutation. pyroB− mutations lead to a block in vitamin B6 biosynthesis which can be supplemented by extremely high concentrations of ammonium. tamA− mutations, possibly as the result of a membrane alteration, reduce the concentration of ammonium required to supplement the pyroB− auxotrophy. There is, however, no evidence that pyroB− or tamA- mutations, alone or in combination, affect the regulation of the levels of a number of enzymes subject to nitrogen metabolite repression. Reversion of pyroB− strains constitutes a powerful positive selection technique for obtaining a wide variety of mutations in glnA, the probable structural gene for glutamine synthetase. We suggest that the nitrogen metabolite derepressed phenotype attributed to “tamAd-1” might have resulted from an extremely leaky glnA− mutation.

Evaluation of photo‐crosslinked fibrinogen as a rapid and strong tissue adhesive

Tissue adhesives and sealants are commonly used in surgery either as an adjunct to, or replacement for, sutures. Previously, we have shown that fibrinogen can be crosslinked rapidly to give a high-strength bond in the presence of a ruthenium(II) complex, a persulfate and irradiation with visible light, and that the crosslinked fibrinogen is nontoxic to cells in vitro. This approach addresses limitations to current fibrin sealants that typically have relatively slow curing times and low bond strengths. In the present study, we have evaluated the efficacy and safety of this new biological scaffold sealant in various animal models. When placed as solid implants into rats, the crosslinked fibrinogen persisted for at least 8 weeks but was fully resorbed by 18 weeks with minimal inflammatory responses. When used as a tissue adhesive for repair of skin incisions in rats or as an arterial haemostat in pig, the photo-crosslinked fibrinogen sealed tissue or arrested bleeding within 20 s of application. For the skin incisions, the fibrinogen sealant promoted rapid tissue vascularization and cellular infiltration with no adverse foreign body cell generation. New collagen deposition occurred and with time the matrix had remodelled to acquire large mature collagen fiber bundles which were accompanied by maximum regenerated tensile strength. This biomaterial system may find useful applications in surgical procedures where rapid curing and/or high strength tissue sealing is required.

Microsatellite detection of donor-derived sperm DNA following germ cell transplantation in cattle

Although autologous and heterologous transplantation has resulted in colonisation of recipient testes in cattle, the ability of the transplanted spermatogonial stem cells to complete spermatogenesis has not yet been determined. The objective of the present study was to identify and validate microsatellite markers that can distinguish the genotype of different individuals and therefore can be used to detect the presence of donor DNA in recipient semen samples. In a previous study by this group, successful colonisation of recipient testes by heterologous transfer using a fluorescent dye was shown. In the present work, some of the same recipient animals were investigated further to monitor donor-derived sperm production. The bovine microsatellite detection method was developed specifically to test the ejaculates of the recipients and can also be used to pre-match individuals before germ cell transplantation. Semen was collected from the recipients 52-98 weeks after transfer and the presence of donor DNA in the samples was determined using microsatellite markers. In one of the recipients, all collected semen samples were shown to be positive for donor-derived cells; however, the percentage of donor spermatozoa in the recipient ejaculate declined with time. The donor DNA was also detected in both single cell suspensions and testis tissue from this recipient. These results demonstrate for the first time that testicular germ cell transplantation between different breeds of cattle is feasible and the recipients thereof are able to produce spermatozoa of donor origin. This technology has potential applications in livestock breeding systems and may provide an alternative to artificial insemination.

Photochemically crosslinked matrices of gelatin and fibrinogen promote rapid cell proliferation

Here we report the use of a facile photochemical crosslinking method to fabricate stable polymer matrices from unmodified gelatin and fibrinogen. Gels were produced by covalent crosslinking of the proteins in a rapid photo‐oxidative process, catalysed by a ruthenium metal complex and irradiation with visible light. For generation of macroporous, spongy matrices, the proteins and crosslinking reagents were mixed with catalase and hydrogen peroxide to achieve a foaming reaction, producing a stable, foamed matrix that was subsequently photo‐crosslinked. C2C12 cells were either seeded onto the matrices after photo‐curing or embedded in the protein matrix prior to foaming and crosslinking. Cells seeded onto scaffolds post‐curing showed high cell viability and rapid proliferation in vitro. For cells embedded in the matrix prior to crosslinking there was some loss of initial viability, but surviving cells were able to proliferate after a period of in vitro cultivation. The matrices were shown to be biocompatible when implanted into nude mice, with evidence of proliferation and differentiation of cells seeded into the scaffolds. The results are promising for further development of tissue‐engineering scaffolds based on this ruthenium‐catalysed photo‐crosslinking method. Copyright

Introduction and expression of the bacterial glyoxylate cycle genes in transgenic mice

The glyoxylate cycle, catalysed by two unique enzymes: isocitrate lyase (ICL; EC 4.1.3.1) and malate synthase (MS; EC 4.1.3.2), is necessary for the net conversion of acetate into glucose. This metabolic pathway operates in microorganisms, higher plants and nematodes. Two bacterial genes, encoding ICL and MS, were modified in order to introduce them into the mouse germ line. The ovine metallothionein-Ia (MT-Ia) promoter-aceB gene-ovine growth hormone (GH) gene (3′ GH sequence) construct was fused to the ovine MT-Ia promoter-aceA gene-ovine GH gene (3′ GH sequence). Therefore, in this single DNA sequence, bothaceA andaceB are under independent MT-Ia promoter control and can be induced by zinc. Transgenic mice were generated by pronuclear microinjection of theaceB-aceA gene construct. We now report the establishment of four mouse lines carying these two transgenes. Studies on the progeny of these lines indicate that one line (No. 91) is expressing both genes at the mRNA and enzyme levels in the liver and intestine, whereas another line (No. 66) has a much lower expression. Both enzyme activities were detected in the liver and intestine at levels up to 25% of those measured in fully derepressedEscherichia coli cells.

Gene expression profiles of BMP4, FGF10 and cognate inhibitors, in the skin of foetal Merino sheep, at the time of secondary follicle branching

Abstract:  The high concentration of secondary branched follicles is a distinctive feature of the Merino sheep. These follicles initiate from 100 days of gestation. Here, we report a transition in abundance of the BMP4 and FGF10 morphogens occurring at this time. At 103 days of gestation, FGF10 gene expression dropped steadily from maximal levels, in a trend that continued until day 143. Conversely, from day 105, BMP4 transcript levels rapidly increased to maximal levels that were maintained until 131 days, before declining. This profile closely matches reported changes in branched follicle numbers, which peak in density at day 134. SPRY4, a known regulator of FGF10, increased to maximal levels concomitant with the fall in FGF10, suggesting a relationship. Levels of the BMP4 inhibitor NOG matched the initial rise of BMP4, with a fivefold spike at 108 days; but consistent with the rise in BMP4, this high level was not sustained.