Enhanced resistance of anaerobic rumen fungi to the ionophores monensin and lasalocid in the presence of methanogenic bacteria

The presence of Methanobrevibacter smithii altered the susceptibility of the anaerobic fungi Neocallimastix frontalis and Piromonas communis to the carboxylic ionophores monensin and lasalocid. The ionophores depressed growth (measured by chitin accretion), the uptake of glucose and the production of H2, formate and acetate by the fungi growing axenically in semi-solid medium. In the presence of M. smithii , the sensitivity of the fungi to monensin and lasalocid was decreased. For example, the uptake of glucose by N. frontalis strain RE1 in the culture was reduced to 50% of the control value by monensin at 0.5 mUg/ml. In the presence of M. smithii strain PS, approximately three tunes as much monensin was needed to bring about the same effect. In similar tests, the sensitivity of strain RE1 to lasalocid was decreased about nine-fold in the presence of M. smithii. The effect was not observed if the methanogens were killed by autoclaving before inoculation. It is suggested that the enhanced resistance to ionophores in the presence of M. smithii is a consequence of changes in the energy metabolism of the fungi growing in co-culture.     

Characterization of proteins secreted by sheep oviduct epithelial cells and their function in embryonic development

The role in early development of proteins secreted by oviduct epithelial cells has been investigated. Secreted proteins devoid of serum contamination have been produced by the surgical removal and immediate incubation of oviduct cells in [35S]methionine-containing medium. After electrophoretic separation, secreted polypeptides could be divided into those that were secreted uniformly throughout the oestrous cycle and a second class that showed a cyclical pattern of secretion. The first class of proteins represented a small proportion of total output whilst the predominant second class was composed mainly of polypeptides of Mr 92 and 46 x 10(3), respectively. Both of these polypeptide species, referred to as sheep oviduct proteins 92 and 46 (SOP 92, SOP 46), are detected only during the first 4 to 5 days after oestrus when the embryos are located in the oviduct. Oviduct cells collected at oestrus and maintained thereafter in culture secrete the same pattern of proteins and follow the same time course as their counterparts in vivo. The interaction between the oviduct proteins and the developing embryo was studied firstly by determining whether any of the secreted proteins bound to the zona pellucida. The results of iodination studies showed that two polypeptides of Mr 92 and 46 x 10(3), respectively, were bound to the zona pellucida of eggs removed from the oviduct but were absent from eggs that had not had contact with the oviduct epithelium. That these newly acquired proteins represent SOP 92 and 46 is suggested by their electrophoretic mobility and their ability to bind to the zona of follicular eggs when added in vitro and by the fact that they both disappear from the zonae of embryos after exit from the oviduct. The collection of unlabelled secreted proteins enabled us to produce a monoclonal antibody, which was used in the second series of experiments on oviduct-embryo interactions. The results confirmed that SOP 92 binds to the zona pellucida and moreover showed that this protein crosses the zona and becomes associated with the individual blastomeres of the developing embryo. These findings provide evidence that the mammalian oviduct probably plays a direct role in supporting embryonic development through specific polypeptides produced by its epithelium.     

PDGF receptors on cells of the oligodendrocyte-type-2 astrocyte (O-2A) cell lineage

It has been shown previously that cultures of rat optic nerve contain three types of macroglial cells--oligodendrocytes and two types of astrocytes. Type-1 astrocytes develop from their own precursor cells beginning before birth, while oligodendrocytes and type-2 astrocytes develop postnatally from a common bipotential precursor called the O-2A progenitor cell. Proliferating O-2A progenitor cells give rise to postmitotic oligodendrocytes beginning around birth, and to type-2 astrocytes beginning in the second postnatal week. Studies in vitro have suggested that platelet-derived growth factor (PDGF), secreted by type-1 astrocytes, plays an important part in timing oligodendrocyte development: PDGF seems to keep O-2A progenitor cells proliferating until an intrinsic clock in the progenitor cells initiates the process leading to oligodendrocyte differentiation. The clock apparently determines when a progenitor cell becomes unresponsive to PDGF, at which point the cell stops dividing and, as a consequence, automatically differentiates into an oligodendrocyte. Here we have used radiolabelled PDGF to show that O-2A progenitor cells have PDGF receptors, suggesting that these cells respond directly to PDGF. The receptors resemble the type A PDGF receptor previously described on human fibroblasts and are initially retained when progenitor cells stop dividing and develop in vitro into oligodendrocytes. The latter finding indicates that receptor loss is not the reason that progenitor cells initially become mitotically unresponsive to PDGF.