EphrinB3 is an anti-apoptotic ligand that inhibits the dependence receptor functions of EphA4 receptors during adult neurogenesis

Eph receptors have been implicated in regulating a diverse array of cellular functions in the developing nervous system. Recently, Eph receptors have been shown to promote cell death in adult germinal zones; however, their mechanisms of action remain ill-defined. In this study, we demonstrate that EphA4 is a new member of the dependence receptors family, which can initiate cell death in the absence of its ligand ephrinB3. Upon removal of its ligand, EphA4 triggers cell death that is dependent on caspase activation as caspase inhibitors prevent cell death. EphA4 itself is cleaved by caspase-3-like caspase in the intracellular domain at position D773/774, which is necessary for cell death initiation as mutation of the cleavage site abolishes apoptosis. In the adult subventricular zone, abolishing ephrinB3 results in increased cell death, while the absence of EphA4 results in excessive numbers of neuroblasts. Furthermore, infusion of soluble ephrinB3 into the lateral ventricle reduced cell death, and together these results support a dependence role for EphA4 in adult neurogenesis.     

DNA synthesis in isolated mitochondria and mitochondrial extracts from wheat embryos

DNA synthesis was studied using purified wheat embryo mitochondria as well as mitochondrial lysates deprived of endogenous DNA. The optimal conditions for DNA synthesis are very similar in both systems: ATP stimulates dramatically mitochondrial DNA synthesis and magnesium is a better co-factor than manganese, contrary to what has been reported in animal mitochondrial systems. Wheat mitochondrial DNA synthesis is resistant to aphidicolin and strongly inhibited by dideoxythymidine triphosphate and ethidium bromide. Thus, the DNA polymerase involved in this system seems to be the same as that previously purified and characterized from wheat embryo mitochondria (Christopheet al., Plant Science Letters 21: 181, 1981). Two different approaches: restriction endonuclease digestion followed by electrophoresis, and autoradiography and cesium chloride equilibrium centrifugation of mitochondrial DNA, where BrdUTP has been incorporated instead of TTP, show that long stretches of the mitochondrial genome have been synthesized.     

Kinetic co-operativity of monomeric mnemonical enzymes. The significance of the kinetic Hill coefficient

The expression of the kinetic Hill coefficient for a two-substrate, two-product mnemonical enzyme has been derived. Its relation with the gamma coefficient, that is the slope of the reciprocal plots for 1/[A]----O, has been established. The variation of this Hill coefficient, as a function of the second substrate and product concentrations, has been studied theoretically. Whereas the gamma coefficient does not vary as a function of the substrate and first product concentrations, the kinetic Hill coefficient does. If the enzyme is positively co-operative, the Hill coefficient increases upon increasing the second substrate concentration and decreases if the first product concentration is increased. The converse is expected to occur if the enzyme displays a negative co-operativity. The last product may either reverse a positive co-operativity into a negative one or, alternatively, strengthen an already negative co-operativity. The co-operativity generated by the mnemonical model has been compared to the kinetic behaviour of a random model. These two models have been shown to be discriminated on the basis of the departure they show with respect to the Michaelis-Menten behaviour. These theoretical considerations have been applied to previously published data, obtained with wheat germ hexokinase LI. This monomeric enzyme has a negative co-operativity with respect to the preferred substrate, glucose. The Hill coefficient decreases with MgATP concentration, increases with MgADP concentration and decreases with glucose-6-phosphate concentration. This is exactly what is to be expected on the basis of the above theory of kinetic co-operativity.     

Temperature-jump method for studying the fast transport of Na+ by (221) C10-cryptand across lipid membranes.

The kinetics of Na+ transport by (221)C10-cryptand through thin lipid membranes were determined by performing temperature-jump relaxation experiments on large unilamellar vesicles (L.U.V.) loaded with a fluorescent pH indicator. Applying temperature jumps of 4 to 7 degrees C to liposomes having phosphate as internal buffer and Tris as external buffer resulted in transmembrane delta pH's of about 0.104 to 0.182. After a temperature-jump, a decay in the delta pH was observed which corresponded to a Na+/H+ exchange occurring through membranes in the simultaneous presence of the cryptand and a proton carrier. The transport of Na+ ions by (221)C10 was found to be a fast kinetic process. Its initial rate increased with both the temperature and the cryptand concentrations. In addition, the temperature-induced changes in the apparent rate constants of the translocation of Na+ by (221)C10 were carrier concentration-dependent, and the apparent activation energy required to activate the transport decreased significantly with increasing cryptand concentrations. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.     

Marine phytoplankton: how many species in the world ocean?

Towards the end of the 1980s, living plankton flora of the worldocean amounted to 474-504 genera and 3444–4375 speciesif one neglects the increase rate of taxa during the latestyears In the above figures, the lower estimate is that of the‘reliable’ taxa (of practical value for identificationtasks), whereas the higher estimate includes the insufficientlyknown or doubtful organisms, and synonyms are excluded. Thefrequency distribution of the numbers of species per genus confirmsthe general hyperbolic law, which implies that a relativelylarge number of genera are uni- or paucispecific.     

pH-induced co-operative effects in hysteretic enzymes. 1. A theoretical model of a new type of co-operative behaviour controlled by pH

A new model which provides an explanation for pH-induced co-operativity of hysteretic enzymes is proposed. The essence of the model is that a region, or a domain, of the enzyme undergoes a spontaneous 'slow' conformational change which does not affect the geometry of the active site. The region which undergoes this spontaneous conformational transition bears an ionizable group. Kinetic co-operativity occurs if the pK of this ionizable group changes upon this conformational transition. Thus co-operativity does not arise from a distortion of the active site. An interesting prediction of the model is that at 'extreme' pH values co-operativity must be suppressed. Although the kinetic equation pertaining to the model is of the 2:2 type, co-operativity is not maximum or minimum at half-saturation of the enzyme by the substrate, as occurs with 2:2 binding isotherms. A new index of maximum or minimum kinetic co-operativity, whether this extreme occurs at half-saturation or not, has been proposed which allows the change of kinetic co-operativity to be followed as a function of pH. It is believed that this model will be useful in explaining the behaviour of enzymes attached to biological polyelectrolytes, such as membranes or cell envelopes.