The modelling of fibre reorientation in soft tissue

In this paper, a hyperelastic and thermodynamically consistent model for soft tissue is developed that is able to describe the change of the initial orientation of the collagen fibres. Full numerical implementation is considered as well. The collagen architecture is assumed to reorient driven by a specific thermodynamical force. The anisotropy is described by a strain energy function, which is decomposed into a part related to the matrix and a part related to the fibres. The initial fibre orientation is defined by a structural tensor, while the current orientation is described by a time-dependent structural tensor, which results from the initial one by a rotational transformation. The rotation tensor is obtained via an integration process of a rate tensor, which depends on an adequately defined thermodynamical force. The integration is achieved via an exponential map algorithm, where it is shown that the rotation is necessarily a two-parametric one. Efficiency of the proposed formulation is demonstrated using some numerical examples.     

Correlation between transglutaminase activity and polyamine levels in human neuroblastoma cells. Effect of retinoic acid and alpha-difluoromethylornithine

The human neuroblastoma cell line SK-N-BE can be induced to differentiate by retinoic acid (RA) or by alpha-difluoromethylornithine (DFMO). The former inducer produces neurite outgrowth, 60% reduction of growth rate, overexpression of neural antigens, and enhanced gamma-aminobutyric acid (GABA) and acetylcholinesterase levels. In contrast, DFMO causes cell body elongation, complete growth inhibition, and higher binding of antibodies directed against neuroectodermal antigens. Polyamine metabolism is also differently affected by the two agents. In particular a large spermine catabolism is induced by RA, while DFMO treatment leads to a small increase in the level of this compound. The neural differentiation induced by RA is accompanied by a marked increase in transglutaminase activity and its induction is paralleled by a transient increase of putrescine and spermidine. The putrescine and spermidine depletion determined by DFMO is accompanied instead by a large inhibition of transglutaminase activity. The inhibiting effect of DFMO treatment on transglutaminase is reversed by the addition of 1 mM putrescine to the culture medium. In the presence of both RA and DFMO a mixed morphological and biochemical pattern is observed. The possibility that the expression of transglutaminase associated to cellular differentiation may be modulated by the level of its substrates is also discussed.     

Mathematical conservation ecology: A one-predator-two-prey system as case study

A method is presented to analyse the long-term stochastic dynamics of a biological population that is at risk of extinction. From the full ecosystem the method extracts the minimal information to describe the long-term dynamics of that population by a stochastic logistic system. The method is applied to a one-predator-two-prey model. The choice of this example is motivated by a study on the near-extinction of a porcupine population by mountain lions whose presence is facilitated by mule deer taking advantage of a change in land use. The risk of extinction is quantified by the expected time of extinction of the population.     

Models for the pressing rate response of rats and pigeons on a certain class of variable interval schedules of behavior

Two mathematical models are proposed for a certain class of variable interval schedules. These schedules are derived from the fixed interval schedule by adding (with probabilityp on any interval) reinforcing stimuli at a fixed point within the interval. The first model is a deterministic one in which the excitation producing the response is treated as a superposition of an excitation generated by the added reinforcements upon that generated by the fixed interval reinforcements. A method for estimating some of the parameters is presented. The model is applied with slight variations to data from performances by pigeons and by normal and brain-operated rats. The second model is a probabilistic one in which the basic entity is the interval between successive responses. This quantity must be represented by a probability distribution which is changing throughout the intervals between reinforcements. A solution is presented for a class of interresponse time distributions. Results from a simulation of a variant of this model are discussed briefly.     

A Slow Rotary Shaker for Embedding in Viscous Media

Embedding vessels are carried by a metal tray which has a tilt of about 20° to the horizontal plane. The tray is supported by a shaft attached perpendicular to its bottom. The shaft passes loosely through a hole in a rigid plate and is attached by a ball joint to a crank, which is driven at about 20 rev/min by a geared-down electric motor. Thus a combined tilting and rotary motion is imparted to the tray to produce a continuous flow of the embedding medium about the specimens.     

Length control of extended protein structures in bacteria and bacteriophages

The length of the tail of bacteriophages is controlled by a protein which acts as a molecular ruler. The needle of the injectisome, which is assembled by the polymerization of subunits that are exported through the nascent injectisome, is functionally related to the tail of bacteriophages. Interestingly, its length is controlled by a protein, which is itself exported and acts as a molecular ruler that is coupled to a substrate specificity switch. The bacterial flagellum is evolutionarily related to the injectisome. The length of the hook is also controlled by a secreted protein. This protein acts as a substrate specificity switch and, possibly, also as a ruler.     

Factors influencing the rate of `aging' of a series of alkyl methylphosphonyl-acetylcholinesterases

1. The progressive development of resistance to reactivation by an oxime (;aging') shown by a series of alkyl methylphosphonyl-acetylcholinesterases is slow when the alkyl group is a primary alcohol, whether or not the carbon chain is branched, but is much more rapid if the alkyl group is a secondary or cyclic alcohol. 2. Aging is accelerated by increase of temperature or decrease of pH. 3. Aging is inhibited by the quaternary amine N-methylpyridinium iodide. 4. The results are discussed in relation to the role played by aging in the therapy of poisoning by organophosphorus compounds.     

The newt ortholog of CD59 is implicated in proximodistal identity during amphibian limb regeneration

The proximodistal identity of a newt limb regeneration blastema is respecified by exposure to retinoic acid, but its molecular basis is unclear. We identified from a differential screen the cDNA for Prod 1, a gene whose expression in normal and regenerating limbs is regulated by proximodistal location and retinoic acid: Prod 1 is the newt ortholog of CD59. Prod 1/CD59 was found to be located at the cell surface with a GPI anchor which is cleaved by PIPLC. A proximal newt limb blastema engulfs a distal blastema after juxtaposition in culture, and engulfment is specifically blocked by PIPLC, and by affinity-purified antibodies to two distinct Prod 1/CD59 peptides. Prod 1 is therefore a cell surface protein implicated in the local cell-cell interactions mediating positional identity.     

Lysine catabolism, an effective versatile regulator of lysine level in plants

Lysine is a nutritionally important essential amino acid, whose synthesis in plants is strongly regulated by the rate of its synthesis. Yet, lysine level in plants is also finely controlled by a super-regulated catabolic pathway that catabolizes lysine into glutamate and acetyl Co-A. The first two enzymes of lysine catabolism are synthesized from a single LKR/SDH gene. Expression of this gene is subject to compound developmental, hormonal and stress-associated regulation. Moreover, the LKR/SDH gene of different plant species encodes up to three distinct polypeptides: (i) a bifunctional enzyme containing the linked lysine-ketoglutarate (LKR) and saccharopine dehydrogenase (SDH) whose LKR activity is regulated by its linked SDH enzyme; (ii) a monofunctional SDH encoded by an internal promoter, which is a part of the coding DNA region of the LKR/SDH gene; and (iii) a monofunctional, highly potent LKR that is formed by polyadenylation within an intron. LKR activity in the bifunctional LKR/SDH polypeptide is also post-translationally regulated by phosphorylation by casein kinase-2 (CK2), but the consequence of this regulation is still unknown. Why is lysine metabolism super-regulated by synthesis and catabolism? A hypothesis addressing this important question is presented, suggesting that lysine may serve as a regulator of plant growth and interaction with the environment.     

Mechanogram profile of the guinea pig intestine. The supertone

The authors detected the possibility, during the phase of low tone contraction of isolated intestine, to develop a further contraction, a supertone (S), simply washing the preparation. It has been evidenced that S appears when the tonic phase is low; it does not disappear if the tone is increased lowering NaCl concentration in the saline plasma; is absent in the KCl induced contraction; is induced also by low doses of atropine; is decreased by digitalic agents. The results obtained lead the authors to conclude that: the reduced tonic phase is due to a positive factor, the Ca++ of the cytoplasm, and to a negative factor, the high level of cytoplasmic Na+. This high level of Na+ is maintained by a dynamic equilibrium with ion input through channels opened by muscarinic agent and output by a Na-K-ATP asi pump. The Na+ output is predominant and fraction of Ca++, no longer counterbalanced by Na+, causes the rise of S.     

Glutamine transport in brain mitochondria

Gln is transported into rat brain synaptic and non-synaptic mitochondria by a protein catalyzed process. The uptake is significantly higher in synaptic than in non-synaptic mitochondria. The transport is inhibited by the amino acids Glu, Asn and Asp, and by the TCA cycle intermediates succinate, malate and 2-OG. The inhibition by 2-OG is counteracted by AOA and is therefore assumed to be due to transamination of 2-OG, whereby Glu is formed. This presumes that Glu also binds to an inhibitory site on the matrix face of the inner membrane. The transport is complex and cannot be explained by the simple uniport mechanism which has been proposed for renal (Schoolwerth and LaNoue, 1985), and liver mitochondria (Soboll et al., 1991). Thus, Gln transport is stimulated by respiration and by the proton electrochemical gradient. Since it is indicated that both the neutral Gln zwitterion and the Gln anion are transported, there are probably different uptake mechanisms, but not necessarily different carriers. Gln may be transported by an electroneutral mechanism as a proton compensated anion, as well as electrophoretically as a zwitterion with a proton, and probably also by diffusion as a zwitterion. The properties of the brain mitochondrial Gln uptake mechanisms are also not identical with those of a purified renal Gln transporter. It is possible that the Gln transport is controlled by more than one protein, which may be situated on distinct species in a heterogeneous mitochondrial population. Since Gln is assumed to participate in energy production as well as in the synthesis of nucleic acid components and proteins in brain mitochondria, the control of Gln uptake in these organelles may be important.     

A novel phosphodiesterase I from the soluble fraction of erythrocytes

A previously unrecognized erythrocyte phosphodiesterase I with activity against thymidine-5'-monophospho-p-nitrophenyl ester is described. The enzyme is present in the soluble fraction of the erythrocyte, and was purified about 500-fold by chromatography using DEAE-cellulose, followed by gel chromatography with Sephadex G-200. Erythrocyte phosphodiesterase I has a molecular weight of about 70 000, when fully active as a monomer. Its pI is 5.4 and the pH optimum is 8.5. The Km value for thymidine-5'-monophospho-p-nitrophenyl ester is rather high, about 4 mmol/l. The enzyme has a barely detectable nucleotide pyrophosphatase activity. It is extremely sensitive to SH-inhibitors such as N-ethyl-maleimide, p-chloromercuribenzoate and disulphides (a reversible 50% inhibition was obtained by cystamine, 0.01 mmol/l). It is a metalloenzyme with loosely bound metal, and is stimulated by Mg2+. This activation by Mg2+ is counteracted by Zn2+. Gel chromatography revealed that the enzyme is a monomer in the presence of Mg2+. When inhibited by Zn2+, it forms polymers that can be reconverted to the monomer by thiols. All of the above properties of the erythrocyte enzyme support the conclusion that it is different from plasma membrane phosphodiesterase I (oligonucleate 5'-nucleotidohydrolase, EC 3.1.4.1).     

Control Mechanisms at the Level of FDP-Aldolases in Algae

Abstract

The FDP-aldolase I from Euglena gracilis is a four-chain enzyme, as shown by the amino acid analysis of the C and N terminals. The protein is dissociated by acid pH to a monomer. The reassociation-dissociation process goes through a dimer stage. The enzyme, inhibited either by mercurials or by ATP, is dissociated to a dimer. This process is readly reversible either by mercaptoethanol and by AMP.     

Time course of color induction in the honeybee

Color induction in the honeybee is investigated in color discrimination experiments. An individual bee walks in a dark arena and is trained to a self-luminant stimulus presented from below. In the dual-choice tests the dark background is replaced by a colored induction stimulus. Choice behavior is recorded by TV camera and analyzed by computer. Successive color induction is separated from simultaneous induction by analysis of the walking paths. Only successive color induction occurs. Simultaneous effects are not observed. That is a stimulus acts as a color inducing stimulus only when the bee crosses this stimulus. Thus, the color perceived by a given eye region is found to be dependent on the viewing history, but not on the stimuli presented simultaneously on neighboring parts of the retina. Color induction in the honeybee described in terms of selective sensitivity decrease (adaptation) does not explain all behavioral effects induced by the stimulus. The time course of successive color induction is calculated from the exposure times to the induction stimulus and from the choice behavior. The data suggest that color induction is complete after a few seconds. Photoreceptor adaptation is sufficient to explain the observed time course.     

Translation mediated by the internal ribosome entry site of the cat-1 mRNA is regulated by glucose availability in a PERK kinase-dependent manner.

The cationic amino acid transporter, Cat-1, is a high affinity transporter of the essential amino acids, arginine and lysine. Expression of the cat-1 gene is known to be regulated by amino acid availability. It is shown here that cat-1 gene expression is also induced by Glc limitation, which causes a 7-fold increase in cat-1 mRNA, a 30-fold induction of Cat-1 protein levels, and a 4-fold stimulation of arginine uptake. Glc limitation is known to induce the unfolded protein response (UPR) by altering protein glycosylation in the endoplasmic reticulum (ER). The studies here demonstrate that synthesis of Cat-1 occurs during the UPR when global protein synthesis is inhibited. The 5'-UTR of the cat-1 mRNA contains an internal ribosomal entry site (IRES) that is activated by amino acid starvation by a mechanism that involves phosphorylation of the translation initiation factor, eukaryotic initiation factor 2alpha, by the GCN2 kinase. It is shown here that translation from the cat-1/IRES is also induced by Glc deprivation in a manner dependent upon phosphorylation of eukaryotic initiation factor 2alpha by the transmembrane ER kinase, PERK. Because PERK is a key constituent of the UPR, it is concluded that induction of cat-1 gene expression is part of the adaptive response of cells to ER stress. These results also demonstrate that regulation of IRES activity in cellular mRNAs is part of the mechanism by which the UPR protects cells from unfolded proteins in the ER.     

Electroinsertion and activation of the C-terminal domain of colicin A, a voltage gated bacterial toxin, into mammalian cell membranes

The C-terminal fragment of colicin, a protein that is highly soluble in aqueous solution, is spontaneously and irreversibly inserted into the membranes of mammalian cells, which are locally permeabilized by a transmembrane voltage increase. Insertion is detected by immunodetection. This is obtained by mixing the protein with electropermeabilized cells. The same result is observed by pulsing the colicin/cell mixture. Electroinsertion is therefore obtained for the first time with a multi-fragment spanning protein. The cell viability is not affected beyond the effect of electropermeabilization. A train of low voltage repetitive transmembrane modulation, which cannot trigger membrane permeabilization, is applied a day after the electroinsertion. This induces no effect on unmodified cells but triggers the lysis of cells in which colicin has been inserted by the first electropulsation. The low-level electrical treatment is high enough to trigger the voltage gated opening of colicin and to induce the associated toxicity. A transmembrane configuration of colicin is therefore obtained by electroinsertion. The toxic effect of their voltage gating is only obtained when a critical number of voltage gated channels are activated.     

Dual regulation of adenylate cyclase and guanylate cyclase: alpha 2-adrenergic signal transduction in adrenocortical carcinoma cells

Isolated adrenocortical carcinoma cells of rat contain alpha 2- and beta-adrenergic receptors. When these cells are incubated with alpha 2-adrenergic agonists, there is a concentration-dependent increase of cyclic GMP that is blocked by the alpha 2-adrenergic antagonist yohimbine but not by the beta-antagonist propranolol. Concomitantly, both p-aminoclonidine (20 microM) and clonidine (100 microM), the alpha 2-adrenergic agonists, stimulate membrane guanylate cyclase activity. In calcium free medium there is no alpha 2-agonist-dependent increase in cyclic GMP. Isoproterenol, a beta-agonist, and forskolin cause an increase in cyclic AMP but not cyclic GMP. The cyclic AMP increase induced by isoproterenol is blocked by propranolol but not by yohimbine. Isoproterenol- and forskolin-dependent increases in cyclic AMP are inhibited by p-aminoclonidine and the inhibition is relieved by yohimbine. These results indicate a dual regulation of guanylate cyclase and adenylate cyclase by the alpha 2-receptor signal: guanylate cyclase is coupled to the receptor in a positive fashion, whereas adenylate cyclase is coupled in a negative fashion. Calcium is obligatory in the cyclic GMP-mediated response.     

Lactose-proton symport by purified lac carrier protein

The lac carrier protein of Escherichia coli was purified by an improved procedure and its activity assayed by a rapid filter method. Following reconstitution of the carrier by octyl glucoside dilution, proteoliposomes were concentrated by filtration on a microporous filter. Lactose accumulation by adsorbed or entrapped proteoliposomes is driven by an artificially imposed pH gradient (interior alkaline), by a membrane potential (interior negative), or by a combination of both forces. Activity is almost completely abolished by the protonophore carbonyl cyanide m-chlorophenylhydrazone or by the competitive inhibitor thiodigalactoside. Addition of lactose to proteoliposomes under appropriate conditions results in alkalinization of the external medium. This effect is not observed with liposomes devoid of lac carrier or in the presence of proton conducting agents. The results provide a strong indication that the lac gamma gene product is the only protein in the cytoplasmic membrane of Escherichia coli required for lactose-proton symport.