Mechanisms for oscillation and frequency control in reciprocally inhibitory model neural networks

We describe four different mechanisms that lead to oscillations in a network of two reciprocally inhibitory cells. In two cases (intrinsic release and intrinsic escape) the frequency of the network oscillation is insensitive to the threshold voltage of the synaptic potentials. In the other two cases (synaptic release and synaptic escape) the network frequency is strongly determined by the threshold voltage of the synaptic connections. The distinction between the different mechanisms blurs as the function describing synaptic activation becomes less steep and as the model neurons are removed from the relaxation regime. These mechanisms provide insight into the parameters that control network frequency in motor systems that depend on reciprocal inhibition.     

Parallel β-domains: a new fold in protein structures

A new type of structural domain, composed of parallel β-strands folded into a coiled structure, has been observed in several protein structures within the past year. An analysis of the basic motif indicates that there are two distinct types, with variations likely to be discovered in the future.     

The Effect of Extracellular Matrix Molecules on the in Vitro Behavior of Bovine Endothelial Cells

Extracellular matrix (ECM) is an important mediator of endothelial functions such as adhesion, spreading, migration, proliferation, and maintenance of differentiated functions. Attachment of cultured cells to tissue culture polystyrene (TCPS) is dependent on vitronectin which adsorbs onto the surface from the serum in the culture medium. Vitronectin (VN) will adsorb efficiently to TCPS even if the latter has been coated with another matrix molecule and blocked with albumin. This means that studies of the interactions of cells with individual coated ECM molecules will be confounded by the presence of adsorbed VN if serum is present in the culture medium. In this study, the adhesion, spreading, growth, and output of endogenous matrix molecules by bovine corneal endothelial (BCE) cells were measured on five different matrix substrates using medium which had been depleted of vitronectin to avoid such confounding effects. The same cell adhesion and spreading maxima were achieved on vitronectin, fibronectin (FN), laminin (LM), and types I and IV collagen (col I, col IV). The coating concentrations required to achieve these maxima, however, differed among the substrates, LM needing considerably higher concentrations than the other substrates for both maximal adhesion and spreading and FN needing higher concentrations for cell spreading. When cells were continuously passaged on each of the five substrates coated at concentrations optimal for cell spreading, no differences in cell proliferation rates or cell morphology were observed. Significant differences, however, were observed in the subcellular output of endogenous matrix molecules (FN, LM, col IV, and thrombospondin) between the different substrates. Col I was a poor substrate for the production of all ECM molecules tested over the 10 passages of the experiment, whereas col IV was a consistently good substrate. LM and FN substrates displayed differential effects on the output of different ECM molecules. VN was unique in that BCE cells at early passage on this substrate produced high levels of endogenous matrix molecules, whereas with continued passage on this substrate, a progressive decline in ECM secretion was observed. These results show that incorporation of individual molecules into the ECM by BCE cells in culture is significantly affected by the nature of the substratum. They further suggest that passage of endothelial cells in media containing serum (which results in coating of VN onto the substrate) may result in a progressive reduction of ECM output.     

Analysis of developmental switching in transgenic mice with 5′ and 3′ deletions in the human β globin gene

Our interest in thecis-acting elements that promote the up-regulation of the globin gene has led to a systematic deletion analysis of portions of the globin gene in the context of the HS2 and globin gene using transgenic mice. In constructs that delete the 5 region to only 265 bp, high-level erythroid-specific expression was observed. Further deletion to 122 bp, however, results in significantly reduced expression levels A substitution of a minilocus control region for the single HS2 site was also produced, resulting in increased globin expression over that seen with the HS2 alone. These results are consistent with the presence of an enhancer-like element between –122 and –265. In addition, a construct in which the entire globin gene promoter was replaced by a thymidine kinase promoter was tested. Interestingly, no expression was detected in these transgenic mice. This may indicate the requirement for an erythroid-specific promoter to drive this gene. Finally, the 3 region of the globin gene was deleted in order to examine the effect of a previously defined 3 enhancer region. With deletion of this region, the expression of the human globin gene in transgenic mice is unchanged relative to the parental constructs.     

The dominant phosphoprotein pp65 (UL83) of human cytomegalovirus is dispensable for growth in cell culture.

The phosphoprotein pp65 (ppUL83) of human cytomegalovirus (HCMV) is abundantly synthesized during lytic infection in cultured fibroblasts. As a major constituent of extracellular particles, it gains entry to infected cells immediately after adsorption and subsequently translocates to the cell nucleus. This efficient transport is mediated by unique nuclear localization signals. To study the function of pp65, a viral deletion mutant was constructed by replacing the pp65 gene with the bacterial neomycin phosphotransferase gene, driven by the simian virus 40 early promoter. The resulting virus, RVAd65, could be grown and selected on human fibroblasts without complementation. The deletion of the pp65 gene in RVAd65 was verified by using Southern blot and PCR analyses. The lack of expression from the gene was investigated by immunoblotting with pp65-specific monoclonal antibodies. Single-cycle growth analyses showed that RVAd65 grew to levels of infectivity comparable to those of the wild-type virus. Therefore, pp65 is nonessential for the growth of HCMV in human fibroblasts. Electron microscopy revealed no differences in the processes of virion morphogenesis, although the maturation appeared to be delayed. However, the kinetics of expression of the immediate-early genes UL122 and UL123, the early gene UL44, and the late gene UL32 were the same in RVAd65-infected cells as in wild-type virus-infected cells in immunoblot analyses. In vitro phosphorylation assays showed that some of the virion proteins were labelled to a markedly reduced extent by virion-associated kinases in RVAd65 compared with wild-type virus. We therefore conclude that although deletion of the pp65 gene does not abolish replication of HCMV, a recombinant virus lacking pp65 displays phenotypic alterations compared with wild-type virus during growth in cultured fibroblasts.     

How Does the Crayfish Swimmeret System Work? Insights from Nearest-Neighbor Coupled Oscillator Models

Rhythmic movements of crayfish swimmerets are coordinated by a neural circuit that links their four abdominal ganglia. Each swimmeret is driven by its own small local circuit, or pattern-generating module. We modeled this networkas a chain of four oscillators, bidirectionally coupled to their nearest neighbors, and tested the models ability to reproduce experimentally observed changes in intersegmental phases and in period caused by differential excitation of selected abdominal ganglia. The choices needed to match the experimental data lead to the followingpredictions: coupling between ganglia is asymmetric; the ascending and descending coupling have approximately equal strengths; intersegmental coupling does not significantly affect the frequency of the system; and excitation affects the intrinsic frequencies of the oscillators and might also change properties ofintersegmental coupling.     

Evolutionarily stable communication between kin: a general model

At present, the most general evolutionary theory of honest communication is Grafen''s model of Zahavi''s ''handicap'' signalling system, in which honesty of signals about the signaller''s quality (e.g. mate suitability or fighting ability) is maintained by the differentially high cost of signals to signallers having lower quality. The latter model is here further generalized to include any communication between signallers and receivers that are genetically related (e.g. parents and begging offspring, cooperative or competing siblings). Signalling systems involving relatives are shown to be evolutionarily stable, despite a potential pay-off for false signalling, if the Zahavian assumption of differential signal costs holds and there are diminishing reproductive returns to the signaller as the receiver''s assessed value of its attribute increases, or if, regardless of whether the Zahavian assumption holds, signallers with high values of the attribute benefit more from a given receiver assessment than signallers with low values (e.g. begging chicks that are hungrier benefit more from being fed). In stable systems of signalling among kin, it is also shown to be generally true that (i) levels of signalling and thus observed signal costs will decline as relatedness increases or as the receiver''s reproductive penalty for erroneous assessment increases, and (ii) receivers will consistently, altruistically overestimate the true value of the signalled attribute.