Liquid-ordered microdomains in lipid rafts and plasma membrane of U-87 MG cells: a time-resolved fluorescence study

Lipid rafts, the functional microdomains in the cell membrane, are believed to exist as liquid-ordered (Lo) phase domains along with the liquid-disordered (Ld) phase of the bulk of the cell membranes. We have examined the lipid order in model and natural membranes by time-resolved fluorescence of trimethylammonium-1,6-diphenylhexatriene incorporated into the membranes. The lipid phases were discerned by the limiting anisotropy, rotational diffusion rate and distribution of the fluorescence lifetime. In dipalmitoylphosphatidylcholine (DPPC)-cholesterol mixtures the gel phase exhibited higher anisotropy and a two-fold slower rotational diffusion rate of the probe as compared to the Ld phase. On the other hand, the Lo phase exhibited higher limiting anisotropy but a rotational diffusion rate comparable to the Ld phase. The Ld and Lo phases elicited unimodal distribution of lifetimes with distinct mean values and their co-existence in phospholipid-cholesterol mixtures was reflected as a biphasic change in the width of the lifetime distribution. Global analysis of the lifetimes yielded a best fit with two lifetimes which were identical to those observed in single Lo or Ld phases, but their fractional contribution varied with cholesterol concentration. Attributing the shorter and longer lifetime components to the Ld and Lo phases, respectively, the extent of the Lo/Ld phase domains in the membranes was estimated by their fractional contribution to the fluorescence decay. In ternary mixtures of egg PC-gangliosides-cholesterol, the gangliosides induced heterogeneity in the membrane but the Ld phase prevailed. The Lo phase properties were observed only in the presence of cholesterol. Results obtained in the plasma membrane and detergent-resistant membrane fractions (DRMs) isolated from U-87 MG cells revealed that DRMs mainly possess the Lo phase; however, a substantially large proportion of plasma membrane also exists in the Lo phase. Our data show that, besides cholesterol, the membrane proteins play a significant role in the organization of lipid rafts and, furthermore, a considerable amount of heterogeneity is present among the lipid rafts.     

Design and synthesis of factor Xa inhibitors and their prodrugs

In addition to our previously reported fluoro acrylamides Xa inhibitors 2 and 3, a series of potent and novel cyclic diimide amidine compounds has been identified. In efforts to improve their oral bioavailability, replacement of the amidine group with methyl amidrazone gives compounds of moderate potency (14, IC(50)=0.028 microM). In the amidoxime prodrug approach, the amidoxime compounds show good oral bioavailability in rats and dogs. High plasma level of prodrug 26 and significant concentration of active drug 26a were obtained upon oral administration of prodrug 26 in rats.     

A proposed structural model for amyloid fibril elongation: domain swapping forms an interdigitating beta-structure polymer

We propose a model illustrating how proteins, which differ in their overall sequences and structures, can form the propagating, twisted beta-sheet conformations, characteristic of amyloids. Some cases of amyloid formation can be explained through a "domain swapping" event, where the swapped segment is either a beta-hairpin or an unstable conformation which can partially unfold and assume a beta-hairpin structure. As in domain swapping, here the swapped beta-hairpin is at the edge of the structure, has few (if any) salt bridges and hydrogen bonds connecting it to the remainder of the structure and variable extents of buried non-polar surface areas. Additionally, in both cases the swapped piece constitutes a transient "building block" of the structure, with a high population time. Whereas in domain swapping the swapped fragment has been shown to be an alpha-helix, loop, strand or an entire domain, but so far not a beta-hairpin, despite the large number of cases in which it was already detected, here swapping may involve such a structural motif. We show how the swapping of beta-hairpins would form an interdigitated, twisted beta-sheet conformation, explaining the remarkable high stability of the protofibril in vitro. Such a swapping mechanism is attractive as it involves a universal mechanism in proteins, critical for their function, namely hinge-bending motions. Our proposal is consistent with structural superpositioning of mutational variants. While the overall r.m.s.d.s of the wild-type and mutants are small, the proposed hinge-bending region consistently shows larger deviations. These larger deviations illustrate that this region is more prone to respond to the mutational changes, regardless of their location in the sequence or in the structure. Nevertheless, above all, we stress that this proposition is hypothetical, since it is based on assumptions lacking definitive experimental support.     

Design, synthesis and structure-activity relationship of a series of arginine aldehyde factor Xa inhibitors. Part 1: structures based on the (D)-Arg-Gly-Arg tripeptide sequence

A series of arginine aldehyde inhibitors was designed as transition state (TS) analogues based on the known factor Xa specific substrate Cbz-D-Arg-Gly-Arg-pNA. BnSO2-(D)Arg-Gly-Arg-H (20) was found to be the most potent and selective inhibitor of factor Xa and prothrombinase activity in this series.     

An overview of telemedicine: the virtual gaze of health care in the next century

This article considers the role of telemedicine in the production of biomedical health care using three specific theoretical constructs as lenses through which to examine this phenomenon: (1) Foucault's medical "gaze"; (2) the political economy of health; and (3) deterritorialization and multisite ethnography. This examination focuses first on the changing corporate structure of health care and changing political attitudes toward telemedicine. Second, it documents the current use of telemedicine in prisons, the military, and in cross-cultural settings. Third, it discusses responses to telemedicine of individual physicians, health care staff, and patients, finding that these responses are broadly conditioned by an individual's mode of articulation with biomedical institutions and that they are mediated by personal experience.     

Factors determining the midday depression of photosynthesis in trees under monsoon climate

 Field studies of gas exchange of Populus deltoides, Prosopis juliflora and Acacia auriculiformis showed large diurnal changes in net photosynthesis (A) and stomatal conductance (g_s) during autumn. P. deltoides and P. juliflora undergo pronounced midday depression in A and g_s while A. auriculiformis showed a one-peak response. Several factors indicative of photosynthetic performance were found to be reversibly affected during afternoon decline. These include (i) decrease in initial slope of the CO₂ response curve (carboxylation efficiency), (ii) substantial increase in CO₂ compensation point and (iii) decrease in overall quantum yield of photosystem II. The phenomenon can be duplicated in potted plants by simulating a typical daily pattern of PPFD and VPD. It is found that high VPD induces significant decline in A and g_s at moderate temperature and saturating PPFD (800 μmol m^–2 s^–1) whereas these parameters are only marginally affected at high PPFD and low VPD. Fluorescence data show that the tree species under study have a high capacity for safe dissipation of excessive excitation energy. The activation of photorespiration, as evident from an increase in CO₂ compensation point, maintains constant internal CO₂ concentration (C_i) which may aid in minimizing photoinhibition during stomatal closure at midday. In case of P. deltoides and P. juliflora the stomata seem to be quite sensitive to the changes in humidity whereas this does not appear to be essential in case of A. auriculiformis because of its phyllode structure that endows it with mechanisms for conserving water without undergoing large-scale stomatal changes. Received: 16 October 1997 / Accepted: 5 March 1998     

Knowledge acquired and decisions made: triadic interactions during allogrooming in wild bonnet macaques,Macaca radiata.

The pressures of developing and maintaining intricate social relationships may have led to the evolution of enhanced cognitive abilities in many nonhuman primates. Knowledge of the dominance ranks and social relationships of other individuals, in particular, is important in evaluating one''s position in the rank hierarchy and affiliative networks. Triadic interactions offer an excellent opportunity to examine whether decisions are taken by individuals on the basis of such knowledge. Allogrooming supplants among wild female bonnet macaques (macaca radiata) usually involved the subordinate female of a grooming dyad retreating at the approach of a female dominant to both members of the dyad. In a few exceptional cases, however, the dominant member of the dyad retreated; simple non-cognitive hypotheses involving dyadic rank differences and agonistic relationships failed to explain this phenomenon. Instead, retreat by the dominant individual was positively correlated with the social attractiveness of her subordinate companion (as measured by the duration of grooming received by the latter from other females in the troop). This suggests that not only does an individual evaluate relationships among other females, but does so on the basis of the amount of grooming received by them. Similarly, the frequency of approaches received by any female was correlated with her social attractiveness when she was the dominant member of the dyad, but not when she was the subordinate. This indicated that approaching females might be aware of the relative dominance ranks of the two allogrooming individuals. In logistic regression analyses, the probability of any individual retreating was found to be influenced more by her knowledge of her rank difference with both the other interactants, rather than by their absolute ranks. Moreover, information about social attractiveness appeared to be used in terms of correlated dominance ranks. The nature of knowledge acquired by bonnet macaque females may thus be egotistical in that other individuals are evaluated relative to oneself, integrative in that information about all other interactants is used simultaneously, and hierarchical in the ability to preferentially use certain categories of knowledge for the storage of related information from other domains.