Infectious Disease Modeling of Social Contagion in Networks

Many behavioral phenomena have been found to spread interpersonally through social networks, in a manner similar to infectious diseases. An important difference between social contagion and traditional infectious diseases, however, is that behavioral phenomena can be acquired by non-social mechanisms as well as through social transmission. We introduce a novel theoretical framework for studying these phenomena (the SISa model) by adapting a classic disease model to include the possibility for ‘automatic’ (or ‘spontaneous’) non-social infection. We provide an example of the use of this framework by examining the spread of obesity in the Framingham Heart Study Network. The interaction assumptions of the model are validated using longitudinal network transmission data. We find that the current rate of becoming obese is 2 per year and increases by 0.5 percentage points for each obese social contact. The rate of recovering from obesity is 4 per year, and does not depend on the number of non-obese contacts. The model predicts a long-term obesity prevalence of approximately 42, and can be used to evaluate the effect of different interventions on steady-state obesity. Model predictions quantitatively reproduce the actual historical time course for the prevalence of obesity. We find that since the 1970s, the rate of recovery from obesity has remained relatively constant, while the rates of both spontaneous infection and transmission have steadily increased over time. This suggests that the obesity epidemic may be driven by increasing rates of becoming obese, both spontaneously and transmissively, rather than by decreasing rates of losing weight. A key feature of the SISa model is its ability to characterize the relative importance of social transmission by quantitatively comparing rates of spontaneous versus contagious infection. It provides a theoretical framework for studying the interpersonal spread of any state that may also arise spontaneously, such as emotions, behaviors, health states, ideas or diseases with reservoirs.National Institutes of Health (U.S.) (grant R01GM078986)National Science Foundation (U.S.)Bill & Melinda Gates FoundationTempleton FoundationNational Institute on Aging (grant P01 AG031093)Framingham Heart Study (contract number N01-HC-25195

Transcriptional repressor DREAM regulates T-lymphocyte proliferation and cytokine gene expression

Downstream Regulatory Element Antagonist Modulator (DREAM) is a Ca(2+)-dependent transcriptional repressor expressed in the brain, thyroid gland and thymus. Here, we analyzed the function of DREAM and the related protein KChIP-2 in the immune system using transgenic (tg) mice expressing a cross-dominant active mutant (EFmDREAM) for DREAM and KChIPs Ca(2+)-dependent transcriptional derepression. EFmDREAM tg mice showed reduced T-cell proliferation. Tg T cells exhibited decreased interleukin (IL)-2, -4 and interferon (IFN)γ production after polyclonal activation and following antigen-specific response. Chromatin immunoprecipitation and transfection assays showed that DREAM binds to and represses transcription from these cytokine promoters. Importantly, specific transient knockdown of DREAM or KChIP-2 induced basal expression of IL-2 and IFNγ in wild-type splenocytes. These data propose DREAM and KChIP-2 as Ca(2+)-dependent repressors of the immune response

Ecophysiology of desiccation/rehydration cycles in mosses and lichens

Although both lichens and bryophytes are all poikilohydric the groups seem to behave very differently. Bryophytes also show a clear preference for wetter areas and this seems to be a result of the different structures of the organisms. A lichen is algae (or cyanobacteria) suspended in a mycobiont with excess water often having a negative effect on photosynthesis. Bryophytes, in contrast, are true multicellular plants and can construct photosynthetic tissues that can effectively separate their photosynthetic and water storage functions. Under dry atmospheric conditions lichens and bryophytes will desiccate to low water contents and they become dormant. Ability to tolerate desiccation varies considerably both between and within the groups. Somewhat surprisingly, lichens appear to show less ability to tolerate long periods of desiccation than bryophytes, and even some vascular plants. Actual mechanisms of desiccation have been best studied in bryophytes and appear to be constitutive, no protein synthesis is required on rehydration to enable the commencement of metabolism and the necessary protection appears to be always present. Consistently high sucrose levels, for instance are reported from bryophytes. Cellular structure is often maintained when desiccated. Recovery from dryness also differs between the groups with bryophytes generally hydrating more slowly but there are large species differences. In general, rate of recovery may be related to the length of the hydrated activity period, species that hydrate and then dry rapidly, as on rock surfaces, recover rapidly. Species in habitats that remain wet for long periods once hydrated appear to recover more slowly from dryness. In addition to a photosynthetic response to light and temperature, the poikilohydric lichens and bryophytes also have a photosynthetic response to thallus water content. Starting with a dry thallus, addition of water will both increase the thallus water content and also allow photosynthesis and respiration to commence. Both processes increase almost linearly with further hydration at low water contents. Photosynthesis reaches a maximum at an optimal thallus water content (WCopt) that is strongly species dependant. In both groups this photosynthetic optimum represents full cellular turgor. At water contents above this optimum surface or external water can interfere with carbon dioxide uptake and can severely limit photosynthetic rates, especially in lichens. When thallus water contents are normalised to WCopt = 1, then the net photosynthesis (NP) response curves at water contents below WCopt are very similar for liverworts, mosses and higher plants, suggesting a common mechanism in controlling NP. It is suggested that this might be an inhibitor acting on Rubisco activity. In contrast to vascular plants both groups can carry out photosynthesis at lower, suboptimal thallus water contents and very low water potentials but the contribution that this makes to total carbon budget appears to be a major difference between the groups. Bryophytes seem to pass rapidly through this water content range when both drying and hydrating for tens of minutes are often enough. In contrast, it is now apparent that lichens are often active at low thallus water contents. They can not only hydrate from humid air alone, or from dew and fog, but can use these water sources very effectively, often achieving a major part of their annual carbon gain. Information on when the lichens and bryophytes are actually active is only recently starting to appear but, again, the groups seem to differ. Bryophytes strongly prefer wetter habitats and can be active and fully hydrated for long periods and seem to have excellent capacity to tolerate high light and UV radiation when wet. In contrast many lichens, in particular those with green algal symbionts, rarely seem to be hydrated for long periods, especially in high light conditions, and rapidly dry out. Lichens seem to be active mainly under suboptimal conditions one of which is suboptimal water content