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1. Chaperone-Mediated Stress Sensing in Mycobacterium tuberculosis Enables Fast Activation and Sustained Response

   https://doi.org/10.1128/MSYSTEMS.00979-20  

   Rao, Satyajit D. ; Datta, Pratik ; Gennaro, Maria Laura ; Igoshin, Oleg A. ( February 2021 , mSystems)

   Singh, Yogendra (Ed.)

   ABSTRACT Dynamical properties of gene regulatory networks are tuned to ensure bacterial survival. In mycobacteria, the MprAB-σ E network responds to the presence of stressors, such as surfactants that cause surface stress. Positive feedback loops in this network were previously predicted to cause hysteresis, i.e., different responses to identical stressor levels for prestressed and unstressed cells. Here, we show that hysteresis does not occur in nonpathogenic Mycobacterium smegmatis but does occur in Mycobacterium tuberculosis . However, the observed rapid temporal response in M. tuberculosis is inconsistent with the model predictions. To reconcile these observations, we implement a recently proposed mechanism for stress sensing, namely, the release of MprB from the inhibitory complex with the chaperone DnaK upon the stress exposure. Using modeling and parameter fitting, we demonstrate that this mechanism can accurately describe the experimental observations. Furthermore, we predict perturbations in DnaK expression that can strongly affect dynamical properties. Experiments with these perturbations agree with model predictions, confirming the role of DnaK in fast and sustained response. IMPORTANCE Gene regulatory networks controlling stress response in mycobacterial species have been linked to persistence switches that enable bacterial dormancy within a host. However, the mechanistic basis of switching and stress sensing is not fully understood. In this paper, combining quantitative experiments and mathematical modeling, we uncover how interactions between two master regulators of stress response—the MprAB two-component system (TCS) and the alternative sigma factor σ E —shape the dynamical properties of the surface stress network. The result show hysteresis (history dependence) in the response of the pathogenic bacterium M. tuberculosis to surface stress and lack of hysteresis in nonpathogenic M. smegmatis . Furthermore, to resolve the apparent contradiction between the existence of hysteresis and fast activation of the response, we utilize a recently proposed role of chaperone DnaK in stress sensing. These result leads to a novel system-level understanding of bacterial stress response dynamics. 

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2. Quantifying the Annular Mode Dynamics in an Idealized Atmosphere

   https://doi.org/10.1175/JAS-D-18-0268.1  

   Hassanzadeh, Pedram ; Kuang, Zhiming ( April 2019 , Journal of the Atmospheric Sciences)

   The linear response function (LRF) of an idealized GCM, the dry dynamical core with Held–Suarez physics, is used to accurately compute how eddy momentum and heat fluxes change in response to the zonal wind and temperature anomalies of the annular mode at the quasi-steady limit. Using these results and knowing the parameterizations of surface friction and thermal radiation in Held–Suarez physics, the contribution of each physical process (meridional and vertical eddy fluxes, surface friction, thermal radiation, and meridional advection) to the annular mode dynamics is quantified. Examining the quasigeostrophic potential vorticity balance, it is shown that the eddy feedback is positive and increases the persistence of the annular mode by a factor of more than 2. Furthermore, how eddy fluxes change in response to only the barotropic component of the annular mode, that is, vertically averaged zonal wind (and no temperature) anomaly, is also calculated similarly. The response of eddy fluxes to the barotropic-only component of the annular mode is found to be drastically different from the response to the full (i.e., barotropic + baroclinic) annular mode anomaly. In the former, the eddy generation is significantly suppressed, leading to a negative eddy feedback that decreases the persistence of the annular mode by nearly a factor of 3. These results suggest that the baroclinic component of the annular mode anomaly, that is, the increased low-level baroclinicity, is essential for the persistence of the annular mode, consistent with the baroclinic mechanism but not the barotropic mechanism proposed in the previous studies.

    

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3. Avenues of reef-building coral acclimatization in response to rapid environmental change

   https://doi.org/10.1242/jeb.239319  

   Putnam, Hollie M. ( February 2021 , Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT The swiftly changing climate presents a challenge to organismal fitness by creating a mismatch between the current environment and phenotypes adapted to historic conditions. Acclimatory mechanisms may be especially crucial for sessile benthic marine taxa, such as reef-building corals, where climate change factors including ocean acidification and increasing temperature elicit strong negative physiological responses such as bleaching, disease and mortality. Here, within the context of multiple stressors threatening marine organisms, I describe the wealth of metaorganism response mechanisms to rapid ocean change and the ontogenetic shifts in organism interactions with the environment that can generate plasticity. I then highlight the need to consider the interactions of rapid and evolutionary responses in an adaptive (epi)genetic continuum. Building on the definitions of these mechanisms and continuum, I also present how the interplay of the microbiome, epigenetics and parental effects creates additional avenues for rapid acclimatization. To consider under what conditions epigenetic inheritance has a more substantial role, I propose investigation into the offset of timing of gametogenesis leading to different environmental integration times between eggs and sperm and the consequences of this for gamete epigenetic compatibility. Collectively, non-genetic, yet heritable phenotypic plasticity will have significant ecological and evolutionary implications for sessile marine organism persistence under rapid climate change. As such, reef-building corals present ideal and time-sensitive models for further development of our understanding of adaptive feedback loops in a multi-player (epi)genetic continuum. 

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4. Different paths, same destination: divergent action potential responses produce conserved cardiac fight‐or‐flight response in mouse and rabbit hearts

   https://doi.org/10.1113/JP278016  

   Wang, Lianguo ; Morotti, Stefano ; Tapa, Srinivas ; Francis Stuart, Samantha D. ; Jiang, Yanyan ; Wang, Zhen ; Myles, Rachel C. ; Brack, Kieran E. ; Ng, G. André ; Bers, Donald M. ; et al ( July 2019 , The Journal of Physiology)

   Cardiac electrophysiology and Ca²⁺handling change rapidly during the fight‐or‐flight response to meet physiological demands.

   Despite dramatic differences in cardiac electrophysiology, the cardiac fight‐or‐flight response is highly conserved across species.

   In this study, we performed physiological sympathetic nerve stimulation (SNS) while optically mapping cardiac action potentials and intracellular Ca²⁺transients in innervated mouse and rabbit hearts.

   Despite similar heart rate and Ca²⁺handling responses between mouse and rabbit hearts, we found notable species differences in spatio‐temporal repolarization dynamics during SNS.

   Species‐specific computational models revealed that these electrophysiological differences allowed for enhanced Ca²⁺handling (i.e. enhanced inotropy) in each species, suggesting that electrophysiological responses are fine‐tuned across species to produce optimal cardiac fight‐or‐flight responses.

   Sympathetic activation of the heart results in positive chronotropy and inotropy, which together rapidly increase cardiac output. The precise mechanisms that produce the electrophysiological and Ca²⁺handling changes underlying chronotropic and inotropic responses have been studied in detail in isolated cardiac myocytes. However, few studies have examined the dynamic effects of physiological sympathetic nerve activation on cardiac action potentials (APs) and intracellular Ca²⁺transients (CaTs) in the intact heart. Here, we performed bilateral sympathetic nerve stimulation (SNS) in fully innervated, Langendorff‐perfused rabbit and mouse hearts. Dual optical mapping with voltage‐ and Ca²⁺‐sensitive dyes allowed for analysis of spatio‐temporal AP and CaT dynamics. The rabbit heart responded to SNS with a monotonic increase in heart rate (HR), monotonic decreases in AP and CaT duration (APD, CaTD), and a monotonic increase in CaT amplitude. The mouse heart had similar HR and CaT responses; however, a pronounced biphasic APD response occurred, with initial prolongation (50.9 ± 5.1 ms att = 0 svs. 60.6 ± 4.1 ms att = 15 s,P < 0.05) followed by shortening (46.5 ± 9.1 ms att = 60 s,P = NSvs. t = 0). We determined the biphasic APD response in mouse was partly due to dynamic changes in HR during SNS and was exacerbated by β‐adrenergic activation. Simulations with species‐specific cardiac models revealed that transient APD prolongation in mouse allowed for greater and more rapid CaT responses, suggesting more rapid increases in contractility; conversely, the rabbit heart requires APD shortening to produce optimal inotropic responses. Thus, while the cardiac fight‐or‐flight response is highly conserved between species, the underlying mechanisms orchestrating these effects differ significantly.

    

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5. Temperature and nutrient conditions modify the effects of phenological shifts in predator–prey communities

   https://doi.org/10.1002/ecy.3704  

   Rudolf, V. H. W. ( May 2022 , Ecology)

   Although there is mounting evidence indicating that the relative timing of predator and prey phenologies determines the outcome of trophic interactions, we still lack a comprehensive understanding of how the environmental context (e.g., abiotic conditions) influences this relationship. Environmental conditions not only frequently drive shifts in phenologies, but they can also affect the very same processes that mediate the effects of phenological shifts on species interactions. Therefore, identifying how environmental conditions shape the effects of phenological shifts is key to predicting community dynamics across a heterogeneous landscape and how they will change with ongoing climate change in the future. Here I tested how environmental conditions shape the effects of phenological shifts by experimentally manipulating temperature, nutrient availability, and relative phenologies in two predator–prey freshwater systems (mole salamander–bronze frog vs. dragonfly larvae–leopard frog). This allowed me to (1) isolate the effects of phenological shifts and different environmental conditions; (2) determine how they interact; and (3) evaluate how consistent these patterns are across different species and environments. I found that delaying prey arrival dramatically increased predation rates, but these effects were contingent on environmental conditions and the predator system. Although nutrient addition and warming both significantly enhanced the effect of arrival time, their effect was qualitatively different across systems: Nutrient addition enhanced the positive effect of early arrival in the dragonfly–leopard frog system, whereas warming enhanced the negative effect of arriving late in the salamander–bronze frog system. Predator responses varied qualitatively across predator–prey systems. Only in the system with a strong gape limitation were predators (salamanders) significantly affected by prey arrival time and this effect varied with environmental context. Correlations between predator and prey demographic rates suggest that this was driven by shifts in initial predator–prey size ratios and a positive feedback between size‐specific predation rates and predator growth rates. These results highlight the importance of accounting for temporal and spatial correlations of local environmental conditions and gape limitation when predicting the effects of phenological shifts and climate change on predator–prey systems.

    

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