Guppies in large groups cooperate more frequently in an experimental test of the group size paradox

The volunteer's dilemma, in which a single individual is required to produce a public good, predicts that individuals in larger groups will cooperate less frequently. Mechanistically, this could result from trade-offs between costs associated with volunteering and costs incurred if the public good is not produced (nobody volunteers). During predator inspection, one major contributor to the cost of volunteering is likely increased probability of predation; however, a predator also poses a risk to all individuals if nobody inspects. We tested the prediction that guppies in larger groups will inspect a predator less than those in smaller groups. We also predicted that individuals in larger groups would perceive less threat from the predator stimulus because of the protective benefits of larger groups (e.g. dilution). Contrary to prediction, we found that individuals in large groups inspected more frequently than those in smaller groups, but (as predicted) spent less time in refuges. There was evidence that individuals in intermediate-sized groups made fewest inspections and spent most time in refuges, suggesting that any link between group size, risk and cooperation is not driven by simple dilution. Extensions of theoretical models that capture these dynamics will likely be broadly applicable to risky cooperative behaviour.

The evolution of dynamic and flexible courtship displays that reveal individual quality

Abstract

Sexual selection is a major force shaping morphological and behavioral diversity. Existing theory focuses on courtship display traits such as morphological ornaments whose costs and benefits are assumed be to fixed across individuals’ lifetimes. In contrast, empirically observed displays are often inherently dynamic, as vividly illustrated by the acrobatic dances, loud vocalizations, and vigorous motor displays involved in courtship behavior across a broad range of taxa. One empirically observed form of display flexibility occurs when signalers adjust their courtship investment based on the number of rival signalers. The predictions of established sexual selection theory cannot readily be extended to such displays because display expression varies between courtship events, such that any given display may not reliably reflect signaler quality. We thus lack an understanding of how dynamic displays coevolve with sexual preferences and how signalers should tactically adjust their display investment across multiple courtship opportunities. To address these questions, we extended an established model of the coevolution of a female sexual preference and a male display trait to allow for flexible, dynamic displays. We find that such a display can coevolve with a sexual preference away from their naturally selected optima, though display intensity is a weaker signal of male quality than for non-flexible displays. Furthermore, we find that males evolve to decrease their display investment when displaying alongside more rivals. This research represents a first step towards generalizing the findings of sexual selection theory to account for the ubiquitous dynamism of animal courtship.

Significance statement

Animal courtship displays are typically costly for survival: songs attract predators; dances are exhausting; extravagant plumage is cumbersome. Because of the trade-off between mating benefits and survival costs, displaying individuals often vary their displays across time, courting more intensely when the potential benefit is higher or the cost is lower. Despite the ubiquity of such adjustment in nature, existing theory cannot account for how this flexibility might affect the coevolution of displays with sexual preferences, nor for the patterns of tactical display adjustment that might result, because those models treat displays as static, with fixed costs and benefits. Generalizing a well-studied model of sexual selection, we find that a static display and a flexible display can evolve under similar conditions. Our model predicts that courtship should be less intense when more competitors are present.

Urban gulls show similar thermographic and behavioral responses to human shouting and conspecific alarm calls

Rapid population growth and the urbanization of modern environments are markedly increasing human-wildlife conflict. Wild animals in urban landscapes can benefit from exploiting human resources, but are also exposed to increased risk of human-caused injury, which should favor the ability to perceive and respond to human cues. Although it is well known that domesticated animals use human cues that may indicate threats, less is known about wild animals living in urban environments. Herring gulls (Larus argentatus) in urban landscapes have adapted kleptoparasitic behaviors to obtain human food, often resulting in negative interactions with humans. Here we quantified both the behavioral and physiological responses of free-living urban herring gulls to human shouting. We presented urban gulls with a fake human food item and played back recordings of either a man shouting, a natural stressor (i.e., conspecific alarm call), or a neutral stimulus (i.e., robin song). We recorded behavioral responses and used non-invasive infrared thermography to measure eye-region surface temperature changes associated with the avian physiological stress response. We found that gulls exposed to shouting and to conspecific alarm calls showed similar changes in behavior (indicating high levels of vigilance) and eye-region surface temperature (indicating physiological stress). Both responses were significantly stronger than the responses to robin song. Additionally, the behavioral and physiological responses were positively correlated across individuals. Our results demonstrate that urban-dwelling gulls respond to human shouting and conspecific alarm calls in a similar way, and suggest that infrared thermography is a viable technique to monitor stress responses in free-living birds.

An evolutionary perspective on stress responses, damage and repair

Variation in stress responses has been investigated in relation to environmental factors, species ecology, life history and fitness. Moreover, mechanistic studies have unravelled molecular mechanisms of how acute and chronic stress responses cause physiological impacts ('damage'), and how this damage can be repaired. However, it is not yet understood how the fitness effects of damage and repair influence stress response evolution. Here we study the evolution of hormone levels as a function of stressor occurrence, damage and the efficiency of repair. We hypothesise that the evolution of stress responses depends on the fitness consequences of damage and the ability to repair that damage. To obtain some general insights, we model a simplified scenario in which an organism repeatedly encounters a stressor with a certain frequency and predictability (temporal autocorrelation). The organism can defend itself by mounting a stress response (elevated hormone level), but this causes damage that takes time to repair. We identify optimal strategies in this scenario and then investigate how those strategies respond to acute and chronic exposures to the stressor. We find that for higher repair rates, baseline and peak hormone levels are higher. This typically means that the organism experiences higher levels of damage, which it can afford because that damage is repaired more quickly, but for very high repair rates the damage does not build up. With increasing predictability of the stressor, stress responses are sustained for longer, because the animal expects the stressor to persist, and thus damage builds up. This can result in very high (and potentially fatal) levels of damage when organisms are exposed to chronic stressors to which they are not evolutionarily adapted. Overall, our results highlight that at least three factors need to be considered jointly to advance our understanding of how stress physiology has evolved: (i) temporal dynamics of stressor occurrence; (ii) relative mortality risk imposed by the stressor itself versus damage caused by the stress response; and (iii) the efficiency of repair mechanisms.

Hot-headed peckers: thermographic changes during aggression among juvenile pheasants (Phasianus colchicus)

In group-living vertebrates, dominance status often covaries with physiological measurements (e.g. glucocorticoid levels), but it is unclear how dominance is linked to dynamic changes in physiological state over a shorter, behavioural timescale. In this observational study, we recorded spontaneous aggression among captive juvenile pheasants (Phasianus colchicus) alongside infrared thermographic measurements of their external temperature, a non-invasive technique previously used to examine stress responses in non-social contexts, where peripheral blood is redirected towards the body core. We found low but highly significant repeatability in maximum head temperature, suggesting individually consistent thermal profiles, and some indication of lower head temperatures in more active behavioural states (e.g. walking compared to resting). These individual differences were partly associated with sex, females being cooler on average than males, but unrelated to body size. During pairwise aggressive encounters, we observed a non-monotonic temperature change, with head temperature dropping rapidly immediately prior to an attack and increasing rapidly afterwards, before returning to baseline levels. This nonlinear pattern was similar for birds in aggressor and recipient roles, but aggressors were slightly hotter on average. Our findings show that aggressive interactions induce rapid temperature changes in dominants and subordinates alike, and highlight infrared thermography as a promising tool for investigating the physiological basis of pecking orders in galliforms. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Towards an Evolutionary Theory of Stress Responses

All organisms have a stress response system to cope with environmental threats, yet its precise form varies hugely within and across individuals, populations, and species. While the physiological mechanisms are increasingly understood, how stress responses have evolved remains elusive. Here, we show that important insights can be gained from models that incorporate physiological mechanisms within an evolutionary optimality analysis (the 'evo-mecho' approach). Our approach reveals environmental predictability and physiological constraints as key factors shaping stress response evolution, generating testable predictions about variation across species and contexts. We call for an integrated research programme combining theory, experimental evolution, and comparative analysis to advance scientific understanding of how this core physiological system has evolved.

Attractiveness is positively related to World Cup performance in male, but not female, biathletes

Whole-organism performance capacity is thought to play a key role in sexual selection, through its impacts on both intrasexual competition and intersexual mate choice. Based on data from elite sports, several studies have reported a positive association between facial attractiveness and athletic performance in humans, leading to claims that facial correlates of sporting prowess in men reveal heritable or nonheritable mate quality. However, for most of the sports studied (soccer, ice hockey, American football, and cycling), it is not possible to separate individual performance from team performance. Here, using photographs of athletes who compete annually in a multi-event World Cup, we examine the relationship between facial attractiveness and individual career-best performance metrics in the biathlon, a multidisciplinary sport that combines target shooting and cross-country skiing. Unlike all previous studies, which considered only male athletes, we report relationships for both sportsmen and sportswomen. As predicted by evolutionary arguments, we found that male biathletes were judged more attractive if (unknown to the raters) they had achieved a higher peak performance (World Cup points score) in their career, whereas there was no significant relationship for female biathletes. Our findings show that elite male athletes display visible, attractive cues that reliably reflect their athletic performance.

Trust your gut: using physiological states as a source of information is almost as effective as optimal Bayesian learning

Approaches to understanding adaptive behaviour often assume that animals have perfect information about environmental conditions or are capable of sophisticated learning. If such learning abilities are costly, however, natural selection will favour simpler mechanisms for controlling behaviour when faced with uncertain conditions. Here, we show that, in a foraging context, a strategy based only on current energy reserves often performs almost as well as a Bayesian learning strategy that integrates all previous experiences to form an optimal estimate of environmental conditions. We find that Bayesian learning gives a strong advantage only if fluctuations in the food supply are very strong and reasonably frequent. The performance of both the Bayesian and the reserve-based strategy are more robust to inaccurate knowledge of the temporal pattern of environmental conditions than a strategy that has perfect knowledge about current conditions. Studies assuming Bayesian learning are often accused of being unrealistic; our results suggest that animals can achieve a similar level of performance to Bayesians using much simpler mechanisms based on their physiological state. More broadly, our work suggests that the ability to use internal states as a source of information about recent environmental conditions will have weakened selection for sophisticated learning and decision-making systems.

Adaptive and non-adaptive models of depression: A comparison using register data on antidepressant medication during divorce

Divorce is associated with an increased probability of a depressive episode, but the causation of events remains unclear. Adaptive models of depression propose that depression is a social strategy in part, whereas non-adaptive models tend to propose a diathesis-stress mechanism. We compare an adaptive evolutionary model of depression to three alternative non-adaptive models with respect to their ability to explain the temporal pattern of depression around the time of divorce. Register-based data (304,112 individuals drawn from a random sample of 11% of Finnish people) on antidepressant purchases is used as a proxy for depression. This proxy affords an unprecedented temporal resolution (a 3-monthly prevalence estimates over 10 years) without any bias from non-compliance, and it can be linked with underlying episodes via a statistical model. The evolutionary-adaptation model (all time periods with risk of divorce are depressogenic) was the best quantitative description of the data. The non-adaptive stress-relief model (period before divorce is depressogenic and period afterwards is not) provided the second best quantitative description of the data. The peak-stress model (periods before and after divorce can be depressogenic) fit the data less well, and the stress-induction model (period following divorce is depressogenic and the preceding period is not) did not fit the data at all. The evolutionary model was the most detailed mechanistic description of the divorce-depression link among the models, and the best fit in terms of predicted curvature; thus, it offers most rigorous hypotheses for further study. The stress-relief model also fit very well and was the best model in a sensitivity analysis, encouraging development of more mechanistic models for that hypothesis.

Risk attitudes in a changing environment: An evolutionary model of the fourfold pattern of risk preferences

A striking feature of human decision making is the fourfold pattern of risk attitudes, involving risk-averse behavior in situations of unlikely losses and likely gains, but risk-seeking behavior in response to likely losses and unlikely gains. Current theories to explain this pattern assume particular psychological processes to reproduce empirical observations, but do not address whether it is adaptive for the decision maker to respond to risk in this way. Here, drawing on insights from behavioral ecology, we build an evolutionary model of risk-sensitive behavior, to investigate whether particular types of environmental conditions could favor a fourfold pattern of risk attitudes. We consider an individual foraging in a changing environment, where energy is needed to prevent starvation and build up reserves for reproduction. The outcome, in terms of reproductive value (a rigorous measure of evolutionary success), of a one-off choice between a risky and a safe gain, or between a risky and a safe loss, determines the risk-sensitive behavior we should expect to see in this environment. Our results show that the fourfold pattern of risk attitudes may be adaptive in an environment in which conditions vary stochastically but are autocorrelated in time. In such an environment the current options provide information about the likely environmental conditions in the future, which affect the optimal pattern of risk sensitivity. Our model predicts that risk preferences should be both path dependent and affected by the decision maker's current state.

Adaptive explanations for sensitive windows in development

Development in many organisms appears to show evidence of sensitive windows—periods or stages in ontogeny in which individual experience has a particularly strong influence on the phenotype (compared to other periods or stages). Despite great interest in sensitive windows from both fundamental and applied perspectives, the functional (adaptive) reasons why they have evolved are unclear. Here we outline a conceptual framework for understanding when natural selection should favour changes in plasticity across development. Our approach builds on previous theory on the evolution of phenotypic plasticity, which relates individual and population differences in plasticity to two factors: the degree of uncertainty about the environmental conditions and the extent to which experiences during development (‘cues’) provide information about those conditions. We argue that systematic variation in these two factors often occurs within the lifetime of a single individual, which will select for developmental changes in plasticity. Of central importance is how informational properties of the environment interact with the life history of the organism. Phenotypes may be more or less sensitive to environmental cues at different points in development because of systematic changes in (i) the frequency of cues, (ii) the informativeness of cues, (iii) the fitness benefits of information and/or (iv) the constraints on plasticity. In relatively stable environments, a sensible null expectation is that plasticity will gradually decline with age as the developing individual gathers information. We review recent models on the evolution of developmental changes in plasticity and explain how they fit into our conceptual framework. Our aim is to encourage an adaptive perspective on sensitive windows in development.

Adaptive learning can result in a failure to profit from good conditions: implications for understanding depression

BACKGROUND AND OBJECTIVES

Depression is a major medical problem diagnosed in an increasing proportion of people and for which commonly prescribed psychoactive drugs are frequently ineffective. Development of treatment options may be facilitated by an evolutionary perspective; several adaptive reasons for proneness to depression have been proposed. A common feature of many explanations is that depressive behaviour is a way to avoid costly effort where benefits are small and/or unlikely. However, this viewpoint fails to explain why low mood persists when the situation improves. We investigate whether a behavioural rule that is adapted to a stochastically changing world can cause inactivity which appears similar to the effect of depression, in that it persists after the situation has improved.

METHODOLOGY

We develop an adaptive learning model in which an individual has repeated choices of whether to invest costly effort that may result in a net benefit. Investing effort also provides information about the current conditions and rates of change of the conditions.

RESULTS

An individual following the optimal behavioural strategy may sometimes remain inactive when conditions are favourable (i.e. when it would be better to invest effort) when it is poorly informed about the current environmental state. Initially benign conditions can predispose an individual to inactivity after a relatively brief period of negative experiences.

CONCLUSIONS AND IMPLICATIONS

Our approach suggests that the antecedent factors causing depressed behaviour could go much further back in an individual s history than is currently appreciated. The insights from our approach have implications for the ongoing debate about best treatment options for patients with depressive symptoms.

Evolution of a flexible rule for foraging that copes with environmental variation

Models of adaptive behaviour typically assume that animals behave as though they have highly complex, detailed strategies for making decisions. In reality, selection favours the optimal balance between the costs and benefits of complexity. Here we investigate this trade-off for an animal that has to decide whether or not to forage for food – and so how much energy reserves to store – depending on the food availability in its environment. We evolve a decision rule that controls the target reserve level for different ranges of food availability, but where increasing complexity is costly in that metabolic rate increases with the sensitivity of the rule. The evolved rule tends to be much less complex than the optimal strategy but performs almost as well, while being less costly to implement. It achieves this by being highly sensitive to changing food availability at low food abundance – where it provides a close fit to the optimal strategy – but insensitive when food is plentiful. When food availability is high, the target reserve level that evolves is much higher than under the optimal strategy, which has implications for our understanding of obesity. Our work highlights the important principle of generalisability of simple decision-making mechanisms, which enables animals to respond reasonably well to conditions not directly experienced by themselves or their ancestors.

An adaptive response to uncertainty generates positive and negative contrast effects

Successive contrast effects, in which behavior is dependent on whether conditions are currently better or worse than they were before, are a striking illustration of the fact that animals evaluate the world in relative terms. Existing explanations for these effects are based on descriptive models of psychological and physiological processes, but little attention has been paid to the factors promoting their evolution. Using a simple and general optimality model, we show that contrast effects can result from an adaptive response to uncertainty in a changing, unpredictable world. A wide range of patterns of environmental change will select for sensitivity to past conditions, generating positive and negative contrast effects. Our analysis reveals the importance of incorporating uncertainty and environmental stochasticity into models of adaptive behavior.

Comparing pleasure and pain: the fundamental mathematical equivalence of reward gain and shock reduction under variable interval schedules

The relationship between positive and negative reinforcement and the symmetry of Thorndike's law of effect are unresolved issues in operant psychology. Here we show that, for a given pattern of responding on variable interval (VI) schedules with the same programmed rate of food rewards (positive reinforcement VI) or electric shocks (negative reinforcement VI), there is a fundamental mathematical equivalence between reward gain and shock reduction. We also provide the first normative account of how animals should respond on a negative VI schedule, showing that it is better to space responses evenly than to respond with a variable interresponse time (IRT). Published data from rats, however, indicate that these animals respond irregularly, often with a burst of activity immediately following a shock. While this is irrational in the experimental setting, it may represent an appropriate response to the heterogeneity of stimuli commonly encountered in natural environments. We discuss the broader implications of our analysis for understanding how animals evaluate appetitive and aversive stimuli.

Individual variation and the resolution of conflict over parental care in penduline tits

Eurasian penduline tits (Remiz pendulinus) have an unusually diverse breeding system consisting of frequent male and female polygamy, and uniparental care by the male or the female. Intriguingly, 30 to 40 per cent of all nests are deserted by both parents. To understand the evolution of this diverse breeding system and frequent clutch desertion, we use 6 years of field data to derive fitness expectations for males and females depending on whether or not they care for their offspring. The resulting payoff matrix corresponds to an asymmetric Snowdrift Game with two alternative evolutionarily stable strategies (ESSs): female-only and male-only care. This, however, does not explain the polymorphism in care strategies and frequent biparental desertion, because theory predicts that one of the two ESSs should have spread to fixation. Using a bootstrapping approach, we demonstrate that taking account of individual variation in payoffs explains the patterns of care better than a model based on the average population payoff matrix. In particular, a model incorporating differences in male attractiveness closely predicts the observed frequencies of male and female desertion. Our work highlights the need for a new generation of individual-based evolutionary game-theoretic models.

Is optimism optimal? Functional causes of apparent behavioural biases

We review the use of the terms 'optimism' and 'pessimism' to characterize particular types of behaviour in non-human animals. Animals can certainly behave as though they are optimistic or pessimistic with respect to specific motivations, as documented by an extensive range of examples in the literature. However, in surveying such examples we find that these terms are often poorly defined and are liable to lead to confusion. Furthermore, when considering behaviour within the framework of optimal decision theory using appropriate currencies, it is often misleading to describe animals as optimistic or pessimistic. There are two common misunderstandings. First, some apparent cases of biased behaviour result from misidentifying the currencies and pay-offs the animals should be maximising. Second, actions that do not maximise short-term pay-offs have sometimes been described as optimistic or pessimistic when in fact they are optimal in the long term; we show how such situations can be understood from the perspective of bandit models. Rather than describing suboptimal, unrealistic behaviour, the terms optimism and pessimism are better restricted to informal usage. Our review highlights the importance of choosing the relevant currency when attempting to predict the action of natural selection.

Are high-quality mates always attractive?: State-dependent mate preferences in birds and humans

Sexual selection theory posits that females should choose mates in a way that maximizes their reproductive success. But what exactly is the optimal choice? Most empirical research is based on the assumption that females seek a male of the highest possible quality (in terms of the genes or resources he can provide), and hence show directional preferences for indicators of male quality. This implies that attractiveness and quality should be highly correlated. However, females frequently differ in what they find attractive. New theoretical and empirical insights provide mounting evidence that a female's own quality biases her judgement of male attractiveness, such that male quality and attractiveness do not always coincide. A recent experiment in songbirds demonstrated for the first time that manipulation of female condition can lead to divergent female preferences, with low-quality females actively preferring low-quality males over high-quality males. This result is in line with theory on state-dependent mate choice and is reminiscent of assortative mating preferences in humans. Here we discuss the implications of this work for the study of mate preferences.

Men with elevated testosterone levels show more affiliative behaviours during interactions with women

Testosterone (T) is thought to play a key role in male-male competition and courtship in many vertebrates, but its precise effects are unclear. We explored whether courtship behaviour in humans is modulated and preceded by changes in T. Pairs of healthy male students first competed in a non-physical contest in which their T levels became elevated. Each participant then had a short, informal interaction with either an unfamiliar man or woman. The sex of the stimulus person did not affect the participants' behaviour overall. However, in interactions with women, those men who had experienced a greater T increase during the contest subsequently showed more interest in the woman, engaged in more self-presentation, smiled more and made more eye contact. No such effects were seen in interactions with other men. This is the first study to provide direct evidence that elevating T during male-male competition is followed by increased affiliative behaviour towards women.

Covariation between personalities and individual differences in coping with stress: Converging evidence and hypotheses

In the past decade there has been a profusion of studies highlighting covariation between individual differences in stress physiology and behavioural profiles, here called personalities. Such individual differences in ways of coping with stress are relevant both in biomedicine, since different personalities may experience a different stress and disease vulnerability, and in behavioural ecology, since their adaptive value and evolutionary maintenance are the subject of debate. However, the precise way in which individual stress differences and personalities are linked is unclear. Here we provide an updated overview of this covariation across different species and taxa, consider its functional significance and present working hypotheses for how behavioural and physiological responses to stress might be causally linked, affecting life-history traits such as dispersal and life-span.

Learning your own strength: winner and loser effects should change with age and experience

Winner and loser effects, in which the outcome of an aggressive encounter influences the tendency to escalate future conflicts, have been documented in many taxa, but we have limited understanding of why they have evolved. One possibility is that individuals use previous victories and defeats to assess their fighting ability relative to others. We explored this idea by modelling a population of strong and weak individuals that do not know their own strength, but keep track of how many fights they have won. Under these conditions, adaptive behaviour generates clear winner and loser effects: individuals who win fights should escalate subsequent conflicts, whereas those who lose should retreat from aggressive opponents. But these effects depend strongly on age and experience. Young, naive individuals should show highly aggressive behaviour and pronounced loser effects. For these inexperienced individuals, fighting is especially profitable because it yields valuable information about their strength. Aggression should then decline as an individual ages and gains experience, with those who lose fights becoming more submissive. Older individuals, who have a better idea of their own strength, should be more strongly influenced by victories than losses. In conclusion, we predict that both aggressiveness and the relative magnitude of winner and loser effects should change with age, owing to changes in how individuals perceive their own strength.

Sex-ratio biasing towards daughters among lower-ranking co-wives in Rwanda

There is considerable debate as to whether human females bias the sex ratio of their offspring as a function of their own condition. We apply the Trivers-Willard prediction-that mothers in poor condition will overproduce daughters-to a novel measure of condition, namely wife rank within a polygynous marriage. Using a large-scale sample of over 95 000 Rwandan mothers, we show that lower-ranking polygynous wives do indeed have significantly more daughters than higher-ranking polygynous wives and monogamously married women. This effect remains when controlling for potential confounds such as maternal age. We discuss these results in reference to previous work on sex-ratio adjustment in humans.

Sexual selection: copycat mating in birds

Female zebra finches may be influenced by the choices of other females when selecting mates, challenging the view that mate-choice copying should not occur in species with biparental care.

Optimal assessment of multiple cues

In a wide range of contexts from mate choice to foraging, animals are required to discriminate between alternative options on the basis of multiple cues. How should they best assess such complex multicomponent stimuli? Here, we construct a model to investigate this problem, focusing on a simple case where a 'chooser' faces a discrimination task involving two cues. These cues vary in their accuracy and in how costly they are to assess. As an example, we consider a mate-choice situation where females choose between males of differing quality. Our model predicts the following: (i) females should become less choosy as the cost of finding new males increases; (ii) females should prioritize cues differently depending on how choosy they are; (iii) females may sometimes prioritize less accurate cues; and (iv) which cues are most important depends on the abundance of desirable mates. These predictions are testable in mate-choice experiments where the costs of choice can be manipulated. Our findings are applicable to other discrimination tasks besides mate choice, for example a predator's choice between palatable and unpalatable prey, or an altruist's choice between kin and non-kin.

Activation of the cholesterol pathway and Ras maturation in response to stress

All cells depend on sterols and isoprenoids derived from mevalonate (MVA) for growth, differentiation, and maintenance of homeostatic functions. In plants, environmental insults like heat and sunlight trigger the synthesis of isoprene, also derived from MVA, and this phenomenon has been associated with enhanced tolerance to heat. Here, we show that in human prostate adenocarcinoma PC-3M cells heat shock leads to activation of the MVA pathway. This is characterized by a dose- and time-dependent elevation in 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) activity, enhanced sterol and isoprenoid synthesis, and increased protein prenylation. Furthermore, prenylation and subsequent membrane localization of Ras, a central player in cell signaling, was rapidly induced following heat stress. These effects were dose-dependent, augmented with repeated insults, and were prevented by culturing cells in the presence of lovastatin, a competitive inhibitor of HMGR. Enhanced Ras maturation by heat stress was also associated with a heightened activation of extracellular signal-regulated kinase (ERK), a key mediator of both mitogenic and stress signaling pathways, in response to subsequent growth factor stimulation. Thus, activation of the MVA pathway may constitute an important adaptive host response to stress, and have significant implications to carcinogenesis.

Complexes containing activating transcription factor (ATF)/cAMP-responsive-element-binding protein (CREB) interact with the CCAAT/enhancer-binding protein (C/EBP)-ATF composite site to regulate Gadd153 expression during the stress response

Gadd153, also known as chop, encodes a member of the CCAAT/enhancer-binding protein (C/EBP) transcription factor family and is transcriptionally activated by cellular stress signals. We recently demonstrated that arsenite treatment of rat pheochromocytoma PC12 cells results in the biphasic induction of Gadd153 mRNA expression, controlled in part through binding of C/EBPbeta and two uncharacterized protein complexes to the C/EBP-ATF (activating transcription factor) composite site in the Gadd153 promoter. In this report, we identified components of these additional complexes as two ATF/CREB (cAMP-responsive-element-binding protein) transcription factors having differential binding activities dependent upon the time of arsenite exposure. During arsenite treatment of PC12 cells, we observed enhanced binding of ATF4 to the C/EBP-ATF site at 2 h as Gadd153 mRNA levels increased, and enhanced binding of ATF3 complexes at 6 h as Gadd153 expression declined. We further demonstrated that ATF4 activates, while ATF3 represses, Gadd153 promoter activity through the C/EBP-ATF site. ATF3 also repressed ATF4-mediated transactivation and arsenite-induced activation of the Gadd153 promoter. Our results suggest that numerous members of the ATF/CREB family are involved in the cellular stress response, and that regulation of stress-induced biphasic Gadd153 expression in PC12 cells involves the ordered, sequential binding of multiple transcription factor complexes to the C/EBP-ATF composite site.

Molecular mechanisms of impaired stimulation of DNA synthesis in cultured hepatocytes of aged rats

We examined epidermal growth factor (EGF)- and epinephrine-stimulated mitogen-activated protein kinase kinase (MEK) 1 and MEK2 activities, DNA polymerase alpha activity, and EGF-stimulated E2F DNA binding activity in primary cultured hepatocytes from 6- and 24-mo-old rats. MEK stimulation by either EGF or epinephrine was not altered with aging. However, stimulation of DNA polymerase alpha activity by these agents was 70% and 50% lower, respectively, in cells of aged compared with cells of young rats, consistent with a lesser increase in [3H]thymidine incorporation. EGF-stimulated E2F (a transcription factor that regulates expression of the DNA polymerase alpha gene) binding to DNA was reduced with age. PD-098059, a specific inhibitor of MEK, inhibited EGF-stimulated MEK1 and MEK2 activities in hepatocytes from 6- and 24-mo-old rats. Although PD-098059 inhibited EGF-stimulated DNA synthesis in hepatocytes from 6-mo-old rats, it had no effect in 24-mo-old rats. Thus the age-related impairment appears to occur before E2F activation, and signal transduction sequences other than the mitogen-activated protein kinase pathway may be involved in stimulated DNA synthesis in hepatocytes from old rats.

On the feasibility of MRI-guided focused ultrasound for local induction of gene expression

Gene therapy is a promising approach to the treatment of many forms of disease, including cancer. Of critical concern in its implementation is the ability to control the location, duration, and level of expression of the therapeutic gene. Here, we propose the use of local heat in combination with a heat-sensitive promoter to help accomplish this. Certain members of the family of heat shock protein (hsp) promoters display a regulation that depends strongly on temperature. We present a study of natural hsp70 induction in rat leg by MRI-guided focused ultrasound to investigate the hsp70 promoter as a possible candidate for use in control of gene expression with local heat. A temperature increase of 5-8 degrees C in the focal region for 45 minutes led to a differential expression of the hsp70 mRNA between the focal region and the surrounding tissue ranging from a factor of 3 to 67.

Induction of mitogen-activated protein kinase phosphatase-1 during acute hypertension

Recently, we demonstrated that elevated blood pressure activates mitogen-activated protein (MAP) kinases in rat aorta. Here we provide evidence that the vascular response to acute hypertension also includes induction of MAP kinase phosphatase-1 (MKP-1), which has been shown to function in the dephosphorylation and inactivation of MAP kinases. Restraint or immobilization stress, which leads to a rapid rise in blood pressure, resulted in a rapid and transient induction of MKP-1 mRNA followed by elevated MKP-1 protein expression in rat aorta. That the induction of MKP-1 by restraint was due to the rise in blood pressure was supported by the finding that several different hypertensive agents (phenylephrine, vasopressin, and angiotensin II) were likewise capable of eliciting the response, and sodium nitroprusside, a nonspecific vasodilator agent that prevented the acute rise in blood pressure in response to the hypertensive agents, abrogated MKP-1 mRNA induction. The in vivo effects could not be mimicked by treatment of cultured aortic smooth muscle cells with similar doses of the hypertensive agents. These findings support a role for MKP-1 in the in vivo regulation of MAP kinase activity during hemodynamic stress.

Potentiation of heat stress-induced hsp70 expression in vivo by aspirin

Studies in cultured cells have demonstrated that non-steroidal anti-inflammatory agents can potentiate heat-induced hsp70 expression through activation of HSF1 to a DNA binding state. We investigated the influence of aspirin on hsp70 expression in intact rats subjected to heat stress. Rats were injected intraperitoneally either with aspirin (100 mg/kg) or vehicle alone, 60 min prior to their placement at 37 degrees C or room temperature for 30 min. hsp70 mRNA expression was analyzed in lung, liver and kidney isolated from animals assigned to one of four different treatment paradigms; untreated controls, heat, aspirin, and aspirin-plus-heat. Comparison of hsp70 expression in the treatment groups revealed that in all tissues examined, aspirin-plus-heat treatment resulted in 3-4 fold higher levels of hsp70 mRNA relative to those seen with heat treatment alone. Little or no hsp70 mRNA expression was detected in the unheated groups, regardless of aspirin treatment. In keeping with the mRNA expression, Hsp70 protein levels were also elevated in aspirin-plus-heat treated animals. Aspirin treatment did not alter hsp70 protein expression in the absence of heat. In contrast to in vitro observations, aspirin treatment in vivo did not alter HSF1 DNA binding properties. Core body temperature measurements revealed that aspirin pretreatment enhanced the rise in body temperature seen in response to heat treatment. This increased hyperthermic response to heat stress probably accounts for the potentiation of hsp70 expression observed in aspirin-plus-heat treated rats. Given the widespread use of aspirin in humans within a dose range comparable to that used here, our findings are likely to have important physiological consequences.

Effects of perturbations of the hypothalamic-pituitary-adrenal axis on the acute phase response: altered C/EBP and acute phase response gene expression in lipopolysaccharide-treated rats

In this study, we investigated the influence of long term perturbations of the hypothalamicpituitary-adrenal axis on the acute phase response elicited following lipopolysaccharide (LPS) challenge in rats. Specifically, we examined the effects of either long term absence of glucocorticoids (adrenalectomized rats treated with placebo chronic release pellets) or extended exposure to pharmacologic levels of glucocorticoids (adrenalectomized rats treated with dexamethasone chronic release pellets) on the expression of selected acute phase proteins and various members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors. Both hypothalamic-pituitary-adrenal axis manipulations resulted in a reduction of the acute phase response as assessed by the LPS-mediated induction of acute phase proteins and C/EBP gene expression, with dexamethasone treatment exhibiting a greater inhibitory effect than adrenalectomy. Induction of hemopexin, alpha 1-acid glycoprotein, alpha 2-macroglobulin, GADD153, C/EBP beta, and C/EBP delta mRNAs by LPS were all abolished in dexamethasone-treated rats. These findings have direct implications for patients undergoing chronic high dose glucocorticoid therapy.

Activation of heat shock transcription factor 1 in rat aorta in response to high blood pressure

We have previously demonstrated that acute hypertension induces heat shock protein gene expression in rat arterial wall. Here we provide evidence that this induction is mediated through the activation of heat shock transcription factor 1 in response to high blood pressure. Rats subjected to restraint or immobilization stress displayed an acute elevation in systolic pressure accompanied by an increase in heat shock protein 70 mRNA expression. Consistent with the rapid time course of mRNA induction, an increase in binding activity to an oligonucleotide encompassing a consensus heat shock element sequence was seen in protein extracts from aorta of restrained rats as assessed with gel mobility shift assays. A similar increase in DNA binding activity was also observed in aortic extracts from rats treated with various hypertensive agents, including phenylephrine, angiotensin II, and vasopressin. That the DNA binding activity was attributed to heat shock factor 1 was shown through use of antibodies to the transcription factor that retarded the DNA-protein complexes in gel mobility supershift assays. Western blot analysis of heat shock factor 1 protein expression in aortic extracts showed a slower mobility form of the protein in hypertensive rats, indicative of an activated, presumably phosphorylated, form of the transcription factor. These findings support the view that heat shock factor 1 is responsible for induction of heat shock protein 70 in the arterial wall during acute hypertension, a response that is likely to play an important role in protecting arteries during hemodynamic stress.

Physical and functional association between GADD153 and CCAAT/enhancer-binding protein beta during cellular stress

GADD153, a ubiquitously expressed member of the CCAAT/enhancer-binding protein (C/EBP) family is induced by a wide variety of growth-arresting and DNA-damaging agents. Functionally, GADD153 has been postulated to act as a dominant-negative regulator of C/EBPs. Therefore we sought to gain evidence for interactions between GADD153 and other C/EBPs during cellular responses to stress. In this report we have demonstrated that treatment of rat pheochromocytoma PC12 cells with sodium arsenite leads to enhanced expression of C/EBP-beta and GADD153 (growth arrest and DNA damage inducible gene 153) but not other C/EBPs. Coimmunoprecipitation experiments provided evidence for the formation of endogenous GADD153-C/EBP-beta complexes in arsenite-treated cells. Additional experiments were performed to determine the role of such complexes in regulating GADD153 expression. Previous studies in our laboratory demonstrated that the GADD153 promoter contains a C/EBP binding site through which other C/EBPs interact to transactivate GADD153 expression in liver hepatoma cells. Here, we demonstrate that extracts prepared from arsenite-treated PC12 cells likewise show increased amounts of factors capable of binding to the GADD153-C/EBP site and that these complexes are comprised at least in part of C/EBP-beta. Forced expression of C/EBP-beta was found to be capable of transactivating the GADD153 promoter in PC12 cells cotransfected with plasmids expressing a GADD153 reporter gene and C/EBP-beta protein. However, overexpression of GADD153 inhibited the transactivation of the GADD153 promoter by C/EBP-beta. These findings provide evidence for an autoregulatory loop in which stress-induced GADD153 feeds back to attenuate GADD153 expression during the cellular response to stress.

Vasopressin-induced heat shock protein expression in renal tubular cells

Heat shock proteins (HSPs), which have been shown to be induced in the kidney by a variety of stress conditions, including ischemia, inflammation, oxidative stress, and toxin exposure, are believed to protect the cells from injury. In the present study, we demonstrated that administration of vasopressin i.v. to Wistar rats leads to HSP70 induction in the kidney. The effect was specific to the kidney (i.e., absent in brain, heart, lung, muscle, etc.) and selective for the HSP70 gene family (HSP27, HSP60, and HSP90 were not induced). Western blot analysis demonstrated that HSP70 protein expression peaked between 6 and 12 hours after vasopressin administration. Immunohistochemical staining revealed that induction was localized to renal tubule lining cells, with no expression seen in glomerular or interstitial regions. The elevated protein levels were preceded by the induction of HSP70 mRNA within 30 minutes after vasopression injection. The induction of HSP70 mRNA was associated with the activation of heat shock transcription factor 1 (HSF1), suggesting that the response was regulated at the level of transcription. This HSP70 expression was completely blocked in the presence of both a general vasopressin receptor antagonist (V1 and V2 receptors) and an antidiuretic antagonist (V2), but not in the presence of a vasopressor antagonist (V1). These observations could be significant for understanding the possible involvement of HSP70 in physiological processes of the kidney, as well as pathophysiologic conditions associated with either elevated or deficient levels of vasopressin.

Effects of neurohormonal stress and aging on the activation of mammalian heat shock factor 1

The mammalian heat shock response has been investigated extensively using tissue culture cells with only a limited amount of information available on animals and intact tissues. The neurohormonal stress response mediated by the hypothalamic-pituitary-adrenal axis leads to the activation of heat shock factor (HSF) in rat adrenal tissue. Here we show through the use of antibodies specific to each member of the HSF family that restraint-induced stress in intact Wistar rats and adrenocorticotropic hormone treatment of hypophysectomized animals leads to the activation of HSF1 monomers to trimers with DNA-binding activity. Because HSF1 is also the target factor for metabolic and environmental stress, these data reveal an intersection of pathways leading to HSF1 activation. Comparison of the biochemical properties and levels of HSF1 in the Wistar and Fischer 344 rat strains reveals that HSF1 is constitutively present in an activated DNA-binding state in the adrenals of Fischer 344 rats. During aging, the levels of HSF1 remain constant, yet the transcription factor from aged animals exhibits a decreased ability to bind DNA.

Regulation of the C/EBP-related gene gadd153 by glucose deprivation

gadd153 encodes a CCAAT/enhancer-binding protein (C/EBP)-related protein that lacks a functional DNA-binding domain. Since the gadd153 protein is capable of heterodimerizing with other C/EBPs, gadd153 may function as a negative regulator of these transcription factors. Here we examined the role of glucose in regulating gadd153 expression. We found that glucose deprivation markedly induces gadd153 mRNA levels in both HeLa and 3T3-L1 cells and that addition of D-(+)-glucose resulted in a rapid decrease of gadd153 mRNA. Similar induction and reversal of gadd153 expression were observed at the protein level. Because C/EBP alpha appears to play an important role in regulating genes involved in adipogenesis and energy metabolism, we examined gadd153 expression during the differentiation of 3T3-L1 preadipocytes and as a function of glucose utilization in differentiated adipocytes. Using a standard differentiation protocol that consisted of hormonal stimulation for 2 days followed by medium changes every 2 days thereafter, we observed that both C/EBP alpha and gadd153 mRNAs were elevated. However, C/EBP alpha induction occurred on day 3, while gadd153 expression was not seen until day 4, when the cells were fully differentiated. Frequent addition of fresh medium to the cells during the differentiation process, as well as supplementation of medium with glucose, reduced gadd153 expression without preventing C/EBP alpha expression or interfering with cellular differentiation. Thus, gadd153 expression is not essential for the process of adipocyte differentiation but is significantly influenced by the availability of glucose to the cell.

Plant organelles contain distinct peptidylprolyl cis,trans-isomerases

Peptidylprolyl cis,trans-isomerase (PPIase) activity was detected in the cytosol, mitochondria, and chloroplast of pea plants. Cyclosporin A inhibited the activity largely localized to the mitochondrial matrix while rapamycin inhibited the PPIase activity associated with the mitochondrial membranes. Differential inhibition by the two immunosuppressive drugs, the specific binding of these drugs to different mitochondrial fractions, and the immunological detection of a putative 25-kDa rapamycin-binding protein (RBP) in mitochondrial extracts attests to the presence in plant mitochondria of both cyclophilin and RBP classes of PPIases. Cyclosporin A-sensitive PPIase detected in the chloroplast was mostly localized to the thylakoids, which is suggestive of its function in the folding of membranal proteins. PPIase associated with the chloroplast stroma and the thylakoids was not inhibited by rapamycin nor was any cross-reactive RBP detected in chloroplast extracts. These results demonstrate the presence of distinct classes of PPIases in the mitochondria and the chloroplasts of plants.

Oxidative stress causes rapid membrane translocation and in vivo degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase

We have studied the turnover of an abundant chloroplast protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rbu-P2 carboxylase/oxygenase), in plants (Spirodela oligorrhiza and Triticum aestivum L.) and algae (Chlamydomonas reinhardtii and C. moewusii) induced to senesce under oxidative conditions. Rbu-P2 carboxylase/oxygenase activity and stability in vivo were found to be highly susceptible to oxidative stress, resulting in intermolecular cross-linking of large subunits by disulfide bonds within the holoenzyme, rapid and specific translocation of the soluble enzyme complex to the chloroplast membranes, and finally protein degradation. The redox state of Cys-247 in Rbu-P2 carboxylase/oxygenase large subunit seems involved in the sensitivity of the holoenzyme to oxidative inactivation and cross-linking. However, this process did not drive membrane attachment or degradation of Rbu-P2 carboxylase/oxygenase in vivo. Translocation of oxidized Rbu-P2 carboxylase/oxygenase to chloroplast membranes may be a necessary step in its turnover, particularly during leaf senescence. Thus, processes that regulate the redox state of plant cells seem closely intertwined with cellular switches shifting the leaf from growth and maturation to senescence and death.