Circadian glucose homeostasis requires compensatory interference between brain and liver clocks

Stiffness Modulation and Pulsatile Release in Dual Responsive Hydrogels

Inspired by nature, self-regulation can be introduced in synthetic hydrogels by incorporating chemo-mechanical signals or coupled chemical reactions to maintain or adapt the material's physico-chemical properties when exposed to external triggers. In this work, we present redox and light dual stimuli responsive hydrogels capable of rapidly adapting the polymer crosslinking network while maintaining hydrogel stability. Upon irradiation with UV light, polymer hydrogels containing redox responsive disulfide crosslinks and light responsive ortho-nitrobenzyl moieties show a release of payload accompanied by adaptation of the hydrogel network towards higher stiffness due to in situ crosslinking by S-nitrosylation. Whereas the hydrogel design allows the network to either become softer in presence of reducing agent glutathione or stiffer upon UV irradiation, simultaneous application of both stimuli induces network self-regulation resulting in a pulsatile form of payload release from the hydrogel. Finally, adaptive stiffness was used to make tunable hydrogels as substrates for different cell lines.

Circadian modulation of glucose utilization via CRY1-mediated repression of Pdk1 expression

Life adapts to daily environmental changes through circadian rhythms, exhibiting spontaneous oscillations of biological processes. These daily functional oscillations must match the metabolic requirements responding to the time of the day. We focus on the molecular mechanism of how the circadian clock regulates glucose, the primary resource for energy production and other biosynthetic pathways. The complex regulation of the circadian rhythm includes many proteins that control this process at the transcriptional and translational levels and by protein-protein interactions. We have investigated the action of one of these proteins, cryptochrome (CRY), whose elevated mRNA and protein levels repress the function of an activator in the transcription-translation feedback loop, and this activator causes elevated Cry1 mRNA. We used a genome-edited cell line model to investigate downstream genes affected explicitly by the repressor CRY. We found that CRY can repress glycolytic genes, particularly that of the gatekeeper, pyruvate dehydrogenase kinase 1 (Pdk1), decreasing lactate accumulation and glucose utilization. CRY1-mediated decrease of Pdk1 expression can also be observed in a breast cancer cell line MDA-MB-231, whose glycolysis is associated with Pdk1 expression. We also found that exogenous expression of CRY1 in the MDA-MB-231 decreases glucose usage and growth rate. Furthermore, reduced CRY1 levels and the increased phosphorylation of PDK1 substrate were observed when cells were grown in suspension compared to cells grown in adhesion. Our data supports a model that the transcription-translation feedback loop can regulate the glucose metabolic pathway through Pdk1 gene expression according to the time of the day.

Circadian regulation of liver function: from molecular mechanisms to disease pathophysiology

A wide variety of liver functions are regulated daily by the liver circadian clock and via systemic circadian control by other organs and cells within the gastrointestinal tract as well as the microbiome and immune cells. Disruption of the circadian system, as occurs during jetlag, shift work or an unhealthy lifestyle, is implicated in several liver-related pathologies, ranging from metabolic diseases such as obesity, type 2 diabetes mellitus and nonalcoholic fatty liver disease to liver malignancies such as hepatocellular carcinoma. In this Review, we cover the molecular, cellular and organismal aspects of various liver pathologies from a circadian viewpoint, and in particular how circadian dysregulation has a role in the development and progression of these diseases. Finally, we discuss therapeutic and lifestyle interventions that carry health benefits through support of a functional circadian clock that acts in synchrony with the environment.

The circadian rhythm: an influential soundtrack in the diabetes story

Type 2 Diabetes Mellitus (T2DM) has been the main category of metabolic diseases in recent years due to changes in lifestyle and environmental conditions such as diet and physical activity. On the other hand, the circadian rhythm is one of the most significant biological pathways in humans and other mammals, which is affected by light, sleep, and human activity. However, this cycle is controlled via complicated cellular pathways with feedback loops. It is widely known that changes in the circadian rhythm can alter some metabolic pathways of body cells and could affect the treatment process, particularly for metabolic diseases like T2DM. The aim of this study is to explore the importance of the circadian rhythm in the occurrence of T2DM via reviewing the metabolic pathways involved, their relationship with the circadian rhythm from two perspectives, lifestyle and molecular pathways, and their effect on T2DM pathophysiology. These impacts have been demonstrated in a variety of studies and led to the development of approaches such as time-restricted feeding, chronotherapy (time-specific therapies), and circadian molecule stabilizers.

The liver-clock coordinates rhythmicity of peripheral tissues in response to feeding

The mammalian circadian system consists of a central clock in the brain that synchronizes clocks in the peripheral tissues. Although the hierarchy between central and peripheral clocks is established, little is known regarding the specificity and functional organization of peripheral clocks. Here, we employ altered feeding paradigms in conjunction with liver-clock mutant mice to map disparities and interactions between peripheral rhythms. We find that peripheral clocks largely differ in their responses to feeding time. Disruption of the liver-clock, despite its prominent role in nutrient processing, does not affect the rhythmicity of clocks in other peripheral tissues. Yet, unexpectedly, liver-clock disruption strongly modulates the transcriptional rhythmicity of peripheral tissues, primarily on daytime feeding. Concomitantly, liver-clock mutant mice exhibit impaired glucose and lipid homeostasis, which are aggravated by daytime feeding. Overall, our findings suggest that, upon nutrient challenge, the liver-clock buffers the effect of feeding-related signals on rhythmicity of peripheral tissues, irrespective of their clocks.

Importance of circadian timing for aging and longevity

Dietary restriction (DR) decreases body weight, improves health, and extends lifespan. DR can be achieved by controlling how much and/or when food is provided, as well as by adjusting nutritional composition. Because these factors are often combined during DR, it is unclear which are necessary for beneficial effects. Several drugs have been utilized that target nutrient-sensing gene pathways, many of which change expression throughout the day, suggesting that the timing of drug administration is critical. Here, we discuss how dietary and pharmacological interventions promote a healthy lifespan by influencing energy intake and circadian rhythms.

Circadian clocks link physiologic processes to environmental conditions and a mismatch between internal and external rhythms has negative effects on organismal health. In this review, the authors discuss the interactions between circadian clocks and dietary interventions targeted to promote healthy aging.

Unique adaptations in neonatal hepatic transcriptome, nutrient signaling, and one-carbon metabolism in response to feeding ethyl cellulose rumen-protected methionine during late-gestation in Holstein cows

BACKGROUND

Methionine (Met) supply during late-pregnancy enhances fetal development in utero and leads to greater rates of growth during the neonatal period. Due to its central role in coordinating nutrient and one-carbon metabolism along with immune responses of the newborn, the liver could be a key target of the programming effects induced by dietary methyl donors such as Met. To address this hypothesis, liver biopsies from 4-day old calves (n = 6/group) born to Holstein cows fed a control or the control plus ethyl-cellulose rumen-protected Met for the last 28 days prepartum were used for DNA methylation, transcriptome, metabolome, proteome, and one-carbon metabolism enzyme activities.

RESULTS

Although greater withers and hip height at birth in Met calves indicated better development in utero, there were no differences in plasma systemic physiological indicators. RNA-seq along with bioinformatics and transcription factor regulator analyses revealed broad alterations in 'Glucose metabolism', 'Lipid metabolism, 'Glutathione', and 'Immune System' metabolism due to enhanced maternal Met supply. Greater insulin sensitivity assessed via proteomics, and efficiency of transsulfuration pathway activity suggested beneficial effects on nutrient metabolism and metabolic-related stress. Maternal Met supply contributed to greater phosphatidylcholine synthesis in calf liver, with a role in very low density lipoprotein secretion as a mechanism to balance metabolic fates of fatty acids arising from the diet or adipose-depot lipolysis. Despite a lack of effect on hepatic amino acid (AA) transport, a reduction in metabolism of essential AA within the liver indicated an AA 'sparing effect' induced by maternal Met.

CONCLUSIONS

Despite greater global DNA methylation, maternal Met supply resulted in distinct alterations of hepatic transcriptome, proteome, and metabolome profiles after birth. Data underscored an effect on maintenance of calf hepatic Met homeostasis, glutathione, phosphatidylcholine and taurine synthesis along with greater efficiency of nutrient metabolism and immune responses. Transcription regulators such as FOXO1, PPARG, E2F1, and CREB1 appeared central in the coordination of effects induced by maternal Met. Overall, maternal Met supply induced better immunometabolic status of the newborn liver, conferring the calf a physiologic advantage during a period of metabolic stress and suboptimal immunocompetence.

Paraventricular hypothalamus mediates diurnal rhythm of metabolism

Defective rhythmic metabolism is associated with high-fat high-caloric diet (HFD) feeding, ageing and obesity; however, the neural basis underlying HFD effects on diurnal metabolism remains elusive. Here we show that deletion of BMAL1, a core clock gene, in paraventricular hypothalamic (PVH) neurons reduces diurnal rhythmicity in metabolism, causes obesity and diminishes PVH neuron activation in response to fast-refeeding. Animal models mimicking deficiency in PVH neuron responsiveness, achieved through clamping PVH neuron activity at high or low levels, both show obesity and reduced diurnal rhythmicity in metabolism. Interestingly, the PVH exhibits BMAL1-controlled rhythmic expression of GABA-A receptor γ2 subunit, and dampening rhythmicity of GABAergic input to the PVH reduces diurnal rhythmicity in metabolism and causes obesity. Finally, BMAL1 deletion blunts PVH neuron responses to external stressors, an effect mimicked by HFD feeding. Thus, BMAL1-driven PVH neuron responsiveness in dynamic activity changes involving rhythmic GABAergic neurotransmission mediates diurnal rhythmicity in metabolism and is implicated in diet-induced obesity.

Beneficial Effects of Time-Restricted Eating on Metabolic Diseases: A Systemic Review and Meta-Analysis

Various behavioral and physiological pathways follow a pre-determined, 24 hour cycle known as the circadian rhythm. Metabolic homeostasis is regulated by the circadian rhythm. Time-restricted eating (TRE) is a type of intermittent fasting based on the circadian rhythm. In this study, we aim to analyze systemically the effects of TRE on body weight, body composition, and other metabolic parameters. We reviewed articles from PubMed, EMBASE, and the Cochrane Library to identify clinical trials that compared TRE to a regular diet. We included 19 studies for meta-analysis. Participants following TRE showed significantly reduced body weight (mean difference (MD), -0.90; 95% confidence interval (CI): -1.71 to -0.10) and fat mass (MD: -1.58, 95% CI: -2.64 to -0.51), while preserving fat-free mass (MD, -0.24; 95% CI: -1.15 to 0.67). TRE also showed beneficial effects on cardiometabolic parameters such as blood pressure (systolic BP, MD, -3.07; 95% CI: -5.76 to -0.37), fasting glucose concentration (MD, -2.96; 95% CI, -5.60 to -0.33), and cholesterol profiles (triglycerides, MD: -11.60, 95% CI: -23.30 to -0.27). In conclusion, TRE is a promising therapeutic strategy for controlling weight and improving metabolic dysfunctions in those who are overweight or obese. Further large-scale clinical trials are needed to confirm these findings and the usefulness of TRE.

Spinal Cord Injury in Rats Dysregulates Diurnal Rhythms of Fecal Output and Liver Metabolic Indicators

Spinal cord injury (SCI) dysregulates metabolic homeostasis. Metabolic homeostasis is optimized across the day by the circadian system. Despite the prevalence of metabolic pathologies after SCI, post-SCI circadian regulation of metabolism remains understudied. Here, we hypothesized that SCI in rats would disrupt circadian regulation of key metabolic organs, leading to metabolic dysregulation. Female and male Sprague-Dawley rats received moderate thoracic (T)-9 contusion SCI (or sham surgery). First, SCI disrupted diurnal rhythms in two metabolic behaviors: fecal production and food intake rhythms were ablated acutely. SCI also expedited whole-gut transit time. In parallel, acute SCI increased plasma glucose. Diurnal glucose storage-release cycles regulated by the liver were disrupted by SCI, which also increased liver glucose metabolism messenger RNAs (mRNAs). Further, SCI disrupted liver clock gene expression and suppressed inflammatory gene rhythms. Together, our novel data suggest that SCI disrupts typical metabolic and circadian function. Improving post-SCI metabolic function could enhance recovery of homeostasis.

Microbiome diurnal rhythmicity and its impact on host physiology and disease risk

Host-microbiome interactions constitute key determinants of host physiology, while their dysregulation is implicated in a wide range of human diseases. The microbiome undergoes diurnal variation in composition and function, and this in turn drives oscillations in host gene expression and functions. In this review, we discuss the newest developments in understanding circadian host-microbiome interplays, and how they may be relevant in health and disease contexts. We summarize the molecular mechanisms by which the microbiome influences host function in a diurnal manner, and inversely describe how the host orchestrates circadian rhythmicity of the microbiome. Furthermore, we highlight the future perspectives and challenges in studying this new and exciting facet of host-microbiome interactions. Finally, we illustrate how the elucidation of the microbiome chronobiology may pave the way for novel therapeutic approaches.

Mice under Caloric Restriction Self-Impose a Temporal Restriction of Food Intake as Revealed by an Automated Feeder System

Caloric restriction (CR) extends lifespan in mammals, yet the mechanisms underlying its beneficial effects remain unknown. The manner in which CR has been implemented in longevity experiments is variable, with both timing and frequency of meals constrained by work schedules. It is commonplace to find that nocturnal rodents are fed during the daytime and meals are spaced out, introducing prolonged fasting intervals. Since implementation of feeding paradigms over the lifetime is logistically difficult, automation is critical, but existing systems are expensive and not amenable to scale. We have developed a system that controls duration, amount, and timing of food availability and records feeding and voluntary wheel-running activity in mice. Using this system, mice were exposed to temporal or caloric restriction protocols. Mice under CR self-imposed a temporal component by consolidating food intake and unexpectedly increasing wheel-running activity during the rest phase, revealing previously unrecognized relationships among feeding, metabolism, and behavior.

Clock Genes, Metabolism, and Cardiovascular Risk

The molecular clockwork drives rhythmic oscillations of signaling pathways managing intermediate metabolism; the circadian timing system synchronizes behavioral cycles and anabolic/catabolic processes with environmental cues, mainly represented by light/darkness alternation. Metabolic pathways, bile acid synthesis, and autophagic and immune/inflammatory processes are driven by the biological clock. Proper timing of hormone secretion, metabolism, bile acid turnover, autophagy, and inflammation with behavioral cycles is necessary to avoid dysmetabolism. Disruption of the biological clock and mistiming of body rhythmicity with respect to environmental cues provoke loss of internal synchronization and metabolic derangements, causing liver steatosis, obesity, metabolic syndrome, and diabetes mellitus.

Effects of Intracerebroventricular Administration of Neuropeptide Y on Metabolic Gene Expression and Energy Metabolism in Male Rats

Neuropeptide Y (NPY) is an important neurotransmitter in the control of energy metabolism. Several studies have shown that obesity is associated with increased levels of NPY in the hypothalamus. We hypothesized that the central release of NPY has coordinated and integrated effects on energy metabolism in different tissues, resulting in increased energy storage and decreased energy expenditure (EE). We first investigated the acute effects of an intracerebroventricular (ICV) infusion of NPY on gene expression in liver, brown adipose tissue, soleus muscle, and sc and epididymal white adipose tissue (WAT). We found increased expression of genes involved in gluconeogenesis and triglyceride secretion in the liver already 2-hour after the start of the NPY administration. In brown adipose tissue, the expression of thermogenic genes was decreased. In sc WAT, the expression of genes involved in lipogenesis was increased, whereas in soleus muscle, the expression of lipolytic genes was decreased after ICV NPY. These findings indicate that the ICV infusion of NPY acutely and simultaneously increases lipogenesis and decreases lipolysis in different tissues. Subsequently, we investigated the acute effects of ICV NPY on locomotor activity, respiratory exchange ratio, EE, and body temperature. The ICV infusion of NPY increased locomotor activity, body temperature, and EE as well as respiratory exchange ratio. Together, these results show that an acutely increased central availability of NPY results in a shift of metabolism towards lipid storage and an increased use of carbohydrates, while at the same time increasing activity, EE, and body temperature.

Adaptation of molecular circadian clockwork to environmental changes: a role for alternative splicing and miRNAs

Circadian (24 h) clocks provide a source of internal timing in most living organisms. These clocks keep time by using complex transcriptional/post-translational feedback loops that are strikingly resilient to changes in environmental conditions. In the last few years, interest has increased in the role of post-transcriptional regulation of circadian clock components. Post-transcriptional control plays a prominent role in modulating rapid responses of the circadian system to environmental changes, including light, temperature and general stress and will be the focus of this review.

Metabolic compensation of the Neurospora clock by a glucose-dependent feedback of the circadian repressor CSP1 on the core oscillator

Conidial separation 1 (CSP1) is a global transcription repressor. It is expressed under control of the white collar complex (WCC), the core transcription factor of the circadian clock of Neurospora. Here we report that the length of the circadian period decreases with increasing glucose concentrations in csp1 mutant strains, while the period is compensated for changes in glucose concentration in wild-type strains. Glucose stimulated CSP1 expression. Overexpression of CSP1 caused period lengthening and, eventually, complete dampening of the clock rhythm. We show that CSP1 inhibits expression of the WHITE COLLAR 1 (WC1) subunit of the WCC by repressing the wc1 promoter. Glucose-dependent repression of wc1 transcription by CSP1 compensated for the enhanced translation of WC1 at high glucose levels, resulting in glucose-independent expression of the WCC and, hence, metabolic compensation that maintained a constant circadian period. Thus, the negative feedback of CSP1 on WC1 expression constitutes a molecular pathway that coordinates energy metabolism and the circadian clock.

Evidence for possible period 2 gene mediation of the effects of alcohol exposure during the postnatal period on genes associated with maintaining metabolic signaling in the mouse hypothalamus

BACKGROUND

Animals exposed to alcohol during the developmental period develop circadian disturbances and metabolic problems that often persist during their adult period. In order to study whether alcohol and the circadian clock interact to alter metabolic signaling in the hypothalamus, we determined whether postnatal alcohol feeding in mice permanently alters metabolic sensing in the hypothalamus. Furthermore, we evaluated whether the effect of circadian disruption via Period 2 (Per2) gene mutation prevents alcohol's effects on metabolic signaling in the hypothalamus.

METHODS

Per2 mutant and wild-type male and female mice of the same genetic background were given a milk formula containing ethanol (EtOH; 11.34% vol/vol) from postnatal day (PD) 2 to 7 and used for gene expression and peptide level determinations in the hypothalamus at PD7 and PD90.

RESULTS

We report here that postnatal alcohol feeding reduces the expression of proopiomelanocortin (Pomc) gene and production of β-endorphin and α-melanocyte stimulating hormone (α-MSH) in the hypothalamus that persists into adulthood. In addition, expressions of metabolic sensing genes in the hypothalamus were also reduced as a consequence of postnatal alcohol exposure. These effects were not sex-specific and were observed in both males and females. Mice carrying a mutation of the Per2 gene did not show any reductions in hypothalamic levels of Pomc and metabolic genes and β-endorphin and α-MSH peptides following alcohol exposure.

CONCLUSIONS

These data suggest that early-life exposure to alcohol alters metabolic sensing to the hypothalamus possibly via regulating Per2 gene and/or the cellular circadian clock mechanism.

Clock genes and clock-controlled genes in the regulation of metabolic rhythms

Daily rotation of the Earth on its axis and yearly revolution around the Sun impose to living organisms adaptation to nyctohemeral and seasonal periodicity. Terrestrial life forms have developed endogenous molecular circadian clocks to synchronize their behavioral, biological, and metabolic rhythms to environmental cues, with the aim to perform at their best over a 24-h span. The coordinated circadian regulation of sleep/wake, rest/activity, fasting/feeding, and catabolic/anabolic cycles is crucial for optimal health. Circadian rhythms in gene expression synchronize biochemical processes and metabolic fluxes with the external environment, allowing the organism to function effectively in response to predictable physiological challenges. In mammals, this daily timekeeping is driven by the biological clocks of the circadian timing system, composed of master molecular oscillators within the suprachiasmatic nuclei of the hypothalamus, pacing self-sustained and cell-autonomous molecular oscillators in peripheral tissues through neural and humoral signals. Nutritional status is sensed by nuclear receptors and coreceptors, transcriptional regulatory proteins, and protein kinases, which synchronize metabolic gene expression and epigenetic modification, as well as energy production and expenditure, with behavioral and light-dark alternance. Physiological rhythmicity characterizes these biological processes and body functions, and multiple rhythms coexist presenting different phases, which may determine different ways of coordination among the circadian patterns, at both the cellular and whole-body levels. A complete loss of rhythmicity or a change of phase may alter the physiological array of rhythms, with the onset of chronodisruption or internal desynchronization, leading to metabolic derangement and disease, i.e., chronopathology.

The daily rhythm of mice

The house mouse Mus musculus represents a valuable tool for the analysis and the understanding of the mammalian circadian oscillator. Forward and reverse genetics allowed the identification of clock components and the verification of their function within the circadian clockwork. In many cases unforeseen links were discovered between a particular circadian regulatory protein and various diseases or syndromes. Thus, this model system is not only perfectly suited to pinpoint the components of the mammalian circadian clock, but also to unravel metabolic, physiological, and pathological processes linked to the circadian timing system.

Heme-binding characteristics of the isolated PAS-B domain of mouse Per2, a transcriptional regulatory factor associated with circadian rhythms

Mouse period homolog 2 (mPer2), an important transcriptional regulatory factor associated with circadian rhythms, is composed of two N-terminal PAS (PAS-A and PAS-B) domains and a C-terminal domain. The PAS-A domain of mPer2 binds the heme iron via a Cys axial ligand. A corresponding transcriptional regulatory factor, neuronal PAS 2 protein (NPAS2), also contains PAS-A and PAS-B domains at the N-terminus with heme-binding capability. In particular, the PAS-B domain appears important for protein-protein interactions critical for transcriptional regulation. In the present study, we examined the heme-binding characteristics of the isolated PAS-B domain of mPer2. Our experiments show that the Fe(III) heme binds the isolated PAS-B domain with a heme to protein stoichiometry of 1:1. The Fe(III) protein complex is suggested to consist of an admixture of 6-coordinated His-bound high-spin and low-spin complexes. Marked pH-dependent spectral changes were observed, in contrast to the spectrum of the Fe(III) bound PAS-A domain of mPer2, which appeared pH-resistant. Treatment with diethylpyrocarbonate abolished the heme-binding ability of this protein, supporting the proposal that His is the axial ligand. Heme dissociation was composed of two phases with rate constants of 4.3 × 10⁻⁴ s⁻¹ (50%) and 4.0 × 10⁻³ s⁻¹ (50%), which were markedly higher than that (1.5 × 10⁻⁷ s⁻¹) of the prototype heme protein, myoglobin. The Soret CD band of the H454A PAS-B mutant was significantly different from those of wild-type and other His mutant proteins, strongly suggesting that His454 is one of the axial ligands for the Fe(III) complex.

The role of melanocortin neuronal pathways in circadian biology: a new homeostatic output involving melanocortin-3 receptors?

Obesity, insulin resistance and increased propensity for type 2 diabetes and cardiovascular disease result from an imbalance between energy intake and expenditure. The cloning of genes involved in energy homeostasis produced a simple feedback model for the homeostatic regulation of adipose mass. Serum leptin secreted from adipocytes signals nutrient sufficiency, curbing appetite and supporting energy expenditure. A rapid decline in leptin during nutrient scarcity instigates adaptive mechanisms, including increased appetite and reduced energy expenditure. Hypothalamic melanocortin neurons are important mediators of this response, integrating inputs of energy status from leptin with other peripheral signals. While this feedback response prolongs survival during fasting, other mechanisms allowing the prediction of nutrient availability also confer a selective advantage. This adaptation has been commonly studied in rodents using restricted feeding paradigms constraining food intake to limited periods at 24-h intervals. Restricted feeding rapidly elicits rhythmic bouts of activity and wakefulness anticipating food presentation. While the response exhibits features suggesting a clock-like mechanism, the neuromolecular mechanisms governing expression of food anticipatory behaviours are poorly understood. Here we discuss a model whereby melanocortin neurons regulating the homeostatic adaptation to variable caloric availability also regulate inputs into neural networks governing anticipatory rhythms in wakefulness, activity and metabolism.

[Pathological anatomy of the vinylchloride-disease (author's transl)]

A letal case of angiosarcoma of the liver is reported. The 38 years old chemical laboratory assistant was exposed in vinylchloride medium about 3 1/2 years. The sarcoma metastasised into many other organs. This case is the fifth one reported in the German Federal Republic, in four of the cases the patient died. VC concentration at the work place was not measured. Thrombocytopenia observed in about 80% of the VC cases was not seen. There will be possibly observed further VC-caused tumors in the population. Nevertheless there might be other even extrahepatic VC-caused tumors in man, which could be concluded by animal experimental investigations.