Hepatic vagal afferents convey clock-dependent signals to regulate circadian food intake

Reduced liver mitochondrial energy metabolism impairs food intake regulation following gastric preloads and fasting

OBJECTIVE

The capacity of the liver to serve as a peripheral sensor in the regulation of food intake has been debated for over half a century. The anatomical position and physiological roles of the liver suggest it is a prime candidate to serve as an interoceptive sensor of peripheral tissue and systemic energy state. Importantly, maintenance of liver ATP levels and within-meal food intake inhibition is impaired in human subjects with obesity and obese pre-clinical models. Previously, we have shown decreased hepatic mitochondrial energy metabolism (i.e., oxidative metabolism & ADP-dependent respiration) in male liver-specific, heterozygous PGC1a mice results in increased short-term diet-induced weight gain with increased within meal food intake. Herein, we tested the hypothesis that decreased liver mitochondrial energy metabolism impairs meal termination following nutrient oral pre-loads.

METHODS

Liver mitochondrial respiratory response to changes in ΔGATP and adenine nucleotide concentration following fasting were examined in male liver-specific, heterozygous PGC1a mice. Further, food intake and feeding behavior during basal conditions, following nutrient oral pre-loads, and following fasting were investigated.

RESULTS

We observed male liver-specific, heterozygous PGC1a mice have reduced mitochondrial response to changes in ΔGATP and tissue ATP following fasting. These impairments in liver energy state are associated with larger and longer meals during chow feeding, impaired dose-dependent food intake inhibition in response to mixed and individual nutrient oral pre-loads, and greater acute fasting-induced food intake.

CONCLUSIONS

These data support previous work proposing liver-mediated food intake regulation through modulation of peripheral satiation signals.

Genome-wide association study revealed candidate genes associated with egg-laying time traits in layer chicken

In modern intensive caged laying hen production, variations in egg-laying time (ELT) among layers often increase the workload for egg collection, thereby raising the costs of labor or power and reducing overall efficiency. For management purpose, early and synchronized ELT is also advantageous, particularly to large-scale layer farm. However, the underlying genetic mechanisms of ELT remain unclear. In this study, through the development of video and artificial intelligence-based software, ELT records during the peak laying period (27-32 weeks) from 507 layers, and their earlier laying performance (21-32 weeks) were collected. Via whole genome sequencing data of all the individuals, the estimated heritabilities of traditional egg production traits ranged from 0.23 to 0.36, consistent with previous reports. The heritability of average egg-laying time (AELT) was estimated as 0.46. Furthermore, individuals with earlier AELT tended to exhibit superior egg production performance. Genome-wide association study revealed three SNPs associated with AELT traits, located at 170,867,650 bp on chromosome 1, at 5,548,087 and 5,817,488 bp on chromosome 9. Across the region of 5.4 to 7.0 Mb on chromosome 9, mutations were also identified to be strongly linked with the two AELT-associated SNPs. Genes located in this region may be responsible for the differences in AELT among hens. These results indicate that ELT has the potential to be integrated into the production system of caged layers. If ELT is to be included as a breeding objective in the future, its reliability needs to be validated in larger populations and over longer periods.

Effect of Anesthesia and Diurnal Variation on Chronic Vagus Nerve Activity in Rats

The vagus nerve, serving as a pivotal link between the brain and vital organs, regulates crucial physiological functions. It plays a central role in maintaining homeostasis within the body and must dynamically adapt to changing conditions such as anesthesia or sleep. While vagal tone, typically estimated indirectly from heart rate variability, has been extensively studied, direct measurement of vagal activity during sleep and anesthesia remains unreported to date. Recent technological advancements have facilitated the recording of vagus nerve activity in freely moving rodents using small, highly flexible carbon nanotube yarns. Consequently, it is now feasible to directly investigate vagal activity during events known to impact homeostasis, such as diurnal variations and anesthesia. In this study, we explore the relationship between anesthesia and vagus nerve activity by comparing the effects of 2% isoflurane anesthesia with activity in freely moving male Sprague Dawley rats. The findings reveal that 2% isoflurane anesthesia significantly suppresses vagus nerve activity, and normal activity levels do not resume until 2 h after the termination of the anesthesia supply. Additionally, we examine the influence of diurnal variations on vagus nerve activity and observe a notable presence of diurnal variations in vagal activity patterns. These results provide insights into the interaction among anesthesia, diurnal variations, and vagal tone, offering valuable understanding of the autonomic nervous system during critical physiological states.

From Physiology to Psychiatry: Key role of vagal interoceptive pathways in emotional control

Interoception is the awareness of bodily sensations, conveyed by both hormonal and neural signals. The vagus nerve is the primary neural interoceptive conduit, responsible for transmitting information from peripheral organs to the brain. It is widely accepted that vagal signals are essential for purely physiological functions like blood pressure maintenance, or nutrient intake homeostasis. However, a growing body of evidence, taking advantage of new technological advances, suggests that the vagus nerve also orchestrates or tunes emotions. Disruption of vagal interoceptive feedback prevents normal emotional control in rodents. Importantly, accumulating evidence indicates that pathological disruption of vagal afferent signals also occurs in humans and may constitute an important risk factor for emotional disorders. Hence, alleviating vagal interoceptive deficits may constitute a new therapeutic avenue for neurotic and affective disorders. Considering the technical and safety challenges for direct stimulation of brain regions relevant to emotionality disorders, the vagus nerve offers a safer and more practical route of potentially achieving similar outcomes. Here we will highlight the earliest studies which examined the consequences of manipulations of the vagal afferent neurons on anxiety, fear, and mood, and integrate these older findings with new research investigating the necessity of vagal afferent neurons in mediating the anxiety or mood-altering effects of physiological signals. We will also discuss the evolutionary significance of vagal control over emotional states within the boundaries of "normal" physiology and conclude by discussing the challenges of engaging the vagal interoception as novel therapeutics in mental health disorders.

Liver Neurobiology: Regulation of Liver Functions by the Nervous System

The nervous system plays an important role in the regulation of liver functions during physiological as well as pathological conditions. This regulatory effect is based on the processing of signals transmitted to the brain by sensory nerves innervating the liver tissue and other visceral organs and by humoral pathways transmitting signals from peripheral tissues and organs. Based on these signals, the brain modulates metabolism, detoxification, regeneration, repair, inflammation, and other processes occurring in the liver. The nervous system thus determines the functional and morphological characteristics of the liver. Liver innervation also mediates the influence of psychosocial factors on liver functions. The aim of this review is to describe complexity of bidirectional interactions between the brain and liver and to characterize the mechanisms and pathways through which the nervous system influences liver function during physiological conditions and maintains liver and systemic homeostasis.

Potential bidirectional communication between the liver and the central circadian clock in MASLD

Most aspects of physiology and behaviour fluctuate every 24 h in mammals. These circadian rhythms are orchestrated by an autonomous central clock located in the suprachiasmatic nuclei that coordinates the timing of cellular clocks in tissues throughout the body. The critical role of this circadian system is emphasized by increasing evidence associating disruption of circadian rhythms with diverse pathologies. Accordingly, mounting evidence suggests a bidirectional relationship where disruption of rhythms by circadian misalignment may contribute to liver diseases while liver diseases alter the central clock and circadian rhythms in other tissues. Therefore, liver pathophysiology may broadly impact the circadian system and may provide a mechanistic framework for understanding and targeting metabolic diseases and adjust metabolic setpoints.

Integrating cancer medicine into metabolic rhythms

Circadian rhythms are cell-intrinsic time-keeping mechanisms that allow organisms to adapt to 24-h environmental changes, ensuring coordinated physiological functions by aligning internal metabolic oscillations with external timing cues. Disruption of daily metabolic rhythms is associated with pathological events such as cancer development, yet the mechanisms by which perturbed metabolic rhythms contribute to tumorigenesis remain unclear. Herein we review how circadian clocks drive balanced rhythmic metabolism which in turn governs physiological functions of locomotor, immune, and neuroendocrine systems. Misaligned metabolic rhythms cause pathological states which further drive cancer initiation, progression, and metastasis. Restoring the balance of metabolic rhythms with chemical, hormonal, and behavioral interventions serves as a promising strategy for cancer therapy.

Enhancer binding as a KEysTONE of fasting response

Fasting is a recurrent daily energy stress that benefits healthspan and lifespan. While ketones fuel fasting in vertebrates, the underlying transcriptional mechanism remains incompletely understood. Recently, Korenfeld et al. revealed peroxisome proliferator-activated receptor alpha (PPARα)-dependent enhancer priming as a keystone for ketone production, increasing our understanding of mechanisms underlying metabolic benefits of alternate-day fasting (ADF).

Dietary timing enhances exercise by modulating fat-muscle crosstalk via adipocyte AMPKα2 signaling

Feeding rhythms regulate exercise performance and muscle energy metabolism. However, the mechanisms regulating adipocyte functions remain unclear. Here, using multi-omics analyses, involving (phospho-)proteomics and lipidomics, we found that day-restricted feeding (DRF) regulates diurnal rhythms of the mitochondrial proteome, neutral lipidome, and nutrient-sensing pathways in mouse gonadal white adipose tissue (GWAT). Adipocyte-specific knockdown of Prkaa2 (the gene encoding AMPKα2) impairs physical endurance. This defect is associated with altered rhythmicity in acyl-coenzyme A (CoA) metabolism-related genes, a loss of rhythmicity in the GWAT lipidome, and circadian remodeling of serum metabolites-in particular, lactate and succinate. We also found that adipocyte Prkaa2 regulates muscle clock genes during DRF. Notably, oral administration of the AMPK activator C29 increases endurance and muscle functions in a time-of-day manner, which requires intact adipocyte AMPKα2 signaling. Collectively, our work defines adipocyte AMPKα2 signaling as a critical regulator of circadian metabolic coordination between fat and muscle, thereby enhancing exercise performance.

Liver-innervating vagal sensory neurons are indispensable for the development of hepatic steatosis and anxiety-like behavior in diet-induced obese mice

The visceral organ-brain axis, mediated by vagal sensory neurons, is essential for maintaining various physiological functions. Here, we investigate the impact of liver-projecting vagal sensory neurons on energy balance, hepatic steatosis, and anxiety-like behavior in mice under obesogenic conditions. A small subset of vagal sensory neurons innervate the liver and project centrally to the nucleus of the tractus solitarius, area postrema, and dorsal motor nucleus of the vagus, and peripherally to the periportal areas in the liver. The loss of these neurons prevents diet-induced obesity, and these outcomes are associated with increased energy expenditure. Although males and females exhibit improved glucose homeostasis following disruption of liver-projecting vagal sensory neurons, only male mice display increased insulin sensitivity. Furthermore, the loss of liver-projecting vagal sensory neurons limits the progression of hepatic steatosis. Intriguingly, mice lacking liver-innervating vagal sensory neurons also exhibit less anxiety-like behavior compared to control mice. Modulation of the liver-brain axis may aid in designing effective treatments for both psychiatric and metabolic disorders associated with obesity and MAFLD.

Genetic synchronization of the brain and liver molecular clocks defend against chrono-metabolic disease

Nearly every cell of the body contains a circadian clock mechanism that is synchronized with the light-entrained clock in the suprachiasmatic nucleus (SCN). Desynchrony between the SCN and the external environment leads to metabolic dysfunction in shift workers. Similarly, mice with markedly shortened endogenous period due to the deletion of circadian REV-ERBα/β nuclear receptors in the SCN (SCN DKO) exhibit increased sensitivity to diet-induced obesity (DIO) on a 24 h light:dark cycle while mice with REV-ERBs deleted in hepatocytes (HepDKO) display exacerbated hepatosteatosis in response to a high-fat diet. Here, we show that inducing deletion of hepatocyte REV-ERBs in SCN DKO mice (Hep-SCN DDKO) rescued the exacerbated DIO and hepatic triglyceride accumulation, without affecting the shortened behavioral period. These findings suggest that metabolic disturbances due to environmental desynchrony with the central clock are due to effects on peripheral clocks which can be mitigated by matching peripheral and central clock periods even in a desynchronous environment. Thus, maintaining synchrony within an organism, rather than between endogenous and exogenous clocks, may be a viable target for the treatment of metabolic disorders associated with circadian disruption.