Regulation of the central melanocortin system on energy balance in mammals and birds

Drosophila as a Genetic Model System to Study Organismal Energy Metabolism

Metabolism is the essential process by which an organism converts nutrients into energy to fuel growth, development, and repair. Metabolism at the level of a multicellular, multi-organ animal is inherently more complex than metabolism at the single-cell level. Indeed, each organ also must maintain its own homeostasis to function. At all three scales, homeostasis is a defining feature: as energy sources and energetic demands wax and wane, the system must be robust. While disruption of organismal energy homeostasis can be manifested in different ways in humans, obesity (defined as excess body fat) is an increasingly common outcome of metabolic imbalance. Here we will discuss the genetic basis of metabolic dysfunction that underlies obesity. We focus on what we are learning from Drosophila melanogaster as a model organism to explore and dissect genetic causes of metabolic dysfunction in the context of a whole organism.

Urbanization's hidden influence: Linking landscape alterations and feather coloration with pigeon's cholesterol levels

Urbanization significantly impacts synanthropic birds, influencing their physiology and appearance. For instance, urban environments are associated with higher plasma cholesterol levels in birds due to human-derived food sources. Additionally, landscape changes create environmental pressures, which favor more melanic plumages. Current evidence suggests that urbanization may influence bird cholesterol through mechanisms beyond diet, possibly via the pleiotropic effects of the melanocortin system. In this study, we examine how cholesterolemia varies in urban pigeons (Columba livia), a species with polymorphic plumage, across different levels of urbanization and degrees of melanism. To investigate this, we analyzed pigeons along urbanization gradients and sampled birds from various locations in Santiago, a South American metropolis. Plumage color was characterized using image analysis, and the expression of the MITF and MC1R genes in the skin was measured. Blood samples were analyzed for biochemical parameters, genetic sex determination, and nitrogen isotopic signatures (δ15N) in red blood cells. We found correlations between human-induced spatial heterogeneity indexes (e.g., Impervious Surface, Global Human Modification, and Land Surface Temperature) and plumage melanism. However, no direct relationship was found between these landscape features and cholesterol levels, although there were links to glucose and triglycerides. Interestingly, plasma cholesterol levels strongly correlated with plumage melanism features independent of δ15N, suggesting a non-trophic origin of elevated cholesterol. Furthermore, we identified and quantified the indirect effects of urbanization on cholesterolemia using structural equation modeling. This evidence highlights the interplay between urban stressors and bird melanism, underscoring the importance of pleiotropic phenomena in socio-eco-evolutionary dynamics of urban ecosystems.

Insulin and leptin acutely modulate the energy metabolism of primary hypothalamic and cortical astrocytes

Astrocytes constitute a heterogeneous cell population within the brain, contributing crucially to brain homeostasis and playing an important role in overall brain function. Their function and metabolism are not only regulated by local signals, for example, from nearby neurons, but also by long-range signals such as hormones. Thus, two prominent hormones primarily known for regulating the energy balance of the whole organism, insulin, and leptin, have been reported to also impact astrocytes within the brain. In this study, we investigated the acute regulation of astrocytic metabolism by these hormones in cultured astrocytes prepared from the mouse cortex and hypothalamus, a pivotal region in the context of nutritional regulation. Utilizing genetically encoded, fluorescent nanosensors, the cytosolic concentrations of glucose, lactate, and ATP, along with glycolytic rate and the NADH/NAD+ redox state were measured. Under basal conditions, differences between the two populations of astrocytes were observed for glucose and lactate concentrations as well as the glycolytic rate. Additionally, astrocytic metabolism responded to insulin and leptin in both brain regions, with some unique characteristics for each cell population. Finally, both hormones influenced how cells responded to elevated extracellular levels of potassium ions, a common indicator of neuronal activity. In summary, our study provides evidence that insulin and leptin acutely regulate astrocytic metabolism within minutes. Additionally, while astrocytes from the hypothalamus and cortex share similarities in their metabolism, they also exhibit distinct properties, further underscoring the growing recognition of astrocyte heterogeneity.

Perinatal metabolic inflammation in the hypothalamus impairs the development of homeostatic feeding circuitry

Over the past few decades, there has been a global increase in childhood obesity. This rise in childhood obesity contributes to the susceptibility of impaired metabolism during both childhood and adulthood. The hypothalamus, specifically the arcuate nucleus (ARC), houses crucial neurons involved in regulating homeostatic feeding. These neurons include proopiomelanocortin (POMC) and agouti-related peptide (AGRP) secreting neurons. They play a vital role in sensing nutrients and metabolic hormones like insulin, leptin, and ghrelin. The neurogenesis of AGRP and POMC neurons completes at birth; however, axon development and synapse formation occur during the postnatal stages in rodents. Insulin, leptin, and ghrelin are the essential regulators of POMC and AGRP neurons. Maternal obesity and postnatal overfeeding or a high-fat diet (HFD) feeding cause metabolic inflammation, disrupted signaling of metabolic hormones, netrin-1, and neurogenic factors, neonatal obesity, and defective neuronal development in animal models; however, the mechanism is unclear. Within the hypothalamus and other brain areas, there exists a wide range of interconnected neuronal populations that regulate various aspects of feeding. However, this review aims to discuss how perinatal metabolic inflammation influences the development of POMC and AGRP neurons within the hypothalamus.

Selenium in Bodily Homeostasis: Hypothalamus, Hormones, and Highways of Communication

The ability of the body to maintain homeostasis requires constant communication between the brain and peripheral tissues. Different organs produce signals, often in the form of hormones, which are detected by the hypothalamus. In response, the hypothalamus alters its regulation of bodily processes, which is achieved through its own pathways of hormonal communication. The generation and transmission of the molecules involved in these bi-directional axes can be affected by redox balance. The essential trace element selenium is known to influence numerous physiological processes, including energy homeostasis, through its various redox functions. Selenium must be obtained through the diet and is used to synthesize selenoproteins, a family of proteins with mainly antioxidant functions. Alterations in selenium status have been correlated with homeostatic disturbances in humans and studies with animal models of selenoprotein dysfunction indicate a strong influence on energy balance. The relationship between selenium and energy metabolism is complicated, however, as selenium has been shown to participate in multiple levels of homeostatic communication. This review discusses the role of selenium in the various pathways of communication between the body and the brain that are essential for maintaining homeostasis.

Agouti-related protein as the glucose signaling sensor in the central melanocortin circuits in regulating fish food intake

Agouti-related protein (AgRP) is a neuropeptide synthesized by AgRP/NPY neurons and transcribed as 132 amino acids in humans and 142 amino acids (AgRP1) in Japanese seabass (Lateolabrax maculatus) fish. AgRP neurons are activated by hormonal signals of energy deficits and inhibited by signals of energy surpluses and have been demonstrated to have the ability to sense the dynamics of blood glucose concentrations as the "glucose sensor" in mammals. It is widely recognized that AgRP is an endogenous antagonist of the melanocortin-3 and -4 receptors (MC3R and MC4R) in the hypothalamus, exhibiting potent orexigenic activity and control of energy homeostasis. Most fish, especially carnivorous fish, cannot make efficient use of carbohydrates. When carbohydrates like corn or wheat bran are added as energy sources, they often cause feeding inhibition and metabolic diseases. When fishmeal is replaced by plant protein, this does not completely eliminate carbs, limiting the utilization of carbohydrates and plant proteins in aquaculture. Our previous study showed that AgRP, and not neuropeptide Y (NPY) is the principal protein molecule that correlates well with feeding behavior in Japanese seabass from anorexia to adaptation. The Ghrelin/Leptin-mTOR-S6K1-NPY/AgRP/POMC feed intake regulatory pathway responds to the plant-oriented protein which contains glucose. However, its regulatory function and mechanism are still not clear. This review offers an integrative overview of how glucose signals converge on a molecular level in AgRP neurons of the arcuate nucleus of the hypothalamus. This is in order to control fish food intake and energy homeostasis.