Rhythmic clock and neuropeptide gene expression in hypothalamic mHypoE-44 neurons

Nutrition, metabolism, and epigenetics: pathways of circadian reprogramming

Food intake profoundly affects systemic physiology. A large body of evidence has indicated a link between food intake and circadian rhythms, and ~24-h cycles are deemed essential for adapting internal homeostasis to the external environment. Circadian rhythms are controlled by the biological clock, a molecular system remarkably conserved throughout evolution. The circadian clock controls the cyclic expression of numerous genes, a regulatory program common to all mammalian cells, which may lead to various metabolic and physiological disturbances if hindered. Although the circadian clock regulates multiple metabolic pathways, metabolic states also provide feedback on the molecular clock. Therefore, a remarkable feature is reprogramming by nutritional challenges, such as a high-fat diet, fasting, ketogenic diet, and caloric restriction. In addition, various factors such as energy balance, histone modifications, and nuclear receptor activity are involved in the remodeling of the clock. Herein, we review the interaction of dietary components with the circadian system and illustrate the relationships linking the molecular clock to metabolism and critical roles in the remodeling process.

Mechanisms Driving Palmitate-Mediated Neuronal Dysregulation in the Hypothalamus

The hypothalamus maintains whole-body homeostasis by integrating information from circulating hormones, nutrients and signaling molecules. Distinct neuronal subpopulations that express and secrete unique neuropeptides execute the individual functions of the hypothalamus, including, but not limited to, the regulation of energy homeostasis, reproduction and circadian rhythms. Alterations at the hypothalamic level can lead to a myriad of diseases, such as type 2 diabetes mellitus, obesity, and infertility. The excessive consumption of saturated fatty acids can induce neuroinflammation, endoplasmic reticulum stress, and resistance to peripheral signals, ultimately leading to hyperphagia, obesity, impaired reproductive function and disturbed circadian rhythms. This review focuses on the how the changes in the underlying molecular mechanisms caused by palmitate exposure, the most commonly consumed saturated fatty acid, and the potential involvement of microRNAs, a class of non-coding RNA molecules that regulate gene expression post-transcriptionally, can result in detrimental alterations in protein expression and content. Studying the involvement of microRNAs in hypothalamic function holds immense potential, as these molecular markers are quickly proving to be valuable tools in the diagnosis and treatment of metabolic disease.

Selenoprotein M Promotes Hypothalamic Leptin Signaling and Thioredoxin Antioxidant Activity

Aims: Selenoproteins are an essential class of proteins involved in redox signaling and energy metabolism. However, the functions of many selenoproteins are not clearly established. Selenoprotein M (SELENOM), an endoplasmic reticulum (ER)-resident oxidoreductase bearing structural similarity to thioredoxin (TXN), is among those yet to be fully characterized. This protein is highly expressed in hypothalamic regions involved in leptin signaling and has been previously linked to energy metabolism. Herein, we performed a series of studies using in vivo and in vitro models to probe the specific influence of SELENOM on hypothalamic leptin signaling and assess SELENOM-regulated pathways. Innovation and Results: Our initial experiment in vivo demonstrated that (i) leptin promotes hypothalamic expression of SELENOM and (ii) leptin-induced STAT3 phosphorylation is impeded by SELENOM deficiency. Additional in vitro studies using mHypoE-44 immortalized hypothalamic neurons corroborated these findings, as SELENOM deficiency obstructed downstream STAT3 phosphorylation and cytosolic calcium responses evoked by leptin treatment. Correspondingly, SELENOM overexpression enhanced leptin sensitivity. Microarray analysis conducted in parallel on hypothalamic tissue and mHypoE-44 cells revealed multiple genes significantly affected by SELENOM deficiency, including thioredoxin interacting protein, a negative regulator of the TXN system. Further analysis determined that (i) SELENOM itself possesses intrinsic TXN activity and (ii) SELENOM deficiency leads to a reduction in overall TXN activity. Finally, mHypoE-44 cells lacking SELENOM displayed diminished activation of the nuclear factor kappa-light-chain enhancer of activated B-cells (NF-κB) signaling pathway and increased susceptibility to ER stress-mediated cell death. Conclusion: In sum, these findings establish SELENOM as a positive regulator of leptin signaling and TXN antioxidant activity in the hypothalamus. Antioxid. Redox Signal. 35, 775-787.

Generation of Mouse Primary Hypothalamic Neuronal Cultures for Circadian Bioluminescence Assays

An endogenous circadian clock system enables organisms to adapt to time-of-day dependent environmental changes. In consequence, most physiological processes exhibit daily rhythms of, e.g., energy metabolism, immune function, sleep, or hormone production. Hypothalamic circadian clocks have been identified to play a particular role in coordinating many of these processes. Primary neuronal cultures are widely used as a physiologically relevant model to study molecular events within neurons. However, as circadian rhythms include dynamic molecular changes over longer timescales that vary between individual cells, longitudinal measurement methods are essential to investigate the regulation of circadian clocks of hypothalamic neurons. Here we provide a protocol for generating primary hypothalamic neuronal cultures expressing a circadian luciferase reporter. Such reporter cells can be used to longitudinally monitor cellular circadian rhythms at high temporal resolution by performing bioluminescence measurements.

Analysis of Western diet, palmitate and BMAL1 regulation of neuropeptide Y expression in the murine hypothalamus and BMAL1 knockout cell models

Western diets that are high in saturated fat and sugar disrupt circadian rhythms, induce weight gain, and lead to metabolic diseases including obesity. However, the mechanistic link between altered circadian rhythms and energy homeostasis remains poorly understood. In C57BL/6J mice, consuming a Western diet for 16 weeks significantly reduced food intake (at zeitgeber 12-16), in association with decreases in hypothalamic expression of the orexigenic neuropeptides, neuropeptide Y (Npy) and agouti-related peptide (AgRP). To examine the acute effects of the most prevalent saturated fatty acid in a Western diet, palmitate, and the role of the core clock gene, Bmal1, in the regulation of hypothalamic feeding neuropeptides, we used heterogeneous and clonal BMAL1 knockout (KO) immortalized hypothalamic cell lines, expressing specific neuropeptides, derived from male (M) and female (F) mice. Both mHypoA-BMAL1-KO/F and mHypoA-BMAL1-KO/M cells demonstrated a loss of circadian rhythmicity in expression of the clock gene, Per2, as compared to wild-type (control) cultures. Loss of BMAL1 also altered the time-dependent expression of Npy and proopiomelanocortin, and disrupted AgRP rhythmicity. Furthermore, palmitate increased BMAL1 binding to the Npy promotor region, and palmitate treatment (50 μM for 24 h) stimulated Npy expression in a BMAL1-dependent manner in both heterogeneous and clonal NPY-expressing female-derived cell models. The results of this study demonstrate that circadian expression of Bmal1 serves as a mechanistic link between Western diet- and palmitate-induced disruptions of the normal rhythmic patterns in hypothalamic feeding-related neuropeptides.

An adipokine feedback regulating diurnal food intake rhythms in mice

Endogenous circadian clocks have evolved to anticipate 24 hr rhythms in environmental demands. Recent studies suggest that circadian rhythm disruption is a major risk factor for the development of metabolic disorders in humans. Conversely, alterations in energy state can disrupt circadian rhythms of behavior and physiology, creating a vicious circle of metabolic dysfunction. How peripheral energy state affects diurnal food intake, however, is still poorly understood. We here show that the adipokine adiponectin (ADIPOQ) regulates diurnal feeding rhythms through clocks in energy regulatory centers of the mediobasal hypothalamus (MBH). Adipoq-deficient mice show increased rest phase food intake associated with disrupted transcript rhythms of clock and appetite-regulating genes in the MBH. ADIPOQ regulates MBH clocks via AdipoR1-mediated upregulation of the core clock gene Bmal1. BMAL1, in turn, controls expression of orexigenic neuropeptide expression in the MBH. Together, these data reveal a systemic metabolic circuit to regulate central circadian clocks and energy intake.

Bisphenol A Alters Bmal1, Per2, and Rev-Erba mRNA and Requires Bmal1 to Increase Neuropeptide Y Expression in Hypothalamic Neurons

Bisphenol A (BPA), a ubiquitous environmental endocrine disruptor, is considered an obesogen. However, its role in the hypothalamic control of energy balance remains largely unexplored. Because disruption of the circadian clock is tightly associated with metabolic consequences, we explored how BPA affects the components of the molecular circadian clock in the feeding-related neurons of the hypothalamus. In immortalized POMC and NPY/AgRP-expressing hypothalamic cell lines and primary culture, we describe how BPA significantly alters mRNA expression of circadian clock genes Bmal1,Per2, and Rev-Erbα. Furthermore, we use newly generated Bmal1-knockout (KO) hypothalamic cell lines to link the BPA-induced neuropeptide dysregulation to the molecular clock. Specifically, BPA increased Npy, Agrp, and Pomc mRNA expression in wild type hypothalamic cells, whereas the increase in Npy, but not Agrp or Pomc, was abolished in cell lines lacking BMAL1. In line with this increase, BPA led to increased BMAL1 binding to the Npy promotor, potentially increasing Npy transcription. In conclusion, we show that BPA-mediated dysregulation of the circadian molecular clock is linked to the deleterious effects of BPA on neuropeptide expression. Furthermore, we describe hypothalamic Bmal1-KO cell lines to study the role of BMAL1 in hypothalamic responses to metabolic, hormonal, and environmental factors.

Role of the saturated fatty acid palmitate in the interconnected hypothalamic control of energy homeostasis and biological rhythms

The brain, specifically the hypothalamus, controls whole body energy and glucose homeostasis through neurons that synthesize specific neuropeptides, whereas hypothalamic dysfunction is linked directly to insulin resistance, obesity, and type 2 diabetes mellitus. Nutrient excess, through overconsumption of a Western or high-fat diet, exposes the hypothalamus to high levels of free fatty acids, which induces neuroinflammation, endoplasmic reticulum stress, and dysregulation of neuropeptide synthesis. Furthermore, exposure to a high-fat diet also disrupts normal circadian rhythms, and conversely, clock gene knockout models have symptoms of metabolic disorders. While whole brain/animal studies have provided phenotypic end points and important clues to the genes involved, there are still major gaps in our understanding of the intracellular pathways and neuron-specific components that ultimately control circadian rhythms and energy homeostasis. Because of its complexity and heterogeneous nature, containing a diverse mix cell types, it is difficult to dissect the critical hypothalamic components involved in these processes. Of significance, we have the capacity to study these individual components using an extensive collection of both embryonic- and adult-derived, immortalized hypothalamic neuronal cell lines from rodents. These defined neuronal cell lines have been used to examine the impact of nutrient excess, such as palmitate, on circadian rhythms and neuroendocrine signaling pathways, as well as changes in vital neuropeptides, leading to the development of neuronal inflammation; the role of proinflammatory molecules in this process; and ultimately, restoration of normal signaling, clock gene expression, and neuropeptide synthesis in disrupted states by beneficial anti-inflammatory compounds in defined hypothalamic neurons.

Effects of continuous white light and 12h white-12h blue light-cycles on the expression of clock genes in diencephalon, liver, and skeletal muscle in chicks

The core circadian clock mechanism relies on a feedback loop comprised of clock genes, such as the brain and muscle Arnt-like 1 (Bmal1), chriptochrome 1 (Cry1), and period 3 (Per3). Exposure to the light-dark cycle synchronizes the master circadian clock in the brain, and which then synchronizes circadian clocks in peripheral tissues. Birds have long been used as a model for the investigation of circadian rhythm in human neurobiology. In the present study, we examined the effects of continuous light and the combination of white and blue light on the expression of clock genes (Bmal1, Cry1, and Per3) in the central and peripheral tissues in chicks. Seventy two day-old male chicks were weighed, allocated to three groups and maintained under three light schedules: 12h white light-12h dark-cycles group (control); 24h white light group (WW group); 12h white light-12h blue light-cycles group (WB group). The mRNA levels of clock genes in the diencephalon were significantly different between the control and WW groups. On the other hand, the alteration in the mRNA levels of clock genes was similar between the control and WB groups. Similar phenomena were observed in the liver and skeletal muscle (biceps femoris). These results suggest that 12h white-12h blue light-cycles did not disrupt the circadian rhythm of clock gene expression in chicks.

Differential effects of omega-3 fatty acid docosahexaenoic acid and palmitate on the circadian transcriptional profile of clock genes in immortalized hypothalamic neurons

Diets high in saturated fatty acids (SFAs) are associated with the development of circadian dysregulation, obesity, and Type 2 diabetes mellitus. Conversely, polyunsaturated fatty acids (PUFAs) have recently been identified to improve insulin sensitivity, reduce weight gain, and relieve obesity-induced inflammation. While saturated fatty acids, such as the prevalent dietary fatty acid palmitate, have been implicated in circadian disruption, there is a paucity of studies regarding the effects of PUFAs on circadian parameters. Therefore, the immortalized murine neuronal model, mHypoE-37, was utilized to examine the effects of the SFA palmitate and omega-3 PUFA docosahexaenoic acid (DHA) on circadian rhythms. The mHypoE-37 neurons express the core clock genes, Bmal1, Per2, and Rev-erbα, in a circadian manner. 25 μM of palmitate significantly increased the transcriptional expression of Bmal1, without altering the expression of inflammatory markers TLR4, IκBα, and IL-6, nor the orexigenic neuropeptide AgRP, suggesting that the observed disruption of the molecular clock is the result of a mechanism distinct from that of hypothalamic cellular inflammation. Furthermore, treatment with the PUFA DHA resulted in alterations in the circadian expression profile of Bmal1, although differentially from the effects of palmitate. In the presence of DHA, the disruptive effects of palmitate on Bmal1 were less pronounced, suggesting a protective effect of DHA. These studies are the first to identify the potential for omega-3 PUFAs to protect against palmitate-mediated dysregulation of circadian parameters and will ultimately improve the understanding of circadian control mechanisms.

Gene array analysis of embryonic- versus adult-derived hypothalamic NPY-expressing cell lines

Few studies have utilized microarray analysis to understand the genome wide changes involved in the development of the hypothalamus despite its overall importance to basic physiology. Gene expression profiling of immortalized, clonal hypothalamic neurons, embryonic-derived mHypoE-46 and adult-derived mHypoA-2/12, reveals that the expression of 1225 probes was significantly changed between the two neuronal models. Further comparison of the gene expression profiles identified two categories of genes that were confirmed with qRT-PCR: (i) genes implicated in the Wnt signaling pathway; and (ii) transcription factors previously implicated in the development of the central nervous system. Yet, functional analysis of the two cell lines, including hormonal responses and secretion, indicate that they are comparable despite their developmental origin. This study provides a comprehensive analysis of embryonic- and adult-derived hypothalamic neuronal cell models that both express neuropeptide Y, and identifies novel genes as candidates for mediating the development of specific hypothalamic neurons.

Palmitate alters the rhythmic expression of molecular clock genes and orexigenic neuropeptide Y mRNA levels within immortalized, hypothalamic neurons

The control of energy homeostasis within the hypothalamus is under the regulated control of homeostatic hormones, nutrients and the expression of neuropeptides that alter feeding behavior. Elevated levels of palmitate, a predominant saturated fatty acid in diet and fatty acid biosynthesis, alter cellular function. For instance, a key mechanism involved in the development of insulin resistance is lipotoxicity, through increased circulating saturated fatty acids. Although many studies have begun to determine the underlying mechanisms of lipotoxicity in peripheral tissues, little is known about the effects of excess lipids in the brain. To determine these mechanisms we used an immortalized, clonal, hypothalamic cell line, mHypoE-44, to demonstrate that palmitate directly alters the expression of molecular clock components, by increasing Bmal1 and Clock, or by decreasing Per2, and Rev-erbα, their mRNA levels and altering their rhythmic period within individual neurons. We found that these neurons endogenously express the orexigenic neuropeptides NPY and AgRP, thus we determined that palmitate administration alters the mRNA expression of these neuropeptides as well. Palmitate treatment causes a significant increase in NPY mRNA levels and significantly alters the phase of rhythmic expression. We explored the link between AMPK and the expression of neuropeptide Y using the AMPK inhibitor compound C and the AMP analog AICAR. AMPK inhibition decreased NPY mRNA. AICAR also elevated basal NPY, but prevented the palmitate-mediated increase in NPY mRNA levels. We postulate that this palmitate-mediated increase in NPY and AgRP synthesis may initiate a detrimental positive feedback loop leading to increased energy consumption.

Investigation of the presence of ghrelin in the central nervous system of the rat and mouse

Ghrelin and ghrelin receptor agonist have effects on central neurons in many locations, including the hypothalamus, caudal brain stem, and spinal cord. However, descriptions of the distributions of ghrelin-like immunoreactivity in the CNS in published work are inconsistent. We have used three well-characterized anti-ghrelin antibodies, an antibody to the unacylated form of ghrelin, and a ghrelin peptide assay in rats, mice, ghrelin knockout mice, and ghrelin receptor reporter mice to re-evaluate ghrelin presence in the rodent CNS. The stomach served as a positive control. All antibodies were effective in revealing gastric endocrine cells. However, no specific staining could be found in the brain or spinal cord. Concentrations of antibody 10 to 30 times those effective in the stomach bound to nerve cells in rat and mouse brain, but this binding was not reduced by absorbing concentrations of ghrelin peptide, or by use of ghrelin gene knockout mice. Concentrations of ghrelin-like peptide, detected by enzyme-linked immunosorbent assay in extracts of hypothalamus, were 1% of gastric concentrations. Ghrelin receptor-expressing neurons had no adjacent ghrelin immunoreactive terminals. It is concluded that there are insignificant amounts of authentic ghrelin in neurons in the mouse or rat CNS and that ghrelin receptor-expressing neurons do not receive synaptic inputs from ghrelin-immunoreactive nerve terminals in these species.