Molecular control of the development of hypothalamic neurons involved in metabolic regulation.

The hypothalamus is a large brain region made of nuclei and areas involved in the control of behaviors and physiological regulations. Among them, the arcuate nucleus (ARH) and the lateral hypothalamic area (LHA) contain key neuronal populations expressing the pro-opiomelanocortin (POMC), the agouti-related peptide (AgRP), and the melanin-concentrating hormone (MCH), respectively, that are involved in goal-oriented behaviors (such as feeding behavior) and glucose homeostasis. These neuronal populations are generated from distinct parts of the germinative neuroepithelium during embryonic life, and acquire their cell fate under the influence of morphogen proteins, specific transcription factors, and epigenetic modulators. POMC and MCH neuronal development continues by sending long descending axonal projections before birth under the control of axon guidance molecules such as Netrin1 and Slit2. Later, during the postnatal period, POMC and AgRP neurons develop intra-hypothalamic projections notably to the paraventricular nucleus of the hypothalamus through the influence of other axon guidance cues such as the class3 Semaphorins. Other cellular processes, such as autophagy and primary cilia function, and hormonal cues also appear critical for the proper development of POMC neurons.

Hypothalamic Irak4 is a genetically controlled regulator of hypoglycemia-induced glucagon secretion.

OBJECTIVES: Glucagon secretion to stimulate hepatic glucose production is the first line of defense against hypoglycemia. This response is triggered by so far incompletely characterized central hypoglycemia-sensing mechanisms, which control autonomous nervous activity and hormone secretion. The objective of this study was to identify novel hypothalamic genes controlling insulin-induced glucagon secretion. METHODS: To obtain new information on the mechanisms of hypothalamic hypoglycemia sensing, we combined genetic and transcriptomic analysis of glucagon response to insulin-induced hypoglycemia in a panel of BXD recombinant inbred mice. RESULTS: We identified two QTLs on chromosome 8 and chromosome 15. We further investigated the role of Irak4 and Cpne8, both located in the QTL on chromosome 15, in C57BL/6J and DBA/2J mice, the BXD mouse parental strains. We found that the poor glucagon response of DBA/2J mice was associated with higher hypothalamic expression of Irak4, which encodes a kinase acting downstream of the interleukin-1 receptor (Il-1R), and of Il-ß when compared with C57BL/6J mice. We showed that intracerebroventricular administration of an Il-1R antagonist in DBA/2J mice restored insulin-induced glucagon secretion; this was associated with increased c-fos expression in the arcuate and paraventricular nuclei of the hypothalamus and with higher activation of both branches of the autonomous nervous system. Whole body inactivation of Cpne8, which encodes a Ca++-dependent regulator of membrane trafficking and exocytosis, however, had no impact on insulin-induced glucagon secretion. CONCLUSIONS: Collectively, our data identify Irak4 as a genetically controlled regulator of hypoglycemia-activated hypothalamic neurons and glucagon secretion.

Structural and molecular characterization of paraventricular thalamic glucokinase-expressing neuronal circuits in the mouse.

The thalamic paraventricular nucleus (PVT) is a structure highly interconnected with several nuclei ranging from forebrain to hypothalamus and brainstem. Numerous rodent studies have examined afferent and efferent connections of the PVT and their contribution to behavior, revealing its important role in the integration of arousal cues. However, the majority of these studies used a region-oriented approach, without considering the neuronal subtype diversity of the nucleus. In the present study, we provide the anatomical and transcriptomic characterization of a subpopulation of PVT neurons molecularly defined by the expression of glucokinase (Gck). Combining a genetically modified mouse model with viral tracing approaches, we mapped both the anterograde and the retrograde projections of Gck-positive neurons of the anterior PVT (GckaPVT ). Our results demonstrated that GckaPVT neurons innervate several nuclei throughout the brain axis. The strongest connections are with forebrain areas associated with reward and stress and with hypothalamic structures involved in energy balance and feeding regulation. Furthermore, transcriptomic analysis of the Gck-expressing neurons revealed that they are enriched in receptors for hypothalamic-derived neuropeptides, adhesion molecules, and obesity and diabetes susceptibility transcription factors. Using retrograde labeling combined with immunohistochemistry and in situ hybridization, we identify that GckaPVT neurons receive direct inputs from well-defined hypothalamic populations, including arginine-vasopressin-, melanin-concentrating hormone-, orexin-, and proopiomelanocortin-expressing neurons. This detailed anatomical and transcriptomic characterization of GckaPVT neurons provides a basis for functional studies of the integration of homeostatic and hedonic aspects of energy homeostasis, and for deciphering the potential role of these neurons in obesity and diabetes development.

The distribution of Dlx1-2 and glutamic acid decarboxylase in the embryonic and adult hypothalamus reveals three differentiated LHA subdivisions in rodents.

The lateral hypothalamus (LHA) is still a poorly understood brain region. Based on published Dlx and Gad gene expression patterns in the embryonic and adult hypothalamus respectively, three large areas are identified in the LHA. A central tuberal LHA region is already well described as it contains neurons producing the peptides melanin-concentrating hormone or hypocretin. This region is rich in GABAergic neurons and is specified by Dlx gene expression in the rodent embryo. Rostrally and caudally bordering the tuberal LHA, two Dlx-GAD-GABA poor regions are then easily delineated. The three regions show different organizational schema. The tuberal region is reticularly organized, connected with the cerebral cortex and the spinal cord, and its embryonic development occurs along the tractus postopticus. The region anterior to it is associated with the stria medullaris in both embryonic and adult subjects. The posterior LHA region is made of differentiated nuclei and includes the subthalamic nucleus. Therefore, the LHA is divided into three distinct parts: in addition to the well-known tuberal LHA, caudal and anterior LHA regions exist that have specific anatomical and functional characteristics. The hypothalamus is made up of several dozens of nuclei or areas that are more or less well differentiated and whose boundaries and arrangements are drawn differently according to authors and atlases (Allen Institute, 2004; Paxinos and Franklin, 2019; Paxinos and Watson, 2013; Swanson, 2004). The dominant hypothesis for more than 50 years is that these structures are distributed within three antero-posterior areas (anterior, tuberal, posterior) and more or less three longitudinal zones (lateral, medial and periventricular) (Fig. 1). In addition to these regions, several adjacent territories are often associated to the hypothalamus. The preoptic area is functionally related to the hypothalamus, but it is better seen as a telencephalic structure based on developmental data (Croizier et al., 2015; Puelles and Rubenstein, 2015). Lately, the zona incerta and the subthalamic nucleus (STN) have also been associated to the hypothalamus on the basis of their connections and development for the STN (Altman and Bayer, 1986; Barbier and Risold, 2021; Swaab et al., 2021). However, the zona incerta is still included in the 'pre-thalamus' or "ventral thalamus" in the embryo (Puelles and Rubenstein, 2015). Thus, the boundaries of the hypothalamus remain blurred around what we can call a 'core' made of the anterior to posterior regions (Brooks, 1988). In addition, unlike other large brain regions that are characterized early on by a molecular signature, i.e. by the embryonic expression of specific molecular markers, data illustrating the distribution of dozens of transcription factors involved in brain patterning and cell lineage specification confirmed the extremely heterogeneous and mosaic nature of the anterior and posterior regions of the hypothalamus (Alvarez-Bolado, 2019; Puelles et al., 2013; Puelles and Rubenstein, 2015). The rich nuclear organization of the medial and periventricular zones of the hypothalamus is consistent with the mosaic expression of developmental genes. The LHA, however, is often perceived as much more homogeneous in its cytoarchitectural organization. At the same time, there is little information regarding the expression of developmental genes in the anterior and posterior territories of the LHA. Most studies focus on the tuberal LHA which expresses many of these genes. Admittedly, even in the adult hypothalamus, the internal boundaries of the LHA are difficult to identify and the same is true in the embryo. Developmental data alone are insufficient to achieve a better understanding of the LHA anatomical organization and for this region as for medial and periventricular zones, a coherence must be established between development and adult anatomical organization. Among the most useful neurochemical markers to identify large regions of the forebrain, those involved in the identification of GABAergic and glutamatergic neurons have proven to be particularly efficient. Indeed, GABAergic neurons are not ubiquitously distributed. Large regions of the forebrain are rich in such cells, including the basal telencephalon, but others contain few or no GABAergic cells and are rich in glutamatergic neurons instead (for example the dorsal thalamus that is free of GABA-neurons in rodents). The same applies for the hypothalamus: several structures of the hypothalamus are free of GABAergic neurons, as, for example, the mammillary nuclei (Hahn et al., 2019). Recently, we also identified a GABA-poor posterior LHA territory that includes the (STN), and is localized caudal to the GABA-rich tuberal LHA (Barbier et al., 2020; Barbier and Risold, 2021; Chometton et al., 2016b). Therefore, the LHA seems partitioned into GABA-rich/GABA-poor regions. However, to define or confirm distinct neuroanatomical entities, these regions must have a specific embryological origin, and show specific hodological patterns and functions. Hence, the purpose of this short review is to identify divisions of the LHA based on developmental and neurochemical criteria. Such an analysis seems to us relevant in order to allow later functional studies on regions whose boundaries will be based on objective criteria.

Ontogeny of ependymoglial cells lining the third ventricle in mice.

Introduction: During hypothalamic development, the germinative neuroepithelium gives birth to diverse neural cells that regulate numerous physiological functions in adulthood. Methods: Here, we studied the ontogeny of ependymal cells in the mouse mediobasal hypothalamus using the BrdU approach and publicly available single-cell RNAseq datasets. Results: We observed that while typical ependymal cells are mainly produced at E13, tanycyte birth depends on time and subtypes and lasts up to P8. Typical ependymocytes and ß tanycytes are the first to arise at the top and bottom of the dorsoventral axis around E13, whereas α tanycytes emerge later in development, generating an outside-in dorsoventral gradient along the third ventricle. Additionally, α tanycyte generation displayed a rostral-to-caudal pattern. Finally, tanycytes mature progressively until they reach transcriptional maturity between P4 and P14. Discussion: Altogether, this data shows that ependyma generation differs in time and distribution, highlighting the heterogeneity of the third ventricle.

Ontogeny of the Projections From the Dorsomedial Division of the Anterior Bed Nucleus of the Stria Terminalis to Hypothalamic Nuclei.

The bed nucleus of the stria terminalis (BNST) is a telencephalic structure well-connected to hypothalamic regions known to control goal-oriented behaviors such as feeding. In particular, we showed that the dorsomedial division of the anterior BNST innervate neurons of the paraventricular (PVH), dorsomedial (DMH), and arcuate (ARH) hypothalamic nuclei as well as the lateral hypothalamic area (LHA). While the anatomy of these projections has been characterized in mice, their ontogeny has not been studied. In this study, we used the DiI-based tract tracing approach to study the development of BNST projections innervating several hypothalamic areas including the PVH, DMH, ARH, and LHA. These results indicate that projections from the dorsomedial division of the anterior BNST to hypothalamic nuclei are immature at birth and substantially reach the PVH, DMH, and the LHA at P10. In the ARH, only sparse fibers are observed at P10, but their density increased markedly between P12 and P14. Collectively, these findings provide new insight into the ontogeny of hypothalamic circuits, and highlight the importance of considering the developmental context as a direct modulator in their proper formation.

Glucokinase neurons of the paraventricular nucleus of the thalamus sense glucose and decrease food consumption.

The paraventricular nucleus of the thalamus (PVT) controls goal-oriented behavior through its connections to the nucleus accumbens (NAc). We previously characterized Glut2aPVT neurons that are activated by hypoglycemia, and which increase sucrose seeking behavior through their glutamatergic projections to the NAc. Here, we identified glucokinase (Gck)-expressing neurons of the PVT (GckaPVT) and generated a mouse line expressing the Cre recombinase from the glucokinase locus (Gck Cre/+ mice). Ex vivo calcium imaging and whole-cell patch clamp recordings revealed that GckaPVT neurons that project to the NAc were mostly activated by hyperglycemia. Their chemogenetic inhibition or optogenetic stimulation, respectively, enhanced food intake or decreased sucrose-seeking behavior. Collectively, our results describe a neuronal population of Gck-expressing neurons in the PVT, which has opposite glucose sensing properties and control over feeding behavior than the previously characterized Glut2aPVT neurons. This study allows a better understanding of the complex regulation of feeding behavior by the PVT.

Fgf15 Neurons of the Dorsomedial Hypothalamus Control Glucagon Secretion and Hepatic Gluconeogenesis.

The counterregulatory response to hypoglycemia is an essential survival function. It is controlled by an integrated network of glucose-responsive neurons, which trigger endogenous glucose production to restore normoglycemia. The complexity of this glucoregulatory network is, however, only partly characterized. In a genetic screen of a panel of recombinant inbred mice we previously identified Fgf15, expressed in neurons of the dorsomedial hypothalamus (DMH), as a negative regulator of glucagon secretion. Here, we report on the generation of Fgf15CretdTomato mice and their use to further characterize these neurons. We show that they were glutamatergic and comprised glucose-inhibited and glucose-excited neurons. When activated by chemogenetics, Fgf15 neurons prevented the increase in vagal nerve firing and the secretion of glucagon normally triggered by insulin-induced hypoglycemia. On the other hand, they increased the activity of the sympathetic nerve in the basal state and prevented its silencing by glucose overload. Higher sympathetic tone increased hepatic Creb1 phosphorylation, Pck1 mRNA expression, and hepatic glucose production leading to glucose intolerance. Thus, Fgf15 neurons of the DMH participate in the counterregulatory response to hypoglycemia by a direct adrenergic stimulation of hepatic glucose production while suppressing vagally induced glucagon secretion. This study provides new insights into the complex neuronal network that prevents the development of hypoglycemia.

Projections from the dorsomedial division of the bed nucleus of the stria terminalis to hypothalamic nuclei in the mouse.

As stressful environment is a potent modulator of feeding, we seek in the present work to decipher the neuroanatomical basis for an interplay between stress and feeding behaviors. For this, we combined anterograde and retrograde tracing with immunohistochemical approaches to investigate the patterns of projections between the dorsomedial division of the bed nucleus of the stria terminalis (BNST), well connected to the amygdala, and hypothalamic structures such as the paraventricular (PVH) and dorsomedial (DMH), the arcuate (ARH) nuclei and the lateral hypothalamic areas (LHA) known to control feeding and motivated behaviors. We particularly focused our study on afferences to proopiomelanocortin (POMC), agouti-related peptide (AgRP), melanin-concentrating-hormone (MCH) and orexin (ORX) neurons characteristics of the ARH and the LHA, respectively. We found light to intense innervation of all these hypothalamic nuclei. We particularly showed an innervation of POMC, AgRP, MCH and ORX neurons by the dorsomedial and dorsolateral divisions of the BNST. Therefore, these results lay the foundation for a better understanding of the neuroanatomical basis of the stress-related feeding behaviors.

EphrinB1 modulates glutamatergic inputs into POMC-expressing progenitors and controls glucose homeostasis.

Proopiomelanocortin (POMC) neurons are major regulators of energy balance and glucose homeostasis. In addition to being regulated by hormones and nutrients, POMC neurons are controlled by glutamatergic input originating from multiple brain regions. However, the factors involved in the formation of glutamatergic inputs and how they contribute to bodily functions remain largely unknown. Here, we show that during the development of glutamatergic inputs, POMC neurons exhibit enriched expression of the Efnb1 (EphrinB1) and Efnb2 (EphrinB2) genes, which are known to control excitatory synapse formation. In vivo loss of Efnb1 in POMC-expressing progenitors decreases the amount of glutamatergic inputs, associated with a reduced number of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits and excitability of these cells. We found that mice lacking Efnb1 in POMC-expressing progenitors display impaired glucose tolerance due to blunted vagus nerve activity and decreased insulin secretion. However, despite reduced excitatory inputs, mice lacking Efnb2 in POMC-expressing progenitors showed no deregulation of insulin secretion and only mild alterations in feeding behavior and gluconeogenesis. Collectively, our data demonstrate the role of ephrins in controlling excitatory input amount into POMC-expressing progenitors and show an isotype-specific role of ephrins on the regulation of glucose homeostasis and feeding.

Neuropilin-1 expression in GnRH neurons regulates prepubertal weight gain and sexual attraction.

Hypothalamic neurons expressing gonadotropin-releasing hormone (GnRH), the "master molecule" regulating reproduction and fertility, migrate from their birthplace in the nose to their destination using a system of guidance cues, which include the semaphorins and their receptors, the neuropilins and plexins, among others. Here, we show that selectively deleting neuropilin-1 in new GnRH neurons enhances their survival and migration, resulting in excess neurons in the hypothalamus and in their unusual accumulation in the accessory olfactory bulb, as well as an acceleration of mature patterns of activity. In female mice, these alterations result in early prepubertal weight gain, premature attraction to male odors, and precocious puberty. Our findings suggest that rather than being influenced by peripheral energy state, GnRH neurons themselves, through neuropilin-semaphorin signaling, might engineer the timing of puberty by regulating peripheral adiposity and behavioral switches, thus acting as a bridge between the reproductive and metabolic axes.

Hypothalamic CDK4 regulates thermogenesis by modulating sympathetic innervation of adipose tissues.

This study investigated the role of CDK4 in the oxidative metabolism of brown adipose tissue (BAT). BAT from Cdk4-/- mice exhibited fewer lipids and increased mitochondrial volume and expression of canonical thermogenic genes, rendering these mice more resistant to cold exposure. Interestingly, these effects were not BAT cell-autonomous but rather driven by increased sympathetic innervation. In particular, the ventromedial hypothalamus (VMH) is known to modulate BAT activation via the sympathetic nervous system. We thus examined the effects of VMH neuron-specific Cdk4 deletion. These mice display increased sympathetic innervation and enhanced cold tolerance, similar to Cdk4-/- mice, in addition to browning of scWAT. Overall, we provide evidence showing that CDK4 modulates thermogenesis by regulating sympathetic innervation of adipose tissue depots through hypothalamic nuclei, including the VMH. This demonstrates that CDK4 not only negatively regulates oxidative pathways, but also modulates the central regulation of metabolism through its action in the brain.

Human Semaphorin 3 Variants Link Melanocortin Circuit Development and Energy Balance.

Hypothalamic melanocortin neurons play a pivotal role in weight regulation. Here, we examined the contribution of Semaphorin 3 (SEMA3) signaling to the development of these circuits. In genetic studies, we found 40 rare variants in SEMA3A-G and their receptors (PLXNA1-4; NRP1-2) in 573 severely obese individuals; variants disrupted secretion and/or signaling through multiple molecular mechanisms. Rare variants in this set of genes were significantly enriched in 982 severely obese cases compared to 4,449 controls. In a zebrafish mutagenesis screen, deletion of 7 genes in this pathway led to increased somatic growth and/or adiposity demonstrating that disruption of Semaphorin 3 signaling perturbs energy homeostasis. In mice, deletion of the Neuropilin-2 receptor in Pro-opiomelanocortin neurons disrupted their projections from the arcuate to the paraventricular nucleus, reduced energy expenditure, and caused weight gain. Cumulatively, these studies demonstrate that SEMA3-mediated signaling drives the development of hypothalamic melanocortin circuits involved in energy homeostasis.

Central Dicer-miR-103/107 controls developmental switch of POMC progenitors into NPY neurons and impacts glucose homeostasis.

Proopiomelanocortin (POMC) neurons are major negative regulators of energy balance. A distinct developmental property of POMC neurons is that they can adopt an orexigenic neuropeptide Y (NPY) phenotype. However, the mechanisms underlying the differentiation of Pomc progenitors remain unknown. Here, we show that the loss of the microRNA (miRNA)-processing enzyme Dicer in POMC neurons causes metabolic defects, an age-dependent decline in the number of PomcmRNA-expressing cells, and an increased proportion of Pomc progenitors acquiring a NPY phenotype. miRNome microarray screening further identified miR-103/107 as candidates that may be involved in the maturation of Pomc progenitors. In vitro inhibition of miR-103/107 causes a reduction in the number of Pomc-expressing cells and increases the proportion of Pomc progenitors differentiating into NPY neurons. Moreover, in utero silencing of miR-103/107 causes perturbations in glucose homeostasis. Together, these data suggest a role for prenatal miR-103/107 in the maturation of Pomc progenitors and glucose homeostasis.

A Transcriptomic Signature of the Hypothalamic Response to Fasting and BDNF Deficiency in Prader-Willi Syndrome.

Transcriptional analysis of brain tissue from people with molecularly defined causes of obesity may highlight disease mechanisms and therapeutic targets. We performed RNA sequencing of hypothalamus from individuals with Prader-Willi syndrome (PWS), a genetic obesity syndrome characterized by severe hyperphagia. We found that upregulated genes overlap with the transcriptome of mouse Agrp neurons that signal hunger, while downregulated genes overlap with the expression profile of Pomc neurons activated by feeding. Downregulated genes are expressed mainly in neuronal cells and contribute to neurogenesis, neurotransmitter release, and synaptic plasticity, while upregulated, predominantly microglial genes are involved in inflammatory responses. This transcriptional signature may be mediated by reduced brain-derived neurotrophic factor expression. Additionally, we implicate disruption of alternative splicing as a potential molecular mechanism underlying neuronal dysfunction in PWS. Transcriptomic analysis of the human hypothalamus may identify neural mechanisms involved in energy homeostasis and potential therapeutic targets for weight loss.

Loss of Magel2 impairs the development of hypothalamic Anorexigenic circuits.

Prader-Willi syndrome (PWS) is a genetic disorder characterized by a variety of physiological and behavioral dysregulations, including hyperphagia, a condition that can lead to life-threatening obesity. Feeding behavior is a highly complex process with multiple feedback loops that involve both peripheral and central systems. The arcuate nucleus of the hypothalamus (ARH) is critical for the regulation of homeostatic processes including feeding, and this nucleus develops during neonatal life under of the influence of both environmental and genetic factors. Although much attention has focused on the metabolic and behavioral outcomes of PWS, an understanding of its effects on the development of hypothalamic circuits remains elusive. Here, we show that mice lacking Magel2, one of the genes responsible for the etiology of PWS, display an abnormal development of ARH axonal projections. Notably, the density of anorexigenic α-melanocyte-stimulating hormone axons was reduced in adult Magel2-null mice, while the density of orexigenic agouti-related peptide fibers in the mutant mice appeared identical to that in control mice. On the basis of previous findings showing a pivotal role for metabolic hormones in hypothalamic development, we also measured leptin and ghrelin levels in Magel2-null and control neonates and found that mutant mice have normal leptin and ghrelin levels. In vitro experiments show that Magel2 directly promotes axon growth. Together, these findings suggest that a loss of Magel2 leads to the disruption of hypothalamic feeding circuits, an effect that appears to be independent of the neurodevelopmental effects of leptin and ghrelin and likely involves a direct neurotrophic effect of Magel2.

Leptin Controls Parasympathetic Wiring of the Pancreas during Embryonic Life.

The autonomic nervous system plays a critical role in glucose metabolism through both its sympathetic and parasympathetic branches, but the mechanisms that underlie the development of the autonomic innervation of the pancreas remain poorly understood. Here, we report that cholinergic innervation of pancreatic islets develops during mid-gestation under the influence of leptin. Leptin-deficient mice display a greater cholinergic innervation of pancreatic islets beginning in embryonic life, and this increase persists into adulthood. Remarkably, a single intracerebroventricular injection of leptin in embryos caused a permanent reduction in parasympathetic innervation of pancreatic ß cells and long-term impairments in glucose homeostasis. These developmental effects of leptin involve a direct inhibitory effect on the outgrowth of preganglionic axons from the hindbrain. These studies reveal an unanticipated regulatory role of leptin on the parasympathetic nervous system during embryonic development and may have important implications for our understanding of the early mechanisms that contribute to diabetes.

The MCH neuron population as a model for the development and evolution of the lateral and dorsal hypothalamus.

The LHA contains neurons producing melanin-concentrating hormone (MCH) or hypocretin (Hcrt) that have emerged as being more conspicuous and representative of the posterior LHA. In this review, we focus on MCH neurons and show that they have unique qualities. Their distribution is conserved in the posterior hypothalamus of all vertebrates and their ontogenetic differentiation is very precocious in the rodent embryo. In mammals, interspecific differences in their medio-lateral distribution suggest that the LHA differentiation may follow species specific strategies. These characteristics make a very valuable tool of MCH neurons to study the development as well as the phylogenetical origin and differentiation of the LHA.

Neonatal ghrelin programs development of hypothalamic feeding circuits.

A complex neural network regulates body weight and energy balance, and dysfunction in the communication between the gut and this neural network is associated with metabolic diseases, such as obesity. The stomach-derived hormone ghrelin stimulates appetite through interactions with neurons in the arcuate nucleus of the hypothalamus (ARH). Here, we evaluated the physiological and neurobiological contribution of ghrelin during development by specifically blocking ghrelin action during early postnatal development in mice. Ghrelin blockade in neonatal mice resulted in enhanced ARH neural projections and long-term metabolic effects, including increased body weight, visceral fat, and blood glucose levels and decreased leptin sensitivity. In addition, chronic administration of ghrelin during postnatal life impaired the normal development of ARH projections and caused metabolic dysfunction. Consistent with these observations, direct exposure of postnatal ARH neuronal explants to ghrelin blunted axonal growth and blocked the neurotrophic effect of the adipocyte-derived hormone leptin. Moreover, chronic ghrelin exposure in neonatal mice also attenuated leptin-induced STAT3 signaling in ARH neurons. Collectively, these data reveal that ghrelin plays an inhibitory role in the development of hypothalamic neural circuits and suggest that proper expression of ghrelin during neonatal life is pivotal for lifelong metabolic regulation.

Hippocampal lipoprotein lipase regulates energy balance in rodents.

Brain lipid sensing is necessary to regulate energy balance. Lipoprotein lipase (LPL) may play a role in this process. We tested if hippocampal LPL regulated energy homeostasis in rodents by specifically attenuating LPL activity in the hippocampus of rats and mice, either by infusing a pharmacological inhibitor (tyloxapol), or using a genetic approach (adeno-associated virus expressing Cre-GFP injected into Lpl (lox/lox) mice). Decreased LPL activity by either method led to increased body weight gain due to decreased locomotor activity and energy expenditure, concomitant with increased parasympathetic tone (unchanged food intake). Decreased LPL activity in both models was associated with increased de novo ceramide synthesis and neurogenesis in the hippocampus, while intrahippocampal infusion of de novo ceramide synthesis inhibitor myriocin completely prevented body weight gain. We conclude that hippocampal lipid sensing might represent a core mechanism for energy homeostasis regulation through de novo ceramide synthesis.