Hypothalamic astrocyte NAD+ salvage pathway mediates the coupling of dietary fat overconsumption in a mouse model of obesity.

Nicotinamide adenine dinucleotide (NAD)+ serves as a crucial coenzyme in numerous essential biological reactions, and its cellular availability relies on the activity of the nicotinamide phosphoribosyltransferase (NAMPT)-catalyzed salvage pathway. Here we show that treatment with saturated fatty acids activates the NAD+ salvage pathway in hypothalamic astrocytes. Furthermore, inhibition of this pathway mitigates hypothalamic inflammation and attenuates the development of obesity in male mice fed a high-fat diet (HFD). Mechanistically, CD38 functions downstream of the NAD+ salvage pathway in hypothalamic astrocytes burdened with excess fat. The activation of the astrocytic NAMPT-NAD+-CD38 axis in response to fat overload induces proinflammatory responses in the hypothalamus. It also leads to aberrantly activated basal Ca2+ signals and compromised Ca2+ responses to metabolic hormones such as insulin, leptin, and glucagon-like peptide 1, ultimately resulting in dysfunctional hypothalamic astrocytes. Our findings highlight the significant contribution of the hypothalamic astrocytic NAD+ salvage pathway, along with its downstream CD38, to HFD-induced obesity.

Hypothalamic AMP-Activated Protein Kinase as a Whole-Body Energy Sensor and Regulator.

5´-Adenosine monophosphate (AMP)-activated protein kinase (AMPK), a cellular energy sensor, is an essential enzyme that helps cells maintain stable energy levels during metabolic stress. The hypothalamus is pivotal in regulating energy balance within the body. Certain neurons in the hypothalamus are sensitive to fluctuations in food availability and energy stores, triggering adaptive responses to preserve systemic energy equilibrium. AMPK, expressed in these hypothalamic neurons, is instrumental in these regulatory processes. Hypothalamic AMPK activity is modulated by key metabolic hormones. Anorexigenic hormones, including leptin, insulin, and glucagon-like peptide 1, suppress hypothalamic AMPK activity, whereas the hunger hormone ghrelin activates it. These hormonal influences on hypothalamic AMPK activity are central to their roles in controlling food consumption and energy expenditure. Additionally, hypothalamic AMPK activity responds to variations in glucose concentrations. It becomes active during hypoglycemia but is deactivated when glucose is introduced directly into the hypothalamus. These shifts in AMPK activity within hypothalamic neurons are critical for maintaining glucose balance. Considering the vital function of hypothalamic AMPK in the regulation of overall energy and glucose balance, developing chemical agents that target the hypothalamus to modulate AMPK activity presents a promising therapeutic approach for metabolic conditions such as obesity and type 2 diabetes mellitus.

Rosa davurica inhibits skin photoaging via regulating MAPK/AP-1, NF-κB, and Nrf2/HO-1 signaling in UVB-irradiated HaCaTs.

Rosa davurica is widely used to treat various kinds of diseases because of its high antioxidant, antiviral and anti-inflammatory activities. This use of plant-based materials as medicine is called phytomedicine and has been widely practiced since time immemorial. However, the pharmacological mechanism of R. davurica in skin photoaging is not yet fully understood. Therefore, this study was carried out to evaluate the recovery effects of R. davurica leaf extracts (RDE) in UVB-irradiated human skin keratinocytes (HaCaTs) and investigate whether RDE is a potential therapeutic agent against skin photoaging. The expression of aging-related markers including mitogen-activated protein kinases/activator protein 1 (MAPK/AP-1), nuclear factor-κB (NF-κB), and nuclear factor E2-related factor 2/heme oxygenase-1 (Nrf2/HO-1) was evaluated using Western blot analysis. The reactive oxygen species (ROS) was also used by FACS in HaCaTs. Findings indicated that RDE is efficient in scavenging free radicals and dose-dependently reducing ROS generation. Furthermore, RDE notably decreased UVB-induced matrix metalloproteinase-1 (MMP-1) expression through inhibition of MAPK/AP-1 and NF-κB signaling pathways as well as induced blocking of extracellular matrix (ECM) degradation in UVB-irradiated HaCaTs. In addition, RDE improved Nrf2/HO-1 signaling that increases oxidative defense capacity and enhances transforming growth factor-beta (TGF-ß) signaling activation to promote procollagen type I synthesis, relieving UVB-induced skin cell damage. In conclusion, the protective effects of RDE on skin cellular components suggest that it has a high biological potential for skin protection from UVB-induced skin photoaging and is a good candidate for drug and cosmetic application.

Mechanisms of Weight Control by Primary Cilia.

A primary cilium, a hair-like protrusion of the plasma membrane, is a pivotal organelle for sensing external environmental signals and transducing intracellular signaling. An interesting linkage between cilia and obesity has been revealed by studies of the human genetic ciliopathies Bardet-Biedl syndrome and Alström syndrome, in which obesity is a principal manifestation. Mouse models of cell type-specific cilia dysgenesis have subsequently demonstrated that ciliary defects restricted to specific hypothalamic neurons are sufficient to induce obesity and hyperphagia. A potential mechanism underlying hypothalamic neuron cilia-related obesity is impaired ciliary localization of G protein-coupled receptors involved in the regulation of appetite and energy metabolism. A well-studied example of this is melanocortin 4 receptor (MC4R), mutations in which are the most common cause of human monogenic obesity. In the paraventricular hypothalamus neurons, a blockade of ciliary trafficking of MC4R as well as its downstream ciliary signaling leads to hyperphagia and weight gain. Another potential mechanism is reduced leptin signaling in hypothalamic neurons with defective cilia. Leptin receptors traffic to the periciliary area upon leptin stimulation. Moreover, defects in cilia formation hamper leptin signaling and actions in both developing and differentiated hypothalamic neurons. The list of obesity-linked ciliary proteins is expending and this supports a tight association between cilia and obesity. This article provides a brief review on the mechanism of how ciliary defects in hypothalamic neurons facilitate obesity.

Camu-Camu Fruit Extract Inhibits Oxidative Stress and Inflammatory Responses by Regulating NFAT and Nrf2 Signaling Pathways in High Glucose-Induced Human Keratinocytes.

Myrciaria dubia (HBK) McVaugh (camu-camu) belongs to the family Myrtaceae. Although camu-camu has received a great deal of attention for its potential pharmacological activities, there is little information on the anti-oxidative stress and anti-inflammatory effects of camu-camu fruit in skin diseases. In the present study, we investigated the preventative effect of 70% ethanol camu-camu fruit extract against high glucose-induced human keratinocytes. High glucose-induced overproduction of reactive oxygen species (ROS) was inhibited by camu-camu fruit treatment. In response to ROS reduction, camu-camu fruit modulated the mitogen-activated protein kinases (MAPK)/activator protein-1 (AP-1), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and nuclear factor of activated T cells (NFAT) signaling pathways related to inflammation by downregulating the expression of proinflammatory cytokines and chemokines. Furthermore, camu-camu fruit treatment activated the expression of nuclear factor E2-related factor 2 (Nrf2) and subsequently increased the NAD(P)H:quinone oxidoreductase1 (NQO1) expression to protect keratinocytes against high-glucose-induced oxidative stress. These results indicate that camu-camu fruit is a promising material for preventing oxidative stress and skin inflammation induced by high glucose level.

Mitohormesis in Hypothalamic POMC Neurons Mediates Regular Exercise-Induced High-Turnover Metabolism.

Low-grade mitochondrial stress can promote health and longevity, a phenomenon termed mitohormesis. Here, we demonstrate the opposing metabolic effects of low-level and high-level mitochondrial ribosomal (mitoribosomal) stress in hypothalamic proopiomelanocortin (POMC) neurons. POMC neuron-specific severe mitoribosomal stress due to Crif1 homodeficiency causes obesity in mice. By contrast, mild mitoribosomal stress caused by Crif1 heterodeficiency in POMC neurons leads to high-turnover metabolism and resistance to obesity. These metabolic benefits are mediated by enhanced thermogenesis and mitochondrial unfolded protein responses (UPRmt) in distal adipose tissues. In POMC neurons, partial Crif1 deficiency increases the expression of ß-endorphin (ß-END) and mitochondrial DNA-encoded peptide MOTS-c. Central administration of MOTS-c or ß-END recapitulates the adipose phenotype of Crif1 heterodeficient mice, suggesting these factors as potential mediators. Consistently, regular running exercise at moderate intensity stimulates hypothalamic MOTS-c/ß-END expression and induces adipose tissue UPRmt and thermogenesis. Our findings indicate that POMC neuronal mitohormesis may underlie exercise-induced high-turnover metabolism.

Crataegus laevigata Suppresses LPS-Induced Oxidative Stress during Inflammatory Response in Human Keratinocytes by Regulating the MAPKs/AP-1, NFκB, and NFAT Signaling Pathways.

Crataegus laevigata belongs to the family Rosaceae, which has been widely investigated for pharmacological effects on the circulatory and digestive systems. However, there is limited understanding about its anti-oxidative stress and anti-inflammatory effects on skin. In this study, 70% ethanol C. laevigata berry extract (CLE) was investigated on lipopolysaccharide (LPS)-stimulated keratinocytes. The LPS-induced overproduction of reactive oxygen species (ROS) was suppressed by the treatment with CLE. In response to ROS induction, the overexpression of inflammatory regulating signaling molecules including mitogen-activated protein kinases (MAPK)/activator protein-1 (AP-1), nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB), and nuclear factor of activated T-cells (NFAT) were reduced in CLE-treated human keratinocytes. Consequently, CLE significantly suppressed the mRNA levels of pro-inflammatory chemokines and interleukins in LPS-stimulated cells. Our results indicated that CLE has protective effects against LPS-induced injury in an in vitro model and is a potential alternative agent for inflammatory treatment.

Primary cilia mediate early life programming of adiposity through lysosomal regulation in the developing mouse hypothalamus.

Hypothalamic neurons including proopiomelanocortin (POMC)-producing neurons regulate body weights. The non-motile primary cilium is a critical sensory organelle on the cell surface. An association between ciliary defects and obesity has been suggested, but the underlying mechanisms are not fully understood. Here we show that inhibition of ciliogenesis in POMC-expressing developing hypothalamic neurons, by depleting ciliogenic genes IFT88 and KIF3A, leads to adulthood obesity in mice. In contrast, adult-onset ciliary dysgenesis in POMC neurons causes no significant change in adiposity. In developing POMC neurons, abnormal cilia formation disrupts axonal projections through impaired lysosomal protein degradation. Notably, maternal nutrition and postnatal leptin surge have a profound impact on ciliogenesis in the hypothalamus of neonatal mice; through these effects they critically modulate the organization of hypothalamic feeding circuits. Our findings reveal a mechanism of early life programming of adult adiposity, which is mediated by primary cilia in developing hypothalamic neurons.

Cellular Contributors to Hypothalamic Inflammation in Obesity.

The hypothalamus is a crucial organ for the maintenance of appropriate body fat storage. Neurons in the hypothalamic arcuate nucleus (ARH) detect energy shortage or surplus via the circulating concentrations of metabolic hormones and nutrients, and then coordinate energy intake and expenditure to maintain energy homeostasis. Malfunction or loss of hypothalamic ARH neurons results in obesity. Accumulated evidence suggests that hypothalamic inflammation is a key pathological mechanism that links chronic overconsumption of a high-fat diet (HFD) with the development of obesity and related metabolic complications. Interestingly, overnutrition-induced hypothalamic inflammation occurs specifically in the ARH, where microglia initiate an inflammatory response by releasing proinflammatory cytokines and chemokines in response to excessive fatty acid flux. Upon more prolonged HFD consumption, astrocytes and perivascular macrophages become involved and sustain hypothalamic inflammation. ARH neurons are victims of hypothalamic inflammation, but they may actively participate in hypothalamic inflammation by sending quiescence or stress signals to surrounding glia. In this mini-review, we describe the current state of knowledge regarding the contributions of neurons and glia, and their interactions, to HFD-induced hypothalamic inflammation.

Del-1, an Endogenous Inhibitor of TGF-ß Activation, Attenuates Fibrosis.

Uncontrolled activation of transforming growth factor (TGF)-ß results in a wide range of pathologic conditions. Therapeutic interventions to regulate TGF-ß signaling during fibrosis have been developed but the effectiveness is still limited. Here, we show that developmental endothelial locus-1 (Del-1) ameliorates fibrosis in mice by inhibiting αv integrin-mediated activation of TGF-ß. Del-1 bound to αvß6 integrin, an important activator of TGF-ß, and inhibited the binding of αvß6 integrin to the latency-associated peptide (LAP), thereby suppressing αv integrin-mediated activation of TGF-ß. Lack of Del-1 increased colocalization of αv integrin and LAP in the lungs, which was reversed by Del-1 supplementation. The crucial role of Del-1 in regulating TGF-ß activity was recapitulated in a mouse model of fibrosis using an adenovirus expressing inactive TGF-ß1. Del-1 supplementation improved the pathological characteristics of the mice and reduced mortality. Thus, we propose that Del-1 is a negative regulator of TGF-ß activation and a potential anti-fibrotic factor.

Hypothalamic NAD+-Sirtuin Axis: Function and Regulation.

The rapidly expanding elderly population and obesity endemic have become part of continuing global health care problems. The hypothalamus is a critical center for the homeostatic regulation of energy and glucose metabolism, circadian rhythm, and aging-related physiology. Nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase sirtuins are referred to as master metabolic regulators that link the cellular energy status to adaptive transcriptional responses. Mounting evidence now indicates that hypothalamic sirtuins are essential for adequate hypothalamic neuronal functions. Owing to the NAD+-dependence of sirtuin activity, adequate hypothalamic NAD+ contents are pivotal for maintaining energy homeostasis and circadian physiology. Here, we comprehensively review the regulatory roles of the hypothalamic neuronal NAD+-sirtuin axis in a normal physiological context and their changes in obesity and the aging process. We also discuss the therapeutic potential of NAD+ biology-targeting drugs in aging/obesity-related metabolic and circadian disorders.

Cellular source of hypothalamic macrophage accumulation in diet-induced obesity.

BACKGROUND: Obese mice on a high-fat diet (HFD) display signs of inflammation in the hypothalamic arcuate nucleus (ARC), a critical area for controlling systemic energy metabolism. This has been suggested as a key mechanism of obesity-associated hypothalamic dysfunction. We reported earlier that bone marrow-derived macrophages accumulate in the ARC to sustain hypothalamic inflammation upon chronic exposure to an HFD. However, the mechanism underlying hypothalamic macrophage accumulation has remained unclear. METHODS: We investigated whether circulating monocytes or myeloid precursors contribute to hypothalamic macrophage expansion during chronic HFD feeding. To trace circulating myeloid cells, we generated mice that express green fluorescent protein (GFP) in their lysozyme M-expressing myeloid cells (LysMGFP mice). We conducted parabiosis and bone marrow transplantation experiments using these animals. Mice received an HFD for 12 or 30 weeks and were then sacrificed to analyze LysMGFP cells in the hypothalamus. Hypothalamic vascular permeability in the HFD-fed obese mice was also tested by examining the extravascular leakage of Evans blue and fluorescence-labeled albumin. The timing of LysMGFP cell entry to the hypothalamus during development was also evaluated. RESULTS: Our parabiosis and bone marrow transplantation experiments revealed a significant infiltration of circulating LysMGFP cells into the liver, skeletal muscle, choroid plexus, and leptomeninges but not in the hypothalamic ARC during chronic HFD feeding, despite increased hypothalamic vascular permeability. These results suggested that the recruitment of circulating monocytes is not a major mechanism for maintaining and expanding the hypothalamic macrophage population in diet-induced obesity. We demonstrated instead that LysMGFP cells infiltrate the hypothalamus during its development. LysMGFP cells appeared in the hypothalamic area from the late embryonic period. This cellular pool suddenly increased immediately after birth, peaked at the postnatal second week, and adopted an adult pattern of distribution after weaning. CONCLUSIONS: Bone marrow-derived macrophages mostly populate the hypothalamus in early postnatal life and may maintain their pool without significant recruitment of circulating monocytes throughout life, even under conditions of chronic HFD feeding.

Hypothalamic Macrophage Inducible Nitric Oxide Synthase Mediates Obesity-Associated Hypothalamic Inflammation.

Obesity-associated metabolic alterations are closely linked to low-grade inflammation in peripheral organs, in which macrophages play a central role. Using genetic labeling of myeloid lineage cells, we show that hypothalamic macrophages normally reside in the perivascular area and circumventricular organ median eminence. Chronic consumption of a high-fat diet (HFD) induces expansion of the monocyte-derived macrophage pool in the hypothalamic arcuate nucleus (ARC), which is significantly attributed to enhanced proliferation of macrophages. Notably, inducible nitric oxide synthase (iNOS) is robustly activated in ARC macrophages of HFD-fed obese mice. Hypothalamic macrophage iNOS inhibition completely abrogates macrophage accumulation and activation, proinflammatory cytokine overproduction, reactive astrogliosis, blood-brain-barrier permeability, and lipid accumulation in the ARC of obese mice. Moreover, central iNOS inhibition improves obesity-induced alterations in systemic glucose metabolism without affecting adiposity. Our findings suggest a critical role for hypothalamic macrophage-expressed iNOS in hypothalamic inflammation and abnormal glucose metabolism in cases of overnutrition-induced obesity.

Effects of Chronic NAD Supplementation on Energy Metabolism and Diurnal Rhythm in Obese Mice.

OBJECTIVE: Adequate nicotinamide adenine dinucleotide (NAD) content in hypothalamic neurons is critical for the maintenance of normal energy balance and circadian rhythm. In this study, the beneficial metabolic effects of chronic NAD supplementation on diet-induced obesity and obesity-related disruption of diurnal rhythms were examined. METHODS: C57BL/6 mice were fed a high-fat diet (HFD) for 12 weeks and received an intraperitoneal injection of either saline or NAD (1 mg/kg/day) for the last 4 weeks. The control mice were fed a chow diet and injected with saline for the same period. Body weights were monitored daily. Daily rhythms of food intake, energy expenditure, and locomotor activity were measured at the end of NAD treatment. The effect of NAD treatment on the clock gene Period 1 (PER1) transcription was also studied. RESULTS: Chronic NAD supplementation significantly attenuated weight gain in HFD-fed obese mice. Furthermore, NAD treatment recovered the suppressed rhythms in the diurnal locomotor activity patterns in obese mice. In addition, exogenous NAD supply rescued cellular NAD depletion-induced suppression of PER1 transcriptional activity in hypothalamic neuron cells as well as blunted daily fluctuations of hypothalamic arcuate nucleus PER1 expression in obese mice. CONCLUSIONS: NAD supplementation showed therapeutic effects in obese mice with altered diurnal behaviors.

Exogenous nicotinamide adenine dinucleotide regulates energy metabolism via hypothalamic connexin 43.

BACKGROUND: Nicotinamide adenine dinucleotide (NAD)-dependent deacetylase SIRT1 is an important regulator of hypothalamic neuronal function. Thus, an adequate hypothalamic NAD content is critical for maintaining normal energy homeostasis. METHODS: We investigated whether NAD supplementation increases hypothalamic NAD levels and affects energy metabolism in mice. Furthermore, we investigated the mechanisms underlying the effects of exogenous NAD on central metabolism upon entering the hypothalamus. RESULTS: Central and peripheral NAD administration suppressed fasting-induced hyperphagia and weight gain in mice. Extracellular NAD was imported into N1 hypothalamic neuronal cells in a connexin 43-dependent and CD73-independent manner. Consistent with the in vitro data, inhibition of hypothalamic connexin 43 blocked hypothalamic NAD uptake and NAD-induced anorexia. Exogenous NAD suppressed NPY and AgRP transcriptional activity, which was mediated by SIRT1 and FOXO1. CONCLUSIONS: Exogenous NAD is effectively transported to the hypothalamus via a connexin 43-dependent mechanism and increases hypothalamic NAD content. Therefore, NAD supplementation is a potential therapeutic method for metabolic disorders characterized by hypothalamic NAD depletion.

Leptin directly regulate intrinsic neuronal excitability in hypothalamic POMC neurons but not in AgRP neurons in food restricted mice.

Leptin plays a pivotal role in the central control of energy balance through leptin receptors expressed on specific hypothalamic nuclei. Leptin suppresses food intake and body weight and ameliorates hyperglycemia by acting on the AgRP and POMC neurons of the arcuate nucleus. Leptin action on POMC neurons are essential for control of body weight and blood glucose levels and are known to be mediated by JAK-STAT3 and PI3K signalling pathway thus increase POMC mRNA and intrinsic excitability. The effects of leptin on AgRP neurons are not as clear although it has been reported to hyperpolarize AgRP neurons through change of K+ conductance. Using cell-attached patch and whole cell patch configuration, we directly assessed neuronal response to leptin in GFP labelled AgRP or POMC neurons in mice after 18 h of food deprivation. We found leptin has a direct effect on POMC neuron through increased intrinsic excitability and decreased inhibitory synaptic inputs. However, leptin does not have any effect on intrinsic excitability of AgRP neurons in fasted condition although food deprivation induced increase of firing frequency of AgRP neurons. In conclusion, leptin probably has a direct and acute effect on POMC neurons but not on AgRP neurons to control their excitability within feeding-regulatory neuronal circuitry.

Quercetin Protects Obesity-Induced Hypothalamic Inflammation by Reducing Microglia-Mediated Inflammatory Responses via HO-1 Induction.

Obesity-induced hypothalamic inflammation is characterized by activation of microglia, which are resident macrophages of the central nervous system, and is implicated in the derangement of energy homeostasis, metabolic complications, and neurodegenerative diseases. Quercetin, a naturally occurring flavonoid, is known to protect against oxidative stress and inflammation-related metabolic complications. Here, we demonstrate that quercetin reduces obesity-induced hypothalamic inflammation by inhibiting microglia-mediated inflammatory responses, and the beneficial action of quercetin is associated with heme oxygenase (HO-1) induction. Quercetin markedly reduced the production of inflammatory mediators (monocyte chemoattractant protein (MCP)-1, interleukin (IL-6), IL-1ß, nitric oxide) by microglia stimulated with saturated fatty acid palmitate and/or lipid-laden microglia-conditioned medium. Quercetin also upregulated the expression of HO-1 in palmitate-treated lipid-laden microglia, and the actions of quercetin against microglia activation accompanied by IκBα degradation were abolished by a HO-1 inhibitor. Moreover, quercetin supplementation reduced the levels of inflammatory cytokines and microglia activation markers in the hypothalamus of high fat diet (HFD)-fed obese mice, which was accompanied by upregulation of HO-1. These findings indicate that quercetin suppresses microglia-mediated inflammatory responses via the induction of HO-1, and hence protects against obesity-induced hypothalamic inflammation.

Hypothalamic lipid-laden astrocytes induce microglia migration and activation.

Obesity-induced hypothalamic inflammation is closely associated with various metabolic complications and neurodegenerative disorders. Astrocytes, the most abundant glial cells in the central nervous system, play a crucial role in pathological hypothalamic inflammatory processes. Here, we demonstrate that hypothalamic astrocytes accumulate lipid droplets under saturated fatty acid-rich conditions, such as obese environment, and that the lipid-laden astrocytes increase astrogliosis markers and inflammatory cytokines (TNFα, IL-1ß, IL-6, MCP-1) at the transcript and/or protein level. Medium conditioned by the lipid-laden astrocytes stimulate microglial chemotactic activity and upregulate transcripts of the microglia activation marker Iba-1 and inflammatory cytokines. These findings indicate that the lipid-laden astrocytes formed in free fatty acid-rich obese condition may participate in obesity-induced hypothalamic inflammation through promoting microglia migration and activation.

Leptin signalling pathways in hypothalamic neurons.

Leptin is the most critical hormone in the homeostatic regulation of energy balance among those so far discovered. Leptin primarily acts on the neurons of the mediobasal part of hypothalamus to regulate food intake, thermogenesis, and the blood glucose level. In the hypothalamic neurons, leptin binding to the long form leptin receptors on the plasma membrane initiates multiple signaling cascades. The signaling pathways known to mediate the actions of leptin include JAK-STAT signaling, PI3K-Akt-FoxO1 signaling, SHP2-ERK signaling, AMPK signaling, and mTOR-S6K signaling. Recent evidence suggests that leptin signaling in hypothalamic neurons is also linked to primary cilia function. On the other hand, signaling molecules/pathways mitigating leptin actions in hypothalamic neurons have been extensively investigated in an effort to treat leptin resistance observed in obesity. These include SOCS3, tyrosine phosphatase PTP1B, and inflammatory signaling pathways such as IKK-NFκB and JNK signaling, and ER stress-mitochondrial signaling. In this review, we discuss leptin signaling pathways in the hypothalamus, with a particular focus on the most recently discovered pathways.

Leptin Elongates Hypothalamic Neuronal Cilia via Transcriptional Regulation and Actin Destabilization.

Terminally differentiated neurons have a single, primary cilium. The primary cilia of hypothalamic neurons play a critical role in sensing metabolic signals. We recently showed that mice with leptin deficiency or resistance have shorter cilia in the hypothalamic neurons, and leptin treatment elongates cilia in hypothalamic neurons. Here, we investigated the molecular mechanisms by which leptin controls ciliary length in hypothalamic neurons. In N1 hypothalamic neuronal cells, leptin treatment increased the expression of intraflagellar transport proteins. These effects occurred via phosphatase and tensin homolog/glycogen synthase kinase-3ß-mediated inhibition of the transcriptional factor RFX1. Actin filament dynamics were also involved in leptin-promoted ciliary elongation. Both leptin and cytochalasin-D treatment induced F-actin disruption and cilium elongation in hypothalamic neurons that was completely abrogated by co-treatment with the F-actin polymerizer phalloidin. Our findings suggest that leptin elongates hypothalamic neuronal cilia by stimulating the production of intraflagellar transport proteins and destabilizing actin filaments.