Insufficiency of Janus kinase 2-autonomous leptin receptor signals for most physiologic leptin actions

Roles of Gangliosides in Hypothalamic Control of Energy Balance: New Insights

Gangliosides are essential components of cell membranes and are involved in a variety of physiological processes, including cell growth, differentiation, and receptor-mediated signal transduction. They regulate functions of proteins in membrane microdomains, notably receptor tyrosine kinases such as insulin receptor (InsR) and epidermal growth factor receptor (EGFR), through lateral association. Studies during the past two decades using knockout (KO) or pharmacologically inhibited cells, or KO mouse models for glucosylceramide synthase (GCS; Ugcg), GM3 synthase (GM3S; St3gal5), and GD3 synthase (GD3S; St8sia1) have revealed essential roles of gangliosides in hypothalamic control of energy balance. The a-series gangliosides GM1 and GD1a interact with leptin receptor (LepR) and promote LepR signaling through activation of the JAK2/STAT3 pathway. Studies of GM3S KO cells have shown that the extracellular signal-regulated kinase (ERK) pathway, downstream of the LepR signaling pathway, is also modulated by gangliosides. Recent studies have revealed crosstalk between the LepR signaling pathway and other receptor signaling pathways (e.g., InsR and EGFR pathways). Gangliosides thus have the ability to modulate the effects of leptin by regulating functions of such receptors, and by direct interaction with LepR to control signaling.

Identification of the leptin receptor sequences crucial for the STAT3-Independent control of metabolism

BACKGROUND

Leptin acts via its receptor, LepRb, on specialized neurons in the brain to modulate energy balance and glucose homeostasis. LepRb→STAT3 signaling plays a crucial role in leptin action, but LepRb also mediates an additional as-yet-unidentified signal (Signal 2) that is important for leptin action. Signal 2 requires LepRb regions in addition to those required for JAK2 activation but operates independently of STAT3 and LepRb phosphorylation sites.

METHODS

To identify LepRb sequences that mediate Signal 2, we used CRISPR/Cas9 to generate five novel mouse lines containing COOH-terminal truncation mutants of LepRb. We analyzed the metabolic phenotype and measures of hypothalamic function for these mouse lines.

RESULTS

We found that deletion of LepRb sequences between residues 921 and 960 dramatically worsens metabolic control and alters hypothalamic function relative to smaller truncations. We also found that deletion of the regions including residues 1013-1053 and 960-1013 each decreased obesity compared to deletions that included additional COOH-terminal residues.

CONCLUSIONS

LepRb sequences between residues 921 and 960 mediate the STAT3 and LepRb phosphorylation-independent second signal that contributes to the control of energy balance and metabolism by leptin/LepRb. In addition to confirming the inhibitory role of the region (residues 961-1013) containing Tyr985, we also identified the region containing residues 1013-1053 (which contains no Tyr residues) as a second potential mediator of LepRb inhibition. Thus, the intracellular domain of LepRb mediates multiple Tyr-independent signals.

Hypothalamic neuronal circuits regulating hunger-induced taste modification

The gustatory system plays a critical role in sensing appetitive and aversive taste stimuli for evaluating food quality. Although taste preference is known to change depending on internal states such as hunger, a mechanistic insight remains unclear. Here, we examine the neuronal mechanisms regulating hunger-induced taste modification. Starved mice exhibit an increased preference for sweetness and tolerance for aversive taste. This hunger-induced taste modification is recapitulated by selective activation of orexigenic Agouti-related peptide (AgRP)-expressing neurons in the hypothalamus projecting to the lateral hypothalamus, but not to other regions. Glutamatergic, but not GABAergic, neurons in the lateral hypothalamus function as downstream neurons of AgRP neurons. Importantly, these neurons play a key role in modulating preferences for both appetitive and aversive tastes by using distinct pathways projecting to the lateral septum or the lateral habenula, respectively. Our results suggest that these hypothalamic circuits would be important for optimizing feeding behavior under fasting.

Hunger modulates perception of good and bad tastes. Here, the authors report that orexigenic AgRP neurons in the hypothalamus mediate these effects through glutamatergic lateral hypothalamic neurons that send distinct projections to the lateral septum and lateral habenula.

Hypothalamic neuronal circuits regulating hunger-induced taste modification

The gustatory system plays a critical role in sensing appetitive and aversive taste stimuli for evaluating food quality. Although taste preference is known to change depending on internal states such as hunger, a mechanistic insight remains unclear. Here, we examine the neuronal mechanisms regulating hunger-induced taste modification. Starved mice exhibit an increased preference for sweetness and tolerance for aversive taste. This hunger-induced taste modification is recapitulated by selective activation of orexigenic Agouti-related peptide (AgRP)-expressing neurons in the hypothalamus projecting to the lateral hypothalamus, but not to other regions. Glutamatergic, but not GABAergic, neurons in the lateral hypothalamus function as downstream neurons of AgRP neurons. Importantly, these neurons play a key role in modulating preferences for both appetitive and aversive tastes by using distinct pathways projecting to the lateral septum or the lateral habenula, respectively. Our results suggest that these hypothalamic circuits would be important for optimizing feeding behavior under fasting.

Leptin and the endocrine control of energy balance

The discovery of leptin changed the view of adipose tissue from that of a passive vessel that stores fat to that of a dynamic endocrine organ that actively regulates behaviour and metabolism. Secreted by adipose tissue, leptin functions as an afferent signal in a negative feedback loop, acting primarily on neurons in the hypothalamus and regulating feeding and many other functions. The leptin endocrine system serves a critical evolutionary function by maintaining the relative constancy of adipose tissue mass, thereby protecting individuals from the risks associated with being too thin (starvation and infertility) or too obese (predation). In this Review, the biology of leptin is summarized, and a conceptual framework is established for studying the pathogenesis of obesity, which, analogously to diabetes, can result from either leptin hyposecretion or leptin resistance. Herein, these two states are distinguished with the terms 'type 1 obesity' and 'type 2 obesity': type 1 obesity describes a subset of obese individuals with low endogenous plasma leptin levels who respond to leptin therapy, whereas type 2 obesity describes most obese individuals, who are leptin resistant but might respond to leptin therapy in combination with other drugs, such as leptin sensitizers.

Transcriptional and physiological roles for STAT proteins in leptin action

OBJECTIVES

Leptin acts via its receptor LepRb on specialized neurons in the brain to modulate food intake, energy expenditure, and body weight. LepRb activates signal transducers and activators of transcription (STATs, including STAT1, STAT3, and STAT5) to control gene expression.

METHODS

Because STAT3 is crucial for physiologic leptin action, we used TRAP-seq to examine gene expression in LepRb neurons of mice ablated for Stat3 in LepRb neurons (Stat3LepRbKO mice), revealing the STAT3-dependent transcriptional targets of leptin. To understand roles for STAT proteins in leptin action, we also ablated STAT1 or STAT5 from LepRb neurons and expressed a constitutively-active STAT3 (CASTAT3) in LepRb neurons.

RESULTS

While we also found increased Stat1 expression and STAT1-mediated transcription of leptin-regulated genes in Stat3LepRbKO mice, ablating Stat1 in LepRb neurons failed to alter energy balance (even on the Stat3LepRbKO background); ablating Stat5 in LepRb neurons also failed to alter energy balance. Importantly, expression of a constitutively-active STAT3 (CASTAT3) in LepRb neurons decreased food intake and body weight and improved metabolic parameters in leptin-deficient (ob/ob) mice, as well as in wild-type animals.

CONCLUSIONS

Thus, STAT3 represents the unique STAT protein required for leptin action and STAT3 suffices to mediate important components of leptin action in the absence of other LepRb signals.

Novel leptin receptor signaling mutants identify location and sex-dependent modulation of bone density, adiposity, and growth

Leptin, a hormone primarily produced by adipocytes, contributes to the regulation of bone health by modulating bone density, growth and adiposity. Upon leptin binding, multiple sites of the long form of the leptin receptor (LepRb) are phosphorylated to trigger activation of downstream signaling pathways. To address the role of LepRb-signaling pathways in bone health, we compared the effects of three LepRb mutations on bone density, adiposity, and growth in male and female mice. The ∆65 mutation, which lacks the known tyrosine phosphorylation sites, caused obesity and the most dramatic bone phenotype marked by excessive bone adiposity, osteoporosis, and decreased growth, consistent with the phenotype of db/db and ob/ob mice that fully lack leptin receptor signaling. Mutation of LepRb Tyr ₁₁₃₈ , which results in an inability to recruit and phosphorylate signal transducer and activator of transcription 3, also caused obesity, but bone loss and adiposity were more dominant in male mice and no growth defect was observed. In contrast, mutation of LepRb Tyr ₉₈₅ , which blocks SHP2/SOCS3 recruitment to LepRb and contributes to leptin hypersensitivity, promoted increased femur bone density only in male mice, while marrow adiposity and bone growth were not affected. Additional analyses of vertebral trabecular bone volume indicate that only the Tyr ₁₁₃₈ mutant mice exhibit bone loss in vertebrae. Together, our findings suggest that the phosphorylation status of specific sites of the LepRb contribute to the sex- and location-dependent bone responses to leptin. Unraveling the mechanisms by which leptin responses are sex- and location-dependent can contribute to the development of uniquely targeted osteoporosis therapies.

Trimetazidine therapy for diabetic mouse hearts subjected to ex vivo acute heart failure

Acute heart failure (AHF) is the most common primary diagnosis for hospitalized heart diseases in Africa. As increased fatty acid β-oxidation (FAO) during heart failure triggers detrimental effects on the myocardium, we hypothesized that trimetazidine (TMZ) (partial FAO inhibitor) offers cardioprotection under normal and obese-related diabetic conditions. Hearts were isolated from 12-14-week-old obese male and female diabetic (db/db) mice versus lean non-diabetic littermates (db/+) controls. The Langendorff retrograde isolated heart perfusion system was employed to establish an ex vivo AHF model: a) Stabilization phase-Krebs Henseleit buffer (10 mM glucose) at 100 mmHg (25 min); b) Critical Acute Heart Failure (CAHF) phase-(1.2 mM palmitic acid, 2.5 mM glucose) at 20 mmHg (25 min); and c) Recovery Acute Heart Failure phase (RAHF)-(1.2 mM palmitic acid, 10 mM glucose) at 100 mmHg (25 min). Treated groups received 5 μM TMZ in the perfusate during either the CAHF or RAHF stage for the full duration of each respective phase. Both lean and obese males benefited from TMZ treatment administered during the RAHF phase. Sex differences were observed only in lean groups where the phases of the estrous cycle influenced therapy; only the lean follicular female group responded to TMZ treatment during the CAHF phase. Lean luteal females rather displayed an inherent cardioprotection (without treatments) that was lost with obesity. However, TMZ treatment initiated during RAHF was beneficial for obese luteal females. TMZ treatment triggered significant recovery for male and obese female hearts when administered during RAHF. There were no differences between lean and obese male hearts, while lean females displayed a functional recovery advantage over lean males. Thus TMZ emerges as a worthy therapeutic target to consider for AHF treatment in normal and obese-diabetic individuals (for both sexes), but only when administered during the recovery phase and not during the very acute stages.

The Leptin Receptor Complex: Heavier Than Expected?

Under normal physiological conditions, leptin and the leptin receptor (ObR) regulate the body weight by balancing food intake and energy expenditure. However, this adipocyte-derived hormone also directs peripheral processes, including immunity, reproduction, and bone metabolism. Leptin, therefore, can act as a metabolic switch connecting the body's nutritional status to high energy consuming processes. We provide an extensive overview of current structural insights on the leptin-ObR interface and ObR activation, coupling to signaling pathways and their negative regulation, and leptin functioning under normal and pathophysiological conditions (obesity, autoimmunity, cancer, … ). We also discuss possible cross-talk with other receptor systems on the receptor (extracellular) and signaling cascade (intracellular) levels.

The role of leptin in central nervous system diseases

Leptin is a peptide hormone produced by adipose tissue and acts in brain centers to control critical physiological functions. Leptin receptors are especially abundant in the hypothalamus and trigger specific neuronal subpopulations, and activate several intracellular signaling events, including the JAK/STAT, MAPK, PI3K, and mTOR pathway. Although most studies focus on its role in energy intake and expenditure, leptin also plays a critical role in many central nervous system diseases.

Euglycemia Restoration by Central Leptin in Type 1 Diabetes Requires STAT3 Signaling but Not Fast-Acting Neurotransmitter Release

Central leptin action is sufficient to restore euglycemia in insulinopenic type 1 diabetes (T1D); however, the underlying mechanism remains poorly understood. To examine the role of intracellular signal transducer and activator of transcription 3 (STAT3) pathways, we used LepRs/s mice with disrupted leptin-phosphorylated STAT3 signaling to test the effect of central leptin on euglycemia restoration. These mice developed streptozocin-induced T1D, which was surprisingly not associated with hyperglucagonemia, a typical manifestation in T1D. Further, leptin action on euglycemia restoration was abrogated in these mice, which was associated with refractory hypercorticosteronemia. To examine the role of fast-acting neurotransmitters glutamate and γ-aminobutyric acid (GABA), two major neurotransmitters in the brain, from leptin receptor (LepR) neurons, we used mice with disrupted release of glutamate, GABA, or both from LepR neurons. Surprisingly, all mice responded normally to leptin-mediated euglycemia restoration, which was associated with expected correction from hyperglucagonemia and hyperphagia. In contrast, mice with loss of glutamate and GABA appeared to develop an additive obesity effect over those with loss of single neurotransmitter release. Thus, our study reveals that STAT3 signaling, but not fast-acting neurotransmitter release, is required for leptin action on euglycemia restoration and that hyperglucagonemia is not required for T1D.

20 years of leptin: connecting leptin signaling to biological function

Hypothalamic leptin action promotes negative energy balance and modulates glucose homeostasis, as well as serving as a permissive signal to the neuroendocrine axes that control growth and reproduction. Since the initial discovery of leptin 20 years ago, we have learned a great deal about the molecular mechanisms of leptin action. An important aspect of this has been the dissection of the cellular mechanisms of leptin signaling, and how specific leptin signals influence physiology. Leptin acts via the long form of the leptin receptor LepRb. LepRb activation and subsequent tyrosine phosphorylation recruits and activates multiple signaling pathways, including STAT transcription factors, SHP2 and ERK signaling, the IRS-protein/PI3Kinase pathway, and SH2B1. Each of these pathways controls specific aspects of leptin action and physiology. Important inhibitory pathways mediated by suppressor of cytokine signaling proteins and protein tyrosine phosphatases also limit physiologic leptin action. This review summarizes the signaling pathways engaged by LepRb and their effects on energy balance, glucose homeostasis, and reproduction. Particular emphasis is given to the multiple mouse models that have been used to elucidate these functions in vivo.

The hypothalamic ventral premammillary nucleus: A key site in leptin's regulation of reproduction

Reproduction is an energy-expensive process that relies on indicators of energy availability to adjust its proper functioning. The adipokine leptin provides one such metabolic signal, with leptin receptor-expressing neurons at sites widespread within the CNS, including regions associated with the neuroendocrine reproductive axis. One substantial population lies within the hypothalamic ventral premammillary nucleus (PMv), a region itself linked to reproductive control, which may provide a strategic site for the integration of energy availability, sensory and gonadal cues. Here we review our current understanding of leptin and PMv regulation of reproduction, including emerging details about intracellular mechanisms of leptin action at this site.

Phenotypic effects of an induced mutation of the ObRa isoform of the leptin receptor

Leptin receptors play critical roles in mediating leptin's pleiotropic effects on mammalian physiology. To date, six splice variants of the leptin receptor gene have been identified [1-3]. These splice variants have identical extracellular leptin binding motifs but different intracellular C termini. The finding that mutations specifically ablating the function of ObRb cause obesity has established a critical role for this isoform in leptin signaling [1,7]. ObRa is the most abundant splicing isoform with a broad tissue distribution [5], and it has been proposed to play roles in regulating leptin bioavailability, CSF (cerebrospinal fluid) transport and function by forming heterodimers with ObRb and also activating signal transduction via JAK2 in-vitro [5-10]. To assess the in-vivo role of ObRa, we generated an ObRa KO mouse by deleting the ObRa-specific exon 19a. Homozygous mutant mice breed normally and are indistinguishable from wild-type mice on regular chow diet, but show a slightly increased basal plasma leptin, a slight improvement of their GTT and a slightly reduced response to systemic leptin administration. These mice also show a modest but statistically significant increase in weight when placed on a high fat diet with a slightly reduced CSF/plasma ratio of leptin. These data suggest that ObRa plays a role in mediating some of leptin's effects but that the phenotypic consequences are modest compared to a deletion of ObRb.

Leptin revisited: its mechanism of action and potential for treating diabetes

Since the discovery of leptin in 1994, we now have a better understanding of the cellular and molecular mechanisms underlying its biological effects. In addition to its established anti-obesity effects, leptin exerts antidiabetic actions that are independent of its regulation of body weight and food intake. In particular, leptin can correct diabetes in animal models of type 1 and type 2 diabetes. In addition, long-term leptin replacement therapy improves glycaemic control, insulin sensitivity and plasma triglycerides in patients with severe insulin resistance due to lipodystrophy. These results have spurred enthusiasm for the use of leptin therapy to treat diabetes. Here, we review the current understanding of the glucoregulatory functions of leptin, emphasizing its central mechanisms of action and lessons learned from clinical studies, and discuss possible therapeutic applications of leptin in the treatment of type 1 and type 2 diabetes.

Leptin action via LepR-b Tyr1077 contributes to the control of energy balance and female reproduction

Leptin action in the brain signals the repletion of adipose energy stores, suppressing feeding and permitting energy expenditure on a variety of processes, including reproduction. Leptin binding to its receptor (LepR-b) promotes the tyrosine phosphorylation of three sites on LepR-b, each of which mediates distinct downstream signals. While the signals mediated by LepR-b Tyr1138 and Tyr985 control important aspects of energy homeostasis and LepR-b signal attenuation, respectively, the role of the remaining LepR-b phosphorylation site (Tyr1077) in leptin action has not been studied. To examine the function of Tyr1077, we generated a "knock-in" mouse model expressing LepR-b (F1077), which is mutant for LepR-b Tyr1077. Mice expressing LepR-b (F1077) demonstrate modestly increased body weight and adiposity. Furthermore, females display impairments in estrous cycling. Our results suggest that signaling by LepR-b Tyr1077 plays a modest role in the control of metabolism by leptin, and is an important link between body adiposity and the reproductive axis.

Nuclear JAK2: form and function in cancer

The conventional view of Janus kinase 2 (JAK2) is a nonreceptor tyrosine kinase which transmits information to the nucleus via the signal transducer and activator of transcriptions (STATs) without leaving the cytoplasm. However, accumulating data suggest that JAK2 may signal by exporting from cytoplasm to nucleus, where it guides the transcriptional machinery independent of STATs protein. Recent studies demonstrated that JAK2 is a crucial component of signaling pathways operating in the nucleus. Especially the latest landmark discovery confirmed that JAK2 goes into the nucleus and directly interacts with nucleoproteins, such as histone H3 at tyrosine 41 (H3Y41), nuclear factor 1-C2 (NF1-C2) and SWI/SNF-related helicases/ATPases (RUSH)-1α, indicating that JAK2 has a fresh nuclear function. Nuclear JAK2 is linked to a variety of cellular functions, such as cell cycle progression, apoptosis and genetic instability. The balance between these functions is an essential factor in determining whether a cell remains benign or becomes malignant. The aim of this review is intended to summarize the state of our knowledge on nuclear localization of JAK2 and nuclear JAK2 pathways, and to highlight the emerging roles for nuclear JAK2 in carcinogenesis.

Leptin and the central nervous system control of glucose metabolism

The regulation of body fat stores and blood glucose levels is critical for survival. This review highlights growing evidence that leptin action in the central nervous system plays a key role in both processes. Investigation into underlying mechanisms has begun to clarify the physiological role of leptin in the control of glucose metabolism and raises interesting new possibilities for the treatment of diabetes and related disorders.

Disruption of leptin receptor-STAT3 signaling enhances leukotriene production and pulmonary host defense against pneumococcal pneumonia

The adipocyte-derived hormone leptin regulates energy homeostasis and the innate immune response. We previously reported that leptin plays a protective role in bacterial pneumonia, but the mechanisms by which leptin regulates host defense remain poorly understood. Leptin binding to its receptor, LepRb, activates multiple intracellular signaling pathways, including ERK1/2, STAT5, and STAT3. In this study, we compared the responses of wild-type and s/s mice, which possess a mutant LepRb that prevents leptin-induced STAT3 activation, to determine the role of this signaling pathway in pneumococcal pneumonia. Compared with wild-type animals, s/s mice exhibited greater survival and enhanced pulmonary bacterial clearance after an intratracheal challenge with Streptococcus pneumoniae. We also observed enhanced phagocytosis and killing of S. pneumoniae in vitro in alveolar macrophages (AMs) obtained from s/s mice. Notably, the improved host defense and AM antibacterial effector functions in s/s mice were associated with increased cysteinyl-leukotriene production in vivo and in AMs in vitro. Augmentation of phagocytosis in AMs from s/s mice could be blocked using a pharmacologic cysteinyl-leukotriene receptor antagonist. Phosphorylation of ERK1/2 and cytosolic phospholipase A(2) α, known to enhance the release of arachidonic acid for subsequent conversion to leukotrienes, was also increased in AMs from s/s mice stimulated with S. pneumoniae in vitro. These data indicate that ablation of LepRb-mediated STAT3 signaling and the associated augmentation of ERK1/2, cytosolic phospholipase A(2) α, and cysteinyl-leukotriene synthesis confers resistance to s/s mice during pneumococcal pneumonia. These data provide novel insights into the intracellular signaling events by which leptin contributes to host defense against bacterial pneumonia.