The adipocyte hormone leptin sets the emergence of hippocampal inhibition in mice

Leptin Promotes the Proliferation and Neuronal Differentiation of Neural Stem Cells through the Cooperative Action of MAPK/ERK1/2, JAK2/STAT3 and PI3K/AKT Signaling Pathways

The potential of neural stem cells (NSCs) for neurological disorders the treatment has relied in large part upon identifying the NSCs fate decision. The hormone leptin has been reported to be a crucial regulator of brain development, able to influence the glial and neural development, yet, the underlying mechanism of leptin acting on NSCs' biological characteristics is still poorly understood. This study aims to investigate the role of leptin in the biological properties of NSCs. In this study, we investigate the possibility that leptin may regulate the NSCs' fate decision, which may promote the proliferation and neuronal differentiation of NSCs and thus act positively in neurological disorders. NSCs from the embryonic cerebral cortex were used in this study. We used CCK-8 assay, ki67 immunostaining, and FACS analysis to confirm that 25-100 ng/mL leptin promotes the proliferation of NSCs in a concentration-dependent pattern. This change was accompanied by the upregulation of p-AKT and p-ERK1/2, which are the classical downstream signaling pathways of leptin receptors b (LepRb). Inhibition of PI3K/AKT or MAPK/ERK signaling pathways both abolished the effect of leptin-induced proliferation. Moreover, leptin also enhanced the directed neuronal differentiation of NSCs. A blockade of the PI3K/AKT pathway reversed leptin-stimulated neurogenesis, while a blockade of JAK2/STAT3 had no effect on it. Taken together, our results support a role for leptin in regulating the fate of NSCs differentiation and promoting NSCs proliferation, which could be a promising approach for brain repair via regulating the biological characteristics of NSCs.

K+/Cl- cotransporter 2 (KCC2) and Na+/HCO3- cotransporter 1 (NBCe1) interaction modulates profile of KCC2 phosphorylation

K+/Cl- cotransporter 2 (KCC2) is a major Cl- extruder in mature neurons and is responsible for the establishment of low intracellular [Cl-], necessary for fast hyperpolarizing GABAA-receptor mediated synaptic inhibition. Electrogenic sodium bicarbonate cotransporter 1 (NBCe1) is a pH regulatory protein expressed in neurons and glial cells. An interactome study identified NBCe1 as a possible interaction partner of KCC2. In this study, we investigated the putative effect of KCC2/NBCe1 interaction in baseline and the stimulus-induced phosphorylation pattern and function of KCC2. Primary mouse hippocampal neuronal cultures from wildtype (WT) and Nbce1-deficient mice, as well as HEK-293 cells stably transfected with KCC2WT, were used. The results show that KCC2 and NBCe1 are interaction partners in the mouse brain. In HEKKCC2 cells, pharmacological inhibition of NBCs with S0859 prevented staurosporine- and 4-aminopyridine (4AP)-induced KCC2 activation. In mature cultures of hippocampal neurons, however, S0859 completely inhibited postsynaptic GABAAR and, thus, could not be used as a tool to investigate the role of NBCs in GABA-dependent neuronal networks. In Nbce1-deficient immature hippocampal neurons, baseline phosphorylation of KCC2 at S940 was downregulated, compared to WT, and exposure to staurosporine failed to reduce pKCC2 S940 and T1007. In Nbce1-deficient mature neurons, baseline levels of pKCC2 S940 and T1007 were upregulated compared to WT, whereas after 4AP treatment, pKCC2 S940 was downregulated, and pKCC2 T1007 was further upregulated. Functional experiments showed that the levels of GABAAR reversal potential, baseline intracellular [Cl-], Cl- extrusion, and baseline intracellular pH were similar between WT and Nbce1-deficient neurons. Altogether, our data provide a primary description of the properties of KCC2/NBCe1 protein-protein interaction and implicate modulation of stimulus-mediated phosphorylation of KCC2 by NBCe1/KCC2 interaction-a mechanism with putative pathophysiological relevance.

Peripubertal Alterations of Leptin Levels in Patients with Autism Spectrum Disorder and Elevated or Normal Body Weight

Leptin, which plays a key role in energy homeostasis, is known as a neurotrophic factor possibly linking nutrition and neurodevelopment. Available data on the association between leptin and autism spectrum disorder (ASD) are confusing. The aim of this study was to explore whether plasma levels of leptin in pre- and post-pubertal children with ASD and/or overweightness/obesity differ from those of BMI- and age-matched healthy controls. Leptin levels were determined in 287 pre-pubertal children (mean age 8.09 years), classified as follows: ASD with overweightness/obesity (ASD+/Ob+); ASD without overweightness/obesity (ASD+/Ob-); non-ASD with overweightness/obesity (ASD-/Ob+); non-ASD without overweightness/obesity (ASD-/Ob-). The assessment was repeated in 258 of the children post-pubertally (mean age 14.26 years). There were no significant differences in leptin levels either before or after puberty between ASD+/Ob+ and ASD-/Ob+ or between ASD+/Ob- and ASD-/Ob-, although there was a strong trend toward significance for higher pre-pubertal leptin levels in ASD+/Ob- than in ASD-/Ob-. Post-pubertal leptin levels were significantly lower than pre-pubertal levels in ASD+/Ob+, ASD-/Ob+, and ASD+/Ob- and higher in ASD-/Ob-. Leptin levels, elevated pre-pubertally in the children with overweightness/obesity as well as in children with ASD and normal BMI, decrease with age, in contrast to the increasing leptin levels in healthy controls.

Leptin antagonism improves Rett syndrome phenotype in symptomatic male Mecp2-null mice

Rett syndrome (RTT) is a severe neurodevelopmental disorder that arise from de novo mutations in the X-linked gene MECP2 (methyl-CpG-binding protein 2). Circulating levels of the adipocyte hormone leptin are elevated in RTT patients and rodent models of the disease. Leptin targets a large number of brain structures and regulates a wide range of developmental and physiological functions which are altered in RTT. We hypothesized that elevated leptin levels might contribute to RTT pathogenesis. Accordingly, we show that pharmacological antagonism of leptin or genetic reduction of leptin production prevents the degradation of health status, weight loss and the progression of breathing and locomotor deficits. At the neuronal level, the anti-leptin strategies rescue the hippocampal excitatory/inhibitory imbalance and synaptic plasticity impairment. Targeting leptin might therefore represent a new approach for RTT treatment.

Contribution of Smoothened Receptor Signaling in GABAergic Neurotransmission and Chloride Homeostasis in the Developing Rodent Brain

In the early stages of the central nervous system growth and development, γ-aminobutyric acid (GABA) plays an instructive trophic role for key events including neurogenesis, migration, synaptogenesis, and network formation. These actions are associated with increased concentration of chloride ions in immature neurons [(Cl⁻)_i] that determines the depolarizing strength of ion currents mediated by GABA_A receptors, a ligand-gated Cl⁻ permeable ion channel. During neuron maturation the (Cl⁻)_i progressively decreases leading to weakening of GABA induced depolarization and enforcing GABA function as principal inhibitory neurotransmitter. A neuron restricted potassium-chloride co-transporter KCC2 is a key molecule governing Cl⁻ extrusion and determining the resting level of (Cl⁻)_i in developing and mature mammalian neurons. Among factors controlling the functioning of KCC2 and the maturation of inhibitory circuits, is Smoothened (Smo), the transducer in the receptor complex of the developmental protein Sonic Hedgehog (Shh). Too much or too little Shh-Smo action will have mirror effects on KCC2 stability at the neuron membrane, the GABA inhibitory strength, and ultimately on the newborn susceptibility to neurodevelopmental disorders. Both canonical and non-canonical Shh-Smo signal transduction pathways contribute to the regulation of KCC2 and GABAergic synaptic activity. In this review, we discuss the recent findings of the action of Shh-Smo signaling pathways on chloride ions homeostasis through the control of KCC2 membrane trafficking, and consequently on inhibitory neurotransmission and network activity during postnatal development.

Early life nutrition and neuroendocrine programming

Alterations in the nutritional environment in early life can significantly increase the risk for obesity and a range of development of metabolic disorders in offspring in later life, effects that can be passed onto future generations. This process, termed development programming, provides the framework of the developmental origins of health and disease (DOHaD) paradigm. Early life nutritional compromise including undernutrition, overnutrition or specific macro/micronutrient deficiencies, results in a range of adverse health outcomes in offspring that can be further exacerbated by a poor postnatal nutritional environment. Although the mechanisms underlying programming remain poorly defined, a common feature across the phenotypes displayed in preclinical models is that of altered wiring of neuroendocrine circuits that regulate satiety and energy balance. As such, altered maternal nutritional exposures during critical early periods of developmental plasticity can result in aberrant hardwiring of these circuits with lasting adverse consequences for the offspring. There is also increasing evidence around the role of an altered epigenome and the gut-brain axis in mediating some of the central programming effects observed. Further, although such programming was once considered to result in a permanent change in developmental trajectory, there is evidence, at least from preclinical models, that programming can be reversed via targeted nutritional manipulations during early development. Further work is required at a mechanistic level to allow for identification for early markers of later disease risk, delineation of sex-specific effects and pathways to implementation of strategies aimed at breaking the transgenerational transmission of disease.

A Review of miRNAs as Biomarkers and Effect of Dietary Modulation in Obesity Associated Cognitive Decline and Neurodegenerative Disorders

There has been a progressive increase in the prevalence of obesity and its comorbidities such as type 2 diabetes and cardiovascular diseases worldwide. Recent studies have suggested that the crosstalk between adipose tissue and central nervous system (CNS), through cellular mediators and signaling pathways, may causally link obesity with cognitive decline and give rise to neurodegenerative disorders. Several mechanisms have been proposed in obesity, including inflammation, oxidative stress, insulin resistance, altered lipid and cholesterol homeostasis, which may result in neuroinflammation, altered brain insulin signaling, amyloid-beta (Aβ) deposition and neuronal cell death. Since obesity is associated with functional and morphological alterations in the adipose tissues, the resulting peripheral immune response augments the development and progression of cognitive decline and increases susceptibility of neurodegenerative disorders, such as Alzheimer's Disease (AD) and Parkinson's Disease (PD). Studies have also elucidated an important role of high fat diet in the exacerbation of these clinical conditions. However, the underlying factors that propel and sustain this obesity associated cognitive decline and neurodegeneration, remains highly elusive. Moreover, the mechanisms linking these phenomena are not well-understood. The cumulative line of evidence have demonstrated an important role of microRNAs (miRNAs), a class of small non-coding RNAs that regulate gene expression and transcriptional changes, as biomarkers of pathophysiological conditions. Despite the lack of utility in current clinical practices, miRNAs have been shown to be highly specific and sensitive to the clinical condition being studied. Based on these observations, this review aims to assess the role of several miRNAs and aim to elucidate underlying mechanisms that link obesity with cognitive decline and neurodegenerative disorders. Furthermore, this review will also provide evidence for the effect of dietary modulation which can potentially ameliorate cognitive decline and neurodegenerative diseases associated with obesity.

Leptin increases GABAergic synaptogenesis through the Rho guanine exchange factor β-PIX in developing hippocampal neurons

Developing hippocampal neurons undergo rapid synaptogenesis in response to neurotrophic signals to form and refine circuit connections. The adipokine leptin is a satiety factor with neurotrophic actions, which potentiates both glutamatergic and GABAergic synaptogenesis in the hippocampus during neonatal development. Brief exposure to leptin enhances GABA_A receptor-dependent synaptic currents in hippocampal neurons. Here, using molecular and electrophysiological techniques, we found that leptin increased the surface localization of GABA_A receptors and the number of functional GABAergic synapses in hippocampal cultures from male and female rat pups. Leptin increased the interaction between GABA_A receptors and the Rho guanine exchange factor β-PIX (a scaffolding protein at GABAergic postsynaptic sites) in a manner dependent on the kinase CaMKK. We also found that the leptin receptor and β-PIX formed a complex, the amount of which transiently increased upon leptin receptor activation. Furthermore, Tyr⁹⁸⁵ in the leptin receptor and the SH3 domain of β-PIX are crucial for this interaction, which was required for the developmental increase in GABAergic synaptogenesis. Our results suggest a mechanism by which leptin promotes GABAergic synaptogenesis in hippocampal neurons and reveal further complexity in leptin receptor signaling and its interactome.

Regulation of hippocampal synaptic function by the metabolic hormone leptin: Implications for health and disease

Significant advances have been made in our understanding of the hormone, leptin and its CNS actions in recent years. It is now evident that leptin has a multitude of brain functions, that extend beyond its established role in the hypothalamic control of energy balance. Additional brain regions including the hippocampus are important targets for leptin, with a high density of leptin receptors (LepRs) expressed in specific hippocampal regions and localised to CA1 synapses. Extensive evidence indicates that leptin has pro-cognitive actions, as it rapidly modifies synaptic efficacy at excitatory Schaffer collateral (SC)-CA1 and temporoammonic (TA)-CA1 synapses and enhances performance in hippocampal-dependent memory tasks. There is a functional decline in hippocampal responsiveness to leptin with age, with significant reductions in the modulatory effects of leptin at SC-CA1 and TA-CA1 synapses in aged, compared to adult hippocampus. As leptin has pro-cognitive effects, this decline in leptin sensitivity is likely to have negative consequences for cognitive function during the aging process. Here we review how evaluation of the hippocampal actions of leptin has improved our knowledge of the regulatory brain functions of leptin in health and provided significant insight into the impact of leptin in age-related neurodegenerative disorders linked to cognitive decline.

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl^-), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl^- currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development.

Associations Among Maternal Metabolic Conditions, Cord Serum Leptin Levels, and Autistic Symptoms in Children

INTRODUCTION

Accumulating evidence has shown that maternal metabolic conditions, such as pre-pregnancy overweight, diabetes mellitus, and hypertensive disorders of pregnancy (HDP) are potential risk factors of autism spectrum disorder (ASD). However, it remains unclear how these maternal conditions lead to neurodevelopmental outcomes in the offspring, including autistic symptoms. Leptin, an adipokine that has pro-inflammatory effects and affects fetal neurodevelopment, is a candidate mediator of the association between maternal metabolic factors and an increased risk of ASD. However, whether prenatal exposure to leptin mediates the association between maternal metabolic conditions and autistic symptoms in children has not been investigated yet.

METHODS

This study investigated the associations between mothers' metabolic conditions (pre-pregnancy overweight, diabetes mellitus during or before pregnancy, and HDP), leptin concentrations in umbilical cord serum, and autistic symptoms among 762 children from an ongoing cohort study, using generalized structural equation modeling. We used the Social Responsive Scale, Second Edition (SRS-2) at 8-9 years old to calculate total T-scores. Additionally, we used the T-scores for two subdomains: Social Communication and Interaction (SCI) and Restricted Interests and Repetitive Behavior (RRB).

RESULTS

Umbilical cord leptin levels were associated with pre-pregnancy overweight [coefficient = 1.297, 95% confidence interval (CI) 1.081-1.556, p = 0.005] and diabetes mellitus (coefficient = 1.574, 95% CI 1.206-2.055, p = 0.001). Furthermore, leptin levels were significantly associated with SRS-2 total T-scores (coefficient = 1.002, 95% CI 1.000-1.004, p = 0.023), SCI scores (coefficient = 1.002, 95% CI 1.000-1.004, p = 0.020), and RRB scores (coefficient = 1.001, 95% CI 1.000-1.003, p = 0.044) in children. Associations between maternal metabolic factors and autistic symptoms were not significant.

DISCUSSION

The present study uncovered an association between cord leptin levels and autistic symptoms in children, while maternal metabolic conditions did not have an evident direct influence on the outcome. These results imply that prenatal pro-inflammatory environments affected by maternal metabolic conditions may contribute to the development of autistic symptoms in children. The findings warrant further investigation into the role of leptin in the development of autistic symptoms.

Leptin down-regulates KCC2 activity and controls chloride homeostasis in the neonatal rat hippocampus

The canonical physiological role of leptin is to regulate hunger and satiety acting on specific hypothalamic nuclei. Beyond this key metabolic function; leptin also regulates many aspects of development and functioning of neuronal hippocampal networks throughout life. Here we show that leptin controls chloride homeostasis in the developing rat hippocampus in vitro. The effect of leptin relies on the down-regulation of the potassium/chloride extruder KCC2 activity and is present during a restricted period of postnatal development. This study confirms and extends the role of leptin in the ontogenesis of functional GABAergic inhibition and helps understanding how abnormal levels of leptin may contribute to neurological disorders.

Smoothened receptor signaling regulates the developmental shift of GABA polarity in rat somatosensory cortex

Sonic hedgehog (Shh) and its patched-smoothened receptor complex control a variety of functions in the developing central nervous system, such as neural cell proliferation and differentiation. Recently, Shh signaling components have been found to be expressed at the synaptic level in the postnatal brain, suggesting a potential role in the regulation of synaptic transmission. Using in utero electroporation of constitutively active and negative-phenotype forms of the Shh signal transducer smoothened (Smo), we studied the role of Smo signaling in the development and maturation of GABAergic transmission in the somatosensory cortex. Our results show that enhancing Smo activity during development accelerates the shift from depolarizing to hyperpolarizing GABA in a manner dependent on functional expression of potassium-chloride cotransporter type 2 (KCC2, also known as SLC12A5). On the other hand, blocking Smo activity maintains the GABA response in a depolarizing state in mature cortical neurons, resulting in altered chloride homeostasis and increased seizure susceptibility. This study reveals unexpected functions of Smo signaling in the regulation of chloride homeostasis, through control of KCC2 cell-surface stability, and the timing of the GABA excitatory-to-inhibitory shift in brain maturation.

Leptin stimulates synaptogenesis in hippocampal neurons via KLF4 and SOCS3 inhibition of STAT3 signaling

Normal development of neuronal connections in the hippocampus requires neurotrophic signals, including the cytokine leptin. During neonatal development, leptin induces formation and maturation of dendritic spines, the main sites of glutamatergic synapses in the hippocampal neurons. However, the molecular mechanisms for leptin-induced synaptogenesis are not entirely understood. In this study, we reveal two novel targets of leptin in developing hippocampal neurons and address their role in synaptogenesis. First target is Kruppel-Like Factor 4 (KLF4), which we identified using a genome-wide target analysis strategy. We show that leptin upregulates KLF4 in hippocampal neurons and that leptin signaling is important for KLF4 expression in vivo. Furthermore, KLF4 is required for leptin-induced synaptogenesis, as shKLF4 blocks and upregulation of KLF4 phenocopies it. We go on to show that KLF4 requires its signal transducer and activator of transcription 3 (STAT3) binding site and thus potentially blocks STAT3 activity to induce synaptogenesis. Second, we show that leptin increases the expression of suppressor of cytokine signaling 3 (SOCS3), another well-known inhibitor of STAT3, in developing hippocampal neurons. SOCS3 is also required for leptin-induced synaptogenesis and sufficient to stimulate it alone. Finally, we show that constitutively active STAT3 blocks the effects of leptin on spine formation, while the targeted knockdown of STAT3 is sufficient to induce it. Overall, our data demonstrate that leptin increases the expression of both KLF4 and SOCS3, inhibiting the activity of STAT3 in the hippocampal neurons and resulting in the enhancement of glutamatergic synaptogenesis during neonatal development.

Mothers, Fathers, and Others: Neural Substrates of Parental Care

Parental care is essential for the survival of offspring in altricial mammalian species. However, in most mammals, virgin females tend to avoid or attack infants. Moreover, most males demonstrate avoidance and aggression toward infants, and have little to no involvement in parental care. What mechanisms suppress avoidance, and support approach towards pups, to promote maternal care? In biparental and cooperatively breeding species, what mechanisms allow nonmothers (i.e., fathers and alloparents) to demonstrate parental care? In this review we consider the mechanisms that subserve parental care in mothers, fathers, and others (i.e., alloparents). We emphasize recent discoveries and research trends with particular emphasis on neuroendocrinology, neuroplasticity, transcriptomics, and epigenetics. Finally, we consider outstanding questions and outline opportunities for future research.

Developmental Switch of Leptin Action on Network Driven Activity in the Neonatal Rat Hippocampus

The adipose-derived circulating hormone leptin plays a pivotal role in the control of energy balance and body weight. Sound data indicate that this hormone also acts as an important developmental signal impacting a number of brain regions during fetal and postnatal stages. Leptin levels surge during the two first postnatal weeks of life in rodents. This period is characterized by the presence of early network driven activity in the immature hippocampus, the so-called Giant Depolarizing Potentials (GDPs). GDPs are thought to contribute to the wiring of the hippocampal network. We therefore tested the effect of leptin on GDPs. Leptin increased GDPs frequency between the postnatal days (P) 1 and 3 via a calcium/Calmodulin-dependent kinase (CaMK) and extracellular signal-related kinase (ERK) dependent pathways. Between P6 and P7, leptin inhibited the frequency of GDPs through the activation of large conductance Ca²⁺ activated K⁺ (BK) channels driven by a phosphoinositol-3 kinase (PI3K) dependent pathway. These results show that leptin exerts a bi-directional and age-dependent control of GDPs and extends the scope of leptin’s action in the developing brain.