Region-specific mRNA expression of phosphatidylinositol 3-kinase regulatory isoforms in the central nervous system of C57BL/6J mice

Exploring the Pathophysiology of ATP-Dependent Potassium Channels in Insulin Resistance

Ionic channels are present in eucaryotic plasma and intracellular membranes. They coordinate and control several functions. Potassium channels belong to the most diverse family of ionic channels that includes ATP-dependent potassium (KATP) channels in the potassium rectifier channel subfamily. These channels were initially described in heart muscle and then in other tissues such as pancreatic, skeletal muscle, brain, and vascular and non-vascular smooth muscle tissues. In pancreatic beta cells, KATP channels are primarily responsible for maintaining the membrane potential and for depolarization-mediated insulin release, and their decreased density and activity may be related to insulin resistance. KATP channels' relationship with insulin resistance is beginning to be explored in extra-pancreatic beta tissues like the skeletal muscle, where KATP channels are involved in insulin-dependent glucose recapture and their activation may lead to insulin resistance. In adipose tissues, KATP channels containing Kir6.2 protein subunits could be related to the increase in free fatty acids and insulin resistance; therefore, pathological processes that promote prolonged adipocyte KATP channel inhibition might lead to obesity due to insulin resistance. In the central nervous system, KATP channel activation can regulate peripheric glycemia and lead to brain insulin resistance, an early peripheral alteration that can lead to the development of pathologies such as obesity and Type 2 Diabetes Mellitus (T2DM). In this review, we aim to discuss the characteristics of KATP channels, their relationship with clinical disorders, and their mechanisms and potential associations with peripheral and central insulin resistance.

PI3K signalling in leptin receptor cells: Role in growth and reproduction

Nutrition and growth are important signals for pubertal development, although how they are perceived and integrated in brain circuits has not been well defined. Growth hormones and metabolic cues both recruit phosphatidylinositol 3-kinase (PI3K) signalling in hypothalamic sites, although whether they converge into the same neuronal population(s) is also not known. In this review, we discuss recent findings from our laboratory showing the role of PI3K subunits in cells directly responsive to the adipocyte-derived hormone leptin in the coordination of growth, pubertal development and fertility. Mice with deletion of PI3K p110α and p110β catalytic subunits in leptin receptor cells (LR^Δα+β ) have a lean phenotype associated with increased energy expenditure, locomotor activity and thermogenesis. The LR^Δα+β mice also show deficient growth and delayed puberty. Deletion of a single subunit (ie, p110α) in LR cells (LR^Δα ) causes a similar phenotype of increased energy expenditure, deficient growth and delayed pubertal development, indicating that these functions are preferably controlled by p110α. The LR^Δα mice show enhanced leptin sensitivity in metabolic regulation but, remarkably, these mice are unresponsive to the effects of leptin on growth and puberty. PI3K is also recruited by insulin and a subpopulation of LR neurones is responsive to i.c.v. insulin administration. Deletion of insulin receptor in LR cells causes no changes in body weight or linear growth and induces only a mild delay in pubertal completion. Our findings demonstrate that PI3K in LR cells plays an essential role in growth and reproduction. We will also discuss the potential neural pathways underlying these effects.

Alzheimer's disease and metabolic syndrome: A link from oxidative stress and inflammation to neurodegeneration

Alzheimer's disease (AD) is the most common cause of dementia and one of the most important causes of morbidity and mortality among the aging population. AD diagnosis is made post-mortem, and the two pathologic hallmarks, particularly evident in the end stages of the illness, are amyloid plaques and neurofibrillary tangles. Currently, there is no curative treatment for AD. Additionally, there is a strong relation between oxidative stress, metabolic syndrome, and AD. The high levels of circulating lipids and glucose imbalances amplify lipid peroxidation that gradually diminishes the antioxidant systems, causing high levels of oxidative metabolism that affects cell structure, leading to neuronal damage. Accumulating evidence suggests that AD is closely related to a dysfunction of both insulin signaling and glucose metabolism in the brain, leading to an insulin-resistant brain state. Four drugs are currently used for this pathology: Three FDA-approved cholinesterase inhibitors and one NMDA receptor antagonist. However, wide varieties of antioxidants are promissory to delay or prevent the symptoms of AD and may help in treating the disease. Therefore, therapeutic efforts to achieve attenuation of oxidative stress could be beneficial in AD treatment, attenuating Aβ-induced neurotoxicity and improve neurological outcomes in AD. The term inflammaging characterizes a widely accepted paradigm that aging is accompanied by a low-grade chronic up-regulation of certain pro-inflammatory responses in the absence of overt infection, and is a highly significant risk factor for both morbidity and mortality in the elderly.

Neuroendocrine Regulation of Metabolism

Given the current environment in most developed countries, it is a challenge to maintain a good balance between calories consumed and calories burned, although maintenance of metabolic balance is key to good health. Therefore, understanding how metabolic regulation is achieved and how the dysregulation of metabolism affects health is an area of intense research. Most studies focus on the hypothalamus, which is a brain area that acts as a key regulator of metabolism. Among the nuclei that comprise the hypothalamus, the arcuate nucleus is one of the major mediators in the regulation of food intake. The regulation of energy balance is also a key factor ensuring the maintenance of any species as a result of the dependence of reproduction on energy stores. Adequate levels of energy reserves are necessary for the proper functioning of the hypothalamic-pituitary-gonadal axis. This review discusses valuable data presented in the 2015 edition of the International Workshop of Neuroendocrinology concerning the fundamental nature of the hormonal regulation of the hypothalamus and the impact on energy balance and reproduction.

Intranasal Insulin and Insulin-Like Growth Factor 1 as Neuroprotectants in Acute Ischemic Stroke

Treatment options for stroke remain limited. Neuroprotective therapies, in particular, have invariably failed to yield the expected benefit in stroke patients, despite robust theoretical and mechanistic background and promising animal data. Insulin and insulin-like growth factor 1 (IGF-1) play a pivotal role in critical brain functions, such as energy homeostasis, neuronal growth, and differentiation. They may exhibit neuroprotective properties in acute ischemic stroke based upon their vasodilatory, anti-inflammatory and antithrombotic effects, as well as improvements of functional connectivity, neuronal metabolism, neurotransmitter regulation, and remyelination. Intranasally administered insulin has demonstrated a benefit for prevention of cognitive decline in older people, and IGF-1 has shown potential benefit to improve functional outcomes in animal models of acute ischemic stroke. The intranasal route presents a feasible, tolerable, safe, and particularly effective administration route, bypassing the blood-brain barrier and maximizing distribution to the central nervous system (CNS), without the disadvantages of systemic side effects and first-pass metabolism. This review summarizes the neuroprotective potential of intranasally administered insulin and IGF-1 in stroke patients. We present the theoretical background and pathophysiologic mechanisms, animal and human studies of intranasal insulin and IGF-1, and the safety and feasibility of intranasal route for medication administration to the CNS.

Deregulation of brain insulin signaling in Alzheimer's disease

Contrary to the previous belief that insulin does not act in the brain, studies in the last three decades have demonstrated important roles of insulin and insulin signal transduction in various functions of the central nervous system. Deregulated brain insulin signaling and its role in molecular pathogenesis have recently been reported in Alzheimer's disease (AD). In this article, we review the roles of brain insulin signaling in memory and cognition, the metabolism of amyloid β precursor protein, and tau phosphorylation. We further discuss deficiencies of brain insulin signaling and glucose metabolism, their roles in the development of AD, and recent studies that target the brain insulin signaling pathway for the treatment of AD. It is clear now that deregulation of brain insulin signaling plays an important role in the development of sporadic AD. The brain insulin signaling pathway also offers a promising therapeutic target for treating AD and probably other neurodegenerative disorders.

Changes in phosphatidylinositol 3-kinase 55 kDa gamma expression and subcellular localization may be caspase 6 dependent in paraquat-induced SH-SY5Y apoptosis

The neurotoxin paraquat (PQ) causes apoptosis of dopaminergic neurons in mammalian cell culture and animal models, mimicking an important pathological feature of Parkinson's disease (PD). The phosphoinositide 3-kinase (PI3K)/Akt pathway is critical for several major survival signals in central nervous system neurons. Phosphatidylinositol 3-kinase 55 kDa gamma (p55PIK) is a regulatory subunit of PI3Ks with important roles in cell proliferation, antiapoptosis, and cell cycle progression. However, p55PIK involvement in mechanisms regarding progression and maintenance of neurodegenerative diseases is largely undetermined. We used PQ-induced apoptosis in human dopaminergic SH-SY5Y cells to investigate the association between p55PIK expression levels, subcellular location, and apoptosis. p55PIK expression was reduced in SH-SY5Y cells and p55PIK messenger RNA and protein expression levels were decreased after PQ treatment. Apoptosis induced by PQ was associated with caspase activation and decreased p55PIK expression. Restoration of p55PIK expression was observed after coincubation with a caspase inhibitor. Overexpressed full-length p55PIK in SH-SY5Y and human embryonic kidney 293 cells showed specific distribution in the nucleus and was cleaved in vitro by recombinant caspase 6 (C6), but not C3 and C7. A p55PIK construct lacking 24 N-terminal amino acids (N24) was tested for the presence of a potential C6-recognizable sequence and was found to express its proteins outside the nucleus. The results suggest that p55PIK may be involved in PQ-induced apoptosis signal transduction and that N24 is crucial for p55PIK subcellular localization. Thus, p55PIK could be a substrate of activated C6 during apoptosis, leading to loss of original biological functions and redistribution to disturb cell cycle progression.

Insulin in the brain: sources, localization and functions

Historically, insulin is best known for its role in peripheral glucose homeostasis, and insulin signaling in the brain has received less attention. Insulin-independent brain glucose uptake has been the main reason for considering the brain as an insulin-insensitive organ. However, recent findings showing a high concentration of insulin in brain extracts, and expression of insulin receptors (IRs) in central nervous system tissues have gathered considerable attention over the sources, localization, and functions of insulin in the brain. This review summarizes the current status of knowledge of the peripheral and central sources of insulin in the brain, site-specific expression of IRs, and also neurophysiological functions of insulin including the regulation of food intake, weight control, reproduction, and cognition and memory formation. This review also considers the neuromodulatory and neurotrophic effects of insulin, resulting in proliferation, differentiation, and neurite outgrowth, introducing insulin as an attractive tool for neuroprotection against apoptosis, oxidative stress, beta amyloid toxicity, and brain ischemia.

Leptin signaling and circuits in puberty and fertility

Leptin is an adipocyte-derived hormone involved in a myriad of physiological process, including the control of energy balance and several neuroendocrine axes. Leptin-deficient mice and humans are obese, diabetic, and display a series of neuroendocrine and autonomic abnormalities. These individuals are infertile due to a lack of appropriate pubertal development and inadequate synthesis and secretion of gonadotropins and gonadal steroids. Leptin receptors are expressed in many organs and tissues, including those related to the control of reproductive physiology (e.g., the hypothalamus, pituitary gland, and gonads). In the last decade, it has become clear that leptin receptors located in the brain are major players in most leptin actions, including reproduction. Moreover, the recent development of molecular techniques for brain mapping and the use of genetically modified mouse models have generated crucial new findings for understanding leptin physiology and the metabolic influences on reproductive health. In the present review, we will highlight the new advances in the field, discuss the apparent contradictions, and underline the relevance of this complex physiological system to human health. We will focus our review on the hypothalamic circuitry and potential signaling pathways relevant to leptin’s effects in reproductive control, which have been identified with the use of cutting-edge technologies of molecular mapping and conditional knockouts.

The neurodevelopmental implications of PI3K signaling

Our understanding of the mechanisms involved in the formation of the complex arrangement of neurons and their interconnections within the brain has made significant progress in recent years. Current research has uncovered a network of intracellular signaling events that provide precise coordination of a diverse array of cellular responses, including trafficking events, cytoskeletal remodeling, gene transcription, and protein ubiquitination and translation. This chapter considers the specific cellular responses controlled by the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which is instructive with regard to a number of important steps involved in the development of the brain. These range from the mediation of extrinsic signals - such as growth factors, axon guidance cues, and extracellular matrix components - to intrinsic effectors, such as downstream signaling components that act, for example, at the translation level. PI3K signaling is, consequently, at the heart of controlling neuronal migration and neuronal morphogenesis, as well as dendrite and synapse development. Many neurobehavioral disorders arise as a consequence of subtle developmental abnormalities. Unsurprisingly, therefore, aberrant PI3K signaling has been indicated by many studies to be a contributing factor to the pathophysiology of disorders such as schizophrenia and autism. In this chapter, we will focus on the specific, yet divergent, cellular processes that are achieved through PI3K signaling in neurons and are key to brain development.

Male-biased effects of gonadotropin-releasing hormone neuron-specific deletion of the phosphoinositide 3-kinase regulatory subunit p85alpha on the reproductive axis

GnRH neurosecretion is subject to regulation by insulin, IGF-I, leptin, and other neuroendocrine modulators whose effects may be conveyed by activation of phosphoinositide 3-kinase (PI3K)-mediated pathways. It is not known, however, whether any of these regulatory actions are exerted directly, via activation of PI3K in GnRH neurons, or whether they are primarily conveyed via effects on afferent circuitries governing GnRH neurosecretion. To investigate the role of PI3K signaling in GnRH neurons, we used conditional gene targeting to ablate expression of the major PI3K regulatory subunit, p85alpha, in GnRH neurons. Combined in situ hybridization and immunohistochemistry confirmed reduction of p85alpha mRNA expression in GnRH neurons of GnRH-p85alpha knockout (KO) animals. Females of both genotypes exhibited estrous cyclicity and had comparable serum LH, estradiol-17beta, and FSH levels. In male GnRH-p85alphaKO mice, serum LH, testosterone, and sperm counts were significantly reduced compared with wild type. To investigate the role of the other major regulatory subunit, p85beta, on the direct control of GnRH neuronal function, we generated mice with a GnRH-neuron-specific p85alpha deletion on a global betaKO background. No additional reproductive effects in male or female mice were found, suggesting that p85beta does not substitute p85 activity toward PI3K function in GnRH neurons. Our results suggest that p85alpha, and thus PI3K activity, participates in the control of GnRH neuronal activity in male mice. The sex-specific phenotype in these mice raises the possibility that PI3K activation during early development may establish sex differences in GnRH neuronal function.

Regulation of ceramide-induced neuronal death: cell metabolism meets neurodegeneration

The present review explores the role of ceramides in neuronal apoptosis, as well as the recent discovery of the signaling pathways involved in this process placing particular emphasis on the correlation between cellular metabolism and neuronal death. Endogenous levels of ceramides are increased following various pro-apoptotic stimuli which have been identified as potential causes of chronic and acute neurodegenerative diseases. Ceramides induce changes in multiple enzymes and cell signaling components. The early inhibition of the neuronal survival pathway regulated by phosphatidil-inositol-3-kinase/protein kinase B or AKT mediated by ceramide may be a relevant early event in the decision of neuronal survival/death. It may perturb several molecular and metabolic functions. In particular it might decrease glycolysis through rapid modulation of hexokinase activity. This would in turn generate limited amounts of mitochondrial substrates leading to mitochondrial dysfunction and neuronal apoptosis. Subtle and early metabolic alterations caused by inhibition of the PI3K/AKT pathway mediated by ceramide may potentially work with genes associated with neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Together they may be determinant steps in downstream events leading to neuronal apoptosis. Therefore, reinforcement of the PI3K/AKT pathway could constitute an important neuroprotective strategy.

Lower phosphoinositide 3-kinase (PI 3-kinase) activity and differential expression levels of selective catalytic and regulatory PI 3-kinase subunit isoforms in prefrontal cortex and hippocampus of suicide subjects

Phosphoinositide 3 (PI 3)-kinase is one of the key signaling enzymes that participates in a myriad of physiological functions in brain and is utilized by neurotrophins to mediate neuronal plasticity, cell survival, and inhibition of apoptosis for several neuronal subtypes. Our recent demonstration that expression of neurotrophic factors and activation of the receptor tyrosine kinase B are significantly altered in postmortem brain of suicide subjects led us to examine whether suicide brain is associated with alterations in PI 3-kinase signaling. In prefrontal cortex (PFC), hippocampus, and cerebellum of suicide (n=28) and nonpsychiatric control (n=21) subjects we examined catalytic activation of PI 3-kinase, and mRNA and protein levels of regulatory (p85alpha, p85beta) and catalytic (p110alpha, p110beta) subunits of PI 3-kinase. It was observed that the catalytic activity of PI 3-kinase was significantly reduced in PFC and hippocampus of suicide subjects compared with nonpsychiatric control subjects. Competitive PCR analysis revealed significantly reduced mRNA expression of p85beta and p110alpha and increased expression of p85alpha subunit isoforms in PFC and hippocampus of suicide subjects. Alterations in these catalytic and regulatory subunits were accompanied by changes in their respective protein levels. These changes were not present in cerebellum of suicide subjects. Also, these changes were present in all suicide subjects irrespective of psychiatric diagnosis. Our findings of reduced activation and altered expression of specific PI 3-kinase regulatory and catalytic subunit isoforms demonstrate abnormalities in this signaling pathway in postmortem brain of suicide subjects and suggest possible involvement of aberrant PI 3-kinase signaling in the pathogenic mechanisms of suicide.

Acute effects of leptin require PI3K signaling in hypothalamic proopiomelanocortin neurons in mice

Normal food intake and body weight homeostasis require the direct action of leptin on hypothalamic proopiomelanocortin (POMC) neurons. It has been proposed that leptin action requires PI3K activity. We therefore assessed the contribution of PI3K signaling to leptin's effects on POMC neurons and organismal energy balance. Leptin caused a rapid depolarization of POMC neurons and an increase in action potential frequency in patch-clamp recordings of hypothalamic slices. Pharmacologic inhibition of PI3K prevented this depolarization and increased POMC firing rate, indicating a PI3K-dependent mechanism of leptin action. Mice with genetically disrupted PI3K signaling in POMC cells failed to undergo POMC depolarization or increased firing frequency in response to leptin. Insulin's ability to hyperpolarize POMC neurons was also abolished in these mice. Moreover, targeted disruption of PI3K blunted the suppression of feeding elicited by central leptin administration. Despite these differences, mice with impaired PI3K signaling in POMC neurons exhibited normal long-term body weight regulation. Collectively, these results suggest that PI3K signaling in POMC neurons is essential for leptin-induced activation and insulin-induced inhibition of POMC cells and for the acute suppression of food intake elicited by leptin, but is not a major contributor to the regulation of long-term organismal energy homeostasis.

PI3K signaling effects in hypothalamic neurons mediated by estrogen

Multiple mechanisms mediate the effects of estrogen in the central nervous system, including signal transduction pathways such as protein kinase A, protein kinase C, and phosphatidylinositol 3-kinase (PI3K) pathways. Previously we demonstrated that estrogen regulates a number of PI3K-related genes in the hypothalamus, including the PI3K p55gamma regulatory subunit. We hypothesized that PI3K activation is critical for the effects of estrogen and that the p55gamma subunit may be more prevalent than the p85alpha regulatory subunit in the hypothalamus. Therefore, in the present study, we compared the mRNA distribution of the p55gamma and p85alpha regulatory subunits by using in situ hybridization in guinea pig. Expression level of p55gamma mRNA was greater than p85alpha in most hypothalamic nuclei. Twenty-four hours of estrogen treatment increased p55gamma mRNA expression in the paraventricular, suprachiasmatic, arcuate, and ventromedial nuclei, and little or no change was observed for p85alpha mRNA. Quantitative real-time PCR confirmed the in situ hybridization results. Next, we investigated the general role of PI3K signaling in the estrogen-mediated changes of arcuate proopiomelanocortin (POMC) neuronal excitability by using whole-cell recording. One cellular mechanism by which estrogen increases neuronal excitability is to desensitize (uncouple) gamma-aminobutyric acid type B (GABA(B)) receptors from their G-protein-gated inwardly rectifying K(+) channels in hypothalamic neurons. We found that the PI3K inhibitors wortmannin and LY294002 significantly reduced the estrogen-mediated GABA(B) receptor desensitization in POMC arcuate neurons, suggesting that PI3K signaling is a critical downstream mediator of the estrogen-mediated rapid effects. Collectively, these data suggest that the interplay between estrogen and PI3K occurs at multiple levels, including transcriptional and membrane-initiated signaling events that ultimately lead to changes in homeostatic function.

Learning deficits and agenesis of synapses and myelinated axons in phosphoinositide-3 kinase-deficient mice

Although previous studies have reported a role for phosphoinositide-3 kinase (PI3K) in axonal definition and growth in vitro, it is not clear whether PI3K regulates axonal formation and synaptogenesis in vivo. The goal of the present study was to clarify the role of PI3K in behavioral functions and some underlying neuroanatomical structures. Immunohistochemistry, an electron-microscopic analysis and behavioral tests were carried out. Knockout mice lacking the p85alpha regulatory subunit of PI3K (p85alpha-/- mice) significantly showed learning deficits, restlessness and motivation deficit. Expression of phosphorylated Akt, which indirectly shows the activity of PI3K, was high in myelinated axons, especially in axonal bundles in the striatum of wild-type mice, but was significantly low in the striatum, cerebral cortex and the hippocampal CA3 of p85alpha-/- mice. The axonal marker protein level decreased mainly in the striatum and cerebral cortex of p85alpha-/- mice. In these two regions, myelinated axons are rich in the wild-type mice. However, the density of myelinated axons and myelin thickness were significantly low in the striatum and cerebral cortex of p85alpha-/- mice. Synaptic protein level was clearly decreased in the striatum, cerebral cortex, and hippocampus of p85alpha-/- mice when compared with wild mice. The present results suggest that PI3K plays a role in the generation and/or maintenance of synapses and myelinated axons in the brain and that deficiencies in PI3K activity result in abnormalities in several neuronal functions, including learning, restlessness and motivation.

Adipokine gene expression in brain and pituitary gland

The brain has been recognized as a prominent site of peptide biosynthesis for more than 30 years, and many neuropeptides are now known to be common to gut and brain. With these precedents in mind it is remarkable that adipose-derived peptides like leptin have attracted minimal attention as brain-derived putative neuromodulators of energy balance. This review outlines the evidence that several adipose-specific genes are also expressed in the central nervous system and pituitary gland. We, and others, confirmed that the genes for leptin, resistin, adiponectin, FIAF (fasting-induced adipose factor) and adiponutrin are expressed and regulated in these tissues. For example, leptin mRNA was readily detectable in human, rat, sheep and pig brain, but not in the mouse. Leptin expression in rat brain and pituitary was regulated through development, by food restriction, and following traumatic brain injury. In contrast, hypothalamic resistin mRNA was unaffected by age or by fasting, but was significantly depleted by food restriction in mouse pituitary gland. Similar results were seen in the ob/ob mouse, and we noted a marked reduction in resistin-positive hypothalamic nerve fibres. Resistin and fiaf mRNA were also upregulated in hypoxic/ischaemic mouse brain. Our studies on the regulation of neuronal adipokines were greatly aided by the availability of clonal hypothalamic neuronal cell lines. One of these, N-1, expresses both rstn and fiaf together with several other neuropeptides and receptors involved in energy homeostasis. Selective silencing of rstn revealed an autocrine/paracrine regulatory system, mediated through socs-3 expression that may influence the feedback effects of insulin and leptin in vivo. A similar convergence of signals in the pituitary gland could also influence anterior pituitary hormone secretion. In conclusion, the evidence is suggestive that brain and pituitary-derived adipokines represent a local regulatory circuit that may fine tune the feedback effects of adipose hormones in the control of energy balance.

Photoperiodic regulation of insulin receptor mRNA and intracellular insulin signaling in the arcuate nucleus of the Siberian hamster,Phodopus sungorus

During the last 5 years it has been well established that photoperiod-induced changes in body weight in the seasonal hamster, Phodopus sungorus, are accompanied by a marked seasonal cycle in leptin sensitivity. In the present study, we investigated the possible involvement of insulin signaling in seasonal body weight regulation. We analyzed the expression pattern and relative intensity of insulin receptor (IR), phosphatidylinositol 3-kinase (PI3-kinase), and protein tyrosine phosphatase 1B (PTP1B) mRNAs by in situ hybridization in the brains of juvenile female hamsters acclimated to either long- (LD) or short-day length (SD) for 8 wk, with or without superimposed food deprivation for 48 h. Furthermore, the hypothalamic concentration and distribution of phospho-AKT, a marker of PI3-kinase activity was determined by immunoblotting and immunohistochemistry. Eight weeks of acclimation to SD led to a substantial downregulation of IR, PTP1B gene expression, and phospho-AKT concentration in this brain region, whereas PI3-kinase mRNA was unchanged. Food deprivation induced a decrease in PTP1B and a trend toward lowered IR gene expression in LD but not in SD. Additionally, a striking increase in PTP1B gene expression in the thalamus was observed after food deprivation in both photoperiods. The direction of change in neuronal insulin signaling contrasts to the central catabolic nature of this pathway described in other species. SD-induced reduction in insulin signaling may be due to decline in body fat stores mediated by enhanced central leptin sensitivity. Increased anorexigenic tone of leptin may overwrite central insulin signaling to prevent catabolic overdrive.

Insulin signaling in the central nervous system: Learning to survive

Insulin is best known for its role in peripheral glucose homeostasis. Less studied, but not less important, is its role in the central nervous system. Insulin and its receptor are located in the central nervous system and are both implicated in neuronal survival and synaptic plasticity. Interestingly, over the past few years it has become evident that the effects of insulin, on neuronal survival and synaptic plasticity, are mediated by a common signal transduction cascade, which has been identified as "the PI3K route". This route has turned out to be a major integrator of insulin signaling in the brain. A pronounced feature of this insulin-activated route is that it promotes survival by directly inactivating the pro-apoptotic machinery. Interestingly, it is this same route that is required for the induction of long-term potentiation and depression, basic processes underlying learning and memory. This leads to the hypothesis that the PI3K route forms a direct link between learning and memory and neuronal survival. The implications of this hypothesis are far reaching, since it provides an explanation why insulin has beneficial effects on learning and memory and how synaptic activity can prevent cellular degeneration. Applying this knowledge may provide novel therapeutic approaches in the treatment of neurodegenerative diseases such as Alzheimer's disease.

Exercise activates the phosphatidylinositol 3-kinase pathway

Physical exercise is known to enhance psychological well-being and coping capacity. Voluntary physical exercise in rats also robustly and rapidly up-regulates hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels, which are potentiated following a regimen of chronic antidepressant treatment. Increased BDNF levels are associated with enhanced activity of cyclic AMP response element binding protein (CREB). So far, relatively little is known about the intracellular signaling mechanisms mediating this effect of exercise. We wished to explore the possibility that exercise and/or antidepressant treatment activate the hippocampal phosphatidylinositol-3 (PI-3) kinase pathway, which mediates cellular survival. In young male Sprague-Dawley rats, we examined the effects of 2 weeks of daily voluntary wheel-running activity and/or tranylcypromine (n = 7 per group) on the levels of the active forms of protein-dependent kinase-1 (PDK-1), PI-3 kinase, phospho-thr308-Akt, phospho-ser473-Akt, and phospho-glycogen synthase kinase-3beta (GSK3beta; inactive form), as well as BDNF, activated CREB, and the phospho-Trk receptor, in the rat hippocampus, and compared these with sedentary saline-treated controls. Immunoblotting analyses revealed that in exercising rats, there was a significant increase in PI-3 kinase expression (4.61 times that of controls, P = 0.0161) and phosphorylation of PDK-1 (2.73 times that of controls, P = 0.0454), thr308-Akt (2.857 times that of controls, P = 0.0082), CREB (60.27 times that of controls, P = 0.05), and Trk (35.3 times that of controls, P < 0.0001) in the hippocampi of exercising animals; BDNF was also increased (3.2 times that of controls), but this was not statistically significant. In rats receiving both exercise and tranylcypromine, BDNF (4.51 times that of controls, P = 0.0068) and PI-3 kinase (4.88 times that of controls, P = 0.0103), and the phospho- forms of Trk (13.67 times that of controls, P = 0.0278), thr308-Akt (3.644 times that of controls, P = 0.0004), GSK-3beta (2.93 times that of controls, P = 0.026), and CREB (88.97 times that of controls, P = 0.0053) were significantly increased. These results suggest that the exercise-induced expression of BDNF is associated with the increased expression of several key intermediates of the PI-3 kinase/Akt pathway, which is known for its role in enhancing neuronal survival.

The role of insulin receptor signaling in the brain

The insulin receptor (IR) is expressed in various regions of the developing and adult brain, and its functions have become the focus of recent research. Insulin enters the central nervous system (CNS) through the blood-brain barrier by receptor-mediated transport to regulate food intake, sympathetic activity and peripheral insulin action through the inhibition of hepatic gluconeogenesis and reproductive endocrinology. On a molecular level, some of the effects of insulin converge with those of the leptin signaling machinery at the point of activation of phosphatidylinositol 3-kinase (PI3K), resulting in the regulation of ATP-dependent potassium channels. Furthermore, insulin inhibits neuronal apoptosis via activation of protein kinase B in vitro, and it regulates phosphorylation of tau, metabolism of the amyloid precursor protein and clearance of beta-amyloid from the brain in vivo. These findings indicate that neuronal IR signaling has a direct role in the link between energy homeostasis, reproduction and the development of neurodegenerative diseases.

Role of signal transducer and activator of transcription 3 in regulation of hypothalamic proopiomelanocortin gene expression by leptin

Leptin acts on the brain to regulate body weight and neuroendocrine function. Proopiomelanocortin (POMC) neurons in the hypothalamus are important targets of leptin. These cells express the leptin receptor ObRb, and leptin can regulate POMC mRNA levels, but the cellular mechanisms by which this occurs is unknown. Here we show evidence that leptin stimulates pomc gene transcription via activation of intracellular signal transducer and activator of transcription 3 (STAT3) proteins. In pomc-promoter assays using transfected cells, leptin induces pomc promoter activity. Expression of dominant negative STAT3 strongly suppresses this effect. Furthermore, maximal activation requires the presence of the STAT3-binding site, tyrosine 1138, of ObRb. Mutational analysis identifies a 30-bp promoter element that is required for regulation by leptin. In rats, robust leptin-dependent induction of STAT3 phosphorylation is demonstrated in hypothalamic POMC neurons using double immunohistochemistry. In total, approximately 37% of POMC cells are positive for phospho-STAT3 after leptin treatment. Furthermore, leptin-responsive POMC neurons are concentrated in the rostral region of the hypothalamus. Combined, our data show that a subpopulation of POMC neurons is leptin-responsive and suggest that stimulation of hypothalamic pomc gene expression in these cells requires STAT3 activation. We speculate that STAT3 is critical for leptin-dependent effects on energy homeostasis that are mediated by the central melanocortin system.

Altered signaling and cell cycle regulation in embryonal stem cells with a disruption of the gene for phosphoinositide 3-kinase regulatory subunit p85alpha

The p85alpha regulatory subunit of class I(A) phosphoinositide 3-kinases (PI3K) is derived from the Pik3r1 gene, which also yields alternatively spliced variants p50alpha and p55alpha. It has been proposed that excess monomeric p85 competes with functional PI3K p85-p110 heterodimers. We examined embryonic stem (ES) cells with heterozygous and homozygous disruptions in the Pik3r gene and found that wild type ES cells express virtually no monomeric p85alpha. Although, IGF-1-stimulated PI3K activity associated with insulin receptor substrates was unaltered in all cell lines, p85alpha-null ES cells showed diminished protein kinase B activation despite increased PI3K activity associated with the p85beta subunit. Furthermore, p85alpha-null cells demonstrated growth retardation, increased frequency of apoptosis, and altered cell cycle regulation with a G(0)/G(1) cell cycle arrest and up-regulation of p27(KIP), whereas signaling through CREB and MAPK was enhanced. These phenotypes were reversed by re-expression of p85alpha via adenoviral gene transfer. Surprisingly, all ES cell lines could be differentiated into adipocytes. In these differentiated ES cells, however, compensatory p85beta signaling was lost in p85alpha-null cells while increased signaling by CREB and MAPK was still observed. Thus, loss of p85alpha in ES cells induced alterations in IGF-1 signaling and regulation of apoptosis and cell cycle but no defects in differentiation. However, differentiated ES cells partially lost their ability for compensatory signaling at the level of PI3K, which may explain some of the defects observed in mice with homozygous deletion of the Pik3r1 gene.

A Role for phosphoinositide 3-kinase in the control of cell division and survival during retinal development

Neurogenesis in the retina requires the concerted action of three different cellular processes: proliferation, differentiation, and apoptosis. Class IA phosphoinositide 3-kinase (PI3K) is a heterodimer composed of a p85 regulatory and a p110 catalytic subunit. p110alpha has been shown to regulate cell division and survival. Little is known of its function in development, however, as p110alpha knockout mice exhibit CNS defects, but death at early embryonic stages impairs further study. Here, we examine the role of PI3K in mouse retina development by expressing an activating form of PI3K regulatory subunit, p65(PI3K), as a transgene in the retina. Mice expressing p65(PI3K) showed severely disrupted retina morphogenesis, with ectopic cell masses in the neuroepithelium that evolved into infoldings of adult retinal cell layers. These changes correlated with an altered cell proliferation/cell death balance at early developmental stages. Nonetheless, the most affected cell layer in adult retina was that of photoreceptors, which correlated with selectively increased survival of these cells at developmental stages at which cell division has ceased. These results demonstrate the relevance of accurate PI3K regulation for normal retinal development, supporting class IA PI3K involvement in induction of cell division at early stages of neurogenesis. These data also show that, even after cell division decline, PI3K activation mediates survival of differentiated neurons in vivo.

Functions of PI 3-kinase in development of the nervous system

In the nervous system, receptor regulated phosphoinositide (PI) 3-kinases (PI 3-kinases) participate in fundamental cellular activities that underlie development. Activated by trophic factors, growth factors, neuregulins, cytokines, or neurotransmitters, PI 3-kinases have been implicated in neuronal and glial survival and differentiation. PI 3-kinases produce inositol lipid second messengers that bind to pleckstrin homology (PH) domains in diverse groups of signal transduction proteins, and control their enzymatic activities, subcellular membrane localization, or both. Downstream targets of the inositol lipid messengers include protein kinases and regulators of small GTPases. The kinase Akt/PKB functions as a key component of the PI 3-kinase dependent survival pathway through its phosphorylation and regulation of apoptotic proteins and transcription factors. Furthermore, since members of the Rho GTPase and Arf GTPase families have been implicated in regulation of the actin cytoskeleton, vesicular trafficking, and transcription, the downstream targets of PI 3-kinase that control these GTPases are excellent candidates to mediate aspects of PI 3-kinase dependent neuronal and glial differentiation.

Phosphatidylinositol-3-OH kinase regulatory subunits are differentially expressed during development of the rat cerebellum

Recent evidence implicates a central role for PI3K signalling in mediating cell survival during the process of neuronal differentiation. Although PI3K activity is stimulated by a wide range of growth factors and cytokines in different cell lines and tissues, activation of this pathway by insulin-like growth factor I (IGF-I) most likely represents the main survival signal during neuronal differentiation. IGF-I is highly expressed during development of the central nervous system, and thus is a critical factor for the development and maturation of the cerebellum. Upon ligand binding, the IGF-I receptor phosphorylates tyrosine residues in SHC and insulin receptor substrates (IRSs) initiating two main signalling cascades, the MAP kinase and the phosphatidylinositol 3-kinase (PI3K) pathways. Activated PI3K is composed of a catalytic subunit (p110alpha or beta) associated with one of a large family of regulatory subunits (p85alpha, p85beta, p55gamma, p55alpha, and p50alpha). To evaluate the contributions of these various regulatory subunits to neuronal differentiation, we have used antibodies specific for each of the PI3K subunits. Using these antisera, we now demonstrate that PI3K subunits are differentially regulated in cerebellar development, and that the expression level of the p55gamma regulatory subunit reaches a maximum during postnatal development, decreasing thereafter to low levels in the adult cerebellum. Furthermore, our studies reveal that the distribution of the various PI3K regulatory subunits varies during development of the cerebellum. Interestingly, p55gamma is expressed in both glial and neuronal cells; moreover, in Purkinje neurones, this subunit colocalises with the IGF-IR.