Insulin-like growth factor-1 inhibits adult supraoptic neurons via complementary modulation of mechanoreceptors and glycine receptors

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Insulin-like growth factor 1 regulates excitatory synaptic transmission in pyramidal neurons from adult prefrontal cortex

Insulin-like growth factor 1 (IGF1) influences synaptic function in addition to its role in brain development and aging. Although the expression levels of IGF1 and IGF1 receptor (IGF1R) peak during development and decline with age, the adult brain has abundant IGF1 or IGF1R expression. Studies reveal that IGF1 regulates the synaptic transmission in neurons from young animals. However, the action of IGF1 on neurons in the adult brain is still unclear. Here, we used prefrontal cortical (PFC) slices from adult mice (∼8 weeks old) to characterize the role of IGF1 on excitatory synaptic transmission in pyramidal neurons and the underlying molecular mechanisms. We first validated IGF1R expression in pyramidal neurons using translating ribosomal affinity purification assay. Then, using whole-cell patch-clamp recording, we found that IGF1 attenuated the amplitude of evoked excitatory postsynaptic current (EPSC) without affecting the frequency and amplitude of miniature EPSC. Furthermore, this decrease in excitatory neurotransmission was blocked by pharmacological inhibition of IGF1R or conditionally knockdown of IGF1R in PFC pyramidal neurons. In addition, we determined that IGF1-induced decrease of EPSC amplitude was due to postsynaptic effect (internalization of a-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid receptors [AMPAR]) rather than presynaptic glutamate release. Finally, we found that inhibition of metabotropic glutamate receptor subtype-1 (mGluR1) abolished IGF1-induced attenuation of evoked EPSC amplitude and decrease of AMPAR expression at synaptic membrane, suggesting mGluR1-mediated endocytosis of AMPAR was involved. Taken together, these data provide the first evidence that IGF1 regulates excitatory synaptic transmission in adult PFC via the interaction between IGF1R-dependent signaling pathway and mGluR1-mediated AMPAR endocytosis.

Ion Channels of the Islets in Type 2 Diabetes

Ca²⁺ is an essential signal for pancreatic β-cell function. Ca²⁺ plays critical roles in numerous β-cell pathways such as insulin secretion, transcription, metabolism, endoplasmic reticulum function, and the stress response. Therefore, β-cell Ca²⁺ handling is tightly controlled. At the plasma membrane, Ca²⁺ entry primarily occurs through voltage-dependent Ca²⁺ channels. Voltage-dependent Ca²⁺ channel activity is dependent on orchestrated fluctuations in the plasma membrane potential or voltage, which are mediated via the activity of many ion channels. During the pathogenesis of type 2 diabetes the β-cell is exposed to stressful conditions, which result in alterations of Ca²⁺ handling. Some of the changes in β-cell Ca²⁺ handling that occur under stress result from perturbations in ion channel activity, expression or localization. Defective Ca²⁺ signaling in the diabetic β-cell alters function, limits insulin secretion and exacerbates hyperglycemia. In this review, we focus on the β-cell ion channels that control Ca²⁺ handling and how they impact β-cell dysfunction in type 2 diabetes.

Oxytocin is lower in African American men with diabetes and associates with psycho-social and metabolic health factors

Objective

Recently, it has been suggested that oxytocin (OT) has a role in metabolism and neuropsychiatry health and disease, and therefore, it may represent a potential therapeutic target. The current study aimed to investigate relationships between OT and glycemic status along with psycho-social and behavioral factors.

Design and methods

A total of 92 obese or overweight, African American, male subjects were enrolled in the study. Biometric and biochemical data were collected including oral glucose tolerance testing and urinary OT (measured by ELISA). Subjects also completed questionnaires on social and lifestyle factors.

Results

OT levels were found to be significantly lower in subjects with type 2 diabetes mellitus (T2DM) compared to normal glucose tolerance (p<0.05). When stratified by OT tertiles, subjects with higher OT had lower weight, body mass index (BMI) and hemoglobin A1c, but higher eGFR which remained significant after BMI adjustment. The highest OT tertile also had more smokers and more users of psychiatric medications. A stepwise ordered logistic regression supported these findings and could account for 21% of the variation in OT categories (pseudoR² = 0.21).

Conclusions

In this unique population, OT was found lower in subjects with diabetes but higher with better renal function, cigarette smoking and use of psychiatric medications. Future studies are needed to confirm these findings and examine the potential therapeutic role of OT.

Vasopressin Hypersecretion-Associated Brain Edema Formation in Ischemic Stroke: Underlying Mechanisms

BACKGROUND

Brain edema formation is a major cause of brain damages and the high mortality of ischemic stroke. The aim of this review is to explore the relationship between ischemic brain edema formation and vasopressin (VP) hypersecretion in addition to the oxygen and glucose deprivation and the ensuing reperfusion injury.

METHODS

Pertinent studies involving ischemic stroke, brain edema formation, astrocytes, and VP were identified by a search of the PubMed and the Web of Science databases in January 2016. Based on clinical findings and reports of animal experiments using ischemic stroke models, this systematic review reanalyzes the implication of individual reports in the edema formation and then establishes the inherent links among them.

RESULTS

This systematic review reveals that cytotoxic edema and vasogenic brain edema in classical view are mainly under the influence of a continuous malfunction of astrocytic plasticity. Adaptive VP secretion can modulate membrane ion transport, water permeability, and blood-brain barrier integrity, which are largely via changing astrocytic plasticity. Maladaptive VP hypersecretion leads to disruptions of ion and water balance across cell membranes as well as the integrity of the blood-brain barrier. This review highlights our current understandings of the cellular mechanisms underlying ischemic brain edema formation and its association with VP hypersecretion.

CONCLUSIONS

VP hypersecretion promotes brain edema formation in ischemic stroke by disrupting hydromineral balance in the neurovascular unit; suppressing VP hypersecretion has the potential to alleviate ischemic brain edema.

Generating new neurons to circumvent your fears: the role of IGF signaling

Extinction of fear memory is a particular form of cognitive function that is of special interest because of its involvement in the treatment of anxiety and mood disorders. Based on recent literature and our previous findings (EMBO J 30(19):4071-4083, 2011), we propose a new hypothesis that implies a tight relationship among IGF signaling, adult hippocampal neurogenesis and fear extinction. Our proposed model suggests that fear extinction-induced IGF2/IGFBP7 signaling promotes the survival of neurons at 2-4 weeks old that would participate in the discrimination between the original fear memory trace and the new safety memory generated during fear extinction. This is also called "pattern separation", or the ability to distinguish similar but different cues (e.g., context). To understand the molecular mechanisms underlying fear extinction is therefore of great clinical importance.

The many faces of insulin-like peptide signalling in the brain

Central and peripheral insulin-like peptides (ILPs), which include insulin, insulin-like growth factor 1 (IGF1) and IGF2, exert many effects in the brain. Through their actions on brain growth and differentiation, ILPs contribute to building circuitries that subserve metabolic and behavioural adaptation to internal and external cues of energy availability. In the adult brain each ILP has distinct effects, but together their actions ultimately regulate energy homeostasis - they affect nutrient sensing and regulate neuronal plasticity to modulate adaptive behaviours involved in food seeking, including high-level cognitive operations such as spatial memory. In essence, the multifaceted activity of ILPs in the brain may be viewed as a system organization involved in the control of energy allocation.

CD38/cyclic ADP-ribose system: a new player for oxytocin secretion and regulation of social behaviour

Oxytocin is important for regulating a number of physiological processes. Disruption of the secretion, metabolism or action of oxytocin results in an impairment of reproductive function, social and sexual behaviours, and stress responses. This review discusses current views on the regulation and autoregulation of oxytocin release in the hypothalamic-neurohypophysial system, with special focus on the activity of the CD38/cADP-ribose system as a new component in this regulation. Data from our laboratories indicate that an impairment of this system results in alterations of oxytocin secretion and abnormal social behaviour, thus suggesting new clues that help in our understanding of the pathogenesis of neurodevelopmental disorders.

Chronic vs. acute interactions between supraoptic oxytocin neurons and astrocytes during lactation: role of glial fibrillary acidic protein plasticity

In this article, we review studies of astrocytic-neuronal interactions and their effects on the activity of oxytocin (OXT) neurons within the magnocellular hypothalamo-neurohypophysial system. Previous work over several decades has shown that withdrawal of astrocyte processes increases OXT neuron excitability in the hypothalamic supraoptic nucleus (SON) during lactation. However, chronically disabling astrocyte withdrawal does not significantly affect the functioning of OXT neurons during suckling. Nevertheless, acute changes in a cytoskeletal element of astrocytes, glial fibrillary acidic protein (GFAP), occur in concert with changes in OXT neuronal activity during suckling. Here, we compare these changes in GFAP and related proteins with chronic changes that persist throughout lactation. During lactation, a decrease in GFAP levels accompanies retraction of astrocyte processes surrounding OXT neurons in the SON, resulting from high extracellular levels of OXT. During the initial stage of suckling, acute increases in OXT levels further strengthen this GFAP reduction and facilitate the retraction of astrocyte processes. This change, in turn, facilitates burst discharges of OXT neurons and leads to a transient increase in excitatory neurochemicals. This transient neurochemical surge acts to reverse GFAP expression and results in postburst inhibition of OXT neurons. The acute changes in astrocyte GFAP levels seen during suckling likely recur periodically, accompanied by rhythmic changes in glutamate metabolism, water transport, gliotransmitter release, and spatial relationships between astrocytes and OXT neurons. In the neurohypophysis, astrocyte retraction and reversal with accompanying GFAP plasticity also likely occur during lactation and suckling, which facilitates OXT release coordinated with its action in the SON. These studies of the dynamic interactions that occur between astrocytes and OXT neurons mediated by GFAP extend our understanding of astrocyte functions within the central nervous system.

Circulating insulin-like growth factor I and cognitive function: neuromodulation throughout the lifespan

Insulin-like growth factor I (IGF-I) is central to the somatotropic (growth hormone) axis. It promotes tissue growth and continues to have anabolic effects in adulthood. Accumulating evidence from the last decade, however, reveals that circulating levels of IGF-I also significantly affects cognitive brain function. Specifically, the decline of serum IGF-I might be associated with the age-related cognitive decline in elderly people. Moreover, psychiatric and neurological conditions characterized by cognitive impairment may be characterized by altered levels of IGF-I. Some evidence is emerging that interventions that target the GH/IGF-I axis may improve cognitive functioning, at least in deficient states. As there is evidence linking high serum IGF-I levels with cancer risk, these interventions should be carefully evaluated. On a cellular and molecular level, IGF-I may be a crucial component of neural homeostasis since disturbed IGF-I input is inevitably linked to perturbed function. Consistent with this, all nerve cells are potential targets of IGF-I actions, including neurons, glia, endothelial, epithelial, and perivascular cells. Indeed, many key cellular processes in the brain are affected by IGF-I's neurotrophic and modulatory actions. We review the regulation by IGF-I of neurotransmission and neuronal plasticity and conclude that serum IGF-I is an important mediator of neuronal growth, survival and function throughout the lifespan. The role of IGF-I in synaptic plasticity render its neurotrophic potential a key target for remediating the cognitive impairment associated with a range of neurological conditions.

Brain insulin growth factor-I induces diuresis increase through the inhibition of arginin-vasopressin release in aged rats

Normal aging is associated with water homeostasis impairment, arginin-vasopressin (AVP) neuron dysfunction and cerebral insulin growth factor-I (IGF-I) expression deficit. Therefore, we aimed at investigating whether a cerebral chronic treatment of IGF-I in aged rats (26-mo) could restore diuretic function comparable with that observed in adults (3-mo). By using osmotic pumps, we have shown that in aged rats, IGF-I treatment in the third ventricle for four weeks increases water intake and restores diuresis and AVP plasma release similar with that observed in adults. The decrease in AVP plasma release induced by brain IGF-I treatment was also associated with the decrease in urinary osmolality. These results indicate that the age-dependent IGF-I deficit in the brain may be involved in the age-impaired fluid homeostasis in rats.

Insulin increases the potency of glycine at ionotropic glycine receptors

The mechanisms by which insulin modulates neuronal plasticity and pain processes remain poorly understood. Here we report that insulin rapidly increases the function of glycine receptors in murine spinal neurons and recombinant human glycine receptors expressed in human embryonic kidney cells. Whole-cell patch-clamp recordings showed that insulin reversibly enhanced current evoked by exogenous glycine and increased the amplitude of spontaneous glycinergic miniature inhibitory postsynaptic currents recorded in cultured spinal neurons. Insulin (1 microM) also shifted the glycine concentration-response plot to the left and reduced the glycine EC(50) value from 52 to 31 microM. Currents evoked by a submaximal concentration of glycine were increased to approximately 140% of control. The glycine receptor alpha subunit was sufficient for the enhancement by insulin because currents from recombinant homomeric alpha(1) receptors and heteromeric alpha(1)beta receptors were both increased. Insulin acted at the insulin receptor via pathways dependent on tyrosine kinase and phosphatidylinositol 3 kinase because the insulin effect was eliminated by the insulin receptor antagonist, hydroxy-2-naphthalenylmethylphosphonic acid trisacetoxymethyl ester, the tyrosine kinase inhibitor lavendustin A, and the phosphatidylinositol 3 kinase antagonist wortmannin. Together, these results show that insulin has a novel regulatory action on the potency of glycine for ionotropic glycine receptors.

Age-dependent loss of insulin-like growth factor-1 receptor immunoreactive cells in the supraoptic hypothalamus is reduced in calorically restricted mice

Both life-long caloric restriction (CR) and the suppression of insulin-like growth factor-1 (IGF-1) signaling reliably extend the mammalian lifespan. The neuroendocrine system, regulated by the hypothalamus, remains the most convincing site of action for both these modes of life extension. Yet, determining whether CR actions are mediated by the modulation of neuroendocrine IGF-1 signaling remains unclear. Of the hypothalamic nuclei that express the IGF-1 receptor (IGF-1R), the cells of the supraoptic nucleus (SON) display some of the most robust IGF-1R expression. Taking IGF-1R immunoreactivity as an index of sensitivity to IGF-1, we counted IGF-1R immunoreactive and non-immunoreactive cells in the SON of young-ad-libitum fed (young-Al, 6 weeks), old-ad-libitum fed (Old-Al, 22 months), and old-calorie-restricted (Old-CR, 22 months) female B6D2F1 mice. An automated imaging microscopy system (AIMS) was used to generate cell counts for each section of supraoptic hypothalamus. Results show that while the total number of cells in the SON of ad-libitum fed mice does not change significantly with aging, a significant reduction in IGF-1R immunoreactive cells does occur in ad-libitum fed mice with aging. In contrast to this, calorie restricted mice show both a decline in the total number of cells and IGF-1R immunoreactive cells in the SON with age, but with the decrease in the latter being notably attenuated when compared to the degree of loss seen in ad-libitum fed mice. Thus, while CR induces greater loss in the total number of cells in the SON with age, it reduces the degree of age-dependent loss seen in IGF-1R expressing cells. As a result, when compared to Old-AL mice, the SON of Old-CR mice displays a greater proportion of IGF-1R cells and thus possibly enhanced IGF-1 sensitivity with aging.

Insulin-like growth factor actions during development of neural stem cells and progenitors in the central nervous system

Insulin-like growth factor-I (IGF-I) plays a key role in normal development. Recent studies show that IGF-I exerts a wide variety actions in the central nervous system during development as well as in adulthood. This report reviews recent developments on IGF-I actions and its mechanisms in the central nervous system, with a focus on its actions during the development of neural stem cells and progenitors. Available data strongly indicate that IGF-I shortens the length of the cell cycle in neuron progenitors during embryonic life and has an influence on the growth of all neural cell types. The phosphatidylinositol-3 kinase/Akt and mitogen-activated protein kinase pathways seem to be the predominant mediators of IGF-I-stimulated neural cell proliferation and survival. IGF-I actions, however, likely depend on cell type, developmental stage, and microenvironmental milieu.

An N-terminal variant of Trpv1 channel is required for osmosensory transduction

Body fluid homeostasis requires the release of arginine-vasopressin (AVP, an antidiuretic hormone) from the neurohypophysis. This release is controlled by specific and highly sensitive 'osmoreceptors' in the hypothalamus. Indeed, AVP-releasing neurons in the supraoptic nucleus (SON) are directly osmosensitive, and this osmosensitivity is mediated by stretch-inhibited cation channels. However, the molecular nature of these channels remains unknown. Here we show that SON neurons express an N-terminal splice variant of the transient receptor potential vanilloid type-1 (Trpv1), also known as the capsaicin receptor, but not full-length Trpv1. Unlike their wild-type counterparts, SON neurons in Trpv1 knockout (Trpv1(-/-)) mice could not generate ruthenium red-sensitive increases in membrane conductance and depolarizing potentials in response to hyperosmotic stimulation. Moreover, Trpv1(-/-) mice showed a pronounced serum hyperosmolality under basal conditions and severely compromised AVP responses to osmotic stimulation in vivo. These results suggest that the Trpv1 gene may encode a central component of the osmoreceptor.