François P. Pralong

Neuropeptide Y: the universal soldier.

Abstract. The peptidic neurotransmitter neuropeptide Y (NPY) has received great attention because it has been implicated in the regulation of several organ systems. In particular, NPY is involved in the regulatory loops that control food intake in the hypothalamus and appears also to be important for regulating the activity of neuroendocrine axes under poor metabolic conditions. Furthermore, NPY exerts vasoconstrictive action on the vasculature and potentiates the actions of many other vasoconstrictors. In addition, it was demonstrated to have trophic properties and could therefore contribute to cardiovascular remodeling. These various effects plus a number of others make NPY an attractive target for the potential treatment of human diseases, such as obesity, metabolic disorders, hypertension and heart failure.

Evidence for a leptin-neuropeptide Y axis for the regulation of growth hormone secretion in the rat

The obese gene (OB) product, leptin, has been shown to exert control on metabolic processes such as food intake and body weight homeostasis, possibly through a neuropeptide Y (NPY) neurotransmission. More recently, leptin has been shown to control several neuroendocrine axes, modulating pituitary hormone secretions in function of metabolic conditions. Since in the rat growth hormone (GH) secretion is dependent upon prevailing metabolic conditions, and NPY has been shown to be implicated in the feedback mechanisms of this hormone, we reasoned that leptin could also exert control over GH secretion and we examined this hypothesis in male rats submitted to a 3-day fast. Circulating leptin concentrations measured by RIA abruptly fell to low values after 24 h of fasting and remained low thereafter. Upon refeeding, leptin secretion regularly increased. As shown by others, pulsatile GH secretion had disappeared after 3 days of fasting. Centrally administered leptin (10 µg/day, i.c.v. infusion initiated at the beginning of the fasting period) totally prevented the disappearance of pulsatile GH secretion. No leak of centrally administered leptin to the general circulation was observed. Infusing the same amount of leptin intracerebroventricularly to rats receiving ad libitum feeding produced a severe reduction in food intake but maintained a normal GH secretory pattern. In contrast, pair-fed rats, submitted to the same food restriction, exhibited a marked reduction in GH secretion. Hypothalamic NPY gene expression, estimated by Northern blot analysis, was significantly increased in fasting rats, and decreased in leptin-treated, fasting rats. In rats receiving ad libitum feeding, leptin treatment reduced NPY gene expression, consistent with the observed reduction in food intake, whereas pair-fed animals logically exhibited increased NPY gene expression. In both situations with reduced feeding, normal GH secretion was seen in leptin-treated animals exhibiting low NPY gene expression, whereas decreased or abolished GH secretion was seen in animals not receiving leptin and exhibiting increased NPY mRNA levels. Interestingly, despite maintenance of normal GH secretion in leptin-treated, fasting rats, plasma IGF-I levels were low, as in vehicle-treated rats. Indeed, hepatic gene expression for both GH receptor and IGF-I was markedly reduced by fasting, and no correction was seen with leptin treatment. In summary, the regulation of GH secretion, at least the changes linked with malnutrition, appears to be dependent upon a leptin signal, perceived centrally, possibly related to circulating levels of this new hormone. The present data suggest that leptin can rescue normal pulsatile GH secretion by preventing the documented inhibitory action of NPY on GH secretion.

AMP-activated protein kinase mediates glucocorticoid-induced metabolic changes: a novel mechanism in Cushing’s syndrome

Chronic exposure to glucocorticoid hormones, resulting from either drug treatment or Cushings syndrome, results in insulin resistance, central obesity, and symptoms similar to the metabolic syndrome. We hypothesized that the major metabolic effects of corticosteroids are mediated by changes in the key metabolic enzyme adenosine monophosphate‐activated protein kinase (AMPK) activity. Activation of AMPK is known to stimulate appetite in the hypothalamus and stimulate catabolic processes in the periphery. We assessed AMPK activity and the expression of several metabolic enzymes in the hypothalamus, liver, adipose tissue, and heart of a rat glucocorticoid‐excess model as well as in in vitro studies using primary human adipose and primary rat hypothalamic cell cultures, and a human hepatoma cell line treated with dexamethasone and metformin. Glucocorticoid treatment inhibited AMPK activity in rat adipose tissue and heart, while stimulating it in the liver and hypothalamus. Similar data were observed in vitro in the primary adipose and hypothalamic cells and in the liver cell line. Metformin, a known AMPK regulator, prevented the corticosteroidinduced effects on AMPK in human adipocytes and rat hypothalamic neurons. Our data suggest that glucocorticoid‐induced changes in AMPK constitute a novel mechanism that could explain the increase in appetite, the deposition of lipids in visceral adipose and hepatic tissue, as well as the cardiac changes that are all characteristic of glucocorticoid excess. Our data suggest that metformin treatment could be effective in preventing the metabolic complications of chronic glucocorticoid excess.— Christ‐Crain M., Kola, B., Lolli F., Fekete, C., Seboek, D., Wittmann, G., Feltrin, D., Igreja, S. C., Ajodha, S., Harvey‐White, J., Kunos, G., Müller B., Pralong, F., Aubert, G., Arnaldi, G., Giacchetti, G., Boscaro, M., Grossman, A. B., Korbonits M. AMP‐activated protein kinase mediates glucocorticoidinduced metabolic changes: a novel mechanism in Cushings syndrome. FASEB J. 22, 1672–1683 (2008)

Control of GnRH neuronal activity by metabolic factors: the role of leptin and insulin.

Energy balance exerts a critical influence on reproductive function. Leptin and insulin are among the metabolic factors signaling the nutritional status of an individual to the hypothalamus, and their role in the overall modulation of the activity of GnRH neurons is increasingly recognized. The experiments described here were designed to further investigate the central mechanisms of action of these two hormones and the precise hypothalamic pathways implicated in their effects on the reproductive axis. NPY neurons represent a primary target of leptin actions within the hypothalamus We used mice lacking the NPY Y1 receptor (Y1-/- mice) to investigate the physiological importance of the hypothalamic NPY neuronal system and its downstream pathways involving Y1 in the reproductive effects of leptin. Results point to a crucial role for the NPY Y1 receptor in the control of the onset of puberty and the maintenance of reproductive functions by leptin. A striking finding of these experiments was the observation that juvenile Y1-/- mice submitted to food restriction can proceed through puberty like normally fed animals, demonstrating that the absence of Y1 impairs the perception of decreasing energy stores by the gonadotrope axis. Next, we used parallel in vivo and in vitro experiments to delineate the role of insulin in the stimulation and maintenance of the activity of the neuroendocrine reproductive axis. First, we observed that the increase in circulating insulin levels achieved during hyperinsulinemic clamp studies in normal male mice was associated with a significant rise in LH secretion. This effect of insulin is likely mediated at the hypothalamic level, as insulin stimulates the secretion and the expression of GnRH by hypothalamic neurons in culture. Using primary neuronal cultures as well as a novel GnRH neuronal cell line obtained by conditional immortalization of adult rat hypothalamic neurons, we have recently demonstrated that this effect of insulin on GnRH gene expression is probably mediated directly at the level of GnRH neurons, and involves the stimulation of the MAP kinase Erk1/2 pathway. Taken together, these results provide new insights into the mechanisms involved in the regulation of GnRH neuronal activity by metabolic factors.

An Essential Role for Insulin and IGF1 Receptors in Regulating Sertoli Cell Proliferation, Testis Size, and FSH Action in Mice

Testis size and sperm production are directly correlated to the total number of adult Sertoli cells (SCs). Although the establishment of an adequate number of SCs is crucial for future male fertility, the identification and characterization of the factors regulating SC survival, proliferation, and maturation remain incomplete. To investigate whether the IGF system is required for germ cell (GC) and SC development and function, we inactivated the insulin receptor (Insr), the IGF1 receptor (Igf1r), or both receptors specifically in the GC lineage or in SCs. Whereas ablation of insulin/IGF signaling appears dispensable for GCs and spermatogenesis, adult testes of mice lacking both Insr and Igf1r in SCs (SC-Insr;Igf1r) displayed a 75% reduction in testis size and daily sperm production as a result of a reduced proliferation rate of immature SCs during the late fetal and early neonatal testicular period. In addition, in vivo analyses revealed that FSH requires the insulin/IGF signaling pathway to mediate its proliferative effects on immature SCs. Collectively, these results emphasize the essential role played by growth factors of the insulin family in regulating the final number of SCs, testis size, and daily sperm output. They also indicate that the insulin/IGF signaling pathway is required for FSH-mediated SC proliferation.

Dicer1 Depletion in Male Germ Cells Leads to Infertility Due to Cumulative Meiotic and Spermiogenic Defects

Background Spermatogenesis is a complex biological process that requires a highly specialized control of gene expression. In the past decade, small non-coding RNAs have emerged as critical regulators of gene expression both at the transcriptional and post-transcriptional level. DICER1, an RNAse III endonuclease, is essential for the biogenesis of several classes of small RNAs, including microRNAs (miRNAs) and endogenous small interfering RNAs (endo-siRNAs), but is also critical for the degradation of toxic transposable elements. In this study, we investigated to which extent DICER1 is required for germ cell development and the progress of spermatogenesis in mice. Principal Findings We show that the selective ablation of Dicer1 at the early onset of male germ cell development leads to infertility, due to multiple cumulative defects at the meiotic and post-meiotic stages culminating with the absence of functional spermatozoa. Alterations were observed in the first spermatogenic wave and include delayed progression of spermatocytes to prophase I and increased apoptosis, resulting in a reduced number of round spermatids. The transition from round to mature spermatozoa was also severely affected, since the few spermatozoa formed in mutant animals were immobile and misshapen, exhibiting morphological defects of the head and flagellum. We also found evidence that the expression of transposable elements of the SINE family is up-regulated in Dicer1-depleted spermatocytes. Conclusions/Significance Our findings indicate that DICER1 is dispensable for spermatogonial stem cell renewal and mitotic proliferation, but is required for germ cell differentiation through the meiotic and haploid phases of spermatogenesis.

The neuropeptide Y Y1 receptor regulates leptin-mediated control of energy homeostasis and reproductive functions

The orexigenic neurotransmitter neuropeptide Y (NPY) plays a central role in the hypothalamic control of food intake and energy balance. NPY also exerts an inhibition of the gonadotrope axis that could be important in the response to poor metabolic conditions. In contrast, leptin provides an anorexigenic signal to centrally control the body needs in energy. Moreover, leptin contributes to preserve adequate reproductive functions by stimulating the activity of the gonadotrope axis. It is of interest that hypothalamic NPY represents a primary target of leptin actions. To evaluate the importance of the NPY Y1 and Y5 receptors in the downstream pathways modulated by leptin and controlling energy metabolism as well as the activity of the gonadotrope axis, we studied the effects of leptin administration on food intake and reproductive functions in mice deficient for the expression of either the Y1 or the Y5 receptor. Furthermore, the role of the Y1 receptor in leptin resistance was determined in leptin‐deficient ob/ob mice bearing a null mutation in the NPY Y1 locus. Results point to a crucial role for the NPY Y1 receptor in mediating the NPY pathways situated downstream of leptin actions and controlling food intake, the onset of puberty, and the maintenance of reproductive functions.

Dexamethasone treatment increases neuropeptide Y levels in rat hypothalamic neurones

Immunoreactive glucocorticoid receptors (GR) have previously been demonstrated in neuropeptide Y (NPY) neurones of the rat hypothalamus. To determine whether NPY synthesis is influenced by glucocorticoids, the effect of dexamethasone (DEX) on the levels of immunoreactive NPY in rat hypothalamic neurones was investigated in vivo and in vitro. Daily injections of DEX (0.1 mg/day) for 5 days increased the NPY content of the mediobasal hypothalamus in female rats by 117% (p less than 0.002). Primary cultures of hypothalamic neurones were also sensitive to the effect of glucocorticoids. Intracellular NPY levels were significantly increased (p less than 0.001) compared to control values by 151%, 222% and 268% when cultures were maintained in a defined serum free medium containing DEX 10(-9), 10(-8) and 10(-7) M respectively.

Blood-Borne Circadian Signal Stimulates Daily Oscillations in Actin Dynamics and SRF Activity

In peripheral tissues circadian gene expression can be driven either by local oscillators or by cyclic systemic cues controlled by the master clock in the brains suprachiasmatic nucleus. In the latter case, systemic signals can activate immediate early transcription factors (IETFs) and thereby control rhythmic transcription. In order to identify IETFs induced by diurnal blood-borne signals, we developed an unbiased experimental strategy, dubbed Synthetic TAndem Repeat PROMoter (STAR-PROM) screening. This technique relies on the observation that most transcription factor binding sites exist at a relatively high frequency in random DNA sequences. Using STAR-PROM we identified serum response factor (SRF) as an IETF responding to oscillating signaling proteins present in human and rodent sera. Our data suggest that in mouse liver SRF is regulated via dramatic diurnal changes of actin dynamics, leading to the rhythmic translocation of the SRF coactivator Myocardin-related transcription factor-B (MRTF-B) into the nucleus.

Cytokines, Leptin, and the Hypothalamo‐Pituitary‐Adrenal Axis

Abstract: The endocrine and immune systems are linked via an elaborated communication system constituted by an array of cytokines and neuropeptides which interact to modulate the integrated response of an organism to infection. Weight loss and anorexia, probably secondary to cytokine release, frequently accompany infection, but leptin could also play a role. Like cytokines, leptin serves as a peripheral messenger to convey signals to the brain. Expression of leptin is stimulated by glucocorticoids, endotoxins, and cytokines; on the other hand, leptin seems to inhibit the activation of the hypothalamo‐pituitary‐adrenal (HPA) axis. Indeed leptin exerts a direct, dose‐dependent inhibition of stimulated cortisol secretion by normal human and rat adrenal cells in vitro. These effects are mediated by the long isoform of the leptin receptor, because its transcript is expressed in the adrenal tissue. In addition we investigated the role played by the glucocorticoids in the development of tolerance of the hypothalamo‐corticotropic, immune and adipose system responses to repeated endotoxin administration. Unlike that of the corticotropic axis, tolerance of the immune and adipose systems is at least partially glucocorticoid‐independent. This crosstalk between the endocrine, immune, and adipose systems may be of prime importance to homeostasis in pathophysiological events occurring during infection.