Raphe-mediated signals control the hippocampal response to SRI antidepressants via miR-16

Serotonin reuptake inhibitor (SRI) antidepressants such as fluoxetine (Prozac), promote hippocampal neurogenesis. They also increase the levels of the bcl-2 protein, whose overexpression in transgenic mice enhances adult hippocampal neurogenesis. However, the mechanisms underlying SRI-mediated neurogenesis are unclear. Recently, we identified the microRNA miR-16 as an important effector of SRI antidepressant action in serotonergic raphe and noradrenergic locus coeruleus (LC). We show here that miR-16 mediates adult neurogenesis in the mouse hippocampus. Fluoxetine, acting on serotonergic raphe neurons, decreases the amount of miR-16 in the hippocampus, which in turn increases the levels of the serotonin transporter (SERT), the target of SRI, and that of bcl-2 and the number of cells positive for Doublecortin, a marker of neuronal maturation. Neutralization of miR-16 in the hippocampus further exerts an antidepressant-like effect in behavioral tests. The fluoxetine-induced hippocampal response is relayed, in part, by the neurotrophic factor S100β, secreted by raphe and acting via the LC. Fluoxetine-exposed serotonergic neurons also secrete brain-derived neurotrophic factor, Wnt2 and 15-Deoxy-delta12,14-prostaglandin J2. These molecules are unable to mimic on their own the action of fluoxetine and we show that they act synergistically to regulate miR-16 at the hippocampus. Of note, these signaling molecules are increased in the cerebrospinal fluid of depressed patients upon fluoxetine treatment. Thus, our results demonstrate that miR-16 mediates the action of fluoxetine by acting as a micromanager of hippocampal neurogenesis. They further clarify the signals and the pathways involved in the hippocampal response to fluoxetine, which may help refine therapeutic strategies to alleviate depressive disorders.

miR-16 targets the serotonin transporter: a new facet for adaptive responses to antidepressants

The serotonin transporter (SERT) ensures the recapture of serotonin and is the pharmacological target of selective serotonin reuptake inhibitor (SSRI) antidepressants. We show that SERT is a target of microRNA-16 (miR-16). miR-16 is expressed at higher levels in noradrenergic than in serotonergic cells; its reduction in noradrenergic neurons causes de novo SERT expression. In mice, chronic treatment with the SSRI fluoxetine (Prozac) increases miR-16 levels in serotonergic raphe nuclei, which reduces SERT expression. Further, raphe exposed to fluoxetine release the neurotrophic factor S100β, which acts on noradrenergic cells of the locus coeruleus. By decreasing miR-16, S100β turns on the expression of serotonergic functions in noradrenergic neurons. Based on pharmacological and behavioral data, we propose that miR-16 contributes to the therapeutic action of SSRI antidepressants in monoaminergic neurons.

Serotonergic 5-HT(2B) receptor controls tissue-nonspecific alkaline phosphatase activity in osteoblasts via eicosanoids and phosphatidylinositol-specific phospholipase C

In previous studies, we observed that mice knocked out for the serotonin-2B receptor (5-HT(2B)R) show defects in bone homeostasis. The present work focuses on the downstream targets relaying the anabolic function of this receptor in osteoblasts. A functional link between the 5-HT(2B)R and the activity of the tissue-nonspecific alkaline phosphatase (TNAP) is established using the C1 osteoprogenitor cell line. During C1 osteogenic differentiation, both 5-HT(2B)R and TNAP mRNA translations are delayed with respect to extracellular matrix deposition. Once the receptor is expressed, it constitutively controls TNAP activity at a post-translational level along the overall period of mineral deposition. Indeed, pharmacological inhibition of the 5-HT(2B)R intrinsic activity or shRNA-mediated 5-HT(2B)R knockdown prevents TNAP activation, but not its mRNA translation. In contrast, agonist stimulation of the receptor further increases TNAP activity during the initial mineralization phase. Building upon our previous observations that the 5-HT(2B)R couples with the phospholipase A2 pathway and prostaglandin production at the beginning of mineral deposition, we show that the 5-HT(2B)R controls leukotriene synthesis via phospholipase A2 at the terminal stages of C1 differentiation. These two 5-HT(2B)R-dependent eicosanoid productions delineate distinct time windows of TNAP regulation during the osteogenic program. Finally, prostaglandins or leukotrienes are shown to relay the post-translational activation of TNAP via stimulation of the phosphatidylinositol-specific phospholipase C. In agreement with the above findings, primary calvarial osteoblasts from 5-HT(2B)R-null mice exhibit defects in TNAP activity.

The cellular prion protein interacts with the tissue non-specific alkaline phosphatase in membrane microdomains of bioaminergic neuronal cells

Background

The cellular prion protein, PrP^C, is GPI anchored and abundant in lipid rafts. The absolute requirement of PrP^C in neurodegeneration associated to prion diseases is well established. However, the function of this ubiquitous protein is still puzzling. Our previous work using the 1C11 neuronal model, provided evidence that PrP^C acts as a cell surface receptor. Besides a ubiquitous signaling function of PrP^C, we have described a neuronal specificity pointing to a role of PrP^C in neuronal homeostasis. 1C11 cells, upon appropriate induction, engage into neuronal differentiation programs, giving rise either to serotonergic (1C11^5-HT) or noradrenergic (1C11^NE) derivatives.

Methodology/Principal Findings

The neuronal specificity of PrP^C signaling prompted us to search for PrP^C partners in 1C11-derived bioaminergic neuronal cells. We show here by immunoprecipitation an association of PrP^C with an 80 kDa protein identified by mass spectrometry as the tissue non-specific alkaline phosphatase (TNAP). This interaction occurs in lipid rafts and is restricted to 1C11-derived neuronal progenies. Our data indicate that TNAP is implemented during the differentiation programs of 1C11^5-HT and 1C11^NE cells and is active at their cell surface. Noteworthy, TNAP may contribute to the regulation of serotonin or catecholamine synthesis in 1C11^5-HT and 1C11^NE bioaminergic cells by controlling pyridoxal phosphate levels. Finally, TNAP activity is shown to modulate the phosphorylation status of laminin and thereby its interaction with PrP.

Conclusion/Significance

The identification of a novel PrP^C partner in lipid rafts of neuronal cells favors the idea of a role of PrP in multiple functions. Because PrP^C and laminin functionally interact to support neuronal differentiation and memory consolidation, our findings introduce TNAP as a functional protagonist in the PrP^C-laminin interplay. The partnership between TNAP and PrP^C in neuronal cells may provide new clues as to the neurospecificity of PrP^C function.

PKBα is required for adipose differentiation of mouse embryonic fibroblasts

Protein kinase Bα (PKBα) is a key regulator of metabolism, proliferation and differentiation. We have explored the role of PKBα in adipogenesis using wild-type and PKBα-knockout mouse embryonic fibroblasts (MEFs) and show that lack of PKBα prevents MEF differentiation into adipocytes. Expression of ectopic PKBα in PKBα-deficient cells restores adipogenesis. We identified 80 genes whose expression was upregulated in wild-type MEFs during adipogenesis but whose expression was significantly reduced in PKBα-deficient MEFs under the same conditions. Significantly, the regulator of adipogenesis Krüppel-like transcription factor 15 gene expression was downregulated in PKBα-deficient MEFs but could be restored by expressing an active PKBα in the deficient cells. The level of lipocalin 2, renin 1 and receptor-activity-modifying protein 3 genes expressed by adipose cells was also decreased in PKBα-deficient MEFs, and are inhibited by LY294002 treatment during early adipocyte differentiation of 3T3-L1 cells. The results underscore an essential role for PKBα in the transcriptional program required for adipogenesis.

Dosage-dependent effects of Akt1/protein kinase Balpha (PKBalpha) and Akt3/PKBgamma on thymus, skin, and cardiovascular and nervous system development in mice

Akt/protein kinase B (PKB) plays a critical role in the regulation of metabolism, transcription, cell migration, cell cycle progression, and cell survival. The existence of viable knockout mice for each of the three isoforms suggests functional redundancy. We generated mice with combined mutant alleles of Akt1 and Akt3 to study their effects on mouse development. Here we show that Akt1-/- Akt3+/- mice display multiple defects in the thymus, heart, and skin and die within several days after birth, while Akt1+/- Akt3-/- mice survive normally. Double knockout (Akt1-/-) Akt3-/-) causes embryonic lethality at around embryonic days 11 and 12, with more severe developmental defects in the cardiovascular and nervous systems. Increased apoptosis was found in the developing brain of double mutant embryos. These data indicate that the Akt1 gene is more essential than Akt3 for embryonic development and survival but that both are required for embryo development. Our results indicate isoform-specific and dosage-dependent effects of Akt on animal survival and development.

Physiological functions of protein kinase B/Akt

The genetic manipulation of mice has become an essential and elegant method for studying the function of proteins in physiology, and for testing the veracity of information obtained from cell culture experiments. During the past few years, a variety of transgenic and knockout mouse models of PKB (protein kinase B)/Akt have been generated and investigated. In this paper, we focus on the phenotypes of these PKB/Akt overexpression and mutant mice that may help to elucidate the functions exerted by PKB/Akt in mammals.

First Report of Bacterial Canker of Walnut Caused by Brenneria nigrifluens in France

Since the summer of 2000, vertical oozing cankers have been observed on trunks and branches of Persian walnut trees (Juglans regia). Cvs. Fernor, Chandler, Mayette, and Hartley were the most frequently affected, but cvs. Lara and Franquette could also be affected. Brenneria nigrifluens (synonym Erwinia nigrifluens) (3) was isolated from diseased trees from 13 orchards and nurseries in southwestern (Aquitaine, Périgord, Charentes, and Quercy), southeastern (Grenoble areas), and western (near Angers) France. Cankers were observed on trunks and branches where brown-to-black exudates staining the bark appeared mainly in the summer. Isolations were performed primarily from exudates but also from infected tissues by using King's medium B. Colonies similar in appearance to Brenneria nigrifluens (1) were purified and characterized. Gram reaction, Kovac's oxidase, oxidative-fermentative metabolism, and urease activity were assayed for all isolates. API Biotype 100 kits (BioMérieux, Marcy l'Etoile, France) were used as recommended, except that incubations were at 28°C for 4 days. When compared with the reference strain (French Collection of Plant Pathogenic Bacteria (CFBP) 4998^T = National Collection of Plant Pathogenic Bacteria (NCPPB) 564^T = American Type Culture Collection (ATCC) 13028^T) from California, 14 isolated strains were identified as B. nigrifluens on the basis of physiological and biochemical characteristics. These 14 strains were deposited in the CFBP under Accession Nos. 6746 to 6759. Pathogenicity of three selected strains (CFBP 6746, 6747, and 6758) was confirmed by inoculating branches of 7-year-old walnut trees with 10⁸ CFU of each isolate introduced in wounds (2). The reference strain (CFBP 4998^T) and water were similarly inoculated as controls. Two and five months later, necrotic lesions were observed in the inner bark and dark lines were observed in internal wood, but no external cankers were observed on any trees inoculated with the local and reference strains. B. nigrifluens was reisolated from the dark lines in internal wood up to approximately 10 cm from the inoculation site. To our knowledge, this is the first report of this bacterium in France. References: (1) L. Hauben et al. Syst. Appl. Microbiol. 21:384, 1998. (2) M. Ridé and S. Ridé. Proc. Int. Conf. Plant Pathogenic Bacteria, 4th, 2:957, 1978. (3) E. E. Wilson et al. Phytopathology 47:669, 1957.

Partial rescue of insulin receptor-deficient mice by transgenic complementation with an activated insulin receptor in the liver

Insulin receptor (IR)-deficient mice develop severe diabetes mellitus, diabetic ketoacidosis (DKA) and liver steatosis and die within 1 week after birth. We examined in this work whether the metabolic phenotype of IR(-/-) mutants could be improved by transgenic complementation with IR selectively in the liver. We first generated transgenic mice expressing a human DNA complementary to RNA encoding a truncated constitutively activated form of IR (IRdelta) under the control of liver-specific phenylalanine hydroxylase (PAH) gene promoter. These mice presented more pronounced fasting hypoglycemia and showed slightly improved glucose tolerance as compared to controls. The transgenic mice were crossed with IR(+/-) mutants to generate IR(-/-) mice carrying the PAH-IRDelta transgene. Although such mutants developed glycosuria, DKA was delayed by more than 1 week and survival was prolonged to 8-20 days in approximately 10% of mice. In these partially rescued pups, serum glucose and triglyceride levels were lowered, hepatic glycogen stores were reconstituted and liver steatosis was absent as compared with pups which developed strong DKA and died earlier. Thus, lack of insulin action in the liver is responsible in large part for the metabolic disorders seen in IR(+/-) mice. This study should stimulate interest in therapeutic strategies aimed at improving hepatic function in diabetes.

Improved metabolic disorders of insulin receptor-deficient mice by transgenic overexpression of glucokinase in the liver

AIMS/HYPOTHESIS

Insulin receptor null mutant mice develop severe diabetes, ketoacidosis and liver steatosis and die within 1 week after birth. Since the liver plays an essential role in the control of glucose homeostasis, we examined in this work whether the metabolic disorders of insulin receptor-deficient mice could be improved upon restoration of hepatic glucose metabolism by transgenic constitutive overexpression of glucokinase selectively in the liver.

METHODS

We first generated transgenic mice overexpressing rat glucokinase cDNA under control of the liver-specific phenylalanine hydroxylase gene promoter. These transgenic mice were crossed with heterozygous insulin-receptor-null mutants to produce homozygous insulin-receptor-null mice overexpressing glucokinase in the liver.

RESULTS

The transgenic mice overexpressing glucokinase in the liver showed improved glucose tolerance and were mildly hypoglycaemic and hyperlipidaemic under starved conditions. The introduction of the glucokinase transgene in insulin receptor null mice did not prevent the development of glycosuria. However, ketoacidosis was delayed by more than 1 week and survival was prolonged to 10 to 16 days in 16% of the pups. In these longer surviving pups, serum glucose and triglyceride concentrations were lowered, hepatic glycogen stores were reconstituted and liver steatosis was absent as compared with the pups which had developed strong ketoacidosis and died earlier.

CONCLUSIONS/INTERPRETATION

These results show that overexpression of hepatic glucokinase can compensate, in part, for the metabolic disorders developed by insulin receptor-deficient mice. This shows the importance of improving hepatic function in diabetes and must revive interest in enhancement of glucokinase activity as a therapeutic strategy for the treatment of diabetes.

Genetic manipulation of insulin signaling, action and secretion in mice. Insights into glucose homeostasis and pathogenesis of type 2 diabetes

Non-insulin-dependent diabetes mellitus (NIDDM) is a complex heterogeneous polygenic disease characterized mainly by insulin resistance and pancreatic beta-cell dysfunction. In recent years, several genetically engineered mouse models have been developed for the study of the pathophysiological consequences of defined alterations in a single gene or in a set of candidate diabetogenes. These represent new tools that are providing invaluable insights into NIDDM pathogenesis. In this review, we highlight the lessons emerging from the study of some of the transgenic or knockout mice in which the expression of key actors in insulin signaling, action or secretion has been manipulated. In addition to contributing to our knowledge of the specific roles of individual genes in the control of glucose homeostasis, these studies have made it possible to address several crucial issues in NIDDM that have remained controversial or unanswered for a number of years.

Endocrine pancreas in insulin receptor-deficient mouse pups

Insulin receptor (IR)-deficient pups rapidly become hyperglycemic and hyperinsulinemic and die of diabetic ketoacidosis within a few days. Immunocytochemical analysis of the endocrine pancreas revealed that IR deficiency did not alter islet morphology or the number of beta-, alpha-, delta-, and pancreatic polypeptide (PP) cells. The lack of IR did not result in major changes in the expression of islet hormone genes or of beta-cell-specific marker genes encoding pancreas duodenum homeobox-containing transcription factor-1 (PDX-1), glucokinase (GCK), and GLUT2, as shown by reverse transcriptase-polymerase chain reaction analysis. The serum glucagon levels in IR-deficient and nondiabetic littermates were comparable. Finally, total insulin content in the pancreas of IR-deficient pups was gradually depleted, indicating sustained insulin secretion, not compensated for by increased insulin biosynthesis. These findings are discussed in light of recent results suggesting a role of IR in beta-cell function.

IGF-1 receptor as an alternative receptor for metabolic signaling in insulin receptor-deficient muscle cells

We have derived skeletal muscle cell lines from wild-type (wt) and insulin receptor (IR) knockout mice to unravel the metabolic potential of IGF-1 receptor (IGF-1R). Both wt and IR(-/-) myoblasts differentiated into myotubes with similar patterns of expression of muscle-specific genes such as MyoD, myogenin and MLC1A indicating that IR is not required for this process. Binding of 125I-IGF-1 on wt and IR(-/-) myotubes was similar showing that IGF-1R was not upregulated in the absence of IR. Stimulation of IR(-/-) myotubes with IGF-1 (10(-10) to 10(-7) M) increased glucose uptake and incorporation into glycogen, induced IRS-1 phosphorylation and activated PI 3-kinase and MAP kinase, two enzymes of major signaling pathways. These effects were comparable to those obtained with wt myotubes using insulin or IGF-1 or with IR(-/-) myotubes using insulin at higher concentrations. This study provides a direct evidence that IGF-1R can represent an alternative receptor for metabolic signaling in muscle cells.

Genetic engineering in mice: impact on insulin signalling and action

The expression of a number of genes encoding key players in insulin signalling and action, including insulin, insulin receptor (IR), downstream signalling molecules such as insulin receptor substrate-1 (IRS-1) and IRS-2, glucose transporters (GLUT4, GLUT2) and important metabolic enzymes such as glucokinase, has now been altered in transgenic or knockout mice. Such mice presented with phenotypes ranging from mild defects, revealing complementarity between key molecules or pathways, to severe diabetes with ketoacidosis and early postnatal death. Insulin action could also be improved by overproduction of proteins acting at regulatory steps. The development of diabetes by combining mutations, which alone do not lead to major metabolic alterations, validated the 'diabetogenes' concept of non-insulin-dependent diabetes mellitus. Genes encoding insulin-like growth factors (IGF-I and IGF-II) and their type I receptor (IGF-IR) have also been disrupted. It appears that although IR and IGF-IR are both capable of metabolic and mitogenic signalling, they are not fully redundant. However, IR could replace IGF-IR if efficiently activated by IGF-II. Studies with cell lines lacking IR or IGF-IR lend support to such conclusions. Concerning the issues of specificity and redundancy, studies with cell lines derived from IRS-1-deficient mice showed that IRS-1 and IRS-2 are also not completely interchangeable.

Genetic manipulation of insulin action and beta-cell function in mice

Transgenic and gene targeting approaches have now been applied to a number of genes in order to investigate the metabolic disorders that would result by manipulating insulin action or pancreatic beta-cell function in the mouse. The availability of such mutant mice will allow in the future to develop animal models in which the pathophysiologies resulting from polygenic defects might be reconstituted and studied in detail. Such animal models hopefully will lead to better understanding of complex polygenic diseases such as non-insulin-dependent diabetes mellitus (NIDDM).

Insulin receptor-deficient cells as a new tool for dissecting complex interplay in insulin and insulin-like growth factors

Cell systems derived from knockout mice for the insulin receptor (IR) or the IGF-1 receptor (IGF-1R) represent unique tools for dissecting complex interplay in the actions of insulin and insulin-like growth factors through their cognate versus non-cognate receptor. In this study, we used a fibroblast cell line derived from IR-deficient mice to investigate metabolic and mitogenic effects of IGF-1 and insulin. IGF-1 was able to stimulate glucose uptake, glucose incorporation into glycogen and thymidine incorporation in such cells. Phosphatidylinositol 3-kinase and mitogen-activated protein kinase, two enzymes of major metabolic-mitogenic signaling pathways, were activated upon stimulating these cells with IGF-1. All these effects were also achieved when IR-deficient cells were stimulated with insulin. Thus, IGF-1R can represent an alternative receptor through which insulin might exert some of its effects.