Kim A. Heidenreich

FoxO1 Regulates Multiple Metabolic Pathways in the Liver EFFECTS ON GLUCONEOGENIC, GLYCOLYTIC, AND LIPOGENIC GENE EXPRESSION

FoxO transcription factors are important targets of insulin action. To better understand the role of FoxO proteins in the liver, we created transgenic mice expressing constitutively active FoxO1 in the liver using the α1-antitrypsin promoter. Fasting glucose levels are increased, and glucose tolerance is impaired in transgenic (TGN) versus wild type (WT) mice. Interestingly, fasting triglyceride and cholesterol levels are reduced despite hyperinsulinemia, and post-prandial changes in triglyceride levels are markedly suppressed in TGN versus WT mice. Activation of pro-lipogenic signaling pathways (atypical protein kinase C and protein kinase B) and the ability to suppress β-hydroxybutyrate levels are not impaired in TGN. In contrast, de novo lipogenesis measured with 3H2O is suppressed by ∼70% in the liver of TGN versus WT mice after refeeding. Gene-array studies reveal that the expression of genes involved in gluconeogenesis, glycerol transport, and amino acid catabolism is increased, whereas genes involved in glucose utilization by glycolysis, the pentose phosphate shunt, lipogenesis, and sterol synthesis pathways are suppressed in TGN versus WT. Studies with adenoviral vectors in isolated hepatocytes confirm that FoxO1 stimulates expression of gluconeogenic genes and suppresses expression of genes involved in glycolysis, the shunt pathway, and lipogenesis, including glucokinase and SREBP-1c. Together, these results indicate that FoxO proteins promote hepatic glucose production through multiple mechanisms and contribute to the regulation of other metabolic pathways important in the adaptation to fasting and feeding in the liver, including glycolysis, the pentose phosphate shunt, and lipogenic and sterol synthetic pathways.

Cyclic AMP Promotes Neuronal Survival by Phosphorylation of Glycogen Synthase Kinase 3β

ABSTRACT Agents that elevate intracellular cyclic AMP (cAMP) levels promote neuronal survival in a manner independent of neurotrophic factors. Inhibitors of phosphatidylinositol 3 kinase and dominant-inactive mutants of the protein kinase Akt do not block the survival effects of cAMP, suggesting that another signaling pathway is involved. In this report, we demonstrate that elevation of intracellular cAMP levels in rat cerebellar granule neurons leads to phosphorylation and inhibition of glycogen synthase kinase 3β (GSK-3β). The increased phosphorylation of GSK-3β by protein kinase A (PKA) occurs at serine 9, the same site phosphorylated by Akt. Purified PKA is able to phosphorylate recombinant GSK-3β in vitro. Inhibitors of GSK-3 block apoptosis in these neurons, and transfection of neurons with a GSK-3β mutant that cannot be phosphorylated interferes with the prosurvival effects of cAMP. These data suggest that activated PKA directly phosphorylates GSK-3β and inhibits its apoptotic activity in neurons.

Glycogen synthase kinase-3beta phosphorylates Bax and promotes its mitochondrial localization during neuronal apoptosis.

Glycogen synthase kinase-3β (GSK-3β) is a critical activator of neuronal apoptosis induced by a diverse array of neurotoxic insults. However, the downstream substrates of GSK-3β that ultimately induce neuronal death are unknown. Here, we show that GSK-3β phosphorylates and regulates the activity of Bax, a pro-apoptotic Bcl-2 family member that stimulates the intrinsic (mitochondrial) death pathway by eliciting cytochrome c release from mitochondria. In cerebellar granule neurons undergoing apoptosis, inhibition of GSK-3β suppressed both the mitochondrial translocation of an expressed green fluorescent protein (GFP)-Baxα fusion protein and the conformational activation of endogenous Bax. GSK-3β directly phosphorylated Baxα on Ser163, a residue found within a species-conserved, putative GSK-3β phosphorylation motif. Coexpression of GFP-Baxα with a constitutively active mutant of GSK-3β, GSK-3β(Ser9Ala), enhanced the in vivo phosphorylation of wild-type Baxα, but not a Ser163Ala mutant of Baxα, in transfected human embryonic kidney 293 (HEK293) cells. Moreover, cotransfection with constitutively active GSK-3β promoted the localization of Baxα to mitochondria and induced apoptosis in both transfected HEK293 cells and cerebellar granule neurons. In contrast, neither a Ser163Ala point mutant of Baxα nor a naturally occurring splice variant that lacks 13 amino acids encompassing Ser163 (Baxσ) were driven to mitochondria in HEK293 cells coexpressing constitutively active GSK-3β. In a similar manner, either mutation or deletion of the identified GSK-3β phosphorylation motif prevented the localization of Bax to mitochondria in cerebellar granule neurons undergoing apoptosis. Our results indicate that GSK-3β exerts some of its pro-apoptotic effects in neurons by regulating the mitochondrial localization of Bax, a key component of the intrinsic apoptotic cascade.

Insulin-like growth factor-I induces bcl-2 promoter through the transcription factor cAMP-response element-binding protein.

Insulin-like growth factor-I (IGF-I) is known to prevent apoptosis induced by diverse stimuli. The present study examined the effect of IGF-I on the promoter activity ofbcl-2, a gene with antiapoptotic function. A luciferase reporter driven by the promoter region of bcl-2 from −1640 to −1287 base pairs upstream of the translation start site containing a cAMP-response element was used in transient transfection assays. Treatment of PC12 cells with IGF-I enhanced the bcl-2promoter activity by 2.3-fold, which was inhibited significantly (p < 0.01) by SB203580, an inhibitor of p38 mitogen-activated protein kinase (MAPK). Cotransfection of thebcl-2 promoter with MAPK kinase 6 and the β isozyme of p38 MAPK resulted in 2–3-fold increase in the reporter activity. The dominant negative form of MAPKAP-K3, a downstream kinase activated by p38 MAPK, and the dominant negative form of cAMP-response element-binding protein, inhibited the reporter gene activation by IGF-I and p38β MAPK significantly (p < 0.01). IGF-I increased the activity of p38β MAPK introduced into the cells by adenoviral infection. Thus, we have characterized a novel signaling pathway (MAPK kinase 6/p38β MAPK/MAPKAP-K3) that defines a transcriptional mechanism for the induction of the antiapoptotic protein Bcl-2 by IGF-I through the nuclear transcription factor cAMP-response element-binding protein in PC12 cells.

Inactivation of the Myocyte Enhancer Factor-2 Repressor Histone Deacetylase-5 by Endogenous Ca2//Calmodulin-dependent Kinase II Promotes Depolarization-mediated Cerebellar Granule Neuron Survival

Cerebellar granule neuron (CGN) survival depends on activity of the myocyte enhancer factor-2 (MEF2) transcription factors. Neuronal MEF2 activity is regulated by depolarization via a mechanism that is presently unclear. Here, we show that depolarization-mediated MEF2 activity and CGN survival are compromised by overexpression of the MEF2 repressor histone deacetylase-5 (HDAC5). Furthermore, removal of depolarization induced rapid cytoplasm-to-nuclear translocation of endogenous HDAC5. This effect was mimicked by addition of the calcium/calmodulin-dependent kinase (CaMK) inhibitor KN93 to depolarizing medium. Removal of depolarization or KN93 addition resulted in dephosphorylation of HDAC5 and its co-precipitation with MEF2D. HDAC5 nuclear translocation triggered by KN93 induced a marked loss of MEF2 activity and subsequent apoptosis. To selectively decrease CaMKII, CGNs were incubated with an antisense oligonucleotide to CaMKIIα. This antisense decreased CaMKIIα expression and induced nuclear shuttling of HDAC5 in CGNs maintained in depolarizing medium. Selectivity of the CaMKIIα antisense was demonstrated by its lack of effect on CaMKIV-mediated CREB phosphorylation. Finally, antisense to CaMKIIα induced caspase-3 activation and apoptosis, whereas a missense control oligonucleotide had no effect on CGN survival. These results indicate that depolarization-mediated calcium influx acts through CaMKII to inhibit HDAC5, thereby sustaining high MEF2 activity in CGNs maintained under depolarizing conditions.

Novel Mechanism for Gonadotropin-Releasing Hormone Neuronal Migration Involving Gas6/Ark Signaling to p38 Mitogen-Activated Protein Kinase

ABSTRACT Gonadotropin-releasing hormone (GnRH) is the central regulator of the reproductive axis. Normal sexual maturation depends on the migration of GnRH neurons from the olfactory placode to the hypothalamus during development. Previously, we showed restricted expression of the membrane receptor adhesion-related kinase (Ark) in immortalized cell lines derived from migratory but not postmigratory GnRH neurons. In addition, Ark and GnRH transcripts were detected along the GnRH neuron migratory route in the E13 mouse cribriform plate. In the present study, we examined the role of Ark and its ligand, Gas6 (encoded by growth arrest-specific gene 6), in GnRH neuron migration. Gas6 stimulated lamellipodial extension, membrane ruffling, and chemotaxis of immortalized NLT GnRH neuronal cells via the Ark receptor. Gas6/Ark signaling promoted activation of the Rho family GTPase Rac, and adenoviral-mediated expression of dominant negative N17Rac abolished Gas6/Ark-induced actin cytoskeletal reorganization and migration of GnRH neuronal cells. In addition, p38 MAPK was activated downstream of Ark and Rac, and inhibition of p38 MAPK with either SB203580 or adenoviral dominant negative p38α also blocked Gas6/Ark-mediated migration. Finally, downstream of Rac and p38 mitogen-activated protein kinase (MAPK), Gas6/Ark signaling promoted activation of MAPK-activated protein kinase 2 and induced phosphorylation of HSP25, a known regulator of cortical actin remodeling. The data are the first to demonstrate a migratory signaling pathway downstream of Ark/Axl family receptors and suggest a previously unidentified role for p38 MAPK in neuronal migration. Furthermore, these studies support a potential role for Ark in the regulation of GnRH neuronal migration.

Insulin-like Growth Factor I-mediated Activation of the Transcription Factor cAMP Response Element-binding Protein in PC12 Cells INVOLVEMENT OF p38 MITOGEN-ACTIVATED PROTEIN KINASE-MEDIATED PATHWAY

IGF-I is known to support growth and to prevent apoptosis in neuronal cells. Activation of the nuclear transcription factor cAMP response element-binding protein (CREB) has emerged as a central determinant in neuronal functions. In the present investigation, we examined the IGF-I-mediated phosphorylation and transcriptional activation of CREB in rat pheochromocytoma (PC12) cells, a cellular model for neuronal differentiation, and defined three distinct postreceptor signaling pathways important for this effect including the p38 mitogen-activated protein kinase (MAPK) pathway. CREB phosphorylation at serine 133 and its transcriptional activation as measured by a CREB-specific Gal4-CREB reporter and the neuroendocrine-specific gene chromogranin A was induced 2–3.3-fold by insulin-like growth factor (IGF)-I. This activation was significantly blocked (p < 0.001) by the dominant negative K-CREB or by mutation of the CRE site. IGF-I stimulated chromogranin A gene expression by Northern blot analysis 3.7-fold. Inhibition of MAPK kinase with PD98059, PI 3-kinase with wortmannin, and p38 MAPK with SB203580 blocked IGF-I-mediated phosphorylation and transcriptional activation of CREB by 30–50% (p < 0.001). Constitutively active and dominant negative regulators of the Ras and PI 3-kinase pathways confirmed the contribution of these pathways for CREB regulation by IGF-I. Cotransfection of PC12 cells with p38β and constitutively active MAPK kinase 6 resulted in enhanced basal as well as IGF-I-stimulated chromogranin A promoter. IGF-I activated p38 MAPK, which was blocked by the inhibitor SB203580. This is the first description of a p38 MAPK-mediated nuclear signaling pathway for IGF-I leading to CREB-dependent neuronal specific gene expression.

The p75 Neurotrophin Receptor Can Induce Autophagy and Death of Cerebellar Purkinje Neurons

The cellular mechanisms underlying Purkinje neuron death in various neurodegenerative disorders of the cerebellum are poorly understood. Here we investigate an in vitro model of cerebellar neuronal death. We report that cerebellar Purkinje neurons, deprived of trophic factors, die by a form of programmed cell death distinct from the apoptotic death of neighboring granule neurons. Purkinje neuron death was characterized by excessive autophagic-lysosomal vacuolation. Autophagy and death of Purkinje neurons were inhibited by nerve growth factor (NGF) and were activated by NGF-neutralizing antibodies. Although treatment with antisense oligonucleotides to the p75 neurotrophin receptor (p75ntr) decreased basal survival of cultured cerebellar neurons, p75ntr-antisense decreased autophagy and completely inhibited death of Purkinje neurons induced by trophic factor withdrawal. Moreover, adenoviral expression of a p75ntr mutant lacking the ligand-binding domain induced vacuolation and death of Purkinje neurons. These results suggest that p75ntr is required for Purkinje neuron survival in the presence of trophic support; however, during trophic factor withdrawal, p75ntr contributes to Purkinje neuron autophagy and death. The autophagic morphology resembles that found in neurodegenerative disorders, suggesting a potential role for this pathway in neurological disease.

The pesticide rotenone induces caspase‐3‐mediated apoptosis in ventral mesencephalic dopaminergic neurons

In vivo, the pesticide rotenone induces degeneration of dopamine neurons and parkinsonian‐like pathology in adult rats. In the current study, we utilized primary ventral mesencephalic (VM) cultures from E15 rats as an in vitro model to examine the mechanism underlying rotenone‐induced death of dopamine neurons. After 11 h of exposure to 30 nm rotenone, the number of dopamine neurons identified by tyrosine hydroxylase (TH) immunostaining declined rapidly with only 23% of the neurons surviving. By contrast, 73% of total cells survived rotenone treatment, indicating that TH+ neurons are more sensitive to rotenone. Examination of the role of apoptosis in TH+ neuron death, revealed that 10 and 30 nm rotenone significantly increased the number of apoptotic TH+ neurons from 7% under control conditions to 38 and 55%, respectively. The increase in apoptotic TH+ neurons correlated with an increase in immunoreactivity for active caspase‐3 in TH+ neurons. The caspase‐3 inhibitor, DEVD, rescued a significant number of TH+ neurons from rotenone‐induced death. Furthermore, this protective effect lasted for at least 32 h post‐rotenone and DEVD exposure, indicating lasting neuroprotection achieved with an intervention prior to the death commitment point. Our results show for the first time in primary dopamine neurons that, at low nanomolar concentrations, rotenone induces caspase‐3‐mediated apoptosis. Understanding the mechanism of rotenone‐induced apoptosis in dopamine neurons may contribute to the development of new neuroprotective strategies against Parkinsons disease.

MALDI mass spectrometric imaging of lipids in rat brain injury models.

Matrix-assisted laser desorption ionization/ionization imaging mass spectrometry (MALDI IMS) with a time-of-flight analyzer was used to characterize the distribution of lipid molecular species in the brain of rats in two injury models. Ischemia/reperfusion injury of the rat brain after bilateral occlusion of the carotid artery altered appearance of the phospholipids present in the hippocampal region, specifically the CA1 region. These brain regions also had a large increase in the ion abundance at m/z 548.5 and collisional activation supported identification of this ion as arising from ceramide (d18:1/18:0), a lipid known to be associated with cellular apoptosis. Traumatic brain injury model in the rat was examined by MALDI IMS and the area of damage also showed an increase in ceramide (d18:1/18:0) and a remarkable loss of signal for the potassium adduct of the most abundant phosphocholine molecular species 16:0/18:1 (PC) with a corresponding increase in the sodium adduct ion. This change in PC alkali attachment ion was suggested to be a result of edema and influx of extracellular fluid likely through a loss of Na/K-ATPase caused by the injury. These studies reveal the value of MALDI IMS to examine tissues for changes in lipid biochemistry and will provide data needed to eventually understand the biochemical mechanisms relevant to tissue injury.