Expression of inducible membrane-anchored insulin receptor kinase enhances deoxyglucose uptake

Molecular elevation of insulin receptor signaling improves memory recall in aged Fischer 344 rats

As demonstrated by increased hippocampal insulin receptor density following learning in animal models and decreased insulin signaling, receptor density, and memory decline in aging and Alzheimer's diseases, numerous studies have emphasized the importance of insulin in learning and memory processes. This has been further supported by work showing that intranasal delivery of insulin can enhance insulin receptor signaling, alter cerebral blood flow, and improve memory recall. Additionally, inhibition of insulin receptor function or expression using molecular techniques has been associated with reduced learning. Here, we sought a different approach to increase insulin receptor activity without the need for administering the ligand. A constitutively active, modified human insulin receptor (IRβ) was delivered to the hippocampus of young (2 months) and aged (18 months) male Fischer 344 rats in vivo. The impact of increasing hippocampal insulin receptor expression was investigated using several outcome measures, including Morris water maze and ambulatory gait performance, immunofluorescence, immunohistochemistry, and Western immunoblotting. In aged animals, the IRβ construct was associated with enhanced performance on the Morris water maze task, suggesting that this receptor was able to improve memory recall. Additionally, in both age-groups, a reduced stride length was noted in IRβ-treated animals along with elevated hippocampal insulin receptor levels. These results provide new insights into the potential impact of increasing neuronal insulin signaling in the hippocampus of aged animals and support the efficacy of molecularly elevating insulin receptor activity in vivo in the absence of the ligand to directly study this process.

Elevating Insulin Signaling Using a Constitutively Active Insulin Receptor Increases Glucose Metabolism and Expression of GLUT3 in Hippocampal Neurons

Insulin signaling is an integral component of healthy brain function, with evidence of positive insulin-mediated alterations in synaptic integrity, cerebral blood flow, inflammation, and memory. However, the specific pathways targeted by this peptide remain unclear. Previously, our lab used a molecular approach to characterize the impact of insulin signaling on voltage-gated calcium channels and has also shown that acute insulin administration reduces calcium-induced calcium release in hippocampal neurons. Here, we explore the relationship between insulin receptor signaling and glucose metabolism using similar methods. Mixed, primary hippocampal cultures were infected with either a control lentivirus or one containing a constitutively active human insulin receptor (IRβ). 2-NBDG imaging was used to obtain indirect measures of glucose uptake and utilization. Other outcome measures include Western immunoblots of GLUT3 and GLUT4 on total membrane and cytosolic subcellular fractions. Glucose imaging data indicate that neurons expressing IRβ show significant elevations in uptake and rates of utilization compared to controls. As expected, astrocytes did not respond to the IRβ treatment. Quantification of Western immunoblots show that IRβ is associated with significant elevations in GLUT3 expression, particularly in the total membrane subcellular fraction, but did not alter GLUT4 expression in either fraction. Our work suggests that insulin plays a significant role in mediating neuronal glucose metabolism, potentially through an upregulation in the expression of GLUT3. This provides further evidence for a potential therapeutic mechanism underlying the beneficial impact of intranasal insulin in the clinic.

Broadening the definition of brain insulin resistance in aging and Alzheimer's disease

It has been >20 years since studies first revealed that the brain is insulin sensitive, highlighted by the expression of insulin receptors in neurons and glia, the presence of circulating brain insulin, and even localized insulin production. Following these discoveries, evidence of decreased brain insulin receptor number and function was reported in both clinical samples and animal models of aging and Alzheimer's disease, setting the stage for the hypothesis that neuronal insulin resistance may underlie memory loss in these conditions. The development of therapeutic insulin delivery to the brain using intranasal insulin administration has been shown to improve aspects of memory or learning in both humans and animal models. However, whether this approach functions by compensating for poorly signaling insulin receptors, for reduced insulin levels in the brain, or for reduced trafficking of insulin into the brain remains unclear. Direct measures of insulin's impact on cellular physiology and metabolism in the brain have been sparse in models of Alzheimer's disease, and even fewer studies have analyzed these processes in the aged brain. Nevertheless, recent evidence supports the role of brain insulin as a mediator of glucose metabolism through several means, including altering glucose transporters. Here, we provide a review of contemporary literature on brain insulin resistance, highlight the rationale for improving memory function using intranasal insulin, and describe initial results from experiments using a molecular approach to more directly measure the impact of insulin receptor activation and signaling on glucose uptake in neurons.

Expression of a Constitutively Active Human Insulin Receptor in Hippocampal Neurons Does Not Alter VGCC Currents

Memory and cognitive decline are the product of numerous physiological changes within the aging brain. Multiple theories have focused on the oxidative, calcium, cholinergic, vascular, and inflammation hypotheses of brain aging, with recent evidence suggesting that reductions in insulin signaling may also contribute. Specifically, a reduction in insulin receptor density and mRNA levels has been implicated, however, overcoming these changes remains a challenge. While increasing insulin receptor occupation has been successful in offsetting cognitive decline, alternative molecular approaches should be considered as they could bypass the need for brain insulin delivery. Moreover, this approach may be favorable to test the impact of continued insulin receptor signaling on neuronal function. Here we used hippocampal cultures infected with lentivirus with or without IRβ, a constitutively active, truncated form of the human insulin receptor, to characterize the impact continued insulin receptor signaling on voltage-gated calcium channels. Infected cultures were harvested between DIV 13 and 17 (48 h after infection) for Western blot analysis on pAKT and AKT. These results were complemented with whole-cell patch-clamp recordings of individual pyramidal neurons starting 96 h post-infection. Results indicate that while a significant increase in neuronal pAKT/AKT ratio was seen at the time point tested, effects on voltage-gated calcium channels were not detected. These results suggest that there is a significant difference between constitutively active insulin receptors and the actions of insulin on an intact receptor, highlighting potential alternate mechanisms of neuronal insulin resistance and mode of activation.

Susi, a negative regulator of Drosophila PI3-kinase

The Phosphatidylinositol-3 kinase/Protein Kinase B (PI3K/PKB) signaling pathway controls growth, metabolism, and lifespan in animals, and deregulation of its activity is associated with diabetes and cancer in humans. Here, we describe Susi, a coiled-coil domain protein that acts as a negative regulator of insulin signaling in Drosophila. Whereas loss of Susi function increases body size, overexpression of Susi reduces growth. We provide genetic evidence that Susi negatively regulates dPI3K activity. Susi directly binds to dP60, the regulatory subunit of dPI3K. Since Susi has no overt similarity to known inhibitors of PI3K/PKB signaling, it defines a novel mechanism by which this signaling cascade is kept in check. The fact that Susi is expressed in a circadian rhythm, with highest levels during the night, suggests that Susi attenuates insulin signaling during the fasting period.

Sequence analysis of bacterial redox enzyme maturation proteins (REMPs)

The twin-arginine protein transport (Tat) system is a remarkable molecular machine dedicated to the translocation of fully folded proteins across energy-transducing membranes. Complex cofactor-containing Tat substrates acquire their cofactors prior to export, and substrate proteins actually require to be folded before transport can proceed. Thus, it is very likely that mechanisms exist to prevent wasteful export of immature Tat substrates or to curb competition between immature and mature substrates for the transporter. Here we assess the primary sequence relationships between the accessory proteins implicated in this process during assembly of key respiratory enzymes in the model prokaryote Escherichia coli. For each respiratory enzyme studied, a redox enzyme maturation protein (REMP) was assigned. The main finding from this review was the hitherto unexpected link between the Tat-linked REMP DmsD and the nitrate reductase biosynthetic protein NarJ. The evolutionary link between Tat transport and cofactor insertion processes is discussed.Key words: Tat translocase, twin-arginine leader, hydrogenase, nitrate reductase, TMAO reductase, DMSO reductase, formate dehydrogenase, Tor, Dms, Hya, Hyb, Fdh, Nap.

Reconstitution of phosphoinositide 3-kinase-dependent insulin signaling in a cell-free system

Early insulin signaling events were examined in a novel cell-free assay utilizing subcellular fractions derived from 3T3-L1 adipocytes. The following cellular processes were observed in vitro in a manner dependent on insulin, time of incubation, and exogenous ATP: 1) autophosphorylation and activation of the insulin receptor; 2) tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1); 3) association of tyrosine-phosphorylated IRS-1 with phosphoinositide 3-kinase; 4) activation of the kinase Akt via its phosphorylation on Thr-308 and Ser-473; and 5) phosphorylation of glycogen synthase kinase-3 by activated Akt. The activation of Akt in vitro was abolished in the presence of the phosphoinositide 3-kinase inhibitor, wortmannin, thus recapitulating the most notable regulatory feature of Akt observed in vivo. Evidence is presented indicating that the critical spatial compartmentalization of signaling molecules necessary for efficient signal transduction is likely to be preserved in the cell-free system. Additionally, data are provided demonstrating that full Akt activation in this system is dependent on plasma membrane-associated IRS-1, cannot be mediated by robust cytosol-specific tyrosine phosphorylation of IRS-1, and occurs in the complete absence of detectable IRS-2 phosphorylation in the cytosol and plasma membrane.

The carboxyl terminal extension of the Drosophila insulin receptor homologue binds IRS-1 and influences cell survival

The Drosophila insulin receptor (INR) homolog includes an extension of approximately 400 amino acids at the carboxyl-terminal end of its beta subunit containing several tyrosine-based motifs known to mediate interactions with signaling proteins. In order to explore the role of this extension in INR function, mammalian expression vectors encoding either the complete INR beta subunit (beta-Myc) or the INR beta subunit without the carboxyl-terminal extension (betaDelta) were constructed, and the membrane-bound beta subunits were expressed in 293 and Madin-Darby canine kidney cells in the absence of the ligand-binding alpha subunits. beta-Myc and betaDelta proteins were constitutively active tyrosine kinases of 180 and 102 kDa, respectively. INR beta-Myc co-immunoprecipitated a phosphoprotein of 170 kDa identified as insulin receptor substrate-1 (IRS-1), whereas INR betaDelta did not, suggesting that the site of interaction was within the carboxyl-terminal extension. IRS-1 was phosphorylated on tyrosine to a much greater extent in cells expressing INR beta-Myc than in parental or INR betaDelta cells. Despite this, a variety of PTB or SH2 domain-containing signaling proteins, including IRS-2, mSos-1, Shc, p85 subunit of phosphatidylinositol 3-kinase, SHP-2, Raf-1, and JAK2, were not associated with the INR beta-Myc.IRS-1 complex. Overexpression of INR beta-Myc and betaDelta kinases conferred an equivalent increase in cell proliferation in both 293 and Madin-Darby canine kidney cells, indicating that this growth response is independent of the carboxyl-terminal extension. However, INR beta-Myc-expressing cells exhibited enhanced survival relative to parental and betaDelta cells, suggesting that the carboxyl-terminal extension, through its interaction with IRS-1, plays a role in the regulation of cell death.

Role of arginine 86 of the insulin receptor in insulin binding and activation of glucose transport

Mutations in the insulin receptor gene cause the inherited insulin resistant syndrome leprechaunism. Patient Atl-1 with leprechaunism was homozygous for the substitution of Arg-86 with Pro (R86P) in the alpha subunit of the insulin receptor. Fibroblasts homozygous for the mutant receptor had defective insulin binding, but increased glucose transport and receptor kinase activity. The R86P mutation is located in a putative beta turn N-terminal to a proposed insulin binding domain of the receptor [P. DeMeyts, J.L. Gu, R.M. Shymko, B.E. Kaplan, G.I. Bell, J. Whittaker, Mol. Endocrinol. 4 (1990) 409-416]. To get further insight into the mechanism of the paradoxical activation of receptor signalling by the R86P mutation, the codons for proline, alanine, and glycine were substituted in the R86 position of the insulin receptor cDNA by PCR-mediated mutagenesis and stably transfected into Chinese hamster ovary (CHO) cells. Insulin binding increased 10-20 fold in CHO cells transfected with the wild type, the R86A, and the R86G insulin receptor cDNA, but did not increase in cells expressing the R86P mutation. The R86P mutation caused a constitutive activation of insulin receptor phosphorylation in CHO cells, but did not increase basal glucose transport or its sensitivity to insulin stimulation. By contrast, transfection with the wild type and the R86A receptors increased 20-30 fold the sensitivity of glucose transport to stimulation by insulin. The R86G insulin receptor bound insulin normally, but was four times less efficient than the wild type or R86A insulin receptor in increasing the sensitivity for insulin stimulation of glucose transport. These results indicate that position 86 of the insulin receptor alpha subunit is tolerant to substitution by alanine, but not by proline. Substitution with glycine allows insulin binding, but does not activate normally glucose transport, further supporting an essential role of this position in the initiation of insulin receptor signalling of glucose transport.

Constitutive proteolysis of the ErbB-4 receptor tyrosine kinase by a unique, sequential mechanism

The heregulin receptor tyrosine kinase ErbB-4 is constitutively cleaved, in the presence or absence of ligand, by an exofacial proteolytic activity producing a membrane-anchored cytoplasmic domain fragment of 80 kD. Based on selective sensitivity to inhibitors, the proteolytic activity is identified as that of a metalloprotease. The 80-kD product is tyrosine phosphorylated and retains tyrosine kinase activity. Importantly, the levels of this fragment are controlled by proteasome function. When proteasome activity is inhibited for 6 h, the kinase-active 80-kD ErbB-4 fragment accumulates to a level equivalent to 60% of the initial amount of native ErbB-4 (approximately 10(6) receptors per cell). Hence, proteasome activity is essential to prevent the accumulation of a significant level of ligand-independent, active ErbB-4 tyrosine kinase generated by metalloprotease activity. Proteasome activity, however, does not act on the native ErbB-4 receptor before the metalloprotease-mediated cleavage, as no ErbB-4 fragments accumulate when metalloprotease activity is blocked. Although no ubiquitination of the native ErbB-4 is detected, the 80-kD fragment is polyubiquitinated. The data, therefore, describe a unique pathway for the processing of growth factor receptors, which involves the sequential function of an exofacial metalloprotease and the cytoplasmic proteasome.

An aspartate/insulin receptor chimera mitogenically activates fibroblasts

A gene encoding the ligand-binding domain of the Escherichia coli aspartate receptor fused to the cytoplasmic domain of the insulin receptor tyrosine kinase to produce the chimeric aspartate insulin receptor (AIR) was expressed in mammalian cells. A murine fibroblast transfectant line designated CA3 was generated that stably expressed the AIR receptor. This 70,000 Mr receptor containing the tyrosine kinase of the insulin receptor was recognized by aspartate receptor-specific antisera. When isolated in cellular membrane preparations, AIR was found to be capable of autophosphorylation and phosphorylation of histone H2B on tyrosine. The receptor was found to be predominately cytoplasmic and to be situated in the endoplasmic reticulum and Golgi membranes by immunofluorescence imaging of CA3 cells. Mitogenic effects of AIR were observed; CA3 cells continued DNA synthesis under serum deprivation conditions that prevented parental cells from cycling. These results demonstrate that a chimeric receptor containing procaryotic transmembrane sequences is expressed by a eucaryotic cell in intracellular membranes and functionally couples to cellular signaling pathways.

Genistein is a natural inhibitor of hexose and dehydroascorbic acid transport through the glucose transporter, GLUT1

Genistein is a dietary-derived plant product that inhibits the activity of protein-tyrosine kinases. We show here that it is a potent inhibitor of the mammalian facilitative hexose transporter GLUT1. In human HL-60 cells, which express GLUT1, genistein inhibited the transport of dehydroascorbic acid, deoxyglucose, and methylglucose in a dose-dependent manner. Transport was not affected by daidzein, an inactive genistein analog that does not inhibit protein-tyrosine kinase activity, or by the general protein kinase inhibitor staurosporine. Genistein inhibited the uptake of deoxyglucose and dehydroascorbic acid in Chinese hamster ovary (CHO) cells overexpressing GLUT1 in a similar dose-dependent manner. Genistein also inhibited the uptake of deoxyglucose in human erythrocytes indicating that its effect on glucose transporter function is cell-independent. The inhibitory action of genistein on transport was instantaneous, with no additional effect observed in cells preincubated with it for various periods of time. Genistein did not alter the uptake of leucine by HL-60 cells, indicating that its inhibitory effect was specific for the glucose transporters. The inhibitory effect of genistein was of the competitive type, with a Ki of approximately 12 microM for inhibition of the transport of both methylglucose and deoxyglucose. Binding studies showed that genistein inhibited glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts in a competitive manner, with a Ki of 7 microM. These data indicate that genistein inhibits the transport of dehydroascorbic acid and hexoses by directly interacting with the hexose transporter GLUT1 and interfering with its transport activity, rather than as a consequence of its known ability to inhibit protein-tyrosine kinases. These observations indicate that some of the many effects of genistein on cellular physiology may be related to its ability to disrupt the normal cellular flux of substrates through GLUT1, a hexose transporter universally expressed in cells, and is responsible for the basal uptake of glucose.

Effects of tyrosine-->phenylalanine mutations on auto- and trans-phosphorylation reactions catalyzed by the insulin receptor beta-subunit cytoplasmic domain

Activation of the insulin receptor kinase is closely associated with autophosphorylation of several tyrosine residues in the cytoplasmic domain of the receptor's two beta-subunits. To determine the contribution of these tyrosine phosphorylations to autoactivation of the receptor kinase, we have blocked phosphorylation at specific tyrosine by replacing these tyrosine residues, individually and in combination, with phenylalanine in a soluble 45-kD analog of the cytoplasmic insulin receptor kinase domain (CIRK). Kinetic studies of auto- and transphosphorylation with this panel of mutated CIRKs indicate that: (i) None of the tyrosines (953, 960, 1,146, 1,150, 1,151, 1,316, or 1,322) are necessary for catalysis: all single Y-->F mutants retain the ability to autoactivate comparable to the parent CIRK. (ii) Two of the tyrosine autophosphorylation sites, either tyrosine 1,150 or 1,151, contribute most (70-80%) of the autoactivation, because replacement of these two tyrosines by phenylalanine was the minimal change that abolishes autoactivation. (iii) A mutant CIRK having all seven reported tyrosine phosphorylation sites replaced by phenylalanine retained basal kinase activity but was incapable of autoactivation. These findings imply that autoactivation can occur without phosphorylation having occurred at any single site (953, 960, 1,146, 1,150, 1,151, 1,316, or 1,322), and autophosphorylation need not follow an ordered, sequential pathway beginning, for example, at tyrosine 1,146 as proposed for the intact insulin receptor.