AMP-activated protein kinase (AMPK) molecular crossroad for metabolic control and survival of neurons

The biguanide metformin alters phosphoproteomic profiling in mouse brain

Metformin, a potent antihyperglycemic agent is recommended as the first-line oral therapy for type 2 diabetes (T2D). Recently, metformin has been reported to be beneficial to neurodegenerative disease models. However, the putative mechanisms underlying the neuroprotective effects of metformin in disease models are unknown. Thus, we applied LC-MS/MS-based pattern analysis and two-dimensional electrophoresis (2DE)-based proteomic approach to understand the global phosphoproteomic alteration in the brain of metformin-administrated mice. Collectively, LC-MS/MS-based pattern analysis reveals that 41 phosphoproteins were upregulated and 22 phosphoproteins were downregulated in the brain of metformin-administrated mice. In addition, 5 differentially expressed phosphoproteins were identified upon metformin administration by 2DE coupled with mass spectrometry. The phosphorylation status of metabolic enzymes was decreased while that of mitochondrial proteins was increased by metformin. Interestingly, phosphorylated α-synuclein was significantly decreased by metformin administration. Taken together, our results might provide potential pathways to understand the pharmacological effect of metformin on neuroprotection.

Activators of AMPK: not just for Type II diabetes

Recent discoveries of AMPK activators point to the large number of therapeutic candidates that can be transformed to successful designs of novel drugs. AMPK is a universal energy sensor and influences almost all physiological processes in the cells. Thus, regulation of the cellular energy metabolism can be achieved in selective tissues via the artificial activation of AMPK by small molecules. Recently, special attention has been given to direct activators of AMPK that are regulated by several nonspecific upstream factors. The direct activation of AMPK, by definition, should lead to more specific biological activities and as a result minimize possible side effects.

Unpredictable chronic mild stress induces anxiety and depression-like behaviors and inactivates AMP-activated protein kinase in mice

The unpredictable chronic mild stress (UCMS) model was developed based upon the stress-diathesis hypothesis of depression. Most effects of UCMS can be reversed by antidepressants, demonstrating a strong predictive validity of this model for depression. However, the mechanisms underlying the effects induced by UCMS remain incompletely understood. Increasing evidence has shown that AMP-activated protein kinase (AMPK) regulates intracellular energy metabolism and is especially important for neurons because neurons are known to have small energy reserves. Abnormalities in the AMPK pathway disturb normal brain functions and synaptic integrity. In the present study, we first investigated the effects of UCMS on a battery of different tests measuring anxiety and depression-like behaviors in female C57BL/6N mice after 4 weeks of UCMS exposure. Stressed mice showed suppressed body weight gain, heightened anxiety, and increased immobility in the forced swim and tail suspension tests. These results are representative of some of the core symptoms of depression. Simultaneously, we observed decrease of synaptic proteins in the cortex of mice subjected to UCMS, which is associated with decreased levels of phosphorylated AMP-activated protein kinase α (AMPKα) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase). Our findings suggest that AMPKα inactivation might be a mechanism by which UCMS causes anxiety/depression-like behaviors in mice.

Activation of AMP-activated protein kinase by metformin protects against global cerebral ischemia in male rats: interference of AMPK/PGC-1α pathway

Here, we have investigated the effect of metformin pretreatment in the rat models of global cerebral ischemia. Cerebral ischemia which leads to brain dysfunction is one of the main causes of neurodegeneration and death worldwide. Metformin is used in clinical drug therapy protocols of diabetes. It is suggested that metformin protects cells under hypoxia and ischemia in non-neuronal contexts. Protective effects of metformin may be modulated via activating the AMP activated protein kinase (AMPK). Our results showed that induction of 30 min global cerebral I/R injury using 4-vesseles occlusion model led to significant cell death in the rat brain. Metformin pretreatment (200 mg kg/once/day, p.o., 2 weeks) attenuated apoptotic cell death and induced mitochondrial biogenesis proteins in the ischemic rats, analyzed using histological and Western blot assays. Besides, inhibition of AMPK by compound c showed that metformin resulted in apoptosis attenuation via AMPK activation. Interestingly, AMPK activation was also involved in the induction of mitochondrial biogenesis proteins using metformin, inhibition of AMPK by compound c reversed such effect, further supporting the role of AMPK upstream of mitochondrial biogenesis proteins. In summary, Metformin pretreatment is able to modulate mitochondrial biogenesis and apoptotic cell death pathways through AMPK activation in the context of global cerebral ischemia, conducting the outcome towards neuroprotection.

Increased actin polymerization and stabilization interferes with neuronal function and survival in the AMPKγ mutant Loechrig

loechrig (loe) mutant flies are characterized by progressive neuronal degeneration, behavioral deficits, and early death. The mutation is due to a P-element insertion in the gene for the γ-subunit of the trimeric AMP-activated protein kinase (AMPK) complex, whereby the insertion affects only one of several alternative transcripts encoding a unique neuronal isoform. AMPK is a cellular energy sensor that regulates a plethora of signaling pathways, including cholesterol and isoprenoid synthesis via its downstream target hydroxy-methylglutaryl (HMG)-CoA reductase. We recently showed that loe interferes with isoprenoid synthesis and increases the prenylation and thereby activation of RhoA. During development, RhoA plays an important role in neuronal outgrowth by activating a signaling cascade that regulates actin dynamics. Here we show that the effect of loe/AMPKγ on RhoA prenylation leads to a hyperactivation of this signaling pathway, causing increased phosphorylation of the actin depolymerizating factor cofilin and accumulation of filamentous actin. Furthermore, our results show that the resulting cytoskeletal changes in loe interfere with neuronal growth and disrupt axonal integrity. Surprisingly, these phenotypes were enhanced by expressing the Slingshot (SSH) phosphatase, which during development promotes actin depolymerization by dephosphorylating cofilin. However, our studies suggest that in the adult SSH promotes actin polymerization, supporting in vitro studies using human SSH1 that suggested that SSH can also stabilize and bundle filamentous actin. Together with the observed increase in SSH levels in the loe mutant, our experiments suggest that in mature neurons SSH may function as a stabilization factor for filamentous actin instead of promoting actin depolymerization.

Reduction in neural performance following recovery from anoxic stress is mimicked by AMPK pathway activation

Nervous systems are energetically expensive to operate and maintain. Both synaptic and action potential signalling require a significant investment to maintain ion homeostasis. We have investigated the tuning of neural performance following a brief period of anoxia in a well-characterized visual pathway in the locust, the LGMD/DCMD looming motion-sensitive circuit. We hypothesised that the energetic cost of signalling can be dynamically modified by cellular mechanisms in response to metabolic stress. We examined whether recovery from anoxia resulted in a decrease in excitability of the electrophysiological properties in the DCMD neuron. We further examined the effect of these modifications on behavioural output. We show that recovery from anoxia affects metabolic rate, flight steering behaviour, and action potential properties. The effects of anoxia on action potentials can be mimicked by activation of the AMPK metabolic pathway. We suggest this is evidence of a coordinated cellular mechanism to reduce neural energetic demand following an anoxic stress. Together, this represents a dynamically-regulated means to link the energetic demands of neural signaling with the environmental constraints faced by the whole animal.

The protective role of AMP-activated protein kinase in alpha-synuclein neurotoxicity in vitro

In the present study, we investigated the role of the main intracellular energy sensor, AMP-activated protein kinase (AMPK), in the in vitro neurotoxicity of α-synuclein (ASYN), one of the key culprits in the pathogenesis of Parkinson's disease. The loss of viability in retinoic acid-differentiated SH-SY5Y human neuroblastoma cells inducibly overexpressing wild-type ASYN was associated with the reduced activation of AMPK and its activator LKB1, as well as AMPK target Raptor. ASYN-overexpressing rat primary neurons also displayed lower activity of LKB1/AMPK/Raptor pathway. Restoration of AMPK activity by metformin or AICAR reduced the in vitro neurotoxicity of ASYN overexpression, acting independently of the prosurvival kinase Akt or the induction of autophagic response. The conditioned medium from ASYN-overexpressing cells, containing secreted ASYN, as well as dopamine-modified or nitrated recombinant ASYN oligomers, all inhibited AMPK activation in differentiated SH-SY5Y cells and reduced their viability, but not in the presence of metformin or AICAR. The RNA interference-mediated knockdown of AMPK increased the sensitivity of SH-SY5Y cells to the harmful effects of secreted ASYN. AMPK-dependent protection from extracellular ASYN was also observed in rat neuron-like pheochromocytoma cell line PC12. These data demonstrate the protective role of AMPK against the toxicity of both intracellular and extracellular ASYN, suggesting that modulation of AMPK activity may be a promising therapeutic strategy in Parkinson's disease.

Revisiting the relationship between autism and schizophrenia: toward an integrated neurobiology

Schizophrenia and autism have been linked since their earliest descriptions. Both are disorders of cerebral specialization originating in the embryonic period. Genetic, molecular, and cytologic research highlights a variety of shared contributory mechanisms that may lead to patterns of abnormal connectivity arising from altered development and topology. Overt behavioral pathology likely emerges during or after neurosensitive periods in which resource demands overwhelm system resources and the individual's ability to compensate using interregional activation fails. We are at the threshold of being able to chart autism and schizophrenia from the inside out. In so doing, the door is opened to the consideration of new therapeutics that are developed based upon molecular, synaptic, and systems targets common to both disorders.

Neuroprotective Effects of AMP-Activated Protein Kinase on Scopolamine Induced Memory Impairment

AMP-activated protein kinase (AMPK), an important regulator of energy metabolism, is activated in response to cellular stress when intracellular levels of AMP increase. We investigated the neuroprotective effects of AMPK against scopolamine-induced memory impairment in vivo and glutamate-induced cytotoxicity in vitro. An adenovirus expressing AMPK wild type alpha subunit (WT) or a dominant negative form (DN) was injected into the hippocampus of rats using a stereotaxic apparatus. The AMPK WT-injected rats showed significant reversal of the scopolamine induced cognitive deficit as evaluated by escape latency in the Morris water maze. In addition, they showed enhanced acetylcholinesterase (AChE)-reactive neurons in the hippocampus, implying increased cholinergic activity in response to AMPK. We also studied the cellular mechanism by which AMPK protects against glutamate-induced cell death in primary cultured rat hippocampal neurons. We further demonstrated that AMPK WT-infected cells increased cell viability and reduced Annexin V positive hippocampal neurons. Western blot analysis indicated that AMPK WT-infected cells reduced the expression of Bax and had no effects on Bcl-2, which resulted in a decreased Bax/Bcl-2 ratio. These data suggest that AMPK is a useful cognitive impairment treatment target, and that its beneficial effects are mediated via the protective capacity of hippocampal neurons.

The activation of adenosine monophosphate-activated protein kinase in rat hippocampus contributes to the rapid antidepressant effect of ketamine

Recent studies have shown a rapid, robust, and lasting antidepressant effect of ketamine that makes ketamine a promising antidepressant drug. However, the mechanisms underlying this rapid antidepressant effect remain incompletely understood. The goal of the present study was to determine whether adenosine monophosphate-activated protein kinase (AMPK) was involved in ketamine's rapid antidepressant effect during the forced swimming test (FST). In the first stage of experiment, a lower level of phosphorylated form of AMPK (p-AMPK) in the hippocampus and a longer immobility time were observed in the depressed rats during FST; whereas ketamine reversed these changes at 30min after the administration. In the second stage of experiment, we observed that, ketamine up-regulated the levels of p-AMPK and brain-derived neurotrophic factor (BDNF) in the hippocampus of the depressed rats. Moreover, AMPK agonist strengthened the antidepressant effect of ketamine with an up-regulation of BDNF, while AMPK antagonist attenuated the antidepressant effect of ketamine with a down-regulation of BDNF. In conclusion, our results suggest that the activation of AMPK in rat hippocampus is involved in the procedure of ketamine exerting rapid antidepressant effect through the up-regulation of BDNF.

AMPK: An emerging target for modification of injury-induced pain plasticity

Chronic pain is a critical medical problem afflicting hundreds of millions of people worldwide with costly effects on society and health care systems. Novel therapeutic avenues for the treatment of pain are needed that are directly targeted to the molecular mechanisms that promote and maintain chronic pain states. Recent evidence suggests that peripheral pain plasticity is promoted and potentially maintained via changes in translation control that are mediated by mTORC1 and MAPK pathways. While these pathways can be targeted individually, stimulating the AMPK pathway with direct or indirect activators achieves inhibition of these pathways via engagement of a single kinase. Here we review the form, function and pharmacology of AMPK with special attention to its emerging role as a potential target for pain therapeutics. We present the existing evidence supporting a role of AMPK activation in alleviating symptoms of peripheral nerve injury- and incision-induced pain plasticity and the blockade of the development of chronic pain following surgery. We argue that these preclinical findings support a strong rationale for clinical trials of currently available AMPK activators and further development of novel pharmacological strategies for more potent and efficacious manipulation of AMPK in the clinical setting. Finally, we posit that AMPK represents a unique opportunity for drug development in the kinase area for pain because it is pharmacologically manipulated via activation rather than inhibition potentially offering a wider therapeutic window with interesting additional pharmacological opportunities. Altogether, the physiology, pharmacology and therapeutic opportunities surrounding AMPK make it an attractive target for novel intervention for chronic pain and its prevention.

Activation of AMP-activated protein kinase alleviates homocysteine-mediated neurotoxicity in SH-SY5Y cells

Mammalian AMP-activated protein kinase (AMPK) acts as a metabolite-sensing protein kinase in multiple tissues. Recent studies have shown that AMPK activation also regulates intracellular signaling pathways involved in cellular survival and apoptosis. Previously, we have reported that AMPK activation alleviates the endoplasmic reticulum (ER) stress-mediated neurotoxicity and tau hyperphosphorylation caused by palmitate. Therefore, we investigated whether AMPK activation alleviates ER stress-mediated neurotoxicity in SH-SY5Y human neuroblastoma cells incubated with homocysteine. Regulation of AMPK activity by isoflavone was also determined to investigate the underlying mechanism of its neuroprotective effect. Treatment of SH-SY5Y human neuroblastoma cells with N (1)-(β-D-ribofuranosyl)-5-aminoimidazole-4-carboxamide (AICAR), a pharmacological activator of AMPK, significantly protected cells against cytotoxicity imposed by tunicamycin and homocysteine. Homocysteine significantly suppressed AMPK activation, which was alleviated by AICAR. We observed a significant inhibition of the unfolded protein response by AICAR in cells incubated with homocysteine, suggesting a protective role of AMPK activation against ER stress-mediated neurotoxicity. AICAR also significantly reduced tau hyperphosphorylation by inactivating glycogen synthase kinase-3β and c-Jun N-terminal kinase in cells incubated with homocysteine. Furthermore, treatment of cells with soy isoflavone, genistein and daidzein significantly activated AMPK, which was repressed by tunicamycin and homocysteine. Therefore, our results suggest that AMPK activation by isoflavone as well as AICAR alleviates homocysteine-mediated neurotoxicity in SH-SY5Y cells.

Metformin: an old but still the best treatment for type 2 diabetes

The management of T2DM requires aggressive treatment to achieve glycemic and cardiovascular risk factor goals. In this setting, metformin, an old and widely accepted first line agent, stands out not only for its antihyperglycemic properties but also for its effects beyond glycemic control such as improvements in endothelial dysfunction, hemostasis and oxidative stress, insulin resistance, lipid profiles, and fat redistribution. These properties may have contributed to the decrease of adverse cardiovascular outcomes otherwise not attributable to metformin's mere antihyperglycemic effects. Several other classes of oral antidiabetic agents have been recently launched, introducing the need to evaluate the role of metformin as initial therapy and in combination with these newer drugs. There is increasing evidence from in vivo and in vitro studies supporting its anti-proliferative role in cancer and possibly a neuroprotective effect. Metformin's negligible risk of hypoglycemia in monotherapy and few drug interactions of clinical relevance give this drug a high safety profile. The tolerability of metformin may be improved by using an appropiate dose titration, starting with low doses, so that side-effects can be minimized or by switching to an extended release form. We reviewed the role of metformin in the treatment of patients with type 2 diabetes and describe the additional benefits beyond its glycemic effect. We also discuss its potential role for a variety of insulin resistant and pre-diabetic states, obesity, metabolic abnormalities associated with HIV disease, gestational diabetes, cancer, and neuroprotection.

AMP kinase activation mitigates dopaminergic dysfunction and mitochondrial abnormalities in Drosophila models of Parkinson's disease

Mutations in parkin and LRRK2 together account for the majority of familial Parkinson's disease (PD) cases. Interestingly, recent evidence implicates the involvement of parkin and LRRK2 in mitochondrial homeostasis. Supporting this, we show here by means of the Drosophila model system that, like parkin, LRRK2 mutations induce mitochondrial pathology in flies when expressed in their flight muscles, the toxic effects of which can be rescued by parkin coexpression. When expressed specifically in fly dopaminergic neurons, mutant LRRK2 results in the appearance of significantly enlarged mitochondria, a phenotype that can also be rescued by parkin coexpression. Importantly, we also identified in this study that epigallocatechin gallate (EGCG), a green tea-derived catechin, acts as a potent suppressor of dopaminergic and mitochondrial dysfunction in both mutant LRRK2 and parkin-null flies. Notably, the protective effects of EGCG are abolished when AMP-activated protein kinase (AMPK) is genetically inactivated, suggesting that EGCG-mediated neuroprotection requires AMPK. Consistent with this, direct pharmacological or genetic activation of AMPK reproduces EGCG's protective effects. Conversely, loss of AMPK activity exacerbates neuronal loss and associated phenotypes in parkin and LRRK mutant flies. Together, our results suggest the relevance of mitochondrial-associated pathway in LRRK2 and parkin-related pathogenesis, and that AMPK activation may represent a potential therapeutic strategy for these familial forms of PD.

Adiponectin is protective against oxidative stress induced cytotoxicity in amyloid-beta neurotoxicity

Beta-amyloid (Aβ ) neurotoxicity is important in Alzheimer’s disease (AD) pathogenesis. Aβ neurotoxicity causes oxidative stress, inflammation and mitochondrial damage resulting in neuronal degeneration and death. Oxidative stress, inflammation and mitochondrial failure are also pathophysiological mechanisms of type 2 diabetes (T₂DM) which is characterized by insulin resistance. Interestingly, T₂DM increases risk to develop AD which is associated with reduced neuronal insulin sensitivity (central insulin resistance). We studied the potential protective effect of adiponectin (an adipokine with insulin-sensitizing, anti-inflammatory and anti-oxidant properties) against Aβ neurotoxicity in human neuroblastoma cells (SH-SY5Y) transfected with the Swedish amyloid precursor protein (Sw-APP) mutant, which overproduced Aβ with abnormal intracellular Aβ accumulation. Cytotoxicity was measured by assay for lactate dehydrogenase (LDH) released upon cell death and lysis. Our results revealed that Sw-APP transfected SH-SY5Y cells expressed both adiponectin receptor 1 and 2, and had increased AMP-activated protein kinase (AMPK) activation and enhanced nuclear factor-kappa B (NF-κB) activation compared to control empty-vector transfected SH-SY5Y cells. Importantly, adiponectin at physiological concentration of 10 µg/ml protected Sw-APP transfected SH-SY5Y cells against cytotoxicity under oxidative stress induced by hydrogen peroxide. This neuroprotective action of adiponectin against Aβ neurotoxicity-induced cytotoxicity under oxidative stress involved 1) AMPK activation mediated via the endosomal adaptor protein APPL1 (adaptor protein with phosphotyrosine binding, pleckstrin homology domains and leucine zipper motif) and possibly 2) suppression of NF-κB activation. This raises the possibility of novel therapies for AD such as adiponectin receptor agonists.

Copper signaling in the mammalian nervous system: synaptic effects

Copper is an essential metal present at high levels in the CNS. Its role as a cofactor in mitochondrial ATP production and in essential cuproenzymes is well defined. Menkes and Wilson's diseases are severe neurodegenerative conditions that demonstrate the importance of Cu transport into the secretory pathway. In the brain, intracellular levels of Cu, which is almost entirely protein bound, exceed extracellular levels by more than 100-fold. Cu stored in the secretory pathway is released in a Ca(2+)-dependent manner and can transiently reach concentrations over 100 μM at synapses. The ability of low micromolar levels of Cu to bind to and modulate the function of γ-aminobutyric acid type A (GABA(A)) receptors, N-methyl-D-aspartate (NMDA) receptors, and voltage-gated Ca(2+) channels contributes to its effects on synaptic transmission. Cu also binds to amyloid precursor protein and prion protein; both proteins are found at synapses and brain Cu homeostasis is disrupted in mice lacking either protein. Especially intriguing is the ability of Cu to affect AMP-activated protein kinase (AMPK), a monitor of cellular energy status. Despite this, few investigators have examined the direct effects of Cu on synaptic transmission and plasticity. Although the variability of results demonstrates complex influences of Cu that are highly method sensitive, these studies nevertheless strongly support important roles for endogenous Cu and new roles for Cu-binding proteins in synaptic function/plasticity and behavior. Further study of the many roles of Cu in nervous system function will reveal targets for intervention in other diseases in which Cu homeostasis is disrupted.

Transcriptional insights on the regenerative mechanics of axotomized neurons in vitro

Axotomized neurons have the innate ability to undergo regenerative sprouting but this is often impeded by the inhibitory central nervous system environment. To gain mechanistic insights into the key molecular determinates that specifically underlie neuronal regeneration at a transcriptomic level, we have undertaken a DNA microarray study on mature cortical neuronal clusters maintained in vitro at 8, 15, 24 and 48 hrs following complete axonal severance. A total of 305 genes, each with a minimum fold change of ±1.5 for at least one out of the four time points and which achieved statistical significance (one-way ANOVA, P < 0.05), were identified by DAVID and classified into 14 different functional clusters according to Gene Ontology. From our data, we conclude that post-injury regenerative sprouting is an intricate process that requires two distinct pathways. Firstly, it involves restructuring of the neurite cytoskeleton, determined by compound actin and microtubule dynamics, protein trafficking and concomitant modulation of both guidance cues and neurotrophic factors. Secondly, it elicits a cell survival response whereby genes are regulated to protect against oxidative stress, inflammation and cellular ion imbalance. Our data reveal that neurons have the capability to fight insults by elevating biological antioxidants, regulating secondary messengers, suppressing apoptotic genes, controlling ion-associated processes and by expressing cell cycle proteins that, in the context of neuronal injury, could potentially have functions outside their normal role in cell division. Overall, vigilant control of cell survival responses against pernicious secondary processes is vital to avoid cell death and ensure successful neurite regeneration.

Life or death: neuroprotective and anticancer effects of quercetin

ETHNOPHARMACOLOGICAL RELEVANCE

Quercetin is a ubiquitous flavonoid that is present in numerous plants that are utilized in many different cultures for their nervous system and anticancer effects. To better understand the neuroprotective and antiproliferative activities of quercetin, we present a comprehensive review of the divergent actions that contribute to the ethnopharmacological profile of these plants.

RESULTS

The pharmacological activities of quercetin that modulate antioxidation/oxidation/kinase-signaling pathways might be differentially elicited in neurons compared with malignant cells, ultimately promoting cell survival or death in a cell type- and metabolism-specific manner. Whereas the broad antioxidation and anti-inflammatory activities of quercetin are important for neuronal survival, the oxidative, kinase- and cell cycle-inhibitory, apoptosis-inducing effects of quercetin are essential for its anticancer effects. The diverse mechanistic interactions and activities of quercetin that modulate the phosphorylation state of molecules as well as gene expression would alter the interconnected and concerted intracellular signaling equilibrium, either inhibiting or strengthening survival signals. These mechanisms, which have been mainly observed in in vitro studies, cannot be easily translated into an explanation of the divergent simultaneous neuroprotective and anticancer effects observed in vivo. This is in part due to low bioavailability in plasma and in the brain, as well as the nature of the actual active molecules.

CONCLUSIONS

Numerous studies have demonstrated the beneficial effects of chronic quercetin intake, which is ethnopharmacologically meaningful, as many plants that are chronically ingested by people contain quercetin. Although quercetin and quercetin-containing plants exhibit potential as therapeutic modalities in neuropathology and in cancer, the data collectively highlight the need to elucidate issues such as bioavailability as well as its correlation with effectiveness at biomarkers in vivo. There would be an increased potentential of these plants for chemoprevention and neuropathology prevention.

Resveratrol promotes cellular glucose utilization in primary cultured cortical neurons via calcium-dependent signaling pathway

BACKGROUND AND PURPOSE

Impairment of glucose utilization contributes to neuronal degeneration of Alzheimer's disease patients. Cellular glucose utilization can be regulated by calcium-dependent signaling pathways. Resveratrol (RSV) is a plant-derived polyphenol with multiple beneficial effects, including neuroprotection and metabolic improvement. Here, we investigated the effect of RSV on neuronal calcium signal and glucose utilization.

EXPERIMENTAL METHODS

Primary culture of cortical neurons, calcium imaging, 2-NBDG assay and western blotting were employed to investigate RSV-mediated effects on neuronal calcium signal and glucose utilization.

RESULTS

RSV elevated intracellular calcium in cortical neurons via modulation of secondary messenger system including nitrous oxide, cGMP and cAMP. Secondarily, a calcium-dependent enhancement of neuronal glucose utilization after RSV treatment was observed. The effects on neuronal glucose utilization are largely dependent on RSV-induced calcium-dependent AMP-activated protein kinase activation.

CONCLUSION

Our findings show that activation of calcium-dependent signaling pathways by RSV may convey improvements of neuronal glucose utilization.

AMPK γ2 subunit gene PRKAG2 polymorphism associated with cognitive impairment as well as diabetes in old age

Metabolic and cognitive disorders are closely related. However, the molecular mechanism underlying this association is still elusive. Given the importance of energy metabolism in neuronal cells, AMP-activated protein kinase (AMPK), a master switch of energy metabolism, could be an independent factor affecting cognitive as well as metabolic functions. Therefore, we examined the relationship between the AMPK γ2 gene, the PRKAG2 -26C/T polymorphism and cognitive impairment or diabetes in 1609 subjects aged from 60 to 80. We performed multivariate logistic regression analyses with adjustment for age, gender, education, smoking, alcohol, depression, waist circumference, APOE e4, and stroke history. We found a significant association between the -26C/T polymorphism (CC vs. CT/TT) and cognitive impairment (OR, 1.6; 95% CI, 1.1-2.3). Moreover, this polymorphism (CC/CT vs. TT) was also related to the presence of diabetes (OR, 1.8; 95% CI, 1.2-2.8). Importantly, the relationship with cognitive impairment was still significant in non-diabetic individuals (OR, 1.6; 95% CI, 1.1-2.4). Further analyses with a subpopulation (n=611) revealed that CC homozygotes relative to T-allele carriers had significantly better performances in verbal memory and attentional tasks. These findings collectively support a hypothesis that AMPK has a role not only in metabolic functioning but also in cognitive functioning in humans. Extended longitudinal study with a larger number of samples is warranted.

Partial rescue of retinal function in chronically hypoglycemic mice

PURPOSE

Mice rendered hypoglycemic by a null mutation in the glucagon receptor gene Gcgr display late-onset retinal degeneration and loss of retinal sensitivity. Acute hyperglycemia induced by dextrose ingestion does not restore their retinal function, which is consistent with irreversible loss of vision. The goal of this study was to establish whether long-term administration of high dietary glucose rescues retinal function and circuit connectivity in aged Gcgr-/- mice.

METHODS

Gcgr-/- mice were administered a carbohydrate-rich diet starting at 12 months of age. After 1 month of treatment, retinal function and structure were evaluated using electroretinographic (ERG) recordings and immunohistochemistry.

RESULTS

Treatment with a carbohydrate-rich diet raised blood glucose levels and improved retinal function in Gcgr-/- mice. Blood glucose increased from moderate hypoglycemia to euglycemic levels, whereas ERG b-wave sensitivity improved approximately 10-fold. Because the b-wave reflects the electrical activity of second-order cells, we examined for changes in rod-to-bipolar cell synapses. Gcgr-/- retinas have 20% fewer synaptic pairings than Gcgr+/- retinas. Remarkably, most of the lost synapses were located farthest from the bipolar cell body, near the distal boundary of the outer plexiform layer (OPL), suggesting that apical synapses are most vulnerable to chronic hypoglycemia. Although treatment with the carbohydrate-rich diet restored retinal function, it did not restore these synaptic contacts.

CONCLUSIONS

Prolonged exposure to diet-induced euglycemia improves retinal function but does not reestablish synaptic contacts lost by chronic hypoglycemia. These results suggest that retinal neurons have a homeostatic mechanism that integrates energetic status over prolonged periods of time and allows them to recover functionality despite synaptic loss.

Role of 5'-adenosine monophosphate-activated protein kinase in cell survival and death responses in neurons

5'-Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a key sensor of cellular energy status. AMPK signaling regulates energy balance at the cellular, organ, and whole-body level. More recently, it has become apparent that AMPK plays also an important role in long-term decisions that determine cell fate, in particular cell cycle progression and apoptosis activation. Here, we describe the diverse mechanisms of AMPK activation and the role of AMPK in the regulation of cellular energy balance. We summarize recent studies implicating AMPK activation in the regulation of neuronal survival and as a key player during ischemic stroke. We also suggest that AMPK activation may have dual functions in the regulation of neuronal survival: AMPK provides a protective effect during transient energy depletion as exemplified in a model of neuronal Ca(2+) overloading, and this effect is partially mediated by the activation of neuronal glucose transporter 3. Prolonged AMPK activation, on the contrary, can lead to neuronal apoptosis via the transcriptional activation of the proapoptotic Bcl-2 family member, bim. Molecular switches that determine the protective versus cell death-inducing effects of AMPK activation are discussed.

Metabolic stress modulates motor patterning via AMP-activated protein kinase

Neural circuits are especially vulnerable to metabolic stress. The locust (Locusta migratoria) responds to anoxia by entering a coma during which neural and muscular systems shut down. During anoxic coma, arrest of the ventilatory central pattern generator is tightly correlated with an abrupt spreading depression (SD)-like increase in extracellular potassium concentration within the metathoracic neuropile. We examined the role of the AMP-activated protein kinase (AMPK), an evolutionarily conserved sensor of cellular energy status, in anoxia-induced ventilatory arrest and SD-like events in the locust. Perfusion of sodium azide (NaN(3); mitochondrial toxin) induced SD, temporary coma, and profound changes in the ventilatory motor pattern characterized as a rapid rhythm before coma and a slower rhythm following recovery. AMPK activation using 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) mimicked the motor pattern changes induced by NaN(3) but did not induce SD and coma. The effects of NaN(3) on the ventilatory rhythm were reversed by perfusion of compound-C (AMPK inhibitor) or glucose, and the effects of AICAR were also reversed by compound-C, confirming the modulatory roles of AMPK and energy status. Ouabain-induced recurring SD was suppressed by inhibition of AMPK and exacerbated by its activation. We show that the motor pattern changes induced by metabolic stress are not the result of SD alone, but that AMPK is necessary and sufficient for these changes and that AMPK activity strongly influences susceptibility to SD.

AICAR induced AMPK activation potentiates neuronal insulin signaling and glucose uptake

Insulin signaling is extensively studied in peripheral tissues while comparatively understudied in neuronal cells. AMPK is considered to be a fuel gauge of our body and activation of the same has been reported to increase insulin sensitivity in skeletal muscles thereby increasing glucose transport. However its role in neuronal insulin signaling is not established yet. Here we report positive regulation of insulin signaling as well as glucose uptake by AICAR, a pharmacological activator of AMPK, in cultured Neuro-2a cells in vitro. Compound C, a specific AMPK inhibitor, completely blocked the potentiating effects of AICAR on insulin signaling and glucose uptake, thus suggesting that AMPK mediates effects of AICAR on insulin signaling. Our study provides valuable insight in understanding the role of AMPK in neuronal insulin signal transduction.

Altered energy status of primary cerebellar granule neuronal cultures from rats exposed to lead in the pre- and neonatal period

This paper examines the effect of pre- and neonatal exposure of rats to lead (0.1% lead acetate in drinking water, resulting in rat offspring whole blood lead concentration (Pb-B) 4μg/dL) on the energy status of neuronal mitochondria by measuring changes in ATP, ADP, AMP, adenosine, TAN concentration, adenylate energy charge value (AEC) and mitochondrial membrane potential in primary cerebellar granule neurons (CGC) in dissociated cultures. Fluorescence studies were performed to imaging and evaluate mitochondria mass, mitochondrial membrane potential, intracellular and mitochondrial reactive oxygen species (ROS) production. The Na(+)/K(+) ATPase activity in intact CGC was measured spectrophotometrically. Our data shows that pre- and neonatal exposure of rats to Pb, even below the threshold of whole blood Pb value considered safe for people, affects the energy status of cultured primary cerebellar granule neurons through a decrease in ATP and TAN concentrations and AEC value, inhibition of Na(+)/K(+) ATPase, and increase in intracellular and mitochondrial ROS concentration. These observations suggest that even these low levels of Pb are likely to induce important alterations in neuronal function that could play a role in neurodegeneration.

Regulation of bile canalicular network formation and maintenance by AMP-activated protein kinase and LKB1

AMP-activated protein kinase (AMPK), a cellular metabolic sensor, is essential in energy regulation and metabolism. Hepatocyte polarization during liver development and regeneration parallels increased metabolism. The current study investigates the effects of AMPK and its upstream activator LKB1 on polarity and bile canalicular network formation and maintenance in collagen sandwich cultures of rat hepatocytes. Immunostaining for the apical protein ABCB1 and the tight junction marker occludin demonstrated that canalicular network formation is sequential and is associated with activation of AMPK and LKB1. AMPK and LKB1 activators accelerated canalicular network formation. Inhibition of AMPK or LKB1 by dominant-negative AMPK or kinase-dead LKB1 constructs blocked canalicular network formation. AICAR and 2-deoxyglucose, which activate AMPK, circumvented the inhibitory effect of kinase-dead LKB1 on canalicular formation, indicating that AMPK directly affects canalicular network formation. After the canalicular network was formed, inhibition of AMPK and LKB1 by dominant-negative AMPK or kinase-dead LKB1 constructs resulted in loss of canalicular network, indicating that AMPK and LKB1 also participate in network maintenance. In addition, activation of AMPK and LKB1 prevented low-Ca(2+)-mediated disruption of the canalicular network and tight junctions. These studies reveal that AMPK and its upstream kinase, LKB1, regulate canalicular network formation and maintenance.

AMPK-associated signaling to bridge the gap between fuel metabolism and hepatocyte viability

The adenosine monophosphate-activated protein kinase (AMPK) and p70 ribosomal S6 kinase-1 pathway may serve as a key signaling flow that regulates energy metabolism; thus, this pathway becomes an attractive target for the treatment of liver diseases that result from metabolic derangements. In addition, AMPK emerges as a kinase that controls the redox-state and mitochondrial function, whose activity may be modulated by antioxidants. A close link exists between fuel metabolism and mitochondrial biogenesis. The relationship between fuel metabolism and cell survival strongly implies the existence of a shared signaling network, by which hepatocytes respond to challenges of external stimuli. The AMPK pathway may belong to this network. A series of drugs and therapeutic candidates enable hepatocytes to protect mitochondria from radical stress and increase cell viability, which may be associated with the activation of AMPK, liver kinase B1, and other molecules or components. Consequently, the components downstream of AMPK may contribute to stabilizing mitochondrial membrane potential for hepatocyte survival. In this review, we discuss the role of the AMPK pathway in hepatic energy metabolism and hepatocyte viability. This information may help identify ways to prevent and/or treat hepatic diseases caused by the metabolic syndrome. Moreover, clinical drugs and experimental therapeutic candidates that directly or indirectly modulate the AMPK pathway in distinct manners are discussed here with particular emphasis on their effects on fuel metabolism and mitochondrial function.

Activation of kinase phosphorylation by heat-shift and mild heat-shock

Most cells activate intracellular signalling to recover from heat damage. An increase of temperature, known as HS (heat shock), induces two major signalling events: the transcriptional induction of HSPs (heat-shock proteins) and the activation of the MAPK (mitogen-activated protein kinase) cascade. We performed the present study to examine the effects of HS, induced by different experimental conditions, on various kinases [ERK (extracellular-signal-regulated kinase), JNK (c-Jun N-terminal kinase), p38, Akt, AMPK (AMP-activated protein kinase) and PKC (protein kinase C)]. We investigated by Western blot analysis the phosphorylation of MAPK as a measure of cellular responsiveness to heat shift (37°C) and mild HS (40°C) in different cell lines. The results of the study indicate that every cell line responded to heat shift, and to a greater extent to HS, increasing ERK and JNK phosphorylation, whereas variable effects on activation or inhibition of PKC, AMPK, Akt and p38 were observed. Besides the implications of intracellular signalling activated by heat variations, these data may be of technical relevance, indicating possible sources of error due to different experimental temperature conditions.

AMP-activated protein kinase mediates activity-dependent regulation of peroxisome proliferator-activated receptor gamma coactivator-1alpha and nuclear respiratory factor 1 expression in rat visual cortical neurons

Nuclear respiratory factor 1 (NRF-1) is one of the key transcription factors implicated in mitochondrial biogenesis by activating the transcription of mitochondrial transcription factor A (mtTFA) and subunit genes of respiratory enzymes. NRF-1 transactivation activity can be enhanced by interaction with transcription coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). The expression of PGC-1alpha, NRF-1 and mtTFA in neurons is known to be tightly regulated by neuronal activity. However, the coupling signaling mechanism is poorly understood. Here, we use primary cultures of rat visual cortical neurons and a rat model of monocular deprivation (MD) to investigate whether AMP-activated protein kinase (AMPK) is implicated in mediating activity-dependent regulation of PGC-1alpha and NRF-1 expression in neurons. We find that KCl depolarization rapidly activates AMPK and significantly increases PGC-1alpha, NRF-1, and mtTFA levels with increased ATP production in neuron cultures. Similarly, pharmacological activation of AMPK with 5'-aminoimidazole-4-carboxamide riboside (AICAR) or resveratrol also markedly increases PGC-1alpha and NRF-1 mRNA levels in neuron cultures. All these effects can be completely blocked by an AMPK inhibitor, Compound C. Conversely, 1 week of MD significantly reduces AMPK phosphorylation and activity, dramatically down-regulates PGC-1alpha and NRF-1 expression in deprived primary visual cortex. Administration of resveratrol in vivo significantly activates AMPK activity and attenuates the effects of MD on mitochondria by significant increase in PGC-1alpha and NRF-1 levels, mitochondria amount, and coupled respiration. These results strongly indicate that AMPK is an essential upstream mediator that couples neuronal activity to mitochondrial energy metabolism by regulation of PGC-1alpha-NRF-1 pathway in neurons.

Functional modulation of GABAB receptors by protein kinases and receptor trafficking

GABA(B) receptors (GABA(B)R) are heterodimeric G protein-coupled receptors (GPCRs) that mediate slow and prolonged inhibitory signals in the central nervous system. The signaling of GPCRs is under stringent control and is subject to regulation by multiple posttranslational mechanisms. The beta-adrenergic receptor is a prototypic GPCR. Like most GPCRs, prolonged exposure of this receptor to agonist induces phosphorylation of multiple intracellular residues that is largely dependent upon the activity of G protein-coupled receptor kinases (GRKs). Phosphorylation terminates receptor-effector coupling and promotes both interaction with beta-arrestins and removal from the plasma membrane via clathrin-dependent endocytosis. Emerging evidence for GABA(B)Rs suggests that these GPCRs do not conform to this mode of regulation. Studies using both native and recombinant receptor preparations have demonstrated that GABA(B)Rs do not undergo agonist-induced internalization and are not GRK substrates. Moreover, whilst GABA(B)Rs undergo clathrin-dependent constitutive endocytosis, it is generally accepted that their rates of internalization are not modified by prolonged agonist exposure. Biochemical studies have revealed that GABA(B)Rs are phosphorylated on multiple residues within the cytoplasmic domains of both the R1 and R2 subunits by cAMP-dependent protein kinase and 5'AMP-dependent protein kinase (AMPK). Here we discuss the role that this phosphorylation plays in determining GABA(B)R effector coupling and their trafficking within the endocytic pathway and go on to evaluate the significance of GABA(B)R phosphorylation in controlling neuronal excitability under normal and pathological conditions.