The protein phosphatases involved in cellular regulation. 6. Measurement of type-1 and type-2 protein phosphatases in extracts of mammalian tissues; an assessment of their physiological roles

Biomarkers and Target-Specific Small-Molecule Drugs in Alzheimer's Diagnostic and Therapeutic Research: From Amyloidosis to Tauopathy

Alzheimer's disease (AD) is the most common type of human dementia and is responsible for over 60% of diagnosed dementia cases worldwide. Abnormal deposition of β-amyloid and the accumulation of neurofibrillary tangles have been recognised as the two pathological hallmarks targeted by AD diagnostic imaging as well as therapeutics. With the progression of pathological studies, the two hallmarks and their related pathways have remained the focus of researchers who seek for AD diagnostic and therapeutic strategies in the past decades. In this work, we reviewed the development of the AD biomarkers and their corresponding target-specific small molecule drugs for both diagnostic and therapeutic applications, underlining their success, failure, and future possibilities.

A spatial model of autophosphorylation of Ca 2+ /calmodulin-dependent protein kinase II in a glutamatergic spine reveals dynamics of kinase activation in the first several seconds after a complex synaptic stimulus

Long-term potentiation (LTP) is a biochemical process in excitatory glutamatergic synapses in the Central Nervous System (CNS). It is initiated by a bout of synaptic activation that is strong enough to contribute to production of an action potential in the axon of the postsynaptic neuron, and it results in an increase in the size of postsynaptic depolarization during subsequent activity. The first step leading to LTP is activation and autophosphorylation of an abundant postsynaptic enzyme, Ca 2+ /calmodulin-dependent protein kinase II (CaMKII). We use simulation of activation of CaMKII holoenzymes in a realistic spatial model of a spine synapse, created in MCell4, to test three hypotheses about how the autophosphorylation response of CaMKII is shaped during a repeated high-frequency stimulus. First, the simulation results indicate that autophosphorylation of CaMKII does not constitute a bistable switch under biologically realistic conditions. Instead, prolonged autophosphorylation of CaMKII may contribute to a biochemical "kinetic proof-reading" mechanism that controls induction of synaptic plasticity. Second, concentration of CaMKII near the postsynaptic membrane increases the local concentration of kinase activity. However, neither localization nor "Ca 2+ -calmodulin-trapping (CaM-trapping)" increase the proportion of autophosphorylated subunits in holoenzymes after a complex stimulus, as previously hypothesized. Finally, we show that, as hypothesized, the amplitude of autophosphorylation in the first 30 seconds after a stimulus is extremely sensitive to the level and location of PP1 activity when PP1 is present in biologically accurate amounts. We further show that prolonged steric hindrance of dephosphorylation of CaMKII, caused by CaM-trapping, can increase the amplitude of autophosphorylation after a complex stimulus. These simulation results sharpen our quantitative understanding of the early events leading to LTP at excitatory synapses.

Author Summary

Neurons in the brain are interconnected in an organized fashion by synapses that transmit neuronal activity from one neuron to another. Most of the billions of neurons in the brain have about 10,000 synapses spread over the neuronal membrane. Information is stored in the brain when the ability of specific synapses to pass along neuronal activity is strengthened resulting in formation of new networks. The increase in strength of a synapse is tightly controlled by the frequency and amplitude of its activity, and by neurohormonal signals, which, in combination, can cause long-lasting biochemical changes at the synapse that underlie learning and memory. Defects in these biochemical pathways cause mental and neurological diseases. To develop treatments, we need to understand the precise choreography of these critical biochemical changes. However, the tiny size of the synaptic compartment makes precise measurements of the biochemical reactions impossible. We have used computer simulation techniques and information gathered from experiments on purified synaptic proteins to simulate, within a single synapse, the choreography of the first biochemical step in synaptic strengthening: activation of the enzyme Ca 2+ / calmodulin-dependent protein kinase II. Our results provide insights that can be used in future studies to develop treatments for neuronal diseases.

Knockout of protein phosphatase 1 in Leishmania major reveals its role during RNA polymerase II transcription termination

The genomes of kinetoplastids are organized into polycistronic transcription units that are flanked by a modified DNA base (base J, beta-D-glucosyl-hydroxymethyluracil). Previous work established a role of base J in promoting RNA polymerase II (Pol II) termination in Leishmania major and Trypanosoma brucei. We recently identified a PJW/PP1 complex in Leishmania containing a J-binding protein (JBP3), PP1 phosphatase 1, PP1 interactive-regulatory protein (PNUTS) and Wdr82. Analyses suggested the complex regulates transcription termination by recruitment to termination sites via JBP3-base J interactions and dephosphorylation of proteins, including Pol II, by PP1. However, we never addressed the role of PP1, the sole catalytic component, in Pol II transcription termination. We now demonstrate that deletion of the PP1 component of the PJW/PP1 complex in L. major, PP1-8e, leads to readthrough transcription at the 3'-end of polycistronic gene arrays. We show PP1-8e has in vitro phosphatase activity that is lost upon mutation of a key catalytic residue and associates with PNUTS via the conserved RVxF motif. Additionally, purified PJW complex with associated PP1-8e, but not complex lacking PP1-8e, led to dephosphorylation of Pol II, suggesting a direct role of PNUTS/PP1 holoenzymes in regulating transcription termination via dephosphorylating Pol II in the nucleus.

The role of glycogen phosphorylase in glycogen biogenesis in skeletal muscle after exercise

Initially it was believed that phosphorylase was responsible for both glycogen breakdown and synthesis in the living cell. The discovery of glycogen synthase and McArdle's disease (lack of phosphorylase activity), together with the high P_i/glucose 1-P ratio in skeletal muscle, demonstrated that glycogen synthesis could not be attributed to reversal of the phosphorylase reaction. Rather, glycogen synthesis was attributable solely to the activity of glycogen synthase, subsequent to the transport of glucose into the cell. However, the well-established observation that phosphorylase was inactivated (i.e., dephosphorylated) during the initial recovery period after prior exercise, when the rate of glycogen accumulation is highest and independent of insulin, suggested that phosphorylase could play an active role in glycogen accumulation. But the quantitative contribution of phosphorylase inactivation was not established until recently, when studying isolated murine muscle preparations during recovery from repeated contractions at temperatures ranging from 25 to 35 °C. Thus, in both slow-twitch, oxidative and fast-twitch, glycolytic muscles, inactivation of phosphorylase accounted for 45%-75% of glycogen accumulation during the initial hours of recovery following repeated contractions. Such data indicate that phosphorylase inactivation may be the most important mechanism for glycogen accumulation under defined conditions. These results support the initial belief that phosphorylase plays a quantitative role in glycogen formation in the living cell. However, the mechanism is not via activation of phosphorylase, but rather via inactivation of the enzyme.

Epigallocatechine-3-gallate Inhibits the Adipogenesis of Human Mesenchymal Stem Cells via the Regulation of Protein Phosphatase-2A and Myosin Phosphatase

Epigallocatechin-3-gallate (EGCG) has widespread effects on adipocyte development. However, the molecular mechanisms of EGCG are not fully understood. We investigate the adipogenic differentiation of human-derived mesenchymal stem cells, including lipid deposition and changes in the expression and phosphorylation of key transcription factors, myosin, protein phosphatase-2A (PP2A), and myosin phosphatase (MP). On day 6 of adipogenic differentiation, EGCG (1–20 µM) suppressed lipid droplet formation, which was counteracted by an EGCG-binding peptide for the 67 kDa laminin receptor (67LR), suggesting that EGCG acts via 67LR. EGCG decreased the phosphorylation of CCAAT-enhancer-binding protein beta via the activation of PP2A in a protein kinase A (PKA)-dependent manner, leading to the partial suppression of peroxisome proliferator-activated receptor gamma (PPARγ) and adiponectin expression. Differentiated cells exhibited a rounded shape, cortical actin filaments, and lipid accumulation. The EGCG treatment induced cell elongation, stress fiber formation, and less lipid accumulation. These effects were accompanied by the degradation of the MP target subunit-1 and increased the phosphorylation of the 20 kDa myosin light chain. Our results suggest that EGCG acts as an agonist of 67LR to inhibit adipogenesis via the activation of PP2A and suppression of MP. These events are coupled with the decreased phosphorylation and expression levels of adipogenic transcription factors and changes in cell shape, culminating in curtailed adipogenesis.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Proteome and phosphoproteome profiling reveals the regulation mechanism of hibernation in a freshwater leech (Whitmania pigra)

Hibernation is an energy-saving and adaptive strategy adopted by leech, an important medicinal resource in Asia, to survive low temperature. Reversible protein phosphorylation (RPP) plays a key role in the regulation of mammalian hibernation processes but has never been documented in freshwater invertebrate such as leech. In this study, we detected the effects of hibernation on the proteome and phosphoproteome of the leech Whitmania pigra. A total of 2184 proteins and 2598 sites were quantified. Deep-hibernation resulted in 85 up-regulated and 107 down-regulated proteins and 318 up-regulated and 204 down-regulated phosphosites using a 1.5-fold threshold (P<0.05). Proteins involved in protein digestion and absorption, amino acid metabolism and N-glycan biosynthesis were significantly down-regulated during deep-hibernation. However, proteins involved in maintaining cell structure stability in hibernating animals were up-regulated. Differentially phosphorylated proteins provided the first global picture of a shift in energy metabolism, protein synthesis, cytoprotection and signaling during deep hibernation. Furthermore, AMP-activated protein kinase and protein kinase C play major roles in the regulation of these functional processes. These data significantly improve our understanding of the regulatory mechanisms of leech hibernation processes and provides substantial candidate phosphorylated proteins that could be important for functionally adapt in freshwater animals. SIGNIFICANCE: The leech Whitmania pigra as an important medicinal resource in Asia is an excellent model freshwater invertebrate for studies of environmentally-induced hibernation. The present study provides the first quantitative proteomics and phosphoproteomic analysis of leech hibernation using isobaric tag based TMT labeling and high-resolution mass spectrometry. These data significantly improve our understanding of the regulatory mechanisms when ectotherm animals face environmental stress and provides substantial candidate phosphorylated proteins that could be important for functionally adapt in freshwater animals.

Calmodulin-binding proteins: A journey of 40 years

The proteins which bind to calmodulin in a Ca²⁺-dependent and reversible manner are known as calmodulin-binding proteins. These proteins are involved in a multitude of processes in which Ca²⁺ and calmodulin play crucial roles. Our group elucidated the mechanism and importance of these proteins in normal and diseased conditions. Various calmodulin-binding proteins were discovered and purified from bovine tissue including a heat stable calmodulin-binding protein 70, calmodulin-dependent protein kinase VI and a high molecular weight calmodulin-binding protein (HMWCaMBP). We observed a complex interplay occurs between these and other Ca²⁺ and calmodulin-binding proteins during cardiac ischemia and reperfusion. Purified cardiac HMWCaMBP is a homolog form of calpastatin and an inhibitor of the Ca²⁺-activated cysteine proteases, calpains and therefore can have cardioprotective role in ischemic conditions. Calcineurin is a Ca²⁺ and calmodulin-dependent serine/threonine protein phosphatase showed increased phosphatase activity in ischemic heart through its direct interaction with Hsp70 and expression of calcineurin following ischemia suggests self-repair and favorable survival outcomes. Calcineurin was also found to be present in other tissues including the eye; where its expression and calcineurin phosphatase activity varied. In neurons, calcineurin may play a key role in initiating apoptosis-related pathways especially in epilepsy. In colorectal cancer we demonstrated high calcineurin phosphatase activity and simultaneous overexpression of calcineurin. The impact of calcineurin signaling on neuronal apoptosis in epilepsy and its use as a diagnostic marker for colorectal cancer requires in-depth study.

The targeting effect of Hm2E8b-NCTD-liposomes on B-lineage leukaemia stem cells is associated with the HLF-SLUG axis

To identify an agent with specific activity against B-lineage leukaemia stem cells (B-LSCs), we generated norcantharidin (NCTD)-encapsulated liposomes modified with a novel humanised anti-human CD19 monoclonal antibody, Hm2E8b (Hm2E8b-NCTD-liposomes). These liposomes were specially designed to recognise and kill B-LSCs in vitro, and to decrease non-specific cytotoxicity to untargeted cells. Hm2E8b-NCTD-liposomes selectively ablated B-LSCs through targeting hepatic leukaemia factor (HLF), which is implicated in haematopoietic stem cell regulation and is overexpressed in LSCs. Hm2E8b-NCTD-liposomes decreased HLF protein levels and induced apoptosis in the HAL-01 cell line harbouring the oncoprotein E2A-HLF. This resulted in modulation of the expression of several molecules that govern survival pathways, including HLF, SLUG, NFIL3 and C-Myc, thereby causing the induction of p53 and the mitochondrial caspase cascade. Therefore, the potent in vitro effect of Hm2E8b-NCTD-liposomes on B-LSC activity and survival pathways have the potential to be exploited clinically with appropriate drug combinations.

HIGD1A Regulates Oxygen Consumption, ROS Production, and AMPK Activity during Glucose Deprivation to Modulate Cell Survival and Tumor Growth

Hypoxia-inducible gene domain family member 1A (HIGD1A) is a survival factor induced by hypoxia-inducible factor 1 (HIF-1). HIF-1 regulates many responses to oxygen deprivation, but viable cells within hypoxic perinecrotic solid tumor regions frequently lack HIF-1α. HIGD1A is induced in these HIF-deficient extreme environments and interacts with the mitochondrial electron transport chain to repress oxygen consumption, enhance AMPK activity, and lower cellular ROS levels. Importantly, HIGD1A decreases tumor growth but promotes tumor cell survival in vivo. The human Higd1a gene is located on chromosome 3p22.1, where many tumor suppressor genes reside. Consistent with this, the Higd1a gene promoter is differentially methylated in human cancers, preventing its hypoxic induction. However, when hypoxic tumor cells are confronted with glucose deprivation, DNA methyltransferase activity is inhibited, enabling HIGD1A expression, metabolic adaptation, and possible dormancy induction. Our findings therefore reveal important new roles for this family of mitochondrial proteins in cancer biology.

PP2A inhibition results in hepatic insulin resistance despite Akt2 activation

In the liver, insulin suppresses hepatic gluconeogenesis by activating Akt, which inactivates the key gluconeogenic transcription factor FoxO1 (Forkhead Box O1). Recent studies have implicated hyperactivity of the Akt phosphatase Protein Phosphatase 2A (PP2A) and impaired Akt signaling as a molecular defect underlying insulin resistance. We therefore hypothesized that PP2A inhibition would enhance insulin-stimulated Akt activity and decrease glucose production. PP2A inhibitors increased hepatic Akt phosphorylation and inhibited FoxO1 in vitro and in vivo, and suppressed gluconeogenesis in hepatocytes. Paradoxically, PP2A inhibition exacerbated insulin resistance in vivo. This was explained by phosphorylation of both hepatic glycogen synthase (GS) (inactivation) and phosphorylase (activation) resulting in impairment of glycogen storage. Our findings underline the significance of GS and Phosphorylase as hepatic PP2A substrates and importance of glycogen metabolism in acute plasma glucose regulation.

Functional analysis of the glycogen binding subunit CG9238/Gbs-70E of protein phosphatase 1 in Drosophila melanogaster

The product of the CG9238 gene that we termed glycogen binding subunit 70E (Gbs-70E) was characterized by biochemical and molecular genetics methods. The interaction between Gbs-70E and all catalytic subunits of protein phosphatase 1 (Pp1-87B, Pp1-9C, Pp1-96A and Pp1-13C) of Drosophila melanogaster was confirmed by pairwise yeast two-hybrid tests, co-immunoprecipitation and pull down experiments. The binding of Gbs-70E to glycogen was demonstrated by sedimentation analysis. With RT-PCR we found that the mRNAs coding for the longer Gbs-70E PB/PC protein were expressed in all developmental stages of the fruit flies while the mRNA for the shorter Gbs-70E PA was restricted to the eggs and the ovaries of the adult females. The development specific expression of the shorter splice variant was not conserved in different Drosophila species. The expression level of the gene was manipulated by P-element insertions and gene deletion to analyze the functions of the gene product. A small or moderate reduction in the gene expression resulted in no significant changes, however, a deletion mutant expressing very low level of the transcript lived shorter and exhibited reduced glycogen content in the imagos. In addition, the gene deletion decreased the fertility of the fruit flies. Our results prove that Gbs-70E functions as the glycogen binding subunit of protein phosphatase 1 that regulates glycogen content and plays a role in the development of eggs in D. melanogaster.

Greatwall-phosphorylated Endosulfine is both an inhibitor and a substrate of PP2A-B55 heterotrimers

During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55's action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55's attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55. DOI: http://dx.doi.org/10.7554/eLife.01695.001.

Integration of biochemical and electrical signaling-multiscale model of the medium spiny neuron of the striatum

Neuron behavior results from the interplay between networks of biochemical processes and electrical signaling. Synaptic plasticity is one of the neuronal properties emerging from such an interaction. One of the current approaches to study plasticity is to model either its electrical aspects or its biochemical components. Among the chief reasons are the different time scales involved, electrical events happening in milliseconds while biochemical cascades respond in minutes or hours. In order to create multiscale models taking in consideration both aspects simultaneously, one needs to synchronize the two models, and exchange relevant variable values. We present a new event-driven algorithm to synchronize different neuronal models, which decreases computational time and avoids superfluous synchronizations. The algorithm is implemented in the TimeScales framework. We demonstrate its use by simulating a new multiscale model of the Medium Spiny Neuron of the Neostriatum. The model comprises over a thousand dendritic spines, where the electrical model interacts with the respective instances of a biochemical model. Our results show that a multiscale model is able to exhibit changes of synaptic plasticity as a result of the interaction between electrical and biochemical signaling. Our synchronization strategy is general enough to be used in simulations of other models with similar synchronization issues, such as networks of neurons. Moreover, the integration between the electrical and the biochemical models opens up the possibility to investigate multiscale process, like synaptic plasticity, in a more global manner, while taking into account a more realistic description of the underlying mechanisms.

AMP-activated protein kinase: a target for old drugs against diabetes and cancer

The AMP-activated protein kinase (AMPK) is considered a key checkpoint to ensure energy balance in both cells and organisms. AMPK is an αβγ heterotrimer controlled by allosteric regulation by AMP, ADP and ATP, auto-inhibitory features and phosphorylation, with the threonine-172 phosphorylation on the catalytic α-subunit by LKB1, CaMKKβ or Tak1 being essential for its fully activation. AMPK acts as a protective response to energy stress in numerous systems, but it is also a key player in diabetes and related metabolic diseases and cancer. Pharmacological activation of AMPK by metformin or other compounds holds a considerable potential to reverse the metabolic abnormalities associated with type 2 diabetes. In cancer, correction of the dysregulated metabolic pathway LKB1/AMPK/mTORC1 can lower the Warburg effect, suggesting AMPK as a potential target for cancer prevention and/or treatment. In this commentary, we review recent findings that support the role and function of AMPK in normal and pathological conditions. We also discuss how the activation of AMPK by naturally occurring compounds could help to prevent the development of numerous chronic diseases contributing in such a way to the well-being of ageing population.

Protein phosphatase 2A dephosphorylates CaBP4 and regulates CaBP4 function

PURPOSE

CaBP4 is a neuronal Ca(2+)-binding protein that is expressed in the retina and in the cochlea, and is essential for normal photoreceptor synaptic function. CaBP4 is phosphorylated by protein kinase C zeta (PKCζ) in the retina at serine 37, which affects its interaction with and modulation of voltage-gated Ca(v)1 Ca(2+) channels. In this study, we investigated the potential role and functional significance of protein phosphatase 2A (PP2A) in CaBP4 dephosphorylation.

METHODS

The effect of protein phosphatase inhibitors, light, and overexpression of PP2A subunits on CaBP4 dephosphorylation was measured in in vitro assays. Pull-down experiments using retinal or transfected HEK293 cell lysates were used to investigate the association between CaBP4 and PP2A subunits. Electrophysiologic recordings of cotransfected HEK293 cells were performed to analyze the effect of CaBP4 dephosphorylation in modulating Ca(v)1.3 currents.

RESULTS

PP2A inhibitors, okadaic acid (OA), and fostriecin, but not PP1 selective inhibitors, NIPP-1, and inhibitor 2, block CaBP4 dephosphorylation in retinal lysates. Increased phosphatase activity in light-dependent conditions reverses phosphorylation of CaBP4 by PKCζ. In HEK293 cells, overexpression of PP2A enhances the rate of dephosphorylation of CaBP4. In addition, inhibition of protein phosphatase activity by OA increases CaBP4 phosphorylation and potentiates the modulatory effect of CaBP4 on Ca(v)1.3 Ca(2+) channels in HEK293T cells.

CONCLUSIONS

This study provides evidence that CaBP4 is dephosphorylated by PP2A in the retina. Our findings reveal a novel role for protein phosphatases in regulating CaBP4 function in the retina, which may fine tune presynaptic Ca(2+) signals at the photoreceptor synapse.

AMPK activity is regulated by calcium-mediated protein phosphatase 2A activity

AMP-activated protein kinase (AMPK) is activated by upstream kinases and negatively regulated by protein phosphatases. Intracellular calcium mediates protein phosphatase 2A (PP2A), which is in a heterotrimeric complex with the PR72 subunit. The PR72 subunit contains two calcium-binding sites formed by EF hands. Our previous study has shown that chronic calcium exposure decreases AMPK activity. To define the specific molecular mechanism whereby calcium can deactivate AMPK, activities of AMPK and PP2A were analyzed in C2C12 muscle cell cultures and skeletal muscle tissues from mutant pigs possessing the AMPKγ3-mutation or the ryanodine receptor (RyR1) calcium gating mutation, or both. C2C12 myotubes treated with calcium releasing agent (caffeine) for 10h decreased (P<0.05) AICAR-induced AMPK activity to control levels and this negative effect was eliminated by ryanodine receptor stabilizer, dantrolene. Interestingly, muscle from pigs with the RyR1 mutation and C2C12 cells administered with 10h caffeine showed higher (P<0.05) PP2A activity compared to controls. More importantly, the inhibitory effect of caffeine on AMPK activity was attenuated by the PP2A inhibitor, calyculin A or siRNA induced knockdown of PP2A. These data show the inhibitory effect of chronic calcium on AMPK activity is exerted through the activation of PP2A.

Calcium input frequency, duration and amplitude differentially modulate the relative activation of calcineurin and CaMKII

NMDA receptor dependent long-term potentiation (LTP) and long-term depression (LTD) are two prominent forms of synaptic plasticity, both of which are triggered by post-synaptic calcium elevation. To understand how calcium selectively stimulates two opposing processes, we developed a detailed computational model and performed simulations with different calcium input frequencies, amplitudes, and durations. We show that with a total amount of calcium ions kept constant, high frequencies of calcium pulses stimulate calmodulin more efficiently. Calcium input activates both calcineurin and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) at all frequencies, but increased frequencies shift the relative activation from calcineurin to CaMKII. Irrespective of amplitude and duration of the inputs, the total amount of calcium ions injected adjusts the sensitivity of the system to calcium input frequencies. At a given frequency, the quantity of CaMKII activated is proportional to the total amount of calcium. Thus, an input of a small amount of calcium at high frequencies can induce the same activation of CaMKII as a larger amount, at lower frequencies. Finally, the extent of activation of CaMKII signals with high calcium frequency is further controlled by other factors, including the availability of calmodulin, and by the potency of phosphatase inhibitors.

Sustained postexercise increases in AS160 Thr642 and Ser588 phosphorylation in skeletal muscle without sustained increases in kinase phosphorylation

Prior exercise by rats can induce a sustained increase in muscle Akt substrate of 160 kDa (AS160) phosphorylation on Thr(642) (pAS160(Thr642)). Because phosphorylation of AS160 on both AS160(Thr642) and AS160(Ser588) is important for insulin-stimulated glucose transport (GT), we determined if exercise would also induce a sustained increase in pAS160(Ser588) concomitant with persistently elevated pAS160(Thr642) and GT. Given that the mechanisms for sustained postexercise (PEX) effects on pAS160 were uncertain, we also studied the four kinases known to phosphorylate AS160 (Akt, AMPK, RSK, and SGK1). In addition, because the serine/threonine phosphatase(s) that dephosphorylate muscle AS160 were previously unidentified, we assessed the ability of four serine/threonine phosphatases (PP1, PP2A, PP2B, and PP2C) to dephosphorylate AS160. We also evaluated exercise effects on posttranslational modifications (Tyr(307) and Leu(309)) that regulate PP2A. In isolated epitrochlearis muscles from rats, GT at 3hPEX with insulin significantly (P < 0.05) exceeded SED controls. Muscles from 0hPEX vs. 0hSED and 3hPEX vs. 3hSED rats had greater pAS160(Thr642) and pAS160(Ser588). AMPK was the only kinase with greater phosphorylation at 0hPEX vs. 0hSED, and none had greater phosphorylation at 3hPEX vs. 3hSED. Each phosphatase was able to dephosphorylate pAS160(Thr642) and pAS160(Ser588) in cell-free assays. Exercise did not alter posttranslational modifications of PP2A. Our results revealed: 1) pAMPK as a potential trigger for increased pAS160(Thr642) and pAS160(Ser588) at 0hPEX; 2) PP1, PP2A, PP2B, and PP2C were each able to dephosphorylate AS160; and 3) sustained PEX-induced elevations of pAS160(Thr642) and pAS160(Ser588) were attributable to mechanisms other than persistent phosphorylation of known AS160 kinases or altered posttranslational modifications of PP2A.

Role of calcineurin in neurodegeneration produced by misfolded proteins and endoplasmic reticulum stress

A hallmark event in neurodegenerative diseases is the accumulation of misfolded aggregated proteins in the brain leading to neuronal dysfunction and disease. Compelling evidence suggests that misfolded proteins damage cells by inducing endoplasmic reticulum (ER) stress and alterations in calcium homeostasis. Changes in cytoplasmic calcium concentration lead to unbalances on several signaling pathways. Recent data suggest that calcium-mediated hyperactivation of calcineurin (CaN), a key phosphatase in the brain, triggers synaptic dysfunction and neuronal death, the two central events responsible for brain degeneration in neurodegenerative diseases. Therefore, blocking CaN hyper-activation might be a promising therapeutic strategy to prevent brain damage in neurodegenerative diseases.

Dopamine-dependent tuning of striatal inhibitory synaptogenesis

Dopaminergic projections to the striatum, crucial for the correct functioning of this brain region in adulthood, are known to be established early in development, but their role is currently uncharacterized. We demonstrate here that dopamine, by activating D(1)- and/or D(2)-dopamine receptors, decreases the number of functional GABAergic synapses formed between the embryonic precursors of the medium spiny neurons, the principal output neurons of the striatum, with associated changes in spontaneous synaptic activity. Activation of these receptors reduces the size of postsynaptic GABA(A) receptor clusters and their overall cell-surface expression, without affecting the total number of clusters or the size or number of GABAergic nerve terminals. These changes result from an increased internalization of GABA(A) receptors, and are mediated by distinct signaling pathways converging at the level of GABA(A) receptors to cause a transient PP2A/PP1-dependent dephosphorylation. Thus, tonic D(1)- and D(2)-receptor activity limits the extent of collateral inhibitory synaptogenesis between medium spiny neurons, revealing a novel role of dopamine in controlling the development of intrinsic striatal microcircuits.

Kinases and phosphatases in ischaemic preconditioning: a re-evaluation

Activation of several protein kinases occurs during myocardial ischaemia and during subsequent reperfusion. In contrast to the intensive investigation into the significance of kinase activation in cardioprotection, relatively little is known about the role of the phosphatases in this regard. The aim of this study was to re-evaluate the putative roles of PP1 and PP2A in ischaemia/reperfusion and in triggering ischaemic preconditioning. Isolated perfused working rat hearts were subjected to sustained global (15 or 20 min) or regional ischaemia (35 min), followed by reperfusion. Hearts were preconditioned using global ischaemia (1 x 5 or 3 x 5 min, alternated with 5 min reperfusion). To inhibit both PP1 and PP2A cantharidin (5 muM) was used. To inhibit PP2A only, okadaic acid (7.5 nM) was used. The drugs were administered during the preconditioning protocol, before onset of sustained ischaemia (pretreatment) or during reperfusion. Endpoints were mechanical recovery during reperfusion, infarct size and activation of PKB/Akt, p38 MAPK and ERK p42/p44, as determined by Western blot. Pretreatment of hearts with okadaic acid or cantharidin caused a significant reduction in mechanical recovery after 15 or 20 min global ischaemia. Administration of the drugs during an ischaemic preconditioning protocol abolished functional recovery during reperfusion and significantly increased infarct size. Administration of the drugs during reperfusion had no deleterious effects and increased functional recovery in 3 x PC hearts. To find an explanation for the differential effects of the inhibitors depending on the time of administration, hearts were freeze-clamped at different time points during the perfusion protocol. Administration of cantharidin before 5 min ischaemia activated all kinases. Subsequent reperfusion for 5 min without the drug maintained activation of the kinases until the onset of sustained ischaemia. Cantharidin given during preconditioning was associated with activation of p38MAPK and PKB/Akt during reperfusion after sustained ischaemia. However, administration of the drug during reperfusion only after sustained ischaemia caused activation of both PKB/Akt and ERK p42/p44. Phosphatase inhibition immediately prior to the onset of sustained ischaemia or during preconditioning abolishes protection during reperfusion, while inhibition of these enzymes during reperfusion either had no effect or enhanced the cardioprotective effects of preconditioning. It is proposed that inhibition of phosphatases during reperfusion may prolong the period of RISK activation and hence protect the heart.

The regulation of AMPK signaling in a natural state of profound metabolic rate depression

In response to energy stress (and elevated AMP), the AMP-activated protein kinase (AMPK) coordinates the restoration of energy homeostasis. We determined that AMPK is activated in a model system (desert snail Otala lactea) during a physiological state of profound metabolic rate depression (estivation) in the absence of a rise in AMP. Kinetic characterization indicated a strong increase in AMPK activity and phosphorylation in estivation, consistent with an increase in P-Ser428 LKB, an established regulator of AMPK. Accordingly, approximately 2-fold increases in AMPKalpha1 protein and activity were observed with LKB1 immunoprecipitates from estivating snails. In vitro studies determined that AMPK in crude extracts was activated in the presence of cGMP and deactivated in conditions that permitted protein phosphatase type-2A (PP2A) activity. Furthermore, AMPKalpha1 protein and activity increased in PKG immunoprecipitates from estivating tissues, suggesting a novel role for PKG in the regulation of AMPK in vivo. We evaluated several downstream targets of AMPK. Acetyl-CoA carboxylase (ACC) activity was strongly inhibited in estivation, consistent with increased P-Ser79 content, and in vitro stimulation of AMPK negated citrate's ability to stimulate ACC aggregation. Analysis of other targets revealed a strong decrease in PPARgamma-coactivator 1alpha expression in both tissues, which was related to decreased gluconeogenic protein expression in hepatic tissue, but no changes in mitochondrial biogenesis markers in muscle. We concluded that AMPK activation in O. lactea aids in facilitating the suppression of anabolic pathways, without necessarily activating ATP-generating catabolism.

Using protein kinase and protein phosphatase inhibitors to dissect protein phosphorylation pathways

It is often desirable to attempt to measure and to modulate the activity of the protein kinases and phosphatases responsible for a given phenotypic change. The protocols in this unit describe how to assay a variety of protein kinase activities in complex mixtures, such as whole cell lysates, in order to screen compounds as potential inhibitors of these activities. The effects of known or potential inhibitors can be evaluated by monitoring phenotypic changes in intact cells following treatment with the inhibitor. The activities of the major serine/threonine-specific protein phosphatases in most mammalian cells can be distinguished using a selective inhibitor, okadaic acid. Support protocols describe preparation of cell extracts needed for the kinase assays, and of (32)P-labeled phosphorylase used in the inhibition assays.

Protein phosphatase-2A is activated in pig brain following cardiac arrest and resuscitation

Protein phosphatase-2A (PP-2A) interacts with several regulators of cell death pathways and is therefore a potential component of signaling pathways linking global cerebral ischemia to cell death. Using a novel procedure to quantify PP-2A activity, we find that cardiac arrest with resuscitation and reperfusion leads to activation of PP-2A by 1.6-fold in pig brain extract and by 3.4-fold after partial purification of PP-2A. This is the first demonstration of PP-2A activation in a clinically relevant model of transient global cerebral ischemia. These results suggest that inhibition of PP-2A activity may be neuroprotective in global cerebral ischemia.

Cellular energy sensing and signaling by AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is an energy sensing/signaling protein that, when activated, increases ATP production by stimulating glucose uptake and fatty acid oxidation while at the same time inhibiting ATP = consuming processes such as protein synthesis. Chronic activation of AMPK inhibits expression of lipogenic enzymes in the liver and enhances expression of mitochondrial oxidative enzymes in skeletal muscle. Deficiency of muscle LKB1, the upstream kinase of AMPK, results in greater fluctuation in energy charge during muscle contraction and decreased capacity for exercise at higher work rates. Because AMPK enhances both glucose uptake and fatty acid oxidation in skeletal muscle, it has become a target for prevention and treatment of type 2 diabetes and obesity.

AMP-activated protein kinase activation as a strategy for protecting vascular endothelial function

1. AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase involved in the regulation of cellular and organismal metabolism. AMPK has a heterotrimeric structure, consisting of a catalytic alpha-subunit and regulatory beta- and gamma-subunits, each of which has two or more isoforms that are differentially expressed in various tissues and that arise from distinct genes. The AMPK system acts as a sensor of cellular energy status that is conserved in all eukaryotic cells. In addition, AMPK is activated by physiological stimuli and oxidants. 2. The importance of AMPK in cardiovascular functions is best demonstrated by recent studies showing that widely used drugs, including statins, metformin and rosiglitazone, execute cardiovascular protective effects at least partly through the activation of AMPK. As a consequence, AMPK has been proposed as a candidate target for therapeutic intervention in the treatment of both Type 2 diabetes and metabolic syndrome owing to its central role in the regulation of energy balance; it may also have a role in weight control. 3. In the present brief review, we summarize the recent progress of AMPK signalling and regulation focusing on vascular endothelial cells. We further hypothesize that AMPK is a dual sensor for energy and redox status within a cell and AMPK may be a therapeutic target for protecting vascular endothelial function.

Protein phosphatase 2A plays an important role in stromal cell-derived factor-1/CXC chemokine ligand 12-mediated migration and adhesion of CD34+ cells

Migration of hemopoietic stem and progenitor cells (HSPC) is required for homing to bone marrow following transplantation. Therefore, it is critical to understand signals underlying directional movement of HSPC. Stromal cell-derived factor-1 (SDF-1)/CXCL12 is a potent chemoattractant for HSPC. In this study, we demonstrate that the serine-threonine protein phosphatase (PP)2A plays an important role in regulation of optimal level and duration of Akt/protein kinase B activation (a molecule important for efficient chemotaxis), in response to SDF-1. Inhibition of PP2A, using various pharmacological inhibitors of PP2A including okadaic acid (OA) as well as using genetic approaches including dominant-negative PP2A-catalytic subunit (PP2A-C) or PP2A-C small interfering RNA, in primary CD34(+) cord blood (CB) cells led to reduced chemotaxis. This was associated with impairment in polarization and slower speed of movement in response to SDF-1. Concomitantly, SDF-1-induced Akt phosphorylation was robust and prolonged. Following SDF-1 stimulation, Akt and PP2A-C translocate to plasma membrane with enhanced association of PP2A-C with Akt observed at the plasma membrane. Inhibition of PI3K by low-dose LY294002 partially recovered chemotactic activity of cells pretreated with OA. In addition to chemotaxis, adhesion of CD34(+) cells to fibronectin was impaired by OA pretreatment. Our study demonstrates PP2A plays an important role in chemotaxis and adhesion of CD34(+) CB cells in response to SDF-1. CD34(+) CB cells pretreated with OA showed impaired ability to repopulate NOD-SCID mice in vivo, suggesting physiological relevance of these observations.

Pharmacological evidence for altered src kinase regulation of I (Ca,L) in patients with chronic atrial fibrillation

A reduction in L-type Ca(2+) current (I (Ca,L)) contributes to electrical remodeling in chronic atrial fibrillation (AF). Whether the decrease in I (Ca,L) is solely due to a reduction in channel proteins remains controversial. Protein tyrosine kinases (PTK) have been described as potent modulators of I (Ca,L) in cardiomyocytes. We studied alpha(1C) L-type Ca(2+) channel subunit expression and the regulation of I (Ca,L) by PTK in chronic AF using PTK inhibitors: genistein, a nonselective inhibitor of PTK, and 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo-3,4-d-pyrimidine (PP1), a selective inhibitor of src kinases. Furthermore, type-1 and type-2A protein phosphatase activity was measured with phosphorylase as substrate in whole-cell lysates derived from atrial tissue of AF patients. Right atrial appendages were obtained from patients undergoing open-heart surgery. Protein levels of alpha(1C) L-type Ca(2+) channel subunit were determined using Western blot analysis and normalized to the protein amounts of calsequestrin as internal control. The protein concentrations of alpha(1C) did not differ between AF and sinus rhythm (SR; alpha(1C)/calsequestrin: 1.0 +/- 0.1 and 1.2 +/- 0.2, respectively, n = 8 patients). In cardiomyocytes from patients in SR (n = 20 patients), genistein and PP1 both evoked similar increases in I (Ca,L) from 3.0 +/- 0.3 to 6.1 +/- 0.8 pA/pF and from 2.8 +/- 0.4 to 6.1 +/- 0.6 pA/pF, respectively. In cells from AF patients (n = 10 patients), basal I (Ca,L) was significantly lower. In this case, genistein lead to the same relative increase in I (Ca,L) as in SR cells (from 1.46 +/- 0.30 to 3.2 +/- 1.0 pA/pF), whereas no increase was elicited by PP1 suggesting impaired regulation of I (Ca,L) by src kinases in AF. Total and type 1 and type 2A-related phosphatase activities were higher in tissue from patients with chronic AF compared to SR (4.8 +/- 0.4, 2.1 +/- 0.2, and 2.7 +/- 0.4 nmol/mg/min and 3.6 +/- 0.4, 1.3 +/- 0.2, and 2.4 +/- 0.3 nmol/mg/min, respectively, n = 7 patients per group). Downregulation of I (Ca,L) in AF is not due to a reduction in L-type Ca(2+) channel protein expression. Indirect evidence for an impaired src kinase regulation of I (Ca,L) together with an increased phosphatase activity suggests that a complex alteration in the kinase/phosphatase balance leads to I (Ca,L) dysregulation in chronic AF.

Proper Chromatin Condensation and Maintenance of Histone H3 Phosphorylation During Mouse Oocyte Meiosis Requires Protein Phosphatase Activity1

We have shown okadaic acid (OA) and calyculin-A (CLA) inhibition of mouse oocyte phosphoprotein phosphatase 1 (PPP1C) and/or phosphoprotein phosphatase 2A (PPP2CA) results in aberrant chromatin condensation, as evidenced by the inability to resolve bivalents. Phosphorylation of histone H3 at specific residues is thought to regulate chromatin condensation. Therefore, we examined changes in histone H3 phosphorylation during oocyte meiosis and the potential regulation by protein PPPs. Western blot and immunocytochemical analysis revealed histone H3 phosphorylation changed during mouse oocyte meiosis, with changes in chromatin condensation. Germinal vesicle-intact (GV-intact; 0 h) oocytes had no phospho-Ser10 but did have phospho-Ser28 histone H3. Oocytes that had undergone germinal vesicle breakdown (GVBD; 2 h) and progressed to metaphase I (MI; 7 h) and MII (16 h) had phosphorylated Ser10 and Ser28 histone H3 associated with condensed chromatin. To determine whether OA-induced aberrations in chromatin condensation were due to alterations in levels of histone H3 phosphorylation, we assessed phosphorylation of Ser10 and Ser28 residues following PPP inhibition. Oocytes treated with OA (1 microM) displayed increased phosphorylation of histone H3 at both Ser10 and Ser28 compared with controls. To begin to elucidate which OA-sensitive PPP is responsible for regulating chromatin condensation and histone H3 phosphorylation, we examined spatial and temporal localization of OA-sensitive PPPs, PPP1C, and PPP2CA. PPPC2A did not localize to condensed chromatin, whereas PPP1beta (PPP1CB) associated with condensing chromatin in GVBD, MI, and MII oocytes. Additionally, Western blot and immunocytochemistry confirmed presence of the PPP1C regulatory inhibitor subunit 2 (PPP1R2) in oocytes at condensed chromatin during meiosis and indicated a change in PPP1R2 phosphorylation. Inhibition of oocyte glycogen synthase kinase 3 (GSK3) appeared to regulate phosphorylation of PPP1R2. Furthermore, inhibition of GSK3 resulted in aberrant oocyte bivalent formation similar to that observed following PPP inhibition. These data suggest that PPP1CB is the OA/CLA-sensitive PPP that regulates oocyte chromatin condensation through regulation of histone H3 phosphorylation. Furthermore, GSK3 inhibition results in aberrant chromatin condensation and appears to regulate phosphorylation of PPP1R2.

Activation of protein phosphatase 2A by palmitate inhibits AMP-activated protein kinase

Elevated levels of free fatty acids contribute to cardiovascular diseases, but the mechanisms remain poorly understood. The present study was aimed to determine if free fatty acid inhibits the AMP-activated kinase (AMPK). Exposure of cultured bovine aortic endothelial cells (BAECs) to palmitate (0.4 mM) but not to palmitoleic or oleic acid (0.4 mM) for 40 h significantly reduced the Thr(172) phosphorylation of AMPK-alpha without altering its protein expression or the phosphorylation of LKB1-Ser(428), a major AMPK kinase in BAECs. Further, in LKB1-deficient cells, palmitate suppressed AMPK-Thr(172) implying that the inhibitory effects of palmitate on AMPK might be independent of LKB1. In contrast, 2-bromopalmitate, a non-metabolizable analog of palmitate, did not alter the phosphorylation of AMPK and acetyl-CoA carboxylase. Further, palmitate significantly increased the activity of protein phosphatase (PP)2A. Inhibition of PP2A with either okadaic acid, a selective PP2A inhibitor, or PP2A small interference RNA abolished palmitate-induced inhibition on AMPK-Thr(172) phosphorylation. Exposure of BAECs to C(2)-ceramide, a cell-permeable analog of ceramide, mimicked the effects of palmitate. Conversely, fumonisin B1, which selectively inhibits ceramide synthase and decreases de novo formation of ceramide, abolished the effects of palmitate on both PP2A and AMPK. Inhibition of AMPK in parallel with increased PP2A activity was founded in C57BL/6J mice fed with high fat diet (HFD) rich in palmitate but not in mice fed with HFD rich in oleate. Moreover, inhibition of PP2A with PP2A-specific siRNA but not scrambled siRNA reversed HFD-induced inhibition on the phosphorylation of AMPK-Thr(172) and endothelial nitric-oxide synthase (eNOS)-Ser(1177) in mice fed with high fat diets. Taken together, we conclude that palmitate inhibits the phosphorylation of both AMPK and endothelial nitric-oxide synthase in endothelial cells via ceramide-dependent PP2A activation.

Purification and characterization of butyrate-induced protein phosphatase involved in apoptosis of Ehrlich ascites tumor cells

Short chain fatty acids including butyrate exhibit wide variety of biological effects towards cell growth, morphology and gene expression. In this report, we study the mechanism by which butyrate (BuA) modulates the expression of protein phosphatase when treated to the cells. As a model system, we used Ehrlich Ascites Tumor (EAT) cells in which BuA-treatment induces expression of a protein phosphatase enzyme. Subsequently, BuA-induced protein phosphatase has been biochemically purified and characterized. Further, pretreatment of caspase-3 inhibitor abolished the activity of BuA-induced protein phosphatase indicating the involvement of caspase-3 in the activation of BuA-induced protein phosphatase. In addition, the relationship between BuA-induced protein phosphatase and apoptosis has been verified. Activation of endonuclease-II has been shown in BuA-treated EAT cells and that activity was completely inhibited by sodium orthovanadate, a tyrosine phosphatase inhibitor suggesting that endonuclease-II may serve as a possible down-stream target for BuA-induced protein phosphatase. Together, the data suggest that activation of protein phosphatase may be an early and essential step in BuA-mediated apoptotic signaling pathway in EAT cells.

Loss of phosphatase activity in Ptp69D alleles supporting axon guidance defects

PTP69D is a receptor protein tyrosine phosphatase that was identified as a key regulator of neuromuscular axon guidance in Drosophila, and has subsequently been shown to play a similar role in the central nervous system and retina. Three Ptp69D alleles with mutations involving catalytically important residues exhibit a high degree of phenotypic variation with viability of mutant adult flies ranging from 0 to 96%, and ISNb motor nerve defects ranging from 11 to 57% [Desai and Purdy, 2003]. To determine whether mutations in Ptp69D affecting axon guidance and viability demonstrate losses of phosphatase activity and whether differences in catalytic potential underlie phenotypic variability, we expressed full-length wild-type and mutant PTP69D protein in Schneider 2 cells, and assessed phosphatase activity using the fluorogenic substrate 6,8-difluoro-4-methylumbelliferone phosphate (DiFMUP). Detailed biochemical characterization of wild-type PTP69D, including an examination of sensitivity to various inhibitors, in vitro catalytic efficiency, and the pH-k(cat) profile of the enzyme, suggests a common tyrosine phosphatase reaction mechanism despite lack of sequence conservation in the WPD loop. Analysis of mutant proteins revealed that every mutant had less than 1% activity relative to the wild-type enzyme, and these rates did not differ significantly from one another. These results indicate that mutations in Ptp69D resulting in axon guidance defects and lethality significantly compromise catalytic activity, yet the range of biological activity exhibited by Ptp69D mutants cannot be explained by differences in catalytic activity, as gauged by their ability to hydrolyze the substrate DiFMUP.

Role of protein phosphatase 2A in calcium-dependent chloride secretion by human colonic epithelial cells

EGF inhibits carbachol-induced chloride secretion by regulating a basolateral potassium channel via phosphatidylinositol 3-kinase (PI 3-kinase) and PKCepsilon activation. Although both EGF and carbachol cause tyrosine phosphorylation of p85 of PI 3-kinase, only EGF activates the enzyme. Serine phosphorylation of p85 is thought to suppress the lipid kinase of PI 3-kinase. Our present study examined whether the differential effects of carbachol and EGF on PI 3-kinase activity correspond to varying phosphorylation of p85, and the mechanisms and consequences. T(84) colonic epithelial cells were treated with either EGF or carbachol. Cell lysates were immunoprecipitated with p85 antibody and blotted with either phosphotyrosine or phosphoserine antibodies. Protein phosphatase (PP) 1 and 2A activities were also measured. Both tyrosine and serine residues of p85 were phosphorylated by carbachol, whereas EGF induced only tyrosine phosphorylation. Moreover, EGF abolished carbachol-induced serine phosphorylation of p85 and activated PP2A without affecting PP1. Carbachol did not affect either phosphatase. Calyculin A or okadaic acid pretreatment reversed the inhibitory action of EGF on carbachol-induced chloride secretion and restored serine phosphorylation of p85. Although carbachol recruits p85, it phosphorylates both serine and tyrosine residues so that the lipid kinase of PI 3-kinase is inhibited. EGF results in p85 tyrosine phosphorylation as well as dephosphorylation of serine residues via the activation of PP2A. This explains the differential induction of PI 3-kinase enzyme activity in response to EGF and/or carbachol and has functional implications. Our data provide further insights into negative signals that regulate chloride secretion and into the molecular basis of signaling diversification in the intestinal epithelium.

Quantification of the glycogen cascade system: the ultrasensitive responses of liver glycogen synthase and muscle phosphorylase are due to distinctive regulatory designs

Background

Signaling pathways include intricate networks of reversible covalent modification cycles. Such multicyclic enzyme cascades amplify the input stimulus, cause integration of multiple signals and exhibit sensitive output responses. Regulation of glycogen synthase and phosphorylase by reversible covalent modification cycles exemplifies signal transduction by enzyme cascades. Although this system for regulating glycogen synthesis and breakdown appears similar in all tissues, subtle differences have been identified. For example, phosphatase-1, a dephosphorylating enzyme of the system, is regulated quite differently in muscle and liver. Do these small differences in regulatory architecture affect the overall performance of the glycogen cascade in a specific tissue? We address this question by analyzing the regulatory structure of the glycogen cascade system in liver and muscle cells at steady state.

Results

The glycogen cascade system in liver and muscle cells was analyzed at steady state and the results were compared with literature data. We found that the cascade system exhibits highly sensitive switch-like responses to changes in cyclic AMP concentration and the outputs are surprisingly different in the two tissues. In muscle, glycogen phosphorylase is more sensitive than glycogen synthase to cyclic AMP, while the opposite is observed in liver. Furthermore, when the liver undergoes a transition from starved to fed-state, the futile cycle of simultaneous glycogen synthesis and degradation switches to reciprocal regulation. Under such a transition, different proportions of active glycogen synthase and phosphorylase can coexist due to the varying inhibition of glycogen-synthase phosphatase by active phosphorylase.

Conclusion

The highly sensitive responses of glycogen synthase in liver and phosphorylase in muscle to primary stimuli can be attributed to distinctive regulatory designs in the glycogen cascade system. The different sensitivities of these two enzymes may exemplify the adaptive strategies employed by liver and muscle cells to meet specific cellular demands.

Endogenous regulators of protein phosphatase-1 during mouse oocyte development and meiosis

Reversible phosphorylation, involving protein kinases and phosphatases (PP), is important in regulating oocyte meiosis. Okadaic acid (OA) inhibition of PP1 and/or PP2A stimulates oocyte germinal vesicle breakdown (GVB). In oocytes, PP1 is localized in the cytoplasm and nucleus, yet endogenous regulation of oocyte PP1 has not been investigated. The objectives of the study were to identify intra-oocyte mechanisms regulating PP1 during acquisition of OA-sensitive meiotic competence and meiotic resumption. Immunohistochemical studies revealed that GVB-incompetent oocytes contained equivalent cytoplasmic and nuclear PP1. Upon development of OA-sensitive meiotic competence, PP1 displayed differential intracellular localization with significantly greater nuclear staining with distinct nucleolar rimming compared with cytoplasmic staining. Germinal vesicle-intact oocytes contained neither nuclear inhibitor of PP1, nor PP1 cytoplasmic inhibitor-1 transcripts or proteins. Reverse transcription-PCR with PP1 cytoplasmic inhibitor-2 (I2) primers and oocyte RNA amplified a predicted 330-bp product with the identical sequence to mouse liver I2. Oocytes contained a heat-stable PP1 inhibitor with biochemical properties of I2. Phosphorylation of PP1 at Thr320 by cyclin dependent kinase-1 (CDK1) causes PP1 inactivation. Germinal vesicle-intact oocytes did not contain phospho-Thr320-PP1. Upon GVB, PP1 became phosphorylated at Thr320 and this phosphorylation did not occur if GVB was blocked with the CDK1 inhibitor, roscovitine (ROSC). Inhibition of oocyte GVB with ROSC was reversible and coincided with PP1 phosphorylation at Thr320. Increased oocyte staining of nuclear PP1 compared with cytoplasmic staining at a chronological stage when oocytes gain meiotic competence, and phosphorylation and inhibition of PP1 by CDK1 at or around GVB appear to be important mechanisms in regulating oocyte PP1 activity and meiosis. In addition, these studies provide further support for PP1 being the OA-sensitive PP important in the regulation of the acquisition of meiotic competence, nuclear events during meiotic arrest, and GVB.

Activation of calcineurin expression in ischemia-reperfused rat heart and in human ischemic myocardium

Calcineurin (CaN) has been reported as a critical mediator for cardiac hypertrophy and cardiac myocyte apoptosis. In the present study, we investigated the activity and expression of CaN and the effect of calpain in rat heart after ischemia and reperfusion. Rat ischemic heart showed significant increase in CaN activity. Western blot analysis of normal rat heart extract with a polyclonal antibody raised against bovine CaN indicated a prominent immunoreactive band of 60 kDa (CaN A). In ischemic-reperfused hearts, the expression of CaN A was significantly low and immunoreactivity was observed in proteolytic bands of 46 kDa. This may be due to the proteolytic degradation of CaN A in ischemic tissues by m-calpain. We also noticed in vitro proteolysis of bovine cardiac CaN A by m-calpain. Immunohistochemical studies showed strong staining of immunoreactivity in rat hearts that had gone under 30 min ischemia followed by 30 min reperfusion similar to that found in human ischemic heart. Ischemia is associated with multiple alterations in the extracellular and intracellular signaling of cardiomyocytes and may act as an inducer of apoptosis. The increase in CaN activity and strong immunostaining observed in ischemic/perfused rat heart may be due to the calpain-mediated proteolysis of this phosphatase.

Function and structure of recombinant single chain calcineurin

Calcineurin (CN) is a Ca(2+)/calmodulin(CaM)-dependent serine/threonine protein phosphatase which is a heterodimer composed of a 61 kDa catalytic subunit (CNA) and a 19 kDa regulatory subunit (CNB). The enzyme is critical for several important intracellular signal-transducing pathways, including T-cell activation. Its crystal structure reveals that the C-terminal of CNB lies in close vicinity of the N-terminal of CNA and each end has a long arm not involved in the active site. After fusing two subunits, it was determined that folding and function of the protein were not affected by the fusion. We amplified a fused gene of A and B subunits using a pair of linker primers including six codons of glycine. A single chain calcineurin was constructed and purified to near-homogeneity. The recombinant enzyme was fully soluble, displayed high specific activity with substrate, and exhibited biochemical properties and kinetic parameters similar to those of the native enzyme from the bovine brain. It was still activated by Ca(2+)/calmodulin but was not regulated by extra CNB and was still strongly stimulated by Mn(2+) and Ni(2+) divalent metal ions. The solution conformations of both recombinant enzyme and bovine calcineurin were assayed under the same conditions using intrinsic fluorescence spectroscopy and circular dichroism spectropolarimetry, and results showed their graphs are approximately identical. Our findings suggested that the fusion of A and B subunits of calcineurin does not affect their folding pathways and structural changes involved in their function, furthermore, they are bound to the correct binding site.

Elevated levels of protein phosphatase 1 and phosphatase 2A may contribute to cardiac dysfunction in diabetes

Although protein phosphorylation and dephosphorylation are known to regulate the activities of different enzymes, sufficient information on the role of dephosphorylation in cardiac function is not available. Since protein phosphatases mediate dephosphorylation, it is possible that cardiac dysfunction induced by diabetes may be due to alterations in the activities of these enzymes. We therefore determined cardiac protein phosphatase activity as well as protein contents of phosphatase 1 and phosphatase 2A in diabetic animals. For this purpose, rats were made diabetic by administering a single intravenous injection of streptozotocin (65 mg/kg body weight) and hearts were examined after 1, 2, 3, 4 and 8 weeks. Some of the 4-week diabetic animals received subcutaneous injections of insulin (3 U/day) for a further period of 4 weeks. Cardiac dysfunction was evident after 2 weeks of inducing diabetes and deteriorated further with time. A significant increase in protein phosphatase activity appeared after 1 week and persisted until 8 weeks. Increased protein phosphatase activity in the diabetic heart was associated with a corresponding increase in the protein contents of both phosphatase 1 and phosphatase 2A. Insulin treatment partly prevented the changes observed in diabetic animals. The results suggest that increased protein phosphatase activities and subsequent enhanced protein dephosphorylation may play a role in diabetes-induced cardiac dysfunction.

Bidirectional synaptic plasticity as a consequence of interdependent Ca2+-controlled phosphorylation and dephosphorylation pathways

Postsynaptic Ca2+ signals of different amplitudes and durations are able to induce either long-lasting potentiation (LPT) or depression (LTD). The bidirectional character of synaptic plasticity may result at least in part from an increased or decreased responsiveness of the glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R) due to the modification of conductance and/or channel number, and controlled by the balance between the activities of phosphorylation and dephosphorylation pathways. AMPA-R depression can be induced by a long-lived Ca2+ signal of moderate amplitude favouring the activation of the dephosphorylation pathway, whereas a shorter but higher Ca2+ signal would induce AMPA-R potentiation resulting from the preferential activation of the phosphorylation pathway. Within the framework of a model involving calcium/calmodulin-dependent protein kinase II (CaMKII), calcineurin (PP2B) and type 1 protein phosphatase (PP1), we aimed at delineating the conditions allowing a biphasic U-shaped relationship between AMPA-R and Ca2+ signal amplitude, and thus bidirectional plasticity. Our theoretical analysis shows that such a property may be observed if the phosphorylation pathway: (i) displays higher cooperativity in its Ca2+-dependence than the dephosphorylation pathway; (ii) displays a basal Ca2+-independent activity; or (iii) is directly inhibited by the dephosphorylation pathway. Because the experimentally observed inactivation of CaMKII by PP1 accounts for this latter characteristic, we aimed at verifying whether a realistic model using reported parameters values can simulate the induction of either LTP or LTD, depending on the time and amplitude characteristics of the Ca2+ signal. Our simulations demonstrate that the experimentally observed bidirectional nature of Ca2+-dependent synaptic plasticity could be the consequence of the PP1-mediated inactivation of CaMKII.

Degeneracy and function of the ubiquitous RVXF motif that mediates binding to protein phosphatase-1

Most interactors of protein phosphatase-1 (PP1) contain a variant of a so-called "RVXF" sequence that binds to a hydrophobic groove of the catalytic subunit. A combination of sequence alignments and site-directed mutagenesis has enabled us to further define the consensus sequence for this degenerate motif as [RK]-X(0-1)-[VI]-[P]-[FW], where X denotes any residue and [P] any residue except Pro. Naturally occurring RVXF sequences differ in their affinity for PP1, and we show by swapping experiments that this binding affinity is an important determinant of the inhibitory potency of the regulators NIPP1 and inhibitor-1. Also, inhibition by NIPP1-(143-224) was retained when the RVXF motif (plus the preceding Ser) was swapped for either of two unrelated PP1-binding sequences from human inhibitor-2, i.e. KGILK or RKLHY. Conversely, the KGILK motif of inhibitor-2 could be functionally replaced by the RVXF motif of NIPP1. Our data provide additional evidence for the view that the RVXF and KGILK motifs function as anchors for PP1 and thereby promote the interaction of secondary binding sites that determine the activity and substrate specificity of the enzyme.