Identification of filamin C as a new physiological substrate of PKBalpha using KESTREL

Phenotypic variability of filamin C-related cardiomyopathy: Insights from a novel Dutch founder variant

BACKGROUND

Dilated cardiomyopathy (DCM) can be caused by truncating variants in the filamin C gene (FLNC). A new pathogenic FLNC variant, c.6864_6867dup, p.(Val2290Argfs∗23), was recently identified in Dutch patients with DCM.

OBJECTIVES

The report aimed to evaluate the phenotype of FLNC variant carriers and to determine whether this variant is a founder variant.

METHODS

Clinical and genetic data were retrospectively collected from variant carriers. Cardiovascular magnetic resonance studies were reassessed. Haplotypes were reconstructed to determine a founder effect. The geographical distribution and age of the variant were determined.

RESULTS

Thirty-three individuals (of whom 23 [70%] were female) from 9 families were identified. Sudden cardiac death was the first presentation in a carrier at the age of 28 years. The median age at diagnosis was 41 years (range 19-67 years). The phenotype was heterogeneous. DCM with left ventricular dilation and reduced ejection fraction (<45%) was present in 11 (33%) individuals, 3 (9%) of whom underwent heart transplantation. Cardiovascular magnetic resonance showed late gadolinium enhancement in 13 (65%) of the assessed individuals, primarily in a ringlike distribution. Nonsustained ventricular arrhythmias were detected in 6 (18%), and 5 (15%) individuals received an implantable cardioverter-defibrillator. A shared haplotype spanning 2.1 Mb was found in all haplotyped individuals. The variant originated between 275 and 650 years ago.

CONCLUSION

The pathogenic FLNC variant c.6864_6867dup, p.(Val2290Argfs∗23) is a founder variant originating from the south of the Netherlands. Carriers are susceptible to developing heart failure and ventricular arrhythmias. The cardiac phenotype is characterized by ringlike late gadolinium enhancement, even in individuals without significantly reduced left ventricular function.

Phosphoproteomics Profiling Defines a Target Landscape of the Basophilic Protein Kinases AKT, S6K, and RSK in Skeletal Myotubes

Phosphorylation-dependent signal transduction plays an important role in regulating the functions and fate of skeletal muscle cells. Central players in the phospho-signaling network are the protein kinases AKT, S6K, and RSK as part of the PI3K-AKT-mTOR-S6K and RAF-MEK-ERK-RSK pathways. However, despite their functional importance, knowledge about their specific targets is incomplete because these kinases share the same basophilic substrate motif RxRxxp[ST]. To address this, we performed a multifaceted quantitative phosphoproteomics study of skeletal myotubes following kinase inhibition. Our data corroborate a cross talk between AKT and RAF, a negative feedback loop of RSK on ERK, and a putative connection between RSK and PI3K signaling. Altogether, we report a kinase target landscape containing 49 so far unknown target sites. AKT, S6K, and RSK phosphorylate numerous proteins involved in muscle development, integrity, and functions, and signaling converges on factors that are central for the skeletal muscle cytoskeleton. Whereas AKT controls insulin signaling and impinges on GTPase signaling, nuclear signaling is characteristic for RSK. Our data further support a role of RSK in glucose metabolism. Shared targets have functions in RNA maturation, stability, and translation, which suggests that these basophilic kinases establish an intricate signaling network to orchestrate and regulate processes involved in translation.

Proximity labeling of endogenous RICTOR identifies mTOR Complex 2 regulation by ADP ribosylation factor ARF1

Mechanistic target of rapamycin (mTOR) complex 2 (mTORC2) regulates metabolism, cell proliferation, and cell survival. mTORC2 activity is stimulated by growth factors, and it phosphorylates the hydrophobic motif site of the AGC kinases AKT, SGK, and PKC. However, the proteins that interact with mTORC2 to control its activity and localization remain poorly defined. To identify mTORC2-interacting proteins in living cells, we tagged endogenous RICTOR, an essential mTORC2 subunit, with the modified BirA biotin ligase BioID2 and performed live-cell proximity labeling. We identified 215 RICTOR-proximal proteins, including proteins with known mTORC2 pathway interactions, and 135 proteins (63%) not previously linked to mTORC2 signaling, including nuclear and cytoplasmic proteins. Our imaging and cell fractionation experiments suggest nearly 30% of RICTOR is in the nucleus, hinting at potential nuclear functions. We also identified 29 interactors containing RICTOR-dependent, insulin-stimulated phosphorylation sites, thus providing insight into mTORC2-dependent insulin signaling dynamics. Finally, we identify the endogenous ADP ribosylation factor 1 (ARF1) GTPase as an mTORC2-interacting protein. Through gain-of-function and loss-of-function studies, we provide functional evidence that ARF1 may negatively regulate mTORC2. In summary, we present a new method of studying endogenous mTORC2, a resource of RICTOR/mTORC2 protein interactions in living cells, and a potential mechanism of mTORC2 regulation by the ARF1 GTPase.

Filamin C in cardiomyopathy: from physiological roles to DNA variants

Cardiomyopathy affects approximately 1 in 500 adults and is the leading cause of death. Familial cases are common, and mutations in many genes are involved in cardiomyopathy, especially those in genes encoding cytoskeletal, sarcomere, and nuclear envelope proteins. Filamin C is an actin-binding protein encoded by filamin C (FLNC) gene and participates in sarcomere stability maintenance. FLNC was first demonstrated to be a causal gene of myofibrillar myopathy; recently, it has been found that FLNC mutation plays a critical role in the pathogenesis of cardiomyopathy. In this review, we summarized the physiological roles of filamin C in cardiomyocytes and the genetic evidence for links between FLNC mutations and cardiomyopathies. Truncated FLNC is enriched in dilated cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy. Non-truncated FLNC is enriched in hypertrophic cardiomyopathy and restrictive cardiomyopathy. Two major pathomechanisms in FLNC-related cardiomyopathy have been described: protein aggregation resulting from non-truncating mutations and haploinsufficiency triggered by filamin C truncation. Therefore, it is important to understand the cellular biology and molecular regulation of FLNC to design new therapies to treat patients with FLNC-related cardiomyopathy.

Phosphoproteomics identifies dual-site phosphorylation in an extended basophilic motif regulating FILIP1-mediated degradation of filamin-C

The PI3K/Akt pathway promotes skeletal muscle growth and myogenic differentiation. Although its importance in skeletal muscle biology is well documented, many of its substrates remain to be identified. We here studied PI3K/Akt signaling in contracting skeletal muscle cells by quantitative phosphoproteomics. We identified the extended basophilic phosphosite motif RxRxxp[S/T]xxp[S/T] in various proteins including filamin-C (FLNc). Importantly, this extended motif, located in a unique insert in Ig-like domain 20 of FLNc, is doubly phosphorylated. The protein kinases responsible for this dual-site phosphorylation are Akt and PKCα. Proximity proteomics and interaction analysis identified filamin A-interacting protein 1 (FILIP1) as direct FLNc binding partner. FILIP1 binding induces filamin degradation, thereby negatively regulating its function. Here, dual-site phosphorylation of FLNc not only reduces FILIP1 binding, providing a mechanism to shield FLNc from FILIP1-mediated degradation, but also enables fast dynamics of FLNc necessary for its function as signaling adaptor in cross-striated muscle cells.

Reimann, Schwäble et al. perform quantitative proteomics to study PI3K/Akt signaling in contracting myotubes. They identify a dual-site phosphorylation motif in the actin cross-linker and signaling adaptor filamin C, which regulates its degradation and mobility, suggesting the importance of dual phosphorylation for filamin C function in striated muscle cells.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

Drosophila NUAK functions with Starvin/BAG3 in autophagic protein turnover

The inability to remove protein aggregates in post-mitotic cells such as muscles or neurons is a cellular hallmark of aging cells and is a key factor in the initiation and progression of protein misfolding diseases. While protein aggregate disorders share common features, the molecular level events that culminate in abnormal protein accumulation cannot be explained by a single mechanism. Here we show that loss of the serine/threonine kinase NUAK causes cellular degeneration resulting from the incomplete clearance of protein aggregates in Drosophila larval muscles. In NUAK mutant muscles, regions that lack the myofibrillar proteins F-actin and Myosin heavy chain (MHC) instead contain damaged organelles and the accumulation of select proteins, including Filamin (Fil) and CryAB. NUAK biochemically and genetically interacts with Drosophila Starvin (Stv), the ortholog of mammalian Bcl-2-associated athanogene 3 (BAG3). Consistent with a known role for the co-chaperone BAG3 and the Heat shock cognate 71 kDa (HSC70)/HSPA8 ATPase in the autophagic clearance of proteins, RNA interference (RNAi) of Drosophila Stv, Hsc70-4, or autophagy-related 8a (Atg8a) all exhibit muscle degeneration and muscle contraction defects that phenocopy NUAK mutants. We further demonstrate that Fil is a target of NUAK kinase activity and abnormally accumulates upon loss of the BAG3-Hsc70-4 complex. In addition, Ubiquitin (Ub), ref(2)p/p62, and Atg8a are increased in regions of protein aggregation, consistent with a block in autophagy upon loss of NUAK. Collectively, our results establish a novel role for NUAK with the Stv-Hsc70-4 complex in the autophagic clearance of proteins that may eventually lead to treatment options for protein aggregate diseases.

When signalling goes wrong: pathogenic variants in structural and signalling proteins causing cardiomyopathies

Cardiomyopathies are a diverse group of cardiac disorders with distinct phenotypes, depending on the proteins and pathways affected. A substantial proportion of cardiomyopathies are inherited and those will be the focus of this review article. With the wide application of high-throughput sequencing in the practice of clinical genetics, the roles of novel genes in cardiomyopathies are recognised. Here, we focus on a subgroup of cardiomyopathy genes [TTN, FHL1, CSRP3, FLNC and PLN, coding for Titin, Four and a Half LIM domain 1, Muscle LIM Protein, Filamin C and Phospholamban, respectively], which, despite their diverse biological functions, all have important signalling functions in the heart, suggesting that disturbances in signalling networks can contribute to cardiomyopathies.

Myofibrillar Z-discs Are a Protein Phosphorylation Hot Spot with Protein Kinase C (PKCα) Modulating Protein Dynamics

The Z-disc is a protein-rich structure critically important for the development and integrity of myofibrils, which are the contractile organelles of cross-striated muscle cells. We here used mouse C2C12 myoblast, which were differentiated into myotubes, followed by electrical pulse stimulation (EPS) to generate contracting myotubes comprising mature Z-discs. Using a quantitative proteomics approach, we found significant changes in the relative abundance of 387 proteins in myoblasts versus differentiated myotubes, reflecting the drastic phenotypic conversion of these cells during myogenesis. Interestingly, EPS of differentiated myotubes to induce Z-disc assembly and maturation resulted in increased levels of proteins involved in ATP synthesis, presumably to fulfill the higher energy demand of contracting myotubes. Because an important role of the Z-disc for signal integration and transduction was recently suggested, its precise phosphorylation landscape further warranted in-depth analysis. We therefore established, by global phosphoproteomics of EPS-treated contracting myotubes, a comprehensive site-resolved protein phosphorylation map of the Z-disc and found that it is a phosphorylation hotspot in skeletal myocytes, underscoring its functions in signaling and disease-related processes. In an illustrative fashion, we analyzed the actin-binding multiadaptor protein filamin C (FLNc), which is essential for Z-disc assembly and maintenance, and found that PKCα phosphorylation at distinct serine residues in its hinge 2 region prevents its cleavage at an adjacent tyrosine residue by calpain 1. Fluorescence recovery after photobleaching experiments indicated that this phosphorylation modulates FLNc dynamics. Moreover, FLNc lacking the cleaved Ig-like domain 24 exhibited remarkably fast kinetics and exceedingly high mobility. Our data set provides research community resource for further identification of kinase-mediated changes in myofibrillar protein interactions, kinetics, and mobility that will greatly advance our understanding of Z-disc dynamics and signaling.

Filamin C is a highly dynamic protein associated with fast repair of myofibrillar microdamage

Filamin c (FLNc) is a large dimeric actin-binding protein located at premyofibrils, myofibrillar Z-discs and myofibrillar attachment sites of striated muscle cells, where it is involved in mechanical stabilization, mechanosensation and intracellular signaling. Mutations in the gene encoding FLNc give rise to skeletal muscle diseases and cardiomyopathies. Here, we demonstrate by fluorescence recovery after photobleaching that a large fraction of FLNc is highly mobile in cultured neonatal mouse cardiomyocytes and in cardiac and skeletal muscles of live transgenic zebrafish embryos. Analysis of cardiomyocytes from Xirp1 and Xirp2 deficient animals indicates that both Xin actin-binding repeat-containing proteins stabilize FLNc selectively in premyofibrils. Using a novel assay to analyze myofibrillar microdamage and subsequent repair in cultured contracting cardiomyocytes by live cell imaging, we demonstrate that repair of damaged myofibrils is achieved within only 4 h, even in the absence of de novo protein synthesis. FLNc is immediately recruited to these sarcomeric lesions together with its binding partner aciculin and precedes detectable assembly of filamentous actin and recruitment of other myofibrillar proteins. These data disclose an unprecedented degree of flexibility of the almost crystalline contractile machinery and imply FLNc as a dynamic signaling hub, rather than a primarily structural protein. Our myofibrillar damage/repair model illustrates how (cardio)myocytes are kept functional in their mechanically and metabolically strained environment. Our results help to better understand the pathomechanisms and pathophysiology of early stages of FLNc-related myofibrillar myopathy and skeletal and cardiac diseases preceding pathological protein aggregation.

Calorie restriction leads to greater Akt2 activity and glucose uptake by insulin-stimulated skeletal muscle from old rats

Skeletal muscle insulin resistance is associated with many common age-related diseases, but moderate calorie restriction (CR) can substantially elevate glucose uptake by insulin-stimulated skeletal muscle from both young and old rats. The current study evaluated the isolated epitrochlearis muscle from ∼24.5-mo-old rats that were either fed ad libitum (AL) or subjected to CR (consuming ∼65% of ad libitum, AL, intake beginning at ∼22.5 mo old). Some muscles were also incubated with MK-2206, a potent and selective Akt inhibitor. The most important results were that in isolated muscles, CR vs. AL resulted in 1) greater insulin-stimulated glucose uptake 2) that was accompanied by significantly increased insulin-mediated activation of Akt2, as indicated by greater phosphorylation on both Thr(309) and Ser(474) along with greater Akt2 activity, 3) concomitant with enhanced phosphorylation of several Akt substrates, including an Akt substrate of 160 kDa on Thr(642) and Ser(588), filamin C on Ser(2213) and proline-rich Akt substrate of 40 kDa on Thr(246), but not TBC1D1 on Thr(596); and 4) each of the CR effects was eliminated by MK-2206. These data provide compelling new evidence linking greater Akt2 activation to the CR-induced elevation of insulin-stimulated glucose uptake by muscle from old animals.

Investigating the role of filamin C in Belgian patients with frontotemporal dementia linked to GRN deficiency in FTLD-TDP brains

TAR DNA-binding protein 43 (TDP-43) inclusions are pathological hallmarks of patients with frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Loss of TDP-43 in zebrafish engenders a severe muscle and vascular phenotype with a concomitant elevation of filamin C (FLNC) levels, an observation confirmed in the frontal cortex of FTLD-TDP patients. Here, we aimed to further assess the contribution of FLNC to frontotemporal dementia (FTD) etiology. We conducted a mutational screening of FLNC in a cohort of 529 unrelated Belgian FTD and FTD-ALS patients, and a control cohort of 920 unrelated and age-matched individuals. Additionally we performed an in-depth characterization of FLNC expression levels in FTD patients and a murine FTD model.

In total 68 missense variants were identified of which 19 (MAF < 1 %) were patient-only. Gene burden analysis demonstrated a significant association between the presence of rare variants in FLNC and disease (P = 0.0349, RR = 1.46 [95 % CI 1.03–2.07]). Furthermore, elevated FLNC expression levels, observed previously in FTLD-TDP patients, were mainly attributable to FTD patients with the progranulin (GRN) p.0(IVS1 + 5G > C) loss-of-function mutation. Increased FLNC levels were, to a lesser extent, also identified in a FLNC p.V831I variant carrier and in FTD patients with the p.R159H mutation in valosin-containing protein (VCP). The GRN-associated increase of FLNC was confirmed in the frontal cortex of aged Grn knockout mice starting at 16–18 months of age. Combined quantitative proteomic and bioinformatic analyses of the frontal cortex of FTD patients possessing elevated FLNC levels, identified multiple altered protein factors involved in accelerated aging, neurodegeneration and synaptogenesis.

Our findings further support the involvement of aberrant FLNC expression levels in FTD pathogenesis. Identification of increased FLNC levels in aged Grn mice and impaired pathways related to aging and neurodegeneration, implies a potential role for FLNC in mediating or accelerating the aging process.

Insulin Signaling and Glucose Uptake in the Soleus Muscle of 30-Month-Old Rats After Calorie Restriction With or Without Acute Exercise

Exercise and calorie restriction (CR) can each improve insulin sensitivity in older individuals, but benefits of combining these treatments on skeletal muscle insulin signaling and glucose uptake are poorly understood, especially in predominantly slow-twitch muscles (eg, soleus). Accordingly, our purpose was to determine independent and combined effects of prior acute exercise and CR (beginning at 14 weeks old) on insulin signaling and glucose uptake in insulin-stimulated soleus muscles of 30-month-old rats. CR alone (but not exercise alone) versus ad libitum sedentary controls induced greater insulin-stimulated glucose uptake. There was a main effect of diet (CR > ad libitum) for insulin-stimulated Akt(Ser473) and Akt(Thr308) phosphorylation. CR alone versus ad libitum sedentary increased Akt substrate of 160 kDa (AS160) Ser(588) phosphorylation and TBC1D1 Thr(596), but not AS160 Thr(642) phosphorylation or abundance of GLUT4, GLUT1, or hexokinase II proteins. Combined CR and exercise versus CR alone did not further increase insulin-stimulated glucose uptake although phosphorylation of Akt(Ser473), Akt(Thr308), TBC1D1(Thr596), and AMPK(Thr172) for the combined group exceeded values for CR and/or exercise alone. These results revealed that although the soleus was highly responsive to a CR-induced enhancement of insulin-stimulated glucose uptake, the exercise protocol did not elevate insulin-stimulated glucose uptake, either alone or when combined with CR.

Mechanisms for independent and combined effects of calorie restriction and acute exercise on insulin-stimulated glucose uptake by skeletal muscle of old rats

Either calorie restriction [CR; consuming 60-65% of ad libitum (AL) intake] or acute exercise can independently improve insulin sensitivity in old age, but their combined effects on muscle insulin signaling and glucose uptake have previously been unknown. Accordingly, we assessed the independent and combined effects of CR (beginning at 14 wk old) and acute exercise (3-4 h postexercise) on insulin signaling and glucose uptake in insulin-stimulated epitrochlearis muscles from 30-mo-old rats. Either CR alone or exercise alone vs. AL sedentary controls induced greater insulin-stimulated glucose uptake. Combined CR and exercise vs. either treatment alone caused an additional increase in insulin-stimulated glucose uptake. Either CR or exercise alone vs. AL sedentary controls increased Akt Ser(473) and Akt Thr(308) phosphorylation. Combined CR and exercise further elevated Akt phosphorylation on both sites. CR alone, but not exercise alone, vs. AL sedentary controls significantly increased Akt substrate of 160 kDa (AS160) Ser(588) and Thr(642) phosphorylation. Combined CR and exercise did not further enhance AS160 phosphorylation. Exercise alone, but not CR alone, modestly increased GLUT4 abundance. Combined CR and exercise did not further elevate GLUT4 content. These results suggest that CR or acute exercise independently increases insulin-stimulated glucose uptake via overlapping (greater Akt phosphorylation) and distinct (greater AS160 phosphorylation for CR, greater GLUT4 for exercise) mechanisms. Our working hypothesis is that greater insulin-stimulated glucose uptake in the combined CR and exercise group vs. CR or exercise alone relies on greater Akt activation, leading to greater phosphorylation of one or more Akt substrates other than AS160.

Effects of Caenorhabditis elegans sgk-1 mutations on lifespan, stress resistance, and DAF-16/FoxO regulation

The AGC family serine–threonine kinases Akt and Sgk are similar in primary amino acid sequence and in vitro substrate specificity, and both kinases are thought to directly phosphorylate and inhibit FoxO transcription factors. In the nematode Caenorhabditis elegans, it is well established that AKT-1 controls dauer arrest and lifespan by regulating the subcellular localization of the FoxO transcription factor DAF-16. SGK-1 is thought to act similarly to AKT-1 in lifespan control by phosphorylating and inhibiting the nuclear translocation of DAF-16/FoxO. Using sgk-1 null and gain-of-function mutants, we now provide multiple lines of evidence indicating that AKT-1 and SGK-1 influence C. elegans lifespan, stress resistance, and DAF-16/FoxO activity in fundamentally different ways. Whereas AKT-1 shortens lifespan, SGK-1 promotes longevity in a DAF-16-/FoxO-dependent manner. In contrast to AKT-1, which reduces resistance to multiple stresses, SGK-1 promotes resistance to oxidative stress and ultraviolet radiation but inhibits thermotolerance. Analysis of several DAF-16/FoxO target genes that are repressed by AKT-1 reveals that SGK-1 represses a subset of these genes while having little influence on the expression of others. Accordingly, unlike AKT-1, which promotes the cytoplasmic sequestration of DAF-16/FoxO, SGK-1 does not influence DAF-16/FoxO subcellular localization. Thus, in spite of their similar in vitro substrate specificities, Akt and Sgk influence longevity, stress resistance, and FoxO activity through distinct mechanisms in vivo. Our findings highlight the need for a re-evaluation of current paradigms of FoxO regulation by Sgk.

Greater filamin C, GSK3α, and GSK3β serine phosphorylation in insulin-stimulated isolated skeletal muscles of calorie restricted 24 month-old rats

Moderate calorie restriction (CR) can improve insulin-stimulated Akt phosphorylation and glucose uptake in muscles from 24 month-old rats, but the specific Akt substrates linking CR-effects on Akt to glucose uptake and other cellular processes are uncertain. We probed CR's influence on site-specific phosphorylation of five Akt substrates (AS160(Ser588), TBC1D1(Thr596), FLNc(Ser2213), GSK3α(Ser21), and GSK3β(Ser9)) in predominantly fast-twitch (epitrochlearis) and predominantly slow-twitch (soleus) muscles. We observed no CR-effect on phosphorylation of AS160(Ser588) or TBC1D1(Thr596), but there was a CR-induced increase in insulin-stimulated FLNc(Ser2213), GSK3α(Ser21), and GSK3β(Ser9) phosphorylation for both muscles. These results indicate that CR does not uniformly affect insulin-mediated phosphorylation of Akt substrates in fast- or slow-twitch muscles from 24 month-old rats.

Greater insulin-mediated Akt phosphorylation concomitant with heterogeneous effects on phosphorylation of Akt substrates in soleus of calorie-restricted rats

Akt is a serine/threonine kinase that plays a key role in numerous cellular functions including metabolism, growth, protein synthesis, apoptosis, and cell proliferation. The most consistent and robust effect of moderate calorie restriction (CR; ~60% of ad libitum, AL, food consumption) on insulin signaling in rodent muscle has been enhanced insulin-induced phosphorylation of Akt (pAkt). However, there is limited knowledge regarding the mechanism for this enhancement and its consequences in predominantly slow-twitch muscle. Accordingly, in soleus muscle of 9-mo-old rats, we analyzed the effect of CR and insulin on important signaling events that are proximal to Akt activation including: pIR(Tyr1162/1163), pIRS1(Tyr), pIRS1(Ser312), IRS1-associated phosphatidylinositol 3-kinase activity, or pPTEN(Ser380). In addition, we analyzed the effect of CR and insulin on Akt substrates that have established or putative roles in glucose metabolism, cellular growth, maintenance of muscle structure, or protein synthesis including pGSK3α(Ser21), pGSK3β(Ser9), pTSC2(Ser939), pP70S6K(Thr412), pAS160(Thr642), and pFLNc(Ser2213). The current study demonstrated that the CR-induced increase in pAkt in isolated soleus muscles from 9-mo-old rats can occur without concomitant enhancement of several important insulin signaling events that are proximal to Akt activation. These results suggest that the greater pAkt in the soleus muscles from CR rats was attributable to an alternative mechanism. We also observed that the effects of CR were not uniform for phosphorylation of six insulin-regulated Akt substrates in the soleus. The differential response in phosphorylation by Akt substrates likely has important implications for explaining the complex effect of CR diverse cellular functions.

Profiling the kinome: current capabilities and future challenges

Protein kinases are the second largest human protein family, but in terms of research interest, both basic and applied, they are surely the most popular. Over the past decade, many techniques and approaches for studying the kinome have been described and the pace of development is ever increasing. Presently, a molecular biologist can approach the kinome from many different angles: what kinases are active during a specific cell state of interest or become activated in response to a specific stimulus? What are the effects of controlling the activation status of an individual kinase? What substrates are targeted by a particular kinase, either in general or under particular conditions? And what kinase is responsible for targeting a specific phosphorylation site of interest? These are some of the more commonly asked questions during any kinase-centric research project and different strategies have been devised for answering such queries. In this review, we outline the most promising of these approaches, particularly those with a capacity for high-throughput studies. This article is part of a Special Issue entitled: From protein structures to clinical applications.

Preventing the calorie restriction-induced increase in insulin-stimulated Akt2 phosphorylation eliminates calorie restriction's effect on glucose uptake in skeletal muscle

Calorie restriction (CR; ~60% of ad libitum, AL, consumption) improves insulin-stimulated glucose uptake in skeletal muscle. The precise cellular mechanism for this healthful outcome is unknown, but it is accompanied by enhanced insulin-stimulated activation of Akt. Previous research using Akt2-null mice demonstrated that Akt2 is essential for the full CR-effect on insulin-stimulated glucose uptake by muscle. However, because Akt2-null mice were completely deficient in Akt2 in every cell throughout life, it would be valuable to assess the efficacy of transient, muscle-specific Akt inhibition for attenuation of CR-effects on glucose uptake. Accordingly, we used a selective Akt inhibitor (MK-2206) to eliminate the CR-induced elevation in insulin-stimulated Akt2 phosphorylation and determined the effects on Akt substrates and glucose uptake. We incubated isolated epitrochlearis muscles from 9-month-old AL and CR (~60-65% of AL intake for 6months) rats with or without MK-2206 and measured insulin-stimulated (1.2nM) glucose uptake and phosphorylation of the insulin receptor (Tyr1162/1163), pan-Akt (Thr308 and Ser473), Akt2 (Thr308 and Ser473), AS160/TBC1D4 (Thr642), and Filamin C (Ser2213). Incubation of isolated skeletal muscles with a dose of a selective Akt inhibitor that eliminated the CR-induced increases in Akt2 phosphorylation prevented CR's effects on insulin-stimulated glucose uptake, pAS160(Thr642) and pFilamin C(Ser2213) without altering pIR(Tyr1162/1163). These data provide compelling new evidence linking the CR-induced increase in insulin-stimulated Akt2 phosphorylation to CR's effects on insulin-mediated phosphorylation of Akt substrates and glucose uptake in skeletal muscle.

Phosphoinositide 3-kinase (PI3K(p110alpha)) directly regulates key components of the Z-disc and cardiac structure

Maintenance of cardiac structure and Z-disc signaling are key factors responsible for protecting the heart in a setting of stress, but how these processes are regulated is not well defined. We recently demonstrated that PI3K(p110α) protects the heart against myocardial infarction. The aim of this study was to determine whether PI3K(p110α) directly regulates components of the Z-disc and cardiac structure. To address this question, a unique three-dimensional virtual muscle model was applied to gene expression data from transgenic mice with increased or decreased PI3K(p110α) activity under basal conditions (sham) and in a setting of myocardial infarction to display the location of structural proteins. Key findings from this analysis were then validated experimentally. The three-dimensional virtual muscle model visually highlighted reciprocally regulated transcripts associated with PI3K activation that encoded key components of the Z-disc and costamere, including melusin. Studies were performed to assess whether PI3K and melusin interact in the heart. Here, we identify a novel melusin-PI3K interaction that generates lipid kinase activity. The direct impact of PI3K(p110α) on myocyte structure was assessed by treating neonatal rat ventricular myocytes with PI3K(p110α) inhibitors and examining the myofiber morphology of hearts from PI3K transgenic mice. Results demonstrate that PI3K is critical for myofiber maturation and Z-disc alignment. In summary, PI3K regulates the expression of genes essential for cardiac structure and Z-disc signaling, interacts with melusin, and is critical for Z-disc alignment.

Defining the substrate specificity determinants recognized by the active site of C-terminal Src kinase-homologous kinase (CHK) and identification of β-synuclein as a potential CHK physiological substrate

C-Terminal Src kinase-homologous kinase (CHK) exerts its tumor suppressor function by phosphorylating the C-terminal regulatory tyrosine of the Src-family kinases (SFKs). The phosphorylation suppresses their activity and oncogenic action. In addition to phosphorylating SFKs, CHK also performs non-SFK-related functions by phosphorylating other cellular protein substrates. To define these non-SFK-related functions of CHK, we used the "kinase substrate tracking and elucidation" method to search for its potential physiological substrates in rat brain cytosol. Our search revealed β-synuclein as a potential CHK substrate, and Y127 in β-synuclein as the preferential phosphorylation site. Using peptides derived from β-synuclein and positional scanning combinatorial peptide library screening, we defined the optimal substrate phosphorylation sequence recognized by the CHK active site to be E-x-[Φ/E/D]-Y-Φ-x-Φ, where Φ and x represent hydrophobic residues and any residue, respectively. Besides β-synuclein, cellular proteins containing motifs resembling this sequence are potential CHK substrates. Intriguingly, the CHK-optimal substrate phosphorylation sequence bears little resemblance to the C-terminal tail sequence of SFKs, indicating that interactions between the CHK active site and the local determinants near the C-terminal regulatory tyrosine of SFKs play only a minor role in governing specific phosphorylation of SFKs by CHK. Our results imply that recognition of SFKs by CHK is mainly governed by interactions between motifs located distally from the active site of CHK and determinants spatially separate from the C-terminal regulatory tyrosine in SFKs. Thus, besides assisting in the identification of potential CHK physiological substrates, our findings shed new light on how CHK recognizes SFKs and other protein substrates.

Research resource: New and diverse substrates for the insulin receptor isoform A revealed by quantitative proteomics after stimulation with IGF-II or insulin

The isoform A of the insulin receptor (IR) (IR-A) is a bifunctional receptor, because it binds both insulin and IGF-II. IR-A activation by IGF-II plays a role in development, but its physiological role in adults is unknown. IGF-II signaling through IR-A is deregulated in cancer and favors tumor progression. We hypothesized that IGF-II binding to the IR-A elicits a unique signaling pathway. In order to obtain an unbiased evaluation of IR-A substrates differentially involved after IGF-II and insulin stimulation, we performed quantitative proteomics of IR-A substrates recruited to tyrosine-phosphorylated protein complexes using stable isotope labeling with amino acids in cell culture in combination with antiphosphotyrosine antibody pull down and mass spectrometry. Using cells expressing only the human IR-A and lacking the IGF-I receptor, we identified 38 IR-A substrates. Only 10 were known IR mediators, whereas 28 substrates were not previously related to IR signaling. Eleven substrates were recruited by stimulation with both ligands: two equally recruited by IGF-II and insulin, three more strongly recruited by IGF-II, and six more strongly recruited by insulin. Moreover, 14 substrates were recruited solely by IGF-II and 13 solely by insulin stimulation. Interestingly, discoidin domain receptors, involved in cell migration and tumor metastasis, and ephrin receptor B4, involved in bidirectional signaling upon cell-cell contact, were predominantly activated by IGF-II. These findings indicate that IR-A activation by IGF-II elicits a unique signaling pathway that may play a distinct role in physiology and in disease.

Pathway-based identification of biomarkers for targeted therapeutics: personalized oncology with PI3K pathway inhibitors

Although we have made great progress in understanding the complex genetic alterations that underlie human cancer, it has proven difficult to identify which molecularly targeted therapeutics will benefit which patients. Drug-specific modulation of oncogenic signaling pathways in specific patient subpopulations can predict responsiveness to targeted therapy. Here, we report a pathway-based phosphoprofiling approach to identify and quantify clinically relevant, drug-specific biomarkers for phosphatidylinositol 3-kinase (PI3K) pathway inhibitors that target AKT, phosphoinositide-dependent kinase 1 (PDK1), and PI3K-mammalian target of rapamycin (mTOR). We quantified 375 nonredundant PI3K pathway-relevant phosphopeptides, all containing AKT, PDK1, or mitogen-activated protein kinase substrate recognition motifs. Of these phosphopeptides, 71 were drug-regulated, 11 of them by all three inhibitors. Drug-modulated phosphoproteins were enriched for involvement in cytoskeletal reorganization (filamin, stathmin, dynamin, PAK4, and PTPN14), vesicle transport (LARP1, VPS13D, and SLC20A1), and protein translation (S6RP and PRAS40). We then generated phosphospecific antibodies against selected, drug-regulated phosphorylation sites that would be suitable as biomarker tools for PI3K pathway inhibitors. As proof of concept, we show clinical translation feasibility for an antibody against phospho-PRAS40(Thr246). Evaluation of binding of this antibody in human cancer cell lines, a PTEN (phosphatase and tensin homolog deleted from chromosome 10)-deficient mouse prostate tumor model, and triple-negative breast tumor tissues showed that phospho-PRAS40(Thr246) positively correlates with PI3K pathway activation and predicts AKT inhibitor sensitivity. In contrast to phosphorylation of AKT(Thr308), the phospho-PRAS40(Thr246) epitope is highly stable in tissue samples and thus is ideal for immunohistochemistry. In summary, our study illustrates a rational approach for discovery of drug-specific biomarkers toward development of patient-tailored treatments.

Novel molecular imaging platform for monitoring oncological kinases

Recent advances in oncology have lead to identification of a plethora of alterations in signaling pathways that are critical to oncogenesis and propagation of malignancy. Among the biomarkers identified, dysregulated kinases and associated changes in signaling cascade received the lion's share of scientific attention and have been under extensive investigations with goal of targeting them for anti-cancer therapy. Discovery of new drugs is immensely facilitated by molecular imaging technology which enables non-invasive, real time, dynamic imaging and quantification of kinase activity. Here, we review recent development of novel kinase reporters based on conformation dependent complementation of firefly luciferase to monitor kinase activity. Such reporter system provides unique insights into the pharmacokinetics and pharmacodynamics of drugs that modulate kinase signaling and have a huge potential in drug discovery, validation, and drug-target interactions.

A novel pleckstrin homology domain-containing protein enhances insulin-stimulated Akt phosphorylation and GLUT4 translocation in adipocytes

Protein kinase B/Akt protein kinases control an array of diverse functions, including cell growth, survival, proliferation, and metabolism. We report here the identification of pleckstrin homology-like domain family B member 1 (PHLDB1) as an insulin-responsive protein that enhances Akt activation. PHLDB1 contains a pleckstrin homology domain, which we show binds phosphatidylinositol PI(3,4)P₂, PI(3,5)P₂, and PI(3,4,5)P₃, as well as a Forkhead-associated domain and coiled coil regions. PHLDB1 expression is increased during adipocyte differentiation, and it is abundant in many mouse tissues. Both endogenous and HA- or GFP-tagged PHLDB1 displayed a cytoplasmic disposition in unstimulated cultured adipocytes but translocated to the plasma membrane in response to insulin. Depletion of PHLDB1 by siRNA inhibited insulin stimulation of Akt phosphorylation but not tyrosine phosphorylation of IRS-1. RNAi-based silencing of PHLDB1 in cultured adipocytes also attenuated insulin-stimulated deoxyglucose transport and Myc-GLUT4-EGFP translocation to the plasma membrane, whereas knockdown of the PHLDB1 isoform PHLDB2 failed to attenuate insulin-stimulated deoxyglucose transport. Furthermore, adenovirus-mediated expression of PHLDB1 in adipocytes enhanced insulin-stimulated Akt and p70 S6 kinase phosphorylation, as well as GLUT4 translocation. These results indicate that PHLDB1 is a novel modulator of Akt protein kinase activation by insulin.

Filamins in cardiovascular development

Filamins are classically recognized as large cytoplasmic proteins that cross-link cortical actin into dynamic 3-dimensional structures and transmit extracellular signals through integrin receptors into the cytoplasm. However, recent reports indicate that filamins interact with a large number of other proteins with diverse functions, including transcriptional factors and cellular molecules involved in signaling, adhesion, and cellular motility, and are also present in the cell nucleus. In addition, genetic mutations in filamins have been linked to a wide range of human genetic disorders, including skeletal, central nervous system, and cardiovascular malformations, highlighting distinct filamin interactions. Here, we update the cardiovascular phenotypes of patients with mutations in filamin genes and mice deficient in filamins and filamin-interacting proteins.

A 90 kDa fragment of filamin A promotes Casodex-induced growth inhibition in Casodex-resistant androgen receptor positive C4-2 prostate cancer cells

Prostate tumors are initially dependent on androgens for growth, but the majority of patients treated with anti-androgen therapy progress to androgen-independence characterized by resistance to such treatment. This study investigates a novel role for filamin A (FlnA), a 280 kDa cytoskeletal protein (consisting of an actin-binding domain (ABD) followed by 24 sequential repeats), in androgen-independent (AI) growth. Full-length FlnA is cleaved to 170 kDa (ABD+FlnA1-15) and 110 kDa fragments (FlnA16-24); the latter is further cleaved to a 90 kDa fragment (repeats 16-23) capable of nuclear translocation and androgen receptor (AR) binding. Here, we demonstrate that in androgen-dependent LNCaP prostate cancer cells, the cleaved 90 kDa fragment is localized to the nucleus, whereas in its AI subline C4-2, FlnA failed to cleave and remained cytoplasmic. Transfection of FlnA16-24 cDNA in C4-2 cells restored expression and nuclear localization of 90 kDa FlnA. Unlike LNCaP, C4-2 cells proliferate in androgen-reduced medium and in the presence of the AR-antagonist Casodex. They also exhibit increased Akt phosphorylation compared to LNCaP, which may contribute to their AI phenotype. Nuclear expression of 90 kDa FlnA in C4-2 cells decreased Akt phosphorylation, prevented proliferation in androgen-reduced medium and restored Casodex sensitivity. This effect was inhibited by constitutive activation of Akt indicating that FlnA restored Casodex sensitivity in C4-2 cells by decreasing Akt phosphorylation. In addition, FlnA-specific siRNA which depleted FlnA levels, but not control siRNA, induced resistance to Casodex in LNCaP cells. Our results demonstrate that expression of nuclear FlnA is necessary for androgen dependence in these cells.

Structural determinants of LL5beta subcellular localisation and association with filamin C

PI3K signalling pathways link cell surface receptors to the control of several intracellular functions including cell growth, survival and movement. Filamins are important regulators of cortical actin structure and function. LL5beta is a filamin binding protein that is an effector of the PI3K signalling pathway. We define an N-terminal region of LL5beta that is responsible for binding to the C-terminus of filamins. Under conditions of very low PI3K activity, we show that this region, together with an additional domain of the protein, is responsible for localising the complex to punctate structures that are also decorated by L-FILIP (a protein previously characterised to bind filamin and accelerate its destruction). Under conditions of significant PI3K activity, PtdIns(3,4,5)P(3) binding to the C-terminal PH domain in LL5beta prevents localisation to these structures. These observations start to define the basis for PI3K regulation of filamin through LL5beta.

(Patho)physiological significance of the serum- and glucocorticoid-inducible kinase isoforms

The serum- and glucocorticoid-inducible kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress (including cell shrinkage) and hormones (including gluco- and mineralocorticoids). Similar to its isoforms SGK2 and SGK3, SGK1 is activated by insulin and growth factors via phosphatidylinositol 3-kinase and the 3-phosphoinositide-dependent kinase PDK1. SGKs activate ion channels (e.g., ENaC, TRPV5, ROMK, Kv1.3, KCNE1/KCNQ1, GluR1, GluR6), carriers (e.g., NHE3, GLUT1, SGLT1, EAAT1-5), and the Na+-K+-ATPase. They regulate the activity of enzymes (e.g., glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, phosphomannose mutase-2) and transcription factors (e.g., forkhead transcription factor FKHRL1, beta-catenin, nuclear factor kappaB). SGKs participate in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis. SGK1 contributes to Na+ retention and K+ elimination of the kidney, mineralocorticoid stimulation of salt appetite, glucocorticoid stimulation of intestinal Na+/H+ exchanger and nutrient transport, insulin-dependent salt sensitivity of blood pressure and salt sensitivity of peripheral glucose uptake, memory consolidation, and cardiac repolarization. A common ( approximately 5% prevalence) SGK1 gene variant is associated with increased blood pressure and body weight. SGK1 may thus contribute to metabolic syndrome. SGK1 may further participate in tumor growth, neurodegeneration, fibrosing disease, and the sequelae of ischemia. SGK3 is required for adequate hair growth and maintenance of intestinal nutrient transport and influences locomotive behavior. In conclusion, the SGKs cover a wide variety of physiological functions and may play an active role in a multitude of pathophysiological conditions. There is little doubt that further targets will be identified that are modulated by the SGK isoforms and that further SGK-dependent in vivo physiological functions and pathophysiological conditions will be defined.

Exercise-induced phosphorylation of the novel Akt substrates AS160 and filamin A in human skeletal muscle

Skeletal muscle contraction stimulates multiple signaling cascades that govern a variety of metabolic and transcriptional events. Akt/protein kinase B regulates metabolism and growth/muscle hypertrophy, but contraction effects on this target and its substrates are varied and may depend on the mode of the contractile stimulus. Accordingly, we determined the effects of endurance or resistance exercise on phosphorylation of Akt and downstream substrates in six trained cyclists who performed a single bout of endurance or resistance exercise separated by approximately 7 days. Muscle biopsies were taken from the vastus lateralis at rest and immediately after exercise. Akt Ser(473) phosphorylation was increased (1.8-fold; P=0.011) after endurance but was unchanged after resistance exercise. Conversely, Akt Thr(308) phosphorylation was unaltered after either bout of exercise. Several exercise-responsive phosphoproteins were detected by immunoblot analysis with a phospho-Akt substrate antibody. pp160 and pp300 were identified as AS160 and filamin A, respectively, with increased phosphorylation (2.0- and 4.9-fold, respectively; P<0.05) after endurance but not resistance exercise. In conclusion, AS160 and filamin A may provide an important link to mediate endurance exercise-induced bioeffects in skeletal muscle.

KESTREL: a powerful method for identifying the physiological substrates of protein kinases

The identification of all the substrates of every protein kinase is one of the major challenges of post-genomic research. Here we review a powerful method for tackling this problem that we have developed over the last 5 years. The method has so far been used to identify novel substrates for eight different protein kinases, demonstrating that it is of general utility. Importantly, the method can be used to identify distinct physiological substrates of protein kinases, such as PKB (protein kinase B) and SGK (serum- and glucocorticoid-induced kinase), that are closely related in structure and have similar specificity determinants.

Identification of calcium-regulated heat-stable protein of 24 kDa (CRHSP24) as a physiological substrate for PKB and RSK using KESTREL

A substrate for PKBalpha (protein kinase Balpha) was detected in liver extracts, and was purified and identified as CRHSP24 (calcium-regulated heat-stable protein of apparent molecular mass 24 kDa). PKBalpha, as well as SGK1 (serum- and glucocorticoid-induced protein kinase 1) and RSK (p90 ribosomal S6 kinase), phosphorylated CRHSP24 stoichiometrically at Ser52 in vitro and its brain-specific isoform PIPPin at the equivalent residue (Ser58). CRHSP24 became phosphorylated at Ser52 when HEK-293 (human embryonic kidney) cells were stimulated with IGF-1 (insulin-like growth factor-1) and this was prevented by inhibitors of PI3K (phosphoinositide 3-kinase), but not by rapamycin [an inhibitor of mTOR (mammalian target of rapamycin)] or PD 184352, an inhibitor of the classical MAPK (mitogen-activated protein kinase) cascade and hence the activation of RSK. IGF-1 induced a similar phosphorylation of CRHSP24 in ES (embryonic stem) cells from wild-type mice or mice that express the PDK1 (3-phosphoinositide-dependent kinase 1) mutant (PDK1[L155E]) that activates PKBalpha normally, but cannot activate SGK. CRHSP24 also became phosphorylated at Ser52 in response to EGF (epidermal growth factor) and this was prevented by blocking activation of both the classical MAPK cascade and the activation of PKBalpha, but not if just one of these pathways was inhibited. DYRK2 (dual-specificity tyrosine-phosphorylated and -regulated protein kinase 2) phosphorylated CRHSP24 at Ser30, Ser32 and Ser41 in vitro, and Ser41 was identified as a site phosphorylated in cells. These and other results demonstrate that CRHSP24 is phosphorylated at Ser52 by PKBalpha in response to IGF-1, at Ser52 by PKBalpha and RSK in response to EGF, and at Ser41 in the absence of IGF-1/EGF by a DYRK isoform or another proline-directed protein kinase(s).

Regulation of microfilament organization by Kaposi sarcoma-associated herpes virus-cyclin.CDK6 phosphorylation of caldesmon

Kaposi sarcoma-associated herpes virus (KSHV) encodes a D-like cyclin (K-cyclin) that is thought to contribute to the viral oncogenicity. K-cyclin activates cellular cyclin-dependent kinases (CDK) 4 and 6, generating enzymes with a substrate selectivity deviant from CDK4 and CDK6 activated by D-type cyclins, suggesting different biochemical and biological functions. Here we report the identification of the actin- and calmodulin-binding protein caldesmon (CALD1) as a novel K-cyclin.CDK substrate, which is not phosphorylated by D.CDK. CALD1 plays a central role in the regulation of microfilament organization, consequently controlling cell shape, adhesion, cytokinesis and motility. K-cyclin.CDK6 specifically phosphorylates four Ser/Thr sites in the human CALD1 carboxyl terminus, abolishing CALD1 binding to its effector protein, actin, and its regulator protein, calmodulin. CALD1 is hyperphosphorylated in cells following K-cyclin expression and in KSHV-transformed lymphoma cells. Moreover, expression of exogenous K-cyclin results in microfilament loss and changes in cell morphology; both effects are reliant on CDK catalysis and can be reversed by the expression of a phosphorylation defective CALD1. Together, these data strongly suggest that K-cyclin expression modulates the activity of caldesmon and through this the microfilament functions in cells. These results establish a novel link between KSHV infection and the regulation of the actin cytoskeleton.

The phosphorylation of CapZ-interacting protein (CapZIP) by stress-activated protein kinases triggers its dissociation from CapZ

A protein expressed in immune cells and muscle was detected in muscle extracts as a substrate for several SAPKs (stress-activated protein kinases). It interacted specifically with the F-actin capping protein CapZ in splenocytes, and was therefore termed 'CapZIP' (CapZ-interacting protein). Human CapZIP was phosphorylated at Ser-179 and Ser-244 by MAPKAP-K2 (mitogen-activated protein kinase-activated protein kinase 2) or MAPKAP-K3 in vitro. Anisomycin induced the phosphorylation of CapZIP at Ser-179 in Jurkat cells, which was prevented by SB 203580, consistent with phosphorylation by MAPKAP-K2 and/or MAPKAP-K3. However, osmotic shock-induced phosphorylation of Ser-179 was unaffected by SB 203580. These and other results suggest that CapZIP is phosphorylated at Ser-179 in cells by MAPKAP-K2/MAPKAP-K3, and at least one other protein kinase. Stress-activated MAP kinase family members phosphorylated human CapZIP at many sites, including Ser-68, Ser-83, Ser-108 and Ser-216. Ser-108 became phosphorylated when Jurkat cells were exposed to osmotic shock, which was unaffected by SB 203580 and/or PD 184352, or in splenocytes from mice that do not express either SAPK3/p38gamma or SAPK4/p38delta. Our results suggest that CapZIP may be phosphorylated by JNK (c-Jun N-terminal kinase), which phosphorylates CapZIP to >5 mol/mol within minutes in vitro. Osmotic shock or anisomycin triggered the dissociation of CapZIP from CapZ in Jurkat cells, suggesting that phosphorylation of CapZIP may regulate the ability of CapZ to remodel actin filament assembly in vivo.

Exploitation of KESTREL to identify NDRG family members as physiological substrates for SGK1 and GSK3

We detected a protein in rabbit skeletal muscle extracts that was phosphorylated rapidly by SGK1 (serum- and glucocorticoid-induced kinase 1), but not by protein kinase Ba, and identified it as NDRG2 (N-myc downstream-regulated gene 2). SGK1 phosphorylated NDRG2 at Thr330, Ser332 and Thr348 in vitro. All three residues were phosphorylated in skeletal muscle from wild-type mice, but not from mice that do not express SGK1. SGK1 also phosphorylated the related NDRG1 isoform at Thr328, Ser330 and Thr346 (equivalent to Thr330, Ser332 and Thr348 of NDRG2), as well as Thr356 and Thr366. Residues Thr346, Thr356 and Thr366 are located within identical decapeptide sequences GTRSRSHTSE, repeated three times in NDRG1. These threonines were phosphorylated in NDRG1 in the liver, lung, spleen and skeletal muscle of wild-type mice, but not in SGK1-/- mice. Knock-down of SGK1 in HeLa cells using small interfering RNA also suppressed phosphorylation of the threonine residues in the repeat region of NDRG1. The phosphorylation of NDRG1 by SGK1 transformed it into an excellent substrate for GSK3 (glycogen synthase kinase 3), which could then phosphorylate Ser342, Ser352 and Ser362 in the repeat region. Incubation of HeLa cells with the specific GSK3 inhibitor CT 99021 increased the electrophoretic mobility of NDRG1 in HeLa cells, demonstrating that this protein is phosphorylated by GSK3 in cells. Our results identify NDRG1 and NDRG2 as physiological substrates for SGK1, and demonstrate that phosphorylation of NDRG1 by SGK1 primes it for phosphorylation by GSK3.

The tRNA methylase METTL1 is phosphorylated and inactivated by PKB and RSK in vitro and in cells

A substrate for protein kinase B (PKB)alpha in HeLa cell extracts was identified as methyltransferase-like protein-1 (METTL1), the orthologue of trm8, which catalyses the 7-methylguanosine modification of tRNA in Saccharomyces cerevisiae. PKB and ribosomal S6 kinase (RSK) both phosphorylated METTL1 at Ser27 in vitro. Ser27 became phosphorylated when HEK293 cells were stimulated with insulin-like growth factor-1 (IGF-1) and this was prevented by inhibition of phosphatidyinositol 3-kinase. The IGF-1-induced Ser27 phosphorylation did not occur in 3-phosphoinositide-dependent protein kinase-1 (PDK1)-deficient embryonic stem cells, but occurred normally in PDK1[L155E] cells, indicating that the effect of IGF-1 is mediated by PKB. METTL1 also became phosphorylated at Ser27 in response to phorbol-12-myristate 13-acetate and this was prevented by PD 184352 or pharmacological inhibition of RSK. Phosphorylation of METTL1 by PKB or RSK inactivated METTL1 in vitro, as did mutation of Ser27 to Asp or Glu. Expression of METTL1[S27D] or METTL1[S27E] did not rescue the growth phenotype of yeast lacking trm8. In contrast, expression of METTL1 or METTL1[S27A] partially rescued growth. These results demonstrate that METTL1 is inactivated by PKB and RSK in cells, and the potential implications of this finding are discussed.

Identification of different specificity requirements between SGK1 and PKBalpha

NDRG1 is phosphorylated by SGK1 (but not PKB) in vivo at three residues each contained within three nonapeptide repeats. Here, we demonstrate that this nonapeptide, like the NDRG1 protein, is phosphorylated by SGK1, but not by PKBalpha or RSK1 in vitro. The inability of PKBalpha and RSK1 to phosphorylate the nonapeptide was traced to residues n+1, n+2 and n-4 (where n is the phosphorylation site). Changing them from Ser, Glu and Ser to Phe, Ala and Pro, respectively, transformed the nonapeptide into an excellent substrate for PKBalpha and RSK1. Our results identify a specific substrate for SGK1 and may facilitate detection of additional physiological substrates for this enzyme.