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1.  Optimizing small molecule inhibitors of calcium-dependent protein kinase 1 to prevent infection by Toxoplasma gondii 
Journal of medicinal chemistry  2013;56(7):3068-3077.
Toxoplasma gondii is sensitive to bulky pyrazolo [3,4-d] pyrimidine (PP) inhibitors due to the presence of a Gly gatekeeper in the essential calcium dependent protein kinase 1 (CDPK1). Here we synthesized a number of new derivatives of 3-methyl-benzyl-PP (3-MB-PP, or 1). The potency of PP analogs in inhibiting CDPK1 enzyme activity in vitro (low nM IC50 values) and blocking parasite growth in host cell monolayers in vitro (low μM EC50 values) were highly correlated and occurred in a CDPK1-specific manner. Chemical modification of the PP scaffold to increase half-life in the presence of microsomes in vitro led to identification of compounds with enhanced stability while retaining activity. Several of these more potent compounds were able to prevent lethal infection with T. gondii in the mouse model. Collectively the strategies outlined here provide a route for development of more effective compounds for treatment of toxoplasmosis, and perhaps related parasitic diseases.
doi:10.1021/jm4001314
PMCID: PMC3625458  PMID: 23470217
Serine/threonine protein kinase; gatekeeper; calcium signaling; toxoplasmosis; chemotherapy
2.  A neo-substrate that amplifies catalytic activity of Parkinson’s disease related kinase PINK1 
Cell  2013;154(4):737-747.
Summary
Mitochondria have long been implicated in the pathogenesis of Parkinson’s disease (PD). Mutations in the mitochondrial kinase PINK1 that reduce kinase activity are associated with mitochondrial defects and result in an autosomal recessive form of early onset PD. Therapeutic approaches for enhancing the activity of PINK1 have not been considered since no allosteric regulatory sites for PINK1 are known. Here we show that an alternative strategy, a neo-substrate approach involving the ATP analog kinetin triphosphate (KTP), can be used to increase the activity of both PD related mutant PINK1G309D and PINK1wt. Moreover, we show that application of the KTP precursor kinetin to cells results in biologically significant increases in PINK1 activity, manifest as higher levels of Parkin recruitment to depolarized mitochondria, reduced mitochondrial motility in axons, and lower levels of apoptosis. Discovery of neo-substrates for kinases could provide a heretofore-unappreciated modality for regulating kinase activity.
doi:10.1016/j.cell.2013.07.030
PMCID: PMC3950538  PMID: 23953109
3.  Glucose Sensor O-GlcNAcylation Coordinates with Phosphorylation to Regulate Circadian Clock 
Cell metabolism  2013;17(2):291-302.
SUMMARY
Post-translational modifications play central roles in myriad biological pathways including circadian regulation. We employed a circadian proteomic approach to demonstrate that circadian timing of phosphorylation is a critical factor in regulating complex GSK3β dependent pathways and identified O-GlcNAc transferase (OGT) as a substrate of GSK3β. Interestingly, OGT activity is regulated by GSK3β, hence OGT and GSK3β exhibit reciprocal regulation. Modulating OGlcNAcylation levels alter circadian period length in both mice and Drosophila, and conversely protein O-GlcNAcylation is circadianly regulated. Central clock proteins, Clock and Period, are reversibly modified by O-GlcNAcylation to regulate their transcriptional activities. In addition, O-GlcNAcylation of a region in PER2 known to regulate human sleep phase (S662–S674) competes with phosphorylation of this region, and this interplay is at least partly mediated by glucose levels. Together, these results indicate that O-GlcNAcylation serves as a metabolic sensor for clock regulation and works coordinately with phosphorylation to fine tune circadian clock.
doi:10.1016/j.cmet.2012.12.017
PMCID: PMC3597447  PMID: 23395175
4.  The In Vivo Response of Stem and Other Undifferentiated Spermatogonia to the Reversible Inhibition of GDNF Signaling in the Adult 
Stem cells (Dayton, Ohio)  2012;30(4):732-740.
Maintaining adequate numbers of spermatogonial stem cells is required for the production of the millions of sperm required for male fertility. To date, however, the mechanisms that regulate the size of this pool in the adult are poorly defined. Glial cell line-derived neurotrophic factor (GDNF) is required for establishing this pool in the prepubertal animal, but its in vivo function in the normal adult testis has never been examined directly. We used a chemical-genetic approach to address this issue. We generated mice carrying a single amino acid mutation (V805A) in Ret, the kinase subunit of the GDNF receptor. This mutation does not affect normal GDNF signaling, but renders it susceptible to inhibition by the ATP competitive inhibitor, NA-PP1. When GDNF signaling was blocked in adults for 11 days, only a few cells remained that expressed the stem spermatogonial markers, Gfrα1 and Zbtb16 and testicular Ret mRNA content was reduced markedly. These decreases were associated with depletion of functional stem spermatogonia; some were lost when GDNF signaling was inhibited for only 2 days while others survived for up to 11 days. However, when signaling was restored, the remaining stem cells proliferated, initiating tissue restoration. In conclusion, these results provide the first direct proof that GDNF acutely regulates the numbers of spermatogonial stem cells in the normal adult testis. Additionally, these results demonstrate different sensitivities among subpopulations of these stem cells to inhibition of GDNF signaling.
doi:10.1002/stem.1028
PMCID: PMC3894909  PMID: 22232066
stem spermatogonia; GDNF; Sertoli cell; Ret; spermatogenesis
5.  Chemical Genetic Approach for Kinase-Substrate Mapping by Covalent Capture of Thiophosphopeptides and Analysis by Mass Spectrometry 
Mapping kinase-substrate interactions demands robust methods to rapidly and unequivocally identify substrates from complex protein mixtures. Towards this goal we present a method in which a kinase, engineered to utilize synthetic ATPγS analogs, specifically thiophosphorylates its substrates in a complex lysate. The thiophosphate label provides a bio-orthogonal tag that can be used to affinity purify and identify labeled proteins. Following the labeling reaction proteins are digested with trypsin, thiol containing peptides are then covalently captured and non-thiol containing peptides are washed from the resin. Oxidation promoted hydrolysis, at sites of thiophosphorylation, releases phosphopeptides for analysis by tandem mass spectrometry. By incorporating two specificity gates: kinase engineering and peptide affinity purification, this method yields high confidence substrate identifications. This method gives both the identity of the substrates and phosphorylation site localization. With this information investigators can analyze the biological significance of the phosphorylation mark immediately following confirmation of the kinase-substrate relationship. Here we provide an optimized version of this technique to further enable widespread utilization of this technology.
doi:10.1002/9780470559277.ch090201
PMCID: PMC3894912  PMID: 23836541
phosphorylation; chemical genetics; analog specific kinase; kinase substrate identification; thiophosphate labeling
6.  Cyclin-dependent kinase control of the initiation-to-elongation switch of RNA polymerase II 
Nature structural & molecular biology  2012;19(11):1108-1115.
Promoter-proximal pausing by RNA polymerase II (Pol II) ensures both gene-specific regulation and RNA quality control. Structural considerations suggested initiation factor eviction would be required for elongation factor engagement and pausing of transcription complexes. Here we show that selective inhibition of Cdk7—part of TFIIH—increases TFIIE retention, prevents DRB-sensitivity inducing factor (DSIF) recruitment and attenuates pausing in human cells. Pause release depends on Cdk9—cyclin T1 (P-TEFb); Cdk7 is also required for Cdk9-activating phosphorylation and Cdk9-dependent downstream events—Pol II carboxyl-terminal domain Ser2 phosphorylation and histone H2B ubiquitylation—in vivo. Cdk7 inhibition, moreover, impairs Pol II transcript 3′-end formation. Cdk7 thus acts through TFIIE and DSIF to establish and through P-TEFb to relieve barriers to elongation: incoherent feedforward that might create a window to recruit RNA-processing machinery. Therefore, cyclin-dependent kinases govern Pol II handoff from initiation to elongation factors and co-transcriptional RNA maturation.
doi:10.1038/nsmb.2399
PMCID: PMC3746743  PMID: 23064645
7.  PTEN expression is consistent in colorectal cancer primaries and metastases and associates with patient survival 
Cancer Medicine  2013;2(4):496-506.
Abstract
Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) negatively regulates the phosphoinositide-3-kinase (PI3K) signaling pathway. In colorectal cancer (CRC), observed frequencies of loss of PTEN expression, concordant expression in primary tumors and metastases, and the association of PTEN status with outcome vary markedly by detection method. We determined the degree to which PTEN expression is consistent in 70 matched human CRC primaries and liver metastases using a validated immunohistochemistry assay. We found loss of PTEN expression in 12.3% of assessable CRC primaries and 10.3% of assessable liver metastases. PTEN expression (positive or negative) was concordant in 98% of matched colorectal primaries and liver metastases. Next we related PTEN status to mutations in RAS and PI3K pathway genes (KRAS, NRAS, BRAF, and PIK3CA) and to overall survival (OS). PTEN expression was not significantly associated with the presence or absence of mutations in RAS or PI3K pathway genes. The median OS of patients whose tumors did not express PTEN was 9 months, compared to 49 months for patients whose tumors did express PTEN (HR = 6.25, 95% confidence intervals (CI) (1.98, 15.42), P = 0.0017). The association of absent PTEN expression with increased risk of death remained significant in multivariate analysis (HR = 6.31, 95% CI (2.03, 17.93), P = 0.0023). In summary, PTEN expression was consistent in matched CRC primaries and in liver metastases. Therefore, future investigations of PTEN in metastatic CRC can use primary tumor tissue. In patients with liver-only metastases, loss of PTEN expression predicted poor OS.
We observed concordant PTEN expression in 98% of colorectal cancer (CRC) primary and liver metastasis pairs using a validated immunohistochemistry assay. Consistent PTEN expression at both disease sites is significant because tumor tissue is usually available from CRC primaries but not metastases. Loss of PTEN expression associated with poor survival of CRC patients with liver-only metastases.
doi:10.1002/cam4.97
PMCID: PMC3799284  PMID: 24156022
Biomarker; colorectal cancer; concordance; PTEN; survival
8.  A genetically selective inhibitor reveals a ZAP-70 catalytic-independent function in TREG cells 
Nature immunology  2010;11(12):1085-1092.
To investigate the role of the kinase zeta-associated protein of 70 kDa (ZAP-70) in T cells, we generated mice expressing a ZAP-70 mutant whose catalytic activity can be selectively blocked by a small molecule inhibitor. Conventional naïve, effector and memory T cells were dependent on ZAP-70 kinase activity for their activation, demonstrating a non-redundant role for ZAP-70 in TCR-induced signals. In contrast, ZAP-70 catalytic activity was not required for activation of the GTPase Rap1 and inside-out signals that promote integrin adhesion. This ZAP-70 kinase-independent pathway is sufficient for regulatory T (TREG) cell suppressive activity, which was unperturbed by ZAP-70 catalytic inhibition. Our results implicate ZAP-70 as an attractive therapeutic target.
doi:10.1038/ni.1955
PMCID: PMC3711183  PMID: 21037577
9.  Chemical genetic discovery of targets and anti-targets for cancer polypharmacology 
Nature  2012;486(7401):80-84.
The complexity of cancer has led to recent interest in polypharmacological approaches for developing kinase-inhibitor drugs; however, optimal kinase-inhibition profiles remain difficult to predict. Using a Ret-kinase-driven Drosophila model of multiple endocrine neoplasia type 2 and kinome-wide drug profiling, here we identify that AD57 rescues oncogenic Ret-induced lethality, whereas related Ret inhibitors imparted reduced efficacy and enhanced toxicity. Drosophila genetics and compound profiling defined three pathways accounting for the mechanistic basis of efficacy and dose-limiting toxicity. Inhibition of Ret plus Raf, Src and S6K was required for optimal animal survival, whereas inhibition of the ‘anti-target’ Tor led to toxicity owing to release of negative feedback. Rational synthetic tailoring to eliminate Tor binding afforded AD80 and AD81, compounds featuring balanced pathway inhibition, improved efficacy and low toxicity in Drosophila and mammalian multiple endocrine neoplasia type 2 models. Combining kinase-focused chemistry, kinome-wide profiling and Drosophila genetics provides a powerful systems pharmacology approach towards developing compounds with a maximal therapeutic index.
doi:10.1038/nature11127
PMCID: PMC3703503  PMID: 22678283
10.  IRE1 Signaling Affects Cell Fate During the Unfolded Protein Response 
Science (New York, N.Y.)  2007;318(5852):944-949.
Endoplasmic reticulum (ER) stress activates a set of signaling pathways, collectively termed the unfolded protein response (UPR). The three UPR branches (IRE1, PERK, and ATF6) promote cell survival by reducing misfolded protein levels. UPR signaling also promotes apoptotic cell death if ER stress is not alleviated. How the UPR integrates its cytoprotective and proapoptotic outputs to select between life or death cell fates is unknown. We found that IRE1 and ATF6 activities were attenuated by persistent ER stress in human cells. By contrast, PERK signaling, including translational inhibition and proapoptotic transcription regulator Chop induction, was maintained. When IRE1 activity was sustained artificially, cell survival was enhanced, suggesting a causal link between the duration of UPR branch signaling and life or death cell fate after ER stress. Key findings from our studies in cell culture were recapitulated in photoreceptors expressing mutant rhodopsin in animal models of retinitis pigmentosa.
doi:10.1126/science.1146361
PMCID: PMC3670588  PMID: 17991856
11.  A Chemical Genetic Approach Reveals Distinct Mechanisms of EphB Signaling During Brain Development 
Nature neuroscience  2012;15(12):1645-1654.
EphB receptor tyrosine kinases control multiple steps in nervous system development. However, it remains unclear whether EphBs regulate these different developmental processes directly or indirectly. In addition, as EphBs signal through multiple mechanisms, it has been challenging to define which signaling functions of EphBs regulate particular developmental events. To address these issues, we engineered triple knockin mice in which the kinase activity of three neuronally expressed EphBs can be rapidly, reversibly, and specifically blocked. Using these mice we demonstrate that the tyrosine kinase activity of EphBs is required for axon guidance in vivo. By contrast, EphB-mediated synaptogenesis occurs normally when the kinase activity of EphBs is inhibited suggesting that EphBs mediate synapse development by an EphB tyrosine kinase-independent mechanism. Taken together, these experiments reveal that EphBs control axon guidance and synaptogenesis by distinct mechanisms, and provide a new mouse model for dissecting EphB function in development and disease.
doi:10.1038/nn.3249
PMCID: PMC3509236  PMID: 23143520
12.  The translational landscape of mTOR signalling steers cancer initiation and metastasis 
Nature  2012;485(7396):55-61.
The mammalian target of rapamycin (mTOR) kinase is a master regulator of protein synthesis that couples nutrient sensing to cell growth and cancer. However, the downstream translationally regulated nodes of gene expression that may direct cancer development are poorly characterized. Using ribosome profiling, we uncover specialized translation of the prostate cancer genome by oncogenic mTOR signalling, revealing a remarkably specific repertoire of genes involved in cell proliferation, metabolism and invasion. We extend these findings by functionally characterizing a class of translationally controlled pro-invasion messenger RNAs that we show direct prostate cancer invasion and metastasis downstream of oncogenic mTOR signalling. Furthermore, we develop a clinically relevant ATP site inhibitor of mTOR, INK128, which reprograms this gene expression signature with therapeutic benefit for prostate cancer metastasis, for which there is presently no cure. Together, these findings extend our understanding of how the ‘cancerous’ translation machinery steers specific cancer cell behaviours, including metastasis, and may be therapeutically targeted.
doi:10.1038/nature10912
PMCID: PMC3663483  PMID: 22367541
13.  Kinetics of inhibitor cycling underlie therapeutic disparities between EGFR-driven lung and brain cancers 
Cancer discovery  2012;2(5):450-457.
While mutational activation of the Epidermal Growth Factor Receptor (EGFR) features prominently in glioma and non-small-cell lung cancer (NSCLC), inhibitors of EGFR improve survival only in NCSLC. To understand how mutations in EGFR influence response to therapy, we generated glioma cells expressing either glioma- or NSCLC-derived alleles, quantifying kinase site occupancy by clinical inhibitors using novel affinity probe and kinetic methodology. At equivalent doses, erlotinib achieved lower kinase site occupancy in glioma-derived EGFRvIII, compared to NSCLC-derived EGFR mutants. Kinase site occupancy correlated directly with cell cycle arrest. EGFRvIII released erlotinib rapidly compared to wild-type EGFR, whereas NSCLC-derived mutants released erlotinib slowly.
doi:10.1158/2159-8290.CD-11-0287
PMCID: PMC3354705  PMID: 22588882
Erlotinib; EGFR; malignant glioma; EGFRvIII; kinase site occupancy
14.  Separate Domains of Fission Yeast Cdk9 (P-TEFb) Are Required for Capping Enzyme Recruitment and Primed (Ser7-Phosphorylated) Rpb1 Carboxyl-Terminal Domain Substrate Recognition 
Molecular and Cellular Biology  2012;32(13):2372-2383.
In fission yeast, discrete steps in mRNA maturation and synthesis depend on a complex containing the 5′-cap methyltransferase Pcm1 and Cdk9, which phosphorylates the RNA polymerase II (Pol II) carboxyl-terminal domain (CTD) and the processivity factor Spt5 to promote transcript elongation. Here we show that a Cdk9 carboxyl-terminal extension, distinct from the catalytic domain, mediates binding to both Pcm1 and the Pol II CTD. Removal of this segment diminishes Cdk9/Pcm1 chromatin recruitment and Spt5 phosphorylation in vivo and leads to slow growth and hypersensitivity to cold temperature, nutrient limitation, and the IMP dehydrogenase inhibitor mycophenolic acid (MPA). These phenotypes, and the Spt5 phosphorylation defect, are suppressed by Pcm1 overproduction, suggesting that normal transcript elongation and gene expression depend on physical linkage between Cdk9 and Pcm1. The extension is dispensable, however, for recognition of CTD substrates “primed” by Mcs6 (Cdk7). On defined peptide substrates in vitro, Cdk9 prefers CTD repeats phosphorylated at Ser7 over unmodified repeats. In vivo, Ser7 phosphorylation depends on Mcs6 activity, suggesting a conserved mechanism, independent of chromatin recruitment, to order transcriptional CDK functions. Therefore, fission yeast Cdk9 comprises a catalytic domain sufficient for primed substrate recognition and a multivalent recruitment module that couples transcription with capping.
doi:10.1128/MCB.06657-11
PMCID: PMC3434489  PMID: 22508988
15.  Chemical genetic screen for AMPKα2 substrates uncovers a network of proteins involved in mitosis 
Molecular cell  2011;44(6):878-892.
SUMMARY
The energy-sensing AMP-activated protein kinase (AMPK) is activated by low nutrient levels. Functions of AMPK, other than its role in cellular metabolism, are just beginning to emerge. Here we use a chemical genetics screen to identify direct substrates of AMPK in human cells. We find that AMPK phosphorylates 28 previously unidentified substrates, several of which are involved in mitosis and cytokinesis. We identify the residues phosphorylated by AMPK in vivo in several substrates, including protein phosphatase 1 regulatory subunit 12C (PPP1R12C) and p21 -activated protein kinase (PAK2). AMPK-induced phosphorylation is necessary for PPP1R12C interaction with 14-3-3 and phosphorylation of myosin regulatory light chain. Both AMPK activity and PPP1R12C phosphorylation are increased in mitotic cells and are important for mitosis completion. These findings suggest that AMPK coordinates nutrient status with mitosis completion, which may be critical for the organism’s response to low nutrients during development, or in adult stem and cancer cells.
doi:10.1016/j.molcel.2011.11.005
PMCID: PMC3246132  PMID: 22137581
16.  Chemical genetic identification of NDR1/2 kinase substrates AAK1 and Rabin8 uncovers their roles in controlling dendrite arborization and spine development 
Neuron  2012;73(6):1127-1142.
Summary
Dendrite arborization and synapse formation are essential for wiring the neural circuitry. The evolutionarily conserved NDR1/2 kinase pathway, important for polarized growth from yeast to mammals, controls dendrite growth and morphology in worm and fly. Whether NDR1/2 kinases regulate dendrite and synapse development in mammals was not known. Nor have their phosphorylation targets been identified. Here we show that expression of dominant negative (kinase dead) NDR1/2 mutants or siRNA increase dendrite length and proximal branching of mammalian pyramidal neurons in cultures and in vivo, whereas expression of constitutively active NDR1/2 has the opposite effects. Moreover, NDR1/2 contributes to dendritic spine development and excitatory synaptic function. We further employed chemical genetics and identified NDR1/2 substrates in the brain, including two proteins involved in intracellular vesicle trafficking: AAK1 (AP-2 associated kinase) and Rabin8, a GDP/GTP exchange factor (GEF) of Rab8 GTPase. We finally show that AAK1 contributes to dendrite growth regulation and Rabin8 regulates spine development.
doi:10.1016/j.neuron.2012.01.019
PMCID: PMC3333840  PMID: 22445341
17.  Orm protein phosphoregulation mediates transient sphingolipid biosynthesis response to heat stress via the Pkh-Ypk and Cdc55-PP2A pathways 
Molecular Biology of the Cell  2012;23(12):2388-2398.
This study reveals the basis for how temporal phosphoregulation of Orm protein controls sphingolipid production in response to stress. Orm protein phosphorylation is highly responsive to sphingoid bases, and Ypk1 protein kinase transmits heat stress signals to the sphingolipid biosynthesis pathway via Orm phosphorylation.
Sphingoid intermediates accumulate in response to a variety of stresses, including heat, and trigger cellular responses. However, the mechanism by which stress affects sphingolipid biosynthesis has yet to be identified. Recent studies in yeast suggest that sphingolipid biosynthesis is regulated through phosphorylation of the Orm proteins, which in humans are potential risk factors for childhood asthma. Here we demonstrate that Orm phosphorylation status is highly responsive to sphingoid bases. We also demonstrate, by monitoring temporal changes in Orm phosphorylation and sphingoid base production in cells inhibited for yeast protein kinase 1 (Ypk1) activity, that Ypk1 transmits heat stress signals to the sphingolipid biosynthesis pathway via Orm phosphorylation. Our data indicate that heat-induced sphingolipid biosynthesis in turn triggers Orm protein dephosphorylation, making the induction transient. We identified Cdc55–protein phosphatase 2A (PP2A) as a key phosphatase that counteracts Ypk1 activity in Orm-mediated sphingolipid biosynthesis regulation. In total, our study reveals a mechanism through which the conserved Pkh-Ypk kinase cascade and Cdc55-PP2A facilitate rapid, transient sphingolipid production in response to heat stress through Orm protein phosphoregulation. We propose that this mechanism serves as the basis for how Orm phosphoregulation controls sphingolipid biosynthesis in response to stress in a kinetically coupled manner.
doi:10.1091/mbc.E12-03-0209
PMCID: PMC3374756  PMID: 22535525
18.  Chemical Genetics Reveals a Specific Requirement for Cdk2 Activity in the DNA Damage Response and Identifies Nbs1 as a Cdk2 Substrate in Human Cells 
PLoS Genetics  2012;8(8):e1002935.
The cyclin-dependent kinases (CDKs) that promote cell-cycle progression are targets for negative regulation by signals from damaged or unreplicated DNA, but also play active roles in response to DNA lesions. The requirement for activity in the face of DNA damage implies that there are mechanisms to insulate certain CDKs from checkpoint inhibition. It remains difficult, however, to assign precise functions to specific CDKs in protecting genomic integrity. In mammals, Cdk2 is active throughout S and G2 phases, but Cdk2 protein is dispensable for survival, owing to compensation by other CDKs. That plasticity obscured a requirement for Cdk2 activity in proliferation of human cells, which we uncovered by replacement of wild-type Cdk2 with a mutant version sensitized to inhibition by bulky adenine analogs. Here we show that transient, selective inhibition of analog-sensitive (AS) Cdk2 after exposure to ionizing radiation (IR) enhances cell-killing. In extracts supplemented with an ATP analog used preferentially by AS kinases, Cdk2as phosphorylated the Nijmegen Breakage Syndrome gene product Nbs1—a component of the conserved Mre11-Rad50-Nbs1 complex required for normal DNA damage repair and checkpoint signaling—dependent on a consensus CDK recognition site at Ser432. In vivo, selective inhibition of Cdk2 delayed and diminished Nbs1-Ser432 phosphorylation during S phase, and mutation of Ser432 to Ala or Asp increased IR–sensitivity. Therefore, by chemical genetics, we uncovered both a non-redundant requirement for Cdk2 activity in response to DNA damage and a specific target of Cdk2 within the DNA repair machinery.
Author Summary
Multiple cyclin-dependent kinases (CDKs) control human cell proliferation, but it remains unclear how functions of different CDKs are coordinated during unperturbed cell division or after dividing cells incur DNA damage. DNA lesions activate checkpoint signaling pathways to inhibit CDK activity, arrest the cell division cycle, and thus prevent loss of genetic information; but an effective response to damage also requires CDK activity to modify components of repair and checkpoint pathways. We took a chemical-genetic approach to ask if a specific CDK, Cdk2, played a specialized, non-redundant role in protecting genomic integrity of human cells. By sensitizing Cdk2 to chemical inhibition, we were able to detect a specific requirement for its catalytic activity in survival of cells after exposure to ionizing radiation (IR). We identified Nbs1, product of the gene mutated in the cancer-predisposing Nijmegen Breakage Syndrome, as a Cdk2 substrate and showed that mutant forms of Nbs1 that cannot be modified by Cdk2 are defective in protecting cells from death due to IR–induced DNA damage. Therefore, our work defines a DNA damage response pathway that depends on catalytic activity of a specific CDK in human cells and suggests a mechanism to promote efficient repair without triggering inappropriate cell division.
doi:10.1371/journal.pgen.1002935
PMCID: PMC3426557  PMID: 22927831
19.  Integrin α9β1 in airway smooth muscle suppresses exaggerated airway narrowing  
The Journal of Clinical Investigation  2012;122(8):2916-2927.
Exaggerated contraction of airway smooth muscle is the major cause of symptoms in asthma, but the mechanisms that prevent exaggerated contraction are incompletely understood. Here, we showed that integrin α9β1 on airway smooth muscle localizes the polyamine catabolizing enzyme spermidine/spermine N1-acetyltransferase (SSAT) in close proximity to the lipid kinase PIP5K1γ. As PIP5K1γ is the major source of PIP2 in airway smooth muscle and its activity is regulated by higher-order polyamines, this interaction inhibited IP3-dependent airway smooth muscle contraction. Mice lacking integrin α9β1 in smooth muscle had increased airway responsiveness in vivo, and loss or inhibition of integrin α9β1 increased in vitro airway narrowing and airway smooth muscle contraction in murine and human airways. Contraction was enhanced in control airways by the higher-order polyamine spermine or by cell-permeable PIP2, but these interventions had no effect on airways lacking integrin α9β1 or treated with integrin α9β1–blocking antibodies. Enhancement of SSAT activity or knockdown of PIP5K1γ inhibited airway contraction, but only in the presence of functional integrin α9β1. Therefore, integrin α9β1 appears to serve as a brake on airway smooth muscle contraction by recruiting SSAT, which facilitates local catabolism of polyamines and thereby inhibits PIP5K1γ. Targeting key components of this pathway could thus lead to new treatment strategies for asthma.
doi:10.1172/JCI60387
PMCID: PMC3448403  PMID: 22772469
20.  Switching Cdk2 On or Off with Small Molecules to Reveal Requirements in Human Cell Proliferation 
Molecular cell  2011;42(5):624-636.
Summary
Multiple cyclin-dependent kinases (CDKs) control eukaryotic cell division, but assigning specific functions to individual CDKs remains a challenge. During the mammalian cell cycle, Cdk2 forms active complexes before Cdk1, but lack of Cdk2 protein does not block cell-cycle progression. To detect requirements and define functions for Cdk2 activity in human cells when normal expression levels are preserved, and non-physiologic compensation by other CDKs is prevented, we replaced the wild-type kinase with a version sensitized to specific inhibition by bulky adenine analogs. The sensitizing mutation also impaired a non-catalytic function of Cdk2 in restricting assembly of cyclin A with Cdk1, but this defect could be corrected by both inhibitory and non-inhibitory analogs. This allowed either chemical rescue or selective antagonism of Cdk2 activity in vivo, to uncover a requirement in cell proliferation, and non-redundant, rate-limiting roles in restriction point passage and S-phase entry.
doi:10.1016/j.molcel.2011.03.031
PMCID: PMC3119039  PMID: 21658603
21.  TrkB and PKMζ regulate synaptic localization of PSD-95 in developing cortex 
Post-synaptic density 95 (PSD-95), the major scaffold at excitatory synapses, is critical for synapse maturation and learning. In rodents, eye opening, the onset of pattern vision, triggers a rapid movement of PSD-95 from visual neuron somata to synapses. We previously showed that the PI3 kinase-Akt pathway downstream of BDNF/TrkB signaling stimulates synaptic delivery of PSD-95 via vesicular transport. However, vesicular transport requires PSD-95 palmitoylation to attach it to a lipid membrane. Also PSD-95 insertion at synapses is known to require this lipid modification. Here, we show that BDNF/TrkB signaling is also necessary for PSD-95 palmitoylation and its transport to synapses in mouse visual cortical layer 2/3 neurons. However, palmitoylation of PSD-95 requires the activation of another pathway downstream of BDNF/TrkB, namely signaling through PLCγ and the brain-specific PKC variant PKMζ. We find that PKMζ selectively regulates phosphorylation of the palmitoylation enzyme ZDHHC8. Inhibition of PKMζ results in a reduction of synaptic PSD-95 accumulation in vivo, which can be rescued by over-expression ZDHHC8. Therefore, TrkB and PKMζ, two critical regulators of synaptic plasticity, facilitate PSD-95 targeting to synapses. These results also indicate that palmitoylation can be regulated by a trophic factor. Our findings have implications for neurodevelopmental disorders as well as ageing brains.
doi:10.1523/JNEUROSCI.2190-11.2011
PMCID: PMC3158490  PMID: 21849550
22.  Generation of a Novel System for Studying Spleen Tyrosine Kinase Function in Macrophages and B Cells1 
Spleen tyrosine kinase (Syk) is a nonreceptor tyrosine kinase that is expressed primarily in hematopoietic cells. Because this protein has been implicated in processes such as Fc-mediated phagocytosis, BCR signaling, oxidative burst, degranulation, cytokine secretion, and integrin-mediated outside-in signaling, it is hypothesized that Syk may be a viable target in the treatment of a variety of autoimmune and inflammatory diseases. Because efforts to design a small-molecule therapeutic that specifically inhibits Syk have been largely unsuccessful, and genetic studies of Syk have been hampered by the fact that syk−/− mice die in utero, we have taken a chemical genetic approach to study the function of Syk. Specifically, we have created a mutant form of Syk that retains its wild-type function, but is susceptible to inhibition by enlarged derivatives of the tyrosine kinase inhibitor, PP1. We report in this study that Syk M442A S505A reconstituted wild-type function when introduced into murine syk−/− bone marrow-derived macrophages and syk−/− DT40 chicken B cells, as determined by functional and biochemical assays. Furthermore, after screening a series of PP1 derivatives, we identified one compound, namely 2,3-DMB-PP1, that specifically inhibited Syk M442A S505A, but not wild-type Syk. This system provides us with the power to characterize immune functions that are Syk specific, and furthermore, it provides us with a tool to assess how inhibition of Syk may alter an immune response and influence disease pathogenesis and/or progression.
PMCID: PMC3248399  PMID: 19124742
23.  Access Denied: Snf1 Activation Loop Phosphorylation Is Controlled by Availability of the Phosphorylated Threonine 210 to the PP1 Phosphatase* 
The Journal of biological chemistry  2007;283(1):222-230.
Phosphorylation of the Saccharomyces cerevisiae Snf1 kinase activation loop is determined by the integration of two reaction rates: the rate of phosphorylation by upstream kinases and the rate of dephosphorylation by Glc7. The activities of the Snf1-activating kinases do not appear to be glucose-regulated, since immune complex kinase assays with each of the three Snf1-activating kinases show similar levels of activity when prepared from cells grown in either high or low glucose. In contrast, the dephosphorylation of the Snf1 activation loop was strongly regulated by glucose. When de novo phosphorylation of Snf1 was inhibited, phosphorylation of the Snf1 activation loop was found to be stable in low glucose but rapidly lost upon the addition of glucose. A greater than 10-fold difference in the rates of Snf1 activation loop dephosphorylation was detected. However, the activity of the Glc7-Reg1 phosphatase may not itself be directly regulated by glucose, since the Glc7-Reg1 enzyme was active in low glucose toward another substrate, the transcription factor Mig1. Glucose-mediated regulation of Snf1 activation loop dephosphorylation is controlled by changes in the ability of the Snf1 activation loop to act as a substrate for Glc7.
doi:10.1074/jbc.M707957200
PMCID: PMC3244878  PMID: 17991748
24.  Generation of a set of conditional analog-sensitive alleles of essential protein kinases in the fission yeast Schizosaccharomyces pombe 
Cell Cycle  2011;10(20):3527-3532.
The genome of the fission yeast Schizosaccharomyces pombe encodes for 17 protein kinases that are essential for viability. Studies of the essential kinases often require the use of mutant strains carrying conditional alleles. To inactivate these kinases conditionally, we applied a recently developed chemical genetic strategy. The mutation of a single residue in the ATP-binding pocket confers sensitivity to small-molecule inhibitors, allowing for specific inactivation of the modified kinase. Using this approach, we constructed conditional analog-sensitive alleles of 13 essential protein kinases in the fission yeast S. pombe.
doi:10.4161/cc.10.20.17792
PMCID: PMC3266181  PMID: 22030861
kinase; analog-sensitive; conditional allele; fission yeast; phosphorylation

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