Key Points AZD1208 is a selective pan-Pim kinase inhibitor with efficacy in AML cells, xenografts, and Flt3-internal tandem duplication or Flt3 wild-type patient samples. AML cell growth inhibition is associated with suppression of p70S6K, 4EBP1 phosphorylation, and messenger RNA translation.
We report the X-ray crystal structure of a phosphodiesterase (PDE) that includes both catalytic and regulatory domains. PDE2A (215-900) crystallized as a dimer in which each subunit had an extended organization of regulatory GAF-A and GAF-B and catalytic domains connected by long ␣-helices. The subunits cross at the GAF-B/ catalytic domain linker, and each side of the dimer contains in series the GAF-A and GAF-B of one subunit and the catalytic domain of the other subunit. A dimer interface extends over the entire length of the molecule. The substrate binding pocket of each catalytic domain is occluded by the H-loop. We deduced from comparisons with structures of isolated, ligand-bound catalytic subunits that the H-loop swings out to allow substrate access. However, in dimeric PDE2A (215-900), the H-loops of the two catalytic subunits pack against each other at the dimer interface, necessitating movement of the catalytic subunits to allow for H-loop movement. Comparison of the unliganded GAF-B of PDE2A (215-900) with previous structures of isolated, cGMP-bound GAF domains indicates that cGMP binding induces a significant shift in the GAF-B/catalytic domain linker. We propose that cGMP binding to GAF-B causes movement, through this linker region, of the catalytic domains, such that the H-loops no longer pack at the dimer interface and are, instead, free to swing out to allow substrate access. This increase in substrate access is proposed as the basis for PDE2A activation by cGMP and may be a general mechanism for regulation of all PDEs.cGMP activation ͉ GAF domains ͉ PDE-2A T he cyclic nucleotides cAMP and cGMP are ubiquitous intracellular signaling molecules that mediate a vast array of biological processes throughout the body. The means by which these two molecules participate in diverse functions, in different cell types and within single cells, involves tight regulation of the spatial and temporal residence of their concentrations. The phosphodiesterases (PDEs) are a superfamily of enzymes that metabolically inactivate cAMP and cGMP (1) to play key roles in both these aspects of regulation. The PDEs are modular enzymes characterized by a relatively conserved C-terminal catalytic domain and a more variable N-terminal domain involved in regulation of activity, subcellular localization, and interactions with other proteins. There are 11 PDE gene families, with different families encoded by one to four genes, and further diversity derived from alternative splicing. The PDE families differ broadly in specificity and affinity for cAMP and cGMP. Much has been learned about the molecular bases for these differences from studies of X-ray crystal structures of the catalytic domains of
Chronic activity blockade increases synaptic levels of NMDA receptor immunoreactivity in hippocampal neurons. We show here that blockade-induced synaptic NMDA receptors are functional and mediate enhanced excitotoxicity in response to synaptically released glutamate. Activity blockade increased the cell surface association of NMDA receptors. Blockade-induced synaptic targeting of NMDA receptors did not require protein synthesis but required phosphorylation and specifically cAMPdependent protein kinase (PKA). Furthermore, activation of PKA was sufficient to induce synaptic targeting of NMDA receptors regardless of receptor activity status. These results implicate PKA activity downstream of receptor blockade as a mediator of enhanced synaptic transport or stabilization of NMDA receptors. Synaptic clustering of NR1-green fluorescent protein was observed in living neurons in response to NMDA receptor and cAMP phosphodiesterase antagonists and occurred gradually over the course of a day. This pathway represents a cellular mechanism for synaptic homeostasis and is likely to function in metaplasticity, long-term regulation of the ability of a synapse to undergo potentiation or depression. Key words: NMDA receptor; synaptogenesis; activity; synaptic clustering; excitotoxicity; subcellular localization; hippocampus; NR1-GFPThe NMDA-type glutamate receptor plays a central role in circuit development, memory formation, and many forms of synaptic plasticity in the mammalian brain. The NMDA receptor is composed of the essential NR1 subunit and one or more of the modulatory NR2A-D and NR3 subunits (Nakanishi, 1992;Seeburg, 1993;Mori and Mishina, 1995). NMDA receptor channel opening requires ligand binding (by presynaptic glutamate release) and removal of Mg 2ϩ block (by postsynaptic depolarization), thus conferring on the NMDA receptor the ability to function as a molecular coincidence detector (Mayer et al., 1984). Through its Ca 2ϩ permeability, NMDA receptor function is linked with many downstream signal transducing pathways in the neuron. The magnitude and kinetics of calcium elevation at the synapse are thought to be major determinants of long-term effects on synaptic efficacy (Lisman, 1989;Abraham and Bear, 1996).The level of NMDA receptor function at the synapse critically regulates brain function and cell survival. Mice expressing 5% of normal levels of NR1 exhibit increased motor activity, stereotypy, and deficits in social and sexual interactions, behaviors associated with schizophrenia (Mohn et al., 1999). Deletion of NR1 targeted postnatally selectively to CA1 of the hippocampus results in mice that are viable but deficient in spatial learning and formation of temporal memory (Tsien et al., 1996;Huerta et al., 2000). In contrast, overactivation of NMDA receptors contributes substantially to neuronal death during epilepsy, stroke, trauma, and neurodegenerative disorders (McDonald and Johnston, 1990;Choi, 1994;Rothman and Olney, 1995;During et al., 2000).NMDA receptor function is regulated during development and by ...
The phosphodiesterases (PDEs) are metal ion-dependent enzymes that regulate cellular signaling by metabolic inactivation of the ubiquitous second messengers cAMP and cGMP. In this role, the PDEs are involved in many biological and metabolic processes and are proven targets of successful drugs for the treatments of a wide range of diseases. However, because of the rapidity of the hydrolysis reaction, an experimental knowledge of the enzymatic mechanisms of the PDEs at the atomic level is still lacking. Here, we report the structures of reaction intermediates accumulated at the reaction steady state in PDE9/crystal and preserved by freeze-trapping. These structures reveal the catalytic process of a PDE and explain the substrate specificity of PDE9 in an actual reaction and the cation requirements of PDEs in general.crystallography ͉ enzyme mechanism ͉ reaction intermediates ͉ freeze-trapping T he phosphodiesterases (PDEs) are a superfamily of enzymes that metabolically inactivate the ubiquitous intracellular messengers cAMP and cGMP. This function involves the PDEs in a broad range of important cellular functions, such as immune response, memory, and vision (1-4). The human genome encodes for 21 PDEs that are categorized into 11 families (2). Alternative splicing results in the generation of Ͼ60 identified isoforms. These enzymes share a conserved catalytic domain of approximately 300 aa that is located in the C-terminal region of the protein. The N-terminal regions, which vary among different PDEs, serve regulatory functions including autoinhibition of the catalytic domains or control of subcellular localization (2). The PDEs have different substrate preferences: PDE 4, -7, and -8 preferentially hydrolyze cAMP; PDE5, -6, and -9 are cGMP specific. PDE1, -2, -3, -10, and -11 can hydrolyze both cyclic nucleotides (2). The different substrate preferences, combined with different expression profiles, cellular compartmentalization, and regulation, allow the PDEs to play a very versatile role in cell signal transduction (5).It is becoming increasingly clear that the physiological role of PDEs is the temporal and spatial control of cyclic nucleotide signaling, not simply inactivation (1, 2, 6). The clearest example of this control is in the fast action of PDE6 required for the temporal resolution of human vision (4). However, a complete understanding of the catalytic mechanism of these enzymes that accounts for the substrate specificity and reaction kinetics at the atomic level is lacking. Crystal structures of the catalytic domains of PDE1B, -2, -3, -4, -5, -7, -9, and -10, by themselves or in complex with inhibitors, substrates, or products, have been reported (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Generally, the active sites of PDEs can be divided into 2 parts: a nucleotide recognition pocket and a hydrolysis center (7,19). Each PDE has its unique nucleotide recognition pocket, resulting in different substrate specificity and inhibition profiles (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Based on t...
Thymidylate kinase (TMK) is a potential chemotherapeutic target because it is directly involved in the synthesis of an essential component, thymidine triphosphate, in DNA replication. All reported TMK inhibitors are thymidine analogs, which might retard their development as potent therapeutics due to cell permeability and off-target activity against human TMK. A small molecule hit (1, IC50 = 58 μM), which has reasonable inhibition potency against Pseudomonas aeruginosa TMK (PaTMK), was identified by the analysis of the binding mode of thymidine or TP5A in a PaTMK homology model. This hit (1) was co-crystallized with PaTMK, and several potent PaTMK inhibitors (leads, 46, 47, 48, and 56, IC50 = 100–200 nM) were synthesized using computer aided design approaches including virtual synthesis/screening, which was used to guide the design of inhibitors. The binding mode of the optimized leads in PaTMK overlaps with that of other bacterial TMKs, but not with human TMK which shares few common features with the bacterial enzymes. Therefore, the optimized TMK inhibitors described here should be useful for the development of antibacterial agents targeting TMK without undesired off-target effects. In addition, an inhibition mechanism associated with the LID loop, which mimics the process of phosphate transfer from ATP to dTMP, was proposed based on X-ray co-crystal structures, homology models, and SAR results.
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