Phosphatidylinositol 5-phosphate (PtdIns5P), a novel inositol lipid, has been shown to be the major substrate for the type II PtdInsP kinases (PIPkins) ¿Rameh et al. (1997) Nature 390, 192-196. A PtdInsP fraction was prepared from cell extracts by neomycin chromatography, using a protocol devised to eliminate the interaction of acidic solvents with plasticware, since this was found to inhibit the enzyme. The PtdIns5P in this fraction was measured by incubating with ¿gamma-(32)PATP and recombinant PIPkin IIalpha, and quantifying the radiolabelled PtdInsP(2) formed. This assay was used on platelets to show that during 10 min stimulation with thrombin, the mass level of PtdIns5P increases, implying the existence of an agonist-stimulated synthetic mechanism.
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"c o m / l o c a t e / b b a m c r . Studies revealed that the separate administration of either plateletderived growth factor, vasopressin, prostaglandin, bombesin or EGF evokes Ca 2+ transients and also induces inositol trisphosphate (InsP 3 ) production  . In Swiss 3T3 cells, serum-induced Ca 2+ increases are essential, but not sufficient to induce NF-κB activation and subsequent DNA synthesis . "
"A binding assay and an enzymatic assay that utilized an elaborate liposome-based substrate preparation have been previously reported , . Additionally, low-throughput radiolabeled enzymatic thin layer chromatography (TLC) assays were developed where the substrate was prepared in an isotonic KCl solution or as a liposome mixture , . The radiometric assay uses γ-32P-ATP and PI5P and measures radiolabeled enzymatic product, PI(4,5)P2 after the separation by TLC , , . "
[Show abstract][Hide abstract] ABSTRACT: Phosphoinositide kinases regulate diverse cellular functions and are important targets for therapeutic development for diseases, such as diabetes and cancer. Preparation of the lipid substrate is crucial for the development of a robust and miniaturizable lipid kinase assay. Enzymatic assays for phosphoinositide kinases often use lipid substrates prepared from lyophilized lipid preparations by sonication, which result in variability in the liposome size from preparation to preparation. Herein, we report a homogeneous 1536-well luciferase-coupled bioluminescence assay for PI5P4Kα. The substrate preparation is novel and allows the rapid production of a DMSO-containing substrate solution without the need for lengthy liposome preparation protocols, thus enabling the scale-up of this traditionally difficult type of assay. The Z'-factor value was greater than 0.7 for the PI5P4Kα assay, indicating its suitability for high-throughput screening applications. Tyrphostin AG-82 had been identified as an inhibitor of PI5P4Kα by assessing the degree of phospho transfer of γ-(32)P-ATP to PI5P; its inhibitory activity against PI5P4Kα was confirmed in the present miniaturized assay. From a pilot screen of a library of bioactive compounds, another tyrphostin, I-OMe tyrphostin AG-538 (I-OMe-AG-538), was identified as an ATP-competitive inhibitor of PI5P4Kα with an IC(50) of 1 µM, affirming the suitability of the assay for inhibitor discovery campaigns. This homogeneous assay may apply to other lipid kinases and should help in the identification of leads for this class of enzymes by enabling high-throughput screening efforts.
"However, specific mass assays and imaging using high affinity probes like the plant homeodomain (PHD) of the inhibitor of growth 2 (ING2) lead several groups to suggest that PtdIns5P may act as a second messenger both in the cytoplasm and the nucleus (Gozani et al., 2005; Gozani et al., 2003; Morris et al., 2000). The amount of PtdIns5P can be increased by different stimuli such as thrombin in platelets, insulin in 3T3-L1 cells, T cell receptor engagement, increased tyrosine kinase activity, and stress signals (Grainger et al., 2011; Morris et al., 2000; Pizarro-Cerda and Cossart, 2004; Sbrissa et al., 2001; Sbrissa et al., 2002). In addition, an increase in PtdIns5P was reported during the G1 phase of the cell cycle (Choudhury et al., 2006). "
[Show abstract][Hide abstract] ABSTRACT: The level and turnover of phosphoinositides (PIs) are tightly controlled by a large set of PI-specific enzymes (PI kinases and phosphatases). Mammalian PI phosphatases are conserved through evolution and among this large family the dual-specificity phosphatase (PTP/DSP) are metal-independent enzymes displaying the amino acid signature Cys-X5-Arg-Thr/Ser (CX5RT/S) in their active site. Such catalytic site characterizes the myotubularin 3-phosphatases that dephosphorylate PtdIns3P and PtdIns(3,5)P
2 and produce PtdIns5P. Substrates of myotubularins have been implicated in endocytosis and membrane trafficking while PtdIns5P may have a role in signal transduction. As a paradox, 6 of the 14 members of the myotubularin family lack enzymatic activity and are considered as dead phosphatases. Several myotubularins have been genetically linked to human diseases: MTM1 is mutated in the congenital myopathy X-linked centronuclear or myotubular myopathy (XLCNM) and MTMR14 (JUMPY) has been linked to an autosomal form of such disease, while MTMR2 and MTMR13 are mutated in Charcot-Marie-Tooth (CMT) neuropathies. Furthermore, recent evidences from genetic association studies revealed that several other myotubularins could be associated to chronic disorders such as cancer and obesity, highlighting their importance for human health. Here, we discuss cellular and physiological roles of myotubularins and their implication in human diseases, and we present potential pathological mechanisms affecting specific tissues in myotubularin-associated diseases.
Full-text · Article · Oct 2012 · Current topics in microbiology and immunology