Protocols for regulation and study of diphosphoinositol polyphosphates

The inositol pyrophosphate pathway in health and diseases

Inositol pyrophosphates (IPPs) are present in organisms ranging from plants, slime moulds and fungi to mammals. Distinct classes of kinases generate different forms of energetic diphosphate-containing IPPs from inositol phosphates (IPs). Conversely, polyphosphate phosphohydrolase enzymes dephosphorylate IPPs to regenerate the respective IPs. IPPs and/or their metabolizing enzymes regulate various cell biological processes by modulating many proteins via diverse mechanisms. In the last decade, extensive research has been conducted in mammalian systems, particularly in knockout mouse models of relevant enzymes. Results obtained from these studies suggest impacts of the IPP pathway on organ development, especially of brain and testis. Conversely, deletion of specific enzymes in the pathway protects mice from various diseases such as diet-induced obesity (DIO), type-2 diabetes (T2D), fatty liver, bacterial infection, thromboembolism, cancer metastasis and aging. Furthermore, pharmacological inhibition of the same class of enzymes in mice validates the therapeutic importance of this pathway in cardio-metabolic diseases. This review critically analyses these findings and summarizes the significance of the IPP pathway in mammalian health and diseases. It also evaluates benefits and risks of targeting this pathway in disease therapies. Finally, future directions of mammalian IPP research are discussed.

The enzymes of human diphosphoinositol polyphosphate metabolism

Diphospho-myo-inositol polyphosphates have many roles to play, including roles in apoptosis, vesicle trafficking, the response of cells to stress, the regulation of telomere length and DNA damage repair, and inhibition of the cyclin-dependent kinase Pho85 system that monitors phosphate levels. This review focuses on the three classes of enzymes involved in the metabolism of these compounds: inositol hexakisphosphate kinases, inositol hexakisphosphate and diphosphoinositol-pentakisphosphate kinases and diphosphoinositol polyphosphate phosphohydrolases. However, these enzymes have roles beyond being mere catalysts, and their interactions with other proteins have cellular consequences. Through their interactions, the three inositol hexakisphosphate kinases have roles in exocytosis, diabetes, the response to infection, and apoptosis. The two inositol hexakisphosphate and diphosphoinositol-pentakisphosphate kinases influence the cellular response to phosphatidylinositol (3,4,5)-trisphosphate and the migration of pleckstrin homology domain-containing proteins to the plasma membrane. The five diphosphoinositol polyphosphate phosphohydrolases interact with ribosomal proteins and transcription factors, as well as proteins involved in membrane trafficking, exocytosis, ubiquitination and the proteasomal degradation of target proteins. Possible directions for future research aiming to determine the roles of these enzymes are highlighted.

Inositol pyrophosphates as mammalian cell signals

Inositol pyrophosphates are highly energetic inositol polyphosphate molecules present in organisms from slime molds and yeast to mammals. Distinct classes of enzymes generate different forms of inositol pyrophosphates. The biosynthesis of these substances principally involves phosphorylation of inositol hexakisphosphate (IP₆) to generate the pyrophosphate IP₇. Initial insights into functions of these substances derived primarily from yeast, which contain a single isoform of IP₆ kinase (yIP₆K), as well as from the slime mold Dictyostelium. Mammalian functions for inositol pyrophosphates have been investigated by using cell lines to establish roles in various processes, including insulin secretion and apoptosis. More recently, mice with targeted deletion of IP₆K isoforms as well as the related inositol polyphosphate multikinase (IPMK) have substantially enhanced our understanding of inositol polyphosphate physiology. Phenotypic alterations in mice lacking inositol hexakisphosphate kinase 1 (IP₆K1) reveal signaling roles for these molecules in insulin homeostasis, obesity, and immunological functions. Inositol pyrophosphates regulate these processes at least in part by inhibiting activation of the serine-threonine kinase Akt. Similar studies of IP₆K2 establish this enzyme as a cell death inducer acting by stimulating the proapoptotic protein p53. IPMK is responsible for generating the inositol phosphate IP₅ but also has phosphatidylinositol 3-kinase activity--that participates in activation of Akt. Here, we discuss recent advances in understanding the physiological functions of the inositol pyrophosphates based in substantial part on studies in mice with deletion of IP₆K isoforms. These findings highlight the interplay of IPMK and IP₆K in regulating growth factor and nutrient-mediated cell signaling.

The nucleolus exhibits an osmotically regulated gatekeeping activity that controls the spatial dynamics and functions of nucleolin

We demonstrate that physiologically relevant perturbations in the osmotic environment rheostatically regulate a gatekeeping function for the nucleolus that controls the spatial dynamics and functions of nucleolin. HeLa cells and U2-OS osteosarcoma cells were osmotically challenged with 100-200 mm sorbitol, and the intranuclear distribution of nucleolin was monitored by confocal microscopy. Nucleolin that normally resides in the innermost fibrillar core of the nucleolus, where it assists rDNA transcription and replication, was expelled within 30 min of sorbitol addition. The nucleolin was transferred into the nucleoplasm, but it distributed there non-uniformly; locally high levels accumulated in 4',6-diamidino-2-phenylindole-negative zones containing euchromatic (transcriptionally active) DNA. Inositol pyrophosphates also responded within 30 min of hyperosmotic stress: levels of bisdiphosphoinositol tetrakisphosphate increased 6-fold, and this was matched by decreased levels of its precursor, diphosphoinositol pentakisphosphate. Such fluctuations in inositol pyrophosphate levels are of considerable interest, because, according to previously published in vitro data, they regulate the degree of phosphorylation of nucleolin through a novel kinase-independent phosphotransferase reaction ( Saiardi, A., Bhandari, A., Resnick, R., Cain, A., Snowman, A. M., and Snyder, S. H. (2004) Science 306, 2101-2105 ). However, by pharmacologically intervening in inositol pyrophosphate metabolism, we found that it did not supervise the osmotically driven switch in the biological activities of nucleolin in vivo.

Purification, sequencing, and molecular identification of a mammalian PP-InsP5 kinase that is activated when cells are exposed to hyperosmotic stress

Mammalian cells utilize multiple signaling mechanisms to protect against the osmotic stress that accompanies plasma membrane ion transport, solute uptake, and turnover of protein and carbohydrates (Schliess, F., and Haussinger, D. (2002) Biol. Chem. 383, 577-583). Recently, osmotic stress was found to increase synthesis of bisdiphosphoinositol tetrakisphosphate ((PP)2-InsP4), a high energy inositol pyrophosphate (Pesesse, X., Choi, K., Zhang, T., and Shears, S. B. (2004) J. Biol. Chem. 279, 43378-43381). Here, we describe the purification from rat brain of a diphosphoinositol pentakisphosphate kinase (PPIP5K) that synthesizes (PP)2-InsP4. Partial amino acid sequence, obtained by mass spectrometry, matched the sequence of a 160-kDa rat protein containing a putative ATP-grasp kinase domain. BLAST searches uncovered two human isoforms (PPIP5K1 (160 kDa) and PPIP5K2 (138 kDa)). Recombinant human PPIP5K1, expressed in Escherichia coli, was found to phosphorylate diphosphoinositol pentakisphosphate (PP-InsP5) to (PP)2-InsP4 (Vmax = 8.3 nmol/mg of protein/min; Km = 0.34 microM). Overexpression in human embryonic kidney cells of either PPIP5K1 or PPIP5K2 substantially increased levels of (PP)2-InsP4, whereas overexpression of a catalytically dead PPIP5K1(D332A) mutant had no effect. PPIP5K1 and PPIP5K2 were more active against PP-InsP5 than InsP6, both in vitro and in vivo. Analysis by confocal immunofluorescence showed PPIP5K1 to be distributed throughout the cytoplasm but excluded from the nucleus. Immunopurification of overexpressed PPIP5K1 from osmotically stressed HEK cells (0.2 M sorbitol; 30 min) revealed a persistent, 3.9 +/- 0.4-fold activation when compared with control cells. PPIP5Ks are likely to be important signaling enzymes.

Signal transduction during environmental stress: InsP8 operates within highly restricted contexts

Genetic manipulation of diphosphoinositol polyphosphate synthesis impacts many biological processes (reviewed in S.B. Shears, Biochem. J. 377, 2004, 265-280). These observations lacked a cell-signalling context, until the recent discovery that bis-diphosphoinositol tetrakisphosphate ([PP]2-InsP4 or "InsP8") accumulates rapidly in mammalian cells in response to hyperosmotic stress (X. Pesesse, K. Choi, T. Zhang, and S. B. Shears J. Biol. Chem. 279, 2004, 43378-43381). We now investigate how widely applicable is this new stress-response. [PP]2-InsP4 did not respond to mechanical strain or oxidative stress in mammalian cells. Furthermore, despite tight conservation of many molecular stress responses across the phylogenetic spectrum, we show that cellular [PP]2-InsP4 levels do not respond significantly to osmotic imbalance, heat stress and salt toxicity in Saccharomyces cerevisiae. In contrast, we show that [PP]2-InsP4 is a novel sensor of mild thermal stress in mammalian cells: [PP]2-InsP4 levels increased 3-4 fold when cells were cooled from 37 to 33 degrees C, or heated to 42 degrees C. Increases in [PP]2-InsP4 levels following heat-shock were evident <5 min, and reversible (t(1/2)=7 min) once cells were returned to 37 degrees C. These responses were blocked by pharmacological inhibition of the ERK/MEK pathway. Additional control processes may lie upstream of [PP]2-InsP4 synthesis, which was synergistically activated when heat stress and osmotic stress were combined. Our data add to the repertoire of signaling responses following thermal challenges, a topic of current interest for its possible therapeutic value.