Roles for the phosphatidylinositol cycle in early development

Developmental expression of the inositol 1,4,5-trisphosphate receptor and localization of inositol 1,4,5-trisphosphate during early embryogenesis in Xenopus laevis

To study the role of inositol 1,4,5-trisphosphate (IP3) receptors during early embryogenesis in Xenopus, we examined the temporal-spatial localization of Xenopus IP3 receptor (XIP3R). XIP3R protein is enriched in the animal hemisphere of early cleavage stage embryos and becomes localized in the ectoderm and involuted mesoderm in gastrula stage embryos. Up to tailbud stages, expression of XIP3R is observed in the mesodermal tissues and in most subregions of the central nervous system. A quantitative analysis of endogenous IP3 mass during normal early embryogenesis revealed an increase in IP3 mass first observed at early gastrula stage of 10.5 with an enrichment in the ectoderm throughout the gastrula stages, implying a potential role during gastrulation.

Lithium blocks cell cycle transitions in the first cell cycles of sea urchin embryos, an effect rescued by myo-inositol

Lithium is a classical inhibitor of the phosphoinositide pathway and is teratogenic. We report the effects of lithium on the first cell cycles of sea urchin (Lytechinus pictus) embryos. Embryos cultured in 400 mM lithium chloride sea water showed marked delay to the cell cycle and a tendency to arrest prior to nuclear envelope breakdown, at metaphase and at cytokinesis. After removal of lithium, the block was reversed and embryos developed to form normal late blastulae. The lithium-induced block was also reversed by myo- but not epi-inositol, indicating that lithium was acting via the phosphoinositide pathway. Lithium microinjection before fertilization caused arrest prior to nuclear envelope breakdown at much lower concentrations (3-5 mM). Co-injection of myo-inositol prevented the block. Microinjection of 1–2 mM lithium led to block at the cleavage stage. This was also reversed by coinjection of myo-inositol. Embryos blocked by lithium microinjection proceeded rapidly into mitosis after photolysis of caged inositol 1,4,5-trisphosphate. These data demonstrate that a patent phosphoinositide signalling pathway is essential for the proper timing of cell cycle transitions and offer a possible explanation for lithium's teratogenic effects.

Protein kinase C in hydrozoans: involvement in metamorphosis of Hydractinia and in pattern formation of Hydra

A wealth of information has suggested the involvement of protein kinase C (PKC) in metamorphosis of Hydractinia echinata and in pattern formation of Hydra magnipapillata. We have identified a Ca²⁺- and phospholipid-dependent kinase activity in extracts of both species. The enzyme was characterized as being similar to mammalian PKC by ion exchange chromatography. Gel filtration experiments revealed a molecular weight of about 70 kD. In phosphorylation assays of endogenous Hydractinia proteins, a protein with a molecular weight of 22.5 kD was found to be phoshorylated upon addition of phosphatidylserine. Bacterial induction of metamorphosis of Hydractinia echinata caused an increase in endogenous diacylglycerol, the physiological activator of PKC, suggesting that the bacterial inducer acts by activating receptor-regulated phospholipid metabolism. Exogenous diacylglycerol leads to membrane translocation of PKC, indicative of an activation. On the basis of our results and those of Freeman and Ridgway (1990) a model for the biochemical events during metamorphosis is presented.

Glucose and scyllo-inositol impair phosphoinositide hydrolysis in the 10.5-day cultured rat conceptus: a role in dysmorphogenesis?

Culture of the postimplantation rat conceptus from gestational day 9.5-10.5 in media supplemented with d-glucose or scyllo-inositol decreases tissue myo-inositol and phosphoinositides with a concomitant increase in dysmorphogenesis. A number of mitogenic agents initiate cellular proliferation and differentiation through receptors coupled to phosphoinositide hydrolysis. To test whether the decrease in conceptus phosphoinositides is associated with a reduced phosphoinositide hydrolytic response, we developed a protocol to stimulate phosphoinositide hydrolysis. Phosphoinositide hydrolysis was monitored by measurement of [3H]inositol phosphates after preincubation in serum free media. We examined the ability of serum, platelet-derived growth factor (PDGF), epidermal-derived growth factor (EGF), insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), endothelin-1 (ET-1), and endothelin-2 (ET-2), to stimulate phosphoinositide hydrolysis. As measured by [3H]inositol monophosphate ([3H]InsP1) accumulation, normal rat seru, ET-1, and ET-2 stimulated phosphoinositide hydrolysis 47%, 420%, and 154% above the basal rate observed in serum free controls. EGF stimulated a statistically insignificant 15% increase while PDGF, IGF-1, or IGF-2 were without effect. We further characterized ET-1 stimulated phosphoinositide hydrolysis. Dose-response studies disclosed that incremental increases in [3H]InsP1 (129-420%) are observed over a concentration range of 10-1,000 nM. Maximal stimulation was not reached even at 1,000 nM. Temporally [3H]InsP1 and [3H]InsP3 levels increased linearly during incubation periods of 15-60 min. We further analyzed ET-1 stimulated phosphoinositide hydrolysis in 10.5-day conceptuses cultured for 24 hr in media containing high concentrations of glucose (23.3-56.6 mM) or scyllo-inositol (0.55, 5.5 mM). Under these dysmorphogenic conditions that concomitantly decrease the phosphoinositide precursor pool the response to ET-1 was blunted 28-76% for glucose and 29-65% for scyllo-inositol. This suggests that the effect of glucose and scyllo-inositol on lowering phosphoinositide precursor pools also results in a decrease in the response to agonists using the inositol/lipid intracellular pathway. This impaired signaling response may contribute to initiating dysmorphogenic events in diabetic embryopathy.

Improving medical approaches to primary CNS malignancies--retinoid therapy and more

Successfully inducing differentiation in ectodermal diseases, retinoids harbour considerable therapeutic potential in the treatment of neuroectodermal-neuroepithelial malignancies. The principal tissue retinoid, retinoic acid, can be potently upregulated in vivo by a relatively specific catabolic inhibitor, R75251 (liarozole). Both substances have been given orally over 2 years in addition to standard treatment, and have been well tolerated. Corresponding closely to plasma retinoid levels, cutaneous side effects facilitate individual dosing. We evaluate this adjuvant retinoid approach and additional efforts to improve therapy of primary CNS malignancies, including the topical administration of retinoids in gamma linolenic acid.

The Xenopus IP3 receptor: structure, function, and localization in oocytes and eggs

To study the role of the IP3 receptor (IP3R) upon egg activation, cDNA clones encoding IP3R expressed in the Xenopus oocytes were isolated. By analyses of the primary structure and functional expression of the cDNA, Xenopus IP3R (XIP3R) was shown to have an IP3-binding domain and a putative Ca2+ channel region. Immunocytochemical studies revealed polarized distribution of XIP3R in the cytoplasm of the animal hemisphere in a well-organized endoplasmic reticulum-like structure and intensive localization in the perinuclear region of stage VI immature oocytes. In ovulated unfertilized eggs, XIP3R was densely enriched in the cortical region of both hemispheres in addition to its polarized localization. After fertilization, XIP3R showed a drastic change in its distribution in the cortical region. These results imply the predominant role of the XIP3R in both the formation and propagation of Ca2+ waves at fertilization.

Inositol 1,4,5-trisphosphate mass changes from fertilization through first cleavage in Xenopus laevis

After fertilization in Xenopus laevis, inositol 1,4,5-trisphosphate (IP3) mass increased from 53 to 261 fmol/cell and returned to near basal by 10 min after insemination. IP3 was also elevated over control egg levels during first mitosis and first cleavage. Because IP3 levels and the fertilization calcium wave decline at about the same time and because calcium ionophore or pricking the egg increased IP3, the fertilization calcium wave may be due to calcium-induced IP3 production. In addition, the onset of sperm motility was associated with an increase, whereas the acrosomal reaction was accompanied by a decrease in IP3 mass. Combining our published data with this report, the first chronology of the levels of IP3 from the induction of meiosis (maturation) through fertilization and cleavage in one cellular system is summarized. These data suggest an in vivo dose response for IP3 and calcium release. A small (17 fmol/cell) IP3 change during the induction of meiosis may not be associated with a calcium change. Larger IP3 changes at cleavage (40 fmol/cell) and mitosis (125 fmol/cell) are associated with localized small calcium increases, whereas the largest IP3 change (208 fmol/cell) is associated with the large calcium increase at fertilization.

Inositol monophosphatase is a highly conserved enzyme having localized structural similarity to both glycerol 3-phosphate dehydrogenase and haemoglobin

The cDNA coding for an inositol monophosphatase in the oocytes of the African clawed frog, Xenopus laevis, has been isolated and sequenced. The predicted primary structure of this enzyme is markedly conserved when it is compared with its mammalian functional homologues; up to 84% of the amino acid residues are identical, and conservative substitutions increase the similarity to 95%, suggesting that this sequence represents the most parsimonious primary structure for the protein to maintain not only catalytic activity but also perhaps the facility to interact with other macromolecules. Two regions of the protein, each of about 11 residues and separated by about 90 residues, have been identified as a consensus found also in glycerol 3-phosphate dehydrogenase (EC 1.1.1.8). One of these regions is also found to be particularly conserved in the alpha-globin of birds and reptiles; birds and some turtles are known to modulate the oxygen affinity of their haemoglobin with inositol polyphosphate in the same way as with 2,3-bisphosphoglycerate in other species. This region is also conserved in the beta-globin of most species, beginning with lysine-82, which is known to participate in the binding of organic phosphates. These regions of the inositol monophosphatase may represent motifs for the binding of its substrate.

Down's syndrome fibroblasts exhibit enhanced inositol uptake

The inositol metabolism of Down's syndrome (DS, trisomy 21) skin fibroblasts was examined. We report that DS cells accumulated [3H]inositol 2-3-fold faster than did other aneuploid or diploid controls. In contrast, trisomy 21 did not affect the uptake of choline, serine or glucose. Kinetic analysis demonstrated an increased maximal velocity of high-affinity, Na(+)-dependent, inositol transport, consistent with the expression of higher numbers of transporters by DS cells. Enhanced uptake was accompanied by a proportional increase in the incorporation of radiolabelled inositol into phospholipid. We suggest that an imbalance of inositol metabolism may contribute to plasma membrane abnormalities characteristic of DS cells.

Effect of prolonged in vitro lithium treatment on calcium transients in frog twitch muscle fibres and its reversal by exogenous myo-inositol

Using arsenazo III as an intracellular indicator to monitor the calcium transients elicited by voltage-clamp depolarizing pulse, the effect of prolonged in vitro lithium treatment on excitation--contraction coupling in frog twitch muscle fibres was investigated. Incubation in 10 mM Li+ Ringer's solution for 2 days caused a 46% increase in the amplitude of the calcium transients, while treatment with 30 mM Li+ for 2 days produced a depression of 44%. Shortening the bathing time to 1 day, the decrease of calcium transients caused by 30 mM Li+ was reversed to a small increase. For the 2-day incubation, both the increase in the amplitude with 10 mM and the decrease with 30 mM Li+ were abolished by the presence of 1 mM myo-inositol in the bathing medium. These results imply that the turnover of inositol phospholipids is involved in regulating excitation-contraction coupling in skeletal muscle fibres.

Inositol phosphates and cell signalling

Inositol 1,4,5-trisphosphate is a second messenger which regulates intracellular calcium both by mobilizing calcium from internal stores and, perhaps indirectly, by stimulating calcium entry. In these actions it may function with its phosphorylated metabolite, inositol 1,3,4,5-tetrakisphosphate. The subtlety of calcium regulation by inositol phosphates is emphasized by recent studies that have revealed oscillations in calcium concentration which are perhaps part of a frequency-encoded second-messenger system.

Lithium-induced teratogenesis in frog embryos prevented by a polyphosphoinositide cycle intermediate or a diacylglycerol analog

Microinjection of LiCl into prospective ventral blastomeres of the 32-cell Xenopus embryo gives rise to duplication of dorsoanterior structures such as the notochord, neural tube, eyes, and cement gland. We report here that this teratogenic effect of Li+ is prevented by coinjection of equimolar myo-inositol, an intermediate of the polyphosphoinositide cycle. In contrast, epi-inositol, a nonbiological positional isomer of inositol not employed in this cycle, is ineffective at rescuing Li+-injected embryos. Treatment of embryos at stage 7 with the tumor promoter, phorbol myristate acetate (an analog of the polyphosphoinositide cycle-derived second messenger, diacylglycerol), also prevents dorsoanterior duplication of Li+ embryos, while the nontransforming analog, phorbol myristate acetate-4-O-methyl ether, is without effect. Both of these rescuing agents are without obvious effects on development when administered alone (i.e., without Li+). Li+-selective microelectrode measurements demonstrate that intracellular Li+ levels are identical when Li+ is injected with or without myo-inositol. Clonal analysis shows that blastomeres injected with Li+ plus myo-inositol make a normal contribution of progeny to the later embryo. Because Li+ is a well-established inhibitor of the polyphosphoinositide cycle and can thereby have profound effects on cellular myo-inositol and diacylglycerol levels, these observations concerning inositol-mediated rescue suggest a role for altered polyphosphoinositide cycle activity in lithium-induced teratogenesis.

The Croonian lecture, 1988. Inositol lipids and calcium signalling

The response of cells to many external stimuli requires a decoding process at the membrane to transduce information into intracellular messengers. A major decoding mechanism employed by a variety of hormones, neurotransmitters and growth factors depends on the hydrolysis of a unique inositol lipid to generate two key second messengers, diacylglycerol and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). Here I examine the second messenger function of Ins(1,4,5)P3 in controlling the mobilization of calcium. We know most about how this messenger releases calcium from internal reservoirs but less is known concerning the entry of external calcium. One interesting possibility is that Ins(1,4,5)P3 might function in conjunction with its metabolic product Ins(1,3,4,5)P4 to control calcium entry through a mechanism employing a region of the endoplasmic reticulum as a halfway house during the transfer of calcium from outside the cell into the cytoplasm. The endoplasmic reticulum interposed between the plasma membrane and the cytosol may function as a capacitor to insure against the cell being flooded with external calcium. When stimulated, cells often display remarkably uniform oscillations in intracellular calcium. At least two oscillatory patterns have been recognized suggesting the existence of separate mechanisms both of which may depend upon Ins(1,4,5)P3. In one mechanism, oscillations may be driven by periodic pulses of Ins(1,4,5)P3 produced by receptors under negative feedback control of protein kinase C. The other oscillatory mechanism may depend upon Ins(1,4,5)P3 unmasking a process of calcium-induced calcium release from the endoplasmic reticulum. The function of these calcium oscillations is still unknown. This Ins(1,4,5)P3/calcium signalling system is put to many uses during the life history of a cell.(ABSTRACT TRUNCATED AT 250 WORDS)