Lithium respecifies cyclic AMP-induced cell-type specific gene expression in Dictyostelium

Lithium and Therapeutic Targeting of GSK-3

Lithium salts have been in the therapeutic toolbox for better or worse since the 19th century, with purported benefit in gout, hangover, insomnia, and early suggestions that lithium improved psychiatric disorders. However, the remarkable effects of lithium reported by John Cade and subsequently by Mogens Schou revolutionized the treatment of bipolar disorder. The known molecular targets of lithium are surprisingly few and include the signaling kinase glycogen synthase kinase-3 (GSK-3), a group of structurally related phosphomonoesterases that includes inositol monophosphatases, and phosphoglucomutase. Here we present a brief history of the therapeutic uses of lithium and then focus on GSK-3 as a therapeutic target in diverse diseases, including bipolar disorder, cancer, and coronavirus infections.

Inhibitors of Brassinosteroid Biosynthesis and Signal Transduction

Chemical inhibitors are invaluable tools for investigating protein function in reverse genetic approaches. Their application bears many advantages over mutant generation and characterization. Inhibitors can overcome functional redundancy, their application is not limited to species for which tools of molecular genetics are available and they can be applied to specific tissues or developmental stages, making them highly convenient for addressing biological questions. The use of inhibitors has helped to elucidate hormone biosynthesis and signaling pathways and here we review compounds that were developed for the plant hormones brassinosteroids (BRs). BRs are steroids that have strong growth-promoting capacities, are crucial for all stages of plant development and participate in adaptive growth processes and stress response reactions. In the last two decades, impressive progress has been made in BR inhibitor development and application, which has been instrumental for studying BR modes of activity and identifying and characterizing key players. Both, inhibitors that target biosynthesis, such as brassinazole, and inhibitors that target signaling, such as bikinin, exist and in a comprehensive overview we summarize knowledge and methodology that enabled their design and key findings of their use. In addition, the potential of BR inhibitors for commercial application in plant production is discussed.

New insights into negative effects of lithium on sea urchin Paracentrotus lividus embryos

The diffuse use of lithium in a number of industrial processes has produced a significant contamination of groundwater and surface water with it. The increased use of lithium has generated only scarce studies on its concentrations in ambient waters and on its effects on aquatic organisms. Only few contributions have focused on the toxicity of lithium in marine organisms (such as marine animals, algae and vegetables), showing that the toxic effect depends on the animal species. In the present study we describe the morphological and the molecular effects of lithium chloride (LiCl), using the sea urchin Paracentrotus lividus as a model organism. We show that LiCl, if added to the eggs before fertilization, induces malformations in the embryos in a dose-dependent manner. We have also followed by RT qPCR the expression levels of thirty seven genes (belonging to different classes of functional processes, such as stress, development, differentiation, skeletogenesis and detoxifications) to identify the molecular targets of LiCl. This study opens new perspectives for the understanding of the mechanism of action of lithium on marine organisms. The findings may also have relevance outside the world of marine organisms since lithium is widely prescribed for the treatment of human bipolar disorders.

Lithium-induced developmental anomalies in the spirotrich ciliate Stylonychia lemnae (Ciliophora, Hypotrichida)

Lithium is known to have profound biological effects of varying intensity in different life forms. In the present investigation, the effect of lithium was studied on the spirotrich ciliate Stylonychia lemnae. Lithium treatment brings about quantitative changes in the patterning of ciliary structures in S. lemnae. The dorsal surface of the affected cells develops supernumerary ciliary kineties due to excessive proliferation of the kinetosomes. The ventral surface on the other hand develops fewer than normal cirri formed from reduced numbers of ciliary primordia. The adoral zone of membranelles (AZM) fails to remodel properly as, in certain segments, membranelles become disarranged and misaligned. Lithium-induced changes are transitory as the normal pattern is restored during recovery after the cells are shifted to normal medium, suggesting non-genic regulation of cortical pattern. Lithium also affects the process of cell proliferation as the number of cells undergoing division is negligible as compared to reorganizing cells. The results point to the extremely complex and heterogeneous organization of the cellular cortex (plasma membrane and cytoskeleton) which is capable of exerting autonomous control over the phenotype and cortical pattern.

Influence of retinoic acid and lithium on proliferation and dopaminergic potential of human NT2 cells

Our laboratory is working with the human NTera2/D1 (NT2) cell line, which has properties similar to those of progenitor cells in the central nervous system (CNS). These neural-like precursor cells can differentiate into all three major lineages, neurons, astrocytes, and oligodendrocytes. The pure neuronal population, hNT neurons, possess characteristics of dopamine (DA) cells. First, we analyzed whether the retinoic acid (RA)-treated hNT neurons and the NT2 precursor cells expressed two transcription factors required for development of the midbrain DA neurons. We report that NT2 cells endogenously expressed Engrailed-1 and Ptx3, whereas RA-treated hNT neurons did not express Engrailed-1 or Ptx3. Next we examined the influence of lithium treatment on Engrailed-1 and Ptx3 as well as another critical transcription factor, Nurr1. Previous research has shown that lithium can mimic the Wnt pathway, which is important for the induction of these transcription factors. Finally, we investigated the effect of lithium treatment on the viability and proliferation of NT2 cells, because lithium has been shown to stimulate neurogenesis in adult neural precursors. Lithium treatment increased the viability and proliferation of NT2 cells. The expression of transcription factors essential for the induction and maintenance of the DA phenotype was not increased in NT2 after lithium treatment. We conclude that the NT2 cell line is an excellent in vitro model system for studying the influence of pharmalogical agents on proliferation, differentiation, and apoptosis of a human neural progenitor cell line.

Wnt signalling in osteoblasts regulates expression of the receptor activator of NFkappaB ligand and inhibits osteoclastogenesis in vitro

Reports implicating Wnt signalling in the regulation of bone mass have prompted widespread interest in the use of Wnt mimetics for the treatment of skeletal disorders. To date much of this work has focused on their anabolic effects acting on cells of the osteoblast lineage. In this study we provide evidence that Wnts also regulate osteoclast formation and bone resorption, through a mechanism involving transcriptional repression of the gene encoding the osteoclastogenic cytokine receptor activator of NFkappaB ligand (RANKL or TNFSF11) expressed by osteoblasts. In co-cultures of mouse mononuclear spleen cells and osteoblasts, inhibition of GSK3beta with LiCl or exposure to Wnt3a inhibited the formation of tartrate-resistant acid phosphatase-positive multinucleated cells compared with controls. However, these treatments had no consistent effect on the differentiation, survival or activity of osteoclasts generated in the absence of supporting stromal cells. Activation of Wnt signalling downregulated RANKL mRNA and protein expression, and overexpression of fulllength beta-catenin, but not transcriptionally inactive beta-catenin DeltaC(695-781), inhibited RANKL promoter activity. Since previous studies have demonstrated an absence of resorptive phenotype in mice lacking LRP5, we determined expression of a second Wnt co-receptor LRP6 in human osteoblasts, CD14(+) osteoclast progenitors and mature osteoclasts. LRP5 expression was undetectable in CD14-enriched cells and mature human osteoclasts, although LRP6 was expressed at high levels by these cells. Our evidence of Wnt-dependent regulation of osteoclastogenesis adds to the growing complexity of Wnt signalling mechanisms that are now known to influence skeletal function and highlights the requirement to develop novel therapeutics that differentially target anabolic and catabolic Wnt effects in bone.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Molecular targets of lithium action

Lithium is highly effective in the treatment of bipolar disorder and also has multiple effects on embryonic development, glycogen synthesis, hematopoiesis, and other processes. However, the mechanism of lithium action is still unclear. A number of enzymes have been proposed as potential targets of lithium action, including inositol monophosphatase, a family of structurally related phosphomonoesterases, and the protein kinase glycogen synthase kinase-3. These potential targets are widely expressed, require metal ions for catalysis, and are generally inhibited by lithium in an uncompetitive manner, most likely by displacing a divalent cation. Thus, the challenge is to determine which target, if any, is responsible for a given response to lithium in cells. Comparison of lithium effects with genetic disruption of putative target molecules has helped to validate these targets, and the use of alternative inhibitors of a given target can also lend strong support for or against a proposed mechanism of lithium action. In this review, lithium sensitive enzymes are discussed, and a number of criteria are proposed to evaluate which of these enzymes are involved in the response to lithium in a given setting.

Wnt signaling to beta-catenin involves two interactive components. Glycogen synthase kinase-3beta inhibition and activation of protein kinase C

Wnt signaling involves inhibition of glycogen synthase kinase-3beta (GSK-3beta) and elevation of cytoplasmic beta-catenin. This pathway is essential during embryonic development and oncogenesis. Previous studies on both Xenopus and mammalian cells indicate that lithium mimics Wnt signaling by inactivating GSK-3beta. Here we show that serum enhances accumulation of cytoplasmic beta-catenin induced by lithium in both 293 and C57MG cell lines and that growth factors are responsible for this enhancing activity. Growth factors mediate this effect through activation of protein kinase C (PKC), not through Ras or phosphatidylinositol 3-kinase. In addition, Wnt-induced accumulation of cytoplasmic beta-catenin is partially inhibited by PKC inhibitors and by chronic treatment of cells with phorbol ester. Both calphostin C, a PKC inhibitor, and a dominant negative PKC exhibit partial inhibition on Wnt-mediated transcriptional activation. We therefore propose that Wnt signaling to beta-catenin consists of two interactive components: one involves inhibition of GSK-3beta and is mimicked by lithium, and the other involves PKC and serves to augment the effects of GSK-3beta inhibition.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.

Inhibition of cell proliferation by lithium is associated with interference in cdc2 activation

Lithium can interfere with embryonal development in a variety of organisms. We investigated the effect of lithium on the proliferation of early embryonal cells. [3H]Thymidine incorporation of non-committed mouse P19 embryonal carcinoma cells was inhibited by lithium treatment. Similar effects were seen in a variety of other cells. This growth inhibition occurred in the G2 phase, since cells accumulated with a 4N DNA content, but the appearance of mitotic cells was blocked. Lithium could also prevent the activation of cdc2, thereby inhibiting cyclin B/cdc2 kinase activity. These data indicate that lithium might disturb embryonal development through interference in embryonal cell cycle regulation.

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

Lithium inhibits glycogen synthase kinase-3 activity and mimics wingless signalling in intact cells

BACKGROUND

Exposing eukaryotic cells to lithium ions (Li+) during development has marked effects on cell fate and organization. The phenotypic consequences of Li+ treatment on Xenopus embryos and sporulating Dictyostelium are similar to the effects of inhibition or disruption, respectively, of a highly conserved protein serine/threonine kinase, glycogen synthase kinase-3 (GSK-3). In Drosophila, the GSK-3 homologue is encoded by zw3sgg, a segment-polarity gene involved in embryogenesis that acts downstream of wg. In higher eukaryotes, GSK-3 has been implicated in signal transduction pathways downstream of phosphoinositide 3-kinase and mitogen-activated protein kinases.

RESULTS

We investigated the effect of Li+ on the activity of the GSK-3 family. At physiological doses, Li+ inhibits the activity of human GSK-3 beta and Drosophila Zw3Sgg, but has no effect on other protein kinases. The effect of Li+ on GSK-3 is reversible in vitro. Treatment of cells with Li+ inhibits GSK-3-dependent phosphorylation of the microtubule-associated protein Tau. Li+ treatment of Drosophila S2 cells and rat PC12 cells induces accumulation of cytoplasmic Armadillo/beta-catenin, demonstrating that Li+ can mimic Wingless signalling in intact cells, consistent with its inhibition of GSK-3.

CONCLUSIONS

Li+ acts as a specific inhibitor of the GSK-3 family of protein kinases in vitro and in intact cells, and mimics Wingless signalling. This reveals a possible molecular mechanism of Li+ action on development and differentiation.

A molecular mechanism for the effect of lithium on development

Lithium, one of the most effective drugs for the treatment of bipolar (manic-depressive) disorder, also has dramatic effects on morphogenesis in the early development of numerous organisms. How lithium exerts these diverse effects is unclear, but the favored hypothesis is that lithium acts through inhibition of inositol monophosphatase (IMPase). We show here that complete inhibition of IMPase has no effect on the morphogenesis of Xenopus embryos and present a different hypothesis to explain the broad action of lithium. Our results suggest that lithium acts through inhibition of glycogen synthase kinase-3 beta (GSK-3 beta), which regulates cell fate determination in diverse organisms including Dictyostelium, Drosophila, and Xenopus. Lithium potently inhibits GSK-3 beta activity (Ki = 2 mM), but is not a general inhibitor of other protein kinases. In support of this hypothesis, lithium treatment phenocopies loss of GSK-3 beta function in Xenopus and Dictyostelium. These observations help explain the effect of lithium on cell-fate determination and could provide insights into the pathogenesis and treatment of bipolar disorder.

Lithium, an inhibitor of cAMP-induced inositol 1,4,5-trisphosphate accumulation in Dictyostelium discoideum, inhibits activation of guanine-nucleotide-binding regulatory proteins, reduces activation of adenylylcyclase, but potentiates activation of guanylyl cyclase by cAMP

Li+ drastically alters pattern formation in Dictyostelium by inhibiting cAMP-induced prespore-gene expression and promoting cAMP-induced prestalk-gene expression. We reported previously that Li+ inhibits inositol monophosphatases in this organism and strongly reduces basal and cAMP-stimulated inositol 1,4,5-trisphosphate levels. We show here that Li+ also reduces cAMP-induced accumulation of cAMP, but promotes cAMP-induced accumulation of cGMP. This effect is not due to inhibition of cGMP hydrolysis or inhibition of adaptation and may therefore reflect stimulation of guanylyl-cyclase activation. Li+ does not affect the binding of cAMP to surface receptors but interferes with the interaction between receptors and guanine-nucleotide-binding regulatory (G) proteins. These effects are complex; in the absence of Mg2+, Li+ increases guanosine 5'-[gamma-thio]triphosphate(GTP[S])-binding activity to similar levels as 1 mM Mg2+. However, while Mg2+ potentiates cAMP-induced stimulation of GTP[S]-binding activity, Li+ effectively inhibits stimulation. Li+ also inhibits cAMP-stimulated, but not basal high-affinity GTP-ase activity, indicating an inhibitory effect on cAMP-induced activation of G-proteins. Our data suggest that in addition to inositolphosphate metabolism, the activation of G-proteins may be a second biochemical target for Li+ effects on pattern formation and signal transduction in Dictyostelium.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

Control of cAMP-induced gene expression by divergent signal transduction pathways

A compilation of literature data and recent experiments led to the following conclusions regarding cyclic adenosine 3':5' monophosphate (cAMP) regulation of gene expression. Several classes of cAMP-induced gene expression can be discriminated by sensitivity to stimulation kinetics. The aggregation-related genes respond only to nanomolar cAMP pulses. The prestalk-related genes respond both to nanomolar pulses and persistent micromolar stimulation. The prespore specific genes respond only to persistent micromolar stimulation. The induction of the aggregation- and prestalk-related genes by nanomolar cAMP pulses may share a common transduction pathway, which does not involve cAMP, while involvement of the inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway is unlikely. Induction of the expression of prespore and prestalk-related genes by micromolar cAMP stimuli utilizes divergent signal processing mechanisms. cAMP-induced prespore gene expression does not involve cAMP and probably also not cyclic guanosine 3'.5' monophosphate (cGMP) as intracellular intermediate. Involvement of cAMP-induced phospholipase C (PLC) activation in this pathway is suggested by the observation that IP3 and 1,2-diacylglycerol (DAG) can induce prespore gene expression, albeit in a somewhat indirect manner and by the observation that Li+ and Ca2+ antagonists inhibit prespore gene expression. Cyclic AMP induction of prestalk-related gene expression is inhibited by IP3 and DAG and promoted by Li+, and is relatively insensitive to Ca2+ antagonists, which indicates that PLC activation does not mediate prestalk-related gene expression. Neither prespore nor prestalk-related gene expression utilizes the sustained cAMP-induced pHi increase as intracellular intermediate.

Cell-type-specific genes expressed late in Dictyostelium development show markedly different responses to 3'5' cyclic AMP

The maintenance of late gene expression by 3'5' cyclic AMP was re-examined using several newly isolated cell-type-specific genes. Expression of all the prespore-enriched genes ceased immediately upon disaggregation of developing cells and pre-existing mRNA was rapidly degraded. For most genes, cAMP had little or no effect either alone or in combination with conditioned medium factors. The expression of the non-cell-type-specific genes 7E and 2C also ceased upon cell disaggregation but cAMP triggered a full re-induction of expression although the timing of the response differed markedly between these two genes. In contrast to earlier interpretations, these data argue that for none of these late prespore genes is cAMP alone sufficient for the maintenance of expression. The responses of the two prestalk mRNAs examined were gene-specific. Prestalk 5D mRNA decayed slowly upon disaggregation and was partially stabilized by cAMP whereas prestalk 5G mRNA increased upon disaggregation and was inhibited by cAMP.

Inositol trisphosphate and diacylglycerol can differentially modulate gene expression in Dictyostelium

We have previously shown that several genes expressed during Dictyostelium development could be induced in shaking culture by exogenous cAMP, even though the accumulation of intracellular cAMP was inhibited. The use of selected cAMP analogs indicated that the exogenous cAMP functioned by activating the cell surface cAMP receptor and not by interacting with the regulatory subunit of the intracellular cAMP-dependent protein kinase. Although some genes in Dictyostelium appear to be regulated by intracellular cAMP, these data suggest that this is not the case for all genes regulated by cAMP. Intracellular second messengers other than cAMP may, therefore, promote the expression of these other genes. Here, we have examined inositol trisphosphate and diacylglycerol as candidates for such mediators of signal transduction. We have studied three genes that exhibit disparate modes of temporal and spatial expression during development of Dictyostelium. In shaking cultures, maximal levels of expression of each are dependent on the accumulation of or exposure to extracellular cAMP. We show that the addition of inositol trisphosphate and/or diacylglycerol to cells in shaking culture has distinct effects on the expression of each gene and, under specific conditions, can bypass the requirement for extracellular cAMP. These data suggest that extracellular cAMP interacting with its cell surface receptor may promote synthesis of inositol trisphosphate and diacylglycerol to regulate gene expression and aspects of differentiation in Dictyostelium.