Functional expression of Ca2+ signaling pathways in mouse embryonic stem cells

Integrated chemical genomics reveals modifiers of survival in human embryonic stem cells

Understanding how survival is regulated in human embryonic stem cells (hESCs) could improve expansion of stem cells for production of cells for regenerative therapy. There is great variability in comparing the differentiation potential of multiple hESC lines. One reason for this is poor survival upon dissociation, which limits selection of homogeneous populations of cells. Understanding the complexity of survival signals has been hindered by the lack of a reproducible system to identify modulators of survival in pluripotent cells. We therefore developed a high-content screening approach with small molecules to examine hESC survival. We have identified novel small molecules that improve survival by inhibiting either Rho-kinase or protein kinase C. Importantly, small molecule targets were verified using short hairpin RNA. Rescreening with stable hESCs that were genetically altered to have increased survival enabled us to identify groups of pathway targets that are important for modifying survival. Understanding how survival is regulated in hESCs could overcome severe technical difficulties in the field, namely expansion of stem cells to improve production of cells and tissues for regenerative therapy.

Pharmacological properties of purinergic receptors and their effects on proliferation and induction of neuronal differentiation of P19 embryonal carcinoma cells

We have used P19 embryonal carcinoma cells as in vitro model for early neurogenesis to study ionotropic P2X and metabotropic P2Y receptor-induced Ca(2+) transients and their participation in induction of proliferation and differentiation. In embryonic P19 cells, P2Y(1), P2Y(2) and P2X(4) receptors or P2X-heteromultimers with similar P2X(4) pharmacology were responsible for ATP and ATP analogue-induced Ca(2+) transients. In neuronal-differentiated cells, P2Y(2,) P2Y(6), P2X(2) and possibly P2X(2)/P2X(6) heteromeric receptors were the major mediators of the elevations in intracellular free calcium concentration [Ca(2+)](i). We have collected evidence for the involvement of metabotropic purinergic receptors in proliferation induction of undifferentiated and neural progenitor cells by using a BrdU-incorporation assay. ATP-, UTP-, ADP-, 2-MeS-ATP- and ADP-betaS-induced proliferation in P19 cells was mediated by P2Y(1) and P2Y(2) receptors as judged from pharmacological profiles of receptor responses. ATP-provoked acceleration of neuronal differentiation, determined by analysis of nestin and neuron-specific enolase gene and protein expression, also resulted from P2Y(1) and P2Y(2) receptor activation. Proliferation- and differentiation-induction involved the activation of inositol-trisphosphate sensitive intracellular Ca(2+) stores.

Caloxins: a novel class of selective plasma membrane Ca2+ pump inhibitors obtained using biotechnology

Plasma membrane Ca2+ pumps (PMCA) extrude cellular Ca2+ with a high affinity and hence play a major role in Ca2+ homeostasis and signaling. Caloxins (selective extracellular PMCA inhibitors) would aid in elucidating the physiology of PMCA. PMCA proteins have five extracellular domains (exdoms). Our hypotheses are: 1) peptides that bind selectively to each exdom can be invented by screening a random peptide library, and 2) a peptide can modulate PMCA activity by binding to one of the exdoms. The first caloxin 2a1, selected for binding exdom 2 was selective for PMCA (Ki=529 microM). It has been used to examine the physiological role of PMCA. PMCA isoforms are encoded by four genes. PMCA isoform expression differs in various cell types, with PMCA1 and 4 being the most widely distributed. There are differences between PMCA1-4 exdom 1 sequences, which may be exploited for inventing isoform selective caloxins. Using exdom 1 of PMCA4 as a target, modified screening procedures and mutagenesis led to the high-affinity caloxin 1c2 (Ki=2.3 microM for PMCA4). It is selective for PMCA4 over PMCA1, 2, or 3. We hope that caloxins can be used to discern the roles of individual PMCA isoforms in Ca2+ homeostasis and signaling. Caloxins may also become clinically useful in cardiovascular diseases, neurological disorders, retinopathy, cancer, and contraception.

Expression and function of cannabinoid receptors CB1 and CB2 and their cognate cannabinoid ligands in murine embryonic stem cells

BACKGROUND

Characterization of intrinsic and extrinsic factors regulating the self-renewal/division and differentiation of stem cells is crucial in determining embryonic stem (ES) cell fate. ES cells differentiate into multiple hematopoietic lineages during embryoid body (EB) formation in vitro, which provides an experimental platform to define the molecular mechanisms controlling germ layer fate determination and tissue formation.

METHODS AND FINDINGS

The cannabinoid receptor type 1 (CB1) and cannabinoid receptor type 2 (CB2) are members of the G-protein coupled receptor (GPCR) family, that are activated by endogenous ligands, the endocannabinoids. CB1 receptor expression is abundant in brain while CB2 receptors are mostly expressed in hematopoietic cells. However, the expression and the precise roles of CB1 and CB2 and their cognate ligands in ES cells are not known. We observed significant induction of CB1 and CB2 cannabinoid receptors during the hematopoietic differentiation of murine ES (mES)-derived embryoid bodies. Furthermore, mES cells as well as ES-derived embryoid bodies at days 7 and 14, expressed endocannabinoids, the ligands for both CB1 and CB2. The CB1 and CB2 antagonists (AM251 and AM630, respectively) induced mES cell death, strongly suggesting that endocannabinoids are involved in the survival of mES cells. Treatment of mES cells with the exogenous cannabinoid ligand Delta(9)-THC resulted in the increased hematopoietic differentiation of mES cells, while addition of AM251 or AM630 blocked embryoid body formation derived from the mES cells. In addition, cannabinoid agonists induced the chemotaxis of ES-derived embryoid bodies, which was specifically inhibited by the CB1 and CB2 antagonists.

CONCLUSIONS

This work has not been addressed previously and yields new information on the function of cannabinoid receptors, CB1 and CB2, as components of a novel pathway regulating murine ES cell differentiation. This study provides insights into cannabinoid system involvement in ES cell survival and hematopoietic differentiation.

Cell cycle-dependent calcium oscillations in mouse embryonic stem cells

During cell cycle progression, somatic cells exhibit different patterns of intracellular Ca2+signals during the G0phase, the transition from G1to S, and from G2to M. Because pluripotent embryonic stem (ES) cells progress through cell cycle without the gap phases G1and G2, we aimed to determine whether mouse ES (mES) cells still exhibit characteristic changes of intracellular Ca2+concentration during cell cycle progression. With confocal imaging of the Ca2+-sensitive dye fluo-4 AM, we identified that undifferentiated mES cells exhibit spontaneous Ca2+oscillations. In control cultures where 50.4% of the cells reside in the S phase of the cell cycle, oscillations appeared in 36% of the cells within a colony. Oscillations were not initiated by Ca2+influx but depended on inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+release and the refilling of intracellular stores by a store-operated Ca2+influx (SOC) mechanism. Using cell cycle synchronization, we determined that Ca2+oscillations were confined to the G1/S phase (∼70% oscillating cells vs. G2/M with ∼15% oscillating cells) of the cell cycle. ATP induced Ca2+oscillations, and activation of SOC could be induced in G1/S and G2/M synchronized cells. Intracellular Ca2+stores were not depleted, and all three IP3receptor isoforms were present throughout the cell cycle. Cell cycle analysis after EGTA, BAPTA-AM, 2-aminoethoxydiphenyl borate, thapsigargin, or U-73122 treatment emphasized that IP3-mediated Ca2+release is necessary for cell cycle progression through G1/S. Because the IP3receptor sensitizer thimerosal induced Ca2+oscillations only in G1/S, we propose that changes in IP3receptor sensitivity or basal levels of IP3could be the basis for the G1/S-confined Ca2+oscillations.

Physical manipulation of calcium oscillations facilitates osteodifferentiation of human mesenchymal stem cells

The role of cytosolic calcium oscillation has long been recognized in the regulation of cellular and molecular interactions. Information embedded in calcium oscillation can provide molecular cues for cell behavior such as cell differentiation. Although calcium dynamics are versatile and likely to depend on the cell type, the calcium dynamics in human mesenchymal stem cells (hMSCs) and its role in differentiation are yet to be fully elucidated. In the present study we characterized the calcium oscillation profiles in hMSCs before and after subjecting the cells to the osteoinductive factors. Our findings indicate that the calcium spikes decreased rapidly with osteodifferentiation to a level observed in terminally differentiated human osteoblasts. In addition, the calcium oscillations appear to serve as a bidirectional signal during hMSC differentiation. While an altered calcium oscillation pattern may be an indicator for hMSC differentiation, it is also likely to be involved in directing hMSC differentiation. Treatment of hMSCs with a noninvasive electrical stimulation, for example, not only altered the calcium oscillations but also facilitated osteodifferentiation. Regulation of calcium oscillation by external physical stimulation could amplify hMSC differentiation into a tissue-specific lineage and may offer an alternate biotechnology to harness the unique properties of stem cells.

ATP stimulates mouse embryonic stem cell proliferation via protein kinase C, phosphatidylinositol 3-kinase/Akt, and mitogen-activated protein kinase signaling pathways

This study investigated the effect of ATP and its related signal cascades on the proliferation of mouse ESCs. ATP increased the level of [(3)H]thymidine/5-bromo-2'-deoxyuridine incorporation and the number of cells in both a time- and dose-dependent manner. AMP-CPP (a P2X(1) and P2X(3) agonist), ATP-gammaS (a P2Y agonist), and 2-methylthio-ATP (a P2X and P2Y agonist) stimulated [(3)H]thymidine incorporation. P2 purinoceptor antagonists (suramin, reactive blue 2) inhibited the ATP-induced increase in [(3)H]thymidine incorporation. Reverse transcription-polymerase chain reaction analysis revealed P2X(3), P2X(4), P2Y(1), and P2Y(2) expression in mouse ESCs. Adenylate cyclase inhibitor (SQ 22536), phospholipase C inhibitors (neomycin or U 73122), and protein kinase C (PKC) inhibitors (bisindolylmaleimide I or staurosporine) inhibited the ATP-induced increase in [(3)H]thymidine incorporation. ATP increased the level of intracellular cAMP and inositol phosphates. ATP translocated PKC alpha, delta, and zeta from the cytosol to the membrane compartment. ATP and its agonists increased [Ca(2+)](i). In addition, the ATP-induced increase in [(3)H]thymidine incorporation was completely inhibited by a combination of EGTA (extracellular Ca(2+) chelator) and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM (intracellular Ca(2+) chelator). ATP phosphorylated Akt and p44/42 mitogen-activated protein kinases (MAPKs) in a time-dependent manner, and either suramin or reactive blue 2 (RB2) blocked the ATP-induced phosphorylation of Akt. Suramin, RB2, the phosphatidylinositol 3-kinase (PI3K) inhibitor (wortmannin), or the Akt inhibitor inhibited the phosphorylation of p44/42 MAPKs. The ATP-induced increase in [(3)H]thymidine incorporation was inhibited by wortmannin, the Akt inhibitor, and the MAPK kinase inhibitor (PD 98059). Suramin, RB2, PD 98059, and wortmannin blocked the ATP-induced increase in the cyclin D1, cyclin E, cyclin-dependent kinase (CDK) 2, and CDK4 levels. In conclusion, ATP stimulates mouse ESC proliferation through PKC, PI3K/Akt, and MAPKs via the P2 purinoceptors.

Inositol 1,4,5-triphosphate-mediated shuttling between intracellular stores and the cytosol contributes to the sustained elevation in cytosolic calcium in FMLP-activated human neutrophils

The current study was designed to probe Ca2+ shuttling between intracellular stores and the cytosol as a potential mechanism contributing to the prolongation of elevated Ca2+ transients in N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-activated human neutrophils. Cytosolic Ca2+ concentrations and transmembrane fluxes of the cation were measured using spectrofluorimetric and radiometric procedures, respectively, while inositol 1,4,5-triphosphate (IP3) was measured using a radioreceptor assay. The Ca2+-chelating agent, ethylene glycol-bis (beta-aminoethyl ether) N,N,N'N'-tetraacetic acid (EGTA; 10mM), was used to exclude store-operated influx of Ca2+ into neutrophils, while the IP3 receptor antagonist, 2-aminoethoxydiphenyl borate (2-APB, 100 microM), added to the cells 10s after FMLP (0.01 and 1 microM), at which time the increases in IP3 and cytosolic Ca2+ were maximal, was used to eliminate both sustained release from stores and influx of Ca2+. Addition of FMLP at 0.01 or 1 microM resulted in equivalent peak increases in cytosolic Ca2+, while the increase in IP3 was greater and the rate of clearance of Ca2+ from the cytosol slower, in cells activated with 1 microM FMLP. Treatment of the cells with either EGTA or 2-APB following addition of 1 microM FMLP, completely (EGTA) or almost completely (2-APB) abolished the influx of Ca2+ and accelerated the rate of clearance of the cation from the cytosol. Post-peak cytosolic Ca2+ concentrations were lower, and the Ca2+ content of the stores higher, in cells treated with 2-APB. The involvement of IP3 was confirmed by similar findings in cells treated with U-73122 (1 microM), a selective inhibitor of phospholipase C. Taken together, these observations are compatible with IP3-mediated Ca2+ shuttling in neutrophils activated with FMLP.

Na+/K+ ATPase and its functional coupling with Na+/Ca2+ exchanger in mouse embryonic stem cells during differentiation into cardiomyocytes

Cardiomyocytes derived from mouse embryonic stem (mES) cells have been demonstrated to exhibit a time-dependent expression of ion channels and signal transduction pathways in electrophysiological studies. However, ion transporters, such as Na+/K+ ATPase (Na+ pump) or Na+/Ca2+ exchanger, which play crucial roles for cardiac function, have not been well studied in this system. In this study, we investigated the functional expression of Na+/K+ ATPase and Na+/Ca2+ exchanger in mES cells during in vitro differentiation into cardiomyocytes, as well as the functional coupling between the two transporters. By measuring [Na+]i and Na+ pump current (Ip), it was shown that an ouabain-high sensitive Na+/K+ ATPase was expressed functionally in undifferentiated mES cells and these activities increased during a time course of differentiation. Using RT-PCR, the expression of mRNA for alpha1-subunit and alpha3-subunit of the Na+/K+ ATPase could be detected in both undifferentiated mES cells and derived cardiomyocytes. In contrast alpha2-subunit mRNA could be detected only in derived cardiomyocytes but not in undifferentiated mES cells. mRNA for the Na+/Ca2+ exchanger 1 isoform (NCX1) could be detected in undifferentiated mES cells and its expression levels seemed to gradually increase throughout the differentiation accompanied by increasing its Ca2+ extrusion function. At the middle stages of differentiation (after 10-day induction), more than 75% derived cardiomyocytes exhibited [Ca2+]i oscillations by blocking of Na+/K+ ATPase, suggesting the functional coupling with Na+/Ca2+ exchanger. From these results and RT-PCR analysis, we conclude that alpha2-subunit Na+/K+ ATPase mainly contributes to establish the functional coupling with NCX1 at the middle stages of differentiation of cardiomyocytes.