Changes in cytoplasmic pH are involved in the cell type regulation of Dictyostelium

Ion Signaling in Cell Motility and Development in Dictyostelium discoideum

Cell-to-cell communication is fundamental to the organization and functionality of multicellular organisms. Intercellular signals orchestrate a variety of cellular responses, including gene expression and protein function changes, and contribute to the integrated functions of individual tissues. Dictyostelium discoideum is a model organism for cell-to-cell interactions mediated by chemical signals and multicellular formation mechanisms. Upon starvation, D. discoideum cells exhibit coordinated cell aggregation via cyclic adenosine 3',5'-monophosphate (cAMP) gradients and chemotaxis, which facilitates the unicellular-to-multicellular transition. During this process, the calcium signaling synchronizes with the cAMP signaling. The resulting multicellular body exhibits organized collective migration and ultimately forms a fruiting body. Various signaling molecules, such as ion signals, regulate the spatiotemporal differentiation patterns within multicellular bodies. Understanding cell-to-cell and ion signaling in Dictyostelium provides insight into general multicellular formation and differentiation processes. Exploring cell-to-cell and ion signaling enhances our understanding of the fundamental biological processes related to cell communication, coordination, and differentiation, with wide-ranging implications for developmental biology, evolutionary biology, biomedical research, and synthetic biology. In this review, I discuss the role of ion signaling in cell motility and development in D. discoideum.

Non-genetic heterogeneity and cell fate choice in Dictyostelium discoideum

From microbes to metazoans, it is now clear that fluctuations in the abundance of mRNA transcripts and protein molecules enable genetically identical cells to oscillate between several distinct states (Kaern et al. 2005). Since this cell-cell variability does not derive from physical differences in the genetic code it is termed non-genetic heterogeneity. Non-genetic heterogeneity endows cell populations with useful capabilities they could never achieve if each cell were the same as its neighbors (Raj & van Oudenaarden 2008; Eldar & Elowitz 2010). One such example is seen during multicellular development and "salt and pepper" cell type differentiation. In this review, we will first examine the importance of non-genetic heterogeneity in initiating "salt and pepper" pattern formation during Dictyostelium discoideum development. Second, we will discuss the various ways in which non-genetic heterogeneity might be generated, as well as recent advances in understanding the molecular basis of heterogeneity in this system.

The glycogen phosphorylase-2 promoter binding protein in Dictyostelium is replication protein A

During Dictyostelium development, glycogen degradation is a crucial event that provides glucose monomers used in the synthesis of the essential structural components for cellular differentiation. The product of the developmentally regulated glycogen phosphorylase-2 gene (gp2) catalyzes the degradation. DNA-binding proteins were found to bind to a regulatory site of the gp2 gene in a stage-dependent pattern. Gel-shift analysis of undifferentiated amoebae cell extract revealed a protein migrating at 0.40 Rf, while 17 hour differentiated cell extract produced a species migrating at 0.32 Rf. Both the 0.32 and 0.40 Rf proteins were purified and found to consist of three subunits of 18, 35 and 62 kDa (for 0.40 Rf) or 81 kDa (for 0.32 Rf). Data base searches identified the protein as the Dictyostelium homologue of replication protein A (DdRPA). Amino acid sequence analysis showed identity between the 62 and 81 kDa subunits. Incubation of cell-free extracts under appropriate conditions at low pH, resulted in conversion of the 81 kDa to the 62 kDa subunit. Northern blot analysis revealed that the levels of expression of the large subunit of DdRPA were constant throughout differentiation and the size of the mRNA was the same at all stages of development. The results raise the possibility that pH induced post-translational modifications of DdRPA are involved in events that halt cell proliferation and induce differentiation in Dictyostelium.

The patB gene of Dictyostelium discoideum encodes a P-type H(+)-ATPase isoform essential for growth and development under acidic conditions

During growth and early development of Dictyostelium discoideum, the amoebae exhibit transient pH changes in their cytosol (pHi) and external medium which correlate with the extrusion of H+ from the cell by a plasma membrane pump. Moreover, the changes in pHi have been postulated to influence early prestalk/prespore differentiation during development. To learn more about the role of H+ fluxes in Dictyostelium, we cloned and analysed cDNAs of the gene patB, which appears to encode a P-type H(+)-ATPase. The patB ORF encodes a protein (termed PAT2) of 1058 amino acids with a calculated molecular mass of 117,460 Da. When aligned with other P-type ion-transport ATPases, PAT2 showed the greatest amino acid sequence identity with plasma membrane H(+)-ATPases of plants and fungi and considerably lower identity with other monovalent cation pumps and with Ca2+ pumps. Northern and Western analyses revealed that patB is expressed at very low levels in cells growing at neutral pH, but it is up-regulated rapidly and dramatically when the cells are shifted to an acidic medium. Immunofluorescence analysis indicated that PAT2 resides on the plasma membrane. When patB was disrupted by homologous recombination, the cells grew and developed normally at neutral and slightly alkaline pHs but they were unable to grow or develop at pH 5.0, and they slowly died. In growth medium at pH 6.8, patB+ and patB cells exhibited similar levels of vanadate-sensitive ATPase activity. However, when the cells were shifted to pH 5.0, this activity rapidly increased about twofold in the control cells but not in the mutant cells. Despite the lower ATPase activity in patB cells, they showed relatively normal H+ fluxes and only a slight decrease in pHi when incubated in acidic medium. Together, these results suggest that patB encodes an acid-inducible P-type H(+)-ATPase which is indispensable for the survival of Dictyostelium cells in moderately acidic external environments.

Developmental decisions in Dictyostelium discoideum

A few hours after the onset of starvation, amoebae of Dictyostelium discoideum start to form multicellular aggregates by chemotaxis to centers that emit periodic cyclic AMP signals. There are two major developmental decisions: first, the aggregates either construct fruiting bodies directly, in a process known as culmination, or they migrate for a period as "slugs." Second, the amoebae differentiate into either prestalk or prespore cells. These are at first randomly distributed within aggregates and then sort out from each other to form polarized structures with the prestalk cells at the apex, before eventually maturing into the stalk cells and spores of fruiting bodies. Developmental gene expression seems to be driven primarily by cyclic AMP signaling between cells, and this review summarizes what is known of the cyclic AMP-based signaling mechanism and of the signal transduction pathways leading from cell surface cyclic AMP receptors to gene expression. Current understanding of the factors controlling the two major developmental choices is emphasized. The weak base ammonia appears to play a key role in preventing culmination by inhibiting activation of cyclic AMP-dependent protein kinase, whereas the prestalk cell-inducing factor DIF-1 is central to the choice of cell differentiation pathway. The mode of action of DIF-1 and of ammonia in the developmental choices is discussed.

A mechanism for the intracellular localization of myosin II filaments in the Dictyostelium amoeba

When ATP is added to membrane-cytoskeletons prepared from Dictyostelium amoebae by the method described previously (S. Yumura and T. Kitanishi-Yumura, Cell Struct. Funct. 15, 355–364, 1990), myosin II is released from the membrane-cytoskeletons after contraction. Simultaneously, the heavy chains of myosin II are phosphorylated by a putative myosin II heavy-chain kinase, at foci within the actin network, with the resultant disassembly of filaments. In this study, we examined factors that control the release of myosin II from the membrane-cytoskeletons, on the assumption that inhibition of the release of myosin II keeps the myosin II in the cortical region, and is responsible for the localization of myosin II in the cortical region. The release of myosin II is inhibited at pH values below 6.5. This effect is not due to the inhibition of heavy-chain phosphorylation but is due to the suppression of disassembly of the filaments. In the membrane-cytoskeletons of aggregating cells, the release of myosin II is inhibited by Ca2+, and this effect is enhanced by pretreatment with calmodulin. In the membrane-cytoskeletons of vegetative cells, the release of myosin II is inhibited by pretreatment with calmodulin, and this effect is Ca2+-independent. The inhibition of the release of myosin II by Ca2+ and/or calmodulin is due to the inhibition of heavy-chain phosphorylation, and calmodulin is associated with the foci within the actin network. These results represent a possible mechanism for the intracellular localization of myosin II via regulation of the release of myosin from the cortical region by changes in intracellular pH and/or intracellular concentrations of Ca2+.

Hyperoxia induces alkalinization and dome formation in MDCK epithelial cells

We observed that confluent Madin-Darby canine kidney (MDCK) epithelial cells exposed to 95% O2 showed intensive dome formation, a manifestation of cell differentiation and transepithelial fluid transport, whereas cells exposed to 40% O2 or to normoxia did not. Hyperoxia-induced dome formation (HIDF) was preceded and accompanied by a significant rise in intracellular pH (pHi) on days 2 (7.53 vs. 7.42) and 3 (7.62 vs. 7.45), as compared with controls. Inhibition of the Na(+)-H+ exchanger by 0.1 or 1.0 mM amiloride caused 29 or 69% reduction of HIDF and completely abolished hyperoxia-induced alkalinization of the cells. HIDF was altered by modification of extracellular pH (pHo); there was a decrease by 84% with pHo 6.8, while pHo 7.8 led to earlier and more intensive dome formation (day 2, +472%; day 3, +27%). We also found that adenosine 3',5'-cyclic monophosphate (cAMP) was increased in hyperoxic cells, a change that was independent from the rise of pHi. We conclude that high-level hyperoxia induces dome formation in MDCK epithelial monolayers by a process involving activation of the Na(+)-H+ exchanger, together with increased intracellular cAMP.

Cytoplasmic pH of Dictyostelium discoideum amebae during early development: identification of two cell subpopulations before the aggregation stage

Development of the cellular slime mold Dictyostelium discoideum is initiated by the removal of nutrients, and results in formation of a mature fruiting body composed of two cell types, the stalk and spore cells. A considerable body of evidence supports the hypothesis that cytoplasmic pH may be an essential regulator of the choice to differentiate in either the prestalk or prespore pathway. We have devised methods for measurement and analysis of intracellular pH in developing Dictyostelium amebae in order to assess directly the potential role of cytoplasmic pH in regulating the pathway of differentiation. The intracellular pH of single D. discoideum amebae during development and in intact slugs has been measured using the pH-sensitive indicator pyranine in a low light level microspectrofluorometer. We have used the ATP-mediated loading method to introduce pyranine into these cells. Cells loaded by the ATP method appear healthy, have no detectable defects in development, and exhibit a similar population distribution of intracellular pH to those loaded by sonication. The intracellular pH of populations comprised of single amebae was found to undergo a transient acidification during development resulting in a bimodal distribution of intracellular pH. The subpopulations were characterized by fitting two gaussian distributions to the data. The number of cells in the acidic intracellular pH subpopulation reached a maximum 4 h after initiation of development, and had returned to a low level by 7 h of development. In addition, a random sample of single amebae within a slug had a median intracellular pH of 7.2, nearly identical to the median pH (7.19) of similarly treated vegetative cells. No gradient of intracellular pH along the anterior to posterior axis of the slug was detected. Our data demonstrate the existence of two distinct subpopulations of cells before the aggregation stage of development in Dictyostelium, and offers support for the hypothesis that changes in intracellular pH contribute to development in D. discoideum.

The effect of ammonia on cell-type-specific enzyme accumulation in Dictyostelium discoideum

We have shown that ammonia inhibits stalk cell formation and promotes spore formation during cyclic AMP induced differentiation of monolayers of sporogenous amoebae (Gross, J.D. et al., Nature (London), 303, 244-245, 1983). Here we show that exposure to ammonia favours the accumulation of prespore- over prestalk-specific enzymes and related products. We conclude that ammonia switches cells from the prestalk to the prespore pathway rather than simply inhibiting stalk cell maturation and hence may act as a morphogen during development.

Studies on the accumulation of the differentiation-inducing factor (DIF) in high-cell-density monolayers of Dictyostelium discoideum

A number of factors that have been shown to influence cell type determination in Dictyostelium discoideum were assessed for their effects on the accumulation of the stalk cell differentiation-inducing factor (DIF) in high-cell-density monolayers of strain V12-M2. DIF accumulation is markedly enhanced by low pH, butyrate, and the proton pump inhibitor diethylstilbestrol (DES), conditions that induce stalk cell formation in low-cell-density monolayers in the absence of added DIF. These results are discussed in relation to a model for cell type determination recently proposed by (J.D. Gross, M.J. Peacey, and R. Pogge Von Strandmann (1988, Differentiation, 38: 91-98). DIF accumulates in high-cell-density monolayers after the cells have become independent of cyclic AMP for stalk cell formation. This accumulation is greatly enhanced by the addition of cyclic AMP. This result may explain why cyclic AMP stimulates stalk cell formation in low-density monolayers in the presence of suboptimal levels of DIF, following preincubation in the presence of saturating levels of cyclic AMP (L. Kwong, A. Sobolewski, and G. Weeks, 1988, Differentiation 37, 1-6). Adenosine has no effect on DIF accumulation in high-cell-density monolayers.

Changes in internal and external pH accompanying growth of Candida albicans: studies of non-dimorphic variants

Non-dimorphic variants of Candida albicans, which were unable to form germ tubes or mature hyphae in media containing amino acids, glucose and salts or N-acetylglucosamine or serum, were prepared from two hyphal positive laboratory strains using a physical separation method. The hyphal-minus phenotype was stable and may be due to mutations or phenotypic variation. The variant strains maintained their internal pH within narrower bounds as compared to their parental wild-types. When exposed to conditions that normally supported the induction of germ tubes the cytoplasmic pH of the wild type strains increased from 6.8 to over pH 8.0 within 5 min while in the variants the rise in internal pH was only about 0.3 pH units. The wild type strains acidified the growth medium more rapidly than the variants. The results suggest that the control of internal pH is directly or indirectly associated with the regulation of dimorphism. The variants had unaltered cell volumes and specific growth rates. The hyphal-minus phenotype was however fully reversible since revertants occurred spontaneously on serum containing agar.

Measurement of the cytoplasmic pH of Dictyostelium discoideum using a low light level microspectrofluorometer

Pyranine was employed as a sensitive pH indicator in a low light level microspectrofluorometer. The in vivo and in vitro standard curves of the 460/410-nm fluorescence excitation ratio of pyranine as a function of pH are identical. Therefore, pyranine is specifically sensitive to cytoplasmic pH in Dictyostelium. The cytoplasmic pH of single cells in a population of Dictyostelium discoideum amoebae was obtained for the first time. The median cytoplasmic pH of vegetative amoebae was 7.19. Carbonyl cyanide m-chlorophenylhydrazone, a mitochondrial uncoupler and a protonophore, lowered the median cytoplasmic pH to 6.12 when the extracellular pH was 6.1. This result is in accord with the protonophore activity of carbonyl cyanide m-chlorophenylhydrazone. Interest in the cytoplasmic pH of Dictyostelium has been greatly stimulated by the theory that cytoplasmic acidification promotes development of pre-stalk cells, while cytoplasmic alkalinization favors the pre-spore pathway (Gross, J. D., J. Bradbury, R. R. Kay, M. J. Peacey. 1983. Nature (Lond.). 303:244-245). The theory postulates that diethylstilbestrol (DES), an inducer of stalk cell differentiation and a plasma membrane proton translocating ATPase inhibitor, should cause acidification of the cytosol. Previous measurements of the effects of stalk cell inducers including DES on intracellular pH using 31P nuclear magnetic resonance measurements have failed to confirm the predictions of the theory, and have suggested that significant modification of the model may be required. Using pyranine as the pH indicator, we find that the median cytoplasmic pH in cells treated with 10 microM DES dropped from 7.19 to pH 6.02. This effect is consistent with the pharmacological action of DES and with the proposal that DES, a stalk cell inducer, should acidify the cytosol. These results provide direct support for the theory that cytoplasmic pH is an essential regulator of the developmental pathway in Dictyostelium.