Correlations between tip dominance, prestalk/prespore pattern, and CAMP-relay efficiency in slugs of Dictyostelium discoideum

Evidence for positional differentiation of prestalk cells and for a morphogenetic gradient in Dictyostelium

We present evidence that Dictyostelium slug tip cells, the pstA cells, may arise by positional differentiation, but at a site remote from that which they will eventually occupy. When first detectable, the pstA cells form a peripheral ring surrounding the other prestalk cell subtype, the pstO cells, but subsequently move above the pstO cells to form the tip. Because pstA cell differentiation requires a 10-fold higher concentration of differentiation-inducing factor, the stalk cell inducer, the initial patterning seems likely to reflect the existence of a morphogenetic gradient. The subsequent redistribution of the two cell types is explicable by their different rates of chemotaxis to cyclic AMP. These results help reconcile the two apparently opposing views of pattern formation in Dictyostelium, that there is positional differentiation and that pattern formation occurs by cell sorting.

Patterns of free calcium in multicellular stages of Dictyostelium expressing jellyfish apoaequorin

To examine the patterns of high free cytosolic calcium or [Ca2+]i during Dictyostelium's development, we expressed apoaequorin in D. discoideum, reconstituted aequorin and observed the resultant patterns of calcium-dependent luminescence. Specific, high calcium zones are seen throughout normal multicellular development and are roughly coincident with those regions that later differentiate into stalk or stalk-like cells. A slug, for example, shows a primary high calcium zone within its front quarter and a secondary one around its tail; while a mound shows such a zone around the periphery of its base. Combined with previous evidence, our findings support the hypothesis that high [Ca2+]i feeds back to favor the stalk pathway. We also discovered several high calcium zones within the mound's base that do not coincide with any known prepatterns in D. discoideum. These include two, relatively persistent, antipodal strips along the mound's periphery. These various persistent zones of high calcium are largely made up of frequent, 10 to 30 second long, semiperiodic calcium spikes. Each of these spikes generates a correspondingly short-lived, 200 to 500 microns long, high calcium band which extends along the nearby surface. Similar, but relatively large and infrequent, spikes generate cross bands which extend across migrating slugs and just behind their advancing tips as well as across the peripheries of rotating mounds and midway between their antipodal strips. Moreover, calcium has a doubling time of about a second as various spikes rise. This last observation suggests that the calcium bands seen in Dictyostelium may be generated by so-called fast calcium waves.

Integration of signaling information in controlling cell-fate decisions in Dictyostelium

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Length regulation in the Dictyostelium discoideum slug is a late event

Time-lapse video light microscopy was used to study the emergence and maturation of the migratory slug from a D. discoideum aggregate. The anterior part, the tip of this simple multicellular organism, establishes migration prior to the definition of the rear, and hence the length of the slug. It was found that newly formed slugs of wild-type strain WS380B can reach lengths greater than 1 cm, yet mature slugs of this strain are rarely longer than 2-3 mumm. Often the tip extended out of the aggregation mound upon an arching pillar of cells. After the tip first touched the substratum, it commenced migration with a rapid succession of movement steps. Here we show that at the initiation of migration, a differential rate of cell movement along the developing slug axis results in a series of complicated changes, before the stable and mature shape of the slug is formed. Our results lead to new conclusions about D. discoideum slug formation and shape maintenance. Evidence is presented for regulation of slug length.

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.

Conditions that elevate intracellular cyclic AMP levels promote spore formation in Dictyostelium

We have been using sporogenous mutants of Dictyostelium discoideum strain V12M2 to study regulation of cell fate during terminal differentiation of spores and stalk cells. Analyses of intracellular cAMP accumulation, cAMP secretion, cAMP binding to cell surface receptors, and chemotactic sensitivity to exogenous cAMP during aggregation showed that all of these functions were identical in V12M2 and HB200, a sporogenous mutant. We used several methods of altering intracellular cAMP levels in HB200 cells to test the hypothesis that intracellular cAMP levels affect cell fate. First, HB200 amoebae were treated with 5 mM caffeine for 4 h during growth, washed, and allowed to develop in the absence of caffeine. Treated cells had normal levels of intracellular cAMP and adenylate cyclase activities at the beginning of differentiation; by 6 h development, they contained two to three times more intracellular cAMP and two times more GTP-dependent adenylate cyclase activity than untreated cells. However, their level of basal Mn++-dependent adenylate cyclase activity was the same as untreated controls. Thus, treatment of growing HB200 amoebae with caffeine for only 4 h leads to hyperinduction of a GTP-dependent regulator (or inhibition of a negative regulator) of adenylate cyclase during subsequent differentiation, without induction of basal activity. The fraction of amoebae forming spores increased twofold when HB200 amoebae were treated with caffeine during growth. Spore (but not stalk cell) differentiation by such treated cells was blocked by inhibitors of cAMP accumulation. Second, cells grown on nutrient agar accumulated higher levels of intracellular cAMP and formed more spores in vitro than cells grown in shaken suspension.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced cAMP secretion in Dictyostelium discoideum mutant HB3

Extracellular cAMP induces the intracellular synthesis and subsequent secretion of cAMP in Dictyostelium discoideum (relay). cAMP relay was strongly diminished in mutant HB3 which shows abnormal development by making very small fruiting bodies. Extracellular cAMP binds to receptors on the surface of mutant cells and induces the rapid activation of adenylate cyclase. Intracellular cAMP rises to a concentration as high as that in wild-type cells but only a very small amount of cAMP is secreted. cAMP secretion in wild-type cells starts immediately after cAMP production, and is proportional to the intracellular cAMP concentration. In the mutant cells cAMP secretion starts a few minutes after cAMP production; by that time most of the intracellular cAMP is already degraded by phosphodiesterase and little cAMP is available for secretion. We conclude that mutant HB3 has a defect in the mechanism by which Dictyostelium cells secrete cAMP.

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

The cytoplasmic pH (pHi) of populations of developing Dictyostelium discoideum cells was determined by means of two independent pH null-point methods. Both methods reveal in populations containing 75-80% prespore cells a pHi value of about 0.2 pH units higher than in populations containing 50% prespore cells. During the process of cell type regulation, decreases and increases in the percentage of prespore cells of about 15-20% are accompanied by decreases and increases in pHi of about 0.2 pH units. Abolition of these changes in pHi by means of a weak base or acid also prevents the regulation process. It is concluded that changes in pHi are involved in the prespore cell type regulation in D. discoideum.

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

We investigated the effect of LiCl on pattern formation and cAMP-regulated gene expression in Dictyostelium discoideum. In intact slugs, 5 mM LiCl induces an almost complete redifferentiation of prespore into prestalk cells. We found that LiCl acts by interfering with the transduction of extracellular cAMP to cell-type-specific gene expression; LiCl inhibits the induction of prespore-specific gene expression by cAMP, while it promotes the induction of prestalk-associated gene expression by cAMP. Our results indicate that two divergent pathways transduce the extracellular cAMP signal to, respectively, prestalk and prespore gene expression.

Altered cyclic-AMP receptor activity and morphogenesis in a chemosensory mutant of Dictyostelium discoideum

The functional properties of the cell-surface cyclic-AMP receptor that controls chemotaxis were found to be altered in an aggregation mutant of Dictyostelium discoideum. The mutant aggregated without stream formation and had a tenfold increased cell-density requirement for the initiation of aggregation. After aggregation, mounds formed multiple tips and subsequently subdivided to give multiple fruits that were small and abnormally proportioned. Cyclic-AMP-induced light-scattering changes in cell suspensions indicated that the mutant had a diminished response to external cyclic-AMP signals. Associated with these altered functional responses was a physical change in the cyclic-AMP sensory system. Cyclic-AMP-binding studies showed that the parent had two classes of cyclic-AMP binding sites, i.e., Kd = 32 and 110 nM. In contrast, the mutant had two- to threefold or more high-affinity sites (Kd = 25 nM) and altered low-affinity sites (Kd less than 3 microM). These results indicate that both affinity classes of binding site are independently mutable. This observation suggests that the two affinity classes can be interconverted by mutation, or the mutation alters a single molecular species and its equilibrium between binding sites with different affinities for cyclic AMP, as postulated in receptor cycling models.

Production and turnover of cAMP signals by prestalk and prespore cells in Dictyostelium discoideum cell aggregates

Dictyostelium discoideum prestalk cells and prespore cells from migrating slugs and culminating cell aggregates were isolated by Percoll density centrifugation. Several activities relevant to the generation, detection, and turnover of extracellular cyclic AMP (cAMP) signals were determined. It was found that: the two cell types have the same basal adenylate cyclase activity; prespore cells and prestalk cells are able to relay the extracellular cAMP signal equally well; intact prestalk cells show a threefold higher cAMP phosphodiesterase activity on the cell surface than prespore cells, whereas their cytosolic activity is the same; intact prestalk cells bind three to four times more cAMP than prespore cells; no large differences in cAMP metabolism and detection were observed between cells derived from migrating slugs and culminating aggregates. The results are discussed in relation to the possible morphogenetic role of extracellular cAMP in Dictyostelium cell aggregates. On the basis of the properties of the isolated cells we assume that a gradient of extracellular cAMP exists in Dictyostelium aggregates. This gradient appears to be involved in the formation and stabilization of the prestalk-prespore cell pattern.

Postaggregative differentiation induction by cyclic AMP in Dictyostelium: intracellular transduction pathway and requirement for additional stimuli

Cyclic AMP induces postaggregative differentiation in aggregation competent cells of Dictyostelium by interacting with cell surface cAMP receptors. We investigated the transduction pathway of this response and additional requirements for the induction of postaggregative differentiation. Optimal induction of postaggregative gene expression requires that vegetative cells are first exposed to 2-4 hr of nanomolar cAMP pulses, and subsequently for 4-6 hr to steady-state cAMP concentrations in the micromolar range. Cyclic AMP pulses, which are endogenously produced before and during aggregation, induce full responsiveness to cAMP as a morphogen. The transduction pathway from the cell surface cAMP receptor to postaggregative gene expression may involve Ca2+ ions as intracellular messengers. A cAMP-induced increase in intracellular cAMP or cGMP levels is not involved in the transduction pathway.

Interactions between adenosine and oscillatory cAMP signaling regulate size and pattern in Dictyostelium

We present evidence for the hypothesis that in multicellular structures of Dictyostelium, production of adenosine by hydrolysis of cAMP near the tip region prevents both generation of competing tips and differentiation of prespore cells near the tip, and thus establishes a "prestalk" region. We demonstrate that adenosine affects the immunological prespore specific staining pattern in slugs in a manner opposite to cAMP:cAMP induces an increase of prespore antigen; adenosine induces a decrease. When endogenous adenosine is removed from slugs, prespore vacuoles are synthesized throughout the prestalk region. Adenosine was found to inhibit the induction of prespore differentiation by cAMP in an apparently competitive manner. It was also found that adenosine specifically increased the amount of tissue controlled by one tip, probably by inhibiting generation of competing oscillators. Removing endogenous adenosine from slugs resulted in a decrease of tip dominance.