Evolving Concept of Aortic Valve Replacement in Asymptomatic Patients With Severe Aortic Stenosis

BACKGROUND

Surgical treatment of asymptomatic severe aortic stenosis (AS) has been gaining attention ever since the results of the Early Surgery Versus Conventional Treatment in Very Severe Aortic Stenosis (RECOVERY) and Aortic Valve replacement versus conservative treatment in Asymptomatic seveRe aortic stenosis (AVATAR) trials showed survival benefits with early surgical aortic valve replacement (SAVR). This study analyzed the long-term clinical and echocardiographic outcomes of SAVR in asymptomatic severe AS.

METHODS

Between 2002 and 2020, 272 patients with asymptomatic severe AS and a left ventricular ejection fraction ≥50% underwent SAVR with or without concomitant aortic surgery and met the study criteria. The median follow-up was 8.5 years (interquartile range, 6-12.8 years), for a total of 2584 patient-years. The time course of the left ventricular mass index (LVMI) and the average E/E' (ratio of the Doppler-derived E wave to the tissue Doppler-derived E' wave) were assessed using 594 postoperative echocardiograms. The association of preoperative LVMI and average E/E' with survival was assessed using Cox proportional hazards.

RESULTS

There was no operative mortality. On longitudinal analyses, LVMI improved in patients who presented with moderate or severe preoperative left ventricular hypertrophy (LVH). However, after the early decline in average E/E', there was a late increase to greater than upper limit normal, particularly in patients with a preoperative average E/E'≥14. Postoperative survival was 100%, 94%, 84%, and 76% at 1, 5, 10, and 15 years, respectively, comparable to age- and sex-matched expected survival on the basis of the US general population. On adjusted Cox survival analysis, only moderate to severe LVH was associated with a survival penalty (hazard ratio], 2.32; 95% CI, 1.02-5.27; P = .045).

CONCLUSIONS

In asymptomatic patients with AS, SAVR restores survival and improves LVH, but patients with diastolic dysfunction are left with persistent dysfunction. Presentation with moderate or severe LVH at the time of surgery translated to a survival penalty. This observational study supports early SAVR in this population before development of LVH, although further investigation is needed.

Minimally invasive, surgical, and transcatheter aortic valve replacement: A network meta-analysis

BACKGROUND

Transcatheter aortic valve replacement (TAVR) has evolved as an alternative to surgical aortic valve replacement (SAVR). In addition to full-sternotomy (FS), recent reports have shown successful minimally-invasive SAVR approaches, including mini-sternotomy (MS) and mini-thoracotomy (MT). This network-meta-analysis (NMA) seeks to provide an outcomes comparison based on these different modalities (MS, MT, TAVR) compared with FS as a reference arm for the management of aortic valve disease.

METHODS

A comprehensive literature search was performed to identify studies that compared minimally-invasive SAVR (MS/MT) to conventional FS-SAVR, and/or TAVR. Bayesian NMA was performed using the random effects model. Outcomes were pooled as risk ratios (RR) with their 95 % confidence intervals (CIs). Our primary outcomes included 30-day mortality, stroke, acute kidney injury (AKI), major bleeding, new permanent pacemaker (PPM), and paravalvular leak (PVL). We also assessed long-term mortality at the latest follow-up.

RESULTS

A total of 27,117 patients (56 studies) were included; 10,397 patients had FS SAVR, 9523 had MS, 5487 had MT, and 1710 had TAVR. Compared to FS, MS was associated with statistically-significantly lower rates of 30-day mortality (RR, 0.76, 95%CI 0.59-0.98), stroke (RR, 0.84, 95%CI 0.72-0.97), AKI (RR, 0.76, 95%CI 0.61-0.94), and long-term mortality (RR 0.84, 95%CI 0.72-0.97) at a weighted mean follow-up duration of 10.4 years, while MT showed statistically-significantly higher rates of 30-day PVL (RR, 3.76, 95%CI 1.31-10.85) and major bleeding (RR 1.45; 95%CI 1.08-1.94). TAVR had statistically significant lower rates of 30-day AKI (RR 0.49, 95%CI 0.31-0.77), but showed statistically-significantly higher PPM (RR 2.50; 95%CI 1.60-3.91) and 30-day PVL (RR 12.85, 95%CI 5.05-32.68) compared to FS.

CONCLUSIONS

MS was protective against 30-day mortality, stroke, AKI, and long-term mortality compared to FS; TAVR showed higher rates of 30-day PVL and PPM but was protective against AKI. Conversely, MT showed higher rates of 30-day PVL and major bleeding. With the emergence of TAVR, the appropriate benchmarks for SAVR comparison in future trials should be the minimally-invasive SAVR approaches to provide clinical equipoise.

Incidence, characteristics, and outcomes of reintervention after mitral transcatheter edge-to-edge repair

OBJECTIVE

The use of transcatheter edge-to-edge repair (TEER) is growing substantially, and reintervention after TEER by way of repeat TEER or mitral valve surgery (MVS) is increasing as a result. In this nationally representative study we examined the incidence, characteristics, and outcomes of reintervention after index TEER.

METHODS

Between July 2013 and November 2017, we reviewed 11,396 patients who underwent index TEER using Medicare beneficiary data. These patients were prospectively tracked and identified as having repeat TEER or MVS. Primary outcomes included 30-day mortality, 30-day readmission, 30-day composite morbidity, and cumulative survival.

RESULTS

Among 11,396 patients who underwent TEER, 548 patients (4.8%) required reintervention after a median time interval of 4.5 months. Overall 30-day mortality was 8.6%, 30-day readmission was 20.9%, and 30-day composite morbidity was 48.2%. According to reintervention type, 294 (53.7%) patients underwent repeat TEER, and 254 (46.3%) underwent MVS. Patients who underwent MVS were more likely to be younger and female, but had a similar comorbidity burden compared with the repeat TEER cohort. After adjustment, there were no differences in 30-day mortality (adjusted odds ratio [AOR], 1.26 [95% CI, 0.65-2.45]) or 30-day readmission (AOR, 1.14 [95% CI, 0.72-1.81]). MVS was associated with higher 30-day morbidity (AOR, 4.76 [95% CI, 3.17-7.14]) compared with repeat TEER. Requirement for reintervention was an independent risk factor for long-term mortality in a Cox proportional hazard model (hazard ratio, 3.26 [95% CI, 2.53-4.20]).

CONCLUSIONS

Reintervention after index TEER is a high-risk procedure that carries a significant mortality burden. This highlights the importance of ensuring procedural success for index TEER to avoid the morbidity of reintervention altogether.

Comparison of Postdischarge Outcomes Between Valve-in-Valve Transcatheter Mitral Valve Replacement and Reoperative Surgical Mitral Valve Replacement

Limited data are available comparing the postdischarge perioperative outcomes of isolated valve-in-valve transcatheter mitral valve replacement (VIV-TMVR) versus surgical reoperative mitral valve replacement (re-SMVR) on a nationwide scale. The objective of this study was to perform a robust head-to-head assessment of contemporary postdischarge outcomes between isolated VIV-TMVR and re-SMVR using a large national multicenter longitudinal database. Adult patients aged ≥18 years with failed/degenerated bioprosthetic mitral valves who underwent either isolated VIV-TMVR or re-SMVR were identified in the 2015 to 2019 Nationwide Readmissions Database. The risk-adjusted differences in 30-, 90-, and 180-day outcomes were compared using propensity score weighting with overlap weights to mimic the results of a randomized controlled trial. The differences between a transeptal and transapical VIV-TMVR approach were also compared. A total of 687 patients with VIV-TMVR and 2,047 patients with re-SMVR were included. After the overlap weighting to attain balance between treatment groups, VIV-TMVR was associated with significantly lower major morbidity within 30 (odds ratio [95% confidence interval (CI)] 0.0.31 [0.22 to 0.46]), 90 (0.34 [0.23 to 0.50]), and 180 (0.35 [0.24 to 0.51]) days. The differences in major morbidity were primarily driven by less major bleeding (0.20 [0.14 to 0.30]), new onset complete heart block (0.48 [0.28 to 0.84]) and need for permanent pacemaker placement (0.26 [0.12 to 0.55]). The differences in renal failure and stroke were not significant. VIV-TMVR was also associated with shorter index hospital stays (median difference [95% CI] -7.0 [4.9 to 9.1] days) and an increased ability for patients to be discharged home (odds ratio [95% CI] 3.35 [2.37 to 4.72]). There were no significant differences in total hospital costs; in-hospital or 30-, 90-, and 180-day mortality; or readmission. The findings remained similar when stratifying the VIV-TMVR access using a transeptal versus a transapical approach. The changes in outcomes over time suggest marked improvements for patients with VIV-TMVR relative to stagnant results for patients with re-SMVR from 2015 to 2019. In this large nationally representative cohort of patients with failed/degenerated bioprosthetic mitral valves, VIV-TMVR appears to confer a short-term advantage over re-SMVR in terms of morbidity, discharge home, and length of stay. It yielded equivalent outcomes for mortality and readmission. Longer-term studies are needed to assess further follow-up beyond 180 days.

Calcium induces cyclic GMP formation in Dictyostelium

Cyclic GMP is rapidly formed a few seconds after binding of chemotactic signalling molecules to specific receptors on the cell surface of Dictyostelium amoebae. This phenomenon could be mimicked by addition of a pulse of Ca2+ to permeabilised amoebae. The concentration of Ca2+ for half-maximal response was 60 microM. Other ions (K+, Na+, Mg+ or Mn+) had no effect. A pulse of 5 microM IP3 produced a cyclic GMP response of similar magnitude but IP2 elicited no response. The data provide strong support for the hypothesis that cell surface receptor binding induces cyclic GMP formation by liberating Ca2+ from internal stores.

Inhibition of cyclic GMP formation and aggregation inDictyosteliumby the intracellular Ca2+antagonist TMB-8

Aggregation in Dictyostelium discoideum was shown in previous studies employing EGTA to require Ca2+, but the intra- or extracellular site of action of this ion and its role in chemotaxis were not determined [1]. In this investigation we show that the intracellular Ca2+ immobilising agent TMB-8 does not affect binding of the signalling nucleotide, cAMP, to the cell surface receptors but abolishes the rapid accumulation of intracellular cGMP and subsequent chemotactic aggregation. We infer that movement of Ca2+ from membrane-bound stores is triggered by binding of cAMP to the cell-surface receptor and that this plays a primary role in stimulating cGMP formation and chemotaxis.

Calcium transport in the cellular slime mould Dictyostelium discoideum

Transport of Ca2+ into amoebae of Dictyostelium discoideum was studied using 45Ca and a lanthanum stopping technique. Ca2 uptake was found to be rapid and showed saturation kinetics. No difference was found in Ca2+ uptake between vegetative and aggregation competent cells, the V(max) for unstimulated amoebae being approx. 10 nmol/10(7) cells per min. Ca2+ uptake had the characteristics of passive facilitated diffusion using a saturatable carrier and NaN3 and ouabain were not inhibitory. The chemoattractants cAMP and folate, previously reported to stimulate the uptake of Ca2+ into amoebae, did not stimulate the rate of Ca2+ uptake by this carrier but increased the extent of Ca2+ taken up over the period 10-30 s after chemotactic stimulation. The significance of these findings for the function of Ca2+ in chemotactic signalling is discussed.

Identification of STKA-dependent genes in Dictyostelium discoideum

During culmination of Dictyostelium aggregates, prespore and prestalk cells undergo terminal differentiation to form spores and a cellular stalk. Disruption of the cell-fate gene stkA leads to a phenotype in which all the cells destined to become spores end up as stalk cells. 'Stalky' mutants express normal levels of prespore cell transcripts but fail to produce the culmination-stage spore transcript spiA. The stkA gene encodes a putative GATA-type transcription factor (STKA). In order to identify possible downstream targets of STKA we used the technique of mRNA differential display and isolated four cDNA fragments that hybridise to mRNAs present during the later stages of development. All four gene tags were cloned and sequenced. mRNAs represented by these four sequence tags do not accumulate during culmination of 'stalky' cells and therefore must be specific to the spore pathway. By screening a cDNA library, longer cDNAs for all four were cloned and sequenced. Three of these contained complete protein-coding regions while only a partial cDNA was recovered for the fourth. One of the corresponding proteins has significant homology to a surface zinc metalloproteinase (GP63) of the protozoan parasite Leishmania, while another is closely related to a human pre-RNA binding protein (hnRNP R).

Pyridoxal kinase knockout of Dictyostelium complemented by the human homologue

The gene (pykA) encoding pyridoxal kinase which converts pyridoxal (vitamin B(6)) to pyridoxal phosphate was isolated from Dictyostelium discoideum using insertional mutagenesis. Cells of a pykA gene knockout grew poorly in axenic medium with low yield but growth was restored by the addition of pyridoxal phosphate. Sequencing indicated a gene, with one intron, encoding a predicted protein of 301 amino acids that was 42% identical in amino acid sequence to human pyridoxal kinase. After expression of the wild-type gene in Escherichia coli, the purified PykA protein product was shown to have pyridoxal kinase enzymatic activity with a K(m) of 8.7 microM for pyridoxal. Transformation of the Dictyostelium knockout mutant with the human pyridoxal kinase gene gave almost the same level of complementation as that seen using transformation with the wild-type Dictyostelium gene. Phylogenetic analysis indicated that the Dictyostelium amino acid sequence was closer to human pyridoxal kinase than to pyridoxal kinases of lower eukaryotes.

A negative regulatory element in a prespore-specific promoter of dictyostelium discoideum(1)

We previously isolated several 'promoter-trap' transformants in which insertion of a promoterless beta-galactosidase gene into the genome caused expression of beta-galactosidase in specific cell types. The upstream flanking region was rescued from one transformant specifically expressing beta-galactosidase in prespore cells. We sequenced the promoter of the gene that is fused in-frame with lacZ and characterised a negative element that inhibits expression in pstO cells (a subtype of prestalk cells). Gel-retardation assays show that a developmentally regulated factor(s) recognises and binds to this element.

A myb-related protein required for culmination in Dictyostelium

The avian retroviral v-myb gene and its cellular homologues throughout the animal and plant kingdoms contain a conserved DNA binding domain. We have isolated an insertional mutant of Dictyostelium unable to switch from slug migration to fruiting body formation i.e. unable to culminate. The gene that is disrupted, mybC, codes for a protein with a myb-like domain that is recognized by an antibody against the v-myb repeat domain. During development of myb+ cells, mybC is expressed only in prestalk cells. When developed together with wild-type cells mybC- cells are able to form both spores and stalk cells very efficiently. Their developmental defect is also bypassed by overexpressing cAMP-dependent protein kinase. However even when their defect is bypassed, mybC null slugs and culminates produce little if any of the intercellular signalling peptides SDF-1 and SDF-2 that are believed to be released by prestalk cells at culmination. We propose that the mybC gene product is required for an intercellular signaling process controlling maturation of stalk cells and spores and that SDF-1 and/or SDF-2 may be implicated in this process.

Isolation of spermidine synthase gene (spsA) of Dictyostelium discoideum

The gene encoding spermidine synthase (spsA) was isolated from Dictyostelium discoideum using the technique of insertional mutagenesis. Northern blot analysis showed that the spsA mRNA is expressed maximally during the vegetative stage and decreases gradually during the 24 h of development. Sequencing of the genomic DNA and a full-length cDNA clone indicated the presence of one intron in a gene coding for a predicted protein (SpsA) with 284 amino acids. The sequence is highly conserved, with amino acid identities compared to spermidine synthases of humans, 59.5%, to mouse, 61.3%, and to yeast, 58.1%. A null mutant of the spsA gene is unable to grow in the absence of exogenous spermidine. Development of spsA null cells grown in the absence of spermidine produced fruiting bodies that have abnormally short stalks.

An IgG monoclonal antibody against Dictyostelium discoideum glycoproteins specifically recognizes Fucalpha1,6GlcNAcbeta in the core of N-linked glycans. Localized expression of core-fucosylated glycoconjugates in human tissues

Core fucosylation of N-linked oligosaccharides (GlcNAcbeta1, 4(Fucalpha1,6)GlcNAcbeta1-Asn) is a common modification in animal glycans, but little is known about the distribution of core-fucosylated glycoproteins in mammalian tissues. Two monoclonal antibodies, CAB2 and CAB4, previously raised against carbohydrate epitopes of Dictyostelium discoideum glycoproteins (Crandall, I. E. and Newell, P. C. (1989) Development 107, 87-94), specifically recognize fucose residues in alpha1,6-linkage to the asparagine-bound GlcNAc of N-linked oligosaccharides. These IgG3 antibodies do not cross-react with glycoproteins containing alpha-fucoses in other linkages commonly seen in N- or O-linked sugar chains. CAB4 recognizes core alpha1,6 fucose regardless of terminal sugars, branching pattern, sialic acid linkage, or polylactosamine substitution. This contrasts to lentil and pea lectins that recognize a similar epitope in only a subset of these structures. Additional GlcNAc residues found in the core of N-glycans from dominant Chinese hamster ovary cell mutants LEC14 and LEC18 progressively decrease binding. These antibodies show that many proteins in human tissues are core-fucosylated, but their expression is localized to skin keratinocytes, vascular and visceral smooth muscle cells, epithelia, and some extracellular matrix-like material surrounding subpopulations of lymphocytes. The availability of these antibodies now allows for an extended investigation of core fucose epitope expression in development and malignancy and in genetically manipulated mice.

Identification of the cell fate gene stalky in Dictyostelium

Using insertional mutagenesis, we have isolated a "stalky" mutant in which cells destined to become spores end up as stalk cells. Similar mutants were previously observed after chemical mutagenesis, but the affected gene could not be isolated. Our mutant, like the previous ones, is in stkA. Its defect is cell-autonomous and not overcome by overexpressing cAMP-dependent protein kinase. stkA is strongly expressed in the prespore region of aggregates but not in the anterior prestalk zone. The mutant expresses normal levels of prespore-cell transcripts but fails to produce the spore transcript spiA. stkA encodes a predicted 99 kDa protein (STKA) with two putative C4 zinc fingers, one of which is a GATA-type finger, indicating that it may be a transcription factor. This conclusion is supported by localization of STKA in the nucleus.

The role of calcium in aggregation and development of Dictyostelium

Changes in cytosolic Ca2+ play an important role in a wide array of cell types and the control of its concentration depends upon the interplay of many cellular constituents. Resting cells maintain cytosolic calcium ([Ca2+]i) at a low level in the face of steep gradients of extracellular and sequestered Ca2+. Many different signals can provoke the opening of calcium channels in the plasma membrane or in intracellular compartments and cause rapid influx of Ca2+ into the cytosol and elevation of [Ca2+]i. After such stimulation Ca2+ ATPases located in the plasma membrane and in the membranes of intracellular stores rapidly return [Ca2+]i to its basal level. Such responses to elevation of [Ca2+]i are a part of an important signal transduction mechanism that uses calcium (often via the binding protein calmodulin) to mediate a variety of cellular actions responsive to outside influences.

Signal transduction and motility of Dictyostelium

This review is concerned with the roles of cyclic GMP and Ca(2+) ions in signal transduction for chemotaxis of Dictyostelium. These molecules are involved in signalling between the cell surface cyclic AMP receptors and cytoskeletal myosin II involved in chemotactic cell movement. Evidence is presented for uptake and/or efflux of Ca(2+) being regulated by cyclic GMP. The link between Ca(2+), cyclic GMP and chemotactic cell movement has been explored using "streamer F" mutants whose primary defect is in the structural gene for the cyclic GMP-specific phosphodiesterase. This mutation causes the mutants to produce an abnormally prolonged peak of cyclic GMP accumulation in response to stimulation with the chemoattractant cyclic AMP. The production and relay of cyclic AMP signals is normal in these mutants, but certain events associated with movement are (like the cyclic GMP response) abnormally prolonged in the mutants. These events include Ca(2+) uptake, myosin II association with the cytoskeleton and regulation of both myosin heavy and light chain phosphorylation. These changes can be correlated with changes in the shape of the amoebae after chemotactic stimulation. Other mutants in which the accumulation of cyclic GMP in response to cyclic AMP stimulation was absent produced no myosin II responses. A model is described in which cyclic GMP (directly or indirectly via Ca(2+) regulates accumulation of myosin II on the cytoskeleton by regulating phosphorylation of the myosin heavy and light chain kinases.

Trapping developmental promoters in Dictyostelium

Recently an insertional mutagenesis procedure has been developed to permit cloning of genes affected in developmental mutants of Dictyostelium discoideum (Kuspa and Loomis, Proc. Natl. Acad. Sci. USA 89, 8803-8807, 1992). In this procedure a plasmid bearing the URA (pyr5-6) gene is linearized with a restriction enzyme and electroporated into URA- amoebae (auxotrophic for uracil) together with the corresponding restriction enzyme. Transformants that can grow without uracil are screened for developmental defects resulting from insertion of the plasmid into a gene of developmental importance. We have modified this procedure to permit characterization of the promoters and structural sequences of genes that would be missed by the standard procedure because their disruption produces no obvious phenotype. Constructs carrying a promoter-less Escherichia coli lacZ gene were designed so that expression of lacZ requires insertion into an active host transcription unit. By screening restriction enzyme-generated transformants we have isolated several strains in which lacZ is under the control of a developmentally activated promoter and have cloned the 5' flanking DNA adjacent to the insertion site. Sequencing the junction between plasmid and host genome has confirmed in-frame fusion with the lacZ gene, and reintroduction of the cloned plasmids into parental cells has shown that the cloned sequences do actually contain the relevant promoters. This procedure should give ready access to a wide range of developmental promoters without the need for prior identification of the developmental genes involved.

Regulation of myosin regulatory light chain phosphorylation via cyclic GMP during chemotaxis of Dictyostelium

Previous studies on the chemotactic movement of Dictyostelium have indicated a role for cyclic GMP in regulating the association of myosin II with the cytoskeleton. In this study we have examined the part played by phosphorylation of the 18 kDa myosin regulatory light chain in this event. Using streamer F mutant NP368 (which is deficient in the structural gene for cyclic GMP-specific phosphodiesterase) we find that, for the regulatory light chain kinase, the major peak of phosphorylation is delayed compared to the parental control strain XP55, occurring at 80 seconds rather than about 30 seconds in XP55. In two independently derived mutants that are unable to increase their cellular concentration of cyclic GMP (above basal levels) in response to a chemotactic stimulus of cyclic AMP (KI-10 and SA219), no increase in the phosphorylation of the light chain occurred, or movement of myosin II to the cytoskeleton. We also find a smaller peak of light chain phosphorylation that occurs within 10 seconds of cyclic AMP stimulation of the amoebae, and which is absent in the cyclic GMP-unresponsive strains. We conclude that cyclic GMP is involved in regulating light chain phosphorylation in this system. The possible significance of these findings is discussed and a model that relates these findings to published data on cytoskeletal myosin changes during chemotaxis is presented.

The role of cyclic GMP in regulating myosin during chemotaxis of Dictyostelium: evidence from a mutant lacking the normal cyclic GMP response to cyclic AMP

Evidence has previously been reported that, during chemotaxis of the cellular slime mould Dictyostelium discoideum, cyclic GMP regulates the association of myosin II with the cytoskeleton and that this regulation is effected by inhibiting myosin II heavy chain phosphorylation (Liu and Newell, J. Cell Sci., 90, 123–129, 1988; 98, 483–490, 1991). Here we provide further evidence in support of this hypothesis using a mutant (KI-10) that is defective in chemotaxis and lacks the normal cyclic AMP-induced cyclic GMP response. We found that the cyclic AMP-induced cytoskeletal actin response was similar to that of the parental strain in this mutant (although showing a slight displacement in the dose-response curve) but the cytoskeletal myosin II heavy chain response was abolished. Moreover, the mutant showed no phosphorylation of myosin II heavy chain in response to cyclic AMP. Compared to the parental strain XP55, the mutant cells contained approximately 40% more protein and their doubling time was 30% longer. These differences could be due to differences in the efficiency of cell division, a process in which the proper regulation of myosin function is essential and in which cyclic GMP may therefore play a role.

Streamer F mutants and chemotaxis of Dictyostelium

Streamer F mutants have been found to be useful tools for studying the pathway of signal transduction leading to chemotactic cell movement. The primary defect in these mutants is in the structural gene for the cyclic GMP specific phosphodiesterase. This defect allows a larger and prolonged peak of cyclic GMP to be formed in response to the chemotactic stimulus, cyclic AMP. This characteristic aberrant pattern of cyclic GMP accumulation in the streamer F mutants has been correlated with similar patterns of changes in the influx of calcium from the medium, myosin II association with the cytoskeleton, myosin phosphorylation and a decrease in speed of movement of the amoebae. From these studies a sequence of events can be deduced that leads from cell surface cyclic AMP stimulation to cell polarization prior to movement of the amoebae in response to the chemotactic stimulus.

Accumulation of [3H]-inositol into inositol polyphosphates during development of Dictyostelium

Inositol hexakisphosphate (InsP6) is present in large amounts during the development of Dictyostelium discoideum although its function is unknown. We have investigated the accumulation of [3H]-labelled inositol into both InsP6 and a more highly charged inositol species called InsPY during development of amoebae on filters. We report that the pattern of [3H]-labelled inositol incorporation into InsP6 and InsPY differs markedly from the pattern seen for inositol phospholipids and inositol(1,4,5)trisphosphate. Incorporation into PtdIns, PtdInsP, PtdInsP2 and Ins(1,4,5)P3 reached plateau values within 4 h. In contrast, incorporation into InsP6 continued in an approximately linear manner for the first eight hours of development. No incorporation into InsPY was seen during the first three hours of development at which time accumulation of [3H] continued linearly in a similar manner as for InsP6. Because the total cell concentration of InsP6 (measured by chemical assay) changes very little during development it seems probable that incorporation of label into InsPY after 3 h is due to a developmentally controlled metabolic switch rather than a requirement for a threshold level of its probable precursor, InsP6.

Evidence of cyclic GMP may regulate the association of myosin II heavy chain with the cytoskeleton by inhibiting its phosphorylation

Previous studies have implicated cyclic GMP in the regulation of myosin II heavy chain (MHC) association with the cytoskeleton in Dictyostelium discoideum. Here we provide evidence that cyclic GMP may regulate MHC association with the cytoskeleton through MHC phosphorylation. Comparative data are presented of MHC phosphorylation in the wild-type strain NC4, the parental strain XP55 and streamer mutants NP368 and NP377. Using an anti-MHC monoclonal antibody to immunoprecipitate MHC from [32P]phosphate-labelled developing cells, we found that cyclic AMP stimulation of the wild-type strain NC4 and parental strain XP55 induced MHC phosphorylation in vivo. A peak of phosphorylation was observed at 30–40 s, followed by a gradual decrease to basal level at 160 s. In contrast, in both of the streamer mutants NP368 and NP377 (which have prolonged cyclic GMP accumulation and prolonged MHC association with the cytoskeleton), the phosphorylation of MHC was delayed and did not form a peak until 60–80 s after cyclic AMP stimulation. We also found that cytoskeletal MHC showed only minor phosphorylation, the majority of the phosphorylated MHC being found in the cytosol. We present a model to account for these results in which cyclic GMP regulates MHC association with the cytoskeleton by regulating the phosphorylation/dephosphorylation cycle of MHC in these cells.

Effects of normal and mutant ras genes on inositol lipid metabolism in Dictyostelium

Dictyostelium cells transformed with multiple copies of a mutant Dictyostelium ras gene (ras-Thr12 that gave a Gly to Thr substitution at position 12 of the ras protein, showed 2 to 3 times greater incorporation of 32P into PtdInsP and PtdInsP2 (without changing the specific radioactivity) compared to the untransformed strain or a strain transformed with multiple copies of the normal ras-Gly12 gene. The ratio of labelled PtdInsP2/PtdInsP, however, was not affected by the ras-Thr12 gene. Stimulation with the chemoattractant, cyclic AMP, caused a rapid but transient decrease in the levels of labelled PtdInsP and PtdInsP2 in the normal and ras-Gly12-transformed strains but ras-Thr12-transformed strains failed to respond. In untransformed cells a small, very rapid rise in the level of labelled PtdInsP and PtdInsP2 was seen immediately after stimulation of the cells with cyclic AMP (before the transient decrease) and this rise was greatly accentuated in cells transformed with multiple copies of the normal ras-Gly12 gene. Agents that induce prolonged activation of phosphoinositidase C such as AlF4- or GTPYS gave a lowered steady-state level of incorporation of 32P into PtdInsP and PtdInsP2 in all strains. The results indicate that the enzyme in the inositol phosphate pathway that is affected by the ras gene is not phosphoinositidase C, but is an enzyme before PtdInsP kinase, possibly PtdIns kinase.

Membrane-associated phosphoinositidase C activity in Dictyostelium discoideum

Membrane-associated phosphoinositidase C activity has been identified in Dictyostelium discoideum using phosphatidylinositol 4,5-bisphosphate as exogenous substrate. Maximal activity was observed with 0.4 mM phosphatidylinositol 4,5-bisphosphate at pH 7.0. The enzyme was stimulated by micromolar concentrations of free calcium with maximal activity at 100 microM.

Signal transduction for chemotaxis in Dictyostelium amoebae

The signal for chemotaxis in D. discoideum is cyclic AMP. This molecule binds to cell surface receptors and triggers the production of inositol (1,4,5)trisphosphate which releases Ca2+ from non-mitochondrial stores. The subsequent chain of signal transduction events brings about the polymerization of cytoskeletal actin (associated with pseudopodium formation) within five seconds and the formation of a peak of cyclic GMP within 10 s. Evidence from streamer F mutants indicates that the cyclic GMP regulates the association of myosin with the cytoskeleton that occurs at 25-50 s and that this phenomenon is concerned with elongation of the amoebae during chemotactic movement.

Changes in cell surface glycoproteins during Dictyostelium development analysed using monoclonal antibodies

We have produced a series of monoclonal antibodies that recognize carbohydrate epitopes on cell surface glycoproteins of developing amoebae of Dictyostelium discoideum. The antibodies were found to have differential specificity for amoebae at different stages of development and were classified into types A to E on the basis of their temporal pattern of reactivity with the developing amoebal cell surface. Evidence from Western Blots and digestion of the glycoproteins with alkaline phosphatase were consistent with previous reports that the cell surface glycoproteins are extensively processed during development, leading at 16 h of development to the exposure of a highly antigenic core recognized by antibodies in group E. The nature of this core structure is indicated by the finding that antibodies in group E were found also to bind with high avidity to the plant glycoprotein horse radish peroxidase.

Inositol tris- and polyphosphate formation during chemotaxis of Dictyostelium

Using the technique of HPLC with Partisil SAX columns, we have found that stimulation of amoebae of Dictyostelium discoideum with the chemoattractant cyclic AMP induces the rapid accumulation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), with a peak at 5 s. A smaller HPLC peak (designated P3) that elutes just after the Ins(1,4,5)P3 peak accumulates more slowly to a maximum at 20 s. In control studies, the changes in Ins(1,4,5)P3 were shown not to be due to varying recovery from the cell extracts and a comparison of reverse-phase and Partisil SAX HPLC columns showed similar values for determinations by either method. The involvement of a G-protein in this chemotactic system was confirmed by the finding that accumulation of Ins(1,4,5)P3 was elicited by the addition of GTP gamma S (5′-[gamma-thio]triphosphate) to saponin-permeabilized amoebae. A study of the changes in the lipid-soluble phosphatidyl inositol phosphates demonstrated that cyclic AMP also stimulated a rapid loss of radioactivity from 32P-labelled phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), which corresponded in its timing to the rise in Ins(1,4,5)P3, indicating that a phosphoinositidase C (phospholipase C) is present that can be stimulated by occupation of the cell surface cyclic AMP receptors.

Evidence that cyclic GMP regulates myosin interaction with the cytoskeleton during chemotaxis of Dictyostelium

Amoebae of Dictyostelium discoideum respond to a chemotactic cyclic AMP stimulus within 10 s by the formation of an intracellular peak of cyclic GMP. In wild-type cells the cyclic GMP is rapidly degraded by a cyclic GMP-specific phosphodiesterase. In ‘streamer F’ mutants this enzyme is defective, due to mutation in the enzyme structural gene, and the cyclic GMP persists several times longer than the normal period, an effect that is correlated with a persistence in cell elongation during the chemotactic movement phase.

In this study we have used the streamer mutants NP368 and NP377, and their parental strain XP55, to study changes in cytoskeletal proteins during the chemotactic response. We have studied three proteins that change their association with the cytoskeleton after stimulation of amoebae with the chemoattractant cyclic AMP: (1) actin, (2) a protein with an apparent Mr of 190x103 and (3) myosin heavy chain. Both actin and the 190x103Mr protein were found to accumulate rapidly in the cytoskeleton after cyclic AMP stimulation, with a sharp peak at 5 s, and showed similar changes in the parental and streamer mutants. However, the cytoskeletal level of myosin heavy chain showed a different pattern of changes, which also clearly differed in the streamer mutants compared with the parental strain XP55. In XP55 myosin heavy chain showed an initial drop after cyclic AMP stimulation, with a trough at 3–10 s followed by a rapid rise to a sharp peak at 20–25 s. In contrast, the myosin heavy chain in the streamer mutants produced a broad peak that persisted several times longer than the parental strain.

We conclude that in the streamer mutants the defect in cyclic GMP phosphodiesterase that produces the broad peak of cyclic GMP is causally correlated with the broad peak of cytoskeletal myosin, and we suggest that this is connected with the observed phenotype of prolonged cell elongation during chemotaxis in these mutants.

Mutant ras gene induces elevated levels of inositol tris- and hexakisphosphates in Dictyostelium

Previous studies of Europe-Finner & Newell indicated that in amoebae of Dictyostelium discoideum, signal transduction used for chemotaxis to cyclic AMP involved transient formation of inositol tris- and polyphosphates. Evidence was also presented for the involvement of a GTP-binding G-protein. Here we report evidence for the involvement of a ras gene product in the D. discoideum inositol phosphate pathway. Use was made of strains of Dictyostelium transformed with a wild-type D. discoideum ras gene (ras-Gly12) or a mutant form of the gene (ras-Thr12). Experiments using separation of soluble inositol phosphates by Dowex anion-exchange resin chromatography indicated that cells transformed with the wild-type ras-Gly12 gene were unaffected in their basal levels of inositol polyphosphates and in the inositol phosphates formed in response to stimulation with the chemotactic agent cyclic AMP. In contrast, cells transformed with the mutant ras-Thr12 gene showed a basal level of inositol polyphosphate that was several-fold elevated over the controls and stimulation of these cells with cyclic AMP produced only a small further elevation. When the inositol phosphates were analysed by h.p.l.c. it was found that the basal level of inositol 1,4,5-trisphosphate was raised three- to fivefold in the ras-Thr12 strain compared to the strain transformed with ras-Gly12, and that inositol hexakisphosphate (which was found to be present in large amounts relative to other inositol phosphates in D. discoideum cells) was also raised to a similar extent in the ras-Thr12-transformed cells. We propose that the Dictyostelium ras gene product codes for a regulatory protein involved in the inositol phosphate chemotactic signal-transduction pathway.

Adaptation to chemotactic cyclic AMP signals in Dictyostelium involves the G-protein

Amoebae of Dictyostelium discoideum show adaptation towards a chemotactic cyclic AMP signal. Within a few seconds of receipt of the signal they are inhibited for a period of 1–2 min from further chemotactic responses to subsequent cyclic AMP signals of similar or smaller magnitude. The site of this adaptation mechanism in the chemotactic transduction pathway was investigated by addition of components of the transduction chain (GTP analogues, myo-inositol-1,4,5-trisphosphate (InsP3) and Ca2+) to permeabilized cells followed by determination of the amount of cyclic GMP formed as a measure of the chemotactic response. This approach was made possible by finding that permeabilization of amoebae with saponin did not uncouple the cell surface cyclic AMP receptors from stimulation of cyclic GMP formation. It was found that InsP3 and Ca2+ were ‘downstream’ from the adaptation mechanism: they could trigger a cyclic GMP response in cyclic AMP-adapted amoebae but could not themselves induce adaptation. In contrast, GTP gamma S was unable to trigger a cyclic GMP response in cyclic AMP-adapted cells, although it could trigger multiple cyclic GMP responses in non-adapted cells. We deduce that the site of adaptation to cyclic AMP stimulation is at the G-protein involved in this signalling pathway. Moreover, as GTP gamma S was found to be unable to induce adaptation, we conclude that the mechanism of adaptation involves an action of the cyclic AMP receptor on the G-protein that is distinct from its commonly reported action of stimulating G-protein binding of GTP.

GTP analogues stimulate inositol trisphosphate formation transiently in Dictyostelium

Permeabilization of amoebae of Dictyostelium discoideum with saponin was found not to uncouple the chemotactic cell surface cyclic AMP receptors from inositol trisphosphate (IP3) formation, and stimulation of permeabilized amoebae with 50 nM-cyclic AMP produced peaks of IP3 at 5, 15 and 30 s in a manner comparable to that seen previously in non-permeabilized cells. The possible involvement of a GTP-binding protein (G-protein) in this IP3 signal transduction pathway was investigated by studying the effects on such permeabilized amoebae of added GTP and non-hydrolysable GTP analogues. While GDP produced only very minor effects, stimulation of the amoebae (in the absence of added cyclic AMP) with GTP or the non-hydrolysable GTP analogues GTP gamma S (guanosine 5′-O-(3-thio-triphosphate] and Gpp(NH)p (5′-guanylylimidodiphosphate) induced transient formation of IP3 in an oscillatory manner, with peaks similar in magnitude and timing to those elicited by cyclic AMP. A dose-response curve for GTP gamma S indicated a concentration for half-maximal stimulation of approximately 8 microM. When tested at 300 s after addition of GTP gamma S, the basal level of IP3 was found to be twofold elevated with shallow (presumably asynchronous) oscillations still just discernible. The significance of the IP3 oscillations elicited by GTP and its analogues is discussed in relation to the mechanism of signal adaptation and the presumed role of G-proteins.

Cyclic AMP stimulates accumulation of inositol trisphosphate in Dictyostelium

Previous studies indicated that in Dictyostelium amoebae signal transmission from cell surface cyclic AMP receptors to intracellular events concerned with chemotaxis involves inositol 1,4,5-trisphosphate (1,4,5-IP3): micromolar amounts of 1,4,5-IP3 or Ca2+ were found to mimic the effects of chemoattractants and 1,4,5-IP3 triggered release of Ca2+ from non-mitochondrial stores. Here we report a more direct test of the involvement of inositol phosphates. Intact amoebae were labelled with high specific activity [3H]inositol, then stimulated with the chemoattractant cyclic AMP at 22 degrees C and rapidly assayed for phosphorylated inositol products formed. Labelled IP3 was found to accumulate transiently after a pulse of 50 nM-cyclic AMP, with a peak at 15 s after stimulation and some (inconclusive) evidence for a more rapidly formed peak at 5 s or less. Inositol bisphosphate (IP2) showed a transient shallow peak at about 20 s. When the events of signal transmission were slowed down by incubation at 4 degrees C, the rapidly formed IP3 peak could be consistently seen at 5 s after stimulation and the second peak at 25–30 s. Further resolution of the IP3 peaks indicated the presence of IP4, which represented a major fraction of the peak accumulated at 5 s (4 degrees C). The results provide an important link in the chain of evidence connecting the cell surface cyclic AMP receptors, via IP3, with the Ca2+-activated events of chemotaxis.

Chemotaxis towards pteridines during development of Dictyostelium

Following a previous study indicating a sensitivity to folate during the developmental phase of Dictyostelium discoideum, a series of pteridines were investigated for their ability to induce amoebal chemotaxis during development of this organism. Several compounds were found to resemble folate in their ability to induce chemotaxis of both vegetative amoebae and amoebae developing during the first few hours of starvation. One compound, L-monapterin, was distinct in showing chemotactic activity only during the developmental phase after approximately 12 h of starvation. Tests using the polymerization of cytoskeletal actin as an assay for a cellular response correlated with chemotaxis showed that 10 nM-L-monapterin was a potent inducer of this response and that responsiveness appeared only after 12 h of development. The timing of these events may be correlated with the formation of tips containing stalk cells that occurs in multicellular aggregates at approximately 12 h, and suggests a role for L-monapterin (or a naturally occurring, closely related pteridine) in cell sorting. The evolutionary significance of the roles of pteridines is discussed.

Signal transduction during amoebal chemotaxis of Dictyostelium discoideum

Amoebae of the cellular slime mould Dictyostelium are chemotactically responsive to pulses of cyclic AMP produced by aggregation centres. Pulses of this nucleotide bind to the cell surface cyclic AMP receptors and induce a chain of intracellular events leading to actin polymerization, formation of cyclic GMP and specific induction of gene activity. An intermediary messenger is Ca2+ liberated from internal stores by inositol trisphosphate (IP3).

Inositol 1,4,5-triphosphate induces calcium release from a non- mitochondrial pool in amoebae of Dictyostelium

Evidence is presented for two distinct CA2+ pools in amoebae of Dictyostelium discoideum. One pool, presumably mitochondrial, was sensitive to the mitochondrial inhibitors oligomycin and dinitrophenol and showed an affinity for Ca2+ in the micro M concentration range. The other Ca2+ pool, which was insensitive to these inhibitors, was of lower capacity but had higher affinity (in the nM range). Inositol 1,4,5-trisphosphate (5 micro M) added to saponin-permeabilized amoebae induced a rapid release of Ca2+ from the latter pool but had no effect on the presumed mitochondrial pool. Controls using addition of inositol 1,4-bisphosphate (the hydrolytic product of IP3) induced no such CA2+ release. The results provide strong support for the involvement of IP3 in signal transmission during chemotaxis of D. discoideum.

Inositol 1,4,5-trisphosphate and calcium stimulate actin polymerization in Dictyostelium discoideum

The effect of chemoattractants such as cyclic AMP and folate on amoebae of the cellular slime mould Dictyostelium discoideum is to cause a series of rapid intracellular responses. One of the most rapid of these responses is the polymerization of actin associated with the cytoskeleton, an event correlated with pseudopodium formation, which occurs within 3–5 s of chemotactic receptor stimulation. We report that this response can be mimicked by addition of 5 microM-inositol 1,4,5-triphosphate (IP3) or by addition of 100 microM-Ca2+ to saponin-permeabilized amoebae. The data suggest that cytoskeletal actin polymerization occurs in normal cells as a result of IP3 formation in response to cell surface receptor stimulation and the consequent release of Ca2+ from internal stores.

TMB-8 inhibits respiration and cyclic GMP formation in Dictyostelium discoideum

The putative inhibitor of intracellular calcium mobilization, TMB-8 was found to be a powerful inhibitor of respiration in amoebae of Dictyostelium discoideum. Consequently, the previously reported effects of this drug on cyclic GMP formation induced by chemoattractants were reassessed. It was found that TMB-8 abolished both folate and cyclic AMP-mediated accumulation of cyclic GMP in D. discoideum amoebae and that addition of Ca2+ completely restored this response. The Ca2+ chelating agent EGTA did not mimic the effect of TMB-8. The effect on cyclic GMP formation, however, occurred only at a concentration of TMB-8 that was ten times that causing maximal inhibition of respiration, and inhibition of cyclic GMP formation was completely restored by addition of excess Ca2+, whereas inhibition of respiration was only partially restored. The data suggest that TMB-8 has more than one inhibitory action, and because of the differential sensitivity of respiration and cyclic GMP formation to this drug, and the differential antagonism of excess Ca2+, we conclude that the effect of TMB-8 on the cyclic GMP response is probably due to its effect on Ca2+ mobilization, rather than indirectly via its effects on respiration. However, we advise caution in interpretation of data using this inhibitor where the responses measured are prolonged, are energy-requiring or are not freely reversible by excess Ca2+.

Effects of cytochalasin B on cell movements and chemoattractant-elicited actin changes of Dictyostelium

The actin-binding drug cytochalasin B (CB) was employed to study the stability and role of cytoskeletal actin following chemotactic stimulation of Dictyostelium discoideum. Intact amoebae were found to be impermeable to this drug, as shown by lack of inhibition of chemotactic movement in its presence and failure of [3H]CB to bind to intact amoebae. However, there were approx. 150 000 high affinity CB-binding sites per cell detectable after cell breakage and preparation of Triton-insoluble cytoskeletons. The effect of CB on cytoskeletons was to destabilize the second (25-45 sec) and third (60 sec) chemotactically-induced peaks of cytoskeletal actin accumulation and to reduce the actin levels to the low prestimulus amount. In contrast, the drug had no such action on the rapid (3-5 sec) actin peak. This peak appeared to be stable in the presence of CB added before or simultaneously with lysis of the cell. It was also observed that the instability of the second and third peaks to CB gradually decreased after cell lysis (as did the number of CB binding sites) such that if CB was added 5 min after lysis of the chemotactically stimulated amoebae it had no destabilizing effect. Evidence was obtained from experiments employing centrifugation of cytoskeletons at 100 000 g and from the use of the DNase I inhibition assay for G-actin, that the first (3-5 sec) actin peak of accumulation involved polymerization rather than just cross-linking of short filamentous actin fragments. The significance of these actin accumulation peaks is discussed and their timing correlated with events involved in chemotaxis.

Inositol 1,4,5-trisphosphate induces cyclic GMP formation in Dictyostelium discoideum

Chemotactic signalling in the cellular slime mould Dictyostelium discoideum employs signalling molecules such as folate and cyclic AMP. These bind to specific cell surface receptors and rapidly trigger internal responses that induce chemotactic movement of the amoebae. Previous studies have shown that actin is polymerised within 3-5 sec of cyclic AMP or folate binding and that a peak of cyclic GMP is formed within 9-12 sec. Release of Ca2+ from intracellular stores has been implicated as a secondary messenger. Here we present evidence that D-myo-inositol 1,4,5-trisphosphate, when added to permeabilized amoebae of Dictyostelium, can mimic the action of chemoattractants on normal intact amoebae in inducing cyclic GMP formation. Our data suggest that IP3, which is known to act as an intermediary messenger between cell surface hormone receptors and release of Ca2+ from internal stores in mammalian cells, functions in a similar capacity during chemotaxis of this primitive eukaryote.

A new model for chemotactic signal transduction in Dictyostelium discoideum

Dictyostelium discoideum amoebae were employed to study the refractoriness and adaptation of the rapid (5sec) accumulation of actin in their Triton-insoluble cytoskeletons following stimulation with specific chemoattractants. Amoebae became refractory within 10sec for this response but no adaptation occurred during this period. Amoebae desensitized for one attractant were not desensitized for another and responses to stimulation with a mixture of attractants were approximately additive. The characteristics of these processes are compared to published studies of adaptation in other chemoattractant-induced responses and a new model for the chemotactic signal transduction pathway is formulated. We conclude that intracellular cGMP accumulation may be on a separate branch of the pathway from the actin response.

Monoclonal antibodies specific for stalk differentiation in Dictyostelium discoideum

We have produced two monoclonal antibodies specific to the stalk cells of Dictyostelium discoideum fruiting bodies. Both monoclonal antibodies react with high molecular weight proteins previously found to be stalk-specific by two-dimensional gel analysis. One antibody (JAb 1) is specific for a single protein of apparent molecular weight 310 000 which first appears when overt stalk differentiation begins at 20 h. The other monoclonal antibody (JAb 2) is also stalk-specific, though earlier in development it binds to proteins extracted from both prestalk and prespore cells of the migrating slug. It reacts with two proteins in stalks, one of apparent molecular weight 430 000 which is first detected during tip formation at 12 h and a lower molecular weight protein (310 000) detected from 20 h. Although several markers are available for the investigation of prespore/spore differentiation there is a distinct lack of suitable prestalk/stalk markers. The monoclonal antibodies described here are highly specific stalk markers and should prove useful in the study of cell proportioning and terminal differentiation.

Chemoattractant-mediated changes in cytoskeletal actin of cellular slime moulds

We report changes in actin associated with the Triton-insoluble cytoskeleton following chemotactic stimulation in five species of cellular slime mould, Dictyostelium discoideum, D. mucoroides, D. purpureum, D. lacteum and Polysphondylium violaceum. Stimulation of amoebae with the chemoattractant specific for each species elicits a rapid increase in cytoskeletal actin content with a peak at 5 s, probably corresponding to pseudopodium formation. Subsequent changes consist of a second discrete peak of actin accumulation and, in at least two species, D. discoideum and P. violaceum, a third peak. We correlate these later changes in actin content with the changes in amoebal shape that have been reported to be a consequence of chemotactic stimulation. We have also investigated species variation in actin content and find D. purpureum to have a remarkably low cytoskeletal actin content in comparison to other species. Whole cell and cytoskeletal actin levels during the first 12 h of D. discoideum development have also been analysed. While little change is found in total protein content, whole cell actin shows an abrupt decline during aggregation and this is accompanied by an increase in cytoskeletal actin, amounting to 100% over 0-12 h of development. These results are discussed in the light of a possible changing role for actin during the course of development, from being involved in the dynamic events of chemotaxis to assuming a more structural role in the multicellular aggregate.

Inhibition of aggregation inDictyosteliumby EGTA-induced depletion of calcium

During aggregation of Dictyostelium discoideum, amoebae move towards collecting centres emitting cyclic AMP. Previous work has indicated that Ca2+ can affect the operation of various parts of the chemotactic mechanism but there are contradictory reports about the role or requirement for Ca2+ during aggregation of intact amoebae. In this study we show that there is a requirement for Ca2+ during this aggregation process. Addition of EGTA (7 mM) and in some cases extensive washing with EGTA is needed to see inhibition of aggregation. Considerable stores of Ca2+ present within cells may explain the failure of some previous attempts to see a Ca2+ requirement during aggregation.

Changes in actin associated with the cytoskeleton following chemotactic stimulation of dictyostelium discoideum

Chemotactic stimulation of Dictyostelium discoideum amoebae with pulses of cAMP or folate causes a series of rapid changes in the amount of actin protein associated with the Triton-insoluble cytoskeleton. The first of these changes occurs within 3 sec. of stimulation. The changes are dose-dependent and are within the physiological range of concentrations of cAMP or folate eliciting chemotaxis. These effects on the cytoskeleton show a pattern of regulation during development matching the respective chemotactic sensitivities of D.discoideum to cAMP (most sensitive at 4-8 hr of development) and to folate (rapidly decreasing sensitivity over 0-4 hr). At twelve hr, however, the responsiveness to folate unexpectedly reappears, suggesting a function of folate later in development than previously reported.

Genetic analysis by mitotic recombination in Dictyostelium discoideum of growth and developmental loci on linkage group VII

A genetic study was made of two developmental loci, stmA (mutations of which give 'streamer' mutants with large aggregation territories) and fgdB (mutations of which give a class of 'aggregateless' mutant unable to respond to exogenous cyclic AMP signals), and three growth-related loci, couA, tsgK and bsgB (sensitivity to growth with coumarin, temperature sensitivity for growth and inability to grow on Bacillus subtilis, respectively). These loci, which have previously been located on linkage group VII, were found by mitotic recombination studies to lie proximal to the recombinant selector cobAI (resistance to cobaltous chloride) with the order: centromere-couA-tsgK-(stmA-bsgB)-fgdB-cobA. Analysis of haploid segregants derived from 374 recombinant diploids allowed relative map distances to be calculated and revealed that tsgK, stmA, bsgB and fgdB lay as a loosely clustered group close to cobA. Two independently isolated mutations at another streamer locus, stmF, were found to be associated with a diploid instability phenomenon such that heterozygous (stmF/+) diploids which became homozygous at the cobA locus on linkage group VII by mitotic recombination reverted very rapidly to the haploid state. Reversion studies with the tsgK21 marker revealed that partial revertants (or suppressors) at this locus frequently introduced secondary developmental or growth-related mutations by an unexplained mechanism.

Streamers: chemotactic mutants of Dictyostelium discoideum with altered cyclic GMP metabolism

Mutants of Dictyostelium discoideum that developed huge aggregation streams in expanding clones were investigated using optical and biochemical techniques. Representatives of the six complementation groups previously identified (stmA-stmF) were found to be similar to the parental wild-type strain XP55 in both the extent and timing of their ability to initiate and relay chemotactic signals and in the formation of cyclic AMP receptors and phosphodiesterases. The mutants differed from the wild-type in producing an abnormal chemotactic (movement) response visible using both dark-field optics with synchronously aggregating amoebae on solid substrata and light scattering techniques with oxygenated cell suspensions. Mutants of complementation group stmF showed chemotactic movement responses lasting up to 520 s, rather than 100 s as seen in the parental and other strains. Measurements of cyclic GMP formed intracellularly in response to chemotactic pulses of cyclic AMP in stmF mutants showed that abnormally high concentrations of this nucleotide were formed within 10 s and were not rapidly degraded. A causal correlation between defective cyclic GMP metabolism and the altered chemotactic response is suggested, and a model is proposed that accounts for the formation of huge aggregation streams in clones of these mutants.U

Genetics of aggregation pattern mutations in the cellular slime mould Dictyostelium discoideum

A class of aggregation pattern mutants called 'streamers' have been isolated from Dictyostelium discoideum and analysed genetically. The streamer phenotype is the formation of very large streams of centripetally moving amoebae which are collected from abnormally large territories during the aggregation phase of this organism. Such mutants do not show the pleiotropic developmental defects seen with most other classes of aggregation mutants and after the abnormal aggregation phase they develop into normally differentiated stalk cells and spores. Twenty-four haploid streamers were isolated and assigned to seven complementation groups, stmA to stmG, after selecting diploids formed between pairs of the mutants. The complementation loci were assigned to the following linkage groups using parasexual genetic techniques: stmA and stmF, linkage group VII; stmB, stmD and stmG, linkage group II; stmC and stmE, linkage group III. Use was made of a new temperature sensitive for growth marker, tsgK21, which was assigned to linkage group VII. The total number of complementation groups giving the streamer phenotype is estimated from statistical calculation, based on the frequency of allelism, to be between seven and nine.

Linkage analysis in Dictyostelium discoideum using multiply marked tester strains: establishment of linkage group VII and the reassessment of earlier linkage data

To aid linkage analysis and mapping studies in Dictyostelium discoideum, we have constructed several tester strains with easily scored mutations characterizing the six currently identified linkage groups. Use has been made of conditionally lethal mutants unable to grow upon Bacillus subtilis, and the locus of the mutation involved (bsgA) has been assigned to linkage group III. The mutation cobA1, which confers resistance to cobaltous chloride, has been assigned to a previously unidentified linkage group (VII). The temperature-sensitive growth mutation tsgC7, previously reported to define linkage group V, has been reassigned to group III, leaving linkage group V presently unmarked. The further use of genetic tester strains is described.