Secreted Cyclic Di-GMP Induces Stalk Cell Differentiation in the Eukaryote Dictyostelium discoideum

Visualization of c-di-GMP in multicellular Dictyostelium stages

The bacterial signaling molecule cyclic diguanosine monophosphate (c-di-GMP) is only synthesized and utilized by the cellular slime mold Dictyostelium discoideum among eukaryotes. Dictyostelium cells undergo a transition from a unicellular to a multicellular state, ultimately forming a stalk and spores. While Dictyostelium is known to employ c-di-GMP to induce differentiation into stalk cells, there have been no reports of direct observation of c-di-GMP using fluorescent probes. In this study, we used a fluorescent probe used in bacteria to visualize its localization within Dictyostelium multicellular bodies. Cytosolic c-di-GMP concentrations were significantly higher at the tip of the multicellular body during stalk formation.

Extracellular c-di-GMP Plays a Role in Biofilm Formation and Dispersion of Campylobacter jejuni

Cyclic diguanosine monophosphate (c-diGMP) is a ubiquitous second messenger involved in the regulation of many signalling systems in bacteria, including motility and biofilm formation. Recently, it has been reported that c-di-GMP was detected in C. jejuni DRH212; however, the presence and the role of c-di-GMP in other C. jejuni strains are unknown. Here, we investigated extracellular c-di-GMP as an environmental signal that potentially triggers biofilm formation in C. jejuni NCTC 11168 using a crystal violet-based assay, motility-based plate assay, RT-PCR and confocal laser scanning microscopy (CLSM). We found that, in presence of extracellular c-di-GMP, the biofilm formation was significantly reduced (>50%) and biofilm dispersion enhanced (up to 60%) with no effect on growth. In addition, the presence of extracellular c-di-GMP promoted chemotactic motility, inhibited the adherence of C. jejuni NCTC 11168-O to Caco-2 cells and upregulated the expression of Cj1198 (luxS, encoding quarum sensing pathway component, autoinducer-2), as well as chemotaxis genes Cj0284c (cheA) and Cj0448c (tlp6). Unexpectedly, the expression of Cj0643 (cbrR), containing a GGDEF-like domain and recently identified as a potential diguanylate cyclase gene, required for the synthesis of c-di-GMP, was not affected. Our findings suggest that extracellular c-di-GMP could be involved in C. jejuni gene regulation, sensing and biofilm dispersion.

The potential of amoeba-based processes for natural product syntheses

The identification of novel platform organisms for the production and discovery of small molecules is of high interest for the pharmaceutical industry. In particular, the structural complexity of most natural products with therapeutic potential restricts an industrial production since chemical syntheses often require complex multistep routes. The amoeba Dictyostelium discoideum can be easily cultivated in bioreactors due to its planktonic growth behavior and contains numerous polyketide and terpene synthase genes with only a few compounds being already elucidated. Hence, the amoeba both bears a wealth of hidden natural products and allows for the development of new bioprocesses for existing pharmaceuticals. In this mini review, we present D. discoideum as a novel platform for the production of complex secondary metabolites and discuss its suitability for industrial processes. We also provide initial insights into future bioprocesses, both involving bacterial coculture setups and for the production of plant-based pharmaceuticals.

The Cellular and Developmental Roles of Cullins, Neddylation, and the COP9 Signalosome in Dictyostelium discoideum

Cullins (CULs) are a core component of cullin-RING E3 ubiquitin ligases (CRLs), which regulate the degradation, function, and subcellular trafficking of proteins. CULs are post-translationally regulated through neddylation, a process that conjugates the ubiquitin-like modifier protein neural precursor cell expressed developmentally downregulated protein 8 (NEDD8) to target cullins, as well as non-cullin proteins. Counteracting neddylation is the deneddylase, COP9 signalosome (CSN), which removes NEDD8 from target proteins. Recent comparative genomics studies revealed that CRLs and the CSN are highly conserved in Amoebozoa. A well-studied representative of Amoebozoa, the social amoeba Dictyostelium discoideum, has been used for close to 100 years as a model organism for studying conserved cellular and developmental processes owing to its unique life cycle comprised of unicellular and multicellular phases. The organism is also recognized as an exceptional model system for studying cellular processes impacted by human diseases, including but not limited to, cancer and neurodegeneration. Recent work shows that the neddylation inhibitor, MLN4924 (Pevonedistat), inhibits growth and multicellular development in D. discoideum, which supports previous work that revealed the cullin interactome in D. discoideum and the roles of cullins and the CSN in regulating cellular and developmental processes during the D. discoideum life cycle. Here, we review the roles of cullins, neddylation, and the CSN in D. discoideum to guide future work on using this biomedical model system to further explore the evolutionarily conserved functions of cullins and neddylation.

Cancer and the breakdown of multicellularity: What Dictyostelium discoideum, a social amoeba, can teach us

Ancient pathways promoting unicellularity and multicellularity are associated with cancer, the former being pro-oncogenic and the latter acting to suppress oncogenesis. However, there are only a limited number of non-vertebrate models for studying these pathways. Here, we review Dictyostelium discoideum and describe how it can be used to understand these gene networks. D. discoideum has a unicellular and multicellular life cycle, making it possible to study orthologs of cancer-associated genes in both phases. During development, differentiated amoebae form a fruiting body composed of a mass of spores that are supported atop a stalk. A portion of the cells sacrifice themselves to become non-reproductive stalk cells. Cheating disrupts the principles of multicellularity, as cheater cells alter their cell fate to preferentially become spores. Importantly, D. discoideum has gene networks and several strategies for maintaining multicellularity. Therefore, D. discoideum can help us better understand how conserved genes and pathways involved in multicellularity also influence cancer development, potentially identifying new therapeutic avenues.

Extracellular signaling in Dictyostelium

In the last few decades, we have learned a considerable amount about how eukaryotic cells communicate with each other, and what it is the cells are telling each other. The simplicity of Dictyostelium discoideum, and the wide variety of available tools to study this organism, makes it the equivalent of a hydrogen atom for cell and developmental biology. Studies using Dictyostelium have pioneered a good deal of our understanding of eukaryotic cell communication. In this review, we will present a brief overview of how Dictyostelium cells use extracellular signals to attract each other, repel each other, sense their local cell density, sense whether the nearby cells are starving or stressed, count themselves to organize the formation of structures containing a regulated number of cells, sense the volume they are in, and organize their multicellular development. Although we are probably just beginning to learn what the cells are telling each other, the elucidation of Dictyostelium extracellular signals has already led to the development of possible therapeutics for human diseases.

Calmodulin-mediated events during the life cycle of the amoebozoan Dictyostelium discoideum

This review focusses on the functions of intracellular and extracellular calmodulin, its target proteins and their binding proteins during the asexual life cycle of Dictyostelium discoideum. Calmodulin is a primary regulatory protein of calcium signal transduction that functions throughout all stages. During growth, it mediates autophagy, the cell cycle, folic acid chemotaxis, phagocytosis, and other functions. During mitosis, specific calmodulin-binding proteins translocate to alternative locations. Translocation of at least one cell adhesion protein is calmodulin dependent. When starved, cells undergo calmodulin-dependent chemotaxis to cyclic AMP generating a multicellular pseudoplasmodium. Calmodulin-dependent signalling within the slug sets up a defined pattern and polarity that sets the stage for the final events of morphogenesis and cell differentiation. Transected slugs undergo calmodulin-dependent transdifferentiation to re-establish the disrupted pattern and polarity. Calmodulin function is critical for stalk cell differentiation but also functions in spore formation, events that begin in the pseudoplasmodium. The asexual life cycle restarts with the calmodulin-dependent germination of spores. Specific calmodulin-binding proteins as well as some of their binding partners have been linked to each of these events. The functions of extracellular calmodulin during growth and development are also discussed. This overview brings to the forefront the central role of calmodulin, working through its numerous binding proteins, as a primary downstream regulator of the critical calcium signalling pathways that have been well established in this model eukaryote. This is the first time the function of calmodulin and its target proteins have been documented through the complete life cycle of any eukaryote.

An Improved Strategy for the Chemical Synthesis of 3',5'-Cyclic Diguanylic Acid

The physiological functions of c-di-GMP and its involvement in many key processes led to its recognition as a major and ubiquitous bacterial second messenger. Aside from being a bacterial signaling molecule, c-di-GMP is also an immunostimulatory molecule capable of inducing innate and adaptive immune responses through maturation of immune mammalian cells. Given the broad biological functions of c-di-GMP and its potential applications as a nucleic-acid-based drug, the chemical synthesis of c-di-GMP has drawn considerable interest. An improved phosphoramidite approach to the synthesis of c-di-GMP is reported herein. The synthetic approach is based on the use of a 5'-O-formyl protecting group, which can be rapidly and chemoselectively cleaved from a key dinucleotide phosphoramidite intermediate to enable a cyclocondensation reaction leading to a fully protected c-di-GMP product in a yield ∼80%. The native c-di-GMP is isolated, after complete deprotection, in an overall yield of 36% based on the commercial ribonucleoside used as starting material. © 2019 by John Wiley & Sons, Inc.

Autophagy-dependent cell death – where, how and why a cell eats itself to death

Autophagy as a means of cell killing was first advanced by Clark's phenotypic description of ‘Type II autophagic cell death’ in 1990. However, this phenomenon later came into question, because the presence of autophagosomes in dying cells does not necessarily signify that autophagy is the cause of demise, but rather may reflect the efforts of the cell to prevent it. Resolution of this issue comes from a more careful definition of autophagy-dependent cell death (ADCD) as a regulated cell death that is shown experimentally to require different components of the autophagy machinery without involvement of alternative cell death pathways. Following these strict criteria, ADCD has been validated in both lower model organisms and mammalian cells, highlighting its importance for developmental and pathophysiological cell death. Recently, researchers have defined additional morphological criteria that characterize ADCD and begun to explore how the established, well-studied autophagy pathway is subverted from a survival to a death function. This Review explores validated models of ADCD and focuses on the current understanding of the mechanisms by which autophagy can kill a cell.

c-di-GMP Regulates Various Phenotypes and Insecticidal Activity of Gram-Positive Bacillus thuringiensis

C-di-GMP has been well investigated to play significant roles in the physiology of many Gram-negative bacteria. However, its effect on Gram-positive bacteria is less known. In order to more understand the c-di-GMP functions in Gram-positive bacteria, we have carried out a detailed study on the c-di-GMP-metabolizing enzymes and their physiological functions in Bacillus thuringiensis, a Gram-positive entomopathogenic bacterium that has been applied as an insecticide successfully. We performed a systematic study on the ten putative c-di-GMP-synthesizing enzyme diguanylate cyclases (DGCs) and c-di-GMP-degrading enzyme phosphodiesterases (PDEs) in B. thuringiensis BMB171, and artificially elevated the intracellular c-di-GMP level in BMB171 by deleting one or more pde genes. We found increasing level of intracellular c-di-GMP exhibits similar activities as those in Gram-negative bacteria, including altered activities in cell motility, biofilm formation, and cell-cell aggregation. Unexpectedly, we additionally found a novel function exhibited by the increasing level of c-di-GMP to promote the insecticidal activity of this bacterium against Helicoverpa armigera. Through whole-genome transcriptome profile analyses, we found that 4.3% of the B. thuringiensis genes were differentially transcribed when c-di-GMP level was increased, and 77.3% of such gene products are involved in some regulatory pathways not reported in other bacteria to date. In summary, our study represents the first comprehensive report on the c-di-GMP-metabolizing enzymes, their effects on phenotypes, and the transcriptome mediated by c-di-GMP in an important Gram-positive bacterium.

Autophagy in Dictyostelium: Mechanisms, regulation and disease in a simple biomedical model

Autophagy is a fast-moving field with an enormous impact on human health and disease. Understanding the complexity of the mechanism and regulation of this process often benefits from the use of simple experimental models such as the social amoeba Dictyostelium discoideum. Since the publication of the first review describing the potential of D. discoideum in autophagy, significant advances have been made that demonstrate both the experimental advantages and interest in using this model. Since our previous review, research in D. discoideum has shed light on the mechanisms that regulate autophagosome formation and contributed significantly to the study of autophagy-related pathologies. Here, we review these advances, as well as the current techniques to monitor autophagy in D. discoideum. The comprehensive bioinformatics search of autophagic proteins that was a substantial part of the previous review has not been revisited here except for those aspects that challenged previous predictions such as the composition of the Atg1 complex. In recent years our understanding of, and ability to investigate, autophagy in D. discoideum has evolved significantly and will surely enable and accelerate future research using this model.

Stalk cell differentiation without polyketides in the cellular slime mold

Polyketides induce prestalk cell differentiation in Dictyostelium. In the double-knockout mutant of the SteelyA and B polyketide synthases, most of the pstA cells-the major part of the prestalk cells-are lost, and we show by whole mount in situ hybridization that expression of prestalk genes is also reduced. Treatment of the double-knockout mutant with the PKS inhibitor cerulenin gave a further reduction, but some pstA cells still remained in the tip region, suggesting the existence of a polyketide-independent subtype of pstA cells. The double-knockout mutant and cerulenin-treated parental Ax2 cells form fruiting bodies with fragile, single-cell layered stalks after cerulenin treatment. Our results indicate that most pstA cells are induced by polyketides, but the pstA cells at the very tip of the slug are induced in some other way. In addition, a fruiting body with a single-cell layered, vacuolated stalk can form without polyketides.