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Dictyostelium

2010; Elsevier BV; Volume: 20; Issue: 23 Linguagem: Inglês

10.1016/j.cub.2010.09.051

1879-0445

Louise Fets, Robert R. Kay, Francisco Velázquez,

3D Printing in Biomedical Research

What is Dictyostelium? Dictyostelium discoideum (Dicty) is a social amoeba that lives in the soil and feeds on bacteria and other microbes. Dictyosteliida is a distinct branch of the eukaryotes, separate from plants, fungi and animals. The cells lack a cell wall and resemble animal cells in organisation, except for the presence of a contractile vacuole. How can an amoeba be ‘social’? Dicty is described as social because in times of starvation, individual amoebae aggregate to form a multicellular mound, containing up to a hundred thousand cells. The aggregate undergoes differentiation and morphogenic changes before maturing into a fruiting body which consists of two main cell types: spore cells, which are resistant to temperature extremes, desiccation and digestion, and stalk cells, which form the ancillary structures supporting the spore head (Figure 1). One interesting intermediary structure is the slug; during this stage, the aggregate moves collectively, responding to light and heat stimuli in order to find favourable conditions for fruiting body formation. This response to starvation is referred to as development and by going through this social, multicellular phase, the population dramatically increases their chances of surviving unfavourable environmental conditions. So how do the cells move? Dicty amoebae are intrinsically motile and generally move using what is appropriately termed as amoeboid movement, producing actin-rich pseudopods at the front of the cell and using myosin to contract the rear. Amoeboid motility is also seen in neutrophils and tumour cells in animals; however, Dicty is flexible: it can also move using hydrostatic pressure-driven, actin-free extensions (blebs), or in a keratocyte-like manner, with a single, flat, actin-rich lamellipod extending in the direction of movement. It thus displays, in one cell type, three of the major ways in which animals cells move. How do the amoebae know where to go? Amoebae are chemotactic: they can sense gradients of certain chemicals and move along them. Dicty is known to chemotax to two chemicals: folic acid, which is released by bacteria and used in the hunt for food, and cAMP, which is released by amoebae during starvation and used to find each other during aggregation. Cells have evolved a relay mechanism in which cAMP stimulates its own release, thus forming waves that can propagate through a field of responsive amoebae (Figure 2). Amoebae respond to cAMP gradients by polarising: creating a leading edge and a rear with different sets of lipids and proteins defining each pole. A classic example of this is the accumulation of PI(3,4,5)P3 at the leading edge. After polarising, amoebae begin to move up the gradient of cAMP and are extremely sensitive to even shallow concentration changes — they can detect as little as 2% difference across their length. How cells are able to sense and interpret a gradient is a major question in biology. It is widely accepted that the core features of the chemotactic signalling process and machinery are conserved from Dicty to mammals. Dicty has therefore become a very popular model for studying chemotaxis, because findings in Dicty often translate to the directed migration seen during the immune response, wound healing, embryogenesis and in tumour cell metastasis. What can we learn from its development? In Dicty development, multicellularity is achieved by aggregation of pre-existing cells and not by division of a zygote or precursor cell, which allows the study of development in isolation from the cell cycle and cell division. Cell fate is first determined early in development, with pre-stalk and pre-spore cells arising randomly in a ‘salt and pepper’ pattern at the mound stage. Fascinatingly, this occurs without any apparent positional information, in stark contrast to major mechanisms of multicellular development, which mainly rely on gradients of morphogens. After differentiation, the different cell type precursors sort to different regions of the mound. This differentiation-followed-by-sorting mechanism is not exclusive to Dictyostelium — similar processes are now being reported in chick and mouse development. Despite their early determination, cells are not fully committed and can transdifferentiate until the late stages of development. For example, at the slug stage, pre-stalk cells are clustered at the front of the structure and the rear consists of pre-spore cells. If a slug is cut and the pre-stalk containing front half removed, pre-spore cells in the rear portion will rapidly transdifferentiate to produce a new population of pre-stalk cells to restore proportionality, before the slug quickly continues on its developmental course. How this is achieved with such accuracy is still far from understood. How to survive in the soil. Dicty is predicted to produce a plethora of natural products — the Dicty genome contains genes for a surprising number of polyketide synthases, which can contort carbon backbones into all sorts of configurations. The majority of these enzymes are uncharacterised but their products are of great interest since they are presumably the arsenal that allows Dicty to survive in an environment filled with hostile bacteria, fungi and nematodes. The social interactions between Dicty cells during development are also important for survival. Development is an altruistic process and, for each fruiting body formed, 20% of the cells entering into the aggregate will be sacrificed to form stalk cells. Altruism is very intriguing from an evolutionary perspective, and our understanding of this aspect of Dicty biology has been improved by the discovery of ‘cheater mutants’. When a cheater develops in a mixed population, it puts less than its fair share of stalk cells into the fruiting body, and so benefits from the altruistic nature of its unwitting cohabiting strain. Many cheater mutants have been isolated in the laboratory and the challenge now is to understand why they have not taken over in the wild. Can Dicty be used for medical research? Because so many pathways and processes are well conserved from Dicty through to mammals, Dicty is often a great place to start basic research into disease mechanisms. Even the mechanism of psychiatric drugs such as lithium — a major treatment for bipolar disorder — can be investigated because many of these drugs appear to affect basic cellular processes, rather than neuronal-specific ones. As Dicty is a professional phagocyte, engulfing bacteria in an analogous mechanism to macrophages, it can be infected with intracellular pathogens, including Legionella, Mycobacterium and Salmonella. Host–pathogen interactions are therefore also being widely investigated. What techniques can be used with Dicty? Although wild-type isolates can only thrive on bacteria, lab strains have been selected that grow in a simple defined media. They can therefore be grown as adherent cells on tissue cultures plates (mammalian cell culture style), on a bacterial lawn (Caenorhabditis elegans style) or in shaken suspension (Escherichia coli style). Dicty has been studied for decades, and so virtually every molecular tool required for genetics has been developed for use in this system, for example: gene knock-outs and knock-ins; insertional mutagenesis (REMI); cell-type-specific or inducible expression systems; replicative and integrative plasmids. Cell biological tools such as protein tagging, organelle markers, and immunostaining are also available. All strains generated can be easily frozen down in liquid nitrogen for long-term storage. What resources are there? Since the completion of the genome in 2005, it has become easy to search for gene homologues in Dicty. The genome is very simple to browse thanks to dictyBase, a website that provides access to all sorts of information from movies, to techniques, to mutant phenotypes, to gene expression data. The stock centre is another fantastic resource: a central repository where wild-type strains, mutants, cDNA libraries and constructs are maintained and available to order. Anything else? Dicty is being used to study so much more than has been possible to mention here! Many other researchers have taken advantage of Dicty's genetic tractability and conserved machinery to investigate other biological processes such as histone modification, DNA repair pathways, mitochondrial diseases and nuclear architecture. Others are interested in Dicty biology itself and how Dictyosteliida do things such as sexual reproduction; how this is achieved in lower eukaryotes can tell us much about evolution of such fundamental processes.