Trends in Cell Biology
ReviewChemotaxis: finding the way forward with Dictyostelium
Introduction
All eukaryotes depend on cell motility at some point in their life cycle, and the ability to migrate, adhere, and change shape is of fundamental importance to a wide range of basic cellular processes. Cell motility is most commonly studied in higher eukaryotes where it is essential for cell division and embryogenesis, as well as for wound-healing, the hunting and killing of pathogens by immune cells, and for a host of other aspects of cell physiology. While a few cells respond primarily to internal cues, for example during cytokinesis or to change shape, most need to move in response to external signals. One of the most interesting and important responses is chemotaxis, where cells move towards (or occasionally away from) a diffusible chemical signal.
Cell migration is a complex process, or to be more accurate, a complex set of interacting processes. Cells must solve a number of issues in order to move towards an attractant source. They must detect the attractant, often present as only a small number of molecules, against a background of other signals; they must transmit the information to the motile machinery inside the cell, and they must somehow extract and integrate the information about the source's direction so as to migrate in a co-ordinated fashion up the attractant gradient. Alteration of any of these processes leads to changes in the chemotactic behaviour of cells. Mostly this leads to diminished chemotaxis, but inappropriate gains in chemotaxis are also harmful, for example during cancer metastasis, when cancer cells migrate into the circulation and escape from the primary tumour to colonise other sites [1]. The development of secondary tumours is often the most difficult aspect of cancer to treat and has led to increased interest in the study of cell migration as a target for anti-metastatic therapy.
Although the importance of cell migration in mammalian cells is obvious, it is often difficult to address simple questions within the complexity of a whole organism. Even in tissue culture, changes in many of the processes underlying chemotaxis often lead to secondary effects such as apoptosis. This has led to the use of several alternative organisms to study the basic underlying principles and mechanisms behind cell movement. Prominent amongst these has been the genetically tractable social amoeba Dictyostelium discoideum; this organism has allowed us to ask fundamental questions about the biology of cell migration in a relatively controlled and amenable way. The genetics of Dictyostelium has been developed for over 50 years and genetic approaches have been used to study a number of processes. Because the lifestyle of Dictyostelium is totally dependent on efficient motility, this organism is particularly suited for the study of cell movement.
Dictyostelium lives in the soil, catching and eating bacteria. This requires the cells to be constitutively motile, and they are able to move towards folate secreted by bacteria and to move rapidly, in a manner similar to some cells of the mammalian immune system (Box 1) [2]. In addition, Dictyostelium also has a developmental cycle that is initiated in order to survive starvation, and which relies heavily upon chemotaxis. During Dictyostelium development, starving cells begin to secrete waves of cyclic adenosine monophosphate (cAMP) that acts as a chemoattractant for neighbouring cells; these then rapidly move towards each other to form aggregates of tens or hundreds of thousands of cells (Box 1). These aggregates subsequently differentiate into a number of distinct cell types, eventually resulting in the formation of a fruiting body consisting of a ball of spore cells held up by a stalk so that they can be dispersed and find a new food supply.
There are a number of reasons why Dictyostelium is an attractive organism for basic research, but by far the biggest is the ease with which it can be grown and genetically manipulated. Dictyostelium cells are haploid and therefore it is relatively simple to disrupt genes and generate a knockout of a gene of interest as well as to generate libraries of mutants for genetic screens. In addition, the Dictyostelium genome is relatively small and compact, often containing only a single gene for proteins that are present in multiple isoforms in mammalian cells. It is this exceptional combination of highly motile cells that can be easily manipulated which has led to the exploitation of Dicytostelium to understand both the signalling pathways and the mechanical processes that underlie chemotaxis, providing insights that would otherwise be hard to obtain.
A number of recent studies have improved our understanding of the processes directing chemotaxis because they implicate new signalling pathways. In addition, the underlying processes that generate cell protrusions are being re-evaluated and the role of signals such as PIP3 now seems far less simple than before. This has led to the emergence of a more complex picture of chemotaxis that will be discussed below.
Section snippets
The generation of pseudopods
The general model of eukaryotic cell migration is that the motile force is generated by the local polymerisation of actin at the leading edge – pushing the membrane forward and generating protrusions (generally known as pseudopods; mammalian cells frequently use a specialised subset of flat, sheet-like pseudopods named lamellipodia) [3]. In general this process involves the nucleation of new actin filaments mediated by the Arp2/3 complex [4]. The Arp2/3 complex itself is strongly activated by
Signalling responses to chemoattractants
In order to move towards a chemical signal the cell must read the extracellular attractant gradient and transmit this information inside the cell to elicit changes in cell morphology and produce motility in the direction of the gradient. Dictyostelium cells, unlike many mammalian cells, are constitutively motile. This means that, in the absence of a chemotactic signal, they randomly produce pseudopods and ‘walk’ around searching for one [16]. When a signal is encountered, cells are able to
The evolving story of PIP3
A popular hypothesis is the idea that the cell has some sort of chemical ‘compass’, strongly orientated in the direction of the chemoattractant, that locally induces actin polymerisation and the production of new pseudopods at the part of the cell closest the signal [18]. The first molecule to be identified that fitted the bill as the needle of an internal compass was phosphatidylinositol (3,4,5)-trisphosphate (PIP3), a molecule that recruits certain proteins containing a specific type of PH
What does and doesn’t PIP3 do?
The realisation that PIP3 is not essential for chemotaxis has led to reassessment of its exact role in migration. Careful observation of the effects of PI3K inhibitors has indicated that the localisation and functional role of PIP3 may differ depending on how strong a signal the cell experiences. For example, the strong localisation of PIP3 at the leading edge is actually seen only in steep gradients and at cell-cell junctions [32]; when the signal is much weaker PIP3 is almost uniformly
Alternative intracellular signals
The realisation that cells are capable of chemotaxis without PIP3 has led to a renewed search for other signalling pathways that orientate the cell. One important intracellular pathway induced upon receptor activation involves the conversion of members of the Ras family of small GTPases into their active GTP-bound state 20, 40. The Dictyostelium genome contains an unusually large number of small GTPases, including 15 Ras family members (reviewed in Ref. [41]), although rasG and rasC appear to
Integration of signals and cytoskeleton: can chemotaxis be explained using linear pathways?
Dictyostelium has been a successful and productive tool for experiments in chemotaxis. Researchers throughout the world have identified a number of genes and associated pathways that are important for efficient chemotaxis (Figure 2). Nevertheless, many of the pathways discussed above do not seem to generate signals that are spatially localised in the manner expected for compass-based models. It may be that by disrupting them we are merely interfering with normal cell movement, rather than
Prospects and unanswered questions
Our picture of the processes driving cell migration has rapidly evolved over the past few years, but a number of major questions remain. Primarily, we need to know how the multiple signalling pathways involved in directing the cell are integrated, and to clarify which of these actually change the directionality of the cell, and which simply alter its general underlying motility. This has started to be done with the TORC2 pathway where a number of PKBR1 targets have been identified [48]. These
Conclusion
In this review we have outlined the ways in which Dictyostelium can be used to study cell migration. Although some of the intricacies of intracellular signalling in mammalian tissue cells are absent, Dictyostelium provides a relatively simple experimental target in which to understand fundamental principles of cell migration that can be generally extrapolated.
Recently there has been considerable change in our understanding of both the underlying principles of directed migration and of the
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