Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations. Errors during this process have serious consequences, including mental retardation, vascular disease, tumor formation and metastasis. An understanding of the mechanism by which cells migrate may lead to the development of novel therapeutic strategies for controlling, for example, invasive tumour cells. Cells often migrate in response to, and toward, specific external signals, a process called chemotaxis.
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The migration of single mammalian cells is usually viewed in the microscope as the cells move randomly on a glass slide. As the actual movement is very slow — usually a few micrometers/minute — time-lapse films are taken so that a speeded up movie can be viewed.[1] This shows that, although the shape of a moving cell varies considerably, its leading front has a characteristic behaviour. This region of the cell is highly active, sometimes spreading forward quickly, sometimes retracting, sometimes ruffling or bubbling. It is generally accepted that the leading front is the main motor that pulls the cell forward.
There is still great uncertainty of how cell migration really works. However, because the locomotion of all mammalian cells (except sperm) has several common features, the underlying processes are believed to be similar. The two main constant features are:
The latter feature is most easily observed when aggregates of a surface molecule are cross-linked with a fluorescent antibody (see cap formation) or when small beads become artificially bound to the front of the cell.[2]
Besides mammalian cells, many other eukaryotic cells appear to move in a similar way. One of the most valuable model creatures for studying locomotion and chemotaxis is the amoeba Dictyostelium discoideum, because they move more quickly than most mammalian cells grown in the lab, and they chemotax toward cyclic AMP. In addition, they have a haploid genome that assists understanding the role of a particular gene product in movement.
There are two main theories for how the cell advances its front edge: the cytoskeletal model and membrane flow model. It is possible that both underlying processes contribute to cell extension.
Through experiment, it is found that the cell's front is a site of rapid actin polymerisation: soluble actin monomers polymerise there to form filaments.[3] This has led to the view that it is the formation of these actin filaments, which pushes the leading front forward and is the main motile force for advancing the cell’s front.[4][5] In addition, cytoskeletal elements are able to interact extensively and intimately with a cell's plasma membrane.[6]
Studies have also shown that the front is the site at which membrane is returned to the cell surface from internal membrane pools at the end of the endocytic cycle.[7] This has led to the view that extension of the leading edge occurs primarily by addition of membrane at the front of the cell. If so, the actin filaments that form at the front might stabilize the added membrane so that a structured extension, or lamella, is formed rather than the cell's blowing bubbles (or "blebs") at its front.[8] For a cell to move, it is necessary to bring a fresh supply of "feet" (those molecules called integrins, which attach a cell to the surface on which it is crawling) to the front. It is likely that these feet are endocytosed toward the rear of the cell and brought to the cell's front by exocytosis, to be reused to form new attachments to the substrate.
Given that a cell’s front advances, what about the rest of the cell? Is it simply dragged forward, like a sack? This information is not known, but there are suggestions that the nucleus and perhaps other large structures inside the cell may also be pulled forward by actin filaments. In addition, it may be that the rear of the cell actively contracts, as it is here that, in some cells, the major contractile protein myosin is found.
Insight into how complex biological processes work can often be gleaned from a study of mutations. In the case of the intracellular mechanisms underlying cell movement, this has been largely unsuccessful. Thus, although many mutants are known in Drosophila, which affect migratory processes, these tend to fall into two groups: transcription factors (such as slow border cells (slbo), which affects the migration of the border cells) or key regulator proteins (such as C-Jun N-terminal kinases (JNK), which controls dorsal closure). These, however, tell us little about how cells actually move.
Another major source of mutants is the haploid amoeba Dictyostelium. Many single-copy genes associated with cytoskeletal function have been deleted: These mutants usually have only a weak phenotype, suggesting either that these genes are not required for locomotion, or that there are multiple mechanisms by which cells can move.[9] However, temperature-sensitive mutants in the genes for N-ethylmaleimide sensitive fusion protein (NSF) and Sec1 rapidly block cell migration[10][11] indicating that the NSF protein and Sec1p are both required for some aspects of cell movement. NSF is known to function in intracellular membrane fusion;[10] Sec1p in yeast is required for polarised exocytosis.[11]
Migrating cells have a polarity—a front and a back. Without it, they would move in all directions at once, i.e. spread. How this arrow is formulated at a molecular level inside a cell is unknown. In a cell that is meandering in a random way, the front can easily give way to become passive as some other region, or regions, of the cell form(s) a new front. In chemotaxing cells, the stability of the front appears enhanced as the cell advances toward a higher concentration of the stimulating chemical. This polarity is reflected at a molecular level by a restriction of certain molecules to particular regions of the inner cell surface. Thus, the phospholipid PIP3 and activated Rac and CDC42 are found at the front of the cell, whereas Rho GTPase and PTEN are found toward the rear.[12][13]
It is believed that microtubules and filamentous actin are important for establishing and maintaining a cell’s polarity. Thus, drugs that destroy microtubules disrupt the polarity of many cells: If the cell is attached to a substratum, they often become round and flat. Drugs that destroy actin filaments have multiple and complex effects, reflecting the wide role that these filaments play in many cell processes. It may be that, as part of the locomotory process, membrane vesicles are transported along these filaments to the cell’s front. In chemotaxing cells, the increased persistence of migration toward the target may result from an increased stability of the arrangement of the filamentous structures inside the cell and determine its polarity. In turn, these filamentous structures may be arranged inside the cell according to how molecules like PIP3 and PTEN are arranged on the inner cell surface. And where these are located appears in turn to be determined by the chemoattractant signals as these impinge on specific receptors on the cell’s outer surface.
Although the movement of animal cells is usually studied as they migrate, it seems likely that many motile cells can also swim[14]. Thus, human neutrophils are able to migrate towards a source of a chemoattractant, the tripeptide FMLP, whilst suspended in an isodense medium. They swim at the same speed as they would crawl on a solid surface. Likewise, Dictyostelium amoebae swim towards a chemical attractant, in this case cyclic AMP. The actual mechanism that these neutrophils or amoebae use to produce a thrust against the medium to propel themselves is uncertain; however, how they do so must be consistent with physical principles. To swim they must transmit a force against the viscous fluid in order to propel themselves forward. Different mechanisms by which they might do so were presented by Ed Purcell [15] in a famous talk he gave celebrating the 80th birthday of his friend Viki Weisskopf.
In this he developed his “scallop theorem”: a normal scallop moves by opening its shells slowly and shutting them quickly. In the latter step it quickly squeezes the fluid between the shells backwards and, using the momentum of the water, pushes itself forward. Purcell realised that a microorganism trying to do the same would simply move forwards on shutting its shells and move backwards to its original position on opening them. The set of movements is “reciprocal”: it appears the same if viewed forwards or backwards in time. He concluded that microorganisms cannot move by a reciprocal mechanism: to move, they must exert some thrust against the medium and do so in a non-reciprocal manner. He suggested various ways in which an organism could swim:
— They could do so with a flagellum, which rotates, pushing the medium backwards — and the cell forwards — in much the same way that a ship’s screw moves a ship. This is how some bacteria move; the flagellum is attached at one end to a complex rotating motor held rigidly in the bacterial cell surface[16] [17]
—They could use a flexible arm: this could be done in many different ways. For example, mammalian sperm have a flagellum which, whip-like, wriggles at the end of the cell, pushing the cell forward[18]. Cilia are quite similar structures to mammalian flagella; they can advance a cell like paramecium by a complex motion not dissimilar to breast stroke.
— A hypothetical toroidal cell could move by rotating its surface through the central hole, thereby creating a surface flow. The surface drag on the outer edges of the cell could provide the thrust against the medium needed to move the cell forward. This is related to the membrane flow model B, above, except in that scheme the surface flow is achieved by removing surface from the rearward end of the cell and transporting it as vesicles through the cell interior to the cell's front.
The manner in which cells swim, and therefore move, suggests that it is membrane flow which is the motor for movement[14].