Effective cell migration requires the seamless integration of localized, transient signalling events with changes in cellular architecture. Migration is a cyclical process in which a cell extends protrusions at its front and retracts its trailing end. It is spurred into action by migration-promoting or chemotactic agents that induce an initial polarization.

Polarization.

In a quiescent non-migrating cell, adhesive forces maintain the attachment to neighbouring cells and the extracellular matrix. Upon encountering chemotactic molecules, cells re-organize into a predominantly ‘front’, forward moving portion and a predominantly ‘back’, retracting rear portion that are defined by distinct signalling events; this process is known as polarization. Cdc42, PAR proteins (PAR6 and PAR3) and atypical protein kinase (aPKC) are involved in the initial polarization process. Changes in cellular architecture, such as the re-organization of the microtubule-organizing centre (MTOC), microtubules and Golgi apparatus to the front of the nucleus, accompany polarization. The production of phosphatidylinositol trisphosphate (PIP 3) at the leading edge by the action of phosphoinositide 3-kinase (PI3K) is also implicated in polarization. PTEN, a PIP 3 phosphatase, localized to the rear and sides of the cell, helps to regulate the levels of PIP 3. Once polarization is initiated it is maintained by a set of overlapping positive feedback loops involving PI3K, microtubules, Rho family GTPases, integrins and vesicular transport.

Protrusion.

The process of protrusion marks the start of the migration cycle. The actin cytoskeleton provides the basic machinery for protrusion. Actin filaments themselves are intrinsically polarized; fast-growing “barbed” ends and slow-growing “pointed” ends provide an inherent drive for membrane protrusion. This can take the form of spike-like filopodia, in which actin filaments form long parallel bundles, or large, broad lamellipodia, where actin filaments form a branching ‘dendritic’ network. These two forms of protrusion appear to have very distinct functions. The parallel bundle design of filopodia means they are well suited to act as sensors and explore the local environment, whereas the branched design of lamellipodia forms broad protrusions in the direction of migration, providing a strong foundation over which the cell can move forward.

Actin filaments in filopodia elongate at their barbed ends and disassemble releasing actin monomers at their pointed ends, thus, a filament tread-milling mechanism rather than a branched nucleation mechanism is believed to be responsible for filopodial protrusion. The enrichment of proteins such as Ena/VASP, which binds barbed ends and antagonizes capping and branching, and the bundling protein fascin at filopodial tips greatly increases the ability of filopodia to push the plasma membrane forward.

In lamellipodial protrusions a branched nucleation mechanism forms a wide net-like structure which pushes the plasma membrane forward. The Arp2/3 complex induces the formation of new actin filaments by binding to the sides or tip of pre-existing ‘mother’ filaments. The Arp2/3 complex is activated locally at the cell membrane by Wasp/Wave family members which are themselves major targets for Cdc42 and Rac.

The rate and organization of actin polymerization in protrusions is regulated by several actin-binding proteins that affect the pool of available actin monomers and free ends. Profilin binds and targets actin monomers to barbed ends thereby preventing self-nucleation. Capping proteins terminate elongation and limit polymerization to new filaments at the plasma membrane, whereas proteins of the ADF/cofilin family sever filaments and promote actin dissociation from the pointed end. Within the dendritic network of lamellipodial structures other proteins such as cortactin, filamin A and ?-actinin help to stabilize branches and cross-link actin filaments.

Traction.

For a cell to advance, newly extended protrusions must attach to the surroundings and stabilize, providing a means of traction for the cell to pull itself forward. The physical component of traction is provided by the action of integrins in adhesions. Tractional force is created at sites of adhesion by the contractile properties of myosin II interacting with actin filaments attached indirectly to integrins at these adhesion sites.

Integrins are a major family of migration-promoting receptors that enable the outer surface of a protruding cell to adhere to the surface over which it is moving. By linking via adaptor proteins to actin filaments, integrins facilitate the cell to pull the bulk of its body forward. Integrins are heterodimeric receptors (comprised of ? and ? chains) with a short domain projecting into the cytoplasm and a large extracellular domain. They act as both traction sites and mechanosensors transmitting information about the physical state of the surface over which the cell is moving and enabling it to make the necessary cytoskeletal adjustments. Ligand binding induces conformational changes that are transmitted to the cytoplasmic domains, integrin clustering and the activation of intracellular signalling cascades that lead to changes in the rate of phosphoinositol lipid synthesis, protein phosphorylation and activation of small GTPases. These signalling pathways are involved in establishing and maintaining cell polarity, regulating the formation and strength of adhesions as well as in modulating the organization and dynamics of the cytoskeleton. Activated integrins localize to the leading edge of migrating cells and are associated with the formation of new adhesions. While PKC or the GTPase, Rap1 act through talin to promote integrin activation and increase integrin affinity, leading to adhesion assembly, the kinase Raf-1 suppresses integrin activation and favours disassembly.

The turnover of adhesions is critical for effective migration. To extend a protrusion, the adhesions in that portion of the cell must disassemble. Once the protrusion has been extended, adhesions are re-assembled to provide the required traction for the cell to pull forward. At the same time, disassembly of adhesions at the cell rear is necessary to enable forward progress.

The mechanisms of adhesion assembly and disassembly are still rather poorly understood, but clearly require tight spatial and temporal regulation. The initial clustering of integrins, stimulated by the multivalent nature of the extracellular matrix to which the cell is adhering, is thought to initiate adhesion assembly, but the types of adhesions which form vary greatly depending on the cell type and pliability of the substratum. Rapidly migrating blood cells, such as leukocytes for example, display few integrin clusters and their submicroscopic adhesions facilitate rapid movement. Normally non-migratory or slow moving cells display focal adhesions, large integrin clusters that are tightly adherent and appear to be dependant on Rho-stimulated myosin contractility. Small adhesions, also known as focal complexes, are commonly observed at the leading edge of migrating cells and have been shown to be dependent on Rac and Cdc42. These types of adhesions tend to stabilize lamellipodia and contribute to efficient migration.

Retraction.

The disassembly of adhesions at the rear of the cell and the retraction of the cell’s tail complete the cycle of migration and enable cell translocation. Myosin II is crucial for retraction and the development of tension between adhesions at the rear and the retraction machinery. This tension can be sufficient to open stretch-activated calcium channels and lead to the activation of calpain. This protease contributes to adhesion disassembly at the cell rear by cleaving a number of focal adhesion proteins, including integrins, talin, vinculin and FAK. Disruption of PAKa and Rho/Rho kinase signalling, which regulates myosin II, severely impairs retraction in migrating cells. The release of adhesions at the rear of the cell facilitates protrusive activity at the front of the cell, contributes to the overall polarization and provides positive feedback for the continued cycle of migration.

Directed cell migration accompanies us from conception to death. This integrated process choreographs the morphogenesis of the embryo during its development. Failure of cells to migrate, or migration of cells to inappropriate locations, can result in life threatening consequences such as the congenital brain defects. In the adult, cell migration is central to homeostatic processes such as mounting an effective immune response and the repair of injured tissues. Furthermore, it contributes to pathologies including vascular disease, chronic inflammatory diseases, tumour formation and metastasis. Understanding the mechanisms underlying cell migration is also important to emerging areas of biotechnology which focus on cellular transplantation and the manufacture of artificial tissues, as well as for the development of new therapeutic strategies for controlling invasive tumour cells.

We all began life as a single, genetically complete cell (the zygote, or “fertilized egg”), resulting from the union of a sperm and egg cell. After conception, the zygote divides to form a ball of rapidly multiplying cells called a blastocyst. When the blastocyst arrives in the uterus, it migrates into the uterine wall so that a placenta may develop to nourish the developing embryo. While this is occurring, large groups of cells inside the blastocyst migrate to form layers in a process called gastrulation. Cells within these layers eventually migrate to their target destinations in the developing embryo and specialize to become components of arms, legs, liver, heart, brain and other organs. In the developing brain, for example, primitive neuronal cells migrate out of the neural tube and take up residence in distinct layers, where they send projections (axons and dendrites) through the layers of developing cells to their final targets with which they form specific connections, called synapses that allow complex functions such as learning and memory. It is important to realize that developmental processes continue throughout our lives as some cells in our bodies are born, migrate, mature and die on a daily basis. The continuous renewal of skin cells (keratinocytes) and intestinal (epithelial) cells are two of many examples. Thus, the process of cell migration literally accompanies us from conception to death.

Immunity and wound healing are two homeostatic processes that often occur together and rely on the ability of cells to migrate. For example, when you cut yourself, the process of wound healing is initiated to repair the damage. This entails the proliferation and migration of nearby resident cells to fill the wound, as well as the recruitment of immune cells to dispose of invading bacteria and other microorganisms. Bacteria that enter through the wound are engulfed by white blood cells (leukocytes) and are destroyed by their potent digestive enzymes. Immune cells are constantly on surveillance duty, circulating throughout the body and migrating through tissue looking for foreign material to attack and destroy. It is important for these cells to develop a sense of ‘self’ so that they can recognize the body's own cells and not destroy them. This sense of self is established early on in their development as they migrate through the primary lymphoid tissues of the bone marrow and thymus.

Several proteins that regulate cell migration are critical for embryonic and foetal development. Defects in these proteins can be manifest very early on and can lead to the failure of blastocyst implantation into the uterus, resulting in early loss of pregnancy. At later stages in development, defects in proteins regulating migration can result in malformed embryos with disorganized tissues because their component cells have failed to travel to the correct location or, despite having travelled along the right path, they fail to form the appropriate connections with neighbouring cells and their surroundings. Abnormalities that do not result in early foetal death can lead to a number of congenital abnormalities in brain development — such as epilepsy, focal neurological deficits and mental retardation— and heart development.

Cell migration also has a central role in chronic inflammatory conditions. Asthma is a chronic inflammation of the airways which results from an ongoing immune response to foreign materials (allergens) inhaled from the environment. The constant presence and activation of white blood cells in the airways of asthmatics causes tissue damage, leading to hyper-reactivity of the airways to otherwise innocuous stimuli such as exercise, stress and cold air. In rheumatoid arthritis, the constant destruction of joint tissue by inflammatory cells migrating into these compartments results in compromised limb function and crippling pain.

Cell migration also contributes to the formation of metastasis. Cancer cells migrate as single cells or in small groups to spread from the initial site of tumour growth. They acquire an invasive phenotype characterized by both the loss of cell-cell interactions and increased cell motility. These cells are able to enter the blood or lymph vessels (intravasation) and cross the vessel wall to exit the vasculature (extravasation) in distal organs where they can continue to proliferate forming a second tumour mass. Cancer cell migration is typically regulated by integrins, matrix-degrading enzymes, and cell–cell adhesion molecules. Several cytokines and growth factors have been shown stimulate invasion and to be upregulated in a variety of tumour types.

Finally, the migration and proliferation of vascular smooth muscle cells is a key event in progressive vessel thickening leading to atherosclerosis and other vascular diseases. Vascular injury leads to endothelial dysfunction, which in turn promotes the expression of inflammatory markers and transendothelial leukocyte migration. Recruitment of leukocytes from the blood stream into the vessel intima is a crucial step for the development of fibrous plaques. Cytokines are among the molecules known to upregulate endothelial cell adhesion molecules, recruit leukocytes and induce smooth muscle cell migration and proliferation.

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