Amoeboid movement

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Two common modes of amoeboid motility WikiPic4.png
Two common modes of amoeboid motility

Amoeboid movement is the most typical mode of locomotion in adherent eukaryotic cells. [1] It is a crawling-like type of movement accomplished by protrusion of cytoplasm of the cell involving the formation of pseudopodia ("false-feet") and posterior uropods. One or more pseudopodia may be produced at a time depending on the organism, but all amoeboid movement is characterized by the movement of organisms with an amorphous form that possess no set motility structures. [2]

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Movement occurs when the cytoplasm slides and forms a pseudopodium in front to pull the cell forward. Some examples of organisms that exhibit this type of locomotion are amoebae (such as Amoeba proteus and Naegleria gruberi , [2] ) and slime molds, as well as some cells in humans such as leukocytes. Sarcomas, or cancers arising from connective tissue cells, are particularly adept at amoeboid movement, thus leading to their high rate of metastasis.

This type of movement has been linked to changes in action potential. While several hypotheses have been proposed to explain the mechanism of amoeboid movement, its exact mechanisms are not yet well understood. [3] [4] Assembly and disassembly of actin filaments in cells may be important to the biochemical and biophysical mechanisms that contribute to different types of cellular movements in both striated muscle structures and nonmuscle cells. [5] [6] Polarity gives cells distinct leading and lagging edges through the shifting of proteins selectively to the poles, and may play an important role in eukaryotic chemotaxis. [7] [8]

Types of amoeboid motion

Diagram of the three main kinds of amoeboid cell movement Types of amoeboid movement.tif
Diagram of the three main kinds of amoeboid cell movement

Crawling

Crawling is one form of amoeboid movement which starts when an extension of the moving cell (pseudopod) binds tightly to the surface. [9] [10] The main bulk of the cell pulls itself toward the bound patch. By repeating this process the cell can move until the first bound patch is at the very end of the cell, at which point it detaches. [9] [10] The speed at which cells crawl can vary greatly, but generally crawling is faster than swimming, but slower than gliding on a smooth surface. [9] However, crawling does not become notably slower on uneven and irregular surfaces, while gliding becomes much slower under such conditions. [9] It seems that crawling can be either bleb-driven or actin-driven (see sections below), depending on the nature of the surface. [10]

Gliding

Gliding is similar to crawling, but is characterized by much less adhesion to the surface, making it faster on smoother surfaces which require less traction but slower on more difficult and complicated surfaces. [9] Some cells glide with the same mechanism as crawling, but with larger pseudopods and less surface adhesion. [9] Other cells use a different method to glide: a small patch of the cell already touching the surface binds to the surface, after which the cytoskeleton pushes or pulls on the anchored patch to slide the cell forward. [11] This differs from the aforementioned mechanism in that the cell does not extend a pseudopod, so there is relatively little deformation of the cell as it progresses. [11]

Swimming

Many different prokaryotic and eukaryotic cells can swim and many of these have either flagella or cilia for that purpose. These dedicated structures are not necessary for swimming, though, as there are amoeba and other eukaryotic cells which lack flagella and cilia but can still swim, although it is slower than crawling or gliding. [9] [10] [12] There are two different proposed mechanisms for amoeboid swimming. In the first the cell extends small pseudopods which then move down the sides of the cell, acting like paddles. [9] [10] [12] In the second the cell generates an internal flow cycle, with the cytoplasm flowing backward along the membrane edge and forward through the middle, generating a force on the membrane which moves the cell forward. [10] [12]

Molecular mechanism of cell motion

Sol-gel theory

The protoplasm of an amoeba is made up of an outer layer termed the ectoplasm which surrounds an inner portion called the endoplasm. The ectoplasm consists of a gelatinous semisolid called plasma gel whereas the endoplasm is made up of a less viscous fluid called plasma sol. The ectoplasm owes its highly viscous state, in part, to the cross-linking actomyosin complex. Locomotion of an amoeba is thought to occur due to the sol-gel conversion of the protoplasm within its cell. 'Sol-gel conversion describes the contraction and relaxation events which are enforced by osmotic pressure and other ionic charges.' [13]

For example, when an amoeba moves, it extends a gelatinous, cytosolic pseudopodium, which then results in the more fluid cytosol (plasma sol) flowing after the gelatinous portion (plasma gel) where it congeals at the end of the pseudopodium. This results in the extension of this appendage. On the opposite (posterior) end of the cell, plasma gel is then converted to plasma sol and streamed toward the advancing pseudopodium. As long as the cell has a way to grapple the substratum, repetition of this process guides the cell forward. Inside the amoeba, there are proteins that can be activated to convert the gel into the more liquid sol state.

Cytoplasm consist largely of actin and actin is regulated by actin-binding protein. Actin binding proteins are in turn regulated by calcium ions; hence, calcium ions are very important in the sol-gel conversion process. [1] [13]

Amoeboid movement modalities

Actin-driven motility

Based on some mathematical models, recent studies hypothesize a novel biological model for collective biomechanical and molecular mechanisms of cellular motion. [14] It is proposed that microdomains weave the texture of cytoskeleton and their interactions mark the location for formation of new adhesion sites. According to this model, microdomain signaling dynamics organize the cytoskeleton and its interaction with the substratum. As microdomains trigger and maintain active polymerization of actin filaments, their propagation and zigzagging motion on the membrane generate a highly interlinked network of curved or linear filaments oriented at a wide spectrum of angles to the cell boundary. It has also been proposed that microdomain interaction marks the formation of new focal adhesion sites at the cell periphery. The interaction of myosin with the actin network then generates membrane retraction/ruffling, retrograde flow, and contractile forces for forward motion. Finally, continuous application of stress on the old focal adhesion sites could result in the calcium-induced activation of calpain, and consequently the detachment of focal adhesions which completes the cycle.

In addition to actin polymerization, microtubules may also play an important role in cell migration where the formation of lamellipodia is involved. One experiment showed that although microtubules are not required for actin polymerization to create lamellipodial extensions, they are needed in order to afford cellular movement. [15]

Bleb-driven motility

Another such proposed mechanism, the 'bleb-driven amoeboid locomotion' mechanism, suggests that the cell cortex actomyosin contracts to increase hydrostatic pressure inside the cell. Blebbing occurs in amoeboid cells when there is a roughly spherical protrusion in the cell membrane characterized by detachment from the actomyosin cortex. This mode of amoeboid movement requires that myosin II play a role in generating the hydrostatic pressure that causes the bleb to extend. [16]  This is different from actin-driven locomotion where the protrusion created is by the actin polymerizing while remaining attached to the actomyosin cortex and physically pushing against the cell's barrier. During the bleb-driven amoeboid movement, the cytoplasmic sol-gel state is regulated. [1]

Blebbing can also be a sign of when a cell is undergoing apoptosis. [17]

It has also been observed that the blebs formed by motile cells undergo a roughly uniform life cycle that lasts approximately one minute. This includes a phase involving the initial outward expansion where the membrane breaks away from the membranous cytoskeleton. This is then followed by a short static phase where the hydrostatic pressure that has built up is just enough to maintain the size of the bleb. Following this is the last phase characterized by the bleb slowly retracting and the membrane being reintroduced to the cytoskeleton infrastructure. [18]

Cells may undergo fast transitions between blebbing and lamellipodium-based motility as a means of migration. However, the rate at which these transitions are made is still unknown. Tumor cells may also exhibit rapid transitions between amoeboid motility and mesenchymal motility, another form of cellular movement. [19]

Dictyostelium cells and neutrophils can also swim, using a similar mechanism as for crawling. [9] [20]

Another unicellular form of movement shown in Euglena is known as metaboly. The basis of sol gel theory is interconversion of sol and gel.

See also

Related Research Articles

<span class="mw-page-title-main">Pseudopodia</span> False leg found on slime molds, archaea, protozoans, leukocytes and certain bacteria

A pseudopod or pseudopodium is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and may also contain microtubules and intermediate filaments. Pseudopods are used for motility and ingestion. They are often found in amoebas.

<span class="mw-page-title-main">Cytoskeleton</span> Network of filamentous proteins that forms the internal framework of cells

The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. It is composed of three main components, microfilaments, intermediate filaments and microtubules, and these are all capable of rapid growth or disassembly dependent on the cell's requirements.

<span class="mw-page-title-main">Microfilament</span> Filament in the cytoplasm of eukaryotic cells

Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other proteins in the cell. Microfilaments are usually about 7 nm in diameter and made up of two strands of actin. Microfilament functions include cytokinesis, amoeboid movement, cell motility, changes in cell shape, endocytosis and exocytosis, cell contractility, and mechanical stability. Microfilaments are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. In inducing cell motility, one end of the actin filament elongates while the other end contracts, presumably by myosin II molecular motors. Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement. Microfilaments have a tough, flexible framework which helps the cell in movement.

Mechanotaxis refers to the directed movement of cell motility via mechanical cues. In response to fluidic shear stress, for example, cells have been shown to migrate in the direction of the fluid flow. Mechanotaxis is critical in many normal biological processes in animals, such as gastrulation, inflammation, and repair in response to a wound, as well as in mechanisms of diseases such as tumor metastasis.

<span class="mw-page-title-main">Motility</span> Ability to move using metabolic energy

Motility is the ability of an organism to move independently, using metabolic energy.

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. Cells often migrate in response to specific external signals, including chemical signals and mechanical signals. Errors during this process have serious consequences, including intellectual disability, 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.

<span class="mw-page-title-main">Focal adhesion</span>

In cell biology, focal adhesions are large macromolecular assemblies through which mechanical force and regulatory signals are transmitted between the extracellular matrix (ECM) and an interacting cell. More precisely, focal adhesions are the sub-cellular structures that mediate the regulatory effects of a cell in response to ECM adhesion.

<span class="mw-page-title-main">Podosome</span>

Podosomes are conical, actin-rich structures found on the outer surface of the plasma membrane of animal cells. Their size ranges from approximately 0.5 µm to 2.0 µm in diameter. While usually situated on the periphery of the cellular membrane, these unique structures display a polarized pattern of distribution in migrating cells, situating at the front border between the lamellipodium and lamellum. Their primary purpose is connected to cellular motility and invasion; therefore, they serve as both sites of attachment and degradation along the extracellular matrix. Many different specialized cells exhibit these dynamic structures such as invasive cancer cells, osteoclasts, vascular smooth muscle cells, endothelial cells, and certain immune cells like macrophages and dendritic cells.

<span class="mw-page-title-main">Endoplasm</span> Also known as entoplasm

Endoplasm generally refers to the inner, dense part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which is typically watery and immediately adjacent to the plasma membrane. The nucleus is separated from the endoplasm by the nuclear envelope. The different makeups/viscosities of the endoplasm and ectoplasm contribute to the amoeba's locomotion through the formation of a pseudopod. However, other types of cells have cytoplasm divided into endo- and ectoplasm. The endoplasm, along with its granules, contains water, nucleic acids, amino acids, carbohydrates, inorganic ions, lipids, enzymes, and other molecular compounds. It is the site of most cellular processes as it houses the organelles that make up the endomembrane system, as well as those that stand alone. The endoplasm is necessary for most metabolic activities, including cell division.

<span class="mw-page-title-main">Spectrin</span>

Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffold and playing an important role in maintenance of plasma membrane integrity and cytoskeletal structure. The hexagonal arrangements are formed by tetramers of spectrin subunits associating with short actin filaments at either end of the tetramer. These short actin filaments act as junctional complexes allowing the formation of the hexagonal mesh. The protein is named spectrin since it was first isolated as a major protein component of human red blood cells which had been treated with mild detergents; the detergents lysed the cells and the hemoglobin and other cytoplasmic components were washed out. In the light microscope the basic shape of the red blood cell could still be seen as the spectrin-containing submembranous cytoskeleton preserved the shape of the cell in outline. This became known as a red blood cell "ghost" (spectre), and so the major protein of the ghost was named spectrin.

<span class="mw-page-title-main">Cell cortex</span> Layer on the inner face of a cell membrane

The cell cortex, also known as the actin cortex, cortical cytoskeleton or actomyosin cortex, is a specialized layer of cytoplasmic proteins on the inner face of the cell membrane. It functions as a modulator of membrane behavior and cell surface properties. In most eukaryotic cells lacking a cell wall, the cortex is an actin-rich network consisting of F-actin filaments, myosin motors, and actin-binding proteins. The actomyosin cortex is attached to the cell membrane via membrane-anchoring proteins called ERM proteins that plays a central role in cell shape control. The protein constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic, two properties essential to its function. In most cases, the cortex is in the range of 100 to 1000 nanometers thick.

<span class="mw-page-title-main">Major sperm protein</span>

Major sperm protein (MSP) is a nematode specific small protein of 126 amino acids with a molecular weight of 14 kDa. It is the key player in the motility machinery of nematodes that propels the crawling movement/motility of nematode sperm. It is the most abundant protein present in nematode sperm, comprising 15% of the total protein and more than 40% of the soluble protein. MSP is exclusively synthesized in spermatocytes of the nematodes. The MSP has two main functions in the reproduction of the helminthes: i) as cytosolic component it is responsible for the crawling movement of the mature sperm, and ii) once released, it acts as hormone on the female germ cells, where it triggers oocyte maturation and stimulates the oviduct wall to contract to bring the oocytes into position for fertilization. MSP has first been identified in Caenorhabditis elegans.

When molecules on the surface of a motile eukaryotic cell are crosslinked, they are moved to one end of the cell to form a "cap". This phenomenon, the process of which is called cap formation, was discovered in 1971 on lymphocytes and is a property of amoebae and all locomotory animal cells except sperm. The crosslinking is most easily achieved using a polyvalent antibody to a surface antigen on the cell. Cap formation can be visualised by attaching a fluorophore, such as fluorescein, to the antibody.

<span class="mw-page-title-main">Bleb (cell biology)</span> Bulge in the plasma membrane of a cell

In cell biology, a bleb is a bulge of the plasma membrane of a cell, characterized by a spherical, "blister-like", bulky morphology. It is characterized by the decoupling of the cytoskeleton from the plasma membrane, degrading the internal structure of the cell, allowing the flexibility required for the cell to separate into individual bulges or pockets of the intercellular matrix. Most commonly, blebs are seen in apoptosis but are also seen in other non-apoptotic functions. Blebbing, or zeiosis, is the formation of blebs.

The Rho family of GTPases is a family of small signaling G proteins, and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found in all eukaryotic kingdoms, including yeasts and some plants. Three members of the family have been studied in detail: Cdc42, Rac1, and RhoA. All G proteins are "molecular switches", and Rho proteins play a role in organelle development, cytoskeletal dynamics, cell movement, and other common cellular functions.

<span class="mw-page-title-main">Gliding motility</span>

Gliding motility is a type of translocation used by microorganisms that is independent of propulsive structures such as flagella, pili, and fimbriae. Gliding allows microorganisms to travel along the surface of low aqueous films. The mechanisms of this motility are only partially known.

<span class="mw-page-title-main">Rho-associated protein kinase</span>

Rho-associated protein kinase (ROCK) is a kinase belonging to the AGC family of serine-threonine specific protein kinases. It is involved mainly in regulating the shape and movement of cells by acting on the cytoskeleton.

<span class="mw-page-title-main">Cell membrane</span> Biological membrane that separates the interior of a cell from its outside environment

The cell membrane is a biological membrane that separates and protects the interior of a cell from the outside environment. The cell membrane consists of a lipid bilayer, made up of two layers of phospholipids with cholesterols interspersed between them, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins, including integral proteins that span the membrane and serve as membrane transporters, and peripheral proteins that loosely attach to the outer (peripheral) side of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose. The cell membrane controls the movement of substances in and out of a cell, being selectively permeable to ions and organic molecules. In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity, and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall and the carbohydrate layer called the glycocalyx, as well as the intracellular network of protein fibers called the cytoskeleton. In the field of synthetic biology, cell membranes can be artificially reassembled.

Uropods, in immunology, refer to the hind part of polarized cells during cell migration that stabilize and move the cell. Polarized leukocytes move using amoeboid cell migration mechanisms, with a small leading edge, main cell body, and posterior uropod protrusion. Cytoskeleton contraction and extension, controlled by various polarized signals, helps propel the cell body forward. Leukocyte polarization is an important requirement for migration, activation and apoptosis in the adaptive and innate immune systems; most leukocytes, including monocytes, granulocytes, and T and B lymphocytes migrate to and from primary and secondary lymphoid organs to tissues to initiate immune responses to pathogens.

<span class="mw-page-title-main">Amoeba</span> Polyphyletic group of unicellular eukaryotes with the ability to shapeshift

An amoeba, often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebae do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.

References

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