Pseudopodia

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This article is about eukaryotic cells. For the band, see Pseudopod (band). For the podcast, see Pseudopod (podcast). For the structure in insect anatomy, see Proleg.
Amoeba proteus extending lobose pseudopodia

A pseudopod or pseudopodium (pl.: pseudopods or pseudopodia) is a temporary arm-like projection of a

actin filaments and may also contain microtubules and intermediate filaments.[1][2] Pseudopods are used for motility and ingestion. They are often found in amoebas
.

Different types of pseudopodia can be classified by their distinct appearances.

Reticulopodia are complex structures bearing individual pseudopodia which form irregular nets. Axopodia are the phagocytosis type with long, thin pseudopods supported by complex microtubule arrays enveloped with cytoplasm; they respond rapidly to physical contact.[4]

Generally, several pseudopodia arise from the surface of the body, (polypodial, for example, Amoeba proteus), or a single pseudopod may form on the surface of the body (monopodial, such as Entamoeba histolytica).[5]

Formation

Cells which make pseudopods are generally referred to as

amoeboids.[6]

Via extracellular cue

To move towards a target, the cell uses chemotaxis. It senses extracellular signalling molecules, chemoattractants (e.g. cAMP for Dictyostelium cells),[7] to extend pseudopodia at the membrane area facing the source of these molecules.

The chemoattractants bind to G protein-coupled receptors, which activate GTPases of the Rho family (e.g. Cdc42, Rac) via G proteins.

Rho GTPases are able to activate WASp which in turn activate Arp2/3 complex which serve as nucleation sites for actin polymerization.[8] The actin polymers then push the membrane as they grow, forming the pseudopod. The pseudopodium can then adhere to a surface via its adhesion proteins (e.g. integrins), and then pull the cell's body forward via contraction of an actin-myosin complex in the pseudopod.[9][10] This type of locomotion is called amoeboid movement.

Rho GTPases can also activate

phosphatidylinositol 3-kinase (PI3K) which recruit PIP3 to the membrane at the leading edge and detach the PIP3-degrading enzyme PTEN from the same area of the membrane. PIP3 then activate GTPases back via GEF stimulation. This serves as a feedback loop to amplify and maintain the presence of local GTPase at the leading edge.[8]

Otherwise, pseudopodia cannot grow on other sides of the membrane than the leading edge because myosin filaments prevent them to extend. These myosin filaments are induced by

Rho kinase in neutrophils for example.[8]

Different physical parameters were shown to regulate the length and time-scale of pseudopodia formation. For example, an increase in membrane tension inhibits actin assembly and protrusion formation.[11] It was demonstrated that the lowered negative surface charge on the inner surface of the plasma membrane generates protrusions via activation of the Ras-PI3K/AKT/mTOR signalling pathway.[12]

Without extracellular cue

In the case there is no extracellular cue, all moving cells navigate in random directions, but they can keep the same direction for some time before turning. This feature allows cells to explore large areas for colonization or searching for a new extracellular cue.

In Dictyostelium cells, a pseudopodium can form either de novo as normal, or from an existing pseudopod, forming a Y-shaped pseudopodium.

The Y-shaped pseudopods are used by Dictyostelium to advance relatively straight forward by alternating between retraction of the left or right branch of the pseudopod. The de novo pseudopodia form at different sides than pre-existing ones, they are used by the cells to turn.

Y-shaped pseudopods are more frequent than de novo ones, which explain the preference of the cell to keep moving to the same direction. This persistence is modulated by PLA2 and cGMP signalling pathways.[7]

Functions

The functions of pseudopodia include locomotion and ingestion:

  • Pseudopodia are critical in sensing targets which can then be engulfed; the engulfing pseudopodia are called phagocytosis pseudopodia. A common example of this type of amoeboid cell is the macrophage.
  • They are also essential to amoeboid-like locomotion. Human
    trilaminar germ disc during gastrulation.[13]

Morphology

The forms of pseudopodia, from left: polypodial and lobose; monopodial and lobose; filose; conical; reticulose; tapering actinopods; non-tapering actinopods

Pseudopods can be classified into several varieties according to the number of projections (monopodia and polypodia), and according to their appearance.

Some pseudopodial cells are able to use multiple types of pseudopodia depending on the situation. Most use a combination of

lamellipodia and filopodia to migrate[14] (e.g. metastatic cancer cells).[15] Human foreskin fibroblasts can either use lamellipodia- or lobopodia-based migration in a 3D matrix depending on the matrix elasticity.[16]

Lamellipodia

Lamellipodia are broad and flat pseudopodia used in locomotion.[4] They are supported by microfilaments which form at the leading edge, creating a mesh-like internal network.[17]

Filopodia

Filopodia (or filose pseudopods) are slender and filiform with pointed ends, consisting mainly of ectoplasm. These formations are supported by microfilaments which, unlike the filaments of lamellipodia with their net-like actin, form loose bundles by cross-linking. This formation is partly due to bundling proteins such as fimbrins and fascins.[17][18] Filopodia are observed in some animal cells: in part of

Opisthokonta).[4]

Lobopodia

Lobopodia (or lobose pseudopods) are bulbous, short, and blunt in form.

Heterolobosea (Excavata
).

High-pressure lobopodia can also be found in human

RhoA, ROCK and myosin II
. Otherwise, lobopods are often accompanied with small lateral blebs forming along the side of the cell, probably due to the high intracellular pressure during lobopodia formation increasing the frequency of plasma membrane-cortex rupture.[20][16][21]

Reticulopodia

Reticulopodia (or reticulose pseudopods),

Filoreta (Rhizaria).[4]

Axopodia

Axopodia (also known as actinopodia) are narrow pseudopodia containing complex arrays of microtubules enveloped by cytoplasm. Axopodia are mostly responsible for phagocytosis by rapidly retracting in response to physical contact. These pseudopodia are primarily food-collecting structures, but also provide a means of hydrological transportation via the expansion of their surface areas. They are observed in "Radiolaria" and "Heliozoa".[4]

References

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  3. ^ Patterson, David J. "Amoebae: Protists Which Move and Feed Using Pseudopodia". tolweb.org. Tree of Life Web Project. Retrieved 2017-11-12.
  4. ^ a b c d e "Pseudopodia". Arcella.nl. May 23, 2017. Archived from the original on 2018-12-16. Retrieved 2018-12-16.{{cite web}}: CS1 maint: unfit URL (link)
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  6. ^ "Pseudopodia". Encyclopedia.com. Retrieved 2018-12-16.
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  13. ^ Schoenwolf, Gary (2009). Larsen's Human Embryology (4th ed.). Churchill Livingstone Elsevier.
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  15. S2CID 46438967
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  16. ^
    PMID 22547408
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  17. ^ a b Bray, Dennis (2001). Cell Movements: From molecules to motility second edition.
  18. PMID 16966425
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  19. ^ "Pseudopodium | cytoplasm". Encyclopedia Britannica. Retrieved 2018-12-16.
  20. PMID 29551161
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  21. PMID 27998990
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  22. ^ "Reticulopodia". eForams. Archived from the original on 2007-07-17. Retrieved 2005-12-30.
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