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Filopodia

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"Microspikes" redirects here. For the hiking footgear, see Crampons § Microspikes.
This electron micrograph shows exaggerated filopodia with club-like shape induced by formin mDia2 in cultured cells. These filopodia are filled with bundled actin filaments which were born in and converged from the lamellipodial network.

Filopodia (sg.: filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells.[1] Within the lamellipodium, actin ribs are known as microspikes, and when they extend beyond the lamellipodia, they're known as filopodia.[2] They contain microfilaments (also called actin filaments) cross-linked into bundles by actin-bundling proteins,[3] such as fascin and fimbrin.[4] Filopodia form focal adhesions with the substratum, linking them to the cell surface.[5] Many types of migrating cells display filopodia, which are thought to be involved in both sensation of chemotropic cues, and resulting changes in directed locomotion.

Activation of the Rho family of GTPases, particularly Cdc42 and their downstream intermediates, results in the polymerization of actin fibers by Ena/Vasp homology proteins.[6] Growth factors bind to receptor tyrosine kinases resulting in the polymerization of actin filaments, which, when cross-linked, make up the supporting cytoskeletal elements of filopodia. Rho activity also results in activation by phosphorylation of ezrin-moesin-radixin family proteins that link actin filaments to the filopodia membrane.[6]

Filopodia have roles in sensing, migration, neurite outgrowth, and cell-cell interaction.[1][further explanation needed] To close a wound in vertebrates, growth factors stimulate the formation of filopodia in fibroblasts to direct fibroblast migration and wound closure.[7] In macrophages, filopodia act as phagocytic tentacles, pulling bound objects towards the cell for phagocytosis.[8]

Functions and variants

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Many cell types have filopodia.[citation needed] The functions of filopodia have been attributed to pathfinding of neurons,[9] early stages of synapse formation,[10] antigen presentation by dendritic cells of the immune system,[11] force generation by macrophages[12] and virus transmission.[13] They have been associated with wound closure,[14] dorsal closure of Drosophila embryos,[15] chemotaxis in Dictyostelium,[16] Delta-Notch signaling,[17][18] vasculogenesis,[19] cell adhesion,[20] cell migration, and cancer metastasis. Specific kinds of filopodia have been given various names:[citation needed] microspikes, pseudopods, thin filopodia,[21] thick filopodia,[22] gliopodia,[23] myopodia,[24] invadopodia,[25] podosomes,[26] telopodes,[27] tunneling nanotubes[28] and dendrites.

In infections

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Filopodia are also used for movement of bacteria between cells, so as to evade the host immune system. The intracellular bacteria Ehrlichia are transported between cells through the host cell filopodia induced by the pathogen during initial stages of infection.[29] Filopodia are the initial contact that human retinal pigment epithelial (RPE) cells make with elementary bodies of Chlamydia trachomatis, the bacteria that causes chlamydia.[30]

Viruses have been shown to be transported along filopodia toward the cell body, leading to cell infection.[31] Directed transport of receptor-bound epidermal growth factor (EGF) along filopodia has also been described, supporting the proposed sensing function of filopodia.[32]

SARS-CoV-2, the strain of coronavirus responsible for COVID-19, produces filopodia in infected cells.[33]

In brain cells

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In developing neurons, filopodia extend from the growth cone at the leading edge. In neurons deprived of filopodia by partial inhibition of actin filaments polymerization, growth cone extension continues as normal, but direction of growth is disrupted and highly irregular.[7] Filopodia-like projections have also been linked to dendrite creation when new synapses are formed in the brain.[34][35]

A study deploying protein imaging of adult mice showed that filopodia in the explored regions were by an order of magnitude more abundant than previously believed, comprising about 30% of all dendritic protrusions. At their tips, they contain "silent synapses" that are inactive until recruited as part of neural plasticity and flexible learning or memories, previously thought to be present mainly in the developing pre-adult brain and to die off with time.[36][37][further explanation needed]

References

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  2. ^ Small JV, Stradal T, Vignal E, Rottner K (March 2002). "The lamellipodium: where motility begins". Trends in Cell Biology. 12 (3): 112–120. doi:10.1016/S0962-8924(01)02237-1. PMID 11859023.
  3. ^ Khurana S, George SP (September 2011). "The role of actin bundling proteins in the assembly of filopodia in epithelial cells". Cell Adhesion & Migration. 5 (5): 409–420. doi:10.4161/cam.5.5.17644. PMC 3218608. PMID 21975550.
  4. ^ Hanein D, Matsudaira P, DeRosier DJ (October 1997). "Evidence for a conformational change in actin induced by fimbrin (N375) binding". The Journal of Cell Biology. 139 (2): 387–396. doi:10.1083/jcb.139.2.387. PMC 2139807. PMID 9334343.
  5. ^ Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J, eds. (2004). Molecular Cell Biology (fifth ed.). W.H. Freeman and Company. pp. 821, 823.
  6. ^ a b Ohta Y, Suzuki N, Nakamura S, Hartwig JH, Stossel TP (March 1999). "The small GTPase RalA targets filamin to induce filopodia". Proceedings of the National Academy of Sciences of the United States of America. 96 (5): 2122–2128. Bibcode:1999PNAS...96.2122O. doi:10.1073/pnas.96.5.2122. PMC 26747. PMID 10051605.
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  8. ^ Kress H, Stelzer EH, Holzer D, Buss F, Griffiths G, Rohrbach A (July 2007). "Filopodia act as phagocytic tentacles and pull with discrete steps and a load-dependent velocity". Proceedings of the National Academy of Sciences of the United States of America. 104 (28): 11633–11638. Bibcode:2007PNAS..10411633K. doi:10.1073/pnas.0702449104. PMC 1913848. PMID 17620618.
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  17. ^ Cohen M, Georgiou M, Stevenson NL, Miodownik M, Baum B (July 2010). "Dynamic filopodia transmit intermittent Delta-Notch signaling to drive pattern refinement during lateral inhibition". Developmental Cell. 19 (1): 78–89. doi:10.1016/j.devcel.2010.06.006. PMID 20643352.
  18. ^ Berkemeier F, Page, KM (June 2023). "Coupling dynamics of 2D Notch-Delta signalling". Mathematical Biosciences. 360 (1). doi:10.1016/j.mbs.2023.109012. PMID 37142213.
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  21. ^ Miller J, Fraser SE, McClay D (August 1995). "Dynamics of thin filopodia during sea urchin gastrulation". Development. 121 (8): 2501–11. doi:10.1242/dev.121.8.2501. PMID 7671814.
  22. ^ McClay DR (December 1999). "The role of thin filopodia in motility and morphogenesis". Experimental Cell Research. 253 (2): 296–301. doi:10.1006/excr.1999.4723. PMID 10585250.
  23. ^ Vasenkova I, Luginbuhl D, Chiba A (January 2006). "Gliopodia extend the range of direct glia-neuron communication during the CNS development in Drosophila". Molecular and Cellular Neurosciences. 31 (1): 123–30. doi:10.1016/j.mcn.2005.10.001. PMID 16298140. S2CID 39541898.
  24. ^ Ritzenthaler S, Suzuki E, Chiba A (October 2000). "Postsynaptic filopodia in muscle cells interact with innervating motoneuron axons". Nature Neuroscience. 3 (10): 1012–7. doi:10.1038/79833. PMID 11017174. S2CID 23718828.
  25. ^ Chen WT (August 1989). "Proteolytic activity of specialized surface protrusions formed at rosette contact sites of transformed cells". The Journal of Experimental Zoology. 251 (2): 167–85. doi:10.1002/jez.1402510206. PMID 2549171.
  26. ^ Tarone G, Cirillo D, Giancotti FG, Comoglio PM, Marchisio PC (July 1985). "Rous sarcoma virus-transformed fibroblasts adhere primarily at discrete protrusions of the ventral membrane called podosomes". Experimental Cell Research. 159 (1): 141–57. doi:10.1016/S0014-4827(85)80044-6. PMID 2411576.
  27. ^ Popescu LM, Faussone-Pellegrini MS (April 2010). "TELOCYTES - a case of serendipity: the winding way from Interstitial Cells of Cajal (ICC), via Interstitial Cajal-Like Cells (ICLC) to TELOCYTES". Journal of Cellular and Molecular Medicine. 14 (4): 729–40. doi:10.1111/j.1582-4934.2010.01059.x. PMC 3823108. PMID 20367664.
  28. ^ Rustom A, Saffrich R, Markovic I, Walther P, Gerdes HH (February 2004). "Nanotubular highways for intercellular organelle transport". Science. 303 (5660): 1007–10. Bibcode:2004Sci...303.1007R. doi:10.1126/science.1093133. PMID 14963329. S2CID 37863055.
  29. ^ Thomas S, Popov VL, Walker DH (December 2010). "Exit mechanisms of the intracellular bacterium Ehrlichia". PLOS ONE. 5 (12): e15775. Bibcode:2010PLoSO...515775T. doi:10.1371/journal.pone.0015775. PMC 3004962. PMID 21187937.
  30. ^ Ford C, Nans A, Boucrot E, Hayward RD (May 2018). Welch MD (ed.). "Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry". PLOS Pathogens. 14 (5): e1007051. doi:10.1371/journal.ppat.1007051. PMC 5955597. PMID 29727463.
  31. ^ Lehmann MJ, Sherer NM, Marks CB, Pypaert M, Mothes W (July 2005). "Actin- and myosin-driven movement of viruses along filopodia precedes their entry into cells". The Journal of Cell Biology. 170 (2): 317–325. doi:10.1083/jcb.200503059. PMC 2171413. PMID 16027225.
  32. ^ Lidke DS, Lidke KA, Rieger B, Jovin TM, Arndt-Jovin DJ (August 2005). "Reaching out for signals: filopodia sense EGF and respond by directed retrograde transport of activated receptors". The Journal of Cell Biology. 170 (4): 619–626. doi:10.1083/jcb.200503140. PMC 2171515. PMID 16103229.
  33. ^ Bouhaddou M, Memon D, Meyer B, White KM, Rezelj VV, Correa Marrero M, et al. (August 2020). "The Global Phosphorylation Landscape of SARS-CoV-2 Infection". Cell. 182 (3): 685–712.e19. doi:10.1016/j.cell.2020.06.034. PMC 7321036. PMID 32645325.
  34. ^ Beardsley J (June 1999). "Getting Wired". Scientific American. 280 (6): 24. Bibcode:1999SciAm.280f..24B. doi:10.1038/scientificamerican0699-24b (inactive 1 November 2024).{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
  35. ^ Maletic-Savatic M, Malinow R, Svoboda K (March 1999). "Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity". Science. 283 (5409): 1923–1927. doi:10.1126/science.283.5409.1923. PMID 10082466.
  36. ^ Lloreda, Claudia López (16 December 2022). "Adult mouse brains are teeming with 'silent synapses'". Retrieved 18 December 2022.
  37. ^ Vardalaki, Dimitra; Chung, Kwanghun; Harnett, Mark T. (December 2022). "Filopodia are a structural substrate for silent synapses in adult neocortex". Nature. 612 (7939): 323–327. Bibcode:2022Natur.612..323V. doi:10.1038/s41586-022-05483-6. ISSN 1476-4687. PMID 36450984. S2CID 254122483.
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