Biology:Filopodia

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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

Many cell types have filopodia. 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|date=September 2024} opodia,[21] thick filopodia,[22] gliopodia,[23] myopodia,[24] invadopodia,[25] podosomes,[26] telopodes,[27] tunneling nanotubes[28] and dendrites.

In infections

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

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]

References

  1. ↑ 1.0 1.1 "Filopodia: molecular architecture and cellular functions". Nature Reviews. Molecular Cell Biology 9 (6): 446–454. June 2008. doi:10.1038/nrm2406. PMID 18464790. https://www.utupub.fi/handle/10024/159021. 
  2. ↑ "The lamellipodium: where motility begins". Trends in Cell Biology 12 (3): 112–120. March 2002. doi:10.1016/S0962-8924(01)02237-1. PMID 11859023. 
  3. ↑ "The role of actin bundling proteins in the assembly of filopodia in epithelial cells". Cell Adhesion & Migration 5 (5): 409–420. September 2011. doi:10.4161/cam.5.5.17644. PMID 21975550. 
  4. ↑ "Evidence for a conformational change in actin induced by fimbrin (N375) binding". The Journal of Cell Biology 139 (2): 387–396. October 1997. doi:10.1083/jcb.139.2.387. PMID 9334343. 
  5. ↑ Molecular Cell Biology (fifth ed.). W.H. Freeman and Company. 2004. pp. 821, 823. 
  6. ↑ 6.0 6.1 "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. March 1999. doi:10.1073/pnas.96.5.2122. PMID 10051605. Bibcode: 1999PNAS...96.2122O. 
  7. ↑ 7.0 7.1 "Disoriented pathfinding by pioneer neurone growth cones deprived of filopodia by cytochalasin treatment". Nature 323 (6090): 712–715. 1986. doi:10.1038/323712a0. PMID 3773996. Bibcode: 1986Natur.323..712B. 
  8. ↑ "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. July 2007. doi:10.1073/pnas.0702449104. PMID 17620618. Bibcode: 2007PNAS..10411633K. 
  9. ↑ "Disoriented pathfinding by pioneer neurone growth cones deprived of filopodia by cytochalasin treatment". Nature 323 (6090): 712–5. 1986. doi:10.1038/323712a0. PMID 3773996. Bibcode: 1986Natur.323..712B. 
  10. ↑ "Genesis of dendritic spines: insights from ultrastructural and imaging studies". Nature Reviews. Neuroscience 5 (1): 24–34. January 2004. doi:10.1038/nrn1300. PMID 14708001. 
  11. ↑ "Functional analysis of B144/LST1: a gene in the tumor necrosis factor cluster that induces formation of long filopodia in eukaryotic cells". Experimental Cell Research 268 (2): 230–44. August 2001. doi:10.1006/excr.2001.5290. PMID 11478849. 
  12. ↑ "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): 11633–8. July 2007. doi:10.1073/pnas.0702449104. PMID 17620618. Bibcode: 2007PNAS..10411633K. 
  13. ↑ "Actin- and myosin-driven movement of viruses along filopodia precedes their entry into cells". The Journal of Cell Biology 170 (2): 317–25. July 2005. doi:10.1083/jcb.200503059. PMID 16027225. 
  14. ↑ "Epithelial wound closure in the rabbit cornea. A biphasic process". Investigative Ophthalmology & Visual Science 27 (4): 464–73. April 1986. PMID 3957565. http://iovs.arvojournals.org/article.aspx?volume=27&page=464. 
  15. ↑ "Dynamic actin-based epithelial adhesion and cell matching during Drosophila dorsal closure". Current Biology 10 (22): 1420–6. November 2000. doi:10.1016/S0960-9822(00)00796-X. PMID 11102803. Bibcode: 2000CBio...10.1420J. 
  16. ↑ "Requirement of a vasodilator-stimulated phosphoprotein family member for cell adhesion, the formation of filopodia, and chemotaxis in dictyostelium". The Journal of Biological Chemistry 277 (51): 49877–87. December 2002. doi:10.1074/jbc.M209107200. PMID 12388544. 
  17. ↑ "Dynamic filopodia transmit intermittent Delta-Notch signaling to drive pattern refinement during lateral inhibition". Developmental Cell 19 (1): 78–89. July 2010. doi:10.1016/j.devcel.2010.06.006. PMID 20643352. 
  18. ↑ "Coupling dynamics of 2D Notch-Delta signalling". Mathematical Biosciences 360 (1). June 2023. doi:10.1016/j.mbs.2023.109012. PMID 37142213. 
  19. ↑ "In vivo imaging of embryonic vascular development using transgenic zebrafish". Developmental Biology 248 (2): 307–18. August 2002. doi:10.1006/dbio.2002.0711. PMID 12167406. 
  20. ↑ "Directed actin polymerization is the driving force for epithelial cell-cell adhesion". Cell 100 (2): 209–19. January 2000. doi:10.1016/S0092-8674(00)81559-7. PMID 10660044. 
  21. ↑ "Dynamics of thin filopodia during sea urchin gastrulation". Development 121 (8): 2501–11. August 1995. doi:10.1242/dev.121.8.2501. PMID 7671814. http://dev.biologists.org/cgi/pmidlookup?view=long&pmid=7671814. 
  22. ↑ "The role of thin filopodia in motility and morphogenesis". Experimental Cell Research 253 (2): 296–301. December 1999. doi:10.1006/excr.1999.4723. PMID 10585250. 
  23. ↑ "Gliopodia extend the range of direct glia-neuron communication during the CNS development in Drosophila". Molecular and Cellular Neurosciences 31 (1): 123–30. January 2006. doi:10.1016/j.mcn.2005.10.001. PMID 16298140. 
  24. ↑ "Postsynaptic filopodia in muscle cells interact with innervating motoneuron axons". Nature Neuroscience 3 (10): 1012–7. October 2000. doi:10.1038/79833. PMID 11017174. 
  25. ↑ "Proteolytic activity of specialized surface protrusions formed at rosette contact sites of transformed cells". The Journal of Experimental Zoology 251 (2): 167–85. August 1989. doi:10.1002/jez.1402510206. PMID 2549171. Bibcode: 1989JEZ...251..167C. 
  26. ↑ "Rous sarcoma virus-transformed fibroblasts adhere primarily at discrete protrusions of the ventral membrane called podosomes". Experimental Cell Research 159 (1): 141–57. July 1985. doi:10.1016/S0014-4827(85)80044-6. PMID 2411576. 
  27. ↑ "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. April 2010. doi:10.1111/j.1582-4934.2010.01059.x. PMID 20367664. 
  28. ↑ "Nanotubular highways for intercellular organelle transport". Science 303 (5660): 1007–10. February 2004. doi:10.1126/science.1093133. PMID 14963329. Bibcode: 2004Sci...303.1007R. 
  29. ↑ "Exit mechanisms of the intracellular bacterium Ehrlichia". PLOS ONE 5 (12). December 2010. doi:10.1371/journal.pone.0015775. PMID 21187937. Bibcode: 2010PLoSO...515775T. 
  30. ↑ "Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry". PLOS Pathogens 14 (5). May 2018. doi:10.1371/journal.ppat.1007051. PMID 29727463. 
  31. ↑ "Actin- and myosin-driven movement of viruses along filopodia precedes their entry into cells". The Journal of Cell Biology 170 (2): 317–325. July 2005. doi:10.1083/jcb.200503059. PMID 16027225. 
  32. ↑ "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. August 2005. doi:10.1083/jcb.200503140. PMID 16103229. 
  33. ↑ "The Global Phosphorylation Landscape of SARS-CoV-2 Infection". Cell 182 (3): 685–712.e19. August 2020. doi:10.1016/j.cell.2020.06.034. PMID 32645325. 
  34. ↑ "Getting Wired". Scientific American 280 (6): 24. June 1999. doi:10.1038/scientificamerican0699-24b. Bibcode: 1999SciAm.280f..24B. 
  35. ↑ "Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity". Science 283 (5409): 1923–1927. March 1999. doi:10.1126/science.283.5409.1923. PMID 10082466. 

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